Introduction: Microsatellite instability (MSI), caused by defects in DNA mismatch repair genes, including MLH1 and MSH2, occur in 30% of endometrial cancers (EC). High mutational loads associated with MSI have been associated with greater immunogenicity, and an increase in immune inhibitory factors such as PD-1 and PD-L1. While immunotherapy with anti-PD-L1 has shown promising results in MSI ECs, up to 40% did not respond, suggesting there are alternate immunosuppressive mechanisms. To improve understanding of response/resistance to immunotherapy, we evaluated immune activation and evasion using a novel immunocompetent mouse model of MSI EC. Methods: Endometrial lesions were evaluated in mice with MSH2 loss targeted to the endometrium (PRCre+MSH2flox/flox). When aged to 12-16 months, 22% develop hyperplasia and 18% develop spontaneous EC. MSI status was confirmed by PCR. Immune infiltration and function were assessed by immunohistochemistry (IHC) and Nanostring digital spatial profiling. Tumors were evaluated for tumor infiltrating lymphocytes (TIL) based on CD8a expression and found to separate into TILHIGH and TILLOW groups and further characterized to determine immune activation/resistance mechanisms using transcriptome analysis with Affymetrix Clariom D assay and validation by RT-PCR. A gene signature score approach was used to quantify immune-related expression changes for tumors from PRCre+MSH2flox/flox mice and MSI EC patient data from TCGA. Cell lines were generated from tumors and used to evaluate a syngeneic orthotopic mouse model for immunotherapy studies, where immune infiltration was characterized. Results: All endometrial tumors were MSI and included endometrioid, serous, and mixed histologies. Tumors showed varying degrees of CD8+ T cell infiltration, irrespective of histology. TILHIGH tumors had increased infiltration of dendritic cells and Type I interferon response, including genes IFI203 (fold-change, FC=18.5), IFI204 (FC=11.1) and CXCL9 (FC=5.7). There was no significant difference in type II interferon response. TILLOW tumors had low expression of IFN-associated genes. TILHIGH tumors also exhibited increased immune exhaustion markers such as TIM3 (FC=2.4) and LAG3 (FC=3.2). Tumoral PD-L1 expression was negative (HIGH MSI uterine cancer patients from TCGA also showed activation of type I interferon, increased dendritic cell infiltration, and increased T cell exhaustion markers. Primary cell lines used in an orthotopic mouse model retained the immune profile of the original tumor. Conclusions: Elucidating immune activation and evasion mechanisms is critical for improving immunotherapies. The PRCre+MSH2flox/flox mouse model reflects the spectrum of immunogenicity observed in patients. A panel of primary cell lines generated from this model are an important tool that enable studies of immunotherapy efficacy and resistance mechanisms, in both immunogenic and non-immunogenic tumors. Citation Format: Brenda Melendez, Emily Hinchcliff, Nisha Gokul, Xiaoping Su, Daniel McGrail, Elizabeth Whitley, Russell Broaddus, Rosemarie Schmandt, Karen Lu, Melinda Yates. Identifying mechanisms of immune evasion in microsatellite instable endometrial cancer mouse models [abstract]. In: Proceedings of the AACR Virtual Special Conference: Endometrial Cancer: New Biology Driving Research and Treatment; 2020 Nov 9-10. Philadelphia (PA): AACR; Clin Cancer Res 2021;27(3_Suppl):Abstract nr PR008.