Defining the determinants of immune response in DNA homologous recombination deficient tumors 3797

Abstract Description Studies by our lab have demonstrated that DNA Damage Repair (DDR) pathways play a key role in determining response to ICB. To better characterize the effect of these alternations on the tumor immune microenvironment (TME) and define key determinants of ICB response, we created isogenic knockouts of BRCA2 and BRCA1 in a murine 4T1 metastatic TNBC background to assess how these tumor-intrinsic programs influence the TME and poise tumors for immunotherapy response. Differential immune landscapes identified via scRNAseq in Brca2-mutant tumors at baseline showed key differences in the myeloid compartment and interferon-stimulated gene (ISG) expression in tumor-infiltrating myeloid cells from Brca2 mutant tumors. Bulk RNAseq analysis showed increased production of T cell trafficking chemokines that could poise the tumor for response to ICB. Interferon reporter assay results suggested tumor-intrinsic cGAS drives trans-activation of myeloid STING and IFNb1. Further assessment of tumor cell lines via cellular fractionation experiments identified the presence of both DNA and R-loops in the cytoplasm of BRCA2-mutant cell lines that serve to active cGAS/STING. In vivo experiments with tumor intrinsic knockouts showed that tumor STING signaling and cytokine production were dispensable for ICB response while depletion of monocytes via CSF1R blockade showed a complete reversal of ICB response, underlining the essential role of monocytes in poising the tumor for ICB response. Funding Sources Supported by NIH Director’s Early Independence Award, the Parker Institute for Cancer Immunotherapy, Burrough’s Welcome Fund, and Mount Sinai’s Cancer Biology T32 Training Grant. Topic Categories Tumor Immunology: Cellular Responses and Tumor Microevironment (TIME)