BACKGROUND: The human lectin complement pathway (LCP) involves circulating 
complexes consisting of mannose-binding lectin (MBL) or ficolins in association 
with serine proteases named MASP-1, -2 and -3 and a non-enzymatic protein, sMAP. 
MASP-3 originates from the MASP1 gene through differential splicing and little 
is known about its biological characteristics. For this reason we expressed 
recombinant MASP-3 and generated specific monoclonal antibodies to establish 
biochemical characteristics and to determine the serum levels, the interactions 
with the LCP recognition molecules and the influence on complement activation of 
MASP-3.
METHODS: We expressed rMASP-3 in CHO-DG44 cells and used SDS-PAGE and Western 
blotting for biochemical characterization. We generated monoclonal antibodies 
against MASP-3 and developed a quantitative MASP-3 assay to establish the serum 
levels in 100 Danish blood donors. In addition we assessed the association 
levels between MASP-3 and Ficolin-2, -3 and MBL using both ELISA and 
immunoprecipitation techniques. Moreover, we assessed the influence on 
complement factor C4 deposition.
RESULTS: We found the mean serum MASP-3 concentration to be 6.4mg/l (range: 
2-12.9mg/l) and that MASP-3 in serum is primarily found in complex with 
Ficolin-3. In contrast to this the MASP-3 association with Ficolin-2 and 
especially with MBL seems to be less evident. rMASP-3 significantly inhibited 
Ficolin-3 mediated C4 deposition, while the opposite was the case for rMASP-1.
CONCLUSION: Our results show that MASP-3 is present in relatively high serum 
concentrations. Moreover, Ficolin-3 is the primary acceptor molecule of MASP-3 
among the LCP activator molecules, but MASP-3 appears to down-regulate Ficolin-3 
mediated complement activation through the lectin pathway.