The long-term goal of this research program is to understand the role of key hemostatic factors in inflammatory joint disease. The widespread deposition of fibrin (ogen) and fibrin degradation products within joints is one of the most conspicuous features of rheumatoid arthritis. Given that fibrin (ogen), plasmin(ogen), fibrin degradation products (FDPs), and their cell surface receptors have been shown to be important in both the inflammatory response and tissue remodeling/repair, hemostasis-related proteins are prime candidates to be critical determinants of inflammatory joint disease. The aims of this project center on the following specific hypotheses: 1) fibrin (ogen) and plasmin-mediated proteolysis are important in the inflammatory processes leading to the synovial remodeling, pannus formation, neovascularization, cartilage destruction and bone erosion; 2) the mechanism by which fibrinogen contributes to the inopportune inflammatory and synovial cell activity leading progressive joint degeneration is coupled to: i) the local formation of provisional fibrin matrices, and ii) the local cellular engagement of fibrin(ogen) through specific integrin receptors; and 3) thrombin, the enzyme that proteolytically converts fibrinogen to fibrin, contributes to inflammatory joint disease through both fibrinogen-dependent and fibrinogen-independent mechanisms. These hypotheses will be tested by exploring the impact of genetic deficits and functional disorders in fibrinogen, plasminogen, and plasminogen activators on cytokine-driven and adaptive immunity-driven arthritis induced by either TNF-alpha transgene expression (Aim 1A) or immunization with type II collagen (Aim 1B). The specific importance of fibrinogen engagement of leukocyte and platelet integrin receptors in arthritis will be defined by comparative studies of arthritis in mice expressing mutant forms of fibrinogen lacking either the alpha-M-Beta-2 or alpha-IIb-beta-3 integrin binding motifs (Aim 2). Similarly, the importance of local thrombin-mediated conversion of fibrinogen to insoluble fibrin matrices in the pathogenesis of inflammatory joint disease will be defined in mice where thrombin activity is pharmacologically and/or genetically blocked (Aim 3). The proposed studies will provide a more detailed understanding of the role of hemostatic factors in the pathogenesis of arthritic disease with distinct etiologies, and could ultimately illuminate new therapeutic strategies for treating joint disease.