Interleukin-33/ Suppression of Tumorigenicity 2 in Renal Fibrosis: Emerging Roles in Prognosis and Treatment

Chronic kidney disease (CKD) is a major public health problem that affects more than 10% of the population worldwide and has a high mortality rate. Therefore, it is necessary to identify novel treatment strategies for CKD. Incidentally, renal fibrosis plays a central role in the progression of CKD to end-stage renal disease (ESRD). The activation of inflammatory pathways leads to the development of renal fibrosis. In fact, interleukin-33 (IL-33), a newly discovered member of the interleukin 1 (IL-1) cytokine family, is a crucial regulator of the inflammatory process. It exerts pro-inflammatory and pro-fibrotic effects via the suppression of tumorigenicity 2 (ST2) receptor, which, in turn, activates other inflammatory pathways. Although the role of this pathway in cardiac, pulmonary, and hepatic fibrotic diseases has been extensively studied, its precise role in renal fibrosis has not yet been completely elucidated. Recent studies have shown that a sustained activation of IL-33/ST2 pathway promotes the development of renal fibrosis. However, with prolonged research in this field, it is expected that the IL-33/ST2 pathway will be used as a diagnostic and prognostic tool for renal diseases. In addition, the IL-33/ST2 pathway seems to be a new target for the future treatment of CKD. Here, we review the mechanisms and potential applications of the IL-33/ST2 pathway in renal fibrosis; such that it can help clinicians and researchers to explore effective treatment options and develop novel medicines for CKD patients.

Allotransplantation Is Associated With Exacerbation of CD8 T-Cell Senescence: The Particular Place of the Innate CD8 T-Cell Component

Immunosenescence is a physiological process that is associated with changes in the immune system, particularly among CD8 T-cells. Recent studies have hypothesized that senescent CD8 T-cells are produced with chronologic age by chronic stimulation, leading to the acquisition of hallmarks of innate-like T-cells. While conventional CD8 T-cells are quite well characterized, CD8 T-cells sharing features of NK cells and memory CD8 T-cells, are a newly described immune cell population. They can be distinguished from conventional CD8 T-cells by their combined expression of panKIR/NKG2A and Eomesodermin (E), a unique phenotype closely associated with IFN-γ production in response to innate stimulation. Here, we first provided new evidence in favor of the innate character of panKIR/NKG2A(+) E(+) CD8 T-cells in normal subjects, documenting their position at an intermediate level in the innateness gradient in terms of both innate IFN-γ production and diminished mitochondrial mass. We also revealed that CD8 E(+) panKIR/NKG2A(+) T-cells, hereafter referred to as Innate E(+) CD8 T-cells, exhibit increased senescent (CD27(-) CD28(-)) phenotype, compared to their conventional memory counterparts. Surprisingly, this phenomenon was not dependent on age. Given that inflammation related to chronic viral infection is known to induce NK-like marker expression and a senescence phenotype among CD8 T-cells, we hypothesized that innate E(+) CD8 T-cells will be preferentially associated with exacerbated cellular senescence in response to chronic alloantigen exposure or CMV infection. Accordingly, in a pilot cohort of stable kidney allotransplant recipients, we observed an increased frequency of the Innate E(+) CD8 T-cell subset, together with an exacerbated senescent phenotype. Importantly, this phenotype cannot be explained by age alone, in clear contrast to their conventional memory counterparts. The senescent phenotype in CD8 T-cells was further increased in cytomegalovirus (CMV) positive serology transplant recipients, suggesting that transplantation and CMV, rather than aging by itself, may promote an exacerbated senescent phenotype of innate CD8 T-cells. In conclusion, we proposed that kidney transplantation, via the setting of inflammatory stimuli of alloantigen exposure and CMV infection, may exogenously age the CD8 T-cell compartment, especially its innate component. The physiopathological consequences of this change in the immune system remain to be elucidated.

