Identification of a Critical Ig-Like Domain in IL-18 Receptor α and Characterization of a Functional IL-18 Receptor Complex

Functional characterization of IL-18 receptor subunits, IL-18Rα and IL-18Rβ, and its natural inhibitor, IL-18 binding protein (IL-18BP) in rainbow trout Oncorhynchus mykiss

As an important proinflammation and immunomodulatory cytokine, IL-18 has been reported in several species of fish, but its receptor subunits, IL-18Rα and IL-18Rβ, and its decoy receptor, IL-18BP, have not been functionally characterized in fish. In the present study, IL-18Rα, IL-18Rβ and IL-18BP were cloned from rainbow trout Oncorhynchus mykiss, and they possess common conserved domains with their mammalian orthologues. In tested organs/tissues, IL-18Rα and IL-18Rβ exhibit basal expression levels, and IL-18BP has a pattern of constitutive expression. When transfected with different combinations of chimeric receptors in HEK293T cells, recombinant IL-18 (rIL-18) can induce the activation of NF-κB only when pcDNA3.1-IL-18Rα/IL-1R1 and pcDNA3.1-IL-18Rβ/IL-1RAP were both expressed. On the other hand, recombinant receptors, including rIL-18BP, rIL-18Rα-ECD-Fc and rIL-18Rβ-ECD-Fc can down-regulate significantly the activity of NF-κB, suggesting the participation of IL-18Rα, IL-18Rβ and IL-18BP in rainbow trout IL-18 signal transduction. Co-IP assays indicated that IL-18Rβ may form a complex with MyD88, IRAK4, IRAK1, TRAF6 and TAB2 in HEK293T cells, indicating that IL-18Rβ, in IL-18 signalling pathway, is associated with these signalling molecules. In conclusion, IL-18Rα, IL-18Rβ and IL-18BP in rainbow trout are conserved in function and signalling pathway with their mammalian orthologues.

Epigenetic and inflammatory events in experimental periodontitis following systemic microbial challenge

AIM

The purpose of this study was to determine inflammatory and epigenetic features following induction of oral and gut dysbiosis in experimental periodontitis in order to examine the interplay between oral and systemic infection.

MATERIALS AND METHODS

Periodontitis was induced in 6- to 8-week-old C57BL/6 mice by (a) Ligature placement (Lig group) (oral challenge); (b) P. gingivalis gavage (Pg group) (systemic challenge); and (c) the combination of the two models oral and systemic challenge (Pg + Lig). The duration of the experiment was 60 days, and the animals were then sacrificed for analyses. Alveolar bone loss was assessed, and a multiplex immunoassay was performed. Maxillae and gut tissues were immunostained for DNMT3b (de novo methylation marker), B and T lymphocyte attenuator (BTLA) and IL-18R1 (inflammation markers).

RESULTS

Pg and Pg + Lig groups exhibited higher bone loss when compared to Sham. BAFF, VEGF, RANKL, RANTES and IP-10 were significantly higher with Pg gavage. Likewise, DNMT3b was overexpressed in both gut and maxilla after the Pg administration. The same pattern was observed for BTLA and IL-18R1 in gut tissues.

CONCLUSIONS

The systemic microbial challenge either alone or in combination with local challenge leads to distinct patterns of inflammatory and epigenetic features when compared to simply locally induced experimental periodontitis.

Theoretical Studies on the Engagement of Interleukin 18 in the Immuno-Inflammatory Processes Underlying Atherosclerosis

Interleukin 18 (IL-18) is one of the pro-inflammatory cytokines expressed by macrophages, suggesting that it plays important physiological and immunological functions, among the others: stimulation of natural killers (NKs) and T cells to interferon gamma (IFN- γ ) synthesis. IL-18 was originally identified as interferon gamma inducing factor and now it is recognized as multifunctional cytokine, which has a role in regulation of innate and adaptive immune responses. Therefore, in order to investigate IL-18 contribution to the immuno-inflammatory processes underlying atherosclerosis, a systems approach has been used in our studies. For this purpose, a model of the studied phenomenon, including selected pathways, based on the Petri-net theory, has been created and then analyzed. Two pathways of IL-18 synthesis have been distinguished: caspase 1-dependent pathway and caspase 1-independent pathway. The analysis based on t-invariants allowed for determining interesting dependencies between IL-18 and different types of macrophages: M1 are involved in positive regulation of IL-18, while M2 are involved in negative regulation of IL-18. Moreover, the obtained results showed that IL-18 is produced more often via caspase 1-independent pathway than caspase 1-dependent pathway. Furthermore, we found that this last pathway may be associated with caspase 8 action.

