Gene expression, synthesis, and secretion of interleukin 18 and interleukin 1beta are differentially regulated in human blood mononuclear cells and mouse spleen cells

Identification of IL-18RAP mRNA truncated splice variants in human testis and the other human tissues

IL-18 is a pleiotropic cytokine involved in the regulation of both innate and adaptive immunity. It plays a key role in the autoimmune, inflammatory and infectious diseases. IL-18 acts via a receptor complex that closely resembles that of IL-1, consisting of a ligand binding protein, IL-18Ralpha, and an accessory protein, IL-18RAP (IL-18Rbeta). IL-18RAP is essential for IL-18 signal transduction and ligand binding affinity to IL-18Ralpha receptor chain. mRNA of gene coding for IL-18RAP in human testicular tissue and the nucleotide sequence of splice variants was carefully examined. We have found for the first time ever, IL-18RAP mRNA in studied tissue samples of physiological testis. Using the RT-PCR technique, the whole coding sequence of this gene was amplified. An alternative splicing of mRNA for IL-18RAP was then discovered and subsequently confirmed by cDNA sequencing. The putative amino acid content was predicted and a computer modeling was performed. It might be hypothesized that the truncated forms of IL-18RAP can be involved in the complex mechanism of IL-18 activity regulation.

The expanding family of interleukin-1 cytokines and their role in destructive inflammatory disorders

Understanding cytokine immunobiology is central to the development of rational therapies for destructive inflammatory diseases such as rheumatoid arthritis (RA) and periodontitis. The classical interleukin-1 (IL-1) family cytokines, IL-1alpha and IL-1beta, as well as IL-18, play key roles in inflammation. Recently, other members of the IL-1 family have been identified. These include six cytokines whose genes are located downstream of the genes for IL-1alpha and IL-1beta on chromosome 2 (IL-1F5-10) and also IL-33, which is the ligand for ST2, a member of the IL-1R/Toll-like receptor (TLR) receptor superfamily. IL-1F6, IL-1F8 and Il-1F9 are agonists and, along with their receptor IL-1Rrp2, are highly expressed in epithelial cells suggesting a role in immune defence in the skin and the gastrointestinal (GI) tract including the mouth. Synovial fibroblasts and articular chondrocytes also express IL-1Rrp2 and respond to IL-1F8, indicating a possible role in RA. IL-33 is associated with endothelial cells in the inflamed tissues of patients with RA and Crohn's disease, where it is a nuclear factor which regulates transcription. IL-33 is also an extracellular cytokine: it induces the expression of T helper 2 (Th2) cytokines in vitro and in vivo as well as histopathological changes in the lungs and GI tract of mice. Therapeutic agents which modify IL-1 cytokines (e.g. recombinant IL-1Ra) have been used clinically and others are at various stages of development (e.g. anti-IL-18 antibodies). This review highlights the emerging data on these novel IL-1 cytokines and assesses their possible role in the pathogenesis and therapy of destructive inflammatory disorders such as RA and periodontitis.

Development of PCR-ELISA for the detection of hepatitis B virus x gene expression and clinical application

The presence of hepatitis B virus x (HBx) antigen/antibody is known to correlate with the well-established serological markers of ongoing viral replication in the chronic phase of HBV infection, and strongly suggests that the level and duration of HBx expression may influence the outcome of the chronic infection. In this research, we developed a polymerase chain reaction enzyme-linked immunosorbent assay (PCR-ELISA) method for the detection of HBx gene expression. We also investigated its relationship to the progress of the disease in HBV-related patients. Peripheral blood mononuclear cells (PBMCs) were purely isolated, and reverse transcription-PCR (RT-PCR) was performed for improved sensitivity. The PCR products were determined by ELISA, and we investigated the relationship of the proposed method to the clinical status of the patients. The PCR-ELISA used in this work was found to be at least 100 times more sensitive than the conventional PCR method, and even 8,000-fold diluted PCR products could be detected. The HBx concentrations significantly differed among control subjects (0.36+/-0.09, [P<0.01] and patients with chronic hepatitis (1.13+/-0.34 [P<0.01 compared to control]), liver cirrhosis (LC; 1.37+/-0.28 [P<0.01 compared to control]), and hepatocellular carcinoma (HCC; 1.48+/-0.95 [P<0.01 compared to control]). These findings suggest that monitoring of HBx could be useful for early diagnosis and prognosis in patients with chronic HBV infection, LC, and HCC.

Endogenous interleukin-18 improves the early antimicrobial host response in severe melioidosis

Melioidosis is caused by the soil saprophyte Burkholderia pseudomallei and is endemic in Southeast Asia. The pathogenesis of melioidosis is still largely unknown, although gamma interferon (IFN-gamma) seems to play an obligatory role in host defense. Previously, we have shown that IFN-gamma production in melioidosis is controlled in part by interleukin-18 (IL-18). The aim of the present study was to determine the role of IL-18 in the immune response to B. pseudomallei. For this the following investigations were performed. (i) Plasma IL-18 and blood monocyte IL-18 mRNA levels were elevated in 34 patients with culture-proven melioidosis compared to the levels in 32 local healthy controls; in addition, IL-18 binding protein levels were markedly elevated in patients, strongly correlating with mortality. (ii) IL-18 gene-deficient (IL-18 knockout [KO]) mice showed accelerated mortality after intranasal infection with a lethal dose of B. pseudomallei, which was accompanied by enhanced bacterial growth in their lungs, livers, spleens, kidneys, and blood at 24 and 48 h postinfection, compared to wild-type mice. In addition, IL-18 KO mice displayed evidence of enhanced hepatocellular injury and renal insufficiency. Together, these data indicate that the enhanced production of IL-18 in melioidosis is an essential part of a protective immune response to this severe infection.

Interleukin-18 and the pathogenesis of inflammatory diseases

Several autoimmune diseases are thought to be mediated in part by interleukin (IL)-18. Many are those with associated increased interferon-gamma (IFNgamma) levels such as systemic lupus erythematosus, macrophage activation syndrome, rheumatoid arthritis, Crohn's disease, psoriasis, and graft-versus-host disease. In addition, ischemia, including acute renal failure in human beings, appears to involve IL-18. Animal studies also support the concept that IL-18 is a key player in models of lupus erythematosus, atherosclerosis, graft-versus-host disease, and hepatitis. Unexpectedly, IL-18 plays a role in appetite control and the development of obesity. IL-18 is a member of the IL-1 family; IL-1beta and IL-18 are related closely, and both require the intracellular cysteine protease caspase-1 for biological activity. The IL-18 binding protein, a naturally occurring and specific inhibitor of IL-18, neutralizes IL-18 activities and has been shown to be safe in patients. Other options for reducing IL-18 activities are inhibitors of caspase-1, human monoclonal antibodies to IL-18, soluble IL-18 receptors, and anti-IL-18 receptor monoclonal antibodies.

IL-18 expression in pigs following infection with Mycoplasma hyopneumoniae

Little is known about the detail of the immune response during infection of pigs with Mycoplasma hyopneumoniae (Mhp). To further understand this important porcine pathogen, we examined the interleukin-18 (IL- 18) response in experimentally infected piglets. We found that large amounts of IL-18 were produced in the bronchoalveolar lavage fluids (BALF) of pigs experimentally infected with Mhp. However, the concentration of interferon-gamma (IFN-gamma) in the same BALF was negatively correlated with that of IL-18. The antibody response against Mhp was found to be associated with the IL-18 concentration in the BALF. Immunohistochemical staining revealed that both IL-18 and IL-18 receptor alpha chain (IL-18Ralpha) were present in macrophages and plasma cells in the lungs of Mhp-infected pigs. Lung mononuclear cells isolated from pneumonic lesions secreted IL-18 and prostaglandin E(2) (PGE(2)) in vitro, and PGE(2) production was enhanced by stimulation with IL-18. These results indicate that IL-18 produced in the pig lung contributes to the development of innate and acquired immune responses against Mhp as a proinflammatory cytokine rather than as an IFN-gamma-inducing factor and may be involved in immunomodulation in pigs.

