1
IL-25
Discovery and structure
IL-25 was first described in 2001 by Hurst et al.1 Due to its homology to the IL17 cytokine family (16-18% homology), it has also been named IL17E.1, 2 The protein consists of 177 amino acids and has a molecular mass of 16.7 kDa in humans and 17.6 kDa in mice. It is encoded on chromosome 14q11.1 and little is known so far about its structure. The strongest identity exists at the C-terminal part (20-30%).
Receptor and signaling
IL-25 binds to the IL17RB and IL-17RA to form a heteromeric receptor complex.3 It was previously shown that IL17RA-/- and IL17RB-/- mice are phenotypically normal. However, splenocytes from IL17RB-/- mice do not produce IL-5 or IL-13 in response to IL-25 stimulation, in contrast to wild type.3 Intranasal challenge with IL-25 increases the numbers of eosinophils, neutrophils, lymphocytes, and expression of CCL2, CCL11, IL-5, -13, -9 and IL-10. This response in not visible in IL-17RA-/- and IL17RB-/- mice, since both receptors subunits are needed for IL-25 signaling.
IL-25 activates NF-kB and members of the MAP kinase transcription factor family.4 TNFR-associated factor (TRAF) 6 directly associates with the cytoplasmic region of IL-17RB and mediates NF-kB, but not MAP kinase, activation upon ligand binding.5 In contrast, IL-17RA does not contain a TRAF6-binding motif, but can recruit TRAF6 by directly associating with the adaptor protein Act1.6 IL-17RA and Act1 interact through homotypic interaction of their SEFIR (similar expression to fibroblast growth factor genes and IL-17R) domains, which share residues with two of the three conserved motifs present in TIR (Toll/IL-1R) domains.7 Act1 is also recruited to IL-17RB through the SEFIR domain, and Act1 deficiency reduces IL-25-mediated eosinophilia and Th2 responses in the airway.8There is an interaction interface (CC′ loop) in the SEFIR domains of several interacting SEFIR family members, including IL-17RA, IL-17RB, IL-17RC, and Act1. Deletion of the CC′ loop from either Act1 or IL-17RA disrupted the interaction between these two proteins.9IL-25 can directly activate STAT5 through a Act1-independent pathway.10
Cellular sources and targets
IL-25 is a product of several cell types and can regulate the function of both immune and non-immune cells. IL-25 is produced by Th2 cells11, by in vitro cultured mast cells12 and epithelial cells.13, 14 Other cell types can also produce IL-25, including alveolar macrophages after solid particle inhalation15, intestinal epithelial cells16 and brain capillary endothelial cells.17 Eosinophil and basophil granulocytes from atopic individuals have also been described as sources of IL-25. RAG KO mice treated with IL-25 have the same hematological and histological changes as the WT mice. Because IL-25-treated RAG KO mice have increased gene expression of IL-5 and IL-13, it seems that there are non-T/non-B cells that responded to IL-25 by producing Th2-associated cytokines.11IL17RA and IL17RB are expressed on innate and adaptive immune cells such as CD4+Th2 cells, fibroblasts, basophils, invariant natural killer cells (iNKT cells), macrophages and innate lymphoid cells type II (ILC2s).18,20,19,20Mucosal CD14+ cells, natural killer T (NKT) cells21-24are additional cell targets of IL-25. Th9 cells express IL-17RB and are able to respond to IL-25 by enhancing the production of IL-9.25Type 2 myeloid (T2M) cells have IL-25 receptor subunit IL-17RB, which is a key mediator of both innate and adaptive pulmonary type 2 immune responses.26 IL-25 is found at very low levels in various tissues (brain, kidney, lung, prostate, testis, spinal cord, adrenal gland, trachea at an mRNA-level). Highest expression levels were detected in the gastrointestinal-tract and uterus.
