Generation of Tissue factor-rich microparticles in an ex vivo whole blood model

Sensitive and Specific Detection of Platelet-Derived and Tissue Factor-Positive Extracellular Vesicles in Plasma Using Solid-Phase Proximity Ligation Assay

Extracellular vesicles (EVs) derived from blood cells are promising biomarkers for various diseases. However, they are difficult to measure accurately in plasma due to their small size. Here, we demonstrate that platelet-derived EVs in plasma can be measured using solid-phase proximity ligation assay with high sensitivity and specificity using very small sample volume of biological materials. The results correlate well with high-sensitivity flow cytometry with the difference that the smallest EVs are detected. Briefly, the EVs are first captured on a solid phase, using lactadherin binding, and detection requires recognition with two antibodies followed by qPCR. The assay, using cholera toxin subunit-B or lactadherin as capture agents, also allowed detection of the more rare population of tissue factor (TF)-positive EVs at a concentration similar to sensitive TF activity assays. Thus, this assay can detect different types of EVs with high specificity and sensitivity, and has the potential to be an attractive alternative to flow cytometric analysis of preclinical and clinical samples. Improved techniques for measuring EVs in plasma will hopefully contribute to the understanding of their role in several diseases.

Peripheral blood microvesicles secretion is influenced by storage time, temperature, and anticoagulants

Microvesicles (MVs) are small membrane bound vesicles released from various cell types after activation or apoptosis. In the last decades, MVs received an increased interest as biomarkers in inflammation, coagulation and cancer. However, standardized pre-analytical steps are crucial for the minimization of artifacts in the MV analysis. Thus, this study evaluated the MV release in whole blood samples under the influence of different anticoagulants, storage time and various temperature conditions. Samples were collected from healthy probands and processed immediately, after 4, 8, 24 and 48 hours at room temperature (RT) or 4°C. To identify MV subpopulations, platelet free plasma (PFP) was stained with Annexin V, calcein AM, CD15, CD41 and CD235a. Analysis was performend on a CytoFLEX flow cytometer. Procoagulatory function of MVs was measured using a phospholipid dependent activity and a tissue factor MVactivity assay. Without prior storage, sodium citrate showed the lowest MV count compared to heparin and EDTA. Interestingly, EDTA showed a significant release of myeloid-derived MVs (MMVs) compared to sodium citrate. Sodium citrate showed a stable MV count at RT in the first 8 hours after blood collection. Total MV counts increased after 24 hours in sodium citrated or heparinzed blood which was related to all subpopulations. Interestingly, EDTA showed stable platelet-derived MV (PMV) and erythrocyte-derived MV (EryMV) count at RT over a 48 h period. In addition, the procoagulatory potential increased significantly after 8-hour storage. Based on both, this work and literature data, the used anticoagulant, storage time and storage temperature differently influence the analysis of MVs within 8 hours. To date, sodium citrated tubes are recommended for MV enumeration and functional analysis. EDTA tubes might be an option for the clinical routine due to stable PMV and EryMV counts. These new approaches need to be validated in a clinical laboratory setting before being applied to patient studies. © 2016 International Society for Advancement of Cytometry.

Circulating cell-derived microparticles as biomarkers in cardiovascular disease

Cardiovascular diseases (CVDs) are a common cause of death, and a search for biomarkers for risk stratification is warranted. Elevated levels of cell-derived microparticles (MPs) are found in patients with CVD and in groups with risk factors for CVD. Subpopulations of MPs are promising biomarkers for improving risk prediction, as well as monitoring treatment. However, the field has been hampered by technical difficulties, and the ongoing development of sensitive standardized techniques is crucial for implementing MP analyses in the clinic. Large prospective studies are required to establish which MPs are of prognostic value in different patient groups. In this review, we discuss methodological challenges and progress in the field, as well as MP populations that are of interest for further clinical evaluation.

Analysis of the potential of cancer cell lines to release tissue factor-containing microvesicles: correlation with tissue factor and PAR2 expression

BACKGROUND

Despite the association of cancer-derived circulating tissue factor (TF)-containing microvesicles and hypercoagulable state, correlations with the incidence of thrombosis remain unclear.

METHODS

In this study the upregulation of TF release upon activation of various cancer cell lines, and the correlation with TF and PAR2 expression and/or activity was examined. Microvesicle release was induced by PAR2 activation in seventeen cell lines and released microvesicle density, microvesicle-associated TF activity, and phoshpatidylserine-mediated activity were measured. The time-course for TF release was monitored over 90 min in each cell line. In addition, TF mRNA expression, cellular TF protein and cell-surface TF activities were quantified. Moreover, the relative expression of PAR2 mRNA and cellular protein were analysed. Any correlations between the above parameters were examined by determining the Pearson's correlation coefficients.

