The association of the lipid profile with knee and hand osteoarthritis severity: the IMI-APPROACH cohort.

OBJECTIVE: To investigate the association of the lipidomic profile with osteoarthritis (OA) severity, considering the outcomes radiographic knee and hand OA, pain and function. DESIGN: We used baseline data from the Applied Public-Private Research enabling OsteoArthritis Clinical Headway (APPROACH) cohort, comprising persons with knee OA fulfilling the clinical American College of Rheumatology classification criteria. Radiographic knee and hand OA severity was quantified with Kellgren-Lawrence sum scores. Knee and hand pain and function were assessed with validated questionnaires. We quantified fasted plasma higher order lipids and oxylipins with liquid chromatography with tandem mass spectrometry (LC-MS/MS)-based platforms. Using penalised linear regression, we assessed the variance in OA severity explained by lipidomics, with adjustment for clinical covariates (age, sex, body mass index (BMI) and lipid lowering medication), measurement batch and clinical centre. RESULTS: In 216 participants (mean age 66 years, mean BMI 27.3 kg/m2, 75% women) we quantified 603 higher order lipids (triacylglycerols, diacylglycerols, cholesteryl esters, ceramides, free fatty acids, sphingomyelins, phospholipids) and 28 oxylipins. Lipidomics explained 3% and 2% of the variance in radiographic knee and hand OA severity, respectively. Lipids were not associated with knee pain or function. Lipidomics accounted for 12% and 6% of variance in hand pain and function, respectively. The investigated OA severity outcomes were associated with the lipidomic fraction of bound and free arachidonic acid, bound palmitoleic acid, oleic acid, linoleic acid and docosapentaenoic acid. CONCLUSIONS: Within the APPROACH cohort lipidomics explained a minor portion of the variation in OA severity, which was most evident for the outcome hand pain. Our results suggest that eicosanoids may be involved in OA severity.

The association of plasma fatty acids with hand and knee osteoarthritis: the NEO study.

OBJECTIVE: To investigate the association of postprandial and fasting plasma saturated fatty acid (SFAs), monounsaturated fatty acid (MUFAs) and polyunsaturated fatty acid (PUFAs) concentrations with hand and knee osteoarthritis (OA). DESIGN: In the population-based NEO study clinical hand and knee OA were defined by the ACR classification criteria. Structural knee OA was defined on MRI. Hand and knee pain was determined by Australian/Canadian Hand Osteoarthritis Index (AUSCAN) and KOOS, respectively. Plasma was sampled fasted and 150 min after a standardized meal, and subsequently analysed using a nuclear magnetic resonance platform. Logistic regression analyses were used to investigate the association of total fatty acid, SFA, MUFA, total PUFA, omega-3 PUFA and omega-6 PUFA concentrations with clinical hand and knee OA, structural knee OA and hand and knee pain. Fatty acid concentrations were standardized (mean 0, SD 1). Analyses were stratified by sex and corrected for age, education, ethnicity and total body fat percentage. RESULTS: Of the 5,328 participants (mean age 56 years, 58% women) 7% was classified with hand OA, 10% with knee OA and 4% with concurrent hand and knee OA. In men, postprandial SFAs (OR (95% CI)) 1.23 (1.00; 1.50), total PUFAs 1.26 (1.00; 1.58) and omega-3 PUFAs 1.24 (1.01; 1.52) were associated with hand OA. SFAs and PUFAs were associated with structural, but not clinical knee OA. Association of fasting fatty acid concentrations were weaker than postprandial concentrations. CONCLUSION: Plasma postprandial SFA and PUFA levels were positively associated with clinical hand and structural knee OA in men, but not in women.

Complement component C1q is produced by isolated articular chondrocytes.

