Abstract
Interleukin 33 (IL-33) is a newly described member of the IL-1 superfamily of cytokines. Through activation of the ST2 receptor, which is widely expressed particularly by helper T 2 cells and mast cells, IL-33 is involved in T-cell–mediated immune responses. Many previous studies have demonstrated that IL-33 may have a pleiotropic function in different diseases, and it could represent a novel target for the treatment of a range of diseases. Recent works have explored the role of IL-33 in chronic autoimmune diseases, such as systemic sclerosis, inflammatory bowel disease, rheumatoid arthritis, and systemic lupus erythematosus. These results indicate that IL-33 may contribute to the pathogenesis of chronic autoimmune diseases. Hence, in this review, we discuss the biological features of IL-33 and summarize recent advances on the role of IL-33 in the pathogenesis and treatment of autoimmune diseases.
Interleukin (IL)-33 is a newly identified cytokine of the IL-1 family, which has been identified as a ligand of the orphan receptor ST2. ST2 is formed from heterodimeric molecules composed of ST2 and IL-1R accessory protein (IL-1RAcP).1,2IL-33 has been proposed to be a regulator of inflammation and helper T 1 (TH1)/TH2 balance. In fact, cytokines that mediate TH1 type of response have been revealed to contribute to the induction of pathogenic autoantibody isotypes leading to tissue pathology. At the same time, TH2 type of response may be involved in driving B-cell hyperactivity.3–8In addition, the antibodies have an important pathogenic role in autoimmune diseases and may be a useful parameter for diagnosis and follow-up of autoimmune diseases. The role of antibody may be associated with cytokines.
Nevertheless, whether IL-33 plays a role in the pathogenesis of autoimmune diseases, and which property it works with are still unclear. Therefore, in this review, we will discuss the IL-33 pathway and the therapeutic potential of modulating the pathway in autoimmune diseases.
INTERLEUKIN 33
IL-33 is the 11th described member of the IL-1 family of immunoregulatory cytokines in human, and the group of cytokines comprises IL-1α, IL-1β, IL-18, and IL-1Ra. In 1999, IL-33 (also known as C9ORF26, DVS27, NF-HEV, and IL-1F11) was first cloned by Onda et al.9and Boraschi and Tagliabue10from canine vasospastic arteries induced by subarachnoid hemorrhage. Researchers deduced the sequences of the gene by expressed sequence tag alignment, which mapped to human chromosome 9p24.1 and mouse chromosome 19qC1.1The translated protein pro-IL-33 (270 and 266 amino acids) is a 30-kd prodomain,1and it indeed adopts αβ-trefoil fold, which is similar to that described (R1-q1) for IL-1α, IL-1β, IL-1Ra, and IL-18.11,12IL-33 messenger RNA (mRNA) was found in human endothelial cells, high endothelial venules, and in adipocytes and synovial fibroblasts.13–15At the beginning, it was not very clear about cellular sources of IL-33 protein in vivo. However, recent experiments demonstrated that the vasculature is the dominant source, especially the endothelial cells. In addition, IL-33 has also been demonstrated in macrophages,1,16mast cells,1,17dendritic cells (DCs),1skin keratinocytes, epithelial cells, cardiac fibroblasts, cardiomyocytes, and fibroblastic reticular cells of lymphoid tissues.18,19Endothelial cells and epithelial cells constitute major sources of IL-33 in vivo. Moussion et al.19speculated that IL-33 may function as a novel "alarmin" (an endogenous danger signal to alert the immune system after endothelial or epithelial cell damage during trauma or infection). IL-33 was released by necrotic, but not apoptotic, cells and can recruit and activate immune cells.19Inside most cells, IL-33 is more concentrated in the nucleus of cells rather than in the cytoplasm, and it associates with heterochromatin and mitotic chromosomes with transcriptional repressor properties.16Therefore, it has been shown that IL-33 played a role in the intracellular transcriptional regulation, and IL-33 has been always depicted as a nuclear factor. Furthermore, a number of researches have mentioned that the secretion of mature IL-33 in a monocyte cell line THP-1 cells in response to certain stimuli: lipopoly- saccharide (LPS), the adjuvant alum and LPS, and infection (Listeria monocytogenes and Salmonella typhimurium).20,21IL-33 were released from cultured astrocytes activated by LPS and adenosine triphosphate,22and it has also been described in cardiac fibroblasts stimulated with phorbol 12-myristate 13-acetate.23
Of note, IL-33 is considered unique in the IL-1 family, such as processing and biologically active domains. Although IL-1β and IL-18 require maturation by caspase-1 for optimal biological activity, caspase-1 processing could lead to inactivation of IL-33, rather than activation. Caspase-1 cleaves at residue 178 and inactivates IL-33. Mature IL-33 is the form cleaved at 112. Thus, caspase-1 cleavage is not required for IL-33 secretion and bioactivity.20,24Moreover, caspase-3 and caspase-7 could cleave pro-IL-33 during apoptosis, whereas IL-33 processed by these caspases does not express biological activities.24,25In a recent study, Hayakawa et al.26demonstrated that calpain could mediate pro-IL-33 processing in human epithelial cells, suggesting that calpain may be a candidate to mediate pro-IL-33 processing in vivo.
