Pathophysiological roles for IL-18 in inflammatory arthritis

Pyroptosis in Osteoarthritis: Molecular Mechanisms and Therapeutic Implications

Osteoarthritis (OA) is a chronic disease that causes pain and functional impairment by affecting joint tissue. Its global impact is noteworthy, causing significant economic losses and property damage. Despite extensive research, the underlying pathogenesis of OA remain an area of ongoing investigation. It has recently been discovered that the OA progression is significantly influenced by pyroptosis. Pyroptosis is a complex process that involves three pathways culminating in the assembly of Gasdermin-D (GSDMD)-N-terminal (GSDMD-NT) into pores through aggregation on the plasma membrane. The aggregation of GSDMD-NT proteins stimulates the release of inflammatory mediators, such as Interleukin-1β (IL-1β), Interleukin-18 (IL-18), and Matrix Metallopeptidase 13 (MMP13), ultimately leading to cellular lysis. The pyroptosis process in specific cells, including synovial macrophages, fibroblast-like synoviocytes (FLS), chondrocytes, and subchondral osteoblasts, contributs factor to the development of OA. Currently, the specific cells that undergo pyroptosis first are not yet fully understood, and it remains unknown whether pyroptosis in one cell can trigger the same process in other cells. Therefore, targeting pyroptosis could potentially offer a novel treatment approach for OA patients. We present a comprehensive analysis of the molecular mechanisms and key features of pyroptosis. We also outline the current research progress on various aspects, including synovial tissue, articular cartilage, extracellular matrix (ECM), and subchondral bone, with a focus on pyroptosis. The aim is to provide theoretical references for the effective management of OA.

The Role of Inflammasomes in Osteoarthritis and Secondary Joint Degeneration Diseases

Osteoarthritis is age-related and the most common form of arthritis. The main characteristics of the disease are progressive loss of cartilage and secondary synovial inflammation, which finally result in pain, joint stiffness, and functional disability. Similarly, joint degeneration is characteristic of systemic inflammatory diseases such as rheumatoid arthritis and gout, with the associated secondary type of osteoarthritis. Studies suggest that inflammation importantly contributes to the progression of the disease. Particularly, cytokines TNFα and IL-1β drive catabolic signaling in affected joints. IL-1β is a product of inflammasome activation. Inflammasomes are inflammatory multiprotein complexes that propagate inflammation in various autoimmune and autoinflammatory conditions through cell death and the release of inflammatory cytokines and damage-associated molecule patterns. In this article, we review genetic, marker, and animal studies that establish inflammasomes as important drivers of secondary arthritis and discuss the current evidence for inflammasome involvement in primary osteoarthritis. The NLRP3 inflammasome has a significant role in the development of secondary osteoarthritis, and several studies have provided evidence of its role in the development of primary osteoarthritis, while other inflammasomes cannot be excluded. Inflammasome-targeted therapeutic options might thus provide a promising strategy to tackle these debilitating diseases.

Associations between knee structural measures, circulating inflammatory factors and MMP13 in patients with knee osteoarthritis

OBJECTIVE

To investigate cross-sectional associations between serum level of Matrix metalloproteinase (MMP)13 and knee structural measures and circulating inflammatory factors in patients with symptomatic knee osteoarthritis (OA).

DESIGN

A total of 149 subjects with symptomatic knee OA were included. Magnetic resonance imaging was used to measure infrapatellar fat pad (IPFP) volume, IPFP signal intensity alternation, cartilage volume and cartilage defects. Knee radiography was used to assess radiographic OA using the Kellgren-Lawrence (K-L) grading system. Enzyme-linked immunosorbent assay was used to measure the serum levels of inflammatory factors and MMP13.

RESULTS

In multivariable analyses, serum MMP13 was negatively associated with cartilage volume at patellar site (β: -32.94 mm³ per 10 ng/ml, P < 0.05), and positively associated with cartilage defect at medial femoral site (OR: 1.13 per 10 ng/ml, P < 0.05). Also, MMP13 was positively associated with K-L grading and IPFP signal intensity alteration (OR: 1.14 and 1.15 per 10 ng/ml, respectively, both P < 0.05), and negatively associated with IPFP volume (β: -0.34 cm³ per 10 ng/ml, P < 0.05). Furthermore, serum level of adiponectin was negatively associated serum MMP13 quartiles (OR: 0.66 per 10 μg/ml, P < 0.05), and serum levels of tumor necrosis factor (TNF)-α, interleukin (IL)-8 and IL-18 were positively associated with serum MMP13 quartiles (ORs: 1.01-1.18 per 10 pg/ml, all P < 0.05).

CONCLUSIONS

Serum level of MMP13 was associated with knee structural abnormalities as well as serum inflammatory factors. These suggest that systemic MMP13 may play a role in knee OA, and could be regulated by inflammatory factors.

