Tatsukuni Ohno

IL-33 is a crucial amplifier of innate rather than acquired immunity

IL-33, a member of the IL-1-related cytokines, is considered to be a proallergic cytokine that is especially involved in Th2-type immune responses. Moreover, like IL-1α, IL-33 has been suggested to act as an “alarmin” that amplifies immune responses during tissue injury. In contrast to IL-1, however, the precise roles of IL-33 in those settings are poorly understood. Using IL-1- and IL-33-deficient mice, we found that IL-1, but not IL-33, played a substantial role in induction of T cell-mediated type IV hypersensitivity such as contact and delayed-type hypersensitivity and autoimmune diseases such as experimental autoimmune encephalomyelitis. Most notably, however, IL-33 was important for innate-type mucosal immunity in the lungs and gut. That is, IL-33 was essential for manifestation of T cell-independent protease allergen-induced airway inflammation as well as OVA-induced allergic topical airway inflammation, without affecting acquisition of antigen-specific memory T cells. IL-33 was significantly involved in the development of dextran-induced colitis accompanied by T cell-independent epithelial cell damage, but not in streptozocin-induced diabetes or Con A-induced hepatitis characterized by T cell-mediated apoptotic tissue destruction. In addition, IL-33-deficient mice showed a substantially diminished LPS-induced systemic inflammatory response. These observations indicate that IL-33 is a crucial amplifier of mucosal and systemic innate, rather than acquired, immune responses.

IL-33 can promote survival, adhesion and cytokine production in human mast cells

IL-33 is a recently identified member of the IL-1 family of molecules, which also includes IL-1 and IL-18. IL-33 binds to the receptor, T1/ST2/IL-1R4, and can promote cytokine secretion by Th2 cells and NF-κB phosphorylation in mouse mast cells. However, the effects of these molecules, especially IL-33, in human mast cells are poorly understood. Expression of the receptors for IL-1 family molecules, specifically, IL-1R1, IL-18R and T1/ST2, was detectable intracellularly in human umbilical cord blood-derived mast cells (HUCBMCs) by flow cytometry, but was scarcely detectable on the cells’ surface. However, IL-1β, IL-18 or IL-33 induced phosphorylation of Erk, p38 and JNK in naïve HUCBMCs, and IL-33 or IL-1β, but not IL-18, enhanced the survival of naive HUCBMCs and promoted their adhesion to fibronectin. IL-33 or IL-1β also induced IL-8 and IL-13 production in naïve HUCBMCs, and enhanced production of these cytokines in IgE/anti-IgE-stimulated HUCBMCs, without enhancing secretion of either PGD2 or histamine. Moreover, IL-33-mediated IL-8 production by HUCBMCs was markedly reduced by the p38 MAPK inhibitor, SB203580. In contrast to findings with mouse mast cells, IL-18 neither induced nor enhanced secretion of the mediators PGD2 or histamine by HUCBMCs. Our findings identify previously unknown functions of IL-33 in human mast cells. One of these is that IL-33, like IL-1β, can induce cytokine production in human mast cells even in the absence of stimuli of FcɛRI aggregation. Our findings thus support the hypothesis that IL-33 may enhance mast cell function in allergic disorders and other settings, either in the presence or absence of co-stimulation of mast cells via IgE/antigen–FcɛRI signals.

Th17 and allergy.

The identification of novel helper T (Th) cell subsets, i.e., IL-17-producing Th cells (Th17 cells) and regulatory T cells (Treg cells), provided new insight into our understanding of the molecular mechanisms involved in the development of infectious and autoimmune diseases as well as immune responses, and thus led to revision of the classic Th1/Th2 paradigm. Several current lines of evidence from gene-deficient mice indicate that IL-17 and Th17 cells, but not IFN-gamma and Th1 cells, are responsible for the development of autoimmune diseases such as murine arthritis and encephalomyelitis, which have classically been considered to be Th1-mediated disorders. Th17 cells may also contribute to the pathogenesis of classically recognized Th2-mediated allergic disorders. In this review, we summarize the current knowledge regarding IL-17 and Th17 cells and discuss their potential roles in the pathogenesis of allergic disorders.

