IL-33 induces nuocytes and modulates liver injury in viral hepatitis

The alarmin IL-33 exacerbates pulmonary inflammation and immune dysfunction in SARS-CoV-2 infection

Dysregulated host immune responses contribute to disease severity and worsened prognosis in COVID-19 infection and the underlying mechanisms are not fully understood. In this study, we observed that IL-33, a damage-associated molecular pattern molecule, is significantly increased in COVID-19 patients and in SARS-CoV-2-infected mice. Using IL-33-/- mice, we demonstrated that IL-33 deficiency resulted in significant decreases in bodyweight loss, tissue viral burdens, and lung pathology. These improved outcomes in IL-33-/- mice also correlated with a reduction in innate immune cell infiltrates, i.e., neutrophils, macrophages, natural killer cells, and activated T cells in inflamed lungs. Lung RNA-seq results revealed that IL-33 signaling enhances activation of inflammatory pathways, including interferon signaling, pathogen phagocytosis, macrophage activation, and cytokine/chemokine signals. Overall, these findings demonstrate that the alarmin IL-33 plays a pathogenic role in SARS-CoV-2 infection and provides new insights that will inform the development of effective therapeutic strategies for COVID-19.

Type 2 innate lymphoid cells are protective against hepatic ischaemia/reperfusion injury

Background and Aims

Although type 2 innate lymphoid cells (ILC2s) were originally found to be liver-resident lymphocytes, the role and importance of ILC2 in liver injury remains poorly understood. In the current study, we sought to determine whether ILC2 is an important regulator of hepatic ischaemia/reperfusion injury (IRI).

Methods

ILC2-deficient mice (ICOS-T or NSG) and genetically modified ILC2s were used to investigate the role of ILC2s in murine hepatic IRI. Interactions between ILC2s and eosinophils or macrophages were studied in coculture. The role of human ILC2s was assessed in an immunocompromised mouse model of hepatic IRI.

Results

Administration of IL-33 prevented hepatic IRI in association with reduction of neutrophil infiltration and inflammatory mediators in the liver. IL-33-treated mice had elevated numbers of ILC2s, eosinophils, and regulatory T cells. Eosinophils, but not regulatory T cells, were required for IL-33-mediated hepatoprotection in IRI mice. Depletion of ILC2s substantially abolished the protective effect of IL-33 in hepatic IRI, indicating that ILC2s play critical roles in IL-33-mediated liver protection. Adoptive transfer of ex vivo-expanded ILC2s improved liver function and attenuated histologic damage in mice subjected to IRI. Mechanistic studies combining genetic and adoptive transfer approaches identified a protective role of ILC2s through promoting IL-13-dependent induction of anti-inflammatory macrophages and IL-5-dependent elevation of eosinophils in IRI. Furthermore, in vivo expansion of human ILC2s by IL-33 or transfer of ex vivo-expanded human ILC2s ameliorated hepatic IRI in an immunocompromised mouse model of hepatic IRI.

Conclusions

This study provides insight into the mechanisms of ILC2-mediated liver protection that could serve as therapeutic targets to treat acute liver injury.

Impact and Implications

We report that type 2 innate lymphoid cells (ILC2s) are important regulators in a mouse model of liver ischaemia/reperfusion injury (IRI). Through manipulation of macrophage and eosinophil phenotypes, ILC2s mitigate liver inflammation and injury during liver IRI. We propose that ILC2s have the potential to serve as a therapeutic tool for protecting against acute liver injury and lay the foundation for translation of ILC2 therapy to human liver disease.

Interleukin-33 deficiency prevents biliary injuries and repairments caused by Clonorchis sinensis via restraining type 2 cytokines

Background

Clonorchiasis caused by Clonorchis sinensis is a zoonotic parasitic disease characterized by cholangitis, biliary proliferation, biliary fibrosis, and even cholangiocarcinoma. Our previous study showed that the expression of interleukin (IL)-33 is increased in both humans and mice infected by C. sinensis, suggesting that IL-33 is potentially involved in the pathogenesis of clonorchiasis. However, the roles and potential mechanism of IL-33 underlying remain unknown.

Methods

Wild-type (WT) and IL-33 knockout (KO) mice (BALB/c female mice) were orally infected with 45 metacercariae of C. sinensis for 8 weeks. Biliary injuries and fibrosis were extensively evaluated. Hepatic type II cytokines (IL-4, IL-13, and IL-10) were detected by ELISA.

Results

For wild-type mice, we found that the mice infected with C. sinensis showed severe biliary injuries and fibrosis compared with the normal mice that were free from worm infection. In addition, the levels of type II cytokines such as IL-4, IL-13, and IL-10 in infected wild-type mice were significantly higher than in the control mice without infection (P < 0.05). However, IL-33 deficiency (IL-33 KO) prevents the augmentation of biliary injuries and fibrosis caused by C. sinensis infection. Furthermore, the increased levels of these type II cytokines induced by worm infection were also reversed in IL-33 KO mice.

Conclusion

Our present study demonstrates that IL-33 contributes to the pathogenesis of C. sinensis-induced biliary injuries and repair, which can potentially orchestrate type 2 responses. These findings highlight the pathophysiological role of IL-33 in the progression of clonorchiasis.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13071-022-05490-6.

Heterogeneity of ILC2s in the Lungs

Group 2 innate lymphoid cells (ILC2s) are GATA3-expressing type 2 cytokine-producing innate lymphocytes that are present in various organs throughout the body. Basically, ILC2s are tissue-resident cells associated with a variety of pathological conditions in each tissue. Differences in the tissue-specific properties of ILC2s are formed by the post-natal tissue environment; however, diversity exists among ILC2s within each localized tissue due to developmental timing and activation. Diversity between steady-state and activated ILC2s in mice and humans has been gradually clarified with the advancement of single-cell RNA-seq technology. Another layer of complexity is that ILC2s can acquire other ILC-like functions, depending on their tissue environment. Further, ILC2s with immunological memory and exhausted ILC2s are both present in tissues, and the nature of ILC2s varies with senescence. To clarify how ILC2s affect human diseases, research should be conducted with a comprehensive understanding of ILC2s, taking into consideration the diversity of ILC2s rather than a snapshot of a single section. In this review, we summarize the current understanding of the heterogeneity of ILC2s in the lungs and highlight a novel field of immunology.

Tollip Inhibits IL-33 Release and Inflammation in Influenza A Virus-Infected Mouse Airways

Respiratory influenza A virus (IAV) infection continues to pose significant challenges in healthcare of human diseases including asthma. IAV infection in mice was shown to increase IL-33, a key cytokine in driving airway inflammation in asthma, but how IL-33 is regulated during viral infection remains unclear. We previously found that a genetic mutation in Toll-interacting protein (Tollip) was linked to less airway epithelial Tollip expression, increased neutrophil chemokines, and lower lung function in asthma patients. As Tollip is involved in maintaining mitochondrial function, and mitochondrial stress may contribute to extracellular ATP release and IL-33 secretion, we hypothesized that Tollip downregulates IL-33 secretion via inhibiting ATP release during IAV infection. Wild-type and Tollip knockout (KO) mice were infected with IAV and treated with either an ATP converter apyrase or an IL-33 decoy receptor soluble ST2 (sST2). KO mice significantly lost more body weight and had increased extracellular ATP, IL-33 release, and neutrophilic inflammation. Apyrase treatment reduced extracellular ATP levels, IL-33 release, and neutrophilic inflammation in Tollip KO mice. Excessive lung neutrophilic inflammation in IAV-infected Tollip KO mice was reduced by sST2, which was coupled with less IL-33 release. Our data suggest that Tollip inhibits IAV infection, potentially by inhibiting extracellular ATP release and reducing IL-33 activation and lung inflammation. In addition, sST2 may serve as a potential therapeutic approach to mitigate respiratory viral infection in human subjects with Tollip deficiency.

