A serum lectin (mannan-binding protein) has complement-dependent bactericidal activity

Serum mannan-binding protein (MBP), which is a lectin specific for mannose and N-acetylglucosamine and is known to activate complement via the classical pathway, has been revealed to have a complement-dependent bactericidal activity, as tested on rough strains of Escherichia coli, K-12 and B. The bacteria, which had been sensitized with purified human serum MBP in the presence of Ca2+, followed by incubation with guinea pig complement, showed a marked decrease of colony forming ability compared with those not sensitized with the lectin. The bactericidal effect depended on the concentrations of the lectin and complement. The C4-dependency of the reaction indicated that the complement-dependent bactericidal action by MBP is expressed through the classical pathway. The bacteria were aggregated by the lectin. Scatchard plot analysis of 125I-labeled MBP binding to the bacteria showed that the dissociation constant (Kd) and the maximum binding capacity were 6 x 10(-9) M and 30,000 molecules of MBP per cell, respectively. The binding was inhibited by mannose, N-acetylglucosamine, N-acetylmannosamine, L-fucose, manno-heptulose, and sedoheptulose, suggesting that MBP recognized L-glycero-D-manno-heptose and N-acetylglucosamine constituting the core oligosaccharide of the E. coli K-12 cell wall, and L-glycero-D-manno-heptose for E. coli B. These findings suggest the physiological significance of the serum lectin in host defense, being consistent with the avirulence of E. coli rough strains in mammals.