An IL-1 cytokine member, IL-33, induces human basophil activation via its ST2 receptor

Interleukin-33 stimulates GM-CSF and M-CSF production by human endothelial cells

Interleukin (IL)-33, a member of the IL-1 family of cytokines, is involved in various inflammatory conditions targeting amongst other cells the endothelium. Besides regulating the maturation and functions of myeloid cells, granulocyte macrophage-colony stimulating factor (GM-CSF) and macrophage-CSF (M-CSF) have been shown to play a role in such pathologies too. It was the aim of our study to investigate a possible influence of IL-33 on GM-CSF and M-CSF production by human endothelial cells. IL-33, but not IL-18 or IL-37, stimulated GM-CSF and M-CSF mRNA expression and protein production by human umbilical vein endothelial cells (HUVECs) and human coronary artery ECs (HCAECs) through the nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB) pathway in an IL-1-independent way. This effect was inhibited by the soluble form of ST2 (sST2), which is known to act as a decoy receptor for IL-33. The 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase inhibitor fluvastatin could also be shown to moderately reduce the IL-33-mediated effect on M-CSF, but not on GM-CSF expression. In addition, IL-33, IL-1β, GM-CSF and M-CSF were detected in endothelial cells of human carotid atherosclerotic plaques using immunofluorescence. Upregulation of GM-CSF and M-CSF production by human endothelial cells, an effect that appears to be mediated by NF-κB and to be independent of IL-1, may be an additional mechanism through which IL-33 contributes to inflammatory activation of the vessel wall.

Regulatory roles of mast cells in immune responses

Mast cells are important immune cells for host defense through activation of innate immunity (via toll-like receptors or complement receptors) and acquired immunity (via FcεRI). Conversely, mast cells also act as effector cells that exacerbate development of allergic or autoimmune disorders. Yet, several lines of evidence show that mast cells act as regulatory cells to suppress certain inflammatory diseases. Here, we review the mechanisms by which mast cells suppress diseases.

IL-1 is a critical regulator of group 2 innate lymphoid cell function and plasticity

Group 2 innate lymphoid cells (ILC2 cells) are important for type 2 immune responses and are activated by the epithelial cytokines interleukin 33 (IL-33), IL-25 and thymic stromal lymphopoietin (TSLP). Here we demonstrated that IL-1β was a critical activator of ILC2 cells, inducing proliferation and cytokine production and regulating the expression of epithelial cytokine receptors. IL-1β also governed ILC2 plasticity by inducing low expression of the transcription factor T-bet and the cytokine receptor chain IL-12Rβ2, which enabled the conversion of these cells into an ILC1 phenotype in response to IL-12. This transition was marked by an atypical chromatin landscape characterized by the simultaneous transcriptional accessibility of the locus encoding interferon-γ (IFN-γ) and the loci encoding IL-5 and IL-13. Finally, IL-1β potentiated ILC2 activation and plasticity in vivo, and IL-12 acted as the switch that determined an ILC2-versus-ILC1 response. Thus, we have identified a previously unknown role for IL-1β in facilitating ILC2 maturation and plasticity.

Platelets constitutively express IL-33 protein and modulate eosinophilic airway inflammation

BACKGROUND

Although platelets play a key role in allergic inflammation in addition to their well-established role in hemostasis, the precise mechanisms of how platelets modulate allergic inflammation are not fully understood. IL-33 is an essential regulator of innate immune responses and allergic inflammation.

OBJECTIVE

We sought to determine the expression of IL-33 protein by platelets and its functional significance in airway inflammation.

METHODS

IL-33 protein in human platelets, the human megakaryocyte cell line MEG-01, and bone marrow-derived mouse megakaryocytes was detected by using Western blot analysis and fluorescent immunostaining. We examined the functional relevance of IL-33 protein in platelets by comparing platelet-intact and platelet-depleted groups in a murine model of IL-33-dependent airway eosinophilia elicited by intranasal administration of papain. We further compared the additive effect of administration of platelets derived from wild-type versus IL-33-deficient mice on the papain-induced eosinophilia.

RESULTS

Platelets and their progenitor cells, megakaryocytes, constitutively expressed IL-33 protein (31 kDa). Papain-induced IL-33-dependent airway eosinophilia in mice was significantly attenuated by platelet depletion. Conversely, concomitant administration of platelets derived from wild-type mice but not IL-33-deficient mice enhanced the papain-induced airway eosinophilia.

CONCLUSIONS

Our novel findings suggest that platelets might be important cellular sources of IL-33 protein in vivo and that platelet-derived IL-33 might play a role in airway inflammation. Therefore platelets might become an attractive novel therapeutic target for asthma and probably allergic inflammation.

Reconstitution of ST2 (IL-1R4) specific for IL-33 activity; no suppression by IL-1Ra though a common chain IL-1R3 (IL-1RAcP) shared with IL-1

Interleukin-33 (IL-33) receptors are composed of ST2 (also known as IL-1R4), a ligand binding chain, and IL-1 receptor accessory protein (IL-1RAcP, also known as IL-1R3), a signal transducing chain. IL-1R3 is a common receptor for IL-1α, and IL-1β, IL-33, and three IL-36 isoforms. A549 human lung epithelial cells are highly sensitive to IL-1α and IL-1β but not respond to IL-33. The lack of responsiveness to IL-33 is due to ST2 expression. ST2 was stably transfected into A549 cells to reconstitute its activity. RT-PCR and FACS analysis confirmed ST2 expression on the cell surface of A549/ST2 cells. Upon IL-33 stimulation, A549/ST2 cells induced IL-8 and IL-6 production in a dose dependent manner while A549/mock cells remained unresponsive. There was no difference in IL-1α and IL-1β activity in A549/ST2 cells compared to A549/mock cells despite the fact that IL-33 shares IL-1R3 with IL-1α/β. IL-33 activated inflammatory signaling molecules in a time- and dose-dependent manner. Anti-ST2 antibody and soluble recombinant ST2-Fc abolished IL-33-induced IL-6 and IL-8 production in A549/ST2 cells but the IL-1 receptor antagonist failed to block IL-33-induced cytokines. This result demonstrates for the first time the reconstitution of ST2 in A549 human lung epithelial cell line and verified its function in IL-33-mediated cytokine production and signal transduction.

IL-25 and IL-33 induce Type 2 inflammation in basophils from subjects with allergic asthma

BACKGROUND

The alarmin cytokines IL-25 and IL-33 are key promoters of type 2 inflammation. Basophils respond to alarmin cytokines, however the relationship of these cytokines with basophil activation and recruitment in human studies of allergic asthma has not been well characterized. This study investigated the effect of IL-25 and IL-33 on basophils in a model of allergic asthma.

