Pathophysiological roles of interleukin-18 in inflammatory liver diseases

Significant elevation of serum interleukin-18 levels in patients with acute pancreatitis

BACKGROUND

We have reported that peripheral lymphocyte reduction due to apoptosis is linked to the development of subsequent infectious complications in patients with severe acute pancreatitis and that Th1 (helper T cell type 1)/Th2 (helper T cell type 2) balance tends to cause Th1 suppression in experimental severe acute pancreatitis. It has been reported that interleukin (IL)-18 is a cytokine produced from Kupffer cells and activated macrophages, and that IL-18 acts on Th1 cells and in combination with IL-12 strongly induces production of interferon-gamma. However, the role of IL-18 in acute pancreatitis has not yet been fully understood.

METHODS

Serum IL-18 concentrations were determined by an enzyme-linked immunosorbent assay in 43 patients with acute pancreatitis at the time of admission. The relationships with etiology, pancreatic necrosis, severity, blood biochemical parameters on admission, infection, and organ dysfunction during the clinical course and prognosis were analyzed.

RESULTS

Serum IL-18 levels in patients with acute pancreatitis (656+/- 11pg/ml) were significantly higher than those in healthy volunteers (126+/- pg/ml). Serum IL-18 levels were significantly positively correlated with the Ranson score and Japanese severity score. Among the blood biochemical parameters on admission, base excess and total protein were significantly negatively correlated with serum IL-18 levels. Moreover, the CD4/CD8 rate of lymphocytes, serum IL-6 levels, and serum IL-8 levels were significantly positively correlated with serum IL-18 levels. On day 7 after admission, the CD4/CD8 rate of lymphocytes and the rate of CD4-positive lymphocytes were significantly positively correlated with serum IL-18 levels. Furthermore, serum IL-18 levels in patients with hepatic dysfunction (980+/- 25pg/ml) were significantly higher than those without hepatic dysfunction (464+/- 8pg/ml). Serum IL-18 levels were not related to infection or prognosis. Elevation of serum IL-18 levels continued during 4 weeks after admission.

CONCLUSIONS

These results suggest that serum IL-18 levels are significantly elevated and are correlated with severity in patients with acute pancreatitis and that IL-18 may be closely related to helper T cell response and hepatic dysfunction in this disease.

Involvement of neutrophil recruitment and protease-activated receptor 2 activation in the induction of IL-18 in mice

Activated neutrophils produce serine proteases, which activate cells through protease-activated receptor 2 (PAR2). As proteinase 3 (PR3) induces the secretion of interleukin (IL)-18 from epithelial cells in combination with lipopolysaccharide (LPS) in vitro, we examined whether neutrophils, serine proteases, and PAR2 are involved in the induction of serum IL-18 and IL-18-dependent liver injury in mice treated with heat-killed Propionibacterium acnes and LPS. LPS-induced serum IL-18 levels in P. acnes-primed mice were reduced significantly by anti-Gr-1 injection (depletion of neutrophils and macrophages) but not by a macrophage "suicide" technique, using liposomes encapsulating clodronate. The IL-18 induction was decreased significantly by coadministration of a serine protease inhibitor [Nafamostat mesilate (FUT-175)] with LPS. Serum levels of tumor necrosis factor alpha and liver enzymes induced by P. acnes and LPS were abolished by anti-Gr-1 treatment, and concomitantly, liver injury (necrotic change and granuloma formation) and Gr-1(+) cell infiltration into the liver were prevented by the treatment. A deficiency of PAR2 in mice significantly impaired IL-18 induction by treatment with P. acnes and LPS, and only slight pathological changes in hepatic tissues occurred in the PAR2-deficient mice treated with P. acnes and LPS. Furthermore, coadministration of exogenous murine PR3 or a synthetic PAR2 agonist (ASKH95) with LPS in the anti-Gr-1-treated mice restored the serum IL-18 levels to those in control mice treated with P. acnes and LPS. These results indicate that neutrophil recruitment and PAR2 activation by neutrophil serine proteases are critically involved in the induction of IL-18 and IL-18-dependent liver injury in vivo.

Acute liver failure: from bench to bedside

Acute liver failure constitutes a challenge to clinicians and scientists alike. The course of the disease, usually unpredictable and polarizing, is associated with a high mortality unless liver transplantation is feasible, but can end in a spontaneous restitution. It poses many scientific questions regarding the mechanisms of liver cell damage and regeneration and the possibility of new therapeutic approaches. However, the performance of clinical studies in patients in acute liver failure presents problems because of the varied etiology, the small number of cases, and furthermore due to ethical and logistical difficulties. For this reason experimental investigations have gained a special importance. Arising from the improved understanding of the mechanisms of liver cell damage in acute liver failure, which may be primarily due not to the initial noxious agent alone but may also be triggered secondarily by the release of proinflammatory mediators, there are numerous options for liver cell protection, some of which have already proved successful in experimental studies. New insights into the mechanisms of regulation of liver regeneration and the physiological liver mass, gathered in particular from experimental models of partial hepatectomy and by the use of gene-manipulated animals, have contributed to the development of new therapeutic approaches for the stimulation of liver cell regeneration. Temporary liver support systems have already been successfully employed in some cases under clinical conditions. Although the systematic experimental investigation of many of the questions of acute liver failure has significantly contributed to a better understanding of liver cell damage and regeneration, the application of this new knowledge to clinical practice is to some extent made difficult by the artificial simplification that experimental studies inevitably entail and needs to be validated by controlled clinical studies.

Recombinant HBsAg inhibits LPS-induced COX-2 expression and IL-18 production by interfering with the NFkappaB pathway in a human monocytic cell line, THP-1

BACKGROUND/AIMS

Hepatitis B virus suppresses the human immune-system and HBsAg inhibits the induction of cytokines by LPS in human macrophages, but the mechanisms involved remain unclear. COX-2 and its product, PGE2, play a role in hepatitis B and IL-18 has also been shown to inhibit HBV infection in vivo. We investigated whether rHBsAg affects induction of COX-2 and IL-18 by LPS and, if so, which signal pathways are involved in a human monocytic cell line, THP-1.

METHODS

Cell culture, Western blotting for COX-2, ERK and IKB-alpha, immunofluorescence for HBsAg and NFkappaB protein and ELISA for PGE2, IL-18 and IL-12 were performed.

RESULTS

rHBsAg inhibits LPS-induced COX-2 expression in a time- and dose-dependent manner by blocking the ERK and NFkappaB pathways. LPS-induced IL-18 production was also down-regulated by rHBsAg by interfering mainly with the NFkappaB pathway. PGE2 reversed the inhibition of LPS-induced IL-18 production by rHBsAg. rHBsAg was also found to inhibit the induction of IL-12 by LPS in THP-1 cells.

CONCLUSIONS

These results showed a novel anti-inflammatory property of rHBsAg which involves inhibition of COX-2 and suggested that hepatitis B virus may regulate IFN-gamma production by inhibiting IL-18 and IL-12 production.