Role in immune regulation and cellular networks
IL-25 initiates, promotes and augments Th2 cell-mediated immune responses, and inhibits both Th1 and Th17 cell responses.11, 13, 16, 21 IL-25 seems to have two distinct functions in immune responses: one as an initiator of responses causing allergic diseases, by increasing the expression of IL-4, IL-5 and IL-1311, and the other as an attenuator of the destructive inflammation found in diseases such as inflammatory bowel disease, diabetes and multiple sclerosis. Intra-peritoneal injection of IL-25 in mice leads to an increase in eosinophilia, splenomegaly and splenic plasma cell numbers. Moreover, IgE and IgG1 increase.11 IL-25 induces Th2 associated cytokines (IL-13 all tissues; IL-4, IL-5 spleen). Nevertheless, it is important to mention that IL-25 leads to pathological changes solely in mucosal tissues, despite of gene expression in various other tissues. IL-25 administration induced epithelial hyperplasia in the esophagus and the non-glandular part of the stomach. Epithelial cells contained eosinophilic cytoplasmatic inclusions in gastric glands and pyloric epithelial cells. Furthermore, inflammatory infiltrates of eosinophils, neutrophils and mononuclear cells in epithelium and lamina propria of esophagus and stomach, sometimes in the serosa and submucosa are described. Within the small and large intestine: goblet cell hypertrophy and hyperplasia occurs and epithelia of the large biliary tract in the liver and the large pancreatic ducts were often vacuolated and contained eosinophilic cytoplasmic inclusions.11 Administration of IL-25 does not result in an increased IL-6 expression in the spleen, stomach or small intestine. No changes of IL-1α, TNF-α; IL-10; IFN-γ are described. In addition, IL-25 mRNA expression is not up-regulated upon IL-25 treatment. IL-25 had no impact on IL-4, IL-5 and IL-13 production of naïve T-cells or B-cells in the presence of LPS or CD40L.
IL-25 dependent actions are attributed to an IL-4, -5, -13 producing non T/non B-cell population (NTNB) that is ckitpos FcR1neg. The NTNB ckitpos FcR1neg cells were described as small agranular cells with limited cytoplasm with no marker expression of mature NK cells, mast cells, basophils or eosinophils. IL-25 has a key role in the initiation of Th2 type responses.27 Saenz et al. reported a lineage negative multi-potent progenitor cell population they called MPPtype2. This subset is Linneg c-Kitint Sca-1pos, expands upon IL-25 administration in the gastric lymphatic tissue and can give rise to cells of the monocyte/macrophage and granulocyte lineage. MPPtype2 cells lead to Th2 type responses and favor helminth expulsion.28 A similar fraction that also responds to IL-25 has been described in the fat associated lymphoid cluster.29Epithelial-specific Act1 mediates the expansion of the Lin-c-Kit+ innate cell population through the positive-feedback loop of IL-25, initiating the type 2 immunity against helminth infection.30
Neill and Wong et al defined a Linneg ICOSpos, ST2pos, IL17RBpos, IL7Rαpos, MHCIIpos IL-13-producing leukocyte subset that expanded upon IL-25 and/or IL33 administration. They were named nuocytes according to their cytokine predominance. Nuocytes represent the predominant IL-13 producing subset 5 days after N. brasiliensis infection and are crucial for worm expulsion. Transfer of nuocytes into IL17RA-/- established features of IL-25-evoked Th2 type responses and established an IL-25 response in the recipient mice.31
Human lineage-negative CD127+ CD161+ and CRTh2+ type 2 innate lymphoid (ILC2) cells are able to respond to IL-25 with T helper type 2 cytokine production and play important role in the pathogenesis of allergic diseases.20 Huang et al., described a different developmental pathway for ILC2 responsive to IL-25, they called it the inflammatory ILC2 pathway.18
A crucial role of IL-25 on Th9 cells has been suggested. IL-17RB is up-regulated on Th9 cells and IL-25 is capable of IL-9 induction in differentiated Th9 cells, but not naïve cells.32 In addition, IL-25 may interact with the IL-17A pathway. IL25-/- mice did not only display problems in reducing T. muris burden, but increased their IL-17A and IFN- levels.33 Moreover, IL25-/- mice are highly susceptible to experimental autoimmune encephalomyelitis (EAE) most likely via an up-regulation of IL-23.34 Treatment with IL-25 resulted in increased IL-13 secretion that opposed IL-17A induced EAE.