Effect of the conceptus on uterine natural killer cell numbers and function in the mouse uterus during decidualization

Uterine natural killer (uNK) cells are the most abundant lymphocytes in the uterus during early pregnancy and play a role in spiral arteriole modifications. In the present study, we investigated whether uNK cell populations differed between mouse decidua and deciduoma. Histochemical staining using the Dolichos biflorus agglutinin (DBA) lectin was used to identify uNK cells and classify their stages of maturation. We found differences in the pattern of localization and density of uNK cells between the decidua and deciduoma at Days 2-4 after the onset of decidualization. The cells were more distributed and the densities were significantly greater in the mesometrial region of the decidua than in the deciduoma. Using double-labeling for DBA lectin binding and bromodeoxyuridine incorporation, we found that the higher number of uNK cells in the decidua was not due to an increase in uNK cell proliferation. Western blot analyses revealed that the increase in uNK cell number was accompanied by significant increases in the levels of interferon gamma (IFNG) and prointerleukin 18 when a conceptus was present. Vascular morphometry revealed that modifications of the spiral arterioles occurred in the mesometrial decidua but not in the deciduoma, which could be attributed to the differences observed in uNK cell number and IFNG production. The present study demonstrates that differences exist in uNK cell populations between the decidua and deciduoma, providing evidence that the conceptus generates signals that regulate uNK cell number and function in the uterus during implantation.

Opposite Regulation of IL-1β and Secreted IL-1 Receptor Antagonist Production by Phosphatidylinositide-3 Kinases in Human Monocytes Activated by Lipopolysaccharides or Contact with T Cells

The unbalanced production of IL-1beta and its natural, specific inhibitor, the secreted IL-1R antagonist (sIL-1Ra), plays an important role in chronic/sterile inflammation. Relevant to this condition is direct cellular contact with stimulated T cells which is a potent inducer of cytokine production in human monocytes/macrophages. We previously demonstrated that activation of PI3Ks is a prerequisite of the transcription of the sIL-1Ra gene in human monocytes activated by IFN-beta. In this study, we addressed the question of PI3K involvement in the production of IL-1beta and sIL-1Ra in monocytes activated by cellular contact with stimulated T cells (mimicked by CHAPS-solubilized membranes of stimulated T cells (CE(sHUT))), and a crude preparation of LPS, to compare stimuli relevant to chronic/sterile and acute/infectious inflammation, respectively. In monocytes activated by either CE(sHUT) or LPS, the inhibition of PI3Ks abrogated sIL-1Ra transcript expression and sIL-1Ra production, demonstrating that PI3Ks control the induction of sIL-1Ra gene transcription. In contrast, PI3K inhibition increased the production of IL-1beta protein in both CE(sHUT)- and LPS-activated monocytes, the enhancement being drastically higher in the former. This was not due to changes in IL-1beta mRNA steady-state levels or transcript stability, but to the involvement of PI3Ks in the repression of IL-1beta secretion. The downstream PI3K effector, Akt, was implicated in this process. The present results demonstrate that PI3Ks are involved in the inhibition of IL-1beta secretion and in the induction of sIL-1Ra production in human blood monocytes by controlling different mechanisms in conditions mimicking chronic/sterile (CE(sHUT)) and acute/infectious (LPS) inflammation.

Responses of IL-18- and IL-18 receptor-deficient pancreatic islets with convergence of positive and negative signals for the IL-18 receptor

Pancreatic islets contain cells that produce IL-18 and cells that express IL-18 receptors. In experimentally induced diabetes, islet failure correlates with IL-18 levels and diabetes is delayed with blockade of endogenous IL-18. We studied islet-derived IL-18 and responses to IL-18 in a mouse model of islet allograft transplantation. In vitro, IL-18-stimulated islets produced nitric oxide, which closely matched islet apoptosis. By neutralizing IL-18 activity with IL-18 binding protein (IL-18BP), we observed that islets produce bioactive IL-18. In vivo, transgenic mice overproducing IL-18BP (IL-18BP-Tg) exhibited delayed hyperglycemia induced by beta cell toxic streptozotocin. Similarly, cultured IL-18BP-Tg islets were protected from streptozotocin-induced apoptosis. In the transplant model, islets grafted from WT to IL-18BP-Tg mice achieved prolonged normoglycemia (P = 0.031). Improved graft function was also observed by using IL-18-deficient islets transplanted into WT recipients, demonstrating that endogenous, islet-derived IL-18 mediates IL-18-driven graft damage. Unexpectedly, islets from mice deficient in IL-18 receptor alpha chain (IL-18R) exhibited rapid graft failure (P = 0.024; IL-18- versus IL-18R-deficient grafts in WT recipients). In related studies, IL-18R-deficient splenocytes and macrophages produced 2- to 3-fold greater amounts of IL-18, TNFalpha, macrophage inflammatory protein 1, macrophage inflammatory protein 2, and IFNgamma upon stimulation with Con A, Toll-like receptor 2 agonist, or anti-CD3 antibodies. These data reveal a role for islet-derived IL-18 activity during inflammation-mediated islet injury. Importantly, discrepancies between IL-18- and IL-18R-deficient cells suggest that IL-18Ralpha chain is used by an inflammation-suppressing signal.

Interleukin-18 (IL-18) mRNA expression and localization of IL-18 mRNA-expressing cells in the mouse uterus

Interleukin-18 (IL-18) belongs to the interleukin-1 family and was identified as an interferon-gamma inducing factor. We investigated IL-18 mRNA-expressing cells in the mouse uterus. By RNase protection assay, IL-18 mRNA and alpha subunit of IL-18 receptor mRNA were detected in the uterus. In the uterus, IL-18 mRNA levels increased during sexual maturation. In situ hybridization analysis demonstrated IL-18 mRNA-expressing cells in the mouse uterus of different ages. At 21 days of age, IL-18 mRNA-expressing cells were detected in the luminal epithelial cells and stromal cells although the IL-18 mRNA signal was weak. At 42 days of age, IL-18 mRNA signal was mainly detected in the stromal cells located near the myometrium, and in some of the luminal and glandular epithelial cells. In the uterus of 63-day-old adult mice, a strong hybridization signal for IL-18 mRNA was detected at estrus, but was weak at diestrus. IL-18 mRNA was mainly detected in the glandular epithelial cells and stromal cells. The effect of estradiol-17beta (E(2)) on IL-18 mRNA-expressing cells in the uterus was examined in ovariectomized mice. In oil-treated mice IL-18 mRNA signal was localized in luminal epithelial cells and stromal cells, while in E(2)-treated mice IL-18 mRNA signal was localized in stromal cells alone. These results suggest that the mouse uterus has an IL-18 system, and IL-18 exerts a physiological role within the uterus in a paracrine manner, and that IL-18 gene expression is regulated by estrogen.

Interleukin-18 secretion and Th1-like cytokine responses in human peripheral blood mononuclear cells under the influence of the toll-like receptor-5 ligand flagellin

Flagellin is the major protein component of the flagella from motile bacteria and was identified as the ligand for toll-like receptor (TLR)-5. Whereas its effects on epithelial cells have been studied in detail, activation of human peripheral blood mononuclear cells (PBMC) by flagellin is characterized only partially. By using the recombinant protein of Salmonella muenchen we confirm the proinflammatory nature of flagellin as detected by nuclear factor-kappaB activation and interleukin (IL)-8 production. Aim of the current study was to elucidate in PBMC effects of flagellin on IL-18 and Th1-like cytokine responses. We report that flagellin in pathophysiologically relevant concentrations augmented release of mature IL-18 by THP-1 monocytes, PBMC, and whole blood stimulated with nigericin or by ATP-mediated P2X7 purinergic receptor activation. Further key functions of the IL-18/IL-12/interferon-gamma (IFNgamma) pathway were upregulated by flagellin. Flagellin synergized with IL-12 for production of IFN-gamma and augmented secretion of interferon-inducible protein-10, a CXC-chemokine that is key to the generation of Th1-type responses. In contrast, neither IL-18-binding protein nor IL-4 was affected. Taken together, the present data demonstrate for the first time that flagellin at concentrations that are detectable in the blood compartment during sepsis efficiently enhances the IL-18/IL-12/IFNgamma pathway and thus Th1-like cytokine responses in PBMC.

