Reversible phosphatidylserine expression on blood granulocytes related to membrane perturbation but not DNA strand breaks

Exposure of phosphatidylserine (PS) on the outer leaflet of the cell membrane is recognized as an early indicator of programmed cell death (apoptosis) in plant and mammalian cells. Currently, there is no literature describing that PS expression on the surface of white blood cells is reversible. We found that a hypotonic 0.2% NaCl or NH₄Cl lysing solution used to separate white blood cells from red blood cells induced a reversible PS expression on the cell surface of granulocytes and monocytes but not lymphocytes. This reversible PS expression was associated with change of plasma membrane potential but not degranulation-associated membrane mobilization or DNA fragmentation. In contrast, TNF-α induced an irreversible PS expression, associated with apoptotic DNA fragmentation shown on gel electrophoresis. The fact that hypotonic shock induced a reversible PS expression on granulocytes, and TNF-α induced an irreversible PS expression associated with apoptotic DNA fragmentation indicate the new insight that expression of PS on the outer cell surface does not always represent cell apoptosis. Also, the reversible PS expression was associated with altered plasma-membrane potential but not DNA strand breaks, indicating that early PS expression may be related to the membrane perturbation but not directly related to DNA fragmentation in certain types of cells.