A fibrin based model for rheumatoid synovitis

Inhibitory effects of Acanthopanax sessiliflorus Harms extract on the etiology of rheumatoid arthritis in a collagen-induced arthritis mouse model

BACKGROUND

The biological function of Acanthopanax sessiliflorus Harm (ASH) has been investigated on various diseases; however, the effects of ASH on arthritis have not been investigated so far. This study investigates the effects of ASH on rheumatoid arthritis (RA).

METHODS

Supercritical carbon dioxide (CO2) was used for ASH extract preparation, and its primary components, pimaric and kaurenoic acids, were identified using gas chromatography-mass spectrometer (GC-MS). Collagenase-induced arthritis (CIA) was used as the RA model, and primary cultures of articular chondrocytes were used to examine the inhibitory effects of ASH extract on arthritis in three synovial joints: ankle, sole, and knee.

RESULTS

Pimaric and kaurenoic acids attenuated pro-inflammatory cytokine-mediated increase in the catabolic factors and retrieved pro-inflammatory cytokine-mediated decrease in related anabolic factors in vitro; however, they did not affect pro-inflammatory cytokine (IL-1β, TNF-α, and IL-6)-mediated cytotoxicity. ASH effectively inhibited cartilage degradation in the knee, ankle, and toe in the CIA model and decreased pannus development in the knee. Immunohistochemistry demonstrated that ASH mostly inhibited the IL-6-mediated matrix metalloproteinase. Gene Ontology and pathway studies bridge major gaps in the literature and provide insights into the pathophysiology and in-depth mechanisms of RA-like joint degeneration.

CONCLUSIONS

To the best of our knowledge, this is the first study to conduct extensive research on the efficacy of ASH extract in inhibiting the pathogenesis of RA. However, additional animal models and clinical studies are required to validate this hypothesis.

Blood-Induced Arthropathy: A Major Disabling Complication of Haemophilia

Haemophilic arthropathy (HA) is one of the most serious complications of haemophilia. It starts with joint bleeding, leading to synovitis which, in turn, can cause damage to the cartilage and subchondral bone, eventually inducing degenerative joint disease. Despite significant improvements in haemophilia treatment over the past two decades and recent guidelines from ISTH and WFH recommending FVIII trough levels of at least 3 IU/dL during prophylaxis, patients with haemophilia still develop joint disease. The pathophysiology of HA is complex, involving both inflammatory and degenerative components. Early diagnosis is key for proper management. Imaging can detect joint subclinical changes and influence prophylaxis. Magnetic resonance imagining (MRI) and ultrasound are the most frequently used methods in comprehensive haemophilia care centres. Biomarkers of joint health have been proposed to determine osteochondral joint deterioration, but none of these biomarkers has been validated or used in clinical practice. Early prophylaxis is key in all severe haemophilia patients to prevent arthropathy. Treatment is essentially based on prophylaxis intensification and chronic joint pain management. However, there remain significant gaps in the knowledge of the mechanisms responsible for HA and prognosis-influencing factors. Better understanding in this area could produce more effective interventions likely to ultimately prevent or attenuate the development of HA.

High-Sucrose Diet Accelerates Arthritis Progression in a Collagen-Induced Rheumatoid Arthritis Model

SCOPE

High dietary sugar and sweeteners are suspected to cause the development of rheumatoid arthritis (RA) symptoms through the induction of proinflammatory cytokine release. However, the mechanisms by which increased dietary sugar affects RA etiology are not yet fully understood. The study uses a mouse model of collagen-induced RA (CIA) to investigate the relationship between excessive sugar consumption and RA risk.

METHODS AND RESULTS

RA-associated pathological features are assessed in the nonimmunized (NI) control group, the CIA-positive control group, and the CIA + high-sucrose diet (CIA+HS, 63% calories from sucrose) group. Compared with the CIA group, the CIA+HS group shows a greater increase in paw thickness and clinical scores, as well as, a higher degree of pannus formation and inflammation in the knee, ankle, and sole tissues. Moreover, the infiltration of immune cells is increased in the CIA+HS group. Although the expression of hepatic lipogenic genes, is not altered, that of toll-like receptor (TLR4) and IL-1β is considerably elevated in the CIA+HS group.

CONCLUSIONS

These findings suggest that excessive sucrose consumption causes hepatic fibrosis and inflammation, contributing to the pathophysiology of RA.

Fibrin deposition associates with cartilage degeneration in arthritis

Background

Cartilage damage in inflammatory arthritis is attributed to inflammatory cytokines and pannus infiltration. Activation of the coagulation system is a well known feature of arthritis, especially in rheumatoid arthritis (RA). Here we describe mechanisms by which fibrin directly mediates cartilage degeneration.

Methods

Fibrin deposits were stained on cartilage and synovial tissue of RA and osteoarthritis (OA) patients and in murine adjuvant-induced arthritis (AIA) in wild-type or fibrinogen deficient mice. Fibrinogen expression and procoagulant activity in chondrocytes were evaluated using qRT-PCR analysis and turbidimetry. Chondro-synovial adhesion was studied in co-cultures of human RA cartilage and synoviocytes, and in the AIA model. Calcific deposits were stained in human RA and OA cartilage and in vitro in fibrinogen-stimulated chondrocytes.

Findings

Fibrin deposits on cartilage correlated with the severity of cartilage damage in human RA explants and in AIA in wild-type mice, whilst fibrinogen deficient mice were protected. Fibrin upregulated Adamts5 and Mmp13 in chondrocytes. Chondro-synovial adhesion only occurred in fibrin-rich cartilage areas and correlated with cartilage damage. In vitro, autologous human synoviocytes, cultured on RA cartilage explants, adhered exclusively to fibrin-rich areas. Fibrin co-localized with calcification in human RA cartilage and triggered chondrocyte mineralization by inducing pro-calcification genes (Anx5, Pit1, Pc1) and the IL-6 cytokine. Similar fibrin-mediated mechanisms were observed in OA models, but to a lesser extent and without pseudo-membranes formation.

Interpretation

In arthritis, fibrin plaques directly impair cartilage integrity via a triad of catabolism, adhesion, and calcification.

Funding

None.

Detection of Citrullinated Fibrin in Plasma Clots of Rheumatoid Arthritis Patients and Its Relation to Altered Structural Clot Properties, Disease-Related Inflammation and Prothrombotic Tendency

Aims

The risk of cardiovascular events in patients with Rheumatoid Arthritis (RA) is disproportionately heightened as a result of systemic inflammation. The relative effect of autoimmune-associated citrullination on the structure and thrombotic potential of fibrin(ogen) remains unknown. We therefore compared indices of vascular function, inflammation, coagulation and fibrin clot composition in RA patients with healthy controls and evaluated parameter association with disease presence.