Macrophage roles in the clearance of apoptotic cells and control of inflammation in the prostate gland after castration

BACKGROUND

Androgen deprivation results in massive apoptosis in the prostate gland. Macrophages are actively engaged in phagocytosing epithelial cell corpses. However, it is unknown whether microtubule-associated protein 1 light chain 3 alpha (LC3)-associated phagocytosis (LAP) is involved and contribute to prevent inflammation.

METHODS

Flow cytometry, RT-PCR and immunohistochemistry were used to characterize the macrophage subpopulation residing in the epithelial layer of the rat ventral prostate (VP) after castration. Stereology was employed to determine variations in the number of ED1 and ED2. Mice were treated with either chloroquine or L-asparagine to block autophagy.

RESULTS

M1 (iNOS-positive) and M2 macrophages (MRC1+ and ARG1+) were not found in the epithelium at day 5 after castration. The percentage of CD68⁺ (ED1) and CD163⁺ (ED2) phenotypes increased after castration but only CD68⁺ cells were present in the epithelium. RT-PCR showed increased content of the autophagy markers Bcl1 and LC3 after castration. In addition, immunohistochemistry showed the presence of LC3⁺ and ATG5⁺ cells in the epithelium. Double immunohistochemistry showed these cells to be CD68⁺ /LC3⁺ , compatible with the LAP phenotype. LC3⁺ cells accumulate significantly after castration. Chloroquine and L-asparagine administration caused inflammation of the glands at day 5 after castration.

CONCLUSIONS

CD68⁺ macrophages phagocytose apoptotic cell corpses and activate the LAP pathway, thereby contributing to the preservation of a non-inflammed microenvironment. Marked inflammation was detected when autophagy blockers were administered to castrated animals.

IL-18BP is decreased in osteoporotic women: Prevents Inflammasome mediated IL-18 activation and reduces Th17 differentiation

IL-18BP is a natural antagonist of pro-inflammatory IL-18 cytokine linked to autoimmune disorders like rheumatoid arthritis. However, its role in post menopausal osteoporosis is still unknown. In this study, we investigated the role of IL-18BP on murine osteoblasts, its effect on osteoblasts-CD4+ T cells and osteoblasts-CD11b+ macrophage co-culture. mIL-18BPd enhances osteoblast differentiation and inhibits the activation of NLRP3 inflammasome and caspase-1 which process IL-18 to its active form. Using estrogen deficient mice, we also determined the effect of mIL-18BP on various immune and skeletal parameters. Ovariectomized mice treated with mIL-18BPd exhibited decrease in Th17/Treg ratio and pro-inflammatory cytokines. mIL-18BPd treatment restored trabecular microarchitecture, preserved cortical bone parameters likely attributed to an increased number of bone lining cells and reduced osteoclastogenesis. Importantly, these results were corroborated in female osteoporotic subjects where decreased serum IL-18BP levels and enhanced serum IL-18 levels were observed. Our study forms a strong basis for using humanized IL-18BP towards the treatment of postmenopausal osteoporosis.

UP1304, a Botanical Composition Containing Two Standardized Extracts of Curcuma longa and Morus alba, Mitigates Pain and Inflammation in Adjuvant-induced Arthritic Rats

BACKGROUND

Though, the initial etiologies of arthritis are multifactorial, clinically, patients share pain as the prime complaints. Present day pain relief therapeutics heavily relies on the use of prescription and over the counter nonsteroidal anti-inflammatory drugs as the first line of defense where their long-term usage causes gastrointestinal and cardiovascular-related side effects. Hence, the need for evidence-based safer and efficacious alternatives from natural sources to overcome the most prominent and disabling symptoms of arthritis is an overdue. Here, we evaluated the anti-inflammatory and analgesic effect of UP1304, a composition that contains a standardized blend of two extracts from the rhizome of Curcuma longa and the root bark of Morus alba in adjuvant-induced arthritis models in rats.

MATERIALS AND METHODS

The anti-inflammatory and analgesic effects of the botanical composition were demonstrated in adjuvant-induced arthritis models in rats with oral dose ranges of 50-200 mg/kg. Ibuprofen at a dose of 100 mg/kg was used as a reference compound. Ex vivo sulfated glycosaminoglycan inhibition assays were performed.

RESULTS

Statistically significant improvements in pain resistance, suppression of paw edema and ankle thickness were observed in animals treated with UP1304 compared to vehicle-treated diseased rats. These results were similar to those achieved by ibuprofen treatment. Inhibitions of proteoglycan degradation were observed in a range of 37.5-61.7% for concentration of UP1304 at 50-200 μg/mL when compared to interleukin-1α-exposed untreated explants.

CONCLUSIONS

These data suggest that UP1304, for its analgesic and anti-inflammatory effects, could potentially be considered agent of botanical origin for the improvement of arthritis associated symptoms.

