Fibrin(ogen) exacerbates inflammatory joint disease through a mechanism linked to the integrin αMβ2 binding motif

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Abstract
Fibrin deposition within joints is a prominent feature of arthritis, but the precise contribution of fibrin(ogen) to inflammatory events that cause debilitating joint damage remains unknown. To determine the importance of fibrin(ogen) in arthritis, gene-targeted mice either deficient in fibrinogen (Fib–) or expressing mutant forms of fibrinogen, lacking the leukocyte receptor integrin αMβ2 binding motif (Fibγ390–396A) or the αIIbβ3 platelet integrin-binding motif (FibγΔ5), were challenged with collagen-induced arthritis (CIA). Fib– mice exhibited fewer affected joints and reduced disease severity relative to controls. Similarly, diminished arthritis was observed in Fibγ390–396A mice, which retain full clotting function. In contrast, arthritis in FibγΔ5 mice was indistinguishable from that of controls. Fibrin(ogen) was not essential for leukocyte trafficking to joints, but appeared to be involved in leukocyte activation events. Fib– and Fibγ390–396A mice with CIA displayed reduced local expression of TNF-α, IL-1β, and IL-6, which suggests that αMβ2-mediated leukocyte engagement of fibrin is mechanistically upstream of the production of proinflammatory mediators. Supporting this hypothesis, arthritic disease driven by exuberant TNF-α expression was not impeded by fibrinogen deficiency. Thus, fibrin(ogen) is an important, but context-dependent, determinant of arthritis, and one mechanism linking fibrin(ogen) to joint disease is coupled to αMβ2-mediated inflammatory processes.