Methods

Blood samples were collected from 30 RA patients and 30 age- and gender-matched healthy volunteers. Levels of serum amyloid A (SAA), c-reactive protein (CRP), soluble intercellular adhesion molecule 1 (sICAM-1) and soluble vascular cell adhesion molecule 1 (sVCAM-1) was measured using a sandwich immunoassay. Whole blood coagulation was assessed using Thromboelastography (TEG^®). Fibrin clot networks and fiber structure was investigated using Scanning Electron Microscopy. The detection and quantification of citrullination in formed fibrin clots was performed using a fluorescently labeled Citrulline monoclonal antibody with Fluorescence Wide Field Microscopy.

Results

Concentrations of SAA, CRP and ICAM-1 were significantly elevated in RA patients compared to controls. TEG parameters relating to coagulation initiation, rate of fibrin cross-linking, and time to reach maximum thrombus generation were attenuated in RA patients. Microscopic analysis revealed denser networks of thicker fibrin fibers in RA patients compared to controls and multiple citrullinated regions within fibrin clot structures in RA patients were present.

Conclusion

Our findings provide novel evidence for the citrullination of fibrin within vasculature is more prominent in RA plasma compared to control plasma and plasma is more accessible than synovial fluid. Citrullinated fibrinogen could play a role as a determinant of thrombotic risk in RA patients.

Fibrinogen, factor XIII and α2-antiplasmin genotypes are associated with inflammatory activity and anti-citrullinated protein antibodies

BACKGROUND

Fibrin(ogen) derivatives, crosslinked fibrin and fibrinolysis play important roles in inflammation and are involved in pathogenesis of rheumatoid arthritis (RA). About 2/3 of RA patients exhibit anti-citrullinated protein antibodies (ACPA) that target deiminated fibrinogen. Genetic variants of β-fibrinogen (FGB) (rs1800790G>A) and factor XIII A-subunit (F13A) Val34Leu (rs5985) are known to influence interactively inflammatory processes. It is hypothesized that predisposition for dense fibrin clots is related to better inflammation control.

METHODS

To test this hypothetical model a cohort of 924 patients (288 RA and 636 non-RA patients) (3545 observations) was genotyped for FGB (rs1800790G>A, rs1800788C>T), α-fibrinogen (FGA) (rs6050A>G, rs2070006G>A, rs2070016T>C), γ-fibrinogen (FGG) (rs1049636T>C), F13A Val34Leu (rs5985) and α₂-antiplasmin (A2AP) Arg6Trp (rs2070863). Genotype constellations potentially predisposing for dense fibrin clots were defined and their relation to inflammatory activity as measured by C-reactive protein (CRP) and disease activity score of 28 joints (DAS28) was assessed in univariate and multivariate analyses. The relation of these genotype constellations with presence of ACPA was tested.

RESULTS

Genotype constellations involving FGB rs1800790G>A and FGA rs2070016T>C were inversely associated with CRP levels (≥10 mg/L) (OR: 0.49, P < 10^-8/7_adj = 0.0001; OR: 0.52, P < 0.0005/P_adj = 0.01). In RA, both genotype constellations were observed with higher frequencies of low disease activity (DAS28 ≤ 3.2) (OR: 2.66, P = .009; OR 2.78, P = .01) and lower frequencies of high disease activity (DAS28>5.1) (OR: 0.52, P < .03, OR: 0.42, P = .01). Associations with CRP depended on A2AP 6Arg/Arg genotype known to be necessary for optimal anti-fibrinolytic capacity (P = .001). Finally, Genotype constellations involving FGB rs1800790G>A and FGA rs2070016T>C were found to be associated with ACPA-positivity in RA (OR: 2.18, P < .03; OR: 1.95, P = .09).

CONCLUSIONS

These results support the hypothesis that genotypes, which increase fibrin clot density and anti-fibrinolytic capacity, reduce inflammatory activity and are related to humoral autoimmunity in RA.

Expanding the citrullinome of synovial fibrinogen from rheumatoid arthritis patients

Citrullination is a post-translational protein modification, which is associated with inflammation in general and is thought to play an important pathogenic role in rheumatoid arthritis (RA). Here a mass spectrometry-based proteomics approach was applied to identify citrullination sites in synovial fluid fibrinogen from four RA patients. In general, high disease activity correlated with increased number of identified citrullination sites and higher relative citrulline occupancy. Altogether, 23 sites were identified, of which 9 have not been previously reported to be citrullinated in vivo. Citrullination at site α84, α123, α129, α547, α573, α591, β334 and γ134 was identified in more than one patient, and these positions were therefore regarded as hotspots. Following citrullination of fibrinogen in vitro using human recombinant peptidylarginine deiminase 2 (PAD2), a total of 46 citrullination sites were identified, including 6 hitherto unreported in vitro citrullination sites. Twenty-two out of the 23 citrullination sites identified in vivo were also detected in vitro, supporting the validity of the identifications. SIGNIFICANCE: This work provides information about previously uncharacterized citrullination sites in synovial fluid fibrinogen from rheumatoid arthritis patients. Detection of these novel citrullination sites may prove to have diagnostic or prognostic value in RA and enhance our understanding of the immune pathogenesis.

Pathophysiological significance of protein hydrophobic interactions: An emerging hypothesis

Fibrinogen is a unique protein that is converted into an insoluble fibrin in a single enzymatic event, which is a characteristic feature of fibrinogen due to its susceptibility to fibrinolytic degradation and dissolution. Although thrombosis is a result of activated blood coagulation, no explanation is being offered for the persistent presence of fibrin deposits in the affected organs. A classic example is stroke, in which the thrombolytic therapy is effective only during the first 3-4 h after the onset of thrombosis. This phenomenon can now be explained in terms of the modification of fibrinogen structure induced by hydroxyl radicals generated during the period of ischemia caused, in turn, by the blocking of the blood flow within the obstructed vessels. Fibrinogen modification involves intra-to intermolecular disulfide rearrangement induced by the reductive power of hydroxyl radicals that result in the exposition of buried hydrophobic epitopes. Such epitopes react readily with each other forming linkages stronger than the peptide covalent bonds, thus rendering them resistant to the proteolytic degradation. Also, limited reduction of human serum albumin (HSA) generates hydrophobic polymers that form huge insoluble complexes with fibrinogen. Consequently, such insoluble copolymers can be deposited within the circulation of various organs leading to their dysfunction. In conclusion, the study of protein hydrophobic interactions induced by a variety of nutritional and/or environmental factors can provide a rational explanation for a number of pathologic conditions including cardiovascular, neurologic, and other degenerative diseases including cancer.

