PDCD1 and cancer: Cancer immunotherapy using anti-PD-1 antibodies (PD-1 therapy) has been thought to enhance antitumor immunity by reinvigorating the functionality of tumor-infiltrating (TI) PD-1+CD8+ T cells (Wherry and Kurachi, 2015; Thommen et al., 2018; Kim K. H. et al., 2019).