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Citrullinated calreticulin potentiates rheumatoid arthritis shared epitope signaling.

ABSTRACT: Citrullinated proteins are immunogenic in rheumatoid arthritis (RA), particularly in patients who carry shared epitope (SE)-coding HLA-DRB1 alleles. The mechanism underlying this association is unknown. We have previously identified the SE as a ligand that interacts with cell surface calreticulin (CRT) and activates immune dysregulation. This study was undertaken to determine the effect of CRT citrullination on SE signaling.CRT-SE binding affinity was measured by surface plasmon resonance. The role of individual CRT arginine residues was determined by site-directed mutagenesis, and nitric oxide levels were measured using a fluorochrome-based assay. CRT citrullination in synovial tissue samples and cell cultures was determined by 2-dimensional gel electrophoresis, immunoblotting, and mass spectrometry techniques.Synovial tissue and fibroblast-like synoviocytes from RA patients were found to express a higher abundance of citrullinated CRT than samples from osteoarthritis patients. Citrullinated CRT showed more robust interaction with the SE ligand, and transduced SE signaling at a 10,000-fold higher potency, compared to noncitrullinated CRT. Site-directed mutation analysis identified Arg(205), which is spatially adjacent to the SE binding site in the CRT P-domain, as a dominant inhibitor of SE-CRT interaction and signaling, while a more remote arginine residue, Arg(261), was found to enhance these SE functions.Our findings indicate that citrullinated CRT is overabundant in the RA synovium and potentiates SE-activated signaling in vitro. These findings could introduce a new mechanistic model of gene-environment interaction in RA.

SUBMITTER: Ling S

PROVIDER: S-EPMC3582785 | biostudies-literature | 2013 Mar

REPOSITORIES: biostudies-literature

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Citrullinated calreticulin potentiates rheumatoid arthritis shared epitope signaling.

Ling Song S Cline Erika N EN Haug Timothy S TS Fox David A DA Holoshitz Joseph J

Arthritis and rheumatism 20130301 3

<h4>Objective</h4>Citrullinated proteins are immunogenic in rheumatoid arthritis (RA), particularly in patients who carry shared epitope (SE)-coding HLA-DRB1 alleles. The mechanism underlying this association is unknown. We have previously identified the SE as a ligand that interacts with cell surface calreticulin (CRT) and activates immune dysregulation. This study was undertaken to determine the effect of CRT citrullination on SE signaling.<h4>Methods</h4>CRT-SE binding affinity was measured b ...[more]

PMID: 23233327

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Project description:INTRODUCTION: Fibronectin is one of the most abundant proteins present in the inflamed joint. Here, we characterized the citrullination of fibronectin in the joints of rheumatoid arthritis (RA) patients and studied the prevalence, epitope specificity and human leukocyte antigen (HLA) association of autoantibodies against citrullinated fibronectin in RA. METHODS: Citrullinated residues in fibronectin isolated from RA patient synovial fluid were identified by mass spectrometry. The corresponding citrullinated and non-citrullinated peptides were synthesized and used to analyze the presence of autoantibodies to these peptides in RA sera and sera from other diseases and healthy controls by ELISA. The data were compared with risk factors like shared epitope HLA alleles and smoking, and with clinical features. RESULTS: Five citrullinated residues were identified in fibronectin from RA synovial fluid. RA sera reacted in a citrulline-dependent manner with two out of four citrullinated fibronectin peptides, one of which contains two adjacent citrulline residues, in contrast to non-RA sera, which were not reactive. The most frequently recognized peptide (FN-Cit1035,1036, LTVGLTXXGQPRQY, in which × represents citrulline) was primarily targeted by anti-CCP (cyclic citrullinated peptide) 2-positive RA patients. Anti-FN-Cit1035,1036 autoantibodies were detected in 50% of established anti-CCP2-positive RA patients and in 45% of such patients from a early arthritis clinic. These antibodies appeared to be predominantly of the immunoglobulin G (IgG) isotype and to be associated with HLA shared epitope alleles (odds ratio = 2.11). CONCLUSIONS: Fibronectin in the inflamed synovia of RA patients can be citrullinated at least at five positions. Together with the flanking amino acids, three of these citrullinated residues comprise two epitopes recognized by RA autoantibodies. Anti-citrullinated fibronectin peptide antibodies are associated with HLA shared epitope alleles.

