Lactadherin and clearance of platelet-derived microvesicles

The transbilayer movement of phospha-tidylserine from the inner to the outer leaflet of the membrane bilayer during platelet activation is associated with the release of procoagulant phosphatidylserine-rich small membrane vesicles called platelet-derived microvesicles. We tested the effect of lactadherin, which promotes the phagocytosis of phosphatidylserine-expressing lymphocytes and red blood cells, in the clearance of platelet mi-crovesicles. Platelet-derived micro-vesicles were labeled with BODIPY-maleimide and incubated with THP-1-derived macrophages. The extent of phagocytosis was quantified by flow cytometry. Lactadherin promoted phagocytosis in a concentration-dependent manner with a half-maximal effect at approximately 5 ng/mL Lactadherin-deficient mice had increased number of platelet-derived microvesicles in their plasma compared with their wild-type littermates (950 ± 165 vs 4760 ± 650; P = .02) and generated 2-fold more thrombin. In addition, splenic macrophages from lactadherin-deficient mice showed decreased capacity to phagocy-tose platelet-derived microvesicles. In an in vivo model of light/dye-induced endothelial injury/thrombosis in the cremasteric venules, lactadherin-deficient mice had significantly shorter time for occlusion compared with their wild-type littermate controls (5.93 ± 0.43 minutes vs 9.80 ± 1.14 minutes;P = .01). These studies show that lactadherin mediates the clearance of phosphatidylserine-expressing platelet-derived microvesicles from the circulation and that a defective clearance can induce a hypercoagulable state.