Implication of Interleukin (IL)-18 in the pathogenesis of chronic obstructive pulmonary disease (COPD)

Interleukin (IL)-18 is a pro-inflammatory cytokine that was firstly described as an interferon (IFN)-γ-inducing factor. Similar to IL-1β, IL-18 is synthesized as an inactive precursor requiring processing by caspase-1 into an active cytokine. The platform for activating caspase-1 is known as the inflammasome, a multiple protein complex. Macrophages and dendritic cells are the primary sources for the release of active IL-18, whereas the inactive precursor remains in the intracellular compartment of mesenchymal cells. Finally, the IL-18 precursor is released from dying cells and processed extracellularly. IL-18 has crucial host defense and antitumor activities, and gene therapy to increase IL-18 levels in tissues protects experimental animals from infection and tumor growth and metastasis. Moreover, multiple studies in experimental animal models have shown that IL-18 over-expression results to emphysematous lesions in mice. The published data prompt to the hypothesis that IL-18 induces a broad spectrum of COPD-like inflammatory and remodeling responses in the murine lung and also induces a mixed type 1, type 2, and type 17 cytokine responses. The majority of studies identify IL-18 as a potential target for future COPD therapeutics to limit both the destructive and remodeling processes occurring in COPD lungs.

Human plasmacytoid dendritic cells regulate IFN-α production through activation-induced splicing of IL-18Rα

IFN-α production by pDCs regulates host protection against viruses and is implicated in autoimmune pathology. Human pDCs express high levels of IL-18R, but little is known of its role in pDC function. We report that IL-18R signaling negatively regulates IFN-α production through activation-induced splicing of IL-18Rα in human pDCs. Our data reveal two distinct isoforms of IL-18Rα in human pDCs: the known, full-length receptor (IL-18Rα1) and a novel, truncated variant (IL-18Rα2), which functions as a molecular decoy that competitively inhibits the canonical IL-18Rα1/IL-18Rβ signaling pathway. Whereas NK cells and pDCs both express IL-18Rα1, pDCs express significantly higher levels of IL-18Rα2, resulting in differential responses of these populations to IL-18. Flu exposure increases IL-18Rα1 expression in pDCs, and the blocking of IL-18R enhances pDC production of IFN-α and IP-10; thus, pDCs use activation-induced splicing to regulate IFN-α production in response to flu. These data demonstrate that IL-18R modulates IFN-α release by human pDCs and suggest that IL-18R signaling may represent a promising therapeutic target.

Common SNPs/haplotypes in IL18R1 and IL18 genes are associated with variations in humoral immunity to smallpox vaccination in Caucasians and African Americans

BACKGROUND

Identifying genetic factors that influence poxvirus immunity across races may assist in the development of better vaccines and approaches for vaccine development.

METHODS

We performed an extensive candidate-gene genetic screen (across 32 cytokine and cytokine receptor genes) in a racially diverse cohort of 1056 healthy adults after a single dose of smallpox vaccine. Associations between single-nucleotide polymorphisms (SNPs)/haplotypes and vaccinia virus-specific neutralizing antibodies were assessed using linear regression methodologies.

RESULTS

The combined analysis identified 63 associations between candidate SNPs and antibody levels after smallpox vaccination with P < .05. Thirty-one of these were within the IL18R1 and IL18 genes. Five IL18R1 SNPs, including a coding synonymous polymorphism rs1035130 (Phe251Phe) and 2 promoter SNPs (rs6710885, rs2287037), all in linkage disequilibrium, were associated with significant variations in antibody levels in both Caucasians (P ≤ .016) and African Americans (P ≤ .025). Similarly, associations with 2 intronic IL18 SNPs (rs2043055 and rs5744280) were consistent in the Caucasian (P ≤ .023) and African American samples (P ≤ .014). Haplotype analysis revealed highly significant associations between IL18R1 haplotypes and vaccinia virus-specific antibody levels (P < .001, by combined analysis) that were consistent across races.

CONCLUSIONS

Our study provides evidence for IL18 and IL18R1 genes as plausible genes regulating the humoral immune response to smallpox vaccine in both Caucasians and African Americans.

A (99m)Tc-labeled dual-domain cytokine ligand for imaging of inflammation

INTRODUCTION

Interleukin (IL)-1 and IL-18 are potent proinflammatory cytokines in inflammation-related diseases. Their actions are regulated by IL-1 receptor antagonist (IL-1ra) and IL-18 binding protein (IL-18bp). This study was designed to (99m)Tc-radiolabel an IL-1ra and IL-18bp dual-domain cytokine ligand, IL-18bp-Fc-IL-1ra, for specific inflammation targeting.

