FUNCTIONAL-ACTIVITY OF ANTIBODIES TO THE GROUP-B POLYSACCHARIDE OF GROUP-B STREPTOCOCCI ELICITED BY A POLYSACCHARIDE-PROTEIN CONJUGATE VACCINE

Group B streptococci (GBS) are a major cause of sepsis and meningitis in infants. While antibodies directed to the type-specific GBS capsule have been shown to be protective, it is less clear whether antibodies to the group B polysaccharide, a noncapsular, cell wall-associated an tigen, may play a role in immunity. To investigate the functional acti vity of group B polysaccharide-specific antibodies, we tested sera fro m rabbits vaccinated with group B polysaccharide coupled to tetanus to roid (B-TT). Anti-B-TT was weakly opsonic in vitro for a highly encaps ulated type III strain, while antiserum elicited by vaccination with t ype III capsular polysaccharide linked to tetanus toxoid (III-TT) was a very effective opsonin. In contrast to anti-III-TT, anti-B-TT given before or after bacterial challenge was only marginally effective in p rotecting newborn mice against lethal infection with type III GBS. The number of C3 molecules bound to type III GBS was augmented by anti-II I-TT but not by high antibody concentrations of anti-B-TT. These resul ts suggest that the difference in opsonic activity between anti-B-TT a nd anti-III-TT may be due to a difference in their ability to deposit C3. In addition, the maximum number of antibody molecules bound to the bacterial surface was greater for anti-III-TT than for anti-B-TT. Tha t anti-B-TT binds to fewer sites than anti-III-TT may explain the diff erences in complement activation and in opsonic and protective efficac y of antibodies to group B polysaccharide compared with antibodies to the type-specific capsular polysaccharide.