Detection of Subclinical Arthritis in Mice by a Thrombin Receptor-Derived Imaging Agent

OBJECTIVE

Functional imaging of synovitis could improve both early detection of rheumatoid arthritis (RA) and long-term outcomes. Given the intersection of inflammation with coagulation protease activation, this study was undertaken to examine coagulation protease activities in arthritic mice with a dual-fluorescence ratiometric activatable cell-penetrating peptide (RACPP) that has a linker, norleucine (Nle)-TPRSFL, with a cleavage site for thrombin.

METHODS

K/BxN-transgenic mice with chronic arthritis and mice with day 1 passive serum-transfer arthritis were imaged in vivo for Cy5:Cy7 emission ratiometric fluorescence from proteolytic cleavage and activation of RACPP_NleTPRSFL . Joint thickness in mice with serum-transfer arthritis was measured from days 0 to 10. The cleavage-evoked release of Cy5-tagged tissue-adhesive fragments enabled microscopic correlation with immunohistochemistry for inflammatory markers. Thrombin dependence of ratiometric fluorescence was tested by ex vivo application of RACPP_NleTPRSFL and argatroban to cryosections obtained from mouse hind paws on day 1 of serum-transfer arthritis.

RESULTS

In chronic arthritis, RACPP_NleTPRSFL fluorescence ratios of Cy5:Cy7 emission were significantly higher in diseased swollen ankles of K/BxN-transgenic mice than in normal mouse ankles. A high ratio of RACPP_NleTPRSFL fluorescence in mouse ankles and toes on day 1 of serum-transfer arthritis correlated with subsequent joint swelling. Foci of high ratiometric fluorescence localized to inflammation, as demarcated by immune reactivity for citrullinated histones, macrophages, mast cells, and neutrophils, in soft tissue on day 1 of serum-transfer arthritis. Ex vivo application of RACPP_NleTPRSFL to cryosections obtained from mice on day 1 of serum-transfer arthritis produced ratiometric fluorescence that was inhibited by argatroban.

CONCLUSION

RACPP_NleTPRSFL activation detects established experimental arthritis, and the detection of inflammation by RACPP_NleTPRSFL on day 1 of serum-transfer arthritis correlates with disease progression.

Changes in concentrations of haemostatic and inflammatory biomarkers in synovial fluid after intra-articular injection of lipopolysaccharide in horses

Background

Septic arthritis is a common and potentially devastating disease characterized by severe intra-articular (IA) inflammation and fibrin deposition. Research into equine joint pathologies has focused on inflammation, but recent research in humans suggests that both haemostatic and inflammatory pathways are activated in the joint compartment in arthritic conditions. The aim of this study was to characterize the IA haemostatic and inflammatory responses in horses with experimental lipopolysaccharide (LPS)-induced joint inflammation. Inflammation was induced by IA injection of LPS into one antebrachiocarpal joint of six horses. Horses were evaluated clinically with subjective grading of lameness, and blood and synovial fluid (SF) samples were collected at post injection hours (PIH) -120, −96, −24, 0, 2, 4, 8, 16, 24, 36, 48, 72 and 144. Total protein (TP), white blood cell counts (WBC), serum amyloid A (SAA), haptoglobin, iron, fibrinogen, thrombin-antithrombin (TAT) and d-dimer concentrations were assessed in blood and SF.

Results

Intra-articular injection of LPS caused local and systemic signs of inflammation including increased rectal temperature, lameness and increased joint circumference and skin temperature. Most of the biomarkers (TP, WBC, haptoglobin, fibrinogen and TAT) measured in SF increased quickly after LPS injection (at PIH 2–4), whereas SAA and d-dimer levels increased more slowly (at PIH 16 and 144, respectively). SF iron concentrations did not change statistically significantly. Blood WBC, SAA, haptoglobin and fibrinogen increased and iron decreased significantly in response to the IA LPS injection, while TAT and d-dimer concentrations did not change. Repeated pre-injection arthrocenteses caused significant changes in SF concentrations of TP, WBC and haptoglobin.

Conclusion

Similar to inflammatory joint disease in humans, joint inflammation in horses was accompanied by an IA haemostatic response with changes in fibrinogen, TAT and d-dimer concentrations. Inflammatory and haemostatic responses were induced simultaneously and may likely interact. Further studies of interactions between the two responses are needed for a better understanding of pathogenesis of joint disease in horses. Knowledge of effects of repeated arthrocenteses on levels of SF biomarkers may be of value when markers are used for diagnostic purposes.

Fibrinogen and factor XIII at the intersection of coagulation, fibrinolysis and inflammation

Fibrinogen and factor XIII are two essential proteins that are involved directly in fibrin gel formation as the final step of a sequence of reactions triggered by a procoagulant stimulus. Haemostasis is the most obvious function of the resulting fibrin clot. Different variables affect the conversion of fibrinogen to fibrin as well as the mode of fibrin polymerisation and fibrin crosslinking, hereby, critically influencing the architecture of the resulting fibrin network and consequently determining its mechanical strength and resistance against fibrinolysis. Due to fibrinogen's structure with a multitude of domains and binding motifs the fibrin gel allows for complex interactions with other coagulation factors, with profibrinolytic as well as antifibrinolyic proteins, with complement factors and with various cellular receptors. These interactions enable the fibrin network to control its own further state (i. e. expansion or degradation), to influence innate immunity, and to function as a scaffold for cell migration processes. During the whole process of fibrin gel formation biologically active peptides and protein fragments are released that additionally influence cellular processes via chemotaxis or by modulating cell-cell interactions. Thus, it is not surprising that fibrinogen and factor XIII in addition to their haemostatic function influence innate immunity as well as cell-mediated reactions like wound healing, response to tissue injury or inflammatory processes. The present review summarises current knowledge of fibrinogen's and factor XIII's function in coagulation and fibrinolysis giving special emphasis on their relation to inflammation control.

Felty's syndrome and hypofibrinogenemia: an unusual target for anti-cyclic citrullinated peptide antibodies?

BACKGROUND

Rheumatoid arthritis (RA) is a risk factor for the development of Felty's syndrome and large granular lymphocyte (LGL) leukemia. Anti-cyclic citrullinated peptide (CCP) antibodies are considered highly specific for RA and are directed against various citrullinated antigens, including citrullinated fibrinogen. Anti-CCP antibodies may interfere with the detection of citrullinated proteins and their function. In this article, we describe the possible inhibition of fibrinogen by anti-CCP antibodies with clinical consequences which have never been reported in the literature to our best knowledge.

