Interleukin-33 in health and disease. Nat Rev Immunol. 2016;16:676-689. [PMID: 27640624 DOI: 10.1038/nri.2016.95]

Conflicting vascular and metabolic impact of the IL-33/sST2 axis

Interleukin 33 (IL-33), which is expressed by several immune cell types, endothelial and epithelial cells, and fibroblasts, is a cytokine of the IL-1 family that acts both intra- and extracellularly to either enhance or resolve the inflammatory response. Intracellular IL-33 acts in the nucleus as a regulator of transcription. Once released from cells by mechanical stress, inflammatory cytokines, or necrosis, extracellular IL-33 is proteolytically processed to act in an autocrine/paracrine manner as an 'alarmin' on neighbouring or various immune cells expressing the ST2 receptor. Thus, IL-33 may serve an important role in tissue preservation and repair in response to injury; however, the actions of IL-33 are dampened by a soluble form of ST2 (sST2) that acts as a decoy receptor and is produced by endothelial and certain immune cells. Accumulating evidence supports the conclusion that sST2 is a biomarker of vascular health with diagnostic and/or prognostic value in various cardiovascular diseases, including coronary artery disease, myocardial infarction, atherosclerosis, giant-cell arteritis, acute aortic dissection, and ischaemic stroke, as well as obesity and diabetes. Although sST2 levels are positively associated with cardiovascular disease severity, the assumption that IL-33 is always beneficial is naïve. It is increasingly appreciated that the pathophysiological importance of IL-33 is highly dependent on cellular and temporal expression. Although IL-33 is atheroprotective and may prevent obesity and type 2 diabetes by regulating lipid metabolism, IL-33 appears to drive endothelial inflammation. Here, we review the current knowledge of the IL-33/ST2/sST2 signalling network and discuss its pathophysiological and translational implications in cardiovascular diseases.

Role of alarmin cytokines and microRNAs in the host-schistosome interaction

Schistosomiasis is a serious but neglected tropical infectious disease, afflicting more than 240 million people in 78 countries. Lack of an effective vaccine and obscuring disease mechanism could be the main hurdles to effectively control and eradicate this disease. A better understanding of the host-schistosome interaction is the key to clearing these hurdles. Recently, accumulating evidence shows that alarmin cytokines and microRNAs (miRNAs) are crucial regulators in the host-schistosome interaction. Alarmin cytokines are proven to be potent mechanisms driving type 2 immunity, which is the central disease mechanism of schistosomiasis. MiRNA deregulation is a hallmark of a variety of human diseases, including schistosomiasis. In this review, we summarize the research advances on the role of alarmin cytokines and miRNAs in the host-schistosome interaction.

Interleukin-33 and ST2 Signaling in Tumor Microenvironment

Interleukin-33 (IL-33) is one of the members of the IL-1 family of cytokines and a ligand of ST2 and IL-1 receptor accessory protein (IL-1RAcP) that is known to affect Th2 inflammatory response with partial effects on Th1 responses. This cytokine is released by epithelial and smooth muscle cells of the airway system during their injury by several environmental stimuli, such as allergens, viruses, helminths, and pollutants. IL-33 is an alarmin that acts as an endogenous danger signal, and it has been known to affect various types of cells, such as mast cells, basophils, eosinophils, T cells, and specific subsets of innate lymphoid cells (ILCs). In recent findings, this cytokine is believed to have a critical role in several types of cancers, such as lung cancer, liver cancer, and head and neck squamous cell cancer. The expression of IL-33/ST2 in cancer tissues shows a close association with tumor growth and tumor progression in several types of cancer, suggesting the IL-33/ST2 pathway as a potential target for therapy.

IL-33 signalling contributes to pollutant-induced allergic airway inflammation

BACKGROUND

Clinical and experimental studies have identified a crucial role for IL-33 and its receptor ST2 in allergic asthma. Inhalation of traffic-related pollutants, such as diesel exhaust particles (DEP), facilitates the development of asthma and can cause exacerbations of asthma. However, it is unknown whether IL-33/ST2 signalling contributes to the enhancing effects of air pollutants on allergic airway responses.

OBJECTIVE

We aim to investigate the functional role of IL-33/ST2 signalling in DEP-enhanced allergic airway responses, using an established murine model.

METHODS

C57BL/6J mice were exposed to saline, DEP alone, house dust mite (HDM) alone or combined DEP+HDM. To inhibit IL-33 signalling, recombinant soluble ST2 (r-sST2) was given prophylactically (ie, during the whole experimental protocol) or therapeutically (ie, at the end of the experimental protocol). Airway hyperresponsiveness and the airway inflammatory responses were assessed in bronchoalveolar lavage fluid (BALF) and lung.

RESULTS

Combined exposure to DEP+HDM increased IL-33 and ST2 expression in lung, elevated inflammatory responses and bronchial hyperresponsiveness compared to saline, sole DEP or sole HDM exposure. Prophylactic interference with the IL-33/ST2 signalling pathway impaired the DEP-enhanced allergic airway inflammation in the BALF, whereas effects on lung inflammation and airway hyperresponsiveness were minimal. Treatment with r-sST2 at the end of the experimental protocol did not modulate the DEP-enhanced allergic airway responses.

CONCLUSION

Our data suggest that the IL-33/ST2 pathway contributes to the onset of DEP-enhanced allergic airway inflammation.

IL-33 promotes recovery from acute colitis by inducing miR-320 to stimulate epithelial restitution and repair

Defective and/or delayed wound healing has been implicated in the pathogenesis of several chronic inflammatory disorders, including inflammatory bowel disease (IBD). The resolution of inflammation is particularly important in mucosal organs, such as the gut, where restoration of epithelial barrier function is critical to reestablish homeostasis with the interfacing microenvironment. Although IL-33 and its receptor ST2/ILRL1 are known to be increased and associated with IBD, studies using animal models of colitis to address the mechanism have yielded ambiguous results, suggesting both pathogenic and protective functions. Unlike those previously published studies, we focused on the functional role of IL-33/ST2 during an extended (2-wk) recovery period after initial challenge in dextran sodium sulfate (DSS)-induced colitic mice. Our results show that during acute, resolving colitis the normal function of endogenous IL-33 is protection, and the lack of either IL-33 or ST2 impedes the overall recovery process, while exogenous IL-33 administration during recovery dramatically accelerates epithelial restitution and repair, with concomitant improvement of colonic inflammation. Mechanistically, we show that IL-33 stimulates the expression of a network of microRNAs (miRs) in the Caco2 colonic intestinal epithelial cell (IEC) line, especially miR-320, which is increased by >16-fold in IECs isolated from IL-33-treated vs. vehicle-treated DSS colitic mice. Finally, IL-33-dependent in vitro proliferation and wound closure of Caco-2 IECs is significantly abrogated after specific inhibition of miR-320A. Together, our data indicate that during acute, resolving colitis, IL-33/ST2 plays a crucial role in gut mucosal healing by inducing epithelial-derived miR-320 that promotes epithelial repair/restitution and the resolution of inflammation.

NLRP3 Inflammasome and IL-33: Novel Players in Sterile Liver Inflammation

In sterile liver inflammation, danger signals are released in response to tissue injury to alert the immune system; e.g., by activation of the NLRP3 inflammasome. Recently, IL-33 has been identified as a novel type of danger signal or “alarmin”, which is released from damaged and necrotic cells. IL-33 is a pleiotropic cytokine that targets a broad range of immune cells and exhibits pro- and anti-inflammatory properties dependent on the disease. This review summarizes the immunomodulatory roles of the NLRP3 inflammasome and IL-33 in sterile liver inflammation and highlights potential therapeutic strategies targeting these pathways in liver disease.

