ABSTRACT
In this study, we tested the hypothesis that the immunogenicity and protective efficacy of polysaccharide-protein conjugate vaccines are influenced by three variables: (i) molecular size of the conjugate, (ii) molecular size of the polysaccharide used for conjugation, and (iii) extent of polysaccharide-to-protein cross-linking. Type III group B Streptococcus capsular polysaccharide was linked by reductive amination at multiple sites to tetanus toxoid to create a polysaccharide-protein conjugate (III-TT). A single lot of III-TT was fractionated into small, medium, and large M
rpools. Whereas all three conferred protection in a maternal immunization-neonatal challenge model in mice, the smallestM
r conjugate evoked less polysaccharide-specific immunoglobulin G (IgG) than the two largerM
r conjugates. To test whether the molecular size of the polysaccharide used for conjugation also affected the immunogenicity of the conjugate, vaccines were synthesized using capsular polysaccharides with M
rs of 38,000, 105,000, and 349,000. Polysaccharide-specific IgG responses in mice increased with the M
r of the polysaccharides, and protective efficacy was lower for the smallest polysaccharide conjugate compared to the other two vaccines. Immunogenicity testing of a series of vaccines prepared with different degrees of polysaccharide-to-protein cross-linking demonstrated higher polysaccharide-specific antibody responses as the extent of cross-linking increased. However, opsonic activity was greatest in mouse antiserum raised to a moderately cross-linked conjugate, suggesting that some antibodies evoked by highly cross-linked conjugates were directed to a nonprotective epitope. We conclude that conjugate size, polysaccharide size, and degree of polysaccharide-protein cross-linking influence the immunogenicity and protective efficacy of III-TT conjugate vaccines.
Laboratory and preliminary clinical characterization of Vi capsular polysaccharide-protein conjugate vaccines.
