Levels of mannan-binding lectin-associated serine protease-2 in healthy individuals.

The lectin pathway is part of the innate immune system providing a first line of defence against infections. Mannan-binding lectin (MBL) and ficolins, in complex with MBL-associated serine proteases (MASPs), are capable of activating the complement system, thus mediating the destruction of infectious agents. MASP-2 cleaves C4 and C2 and is thus crucial for the activation of downstream complement components. We present an assay for quantifying total MASP-2 in plasma and serum samples. The assay is a sandwich type assay using a combination of two monoclonal anti-MASP-2 antibodies, one directed against the N-terminal part of MASP-2 and the other against its C-terminal part. Based on a population of Danish blood donors, the average MASP-2 concentration was estimated at 534 (S.D.+/-213) ng per ml of plasma. Characterization of the MASP-2 protein in serum showed high stability at 4 degrees C and at ambient temperature but a rapid decline at 37 degrees C. Gel permeation chromatography (GPC) indicated that all MASP-2 in serum is present in complexes with MBL and ficolins.