Role of myeloid-derived suppressor cells in tumor recurrence.

Introduction

When possible, solid tumors are treated with surgery at diagnosis. However, despite resection of the primary tumor, many patients have disseminated tumor cells (DTCs) at diagnosis. Over time, these DTCs become clinically or radiologically detectable as metastatic disease. Further, the majority of patients has detectable metastatic disease at diagnosis or later becomes refractory to intervention. In recent decades, screening programs have resulted in earlier detection and treatment for patients with breast, colorectal, and prostate cancers [ 1 , 2 ]. This has contributed to significant improvement in 5- and 10-year overall survival (OS) rates. In addition, new molecular targeted and immune therapies together with conventional treatments have improved the OS of these and other types of solid tumors [ 3 , 4 ]. However, the fatal outcomes for patients with metastatic disease have not changed. In recent years, research [ 5 – 7 ] has focused on DTCs, which are also known as “minimal residual disease” (MRD), as the origin for clinically overt metastases. The biology of MRD is different relative to that of primary tumors. These lesions often arise following prolonged dormancy and later “awaken” in response to intrinsic and/or tissue specific factors, somatic and therapeutic pressure-induced mutations, and therapeutic resistance. Thus, an improved understanding of the mechanisms underlying the biology of tumor cell release from dormancy is critical to prevent the progression of detectable metastatic disease. Immune surveillance is hypothesized to be one of the principal mechanisms of DTC dormancy and is associated with tumor cell arrest in the G0–G1 state. [ 8 , 9 ] As such, one clinical goal is to identify and develop novel therapeutic strategies targeting the eradication of dormant DTCs or maintaining them in an G0–G1 state. [ 10 ]

During tumor progression, the invasion and metastasis of tumor cells to distant organs can progress to overt metastases resulting in patient mortality. Clinically, it is well established that overt metastases are associated with poor patient outcomes [ 11 ]. However, many patients with no detectable metastases at diagnosis and curative surgery can later develop overt metastases months, years, or even decades later [ 12 ]. This latency period, during which cancer cells do not grow and remain in a quiescent or equilibrium state, is known as “cancer dormancy” [ 13 ]. In these patients, DTCs can remain in a latent state as micro-metastases, designated as “dormant metastases” for variable durations among tumors [ 14 ]. The theory of occult tumors as dormant metastatic lesions in different solid tumors is well-founded [ 15 ], dating back to the 1930s if not earlier. [ 16 , 17 ] In 1934, Australian pathologist Rupert Willis used the concept of DTC dormancy to explain recurrent metastases that occurred long after treatment of a patient’s primary disease. [ 18 ]

Although the concept of dormancy is relatively old, our biological and molecular understanding of this process and the mechanism of cellular release from dormancy is not fully understood. In 1954, Geoffrey Hadfield postulated that tumors that recurred years post excision at a metastatic site must have arisen from DTCs. Further, these DTCs were held in mitotic arrest, which led him to coin the term “dormant cancer cell” [ 19 ]. Multiple, independent lines of evidence support the concept that dormancy is the biological mechanism contributing to relapse. One is the finding that DTCs can be found in bone marrows (BM) of cancer patients at the time of their primary tumor resection [ 20 , 21 ]. Additionally, examples of DTC dormancy have come from organ transplantation studies, in which inadvertent transfer of malignant cells occurred between an organ donor and a recipient, even though the donors were believed to be cured up to a decade prior to transplantation. [ 22 ] Melanoma is one example [ 7 , 23 ]. Notably, the origin of these metastases could be traced back to the donors, revealing that the transplanted organs harbored ~ 15-year-old dormant DTCs that were able to resume proliferation once established in a favorable environment such as immunosuppression. This observation also provided support for a major role of the host immune system in regulating the dormancy colonization switch.

There are two overall mechanisms of dormancy: quiescence and tumor mass dormancy (Fig. 1 ). Quiescence is a state of dormancy in which tumor cells do not proliferate and are maintained by interactions with the extracellular matrix in pre-metastatic niches, hypoxia, and endoplasmic reticulum stress [ 24 ]. Experimental evidence suggest that cellular dormancy or quiescence, as defined by Hadfield and further explored by the Ann Chambers’ group [ 15 ], occurs where tumor cells enter a state of mitotic arrest as solitary cells. This contrasts with micrometastatic dormancy, which occurs at the metastatic tumor mass level and not at a cellular level. In this mechanism of dormancy, cells are proliferative but undergo apoptosis at a similar rate and thus maintain a mass constant. Support for this mechanism of DTC dormancy was provided by studies from the laboratory of Dr. Folkman. They showed that despite being mitotically active, the tumor mass does not progress due to failure in angiogenesis [ 25 , 26 ]. Another line of support was provided by the laboratory of Dr. Chambers, who observed that some liver micro-metastases do not progress to macro-metastases [ 15 ]. Besides angiogenesis, immune-related mechanisms may also maintain the tumor mass in a state of “equilibrium,” which is associated with dormancy when proliferative cells are found but do not progress into larger lesions. These immune-related mechanism have been shown in both the settings of primary and metastatic tumors [ 27 , 28 ]. Thus, micro-metastatic dormancy can be categorized into angiogenic dormancy and immunologic dormancy . As different as these categories are, they can all contribute to clinical latency. In addition, it is possible that both mechanisms may coexist and are not mutually exclusive in cancer patients. Fig. 1 Overview of the process of dormancy. Tumor cells extravasate from the primary tumor as circulating tumor cells (CXTCs) and arrest in the capillary bed of a dis\contiguous organ, becoming disseminated tumor cells (DTCs). At the secondary site, they can proliferate forming a micro metastasis and eventually a gross metastasis. The tumor microenvironment or the intrinsic factors decide the fate of the DTCs either to develop clinical metastasis or to maintain the dormant state. Over time (years), these dormant tumor cells can escape from dormancy via angiogenesis or release from immune control and develop a clinical metastasis. Further, host immunity via tumor specific cytotoxic T-lymphocytes (CTL) can control DTC growth, resulting in a dormant lesion that can be held in stasis for years or decades until released from dormancy in association with increase in myeloid derived suppressor cells (MDSCs) reversing host T-cell responses. Differences in tumor cell colors represent different clonal phenotypes

Timing and processes of tumor metastasis initiation and progression

Metastasis is initiated when tumor cells leave the primary tumor and enter the hematopoietic or lymphatic system as circulating tumor cells (CTCs) or invade through tissue adjacent to the primary tumor. However, the formation of a clinically relevant metastasis occurs following survival of CTCs during innate immune cell interactions, as well as vascular turbulence, vascular arrest, and extravasation. The arrest of CTCs is a mechanical process such that the survival and proliferation of arrested tumor cells are, in part, capillary bed selective, as varying tumor cell phenotypes preferentially become established in distinct secondary organs [ 29 ]. Thus, vascular and lymphatic circulation has an important role in the dissemination and arrest of tumor cells. As such, the arrest of tumor cells in a given organ is a necessary but insufficient portion of the process for the initiation of a metastasis [ 30 ]. For example, breast adenocarcinoma metastases are frequently initially found in axillary lymph nodes, from which they then traffic to the liver, lungs, bones, and, lastly, brain, supporting a progression of lymphatic followed by hematogenous metastatic spread [ 31 , 32 ]. Similarly, lung cancers initially metastasize to the regional lymph nodes, other secondary sites, and then to the brain [ 31 , 33 ]. Thus, when CTCs survive arrest, they may proliferate, become vascularized, and progress into DTCs [ 34 ]. However, due to selective pressures, the presence of CTCs and DTCs is, again, not predictive of the extent of overt metastasis [ 35 ], as most tumor cells that enter the circulation are rapidly eliminated [ 36 ]. Clinically, CTCs can be used to stratify breast, prostate, and colorectal cancer patients to monitor disease progression and provide a prognostic tool [ 37 ] for PFS and OS [ 38 ]. The hypothesis that specific organ microenvironments support secondary tumor foci by providing congenial “soils” integral to metastasis formation was first published by Paget. These “soils” contributed to site specificity and establishment of an appropriate microenvironment to support tumor seeding [ 32 ]. Thus, the selection of metastatic sites is not random but rather due to various affinities of tumor cells for the milieu of specific organs.

The heterogeneity of metastatic lesions is associated, in part, with the progression of cells within early DTCs rather than progression from primary tumors: a process known as parallel progression [ 39 – 41 ]. The circulation and arrest of CTCs early in tumor progression are supported by the finding that DTCs are detected in the BM of patients with early-stage breast cancer [ 42 ], including patients with pre-invasive ductal carcinoma in situ (DCIS) [ 43 , 44 ]. Several studies comparing the genotypes of DTCs with those of primary tumor cells have shown that DTCs can be less evolved than cells in a primary tumor [ 45 ]. This supports the process of parallel evolution with DTCs arising earlier than the majority of primary tumor cells [ 39 , 45 – 47 ]. For example, in breast cancer patients, BM DTCs display fewer chromosomal aberrations, sub-chromosomal allelic losses, and gene amplification events than cells from primary tumors, as documented by comparative genomic hybridization [ 48 ]. Indeed, only half of BM DTCs display abnormal karyograms, as opposed to 100% of cells from primary tumors [ 48 , 49 ]. This divergent molecular progression between primary tumors and DTCs supports a prolonged time frame between CTC arrest, formation of an overt metastasis, and diagnosis of a primary tumor. Furthermore, parallel evolution is supported by microarray studies of primary tumors, including breast tumors, with the identification of expression profiles predicting the risk of developing a metastasis [ 49 – 51 ]. These microarray studies also suggested that DTCs and micro-metastases occurred years before diagnosis.

The above studies on parallel evolution have been confirmed and extended with studies using tumors from colorectal carcinoma (CRC) patients. In these CRC studies, tumor cells in early-stage cancer patients were shown to have established a subset of primary tumor cells with invasive potential [ 52 ]. Genetic mapping of clonal and sub-clonal tumor cell progression, with whole-exome sequencing of 23 metastatic CRC patients, showed [ 53 ] that most single-nucleotide variants were shared between the primary tumor and metastases despite their phenotypic and functional differences. Another study undertaking a phylogenetic analysis of CRC progression documented the divergence of metastatic lineages occurring early in cancer progression [ 54 ], supporting the origin of metastatic cells from one founder cell. This suggests that in most patients, a metastatic lesion maybe derived from a single tumor cell before it is clinically detectable. In addition, it has been reported that the primary tumors of ~ 90% of patients with metastatic CRC have a sub-clonal selection of tumor cells with a selective growth advantage. Together, studies such as these have demonstrate that parallel evolution of primary and secondary tumors occurs in association with host selection pressures [ 55 ] and that metastasis occurs early during tumor progression, frequently long before diagnosis [ 50 , 56 ].

MDSCs and metastasis

Cancer patients who present with a high tumor burden and/or an advanced clinical stage or histologic grade frequently have an increase in circulating myelopoietic progenitor cells (MPCs) and neutrophils (termed neutrophilia [ 57 , 58 ]) in their PB [ 59 , 60 ]. Cancer-associated neutrophilia and the associated circulating MPCs are due in part to tumor secretion [ 57 ] of hematopoietic growth factors, including colony-stimulating factors (CSFs) [ 61 ]. CSFs stimulate the proliferation and mobilization of MPCs from the BM into the circulation [ 62 , 63 ]. Mobilized MPCs can establish sites of extramedullary myelopoiesis (EMM) in the spleen or liver, further increasing the frequency of circulating MPCs and neutrophilia.

A subset of circulating and tumor infiltrating MPCs are identified as MDSCs due to their innate immune suppressive activity [ 64 , 65 ]. Currently, there are three recognized subsets of MDSCs [ 66 – 68 ]: granulocytic (G) or polymorphonuclear (PMN), monocytic (M), and immature or early (i or e). The comparative human phenotype remains somewhat controversial; however, at present, most investigators agree that human MDSCs are lineage-negative (Lin − ), i.e., lacking T-(CD3), B-(CD19/CD20), and natural killer (NK)-(CD56) cell markers and are CD11b + CD33 + HLA-DR −/low69 . These MDSC subsets are differentiated based on their expression of the granulocyte marker CD15 or CD66b, the monocyte marker CD14, and CD33. The third MDSC subset (i-MDSC) lacks markers for granulocytes and monocytes.

MDSCs contribute to immune evasion by inducing T-cell dysfunction through the production of reactive oxygen species (ROS) [ 70 , 71 ], arginase-1 (ARG1) [ 72 ], and nitric oxide synthase (NOS2) [ 73 ]. ARG1 hydrolyzes extracellular L-arginine into urea and ornithine [ 74 ]. L-arginine is required for T-cell proliferation [ 72 ], cytokine production [ 72 ] and expression of the T-cell receptor zeta chain (TCR- ζ) [ 72 ]. Thus, depletion of extracellular L-arginine inhibits T-cell proliferation, function, reduces anti-tumor activity [ 75 – 78 ], and potentially induces cell cycle arrest [ 79 ]. Thus, the downregulation of TCR- ζ chain expression in T-cells is associated with reduced stability of TCR-ζ mRNA [ 80 ],which may also contribute to the inhibition of T-cell proliferation [ 81 ]. Further, L-arginine is a substrate for inducible (i)-NOS [ 74 ], which results in the production of nitric oxide (NO), a potent signaling molecule with lymphotoxic activity [ 82 ]. iNOS also contributes to MDSC immunosuppression by increasing signal transducer and activator of transcription 3 (STAT3) activation, elevating ROS levels via the production of NO [ 83 – 85 ], contributing to the formation of peroxynitrites via interaction between NO and ROS [ 85 , 86 ] and results in nitrosylation of T-cell receptors [ 87 – 90 ]. As such L-arginine has a role in two mechanisms of immunosuppression, ARG1 and iNOS. In addition, MDSCs can secrete immunosuppressive cytokines, most notably IL-10 [ 91 ], which also contribute to the immunosuppressive activity of MDSCs.

MDSCs have been shown to have a role in the process of tumor metastases by suppressing the immune system [ 92 ], contributing to vasculogenesis [ 93 ], and possibly aiding in the formation of a pre-metastatic niche [ 94 ]. A pre-metastatic niche provides a microenvironment consisting of infiltrating inflammatory cells and extracellular matrix proteins that facilitate metastatic cell colonization via the arrest of CTCs and survival of DTC [ 95 ]. Steven Paget stated that metastasizing tumor cells have site-specific preferences in association with the presence of congenial soil such that specific tumor types “seed” to specific organs [ 32 ], a concept that predates and provides a perhaps more accurate mechanism relative to the concept of a pre-metastatic niche based on the timing of tumor progression. Paget posited in this “seed-and-soil” hypothesis that the spread of tumor cells was not random but rather governed by a regulated process [ 32 ]. This hypothesis was elaborated on by Kaplan et al., who observed that a pre-metastatic niche was supported by vascular endothelial growth factor receptor-1 positive (VEGFR1 + ) BM-derived hematopoietic progenitor cells [ 94 ], a cell population now known as MDSCs. MDSCs promote and sustain tumor angiogenesis primarily by the secretion of matrix metalloproteinases (MMPs). Mechanistically, MMP-9 can boost angiogenesis and stimulate tumor neovasculature by increasing the bioavailability of VEGF [ 96 ]. This initiates a positive feedback loop as VEGF can also trigger MDSC recruitment [ 97 ]. The accumulation of tumor-infiltrating MDSCs has also been shown to correlate with intra-tumoral VEGF concentration during tumor progression [ 98 ].

MDSCs are also associated with myelopoiesis, which is frequently dysregulated in cancer patients and is associated with failure of immature myeloid cells to differentiate. This creates a source of immature myeloid cells in the BM that can be mobilized into the circulation with MPCs, arresting in organs that are common sites of metastasis. Hematopoietic progenitor cell (HPC) mobilization occurs through tumor secretion of growth factors, such as granulocyte-colony stimulating factor (G-CSF), or disruption of CXC chemokine receptor 4 (CXCR4) [ 99 ] binding to its ligand, stromal cell-derived factor 1 (SDF-1/CXCL12). CXCR4 is expressed on CD34 + HPC and, when bound to its ligand CXCL12, inhibits CD34 + HPC mobilization [ 99 ]. G-CSF induces progenitor cell mobilization and secretion of proteases that can cleave CXCR4 [ 100 ], making it unable to bind to its ligand (CXCL12) contributing to MPC mobilization [ 100 ]. In cancer patients, this presents clinically as neutrophilia and monocytosis. Further, the administration of myelopoietic growth factors in both cancer patients and normal stem cell donors [ 101 ] has been shown to increase the serum levels of the stromal factor MMP-9. MMP-9 supports MDSC migration through the bone marrow [ 102 , 103 ], as well as tumor progression [ 104 – 106 ]. Circulating MDSCs can then arrest in secondary lymphoid tissues, such as the spleen or liver, and proliferate, i.e., EMM.

Indeed, increased frequencies of myeloid (CD33 + ) and hematopoietic progenitor (CD34 + ) cells have been found in the spleen of cancer patients with solid tumors as compared to patients without neoplastic disease [ 107 , 108 ]. Furthermore, the frequencies of myeloid and MPPs cells in the spleen of cancer patients negatively correlate with OS [ 108 ]. Though rare, patients with solid tumors can develop an enlarged spleen [ 109 ] (splenomegaly). Splenomegaly is caused by increased EMM [ 110 ], which can also occur in the liver [ 111 , 112 ] following dysregulation of the BM microenvironment [ 113 ]. Perturbation of the BM microenvironment during tumor progression, metastasis, and/or chemotherapy can also result in myelofibrosis (MF). This is supported by a case report by Kiely et al. [ 110 ], in which six out of eight patients with metastatic carcinoma experienced bone pain and marrow fibrosis [ 110 ]. Similarly, BM metastases secondary to MF have also been observed in a breast cancer patient [ 114 ]. Recent studies have supported this observation with the finding that pulmonary fibrosis has a pathobiology similar to MDSCs in cancer [ 115 ] based on a common role for CXCR2 in tumor-associated increase in MDSCs [ 116 ]. In the bleomycin model of pulmonary fibrosis, G-MDSCs have been shown to have a key role in the development of pulmonary hypertension via regulation by CXCR2 [ 117 ], an observation that has been extended to patients with idiopathic pulmonary fibrosis that includes increase in PB G-MDSCs via expression of CXCR2. Studies such as these provide an insight into tumor growth regulation and immune dysregulation in the marrow resulting in increased myelopoiesis and MDSC frequencies that can further neoplastic progression and contribute to niche formation.

Dormancy of micro-metastatic foci

Consistent with the chronology of metastasis and parallel evolution is the finding that DTCs can become dormant, a state in which they survive while not increasing in size, allowing DTCs to remain undiagnosed for years or even decades [ 18 ]. However, following release from dormancy, DTCs proliferate, forming overt metastatic foci. Quiescent dormancy has been additionally supported by a meta-analysis with over 60,000 early-stage breast cancer patients given endocrine therapy and documenting that metastases become detectable 5 to 20 years following primary tumor diagnosis, surgical resection, and adjuvant chemotherapy [ 118 ]. However, the relative contribution of quiescent DTC and their progression into micro-metastases in relation to cancer relapse and survival, relative to chemo-resistance or other treatment challenges, remains unclear.

The second mechanism of dormancy, termed tumor mass dormancy [ 24 , 28 ], occurs when balance is achieved between tumor cell proliferation and death, generating a “steady state” in which the DTC size remains constant [ 25 ]. This balance is achieved by angiogenic or immune-mediated regulation. The former is due to the requirement for growth of cancer cells of energy resources, which is provided by vascularity [ 119 ]. Angiogenesis, the process by which additional blood supply (vascularity) is developed, is regulated by pro-angiogenic and anti-angiogenic factors [ 120 ]. By regulating the tumor vascular supply, these factors affect the transport of oxygen and nutrients to the tumor site, supporting tumor growth and reducing apoptosis [ 121 ]. Thus, there may be a trigger for DTC re-emergence that occurs in patients who develop metastases that do not occur in others. Physiological stresses that induce neoangiogenesis such as systemic inflammation or surgery could cause dormant cells to re-enter the cell cycle. This occurrence has been supported by a retrospective study showing that early breast cancer relapse incidents during the 9- to 18-month period after surgery are reduced approximately fivefold in patients taking preoperative nonsteroidal anti-inflammatory drugs [ 122 ], suggesting that systemic inflammation is indeed a trigger.

Immune-mediated tumor mass dormancy can also control the development and growth of dormant tumor lesions. Thus, the survival and continued growth of primary and secondary tumor foci requires the evasion of the host immune response [ 123 , 124 ]. This mechanism of dormancy is evoked when the rate at which immune-mediated cancer cell cytotoxicity equals cancer cell proliferation, resulting in a malignant mass with a stable size [ 125 ]. Immune-mediated dormancy is suggested to be the result of tumor-specific cytostatic and cytolytic CD8 + T-effect or cell activity [ 126 ]. However, T-cell responses that can cause dormancy can also lead to secretion of cytokines that inhibit angiogenesis [ 127 , 128 ]; which would then lead to angiogenic dormancy. This response suggests the potential for crosstalk between these two dormancy mechanisms.

