Use of defined oligosaccharide epitopes in an ELISA for group B streptococcus

A single-site ELISA for group B streptococcal polysaccharide based on a monoclonal antibody against an immunodominant trirhamnoside epitope was inhibited at high capture antibody coating densities. The inhibition was eliminated when less antibody was coated or when high antigen concentrations were tested. This antigen is polyvalent with respect to the terminal trirhamnoside epitope and therefore it appears that closely spaced capture antibodies bound the epitope completely, leaving no sites for attachment of the enzyme-labeled second antibody with the same specificity. To make use of the trirhamnoside epitope feasible, a two-site ELISA was evaluated with this monoclonal antibody and a polyclonal antibody isolated by affinity chromatography. ELISA inhibition studies using oligosaccharides derived from the group B polysaccharide were used to evaluate the specificity of the polyclonal antibody. This showed that the antibody recognized both alpha-L-rhamnose (1----3)-D-galactose and alpha-L-rhamnose(1----3)-D-glucitol side chains, which together represent 30 potential binding sites per antigen molecule. A two-site ELISA with the anti-trirhamnoside monoclonal antibody to capture the antigen and the polyclonal antibody as enzyme conjugated second antibody reacted with only group B streptococci when tested against a panel of bacteria representative of the vaginal flora and was able to detect 3 x 10(4) cfu/test of group B streptococci. This two-site ELISA, based on well defined oligosaccharide epitopes had the sensitivity and specificity necessary to identify women at risk of infecting their newborns with group B streptococcus.