Interleukin-17A (IL-17A), also known as CTLA-8, is a 15-20 kDa glycosylated cytokine that plays an important role in anti-microbial and chronic inflammation. The six IL-17 cytokines (IL-17A-F) are encoded by separate genes but adopt a conserved cystine knot fold [1, 2]. Mature human IL-17A shares 60% amino acid sequence identity with mouse and rat IL-17A [3, 4]. IL-17A is secreted by Th17 cells, gamma /δ T cells, iNKT cells, NK cells, LTi cells, neutrophils, and intestinal Paneth cells [2]. It forms disulfide-linked homodimers as well as disulfide-linked heterodimers with IL-17F [5, 6]. IL-17A exerts its effects through the transmembrane IL-17RA in complex with IL-17RC or IL-17RD [7, 8]. Both IL-17RA and IL-17RC are required for responsiveness to heterodimeric IL-17A/F [7]. IL-17A promotes protective mucosal and epidermal inflammation in response to microbial infection [9-12]. It induces chemokine production, neutrophil influx, and the production of antibacterial peptides [9-11]. IL-17A/F likewise induces neutrophil migration, but IL-17F does not [11]. IL-17A additionally enhances the production of inflammatory mediators by rheumatoid synovial fibroblasts and contributes to TNF-alpha induced shock [4, 13]. In contrast, it can protect against the progression of colitis by limiting chronic inflammation [12]. IL-17A has been shown to exert either tumorigenic or anti-tumor effects [14, 15].
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