IL-33 RECEPTOR AND SIGNALING
The cytokine receptor ST2, which is a member of the IL-1R family, has been proposed to be a receptor for IL-33. ST2 was identified as an orphan receptor in 1989, and the gene encodes at least 3 isoforms by differential splicing—the longest of them is a signal-mediating receptor (ST2L), which contains 3 immunoglobulinlike domains, a transmembrane domain, and a cytoplasmic Toll/IL-1R homologous region (TIR). The soluble receptor (soluble ST2) and a membrane-bound receptor lacking TIR domain (ST2V) are the other variants.27–29Over the last 15 years, investigation into the function of ST2 receptor revealed its participation in inflammatory processes, particularly regarding mast cells, type 2 CD4+ TH cells, and the production of TH2-associated cytokines.30Moreover, a number of studies established ST2 as a specific cellular marker that differentiated TH2 from TH1 cells.2,31
Yanagisawa et al.32found a 5-kb transcript that was identified with a putative transmembrane motif. The protein product of this transcript was confirmed to be the transmembrane receptor ST2L. More experiments indicated that ST2L is restricted to the surface of TH2 and mast cells,31,33and its expression was also described on some mesenchymal and epithelial cell types.23,29IL-33 binds specifically to this ST2L receptor.
The soluble ST2 was obtained by differential mRNA processing. It lacks the transmembrane and cytoplasmic domains contained within the structure of ST2L and includes 9 additional amino acids, which are present at the C-terminus of the molecule.34Although ST2L mediates the effect of IL-33 on TH2-driven immunopathology, the notion that expression of soluble ST2 might serve as a physiological mechanism in the attenuation of TH2 inflammatory response could even down-regulate macrophage-dependent inflammation.35Furthermore, similar to that of IL-1R2 in IL-1 signaling, the role of soluble ST2 was formally demonstrated to act as a decoy receptor for IL-33. The expression of soluble ST2 was readily triggered in fibroblasts, macrophages, and monocytes stimulated with LPS, tumor necrosis factor (TNF)-α or IL-1, as well as in activated TH2 clones.30
Finally, ST2V is a splice variant of ST2 as well. Loss of the third immunoglobulin motif and alternative splicing leads to a change in the C-terminal portion of ST2, causing it to gain a hydrophobic tail instead of the third immunoglobulinlike domain, produces ST2V. However, the functions of ST2V have not been clearly elucidated.28,36
As is well known, the signaling mechanisms of the IL-1 family of cytokines are typically mediated by their binding to a specific receptor followed by the recruitment of a coreceptor.37IL-33 binds a heterodimeric receptor complex consisting of ST2L and the coreceptor IL-1R accessory protein (IL-1RAcP) and recruits a number of signaling molecules through the TIR domain of IL-1RAcP.38–40This receptor complex could recruit myeloid differentiation primary response protein 88, IL-1R–associated kinase 1 (IRAK1), IRAK4, and TNF receptor–associated factor 6 (TRAF6).1These signaling components induce the activation of numerous signaling proteins, such as extracellular signal-regulated kinase 1 (ERK1; also known as mitogen-activated protein kinase [MAPK]3), ERK2 (also known as MAPK1), p38 (also known as MAPK13), Jun N-terminal kinase-1 (JNK1; also known as MAPK8) and nuclear factor-κB.1,41Moreover, TRAF6 is essential for IRAK recruitment and subsequent activation of p38, JNK1, and nuclear factor-κB, but not ERK proteins.42Therefore, there is a potential separate TRAF6-independent pathway.
