The recognition of red blood cells by macrophages: role of phosphatidylserine and possible implications of membrane phospholipid asymmetry.

The recognition of phosphatidylserine (PS) by macrophages was investigated using inside-out (IO) red blood cell (RBC) ghosts and RBC displaying PS in their surface membranes. This was accomplished by employing unmodified pathologic sickle RBC which contain endogenous PS in their outer membrane leaflet, and RBC modified by the transfer of an exogenous fluorescent PS analog. Proper insertion of exogenous PS was confirmed by monitoring the degree to which cell-associated lipid fluorescence was dequenched following transfer of 1-acyl-2-[(N-4-nitro-benzo-2-oxa-1,3 diazole) aminocaproyl] phosphatidylserine (NBD-PS) from a population of self-quenched donor vesicles. Inside-out RBC ghosts were endocytosed approximately 3 times faster than were right side-out control populations. Similarly, using NBD-PS vesicles at concentrations at which dilution of all the cell-associated analog in the recipient RBC could be unequivocally confirmed, we observed that the uptake of NBD-PS treated RBC by macrophages was significantly increased over that of control RBC populations. Fluorescence and electron microscopic observations revealed the formation of typical RBC-macrophage rosettes that were morphologically distinct from opsonized RBC-macrophage rosettes. Enhanced RBC binding to macrophages was also obtained with deoxygenated reversibly sickled cells (RSC); the enhancement correlated with increased exposure of outer leaflet PS in these cells. These findings suggest that PS is recognized by macrophages and that its exposure in the outer leaflet of RBC may have significant pathophysiologic implications.