ST2-deficient Mice Research Articles

Participation of MyD88 and Interleukin-33 as Innate Drivers of Th2 Immunity to Trichinella spiralis

Trichinella spiralis is a highly destructive parasitic nematode that invades and destroys intestinal epithelial cells, injures many different tissues during its migratory phase, and occupies and transforms myotubes during the final phase of its life cycle. We set out to investigate the role in immunity of innate receptors for potential pathogen- or danger-associated molecular patterns (PAMPs or DAMPs). Focusing on the MyD88-dependent receptors, which include Toll-like receptors (TLRs) and interleukin-1 (IL-1) family members, we found that MyD88-deficient mice expelled worms normally, while TLR2/4-deficient mice showed accelerated worm expulsion, suggesting that MyD88 was active in signaling pathways for more than one receptor during intestinal immunity. A direct role for PAMPs in TLR activation was not supported in a transactivation assay involving a panel of murine and human TLRs. Mice deficient in the IL-1 family receptor for the DAMP, IL-33 (called ST2), displayed reduced intestinal Th2 responses and impaired mast cell activation. IL-33 was constitutively expressed in intestinal epithelial cells, where it became concentrated in nuclei within 2 days of infection. Nuclear localization was an innate response to infection that occurred in intestinal regions where worms were actively migrating. Th2 responses were also compromised in the lymph nodes draining the skeletal muscles of ST2-deficient mice, and this correlated with increased larval burdens in muscle. Our results support a mechanism in which the immune system recognizes and responds to tissue injury in a way that promotes Th2 responses.

Lack of ST2 enhances high-fat diet-induced visceral adiposity and inflammation in BALB/c mice

ABSTRACT Obesity and obesity related disorders are strongly associated with chronic low-grade inflammation originating from growing visceral adipose tissue during nutrient excess. Although interleukin (IL)-33 may have protective role in obesity and atherosclerosis, the impact of IL-33/ST2 axis in metabolic disorders needs to be elucidated. In this study, we investigated the role of IL-33/ST2 pathway in high-fat diet (HFD)-induced obesity using ST2-deficient (ST2 -/- ) and wild type mice on BALB/c background. The deletion of ST2 enhanced systemic and visceral adipose tissue (VAT) inflammation and was associated with significantly higher weight gain and amount of total VAT in ST2 -/- mice fed with HFD for 18 weeks. More numerous classically activated M1 macrophages and markedly decreased alternatively activated M2 macrophages were observed in VAT of HFD-fed ST2 -/- mice. Additionally, VAT of ST2 -/- mice fed with HFD had increased percentage of CD3 + T cells with lower incidence of CD4 + CD25 + FoxP3 + T regulatory cells in comparison with low-fat diet fed controls. The incidence of CD3 + IL-17 + and IL-5 positive VAT-derived mononuclear cells was significantly lower in HFD-fed ST2 -/- mice. Serum levels of proinflammatory cytokines IL-1β and IFN-γ were increased in HFD-fed ST2 -/- mice, while the levels of IL-6 and CRP did not differ among groups. Importantly, the levels of anti-inflammatory IL-10 and IL-13 were significantly lower in the sera of ST2 -/- mice compared with wild-type controls. Our findings suggest that protective role of IL33/ST2 signaling in high-fat diet-induced adipose tissue inflammation. ST2 deficiency related to nutrient excess is associated with polarization of macrophages toward M1 phenotype and the induction Th1-mediated immune response. Key words: ST2, obesity, high-fat diet, visceral adipose tissue, inflammation

Neutralisation of the interleukin-33/ST2 pathway ameliorates experimental colitis through enhancement of mucosal healing in mice

ObjectiveInflammatory bowel diseases (IBD) have been intrinsically linked to a deregulated cytokine network, but novel therapeutic principles are urgently needed. Here we identify the interleukin (IL)-33 and its receptor ST2...

