PHOSPHATIDYLSERINE EXPOSURE AND RED-CELL VIABILITY IN RED-CELL AGING AND IN HEMOLYTIC-ANEMIA

Phosphatidylserine (PS) normally localizes to the inner leaflet of cel l membranes but becomes exposed in abnormal or apoptotic cells, signal ing macrophages to ingest them. Along similar lines, it seemed possibl e that the removal of red cells from circulation because of normal agi ng or in hemolytic anemias might be triggered by PS exposure. To inves tigate the role of PS exposure in normal red cell aging, we used N-hyd roxysuccinimide-biotin to tag rabbit red cells in vivo, then used phyc oerythrin-streptavidin to label the biotinylated cells, and annexin V- fluorescein isothiocyanate (FITC) to detect the exposed PS. Flow cytom etric analysis. of these cells drawn at 10-day intervals up to 70 days after biotinylation indicated that older, biotinylated cells expose m ore PS. Furthermore, our data match a simple model of red cell senesce nce that assumes both an age-dependent destruction of senescent red ce lls preceded by several hours of PS exposure and a random destruction of red cells without PS exposure. By using this model, we demonstrated that the exposure of PS parallels the rate at which biotinylated red cells are removed from circulation. On the other hand, using an annexi n V-FITC label and flow cytometry demonstrates that exposed PS does no t cause the reduced red cell life span of patients with hemolytic anem ia, with the possible exception of those with unstable hemoglobins or sickle cell anemia. Thus, in some cases PS exposure on the cell surfac e may signal the removal of red cells from circulation, but in other c ases some other signal must trigger the sequestration of cells.