Pathophysiologic role of Interleukin-33/ST2 in Sjögren's syndrome

Interleukin-33 (IL-33) is a member of the IL-1 family and has dual functions as a nuclear factor as well as a cytokine. The pivotal role of IL-33 as an active player contributing to aberrant local and systemic damage has been highlighted in several inflammatory and autoimmune diseases. Primary Sjögren's syndrome (pSS) is an autoimmune disease characterized by dry eyes and mouth syndrome due to local dysfunctions of exocrine glands, but also accompanied with systemic manifestations. The pathophysiology of pSS has been advocated as a conjecture of activated B and T cells as well as the production of inflammatory cytokines and autoantibodies, driving epithelial tissue damage and disease progression. In pSS, IL-33 is released in the extracellular space from damaged salivary cells upon pro-inflammatory stimuli and/or dysfunction of epithelial barrier. Counter-regulatory mechanisms are initiated to limit the pro-inflammatory actions of IL-33 as portrayed by an increase in the decoy receptor for IL-33, the soluble form of ST2 (sST2). In pSS and associated diseases, the levels of IL-33 are significantly elevated in the serum or tears of patients. Mechanistically, IL-33 acts in synergy with IL-12 and IL-23 on NK and NKT cells to boost the production of IFN-γ contributing to inflammation. TNF-α, IL-1β and IFN-γ in turn further increase the activation of IL-33/ST2 pathway, thereby constituting a vicious inflammatory loop leading to disease exacerbation. IL-33/ST2 axis is involved in Sjögren's syndrome and opens new perspectives as therapeutic target of one of the culprits in the inflammatory perpetuation.

Interleukin-1 Family Cytokines: Keystones in Liver Inflammatory Diseases

The pyrogenic property being the first activity described, members of the interleukin-1 superfamily (IL-1α, IL-1β, IL-18, and the newest members: IL-33, IL-36, IL-37, and IL-38) are now known to be involved in several inflammatory diseases such as obesity, atherosclerosis, cancer, viral and parasite infections, and auto-inflammatory syndromes as well as liver diseases. Inflammation processes are keystones of chronic liver diseases, of which the etiology may be viral or toxic, as in alcoholic or non-alcoholic liver diseases. Inflammation is also at stake in acute liver failure involving massive necrosis, and in ischemia-reperfusion injury in the setting of liver transplantation. The role of the IL-1 superfamily of cytokines and receptors in liver diseases can be either protective or pro-inflammatory, depending on timing and the environment. Our review provides an overview of current understanding of the IL-1 family members in liver inflammation, highlighting recent key investigations, and therapeutic perspectives. We have tried to apply the concept of trained immunity to liver diseases, based on the role of the members of the IL-1 superfamily, first of all IL-1β but also IL-18 and IL-33, in modulating innate lymphoid immunity carried by natural killer cells, innate lymphoid cells or innate T-αβ lymphocytes.

Sulfatide-activated type II NKT cells suppress immunogenic maturation of lung dendritic cells in murine models of asthma

Our previous study showed that sulfatide-activated type II natural killer T (NKT) cells can prevent allergic airway inflammation in an ovalbumin (OVA)-induced murine model of asthma, but the underlying mechanism is unclear. Recently, sulfatide-activated type II NKT cells were shown to modulate the function of dendritic cells in experimental autoimmune encephalomyelitis and nonobese diabetic mice. Thus, it was hypothesized that sulfatide-activated type II NKT cells may modulate the function of lung dendritic cells (LDCs) in asthmatic mice. Our data showed that, in our mouse models, activation of type II NKT cells by sulfatide administration and adoptive transfer of sulfatide-activated type II NKT cells resulted in reduced expression of surface maturation markers and proinflammatory cytokine production of LDCs. LDCs from sulfatide-treated asthmatic mice, in contrast to LDCs from PBS-treated asthmatic mice, significantly reduced allergic airway inflammation in vivo. However, we found no influence of sulfatide-activated type II NKT cells on the phenotypic and functional maturation of bone marrow-derived dendritic cells in vitro. In addition, adoptive transfer of sulfatide-activated type II NKT cells did not influence the phenotypic and functional maturation of LDCs in CD1d^-/- mice, which lack both type I and II NKT cells, immunized and challenged with OVA. Our data reveal that sulfatide-activated type II NKT cells can suppress immunogenic maturation of LDCs to reduce allergic airway inflammation in mouse models of asthma, and it is possible that the immunomodulatory effect needs type I NKT cells.

