|Alt. symbols||IL17, CTLA8|
|Locus||Chr. 6 p12|
|Locus||Chr. 5 q32-34|
|Alt. symbols||, CX2|
|Locus||Chr. 16 q24|
|Locus||Chr. 13 q11|
|Locus||Chr. 14 q11.2|
|PDB||1JPY (RCSB PDB PDBe PDBj)|
|Locus||Chr. 6 p12|
Interleukin 17A (IL-17 or IL-17A), originally identified as a transcript from a rodent T-cell hybridoma by Rouvier et al. in 1993, is the founding member of a group of cytokines called the IL-17 family. Known as CTLA8 in rodents, IL-17 shows high homology to viral IL-17 encoded by an open reading frame of the T-lymphotropic rhadinovirus Herpesvirus saimiri.
Interleukin 17 is a cytokine that acts as a potent mediator in delayed-type reactions by increasing chemokine production in various tissues to recruit monocytes and neutrophils to the site of inflammation, similar to Interferon gamma. IL-17 is produced by T-helper cells and is induced by IL–23 which results in destructive tissue damage in delayed-type reactions. Interleukin 17 as a family functions as a proinflammatory cytokine that responds to the invasion of the immune system by extracellular pathogens and induces destruction of the pathogen’s cellular matrix. Interleukin 17 acts synergistically with tumor necrosis factor and interleukin-1.
In addition to IL-17A, members of the IL-17 family include IL-17B, IL-17C, IL-17D, IL-17E (also called IL-25), and IL-17F. All members of the IL-17 family have a similar protein structure, with four highly conserved cysteine residues critical to their 3-dimensional shape, yet they have no sequence similarity to any other known cytokines. Phylogenetic analysis reveals that among IL-17 family members, the IL-17F isoforms 1 and 2 (ML-1) have the highest homology to IL-17A (sharing 55 and 40% amino acid identity to IL-17A respectively), followed by IL-17B (29%), IL-17D (25%), IL-17C (23%), and IL-17E being most distantly related to IL-17A (17%). These cytokines are all well conserved in mammals, with as much as 62–88% of amino acids conserved between the human and mouse homologs.
Numerous immune regulatory functions have been reported for the IL-17 family of cytokines, presumably due to their induction of many immune signaling molecules. The most notable role of IL-17 is its involvement in inducing and mediating proinflammatory responses. IL-17 is commonly associated with allergic responses. IL-17 induces the production of many other cytokines (such as IL-6, G-CSF, GM-CSF, IL-1β, TGF-β, TNF-α), chemokines (including IL-8, GRO-α, and MCP-1), and prostaglandins (e.g., PGE2) from many cell types (fibroblasts, endothelial cells, epithelial cells, keratinocytes, and macrophages). The release of cytokines causes many functions, such as airway remodeling, a characteristic of IL-17 responses. The increased expression of chemokines attracts other cells including neutrophils but not eosinophils. IL-17 function is also essential to a subset of CD4+ T-Cells called T helper 17 (Th17) cells. As a result of these roles, the IL-17 family has been linked to many immune/autoimmune related diseases including rheumatoid arthritis, asthma, lupus, allograft rejection, anti-tumour immunity and recently Psoriasis.
Role in asthma
The IL-17F gene was discovered in 2001, and is located on chromosome 6p12. Notably, among this family, IL-17F has been well characterized both in vitro and in vivo, and has been shown to have a pro-inflammatory role in asthma. IL-17F is clearly expressed in the airway of asthmatics and its expression level is correlated with disease severity. Moreover, a coding region variant (H161R) of the IL-17F gene is inversely associated with asthma and encodes an antagonist for the wild-type IL-17F. IL-17F is able to induce several cytokines, chemokines and adhesion molecules in bronchial epithelial cells, vein endothelial cells, fibroblasts and eosinophils. IL-17F utilizes IL-17RA and IL-17RC as its receptors, and activates the MAP kinase related pathway. IL-17F is derived from several cell types such as Th17 cells, mast cells and basophils, and shows a wide tissue expression pattern including lung. Overexpression of IL-17F gene in the airway of mice is associated with airway neutrophilia, the induction of many cytokines, an increase in airway hyperreactivity, and mucus hypersecretion. Hence, IL-17F may have a crucial role in allergic airway inflammation, and have important therapeutic implications in asthma.
