Elevated endothelial microparticles in thrombotic thrombocytopenic purpura: findings from brain and renal microvascular cell culture and patients with active disease

Endothelial injury is believed to be a key initiating event in the pathogen esis of thrombotic thrombocytopenic purpura (TTP), leading to platelet acti vation and formation of platelet-rich thrombi in microvasculature. However, the nature of endothelial injury in TTP is poorly defined and clinical ass ays to rapidly and reliably monitor endothelial damage are not readily avai lable. Using flow cytometry, we measured endothelial microparticles (EMPs) generated from cultured renal and brain microvascular endothelial cells (MV ECs) during activation and apoptosis, and evaluated the effect of TTP plasm a on them. EMPs were measured using positivity for monoclonal antibodies (m Abs) CD31 and CD51, and their procoagulant activity was assessed using a Ru ssell viper venom assay. Both cell lines generated procoagulant EMPs when c ultured with inducers of activation (tumour necrosis factor alpha; TNF-alph a) or apoptosis (mitomycin C). TTP plasma induced a five- to sixfold increa se of EMP generation and a two- to threefold increase of procoagulant activ ity in cultured brain and renal MVECs. TTP plasma induced a threefold and 1 3-fold increase of intercellular adhesion molecule-1 (ICAM-1) and vascular cell adhesion molecule-1 (VCAM-1) expression, respectively, on renal MVECs. Procoagulant activity tended to parallel EMP numbers. The effect of TTP pl asma on cell viability was similar to that of TNF-alpha, implying that it i nduced activation rather than apoptosis. Control plasma and idiopathic thro mbocytopenic purpura (ITP) plasma had little effect. In the clinical study, EMP assay of blood from acute TTP patients showed levels markedly elevated compared with normal controls, but values returned to normal in remission. In conclusion, TTP plasma activated and induced injury to MVECs in culture , judged by production of EMP and expression of activation markers. Release d procoagulant EMP may play a role in the pathogenesis of TTP. Assay of EMP may be a useful marker of disease activity and endothelial injury in TTP a nd possibly other thrombotic disorders.