Correlated flow cytometric analysis of terminal events in apoptosis reveals the absence of some changes in some model systems.

Many parallel processes occur during the final stages of apoptosis. It is not clear which of these processes occur in all or most models of apoptosis and which occur only in some. In addition, the temporal relationship of these events is not always well understood. Correlated flow cytometric measurements were used to address these questions. Several models of apoptosis were studied, including thymocytes treated with dexamethasone. MOLT-4 cells treated with etoposide, U937 cells treated with anti-Fas, HL-60 cells treated with camptothecin, Raji cells grown in low serum, and aged neutrophils. All models showed a decrease in LDS-751 and fluorescein diacetate (FDA) staining, an increase in staining with dihydrorhodamine 123 (dhR123) or dihydroethidium, and an acidification of the cytoplasm. In each model, these changes were highly correlated, appearing simultaneously as multiparameter measurements. Changes in membrane status detected with merocyanin 540 (MC540) and annexin V behaved differently. A population with LDS-751 and FDA changes but without annexin V or MC540 changes could be demonstrated in some models. Several models did not show any change in annexin V binding, and HL-60 did not show a change in MC540 binding during apoptosis. The loss of cell surface antigens (CD45 and CD16) from aged neutrophils occurred in the entire LDS-751 and FDA dim population, even though other membrane changes (including the appearance of annexin V binding sites) were only apparent in a subset of these cells. These results suggest a model for the ordering of some of the terminal processes in apoptosis, with annexin V and MC540 changes trailing other events in apoptosis. These results confirm the need for caution in using a single-parameter measurement as an indicator of apoptosis for any new model being studied.