High density of cytotoxic T-lymphocytes is linked to tumoral PD-L1 expression regardless of the mismatch repair status in colorectal cancer

BACKGROUND Immune checkpoint-inhibitors targeting the PD-1/PD-L1 system are FDA approved in microsatellite instable (MSI) or mismatch repair deficient (dMMR) colorectal cancer (CRC). PD-L1 expression is tightly linked to features connected to immune checkpoint inhibitor response, but studies on large subsets of cancers analyzing the correlation between different status of MSI/dMMR, tumor infiltrating lymphocytes and PD-L1 expression are still lacking. METHODS More than 1800 CRC were analyzed for PD-L1 by immunohistochemistry in a tissue microarray format. Data were compared to MMR, the number of intratumoral CD8+ cytotoxic T-cells, and adverse clinico-pathological parameters. Different cutoff levels for defining PD-L1 positivity in tumor cells (1%, 5%, 10%, and 50%) yielded comparable results. RESULTS At a cutoff level of 5%, PD-L1 positivity was seen in 5.1% of tumors. PD-L1 was more often positive in dMMR (18.6%) than in MMR proficient (pMMR) cancers (4.1%; p