SUMMARY

Pain is one of the cardinal signs of arthritis.Long term applications of commonly used non-steroidal anti-inflammatory drugs for pain relief are associated with cardiovascular and gastrointestinal side effects.Cartilage degradation evidenced as glycosaminoglycan loss from articular cartilage into the synovial fluid has been reported in arthritis patients.Adjuvant-induced arthritis model in rats are among the widely used models for efficacy evaluation of nutraceuticals.Efficacy of UP1304, a composition containing a blend of two standardized extracts from the rhizome of Curcuma longa and root bark of Morus alba, was evaluated in adjuvant-induced arthritis model in rats and in glycosaminoglycan releasing inhibition assays.UP1304 demonstrated its enhanced significance by improving the major cardinal signs of arthritis in vivo and ex vivo.UP1304 could potentially be considered as a dietary supplement product for the management of arthritis.

Association between interleukin 18 polymorphisms and alopecia areata in Koreans

Interleukin 18 (interferon gamma-inducing factor) (IL18) is an important proinflammatory cytokine that belongs to the IL1 family. This study investigated whether IL18 single-nucleotide polymorphisms (SNPs) are associated with the susceptibility to alopecia areata (AA) in a Korean population. Two hundred thirty-three AA patients and 243 healthy control subjects were recruited. One promoter SNP (rs187238, -137G/C) and exonic SNP (rs549908, Ser35Ser) in IL18 were genotyped using direct sequencing. SNPStats, SPSS 18.0, and Haploview version 4.2 programs were used to evaluate genetic data. Multiple logistic regression models were used to determine odds ratios, 95% confidence intervals, and P values. Tested 2 SNPs (rs187238 and rs549908) were associated with the development of AA (rs187238, P=0.002 in a codominant model 1, P=0.0048 in a dominant model, P=0.02 in a log-additive model, P=0.023 in allele distribution; rs549908, P=0.003 in a codominant model 1, P=0.0052 in a dominant model, P=0.016 in a log-additive model, P=0.015 in allele distribution). Our data suggest that the IL18 may be a risk factor for AA susceptibility.

Chondrocyte-specific pathology during skeletal growth and therapeutics in a murine model of pseudoachondroplasia

Mutations in the gene encoding cartilage oligomeric matrix protein (COMP) cause pseudoachondroplasia (PSACH), a severe dwarfing condition. Pain, a significant complication, has generally been attributed to joint abnormalities and erosion and early onset osteoarthritis. Previously, we found that the inflammatory-related transcripts were elevated in growth plate and articular cartilages, indicating that inflammation plays an important role in the chondrocyte disease pathology and may contribute to the overall pain sequelae. Here, we describe the effects of D469-delCOMP expression on the skeleton and growth plate chondrocytes with the aim to define a treatment window and thereby reduce pain. Consistent with the human PSACH phenotype, skeletal development of D469del-COMP mice was normal and similar to controls at birth. By postnatal day 7 (P7), the D469del-COMP skeleton, limbs, skull and snout were reduced and this reduction was progressive during postnatal growth, resulting in a short-limbed dwarfed mouse. Modulation of prenatal and postnatal expression of D469del-COMP showed minimal retention/cell death at P7 with some retention/cell death by P14, suggesting that earlier treatment intervention at the time of PSACH diagnosis may produce optimal results. Important and novel findings were an increase in inflammatory proteins generally starting at P21 and that exercise exacerbates inflammation. These observations suggest that pain in PSACH may be related to an intrinsic inflammatory process that can be treated symptomatically and is not related to early joint erosion. We also show that genetic ablation of CHOP dampens the inflammatory response observed in mice expressing D469del-COMP. Toward identifying potential treatments, drugs known to decrease cellular stress (lithium, phenylbutyric acid, and valproate) were assessed. Interestingly, all diminished the chondrocyte pathology but had untoward outcomes on mouse growth, development, and longevity. Collectively, these results define an early treatment window in which chondrocytes can be salvaged, thereby potentially increasing skeletal growth and decreasing pain.

Association of interleukin-18 (-137G/C) polymorphism with rheumatoid arthritis and systemic lupus erythematosus: a meta-analysis

BACKGROUND

Recent studies have suggested that interleukin (IL)-18 gene (-137G/C) polymorphism is associated with rheumatoid arthritis (RA) and systemic lupus erythematosus (SLE). However, other studies did not confirm this correlation.

OBJECTIVE

The objective of this study was to evaluate the relationships of IL-18 -137G/C and RA and SLE using a meta-analysis.

METHODS

Pubmed, Embase and Cochrane library databases were systemically searched. Data were extracted by two independent reviewers and pooled odds ratio (OR) with 95% confidence interval (CI) was calculated.

