PLASMIN: A POTENT PRO-INFLAMMATORY FACTOR By

Plasmin: A Potent Pro-inflammatory Factor Yongzhi Guo, Department of Medical Biochemistry and Biophysics, Umeå University, SE-901 87, Umeå, Sweden Plasmin, the central molecule of the plasminogen activator system, is a broad-spectrum serine protease. Plasmin is important for the degradation of fibrin and other components of the extracellular matrix (ECM) during a number of physiological and pathological processes. The aim of this thesis was to elucidate the functional roles of plasmin during pathological inflammation and infection in autoimmune and non-autoimmune diseases. For this purpose, mouse models of rheumatoid arthritis (RA), bacterial arthritis, infection, and sepsis have been used. Previous studies from our laboratory have shown that plasminogen-deficient mice are resistant to the development of collagen type II-induced arthritis (CIA). In contrast, others have shown that plasmin plays a protective role in antigen-induced arthritis (AIA). To investigate the contrasting roles of plasminogen deficiency in models of CIA and AIA, a new animal model of arthritis called local injection-induced arthritis (LIA) was developed. In this model, we replaced methylated bovine serum albumin, which is normally used as an immunogen in the AIA model, with collagen type II (CII) to induce arthritis. When wild-type and plasminogen-deficient mice were injected intra-articularly with CII or 0.9% NaCl following CIA induction, plasminogen-deficient mice developed typical CIA, but the disease was less severe than in wild-type mice and was restricted to the injected joints. When the AIA model was used, plasminogen-deficient mice developed a much more severe arthritis than the wildtype mice. These results indicate that both the antigen and joint trauma caused by the local injection are critical to explaining the contrasting roles of plasminogen deficiency in CIA and AIA. This indicates that CIA and AIA have distinct pathogenic mechanisms and plasmin plays contrasting roles in different types of arthritis models. To study the functional roles of plasmin in the host inflammatory response during infectious arthritis, a Staphylococcus aureus-induced bacterial arthritis model was established. When wild-type mice were injected intra-articularly with 1 × 10 colony-forming units (CFU) of S. aureus per joint, all the bacteria were completely eliminated from the injected joints in 28 days. However, in the plasminogen-deficient mice, the S. aureus counts were 27-fold higher at day 28 than at day 0. When human plasminogen was given to the plasminogen-deficient mice daily for 7 days, the bacterial clearance was greatly improved and the necrotic tissue in the joint cavity was also completely eliminated. Supplementation of plasminogen-deficient mice with plasminogen also restored the expression level of interleukin-6 (IL-6) in the arthritic joints. In summary, plasmin has protective roles during S. aureus-induced arthritis by enhancing cytokine expression, removing necrotic tissue, and mediating bacterial killing and inflammatory cell activation. The functional roles of plasmin during infection and sepsis were also studied in mice. Infection was induced by injecting 1 × 10 CFU of S. aureus intravenously and the sepsis model was induced by injecting 1.6 × 10 CFU of S. aureus. In the infection model, the wild-type mice had a 25-day survival rate of 86.7%, as compared to 50% in the plasminogen-deficient group. However, when sepsis was induced, the average survival for plasminogen-deficient mice was 3 days longer than for wild-type mice. Twenty-four hours after the induction of sepsis, the serum levels of IL-6 and IL-10 as well as the bacterial counts in all organs investigated were significantly higher in wild-type mice than in plasminogen-deficient mice. In wild-type mice, blockade of IL-6 by intravenous injection of anti-IL-6 antibodies significantly prolonged the onset of mortality and improved the survival rate during sepsis. These data indicate that plasmin plays different roles during infection and sepsis. Furthermore, plasmin appears to be involved in the regulation of inflammatory cytokine expression during sepsis. Taken together, our data indicate that plasmin plays multifunctional pro-inflammatory roles in different autoimmune and non-autoimmune diseases. The pro-inflammatory roles of plasmin include activation of inflammatory cells, regulation of cytokine expression, and enhancement of the bacterial killing ability of the host.