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

Abstract

Objective. It has been suggested that anticitrullinated 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.

Complement System and Rheumatoid Arthritis: Relationships with Autoantibodies, Serological, Clinical Features, and Anti-TNF Treatment

“…This provides a more conclusive evidence for the pathogenic significance ofcomplement activation in RA. Trouw et aI, from in vitro observations, suggested that complement activation could be involved in RA pathogenesis in ACPApositive patients (6). This was not confirmed by our in vivo results that provided evidence that RF, rather than ACPA, plays a role in complement modulation.…”

“…In one study, Trouw and colleagues recently showed that the addition ofACPA to normal human serum can lead to complement activation via both the classical and the alternative pathways (6). However, no studies have been performed in vivo to address this in vitro effect ofACPA.…”

Autoantibodies recognizing carbamylated proteins are present in sera of patients with rheumatoid arthritis and predict joint damage

“…This finding has made considerable impact, as it has opened up the way to relevant and novel insights into RA-diagnosis and etiopathology (1). For example, ACPA are now part of the new American College of Rheumatology/ European League Against Rheumatism criteria for RA (18), and have been implicated in RA-pathogenesis, both in animal models (19)(20)(21) and in ex vivo human studies (22)(23)(24)(25). Importantly, the description of ACPA has led to the realization that RA constitutes at least two clinical syndromes that share many clinical features, but differ with respect to genetic background, predisposing environmental factors and clinical progression/remission (3,4,(26)(27)(28).…”

Genetic variants in the region of the C1q genes are associated with rheumatoid arthritis

“…Immune complexes, formed, for example, by anti-citrullinated protein antibodies (ACPA), can trigger the classical pathway [39]. However, this would occur after the induction of such antibodies [40,41], which would put genetic variants of C1q at the effector-phase of RA rather than at the onset of RA.…”