Deciphering the molecular composition of two independent activation cascades of the lectin pathway of complement

The lectin pathway of complement activation is driven by pattern recognition molecules that direct activation of three different effector enzymes, called MASP-1, MASP-2 and MASP-3 (for Mannan binding lectin Associated Serine Protease). They drive complement activation through two independently operating effector arms. One effector arm (LEA-1) amplifies complement activation through MASP-3 dependent initiation of the alternative pathway amplification loop, while the second effector arm (LEA-2) is MASP-2 dependent and drives complement activation through the lectin pathway C3 and C5 convertases, C4bC2a and C4bC2a (C3b)n. Recently, a residual MASP-2 dependent C4-bypass route to activate C3 has been described in C4 deficient individuals. The first part of this thesis defines the molecular mechanism behind this C4-bypass activation route and demonstrates that MASP-2 can directly cleave native C3 to deposit C3b and iC3b on activator surfaces. The second part of this thesis studied the natural substrates and activators of MASP-3 to elucidate the sequence of molecular events that lead to alternative pathway activation via LEA-1. My results demonstrate that MASP-3 can be activated by both MASP-1 and MASP-2 and that activated MASP- 3 directly cleaves pro-FD, but not zymogen FB. While reconstitution of the deficient alternative pathway functional activity in MASP-1/-3 deficient mice could not be achieved by adding recombinant MASP-1, addition of either enzymatically active MASP-3 or injection of zymogen MASP-3 into these mice restored alternative pathway functional activity, underlining that MASP-3 is the predominant enzyme that drives LEA-1. Finally, the important role of MASP-1 and MASP-3 in the innate immune response to infection was demonstrated through the dramatically increased susceptibility of MASP-1/-3 deficient mice to S. pneumoniae infection. Notably, the defective C3b/iC3b opsonization of S. pneumoniae in MASP-1/-3 deficient mouse serum could be restored by reconstitution with recombinant MASP-3.