METHODS

10 mild allergic asthmatics underwent allergen and diluent inhalation challenges. Bone marrow aspirates were collected at pre-challenge and 24 h (h) post challenge. Peripheral blood and sputum samples were collected at pre-challenge, 7 h, and 24 h post-challenge to measure basophil expression of IL-17RB, ST2, and intracellular IL-25. Freshly isolated peripheral blood basophils from allergic donors were incubated overnight with IL-25 and IL-33, or sputum supernatant collected post-allergen to assess pro-inflammatory effects of mediators released in the airways.

RESULTS

There were increased percentage of basophils expressing IL-17RB, ST2, and intracellular IL-25 collected from bone marrow, peripheral blood, and sputum after allergen inhalation challenge. In vitro stimulation with IL-25 and IL-33 increased the percentage of basophils expressing intracellular type 2 cytokines and surface activation markers, and primed eotaxin-induced migratory potential of basophils, which was mediated directly through IL-17RB and ST2, respectively. Stimulation of basophils with sputum supernatants collected post-allergen challenge up-regulated the percentage of basophils expressing markers of activation and intracellular type 2 cytokines, which was reversed following blockade of the common β chain (βc).

CONCLUSIONS

Our findings indicate that the alarmin cytokines IL-33 and IL-25 increase basophil activation and migratory potential, and may pose as a novel therapeutic targets for the treatment of allergic asthma.

Recent advances in understanding the roles of vascular endothelial cells in allergic inflammation

Allergic disorders commonly involve both chronic tissue inflammation and remodeling caused by immunological reactions to various antigens on tissue surfaces. Due to their anatomical location, vascular endothelial cells are the final responders to interact with various exogenous factors that come into contact with the epithelial surface, such as pathogen-associated molecular patterns (PAMPs) and antigens. Recent studies have shed light on the important roles of endothelial cells in the development and exacerbation of allergic disorders. For instance, endothelial cells have the greatest potential to produce several key molecules that are deeply involved in allergic inflammation, such as periostin and thymus and activation-regulated chemokine (TARC/CCL17). Additionally, endothelial cells were recently shown to be important functional targets for IL-33--an essential regulator of allergic inflammation. Notably, almost all endothelial cell responses and functions involved in allergic inflammation are not suppressed by corticosteroids. These corticosteroid-refractory endothelial cell responses and functions include TNF-α-associated angiogenesis, leukocyte adhesion, IL-33-mediated responses and periostin and TARC production. Therefore, these unique responses and functions of endothelial cells may be critically involved in the pathogenesis of various allergic disorders, especially their refractory processes. Here, we review recent studies, including ours, which have elucidated previously unknown pathophysiological roles of vascular endothelial cells in allergic inflammation and discuss the possibility of endothelium-targeted therapy for allergic disorders.

TCRVγ9 γδ T Cell Response to IL-33: A CD4 T Cell-Dependent Mechanism

The availability of specific stimuli to induce the anticancer cytotoxicity of human TCRVγ9-expressing T lymphocytes has allowed the development of γδ T cell-based cancer immunotherapies. However, the stringent dependence of such strategies on the inherently toxic IL-2 has raised safety concerns for patients, justifying a search for alternative methods for inducing γδ T cell stimulation. IL-33 is a γ-chain receptor-independent cytokine of the IL-1 superfamily that is expressed by endothelial cells from a tumor microenvironment and can sustain Th1 and Th2 immune responses. Therefore, we investigated its ability to support the stimulation of human TCRVγ9(+) γδ T cells. In this study, we report that IL-33 efficiently sustained the in vitro activation of Vγ9 T lymphocytes by synthetic phosphoantigens, zoledronate, and a BTN3A1 Ab in the absence of an exogenous supply of IL-2. IL-33 was as potent as IL-2 in allowing the proliferative amplification of Vγ9 T cells isolated from PBMC following activation by the synthetic phosphoantigen bromohydrin pyrophosphate. IL-33 also induced an identical maturation into TNF-α- and IFN-γ-producing Th1 effector memory cells, and IL-33-stimulated cells showed an equivalent cytotoxicity for various tumor cells in vitro. Finally, we found that the bioactivity of IL-33 on the Vγ9 T cell was indirectly mediated through contact with CD4 T cells and IL-2 production by CD4 T cells and Vγ9 T cells themselves. These data posit IL-33 as an alternative to IL-2 for Vγ9 T cell-based cancer immunotherapies.

Peripheral basophil reactivity, CD203c expression by Cryj1 stimulation, is useful for diagnosing seasonal allergic rhinitis by Japanese cedar pollen

Measuring specific IgE can yield direct, accurate, and objective data. Nevertheless, clinical symptoms of allergy are often inconsistent with these data. Recently, the expression of CD203c, a surface marker of basophils, has been reported as capable of distinguishing allergic patients. This study compared specific IgE in serum and skin tests against antigen to assess CD203c as a biomarker correlated with allergic rhinitis (AR). We asked 3,453 subjects whether they experienced any AR related symptom. All subjects were assessed for six specific IgEs for common aeroallergens. Skin tests were also conducted for six aeroallergens. We observed the reactivity of peripheral basophil by measuring the levels of CD203c by Cryj1 stimulation using flow cytometry. Of the 3,453 participants, 1,987 (57.5%) possessed Japanese cedar pollen (JCP) specific IgE in their serum. Among those 1,987 JCP specific IgE positive participants, 552 (27.8%) had not experienced any allergic symptom during the JCP season. The levels of CD203c in the peripheral basophil by Cryj1 stimulation were significantly higher in SAR-JCP subjects than in non-SAR-JCP subjects (Cryj1 0.5 ng/ml: 2.25 ± 0.90% vs. 60.2 ± 27.4%, p < 0.01, Cryj1 50 ng/ml: 1.89 ± 0.90% vs. 68.0 ± 21.2%, p < 0.01). Our results indicate that the levels of CD203c in peripheral basophils by Cryj1 stimulation is a more objective and reliable marker that better reflects the allergic reaction by SAR-JCP in vivo than measuring specific IgE in serum or skin tests.

