Natural killer function in flow cytometry. II. Evaluation of NK lytic activity by means of target cell morphological changes detected by right angle light scatter.

Morphological changes that occur in K562 cells after natural killing produce profound changes in cellular light scattering properties. The possibility of gating out all the effector cells by thresholding on perpendicular light scatter and the subsequent identification of two distinct clusters of cells, which correspond to dead and viable targets, have permitted the measurement of natural killer activity in vitro. The changes in scattering properties after cell death are mainly determined by the variation of internal refractive index of the dying cell. A comparison of the scattering and propidium iodide staining procedures showed good correlation. The morphological detection and measurement of cellular death is therefore used to estimate NK lytic activity. This methodology permits the measurement of NK activity without staining the target and the measurement of perpendicular light scatter provides an alternative approach to the study of lytic processes in vitro.