RESULTS

In RA, the overall ORs and 95% CIs of -137C were 1.03, 0.88-1.22 (p=0.391); 1.22, 0.89-1.68 (p=0.020) and 1.06, 0.93-1.21 (p=0.110) in dominant, recessive, and additive models, respectively. Furthermore, in SLE, the overall ORs and 95% CIs of -137C were 1.10, 0.94-1.29 (p=0.980); 1.21, 0.91-1.60 (p=0.010) and 1.10, 0.97-1.24 (p=0.454) in dominant, recessive, and additive models, respectively. IL-18 -137G/C could increase the risk of RA and SLE. No publication bias was found in this meta-analysis. After population stratification analysis, under recessive model, the pooled ORs and 95% CIs of -137C were 1.14, 0.82-1.60 (p=0.008) and 1.01, 0.66-1.55 (p=0.004) in European RA patients and Asian SLE patients, respectively.

CONCLUSIONS

This meta-analysis showed that IL-18 -137G/C was a risk factor for RA and SLE, especially for RA in Europeans and SLE in Asians.

Association of IL-18 polymorphisms with rheumatoid arthritis and systemic lupus erythematosus in Asian populations: a meta-analysis

BACKGROUND

Interleukin (IL)-18, an important proinflammatory cytokine, plays a potential pathological role in rheumatoid arthritis (RA) and systemic lupus erythematosus (SLE). Studies on the relationship of IL-18 gene promoter rs1946518 (-607A/C) polymorphism, rs187238 (-137G/C) polymorphism with RA and SLE are inconclusive. The aim of this study was to get a more precise estimation of the relationship in Asian populations.

METHODS

Meta-analysis was conducted on the associations between the IL-18 (-607A/C and -137G/C) polymorphisms and RA and SLE, using; (1) allele contrast, (2) dominant, and (3) recessive models. A total of 11 studies were included in this study.

RESULTS

For the relationship of IL-18 rs1946518 polymorphism with RA (additive model: OR=0.752, 95%CI=0.562-1.006; dominant model: OR=0.730, 95%CI =0.479-1.113; recessive model: OR=0.537, 95%CI=0.271-1.064) and SLE (additive model: OR=0.684, 95%CI=0.455-1.028; dominant model: OR=0.645, 95%CI=0.368-1.130; recessive model: OR=0.672, 95%CI =0.447-1.010), no significant association with RA and SLE risk can be found under all genetic models in Asian populations. However, significant associations were observed in Chinese population for both RA ((OR=0.688, 95%CI =0.532-0.889) and SLE (OR=0.606, 95%CI =0.396-0.930) under additive model. For the relationship between IL-18 rs187238 polymorphism and RA or SLE, there was no significant association detected in all genetic models, even in Chinese population.

CONCLUSIONS

This meta-analysis indicates that the IL-18-607A/C polymorphism may confer susceptibility to RA and SLE in Chinese population, but not all Asians.

Interleukin-18-induced inflammatory responses in synoviocytes and chondrocytes from osteoarthritic patients

The major pathological changes of osteoarthritis (OA) include cartilage degeneration and synovial inflammation. Previous studies confirmed that interleukin-1 (IL-1) stimulates the secretion of multiple inflammatory factors in synoviocytes and chondrocytes. IL-18 is a member of the IL-1 superfamily. In this study, the pro-inflammatory effects of IL-18 on synoviocytes and chondrocytes in patients with OA were investigated. Knee synovial membrane and cartilage samples were obtained from OA patients, then primary cells were cultured. Synoviocytes and primary chondrocytes at different generations (primary, secondary and tertiary), were stimulated with IL-18, then inflammatory marker levels, including tumor necrosis factor-α (TNF-α), prostaglandin E2 (PGE₂) and cyclooxygenase-2 (COX-2), were measured using reverse transcription polymerase chain reaction (RT-PCR) and enzyme-linked immunosorbent assay. IL-1 receptor antagonist (IL-1Ra) was applied to interfere with the IL-18 stimulation of chondrocytes, and then the COX-2 expression in chondrocytes and the PGE2 levels in the medium were measured. The expression of IL-18 receptor α (IL-18Rα) and IL-18 receptor β (IL-18Rβ) in synoviocytes and chondrocytes was assessed, using RT-PCR. Our results showed that IL-18 stimulated the COX-2 and TNF-α expressions in primary synoviocytes, while increasing PGE₂ and TNF-α levels in the supernatant (P<0.05) of the culture medium in primary synoviocytes. IL-18 also induced high PGE₂ level production in second-generation synoviocytes (P<0.05). Moreover, IL-18 upregulated COX-2 and TNF-α mRNA in chondrocytes, while promoting PGE₂ and TNF-α (P<0.05) secretions in a dose-dependent manner. The induced effects were not attenuated by the addition of IL-1Ra (P<0.05). IL-18Rα was expressed in the chondrocytes and synoviocytes of 4/8 patients, while IL-18Rβ was expressed in the chondrocytes of 4/8 patients and in the synoviocytes of 2/8 patients. We conclude that IL-18 induces inflammatory responses in synoviocytes and chondrocytes and that this effect was correlated with, although not entirely dependent on, IL-1β.