IL-33 IN AUTOIMMUNE DISEASES
More recently, there is increasing evidence that IL-33 may play an important role in the development and pathogenesis of chronic autoimmune diseases, such as systemic sclerosis, inflammatory bowel disease, rheumatoid arthritis, and systemic lupus erythematosus.
Inflammatory Bowel Disease
Ulcerative colitis (UC) was characterized to associate with a TH-type cytokine profile. A higher level of mucosal IL-33 mRNA expression was found in patients with active UC.43Beltrán et al.3showed higher expression of mucosal IL-33 protein in active UC than in patients with Crohn disease (CD) and control subjects. Kobori et al.44showed that mucosal IL-33 mRNA levels were significantly increased in patients with active UC. However, it was not detected in inactive UC, active and inactive CD, or infectious or ischemic colitis.43Therefore, they implied that the evaluation of mucosal IL-33 levels could be a potential biomarker for active UC.44Moreover, Pastorelli et al.45reported that IL-33 mRNA levels were correlated with disease severity in SAMP mice, a mixed TH1/TH2 model of inflammatory bowel disease (IBD). They also found increased IL-33 and ST2 expression in inflamed colonic mucosa and serum of patients with IBD. IL-33 seems to play a critical role in IBD and experimental colitis.45
Rheumatoid Arthritis
Collagen-induced arthritis (CIA) in susceptible DBA/1 mice is a predominantly TH1-mediated disease whose immunopathogenesis is similar to clinical rheumatoid arthritis (RA). In the previous study, anti-ST2 antibody exacerbated CIA in DBA/1 mice.2However, the founding was considered as a result of complement-dependent TH2 clone lysis rather than an effect of the antibody on ST2 signaling. Now, IL-33 is detected in the joints of mice with CIA and increased during the early phase of the disease. Treatment with anti-ST2 antibody decreases the severity of CIA. Blockade of the IL-33 pathway significantly attenuated the severity of CIA and decreased the IFN-γ production by draining lymph node cells from anti-ST2 antibody–treated mice induced by costimulatory blockade. In addition, a significantly decreased production of IFN-γ and TNF-α by ex vivo–stimulated spleen cells was observed after treatment of soluble ST2-Fc in mice with CIA.46Leung et al.47have investigated the therapeutic effect of soluble ST2-Fc in the murine model of CIA. In this experiment, administration of soluble ST2-Fc fusion protein reduced serum levels of TNF-α, IL-6, and IL-12 compared with cells from the control mice. They also found that pretreatment with soluble ST2-Fc significantly suppressed the capability of human monocytic THP1 cells to release TNF-α when cultured with peripheral blood T cells from rheumatoid patients.47IL-17 production by draining lymph node cells was reduced in anti-ST2–treated mice, indicating that IL-33 is able to enhance TH17 responses.46TH17 cells have drawn much attention in recent years because they seem to be principal mediators of etiology in human autoimmune diseases, such as RA. Several researches have indicated the development of pathogenesis of RA may be associated with the TH17 cytokines including IL-17, IL-21, and IL-22. Actually, IL-33 is abundantly expressed in rheumatic synovium as measured by in situ hybridization.15Xu et al.48showed that IL-33 was expressed in synovial fibroblasts in RA and that mast cells were linked with IL-33–mediated arthritic disease. It may play a crucial role in mediating antigen-induced mechanical hypernociception and induce articular movement limitation in RA.49IL-33 can exacerbate autoantibody-induced arthritis by inducing mast cell degranulation and proinflammatory cytokine production. It drives both cellular and humoral arthritic responses via an arthritogenic pathway.50Matsuyama et al.51also investigated the existence of IL-33 in patients with RA. The expression levels of IL-33 were increased both in sera and synovial fluid (SF) samples, and associated with disease activity. It was produced mainly in inflamed joints.51
Furthermore, IL-33 is a potent chemoattractant for neutrophils in antigen-induced arthritis. Tumor necrosis factor α seems to contribute to local inflammation by inducing ST2 expression on neutrophils, which then migrate to the site of inflammation in response to IL-33. The study by Verri et al.17implied that suppression of ST2 expression in neutrophils could prevent IL-33–induced neutrophil migration. It may be an important therapy of TNF targeting in RA.17
Systemic Lupus Erythematosus