P095 Neutralizing responsiveness to Interleukin-33 abrogat1es experimental colitis through enhanced mucosal wound healing

Novel therapeutic principles are urgently needed to cure inflammatory bowel diseases (IBD) that have been intrinsically linked to a deregulated inflammatory cytokine network. Here we identify the interleukin (IL)-1-like cytokine IL-33 and its receptor ST2 as novel negative regulators of wound healing in the colon of mice. Study in BL6 and IL-33R/ST2 deficient mice using dextran sodium sulfate (DSS) and 2,4,6-trinitrobenzene sulfonic acid (TNBS) colitis analyzing classical parameters of inflammation. Genetic ablation of ST2 protected the colon from chemical insults by dextran sodium sulfate (DSS) and 2,4,6-trinitrobenzene sulfonic acid (TNBS) that coincided with reduced acute pro-inflammatory response together with sustained expression of cytoprotective factors, including connexin-43. Likewise, administration of an anti-ST2 blocking antibody to mice treated by dextran sodium sulfate improved signs of morbidity, whereas recombinant IL-33 increased the risk for colitis. Consistently, the colonic mucosa of ST2-deficient mice was characterized by an enhanced wound healing response in a model of acute in vivo injury in the colon. In conclusion, we report here that the IL-33/ST2 axis plays a critical role in inflammation resolution by regulating self-renewal of the colonic epithelia, indicating neutralization of ST2 may represent a novel therapeutic target for effective treatment in IBD.

The ST2 Pathway Is Involved in Acute Pancreatitis: A Translational Study in Humans and Mice

Acute pancreatitis (AP) is an inflammatory disease in which the regulatory pathways are not clearly elucidated. Activation of interleukin 1β (IL-1β) and immunomodulation via MyD88, the first signaling molecule in the ST2 pathway, seem to be involved. Because IL-33, the ST2 ligand, is an IL-1 family member and acts as an alarmin, we explored the ST2 pathway in human and mouse AP. Soluble ST2 was assayed by enzyme-linked immunosorbent assay (ELISA) in plasma of 44 patients admitted for AP. The levels of soluble ST2 increased early during AP and correlated with parameters of severity. Under two different experimental models of AP (ie, choline-deficient-ethionine-supplemented diet and cerulein injections), ST2-deficient mice (Il1rl1(-/-)) presented with more severe disease than wild-type mice, with increased activation of mast cells. In vitro, Il1rl1(-/-) bone-marrow-derived mast cells exhibited exacerbated degranulation, compared with the wild type. Flow cytometry identified mast cells as the main peritoneal population expressing ST2. Using immunohistochemistry and ELISA, we showed constitutive expression of IL-33 in murine pancreas and its release during experimental AP. Correlated with AP severity, increased soluble ST2 levels evoke involvement of the ST2 pathway in human AP. Furthermore, our experimental data suggest a protective role for ST2 during AP, highlighting the potential regulatory role of mast cells and the possibility of the ST2 pathway as a new therapeutic target in AP.

IL-33/ST2 axis in inflammation and immunopathology

Interleukin-33 (IL-33), a member of the IL-1 family of cytokines, binds to its plasma membrane receptor, heterodimeric complex consisted of membrane-bound ST2L and IL-1R accessory protein, inducing NFkB and MAPK activation. IL-33 exists as a nuclear precursor and may act as an alarmin, when it is released after cell damage or as negative regulator of NFκB gene transcription, when acts in an intracrine manner. ST2L is expressed on several immune cells: Th2 lymphocytes, NK, NKT and mast cells and on cells of myeloid lineage: monocytes, dendritic cells and granulocytes. IL-33/ST2 axis can promote both Th1 and Th2 immune responses depending on the type of activated cell and microenvironment and cytokine network in damaged tissue. We previously described and discuss here the important role of IL-33/ST2 axis in experimental models of type 1 diabetes, experimental autoimmune encephalomyelitis, fulminant hepatitis and breast cancer. We found that ST2 deletion enhance the development of T cell-mediated autoimmune disorders, EAE and diabetes mellitus type I. Disease development was accompanied by dominantly Th1/Th17 immune response but also higher IL-33 production, which suggest that IL-33 in receptor independent manner could promote the development of inflammatory autoreactive T cells. IL-33/ST2 axis has protective role in Con A hepatitis. ST2-deficient mice had more severe hepatitis with higher influx of inflammatory cells in liver and dominant Th1/Th17 systemic response. Pretreatment of mice with IL-33 prevented Con A-induced liver damage through prevention of apoptosis of hepatocytes and Th2 amplification. Deletion of IL-33/ST2 axis enhances cytotoxicity of NK cells, production of IFN-γ in these cells and systemic production of IFN-γ, IL-17 and TNF-α, which leads to attenuated tumor growth. IL-33 treatment of tumor-bearing mice suppresses activity of NK cells, dendritic cell maturation and enhances alternative activation of macrophages. In conclusion, we observed that IL-33 has attenuated anti-inflammatory effects in T cell-mediated responses and that both IL-33 and ST2 could be further explored as potential therapeutic targets in treatment of immune-mediated diseases.