OBJECTIVE: Inflammation and innate immune responses may contribute to development and progression of Osteoarthritis (OA). Chondrocytes are the sole cell type of the articular cartilage and produce extracellular-matrix molecules. How inflammatory mediators reach chondrocytes is incompletely understood. Previous studies have shown that chondrocytes express mRNA encoding complement proteins such as C1q, suggesting local protein production, which has not been demonstrated conclusively. The aim of this study is to explore C1q production at the protein level by chondrocytes. DESIGN: We analysed protein expression of C1q in freshly isolated and cultured human articular chondrocytes using Western blot, ELISA and flow cytometry. We examined changes in mRNA expression of collagen, MMP-1 and various complement genes upon stimulation with pro-inflammatory cytokines or C1q. mRNA expression of C1 genes was determined in articular mouse chondrocytes. RESULTS: Primary human articular chondrocytes express genes encoding C1q, C1QA, C1QB, C1QC, and secrete C1q to the extracellular medium. Stimulation of chondrocytes with pro-inflammatory cytokines upregulated C1QA, C1QB, C1QC mRNA expression, although this was not confirmed at the protein level. Extracellular C1q bound to the chondrocyte surface dose dependently. In a pilot study, binding of C1q to chondrocytes resulted in changes in the expression of collagens with a decrease in collagen type 2 and an increase in type 10. Mouse articular chondrocytes also expressed C1QA, C1QB, C1QC, C1R and C1S at the mRNA level. CONCLUSIONS: C1q protein can be expressed and secreted by human articular chondrocytes and is able to bind to chondrocytes influencing the relative collagen expression.

Targeted lipidomics reveals activation of resolution pathways in knee osteoarthritis in humans.

OBJECTIVE: To investigate the presence of inflammation and resolution pathways in osteoarthritis (OA). DESIGN: Tissues were obtained from knee OA patients and control rheumatoid arthritis (RA) patients. Cells in synovial fluid (SF) were visualized by flow cytometry. Cytokines and chemokines were measured by multiplex assay. Lipid mediators (LMs) were determined by targeted lipidomics using liquid-chromatography mass spectrometry. RESULTS: SF of OA patients contained less cells, especially neutrophils, less cytokines and comparable levels of chemokines compared to RA controls. Thirty-seven lipids were detected in the soluble fraction of SF, including polyunsaturated fatty acids (PUFAs) and their pro-inflammatory and pro-resolving lipoxygenase (LOX) and cyclooxygenase (COX) pathway markers in both OA and RA patients. Among these, pro-inflammatory LM such as prostaglandin E2 (PGE2) and thromboxane B2, as well as precursors and pathway markers of resolution such as 17-HDHA and 18-HEPE were detected. Interestingly, the pro-resolving lipid RvD2 could also be detected, but only in the insoluble fraction (cells and undigested matrix). Ratios of metabolites to their precursors indicated a lower activity of 5-LOX and 15-LOX in OA compared to RA, with no apparent differences in COX-derived products. Interestingly, synovial tissue and SF cells could produce 5-LOX and 15-LOX metabolites, indicating these cells as possible source of LM. CONCLUSIONS: By using a state-of-the-art technique, we show for the first time that resolution pathways are present in OA patients. A better understanding of these pathways could guide us to more effective therapeutic approaches to inhibit inflammation and further structural damage in OA and RA.

Evolution of synovitis in osteoarthritic knees and its association with clinical features.

OBJECTIVE: To investigate the course of synovitis on contrast-enhanced magnetic resonance images (CE-MRI) in osteoarthritic knees over 2 years, and its association with pain and cartilage deterioration. DESIGN: Consecutive patients (n = 39, mean age 61 years, 79% woman, median (range) body mass index (BMI) 29 (24-48) kg/mm2) with clinical osteoarthritis (OA) were included. Baseline and follow-up CE-MRI (3 T) were scored paired in chronological order for synovitis (semi-quantitatively at 11 sites (range 0-22)), cartilage deterioration and bone marrow lesions (BMLs) (semi-quantitatively according to Knee Osteoarthritis Scoring System (KOSS)). Changes in sum scores were calculated. Cartilage deterioration was defined as change of ≥2 above the smallest detectable change (SDC). Pain was assessed by standardized questionnaires. Analysis of covariance (ANCOVA) and linear regression models were used to investigate association between synovitis change and cartilage deterioration and between synovitis change or cartilage deterioration and change in pain. RESULTS: The total synovitis score did not change over 2 years (mean change 0.2 (standard deviation (SD) 3.2)), although changes in individual patients were observed. Cartilage deterioration was observed in 51% of patients. Synovitis change score was lower in patients without compared to patients with cartilage deterioration, taking BML change in account (mean difference -2.1 (-4.1 to -0.1)). Change in synovitis was not associated with change in pain, whereas cartilage deterioration was associated with change in Intermittent and Constant OsteoArthritis Pain (ICOAP) constant pain in adjusted models (unstandardised coefficient (B) (95% confidence interval (CI)) 2.8 (0.4-5.3)). CONCLUSIONS: In individual patients synovitis fluctuates during disease course. Synovitis change was not associated with change in pain. Increase in synovitis is associated with cartilage deterioration, suggesting a role for synovitis as a target for disease-modifying treatment.

