Human macrophages produce dimeric forms of IL-18 which can be detected with monoclonal antibodies specific for inactive IL-18

Generation and characterization of antagonistic anti-human interleukin (IL)-18 monoclonal antibodies with high affinity: Two types of monoclonal antibodies against full-length IL-18 and the neoepitope of inflammatory caspase-cleaved active IL-18

Interleukin-18 (IL-18) is a pro-inflammatory cytokine that evokes both innate and acquired immune responses. IL-18 is initially synthesized as an inactive precursor and the cleavage for processing into a mature, active molecule is mediated by pro-inflammatory caspases following the activation of inflammasomes. Two types of monoclonal antibodies were raised: anti-IL-18^63-68 antibodies which recognize full-length^1-193 and cleaved IL-18; and anti-IL-18 neoepitope antibodies which specifically recognize the new N-terminal ³⁷YFGKLESK⁴⁴ of IL-18 cleaved by pro-inflammatory caspase-1/4. These mAbs were suitable for Western blotting, capillary Western immunoassay (WES), immunofluorescence, immunoprecipitation, and function-blocking assays. WES analysis of these mAbs allowed visualization of the IL-18 bands and provided a molecular weight corresponding to the pro-inflammatory caspase-1/4 cleaved, active form IL-18^37-193, and not to the inactive precursor IL-18, in the serum of patients with adult-onset Still's disease (6/14, 42%) and hemophagocytic activation syndrome (2/6, 33%). These monoclonal antibodies will be very useful in IL-18 and inflammasome biology and for diagnostic and therapeutic strategies for inflammatory diseases.

New perspectives on IL-33 and IL-1 family cytokines as innate environmental sensors

Interleukin (IL)-1 family cytokines are important initiators of innate immunity and host defence; however, their uncontrolled activities can cause tissue-damaging inflammation. Consequently, IL-1 family cytokines have sophisticated regulatory mechanisms to control their activities including proteolytic processing for their activation and the deployment of soluble receptors and receptor antagonists to limit their activities. IL-33 is a promoter of type 2 immunity and allergic inflammation through its alarmin activity that can rapidly initiate local immune responses by stimulating innate immune cells following exposure to environmental insults, pathogens, or sterile injury. Recent publications have provided new insights into how the range and duration of IL-33 activity is regulated by direct sensing of host-derived and exogenous proteolytic activities as well as oxidative changes during tissue damage. Here, we discuss how this impacts our understanding of the roles of IL-33 in initiating immune responses and the evidence that these sensing mechanisms might regulate the activities of other IL-1 family cytokines and their biological functions. Finally, we discuss translational challenges these discoveries pose for the accurate detection of different forms of these cytokines.

Inflammasome Adaptor ASC Suppresses Apoptosis of Gastric Cancer Cells by an IL18-Mediated Inflammation-Independent Mechanism

Inflammasomes are key regulators of innate immunity in chronic inflammatory disorders and autoimmune diseases, but their role in inflammation-associated tumorigenesis remains ill-defined. Here we reveal a protumorigenic role in gastric cancer for the key inflammasome adaptor apoptosis-related speck-like protein containing a CARD (ASC) and its effector cytokine IL18. Genetic ablation of ASC in the gp130^F/F spontaneous mouse model of intestinal-type gastric cancer suppressed tumorigenesis by augmenting caspase-8-like apoptosis in the gastric epithelium, independently from effects on myeloid cells and mucosal inflammation. This phenotype was characterized by reduced activation of caspase-1 and NF-κB activation and reduced expression of mature IL18, but not IL1β, in gastric tumors. Genetic ablation of IL18 in the same model also suppressed gastric tumorigenesis, whereas blockade of IL1β and IL1α activity upon genetic ablation of the IL1 receptor had no effect. The specific protumorigenic role for IL18 was associated with high IL18 gene expression in the gastric tumor epithelium compared with IL1β, which was preferentially expressed in immune cells. Supporting an epithelial-specific role for IL18, we found it to be highly secreted from human gastric cancer cell lines. Moreover, IL18 blockade either by a neutralizing anti-IL18 antibody or by CRISPR/Cas9-driven deletion of ASC augmented apoptosis in human gastric cancer cells. In clinical specimens of human gastric cancer tumors, we observed a significant positive correlation between elevated mature IL18 protein and ASC mRNA levels. Collectively, our findings reveal the ASC/IL18 signaling axis as a candidate therapeutic target in gastric cancer.Significance: Inflammasome activation that elevates IL18 helps drive gastric cancer by protecting cancer cells against apoptosis, with potential implications for new therapeutic strategies in this setting. Cancer Res; 78(5); 1293-307. ©2017 AACR.

