CAR-T Plus Radiotherapy: A Promising Combination for Immunosuppressive Tumors.

Introduction

Adoptive cell transfer (ACT) has shifted the therapeutic paradigm for cancer patients in recent years. Transducing T cells with a chimeric antigen receptor (CAR) to redirect their antigen specificity against a defined tumor antigen has further broadened the use of ACT. CAR-T cells can recognize tumor-associated surface antigens via the single-chain variable fragment (scFv) and initiate anti-tumor immune responses by intracellular signaling domains, such as CD3ζ and CD28 ( 1 ). CAR-T therapy has demonstrated remarkable anti-cancer activity, achieving long-term remission in patients with refractory B cell malignancies ( 2 ). However, similar success with CAR-T cell therapy has not been achieved in solid tumors ( 3 ).

Solid tumors can be characterized into two distinct subsets based on the inflammatory status of the TME ( 4 ). Tumors (eg. melanoma) with a high inflammation signature tend to respond well to ICB ( 5 ), an effect largely mediated by CD8 + effector T cells ( 6 , 7 ); however, the development of resistance to immunotherapy is common. The presence of liver metastases is also being increasingly recognized as a barrier to ICB efficacy, even in the context of melanoma ( 8 , 9 ). Tumors that support an immune excluded or deserted TMEs, such as prostate and pancreatic cancers, are described as immunosuppressive ( 10 , 11 ), and typically fail to respond to ICB and CAR-T therapy. These immunosuppressive tumors lack T-cell chemokines to drive the recruitment of CD8 + effector T cells or CAR-T cells and are also enriched with suppressor cells that compromise T cell persistence and function. To overcome these immunosuppressive features, the value of TME-altering therapies, such as RT, is actively being explored.

RT is the standard-of-care treatment used for curative or palliative intent in close to 50% of cancer patients ( 12 ). Factors including total dose, fractionation scheme, hypoxia, and the intrinsic radiosensitivity of tumor cells all come into play in influencing the overall impact of RT on the TME ( 13 – 15 ) and its ability to reactivate host anti-tumor immune responses ( 16 ). Evidence to support the rationale for combining CAR-T cells with radiation therapy is growing. The RT effect on the TME, and the mechanisms whereby this occurs, have been widely studied in a variety of tumor models using different dose/fractionation schemes ( Table 1 ). As a TME-modifying therapy, RT can induce the release of chemokines, such as CXCL9, to augment T cell trafficking ( Figure 1 ①/②), increase the expression of adhesion molecules that may promote T cell infiltration ( Figure 1 ③/④), alter the immune cell composition in the TME ( Figure 1 ⑤/⑥), and increase the expression of immune-stimulatory cytokines to enhance the functional activity of effector T cells in the TME ( Figure 1 ⑧/⑨) ( 32 ). By changing the TME and creating a niche for immune cells, these benefits may synergize with immunotherapies, including CAR-T therapy.

In the clinic, patients with immunosuppressive tumors have few therapeutic options, have high morbidity, poor long-term survival, and comprise an urgent unmet clinical need. In this review, we will focus on the potential benefits and challenges for combining RT with CAR-T cells for the treatment of immunosuppressive tumors and provide insights into how to manipulate these two treatments to maximize clinical benefit.

Current Challenges for CAR-T Cells That Can be Targeted by RT

Poor trafficking, tumor penetration, and persistence of CAR-T cells, as well as tumor antigen heterogeneity and immunosuppression, are all major barriers to the success of CAR-T therapy in solid tumors. In this section, we will outline some of the mechanisms by which RT can overcome such challenges.

Insufficient Recruitment of CAR-T Cells

Efficient trafficking of CAR-T cells to solid tumors within peripheral tissue has proven to be a significant challenge. The tumor stroma comprises immunosuppressive cells, cancer-associated fibroblasts (CAFs), epithelium, endothelium, and extracellular matrix (ECM). Together these elements form a micro-environment that inhibits the anti-tumor immune response ( 33 ). CAFs are a heterogeneous cell population commonly present in most tumor stroma. CAF-mediated aberrant high-density ECM contributes to the exclusion of effector T cells by acting as barriers to immune cell infiltration ( 34 ). In addition, CAFs also exhibit pro-tumorigenic capacity by inhibiting effector T cells, polarizing macrophages towards an M2 phenotype, recruiting suppressor cells, and remodeling the ECM ( 35 ). Few studies have explored the impact of radiation on CAFs; however, it was reported that RT could alter the pro-tumorigenic status of the tumor stroma ( 20 ) and potentially increase CAR-T cell infiltration into the tumor ( Figure 1 . Step ③).

Tumor-associated hypoxia and dysregulated vasculature are further barriers to T cell access into tumor stroma. Immunosuppressive tumors, such as prostate cancer, present with a hypoxic environment that excludes the T cells from infiltration. These hypoxic zones in the TME also recruit and harbor suppressive cells such as MDSC ( 36 ). As one of the major influencers in the TME, tumor-associated vasculature also contributes to maintaining an immunosuppressive TME. Pro-angiogenic factors, such as vascular endothelial growth factor A (VEGF-A), can reduce T cell infiltration by disrupting their access to the tumor bed and also inhibit adhesion molecules on endothelium for immune cell extravasation, such as intercellular adhesion molecule (ICAM)-1 and vascular cell adhesion protein (VCAM)-1 ( 37 ). RT contributes to the improved normalization of tumor vasculature in the TME. In prostate cancer, fractionated RT altered the function of tumor vasculature to improve tumor reoxygenation ( 19 ). Real-time imaging analysis in a glioblastoma model revealed that RT also promoted CAR-T cell extravasation and local expansion leading to a synergistic benefit of the combination treatment ( 27 ). In addition, radiation was shown to induce increased expression of the integrins ICAM-1 and VCAM-1 on the endothelium of the vasculature in the TME ( 21 , 22 ), these adhesion molecules are critical for transendothelial migration of CAR-T cells ( Figure 1 . Step ④).

The migratory activity of CAR-T cells is largely influenced by the CXCR3/CXCL9-11 chemokine receptor-chemokine axis ( 38 ). These T cell recruiting chemokines are produced by M1-like immune-stimulatory macrophages in response to the proinflammatory cytokine IFN-γ ( 39 , 40 ). In addition, CD8 + T cell infiltration is also controlled by the CCR5/CCL5 axis ( 40 ). However, immunosuppressive tumors lack the pro-inflammatory environment and immune-stimulatory cells to produce the T cell recruiting chemokines. Some tumors also secrete chemokines that recruit suppressive cells, such as regulatory T cell (Treg)-recruiting CCL17/22 and myeloid-derived suppressor cell (MDSC)-recruiting CCL2 ( 41 ). As a result, the immunosuppressive tumors present a mismatched chemokine signature for T cell recruitment, leading to limited effector T cell homing and tumor infiltration ( 42 ). Radiation-induced inflammation in the TME can promote the recruitment of effector T cells by triggering macrophages, in an IFN-γ dependent manner, to produce increased levels of the CXCR3-reactive T cell chemoattractants CXCL9, CXCL10, and CXCL11 ( 17 , 26 ) ( Figure 1 . Steps ①②). Radiation-induced activation of stimulator of interferon genes (STING) pathway was also shown to increase expression of CXCL10 in a mouse mammary carcinoma model refractory to immune checkpoint inhibitors ( 18 ).

