Sickle cell anemia, the most common serious hemoglobinopathy, is associated with a markedly reduced life span of red blood cells due to their preferential clearance by macrophages. During polymerization of sickle hemoglobin, phosphatidylserine, an anionic phospholipid normally present exclusively on the inner leaflet of the membrane bilayer is exteriorized to outer leaflet. This exposure of phosphatidylserine is thought to be a tag for macrophage recognition. Lactadherin, also known as milk fat globule-EGF factor 8, is a phosphatidylserine-binding glycoprotein secreted by macrophages that promotes the engulfment of apoptotic cells. Here, we investigated the role of lactadherin in the phagocytosis of sickle red blood cells.
The binding of fluorescein-lactadherin to normal and sickle red blood cells was studied by flow cytometry. We quantified the effect of lactadherin on phagocytosis of red blood cells by monocyte-derived macrophage.
In normal individuals, less than 0.5% of red blood cells showed any binding to lactadherin when analyzed by flow cytometry. However, in sickle cell patients, circulating red blood cells showed 2 to 10- fold increase in lactadherin binding (P<0.0002). Lactadherin stimulated the phagocytosis of resting sickle red blood cells by macrophages but had no significant effect on the phagocytosis of normal red blood cells. Deoxygenation of sickle red blood cells further increased the lactadherin binding and phagocytosis. Antibodies to integrin αVβ3 also inhibited macrophage binding and phagocytosis.
These results show lactadherin may play a major role in sickle red cell clearance by anchoring the phosphatidylserine-expressing sickle red blood cells to integrins on tissue macrophages.