Tissue-Resident Lymphocytes in Solid Organ Transplantation: Innocent Passengers or the Key to Organ Transplant Survival?

Short-term outcomes of solid organ transplantation have improved dramatically over the past several decades; however, long-term survival has remained static over the same period, and chronic rejection remains a major cause of graft failure. The importance of donor, or "passenger," lymphocytes to the induction of tolerance to allografts was recognized in the 1990s, but their precise contribution to graft acceptance or rejection has not been elucidated. Recently, specialized populations of tissue-resident lymphocytes in nonlymphoid organs have been described. These lymphocytes include tissue-resident memory T cells, regulatory T cells, γδ T cells, invariant natural killer T cells, and innate lymphoid cells. These cells reside in commonly transplanted solid organs, including the liver, kidneys, heart, and lung; however, their contribution to graft acceptance or rejection has not been examined in detail. Similarly, it is unclear whether tissue-resident cells derived from the pool of recipient-derived lymphocytes play a specific role in transplantation biology. This review summarizes the evidence for the roles of tissue-resident lymphocytes in transplant immunology, focussing on their features, functions, and relevance for solid organ transplantation, with specific reference to liver, kidney, heart, and lung transplantation.

The Impact of Invariant NKT Cells in Sterile Inflammation: The Possible Contribution of the Alarmin/Cytokine IL-33

Although the contribution of iNKT cells to induction of sterile inflammation is now well-established, the nature of the endogenous compounds released early after cellular stress or damage that drive their activation and recruitment remains poorly understood. More precisely, iNKT cells have not been described as being reactive to endogenous non-protein damage-associated molecular-pattern molecules (DAMPs). A second subset of DAMPs, called alarmins, are tissue-derived nuclear proteins, constitutively expressed at high levels in epithelial barrier tissues and endothelial barriers. These potent immunostimulants, immediately released after tissue damage, include the alarmin IL-33. This factor has aroused interest due to its singular action as an alarmin during infectious, allergic responses and acute tissue injury, and as a cytokine, contributing to the latter resolutive/repair phase of sterile inflammation. IL-33 targets iNKT cells, inducing their recruitment in an inflammatory state, and amplifying their regulatory and effector functions. In the present review, we introduce the new concept of a biological axis of iNKT cells and IL-33, involved in alerting and controlling the immune cells in experimental models of sterile inflammation. This review will focus on acute organ injury models, especially ischemia-reperfusion injury, in the kidneys, liver and lungs, where iNKT cells and IL-33 have been presumed to mediate and/or control the injury mechanisms, and their potential relevance in human pathophysiology.

