Platelet- and erythrocyte-derived microparticles trigger thrombin generation via factor XIIa

Interplay between coagulation and vascular inflammation in sickle cell disease

Sickle cell disease is the most common inherited haematological disorder that leads to the irreversible damage of multiple organs. Although sickling of red blood cells and vaso-occlusion are central to the pathophysiology of sickle cell disease, the importance of haemolytic anaemia and vasculopathy has been recently recognized. A hypercoagulable state is another prominent feature of sickle cell disease and is mediated by activation of both intrinsic and extrinsic coagulation pathways. Growing evidence demonstrates that coagulation may not only contribute to the thrombotic complications, but also to vascular inflammation associated with this disease. This article summarizes the role of vascular inflammation and coagulation activation, discusses potential mechanisms responsible for activation of coagulation and reviews recent data demonstrating the crosstalk between coagulation and vascular inflammation in sickle cell disease.

Is a decrease of microparticles related to improvement of hemostasis after FVIII injection in hemophilia A patients treated on demand?

BACKGROUND

Microparticles (MPs) are small membrane vesicles (0.1-1 μm) released from various cells after activation and/or apoptosis. There are limited data about their role in hemophilia A.

PATIENTS AND METHODS

Blood samples were taken before and 30 min after FVIII injection in 18 patients with severe hemophilia A treated on demand. Flow-cytometric determination of total MPs (TMPs) using lactadherin, platelet MPs (PMPs) (CD42a), endothelial MPs (EMPs) (CD144) and leukocyte MPs (LMPs) (CD45) was performed. The results were compared with data on endogenous thrombin potential (ETP), overall hemostatic potential (OHP), fibrin gel permeability and thrombin-activatable fibrinolysis inhibitor (TAFI).

RESULTS AND CONCLUSIONS

TMPs and PMPs decreased after treatment (to 1015 ± 221 [SEM] and 602 ± 134 × 10(6)  L(-1) ) in comparison with values before treatment (2373 ± 618 and 1316 ± 331; P < 0.01). EMPs also decreased after treatment (78 ± 12 vs. 107 ± 13; P < 0.05) while LMPs were not influenced. Both TMP and PMP counts were inversely correlated, moderately but statistically significantly, with data on OHP, ETP, fibrin network permeability and TAFI/TAFIi (P < 0.05 for all). EMP counts were correlated only with ETP (P < 0.05), while LMP counts did not show any correlation. TMP and PMP counts were also inversely correlated with FVIII levels (P < 0.05). TMP, PMP and EMP counts decreased after on-demand treatment with FVIII concentrate in hemophilia A patients. The decrease in circulating MPs, which were inversely correlated with hemostatic activation, may imply that MPs are incorporated in the hemostatic plug formed after FVIII substitution at the site of injury.

Elevated levels of thrombin-generating microparticles in stored red blood cells

BACKGROUND

During storage, red blood cells (RBCs) lose their membrane stability, leading to haemolysis and microparticle (MP) formation. The use of RBCs stored for more than 28 days has been associated with an increased incidence of deep vein thrombosis. However, the exact mechanism by which coagulation activation is enhanced in stored RBCs is still unknown.

OBJECTIVES

To investigate the relevant potential procoagulant activities of MPs and study the relative procoagulant factors for initiating the coagulation on MPs in stored RBCs.

STUDY DESIGN AND METHODS

MPs were isolated from the plasma of RBC units stored in citrate-phosphate-dextrose-adenine. At seven storage time-points (d0, d7, d14, d21, d28, d35 and d42), MPs were morphologically observed, quantified and analysed for tissue factor, factor XI (FXI) and their thrombin-generating potential.

RESULTS

MPs were observed using electron microscopy. The size of the MPs ranged from 0·272 μm to 0·973 μm in diameter. During the storage of RBCs in plastic bags, the MP concentration increased from 3389 ± 218/μl at day 0 to 61 586 ± 2237/μl at d42. Thrombin generation was dependent on the total number of MPs (r = 0·987). Anti-human FXI antibody inhibited thrombin concentrations by 50·3% compared with control plasma, whereas antitissue factor and antitissue factor pathway inhibitor failed to reduce thrombin concentrations.

CONCLUSIONS

Our study provides evidence that MP formation due to RBC storage might propagate coagulation not only by exposing phosphatidylserine, but also by initiating thrombin generation independently of tissue factor in a FXI -dependent manner.

Biomarkers of platelet activation in acute coronary syndromes

The most convincing evidence for the participation of platelets in arterial thrombosis in humans comes from studies of platelet activation in patients with acute coronary syndromes (ACS) and from trials of antiplatelet drugs. Both strongly support the concept that repeated episodes of platelet activation over the thrombogenic surface of a vulnerable plaque may contribute to the risk of death from coronary causes. However, the relation of in vivo platelet activation and adverse clinical events to results of platelet function tests remains largely unknown. A valuable marker of in vivo platelet activation should be specific, unaltered by pre-analytical artefacts and reproducibly measured by easily performed methods. This article describes current biomarkers of platelet activation in ACS, reviews their advantages and disadvantages, discusses their potential pitfalls, and demonstrates emerging data supporting the positive clinical implications of monitoring in vivo platelet activation in the setting of ACS.

The impact of blood coagulability on atherosclerosis and cardiovascular disease

Although the link between blood coagulation and atherogenesis has been long postulated, only recently, and through the extensive work on transgenic mice, crossbred on an atherogenic background, has the direction of this interaction become visible. In general, hypercoagulability in mice tends to increase atherosclerosis, whereas hypocoagulability reduces the atherosclerotic burden, depending on the mouse model used. The information on a direct relationship between coagulation and atherosclerosis in humans, however, is not that clear. Almost all coagulation proteins, including tissue factor, are found in atherosclerotic lesions in humans. In addition to producing local fibrin, a matrix for cell growth, serine proteases such as thrombin may be very important in cell signaling processes, acting through the activation of protease-activated receptors (PARs). Activation of PARs on vascular cells drives many complex processes involved in the development and progression of atherosclerosis, including inflammation, angiogenesis, and cell proliferation. Although current imaging techniques do not allow for a detailed analysis of atherosclerotic lesion phenotype, hypercoagulability, defined either by gene defects of coagulation proteins or elevated levels of circulating markers of activated coagulation, has been linked to atherosclerosis-related ischemic arterial disease. New, high-resolution imaging techniques and sensitive markers of activated coagulation are needed in order to study a causal contribution of hypercoagulability to the pathophysiology of atherosclerosis. Novel selective inhibitors of coagulation enzymes potentially have vascular effects, including inhibition of atherogenesis through attenuation of inflammatory pathways. Therefore, we propose that studying the long-term vascular side effects of this novel class of oral anticoagulants should become a clinical research priority.