IL-13
Discovery and structure
IL-13 was first described in 1989 as P600, a protein expressed by activated mouse Th2 cells1 and has been cloned in 1993.2-4 It has a molecular weight of 10kDa and is encoded in a cytokine gene cluster on chromosome 5 (5q31) that includes genes encoding for IL3, 4, 5, 9 and GM-CSF. The structure of IL-13 consists of a four-helix bundle with a characteristic up-up-down-down topology that includes a b-sheet formed between residues in the AB-loop and CD-loop. 5, 6 It shows considerable similarity to IL-4 and is highly conserved with little species-specificity. 3
Receptor and signaling
IL-13 has two known receptors: IL-13R1a1 and IL-13R1a2.7 It signals, together with IL-4 through the IL-4 receptor complex type II that consists of the IL-4Ra and IL-13Ra1. Interleukin 13 binds with its helical faces to the elbow shaped “cytokine binding homology region”. This driver complex and recruits its trigger complex IL-4Ra. Recently, La Porte et al defined the structural basis of type I and type II ternary signaling complexes and the subtle differences that result IL-4 and IL-13 receptor interactions leading to type II complex formation.8 IL-13Ra1 bears an evolutionary relationship to gc, but it contains an extra N-terminal Ig-like domain (D1) not found in other receptors of the gc subfamily that is required for IL-13, but not for IL-4 signaling. For its binding specificity, the IL-13R1D1 domain has been defined as the crucial part type II complex formation that results in IL-13 signaling. IL-4-mediated signaling via this complex is not affected by mutations of this domain. IL-4 binds to IL4Ra with high affinity whereas the recruitment of either gc or IL-13Ra1 contributes little affinity. On the contrary binding of IL-13 by IL13Ra1 occurs with moderate affinity, but its affinity is significantly enhanced by the presence of its ligand, the IL-4Ra. Consequently the expression patterns of the receptors are decisive to define signaling efficiency of IL-4 and IL-13. In general, IL-4Ra is limited however e.g. in cells in which IL-13Ra1 is limiting the higher affinity for its trigger receptor may lead to more potent IL-13 signaling.8 Another important factor is the presence of the
γ-chain that allows IL-4 to signal via the type I complex. Mainly macrophages and hematopoietic cells express both the γc and IL-13R1α. Consequently γc-expression results in a 10-100 fold higher sensitivity of bone marrow derived macrophages and monocytes to IL-4 than to IL-13.9
After binding of IL-13 to IL13Ra1, the IL-13 type II receptor complex is formed and leads to janus kinase mediated phosphorylation of STAT6.10 This is considered to be the major mode of action. However, recently IL-13 has been demonstrated to act also via the mitogen-activated protein kinase pathway. The activation of ERK 1/2 in mouse model with transgenic IL-13 expression in the lung was demonstrated.11 Inhibition of the ERK 1/2 MAPK pathway decreased the expression of several chemokines MIP-1α/CCL-3, MIP-1β/CCL-4, RANTES/CCL-5, the matrix metalloproteases MMP-2, -9, 12, -14 and cathepsin B and leads to the upregulation of a1-antrypsin. MIP-2/CXCL-1 expression was exclusively ERK 1/2 dependent. Moreover, remodeling and recruitment of eosinophils was also affected by IL-13-dependent activation of ERK 1/2. IL-13-induced eotaxin/CCL-11; MIP-1/CCL-2 and C10/CCL-6 mRNA production, IL-13 receptor regulation,15-lipoxygenase regulation (in vitro experiments) and mucus cell metaplasia (in vivo) in an ERK 1/2 independent manner.11 In addition to ERK1/2, JNK-dependent effects have been suggested to play a role on AHR by impacting Ca++-influx in airway smooth muscle cells.12
The second receptor, IL-13Ra2, is assumed to act as a decoy receptor. It binds IL-13 with a higher affinity than IL-13Ra1 and is considered to serve as potent inhibitor of IL-13 induced actions.13, 14 Indeed, TNF-a/IL-17 synergy inhibits IL-13 bioactivity via IL-13Ra2 induction.15 In a double-knockout asthma model (IL10 -/- IL13Ra2 -/-) an additive inhibitory effect of IL-13Ra2 on IL-13-induced airway hyperreactivity, mucous production, inflammation and fibrosis with IL-10 was observed.16 Furthermore, a role of IL-13Ra2 in IL-13-induced fibrosis by TGF-ß1 in the presence of TNF-a has been demonstrated in mouse models for oxazolone- or TNBS-induced colitis and bleomycin-induced lung fibrosis. After an initial step of upregulation of IL-13Ra2 by IL-13 (via the IL-13Ra1), TNF-a mediates fibrosis, which is induced via IL-13Ra2 in an STAT-6 independent fashion via AP-1.17, 18 There is some evidence that the N-linked glycosylation of IL-13Ra219 plays a decisive role for IL-13Ra2-mediated inhibition.