Impact of inflammation on the osteoarthritic niche: implications for regenerative medicine

Osteoarthritis (OA) is the most common form of arthritis worldwide and is the sixth leading cause of disability. It costs the UK economy approximately 1% of gross national product per annum. With an aging population, the cost of chronic conditions such as OA continues to rise. Historically, treatments for OA have been limited to painkillers, physiotherapy and joint injections. When these fail, patients are referred for joint replacement surgery. With the advent of tissue engineering strategies aimed at generating new bone and cartilage for repair of osteochondral defects, there has been considerable interest in exploiting these techniques to devise new treatments for OA. To date, little consideration has been given to the OA niche and attendant inflammatory milieu for any regenerative skeletal strategy. This review highlights the importance of understanding the osteoarthritic niche in order to modify existing tissue engineering and regenerative medicine strategies for the future treatment of OA.

Cytokines profiling by multiplex analysis in experimental arthritis: which pathophysiological relevance for articular versus systemic mediators?

INTRODUCTION

We have taken advantage of the large screening capacity of a multiplex immunoassay to better define the respective contribution of articular versus systemic cytokines in experimental arthritis.

METHODS

We performed a follow up (from 7 hours to 14 days) multiplex analysis of 24 cytokines in synovial fluid and sera of rats developing Antigen-Induced Arthritis (AIA) and confronted their protein level changes with molecular, biochemical, histological and clinical events occurring in the course of the disease.

RESULTS

The time-scheduled findings in arthritic joints correlated with time-dependent changes of cytokine amounts in joint effusions but not with their blood levels. From seven hours after sensitization, high levels of chemokines (MCP-1, MIP1α, GRO/KC, RANTES, eotaxin) were found in synovial fluid of arthritic knees whereas perivascular infiltration occurred in the synovium; local release of inflammatory cytokines (IFNγ, IL-1β, IL-6) preceded the spreading of inflammation and resulted in progressive degradation of cartilage and bone. Finally a local overexpression of several cytokines/adipocytokines poorly described in arthritis (IL-13, IL-18, leptin) was observed.

CONCLUSIONS

Distinct panels of cytokines were found in arthritic fluid during AIA, and the expected effect of mediators correlated well with changes occurring in joint tissues. Moreover, multiplex analysis could be helpful to identify new pathogenic mediators and to elucidate the mechanisms supporting the efficacy of putative targeted therapies.

IL-33: a promising therapeutic target for rheumatoid arthritis?

Cytokine-mediated immunity plays a crucial role in the pathogenesis of various autoimmune diseases, including rheumatoid arthritis (RA). Recently, the IL-1-family-related cytokine, IL-33, was detected at high levels in experimental inflammatory arthritis and in the early phase of human RA, and was reported to exert profound pro-inflammatory effects in several experimental autoimmune models. Moreover, administration of IL-33 leads to the development of severe inflammatory arthritis, suggesting that IL-33 may be therapeutically relevant in RA, and the targeting of IL-33 or the IL-33 receptor has been proposed as a potential therapeutic approach for autoimmune diseases such as RA. In this article, we discuss the biological features of IL-33 and summarize recent advances in our understanding of the role of IL-33 in the pathogenesis and treatment of RA. It is hoped that this information may aid the development of novel therapeutic strategies for RA.

Uric acid is a danger signal of increasing risk for osteoarthritis through inflammasome activation

Uric acid (UA) is known to activate the NLRP3 (Nacht, leucine-rich repeat and pyrin domain containing protein 3) inflammasome. When activated, the NLRP3 (also known as NALP3) inflammasome leads to the production of IL-18 and IL-1β. In this cohort of subjects with knee osteoarthritis (OA), synovial fluid uric acid was strongly correlated with synovial fluid IL-18 and IL-1β. Synovial fluid uric acid and IL-18 were strongly and positively associated with OA severity as measured by both radiograph and bone scintigraphy, and synovial fluid IL-1β was associated with OA severity but only by radiograph. Furthermore, synovial fluid IL-18 was associated with a 3-y change in OA severity, on the basis of the radiograph. We conclude that synovial fluid uric acid is a marker of knee OA severity. The correlation of synovial fluid uric acid with the two cytokines (IL-18 and IL-1β) known to be produced by uric acid-activated inflammasomes and the association of synovial fluid IL-18 with OA progression, lend strong support to the potential involvement of the innate immune system in OA pathology and OA progression.