Role of interleukin-18 and its receptor in hepatocellular carcinoma associated with hepatitis C virus infection

Interleukin (IL)-18 is a proinflammatory cytokine that is up-regulated in patients with hepatitis C virus (HCV) infection, which is the most common underlying disease in hepatocellular carcinoma (HCC). The purpose of our study was to investigate the role of IL-18 in HCC associated with HCV infection. Sixty-five patients with HCC and HCV infections who received curative surgical resections were examined in our study. The expression of the IL-18 receptor was investigated in HCC tissues obtained from these patients and in 2 HCC cell lines. Nuclear factor (NF)-kappaB activity and the expression of Bcl-xL and xIAP mRNA were tested in the cell lines using recombinant human (rh) IL-18. The IL-18 receptor was expressed in both the HCC tissues and the cell lines. NF-kappaB activation and the expression of Bcl-xL and xIAP mRNA were increased by rhIL-18. Moreover, rhIL-18 suppressed the apoptosis of HCC cells which was induced by etoposide in vitro. The overall survival rate (55.4%) was significantly worse in the IL-18 receptor-positive patients than in the IL-18 receptor-negative patients (p = 0.015). In a Cox multivariate analysis, the expression of the IL-18 receptor was found to be a significant predictor of a poor outcome in HCC patients. The expression of the IL-18 receptor and an antiapoptotic mechanism involving NF-kappaB activation in HCC cells may be implicated in a poor patient outcome.

Cys74 and Cys163 are necessary for IL-18 to elicit IFN-gamma production from peripheral blood lymphoid mononuclear cells

There are four cysteines (Cys74, Cys104, Cys112 and Cys163) in mature human IL-18 (hIL-18). These cysteines are highly conserved in IL-18s of 11 species cloned so far, suggesting that one or more of the cysteines may be important for hIL-18 function. In this study, each cysteine residue was individually replaced with serine by site-directed mutagenesis. The wild type and mutant IL-18s were expressed in Escherichia coli and renatured by two renaturing methods. The purified wild type and mutant rhIL-18s were assayed for their capacity of inducing IFN-gamma and activating NF-kappaB from ConA-stimulated PBMC. DNA binding activity of NF-kappaB was performed by electrophoretic mobility-shift analysis. Our results showed that the mutant rhIL-18C74S and C163S induced much less amount of IFN-gamma from PBMC and the decrement of NF-kappaB DNA binding activity was also observed from C74S and C163S treated PBMC. These results indicate that functional hIL-18 has an absolute requirement for residues Cys74 and Cys163.

ASC is essential for LPS-induced activation of procaspase-1 independently of TLR-associated signal adaptor molecules

Toll-like receptors (TLRs) initiate a signalling cascade via association with an adaptor molecule, myeloid differentiation factor 88 (MyD88) and/or TIR domain-containing adaptor inducing-IFN-beta (Trif), to induce various pro-inflammatory cytokines for microbial eradication. After stimulation of TLR4 with lipopolysaccharide (LPS), both IL-1beta and IL-18 are processed, depending on the activation of caspase-1, although its mechanism remains unclear. ASC is an adapter protein possibly involved in the activation of procaspase-1. To unravel the requirement of ASC, we generated Asc(-/-) mice. Upon stimulation with LPS, Asc(-/-) macrophages failed in the processing of procaspase-1 and maturation of pro-IL-1beta and pro-IL-18, but normally produced other pro-inflammatory cytokines including TNF-alpha and IL-6. MyD88(-/-) and Trif(-/-) macrophages showed normal activation of caspase-1, demonstrating a dispensable role for MyD88 and Trif. After, LPS-challenged Asc(-/-) mice lacked serum elevation of IL-1beta and IL-18. Moreover, the Asc(-/-) mice exhibited neither acute liver injury nor lethal shock. These results demonstrate critical roles for ASC in the release of IL-1beta/IL-18 via activation of caspase-1 and provide new insights into the inflammatory responses for host defence and diseases.

A novel model of drug hapten-induced hepatitis with increased mast cells in the BALB/c mouse

Clinical evidence suggests that idiosyncratic hepatitis following administration of halogenated volatile anesthetics is mediated by autoimmune responses. No murine model to study mechanisms of anesthetic-induced or any other form of drug-induced idiosyncratic hepatitis exists. Anesthetics are believed to trigger hepatitis by covalently linking a trifluoroacetyl (TFA) chloride hapten to hepatic proteins, forming haptenated self-proteins. To test this hypothesis, we developed a hapten-induced model of hepatitis by immunization with syngeneic S100 liver proteins covalently coupled to TFA (TFA-S100). We found that TFA-S100 induced hepatitis was more severe than disease induced by S100 plus adjuvants or by the adjuvant alone and was characterized by neutrophil, mast cell, and eosinophil infiltration. TFA-specific IgG1 antibodies directly correlated with hepatitis, whereas S100 autoantibodies did not. TNF-alpha, IL-1beta, and IL-6 released from splenocytes collected 2 weeks after TFA-S100 inoculation were increased resembling the elevated serum cytokines reported in patients with autoimmune hepatitis (AIH). Three weeks after inoculation, the peak of hepatitis, we noted decreased numbers of Kupffer cells and lower levels of IL-6 and IL-10 in the liver, cytokines produced by Kupffer cells. This is the first report, to our knowledge, of a hapten-induced model of hepatitis with immune and autoimmune features.

Endogenous interleukin-6, but not tumor necrosis factor α, contributes to the development of toll-like receptor 4/myeloid differentiation factor 88-mediated acute arthritis in mice

OBJECTIVE

To generate a mouse model of reactive arthritis (ReA), an aseptic synovitis that develops in joints distant from the primary bacterial infection site, to examine roles for Toll-like receptors (TLRs) that recognize bacterial components involved in the development of this arthritis, and to identify the cytokine(s) relevant to this arthritis.

METHODS

Mice were treated with cell wall extract from Escherichia coli (ECW) gram-negative bacterium by injection into the footpads. Seven days later, the mice were challenged with lipopolysaccharide (LPS), a TLR-4 ligand, which was injected into the knee joint cavity. To investigate the cytokine(s) involved in this arthritis, mice deficient in various arthritogenic cytokines, such as interleukin-6 (IL-6), IL-12, IL-18, interferon-gamma, and tumor necrosis factor alpha (TNFalpha), were sequentially treated with ECW and LPS.

RESULTS

ECW-primed mice manifested acute severe arthritis after intraarticular challenge with ECW or LPS, while unprimed mice exhibited modest changes after these challenges. Mutant mice lacking functional TLR-4 or myeloid differentiation factor 88 (MyD88), an adaptor molecule of TLR-4 signaling, were resistant to this arthritis. Although both TNFalpha and IL-6 were equally expressed in the joint after LPS challenge, Il6(-/-) mice, but not Tnf(-/-) mice, were resistant to ECW/LPS-induced arthritis.

CONCLUSION

Our present results clearly indicate the importance of priming with ECW and the requirement of TLR-4/MyD88-mediated IL-6, but not TNFalpha, for the development of ECW/LPS-induced arthritis. LPS-induced IL-6, in the absence of TNFalpha, mediates LPS-induced arthritis. These results suggest that IL-6 is a rational target for therapeutic regimens for inflammatory arthritis, including ReA and rheumatoid arthritis.

Circulating concentrations of interleukin-18, interleukin-18 binding protein, and gamma interferon in patients with alcoholic hepatitis

BACKGROUND

Alcoholic hepatitis (AH) is associated with dysregulated inflammatory and immune responses. interleukin-18 (IL-18), described as gamma interferon (gammaIFN)-inducible factor, and its natural antagonist, IL-18 binding protein (IL-18 BP), has not been fully studied in patients with AH. Thus, our aim was: (i) to determine plasma values of IL-18, IL-18 BP, gammaIFN, and tumor necrosis factor alpha (TNF)-alpha in patients hospitalized for biopsy-proven AH; (ii) to correlate these cytokines with the severity of AH, as assessed by Maddrey's discriminant function (DF), the degree of liver failure using the Child-Pugh score and blood neutrophils; (iii) to compare cytokines values in survivors and non-survivors.

