Plasminogen Mediates Both Positive and Negative Effects on Disease Severity in a Mouse Model of TNFα-Driven Arthritis

Abstract 854

Rheumatoid arthritis is a chronic inflammatory disease of joint tissue wherein dysregulated, robust hemostatic system activity is a consistent pathological feature. Increased local expression of fibrinolytic system components (i.e., plasminogen activators) and accumulation of fibrin degradation products within arthritic joints suggest that plasmin(ogen) may directly or indirectly participate in joint tissue inflammatory/degradative processes. To test the hypothesis that plasmin-mediated proteolysis drives local events in arthritis pathogenesis, we examined the effect(s) of plasminogen deficiency (Plg⁻) on TNFα-driven arthritis in Tg197 transgenic mice that spontaneously develop a chronic, erosive form of polyarthritis. Comparative macroscopic analysis of the distal joints (fore- and hind-paws) from 10-week old mice revealed that plasminogen deficiency resulted in significantly elevated arthritic disease compared to plasminogen-sufficient control animals. Consistent with overt macroscopic disease, evaluation of distal joint sections using a semi-quantitative histopathological scoring system confirmed that Plg⁻ Tg197 mice developed significantly more advanced arthritic disease relative to controls. Typical disease features included extensive synovial hyperplasia, inflammatory cell infiltration, pannus formation, cartilage degradation and bone loss. Remarkably, histological examination of the proximal joints (knees) from the same set of animals revealed that Plg⁻ Tg197 mice developed markedly diminished arthritic disease relative to controls, suggesting that the impact of plasminogen on the progression of arthritis is dependent on anatomical location. Given that fibrin is a primary substrate for plasmin-mediated proteolysis, we examined joint tissue for evidence of fibrin deposition by immunohistochemistry. In distal joints of the paws, Plg⁻ Tg197 mice displayed robust fibrin deposition throughout the hyperplastic synovial tissue and along the articular surfaces exhibiting evidence of cartilage degradation. The degree of fibrin staining in the distal paw joints appeared to correlate with the disease severity (i.e., more extensive fibrin staining in Plg⁻ Tg197 mice with advanced arthritic disease). Intriguingly, fibrin deposition was also observed in the proximal knee joints of Plg⁻ Tg197 transgenic mice, despite the limited arthritis severity. To determine whether fibrin was the plasmin substrate mediating the distinct differences in TNFα-driven arthritis severity at one or both of the anatomical locations examined (i.e., paw joints or knee joints) in Plg⁻ Tg197 mice, fibrinogen deficiency was superimposed on the Plg⁻ background generating mice with combined plasminogen and fibrinogen deficiencies (e.g., Plg⁻/Fib⁻ mice). Remarkably, comparative macroscopic as well as microscopic analyses revealed that the arthritis phenotypes were reversed in both the paw and the knee joints of Plg⁻/Fib⁻ Tg197 mice relative to Plg⁻/Fib⁺ Tg197 mice. Together, these data strongly suggest that fibrin is a dominant plasmin target that contributes to arthritis pathogenesis. A thorough understanding of the precise mechanisms underlying the plasminogen-dependent, location-specific differences in arthritis progression will likely provide valuable insight into novel therapeutic strategies to effectively treat inflammatory arthritis.