Immunosuppression in the TME

The TME comprises a complex network of tumor cells and the tumor stroma made up of endothelial cells, fibroblasts, extracellular matrix, and immune cells ( 43 ). Despite sufficient trafficking and expansion of CAR-T cells in TME, suppressor immune cell subsets and soluble mediators can render CAR-T cells exhausted and dysfunctional ( 44 ).

Tregs are key contributors to tumor-mediated immune suppression ( 45 ). In patients with recurrent glioblastoma, infiltration of Tregs in the TME dampened immune activity and promoted acquired resistance to CAR-T cell therapy ( 46 ). Other immunosuppressive subsets that have been shown to impact CAR-T cell function are MDSC and tumor-associated macrophages (TAM) ( 47 , 48 ). MDSCs can generate a suppressive milieu of cytokines and metabolites such as IL-10, TGF-β, IL-1 receptor antagonist, nitric oxide, and arginase 1 to hinder the tumoricidal immune response ( 49 – 52 ). Neutralizing MDSCs by immunostimulatory agents, such as all-trans retinoic acid, preserved CAR-T cell proliferation and cytotoxic function, and resulted in reduced tumor burden in CAR-T treated mice in an osteosarcoma model ( 53 ). Similarly, TAM can be co-opted by tumor cells and polarized to an anti-inflammatory M2-like phenotype capable of hindering T cell responses by the production of inhibitory mediators (i.e., TGF-β, indoleamine 2,3-dioxygenase IDO) and expression of PD-L1 ( 54 , 55 ). Direct depletion of TAM has proven ineffective in promoting ACT, however, re-wiring of TAM to a pro-inflammatory phenotype by anti-CD40 agonist or blocking AIM2 inflammasome can improve the performance of immunotherapies including CAR-T cells ( 56 , 57 ).

Cytokines and metabolic factors can also contribute to tumor progression and the loss of immune surveillance. The accumulation of lactate and adenosine, by-products of abnormal cellular metabolism in the TME, favors the infiltration and expansion of suppressive TAM and MDSC and dampens the activity of T cells ( 58 – 61 ). Enrichment of the inhibitory cytokine TGF-β has also been documented in many cancers and exerts profound immunomodulatory properties to attenuate the cytotoxic potential of T cells and accelerate T cell dysfunction ( 62 , 63 ). TGF-β can also polarize myeloid cells and B cells towards an immunosuppressive phenotype ( 64 , 65 ). As such, TGF-β co-opts various immunosuppressive cells to indirectly counteract immune activation in the TME. Neutralizing the TGF-β signal in the TME has been shown to unleash potent T cell responses, thereby rendering tumors susceptible to immunotherapy, including CAR-T cell treatment ( 66 , 67 ).

Taken together, depleting Tregs and MDSC, reprogramming the TAM, and blocking the associated soluble mediators can be crucial to rescuing anti-cancer immune activity. In this regard, RT has been broadly investigated for its TME altering capacity ( 16 , 32 ). This rationale is further supported by the notion that irradiation can alter the phenotype of immunosuppressive cells in the TME ( Figure 1 . Step ⑤). RT has been shown to enrich the TME for M1-like macrophages and reduce the frequency of immunosuppressive M2-like macrophages and MDSC ( 23 , 24 , 68 , 69 ) ( Figure 1 . Step ⑥). Reduction of MDSC in the peripheral blood of patients post conventionally fractionated RT (<2 Gy/fraction) has also been reported ( 70 ). Similarly, hypofractionated RT (>2 Gy/fraction), has also been shown to reduce the influx of MDSC in TME by downregulating the expression of VEGF ( 25 ). Taken together, RT can boost immune activation by altering the immunosuppression status of the TME to enhance CAR-T cell efficacy.

Aside from its direct ability to debulk tumors, RT can also engage host immune defenses by causing immunogenic cell death (ICD) ( 71 ). ICD is associated with the release of danger-associated molecular patterns (DAMP) and increased expression of neoantigens that can help facilitate the recruitment and activation of dendritic cells and subsequent priming of T cell responses with an expanded TCR repertoire ( 72 , 73 ). Radiation-induced activation of the STING pathway induces expression of Type I IFNs and TNF, leading to an inflammatory microenvironment, which facilitates T cell activity ( 22 , 74 ) ( Figure 1 . Step ⑦). Of note, the STING response induced by a single fraction of high-dose radiation is distinct from that when the total dose is fractionated into a series of smaller doses. Demaria and colleagues demonstrated that fractionated radiation schedules of less than 8 Gy/fraction activated the STING pathway and release of Type I IFN, permitting the induction of abscopal response when delivered in combination with ICB therapy. In contrast, a single fraction of 20 Gy RT increased the expression of TREX1 within tumor cells, preventing STING activation and its ability to augment the systemic anti-tumor activity of ICB therapy ( 18 ).

The Effect of RT on CAR-T Cell Function

In addition to the effect of RT on the TME and T cell trafficking, radiation can also promote CAR-T cell function. Radiation can induce tumor cell stress ligands and it is an alternate mechanism through which RT may increase tumor-cell susceptibility to CAR-T cell-mediated killing ( Figure 1 . Step ⑧). DeSelm et al. showed that a single fraction of 2 Gy could augment TRAIL-mediated cytolysis by anti-sialyl Lewis-A CAR-T cells, leading to attenuated tumor growth in mice bearing heterogeneous pancreatic tumors ( 28 ). A similar effect was also observed when radiation enhanced the IFN-γ production of CAR-T cells in a glioblastoma mouse model ( 29 ). In addition, CAR-T cell expansion was correlated with RT in a patient with relapsed diffuse large B cell lymphoma ( Figure 1 . Step ⑨). The CAR-T cell transgene copies dropped initially post-infusion but increased dramatically after RT and persisted for more than 120 days, leading to a complete remission post combination treatment ( 30 ).

Radiation may also induce the expression of stress ligands that can be targeted by CAR-T cells. NKG2D CAR-T cells combined with local intracranial RT significantly reduced tumor burden and prolonged survival, which can be attributed to the upregulation of NKG2DL, such as RAE-1 and MULT-1 post-irradiation ( 29 ).

In addition to the local effects of RT, induction of systemic anti-tumor immune responses that control tumor growth outside the irradiation volume (known as the abscopal effect), was also reported ( 75 ). In a case study of a BCMA CAR-T cell clinical trial, combination with RT showed complete radiographic resolution including the innumerable sites outside the radiation site with no relapse in 9 months post-treatment ( 31 ).

More evidence is emerging that certain dose/fractions of RT can, directly and indirectly, affect the CAR-T efficacy against tumor cells, although further detailed mechanisms to explain the benefits of RT on CAR-T cells are still not clear and should be investigated further.

Challenges for the Combination Therapy of RT and CAR-T Cells

Although RT and CAR-T therapy have shown therapeutic efficacy in treating some tumors, further consideration is needed when designing the combination therapy to gain the maximum clinical benefit.

