Highly selective identification of novel vaccine candidate antigens by immunoprecipitation: the group A streptococcal case

Abstract

Streptococcus pyogenes (group A streptococcus, GAS) is an obligate human pathogen that causes a spectrum of pathologies ranging from superficial infections of the skin and oropharynx to severe invasive infections of the soft tissues. Despite a steady rise in the global rate of severe GAS disease over the past 30 years, there is currently no licenced vaccine available for the prevention of GAS infection. The initial stages of GAS pathogenesis are mediated by a repertoire of proteinaceous virulence factors expressed on the bacterial cell surface that facilitate colonisation and infection through specific interactions with the host extracellular matrix. These proteins are therefore predisposed to recognition by the host immune system, which in turn makes them important targets for the development of a novel subunit vaccine. The thesis describes the development of a novel approach to the isolation and identification of the major surface antigens of GAS using twenty GAS isolates representing four serotypes actively circulating within the United Kingdom (M1, M3, M12 and M89). Antigens were purified by immunoprecipitation using an antibody formulation derived from the human clinical blood product IVIG which has been shown to protect against severe GAS infection both clinically and in vivo. This “enriched” (E-)IVIG was produced by affinity purification using a concentrated GAS cell wall protein fraction and was shown to promote neutrophil uptake in an ex vivo opsonophagocytosis assay, and impair dissemination of GAS from the nidus of infection in a murine model. A total of eight pan-serotype vaccine candidate antigens were identified by E-IVIG immunoprecipitation, seven of which were produced recombinantly for use in murine vaccination experiments. This novel approach to vaccine candidate identification could be applied to other gram positive pathogens including Staphylococcus aureus, and may have wider implications for the field of bacterial vaccinology as a whole.