MDSCs, pre-metastatic niches, and release from metastatic dormancy

The microenvironment of metastatic lesions can contribute to the formation of DTCs by supporting tumor cell survival and proliferation [ 34 ] and may be recognized as metastatic niches [ 129 ]. These niches can contribute to metastatic site specificity due to conditioning by organ-specific secretion of growth factors and hormones that support the growth/survival of tumor cells in a tumor- and organ-specific manner and by the systemic expansion of and infiltration by tumor-associated macrophages (TAM), neutrophils, T-regulatory (T-reg) cells, and MDSCs [ 130 ]. These leukocytes originate from the BM where they can be found in high numbers, as well as from extramedullary sites of hematopoiesis [ 131 ]. This supports in part why bone is a common site for DTCs. Prior to or following CTC arrest, a metastatic niche can provide functional elements critical to tumor cell survival and proliferation, including vascularization, modification of the extracellular matrix (ECM), recruitment and infiltration of MPCs, establishment of hypoxia, and organ-associated epithelial cell secretion of hormones and growth factors [ 132 ]. Indeed, metastases from breast cancers to bone involve cross-talk between tumor and bone cells that include tumor-secreted factors that can stimulate bone marrow cell proliferation, resulting in MDSCs that prevent the death of DTCs from tumor-specific cytotoxic T-lymphocytes (CTLs) [ 133 ]. Additionally, tumor-associated MDSCs, TAM, neutrophils, and fibroblasts can support the infiltration and expansion of non-transformed cells, including endothelial cells and their precursors, contributing to the vascularization of a micro-metastatic site [ 134 ]. Together, these processes support the survival of DTCs and the formation of micro-metastatic foci [ 28 ] by inhibiting tumor-antigen specific CTL infiltration via T-regs (T-regulatory cells) and MDSCs [ 135 ], thereby supporting their release from dormancy and facilitating the survival and growth of tumor cells.

Accumulating evidence suggests that the tumor microenvironment (TME) provides mechanisms that regulate cell dormancy. For example, hematopoietic stem cells (HSCs) are capable of long-term survival within a state of dormancy [ 136 ] and are regulated by interactions with unique tissue microenvironments or “niches” in bone [ 136 ]. DTCs from several tumor pathologies have been reported to hijack these endogenous HSC regulatory mechanisms. Thus, the CXCL12/CXCR4 chemokine signaling axis [ 137 ] contributes to HSC homing and quiescence [ 138 ], and is used by prostate and breast cancer cells during tumor cell arrest and dormancy in bone [ 139 ]. Further, tumor cell arrest occurs as part of an interaction with the surrounding microenvironment (niche) initiating a series of adaptive responses involving cell intrinsic signaling programs. This arrest together with niche-specific paracrine signals can induce cellular dormancy associated with chronic tumor cell survival. However, the process of dormant cell reactivation is less understood and appears to be stochastic, whereby a proportion of tumor cells escape from dormancy and initiate growth into a micro-metastatic focus via mechanisms that remain poorly characterized.

Inflammatory processes by which dormant cancer cells can be reactivated and progress into occult metastases are critical to relapse. During sustained inflammation, lung resident neutrophils can produce extracellular traps, which are a mixture of DNA and cytotoxic proteins and proteases. These extracellular traps can initiate the reactivation of dormant cancer cells by remodeling the surrounding extracellular matrix, resulting in growth into micro-metastases [ 140 ]. Indeed, integrin and extracellular matrix remodeling events have been described to be part of central dormant cell reactivation mechanisms [ 141 ] providing potential therapeutic mechanisms for intervention.

Prostate and luminal breast cancers preferentially metastasize to bones while triple-negative breast cancers have a predilection for visceral organs. Indeed, these niche environments have a series of different functions that regulate tumor cell arrest. The endosteal niche has a role in the arrest of myeloma cells that support the survival and growth of HPCs but may also have a similar function and overlap with DTCs [ 142 ]; inhibiting an analysis of the role for the endosteal niche. For example, MM patients whose HSCs were mobilized with G-CSF were found to have an increase in the mobilization of monoclonal plasma cells [ 143 ]. CXCL12 has been shown to have a role in the regulation of HSC maintenance, proliferation, localization, trafficking/mobilization, and self-renewal, occurring in bone marrow niches within which dormant DTCs reside. [ 144 ] CXCL12 and the αvβ3 integrin aid in binding of DTCs and HSCs to bone matrix proteins, supporting interactions within these niches for prolonged periods [ 145 ]. For breast and prostate cancer tumor cells, their preferential interaction with endosteal niches has been associated with CXCR4 expression as well [ 146 , 147 ], supporting interactions within these niches for prolonged periods. [ 145 ] Other studies with acute lymphoblastic leukemia (ALL) cells have shown that extracellular osteopontin, a matrix protein with diverse biological functions, within the endosteal niche supports the induction of dormancy and cell cycle arrest. [ 148 ]

BM niches provide a complex microenvironment incorporating many cell types including endothelial cells, adipocytes, osteoclasts, myeloid cells, and cells of osteoblastic lineage [ 149 ]. These niches provide homing signals for HSCs in support of their normal function [ 150 ]. It has been demonstrated that increasing HSC niches facilitates an increase in the number of BM DTCs [ 142 ], suggesting that these homing signals can also facilitate PC metastasis. One mediator of HSC arrest and mobilization is the chemo-attractant, CXCL12 [ 151 ]. Recently, it was shown that BM stromal cells with heightened levels of CXCL12 increased the recruitment of PCs into the bone marrow, promoted proliferation of PC cells, and protects PC cells from chemotherapy-induced apoptosis [ 152 ], suggesting that DTCs may be home to the HSC niche using similar mechanisms to HSCs themselves. Further, given that cancer cells secrete CSFs [ 153 ], which can mobilize HSC as well as DTCs, studies have suggested that both DTCs [ 142 ] and HSC mobilizing agents such as G-CSF or CXCR4 inhibitors can mobilize DTCs back into the PB from the BM [ 154 ].

As discussed above, primary tumors can modulate the microenvironment of distant organs and metastases but may not have a direct role in tumor cell arrest via the formation of a metastatic niche. Bulky tumors are associated with neutrophilia, as well as circulating and organ-infiltrating MDSCs, due to tumor secretion of growth factors [ 155 , 156 ]. Consistent with this relationship, the frequency of circulating MDSCs is reduced by surgical debulking or primary tumor resection. [ 157 ] However, MDSCs may not have a critical role in the arrest of CTCs and the formation of DTCs, as the arrest of CTCs can occur early in tumor growth and progression prior to the establishment of a primary tumor of sufficient size to result in neutrophilia. While rodent studies have suggested that MDSCs in premetastatic niches may have a role in tumor cell arrest [ 158 , 159 ], the rapid growth of the primary tumor in animal models may not mimic the timeline that occurs clinically [ 53 , 160 ]. Further, given the immunosuppressive activity of MDSCs, i.e., downregulation of adaptive immunity, we posit that their role in tumor progression may be associated with the release of DTC from dormancy by suppressing adaptive host immunity that we suggest is critical to the development of overt metastatic foci. This may be related to MDSC infiltration of organ capillary beds following growth of the primary tumor to a size sufficient to secrete growth factors sufficient to expand and mobilize MDSCs. Given that the frequency of circulating MDSCs is directly correlated with tumor burden [ 161 ], it is unlikely that a primary tumor early in progression can affect CTC arrest and survival at secondary sites. However, MDSCs may have an important role in sustaining the survival and growth of DTCs and micro-metastatic foci by conferring resistance to host immunosurveillance and immunotherapy. MDSCs may also contribute to the organ specificity of metastasis due to regional support of tumor cell escape from host adoptive immunity [ 69 ], as well as establishing of an organ specificity of CTC arrest by the secretion of growth factors that regulate the survival and growth of tumor cells in DTCs and micro-metastatic foci. Thus, while lesion-infiltrating lymphocytes may contribute to tumor cell dormancy, MDSCs may also release DTCs from dormancy in an organ-specific manner, allowing the outgrowth of micro-metastatic foci. The spleen, arguably a rare site for metastasis formation [ 162 ], is one example of organ-specific controlled tumor cell growth [ 163 ] due in part to it being a relatively hostile environment for tumor cells due to high number of T-cells [ 164 – 166 ]. Thus, T-cells can control tumor cell growth, while metastatic site infiltration by MDSCs, in association with their immunosuppressive activity, may facilitate the survival and growth of DTCs and micro-metastatic foci.

The role of MDSCs in CTC arrest, DTC formation, release of DTCs from dormancy, and tumor progression remains controversial, requiring additional investigation. Sophisticated tumor models and techniques are required to address questions regarding pre-metastatic niche DTC development and maintenance, i.e., dormancy. It is only with such studies that we will be able to identify and develop therapeutic approaches to inhibit neoplastic progression.

Therapeutic regulation of MDSCs in neoplasia

Immunotherapies have been rapidly developing, providing promising therapeutic options for cancers, including patients diagnosed with advanced disease. These novel therapies include immune checkpoint inhibitors (ICIs), cellular immune therapies like adoptive transfer of engineered T-cells, cancer vaccines, and oncolytic virus. However, the immunosuppressive microenvironment mediated or induced by cancers limit the efficacy of these novel immunotherapies. Immunosuppression is associated with innate immune cells such as MDSCs, TAMs, tumor-associated neutrophils (TANs), tumor-associated dendritic cells (DCs), and adoptive immune cells like T-regs that are the main cellular components within the TME. Emerging evidence regarding the role of MDSCs in cancer progression and their association with poor clinical outcomes [ 166 , 167 ] has stimulated an exploitation of MDSCs as targets for cancer treatment often in combination with other forms of immunotherapy including tumor vaccines. Further, cancer tissues with high MDSC infiltration have been shown to be associated with patient resistance to immunotherapies. [ 168 ] Most of the current clinical trials targeting MDSC regulation employ off-label use of Food and Drug Administration (FDA)-approved drugs to assess their efficacy as monotherapeutics or in combination with other immune-modulating therapeutic strategies. Currently, MDSC-targeting modalities include approaches to deplete MDSCs [ 169 ], inhibit MDSC-mediated immunosuppressive mechanisms [ 169 ], induce MDSC differentiation [ 89 ], and block MDSC proliferation and recruitment to tumor sites [ 169 , 170 ]. Based on this, the development of combined immunotherapies that target MDSC-mediated immunosuppressive pathways to improve the antitumor efficacy of immunotherapy has a broad potential. [ 171 ] As part of our discussion focused on clinical approaches to target MDSCs and their bioactivities, we have emphasized clinical studies. The majority of studies on drugs with potential to regulate MDSCs, both off target and novel, have been undertaken using preclinical models; however, we have limited this review largely to therapeutics examined clinically and which have shown provisional activity based on immune surrogates (Table 1 ). Table 1 Therapeutic agents that have demonstrated efficacy against MDSCs or other biomarkers of myeloid-associated immunosuppression in clinical studies Drug class Drug Diseases Findings Cytotoxic agents Capecitabine + bevacizumab (anti-VEGF) GBM ↓ % Circulating MDSCs,↑ intratumoral infiltration by CTLs, ↓ CTLA-4 and PD-1 expression on macrophages and CTLA-4 on lymphocytes [ 167 ] Gemcitabine PDAC ↓ % Circulating G-MDSCs [ 172 ] Gemcitabine + capecitabine ↓ Total circulating MDSCs [ 175 ] FOLFOX CRC ↓ total MDSCs [ 177 ] FOLFOX + bevacizumab ↓ % Circulating G-MDSCs [ 176 ] Everolimus + low-dose cyclophosphamide mRCC ↓ In MDSC, T-regs and DC subsets,↑ CD8 T-cells and DC subsets [ 325 ] Cabozantinib + nivolumab mUC ↓ In MDSC, T-regs and DC subsets and decrease in MDSCs was associated with better PFS and OS [ 326 ] Nivolumab + temozolomide SCLC ↓ In MDSC and ↑ in T-cell functions [ 327 ] Therapeutic liver-X nuclear receptor (LXR) agonist RGX-104 Solid tumors ↓MDSC frequency, increased CTL responses [ 328 ] PDE-5 inhibitors Tadalafil MM ↓ % M-MDSCs, expression of ARG1, and iNOS in BM, ↓ tyrosine nitrosylation, ↑ % and proliferation of CD8 + IFN-γ + and TCR-ζ + CTLs [ 191 ] HNSCC ↓ Expression of ARG1 and iNOS, ↓ % circulating MDSCs [ 189 ] Melanoma ↓ NO production, ↑ T-cell recruitment to tumor sites [ 193 ] Sildenafil MM and HNSCC NCT02466802 , restored T-cell proliferation in PBMC [ 329 ] Arginase inhibitors CB-1158 Lung cancer ↑ T-cell proliferation in patient-derived G-MDSC-conditioned media [ 202 ] CRC Demonstrated minimal treatment-related toxicities [ 203 ] iNOS inhibitors L-NMMA + docetaxel Triple-negative breast cancer Decreased arginase levels and an ORR of 45.8% in patients treated with L-NMMA and taxane NCX-4016 Healthy volunteers ↓ Monocyte activation, ↓ IL-6 levels, ↓ CD11b expression on monocytes (17.5 mg/kg) [ 210 ] CRC (mouse model) ↓ iNOS and ARG1 activity by CD11b monocytes [ 209 ] CSF1R inhibitors GW-2580 AML ↓ CSF1R hi HLA-DR + CD33 + monocytes (unclear whether these are myeloid suppressor cells) [ 213 ] Pexidartinib + paclitaxel Advanced solid tumors Resulted in clinical benefit and ORR 16%, ↓ % circulating CD14dimCD16 + non-classical monocytes [ 330 ] Pexidartinib GBM ↓ % Circulating CD14 dim CD16 + non-classical monocytes, ↓ expression of intratumoral Iba1 + microglia, however, did not improve patient progression-free survival [ 220 ] Imatinib CML ↓ % Circulating G-MDSCs [ 222 ] S100A8/9 inhibition Tasquinimod mCRPC ↓ Recruitment of MDSC, decrease monocytes, M1 polarization and prolong PFS [ 331 ] CD33 BI 836,858 MDS ↓ ROS and MDSC [ 259 ] AMV564 MDS, AML, melanoma ↓ MDSC, increase CTL and reduce tumor burden [ 262 ] Immune checkpoint inhibitors Ipilimumab (CTLA-4 inhibitor) Melanoma Found to result in greater OS in patients with lower MDSC frequency [ 290 ] ↓ NO production by M-MDSCs and ↓ % PD-1 + G-MDSCs were found to be biomarkers of response to therapy [ 291 ] ↓ % Circulating G-MDSCs and ARG1 + myeloid cells [ 292 ] ↓ % G-MDSCs, ↓ iNOS and ARG1 expression [ 294 ] Pembrolizumab (PD-1 inhibitor) Urothelial carcinoma ↓ % PD-1 + M-MDSCs and iMDSCs [ 296 ] Pembrolizumab + chemotherapy NSCLC Significantly improved OS compared to standard of care alone [ 321 ] Atezolizumab, avelumab (PD-L1 inhibitors) Urothelial carcinoma ↓ % PD-L1 + M-MDSCs [ 296 ] Ibrutinib (BTK and ITK inhibitor) MM Inhibits BTK phosphorylation on MDSCs generated from patient PBMCs and MDSC migration, generation, and nitrate production in vitro [ 229 ] CLL ↑ Absolute number of CD4 + and CD8 + T-cells, ↓ number of circulating G-MDSCs [ 230 ] Ibrutinib + nivolumab (PD-1 inhibitor) Metastatic solid tumors ↓ % Circulating MDSCs in the first cycle of therapy, ↑ T-cell function, ↓ plasma chemokines involved in MDSC trafficking (IL-12, CCL2, CCL3, CCL4) [ 235 ] Chemokine inhibitors HuMax-IL8 (anti-IL-8 monoclonal antibody) (BMS-986253) Metastatic or unresectable solid tumors ↓ Serum IL-8 but no significant changes in circulating MDSCs [ 241 ] BL-8040 (CXCR4 inhibitor) + pembrolizumab (PD-1 inhibitor) + chemotherapy PDAC ↑ Intratumoral CD8 + effector T-cells, ↓ metastasis infiltration by MDSCs, ↑ circulating CD4 + and CD8 + T-cells, ↓ circulating T-regs, resulted in an overall response rate of 32% [ 243 ] SX-682 inhibitor of CXCR1/2 signaling Melanoma, PDAC, mCRC, MDS NCT03161431 , NCT04599140 NCT04245397 , and NCT04477343 ; no results published to date. [ 247 ] Reparixin (repertaxin) + / − paclitaxel mBC, TNBC, COVID-19, pancreatic islet auto Tx NCT02370238 , NCT02001974 , NCT01861054 ; no flow results published to date. [ 332 ] Ulocuplumab (BMS-936564) Relapsed MM Safe with acceptable AEs and leads to a high response rate [ 244 ] AZD5069 + enzalutamide mProstate CA NCT03177187 [ 332 ] TK and JAK/STAT inhibition Sunitinib (multi-kinase inhibitor) RCC ↓ % PB, MDSCs and induced MDSC apoptosis in vitro , ↓ % circulating MDSCs (which also correlated with ↑ IFN-γ production) [ 282 ] Sunitinib + stereotactic body radiotherapy Oligometastatic disease ↓ % Circulating MDSCs [ 283 ] Ruxolitinib (JAK1/2 inhibitor) Myeloproliferative neoplasms ↓ % Circulating G-MDSCs, ↓ number of T-regs, ↓ pSTAT5 expression [ 274 ] Acalabrutinib + pembrolizumab Advanced pancreatic CA ↓ % Circulating G-MDSCs, with limited clinical activity [ 333 ] COX-2 inhibitor Celecoxib Lung CA, colon carcinoma, breast CA, cervix CA, endometrium CA, NCT00046839 , NCT03026140 , NCT02429427 , NCT00152828 , NCT03896113 Celecoxib ↓MDSC‐suppressive function and ARG-1 levels [ 334 ] Anti-cancer vaccines and MDSC inhibition Nelipepimut-S Breast cancer ↑ CTL response, ↓ % T-regs [ 335 ] Personalized cancer peptide vaccine + gemcitabine PDAC ↓ Tumor size [ 310 ] INGN-225 (autologous DC vaccine transduced with modified adenoviral p53) + ATRA SCLC 43.4% of patients developed a tumor-specific T-cell response [ 336 ] 52% of patients had p53-specific T-cell response, 61.9% of pts receiving the vaccine achieved complete or partial treatment response, and ↑ IFN-γ production, however, ↑ circulating immature myeloid cells including MDSCs [ 337 ] p53MVA + gemcitabine Platinum-resistant ovarian cancer ↑ p53-reactive CD4 + and CD8 + T-cells but no effect on circulating MDSCs or T-regs [ 338 ] VEGF inhibitors Bevacizumab NSCLC ↓ % Circulating G-MDSCs [ 339 ] Bevacizumab + Robustlyim EGFR mutant lung AdCA ↓ % Circulating S100A9 + M-MDSCs [ 340 ] Modulation of myeloid cell differentiation Zoledronic acid PDAC No effect on MDSCs, thought to be due to low dose [ 227 ] ATRA Metastatic RCC ↓ % Circulating Lin − HLA-DR − CD33 + myeloid cells [ 265 ] ATRA + ipilimumab (CTLA-4 inhibitor) Advanced melanoma ↓ % Circulating MDSCs and expression of immunosuppressive markers [ 267 ] ATRA + atezolizumab mNSCLC ↓ Recruitment of MDSCs [ 341 ] Vitamin D3 HNSCC Inhibited cancer progression and recurrence, ↑ disease-free survival [ 269 ] ↓ % Circulating CD34 + suppressor cells and serum GM-CSF but they increased again after 6 weeks 271

Chemotherapy-associated depletion and inhibition of MDSC proliferation

In addition to tumor cytotoxicity, off-target immune regulation has been documented for some chemotherapeutic agents. However, clinical studies with chemotherapies render it difficult to identify whether the impact of the intervention is due to decrease in MDSCs that is influencing clinical outcomes or whether the reduction in MDSC number and function is due to another therapy or reduced tumor burden secondary to the chemotherapy. In preclinical studies, these activities can be assessed by testing individual therapies versus in combination, an approach not ethically appropriate clinically. Such studies with chemotherapeutics include capecitabine, a 5-fluorouracil (5-FU) prodrug that can significantly decrease the frequency of circulating MDSCs and increase CTL infiltration, as reported with glioblastoma multiforme (GBM) tumors, theoretically enhancing antitumor activity [ 167 ]. Similarly, the antimetabolite gemcitabine has been reported to significantly decrease the frequency of circulating G-MDSCs, but not M-MDSCs, in studies recruiting patients diagnosed with pancreatic ductal adenocarcinoma (PDAC) [ 172 ]. However, in this study, the authors examined MDSC frequency using cryopreserved PB mononuclear cells (PBMC), which is a concern as MDSCs are sensitive to freeze–thaw lysis [ 173 , 174 ]. In a separate study, PDAC patients were administered gemcitabine with capecitabine (GemCap) and documented a decrease in the frequency of circulating MDSCs in 8 out of 19 patients [ 175 ]. In another arm of this study, 21 patients were treated with GemCap, in addition to an anti-cancer vaccine, and granulocyte–macrophage (GM)-CSF to reduce chemotherapy-associated myelosuppression, demonstrating a decreased frequency of circulating MDSCs suggested to be associated with the GemCap chemotherapy. [ 175 ] The decrease in the frequency of MDSCs in both cohorts was associated with reduced serum levels of cytokines and inversely correlated with changes in tumor size [ 175 ]. However, this study again examined MDSC frequencies using cryopreserved peripheral blood mononuclear cells (PBMCs) [ 175 ], limiting conclusions into cytotoxic agent reduction in the frequency of MDSCs.

