CD274 and hepatocellular carcinoma: The engagement of PD-1 expressed on the surface of activated CD8+ T cells with PD-L1 expressed by HCC cells not only averts the excessive activation of T cells, decreasing tumor-killing efficiency by transmitting inhibitory signals, but also weakens proliferation and cytolytic activity, followed by the defects or even deletion of cytokine production, eventually leading to an exhausted T-cell phenotype (Wherry, 2011).