Radiographic progression of knee osteoarthritis is associated with MRI abnormalities in both the patellofemoral and tibiofemoral joint.

OBJECTIVE: To investigate patterns of MRI abnormalities in the patellofemoral (PFJ) and tibiofemoral joint (TFJ) and their association with radiographic progression, using hypothesis free analyses. DESIGN: 205 patients from the GARP study with symptomatic OA at multiple sites (mean age 60 years, 80% woman, median BMI 26 kg/m(2)), underwent knee MRI at baseline. Cartilage damage, osteophytes, cysts, bone marrow lesions (BMLs) and effusion/synovitis were scored according to a validated scoring method. Baseline and 6-year TFJ and PFJ radiographs were scored (0-3) for JSN and osteophytes according to OARSI and Burnett atlases, respectively; progression was defined as ≥1 point increase. Baseline patterns of MRI abnormalities derived from principal component analysis (PCA) were associated with progression using adjusted generalized estimating equations (GEE). RESULTS: PCA resulted in extraction of six components, explaining 69% of variance. In 29% and 29% of 133 patients with follow-up the TFJ progressed, whereas in 15% and 9% the PFJ progressed for osteophytes and JSN, respectively. Component 1 (cartilage damage of the PFJ and osteophytes of both joints) was statistically significant associated with TFJ JSN progression and PFJ osteophyte progression. Component 2 (all lateral PFJ abnormalities except osteophytes) was associated with JSN/osteophyte progression in the PFJ alone, whereas component 3 (all medial TFJ abnormalities except osteophytes) was associated with JSN and osteophyte progression in both PFJ and TFJ. CONCLUSION: Baseline structural damage and bone turnover activity, as reflected by BMLs, seem to be involved in knee OA progression. Moreover, progression in PFJ and TFJ seems to be related.

Characterization of synovial mast cells in knee osteoarthritis: association with clinical parameters.

OBJECTIVE: To investigate the presence of mast cells in the osteoarthritic (OA) synovium and their association with clinical parameters in comparison with rheumatoid arthritis (RA) samples. METHOD: Synovial tissues of 56 symptomatic OA and 49 RA patients were obtained. Two to three paraffin slides were used to quantify inflammation using haematoxylin and eosin (H&E) staining (synovitis score 0-9), and numbers of mast cells (per 10 high-power fields) using double immunofluorescence for CD117 and tryptase. Average scores per patient were used for analysis. Knee radiographs of OA patients were scored according to the Kellgren and Lawrence (KL) system and pain was determined in OA patients at baseline by visual analogue scale (VAS). RESULTS: Median (range) of mast cells was significantly higher in OA samples 45 (1-168) compared to RA samples 4 (1-47) (P-value < 0.001), despite a lower median (range) synovitis score in OA (2.5 (0-6.0)) compared to 4.6 (0-8.0) in RA samples. The synovitis score was significantly correlated with the number of mast cells (in OA Spearman's rho (P-value) 0.3 (0.023) and RA 0.5 (P-value < 0.001)). Interestingly, we observed a trend towards an association between the number of mast cells and an increased KL-grade (P-value 0.05) in OA patients, independently of synovitis. No associations were found with self-reported pain. CONCLUSION: Prevalence of mast cells in OA synovial tissue is relatively high and associates with structural damage in OA patients, suggesting a role of mast cells in this disease.

Association of pain in knee osteoarthritis with distinct patterns of synovitis.

