Heterogeneity of MBL–MASP complexes

In order to study aspects of the stoichiometry and composition of human MBL–MASP complexes in the population, MBL–MASP complexes were bound from sera of 152 healthy individuals onto mannan-coated microtitre plates. Bound mannan-binding lectin (MBL) was measured by ELISA, and the enzyme activities of MBL-bound MASP-1 and MASP-2 were measured by an amidolytic assay and a C4 fixation assay, respectively. MASP-1 activity correlated with MBL concentration, as did MASP-2 activity (in both cases: p < 0.0001). This is expected since MASP-1 and MASP-2 are bound to the mannan via MBL. However, when MASP activities were normalised to MBL concentration (i.e. MASP-1 activity/[MBL], MASP-2 activity/[MBL]) MASP-1 activity was inversely correlated with MASP-2. This means on average that high MASP-1 correlates with low MASP-2 and vice-versa, and confirms the hypothesis that native MBL–MASP complexes on average do not have fixed MBL-(MASP-1)-(MASP-2) stoichiometry. The findings are consistent with separate populations of MBL–MASP-1 complexes and MBL–MASP-2 complexes, the concentrations of which show wide inter-individual variation.