An improved fluorescence probe cytotoxicity assay.

The phenanthridine dye, ethidium bromide, which is actively excluded by viable cells, undergoes a significant fluorescence enhancement at 5900 A upon binding intracellular double-stranded polyribonucleotides. A rapid and sensitive assay of antibody mediated cytotoxicity to cells grown in vitro has been developed using this phenomenon. In this communication, we describe this fluorescence probe cytotoxicity assay and a sensitive electro-optical system designed to measure the fluorescence enhancement of ethidium bromide as it intercalates with intracellular polyribonucleotides. Basic characteristics of the fluorescence enhancement resulting from the interaction of ethidium bromide and non-viable cells are presented as well as examples of this assay as it has been used to study surface membrane neoantigens of cells tranformed by the oncogenic DNA virus, SV40.