Generation and characterization of mice transgenic for human IL-18-binding protein isoform a

Faecalibacterium prausnitzii promotes intestinal epithelial IL-18 production through activation of the HIF1α pathway

Introduction

Intestinal epithelial cells produce interleukin-18 (IL-18), a key factor in promoting epithelial barrier integrity. Here, we analyzed the potential role of gut bacteria and the hypoxia-inducible factor 1α (HIF1α) pathway in regulating mucosal IL18 expression in inflammatory bowel disease (IBD).

Methods

Mucosal samples from patients with IBD (n = 760) were analyzed for bacterial composition, IL18 levels and HIF1α pathway activation. Wild-type Caco-2 and CRISPR/Cas9-engineered Caco-2-HIF1A-null cells were cocultured with Faecalibacterium prausnitzii in a "Human oxygen-Bacteria anaerobic" in vitro system and analyzed by RNA sequencing.

Results

Mucosal IL18 mRNA levels correlated positively with the abundance of mucosal-associated butyrate-producing bacteria, in particular F. prausnitzii, and with HIF1α pathway activation in patients with IBD. HIF1α-mediated expression of IL18, either by a pharmacological agonist (dimethyloxallyl glycine) or F. prausnitzii, was abrogated in Caco-2-HIF1A-null cells.

Conclusion

Butyrate-producing gut bacteria like F. prausnitzii regulate mucosal IL18 expression in a HIF1α-dependent manner that may aid in mucosal healing in IBD.

IL-18/IL-18BP and IL-22/IL-22BP: Two interrelated couples with therapeutic potential

Interleukin (IL)-18 and IL-22 are key components of cytokine networks that play a decisive role in (pathological) inflammation, host defense, and tissue regeneration. Tight regulation of cytokine-driven signaling, inflammation, and immunoactivation is supposed to enable nullification of a given deleterious trigger without mediating overwhelming collateral tissue damage or even activating a cancerous face of regeneration. In fact, feedback regulation by specific cytokine opponents is regarded as a major means by which the immune system is kept in balance. Herein, we shine a light on the interplay between IL-18 and IL-22 and their opponents IL-18 binding protein (IL-18BP) and IL-22BP in order to provide integrated information on their biology, pathophysiological significance, and prospect as targets and/or instruments of therapeutic intervention.

Inherited IL-18BP deficiency in human fulminant viral hepatitis

Fulminant viral hepatitis (FVH) is a devastating and unexplained condition that strikes otherwise healthy individuals during primary infection with common liver-tropic viruses. We report a child who died of FVH upon infection with hepatitis A virus (HAV) at age 11 yr and who was homozygous for a private 40-nucleotide deletion in IL18BP, which encodes the IL-18 binding protein (IL-18BP). This mutation is loss-of-function, unlike the variants found in a homozygous state in public databases. We show that human IL-18 and IL-18BP are both secreted mostly by hepatocytes and macrophages in the liver. Moreover, in the absence of IL-18BP, excessive NK cell activation by IL-18 results in uncontrolled killing of human hepatocytes in vitro. Inherited human IL-18BP deficiency thus underlies fulminant HAV hepatitis by unleashing IL-18. These findings provide proof-of-principle that FVH can be caused by single-gene inborn errors that selectively disrupt liver-specific immunity. They also show that human IL-18 is toxic to the liver and that IL-18BP is its antidote.

Mechanisms by Which the Gut Microbiota Influences Cytokine Production and Modulates Host Inflammatory Responses

The gastrointestinal tract encounters a wide variety of microorganisms, including beneficial symbionts, pathobionts, and pathogens. Recent evidence has shown that the gut microbiota, directly or indirectly through its components, such as metabolites, actively participates in the host inflammatory response by cytokine-microbiota or microbiota-cytokine modulation interactions, both in the gut and systemically. Therefore, further elucidation of host cytokine molecular pathways and microbiota components will provide a novel and promising therapeutic approach to control or prevent inflammatory disease and to maintain host homeostasis. The purpose of this review is to summarize well-established scientific findings and provide an updated overview regarding the direct and indirect mechanisms by which the gut microbiota can influence the inflammatory response by modulating the host's cytokine pathways that are mostly involved, but not exclusively so, with gut homeostasis. In addition, we will highlight recent results from our group, which suggest that the microbiota promotes cytokine release from inflammatory cells though activation of microbial metabolite sensor receptors that are more highly expressed on inflammatory and intestinal epithelial cells.

Inflammasome-derived cytokine IL18 suppresses amyloid-induced seizures in Alzheimer-prone mice

Alzheimer's disease (AD) is characterized by the progressive destruction and dysfunction of central neurons. AD patients commonly have unprovoked seizures compared with age-matched controls. Amyloid peptide-related inflammation is thought to be an important aspect of AD pathogenesis. We previously reported that NLRP3 inflammasome KO mice, when bred into APPswe/PS1ΔE9 (APP/PS1) mice, are completely protected from amyloid-induced AD-like disease, presumably because they cannot produce mature IL1β or IL18. To test the role of IL18, we bred IL18KO mice with APP/PS1 mice. Surprisingly, IL18KO/APP/PS1 mice developed a lethal seizure disorder that was completely reversed by the anticonvulsant levetiracetam. IL18-deficient AD mice showed a lower threshold in chemically induced seizures and a selective increase in gene expression related to increased neuronal activity. IL18-deficient AD mice exhibited increased excitatory synaptic proteins, spine density, and basal excitatory synaptic transmission that contributed to seizure activity. This study identifies a role for IL18 in suppressing aberrant neuronal transmission in AD.