Increased Susceptibility of ST2-Deficient Mice to Polymicrobial Sepsis Is Associated with an Impaired Bactericidal Function

ST2, a member of the Toll/IL-1R superfamily, negatively regulates both TLR2 and TLR4 signaling. In this study, we report that ST2-deficient mice were more susceptible to polymicrobial sepsis than their wild-type littermates, with increased production of proinflammatory cytokines. Bacterial clearance from the circulation and visceral organs following polymicrobial infection was markedly impaired in ST2-deficient mice. This was associated with substantially reduced uptake, phagocytosis, and intracellular killing of both Gram-positive and Gram-negative bacteria by ST2-deficient phagocytes. Consistent with a reduced antimicrobial response, phagocytes lacking ST2 displayed a defect in bactericidal activity in response to bacterial challenges with severely impaired phagosome maturation and NOX2 function. Thus, ST2-deficient mice exhibit an increased susceptibility to polymicrobial infection with impaired bacterial clearance, which is associated with defects in phagosome maturation and NOX2-derived production of reactive oxygen species characterized in ST2-deficient phagocytes.

IL‐33‐activated dendritic cells are critical for allergic airway inflammation

IL-33, a new member of the IL-1 family cytokine, is involved in Th2-type responses in a wide range of diseases and signals through the ST2 receptor expressed on many immune cells. Since the effects of IL-33 on DCs remain controversial, we investigated the ability of IL-33 to modulate DC functions in vitro and in vivo. Here, we report that IL-33 activates myeloid DCs to produce IL-6, IL-1b, TNF, CCL17 and to express high levels of CD40, CD80 OX40L and CCR7. Importantly, IL-33-activated DCs prime naive lymphocytes to produce the Th2 cytokines IL-5 and IL-13, but not IL-4. In vivo, IL-33 exposure induces DC recruitment and activation in the lung. Using an OVA-induced allergic lung inflammation model, we demonstrate that the reduced airway inflammation in ST2-deficient mice correlates with the failure in DC activation and migration to the draining LN. Finally, we show that adoptive transfer of IL-33-activated DCs exacerbates lung inflammation in a DC-driven model of allergic airway inflammation. These data demonstrate for the first time that IL-33 activates DCs during antigen presentation and thereby drives a Th2-type response in allergic lung inflammation.

Protective role of IL-33/ST2 axis in Con A-induced hepatitis

We used Concanavalin A-induced liver injury to study the role of Interleukin 33 and its receptor ST2 in the induction of inflammatory pathology and hepatocellular damage. We tested susceptibility to Concanavalin A induced hepatitis in ST2 deficient and wild type BALB/c mice and analyzed the effects of single injection of Interleukin 33 as evaluated by liver enzyme test, quantitative histology, mononuclear cell infiltration, cytokine production, intracellular staining of immune cells, and markers of apoptosis in the liver. ST2 deficient mice developed significantly more severe hepatitis and had significantly higher number of mononuclear cells in the liver, CD4+ and CD8+ T cells, NKp46+ and CD3+NKp46+ cells, and F4/80+ macrophages. The level of pro-inflammatory cytokines in the sera and number of TNF alpha, IFN gamma, and IL-17 producing cells was higher in ST2 deficient mice. In contrast, number of CD4+Foxp3+ cells was statistically higher in wild type mice. Additionally, treatment of wild type mice with single (1 μg) injection of Interleukin 33 led to attenuation of the liver injury and milder infiltration of mononuclear cells, increase in total number of liver CD4+Foxp3+ cells and IL-4 producing CD4+ T cells. Interleukin 33 also suppressed the activation of caspase 3, prevented the expression of BAX, and enhanced the expression of antiapoptotic Bcl-2 in the liver. We concluded that Interleukin 33/ST2 axis downregulated Concanavalin A-induced liver injury and should be evaluated as potential target in fulminant hepatitis in humans.

