Cutting Edge: A New Player in the Alternative Complement Pathway, MASP-1 is Essential for LPS-Induced, but Not for Zymosan-Induced, Alternative Pathway Activation.

The complement system is a sophisticated network of proteases. In this article, we describe an unexpected link between two linear activation routes of the complement system: the lectin pathway (LP) and the alternative pathway (AP). Mannose-lectin binding-associated serine protease (MASP)-1 is known to be the initiator protease of the LP. Using a specific and potent inhibitor of MASP-1, SGMI-1, as well as other MASP-1 inhibitors with different mechanisms of action, we demonstrated that, in addition to its functions in the LP, MASP-1 is essential for bacterial LPS-induced AP activation, whereas it has little effect on zymosan-induced AP activation. We have shown that MASP-1 inhibition prevents AP activation, as well as attenuates the already initiated AP activity on the LPS surface. This newly recognized function of MASP-1 can be important for the defense against certain bacterial infections. Our results also emphasize that the mechanism of AP activation depends on the activator surface.