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Background:  DNA immunization is a promising strategy for HIV vaccines, with several products now in clinical development. However, DNA vaccination provides limited immunogenicity in larger animals. Electroporation (EP) is an established method for enhancing in vitro and in vivo gene expression. We examined the humoral antibody and cell-mediated immune (CMI) responses to HIV-1 Gag provided by in vivo EP in animal models.

Methods:  BALB/c mice were vaccinated via the intramuscular route with DNA encoding HIV subtype C Gag with or without EP. We chose a dose of 2µg per mouse, which is comparable by body weight to dosage administered in current human DNA vaccine trials. Serum and splenocytes were harvested 10 days post-vaccination. Interferon-γ (INFg) ELISpot responses were examined following overnight stimulation with CD4- and CD8- specific peptides. Antibody responses were examined using a Gag-specific ELISA.

New Zealand white rabbits were used for examination of anti-Gag antibody responses to DNA delivered with EP, versus intramuscular (IM) alone. Rabbits received two administrations of 500µg of DNA per animal at an interval of four weeks. Serum was collected at 2 week intervals and anti-Gag ELISA responses were measured.

Results:  In mice, EP delivery resulted in a 5-fold improvement in CD4 and a 10-fold improvement in CD8 IFN-γ ELISpot responses. Further, significant antibody responses were observed only in the EP group. EP delivery of DNA in both rabbits provided 2-week antibody titers of at least 1x10⁵ while standard IM injection resulted in a titer of 3x10³ in one animal and an undetectable response in another. Following the second injection, animals receiving EP DNA achieved antibody titers as high as 8x10⁵, while standard IM injection yielded a titer of 1x10⁴ in one animal and less than 1:400 in the other. The nonresponding IM animal was then injected with 500µg of DNA via EP. Ten days post-injection, the anti-Gag antibody titer reached a level of 1x10⁴.

Conclusions:  Electroporation provides consistent and significant enhancement of cell-mediated and humoral responses in 2 animal species. Antibody responses, in particular, achieve levels not possible via standard IM delivery. These results support future studies examining the enhancement of DNA vaccination provided by electroporation in humans.

Keywords: electroporation; antibodies; DNA vaccines