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

Elevated serum levels of interleukin-18 discriminate Still's disease from other autoinflammatory conditions: results from the European ImmunAID cohort

OBJECTIVES

Systemic autoinflammatory diseases (SAIDs) represent a set of conditions with exaggerated innate immune responses. IL-1β and IL-18 are key cytokines involved in the pathogenesis of some SAID. We aimed to assess the diagnostic value of serum levels of IL-1β, IL-18, their respective inhibitors IL-1Ra and IL-18 binding protein (IL-18BP), and IFN-γ in SAID.

METHODS

A cohort of patients with active SAID, including monogenic (mSAID) and genetically undiagnosed SAID (guSAID) from different European countries, with active disease at inclusion, was established. Serum levels of cytokines were measured by immunoassays.

RESULTS

Sera from 53 mSAID, 220 guSAID and 49 controls without inflammatory disease were analysed. Serum levels of total and free IL-18 were significantly increased in Still's disease in comparison to most SAID and non-inflammatory controls. Levels of total IL-18 were also elevated in patients with familial Mediterranean fever to a comparable extent as in Still's disease. In contrast, free IL-18 levels were selectively higher in Still's disease. Receiver operating characteristic curve analysis showed that total IL-18 was the most sensitive and specific marker for the diagnosis of Still's disease (area under the curve=0.91). There was a positive correlation between IL-18 and ferritin. In 10 patients with Still's disease who had a second blood collection, we found a significant decrease in serum levels of free IL-18 after treatment.

CONCLUSIONS

Our results show that IL-18 can discriminate Still's disease from other SAID, and free IL-18 levels may be relevant to assess response to therapy in these patients.

Development of anti-murine IL-18 binding protein antibodies to stimulate IL-18 bioactivity

Interleukin (IL)-18 is an immunoregulatory cytokine that acts as a potent inducer of T helper 1 and cytotoxic responses. IL-18 activity is regulated by its decoy receptor IL-18 binding protein (IL-18BP) which forms a high affinity complex with IL-18 to block binding of the cognate receptors. A disbalance between IL-18 and IL-18BP associated with excessive IL-18 signaling can lead to systemic inflammation. Indeed, the severity of CpG-induced macrophage activation syndrome (MAS) is exacerbated in IL-18BP KO mice. On the contrary, targeting IL-18BP can have promising effects to enhance immune responses against pathogens and cancer. We generated monoclonal rabbit anti-mouse IL-18BP antibodies labeled from 441 to 450. All antibodies, except from antibody 443, captured mIL-18BP when used in a sandwich ELISA. Using an IL-18 bioassay, we showed that antibody 441 did not interfere with the regulatory effect of mIL-18BP, whereas all other antibodies displayed different levels of antagonism. Further experiments were performed using antibody 445 endowed with potent neutralizing activity and antibody 441. Despite binding to IL-18BP with the same affinity, antibody 445, but not antibody 441, was able to release IL-18 from preformed IL-18-IL-18BP complexes. Administration of antibody 445 significantly aggravated the severity of CpG-induced MAS as compared to antibody 441. Additional experiments using naïve WT, IL-18BP KO, and IL-18 KO mice confirmed the specificity of the neutralizing effect of antibody 445 towards IL-18BP. Our studies led to the development of a monoclonal anti-IL-18BP antibody with neutralizing activity that results in the promotion of IL-18 activities.

IL-18 signaling is regulated by caspase 6/8 and IL-18BP in turbot (Scophthalmus maximus)

Interleukin (IL)-18 is synthesized as a precursor that requires intracellular processing to become functionally active. In human, IL-18 is processed by caspase 1 (CASP1). In teleost, the maturation and signal transduction mechanisms of IL-18 are unknown. We identified two IL-18 variants, IL-18a and IL-18b, in turbot. IL-18a, but not IL-18b, was processed by CASP6/8 cleavage. Mature IL-18a bound specifically to IL-18 receptor (IL-18R) α-expressing cells and induced IL-18Rα-IL-18Rβ association. Bacterial infection promoted IL-18a maturation in a manner that required CASP6 activation and correlated with gasdermin E activation. The mature IL-18a induced proinflammatory cytokine expression and enhanced bacterial clearance. IL-18a-mediated immune response was suppressed by IL-18 binding protein (IL-18BP), which functioned as a decoy receptor for IL-18a. IL-18BP also functioned as a pathogen pattern recognition receptor and directly inhibited pathogen infection. Our findings revealed unique mechanism of IL-18 maturation and conserved mechanism of IL-18 signaling and regulation in turbot, and provided new insights into the regulation and function of IL-18 related immune signaling.

