 Programmed death 1 PD-1 and its ligand PD-L1 play pivotal roles in regulating host immune responses. Substantial evidence has demonstrated that PD-L1 can deliver an inhibitory signal to PD-1 expressing T cells, leading to suppression of the immune response by inducing apoptosis, energy, unresponsiveness and functional exhaustion of T cells. However, the inhibitory effects of this pathway on the function of cytotoxic T lymphocytes, the main effector cells in HNSCC patients, are not well defined. 