Up-regulation of IL-33 expression in various types of murine cells by IL-3 and IL-4

Adipose-mesenchymal stromal cells suppress experimental Sjögren syndrome by IL-33-driven expansion of ST2+ regulatory T cells

Adipose-derived mesenchymal stromal cells (ADSCs) play important roles in the alleviation of inflammation and autoimmune diseases. Interleukin-33 (IL-33), a member of the IL-1 family, has been shown to regulate innate and adaptive immunity. However, it is still unknown whether ADSCs regulate immune responses via IL-33. We show here that ADSCs produced IL-33 in response to IL-1β stimulation, which depended on TAK1, ERK, and p38 pathways. ADSCs-derived IL-33 drove the proliferation of CD4⁺Foxp3⁺ST2⁺ regulatory T cells (Tregs) and alleviated experimental autoimmune Sjögren syndrome in mice. Importantly, human ADSCs also produced IL-33 in response to IL-1β. Thus, we have revealed a previously unrecognized immunoregulatory function of ADSCs by IL-33 production in experimental autoimmunity, which may have clinical applications for human immunopathology.

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Human and mouse ADSCs express IL-33 in response to IL-β stimulation

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mADSC-derived IL-33 inhibits inflammation in salivary glands in SS model

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mADSC-derived IL-33 expand ST2⁺ Tregs in vitro and in SS model

The blockade of interleukin-33 released by hepatectomy would be a promising treatment option for cholangiocarcinoma

Interleukin-33 (IL-33), an alarmin released during tissue injury, facilitates the development of cholangiocarcinoma (CCA) in a murine model. However, it is unclear whether IL-33 is associated with human CCA. The aim of this study was to support the following hypothesis: IL-33 is released during hepatectomy for CCA, subsequently facilitating the development of subclinical CCA and eventually leading to recurrent disease. IL-33 expression was assessed in various samples from both humans and mice including resected liver and paired plasma samples collected at hepatectomy and after surgery, and its influences on recurrent disease and patient prognosis were determined. Homogenized human liver samples with high or low IL-33 expression were added to the culture medium of human CCA cells, and the changes in proliferation and migration were evaluated. To examine the effects of inhibiting the IL-33 release induced by hepatectomy, syngraft transplantation of murine CCA cells was performed in C57BL/6J mice with or without IL-33 blockade. The amount of IL-33 released into the plasma during hepatectomy correlated with the background liver expression. High expression of IL-33 in the liver was an independent risk factor for recurrence. Homogenized liver tissue strongly expressing IL-33 increased both the proliferation and migration of tumor cells. Mice who underwent hepatectomy exhibited CCA progression in the remnant liver, whereas blockade of IL-33 during hepatectomy inhibited tumor progression. Thus, we concluded that surgery for CCA with curative intent paradoxically induced IL-33 release, which facilitated CCA recurrence, and anti-IL-33 therapy during hepatectomy might reduce the risk of CCA recurrence.

Allergic inflammation is initiated by IL-33-dependent crosstalk between mast cells and basophils

IgE-primed mast cells in peripheral tissues, including the skin, lung, and intestine, are key initiators of allergen-triggered edema and inflammation. Particularly in severe forms of allergy, this inflammation becomes strongly neutrophil dominated, and yet how mast cells coordinate this type of response is unknown. We and others have reported that activated mast cells--a hematopoietic cell type--can produce IL-33, a cytokine known to participate in allergic responses but generally considered as being of epithelial origin and driving Type 2 immune responses (e.g., ILC2 and eosinophil activation). Using models of skin anaphylaxis, our data reveal that mast cell-derived IL-33 also initiates neutrophilic inflammation. We demonstrate a cellular crosstalk mechanism whereby activated mast cells crosstalk to IL-33 receptor-bearing basophils, driving these basophils to adopt a unique response signature rich in neutrophil-associated molecules. We further establish that basophil expression of CXCL1 is necessary for IgE-driven neutrophilic inflammation. Our findings thus unearth a new mechanism by which mast cells initiate local inflammation after antigen triggering and might explain the complex inflammatory phenotypes observed in severe allergic diseases. Moreover, our findings (i) establish a functional link from IL-33 to neutrophilic inflammation that extends IL-33-mediated biology well beyond that of Type 2 immunity, and (ii) demonstrate the functional importance of hematopoietic cell-derived IL-33 in allergic pathogenesis.