Epithelial IL-18 Equilibrium Controls Barrier Function in Colitis

The intestinal mucosal barrier controlling the resident microbiome is dependent on a protective mucus layer generated by goblet cells, impairment of which is a hallmark of the inflammatory bowel disease, ulcerative colitis. Here, we show that IL-18 is critical in driving the pathologic breakdown of barrier integrity in a model of colitis. Deletion of Il18 or its receptor Il18r1 in intestinal epithelial cells (Δ/EC) conferred protection from colitis and mucosal damage in mice. In contrast, deletion of the IL-18 negative regulator Il18bp resulted in severe colitis associated with loss of mature goblet cells. Colitis and goblet cell loss were rescued in Il18bp(-/-);Il18r(Δ/EC) mice, demonstrating that colitis severity is controlled at the level of IL-18 signaling in intestinal epithelial cells. IL-18 inhibited goblet cell maturation by regulating the transcriptional program instructing goblet cell development. These results inform on the mechanism of goblet cell dysfunction that underlies the pathology of ulcerative colitis.

Effect of IL-18 binding protein on hepatic ischemia-reperfusion injury induced by infrarenal aortic occlusion

PURPOSE

Severe local and systemic tissue damage called ischemia/reperfusion (IR) injury occurs during the period of reperfusion. Free oxygen radicals and proinflammatory cytokines are responsible for reperfusion injury. IL-18 binding protein (IL-18BP) is a natural inhibitor of IL-18. The balance between IL-18 and IL-18BP has an important role in the inflammatory setting. The present study aimed to investigate whether IL-18BP had a protective role in remote organ hepatic IR injury.

METHODS

Wistar-Albino rats were divided into three groups that contained seven rats. Group I (sham): Laparotomy and infrarenal abdominal aorta (AA) dissection were done but no clamping was done. Group II (I/R): The infrarenal AA was clamped by atraumatic microvascular clamp for 30 minutes and then was exposed to 90 minutes of reperfusion. Group III (IR + IL-18BP): 75 µg/kg of IL-18BP in 0.9% saline (1 mL) was administered 30 minutes before infrarenal AA dissection and clamping; 30 minutes of ischemia was applied and then was exposed to 90 minutes of reperfusion.

RESULTS

Serum AST, ALT, and LDH levels were remarkably higher in IR group and returned to normal levels in treatment group. The proinflammatory cytokine levels had decreased in treatment group, and was statistically significant compared with the IR group. Serum levels of total oxidant status and oxidative stress index decreased and levels of total antioxidant status increased by IL-18BP.

CONCLUSION

This study suggested that IL-18BP has antioxidant, anti-inflammatory and hepatoprotective effects in cases of IR with infrarenal AA induced liver oxidative damage.

Cross Talk Between Vascular Smooth Muscle Cells and Monocytes Through Interleukin-1β/Interleukin-18 Signaling Promotes Vein Graft Thickening

Objective—

Interleukin (IL)-1β and IL-18 are key proinflammatory cytokines that play important roles in the pathophysiology of vein graft remodeling. However, the mechanism of IL-1β/IL-18 production and its role in the development of graft remodeling remain unclear.

Approach and Results—

IL-1β/IL-18 were rapidly expressed in venous interposition grafts. Vascular smooth muscle cell (VSMC) death and monocytic inflammasome activation occurred in grafted veins. Necrotic VSMCs induced the expression of IL-1β, IL-18, and other inflammasome-associated proteins in monocytes, which was partially inhibited by their antagonist, recombinant IL-1ra-Fc-IL-18bp. Activated monocytes stimulated proliferation of VSMCs by activating cell growth–related signaling molecules (AKT, STAT3, ERK1/2, and mTOR [AKT/protein kinase B, signal transducer and activator of transcription 3, extracellular signal-regulated kinase 1/2, mammalian target of rapamycin]) and increasing production of platelet-derived growth factor-bb; these effects were suppressed by IL-1ra-Fc-IL-18bp. Activated monocytes also promoted migration of VSMCs, which was independent of IL-1β/IL-18 signaling. Importantly, administration of IL-1ra-Fc-IL-18bp inhibited activation of cell growth–related signaling molecules, VSMC proliferation, and vein graft thickening in vivo.

Conclusions—

Our work identified an interaction among necrotic VSMCs, monocytes, and viable VSMCs through IL-1β/IL-18 signaling, which might be exploited as a therapeutic target in vein graft remodeling.

Interleukin-18, more than a Th1 cytokine

Together with IL-12 or IL-15, interleukin-18 (IL-18) plays a major role in the production of interferon-γ from T-cells and natural killer cells; thus, IL-18 is considered to have a major role in the Th1 response. However, without IL-12, IL-18 is proinflammatory in an IFNγ independent manner. IL-18 is a member of the IL-1 family of cytokines and similar to IL-1β, the cytokine is synthesized as an inactive precursor requiring processing by caspase-1 into an active cytokine. IL-18 is also present as an integral membrane protein but requires caspase-1 for full activity in order to induce IFNγ. Uniquely, unlike IL-1β, the IL-18 precursor is constitutively present in nearly all cells in healthy humans and animals. The activity of IL-18 is balanced by the presence of a high-affinity, naturally occurring IL-18 binding protein (IL-18BP). In humans, increased disease severity can be associated with an imbalance of IL-18 to IL-18BP such that the levels of free IL-18 are elevated in the circulation. Increasing number of studies have expanded the role of IL-18 in mediating inflammation in animal models of disease using the IL-18BP, IL-18 deficient mice, neutralization of IL-18 or deficiency in the IL-18 receptor alpha chain. A role for IL-18 has been implicated in several autoimmune diseases, myocardial function, emphysema, metabolic syndromes, psoriasis, inflammatory bowel disease, macrophage activation syndrome, sepsis and acute kidney injury, although paradoxically, in some models of disease, IL-18 is protective. The IL-18BP has been used safely in humans and clinical trials of IL-18BP as well as neutralizing anti-IL-18 antibodies are being tested in various diseases.

