The Group B Streptococcal Capsular Carbohydrate: Immune Response and Molecular Mimicry

Abstract

The group B streptococcus (GBS) is the commonest cause of lethal bacterial infection in the newborn infant.¹ GBS possesses a sialylated carbohydrate capsule which is known to be a major virulence factor.² Mutant GBS which have a non-sialylated capsule are considerably less virulent, fix complement to a greater degree than the isogenic wild type and are more avidly opsonophagocytosed by polymorhonucleoctes.² Antibodies to GBS capsule have been shown to be protective but the majority of the population do not possess these antibodies. For example, 90% of the population lack antibodies to the type III capsule of GBS (GBS III).³ Furthermore, 60% of seronegative individuals do not respond to experimental vaccination with purified GBS III.⁴ There has been considerable interest over the last few years in the development of a vaccine which would prevent neonatal infection with GBS. An appropriate approach would seem to be to conjugate the capsular carbohydrates to proteins as has been successfully achieved for Haemophilus influenzae serotype b. It is envisaged that any such vaccine would be given early in pregnancy and the vaccine induced IgG would subsequently be placentally transported to the fetus. In order to facilitate the development of these new vaccines, it is important to understand the basis of the poor immune response to the native capsular carbohydrate in humans and to document the naturally occurring human antibody response.