Hepatic pannexin-1 mediates ST2+ regulatory T cells promoting resolution of inflammation in lipopolysaccharide-induced endotoxemia

Sepsis remains the most lethal infectious disease and substantially impairs patient prognosis after liver transplantation (LT). Our previous study reported a role of the pannexin 1 (PANX1)–interleukin‐33 (IL‐33) axis in activating innate immunity to protect against methicillin‐resistant Staphylococcus aureus infection; however, the role of PANX1 in regulating adaptive immunity in sepsis and the underlying mechanism are unclear. In this study, we examined the role of the PANX1–IL‐33 axis in protecting against sepsis caused by a gram‐negative bacterial infection in an independent LT cohort. Next, in animal studies, we assessed the immunological state of Panx1^−/‐ mice with lipopolysaccharide (LPS)‐induced endotoxemia and then focused on the cytokine storm and regulatory T cells (Tregs), which are crucial for the resolution of inflammation. To generate liver‐specific Panx1‐deficient mice and mimic clinical LT procedures, a mouse LT model was established. We demonstrated that hepatic PANX1 deficiency exacerbated LPS‐induced endotoxemia and dysregulated the immune response in the mouse LT model. In hepatocytes, we confirmed that PANX1 positively regulated IL‐33 synthesis after LPS administration. We showed that the adenosine triphosphate‐P2X7 pathway regulated the hepatic PANX1–IL‐33 axis during endotoxemia in vitro and in vivo. Recombinant IL‐33 treatment rescued LPS‐induced endotoxemia by increasing the numbers of liver‐infiltrating ST2⁺ Tregs and attenuating the cytokine storm in hepatic PANX1‐deficient mice. In conclusion, our findings revealed that the hepatic PANX1–IL‐33 axis protects against endotoxemia and liver injury by targeting ST2⁺ Tregs and promoting the early resolution of hyperinflammation.

T reg cell-intrinsic requirements for ST2 signaling in health and neuroinflammation

ST2, the receptor for the alarmin IL-33, is expressed by a subset of regulatory T (T reg) cells residing in nonlymphoid tissues, and these cells can potently expand upon provision of exogenous IL-33. Whether the accumulation and residence of T reg cells in tissues requires their cell-intrinsic expression of and signaling by ST2, or whether indirect IL-33 signaling acting on other cells suffices, has been a matter of contention. Here, we report that ST2 expression on T reg cells is largely dispensable for their accumulation and residence in nonlymphoid organs, including the visceral adipose tissue (VAT), even though cell-intrinsic sensing of IL-33 promotes type 2 cytokine production by VAT-residing T reg cells. In addition, we uncovered a novel ST2-dependent role for T reg cells in limiting the size of IL-17A–producing γδT cells in the CNS in a mouse model of neuroinflammation, experimental autoimmune encephalomyelitis (EAE). Finally, ST2 deficiency limited to T reg cells led to disease exacerbation in EAE.

IL-33 activates mTORC1 and modulates glycolytic metabolism in CD8+ T cells

Interleukin (IL)-33, a member in the IL-1 family, plays a central role in innate and adaptive immunity; however, how IL-33 mediates cytotoxic T-cell regulation and the downstream signals remain elusive. In this study, we found increased mouse IL-33 expression in CD8⁺ T cells following cell activation via anti-CD3/CD28 stimulation in vitro or lymphocytic choriomeningitis virus (LCMV) infection in vivo. Our cell adoptive transfer experiment demonstrated that extracellular, but not nuclear, IL-33 contributed to the activation and proliferation of CD8⁺ , but not CD4⁺ T effector cells in LCMV infection. Importantly, IL-33 induced mTORC1 activation in CD8⁺ T cells as evidenced by increased phosphorylated S6 ribosomal protein (p-S6) levels both in vitro and in vivo. Meanwhile, this IL-33-induced CD8⁺ T-cell activation was suppressed by mTORC1 inhibitors. Furthermore, IL-33 elevated glucose uptake and lactate production in CD8⁺ T cells in both dose- and time-dependent manners. The results of glycolytic rate assay demonstrated the increased glycolytic capacity of IL-33-treated CD8⁺ T cells compared with that of control cells. Our mechanistic study further revealed the capacity of IL-33 in promoting the expression of glucose transporter 1 (Glut1) and glycolytic enzymes via mTORC1, leading to accelerated aerobic glucose metabolism Warburg effect and increased effector T-cell activation. Together, our data provide new insights into IL-33-mediated regulation of CD8⁺ T cells, which might be beneficial for therapeutic strategies of inflammatory and infectious diseases in the future.

Migration of Lung Resident Group 2 Innate Lymphoid Cells Link Allergic Lung Inflammation and Liver Immunity

Group 2 innate lymphoid cells (ILC2s) are tissue resident in the lung and activated by inhaled allergens via epithelial-derived alarmins including IL-33. Activated ILC2s proliferate, produce IL-5 and IL-13, and induce eosinophilic inflammation. Here, we report that intranasal IL-33 or the protease allergen papain administration resulted in increased numbers of ILC2s not only in the lung but also in peripheral blood and liver. Analyses of IL-33 treated parabiosis mice showed that the increase in lung ILC2s was due to proliferation of lung resident ILC2s, whereas the increase in liver ILC2s was due to the migration of activated lung ILC2s. Lung-derived ILC2s induced eosinophilic hepatitis and expression of fibrosis-related genes. Intranasal IL-33 pre-treatment also attenuated concanavalin A-induced acute hepatitis and cirrhosis. These results suggest that activated lung resident ILC2s emigrate from the lung, circulate, settle in the liver and promote type 2 inflammation and attenuate type 1 inflammation.

Exogenous oxidative stress suppresses IL-33 -driven proliferation programming in group 2 innate lymphoid cells

Although exogenous oxidative stress has been suggested to promote the pathogenesis of airway inflammation, previous trials using conventional antioxidant therapy in asthma have been largely ineffective, suggesting the complex roles of oxidative stress in the regulation of airway inflammation and of its critical mediating lymphocyte populations. Group 2 innate lymphoid cells (ILC2s) proliferate and induce eosinophilia in response to tissue alarminsin the early phase of airway inflammation. We previously showed that IL-33 -induced endogenous reactive oxygen species is required for optimal metabolic activation of ILC2 functions, however, the role of exogenous oxidative stress in regulating ILC2 functions has not been investigated. Here, we found that exogenous oxidative stress induced by injection of ROS -generating reagent alleviated IL-33 -triggered ILC2 response and inflammation both in the airway and in the liver. Exogenous oxidative stress in ILC2s also compromised IL-33 -mediated accumulation of these cells, as well as subsequent recruitment of eosinophils, after adoptive transferring into ILC2 deficient hosts. Mechanistically, exogenous oxidative stress in ILC2s compromised the proliferation program, while preserving the expression levels of effector molecules in ILC2s. Impaired proliferation under exogenous oxidative stress led to reduced numbers of ILC2s, and an overall decrease in ILC2 response to IL-33 stimulation. Collectively, these data indicate that exogenous oxidative stress suppresses the proliferation program in ILC2s and alleviates IL-33 -triggered inflammation, suggesting that therapeutic induction of oxidative stress might alleviate ILC2 -mediated inflammation in the airway, and possibly also in other organs.

Metformin Modulates T Cell Function and Alleviates Liver Injury Through Bioenergetic Regulation in Viral Hepatitis

Metformin is not only the first-line medication for the treatment of type 2 diabetes, but it is also effective as an anti-inflammatory, anti-oxidative and anti-tumor agent. However, the effect of metformin during viral hepatitis remains elusive. Using an adenovirus (Ad)-induced viral hepatitis mouse model, we found that metformin treatment significantly attenuated liver injury, with reduced serum aspartate transaminase (AST) and alanine transaminase (ALT) levels and liver histological changes, presumably via decreased effector T cell responses. We then demonstrated that metformin reduced mTORC1 activity in T cells from infected mice, as evidenced by decreased phosphorylation of ribosome protein S6 (p-S6). The inhibitory effects on the mTORC1 signaling by metformin was dependent on the tuberous sclerosis complex 1 (TSC1). Mechanistically, metformin treatment modulated the phosphorylation of dynamin-related protein 1 (Drp-1) and mitochondrial fission 1 protein (FIS1), resulting in increased mass in effector T cells. Moreover, metformin treatment promoted mitochondrial superoxide production, which can inhibit excessive T cell activation in viral hepatitis. Together, our results revealed a protective role and therapeutic potential of metformin against liver injury in acute viral hepatitis via modulating effector T cell activation via regulating the mTORC1 pathway and mitochondrial functions.