IL-18 induces profibrotic renal tubular cell injury via STAT3 activation

IL-18 is an important mediator of obstruction-induced renal fibrosis and renal tubular epithelial cell (TEC) injury. IL-18's proinflammatory properties have been attributed, in part, to NF-κB activation and the stimulation of cytokine gene expression; however, STAT3 has increasingly been shown to mediate renal fibrotic injury. We therefore hypothesized that IL-18 mediates profibrotic TEC injury via STAT3 activation. Male C57BL6 wild-type mice and transgenic mice for human IL-18-binding protein were subjected to unilateral ureteral obstruction or sham operation. The kidneys were harvested 1 or 2 wk afterward and analyzed for active STAT3 (p-STAT3) expression (Western blotting, immunohistochemistry) and suppressor of cytokine signaling 3 (SOCS3) expression. In a separate arm, renal tubular cells (HK-2) were directly stimulated with IL-18 for 2 days with or without the STAT3 inhibitor S3I-201 (50 μM). Cell lysates were then analyzed for p-STAT3 and SOCS3 expression, profibrotic cellular changes (collagen and α-SMA expression), and tubular cell apoptosis. p-STAT3 and SOCS3 expression increased significantly in response to obstruction; however, a significant reduction in p-STAT3 and SOCS3 expression occurred following 1 wk, but not 2 wk, of obstruction in the presence of IL-18 neutralization. In vitro results similarly demonstrate increased p-STAT3, SOCS3, α-SMA, and collagen III expression, and increased collagen production and TEC apoptosis in response to IL-18 stimulation, but the response was significantly diminished in the presence of STAT3 inhibition. These results demonstrate that IL-18-induces profibrotic cellular changes and collagen production in TECs via STAT3 activation.

Interleukin-18 binding protein therapy is protective in adriamycin nephropathy

Adriamycin nephropathy (AN) is an experimental model of focal segmental glomerulosclerosis (FSGS) in which macrophages are considered to play a pathogenic role. We hypothesize that interleukin-18 (IL-18), largely derived from macrophages, is a key contributor to kidney injury in AN and a potential therapeutic target. In this study, BALB/c mice received adriamycin (9.6 mg/kg) via tail vein injection and subsequently were treated with either neutralizing IL-18 binding protein (IL-18BP; 250 μg) or normal saline (control). At 5 wk, IL-18 was upregulated in AN, and IL-18BP therapy afforded significant protection against the development of AN, resulting in less proteinuria (P < 0.01), kidney dysfunction (P < 0.01), glomerulosclerosis (P < 0.001), and interstitial accumulation of macrophages and T cells (P < 0.001). Gene expression of IL-18 downstream inflammatory molecules, including inducible nitric oxide synthase (P < 0.001), TNF-α (P < 0.001), and IFN-γ (P < 0.01); IL-17 (P < 0.001) and the chemokines CCL2 (P < 0.01) and CCL5 (P < 0.005), was reduced. We demonstrate that IL-18 plays a significant role in the pathogenesis of AN. The protective effect of IL-18BP therapy illustrates the importance of immune mediators in chronic proteinuric kidney disease and highlights the potential of IL-18BP therapy.

Profibrotic effect of interleukin-18 in HK-2 cells is dependent on stimulation of the Toll-like receptor 4 (TLR4) promoter and increased TLR4 expression

BACKGROUND

IL-18 induces profibrotic changes in TECs independent of TGF-β1 activity.

RESULTS

IL-18 stimulates the TLR4 promoter via AP-1 activation to increase TLR4 expression in TECs and stimulates profibrotic changes in TECs through increased TLR4 expression/signaling.

CONCLUSION

The profibrotic effect of IL-18 in TECs is mediated through stimulation of TLR4 expression via activation of AP-1.

SIGNIFICANCE

This represents a novel fibrotic signaling pathway in TECs independent of TGF-β1. IL-18 is an important mediator of obstruction-induced renal fibrosis and tubular epithelial cell injury independent of TGF-β1 activity. We sought to determine whether the profibrotic effect of IL-18 is mediated through Toll-like receptor 4 (TLR4). Male C57BL6 wild type and mice transgenic for human IL-18-binding protein were subjected to left unilateral ureteral obstruction versus sham operation. The kidneys were harvested 1 week postoperatively and analyzed for IL-18 production and TLR4 expression. In a separate arm, renal tubular epithelial cells (HK-2) were directly stimulated with IL-18 in the presence or absence of a TLR4 agonist, TLR4 antagonist, or TLR4 siRNA knockdown. Cell lysates were analyzed for TLR4, α-smooth muscle actin, and E-cadherin expression. TLR4 promotor activity, as well as AP-1 activation and the effect of AP-1 knockdown on TLR4 expression, was evaluated in HK-2 cells in response to IL-18 stimulation. The results demonstrate that IL-18 induces TLR4 expression during unilateral ureteral obstruction and induces TLR4 expression in HK-2 cells via AP-1 activation. Inhibition of TLR4 or knockdown of TLR4 gene expression in turn prevents IL-18-induced profibrotic changes in HK-2 cells. These results suggest that IL-18 induces profibrotic changes in tubular epithelial cells via increased TLR4 expression/signaling.

Cytosolic flagellin receptor NLRC4 protects mice against mucosal and systemic challenges

Bacterial flagellin is a dominant innate immune activator of the intestine. Therefore, we examined the role of the intracellular flagellin receptor, NLRC4, in protecting the gut and/or driving inflammation. In accordance with NLRC4 acting through transcription-independent pathways, loss of NLRC4 did not reduce the rapid robust changes in intestinal gene expression induced by flagellin administration. Loss of NLRC4 did not alter basal intestinal homeostasis nor predispose mice to development of colitis upon administration of an anti-interleukin (IL)-10R monoclonal antibody. However, epithelial injury induced by dextran sulfate sodium in mice lacking NLRC4 resulted in a more severe disease, indicating a role for NLRC4 in protecting the gut. Moreover, loss of NLRC4 resulted in increased mortality in response to flagellate, but not aflagellate Salmonella infection. Thus, despite not being involved in rapid intestinal gene remodeling upon detection of flagellin, NLRC4-mediated inflammasome activation results in production of IL-1β and IL-18, two cytokines that protect mice from mucosal and systemic challenges.

