An update of knowledge on PD-L1 in head and neck cancers: Physiologic, prognostic and therapeutic perspectives.

Programmed cell death-ligand 1 (PD-L1) is a transmembrane protein that acts as a co-inhibitory factor in the immune response. Its receptor, programmed cell death protein 1 (PD-1), is found on immune cells, where binding to PD-L1 can reduce the proliferation of PD-1-positive cells, inhibit their cytokine secretion and induce apoptosis. PD-L1 in immune-privileged tissue plays a crucial role in peripheral tolerance.