IL-33 – A cytokine which balances on a knife’s edge?
Authors: Annette Erhardt, Gisa Tiegs
Journal: Journal of Hepatology (2011)

Protective role of IL-33/ST2 axis in Con A-induced hepatitisJournal of HepatologyVol. 56Issue 1PreviewWe 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. Full-Text PDF Acute liver failure arises from several causes including intensive alcohol abuse, viral infections, bacterial invasion, drugs, and toxins as well as foreign antigens during transplantation, and autoantigens [[1]Bernal W. Auzinger G. Dhawan A. Wendon J. Acute liver failure.Lancet. 2010; 376: 190-201Abstract Full Text Full Text PDF PubMed Scopus (779) Google Scholar]. These hepatic failures are often associated with an inadequate immune response, since immune cells of the innate and adaptive immune response are activated. Consequently, pro-inflammatory cytokines initiate the inflammation cascade resulting in immune attack against hepatocytes. Finally, liver injury might pass into fibrosis, liver cirrhosis or, in the worst case, hepatocellular carcinoma. Hence, liver failure is still associated with a high mortality rate and the development of novel therapeutic options is necessary. Until now, some meaningful models of immune-mediated liver injury were established to mimic human liver disorders such as steatohepatitis, autoimmune hepatitis, alcohol-induced hepatitis, or ischemia/reperfusion liver injury. It has been demonstrated that a break down of the gastrointestinal barrier and translocation of gut-derived bacterial toxins promote the induction of liver injury in patients with alcohol-induced hepatitis due to the activation of innate immune cells (e.g., Kupffer cells) and release of TNFα [[2]Hines I.N. Wheeler M.D. Recent advances in alcoholic liver disease III. Role of the innate immune response in alcoholic hepatitis.Am J Physiol Gastrointest Liver Physiol. 2004; 287: G310-G314Crossref PubMed Scopus (138) Google Scholar]. However, steatohepatitis and ischemia/reperfusion injury are associated with an adaptive immune response characterized by the release of the Th1 cytokines IFNγ and TNFα. This indicates that NKT and conventional CD4+ T cells are critical for the pathophysiology of these disorders [[3]Shimamura K. Kawamura H. Nagura T. Kato T. Naito T. Kameyama H. et al.Association of NKT cells and granulocytes with liver injury after reperfusion of the portal vein.Cell Immunol. 2005; 234: 31-38Crossref PubMed Scopus (47) Google Scholar]. Therefore, the development of models of T cell-dependent liver damage might be necessary. The experimental model of concanavalin A (ConA) – induced T cell-mediated hepatitis is a well-established mouse model of acute liver damage, to study the mechanisms of immune pathology and cytokine signaling. ConA is a plant lectin, which is widely used to activate T cells in vitro. However, in the last two decades, ConA has been shown to induce an acute hepatitis in mice [[4]Tiegs G. Hentschel J. Wendel A. A T-cell dependent experimental liver injury in mice inducible by concanavalin A.J Clin Invest. 1992; 90: 196-203Crossref PubMed Scopus (973) Google Scholar] and rats [[5]Cao Q. Batey R. Pang G. Russell A. Clancy R. IL-6, IFN-gamma and TNF-alpha production by liver-associated T cells and acute liver injury in rats administered concanavalin A.Immunol Cell Biol. 1998; 76: 542-549Crossref PubMed Scopus (70) Google Scholar] upon a single intravenous injection. This model of immune-mediated hepatitis allows investigation of novel immunosuppressive therapies with respect to an over-activated or misdirected immune response in the liver, as seen e.g., in autoimmune hepatitis. The mouse model resembles the human disease in several aspects: the predominance of CD4+ T cells, strain-, and gender-related differences with respect to susceptibility, immunosuppression in state of remission, or the good responsiveness to immunosuppressive drugs [[6]Erhardt A. Tiegs G. Tolerance induction in response to liver inflammation.Dig Dis. 2010; 28: 86-92Crossref PubMed Scopus (22) Google Scholar]. A single intravenous injection of a sublethal dose of ConA induces an immune-mediated liver damage by local activation of liver-resident NKT cells which mainly secrete IFNγ [[7]Küsters S. Gantner F. Künstle G. Tiegs G. Interferon-γ plays a critical role in T cell-dependent liver injury in mice initiated by concanavalin A.Gastroenterology. 