Endogenous IL-33 Contributes to Kidney Ischemia-Reperfusion Injury as an Alarmin

Inflammation is a prominent feature of ischemia-reperfusion injury (IRI), which is characterized by leukocyte infiltration and renal tubular injury. However, signals that initiate these events remain poorly understood. We examined the role of the nuclear alarmin IL-33 in tissue injury and innate immune response triggered by experimental kidney ischemia-reperfusion. In wild-type mice, we found that IL-33 was constitutively expressed throughout the kidney in peritubular and periglomerular spaces, mainly by microvascular endothelial cells, from which it was released immediately during IRI. Compared with wild-type mice, mice lacking IL-33 (IL-33^Gt/Gt) exhibited reductions in early tubular cell injury and subsequent renal infiltration of IFN-γ/IL-17A-producing neutrophils, with preservation of renal functions. This protection associated with decreased renal recruitment of myeloid dendritic cells, natural killer (NK) cells, and invariant natural killer T (iNKT) cells, the latter of which were reported as deleterious in IRI. Increases in the level of circulating IL-12, a key IL-33 cofactor, and the expression of ST2, an IL-33-specific receptor, on the surface of iNKT cells preceded the IL-33- and iNKT cell-dependent phase of neutrophil infiltration. Furthermore, IL-33 directly targeted iNKT cells in vitro, inducing IFN-γ and IL-17A production. We propose that endogenous IL-33 is released as an alarmin and contributes to kidney IRI by promoting iNKT cell recruitment and cytokine production, resulting in neutrophil infiltration and activation at the injury site. Our findings show a novel molecular mediator contributing to innate immune cell recruitment induced by renal ischemia-reperfusion and may provide therapeutic insights into AKI associated with renal transplantation.

Interleukin-33 in tumorigenesis, tumor immune evasion, and cancer immunotherapy

Interleukin-33 (IL-33) is a member of the IL-1 gene family and mainly expressed in the nucleus of tissue lining cells, stromal cells, and activated myeloid cells. IL-33 is considered a damage-associated molecular pattern (DAMP) molecule and plays an important role in many physiological and pathological settings such as tissue repair, allergy, autoimmune disease, infectious disease, and cancer. The biological functions of IL-33 include maintaining tissue homeostasis, enhancing type 1 and 2 cellular immune responses, and mediating fibrosis during chronic inflammation. IL-33 exerts diverse functions through signaling via its receptor ST2, which is expressed in many types of cells including regulatory T cells (Treg), group 2 innate lymphoid cells (ILC2s), myeloid cells, cytotoxic NK cells, Th2 cells, Th1 cells, and CD8(+) T cells. Tumor development results in downregulation of IL-33 in epithelial cells but upregulation of IL-33 in the tumor stroma and serum. The current data suggest that IL-33 expression in tumor cells increases immunogenicity and promotes type 1 antitumor immune responses through CD8(+) T cells and NK cells, whereas IL-33 in tumor stroma and serum facilitates immune suppression via Treg and myeloid-derived suppressor cell (MDSC). Understanding the role of IL-33 in cancer immunobiology sheds lights on targeting this cytokine for cancer immunotherapy.

Divergent invariant natural killer T-cell response to sepsis of abdominal vs. non-abdominal origin in human beings

BACKGROUND

The etiology of sepsis is broad. The peritoneal cavity displays compartmentalization with respect to inflammatory responses, so peripheral blood responses to sepsis of abdominal vs. non-abdominal origin are expected to be divergent. Lymphocytes and invariant natural killer T (iNKT) cells play important roles in survival from sepsis, as they dampen the neutrophil and macrophage responses. We assessed whether circulating iNKT cells display distinct phenotypic profiles depending on the presence of abdominal vs. non-abdominal infection with sepsis.

METHODS

Patients with sepsis, defined as infection confirmed microbiologically with a systemic inflammatory response syndrome (SIRS), were enrolled prospectively. They were categorized as having either exclusively sepsis of abdominal or exclusively non-abdominal origin. The white blood cell (WBC) count was recorded. Whole-blood staining with monoclonal antibodies to CD3, V-alpha-24 (to identify iNKT cells), and CD69 (marker of early activation) was applied.

