IL-17E Austin L. Gurney* Departments of Molecular Biology and Immunology, Genentech, South San Francisco, CA 94080, USA * corresponding author tel: (650) 225-8996, fax: (650) 225-6497, e-mail:
[email protected] DOI: 10.1006/rwcy.2002.0327.
SUMMARY Interleukin 17E (IL-17E) is a member of the IL-17 family of cytokines. IL-17E induces expression of a variety of other chemokines and cytokines and thereby acts to influence inflammation and hematopoiesis. Overexpression of IL-17E in transgenic mice results in severe chronic inflammation in multiple tissues and a bias towards a TH2-type immune response. IL-17E is a high-affinity ligand for a receptor IL-17Rh1 (also termed evi27, and IL-17BR) that is distantly related to the IL-17 receptor (IL-17R).
BACKGROUND
Discovery IL-17 was first identified as a potent proinflammatory cytokine produced by activated memory T cells (Rouvier et al., 1993; Yao et al., 1995; Fossiez et al., 1996; Kennedy et al., 1996). The large-scale sequencing of the human and other vertebrate genomes has revealed the presence of additional genes encoding proteins clearly related to IL-17, thus defining a new family of cytokines that appears to have been highly conserved across vertebrate evolution (Li et al., 2000; Shi et al., 2000; Hymowitz et al., 2001; Lee et al., 2001; Starnes et al., 2001, 2002; Aggarwal and Gurney, 2002). Initial characterization suggests that IL-17E, like IL-17, has a potent ability to modulate immune function (Fort et al., 2001; Lee et al., 2001; Pan et al., 2001; Kim et al., 2002). Its actions can promote induction of both proinflammatory cytokines such as TNF and co-stimulatory molecules such as ICAM as well as cytokines
Cytokine Reference
most often associated with an antibody-mediated immune response. The actions of IL-17E resemble but are clearly not equivalent to those of IL-17. IL-17E does not appear to be coexpressed with IL-17 in activated T cell populations, and major questions concerning the sources of IL-17E, its regulation, and its normal physiological role remain unresolved.
Alternative names IL-25
Structure IL-17E is a 38 kDa secreted glycoprotein. The mature protein exists as a covalent dimer of 145 amino acid monomers. The six known members of the IL-17 family are all similarly sized secreted proteins of 145±170 amino acids. They bear greatest similarity within the C-terminal 70 amino acids. Although there is not a strict conservation of spacing, there are four well-conserved cysteines and at least two additional cysteines that appear likely to be functionally conserved but with more variable spacing. The crystal structure of one member of the family, IL-17F, has been determined (Hymowitz et al., 2001) and shown to possess a fold similar to the cystine knot superfamily.
Main activities and pathophysiological roles Initial studies suggest that IL-17E is able to stimulate production of proinflammatory and hematopoietic
Copyright # 2002 Published by Elsevier Science Ltd
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cytokines including IL-6, IL-8, G-CSF from cell lines derived from a variety of lineages (Lee et al., 2001). This response is similar to that observed with IL-17 (Yao et al., 1995; Fossiez et al., 1996; Kennedy et al., 1996), although the range of responsive cell lines appears more restricted and the responses less robust. However, the full spectrum of cell types responsive to IL-17E has not been established. The highest levels of the IL-17E receptor (IL-17Rh1) expression are in kidney and liver with expression detectable at lower levels in other tissues. In kidney IL-17Rh1 is expressed principally in the proximal tubules of the renal tubular epithelium. There is some possibility that there is expression within immune cell compartments although which cell types are most relevant to IL-17E biology has not been established. Transgenic mice overexpressing IL-17E from an MLCH (muscle-specific) promoter designed to provide robust systemic expression exhibited chronic inflammation with mixed leukocyte infiltrates of several tissues, notably including the liver, lung, heart, and kidney (Pan et al., 2001). The mice displayed neutrophilia and eosinophilia, and had elevated levels of many cytokines, particularly those associated with a TH2-type immune response. Similar observations were also made when IL-17E was overexpressed from an apolipoprotein E (ApoE) hepatic promoter (Kim et al., 2002).
