[Abnormalities of Major Histocompatibility Complex in Microsatellite Instability-High Colorectal Cancer]

The assessment of the ability of tumor cells to present antigens with major histocompatibility complex(MHC)class Ⅰ is essential for understanding the immune microenvironment of tumors and for the appropriate application of immunotherapy such as immune checkpoint inhibitors(ICIs). ICIs are very effective for microsatellite instability-high colorectal cancer(MSI-H CRC), where tumor-infiltrating lymphocytes are abundant. At the same time, decreased expression of MHC class Ⅰ is frequently observed in MSI-H CRC. It is necessary to understand the genomic and epigenomic abnormalities that lead to decreased expression of MHC class Ⅰ. In order to optimize ICI treatment, biomarker discovery is required to select patients who need combination therapy, to determine the appropriate timing of treatment discontinuation, and to identify patients who are at high risk for serious complications. Recent reports on MHC class Ⅰ abnormalities and exploration of biomarkers with a focus on MSI-H CRC will be reviewed.