Natural helper cells contribute to pulmonary eosinophilia by producing IL-13 via IL-33/ST2 pathway in a murine model of respiratory syncytial virus infection

It has been reported that natural helper cells, which are a non-T, non-B innate lymphoid cell type expressing c-Kit and ST2, mediate influenza-induced airway hyper-reactivity by producing substantial IL-13. However, little is known about natural helper cells for the development of RSV-induced airway inflammation, particularly eosinophilic infiltration. By using BALB/c mice that were infected intranasally with RSV, it became clear that infection with RSV can induce an increase in the absolute number of natural helper cells in the lungs of mice. It seems likely that these natural helper cells contribute to the massive eosinophilic infiltration in an IL-13-dependent manner. In fact, the number of IL-13-producing natural helper cells as well as the expression of IL-13 mRNA in natural helper cells was enhanced significantly during RSV infection, suggesting that natural helper cells might be cellular source of the Th2-type cytokine IL-13. Indeed, adoptive transfer of pulmonary natural helper cells augmented not only the production of IL-13 but also the infiltration of eosinophils in the lungs of transferred mice. Pulmonary natural helper cells can produce IL-13 following response to IL-33, which was increased markedly in the lungs of mice after intranasal RSV infection. The expression of IL-13 mRNA in pulmonary natural helper cells was up-regulated by in vitro IL-33 stimulation. Furthermore, blockade of IL-33 receptor subunit, ST2, diminished the frequency of IL-13-producing natural helper cells. Taken together, these results demonstrate that natural helper cells may play an important role in RSV-induced pulmonary eosinophilia by producing IL-13 via the IL-33/ST2 pathway.

IL-33 Aggravates DSS-Induced Acute Colitis in Mouse Colon Lamina Propria by Enhancing Th2 Cell Responses

Interleukin- (IL-) 33, a member of the IL-1 cytokine family, is an important modulator of the immune system associated with several immune-mediated diseases. IL-33 was expressed in high level on epithelial cells of intestinal tract. It suggested that IL-33 plays a potential role in inflammatory bowel diseases (IBD). We investigated the role of interleukin- (IL-) 33 in dextran sulphate sodium- (DSS-) induced acute colitis in mice using recombinant mouse IL-33 protein (rIL-33). We found that DSS-induced acute colitis was aggravated by rIL-33 treatment. rIL-33-treated DSS mice showed markedly reduced levels of interferon- (IFN-)γ and IL-17A in their colon lamina propria lymphocytes (LPL), but the levels of Th2 cytokines, such as IL-5 and IL-13, in these cells were significantly increased, compared to DSS mice treated with PBS. Our results suggested that IL-33 stimulated CD4⁺T cells and caused the cell to adopt a Th2-type response but at the same time suppressed Th17 and Th1 cell responses. Therefore, IL-33 may be involved in pathogenesis of DSS-induced acute colitis by promoting Th2 cell response in intestinal mucosa of mice. Modulation of IL-33/ST2 signaling by monoclonal antibody (mAb) could be a novel biological therapy in DSS-induced acute colitis.

Differences in Anti-Inflammatory Actions of Intravenous Immunoglobulin between Mice and Men: More than Meets the Eye

Intravenous immunoglobulin (IVIg) is a therapeutic preparation of polyspecific human IgGs purified from plasma pooled from thousands of individuals. When administered at a high dose, IVIg inhibits inflammation and has proven efficacy in the treatment of various autoimmune and systemic inflammatory diseases. Importantly, IVIg therapy can ameliorate both auto-antibody-mediated and T-cell mediated immune pathologies. In the last few decades, extensive research in murine disease models has resulted in the elucidation of two novel anti-inflammatory mechanisms-of-action of IVIg: induction of FcγRIIB expression by sialylated Fc, and stimulation of regulatory T cells. Whereas controversial findings in mice studies have recently inspired intense scientific debate regarding the validity of the sialylated Fc-FcγRIIB model, the most fundamental question is whether these anti-inflammatory mechanisms of IVIg are operational in humans treated with IVIg. In this review, we examine the evidence for the involvement of these anti-inflammatory mechanisms in the therapeutic effects of IVIg in humans. We demonstrate that although several elements of both immune-modulatory pathways of IVIg are activated in humans, incorrect extrapolations from mice to men have been made on the molecular and cellular components involved in these cascades that warrant for critical re-evaluation of these anti-inflammatory mechanisms of IVIg in humans.

Dynamic evaluation of a mouse model of DSS induced ulcerative colitis

AIM: To develop an experimental mouse model of ulcerative colitis (UC) and to dynamically monitor the colonic injures, pathological changes, and contents of interleukin (IL)-1β, IL-18, and IL-33 in plasma and colonic mucosa to evaluate the model.

METHODS: Sixty-four C57BL/6 mice were randomly divided into either a normal group or a model group. The model group was fed 3% dextran sulfate sodium (DSS) solution for a week, followed by normal drinking water for another 3 wk. We then dynamically observed fecal condition, alterations of colon length and weight, pathological changes, colonic mucosa inflammation score and contents of IL-1β, IL-18, and IL-33 in plasma and colonic mucosa every week.

RESULTS: One week after modeling, the mice developed diarrhea, bloody stools, crouching, and huddled together. Pathological examination showed colon epithelial erosion, bleeding, multifocal ulcers and numerous inflammation cells infiltrating the mucosa and submucosa. Compared to the normal group, the colon weight was markedly increased (P < 0.05), the colon length was significantly shortened (P < 0.01), the colonic mucosa inflammation score was significantly increased (P < 0.01), and the contents of IL-1β, IL-18, and IL-33 in both plasma and mucosa were significantly elevated (P < 0.05, P < 0.01) in the model group. At the end of the second week, compared to the normal group, the model group had gradually decreased bleeding, a large number of inflammatory cells in the submucosa, increased colon weight (P < 0.01), shortened colon length (P < 0.01), increased colonic mucosa inflammation score (P < 0.01), and dramatically elevated contents of IL-1β and IL-18 in plasma and IL-18 in colonic mucosa (P < 0.05). At the third and fourth weeks, the mouse activity and fecal condition returned to normal, inflammation cells could be seen in colonic mucosa, muscle fiber was arranged disorderly; compared to normal group, colon weight was increased (P < 0.01), colon length was shortened (P < 0.01), but colonic mucosa inflammation score and the contents of IL-1β, IL-18, and IL-33 showed no significant changes in the model group (P > 0.05).

CONCLUSION: The dynamic changes in the mouse UC model show a shift from acute to chronic disease process, and this mouse model could be used for studying UC mechanism and treatment in the future.