Expression and alternative processing of IL-18 in human neutrophils

Interleukin-18 (IL-18), a member of the IL-1 cytokine superfamily, is an important regulator of both innate and acquired immune responses. We demonstrate here constitutive expression of IL-18 by human neutrophils. Unexpectedly, we observed that neutrophils from peripheral blood or rheumatoid synovial compartments contained not only pro and mature IL-18, but also several novel smaller-molecular-weight IL-18-derived species. Using specific protease inhibitors, and serine protease gene-targeted mice, we demonstrate that these IL-18-derived products arose through caspase-independent cleavage events mediated by the serine proteases, elastase and cathepsin G. Moreover, we report that the net effect of elastase treatment of mature recombinant IL-18 was to reduce its IFN-gamma-inducing activity. Thus, human neutrophils contain IL-18 and IL-18-derived molecular species that can arise through novel enzymatic processing pathways. Through cytosolic, membrane or secretory expression of such processing enzymes, together with generation of IL-18 itself, neutrophils likely play a critical role in regulating IL-18 activities during early innate immune responses.

Bone morphogenetic protein-2 counterregulates interleukin-18 mRNA and protein in MC3T3-E1 mouse osteoblastic cells

Fibroblast growth factors-2 (FGF-2) and bone morphogenetic protein-2 (BMP-2) are two of the main factors that regulate differentiation of osteoblasts. Interleukin-18 (IL-18), originally cloned as an interferon gamma-inducing factor, has been reported to inhibit maturation of osteoclasts by upregulation of osteoprotegerin secreted from osteoblasts. Little is known about the functional relationship between IL-18 and the two growth factors in osteoblast differentiation. To better understand this relationship, we analyzed the effect of BMP-2 and FGF-2 on the mRNA expression levels of IL-18, as well as IL-1alpha and IL-6, in MC3T3-E1 mouse osteoblastic cells. Following this, the effects of BMP-2 on the expression of IL-18 protein and caspase-1 protein were analyzed by immunofluorescence staining. Real-time PCR and immunofluorescence staining analysis showed that FGF-2 had no effect on the expression of IL-18 mRNA and protein, but while BMP-2 reduced IL-18 mRNA levels, increased immunostaining of both IL-18 protein and caspase-1 protein was detected in BMP-2-treated MC3T3-E1 cells. Although the significance and mechanisms of this counterregulation of IL-18 mRNA and protein were not determined in this study, the increase of IL-18 protein suggested that BMP-2 may induce an active form of IL-18.

Cloning and characterization of a new isoform of mouse interleukin-18

Interleukin-18 (IL-18) is a novel proinflammatory cytokine with potent interferon (IFN)-lambda inducing activity that plays an important biological role in the enhancement of the activity of natural killer cells and cytotoxic T lymphocytes. In this study, we have identified a novel short form of IL-18 in mouse, named IL-18s. IL-18s might be an alternative splicing variant of IL-18 and its cDNA contains a 57 bp in-frame deletion. Like IL-18, IL-18s is also widely expressed in mouse tissues. It was suggested that IL-18s might have a caspase-1-dependent mechanism for maturation and secretion similar to that of IL-18: when transfected in COS-7 cells, pro-IL-18s (22 kDa) could be detected, and the mature IL-18s (16 kDa) could also be detected when combined with caspase-1. We observed that recombinant mouse IL-18s did not show any IL-18-like function, and IL-18s could enhance the ability of IL-18 to increase IFN-lambda production by approximately 40% in mouse splenocytes. This effect was observed primarily at relative low concentrations of IL-18, suggesting that IL-18s might regulate the activity of IL-18 in the physiological conditions.

Severe imbalance of IL-18/IL-18BP in patients with secondary hemophagocytic syndrome

Hemophagocytic syndrome (HPS) is characterized by an uncontrolled and poorly understood activation of T-helper 1 (Th-1) lymphocytes and macrophages. We studied 20 patients with HPS secondary to infections, autoimmune disease, lymphoma, or cancer and observed that the concentrations of serum interleukin 18 (IL-18), a strong inducer of Th-1 responses, interferon gamma (IFN-gamma) production, and stimulation of macrophages and natural killer (NK) cells were highly increased in HPS but not in control patients. In contrast, concentrations of its natural inhibitor, the IL-18 binding protein (IL-18BP), were only moderately elevated, resulting in a high level of biologically active free IL-18 in HPS (4.6-fold increase compared with controls; P < .001). Free IL-18 but not IL-12 concentrations significantly correlated with clinical status and the biologic markers of HPS such as anemia (P < .001), hypertriglyceridemia, and hyperferritinemia (P < .01) and also with markers of Th-1 lymphocyte or macrophage activation, such as elevated concentrations of IFN-gamma and soluble IL-2 and tumor necrosis factor alpha (TNF-alpha) receptor concentrations. Despite high IL-18 elevation, in vitro NK-cell cytotoxicity was severely impaired in HPS patients, in part due to NK-cell lymphopenia that was observed in a majority of patients but also secondary to an intrinsic NK-cell functional deficiency. We concluded that a severe IL-18/IL-18BP imbalance results in Th-1 lymphocyte and macrophage activation, which escapes control by NK-cell cytotoxicity and may allow for secondary HPS in patients with underlying diseases.

Nitric oxide inducing function and intracellular movement of chicken interleukin-18 in cultured cells

To evaluate the characteristics of chicken interleukin-18 (ChIL-18) in different forms in vitro, the ChIL-18 full-length gene (ChIL-18-F) and the ChIL-18 presumed mature protein gene (ChIL-18-M) were cloned and inserted into the eukaryotic expression vector pCI, to construct recombinant pCI-ChIL-18-F and pCI-ChIL-18-M. The recombinant plasmids were then transferred into chicken splenic lymphocytes (CSLs). Western blot showed that ChIL-18-F, with a molecular weight of 23.0 kDa, was produced in CSLs transfected by pCI-ChIL-18-F; ChIL-18-M, with a molecular weight of 19.5 kDa, was produced in CSLs transfected by pCI-ChIL-18-M. The nitric oxide (NO) level in the transfected CSLs and the culture medium at different time points was further examined under confocal microscopy using 4,5-diaminofluorescein staining. The results showed that both pCI-ChIL-18-F and pCI-ChIL-18-M groups showed significant increase in intracellular and extracellular NO production compared with pCI transfected control cells. These results suggest that both ChIL-18-F and ChIL-18-M could stimulate NO secretion in CSLs. To characterize the intracellular distribution of ChIL-18, ChIL-18-F and ChIL-18-M were each fused to the enhanced green fluorescent protein gene, and expressed in Vero cells. The results showed that the ChIL-18-F tended to the membranous region in Vero cells, while ChIL-18-M did not. This indicates that the N-terminal 27 amino acid peptide helped ChIL-18 target to Vero cell membranes.

Interleukin-18 is expressed in rat testis and may promote germ cell growth

Although host-defence mechanisms, designed to preserve the integrity of the developing germ cells are operative in the testis, the components of this protective system have yet to be characterised in detail. Here, we report that the cytokine interleukin-18 (IL-18) is expressed in the rat testis and may contribute to these defences. Thus, analysis by RT-PCR and Western blotting revealed pronounced testicular expression of pro-IL-18 from postnatal day 5 and onwards. Expression of both IL-18 mRNA and protein was found to be localised to meiotic and post-meiotic germ cells as evaluated by in situ hybridisation and immunohistochemistry, respectively. The mRNA species coding for the IL-18 receptor and IL-1beta converting enzyme, which activates pro-IL-18, were also shown to be expressed by the seminiferous tubules. Recombinant IL-18 was seen to stimulate spermatogonial DNA synthesis in cultures of staged segments of rat seminiferous tubules, without influencing germ cell apoptosis. These results suggest that IL-18 may have host-protective and growth-promoting functions in the testis, but further investigations need to be done to confirm this.