OBJECTIVE: To determine possible patterns of synovitis on contrast-enhanced magnetic resonance imaging (CE-MRI) and its relation to pain and severity in patients with radiographic knee osteoarthritis (OA). METHODS: In total, 86 patients (mean age 62 years, 66% women, median body mass index 29 kg/m(2) ) with symptomatic knee OA (Kellgren/Lawrence radiographic score 3) were included. T1-weighted, gadolinium-chelate-enhanced MRI with fat suppression was used to semiquantitatively score the extent of synovitis at 11 knee sites (total score range 0-22). Self-reported pain was assessed with 3 standardized questionnaires. Principal components analysis (PCA) was used to investigate patterns (the location and severity) of synovitis. Subsequently, these patterns were assessed for associations with pain measures and radiographic severity in adjusted logistic regression models. RESULTS: Synovitis was observed in 86 patients and was found to be generally mild on CE-MRI (median total synovitis score 7, range 0-16). The median pain scores were 53 (range 0-96) on the visual analog scale for pain, 51.4 (range 2.8-97.2) on the Knee Injury and Osteoarthritis Outcome Score (KOOS) for pain, 35 (range 0-75) on the Intermittent and Constant Osteoarthritis Pain (ICOAP) score for constant pain, and 40.6 (range 0-87.5) on the ICOAP score for intermittent pain. PCA resulted in extraction of 3 components, explaining 53.4% of the variance. Component 1 was characterized by synovitis at 7 sites (mainly medial parapatellar involvement) and was associated with scores on the KOOS pain subscale and the ICOAP constant pain subscale. Component 2 was characterized by synovitis at 4 sites (mainly the site adjacent to the anterior cruciate ligament), but was not associated with pain measures or with radiographic severity. Component 3, characterized by synovitis at 3 sites (mainly at the loose body site), was associated with radiographic severity. CONCLUSION: Different patterns of synovitis in knee OA were observed. The pattern that included several patellar sites was associated with pain, whereas other patterns showed no association, suggesting that pain perception in patients with knee OA is a localized response.

Degree of synovitis on MRI by comprehensive whole knee semi-quantitative scoring method correlates with histologic and macroscopic features of synovial tissue inflammation in knee osteoarthritis.

OBJECTIVE: To evaluate the association between synovitis on contrast enhanced (CE) MRI with microscopic and macroscopic features of synovial tissue inflammation. METHOD: Forty-one patients (mean age 60 years, 61% women) with symptomatic radiographic knee OA were studied: twenty underwent arthroscopy (macroscopic features were scored (0-4), synovial biopsies obtained), twenty-one underwent arthroplasty (synovial tissues were collected). After haematoxylin and eosin staining, the lining cell layer, synovial stroma and inflammatory infiltrate of synovial tissues were scored (0-3). T1-weighted CE-MRI's (3 T) were used to semi-quantitatively score synovitis at 11 sites (0-22) according to Guermazi et al. Spearman's rank correlations were calculated. RESULTS: The mean (SD) MRI synovitis score was 8.0 (3.7) and the total histology grade was 2.5 (1.6). Median (range) scores of macroscopic features were 2 (1-3) for neovascularization, 1 (0-3) for hyperplasia, 2 (0-4) for villi and 2 (0-3) for fibrin deposits. The MRI synovitis score was significantly correlated with total histology grade [r = 0.6], as well as with lining cell layer [r = 0.4], stroma [r = 0.3] and inflammatory infiltrate [r = 0.5] grades. Moreover, MRI synovitis score was also significantly correlated with macroscopic neovascularization [r = 0.6], hyperplasia [r = 0.6] and villi [r = 0.6], but not with fibrin [r = 0.3]. CONCLUSION: Synovitis severity on CE-MRI assessed by a new whole knee scoring system by Guermazi et al. is a valid, non-invasive method to determine synovitis as it is significantly correlated with both macroscopic and microscopic features of synovitis in knee OA patients.

Synovial inflammation, immune cells and their cytokines in osteoarthritis: a review.