At the bench: understanding group 2 innate lymphoid cells in disease

The conventional paradigm of type 2 inflammatory responses is characterized by activation of CD4(+) Th2 cells that produce IL-4, IL-5, and IL-13, resulting in tissue eosinophil infiltration, mucus metaplasia, AHR, and IgE production. However, the recent discovery of ILC2s in mice and humans has brought forth a novel pathway in type 2 immunity that may work independent of, or in concert with, adaptive Th2 responses. ILC2s were described initially as lineage-negative lymphocytes that produce high levels of Th2 cytokines IL-5 and IL-13 in response to IL-25 and IL-33 and promote protection against helminth infections. More recent investigations have identified novel upstream regulators, as well as novel ILC2 products. ILC2s are found in mucosal surfaces, including respiratory tract and skin, and studies from experimental asthma and atopic dermatitis models support a role for ILC2s in promoting type 2 inflammatory responses. There are many unanswered questions about the role of ILC2s in chronic allergic diseases, including how ILC2s or upstream pathways can be targeted for therapy. As ILC2s are not antigen specific and may be activated after exposures to a variety of infectious agents and irritants thought to contribute to respiratory and skin diseases, future strategies to target ILC2 function in human disease may be promising. Our intent is to identify priority areas for ILC2 translational research based on basic research insights.

Increased IL-33 expression in chronic obstructive pulmonary disease

Chronic obstructive pulmonary disease (COPD) is an inflammatory lung disease characterized by inflammatory cell activation and the release of inflammatory mediators. Interleukin-33 (IL-33) plays a critical role in various inflammatory and immunological pathologies, but evidence for its role in COPD is lacking. This study aimed to investigate the expression of IL-33 in COPD and to determine whether IL-33 participates in the initiation and progression of COPD. Levels of serum IL-33 and its receptors were measured by ELISA, and serum levels of IL-33, ST2, and IL-1 receptor accessory protein were elevated in patients with COPD compared with control subjects. Flow cytometry analysis further demonstrated an increase in peripheral blood lymphocytes (PBLs) expressing IL-33 in patients with COPD. Immunofluorescence analysis revealed that the main cellular source of IL-33 in lung tissue was human bronchial epithelial cells (HBEs). Cigarette smoke extract and lipopolysaccharide could enhance the ability of PBLs and HBEs to express IL-33. Furthermore, PBLs from patients with COPD showed greater IL-33 release in response to the stimulus. Collectively, these findings suggest that IL-33 expression levels are increased in COPD and related to airway and systemic inflammation. Therefore, IL-33 might contribute to the pathogenesis and progression of this disease.

Interleukin (IL)-33: new therapeutic target for atopic diseases

Interleukin (IL)-33, a member of the IL-1 family of cytokines, is produced when epithelial and endothelial cells are exposed to stimuli. Hematopoietic cells such as macrophages also produce IL-33. IL-33 is considered to function as an 'alarmin', activating various immune cells through its receptor ST2, which leads to the production of various molecules. The IL-33-induced production of pro-inflammatory cytokines is a critical event that aggravates atopic diseases such as asthma, atopic dermatitis, and pollenosis and suggests that IL-33-blocking agents could represent new therapeutic drugs. The anti-IL-33 antibody was effective in allergic models, whereas the anti-ST2 antibody has yielded controversial results because soluble ST2 functions as a decoy receptor for IL-33. IL-33-mediated pulmonary inflammation may be glucocorticoid-resistant especially when other cytokines act synergistically. Anti-tumor necrosis factor (TNF)-α therapy may also be effective against IL-33-mediated diseases. ERK1/2 inhibitors have also been shown to suppress the production of IL-33. On the other hand, activation of β2-receptors enhanced the expression of IL-33 mRNA in dendritic cells by activating protein kinase A (PKA), suggesting that PKA inhibitors may be candidates for IL-33-blocking agents. The effects of IL-33-blocking agents on atopic diseases need to be pharmacologically assessed in experimental and clinical studies.

Examining IL-33 expression in the cervix of HPV-infected patients: a preliminary study comparing IL-33 levels in different stages of disease and analyzing its potential association with IFN-γ

Research has shown the essential role of interleukin-33 (IL-33) in driving protective anti-viral immunity. IFN-γ has been reported to improve IL-33 expression in cultured epithelial cells. The development of cervical intraepithelial neoplasia (CIN) and carcinogenesis was closely related to human papilloma virus (HPV) infection and defective anti-viral immunity. The aim of this study was to investigate IL-33 expression alternation during the disease progress and its association with IFN-γ in HPV-positive patients. IL-33 was detected in endothelial cells and populations of epithelial cells in cervix. Though there was no statistically significant difference of IL-33 levels in cervical lavage and serum among different stages of disease (P > 0.05), the IL-33 protein and mRNA levels in cervical tissues were significantly lower in severe CIN patients than that of mild CIN or no CIN patients (P < 0.05). In addition, IL-33 protein levels were positively correlated with IFN-γ mRNA levels in all groups except cervical cancer (CA) group (r = 0.546, P < 0.01). In vitro, IFN-γ was also found to upregulate IL-33 expression in human epidermal keratinocytes (NHEKs) in a dose-dependent manner. However, CA tissues did not show further reduced IL-33 protein and mRNA levels compared with severe CIN tissues (P > 0.05). IFN-γ mRNA levels were even higher in CA tissues than in severe CIN tissues (P < 0.05). Therefore, in cervical precancerous tissues, IL-33 levels were lower in more severe lesions and that may be related to diminished local IFN-γ.

The role of the IL-33/IL-1RL1 axis in mast cell and basophil activation in allergic disorders

Interleukin-33 (IL-33) is a recently discovered cytokine that belongs to the IL-1 superfamily and acts as an important regulator in several allergic disorders. It is considered to function as an alarmin, or danger cytokine, that is released upon structural cell damage. IL-33 activates several immune cells, including Th2 cells, mast cells and basophils, following its interaction with a cell surface heterodimer consisting of an IL-1 receptor-related protein ST2 (IL-1RL1) and IL-1 receptor accessory protein (IL-1RAcP). This activation leads to the production of a variety of Th2-like cytokines that mediate allergic-type immune responses. Thus, IL-33 appears to be a double-edged sword because, in addition to its important contribution to host defence, it exacerbates allergic responses, such as allergic rhinitis and asthma. A major purported mechanism of IL-33 in allergy is the activation of mast cells to produce a variety of pro-inflammatory cytokines and chemokines. In this review, we summarize the current knowledge regarding the genetics and physiology of IL-33 and IL-1RL1 and its association with different allergic diseases by focusing on its effects on mast cells and basophils.