β2 adrenergic activation induces the expression of IL-18 binding protein, a potent inhibitor of isoproterenol induced cardiomyocyte hypertrophy in vitro and myocardial hypertrophy in vivo

Both the sympathetic nervous system and the proinflammatory cytokine interleukin-18 (IL-18) play key roles in the pathophysiology of the hypertrophied failing heart. IL-18 binding protein (IL-18BP), a natural inhibitor of IL-18, counters its biological effects. β-AR stimulation induces IL-18 expression, but whether it also regulates IL-18BP is not known. Here we demonstrate that the β-AR agonist isoproterenol (ISO) increases steady state IL-18BP mRNA and protein levels in adult mouse cardiomyocytes in a β(2)-AR-dependent manner. We cloned mouse Il18bp 5'cis-regulatory region, and identified putative CREB and C/EBPβ transcription factor-binding sites. Forced expression of mutant CREB or C/EBPβ knockdown markedly attenuated ISO-induced Il18bp transcription and deletion or mutation of CREB and C/EBP motifs in the Il18bp promoter reduced ISO-induced promoter-reporter gene activity. ISO induced CREB and C/EBPβ activation in cardiomyocytes via PI3K/Akt and ERK1/2. Importantly, ISO-induced hypertrophy in vitro was dependent on IL-18 induction as it was blunted by IL-18 neutralizing antibodies and forced expression of IL-18BP. Moreover, ISO-induced hypertrophy was markedly attenuated in IL-18 null and IL-18BP transgenic mice. These data support the novel concept that β-AR activation, in addition to inducing cardiomyocyte hypertrophy via IL-18, concomitantly induces a countering effect by stimulating IL-18BP expression, and that ISO-induced cardiomyocyte hypertrophy may result from a net effect of IL-18 and IL-18BP induction.

IL-1α and IL-1β recruit different myeloid cells and promote different stages of sterile inflammation

The immune system has evolved to protect the host from invading pathogens and to maintain tissue homeostasis. Although the inflammatory process involving pathogens is well documented, the intrinsic compounds that initiate sterile inflammation and how its progression is mediated are still not clear. Because tissue injury is usually associated with ischemia and the accompanied hypoxia, the microenvironment of various pathologies involves anaerobic metabolites and products of necrotic cells. In the current study, we assessed in a comparative manner the role of IL-1α and IL-1β in the initiation and propagation of sterile inflammation induced by products of hypoxic cells. We found that following hypoxia, the precursor form of IL-1α, and not IL-1β, is upregulated and subsequently released from dying cells. Using an inflammation-monitoring system consisting of Matrigel mixed with supernatants of hypoxic cells, we noted accumulation of IL-1α in the initial phase, which correlated with the infiltration of neutrophils, and the expression of IL-1β correlated with later migration of macrophages. In addition, we were able to show that IL-1 molecules from cells transfected with either precursor IL-1α or mature IL-1β can recruit neutrophils or macrophages, respectively. Taken together, these data suggest that IL-1α, released from dying cells, initiates sterile inflammation by inducing recruitment of neutrophils, whereas IL-1β promotes the recruitment and retention of macrophages. Overall, our data provide new insight into the biology of IL-1 molecules as well as on the regulation of sterile inflammation.

IL-18 mediates proapoptotic signaling in renal tubular cells through a Fas ligand-dependent mechanism

Renal tubular cell apoptosis is a significant component of obstruction-induced renal injury, and it results in a progressive loss in renal parenchymal mass during renal obstruction. Although IL-18 is an important mediator of inflammatory renal disease and renal fibrosis, its role in obstruction-induced renal tubular cell apoptosis remains unclear. To study this, male C57BL6 wild-type mice and C57BL6 mice transgenic for human IL-18-binding protein (IL-18BP Tg) were subjected to renal obstruction vs. sham operation. The kidneys were harvested after 1 or 2 wk and analyzed for IL-18 production, apoptosis, caspase activity, and Fas/Fas Ligand (FasL) expression. HK-2 cells were similarly analyzed for apoptosis and proapoptotic signaling following 3 days of direct exposure to IL-18 vs. control media. Renal obstruction induced a significant increase in IL-18 production, renal tubular cell apoptosis, caspase activation, and FasL expression. IL-18 neutralization, on the other hand, significantly reduced obstruction-induced apoptosis, caspase-8 and caspase-3 activity, and FasL expression. In vitro experiments similarly demonstrate that IL-18 stimulation induces apoptosis, FasL expression, and increases active caspase-8 and caspase-3 expression in a dose-dependent fashion. siRNA knockdown of FasL gene expression, however, significantly reduced IL-18-induced apoptosis. This study reveals that IL-18 is a significant mediator of obstruction-induced tubular cell apoptosis, and it demonstrates that IL-18 stimulates proapoptotic signaling through a FasL-dependent mechanism.

The oral histone deacetylase inhibitor ITF2357 reduces cytokines and protects islet β cells in vivo and in vitro

In type 1 diabetes, inflammatory and immunocompetent cells enter the islet and produce proinflammatory cytokines such as interleukin-1β (IL-1β), IL-12, tumor necrosis factor-α (TNFα) and interferon-γ (IFNγ); each contribute to β-cell destruction, mediated in part by nitric oxide. Inhibitors of histone deacetylases (HDAC) are used commonly in humans but also possess antiinflammatory and cytokine-suppressing properties. Here we show that oral administration of the HDAC inhibitor ITF2357 to mice normalized streptozotocin (STZ)-induced hyperglycemia at the clinically relevant doses of 1.25-2.5 mg/kg. Serum nitrite levels returned to nondiabetic values, islet function improved and glucose clearance increased from 14% (STZ) to 50% (STZ + ITF2357). In vitro, at 25 and 250 nmol/L, ITF2357 increased islet cell viability, enhanced insulin secretion, inhibited MIP-1α and MIP-2 release, reduced nitric oxide production and decreased apoptosis rates from 14.3% (vehicle) to 2.6% (ITF2357). Inducible nitric oxide synthase (iNOS) levels decreased in association with reduced islet-derived nitrite levels. In peritoneal macrophages and splenocytes, ITF2357 inhibited the production of nitrite, as well as that of TNFα and IFNγ at an IC(50) of 25-50 nmol/L. In the insulin-producing INS cells challenged with the combination of IL-1β plus IFNγ, apoptosis was reduced by 50% (P < 0.01). Thus at clinically relevant doses, the orally active HDAC inhibitor ITF2357 favors β-cell survival during inflammatory conditions.

