Abstract 7408: <i>Brca2-</i> deficiency promotes increased cGAS-STING pathway activation and an immunosuppressive tumor immune microenvironment in prostate cancer

Abstract Background: Combined germline and somatic tumor profiling of advanced metastatic prostate cancer has revealed that ∼20% of metastatic tumors have mutations in DNA repair genes such as BRCA2. Germline variants in BRCA2 predispose to aggressive high-risk prostate cancer and more advanced disease at diagnosis. BRCA2 is vital to the repair of DNA double-strand breaks (DSB) by homologous recombination (HR) and deficiency leads to genomic instability which has been shown to drive inflammation via the cGAS-STING pathway. The implications of loss of BRCA2 function in prostate cancer on immune activation and the tumor microenvironment (TME) are unknown. This represents a significant knowledge gap towards devising therapies that effectively target the immunosuppressive TME of HR-deficient prostate cancer. Methods: We have engineered an immunocompetent Brca2-deficient prostate cancer model with loss of function frame shift (fs) mutations (Brca2fs) using CRISPR/Cas9 to address the critical gap in current preclinical models. We validated BRCA2 loss of function and characterized innate immune signaling through the cGAS-STING pathway. We performed spectral flow cytometry and single-cell RNA sequencing (scRNA-seq) on Brca2WT and Brca2fs tumors from immunocompetent mice to assess the TME. Mice bearing Brca2fs tumors were treated with anti-CCR8 monoclonal antibodies with and without anti-PD-1 to test the effectiveness of targeting the CCR8-axis in Brca2-deficient prostate tumors. Results: Brca2fs cells show hallmarks of increased DNA damage and cGAS-STING pathway activation compared to Brca2WT cells. Analysis of implanted Brca2fs tumors demonstrated enrichment of CD4+ T cells and immunosuppressive CCR8+ regulatory T cells (Treg) compared to Brca2WT tumors. scRNA-seq of Brca2fs tumors implanted in immunocompetent mice showed a significant increase in the interferon expression signature that is likely the result of increased cGAS-STING pathway activation in tumor cells. Additionally, tumor associated macrophages (TAMs) from Brca2fs tumors showed a greater immunosuppressive phenotype with an increase in M2 polarization (pro-tumor) and high expression of Ccl8, the CCR8 receptor ligand. Combination anti-CCR8 and anti-PD-1 therapy significantly reduced tumor growth in mice bearing Brca2fs tumors. Conclusions: Our analysis has revealed enrichment of immunosuppressive CCR8+ Tregs and Ccl8-high expressing TAMs in Brca2-deficient but not Brca2-proficient prostate cancer tumors. Anti-CCR8 therapy, to deplete CCR8+ Tregs, in combination with anti-PD-1 therapy significantly reduced Brca2fs tumor growth in our immunocompetent mouse model. These findings present the exciting possibility of targeting the CCR8-axis in BRCA2-deficient prostate tumors to overcome these immunosuppressive mechanisms and enhance activity of tumor-reactive T cells. Citation Format: Kimberly A. Rickman, Irena Q. Sun, Ramya Parameswaran, Xuxu Gou, Morgan E. Diolaiti, Harish N. Vasudevan, Alan Ashworth. Brca2-deficiency promotes increased cGAS-STING pathway activation and an immunosuppressive tumor immune microenvironment in prostate cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2026; Part 1 (Regular Abstracts); 2026 Apr 17-22; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2026;86(7 Suppl):Abstract nr 7408.