CASE REPORT

We present the case of a 79-year-old Caucasian woman with a longstanding history of untreated seropositive RA and who had been investigated for severe neutropenia since several months. The association of splenomegaly led to suspicion of Felty's syndrome. Flux cytometry was compatible with T-cell LGL leukemia. In addition, severe hypofibrinogenemia was detected. The later finding has not been consistently associated with the former clinical entities. Further investigations demonstrated that the anti-CCP antibodies of the patient also recognized the P41 peptide of citrullinated fibrinogen. The patient deceased of intracranial hemorrhage.

CONCLUSION

It is likely, yet not definite, that high anti-citrullinated fibrinogen titers may contribute to low fibrinogen levels and could have contributed to the fatal hemorrhagic event.

Fibrinogen-specific antibody induces abdominal aortic aneurysm in mice through complement lectin pathway activation

Abdominal aortic aneurysm (AAA) is a common vascular disease associated with high mortality rate due to progressive enlargement and eventual rupture. There is currently no established therapy known to alter the rate of aneurysmal expansion. Thus, understanding the processes that initiate and sustain aneurysmal growth is pivotal for the development of medical therapies aimed at halting disease progression. Using an elastase-induced AAA mouse model that recapitulates key features of human AAA, we previously reported that a natural IgG antibody directs alternative pathway complement activation and initiates the inflammatory process that culminates in aneurysmal development. The target of this natural antibody, however, was unknown. Herein we identify a natural IgG that binds to fibrinogen deposited in elastase-perfused aortic tissues, activates the complement lectin pathway (LP), and induces AAA. Moreover, we establish that alterations in the glycosylation patterns of this antibody critically affect its ability to activate the LP in vivo. We find that LP activation precedes the alternative pathway and absence of the LP complement protein mannan-binding lectin abrogates elastase-induced AAA. In human AAA tissues the mouse anti-fibrinogen antibody recognizes epitopes that localize to the same areas that stain positively for mannan-binding lectin, which suggests that the complement LP is engaged in humans as well. Lastly, we demonstrate that circulating antibodies in a subset of AAA patients react against fibrinogen or fibrinogen-associated epitopes in human aneurysmal tissues. Our findings support the concept that an autoimmune process directed at aortic wall self-antigens may play a central role in the immunopathogenesis of AAA.

Anti-citrullinated protein antibodies and their clinical utility in rheumatoid arthritis

One of the most important serological discoveries in rheumatology in recent years has been the characterization of autoantigens in rheumatoid arthritis (RA) containing the amino acid citrulline. There are many citrullinated proteins in the inflamed RA synovium. Rheumatoid factor (RF), which is the immunologic hallmark of RA, is not specific for RA, as it is found in 5% of healthy individuals and in 10-20% of those over the age of 65 years. RFs are of low titer in early disease stages when a clear diagnosis is often not yet possible; But anti-citrullinated protein antibodies (ACPAs) can be found early in the disease course of RA, even years before the onset of clinical symptoms. The identification of citrullinated epitopes led to the development of the first and later second generation anti-cyclic citrullinated peptide (anti-CCP) antibody assays. Anti-CCP2 antibody has shown a specificity of 98% in sera from patients with established RA and 96% in sera from subjects with early RA. Anti-CCP can predict erosive disease, therefore could be a good serological marker for RA diagnosis.

Proteomic analysis of synovial fluid from the osteoarthritic knee: comparison with transcriptome analyses of joint tissues

OBJECTIVE

The pathophysiology of the most common joint disease, osteoarthritis (OA), remains poorly understood. Since synovial fluid (SF) bathes joint cartilage and synovium, we reasoned that a comparative analysis of its protein constituents in health and OA could identify pathways involved in joint damage. We undertook this study to perform a proteomic analysis of knee SF from OA patients and control subjects and to compare the results to microarray expression data from cartilage and synovium.

METHODS

Age-matched knee SF samples from 10 control subjects, 10 patients with early-stage OA, and 10 patients with late-stage OA were compared using 2-dimensional difference-in-gel electrophoresis and mass spectrometry (MS). MS with a multiplexed peptide selected reaction monitoring assay was used to confirm differential expression of a subset of proteins in an independent OA patient cohort. Proteomic results were analyzed by Ingenuity Pathways Analysis and compared to published synovial tissue and cartilage messenger RNA profiles.

RESULTS

Sixty-six proteins were differentially present in healthy and OA SF. Three major pathways were identified among these proteins: the acute-phase response signaling pathway, the complement pathway, and the coagulation pathway. Differential expression of 5 proteins was confirmed by selected reaction monitoring assay. A focused analysis of transcripts corresponding to the differentially present proteins indicated that both synovial and cartilage tissues may contribute to the OA SF proteome.

CONCLUSION

Proteins involved in the acute-phase response signaling pathway, the complement pathway, and the coagulation pathway are differentially regulated in SF from OA patients, suggesting that they contribute to joint damage. Validation of these pathways and their utility as biomarkers or therapeutic targets in OA is warranted.

Citrullination enhances the pro-inflammatory response to fibrin in rheumatoid arthritis synovial fibroblasts

OBJECTIVE

Fibrin deposits are characteristic of the synovial tissues in rheumatoid arthritis (RA). Once citrullinated, fibrin becomes an autoantigen and is thought to contribute in this way to perpetuate the disease. Our study aimed to analyse the responses of RA synovial fibroblasts (RASF) to native and citrullinated fibrin.

METHODS

The transcriptome induced by fibrin in RASF was approached with whole-genome-based gene expression arrays. The upregulation of selected pro-inflammatory genes by fibrin was confirmed in additional primary cell cultures using quantitative PCR and ELISA. Citrullination reactions were carried out with recombinant human peptidylarginine deiminases (PAD) 2 and 4.

RESULTS

In the whole-genome array native fibrin was found to modulate the gene expression profile of RASF, particularly upregulating mRNA levels of several pro-inflammatory cytokines. The induction of interleukin (IL)-6 and IL-8 by fibrin was confirmed in additional samples at both the mRNA and the protein level. Blocking and knockdown experiments showed the participation of toll-like receptor (TLR)4 in the induction of both cytokines. As compared with the native macromolecule, PAD2-citrullinated fibrin induced significantly higher expression of the pro-inflammatory cytokines in these cells.

CONCLUSIONS

Our results suggest that fibrin mediates inflammatory responses in RASF via a TLR4 pathway. In this way, fibrin and particularly its citrullinated form may contribute to sustain the cytokine burst in RA.

