Apoptotic Microparticles Derived from Endothelial Cells, Smooth Muscle Cells and Monocytes Induce Thrombin Generation Via Different Pathways.

Apoptotic microparticles (MPs) have been shown to be produced in atherosclerotic plaques and could be involved in the development of atherothrombosis. In the present study, we compared the procoagulant activities of apoptotic MPs derived from cultured endothelial cells (ECs), smooth muscle cells (SMCs), and monocytes, the major constituents of the plaque formation. Apoptotic MPs were generated from ECs, SMCs, monocytes and monocytic cell line, U937 cells, collected and purified by differential centrifugation. MPs were analyzed for the expression of phosphatidylserine and tissue factor (TF). Procoagulant activities were measured using a novel thrombin generation assay (TECHNOTHROMBIN® TGA) with normal and factor deficient plasmas. All MPs derived from ECs, SMCs, and U937 cells bound with annexin V, expressed TF, and induced thrombin generation (TG) dose dependently, indicating the thormbogenic properties. Among these MPs, SMC-MPs and EC-MPs showed the higher levels of TG activities than those of monocytic MPs. When antibodies against TF were used to inhibit TG generated by MPs, maximum level of TG of monocytic MPs was quenched dose dependently, and that of SMC-MPs was partially inhibited, but that of EC-MPs was not inhibited. To further investigate this phenomenon, factor VII, VIII, IX, XI and XII deficient plasmas were used. Although the initiation of TG induced by all MPs was dependent on factor VII, the amplification mechanisms of TG were different among these MPs. EC-MP induced amplification of TG was dependent on factors VIII, IX, XI and XII, and not dependent on factor VII. SMC-MPs and monocytic MPs induced TG dependently on an amplification loop including factor VII, and independently of factors VIII, IX, XI and XII. From these data we conclude that, apoptotic MPs derived from cultured ECs, SMCs and monocytes could induce TG via different coagulant pathways and could critically contribute to the process leading to thrombus formation after plaque rapture.