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Key Points The molecular mechanisms of cartilage breakdown in rheumatoid arthritis (RA) and osteoarthritis (OA) show considerable overlap, particularly with respect to matrix-degrading enzymes, but also with respect to some inflammatory mediators The loss of phenotypic stability of articular chondrocytes, and initiation of a programme resembling aspects of embryonic endochondral ossification, could explain important features of OA In contrast to OA, RA is associated with a stable, tumour-like activation of fibroblast-like synoviocytes that mediate the destruction of articular cartilage through directed invasion Cartilage has active roles in OA and RA as a signalling scaffold harbouring bioactive matrix components and soluble factors, which interact with embedded chondrocytes and are released upon cartilage degradation Osteoarthritis (OA) and rheumatoid arthritis (RA) are disorders of the joints that involve degradation of the extracellular matrix of cartilage, and proteases and inflammatory mediators are common to both conditions. However, different cells are affected in OA (chondrocytes) and RA (synoviocytes), and treating these diseases requires an understanding of their differences as well as their similarities. Cartilage damage is a key feature of degenerative joint disorders—primarily osteoarthritis (OA)—and chronic inflammatory joint diseases, such as rheumatoid arthritis (RA). Substantial progress has been made towards understanding the mechanisms that lead to degradation of the cartilage matrix in either condition, which ultimately results in the progressive remodelling of affected joints. The available data have shown that the molecular steps in cartilage matrix breakdown overlap in OA and RA. However, they have also, to a great extent, changed our view of the roles of cartilage in the pathogenesis of these disorders. In OA, cartilage loss occurs as part of a complex programme that resembles aspects of embryonic bone formation through endochondral ossification. In RA, early cartilage damage is a key trigger of cellular reactions in the synovium. In a proposed model of RA as a site-specific manifestation of a systemic autoimmune disorder, early cartilage damage in the context of immune activation leads to a specific cellular response within articular joints that could explain not only the organ specificity of RA, but also the chronic nature and perpetuation of the disease.

The interactions of fibroblast-like synoviocyte (FLS)-derived pro-inflammatory cytokines/chemokines and immune cells support the recruitment and activation of inflammatory cells in RA. Here, we show for the first time that the classical myokine myostatin (GDF-8) is involved in the recruitment of Th17 cells to inflammatory sites thereby regulating joint inflammation in a mouse model of TNFalpha-mediated chronic arthritis. Mechanistically, myostatin-deficiency leads to decreased levels of the chemokine CCL20 which is associated with less infiltration of Th17 cells into the inflamed joints. In vitro, myostatin alone or in combination with IL-17A enhances the secretion of CCL20 by FLS whereas myostatin-deficiency reduces CCL20 secretion, associated with an altered transmigration of Th17 cells. Thus, the communication between activated FLS and Th17 cells through myostatin and IL-17A may likely contribute to a vicious cycle of inflammation, accounting for the persistence of joint inflammation in chronic arthritis. Blockade of the CCL20–CCR6 axis by inhibition of myostatin may, therefore, be a promising treatment option for chronic inflammatory diseases such as arthritis.

Myostatin is shown to directly promote osteoclast differentiation, and its inhibition improves arthritic bone loss in two mouse models. Myostatin (also known as growth and differentiation factor 8) is a secreted member of the transforming growth factor-β (TGF-β) family that is mainly expressed in skeletal muscle, which is also its primary target tissue. Deletion of the myostatin gene ( Mstn ) in mice leads to muscle hypertrophy, and animal studies support the concept that myostatin is a negative regulator of muscle growth and regeneration 1 , 2 , 3 , 4 , 5 . However, myostatin deficiency also increases bone formation, mainly through loading-associated effects on bone 6 , 7 , 8 , 9 , 10 , 11 . Here we report a previously unknown direct role for myostatin in osteoclastogenesis and in the progressive loss of articular bone in rheumatoid arthritis (RA). We demonstrate that myostatin is highly expressed in the synovial tissues of RA subjects and of human tumor necrosis factor (TNF)-α transgenic (hTNFtg) mice, a model for human RA 12 . Myostatin strongly accelerates receptor activator of nuclear factor κB ligand (RANKL)-mediated osteoclast formation in vitro through transcription factor SMAD2-dependent regulation of nuclear factor of activated T-cells (NFATC1). Myostatin deficiency or antibody-mediated inhibition leads to an amelioration of arthritis severity in hTNFtg mice, chiefly reflected by less bone destruction. Consistent with these effects in hTNFtg mice, the lack of myostatin leads to increased grip strength and less bone erosion in the K/BxN serum-induced arthritis model in mice. The results strongly suggest that myostatin is a potent therapeutic target for interfering with osteoclast formation and joint destruction in RA.