Development of isoform-specific monoclonal antibodies against human IL-18 binding protein

Interleukin-18 binding protein (IL-18BP) is a soluble antagonist of IL-18 originally discovered while attempting to isolate a soluble receptor by using IL-18-ligand affinity column. IL-18BP has four isoforms (a, b, c, and d) in humans and two isoforms (c and d) in mice. The human isoforms IL-18BPa and IL-18BPc neutralize IL-18 activity sufficiently at an equimolar ratio; however IL-18BPb and IL-18BPd isoforms lack a complete Ig domain at C-terminus and lose the ability to neutralize IL-18 activity. Mouse IL-18BPc and IL-18BPd isoforms, possessing a similar complete Ig domain, also neutralize the biological activity of mouse IL-18 at an equimolar ratio. Here we expressed recombinant proteins of the active human IL-18BP isoforms and developed monoclonal antibodies (MAbs) against human IL-18BP a and c isoforms. We obtained two MAbs (78-4 and 38-3) of human IL-18BPa and two MAbs (18-7 and 29-6) of human IL-18BPc. The MAb clones 18-7 and 29-6 specifically recognized recombinant IL-18BPc in Western blot analyses and ELISA, whereas the MAb clone 78-4 recognized both isoforms in Western blot analyses, but only human IL-18BPa isoform in ELISA. We developed a sandwich ELISA by using the monoclonal antibody specific to human IL-18BPa isoform. The isoform-specific anti-human IL-18BP MAb may be a useful tool in categorizing a distinct group of patients from various autoimmune diseases related to IL-18BP.

Inflammation in the central nervous system and Th17 responses are inhibited by IFN-gamma-Induced IL-18 binding protein

Inflammatory responses are essential for immune protection but may also cause pathology and must be regulated. Both Th1 and Th17 cells are implicated in the pathogenesis of autoimmune inflammatory diseases, such as multiple sclerosis. We show in this study that IL-18-binding protein (IL-18bp), the endogenous inhibitor of the Th1-promoting cytokine IL-18, is upregulated by IFN-gamma in resident microglial cells in the CNS during multiple sclerosis-like disease in mice. Test of function by overexpression of IL-18bp in the CNS using a viral vector led to marked reduction in Th17 responses and robust inhibition of incidence, severity, and histopathology of disease, independently of IFN-gamma. The disease-limiting action of IL-18bp included suppression of APC-derived Th17-polarizing cytokines. IL-18bp thus acts as a sensor for IFN-gamma and can regulate both Th1 and Th17 responses in the CNS.

Interleukin-18 exacerbates pulmonary injury after hepatic ischemia/reperfusion in mice

BACKGROUND

Hepatic ischemia/reperfusion has been shown to cause both local hepatic and distant organ (such as lung) injury caused by accumulation of neutrophils in the local and distant organs, leading to neutrophil-dependent organ injury. Interleukin (IL) -18 is required for facilitating neutrophil-dependent local hepatic injury by suppressing anti-inflammatory cytokine expression, but less is known about the involvement of this cytokine in distant organ injury. The objective of this study was to determine whether IL-18 contributes to pulmonary injury induced by hepatic ischemia/reperfusion.

METHODS

C57BL/6 mice and IL-18 knockout mice (C57BL/6 background) were subjected to 90 min of partial hepatic ischemia and subsequent reperfusion. Neutrophil accumulation in the lung was assessed by pulmonary myeloperoxidase contents. Pulmonary expressions of keratinocyte derived chemokine (KC, CXCL1), macrophage chemoattractant protein-1 (MCP-1, CCL2), tumor necrosis factor-alpha, interferon-gamma, IL-4, and IL-10 were measured by tissue enzyme-linked immunosorbent assay (ELISA). Lung edema was quantified by the pulmonary wet to dry weight ratios.

RESULTS

Hepatic ischemia/reperfusion caused significant increases in pulmonary neutrophil recruitment and lung edema. Also, pulmonary expression of KC and MCP-1 were up-regulated. In the IL-18 knockout mice, hepatic ischemia/reperfusion-induced increases in pulmonary neutrophil recruitment, lung injury defined by lung edema, and pulmonary chemokine expression were attenuated. Furthermore, pulmonary expression of an anti-inflammatory cytokine IL-4 and systemic IL-10 expression were significantly up-regulated in the IL-18 knockout mice.

CONCLUSIONS

The data suggested that IL-18 plays an important role in the development of pulmonary injury after hepatic ischemia/reperfusion by up-regulating proinflammatory mediators and possibly suppressing anti-inflammatory cytokine expression.

IL-18 binding protein-expressing mesenchymal stem cells improve myocardial protection after ischemia or infarction

IL-18 is a proinflammatory cytokine known to cause tissue injury by inducing inflammation and cell death. Increased levels of IL-18 are associated with myocardial injury after ischemia or infarction. IL-18-binding protein (IL-18BP), the naturally occurring inhibitor of IL-18 activity, decreases the severity of inflammation in response to injury. In the present study, mesenchymal stem cells (MSCs) derived from mice transgenic for over expression of human IL-18BP were tested in rat models of global myocardial ischemia and acute myocardial infarction. Improved myocardial function is associated with production of VEGF, and in vitro, IL-18BP MSCs secreted higher levels of constitutive VEGF compared to wild-type MSCs. Whereas IL-18 increased cell death and reduced VEGF in wild-type MSCs, IL-18BP MSCs were protected. In an isolated heart model, intracoronary infusion of IL-18BP MSCs before ischemia increased postischemic left ventricular (LV) developed pressure to 79.5 + or - 9.47 mmHg compared to 59.3 + or - 7.8 mmHg in wild-type MSCs and 37.8 + or - 5 mmHg in the vehicle group. Similarly, using a coronary artery ligation model, intramyocardial injection of IL-18BP MSCs improved LV ejection fraction to 67.8 + or - 1.76% versus wild-type MSCs (57.4 + or - 1.33%) and vehicle (39.2 + or - 2.07%), increased LV fractional shortening 1.25-fold over wild-type MSCs and 1.95-fold over vehicle, decreased infarct size to 38.8 + or - 2.16% compared to 46.4 + or - 1.92% in wild-type MSCs and 60.7 + or - 2.2% in vehicle, reduced adverse ventricular remodeling, increased myocardial VEGF production, and decreased IL-6 levels. This study provides the concept that IL-18BP genetically modified stem cells improve cardioprotection over that observed with unmodified stem cells.

