[Anti-PD-1 antibody: basics and clinical application]

Although the treatment of cancer with monoclonal antibodies has long been pursued, T cell-directed immunotherapy has met with limited success. Recently, much attention has been devoted to the blockade of PD-1 signaling to activate an immune response to cancer. PD-1, a protein expressed on T cells, is a member of the CD28 superfamily, and it transmits coinhibitory signals upon engagement with its ligands PD-L1 and PD-L2. Accumulating evidence suggests that the PD-1 system plays pivotal roles in the regulation of autoimmunity, transplantation immunity, infectious immunity, and tumor immunity. Because the interaction of PD-1 with its ligands occurs in the effector phase of killer T cell responses in peripheral blood, anti-PD-1 and anti-PD-L1 monoclonal antibodies are ideal as specific agents to augment T cell responses to tumors with fewer adverse events than with the inhibition of CTLA-4, because the interaction of CTLA-4 with its ligands occurs in the priming phase of T cell responses within lymph nodes. In recent phase I clinical trials, objective responses were observed in patients with melanoma, renal cell carcinoma, and non-small cell lung cancer who underwent immunotherapy with an anti-PD-1 monoclonal antibody. In addition, the antitumor activity of an anti-PD-L1 monoclonal antibody was observed in patients with melanoma, renal cell carcinoma, non-small cell lung cancer, and ovarian cancer. The next frontier of immunotherapy targeting the PD-1 axis is to define patient selection criteria and explore combination therapy with other therapeutic manipulations such as adoptive immunotherapies.