Cationic liposomal delivery of plasmid to endothelial cells measured by quantitative flow cytometry

Cationic liposomes are potentially important gene transfer vehicles, capable of conjugating with anionic DNA by condensation. Flow cytometry was used to examine quantitatively the incorporation of DNA-liposome complex into murine capillary lung endothelial cells. The plasmid DNA, a pSV-beta-galactosidase vector, was covalently labeled with ethidium monoazide by photoactivation. The cationic liposome consisted of egg phosphatidylcholine (90%), cholesterol (5%), and stearylamine (5%). The number of plasmid molecules contained within each cell as a function of exposure time was estimated from fluorescence intensity. Fluorescently labeled plasmid is detectable after 10 min and increases with continued exposure, but at a decreasing rate, up to 2160 min. After 2160 min each cell, on average, contains approximately 10,000 plasmid molecules. Following transfection, a single cell unimodal population was detected by flow cytometry, suggesting that all cells participate in transfection equally. Furthermore, cell cycle analysis indicates that the entry of DNA-liposome complex is independent of cell cycle.