Aberrant immune responses represent the underlying cause of central nervous system (CNS) autoimmunity, including multiple sclerosis (MS). Recent evidence implicated the crosstalk between coagulation and immunity in CNS autoimmunity. Here we identify coagulation factor XII (FXII), the initiator of the intrinsic coagulation cascade and the kallikrein–kinin system, as a specific immune cell modulator. High levels of FXII activity are present in the plasma of MS patients during relapse. Deficiency or pharmacologic blockade of FXII renders mice less susceptible to experimental autoimmune encephalomyelitis (a model of MS) and is accompanied by reduced numbers of interleukin-17A-producing T cells. Immune activation by FXII is mediated by dendritic cells in a CD87-dependent manner and involves alterations in intracellular cyclic AMP formation. Our study demonstrates that a member of the plasmatic coagulation cascade is a key mediator of autoimmunity. FXII inhibition may provide a strategy to combat MS and other immune-related disorders. Factor XII initiates the intrinsic blood coagulation cascade and the kinin system. Here the authors show that Factor XII is elevated in the blood of multiple sclerosis patients, activates dendritic cells via CD87 and cAMP, and its blockade inhibits immunopathology in a mouse model of the disease.

The LIM and SH3 domain protein 1 (Lasp1) was originally cloned from metastatic breast cancer and characterised as an adaptor molecule associated with tumourigenesis and cancer cell invasion. However, the regulation of Lasp1 and its function in the aggressive transformation of cells is unclear. Here we use integrative epigenomic profiling of invasive fibroblast-like synoviocytes (FLS) from patients with rheumatoid arthritis (RA) and from mouse models of the disease, to identify Lasp1 as an epigenomically co-modified region in chronic inflammatory arthritis and a functionally important binding partner of the Cadherin-11/β-Catenin complex in zipper-like cell-to-cell contacts. In vitro, loss or blocking of Lasp1 alters pathological tissue formation, migratory behaviour and platelet-derived growth factor response of arthritic FLS. In arthritic human TNF transgenic mice, deletion of Lasp1 reduces arthritic joint destruction. Therefore, we show a function of Lasp1 in cellular junction formation and inflammatory tissue remodelling and identify Lasp1 as a potential target for treating inflammatory joint disorders associated with aggressive cellular transformation. Fibroblast-like synoviocytes are important mediators of joint pathology in rheumatoid arthritis (RA). Here the authors show that Lasp1 is epigenetically regulated and highly expressed by these cells in RA and its deletion can limit joint pathology in a mouse model of inflammatory arthritis.

Objective To elucidate the mechanisms involved in cartilage damage in an experimental model of rheumatoid arthritis (RA) by specifically addressing the time course of extracellular matrix degradation and the contribution of cell–matrix interactions for initiation and perpetuation of this process. Methods The human tumour necrosis factor (TNF) transgenic (hTNFtg) mouse model of RA was used to analyse the time course of pannus attachment to the cartilage and cartilage destruction, respectively, and crossed hTNFtg mice with interleukin (IL)-1−/− animals were used to investigate the role of IL-1 on these TNF-induced mechanisms in vivo. In addition, an in vitro attachment assay using synovial fibroblasts (SFs) from hTNFtg mice and freshly isolated articular cartilage was used to determine the role of proteoglycan loss in attachment of SFs and the role of the transmembrane heparan sulfate proteoglycan syndecan-4. Results In vivo analyses of hTNFtg mice showed that proteoglycan loss induced by IL-1 precedes and constitutes an important prerequisite for these processes as, in hTNFtg mice, IL-1 deficiency protected from the loss of cartilage proteoglycans and almost completely prevented the attachment and subsequent invasion of inflamed synovial tissue into cartilage. In vitro studies confirmed that loss of cartilage proteoglycans is required for attachment of SFs and that syndecan-4 is prominently involved in SF attachment and activation. Conclusions The results of this study suggest that the loss of cartilage proteoglycans is an early event in the course of destructive arthritis that facilitates the attachment of the inflamed synovial membrane and also initiates matrix degradation and inflammation through cell–matrix interactions.

