Background：IL-17A is the founding member of the IL-17 family, a group of six structurally related pro-inflammatory cytokines. IL-17A, secreted by activated CD4+ Th17 cell subpopulation, elicits multiple biological activities on a variety of cells including： the induction of IL-6, IL-8, G-CSF, and PGE2 production in epithelial, endothelial or fibroblasts; the enhancement of surface expression of ICAM-1 in fibroblasts; activation of NF-kappaB and costimulation of T cell proliferation. Recent studies demonstrated that, in mice, activated IL-17-secreting CD4+ helper T cells （Th17 cells） mediate an autoimmune arthritis that clinically and immunologically resembles rheumatoid arthritis （RA）. Human IL-17A shows 63％, 63％, and 72％ amino acid sequence identity to rat IL-17A, mouse IL-17A, and a protein encoded by the ORF13 gene of herpesvirus Saimiri （HVS）, respectively.
