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(Circulation. 1999;99:2577-2582.)
© 1999 American Heart Association, Inc.

Basic Science Reports

Platelet Microparticles Promote Platelet Interaction With Subendothelial Matrix in a Glycoprotein IIb/IIIa–Dependent Mechanism

Michael Merten, MD; Rajbabu Pakala, PhD; Perumal Thiagarajan, MD; Claude R. Benedict, MD

From the Department of Internal Medicine, Divisions of Hematology (P.T.) and Cardiology, University of Texas Houston Medical School.

Correspondence to Claude R. Benedict, MD, Department of Internal Medicine, Division of Cardiology, University of Texas Houston Medical School, 6431 Fannin, MSB 6.039, Houston, TX 77030.

Background—Platelets, on activation, release vesicular particles called platelet microparticles. Despite their procoagulant activity, their functional role in platelet–vessel wall interactions is not known.

Methods and Results—We examined the binding of microparticles to vessel wall components in vitro and in vivo. Microparticles bound to fibrinogen-, fibronectin-, and collagen-coated surfaces. Compared with activated platelets, we observed minimal binding of microparticles to vitronectin and von Willebrand factor. The glycoprotein IIb/IIIa (GP IIb/IIIa) inhibitors abciximab and eptifibatide (Integrilin) inhibited the binding to fibrinogen and fibronectin but had minimal effect on binding to collagen. Furthermore, monoclonal antibodies to GP Ib or anionic phospholipid-binding proteins (ß₂-glycoprotein I or annexin V) had no effect in these interactions. Microparticles did not bind to monolayers of resting or stimulated human umbilical vein endothelial cells (HUVECs), even in the presence of fibrinogen or von Willebrand factor. However, under similar conditions, microparticles bound to extracellular matrix produced by cultured HUVECs. Abciximab inhibited this interaction by 50%. In a rabbit model of arterial endothelial injury, the infusion of ⁵¹Cr-labeled microparticles resulted in a 3- to 5-fold increase of microparticle adhesion to the injured site compared with the uninjured site (P<0.05%). Furthermore, activated platelets bound to surface-immobilized microparticles in a GP IIb/IIIa–dependent mechanism. This binding increased in the presence of fibrinogen by 30%.

Conclusions—Platelet microparticles bind to subendothelial matrix in vitro and in vivo and can act as a substrate for further platelet binding. This interaction may play a significant role in platelet adhesion to the site of endothelial injury.

Key Words: platelets • microparticles • endothelium • glycoproteins

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