Therapeutic hepatitis B vaccine employing DNA prime – MVA boost scheme requires additional priming with recombinant HBsAg to elicit an adequate antibody response

Therapeutic vaccines offer hope for a curative treatment of chronic hepatitis B virus (HBV) infection. We developed a therapeutic hepatitis B vaccine, TherVacB , which employs priming with adjuvanted HBV S (HBsAg) and Core antigens of a single genotype, and a boost with a pan-genotypic Modified Vaccinia Ankara vector (MVA-HBVac) expressing several viral proteins that cover more than 95% of HBV circulating strains. Priming immunization with antigens exactly matching those expressed by the MVA may improve the efficacy of TherVacB . However, recombinant protein antigens need to be produced separately and require complex and costly purification, as well as a Th1-inducing adjuvant, to elicit CD8 + T-cell responses. These limitations may be overcome by DNA vaccines, which enable the direct expression of several antigens in vivo . Here, we investigated the potential of DNA vaccination as an alternative to protein priming to broaden and enhance HBV-specific immunity. Immunization of HBV-carrier mice with a DNA prime-MVA boost regimen elicited robust HBV-specific CD8 + T-cell responses, capable of reducing serum HBeAg levels and eliminating HBV-infected hepatocytes. However, it yielded poor neutralizing anti-HBs titers, resulting in an inefficient reduction of circulating HBsAg. Including adjuvanted HBsAg into DNA priming, either simultaneously or sequentially, significantly enhanced HBsAg-to-anti-HBs seroconversion rates. Still, only the sequential application supported vigorous plasmid-mediated CD8 + T-cell immunity. Interestingly, sequential immunization with DNA, followed by HBsAg and MVA, elicited neutralizing anti-HBs antibodies and enhanced HBV-specific CD8 + T-cell responses, resulting in a more potent antiviral effect compared to the reverse priming regimen. Our study demonstrates that although DNA can successfully prime an effective antiviral CD8 + T-cell response, it requires additional immunization with adjuvanted HBsAg to elicit satisfactory neutralizing antibody titers. In addition, it demonstrates that the sequence of immunizations in multicomponent heterologous-prime boost regimens is crucial and can significantly affect the quantity and quality of induced immune responses.