IL-33 ameliorates liver injury and inflammation in Poly I:C and Concanavalin-A induced acute hepatitis

The IL-33/ST2 axis is known to be involved in liver pathologies and IL-33 is over-expressed in mouse hepatitis models. We aimed to investigate the proposed protective effect of IL-33 in murine fulminant hepatitis induced by a Toll like receptor 3 (TLR3) viral mimetic, Poly I:C or by Concanavalin-A (ConA). The Balb/C mice were administered intravenously with ConA (15 mg/kg) or Poly I:C (30 μg/mouse) to induce acute hepatitis along with vehicle control. The recombinant mouse IL-33 (rIL-33) was injected (0.2 μg/mouse) to mice 2 h prior to ConA or Poly I:C injection to check its hepato-protective effects. The gross lesions, level of serum aspartate aminotransferase (AST), alanine aminotransferase (ALT), histopathology (H&E staining) and levels of IFNγ and TNFα were measured by ELISA. The gross pathological liver injury induced by Poly I:C or ConA was reduced by rIL-33 administration in mice. The levels of AST and ALT were significantly (P ≤ 0.05) higher in mice challenged with Poly I:C or ConA in comparison to control mice. The rIL-33 pre-treated mice in both Poly I:C and ConA challenge groups showed significantly (P ≤ 0.05) lower levels of AST and ALT, and decreased liver injury (parenchymal and per-vascular necrotic areas) in histological liver sections. The soluble levels of TNFα and IFNγ were significantly (P ≤ 0.05) raised in Poly I:C or ConA challenged mice than control mice. The levels of TNFα and IFNγ were significantly reduced (P ≤ 0.05) in rIL-33 pre-treated mice. In conclusion, the exogenous IL-33 administration mitigated liver injury and inflammation (decreased levels of IFNγ and TNFα) in Poly I:C and ConA-induced acute hepatitis in mice.

High Expression of Interleukin-33/ST2 Predicts the Progression and Poor Prognosis in Chronic Hepatitis B Patients with Hepatic Flare

BACKGROUND

Interleukin-33 (IL-33), along with its receptor suppression of tumorigenicity 2 (ST2), is capable of regulating immune responses. Immunologically mediated events play a critical role in the acute phase of chronic hepatitis B (CHB) infection. The present study primarily aimed to determine whether the IL-33/ST2 axis could be used as a reliable biomarker to predict disease progression and prognosis.

METHODS

The study included 130 cases of CHB, with 48 cases in stable condition, 50 cases of progression to hepatitis B virus (HBV)-related acute-on-chronic liver failure (ACLF), and 32 cases of progression to HBV related pre-ACLF. The demographic data and laboratory test results were recorded and compared among the groups. The blood samples for the measurement of serum IL-33 and soluble ST2 (sST2) levels were collected at admission and evaluated twice using the ELISA method.

RESULTS

The patients in which the disease progressed to HBV-ACLF had the highest serum IL-33 and sST2 levels among the three groups (p<0.001). The correlation analysis showed that the serum IL-33 levels were associated with the levels of ALT (r = 0.367, p<0.001), AST (r = 0.456, p<0.001) and the MELD score (r = 0.377, p = 0.001). The area under the curve (AUC) of IL-33 and sST2 levels for differentiation of disease progression were 0.861 (95% CI: 0.787-0.934, p<0.001) and 0.788 (95% CI: 0.692-0.884, p<0.001), respectively. The serum IL-33 levels combined with the MELD score had the highest 90-day mortality prediction efficiency, with an AUC of 0.918 (95% CI: 0.859-0.977, p<0.001), a sensitivity of 92.3%, and a specificity of 88.7%.

CONCLUSIONS

The IL-33/sST2 axis could be used to evaluate the progression and mortality in CHB patients with hepatic flare. The combinatorial use of multiple indicators could achieve the highest diagnostic and predictive accuracy.

Mitochondrial DNA from hepatocytes induces upregulation of interleukin-33 expression of macrophages in nonalcoholic steatohepatitis

OBJECTIVE

In the present study, we propose that lipotoxicity induces the release of mitochondrial DNA (mtDNA) from hepatocytes, which in turn upregulates IL-33 expression in macrophages.

METHODS

The mtDNA levels of plasma were determined in methionine- and mholine-deficient diet (MCD)-fed mice and NASH patients. Cultured hepatocytes were pre-incubated with Mito-TEMPO or rapamycin and were then stimulated with palmitic acid. The mtDNA levels in the cytosol were measured. The mtDNA from hepatocytes of mice was added to bone marrow-derived macrophages (BMDMs) in the presence of IRS (TLR9 antagonist). The expression of IL-33 in BMDMs was measured.

RESULTS

Levels of mtDNA were higher in NASH patients and MCD-fed mice. Treatment of hepatocytes with palmitic acid in vitro induced mtDNA release into cytosol, which was attenuated by mito-TEMPO or rapamycin, and aggravated by inhibition of autophagy. Treatment of BMDMs with mtDNA enhanced IL-33 expression, which was attenuated by knockdown of TLR9. Treatment of BMDMs with mtDNA enhanced lipopolysaccharide (LPS)-induced production of IL-1β and TNF-α, which was attenuated by pretreatment with soluble ST2.

CONCLUSION

mtDNA released from injured hepatocytes under lipid overload induced the upregulation of IL-33 expression in macrophages via TLR9, and enhanced LPS-induced inflammatory cytokine production.

Retinoic Acid Modulates Hyperactive T Cell Responses and Protects Vitamin A-Deficient Mice against Persistent Lymphocytic Choriomeningitis Virus Infection

Vitamin A deficiency (VAD) is a major public health problem and is associated with increased host susceptibility to infection; however, how VAD influences viral infection remains unclear. Using a persistent lymphocytic choriomeningitis virus infection model, we showed in this study that although VAD did not alter innate type I IFN production, infected VAD mice had hyperactive, virus-specific T cell responses at both the acute and contraction stages, showing significantly decreased PD-1 but increased cytokine (IFN-γ, TNF-α, and IL-2) expression by T cells. Compared with control mice, VAD mice displayed excessive inflammation and more severe liver pathology, with increased death during persistent infection. Of note, supplements of all-trans retinoic acid (RA), one of the important metabolites of vitamin A, downregulated hyperactive T cell responses and rescued the persistently infected VAD mice. By using adoptive transfer of splenocytes, we found that the environmental vitamin A or its metabolites acted as rheostats modulating antiviral T cells. The analyses of T cell transcriptional factors and signaling pathways revealed the possible mechanisms of RA, as its supplements inhibited the abundance of NFATc1 (NFAT 1), a key regulator for T cell activation. Also, following CD3/CD28 cross-linking stimulation, RA negatively regulated the TCR-proximal signaling in T cells, via decreased phosphorylation of Zap70 and its downstream signals, including phosphorylated AKT, p38, ERK, and S6, respectively. Together, our data reveal VAD-mediated alterations in antiviral T cell responses and highlight the potential utility of RA for modulating excessive immune responses and tissue injury in infectious diseases.

Hepatic ILC2 activity is regulated by liver inflammation-induced cytokines and effector CD4+ T cells

In immune-mediated hepatitis, type 2 innate lymphoid cells (ILC2) as well as effector CD4⁺ T cells have been shown to drive disease pathology. However, less is known about mechanisms involved in the regulation of ILC2 function during liver inflammation. We showed that in homeostasis, hepatic ILC2 constituted a very small population with a naive, inactive phenotype. During immune-mediated hepatitis, the cytokines IL-33 and IFNγ were expressed in liver tissue. IL-33 induced strong activation and expression of type 2 cytokines as well as IL-6 by hepatic ILC2 while IFNγ suppressed cytokine production. Interestingly, this inhibitory effect was overcome by IL-33. The phenotype of activated hepatic ILC2 were stable since they did not show functional plasticity in response to liver inflammation-induced cytokines. Moreover, hepatic ILC2 induced a Th2 phenotype in activated CD4⁺ T cells, which increased ILC2-derived cytokine expression via IL-2. In contrast, Th1 cells inhibited survival of ILC2 by production of IFNγ. Thus, hepatic ILC2 function is regulated by IL-33, IL-2, and IFNγ. While IL-33 and IL-2 support hepatic ILC2 activation, their inflammatory activity in immune-mediated hepatitis might be limited by infiltrating IFNγ-expressing Th1 cells.

Tissue-resident NK cells and other innate lymphoid cells

Natural killer (NK) cells are important innate effectors for their defense against pathogens and tumors without the need of prior sensitization. Along with the growing understanding of basic NK cell biology, it has been widely accepted that NK cells are a heterogeneous population of innate lymphoid cell (ILC) family. Apart from the conventional NK cell (cNK) subset that circulates throughout the body, some non-lymphoid tissues contain tissue-resident NK (trNK) cell subsets, and the composition of NK cell subsets varies greatly with different locations. Except for cNK cells, other ILCs are known as tissue-resident cells. In this review, we summarize the unique properties of trNK cells, discuss their lineage relationship with other ILCs, and highlight recent advances in our understanding of the functions of trNK cells and other ILCs.