Macrophages are not the source of injurious interleukin-18 in ischemic acute kidney injury in mice

We previously reported in ischemic acute kidney injury (AKI) in mice that caspase-1-mediated production of interleukin-18 (IL-18) is pathogenic and that macrophage depletion by liposome-encapsulated clodronate (LEC) is protective. Therefore, our aim was to determine whether macrophages are a source of IL-18 in ischemic AKI in mice. On immunofluorescence staining of the outer stripe of outer medulla, the number of macrophages double stained for CD11b and IL-18 was significantly increased in AKI and significantly decreased by LEC. Adoptive transfer of RAW 264.7 cells, a mouse macrophage line that constitutively expresses IL-18 mRNA, reversed the functional protection against AKI in both LEC-treated wild-type and caspase-1 -/- mice. To test whether IL-18 in macrophages is necessary to cause AKI, we adoptively transferred macrophages in which IL-18 was inhibited. Peritoneal macrophages isolated from wild-type mice, IL-18 binding protein transgenic (IL-18 BP Tg) mice, and IL-18 -/- mice were used. IL-18 BP Tg mice overexpress human IL-18 BP and exhibit decreased biological activity of IL-18. Adoptive transfer of peritoneal macrophages from wild-type as well as IL-18 BP Tg and IL-18 -/- mice reversed the functional protection against AKI in LEC-treated mice. In summary, adoptive transfer of RAW cells, that constitutively express IL-18, reverses the functional protection in macrophage-depleted wild-type and caspase-1 -/- mice with AKI. However, adoptive transfer of peritoneal macrophages in which IL-18 function was inhibited also reverses the functional protection in macrophage-depleted mice. In conclusion, IL-18 from adoptive transfer of macrophages is not sufficient to cause ischemic AKI.

Neutralization of interleukin-18 ameliorates ischemia/reperfusion-induced myocardial injury

Ischemia/reperfusion (I/R) injury is characterized by the induction of oxidative stress and proinflammatory cytokine expression. Recently demonstrating that oxidative stress and TNF-alpha each stimulate interleukin (IL)-18 expression in cardiomyocytes, we hypothesized that I/R also induces IL-18 expression and thus exacerbates inflammation and tissue damage. Neutralization of IL-18 signaling should therefore diminish tissue injury following I/R. I/R studies were performed using a chronically instrumented closed chest mouse model. Male C57BL/6 mice underwent 30 min of ischemia by LAD coronary artery ligation followed by various periods of reperfusion. Sham-operated or ischemia-only mice served as controls. A subset of animals was treated with IL-18-neutralizing antibodies 1 h prior to LAD ligation. Ischemic LV tissue was used for analysis. Our results demonstrate that, compared with sham operation and ischemia alone, I/R significantly increased (i) oxidative stress (increased MDA/4-HNE levels), (ii) neutrophil infiltration (increased MPO activity), (iii) NF-kappaB DNA binding activity (p50, p65), and (iv) increased expression of IL-18Rbeta, but not IL-18Ralpha or IL-18BP transcripts. Administration of IL-18-neutralizing antibodies significantly reduced I/R injury measured by reduced infarct size (versus control IgG). In isolated adult mouse cardiomyocytes, simulated ischemia/reperfusion enhanced oxidative stress and biologically active IL-18 expression via IKK-dependent NF-kappaB activation. These results indicate that IL-18 plays a critical role in I/R injury and thus represents a promising therapeutic target.

IL-18 contributes to renal damage after ischemia-reperfusion

IL-18 is a proinflammatory cytokine produced by macrophages and other cell types present in the kidney during ischemia-reperfusion injury (IRI), but its role in this injury is unknown. Here, compared with wild-type mice, IL-18(-/-) mice subjected to kidney IRI demonstrated better kidney function, less tubular damage, reduced accumulation of neutrophils and macrophages, and decreased expression of proinflammatory molecules that are downstream of IL-18. For determination of the relative contributions of leukocytes and parenchymal cells to IL-18 production and subsequent kidney damage during IRI, bone marrow-chimeric mice were generated. Wild-type mice engrafted with IL-18(-/-) hemopoietic cells showed less kidney dysfunction and tubular damage than IL-18(-/-) mice engrafted with wild-type bone marrow. In vitro, macrophages produced IL-18 mRNA and protein in response to ischemia. These data suggest bone marrow-derived cells are the key contributors to IL-18-mediated effects of renal IRI. Finally, similar to IL-18(-/-) mice, pretreatment of wild-type mice with IL-18-binding protein was renoprotective in this model of IRI. In conclusion, IL-18, derived primarily from cells of bone marrow origin, contributes to the renal damage observed during IRI. IL-18-binding protein may have potential as a renoprotective therapy.