RESULTS

Of the 53 enrolled patients, 18 had abdominal infection. Pneumonia was the most common non-abdominal type. There was no difference in gender, age, Acute Physiology and Chronic Health Evaluation (APACHE) II score, WBC count, or CD3(+) T cells (7.1%±1.6% vs. 6.5%±0.9%; p=0.75) in the two groups. Patients with abdominal infection had a higher proportion of iNKT cells (2.7%±1.1% vs. 0.89%±0.14%; p=0.032). Correcting for WBC count, this translated into a higher absolute number of iNKT cells (3.4±1.8×10(7)/L vs. 0.74±0.15×10(7)/L; p=0.03). Patients with sepsis of abdominal origin had a lower percentage of CD69(+) iNKT cells (9.1%±3.1% vs. 27.2%±5.8%; p=0.028). In patients in shock vs. those who were not, patients with non-abdominal infection exhibited a greater number of iNKT cells (1.47±0.3 v. 0.62±0.1×10(7)/L; p=0.022) and percentage of activated iNKT cells (53±14.5% vs. 17.9±4.8%; p=0.04). Patients with non-abdominal infection who died had a lower absolute number of activated iNKT cells (0.8±1.2×10(7)/L vs. 0.34±0.1×10(7)/L; p=0.023); however, no such shock or death correlation was noted in patients with sepsis of abdominal origin.

CONCLUSIONS

Divergent sepsis etiologies display distinct blood iNKT cell population changes. In non-abdominal infection, this difference was associated with septic shock and death. Elucidating the importance and basis for these changes relative to the response to sources of infection will help clarify appropriate diagnosis and management of the patient with sepsis.

Development of genetically engineered iNKT cells expressing TCRs specific for the M. tuberculosis 38-kDa antigen

Introduction

The invariant natural killer T (iNKT) cell has been shown to play a central role in early stages immune responses against Mycobacterium tuberculosis (Mtb) infection, which become nonresponsive (anergic) and fails to control the growth of Mtb in patients with active tuberculosis. Enhancement of iNKT cell responses to Mtb antigens can help to resist infection.

Study design and methods

In the present study, an Mtb 38-kDa antigen-specific T cell receptor (TCR) was isolated from human CD8⁺ T cells stimulated by 38-kDa antigen in vitro, and then transduced into primary iNKT cells by retrovirus vector.

Results

The TCR gene-modified iNKT cells are endowed with new features to behave as a conventional MHC class I restricted CD8⁺ T lymphocyte by displaying specific antigen recognition and anti-Mtb antigen activity in vitro. At the same time, the engineered iNKT cells retaining its original capacity to be stimulated proliferation by non-protein antigens α-Gal-Cer.

Conclusions

This work is the first attempt to engineer iNKT cells by exogenous TCR genes and demonstrated that iNKT cell, as well as CD4⁺ and CD8⁺ T cells, can be genetically engineered to confer them a defined and alternative specificity, which provides new insights into TCR gene therapy for tuberculosis patients, especially those infected with drug-resistant Mtb.

α-Galactosylceramide suppresses murine eosinophil production through interferon-γ-dependent induction of NO synthase and CD95

BACKGROUND AND PURPOSE

α-Galactosylceramide (α-GalCer), a pleiotropic immunomodulator with therapeutic potential in neoplastic, autoimmune and allergic diseases, activates invariant natural killer T-cells throughCD1-restricted receptors for α-GalCer on antigen-presenting cells, inducing cytokine secretion. However the haemopoietic effects of α-GalCer remain little explored.

EXPERIMENTAL APPROACH

α-GalCer-induced modulation of eosinophil production in IL-5-stimulated bone marrow cultures was examined in wild-type (BALB/c, C57BL/6) mice and their mutants lacking CD1, inducible NOS (iNOS), CD95 and IFN-γ, along with the effects of lymphocytes; IFN-γ; caspase and iNOS inhibitors; non-steroidal anti-inflammatory drugs (NSAIDs) and LTD4 ; and dexamethasone.