IL-33 and IL-33 Receptors in Host Defense and Diseases

Interleukin-33 (IL-33) is a member of the IL-1 cytokine family, which includes IL-1 and IL-18. IL-33 is considered to be crucial for induction of Th2-type cytokine-associated immune responses such as host defense against nematodes and allergic diseases by inducing production of such Th2-type cytokines as IL-5 and IL-13 by Th2 cells, mast cells, basophils and eosinophils. In addition, IL-33 is involved in the induction of non-Th2-type acute and chronic inflammation as a proinflammatory cytokine, similar to IL-1 and IL-18. In this review, we summarize and discuss the current knowledge regarding the roles of IL-33 and IL-33 receptors in host defense and disease development.

Caspase-1, Caspase-8, and Calpain Are Dispensable for IL-33 Release by Macrophages

In addition to IL-1 and IL-18, IL-33 was recently identified as a member of the IL-1 cytokine family. rIL-33 can promote production of Th2-type cytokines by Th2 cells and mast cells in vitro. Administration of rIL-33 to mice results in increases in IgE secretion and eosinophilic inflammation. However, the precise immune cell source of IL-33 remains unclear. Moreover, although recombinant pro-IL-33 is cleaved by recombinant caspase-1 in vitro, as are pro-IL-1β and pro-IL-18, the involvement of caspase-1 in pro-IL-33 cleavage remains controversial. In this study, we show that mouse peritoneal macrophages, but not splenic dendritic cells, produced IL-33 upon stimulation with LPS. Likewise, mouse bone marrow cell-derived cultured mast cells also produced a small, but significant amount of IL-33 via FcεRI cross-linking, but not in response to stimulation with LPS. To our surprise, IL-33 release was found even in caspase-1-deficient, caspase-8 inhibitor-treated, and calpain inhibitor-treated macrophages. These observations suggest that caspase-1-, caspase-8-, and calpain-independent IL-33 production by macrophages and/or mast cells may contribute to the pathogenesis of Th2-type allergic inflammation.

IL-33–Mediated Innate Response and Adaptive Immune Cells Contribute to Maximum Responses of Protease Allergen–Induced Allergic Airway Inflammation

How the innate and adaptive immune systems cooperate in the natural history of allergic diseases has been largely unknown. Plant-derived allergen, papain, and mite allergens, Der f 1 and Der p 1, belong to the same family of cysteine proteases. We examined the role of protease allergens in the induction of Ab production and airway inflammation after repeated intranasal administration without adjuvants and that in basophil/mast cell stimulation in vitro. Papain induced papain-specific IgE/IgG1 and lung eosinophilia. Der f 1 induced Der f 1–specific IgG1 and eosinophilia. Although papain-, Der f 1–, and Der p 1–stimulated basophils expressed allergy-inducing cytokines, including IL-4 in vitro, basophil-depleting Ab and mast cell deficiency did not suppress the papain-induced in vivo responses. Protease inhibitor–treated allergens and a catalytic site mutant did not induce the responses. These results indicate that protease activity is essential to Ab production and eosinophilia in vivo and basophil activation in vitro. IL-33–deficient mice lacked eosinophilia and had reduced papain-specific IgE/IgG1. Coadministration of OVA with papain induced OVA-specific IgE/IgG1, which was reduced in IL-33–deficient mice. We demonstrated IL-33 release, subsequent IL-33–dependent IL-5/IL-13 release, and activation of T1/ST2-expressing lineage−CD25+CD44+ innate lymphoid cells in the lung after papain inhalation, suggesting the contribution of the IL-33–type 2 innate lymphoid cell–IL-5/IL-13 axis to the papain-induced airway eosinophilia. Rag2-deficient mice, which lack adaptive immune cells, showed significant, but less severe, eosinophilia. Collectively, these results suggest cooperation of adaptive immune cells and IL-33–responsive innate cells in protease-dependent allergic airway inflammation.

Epithelial cell-derived IL-25, but not Th17 cell-derived IL-17 or IL-17F, is crucial for murine asthma.

