Mismatch repair–deficient colorectal cancer: a model of immunogenic and immune cell–rich tumor despite nonsignificant PD-L1 expression in tumor cells

Mismatch repair–deficient colorectal cancers (CRC) are good responders to anti–programmed cell death ligand-1 (PD-L1) immunotherapy, but the value of PD-L1 testing remains unclear. We studied PD-L1 expression and the tumor immune microenvironment in mismatch repair–deficient (dMMR) CRC as a model of good responders to immunotherapy. We examined 35 dMMR and 34 mismatch repair–proficient (pMMR) CRC using immune cell markers (CD3, CD4, CD8, CD20, CD68, and FOXP3) as well as PD-1 and PD-L1 immunohistochemistry staining in whole tumor specimens and tissue microarray slides to compare 4r PD-L1 immunohistochemistry clones (SP142, E1L3N, 22C3, and 28.8).

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