Abstract 1593: Genetic and pharmacological modulation of DNA mismatch repair promotes immune surveillance in murine colorectal cancer

Abstract Patients affected by colorectal cancer (CRC) with microsatellite instability (MSI), which is caused by DNA mismatch repair deficiency (MMRd), are eligible for therapies based on immune checkpoint inhibitors (CPI), while microsatellite stable (MSS) tumors are not. However, a subset of MSS CRCs contains variable fractions of MMRd cells. How the presence of MMRd cells in tumors classified as MSS impacts cancer immune surveillance is largely unknown. It is also unclear whether the pharmacological modulation of MMRd cells percentage in MMR heterogeneous tumor can occur and if this might result in the improvement of tumor immune control.To shed light on these aspects we studied isogenic mismatch repair proficient and deficient mouse tumor cells, generated by genetic inactivation of MLH1, a key component of the MMR machinery. We mixed MLH1+/+and MLH1-/-cells at different ratios, injecting the resulting heterogeneous populations in mice. In the presence of a competent immune system, the tumorigenic potential and immune surveillance of MSS/MSI (MLH1+/+/MLH1-/-) heterogeneous tumors was dependent upon the MMRd fraction. Tumor rejection was observed when at least 50% of the cells were MMRd, but tumor growth delay was also evident when as low as 20% of MMRd cells were present in the mixed population and this was paralleled by immune infiltration of the tumor. Molecular profiles of samples from MSS/MSI heterogeneous tumors that evaded immune control, showed enrichment of the MSS fraction. Treatment of MSS/MSI mixed populations with the antimetabolite 6-Thioguanine (6TG) greatly enriched the MMRd fraction and improved immune response. Overall, these results suggest that genetic and/or pharmacological modulation of the DNA mismatch repair machinery can foster immune surveillance of MMR heterogeneous tumors and modulate the cancer immune environment. Citation Format: Vito Amodio, Giovanni Germano, Benedetta Mussolin, Simona Lamba, Rosaria Chilà, Giuseppe Rospo, Caterina Marchiò, Silvia Marsoni, Gianluca Mauri, Federica Di Nicolantonio, Alberto Bardelli. Genetic and pharmacological modulation of DNA mismatch repair promotes immune surveillance in murine colorectal cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 1593.