Ficolins and the lectin complement pathway

Ficolins, found in various tissues, are a group of proteins containing both a collagen-like and a fibrinogen-like domain. Recently, it was shown that ficolins present in serum are lectins with a common binding specificity for N-acetylglucosamine (GlcNAc). The fibrinogen-like domain is responsible for the carbohydrate binding. Mannose-binding lectin (MBL) is also a collagenous lectin in serum that is specific for GlcNAc and mannose binding. Its domain organization is similar to that of ficolins, except that MBL has a carbohydrate-recognition domain instead of a fibrinogen-like domain. MBL plays a role in innate immunity by acting as an opsonin and activating complement in association with MBL-associated serine protease (MASP) via the lectin pathway. Investigations of two types of human serum ficolins, ficolin/P35 and Hakata antigen, revealed that they are associated with MASPs and sMAP, a truncated protein of MASP-2, and that they activate complement. These findings indicate that serum ficolins are structurally and functionally similar to MBL and have the capacity to activate the lectin pathway and thus have a role in innate immunity.