KEY RESULTS

α-GalCer (10(-6) -10(-8) M) suppressed IL-5-stimulated eosinopoiesis by inducing apoptosis. α-GalCer pretreatment in vivo (100 μg·kg(-1) , i.v.) suppressed colony formation by GM-CSF-stimulated bone marrow progenitors in semi-solid cultures. α-GalCer and dexamethasone synergistically promoted eosinophil maturation. Suppression of eosinophil production by α-GalCer was prevented by aminoguanidine and was undetectable in bone marrow lacking iNOS, CD95, CD28; or CD1d. Separation on Percoll gradients and depletion of CD3+ cells made bone marrow precursors unresponsive to α-GalCer. Responsiveness was restored with splenic lymphocytes. Experiments with (i) IFN-γ-deficient bone marrow, alone or co-cultured with spleen T-cells from wild-type, but not from CD1d-deficient, donors; (ii) IFN-γ neutralization; and (iii) recombinant IFN-γ, showed that these effects of α-GalCer were mediated by IFN-γ. Effects of α-GalCer on eosinophil production were blocked by LTD4 and NSAIDs.

CONCLUSIONS AND IMPLICATIONS

α-GalCer activation of IFN-γ-secreting, CD1d-restricted lymphocytes induced iNOS-CD95-dependent apoptosis in developing eosinophils. This pathway is initiated by endogenous regulatory lymphocytes, antagonised by LTD4 , NSAIDs and aminoguanidine, and modified by dexamethasone.

Tumoral expression of IL-33 inhibits tumor growth and modifies the tumor microenvironment through CD8+ T and NK cells

Cancer immunotherapy has shown great promise as a new standard cancer therapeutic modality. However, the response rates are limited for current approach that depends on enhancing spontaneous antitumor immune responses. Therefore, increasing tumor immunogenicity by expressing appropriate cytokines should further improve the current immunotherapy. IL-33 is a member of the IL-1 family of cytokines and is released by necrotic epithelial cells or activated innate immune cells and is thus considered a "danger" signal. The role of IL-33 in promoting type 2 immune responses and tissue inflammation has been well established. However, whether IL-33 drives antitumor immune responses is controversial. Our previous work established that IL-33 promoted the function of CD8(+) T cells. In this study, we showed that the expression of IL-33 in two types of cancer cells potently inhibited tumor growth and metastasis. Mechanistically, IL-33 increased numbers and IFN-γ production by CD8(+) T and NK cells in tumor tissues, thereby inducing a tumor microenvironment favoring tumor eradication. Importantly, IL-33 greatly increased tumor Ag-specific CD8(+) T cells. Furthermore, both NK and CD8(+) T cells were required for the antitumor effect of IL-33. Moreover, depletion of regulatory T cells worked synergistically with IL-33 expression for tumor elimination. Our studies established "alarmin" IL-33 as a promising new cytokine for tumor immunotherapy through promoting cancer-eradicating type 1 immune responses.

The alarmin concept applied to human renal transplantation: evidence for a differential implication of HMGB1 and IL-33

The endogenous molecules high mobility group box 1 (HMGB1) and interleukin-33 (IL-33) have been identified as alarmins, capable of mediating danger signals during tissue damage. Here, we address their possible role as innate-immune mediators in ischemia-reperfusion injury (IRI) following human kidney transplantation. We analysed serum and urinary HMGB1 and IL-33 levels, all determined by enzyme-linked immunosorbent assay, in a cohort of 26 deceased renal transplant recipients. Urinary HMGB1 and IL-33 levels were significantly increased as soon as 30 min after reperfusion, as compared to those before treatment. Moreover, both serum and urinary IL-33 (but not HMGB1) increase was positively correlated with cold ischemia time, from 30 min to 3 days post-transplantation. In vitro, human umbilical vein endothelial cells subjected to hypoxia conditions released both HMGB-1 and IL-33, while only the latter was further increased upon subsequent re-oxygenation. Finally, we postulate that leukocytes from renal recipient patients are targeted by both HMGB1 and IL-33, as suggested by increased transcription of their respective receptors (TLR2/4 and ST2L) shortly after transplantation. Consistent with this view, we found that iNKT cells, an innate-like T cell subset involved in IRI and targeted by IL-33 but not by HMGB1 was activated 1 hour post-transplantation. Altogether, these results are in keeping with a potential role of IL-33 as an innate-immune mediator during kidney IRI in humans.

