Tissue Factor-Exposing Microparticles at the Site of Hemostasis Activation.

Introduction: Tissue factor exposing microparticles (TF⁺ MPs) are capable to activate hemostasis in vitro. The aim of this study was to investigate the impact of TF⁺ MPs during physiologic activation of hemostasis in vivo in human.

Methods: In a double-blind, cross-over study healthy male volunteers (n=13) were randomized to 7 days treatment with 100 mg aspirin and placebo, respectively. At the end of each treatment period TF⁺ MPs were determined in venous blood and in blood emerging from a skin incision reflecting hemostasis in an activated state (shed blood). TF⁺ MPs were analyzed by flow cytometry after staining with annexin V, a TF-antibody and cell-specific antibodies.

Results: Compared to venous blood, the median (interquartile range) number of TF⁺ MPs was significantly higher in shed blood: 16.2 (11.3 – 20.1) k/ml vs. 8.2 (5.9 – 14.8) k/ml (p = 0.004). TF⁺ MPs from platelets were higher in shed blood than in venous blood [4.7 (3.7 – 8.6) k/ml vs. 3.3 (2.0 – 5.0) k/ml, p = 0.03], whereas TF⁺ MPs from endothelial cells were not [1.7 (1.2 – 5.8) k/ml vs. 2.7 (1.7 – 4.5) k/ml, p = 0.9]. Aspirin did not significantly influence the level of TF-MPs both in venous blood and in shed blood.

Conclusion: High levels of TF⁺ MPs can be detected at the site of physiologic activation of hemostasis in vivo in human.