For many cancer patients, RT is given post CAR-T cell infusion so the adoptive transferred cells are potentially vulnerable to radiation-induced induce apoptosis ( 76 ), however, in vivo data relating to this point remains scarce. Notably, antigen-experienced T cells, such as memory CD8 + T cells and tissue-resident memory T cells have been reported to be more resistant to radiation-induced apoptosis compared to naïve T cells ( 77 , 78 ). Thus, the ex vivo culturing processes necessary to generate CAR T cells may aid in strengthening their resilience to direct exposure to external beam RT. In a clinical trial, low dose RT (2 Gy/fraction) was found to induce in vivo expansion of CAR-T cells ( 30 ). Based on our current knowledge of the immunological effects of RT we would expect that low dose RT (<2 - 4Gy) to be a better complement to CAR-T therapy. Low-dose RT is likely to be less impactful on the viability of the immune compartment and has reported positive impacts on TAM and the vasculature as discussed above.

Besides reprogramming the TAM and MDSC in the TME to a proinflammatory phenotype, RT can also induce the wound healing response and the induction of highly suppressive Treg responses ( 79 ). Interestingly, the rate and amplitude of accumulation of Tregs within irradiated tumors were dictated by dose per fraction rather than total dose. In this study, a single fraction of 20Gy generated a more aggressive Treg response compared to a fractionated dose of 9 x 4 Gy, despite sharing the same biological effector dose of 45Gy ( 79 ). Several preclinical studies have demonstrated that the targeted depletion of Tregs is required to induce a durable response to RT and support the anti-cancer actions of immune checkpoint blockade therapy ( 80 , 81 ). Thus, a more precise dose/fraction should be considered, and additional Treg-targeted approaches may be needed to overcome potential immune suppression and acquired resistance. In this regard, other combination treatment strategies could be considered, such as chemotherapy and monoclonal antibodies. Docetaxel was reported to specifically deplete activated Tregs with more IFN-γ and less TGF-β, but not resting Tregs in non-small cell lung cancer ( 82 ). In pancreatic ductal adenocarcinoma, low-dose gemcitabine also induced Treg depletion ( 83 ). Another approach is to use the monoclonal antibodies (mAbs) targeting the key molecules on Treg cells, such as CTLA-4 (Cytotoxic T lymphocyte-associated antigen-4), CD73, and TIGIT (T cell immunoreceptor with Ig and ITIM domains). Some of these mAbs have been approved for treating cancers, such as Ipilimumab (anti-CTLA-4 mAb), and others are being investigated in clinical trials as reviewed elsewhere ( 84 – 86 ). Similarly, upregulation of inhibitory signaling molecules, such as PD-L1, have been observed on tumor cells post-radiation ( 87 – 89 ). Thus, adding immune checkpoint blockade into the combination therapy may resolve the negative effects of RT and promote the efficacy of CAR-T cells, although the safety and synergistic benefit remain to be evaluated in the clinic.

ATP released from irradiated tumor cells can be converted to adenosine by the ectoenzymes CD39 and CD73, which is another potential barrier for CAR-T cell function ( 90 ). Elevated adenosine impedes the anti-tumor response of effector T cells through their surface A2a adenosine receptors (A2aRs) ( 91 – 93 ). Blocking adenosine signaling has been shown to elicit a more potent T-cell response in combination therapy ( 94 , 95 ). Besides therapeutic inhibitors, this strategy can also be achieved by directly modifying the CAR-T cells. The modification includes depleting the A2aRs in CAR-T cells by CRISPR-Cas9 editing and engineering CAR-T cells to carry antagonist nanoparticles. Both approaches showed increased efficacy with radiation in CD73/CD39 induced adenosine-enriched tumors ( 96 , 97 ), but not in tumors with weak CD73/CD39 expression, such as melanoma ( 98 ). Therefore, for radiation-induced adenosine-rich tumors, inhibitors of the adenosine pathway or the engineering of CAR-T cells to resist adenosine-induced suppression may prove beneficial in the context of RT.

Radiation-induced enrichment of TGF-β in the TME is another critical barrier to the effective partnership of RT and CAR-T cells ( 26 , 79 ). The wound healing process and DNA repair programs post-radiation treatment trigger TGF-β signaling in the TME, resulting in epithelial-mesenchymal transition, tissue fibrosis, and the induction of a broad spectrum of immunosuppressive effects on infiltrating immune cells including CAR-T cells ( 99 , 100 ). To overcome this barrier, CAR-T cells can be modified to become resistant to TGF-β mediated immunosuppression. For example, Kloss et al. engineered a dominant-negative TGF-β receptor (dnTGF-βRII) capable of binding TGF-β without directly triggering a T-cell inhibitory signaling event. This approach proved effective in promoting superior anti-tumor efficacy compared to the parental CAR-T cells in a metastatic prostate cancer model ( 101 ). The TGF-β on CAR-T cells can also be blocked by knocking out the endogenous TGF-β type II receptor ( 102 ). However, given the homeostatic function of TGF-β signaling in lymphoid tissues, constitutive TGF-β blockade may result in off-target toxicity. Indeed, following treatment with dnTGFβRII-PSMA-CAR-T cells, a subset of patients developed severe cytokine release syndrome and immune effector cell-associated neurotoxicity syndrome (ICANS) ( 103 ). Based on these findings, efforts are ongoing to refine this approach. Developing a CAR construct to selectively capture and neutralize TGF-β only within the TME may indeed prove to be efficacious and safe.

Conclusion

Combination treatment of RT and CAR-T cells has significant therapeutic potential. Although it is a promising option for patients with immunosuppressive tumors, further considerations on dose/fraction, treatment schedule, immune context, and tumor type should be considered when designing the treatment, and more mechanistic studies are still needed to understand how these therapies will best work in combination. Our increased understanding of the immunomodulatory effects of RT together with the incredible advances being made in the CAR-T cell field, especially with promising molecular engineering of novel CAR constructs, will facilitate the successful implementation of this combination strategy in the clinic.

Author Contributions

Writing-original draft preparation: VQ, NH, and JZ. Writing-review and editing: NH, CD’S, PN, and JZ. Supervision: PN. All authors contributed to the article and approved the submitted version.

Conflict of Interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Publisher’s Note

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.