In patients with metastatic CRC treated with FOLFOX (folic acid, 5-FU, and oxaliplatin) in combination with bevacizumab, a significant decrease in the frequency of circulating G-MDSCs, but not M-MDSCs [ 176 ], and improved progression-free survival (PFS) [ 176 ] was observed. However, some chemotherapeutic protocols have been suggested to increase MDSC frequency, negatively affecting the tumor microenvironment. This difference in response to cytotoxic drugs was found in a study on 23 metastatic CRC patients treated with either FOLFOX or FOLFIRI (folic acid, 5-FU, and irinotecan) chemotherapeutic regimens. FOLFOX-treated patients demonstrated a decrease in the frequency of MDSCs, whereas an increased frequency of MDSCs was observed in the FOLFIRI-treated group [ 177 ]. The increased frequency of MDSCs and chemotherapy regulation was also documented in a clinical study on breast cancer patients given 4 cycles of doxorubicin plus cyclophosphamide (AC) every 2 weeks followed by 4 cycles of dose-dense paclitaxel [ 59 ]. In addition, during all of the cycles of chemotherapy, peg-filgrastim (PEGylated G-CSF) was administered on day 2 to reduce therapy-induced neutropenia [ 178 ]. This is critical as following several cycles of chemotherapy, patients become myelosuppressed, placing them at increased risk of potentially life treating microbial infections, such that administration of myelopoietic growth factors enables continuation of chemotherapy without interruption and dose de-escalation [ 179 ]. The two phases of the AC and paclitaxel protocol resulted in a different outcome in each phase based on the frequency and absolute number of circulating MDSCs versus baseline numbers [ 59 ]. AC administration resulted in significantly increased frequency and absolute number of circulating MDSCs (11.72 and 1157 cells/μL) as compared to paclitaxel (3.65 and 257 cells/μL) [ 59 ]. The increased MDSC frequency was suggested to be due to the use of peg-filgrastim and repeated cycles of chemotherapy [ 59 ]. The response to peg-filgrastim is consistent with prior studies demonstrating an increase in MDSC frequency following growth factor administration [ 180 ], as have repeated cycles of chemotherapy that may be positively associated with increased suppressor cell activity [ 92 ]. Despite the demonstrated association of these various chemotherapeutic agents with MDSC dynamics, it has been difficult to correlate changes in MDSC frequency with clinical efficacy supporting a potential for a poly-therapeutic approach in the regulation of MDSCs.

In patients diagnosed with mesothelioma, a recent study examined the impact of gemcitabine treatment on the frequency of MDSCs and lymphocytes, and progression free survival (PFS). The frequency of MDSCs and lymphocytes was assessed at baseline and 3 weeks following start of gemcitabine therapy as compared to patients receiving best supportive care (BSC) as part of the NVALT19-trial. [ 181 ] Gemcitabine treatment was significantly associated with increased NK and decreased T-reg cell proliferation in the experimental group as compared to the BSC cohort. Further, the frequency of MDSCs was lower in gemcitabine-treated patients and correlated with an increase in T-cell proliferation following treatment. The preliminary analysis suggested that the increase in NK-cell proliferation and PD-1 expression on T cells following gemcitabine therapy was associated with improved PFS and OS. The observations from these studies suggest preferential activation of anti-tumor immune cell populations and inhibition of T-regs and MDSCs by gemcitabine administration.

Attenuation of MDSC function

As mentioned above, ARG1 decreases extracellular L-arginine levels, which is required for T-cell function. [ 182 ] For ARG1 to function, tumor-derived prostaglandin E2 (PGE2), produced by cyclooxygenase-2 (COX2), has an important role. [ 183 ] In addition, ROS production by MDSCs is promoted by COX-2 and PGE2 activities. [ 183 , 184 ] It has been shown in autochthonous tumor models that blockade of PGE2 synthesis by the administration of the COX2 inhibitor celecoxib results in the downregulation of ARG1 expression and ROS production by MDSCs, followed by improved antitumor T-cell function [ 185 ] and cancer chemoprevention. [ 186 ] Clinically, due in part to toxicity concerns (GI and cardiac), COX-2 inhibitors have not been aggressively studied for chemoprevention and cancer therapy [ 187 ]. Recently an adjuvant trial of celecoxib for patients with metastatic CRC was reported [ 188 ]. In this small randomized study, 54 patients with metastatic CRC were randomized into 2 groups: a control group that received 6 cycles of the FOLFIRI regimen as compared to a FOLFIRI and celecoxib group (receiving 6 cycles of FOLFIRI plus 200 mg twice daily of celecoxib over 3 months). Patients were assessed at baseline and at the end of intervention using RECIST criteria (Response Evaluation Criteria in Solid Tumors) and an evaluation of serum VEGF, soluble FAS (sFAS), sFAS ligand (sFAS-L), and epithelial neutrophil-activating peptide -78 (ENA-78/CXCL5) levels. Three months following initiation of therapy, the celecoxib/FOLFIRI cohort had a significantly better ORR, significantly lower serum levels of VEGF, CXCL5, and sFASL, and significantly higher sFAS serum levels. Further, the clinical outcomes of patients in the celecoxib/FOLFIRI cohort, i.e., PFS and 1-year OS, were significantly improved relative to the FOLFIRI cohort.

Phosphodiesterase type 5 (PDE5) inhibitors such as sildenafil (Viagra), tadalafil (Cialis), and vardenafil promote the accumulation of intracellular cyclic guanosine monophosphate (cGMP) and can trigger downstream effects that inhibit MDSC function [ 189 , 190 ]. In early studies with PDE5 inhibitors, a patient with refractory multiple myeloma (MM) was treated with tadalafil and found to have reduced serum monoclonal immunoglobulin (M-protein) levels [ 191 ]. Later studies on MM patients [ 191 ] demonstrated that the addition of tadalafil to concurrent treatment regimens significantly reduced the frequency of BM M-MDSCs (HLA-DR − CD14 + ) at 6 months, but following 6–11 months of treatment, BM M-MDSC frequency was observed to be increased. However, a significant decrease in iNOS and ARG1 levels was observed [ 191 ]. Concomitant with the decrease in intracellular iNOS and ARG1 expression by M-MDSCs in the BM, a significant decrease in tyrosine nitrosylation was observed at 11 + months post-treatment [ 191 ] in association with an increase in the frequency and proliferation of stimulated BM CD8 + IFNγ + (interferon) and TCR-ζ + CTL [ 191 ]. This is expected as iNOS, ROS, and ARG1 promote MDSC-mediated T-cell suppression by contributing to the nitrosylation of the T-cell receptor [ 87 , 88 ], depletion of L-arginine, downregulation of TCR- ζ chain expression [ 192 ], and upregulation of toxic mediators including activated O 2 radicals and NO [ 75 – 77 ]. Thus, tadalafil administration and the associated decrease in iNOS and ARG1 expression have been suggested to result in increased CD8 + T-cell activity by decreased MDSC function and number in MM patients [ 191 ].

The regulation of MDSCs by PDE-5 inhibition has been reported for other cancers including metastatic melanoma patients with stable disease (SD). In these studies, tadalafil treatment did not significantly reduce the frequency of circulating M-MDSCs; however, it did reduce NO production by M-MDSCs in metastatic lesions in 2 out of 3 patients who had SD, which was associated with significant increase in tumor infiltrating CD8 T-cells [ 193 ]. In head and neck squamous cell carcinoma (HNSCC) patients, PDE-5 inhibition was shown to significantly decrease the frequency and intracellular levels of ARG1 and iNOS in circulating MDSCs compared to baseline levels [ 189 ]. Further, T-cell proliferation and activation were significantly increased in tadalafil-treated HNSCC patients [ 189 ]. These studies suggested that PDE-5 inhibitors can mitigate T-cell inhibition and potentially improve tumor-specific CTL responses by reducing MDSC frequency and inhibiting iNOS and ARG1 levels [ 189 , 194 ].

In a recent retrospective cohort analysis of 3100 patients with prostate cancer treated with radical prostatectomy between 2003 and 2015, patients were subset into those receiving a PDE-5 inhibitor or non-recipients (controls) [ 195 ]. Biochemical RFS and OS at 5 and 10 years were reported. The biochemical RFS for control patients at 5 and 10 years was 87.6 and 85.3%, respectively, and the OS at these times was 97.9 and 94.5%. In the cohort that received PDE-5 inhibitors, the RFS and OS significantly increased with a RFS of was 94.3 and 93.2% at 5- and 10-year follow-up, respectively, and OS was 99.2 and 95.8% at these time points. A multivariate analysis was also undertaken documenting that PDE-5 inhibitor administration was associated with lower risk of biochemical recurrence and death when corrected for other variables.

ARG1 expression is frequently increased in myeloid cells of cancer patients, including breast cancer [ 196 , 197 ] and CRC [ 198 ]; however, it is unclear if a decrease in ARG1 expression can contribute to patient survival [ 199 ]. In one study that examined critically ill patients, the frequency of G-MDSCs was negatively correlated with L-arginine plasma concentration. Further, in this study, patient mortality was directly associated with frequency of G-MDSCs (CD15 + CD14 − HLA-DR − and Lin − CD33 + HLA-DR − ) on day 1 of hospital admission [ 200 ]. Based on these and similar observations, it has been suggested that L-arginine depletion by G-MDSCs may contribute to increased risk of patient mortality and higher frequency of G-MDSCs. A number of investigational new drugs targeting ARG1 are under investigation using preclinical models and are currently in clinical trials. One such agent studied in a phase I/II clinical study in combination with gemcitabine for advanced biliary cancers (INCB001158) was reported to result in a median PFS of 8.5 months. [ 201 ] A second arginase inhibitor, CB-1158, has been shown to increase T-cell proliferation in medium conditioned by G-MDSCs that were isolated from lung cancer patients [ 202 ] and clinically has shown minimal toxicities and efficiencies in a cohort of solid tumor patients in an ongoing clinical trial [ 203 , 204 ] ( NCT02903914 ).

Investigational iNOS inhibitors such as N(G)-monomethyl L-arginine (L-NMMA) have been shown to decrease MDSC suppressive activity in murine tumor models [ 205 ] and are currently being investigated in several clinical cancer trials ( NCT04095689 , NCT02834403 , and NCT03236935 ). In a recent report of a phase 1/2 trial of L-NMMA combined with taxane for treating patients with chemo-refractory, locally advanced breast cancer (LABC), or metastatic triple negative breast cancer (TNBC) ( NCT02834403 ) immune cell correlates of response were examined. [ 206 ] In this study on 35 patients, the ORR was 45.8%. Phenotypic studies using mass cytometry time of flight (CyTOF) analysis of PB immune cells at the end of therapy revealed that chemotherapy non-responders had increased expression of markers associated with M2 macrophage polarization and increased concentrations of circulating interleukin (IL)-6 and IL-10. In contrast, chemotherapy responders had increased frequency of CD15 + neutrophils with decreased arginase levels. The investigators concluded that L-NMMA combined with taxane therapy warrants additional investigation in larger clinical studies of breast cancer patients.

In similar studies, NCX-4016, an NO-releasing aspirin derivative, was reported to increase, rather than decrease, plasma NO levels by nearly twofold [ 207 ], thereby inhibiting iNOS expression [ 208 ]. NCX-4016 was observed to inhibit iNOS and ARG1 activity by CD11b + cells [ 209 ], resulting in increased anti-tumor immune activity [ 209 ], elevated numbers of CD8 + T-cells [ 209 ], and increased IFN-γ secretion [ 209 ]. In studies on NCX-4016 administration to healthy volunteers, it was found to inhibit monocyte activation, lower IL-6 levels, and reduce CD11b expression on monocytes, suggesting that it can inhibit MDSC-mediated immunosuppressive activity in humans [ 210 ]. Despite the evidence regarding iNOS inhibition, preclinical studies have indicated that NO and peroxynitrites produced by macrophages can result in cytotoxicity to tumor cells [ 205 ], potentially overcoming the effects of iNOS- and NO-induced inhibition of CTL activity [ 85 , 86 ]. Therefore, it remains unclear whether iNOS inhibition or NO supplementation may be worthwhile in the treatment of cancer given the diverse bioactivities of NO despite their potential to curb tumor immune evasion.

Inhibition of myeloid cell growth factors has been shown to reduce MDSC proliferation, as well as tumor infiltration by MDSCs and TAMs [ 211 ]. We note that studies on circulating and tissue-infiltrating myeloid cells are controversial, as the phenotypes of infiltrating TANs, TAMs, and MDSCs are not clearly differentiated. Thus, in our discussion, we use the nomenclature from each referenced manuscript, with the understanding that the cellular phenotype and function may be independent of terminology used. The most extensively studied biologics have targeted colony stimulating factor-1 receptors (CSF1R), also known as c-fms proto-oncogene and CD115. Several CSF1R inhibitors are currently in clinical trials, providing some encouraging results [ 212 ]. Additionally, their administration has been reported to decrease monocytes and MDSCs and, in some studies, to be positively associated with patient outcomes. In AML patients, the CSF1R inhibitor GW-2580 has been reported to decrease monocyte frequency including the frequency of HLA-DR + CD33 + CSF1R hi monocytes [ 213 ]. Conversely, CSF1R hi monocytes that co-expressed CD16 and CD66b were reported to be less responsive to GW-2580 intervention. While all MDSCs are HLA-DR low/− and express CD33, some reports have further differentiated their phenotypes based on the expression of CD16 − [ 214 ]. Thus, it is unknown whether the CSF1R hi monocyte subsets from this study included all MDSCs. However, in vitro studies with blood samples from chronic lymphocytic leukemia (CLL) patients revealed that GW-2580 co-culture decreased the total number of nurse-like cells (NLCs), a monocyte subset in CLL patients that interacts with TAMs and are associated with tumor progression in CLL patients [ 215 , 216 ].

The small molecular weight CSF1R inhibitor pexidartinib (formerly PLX-3397), which is an inhibitor of tyrosine kinases (TK) involved in hematopoiesis [ 217 ] including not only CSF1R but also c-KIT (stem cell factor receptor), Flt-3 (Fms-related tyrosine kinase 3), and VEGFR, has been studied in patients with advanced solid tumors. The administration of pexidartinib in combination with paclitaxel has shown clinical benefit (CR, P, and SD) in 19 of 38 patients including CRs in 16% of the patients studied [ 218 ]. Furthermore, the administration of pexidartinib was associated with a 57–100% decrease in the frequency of circulating CD14 dim CD16 + (non-classical) monocytes, suggesting a relationship between clinical benefit and CSF1R inhibition, as nonclassical monocytes have high expression levels of CSF1R as compared to other monocyte subsets [ 218 , 219 ]. Similarly, pexidartinib administration to GBM patients was found to reduce the frequency of circulating CD14 dim CD16 + monocytes by nearly 50% and to significantly decrease intratumoral Iba1 + microglia cells, a resident macrophage of neural tissue and a form of TAM in neurologic malignancies [ 220 ]. Unfortunately, pexidartinib has not been reported to improve PFS in this subset of GBM patients [ 220 ]. Another TKI, imatinib, with an inhibitory profile of activity similar to pexidartinib is currently used in the treatment of BCR-ABL + chronic myeloid leukemia (CML) and targets CSF1R [ 221 ]. Imatinib has been shown to significantly reduce the frequency of circulating G-MDSCs (CD11b + CD33 + CD14 − HLA‐DR − ) [ 222 ] in patients with CML.

Zoledronic acid (ZA), a bisphosphonate derivative, is used clinically to inhibit bone-resorbing osteoclasts, which are residential bone macrophages derived from myeloid progenitors, for treatment of osteopenia [ 223 ] and to prolong disease-free survival (DFS) in post-menopausal breast cancer patients [ 224 ]. Studies on tumor models using ZA have demonstrated a decrease in circulating MDSCs and increased T-cell infiltration into tumors [ 225 , 226 ]. To date, one clinical study on 15 PDAC patients with non-metastatic disease reported the regulation of MDSCs by ZA [ 227 ]. This was observed in patients that received ZA 2 weeks prior to surgery and twice, 4 weeks apart, following surgery [ 227 ]. In this study, flow analysis was undertaken on PB samples obtained prior to ZA injection and 3 months after surgery processed by Ficoll-density centrifugation and cryopreservation [ 227 ]. Perhaps due to density gradient separation and freezing, no difference was observed in the frequency of G-MDSCs in the PB or BM following ZA treatment [ 227 ]. Further, the authors suggested that the lack of MDSC response to ZA administration might have been due to insufficient dose or duration of ZA administration [ 227 ]. However, as mentioned previously, MDSCs and granulocytes are sensitive to freeze/thaw lysis [ 173 , 174 ]; potentially obscuring their assessment.

Inhibition of MDSC recruitment

Ibrutinib, a Bruton’s tyrosine kinase (BTK) and interleukin-2 (IL-2)-inducible kinase (ITK) inhibitor, has been shown to have off-target effects on MDSC mobilization, resulting in studies as an immunomodulatory therapy against solid tumors [ 228 ]. Human and murine MDSCs express BTK [ 229 ], and treatment with ibrutinib can inhibit the phosphorylation of BTK in MDSCs based on in vitro studies with PB from metastatic melanoma patients [ 229 ]. In vitro studies using PB samples have revealed that ibrutinib administration inhibits MDSC migration, generation, and nitrate production [ 229 ]. In a clinical study on CLL patients who were administered ibrutiniban, increase in the absolute numbers of CD4 + and CD8 + T-cells and decrease in circulating G-MDSCs were observed [ 230 ]. Despite the rationale for the bioactivity of ibrutinib, current clinical studies have not documented any survival benefit either alone or in combination with other therapies for patients with pancreatic [ 231 ], breast [ 232 , 233 ], or neuroendocrine cancers [ 234 ]. However, preliminary results from an ongoing study in patients with solid tumors that were treated with ibrutinib observed a decrease in plasma levels of chemokines involved in MDSC trafficking [ 235 ] that have stimulated similar clinical studies [ 232 , 236 ]. Thus, an indirect anti-tumor mechanism may be part of on-target inhibition of BTK by ibrutinib and another BTK inhibitor called acalabrutinib through the regulation of cytokines, chemokines, and growth factors. Further, the inhibition of BTK with ibrutinib or acalabrutinib has been shown to impair toll-like receptor (TLR) signaling and inflammasome activation in TAMs, MDSCs, DCs, and B cells, suppressing the secretion of cytokines, chemokines, and growth factors including IL-1b, IL-6, CXCL12, CXCL13, CCL19, and VEGF. Thus, ibrutinib and acalabrutinib appear to have potential clinical benefit to reduce inflammation-associated cancer progression and angiogenesis in the TME. [ 237 ]

Antigen-presenting and tumor cells secrete chemokines that recruit inflammatory cells, as well as myeloid progenitors and MDSCs, into the TME. The inhibition of chemokine functions [ 238 ] has been shown to block MDSC recruitment [ 239 ] and their regulation of tumor progression. One such inhibitor, HuMax-IL8 (also identified as BMS-986253), is a monoclonal antibody that binds IL-8, a chemotactic factor associated with cancer progression and MDSC recruitment to tumor sites [ 240 ]. In a dose escalation study on patients with metastatic or unresectable solid tumors, HuMax-IL8 was found to decrease serum IL-8 levels, resulting in significant reduction in IL-8 on day 3 as compared to baseline levels. In addition, SD was observed in 73% of patients with a median treatment duration of 24 weeks [ 241 ]. In preclinical studies, HuMax-IL8 has been shown to significantly inhibit MDSC recruitment and to improve CTL activity when given in combination with docetaxel [ 242 ]. However, in another clinical study using a mixture of solid tumor patients ( N = 3), no significant differences were observed in the frequency of circulating MDSCs and during dose escalation ( N = 6) in the dose expansion phase [ 241 ]. In contrast to these studies, a combination therapy with a CXCR4 antagonist (BL-8040), a PD-1 inhibitor (pembrolizumab), and chemotherapy in a larger cohort of patients with metastatic PDAC demonstrated an elevated frequency of tumor-infiltrating CD8 + effector T-cells and a reduced infiltration by MDSCs in biopsies from tumor metastases [ 243 ]. Additionally, a significant increase in circulating CD4 + and CD8 + T-cells and a significant decrease in T-regs were observed. In the patient cohort that received BL-8040 and pembrolizumab, a disease control rate (DCR) of 34.5% was observed, while in the cohort given BL-8040, pembrolizumab, and chemotherapy, a DCR of 77% and an ORR of 32% were observed. Studies using the CXCR4 antibody ulocuplumab (BMS-936564) in combination with lenalidomide or bortezomib plus dexamethasone for the treatment of MM was examined. In this study, an ORR of 55.2% and a DCR of 72.4% were observed in patients receiving combined therapy [ 244 ]. Taken together, these results suggest that there may be a potential benefit for combination chemokine and checkpoint inhibitor therapy to mitigate MDSC recruitment and immunosuppression of tumors.