A network map of IL-33 signaling pathway

Interleukin-33 (IL-33) is a member of the IL-1 family of cytokines that play a central role in the regulation of immune responses. Its release from epithelial and endothelial cells is mediated by pro-inflammatory cytokines, cell damage and by recognition of pathogen-associated molecular patterns (PAMPs). The activity of IL-33 is mediated by binding to the IL-33 receptor complex (IL-33R) and activation of NF-κB signaling via the classical MyD88/IRAK/TRAF6 module. IL-33 also induces the phosphorylation and activation of ERK1/2, JNK, p38 and PI3K/AKT signaling modules resulting in the production and release of pro-inflammatory cytokines. Aberrant signaling by IL-33 has been implicated in the pathogenesis of several acute and chronic inflammatory diseases, including asthma, atopic dermatitis, rheumatoid arthritis and ulcerative colitis among others. Considering the biomedical importance of IL-33, we developed a pathway resource of signaling events mediated by IL-33/IL-33R in this study. Using data mined from the published literature, we describe an integrated pathway reaction map of IL-33/IL-33R consisting of 681 proteins and 765 reactions. These include information pertaining to 19 physical interaction events, 740 enzyme catalysis events, 6 protein translocation events, 4 activation/inhibition events, 9 transcriptional regulators and 2492 gene regulation events. The pathway map is publicly available through NetPath ( http://www.netpath.org /), a resource of human signaling pathways developed previously by our group. This resource will provide a platform to the scientific community in facilitating identification of novel therapeutic targets for diseases associated with dysregulated IL-33 signaling. Database URL: http://www.netpath.org/pathways?path_id=NetPath_120 .

Male-specific IL-33 expression regulates sex-dimorphic EAE susceptibility

The cellular and molecular basis of sex-dimorphic autoimmune diseases, such as the CNS demyelinating disease multiple sclerosis (MS), remains unclear. Our studies in the SJL mouse model of MS, experimental autoimmune encephalomyelitis (EAE), reveal that sex-determined differences in Il33 expression by innate immune cells in response to myelin peptide immunization regulate EAE susceptibility. IL-33 is selectively induced in PLP_139-151-immunized males and activates type 2 innate lymphoid cells (ILC2s), cells that promote and sustain a nonpathogenic Th2 myelin-specific response. Without this attenuating IL-33 response, females generate an encephalitogenic Th17-dominant response, which can be reversed by IL-33 treatment. Mast cells are one source of IL-33 and we provide evidence that testosterone directly induces Il33 gene expression and also exerts effects on the potential for Il33 gene expression during mast cell development. Thus, in contrast to their pathogenic role in allergy, we propose a sex-specific role for both mast cells and ILC2s as attenuators of the pathogenic Th response in CNS inflammatory disease.

Wounds that heal and wounds that don't - The role of the IL-33/ST2 pathway in tissue repair and tumorigenesis

IL-33 is a member of the IL-1 family of cytokines. IL-33 is predominantly located within the nucleus of cells where it plays a role in gene regulation. Given the right combination of signals and cellular damage, stored IL-33 is released from the cell where it can interact with its receptor ST2, triggering danger-associated responses and act as a cellular "alarmin". Whilst IL-33/ST2 signalling has been shown to induce potent pro-inflammatory responses that can be detrimental in certain disease states, a dichotomous, protective role of IL-33 in promoting wound healing has also emerged in multiple tissues types. This review will explore the current literature concerning this homeostatic role of IL-33/ST2 in tissue repair and also review its role in uncontrolled wound responses as seen in both fibrosis and tumorigenesis.