The gene for human IL-17 is 1874 base pairs long and was cloned from CD4+ T cells. Each member of the IL-17 family has a distinct pattern of cellular expression. The expression of IL-17A and IL-17F appear to be restricted to a small group of activated T cells, and upregulated during inflammation. IL-17B is expressed in several peripheral tissues and immune tissues. IL-17C is also highly upregulated in inflammatory conditions, although in resting conditions is low in abundance. IL-17D is highly expressed in the nervous system and in skeletal muscle and IL-17E is found at low levels in various peripheral tissues.
Much progress has been made in the understanding of the regulation of IL-17. At first, Aggarwal et al. showed that production of IL-17 was dependent on IL-23. Later, a Korean group discovered that STAT3 and NF-κB signalling pathways are required for this IL-23-mediated IL-17 production. Consistent with this finding, Chen et al. showed that another molecule, SOCS3, plays an important role in IL-17 production. In the absence of SOCS3, IL-23-induced STAT3 phosphorylation is enhanced, and phosphorylated STAT3 binds to the promotor regions of both IL-17A and IL-17F increasing their gene activity. In contrast, some scientists believe IL-17 induction is independent of IL-23. Several groups have identified ways to induce IL-17 production both in vitro and in vivo by distinct cytokines, called TGF-β and IL-6, without the need for IL-23. Although IL-23 is not required for IL-17 expression in this situation, IL-23 may play a role in promoting survival and/or proliferation of the IL-17 producing T-cells. Recently, Ivanov et al. found that the thymus specific nuclear receptor, ROR-γ, directs differentiation of IL-17-producing T cells.
IL-17(A) is a 155-amino acid protein that is a disulfide-linked, homodimeric, secreted glycoprotein with a molecular mass of 35 kDa. Each subunit of the homodimer is approximately 15-20 KDa. The structure of IL-17 consists of a signal peptide of 23 amino acids (aa) followed by a 123-aa chain region characteristic of the IL-17 family. An N-linked glycosylation site on the protein was first identified after purification of the protein revealed two bands, one at 15 KDa and another at 20 KDa. Comparison of different members of the IL-17 family revealed four conserved cysteines that form two disulfide bonds. IL-17 is unique in that it bears no resemblance to other known interleukins. Furthermore, IL-17 bears no resemblance to any other known proteins or structural domains.
The crystal structure of IL-17F, which is 50% homologous to IL-17A, revealed that IL-17F is structurally similar to the cysteine knot family of proteins that includes the neurotrophins. The cysteine knot fold is characterized by two sets of paired β-strands stabilized by three disulfide interactions. However, in contrast to the other cysteine knot proteins, IL-17F lacks the third disulfide bond. Instead, a serine replaces the cysteine at this position. This unique feature is conserved in the other IL-17 family members. IL-17F also dimerizes in a fashion similar to nerve growth factor (NGF) and other neurotrophins.
Because of its involvement in autoimmune conditions, IL-17 inhibitors are being investigated as possible treatments, e.g. vidofludimus in phase IIB for rheumatoid arthritis, psoriasis and inflammatory bowel disease. As of early 2012, clinical trials of IL-17 antibodies against psoriasis have shown extremely promising results in Phase II. IL-17 has also been implicated in multiple sclerosis. The anti-IL-23 antibody ustekinumab can be used to effectively treat psoriasis by reducing IL-17. Based on emerging evidence from animal models, IL-17 has been suggested as a target for anti-inflammatory therapies to improve recovery post-stroke.
See also Interleukin-17 receptors.
