Treatment of atypical hemolytic uremic syndrome and C3 glomerulopathy in mice by hepatic expression of factor D

Atypical hemolytic uremic syndrome (aHUS) and C3 glomerulopathy (C3G) are diseases driven by dysregulation of the alternative pathway (AP) complement. Current treatments of these diseases involve the use of frequent and often large doses of monoclonal antibodies, peptide inhibitors, or chemical inhibitors of complement proteins, with various limitations. In this study, we describe ectopic expression of factor D (FD) in the liver as a potential novel therapy for these indications. Unlike most plasma complement proteins, which are synthesized in the liver, FD, a serine protease in the AP, is synthesized primarily in fat tissues. Newly synthesized FD is released largely as an inactive proenzyme that requires activation by mannose-binding lectin-associated serine protease 3 (MASP3) to form the mature enzyme. We found serendipitously that ectopic expression of mature FD, but not pro-FD, in the mouse liver led to C3-dependent depletion of plasma factor B (FB) and abolished AP complement activity. Coexpression of C3, FB, and mature FD in cultured Hepa1-6 cells caused intracellular FB activation and consumption, probably within the secretory vesicles where all 3 proteins may colocalize. Hepatic FD-mediated local FB activation and depletion effectively prevented disease development in murine models of aHUS and C3G. Our results suggest that segregated tissue production of FD, FB, and C3, as well as FD regulation by MASP3, is necessary to prevent in situ FB activation and depletion. Ectopic expression of mature FD in the liver is an effective way to inhibit FB expression and AP complement activity, with potential therapeutic applications in diseases such as aHUS and C3G.