The IL-1 cytokine family and its role in inflammation and fibrosis in the lung

The IL-1 cytokine family comprises 11 members (7 ligands with agonist activity, 3 receptor antagonists and 1 anti-inflammatory cytokine) and is recognised as a key mediator of inflammation and fibrosis in multiple tissues including the lung. IL-1 targeted therapies have been successfully employed to treat a range of inflammatory conditions such as rheumatoid arthritis and gouty arthritis. This review will introduce the members of the IL-1 cytokine family, briefly discuss the cellular origins and cellular targets and provide an overview of the role of these molecules in inflammation and fibrosis in the lung.

Controlling the burn and fueling the fire: defining the role for the alarmin interleukin-33 in alloimmunity

PURPOSE OF REVIEW

The purpose of this review is to provide a general update on recent developments in the immunobiology of IL-33 and IL-33-targeted immune cells. We also discuss emerging concepts regarding the potential role IL-33 appears to play in altering alloimmune responses mediating host-versus-graft and graft-versus-host alloresponses.

RECENT FINDINGS

Stromal cells and leukocytes display regulated expression of IL-33 and may actively or passively secrete this pleotropic cytokine. Type 2 innate lymphoid cells and a large proportion of tissue resident regulatory T cells (Treg) express membrane-bound suppressor of tumorigenicity 2 (ST2), the IL-33 receptor. Although Treg are appreciated suppressors of the inflammatory function of immune cells, both type 2 innate lymphoid cells and tissue resident Treg could play key roles in tissue repair and homeostasis. The functions of IL-33 in transplantation are poorly understood. However, like other disease models, the functions of IL-33 in alloimmunity appear to be quite pleiotropic. IL-33 is associated with immune regulation and graft protection in cardiac transplant settings. Yet, it is highly proinflammatory and stimulates lethal graft-versus-host disease through its capacity to stimulate type 1 immunity.

SUMMARY

Intensive studies on IL-33/ST2 signaling pathways and ST2 cell populations in solid organ and cell transplantation are warranted. A better understanding of this important pathway will provide promising therapeutic targets controlling pathogenic alloimmune responses, as well as potentially facilitating the function of regulatory and reparative immune cells posttransplantation.

Upregulation of Interleukin-33 in obstructive renal injury

Interstitial fibrosis and loss of parenchymal tubular cells are the common outcomes of progressive renal diseases. Pro-inflammatory cytokines have been known contributing to the damage of tubular cells and fibrosis responses after renal injury. Interleukin (IL)-33 is a tissue-derived nucleus alarmin that drives inflammatory responses. The regulation and function of IL-33 in renal injury, however, is not well understood. To investigate the involvement of cytokines in the pathogenesis of renal injury and fibrosis, we performed the mouse renal injury model induced by unilateral urinary obstruction (UUO) and analyze the differentially upregulated genes between the obstructed and the contralateral unobstructed kidneys using RNA sequencing (RNAseq). Our RNAseq data identified IL33 and its receptor ST2 were upregulated in the UUO kidney. Quantitative analysis confirmed that transcripts of IL33 and ST2 were upregulated in the obstructed kidneys. Immunofluorescent staining revealed that IL-33 was upregulated in Vimentin- and alpha-SMA-positive interstitial cells. By using genetically knockout mice, deletion of IL33 reduced UUO-induced renal fibrosis. Moreover, in combination with BrdU labeling technique, we observed that the numbers of proliferating tubular epithelial cells were increased in the UUO kidneys from IL33-or ST2-deficient mice compared to wild type mice. Collectively, our study demonstrated the upregulation of IL-33/ST2 signaling in the obstructed kidney may promote tubular cell injury and interstitial fibrosis. IL-33 may serve as a biomarker to detect renal injury and that IL-33/ST2 signaling may represent a novel target for treating renal diseases.

