The liver X receptor pathway is highly upregulated in rheumatoid arthritis synovial macrophages and potentiates TLR-driven cytokine release.

OBJECTIVES: Macrophages are central to the inflammatory processes driving rheumatoid arthritis (RA) synovitis. The molecular pathways that are induced in synovial macrophages and thereby promote RA disease pathology remain poorly understood. METHODS: We used microarray to characterise the transcriptome of synovial fluid (SF) macrophages compared with matched peripheral blood monocytes from patients with RA (n=8). RESULTS: Using in silico pathway mapping, we found that pathways downstream of the cholesterol activated liver X receptors (LXRs) and those associated with Toll-like receptor (TLR) signalling were upregulated in SF macrophages. Macrophage differentiation and tumour necrosis factor α promoted the expression of LXRα. Furthermore, in functional studies we demonstrated that activation of LXRs significantly augmented TLR-driven cytokine and chemokine secretion. CONCLUSIONS: The LXR pathway is the most upregulated pathway in RA synovial macrophages and activation of LXRs by ligands present within SF augments TLR-driven cytokine secretion. Since the natural agonists of LXRs arise from cholesterol metabolism, this provides a novel mechanism that can promote RA synovitis.

The parasitic helminth product ES-62 suppresses pathogenesis in collagen-induced arthritis by targeting the interleukin-17-producing cellular network at multiple sites.

OBJECTIVE: Among many survival strategies, parasitic worms secrete molecules that modulate host immune responses. One such product, ES-62, is protective against collagen-induced arthritis (CIA), a model of rheumatoid arthritis (RA). Since interleukin-17 (IL-17) has been reported to play a pathogenic role in the development of RA, this study was undertaken to investigate whether targeting of IL-17 may explain the protection against CIA afforded by ES-62. METHODS: DBA/1 mice progressively display arthritis following immunization with type II collagen. The protective effects of ES-62 were assessed by determination of cytokine levels, flow cytometric analysis of relevant cell populations, and in situ analysis of joint inflammation in mice with CIA. RESULTS: ES-62 was found to down-regulate IL-17 responses in mice with CIA. First, it acted to inhibit priming and polarization of IL-17 responses by targeting a complex IL-17-producing network, involving signaling between dendritic cells and γ/δ or CD4+ T cells. In addition, ES-62 directly targeted Th17 cells by down-regulating myeloid differentiation factor 88 expression to suppress responses mediated by IL-1 and Toll-like receptor ligands. Moreover, ES-62 modulated the migration of γ/δ T cells and this was reflected by direct suppression of CD44 up-regulation and, as evidenced by in situ analysis, dramatically reduced levels of IL-17-producing cells, including lymphocytes, infiltrating the joint. Finally, there was strong suppression of IL-17 production by cells resident in the joint, such as osteoclasts within the bone areas. CONCLUSION: Our findings indicate that ES-62 treatment of mice with CIA leads to unique multisite manipulation of the initiation and effector phases of the IL-17 inflammatory network. ES-62 could be exploited in the development of novel therapeutics for RA.

Phosphodiesterase 4 (PDE4) regulation of proinflammatory cytokine and chemokine release from rheumatoid synovial membrane.

BACKGROUND: The cAMP-metabolising enzyme, phosphodiesterase 4 (PDE4), has been implicated in a number of immune responses, including tumour necrosis factor α (TNFα) production. To date, few data have directly addressed whether synovial cytokine and chemokine production is modified by PDE4. OBJECTIVE: Using specific PDE4 inhibitors, roflumilast plus two novel inhibitors, INH 0061 and INH 0062, the authors studied the effect of PDE4 inhibition on proinflammatory cytokine and chemokine release from primary rheumatoid arthritis (RA) synovial digest suspensions and in a macrophage T cell co-culture assay system. RESULTS: All PDE4 inhibitors dose-dependently reduced the release of TNFα from primary synovial membrane cultures (n=5), half maximal inhibitory concentration (IC(50)) 300-30 nM, p<0.05. Similarly, a significant suppression in the release the proinflammatory chemokines, monocyte chemoattractant protein-1 (MCP-1), macrophage inflammatory protein (MIP)-1α, MIP-1ß (IC(50) 300-30 nM) and regulated upon activation normal T-cell expressed and secreted (RANTES) (IC(50) 3 nM) was also observed, p<0.05. While interleukin 1ß was also reduced, it did not achieve an IC(50). These observations were further confirmed in a macrophage T cell co-culture system, demonstrating the importance of PDE4 pathways in regulating cytokine/chemokine release in a cellular interaction implicated in inflammatory synovitis. Subsequent studies using the human monocytic cell line U937 also demonstrated cytokine regulation with PDE4 knockdown utilising a small interfering RNA approach. CONCLUSION: These data provide direct evidence of PDE4-dependent pathways in human RA synovial inflammatory cytokine and chemokine release and may provide a novel approach in treating chronic autoimmune conditions such as RA.

