Differential contributions of monocyte- and platelet-derived microparticles towards thrombin generation and fibrin formation and stability

Background: Microparticles (MPs) are sub-micron vesicles shed by activated or apoptotic cells, including platelets and monocytes. Increased circulating MPs are associated with thrombosis; however, their role in thrombogenesis is poorly understood. Objective: To determine how MPs promote thrombin generation and modulate fibrin density and stability. Methods: Platelets and monocytes were isolated from healthy donors. Platelets were stimulated with calcium ionophore, thrombin receptor agonist peptide (TRAP) or TRAP/convulxin. Monocytes and human monocytic THP-1 cells were stimulated with lipopolysaccharide (LPS). MPs were isolated, washed by high-speed centrifugation and assessed using the following: transmission electron microscopy (TEM), Nanoparticle Tracking Analysis (NTA), flow cytometry, tissue factor (TF) activity, prothrombinase activity, thrombin generation, and clot formation, density and stability. Results: MPs from monocytes (M-MPs) and platelets (PMPs) had similar shapes and diameters (100-300 nm). M-MPs had TF activity (16.7 +/- 2.4 pM TF per 10(6) MP), supported prothrombinase activity and triggered shorter thrombin generation lag times than buffer controls (5.4 +/- 0.5 vs. 84.2 +/- 4.8 min, respectively). Compared with controls, M-MPs supported faster fibrin formation (0.24 +/- 0.24 vs. 76.7 +/- 15.1 mOD min(-1), respectively), 38% higher fibrin network density and higher clot stability (3.8-fold higher turbidity in the presence of tissue plasminogen activator). In contrast, PMPs did not have TF activity and supported 2.8-fold lower prothrombinase activity than M-MPs. PMPs supported contact-dependent thrombin generation, but did not independently increase fibrin network density or stability. Interestingly, PMPs increased rates of thrombin generation and fibrin formation (1.7- and 1.3-fold, respectively) when mixed with THP-1-derived MPs. Conclusion: MPs from platelets and monocytes differentially modulate clot formation, structure and stability, suggesting unique contributions to thrombosis.