PDCD1 and lung carcinoma: Immune checkpoint inhibitors (ICIs), programmed cell death protein-1/ligand-1 (PD-1/PD-L1) inhibitors and to a lesser extent cytotoxic T-lymphocyte-associated protein 4 (CTLA-4) blockers, have proven to be an effective strategy in the treatment of lung cancer, NSCLC and small cell lung cancer (SCLC), over the past 15 years [36].