Abstract 6420: Harnessing immune responses in prostate MMRd tumors treated with immune checkpoint blockade

Abstract There is increasing evidence correlating tumor mutational burden with response to immunotherapy in many different types of cancer. Some tumors show high microsatellite instability (MSI-H) as a consequence of the loss of function of mismatch repair (MMR) genes, which would resolve DNA damage in normal conditions. MSH2 plays a critical role in MMR, and several studies have identified mutations in MSH2 as key drivers of MSI in tumors. In preclinical mouse models, MMR deficiency and high microsatellite instability have been previously shown to translate into higher sensitivity to anti-PD-1 treatment. Despite the advances led by the recent tumor-agnostic approval of checkpoint inhibitors for highly mutated tumors, response rates among MMR-deficient (MMRd) tumors in humans are variable, and the mechanisms behind such heterogeneity are still poorly understood. We successfully knocked out the MSH2 gene in the murine prostate adenocarcinoma TRAMP-C2 cell line by CRISPR gene editing. By sequential passaging, we generated a MSH2-KO subclonal population that presents MSI and MMR deficiency, validated by increased tumor mutational burden (TMB) observed by exome sequencing. In studies where we challenged immunodeficient and immunocompetent mice with MMR-proficient (MMRp) and MMRd tumors, the latter showed increase immunogenicity, which led us to interrogate and better characterize such immune responses in the context of immune checkpoint blockade by anti-PD-1.In our studies, MMRd tumor-infiltrating T cells presented significantly enhanced effector and cytotoxic profiles, which likely led to improved tumor control with or without anti-PD-1. We observed an intriguingly lower frequency of SPAS-1 (a well-known TRAMP-C2 immunodominant epitope) antigen-specific T cells in the tumor-reactive fraction of MMRd infiltrates compared to MMRp, which prompted us to interrogate tumor-infiltrate gene expression, TCR repertoire and the potential expansion of multiple antigen-reactive subsets against MMRd tumors in successful immune responses by single cell RNA sequencing and functional ex vivo studies. We also interrogated TCR repertoire data from patients harboring MMRd tumors to add translatability to our findings in mice.Our results constitute a relevant addition to the understanding of immune responses in MMRd tumors and the potential features that could be exploited to improve response rates in immunotherapeutic strategies targeting such malignancies. Citation Format: Marcel Arias Badia, Zenghua Fan, Yee May Lwin, PeiXi Chen, Aahir Srinath, Lawrence Fong. Harnessing immune responses in prostate MMRd tumors treated with immune checkpoint blockade [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 6420.