PDCD1 and neoplasm: Microenvironment targeting of checkpoint proteins by programmed death-1 (PD-1) and cytotoxic T lymphocytes-associated antigen-4 (CTLA-4) inhibitors brought more attention to the crosstalk between the immune cell and tumor microenvironment; however, the regulation of immune cell–tumor crosstalk, so far, is not well described [151,152].