METHODS

Cytokines were measured using specific immunoassays within 7 days of admission. The diagnosis of AH was based on histology in all cases. We studied 43 cirrhotic patients with a Maddrey's DF>/=32 (severe AH), 29 patients with a score <32 (non-severe AH), 12 patients with abstinent alcoholic cirrhosis, and 10 healthy subjects.

RESULTS

IL-18 and TNFalpha were increased in severe AH as compared with healthy subjects. Plasma IL-18 BP was elevated in patients with severe and non-severe AH as compared with healthy subjects. gammaIFN did not differ between groups. In patients with severe and non-severe AH, IL-18, IL-18 BP, TNFalpha, but not gammaIFN, were positively correlated to DF and Child-Pugh score. Neither IL-18 nor IL-18 BP correlated to TNFalpha. Patients who died (n=10) during the hospitalization had higher IL-18 BP and TNFalpha at admission as compared with survivors (322 [172-504] vs 222 [109-441] ng/ml; 7.5 [2.2-17.3] vs 3 [0.6-20] pg/ml, P<0.01, respectively).

CONCLUSION

In cirrhotic patients with AH, IL-18, IL-18 BP, and TNFalpha correlate to the hepatitis severity and to the degree of liver failure. High IL-18 BP and TNFalpha at hospital admission in non-survivors suggest it may be of prognostic value.

Inducible histamine protects mice from P. acnes-primed and LPS-induced hepatitis through H2-receptor stimulation

BACKGROUND & AIMS

Inducible histamine and histamine H2-receptors have been suggested to be involved in innate immune response.

METHODS

We examined a functional role of inducible histamine in the protection against hepatic injury and lethality in Propionibacterium acnes -primed and lipopolysaccharide-induced hepatitis, using histidine decarboxylase knockout and H2-receptor knockout mice.

RESULTS

Lipopolysaccharide challenge after Propionibacterium acnes priming increased histidine decarboxylase activity in the liver of wild-type mice, associated with a marked elevation of histamine turnover. Histidine decarboxylase-like immunoreactivity was observed in CD68-positive Kupffer cells/macrophages. Treatment of wild-type mice with famotidine or ranitidine but not d -chlorpheniramine augmented hepatic injury and inhibited the survival rate significantly. The same dose of Propionibacterium acnes and lipopolysaccharide induced severe hepatitis and high lethality in histidine decarboxylase knockout and H2-receptor knockout mice; the former were rescued by the subcutaneous injection of histamine. Immunohistochemical study supported the protective role of histamine against the apoptosis of hepatocytes. Histamine suppressed the expression of IL-18 and tumor necrosis factor alpha in the liver, leading to the reduced plasma levels of cytokines including IL-18, TNF-alpha, IL-12, IFN-gamma, and IL-6.

CONCLUSIONS

These findings as a whole indicated that endogenously produced histamine in Kupffer cells/macrophages plays a very important role in preventing excessive innate immune response in endotoxin-induced fulminant hepatitis through the stimulation of H2-receptors.

Differential Upregulation of Interleukin-18 Receptor α Chain Between CD4+ and CD8+ T Cells During Acute Graft-Versus-Host Disease in Mice

Interleukin-18 (IL-18), a unique cytokine that stimulates both T helper 1 (Th1) and Th2 responses, is associated with acute graft-versus-host disease (aGVHD), the major limiting toxicity following allogeneic stem cell transplantation. Here, we investigated the mechanism underlying the upregulation of IL-18 receptor (IL-18R) expression on T cells in murine aGVHD models. The induction of aGVHD elevated host serum IL-12 levels and increased expression of IL-18Ralpha chain (IL-18Ralpha) on engrafted T cells, in particular on CD8+ T cells. However, IL-18Ralpha expression did not increase on the CD4+ T cells of an IL-12-deficient host, indicating the IL-12-dependent upregulation of IL-18Ralpha expression on CD4+ T cells during aGVHD. Purified donor CD8+ T cells transferred in the host increased IL-18Ralpha expression. In vitro experiments showed that IL-18Ralpha expression upregulated on CD8+ T cells but not on CD4+ T cells on stimulation through the T cell receptor (TCR). These results suggest that IL-18Ralpha expression is differentially upregulated between CD4+ and CD8+ T cells during aGVHD, depending on endogenous IL-12 and TCR engagement, respectively.

Nuclear factor kappa B decoy oligodeoxynucleotides prevent endotoxin-induced fatal liver failure in a murine model

Endotoxin syndrome is a systemic inflammatory response mediated by inflammatory cytokines. Nuclear factor kappa B (NF-kappa B) is the dominant regulator of the production of these cytokines by inflammatory cells. The aim of this study was to assess the efficacy of in vivo transfer of synthetic double-stranded oligodeoxynucleotides (ODN) with high affinity against NF-kappa B (NF-kappa B/decoy/ODN) as a therapeutic strategy for treating endotoxin-induced fatal liver injury. Liver injury was induced by administration of lipopolysaccharide (LPS) to Propionibacterium acnes-primed BALB/C mice. NF-kappa B/decoy/ODN was transferred into the portal vein using a fusigenic liposome with hemagglutinating virus of Japan. NF-kappa B/decoy/ODN was preferentially transferred to Kupffer cells, and activation of NF-kappa B after the LPS challenge was suppressed, leading to decreased inflammatory cytokine production. As a result, the massive necrosis and hepatocyte apoptosis observed in the control mice was dramatically attenuated and the survival rate improved. In conclusion, NF-kappa B/decoy/ODN transfer in vivo effectively suppressed endotoxin-induced fatal liver injury in mice.

Expression of the chemokine IP-10 correlates with the accumulation of hepatic IFN-gamma and IL-18 mRNA in chronic hepatitis C but not in hepatitis B

The pathogenesis of hepatitis C virus-induced chronic liver disease is still poorly understood. Previous studies revealed enhanced hepatic expression of the Th1 prototype cytokine IFN-gamma in individuals with chronic hepatitis C. In accordance with several animal models of experimentally induced hepatitis, a Th1 lymphocyte driven inflammatory process, which involves newly infiltrated as well as resident monocytes/macrophages, has been proposed. An involvement of the interferon-gamma-inducible chemokine IP-10, which is chemoattractive for stimulated Th1 cells and monocytes, is also suggested. Using an HBV transgenic mouse model, a reduction of hepatic infiltration and liver disease was achieved recently by administration of antibodies directed against the interferon-gamma-inducible chemokine Mig and against IP-10. In the present study, expression of IP-10 was investigated both in serum and in the liver of patients with chronic hepatitis C and hepatitis B. Patients with liver diseases of non-viral etiologies served as controls. IP-10 expression was highest in hepatitis C. In chronic hepatitis C, but not in chronic hepatitis B nor in liver disorders unrelated to viral infections, IP-10 expression was strongly correlated with the amount of transcripts for IFN-gamma and to the amount of transcripts for the constitutively expressed macrophage derived cytokine IL-18. Hepatic inflammatory activity, however, was found to be associated more closely with IFN-gamma than with IP-10 or IL-18 mRNA expression. The data support the hypothesis that IP-10 is responsible for the recruitment of Th cells and monocytes in chronic hepatitis C, and suggest that its role in chronic hepatitis B is less determining. Moreover, they deliver additional support for the view that IFN-gamma still has to be considered as a mediator that determines the outcome of inflammation, e.g., via its ability to activate IL-18 expressing cells and to initiate a delayed type hypersensitivity reaction.

