Properties of human anti-group B streptococcal type III capsular IgG antibody

The group B streptococcus is the commonest cause of bacterial infection in the newborn. In an attempt to prevent these infections, various vaccines are in development, most of which contain at least one of the capsular carbohydrates of the bacterium. We present new detailed data on the natural human antibody response to the type III capsular carbohydrate as we believe it is important to ascertain equivalent data for any new candidate vaccine in order to predict efficacy. We demonstrate that naturally occurring IgG is opsonically active in a complement-dependent manner, that fractions of differing avidity isolated from single donors have broadly similar opsonic activity, that the clonotypes from four individuals are restricted in number to a maximum of 15, and that binding kinetics ascertained using a resonant mirror biosensor show that specific antibodies have a moderately high affinity (mean Kd = 1.1e-8 M).