597. DNA Vaccines: Induction of Humoral and Cellular Responses Via Skin or Muscle Immunization Enhanced by Electroporation
Authors: Lei Zhang, Georg Widera, Susan Bleecher, Dietmar Rabussay
Journal: Molecular Therapy (2006)

DNA vaccines are attracting increased attention due to multiple potential advantages over conventional vaccines. To make DNA vaccines effective in humans, improvements in intracellular DNA delivery and identification of the most efficient target tissue continue to be important. Here we compare immune responses in balb/c mice using relative low doses of plasmid DNAs encoding either secreted alkaline phosphatase (SEAP) or the hepatitis B virus surface antigen (HBsAg). Immune responses were determined either after intradermal (i.d.) injection followed by non-invasive skin electroporation (EP), or after i.m. injection followed by EP with two-needle electrodes. After injection of 10 μg SEAP DNA into skin, significant levels of SEAP in serum were only detected in mice receiving EP and then only between day 2 and day 7. All animals (n=6) treated with EP showed an early (week 3) antibody response, which increased for the duration of the experiment (9 weeks). In contrast, only one of the animals immunized by DNA injection alone showed a late and weak response. When SEAP DNA was injected into the muscle with EP, high levels of SEAP were detected from day 2 to day 39 while only negligible antigen levels were observed without EP. Despite the higher and sustained serum SEAP levels obtained after i.m. injection compared with i.d. injection, SEAP antibody levels in i.m. immunized mice were one to two orders of magnitude lower. In a similar experiment, the immune response to DNA encoding HBsAg was measured after DNA delivery to either muscle or skin. At DNA doses which produced very low responses in muscle or no response in skin without EP (40 and 30 μg, respectively), antibody titers against HBsAg increased significantly and to similar levels when EP was applied. Thus, the response to two different antigens (SEAP and HBsAg) differed in their kinetics and amplitude depending on the route of immunization. Antibody isotypes obtained after EP-enhanced DNA immunization indicated predominantly Th1 type responses, independent of the route of DNA administration. To further examine the potential usefulness of the putative cellular response induced by EP augmented immunization we compared the antitumor immunities elicited by i.d. and i.m. injections, respectively. Mice immunized against HBsAg were challenged by injection of either wild-type CT26 cells (murine colon adenocarcinoma) or CT26- F4 cells, which were engineered to stably express HBsAg. After 4 weeks, none of the CT-26-challenged animals survived, whereas the majority of the CT26-F4-challenged mice remained tumor-free after 5 weeks. Moreover, after rechallenge of the tumor-free mice with wild-type CT26 cells, the majority of the animals was completely protected. We conclude that the kinetics and amplitude of the antibody response to different antigens may vary depending on whether DNA is injected into skin or muscle. Furthermore, in the tumor challenge system employed here, both routes of immunization provided a high degree of tumor protection, strongly suggesting the induction of an effective cellular response against the expressed antigen.