IL-25 directly inhibits LPS-induced IL-23 expression by macrophages and reduces the inflammatory cytokine response driven by toll-like receptor ligands in human blood monocytes and intestinal CD14+ cells.16, 21, 23 IL-25 inhibits the production of Th1-associated cytokines, and prevents and cures experimental murine colitis mediated by luminal bacterial-driven Th1 cell responses.21, 35
The importance of IL-25 in the negative control of pathogenic cell responses is also seen in studies of murine diabetes and multiple sclerosis (MS).17, 36 These results indicate that IL-25 may be an important counter-regulator of non-Th2-mediated inflammatory processes in various organs.
IL-25 has high anticancer activity without affecting nonmalignant mammary epithelial cells. Apoptotic activity of IL-25 is mediated by differential expression of its receptor, IL-25R, which was expressed in high amounts in tumors from patients with poor prognoses but was low in nonmalignant breast tissue. In response to IL-25, the IL-25R on the surface of breast cancer cells activated caspase-mediated apoptosis. Thus, the IL-25/IL-25R signaling pathway may serve as a new therapeutic target for advanced breast cancer.37
Role in allergic disease and other pathologic conditions
Recent data suggests a crucial role for IL-25 in asthma. IL-25 is expressed in the lungs of sensitized mice upon antigen inhalation. In contrast, administration of a sIL-25R inhibited antigen-induced eosinophil recruitment, CD4+T-cell recruitment, IL-5 and IL-13 production and goblet cell hyperplasia.38 Administration of anti-IL-25 mAbs reduced IL-5 and IL-13 production, eosinophilic infiltration, goblet hyperplasia and AHR. Interestingly, this antibody successfully prevented AHR during allergen challenge.9 Blocking of IL-25 before sensitization led to an abrogation of AHR after metacholine challenge.39 In a transgenic model with exclusive expression of IL-25 in the lung there is increased recruitment of eosinophils and CD4+ T cells upon allergen-specific stimulation as compared to allergen-challenged wild type, while the neutrophil and macrophage cell counts are similar. IL-4, IL-5 and IL-13 are increased in the BALF, whereas INF-γ is undetectable. Mucus secretion, TARC and eotaxin levels are also enhanced. However, IL-25 expression itself does not induce airway inflammation. Recently, IL17RB expressing CD4+ NKT cells were described to be responsible for airway hyperreactivity in an asthma model in mice.19, 24
IL-25 is elevated in asthma and contributes to angiogenesis, at least partly by increasing endothelial cell VEGF/VEGF receptor expression through PI3K/Akt and Erk/MAPK pathways.40
IL-25 plays an active role in driving the airway remodeling. IL-25 was shown to act directly on human fibroblasts to induce collagen secretion. Recruitment of endothelial progenitor cells to the lung and subsequent neovascularisation was also IL-25 dependent, demonstrating a direct role for IL-25 during angiogenesis in vivo. IL-25 appears to play a critical role in the induction of AHR regardless of the model used to induce allergic airways disease, and this has been shown to be independent of Th2 cytokine production.41
Epithelial overexpression of Smad2 can specifically alter airway hyperreactivity and remodeling in response to an aeroallergen challenge.42
Allergen exposure upregulates IL-25 and induces type 2 cytokine production in a previously undescribed granulocytic population, termed type 2 myeloid (T2M) cells. High-dose dexamethasone treatment did not reduce the IL-25–induced T2M pulmonary response. Similar IL-4– and IL-13–producing granulocytic population was identified in peripheral blood of human subjects with asthma. These data establish IL-25 and its receptor IL-17RB as targets for innate and adaptive immune responses in chronic allergic airway disease and identify T2M cells as a new steroid-resistant cell population.26
Clinical studies have shown reduced numbers of NK cells in infants suffering from a severe RSV infection. The authors demonstrated that NK cell deficiency during primary RSV infection of BALB/c mice results in the suppression of IFN-γ production and the development of an RSV-specific Th2 response and subsequent allergic lung disease. The outgrowth of the Th2 responses was dependent on airway epithelial cell-derived IL-25, which induced the upregulation of the notch ligand Jagged1 on dendritic cells. This study identifies a novel pathway underlying viral-driven Th2 responses that may have functional relevance to viral-associated asthma.43
IL-25 expression is present in nasal polyps of patients with chronic rhinosinusitis. Anti-IL-25 treatment is also suggested to be an effective treatment in this disease, since it reduced the number of polyps, inflammatory infiltrating cells and tissue remodeling in a murine model.44
