PDCD1 and cancer: Chimeric antigen receptor (CAR) T cell-based therapies as well as immune checkpoint inhibitors (ICIs), including antibodies targeting programmed cell death protein-1 (PD-1), programmed death-ligand 1 (PD-L1) and cytotoxic T-lymphocyte-associated antigen-4 (CTLA-4), have revolutionized the cancer treatments and have been approved by the U.S. Food and Drug Administration (FDA) for tackling many tumors (Allison, 2015; Sharma and Allison, 2015; Labanieh et al., 2018).