The Roles of Liver-Resident Lymphocytes in Liver Diseases

Tissue-resident lymphocytes usually reside in barrier sites and are involved in innate and adaptive immunity. In recent years, many studies have shown that multiple types of lymphocytes are resident in the liver, including memory CD8⁺ T (T_RM) cells; "unconventional" T cells, such as invariant natural killer T (iNKT) cells, mucosal associated invariant T (MAIT) cells, and γδT cells; innate lymphoid cells (ILCs) such as natural killer (NK) cells and other ILCs. Although diverse types of tissue-resident lymphocytes share similar phenotypes, functional properties, and transcriptional regulation, the unique microenvironment of the liver can reshape their phenotypic and functional characteristics. Liver-resident lymphocytes serve as sentinels and perform immunosurveillance in response to infection and non-infectious insults, and are involved in the maintenance of liver homeostasis. Under the pathological conditions, distinct liver-resident lymphocytes exert protective or pathological effects in the process of various liver diseases. In this review, we highlight the unique properties of liver-resident lymphocytes, and discuss their functional characteristics in different liver diseases.

Type 2 Innate Lymphoid Cells in Liver and Gut: From Current Knowledge to Future Perspectives

Innate lymphoid cells (ILCs) represent a heterogeneous population of recently discovered immune cells that mirror the functions of adaptive T lymphocytes. However, ILCs are devoid of specific antigen receptors and cellular activation depends on environmental cytokines, rendering them as early regulators of immune responses. Type 2 innate lymphoid cells (ILC2s) respond to alarmins, such as interleukin-25 and -33 and shape Th2-associated immunity by expressing IL-5 and IL-13 in a GATA3-dependent manner. In addition, ILC2s express the epidermal growth factor-like molecule Amphiregulin thereby promoting regeneration of injured tissue during inflammation. The gut and liver confer nutrient metabolism and bidirectional exchange of products, known as the gut-liver axis. Accordingly, both organs are continuously exposed to a large variety of harmless antigens. This requires avoidance of immunity, which is established by a tolerogenic environment in the gut and liver. However, dysregulations within the one organ are assumed to influence vitality of the other and frequently promote chronic inflammatory settings with poor prognosis. Intensive research within the last years has revealed that ILC2s are involved in acute and chronic inflammatory settings of gut and liver. Here, we highlight the roles of ILC2s in intestinal and hepatic inflammation and discuss a regulatory potential.

Dysregulated Th1 Immune and Vascular Responses in Scrub Typhus Pathogenesis

Scrub typhus is an emerging, insect-transmitted disease caused by Orientia tsutsugamushi, a Gram- and LPS-negative bacterium that replicates freely within professional phagocytes and endothelial cells. Scrub typhus is prevalent with high mortality rates, but information regarding its molecular pathogenesis, microbial virulence determinants, and key immune responses is limited. Improved animal models have recently been developed that respectively resemble the pathological features of self-limiting or severe scrub typhus in humans. Strong activation of Th1 and CD8, but not Th2 and regulatory T, immune responses, accompanied by altered angiopoietin/Tie2-related regulation, are hallmarks of lethal infection in murine models. This review, based primarily on recent advances from clinical and experimental studies, highlights tissue- and endothelial cell-specific biomarkers that are indicative of immune dysregulation. The potential roles of neutrophils and damage-associated molecular pattern molecules at late stages of disease are discussed in the context of vascular leakage, pulmonary and renal injury, and scrub typhus pathogenesis.

IL-33 induces immunosuppressive neutrophils via a type 2 innate lymphoid cell/IL-13/STAT6 axis and protects the liver against injury in LCMV infection-induced viral hepatitis

Viral hepatitis is still a public health problem affecting several million people around the world. Neutrophils are polymorphonuclear cells that have a critical role in antibacterial infection. However, the role of neutrophils in viral infection is not fully understood. By using a mouse model of lymphocytic choriomeningitis virus infection-induced viral hepatitis, we observed increased neutrophil recruitment in the liver accompanied by enhanced CD8+ T-cell responses. Liver neutrophils expressed high levels of immunomodulatory cytokines, such as C-X-C chemokine ligand 2, arginase-1, inducible nitric oxide synthase and interleukin (IL)-10, demonstrating immunosuppressive properties. Depletion of neutrophils in vivo by a neutralizing antibody resulted in the exacerbation of liver injury and the promotion of T-cell responses at the immune contraction stage. IL-33 significantly induced neutrophil recruitment in the liver and attenuated liver injury by limiting effector T-cell accumulation. Mechanistically, we found that IL-33 promoted the expression of arginase-1 in neutrophils through the type 2 innate lymphoid cell (ILC2)-derived IL-13. Additionally, IL-13 increased the inhibitory effect of neutrophils on CD8+ T-cell proliferation in vitro, partially through arginase-1. Finally, we found that IL-13 induced arginase-1 expression, depending on signal transducer and activator of transcription factor 6 (STAT6) signaling. Therefore, IL-33 induced immunosuppressive neutrophils via an ILC2/IL-13/STAT6 axis. Collectively, our findings shed new light on the mechanisms associated with IL-33-triggered neutrophils in the liver and suggest potential targets for therapeutic investigation in viral hepatitis.

Innate lymphoid cells in organ fibrosis

Innate lymphoid cells (ILCs) are a recently identified family of lymphoid effector cells. ILCs are mainly clustered into 3 groups based on their unique cytokine profiles and transcription factors typically attributed to the subsets of T helper cells. ILCs have a critical role in the mucosal immune response through promptly responding to pathogens and producing large amount of effector cytokines of type 1, 2, or 3 responses. In addition to the role of early immune responses against infections, ILCs, particularly group 2 ILCs (ILC2), have recently gained attention for modulating remodeling and fibrosis especially in the mucosal tissues. Herein, we overview the current knowledge in this area, highlighting roles of ILCs on fibrosis in the mucosal tissues, especially focusing on the gut and lung. We also discuss some new directions for future research by extrapolating from knowledge derived from studies on Th cells.

In drug-induced, immune-mediated hepatitis, interleukin-33 reduces hepatitis and improves survival independently and as a consequence of FoxP3+ T-cell activity

Immune-mediated, drug-induced hepatitis is a rare complication of halogenated volatile anesthetic administration. IL-4-regulated Th2-polarized reactions initiate this type and other types of hepatitis, while the mechanisms that regulate the severity remain elusive. IL-33 is an innate, IL-4-inducing, Th2-polarizing cytokine that has been detected in patients with liver failure and has been associated with upregulated ST2+Foxp3+CD4+CD25+ T cells; however, roles for IL-33 in drug-induced hepatitis are unclear. We investigated IL-33 in an anesthetic, immune-mediated hepatitis modeled in BALB/c, IL-33-/- and ST2-/- mice, as well as in patients with anesthetic hepatitis. The hepatic IL-33 and ST2 levels were elevated in BALB/c mice (p < 0.05) with hepatitis, and anti-IL-33 diminished hepatitis (p < 0.05) without reducing IL-33 levels. The complete absence of IL-33 reduced IL-10 (p < 0.05) and ST2+Foxp3+CD4+CD25+ T cells (p < 0.05), as well as reduced the overall survival (p < 0.05), suggesting suppressive roles for IL-33 in anesthetic, immune-mediated hepatitis. All of the mice demonstrated similar levels of CD4+ T-cell proliferation following direct T-cell receptor stimulation, but we detected splenic IL-33 and ST2-negative Foxp3+CD4+CD25+ T cells in ST2-/- mice that developed less hepatitis than BALB/c mice (p < 0.05), suggesting that ST2-negative Foxp3+CD4+CD25+ T cells reduced hepatitis. In patients, serum IL-33 and IPEX levels were correlated in controls (r² = 0.5, p < 0.05), similar to the levels in mice, but not in anesthetic hepatitis patients (r² = 0.01), who had elevated IL-33 (p < 0.001) and decreased IPEX (p < 0.01). Our results suggest that, in anesthetic, immune-mediated hepatitis, IL-33 does not regulate the CD4+ T-cell proliferation that initiates hepatitis, but IL-33, likely independent of ST2, reduces hepatitis via upregulation of Foxp3+CD4+CD25+ T cells. Further studies are needed to translate the role of IL-33 to human liver disease.

