PDCD1 and neoplasm: Furthermore, TAMs contribute to the blockade of cytotoxic activities in T cells, natural killer T cells, and natural killer cells on account of the high expression of immune checkpoint ligands on TAMs such as programmed cell death protein ligand 1 (PD-L1), programmed cell death protein 1 (PD-1), B7-H4, and cytotoxic T-lymphocyte-associated protein 4 (CTLA4), which leads to reinforced tumor growth 147, 151-153.