A5.16 TNF-α Response of Synovial Fluid Monocyte-Macrophages to ACPA Immune Complexes. (25th February 2013) - Clavel, Cyril
Abstract : Background: Autoantibodies to citrullinated proteins (ACPA) are specifically associated to rheumatoid arthritis (RA) and produced in the inflamed synovium where citrullinated fibrin, their main antigenic target, is abundant. Using a human in vitro model we showed that macrophages generated by differentiation of blood monocytes from healthy individuals or patients with RA secrete TNF-α in response to immune complexes formed by ACPA and citrullinated fibrinogen (ACPA-IC). Moreover while in both healthy individuals and RA patients the TNF-α response of macrophages was much higher than that of their monocyte precursors, the TNF-α production of blood monocytes and monocyte-derived macrophages from RA patients did not differ from that observed with the healthy controls. Objectives: To further assess the impact of ACPA-IC on joint inflammation, we evaluated the TNF-α response they prompt in monocyte-macrophages isolated from the synovial fluid (SF) of patients with RA and with other arthritides. Materials and Methods: SF samples were obtained from 7 patients with RA (4 ACPA-positive, 3 ACPA-negative) and 8 ACPA-negative control patients with various arthritides. Polymorphonuclear cells were eliminated by centrifugation over Ficoll or capture with CD15-conjugated magnetic beads then monocyte-macrophages further purified using CD14-beads (median purity: 95%). The purified cells were stimulated with IC generated by capture of ACPA from IgG fractions prepared from RA sera, on immobilised citrullinated fibrinogen, as described (Clavel, Arthritis Rheum, 2008). Results: With SF monocyte-macrophages from both RA and control patients, secretion of TNF-α sometimes occurred in the absence of any stimulation. However in RA patients, irrespective of their ACPA status, TNF-α secretion increased when the cells were cultured on ACPA-IC (median (range) = 16 (2–110) pg/ml). Such activation was also observed with the SF monocyte-macrophages from control patients (184 (33–638) pg/ml). In the whole series of SF samples, the increase in TNF-α secretion was found to be highly significant (p < 0.001). Conclusions: In contrast with our previous observations on CD14-positive blood monocytes and on derived macrophages, it appears that CD14-positive monocyte-macrophages can be pre-activated in the SF and secrete TNF-α spontaneously, but that they can nevertheless be further activated by ACPA-IC. These properties are not restricted to RA patients and seem to be characteristic for the SF CD14-positive monocyte-macrophages. Since citrullinated fibrin and ACPA have been described in the SF of RA patients, it is highly probable that ACPA-IC play a direct pro-inflammatory role in the SF by inducing or enhancing TNF-α secretion by the SF monocyte-macrophages.