Interleukin-33 contributes to disease severity in Dengue virus infection in mice

The excessive inflammation often present in patients with severe dengue infection is considered both a hallmark of disease and a target for potential treatments. Interleukin-33 (IL-33) is a pleiotropic cytokine with pro-inflammatory effects whose role in dengue has not been fully elucidated. We demonstrate that IL-33 plays a disease-exacerbating role during experimental dengue infection in immunocompetent mice. Mice infected with dengue virus serotype 2 (DENV2) produced high levels of IL-33. DENV2-infected mice treated with recombinant IL-33 developed markedly more severe disease compared with untreated mice as assessed by mortality, granulocytosis, liver damage and pro-inflammatory cytokine production. Conversely, ST2^-/- mice (deficient in IL-33 receptor) infected with DENV2 developed significantly less severe disease compared with wild-type mice. Furthermore, the increased disease severity and the accompanying pathology induced by IL-33 during dengue infection were reversed by the simultaneous treatment with a CXCR2 receptor antagonist (DF2156A). Together, these results indicate that IL-33 plays a disease-exacerbating role in experimental dengue infection, probably driven by CXCR2-expressing cells, leading to elevated pro-inflammatory response-mediated pathology. Our results also indicate that IL-33 is a potential therapeutic target for dengue infection.

The Role of IL-33/ST2 Pathway in Tumorigenesis

Cancer is initiated by mutations in critical regulatory genes; however, its progression to malignancy is aided by non-neoplastic cells and molecules that create a permissive environment known as the tumor stroma or microenvironment (TME). Interleukin 33 (IL-33) is a dual function cytokine that also acts as a nuclear factor. IL-33 typically resides in the nucleus of the cells where it is expressed. However, upon tissue damage, necrosis, or injury, it is quickly released into extracellular space where it binds to its cognate receptor suppression of tumorigenicity 2 (ST2)L found on the membrane of target cells to potently activate a T Helper 2 (Th2) immune response, thus, it is classified as an alarmin. While its role in immunity and immune-related disorders has been extensively studied, its role in tumorigenesis is only beginning to be elucidated and has revealed opposing roles in tumor development. The IL-33/ST2 axis is emerging as a potent modulator of the TME. By recruiting a cohort of immune cells, it can remodel the TME to promote malignancy or impose tumor regression. Here, we review its multiple functions in various cancers to better understand its potential as a therapeutic target to block tumor progression or as adjuvant therapy to enhance the efficacy of anticancer immunotherapies.

In situ inflammasome activation results in severe damage to the central nervous system in fatal Zika virus microcephaly cases

Zika virus (ZIKV) has caused substantial concern worldwide owing to its association with severe birth defects, such as microcephaly and other congenital malformations. Inflammasomes, i.e., multi-protein complexes that induce inflammation and pyroptosis, are predicted to contribute to the immune response to this flavivirus. Accordingly, in this study, the in situ inflammasome response was evaluated in fatal cases of ZIKV-linked microcephaly. Brain tissue samples were collected from eight babies, including four ZIKV-positive microcephalic neonates who died after birth and four flavivirus-negative neonatal controls who died of other causes and whose central nervous system (CNS) architecture was preserved. In the ZIKV-positive newborn/stillbirth babies, the major histopathological alterations included atrophy of the cortical layer, a predominance of mononuclear cell infiltration in the Virchow-Robin space, neuronal necrosis, vacuolization and neuronal degeneration, neuronophagy, and gliosis. An immunohistochemical analysis of tissues in the neural parenchyma showed significantly higher expression of the receptors NLRP1, NLRP3, and AIM2, cytokines IL-1β, IL-18, and IL-33, and enzymes caspase 1, iNOS, and arginase 1 in ZIKV-positive microcephaly cases than in flavivirus-negative controls. These results suggest that inflammasome activation can aggravate the neuroinflammatory response and consequently increase CNS damage in neonates with fetal neural ZIKV infection and microcephaly.

Regulatory Mechanisms of IL-33-ST2-Mediated Allergic Inflammation

Interleukin-33 (IL-33) plays multiple roles in tissue homeostasis, prevention of parasitic infection, and induction of allergic inflammation. Especially, IL-33-ST2 (IL-1RL1) axis has been regarded as the villain in allergic diseases such as asthma and atopic dermatitis and in autoimmune diseases such as rheumatoid arthritis. Indeed, a number of studies have indicated that IL-33 produced by endothelial cells and epithelial cells plays a critical role in the activation and expansion of group 2 innate lymphoid cells (ILC2s) which cause allergic inflammation by producing large amounts of IL-5 and IL-13. However, mechanisms that antagonize IL-33-ST2-mediated allergic responses remain largely unknown. Recently, several groups including our group have demonstrated cellular and molecular mechanisms that could suppress excessive activation of ILC2s by the IL-33-ST2 axis. In this review, we summarize recent progress in the regulatory mechanisms of IL-33-ST2-mediated allergic responses. Selective targeting of the IL-33-ST2 axis would be a promising strategy in the treatment of allergic diseases.

Interleukin-33 deficiency exacerbated experimental autoimmune encephalomyelitis with an influence on immune cells and glia cells

Interleukin (IL)-33, a member of the IL-1 cytokine family, is highly expressed in central nervous system (CNS), suggesting its potential role in CNS. Although some studies have focused on the role of IL-33 in multiple sclerosis (MS) / experimental autoimmune encephalomyelitis (EAE), an autoimmune disease characterized by demyelination and axonal damage in CNS, the exact role of IL-33 in MS/EAE remains unclear and controversial. Here, we used IL-33 knockout mice to clarify the role of endogenous IL-33 in EAE by simultaneously eliminating its role as a nuclear transcription factor and an extracellular cytokine. We found that the clinical score in IL-33 knockout EAE mice was higher accompanied by more severe demyelination compared with the wild-type (WT) EAE mice. As for the main immune cells participating in EAE in IL-33 knockout mice, pathogenic effector T cells increased both in peripheral immune organs and CNS, while CD4⁺FOXP3⁺ regulatory T cells decreased in spleen and lymph nodes, Th2 cells and natural killer (NK) cells decreased in CNS. Additionally, the populations of microglia/macrophages and CD11C⁺CD11B⁺ dendritic cells (DCs) increased in CNS of IL-33 knockout mice with EAE, among which iNOS-producing microglia/macrophages increased. Moreover, resident astrocytes/microglia were more activated in IL-33 knockout mice with EAE. In vitro, after blocking the IL-33, the proliferation of primary astrocytes, the production of MCP-1/CCL2 and TNF-α by astrocytes, and the production of TNF-α by primary microglia stimulated by the homogenate of the peak stage of EAE were increased. Our results indicate that IL-33 plays a protective role in EAE and exerts extensive influences on multiple immune cells and neural cells involved in EAE.

Immune cells and inflammation in AKI to CKD progression

Acute kidney injury (AKI) is a common clinical state resulting from pathogenic conditions such as ischemic and toxic insults. The pathophysiology of AKI shares common pathogenic denominators including cell death/injury, inflammation, and fibrosis, regardless of the initiating insults. Recent clinical studies have shown that a single episode of AKI can lead to subsequent chronic kidney disease (CKD). Although the involvement of multiple types of cells in the pathophysiology of AKI is becoming increasingly clear, the precise mechanisms for this "AKI to CKD progression" are still unknown, and no drug has been shown to halt this progression. An increasing number of epidemiological studies have also revealed that the presence of aging greatly increases the risk of AKI to CKD progression, and chronic inflammation is increasingly recognized as an important determinant factor for this progression. In this review article, we first describe the current understanding of the pathophysiology of AKI to CKD progression based on multiple types of cells. In particular, we will highlight the recent findings in regard to the mechanisms for chronic inflammation after AKI. Subsequently, we will focus on the mechanisms responsible for the increased risk of AKI to CKD progression in the elderly. Finally, we highlight our recent finding of age-dependent tertiary lymphoid tissue formation and its roles in AKI to CKD progression and speculate on the potential therapeutic opportunities that come from targeting aberrant inflammation after AKI.

