Analysis of the specificity of bactericidal
antibodies in normal, convalescent, and postvaccination human sera is important in understanding human immunity to
meningococcal infections and can aid in the design of an effective group B
vaccine. A collection of human sera, including group C and group B convalescent-phase sera, normal sera with naturally occurring cross-reactive bactericidal activity, and some postvaccination sera, was analyzed to determine the specificity of cross-reactive bactericidal
antibodies. Analysis of human sera using a bactericidal antibody depletion assay demonstrated that a significant portion of the bactericidal activity could be removed by purified
lipopolysaccharide (LPS). LPS homologous to that expressed on the bactericidal test strain was most effective, but partial depletion by heterologous LPS suggested the presence of
antibodies with various degrees of cross-reactivity. Binding of anti-L3,7 LPS bactericidal
antibodies was affected by modification of the core structure, suggesting that these functional
antibodies recognized
epitopes consisting of both core structures and
lacto-N-neotetraose (
LNnT). When the target strain was grown with 5'-cytidinemonophospho-N-acetylneuraminic
acid (
CMP-NANA) to increase LPS sialylation, convalescent-phase serum bactericidal titers were decreased by only 2- to 4-fold, and most remaining bactericidal activity was still depleted by LPS. Highly sialylated LPS was ineffective in depleting bactericidal
antibodies. We conclude that natural
infections caused by strains expressing L3,7 LPS induce persistent, protective bactericidal
antibodies and appear to be directed against nonsialylated bacterial
epitopes. Additionally, subsets of these bactericidal
antibodies are cross-reactive, binding to several different LPS immunotypes, which is a useful characteristic for an effective group B
meningococcal vaccine antigen.