STIMULATION OF PROTECTIVE ANTIBODIES AGAINST TYPE-IA AND TYPE-IB GROUP-B STREPTOCOCCI BY A TYPE-IA POLYSACCHARIDE-TETANUS TOXOID CONJUGATE VACCINE

Antisera elicited by type la group B streptococci (GBS) contain antibo dies that react with both type la and type Ib strains. Previous studie s suggested that antibodies elicited by type Ia organisms recognized a carbohydrate antigen or epitope common to la and lb strains. We now r eport the synthesis and immunogenicity testing of a type Ia polysaccha ride-tetanus toxoid (Ia-TT) conjugate vaccine. Ia-TT elicited type Ia polysaccharide-specific immunoglobulin G antibodies in all three of th e rabbits inoculated. In competitive enzyme-linked immunosorbent assay , these antibodies reacted with high affinity to type la polysaccharid e and with lower affinity to the structurally related GBS type Ib poly saccharide. Despite the lower binding affinity of the Ia-TT-induced an tibodies for the type Ib polysaccharide, Ia-TT antiserum opsonized not only type Ia GBS but also type Ib GBS for killing by human blood leuk ocytes. Ia-TT antiserum was also evaluated in a mouse model designed t o test the efficacy of maternal antibodies in protecting neonates agai nst GBS infection. Pups born to dams that had received Ia-TT antiserum were protected against lethal challenge with either type Ia or Ib GBS . These studies using a polysaccharide-protein conjugate as an immunog en support the view that the carbohydrate immunodeterminant recognized on lb strains by la antisera is a common epitope contained within the structurally related Ia and Ib capsular polysaccharides. Although ant ibodies elicited by Ia-TT had protective activity against both Ia and Ib strains, these antibodies reacted with lower affinity to Ib than to Ia polysaccharide.