A Novel Flow Cytometric Method to Study Cytotoxic Activity in Whole Blood Samples

For several decades, cell‐mediated cytotoxicity has been measured using the 51Cr release assay. This assay, however, has several drawbacks and flow cytometry has been used as an alternative to measure cytotoxic activity. Here, we present a quantitative method for cell‐mediated cytotoxicity studies, preserving cellular function with minimal sample manipulation. Cytotoxic activity is simply and reproducibly measured as the ability of cytotoxic cells to lyse K562 target cells previously loaded with Calcein‐AM vital stain. After spiking a known number of fluorescent viable K562 target cells into whole blood, cell mixtures are incubated for 2 h in a cell incubator and the remaining spiked cells are counted by flow cytometry. In order to discriminate nucleated cells, erythrocytes, and debris, unlysed whole blood is stained with a cell permeable DNA vital fluorescent dye. Cell‐mediated lysis is measured by comparing target counts for different effector‐to‐target ratios. Since the cytotoxicity of these dyes is relatively low, this method can be broadly applied to studies of innate immune response to tumors and infections, especially where target‐killing activity might be compromised by small volume samples or low frequency of cytotoxic cells. © 2020 International Society for Advancement of Cytometry