Cross-activating invariant NKT cells and kupffer cells suppress cholestatic liver injury in a mouse model of biliary obstruction

Both Kupffer cells and invariant natural killer T (iNKT) cells suppress neutrophil-dependent liver injury in a mouse model of biliary obstruction. We hypothesize that these roles are interdependent and require iNKT cell-Kupffer cell cross-activation. Female, wild-type and iNKT cell-deficient C57Bl/6 mice were injected with magnetic beads 3 days prior to bile duct ligation (BDL) in order to facilitate subsequent Kupffer cell isolation. On day three post-BDL, the animals were euthanized and the livers dissected. Necrosis was scored; Kupffer cells were isolated and cell surface marker expression (flow cytometry), mRNA expression (qtPCR), nitric oxide (NO^.) production (Griess reaction), and protein secretion (cytometric bead-array or ELISAs) were determined. To address the potential role of NO^. in suppressing neutrophil accumulation, a group of WT mice received 1400W, a specific inducible nitric oxide synthase (iNOS) inhibitor, prior to BDL. To clarify the mechanisms underlying Kupffer cell-iNKT cell cross-activation, WT animals were administered anti-IFN-γ or anti-lymphocyte function-associated antigen (LFA)-1 antibody prior to BDL. Compared to their WT counterparts, Kupffer cells obtained from BDL iNKT cell-deficient mice expressed lower iNOS mRNA levels, produced less NO^., and secreted more neutrophil chemoattractants. Both iNOS inhibition and IFN-γ neutralization increased neutrophil accumulation in the livers of BDL WT mice. Anti-LFA-1 pre-treatment reduced iNKT cell accumulation in these same animals. These data indicate that the LFA-1-dependent cross-activation of iNKT cells and Kupffer cells inhibits neutrophil accumulation and cholestatic liver injury.

Group 2 innate lymphoid cell production of IL-5 is regulated by NKT cells during influenza virus infection

Respiratory virus infections, such as influenza, typically induce a robust type I (pro-inflammatory cytokine) immune response, however, the production of type 2 cytokines has been observed. Type 2 cytokine production during respiratory virus infection is linked to asthma exacerbation; however, type 2 cytokines may also be tissue protective. Interleukin (IL)-5 is a prototypical type 2 cytokine that is essential for eosinophil maturation and egress out of the bone marrow. However, little is known about the cellular source and underlying cellular and molecular basis for the regulation of IL-5 production during respiratory virus infection. Using a mouse model of influenza virus infection, we found a robust transient release of IL-5 into infected airways along with a significant and progressive accumulation of eosinophils into the lungs, particularly during the recovery phase of infection, i.e. following virus clearance. The cellular source of the IL-5 was group 2 innate lymphoid cells (ILC2) infiltrating the infected lungs. Interestingly, the progressive accumulation of eosinophils following virus clearance is reflected in the rapid expansion of c-kit⁺ IL-5 producing ILC2. We further demonstrate that the enhanced capacity for IL-5 production by ILC2 during recovery is concomitant with the enhanced expression of the IL-33 receptor subunit, ST2, by ILC2. Lastly, we show that NKT cells, as well as alveolar macrophages (AM), are endogenous sources of IL-33 that enhance IL-5 production from ILC2. Collectively, these results reveal that c-kit⁺ ILC2 interaction with IL-33 producing NKT and AM leads to abundant production of IL-5 by ILC2 and accounts for the accumulation of eosinophils observed during the recovery phase of influenza infection.