OBJECTIVE: Although osteoarthritis (OA) is considered a non-inflammatory condition, it is widely accepted that synovial inflammation is a feature of OA. However, the role of immune cells and their cytokines in OA is largely unknown. This narrative systematic review summarizes the knowledge of inflammatory properties, immune cells and their cytokines in synovial tissues (STs) of OA patients. DESIGN: Broad literature search in different databases was performed which resulted in 100 articles. RESULTS: Of 100 articles 33 solely investigated inflammation in OA ST with or without comparison with normal samples; the remaining primarily focussed on rheumatoid arthritis (RA) ST. Studies investigating different severity stages or cellular source of cytokines were sparse. OA ST displayed mild/moderate grade inflammation when investigated by means of haematoxylin and eosin (H&E) staining. Most frequently found cells types were macrophages, T cells and mast cells (MCs). Overall the number of cells was lower than in RA, although the number of MCs was as high as or sometimes even higher than in RA ST. Cytokines related to T cell or macrophage function were found in OA ST. Their expression was overall higher than in normal ST, but lower than in RA ST. Their cellular source remains largely unknown in OA ST. CONCLUSION: Inflammation is common in OA ST and characterized by immune cell infiltration and cytokine secretion. This inflammation seems quantitatively and qualitatively different from inflammation in RA. Further research is needed to clarify the role of inflammation, immune cells and their cytokines in the pathogenesis of OA.

The infrapatellar fat pad of patients with osteoarthritis has an inflammatory phenotype.

OBJECTIVES: Obesity is a risk factor for the development of osteoarthritis (OA) in hands and knees. Adipose tissue can secrete different adipokines with powerful immunomodulatory effects. The infrapatellar fat pad (IFP) is an intra-articular organ in the vicinity of the synovium and cartilage. It is hypothesised that IFP-derived soluble factors could contribute to pathological processes in the knee joint. A study was therefore undertaken to compare the release of inflammatory mediators in the IFP and subcutaneous adipose tissue (ScAT) and to characterise the adipocytes and immune cell infiltrate in these tissues. METHODS: Paired IFP and ScAT samples were obtained from 27 patients with primary OA. The stromal vascular cell fraction (SVF) was isolated and characterised by fluorescence activated cell sorting. Cytokine and adipokine release in fat- and adipocyte-conditioned media was measured by luminex. RESULTS: IFP secreted higher levels of inflammatory mediators such as interleukin 6 (IL-6), adipsin, adiponectin and visfatin than ScAT. This could be due to differences in the phenotype of adipocytes and/or in the composition and phenotype of the SVF cells. IFP adipocyte-conditioned media showed a trend towards more IL-6 and adipsin than ScAT. Moreover, the SVF fraction of IFP contained more cells/g tissue, a lower percentage of T cells and a higher percentage of mast cells than ScAT. In addition, T cells had a predominantly pro-inflammatory phenotype while macrophages had a mixed pro- and anti-inflammatory phenotype in the IFP. CONCLUSION: There are profound differences in secreted inflammatory factors and immune cell composition between the IFP and ScAT. These data indicate that IFP-derived soluble mediators could contribute to pathophysiological processes in the OA knee joint.

Anti-citrullinated protein antibodies have a low avidity compared with antibodies against recall antigens.

OBJECTIVES: Anti-citrullinated protein antibodies (ACPA) are highly specific for rheumatoid arthritis (RA) and have been implicated in disease pathogenesis. Recent ongoing evidence indicates that the ACPA response broadens before precipitation of full-blown RA, as indicated by a more extensive isotype usage and an increased citrullinated epitope recognition profile. Nonetheless, the evolution of the ACPA response is still poorly understood and might intrinsically differ from the protective responses against pathogens. METHODS: The avidity and the avidity maturation of ACPA in relation to the avidity of antibodies against recall antigens were analysed. RESULTS: The avidity of ACPA was significantly lower than the avidity of antibodies to the recall antigens tetanus toxoid and diptheria toxoid. Moreover, ACPA did not show avidity maturation during longitudinal follow-up and ACPA avidity was also relatively low in patients who displayed extensive isotype switching. CONCLUSIONS: These observations indicate that the natural evolution of ACPA differs from the development of antibodies against recall antigens. These data also indicate that ACPA avidity maturation and isotype switching are disconnected, whereby extensive isotype switching occurs in the setting of restricted avidity maturation. Intrinsic differences between the RA-specific autoantibody system and protective antibody responses against pathogens could be of relevance for designing novel B cell-targeted therapies for RA.

Evidence for a functional role of IgE anticitrullinated protein antibodies in rheumatoid arthritis.

