Significance of complement components C1q and C4 bound to circulating immune complexes in juvenile idiopathic arthritis: support for classical complement pathway activation.

OBJECTIVES: Immune complexes (ICs) from sera of juvenile idiopathic arthritis (JIA) patients show increased complement opsonisation; however, a definitive role for involvement of the classical or alternative pathway is not entirely clear. To delineate the role of these pathways, we measured activated complement products bound to circulating IC (CICs) in the sera of JIA patients. METHODS: Sera from 100 JIA patients and 22 healthy children were collected. C1q, C4, C3, C3d, and membrane attack complex (MAC) bound to CICs were measured by enzyme-linked immunosorbent assay. Data was compared to IgM rheumatoid factor (RF), IgG anti-cyclic citrullinated peptide (CCP) antibodies, C-reactive protein (CRP), and erythrocyte sedimentation rate (ESR) levels. RESULTS: Mean levels of C1q, C4, and MAC bound to CICs were significantly elevated in JIA patients compared to healthy children. C1q correlated significantly with C4 and MAC bound to CICs and C4 and MAC also demonstrated significant correlation. No significant differences were noted in complement components bound to CICs when evaluating IgM RF, anti-CCP antibody, and CRP positivity. A significant correlation was noted between MAC bound to CICs and ESR. C1q and MAC bound to CICs mean levels were significantly higher in patients with an elevated ESR compared to those with a normal ESR level. CONCLUSIONS: JIA patients have elevated levels of complement components bound to CICs, particularly from the classical pathway. Moreover, classical pathway components were associated with ESR, a marker of disease activity. MAC bound to CICs also correlated significantly with ESR, further supporting the notion of complement-mediated tissue injury that is triggered by IC-mediated classical pathway activation.

Evidence of fibrinogen as a target of citrullination in IgM rheumatoid factor-positive polyarticular juvenile idiopathic arthritis.

BACKGROUND: Several studies have noted the significance of measuring anti-cyclic citrullinated peptide (CCP) antibodies in juvenile idiopathic arthritis (JIA) as an important indicator for destructive disease, as is the case in rheumatoid arthritis (RA). While the role of anti-CCP antibodies in RA and JIA has become better understood, the identity of the target proteins of this modification has remained elusive. In this study, we evaluated serum from patients with various subtypes of JIA to investigate the presence of anti-deiminated (citrullinated) fibrinogen and anti-citrullinated α-enolase antibodies, and their association with RF and anti-CCP antibody isotypes. METHODS: Sera were obtained from 96 JIA patients, 19 systemic lupus erythematosus (SLE) patients, and 10 healthy children. All sera were measured for antibodies against citrullinated and native fibrinogen and α-enolase by an enzyme linked immunosorbent assay (ELISA). In addition, all sera were assayed for anti-CCP antibody isotypes and rheumatoid factor (RF) isotypes by ELISA. The relationship between anti-citrullinated fibrinogen and anti-α-enolase antibodies and disease activity and joint damage were also investigated. All results were correlated with clinical and laboratory parameters using Spearman's rho correlation coefficient. Multiple logistic regression analysis was utilized to identify which variables were associated with joint erosions and diagnosis of JIA. RESULTS: Thirty-one JIA patients (32%) demonstrated reactivity to citrullinated fibrinogen and 9 (9%) to citrullinated α-enolase. Reactivity to citrullinated fibrinogen and α-enolase was predominantly found in IgM RF-positive polyarthritis patients. Fourteen JIA patients reacted with native α-enolase and a higher percentage of SLE patients reacted with citrullinated α-enolase when compared to JIA patients. Anti-citrullinated fibrinogen antibodies correlated with the presence of IgG anti-CCP antibodies and IgA and IgM RF. The presence of anti-citrullinated α-enolase antibodies correlated with IgA anti-CCP antibodies. IgG anti-CCP antibodies were significantly associated with joint damage and anti-citrullinated fibrinogen antibodies were strongly associated with JIA when compared to control groups. Anti-citrullinated fibrinogen antibodies demonstrated high sensitivity (81%) for IgM RF-positive polyarticular JIA. IgG anti-CCP antibodies had the highest specificity (95%) for JIA, with anti-citrullinated fibrinogen antibodies, IgA anti-CCP antibodies and IgA RF all following at 84%. CONCLUSIONS: JIA patient sera exhibited strong reactivity to anti-citrullinated fibrinogen antibodies and demonstrated high sensitivity and specificity for JIA, primarily in IgM RF-positive polyarthritis patients. Fibrinogen is one of several protein targets for citrullination in JIA.