Fibrin(ogen) Exacerbates Inflammatory Joint Disease Via a Mechanism Linked to Its αMβ2 Binading Motif.

Activation of the hemostatic system is a common pathological feature of severe inflammatory processes. In the context of inflammatory joint disease, articular fibrin deposition is a prominent feature that has been hypothesized to contribute to the derangements in immune/inflammatory processes and tissue reorganization that lead to debilitating and irreversible damage within arthritic joints. We directly examined the role of fibrin(ogen) in the progression of inflammatory joint disease using the well-established experimental setting of collagen-induced arthritis (CIA). Mice with genetically-imposed fibrinogen deficiency (Fib−/−) were found to exhibit significantly reduced macroscopic arthritic disease incidence, progression, and severity in the paws relative to littermate controls throughout the evaluation period. Consistent with the macroscopic observations, qualitative and quantitative histological analysis of distal- and knee-joint sections revealed a significant decrease in all joint disease parameters (e.g., inflammatory cell infiltrates, synovial hyperplasia, edema, and cartilage/bone loss) in Fib−/− mice as compared to fibrinogen-sufficient controls. Fibrin(ogen) has been shown to promote pro-inflammatory activity through engagement of the leukocyte integrin receptor αMβ2 (Mac-1). To determine if fibrinogen/αMβ2 interactions promote the pathogenesis of arthritis, we compared CIA progression in mice expressing fibrinogen that lacks the leukocyte integrin receptor αMβ2 binding motif (i.e., Fibγ390–396A mice). We observed decreased gross and histological inflammatory joint disease in Fibγ390–396A mice relative to control animals. This diminution in arthritic disease severity was comparable to that observed in Fib−/− mice despite the maintenance of normal clotting function in Fibγ390–396A animals. In comparison, CIA studies in mice expressing a mutant form of fibrinogen lacking the platelet integrin receptor αIIbβ3 binding motif showed that this feature of fibrinogen was not an essential driver of arthritis. The reduction in macroscopic and histological arthritic disease observed in CIA-challenged Fibγ390–396A mice was accompanied by reduced local mRNA levels of the pro-inflammatory cytokines TNF α, IL-6, and IL-1β in paws relative to paws from control animals. This data suggests that fibrin(ogen) may function upstream of local production of these critical inflammatory mediators. Consistent with this hypothesis, fibrinogen deficiency did not diminish arthritic disease driven by TNFα-overexpression. Together, these data support the view that local fibrin deposition is an important determinant of inflammatory joint disease and that one mechanism by which fibrin promotes arthritic disease is through leukocyte activation events downstream of αMβ2 engagement.