PDCD1 and neoplasm: However, some tumor cells form an immunosuppressive tumor microenvironment (TME) to avoid attacks from the immune system, which includes downregulation of tumor antigens, upregulation of inhibitory receptors (e.g., programmed cell death protein 1 [PD-1], and programmed cell death protein ligand 1 [PD-L1]), and stimulation of regulatory cells accumulation [3].