Regulation of Energy Expenditure and Brown/Beige Thermogenic Activity by Interleukins: New Roles for Old Actors

Obesity rates and the burden of metabolic associated diseases are escalating worldwide Energy burning brown and inducible beige adipocytes in human adipose tissues (ATs) have attracted considerable attention due to their therapeutic potential to counteract the deleterious metabolic effects of nutritional overload and overweight. Recent research has highlighted the relevance of resident and recruited ATs immune cell populations and their signalling mediators, cytokines, as modulators of the thermogenic activity of brown and beige ATs. In this review, we first provide an overview of the developmental, cellular and functional heterogeneity of the AT organ, as well as reported molecular switches of its heat-producing machinery. We also discuss the key contribution of various interleukins signalling pathways to energy and metabolic homeostasis and their roles in the biogenesis and function of brown and beige adipocytes. Besides local actions, attention is also drawn to their influence in the central nervous system (CNS) networks governing energy expenditure.

Interleukin 33 Selectively Augments Rhinovirus-Induced Type 2 Immune Responses in Asthmatic but not Healthy People

Interleukin- 33 (IL-33) is an epithelial-derived cytokine that initiates type 2 immune responses to allergens, though whether IL-33 has the ability to modify responses to respiratory viral infections remains unclear. This study aimed to investigate the effects of IL-33 on rhinovirus (RV)-induced immune responses by circulating leukocytes from people with allergic asthma, and how this response may differ from non-allergic controls. Our experimental approach involved co-exposing peripheral blood mononuclear cells to IL-33 and RV in order to model how the functions of virus-responsive lymphocytes could be modified after recruitment to an airway environment enriched in IL-33. In the current study, IL-33 enhanced RV-induced IL-5 and IL-13 release by cells from people with allergic asthma, but had no effect on IL-5 and IL-13 production by cells from healthy donors. In asthmatic individuals, IL-33 also enhanced mRNA and surface protein expression of ST2 (the IL-33 receptor IL1RL1), while soluble ST2 concentrations were low. In contrast, IL-33 had no effect on mRNA and surface expression of ST2 in healthy individuals. In people with allergic asthma, RV-activated ST2⁺ innate lymphoid cells (ST2⁺ILC) were the predominant source of IL-33 augmented IL-13 release. In contrast, RV-activated natural killer cells (NK cells) were the predominant source of IL-33 augmented IFNγ release in healthy individuals. This suggests that the effects of IL-33 on the cellular immune response to RV differ between asthmatic and healthy individuals. These findings provide a mechanism by which RV infections and IL-33 might interact in asthmatic individuals to exacerbate type 2 immune responses and allergic airway inflammation.

Chromatin regulates IL-33 release and extracellular cytokine activity

IL-33 is an epithelium-derived, pro-inflammatory alarmin with enigmatic nuclear localization and chromatin binding. Here we report the functional properties of nuclear IL-33. Overexpression of IL-33 does not alter global gene expression in transduced epithelial cells. Fluorescence recovery after photobleaching data show that the intranuclear mobility of IL-33 is ~10-fold slower than IL-1α, whereas truncated IL-33 lacking chromatin-binding activity is more mobile. WT IL-33 is more resistant to necrosis-induced release than truncated IL-33 and has a relatively slow, linear release over time after membrane dissolution as compared to truncated IL-33 or IL-1α. Lastly, IL-33 and histones are released as a high-molecular weight complex and synergistically activate receptor-mediated signaling. We thus propose that chromatin binding is a post-translational mechanism that regulates the releasability and ST2-mediated bioactivity of IL-33 and provide a paradigm to further understand the enigmatic functions of nuclear cytokines.

Increased serum interleukin-33 concentrations predict worse prognosis of aneurysmal subarachnoid hemorrhage

BACKGROUND

Interleukin-33 (IL-33) is an inflammatory biomarker. We elucidated the relationship between serum IL-33 concentrations, severity and prognosis in aneurysmal subarachnoid hemorrhage (aSAH).

METHODS

We prospectively recruited 175 controls and 175 aSAH patients. Serum IL-33 concentrations were gauged using an enzyme-linked immunosorbent assay. Clinical and radiological severity was assessed by World Federation of Neurological Surgeons (WFNS) scale and modified Fisher grading scale respectively. Poor outcome was defined as Glasgow Outcome Scale score of 1-3.

RESULTS

Serum IL-33 concentrations were significantly higher in patients than in controls. IL-33 concentrations were significantly increased with increasing WFNS scores, modified Fisher scores and serum C-reactive protein concentrations. Serum IL-33 emerged as an independent predictor for 6-month mortality and poor outcome. Under receiver operating characteristic curve, the prognostic predictive ability of serum IL-33 was equivalent to those of WFNS scores and modified Fisher scores. Moreover, serum IL-33 significantly improved the prognostic predictive performance of WFNS scores and modified Fisher scores.

CONCLUSIONS

High serum IL-33 concentrations have close relation to the inflammation, severity and poor outcome in aSAH, indicating IL-33 might have the potential to be an inflammatory biomarker for assessing severity and reflecting prognosis of aSAH.

Intestinal Epithelial Cells Regulate Gut Eotaxin Responses and Severity of Allergy

Intestinal epithelial cells (IECs) are known to regulate allergic sensitization. We addressed the role of the intrinsic IKKβ signaling in IECs in the effector phase of allergy following oral allergen challenge and its impact on the severity of responses is poorly. Upon orally sensitization by co-administration of ovalbumin with cholera toxin as adjuvant, wild-type and mice lacking IKKβ in IECs (IKKβ^ΔIEC mice) developed similar levels of serum IgE and allergen-specific secretory IgA in the gut. However, subsequent allergen challenges in the gut promoted allergic lower responses in KKβ^ΔIEC mice. Analysis of cytokines and chemokines in serum and gut tissues after oral allergen challenge revealed impaired eotaxin responses in IKKβ^ΔIEC mice, which correlated with lower frequencies of eosinophils in the gut lamina propria. We also determined that IECs were a major source of eotaxin and that impaired eotaxin production was due to the lack of IKKβ signaling in IECs. Oral administration of CCL11 to IKKβ^ΔIEC mice during oral allergen challenge enhanced allergic responses to levels in wild-type mice, confirming the role of IEC-derived eotaxin as regulator of the effector phase of allergy following allergen challenge. Our results identified targeting IEC-derived eotaxin as potential strategy to limit the severity of allergic responses to food antigens.

From proteomics to discovery of first-in-class ST2 inhibitors active in vivo

Soluble cytokine receptors function as decoy receptors to attenuate cytokine-mediated signaling and modulate downstream cellular responses. Dysregulated overproduction of soluble receptors can be pathological, such as soluble ST2 (sST2), a prognostic biomarker in cardiovascular diseases, ulcerative colitis, and graft-versus-host disease (GVHD). Although intervention using an ST2 antibody improves survival in murine GVHD models, sST2 is a challenging target for drug development because it binds to IL-33 via an extensive interaction interface. Here, we report the discovery of small-molecule ST2 inhibitors through a combination of high-throughput screening and computational analysis. After in vitro and in vivo toxicity assessment, 3 compounds were selected for evaluation in 2 experimental GVHD models. We show that the most effective compound, iST2-1, reduces plasma sST2 levels, alleviates disease symptoms, improves survival, and maintains graft-versus-leukemia activity. Our data suggest that iST2-1 warrants further optimization to develop treatment for inflammatory diseases mediated by sST2.

