Rapid flow cytometric assay for the assessment of natural killer cell activity.

A new assay using flow cytometry has been established to assess natural killer (NK) lytic activity with common bench top instrumentation. This assay uses a cyanine membrane dye to stain live K562 target cells and an iodide nuclear dye to evaluate dead cells, and provides a method of reliably separating target and effector cell populations. Effector cells remain unstained (fluorescent negative) throughout the procedure. The damaged pre-labeled targets appear doubly stained as their membranes become permeable to the nuclear stain during incubation. Percent cytotoxicity of various effector:target cell ratios is discerned using flow cytometric analysis after a 2 h incubation in this new assay, as compared to 4 h with the 51Cr-release 'gold standard' assay for cell-mediated cytotoxicity. Comparisons of normal individuals tested in parallel with the fluorescent dyes and the 51Cr-release assay have shown direct correlations. This new two-color flow cytometric technique has proven to be uncomplicated and reproducible when used in the clinical setting.