Interleukin 17 (also known as CTLA-8) is a T cell-expressed pleiotropic cytokine that exhibits a high degree of homology to a protein encoded by the ORF13 gene of herpesvirus Saimiri. cDNA clones encoding IL-17 have been isolated from activated rat, mouse and human T cells. Human IL-17 cDNA encodes a 155 amino acid (aa) residue precursor protein with a 19 amino acid residue signal peptide that is cleaved to yield the 136 aa residue mature IL-17 containing one potential N-linked glycosylation site. Both recombinant and natural IL-17 have been shown to exist as disulfide linked homodimers. At the amino acid level, human IL-17 shows 72% and 63% sequence identity with herpesvirus and rat IL-17, respectively. An IL-17 specific mouse cell surface receptor (IL-17 R) has recently been cloned. While the expression of IL-17 mRNA is restricted to activated T cells, the expression of mIL-17 R mRNA has been detected in virtually all cells and tissues tested. IL-17 exhibits multiple biological activities on a variety of cells including the induction of IL-6 and IL-8 production in fibroblasts, the enhancement of surface expression of ICAM-1 in fibroblasts, activation of NF-kappaB and costimulation of T cell proliferation.