Endogenous levels of mRNA for IFNs and IFN-related genes in hepatic biopsies of chronic HCV-infected and non-alcoholic steatohepatitis patients

To investigate the intra-hepatic activation of the IFN system in patients affected by chronic HCV-infection in comparison with that observed in a non-infectious liver disease such as non-alcoholic steatohepatitis, we measured the liver steady state mRNA levels of interferon-alpha, interferon-beta and interferon-gamma as well as of IFN-related genes (IFNAR-1, STAT1alpha, PKR, 2-5 AS, IRF-1, ICE and IL-18). In HCV-infected subjects, possible correlations of these parameters with viral load and liver injury were also analyzed. Twenty-four chronic untreated HCV-infected subjects and seven patients with non-alcoholic steatohepatitis were enrolled in the study. Liver biopsies were graded according to Knodell scores. Intra-hepatic mRNA levels of IFNs and related genes were assessed by semi-quantitative RT-PCR. In comparison with non-alcoholic steatohepatitis, in HCV-infected subjects IFN-alpha and -beta mRNA levels were significantly lower, whereas IFN-gamma, IFNAR-1, STAT1alpha IRF-1, and IL-18 mRNA were upregulated. Moreover, IFN-gamma mRNA steady state levels were correlated positively with those of IFNAR-1, IRF-1, and IL-18, suggesting a coordinated induction of these genes. Although plasma viral load was correlated inversely with IL-18-specific mRNA, viral load was not related to liver injury. IFN-gamma and IRF-1 mRNA levels were correlated positively with ALT, but not with the grading or staging. Conversely, IFN-alpha and -beta mRNA levels were higher in livers with lower staging scores. These findings support the hypothesis that in chronic HCV infection there is an imbalance between an upregulated IFN-gamma system and a downregulated IFN-alpha and -beta system, probably due to a mixed effect exerted by HCV-specific and inflammatory non-specific factors.

Interleukin-12 and -18 induce severe liver injury in mice recovered from peritonitis after sublethal endotoxin challenge

BACKGROUND

Postoperative intraabdominal abscess is the major complication after abdominal surgery, and additional infection is often observed and becomes the leading cause of death in septic patients who survive initial resuscitation. Sepsis is initiated and perpetuated by the overzealous systemic production of proinflammatory cytokines-such as tumor necrosis factor (TNF)-alpha, interferon (IFN)-gamma, interleukin (IL)-12, and IL-18-sometimes resulting in excessive tissue injury and death. The purpose of this study was to assess the correlation between liver and spleen innate cytokine responses and organ dysfunction in sepsis syndrome.

METHODS

Peritonitis was induced by cecal ligation and puncture (CLP). All CLP mice survived more than 7 days after the procedure, and serum cytokine (TNF-alpha, IL-12, IL-18, and IL-10) levels peaked 12 hours after CLP; thereafter, they returned to basal levels 7 days after CLP. The mice were injected with a sublethal dose of lipopolysaccharide (LPS) 7 days after CLP. Survival rates, tissue damage, serum cytokine levels, and cytokine production of liver or spleen mononuclear cells (MNCs) were evaluated.

RESULTS

All CLP mice died within 6 hours from liver injury 7 days after LPS challenge, but all sham mice survived. IL-12, IL-18, and IFN-gamma levels in supernatants of the liver MNCs stimulated with LPS in CLP mice were significantly higher than those in sham mice 7 days after the procedure. Furthermore, serum IL-12 and IL-18 levels and liver MNCs IL-12, IL-18, and IFN-gamma production were significantly increased in CLP mice compared with sham mice after LPS challenge. Thereafter, effects of anti-IL-12 and/or anti-IL-18 antibody were evaluated in LPS-injected CLP mice. The survival rate of LPS-injected CLP mice treated with both anti-IL-12 and anti-IL-18 antibody was significantly better than that of untreated mice. Furthermore, liver damage was improved.

CONCLUSION

Mice recovered from mild peritonitis died of severe liver injury by subsequent injection of a sublethal dose of LPS, and this liver injury was related to the collaborating production of IL-12 and IL-18 by liver MNCs.

Differential effect of IL-18 on endothelial cell apoptosis mediated by TNF-alpha and Fas (CD95)

Interleukin-18 (IL-18) is a newly identified cytokine with proinflammatory activity. Numerous studies have shown that proinflammatory cytokines may regulate endothelial cells (EC) apoptosis mediated by members of the tumor necrosis factor (TNF) family, such as TNF-alpha and Fas. In this study we hypothesized that IL-18 may regulate the susceptibility of liver endothelial cells (LEC) to apoptosis induced by TNF and Fas. IL-18 increased the susceptibility of LEC to undergo apoptosis mediated by TNF but not by Fas. Since TNF-induced apoptosis is mediated by the type I TNF receptor (TNFRI), we investigated up-regulation of this receptor in IL-18-treated LEC. IL-18 induced up-regulation of the TNFRI on the surface of LEC. Partial blocking of LEC apoptosis induced by IL-18 and TNF was observed when the cells were pretreated with the broad-spectrum inhibitor of caspases z-VAD-fmk, suggesting involvement of the caspase pathway in apoptosis induced by these cytokines in these cells. Our results show that IL-18 differentially regulates apoptosis mediated by the death-inducing factors, TNF and Fas. To our knowledge, this is the first report that IL-18 may regulate endothelial cell apoptosis mediated by TNF. These results may have clinical implications in those clinical hepatic conditions associated with high levels of IL-18 and TNF.

IL-18 receptor beta-induced changes in the presentation of IL-18 binding sites affect ligand binding and signal transduction

IL-18 is a pleiotropic proinflammatory cytokine that is involved in induction of inflammatory mediators, regulation of the cytotoxic activity of NK cells and T cells, and differentiation and activation of both Th1 and Th2 cells. IL-18 signals through its specific cell surface receptor IL-18R, which comprises two subunits: IL-18R alpha and IL-18R beta. IL-18R alpha alone has a weak affinity for IL-18 binding, while the IL-18R alpha/beta complex has a high affinity. By using several anti-IL-18 mAbs and IL-18 binding protein, we have examined whether these site-specific inhibitors could block the binding of IL-18 to IL-18R alpha and to the IL-18R alpha/beta complex. Here we show that IL-18 binding to IL-18R alpha was inhibited by a neutralizing mAb, 125-2H, while binding of IL-18 to the alpha/beta receptor complex was not. This suggests that IL-18R beta-induced conformational changes may occur in IL-18R alpha upon dimerization, leading to changes in the presentation of IL-18 binding sites. Epitope mapping of 125-2H using human-mouse IL-18 chimeras identified a region in IL-18 that was required for 125-2H recognition. This region, as examined by IL-18R binding and functional analysis, appeared to be critical for triggering signal transduction through the heterodimeric receptor.

Plasma levels of interleukin-18 and interleukin-18 binding protein are elevated in patients with chronic liver disease

Interleukin 18 (IL-18) is a recently described proinflammatory cytokine. In mouse models it has been shown to play a key role in the development of liver injury. IL-18 binding protein (IL-18BP) is a naturally occurring antagonist of IL-18. In this study we investigated whether IL-18/IL-18BP levels are altered in patients with chronic liver disease (CLD). We measured IL-18 and IL-18BP plasma levels in 153 patients with CLD and 41 healthy controls by a specific ELISA. Plasma levels of IL-18 were significantly higher in CLD patients than in healthy controls. Cirrhotics had higher levels than noncirrhotics. IL-18 levels increased with disease progression. IL-18BP plasma levels paralled the increase of IL-18 with disease progression, except in stage Child C cirrhosis. IL-18 and IL-18BP levels were elevated independent of the etiology of CLD. IL-18 and IL-18BP correlated with laboratory parameters of inflammation and liver injury. Plasma levels of IL-18 and its antagonist, IL-18BP, are elevated in CLD and correlate with severity of disease. IL-18BP may not be sufficient to counteract the overwhelming proinflammatory response in end stage liver disease.