Role of IL-33 and its receptor in T cell-mediated autoimmune diseases

Interleukin-33 (IL-33) is a new cytokine of interleukin-1 family, whose specific receptor is ST2. IL-33 exerts its functions via its target cells and plays different roles in diseases. ST2 deletion and exclusion of IL-33/ST2 axis are accompanied by enhanced susceptibility to dominantly T cell-mediated organ-specific autoimmune diseases. It has been reported that IL-33/ST2 pathway plays a key role in host defense and immune regulation in inflammatory and infectious diseases. This review focuses on new findings in the roles of IL-33 and ST2 in several kinds of T cell-mediated autoimmune diseases.

Intravenous immunoglobulin treatment in humans suppresses dendritic cell function via stimulation of IL-4 and IL-13 production

High-dose i.v. Ig (IVIg) is a prominent immunomodulatory therapy for various autoimmune and inflammatory diseases. Recent mice studies suggest that IVIg inhibits myeloid cell function by inducing a cascade of IL-33-Th2 cytokine production causing upregulation of the inhibitory FcγRIIb, as well as by modulating IFN-γ signaling. The purpose of our study was to explore whether and how these mechanisms are operational in IVIg-treated patients. We show that IVIg in patients results in increases in plasma levels of IL-33, IL-4, and IL-13 and that increments in IL-33 levels correlate with rises in plasma IL-4 and IL-13 levels. Strikingly, no upregulation of FcγRIIb expression was found, but instead a decreased expression of the activating FcγRIIa on circulating myeloid dendritic cells (mDCs) after high-dose, but not after low-dose, IVIg treatment. In addition, expression of the signaling IFN-γR2 subunit of the IFN-γR on mDCs was downregulated upon high-dose IVIg therapy. In vitro experiments suggest that the modulation of FcγRs and IFN-γR2 on mDCs is mediated by IL-4 and IL-13, which functionally suppress the responsiveness of mDCs to immune complexes or IFN-γ. Human lymph nodes and macrophages were identified as potential sources of IL-33 during IVIg treatment. Interestingly, stimulation of IL-33 production in human macrophages by IVIg was not mediated by dendritic cell-specific intercellular adhesion molecule-3-grabbing nonintegrin (DC-SIGN). In conclusion, high-dose IVIg treatment inhibits inflammatory responsiveness of mDCs in humans by Th2 cytokine-mediated downregulation of FcγRIIa and IFN-γR2 and not by upregulation of FcγRIIb. Our results suggest that this cascade is initiated by stimulation of IL-33 production that seems DC-SIGN independent.

Interleukin-33 requires CMRF35-like molecule-1 expression for induction of myeloid cell activation

BACKGROUND

IL-33 is a potent activator of various cells involved in allergic inflammation, including eosinophils and mast cells. Despite its critical role in Th2 disease settings, endogenous molecular mechanisms that may regulate IL-33-induced responses remain to be defined. We have recently shown that eosinophils express CMRF35-like molecule (CLM)-1. Yet, the role of CLM-1 in regulating eosinophil functions is still elusive.

METHODS

CLM-1 and CLM-8 expression and cellular localization were assessed in murine bone marrow-derived and/or peritoneal cells at baseline and following IL-33 stimulation (flow cytometry, western blot). IL-33-induced mediator release and signaling were assessed in wild-type (wt) and Clm1(-/-) cells and mice.

RESULTS

BM-derived eosinophils express high levels of glycosylated CLM-1. IL-33 induced a rapid, specific, concentration- and time-dependent upregulation of CLM-1 in eosinophils (in vitro and in vivo). Clm1(-/-) eosinophils secreted less IL-33-induced mediators than wt eosinophils. CLM-1 co-localized to ST2 following IL-33 stimulation and was required for IL-33-induced NFκB and p38 phosphorylation. Th2 cytokine (e.g., IL-5, IL-13) and chemokine (e.g., eotaxins, CCL2) secretion was markedly attenuated in IL-33-treated Clm1(-/-) mice. Subsequently, IL-33-challenged mice displayed reduced infiltration of mast cells, macrophages, neutrophils, and B cells. Despite the markedly impaired IL-33-induced eotaxin expression in Clm1(-/-) mice, eosinophil accumulation was similar in wt and Clm1(-/-) mice, due to hyperchemotactic responses of Clm1(-/-) eosinophils.

CONCLUSIONS

CLM-1 is a novel regulator of IL-33-induced eosinophil activation. These data contribute to the understanding of endogenous molecular mechanisms regulating IL-33-induced responses and may ultimately lead to receptor-based tools for future therapeutic intervention in IL-33-associated diseases.

Introduction to genetics and genomics in asthma: genetics of asthma

While asthma is a heterogeneous disease, a strong genetic basis has been firmly established. Rather than being a single disease entity, asthma consists of related, overlapping syndromes [Barnes (Proc Am Thor Soc 8:143-148, 2011)] including three general domains: variable airway obstruction, airway hyper-responsiveness, and airway inflammation with a considerable proportion, but not all, of asthma being IgE-mediated further adding to its heterogeneity. This chapter reviews the approaches to the elucidation of genetics of asthma from the early evidence of familial clustering to the current state of knowledge with genome-wide approaches. The conclusion is that research efforts have led to a tremendous repository of genetic determinants of asthma, most of which fall into the above phenotypic domains of the syndrome. We now look to future integrative approaches of genetics, genomics (Chap. 10), and epigenetics (Chap. 11) to better understand the causal mechanism through which, these genetic loci act in manifesting asthma.

IL-33 promotes the induction and maintenance of Th2 immune responses by enhancing the function of OX40 ligand

BACKGROUND

In Th2 immune responses, TSLP is a key player by induction of OX40-ligand (OX40L) on dendritic cells (DCs), which is the trigger to induce Th2 cell-mediated allergic cascade. Thus, TSLP-DC-OX40L axis might be the principal pathway in the inflammatory cascades in atopic dermatitis and asthma. IL-33, which is produced by epithelial cells, has been implicated in the Th2 immune responses and pathogenesis of the allergic disorders. However, the role of IL-33 in the Th2-polarizing TSLP-DC-OX40L axis still remains largely elusive. We focused on the ability of IL-33 to promote OX40L-mediated Th2 responses.

METHODS

Purified human naïve or memory CD4+ T cells were stimulated with recombinant OX40L or TSLP-treated DCs (TSLP-DCs) in the presence of IL-33, and the cytokine production by the primed T cells was examined. We also performed immunohistochemical analyses for the expression of IL-33 in specimens of lymph node and skin from the patients with atopic dermatitis.

