Complement mannan-binding lectin associated serine proteases 1 and 3 generated by allo-liver transplantation cleave in vivo pro-Factor D generated by the adipose tissue into active Factor D

Abstract

The alternative pathway (AP) of the complement system plays an important role in human diseases such as rheumatoid arthritis. MBL-associated serine proteases 1 & 3 (MASP-1/3)of the lectin pathway (LP) cleave pro-Factor D (proDf) (inactive) into Df(active), a necessary step to allow the AP to be activated. However, the site(s) of this cleaving activity are unknown; MASP-1/3 are predominantly liver-derived and proDf is produced by adipocytes. We hypothesized that the cleavage event can take place in the circulation, and may be facilitated by a complement activating surface. Liver from fD−/− but MASP-1/3sufficient (donor) was transplanted under the kidney capsule of MASP-1/3−/− (recipient)mice. H & E staining showed the presence of healthy hepatocytes under the kidney capsule of recipient mice. After five weeks of transplantation, the transplanted liver was found to generate MASP-1/3 proteases in the circulation, and concurrently proDf was cleaved into Df. Following liver transplantation, adherent anti-collagen antibodies in vitro also now activated C3 and generated C5a in the serum from transplanted MASP-1/3−/− mouse through the AP. This result illustrates one point of control of the AP by the LP. Furthermore, in vitro mixing and incubation of the sera from mice lacking MASP-1/3 or fD in the absence of anti-collagen antibodies did not cleave proDf. These results show that MASP-1/3 cleaves circulating proDf in a process facilitated by a complement activating target surface. Because the AP of complement is necessary for the development of collagen antibody-induced arthritis (CAIA), MASP-1 and/or MASP-3 inhibition, either at the mRNA or protein activity level, may be used as a new therapeutic approach for CAIA and its deleterious effects.