References

1 Moeller M Haynes NM Trapani JA Teng MW Jackson JT Tanner JE . A Functional Role for CD28 Costimulation in Tumor Recognition by Single-Chain Receptor-Modified T Cells . Cancer Gene Ther ( 2004 ) 11 ( 5 ): 371–9 . doi: 10.1038/sj.cgt.7700710 15060573

2 Park JH Riviere I Gonen M Wang X Senechal B Curran KJ . Long-Term Follow-Up of CD19 CAR Therapy in Acute Lymphoblastic Leukemia . N Engl J Med ( 2018 ) 378 ( 5 ): 449–59 . doi: 10.1056/NEJMoa1709919 PMC6637939 29385376

3 Rodriguez-Garcia A Palazon A Noguera-Ortega E Powell DJ Jr. Guedan S . CAR-T Cells Hit the Tumor Microenvironment: Strategies to Overcome Tumor Escape . Front Immunol ( 2020 ) 11 : 1109 . doi: 10.3389/fimmu.2020.01109 32625204 PMC7311654

4 Galon J Bruni D . Approaches to Treat Immune Hot, Altered and Cold Tumours With Combination Immunotherapies . Nat Rev Drug Discov ( 2019 ) 18 ( 3 ): 197 – 218 . doi: 10.1038/s41573-018-0007-y 30610226

5 Ayers M Lunceford J Nebozhyn M Murphy E Loboda A Kaufman DR . IFN-Gamma-Related mRNA Profile Predicts Clinical Response to PD-1 Blockade . J Clin Invest ( 2017 ) 127 ( 8 ): 2930–40 . doi: 10.1172/JCI91190 PMC5531419 28650338

6 Schadendorf D Hodi FS Robert C Weber JS Margolin K Hamid O . Pooled Analysis of Long-Term Survival Data From Phase II and Phase III Trials of Ipilimumab in Unresectable or Metastatic Melanoma . J Clin Oncol ( 2015 ) 33 ( 17 ): 1889–94 . doi: 10.1200/JCO.2014.56.2736 PMC5089162 25667295

7 Tumeh PC Harview CL Yearley JH Shintaku IP Taylor EJ Robert L . PD-1 Blockade Induces Responses by Inhibiting Adaptive Immune Resistance . Nature ( 2014 ) 515 ( 7528 ): 568–71 . doi: 10.1038/nature13954 PMC4246418 25428505

8 Ma V Griffith K Waninger J Daignault-Newton S Fecher L Alva A . 241 Clinical Outcomes of Metastatic Melanoma Patients With Liver Metastases Treated With Anti-PD-1 Monotherapy Versus Combination Ipilimumab/Nivolumab . J Immuno Therapy Cancer ( 2020 ) 8 : A143–A4 . doi: 10.1136/jitc-2020-SITC2020.0241

9 Pelster MS Gruschkus SK Bassett R Gombos DS Shephard M Posada L . Nivolumab and Ipilimumab in Metastatic Uveal Melanoma: Results From a Single-Arm Phase II Study . J Clin Oncol ( 2021 ) 39 ( 6 ): 599 – 607 . doi: 10.1200/JCO.20.00605 33125309 PMC8257877

10 Strasner A Karin M . Immune Infiltration and Prostate Cancer . Front Oncol ( 2015 ) 5 : 128 . doi: 10.3389/fonc.2015.00128 26217583 PMC4495337

11 Martinez-Bosch N Vinaixa J Navarro P . Immune Evasion in Pancreatic Cancer: From Mechanisms to Therapy . Cancers (Basel) ( 2018 ) 10 ( 1 ): 6 . doi: 10.3390/cancers10010006 PMC5789356 29301364

12 Barton MB Jacob S Shafiq J Wong K Thompson SR Hanna TP . Estimating the Demand for Radiotherapy From the Evidence: A Review of Changes From 2003 to 2012 . Radiother Oncol ( 2014 ) 112 ( 1 ): 140–4 . doi: 10.1016/j.radonc.2014.03.024 24833561

13 Hua L Wang Z Zhao L Mao H Wang G Zhang K . Hypoxia-Responsive Lipid-Poly-(Hypoxic Radiosensitized Polyprodrug) Nanoparticles for Glioma Chemo- and Radiotherapy . Theranostics ( 2018 ) 8 ( 18 ): 5088–105 . doi: 10.7150/thno.26225 PMC6217062 30429888

14 Grapin M Richard C Limagne E Boidot R Morgand V Bertaut A . Optimized Fractionated Radiotherapy With Anti-PD-L1 and Anti-TIGIT: A Promising New Combination . J Immunother Cancer ( 2019 ) 7 ( 1 ): 160 . doi: 10.1186/s40425-019-0634-9 31238970 PMC6593525

15 Morisada M Clavijo PE Moore E Sun L Chamberlin M Van Waes C . PD-1 Blockade Reverses Adaptive Immune Resistance Induced by High-Dose Hypofractionated But Not Low-Dose Daily Fractionated Radiation . Oncoimmunology ( 2018 ) 7 ( 3 ): e1395996 . doi: 10.1080/2162402X.2017.1395996 29399393 PMC5790397

16 McLaughlin M Patin EC Pedersen M Wilkins A Dillon MT Melcher AA . Inflammatory Microenvironment Remodelling by Tumour Cells After Radiotherapy . Nat Rev Cancer ( 2020 ) 20 ( 4 ): 203–17 . doi: 10.1038/s41568-020-0246-1 32161398

17 Lim JY Gerber SA Murphy SP Lord EM . Type I Interferons Induced by Radiation Therapy Mediate Recruitment and Effector Function of CD8(+) T Cells . Cancer Immunol Immunother ( 2014 ) 63 ( 3 ): 259–71 . doi: 10.1007/s00262-013-1506-7 PMC3944132 24357146

18 Vanpouille-Box C Alard A Aryankalayil MJ Sarfraz Y Diamond JM Schneider RJ . DNA Exonuclease Trex1 Regulates Radiotherapy-Induced Tumour Immunogenicity . Nat Commun ( 2017 ) 8 : 15618 . doi: 10.1038/ncomms15618 28598415 PMC5472757

19 Potiron VA Abderrahmani R Clement-Colmou K Marionneau-Lambot S Oullier T Paris F . Improved Functionality of the Vasculature During Conventionally Fractionated Radiation Therapy of Prostate Cancer . PloS One ( 2013 ) 8 ( 12 ): e84076 . doi: 10.1371/journal.pone.0084076 24391887 PMC3877206

20 Grinde MT Vik J Camilio KA Martinez-Zubiaurre I Hellevik T . Ionizing Radiation Abrogates the Pro-Tumorigenic Capacity of Cancer-Associated Fibroblasts Co-Implanted in Xenografts . Sci Rep ( 2017 ) 7 : 46714 . doi: 10.1038/srep46714 28440285 PMC5404232

21 Zhao Y Zhang T Wang Y Lu D Du J Feng X . ICAM-1 Orchestrates the Abscopal Effect of Tumor Radiotherapy . Proc Natl Acad Sci USA ( 2021 ) 118 ( 14 ): e2010333118 . doi: 10.1073/pnas.2010333118 33785590 PMC8040592

22 Lugade AA Sorensen EW Gerber SA Moran JP Frelinger JG Lord EM . Radiation-Induced IFN-Gamma Production Within the Tumor Microenvironment Influences Antitumor Immunity . J Immunol ( 2008 ) 180 ( 5 ): 3132–9 . doi: 10.4049/jimmunol.180.5.3132 18292536

23 Yu J Green MD Li S Sun Y Journey SN Choi JE . Liver Metastasis Restrains Immunotherapy Efficacy via Macrophage-Mediated T Cell Elimination . Nat Med ( 2021 ) 27 ( 1 ): 152–64 . doi: 10.1038/s41591-020-1131-x PMC8095049 33398162