NK/iDC interaction results in IL-18 secretion by DCs at the synaptic cleft followed by NK cell activation and release of the DC maturation factor HMGB1

Interaction of natural killer (NK) cells with autologous immature dendritic cells (DCs) results in reciprocal activation; however, the underlying mechanisms are so far elusive. We show here that NK cells trigger immature DCs to polarize and secrete interleukin 18 (IL-18), a cytokine lacking a secretory leader sequence. This occurs through a Ca2+-dependent and tubulin-mediated recruitment of IL-18-containing secretory lysosomes toward the adhering NK cell. Lysosome exocytosis and IL-18 secretion are restricted at the synaptic cleft, thus allowing activation of the interacting NK cells without spreading of the cytokine. In turn, DC-activated NK cells secrete the proinflammatory cytokine high mobility group B1 (HMGB1), which induces DC maturation and protects DCs from lysis. Also HMGB1 is a leaderless cytokine that undergoes regulated secretion. Differently from IL-18, soluble HMGB1 is consistently detected in NK/DC supernatants. These data point to secretion of leaderless cytokines as a key event for the reciprocal activation of NK cells and DCs. DCs initiate NK cell activation by targeted delivery of IL-18, thus instructing NK cells in the absence of adaptive-type cytokines; in turn, activated NK cells release HMGB1, which promotes inflammation and induces DC maturation, thus favoring the onset of the adaptive immune response.

Interleukin-18 bioactivity: a novel target for immunopharmacological anti-inflammatory intervention

Interleukin-18 is a member of the interleukin-1 family of cytokines with pro-inflammatory and tumor-suppressive properties. Its ability to potently enhance the production of interferon-gamma indicates in particular the crucial function of interleukin-18 as an immunomodulatory molecule. In fact, high levels of interleukin-18 are detected in human diseases associated with immunoactivation including viral or bacterial infections and chronic inflammation. Animal models suggest suppression of interleukin-18 bioactivity as a novel therapeutic concept specifically for the treatment of chronic inflammatory diseases such as rheumatoid arthritis, Crohn's disease, and psoriasis. Here we introduce into the biology of interleukin-18 and review immunopharmacological strategies that aim at reducing interleukin-18 bioactivity in human disease.

Cloning and expression of goat interleukin-18 gene

We isolated and sequenced a 480 bp cDNA encoding mature goat interleukin-18 (gIL-18) from alveolar macrophages and splenocytes activated with LPS by RT-PCR. The gIL-18 gene was cloned into pET32a (+) vectors and sequenced. Nucleotide sequence of gIL-18 shares high homology with cattle. Fusional expression with pET32a (+) of gIL-18 of about 38kD was obtained by SDS-PAGE analysis after induction by IPTG in the E. Coli BL21 expression system. The recombinant protein can induce IFN-gamma production in PBMC. The IL-18 mRNA was constitutively detected in goat alveolar macrophages with or without LPS, While, enhanced expression was detected in splenocytes and liver cells if treated by LPS, and can be weakly detected in Peripheral blood mononuclear cells (PBMCs) treated by activators. Significant deference of IL-18 mRNA level may reflect the capacity to produce mature IL-18 in such tissues.

Disease association of two distinct interleukin-18 promoter polymorphisms in Caucasian rheumatoid arthritis patients

Interleukin (IL)-18 is an important mediator of innate and adaptive immunity. We searched for an association of IL-18 promoter single-nucleotide polymorphisms (SNP) with rheumatoid arthritis (RA) in Caucasians. The entire study population was composed of two independent cohorts from Germany (n=200) and Scotland (n=410). Presence of IL-18 SNP at positions -607 and -137 was determined by allele-specific PCR in 327 RA patients and 283 healthy donors (HD). Diplotype distributions of both loci were in Hardy-Weinberg equilibrium (HWE) in the German and Scottish HD cohorts. In contrast, locus -607 was in HW disequilibrium in German, and locus -137 in Scottish RA patients. Diplotypic exact chi(2) tests suggested that -607CC was overrepresented in German, and -137CC in Scottish RA patients, but conservative chi(2) trend analyses could not prove any significant disease association of these single loci. SNP -607 and -137 were in strong linkage disequilibrium. The -607C(*)-137C haplotype was more prevalent in German RA (3.2 vs 1.2%) and in Scottish RA patients (4.1 vs 0.9%) than in the respective HD cohorts. These observations suggest that SNP of both positions contribute to the genetic background of RA pathogenesis.

The effect of the physical characteristics of hydroxyapatite particles on human monocytes IL-18 production in vitro

Hydroxyapatite (HA) is widely used to coat the metal parts of prosthetic implants in order to improve their biocompatibility and as a bone defect filling material. HA has been demonstrated to produce particles at the prosthetic interface that lead to an activation of phagocytic cells that induce a cascade reaction leading to bone resorption and aseptic loosening. Monocytes/macrophages are commonly observed in the interface tissue, and are among the first cells to colonize the inflammatory site where they play a key role in the immune response. IL-18 is a pro-inflammatory cytokine. Monocytes/macrophages were described as IL-18 producing cells. IL-18 works antagonistically to IL-6, which activates osteoclastogenesis. In the present study, we investigated the ability of HA particles to induce the production of active IL-18 by human monocytes according to particle characteristics (size, sintering temperature and shape). Our study demonstrates, for the first time, that HA particles are capable of stimulating the production of the proinflammatory cytokine IL-18 in human monocytes according to their particle characteristics. The expression and the production of IL-18 was modified by the parameter studied. The difference observed between the expression and the production could be explain by the production of ICE. The needle shaped particles induced the larger production of IL-18.

Enhanced interleukin-18 levels in the peripheral blood of children with coeliac disease

Coeliac disease (CoD) is a small intestinal disorder characterized by villous atrophy, crypt cell hyperplasia and an increased production of T helper cell type 1 (Th1) cytokines. Interleukin (IL)-18 is a pro-inflammatory cytokine that has a crucial role in maintaining the Th1 response. In this study, the serum levels of IL-18 were measured in children with CoD or other gastrointestinal diseases in order to evaluate the possibility of using IL-18 as a disease activity marker. IL-18 levels were higher in samples from CoD patients [median 443 pg/ml (148-885)] compared to healthy controls [median 205 pg/ml (11-379)], P <0.05. In contrast, the levels of IL-18 were not enhanced significantly in the serum from patients with inflammatory bowel disease (IBD) [median 324 pg/ml (207-546)] or in the disease control group [median 303 pg/ml (2-689)]. In CoD patients, after 2 weeks of gluten challenge (GC), serum IL-18 was unchanged [median 268 pg/ml (59-458)] compared to patients on a gluten-free diet [median 220 pg/ml (53-600)], while IL-18 was increased after 12 weeks of GC [median 551 pg/ml (94-952)], P <0.01. The IL-18 levels correlated with IgA anti-transglutaminase antibody levels (rs=0.59, P=0.016) in serum from untreated CoD patients, and IL-18 also followed the degree of small intestinal villous atrophy in 12 out of 19 CoD patients. Our results support the view that serum IL-18 concentrations in children with CoD follow disease activity, suggesting a role for IL-18 in the induction of an inflammatory Th1-response after gluten exposure.

Plasma interleukin-18 is associated with viral load and disease progression in HIV-1-infected patients

Recent studies demonstrate persistent elevation of interleukin-18 (IL-18) concentration in human immunodeficiency virus type 1 (HIV-1)-infected patients. Due to pleiotropic action of IL-18 on the immune system, dysregulation of its synthesis may lead to inappropriate immune activation. The aim of this study was to determine possible correlation between IL-18 levels and the natural stages of HIV-1 infection. IL-18 plasma concentrations were determined in 42 patients in different stages of an HIV-1 infection and in 15 healthy controls. HIV infection resulted in a more than fourfold increase of plasma IL-18 concentration compared to healthy individuals (865 +/- 87 vs. 206 +/- 32 pg/ml, P < 0.001). Moreover, a positive correlation between plasma IL-18 concentration and HIV viral load was found (r = 0.44, P < 0.01). Further analysis showed marked elevation of IL-18 levels in late-stage symptomatic patients. Plasma IL-18 concentrations in patients receiving high-activity antiretroviral treatment (HAART) were significantly lower than in those not undergoing antiretroviral treatment. Individuals who did not reach viral suppression showed higher IL-18 plasma concentration than the group with achieved viral suppression. Excessive production of IL-18 observed in our study may promote viral replication and disease progression in advanced, especially late-stage HIV-infected patients. Furthermore, reduction of IL-18 concentration can be an important step in HAART-related immune restoration.