IL-25-producing cells have been identified within the dermis of AD patients and propose that these cells are dendritic cells (DCs). IL-25 produced by DCs could have dual role as both an inducer of the TH2 response and as an inhibitor of filaggrin synthesis, thereby directly affecting skin barrier function in AD patients.45
Functions as demonstrated in IL-25-deficient mice, receptor-deficient mice and transgenic models
IL-25-deficient mice display a rather normal phenotype. Th2 type effector functions are normal with the exception of eosinophils. However, IL25-/- mice fail to expel Nippostrongylus Brasiliensis efficiently due to subtle changes in the onset of Th2 type cytokine responses.27 In wild type animals, the increase of a non T/non B-cell effector cell population (NTNB) that is ckitpos FcR1neg precedes the increase in Th2 type CD4 cells. Due to the prolonged presence of high Th2 cytokine levels, IgE is higher and the number of mast cells in mice with parasitic infestation is higher in IL-25 -/- deficient mice. IL-25 deficiency does not lead to a delay in goblet cell hyperplasia and has no effect on basophil counts. However, there is a deficit in circulating eosinophils (NBNT, CCR3+, SSC high). The delayed onset of Th2 type response to worms can be restored upon addition of IL-25, but IL-25 itself is not sufficient to expel the worms by its presence in IL-4-/-, -5-/-, -9-/-, -13-/- mice. Response to the parasite is characterized by a delayed onset and permanently high levels of these Th2 type cytokines due to persistent parasitic burden in IL-25-deficient mice.27
IL-25-deficient mice infected with Trichuris muris develop more severe intestinal inflammation than wild-type mice, accompanied by exaggerated production of IL-17A and IFN-γ.33 IL-25-deficient mice exhibit accelerated experimental autoimmune encephalomyelitis onset and enhanced disease severity compared with wild-type mice.34 Eosinophil cell counts and the levels of proinflammatory mediators in bronchoalveolar lavage fluid, airway hyperresponsiveness to methacholine, and the OVA-specific IgG1 and IgE levels in the serum are all significantly suppressed in IL-25–deficient mice during OVA-induced allergic airway inflammation.46
IL-25 over-expressing mice displayed splenomegaly, lymphadenopathy and a strong increase of eosinophils and B-cells in peripheral blood.47 Due to the fact that B-cells decrease in parallel in the bone marrow, it could be assumed that IL-25 contributes to the release of premature B-cells into the periphery. These B-cells do not proliferate upon IL-25 exposure, but they up-regulate their IL-17E receptor expression. In accordance with the increased B-cell frequencies in the periphery, IgM, IgG and IgE levels are elevated. In addition, the levels of various cytokines are substantially increased (IL-2, IL-4, IL-5, IL-6, IL-10, INF-γ, G-CSF, GM-CSF and eotaxin).47 Pan et al. reported similar results. However, their transgenic mice showed marked growth retardation and a more pronounced inflammation of organs.48 Experiments with adenovirus expressing IL-25 or IL-25 alone resulted in Th2-like reactions with elevation of mRNA of IL-4, -5, -13 and production of the chemokines LIX, TARC and eotaxin. In addition, vascular changes (rather distal) in the lung with moderate media hypertrophy, presence of eosinophils in the lumen and infiltrates with eosinophils and monocytes beneath the endothelium within the vessel wall and adjacent to the vessels were described. Bronchi and larger bronchioles epithelium is thickened and contains large amounts of mucus. Epithelial cells sometimes contained eosinophilic inclusions in the cytoplasma.1
In conclusion, IL-25 promotes Th2 cytokine-based immunity, causing asthma and allergy, and at the same time is responsible for airway remodeling and also capable of limiting inflammation associated with debilitating human diseases, such as inflammatory bowel diseases, diabetes and multiple sclerosis.11, 13, 17, 21, 36 This dual role of IL- 25 is strictly linked to its ability to be immune-stimulatory for Th2 type responses and suppressive for the development and/or amplification of Th1/Th17-associated immunity.11, 13, 16, 21IL-25 responses are impaired in Traf4-deficient cells, showing implications in allergic airway inflammation. 49
IL-26
Discovery and structure
IL-26 was discovered in a study investigating the phenotypic changes of T cells after transformation by Herpes saimiri virus.IL-26 was identified and initially termed AK155 by using the technique of subtractive hybridization for cloning cDNA transcripts that are specifically present in transformed human T cells and not in untransformed cells. 50 IL-26 shows 24.7% identity to IL-10 and is suggested to have a closely related structure consisting of six -helices. The observed molecular weight of monomeric IL-26 is 19 kDa. Gel electrophoresis under native conditions revealed that IL-26 spontaneously forms dimers.