Other studies have used inhibitors for chemokine ligands for CXCR2, like CXCL2 or CXCL5, which are essential for PMN-MDSC recruitment. Targeting the chemokine receptor CXCR2 can reduce MDSC populations, promote T-cell infiltration, and improve the efficacy of PD-1 blockade in preclinical studies [ 245 , 246 ]. SX-682, a CXCR1/2 inhibitor, enhanced the T-cell-based immunotherapeutic efficacy in studies using murine tumor models that assessed MDSC-infiltration [ 247 , 248 ]. The CXCR1/2 inhibitor reparixin showed promise in a window-of-opportunity clinical trial for HER-2-negative breast cancer. Reparaxin was safe, well tolerated, and caused reduction in cancer stem cells in patient tumors [ 249 ]. Further, a phase 1b trial in patients with metastatic breast cancer found reparixin to be safe in combination with paclitaxel [ 250 ]. Due to the above studies, reparixin was investigated in combination with paclitaxel as a frontline treatment for metastatic TNBC in a phase 2 trial; however, the primary endpoint of prolonged PFS was not met [ 251 ]. While there are no CXCR2-targeted drugs currently approved for use in cancer treatment, additional CXCR2 antagonists are also in various stages of clinical development for both neoplastic and inflammatory diseases [ 252 ].

S100A8 (calgranulin A, also called myeloid-related protein 8, MRP8) and S100A9 (calgranulin B or myeloid-related protein 14, MRP14) are small molecular calcium-binding proteins with crucial roles in cancer development that can act as diagnostic markers and targets for cancer therapy. [ 253 ] MDSCs express both S100A8 and A9 and the corresponding receptors RAGE, acting as a positive feedback loop to chemo-attract MDSCs and increase their immunosuppressive function. [ 254 ] Tasquinimod, an oral drug that can bind to S100A9 and inhibits interaction between S100A9 and its sensors, including RAGE and TLR4 [ 255 ], has been studied in numerous clinical trials documenting that its administration to cancer patients can decrease blood monocytes, reduce MDSC infiltration of tumors, and polarize TAMs to M1 cells. [ 256 ] In a phase II trial, tasquinimod administration was reported to improve the PFS in patients with metastatic castration-resistant prostate cancer (mCRPC) by reducing the recruitment of MDSCs and inhibiting metastasis [ 257 ]. In a recent phase III clinical trial ( NCT01234311 ), tasquinimod treatment of mCRPC patients resulted in improved radiologic PFS compared with the placebo group but had no impact on OS. [ 258 ]

One marker of G-MDSCs is the expression of CD33 where it has an important role in MDSC-mediated hematopoietic suppressive function following its activation by S100A9. Studies using a fully human, Fc-engineered monoclonal antibody against CD33 (BI 836,858) that suppresses CD33-mediated signal transduction have been shown to improve the bone marrow microenvironment in patients with MDS. [ 259 ] In this study, BI 836,858 was shown to reduce MDSC by antibody-dependent cellular cytotoxicity (ADCC), which correlated with increases in granule mobilization and cell death. BI 836,858 has shown to block CD33 downstream signaling, preventing immune-suppressive cytokine secretion, correlating with a significant increase in the formation of progenitor cells colonies. The activation of the CD33 pathway can also cause reactive oxygen species (ROS)-induced genomic instability. Unconjugated, monospecific anti-CD33 antibodies have had disappointing results clinically, showing only modest activity [ 260 ]. In contrast to monospecific antibodies, T cell engagers that bind to CD3 and CD33 have shown activity in reducing CD33 + malignant clones in AML patients and activation of T cells. [ 261 ] One such bispecific T cell engager, AMV564, is in early-stage clinical trials in patients with advanced solid tumor malignancies, AML, or MDSs ( NCT03144245 , NCT03516591 , and NCT04128423 ). In these clinical studies, in vitro AMV564 has been shown to decrease MDSCs, increase CTLs, and reduce tumor burden. [ 262 ]

Induction of MDSC differentiation

The vitamin A metabolite all- trans retinoic acid (ATRA) is used clinically to induce differentiation of leukemic blasts into mature myeloid cells in patients with acute promyelocytic leukemia [ 263 ] and to induce the differentiation of MDSC into macrophages and/or dendritic cells in vitro. In addition, treatment of tumor-bearing mice with ATRA has been shown to result in significant reduction of MDSC in vivo [ 264 ] . ATRA specifically targets immature myeloid cells, including MDSCs, by inducing their differentiation through the upregulation of glutathione synthase and glutathione production [ 265 , 266 ]. Thus, in addition to reducing MDSCs, especially G–MDSCs, the differentiation of M-MDSCs into DCs has additional potential clinical benefit. In one study, administration of ATRA significantly reduced the frequency of Lin − HLA-DR − CD33 + myeloid cells in the PB of mRCC patients 7 and 14 days post-treatment [ 265 ]. In another study on advanced-stage melanoma patients, the administration of ATRA with ipilimumab significantly reduced the frequency of circulating MDSCs and their expression of immunosuppressive genes [ 267 ]. Similar to ATRA, vitamin D3 has been shown to have anti-inflammatory and immunomodulatory activities [ 268 ]. Furthermore, vitamin D3 has been shown to inhibit cancer progression and recurrence and to improve DFS [ 269 ]. In a recent murine study, it was shown that MDSCs, especially M-MDSCs, express the vitamin D receptor (VDR) and that following ligation had reduced immunosuppressive activity [ 270 ]. In a clinical study on HNSCC patients given escalating doses of vitamin D3, a significant decrease in the frequency of CD34 + suppressor cells, likely MDSCs, was reported in higher dose groups, in association with significant decrease in serum GM-CSF levels [ 271 ]. However, the frequency of CD34 + suppressor cells and serum cytokine levels was observed to increase following 6 weeks of treatment [ 271 ]. The initial reduction followed by the increase in the frequency of circulating suppressor cells may be due to the lower serum GM-CSF levels in the beginning of the study as compared to week 6 [ 271 ]. Based on these early studies, additional clinical research is needed to determine which cancers and associated treatment regimens, as well as patient characteristics, would benefit most from vitamin D3 supplementation.

Tyrosine kinase and JAK inhibitors

The janus kinase (JAK)/STAT, BTK, and mitogen-activated protein kinase (MAPK)-interacting serine/threonine protein kinases 1 and 2 (MNK1/2) pathways have been shown to contribute to the adaptive and innate immune responses that contribute to primary autoimmune disorders and immune response (ir) AEs and myeloplasia [ 272 ]. Further, JAK/STAT signaling pathways engage various TKs and downstream effectors involved in MDSC recruitment, expansion, angiogenesis [ 273 ], as well as, suppression of T-cell function by interactions with tumor-derived signaling molecules including VEGF, G-CSF, and GM-CSF. Ruxolitinib (Rux), a JAK1/2 inhibitor, has been shown to significantly decrease circulating G-MDSCs and T-regs [ 274 ] and the levels of pSTAT5 in patients with myeloproliferative diseases [ 274 ]. However, treatment-induced STAT5 activation and an associated reduction in T-cell numbers and pro-inflammatory cytokines could potentially contribute to loss of T-cell function [ 274 ]. However, Rux treatment has not been documented to provide clinical benefit for solid tumor patients, including early clinical trials of non-small cell lung cancer (NSCLC) [ 275 ] and metastatic pancreatic cancer; [ 276 ] although it has received regulatory approval for the treatment of myelofibrosis [ 277 ]. Indeed, studies using murine tumors and clinical activity for myelofibrosis suggest that JAK/STAT pathway inhibition has a promise as an MDSC-targeting strategy [ 278 ].

Sunitinib, a TKI, has been reported to inhibit the VEGFR-1/2/3, PDGFR-α/β, c-Kit, and FLT-3 and ret proto-oncogene (RET) receptors [ 279 ]; most of which support MDSC proliferation [ 280 ] and are a frontline therapy for renal carcinoma (RCC) [ 281 ]. In a study on RCC patients, a significant decrease in MDSC frequency was observed when patients’ myeloid cells were cultured in vitro with varying concentrations of sunitinib for 48 h, with MDSC apoptosis induced in a dose-dependent manner [ 282 ]. Furthermore, in a clinical trial, a subset of patients treated with two cycles of sunitinib were found to have decreased frequencies of MDSCs relative to baseline, a difference that significantly correlated with increase in IFN-γ production [ 282 ].I n a separate study, cancer patients with oligometastatic disease were treated with sunitinib prior to stereotactic body radiotherapy [ 283 ], and 7 days following sunitinib therapy, patients had a significantly lower frequency of circulating MDSCs (CD33 + HLA-DR −/lo ). The authors of this study concluded that sunitinib increased the efficacy of SBRT in patients with oligometastases by reversing MDSC and T-reg-mediated immune suppression [ 283 ]. Of note, in these studies, M-MDSCs were defined as CD33 + CD14 + CD16 + , differing from most studies, which include HLA-DR low/− and/or CD11b + cells in the M-MDSC phenotype.

Overview of combining MDSC inhibition with immunotherapy

Immunotherapy has provided another novel approach for cancer treatment with numerous immunotherapies being approved by the FDA for routine use or for investigation. By enhancing T-cell-mediated cytotoxicity, drugs and biologics that restore immune response have shown a therapeutic effect on several cancers. [ 284 , 285 ] Tumor cytotoxic activity is induced by an antigen-specific signal by T-cell receptor recognition [ 286 ] or an antigen-independent signal regulated by co-signaling receptors, for which PD-1 and CTLA-4 are crucial co-inhibitors. [ 287 ] Immune checkpoint inhibitors (ICIs) provide a type of immunotherapy that blocks immune checkpoint proteins on tumor cells, including but not limited to programmed death-ligand 1 (PD-L1) and CTL-associated protein 4 (CTLA-4), from binding to their ligands, PD-1 and CD80/86, respectively. These checkpoints limit robust immune responses contributing to tumor evasion by CD8 + T-cell immunosurveillance [ 288 ]. The CTLA-4 inhibitor ipilimumab, which was approved by the FDA for metastatic melanoma [ 289 ], has been shown to improve OS in patients with lower frequencies of MDSCs [ 290 , 291 ]. Furthermore, ipilimumab has been shown to reduce the frequency of M-MDSCs, their production of NO, and the frequency of PD-1 + G-MDSCs [ 291 ]. In a study on melanoma patients treated with ipilimumab over 9 weeks, it was demonstrated that a significant reduction in the PB frequency of G-MDSCs and ARG1 + myeloid cells was induced as compared to baseline levels [ 292 , 293 ]. Further, treatment with ipilimumab was found to significantly decrease iNOS and ARG1 expression relative to baseline [ 294 ].

A low frequency of MDSCs has also been shown to provide a predictive response marker for melanoma patient and a benefit from ipilimumab treatment [ 295 ]. Results from one study suggested that the frequency of M-MDSC provided a predictive response marker, as low frequencies identified patients benefitting from ipilimumab intervention. Interestingly, patients treated with multiple doses of the PD-L1 inhibitors atezolizumab or avelumab have been shown to have a significantly decreased frequency of PB PD-L1 + M-MDSCs [ 296 ]; while multiple doses of the PD-1 inhibitor pembrolizumab have been shown to significantly decrease the frequency of PD-1 + M- and i-MDSCs in the PB [ 2 , 96 ]. Consistent with these observations, several studies have documented a relationship between MDSC infiltration and PD1 blockade resistance, and that selective depletion of MDSCs could restore anti-PD1 therapy efficacy [ 297 , 298 ]. Studies such as these have suggested that administration of ICIs may be effective in reducing the frequency of MDSCs, cells that express ICI-targeted checkpoint markers, as well as a possible use of MDSCs as biomarkers to predict and/or monitor patient outcomes.

Similar to CTLA-4, the interaction of PD-1 and its ligand, PD-L1, is involved in another important immune inhibitory process, which leads to T effector cell exhaustion and their conversion into T-regs [ 299 ]. Clinical trials have documented that blockade of PD-1/PD-L1 signaling can enhance anti-tumor immunoactivity of T cells and clinical outcomes resulting in FDA approval of several biologics. Further, combination therapy with ipilimumab and nivolumab (PD-1 inhibitor) demonstrated additive therapeutic activity against melanoma, advanced UC, mesothelioma, CRC, hepatocellular carcinoma, and NSCLC. [ 300 ] Recently, a biologic against LAG-3 has received FDA approval for the treatment of advanced melanoma [ 301 ]. This immune checkpoint inhibition can also increase responses to other forms of immunotherapy via monoclonal antibodies targeting tumor-associated antigens, cancer vaccines, adoptive immune cells therapies, and unspecific boosting of the immune system with ILs, IFNs, or TLR-ligands. However, the anticancer effects of all these treatments can be limited by MDSCs through dampening the host’s immune responses against tumors. Consequently, alternative strategies targeting MDSCs combined with active or passive immunotherapies are under extensive study in clinical trials. [ 302 ]

The objective of cancer vaccines is to sensitize and activate antigen-specific T-cell responses to kill tumor cells. Despite decades of study, only one vaccine, sipuleucel-T (Provenge®), has received FDA approval, though it has been found to only extend the OS of metastatic prostate cancer patients by 4 months [ 303 ]. An investigational vaccine, nelipepimut-S (NeuVax™), for breast cancer that uses GM-CSF as an adjuvant [ 304 , 305 ] was examined in a phase I/II trial and showed increased CTL responses and decrease in the frequency of T-regs following immunization and indicated favorable responses in patients; however, circulating MDSCs were not assessed [ 306 ]. A subsequent phase III trial of NeuVax™ was recently reported and showed that NeuVax alone had no significant clinical effect on breast cancer patients [ 305 ]. The use of GM-CSF as an adjuvant increases tumor antigen presentation and, thus, potential anti-tumor responses [ 304 ]; however, it also expands MDSCs at higher doses [ 304 , 307 ]. This provides one of the challenges in maximizing antigen presentation using GM-CSF while minimizing MDSC expansion. In a study on relapsed prostate cancer patients given daily high doses of GM-CSF as a vaccine adjuvant, the patients experienced an increase in the absolute number of MDSCs and T-regs, whereas patients given lower, intermittent doses had fewer MDSCs and T-regs [ 308 ]. Additionally, tumor-secreted GM-CSF in mesothelioma patients has been shown to bolster the immunosuppressive activities of G-MDSCs (CD15 + CD33 − HLA-DR low/− CD11b + CD66b + CD16 + ) granulocytic cells [ 309 ]. Similar activities have been observed with G-CSF as discussed in the section of this review titled Chemotherapy-associated depletion and inhibition of MDSC proliferation.

Aside from challenges associated with selecting an appropriate adjuvant to maximize antigen presentation and anti-tumor immunity, chemotherapeutics, as with ICIs, may also minimize the negative impact of inhibitory immune factors and cell populations such as MDSCs on vaccine efficacy. In a clinical trial, patients with advanced pancreatic cancer were given personalized cancer peptide vaccines plus gemcitabine. In this study, 11 of the 13 patients showed clinical responses defined as reduction in tumor size [ 310 ]. In a similar study on small cell lung cancer (SCLC) patients, co-administration of INGN-225 (a vaccine comprised of autologous dendritic cells transduced with a modified p53 adenovirus) and ATRA resulted in the development of a tumor-specific T-cell response in 43.3% of patients versus 20% of patients treated with the vaccine alone [ 311 ]. In another SCLC trial in which patients also received chemotherapy, the administration of INGN-225 resulted in significant p53-specific T-cell response in 52% of patients, along with persistently elevated IFN-γ production 2–3 weeks after the last vaccine cycle; however, a significant elevation in immature myeloid cells (Lin − HLA-DR − CD33 + ) was also observed, which included MDSCs [ 312 ]. Of the patients who received second-line chemotherapy following vaccine administration, 61.9% achieved a complete or partial response to therapy [ 312 ]. In another study, patients with platinum-resistant ovarian cancer were given a non-dendritic cell p53 vaccine (p53MVA; day 15) with gemcitabine (days 1 and 8) over 3 cycles of chemotherapy. Approximately half of the patients had p53-reactive CD4 + and CD8 + T-cells despite no change in the frequency of circulating MDSCs or T-regs after the first treatment cycle [ 313 ]. Although no association was observed between MDSC or T-reg levels and treatment response, an inverse trend in patient PFS and MDSC and T-reg frequency was observed but was not statistically significant [ 313 ]. Cancer vaccines can augment tumor-specific immunity, but it is hypothesized that their use would be most effective in patients with low MDSC frequency [ 303 , 304 , 306 , 307 , 310 – 313 ]. The mixed responses to vaccine-based immunotherapy describe a need for therapeutic strategies involving a combination of vaccine and other immune-modulating agents to abrogate the immunosuppressive effects of MDSCs and ensure the ability to mount sufficient T-cell anti-tumor responses. A phase II study on Sipuleucel-T with indoximod, an indoleamine 2,3-dioxygenase inhibitor (IDO) ( NCT01560923 ) in prostate cancer patients [ 314 ], documented better PFS compared with Sipuleucel-T plus placebo, indicating that the IDO inhibitor may lead to improved vaccine and clinical outcomes. [ 315 ] This is based on the function of the tryptophan catabolic enzyme iIDO1 that can act as a driver of tumor-mediated suppression. Currently, numerous approaches to reduce MDSCs to improve immune response are undergoing, [ 316 ] yet no consensus on the clinical benefit of inhibiting the immunosuppressive effectors of MDSC has been made. More optimized studies are still needed. Other studies demonstrated that the combination of chemotherapeutic drugs with vaccines could decrease MDSC numbers and lead to additive survival benefits in cancer patients [ 11 , 317 ]. Studies on combination trials of chemotherapeutic drugs to decrease MDSCs with vaccines include gemcitabine pretreatment to enhance the efficacy of DC vaccines after tumor resection by eliminating immunosuppressive cells. Additive effects of DC vaccines and gemcitabine are under investigation in adults and children with sarcoma ( NCT01803152 ). Recently, a pilot study was conducted in eight metastatic RCC patients treated with autologous tumor lysate-loaded DC vaccine plus sunitinib. Analysis showed no vaccination-related severe adverse events. Moreover, tumor lysate-reactive T cell responses were observed in five patients, four of whom showed decreased frequencies of MDSCs. [ 318 ]

While ICIs are effective in reducing MDSCs, as demonstrated in patients with metastatic melanoma [ 289 , 290 , 294 ], their efficacy appears to be improved when given in combination with drugs targeting MDSCs. MDSCs inhibit T-cell activation and immune responses to tumor antigens [ 61 , 92 ] despite the use of PD-1 and PD-L1 checkpoint inhibitors [ 169 , 319 ]. Thus, therapeutic strategies promoting anti-tumor immune responses, i.e., ICIs, and ones that deplete or inhibit MDSCs may improve patient outcomes and reduce MDSC-associated ICI resistance [ 320 ]. A meta-analysis of anti-PD-1 therapy studies undertaken in NSCLC patients revealed that the addition of the PD-1 inhibitor pembrolizumab to chemotherapy resulted in significantly improved OS [ 321 ]. Similarly, combining the anti-angiogenic drug bevacizumab with the antimetabolite capecitabine was found to reduce circulating MDSCs following tumor resection [ 167 ]. Furthermore, in these studies, CyTOF analysis of tumor samples determined that adding capecitabine to the treatment regimen supported the induction of tumor-directed immune activation, with a significant reduction in the expression of CTLA-4 and PD-1 on macrophages and CTLA-4 on lymphocytes [ 167 ]. Preliminary studies from a phase I clinical trial using patients with metastatic tumors and the BTK inhibitor ibrutinib in combination with the PD-1 inhibitor nivolumab supported the use of the two inhibitors to significantly decrease plasma chemokine levels (IL-12, CCL2, CCL3, and CCL4) and circulating MDSCs as observed during the first cycle of therapy [ 322 ]. These studies support the efficacy of combining ICIs with immunomodulatory drugs and reinforce the approach of targeting MDSCs to improve the efficacy of ICI therapy.

In MM patients, PD-L1 expression is increased on plasma cells compared to the levels found on plasma cells from healthy donors, and its expression is associated with resistance to a variety of anti-MM treatments. The reported inhibition of the PD-L1/PD-1 pathway in plasma cells by ruxolitinib [ 323 ] and decreased G-MDSC (LOX-1 expression) in Hodgkin’s lymphoma treated with a PD-1 inhibitor plus ruxolitinib suggested that ruxolitinib may be used for combination therapy with ICI therapy in the treatment of myeloproliferative neoplasms (MPNs), as it can reduce MDSC expression of PD-L1. Further, MDSCs in MPN have an increase in PD-L1 expression, and ruxolitinib added to the CPI therapy may provide a promising treatment option. [ 324 ]

Summary

In summary, the goal of adjuvant therapy is to reduce the risk of recurrence and outgrowth of metastases after primary tumor treatment but must also constrain the release of tumor cells from dormancy. Tumor dormancy is a significant contributor to the time between initial diagnosis and relapse/metastasis following primary surgery and adjuvant therapy. Thus, improving our understanding of why DTCs enter dormancy, later reawaken, and proliferate into an occult metastasis and approaches to control this process is vital to developing effective therapeutic strategies against neoplastic disease. Dormant DTCs are rarely detectable with current diagnostic technologies, making it critical to better understand reawakening and preventing their outgrowth. Adaptive anti-tumor immune responses can maintain tumor cell dormancy, whereas chronic inflammation, i.e., circulating MDSCs, can reactivate DTC from immune-mediated dormancy. Thus, we suggest that limiting the expansion and function of MDSCs may maintain DTC dormancy and inhibit outgrowth of micro-metastases, resulting in prolonged survival. In conclusion, disrupting immune TME interactions provides an attractive therapeutic option, bringing us closer to the goal of preventing tumor relapse and improving patient quality of life.

