PDCD1 and neoplasm: These mechanisms also include the upregulation of immune checkpoint receptors, such as cytotoxic T-lymphocyte-associated protein-4 (CTLA-4), programmed cell death protein-1 (PD-1), programmed cell death protein-Ligand 1 (PD-L1) and T-cell immunoglobulin 3 (Tim-3) and the presence of tumor-derived immunosuppressive factors [11,14,22,38,39].