Checkpoint inhibition has revolutionized treatment in cancers that are naturally immunogenic by enabling infiltration of T cells into the tumor microenvironment (TME) and promoting cytotoxic signaling pathways. While this strategy has shown some efficacy in metastatic breast cancer, most breast cancers lack immunogenic potential due to a strictly sustained immunosuppressive microenvironment and a lack of tumor antigen expression and recognition. One strategy to transform the breast TME is to use epigenetic modulation to affect activation and trafficking of myeloid derived suppressor cells (MDSCs), known to alter the immunogenicity of the TME and sensitize tumors to checkpoint modulation. We show that combinations of an epigenetic agent, the histone deacetylase inhibitor entinostat (ENT), checkpoint inhibitors anti -programmed cell death protein (a-PD-1) and anti-cytotoxic T-lymphocyte-associated protein 4 (a-CTLA-4) antibodies, with and without anti-HER2 antibodies significantly improves survival and anti-tumor effects in HER-2/neu transgenic mouse models. Using fluorescence-activated cell sorting, nanostring gene expression profiling, and other functional assays, we have characterized the various types of tumor infiltrating lymphocytes and myeloid cells and their functional capabilities within primary tumors. We show that addition of ENT to checkpoint inhibition leads to significantly decreased suppressive ability of granulocytic-MDSCs into the TME. We also demonstrate an increase in CD8+ T effector cells in mice treated with combination therapy. Flow cytometric evaluation of markers of T cell activation, exhaustion, and MDSC function demonstrate significantly increased T cell activation, exhaustion, and decreased myeloid function. Nanostring gene expression profiling of both MDSCs and lymphocytes infiltrating tumors determined significant changes in immune related pathways that likely lead to our observed outcomes. In summary, addition of ENT to checkpoint inhibition significantly increases infiltration of innate and adaptive immune cells into the highly tolerant neu-N breast tumors and leads to improved survival and decreased tumor burden. Functional assays demonstrate that this drug combination promotes a less suppressive TME and allows a more robust anti-tumor effect of infiltrating cells. It is our hope that these novel findings will provide further rationale for combination therapy and improve the response rate of these immune therapies in patients with breast cancer.

Citation Format: Evanthia T. Roussos Torres, Christine Rafie, Hayley Ma, Brian Christmas, Todd Armstrong, Elizabeth M. Jaffee. Entinostat transforms the suppressive tumor microenvironment of breast cancer and promotes survival and anti-responses when combined with checkpoint inhibition [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 4965.