Paracrine IL-33 Stimulation Enhances Lipopolysaccharide-Mediated Macrophage Activation

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

Cellular reprogramming by bacterial lipoprotein rescues ST2‐deficient mice from lethal sepsis via acceleration of phagosome maturation

BackgroundST2, a member of the Toll‐IL‐1 receptor (TIR) superfamily, negatively regulates both TLR2 and TLR4 signaling. However, it remains undefined whether or not ST2 influences phagocyte‐mediated antimicrobial responses.MethodsPolymicrobial sepsis was induced by cecal ligation and puncture in wild‐type and ST2‐deficient mice. Phagosome maturation was assessed by detecting phagosome acidification and phagosome/lysosome fusion.ResultsST2‐deficient mice were more susceptible to polymicrobial sepsis than their wild‐type littermates, with an increased production of proinflammatory cytokines. Bacterial clearance from the circulation and visceral organs following polymicrobial infection was significantly delayed in ST2‐deficient mice. This was associated with a reduced antimicrobial response and impaired phagosome maturation characterized in ST2‐deficient phagocytes. Cellular reprogramming by bacterial lipoprotein (BLP) tolerisation protected ST2‐deficient mice against polymicrobial sepsis‐related lethality and abrogated the proinflammatory effect of ST2‐deficiency. Moreover, bacterial clearance was accelerated in BLP‐tolerised, ST2‐deficient mice, with substantially enhanced bacterial uptake, phagocytosis and intracellular killing of either gram‐positive S. aureus or gram‐negative S. typhimurium, and accelerated phagosome maturation.ConclusionST2‐deficient mice exhibit an increased susceptibility to polymicrobial infection, and cellular reprogramming by BLP rescues ST2‐deficient mice from lethal sepsis by attenuating the overwhelming proinflammatory response, and simultaneously accelerating bacterial clearance via enhanced phagosome maturation and bactericidal activity.

Paracrine IL-33 stimulation enhances lipopolysaccharide-mediated macrophage activation (117.30)

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

Endogenous IL-33 enhances Th2 cytokine production and T-cell responses during allergic airway inflammation

IL-33 is an IL-1-related cytokine which has been implicated in T(h)2-associated biology and allergic diseases in humans and mice. IL-33 stimulates T(h)2 cells, mast cells, eosinophils, basophils, iNKT cells and circulating CD34(+) stem cells to proliferate and produce pro-allergic cytokines such as IL-5 and IL-13. IL-33 mediates its cytokine effects through a receptor consisting of ST2 and IL-1RAcP. Whereas IL-1RAcP is ubiquitously expressed, ST2 expression is cell-type restricted and determines responsiveness to IL-33. Studies employing ST2-deficient mice have reported variable results on the role of this receptor, and consequently IL-33, with regards to allergic lung inflammation. In this study, we demonstrate that IL-33 is important for allergic lung inflammation. Intra-nasal administration of IL-33 triggered an immediate allergic response in the airways, and more importantly, we show that endogenous IL-33 contributes to airway inflammation and peripheral antigen-specific responses in ovalbumin-induced acute allergic lung inflammation using IL-33-deficient mice. Our results suggest that IL-33 is sufficient and required for severe allergic inflammation in the lung and support the concept of IL-33 as a therapeutic target in allergic lung inflammation.

IL-33 Activates B1 Cells and Exacerbates Contact Sensitivity

B1 B cells produce natural IgM and play a critical role in the early defense against bacterial and viral infection. The polyreactive IgM also contributes to the clearance of apoptotic products and plays an important role in autoimmune pathogenesis. However, the mechanism of activation and proliferation of B1 cells remains obscure. In this study, we report that IL-33, a new member of IL-1 family, activates B1 cells, which express the IL-33 receptor α, ST2. IL-33 markedly activated B1 cell proliferation and enhanced IgM, IL-5, and IL-13 production in vitro and in vivo in a ST2-dependent manner. The IL-33-activated B1 cell functions could be largely abolished by IL-5 neutralization and partially reduced by T cell or mast cell deficiency in vivo. ST2-deficient mice developed less severe oxazolone-induced contact sensitivity (CS) than did wild-type (WT) mice. Furthermore, IL-33 treatment significantly exacerbated CS in WT mice with enhanced B1 cell proliferation and IgM and IL-5 production. Moreover, IL-33-activated B1 cells from WT mice could adoptively transfer enhanced CS in ST2(-/-) mice challenged with IL-33. Thus, we demonstrate, to the best of our knowledge, a hitherto unrecognized mechanism of B1 cell activation and IL-33 function, and suggest that IL-33 may play an important role in delayed-type hypersensitivity.