Type II cytokines impair host defense against an intracellular fungal pathogen by amplifying macrophage generation of IL-33

Interleukin (IL)-4 subverts protective immunity to multiple intracellular pathogens, including the fungus Histoplasma capsulatum. Previously, we reported that H. capsulatum-challenged CCR2(-/-) mice manifest elevated pulmonary fungal burden owing to exaggerated IL-4. Paradoxical to our anticipation in IL-33 driving IL-4, we discovered that the latter prompted IL-33 in mutant mice. In infected CCR2(-/-) animals, amplified IL-33 succeeded the heightened IL-4 response and inhibition of IL-4 signaling decreased IL-33. Moreover, macrophages, but not epithelial cells or dendritic cells, from these mice expressed higher IL-33 in comparison with controls. Dissection of mechanisms that promulgated IL-33 revealed type-II cytokines and H. capsulatum synergistically elicited an IL-33 response in macrophages via signal transducer and activator of transcription factor 6/interferon-regulatory factor-4 and Dectin-1 pathways, respectively. Neutralizing IL-33 in CCR2(-/-) animals, but not controls, enhanced their resistance to histoplasmosis. Thus we describe a previously unrecognized role for IL-4 in instigating IL-33 in macrophages. Furthermore, in the presence of intracellular fungal pathogens, the type-II cytokine-driven IL-33 response impairs immunity.

IL-33/ST2 involves the immunopathology of ocular toxoplasmosis in murine model

Ocular toxoplasmosis (OT) is the major cause of infective uveitis. Since the eye is a special organ protected by immune privilege, its immune response is different from general organs with Toxoplasma gondii infection. Here, we used Kunming outbred mice to establish OT by intravitreal injection of T. gondii RH strain tachyzoites, IL-33 expression in the eyes was localized by immunostaining, the levels of interleukin (IL)-33 and ST2 (IL-33 receptor) and T-helper (Th)1 and Th2-associated cytokines in the eye and cervical lymph nodes (CLNs) of infected mice were measured, and their correlations were analyzed. Our results showed that the pathologies of the eye and CLN tissues and the IL-33 positive cells in the eye tissues of ocular T. gondii-infected mice were all increased at days 2, 6, and 9 postinfection (p.i.), accompanied with significantly increased transcript levels of IL-33, ST2, IL-1β, IFN-γ, IL-12p40, IL-10, and IL-13 in both the eyes and CLNs, and increased IL-4 expressions in the eyes of T. gondii-infected mice. There were significant correlations between the levels of IFN-γ and ST2, IL-4 and ST2, and IL-13 and ST2 in the eye tissues (P < 0.001), significant correlations between the levels of IFN-γ and ST2 (P < 0.001) as well as between IL-13 and ST2 (P < 0.05) in the CLNs, and significant correlations between the levels of IL-1β and IL-33 in the eyes (P < 0.05) and between IL-1β and IL-33/ST2 in the CLNs (P < 0.001 and P < 0.01, respectively). Our data indicated that IL-33/ST2 may involve the regulation of ocular immunopathology induced by T. gondii infection.

IL-33 is an unconventional Alarmin that stimulates IL-2 secretion by dendritic cells to selectively expand IL-33R/ST2+ regulatory T cells

