Deciphering the molecular composition of two independent activation cascades of the lectin pathway of complement
2018-07-10T15:44:46Z (GMT) by
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.