Identification of amino acid residues critical for biological activity in human interleukin-18

Inflammatory Mediators of Endothelial Dysfunction

Endothelial dysfunction (ED) is characterized by imbalanced vasodilation and vasoconstriction, elevated reactive oxygen species (ROS), and inflammatory factors, as well as deficiency of nitric oxide (NO) bioavailability. It has been reported that the maintenance of endothelial cell integrity serves a significant role in human health and disease due to the involvement of the endothelium in several processes, such as regulation of vascular tone, regulation of hemostasis and thrombosis, cell adhesion, smooth muscle cell proliferation, and vascular inflammation. Inflammatory modulators/biomarkers, such as IL-1α, IL-1β, IL-6, IL-12, IL-15, IL-18, and tumor necrosis factor α, or alternative anti-inflammatory cytokine IL-10, and adhesion molecules (ICAM-1, VCAM-1), involved in atherosclerosis progression have been shown to predict cardiovascular diseases. Furthermore, several signaling pathways, such as NLRP3 inflammasome, that are associated with the inflammatory response and the disrupted H2S bioavailability are postulated to be new indicators for endothelial cell inflammation and its associated endothelial dysfunction. In this review, we summarize the knowledge of a plethora of reviews, research articles, and clinical trials concerning the key inflammatory modulators and signaling pathways in atherosclerosis due to endothelial dysfunction.

Surface cysteine to serine substitutions in IL-18 reduce aggregation and enhance activity

Background

Interleukin-18 (IL-18) is prone to form multimers resulting in inactive aggregates, making this cytokine unstable for clinical use. Therefore, mutations have been introduced into recombinant IL-18 to overcome this issue.

Methods

To prevent the formation of disulfide bonds between the IL-18 molecules, multiple mutations targeting surface cysteines (C38, C68, C76, and C127) were introduced into our previously modified human IL-18 double mutant E6K+T63A (IL-18 DM) by direct gene synthesis. The open reading frames of IL-18 wild-type (WT), IL-18 DM, and IL-18 multiple mutant E6K+T63A+C38S+C68S+C76S+C127S (IL-18 DM1234) were inserted in the pET28a expression vector and transformed into Escherichia coli Rosetta2 (DE3) pLysS cells for protein production. The inclusion bodies of WT and mutated IL-18 were extracted by sonication and refolded by stepwise dialysis using 8 M urea as the starting concentration. The refolded IL-18 proteins were tested for aggregation using the ProteoStat protein aggregation assay. Their activity was also investigated by treating NK-92MI cells with each IL-18 at concentrations of 75, 150, and 300 ng/ml with 0.5 ng/ml of human IL-12 and interferon-gamma (IFN-γ) levels in the supernatant were evaluated using ELISA. The structure of modified IL-18 was visualized using molecular dynamics (MD) simulations.

Results

IL-18 DM1234 exhibited the lowest aggregation signal, approximately 1.79- and 1.63-fold less than that of the WT and IL-18 DM proteins. Additionally, the IFN-γ inducing activity of IL-18 DM1234 was about 10 and 2.8 times higher than that of the WT and IL-18 DM, respectively. MD simulations revealed that binding site I of IL-18 DM1234 was altered mainly due to surface cysteine replacement with serine (C-to-S substitution). This is the first report showing that C-to-S substitutions in IL-18 improved its activity and stability, suggesting the use of this modified IL-18 for medical purposes in the future.

Structural basis of human IL-18 sequestration by the decoy receptor IL-18 binding protein (IL-18BP) in inflammation and tumor immunity

Human Interleukin-18 (IL-18) is an omnipresent proinflammatory cytokine of the IL-1 family with central roles in autoimmune and inflammatory diseases and serves as a staple biomarker in the evaluation of inflammation in physiology and disease, including the inflammatory phase of COVID-19. The sequestration of IL-18 by its soluble decoy receptor IL-18-Binding Protein (IL-18BP) is critical to the regulation of IL-18 activity. Since an imbalance in expression and circulating levels of IL-18 is associated with disease, structural insights into how IL-18BP outcompetes binding of IL-18 by its cognate cell-surface receptors are highly desirable; however, the structure of human IL-18BP in complex with IL-18 has been elusive. Here, we elucidate the sequestration mechanism of human IL-18 mediated by IL-18BP based on the crystal structure of the IL-18:IL-18BP complex. These detailed structural snapshots reveal the interaction landscape leading to the ultra-high affinity of IL-18BP toward IL-18 and identify substantial differences with respect to previously characterized complexes of IL-18 with IL-18BP of viral origin. Furthermore, our structure captured a fortuitous higher-order assembly between IL-18 and IL-18BP coordinated by a disulfide-bond distal to the binding surface connecting IL-18 and IL-18BP molecules from different complexes, resulting in a novel tetramer with 2:2 stoichiometry. This tetrapartite assembly was found to restrain IL-18 activity more effectively than the canonical 1:1 complex. Collectively, our findings provide a framework for innovative, structure-driven therapeutic strategies and further functional interrogation of IL-18 in physiology and disease.