Coagulation and the fibrin network in rheumatic disease: a role beyond haemostasis

Activation of the immune system has been increasingly recognised to be associated with procoagulatory status in patients with inflammatory rheumatic disease. Changes in endothelial cell and platelet activation, blood flow, expression and activity of different coagulation factors, and impaired fibrinolysis serve as pathophysiological basis for enhanced risk of venous thromboembolism in inflammatory rheumatic diseases, such as rheumatoid arthritis (RA), connective tissue diseases and vasculitides. Recent studies identifying mechanisms for a functional role of coagulation factors beyond haemostasis have provided examples of interesting links between the coagulation system and innate immune activation. Furthermore, citrullinated fibrinogen is an important and early autoantigen in patients with RA carrying the HLA-DRβ1 shared epitope allele, which demonstrates an adaptive immune response to a coagulation factor in an inflammatory rheumatic disease. Additional studies have provided strong evidence that a multitude of different components of the haemostatic system (such as thrombin, fibrinogen, coagulation factor XIII and factors of the fibrinolytic system) are relevant mediators of inflammatory processes as well as of inflammatory control. Understanding the interactions between coagulation and the immune system in inflammatory rheumatic diseases will not only improve our knowledge of disease mechanisms, but could also permit the development of innovative therapeutic interventions.

Nitric oxide-driven hypoxia initiates synovial angiogenesis, hyperplasia and inflammatory lesions in mice

Background

Rheumatoid arthritis (RA) is an inflammatory articular disease with cartilage and bone damage due to hyperplasic synoviocyte invasion and subsequent matrix protease digestion. Although monoclonal antibodies against tumor necrosis factor alpha (TNFα) have been approved for clinical use in patients with RA, desired therapeutic regimens suitable for non-responders are still unavailable because etiological initiators leading to RA remain enigmatic and unidentified.

Methodology/Principal Findings

Bacteria-induced arthritis (BIA) that simulates collagen-induced arthritis (CIA) is developed in mice upon daily live bacterial feeding. The morphological lesions of paw erythema and edema together with the histological alterations of synovial hyperplasia and lymphocytic infiltration emerge as the early-phase manifestations of BIA and CIA. Bacteria- or collagen-mediated global upregulation of pro-inflammatory cytokines is accompanied by the burst of nitric oxide (NO). Elevation of the serum NO level is correlated with decline of the blood oxygen saturation percentage (SpO2), reflecting a hypoxic consequence during development towards arthritis. NO-driven hypoxia is further evident from a positive relationship between NO and lactic acid (LA), an end product from glycolysis. Upregulation of hypoxia inducible factor 1 alpha (HIF-1α) and vascular endothelial growth factor (VEGF) validates hypoxia-induced angiogenesis in the inflamed synovium of modeling mice. Administration of the NO donor compound sodium nitroprusside (SNP) causes articular inflammation by inducing synovial hypoxia. Anti-bacteria by the antibiotic cefotaxime and/or the immunosuppressant rapamycin or artesunate that also inhibits nitric oxide synthase (NOS) can abrogate NO production, mitigate hypoxia, and considerably ameliorate or even completely abort synovitis, hence highlighting that NO may serve as an initiator of inflammatory arthritis.

Conclusions/Significance

Like collagen, bacteria also enable synovial lesions via upregulating pro-inflammatory cytokines, triggering NO production, driving hypoxic responses, and inducing synovial angiogenesis and hyperplasia, suggesting that sustained infection might be, in part, responsible for the onset of synovitis and arthritis in mice.

Fibrinolysis is down-regulated in mouse collagen-induced arthritis, but its normalization does not alleviate the course of disease

OBJECTIVE

Down-regulation of fibrinolysis and increased fibrin deposition in joints are hallmarks of rheumatoid arthritis (RA), and are believed to be involved in disease progression. The mouse model of collagen-induced arthritis (CIA) closely resembles RA and has been used to explore mechanism and treatments of RA, but neither the fibrinolytic system nor pro-fibrinolytic therapies were investigated in CIA.

MATERIALS AND METHODS

Plasmin activity, levels of plasminogen activator inhibitor (PAI-1), D-dimer, and IL-6 were measured in plasma of CIA mice. Fibrin deposition and PAI-1 levels were also measured in inflamed joints. Mice were treated with plasminogen activators uPA (urokinase-type plasminogen activator) or tPA (tissue-type plasminogen activator). Effects of treatment on disease severity and fibrinolytic system were assessed.

RESULTS

CIA caused decrease in plasmin activity, accompanied by increase in PAI-1 levels, in both blood and inflamed joints. This resulted in massive fibrin deposition in synovium. PAI-1 levels correlated negatively with plasmin activity and positively with IL-6. Treatments with uPA and tPA improved plasmin activity and removed fibrin deposits in inflamed joints. However, disease severity remained unchanged.

CONCLUSIONS

Fibrinolytic changes in CIA parallel changes in RA, making CIA a suitable model to study fibrinolysis in RA. Normalization of plasmin activity in CIA after treatment with plasminogen activators had no effect on disease severity.

Chondroitin sulfate and abnormal contact system in rheumatoid arthritis

Rheumatoid arthritis (RA) is a heterogeneous autoimmune disease that affects 1% of the population worldwide. In the K/BxN mouse model of RA, autoantibodies specific for glucose-6-phosphate isomerase (GPI) from these mice can transfer joint-specific inflammation to normal mice. The binding of GPI/autoantibody to the cartilage surface is a prerequisite for autoantibody-induced joint-specific inflammation in the mouse model. Chondroitin sulfate (CS) on cartilage surface is the long sought high-affinity receptor for GPI. The binding affinity and structural differences between mouse paw/ankle CS and knee/elbow CS correlate with the distal to proximal disease severity in these joints. The data presented in this chapter indicate that autoantigen/autoantibodies in blood circulation activate contact system to produce vasodilators to allow immune complex, protein aggregates, and other plasma proteins to get into the joints. Cartilage surface CS binds and retains autoantigen/autoantibodies. The CS/autoantigen/autoantibody complexes could induce C3a and C5a production through contact system activation. C3a and C5a trigger degranulation of mast cells, which further recruit plasma contact system and complement proteins, immune cells, and immune activation factors to facilitate joint-specific tissue destruction. Therefore, either reducing autoantibody production or inhibiting autoantibody-induced contact system activation might be effective in RA prevention.

