PDCD1 and cancer: Moreover, many cancer types have evolved to express programmed cell death ligand-1 (PD-L1)/PD-L2, the cognate ligands of PD-1, leading to the shut-off of T cell activation in TMEs (Furuse et al., 2020; Rotman et al., 2020; Yamaguchi et al., 2022; Yang et al., 2023).