Role of lactadherin in the clearance of phosphatidylserine-expressing red blood cells.
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.


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.


CONCLUSIONS
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.
Dasgupta
Swapan K
SK
Department of Pathology, Baylor College of Medicine, Houston, Texas, USA.
Abdel-Monem
Hanan
H
Guchhait
Prasenjit
P
Nagata
Shigekazu
S
Thiagarajan
Perumal
P
D000818:Animals; D017304:Annexin A5 / Q000032:analysis; D000954:Antigens, Surface / Q000097:blood / Q000502:physiology*; D017209:Apoptosis; D001793:Blood Preservation; D000001:Calcimycin / Q000494:pharmacology; D002122:Calcium Chloride / Q000494:pharmacology; D002460:Cell Line; D004492:Edetic Acid / Q000494:pharmacology; D004912:Erythrocytes / Q000187:drug effects*; D005033:Ethylmaleimide / Q000494:pharmacology; D016650:Fluorescein-5-isothiocyanate / Q000031:analogs & derivatives / Q000032:analysis; D006801:Humans; D008262:Macrophage Activation; D051379:Mice; D018345:Mice, Knockout; D008894:Milk Proteins / Q000097:blood; D010587:Phagocytosis; D010718:Phosphatidylserines / Q000097:blood; D013997:Time Factors
D017304:Annexin A5; D000954:Antigens, Surface; C492760:FITC-annexin A5; C098942:MFGE8 protein, human; C490919:Mfge8 protein, mouse; D008894:Milk Proteins; D010718:Phosphatidylserines; D000001:Calcimycin; D004492:Edetic Acid; D016650:Fluorescein-5-isothiocyanate; D002122:Calcium Chloride; D005033:Ethylmaleimide