Oncosis is associated with exposure of phosphatidylserine residues on the outside layer of the plasma membrane: A reconsideration of the specificity of the annexin V/Propidium iodide assay

Background: Following a lethal injury, two modes of cell death can be disti nguished, apoptosis and primary necrosis. Cells pass through a prelethal st age characterized by a preservation of membrane integrity, in which they sh rink (apoptosis) or swell (oncosis, the early phase of primary necrosis). D uring apoptosis, a loss of phospholipid asym metry leads to exposure of pho sphatidylserine (PS) residues on the outer leaflet of the plasma membrane. We examined whether the external PS exposure, initially supposed to be spec ific for apoptosis, aas also observed in oncotic cells. Methods: Human peripheral lmphocytes, Jurkat T cells, U937 cells, or HeLa c ells were submitted to either apoptotic or oncotic stimuli. PS external exp osure was assessed after binding of FITC-conjugated annexin V as was the lo ss of membrane integrity after propidium iodide (PI) uptake. Morphological examination a as performed by optical or electron microscopy. Results: Similarly to apoptotic cells, oncotic cells expose external PS res idues while preserving membrane integrity. Consequently, oncotic cells exhi bit the annexin V+ PT phenotype, previously considered to be specific for a poptotic cells. Conclusions: This study concludes that the annexin V/PI assay does not disc riminate between apoptosis and oncosis and that it can be a useful tool to study oncosis by flow cytometry. Cytometry 44:65-72, 2001. (C) 2001 Wiley-L iss, Inc.