ObjectiveTo analyze the IL-6 signaling pathway in SLE to better understand why CRP levels are disproportionally low for the increased IL-6 levels in active SLE, but increase in bacterial infection in the same patients.MethodsPBMC and sera of 41 SLE patients and 71 healthy individuals (HC) were investigated. IL-6 and soluble IL-6 receptor (sIL-6R) were measured by ELISA. CD126 and phosphorylated Stat3 (pStat3) were stained with phycoerythrin (PE)-labelled antibodies. PBMC were incubated short term with IL-6 or for 24 hours with or without IL-6, IL-10, TNF, interferon-α (IFNα), or combinations of these. HEK293T cells transfected with wildtype IL-6R or a shedding resistant IL-6R mutant were incubated with or without IL-6, IFNα, or their combination. Immunoprecipitation (IP) and immunoblotting was used to detect sIL-6R in supernatants. Flow cytometry was used for the other analyses.ResultsIL-6 was increased in SLE (median 3.64 vs 0.89 pg/ml in HC, p<0.0001). ECLAM correlated with IL-6 (Spearman r=0.40, p<0.01), but not CRP (r=0.29). CD126+ lymphocytes were decreased (median 46% SLE vs. 61% for HC, Mann Whitney p<0.0001), and IL-6-induced Stat3 phosphorylation was reduced (Δmfi 14.2 vs. 18.8, p=0.0044). In a mirror image of CD126, sIL-6R was increased in SLE (median 42.2 ng/mL vs. 38.6 ng/mL in HC, p=0.02). Stimulation of healthy PBMC with IL-6 plus IFNα led to a 39±13% reduction (p<0.0001) in CD126+ cells (figure 1A) and to an increase in sIL-6R (p=0.0055) (figure 1B), mimicking the in vivo situation. IL-6-induced increases in supernatant sIL-6R were found for HEK293T transfected with IL-6R, but not those transfected with shedding-resistant IL-6R.ConclusionsThe combination of type I interferon and IL-6, both of which are well known to be increased in SLE, leads to shedding of membrane bound CD126 to sIL-6R. This moves IL-6 effects from the membrane CD126 and gp130 receptors of the liver, which produces CRP, to other cells that bind the IL-6/sIL-6R complex with their gp130. This effectively increases peripheral IL-6 inflammatory effects while limiting CRP levels. As such, the data offer a molecular explanation for the (relatively) low CRP levels in SLE, which have been found associated with the type I interferon signature.AcknowledgementsThis project was in large parts funded by Deutsche Forschungsgemeinschaft (DFG) grant AR-757/1–1 to Martin Aringer.Abstract P6 Figure 1

ObjectiveSyndecan-4 (sdc4) is a cell-anchored proteoglycan that consists of a transmembrane core protein and glucosaminoglycan (GAG) side chains. Binding of soluble factors to the GAG chains of sdc4 may result in the dimerisation of sdc4 and the initiation of downstream signalling cascades. However, the question of how sdc4 dimerisation and signalling affects the response of cells to inflammatory stimuli is unknown.MethodsSdc4 immunostaining was performed on rheumatoid arthritis (RA) tissue sections. Interleukin (IL)-1 induced extracellular signal-regulated kinases (ERK) phosphorylation and matrix metalloproteinase-3 production was investigated. Il-1 binding to sdc4 was investigated using immunoprecipitation. IL-1 receptor (IL1R1) staining on wild-type, sdc4 and IL1R1 knockout fibroblasts was performed in fluorescence-activated cell sorting analyses. A blocking sdc4 antibody was used to investigate sdc4 dimerisation, IL1R1 expression and the histological paw destruction in the human tumour necrosis factor-alpha transgenic mouse.ResultsWe show that in fibroblasts, the loss of sdc4 or the antibody-mediated inhibition of sdc4 dimerisation reduces the cell surface expression of the IL-1R and regulates the sensitivity of fibroblasts to IL-1. We demonstrate that IL-1 directly binds to sdc4 and in an IL-1R-independent manner leads to its dimerisation. IL-1-induced dimerisation of sdc4 regulates caveolin vesicle-mediated trafficking of the IL1R1, which in turn determines the responsiveness to IL-1. Administration of antibodies (Ab) against the dimerisation domain of sdc4, thus, strongly reduces the expression IL1R1 on arthritic fibroblasts both in vitro and an animal model of human RA.ConclusionCollectively, our data suggest that Ab that specifically inhibit sdc4 dimerisation may support anti-IL-1 strategies in diseases such as inflammatory arthritis.

ObjectiveThe aim of this study was to assess the extent and the mechanism by which activin A contributes to progressive joint destruction in experimental arthritis and which activin A-expressing cell type is important for disease progression.MethodsLevels of activin A in synovial tissues were evaluated by immunohistochemistry, cell-specific expression and secretion by PCR and ELISA, respectively. Osteoclast (OC) formation was assessed by tartrat-resistant acid phosphatase (TRAP) staining and activity by resorption assay. Quantitative assessment of joint inflammation and bone destruction was performed by histological and micro-CT analysis. Immunoblotting was applied for evaluation of signalling pathways.ResultsIn this study, we demonstrate that fibroblast-like synoviocytes (FLS) are the main producers of activin A in arthritic joints. Most significantly, we show for the first time that deficiency of activin A in arthritic FLS (ActβAd/d ColVI-Cre) but not in myeloid cells (ActβAd/d LysM-Cre) reduces OC development in vitro, indicating that activin A promotes osteoclastogenesis in a paracrine manner. Mechanistically, activin A enhanced OC formation and activity by promoting the interaction of activated Smad2 with NFATc1, the key transcription factor of osteoclastogenesis. Consistently, ActβAd/d LysM-Cre hTNFtg mice did not show reduced disease severity, whereas deficiency of activin A in ColVI-Cre-expressing cells such as FLS highly diminished joint destruction reflected by less inflammation and less bone destruction.ConclusionsThe results highly suggest that FLS-derived activin A plays a crucial paracrine role in inflammatory joint destruction and may be a promising target for treating inflammatory disorders associated with OC formation and bone destruction like rheumatoid arthritis.