Elevated Procoagulant Microparticles Expressing Endothelial and Platelet Markers in Essential Thrombocythemia

Background: Most cell types, including blood - and vascular cells, produce microparticles (MPs) upon activation. Since cellular MPs are known to be elevated in thromboembolic diseases, we hypothesized a role for MPs in the pathogenesis of thrombosis in Essential Thrombocythemia (ET).

Design and methods: In plasma samples from 21 ET patients and 10 healthy subjects, the levels and the cellular origin of MPs were determined by flowcytometric analysis, while the MP-associated procoagulant activity was measured by the thrombin generation assay.

Results: ET patients had significantly higher numbers of circulating AnnexinV-positive MPs than controls (median 4500 vs 2500×10⁶ events/L; p=0.039), including significantly higher number of MPs positive for the platelet marker CD61 (median 4000 vs 2400×10⁶/L; p=0.043) the endothelial marker CD62E (median 875 vs 14×10⁶/L; p=0.009), and for Tissue Factor (median 1.8 vs 0.9×10⁶/L; p=0.036). CD62E was co-expressed with the platelet marker CD41 on MPs, suggesting a bilineal origin of such MPs, that were observed only in patients with risk factors for thrombosis. ET patients had higher plasma mature von Willebrand factor (vWF) levels (median 50 vs 35 nM, p=0.045) but similar propeptide levels (median 7 vs 5 nM, p=0,07) compared to controls, indicating chronic endothelial activation. In thrombin generation analyses, MP rich plasma from ET patients had a shorter lag time (9.7 min, 95%CI: 8.7–10.7 versus 15.9 min, 95%CI: 10.9–20.9, p=0.001) and higher peak height (215 nM, 95%CI: 189–241 versus 142 nM, 95%CI: 87–189, p=0.038) than from controls. Peak height correlated significantly with the total number of MPs (R=0.634, p<0.001).

Conclusions: ET patients showed higher number of circulating MPs with platelet and endothelial markers, suggesting ongoing platelet and endothelial activation. This is confirmed by an increased mature vWF level and an abnormal mature vWF/propeptide ratio, and a hypercoagulable state reflected in thrombin generation. These findings suggest a role for MPs in thrombosis in ET.