Effects and mechanisms of SGLT2 inhibitors on the NLRP3 inflammasome, with a focus on atherosclerosis

Atherosclerosis is a lipid-driven chronic inflammatory disease that is widespread in the walls of large and medium-sized arteries. Its pathogenesis is not fully understood. The currently known pathogenesis includes activation of pro-inflammatory signaling pathways in the body, increased oxidative stress, and increased expression of cytokines/chemokines. In the innate immune response, inflammatory vesicles are an important component with the ability to promote the expression and maturation of inflammatory factors, release large amounts of inflammatory cytokines, trigger a cascade of inflammatory responses, and clear pathogens and damaged cells. Studies in the last few years have demonstrated that NLRP3 inflammatory vesicles play a crucial role in the development of atherosclerosis as well as its complications. Several studies have shown that NLRP3 binding to ligands promotes inflammasome formation, activates caspase-1, and ultimately promotes its maturation and the maturation and production of IL-1β and IL-18. IL-1β and IL-18 are considered to be the two most prominent inflammatory cytokines in the inflammasome that promote the development of atherosclerosis. SGLT2 inhibitors are novel hypoglycemic agents that also have significant antiatherosclerotic effects. However, their exact mechanism is not yet clear. This article is a review of the literature on the effects and mechanisms of SGLT2 inhibitors on the NLRP3 inflammasome, focusing on their role in antiatherosclerosis.

A new target for the treatment of inflammatory bowel disease: Interleukin-37

Interleukin (IL)-37 belongs to the IL-1 cytokine family. It has anti-inflammatory effects on numerous autoimmune diseases such as asthma, psoriasis, inflammatory bowel disease (IBD), systemic lupus erythematosus (SLE), multiple sclerosis (MS) and rheumatoid arthritis (RA). Mechanistically, IL-37 plays an anti-inflammatory role by regulating the expression of inflammatory factors in two ways: binding extracellular receptors IL-18R or transferring into the nucleus with Smad3. IBD is a kind of idiopathic intestinal inflammatory disease with unknown etiology and pathogenesis. Recent researches had proved that IL-37 is negatively involved in the pathogenesis and development of IBD. Among various inflammatory diseases, IL-37 has been shown to regulate inflammatory development by acting on various immune cells such as neutrophils, macrophages (Mϕ), dendritic cells (DCs), T cells and intestinal epithelial cells. This review summarizes the biological role of IL-37, and its immunoregulatory effects on the immune cells, especially anti-inflammatory function in both human and experimental models of IBD.

Protein modeling, molecular network and molecular dynamics study of newly sequenced interleukin-18 (IL-18) gene in Mus musculus

Interleukin-18 (IL-18) belongs to the superfamily of IL-1 protein and exerts a pleiotropic pro-inflammatory effect on the body. Generally, this protein is significantly involved in immune defense during infection in cells, but sometimes its anomalous activities produce some inflammatory diseases like rheumatoid arthritis and Crohn's disease. In the present study, the IL-18 gene was isolated from mice and was subsequently cloned and sequenced. Further, the network analysis was carried out to explore the functional role of IL-18 protein in animals. The 3D protein structure of the IL-18 protein was generated and docked with appropriate 3-([3-cholamidopropyl]dimethylammonio)-1-propanesulfonate (CPS) ligand. Later the complex structure of the protein was subjected to molecular dynamics simulation (MDS) for 50 ns to determine the effect of ligand on protein. The network analysis explored the correlation of IL-18 protein with others proteins and their involvement in the different significant pathway to defend the cell from various diseases. As confirmed by MDS, the CPS:IL-18 complex was found to be highly stable. Our results further indicated that CPS ligand has the potential to act as a drug molecule, in future, for counteracting IL-18 activity. To date, no structural details were available for animal IL-18. Hence, the finding of this study will be useful in broadening the horizon towards a better understanding of the functional and structural aspects of IL-18 in animals.