Caspase inhibitors as anti-inflammatory and antiapoptotic agents

The striking efficacy of Z-VAD-fmk in the various animal models presented above may reflect its ability to inhibit multiple enzymes including caspases. In accord with this, more selective, reversible inhibitors usually show low efficacy in multifactorial models such as ischaemia, but may offer some protection against NMDA-induced excitotoxicity and hepatitis. Importantly, caspase inhibitors may exhibit significant activity in vivo even when they are applied post insult. As far as the CNS is concerned, the first systemically active inhibitors have emerged. Functional recovery could be achieved in some ischaemia models, but long-term protection by caspase inhibitors is still being questioned. Recent developments in drug design enabled the first caspase inhibitors to enter the clinic. Although initially directed towards peripheral indications such as rheumatoid arthritis, caspase inhibitors will no doubt eventually be used to target CNS disorders. For this purpose the peptidic character of current inhibitors will have to be further reduced. Small molecule, nonpeptidic caspase inhibitors, which have appeared recently, indicate that this goal can be accomplished. Unfortunately, many fundamental questions still remain to be addressed. In particular, the necessary spectrum of inhibitory activity required to achieve the desired effect needs to be determined. There is also a safety aspect associated with prolonged administration. Therefore, the next therapeutic areas for broader-range caspase inhibitors are likely to involve acute treatment. Recent results with synergistic effects between MK-801 and caspase inhibitors in ischaemia suggest that caspase inhibitors may need to be used in conjunction with other drugs. It can be expected that, in the near future, research on caspases and their inhibitors will remain a rapidly developing area of biology and medicinal chemistry. More time, however, may be needed for the first caspase inhibitors to appear on the market.

Inflammatory liver steatosis caused by IL-12 and IL-18

Acute fatty degeneration in the liver is caused by various agents, such as aspirin, valproic acid, and ibuprofen, that directly inhibit mitochondrial beta-oxidation of fatty acid and oxidative phosphorylation. Endogenous molecules, such as cytokines and hormones, are also known to mediate microvesicular steatosis in liver failure. In this study, we examined how interleukin-12 (IL-12) and IL-18 cause steatosis in the liver. Administration of these cytokines in combination caused marked hepatosteatosis and weight loss in mice. There were marked increases in levels of interferon-gamma (IFN-gamma), nitrite (NO(2)/NO(3)), and fibrinogen in the circulation in these mice. On the other hand, the ATP concentration and blood flow in the liver were significantly reduced. These changes, except the production of IFN-gamma and NO, were partially inhibited by Z-VAD-fmk, a synthetic tripeptide inhibitor for NO-induced caspases. These results indicate that IL-12 and IL-18 may mediate inflammatory hepatosteatosis through impairment of the microcirculation, which leads to mitochondrial dysfunction in hepatocytes.

Elevated levels of circulating interleukin-18 in human immunodeficiency virus-infected individuals: role of peripheral blood mononuclear cells and implications for AIDS pathogenesis

Originally identified as the gamma interferon-inducing factor, interleukin-18 (IL-18) was rediscovered as a proinflammatory cytokine related to the IL-1 family of cytokines that plays an important role in both innate and adaptive immune responses against viruses and intracellular pathogens. Despite its importance in inducing and regulating immune responses, relatively little is known about its production in HIV infection. We report here significantly (P < 0.05) elevated levels of this cytokine in the sera of human immunodeficiency virus (HIV)-infected/AIDS patients compared to those of HIV-seronegative healthy persons. Surprisingly, the peripheral blood mononuclear cells (PBMC) from HIV-infected/AIDS patients were compromised in the ability to upregulate IL-18 gene expression and produce this cytokine with and without lipopolysaccharide (LPS) stimulation. A significant positive correlation (P < 0.05) existed between the concentration of IL-18 in serum and its production from PBMC of HIV-seronegative healthy individuals but not those of HIV-infected/AIDS patients. Furthermore, the patients' PBMC expressed relatively reduced levels of activated caspase-1 constitutively as well as in response to LPS stimulation. Our data suggest the involvement of transforming growth factor beta (TGF-beta) in suppressing IL-18 production from the patients' PBMC for the following reasons. (i) In in vitro studies it suppressed the production of IL-18 from PBMC. (ii) Its levels were significantly higher in the plasma of patients compared to that of control subjects. (iii) A significant negative correlation existed between the concentrations of TGF-beta in plasma and of IL-18 in serum of the patients. The elevated levels of IL-18 in the serum of HIV-infected individuals may contribute to AIDS pathogenesis, whereas its compromised production from their PBMC in response to stimuli may reduce their innate defense to opportunistic intracellular pathogens.

Interleukin-18 inhibits hepatitis B virus replication in the livers of transgenic mice

Interleukin-18 (IL-18) produced by activated antigen-presenting cells stimulates natural killer (NK) cells, natural killer T (NKT) cells, and T cells to secrete gamma interferon (IFN-gamma). In this study, injection of a single 10- micro g dose of recombinant murine IL-18 rapidly, reversibly, and noncytopathically inhibited hepatitis B virus (HBV) replication in the livers of HBV transgenic mice. Furthermore, HBV replication was inhibited by as little as 1 micro g of IL-18 injected repetitively, and also by a single 0.1- micro g dose of IL-18 injected together with 1 ng of IL-12, neither of which inhibited HBV replication individually, demonstrating synergy between these cytokines in this system. The antiviral effect of IL-18 was mediated by its ability to activate resident intrahepatic NK cells and NKT cells to produce IFN-gamma and by its ability to induce IFN-alpha/beta production in the liver. These results suggest that IL-18 has the potential to contribute to the control of HBV replication during self-limited infection and that it may have therapeutic value for the treatment of patients with chronic hepatitis.

IL18 polymorphism is associated with an increased risk of Crohn's disease

BACKGROUND

The etiology of inflammatory bowel disease, which includes ulcerative colitis and Crohn's disease, has not yet been made clear. However, inflammatory bowel disease is recognized as a multifactorial disease, and innate genetic factors might contribute to the pathogenesis. Cytokine genes are thought to be important in inflammatory bowel disease. Recently, interleukin 18, cloned as a novel proinflammatory cytokine, has been implicated in inflammatory bowel disease, especially Crohn's disease.

METHODS

To identify germline mutations in patients with inflammatory bowel disease, the entire coding region of IL18 was examined using a DNA sequencing procedure.

RESULTS

No functional mutations were found, but a novel single nucleotide polymorphism (SNP) was identified as TCA/TCC at codon 35. In patients with Crohn's disease, the frequency of TCC allele carriers was significantly higher than in healthy controls (chi2 = 9.35, P = 0.002229, OR = 2.58, 95% CI = 1.39-4.80). Also, the magnitude of the association was more remarkable in females (chi2 = 16.36, P = 0.000052, OR = 8.17, 95% CI = 2.73-24.41). The TCC allele at codon 35 of IL18 may increase the risk for Crohn's disease, especially in females.

CONCLUSIONS

IL18 is probably one of several genes that determine susceptibility to Crohn's disease.