24 Barsoumian HB Ramapriyan R Younes AI Caetano MS Menon H Comeaux NI . Low-Dose Radiation Treatment Enhances Systemic Antitumor Immune Responses by Overcoming the Inhibitory Stroma . J Immunother Cancer ( 2020 ) 8 ( 2 ): e000537 . doi: 10.1136/jitc-2020-000537 33106386 PMC7592253

25 Lan J Li R Yin LM Deng L Gui J Chen BQ . Targeting Myeloid-Derived Suppressor Cells and Programmed Death Ligand 1 Confers Therapeutic Advantage of Ablative Hypofractionated Radiation Therapy Compared With Conventional Fractionated Radiation Therapy . Int J Radiat Oncol Biol Phys ( 2018 ) 101 ( 1 ): 74 – 87 . doi: 10.1016/j.ijrobp.2018.01.071 29619980

26 Keam SP Halse H Nguyen T Wang M Van Kooten Losio N Mitchell C . High Dose-Rate Brachytherapy of Localized Prostate Cancer Converts Tumors From Cold to Hot . J Immunother Cancer ( 2020 ) 8 ( 1 ): e000792 . doi: 10.1136/jitc-2020-000792 32581061 PMC7319782

27 Murty S Haile ST Beinat C Aalipour A Alam IS Murty T . Intravital Imaging Reveals Synergistic Effect of CAR T-Cells and Radiation Therapy in a Preclinical Immunocompetent Glioblastoma Model . Oncoimmunology ( 2020 ) 9 ( 1 ): 1757360 . doi: 10.1080/2162402X.2020.1757360 32923113 PMC7458609

28 DeSelm C Palomba ML Yahalom J Hamieh M Eyquem J Rajasekhar VK . Low-Dose Radiation Conditioning Enables CAR T Cells to Mitigate Antigen Escape . Mol Ther ( 2018 ) 26 ( 11 ): 2542–52 . doi: 10.1016/j.ymthe.2018.09.008 PMC6225039 30415658

29 Weiss T Weller M Guckenberger M Sentman CL Roth P . NKG2D-Based CAR T Cells and Radiotherapy Exert Synergistic Efficacy in Glioblastoma . Cancer Res ( 2018 ) 78 ( 4 ): 1031–43 . doi: 10.1158/0008-5472.CAN-17-1788 29222400

30 Qu C Ping N Kang L Liu H Qin S Wu Q . Radiation Priming Chimeric Antigen Receptor T-Cell Therapy in Relapsed/Refractory Diffuse Large B-Cell Lymphoma With High Tumor Burden . J Immunother ( 2020 ) 43 ( 1 ): 32–7 . doi: 10.1097/CJI.0000000000000284 31219975

31 Smith EL Mailankody S Staehr M Wang X Senechal B Purdon TJ . BCMA-Targeted CAR T-Cell Therapy Plus Radiotherapy for the Treatment of Refractory Myeloma Reveals Potential Synergy . Cancer Immunol Res ( 2019 ) 7 ( 7 ): 1047–53 . doi: 10.1158/2326-6066.CIR-18-0551 PMC6606365 31113804

32 Barker HE Paget JT Khan AA Harrington KJ . The Tumour Microenvironment After Radiotherapy: Mechanisms of Resistance and Recurrence . Nat Rev Cancer ( 2015 ) 15 ( 7 ): 409–25 . doi: 10.1038/nrc3958 PMC4896389 26105538

33 Valkenburg KC de Groot AE Pienta KJ . Targeting the Tumour Stroma to Improve Cancer Therapy . Nat Rev Clin Oncol ( 2018 ) 15 ( 6 ): 366–81 . doi: 10.1038/s41571-018-0007-1 PMC5960434 29651130

34 Henke E Nandigama R Ergun S . Extracellular Matrix in the Tumor Microenvironment and Its Impact on Cancer Therapy . Front Mol Biosci ( 2019 ) 6 : 160 . doi: 10.3389/fmolb.2019.00160 32118030 PMC7025524

35 Monteran L Erez N . The Dark Side of Fibroblasts: Cancer-Associated Fibroblasts as Mediators of Immunosuppression in the Tumor Microenvironment . Front Immunol ( 2019 ) 10 : 1835 . doi: 10.3389/fimmu.2019.01835 31428105 PMC6688105

36 Jayaprakash P Ai M Liu A Budhani P Bartkowiak T Sheng J . Targeted Hypoxia Reduction Restores T Cell Infiltration and Sensitizes Prostate Cancer to Immunotherapy . J Clin Invest ( 2018 ) 128 ( 11 ): 5137–49 . doi: 10.1172/JCI96268 PMC6205399 30188869

37 Schaaf MB Garg AD Agostinis P . Defining the Role of the Tumor Vasculature in Antitumor Immunity and Immunotherapy . Cell Death Dis ( 2018 ) 9 ( 2 ): 115 . doi: 10.1038/s41419-017-0061-0 29371595 PMC5833710

38 Spranger S Dai D Horton B Gajewski TF . Tumor-Residing Batf3 Dendritic Cells Are Required for Effector T Cell Trafficking and Adoptive T Cell Therapy . Cancer Cell ( 2017 ) 31 ( 5 ): 711 – 23 e4 . doi: 10.1016/j.ccell.2017.04.003 28486109 PMC5650691

39 House IG Savas P Lai J Chen AXY Oliver AJ Teo ZL . Macrophage-Derived CXCL9 and CXCL10 Are Required for Antitumor Immune Responses Following Immune Checkpoint Blockade . Clin Cancer Res ( 2020 ) 26 ( 2 ): 487 – 504 . doi: 10.1158/1078-0432.CCR-19-1868 31636098

40 Dangaj D Bruand M Grimm AJ Ronet C Barras D Duttagupta PA . Cooperation Between Constitutive and Inducible Chemokines Enables T Cell Engraftment and Immune Attack in Solid Tumors . Cancer Cell ( 2019 ) 35 ( 6 ): 885 – 900 e10 . doi: 10.1016/j.ccell.2019.05.004 31185212 PMC6961655

41 Kohli K Pillarisetty VG Kim TS . Key Chemokines Direct Migration of Immune Cells in Solid Tumors . Cancer Gene Ther ( 2021 ). doi: 10.1038/s41417-021-00303-x PMC8761573 33603130

42 Sackstein R Schatton T Barthel SR . T-Lymphocyte Homing: An Underappreciated Yet Critical Hurdle for Successful Cancer Immunotherapy . Lab Invest ( 2017 ) 97 ( 6 ): 669–97 . doi: 10.1038/labinvest.2017.25 PMC5446300 28346400

43 Garner H de Visser KE . Immune Crosstalk in Cancer Progression and Metastatic Spread: A Complex Conversation . Nat Rev Immunol ( 2020 ) 20 ( 8 ): 483–97 . doi: 10.1038/s41577-019-0271-z 32024984

44 Moon EK Wang LC Dolfi DV Wilson CB Ranganathan R Sun J . Multifactorial T-Cell Hypofunction That Is Reversible can Limit the Efficacy of Chimeric Antigen Receptor-Transduced Human T Cells in Solid Tumors . Clin Cancer Res ( 2014 ) 20 ( 16 ): 4262–73 . doi: 10.1158/1078-0432.CCR-13-2627 PMC4134701 24919573

