PDCD1 and hepatocellular carcinoma: PD-1, a transmembrane receptor expressed on activated T cells, B cells, and myeloid cells, interacts with its ligand PD-L1—a surface glycoprotein upregulated on tumor cells and tumor-associated macrophages in HCC—to transmit inhibitory signals that suppress cytotoxic T-lymphocyte activity and promote immune tolerance [126].