Expression and proinflammatory role of proteinase-activated receptor 2 in rheumatoid synovium: ex vivo studies using a novel proteinase-activated receptor 2 antagonist.

OBJECTIVE: Serine proteinases activate the G protein-coupled receptor, proteinase-activated receptor 2 (PAR-2), via cleavage and exposure of a tethered ligand. PAR-2 is known to exert proinflammatory actions in a murine model of arthritis, since PAR-2-deficient mice exhibit strikingly reduced articular inflammation. This study was undertaken to examine synovial PAR-2 expression and to determine the effect of a novel PAR-2 antagonist on synovial cytokine production, in order to investigate the hypothesis that PAR-2 plays a critical role in the pathogenesis of rheumatoid arthritis (RA). METHODS: Using a monoclonal antibody to human PAR-2, expression in RA synovium and cultured synovial fibroblasts was characterized. The novel PAR-2 antagonist, ENMD-1068, was added to primary cultures of RA synovial tissue, from which spontaneous cytokine release was measured. RESULTS: PAR-2 was substantially up-regulated in RA synovium compared with control synovial tissue from patients with osteoarthritis or seronegative inflammatory arthritis, neither of which exhibited significant PAR-2 expression. Importantly, spontaneous release of tumor necrosis factor alpha and interleukin-1beta from RA synovium was substantially inhibited by ENMD-1068, in a dose-dependent manner. CONCLUSION: These findings identify PAR-2 as a novel upstream regulator of proinflammatory cytokine production in RA and indicate its potential as a novel therapeutic target in inflammatory arthritis.

Expression and alternative processing of IL-18 in human 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.

Enumeration and phenotypical analysis of distinct dendritic cell subsets in psoriatic arthritis and rheumatoid arthritis.

Dendritic cells (DCs) comprise heterogeneous subsets of professional antigen-presenting cells, linking innate and adaptive immunity. Analysis of DC subsets has been hampered by a lack of specific DC markers and reliable quantitation assays. We characterised the immunophenotype and functional characteristics of psoriatic arthritis (PsA)-derived and rheumatoid arthritis (RA)-derived myeloid DCs (mDCs) and plasmacytoid DCs (pDCs) to evaluate their potential role in arthritis. Circulating peripheral blood (PB) pDC numbers were significantly reduced in PsA patients (P = 0.0098) and RA patients (P = 0.0194), and mDCs were significantly reduced in RA patients (P = 0.0086) compared with healthy controls. The number of circulating mDCs in RA PB was significantly inversely correlated to C-reactive protein (P = 0.021). The phenotype of both DC subsets in PsA PB and RA PB was immature as compared with healthy controls. Moreover, CD62L expression was significantly decreased on both mDCs (PsA, P = 0.0122; RA, P = 0.0371) and pDCs (PsA, P = 0.0373; RA, P = 0.0367) in PB. Both mDCs and pDCs were present in PsA synovial fluid (SF) and RA SF, with the mDC:pDC ratio significantly exceeding that in matched PB (PsA SF, P = 0.0453; RA SF, P = 0.0082). pDCs isolated from RA SF and PsA SF displayed an immature phenotype comparable with PB pDCs. RA and PsA SF mDCs, however, displayed a more mature phenotype (increased expression of CD80, CD83 and CD86) compared with PB mDCs. Functional analysis revealed that both SF DC subsets matured following toll-like receptor stimulation. pDCs from PB and SF produced interferon alpha and tumour necrosis factor alpha on TLR9 stimulation, but only SF pDCs produced IL-10. Similarly, mDCs from PB and SF produced similar tumour necrosis factor alpha levels to TLR2 agonism, whereas SF mDCs produced more IL-10 than PB controls. Circulating DC subset numbers are reduced in RA PB and PsA PB with reduced CD62L expression. Maturation is incomplete in the inflamed synovial compartment. Immature DCs in SF may contribute to the perpetuation of inflammation via sampling of the inflamed synovial environment, and in situ presentation of arthritogenic antigen.

