PDCD1 and viral infectious disease: Programmed death ligand1 (PD-L1) is a Type 1 transmembrane protein, has a MW of 40 kDa, and is known to play an important role in suppressing the immune system when the system is challenged by autoimmune diseases and viral infections [50] PD-L1 is mainly expressed on the cell surface of tumor cells or antigen-presenting cells; the formation of the PD-1/PD-L1 complex transmits an inhibitory signal that reduces the proliferation and activity of killer T lymphocytes [51].