The mannan-binding lectin (MBL) activation pathway of complement plays an important role in the innate immune defense against pathogenic microorganisms. In human serum, two MBL-associated serine proteases (MASP-1, MASP-2) and MBL-associated protein 19 (MAp19) were found to be associated with MBL. With a view to investigate the interaction properties of these proteins, human MASP-1, MASP-2, MAp19, as well as the N-terminal complement subcomponents C1r/C1s, Uegf, and bone morphogenetic protein-1-epidermal growth factor (CUB-EGF) segments of MASP-1 and MASP-2, were expressed in insect or human kidney cells, and MBL was isolated from human serum. Sedimentation velocity analysis indicated that the MASP-1 and MASP-2 CUB-EGF segments and the homologous protein MAp19 all behaved as homodimers (2.8–3.2 S) in the presence of Ca2+. Although the latter two dimers were not dissociated by EDTA, their physical properties were affected. In contrast, the MASP-1 CUB-EGF homodimer was not sensitive to EDTA. The three proteins and full-length MASP-1 and MASP-2 showed no interaction with each other as judged by gel filtration and surface plasmon resonance spectroscopy. Using the latter technique, MASP-1, MASP-2, their CUB-EGF segments, and MAp19 were each shown to bind to immobilized MBL, with KD values of 0.8 nM (MASP-2), 1.4 nM (MASP-1), 13.0 nM (MAp19 and MASP-2 CUB-EGF), and 25.7 nM (MASP-1 CUB-EGF). The binding was Ca2+-dependent and fully sensitive to EDTA in all cases. These data indicate that MASP-1, MASP-2, and MAp19 each associate as homodimers, and individually form Ca2+-dependent complexes with MBL through the CUB-EGF pair of each protein. This suggests that distinct MBL/MASP complexes may be involved in the activation or regulation of the MBL pathway.

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Mannan-binding lectin (MBL) plays a pivotal role in innate immunity by activating complement after binding carbohydrate moieties on pathogenic bacteria and viruses. Structural similarities shared by MBL and C1 complexes and by the MBL- and C1q-associated serine proteases, MBL-associated serine protease (MASP)-1 and MASP-2, and C1r and C1s, respectively, have led to the expectation that the pathways of complement activation by MBL and C1 complexes are likely to be very similar. We have expressed rMASP-2 and show that, whereas C1 complex autoactivation proceeds via a two-step mechanism requiring proteolytic activation of both C1r and C1s, reconstitution with MASP-2 alone is sufficient for complement activation by MBL. The results suggest that the catalytic activities of MASP-2 split between the two proteases of the C1 complex during the course of vertebrate complement evolution.

Complement activation and inflammation have been suggested in the pathogenesis of diabetic vascular lesions. We investigated serum mannose‐binding lectin (MBL) levels and polymorphisms in the MBL gene in type 1 diabetic (T1DM) patients with and without diabetic nephropathy and associated macrovascular complications. Polymorphisms in the MBL gene and serum MBL levels were determined in 199 T1DM patients with overt nephropathy and 192 T1DM patients with persistent normoalbuminuria matched for age, sex and duration of diabetes as well as in 100 healthy control subjects. The frequencies of high and low expression MBL genotypes were similar in patients with T1DM and healthy controls. High MBL genotypes were significantly more frequent in diabetic patients with nephropathy than in the normoalbuminuric group, and the risk of having nephropathy, given a high MBL genotype, assessed by odds ratio was 1.52 (1.02–2.27), P = 0.04. Median serum MBL concentrations were significantly higher in patients with nephropathy than in patients with normoalbuminuria [2306 µg/l (IQR 753–4867 µg/l) versus 1491 µg/l (IQR 577–2944), P = 0.0003], and even when comparing patients with identical genotypes, serum MBL levels were higher in the nephropathy group than in the normoalbuminuric group. Patients with a history of cardiovascular disease had significantly elevated MBL levels independently of nephropathy status [3178 µg/l (IQR 636–5231 µg/l) versus 1741 µg/l (IQR 656–3149 µg/l), P = 0.02]. The differences in MBL levels between patients with and without vascular complications were driven primarily by pronounced differences among carriers of high MBL genotypes (P < 0.0001). Our findings suggest that MBL may be involved in the pathogenesis of microvascular and macrovascular complications in type 1 diabetes and that determination of MBL status might be used to identify patients at increased risk of developing these complications.

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