RESULTS

IL-33 remarkably enhanced TSLP-DCs-driven or OX40L-driven Th2 responses from naïve T cells and the Th2 functional attributes of CRTH2+ CD4+ Th2 memory cells by the increased production of IL-5, IL-9, and IL-13. In addition, IL-33 was expressed in the nuclei of epithelial cells in not only skin lesion but also lymph nodes of the patient with atopic dermatitis, suggesting a specialized role in adaptive T cell-priming phase.

CONCLUSIONS

IL-33 works as a positive regulator of TSLP-DC-OX40L axis that initiates and maintains the Th2 cell-mediated inflammatory responses, and therefore, it would be a new therapeutic target for the treatment of allergic disorders.

Innate-type and acquired-type allergy regulated by IL-33

We propose two types of allergic response: IgE-dependent and IgE-independent, and designate these as 'acquired-type allergy' and 'innate-type allergy', respectively. IL-33 stimulates both innate (basophils, mast cells, or group 2 innate lymphoid cells) and acquired (Th2 cells) allergy-related cells to induce and/or augment Th2 cytokine production, which leads to eosinophilic inflammation in vivo. Thus, IL-33 is an essential regulator for both 'innate-type allergy' and 'acquired-type allergy', and might be an attractive therapeutic target for allergic diseases.

Interleukin 33: a switch-hitting cytokine

For many years IL-33 has been widely studied in the context of T helper type 2 (Th2)-driven inflammatory disorders. Interestingly, IL-33 has now emerged as a cytokine with a plethora of pleiotropic properties. Depending on the immune cells targeted by IL-33, it is reported to not only promote Th2 immunity, but also to induce T helper type 1 (Th1) immunity. Furthermore, recent studies have revealed that IL-33 can activate CD8(+) T cells. These new studies provide evidence for its beneficial role in antiviral and antitumor immunity. Here we review the evidence of IL-33 to drive protective T cell immunity plus its potential use as an adjuvant in vaccination and tumor therapy.

ST2 deficiency does not impair type 2 immune responses during chronic filarial infection but leads to an increased microfilaremia due to an impaired splenic microfilarial clearance

Background

Interactions of the Th2 cytokine IL-33 with its receptor ST2 lead to amplified Type 2 immune responses. As Type 2 immune responses are known to mediate protection against helminth infections we hypothesized that the lack of ST2 would lead to an increased susceptibility to filarial infections.

Methodology/Principal Finding

ST2 deficient and immunocompetent BALB/c mice were infected with the filarial nematode Litomosoides sigmodontis. At different time points after infection mice were analyzed for worm burden and their immune responses were examined within the thoracic cavity, the site of infection, and systemically using spleen cells and plasma. Absence of ST2 led to significantly increased levels of peripheral blood microfilariae, the filarial progeny, whereas L. sigmodontis adult worm burden was not affected. Development of local and systemic Type 2 immune responses were not impaired in ST2 deficient mice after the onset of microfilaremia, but L. sigmodontis infected ST2-ko mice had significantly reduced total numbers of cells within the thoracic cavity and spleen compared to infected immunocompetent controls. Pronounced microfilaremia in ST2-ko mice did not result from an increased microfilariae release by adult female worms, but an impaired splenic clearance of microfilariae.

Conclusions/Significance

Our findings suggest that the absence of ST2 does not impair the establishment of adult L. sigmodontis worms, but is important for the splenic clearance of microfilariae from peripheral blood. Thus, ST2 interactions may be important for therapies that intend to block the transmission of filarial disease.

IL-33 is a mediator rather than a trigger of the acute allergic response in humans

BACKGROUND

IL-33 enhances FcεRI-induced mediator release in human basophils without inducing degranulation itself. In contrast, studies in mice suggested that in the presence of high IgE levels, IL-33 triggers degranulation and anaphylaxis of similar severity as specific allergen. Consistent with this view, sera of atopic patients contain elevated levels of IL-33 after anaphylaxis. In this study, we determined whether IL-33 is potentially anaphylactogenic in humans with high IgE levels by regulating exocytosis independent of FcεRI cross-linking. Furthermore, we investigated whether IL-33 is released upon allergen provocation in vivo.

METHODS

In subjects with high serum IgE levels, we measured IL-33-induced histamine/LTC4 in vitro, CD63 translocation ex vivo, and responsiveness of mast cells in vivo by skin prick test (SPT). In asthma patients, release of IL-33 and its correlation with early (tryptase)- and late-phase markers (IL-13 levels, eosinophil numbers) of the allergic response were assessed in bronchoalveolar lavage fluids (BALFs) after allergen challenge.

RESULTS

IL-33 itself does not trigger basophil degranulation in vitro and ex vivo, even in subjects with high serum IgE levels, and negative SPTs demonstrate that skin mast cells do not degranulate in response to IL-33. However, in response to allergen challenge, IL-33 is rapidly released into BALFs at levels that do not correlate with other immediate- and late-phase parameters.

CONCLUSION

IL-33 is unlikely an independent trigger of anaphylaxis even in subjects with high IgE levels. However, the rapid release of IL-33 upon allergen provocation in vivo supports its role as a mediator of immediate allergic responses.

Role of interleukin 33 in respiratory allergy and asthma

Since the discovery of interleukin 33 as the adopted ligand for the then orphan ST2 receptor, many studies have implicated this cytokine in the pathogenesis of respiratory allergy and asthma. Although some extracellular functions of interleukin 33 have been well defined, many aspects of the regulation and secretion of this cytokine need clarification. Interleukin 33 has been identified as a trigger of T-helper-type-2 cell differentiation, which by interacting with both the innate and the adaptive immune systems, can drive allergy and asthma pathogenesis. However, induction of interleukin 33 by both environmental and endogenous triggers implies a possible role during infection and tissue damage. Further understanding of the biology of interleukin 33 will clarify its possible role in future therapeutic interventions.