This is not an inclusive list of drugs believed to decrease MDSC numbers and function. Rather, it is a list of drugs that have been studied clinically, frequently off label, and have been shown to have or potentially have clinical activity on MDSCs. AE , adverse event; ALVAC , cancer vaccine containing a canary pox virus; AML , acute myeloid leukemia; ARG1 , arginase 1; ATRA , all-trans retinoic acid; BC , breast cancer; BM , bone marrow; BTK , Bruton’s tyrosine kinase; CA , cancer; CCL , chemokine ligand; CD , cluster of differentiation; CCL , chemokine (C–C motif); CLL , chronic lymphocytic leukemia; CML , chronic myeloid leukemia; CRC , colorectal carcinoma; CRPC , castration-resistant prostate cancer; CSF-1R , colony stimulating factor-1 receptor; CRPC , CTL: cytotoxic T-lymphocyte; CTLA-4 , CTL-associated protein 4; CXCR4 , CXC chemokine receptor 4; DC , dendritic cell; DFS , disease-free survival; FOLFOX , folic acid, 5-fluorouracil, oxaliplatin; G , granulocyte; GBM , glioblastoma multiforme; GM , granulocyte–macrophage; HLA-DR , human leukocyte antigen-DR isotype; HNSCC , head and neck squamous cell carcinoma; Iba-1 , ionized calcium-binding adaptor molecule 1; i , immature; IFN-g , interferon-gamma; IL , interleukin; iNOS , inducible nitric oxide synthetase; JAK , Janus kinase; L-NMMA , L-N(G)-monomethyl arginine acetate; Lin , linage; m, metastatic; M , macrophage; MDS , myeloid dysplastic syndrome; MDSC , myeloid-derived suppressor cell; MM , multiple myeloma; MVA , modified vaccinia virus Ankara; NO , nitric oxide; NSCLC , non-small cell lung cancer; ORR , overall response rate; OS , overall survival; PB , peripheral blood; PBL , peripheral blood leukocyte; PBMC, peripheral blood mononuclear cell; PD-1 , programmed cell death; PDAC , pancreatic ductal adenocarcinoma; PDE-5 , phosphodiesterase type 5; PD-L1 , programmed death-ligand 1; PFS , progression-free survival; pt(s) , patient(s); RCC , renal cell carcinoma; SCLC , small cell lung cancer; STAT , signal transducer and activator of transcription; TCR , T-cell receptor; TK , tyrosine kinase; TNBC , triple negative breast cancer; T-reg , T-regulatory cell; TX , transplant; UC , urothelial carcinoma; VEGF , vascular endothelial growth factor; ZA , zoledronic acid.

Abbreviations

5-Fluorouracil

Acute lymphoblastic leukemia

Adenocarcinoma

Adverse events

All-trans retinoic acid

Antibody-dependent cellular cytotoxicity

Arginase-1

Best supportive care

Bone marrow

Bruton’s tyrosine kinase

Colorectal carcinoma

Chronic lymphocytic leukemia

Chronic myeloid leukemia

Circulating tumor cells

Cluster of differentiation

Colony-stimulating factors

CTL-associated protein 4

CXC chemokine receptor 4

Cyclic guanosine monophosphate

Cyclooxygenase-2

Cytotoxic T-lymphocytes

Tumor-associated macrophages

Dendritic cells

Disseminated tumor cells

Doxorubicin plus cyclophosphamide

Ductal carcinoma in situ

Epithelial neutrophil-activating peptide-78

Extracellular matrix

Extramedullary myelopoiesis

Fms-related tyrosine kinase 3

Folic acid, 5-FU, and irinotecan

Folic acid, 5-FU, and oxaliplatin

Food and Drug Administration

Gemcitabine with capecitabine

Glioblastoma multiforme

Granulocyte-colony stimulating factor

Granulocyte macrophage

Granulocytic

Head and neck squamous cell carcinoma

Hematopoietic progenitor cell

Hematopoietic stem cells

IL-2-inducible kinase

Immature or early

Immune checkpoint inhibitors

Immune response

Interferon

Interleukin-2

Janus kinase

Lineage-negative

Lymphocyte activation gene-3

MAPK-interacting serine/threonine protein kinases 1 and 2

Mass cytometry time of flight

Matrix metalloproteinases

Minimal residual disease

Mitogen-activated protein kinase

Monocytic

Multiple myeloma

Myeloid-derived suppressor cells

Myelopoietic progenitor cells

Natural killer

Nitric oxide synthase

Non-small cell lung cancer

Overall survival

Pancreatic ductal adenocarcinoma

Peg-filgrastim

Peripheral blood mononuclear cells

Phosphodiesterase type 5

Polymorphonuclear

Programed death

Programmed death-ligand 1

Progression-free survival

Prostaglandin E2

Reactive oxygen species

Renal carcinoma

Response Evaluation Criteria in Solid Tumors

Ret proto-oncogene

Ruxolitinib

Serum monoclonal immunoglobulin

SFAS ligand

Signal transducer and activator of transcription

Signal transducer and activator of transcription 3

Small cell lung cancer

Soluble FAS

Stromal cell-derived factor 1

T-cell receptor zeta chain

Toll-like receptor

T-regulatory cells

Tumor microenvironment

Tumor-associated neutrophils

Tyrosine kinase inhibitor

Vascular endothelial growth factor

Vascular endothelial growth factor receptor-1 positive

Vitamin D receptor

Zoledronic acid

Publisher's Note

Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Author contribution

All the authors have participated in the writing of this submission and reviewed and approved the submission of this manuscript.

Funding

This work was supported, in whole or in part, by National Institutes of Health Grants for Specialized Programs of Research Excellence (Grant P50 CA127297) and by the Fred & Pamela Buffett Cancer Center Support (Grant P30 CA036727).

Declarations

Ethics approval

As this was a review, no animal or human specimens were involved in the writing of the review.

Conflict of interest

The authors declare no competing interests.

References

1. Coleman C Early detection and screening for breast cancer Seminars in Oncology Nursing 2017 33 141 155 10.1016/j.soncn.2017.02.009 28365057

2. Tsodikov A Reconciling the effects of screening on prostate cancer mortality in the ERSPC and PLCO trials Annals of Internal Medicine 2017 167 449 455 10.7326/M16-2586 28869989 PMC5734053

3. Bivona TG Doebele RC A framework for understanding and targeting residual disease in oncogene-driven solid cancers Nature Medicine 2016 22 472 478 10.1038/nm.4091 27149220 PMC5384713

4. Dasari A Grothey A Kopetz S Circulating tumor DNA-defined minimal residual disease in solid tumors: Opportunities to accelerate the development of adjuvant therapies Journal Clinical Oncology 2018 36 35 3437 10.1200/JCO.2018.78.9032 PMC6286158 30376428

5. Sosa MS Avivar-Valderas A Bragado P Wen HC Aguirre-Ghiso JA ERK1/2 and p38alpha/beta signaling in tumor cell quiescence: Opportunities to control dormant residual disease Clinical Cancer Research 2011 17 5850 5857 10.1158/1078-0432.CCR-10-2574 21673068 PMC3226348

6. Ghajar CM Metastasis prevention by targeting the dormant niche Nature Reviews Cancer 2015 15 238 247 10.1038/nrc3910 25801619 PMC4842412

7. Giancotti FG Mechanisms governing metastatic dormancy and reactivation Cell 2013 155 750 764 10.1016/j.cell.2013.10.029 24209616 PMC4354734

8. Aguirre-Ghiso JA Models, mechanisms and clinical evidence for cancer dormancy Nature Reviews Cancer 2007 7 834 846 10.1038/nrc2256 17957189 PMC2519109

9. Sosa MS Bragado P Aguirre-Ghiso JA Mechanisms of disseminated cancer cell dormancy: An awakening field Nature Reviews Cancer 2014 14 611 622 10.1038/nrc3793 25118602 PMC4230700

10. Nicolini A Ferrari P Rossi G Carpi A Tumour growth and immune evasion as targets for a new strategy in advanced cancer Endocrine-Related Cancer 2018 25 R577 R604 10.1530/ERC-18-0142 30306784

11. DeSantis CE Cancer treatment and survivorship statistics, 2014 CA: A Cancer Journal for Clinicians 2014 64 252 271 24890451 10.3322/caac.21235

12. Jahanban-Esfahlan R Tumor cell dormancy: Threat or opportunity in the fight against cancer Cancers (Basel) 2019 11 1207 10.3390/cancers11081207 31430951 PMC6721805

13. Yuhas JM Tarleton AE Dormancy and spontaneous recurrence of human breast cancer in vitro Cancer Research 1978 38 3584 3589 698921

14. Romero I Garrido F Garcia-Lora AM Metastases in immune-mediated dormancy: A new opportunity for targeting cancer Cancer Research 2014 74 6750 6757 10.1158/0008-5472.CAN-14-2406 25411345

15. Luzzi KJ Multistep nature of metastatic inefficiency: Dormancy of solitary cells after successful extravasation and limited survival of early micrometastases American Journal of Pathology 1998 153 865 873 10.1016/S0002-9440(10)65628-3 9736035 PMC1853000

16. Bissell MJ Hines WC Why don’t we get more cancer? A proposed role of the microenvironment in restraining cancer progression Nature Medicine 2011 17 320 329 10.1038/nm.2328 21383745 PMC3569482

17. Uhr JW Pantel K Controversies in clinical cancer dormancy Proc Natl Acad Sci U S A 2011 108 12396 12400 10.1073/pnas.1106613108 21746894 PMC3145712

18. Willis, R. (1934) The spread of tumours in the human body. By Rupert A. Willis. M.D., B.S., D.Sc. (Melbourne), 1934 London: J. & A. Churchill. 25s. net. BJS (British Journal of Surgery), 22 , 196–196.

19. Hadfield G The dormant cancer cell Br Med 1954 J2 607 610 10.1136/bmj.2.4888.607 PMC2078920 13190204

20. Braun S Cytokeratin-positive cells in the bone marrow and survival of patients with stage I, II, or III breast cancer New England Journal of Medicine 2000 342 525 533 10.1056/NEJM200002243420801 10684910

21. Braun S A pooled analysis of bone marrow micrometastasis in breast cancer New England Journal of Medicine 2005 353 793 802 10.1056/NEJMoa050434 16120859

22. Friberg S Nystrom A Cancer metastases: Early dissemination and late recurrences Cancer Growth Metastasis 2015 8 43 49 10.4137/CGM.S31244 26640389 PMC4664198

23. Morris-Stiff G Transmission of donor melanoma to multiple organ transplant recipients American Journal of Transplantation 2004 4 444 446 10.1111/j.1600-6143.2004.00335.x 14962000

24. Endo H Inoue M Dormancy in cancer Cancer Science 2019 110 474 480 10.1111/cas.13917 30575231 PMC6361606

25. Gimbrone MA Jr Leapman SB Cotran RS Folkman J Tumor dormancy in vivo by prevention of neovascularization Journal of Experimental Medicine 1972 136 261 276 10.1084/jem.136.2.261 5043412 PMC2139203

26. Holmgren L O'Reilly MS Folkman J Dormancy of micrometastases: Balanced proliferation and apoptosis in the presence of angiogenesis suppression Nature Medicine 1995 1 149 153 10.1038/nm0295-149 7585012

27. Stewart, T. H. (1996). Immune mechanisms and tumor dormancy. Medicina (B Aires), 56 : (Suppl 1), 74–82. 9224977

28. Goddard ET Bozic I Riddell SR Ghajar CM Dormant tumour cells, their niches and the influence of immunity Nature Cell Biology 2018 20 1240 1249 10.1038/s41556-018-0214-0 30361702

29. Rusciano D Burger MM Why do cancer cells metastasize into particular organs? BioEssays 1992 14 185 194 10.1002/bies.950140309 1586372

30. Nicolson GL Cancer metastasis: Tumor cell and host organ properties important in metastasis to specific secondary sites Biochimica et Biophysica Acta 1988 948 175 224 3052592 10.1016/0304-419x(88)90010-8

31. Graf AH Buchberger W Langmayr H Schmid KW Site preference of metastatic tumours of the brain Virchows Arch A Pathol Anat Histopathol 1988 412 493 498 10.1007/BF00750584 3128919

32. Paget S The distribution of secondary growths in cancer of the breast 1889 Cancer Metastasis Rev, 1989 8 98 101 2673568

33. Riihimäki M Metastatic sites and survival in lung cancer Lung Cancer 2014 86 78 84 10.1016/j.lungcan.2014.07.020 25130083

34. Talmadge JE Donkor M Scholar E Inflammatory cell infiltration of tumors: Jekyll or Hyde Cancer and Metastasis Reviews 2007 26 373 400 10.1007/s10555-007-9072-0 17717638

35. Tarin D Mechanisms of human tumor metastasis studied in patients with peritoneovenous shunts Cancer Research 1984 44 3584 3592 6744281

36. Fidler IJ Metastasis: Quantitative analysis of distribution and fate of tumor emboli labeled with 125 I-5-iodo-2′-deoxyuridine Journal of the National Cancer Institute 1970 45 773 782 5513503

37. Riethdorf S Detection of circulating tumor cells in peripheral blood of patients with metastatic breast cancer: A validation study of the cell search system Clinical Cancer Research 2007 13 920 928 10.1158/1078-0432.CCR-06-1695 17289886

38. Wang C Improved prognostic stratification using circulating tumor cell clusters in patients with metastatic castration-resistant prostate cancer Cancers (Basel) 2021 13 268 10.3390/cancers13020268 33450815 PMC7828213

39. Klein CA Parallel progression of primary tumours and metastases Nature Reviews Cancer 2009 9 302 312 10.1038/nrc2627 19308069

40. Weiss L Random and nonrandom processes in metastasis, and metastatic inefficiency Invasion and Metastasis 1983 3 193 207 6588046

41. Arnerlöv C Breast carcinoma growth rate described by mammographic doubling time and S-phase fraction Correlations to clinical and histopathologic factors in a screened population Cancer 1992 70 1928 1934 10.1002/1097-0142(19921001)70:7<1928::AID-CNCR2820700720>3.0.CO;2-R 1525768

42. Klein CA Genetic heterogeneity of single disseminated tumour cells in minimal residual cancer Lancet 2002 360 683 689 10.1016/S0140-6736(02)09838-0 12241875

43. Gruber IV Relationship between hematogenous tumor cell dissemination and cellular immunity in DCIS patients Anticancer Research 2016 36 2345 2351 27127142

44. Sänger N Disseminated tumor cells in the bone marrow of patients with ductal carcinoma in situ International Journal of Cancer 2011 129 2522 2526 10.1002/ijc.25895 21207426

45. Hu Z Curtis C Looking backward in time to define the chronology of metastasis Nature Communications 2020 11 3213 10.1038/s41467-020-16995-y 32587245 PMC7316862

46. Alix-Panabieres C Pantel K Challenges in circulating tumour cell research Nature Reviews Cancer 2014 14 623 631 10.1038/nrc3820 25154812

47. Dasgupta A Lim AR Ghajar CM Circulating and disseminated tumor cells: Harbingers or initiators of metastasis? Molecular Oncology 2017 11 40 61 10.1002/1878-0261.12022 28085223 PMC5423226

48. Schardt JA Genomic analysis of single cytokeratin-positive cells from bone marrow reveals early mutational events in breast cancer Cancer Cell 2005 8 227 239 10.1016/j.ccr.2005.08.003 16169467

49. Schmidt-Kittler O From latent disseminated cells to overt metastasis: Genetic analysis of systemic breast cancer progression Proc Natl Acad Sci U S A 2003 100 7737 7742 10.1073/pnas.1331931100 12808139 PMC164657

50. Gray JW Evidence emerges for early metastasis and parallel evolution of primary and metastatic tumors Cancer Cell 2003 4 4 6 10.1016/S1535-6108(03)00167-3 12892707

51. Weigelt B Gene expression profiles of primary breast tumors maintained in distant metastases Proceedings of the National Academy of Sciences 2003 100 15901 15905 10.1073/pnas.2634067100 PMC307665 14665696

52. Magrì A Bardelli A Does early metastatic seeding occur in colorectal cancer? Nature Reviews. Gastroenterology & Hepatology 2019 16 651 653 10.1038/s41575-019-0200-4 31417195

53. Jones S Comparative lesion sequencing provides insights into tumor evolution Proc Natl Acad Sci U S A 2008 105 4283 4288 10.1073/pnas.0712345105 18337506 PMC2393770

54. Hu Z Quantitative evidence for early metastatic seeding in colorectal cancer Nature Genetics 2019 51 1113 1122 10.1038/s41588-019-0423-x 31209394 PMC6982526

55. Stoecklein NH Direct genetic analysis of single disseminated cancer cells for prediction of outcome and therapy selection in esophageal cancer Cancer Cell 2008 13 441 453 10.1016/j.ccr.2008.04.005 18455127

56. Birkbak NJ McGranahan N Cancer genome evolutionary trajectories in metastasis Cancer Cell 2020 37 8 19 10.1016/j.ccell.2019.12.004 31935374

57. Hocking W Goodman J Golde D Granulocytosis associated with tumor cell production of colony-stimulating activity Blood 1983 61 600 603 10.1182/blood.V61.3.600.600 6600635

58. Fredeau L Paraneoplastic neutrophilic leukaemoid reaction in a patient with melanoma: Association between tumour volume and leucocytosis British Journal of Dermatology 2020 183 579 580 10.1111/bjd.19059 32232827

59. Diaz-Montero CM Increased circulating myeloid-derived suppressor cells correlate with clinical cancer stage, metastatic tumor burden, and doxorubicin-cyclophosphamide chemotherapy Cancer Immunology, Immunotherapy 2009 58 49 59 10.1007/s00262-008-0523-4 18446337 PMC3401888

60. Wang L Increased myeloid-derived suppressor cells in gastric cancer correlate with cancer stage and plasma S100A8/A9 proinflammatory proteins The Journal of Immunology 2013 190 794 804 10.4049/jimmunol.1202088 23248262

61. Talmadge JE Pathways mediating the expansion and immunosuppressive activity of myeloid-derived suppressor cells and their relevance to cancer therapy Clinical Cancer Research 2007 13 5243 5248 10.1158/1078-0432.CCR-07-0182 17875751

62. Sieff CA Hematopoietic growth factors The Journal of Clinical Investigation 1987 79 1549 1557 10.1172/JCI112988 3034976 PMC424464

63. Lane TA Mobilization of blood-derived stem and progenitor cells in normal subjects by granulocyte-macrophage- and granulocyte-colony-stimulating factors Transfusion 1999 39 39 47 10.1046/j.1537-2995.1999.39199116893.x 9920165

64. Slavin S Strober S Induction of allograft tolerance after total lymphoid irradiation (TLI): Development of suppressor cells of the mixed leukocyte reaction (MLR) The Journal of Immunology 1979 123 942 946 10.4049/jimmunol.123.2.942 156766

65. Pak AS Mechanisms of immune suppression in patients with head and neck cancer: Presence of CD34(+) cells which suppress immune functions within cancers that secrete granulocyte-macrophage colony-stimulating factor Clinical Cancer Research 1995 1 95 103 9815891

66. Dumitru CA Moses K Trellakis S Lang S Brandau S Neutrophils and granulocytic myeloid-derived suppressor cells: Immunophenotyping, cell biology and clinical relevance in human oncology Cancer Immunology, Immunotherapy 2012 61 1155 1167 10.1007/s00262-012-1294-5 22692756 PMC11028504

67. Poschke I Mougiakakos D Hansson J Masucci GV Kiessling R Immature immunosuppressive CD14+HLA-DR-/low cells in melanoma patients are Stat3hi and overexpress CD80, CD83, and DC-sign Cancer Research 2010 70 4335 4345 10.1158/0008-5472.CAN-09-3767 20484028

68. Rodriguez PC Arginase I-producing myeloid-derived suppressor cells in renal cell carcinoma are a subpopulation of activated granulocytes Cancer Research 2009 69 1553 1560 10.1158/0008-5472.CAN-08-1921 19201693 PMC2900845

69. Talmadge JE Gabrilovich DI History of myeloid-derived suppressor cells Nature Reviews Cancer 2013 13 739 752 10.1038/nrc3581 24060865 PMC4358792

70. Schmielau J Finn OJ Activated granulocytes and granulocyte-derived hydrogen peroxide are the underlying mechanism of suppression of t-cell function in advanced cancer patients Cancer Research 2001 61 4756 4760 11406548

71. Corzo CA Mechanism regulating reactive oxygen species in tumor-induced myeloid-derived suppressor cells The Journal of Immunology 2009 182 5693 5701 10.4049/jimmunol.0900092 19380816 PMC2833019

72. Zea AH Arginase-producing myeloid suppressor cells in renal cell carcinoma patients: A mechanism of tumor evasion Cancer Research 2005 65 3044 3048 10.1158/0008-5472.CAN-04-4505 15833831

73. Khadge S Immune regulation and anti-cancer activity by lipid inflammatory mediators International Immunopharmacology 2018 65 580 592 10.1016/j.intimp.2018.10.026 30447537 PMC6322683

74. Durante W Johnson FK Johnson RA Arginase: A critical regulator of nitric oxide synthesis and vascular function Clinical and Experimental Pharmacology and Physiology 2007 34 906 911 10.1111/j.1440-1681.2007.04638.x 17645639 PMC1955221

75. Czystowska-Kuzmicz M Small extracellular vesicles containing arginase-1 suppress T-cell responses and promote tumor growth in ovarian carcinoma Nature Communications 2019 10 3000 10.1038/s41467-019-10979-3 31278254 PMC6611910

76. Geiger R L-arginine modulates t cell metabolism and enhances survival and anti-tumor activity Cell 2016 167 829 842.e813 10.1016/j.cell.2016.09.031 27745970 PMC5075284

77. Raber P Ochoa AC Rodríguez PC Metabolism of L-arginine by myeloid-derived suppressor cells in cancer: Mechanisms of T cell suppression and therapeutic perspectives Immunological Investigations 2012 41 614 634 10.3109/08820139.2012.680634 23017138 PMC3519282

78. Rodriguez PC Arginase I production in the tumor microenvironment by mature myeloid cells inhibits T-cell receptor expression and antigen-specific T-cell responses Cancer Research 2004 64 5839 5849 10.1158/0008-5472.CAN-04-0465 15313928

79. Rodriguez PC Quiceno DG Ochoa AC l-arginine availability regulates T-lymphocyte cell-cycle progression Blood 2006 109 1568 1573 10.1182/blood-2006-06-031856 17023580 PMC1794048

80. Rodriguez PC Regulation of T cell receptor CD3zeta chain expression by L-arginine Journal of Biological Chemistry 2002 277 21123 21129 10.1074/jbc.M110675200 11950832

81. Bronte V Serafini P Mazzoni A Segal DM Zanovello P L-arginine metabolism in myeloid cells controls T-lymphocyte functions Trends in Immunology 2003 24 302 306 10.1016/S1471-4906(03)00132-7 12810105

82. Eiserich, J. P. (2003). Nitric oxide: A simple free radical with complex chemistry and biology. In M. P. Schneider (ed.), Chemical Probes in Biology (pp. 1-19). © 2003 Kluwer Academic Publishers.

