STRUCTURAL-PROPERTIES OF GROUP-B STREPTOCOCCAL TYPE-III POLYSACCHARIDE CONJUGATE VACCINES THAT INFLUENCE IMMUNOGENICITY AND EFFICACY

In this study, we tested the hypothesis that the immunogenicity and pr otective efficacy of polysaccharide-protein conjugate vaccines are inf luenced by three variables: (i) molecular size of the conjugate, (ii) molecular size of the polysaccharide used for conjugation, and (iii) e xtent of polysaccharide-to-protein cross-linking, Type III group B Str eptococcus capsular polysaccharide was linked by reductive amination a t 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-r pools. Whereas all three conferred protection in a maternal immunization-neonatal challenge model in mice, the small est M-r conjugate evoked less polysaccharide-specific immunoglobulin G (IgG) than the two larger M-r conjugates, To test whether the molecul ar size of the polysaccharide used for conjugation also affected the i mmunogenicity of the conjugate, vaccines were synthesized using capsul ar polysaccharides with M(r)s of 38,000, 105,000, and 349,000, Polysac charide-specific IgG responses in mice increased with the M-r of the p olysaccharides, and protective efficacy was lower for the smallest pol ysaccharide conjugate compared to the other two vaccines. Immunogenici ty testing of a series of vaccines prepared with different degrees of polysaccharide-to-protein cross-linking demonstrated higher polysaccha ride-specific antibody responses as the extent of cross-linking increa sed. However, opsonic activity was greatest in mouse antiserum raised to a moderately cross-linked conjugate, suggesting that some antibodie s evoked by highly cross-linked conjugates were directed to a nonprote ctive epitope, We conclude that conjugate size, polysaccharide size, a nd degree of polysaccharide-protein cross-linking influence the immuno genicity and protective efficacy of III-TT conjugate vaccines.