Fura-10, unlike fura-2, is suitable for long-term calcium imaging in natural killer (NK) cells without compromising cytotoxicity and can be combined with target cell death analysis

There are compelling reasons to opt for primary human natural killer (NK) cells when validating Ca²⁺ indicators. 1.) NK cells exhibit a high degree of vulnerability to stressors such as indicator loading or light exposure. 2.) The lack of research on NK Ca²⁺ signaling underscores the necessity for developing reliable assays. 3.) The increased utilization of NK cell therapies necessitates a more profound comprehension of Ca²⁺ dependent signal transduction. Consequently, an assay was developed to monitor cytosolic Ca²⁺ signals in individual NK cells simultaneously with their cytotoxic function against cancer cells. We used this assay to assess the suitability of fura-2, fura-PE3, fura-8, fura-10 or fura-red for quantifying Ca²⁺ signals in NK cells without compromising their cytotoxic function. In contrast to the widely used fura-2, its red-shifted derivative fura-10 did not interfere with NK cytotoxicity over several hours. It exhibited a superior signal-to-noise ratio and good dynamic range, accompanied by minimal bleaching or leakage. Fura-8 and fura-red also preserved NK cell cytotoxicity, but had other disadvantages compared to fura-10. We successfully used fura-10 to report Ca²⁺ signals in NK cells from blood donors and patients diagnosed with lymphoma and leukemia over several hours at 37 °C during apoptotic or necrotic killing of different cancer cells (K562, THP1, OCI-AML2, and TMD8). Additionally, we show that fura-10 is well suited to report Ca²⁺ signals in intact murine pancreatic islets, another stress-sensitive cell preparation. Consequently, fura-10 is an optimal choice for measuring Ca²⁺ in primary human NK cells and other primary cell preparations.