IL-17, originally identified as mouse cytotoxic T lymphocyte-associated antigen-8 (CTLA-8) (1), is produced by activated T lymphocytes, primarily by memory T cells (1-4). IL-17 appears to mediate communication between the immune system and the hematopoietic system.

IL-17 is a disulfide-linked homodimer (2, 4). Each polypeptide has 155 amino acid (aa) residues (predicted mass = 17.5 kDa), including a 19 aa residue hydrophobic leader sequence (2). There are six cysteines plus one potential N-linked glycosylation site, which is variably glycosylated, at least with recombinant proteins (2, 4). The aa sequence of human IL-17 is 63% and 58% identical to mouse and rat IL-17 and 72% identical to the thirteenth ORF of Herpesvirus saimiri (2, 4). There is at least some species specificity for in vitro action on bone-marrow stromal cells (3).

IL-17 mediation of T cell communication with the hematopoietic system is suggested by two observations. T cell-derived IL-17 induces fibroblasts to produce IL-6, IL-8, ICAM-1 and G-CSF, apparently by an NF-?B-mediated mechanism (5). IL-6 in turn promotes development of granulocyte/macrophage colonies, and G-CSF directs development of neutrophils (4, 6-9). IL-17 also enhances proliferation of partially activated T cells (5) and upregulates nitric oxide (NO) production in osteoarthritic cartilage (10).