Identification of microRNAs that target PD-L1 in mismatch repair-deficient colorectal cancer.

85 Background: Programmed cell death 1 (PD-1) / PD-ligand 1 (PD-L1) immune checkpoint blockade has emerged as a promising therapeutic strategy in various types of cancer. Here we report that certain microRNAs (miRNAs) are involved in immunosuppressive microenvironment by directly targeting PD-L1 in mismatch repair deficient (dMMR) colorectal cancer (CRC). Methods: We identified candidate miRNAs by using RNA-sequence analyses for mRNA and miRNA expression obtained from The Cancer Genome Atlas (TCGA) Colon Adenocarcinoma combined with miRNA target prediction programs. HCT116 and SW837 CRC cell lines were transfected with miRNA mimics and inhibitors, and PD-L1 expression was examined by qRT-PCR, western blotting and flow cytometry. The CRC cell lines were co-cultured with activated T cells and then cytotoxicity for T cells were evaluated. Results: Using miRNA expression profiles of 260 tumors obtained through TCGA Colon Adenocarcinoma, 47 miRNA probes were found to be inversely correlated with PD-L1 expression. Among them, 19 mature miRNAs were down-regulated in dMMR tumors. Furthermore, eight in silico miRNA-target prediction programs were utilized to identify candidate miRNAs that target the 3’UTR of PD-L1 mRNA. We found that forced miRNA expression decreased PD-L1 expression in protein (total and surface) and mRNA levels, and they also dramatically decreased IFN-induced cell surface PD-L1 expression by 32% in CRC cell lines. Furthermore, we found that forced miRNA expression decreased PD-L1 associated apoptotic cell death in co-cultured activated T cells. Conclusions: Our findings suggest that PD-L1 expression is at least in part regulated by miRNAs and may also suggest potential miRNAs to serve as biomarkers and therapeutic targets for cancer immunotherapy.