Targeting cytokines beyond tumor necrosis factor-alpha and interleukin-1 in rheumatoid arthritis.

Targeting tumor necrosis factor-a has proven of considerable value in treatment for rheumatoid arthritis, with substantial benefits achieved in a proportion of treated patients. However, a significant number of patients do not achieve sufficient improvement and as a result there remains considerable unmet clinical need. A number of cytokines have recently been described with proinflammatory activity in rheumatoid arthritis synovitis, including interleukin (IL)-6, IL-12, IL-15, and IL-18. We review recent data that support the notion that some or all of these moieties offer therapeutic potential. The possibility that some may be useful in partial responders to tumor necrosis factor blocking agents or in synergy with the latter is discussed.

Inhibition of neutrophil function by the Kaposi's sarcoma-associated herpesvirus vOX2 protein.

Kaposi's sarcoma is multifactorial, involving Kaposi's sarcoma-associated herpesvirus (KSHV) infection and immune dysfunction. A KSHV protein (vOX2), fused with the Fc domain of human immunoglobulin G1 to create vOX2:Fc, suppressed neutrophil oxidative burst and inhibited the production of pro-inflammatory chemokines (IL-8 and monocyte chemoattractant protein 1) by monocyte/macrophage cells. vOX2:Fc suppressed the acute inflammatory response in mice in which neutrophil-mediated inflammation was induced by carrageenan. The data suggest that vOX2 can contribute to immune dysfunction and could have anti-inflammatory therapeutic potential.

Targeting interleukin-15 in patients with rheumatoid arthritis: a proof-of-concept study.

OBJECTIVE: Interleukin-15 (IL-15) is a proinflammatory, innate response cytokine that mediates pleiotropic effector function in rheumatoid arthritis (RA) inflammatory synovitis. Our objective was to study the ability of HuMax-IL15, a human IgG1 anti-IL-15 monoclonal antibody, to neutralize exogenous and endogenous IL-15 activity in vitro and to perform a phase I-II dose-escalation trial with HuMax-IL15 in patients with active RA. METHODS: Mononuclear cells from blood and synovial fluid (SF) of RA patients were isolated and cultured in vitro under experimental conditions involving the addition of HuMax-IL15. HuMax-IL15 was administered to 30 RA patients who received no other disease-modifying antirheumatic drugs in a 12-week, dose-ascending, placebo-controlled, double-blind, phase I-II proof-of-concept study. RESULTS: In vitro studies showed that HuMax-IL15 suppressed proliferation and induced apoptosis in an IL-15-dependent cell line, BDB2, and was capable of suppressing the release of interferon-gamma by synovial fluid mononuclear cell (SFMC) cultures induced by exogenous IL-15. Furthermore, HuMax-IL15 F(ab')2 fragments suppressed exogenous IL-15-induced CD69 expression in RA peripheral blood mononuclear cells and SFMCs, which indicates that HuMax-IL15 can specifically neutralize several biologic effects of IL-15 in synovial tissue in vitro. In a phase I-II clinical trial, HuMax-IL15 was well tolerated clinically, with no significant effects on T lymphocyte subset and natural killer cell numbers. Substantial improvements in disease activity were observed according to the American College of Rheumatology criteria for 20% improvement (63% of patients), 50% improvement (38%), and 70% improvement (25%). CONCLUSION: These clinical data suggest for the first time that IL-15 could represent a novel therapeutic target in RA.

Prior elevation of IL-18 promotes rapid early IFN-gamma production during staphylococcal infection.

Systemic Staphylococcus aureus infection is associated with significant morbidity and mortality arising from both bacterial and host immune factors. IL-18 is a pro-inflammatory cytokine of the IL-1 superfamily that exhibits broad functional effects in innate and acquired immune responses and which has been found in high levels in several chronic inflammatory and autoimmune diseases. Over-expression of IL-18 may promote early resolution of infection or could promote a detrimental exaggerated immune response. This was explored in a model of S. aureus infection. We report increased mortality in Swiss mice that were given recombinant IL-18 prior to inoculation with S. aureus LS-1. IL-18 administration prior to infection induced preferentially enhanced IFN-gamma mRNA expression in peripheral blood leukocytes and spleen, especially splenic NK cells. This correlated with increased IFN-gamma protein detection in serum, and leukocyte and spleen cultures at subsequent discrete time points. These data suggest that increased mortality following gram-positive infection in autoimmune diseases could in part reflect the impact of high levels of pleiotropic pro-inflammatory cytokines such as IL-18 present prior to the onset of infection.