1996; 111: 462-471Abstract Full Text Full Text PDF PubMed Scopus (402) Google Scholar]. Consequently, CD4+ T cells and polymorphonuclear cells are attracted to the liver. Subsequently, IFNγ activates liver-resident macrophages, namely Kupffer cells (KCs), which produce large amounts of TNFα [[8]Schümann J. Wolf D. Pahl A. Brune K. Papadopoulos T. van Rooijen N. et al.Importance of Kupffer cells for T cell-dependent liver injury in mice.Am J Pathol. 2000; 157: 1671-1683Abstract Full Text Full Text PDF PubMed Scopus (263) Google Scholar] resulting in necrotic cell death of hepatocytes. Additionally, the IFNγ-inducing cytokines IL-12 [[9]Nicoletti F. Di Marco R. Zaccone P. Salvaggio A. Magro G. Bendtzen K. et al.Murine concanavalin A-induced hepatitis is prevented by interleukin 12 (IL-12) antibody and exacerbated by exogenous IL-12 through an interferon-γ-dependent mechanism.Hepatology. 2000; 32: 728-733Crossref PubMed Scopus (87) Google Scholar] and IL-18 [[10]Faggioni R. Jones-Carson J. Reed D.A. Dinarello C.A. Feingold K.R. Grunfeld C. et al.Leptin-deficient (ob/ob) mice are protected from T cell-mediated hepatotoxicity: role of tumor necrosis factor alpha and IL-18.Proc Natl Acad Sci USA. 2000; 97: 2367-2372Crossref PubMed Scopus (300) Google Scholar] are important for disease development. In contrast, the anti-inflammatory cytokine IL-10 is protective in this model [[11]Louis H. Moine O. Peny M.O. Quertinmont E. Fokan D. Goldman M. et al.Production and role of interleukin-10 in concanavalin A-induced hepatitis in mice.Hepatology. 1997; 25: 1382-1389Crossref PubMed Scopus (161) Google Scholar]. Hence, upon ConA challenge, the cytokine profile is shifted to a Th1 immune response, whereas Treg-related and Th2-type cytokine release is reduced. In this context it seems noteworthy, however, that in chronic ConA hepatitis, induced by repeated ConA injections to Th2 biased BALB/c mice, the initial Th1 response shifts to a Th2 and pro-fibrogenic cytokine response with IL-10, IL-4, and transforming growth factor (TGF)- [[12]Louis H. Le Moine A. Quertinmont E. Peny M.O. Geerts A. Goldman M. et al.Repeated concanavalin A challenge in mice induces an interleukin 10-producing phenotype and liver fibrosis.Hepatology. 2000; 31: 381-390Crossref PubMed Scopus (53) Google Scholar]. The indispensability of CD4+ T cells for the development of ConA-induced liver injury has been confirmed by the usage of SCID (Severe Combined Immunodeficiency Disorder) and RAG (Recombinase Activating Gene) KO mice, both lacking T and B cells, athymic nude mice, lacking only T cells, and by experiments with depletion of CD4+ T cells. In contrast, depletion of CD8+ T cells did not prevent ConA-induced liver injury [[4]Tiegs G. Hentschel J. Wendel A. A T-cell dependent experimental liver injury in mice inducible by concanavalin A.J Clin Invest. 1992; 90: 196-203Crossref PubMed Scopus (973) Google Scholar]. NKT cell-deficient CD1dKO mice were also highly resistant to ConA-induced liver damage indicating CD4+ T cells might largely refer to CD4+ NKT cells [13Takeda K. Hayakawa Y. Van Kaer L. Matsuda H. Yagita H. Okumura K. Critical contribution of liver natural killer T cells to a murine model of hepatitis.Proc Natl Acad Sci USA. 2000; 97: 5498-5503Crossref PubMed Scopus (495) Google Scholar, 14Kaneko Y. Harada M. Kawano T. Yamashita M. Shibata Y. Gejyo F. et al.Augmentation of Vα14 NKT cell-mediated cytotoxicity by interleukin 4 in an autocrine mechanism resulting in the development of concanavalin A-induced hepatitis.J Exp Med. 2000; 191: 105-114Crossref PubMed Scopus (376) Google Scholar]. Volarevic and colleagues nicely dissected the role of the recently identified cytokine IL-33 in the animal model of ConA-induced hepatitis. IL-33 is a member of the IL-1 cytokine family and it shares structural relationship with IL-18 and IL-1beta. These two classical cytokines of the IL-1 cytokine family favor the induction of the pro-inflammatory Th1 axis. IL-33 binds to the heterodimeric receptor-complex consisting of IL1R and ST2, in which the latter is expressed on mast cells and Th2 cells [[15]Schmitz J. Owyang A. Oldham E. Song Y. Murphy E. McClanahan T.K. et al.IL-33, an interleukin-1-like cytokine that signals via the IL-1 receptor-related protein ST2 and induces T helper type 2-associated cytokines.Immunity. 