The IL-17 receptor family consists of five, broadly distributed receptors (IL-17RA, B, C, D and E) that present with individual ligand specificities. Within this family of receptors, IL-17RA is the best-described. IL-17RA binds both IL-17A and IL-17F and is expressed in multiple tissues: vascular endothelial cells, peripheral T cells, B cell lineages, fibroblast, lung, myelomonocytic cells, and marrow stromal cells. Signal transduction for both IL-17A and IL-17F requires the presence of a heterodimeric complex consisting of both IL-17RA and IL-17RC and the absence of either receptor results in ineffective signal transduction. This pattern is reciprocated for other members of the IL-17 family such as IL-17E, which requires an IL-17RA-IL-17RB complex (also known as IL-17Rh1, IL-17BR or IL-25R) for effective function.
Another member of this receptor family, IL-17RB, binds both IL-17B and IL-17E. Furthermore, it is expressed in the kidney, pancreas, liver, brain, and intestine. IL-17RC is expressed by the prostate, cartilage, kidney, liver, heart, and muscle, and its gene may undergo alternate splicing to produce a soluble receptor in addition to its cell membrane-bound form. In similar manner, the gene for IL-17RD may undergo alternative splicing to yield a soluble receptor. This feature may allow these receptors to inhibit the stimulatory effects of their yet-undefined ligands. The least-described of these receptors, IL-17RE, is known to be expressed in the pancreas, brain, and prostate.
Signal transduction by these receptors is as diverse as their distribution. These receptors do not exhibit a significant similarity in extracellular or intracellular amino acid sequence when compared to other cytokine receptors. Transcription factors such as TRAF6, JNK, Erk1/2, p38, AP-1 and NF-κB have been implicated in IL-17 mediated signaling in a stimulation-dependent, tissue-specific manner. Other signaling mechanisms have also been proposed, but more work is needed to fully elucidate the true signaling pathways used by these diverse receptors.
- PDB 1JPY; Hymowitz SG, Filvaroff EH, Yin JP, Lee J, Cai L, Risser P, Maruoka M, Mao W, Foster J, Kelley RF, Pan G, Gurney AL, de Vos AM, Starovasnik MA (October 2001). "IL-17s adopt a cystine knot fold: structure and activity of a novel cytokine, IL-17F, and implications for receptor binding". EMBO J. 20 (19): 5332–41. doi:10.1093/emboj/20.19.5332. PMC 125646. PMID 11574464.
- Rouvier E, Luciani MF, Mattéi MG, Denizot F, Golstein P (1993). "CTLA-8, cloned from an activated T cell, bearing AU-rich messenger RNA instability sequences, and homologous to a herpesvirus saimiri gene". Journal of Immunology 150 (12): 5445–56. PMID 8390535.
- Kuby, Janis; Kindt, Thomas J.; Goldsby, Richard A.; Osborne, Barbara A. (2007). Kuby immunology. San Francisco: W.H. Freeman. p. 396. ISBN 1-4292-0211-4.
- Integrative responses to IL-17 and TNF-α in human keratinocytes account for key inflammatory pathogenic circuits in psoriasis. Chiricozzi A, Guttman-Yassky E, Suárez-Fariñas M, Nograles KE, Tian S, Cardinale I, Chimenti S, Krueger JG. J Invest Dermatol. 2011 Mar;131(3):677-87. Epub 2010 Nov 18. PMID 21085185
- Miossec P, Korn T, Kuchroo VK (August 2009). "Interleukin-17 and type 17 helper T cells". N. Engl. J. Med. 361 (9): 888–98. doi:10.1056/NEJMra0707449. PMID 19710487.
- Starnes T, Broxmeyer HE, Robertson MJ, Hromas R (2002). "Cutting edge: IL-17D, a novel member of the IL-17 family, stimulates cytokine production and inhibits hemopoiesis". Journal of Immunology 169 (2): 642–6. PMID 12097364.
- Kolls JK, Lindén A (2004). "Interleukin-17 family members and inflammation". Immunity 21 (4): 467–76. doi:10.1016/j.immuni.2004.08.018. PMID 15485625.
- Aggarwal S, Gurney AL (2002). "IL-17: prototype member of an emerging cytokine family". J. Leukoc. Biol. 71 (1): 1–8. PMID 11781375.
- Kawaguchi M, Kokubu F, Fujita J, Huang SK, Hizawa N (December 2009). "Role of interleukin-17F in asthma". Inflamm Allergy Drug Targets 8 (5): 383–9. PMID 20025586.
