Fluoro-Gold: An alternative viability stain for multicolor flow cytometric analysis

The viability stains propidium iodide (PI) and 7-amino-actinomycin D (7-AAD) are excited at 488 nm, as are the commonly used antibody conjugates fluorescein isothiocyanate (FITC), phycoerythrin (PE), and cyanine 5 dye covalently coupled to R-phycoerythrin (RPE-Cy5). When excited by a single laser, spectral overlap in the emission of PI and 7-AAD with RPE-Cy5 precludes the use of these viability stains for three-color immunophenotyping, particularly when evaluating low levels of marker expression in viable target cells. The ultraviolet excitable dye hydroxystilbamidine methanesulfonate (Fluoro-Gold, or FG) binds to DNA at the A-T-rich regions of the minor groove in permeabilized or dead cells. We assessed the suitability of this dye as a viability stain.

The ability of FG to detect nonviable cells in fresh and cryopreserved human apheresed peripheral blood cells was compared with that of PI and 7-AAD. The stability of FG staining and the effects of dye and cell concentration on the discrimination of nonviable cells was determined by measuring changes in the median fluorescence of viable and nonviable cells.

FG labeling at dye concentrations of 2-8 microM is stable for at least 3 h over a wide range of cell concentrations (4 x 10(5) to 4 x 10(7) cells/ml). Costaining studies and linear regression analysis show that cell viability as determined by FG is strongly correlated with estimates using PI (r = 0.9636) and 7-AAD (r = 0.9879).

FG is a reliable, alternative viability stain that can be used in conjunction with fluorochromes including FITC, PE, and RPE-Cy5 for multicolor analysis using dual-laser instruments.