The family of the interleukin-1 receptors

The extracellular forms of the IL-1 cytokines are active through binding to specific receptors on the surface of target cells. IL-1 ligands bind to the extracellular portion of their ligand-binding receptor chain. For signaling to take place, a non-binding accessory chain is recruited into a heterotrimeric complex. The intracellular approximation of the Toll-IL-1-receptor (TIR) domains of the 2 receptor chains is the event that initiates signaling. The family of IL-1 receptors (IL-1R) includes 10 structurally related members, and the distantly related soluble protein IL-18BP that acts as inhibitor of the cytokine IL-18. Over the years the receptors of the IL-1 family have been known with many different names, with significant confusion. Thus, we will use here a recently proposed unifying nomenclature. The family includes several ligand-binding chains (IL-1R1, IL-1R2, IL-1R4, IL-1R5, and IL-1R6), 2 types of accessory chains (IL-1R3, IL-1R7), molecules that act as inhibitors of signaling (IL-1R2, IL-1R8, IL-18BP), and 2 orphan receptors (IL-1R9, IL-1R10). In this review, we will examine how the receptors of the IL-1 family regulate the inflammatory and anti-inflammatory functions of the IL-1 cytokines and are, more at large, involved in modulating defensive and pathological innate immunity and inflammation. Regulation of the IL-1/IL-1R system in the brain will be also described, as an example of the peculiarities of organ-specific modulation of inflammation.

Reviews of Interleukin-37: Functions, Receptors, and Roles in Diseases

Interleukin-37 (IL-37) is an IL-1 family cytokine discovered in recent years and has 5 different isoforms. As an immunosuppressive factor, IL-37 can suppress excessive immune response. IL-37 plays a role in protecting the body against endotoxin shock, ischemia-reperfusion injury, autoimmune diseases, and cardiovascular diseases. In addition, IL-37 has a potential antitumor effect. IL-37 and its receptors may serve as novel targets for the study, diagnosis, and treatment of immune-related diseases and tumors.

Immunologic Function and Molecular Insight of Recombinant Interleukin-18

In recent years, cytokine-mediated therapy has emerged as further advance alternative in cancer therapy. Interleukin-18 (IL-18) has exhibited interesting anti-cancer properties especially when combined with IL-12. We engineered IL-18 in order to improve its activity using single point mutagenesis. IL-18 mutants were constructed according to binding residues and polarity which we tried to increase polarity in M33Q and M60Q, enhanced cationicity in E6K, and flexibility in T63A. All IL-18 proteins were expressed in Pichia pastoris, purified, and then measured the activity by treating with the NK-92MI cell line to evaluate interferon-γ (IFN-γ) stimulation. The E6K and T63A mutant forms showed higher activity with respect to native proteins at the concentration of 200 ng mL-1 by inducing the expression of IFN-γ, about factors of 9 and 4, respectively. Meanwhile, M33Q and M60Q had no significant activity to induce IFN-γ. Interestingly, the combination of E6K and T63A mutations could synergize the induction activity of IL-18 to be 16 times at 200 ng mL-1. Furthermore, molecular dynamics studies have elucidated the effect due to mutation on conformation of the binding site of IL-18. The results turn out that E6K provides structural perseverance against mutation, while M33Q and M60Q promote vivid overall change in protein conformation, especially at the binding site. For T63A, mutation yields small difference in structure but clearly increases structural flexibility. However, a small structural change was observed when T63A was combined with E6K. Our research resulted in a novel version of IL-18 which could be a new key candidate for cytokine-mediated therapy.

Allergic airway inflammation: unravelling the relationship between IL-37, IL-18Rα and Tir8/SIGIRR

The hallmarks of allergic bronchial asthma arise from chronic airway inflammation. Thus, elucidating the mechanisms regulating the maintenance of this chronic inflammatory response is key to understanding asthma pathogenesis. To date, it is not clear whether a predominance of proinflammatory factors or a reduced capacity of counterbalancing anti-inflammatory mediators is the pivotal factor predisposing individuals towards asthma development. The IL-1 cytokine family and its receptor systems comprise a variety of proinflammatory cytokines like IL-1β and IL-18 and anti-inflammatory molecules such as the Toll/interleukin-1 receptor 8/single Ig IL-1 receptor (IL-R)-related molecule (Tir8/SIGIRR) and the recently established cytokine IL-37. This article reviews the functions of these IL-1 cytokine family members in the regulation of allergic airway inflammation and asthma as they have been assessed clinically, in vitro and in mouse models.

Structural biology of the IL-1 superfamily: key cytokines in the regulation of immune and inflammatory responses

Interleukin-1 superfamily of cytokines (IL-1, IL-18, IL-33) play key roles in inflammation and regulating immunity. The mechanisms of agonism and antagonism in the IL-1 superfamily have been pursued by structural biologists for nearly 20 years. New insights into these mechanisms were recently provided by the crystal structures of the ternary complexes of IL-1β and its receptors. We will review here the structural biology related to receptor recognition by IL-1 superfamily cytokines and the regulation of its cytokine activities by antagonists.

