ACTIVATION OF COMPLEMENT BY MANNOSE-BINDING LECTIN ON ISOGENIC MUTANTS OF NEISSERIA-MENINGITIDIS SEROGROUP-B

Mannose-binding lectin (MBL) is a serum protein that has been demonstr ated to activate the classical complement pathways and to function dir ectly as an opsonin, Although MBL deficiency is associated with a comm on opsonic defect and a predisposition to infection, the role of the p rotein in bacterial infection remains unclear, We have investigated MB L binding to Neisseria meningitidis serogroup B1940 and three isogenic mutants, and the subsequent activation of the two major isoforms of C 4 (C4A and C4B) bg an associated serine protease, MASP. The mutants la cked expression of the capsular polysaccharide (siaD(-)), the lipo-oli gosaccharide (LOS) outer core that prevented LOS sialylation (cpsD(-)) , or both capsule and LOS outer core (eps(-)). Using flow cytometry, i t was possible to detect strong MBL binding to the cps(-) and epsD(-) mutants over a wide range of concentrations. In contrast, minimal or n o MBL binding was detected on the parent organism, with binding to sia D(-) only at higher MBL concentrations, C4 was activated and bound ba mutants that had previously bound MBL/MASP, but there was no significa nt difference in the amounts of C4A and C4B bound, When sialic acid re sidues were removed front the parent organism by neuraminidase treatme nt, the binding of both MBL and C4 increased significantly. Our result s suggest that MBL may bind to and activate complement on these encaps ulated organisms, and the major determinants of these effects are the LOS structure and sialylation.