Essential role of Mannose-binding lectin-associated serine protease-1 in activation of the complement factor D

The complement system is an essential component of innate immunity, participating in the pathogenesis of inflammatory diseases and in host defense. In the lectin complement pathway, mannose-binding lectin (MBL) and ficolins act as recognition molecules, and MBL-associated serine protease (MASP) is a key enzyme; MASP-2 is responsible for the lectin pathway activation. The function of other serine proteases (MASP-1 and MASP-3) is still obscure. In this study, we generated a MASP-1– and MASP-3–deficient mouse model (Masp1/3(−/−)) and found that no activation of the alternative pathway was observed in Masp1/3(−/−) serum. Mass spectrometric analysis revealed that circulating complement factor D (Df) in Masp1/3(−/−) mice is a zymogen (pro-Df) with the activation peptide QPRGR at its N terminus. These results suggested that Masp1/3(−/−) mice failed to convert pro-Df to its active form, whereas it was generally accepted that the activation peptide of pro-Df is removed during its secretion and factor D constitutively exists in an active form in the circulation. Furthermore, recombinant MASP-1 converted pro-Df to the active form in vitro, although the activation mechanism of pro-Df by MASP-1 is still unclear. Thus, it is clear that MASP-1 is an essential protease of both the lectin and alternative complement pathways.