Abstract B049: Modulating macrophages to foster antitumor immunity in breast cancer

Abstract Background: Tumor-associated macrophages (TAMs) are critical mediators of immune evasion in cancer and their abundance in the breast tumor microenvironment often correlates with poor prognosis. We sought to define macrophage intrinsic pathways that regulate antitumor immunity. Specifically, we hypothesized that the transcription factor GATA3 promotes the formation of the alternative pro-tumor phenotype and that myeloid-specific GATA3 ablation would enhance RT-induced antitumor immunity by preventing acquisition of this pro-tumor bioactivity. Methods: Triple-negative breast tumors were established by orthotopic implantation of EO771 cells into wild-type (WT) and myeloid-specific GATA3 knockout (mG3KO) C57BL/6 mice. Localized RT (16 Gy) was delivered to tumors, and tumor growth was assessed every two days by caliper measurements. Tumor-infiltrating leukocytes were then isolated for comprehensive profiling of the tumor microenvironment (TME) via multiparametric flow cytometry and single-cell RNA sequencing (scRNA-seq). Immune cell depletion studies were also performed via intraperitoneal injection of neutralizing antibodies against CD4, CD8, and Ly6G to selectively eliminate helper T cells, cytotoxic T cells, and neutrophils, respectively. Results: mG3KO mice demonstrated significantly delayed tumor growth and dramatically enhanced survival following RT (p