Specificity of antibodies to O-acetyl-positive and O-acetyl-negative group C meningococcal polysaccharides in sera from vaccinees and carriers.

Most group C Neisseria meningitidis strains produce an O-acetyl-positive polysaccharide, a homopolymer of alpha-2----9-linked N-acetylneuraminic acid with O-acetyl groups at the C-7 and C-8 of its sialic acid residues. The majority of disease isolates have been reported to contain this polysaccharide. Some strains produce group C polysaccharide lacking O-acetyl groups. The licensed vaccine contains the O-acetyl-positive polysaccharide. We have measured the antibody specificities to the two polysaccharides in sera from asymptomatic group C meningococcal carriers and vaccinated adults by a new enzyme-linked immunosorbent assay (ELISA) procedure using methylated human serum albumin for coating the group C polysaccharide onto microtiter plates. Inhibition of binding of serum antibodies to polysaccharide-coated plates was measured by ELISA after incubation with O-acetyl-positive and O-acetyl-negative group C polysaccharides. Greater inhibition of binding of carrier sera was observed with the homologous polysaccharide. There was substantial inhibition of binding of vaccinee sera to the O-acetyl-positive polysaccharide-coated plate following preincubation with O-acetyl-positive polysaccharide, but homologous inhibition on plates coated with the O-acetyl-negative polysaccharide required much higher concentrations of polysaccharide. Carrier sera may have a higher proportion of antibodies with greater specificity for the O-acetyl-negative polysaccharide, while vaccinee sera contain antibodies with greater affinity for the O-acetyl-positive polysaccharide. Studies with monoclonal antibodies specific for O-acetyl-positive and O-acetyl-negative polysaccharides reveal that the percentage of group C meningococcal disease caused by O-acetyl-negative strains remains about 15%, as found over 15 years ago.