Rheumatoid arthritis (RA) is a systemic autoimmune disease involving inflammation of the joints. Among the autoantibodies described in RA, anticitrullinated protein antibodies (ACPAs) are highly specific and predictive for RA. In addition, ACPAs have been implicated in the pathogenesis of RA. However, a direct functional response of immune cells from ACPA(+) RA patients toward citrullinated proteins has not been demonstrated. In this study, we show that exposure to citrullinated antigens leads to activation of basophils from ACPA(+) RA patients within 20 minutes. This was not observed after exposure of basophils to noncitrullinated control antigens or after stimulation of basophils from ACPA(-) RA patients and healthy controls. Basophil activation was correlated with the binding of citrullinated proteins to basophils. Furthermore, serum from ACPA(+) RA patients in contrast to that from ACPA(-) RA patients could specifically sensitize human FcepsilonRI expressing rat basophil cells (RBL), enabling activation by citrullinated proteins. Mast cell degranulation products such as histamine levels were enhanced in synovial fluid of ACPA(+) RA patients as compared with ACPA(-) RA and osteoarthritis patients. In addition, histamine levels in synovial fluid from ACPA(+) RA patients correlated with IgE levels, suggesting degranulation of mast cells by cross-linking IgE. Immunohistochemistry on synovial biopsies demonstrated an increased number of degranulated CD117(+) mast cells in ACPA(+) RA patients; IgE and FcepsilonRI expression in synovial mast cells from ACPA(+) RA patients was increased. In conclusion, our results show an immunological response of immune cells from ACPA(+) RA patients in a citrulline-specific manner. Moreover, these data indicate a role for IgE-ACPAs and FcepsilonRI-positive cells in the pathogenesis of RA.

Identification of citrullinated vimentin peptides as T cell epitopes in HLA-DR4-positive patients with rheumatoid arthritis.

OBJECTIVE: Antibodies directed against citrullinated proteins (ACPAs) are highly specific for rheumatoid arthritis (RA). The production of ACPAs is most likely dependent on the presence of T cells, since ACPAs undergo isotype switching and are associated with the shared epitope (SE)-containing HLA-DRB1 alleles. Vimentin is a likely candidate protein for T cell recognition, since >90% of patients positive for ACPAs that are reactive with (peptides derived from) citrullinated vimentin carry SE-containing HLA-DRB1 alleles. The aim of this study was to identify citrullinated vimentin peptides that are presented to HLA-DRB1*0401-restricted T cells. METHODS: HLA-DR4-transgenic mice were immunized with all possible citrulline-containing peptides derived from vimentin, and T cell reactivity was analyzed. Peptides recognized in a citrulline-specific manner by T cells were selected and analyzed for their ability to be processed from the entire vimentin protein. A first inventory of the selected epitopes recognized by T cells was performed using peripheral blood mononuclear cells (PBMCs) from ACPA+, HLA-DR4+ patients with RA. RESULTS: A citrulline-specific response was observed for 2 of the peptides analyzed in DR4-transgenic mice. These peptides were found to be naturally processed from the vimentin protein, since citrullinated vimentin was recognized by peptide-specific T cells. T cell reactivity against these peptides was also observed in cultures of PBMCs from RA patients. CONCLUSION: This study identifies, for the first time, 2 naturally processed peptides from vimentin that are recognized by HLA-DRB1*0401-restricted T cells in a citrulline-specific manner. These peptides can be recognized by T cells in ACPA+, HLA-DR4+ patients with RA, as shown in a first inventory.

Anti-cyclic citrullinated peptide antibodies from rheumatoid arthritis patients activate complement via both the classical and alternative pathways.

OBJECTIVE: It has been suggested that anti-citrullinated protein antibodies (ACPAs) play an important role in the pathogenesis of rheumatoid arthritis (RA). To exert their pathologic effects, ACPAs must recruit immune effector mechanisms such as activation of the complement system. Mouse models of RA have shown that, surprisingly, arthritogenic antibodies activate the alternative pathway of complement rather than the expected classical pathway. This study was undertaken to investigate whether human anti-cyclic citrullinated peptide (anti-CCP) antibodies activate the complement system in vitro and, if so, which pathways of complement activation are used. METHODS: We set up novel assays to analyze complement activation by anti-CCP antibodies, using cyclic citrullinated peptide-coated plates, specific buffers, and normal and complement-deficient sera as a source of complement. RESULTS: Anti-CCP antibodies activated complement in a dose-dependent manner via the classical pathway of complement, and, surprisingly, via the alternative pathway of complement. The lectin pathway was not activated by anti-CCP antibodies. Complement activation proceeded in vitro up to the formation of the membrane attack complex, indicating that all activation steps, including the release of C5a, took place. CONCLUSION: Our findings indicate that anti-CCP antibodies activate the complement system in vitro via the classical and alternative pathways but not via the lectin pathway. These findings are relevant for the design of interventions aimed at inhibition of complement-mediated damage in RA.