Adenovirus-mediated interleukin-18 mutant in vivo gene transfer inhibits tumor growth through the induction of T cell immunity and activation of natural killer cell cytotoxicity

We report here that gene transfer using recombinant adenoviruses encoding interleukin (IL)-18 mutants induces potent antitumor activity in vivo. The precursor form of IL-18 (ProIL-18) is processed by caspase-1 to produce bioactive IL-18, but its cleavage by caspase-3 (CPP32) produces an inactive form. To prepare IL-18 molecules with an effective antitumor activity, a murine IL-18 mutant with the signal sequence of murine granulocyte-macrophage (GM)- colony stimulating factor (CSF) at the 5'-end of mature IL-18 cDNA (GMmIL-18) and human IL-18 mutant with the prepro leader sequence of trypsin (PPT), which is not cleaved by caspase-3 (PPThIL-18CPP32-), respectively, were constructed. Adenovirus vectors carrying GMmIL-18 or PPThIL-18CPP32- produced bioactive IL-18. Ad.GMmIL-18 had a more potent antitumor effect than Ad.mProIL-18 encoding immature IL-18 in renal cell adenocarcinoma (Renca) tumor-bearing mice. Tumor-specific cytotoxic T lymphocytes, the induction of Th1 cytokines, and an augmented natural killer (NK) cell activity were detected in Renca tumor-bearing mice treated with Ad.GMmIL-18. An immunohistological analysis revealed that CD4+ and CD8+ T cells abundantly infiltrated into tumors of mice treated with Ad.GMmIL-18. Huh-7 human hepatoma tumor growth in nude mice with a defect of T cell function was significantly inhibited by Ad.PPThIL-18CPP32- compared with Ad.hProIL-18 encoding immature IL-18. Nude mice treated with Ad.PPThIL-18CPP32- contained NK cells with increased cytotoxicity. The results suggest that the release of mature IL-18 in tumors is required for achieving an antitumor effect including tumor-specific cellular immunity and augmented NK cell-mediated cytotoxicity. These optimally designed IL-18 mutants could be useful for improving the antitumor effectiveness of wild-type IL-18.

Geranylgeraniol regulates negatively caspase-1 autoprocessing: implication in the Th1 response against Mycobacterium tuberculosis

Caspase-1 is a cysteine protease composed by two 20-kDa and two 10-kDa subunits that processes pro-IL-1beta and pro-IL-18 to their mature forms. This enzyme is present in cells as a latent zymogen that becomes active through a tightly regulated proteolytic cascade. Activation is initiated by the oligomerization of an adaptor molecule, or by the formation of a multiprotein complex named inflammasome. Negative regulation of caspase-1 activation is exerted by proteins that compete with the adaptor molecule or with the inflammasome formation. We previously reported that fluvastatin, an inhibitor of 3-hydroxy-3-methylglutaryl coenzyme A reductase, increases caspase-1 activity in PBMC. This effect was strengthened by Mycobacterium tuberculosis, rending an exacerbated IL-1beta, IL-18, and IFN-gamma production. Mevalonate, the product of 3-hydroxy-3-methylglutaryl coenzyme A reductase, is a precursor for both nonsterol isoprenoid and sterol formation. In this study, we studied the involvement of mevalonate derivatives in the regulation of caspase-1 activation. Inhibition of sterol formation by SKF-104976 or haloperidol had no effect on IL-1beta release. However, the isoprenoid geranylgeraniol prevented both caspase-1 activation and the exacerbated IL production induced by fluvastatin. This isoprenoid significantly reduced the release of IL-18 and IFN-gamma by PBMC treated with mycobacteria, even in the absence of fluvastatin. In correlation with the increased caspase-1 activity, fluvastatin stimulated the proforms cleavage, enhancing the formation of active subunit p10. Geranylgeraniol not only prevented this effect, but induced proforms accumulation. Present results suggest that, once the proteolytic cascade is initiated, geranylgeraniol may exert an additional negative regulation on caspase-1 cleavage process.

Interleukin-18: recent advances

PURPOSE OF REVIEW

Interleukin-18 (IL-18) has potent immunomodulatory effects. It is the only cytokine with a unique capacity to induce T helper 1 or T helper 2 polarization, depending on the immunologic context. Serum levels of IL-18 are increased in many human diseases and it has been implicated in the pathogenesis of several immune-mediated processes. Some of the recent key advances in the immunobiology of IL-18 are discussed in this review.

RECENT FINDINGS

Recent data from several laboratories have shed light on the structure of IL-18; the signaling cascades that are initiated; and its role on modulating T cells, dendritic cells, and natural killer cell function. Several new reports have expanded and delineated the role of IL-18 in a multitude of diseases, but only recent advances in the role of IL-18 in three disease processes (acute graft-versus-host disease, insulin-dependent diabetes, and sepsis), where it appears to play paradoxic roles are discussed.

SUMMARY

Although emerging data shed more light on the complex role of IL-18 in immune reactions, they also pose more questions. Given the pleiotropic, complex, and at times paradoxic effects of IL-18 in various disease processes, better understanding of its immunobiology might lead to the development of IL-18 and/or its antagonists as therapeutic agents against immune-mediated diseases.

Decreased interleukin-18 expression in BAL cells and peripheral blood mononuclear cells in adult cystic fibrosis patients

Lymphocytes from cystic fibrosis (CF) patients secrete less interferon-gamma (IFN-gamma) upon stimulation compared to controls. Expression of interleukin (IL)-18 as an IFN-gamma inducing factor and of IL-10 as an IL-18 inhibiting factor were determined in bronchoalveolar lavage (BAL) cells from CF patients (n=5) and from normal control subjects (n=9) as well as in peripheral blood mononuclear cells (PBMC) from patients (n=12) and from control subjects (n=9) with RT-PCR. IL-18 and IL-10 serum protein levels were measured using ELISA. BAL cells and PBMC of CF patients expressed significantly less IL-18 compared to controls (p<0.05). There was no significant difference for IL-10 in BAL cells. However, PBMC from patients expressed significantly more IL-10 mRNA (p<0.05). IL-18 serum protein levels were decreased in the patient group, whereas IL-10 serum concentrations were elevated. Stimulation with rhIL-10 reduced IL-18 expression in PBMC from CF patients. Decreased IL-18 expression in CF patients may contribute to decreased IFN-gamma production. IL-10 may contribute to inhibit IL-18 expression in PBMC in CF.

Changes of inflammation-associated cytokine expressions during early phase of experimental endotoxic shock in macaques

AIM: To study changes of inflammation-associated cytokine expressions during early phase of endotoxic shock in macague.

METHODS: Experiments were performed in Macaque mulatta treated with LPS 2.8 mg/kg in shock model group or with normal saline in control group. Blood samples were collected before, or 60 min, or 120 min after LPS injection, respectively. Liver and spleen tissues were obtained at 120 min after LPS injection. The plasma levels of TNF-α , IL-1 β , IL-10 and IL-12P40 were determined by double-antibody sandwich ELISA with antibodies against human cytokines. The mRNA levels of TNF-α , IL-1 β , and IL-18 in peripheral blood mononuclear cells (PBMCs), liver and spleen were examined by real-time fluorescence semi-quantitative RT-PCR with the primers based on human genes.

RESULTS: Mean systemic arterial pressure (MAP), systemic vascular resistance index (SVRI) and left ventricular work index (LVWI) of macaques were significant declined in shock model group on average 60 min after LPS injection. The plasma levels of TNF-α and IL-10 were significantly increased 60 min after LPS injection and then decreased. The plasma levels of IL-1 β and IL-12P40 were significantly increased at 120 min after LPS injection. The mRNA levels of TNF-α and IL-1 β were significantly increased 60 min after LPS stimulation in PBMCs and 120 min after LPS stimulation in livers. The mRNA level of IL-18 was significantly increased 120 min after LPS stimulation in PBMCs and livers. But in spleen, only TNF-α mRNA level in LPS group was significantly higher 120 min after LPS stimulation, compared with that in control group.

CONCLUSION: An endotoxic shock model of Macaque mulatta was successfully established. Both antibodies for ELISA and PCR primers based on human cytokine assays were successfully applied to detect macaque cytokines. In the model, inflammatory cytokines, such as TNF-α , IL-1 β , IL-12 and IL-18 as well as anti-inflammation cytokine IL-10, were released at very early phase of endotoxic shock within 120 min after LPS injection. PBMCs and liver cells might be the important sources of these cytokines.