GENE AND GENE REGULATION
Accession numbers Human IL-17E mRNA: AF305200 Mouse IL-17E mRNA: NM_080729
Chromosomal location Chromosomal location has not been reported but human IL-17E sequence is contained in BAC C-2201G16 (accession AL132855) derived from chromosome 14.
Cells and tissues that express the gene IL-17E can be detected in various peripheral tissues at very low levels. Regulation of IL-17E expression within the immune system is currently not understood.
PROTEIN
Accession numbers Human IL-17E protein: AF305200 Mouse IL-17E protein: NP_542767.1
Sequence See Figure 1.
Description of protein Two human IL-17E mRNA transcripts occur that differ in the first few N-terminal amino acids. The longer encoded IL-17E that is predicted from the human cDNA sequence contains 177 amino acids. IL17E contains a predicted signal sequence of 32 amino acids and mature IL-17E secreted and purified from baculovirus-infected insect cells begins with amino acid 33. Protein expressed from HEK 293 cells or insect cells runs at a molecular weight of 38 kDa on nonreducing SDS±PAGE gels and approximately 18±19 kDa under reducing conditions, consistent with its existence as a covalent dimer and utilization of the one predicted N-linked glycosylation site per monomer. The crystal structure of one of the IL-17 family members, IL-17F, has recently been determined (Hymowitz et al., 2001). Unexpectedly, the structure of IL-17F reveals that the protein adopts a cystine knot fold and suggests that the family may have relationship to the cystine knot superfamily of proteins (McDonald and Hendrickson, 1993). The protomer structure consists of two pairs of antiparallel strands with two disulfide linkages between strands 2 and 4, as well as a substantial N-terminal extension. In the native dimer, an additional disulfide linkage joins the loop between the third and fourth strand of each monomer with the N-terminal extension of the adjacent monomer. IL-17F folds in a manner quite similar to that of nerve growth factor, NGF, and the other neurotrophins (McDonald et al., 1991; Wiesmann et al., 1999). In addition, IL-17F and NGF both exist as dimers. The IL-17 family does not have any appreciable sequence identity with the neurotrophins. The cystine Figure 1 Amino acid sequence for human IL-17E. MRERPRLGED SSLISLFLQV VAFLAMVMGT HTYSHWPSCC PSKGQDTSEE LLRWSTVPVP PLEPARPNRH PESCRASEDG PLNSRAISPW RYELDRDLNR LPQDLYHARC LCPHCVSLQT GSHMDPRGNS ELLYHNQTVF YRRPCHGEKG THKGYCLERR LYRVSLACVC VRPRVMG
IL-17E 3 knot superfamily is a diverse family, and includes other proteins such as the endocrine glycoprotein hormones (e.g. chorionic gonadotropin), the plateletderived growth factors (PDGFs), and the transforming growth factor (TGF ) family, which also display limited sequence similarity. While IL-17F folds in manner highly analogous to the cystine knot superfamily, one of the canonical cysteine pairs, the pair which links through the ring formed by the other two pairs and thereby forms the `knot', is not present. Instead a spatially distinct pair of cysteines forms a third cysteine bridge. Thus, while the IL-17 family members clearly adopt a cystine knot fold, actual evolutionary membership within the family cannot be assumed. The structure of IL-17E has not yet been determined. The conservation of the four `cystine knot' cysteines implies that it will possess a generally similar structure. Nonetheless, the Nterminal region of the IL-17E possesses little homology to IL-17F and will likely assume a distinct conformation.
Important homologies There are at least six members of the IL-17 family in humans and in mice. The overall sequence identity between IL-17E and other IL-17 family homologs ranges from 15% to 20%, whereas the identity between human and murine IL-17E orthologs is 80%. Neither the IL-17 family nor the cognate receptors that have been identified for these molecules bear obvious sequence similarity to other known families of proteins.