Structural transitions enable interleukin-18 maturation and signaling

Several interleukin-1 (IL-1) family members, including IL-1β and IL-18, require processing by inflammasome-associated caspases to unleash their activities. Here, we unveil, by cryoelectron microscopy (cryo-EM), two major conformations of the complex between caspase-1 and pro-IL-18. One conformation is similar to the complex of caspase-4 and pro-IL-18, with interactions at both the active site and an exosite (closed conformation), and the other only contains interactions at the active site (open conformation). Thus, pro-IL-18 recruitment and processing by caspase-1 is less dependent on the exosite than the active site, unlike caspase-4. Structure determination by nuclear magnetic resonance uncovers a compact fold of apo pro-IL-18, which is similar to caspase-1-bound pro-IL-18 but distinct from cleaved IL-18. Binding sites for IL-18 receptor and IL-18 binding protein are only formed upon conformational changes after pro-IL-18 cleavage. These studies show how pro-IL-18 is selected as a caspase-1 substrate, and why cleavage is necessary for its inflammatory activity.

The pros and confs of IL-18 activation

Interleukin-1 (IL-1) family cytokines are key immunological regulators that achieve their signaling prowess after post-translational proteolytic processing. In this issue of Immunity, Dong et al. reveal the structural consequences of this process on proinflammatory IL-18, demonstrating that pro-IL-18 and mature IL-18 are structurally distinct.

Interleukin‑18 binding protein: Biological properties and roles in human and animal immune regulation (Review)

IL-18 binding protein (IL-18BP) is a natural regulatory molecule of the proinflammatory cytokine IL-18. It can regulate activity of IL-18 by high affinity binding. The present review aimed to highlight developments, characteristics and functions of IL-18BP. IL-18BP serves biological and anti-pathological roles in treating disease. In humans, it modulates progression of a number of chronic diseases, such as adult-onset Still's disease. The present review summarizes molecular structure, role of IL-18BP in disease and interaction with other proteins in important pathological processes.

[Clinical significance of IL-18 and IL-18-binding protein in bone marrow of patients with myelodysplastic syndrome]

Objective: To analyze the level and clinical significance of IL-18 and IL-18-binding protein (BP) in the bone marrow of patients with myelodysplastic syndrome (MDS) . Methods: A total of 43 newly diagnosed patients with MDS who were admitted to the Department of Hematology, Tianjin Medical University General Hospital, from July 2020 to February 2021 were randomly selected. The control group consisted of 14 patients with acute myeloid leukemia (AML) and 25 patients with iron-deficiency anemia (IDA). The levels of IL-18 and IL-18 BP in the bone marrow supernatant were measured, and their correlations with MDS severity, as well as the functionality of CD8(+) T cells and natural killer cells, was analyzed. Results: The levels of IL-18, IL-18 BP, and free IL-18 (fIL-18) in the bone marrow supernatant of patients with MDS were higher than in the IDA group. The level of fIL-18 was linearly and negatively correlated with the MDS-International Prognostic Scoring System (IPSS) score. IL-18 receptor (IL-18Rα) expression on CD8(+) T cells in the MDS group was lower than in the IDA group, and the levels of fIL-18 and IL-18Rα were positively correlated with CD8(+) T-cell function in the MDS group. Conclusion: IL-18 BP antagonizes IL-18, leading to a decrease in fIL-18 in the bone marrow microenvironment of patients with MDS, affecting CD8(+) T-cell function, which is closely related to MDS severity; therefore, it may become a new target for MDS treatment.

Heat Shock Protein SSA1 Enriched in Hypoxic Secretome of Candida albicans Exerts an Immunomodulatory Effect via Regulating Macrophage Function

