PDCD1 and hepatocellular carcinoma: TAMs in the peritumoral stroma of HCC have been shown to secrete multiple key proinflammatory cytokines (e.g. IL-1β, IL-6, IL-23, and TNF-α) and contribute to the expansion of interleukin-17-producing CD4+ T helper 17 cells (Th17), which suppress anti-tumor immunity by over-expressing several activation markers, such as PD-1, CTLA-4, and GITR [40].