Interleukin-18 levels in induced sputum are reduced in asthmatic and normal smokers

BACKGROUND

IL-18 is a cytokine which is known to have an important role in the development of a Th1 lymphocyte response. As such, it may have a regulatory role in asthma by modifying Th2 lymphocyte responses. Cigarette smoking may amplify the airway inflammation associated with asthma.

OBJECTIVE

This study investigated if IL-18 could be detected in induced sputum from asthmatics and normal subjects and if smoking altered IL-18 levels.

METHODS

Induced sputum was obtained from asthmatic (31 smokers, 35 non-smokers) and normal (20 smokers, 20 non-smokers) subjects. All smokers had a smoking history of > or =15 pack years. IL-18 levels in sputum supernatant were measured by ELISA. IL-18 mRNA expression and cellular localization were assessed by quantitative PCR and immunocytochemistry, respectively.

RESULTS

Smoking was associated with a significant reduction in IL-18 levels (median (interquartile range) - smokers 20 (0-102) pg/mL vs. non-smokers 358 (50-876) pg/mL, P<0.001). This was more pronounced in asthmatics (smokers, 47 (40-64) pg/mL vs. non-smokers, 530 (30-1484) pg/mL; P<0.001) than in normal subjects (smokers, 25 (0-78) pg/mL vs. non-smokers, 247 (50-656) pg/mL; P<0.01). Within each of the smoking and non-smoking groups there was no significant difference in IL-18 levels between asthmatic and normal subjects. There was no correlation between sputum IL-18 levels and any specific cell type in the sputum samples nor serum IgE levels. IL-18 mRNA expression was reduced in asthmatic smokers compared with non-smokers. IL-18 production was localized to sputum macrophages by immunocytochemistry.

CONCLUSIONS

IL-18 is detectable in induced sputum samples from both asthmatic and normal subjects. Cigarette smoking significantly reduces sputum IL-18 levels. This effect is more pronounced in asthmatics than in normal subjects.

Evidence for altered interleukin 18 (IL)-18 pathway in human heart failure

Interleukin (IL)-18 is the interferon-gamma-inducing factor and has potent proinflammatory activities. IL-18 has been recently implicated in atherosclerotic plaque instability and myocardial ischemia-reperfusion injury. However, it is unknown whether IL-18 expression is increased in human myocardium or if it has any role in heart failure. We analyzed the expression of IL-18, its receptor IL-18Ralpha, and its endogenous inhibitor, IL-18 binding protein (IL-18BP) in myocardial tissue from patients with end-stage heart failure (ischemic or dilated cardiomyopathy) and controls by use of quantitative real-time reverse transcriptase polymerase chain reaction, Western blot or immunohistochemical techniques. Plasma levels of IL-18 were also determined in 48 patients with heart failure. IL-18 mRNA and protein levels were up-regulated in the myocardium of patients with ischemic cardiomyopathy. Both ischemic and dilated myocardium showed increased IL-18Ralpha levels, suggesting potential biological effects. In addition, mRNA levels of IL-18 BP were down-regulated in the failing myocardium. Finally, plasma IL-18 levels were significantly elevated in patients with heart failure and were higher in those who died at follow-up than in survivors. The results suggest a potential role for the immunoinflammatory IL-18 signaling pathway in the pathophysiology of heart failure and identify novel therapeutic targets for future testing.

Inhibition of lipopolysaccharide/ATP-induced release of interleukin-18 by KN-62 and glyburide

Monocytes release interleukin-18 after activation by lipopolysaccharide/ATP. Since inflammatory conditions such as sepsis are characterized by augmented interleukin-18 in sera of patients, we sought to modulate lipopolysaccharide/ATP-induced interleukin-18 release by pharmacological means. Here we report that 1-[N,O-bis(5-isoquinolinesulfonyl)-N-methyl-L-tyrosyl]-4-phenylpiperazine (KN-62), an inhibitor of ATP-mediated cellular activation by the purinoreceptor subtype P(2x7), potently suppresses interleukin-18 release from peripheral blood mononuclear cells. Interleukin-18 liberation was likewise inhibited by glyburide, a modulator of ion transport and inhibitor of ATP-binding cassette transporter 1. The data presented herein indicate that by pharmacologically interfering with the process of cytokine secretion agents such as KN-62 or glyburide have the potential to curb overproduction of interleukin-18 in septic patients.

Interleukin-18 and the treatment of rheumatoid arthritis

Interleukin (IL)-18 is a new member of the IL-1 family of proinflammatory cytokines. Based on preclinical studies in animals, IL-18 likely plays a role in rheumatoid arthritis, and strategies to block IL-18 activity are underway in clinical trials. In one of these trials,a naturally occurring IL-18 binding protein (IL-18 BP) binds IL-18 with a high affinity and reduces disease severity in models of inflammatory diseases. IL-18 BP is not the soluble receptor for IL-18 but rather a distinct molecule, which appears to be distantly related to the IL-1 receptor type II, both structurally and functionally, and hence represents part of the IL-1 family of receptors.

Regulation of human IL-18 gene expression: interaction of PU.1 with GC-box binding protein is involved in human IL-18 expression in myeloid cells

Interleukin-18 (IL-18) is a pro-inflammatory cytokine which participates in host defense against a variety of infections as well as in chronic inflammation including autoimmune diseases. However, little is known about human IL-18 regulation at the gene level. We have previously demonstrated that sodium butyrate, a bacterial fermentation product, induces IL-18 production via the proximal region of the promoter. In this study we investigated the molecular mechanisms for basal and sodium butyrate-induced expression of IL-18 in human myeloid cells. Two regulatory regions, a consensus binding site for PU.1 and a GC-rich region, are required for basal IL-18 promoter activity in human myeloid cells. PU.1 bound to the PU.1 consensus binding site in electrophoretic mobility shift assays, and overexpression of PU.1 led to activation of the IL-18 promoter through this site. Mutation analysis revealed that the GC-rich region, but not PU.1 site, participates in sodium butyrate-induced transactivation. Furthermore, DNA pull-down experiments and the critical spacing of the two binding sites suggest that formation of a protein complex involving both cis elements and the respective binding proteins might be crucial for human IL-18 expression.

Interleukin 18 and interleukin 18 binding protein: possible role in immunosuppression of chronic renal failure

Although interleukin (IL)-18 is a member of the IL-1 family of ligands, IL-18 appears to have unique characteristics, particularly in the regulation of the T helper type 1 (Th1) response. Th1 responses are required for tumor surveillance, killing intracellular organisms, and to provide help for antibody production. In patients with chronic renal failure, the well-known immunosuppression contributes to a failure to respond to infectious challenges and vaccinations. The most salient biological property of IL-18, linking this cytokine to the Th1 response, is its ability to induce interferon gamma (IFN-gamma). In fact, IL-18 was originally identified as an IFN-gamma-inducing factor, and IFN-gamma production is the hallmark of the Th1 response. Dysregulation of IFN-gamma production resulting from reduced activity of IL-18 would explain one of the mechanisms of immunosuppression in patients with chronic renal failure. The activity of IL-18 can be regulated by the IL-18-binding protein (IL-18BP), a glycoprotein of 40,000 daltons, which is constitutively expressed and appears to be the natural inhibitor of IL-18 activity. Unlike soluble receptors for IL-18, IL-18BP does not have a transmembrane domain; IL-18BP is a secreted protein possessing a high-affinity binding and ability to neutralize IL-18. IL-18BP was discovered in human urine and is excreted in health following glomerular filtration. With decreasing renal function, the concentrations of IL-18BP in the circulation are elevated as compared with subjects with a normal renal function, and these elevated levels may result in a decreased IL-18 activity. Because of the importance of IL-18 and IFN-gamma in the Th1 response, the biology of IL-18 and IL-18BP is reviewed here in the context of the immunosuppression of chronic renal failure.