Hemophagocytic lymphohistiocytosis is associated with deficiencies of cellular cytolysis but normal expression of transcripts relevant to killer-cell-induced apoptosis

In 65 patients with hemophagocytic lymphohistiocytosis (HLH), we found an as yet undescribed heterogeneity of defects in cellular cytotoxicity when assay conditions were modified by the incubation time, the presence of mitogen, or interleukin-2 (IL-2). The standard 4-hour natural killer (NK) test against K562 targets was negative in all patients. In patients deficient in type 1 (n = 21), type 2 (n = 5), and type 4 (n = 8) HLH, negative NK function could be reconstituted by mitogen, by IL-2, or by prolongation of the incubation time (16 hours), respectively. Most patients (n = 31) displayed the type 3 defect, defined by a lack of any cellular cytotoxicity independent of assay variations. The characteristic hypercytokinemia also concerned counterregulatory cytokines, such as proinflammatory interferon-gamma (IFN-gamma), simultaneously elevated with suppressive IL-10 in 38% of types 1-, 2-, and 4-deficient patients and in 71% of type 3-deficient patients. Elevated IFN-gamma alone correlated with high liver enzymes, but sCD95-ligand and sCD25 did not-though these markers were expected to indicate the extent of histiocytic organ infiltration. Outcome analysis revealed more deaths in patients with type 3 deficiency (P =.017). Molecular defects were associated with homozygously mutated perforin only in 4 patients, but other type 3 patients expressed normal transcripts of effector molecules for target-cell apoptosis, including perforin and granzyme family members, as demonstrated by RNase protection analysis. Thus, target-cell recognition or differentiation defects are likely to explain this severe phenotype in HLH. Hyperactive phagocytes combined with NK defects may imply defects on the level of the antigen-presenting cell.

Murine leptin deficiency alters Kupffer cell production of cytokines that regulate the innate immune system

BACKGROUND & AIMS

ob/ob mice are used to study the mechanisms that regulate the progression from steatosis to nonalcoholic steatohepatitis. The livers of ob/ob mice are depleted of CD4-positive natural killer cells, components of the innate immune system that induce anti-inflammatory cytokines. Although this may explain the sensitivity of fatty livers to lipopolysaccharide, why such hepatic CD4-positive natural killer cell depletion occurs is uncertain. Because leptin regulates macrophages, our hypothesis is that leptin deficiency alters Kupffer cell production of cytokines that inhibit (e.g., interleukin [IL]-12) or enhance (e.g., IL-15) hepatic CD4-positive natural killer cell viability.

METHODS

Kupffer cell cytokine production and the hepatic content of CD4-positive natural killer cells were compared in ob/ob and lean mice. ob/ob mice were then treated with IL-15 or leptin to determine whether either factor improved their immunologic abnormalities.

RESULTS

Compared with control Kupffer cells, ob/ob Kupffer cells produced less IL-15 basally and more IL-12 after lipopolysaccharide stimulation. Treatment of ob/ob mice with IL-15 for 1 week normalizes their hepatic CD4-positive natural killer cell content. Leptin increases the hepatic expression of IL-15 in ob/ob mice and partially replenishes their hepatic CD4-positive natural killer cells.

CONCLUSIONS

Leptin deficiency increases hepatic IL-12 and reduces hepatic IL-15 expression. The abnormal production of these Kupffer cell factors promotes hepatic CD4-positive natural killer cell depletion in ob/ob livers.

Interferon-alpha induces interleukin-18 binding protein in chronic hepatitis C patients

Interleukin-18 (IL-18), derived from macrophages and Kupffer cells, is the central pro-inflammatory cytokine leading to experimental liver failure. IL-18 binding protein (IL-18BP) is a circulating protein that binds IL-18 and neutralizes its activity. Since IL-18 production is increased in chronic HCV infection, we asked whether IFN-alpha might act on the IL-18/IL-18BP system in HCV patients. IL-18BP, total and free IL-18 plasma levels were determined in 13 HCV patients receiving 1 x 107 IU IFN-alpha subcutaneously daily for 28 days. The in vitro effects of IFN-alpha on macrophage IL-18BP and IL-18 were studied by enzyme-linked immunosorbent assays and Northern analysis. IFN-alpha administration increased IL-18BP plasma levels 3.24 fold 24 h after institution of therapy, resulting in a 67.4% reduction of free IL-18. Total IL-18 levels decreased from day +24 on. In vitro, IFN-alpha diminished IL-18 release from macrophages of healthy volunteers and chronic HCV patients. On top of its inhibitory effects on IL-1 and TNF-alpha release, IFN-alpha also exerts its anti-inflammatory action in vivo by induction of IL-18BP. These anti-inflammatory properties might account - together with its antiviral action - for its clinical efficacy in chronic hepatitis C.

Lipopolysaccharides in liver injury: molecular mechanisms of Kupffer cell activation

Endogenous gut-derived bacterial lipopolysaccharides have been implicated as important cofactors in the pathogenesis of liver injury. However, the molecular mechanisms by which lipopolysaccharides exert their effect are not entirely clear. Recent studies have pointed to proinflammatory cytokines such as tumor necrosis factor-alpha as mediators of hepatocyte injury. Within the liver, Kupffer cells are major sources of proinflammatory cytokines that are produced in response to lipopolysaccharides. This review will focus on three important molecular components of the pathway by which lipopolysaccharides activate Kupffer cells: CD14, Toll-like receptor 4, and lipopolysaccharide binding protein. Within the liver, lipopolysaccharides bind to lipopolysaccharide binding protein, which then facilitates its transfer to membrane CD14 on the surface of Kupffer cells. Signaling of lipopolysaccharide through CD14 is mediated by the downstream receptor Toll-like receptor 4 and results in activation of Kupffer cells. The role played by these molecules in liver injury will be examined.

Roles of interleukin-18 in tissue destruction and compensatory reactions

Interleukin (IL)-18 is a member of the IL-1 cytokine family. Pro-IL-18 is cleaved by caspase-1 (IL-1beta-converting enzyme) to yield biologically active 18-kDa IL-18. Interleukin-18 is recognized by a heterodimeric receptor, consisting of a ligand-binding alpha-chain (IL-18Ralpha/IL-1Rrp) and an associating beta-chain (IL-18Rbeta/AcPL), which transmits signals through MyD88/IRAK/TRAF-6 molecules. Interleukin-18 is expressed in various types of cells, including macrophages, keratinocytes, intestinal epitherial cells, osteoblastic cells, chondrocytes, and adrenal cortex cells. Interleukin-18 promotes IFN-gamma production and Th1 helper T-cell development, synergistically with IL-12. However, IL-18 itself shows capabilities to induce IL-4, IL-5, IL-10, and IL-13 from T and natural killer cells. It also induces PGE2 production from activated macrophages. Moreover, many diseases are characterized by the production of IL-18 in the lesion. Taking these data together, our working hypothesis on how IL-18 is involved in "destructive" and "compensatory" pathways is proposed in this issue.

