Although DNA vaccines elicit cell mediated immunity in small animal models, they have disappointed in human clinical trials. In an effort to improve their efficacy, we generated a number of DNA vaccines which encode cytoplasmic, membrane or secreted forms of HSP70, recognised as a Damage Associated Molecular Pattern (DAMP). In addition, to mimic the ability of live attenuated virus vaccines which elicit humoral and cellular immunity very effectively, we evaluated the effect of encoding a cytolytic protein in a DNA vaccine along with the immunogenic protein. Necrotic cells release a number of DAMPs which act as natural adjuvants to stimulate dendritic cells (DC) and other antigen presenting cells, and viral antigens which are expressed from the DNA vaccine are phagocytosed and cross presented.
We showed that DNA vaccines which encode membrane or secreted HSP or a vaccine which is designed to induce cell necrosis enhanced DC activation and cross presentation of the HIV gag protein. These vaccines induced a broad, multifunctional T cell response and increased protection, relative to the canonical DNA vaccine, against challenge with a chimeric virus, EcoHIV, which replicates in mice and murine cells due to substitution of the HIV gp120 with the murine leukaemia virus gp80. The enhanced immunity depended on the mechanism and timing of cell death after gag expression, and proved that co-expression of a cytolytic protein and an immunogenic protein can significantly improve the immunogenicity of a DNA vaccine.
We have also vaccinated mice and pigs with a DNA vaccine which encodes the HCV NS3 protein and a cytolytic protein, and confirmed that the cytolytic gene technology is effective in both animal models. Thus, this technology can increase immune responses against immunogenic proteins from two major pathogens, in small and large animal models, and may represent one mechanism to increase the efficacy of DNA vaccines in humans.