AB0092 FIBRIN ADHESION IS A PANNUS-INDEPENDENT MECHANISM OF CARTILAGE DEGENERATION IN RHEUMATOID ARTHRITIS

Background: Current concepts of cartilage destruction in inflammatory arthritis include pannus infiltration by inflamed synovial tissue as well as direct detrimental effects of inflammatory cytokines and proteinases. Fibrin maintains chronic inflammatory processes in arthritis but has never been shown to be directly involved in cartilage damage occurring in rheumatoid arthritis (RA). Objectives: To investigate fibrin-mediated cartilage degradation and the possible underlying mechanisms in arthritis. Methods: Human cartilage samples were obtained from patients with RA undergoing joint replacement and investigated by H.E. and immunohistochemistry for cartilage damage and fibrin deposition. Cartilage explants from RA patients were incubated in vitro with autologous synoviocytes and assessed by immunohistochemistry for cell-adhesion and colocalization with fibrin. Experimental RA was studied in the RA murine model of adjuvant-induced arthritis (AIA), in wildtype (WT) and fibrinogen deficient (Fg-/-) mice. Cartilage damage and chondro-synovial adhesion were analyzed by safranin-O staining and fibrin deposition by immunohistochemistry. Fibrinogen expression (Fgα, Fgβ, Fgɣ) was studied in murine primary chondrocytes by qRT-PCR. Cartilage explants were stained with alizarin-red staining and assessed for colocalization of calcific deposits and fibrin. Calcification of murine primary chondrocytes stimulated with secondary calciprotein particles (CPP) and treated with purified human plasma fibrinogen (100 µg/ml) was assessed by alizarin red staining and gene expression for chondrocytic differentiation (Agg, Coll2, Coll10, Sox9, Runx2), calcification (Alpl, Ank, Anx5, Pc1, Pit1, Pit2), and extracellular matrix remodeling (Adamts4, Adamts5, Mmp3, Mmp13, Comp) by qRT-PCR. Results: Abundant fibrin deposition on cartilage co-localized and positively correlated with cartilage damage in knee joints of patients with RA. In the AIA model, absence of fibrin deposition in Fg-/- mice was accompanied by significantly lower synovial inflammation, chondro-synovial adhesion and cartilage damage than in WT mice. Chondro-synovial adhesion correlated with cartilage damage in the WT and led to apparent mechanical stripping of the superficial cartilage, whilst this phenomenon was not observed in the Fg-/- mice. In vitro, autologous RA synoviocytes adhered to cartilage explants exclusively in the presence of fibrin deposition. Fibrinogen chains were not expressed by primary chondrocytes, indicating passive deposition from synovial fluid or tissue. In human RA cartilage explants, we found colocalization and a significant positive correlation between fibrin and calcific deposits. Fibrinogen caused exacerbated calcification in CPP-treated primary murine chondrocytes and induction of genes involved in chondrocyte calcification (Pc1, Pit1). Cartilage-oligomeric matrix protein (Comp) gene was also highly induced suggesting a pro-catabolic role of fibrinogen. Conclusion: Fibrin deposition is an active trigger of cartilage degeneration in RA via induction of chondro-synovial adhesion (mechanical aspect) and induction of calcification (catabolic aspect). Newer therapeutic approaches may not merely focus on fibrinolysis but protect cartilage from fibrin-induced adhesion or calcification e.g. by fibrin-targeted immunotherapy. Disclosure of Interests: Thomas Hugle Grant/research support from: Abbvie, Novartis, Consultant of: Abbvie, Pfizer, Novartis, Roche, Lilly, BMS, Sonia Nasi: None declared, Driss Ehirchiou: None declared, Alexander So Consultant of: Sobi, Grunenthal, Nathalie Busso: None declared