Ambiguous roles of innate lymphoid cells in chronic development of liver diseases

Innate lymphoid cells (ILCs) are defined as a distinct arm of innate immunity. According to their profile of secreted cytokines and lineage-specific transcriptional factors, ILCs can be categorized into the following three groups: group 1 ILCs (including natural killer (NK) cells and ILC1s) are dependent on T-bet and can produce interferon-γ; group 2 ILCs (ILC2s) are dependent on GATA3 and can produce type 2 cytokines, including interleukin (IL)-5 and IL-13; and, group 3 ILCs (including lymphoid tissue-like cells and ILC3s) are dependent on RORγt and can produce IL-22 and IL-17. Collaborative with adaptive immunity, ILCs are highly reactive innate effectors that promptly orchestrate immunity, inflammation and tissue repair. Dysregulation of ILCs might result in inflammatory disorders. Evidence regarding the function of intrahepatic ILCs is emerging from longitudinal studies of inflammatory liver diseases wherein they exert both physiological and pathological functions, including immune homeostasis, defenses and surveillance. Their overall effect on the liver depends on the balance of their proinflammatory and antiinflammatory populations, specific microenvironment and stages of immune responses. Here, we review the current data about ILCs in chronic liver disease progression, to reveal their roles in different stages as well as to discuss their therapeutic potency as intervention targets.

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.

IL-33 treatment attenuated diet-induced hepatic steatosis but aggravated hepatic fibrosis

The aim of our work was to investigate the role of interleukin-33 (IL-33) and its receptor ST2 in the progression of diet-induced nonalcoholic steatohepatitis (NASH) in mice, and the characteristic expression in livers of patients with NASH. Mice were fed with high-fat diet (HFD) or methionine-choline 4-deficient diet (MCD) and injected intraperitoneally with IL-33. Both mRNA and protein expression levels of IL-33 and ST2 were up-regulated in the livers of mice fed with HFD or MCD. Treatment with IL-33 attenuated diet-induced hepatic steatosis and reduced activities of ALT in serum, as well as ameliorated HFD-induced systemic insulin resistance and glucose intolerance, while aggravated hepatic fibrosis in diet-induced NASH. Furthermore, treatment with IL-33 can also promote Th2 response and M2 macrophage activation and beneficial modulation on expression profiles of fatty acid metabolism genes in livers. ST2 deficiency did not affect hepatic steatosis and fibrosis when fed with controlling diet. IL-33 did not affect diet-induced hepatic steatosis and fibrosis in ST2 knockout mice. Meanwhile, in the livers of patients with NASH, IL-33 was mainly located in hepatic sinusoid, endothelial cells, and hepatic stellate cells. The mRNA expression level of IL-33 and ST2 was elevated with the progression of NASH. In conclusion, treatment with IL-33 attenuated diet-induced hepatic steatosis, but aggravated hepatic fibrosis, in a ST2-dependent manner.

Role of the IL-33/ST2L axis in colorectal cancer progression

Interleukin-33 (IL-33) has been identified as a natural ligand of ST2L. IL-33 primarily acts as a key regulator of Th2 responses through binding to ST2L, which is antagonized by soluble ST2 (sST2). The IL-33/ST2L axis is involved in various inflammatory pathologies, including ulcerative colitis (UC). Several recent investigations have also suggested that the IL-33/ST2L axis plays a role in colorectal cancer (CRC) progression. In CRC, tumor- and stroma-derived IL-33 may activate ST2L on various cell types in an autocrine and paracrine manner. Although several findings support the hypothesis that the IL-33/ST2L axis positively regulates CRC progression, other reports do not; hence, this hypothesis remains controversial. At any rate, recent studies have provided overwhelming evidence that the IL-33/ST2L axis plays important roles in CRC progression. This review summarizes the role of the IL-33/ST2L axis in the UC and CRC microenvironments.

Unconventional Pro-inflammatory CD4+ T Cell Response in B Cell-Deficient Mice Infected with Trypanosoma cruzi

Chagas disease, caused by the parasite Trypanosoma cruzi, is endemic in Latin America but has become a global public health concern by migration of infected people. It has been reported that parasite persistence as well as the intensity of the inflammatory immune response are determinants of the clinical manifestations of the disease. Even though inflammation is indispensable for host defense, when deregulated, it can contribute to tissue injury and organ dysfunction. Here, we report the importance of B cells in conditioning T cell response in T. cruzi infection. Mice deficient in mature B cells (muMT mice) infected with T. cruzi exhibited an increase in plasma TNF concentration, TNF-producing CD4⁺ T cells, and mortality. The increase in TNF-producing CD4⁺ T cells was accompanied by a reduction in IFNγ⁺CD4⁺ T cells and a decrease of the frequency of regulatory Foxp3⁺, IL-10⁺, and IL17⁺CD4⁺ T cells populations. The CD4⁺ T cell population activated by T. cruzi infection, in absence of mature B cells, had a high frequency of Ly6C⁺ cells and showed a lower expression of inhibitory molecules such as CTLA-4, PD-1, and LAG3. CD4⁺ T cells from infected muMT mice presented a high frequency of CD62L^hiCD44^- cells, which is commonly associated with a naïve phenotype. Through transfer experiments we demonstrated that CD4⁺ T cells from infected muMT mice were able to condition the CD4⁺ T cells response from infected wild-type mice. Interestingly, using Blimp-flox/flox-CD23icre mice we observed that in absence of plasmablast/plasma cell T. cruzi-infected mice exhibited a higher number of TNF-producing CD4⁺ T cells. Our results showed that the absence of B cells during T. cruzi infection affected the T cell response at different levels and generated a favorable scenario for unconventional activation of CD4⁺ T cell leading to an uncontrolled effector response and inflammation. The product of B cell differentiation, the plasmablast/plasma cells, could be able to regulate TNF-producing CD4⁺ T cells since their absence favor the increase of the number of TNF⁺ CD4⁺ in T. cruzi-infected mice.

IL-33-ST2 Axis in Liver Disease: Progression and Challenge

The new member of the IL-1 family, interleukin-33 (IL-33), participates in the progression of a variety of diseases through binding with its receptor ST2. Recently, much clinical evidence and experimental data have indicated that IL-33 is associated with various liver diseases. This review primarily addresses the relationship between IL-33 and several hepatic diseases. IL-33 can alleviate high-fat diet- (HFD-) induced hepatic steatosis and insulin resistance, and IL-33 acts as an alarmin, which quickly triggers the immune system to respond to virus invasion and toxic damage to the liver. However, when liver injury is chronic, IL-33 promotes Th2 reactions and hepatic stellate cell (HSC) activity, facilitating progression to liver fibrosis. The complicated functions of IL-33 should be considered before its clinical application.

Emerging concepts in biliary repair and fibrosis

Chronic diseases of the biliary tree (cholangiopathies) represent one of the major unmet needs in clinical hepatology and a significant knowledge gap in liver pathophysiology. The common theme in cholangiopathies is that the target of the disease is the biliary tree. After damage to the biliary epithelium, inflammatory changes stimulate a reparative response with proliferation of cholangiocytes and restoration of the biliary architecture, owing to the reactivation of a variety of morphogenetic signals. Chronic damage and inflammation will ultimately result in pathological repair with generation of biliary fibrosis and clinical progression of the disease. The hallmark of pathological biliary repair is the appearance of reactive ductular cells, a population of cholangiocyte-like epithelial cells of unclear and likely mixed origin that are able to orchestrate a complex process that involves a number of different cell types, under joint control of inflammatory and morphogenetic signals. Several questions remain open concerning the histogenesis of reactive ductular cells, their role in liver repair, their mechanism of activation, and the signals exchanged with the other cellular elements cooperating in the reparative process. This review contributes to the current debate by highlighting a number of new concepts derived from the study of the pathophysiology of chronic cholangiopathies, such as congenital hepatic fibrosis, biliary atresia, and Alagille syndrome.

The Role of Innate Lymphoid Cells in Immune-Mediated Liver Diseases

Innate lymphoid cells (ILCs) are a recently identified group of innate immune cells lacking antigen-specific receptors that can mediate immune responses and regulate tissue homeostasis and inflammation. ILCs comprise group 1 ILCs, group 2 ILCs, and group 3 ILCs. These ILCs usually localize at mucosal surfaces and combat pathogens by the rapid release of certain cytokines. However, the uncontrolled activation of ILCs can also lead to damaging inflammation, especially in the gut, lung, and skin. Although the physiological and pathogenic roles of ILCs in liver diseases have been attracting increasing attention recently, there has been no systematic review regarding the roles of ILCs in immune-mediated liver diseases. Here, we review the relationships between the ILC subsets and their functions in immune-mediated liver diseases, and discuss their therapeutic potential based on current knowledge about the functional roles of these cells in liver diseases.