The precursor form of IL-1alpha is an intracrine proinflammatory activator of transcription

Although most cytokines are studied for biological effects after engagement of their specific cell surface membrane receptors, increasing evidence suggests that some function in the nucleus. In the present study, the precursor form of IL-1alpha was overexpressed in various cells and assessed for activity in the presence of saturating concentrations of IL-1 receptor antagonist to prevent receptor signaling. Initially diffusely present in the cytoplasm of resting cells, IL-1alpha translocated to the to nucleus after activation by endotoxin, a Toll-like receptor ligand. The IL-1alpha precursor, but not the C-terminal mature form, activated the transcriptional machinery in the GAL4 system by 90-fold; a 50-fold increase was observed using only the IL-1alpha propiece, suggesting that transcriptional activation was localized to the N terminus where the nuclear localization sequence resides. Under conditions of IL-1 receptor blockade, intracellular overexpression of the precursor and propiece forms of IL-1alpha were sufficient to activate NF-kappaB and AP-1. Stable transfectants overproducing precursor IL-1alpha released the cytokines IL-8 and IL-6 but also exhibited a significantly lower threshold of activation to subpicomolar concentrations of tumor necrosis factor alpha or IFN-gamma. Thus, intracellular functions of IL-1alpha might play an unforeseen role in the genesis of inflammation. During disease-driven events, the cytosolic precursor moves to the nucleus, where it augments transcription of proinflammatory genes. Because this mechanism of action is not affected by extracellular inhibitors, reducing intracellular functions of IL-1alpha might prove beneficial in some inflammatory conditions.

Human dendritic cells express the IL-18R and are chemoattracted to IL-18

IL-18 is secreted by a variety of cells such as epithelial cells, macrophages, and dendritic cells (DC), in particular, in areas of chronic inflammation. The effects of IL-18 are complex and not fully understood thus far. We sought to explore human DC as a new target for IL-18, since IL-18R expression has been described on myeloid cells such as macrophages and DC are likely to get in contact with IL-18 at sites of inflammatory reactions. We demonstrate the expression of the IL-18R on human DC in peripheral blood and epidermis, as well as monocyte-derived dendritic cells (MoDC). On MoDC, IL-18R expression is up-regulated by IFN-gamma. IL-18 strongly up-regulated CD54 on MoDC, whereas the effect on MHC class II, CD83, and CD86 was only moderate and the expression of CD40 and CD80 was not affected. MoDC primed with IL-18 did not increase their capacity to stimulate the proliferation or IFN-gamma production of autologous T cells. However, IL-18 had a direct migratory effect on MoDC as indicated by induction of filamentous actin polymerization and migration in Boyden chamber experiments. In epidermal DC, IL-18 was also able to induce filamentous actin polymerization. Therefore, IL-18 might represent a novel mechanism to recruit DC to areas of inflammation, in particular under Th1 cytokine conditions where IFN-gamma is increased such as psoriasis or inflammatory bowel diseases.

Anti-inflammatory properties of pro-inflammatory interferon-gamma

Production of interferon-gamma (IFNgamma) in response to infection is a hallmark of innate and adaptive immunity. In addition to the pivotal role of IFNgamma in host defense, its excessive release has been associated with the pathogenesis of chronic inflammatory and autoimmune diseases. In fact, knockout models reveal that IFNgamma plays a key role in mediating a number of pathological processes related to chronic immune activation. On the other hand, evidence has been accumulated in recent years that supports the concept of a dual role of IFNgamma in inflammation. Here, we review anti-inflammatory aspects of IFNgamma in the regulatory network of cytokine biology. These include induction of anti-inflammatory molecules such as interleukin (IL)-1 receptor antagonist (IL-1Ra) and IL-18 binding protein (IL-18BP), modulation of pro-inflammatory cytokine production, activation of apoptosis, and interference with the signal transduction machinery by induction of suppressors of cytokine signaling (SOCS).

Critical role for cathepsin B in mediating caspase-1-dependent interleukin-18 maturation and caspase-1-independent necrosis triggered by the microbial toxin nigericin

The potassium ionophore nigericin induces cell death and promotes the maturation and release of IL-1beta in lipopolysaccharide (LPS)-primed monocytes and macrophages, the latter depending on caspase-1 activation by an unknown mechanism. Here, we investigate the pathway that triggers cell death and activates caspase-1. We show that without LPS priming, nigericin alone triggered caspase-1 activation and IL-18 generation in THP-1 monocytic cells. Simultaneously, nigericin induced caspase-1-independent necrotic cell death, which was blocked by the cathepsin B inhibitor CA-074-Me and other cathepsin inhibitors. Cathepsin B activation after nigericin treatment was determined biochemically and corroborated by rapid lysosomal leakage and translocation of cathepsin B to the cytoplasm. IL-18 maturation was prevented by both caspase-1 and cathepsin B inhibitors in THP-1 cells, primary mouse macrophages and human blood monocytes. Moreover, IL-18 generation was reduced in THP-1 cells stably transformed either with cystatin A (an endogenous cathepsin inhibitor) or antisense cathepsin B cDNA. Collectively, our study establishes a critical role for cathepsin B in nigericin-induced caspase-1-dependent IL-18 maturation and caspase-1-independent necrosis.

Interleukin-18 and host defense against infection

Interferon (IFN)-gamma-inducing factor was previously termed interleukin (IL)-18. Although IL-12 is also an IFN-gamma-inducing factor, the activity of IL-18 (but not IL-12) in models of sepsis and death is dependent on the intracellular cysteine protease IL-1beta converting enzyme (caspase-1). Caspase-1 is required for cleavage of the inactive precursor form of IL-18 into an active cytokine, and caspase-1-deficient mice are resistant to lethal endotoxemia. The absence of IFN-gamma (but not IL-1beta) in caspase-1-deficient mice is responsible for this resistance. However, the role of IFN-gamma in murine defense against gram-negative infection is inconsistent. Mice deficient in IFN-gamma are not resistant to lethal endotoxemia but are resistant when treated with neutralizing antibodies to IL-18 and challenged with a lethal injection of some endotoxins. Anti-IL-18 treatment also reduces neutrophil accumulation in liver and lungs. Neutralizing IL-18 with the IL-18 binding protein protects mice against endotoxin- and ischemia-induced hepatic damage. Thus, blockade of IL-18 appears to be a viable clinical target to combat the pathologic consequences of sepsis via IFN-gamma mechanisms.

Interleukin-18

Interleukin-18 (IL-18), a recently described member of the IL-1 cytokine superfamily, is now recognized as an important regulator of innate and acquired immune responses. IL-18 is expressed at sites of chronic inflammation, in autoimmune diseases, in a variety of cancers, and in the context of numerous infectious diseases. This short review will describe the basic biology of IL-18 and thereafter address its potential effector and regulatory role in several human disease states including autoimmunity and infection. IL-18, previously known as interferon-gamma (IFN-gamma)-inducing factor, was identified as an endotoxin-induced serum factor that stimulated IFN-gamma production by murine splenocytes [(1) ]. IL-18 was cloned from a murine liver cell cDNA library generated from animals primed with heat-killed Propionibacterium acnes and subsequently challenged with lipopolysaccharide [(2) ]. Nucleotide sequencing of murine IL-18 predicted a precursor polypeptide of 192 amino acids lacking a conventional signal peptide and a mature protein of 157 amino acids. Subsequent cloning of human IL-18 cDNA revealed 65% homology with murine IL-18 [(3) ] and showed that both contain an unusual leader sequence consisting of 35 amino acids at their N terminus.

Regulation of Staphylococcus epidermidis-induced IFN-gamma in whole human blood: the role of endogenous IL-18, IL-12, IL-1, and TNF

Interleukin 12 (IL-12) and IL-18 act synergistically to stimulate interferon gamma (IFN-gamma) production; moreover, IL-1 and tumor necrosis factor (TNF) may also augment IFN-gamma synthesis. We have investigated the relative contributions of these cytokines in the production of IFN-gamma and TNF by the Gram-positive bacterium Staphylococcus epidermidis, using the specific cytokine inhibitors IL-18 binding protein (IL-18BP), IL-1 receptor antagonist (IL-1Ra), anti-IL-12 antibodies (anti-IL-12 Ab), and TNF binding protein. Inhibition of caspase-1 reduced IFN-gamma and IL-1beta levels (by 80 and 67%, respectively) when heat-killed S. epidermidis was added to whole human blood cultures. IL-18BP reduced S. epidermidis-induced IFN-gamma (77% maximal suppression). In contrast, blocking IL-1 receptors by IL-1Ra had no effect on IFN-gamma production. Blocking endogenous IL-12 and TNF reduced IFN-gamma production by 69 and 36%. S. epidermidis-induced TNF-alpha was inhibited by IL-18BP and IL-1Ra, but not anti-IL-12 Ab, whereas IL-8 production was unaffected by any of the specific cytokine blocking agents. In conclusion, S. epidermidis stimulates IFN-gamma which is IL-18, IL-12 and TNF-dependent, but IL-1 independent.

