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-κB 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.Both recombinant and natural IL-17A have been shown to exist as a disulfide-linked homodimeric glycoprotein containing two ~16 kD monomers.