Interleukin-18: a therapeutic target in rheumatoid arthritis?

Interleukin 18 (IL-18), a member of the IL-1 superfamily of cytokines has been demonstrated to be an important mediator of both innate and adaptive immune responses. Several reports have implicated its role in the pathogenesis of rheumatoid arthritis (RA). Although biologic therapy is firmly established in the treatment of a number of inflammatory diseases including RA, partial and non-responder patients constitute residual unmet clinical need. The aim of this article is to briefly review the biology of, and experimental approaches to IL-18 neutralisation, together with speculation as to the relative merits of IL-18 as an alternative to existing targets.

Targeting cytokines beyond tumor necrosis factor-alpha and interleukin-1 in rheumatoid arthritis.

Targeting tumor necrosis factor-a has proven of considerable value in treatment for rheumatoid arthritis, with substantial benefits achieved in a proportion of treated patients. However, a significant number of patients do not achieve sufficient improvement and as a result there remains considerable unmet clinical need. A number of cytokines have recently been described with proinflammatory activity in rheumatoid arthritis synovitis, including interleukin (IL) -6, IL-12, IL-15, and IL-18. We review recent data that support the notion that some or all of these moieties offer therapeutic potential. The possibility that some may be useful in partial responders to tumor necrosis factor blocking agents or in synergy with the latter is discussed.

Interleukin-15: a new cytokine target for the treatment of inflammatory diseases.

Interleukin (IL)-15 is a pleiotropic pro-inflammatory cytokine that is expressed in several inflammatory disorders, including rheumatoid arthritis, psoriasis and pulmonary inflammatory diseases. IL-15 promotes activation of T cells, neutrophils and macrophages, and is critical to dendritic cell function in several model systems. Recent emerging data suggest that IL-15 may serve as a useful therapeutic target across a range of disease states. Advances in the past year highlight the beneficial effect of IL-15 neutralisation in models of psoriasis and diabetes. Further evidence for IL-15 expression and effector function has emerged across a range of rheumatic disorders, including juvenile inflammatory arthritis, rheumatoid arthritis and Kawasaki disease. These data hold promise for therapeutic targeting in ongoing human studies and those in the near future.

Inducing experimental arthritis and breaking self-tolerance to joint-specific antigens with trackable, ovalbumin-specific T cells.

The importance of T cell Ag specificity and Th1 vs Th2 phenotype in synovial inflammation remains controversial. Using OVA-specific TCR transgenic T cells from DO11.10 mice, we demonstrate that mice receiving Th1, but not Th2, cells display a transient arthritis following immunization that is characterized by synovial hyperplasia, cellular infiltration, and cartilage erosion. OVA-specific T cells also accumulated in inflamed joints, suggesting that they could exert their inflammatory effect locally in the joint or in the draining lymph node. Importantly, this pathology was accompanied by a breakdown in self-tolerance, as evidenced by the induction of collagen-specific T and B cell responses. This model directly demonstrates a pivotal role for Th1 cells of an irrelevant specificity in the development of inflammatory arthritis. Furthermore, the ability to track these cells in vivo will make feasible studies revealing the dynamic role of T cells in arthritis.

The anti-inflammatory potential of the filarial nematode secreted product, ES-62.

Filarial nematodes achieve long-term infection via modulation of the host immune system. Although human infection can result in severe pathology, the majority of infected individuals exhibit little evidence of this. Analysis of the immune response during infection indicates that the apparently healthy majority have an anti-inflammatory phenotype and it has been speculated that this may contribute to maintenance of host health. Recent data suggest that parasite-derived molecular secretions contribute to the anti-inflammatory phenotype and we have thus characterised a major filarial nematode secreted glycoprotein, ES-62. This molecule has been found to possess broad immunomodulatory activities that are in general, anti-inflammatory. It has long been recognised that several autoimmune disorders including rheumatoid arthritis (RA) exhibit reduced incidence and severity in geographic regions in which filarial nematodes are endemic. Furthermore, it has been speculated that these two observations are causally linked. However, molecular explanations for such an association have not been forthcoming. Although the aetiology of RA is unknown most data suggest that it is mediated via a pro-inflammatory immune response associated with excess cytokine production. Given that ES-62 is anti-inflammatory, we hypothesised that it might possess activity against diseases like RA. Indeed we found that subcutaneous injection of ES-62 prevented initiation of collagen-induced arthritis (CIA) and also suppressed progression of established disease. Ex vivo analyses demonstrated that these effects were due to inhibition of TNF-alpha production and reversal of collagen specific TH-1 responses. The nematode product was also found to inhibit pro-inflammatory cytokine release in vitro in synovial cells derived from RA patients. ES-62 thus represents a parasite-derived immunomodulator with significant therapeutic potential.

