Background: The specific role of apoptosis in human atherosclerosis remains unknown. During apoptotic cell death, phosphatidylserine exposure on the cell surface confers a high tissue-factor (TF)-dependent procoagulant activity.
Methods and results: In this study, we examined the role of apoptotic cell death in the promotion of plaque thrombogenicity. TF expression and its relation to apoptosis was analyzed in 16 human atherosclerotic plaques by the use of immunohistochemical techniques. The presence of shed membrane apoptotic microparticles was analyzed in extracts from 6 human atherosclerotic plaques and 3 underlying arterial walls. The microparticles were captured by annexin V and their amounts estimated with respect to their phospholipid content by use of a prothrombinase assay. The prothrombogenic potential of the microparticles was further assessed by the measurement of total and microparticle-dependent TF activity in the extracts. The cell origin of the microparticles was determined after capture by specific antibodies. We were able to detect marked TF expression in the plaques in close proximity to apoptotic cells and debris, suggesting a potential interaction between TF and the apoptotic cell surfaces. High levels of shed membrane apoptotic microparticles were detected in extracts from atherosclerotic plaques but not in the underlying arterial walls (29.5+/-3.7 nmol/L phosphatidylserine equivalent versus 1.3+/-0.4 nmol/L, respectively, P<0.02). The microparticles were mainly of monocytic and lymphocytic origin and retained 97+/-2% of total TF activity, indicating a direct causal relationship between shed membrane microparticles and procoagulant activity of plaque extracts.
Conclusions: These results indicate that shed membrane microparticles with procoagulant potential are produced in human atherosclerotic plaques. Apoptosis could be a critical determinant of plaque thrombogenicity after plaque rupture.