2005; 23: 479-490Abstract Full Text Full Text PDF PubMed Scopus (2874) Google Scholar]. Upon receptor-binding on Th2 cells, IL-33 induces the release of Th2-type cytokines such as IL-4, IL-13, IL-5, and IL-10. Subsequently, the Th1 immune response is suppressed. In the murine system, activated macrophages, dendritic cells, epithelial cells, and especially endothelial cells express IL-33 [[16]Liew F.Y. Pitman N.I. McInnes I.B. Disease-associated functions of IL-33: the new kid in the IL-1 family.Nat Rev Immunol. 2010; 10: 103-110Crossref PubMed Scopus (833) Google Scholar]. The current study elucidated the role of the IL-33/ST2 axis in the T-cell-mediated model of ConA-induced hepatitis. In contrast to wild-type mice, ST2-deficient mice exhibited an aggravated liver damage upon ConA challenge, which was accompanied by an increased intrahepatic recruitment of macrophages, CD4+ and CD8+ T cells, NK, and NKT cells. Interestingly, the number of CD4+Foxp3+ regulatory T cells (Tregs) was reduced in ST2-deficient mice. Tregs seem to play an important role in regulating the inflammatory response upon ConA challenge, since adoptively transferred Tregs from ConA-treated mice are able to protect from ConA-induced liver injury [17Erhardt A. Biburger M. Papadopoulos T. Tiegs G. IL-10, regulatory T cells, and Kupffer cells mediate tolerance in concanavalin A-induced liver injury in mice.Hepatology. 2007; 45: 475-485Crossref PubMed Scopus (226) Google Scholar, 18Erhardt A. Wegscheid C. Claass B. Carambia A. Herkel J. Mittrucker H.W. et al.CXCR3 deficiency exacerbates liver disease and abrogates tolerance in a mouse model of immune-mediated hepatitis.J Immunol. 2011; 186: 5284-5293Crossref PubMed Scopus (71) Google Scholar, 19Wei H.X. Chuang Y.H. Li B. Wei H. Sun R. Moritoki Y. et al.CD4+ CD25+ Foxp3+ regulatory T cells protect against T cell-mediated fulminant hepatitis in a TGF-beta-dependent manner in mice.J Immunol. 2008; 181: 7221-7229Crossref PubMed Scopus (63) Google Scholar]. Furthermore, in ST2-deficient mice, the cytokine profile was shifted to a Th1/Th17 response, whereas the Th2-type cytokine IL-4 was strongly downregulated. Moreover, in wild-type mice, a prophylactic treatment with exogenously administered IL-33 attenuated ConA-induced hepatitis in contrast to non-treated wild-type mice. IL-33 application reduced the infiltration of mononuclear cells into the liver, which was accompanied by decreased Th1/Th17 immune response and an increased intrahepatic frequency of Tregs. For the future, it might be interesting to investigate the influence of IL-33 with respect to Treg differentiation/generation in more detail. In conclusion, the ST-2/IL-33 pathway seems to modulate the inflammatory immune response during ConA-induced liver damage. A few years ago, the identification of IL-33 raised the question of the biological function of this cytokine in vivo. Therefore, IL-33 protein was injected into wild-type mice. Interestingly, mice exhibited higher levels of IgE and IgA. They also developed splenomegaly, which was a result of increased infiltration of eosinophils and mononuclear cells. Moreover, in several tissues amongst others, IL-13, IL-4, and IL-5 were upregulated upon IL-33 administration in the liver [[15]Schmitz J. Owyang A. Oldham E. Song Y. Murphy E. McClanahan T.K. et al.IL-33, an interleukin-1-like cytokine that signals via the IL-1 receptor-related protein ST2 and induces T helper type 2-associated cytokines.Immunity. 2005; 23: 479-490Abstract Full Text Full Text PDF PubMed Scopus (2874) Google Scholar]. In mucosal tissues such as small intestine and lung, microscopic abnormalities were apparent including the presence of myeloid cells and increased mucus production. Interestingly, IL-33 is highly expressed in experimental models of asthma and also in asthmatic patients. The release of IL-33 by epithelial cells induces an increased eosinophilic inflammation and airway hyperresponsiveness [[16]Liew F.Y. Pitman N.I. McInnes I.B. Disease-associated functions of IL-33: the new kid in the IL-1 family.Nat Rev Immunol. 