The IL-17 family of cytokines includes six members, IL-17/IL-17A, IL-17B, IL-17C, IL-17D, IL-17E/IL-25, and IL-17F, which are produced by multiple cell types. Members of this family have a highly conserved C-terminus containing a cysteine-knot fold structure. Most IL-17 proteins are secreted as disulfide-linked dimers, with the exception of IL-17B, which is secreted as a non-covalent homodimer.
Signaling by IL-17 family cytokines is mediated by members of the IL-17 receptor family, IL-17 R/IL-17 RA, IL-17 B R/IL-17 RB, IL-17 RC, IL-17 RD, and IL-17 RE. Activation of these receptors triggers intracellular pathways that induce the production of pro-inflammatory cytokines and anti-microbial peptides. IL-17A, IL-17F, and IL-17A/F are produced primarily by activated T cells and signal through an oligomerized receptor complex consisting of IL-17 RA and IL-17 RC. Ligand binding to this complex leads to recruitment of the intracellular adaptor proteins, Act1 and TRAF-6, and downstream activation of the transcription factors, NF kappa B, AP-1, and C/EBP. IL-17E activates similar signaling pathways through a receptor complex formed by IL-17 RA and IL-17 B R/IL-17RB. Signaling by IL-17E induces Th2-type immune responses and may be involved in promoting the pathogenesis of asthma. Less is known about the signaling pathways activated by other IL-17 family cytokines. Recent studies suggest that IL-17C is produced primarily by epithelial cells and binds to a receptor complex consisting of IL-17 RA and IL-17 RE. Autocrine signaling by IL-17C in epithelial cells stimulates the production of anti-microbial peptides and pro-inflammatory cytokines, but like IL-17A, overexpression of IL-17C may contribute to the development of autoimmune diseases. Similar to IL-17E, IL-17B binds to IL-17 B R/IL-17 RB, but the major target cells and effects of IL-17B signaling have not been reported. In addition, the receptor for IL-17D and the ligand for IL-17 RD are currently unknown.