Autoimmunity against fibrinogen mediates inflammatory arthritis in mice

Rheumatoid arthritis (RA) is an autoimmune synovitis characterized by the presence of anticitrullinated protein Abs, although the exact targets and role of anticitrullinated protein autoimmunity in the pathogenesis of RA remain to be defined. Fibrinogen, which can be citrullinated, has recently emerged as a candidate autoantigen. To determine whether autoimmunity against fibrinogen can mediate inflammatory arthritis, we immunized a variety of common mouse strains with fibrinogen and found that DBA/1 and SJL mice developed an inflammatory and erosive arthritis. Mice with fibrinogen-induced arthritis (FIA) possess fibrinogen-reactive T cells that produce the proinflammatory cytokines IL-6, IL-17, TNF-alpha, and IFN-gamma. FIA can be adoptively transferred with either plasma or fibrinogen-specific T cells from diseased mice. Mice with FIA possess rheumatoid factor, circulating immune complexes, and anticyclic citrullinated peptide Abs, all of which are characteristic of human RA. These observations demonstrate that fibrinogen is arthritogenic in mice and that the pathogenesis of FIA is mediated by both autoantibodies and fibrinogen-reactive T cells.

Diagnostic and prognostic value of antibodies against chimeric fibrin/filaggrin citrullinated synthetic peptides in rheumatoid arthritis

INTRODUCTION

Evidence suggests that citrullinated fibrin(ogen) may be a potential in vivo target of anticitrullinated protein/peptide antibodies (ACPA) in rheumatoid arthritis (RA). We compared the diagnostic yield of three enzyme-linked immunosorbent assay (ELISA) tests by using chimeric fibrin/filaggrin citrullinated synthetic peptides (CFFCP1, CFFCP2, CFFCP3) with a commercial CCP2-based test in RA and analyzed their prognostic values in early RA.

METHODS

Samples from 307 blood donors and patients with RA (322), psoriatic arthritis (133), systemic lupus erythematosus (119), and hepatitis C infection (84) were assayed by using CFFCP- and CCP2-based tests. Autoantibodies also were analyzed at baseline and during a 2-year follow-up in 98 early RA patients to determine their prognostic value.

RESULTS

With cutoffs giving 98% specificity for RA versus blood donors, the sensitivity was 72.1% for CFFCP1, 78.0% for CFFCP2, 71.4% for CFFCP3, and 73.9% for CCP2, with positive predictive values greater than 97% in all cases. CFFCP sensitivity in RA increased to 80.4% without losing specificity when positivity was considered as any positive anti-CFFCP status. Specificity of the three CFFCP tests versus other rheumatic populations was high (> 90%) and similar to those for the CCP2. In early RA, CFFCP1 best identified patients with a poor radiographic outcome. Radiographic progression was faster in the small subgroup of CCP2-negative and CFFCP1-positive patients than in those negative for both autoantibodies. CFFCP antibodies decreased after 1 year, but without any correlation with changes in disease activity.

CONCLUSIONS

CFFCP-based assays are highly sensitive and specific for RA. Early RA patients with anti-CFFCP1 antibodies, including CCP2-negative patients, show greater radiographic progression.

Targeted Synovial Fluid Proteomics for Biomarker Discovery in Rheumatoid Arthritis

Objective

Rheumatoid arthritis (RA) is an autoimmune disease that targets the synovium. The autoantigens involved in the autoantibody responses in RA are unknown. A targeted proteomics approach was used to identify proteins in RA synovial fluid (SF) that are recognized by autoantibodies in RA sera.

Methods

RA SF, depleted of abundant proteins, was fractionated by two-dimensional liquid chromatography (chromatofocusing followed by reverse phase HPLC). Protein arrays constructed from these fractions were probed with RA and normal control sera, and proteins within reactive fractions were identified by mass spectrometry. The reactivity of RA sera to an identified peptide was confirmed by ELISA.

Results

RA sera specifically reacted to a SF fraction containing fibrin. Mass spectrometry analyses established the presence of a citrullinated arginine at position 271 of the fibrin fragment present in RA SF. A synthetic peptide corresponding to fibrin residues 259–287, containing the citrulline substitution at Arg 271, was recognized by 10 of 12 RA sera, but by two of 18 normal control sera and one of 10 systemic lupus erythematosus sera.

Conclusion

Proteomics methodology can be used to directly characterize post-translational modifications in candidate autoantigens isolated from sites of disease activity. The finding that RA sera contain antibodies to the citrullinated fibrin 259–287 peptide may ultimately lead to improved diagnostic tests for RA and/or biomarkers for disease activity.

Endogenous tissue-type plasminogen activator is protective during ascending urinary tract infection

BACKGROUND

Acute pyelonephritis is one of the most common bacterial infections. Tissue-type plasminogen activator (tPA) is a potent fibrinolytic agent, but can play a role in inflammatory processes as well.

METHODS

We induced pyelonephritis in tPA(-/-) and C57BL/6 wild-type (WT) mice by intravesical inoculation with 10(10) CFU uropathogenic Escherichia coli 1677. The mice were killed after 24 and 48 h, after which bacterial outgrowth and cytokine levels in kidney homogenates were determined. Influx of neutrophils was quantified by myeloperoxidase-ELISA. Neutrophil phagocytosis and oxidative burst were measured.

RESULTS

The tPA(-/-) kidneys contained significantly higher numbers of E. coli CFU, accompanied by higher levels of interleukin-1beta (IL-1beta) and tumour necrosis factor-alpha (TNF-alpha). The number of infiltrating neutrophils was similar in tPA(-/-) and WT mice at both time points, suggesting that tPA(-/-) neutrophils have a lower ability to eliminate E. coli. Phagocytosis of E. coli organisms was not diminished in tPA(-/-) neutrophils. Interestingly, tPA(-/-) neutrophils showed a significantly lower ability to generate an oxidative burst reaction upon stimulation with E. coli than WT neutrophils. Incubation with recombinant tPA reversed this effect completely.

CONCLUSIONS

These results show that deletion of the tPA-gene in mice leads to lower bactericidal potential of tPA(-/-) neutrophils, which results in significantly more bacterial outgrowth during experimental pyelonephritis.

Circulating immune complexes contain citrullinated fibrinogen in rheumatoid arthritis

Introduction

There is increasing evidence that autoantibodies and immune complexes (ICs) contribute to synovitis in rheumatoid arthritis (RA), yet the autoantigens incorporated in ICs in RA remain incompletely characterised.

Methods

We used the C1q protein to capture ICs from plasma derived from human RA and control patients. Antibodies specific for immunoglobulin were used to detect ICs, and fibrinogen antibodies were used to detect fibrinogen-containing ICs. RA and control plasma were separated by liquid chromatography, and fractions then characterised by ELISA, immunoblotting and mass spectrometry. Immunohistochemical staining was performed on rheumatoid synovial tissue.

