Programmed cell death protein 1 (PD-1) is a cell surface receptor that regulates T cell activation through binding to soluble ligands PD-L1 and PD-L2 [15,21]. Programmed cell death protein 1 (PD-1) and programmed death-ligand 1 (PD-L1) play a major role in suppressing the immune system during the formation of the PD-1/PD-L1 pathway, which transmits an inhibitory signal to reduce T cell activity.