Mapping the minimal murine T cell and B cell epitopes within a peptide vaccine candidate from the conserved region of the M protein of group A streptococcus.

The highly conserved C-terminus of the M protein of group A streptococcus (GAS) is a promising vaccine candidate. An epitope within the conserved C-terminus of the M protein, peptide 145 (a 20-mer with the sequence: LRRDLDASREAKKQVEKALE), has been defined which is the target of opsonic antibodies in both humans and mice, and is recognized by the sera of most adults living in areas of high streptococcal exposure. However, due to potential cross-reactivity between T cells stimulated by this region of the M protein and host cardiac myosin, it is critical to define precisely the minimal protective epitopes within p145. Studies have shown that the immunodominant epitope expressed by p145 is conformational, occurring as an alpha-helical coiled-coil. To enable us to map the murine minimal B cell and T cell epitopes within p145, we have used a novel strategy that allowed us to present shorter sequences of p145 in a native-like conformation. The minimal B cell epitope was found to be contained within residues 7-20 of the p145 sequence, and we have shown that mice immunized with this region are able to generate antibodies that bind to and also opsonize the organism GAS. The T cell epitope is located at the N-terminal region of the p145 sequence, residues 3-14. We have managed, therefore, to define a vaccine candidate--a minimal opsonic B cell epitope within the p145 sequence--that does not incorporate a potentially deleterious T cell epitope.