Induction of macrophage secretion of tumor necrosis factor a through Fcγ receptor IIa engagement by rheumatoid arthritis-specific autoantibodies to citrullinated proteins complexed with fibrinogen

Objective. Macrophage-derived tumor necrosis factor a (TNF alpha) is a dominant mediator of synovitis in rheumatoid arthritis (RA). This study was undertaken to assess whether and how immune complexes (ICs) formed by the interaction of disease-specific autoantibodies to citrullinated proteins (ACPAs) with their main synovial target antigen, citrullinated fibrin, contribute to TNF alpha production by macrophages. Methods. An in vitro human model was developed in which monocyte-derived macrophages were stimulated with ACPA-containing ICs that were generated by capturing ACPAs from RA sera on immobilized citrullinated fibrinogen. Cellular activation was evaluated by TNF alpha assay in culture supernatants. Selective blockade of IC interactions with the 3 classes of Fc gamma receptors (Fc gamma R) was used to Assess the contribution of each receptor to macrophage activation. In addition, 2 citrullinated fibrin-derived peptides bearing major ACPA epitopes were tested for their capacity to inhibit formation of macrophage-activating ACPA-containing ICs. Results. ACPA-containing ICs induced a dose-dependent TNF alpha secretion by macrophages from 14 of 20 healthy donors. The macrophage response was systematically higher than that of the paired monocyte. precursors. TNF alpha secretion was not reduced by blockade of Fc gamma RI or Fc gamma RIII, but was strongly repressed when interaction of ICs with Fc gamma RII was prevented. The 2 citrullinated peptides significantly inhibited ACPA reactivity to citrullinated fibrinogen and, when tested together, almost completely abolished formation of macrophage-activating ICs, thereby diminishing the secreted TNF alpha levels. Conclusion. Our model demonstrates the inflammatory potential of ACPA-containing ICs via engagement of Fc gamma RIIa at the surface of macrophages, strongly supporting their pathophysiologic involvement. Continuing dissection of these molecular pathways could open the way to new therapeutic approaches in patients with RA.