Results

C1q-immunoassays demonstrated increased levels of IgG (p = 0.01) and IgM (p = 0.0002) ICs in plasma derived from RA patients possessing anti-cyclic citrullinated peptide (CCP+) autoantibodies as compared with healthy controls. About one-half of the anti-CCP+ RA possessed circulating ICs containing fibrinogen (p = 0.0004). Fractionation of whole RA plasma revealed citrullinated fibrinogen in the high molecular weight fractions that contained ICs. Positive correlations were observed between fibrinogen-containing ICs and anti-citrullinated fibrinogen autoantibodies, anti-CCP antibody, rheumatoid factor and certain clinical characteristics. Immunohistochemical staining demonstrated co-localisation of fibrinogen, immunoglobulin and complement component C3 in RA pannus tissue. Mass spectrometry analysis of immune complexes immunoprecipitated from RA pannus tissue lysates demonstrated the presence of citrullinated fibrinogen.

Conclusion

Circulating ICs containing citrullinated fibrinogen are present in one-half of anti-CCP+ RA patients, and these ICs co-localise with C3 in the rheumatoid synovium suggesting that they contribute to synovitis in a subset of RA patients.

The Involvement of CD44 and Its Novel Ligand Galectin-8 in Apoptotic Regulation of Autoimmune Inflammation

The synovial fluid (SF) cells of rheumatoid arthritis (RA) patients express a specific CD44 variant designated CD44vRA. Using a cellular model of this autoimmune disease, we show in this study that the mammalian lectin, galectin-8 (gal-8), is a novel high-affinity ligand of CD44vRA. By affinity chromatography, flow cytometry, and surface plasmon resonance, we demonstrate that gal-8 interacts with a high affinity (K(d), 6 x 10(-9) M) with CD44vRA. We further demonstrate that SF cells from RA patients express and secrete gal-8, to a concentration of 25-65 nM, well within the concentration of gal-8 required to induce apoptosis of SF cells. We further show that not all gal-8 remains freely soluble in the SF and at least part forms triple complexes with CD44 and fibrinogen that can be detected, after fibrinogen immunoprecipitation, with Abs against fibrinogen, gal-8 and CD44. These triple complexes may therefore increase the inflammatory reaction by sequestering the soluble gal-8, thereby reducing its ability to induce apoptosis in the inflammatory cells. Our findings not only shed light on the receptor-ligand relationships between CD44 and gal-8, but also underline the biological significance of these interactions, which may affect the extent of the autoimmune inflammatory response in the SF of RA patients.

Regulatory polymorphisms in extracellular matrix protease genes and susceptibility to rheumatoid arthritis: a case-control study

Many extracellular matrix (ECM) proteases seem to be important in rheumatoid arthritis (RA) and regulation of their transcription levels is a critical mechanism for controlling their activity. We have investigated, therefore, whether the best-characterized single nucleotide polymorphisms (SNPs) affecting transcription of the ECM proteases that have been related with joint pathology are associated with RA susceptibility. Nine SNPs in eight genes were selected by bibliographic search, including SNPs in the genes encoding matrix metalloproteinase (MMP)1, MMP2, MMP3, MMP7, MMP9, MMP13, plasminogen activator, tissue type (PLAT) and PAI-1. They were studied in a case-control setting that included 550 RA patients and 652 controls of Spanish ancestry from a single center. Genotyping was performed by single-base extension. Only two of the nine SNPs showed significant association with RA susceptibility. RA patients showed increased frequencies of the -7351 T allele of the gene encoding PLAT (36.4% versus 32.1% in controls, p = 0.026) and the -1306 T allele of the gene encoding MMP2 (24.5% versus 20.3% in controls, p = 0.013). These two alleles seemed to cooperate according to an additive model with respect to increased RA susceptibility (p = 0.004), and they were the low-expression alleles of the respective SNPs in a PLAT enhancer and the MMP2 promoter. These findings are in agreement with previous data suggesting that these two ECM proteases have a protective role in RA pathology. Confirmation of these associations will be needed to support these hypotheses. The remaining SNPs did not show association, either individually or collectively. Therefore, although regulatory SNPs in ECM proteases did not show any major effect on RA susceptibility, it was possible to find modest associations that, if replicated, will have interesting implications in the understanding of RA pathology.

Comparison of efficacy of arthroscopic lavage plus administration of corticosteroids, arthroscopic lavage plus administration of placebo, and joint aspiration plus administration of corticosteroids in arthritis of the knee: A randomized controlled trial

OBJECTIVE

To compare the efficacy of arthroscopic lavage plus administration of corticosteroids (ALC), arthroscopic lavage plus administration of placebo (ALP), and joint aspiration plus administration of corticosteroids (JAC) in knee arthritis, and to evaluate whether clinical or histologic characteristics determine outcome.

METHODS

Patients with knee arthritis (not due to gout, osteoarthritis, or septic arthritis) were randomized over 3 treatment arms: ALC, ALP, and JAC. The primary end point was event-free survival, with events defined as 1) recurrence or persistence of symptomatic knee swelling necessitating local re-treatment, or 2) nonimprovement of the knee joint score. Synovial tissue specimens were collected and analyzed histologically to identify predictive factors of responsiveness.

RESULTS

A total of 78 patients were enrolled; 3 patients did not receive their allocated therapy and 3 were lost to followup. The median time until recurrence was 9.6 months after ALC, 3.0 months after JAC, and 1.0 month after ALP, corresponding to a relative risk (RR) of arthritis recurrence of 2.2 for JAC (95% confidence interval [95% CI] 1.2-4.2, P = 0.02) and 4.7 for ALP (95% CI 2.3-9.4, P < 0.0001) compared with ALC. A high versus low synovial extent of fibrosis conferred an RR for recurrence of 5.7 (95% CI 1.6-20.5, P < 0.01) after ALC.

CONCLUSION

Arthroscopic lavage plus administration of corticosteroids was more effective than arthroscopic lavage plus administration of placebo or joint aspiration plus injection of corticosteroids. The absence of fibrosis was a histologic predictor of a beneficial response.

Expression of the peptide C4b-binding protein beta in the arthritic joint

BACKGROUND

C4b-binding protein (C4BP) is a plasma oligomeric glycoprotein that participates in the regulation of complement and haemostasis. Complement-regulatory activity depends on the C4BPalpha-polypeptide, whereas the C4BPbeta-polypeptide inactivates protein S, interfering with the anti-coagulatory protein C-dependent pathway.

OBJECTIVE

To investigate the expression of C4BPbeta in the rheumatoid joint.

METHODS

Expression of C4BP was studied in synovial explants from patients with rheumatoid arthritis, osteoarthritis and healthy controls, using immunohistochemistry and in situ hybridisation. C4BP isoforms and free C4BPbeta were studied in synovial effusions from patients with rheumatoid arthritis, osteoarthritis and microcrystalline arthritis (MCA) by immunoblotting; total and free protein S levels were studied by enzyme immunoassay.

