Effects of Single and Fractionated Irradiation on Natural Killer Cell Populations: Radiobiological Characteristics of Viability and Cytotoxicity In Vitro

Background: Natural killer (NK) cells are important in destroying tumor cells. However, they are damaged by radiation therapy. We studied the effects of single and fractionated irradiation on the viability and cytotoxicity of human non-selected NK cells and sub-groups with cluster of differentiation (CD) CD16(+) and CD56(+) in vitro. Only very few studies dealing with the standard radiobiological parameters for characterizing NK cells exist in the literature. Materials and Methods: NK cell populations were isolated from buffy coats using different methods and irradiated with single doses up to 80 Gy and fractionated doses of 10 or 30 Gy with different numbers of applications and at different intervals. The study end-points were viability using propidium iodide (PI), trypan blue and intracellular adenosine triphosphate (ATP) assays, and cytotoxicity using the Cr-51-release assay. The standard radiobiological parameters alpha and beta of the linear-quadratic (L-Q) model and the mean inactivation dose (D) over bar taken as the area under the curve (AUC) were calculated to characterize the radiosensitivity of different NK cell populations. Results: The AUC values of the 51Cr release data in the dose range of 0-40 Gy were as follows: for non-selected NK cells, 23.6-20.9 Gy; for CD16(+) and CD56(+) cells, 14.5-13.2 Gy. The AUC values of ATP, trypan blue and propidium iodide methods equally well described the viability of irradiated NK cells. The alpha/beta ratio for cytotoxicity and viability data in the L-Q model corresponded to the acutely responding tissues. Splitting a 30-Gy dose into two fractions applied at different intervals caused a significant rise in ATP levels and cytotoxicity. Dividing the total dose into four doses applied at fixed intervals also resulted in significant elevations of ATP content and cytotoxicity of NK cells at 10 Gy. Conclusion: According to the L-Q method, irradiated NK cells behaved similarly to acutely responding human tissues with respect to cytotoxicity and viability. The AUC proved very useful for comparing the effects of irradiation on NK cells.