Graded changes in the response of individual human basophils to stimulation: Distributional behavior of events temporally coincident with degranulation

Human basophils, stimulated with either anti-IgE antibody or formyl-methionine-leucine-phenylalanine, were examined by two measures of the cell response that may reflect degranulation. Flow cytometric measurement of either of these two measures, changes in forward scatter intensity (an indirect measure of the basophil size) or changes in the intensity of acridine orange-loaded cells (which labels basophil granules), allowed an assessment of the distribution of single cell responses. With regard to the latter technique, structures that appeared to be basophil granules were shown to metachromatically label with low concentrations of acridine orange, which has little or no effect on histamine release. During stimulation these labeled granules were lost, leading to decreased fluorescence. Changes in either the forward scatter parameter or acridine orange labeling occurred on the same time scale as histamine release, differentiating these measures of the basophil response from early signal transduction events. Challenging basophils with a combination of phorbol myristate acetate and ionomycin caused 100% histamine release and allowed a measurement of the maximum change in forward scatter intensity or loss of acridine orange labeling. The flow cytometric distributions after this treatment were then compared with the distributions obtained by challenging cells with several concentrations of anti-IgE antibody or formylmethionine-leucine-phenylalanine, which induced various submaximal responses. These flow cytometric distributions demonstrated that single cells could be found in intermediate states of activation, i.e., the response of all cells was graded according to the strength of the stimulus. These studies lead to the general conclusion that all aspects of the basophil response, including those late events in the basophil response we have studied here, as well as early events that we have studied previously, are graded in a continuous manner, according to the magnitude of the stimulus.