Ligand binding to inhibitory receptors on immune cells, such as programmed cell death 1 (PD-1) and cytotoxic T-lymphocyte associated protein 4 (CTLA4), down-regulates the T-cell–mediated immune response (called immune checkpoints). Antibodies that block these receptors increase antitumor immunity in patients with melanoma, non–small-cell lung cancer, and renal cell cancer. Tumor-infiltrating CD4+ and CD8+ T cells in patients with hepatocellular carcinoma (HCC) have been found to be functionally compromised.