The inhibitory function of Fc-ST2 depends on cell type; IL-1RAcP and ST2 are necessary but insufficient for IL-33 activity

IL-33 (IL-1F11) is a member of IL-1 family ligand, which stimulates the production of inflammatory cytokines. IL-33 receptor complex is comprised of IL-1 receptor accessory protein (IL-1RAcP) and ST2 that are activated by IL-33 ligand binding. ST2 is a ligand-binding chain of the IL-33 receptor component, and the soluble ST2 form possesses antagonistic activity. Here, we expressed the extracellular domain of ST2-fused to the immunoglobulin of IgG1 constant region in order to generate a soluble recombinant Fc-ST2. Human and mouse recombinant Fc-ST2 protein were expressed in Chinese hamster ovary cells and purified using a mini-protein A affinity chromatography. The recombinant Fc-ST2 protein was used to examine inhibitory function in IL-33-induced cytokine production in different cell types. The human Fc-ST2 abolished IL-33-induced IL-8 production in human mast cells, but mouse Fc-ST2 failed to inhibit IL-33-induced TNFα production in mouse Raw 264.7 macrophage cells. We further investigated the expression of IL-33 receptor component with various cell lines. IL-33 receptors expression pattern and Fc-ST2 inhibitory activity in different cell types suggest that IL-1RAcP and ST2 are necessary but insufficient for IL-33 activity. Our results suggest that an additional receptor component may participate in the biological activity of IL-33.

Activation of human basophils by combined toll-like receptor- and FcεRI-triggering can promote Th2 skewing of naive T helper cells

Basophils are mostly known for their involvement in allergic reactions. Recent studies in mice indicate a role for basophils in the induction of adaptive immunity, especially T helper 2 (Th2) responses. Therefore, it would be highly important to understand how basophils respond to pathogen-associated molecules, such as ligands for toll-like receptors (TLRs), and if the basophils could promote Th2 responses via these stimuli. To this end, the activation of basophils via TLRs in combination with activation via IgE was studied, as well as its effect on T helper cell skewing. Using quantitative PCR, we demonstrated the presence of mRNA for TLRs 1-8 in human basophils. Basophils responded to TLR triggering with differential cytokine production, but not with degranulation. Simultaneous triggering of TLRs and IgE led to synergy in production of IL-4, IL-8, IL-13, and RANTES. Furthermore, the synergistic effects on basophils mediated by IgE and TLR-4 triggering allowed robust Th2 skewing upon activation of naïve human CD4⁺ T cells. Our data show that human basophils respond to TLR ligands in synergy with IgE-mediated activation and that the cytokines produced can promote Th2 differentiation. These results indicate a role for basophils in the regulation of T-cell responses in humans.

The deleterious role of basophils in systemic lupus erythematosus

Systemic lupus erythematosus is a complex autoimmune disease of multifactorial origins. All compartments of the immune system appear to be affected, at least in some way, and to contribute to disease pathogenesis. Because of an escape from negative selection autoreactive T and B cells accumulate in SLE patients leading to the production of autoantibodies mainly raised against nuclear components and their subsequent deposition into target organs. We recently showed that basophils, in an IgE and IL-4 dependent manner, contribute to SLE pathogenesis by amplifying autoantibody production. Here, we summarize what we have learned about the deleterious role of basophils in lupus both in a mouse model and in SLE patients. We discuss which possible pathways could be involved in basophil activation and recruitment to secondary lymphoid organs during SLE, and how basophils may amplify autoantibody production.

Enhancement of the p38 MAPK and PKA signaling pathways is associated with the pro-melanogenic activity of Interleukin 33 in primary melanocytes

BACKGROUND

Interleukin-33 (IL-33) was recently recognized as a member of the IL-1 cytokine family. The triggers no matter environmental or endogenous that provoke IL-33 cellular release may be associated with inflammation, infection or tissue damage. However, to date, the regulatory role of IL-33 in the control of melanogenesis has not been elucidated.

OBJECTIVE

The present study was designed to investigate the effect of IL-33 on melanogenesis and to explore its underlying mechanisms.

METHODS

Melanocytes were exposed to IL-33. Then cell viabilities were measured by MTT assay. The improving activities of IL-33 were examined by melanin synthesis, tyrosinase (TYR) activity assay. The expressions of relative proteins were assessed by Western blotting.

RESULTS

IL-33 increased the TYR activity and melanin content in a dosage-dependent manner at concentrations of 1-10ng/ml. Treatment with 10ng/ml of IL-33 enhanced the expression of microphthalmia-associated transcription factor (MITF), TYR, tyrosinase-related protein 1 (TRP-1) and dopachrome tautomerase (DCT) in normal human foreskin-derived epidermal melanocytes (NHEM). Furthermore, IL-33 could remarkably promote the phosphorylation levels of p38 mitogen-activated protein kinases (MAPKs) and cyclic AMP-responsive element-binding protein (CREB). This pro-melanogenic effect could be partially reversed by the pre-treatment with the special p38 MAPK inhibitor, SB203580, and protein kinase A (PKA) inhibitor, H89.

CONCLUSIONS

Our results collectively indicated that IL-33 improved melanin biosynthesis in NHEM. This function might be attributed to the fact that IL-33 stimulates the phosphorylation of p38 MAPK and CREB, increases the TYR, TRP-1 and DCT expression through MITF, finally resulting in the augment of melanogenesis.

IL-33 markedly activates murine eosinophils by an NF-κB-dependent mechanism differentially dependent upon an IL-4-driven autoinflammatory loop

Eosinophils are major effector cells in type 2 inflammatory responses and become activated in response to IL-4 and IL-33, yet the molecular mechanisms and cooperative interaction between these cytokines remain unclear. Our objective was to investigate the molecular mechanism and cooperation of IL-4 and IL-33 in eosinophil activation. Eosinophils derived from bone marrow or isolated from Il5-transgenic mice were activated in the presence of IL-4 or IL-33 for 1 or 4 h, and the transcriptome was analyzed by RNA sequencing. The candidate genes were validated by quantitative PCR and ELISA. We demonstrated that murine-cultured eosinophils respond to IL-4 and IL-33 by phosphorylation of STAT-6 and NF-κB, respectively. RNA sequence analysis of murine-cultured eosinophils indicated that IL-33 induced 519 genes, whereas IL-4 induced only 28 genes, including 19 IL-33-regulated genes. Interestingly, IL-33 induced eosinophil activation via two distinct mechanisms, IL-4 independent and IL-4 secretion/autostimulation dependent. Anti-IL-4 or anti-IL-4Rα Ab-treated cultured and mature eosinophils, as well as Il4- or Stat6-deficient cultured eosinophils, had attenuated protein secretion of a subset of IL-33-induced genes, including Retnla and Ccl17. Additionally, IL-33 induced the rapid release of preformed IL-4 protein from eosinophils by a NF-κB-dependent mechanism. However, the induction of most IL-33-regulated transcripts (e.g., Il6 and Il13) was IL-4 independent and blocked by NF-κB inhibition. In conclusion, we have identified a novel activation pathway in murine eosinophils that is induced by IL-33 and differentially dependent upon an IL-4 auto-amplification loop.