ST2/IL-33 signaling promotes malignant development of experimental squamous cell carcinoma by decreasing NK cells cytotoxicity and modulating the intratumoral cell infiltrate

Squamous cell carcinoma (SCC) is the second most common form of skin cancer and the mechanism(s) involved in the progression of this tumor are unknown. Increases in the expression of IL-33/ST2 axis components have been demonstrated to contribute to neoplastic transformation in several tumor models and interleukin-33 is correlated with poor prognosis of patients with squamous cell carcinoma of the tongue. Based on these observations, we sought to determine the role of the IL-33/ST2 pathway during the development of SCC. Our findings show that ST2-deficiency led to a marked decrease in the severity of skin lesions, suggesting that ST2 signaling contributed to tumor development. An analysis of tumor lesions in wild-type and ST2KO mice revealed that a lack of ST2 was associated with specific and significant reductions in the numbers of CD4⁺ T cells, CD8⁺ T cells, dendritic cells, and macrophages. In addition, NK cells that were isolated from ST2KO mice exhibited higher cytotoxic activity than cells isolated from wild-type mice. Notably, ST2 deficiency resulted in lower IFN-γ, TNF-α, IL-10, and IL-17 production in tumor samples. Our findings indicate that the IL-33/ST2 pathway contributes to the development of SCC by affecting leukocyte migration to tumor microenvironment and impairing NK cytotoxic activity.

The family of the interleukin-1 receptors

The extracellular forms of the IL-1 cytokines are active through binding to specific receptors on the surface of target cells. IL-1 ligands bind to the extracellular portion of their ligand-binding receptor chain. For signaling to take place, a non-binding accessory chain is recruited into a heterotrimeric complex. The intracellular approximation of the Toll-IL-1-receptor (TIR) domains of the 2 receptor chains is the event that initiates signaling. The family of IL-1 receptors (IL-1R) includes 10 structurally related members, and the distantly related soluble protein IL-18BP that acts as inhibitor of the cytokine IL-18. Over the years the receptors of the IL-1 family have been known with many different names, with significant confusion. Thus, we will use here a recently proposed unifying nomenclature. The family includes several ligand-binding chains (IL-1R1, IL-1R2, IL-1R4, IL-1R5, and IL-1R6), 2 types of accessory chains (IL-1R3, IL-1R7), molecules that act as inhibitors of signaling (IL-1R2, IL-1R8, IL-18BP), and 2 orphan receptors (IL-1R9, IL-1R10). In this review, we will examine how the receptors of the IL-1 family regulate the inflammatory and anti-inflammatory functions of the IL-1 cytokines and are, more at large, involved in modulating defensive and pathological innate immunity and inflammation. Regulation of the IL-1/IL-1R system in the brain will be also described, as an example of the peculiarities of organ-specific modulation of inflammation.

IL-33 receptor (ST2) deficiency downregulates myeloid precursors, inflammatory NK and dendritic cells in early phase of sepsis

BACKGROUND

Sepsis is a life-threatening disease mediated by profound disturbances in systemic inflammatory response to infection. IL-33 is multifunctional regulator of numerous aspects of innate and adaptive immune response. The aim of this article was to further evaluate the role of IL-33 receptor (ST2) in different pathways of innate immunity during early polymicrobial sepsis.

METHODS

Polymicrobial sepsis was induced using cecal ligation and puncture (CLP) model in ST2 deficient (ST2-/-) and wild type BALB/c mice. Peritoneal and spleen cells were isolated for further phenotyping. Apoptosis was determined by immunohistochemistry and flow cytometry.

RESULTS

Deletion of ST2 leads to increased susceptibility to early manifestations of sepsis as evaluated by clinical signs and survival. These are accompanied by decrease in the total number of neutrophils, eosinophils and mast cells in peritoneal cavity 12 h after CLP. In early sepsis there was also low number of precursors of myeloid cells in particular CD11b+Ly6G+Ly6Clow cells in spleen of ST2-/- mice. Although the number of NK cells in the spleen was similar, there were significant differences in the presence of inflammatory IFN-γ and IL-17 producing NK cells. Further, ST2 deletion affects the phenotype and maturation of dendritic cell in sepsis. The total number of dendritic cells in the spleen was lower as well as IL-12 expressing dendritic cells. Finally, there was higher frequency of active caspase-3 positive and early apoptotic cells, in particular CD11c positive cells, in spleen of septic ST2-/- mice.

CONCLUSION

Taken together, our data provide the evidence that ST2 deficiency in early phase of sepsis downregulates myeloid precursors, inflammatory NK and dendritic cells.

Paracrine effect of regulatory T cells promotes cardiomyocyte proliferation during pregnancy and after myocardial infarction

Cardiomyocyte proliferation stops at birth when the heart is no longer exposed to maternal blood and, likewise, to regulatory T cells (Tregs) that are expanded to promote maternal tolerance towards the fetus. Here, we report a role of Tregs in promoting cardiomyocyte proliferation. Treg-conditioned medium promotes cardiomyocyte proliferation, similar to the serum from pregnant animals. Proliferative cardiomyocytes are detected in the heart of pregnant mothers, and Treg depletion during pregnancy decreases both maternal and fetal cardiomyocyte proliferation. Treg depletion after myocardial infarction results in depressed cardiac function, massive inflammation, and scarce collagen deposition. In contrast, Treg injection reduces infarct size, preserves contractility, and increases the number of proliferating cardiomyocytes. The overexpression of six factors secreted by Tregs (Cst7, Tnfsf11, Il33, Fgl2, Matn2, and Igf2) reproduces the therapeutic effect. In conclusion, Tregs promote fetal and maternal cardiomyocyte proliferation in a paracrine manner and improve the outcome of myocardial infarction.

Regulatory T cells (Tregs) expand during pregnancy to promote tolerance towards the fetus. Here the authors show that Tregs induce proliferation of fetal and maternal cardiomyocytes during pregnancy and enhance myocardial repair via proliferation-promoting paracrine actions.

IL-33/ST2 Pathway and Galectin-3 as a New Analytes in Pathogenesis and Cardiometabolic Risk Evaluation in Psychosis

Schizophrenia and treatment of this disorder are often accompanied with metabolic syndrome and cardiovascular issues. Alterations in the serum level of innate immune mediators, such as interleukin-33 (IL-33) and its receptor IL-33R (ST2) and Galectin-3 (Gal-3) were observed in these conditions. Moreover, these parameters are potential prognostic and therapeutic markers. There is also accumulating evidence that these molecules play a role in neuroinflammation. Therefore, in this study we have investigated the serum level of Gal-3, IL-33 and soluble ST2 (sST2) in different stages of schizophrenia. Gal-3 levels were elevated in remission and lower in schizophrenia exacerbation in comparison with controls. Levels of IL-33 and sST2 are higher in schizophrenia exacerbation in comparison with controls and patients in remission. This initial analysis of new markers of neuroinflammation suggested their involvement in schizophrenia pathophysiology and/or cardiometabolic comorbidity.

IL-33 Initiates Vascular Remodelling in Hypoxic Pulmonary Hypertension by up-Regulating HIF-1α and VEGF Expression in Vascular Endothelial Cells

IL-33 may play a role in the vascular remodelling of hypoxic pulmonary hypertension (PH) but the precise mechanisms are still unclear. We hypothesized that hypoxia promotes expression of IL-33 and its receptor ST2 on vascular endothelial cells, which in turn leads to dysfunction of vascular endothelial cells and smooth muscle cells contributing to PH. Immunohistochemistry showed that immunoreactivity for IL-33 and ST2 was significantly increased in lung tissue of murine model of hypoxia-induced PH (HPH) and of subjects with bronchiectasis-PH. trans-Thoracic echocardiography showed that haemodynamic changes and right ventricular hypertrophy associated with HPH were significantly abrogated in St2^−/− compared with WT mice. Administration of IL-33 further exacerbated these changes in the hypoxia-exposed WT mice. In vitro, hypoxia significantly increased IL-33/ST2 expression by human pulmonary arterial endothelial cells (HPAECs), while exogenous IL-33 enhanced proliferation, adhesiveness and spontaneous angiogenesis of HPAECs. Knockdown of endogenous Il33 or St2 using siRNA transfection significantly suppressed these effects in both normoxic and hypoxic culture-conditions. Deletion of the St2 gene attenuated hypoxia-induced, elevated lung expression of HIF-1α/VEGFA/VEGFR-2/ICAM-1, while administration of exogenous VEGFA partially reversed the attenuation of the haemodynamic indices of PH. Correspondingly, knockdown of the St2 or Hif1α genes almost completely abrogated IL-33-induced expression of HIF-1α/VEGFA/VEGFR-2 by HPAECs in vitro. Further, IL-33-induced angiogenesis by HPAECs was extensively abrogated by knockdown of the Hif1α/Vegfa or Vegfr2 genes. These data suggest that hypoxia induces elevated expression of IL-33/ST2 by HPAECs which, at least partly by increasing downstream expression of HIF-1α and VEGF initiates vascular remodelling resulting in HPH.

