Although it has been reported that activated platelets can adhere to intact endothelium, the receptors involved have not been fully characterized. Also, it is not clear whether activated platelets bind primarily to matrix proteins at sites of endothelial cell denudation or directly to endothelial cells. Thus, this study was designed to further clarify the mechanisms of activated platelet adhesion to endothelium. Unstimulated human umbilical vein endothelial cell (HUVEC) monolayers were incubated with washed, stained, and thrombin-activated human platelets. To exclude matrix involvement, HUVEC were harvested mechanically and platelet binding was measured by flow cytometry. Before the adhesion assay, platelets or HUVEC were treated with different receptor antagonists. Whereas blockade of platelet β1 integrins, GPIbα, GPIV, P-selectin, and platelet-endothelial cell adhesion molecule (PECAM)-1 did not reduce platelet adhesion to HUVEC, blockade of platelet GPIIbIIIa by antibodies or Arg-Gly-Asp (RGD) peptides markedly decreased adhesion. Moreover, when platelets were treated with blocking antibodies to GPIIbIIIa-binding adhesive proteins, including fibrinogen and fibronectin, and von Willebrand factor (vWF), platelet binding was also reduced markedly. Addition of fibrinogen, fibronectin, or vWF further increased platelet adhesion, indicating that both endogenous platelet-exposed and exogenous adhesive proteins can participate in the binding process. Evaluation of the HUVEC receptors revealed predominant involvement of intercellular adhesion molecule (ICAM)-1 and αvβ3 integrin. Blockade of these two receptors by antibodies decreased platelet binding significantly. Also, there was evidence that a component of platelet adhesion was mediated by endothelial GPIbα. Blockade of β1 integrins, E-selectin, P-selectin, PECAM-1, vascular cell adhesion molecule (VCAM)-1 and different matrix proteins on HUVEC did not affect platelet adhesion. In conclusion, we show that activated platelet binding to HUVEC monolayers is mediated by a GPIIbIIIa-dependent bridging mechanism involving platelet-bound adhesive proteins and the endothelial cell receptors ICAM-1, αvβ3 integrin, and, to a lesser extent, GPIbα.

Whereas unperturbed endothelial cells provide potent anticoagulant properties, exposure to inflammatory and atherogenic stimuli can rapidly lead to a procoagulant behavior. Because recent studies provide evidence that apoptosis of vascular cells may occur under conditions such as atherosclerosis and inflammation, we investigated whether apoptotic endothelial cells may contribute to the development of a prothrombotic state. In this report, it is shown that both adherent and detached apoptotic human umbilical vein endothelial cells (HUVECs) become procoagulant. Apoptosis was induced by staurosporine, a nonspecific protein kinase inhibitor, or by culture in suspension with serum deprivation. Both methods resulted in similar findings. As assessed by flow cytometric determination of annexin V binding, HUVECs undergoing cell death exhibited typically a more rapid exposure of membrane phosphatidylserine (PS) than DNA fragmentation. Depending on the stage of apoptosis, this redistribution of phospholipids was found to induce an increase of the activity of the intrinsic tenase complex by 25% to 60%. Although apoptotic cells did not show antigenic or functional tissue factor (TF ) activity, when preactivated with lipopolysaccharide, TF procoagulant activity increased by 50% to 70%. At 8 hours after apoptosis induction, antigenic thrombomodulin, heparan sulfates, and TF pathway inhibitor decreased by about 83%, 80%, and 59%, respectively. The functional activity of these components was reduced by about 36%, 52%, and 39%, respectively. Moreover, the presence of apoptotic HUVECs led to a significant increase of thrombin formation in recalcified citrated plasma. In conclusion, apoptotic HUVECs, either adherent or in suspension, become procoagulant by increased expression of PS and the loss of anticoagulant membrane components.

Analysis of native blood yields the most reliable TEG results. Should immediate TEG processing not be possible, citrated blood may be used if recalcified after 1-8 h.

No abstract

The prevalence of hereditary thrombophilia is well known in patients with lower‐extremity thrombosis but only poorly studied in patients with thrombosis at unusual sites. Consequently, it is still unclear whether such patients should generally be screened for hereditary thrombophilia. We retrospectively analyzed 260 patients with thrombosis at unusual sites including thrombosis in portal, cerebral, retinal, and upper‐extremity veins with respect to the prevalence of FV Leiden, prothrombin G20210A, protein C, protein S, and antithrombin deficiency. In addition, all thrombotic episodes were analyzed for circumstantial risk factors. Used as controls, healthy volunteers (120) and patients with lower‐extremity thrombosis (292) showed overall prevalence of hereditary thrombophilia of 9.1% and 39.0%, respectively. The corresponding numbers were 33.3%, 34.3%, and 39.0% in patients with portal vein, upper‐extremity, and lower‐extremity thrombosis, respectively. In patients with cerebral vein thrombosis, however, the prevalence was significantly lower (23.5%). Patients with retinal vein occlusion did not show an increased frequency of thrombophilia at all (5.9%). In all five groups FV Leiden was by far the most frequent defect (4.4–27.1%), while prothrombin G20210A occurred rarer (2.5–7.6%). Protein C, protein S, and antithrombin deficiency were much less prevalent (0–3.1%) except for patients with portal vein thrombosis (4.8–7.1%). Compared to healthy individuals, the relative risk of thrombosis was 4.3 (2.2–8.1), 3.8 (1.8–7.7), 2.5 (1.0–6.1), 3.7 (1.5–8.6), and 0.6 (0.2–2.1) for patients with lower‐extremity, upper‐extremity, cerebral vein, portal vein, and retinal vein thrombosis, respectively. Circumstantial risk factors were more frequent in patients without than with hereditary thrombophilia and were found most often in patients with upper‐extremity thrombosis. In each group the most frequent circumstantial risk factor was different. However, oral contraceptives and cancer were found in all five groups. In conclusion, independent upon the presence of circumstantial risk factors, screening for hereditary thrombophilia is warranted in all patients with thrombosis at unusual sites except in those with retinal vein occlusion. Am. J. Hematol. 70:126–132, 2002. © 2002 Wiley‐Liss, Inc.