- Yao Z, Painter SL, Fanslow WC, Ulrich D, Macduff BM, Spriggs MK, Armitage RJ (1995). "Human IL-17: a novel cytokine derived from T cells". Journal of Immunology 155 (12): 5483–6. PMID 7499828.
- Aggarwal S, Ghilardi N, Xie MH, de Sauvage FJ, Gurney AL (2003). "Interleukin-23 promotes a distinct CD4 T cell activation state characterized by the production of interleukin-17". J. Biol. Chem. 278 (3): 1910–4. doi:10.1074/jbc.M207577200. PMID 12417590.
- Cho ML, Kang JW, Moon YM, Nam HJ, Jhun JY, Heo SB, Jin HT, Min SY, Ju JH, Park KS, Cho YG, Yoon CH, Park SH, Sung YC, Kim HY (2006). "STAT3 and NF-kappaB signal pathway is required for IL-23-mediated IL-17 production in spontaneous arthritis animal model IL-1 receptor antagonist-deficient mice". Journal of Immunology 176 (9): 5652–61. PMID 16622035.
- Chen Z, Laurence A, Kanno Y, Pacher-Zavisin M, Zhu BM, Tato C, Yoshimura A, Hennighausen L, O'Shea JJ (2006). "Selective regulatory function of Socs3 in the formation of IL-17-secreting T cells". Proc. Natl. Acad. Sci. U.S.A. 103 (21): 8137–42. doi:10.1073/pnas.0600666103. PMC 1459629. PMID 16698929.
- Veldhoen M, Hocking RJ, Atkins CJ, Locksley RM, Stockinger B (2006). "TGFbeta in the context of an inflammatory cytokine milieu supports de novo differentiation of IL-17-producing T cells". Immunity 24 (2): 179–89. doi:10.1016/j.immuni.2006.01.001. PMID 16473830.
- Mangan PR, Harrington LE, O'Quinn DB, Helms WS, Bullard DC, Elson CO, Hatton RD, Wahl SM, Schoeb TR, Weaver CT (2006). "Transforming growth factor-beta induces development of the T(H)17 lineage". Nature 441 (7090): 231–4. doi:10.1038/nature04754. PMID 16648837.
- Bettelli E, Carrier Y, Gao W, Korn T, Strom TB, Oukka M, Weiner HL, Kuchroo VK (2006). "Reciprocal developmental pathways for the generation of pathogenic effector TH17 and regulatory T cells". Nature 441 (7090): 235–8. doi:10.1038/nature04753. PMID 16648838.
- Ivanov II, McKenzie B.S., Zhou L., Tadokoro C.E., Lepelley A., Lafaille J.J., Cua D.J., Littman D.R. (2006). "The orphan nuclear receptor ROR-γ directs the differentiation program of proinflammatory IL-17+ T helper cells". Cell 126 (6): 1121–1133. doi:10.1016/j.cell.2006.07.035. PMID 16990136.
- Kawaguchi M, Adachi M, Oda N, Kokubu F, Huang SK (2004). "IL-17 cytokine family". The Journal of Allergy and Clinical Immunology 114 (6): 1265–73; quiz 1274. doi:10.1016/j.jaci.2004.10.019. PMID 15577820.
- Moseley TA, Haudenschild DR, Rose L, Reddi AH (2003). "Interleukin-17 family and IL-17 receptors". Cytokine Growth Factor Rev. 14 (2): 155–74. doi:10.1016/S1359-6101(03)00002-9. PMID 12651226.
- Pappu R, Ramirez-Carrozzi V, Sambandam A (September 2011). "The interleukin-17 cytokine family: critical players in host defence and inflammatory diseases". Immunology 134 (1): 8–16. doi:10.1111/j.1365-2567.2011.03465.x. PMC 3173690. PMID 21726218.
- Ley K, Smith E, Stark MA (2006). "IL-17A-producing neutrophil-regulatory Tn lymphocytes". Immunol. Res. 34 (3): 229–42. doi:10.1385/IR:34:3:229. PMID 16891673.