Strong upregulation of inflammatory genes accompanies photoreceptor demise in canine models of retinal degeneration

We have analyzed the complex pattern of the inflammatory response in early-onset canine models of human retinitis pigmentosa, rcd1, xlpra2 and erd, as well as late-onset xlpra1, in comparative manner. The time course of immune response genes and proteins expression was examined along the timeline of photoreceptors degeneration. Gene expression analysis of the early-onset models prior to and after the peak of photoreceptors death identified the involvement of multiple immune response genes including those encoding constituents of the NLRP3 inflammasome, its substrates, pro-IL1B, pro-IL18, and common components of IL1B, IL18 and TLR4 pathways. Out of two activated caspase-1 cleavage products, IL1B and IL18, only IL1B was detected in rcd1 and xlpra2 while precursor IL18 remained unprocessed in the same protein extract highlighting prominence of IL1B pathway. An overall immune response was most prominent in rcd1 followed by xlpra2 and least prominent in erd. Noticeably, in rcd1 and xlpra2, but not in erd, early induction of the immune response was accompanied by sustained intraretinal migration and activation of retinal microglia. Lastly, delayed activation of the anti-inflammatory factors in all early-onset models was insufficient to counterbalance rapidly progressing inflammation. In contrast to early-onset models, in late-onset xlpra1 retinas a subset of the pro-inflammatory genes was highly upregulated long before any disease-related structural changes occurred, but was counterbalanced by an adequate anti-inflammatory response. Results point out to upregulated immune response accompanying disease progression in animal models of retinal degeneration, and to potential benefits of early anti-inflammatory therapy.

Recombinant interleukin-12 and interleukin-18 antitumor therapy in a guinea-pig hepatoma cell implant model

Interleukin (IL)-12 and IL-18 are secreted by myeloid cells activated with adjuvants such as Bacillus Calmette-Guérin (BCG) cell wall. They induce T-helper 1 polarization in the host immune system and upregulate production of lymphocyte interferon-gamma, which leads to the induction of an antitumor gene program. It has been reported that humans have an immune system that more closely resembles that of the guinea pig in adjuvant-response features rather than the mouse system, which prevents the mouse results being extrapolated to human immunotherapy. Here we have constructed a tumor-implant system in guinea pigs to evaluate the antitumor potential of guinea pig IL-12 (gpIL-12) and guinea pig IL-18 (gpIL-18). Purified recombinant gpIL-12 and gpIL-18 were prepared and applied intraperitoneally to tumor-bearing (line 10 hepatoma) guinea pigs as the basis of the adjuvant immunotherapy. Intraperitoneal administration of gpIL-12 and gpIL-18 led to retardation of primary tumor growth and suppression of lymph-node metastasis in tumor-bearing guinea pigs. The permissible range of IL-12 appeared wider in guinea pigs than in mice. Even at an IL-12 dose higher than that in mice, there was no evidence of side-effects until day 26, when the guinea pigs were killed. gpIL-18 augmented the antitumor effect of gpIL-12 but exerted less ability to suppress lymph-node metastasis. The effects of gpIL-12 and gpIL-18 on the tumors implanted in guinea pigs will encourage us to use IL-12- and IL-18-inducible adjuvants for immunotherapy in human patients with solid cancer.

Generation of highly stable IL-18 based on a ligand–receptor complex structure

Human interleukin-18 (hIL-18), initially cloned as an IFN-gamma-inducing factor, has a key role in many inflammatory diseases. We have previously developed a high production system for correctly folded active hIL-18 protein, leading to the revelation of the 3D-structure and the receptor binding mode. These findings can strongly indicate the experimental and medical applications of IL-18; however, the recombinant protein is prone to be inactivated forming multimers. Recently, therapeutic approaches using recombinant IL-18 have shown the effectiveness for treatment of cancer; indicating the necessity of a more stable protein for therapy with intertrial reliability. Here we have generated a highly stable hIL-18 with replacement of cysteine by serine based on the tertiary structure and the binding mechanism, retaining the biological activity. Similar rational designs can be applied to develop new therapeutic molecules of other cytokines.