Structural insights into the interaction of IL-33 with its receptors

Interleukin (IL)-33 is an important member of the IL-1 family that has pleiotropic activities in innate and adaptive immune responses in host defense and disease. It signals through its ligand-binding primary receptor ST2 and IL-1 receptor accessory protein (IL-1RAcP), both of which are members of the IL-1 receptor family. To clarify the interaction of IL-33 with its receptors, we determined the crystal structure of IL-33 in complex with the ectodomain of ST2 at a resolution of 3.27 Å. Coupled with structure-based mutagenesis and binding assay, the structural results define the molecular mechanism by which ST2 specifically recognizes IL-33. Structural comparison with other ligand-receptor complexes in the IL-1 family indicates that surface-charge complementarity is critical in determining ligand-binding specificity of IL-1 primary receptors. Combined crystallography and small-angle X-ray-scattering studies reveal that ST2 possesses hinge flexibility between the D3 domain and D1D2 module, whereas IL-1RAcP exhibits a rigid conformation in the unbound state in solution. The molecular flexibility of ST2 provides structural insights into domain-level conformational change of IL-1 primary receptors upon ligand binding, and the rigidity of IL-1RAcP explains its inability to bind ligands directly. The solution architecture of IL-33-ST2-IL-1RAcP complex from small-angle X-ray-scattering analysis resembles IL-1β-IL-1RII-IL-1RAcP and IL-1β-IL-1RI-IL-1RAcP crystal structures. The collective results confer IL-33 structure-function relationships, supporting and extending a general model for ligand-receptor assembly and activation in the IL-1 family.

How pathogen-derived cysteine proteases modulate host immune responses

In mammals, cysteine proteases are essential for the induction and development of both innate and adaptive immune responses. These proteases play a role in antigen-and pathogen-recognition and elimination, signal processing and cell homeostasis. Many pathogens also secrete cysteine proteases that often act on the same target proteins as the mammalian proteases and thereby can modulate host immunity from initial recognition to effector mechanisms. Pathogen-derived proteases range from nonspecific proteases that degrade multiple proteins involved in the immune response to enzymes that are very specific in their mode of action. Here, we overview current knowledge of pathogen-derived cysteine proteases that modulate immune responses by altering the normal function of key receptors or pathways in the mammalian immune system.

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.

Structural insights into the assembly and activation of IL-1β with its receptors

Interleukin 1β (IL-1β) is a key orchestrator of inflammation and host defense that exerts its effects through IL-1 receptor type I (IL-1RI) and IL-1 receptor accessory protein (IL-1RAcP). How IL-1RAcP is recruited by IL-1β-IL-1RI to form the signaling-competent complex remains elusive. Here we present the crystal structure of IL-1β bound to IL-1 receptor type II (IL-1RII) and IL-1RAcP. IL-1β-IL-1RII generated a composite binding surface to recruit IL-1RAcP. Biochemical analysis demonstrated that IL-1β-IL-1RI and IL-1β-IL-1RII interacted similarly with IL-1RAcP. It also showed the importance of two loops of IL-1 receptor antagonist (IL-1Ra) in determining its antagonism. Our results provide a structural basis for assembly and activation of the IL-1 receptor and offer a general cytokine-receptor architecture that governs the IL-1 family of cytokines.

Interleukin 18 in the CNS

Interleukin (IL)-18 is a cytokine isolated as an important modulator of immune responses and subsequently shown to be pleiotropic. IL-18 and its receptors are expressed in the central nervous system (CNS) where they participate in neuroinflammatory/neurodegenerative processes but also influence homeostasis and behavior. Work on IL-18 null mice, the localization of the IL-18 receptor complex in neurons and the neuronal expression of decoy isoforms of the receptor subunits are beginning to reveal the complexity and the significance of the IL-18 system in the CNS. This review summarizes current knowledge on the central role of IL-18 in health and disease.

Structure of IL-33 and its interaction with the ST2 and IL-1RAcP receptors--insight into heterotrimeric IL-1 signaling complexes

Members of the interleukin-1 (IL-1) family of cytokines play major roles in host defense and immune system regulation in infectious and inflammatory diseases. IL-1 cytokines trigger a biological response in effector cells by assembling a heterotrimeric signaling complex with two IL-1 receptor chains, a high-affinity primary receptor and a low-affinity coreceptor. To gain insights into the signaling mechanism of the novel IL-1-like cytokine IL-33, we first solved its solution structure and then performed a detailed biochemical and structural characterization of the interaction between IL-33, its primary receptor ST2, and the coreceptor IL-1RAcP. Using nuclear magnetic resonance data, we obtained a model of the IL-33/ST2 complex in solution that is validated by small-angle X-ray scattering (SAXS) data and is similar to the IL-1beta/IL-1R1 complex. We extended our SAXS analysis to the IL-33/ST2/IL-1RAcP and IL-1beta/IL-1R1/IL-1RAcP complexes and propose a general model of the molecular architecture of IL-1 ternary signaling complexes.