2010; 10: 103-110Crossref PubMed Scopus (833) Google Scholar]. Moreover, in patients with atopic dermatitis, IL-33 is also upregulated in the inflamed skin. Subsequently, IL-33 activates mast cells resulting in degranulation and IgE release, which might result in an anaphylactic shock. With respect to inflammatory joint disease, it has been demonstrated that patients with rheumatoid arthritis exhibited increased IL-33 and ST2 levels in the synovium. In mouse models of collagen-induced arthritis, the blockade of the IL-33/ST2 pathway ameliorated disease severity. This suggests a pro-inflammatory role of IL-33 in arthritis [[16]Liew F.Y. Pitman N.I. McInnes I.B. Disease-associated functions of IL-33: the new kid in the IL-1 family.Nat Rev Immunol. 2010; 10: 103-110Crossref PubMed Scopus (833) Google Scholar]. Since IL-33 seems to exert several biological functions and IL-33 dysregulation induces severe pathophysiological changes, IL-33 expression has to be tightly regulated. In this context, the well-known soluble form of ST2 could play an important role in regulating the biological activity of IL-33. Soluble ST2 might act as a decoy receptor to attenuate Th2-driven immune responses. Interestingly, in patients with acute liver failure the serum IL-33 concentration was increased, which also correlated with ALT activity [[20]Roth G.A. Zimmermann M. Lubsczyk B.A. Pilz J. Faybik P. Hetz H. et al.Up-regulation of interleukin 33 and soluble ST2 serum levels in liver failure.J Surg Res. 2010; 163: e79-e83Abstract Full Text Full Text PDF PubMed Scopus (46) Google Scholar]. Accordingly, soluble ST2 serum concentrations were also elevated in these patients. Hence, the authors suggest that elevated soluble ST2 levels could be the result of immune hyperactivation and/or a modulatory mechanism to control inflammation [[20]Roth G.A. Zimmermann M. Lubsczyk B.A. Pilz J. Faybik P. Hetz H. et al.Up-regulation of interleukin 33 and soluble ST2 serum levels in liver failure.J Surg Res. 2010; 163: e79-e83Abstract Full Text Full Text PDF PubMed Scopus (46) Google Scholar]. The function of IL-33 to polarize T cells to Th2 cells plays an important role in the liver, since Th2 cells are known to promote fibrosis by the production of IL-4 and IL-13. Indeed, a recently published study demonstrated that IL-33 and ST2 are overexpressed in murine and human fibrotic liver tissue, but not in human hepatocellular carcinoma. The major sources of intrahepatic IL-33 are liver sinusoidal endothelial cells (LSECs) and activated hepatic stellate cells [[21]Marvie P. Lisbonne M. L’Helgoualc’h A. Rauch M. Turlin B. Preisser L. et al.Interleukin-33 overexpression is associated with liver fibrosis in mice and humans.J Cell Mol Med. 2010; 14: 1726-1739Crossref PubMed Scopus (195) Google Scholar]. Moreover, in the mouse fibrosis model induced by carbon tetrachloride, injection of receptor-activating ST2-fusionprotein led to a accelerated onset and increased level of hepatic fibrosis by favoring a type 2 response [[22]Amatucci A. Novobrantseva T. Gilbride K. Brickelmaier M. Hochman P. Ibraghimov A. Recombinant ST2 boosts hepatic Th2 response in vivo.J Leukoc Biol. 2007; 82: 124-132Crossref PubMed Scopus (21) Google Scholar]. Therefore, measurement of IL-33 in the above mentioned chronic model of ConA hepatitis in Th2-biased mice [[12]Louis H. Le Moine A. Quertinmont E. Peny M.O. Geerts A. Goldman M. et al.Repeated concanavalin A challenge in mice induces an interleukin 10-producing phenotype and liver fibrosis.Hepatology. 2000; 31: 381-390Crossref PubMed Scopus (53) Google Scholar] might be interesting. However, beside the function of Th2 cell priming, Moussion et al. reported that IL-33 is constitutively expressed in the nucleus of endothelial cells from both small and large blood vessels in several organs including the liver. This suggests that IL-33 might function as an ‘alarmin’ belonging to the larger family of damage-associated molecular pattern (DAMP) molecules [[23]Moussion C. Ortega N. Girard J.P. The IL-1-like cytokine IL-33 is constitutively expressed in the nucleus of endothelial cells and epithelial cells in vivo: a novel ‘alarmin’?.PLoS One. 2008; 3: e3331Crossref PubMed Scopus (911) Google Scholar]. Hence, on the one hand, IL-33 exerts cytokine activity and on the other hand, it functions as intracellular nuclear factor. In the latter case, IL-33 might be released from dead cells and might function as an endogenous ‘danger’ signal in order to alert the immune system of tissue injury. The constitutive expression of IL-33 in the nucleus of endothelial cells might be interesting in the model of ConA-induced hepatitis: upon intravenous injection of ConA, the plant lectin preferentially binds to LSECs. Afterwards, CD4+ T cell are recruited to the liver and destroy the LSEC layer. Hence, underlying hepatocytes are exposed to further attack by activated T cells due to the loss of the LSEC barrier function [[24]Knolle P.A. Gerken G. Loser E. Dienes H.P. Gantner F. Tiegs G. et al.Role of sinusoidal endothelial cells of the liver in concanavalin A-induced hepatic injury in mice.Hepatology. 