83. Jayaraman P Tumor-expressed inducible nitric oxide synthase controls induction of functional myeloid-derived suppressor cells through modulation of vascular endothelial growth factor release The Journal of Immunology 2012 188 5365 5376 10.4049/jimmunol.1103553 22529296 PMC3358566

84. Mazzoni A Myeloid suppressor lines inhibit T cell responses by an NO-dependent mechanism The Journal of Immunology 2002 168 689 695 10.4049/jimmunol.168.2.689 11777962

85. Nagaraj S Altered recognition of antigen is a mechanism of CD8+ T cell tolerance in cancer Nature Medicine 2007 13 828 835 10.1038/nm1609 17603493 PMC2135607

86. Kusmartsev S Nefedova Y Yoder D Gabrilovich DI Antigen-specific inhibition of CD8⁺ T cell response by immature myeloid cells in cancer is mediated by reactive oxygen species The Journal of Immunology 2004 172 989 999 10.4049/jimmunol.172.2.989 14707072

87. Markowitz J Nitric oxide mediated inhibition of antigen presentation from DCs to CD4+ T cells in cancer and measurement of STAT1 nitration Scientific Reports 2017 7 15424 10.1038/s41598-017-14970-0 29133913 PMC5684213

88. Nagaraj S Schrum AG Cho H-I Celis E Gabrilovich DI Mechanism of T cell tolerance induced by myeloid-derived suppressor cells The Journal of Immunology 2010 184 3106 3116 10.4049/jimmunol.0902661 20142361 PMC2832724

89. Wesolowski R Markowitz J Carson WE Myeloid derived suppressor cells – a new therapeutic target in the treatment of cancer Journal for ImmunoTherapy of Cancer 2013 1 10 10.1186/2051-1426-1-10 24829747 PMC4019895

90. Feng S Myeloid-derived suppressor cells inhibit T cell activation through nitrating LCK in mouse cancers Proc Natl Acad Sci U S A 2018 115 10094 10099 10.1073/pnas.1800695115 30232256 PMC6176562

91. Yaseen MM Abuharfeil NM Darmani H Daoud A Mechanisms of immune suppression by myeloid-derived suppressor cells: The role of interleukin-10 as a key immunoregulatory cytokine Open Biology 2020 10 200111 10.1098/rsob.200111 32931721 PMC7536076

92. Talmadge JE Immunologic attributes of cytokine mobilized peripheral blood stem cells and recovery following transplantation Bone Marrow Transplantation 1996 17 101 109 8673041

93. Shojaei F Bv8 regulates myeloid-cell-dependent tumour angiogenesis Nature 2007 450 825 831 10.1038/nature06348 18064003

94. Kaplan RN VEGFR1-positive haematopoietic bone marrow progenitors initiate the pre-metastatic niche Nature 2005 438 820 827 10.1038/nature04186 16341007 PMC2945882

95. Nguyen DX Bos PD Massague J Metastasis: From dissemination to organ-specific colonization Nature Reviews Cancer 2009 9 274 284 10.1038/nrc2622 19308067

96. Jacob A Prekeris R The regulation of MMP targeting to invadopodia during cancer metastasis Front Cell Dev Biol 2015 3 4 10.3389/fcell.2015.00004 25699257 PMC4313772

97. Horikawa N Expression of vascular endothelial growth factor in ovarian cancer inhibits tumor immunity through the accumulation of myeloid-derived suppressor cells Clinical Cancer Research 2017 23 587 599 10.1158/1078-0432.CCR-16-0387 27401249

98. Karakhanova S Characterization of myeloid leukocytes and soluble mediators in pancreatic cancer: importance of myeloid-derived suppressor cells Oncoimmunology 2015 4 e998519 10.1080/2162402X.2014.998519 26137414 PMC4485765

99. Hoggatt J Pelus LM Many mechanisms mediating mobilization: An alliterative review Current Opinion in Hematology 2011 18 231 238 10.1097/MOH.0b013e3283477962 21537168 PMC3159187

100. Levesque JP Hendy J Takamatsu Y Simmons PJ Bendall LJ Disruption of the CXCR4/CXCL12 chemotactic interaction during hematopoietic stem cell mobilization induced by GCSF or cyclophosphamide The Journal of Clinical Investigation 2003 111 187 196 10.1172/JCI15994 12531874 PMC151860

101. Saito T Elevated serum levels of human matrix metalloproteinase-9 (MMP-9) during the induction of peripheral blood stem cell mobilization by granulocyte colony-stimulating factor (G-CSF) Journal of Infection and Chemotherapy 2007 13 426 428 10.1007/s10156-007-0553-4 18095095

102. Xu M Constitutive mobilization of CD34+ cells into the peripheral blood in idiopathic myelofibrosis may be due to the action of a number of proteases Blood 2005 105 4508 4515 10.1182/blood-2004-08-3238 15705794

103. Heissig B Recruitment of stem and progenitor cells from the bone marrow niche requires MMP-9 mediated release of kit-ligand Cell 2002 109 625 637 10.1016/S0092-8674(02)00754-7 12062105 PMC2826110

104. Zeng ZS Cohen AM Guillem JG Loss of basement membrane type IV collagen is associated with increased expression of metalloproteinases 2 and 9 (MMP-2 and MMP-9) during human colorectal tumorigenesis Carcinogenesis 1999 20 749 755 10.1093/carcin/20.5.749 10334190

105. Davies B Levels of matrix metalloproteases in bladder cancer correlate with tumor grade and invasion Cancer Research 1993 53 5365 5369 8221672

106. Hamdy FC Matrix metalloproteinase 9 expression in primary human prostatic adenocarcinoma and benign prostatic hyperplasia British Journal of Cancer 1994 69 177 182 10.1038/bjc.1994.30 7506923 PMC1968786

107. Cortez-Retamozo V Origins of tumor-associated macrophages and neutrophils Proc Natl Acad Sci U S A 2012 109 2491 2496 10.1073/pnas.1113744109 22308361 PMC3289379

108. Wu C Spleen mediates a distinct hematopoietic progenitor response supporting tumor-promoting myelopoiesis The Journal of Clinical Investigation 2018 128 3425 3438 10.1172/JCI97973 29771686 PMC6063469

109. Klein B Splenomegaly and solitary spleen metastasis in solid tumors Cancer 1987 60 100 102 10.1002/1097-0142(19870701)60:1<100::AID-CNCR2820600118>3.0.CO;2-9 3581023

110. Kiely JM Silverstein MN Metastatic carcinoma simulating agnogenic myeloid metaplasia and myelofibrosis Cancer 1969 24 1041 1044 10.1002/1097-0142(196911)24:5<1041::AID-CNCR2820240526>3.0.CO;2-C 5353936

111. Schlitt HJ Extramedullary erythropoiesis in human liver grafts Hepatology 1995 21 689 696 10.1002/hep.1840210314 7533123

112. Craig CEH Quaglia A Dhillon AP Extramedullary haematopoiesis in massive hepatic necrosis Histopathology 2004 45 518 525 10.1111/j.1365-2559.2004.01970.x 15500656

113. Mohyuddin GR Yacoub A Primary myelofibrosis presenting as extramedullary hematopoiesis in a transplanted liver graft: Case report and review of the literature Case Rep Hematol 2016 2016 9515404 26885416 10.1155/2016/9515404 PMC4739215

114. Yablonski-Peretz T Secondary myelofibrosis with metastatic breast cancer simulating agnogenic myeloid metaplasia: Report of a case and review of the literature Medical and Pediatric Oncology 1985 13 92 96 10.1002/mpo.2950130210 3982370

115. Oliveira AC Chemokine signaling axis between endothelial and myeloid cells regulates development of pulmonary hypertension associated with pulmonary fibrosis and hypoxia American Journal of Physiology. Lung Cellular and Molecular Physiology 2019 317 L434 L444 10.1152/ajplung.00156.2019 31364370 PMC6842914

116. Ostrand-Rosenberg S Fenselau C Myeloid-derived suppressor cells: Immune-suppressive cells that impair antitumor immunity and are sculpted by their environment The Journal of Immunology 2018 200 422 431 10.4049/jimmunol.1701019 29311384 PMC5765878

117. Bryant AJ Myeloid-derived suppressor cells are necessary for development of pulmonary hypertension American Journal of Respiratory Cell and Molecular Biology 2018 58 170 180 10.1165/rcmb.2017-0214OC 28862882 PMC5805999

118. Pan H 20-year risks of breast-cancer recurrence after stopping endocrine therapy at 5 years New England Journal of Medicine 2017 377 1836 1846 10.1056/NEJMoa1701830 29117498 PMC5734609

119. Folkman J Tumor angiogenesis: Therapeutic implications New England Journal of Medicine 1971 285 1182 1186 10.1056/NEJM197111182852108 4938153

120. Hanahan D Folkman J Patterns and emerging mechanisms of the angiogenic switch during tumorigenesis Cell 1996 86 353 364 10.1016/S0092-8674(00)80108-7 8756718

121. Siemann DW Horsman MR Modulation of the tumor vasculature and oxygenation to improve therapy Pharmacology & Therapeutics 2015 153 107 124 10.1016/j.pharmthera.2015.06.006 26073310 PMC4526350

122. Retsky M Reduction of breast cancer relapses with perioperative non-steroidal anti-inflammatory drugs: New findings and a review Current Medicinal Chemistry 2013 20 4163 4176 10.2174/09298673113209990250 23992307 PMC3831877

123. Farrar JD Cancer dormancy. VII. A regulatory role for CD8+ T cells and IFN-gamma in establishing and maintaining the tumor-dormant state J Immunol 1999 162 2842 2849 10.4049/jimmunol.162.5.2842 10072532

124. Dunn GP Old LJ Schreiber RD The immunobiology of cancer immunosurveillance and immunoediting Immunity 2004 21 137 148 10.1016/j.immuni.2004.07.017 15308095

125. Wheelock EF Weinhold KJ Levich J The tumor dormant state Advances in Cancer Research 1981 34 107 140 10.1016/S0065-230X(08)60240-7 7025590

126. Koebel CM Adaptive immunity maintains occult cancer in an equilibrium state Nature 2007 450 903 907 10.1038/nature06309 18026089

127. Sidky YA Borden EC Inhibition of angiogenesis by interferons: Effects on tumor- and lymphocyte-induced vascular responses Cancer Research 1987 47 5155 5161 2441862

128. Voest EE Inhibition of angiogenesis in vivo by interleukin 12 Journal of the National Cancer Institute 1995 87 581 586 10.1093/jnci/87.8.581 7538593

129. Sleeman JP The metastatic niche and stromal progression Cancer and Metastasis Reviews 2012 31 429 440 10.1007/s10555-012-9373-9 22699312 PMC3470821

130. Fane M Weeraratna AT How the ageing microenvironment influences tumour progression Nature Reviews Cancer 2020 20 89 106 10.1038/s41568-019-0222-9 31836838 PMC7377404

131. Baschuk N Rautela J Parker BS Bone specific immunity and its impact on metastasis Bonekey Rep 2015 4 665 10.1038/bonekey.2015.32 25908968 PMC4398004

132. Nakajima M Morikawa K Fabra A Bucana CD Fidler IJ Influence of organ environment on extracellular matrix degradative activity and metastasis of human colon carcinoma cells Journal of the National Cancer Institute 1990 82 1890 1898 10.1093/jnci/82.24.1890 2174463

133. Casimiro S Guise TA Chirgwin J The critical role of the bone microenvironment in cancer metastases Molecular and Cellular Endocrinology 2009 310 71 81 10.1016/j.mce.2009.07.004 19616059

134. Sceneay J Smyth MJ Möller A The pre-metastatic niche: Finding common ground Cancer and Metastasis Reviews 2013 32 449 464 10.1007/s10555-013-9420-1 23636348

135. Lindau D Gielen P Kroesen M Wesseling P Adema GJ The immunosuppressive tumour network: Myeloid-derived suppressor cells, regulatory T cells and natural killer T cells Immunology 2013 138 105 115 10.1111/imm.12036 23216602 PMC3575763

136. Wilson A Dormant and self-renewing hematopoietic stem cells and their niches Annals of the New York Academy of Sciences 2007 1106 64 75 10.1196/annals.1392.021 17442778

137. Liekens S Schols D Hatse S CXCL12-CXCR4 axis in angiogenesis, metastasis and stem cell mobilization Current Pharmaceutical Design 2010 16 3903 3920 10.2174/138161210794455003 21158728

138. Sugiyama T Kohara H Noda M Nagasawa T Maintenance of the hematopoietic stem cell pool by CXCL12-CXCR4 chemokine signaling in bone marrow stromal cell niches Immunity 2006 25 977 988 10.1016/j.immuni.2006.10.016 17174120

139. Price TT Dormant breast cancer micrometastases reside in specific bone marrow niches that regulate their transit to and from bone Sci Transl Med 2016 8 340ra 373 10.1126/scitranslmed.aad4059 PMC8722465 27225183

140. Albrengues J Neutrophil extracellular traps produced during inflammation awaken dormant cancer cells in mice Science 2018 361 6409 10.1126/science.aao4227 PMC6777850 30262472

141. Aguirre Ghiso JA Kovalski K Ossowski L Tumor dormancy induced by downregulation of urokinase receptor in human carcinoma involves integrin and MAPK signaling J Cell Biol 1999 147 89 104 10.1083/jcb.147.1.89 10508858 PMC2164973

142. Shiozawa Y Human prostate cancer metastases target the hematopoietic stem cell niche to establish footholds in mouse bone marrow The Journal of Clinical Investigation 2011 121 1298 1312 10.1172/JCI43414 21436587 PMC3069764

143. Vora AJ Toh CH Peel J Greaves M Use of granulocyte colony-stimulating factor (G-CSF) for mobilizing peripheral blood stem cells: Risk of mobilizing clonal myeloma cells in patients with bone marrow infiltration British Journal of Haematology 1994 86 180 182 10.1111/j.1365-2141.1994.tb03271.x 7516696

144. Nie Y Han YC Zou YR CXCR4 is required for the quiescence of primitive hematopoietic cells Journal of Experimental Medicine 2008 205 777 783 10.1084/jem.20072513 18378795 PMC2292218

145. McCabe NP De S Vasanji A Brainard J Byzova TV Prostate cancer specific integrin alphavbeta3 modulates bone metastatic growth and tissue remodeling Oncogene 2007 26 6238 6243 10.1038/sj.onc.1210429 17369840 PMC2753215

146. Taichman RS Use of the stromal cell-derived factor-1/CXCR4 pathway in prostate cancer metastasis to bone Cancer Research 2002 62 1832 1837 11912162

147. Muller A Involvement of chemokine receptors in breast cancer metastasis Nature 2001 410 50 56 10.1038/35065016 11242036

148. Boyerinas B Adhesion to osteopontin in the bone marrow niche regulates lymphoblastic leukemia cell dormancy Blood 2013 121 4821 4831 10.1182/blood-2012-12-475483 23589674 PMC3682335

149. Taichman RS Prospective identification and skeletal localization of cells capable of multilineage differentiation in vivo Stem Cells Dev 2010 19 1557 1570 10.1089/scd.2009.0445 20446812 PMC3089003

150. Whetton AD Graham GJ Homing and mobilization in the stem cell niche Trends in Cell Biology 1999 9 233 238 10.1016/S0962-8924(99)01559-7 10354570

151. Jung Y Annexin-2 is a regulator of stromal cell-derived factor-1/CXCL12 function in the hematopoietic stem cell endosteal niche Experimental Hematology 2011 39 151 166.e151 10.1016/j.exphem.2010.11.007 21108988 PMC3026087

152. Jung Y Annexin 2-CXCL12 interactions regulate metastatic cell targeting and growth in the bone marrow Molecular Cancer Research 2015 13 197 207 10.1158/1541-7786.MCR-14-0118 25139998 PMC4297714

153. Waight JD Hu Q Miller A Liu S Abrams SI Tumor-derived G-CSF facilitates neoplastic growth through a granulocytic myeloid-derived suppressor cell-dependent mechanism PLoS One 2011 6 e27690 10.1371/journal.pone.0027690 22110722 PMC3218014

154. van der Toom EE Verdone JE Pienta KJ Disseminated tumor cells and dormancy in prostate cancer metastasis Current Opinion in Biotechnology 2016 40 9 15 10.1016/j.copbio.2016.02.002 26900985 PMC4975648

155. Morris KT G-CSF and G-CSFR are highly expressed in human gastric and colon cancers and promote carcinoma cell proliferation and migration British Journal of Cancer 2014 110 1211 1220 10.1038/bjc.2013.822 24448357 PMC3950854

156. Groblewska M Serum levels of granulocyte colony-stimulating factor (G-CSF) and macrophage colony-stimulating factor (M-CSF) in pancreatic cancer patients Clinical Chemistry and Laboratory Medicine 2007 45 30 34 10.1515/CCLM.2007.025 17243911

157. OuYang LY Tumor-induced myeloid-derived suppressor cells promote tumor progression through oxidative metabolism in human colorectal cancer Journal of Translational Medicine 2015 13 47 10.1186/s12967-015-0410-7 25638150 PMC4357065

158. Yan HH Gr-1+CD11b+ myeloid cells tip the balance of immune protection to tumor promotion in the premetastatic lung Cancer Research 2010 70 6139 6149 10.1158/0008-5472.CAN-10-0706 20631080 PMC4675145

159. Shi H Recruited monocytic myeloid-derived suppressor cells promote the arrest of tumor cells in the premetastatic niche through an IL-1beta-mediated increase in E-selectin expression International Journal of Cancer 2017 140 1370 1383 10.1002/ijc.30538 27885671

160. Talmadge JE Fidler IJ AACR centennial series: The biology of cancer metastasis: Historical perspective Cancer Research 2010 70 5649 5669 10.1158/0008-5472.CAN-10-1040 20610625 PMC4037932

161. Mitchell, K. G., et al. (2020) Neutrophil expansion defines an immunoinhibitory peripheral and intratumoral inflammatory milieu in resected non-small cell lung cancer: A descriptive analysis of a prospectively immunoprofiled cohort. Journal for Immunotherapy of Cancer, 8 , e000405 10.1136/jitc-2019-000405 PMC7213906 32350118

162. Marymont JH Jr Gross S Patterns of metastatic cancer in the spleen American Journal of Clinical Pathology 1963 40 58 66 10.1093/ajcp/40.1.58 13933237

163. Sauer J Sobolewski K Dommisch K Splenic metastases–not a frequent problem, but an underestimate location of metastases: Epidemiology and course Journal of Cancer Research and Clinical Oncology 2009 135 667 671 10.1007/s00432-008-0502-3 18936972 PMC12160167

164. Watson GF Diller IC Ludwick NV Spleen extract and tumor growth Science 1947 106 348 10.1126/science.106.2754.348 17777431

165. Cole KE Comparative phenotypes of peripheral blood and spleen cells from cancer patients International Immunopharmacology 2020 85 106655 10.1016/j.intimp.2020.106655 32521493

166. Cole KE Splenic and PB immune recovery in neoadjuvant treated gastrointestinal cancer patients International Immunopharmacology 2022 106 108628 10.1016/j.intimp.2022.108628 35203041 PMC9009221

167. Peereboom, D. M., et al. (2019). Metronomic capecitabine as an immune modulator in glioblastoma patients reduces myeloid-derived suppressor cells. JCI Insight, 4 , e130748 10.1172/jci.insight.130748 PMC6948860 31600167

168. Draghiciu O Lubbers J Nijman HW Daemen T Myeloid derived suppressor cells-an overview of combat strategies to increase immunotherapy efficacy Oncoimmunology 2015 4 e954829 10.4161/21624011.2014.954829 25949858 PMC4368153

169. Weber R Myeloid-derived suppressor cells hinder the anti-cancer activity of immune checkpoint inhibitors Frontiers in Immunology 2018 9 1310 10.3389/fimmu.2018.01310 29942309 PMC6004385

170. Sun, L., et al. (2019). Inhibiting myeloid-derived suppressor cell trafficking enhances T cell immunotherapy. JCI Insight, 4 , e126853 10.1172/jci.insight.126853 PMC6483637 30944253

