Abstract 5027: Sequencing the tumor microenvironment and myeloid derived suppressor cells to understand response to immunotherapy in primary HER2 positive breast cancer

The purpose of this study is to identify how the epigenetic modulator entinostat reprograms myeloid derived suppressor cells (MDSCs) to prime the tumor microenvironment (TME) and modulate immunotherapy response. Early trials investigating single agent ICIs, anti-CTLA-4 and anti-PD-1/PD-L1, resulted in overall response rates of 5-20% in patients with metastatic HER2 positive breast cancer but little is understood about use of these therapies in patients with early stage/ primary disease. Suboptimal and inconsistent immune responsiveness is likely the result of multiple suppressive signals within the TME that provide a formidable barrier to T cell infiltration leading to an opportunity for TME modulation to potentially improve response. We and others have shown that immunosuppressive factors can be overcome by adding the class 1 histone deacetylase inhibitor, entinostat, to ICIs to reprogram MDSCs and upregulate T cell attracting signals within the TME. We hypothesize that gene expression signatures of reprogrammed MDSCs are predictive of response with subsequent immunotherapy treatment. To test this hypothesis, we used the neu-N syngeneic mouse model of HER2 positive breast cancer. We investigated differences in survival and tumor burden in response to combinations of entinostat with anti-PD-1 and anti-CTLA-4. Having previously identified MDSCs as important targets of entinostat that modulate response to ICIs, we used methods we developed to study MDSC function. Given the broad effects of entinostat on global gene expression including that of MDSCs, we used single cell RNA-seq (scRNA-seq) on primary tumors and isolated intratumoral MDSCs to determine what is driving changes in MDSC function in primary tumors. We plan to use data obtained from scRNA-seq to develop signatures of response and identify changes in responders vs. non-responders to help develop biomarkers of response in primary disease. Our results thus far show that treatment with entinostat + ICIs improves survival, and decreases tumor burden in primary tumors. Ex-vivo studies of MDSCs demonstrate changes in immunosuppressive capabilities of MDSCs and show differences in infiltration and function of granulocytic vs. monocytic MDSCs. We expect that evaluation of the ongoing sequencing data will identify more specific immune targets within primary TME and help us understand the importance of targeting MDSCs in early disease. We predict this work will identify additional candidate targets for therapeutic testing to convert breast cancers into immune responsive tumors, and that this will facilitate improved response to ICIs that will ultimately translate into improved survival of patients with breast cancer. Citation Format: Evanthia T. Roussos Torres, Christine Rafie, Emily Davis, Luciane T. Kagohara, Elana J. Fertig, Elizabeth M. Jaffee. Sequencing the tumor microenvironment and myeloid derived suppressor cells to understand response to immunotherapy in primary HER2 positive breast cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 5027.