45 Paluskievicz CM Cao X Abdi R Zheng P Liu Y Bromberg JS . T Regulatory Cells and Priming the Suppressive Tumor Microenvironment . Front Immunol ( 2019 ) 10 : 2453 . doi: 10.3389/fimmu.2019.02453 31681327 PMC6803384

46 O'Rourke DM Nasrallah MP Desai A Melenhorst JJ Mansfield K Morrissette JJD . A Single Dose of Peripherally Infused EGFRvIII-Directed CAR T Cells Mediates Antigen Loss and Induces Adaptive Resistance in Patients With Recurrent Glioblastoma . Sci Transl Med ( 2017 ) 9 ( 399 ): eaaa0984 . doi: 10.1126/scitranslmed.aaa0984 28724573 PMC5762203

47 Li K Shi H Zhang B Ou X Ma Q Chen Y . Myeloid-Derived Suppressor Cells as Immunosuppressive Regulators and Therapeutic Targets in Cancer . Signal Transduct Target Ther ( 2021 ) 6 ( 1 ): 362 . doi: 10.1038/s41392-021-00670-9 34620838 PMC8497485

48 DeNardo DG Ruffell B . Macrophages as Regulators of Tumour Immunity and Immunotherapy . Nat Rev Immunol ( 2019 ) 19 ( 6 ): 369–82 . doi: 10.1038/s41577-019-0127-6 PMC7339861 30718830

49 Steggerda SM Bennett MK Chen J Emberley E Huang T Janes JR . Inhibition of Arginase by CB-1158 Blocks Myeloid Cell-Mediated Immune Suppression in the Tumor Microenvironment . J Immunother Cancer ( 2017 ) 5 ( 1 ): 101 . doi: 10.1186/s40425-017-0308-4 29254508 PMC5735564

50 Feng S Cheng X Zhang L Lu X Chaudhary S Teng R . Myeloid-Derived Suppressor Cells Inhibit T Cell Activation Through Nitrating LCK in Mouse Cancers . Proc Natl Acad Sci USA ( 2018 ) 115 ( 40 ): 10094–9 . doi: 10.1073/pnas.1800695115 PMC6176562 30232256

51 Lu X Horner JW Paul E Shang X Troncoso P Deng P . Effective Combinatorial Immunotherapy for Castration-Resistant Prostate Cancer . Nature ( 2017 ) 543 ( 7647 ): 728–32 . doi: 10.1038/nature21676 PMC5374023 28321130

52 Bronte V Brandau S Chen SH Colombo MP Frey AB Greten TF . Recommendations for Myeloid-Derived Suppressor Cell Nomenclature and Characterization Standards . Nat Commun ( 2016 ) 7 : 12150 . doi: 10.1038/ncomms12150 27381735 PMC4935811

53 Long AH Highfill SL Cui Y Smith JP Walker AJ Ramakrishna S . Reduction of MDSCs With All-Trans Retinoic Acid Improves CAR Therapy Efficacy for Sarcomas . Cancer Immunol Res ( 2016 ) 4 ( 10 ): 869–80 . doi: 10.1158/2326-6066.CIR-15-0230 PMC5050151 27549124

54 Su S Zhao J Xing Y Zhang X Liu J Ouyang Q . Immune Checkpoint Inhibition Overcomes ADCP-Induced Immunosuppression by Macrophages . Cell ( 2018 ) 175 ( 2 ): 442 – 57 e23 . doi: 10.1016/j.cell.2018.09.007 30290143

55 Lundholm M Hagglof C Wikberg ML Stattin P Egevad L Bergh A . Secreted Factors From Colorectal and Prostate Cancer Cells Skew the Immune Response in Opposite Directions . Sci Rep ( 2015 ) 5 : 15651 . doi: 10.1038/srep15651 26503803 PMC4621525

56 Stromnes IM Burrack AL Hulbert A Bonson P Black C Brockenbrough JS . Differential Effects of Depleting Versus Programming Tumor-Associated Macrophages on Engineered T Cells in Pancreatic Ductal Adenocarcinoma . Cancer Immunol Res ( 2019 ) 7 ( 6 ): 977–89 . doi: 10.1158/2326-6066.CIR-18-0448 PMC6548612 31028033

57 Liu D Xu X Dai Y Zhao X Bao S Ma W . Blockade of AIM2 Inflammasome or Alpha1-AR Ameliorates IL-1beta Release and Macrophage-Mediated Immunosuppression Induced by CAR-T Treatment . J Immunother Cancer ( 2021 ) 9 ( 1 ): e001466 . doi: 10.1136/jitc-2020-001466 33414262 PMC7797290

58 Yang R Elsaadi S Misund K Abdollahi P Vandsemb EN Moen SH . Conversion of ATP to Adenosine by CD39 and CD73 in Multiple Myeloma can be Successfully Targeted Together With Adenosine Receptor A2A Blockade . J Immunother Cancer ( 2020 ) 8 ( 1 ): e000610 . doi: 10.1136/jitc-2020-000610 32409420 PMC7239696

59 Ryzhov S Novitskiy SV Goldstein AE Biktasova A Blackburn MR Biaggioni I . Adenosinergic Regulation of the Expansion and Immunosuppressive Activity of CD11b+Gr1+ Cells . J Immunol ( 2011 ) 187 ( 11 ): 6120–9 . doi: 10.4049/jimmunol.1101225 PMC3221925 22039302

60 Brand A Singer K Koehl GE Kolitzus M Schoenhammer G Thiel A . LDHA-Associated Lactic Acid Production Blunts Tumor Immunosurveillance by T and NK Cells . Cell Metab ( 2016 ) 24 ( 5 ): 657–71 . doi: 10.1016/j.cmet.2016.08.011 27641098

61 Wang H Franco F Tsui YC Xie X Trefny MP Zappasodi R . CD36-Mediated Metabolic Adaptation Supports Regulatory T Cell Survival and Function in Tumors . Nat Immunol ( 2020 ) 21 ( 3 ): 298 – 308 . doi: 10.1038/s41590-019-0589-5 32066953 PMC7043937

62 Park BV Freeman ZT Ghasemzadeh A Chattergoon MA Rutebemberwa A Steigner J . TGFbeta1-Mediated SMAD3 Enhances PD-1 Expression on Antigen-Specific T Cells in Cancer . Cancer Discov ( 2016 ) 6 ( 12 ): 1366–81 . doi: 10.1158/2159-8290.CD-15-1347 PMC5295786 27683557

63 Donkor MK Sarkar A Savage PA Franklin RA Johnson LK Jungbluth AA . T Cell Surveillance of Oncogene-Induced Prostate Cancer is Impeded by T Cell-Derived TGF-Beta1 Cytokine . Immunity ( 2011 ) 35 ( 1 ): 123–34 . doi: 10.1016/j.immuni.2011.04.019 PMC3430371 21757379

64 Shalapour S Font-Burgada J Di Caro G Zhong Z Sanchez-Lopez E Dhar D . Immunosuppressive Plasma Cells Impede T-Cell-Dependent Immunogenic Chemotherapy . Nature ( 2015 ) 521 ( 7550 ): 94–8 . doi: 10.1038/nature14395 PMC4501632 25924065

