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 α M β 2 binding motif (Fibγ 390-396A ) or the α IIb β 3 platelet integrin-binding motif (Fibγ Δ5 ), 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 Fibγ 390-396A mice, which retain full clotting function. In contrast, arthritis in Fibγ Δ5 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 Fibγ 390-396A mice with CIA displayed reduced local expression of TNF-α, IL-1β, and IL-6, which suggests that α M β 2 -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 α M β 2 -mediated inflammatory processes.

The local activation of coagulation system proteases and secondary deposition of fibrin may constitute important “danger” or “damage” signals that promote leukocyte activation events at sites of infection or injury. Consistent with this general hypothesis, we previously showed that mice expressing a mutant form of fibrinogen lacking the leukocyte integrin receptor αMβ2-binding motif displayed compromised clearance of bacteria in a model of S. aureus-induced peritonitis. Here, we show that mice with a complete genetic deficiency in fibrinogen (Fib−/−) show an even more profound sensitivity to S. aureus peritonitis. Fib−/− mice were found to succumb to bacterial doses that were rarely fatal in wildtype mice following intraperitoneal injection of wildtype S. aureus. This profound difference in survival profile was also seen when mice were challenged with S. aureus lacking either the bacterial fibrinogen-receptor (ClfA) or the bacterial prothrombin activator, coagulase. These results indicate that efficient bacterial clearance in control animals is related to host leukocyte engagement of fibrin(ogen) as opposed to bacterial engagement of this hemostatic factor. Following intraperitoneal inoculation with one billion CFU of S. aureus, control mice successful cleared >99% of the bacteria within 1 hour, whereas the number of bacteria retrieved within peritoneal lavage fluid of Fib−/− mice remained similar to the original input CFU. Microscopy studies and bacterial fate studies revealed that the rapid loss of viable bacteria in control animals was not due to either fibrinogen-dependent dissemination to other tissues nor the simple association of live bacteria with peritoneal surfaces. Neutrophils and macrophages appear to participate in early bacterial clearance in animals of both genotypes, but these cells appear to be overwhelmed and engorged with microbes in Fib-/− mice. Leukocyte composition and trafficking into the peritoneal cavity was not appreciably altered in fibrin(ogen)-deficient mice. Furthermore, B cells, T cells, mast cells and complement factors C3 and C5 were not found to be major factors in the fibrinogen-dependent bacterial clearance. Taken together, the available data indicate that a failure to fully implement leukocyte-mediated antimicrobial mechanisms is the primary impediment to bacterial clearance in Fib−/− mice. The findings are consistent with the hypothesis that leukocyte engagement of provisional fibrin matrices within challenged tissues is a critical parameter in leukocyte “target recognition” in vivo.