PDCD1 and head and neck squamous cell carcinoma: Dysfunctional tumor infiltrating lymphocytes (TILs) in HNSCC is characterized by the upregulation of several immune checkpoint markers, such as cytotoxic T lymphocyte antigen-4 (CTLA-4) [61], programmed cell death protein-1 (PD-1) [62], T cell immunoglobulin mucin-3 (TIM-3) [63], lymphocyte activated gene-3 (LAG-3) [64], fibrinogen-like protein 1 (FGL1) [65], Glucocorticoid-induced tumor necrosis factor receptor family-related protein (GITR) [66], and V-domain Ig suppressor of T cell activation (VISTA) [64].