Fine specificity of the anti-citrullinated protein antibody response is influenced by the shared epitope alleles.

OBJECTIVE: In classic studies on the genetic background of antibody production, the major histocompatibility complex (MHC) has been shown to act as the most prominent immune response gene that controls the magnitude and the specificity of antibody production. The strongest genetic risk factor for rheumatoid arthritis (RA), the human MHC HLA-DRB1 shared epitope (SE) alleles, predisposes for antibodies against citrullinated proteins (ACPAs). ACPA levels are higher in SE-positive patients with RA than in SE-negative patients with RA. The aim of the present study was to determine whether SE influences not only the magnitude but also the specificity of the ACPA response. METHODS: In 2 cohorts of anti-citrullinated peptide 2-positive patients with RA, one from a study of recent-onset arthritis (n = 206) and the other from a treatment strategy study (n = 141), serum antibodies against a citrullinated peptide derived from vimentin (cVim) and antibodies against a citrullinated fibrinogen peptide (cFibr) were determined by enzyme-linked immunosorbent assay. HLA-DRB1 genotyping was performed. RESULTS: In the first cohort, SE alleles were significantly associated with the presence of antibodies against cVim (odds ratio [OR] 4.95, 95% confidence interval [95% CI] 1.87-15.3) and were not significantly associated with the presence of antibodies against cFibr (OR 1.71, 95% CI 0.70-4.14). These results were replicated in the second cohort (OR 5.05, 95% CI 1.92-13.6 and OR 1.19, 95% CI 0.30-3.97, respectively). CONCLUSION: In 2 cohorts of ACPA-positive patients with RA, SE alleles predisposed for the development of antibodies against cVim but not for the development of antibodies against cFibr. These data indicate that SE alleles act as "classic" immune response genes in the ACPA response, because they influence both the magnitude and the specificity of this RA-specific antibody response.

Isotype distribution of anti-cyclic citrullinated peptide antibodies in undifferentiated arthritis and rheumatoid arthritis reflects an ongoing immune response.

OBJECTIVE: The evolution of the rheumatoid arthritis (RA)-specific anti-cyclic citrullinated peptide (anti-CCP) antibody response, as measured by the isotypes of anti-CCP, has not been described. This study was undertaken to determine anti-CCP isotype usage in patients with undifferentiated arthritis (UA), patients with recent-onset RA, and patients with RA of long duration. METHODS: IgA, IgM, and IgG subclasses of anti-CCP were measured by enzyme-linked immunosorbent assay in serum samples that were obtained from IgG anti-CCP antibody-positive patients with UA (n = 110) and IgG anti-CCP antibody-positive patients with RA (n = 152) early after the onset of arthritis. Patients with UA in whom RA developed within 1 year (UA-->RA) were compared with patients with UA in whom RA did not develop within 1 year (UA-->UA). In addition, baseline serum samples obtained from a subset of patients with RA (n = 64) were compared with sera obtained from the same patients a median of 7 years later. RESULTS: IgM anti-CCP was present in early samples from both patients with UA and patients with RA and in followup samples from patients with RA. Several IgG anti-CCP antibody-positive patients who did not have IgM anti-CCP early after disease onset did display IgM anti-CCP later in the course of the arthritis. A diverse pattern of isotype usage was detected in early samples, with a trend toward lower frequencies of all isotypes of anti-CCP in patients with UA compared with patients with RA and in UA-->UA patients compared with UA-->RA patients. Levels of all isotypes except IgG1 had decreased after 7 years. CONCLUSION: These data indicate development of the anti-CCP isotype repertoire into full usage early in the course of arthritis. The sustained presence of IgM anti-CCP indicates ongoing recruitment of new B cells into the anti-CCP response, reflecting a continuous (re)activation of the RA-specific anti-CCP response during the course of anti-CCP-positive arthritis.