CELLULAR SOURCES AND TISSUE EXPRESSION
Cellular sources that produce IL-17E was identified based on genomic sequence. The physiologically relevant cellular source of IL-17E is currently not known. Very low levels of transcripts can be detected in various peripheral tissues (Lee et al., 2001; Kim et al., 2002). Low but detectable levels of IL-17E can be detected in stimulated splenocyte cultures (unpublished observations).
RECEPTOR UTILIZATION IL-17E is a ligand for IL-17Rh1 (Lee et al., 2001) (also referred to as IL-17B-R and Evi-27) (Shi et al., 2000;
Tian et al., 2000). Binding studies suggest that this is a relatively high affinity interaction with a Kd of 1 nM, higher than the roughly 50 nM affinity reported for the interaction between IL-17 and IL-17R (Yao et al., 1995; Hymowitz et al., 2001). IL-17Rh1 is distantly related to IL-17R and is the second member of the IL-17R family of proteins to be described. The murine ortholog of IL-17Rh1 was identified at Evi27, a common site of retroviral integration in BXH2 murine leukemias (Tian et al., 2000). The viral integration resulted in increased receptor expression and has thereby suggested the potential for a role of IL-17Rh1 in myeloid leukemia and growth and/or differentiation of hematopoietic cells.
IN VITRO ACTIVITIES
In vitro findings IL-17E is able to stimulate production of IL-6, IL-8, and G-CSF from various cell lines (Lee et al., 2001). However, the modest effects seen in cell lines to date do not seem likely to account for the profound inflammation seen in IL-17E transgenic animals, suggesting that there are other, as yet unidentified, target cell populations that respond to IL-17E.
IN VIVO BIOLOGICAL ACTIVITIES OF LIGANDS IN ANIMAL MODELS
Normal physiological roles The normal physiological function of IL-17E is not yet understood, but likely involves the fine tuning and regulation of aspects of the inflammatory response and host defense.
Transgenic overexpression Murine IL-17E has been overexpressed in transgenic mice using the rat skeletal myosin light chain 2 promoter, presumably giving rise to circulating mIL-17E (Pan et al., 2001) and apolipoprotein E (ApoE) hepatic promoter (Kim et al., 2002). Transgenic mice had chronic inflammation in multiple tissues including liver, heart, lungs, lymph node, kidneys, spleen, and urinary bladder. Inflammation in these tissues was composed of mixed infiltrates of neutrophils, eosinophils, lymphocytes, plasma cells,
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and macrophages. At 6 weeks of age, most of the transgenic founder mice were jaundiced with elevated serum levels of bilirubin and liver enzymes, suggestive of liver damage. Message levels of many important cytokines are greatly elevated in these mice. Striking were the high levels of TH2 cytokines including IL-4, IL-5, IL-10, and particularly IL-13 in several tissues including spleen, suggesting a TH2 bias to the immune response. Consistent with this, the mice exhibited significantly elevated levels of both serum IgE and IgG1. However, elevated levels of cytokines not considered characteristic of the TH2 paradigm (e.g. interferon , TNF and IL-1 ) were observed in several tissues. Thus the inflammatory response displays tissue-specific variation and may not be strictly TH2 in character. In addition the mice displayed neutrophilia and eosinophilia, likely due in part to elevated levels of G-CSF and IL-5. Taken together these findings suggest that IL-17E is a unique pleiotrophic cytokine that engages a systemic response with both TH2-like and proinflammatory character capable of inducing tissuespecific immunological and pathological changes.
PATHOPHYSIOLOGICAL ROLES IN NORMAL HUMANS AND DISEASE STATES AND DIAGNOSTIC UTILITY
Normal levels and effects No pathological roles for IL-17E in human disease have been defined. The potent ability of IL-17E to induce and modify the inflammatory response raises the possibility that IL-17E may play a role in human diseases of chronic inflammation and autoimmunity.
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