PDCD1 and head and neck squamous cell carcinoma: However, because of the low response rate to anti-PD1 therapy in HNSCC patients [3], it is important to better understand the tumor microenvironment of HNSCC in order to elucidate precise mechanisms of resistance to α-PD-1 therapy, and to design supplemental treatments or combination strategies to reverse α-PD-1 non-responsiveness and improve clinical outcome.