Interleukin-18 binding protein transgenic mice are protected against ischemic acute kidney injury

IL-18 function is neutralized in IL-18 binding protein transgenic (IL-18BP Tg) mice. First, we determined whether IL-18BP Tg mice are protected against ischemic acute kidney injury (AKI). Ischemic AKI was induced by bilateral renal pedicle clamping. IL-18BP Tg mice were functionally and histologically protected against ischemic AKI as determined by blood urea nitrogen, serum creatinine, and acute tubular necrosis score. We have demonstrated that the injurious effect of IL-18 in the kidney is independent of neutrophils and lymphocytes. Thus the effect of IL-18 inhibition on renal macrophage infiltration was determined. The number of macrophages was significantly reduced in IL-18BP Tg compared with wild-type kidneys. To determine the cytokines and chemokines that are dependent on IL-18, we performed flow cytometry based assays. Multiple chemokines/cytokines, IL-3, IL-6, IL-15, IL-18, leukemia inhibitory factor, macrophage colony-stimulating factor, macrophage inflammatory protein-2, granulocyte-macrophage colony-stimulating factor, and monocyte chemotactic protein-1 were significantly increased in AKI vs. sham kidneys. Only CXCL1 (also known as KC or IL-8) was significantly increased in AKI vs. sham kidneys and significantly reduced in IL-18BP Tg AKI vs. wild-type AKI kidneys. To determine whether macrophages are the source of CXCL1 in the kidney, we depleted macrophages with liposomal encapsulated clodronate. CXCL1 was significantly decreased in macrophage-depleted vs. control AKI mice. In summary, in ischemic AKI in mice, 1) IL-18BP Tg mice are functionally and histologically protected, 2) macrophage infiltration in the kidney and CXCL1 are significantly reduced in IL-18BP Tg mice, and 3) macrophage depletion significantly reduces CXCL1 in the kidney. In conclusion, protection against ischemic AKI in IL-18BP Tg mice is associated with less macrophage infiltration and less production of CXCL1 in the kidney.

Molecular mechanisms of soluble cytokine receptor generation

Soluble cytokine receptors play key roles in regulating cytokine-mediated biological events by binding and modulating the activity of target ligands in either an antagonistic or agonistic fashion. This Minireview will provide an overview of the molecular mechanisms mediating the generation of soluble cytokine receptors, which include sheddase-mediated proteolytic cleavage of cell-surface receptors, generation of soluble receptors by alternative gene splicing, transcription and translation of cytokine-binding genes, and extracellular release of membrane-bound receptors within vesicles such as exosomes.

Interleukin-18: a pro-inflammatory cytokine that plays an important role in acute pancreatitis

A large body of clinical and experimental evidence suggests that cytokines play a key role in the pathogenesis of local and systemic complications of acute pancreatitis. IL-18 is a pro-inflammatory cytokine that plays a key role in many human diseases, including acute pancreatitis. This review focuses on the present understanding in IL-18 and its potential role in acute pancreatitis. IL-18 levels reflect the severity of acute pancreatitis and display a significant negative correlation with the concentrations of antioxidative damage factors, serum selenium and glutathione peroxidases (GPx). The relationship between IL-18 and other pro-inflammatory cytokines shows that IL-18 is one of the key mediators of inflammation in the pathogenesis of acute pancreatitis. Elevation of serum IL-18 levels may mediate acute pancreatitis associated liver injury. The use of IL-18 antagonists as direct routes to block IL-18 activity and P2X7 receptor antagonists and interleukin-1beta-converting enzyme (ICE) inhibitors as indirect routes to block IL-18 activity suggest that specific therapeutic inhibition of IL-18 is a promising therapeutic approach for acute pancreatitis.

Gene transfer of interleukin-18-binding protein attenuates cardiac allograft rejection

Interleukin (IL) 18 is a potent pro-inflammatory Th1 cytokine that exerts pleiotropic effector functions in both innate and acquired immune responses. Increased IL-18 production during acute rejection has been reported in experimental heart transplantation models and in kidney transplant recipients. IL-18-binding protein (IL-18BP) binds IL-18 with high affinity and neutralizes its biologic activity. We have analyzed the efficacy of an adenoviral vector expressing an IL-18BP-Ig fusion protein in a rat model of heart transplantation. IL-18BP-Ig gene transfer into Fisher (F344) rat donor hearts resulted in prolonged graft survival in Lewis recipients (15.8 +/- 1.4 days vs. 10.3 +/- 2.5 and 10.1 +/- 2.1 days with control virus and buffer solution alone, respectively; P < 0.001). Immunohistochemical analysis revealed decreased intra-graft infiltrates of monocytes/macrophages, CD4(+), CD8alpha(+) and T-cell receptor alphabeta(+) cells after IL-18BP-Ig versus mock gene transfer (P < 0.05). Real-time reverse transcriptase polymerase chain reaction analysis showed decreased cytokine transcripts for the RANTES chemokine and transforming growth factor-beta after IL-18BP-Ig gene transfer (P < 0.05). IL-18BP-Ig gene transfer attenuates inflammatory cell infiltrates and prolongs cardiac allograft survival in rats. These results suggest a contributory role for IL-18 in acute rejection. Further studies aiming at defining the therapeutic potential of IL-18BP are warranted.

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.

Cisplatin-induced acute renal failure is associated with an increase in the cytokines interleukin (IL)-1beta, IL-18, IL-6, and neutrophil infiltration in the kidney

We have demonstrated that caspase-1-deficient (caspase-1(-/-)) mice are functionally and histologically protected against cisplatin-induced acute renal failure (ARF). Caspase-1 exerts proinflammatory effects via the cytokines interleukin (IL)-1beta, IL-18, IL-6, and neutrophil recruitment. We sought to determine the role of the cytokines IL-1beta, IL-18, and IL-6 and neutrophil recruitment in cisplatin-induced ARF. We first examined IL-1beta; renal IL-1beta increased nearly 2-fold in cisplatin-induced ARF and was reduced in the caspase-1(-/-) mice. However, inhibition with IL-1 receptor antagonist (IL-1Ra) did not attenuate cisplatin-induced ARF. Renal IL-18 increased 2.5-fold; however, methods to inhibit IL-18 using IL-18 antiserum and transgenic mice that overproduce IL-18-binding protein (a natural inhibitor of IL-18) did not protect. Renal IL-6 increased 3-fold; however, IL-6-deficient (IL-6(-/-)) mice still developed cisplatin-induced ARF. We next examined neutrophils; blood neutrophils increased dramatically after cisplatin injection; however, prevention of peripheral neutrophilia and renal neutrophil infiltration with the neutrophil-depleting antibody RB6-8C5 did not protect against cisplatin-induced ARF. In summary, our data demonstrated that cisplatin-induced ARF is associated with increases in the cytokines IL-1beta, IL-18, and IL-6 and neutrophil infiltration in the kidney. However, inhibition of IL-1beta, IL-18, and IL-6 or neutrophil infiltration in the kidney is not sufficient to prevent cisplatin-induced ARF.