Seeligeriolysin O, a cholesterol-dependent cytolysin of Listeria seeligeri, induces gamma interferon from spleen cells of mice

Seeligeriolysin O (LSO), one of the cholesterol-dependent cytolysins produced by Listeria seeligeri, shows 80% homology to listeriolysin O (LLO) produced by Listeria monocytogenes at the amino acid sequence level. In addition to cytolytic activity, LLO has been shown to exhibit cytokine-inducing activity. In order to determine whether LSO is also capable of exhibiting these two different activities, we constructed a recombinant full-length LSO (rLSO530) and a noncytolytic truncated derivative with a C-terminal deletion (rLSO483) and compared these molecules with recombinant LLO. The cytolytic rLSO530 molecule could induce gamma interferon (IFN-gamma) production in spleen cells when the cytolytic activity was blocked by treatment with cholesterol. The noncytolytic truncated rLSO483 molecule also induced IFN-gamma production. Anti-LLO polyclonal antibody inhibited not only LLO-induced IFN-gamma production but also LSO-induced IFN-gamma production. Both NK cells and CD11b(+) cells were required for LSO-induced IFN-gamma production. Among the various cytokines expressed in CD11b(+) cells, interleukin-12 (IL-12) and IL-18 appeared to be essential. We concluded that LSO exhibits the same biological activity as LLO.

Evolution of interleukin-1beta

All jawed vertebrates possess a complex immune system, which is capable of anticipatory and innate immune responses. Jawless vertebrates possess an equally complex immune system but with no evidence of an anticipatory immune response. From these findings it has been speculated that the initiation and regulation of the immune system within vertebrates will be equally complex, although very little has been done to look at the evolution of cytokine genes, despite well-known biological activities within vertebrates. In recent years, cytokines, which have been well characterised within mammals, have begun to be cloned and sequenced within non-mammalian vertebrates, with the number of cytokine sequences available from primitive vertebrates growing rapidly. The identification of cytokines, which are mammalian homologues, will give a better insight into where immune system communicators arose and may also reveal molecules, which are unique to certain organisms. Work has focussed on interleukin-1 (IL-1), a major mediator of inflammation which initiates and/or increases a wide variety of non-structural, function associated genes that are characteristically expressed during inflammation. Other than mammalian IL-1beta sequences there are now full cDNA sequences and genomic organisations available from bird, amphibian, bony fish and cartilaginous fish, with many of these genes having been obtained using an homology cloning approach. This review considers how the IL-1beta gene has changed through vertebrate evolution and whether its role and regulation are conserved within selected non-mammalian vertebrates.

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.

Correlation of renal tubular epithelial cell-derived interleukin-18 up-regulation with disease activity in MRL-Faslpr mice with autoimmune lupus nephritis

OBJECTIVE

MRL-Fas(lpr) mice spontaneously develop an autoimmune disease that mimics systemic lupus erythematosus in humans. Infiltrating T cells expressing interferon-gamma (IFNgamma) are responsible for the autoimmune kidney destruction in MRL-Fas(lpr) mice, and interleukin-18 (IL-18) released by mononuclear phagocytes stimulates T cells to produce the IFNgamma. Since MRL-Fas(lpr) T cells are characterized by an overexpression of the IL-18 receptor accessory chain, we sought to determine the impact of IL-18 on the progression of lupus nephritis in MRL-Fas(lpr) mice.

METHODS

IL-18 expression in sera and kidney tissues from MRL-Fas(lpr) mice was determined by enzyme-linked immunosorbent assay (ELISA), reverse transcription-polymerase chain reaction (RT-PCR), immunohistochemistry, and Western blotting. IL-18 production by primary cultured tubular epithelial cells (TECs) from MRL-Fas(lpr) and BALB/c mice were examined by RT-PCR, ELISA, and Western blotting. The interactions of TEC-derived IL-18 and MRL-Fas(lpr) T cells were studied in coculture assays. IL-18-related effects on TEC viability and adhesion molecule expression were determined by fluorescence-activated cell sorting and cell proliferation assays.

RESULTS

Up-regulation of mature IL-18 was restricted to nephritic MRL-Fas(lpr) kidneys and increased in parallel with the severity of lupus nephritis. IL-18 expression was not confined to infiltrating monocytes but was primarily detected in TECs. Similarly, interleukin-1beta-converting enzyme expression, which is required for the processing of precursor IL-18, was localized in TECs. De novo synthesis of IL-18 by MRL-Fas(lpr) TECs was confirmed by RT-PCR and Western blotting. Functional assays revealed that activated TECs induced IFNgamma production in MRL-Fas(lpr) T cells through IL-18. IL-18, in turn, increased apoptotic TEC death and up-regulation of intercellular adhesion molecule 1 and vascular cell adhesion molecule 1.

CONCLUSION

Taken together, our findings suggest that IL-18-producing TECs may directly be involved in the pathogenesis of lupus nephritis.

Functional IL-18 Is produced by primary pancreatic mouse islets and NIT-1 beta cells and participates in the progression towards destructive insulitis

BACKGROUND

Preclinical stages of type 1 diabetes are characterized by infiltrating T cells and by the peri- and intra-islet accumulation of pro-inflammatory mediators, such as IFNgamma.

METHODS/RESULTS

Using quantitative PCR we demonstrated that mRNA specific for the IFNgamma-inducing cytokine IL-18 is upregulated in NIT-1 beta cells and intact mouse islets upon exposure to IL-1beta, IFNgamma and TNFalpha. The biological activity of IL-18 was blocked using caspase inhibitors and anti-IL-18 antibodies. Increased IL-18 expression was also detected in islets during advanced stages of insulitis and correlated with elevated transcripts for IFNgamma and for the IL-18 receptor.

CONCLUSION

Thus, beta cells produce bioactive IL-18 in the course of insulitis and actively contribute to the exacerbation of inflammation leading to their own demise.

Modulators of inflammation use nuclear factor-kappa B and activator protein-1 sites to induce the caspase-1 and granzyme B inhibitor, proteinase inhibitor 9

Proteinase inhibitor 9 (PI-9) inhibits caspase-1 (interleukin (IL)-1beta-converting enzyme) and granzyme B, thereby regulating production of the pro-inflammatory cytokine IL-1beta and susceptibility to granzyme B-induced apoptosis. We show that cellular PI-9 mRNA and protein are induced by IL-1beta, lipopolysaccharide, and 12-O-tetradecanoylphorbol-13-acetate. We identified functional imperfect nuclear factor-kappaB (NF-kappaB) sites at -135 and -88 and a consensus activator protein-1 (AP-1) site at -308 in the PI-9 promoter region. Using transient transfections in HepG2 cells to assay PI-9 promoter mutations, we find that mutational ablation of the AP-1 site or of either NF-kappaB site reduces IL-1beta-induced expression of PI-9 by approximately 60%. Mutational ablation of the two NF-kappaB sites and of the AP-1 site nearly abolishes both basal and IL-1beta-induced expression of PI-9. Nuclear extracts from IL-1beta-treated HepG2 cells exhibited strong, IL-1beta-inducible binding to the NF-kappaB sites and to the AP-1 site. Electrophoretic mobility shift assays show that after IL-1beta treatment c-Jun/c-Fos and JunD bind to the AP-1 site, whereas the p50/p65 heterodimer binds to the two NF-kappaB sites. Estrogens induce PI-9, but induction of PI-9 by estrogens and IL-1beta is not synergistic. In transiently transfected, estrogen receptor-positive HepG2ER7 cells, estrogens do not interfere with IL-1beta induction, whereas IL-1beta exhibits dose-dependent repression of estrogen-inducible PI-9 expression. Our surprising finding that the pro-inflammatory cytokine IL-1beta strongly induces PI-9 suggests a novel mechanism for regulating inflammation and apoptosis through a negative feedback loop controlling expression of the anti-inflammatory and anti-apoptotic protein, PI-9.