Impact of mismatch-repair deficiency on the colorectal cancer immune microenvironment

Colorectal cancer patients respond inconsistently to immunotherapies, likely due to the immune microenvironments around their tumors. We analyzed the relationship between deficient mismatch repair (dMMR) and the colorectal cancer immune microenvironment to identify predictors of effective immunotherapy. Colorectal cancer patients (n=113) who had undergone surgical resection were divided into dMMR and proficient mismatch repair (pMMR) groups. The levels of immune checkpoint proteins, including programmed cell death 1 (PD-1), programmed cell death 1 ligand 1 (PD-L1), indoleamine 2,3 dioxygenase and CD8 were assessed immunohistochemically. The percentage of tumor-infiltrating lymphocytes strongly positive for PD-1 (score=3) was higher in the dMMR than pMMR group (79.3% vs. 41.7%; p=0.003). The groups showed similar tumor cell PD-L1 positivity rates (34.5% vs. 35.7%, p=0.905) and PD-L1 intensity levels on immune cell infiltrates (86.2% vs. 84.5%, p=0.964). However, when a cut-off value of 80% was used for PD-L1 positivity, the rate of PD-L1 positivity on immune cell infiltrates differed between the groups (51.7% vs. 22.6%, p=0.003). The rate of high indoleamine 2,3 dioxygenase expression was greater in the dMMR than pMMR group (55.2% vs. 36.9%, p=0.026). CD8+ T cells were elevated in the dMMR group in both compartments (p=0.017 for tumor-infiltrating lymphocytes and stroma; p=0.038 for invasive front). Thus the immune microenvironment of dMMR colorectal cancer differs from that of pMMR colorectal cancer.