The TLR4/TRIF-Mediated Activation of NLRP3 Inflammasome Underlies Endotoxin-Induced Liver Injury in Mice

Administration of heat-killed Propionibacterium acnes renders mice highly susceptible to LPS. After LPS challenge P. acnes-primed mice promptly show hypothermia, hypercoagulation (disseminated intravascular coagulation), elevation of serum proinflammatory cytokine levels, and high mortality. The surviving mice develop liver injury. As previously reported, IL-18 plays a pivotal role in the development of this liver injury. Many cell types including macrophages constitutively store IL-18 as biologically inactive precursor (pro) form. Upon appropriate stimulation exemplified by TLR4 engagement, the cells secrete biologically active IL-18 by cleaving pro-IL-18 with caspase-1. Caspase-1 is also constitutively produced as a zymogen in macrophages. Recently, NLRP3, a cytoplasmic pathogen sensor, has been demonstrated to be involved in the activation of caspase-1. Here, we review the molecular mechanisms for the liver injuries, particularly focusing on the TLR4/NLRP3-mediated caspase-1 activation process, with a brief introduction of the mechanism underlying P. acnes-induced sensitization to LPS.

Cells of the synovium in rheumatoid arthritis. Chondrocytes

Rheumatoid arthritis (RA) is one of the inflammatory joint diseases in a heterogeneous group of disorders that share features of destruction of the extracellular matrices of articular cartilage and bone. The underlying disturbance in immune regulation that is responsible for the localized joint pathology results in the release of inflammatory mediators in the synovial fluid and synovium that directly and indirectly influence cartilage homeostasis. Analysis of the breakdown products of the matrix components of joint cartilage in body fluids and quantitative imaging techniques have been used to assess the effects of the inflammatory joint disease on the local remodeling of joint structures. The role of the chondrocyte itself in cartilage destruction in the human rheumatoid joint has been difficult to address but has been inferred from studies in vitro and in animal models. This review covers current knowledge about the specific cellular and biochemical mechanisms that account for the disruption of the integrity of the cartilage matrix in RA.

Critical involvement of pneumolysin in production of interleukin-1alpha and caspase-1-dependent cytokines in infection with Streptococcus pneumoniae in vitro: a novel function of pneumolysin in caspase-1 activation

Pneumolysin is a pore-forming cytolysin known as a major virulence determinant of Streptococcus pneumoniae. This protein toxin has also been shown to activate the Toll-like receptor 4 (TLR4) signaling pathway. In this study, a mutant S. pneumoniae strain deficient in pneumolysin (Deltaply) and a recombinant pneumolysin protein (rPLY) were constructed. Upon infection of macrophages in vitro, the ability to induce the production of interleukin-1alpha (IL-1alpha), IL-1beta, and IL-18 was severely impaired in the Deltaply mutant, whereas there was no marked difference in the induction of tumor necrosis factor alpha (TNF-alpha) and IL-12p40 between the wild type and the Deltaply mutant of S. pneumoniae. When macrophages were stimulated with rPLY, the production of IL-1alpha, IL-1beta, and IL-18 was strongly induced in a TLR4-dependent manner, whereas lipopolysaccharide, a canonical TLR4 agonist, hardly induced these cytokines. In contrast, lipopolysaccharide was more potent than rPLY in inducing the production of TNF-alpha, IL-6, and IL-12p40, the cytokines requiring no caspase activation. Activation of caspase-1 was observed in macrophages stimulated with rPLY but not in those stimulated with lipopolysaccharide, and the level of activation was higher in macrophages infected with wild-type S. pneumoniae than in those infected with the Deltaply mutant. These results clearly indicate that pneumolysin plays a key role in the host response to S. pneumoniae, particularly in the induction of caspase-1-dependent cytokines.

Free fucose is a danger signal to human intestinal epithelial cells

Fucose is present in foods, and it is a major component of human mucin glycoproteins and glycolipids. l-Fucose can also be found at the terminal position of many cell-surface oligosaccharide ligands that mediate cell-recognition and adhesion-signalling pathways. Mucin fucose can be released through the hydrolytic activity of pathogens and indigenous bacteria, leading to the release of free fucose into the intestinal lumen. The immunomodulating effects of free fucose on intestinal epithelial cells (enterocyte-like Caco-2) were investigated. It was found that the presence of l-fucose up regulated genes and secretion of their encoded proteins that are involved in both the innate and adaptive immune responses, possibly via the toll-like receptor-2 signalling pathway. These include TNFSF5, TNFSF7, TNF-alpha, IL12, IL17 and IL18. Besides modulating immune reactions in differentiated Caco-2 cells, fucose induced a set of cytokine genes that are involved in the development and proliferation of immune cells. These include the bone morphogenetic proteins (BMP) BMP2, BMP4, IL5, thrombopoietin and erythropoietin. In addition, the up regulated gene expression of fibroblast growth factor-2 may help to promote epithelial cell restitution in conjunction with the enhanced expression of transforming growth factor-beta mRNA. Since the exogenous fucose was not metabolised by the differentiated Caco-2 cells as a carbon source, the reactions elicited were suggested to be a result of the direct interaction of fucose and differentiated Caco-2 cells. The presence of free fucose may signal the invasion of mucin-hydrolysing microbial cells and breakage of the mucosal barrier. The intestinal epithelial cells respond by up regulation and secretion of cytokines, pre-empting the actual invasion of pathogens.

