Evaluation of cytotoxic function and apoptosis in interleukin (IL)-12/IL-15-treated umbilical cord or adult peripheral blood natural killer cells by a propidium-iodide based flow cytometry

Both deficient natural killer (NK) cell effector function and increased propensity to apoptosis of neonatal NK cells contribute to the increased susceptibility to infection in the neonates. Interleukin (IL)-12 and IL-15 are two immunoregulatory cytokines known to enhance cytolytic function of neonatal NK cells. The present study aims to simultaneously investigate the effect of IL-12/IL-15 on K562 cytotoxicity as well as NK cells apoptosis of enriched umbilical cord blood (CB) and adult peripheral blood (APB) NK cells, using flow cytometric cytotoxicity assays. The results indicated that (i) prior to cytotoxicity assays, CB NK cells underwent greater degree of spontaneous apoptosis than did APB NK cells; (ii) both IL-12 and IL-15 inhibited the spontaneous apoptosis of CB NK cells, while IL-15 promoted the apoptosis in APB NK cells; (iii) the deficient K562 cytotoxicity of CB NK cells could be enhanced to levels comparable with that of APB NK cells by IL-15; (iv) IL-15 increased the percentages of apoptosis in NK–K562 conjugates in a dose-dependant manner in both CB and APB with a greater effect seen with APB NK cells; (v) target-induced apoptosis was observed with APB NK cells which were further enhanced with IL-15. However, CB NK cells, unstimulated or IL-15-activated, were resistant to K562-induced apoptosis. Thus, the multi-parameter flow cytometry analysis not only demonstrates better for the deficient CB NK function but also provides greater details for cytotoxic mechanisms of NK cells.