Abstract LB082: Disrupting mismatch repair makes mismatch repair-proficient tumors immunogenic and induces CD8 T cell-dependent, but T cell receptor-independent, immune control of heterologous tumors in mice

Abstract Mismatch repair-deficient (MMRd) tumors respond well to immune checkpoint blockade (ICB) while MMR-proficient (MMRp) tumors from the same cell type do not. We hypothesized that disrupting MMR would make immunologically cold MMRp tumors more immunogenic and responsive to ICB. Initial attempts to knockout individual MMR genes had a significant but weak impact on growth of implanted tumors because the development of microsatellite instability (MSI) and chromosomal instability (CIN) was slow relative to tumor growth and progression. However, simultaneous knockout of two MMR genes, Mlh1 and Msh2, in multiple mouse cancer cell lines and organoids (colorectal (CRC), breast (TNBC), and melanoma) led to rapid CD8⁺ T cell-mediated tumor growth inhibition or even rejection of subcutaneous and orthotopic tumor implants. Tumors that were not rejected could be ablated by treatment with anti-PD1. Mlh1-/-Msh2-/- tumors proliferated like wild-type tumors in vitro. In vivo tumor control was immune-mediated and depended on CD8 T cells since it was abrogated in immunodeficient NSG mice or by depletion of CD8 T cells, but not CD4 T cells or NK cells. Delayed or concurrent challenge of mice that reject Mlh1-/-Msh2-/- tumors with MMRp tumors also led to their rejection, indicating that exposure to MMRd tumors induces immune reactivity and memory to the MMRp tumor. Surprisingly, mice that rejected Mlh1-/-Msh2-/- CRC or TNBC tumors were also protected against heterologous CRC or TNBC cancers and even against B16F10 implants, suggesting that exposure to MMRd tumors led to CD8 T cell immunity that was independent of tumor antigens. In fact, knockout of B2m in Mlh1-/-Msh2-/- CRC tumors, which eliminated MHC class I expression and antigen presentation to CD8 T cells, did not affect tumor rejection, suggesting that immune protection was not T cell receptor-mediated. scRNA-seq, qRT-PCR and flow cytometry revealed profound remodeling of the tumor immune microenvironment in Mlh1-/-Msh2-/- tumors, characterized by up-regulation of interferon response genes by the tumor, an expansion of cytotoxic CD8⁺ T cells that express NK activating receptors, reduction in infiltrating neutrophils and a shift in myeloid populations toward less immunosuppressive, but more phagocytic, myeloid cells. Antitumor immunity generated by exposure to Mlh1-/-Msh2-/- tumors was driven by NK activating receptor recognition by CD8⁺ T cells of NK ligands on tumor cells, since protection was abrogated by treating tumor-bearing mice with an antibody to an NK activating receptor. Although no small molecule inhibitors of MMR have been identified, in vivo tumor-targeted knockdown of Mlh1 and Msh2 using EpCAM-targeted aptamer-siRNAs suppressed tumor growth. Thus, disrupting MMR in MMRp cancer cells induces a novel, potent CD8 T cell protective immune response that is mediated by an NK activating receptor that recognizes the genotoxic stress of unrepaired DNA damage in the tumor, rather than by T cell receptor recognition of tumor antigens, that strongly protects mice from MMRp and even unrelated tumors. These results in mice suggest that therapeutic strategies that make MMRp tumors MMRd could be developed to treat immunologically cold tumors or sensitize them to ICB. Citation Format: Haiwei Zhang, Ayijiang Yisimayi, Bowen Gu, Yuting Wang, Wayne M. Yokoyama, Sytse J. Piersma, Eduard Batlle, Daniele V. Tauriello, Judy Lieberman. Disrupting mismatch repair makes mismatch repair-proficient tumors immunogenic and induces CD8 T cell-dependent, but T cell receptor-independent, immune control of heterologous tumors in mice [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2026; Part 2 (Late-Breaking, Clinical Trial, and Invited Abstracts); 2026 Apr 17-22; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2026;86(8_Suppl):Abstract nr LB082.