IL-33 is a recently characterized IL-1 family member that is proposed to function as an alarmin, or endogenous signal of cellular damage, as well as act as a pleiotropic cytokine. The ability of IL-33 to potentiate both Th1 and Th2 immunity supports its role in pathogen clearance and disease immunopathology. Yet, IL-33 restrains experimental colitis and transplant rejection by expanding regulatory T cells (Treg) via an undefined mechanism. We sought to determine the influence of IL-33 on hematopoietic cells that drives Treg expansion and underlies the therapeutic benefit of IL-33 administration. In this study, we identify a feedback loop in which conventional mouse CD11c(+) dendritic cells (DC) stimulated by IL-33 secrete IL-2 to selectively expand IL-33R(ST2(+))- suppressive CD4(+)Foxp3(+) Treg. Interestingly, this occurs in the absence of classical DC maturation, and DC-derived (innate) IL-2 increases ST2 expression on both DC and interacting Treg. ST2(+) Treg represent an activated subset of Foxp3(+) cells, demonstrated to be ICOS(high)CD44(high) compared with their ST2(-) counterparts. Furthermore, although studies have shown that IL-33-exposed DC promote Th2 responses, we reveal that ST2(+) DC are required for IL-33-mediated in vitro and in vivo Treg expansion. Thus, we have uncovered a relationship between IL-33 and innate IL-2 that promotes the selective expansion of ST2(+) Treg over non-Treg. These findings identify a novel regulatory pathway driven by IL-33 in immune cells that may be harnessed for therapeutic benefit or for robust expansion of Treg in vitro and in vivo.

IFN-γ directly controls IL-33 protein level through a STAT1- and LMP2-dependent mechanism

IL-33 contributes to disease processes in association with Th1 and Th2 phenotypes. IL-33 mRNA is rapidly regulated, but the fate of synthesized IL-33 protein is unknown. To understand the interplay among IL-33, IFN-γ, and IL-4 proteins, recombinant replication-deficient adenoviruses were produced and used for dual expression of IL-33 and IFN-γ or IL-33 and IL-4. The effects of such dual gene delivery were compared with the effects of similar expression of each of these cytokines alone. In lung fibroblast culture, co-expression of IL-33 and IFN-γ resulted in suppression of the levels of both proteins, whereas co-expression of IL-33 and IL-4 led to mutual elevation. In vivo, co-expression of IL-33 and IFN-γ in the lungs led to attenuation of IL-33 protein levels. Purified IFN-γ also attenuated IL-33 protein in fibroblast culture, suggesting that IFN-γ controls IL-33 protein degradation. Specific inhibition of caspase-1, -3, and -8 had minimal effect on IFN-γ-driven IL-33 protein down-regulation. Pharmacological inhibition, siRNA-mediated silencing, or gene deficiency of STAT1 potently up-regulated IL-33 protein expression levels and attenuated the down-regulating effect of IFN-γ on IL-33. Stimulation with IFN-γ strongly elevated the levels of the LMP2 proteasome subunit, known for its role in IFN-γ-regulated antigen processing. siRNA-mediated silencing of LMP2 expression abrogated the effect of IFN-γ on IL-33. Thus, IFN-γ, IL-4, and IL-33 are engaged in a complex interplay. The down-regulation of IL-33 protein levels by IFN-γ in pulmonary fibroblasts and in the lungs in vivo occurs through STAT1 and non-canonical use of the LMP2 proteasome subunit in a caspase-independent fashion.

Implications for Interleukin-33 in solid organ transplantation

Interleukin(IL)-33 is a member of the IL-1 cytokine family that has been attributed T helper (Th) type 2 immunity-promoting capacity. However, new studies indicate that IL-33 is a multifunctional protein that acts as transcriptional/signaling repressor, functions as an alarmin alerting the immune system to necrosis, as well as serves as a cytokine that targets cells expressing ST2, the IL-33 receptor. Interestingly, IL-33 is also emerging as a pleiotropic cytokine. Depending on the innate or adaptive immune cells targeted by IL-33, it can not only promote type 2, but also IFN-γ dominated type 1 immunity. In addition, IL-33 expands regulatory T cells. In this review, we assimilate the current knowledge of IL-33 immunobiology and discuss how IL-33 may mediate such diverse roles in the immune response to pathogens and development of immune-mediated pathologies. The function of IL-33 in shaping alloimmune responses to transplanted organs is poorly explored, but a particularly beneficial role of IL-33 in experimental heart transplant models is summarized. Finally, given the implication of IL-33 in pathologies of the lung and intestine, we discuss how IL-33 may contribute to the comparatively poor outcomes following transplantation of these two organs.