Background: In red blood cells (RBCs) anionic phospholipids, such as phosphatidylserine, are present in the inner leaflet of the membrane bilayer. Exposure of phosphatidylserine occurs during senescence and during long-term storage of RBCs and is considered as the tag for removal from the circulation by macrophages. Lactadherin is a phosphatidylserine-binding glycoprotein secreted by macrophages that promotes the engulfment of phosphatidylserine-expressing apoptotic lymphocytes. This study investigates the role of lactadherin in the phagocytosis of phosphatidylserine-expressing RBCs.
Study design and methods: Transbilayer movement of phosphatidylserine was induced in RBCs either by storage beyond 30 days or by treatment with calcium ionophore A23187 and N-ethylmaleimide. Phosphatidylserine-expressing RBCs were incubated with phorbol ester-stimulated THP-1, and phagocytosis was determined by measuring the pseudoperoxidase activity of hemoglobin. The in vivo clearance of phosphatidylserine-enriched RBCs was measured in lactadherin-deficient mice and in their littermate controls.
Results: Lactadherin promoted phagocytosis of phosphatidylserine-expressing RBCs by macrophages in a concentration-dependent manner. Splenic macrophages from lactadherin-deficient mice had diminished capacity to phagocytose phosphatidylserine-expressing RBCs. The life span of RBCs in lactadherin-deficient mice was similar to wild-type littermate controls in vivo. However, when an excess of phosphatidylserine-expressing RBCs were infused, there was only a mild impairment in the clearance in lactadherin-deficient mice compared to wild-type littermate controls.
Conclusion: These results show that clearance of phosphatidylserine-expressing RBCs is not diminished in a significant way in lactadherin-deficient mice under physiologic conditions and suggest the presence of redundant pathways.