T helper 17 (Th17) cells and interleukin-17 (IL-17) in cancer Abstract Th17 cells are a specialized subset of CD4+ T cells that are essential in driving inflammation during autoimmune disease and infection through a signature cytokine IL-17. Th17 cells have been found in various human cancers. The function of these cells in cancers is highly context-dependent; both tumor-promoting and tumor-suppressing activity have been reported. IL-17 and IL-22, Th17-derived cytokines, influence the tumor microenvironment by directly promoting transformed cell properties and neighboring stromal cell activity. These cytokines are also involved in regulation of the immune system by modulating the activities of myeloid cells and T cells. These findings suggest that Th17 cells and their cytokines are a key mediator of cancer development, representing a potential target for cancer therapy. Herein, I review recent preclinical studies on the function of Th17 cells and IL-17 in cancer and discuss possible therapeutic approaches to harness Th17 cells for cancer immunotherapy.