MEMBRANE FLUIDITY INCREASES DURING APOPTOSIS OF SHEEP ILEAL PEYERS PATCH B-CELLS

To investigate specific plasma membrane structural changes associated with apoptosis, whole cells and purified plasma membranes of apoptotic B cells from the ileal Peyer's patch of sheep were analyzed for their ''membrane fluidity.'' The ileal Peyer's patch of sheep provided a la rge number of B cells required for plasma membrane isolation (>5 x 10( 9)). As the incidence of apoptosis increased with time of culture, the fluidity of purified plasma membranes, as measured with the fluoropho re DPH (diphenylhexatriene), increased. To evaluate this phenomenon wi th intact cells, B cells at different apoptotic stages were fractionat ed on discontinuous Percoll gradients. Similar results were obtained u sing the fluorophore TMA-DPH (trimethylammoniumdiphenylhexatriene), wh ich has been shown to localize specifically to the plasma membrane. Fu nctionally, the increase in plasma membrane fluidity associated with a poptosis may represent either a mechanism to cycle phosphatidylserine to the outer leaflet, mediating phagocytic recognition of apoptotic ce lls, or a consequence of this event.