171. Mitchell TC Epacadostat plus pembrolizumab in patients with advanced solid tumors: Phase I results from a multicenter, open-label phase I/II trial (ECHO-202/KEYNOTE-037) Journal of Clinical Oncology 2018 36 3223 3230 10.1200/JCO.2018.78.9602 30265610 PMC6225502

172. Eriksson E Wenthe J Irenaeus S Loskog A Ullenhag G Gemcitabine reduces MDSCs, tregs and TGFβ-1 while restoring the teff/treg ratio in patients with pancreatic cancer Journal of Translational Medicine 2016 14 282 10.1186/s12967-016-1037-z 27687804 PMC5041438

173. Kotsakis A Myeloid-derived suppressor cell measurements in fresh and cryopreserved blood samples Journal of Immunological Methods 2012 381 14 22 10.1016/j.jim.2012.04.004 22522114 PMC3385927

174. Trellakis S Granulocytic myeloid-derived suppressor cells are cryosensitive and their frequency does not correlate with serum concentrations of colony-stimulating factors in head and neck cancer Innate Immunity 2013 19 328 336 10.1177/1753425912463618 23160385

175. Annels NE The effects of gemcitabine and capecitabine combination chemotherapy and of low-dose adjuvant GM-CSF on the levels of myeloid-derived suppressor cells in patients with advanced pancreatic cancer Cancer Immunology, Immunotherapy 2014 63 175 183 10.1007/s00262-013-1502-y 24292263 PMC11028876

176. Limagne E Accumulation of MDSC and Th17 cells in patients with metastatic colorectal cancer predicts the efficacy of a FOLFOX–bevacizumab drug treatment regimen Cancer Research 2016 76 5241 5252 10.1158/0008-5472.CAN-15-3164 27496709

177. Kanterman J Adverse immunoregulatory effects of 5FU and CPT11 chemotherapy on myeloid-derived suppressor cells and colorectal cancer outcomes Cancer Research 2014 74 6022 6035 10.1158/0008-5472.CAN-14-0657 25209187

178. Crawford J Dale DC Lyman GH Chemotherapy-induced neutropenia Cancer 2004 100 228 237 10.1002/cncr.11882 14716755

179. Li X Is PEGylated G-CSF superior to G-CSF in patients with breast cancer receiving chemotherapy? A systematic review and meta-analysis Supportive Care in Cancer 2020 28 5085 5097 10.1007/s00520-020-05603-w 32621264 PMC7333975

180. Wesolowski R Circulating myeloid-derived suppressor cells increase in patients undergoing neo-adjuvant chemotherapy for breast cancer Cancer Immunology, Immunotherapy 2017 66 1437 1447 10.1007/s00262-017-2038-3 28688082 PMC5647220

181. Dammeijer F Immune monitoring in mesothelioma patients identifies novel immune-modulatory functions of gemcitabine associating with clinical response EBioMedicine 2021 64 103160 10.1016/j.ebiom.2020.103160 33516644 PMC7910686

182. Ochoa JB Effects of L-arginine on the proliferation of T lymphocyte subpopulations JPEN. Journal of Parenteral and Enteral Nutrition 2001 25 23 29 10.1177/014860710102500123 11190986

183. Rodriguez PC Arginase I in myeloid suppressor cells is induced by COX-2 in lung carcinoma Journal of Experimental Medicine 2005 202 931 939 10.1084/jem.20050715 16186186 PMC2213169

184. Sinha P Clements VK Fulton AM Ostrand-Rosenberg S Prostaglandin E2 promotes tumor progression by inducing myeloid-derived suppressor cells Cancer Research 2007 67 4507 4513 10.1158/0008-5472.CAN-06-4174 17483367

185. Chesney JA Mitchell RA Yaddanapudi K Myeloid-derived suppressor cells-a new therapeutic target to overcome resistance to cancer immunotherapy Journal of Leukocyte Biology 2017 102 727 740 10.1189/jlb.5VMR1116-458RRR 28546500 PMC6608049

186. Talmadge JE Chemoprevention by cyclooxygenase-2 inhibition reduces immature myeloid suppressor cell expansion International Immunopharmacology 2007 7 140 151 10.1016/j.intimp.2006.09.021 17178380

187. El-Malah AA Selective COX-2 inhibitors: Road from success to controversy and the quest for repurposing Pharmaceuticals (Basel) 2022 15 827 10.3390/ph15070827 35890126 PMC9318302

188. Mostafa TM Alm El-Din MA Rashdan AR Celecoxib as an adjuvant to chemotherapy for patients with metastatic colorectal cancer: A randomized controlled clinical study Saudi Medicine Journal 2022 43 37 44 10.15537/smj.2022.43.1.20210574 PMC9280566 35022282

189. Califano JA Tadalafil augments tumor specific immunity in patients with head and neck squamous cell carcinoma Clinical Cancer Research 2015 21 30 38 10.1158/1078-0432.CCR-14-1716 25564570 PMC4329916

190. Serafini P Phosphodiesterase-5 inhibition augments endogenous antitumor immunity by reducing myeloid-derived suppressor cell function Journal of Experimental Medicine 2006 203 2691 2702 10.1084/jem.20061104 17101732 PMC2118163

191. Noonan KA Ghosh N Rudraraju L Bui M Borrello I Targeting immune suppression with PDE5 inhibition in end-stage multiple myeloma Cancer Immunology Research 2014 2 725 731 10.1158/2326-6066.CIR-13-0213 24878583 PMC4152913

192. Ezernitchi AV TCR zeta down-regulation under chronic inflammation is mediated by myeloid suppressor cells differentially distributed between various lymphatic organs The Journal of Immunology 2006 177 4763 4772 10.4049/jimmunol.177.7.4763 16982917

193. Hassel JC Tadalafil has biologic activity in human melanoma. Results of a pilot trial with tadalafil in patients with metastatic melanoma (TaMe) Oncoimmunology 2017 6 e1326440 10.1080/2162402X.2017.1326440 28932631 PMC5599085

194. Weed DT Tadalafil reduces myeloid-derived suppressor cells and regulatory T cells and promotes tumor immunity in patients with head and neck squamous cell carcinoma Clinical Cancer Research 2015 21 39 48 10.1158/1078-0432.CCR-14-1711 25320361 PMC4322895

195. Danley KT The association of phosphodiesterase-5 inhibitors with the biochemical recurrence-free and overall survival of patients with prostate cancer following radical prostatectomy Urol Oncol 2022 40 57 e51 57 10.1016/j.urolonc.2021.05.031 34284930

196. Poschke I De Boniface J Mao Y Kiessling R Tumor-induced changes in the phenotype of blood-derived and tumor-associated T cells of early stage breast cancer patients International Journal of Cancer 2012 131 1611 1620 10.1002/ijc.27410 22190148

197. Avtandilyan N Javrushyan H Petrosyan G Trchounian A The involvement of arginase and nitric oxide synthase in breast cancer development: Arginase and NO synthase as therapeutic targets in cancer BioMed Research International 2018 2018 8696923 10.1155/2018/8696923 29854802 PMC5952510

198. Ma Z Overexpression of arginase-1 is an indicator of poor prognosis in patients with colorectal cancer Pathology, Research and Practice 2019 215 152383 10.1016/j.prp.2019.03.012 30890279

199. Grzywa TM Myeloid cell-derived arginase in cancer immune response Frontiers in Immunology 2020 11 938 10.3389/fimmu.2020.00938 32499785 PMC7242730

200. Gey A Granulocytic myeloid-derived suppressor cells inversely correlate with plasma arginine and overall survival in critically ill patients Clinical and Experimental Immunology 2015 180 280 288 10.1111/cei.12567 25476957 PMC4408163

201. Lemberg, K. M., Gori, S. S., Tsukamoto, T., Rais, R., & Slusher, B. S. (2022). Clinical development of metabolic inhibitors for oncology. Journal Clinical Investigacion, 132 , e148550 10.1172/JCI148550 PMC8718137 34981784

202. Steggerda SM Inhibition of arginase by CB-1158 blocks myeloid cell-mediated immune suppression in the tumor microenvironment Journal for Immunotherapy of Cancer 2017 5 101 10.1186/s40425-017-0308-4 29254508 PMC5735564

203. Naing A Phase I study of the arginase inhibitor INCB001158 (1158) alone and in combination with pembrolizumab (PEM) in patients (Pts) with advanced/metastatic (adv/met) solid tumours Annals of Oncology 2019 30 v160 10.1093/annonc/mdz244.002

204. Niu F Arginase: An emerging and promising therapeutic target for cancer treatment Biomedicine & Pharmacotherapy 2022 149 112840 10.1016/j.biopha.2022.112840 35316752

205. Fauskanger M Haabeth OAW Skjeldal FM Bogen B Tveita AA Tumor killing by CD4(+) T cells is mediated via induction of inducible nitric oxide synthase-dependent macrophage cytotoxicity Frontiers in Immunology 2018 9 1684 10.3389/fimmu.2018.01684 30083157 PMC6064871

206. Chung AW A phase 1/2 clinical trial of the nitric oxide synthase inhibitor L-NMMA and taxane for treating chemoresistant triple-negative breast cancer Science Translation Medicine 2021 13 eabj5070 10.1126/scitranslmed.abj5070 34910551

207. Carini M Chemiluminescence and LC–MS/MS analyses for the study of nitric oxide release and distribution following oral administration of nitroaspirin (NCX 4016) in healthy volunteers Journal of Pharmaceutical and Biomedical Analysis 2004 35 277 287 10.1016/S0731-7085(03)00531-4 15063462

208. Chang K Nitric oxide suppresses inducible nitric oxide synthase expression by inhibiting post-translational modification of IkappaB Experimental & Molecular Medicine 2004 36 311 324 10.1038/emm.2004.42 15365250

209. De Santo C Nitroaspirin corrects immune dysfunction in tumor-bearing hosts and promotes tumor eradication by cancer vaccination Proc Natl Acad Sci U S A 2005 102 4185 4190 10.1073/pnas.0409783102 15753302 PMC554823

210. Fiorucci S Co-administration of nitric oxide-aspirin (NCX-4016) and aspirin prevents platelet and monocyte activation and protects against gastric damage induced by aspirin in humans Journal of the American College of Cardiology 2004 44 635 641 10.1016/j.jacc.2004.03.079 15358033

211. Li W G-CSF is a key modulator of MDSC and could be a potential therapeutic target in colitis-associated colorectal cancers Protein & Cell 2016 7 130 140 10.1007/s13238-015-0237-2 26797765 PMC4742385

212. Cannarile MA Colony-stimulating factor 1 receptor (CSF1R) inhibitors in cancer therapy Journal for Immunotherapy of Cancer 2017 5 53 10.1186/s40425-017-0257-y 28716061 PMC5514481

213. Edwards DKt CSF1R inhibitors exhibit antitumor activity in acute myeloid leukemia by blocking paracrine signals from support cells Blood 2019 133 588 599 10.1182/blood-2018-03-838946 30425048 PMC6367650

214. Apodaca MC Characterization of a whole blood assay for quantifying myeloid-derived suppressor cells Journal for Immunotherapy of Cancer 2019 7 230 10.1186/s40425-019-0674-1 31462270 PMC6714080

215. Tsukada N Burger JA Zvaifler NJ Kipps TJ Distinctive features of “nurselike” cells that differentiate in the context of chronic lymphocytic leukemia Blood 2002 99 1030 1037 10.1182/blood.V99.3.1030 11807009

216. Edwards VD Targeting of colony-stimulating factor 1 receptor (CSF1R) in the CLL microenvironment yields antineoplastic activity in primary patient samples Oncotarget 2018 9 24576 24589 10.18632/oncotarget.25191 29872489 PMC5973855

217. Curfs JH Meis JF Hoogkamp-Korstanje JA A primer on cytokines: Sources, receptors, effects, and inducers Clinical Microbiology Reviews 1997 10 742 780 10.1128/CMR.10.4.742 9336671 PMC172943

218. Wesolowski R Phase Ib study of the combination of pexidartinib (PLX3397), a CSF-1R inhibitor, and paclitaxel in patients with advanced solid tumors Therapeutic Advances in Medical Oncology 2019 11 1758835919854238 10.1177/1758835919854238 31258629 PMC6589951

219. Olingy CE Dinh HQ Hedrick CC Monocyte heterogeneity and functions in cancer Journal of Leukocyte Biology 2019 106 309 322 10.1002/JLB.4RI0818-311R 30776148 PMC6658332

220. Butowski N Orally administered colony stimulating factor 1 receptor inhibitor PLX3397 in recurrent glioblastoma: An Ivy Foundation Early Phase Clinical Trials Consortium phase II study Neuro-Oncology 2016 18 557 564 10.1093/neuonc/nov245 26449250 PMC4799682

221. Dewar AL Macrophage colony-stimulating factor receptor c-fms is a novel target of imatinib Blood 2005 105 3127 3132 10.1182/blood-2004-10-3967 15637141

222. Giallongo C Monocytic myeloid-derived suppressor cells as prognostic factor in chronic myeloid leukaemia patients treated with dasatinib Journal of Cellular and Molecular Medicine 2018 22 1070 1080 29218828 10.1111/jcmm.13326 PMC5783858

223. Hines SL Zoledronic acid for treatment of osteopenia and osteoporosis in women with primary breast cancer undergoing adjuvant aromatase inhibitor therapy Breast 2010 19 92 96 10.1016/j.breast.2009.12.001 20079640 PMC2854212

224. Coleman R Adjuvant zoledronic acid in patients with early breast cancer: Final efficacy analysis of the AZURE (BIG 01/04) randomised open-label phase 3 trial The Lancet Oncology 2014 15 997 1006 10.1016/S1470-2045(14)70302-X 25035292

225. Melani C Sangaletti S Barazzetta FM Werb Z Colombo MP Amino-biphosphonate-mediated MMP-9 inhibition breaks the tumor-bone marrow axis responsible for myeloid-derived suppressor cell expansion and macrophage infiltration in tumor stroma Cancer Research 2007 67 11438 11446 10.1158/0008-5472.CAN-07-1882 18056472 PMC2646404

226. Porembka MR Pancreatic adenocarcinoma induces bone marrow mobilization of myeloid-derived suppressor cells which promote primary tumor growth Cancer Immunology, Immunotherapy 2012 61 1373 1385 10.1007/s00262-011-1178-0 22215137 PMC3697836

227. Sanford DE A study of zoledronic acid as neo-adjuvant, perioperative therapy in patients with resectable pancreatic ductal adenocarcinoma Journal of Cancer Therapy 2013 4 797 803 10.4236/jct.2013.43096 24089656 PMC3786568

228. Molina-Cerrillo J Alonso-Gordoa T Gajate P Grande E Bruton’s tyrosine kinase (BTK) as a promising target in solid tumors Cancer Treatment Reviews 2017 58 41 50 10.1016/j.ctrv.2017.06.001 28641100

229. Stiff A Myeloid-derived suppressor cells express Bruton’s tyrosine kinase and can be depleted in tumor-bearing hosts by ibrutinib treatment Cancer Research 2016 76 2125 2136 10.1158/0008-5472.CAN-15-1490 26880800 PMC4873459

230. Ferrer G Ibrutinib treatment reduces myeloid derived suppressor cell numbers and function in chronic lymphocytic leukemia Blood 2018 132 239 239 10.1182/blood-2018-99-114799

231. Tempero M Ibrutinib in combination with nab-paclitaxel and gemcitabine as first-line treatment for patients with metastatic pancreatic adenocarcinoma: Results from the phase 3 RESOLVE study Annals of Oncology 2019 30 iv126 10.1093/annonc/mdz154.001 33539945

232. Metzler JM Burla L Fink D Imesch P Ibrutinib in gynecological malignancies and breast cancer: A systematic review International Journal of Molecular Sciences 2020 21 4154 10.3390/ijms21114154 32532074 PMC7312555

233. Hong D A phase 1b/2 study of the Bruton tyrosine kinase inhibitor ibrutinib and the PD-L1 inhibitor durvalumab in patients with pretreated solid tumors Oncology 2019 97 102 111 10.1159/000500571 31230047

234. Al-Toubah T A phase ii study of ibrutinib in advanced neuroendocrine neoplasms Neuroendocrinology 2020 110 377 383 10.1159/000502383 31357193 PMC7771542

235. Benner B A pilot study of Bruton’s tyrosine kinase inhibitor ibrutinib alone and in combination with PD-1 inhibitor nivolumab in patients with metastatic solid tumors Journal of Clinical Oncology 2020 38 3111 3111 10.1200/JCO.2020.38.15_suppl.3111

236. Massó-Vallés D Jauset T Soucek L Ibrutinib repurposing: From B-cell malignancies to solid tumors Oncoscience 2016 3 147 148 10.18632/oncoscience.310 27489860 PMC4965256

237. Zhu S Clinical trials of the BTK inhibitors ibrutinib and acalabrutinib in human diseases beyond B cell malignancies Frontiers in Oncology 2021 11 737943 10.3389/fonc.2021.737943 34778053 PMC8585514

238. Yokochi S An anti-inflammatory drug, propagermanium, may target GPI-anchored proteins associated with an MCP-1 receptor, CCR2 Journal of Interferon and Cytokine Research 2001 21 389 398 10.1089/107999001750277862 11440636

239. Masuda T Phase I dose-escalation trial to repurpose propagermanium, an oral CCL2 inhibitor, in patients with breast cancer Cancer Science 2020 111 924 931 10.1111/cas.14306 31943636 PMC7060487

240. Tobin RP IL-6 and IL-8 are linked with myeloid-derived suppressor cell accumulation and correlate with poor clinical outcomes in melanoma patients Frontiers in Oncology 2019 9 1223 10.3389/fonc.2019.01223 31781510 PMC6857649

241. Bilusic M Phase I trial of HuMax-IL8 (BMS-986253), an anti-IL-8 monoclonal antibody, in patients with metastatic or unresectable solid tumors Journal for Immunotherapy of Cancer 2019 7 240 10.1186/s40425-019-0706-x 31488216 PMC6729083

242. Dominguez, C., McCampbell, K. K., David, J. M., & Palena, C. (2017). Neutralization of IL-8 decreases tumor PMN-MDSCs and reduces mesenchymalization of claudin-low triple-negative breast cancer. JCI Insight, 2: e94296 10.1172/jci.insight.94296 PMC5752275 29093275

243. Bockorny B BL-8040, a CXCR4 antagonist, in combination with pembrolizumab and chemotherapy for pancreatic cancer: The COMBAT trial Nature Medicine 2020 26 878 885 10.1038/s41591-020-0880-x 32451495

244. Ghobrial IM A phase ib/ii trial of the first-in-class anti-CXCR4 antibody ulocuplumab in combination with lenalidomide or bortezomib plus dexamethasone in relapsed multiple myeloma Clinical Cancer Research 2020 26 344 353 10.1158/1078-0432.CCR-19-0647 31672767 PMC11753616

245. Katoh H CXCR2-expressing myeloid-derived suppressor cells are essential to promote colitis-associated tumorigenesis Cancer Cell 2013 24 631 644 10.1016/j.ccr.2013.10.009 24229710 PMC3928012

246. Steele CW CXCR2 inhibition profoundly suppresses metastases and augments immunotherapy in pancreatic ductal adenocarcinoma Cancer Cell 2016 29 832 845 10.1016/j.ccell.2016.04.014 27265504 PMC4912354

247. Park K Veena MS Shin DS Key players of the immunosuppressive tumor microenvironment and emerging therapeutic strategies Front Cell Dev Biol 2022 10 830208 10.3389/fcell.2022.830208 35345849 PMC8957227

248. Greene S Inhibition of MDSC trafficking with SX-682, a CXCR1/2 inhibitor, enhances NK-cell immunotherapy in head and neck cancer models Clinical Cancer Research 2020 26 1420 1431 10.1158/1078-0432.CCR-19-2625 31848188 PMC7073293

249. Goldstein LJ A window-of-opportunity trial of the CXCR1/2 inhibitor reparixin in operable HER-2-negative breast cancer Breast Cancer Research 2020 22 4 10.1186/s13058-019-1243-8 31924241 PMC6954543

250. Schott AF Phase Ib pilot study to evaluate reparixin in combination with weekly paclitaxel in patients with HER-2-negative metastatic breast cancer Clinical Cancer Research 2017 23 5358 5365 10.1158/1078-0432.CCR-16-2748 28539464 PMC5600824

251. Goldstein LJ A randomized, placebo-controlled phase 2 study of paclitaxel in combination with reparixin compared to paclitaxel alone as front-line therapy for metastatic triple-negative breast cancer (fRida) Breast Cancer Research and Treatment 2021 190 265 275 10.1007/s10549-021-06367-5 34476645 PMC8558154

252. Propper DJ Balkwill FR Harnessing cytokines and chemokines for cancer therapy Nature Reviews. Clinical Oncology 2022 19 237 253 10.1038/s41571-021-00588-9 34997230

253. Bresnick AR Weber DJ Zimmer DB S100 proteins in cancer Nature Reviews Cancer 2015 15 96 109 10.1038/nrc3893 25614008 PMC4369764

254. Sinha P Proinflammatory S100 proteins regulate the accumulation of myeloid-derived suppressor cells The Journal of Immunology 2008 181 4666 4675 10.4049/jimmunol.181.7.4666 18802069 PMC2810501

255. Gupta N Al Ustwani O Shen L Pili R Mechanism of action and clinical activity of tasquinimod in castrate-resistant prostate cancer Oncology Targets Theraeutics 2014 7 223 234 10.2147/OTT.S53524 PMC3928061 24600234

