Interaction of meningococcal group B monoclonal antibody and its Fab fragment with alpha 2-8-linked sialic acid polymers: requirement of a long oligosaccharide segment for binding.

Mouse monoclonal IgG2a antibody (735D4) and other antibodies to the capsular polysaccharide of group B meningococci have been shown to require an unusually long segment of the alpha 2-8-linked N-acetylneuraminic acid polymer for binding. This property may be due to a conformational nature of the polysaccharide epitope recognized, or alternatively due to the requirement of bivalent binding of the antibody to the polysaccharide. In order to study the binding requirements, Fab fragments were prepared from the monoclonal antibody and their binding to alpha 2-8-linked sialic acid polymers of different lengths was studied. Both the intact antibody and its Fab fragment bound to sialic acid poly- and oligomers to similar extents, the critical chain length being about 10 sialyl units for both molecules. This excluded bivalency as the explanation for the requirement of a long oligosaccharide segment for binding. Although the binding was enhanced with increasing chain length, the first 10 monosaccharides were calculated to contribute to more than 90% of the total binding energy. This is in agreement with an oligosaccharide segment with defined conformational epitope binding to the antibody combining site. The antibody preparations also bound polysialic acid containing glycopeptides isolated from developing human and rat brain, suggesting, in quantitative binding assay, an average chain length of 10 or more sialic acid residues. The interaction of the antibody with both the bacterial and the tissue derived polysialic acids suggests that the conformational epitope critical for the interaction is formed by both classes of compounds.