PDCD1 and hepatocellular carcinoma: It can be affected by multiple regulatory procedures in HCC TME, including the secretion of transforming growth factor β (TGF-β) and interleukin 10 (IL-10), the recruitment of regulatory T cells (Tregs) and myeloid-derived suppressor cells (MDSCs), high levels of programmed cell death 1 (PD-1), programmed death ligand-1 (PD-L1) and so on (4, 5, 8, 9).