Cytokine-mediated immunity contributes to the pathogenesis of systemic lupus erythematosus (SLE) and other autoimmune diseases. IL-33 may be involved in the release of proinflammatory cytokines, which have been confirmed as major effector molecules in SLE. For example, IFN-γ has been confirmed that its hyperproduction plays an important role in lupus development. The complex mechanism might be involved in increasing major histocompability complex expression, promoting local immune and inflammatory processes.52–54In a recent work, comparing with wild-type mice, ST2-/- mice developed significantly attenuated disease with reduced production of inflammatory cytokines IFN-γ, TNF-α, and IL-17 in T-cell recall responses.48Anti-ST2 antibody treatment was associated with a marked decrease in IFN-γ production. Furthermore, depending on the proinflammatory cytokine IL-12, IL-33 induced a preferential increase in IFN-γ rather than IL-4 production by both invariant natural killer T (iNKT) and NK cells.55Meanwhile, as a potent inducer of adaptive immunity to intestinal nematodes, IL-33 was also characterized to increase the production of IFN-γ by NK cells in a model of parasitic infection in severe combined immunodeficient mice.56Indirectly via TNF-α and IL-1β, IL-33 can induce the production of IFN-γ in vivo in a model of antigen-induced arthritis.49Smithgall et al.57showed that IL-33 was able to enhance the production of IFN-γ in an antigen-dependent and antigen-independent manner in human NK and iNKT cells. Besides, IL-33 was shown to promote myeloid DC generation in vitro by triggering GM-CSF production by other BM cells, likely basophils.58Dendritic cells constitutively express ST2 and are activated by IL-33.59IL-33–activated DC may promote naive CD4+ T cells to express ST2, allowing them to respond directly and robustly to IL-33. The ability of IL-33 to target DC highlights its additional indirect role in the induction of adaptive immune responses. In summary, IL-33 may not only act through a TH2-oriented pathway, but it can also amplify TH1-type response under certain conditions. Moreover, in the IL-33–treated mice, Xu et al.48showed that there was an increase in IL-17 production from antigen-stimulated draining lymph node, and antigen-specific antibodies are increased in the serum. Palmer et al.46reported that IL-17 production by draining lymph node cells was inconspicuously reduced in anti-ST2–treated mice. However, IL-33 had no direct effect on the induction of TH17 cells in vitro, so further studies are imperative to investigate the functional role of IL-33 on the induction of TH17 cells in SLE.46,48Several studies found association of genetic polymorphism, which can alter the TH1/TH2 balance, with susceptibility to SLE.60The hypothesis that individual TH1/TH2 balance may play a critical role for histopathology of lupus nephritis has been previously reported.61More recently, Shah et al.62showed that TH17 cells were increased in the peripheral blood of patients with SLE, and the frequencies of these cells correlated with disease activity. They indicated that the balance of TH17 and TH1 cell responses is dysregulated in SLE, leading to enhanced TH17 cell response. The authors deduced the factors of the mechanism involved in balancing both types of cell response in SLE.62Overall, IL-33 may have a potential role in the pathogenesis of SLE.
Systemic lupus erythematosus is a complex chronic inflammatory autoimmune diseases, which causes multiple tissue and organ damage. Recent researches indicate that IL-33 may play an important role in the induction and development of complications. For example, neuropsychiatric SLE (NPSLE) involves the clinical neurologic manifestation of 19 diverse syndromes likely affecting diverse central nervous system (CNS) functions.63,64A variety of mechanisms such as hemorrhage, vasculitis, thrombosis, and cytokine-mediated damage might contribute to the pathogenesis of NPSLE.65Recently, cerebrospinal fluid (CSF) levels of IL-6 and monocyte chemoattractant protein (MCP)-1 (chemotactic cytokine ligand [CCL]-2) have been reported to be elevated in CSF from patients with NPSLE. Iikuni et al.66investigated the level of MCP-1/CCL2 in CSF and reported it to be significantly increased in patients with NPSLE. In addition, it has been reported that CSF inducible protein-10/MCP-1 ratios were higher in patients with NPSLE than in those without NPSLE, and it may be as a diagnostic marker of NPSLE.67Meanwhile, Trysberg et al.68reported neuronal and astrocytic damage in patients with SLE with CNS involvement, and they found a significant correlation between IL-6 levels and neurofilament triplet protein. They also suggested that IL-6 plays a potential function in vivo neuron damaging.68Therefore, IL-6 and MCP-1 are associated with NP manifestations in patients with SLE. Nevertheless, similar to ST2, as a cytokine, IL-33 has been reported to be expressed by varied cells and tissues of the CNS.1,69IL-33 exerts its influence on CNS glia, as it induces the secretion of cytokines including IL-6, MCP-1 (CCL2), and TNF.22These findings suggest that IL-33 induced the secretion of proinflammatory cytokines, which may play a potential function during the course of CNS lupus.