Phenotypic differences between Th1 and Th17 cells and negative regulation of Th1 cell differentiation by IL-17

Recent evidence from several groups indicates that IL-17-producing Th17 cells, rather than, as once was thought, IFN-gamma-producing Th1 cells, can represent the key effector cells in the induction/development of several autoimmune and allergic disorders. Although Th17 cells exhibit certain phenotypic and developmental differences from Th1 cells, the extent of the differences between these two T cell subsets is still not fully understood. We found that the expression profile of cell surface molecules on Th17 cells has more similarities to that of Th1 cells than Th2 cells. However, although certain Th1-lineage markers [i.e., IL-18 receptor alpha, CXCR3, and T cell Ig domain, mucin-like domain-3 (TIM-3)], but not Th2-lineage markers (i.e., T1/ST2, TIM-1, and TIM-2), were expressed on Th17 cells, the intensity of expression was different between Th17 and Th1 cells. Moreover, the expression of CTLA-1, ICOS, programmed death ligand 1, CD153, Fas, and TNF-related activation-induced cytokine was greater on Th17 cells than on Th1 cells. We found that IL-23 or IL-17 can suppress Th1 cell differentiation in the presence of exogenous IL-12 in vitro. We also confirmed that IL-12 or IFN-gamma can negatively regulate Th17 cell differentiation. However, these cytokines could not modulate such effects on T cell differentiation in the absence of APC.

MyD88 deficiency results in tissue-specific changes in cytokine induction and inflammation in interleukin-18-independent mice infected with Borrelia burgdorferi

Toll-like receptors (TLRs) play an important role in the control of infection with Borrelia burgdorferi. Deficiencies in TLR-2 or the shared TLR adapter molecule MyD88 have been shown to result in greatly increased bacterial burdens in mice. However, although in vitro studies have shown that the activation of TLR pathways by B. burgdorferi results in the release of inflammatory cytokines, studies in deficient mice have shown either no change or increased rather than decreased inflammation in infected animals. In this study, we looked at mechanisms to explain the increase in inflammation in the absence of MyD88. We found that MyD88-deficient mice infected with B. burgdorferi did not show increased inflammation at sites typically associated with Lyme disease (joints and heart). However, there was markedly increased inflammation in the muscles, kidneys, pancreas, and lungs of deficient animals. Muscle inflammation was typically seen perivascularly and perineuronally similar to that seen in infected humans. Chemotactic chemokines and cytokines were greatly increased in the muscle and kidneys of MyD88-deficient animals but not in the joints or heart tissue, suggesting that MyD88-independent pathways for recognizing B. burgdorferi and inducing these chemokines are present in the muscle and kidneys. Interleukin-18 signaling through MyD88 does not appear to play a role in either control of infection or inflammation.

Interleukin-18 promoter polymorphism in patients with rheumatoid arthritis

Rheumatoid arthritis (RA) is a chronic inflammatory disease in which interleukin (IL)-18 plays an important role. However, there are controversial reports on IL-18 promoter polymorphism as an independent marker of RA susceptibility. The aim of present study was to examine the IL-18 promoter polymorphism in patients with RA, and its association with disease susceptibility, activity and severity. We examined 309 patients with RA from a Polish population diagnosed according to the criteria of American College of Rheumatology. An allele-specific polymerase chain reaction was used for analysis of the polymorphisms in positions - 137 and - 607 in promoter region of IL-18 gene. A significantly decreased number of subjects with AC/AC and AG/AG diplotypes was observed among RA patients as compared with healthy controls (OR - 0.51, 95%CI 0.28-0.95, P = 0.045) and (OR - 0.12, 95% CI 0.02-0.97, P = 0.042), respectively. Nevertheless, there was no significant association with disease activity, joint erosions, extra-articular manifestations, rheumatoid factor. Above results suggest that IL-18-137 and - 607 promoter polymorphisms are not the significant factors influencing RA course and severity in a Polish population.

Is interleukin-18 useful for monitoring rheumatoid arthritis?

OBJECTIVE

Interleukin-18 (IL-18) is a proinflammatory regulator of immune responses. Its similarities to IL-1beta and ability to induce tumour necrosis factor-alpha (TNF-alpha) make it potentially important in the pathogenesis of rheumatoid arthritis (RA).

METHODS

The level of IL-18 was assessed in matched pairs of blood and synovial fluid samples from 90 RA patients (47 erosive, 43 non-erosive) by an enzyme-linked immunosorbent assay (ELISA), and the results compared to 40 healthy controls.