Mechanisms of liver injury relevant to pediatric hepatology

Hepatocyte injury and necrosis from many causes may result in pediatric liver disease. Influenced by other cell types in the liver, by its unique vascular arrangements, by lobular zonation, and by contributory effects of sepsis, reactive oxygen species and disordered hepatic architecture, the hepatocyte is prone to injury from exogenous toxins, from inborn errors of metabolism, from hepatotrophic viruses, and from immune mechanisms. Experimental studies on cultured hepatocytes or animal models must be interpreted with caution. Having discussed general concepts, this review describes immune mechanisms of liver injury, as seen in autoimmune hepatitis, hepatitis B and C infection, the anticonvulsant hypersensitivity syndrome, and autoimmune polyendocrinopathy. Of the monogenic disorders causing significant liver injury in childhood, alpha-1 antitrypsin deficiency and Niemann-Pick C disease demonstrate the effect of endoplasmic or endosomal retention of macromolecules. Tyrosinemia illustrates how understanding the biochemical defect leads to understanding cell injury, extrahepatic porphyric effects, oncogenesis, pharmacological intervention, and possible stem cell therapy. Pathogenesis of cirrhosis in galactosemia remains incompletely understood. In hereditary fructose intolerance, phosphate sequestration causes ATP depletion. Recent information about mitochondrial disease, NASH, disorders of glycosylation, Wilson's disease, and the progressive familial intrahepatic cholestases is discussed.

IL-18-binding protein protects against lipopolysaccharide- induced lethality and prevents the development of Fas/Fas ligand-mediated models of liver disease in mice

IL-18-binding protein (IL-18BP) is a natural IL-18 inhibitor. Human IL-18BP isoform a was produced as fusion construct with human IgG1 Fc and assessed for binding and neutralizing IL-18. IL-18BP-Fc binds human, mouse, and rat IL-18 with high affinity (K(D) 0.3-5 nM) in a BIAcore-based assay. In vitro, IL-18BP-Fc blocks IL-18 (100 ng/ml)-induced IFN-gamma production by KG1 cells (EC(50) = 0.3 microg/ml). In mice challenged with an LD(90) of LPS (15 mg/kg), IL-18BP-Fc (5 mg/kg) administered 10 min before LPS blocks IFN-gamma production and protects against lethality. IL-18BP-Fc administered 10 min before LPS blocks IFN-gamma production induced by LPS (5 mg/kg) with ED(50) of 0.005 mg/kg. Furthermore, IL-18BP-Fc (5 mg/kg) abrogates LPS (5 mg/kg)-induced IFN-gamma production even when administered 6 days before LPS but shows no effect when administered 9 or 12 days before LPS. Given 10 min before LPS challenge to mice primed 12 days in advance with heat-killed Propionibacterium acnes, IL-18BP-Fc prevents LPS-induced liver damage and IFN-gamma and Fas ligand expression. Given at the moment of priming with P. acnes, IL-18BP-Fc decreases P. acnes-induced granuloma formation, macrophage-inflammatory protein-1alpha and macrophage-inflammatory protein-2 production and prevents sensitization to LPS. IL-18BP-Fc also prevents Con A-induced liver damage and IFN-gamma and Fas ligand expression as well as liver damage induced by Pseudomonas aeruginosa exotoxin A or by anti-Fas agonistic Ab. In conclusion, IL-18BP can be engineered and produced in recombinant form to generate an IL-18 inhibitor, IL-18BP-Fc, endowed with remarkable in vitro and in vivo properties of binding and neutralizing IL-18.

Plasmodium berghei infection in mice induces liver injury by an IL-12- and toll-like receptor/myeloid differentiation factor 88-dependent mechanism

Malaria, caused by infection with Plasmodium spp., is a life cycle-specific disease that includes liver injury at the erythrocyte stage of the parasite. In this study, we have investigated the mechanisms underlying Plasmodium berghei-induced liver injury, which is characterized by the presence of apoptotic and necrotic hepatocytes and dense infiltration of lymphocytes. Although both IL-12 and IL-18 serum levels were elevated after infection, IL-12-deficient, but not IL-18-deficient, mice were resistant to liver injury induced by P. berghei. Neither elevation of serum IL-12 levels nor liver injury was observed in mice deficient in myeloid differentiation factor 88 (MyD88), an adaptor molecule shared by Toll-like receptors (TLRs). These results demonstrated a requirement of the TLR-MyD88 pathway for induction of IL-12 production during P. berghei infection. Hepatic lymphocytes from P. berghei-infected wild-type mice lysed hepatocytes from both uninfected and infected mice. The hepatocytotoxic action of these cells was blocked by a perforin inhibitor but not by a neutralizing anti-Fas ligand Ab and was up-regulated by IL-12. Surprisingly, these cells killed hepatocytes in an MHC-unrestricted manner. However, CD1d-deficient mice that lack CD1d-restricted NK T cells, were susceptible to liver injury induced by P. berghei. Collectively, our results indicate that the liver injury induced by P. berghei infection of mice induces activation of the TLR-MyD88 signaling pathway which results in IL-12 production and activation of the perforin-dependent cytotoxic activities of MHC-unrestricted hepatic lymphocytes.

Cytokine expression by murine DX5+ cells in response to IL-12, IL-18, or the combination of IL-12 and IL-18

In this study we analyzed the response of DX5+ NK and NK T cells to in vitro stimulation with IL-12 or IL-18. Production of IFN-gamma in response to either IL-12 or IL-18 was dependent upon costimulation with either IL-2 or IL-15. DX5+ splenocytes showed a rapid (6 h) and sustained (6-72 h) accumulation of IFN-gamma transcripts followed by a delayed (12-24 h) up-regulation of IL-10 or IL-13 expression in response to IL-2 + IL-12 or IL-2 + IL-18, respectively. Incubation of DX5+ splenocytes with the combination of IL-2 + IL-12 + IL-18 resulted in up-regulation of IFN-gamma and IL-13 transcripts but down-regulation of IL-10 expression. Furthermore, two distinct populations of cells producing differing amounts of IFN-gamma were observed by intracellular staining after stimulation with IL-2 + IL-12 + IL-18. Last, we demonstrate that DX5+ cells respond to IL-18 independently of IL-12, as cells from both wild-type and IL-12Rbeta2KO mice produce IFN-gamma and IL-13 in response to IL-2 + IL-18.

Interleukin-18 regulates both Th1 and Th2 responses

Although interleukin-18 is structurally homologous to IL-1 and its receptor belongs to the IL-1R/Toll-like receptor (TLR) superfamily, its function is quite different from that of IL-1. IL-18 is produced not only by types of immune cells but also by non-immune cells. In collaboration with IL-12, IL-18 stimulates Th1-mediated immune responses, which play a critical role in the host defense against infection with intracellular microbes through the induction of IFN-gamma. However, the overproduction of IL-12 and IL-18 induces severe inflammatory disorders, suggesting that IL-18 is a potent proinflammatory cytokine that has pathophysiological roles in several inflammatory conditions. IL-18 mRNA is expressed in a wide range of cells including Kupffer cells, macrophages, T cells, B cells, dendritic cells, osteoblasts, keratinocytes, astrocytes, and microglia. Thus, the pathophysiological role of IL-18 has been extensively tested in the organs that contain these cells. Somewhat surprisingly, IL-18 alone can stimulate Th2 cytokine production as well as allergic inflammation. Therefore, the functions of IL-18 in vivo are very heterogeneous and complicated. In principle, IL-18 enhances the IL-12-driven Th1 immune responses, but it can also stimulate Th2 immune responses in the absence of IL-12.

Fas ligand-induced caspase-1-dependent accumulation of interleukin-18 in mice with acute graft-versus-host disease

Acute graft-versus-host disease (aGVHD), the fatal side effects of bone marrow transplantation, was shown to be accompanied by elevation of serum levels of interleukin 18 (IL-18). In this study, the mechanism underlying the accumulation of IL-18 in aGVHD in mice was investigated. Lethally irradiated recipients having transplantation with H-2 disparate donor splenocytes demonstrated aGVHD and contained markedly elevated serum levels of IL-18. In contrast, recipients having transplantation with gld/gld spleen cells, which lack functional Fas ligand (FasL), contained only normal ranges of IL-18, indicating FasL-mediated IL-18 release in aGVHD. The wild-type hosts engrafted with caspase-1-deficient cells revealed marked increases of IL-18 similar to those engrafted with wild-type cells, whereas caspase-1-deficient recipients engrafted with wild-type cells showed only a slight elevation of serum IL-18, indicating that IL-18 elevation is derived from host cells in a caspase-1-dependent manner. These results suggest FasL-mediated caspase-1-dependent IL-18 secretion in aGVHD in mice.

