Crosstalk between the MSI status and tumor microenvironment in colorectal cancer.

e15236 Background: In recent years, cancer immunotherapy has been extensively studied, and colorectal cancer (CRC) patients have also derived clinical benefits from immunotherapy, especially CRC patients with mismatch repair deficiency (dMMR)/microsatellite instability-high (MSI-H), whose sensitivity to immune checkpoint inhibitors (ICIs) is significantly higher than that of patients with microsatellite-stable (MSS)/microsatellite instability-low (MSI-L) disease. This study suggests that patients with MSI-H CRC have a higher mutational burden and more immune cell infiltration than those with MSS/MSI-L disease. However, most studies have not systematically evaluated the immune characteristics and immune microenvironments of MSI-H and MSS/MSI-L CRC. Methods: A published CRC cohort with mutation and immunotherapy-related prognostic data was collected. We analyzed the relationship between the MSI status and prognosis of ICI treatment in an immunotherapy cohort. We then further used mutation data for the immunotherapy and The Cancer Genome Atlas (TCGA)-CRC (colon adenocarcinoma (COAD) + rectum adenocarcinoma (READ) cohorts. For mRNA expression, mutation data analysis of the immune microenvironment and immunogenicity under different MSI status was performed. Results: Compared with MSS/MSI-L CRC patients, patients with MSI-H CRC significantly benefited from ICI treatment. We found that MSI-H CRC had more immune cell infiltration, higher expression of immune-related genes and higher immunogenicity than MSS/MSI-L disease. The MANTIS score used to predict the MSI status was positively correlated with immune cells, immune-related genes, and immunogenicity. In addition, subtype analysis showed that COAD and READ might have different tumor immune microenvironments. Conclusions: MSI-H CRC may have an inflammatory tumor microenvironment and increased sensitivity to ICIs. Unlike those of MSI-H READ, the immune characteristics of MSI-H COAD may be consistent with those of MSI-H CRC. Furthermore, the possible mechanism underlying the prognostic differences among CRC patients receiving ICIs in relation to the immune microenvironment were elucidated to provide theoretical guidance for further improving the curative effect of ICIs treatment on MSI-H CRC patients in the future and solve the problems underlying why MSS/MSI-L CRC patients do not benefit from ICIs treatment.