IL-33 protects murine viral fulminant hepatitis by targeting coagulation hallmark protein FGL2/fibroleukin expression

Fulminant hepatitis (FH) is characterized by rapid liver failure and high mortality. The pathogenesis of viral FH includes virus-induced immune activation, inflammation, and subsequent hepatic apoptosis and necrosis. However, the mechanisms that underlie FH progression are unclear. IL-33 is a member of the IL-1-related cytokines, considered to be an "alarmin" that participates in various diseases, but its precise role in the coagulation of FH is not very clear. In our study, we found that IL-33 is significantly elevated in mice infected with murine hepatitis virus strain 3 (MHV-3). This is accompanied by an increase in pro-coagulant fibrinogen-like protein 2 (FGL2) in the liver. Previous studies have suggested that an increase in FGL2 is diagnostic of FH and liver necrosis, and animals with no FGL2 had better survivorship during FH. Our studies showed that IL-33 administration in a MHV-3 infection promoted survival during FH, with a significant reduction in FGL2 expression and liver inflammation. In vitro IL-33 treatment abrogated MHV-3 and IFN-γ induced FGL2 expression in RAW264.7 and THP-1 cells, respectively. In conclusion, our research suggests that IL-33 protects against viral fulminant hepatitis in mice by antagonizing expression of the pro-coagulant protein FGL2.

Endogenous IL-33 Deficiency Exacerbates Liver Injury and Increases Hepatic Influx of Neutrophils in Acute Murine Viral Hepatitis

The alarmin IL-33 has been described to be upregulated in human and murine viral hepatitis. However, the role of endogenous IL-33 in viral hepatitis remains obscure. We aimed to decipher its function by infecting IL-33-deficient mice (IL-33 KO) and their wild-type (WT) littermates with pathogenic mouse hepatitis virus (L2-MHV3). The IL-33 KO mice were more sensitive to L2-MHV3 infection exhibiting higher levels of AST/ALT, higher tissue damage, significant weight loss, and earlier death. An increased depletion of B and T lymphocytes, NKT cells, dendritic cells, and macrophages was observed 48 h postinfection (PI) in IL-33 KO mice than that in WT mice. In contrast, a massive influx of neutrophils was observed in IL-33 KO mice at 48 h PI. A transcriptomic study of inflammatory and cell-signaling genes revealed the overexpression of IL-6, TNFα, and several chemokines involved in recruitment/activation of neutrophils (CXCL2, CXCL5, CCL2, and CCL6) at 72 h PI in IL-33 KO mice. However, the IFNγ was strongly induced in WT mice with less profound expression in IL-33 KO mice demonstrating that endogenous IL-33 regulated IFNγ expression during L2-MHV3 hepatitis. In conclusion, we demonstrated that endogenous IL-33 had multifaceted immunoregulatory effect during viral hepatitis via induction of IFNγ, survival effect on immune cells, and infiltration of neutrophils in the liver.

Retinoic Acid Regulates Immune Responses by Promoting IL-22 and Modulating S100 Proteins in Viral Hepatitis

Although large amounts of vitamin A and its metabolite all-trans retinoic acid (RA) are stored in the liver, how RA regulates liver immune responses during viral infection remains unclear. In this study, we demonstrated that IL-22, mainly produced by hepatic γδ T cells, attenuated liver injury in adenovirus-infected mice. RA can promote γδ T cells to produce mTORC1-dependent IL-22 in the liver, but inhibits IFN-γ and IL-17. RA also affected the aptitude of T cell responses by modulating dendritic cell (DC) migration and costimulatory molecule expression. These results suggested that RA plays an immunomodulatory role in viral infection. Proteomics data revealed that RA downregulated S100 family protein expression in DCs, as well as NF-κB/ERK pathway activation in these cells. Furthermore, adoptive transfer of S100A4-repressed, virus-pulsed DCs into the hind foot of naive mice failed to prime T cell responses in draining lymph nodes. Our study has demonstrated a crucial role for RA in promoting IL-22 production and tempering DC function through downregulating S100 family proteins during viral hepatitis.

Serum soluble ST2 is a promising prognostic biomarker in HBV-related acute-on-chronic liver failure

BACKGROUND

The IL-33/ST2 axis is involved in the pathogenesis of many diseases such as autoimmune diseases, cancer, and heart failure. However, studies of the IL-33/ST2 pathway in HBV-related acute-on-chronic liver failure (HBV-ACLF) are lacking. The present study aimed to determine the prognostic role of serum IL-33/soluble ST2 (sST2) in HBV-ACLF.

METHODS

Serum levels of IL-33 and sST2 in healthy controls (HC, n=18), chronic hepatitis B (CHB, n=27) and HBV-ACLF (n=51) patients at the 1st and 4th week after enrollment were detected using ELISA, and clinical data were collected. The follow-up of HBV-ACLF patients lasted for 6 months at least.

RESULTS

There was no significant difference of serum IL-33 level among HC, CHB and HBV-ACLF patients at week 1. However, serum sST2 level differed significantly among the three groups: highest in the HBV-ACLF group, moderate in the CHB group and lowest in the HC group. There was a reverse correlation between serum sST2 level and the survival of HBV-ACLF patients. The level of serum sST2 in HBV-ACLF survivors was significantly declined from week 1 to week 4 following the treatment, whereas that in HBV-ACLF non-survivors remained at a high level during the same period. Furthermore, serum sST2 level was significantly correlated with laboratory parameters and the most updated prognostic scores (CLIF-C OF score, CLIF-C ACLF score and ACLF grades). The receiver operating characteristics curves demonstrated that serum sST2 level was a good diagnostic marker for predicting the 6-month mortality in HBV-ACLF patients, comparable to the most updated prognostic scores. Serum sST2 cut-off points for predicting prognosis in HBV-ACLF patients were 76 ng/mL at week 1 or 53 ng/mL at week 4, respectively. HBV-ACLF patients with serum sST2 level above the cut-off point often had a worse prognosis than those below the cut-off point.

CONCLUSION

Serum sST2 may act as a promising biomarker to assess severity and predict prognosis of patients with HBV-ACLF and help for the early identification and optimal treatment of HBV-ACLF patients at high risk of mortality.

The Role of IL-33-Dependent Inflammation in the Tumor Microenvironment

There is compelling evidence that inflammation contributes to tumorigenesis. Inflammatory mediators within the tumor microenvironment can either promote an antitumor immune response or support tumor pathogenesis. Therefore, it is critical to determine the relative contribution of tumor-associated inflammatory pathways to cancer development. Interleukin-33 (IL-33) is a member of the IL-1 family of cytokines that is released upon tissue stress or damage to operate as an alarmin. IL-33 has been primarily implicated in the induction of type-2 immune responses. However, recent findings have shown a role of IL-33 in several cancers where it may exert multiple functions. In this review, we will present the current knowledge on the role of IL-33 in the microenvironment of different tumors. We will highlight which cells produce and which cells are activated by IL-33 in cancer. Furthermore, we will explain how IL-33 modulates the tumor-associated inflammatory microenvironment to restrain or promote tumorigenesis. Finally, we will discuss the issues to be addressed first before potentially targeting the IL-33 pathway for cancer therapy.

Adverse outcome pathway development from protein alkylation to liver fibrosis

In modern toxicology, substantial efforts are undertaken to develop alternative solutions for in vivo toxicity testing. The adverse outcome pathway (AOP) concept could facilitate knowledge-based safety assessment of chemicals that does not rely exclusively on in vivo toxicity testing. The construction of an AOP is based on understanding toxicological processes at different levels of biological organisation. Here, we present the developed AOP for liver fibrosis and demonstrate a linkage between hepatic injury caused by chemical protein alkylation and the formation of liver fibrosis, supported by coherent and consistent scientific data. This long-term process, in which inflammation, tissue destruction, and repair occur simultaneously, results from the complex interplay between various hepatic cell types, receptors, and signalling pathways. Due to the complexity of the process, an adequate liver fibrosis cell model for in vitro evaluation of a chemical's fibrogenic potential is not yet available. Liver fibrosis poses an important human health issue that is also relevant for regulatory purposes. An AOP described with enough mechanistic detail might support chemical risk assessment by indicating early markers for downstream events and thus facilitating the development of an in vitro testing strategy. With this work, we demonstrate how the AOP framework can support the assembly and coherent display of distributed mechanistic information from the literature to support the use of alternative approaches for prediction of toxicity. This AOP was developed according to the guidance document on developing and assessing AOPs and its supplement, the users' handbook, issued by the Organisation for Economic Co-operation and Development.