256. Shen L Pili R Tasquinimod targets suppressive myeloid cells in the tumor microenvironment Oncoimmunology 2019 8 e1072672 10.1080/2162402X.2015.1072672 31646064 PMC6791429

257. Armstrong AJ Long-term survival and biomarker correlates of tasquinimod efficacy in a multicenter randomized study of men with minimally symptomatic metastatic castration-resistant prostate cancer Clinical Cancer Research 2013 19 6891 6901 10.1158/1078-0432.CCR-13-1581 24255071 PMC4251453

258. Sternberg C Randomized, double-blind, placebo-controlled phase iii study of tasquinimod in men with metastatic castration-resistant prostate cancer Journal of Clinical Oncology 2016 34 2636 2643 10.1200/JCO.2016.66.9697 27298414

259. Eksioglu EA Novel therapeutic approach to improve hematopoiesis in low risk MDS by targeting MDSCs with the Fc-engineered CD33 antibody BI 836858 Leukemia 2017 31 2172 2180 10.1038/leu.2017.21 28096534 PMC5552472

260. Laszlo GS Estey EH Walter RB The past and future of CD33 as therapeutic target in acute myeloid leukemia Blood Reviews 2014 28 143 153 10.1016/j.blre.2014.04.001 24809231

261. Liu Y Bewersdorf JP Stahl M Zeidan AM Immunotherapy in acute myeloid leukemia and myelodysplastic syndromes: The dawn of a new era? Blood Reviews 2019 34 67 83 10.1016/j.blre.2018.12.001 30553527

262. Cheng P Immunodepletion of MDSC by AMV564, a novel bivalent, bispecific CD33/CD3 T cell engager, ex vivo in MDS and melanoma Molecular Therapy 2022 30 2315 2326 10.1016/j.ymthe.2022.02.005 35150889 PMC9171150

263. Warrell RP Jr Differentiation therapy of acute promyelocytic leukemia with tretinoin (all-trans-retinoic acid) New England Journal of Medicine 1991 324 1385 1393 10.1056/NEJM199105163242002 1850498

264. Nefedova Y Mechanism of all-trans retinoic acid effect on tumor-associated myeloid-derived suppressor cells Cancer Research 2007 67 11021 11028 10.1158/0008-5472.CAN-07-2593 18006848

265. Mirza N <em>All-trans</em>-retinoic acid improves differentiation of myeloid cells and immune response in cancer patients Cancer Research 2006 66 9299 9307 10.1158/0008-5472.CAN-06-1690 16982775 PMC1586106

266. Nefedova Y Mechanism of all-<em>trans</em> retinoic acid effect on tumor-associated myeloid-derived suppressor cells Cancer Research 2007 67 11021 11028 10.1158/0008-5472.CAN-07-2593 18006848

267. Tobin RP Targeting myeloid-derived suppressor cells using all-trans retinoic acid in melanoma patients treated with Ipilimumab International Immunopharmacology 2018 63 282 291 10.1016/j.intimp.2018.08.007 30121453 PMC6134177

268. Liu W The anti-inflammatory effects of vitamin D in tumorigenesis International Journal Molecular Science 2018 19 2736 10.3390/ijms19092736 PMC6164284 30216977

269. Walsh JE Clark A-M Day TA Gillespie MB Young MRI Use of α,25-dihydroxyvitamin D3 treatment to stimulate immune infiltration into head and neck squamous cell carcinoma Human Immunology 2010 71 659 665 10.1016/j.humimm.2010.04.008 20438786 PMC3337687

270. Fleet JC Burcham GN Calvert R Ratliff TL Abstract 2364: 1α, 25 dihydroxyvitamin D (1,25(OH)₂D) inhibits the T cell suppressive function of myeloid derived suppressor sells (MDSC) Cancer Research 2019 79 2364 2364 10.1158/1538-7445.AM2019-2364 PMC8041088 31783150

271. Lathers DMR Clark JI Achille NJ Young MRI Phase 1B study to improve immune responses in head and neck cancer patients using escalating doses of 25-hydroxyvitamin D3 Cancer Immunology, Immunotherapy 2004 53 422 430 10.1007/s00262-003-0459-7 14648070 PMC11032934

272. Henderson Berg, M. H., Del Rincon, S. V. & Miller, W. H. (2022). Potential therapies for immune-related adverse events associated with immune checkpoint inhibition: From monoclonal antibodies to kinase inhibition. Journal Immunotherapy Cancer, 10 , e003551 10.1136/jitc-2021-003551 PMC8796266 35086945

273. Liu JF Inhibition of JAK2/STAT3 reduces tumor-induced angiogenesis and myeloid-derived suppressor cells in head and neck cancer Molecular Carcinogenesis 2018 57 429 439 10.1002/mc.22767 29215754

274. Parampalli Yajnanarayana S JAK1/2 inhibition impairs T cell function in vitro and in patients with myeloproliferative neoplasms British Journal of Haematology 2015 169 824 833 10.1111/bjh.13373 25824483

275. Giaccone G A placebo-controlled phase ii study of ruxolitinib in combination with pemetrexed and cisplatin for first-line treatment of patients with advanced nonsquamous non–small-cell lung cancer and systemic inflammation Clinical Lung Cancer 2018 19 e567 e574 10.1016/j.cllc.2018.03.016 29681434

276. Hurwitz HI Randomized, double-blind, phase ii study of ruxolitinib or placebo in combination with capecitabine in patients with metastatic pancreatic cancer for whom therapy with gemcitabine has failed Journal of Clinical Oncology 2015 33 4039 4047 10.1200/JCO.2015.61.4578 26351344 PMC5089161

277. Verstovsek S A double-blind, placebo-controlled trial of ruxolitinib for myelofibrosis New England Journal of Medicine 2012 366 799 807 10.1056/NEJMoa1110557 22375971 PMC4822164

278. de Haas N de Koning C Spilgies L de Vries IJM Hato SV Improving cancer immunotherapy by targeting the STATe of MDSCs OncoImmunology 2016 5 e1196312 10.1080/2162402X.2016.1196312 27622051 PMC5006927

279. Motzer RJ Escudier B Gannon A Figlin RA Sunitinib: Ten years of successful clinical use and study in advanced renal cell carcinoma The Oncologist 2017 22 41 52 10.1634/theoncologist.2016-0197 27807302 PMC5313263

280. Finke J MDSC as a mechanism of tumor escape from sunitinib mediated anti-angiogenic therapy International Immunopharmacology 2011 11 856 861 10.1016/j.intimp.2011.01.030 21315783 PMC3109226

281. Jonasch E NCCN guidelines updates: Management of metastatic kidney cancer Journal of the National Comprehensive Cancer Network 2019 17 587 589 31117033 10.6004/jnccn.2019.5008

282. Ko JS Sunitinib mediates reversal of myeloid-derived suppressor cell accumulation in renal cell carcinoma patients Clinical Cancer Research 2009 15 2148 2157 10.1158/1078-0432.CCR-08-1332 19276286

283. Chen HM Myeloid-derived suppressor cells as an immune parameter in patients with concurrent sunitinib and stereotactic body radiotherapy Clinical Cancer Research 2015 21 4073 4085 10.1158/1078-0432.CCR-14-2742 25922428 PMC4720266

284. French JD Programmed death-1+ T cells and regulatory T cells are enriched in tumor-involved lymph nodes and associated with aggressive features in papillary thyroid cancer Journal of Clinical Endocrinology and Metabolism 2012 97 E934 943 10.1210/jc.2011-3428 22466343 PMC3387418

285. Severson JJ PD-1+Tim-3+ CD8+ T lymphocytes display varied degrees of functional exhaustion in patients with regionally metastatic differentiated thyroid cancer Cancer Immunology Research 2015 3 620 630 10.1158/2326-6066.CIR-14-0201 25701326 PMC4457654

286. Huse M The T-cell-receptor signaling network Journal of Cell Science 2009 122 1269 1273 10.1242/jcs.042762 19386893

287. Chen L Flies DB Molecular mechanisms of T cell co-stimulation and co-inhibition Nature Reviews Immunology 2013 13 227 242 10.1038/nri3405 23470321 PMC3786574

288. Granier C Mechanisms of action and rationale for the use of checkpoint inhibitors in cancer ESMO Open 2017 2 e000213 10.1136/esmoopen-2017-000213 28761757 PMC5518304

289. Robert C Ipilimumab plus dacarbazine for previously untreated metastatic melanoma New England Journal of Medicine 2011 364 2517 2526 10.1056/NEJMoa1104621 21639810

290. Martens A Baseline peripheral blood biomarkers associated with clinical outcome of advanced melanoma patients treated with ipilimumab Clinical Cancer Research 2016 22 2908 2918 10.1158/1078-0432.CCR-15-2412 26787752 PMC5770142

291. Gebhardt C Myeloid cells and related chronic inflammatory factors as novel predictive markers in melanoma treatment with ipilimumab Clinical Cancer Research 2015 21 5453 5459 10.1158/1078-0432.CCR-15-0676 26289067

292. de Coaña YP Ipilimumab treatment results in an early decrease in the frequency of circulating granulocytic myeloid-derived suppressor cells as well as their arginase1 production Cancer Immunology Research 2013 1 158 162 10.1158/2326-6066.CIR-13-0016 24777678

293. Pico de Coaña Y Masucci G Hansson J Kiessling R Myeloid-derived suppressor cells and their role in CTLA-4 blockade therapy Cancer Immunology, Immunotherapy 2014 63 977 983 10.1007/s00262-014-1570-7 24966003 PMC11029783

294. Pico de Coana Y Ipilimumab treatment decreases monocytic MDSCs and increases CD8 effector memory T cells in long-term survivors with advanced melanoma Oncotarget 2017 8 21539 10.18632/oncotarget.15368 28423487 PMC5400604

295. Meyer C Frequencies of circulating MDSC correlate with clinical outcome of melanoma patients treated with ipilimumab Cancer Immunology, Immunotherapy 2014 63 247 257 10.1007/s00262-013-1508-5 24357148 PMC11029062

296. Tzeng A Immunological correlates of response to immune checkpoint inhibitors in metastatic urothelial carcinoma Targeted Oncology 2018 13 599 609 10.1007/s11523-018-0595-9 30267200

297. Highfill SL Disruption of CXCR2-mediated MDSC tumor trafficking enhances anti-PD1 efficacy Sci Transl Med 2014 6 237 267 10.1126/scitranslmed.3007974 PMC6980372 24848257

298. De Henau O Overcoming resistance to checkpoint blockade therapy by targeting PI3Kγ in myeloid cells Nature 2016 539 443 447 10.1038/nature20554 27828943 PMC5634331

299. Amarnath S The PDL1-PD1 axis converts human TH1 cells into regulatory T cells Sci Transl Med 2011 3 111 120 10.1126/scitranslmed.3003130 PMC3235958 22133721

300. Naimi A Tumor immunotherapies by immune checkpoint inhibitors (ICIs); the pros and cons Cell Communication and Signaling: CCS 2022 20 44 10.1186/s12964-022-00854-y 35392976 PMC8991803

301. Andrews LP Molecular pathways and mechanisms of LAG-3 in cancer therapy Clinical Cancer Research 2022 28 5030 10.1158/1078-0432.CCR-21-2390 35579997 PMC9669281

302. Toor SM Elkord E Therapeutic prospects of targeting myeloid-derived suppressor cells and immune checkpoints in cancer Immunology and Cell Biology 2018 96 888 897 10.1111/imcb.12054 29635843

303. Fernández A Adjuvants and myeloid-derived suppressor cells: Enemies or allies in therapeutic cancer vaccination Human Vaccines & Immunotherapeutics 2014 10 3251 3260 10.4161/hv.29847 25483674 PMC4514045

304. Dranoff G GM-CSF-secreting melanoma vaccines Oncogene 2003 22 3188 3192 10.1038/sj.onc.1206459 12789295

305. Mittendorf EA Efficacy and safety analysis of nelipepimut-S vaccine to prevent breast cancer recurrence: A randomized, multicenter, phase III clinical trial Clinical Cancer Research 2019 25 4248 4254 10.1158/1078-0432.CCR-18-2867 31036542

306. Schneble EJ The HER2 peptide nelipepimut-S (E75) vaccine (NeuVax™) in breast cancer patients at risk for recurrence: Correlation of immunologic data with clinical response Immunotherapy 2014 6 519 531 10.2217/imt.14.22 24896623

307. Serafini P High-dose granulocyte-macrophage colony-stimulating factor-producing vaccines impair the immune response through the recruitment of myeloid suppressor cells Cancer Research 2004 64 6337 6343 10.1158/0008-5472.CAN-04-0757 15342423

308. Triozzi PL Differential immunologic and microRNA effects of 2 dosing regimens of recombinant human granulocyte/macrophage colony stimulating factor Journal of Immunotherapy 2012 35 587 594 10.1097/CJI.0b013e31826b20b6 22892455

309. Khanna S Tumor-derived GM-CSF promotes granulocyte immunosuppression in mesothelioma patients Clinical Cancer Research 2018 24 2859 2872 10.1158/1078-0432.CCR-17-3757 29602801 PMC6601632

310. Yanagimoto H Immunological evaluation of personalized peptide vaccination with gemcitabine for pancreatic cancer Cancer Science 2007 98 605 611 10.1111/j.1349-7006.2007.00429.x 17309598 PMC11158493

311. Chiappori AA Soliman H Janssen WE Antonia SJ Gabrilovich DI INGN-225: A dendritic cell-based p53 vaccine (Ad.p53-DC) in small cell lung cancer: Observed association between immune response and enhanced chemotherapy effect Expert Opinion on Biological Therapy 2010 10 983 991 10.1517/14712598.2010.484801 20420527 PMC3146364

312. Antonia SJ Combination of p53 cancer vaccine with chemotherapy in patients with extensive stage small cell lung cancer Clinical Cancer Research 2006 12 878 887 10.1158/1078-0432.CCR-05-2013 16467102

313. Hardwick NR p53-Reactive T cells are associated with clinical benefit in patients with platinum-resistant epithelial ovarian cancer after treatment with a p53 vaccine and gemcitabine chemotherapy Clinical Cancer Research 2018 24 1315 1325 10.1158/1078-0432.CCR-17-2709 29301826 PMC5856606

314. Prendergast GC Malachowski WP DuHadaway JB Muller AJ Discovery of IDO1 inhibitors: From bench to bedside Cancer Research 2017 77 6795 6811 10.1158/0008-5472.CAN-17-2285 29247038 PMC6021761

315. Gamat-Huber M Treatment combinations with DNA vaccines for the treatment of metastatic castration-resistant prostate cancer (mCRPC) Cancers (Basel) 2020 12 2831 10.3390/cancers12102831 33008010 PMC7601088

316. Park JA Wang L Cheung NV Modulating tumor infiltrating myeloid cells to enhance bispecific antibody-driven T cell infiltration and anti-tumor response Journal of Hematology & Oncology 2021 14 142 10.1186/s13045-021-01156-5 34496935 PMC8424962

317. Zitvogel L Apetoh L Ghiringhelli F Kroemer G Immunological aspects of cancer chemotherapy Nature Reviews Immunology 2008 8 59 73 10.1038/nri2216 18097448

318. Matsushita H A pilot study of autologous tumor lysate-loaded dendritic cell vaccination combined with sunitinib for metastatic renal cell carcinoma Journal for Immunotherapy of Cancer 2014 2 30 10.1186/s40425-014-0030-4 25694811 PMC4331924

319. Fleming V Targeting myeloid-derived suppressor cells to bypass tumor-induced immunosuppression Frontiers in Immunology 2018 9 398 10.3389/fimmu.2018.00398 29552012 PMC5840207

320. Hou A Hou K Huang Q Lei Y Chen W Targeting myeloid-derived suppressor cell, a promising strategy to overcome resistance to immune checkpoint inhibitors Frontiers in Immunology 2020 11 783 10.3389/fimmu.2020.00783 32508809 PMC7249937

321. Parker N Al-Obaidi A Truong QV Badgett R Pembrolizumab versus the standard of care for cancer therapy: A meta-analysis of 12 KEYNOTE trials comparing overall survival Journal of Clinical Oncology 2019 37 e14159 e14159 10.1200/JCO.2019.37.15_suppl.e14159

322. Benner B A pilot study of Bruton’s tyrosine kinase inhibitor ibrutinib alone and in combination with PD-1 inhibitor nivolumab in patients with metastatic solid tumors Journal of Clinical Oncology 2020 38 3111 3111 10.1200/JCO.2020.38.15_suppl.3111

323. Chen H Ruxolitinib reverses checkpoint inhibition by reducing programmed cell death ligand-1 (PD-L1) expression and increases anti-tumour effects of T cells in multiple myeloma British Journal of Haematology 2021 192 568 576 10.1111/bjh.17282 33341940

324. Wang JC Sun L PD-1/PD-L1, MDSC pathways, and checkpoint inhibitor therapy in Ph(-) myeloproliferative neoplasm: A review International Journal of Molecular Sciences 2022 23 5837 10.3390/ijms23105837 35628647 PMC9143160

325. Huijts CM The effect of everolimus and low-dose cyclophosphamide on immune cell subsets in patients with metastatic renal cell carcinoma: Results from a phase I clinical trial Cancer Immunology, Immunotherapy 2019 68 503 515 10.1007/s00262-018-2288-8 30652208 PMC6426984

326. Girardi DM Cabozantinib plus nivolumab phase i expansion study in patients with metastatic urothelial carcinoma refractory to immune checkpoint inhibitor therapy Clinical Cancer Research 2022 28 1353 1362 10.1158/1078-0432.CCR-21-3726 35031545 PMC9365339

327. Owen DH Deep and durable response to nivolumab and temozolomide in small-cell lung cancer associated with an early decrease in myeloid-derived suppressor cells Clinical Lung Cancer 2021 22 e487 e497 10.1016/j.cllc.2020.10.018 33234490 PMC8088448

328. Tavazoie MF LXR/ApoE activation restricts innate immune suppression in cancer Cell 2018 172 825 840 10.1016/j.cell.2017.12.026 29336888 PMC5846344

329. Serafini P Phosphodiesterase-5 inhibition augments endogenous antitumor immunity by reducing myeloid-derived suppressor cell function Journal of Experimental Medicine 2006 203 2691 2702 10.1084/jem.20061104 17101732 PMC2118163

330. Wesolowski R Phase Ib study of the combination of pexidartinib (PLX3397), a CSF-1R inhibitor, and paclitaxel in patients with advanced solid tumors Ther Adv Med Oncol 2019 11 1758835919854238 10.1177/1758835919854238 31258629 PMC6589951

331. Mehta AR Armstrong AJ Tasquinimod in the treatment of castrate-resistant prostate cancer - current status and future prospects Therapeutic Advances in Urology 2016 8 9 18 10.1177/1756287215603558 26834836 PMC4707420

332. Bullock K Richmond A Suppressing MDSC recruitment to the tumor microenvironment by antagonizing CXCR2 to enhance the efficacy of immunotherapy Cancers (Basel) 2021 13 6293 10.3390/cancers13246293 34944914 PMC8699249

333. Overman, M., et al. (2020). Randomized phase II study of the Bruton tyrosine kinase inhibitor acalabrutinib, alone or with pembrolizumab in patients with advanced pancreatic cancer. Journal Immunotherapy Cancer, 8 , e000587 10.1136/jitc-2020-000587 PMC7057435 32114502

334. Jontvedt Jorgensen M Monocytic myeloid-derived suppressor cells reflect tuberculosis severity and are influenced by cyclooxygenase-2 inhibitors Journal Leukocyte Biology 2021 110 177 186 10.1002/JLB.4A0720-409RR PMC8359170 33155730

335. Schneble EJ The HER2 peptide nelipepimut-S (E75) vaccine (NeuVax) in breast cancer patients at risk for recurrence: Correlation of immunologic data with clinical response Immunotherapy 2014 6 519 531 10.2217/imt.14.22 24896623

336. Chiappori AA Soliman H Janssen WE Antonia SJ Gabrilovich DI INGN-225: A dendritic cell-based p53 vaccine (Ad.p53-DC) in small cell lung cancer: Observed association between immune response and enhanced chemotherapy effect Expert Opinion Biology Therapeutics 2010 10 983 991 10.1517/14712598.2010.484801 PMC3146364 20420527

337. Antonia SJ Combination of p53 cancer vaccine with chemotherapy in patients with extensive stage small cell lung cancer Clinical Cancer Research 2006 12 878 887 10.1158/1078-0432.CCR-05-2013 16467102

338. Hardwick NR p53-Reactive t cells are associated with clinical benefit in patients with platinum-resistant epithelial ovarian cancer after treatment with a p53 vaccine and gemcitabine chemotherapy Clinical Cancer Research 2018 24 1315 1325 10.1158/1078-0432.CCR-17-2709 29301826 PMC5856606

339. Koinis F Effect of first-line treatment on myeloid-derived suppressor cells’ subpopulations in the peripheral blood of patients with non-small cell lung cancer Journal of Thoracic Oncology 2016 11 1263 1272 10.1016/j.jtho.2016.04.026 27178984

340. Feng PH Bevacizumab reduces S100A9-positive MDSCs linked to intracranial control in patients with EGFR-mutant lung adenocarcinoma Journal of Thoracic Oncology 2018 13 958 967 10.1016/j.jtho.2018.03.032 29684573

341. Clay TM Efficient transfer of a tumor antigen-reactive TCR to human peripheral blood lymphocytes confers anti-tumor reactivity The Journal of Immunology 1999 163 507 513 10.4049/jimmunol.163.1.507 10384155