A novel role for IkappaBzeta in the regulation of IFNgamma production

IkappaBzeta is a novel member of the IkappaB family of NFkappaB regulators, which modulates NFkappaB activity in the nucleus, rather than controlling its nuclear translocation. IkappaBzeta is specifically induced by IL-1beta and several TLR ligands and positively regulates NFkappaB-mediated transcription of genes such as IL-6 and NGAL as an NFkappaB binding co-factor. We recently reported that the IL-1 family cytokines, IL-1beta and IL-18, strongly synergize with TNFalpha for IFNgamma production in KG-1 cells, whereas the same cytokines alone have minimal effects on IFNgamma production. Given the striking similarities between the IL-1R and IL-18R signaling pathways we hypothesized that a common signaling event or gene product downstream of these receptors is responsible for the observed synergy. We investigated IkappaBzeta protein expression in KG-1 cells upon stimulation with IL-1beta, IL-18 and TNFalpha. Our results demonstrated that IL-18, as well as IL-1beta, induced moderate IkappaBzeta expression in KG-1 cells. However, TNFalpha synergized with IL-1beta and IL-18, whereas by itself it had a minimal effect on IkappaBzeta expression. NFkappaB inhibition resulted in decreased IL-1beta/IL-18/TNFalpha-stimulated IFNgamma release. Moreover, silencing of IkappaBzeta expression led to a specific decrease in IFNgamma production. Overall, our data suggests that IkappaBzeta positively regulates NFkappaB-mediated IFNgamma production in KG-1 cells.

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.

A synergistic role for IL-1beta and TNFalpha in monocyte-derived IFNgamma inducing activity

Although much is known about classic IFNgamma inducers, little is known about the IFNgamma inducing capability of inflammasome-activated monocytes. In this study, supernatants from LPS/ATP-stimulated human monocytes were analyzed for their ability to induce IFNgamma production by KG-1 cells. Unexpectedly, monocyte-derived IFN inducing activity was detected, but it was completely inhibited by IL-1beta, not IL-18 blockade. Moreover, size-fractionation of the monocyte conditioned media dramatically reduced the IFNgamma inducing activity of IL-1beta, suggesting that IL-1beta requires a cofactor to induce IFNgamma production in KG-1 cells. Because TNFalpha is known to synergize with IL-1beta for various gene products, it was studied as the putative IL-1beta synergizing factor. Although recombinant TNFalpha (rTNFalpha) alone had no IFNgamma inducing activity, neutralization of TNFalpha in the monocyte conditioned media inhibited the IFNgamma inducing activity. Furthermore, rTNFalpha restored the IFNgamma inducing activity of the size-fractionated IL-1beta. Finally, rTNFalpha synergized with rIL-1beta, as well as with rIL-1alpha and rIL-18, for KG-1 IFNgamma release. These studies demonstrate a synergistic role between TNFalpha and IL-1 family members in the induction of IFNgamma production and give caution to interpretations of KG-1 functional assays designed to detect functional IL-18.

Interleukin 18 in the heart

IL-18, originally termed as interferon gamma (IFN-gamma) inducing factor, is a proinflammatory cytokine that belongs to the IL-1 cytokine superfamily. IL-18 plays an important role in immune, infectious, and inflammatory diseases due to its induction of IFN-gamma. However, accumulated evidence has demonstrated that other effects of IL-18 are independent of IFN-gamma. Here, we reviewed the current literatures regarding the role of IL-18 in the heart and cardiovascular system. Infiltrated neutrophils, resident macrophages, endothelial cells, smooth muscle cells, and cardiomyocytes in the heart are able to produce IL-18 in response to injury. IL-18 is produced as a biologically inactive precursor (pro-IL-18) that is activated by caspase 1 (the IL-1beta converting enzyme). Elevated IL-18 levels have been observed in cardiac tissue and circulation after myocardial I/R and sepsis. The possible cellular and molecular mechanisms concerning IL-18-induced myocardial injury include induction of inflammation, increased apoptosis, a cardiac hypertrophy effect, modulation of mitogen activated protein kinase activation, and changes in intracellular calcium. Finally, we briefly reviewed the therapeutic strategies for inhibiting IL-18's biological activity to protect cardiac tissue from injury.

Identification of the most active interleukin-32 isoform

Cytokines are crucial in host defence against pathogens such as bacteria, viruses, fungi and parasites. A newly described cytokine, interleukin-32 (IL-32), induces various proinflammatory cytokines (tumour necrosis factor-alpha, IL-1beta, IL-6) and chemokines in both human and mouse cells through the nuclear factor-kappaB and p38 mitogen-activated protein kinase inflammatory signal pathway. The IL-32 primarily acts on monocytic cells rather than T cells. In an attempt to isolate the IL-32 soluble receptor, we used an IL-32 ligand-affinity column to purify neutrophil proteinase 3, which is a serine proteinase involved in many inflammatory diseases. IL-32 has biological activity associated with Mycobacterium tuberculosis and chronic proinflammatory diseases such as rheumatoid arthritis. IL-32 is transcribed as six alternative splice variants and the biological activity of each individual isoform remains unknown. Here, we cloned the complementary DNA of the four IL-32 isoforms (alpha, beta, gamma and delta) that are the most representative IL-32 transcripts. To produce recombinant protein with a high yield, the amino acids of two cysteine residues were mutated to serine residues, because serine residues are not conserved among different species. The multi-step purified recombinant IL-32 isoform proteins were assessed for their biological activities with different cytokine assays. The gamma isoform of IL-32 was the most active, although all isoforms were biologically active. The present study will provide a specific target to neutralize endogenous IL-32, which may contribute to basic and clinical immunology.