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Evidence before this study

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We have been focusing on the role of cytokines in the pathogenesis of chronic pulmonary diseases for a long time, including asthma, COPD, fibrosis and bronchiectasis. We and others found that IL-33 might contribute to the occurrence and prognosis of many other diseases through binding its receptor ST2. Based on these findings, we were very eager to know whether IL-33/ST2 axis also exerts a role in hypoxia-induced pulmonary hypertension (HPH), a complication of many chronic respiratory diseases. Although it is well known that HIF-1α and VEGF play critical role in this complication, it is still unclear what the upstream of HIF-1α and VEGF is. Therefore, we first tested immunoreactivity for IL-33 and its receptor ST2 in the lung tissue sections derived from surgical specimens and from our established murine models of HPH. Surprisingly, we noted the increased immunoreactivity for both targets in these tissue sections. These findings inspired us to further explore the details of IL-33/ST2 in the pathogenesis of HPH.

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Added value of this study

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HPH is a life-threatening complication because there is lack of effective treatment. Although pulmonary arteries and ventricular remodelling might be mainly involved in the pathogenesis of the disease, the precise mechanisms are largely unknown. In the present study, we showed that hypoxia is a critical driver which induced expression of IL-33 and ST2 by endothelial cells. These factors, in turn triggered expression of HIF-1α and VEGF by endothelial cells and led to proliferation, adhesion and tube formation of these cells. We also showed that in the presence of IL-33, endothelial cells were able to affect proliferation and migration of artery smooth muscle cells, although IL-33 alone did not have such effects. These findings suggest that hypoxia and IL-33/ST2 might be initiators for HPH, through regulating downstream factors HIF-1α and VEGF.

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Implications of all the available evidence

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Our data suggest that IL-33/ST2 axis plays critical role in the pathogenesis of hypoxia-induced pulmonary hypertension because depletion of these molecules much remitted the phenomenon of complication. These observations might provide alternative therapeutic strategy for clinical treatment of HPH.

Mast Cell Proteases Activate Astrocytes and Glia-Neurons and Release Interleukin-33 by Activating p38 and ERK1/2 MAPKs and NF-κB

Inflammatory mediators released from activated microglia, astrocytes, neurons, and mast cells mediate neuroinflammation. Parkinson's disease (PD) is characterized by inflammation-dependent dopaminergic neurodegeneration in substantia nigra. 1-Methyl-4-phenylpyridinium (MPP⁺), a metabolite of parkinsonian neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), induces inflammatory mediators' release from brain cells and mast cells. Brain cells' interaction with mast cells is implicated in neuroinflammation. However, the exact mechanisms involved are not yet clearly understood. Mouse fetal brain-derived cultured primary astrocytes and glia-neurons were incubated with mouse mast cell protease-6 (MMCP-6) and MMCP-7, and mouse bone marrow-derived mast cells (BMMCs) were incubated with MPP⁺ and brain protein glia maturation factor (GMF). Interleukin-33 (IL-33) released from these cells was quantitated by enzyme-linked immunosorbent assay. Both MMCP-6 and MMCP-7 induced IL-33 release from astrocytes and glia-neurons. MPP⁺ and GMF were used as a positive control-induced IL-33 and reactive oxygen species expression in mast cells. Mast cell proteases and MPP⁺ activate p38 and extracellular signal-regulated kinases 1/2 (ERK1/2), mitogen-activated protein kinases (MAPKs), and transcription factor nuclear factor-kappa B (NF-κB) in astrocytes, glia-neurons, or mast cells. Addition of BMMCs from wt mice and transduction with adeno-GMF show higher chemokine (C-C motif) ligand 2 (CCL2) release. MPP⁺ activated glial cells and reduced microtubule-associated protein 2 (MAP-2) expression indicating neurodegeneration. IL-33 expression increased in the midbrain and striatum of PD brains as compared with age- and sex-matched control subjects. Glial cells and neurons interact with mast cells and accelerate neuroinflammation and these interactions can be explored as a new therapeutic target to treat PD.

Insights in particulate matter-induced allergic airway inflammation: Focus on the epithelium

Outdoor air pollution is a major environmental health problem throughout the world. In particular, exposure to particulate matter (PM) has been associated with the development and exacerbation of several respiratory diseases, including asthma. Although the adverse health effects of PM have been demonstrated for many years, the underlying mechanisms have not been fully identified. In this review, we focus on the role of the lung epithelium and specifically highlight multiple cytokines in PM-induced respiratory responses. We describe the available literature on the topic including in vitro studies, findings in humans (ie observations in human cohorts, human controlled exposure and ex vivo studies) and in vivo animal studies. In brief, it has been shown that exposure to PM modulates the airway epithelium and promotes the production of several cytokines, including IL-1, IL-6, IL-8, IL-25, IL-33, TNF-α, TSLP and GM-CSF. Further, we propose that PM-induced type 2-promoting cytokines are important mediators in the acute and aggravating effects of PM on airway inflammation. Targeting these cytokines could therefore be a new approach in the treatment of asthma.

Macrophages-common culprit in obesity and asthma

Macrophages are essential innate immune cells that also regulate local metabolism. Endogenous or exogenous stimuli may polarize macrophages toward phenotypes that serve distinct innate immunological metabolic functions. IFN-γ or lipopolysaccharide (LPS) polarizes macrophages toward the M1, or "classically activated" phenotype that participates in defense against intracellular pathogens. IL-4, IL-13, or chitin polarizes macrophages toward the M2, or "alternatively activated" phenotype, which defends against multicellular nematodes and fungi. As macrophages polarize in local environments, M1 and M2 macrophages may coexist in different organs and may differentially affect asthma and obesity, two comorbid diseases where polarized macrophages contribute to their pathogenesis. While M1 macrophages are considered beneficial in asthma and contribute to the pathology of obesity, M2 macrophages contribute to the pathology of asthma, but limit metabolic syndrome associated with obesity. Here, we discuss the roles for M1 and M2 macrophages in asthma and obesity, and propose a model by which M1-mediated inflammation in adipose tissue enhances M2-mediated inflammation in the asthmatic lung.

Cigarette smoke extract interferes with placenta macrophage functions: A new mechanism to compromise placenta functions?

The success of pregnancy depends on the maternal immune system's ability to promote tolerance and host defense. This equilibrium is compromised in inflammatory and infectious impairment of placenta. Smoking during pregnancy exposes the fetus to severe complications which might result from an alteration in placenta macrophages (pMφ) functions. In this study, we assessed the effect of cigarette smoke extract (CSE) on the functions of third trimester pMφs.CSE inhibited particles uptake and the formation of multinucleated giant cells, a recently reported property of pMφs based on their ability to fuse in vitro. These alterations were associated with a CSE-induced abnormal activation of pMφs, which was characterized by an increased release of TNF, interleukin (IL)-33, and decreased IL-6 and IL-10 release. Furthermore, CSE enhanced the expression of metalloproteinase genes known to be involved in tissue remodeling. This effect of CSE on pMφs was specific because CSE affected circulating monocytes in a different way. Finally, we showed that nicotine affected in part the functional properties of pMφs. Taken together, these results showed that CSE modulated the functional activity of pMφs, which may compromise pregnancy.