IL-17A, IL-17F, and IL-25 are ligands for IL-17RA. In the current study, we demonstrated that IL-25–deficient mice—but not IL-17A–, IL-17F–, IL-17A/F–, IL-23p19–, or retinoic acid-related orphan receptor (ROR)-γt–deficient mice—showed significant suppression of 1) the number of eosinophils and the levels of proinflammatory mediators in bronchoalveolar lavage fluids, 2) airway hyperresponsiveness to methacholine, and 3) OVA-specific IgG1 and IgE levels in the serum during OVA-induced Th2-type/eosinophilic airway inflammation. The IL-25 deficiency did not affect lung dendritic cell migration or Ag-specific memory–Th2 cell expansion during Ag sensitization. Adoptive transfer of T cells, mast cells, or bone marrow cells from IL-25–deficient mice revealed that induction of Th2-type/eosinophilic airway inflammation was dependent on activation of lung epithelial cells and eosinophils by IL-25 produced by airway structural cells such as epithelial cells but not by such hematopoietic stem-cell-origin immune cells as T cells and mast cells. Therefore, airway structural cell-derived IL-25—rather than Th17 cell-derived IL-17A and IL-17F—is responsible for induction of local inflammation by promoting activation of lung epithelial cells and eosinophils in the elicitation phase of Th2-type/eosinophilic airway inflammation. It is not required for Ag-specific Th2 cell differentiation in the sensitization phase.

Interleukin‐33 in allergy

Interleukin‐33 (IL‐33) is a member of the IL‐1 cytokine family, which includes IL‐1 and IL‐18, and is considered to be important for host defense against nematodes by inducing Th2 cytokine production via the IL‐33 receptor. IL‐33 receptor is a heterodimer of IL‐1 receptor‐like 1 (IL‐1RL1; also called ST2, T1, Der4, and fit‐1) and IL‐1 receptor accessory protein (IL‐1RAcP). On the other hand, excessive and/or inappropriate production of IL‐33 is considered to be involved in the development of various disorders, such as allergic and autoimmune diseases. Unlike IL‐1β and IL‐18, IL‐33 does not seem to be secreted through the activation of inflammasomes in events such as apoptosis. However, IL‐33 is localized in the nucleus of cells and is released during tissue injury associated with necrosis. This suggests that it acts as an alarmin, like IL‐1α and high‐mobility group box chromosomal protein‐1 (HMGB‐1). This review summarizes current knowledge regarding the roles of IL‐33 in the functions of various cell types and the pathogenesis of allergy.

Paracrine IL-33 Stimulation Enhances Lipopolysaccharide-Mediated Macrophage Activation

Background IL-33, a member of the IL-1 family of cytokines, provokes Th2-type inflammation accompanied by accumulation of eosinophils through IL-33R, which consists of ST2 and IL-1RAcP. We previously demonstrated that macrophages produce IL-33 in response to LPS. Some immune responses were shown to differ between ST2-deficient mice and soluble ST2-Fc fusion protein-treated mice. Even in anti-ST2 antibody (Ab)-treated mice, the phenotypes differed between distinct Ab clones, because the characterization of such Abs (i.e., depletion, agonistic or blocking Abs) was unclear in some cases. Methodology/Principal Findings To elucidate the precise role of IL-33, we newly generated neutralizing monoclonal Abs for IL-33. Exogenous IL-33 potentiated LPS-mediated cytokine production by macrophages. That LPS-mediated cytokine production by macrophages was suppressed by inhibition of endogenous IL-33 by the anti-IL-33 neutralizing mAbs. Conclusions/Significance Our findings suggest that LPS-mediated macrophage activation is accelerated by macrophage-derived paracrine IL-33 stimulation.

Development of IL-17-mediated Delayed-Type Hypersensitivity Is Not Affected by Down-Regulation of IL-25 Expression

BACKGROUND IL-25, which is a member of the IL-17 family, induces Th2 cell differentiation and Th2 cytokine production, contributing to induction of Th2-type immune responses and diseases, as a result of which it suppresses Th1- and Th17-type immune responses. METHODS To elucidate the role of IL-25 in the pathogenesis of IL-17-mediated delayed-type hypersensitivity (DTH), IL-25-deficient mice were sensitized with methylated BSA (mBSA), and then a DTH reaction was induced by mBSA challenge. mBSA-specific T-cell induction was assessed on the basis of cell proliferation and cytokine production. The DTH reaction was evaluated on the basis of tissue swelling, histology and inflammatory mediator expression. RESULTS IL-25 expression was markedly reduced in local DTH lesions. However, mBSA-specific Th1, Th2 and Th17 cell induction, and the mBSA-induced DTH reaction were comparable in IL-25-deficient and wild-type mice. CONCLUSIONS IL-25 is not essential for differentiation of Th1, Th2 and Th17 cells in the sensitization phase or induction of local inflammation in the elicitation phase of the mBSA-induced DTH reaction.