Increased cell surface exposure of phosphatidylserine on propidium iodide negative thymocytes undergoing death by necrosis

Phosphatidylserine (PS) exposure on propidium iodide negative cells using F ITC labelled annexin-V has been used to quantify apoptosis in vitro and in vivo. Detection of PS within cells undergoing necrosis is also possible if labelled annexin-V specific for PS enters the cell following early membrane damage. Necrotic or late apoptotic cells can be excluded from flow cytomet ric analysis using propidium iodide which enters and stains cells with comp romised membrane integrity. Here we show that thymocytes undergoing death e xclusively by necrosis show early exposure of PS prior to loss of membrane integrity. This early exposure of PS occurs in cells treated with agents wh ich both raise intracellular calcium levels and are also capable of interac ting with protein thiol groups. We also demonstrate that PS exposure in thy mocytes induced to undergo apoptosis by three different agents does not cor relate with calcium rises but correlates with and precedes DNA fragmentatio n.