Group A Streptococcus (GAS) is a globally important pathogen causing a broad range of human disease and significant morbidity and mortality. There are no vaccines for GAS currently available but vaccine candidates based on the T-antigen, which forms the pilus backbone, are in pre-clinical development. T-antigens have previously been shown to have protective properties in mice and relatively low antigenic variation (GAS cluster into 18 major tee-types). This project aims to investigate antibody-T-antigen interactions and provide 3-dimensional structural data of T-antigens to inform structure-led, vaccine design efforts.
FVB/n Mice and NZ white rabbits were vaccinated with recombinant T18 antigen (found in M18, M71 and M217 strains). Sera from these animals were screened by ELISA using an array of T-antigens that covers all major circulating tee-types to elucidate cross-reactivity patterns. The dominant antibody cross-reactivity observed was to T3.2 and T13. To visualise antibody cross-reactivity at a molecular level the structures of the three T-antigens (T3.2, T13 and T18.1) were solved using X-ray crystallography. Each T-antigen features two immunoglobulin-like domains and, despite low overall sequence identity, show significant structural homology with the previously published T1 antigen. Structural overlays reveal that T-antigens share a highly conserved core decorated with variable loop regions. Purification of T-antigen specific IgG from the animal sera, together with the isolation of high affinity (<50 nanomolar) monoclonal antibodies from the same animals, has enabled the patterns of antibody specificity to be mapped onto T-antigen structures. These structural maps of antibody-T-antigen interactions will inform the design of T-antigen based vaccines.