65 Gonzalez-Junca A Driscoll KE Pellicciotta I Du S Lo CH Roy R . Autocrine TGFbeta Is a Survival Factor for Monocytes and Drives Immunosuppressive Lineage Commitment . Cancer Immunol Res ( 2019 ) 7 ( 2 ): 306–20 . doi: 10.1158/2326-6066.CIR-18-0310 PMC6828175 30538091

66 Tauriello DVF Palomo-Ponce S Stork D Berenguer-Llergo A Badia-Ramentol J Iglesias M . TGFbeta Drives Immune Evasion in Genetically Reconstituted Colon Cancer Metastasis . Nature ( 2018 ) 554 ( 7693 ): 538–43 . doi: 10.1038/nature25492 29443964

67 Stuber T Monjezi R Wallstabe L Kuhnemundt J Nietzer SL Dandekar G . Inhibition of TGF-Beta-Receptor Signaling Augments the Antitumor Function of ROR1-Specific CAR T-Cells Against Triple-Negative Breast Cancer . J Immunother Cancer ( 2020 ) 8 ( 1 ): e000676 . doi: 10.1136/jitc-2020-000676 32303620 PMC7204619

68 Klug F Prakash H Huber PE Seibel T Bender N Halama N . Low-Dose Irradiation Programs Macrophage Differentiation to an iNOS(+)/M1 Phenotype That Orchestrates Effective T Cell Immunotherapy . Cancer Cell ( 2013 ) 24 ( 5 ): 589 – 602 . doi: 10.1016/j.ccr.2013.09.014 24209604

69 Prakash H Klug F Nadella V Mazumdar V Schmitz-Winnenthal H Umansky L . Low Doses of Gamma Irradiation Potentially Modifies Immunosuppressive Tumor Microenvironment by Retuning Tumor-Associated Macrophages: Lesson From Insulinoma . Carcinogenesis ( 2016 ) 37 ( 3 ): 301–13 . doi: 10.1093/carcin/bgw007 26785731

70 Navarro-Martin A Galiana IL Berenguer Frances MA Cacicedo J Canas Cortes R Comas Anton S . Preliminary Study of the Effect of Stereotactic Body Radiotherapy (SBRT) on the Immune System in Lung Cancer Patients Unfit for Surgery: Immunophenotyping Analysis . Int J Mol Sci ( 2018 ) 19 ( 12 ): 3963 . doi: 10.3390/ijms19123963 PMC6320787 30544863

71 Golden EB Frances D Pellicciotta I Demaria S Helen Barcellos-Hoff M Formenti SC . Radiation Fosters Dose-Dependent and Chemotherapy-Induced Immunogenic Cell Death . Oncoimmunology ( 2014 ) 3 : e28518 . doi: 10.4161/onci.28518 25071979 PMC4106151

72 Apetoh L Ghiringhelli F Tesniere A Obeid M Ortiz C Criollo A . Toll-Like Receptor 4-Dependent Contribution of the Immune System to Anticancer Chemotherapy and Radiotherapy . Nat Med ( 2007 ) 13 ( 9 ): 1050–9 . doi: 10.1038/nm1622 17704786

73 Lhuillier C Rudqvist NP Yamazaki T Zhang T Charpentier M Galluzzi L . Radiotherapy-Exposed CD8+ and CD4+ Neoantigens Enhance Tumor Control . J Clin Invest ( 2021 ) 131 ( 5 ): e138740 . doi: 10.1172/JCI138740 PMC7919731 33476307

74 Wei S Egenti MU Teitz-Tennenbaum S Zou W Chang AE . Effects of Tumor Irradiation on Host T-Regulatory Cells and Systemic Immunity in the Context of Adoptive T-Cell Therapy in Mice . J Immunother ( 2013 ) 36 ( 2 ): 124–32 . doi: 10.1097/CJI.0b013e31828298e6 PMC3607501 23377667

75 Ngwa W Irabor OC Schoenfeld JD Hesser J Demaria S Formenti SC . Using Immunotherapy to Boost the Abscopal Effect . Nat Rev Cancer ( 2018 ) 18 ( 5 ): 313–22 . doi: 10.1038/nrc.2018.6 PMC5912991 29449659

76 Schnarr K Boreham D Sathya J Julian J Dayes IS . Radiation-Induced Lymphocyte Apoptosis to Predict Radiation Therapy Late Toxicity in Prostate Cancer Patients . Int J Radiat Oncol Biol Phys ( 2009 ) 74 ( 5 ): 1424–30 . doi: 10.1016/j.ijrobp.2008.10.039 19167839

77 Grayson JM Harrington LE Lanier JG Wherry EJ Ahmed R . Differential Sensitivity of Naive and Memory CD8+ T Cells to Apoptosis In Vivo . J Immunol ( 2002 ) 169 ( 7 ): 3760–70 . doi: 10.4049/jimmunol.169.7.3760 12244170

78 Arina A Beckett M Fernandez C Zheng W Pitroda S Chmura SJ . Tumor-Reprogrammed Resident T Cells Resist Radiation to Control Tumors . Nat Commun ( 2019 ) 10 ( 1 ): 3959 . doi: 10.1038/s41467-019-11906-2 31477729 PMC6718618

79 Sia J Hagekyriakou J Chindris I Albarakati H Leong T Schlenker R . Regulatory T Cells Shape the Differential Impact of Radiation Dose-Fractionation Schedules on Host Innate and Adaptive Antitumor Immune Defenses . Int J Radiat Oncol Biol Phys ( 2021 ) 111 ( 2 ): 502–14 . doi: 10.1016/j.ijrobp.2021.05.014 34023423

80 Oweida A Hararah MK Phan A Binder D Bhatia S Lennon S . Resistance to Radiotherapy and PD-L1 Blockade Is Mediated by TIM-3 Upregulation and Regulatory T-Cell Infiltration . Clin Cancer Res ( 2018 ) 24 ( 21 ): 5368–80 . doi: 10.1158/1078-0432.CCR-18-1038 PMC6886391 30042205

81 Oweida AJ Darragh L Phan A Binder D Bhatia S Mueller A . STAT3 Modulation of Regulatory T Cells in Response to Radiation Therapy in Head and Neck Cancer . J Natl Cancer Inst ( 2019 ) 111 ( 12 ): 1339–49 . doi: 10.1093/jnci/djz036 PMC6910208 30863843

82 Li JY Duan XF Wang LP Xu YJ Huang L Zhang TF . Selective Depletion of Regulatory T Cell Subsets by Docetaxel Treatment in Patients With Nonsmall Cell Lung Cancer . J Immunol Res ( 2014 ) 2014 : 286170 . doi: 10.1155/2014/286170 24868562 PMC4020463

83 Shevchenko I Karakhanova S Soltek S Link J Bayry J Werner J . Low-Dose Gemcitabine Depletes Regulatory T Cells and Improves Survival in the Orthotopic Panc02 Model of Pancreatic Cancer . Int J Cancer ( 2013 ) 133 ( 1 ): 98 – 107 . doi: 10.1002/ijc.27990 23233419