Cellular sources and targets
IL-13 is produced by T cells, mast cells, basophils, eosinophils and NKT cells, Th2 cells.20, 21 IL-13 production by Th2 cells is dependent on the transcription factor GATA-3.21 During chronic infections IL-13 is expressed by Th1 cells and the plasticity of the cytokine is regulate by transcription factor E4BP4.22 Major target cells are B cells, mast cells, epithelial cells, eosinophils, smooth muscle cells, fibroblasts, and macrophages.23, 24
Role in immune regulation and cellular networks
IL-13 is antagonized via the Th1 type cytokines INF-γ, IL-12, IL-18, TNF-α and the regulatory cytokine IL-10. IL-13 can induce class switching to IgG4 and IgE in combination with CD40 stimulation.25 The expression of CD23, MHC II is up-regulated on B cells in the presence of IL-13. In addition, the induction of the adhesion molecules CD11b, CD11c, CD18, CD29, CD23, and MHC II on monocytes takes place. Moreover IL-13 activates eosinophils and mast cells, recruits eosinophils and prolongs their survival.26
IL-13Rα1 is expressed on human CD4+Th17 cell, and IL-13 attenuates IL-17A production at polarization and restimulation. Although IL-13 is an attractive therapeutic target for decreasing symptoms associated with asthma, suggest that therapies inhibiting IL-13 production could have adverse side effects by increasing
IL-17A production.27, 28
Let-7 family of microRNAs inhibit IL-13 expression and represent a major regulatory mechanism for modulating IL-13 secretion in IL-13-producing cell types.29
Role in allergic disease
OVA-transgenic Th2-type T cells from IL-13-deficient mice fail to induce AHR in B and T cell deficient mice in the presence of airway eosinophilic infiltrates, IL-4 and IL-5.30 Thus, IL-13 itself is sufficient to induce AHR. IL-13 dependent AHR and mucous secretion was induced independently of IL-5 or ECP. However, the presence of IL-5 and ECP seems to be crucial for maintenance of IL-13 production of Th2 cells. Chiba et al reported an IL-13-induced upregulation of a monomeric GTP-binding protein called RhoA, which is supposed to be involved in increasing the sensitivity of myofilaments to Ca++.31 Interestingly, both, an IL4Ra and even a STAT6 independent pathway through which IL-13 induces AHR and mucous secretion have been suggested.32, 33
The role of IL-13 on asthma is supported by epidemiological data. A combination of polymorphisms of the IL4/IL13 pathway increased the risk to develop asthma 16.8-fold.34 Polymorphisms restricted to IL-13 itself lead to a higher frequency of asthma exacerbations in childhood and elevated total IgE and blood eosinophilia.35 In addition, the IL13 -1112C/T and +2044A/G polymorphisms are risk factors for asthma. IL-13 also contributes to allergic rhinitis late phase responses, whereas its impact on acute response appears to be limited in vivo.36
In addition, IL-13 plays an important role in tissue remodeling and fibrosis.37-39 In a S. mansoni infection model IL4R-/- and STAT6-/- mice had a significantly reduced granuloma size and reduced fibrosis, whereas ablation of IL-4 had no effect. Thus, suggesting a key role of IL-13 in this process. Moreover, IL-13 is a potent inducer of collagen production in fibroblasts. Furthermore, it is able to induce Arginase 1 in macrophages that are termed alternatively activated. This Arginase uses L-arginine as a substrate to make L-ornithine and is converted to Proline and polyamines. Proline is known to be necessary for the development of fibrosis. Increased polyamine production in the presence of IL-13 leads to rat aortic smooth muscle cell proliferation that can be inhibited via dexamethasone treatment.40
Role in host defense or other immune regulatory conditions
IL-13 plays a unique role in parasite defense. IL-13-deficient mice are unable to expel Nippostrongylus brasiliensis. Importantly, this defect is more pronounced in IL-13-/- mice than in IL4 deficient mice, with comparable IL-5 levels as wild type mice.41, 42
Anti-proliferative effects (human breast cancer cells, renal cell carcinoma, B-All), as well as promotion of proliferation by inhibition of Th1 type tumor rejection via the STAT6 pathway, were reported for IL-13. Thus, blocking or antagonizing IL-13 or targeting of IL-13Ra1-expresssing cells may serve as a promising anti-cancer immunotherapy.43, 44 In addition, targeting of IL-13Ra2 could be a useful treatment for inhibiting squamous cell carcinoma.45 However, a detailed understanding of IL-13 in case of malignancies is needed.