Candida albicans is an opportunistic pathogenic yeast that can survive in both normoxic and hypoxic environments. The involvement of C. albicans secretome on host biological processes has been demonstrated. However, the immunoregulatory function of C. albicans secretome released under hypoxic condition remains unclear. This study demonstrated the differences in cytokine responses and protein profiles between secretomes prepared under normoxic and hypoxic conditions. Furthermore, the immunoregulatory effects of heat shock protein SSA1(Ssa1), a protein candidate enriched in the hypoxic secretome, were investigated. Stimulation of mouse bone marrow-derived macrophages (BMMs) with Ssa1 resulted in the significant production of interleukin (IL)-10, IL-6, and tumor necrosis factor (TNF)-α as well as the significant expression of M2b macrophage markers (CD86, CD274 and tumor necrosis factor superfamily member 14), suggesting that C. albicans Ssa1 may promote macrophage polarization towards an M2b-like phenotype. Proteomic analysis of Ssa1-treated BMMs also revealed that Ssa1 reduced inflammation-related factors (IL-18-binding protein, IL-1 receptor antagonist protein, OX-2 membrane glycoprotein and cis-aconitate decarboxylase) and enhanced the proteins involved in anti-inflammatory response (CMRF35-like molecule 3 and macrophage colony-stimulating factor 1 receptor). Based on these results, we investigated the effect of Ssa1 on C. albicans infection and showed that Ssa1 inhibited the uptake of C. albicans by BMMs. Taken together, our results suggest that C. albicans alters its secretome, particularly by promoting the release of Ssa1, to modulate host immune response and survive under hypoxic conditions.

Molecular cloning, characterization and gene expression analysis of twelve interleukins in obscure puffer Takifugu obscurus

Interleukins (ILs) are a subgroup of secreted cytokines, which are molecules involved in the intercellular regulation of the immune system. In this study, 12 IL homologs were cloned and functionally identified from obscure puffer Takifugu obscurus, and they were termed as ToIL-1β, ToIL-1, ToIL-6, ToIL-10, ToIL-11, ToIL-12, ToIL-17, ToIL-18, ToIL-20, ToIL-24, ToIL-27, and ToIL-34. Multiple alignment results showed that except for ToIL-24 and ToIL-27, other deduced ToIL proteins shared typical characteristics and structure with other known fish ILs. Phylogenetic analysis revealed that 12 ToILs were evolutionarily closely related to their counterparts in other selected vertebrates. Tissue distribution assay demonstrated that the mRNA transcripts of most ToIL genes were constitutively expressed in all tissues examined, with relatively high expression in immune tissues. Following Vibrio harveyi and Staphylococcus aureus infection, the expression levels of 12 ToILs in the spleen and liver were significantly upregulated, and their response over time varied. Taken together, these data were discussed accordingly with the ToIL expression and the immune response under the different situations tested. The results suggest that the 12 ToIL genes are involved in the antibacterial immune response in T. obscurus.

Recognition and maturation of IL-18 by caspase-4 noncanonical inflammasome

The canonical (caspase-1) and noncanonical (comprising caspases 4, 5 and 11; hereafter, caspase-4/5/11) inflammasomes both cleave gasdermin D (GSDMD) to induce pyroptosis1,2. Whereas caspase-1 processes IL-1β and IL-18 for maturation3-6, no cytokine target has been firmly established for lipopolysaccharide-activated caspase-4/5/117-9. Here we show that activated human caspase-4, but not mouse caspase-11, directly and efficiently processes IL-18 in vitro and during bacterial infections. Caspase-4 cleaves the same tetrapeptide site in pro-IL-18 as caspase-1. The crystal structure of the caspase-4-pro-IL-18 complex reveals a two-site (binary) substrate-recognition mechanism; the catalytic pocket engages the tetrapeptide, and a unique exosite that critically recognizes GSDMD10 similarly binds to a specific structure formed jointly by the propeptide and post-cleavage-site sequences in pro-IL-18. This binary recognition is also used by caspase-5 as well as caspase-1 to process pro-IL-18. In caspase-11, a structural deviation around the exosite underlies its inability to target pro-IL-18, which is restored by rationally designed mutations. The structure of pro-IL-18 features autoinhibitory interactions between the propeptide and the post-cleavage-site region, preventing recognition by the IL-18Rα receptor. Cleavage by caspase-1, -4 or -5 induces substantial conformational changes of IL-18 to generate two critical receptor-binding sites. Our study establishes IL-18 as a target of lipopolysaccharide-activated caspase-4/5. The finding is paradigm shifting in the understanding of noncanonical-inflammasome-mediated defences and also the function of IL-18 in immunity and disease.

