Altered lipid packing identifies apoptotic thymocytes.

To test whether apoptotic thymocytes can be identified by altered packing of lipids in their plasma membranes, thymocytes were isolated from mice injected with hydrocortisone and stained with merocyanine 540 (MC540), a fluorescent probe sensitive to lipid packing. At 9-10 h after injection, a subpopulation of H2-Klo cells with increased MC540 staining was clearly discernable. When DNA degradation was assessed by staining fixed cells with propidium iodide (PI), the fraction of cells with increased MC540 staining corresponded to the fraction with reduced PI staining. In addition, enriching for the former by fluorescence-activated cell sorting enriched for the latter. These results indicate that living apoptotic thymocytes can be identified and separated on the basis of altered lipid packing and increased staining with MC540.