CD274 and neoplasm: PD-1 and its ligand inhibit antitumor T-cell immunity in the following two ways: 1) PD-L1 downregulates MHC-I, which decreases antigen presentation in tumor cells and dampens CD8+ T-cell killing ability; 2) PD-1-expressing CD8+ T cells are contributes to T cell exhaustion [13–15] .