The microsatellite instable subset of colorectal cancer is a particularly good candidate for checkpoint blockade immunotherapy.

The microsatellite instable (MSI) subset of colorectal cancer exhibits an active Th1/CTL immune microenvironment, probably due to recognition of a high number of tumor neoantigens. However, the high expression of checkpoint molecules PD-1, PD-L1, CTLA-4, LAG-3, and IDO in MSI colorectal cancer distinguishes MSI from microsatellite stable colorectal cancer and creates an immunosuppressive microenvironment that may help MSI tumors evade immune destruction by the infiltrating immune cells. Though colorectal cancer does not have a good response rate to PD-1 pathway immunotherapy, these results suggest that the MSI subset of colorectal cancer is a particularly good candidate for checkpoint immunotherapy. ©2015 American Association for Cancer Research.