Integrated Expression Profiles Analysis Reveals Correlations Between the IL-33/ST2 Axis and CD8+ T Cells, Regulatory T Cells, and Myeloid-Derived Suppressor Cells in Soft Tissue Sarcoma

Soft tissue sarcoma (STS) is a rare solid malignant cancer, and there are few effective treatment options for advanced disease. Cancer immunotherapy is a promising new strategy for STS treatment. IL-33 is a candidate cytokine for immunotherapy that can activate T lymphocytes and modulate antitumor immunity in some cancers. However, the expression and biological role of IL-33 in STS are poorly understood. In this study, we found that the expression of IL-33 and its receptor ST2 was decreased in STS using real-time PCR assays. By analyzing sarcoma data from The Cancer Genome Atlas, we found that higher transcriptional levels of IL-33 and ST2 were associated with a favorable outcome. There were positive correlations between the expression levels of ST2 and CD3E, CD4, CD8A, CD45RO, FOXP3, CD11B, CD33, and IFN-γ. Strong positive correlations between the expression of IFN-γ and CD3E and CD8A were also observed. Moreover, the expression levels of both IL-33 and ST2 were positively correlated with those of CD3E, CD8A, and chemokines that recruit CD8⁺ T cells, indicating that the IL-33/ST2 axis may play an important role in recruiting and promoting the immune response of type 1-polarized CD8⁺ T cells in STS. Meanwhile, we also found that the expression of IL-33 was negatively correlated with that of TGF-β1 and chemokines that recruit regulatory T cells (Tregs) and myeloid-derived suppressor cells (MDSCs), indicating that the IL-33/ST2 axis may also contribute to antagonizing Tregs, MDSCs, and TGF-β1-mediated immunosuppression in STS. The correlations between the IL-33/ST2 axis and CD8⁺ T cells and IFN-γ, as well as Tregs, MDSCs, and TGF-β1 were validated by additional analyses using three other independent GEO datasets of sarcoma. Our results implicate the possible role of the IL-33/ST2 axis in modulating antitumor immunity in STS. IL-33 may not only serve as a useful prognostic biomarker for STS but also as a potential therapeutic target for STS immunotherapy and worth further investigation.

The IL-33/ST2 pathway, inflammation and atherosclerosis: Trigger and target?

The "inflammatory hypothesis" of atherosclerosis postulates that inflammatory cell signalling drives the formation, growth and ultimately the instability of atherosclerotic plaques, setting up the substrate for the thrombotic response that causes myocardial damage or infarction. The recent Canakinumab Antiinflammatory Thrombosis Outcome Study (CANTOS) trial has been hailed as the first demonstration, ex iuvantibus, of the inflammatory hypothesis. Indeed, interleukin (IL)-1β inhibition was found to reduce cardiovascular events in patients with previous myocardial infarction and raised high-sensitivity C-reactive protein, despite no effects on the lipid profile. These results prompt a dissection of inflammatory mechanisms of atherosclerosis in order to search for specific biomarkers with prognostic value and/or therapeutic targets. Under this respect, the IL-33/suppression of tumorigenesis 2 (ST2) pathway deserves consideration. Indeed, its elements are particularly expressed in the endothelium of arterial vessels, and the interaction between IL-33 and the ST2 receptor blunts the immune response characteristic of atherosclerosis. By contrast, soluble ST2 (sST2) acts as a decoy receptor for IL-33, thus blocking its protective effects. Despite a solid theoretical framework, no definite demonstration of an involvement of the IL-33/ST2 pathway in atherosclerosis has been provided. Therefore, further studies are warranted to verify if elements of the IL-33/ST2 pathway may be proposed as markers of plaque burden and predictors of future cardiovascular events, and to explore the potential clinical benefit of enhanced IL-33/ST2 signalling in atherosclerosis.

Interleukin-33 Receptor (ST2) Deficiency Improves the Outcome of Staphylococcus aureus-Induced Septic Arthritis

The ST2 receptor is a member of the Toll/IL-1R superfamily and interleukin-33 (IL-33) is its agonist. Recently, it has been demonstrated that IL-33/ST2 axis plays key roles in inflammation and immune mediated diseases. Here, we investigated the effect of ST2 deficiency in Staphylococcus aureus-induced septic arthritis physiopathology. Synovial fluid samples from septic arthritis and osteoarthritis individuals were assessed regarding IL-33 and soluble (s) ST2 levels. The IL-33 levels in samples from synovial fluid were significantly increased, whereas no sST2 levels were detected in patients with septic arthritis when compared with osteoarthritis individuals. The intra-articular injection of 1 × 10⁷ colony-forming unity/10 μl of S. aureus American Type Culture Collection 6538 in wild-type (WT) mice induced IL-33 and sST2 production with a profile resembling the observation in the synovial fluid of septic arthritis patients. Data using WT, and ST2 deficient (^−/−) and interferon-γ (IFN-γ)^−/− mice showed that ST2 deficiency shifts the immune balance toward a type 1 immune response that contributes to eliminating the infection due to enhanced microbicide effect via NO production by neutrophils and macrophages. In fact, the treatment of ST2^−/− bone marrow-derived macrophage cells with anti-IFN-γ abrogates the beneficial phenotype in the absence of ST2, which confirms that ST2 deficiency leads to IFN-γ expression and boosts the bacterial killing activity of macrophages against S. aureus. In agreement, WT cells achieved similar immune response to ST2 deficiency by IFN-γ treatment. The present results unveil a previously unrecognized beneficial effect of ST2 deficiency in S. aureus-induced septic arthritis.

IL-33 stimulates expression of the GPR84 (EX33) fatty acid receptor gene and of cytokine and chemokine genes in human adipocytes

Expression of GPCR fatty acid sensor/receptor genes in adipocytes is modulated by inflammatory mediators, particularly IL-1β. In this study we examined whether the IL-1 gene superfamily member, IL-33, also regulates expression of the fatty acid receptor genes in adipocytes. Human fat cells, differentiated from preadipocytes, were incubated with IL-33 at three different dose levels for 3 or 24 h and mRNA measured by qPCR. Treatment with IL-33 induced a dose-dependent increase in GPR84 mRNA at 3 h, the level with the highest dose being 13.7-fold greater than in controls. Stimulation of GPR84 expression was transitory; the mRNA level was not elevated at 24 h. In contrast to GPR84, IL-33 had no effect on GPR120 expression. IL-33 markedly stimulated expression of the IL1B, CCL2, IL6, CXCL2 and CSF3 genes, but there was no effect on ADIPOQ expression. The largest effect was on CSF3, the mRNA level of which increased 183-fold over controls at 3 h with the highest dose of IL-33; there was a parallel increase in the secretion of G-CSF protein into the medium. It is concluded that in human adipocytes IL-33, which is synthesised in adipose tissue, has a strong stimulatory effect on the expression of cytokine and chemokine genes, particularly CSF3, and on the expression of GPR84, a pro-inflammatory fatty acid receptor.

Helicobacter pylori-induced IL-33 modulates mast cell responses, benefits bacterial growth, and contributes to gastritis

Interleukin (IL)-induced inflammatory responses are critical for the pathogenesis of Helicobacter pylori (H. pylori)-induced gastritis. IL-33 represents a recently discovered proinflammatory cytokine involved in inflammatory diseases, but its relevance to H. pylori-induced gastritis is unknown. Here, we found that gastric IL-33 mRNA and protein expression were elevated in gastric mucosa of both patients and mice infected with H. pylori, which is positively correlated with bacterial load and the degree of gastritis. IL-33 production was promoted via extracellular regulated protein kinases (ERK) signaling pathway activation by gastric epithelial cells in a cagA-dependent manner during H. pylori infection, and resulted in increased inflammation and bacteria burden within the gastric mucosa. Gastric epithelial cell-derived IL-33 promoted TNF-α production from mast cells in vitro, and IL-33 increased TNF-α production in vivo. Increased TNF-α inhibited gastric epithelial cell proliferation, conducing to the progress of H. pylori-associated gastritis and bacteria colonization. This study defined a patent regulatory networks involving H. pylori, gastric epithelial cell, IL-33, mast cell, and TNF-α, which jointly play a pathological effect within the gastric circumstances. It may be a valuable strategy to restrain this IL-33-dependent pathway in the treatment of H. pylori-associated gastritis.