The interleukin-1 receptor family

The cytokines IL-1 and IL-18 are key molecules both in the innate and in the adaptive immune response. Their activity is mediated by specific receptors present on the membrane of target cells. It has become apparent that these receptors are members of a larger family of related receptors, most of which are apparently involved in the mechanisms of host defense. Thus, the large Toll/IL-1R (TIR) superfamily encompasses the Ig domain family (IL-1 receptors, IL-18 receptors, and IL-1R-like receptors), the leucine-rich domain family [the Toll-like receptors (TLR) and similar receptors], and a series of TIR domain-containing intracellular adapter molecules. The TIR superfamily is defined by a common intracellular TIR domain, involved in the initiation of signaling. A group of TIR domain-containing adapters (MyD88, TIRAP, TRIF, and TRAM) are differentially recruited to the Toll/IL-1 receptors, contributing to the specificity of signaling. Recent studies have also begun to unravel the mechanisms of negative regulation of the Toll/IL-1 receptors. The orphan receptor TIR8/SIGIRR, a member of TIR superfamily, while unable to initiate signaling, can negatively modulate the TIR-mediated responses. Other negative regulators of the Toll/IL-1R family include T1/ST2, some soluble forms of TLR, and MyD88s. The coordinated positive and negative regulation of the TIR activation ensures the appropriate modulation of the innate and inflammatory responses and avoids the risk of pathological derangement. This chapter will consider in detail the characteristics and functional role of the Ig domain receptor subfamily in the regulation of host defense and their possible role in pathology.

Expression and alternative processing of IL-18 inhuman 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.

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.

IL-18 E42A mutant is resistant to the inhibitory effects of HPV-16 E6 and E7 oncogenes on the IL-18-mediated immune response

Our previous studies showed that the down-modulation of IL-18-induced immune response caused by oncoproteins E6 and E7 as one of the mechanisms underlying immune escape in HPV-induced cervical cancer cells. E42 residue of IL-18 also appears to be critical in the activity of IL-18. Single point mutation E42 in IL-18 show promise in the study of IL-18 binding motifs for HPV oncoproteins. We attempted to ascertain whether site-specific IL-18 mutant E42A would modulate the inhibitory effects of IL-18-induced immune responses via the HPV 16 E6 and E7 oncoproteins. Compared to wild type IL-18, E42A-induced IFN-gamma production was not inhibited by HPV 16 E6 and E7. In vitro and in vivo binding assays have also revealed that E6 and E7 do not result in the inhibition of the binding of E42A to its IL-18 receptor alpha chain. There were no effects on the E42A-induced phosphorylations of p38 and JNK observed in the presence of E6 or E7. The degradation of IkappaB by E42A was not affected by E6 or E7 in NK0 cells. Moreover, E42A-induced NF-kappaB activation was also not inhibited by these oncoproteins. These results suggest that E42A is a stronger activator than wild type IL-18, and is not susceptible to inhibition by the HPV oncoproteins E6 and E7. Thus, it is suggested that E42A could be used in immunotherapy for patients with cervical cancer.

Molecular characterization and expression of caprine (Capra hircus) interleukin-18 cDNA

Interleukin 18 (IL-18) has been identified as a potent upstream cytokine required for upregulation of IFN-gamma secretion that plays a crucial role in polarization of Th1 type of immune response. Considering the potential applications of the cytokine in immunomodulation, it has been characterized in many livestock species including cattle, equines, canines, felines and porcines. In this paper we report the isolation, cloning sequencing and expression of caprine precursor IL-18. Full-length caprine IL-18 cDNA was isolated from mitogen-stimulated adherent peripheral blood mononuclear cells using reverse transcription polymerase chain reaction (RT-PCR). The cDNA contained an open reading frame of 579 bp encoding a putative polypeptide of 192 amino acids. Deduced amino acid sequence of caprine IL-18 showed varying amino acid identity with the published sequences of other domestic ruminant species ranging from 94.3% to 96.9%, while it shared over 78% aa identity with other domestic animals. Pairwise multiple aligned sequences showed a deletion of Glu31in caprine IL-18 unlike in other species. Recombinant caprine IL-18 was produced in Escherichia coli, which cross-reacted with two antiporcine IL-18 monoclonal antibodies.