RESULTS

In RA patients with erosive joint disease, the IL-18 level was higher than that in non-erosive RA [(median+/-QR) blood: 385+/-200 vs. 235+/-183 pg/mL, p = 0.02; synovial fluid: 392+/-392 vs. 224+/-324 pg/mL, p = 0.05]. IL-18 levels in blood of RA patients were similar and closely related to the local, intra-articular level (r = 0.96). The IL-18 level was not related to other markers of inflammation, to the duration of RA, or to the treatment modality. The IL-18 level in RA patients was similar to that of the controls (278+/-234 vs. 344+/-179 pg/mL, not significant).

CONCLUSIONS

An increased IL-18 level is associated with erosive joint disease, but the measurement of IL-18 does not help to distinguish between RA patients and healthy controls.

Serum interleukin-18 levels in patients with Behçet’s disease. Is its expression associated with disease activity or clinical presentations?

Interleukin (IL)-18 is a proinflammatory cytokine which plays a crucial role in T helper (Th)1 type immune response. The aim of this study is to investigate the relationship of serum levels of IL-18 with disease activity and clinical presentations in patients with Behcet's disease (BD). Sixty patients with BD and 20 healthy controls were included in the study. Patients were grouped as having active or inactive disease according to the Leeds activity score. They were also separated as a systemic involvement or mucocutaneous symptoms only. Patients with systemic involvement were further grouped according to the presence of ocular, articular and vascular involvement. IL-18 levels were significantly higher in all patient subgroups as compared to healthy controls and found to be correlated with the activity score in patients having active disease. In conclusion, this cytokine participates in the pathogenesis of BD and its levels are correlated with the disease activity. Detection of increased levels of IL-18 in patients with inactive disease implies that Th1 activation and subclinical inflammation persist during the inactive period of the disease.

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.

Induction of allergic inflammation by interleukin-18 in experimental animal models

Interleukin-18 (IL-18) has been regarded as a proinflammatory cytokine because of its potent interferon-gamma-inducing activity. However, mutant mice that release excess amounts of IL-18 spontaneously develop pruritic chronic dermatitis-like atopic dermatitis (AD), suggesting the importance of IL-18 for the development of AD. Intriguingly, depletion of il-18 but not stat6, an essential transcriptional factor for IL-4 signaling, rescues the mice from AD, indicating IL-18-dependent, T-helper 2 (Th2) cell-independent AD. This type of AD is classified as innate-type allergy in contrast to Th2 cell-dependent ordinary allergy. Consistent with the previous studies, mice transferred with antigen-specific Th1 cells exhibit no airway hyperresponsiveness and respiratory eosinophilic inflammation after challenge with antigen alone. However, they suffer from asthma upon challenge with antigen plus IL-18, with comparable levels of both the alterations as in those transferred with Th2 cells following challenge with antigen. The former type of asthma is categorized as Th1-associated allergy. Therefore, it is definitely necessary to evaluate whether individual allergic disorders involve either of these IL-18-mediated pathways or a Th2-mediated one.

Endogenous interleukin-6, but not tumor necrosis factor α, contributes to the development of toll-like receptor 4/myeloid differentiation factor 88-mediated acute arthritis in mice

OBJECTIVE

To generate a mouse model of reactive arthritis (ReA), an aseptic synovitis that develops in joints distant from the primary bacterial infection site, to examine roles for Toll-like receptors (TLRs) that recognize bacterial components involved in the development of this arthritis, and to identify the cytokine(s) relevant to this arthritis.

METHODS

Mice were treated with cell wall extract from Escherichia coli (ECW) gram-negative bacterium by injection into the footpads. Seven days later, the mice were challenged with lipopolysaccharide (LPS), a TLR-4 ligand, which was injected into the knee joint cavity. To investigate the cytokine(s) involved in this arthritis, mice deficient in various arthritogenic cytokines, such as interleukin-6 (IL-6), IL-12, IL-18, interferon-gamma, and tumor necrosis factor alpha (TNFalpha), were sequentially treated with ECW and LPS.

RESULTS

ECW-primed mice manifested acute severe arthritis after intraarticular challenge with ECW or LPS, while unprimed mice exhibited modest changes after these challenges. Mutant mice lacking functional TLR-4 or myeloid differentiation factor 88 (MyD88), an adaptor molecule of TLR-4 signaling, were resistant to this arthritis. Although both TNFalpha and IL-6 were equally expressed in the joint after LPS challenge, Il6(-/-) mice, but not Tnf(-/-) mice, were resistant to ECW/LPS-induced arthritis.

CONCLUSION

Our present results clearly indicate the importance of priming with ECW and the requirement of TLR-4/MyD88-mediated IL-6, but not TNFalpha, for the development of ECW/LPS-induced arthritis. LPS-induced IL-6, in the absence of TNFalpha, mediates LPS-induced arthritis. These results suggest that IL-6 is a rational target for therapeutic regimens for inflammatory arthritis, including ReA and rheumatoid arthritis.