Intrahepatic regulation of antiviral T cell responses at initial stages of viral infection

It is generally accepted that the appropriate boost of early immune response will control viral replications and limit the immune-mediated pathology in viral hepatitis. However, poor immunity results in viral persistence, chronic inflammation and finally liver cirrhosis and carcinoma. As a peripheral non-lymphoid organ of immune surveillance, the liver continually encounters hundreds of molecules from the blood, including nutrients, toxins and pathogens. In this way, the liver maintains immune tolerance under healthy conditions, but responds quickly to the hepatotropic pathogens during the early stages of an infection. Although our knowledge of liver cell compositions and functions has been improved significantly in recent years, the intrahepatic immune regulation of antiviral T cells at the initial stage is complex and not well elucidated. Here, we summarize the role of liver cell subpopulations in regulating antiviral T cell response at the initial stages of viral infection. A better understanding of early hepatic immune regulation will pave the way for the development of novel therapies and vaccine design for human viral hepatitis.

The Dynamic Role of the IL-33/ST2 Axis in Chronic Viral-infections: Alarming and Adjuvanting the Immune Response

Interleukin 33 (IL-33), a member of the IL-1 family, is constitutively expressed in epithelial and in endothelial cells at barrier sites, acting as a danger signal and adjuvanting the immune response following tissue damage and infection. Originally implicated in allergy, IL-33 is also known to be involved in innate and adaptive immune responses by enhancing natural killer, Th1, and CD4 and CD8 T-cell functions. The nature of the antiviral immune response orchestrated by IL-33 depends on the site of infection, the duration of the disease and the cytokine milieu. In this review, we focus on the distinctive contribution of IL-33 as an anti-infective and proinflammatory cytokine in response to cell death and viral infections. The dynamic role of IL-33 in the acute and chronic phases of infection with HIV, hepatitis B and C viruses, and with CMV is highlighted. This review will also discuss the potential immunotherapeutic and adjuvant roles of IL-33.

Search Strategy and Selection Criteria

English language, indexed publications in PubMed were searched using combinations of following key words: “interleukin-33”, “IL-33”, “suppression of tumorigenicity 2”, ST2”, “sST2”, “HIV”, “HBV”, “HCV”, “CMV”, “HPV”, “immunotherapy” and “vaccine”. Except for seminal studies, only articles published between 2010 and 2016 were included.

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IL-33, a guardian of barriers, acts as an alarmin and as an enhancer of immune responses following injury or infection.

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sST2, the IL-33 decoy receptor, is considered as a biomarker for allergies, cardiac conditions and infections.

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IL-33 has immunotherapeutic and/or adjuvant potential.

Regulation of metabolic health and adipose tissue function by group 2 innate lymphoid cells

Adipose tissue (AT) is home to an abundance of immune cells. With chronic obesity, inflammatory immune cells accumulate and promote insulin resistance and the progression to type 2 diabetes mellitus. In contrast, recent studies have highlighted the regulation and function of immune cells in lean, healthy AT, including those associated with type 2 or "allergic" immunity. Although traditionally activated by infection with multicellular helminthes, AT type 2 immunity is active independently of infection, and promotes tissue homeostasis, AT "browning," and systemic insulin sensitivity, protecting against obesity-induced metabolic dysfunction and type 2 diabetes mellitus. In particular, group 2 innate lymphoid cells (ILC2s) are integral regulators of AT type 2 immunity, producing the cytokines interleukin-5 and IL-13, promoting eosinophils and alternatively activated macrophages, and cooperating with and promoting AT regulatory T (Treg) cells. In this review, we focus on the recent developments in our understanding of group 2 innate lymphoid cell cells and type 2 immunity in AT metabolism and homeostasis.

Overexpression of interleukin-33 is associated with poor prognosis of patients with glioma

Interleukin-33 (IL-33) has shown diagnostic and prognostic values in multiple human cancers. However, there is little knowledge on the role of IL-33 in human gliomas and its association with disease prognosis. This study aimed to evaluate the value of IL-33 in the prognosis of glioma patients. The expression of IL-33 was determined and compared in surgical specimens from 86 glioma patients and 16 normal brain tissues. The associations of IL-33 expression with the clinicopathological features and prognosis of glioma patients were assessed. qRT-PCR assay showed higher IL-33 mRNA expression in glioma tissues than in normal brain tissue ( p < 0.001), and significantly higher IL-33 mRNA expression was detected in both low- and high-grade glioma tissues relative to normal brain tissues ( p < 0.001). Western blotting revealed elevated IL-33 protein levels in glioma tissues compared to those in normal brain tissues, and immunohistochemical staining showed higher IL-33 protein expression in glioma tissues than in normal brain tissues. IL-33 expression correlated with the glioma grade ( p < 0.001) and Karnofsky performance status score ( p = 0.024), and the glioma patients with high IL-33 expression had a shorter progression-free survival ( p < 0.001) and overall survival ( p < 0.001) than those with low IL-33 expression. The univariate and multivariate analyses showed that IL-33 overexpression and the glioma grade were independent factors of a poor prognosis in glioma patients. Therefore, IL-33 may be a promising biomarker for the detection of gliomas, and IL-33 expression is useful for predicting the prognosis of the disease.

Potential Therapeutic Aspects of Alarmin Cytokine Interleukin 33 or Its Inhibitors in Various Diseases

PURPOSE

The purpose of this review was to examine the comprehensively accumulated data regarding potential therapeutic aspects of exogenous administration of interleukin 33 (IL-33) or its antagonists in allergic, cancerous, infectious, and inflammatory diseases.

METHODS

A selected review was undertaken of publications that examined the protective and exacerbating effects of IL-33 or its inhibitors in different diseases. Mechanisms of action are summarized to examine the putative role of IL-33 in various diseases.

FINDINGS

IL-33 promoted antibacterial, antiviral, anti-inflammatory, and vaccine adjuvant functions. However, in TH2-biased respiratory, allergic, parasitic, and inflammatory conditions, IL-33 exhibited disease-sensitizing effects. The alarmin cytokine IL-33 induced protective effects in diseases via recruitment of regulatory T cells; antiviral CD8(+) cells, natural killer cells, γδ T cells, and nuocytes; antibacterial and antifungal neutrophils or macrophages; vaccine-associated B/T cells; and inhibition of nuclear factor-κB-mediated gene transcription. In contrast, IL-33 exacerbated the disease process by increasing TH2 cytokines, IgE and eosinophilic immune responses, and inhibition of leukocyte recruitment in various diseases.

IMPLICATIONS

The protective or exacerbated aspects of use of IL-33 or its inhibitors are dependent on the type of infection or inflammatory condition, duration of disease (acute or chronic), organ involved, cytokine microenvironment, dose or kinetics of IL-33, and genetic predisposition. The alarmin cytokine IL-33 acts at cellular, molecular, and transcriptional levels to mediate pluripotent functions in various diseases and has potential therapeutic value to mitigate the disease process.

Expression and potential roles of IL-33/ST2 in the immune regulation during Clonorchis sinensis infection

During clonorchiasis, immune responses of hosts are responsible for the removal of the worms and also are involved in the progress of the pathological damage caused by Clonorchis sinensis. Interleukin-33 (IL-33), a recently described cytokine signaling through the ST2 receptor, has emerged as a potent inducer to bile duct proliferation and fibrosis; however, little is known of this signaling in the pathogen-caused periductal inflammation and fibrosis. In the present study, using immunohistochemistry, real-time PCR, enzyme-linked immunosorbent assay (ELISA), and flow cytometry, we studied the expression of IL-33/ST2 during C. sinensis infection, as well as their potential roles in C. sinensis-induced host immune responses. The results showed that a higher level of IL-33 was detected in the sera of patients of clonorchiasis (n = 45), compared with in those of healthy donors (n = 16). Similarly, in FVB mice experimentally infected with C. sinensis, a higher level of IL-33 was detected at latent stage both in the serum and in the liver, as well as the up-regulated expression of ST2 receptor on the inflammatory cells, especially on CD4(+) T cells in the liver of infected mice. Our results, for the first time, indicated that the increased IL-33/ST2 may be involved in the regulation of immunopathology induced by C. sinensis.

