Multifunctional roles of plasmin in inflammation Studies of animal models on rheumatoid arthritis, multiple sclerosis, wound healing and infection
Abstract: Plasmin has been suggested to be involved in degradation of extracellular matrix (ECM) and tissue remodeling during a number of physiological and pathological processes. The aims of this thesis were to study the functional roles of plasmin during pathological inflammation in autoimmune and nonautoimmune disease models of rheumatoid arthritis (RA), multiple sclerosis (MS), wound healing and infection. In order to explain the obtained results in our functional studies as well as some previous results on the functional roles of plasmin during different tissue remodeling processes, I propose that there is a functional correlation between absence of plasmin and an inability to activate complement.The role of plasminogen during autoimmune collagen type II-induced arthritis (CIA) was studied first. The data revealed that whereas 83% of wild-type (plg+/+) mice developed CIA, none of the plasminogendeficient (plg-/-) mice got arthritis within a 40-day period. When plg+/+ mice were injected with a mixture of monoclonal antibodies against collagen type II they developed arthritis within a 5-day period, whereas no arthritis could be seen in plg-/- mice, although these mice had normal binding of antibody to the cartilage surface. These data suggest that plasmin plays an essential role in the step between antibody binding and inflammatory cell infiltration during CIA, probably during the step of complement activation. When plg+/+ and plg-/- mice were injected intra-articularly with collagen type II or 0.9% NaCl following CIA induction, plg-/- mice developed typical CIA, but the disease was less severe than in the plg+/+ mice and restricted to the injected joints. Sustained tissue necrosis was found only in the plg-/- mice after the local injection. When the antigen-induced arthritis (AIA) model was used, plg-/- mice developed a much more severe arthritis than the plg+/+ mice. These results indicate that different forms of pathogenesis exist for CIA and AIA, and further emphasize the importance of trauma in the induction of CIA in plg-/- mice.We further investigated the role of plasmin in experimental autoimmune encephalomyelitis (EAE), which is an autoimmune disease model for MS. During a 2-month period, the severity, incidence, mean onset day, mean maximal score and mean accumulative score of EAE were essentially identical in plg-/- and plg+/+ mice of B10.Q background. Histopathological studies revealed similar levels of inflammation and demyelination in plg-/- and plg+/+ mice. These data indicate that plasmin does not play an essential role in the development of EAE. The findings that plasmin is essential for the development of CIA but not needed for the development of EAE suggest that plasmin may play a pivotal role in autoimmune diseases where complement activation is critically involved in the pathogenesis.The role of plasmin was also studied in a tympanic membrane (TM) wound healing model. After TM perforations were performed, the plg+/+ TMs had all healed by day 11, whereas TM healing was completely arrested in plg-/- mice even as late as day 143. Immunohistochemical studies revealed a disturbed inflammation and tissue remodeling pattern in plg-/- mice. These data indicate that plasmin plays a central role in the healing of TM perforations.The involvement of plasminogen in ear infections was also investigated in plg-/- mice. During an 18-week experimental period, spontaneous otitis media (OM) was essentially developed in all of the plg-/- mice, whereas all of the plg+/+ mice kept a normal TM status. Positive bacterial growth was found in 5 out of 6 plg-/- mice, but only in 1 out of 6 plg+/+ mice. Immunohistochemical studies showed an accumulation of inflammatory cells, fibrin and also other extracellular matrix in the middle-ear cavity and the external-ear canal of plg-/- mice. These results show a spontaneous development of OM in plg-/- mice, but not in plg+/+ controls, suggesting that plasmin plays a critical role in the defense mechanisms during ear infections. Taken together, plasmin appears to play essential roles during autoimmune and non-autoimmune diseases in which complement activation is critical in the pathogenesis.
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