Mannan‐binding lectin (MBL)‐associated serine protease‐1 (MASP‐1), a serine protease associated with humoral pattern‐recognition molecules: normal and acute‐phase levels in serum and stoichiometry of lectin pathway components

Summary The pattern‐recognition molecules mannan‐binding lectin (MBL) and the three ficolins circulate in blood in complexes with MBL‐associated serine proteases (MASPs). When MBL or ficolin recognizes a microorganism, activation of the MASPs occurs leading to activation of the complement system, an important component of the innate immune system. Three proteins are produced from the MASP1 gene: MASP‐1 and MASP‐3 and MAp44. We present an assay specific for MASP‐1, which is based on inhibition of the binding of anti‐MASP‐1‐specific antibody to MASP‐1 domains coated onto microtitre wells. MASP‐1 was found in serum in large complexes eluting in a position corresponding to ∼600 kDa after gel permeation chromatography in calcium‐containing buffer and as monomers of ∼75 kDa in dissociating buffer. The concentration of MASP‐1 in donor sera ( n  = 105) was distributed log‐normally with a median value of 11 µg/ml (range 4–30 µg/ml). Serum and citrate plasma levels were similar, while the values in ethylenediamine tetraacetic acid plasma were slightly lower and in heparin plasma were 1·5 times higher than in serum. MASP‐1 was present at adult level at 1 year of age, while it was 60% at birth. In normal healthy individuals the level of MASP‐1 was stable throughout a 2‐month period. After induction of an acute‐phase reaction by operation we found an initial short decrease, concomitant with an increase in C‐reactive protein levels, followed by an increase, doubling the MASP‐1 concentration after 2 days. The present data prepare the ground for studies on the associations of MASP‐1 levels with disease.