| Fgl2, also named as fibroleukin, is a member of the fibrinogen-related protein superfamily. The pivotal role of fgl2 in the pathogenesis of diseases (such as MHV-3-induced fulminant hepatitis, Th1 cytokine-induced fetal loss syndrome, and xenotransplantation) whose prominent feature involves fibrin deposition has been demonstrated. Several lines of evidence indicate that fgl2 exhibits a coagulation activity by being able to cleave prothrombin to thrombin; therefore, the induction of fgl2 during immune challenge may represent a novel mechanism whereby macrophages and endothelial cells activate the coagulation cascade. Here I expressed fgl2 protein in a baculovirus expression system and purified. The expressed protein exhibited similar biochemical property with the reported fgl2 protein detected previously in MHV-3 infected macrophages, verifying the efficacy of this expression system. The molecular basis of fgl2 prothrombinase activity was further explored. The kinetics of the enzymatic reaction of fgl2 to cleave human prothrombin to thrombin was examined. I found that the prothrombinase activity was detected only when fgl2 protein was reconstituted into negatively-charged phospholipids, and that Ser89 was critical for its activity. I also identified a novel immunomodulatory property of the soluble/non-membrane-associated fgl2, and further demonstrated that soluble fgl2 exerts immunosuppressive effects on T cell proliferation and LPS-induced DC maturation. The role of fgl2 and fibrosis in chronic hepatitis B infection were examined using an established HBV transgenic SCID mouse model. Collectively, my findings provide novel structural and functional insights into fgl2, which may play an important role in both innate and acquired immunity. |