Molecular cloning and expression profile analysis of interleukin-10 and interleukin-18 cDNA of Indian water buffalo (Bubalus bubalis)

The cDNAs encoding the interleukin-10 and interleukin-18 of Indian water buffalo (Bubalus bubalis) were cloned and sequenced. A 537 bp IL-10 cDNA fragment and a 623 bp IL-18 cDNA fragment were amplified by reverse transcriptase polymerase chain reaction (RT-PCR) from concanavalin A stimulated splenocytes. Sequence analysis of these cytokines revealed high level conservation at nucleic acid and protein level. Both these cytokines also showed strict conservation in the predicted secondary structure and critical amino acid residues compared to the ruminant homologues. Basal level expression of both IL-10 and IL-18 was observed in liver, lung and spleen. The expression level of IL-10 was not affected by mitogenic stimulation, whereas IL-18 was up regulated upon stimulation. The availability of these cytokine molecules will aid in the study of their role in the immunology and pathogenesis of infections in water buffalo.

Effect of bacterial virulence on IL-18 expression in the amnion infected with Escherichia coli

PROBLEM

The upregulation of inflammatory substances threatens pregnancy. Interleukin-18 (IL-18) is elevated in women who miscarried. The purpose of this study was to develop a pig model of chorioamnionitis to study the effect of bacterial virulence on IL-18 response in experimentally infected amnion.

METHOD OF STUDY

A total of 20,000 colony-forming units of Escherichia coli (an enteropathogenic O55 strain, EPEC or O86 non-pathogenic strain) were administered into the amniotic cavity of pig fetuses at 70% of gestation for 10 hr. Fetal amniotic fluid samples were analyzed for IL-18 levels by enzyme-linked immunosorbent assay. The expression of IL-18 was studied also by immunohistochemistry on cryostat sections through amniotic membranes and pathological changes were observed by electron microscopy.

RESULTS

Both E. coli strains propagated in amniotic fluids and reached similar counts. Only EPEC, however, caused a significant increase of IL-18 amniotic fluid levels (P < 0.001) and cytokine expression in the amniotic epithelium.

CONCLUSIONS

The levels of IL-18 in infected amniotic fluids correlated with bacterial virulence and pathological changes in the amnion.

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-1 homologues IL-1F7b and IL-18 contain functional mRNA instability elements within the coding region responsive to lipopolysaccharide

IL-1F7b, a novel homologue of the IL-1 (interleukin 1) family, was discovered by computational cloning. We demonstrated that IL-1F7b shares critical amino acid residues with IL-18 and binds to the IL-18-binding protein enhancing its ability to inhibit IL-18-induced interferon-gamma. We also showed that low levels of IL-1F7b are constitutively present intracellularly in human blood monocytes. In this study, we demonstrate that similar to IL-18, both mRNA and intracellular protein expression of IL-1F7b are up-regulated by LPS (lipopolysaccharide) in human monocytes. In stable transfectants of murine RAW264.7 macrophage cells, there was no IL-1F7b protein expression despite a highly active CMV promoter. We found that IL-1F7b-specific mRNA was rapidly degraded in transfected cells, via a 3'-UTR (untranslated region)-independent control of IL-1F7b transcript stability. After LPS stimulation, there was a rapid transient increase in IL-1F7b-specific mRNA and concomitant protein levels. Using sequence alignment, we found a conserved ten-nucleotide homology box within the open reading frame of IL-F7b, which is flanking the coding region instability elements of some selective genes. In-frame deletion of downstream exon 5 from the full-length IL-1F7b cDNA markedly increased the levels of IL-1F7b mRNA. A similar coding region element is located in IL-18. When transfected into RAW264.7 macrophages, IL-18 mRNA was also unstable unless treated with LPS. These results indicate that both IL-1F7b and IL-18 mRNA contain functional instability determinants within their coding region, which influence mRNA decay as a novel mechanism to regulate the expression of IL-1 family members.

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.

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.

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

Interleukin (IL)-18 binding protein (IL-18BP) is a natural inhibitor of the pleiotropic cytokine IL-18. To study the role of IL-18BP in modulating inflammatory responses in vivo, mice transgenic for human IL-18BP isoform a (IL-18BP-Tg) were generated. The transgene was expressed at high levels in each organ examined. High levels of bioactive human IL-18BPa were detectable in the circulation of IL-18BP-Tg mice, which were viable, fertile, and had no tissue or organ abnormality. The high levels of IL-18BP in the transgenic mice were able to completely neutralize the interferon-gamma (IFN-gamma)-inducing activity of exogenously administered IL-18. Following administration of endotoxin, with or without prior sensitization with heat-inactivated Propionibacterium acnes, IL-18BP-Tg mice produced significantly lower serum levels of IFN-gamma and macrophage-inflammatory protein-2 compared with nontransgenic littermates. Significantly reduced production of IFN-gamma in response to endotoxin was also observed in cultures of IL-18BP-Tg splenocytes. Finally, IL-18BP-Tg mice were completely protected in a model of hepatotoxicity induced by administration of concanavalin A. These results indicate that high endogenous levels of IL-18BP in trangenic mice effectively neutralize IL-18 and are protective in response to different inflammatory stimuli.

