M-ficolin, an innate immune defence molecule, binds patterns of acetyl groups and activates complement.

Ficolins play a role in the innate immune defence as pathogen-associated molecular pattern recognition molecules. Three ficolins are found in humans: H-ficolin, L-ficolin and M-ficolin. L-ficolin and H-ficolin circulate in blood in complexes with mannan-binding lectin-associated serine proteases (MASPs) and are capable of activating the complement system. L-ficolin shows affinity for acetylated compounds and binds to various capsulated strains of bacteria. H-ficolin has been shown to bind Aerococcus viridans. Less is known about M-ficolin, but it is thought to be present only on monocytes. We have synthesized recombinant M-ficolin and find that it, in a manner similar to L-ficolin, is able to bind to acetylated compounds and to associate with recombinant MASP-2. Upon binding to M-ficolin ligands, the associated MASP-2 zymogen is activated and cleaves C4, thus triggering the complement system. We developed a monoclonal rat anti-human-M/L-ficolin antibody and verified by flow cytometric analysis the presence of ficolin on the surface of peripheral blood monocytes.