Project description:IntroductionShared epitope (SE) is present in high proportions of anti-citrullinated protein antibody (ACPA) + patients with rheumatoid arthritis (RA) and is associated with poor prognosis. We assessed the role of SE in RA prognosis, in relation to ACPA positivity.MethodsPatients enrolled in the Brigham and Women's RA Sequential Study were included. Changes from baseline in disease activity (Disease Activity Score in 28 joints using C-reactive protein [DAS28 (CRP)], Clinical Disease Activity Index [CDAI], Simplified Disease Activity Index [SDAI]) to 1 year were assessed. Baseline characteristics were compared by SE and ACPA status (±; chi-squared, Kruskal-Wallis). Association between number of SE alleles and ACPA status (logistic regression models), relationships between baseline characteristics and changes in disease activity (adjusted linear regression model), and effect of ACPA on the association between SE and changes in disease activity (mediation analysis) were studied.ResultsNine hundred twenty-six patients were included. SE + versus SE - patients had significantly longer disease duration and higher disease activity scores and were more likely to have erosive disease, have higher comorbidity burden, and be RF + (all p < 0.05). Among patients with one or two SE alleles (vs. 0), odds of being ACPA + were 1.97 (p = 0.0003) and 3.82 (p < 0.0001), respectively. SE + versus SE - patients had worse disease activity scores as indicated by mean increases in DAS28 (CRP) of 0.22, CDAI of 2.07, and SDAI of 2.43 over 1 year (all p < 0.05). Direct effect of SE + accounted for 76.4-80.1% of total effect in disease activity increases.ConclusionsSE is strongly associated with ACPA positivity and higher disease activity in patients with RA. SE was associated with greater increases in disease activity over 1 year, which was partially mediated by the presence of ACPA.Trial registrationClinicalTrials.gov identifier: NCT01793103; registration date: February 15, 2013, retrospectively registered.

Project description:ObjectiveAnti-citrullinated protein antibodies (ACPAs) and rheumatoid factor (RF) are commonly present in rheumatoid arthritis (RA) without a clear rationale for their coexistence. Moreover, autoantibodies develop against proteins with different posttranslational modifications and native proteins without obvious unifying characteristics of the antigens. We undertook this study to broadly evaluate autoantibody binding in seronegative and seropositive RA to identify novel features of reactivity.MethodsAn array was created using a total of 172,828 native peptides, citrulline-containing peptides, and homocitrulline-containing peptides derived primarily from proteins citrullinated in the rheumatoid joint. IgG and IgM binding to peptides were compared between cyclic citrullinated peptide (CCP)-positive RF+, CCP+RF-, CCP-RF+, and CCP-RF- serum from RA patients (n = 48) and controls (n = 12). IgG-bound and endogenously citrullinated peptides were analyzed for amino acid patterns and predictors of intrinsic disorder, i.e., unstable 3-dimensional structure. Binding to IgG-derived peptides was specifically evaluated. Enzyme-linked immunosorbent assay confirmed key results.ResultsBroadly, CCP+RF+ patients had high citrulline-specific IgG binding to array peptides and CCP+RF- and CCP-RF+ patients had modest citrulline-specific IgG binding (median Z scores 3.02, 1.42, and 0.75, respectively; P < 0.0001). All RA groups had low homocitrulline-specific binding. CCP+RF+ patients had moderate IgG binding to native peptides (median Z score 2.38; P < 0.0001). The highest IgG binding was to citrulline-containing peptides, irrespective of protein identity, especially if citrulline was adjacent to glycine or serine, motifs also seen in endogenous citrullination in the rheumatoid joint. Highly bound peptides had multiple features predictive of disorder. IgG from CCP+RF+ patients targeted citrulline-containing IgG-derived peptides.ConclusionDisordered antigens, which are frequently citrullinated, and common epitopes for ACPAs and RF are potentially unifying features for RA autoantibodies.

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Citrullinated calreticulin potentiates rheumatoid arthritis shared epitope signaling.

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Citrullinated calreticulin potentiates rheumatoid arthritis shared epitope signaling.

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