1996; 24: 824-829Crossref PubMed Google Scholar]. Subsequently, IL-33, which is released by damaged LSECs, binds to the ST2 receptor on immune cells (predominantly on CD4+ Th2 cells or activated macrophages). This might alert the immune system of liver injury and might promote the healing and regeneration process (Fig. 1). In the present study of Volarevic and colleagues, it might be interesting to measure the endogenous IL-33 serum levels upon ConA challenge. Moreover, the determination of IL-33 expression in MACS-sorted LSECs isolated from ConA-treated mice might be important in order to emphasize the immune-modulatory role of LSEC-derived IL-33. Nevertheless, the authors have identified a protective role of exogenously administered IL-33 in a prophylactic experimental setup. It might be worthwhile, whether a therapeutic treatment with IL-33 will also result in a less severe liver damage upon ConA challenge. However, it has to be kept in mind, that the mouse model of ConA-induced liver injury represents a very acute and short-term model of immune-mediated liver damage with increased ALT activities between 8 and 24 h after ConA injection. Therefore, IL-33 treatment in a therapeutic manner, namely after the ConA challenge, might be not successful, since there is a short time frame of potential interference. Surprisingly, Oboki et al. have recently demonstrated that ALT activity was comparable in IL-33-deficient and wildtype mice upon ConA challenge, indicating that IL-33 signaling is not essential for the development of T/NKT cell-mediated liver damage [[25]Oboki K. Ohno T. Kajiwara N. Arae K. Morita H. Ishii A. et al.IL-33 is a crucial amplifier of innate rather than acquired immunity.Proc Natl Acad Sci USA. 2010; 107: 18581-18586Crossref PubMed Scopus (531) Google Scholar]. This contradictory result might be explained by the usage of different mouse strains in the two studies: Oboki et al. used IL-33-deficient mice on the Th1-primed C57BL/6 background, whereas Volarevic and colleagues used ST2-deficient mice on the Th2-prone BALB/c background. In conclusion, IL-33 treatment as a therapeutic option of acute liver failure might be beneficial; however, it has to be kept in mind that the administration of IL-33 is also associated with side effects in both the liver and other target organs. In the liver, IL-33 might promote fibrosis [21Marvie P. Lisbonne M. L’Helgoualc’h A. Rauch M. Turlin B. Preisser L. et al.Interleukin-33 overexpression is associated with liver fibrosis in mice and humans.J Cell Mol Med. 2010; 14: 1726-1739Crossref PubMed Scopus (195) Google Scholar, 22Amatucci A. Novobrantseva T. Gilbride K. Brickelmaier M. Hochman P. Ibraghimov A. Recombinant ST2 boosts hepatic Th2 response in vivo.J Leukoc Biol. 2007; 82: 124-132Crossref PubMed Scopus (21) Google Scholar]; in the lung and small intestine, microscopic abnormalities were demonstrated after injection of IL-33 on seven consecutive days [[15]Schmitz J. Owyang A. Oldham E. Song Y. Murphy E. McClanahan T.K. et al.IL-33, an interleukin-1-like cytokine that signals via the IL-1 receptor-related protein ST2 and induces T helper type 2-associated cytokines.Immunity. 2005; 23: 479-490Abstract Full Text Full Text PDF PubMed Scopus (2874) Google Scholar]. Moreover, IL-33 might exert adverse effects in patients with rheumatoid arthritis and asthma [[16]Liew F.Y. Pitman N.I. McInnes I.B. Disease-associated functions of IL-33: the new kid in the IL-1 family.Nat Rev Immunol. 2010; 10: 103-110Crossref PubMed Scopus (833) Google Scholar]. Due to this double edged role of IL-33, further investigations have to be performed in order to prevent detrimental consequences. Therefore, the local source of IL-33 and the functional role of IL-33 and soluble ST2 have to be examined in detail with respect to the target organ. The authors declared that they do not have anything to disclose regarding funding or conflict of interest with respect to this manuscript.