PDCD1 and neoplasm: Importantly, TIGIT and programmed death-ligand 1 (PD-1) have been shown to be upregulated on tumour antigen-specific CD8+ T cells and CD8+ tumour infiltrating lymphocytes (TILs) in both HNSCC patients and mouse models; in addition, their level was correlated with other immune-checkpoint molecules, i.e., PD-1, TIM-3 and LAG-3 [409,410,411].