IL-33-Dependent Endothelial Activation Contributes to Apoptosis and Renal Injury in Orientia tsutsugamushi-Infected Mice

Endothelial cells (EC) are the main target for Orientia tsutsugamushi infection and EC dysfunction is a hallmark of severe scrub typhus in patients. However, the molecular basis of EC dysfunction and its impact on infection outcome are poorly understood. We found that C57BL/6 mice that received a lethal dose of O. tsutsugamushi Karp strain had a significant increase in the expression of IL-33 and its receptor ST2L in the kidneys and liver, but a rapid reduction of IL-33 in the lungs. We also found exacerbated EC stress and activation in the kidneys of infected mice, as evidenced by elevated angiopoietin (Ang) 2/Ang1 ratio, increased endothelin 1 (ET-1) and endothelial nitric oxide synthase (eNOS) expression. Such responses were significantly attenuated in the IL-33-/- mice. Importantly, IL-33-/- mice also had markedly attenuated disease due to reduced EC stress and cellular apoptosis. To confirm the biological role of IL-33, we challenged wild-type (WT) mice with a sub-lethal dose of O. tsutsugamushi and gave mice recombinant IL-33 (rIL-33) every 2 days for 10 days. Exogenous IL-33 significantly increased disease severity and lethality, which correlated with increased EC stress and activation, increased CXCL1 and CXCL2 chemokines, but decreased anti-apoptotic gene BCL-2 in the kidneys. To further examine the role of EC stress, we infected human umbilical vein endothelial cells (HUVEC) in vitro. We found an infection dose-dependent increase in the expression of IL-33, ST2L soluble ST2 (sST2), and the Ang2/Ang1 ratio at 24 and 48 hours post-infection. This study indicates a pathogenic role of alarmin IL-33 in a murine model of scrub typhus and highlights infection-triggered EC damage and IL-33-mediated pathological changes during the course of Orientia infection.

Fingolimod targeting protein phosphatase 2A differently affects IL-33 induced IL-2 and IFN-γ production in CD8(+) lymphocytes

Multiple sclerosis patients are treated with fingolimod (FTY720), a prodrug that acts as an immune modulator. FTY720 is first phosphorylated to FTY720-P and then internalizes sphingosine-1-phosphate receptors, preventing lymphocyte sequestration. IL-33 is released from necrotic endothelial cells and contributes to MS severity by coactivating T cells. Herein we analyzed the influence of FTY720, FTY720-P, and S1P on IL-33 induced formation of IL-2 and IFN-γ, by using IL-33 receptor overexpressing EL4 cells, primary CD8(+) T cells, and splenocytes. EL4-ST2 cells released IL-2 after IL-33 stimulation that was inhibited dose-dependently by FTY720-P but not FTY720. In this system, S1P increased IL-2, and accordingly, inhibition of S1P producing sphingosine kinases diminished IL-2 release. In primary CD8(+) T cells and splenocytes IL-33/IL-12 stimulation induced IFN-γ, which was prevented by FTY720 but not FTY720-P, independently from intracellular phosphorylation. The inhibition of IFN-γ by nonphosphorylated FTY720 was mediated via the SET/protein phosphatase 2A (PP2A) pathway, since a SET peptide antagonist also prevented IFN-γ formation and the inhibition of IFN-γ by FTY720 was reversible by a PP2A inhibitor. While our findings directly improve the understanding of FTY720 therapy in MS, they could also contribute to side effects of FTY720 treatment, like progressive multifocal leukoencephalopathy, caused by an insufficient immune response to a viral infection.

Oncostatin M induces IL-33 expression in liver endothelial cells in mice and expands ST2+CD4+ lymphocytes

Interleukin (IL)-33 is crucially involved in liver pathology and drives hepatoprotective functions. However, the regulation of IL-33 by cytokines of the IL-6 family, including oncostatin M (OSM) and IL-6, is not well studied. The aim of the present study was to determine whether OSM mediates regulation of IL-33 expression in liver cells. Intramuscular administration in mice of an adenovirus encoding OSM (AdOSM) leads to increase in expression of OSM in muscles, liver, and serum of AdOSM-infected mice compared with control mice. The increase of circulating OSM markedly regulated mRNA of genes associated with blood vessel biology, chemotaxis, cellular death, induction of cell adhesion molecules, and the alarmin cytokine IL-33 in liver. Steady-state IL-33 mRNA was upregulated by OSM at an early phase (8 h) following AdOSM infection. At the protein level, the expression of IL-33 was significantly induced in liver endothelial cells [liver sinusoidal endothelial cells (LSEC) and vascular endothelial cells] with a peak at 8 days post-AdOSM infection in mice. In addition, we found OSM-stimulated human microvascular endothelial HMEC-1 cells and human LSEC/TRP3 cells showed a significant increase in expression of IL-33 mRNA in a dose-dependent manner in cell culture. The OSM-mediated overexpression of IL-33 was associated with the activation/enrichment of CD4(+)ST2(+) cells in liver of AdOSM-infected mice compared with adenovirus encoding green fluorescent protein-treated control mice. In summary, these data suggest that the cytokine OSM regulates the IL-33 expression in liver endothelial cells in vivo and in HMEC-1/TRP3 cells in vitro and may specifically expand the target CD4(+)ST2(+) cells in liver.

Interleukin-33 in Tissue Homeostasis, Injury, and Inflammation

Interleukin-33 (IL-33) is a nuclear-associated cytokine of the IL-1 family originally described as a potent inducer of allergic type 2 immunity. IL-33 signals via the receptor ST2, which is highly expressed on group 2 innate lymphoid cells (ILC2s) and T helper 2 (Th2) cells, thus underpinning its association with helminth infection and allergic pathology. Recent studies have revealed ST2 expression on subsets of regulatory T cells, and for a role for IL-33 in tissue homeostasis and repair that suggests previously unrecognized interactions within these cellular networks. IL-33 can participate in pathologic fibrotic reactions, or, in the setting of microbial invasion, can cooperate with inflammatory cytokines to promote responses by cytotoxic NK cells, Th1 cells, and CD8(+) T cells. Here, we highlight the regulation and function of IL-33 and ST2 and review their roles in homeostasis, damage, and inflammation, suggesting a conceptual framework for future studies.

IL-33 promotes innate IFN-γ production and modulates dendritic cell response in LCMV-induced hepatitis in mice

Recent studies have revealed IL-33 as a key factor in promoting antiviral T-cell responses. However, it is less clear as to how IL-33 regulates innate immunity. In this study, we infected wild-type (WT) and IL-33(-/-) mice with lymphocytic choriomeningitis virus and demonstrated an essential role of infection-induced IL-33 expression for robust innate IFN-γ production in the liver. We first show that IL-33 deficiency resulted in a marked reduction in the number of IFN-γ(+) γδ T and NK cells, but an increase in that of IL-17(+) γδ T cells at 16 h postinfection. Recombinant IL-33 (rIL-33) treatment could reverse such deficiency via increasing IFN-γ-producing γδ T and NK cells, and inhibiting IL-17(+) γδ T cells. We also found that rIL-33-induced type 2 innate lymphoid cells were not involved in T-cell responses and liver injury, since the adoptive transfer of type 2 innate lymphoid cells neither affected the IFN-γ and TNF-α production in T cells, nor liver transferase levels in lymphocytic choriomeningitis virus infected mice. Interestingly, we found that while IL-33 was not required for costimulatory molecule expression, it was critical for DC proliferation and cytokine production. Together, this study highlights an essential role of IL-33 in regulating innate IFN-γ-production and DC function during viral hepatitis.

Group 2 innate lymphoid cells in disease

Group 2 innate lymphoid cells (ILC2) are now recognized as an important innate source of type-2 effector cytokines. Although initially associated with mucosal tissues, it is clear that ILC2 are present in diverse anatomical locations. The function of ILC2 at these sites is equally varied, and although ILC2 represent a relatively minor population, they are fundamentally important regulators of innate and adaptive immune processes. As such, there is much interest to understand the role of ILC2 in diseases with a type-2 inflammatory component. This review explores the known roles of ILC2 in disease, and the diseases that show associations or other strong evidence for the involvement of ILC2.

Interleukin-33 and Interferon-γ Counter-Regulate Group 2 Innate Lymphoid Cell Activation during Immune Perturbation

Group 2 innate lymphoid cells (ILC2s) and regulatory T (Treg) cells are systemically induced by helminth infection but also sustain metabolic homeostasis in adipose tissue and contribute to tissue repair during injury. Here we show that interleukin-33 (IL-33) mediates activation of ILC2s and Treg cells in resting adipose tissue, but also after helminth infection or treatment with IL-2. Unexpectedly, ILC2-intrinsic IL-33 activation was required for Treg cell accumulation in vivo and was independent of ILC2 type 2 cytokines but partially dependent on direct co-stimulatory interactions via ICOSL-ICOS. IFN-γ inhibited ILC2 activation and Treg cell accumulation by IL-33 in infected tissue, as well as adipose tissue, where repression increased with aging and high-fat diet-induced obesity. IL-33 and ILC2s are central mediators of type 2 immune responses that promote tissue and metabolic homeostasis, and IFN-γ suppresses this pathway, likely to promote inflammatory responses and divert metabolic resources necessary to protect the host.