The IL-17 family of cytokines consists of IL-17A-F, and their receptors include IL-17RA-RE. IL-17 cytokines are produced by a variety of cell types including the Th17 subset of CD4+ T cells, as well as subsets of gammadelta T cells, NK cells, and NKT cells. IL-17A and IL-17F, the most well-studied of the IL-17 cytokines, contribute to fungal and bacterial immunity by inducing expression of proinflammatory cytokines, chemokines, and antimicrobial peptides. In addition, IL-17A contributes to the pathogenesis of several autoimmune disease. IL-17E promotes Th2 cell responses. The roles of IL-17B, IL-17C, and IL-17D are less clear, however these family members also appear to have the capacity to induce proinflammatory cytokines. IL-17 receptors have an extracellular domain, a transmembrane domain, and a SEFIR domain. They are believed to signal as homodimers, heterodimers, or multimers through their SEFIR domain by recruiting the SEFIR domain-containing adaptor Act1. Unlike most cytokines that signal through Jak/STAT pathways, IL-17 signaling results in NF-kappaB activation. IL-17RA associates with IL-17RC to mediate signaling by homodimers and heterodimers of IL-17A and IL-17. IL-17RA is broadly expressed with highest expression in hematopoietic cells.