The structure and binding mode of interleukin-18

Interleukin-18 (IL-18), a cytokine formerly known as interferon-gamma- (IFN-gamma-) inducing factor, has pleiotropic immunoregulatory functions, including augmentation of IFN-gamma production, Fas-mediated cytotoxicity and developmental regulation of T-lymphocyte helper type I. We determined the solution structure of IL-18 as a first step toward understanding its receptor activation mechanism. It folds into a beta-trefoil structure that resembles that of IL-1. Extensive mutagenesis revealed the presence of three sites that are important for receptor activation: two serve as binding sites for IL-18 receptor alpha (IL-18Ralpha), located at positions similar to those of IL-1 for IL-1 receptor type I (IL-1RI), whereas the third site may be involved in IL-18 receptor beta (IL-18Rbeta) binding. The structure and mutagenesis data provide a basis for understanding the IL-18-induced heterodimerization of receptor subunits, which is necessary for receptor activation.

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.

A surface amebic cysteine proteinase inactivates interleukin-18

Amebiasis is a major cause of morbidity and mortality worldwide. Invasion by Entamoeba histolytica trophozoites causes secretion of proinflammatory cytokines from host epithelial cells, leading to a local acute inflammatory response, followed by lysis of colonic cells. Extracellular cysteine proteinases from amebic trophozoites are key virulence factors and have a number of important interactions with host defenses, including cleavage of immunoglobulin G (IgG), IgA, and complement components C3 and C5. Amebic lysates have also been shown to activate the precursor to interleukin 1-beta (proIL-1beta), mimicking the action of caspase-1. IL-18 is also a central cytokine, which induces gamma interferon (IFN-gamma) and activates macrophages, one of the main host defenses against invading trophozoites. Because proIL-18 is also activated by caspase-1, we evaluated whether amebic proteinases had a similar effect. Instead, we found that recombinant proIL-18 was cleaved into smaller fragments by the complex of surface-associated and released amebic proteinases. To evaluate the function of an individual proteinase from the complex pool, we expressed an active surface proteinase, EhCP5, which is functional only in E. histolytica. Recombinant EhCP5 expressed in Pichia pastoris had kinetic properties similar to those of the native enzyme with respect to substrate specificity and sensitivity to proteinase inhibitors. In contrast to the activation of proIL-1beta by amebic lysates, the purified proteinase cleaved proIL-18 and mature IL-18 to biologically inactive fragments. These studies suggest that the acute host response and amebic invasion result from a complex interplay of parasite virulence factors and host defenses. E. histolytica may block the host inflammatory response by a novel mechanism, inactivation of IL-18.

A complex of the IL-1 homologue IL-1F7b and IL-18-binding protein reduces IL-18 activity

IL-1F7 was discovered in expressed sequence tag databases as a member of the increasing family of proteins sharing sequence homology to IL-1alpha/beta, IL-1Ra, and IL-18. In the present study using immunohistochemical staining, IL-1F7 was localized in human peripheral monocytic cells, suggesting its role in immune regulation. Recombinant human IL-1F7b was shown to bind to the IL-18Ralpha but without IL-18 agonistic or antagonistic function. Using chemical cross-linking, we observed that, unlike IL-18, IL-1F7b fails to recruit the IL-18Rbeta chain to form a functionally active, ternary complex with the IL-18Ralpha chain. IL-1F7b shares two conserved amino acids with IL-18 (Glu-35 and Lys-124), which participate in the interaction of IL-18 with the IL-18Ralpha chain as well as the IL-18-binding protein (IL-18BP), a secreted protein that neutralizes IL-18 activity. In testing whether IL-1F7b interacts with IL-18BP, we unexpectedly observed that IL-1F7b enhanced the ability of IL-18BP to inhibit IL-18-induced IFNgamma by 25-30% in a human natural killer cell line. This effect was observed primarily at limiting concentrations of IL-18BP (3.12-12.5 ng/ml) and at a 50- to 100-fold molar excess of IL-1F7b. Similar results were obtained by using isolated human peripheral blood mononuclear cells. To study the molecular basis of this effect we performed binding studies of IL-1F7b and IL-18BP. After cross-linking, a high molecular weight complex consisting of IL-1F7b and IL-18BP was observed on SDS/PAGE. We propose that after binding to IL-18BP, IL-1F7b forms a complex with IL-18Rbeta, depriving the beta-chain of forming a functional receptor complex with IL-18Ralpha and thus inhibiting IL-18 activity.