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Activation of complement by mannose-binding lectin on isogenic mutants of Neisseria meningitidis serogroup B
1Immunobiology Unit, Institute of Child Health, London, United Kingdom.
Abstract:
Mannose-binding lectin (MBL) is a serum protein that has been demonstrated to activate the classical complement pathway and to function directly as an opsonin. Although MBL deficiency is associated with a common opsonic defect and a predisposition to infection, the role of the protein in bacterial infection remains unclear. We have investigated MBL binding to Neisseria meningitidis serogroup B1940 and three isogenic mutants, and the subsequent activation of the two major isoforms of C4 (C4A and C4B) by an associated serine protease, MASP. The mutants lacked expression of the capsular polysaccharide (siaD-), the lipo-oligosaccharide (LOS) outer core that prevented LOS sialylation (cpsD-), or both capsule and LOS outer core (cps-). Using flow cytometry, it was possible to detect strong MBL binding to the cps- and cpsD- mutants over a wide range of concentrations. In contrast, minimal or no MBL binding was detected on the parent organism, with binding to siaD- only at higher MBL concentrations. C4 was activated and bound by mutants that had previously bound MBL/MASP, but there was no significant difference in the amounts of C4A and C4B bound. When sialic acid residues were removed from the parent organism by neuraminidase treatment, the binding of both MBL and C4 increased significantly. Our results suggest that MBL may bind to and activate complement on these encapsulated organisms, and the major determinants of these effects are the LOS structure and sialylation.
Insights
Mannose-binding lectin (MBL) binds to Neisseria meningitidis mutants lacking specific surface structures. This binding activates complement, suggesting MBL
Area of Science:
- Immunology
- Microbiology
- Biochemistry
Background:
- Mannose-binding lectin (MBL) is a key serum protein in innate immunity.
- MBL activates the complement system and acts as an opsonin.
- The role of MBL in bacterial infections, particularly with Neisseria meningitidis, requires further elucidation.
Purpose of the Study:
- To investigate MBL binding to Neisseria meningitidis serogroup B1940 and its mutants.
- To determine the subsequent activation of complement component C4 by MBL-associated serine protease (MASP).
- To identify the bacterial surface structures influencing MBL-MASP complex binding and complement activation.
Main Methods:
- Flow cytometry was used to quantify MBL binding to bacterial strains.
- Analysis of complement activation by measuring C4 deposition.
- Neuraminidase treatment to remove sialic acid residues from bacterial surfaces.
Main Results:
- MBL strongly bound to mutants lacking capsular polysaccharide (siaD-) and lipo-oligosaccharide (LOS) outer core (cpsD-).
- Minimal MBL binding occurred on the parent strain and the siaD- mutant.
- Complement component C4 was activated on MBL-binding mutants, with no significant difference between C4A and C4B.
- Sialic acid removal from the parent strain significantly increased MBL and C4 binding.
Conclusions:
- MBL can bind to and activate complement on encapsulated Neisseria meningitidis.
- LOS structure and sialylation are critical determinants of MBL binding and complement activation.
- These findings highlight the complex interplay between MBL, bacterial surface components, and complement activation in host defense.