Neisserial lipooligosaccharide is a target for complement component C4b - Inner core phosphoethanolamine residues define C4b linkage specificity

We identified Neisseria meningitidis lipooligosaccharide ( LOS) as an acceptor for complement component C4b ( C4b). Phosphoethanolamine ( PEA) residues on the second heptose ( HepII) residue in the LOS core structure formed amide linkages with C4b. PEA at the 6- position of HepII ( 6- PEA) was more efficient than 3- PEA in binding C4b. Strains bearing 6- PEA bound more C4b than strains with 3- PEA and were more susceptible to complement-mediated killing in serum bactericidal assays. Deleting 3- PEA from a strain that expressed both 3- and 6- PEA simultaneously on HepII did not decrease C4b binding. Glycose chain extension of the first heptose residue ( HepI) influenced the nature of the C4b- LOS linkage. Predominantly ester C4b- LOS bonds were seen when lacto-N- neotetraose formed the terminus of the glycose chain extension of HepI with 3- PEA on HepII in the LOS core. Related LOS species with more truncated chain extensions from HepI bound C4b via amide linkages to 3- PEA on HepII. However, 6- PEA in the LOS core bound C4b even when the glycose chain from HepI bore lacto-N-neotetraose at the terminus. The C4A isoform exclusively formed amide linkages, whereas C4B bound meningococci preferentially via ester linkages. These data may serve to explain the preponderance of 3- PEA- bearing meningococci among clinical isolates, because 6- PEA enhances C4b binding that may facilitate clearance of 6- PEA- bearing strains resulting from enhanced serum killing by the classical pathway of complement.