84 Sobhani N Tardiel-Cyril DR Davtyan A Generali D Roudi R Li Y . CTLA-4 in Regulatory T Cells for Cancer Immunotherapy . Cancers (Basel) ( 2021 ) 13 ( 6 ): 1440 . doi: 10.3390/cancers13061440 33809974 PMC8005092

85 Roh M Wainwright DA Wu JD Wan Y Zhang B . Targeting CD73 to Augment Cancer Immunotherapy . Curr Opin Pharmacol ( 2020 ) 53 : 66 – 76 . doi: 10.1016/j.coph.2020.07.001 32777746 PMC7669683

86 Rotte A Sahasranaman S Budha N . Targeting TIGIT for Immunotherapy of Cancer: Update on Clinical Development . Biomedicines ( 2021 ) 9 ( 9 ): 1277 . doi: 10.3390/biomedicines9091277 34572463 PMC8472042

87 Sato H Niimi A Yasuhara T Permata TBM Hagiwara Y Isono M . DNA Double-Strand Break Repair Pathway Regulates PD-L1 Expression in Cancer Cells . Nat Commun ( 2017 ) 8 ( 1 ): 1751 . doi: 10.1038/s41467-017-01883-9 29170499 PMC5701012

88 Dovedi SJ Adlard AL Lipowska-Bhalla G McKenna C Jones S Cheadle EJ . Acquired Resistance to Fractionated Radiotherapy Can be Overcome by Concurrent PD-L1 Blockade . Cancer Res ( 2014 ) 74 ( 19 ): 5458–68 . doi: 10.1158/0008-5472.CAN-14-1258 25274032

89 Liu M Wang X Li W Yu X Flores-Villanueva P Xu-Monette ZY . Targeting PD-L1 in Non-Small Cell Lung Cancer Using CAR T Cells . Oncogenesis ( 2020 ) 9 ( 8 ): 72 . doi: 10.1038/s41389-020-00257-z 32792499 PMC7426958

90 Wirsdorfer F de Leve S Cappuccini F Eldh T Meyer AV Gau E . Extracellular Adenosine Production by Ecto-5'-Nucleotidase (CD73) Enhances Radiation-Induced Lung Fibrosis . Cancer Res ( 2016 ) 76 ( 10 ): 3045–56 . doi: 10.1158/0008-5472.CAN-15-2310 PMC4960984 26921334

91 Jin H Lee JS Kim DC Ko YS Lee GW Kim HJ . Increased Extracellular Adenosine in Radiotherapy-Resistant Breast Cancer Cells Enhances Tumor Progression Through A2AR-Akt-Beta-Catenin Signaling . Cancers (Basel) ( 2021 ) 13 ( 9 ): 2105 . doi: 10.3390/cancers13092105 33925516 PMC8123845

92 Sorrentino C Hossain F Rodriguez PC Sierra RA Pannuti A Osborne BA . Adenosine A2A Receptor Stimulation Inhibits TCR-Induced Notch1 Activation in CD8+T-Cells . Front Immunol ( 2019 ) 10 : 162 . doi: 10.3389/fimmu.2019.00162 30792717 PMC6374329

93 Young A Ngiow SF Gao Y Patch AM Barkauskas DS Messaoudene M . A2AR Adenosine Signaling Suppresses Natural Killer Cell Maturation in the Tumor Microenvironment . Cancer Res ( 2018 ) 78 ( 4 ): 1003–16 . doi: 10.1158/0008-5472.CAN-17-2826 29229601

94 Wennerberg E Spada S Rudqvist NP Lhuillier C Gruber S Chen Q . CD73 Blockade Promotes Dendritic Cell Infiltration of Irradiated Tumors and Tumor Rejection . Cancer Immunol Res ( 2020 ) 8 ( 4 ): 465–78 . doi: 10.1158/2326-6066.CIR-19-0449 PMC7125001 32047024

95 Tsukui H Horie H Koinuma K Ohzawa H Sakuma Y Hosoya Y . CD73 Blockade Enhances the Local and Abscopal Effects of Radiotherapy in a Murine Rectal Cancer Model . BMC Cancer ( 2020 ) 20 ( 1 ): 411 . doi: 10.1186/s12885-020-06893-3 32397971 PMC7218548

96 Siriwon N Kim YJ Siegler E Chen X Rohrs JA Liu Y . CAR-T Cells Surface-Engineered With Drug-Encapsulated Nanoparticles Can Ameliorate Intratumoral T-Cell Hypofunction . Cancer Immunol Res ( 2018 ) 6 ( 7 ): 812–24 . doi: 10.1158/2326-6066.CIR-17-0502 29720380

97 Giuffrida L Sek K Henderson MA Lai J Chen AXY Meyran D . CRISPR/Cas9 Mediated Deletion of the Adenosine A2A Receptor Enhances CAR T Cell Efficacy . Nat Commun ( 2021 ) 12 ( 1 ): 3236 . doi: 10.1038/s41467-021-23331-5 34050151 PMC8163771

98 Mayer A Haist M Loquai C Grabbe S Rapp M Roth W . Role of Hypoxia and the Adenosine System in Immune Evasion and Prognosis of Patients With Brain Metastases of Melanoma: A Multiplex Whole Slide Immunofluorescence Study . Cancers (Basel) ( 2020 ) 12 ( 12 ): 3753 . doi: 10.3390/cancers12123753 PMC7763902 33322215

99 Derynck R Turley SJ Akhurst RJ . TGFbeta Biology in Cancer Progression and Immunotherapy . Nat Rev Clin Oncol ( 2021 ) 18 ( 1 ): 9 – 34 . doi: 10.1038/s41571-020-0403-1 32710082 PMC9721352

100 Vanpouille-Box C Diamond JM Pilones KA Zavadil J Babb JS Formenti SC . TGFbeta Is a Master Regulator of Radiation Therapy-Induced Antitumor Immunity . Cancer Res ( 2015 ) 75 ( 11 ): 2232–42 . doi: 10.1158/0008-5472.CAN-14-3511 PMC4522159 25858148

101 Kloss CC Lee J Zhang A Chen F Melenhorst JJ Lacey SF . Dominant-Negative TGF-Beta Receptor Enhances PSMA-Targeted Human CAR T Cell Proliferation And Augments Prostate Cancer Eradication . Mol Ther ( 2018 ) 26 ( 7 ): 1855–66 . doi: 10.1016/j.ymthe.2018.05.003 PMC6037129 29807781

102 Tang N Cheng C Zhang X Qiao M Li N Mu W . TGF-Beta Inhibition via CRISPR Promotes the Long-Term Efficacy of CAR T Cells Against Solid Tumors . JCI Insight ( 2020 ) 5 ( 4 ): e133977 . doi: 10.1172/jci.insight.133977 PMC7101140 31999649

103 Narayan V Barber-Rotenberg J Fraietta J Hwang W-T Lacey SF Plesa G . A Phase I Clinical Trial of PSMA-Directed/Tgfβ-Insensitive CAR-T Cells in Metastatic Castration-Resistant Prostate Cancer . J Clin Oncol ( 2021 ) 39 ( 6_suppl ): 125– . doi: 10.1200/JCO.2021.39.6_suppl.125