METHODS

The (99m)Tc-IL-18bp-Fc-IL-1ra was obtained by direct labeling via 2-iminothiolane reduction. Competitive binding of (99m)Tc-labeled and unlabeled IL-18bp-Fc-IL-1ra to rat polymorphonuclear leukocytes was assessed in vitro. A mouse ear edema model was used to evaluate specific targeting properties of (99m)Tc-IL-18bp-Fc-IL1ra in vivo. The correlation between (99m)Tc-IL-18bp-Fc-IL-1ra uptake and (111)In-labeled polymorphonuclear neutrophil infiltration was studied using ischemic-reperfused rat hearts.

RESULTS

Direct (99m)Tc-labeling yielded a stable dual-domain cytokine radioligand with radiochemical purity greater than 95% after gel filtration. Competitive binding studies showed specific targeting of (99m)Tc-IL-18bp-Fc-IL-1ra to inflammatory cells. The (99m)Tc-IL-18bp-Fc-IL-1ra uptake was 1.80±0.17 % injected dose per gram (%ID/g) in the inflamed ear without blocking, whereas uptake in the presence of IL-18bp-Fc-IL-1ra was 1.09±0.08 %ID/g (P<.05). The amounts of IL-1β and IL-18 were significantly increased in the inflamed ears compared to the vehicle controls. A significant correlation of (99m)Tc-IL-18bp-Fc-IL-1ra with (111)In-labeled neutrophil distribution was observed in the ischemic-reperfused hearts (P<.001).

CONCLUSION

Targeting proinflammatory cytokines with (99m)Tc-IL-18bp-Fc-IL-1ra may provide a suitable approach for specific detection of inflammatory sites.

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.

Unusual water-mediated antigenic recognition of the proinflammatory cytokine interleukin-18

The unique cytokine interleukin-18 (IL-18) acts synergistically with IL-12 to regulate T-helper 1 and 2 lymphocytes and, as such, seems to underlie the pathogenesis of various autoimmune and allergic diseases. Several anti-IL-18 agents are in clinical development, including the recombinant human antibody ABT-325, which is entering trials for autoimmune diseases. Given competing cytokine/receptor and cytokine/receptor decoy interactions, understanding the structural basis for recognition is critical for effective development of anti-cytokine therapies. Here we report three crystal structures: the murine antibody 125-2H Fab fragment bound to human IL-18, at 1.5 A resolution; the 125-2H Fab (2.3 A); and the ABT-325 Fab (1.5 A). These structures, along with human/mouse IL-18 chimera binding data, allow us to make three key observations relevant to the biology and antigenic recognition of IL-18 and related cytokines. First, several IL-18 residues shift dramatically (> 10 A) upon binding 125-2H, compared with unbound IL-18 (Kato, Z., Jee, J., Shikano, H., Mishima, M., Ohki, I., Ohnishi, H., Li, A., Hashimoto, K., Matsukuma, E., Omoya, K., Yamamoto, Y., Yoneda, T., Hara, T., Kondo, N., and Shirakawa, M. (2003) Nat. Struct. Biol. 10, 966-971). IL-18 thus exhibits plasticity that may be common to its interactions with other receptors. Related cytokines may exhibit similar plasticity. Second, ABT-325 and 125-2H differ significantly in combining site character and architecture, thus explaining their ability to bind IL-18 simultaneously at distinct epitopes. These data allow us to define the likely ABT-325 epitope and thereby explain the distinct neutralizing mechanisms of both antibodies. Third, given the high 125-2H potency, 10 well ordered water molecules are trapped upon complex formation in a cavity between two IL-18 loops and all six 125-2H complementarity-determining regions. Thus, counterintuitively, tight and specific antibody binding may in some cases be water-mediated.

Immunological and inflammatory functions of the interleukin-1 family

More than any other cytokine family, the interleukin (IL)-1 family is closely linked to the innate immune response. This linkage became evident upon the discovery that the cytoplasmic domain of the IL-1 receptor type I is highly homologous to the cytoplasmic domains of all Toll-like receptors (TLRs). Thus, fundamental inflammatory responses such as the induction of cyclooxygenase type 2, increased expression of adhesion molecules, or synthesis of nitric oxide are indistinguishable responses of both IL-1 and TLR ligands. Both families nonspecifically affect antigen recognition and lymphocyte function. IL-1beta is the most studied member of the IL-1 family because of its role in mediating autoinflammatory diseases. Although the TLR and IL-1 families evolved to assist in host defense against infection, unlike the TLR family, the IL-1 family also includes members that suppress inflammation, both specifically within the IL-1 family but also nonspecifically for TLR ligands and the innate immune response.