Interleukin-1F7B (IL-1H4/IL-1F7) is processed by caspase-1 and mature IL-1F7B binds to the IL-18 receptor but does not induce IFN-gamma production

We have recently reported the identification of four novel members of the interleukin-1 (IL-1) family which we designated as IL-1 homologue 1-4 (IL-1H1-4). These proteins exhibit significant sequence homology to other members of the IL-1 family. Of these homologues, only IL-1H4 (renamed IL-1F7b) was predicted to contain a propeptide domain and a caspase cleavage site. We now report that caspase-1 cleaves IL-1F7b at the predicted site to generate mature IL-1F7b. Caspase-4 was also able to process IL-1F7b, albeit inefficiently. Other caspases and Granzyme-B did not cleave IL-1F7b. Furthermore, adenovirus-mediated expression of IL-1F7b in HEK 293 cells led to in situ processing and secretion of mature IL-1F7b. In a screen to identify a potential receptor, both pro and mature IL-1F7b bound to the soluble IL-18 receptor alpha-Fc (IL-18Ralpha-Fc) but not to the soluble IL-1R-Fc or ST2R-Fc fusion proteins. Mature IL-1F7b bound to the IL-18Ralpha-Fc protein with higher affinity than the pro form, although the affinities for both proteins were significantly lower than that observed for IL-18. Consistent with this observation, only IL-18 and not IL-1F7b induced IFN-gamma production by KG1a cells. We also report that pro and mature IL-1F7b form homodimers with association constants of 4 microM and 5 nM, respectively, suggesting biological relevance to IL-1F7b processing. Finally, we have localized the expression of IL-1F7b protein in discrete cell populations including plasma cells and tumor cells. These data suggest that IL-1F7b may be involved in immune response, inflammatory diseases and/or cancer.

High serum levels of additional IL-18 forms may be reciprocally correlated with IgE levels in patients with atopic dermatitis

We established an ELISA system for determination of as yet unidentified species of interleukin 18 (IL-18), named IL-18 type 2, in human serum. Serum IL-18 levels and their effect on IgE levels were examined in 18 patients with atopic dermatitis (AD) with no other allergic symptoms. Three of these patients showed high IL-18 type 2 concentrations (25-100 ng/ml) in their blood serum, and this IL-18 type 2 was detectable only with our established ELISA system. In contrast, the level of the conventional form of IL-18 (type 1) was found to be 50-400 pg/ml in all patients by the commercially available ELISA. The levels of type 1 IL-18 showed no correlation with those of type 2 and approximately 2-fold higher in AD patients than in normal subjects. IL-12 p40 and IgE levels were correlated in the patients with no IL-18 type 2, and interestingly, relatively low IgE concentrations were detected in the three IL-18 type 2-positive patients. They showed considerable levels of IL-12 p40 unlike normal subjects. The IFNgamma-inducing activity of IL-18 type 2 was >100-fold less potent by weight ratio than that of a recombinant 'active' IL-18 preparation, even after the treatment with Caspase 1. Although the relationship between AD and serum IgE levels is not clear cut, IL-18 type 2 appears to play some roles in the Th2-polarization involving IgE production in association with immune responses occurring in local inflammatory milieu such as atopic lesions.

An Alternative Form of IL-18 in Human Blood Plasma: Complex Formation with IgM Defined by Monoclonal Antibodies

Monoclonal Abs 21 and 132 were raised against human functionally inactive rIL-18, and plasma IL-18 levels were determined by the sandwich ELISA established with these mABS: Plasma IL-18, designated type 2, was detected by this ELISA, and the levels found were not consistent with those obtained with the commercially available kit for determination of functionally active IL-18 (type 1). Type 1 was detected in all volunteers, whereas type 2 was detected in approximately 30% of healthy subjects, and the levels of type 2 in their blood plasma were high (25-100 ng/ml) compared with those of type 1 (0.05-0.3 ng/ml). We purified IL-18 type 2 from blood plasma of volunteers with high IL-18 type 2 concentrations, and its M(r) was determined to be 800 kDa by SDS-PAGE and molecular sieve HPLC. The purified 800-kDa protein, either caspase-1-treated or untreated, expressed no or marginal IL-18 function in terms of potentiation of NK-mediated cytolysis and IFN-gamma induction, and it barely bound IL-18R-positive cells. N-terminal amino acid analysis indicated that the purified protein was IgM containing a minimal amount of IL-18 proform and its fragment. Again, the purified IgM from IL-18 type2-positive volunteers exhibited cross-reaction with mAb 21 against IL-18. This band was not detected with 125-2H, an mAb against functionally active IL-18. Hence, human IgM carries functionally inactive IL-18 forming a disulfide-bridged complex, and this IL-18 moiety is from 10- to 100-fold higher than the conventional type 1 IL-18 in blood circulation in approximately 30% normal subjects.