IL-13-induced IRAK-M suppress airway epithelial TLR2 signaling, partly through inhibiting activation of nuclear factor kB.46
Recently, signaling pathway from chloride channel calcium-activated 1(CLCA1) to MAPK13 was define, that is responsible for IL-13-driven mucus production human airway epithelial cells. The same pathway is also highly activated in the lungs of humans with excess mucus production due to COPD. Development of inhibitors guided by structural analysis of MAPS13-inhibitor cocrystals and scaffold offers the opportunity to better define the regulation of a major disease end point and to respond to the unmet need for an antimucus therapeutic for inflammatory airway disease.47
Functions as demonstrated in IL-13-deficient mice, receptor-deficient mice and transgenic models.
IL-13-deficient mice produce reduced levels of IL-4, -5, -10 and IgE. They are unable to expel Nippostrongylus brasiliensis. Moreover IL-13-/- mice fail to mount a profound goblet cell hyperplasia although IL-4- and IL-5-producing cells are present. Mast cell cytokine production is unaffected in knockout mice upon stimulation with PMA and Ionomycin.41, 42 Interestingly, IgE levels do not change in IL-13-deficient mice.
Recently, IL-13Ra1-deficient mice have been generated.48, 49 Mice were healthy and showed no fundamental changes in the lymphocyte compartment.
IL-13Ra1 -/- mice do not develop airway hyper-reactivity and mucous hyper-secretion, whereas soluble IL-13Ra2 and IL-13 were upregulated.48 Surprisingly, the percentage of CD4+ Th2 cells were reported to be significantly higher upon
S. mansonii infection in IL13Ra1-/- mice.49 In addition, significantly less hepatic fibrosis and a modestly increased frequency of eosinophils in granuloma related to a higher IL-5 production were reported.
In mouse model of IL-13-induced AD, IL-13 is a potent mediator in generating pathologic fibrosis in AD, and the TSLP and TSLPR signaling pathway is critical in the pathogenesis of the detrimental tissue remodeling and skin fibrosis in AD.50 Treatment of Id3-/- mice, which represent a model for T cell mediated primary Sjogren’s syndrome, with anti IL-13 antibodies over a two-month period resulted in a reduction of both serum IL-13 levels and the number of mast cells in the salivary gland tissue.51
TGF-β has been linked to IL-13-related fibrosis. TGFß1 and MMP-9 were increased in IL-13Ra1-/- mice when challenged with schistosoma antigen.49 Thus,
IL-4R complex II seems to be involved in the activation of fibroblasts and epithelial cells. Moreover, IL-13Ra1 knockout mice had lower49 or non-detectable48 serum IgE levels without any changes of other Ig titers.