The pathogenicity of IL-33 on steroid-resistant eosinophilic inflammation via the activation of memory-type ST2+ CD4+ T cells

The lungs are the primary organs of the respiratory system in many animals and have unique epithelial barrier systems to protect the host from continuous invasion of various harmful particles, such as viruses and bacteria. IL-33, a member of the IL-1 family of cytokines, is released from epithelial cells in the mucosal organs and drives the type 2 immune response by activating a number of immune cells in cases of helminth infection. However, IL-33 derived from epithelial cells also causes various allergic diseases via the activation of ST2-positive immune cells, including memory-type (CD62L^low CD44^hi ) ST2⁺ CD4⁺ T cells in the lung. Recent studies have revealed that the type 2 inflammation induced by IL-33 is steroid resistant. Steroid resistance causes severe chronic inflammatory diseases, such as intractable asthma. In this review, we will discuss the impact of ST2⁺ CD4⁺ T cells on shaping the pathology of IL-33-induced eosinophilic inflammation. We will also highlight the mechanism underlying steroid resistance in eosinophilic pneumonia. A better understanding of the cellular and molecular mechanisms underlying steroid resistance is crucial for the development of new therapeutic strategies for intractable allergic diseases.

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 .

S1P₄ Regulates Passive Systemic Anaphylaxis in Mice but Is Dispensable for Canonical IgE-Mediated Responses in Mast Cells

Mast cells are key players in the development of inflammatory allergic reactions. Cross-linking of the high-affinity receptor for IgE (FcεRI) on mast cells leads to the generation and secretion of the sphingolipid mediator, sphingosine-1-phosphate (S1P) which is able, in turn, to transactivate its receptors on mast cells. Previous reports have identified the expression of two of the five receptors for S1P on mast cells, S1P₁ and S1P₂, with functions in FcεRI-mediated chemotaxis and degranulation, respectively. Here, we show that cultured mouse mast cells also express abundant message for S1P₄. Genetic deletion of S1pr4 did not affect the differentiation of bone marrow progenitors into mast cells or the proliferation of mast cells in culture. A comprehensive characterization of IgE-mediated responses in S1P₄-deficient bone marrow-derived and peritoneal mouse mast cells indicated that this receptor is dispensable for mast cell degranulation, cytokine/chemokine production and FcεRI-mediated chemotaxis in vitro. However, interleukin-33 (IL-33)-mediated enhancement of IgE-induced degranulation was reduced in S1P₄-deficient peritoneal mast cells, revealing a potential negative regulatory role for S1P₄ in an IL-33-rich environment. Surprisingly, genetic deletion of S1pr4 resulted in exacerbation of passive systemic anaphylaxis to IgE/anti-IgE in mice, a phenotype likely related to mast cell-extrinsic influences, such as the high circulating levels of IgE in these mice which increases FcεRI expression and consequently the extent of the response to FcεRI engagement. Thus, we provide evidence that S1P₄ modulates anaphylaxis in an unexpected manner that does not involve regulation of mast cell responsiveness to IgE stimulation.

Regulatory T Lymphocytes in Periodontitis: A Translational View

Periodontitis is a chronic immuno-inflammatory disease in which the disruption of the balance between host and microbiota interactions is key to the onset and progression of the disease. The immune homeostasis associated with periodontal health requires a regulated immuno-inflammatory response, during which the presence of regulatory T cells (Tregs) is essential to ensure a controlled response that minimizes collateral tissue damage. Since Tregs modulate both innate and adaptive immunity, pathological conditions that may resolve by the acquisition of immuno-tolerance, such as periodontitis, may benefit by the use of Treg immunotherapy. In recent years, many strategies have been proposed to take advantage of the immuno-suppressive capabilities of Tregs as immunotherapy, including the ex vivo and in vivo manipulation of the Treg compartment. Ongoing research in both basic and translational studies let us gain a better understanding of the diversity of Treg subsets, their phenotypic plasticity, and suppressive functions, which can be used as a substrate for new immunotherapies. Certainly, as our knowledge of Treg biology increases, we will be capable to develop new therapies designed to enhance the stability and function of Tregs during periodontitis.

Group 2 innate lymphocytes at the interface between innate and adaptive immunity

Group 2 innate lymphoid cells (ILC2) are innate immune cells that respond rapidly to their environment through soluble inflammatory mediators and cell-to-cell interactions. As tissue-resident sentinels, ILC2 help orchestrate localized type 2 immune responses. These ILC2-driven type 2 responses are now recognized in diverse immune processes, different anatomical locations, and homeostatic or pathological settings. ILC2-derived cytokines and cell surface signaling molecules function as key regulators of innate and adaptive immunity. Conversely, ILC2 are governed by their environment. As such, ILC2 form an important nexus of the immune system and may present an attractive target for immune modulation in disease.

Phenotyping acute and chronic atopic dermatitis-like lesions in Stat6VT mice identifies a role for IL-33 in disease pathogenesis

The Stat6VT mouse model of atopic dermatitis (AD) is induced by T-cell-specific expression of a constitutively active form of the protein signal transducer and activator of transcription 6 (STAT6). Although AD-like lesions are known to develop in Stat6VT mice, this study was designed to determine if these mice develop acute and chronic phases of disease similar to humans. To address this, AD-like lesions from Stat6VT mice were harvested at two different timepoints relative to their onset. Lesions harvested within 1 week after development were defined as acute lesions, and those present for 1 month or more were defined as chronic lesions. Acute and chronic AD-like lesions from Stat6VT mice exhibited histologic findings and cytokine expression patterns similar to acute and chronic AD lesions in humans. Further analysis revealed increased levels of interleukin (IL)-33 transcripts in AD-like lesions compared to Stat6VT nonlesional and wild-type skin controls. Immunofluorescence also revealed increased numbers of IL-33+ keratinocytes in Stat6VT lesional skin and localized IL-33+ keratinocytes to a keratin 5+ subset. Furthermore, AD-like disease was more severe in IL-33-deficient Stat6VT mice compared to IL-33-sufficient Stat6VT mice. These studies suggest that Stat6VT mice can serve as a model of acute and chronic AD and that IL-33 may attenuate inflammation in this system.

Oxidative stress enhances the expression of IL-33 in human airway epithelial cells

BACKGROUND

Interleukin-33 (IL-33) is a cytokine belonging to the IL-1 family, and its possible involvement in the pathophysiology of COPD and viral-induced exacerbations has been demonstrated. IL-33 has been shown to be increased in the airway epithelial cells from COPD patients, but the regulating mechanism of IL-33 expression in airway epithelial cells remains largely unknown. In the current study, we examined whether oxidative stress, which participates in the pathogenesis of COPD, affects the expression of IL-33 in airway epithelial cells and also evaluated the effect during viral infection.

METHODS

The involvement of oxidative stress in the expression of IL-33, and its signal pathway was examined after stimulation with hydrogen peroxide (H2O2), with or without stimulation by polyinosinic-polycytidylic acid [poly (I:C)], a synthetic analogue of dsRNA that mimics viral infection, or rhinovirus infection in NCI-H292 cells and primary human bronchial epithelial cells (HBECs). In addition, the effect of antioxidant, N-acetylcysteine (NAC) in the expression of IL-33 was compared between HBECs from healthy subjects and those from COPD patients.

