PDCD1 and neoplasm: The immunosuppressive TME is characterized by the presence of various immunosuppressive cells such as regulatory T cells (Treg cells), myeloid-derived suppressor cells (MDSC) and tumor-associated macrophages, as well as the upregulated expression of immunosuppressive molecules such as programmed cell death protein 1 (PD-1) and programmed death-ligand 1 (PD-L1), making this environment an important barrier for an effective antitumor immune response (276–278).