Specific over-expression of IL-13 in the lung leads to typical features of asthma including pulmonary eosinophilia, airway epithelial hyperplasia, mucous cell metaplasia, sub-epithelial fibrosis, Charcot-Leyden like crystals, airway obstruction, non-specific airway hyper-responsiveness to cholinergic stimulation.52
Development of peanut-induced intestinal allergy in mice is mediated through a mast cell-dependent, IgE-FcεRI-IL-13 pathway. Targeting IL-13 may be a potential treatment for IgE-mediated peanut allergic responses in the intestine.53
IL-4 and IL-13 together account for most allergen–induced pulmonary genes in the mouse lung. The identification of genes selectively induced by individual cytokines, especially IL-13, may provide novel therapeutic targets for the treatment of asthma.54-56
In monkey model of asthma, the dual IL-4/IL-13 antagonist, pitrakinra, significantly reduces allergen-induced AHR with only limited inhibition of allergen-induced airway eosinophilia, indicating the therapeutic potential in the treatment of atopic asthma.57
Clinical use
CAT-354 is a potent and selective IL-13-neutralizing IgG4 mAb. In vitro CAT-354 functionally inhibit IL-13 induced eotaxin production, an analogue of smooth muscle airways hyperresponsiveness, CD23 upregulation and IgE production. CAT-354 inhibited airways hyperresponsiveness and bronchoalveolar lavage eosinophilia in humanized mouse and cynomolgus monkey antigen challenge models.58
Humanized neutralizing mAb blocks IL-13 activity in patients with mild atopic asthma (IMA-638 and IMA-026, Pfizer, New York, NY). IMA-638 specifically prevents recruitment of the IL-4Rα subunit to complex with IL-13 Rα1 after initial binding of
IL-13 to the IL-13Rα1 subunit. IMA-026 inhibits the initial interaction between IL-13 and the IL-13Rα1 and IL-13Rα2 subunits. The effect of antibodies might have significant clinical implications in human asthma and eosinophilic esophagitis.59-62 Studies of both tralokinumab and lebrikizumab suggest that IL-13 inhibition might be most effective in patients who have evidence of residual IL-13 activity with persistent Th2 inflammation.63, 64 However, anti-IL-13 mAb (GSK679586) did not demonstrate clinically meaningful improvements in patients with severe asthma.65 Therefore, further studies are needed to determine whether therapies targeting IL-13 can provide clinical benefit in patients with severe refractory asthma.
IL-14
Alternative names: high molecular weight B cell growth factor (HMW-BCGF, BCGF-H)
Discovery and structure
Several years ago, IL-14 was originally discovered as a high molecular weight B cell growth factor with a molecular size of 60 kDa 66 which differs from the human low molecular weight B cell growth factor (LMW-BCGF). Human IL-14 is a member of B cell growth factors including IL-2, IL-4, IFN-α, IFN-β, IFN-γ, TNF-α and LMW-BCGF. There is evidence that IL-14 is eventually a precursor for the low molecular weight B cell growth factor, which has a molecular weight of 12 to 14 kDa.67 The human IL-14 gene maps on chromosome 1p34-6, the murine IL-14 gene is located on chromosome 4. Two transcripts are produced from opposite strands of the IL-14 gene termed IL-14a and IL-14b.68
IL-14 as originally cloned from a Burkitt lymphoma cell line and cloning of IL-14 cDNA revealed a 53 kDa protein composed of 498 amino acids including a signal peptide of 15 amino acids and three potential N-linked glycosylation sites. The sequence is rich in cysteines.69 Concerning the sequence, no homology to other interleukins and cytokines has been observed and its 3D structure has not been yet solved.
Receptor and signaling
IL-14 binds to a 90 kDa receptor, identified by using a monoclonal antibody (BA5). This monoclonal antibody recognizes a binding site for IL-14 on solubilized membranes of activated B cells. A high correlation between the expression of the receptor and capacity of the cells to react on stimulation was seen. The expression of IL-14R is very low on resting normal tonsillary or peripheral blood B lymphocytes (50-350 IL-14R/cell), hence these cells do not respond efficiently to IL-14, whereas activated normal tonsillary or peripheral blood B lymphocytes express much more IL-14R (1-5x104 IL-14R/cell) and proliferate in the presence of IL-14 .70 However, it is still unknown in which stage of human B cell ontogeny the B cell lineage cells express the receptor and are able to respond to IL-14 stimuli.