RESULTS

Treatment with H2O2 significantly potentiated IL-33 expression in NCI-H292 cells, and the potentiation was reversed by NAC treatment. Mitogen-activated protein kinase (MAPK) inhibitors, but not nuclear factor-kappa B inhibitors, also significantly decreased the H2O2-potentiated IL-33 expression. In addition, H2O2 significantly potentiated the poly (I:C)- or rhinovirus-stimulated IL-33 expression. In HBECs from healthy subjects, H2O2-potentiated IL-33 expression and its reversal by NAC was also confirmed. Under the condition without H2O2-stimulation, treatment with NAC significantly decreased the expression of IL-33 in HBECs from COPD patients, but not in those from healthy subjects.

CONCLUSIONS

These results demonstrate that oxidative stress involves in the expression of IL-33 in airway epithelial cells via MAPK signal pathway and it augments IL-33 expression during viral infection. This mechanism may participate in the regulation of IL-33 expression in airway epithelial cells in COPD and the viral-induced exacerbations. Modulation of this pathway could become a therapeutic target for viral-induced exacerbations of COPD.

IL-33: biological properties, functions, and roles in airway disease

Interleukin (IL)-33 is a key cytokine involved in type 2 immunity and allergic airway diseases. Abundantly expressed in lung epithelial cells, IL-33 plays critical roles in both innate and adaptive immune responses in mucosal organs. In innate immunity, IL-33 and group 2 innate lymphoid cells (ILC2s) provide an essential axis for rapid immune responses and tissue homeostasis. In adaptive immunity, IL-33 interacts with dendritic cells, Th2 cells, follicular T cells, and regulatory T cells, where IL-33 influences the development of chronic airway inflammation and tissue remodeling. The clinical findings that both the IL-33 and ILC2 levels are elevated in patients with allergic airway diseases suggest that IL-33 plays an important role in the pathogenesis of these diseases. IL-33 and ILC2 may also serve as biomarkers for disease classification and to monitor the progression of diseases. In this article, we reviewed the current knowledge of the biology of IL-33 and discussed the roles of the IL-33 in regulating airway immune responses and allergic airway diseases.

Mast Cells and Innate Lymphoid Cells: Underappreciated Players in CNS Autoimmune Demyelinating Disease

Multiple sclerosis (MS) and its mouse model, experimental autoimmune encephalomyelitis, are autoimmune CNS inflammatory diseases. As a result of a breakdown in the relatively impermeable blood–brain barrier (BBB) in affected individuals, myelin-specific CD4⁺ and CD8⁺ T cells gain entry into the immune privileged CNS and initiate myelin, oligodendrocyte, and nerve axon destruction. However, despite the absolute requirement for T cells, there is increasing evidence that innate immune cells also play critical amplifying roles in disease pathogenesis. By modulating the character and magnitude of the myelin-reactive T cell response and regulating BBB integrity, innate cells affect both disease initiation and progression. Two classes of innate cells, mast cells and innate lymphoid cells (ILCs), have been best studied in models of allergic and gastrointestinal inflammatory diseases. Yet, there is emerging evidence that these cell types also exert a profound influence in CNS inflammatory disease. Both cell types are residents within the meninges and can be activated early in disease to express a wide variety of disease-modifying cytokines and chemokines. In this review, we discuss how mast cells and ILCs can have either disease-promoting or -protecting effects on MS and other CNS inflammatory diseases and how sex hormones may influence this outcome. These observations suggest that targeting these cells and their unique mediators can be exploited therapeutically.

Environmental allergens induce allergic inflammation through proteolytic maturation of IL-33

Allergic inflammation has crucial roles in allergic diseases such as asthma. It is therefore important to understand why and how the immune system responds to allergens. Here we found that full-length interleukin 33 (IL-33_FL), an alarmin cytokine with critical roles in type 2 immunity and asthma, functioned as a protease sensor that detected proteolytic activities associated with various environmental allergens across four kingdoms, including fungi, house dust mites, bacteria and pollens. When exposed to allergen proteases, IL-33_FL was rapidly cleaved in its central 'sensor' domain, which led to activation of the production of type 2 cytokines in group 2 innate lymphoid cells. Preventing cleavage of IL-33_FL reduced allergic airway inflammation. Our findings reveal a molecular mechanism for the rapid induction of allergic type 2 inflammation following allergen exposure, with important implications for allergic diseases.

Decreased IL-33 Production Contributes to Trophoblast Cell Dysfunction in Pregnancies with Preeclampsia

Preeclampsia (PE) is a life-threatening pregnancy complication which is related to aggradation of risk regarding fetal and maternal morbidity and mortality. Dysregulation of systemic inflammatory response and dysfunction of trophoblast cells have been proposed to be involved in the development and progression of PE. Some studies have demonstrated that interleukin-33 (IL-33) is an immunomodulatory cytokine that is associated with the immune regulation of tumor cells. However, little is known whether IL-33 and its receptor ST2/IL-1 R4 could regulate trophoblast cells, which are associated with the pathogenesis of PE. In this study, our target is to explore the impact of IL-33 on trophoblast cells and elucidate its underlying pathophysiological mechanisms. Placental tissues from the severe PE group (n = 11) and the normotensive pregnant women's group (n = 11) were collected for the protein expression and distribution of IL-33 along with its receptor ST2/IL-1 R4 via Western blot analysis and immunohistochemistry, respectively. We discovered that the level of IL-33 was decreased in placental tissues of pregnant women with PE, while no distinction was observed in the expression of ST2/IL-1 R4. These results were further verified in villous explants which were treated with sodium nitroprusside with different concentrations, to simulate the pathological environment of PE. To investigate IL-33 effects on trophoblast cells separately, IL-33 shRNA was introduced into HTR8/SVneo cells and villi. IL-33 shRNA weakened the proliferation, migration, and invasion capacity of HTR8/SVneo cells. The migration distance of villous explants was also markedly decreased. The reduced invasion of trophoblast cells is a result of IL-33 knockdown which could be related to the decline of MMP2/9 activity and the increased utterance of TIMP1/2. Overall, our findings demonstrated that the reduction of IL-33 production was connected with the reduced functional capability of trophoblast cells, thus inducing placental insufficiency that has been linked to the development of PE.

IL-33/ST2 signalling and crosstalk with FcεRI and TLR4 is targeted by the parasitic worm product, ES-62

ES-62 is a secreted parasitic worm-derived immunomodulator that exhibits therapeutic potential in allergy by downregulating aberrant MyD88 signalling to normalise the inflammatory phenotype and mast cell responses. IL-33 plays an important role in driving mast cell responses and promoting type-2 allergic inflammation, particularly with respect to asthma, via MyD88-integrated crosstalk amongst the IL-33 receptor (ST2), TLR4 and FcεRI. We have now investigated whether ES-62 targets this pathogenic network by subverting ST2-signalling, specifically by characterising how the functional outcomes of crosstalk amongst ST2, TLR4 and FcεRI are modulated by the worm product in wild type and ST2-deficient mast cells. This analysis showed that whilst ES-62 inhibits IL-33/ST2 signalling, the precise functional modulation observed varies with receptor usage and/or mast cell phenotype. Thus, whilst ES-62’s harnessing of the capacity of ST2 to sequester MyD88 appears sufficient to mediate its inhibitory effects in peritoneal-derived serosal mast cells, downregulation of MyD88 expression appears to be required to dampen the higher levels of cytokine production typically released by bone marrow-derived mucosal mast cells.

Innate immune responses to trauma

Trauma can affect any individual at any location and at any time over a lifespan. The disruption of macrobarriers and microbarriers induces instant activation of innate immunity. The subsequent complex response, designed to limit further damage and induce healing, also represents a major driver of complications and fatal outcome after injury. This Review aims to provide basic concepts about the posttraumatic response and is focused on the interactive events of innate immunity at frequent sites of injury: the endothelium at large, and sites within the lungs, inside and outside the brain and at the gut barrier.