PDCD1 and lung cancer: The therapeutic paradigm of targeting immune checkpoint by using monoclonal antibodies to inhibit cytotoxic T-lymphocyte-associated antigen 4 (CTLA-4), programmed cell death protein-1 (PD-1), and programmed cell death protein Ligand-1 (PD-L1) has achieved encouraging results in melanoma, lung cancer, and other malignancies (17).