ST2 Negatively Regulates TLR2 Signaling, but Is Not Required for Bacterial Lipoprotein-Induced Tolerance

Activation of TLR signaling is critical for host innate immunity against bacterial infection. Previous studies reported that the ST2 receptor, a member of the Toll/IL-1 receptor superfamily, functions as a negative regulator of TLR4 signaling and maintains LPS tolerance. However, it is undetermined whether ST2 negatively regulates TLR2 signaling and furthermore, whether a TLR2 agonist, bacterial lipoprotein (BLP)-induced tolerance is dependent on ST2. In this study, we show that BLP stimulation-induced production of proinflammatory cytokines and immunocomplex formation of TLR2-MyD88 and MyD88-IL-1R-associated kinase (IRAK) were significantly enhanced in ST2-deficient macrophages compared with those in wild-type controls. Furthermore, overexpression of ST2 dose-dependently attenuated BLP-induced NF-kappaB activation, suggesting a negative regulatory role of ST2 in TLR2 signaling. A moderate but significantly attenuated production of TNF-alpha and IL-6 on a second BLP stimulation was observed in BLP-pretreated, ST2-deficient macrophages, which is associated with substantially reduced IRAK-1 protein expression and downregulated TLR2-MyD88 and MyD88-IRAK immunocomplex formation. ST2-deficient mice, when pretreated with a nonlethal dose of BLP, benefitted from an improved survival against a subsequent lethal BLP challenge, indicating BLP tolerance develops in the absence of the ST2 receptor. Taken together, our results demonstrate that ST2 acts as a negative regulator of TLR2 signaling, but is not required for BLP-induced tolerance.

Regulatory T cells and ST2 signaling control diabetes induction with multiple low doses of streptozotocin

Several peripheral mechanisms appear to be operational in limiting autoimmune damage of the islets of Langerhans and organ-specific T cell-mediated autoimmunity in general. These include cyclophosphamide sensitive T regulatory cells (Treg cells) and Th2 derived cytokine downregulation. We used the model of multiple low doses of streptozotocin (MLD-STZ) induced diabetes in susceptible C57BL/6 mice and resistant BALB/c mice to study these regulatory mechanisms. We show that low dose cyclophosphamide (CY) sensitive CD4 +CD25 +FoxP3 + Treg cell-dependent mechanisms can be demonstrated in C57Bl/6 mice susceptible to MLD-STZ diabetes induction. CY pretreatment decreased Foxp3 + cell count, glycemia, glycosuria and insulitis. In contrast, CY did not overcome resistance to diabetes induction in BALB/c mice. However, in BALB/c mice, deletion of ST2, an orphan member of the IL-1R family responsible for Th2 cell signaling leads to enhanced susceptibility to diabetes induction as evaluated by level of glycemia and glycosuria, number of infiltrating cells and β cell loss. RT-PCR analysis of mRNA transcripts of diabetogenic cytokines revealed that the expression of TNF-α, and IFN-γ was significantly enhanced in pancreatic lymph nodes by day 10 after diabetes induction in ST2-deficient mice in comparison with wild type BALB/c mice while IL-17 was detected only in ST2 −/− mice by day 21. Our results are compatible with the notion that Treg cells are involved in MLD-STZ diabetes in susceptible mice and demonstrate that ST2-mediated signaling may also be involved in limiting Th1/Th17-mediated autoimmune pathology in diabetes resistant strain.

ST2 deficient mice display a normal host defense against pulmonary infection with Mycobacterium tuberculosis

Tuberculosis, caused by Mycobacterium ( M.) tuberculosis, is a devastating infectious disease causing many deaths world-wide every year. Successful host defense mainly depends on a strong Th type 1 response. We investigated the role of T1/ST2 (recently identified as the receptor for IL-33), a typical Th2 marker in the assumption that a shift towards a beneficial Th1 response would occur in the absence of ST2. For this, ST2 KO and WT mice were intranasally infected with a virulent strain of M. tuberculosis (150 CFU). In line with our hypothesis, ST2 KO animals displayed increased numbers of lymphocytes infiltrating the lung after 2 weeks of infection, increased IFNγ production by splenocytes in ST2 KO mice early in infection and enhanced lung IFNγ levels at the chronic phase of the disease. However, we did not detect any differences between ST2 KO and WT mice in mycobacterial loads in lungs or liver after M. tuberculosis infection. The pulmonary inflammatory response, as measured by relative lung weights, cytokine and chemokine levels as well as histopathological analysis, was similar in ST2 KO and WT mice. These data suggest that apart from inducing a modest shift towards the Th1 response, the role of ST2 during murine M. tuberculosis infection is limited.

