Determination of natural killer cell function by flow cytometry

Natural killer cells (NK cells) are a subset of peripheral blood lymphocytes that mediate non-major histocompatibility complex-restricted cytotoxicity of foreign target cells. The "gold standard" assay for NK cell activity has been the chromium release assay. This method is not easily performed in the clinical laboratory because of difficulties with disposal of radioactive and hazardous materials, short reagent half-lives, expense, and difficulties with assay standardization. We describe a flow cytometric assay for the clinical measurement of NK cell activity. This study compared the chromium release assay and the flow cytometric assay by using clinically relevant specimens. There were no significant differences between the two assays in the measurement of lytic activity for 17 peripheral blood specimens or in reproducibility in repeated samplings of healthy individuals. We also established a normal range of values for NK activity in healthy adults and identified a small cluster of individuals who have exceptionally high or low levels of NK activity. The flow cytometric assay was validated by testing specimens from subjects expected to have abnormally low levels of NK activity (pregnant women) and specimens from healthy individuals in whom the activity of NK cells was enhanced by exposure to interleukin-2 or alpha interferon. Treatment with these agents was associated with a significant increase in NK activity. These results confirm and extend those of others, showing that the flow cytometric assay is a viable alternative to the chromium release assay for measuring NK cell activity.