Constitutive expression of IL-18 and IL-18R in differentiated IEC-6 cells: effect of TNF-alpha and IFN-gamma treatment

The multifunctional cytokine interleukin-18 (IL-18) is an important mediator in intestinal inflammatory processes. The aim of this study was to evaluate the constitutive expression of IL-18 and its receptors (IL-18Ralpha and IL-18Rbeta) in intestinal epithelial cells (IEC) stimulated by tumor necrosis factor-alpha (TNF-alpha) and interferon-gamma (IFN-gamma). In addition, cellular proliferation and evaluation of brush border enzymes as differentiation markers were studied. Nontransformed rat intestinal epithelial IEC-6 cells were grown on an extracellular matrix (ECM) in medium with or without TNF-alpha, IFN-gamma, or a combination of both. Gene expression of IL-18, its receptors and apoptotic markers was evaluated using real-time PCR. Expression of IL-18Ralpha protein was demonstrated by flow cytometry and Western blot. Enzymatic activities of brush border enzymes and caspase-1 were determined. The constitutive expression of IL-18, IL-18Ralpha and IL-18Rbeta mRNAs and proteins were detected in IEC-6 cells. The biologically active form of IL-18 was released in response to TNF-alpha and IFN-gamma treatment. Exogenous IL-18 had no effect on cellular proliferation, brush border enzyme activities, and gene expression of apoptotic markers. However, the addition of IL-18 stimulated production and release of the chemokine IL-8. These data suggest that IEC-6 cells may be not only a source of IL-18 but also a target for its action.

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.

Clinical and biological effects of recombinant human interleukin-18 administered by intravenous infusion to patients with advanced cancer

PURPOSE

Interleukin-18 (IL-18) is an immunostimulatory cytokine with antitumor activity in preclinical animal models. A phase I study of recombinant human IL-18 (rhIL-18) was done to determine the toxicity, pharmacokinetics, and biological activities of rhIL-18 in patients with advanced cancer.

EXPERIMENTAL DESIGN

Cohorts of patients were given escalating doses of rhIL-18, each administered as a 2-hour i.v. infusion on 5 consecutive days. Toxicities were graded using standard criteria. Serial blood samples were obtained for pharmacokinetic and pharmacodynamic measurements.

RESULTS

Twenty-eight patients (21 with renal cell cancer, 6 with melanoma, and 1 with Hodgkin's lymphoma) were given rhIL-18 in doses ranging from 3 to 1,000 microg/kg. Common side effects included chills, fever, nausea, headache, and hypotension. Common laboratory abnormalities included transient, asymptomatic grade 1 to 2 neutropenia, thrombocytopenia, anemia, hypoalbuminemia, hyponatremia, and elevations in liver transaminases. One patient in the 100 microg/kg cohort experienced transient grade 3 hypotension and grade 2 bradycardia during the first infusion of rhIL-18. No other dose-limiting toxicities were observed. Plasma concentrations of rhIL-18 increased with increasing dose, and 2.5-fold accumulation was observed with repeated dosing. Biological effects of rhIL-18 included transient lymphopenia and increased expression of activation antigens on lymphocytes and monocytes. Increases in serum concentrations of IFN-gamma, granulocyte macrophage colony-stimulating factor, IL-18 binding protein, and soluble Fas ligand were observed. Two patients experienced unconfirmed partial responses after rhIL-18 treatment.

CONCLUSIONS

rhIL-18 can be safely given in biologically active doses to patients with advanced cancer. A maximum tolerated dose of rhIL-18 was not determined. Further clinical studies of rhIL-18 are warranted.

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.

Human Anti–Human IL-18 Antibody Recognizing the IL-18–Binding Site 3 with IL-18 Signaling Blocking Activity

IL-18 is an important regulator in both innate and acquired immune responses. The aberrant expression of IL-18 is associated with severe inflammatory conditions, such as autoimmune diseases and allergies. Thus, human antibodies with inhibitory activity on IL-18 signaling may be useful for therapeutic applications. We report here the first establishment of an antagonistic anti-IL-18 complete human antibody, h18-108, employing a human single chain antibody (scFv)-displaying phage library. The h18-108 scFv inhibited the IFN-gamma production of a human myelomonocytic cell line, KG-1. Flow cytometry analysis showed that h18-108 blocked the binding of IL-18 to KG-1 cells. Epitope mapping analysis using two kinds of random peptide-displaying phage libraries and an IL-18 alanine mutant (D98A) demonstrated that the h18-108 scFv binds to the site 3 of IL-18, which is suggested to be an association site with the IL-18 receptor beta. The complete human Fab and IgG forms of h18-108 have been successfully constructed to attain increases in both binding affinity and inhibitory activity.

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.