Adjuvant effect of B domain of staphyloccocal protein A displayed on the surface of hepatitis B virus capsid

The hepatitis B virus (HBV) capsid-based recombinant particles, which display both major hydrophilic region of HBV surface antigen (HBV-MHR) and B domain of Staphylococcal protein A (SPA_B), were produced using Escherichia coli as expression host. SPA_B was used as an adjuvant to elicit the immune response to HBV-MHR, and its adjuvant effect in the immunized mice was estimated with varying the position and amount of SPA_B on the HBV capsid particles. Compared to the emulsified aluminum gel (alum gel) that is a currently commercialized vaccine adjuvant, SPA_B caused the significantly higher level of anti-HBV immunoglobulin G (IgG) titer and seroconversion rate, and notably SPA_B at the most surface-exposed position on the recombinant particle led to the highest immune response. Moreover, SPA_B caused much lower ratio of IgG1 to IgG2a compared to alum gel, indicating that helper T-cell 1-mediated immune response (responsible for cytotoxic T-cell stimulation) is relatively more stimulated by SPA_B, unlike alum gel that mainly stimulates helper T-cell 2-mediated immune response (responsible for B-cell stimulation). Although HBV-MHR and HBV capsid particle were used as proof-of-concept in this study, SPA_B can be used as a highly effective adjuvant with other disease-specific antigens on the surface of other virus-like particles to produce various recombinant vaccines with high potency.