f Vaccination using "naked" DNA is a highly attractive strategy for induction of pathogen-specific immune responses; however, it has been only weakly immunogenic in humans. Previously, we constructed DNA-launched Semliki Forest virus replicons (DREP), which stimulate pattern recognition receptors and induce augmented immune responses. Also, in vivo electroporation was shown to enhance immune responses induced by conventional DNA vaccines. Here, we combine these two approaches and show that in vivo electroporation increases CD8 ؉ T cell responses induced by DREP and consequently decreases the DNA dose required to induce a response. The vaccines used in this study encode the multiclade HIV-1 T cell immunogen HIVconsv, which is currently being evaluated in clinical trials. Using intradermal delivery followed by electroporation, the DREP.HIVconsv DNA dose could be reduced to as low as 3.2 ng to elicit frequencies of HIV-1-specific CD8 ؉ T cells comparable to those induced by 1 g of a conventional pTH.HIVconsv DNA vaccine, representing a 625-fold molar reduction in dose. Responses induced by both DREP.HIVconsv and pTH.HIVconsv were further increased by heterologous vaccine boosts employing modified vaccinia virus Ankara MVA.HIVconsv and attenuated chimpanzee adenovirus ChAdV63.HIVconsv. Using the same HIVconsv vaccines, the mouse observations were supported by an at least 20-fold-lower dose of DNA vaccine in rhesus macaques. These data point toward a strategy for overcoming the low immunogenicity of DNA vaccines in humans and strongly support further development of the DREP vaccine platform for clinical evaluation. " N aked" DNA constitutes an attractive vaccine platform for delivery of pathogen-or cancer-derived immunogens. Three veterinary DNA vaccines have been licensed for use in dogs, salmon, and horses, demonstrating that DNA vaccines are capable of inducing protective immunity (8, 48). In humans, plasmid DNA vaccines have been shown to be safe and well tolerated in thousands of volunteers (14,41,48); however, their strong immunogenicity observed in smaller animals does not transfer to primates, including humans. Thus, the potency of DNA vaccines in humans must be significantly improved for the vaccines to become a practical and commercially attractive health care tool.The use of in vivo electroporation (EP) was shown to improve DNA transfection into cells at the site of injection and to promote local inflammation, thereby increasing the immunogenicity of DNA vaccines (7,18,37,39). Safety and tolerability of intramuscular (i.m.) and intradermal (i.d.) DNA EP were demonstrated in both preclinical and clinical settings (28,49,51). Although the i.m. delivery route has been more extensively studied, i.d. EP offers an attractive route of delivery due to the characteristics of the skin, with many resident antigen-presenting cells such as Langerhans and dermal dendritic cells. Intradermal EP is also considerably less painful, and any possible residual pain may be further controlled by topical anesthetics (38,48).The D...
