Epigenetic Priming Sensitizes Metastatic Breast Cancer to Checkpoint Blockade by Rewiring the Tumor-Immune Microenvironment.

Immune checkpoint blockade (ICB) has shown limited efficacy in metastatic breast cancer. Resistance mechanisms involve suppressive myeloid and stromal populations in the tumor microenvironment (TME). In this issue of Cancer Research, Gonzalez and colleagues integrate single-cell transcriptomics, spatial proteomics, functional immunology, and mathematical modeling to dissect how combined treatment with the histone deacetylase (HDAC) inhibitor entinostat and anti-PD-1/anti-CTLA-4 blockade (EPC) sensitizes these refractory tumors. Work using metastatic mouse models shows that the antitumor response is multifactorial, comprising downregulation of myeloid-mediated CD8+ T-cell suppression (via ICAM-1 and IFNγ signaling) along with increased B-cell and plasma cell activation, antibody production, and tertiary lymphoid structure (TLS) maturation. Some of these findings are further validated in biospecimens from patients on a phase Ib trial treated with the EPC combination treatment. Mathematical modeling formalizes this insight, demonstrating that simultaneous modulation of T-cell activation and myeloid suppression, rather than either alone, is required to drive tumor eradication in silico. This work highlights the increasingly accepted idea that the answer to how current treatments modify and shape tumors is multifactorial. Thus, diversified strategies that include comprehensive analysis of the TME, mathematical modeling tools, and validation in human specimens are required to optimize treatment efficacy. See related article by Gonzalez et al., p. XX .