RESULTS

C4BPbeta was overexpressed in the synovial membranes of patients with rheumatoid arthritis, in close association with the severity of synovitis and the extension of interstitial fibrin deposits. As many as 85% fluids from patients with rheumatoid arthritis contained free C4BPbeta, whereas this unusual polypeptide was present in 50% fluids from patients with MCA and 40% fluids from patients with osteoarthritis. Free protein S at the effusions was pathologically reduced in patients with rheumatoid arthritis and MCA, and remained normal in patients with osteoarthritis.

CONCLUSION

C4BPbeta is expressed by the inflamed synovial tissue, where it can participate in processes of tissue remodelling associated with invasive growth.

Citrullinated fibrinogen detected as a soluble citrullinated autoantigen in rheumatoid arthritis synovial fluids

BACKGROUND

Anti-citrullinated protein antibodies (ACPA) are specifically and frequently detected in sera of patients with rheumatoid arthritis (RA). Citrullinated fibrin or fibrinogen is a candidate autoantigen of such antibodies.

OBJECTIVE

To investigate the presence of citrullinated fibrinogen (cFBG) in the plasma or synovial fluid of patients with RA and control patients, and to determine cFBG levels and their relationship with serum markers for RA if it is present.

METHODS

A sandwich enzyme linked immunosorbent assay (ELISA) to measure cFBG was established using monoclonal antibodies cF16.1 and cF252.1, generated by immunising mice with R16Cit and R252Cit, the fibrinogen Aalpha chain derived sequences with citrulline at position 16 and 252, respectively, and the presence of cFBG was further investigated with immunoprecipitation-western blotting.

RESULTS

Positive signals were detected in 11/15 RA synovial fluids (RASFs), but not in osteoarthritis synovial fluids or RA plasma with sandwich ELISA for cFBG using cF16.1 and an anti-modified citrulline (AMC) antibody. The presence of cFBG in RASFs was confirmed by immunoprecipitation-western blotting. Furthermore, most RA sera strongly reacted against R16Cit. No relationship was seen between RASF cFBG levels and C reactive protein or anti-cyclic citrullinated peptide antibody levels of the paired sera.

CONCLUSION

cFBG is detected as a soluble citrullinated autoantigen in RASFs and may therefore be a genuine candidate antigen for ACPA in patients with RA.

Hypothesis: the humoral immune response to oral bacteria provides a stimulus for the development of rheumatoid arthritis

Rheumatoid arthritis (RA) and adult periodontitis share common pathogenetic mechanisms and immunologic and pathological findings. One oral pathogen strongly implicated in the pathogenesis of periodontal disease, Porphyromonas gingivalis, possesses a unique microbial enzyme, peptidylarginine deiminase (PAD), the human equivalent of which has been identified as a susceptibility factor for RA. We suggest that individuals predisposed to periodontal infection are exposed to antigens generated by PAD, with de-iminated fibrin as a likely candidate, which become systemic immunogens and lead to intraarticular inflammation. PAD engendered antigens lead to production of rheumatoid factor-containing immune complexes and provoke local inflammation, both in gingiva and synovium via Fc and C5a receptors.

Tissue-type plasminogen activator modulates inflammatory responses and renal function in ischemia reperfusion injury

Acute renal failure is often the result of ischemia-reperfusion (I/R) injury. Neutrophil influx is an important damaging event in I/R. Tissue-type plasminogen activator (tPA) not only is a major fibrinolytic agent but also is involved in inflammatory processes. A distinct upregulation of tPA after I/R, with de novo tPA production by proximal renal tubules, was found. For investigating the role of tPA in I/R, renal ischemia was induced in tPA-/- and wild-type (WT) mice by clamping both renal arteries for 35 min followed by reperfusion. Mice were killed 1, 5, and 10 d after reperfusion. After 1 d, tPA-/- mice displayed significantly less neutrophil influx into the interstitial area compared with WT mice. In addition, tPA-/- mice showed quicker recovery of renal function than WT mice. The protocol was repeated after injection of tPA-antisense oligonucleotides into WT mice, leading to even more explicit results: Antisense-treated mice showed less histologic damage, better renal function, and less neutrophil influx than control mice. Surprising, complement C3 concentration, levels of proinflammatory cytokines and chemokines, intercellular adhesion molecule-1 expression, and matrix metalloproteinase activity were similar in WT and tPA-/- mice. Plasmin activity levels in WT and tPA-/- kidneys were also comparable, indicating that tPA influences neutrophil influx into ischemic renal tissue independent from plasmin generation. This study shows that targeting tPA could be of therapeutic importance in treating I/R injury by diminishing neutrophil influx and preserving renal function.

Citrullination of fibronectin in rheumatoid arthritis synovial tissue

OBJECTIVES

Citrullination, catalysed by peptidylarginine deiminase (PAD), is the post-translational modification of peptidylarginine to citrulline, which is intimately involved in the pathogenesis of rheumatoid arthritis (RA). Fibronectin (Fn), a large glycoprotein, is expressed at high levels in arthritic joints and it mediates various physiological processes through interactions with cell-surface integrin receptors and growth factors. We investigated the citrullination of Fn and its potential contribution to the pathogenesis of RA.

METHODS

We localized Fn expression and citrullination in RA synovial tissue by immunohistochemistry, immunoprecipitation and western blotting. We also determined levels of citrullinated Fn in plasma from RA patients using sandwich enzyme-linked immunosorbent assay (ELISA). After incubating Fn with rabbit skeletal muscle PAD, we examined the binding ability of citrullinated Fn to vascular endothelial growth factor (VEGF) and integrin beta1 using a solid-phase receptor binding assay as well as the effect of the citrullinated Fn on apoptosis using cultured HL-60 cells.

RESULTS

Immunohistochemistry and western blotting analysis indicated that Fn formed extracellular aggregates that were specifically citrullinated in RA synovial tissue. No Fn deposits were observed in synovial tissues of osteoarthritis (OA). Sandwich ELISA detected higher levels of citrullinated Fn in plasma from patients with RA than from healthy controls or those with systemic lupus erythematosus. Following citrullination in vitro, the affinity of Fn for VEGF increased, but binding activity to integrin beta1 decreased and Fn no longer stimulated the apoptosis of monocytes induced from cultured HL-60 cells.

CONCLUSIONS

Our results suggest that the citrullination of Fn is a specific event for RA synovium, although others have detected citrullinated total proteins in inflamed synovial tissue of RA and non-RA patients. Citrullination of Fn could alter interactions between Fn and its receptors and growth factors, consequently contributing to mechanisms of RA pathogenesis such as perturbed angiogenesis and apoptosis.