PDCD1 and neoplasm: Exhausted CD8+ T cells (Tex), characterized by the loss of effector cytokine production (IFN-γ, TNF-α, IL-2), reduced proliferative capacity, and elevated cell surface expression of various co-inhibitory receptors (such as PD1, CTLA4, Tim3, Lag3, TIGIT), are unable to mount an effective anti-tumor response (3).