Chimeric hepatitis B surface antigen virus-like particles expressing the strep A epitope p*17 elicit a humoral immune response in mice.

To optimize immunogenicity, bacterial epitopes in putative vaccine constructs can be presented to immune cells as multiple repeated structures on a defined nanoparticle. Virus-like particles (VLPs) are viral capsid proteins that self-assemble to form compact and highly ordered nanoparticles that are within the optimal size range for uptake by dendritic cells. VLPs mimic the live virus in size and form but contain no viral genetic material, are therefore noninfective and are the basis of safe and effective vaccines against hepatitis B virus (HBV) and human papillomavirus (HPV). Due to their particulate nature, molecular stability, and expression of high density and repetitive antigen displays, recombinant cell culture-derived VLPs are ideal platforms for the delivery of small molecules, including bacterial epitopes. We developed a putative vaccine by expressing a minimal epitope from the bacterium (Strep A) on the surface of a recombinant VLP comprising multiple copies of HBV small envelope protein (HBsAg-S). Strep A is responsible for a wide spectrum of human infections and postinfectious diseases that disproportionately affect children and young adults living in resource-poor communities. No vaccine is currently available to offer sufficiently broad protection from the numerous and diverse strains of Strep A endemic in these at-risk populations. The Strep A antigen targeted by our vaccine construct is p*17, a cryptic epitope from a highly conserved region of the Strep A M-protein with demonstrated enhanced immunogenicity and broad protective potential against Strep A. To ensure surface expression and optimal immunogenicity, we expressed p*17 within the immunodominant "a" determinant of HBsAg-S. The recombinant VLPs (VLP-p*17) expressed in HEK293T cells spontaneously formed 22 nm particles and induced the production of high titers of p*17-specific IgG in BALB/c mice immunized with three 0.5 μg doses of VLP-p*17 formulated with adjuvant.