PDCD1 and neoplasm: The interaction of programmed cell death protein-1 (PD-1) on tumor-specific T cells with programmed cell death-ligand 1 (PD-L1) or PD-L2 on TAMs, MDSCs, or cancer-associated fibroblasts in the TME prompts cell death and reduces the number of cytotoxic T cells in the tumor tissue (46).