Colorectal cancer (CRC) with high-level microsatellite instability (MSI-H) tends to be associated with a better response to programmed death receptor-1 (PD-1) blockade than does microsatellite stable CRC. However, emerging evidence makes the use of programmed death ligand-1 (PD-L1) as a biomarker problematic. Here, we sought to characterize the interactions between PD-L1 expression and the response to PD-1 blockade therapy in BALB/c mice with a subcutaneous tumor challenge. We further focused on interferon gamma (IFNγ)-induced PD-L1 expression in an in vitro setting to evaluate the responsiveness to IFNγ exposure and the specific signaling of PD-1 in HCT116 and SW480 cell lines. In this study, enhanced PD-L1 expression increased survival in CT26 cells, and PD-1 blockade increased the CTL profile and apoptotic cells in mice with CRC. Our in vitro findings showed that PD-L1 expression was significantly upregulated by a low-dose IFNγ treatment, and the MSI-H cell line might exhibit hyperresponsiveness to IFNγ exposure partly through the JAK-STAT pathway. These results suggest that intrinsic PD-L1 in cooperation with extrinsic IFNγ exposure in CRC may be more responsive to anti-PD-1 therapy, mainly through the CTL profile in the tumor microenvironment.
Keywords: CTL; Colorectal cancer; IFNγ; Microsatellite instability; PD-L1.