Analysis of humoral immunity of hepatitis D virus DNA vaccine generated in mice by using different dosage, gene gun immunization, and in vivo electroporation.

BACKGROUND Hepatitis D virus (HDV) DNA vaccine can produce Th1 and cytotoxic T-cell immune responses but only a low anti-HDV antibody titer is generated with a large hepatitis D antigen (L-HDAg) construct. In contrast, DNA vaccine expressing small hepatitis D antigen (S-HDAg) can generate a high titer of anti-HDV antibodies. Whether the low humoral immunity of L-HDAg DNA vaccine is due to inadequate dosage or can be ameliorated by other modes of immunization needs further evaluation. METHODS Plasmid (p25L) encoding L-HDAg and plasmid (pS/p25L) coexpressing hepatitis B surface antigen (HBsAg) and L-HDAg were used in this study. We compared the humoral response generated in mice using different plasmid DNA dosages and modes of immunization, including gene gun and in vivo electroporation (EP). RESULTS Intramuscular injection with a high dose of plasmid DNA (10 mg/kg) produced strong antibodies to HBsAg earlier than the usual dose did, but did not augment the anti-HDV response. Gene gun DNA immunization could not provide a better humoral immune response to HDV. EP DNA immunization had a higher anti-HDV seroconversion rate of 80%, but the anti-HDV antibody responses were generally weak (titer < or = 400:1). CONCLUSION The low humoral immunogenicity of DNA vaccine with L-HDAg cannot be ameliorated by different dosage, gene gun immunization, or in vivo EP intramuscular injection. DNA vaccine with a L-HDAg construct may not be a candidate HDV vaccine to generate anti-HDV humoral immunity.