CTLA4 and hepatocellular carcinoma: The role of several immune checkpoints including programmed cell death 1/programmed death-ligand 1 (PD-1/PD-L1), cytotoxic T-lymphocyte-associated protein 4 (CTLA-4), lymphocyte-activation gene 3 (LAG-3), adenosine A2a receptor (A2aR), or T-cell immunoglobulin and mucin domain-3 (TIM-3) has been highlighted in the immune tolerance for HCC (66).