SIRT1 regulates hepatocyte programmed cell death via GSDME - IL18 axis in human and mouse liver transplantation

Sirtuin 1 (SIRT1) is a histone/protein deacetylase in the cellular response to inflammatory, metabolic, and oxidative stressors. We previously reported that myeloid SIRT1 regulates the inflamed liver's canonical pyroptosis cell death pathway. However, whether/how hepatocyte SIRT1 is engaged in programmed cell death in the cold-stressed liver remains uncertain. Here, we undertook translational studies in human and mouse orthotopic liver transplantation (OLT) to interrogate the significance of hepatocyte-specific SIRT1 in cold-stored donor livers and liver grafts after reperfusion. In the clinical arm of sixty human OLT patients, hepatic SIRT1 levels in cold-preserved donor livers correlated with the anti-apoptotic Bcl-2 expression. After reperfusion, improved OLT function was accompanied by hepatic SIRT1 levels negatively associated with cleaved caspase-3 expression. In the experimental arm, we compared FLOX-control with hepatocyte-specific SIRT1-KO livers after orthotopic transplantation into WT mouse recipients, parallel with primary murine hepatocyte cultures subjected to cold activation with/without knockdown of SIRT1, GSDME, and IL18Rβ. Indeed, hepatocyte SIRT1 deficiency upregulated apoptosis and GSDME-mediated programmed cell death, deteriorating hepatocellular function and shortening OLT survival. Augmented GSDME processing, accompanied by increased secretion of IL18 by stressed hepatocytes, was prominent in SIRT1-deficient, cold-stored livers. Hepatocyte SIRT1 expression regulated anti-apoptotic Bcl-2/XIAP proteins, suppressed cold stress-triggered apoptosis, and mitigated GSDME licensing to release IL18. Notably, consistent with the ability of IL18 to depress hepatocyte SIRT1 and Bcl-2/XIAP in vitro, IL18 neutralization in vivo prevented hepatocellular damage and restored the anti-apoptotic phenotype in otherwise injury-prone SIRT1-deficient OLTs. In conclusion, this translational study identifies a novel hepatocyte SIRT1-IL18 molecular circuit as a therapeutic target in the mechanism underpinning hepatocyte death pathways in human and mouse liver transplantation.

SIRT1 Regulates Hepatocyte Programmed Cell Death via GSDME - IL18 Axis in Human and Mouse Liver Transplantation

Sirtuin 1 (SIRT1) is a histone/protein deacetylase involved in cellular senescence, inflammation, and stress resistance. We previously reported that myeloid SIRT1 signaling regulates the inflamed liver's canonical pyroptosis cell death pathway. However, whether/how hepatocyte SIRT1 is engaged in programmed cell death in the cold-stressed liver remains uncertain. Here, we undertook translational studies in human and mouse orthotopic liver transplantation (OLT) to interrogate the significance of hepatocyte-specific SIRT1 signaling in cold-stored donor livers and liver grafts after reperfusion. In the clinical arm of sixty human OLT patients, hepatic SIRT1 levels in cold-preserved donor livers correlated with anti-apoptotic Bcl-2 expression. After reperfusion, improved OLT function was accompanied by hepatic SIRT1 levels negatively associated with cleaved caspase-3 expression. In the experimental arm, we compared FLOX-control with hepatocyte-specific SIRT1-KO livers after orthotopic transplantation into WT mouse recipients, parallel with primary murine hepatocyte cultures subjected to cold activation with/without knockdown of SIRT1, GSDME, and IL18Rβ signaling. Hepatocyte SIRT1 deficiency upregulated apoptosis and GSDME-mediated programmed cell death, which in turn deteriorated the hepatocellular function and shortened OLT survival. Augmented GSDME processing, accompanied by increased secretion of IL18 by stressed hepatocytes, was prominent in SIRT1-deficient, cold-stored livers. Hepatocyte SIRT1 signaling regulated anti-apoptotic Bcl-2/XIAP proteins, suppressed cold stress-triggered apoptosis, and mitigated GSDME licensing to release IL18. Notably, while crosslinking IL18R depressed SIRT1 and Bcl-2/XIAP signaling in vitro, IL18 neutralization in vivo prevented hepatocellular damage and restored the anti-apoptotic phenotype in otherwise injury-prone SIRT1-deficient OLTs. In conclusion, this translational study identifies a novel hepatocyte SIRT1-IL18 signaling circuit as a therapeutic target in the mechanism underpinning hepatocyte death in human and mouse liver transplantation.

mRNAs encoding IL-12 and a decoy-resistant variant of IL-18 synergize to engineer T cells for efficacious intratumoral adoptive immunotherapy