The protective role of interleukin-18 in Salmonella infection

Since its derivation, much has been learned about the proinflammatory inducing cytokine interleukin-18, and its role in resolving infectious diseases. Studies to date that examined interleukin-18 have shown that endogenously induced interleukin-18 plays an important protective role in some viral and bacterial infections. However, interleukin-18 has a limited role in protective immunity to Salmonella infections, and is secondary to the protective role of interleukin-12. This lack of sensitivity to interleukin-18 may be in part related to the dependence on interleukin-12 for interleukin-18 receptor expression and to the nanomolar concentrations that are required. Nonetheless, an understanding of how some pathogens have evolved to circumvent interleukin-18 and of how interleukin-18 can be effectively induced are essential to our ability to develop better vaccines against enteric pathogens.

Determination of the functional epitopes of human interleukin-18-binding protein by site-directed mutagenesis

The human interleukin (IL)-18-binding protein (hIL-18BP) is a naturally occurring antagonist of IL-18, a proinflammatory cytokine that is related to IL-1beta and has an important role in defense against microbial invaders. As its name implies, the hIL-18BP binds to IL-18 with high affinity and prevents the interaction of IL-18 with its receptor. We genetically modified the C terminus of hIL-18BP by appending a 15-amino acid biotinylation recognition site and a six-histidine tag and then performed site-directed mutagenesis to determine the functional epitopes that mediate efficient binding to IL-18. The mutated IL-18BPs were secreted from mammalian cells, captured by metal affinity chromatography, biotinylated in situ, eluted, and immobilized on streptavidin-coated chips. Using surface plasmon resonance, we identified seven amino acids of hIL-18BP which, when changed individually to alanine, caused an 8-750-fold decrease in binding affinity, largely because of increased off-rates. These seven amino acids localized to the predicted beta-strand c and d of hIL-18BP immunoglobulin-like domain, and most had hydrophobic side chains. Just two amino acids, tyrosine 97 and phenylalanine 104, contributed approximately 50% of the binding free energy. Information obtained from these studies could contribute to the design of molecular antagonists of IL-18 for treatment of inflammatory diseases.

Lethal hepatitis after gene transfer of IL-4 in the liver is independent of immune responses and dependent on apoptosis of hepatocytes: a rodent model of IL-4-induced hepatitis

The putative role of IL-4 in human and animal models of hepatitis has not yet been directly determined. We now report that direct expression of IL-4 in the liver of rats or mice using recombinant adenoviruses coding for rat or mouse IL-4 (AdrIL-4 and AdmIL-4, respectively) results in a lethal, dose-dependent hepatitis. The hepatitis induced by IL-4 was characterized by hepatocyte apoptosis and a massive monocyte/macrophage infiltrate. IL-4-induced hepatitis was independent of T cell-mediated immune responses. Hepatitis occurred even after gene transfer of IL-4 into nude rats, CD8-depleted rats, cyclosporine A-treated rats, or recombinase-activating gene 2(-/-) immunodeficient mice. Peripheral depletion of leukocytes using high doses of cyclophosphamide, and/or the specific depletion of liver macrophages with liposome-encapsulated dichloromethylene diphosphonate in rats did not block lethal IL-4-induced hepatitis. Direct transduction of hepatocytes with adenoviruses was not essential, since injection of AdrIL-4 into the hind limb induced an identical hepatitis. Finally, primary rat hepatocytes in culture also showed apoptosis when cultured in the presence of rIL-4. IL-4-dependent hepatitis was associated with increases in the intrahepatic levels of IFN-gamma, TNF-alpha, and Fas ligand. Administration of AdmIL-4 to IFN-gamma, TNF-alpha receptor type I, or TNF-alpha receptor type II knockout mice also resulted in lethal hepatitis, whereas a moderate protection was observed in Fas-deficient lpr mice. IL-4-dependent hepatocyte apoptosis could be abolished by treatment with caspase inhibitory peptides. Our results thus demonstrate that IL-4 causes hepatocyte apoptosis, which is only partially dependent on the activation of Apo-1-Fas signaling and is largely independent of any immune cells in the liver.

Lipopolysaccharide-induced IL-18 secretion from murine Kupffer cells independently of myeloid differentiation factor 88 that is critically involved in induction of production of IL-12 and IL-1beta

IL-18, produced as biologically inactive precursor, is secreted from LPS-stimulated macrophages after cleavage by caspase-1. In this study, we investigated the mechanism underlying caspase-1-mediated IL-18 secretion. Kupffer cells constantly stored IL-18 and constitutively expressed caspase-1. Inhibition of new protein synthesis only slightly reduced IL-18 secretion, while it decreased and abrogated their IL-1beta and IL-12 secretion, respectively. Kupffer cells deficient in Toll-like receptor (TLR) 4, an LPS-signaling receptor, did not secrete IL-18, IL-1beta, and IL-12 upon LPS stimulation. In contrast, Kupffer cells lacking myeloid differentiation factor 88 (MyD88), an adaptor molecule for TLR-mediated-signaling, secreted IL-18 without IL-1beta and IL-12 production in a caspase-1-dependent and de novo synthesis-independent manner. These results indicate that MyD88 is essential for IL-12 and IL-1beta production from Kupffer cells while their IL-18 secretion is mediated via activation of endogenous caspase-1 without de novo protein synthesis in a MyD88-independent fashion after stimulation with LPS. In addition, infection with Listeria monocytogenes, products of which have the capacity to activate TLR, increased serum levels of IL-18 in wild-type and MyD88-deficient mice but not in caspase-1-deficient mice, whereas it induced elevation of serum levels of IL-12 in both wild-type and caspase-1-deficient mice but not in MyD88-deficient mice. Taken together, these results suggested caspase-1-dependent, MyD88-independent IL-18 release in bacterial infection.

Skin-specific caspase-1-transgenic mice show cutaneous apoptosis and pre-endotoxin shock condition with a high serum level of IL-18

To study the pathophysiological roles of overexpressed caspase-1 (CASP1), originally designated as IL-1 beta-converting enzyme, we generated transgenic mice in which human CASP1 is overexpressed in their keratinocytes. The transgenic mice spontaneously developed recalcitrant dermatitis and skin ulcers, characterized by the presence of massive keratinocyte apoptosis. The skin of the mice contained the active form of human CASP1 and expressed mRNA for caspase-activated DNase, an effector endonuclease responsible for DNA fragmentation. Their skin and sera showed elevated levels of mature IL-18 and IL-1 beta, but not of IFN-gamma. The plasma from these animals induced IFN-gamma production by IL-18-responsive NK cells. Administration of heat-killed Propionibacterium acnes, a potent in vivo type 1 cell inducer, caused IFN-gamma-mediated lethal liver injury in the transgenic mice, which was completely inhibited by treatment with neutralizing anti-IL-18 Ab. These results indicated that in vivo overexpression of CASP1 caused spontaneous apoptotic tissue injury and rendered mice highly susceptible to exogenous type 1 cell-inducing condition in collaboration with endogenously accumulated proinflammatory cytokines.