Standardised opsonophagocytosis killing assays (OPkA) for assessment of functional immunity to Streptococcus agalactiae (Group B Streptococcus)

Group B Streptococcus (GBS) is a leading cause of neonatal mortality and morbidity A maternal vaccine could be effective in preventing early and late onset GBS disease in infants. Several vaccines are in development based on capsular polysaccharides or protein antigens. Assessment of these new vaccines will greatly benefit from standardised immunoassays to quantify both natural and vaccine-induced immunity. ELISA-type assays are being developed to quantify IgG concentrations elicited by the vaccine antigens. In addition, it is important to quantify antibody function with antibody and complement-mediated opsonophagocytic killing the likely mechanism of protection. In a study funded by the Bill and Melinda Gates Foundation, we have characterised opsonophagocytic killing of a panel of strains ( historical and recent isolates) representing each of the vaccine serotypes using standard human reference sera developed in studies of monovalent capsular polysaccharide conjugate vaccines by Carol Baker (Baylor College of Medicine, Texas). Thus we have determined strains that are representative of the majority of recent isolates in their behaviour in the OPkA. One representative recent clinical isolate of each vaccine serotype has been selected as the strain to use in the standardised OPkA. These strains are UK clinical isolates and are designated OPkA ST-Ia, OPkA ST-Ib, OPkA ST-II, OPkA ST-III and OPkA ST-V.