IL-5 is a cytokine that is typically associated with parasitic infections and allergic reactions. The primary role of IL-5 is thought to be for the development and maturation of an innate immune cell type, the eosinophil, which is also a culprit in allergic diseases such as asthma. During respiratory virus infection, such as influenza infection, IL-5 and eosinophils are not thought to play a major role in host defense. Here we show that IL-5 is produced in response to influenza infection and results in the progressive accumulation of eosinophils in the lung. We show that a newly discovered cell type, the group 2 innate lymphoid cell (ILC2), is responsible for IL-5 production during influenza infection and that the capacity of ILC2 to make IL-5 is greatly increased following virus clearance, i.e. during the recovery phase. The production of IL-5 by ILC2 is in part regulated by NKT cells and IL-33 produced by this cell type during the recovery phase of influenza infection.

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.

Invariant natural killer T cells as sensors and managers of inflammation

Invariant natural killer T (iNKT) cells are a subset of innate-like lymphocytes that recognize glycolipid antigens bound by the major histocompatibility complex (MHC)-class-I-related protein CD1d. iNKT cells are activated early during a variety of infections and inflammatory diseases and contribute to the subsequent development of adaptive immune responses. Consequently, iNKT cells play a critical role in the development and resolution of inflammatory diseases and represent attractive targets for the development of immunotherapies. Recent studies have provided important insight into the mechanisms by which iNKT cells become activated in response to diverse inflammatory stimuli. These new findings should be instrumental to promote the immunomodulatory properties of iNKT cells for treatment of inflammatory diseases.

Asthma phenotypes: the evolution from clinical to molecular approaches

Although asthma has been considered as a single disease for years, recent studies have increasingly focused on its heterogeneity. The characterization of this heterogeneity has promoted the concept that asthma consists of multiple phenotypes or consistent groupings of characteristics. Asthma phenotypes were initially focused on combinations of clinical characteristics, but they are now evolving to link biology to phenotype, often through a statistically based process. Ongoing studies of large-scale, molecularly and genetically focused and extensively clinically characterized cohorts of asthma should enhance our ability to molecularly understand these phenotypes and lead to more targeted and personalized approaches to asthma therapy.

Dynamic role of epithelium-derived cytokines in asthma

Asthma is an inflammatory disorder of the airways, characterized by infiltration of mast cells, eosinophils, and Th2-type CD4+ T cells in the airway wall. Airway epithelium constitutes the first line of interaction with our atmospheric environment. The protective barrier function of the airway epithelium is likely impaired in asthma. Furthermore, recent studies suggest critical immunogenic and immunomodulatory functions of airway epithelium. In particular, a triad of cytokines, including IL-25, IL-33 and TSLP, is produced and released by airway epithelial cells in response to various environmental and microbial stimuli or by cellular damage. These cytokines induce and promote Th2-type airway inflammation and cause remodeling and pathological changes in the airway walls, suggesting their pivotal roles in the pathophysiology of asthma. Thus, the airway epithelium can no longer be regarded as a mere structural barrier, but must be considered an active player in the pathogenesis of asthma and other allergic disorders.

IL-33 synergizes with TCR and IL-12 signaling to promote the effector function of CD8+ T cells

The effector functions of CD8(+) T cells are influenced by tissue inflammatory microenvironments. IL-33, a member of the IL-1 family, acts as a danger signal after its release during cell necrosis. The IL-33/ST2 axis has been implicated in various Th2 responses. Its role in CD8(+) T-cell-mediated immune response is, however, not known. Here we find that type 1 cytotoxic T (Tc1) cells cultured in vitro unexpectedly express high levels of the IL-33 receptor ST2. Interestingly, the expression of ST2 in Tc1 cells is dependent on T-bet, a master Th1/Tc1 transcription factor. In addition, IL-33 enhances TCR-triggered IFN-γ production. IL-33 together with IL-12 can stimulate IFN-γ production in Tc1 cells. Moreover, IL-33 synergizes with IL-12 to promote CD8(+) T-cell effector function. The synergistic effect of IL-33 and IL-12 is partly mediated by Gadd45b. Together, these in vitro data establish a novel role of IL-33 in promoting effector type 1 adaptive immune responses.