The gene encoding IL-26 is located on chromosome 12q15, in close proximity to the genes for IFN- and IL-22. Therefore, common regulatory mechanisms can be expected. Interestingly, several groups failed to identify a murine homolog of IL-26 and apparently this gene is missing in mice and rat. Only for exon 5, a section sharing weak homology with human IL-26 could be detected. This sequence is disrupted by several stop codons. Unexpectedly, IL-26 has been found in zebra fish, chicken and frog, suggesting an evolutionary conservation despite the lack of IL-26 in rodents. 51, 52
Receptor and signalling
The receptor for IL-26 consists of two chains. 53, 54 One chain is the IL-10R2, which also belongs to other receptor complexes of this cytokine family, namely the receptors for IL-10, IL-22, IL-28 and IL-29. The second chain is the IL-20R1, which is also required for binding the cytokines IL-19, IL-20 and IL-24. As IL-26 is suggested to form dimers, it is likely to bind to two IL-10R2 and two IL-20R1 chains, although this has not been proven yet. IL-26 seems to bind first to IL-20R1, thereby inducing dimerisation of the receptor chains. This dimerisation activates the Jak/STAT signalling pathway, resulting in rapid phosphorylation of STAT1 and STAT3.
Cellular sources and targets
IL-26 expression has been found to be restricted to memory T cells after T cell receptor stimulation, and to NK cells. 55 Among the T cells, CD4+ cells produced higher levels of IL-26 than CD8+ cells. Recently, IL-26 has been found to be specifically expressed by the Th17 subset of T cells 56,57, a subset involved in many inflammatory and autoimmune disorders. The finding that IL-26 is expressed by these cells expands the research of Th17-dominated diseases and underlines a difference between human and mouse Th17 cells, as IL-26 is not present in mice. Similar things have been observed in NK cells. A recent study identified a distinct subset of human NK cells that specifically express IL-22, and that have therefore been termed NK-22 by the authors. 58 This subset, located in mucosa-associated lymphoid tissues, such as tonsils and Peyer's patches, also produces IL-26, but in contrast to Th17 cells no IL-17A and IL-17F. The co-expression of IL-22 and IL-26 is therefore commonly observed and not surprising, considering the close chromosomal location of these genes.
As the IL-10R2 chain is ubiquitously expressed, fitting to its role as common part of several cytokine receptors, it is likely that the IL-20R1 chains account for the cell specificity of IL-26 signalling. 55 In contrast to IL-10R2, IL-20R1 is not detected in immune cells, but on several epithelial cell lines and many tissues like skin, testis, heart, placenta, salivary gland, and prostate. 59, 60 Activation of STAT1 or STAT3 upon IL-26 stimulation has been observed for example in the colorectal adenocarcinoma cell line HT-29 or the epithelial keratinocyte line HaCaT. 53, 54 Therefore it seems that while immune cells are major producers of IL-26, its targets are non-immune cells like epithelial cells from various tissues.