Innate immune cells in asthma

Asthma is an inflammatory disease of the airways associated with a T helper (Th)2 response. Such a response in the lungs requires complex interactions between innate cells and structural cells. Dendritic cells (DCs) are pivotal during sensitization to allergens but clearly require epithelium-derived signals to become activated. Epithelial cells also contribute to the activation and the survival of mast cells (MCs), basophils, and eosinophils and group 2 innate lymphoid cells (ILC2s). In turn, these innate cells can activate DCs to sustain Th2 immunity. Here, we review the role played by these different populations of immune cells in the pathogenesis of asthma and how they interact to orchestrate Th2 immunity.

Th2-type innate immune responses mediated by natural helper cells

Natural helper (NH) cells are a newly identified innate lymphocyte population that responds to a combination of interleukin (IL)-2 and either IL-25 or IL-33 to produce large amounts of T helper cell type 2 (Th2) cytokines. NH cells have been identified in fat-associated lymphoid clusters (FALCs), produce Th2 cytokines constitutively without any stimulation, and support the self-renewal of B1 cells and IgA production by B cells. Large amounts of IL-5 and IL-13 produced upon helminth infection or in response to IL-33 can induce eosinophilia and goblet cell hyperplasia in the lung and intestine; these cytokines, which activate NH cells, play important roles in antihelminth immunity and allergic diseases such as asthma.

The IL-33/ST2 pathway controls coxsackievirus B5-induced experimental pancreatitis

Coxsackievirus B (CVB) is a common cause of acute and chronic infectious myocarditis and pancreatitis. Th1 cells producing IFN-γ and TNF-α are important for CVB clearance, but they are also associated with the pathogenesis of inflammatory lesions, suggesting that the modulation of Th1 and Th2 balance is likely important in controlling CVB-induced pancreatitis. We investigated the role of IL-33, which is an important recently discovered cytokine for induction of Th2-associated responses, in experimental CVB5 infection. We found that mice deficient in IL-33R, T1/ST2, significantly developed more severe pancreatitis, had greater weight loss, and contained higher viral load compared with wild-type (WT) mice when infected with CVB5. Conversely, WT mice treated with rIL-33 developed significantly lower viral titers, and pancreatitis was attenuated. Mechanistic studies demonstrated that IL-33 enhances the degranulation and production of IFN-γ and TNF-α by CD8(+) T and NK cells, which is associated with viral clearance. Furthermore, IL-33 triggers the production of IL-4 from mast cells, which results in enhanced differentiation of M2 macrophages and regulatory T cells, leading to the attenuation of inflammatory pancreatitis. Adoptively transferred mast cells or M2 macrophages reversed the heightened pancreatitis in the T1/ST2(-/-) mice. In contrast, inhibition of regulatory T cells exacerbated the disease in WT mice. Together, our findings reveal an unrecognized IL-33/ST2 functional pathway and a key mechanism for CVB5-induced pancreatitis. These data further suggest a novel approach in treating virus-induced pancreatitis, which is a major medical condition with unmet clinical needs.

Interleukin-33: a mediator of inflammation targeting hematopoietic stem and progenitor cells and their progenies

Inflammation is defined as a physiological response initiated by a variety of conditions that cause insult to the body, such as infection and tissue injury. Inflammation is triggered by specialized receptors in the innate immune system, which recognize microbial components known as pathogen-associated molecular patterns or endogenous signals produced by damaged cells (damage-associated molecular patterns). IL-33 is a cytokine that is released predominantly at the epithelial barrier when it is exposed to pathogens, allergens, or injury-inducing stimuli. IL-33 target cells are various, ranging from hematopoietic stem and progenitor cells (HSPCs) and essentially all types of their progeny to many non-hematopoietic cells. The pleiotropic actions of IL-33 suggest that IL-33 is involved in every phase of the inflammatory process. In this review, we discuss recent advances in the understanding of how IL-33 orchestrates inflammatory responses by regulating HSPCs and innate immune cells.

The role of interleukin-33 in rhinitis

IL-33, a member of the IL-1 cytokine family and a ligand to receptor ST2, has great potential to induce a T helper 2-type inflammatory response. IL-33 is proven to be released by epithelial cells during their injury by different environmental stimuli such as airborne allergens, viruses, and air pollutants. IL-33 acting as an endogenous danger signal is termed an alarmin. As such, this cytokine is considered to play a crucial role in an allergic inflammatory disease such as rhinitis. Recent investigations regarding the IL-33/ST2 axis involvement in Th2 inflammatory response and pathogenesis of rhinitis have been reviewed. The role of IL-33 as a novel promising therapeutic target has also been discussed.

A novel interleukin 33/ST2 signaling regulates inflammatory response in human corneal epithelium

Interleukin (IL) 33, a member of IL-1 cytokine family, is well known to promote Th2 type immune responses by signaling through its receptor ST2. However, it is not clear whether ST2 is expressed by mucosal epithelium, and how it responds to IL-33 to induce inflammatory mediators. This study was to identify the presence and function of ST2 and explore the role of IL-33/ST2 signaling in regulating the inflammatory cytokine production in corneal epithelial cells. Human corneal tissues and cultured primary human corneal epithelial cells (HCECs) were treated with IL-33 in different concentrations without or with different inhibitors to evaluate the expression, location and signaling pathways of ST2 in regulating production of inflammatory cytokine and chemokine. The mRNA expression was determined by reverse transcription and real time PCR, and protein production was measured by enzyme-linked immunosorbent assay (ELISA), immunohistochemical and immunofluorescent staining. ST2 mRNA and protein were detected in donor corneal epithelium and cultured HCECs, and ST2 signal was enhanced by exposure to IL-33. IL-33 significantly stimulated the production of inflammatory cytokines (TNF-α, IL-1β and IL-6) and chemokine IL-8 by HCECs at both mRNA and protein levels. The stimulated production of inflammatory mediators by IL-33 was blocked by ST2 antibody or soluble ST2 protein. Interestingly, the IκB-α inhibitor BAY11-7082 or NF-κB activation inhibitor quinazoline blocked NF-κB p65 protein phosphorylation and nuclear translocation, and also suppressed the production of these inflammatory cytokines and chemokine induced by IL-33. These findings demonstrate that ST2 is present in human corneal epithelial cells, and IL-33/ST2 signaling plays an important role in regulating IL-33 induced inflammatory responses in ocular surface.