Effect of cationic liposomes/DNA charge ratio on gene expression and antibody response of a candidate DNA vaccine against Maedi Visna virus

Maedi Visna virus (MVV) is an ovine lentivirus with high prevalence all over the world. Since conventional vaccines had failed in protecting animals against the infection, the development of a DNA vaccine can be an alternative. The candidate vaccine was constructed by cloning the sequence encoding MVV p25 protein and was tested both in vitro and in vivo experiments associated with cationic liposomes. The lipoplexes (plasmid DNA–liposome complexes) with charge ratios ranging from 0 to 18 were prepared in physiological saline solution and characterized at a physical-chemistry level. Agarose gel electrophoresis was used as a first approach to evaluate qualitatively the amount of unbounded DNA by the liposomes. Dynamic light scattering measurements revealed that under the studied conditions lipoplexes with theoretical charge ratios (+/−) from 3 to 6 are unstable and prone to aggregation displaying sizes higher than 1 μm. At lower and higher charge ratios lipoplex size range from 200 to 500 nm. Using a Foster Resonance Energy Transfer methodology previously reported by us, complexation efficiency of the same complexes was related to in vitro and in vivo results.Higher transfection efficiencies were obtained in vitro with lipoplexes with charge ratio (+/−) = 10, where 97% of the DNA were protected by the liposomes. However, the subcutaneous immunization of mice induced higher antibody titers with lipoplexes at charge ratio (+/−) = 1, in which only 23% DNA is protected by the liposomes. Moreover, use of cationic liposomes has shown an increased antibody response when compared with a naked DNA immunization.