However, the exact role of IL-33 in the development of SLE has not been clearly defined. Mok et al.70detected a higher expression of soluble ST2 in patients with SLE, and it was found to correlate with disease activity and was sensitive to change. Soluble ST2 may act as a potential marker of disease activity. Nevertheless, serum IL-33 levels were not significantly different between the patients with SLE and the controls group. It was not related to soluble ST2 level, lupus disease activity, or specific organ involvement as well.70These findings seem to contradict with our viewpoint. However, most recently, Yang et al.71detected the serum IL-33 levels in Chinese population; although serum IL-33 levels did not correlate with most clinical and laboratory characteristics, it was obviously increased in patients with SLE when compared with healthy donors. This was contrary to the observation of Mok et al. The cause of this paradoxical pattern of change in IL-33 expression remains elusive; different assay kits and different populations may contribute to the disparity. Therefore, further studies with a large sample size and reliable method are required. Moreover, lupus-model mice should be used for investigating precise mechanism of IL-33.
Other Autoimmune Diseases
Systemic sclerosis (SSc) is a generalized connective tissue disorder characterized by sclerotic changes in the skin and internal organs. Systemic sclerosis is also generally a TH2-dominant autoimmune disease. Manetti et al.72have investigated the effects of IL-33 and ST2 on SSc and found that IL-33 and ST2 are abnormally expressed in SSc. Afterward, Yanaba et al.73showed that IL-33 levels were increased in patients with SSc compared with healthy individuals. Furthermore, they found that patients with diffuse cutaneous SSc had higher levels of IL-33 than those with limited cutaneous SSc. Moreover, pulmonary fibrosis and decreased forced vital capacity were more commonly found in patients with elevated IL-33 levels. Collectively, these findings suggest that IL-33 possibly plays a role in pathogenesis of SSc.73Psoriasis is an immune-mediated chronic inflammatory disease characterized by the growth and dilation of blood vessels and the infiltration of leukocytes into the dermis and epidermis. Vascular endothelial growth factor (VEGF) plays an important role in the angiogenesis, which is at the core of psoriasis pathogenesis. Furthermore, the higher VEGF expression associates with the clinical severity of psoriasis. Theoharides et al.74showed that IL-33 could augment the substance P-induced VEGF mRNA expression and VEGF protein secretion from mast cell, and the function contributed to the psoriatic skin.
Experimental autoimmune encephalomyelitis (EAE) is an inflammatory demyelinating disease of the CNS that serves as an animal model for human multiple sclerosis. Li et al.75reported that the expression of IL-33 and ST2 was markedly increased in the spinal cord of mice during myelin oligodendrocyte glycoprotein 35–55 peptide-induced EAE. They found that administration of a blocking anti–IL-33 antibody in mice of EAE during the induction phase significantly inhibited the onset and severity of EAE and reduced myelin oligodendrocyte glycoprotein–specific IL-17 and IFN-γproduction. As expected, treatment with recombinant IL-33 worsened the disease course of EAE in association with increased induction of both IL-17 and IFN-γ. These results demonstrate that blockade of IL-33 has a significant protective effect against EAE.75
CONCLUSIONS
Based on the discussion earlier, IL-33 pathway may play an important role in the etiology of autoimmune diseases (Fig. 1, Table 1). It is hoped that within the next few years, newly developed biological agents, including stimulating antibodies, ST2, or artificial agonists, will be available for use in daily clinical practice. Nevertheless, at present, the relevant data are very limited, and much of the data regarding IL-33 have been generated using animal models, which may not be applicable to human. Therefore, further studies on the functional role and molecular mechanism of this new cytokine are required, especially in human systems, to comprehensively explore the therapeutic potential of IL-33 in autoimmune diseases.