QS405. ST2-Deficient Mice Exhibit a Defect in Bacterial Phagocytosis and Phagosome Maturation, and are More Susceptible to Polymicrobial Sepsis

QS405. ST2-Deficient Mice Exhibit a Defect in Bacterial Phagocytosis and Phagosome Maturation, and are More Susceptible to Polymicrobial Sepsis

Adaptor Wars

Fas, a receptor associated with apoptosis triggering, is reported to be an enhancer of signaling through members of the Toll-interleukin 1 (TIR) family (Ma et al. ), and ST2, which is a member of the TIR family, is reported to be an inhibitor of TIR signaling (Brint et al. ). Cytokine production by joint macrophages contributes to chronic inflammation associated with rheumatoid arthritis. Ma et al. showed that collagen-induced arthritis symptoms were reduced in mice deficient for Fas (DBA/1 lpr/lpr mice) compared with heterozygous littermates. Despite having increased levels of interleukin 1β (IL-1β) and its receptor IL-1R in the paws of the Fas-deficient animals, there was less IL-6, an inflammatory mediator produced in response to IL-1β. Disruption of Fas activation in isolated macrophages resulted in decreased production of IL-6 and activation of nuclear factor κB (NF-κB) in response to IL-1R activation by IL-1β or TLR4 (Toll-like receptor 4) in response to lipopolysaccharide (LPS). Crosstalk between Fas signaling and TIR signaling occurred at the level of the adaptor proteins. MyD88, which mediates IL-R1 and TLR4 signaling, and FADD (Fas-associated protein with death domain), which mediates Fas signaling, interact and the outcome is that in the absence of Fas signaling, FADD sequesters MyD88, making it less available for TIR signaling. In the presence of Fas activation, FADD is recruited to the Fas signaling complex and MyD88 is free to mediate TIR signaling. Brint et al. reported that ST2 also acts to sequester the TIR adaptors MyD88 and Mal. In macrophages from ST2-deficient mice, there was enhanced cytokine production in response to stimuli that activate TLR4, TLR9, TLR2, or IL-R1. The TIR domain of ST2 binds the adaptors MyD88 and Mal, which results in decreased availability of these adaptors for mediating TLR signaling. ST2 is not abundant in macrophages from unchallenged mice, and ST2-deficient mice showed the same mortality to LPS as wild-type mice. However, cytokine production and mortality were decreased in ST2-deficient macrophages and mice, respectively, compared with wild type if there was a sublethal LPS exposure before the LPS challenge. Thus, ST2 is stimulated in response to TLR signaling and can then attenuate subsequent responses, which contributes to endotoxin tolerance. Y. Ma, H. Liu, H. Tu-Rapp, H.-J. Thiesen, S. M. Ibrahim, S. M. Cole, R. M. Pope, Fas ligation on macrophages enhances IL-1R1-Toll-like receptor 4 signaling and promotes chronic inflammation. Nat. Immunol. 5 , 380-387 (2004). [Online Journal] E. K. Brint, D. Xu, H. Liu, A. Dunne, A. N. J. McKenzie, L. A. J. O'Neill, F. Y. Liew, ST2 is an inhibitor of interleukin 1 receptor and Toll-like receptor 4 signaling and maintains endotoxin tolerance. Nat. Immunol. 5 , 373-379 (2004). [Online Journal]

ST2 is an inhibitor of interleukin 1 receptor and Toll-like receptor 4 signaling and maintains endotoxin tolerance.

The Toll-interleukin 1 receptor (TIR) superfamily, defined by the presence of an intracellular TIR domain, initiates innate immunity through activation of the transcription factor NF-kappa B, leading to the production of proinflammatory cytokines. ST2 is a member of the TIR family that does not activate NF-kappa B and has been suggested as an important effector molecule of T helper type 2 (T(H)2) responses. We show here that the membrane-bound form of ST2 negatively regulated type I interleukin 1 receptor (IL-1RI) and Toll-like receptor 4 (TLR4) but not TLR3 signaling by sequestrating the adaptors MyD88 and Mal. In contrast to wild-type mice, ST2-deficient mice failed to develop endotoxin tolerance. Thus, these results provide a molecular explanation for the function of ST2 in T(H)2 responses, as inhibition of TLRs promotes a T(H)2 response, and also identify ST2 as a key regulator of endotoxin tolerance.