Structural optimisation of a conformational epitope improves antigenicity when expressed as a recombinant fusion protein

A conformationally restricted B cell epitope has been identified as a potential safe vaccine candidate from the major group A streptococcal virulence factor, the M protein. To maintain α-helical secondary structure, the minimal epitope is flanked with heterologous sequences to produce the chimeric vaccine candidate called J14. As a strategy toward developing an affordable multivalent GAS vaccine, we have expressed J14 recombinantly with a second GAS protective antigen H12 (rJ14H12). When administered to mice sub-cutaneously, the fusion protein stimulated a strong serum IgG response to the H12 component, but J14 was poorly immunogenic. To increase the immunogenicity of J14 when expressed with the model fusion partner, amino acid modifications were made to the initial recombinant J14 construct to produce rJJo. These changes stabilised the α-helical conformation of the recombinant antigen as assessed by circular dichroism. Mice immunised with rJJoH12, the fusion protein incorporating JJo, effectively stimulated a humoral response to both of the included antigens. These data support the feasibility of developing a multivalent vaccine incorporating the conformationally restricted protective antigen J14.