A novel therapeutic approach targeting articular inflammation using the filarial nematode-derived phosphorylcholine-containing glycoprotein ES-62.

Understanding modulation of the host immune system by pathogens offers rich therapeutic potential. Parasitic filarial nematodes are often tolerated in human hosts for decades with little evidence of pathology and this appears to reflect parasite-induced suppression of host proinflammatory immune responses. Consistent with this, we have previously described a filarial nematode-derived, secreted phosphorylcholine-containing glycoprotein, ES-62, with immunomodulatory activities that are broadly anti-inflammatory in nature. We sought to evaluate the therapeutic potential of ES-62 in vitro and in vivo in an autoimmune disease model, namely, collagen-induced arthritis in DBA/1 mice. ES-62 given during collagen priming significantly reduced initiation of inflammatory arthritis. Crucially, ES-62 was also found to suppress collagen-induced arthritis severity and progression when administration was delayed until after clinically evident disease onset. Ex vivo analyses revealed that in both cases, the effects were associated with inhibition of collagen-specific pro-inflammatory/Th1 cytokine (TNF-alpha, IL-6, and IFN-gamma) release. In parallel in vitro human tissue studies, ES-62 was found to significantly suppress macrophage activation via cognate interaction with activated T cells. Finally, ES-62 suppressed LPS-induced rheumatoid arthritis synovial TNF-alpha and IL-6 production. Evolutionary pressure has promoted the generation by pathogens of diverse mechanisms enabling host immune system evasion and induction of "tolerance." ES-62 represents one such mechanism. We now provide proof of concept that parasite-derived immunomodulatory strategies offer a novel therapeutic opportunity in inflammatory arthritis.

Chemoattraction of human T cells by IL-18.

Cell locomotion is crucial to the induction of an effective immune response. We report here the chemoattraction of CD4(+) T cells by IL-18, a member of the IL-1 cytokine family. Recombinant IL-18 increased the proportion of T cells in polarized morphology in vitro and stimulated their subsequent invasion into collagen gels in an IL-18 concentration gradient-dependent manner. Immunofluorescent microscopy studies determined that the major cell type responding to IL-18 was IL-18R(+)CD4(+). Importantly, synovial CD4(+) T cells from patients with rheumatoid arthritis responded to IL-18, adopting polarized morphology and gel invasion without further activation ex vivo, indicating the physiologic relevance of our observations. Finally, injection of rIL-18 into the footpad of DBA/1 mice led to local accumulation of inflammatory cells. These data therefore demonstrate for the first time lymphocyte chemoattractant properties of a member of the IL-1 cytokine family and its relevance in inflammatory diseases.

Interleukin-18.

Interleukin-18 (IL-18), a recently described member of the IL-1 cytokine superfamily, is now recognized as an important regulator of innate and acquired immune responses. IL-18 is expressed at sites of chronic inflammation, in autoimmune diseases, in a variety of cancers, and in the context of numerous infectious diseases. This short review will describe the basic biology of IL-18 and thereafter address its potential effector and regulatory role in several human disease states including autoimmunity and infection. IL-18, previously known as interferon-gamma (IFN-gamma)-inducing factor, was identified as an endotoxin-induced serum factor that stimulated IFN-gamma production by murine splenocytes [(1) ]. IL-18 was cloned from a murine liver cell cDNA library generated from animals primed with heat-killed Propionibacterium acnes and subsequently challenged with lipopolysaccharide [(2) ]. Nucleotide sequencing of murine IL-18 predicted a precursor polypeptide of 192 amino acids lacking a conventional signal peptide and a mature protein of 157 amino acids. Subsequent cloning of human IL-18 cDNA revealed 65% homology with murine IL-18 [(3) ] and showed that both contain an unusual leader sequence consisting of 35 amino acids at their N terminus.