T-cell activation in the intestinal mucosa

The vast majority of peripheral T cells exist as resting lymphocytes until a signal for activation has been received. In response to antigen, this activation involves ligation of the T-cell receptor (TCR) and signal transmission through the CD3 complex, which then initiates a cascade of intracellular events that lead to the expression of genes used in T-cell activation. T-cell activation also requires soluble mediators in the form of cytokines and chemokines that regulate the process in both positive and negative ways, and costimulatory signals received in conjunction with TCR/CD3 signaling are important in the activation of T cells. Unlike T cells in other peripheral immune compartments, small and large intestinal intraepithelial lymphocytes (IELs) bear some but not all properties of activated T cells, suggesting that they constitute a large population of 'partially activated' effector cells. Thus, regulation of the IEL activation process must be held in tight check, yet it must be ready to respond to foreign antigen rapidly and effectively. We discuss how costimulatory molecules may hold the key to controlling IEL activation through a multiphase process beginning with cells that have already entered into the early stage of activation.

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.

Disruption of interleukin-18, but not interleukin-1, increases vulnerability to preterm delivery and fetal mortality after intrauterine inflammation

Preterm birth is a major contributor of adverse perinatal outcome. Clinical data suggest that an inflammatory response is important in the process leading to preterm labor. By using a recently introduced mouse model of localized intrauterine lipopolysaccharide-induced inflammation, the effect of interleukin (IL)-18 gene disruption and/or IL-18 neutralization as well as combined IL-1alpha/beta gene disruption on inflammation-induced fetal loss was investigated. The frequency of preterm fetal loss was significantly higher in IL-18 knockout mice (58.9%) and in mice administered IL-18-binding protein (59.7%) compared to wild-type controls (34.7%). The rate of fetal loss was not affected by IL-1alpha/beta gene deficiency (38.7%). Decreased IL-18 protein expression combined with elevated IL-12 protein expression in uterine tissue of IL-18 knockout mice and IL-18-binding protein-treated animals was noticed. These data demonstrate that preterm pregnancy loss in response to intrauterine inflammation was enhanced by disruption of the IL-18 gene and/or IL-18 neutralization, events that may relate to exaggerated Th1 responses because of an increased IL-12/IL-18 ratio.

Interleukin 1 and interleukin 18 as mediators of inflammation and the aging process

In this review, 2 cytokines are discussed with respect to the inflammatory processes that are fundamental to aging and mortality. Both interleukin (IL)-1 and IL-18 are members of the same structural family (IL-1 family, or IL-F); there are presently 9 members of this family, but with the exception of IL-1alpha, IL-1beta, and IL-18, the others are antagonists or remain without known function. IL-1alpha is an intracellular cytokine with properties of both a cytokine and a transcription factor. IL-1beta and IL-18 are closely related; both possess a similar three-dimensional structure, and their respective precursor forms are inactive until cleaved by the intracellular cysteine protease caspase-1. Patients with mutations in the NALP3 gene, which controls the activity of caspase-1, readily secrete more IL-1beta and IL-18 and suffer from systemic inflammatory diseases. Patients with defects in this gene have high circulating concentrations of IL-6, serum amyloid A, and C-reactive protein, each of which decrease rapidly upon blockade of the IL-1 receptor, which suggests that IL-1beta contributes to the elevation of these markers of the inflammatory mechanisms of aging. Animal studies support the concept that IL-1beta and IL-18 participate in the pathogenesis of atherosclerosis. For example, overexpression of the IL-18 binding protein, a naturally occurring, specific inhibitor of IL-18, prevents the spontaneous development of atherosclerosis in apolipoprotein E-deficient mice. From human and animal studies, one may conclude that IL-1beta and IL-18 participate in fundamental inflammatory processes that increase during the aging process.

Inhibitors of histone deacetylases as anti-inflammatory drugs

This review addresses the issue of histone deacetylase (HDAC) inhibitors as developed for the treatment of cancer and for the investigation of the inhibition of inflammation. The review focuses on both in vitro and in vivo models of inflammation and autoimmunity. Of particular interest is the inhibition of pro-inflammatory cytokines. Although the reduction in cytokines appears paradoxical at first, upon examination, some genes that are anti-inflammatory are upregulated by inhibition of HDAC. Whether skin diseases will be affected by inhibitors of HDAC remains to be tested.

Interleukin-32: a cytokine and inducer of TNFalpha

We describe the gene structure, regulation, signal transduction. and functions of a cytokine, interleukin (IL)-32. An IL-18 unresponsive cell was converted to a responsive cell by transfection of the IL-18 receptor beta chain, and IL-18-induced microarray revealed high expression of a cytokine-like gene. Although IL-32 does not share sequence homology with known cytokine families, IL-32 induces various cytokines, human TNFalpha, and IL-8 in THP-1 monocytic cells as well as mouse TNFalpha and MIP-2 in Raw macrophage cells. IL-32 activates typical cytokine signal pathways of nuclear factor-kappa B (NF-kappaB) and p38 mitogen-activated protein kinase. IL-32 mRNA is highly expressed in immune tissue rather than other tissues. Human IL-32 exists as four splice variants, and IL-32 from other species were found as expressed sequence tag clones in the databank. Induced in human peripheral lymphocyte cells after mitogen stimulation, in human epithelial cells by IFNgamma, and in NK cells after exposure to the combination of IL-12 plus IL-18, IL-32 may play a role in inflammatory/autoimmune diseases.

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.