Determinant specificities of the groups B and C polysaccharides of Neisseria meningitidis

A meningococcal group B-specific horse antiserum contains at least two distinct populations of antibodies with specificities for determinants on the group B capsular polysaccharide antigen. These two populations were differentiated on the basis of the ability of only one of them to be absorbed from the antiserum by the structurally related colominic acid. The nature of the colominic acid-specific determinant was elucidated by a radioimmunoassay inhibition technique with the use of a series of linear alpha-(2----8)-linked oligomers of sialic acid as inhibitors. Colominic acid was labeled by prior removal of its N-acetyl groups, followed by their replacement with the use of [3H]acetic anhydride. The conformational nature of the determinant was proposed because of the unusually large size (10 sialic acid residues) of the oligomer required to function as an efficient inhibitor. The structure of the determinant responsible for the second population of group B-specific antibodies has not been determined, but it is obviously based on an as yet undefined conformational or structural feature peculiar to the group B meningococcal polysaccharide. In contrast to the colominic acid-specific group B determinant, the determinant responsible for the group C polysaccharide-specific rabbit antibodies proved to be more conventional. Inhibitory properties of the alpha-(2----9)-linked oligomers maximized with those containing four or five sialic acid residues, which is consistent with the approximate estimated maximal size of an antibody site.