PDCD1 and hepatocellular carcinoma: To date, it is accepted that during HCC progression, the proportion of tumor-associated macrophages (TAM) displaying an M2 phenotype and expressing the immunosuppressive cytokines interleukin (IL)-10 and transforming growth factor-β (TGF-β) is increased, while cytotoxic natural killer (NK) and CD8 T cells are both decreased in number and inhibited through activation of immune checkpoints signaling (i.e., PD-1, CTLA4, TIGIT and LAG3) [7,8,9,10,11].