Interleukin-12 (IL-12) gene transfer enhances the therapeutic potency of adoptive T cell therapies. We previously reported that transient engineering of tumor-specific CD8 T cells with IL-12 mRNA enhanced their systemic therapeutic efficacy when delivered intratumorally. Here, we mix T cells engineered with mRNAs to express either single-chain IL-12 (scIL-12) or an IL-18 decoy-resistant variant (DRIL18) that is not functionally hampered by IL-18 binding protein (IL-18BP). These mRNA-engineered T cell mixtures are repeatedly injected into mouse tumors. Pmel-1 T cell receptor (TCR)-transgenic T cells electroporated with scIL-12 or DRIL18 mRNAs exert powerful therapeutic effects in local and distant melanoma lesions. These effects are associated with T cell metabolic fitness, enhanced miR-155 control on immunosuppressive target genes, enhanced expression of various cytokines, and changes in the glycosylation profile of surface proteins, enabling adhesiveness to E-selectin. Efficacy of this intratumoral immunotherapeutic strategy is recapitulated in cultures of tumor-infiltrating lymphocytes (TILs) and chimeric antigen receptor (CAR) T cells on IL-12 and DRIL18 mRNA electroporation.

Role and Potential of Different T Helper Cell Subsets in Adoptive Cell Therapy

Historically, CD8⁺ T cells have been considered the most relevant effector cells involved in the immune response against tumors and have therefore been the focus of most cancer immunotherapy approaches. However, CD4⁺ T cells and their secreted factors also play a crucial role in the tumor microenvironment and can orchestrate both pro- and antitumoral immune responses. Depending on the cytokine milieu to which they are exposed, CD4⁺ T cells can differentiate into several phenotypically different subsets with very divergent effects on tumor progression. In this review, we provide an overview of the current knowledge about the role of the different T helper subsets in the immune system, with special emphasis on their implication in antitumoral immune responses. Furthermore, we also summarize therapeutic applications of each subset and its associated cytokines in the adoptive cell therapy of cancer.

Interleukin-18 cytokine in immunity, inflammation, and autoimmunity: Biological role in induction, regulation, and treatment

Interleukin-18 (IL-18) is a potent pro-inflammatory cytokine involved in host defense against infections and regulates the innate and acquired immune response. IL-18 is produced by both hematopoietic and non-hematopoietic cells, including monocytes, macrophages, keratinocytes and mesenchymal cell. IL-18 could potentially induce inflammatory and cytotoxic immune cell activities leading to autoimmunity. Its elevated levels have been reported in the blood of patients with some immune-related diseases, including rheumatoid arthritis, systemic lupus erythematosus, type I diabetes mellitus, atopic dermatitis, psoriasis, and inflammatory bowel disease. In the present review, we aimed to summarize the biological properties of IL-18 and its pathological role in different autoimmune diseases. We also reported some monoclonal antibodies and drugs targeting IL-18. Most of these monoclonal antibodies and drugs have only produced partial effectiveness or complete ineffectiveness in vitro, in vivo and human studies. The ineffectiveness of these drugs targeting IL-18 may be largely due to the loophole caused by the involvement of other cytokines and proteins in the signaling pathway of many inflammatory diseases besides the involvement of IL-18. Combination drug therapies, that focus on IL-18 inhibition, in addition to other cytokines, are highly recommended to be considered as an important area of research that needs to be explored.

Interleukin-18 Binding Protein in Immune Regulation and Autoimmune Diseases

Natural soluble antagonist and decoy receptor on the surface of the cell membrane are evolving as crucial immune system regulators as these molecules are capable of recognizing, binding, and neutralizing (so-called inhibitors) their targeted ligands. Eventually, these soluble antagonists and decoy receptors terminate signaling by prohibiting ligands from connecting to their receptors on the surface of cell membrane. Interleukin-18 binding protein (IL-18BP) participates in regulating both Th1 and Th2 cytokines. IL-18BP is a soluble neutralizing protein belonging to the immunoglobulin (Ig) superfamily as it harbors a single Ig domain. The Ig domain is essential for its binding to the IL-18 ligand and holds partial homology to the IL-1 receptor 2 (IL-1R2) known as a decoy receptor of IL-1α and IL-1β. IL-18BP was defined as a unique soluble IL-18BP that is distinct from IL-18Rα and IL-18Rβ chain. IL-18BP is encoded by a separated gene, contains 8 exons, and is located at chr.11 q13.4 within the human genome. In this review, we address the difference in the biological activity of IL-18BP isoforms, in the immunity balancing Th1 and Th2 immune response, its critical role in autoimmune diseases, as well as current clinical trials of recombinant IL-18BP (rIL-18BP) or equivalent.