In vivo phagocytosis of senescent red blood cells (RBCs) by macrophages occurs 120 days after their release into the circulation. It depends on two sequential signals that trigger phagocytosis: (1) desialylation of membrane glycoconjugates with the exposure of the penultimate beta-galactosyl residues and (2) exposure of phosphatidylserine in the membrane outer leaflet. Leukodepleted and nonleukodepleted RBCs were compared using flow cytometric procedures to determine whether the in vitro deterioration of RBCs during storage might be attributable to an identical mechanism of desialylation induced by leukocyte neuraminidases, resulting in exposure of beta-galactosyl and subsequently phosphatidylserine residues - signals of senescent RBCs.
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