The complement system is an essential part of the innate immune response. It consists of more than 30 proteins which act as a proteolytic cascade as first defense against pathogens. Complement modulates also B- and T-cell responses thereby bridging innate and adaptive immunity. Complement is activated in three ways, the classical, alternative and lectin pathways. The lectin pathway is initiated by pathogen-recognition receptors mannan-binding lectin (MBL), H-ficolin, L-ficolin, M-ficolin or collectin-K1 which recognize a broad range of pathogens or apoptotic cells. All molecules form complexes with MBL-associated serine proteases (MASPs). Five MASP proteins have thus far been identified: MASP1, MASP2, MASP3 and their truncated proteins MAp44 (also called MAp1) and MAp19 (or sMAP). MASP1&3 and MAp44 are all derived from MASP1 gene. MAp44 mRNA consists of the first 8 exons encoding the first four domains identical to those of MASP-1 and MASP-3 and an extra exon encoding 17 amino acids unique to MAp44. Deficiencies in the lectin pathway are associated with sensitivity for infection and the lectin-MASP complex is also involved in activation of the coagulation system. The function of MASP3, MAp44 and MAp19 in the lectin pathway are largely unknown. Competition of recombinant MAp44 in complex with either recombinant MBL or recombinant H-ficolin causes decreased levels of cleavage of C4 in sera depleted of MBL and H-ficolin, respectively, suggesting that MAp44 has an inhibition effect on activation of complement system. MAp44 may represent a tissue-specific protective mechanism by which local inflammation is prevented. MBL-associated protein MAp44 has shown cardio-protective effects in murine models. Although associated with cardiovascular risk factors, MAp44 levels were not associated with severity of disease in humans. In human plasma concentrations are 0.8-3.2 µg/ml. The assay captures MAp44 with a monoclonal antibody recognizing the C-terminus and uses a detection antibody reacting with the CCP-1 domain.