RESULTS

TF release as microvesicles peaked between 30-60 min post-activation in the majority of cell lines tested. The magnitude of the maximal TF release positively correlated with TF mRNA (c = 0.717; p < 0.001) and PAR2 mRNA (c = 0.770; p < 0.001) expressions while the percentage increase correlated with PAR2 mRNA (c = 0.601; p = 0.011) and protein (c = 0.714; p < 0.001). There was only a weak correlation between resting TF release, and microvesicle release. However, TF release in resting cells did not significantly correlate with any of the parameters examined. Furthermore, TF mRNA expression correlated with PAR2 mRNA expression (c = 0.745; p < 0.001).

DISCUSSION AND CONCLUSIONS

In conclusion, our data suggest that TF and PAR2 mRNA, and PAR2 protein are better indicators of the ability of cancer cells to release TF and may constitute more accurate predictors of risk of thrombosis.

Impact of collection, isolation and storage methodology of circulating microvesicles on flow cytometric analysis

Microvesicles (MVs) in body fluids participate in a variety of physical and pathological processes, and are regarded as potential biomarkers for numerous diseases. Flow cytometry (FCM) is among the most frequently used techniques for MV detection. However, different handling methods unavoidably cause pre-analytical variations in the counts and sizes of MVs determined by FCM. The aim of the present study was to investigate the effect of centrifugation, storage conditions and anticoagulant on MV measurements. Blood samples were obtained from 13 healthy donors, including 4 women and 9 men. Calcein-AM staining was used to label MVs and assess the impact of pre-analytical preparation, including centrifugation, and storage conditions on MV measurements obtained using FCM. The range of factors investigated for comparison included: Platelet-free plasma (PFP) stored at −80°C for 1 or 4 weeks; MVs stored at 4°C for 3–4 days or 1 week; MVs frozen at −80°C for 1 or 4 weeks; and anticoagulants, either heparin or ethylenediaminetetraacetic acid (EDTA). No statistically significant differences in MV counts were detected between the two centrifugation speeds (16,000 and 20,500 × g) or among the three centrifugation times (15, 30 and 60 min) investigated. Similarly, no significant differences were noted in MV counts between the two anticoagulants tested (heparin and EDTA). However, the storage of PFP or MVs in heparin-anticoagulated plasma for different periods markedly affected the detected MV counts and size distribution. The counts and sizes of MVs from EDTA-anticoagulated plasma were only affected when the MVs were frozen at −80°C for 4 weeks. In conclusion, calcein-AM is able to efficiently identify MVs from plasma and may be an alternative to Annexin V for MV staining. EDTA preserves the MV counts and size more accurately compared with heparin under calcein-AM staining. PFP centrifuged at 16,000 × g for 15 min is sufficient to isolate MVs, which enables the batch processing of samples. PFP, rather than MVs alone, appears to be the preferable mode of sample storage, as MVs stored in PFP were less affected by storage temperature and duration. The present study provides a methodology for MV collection, storage and isolation, to facilitate further investigation of MVs as biomarkers in disease.

Tissue factor structure and function

Tissue factor (TF) is an integral membrane protein that is essential to life. It is a component of the factor VIIa-TF complex enzyme and plays a primary role in both normal hemostasis and thrombosis. With a vascular injury, TF becomes exposed to blood and binds plasma factor VIIa, and the resulting complex initiates a series of enzymatic reactions leading to clot formation and vascular sealing. Many cells, both healthy, and tumor cells, produce detectable amounts of TF, especially when they are stimulated by various agents. Despite the relative simplicity and small size of TF, there are numerous contradictory reports about the synthesis and presentation of TF on blood cells and circulation in normal blood either on microparticles or as a soluble protein. Another subject of controversy is related to the structure/function of TF. It has been almost commonly accepted that cell-surface-associated TF has low (if any) activity, that is, is "encrypted" and requires specific conditions/reagents to become active, that is, "decrypted." However there is a lack of agreement related to the mechanism and processes leading to alterations in TF function. In this paper TF structure, presentation, and function, and controversies concerning these features are discussed.

Microparticles and acute lung injury

The pathophysiology of acute lung injury (ALI) and its most severe form, acute respiratory distress syndrome (ARDS), is characterized by increased vascular and epithelial permeability, hypercoagulation and hypofibrinolysis, inflammation, and immune modulation. These detrimental changes are orchestrated by cross talk between a complex network of cells, mediators, and signaling pathways. A rapidly growing number of studies have reported the appearance of distinct populations of microparticles (MPs) in both the vascular and alveolar compartments in animal models of ALI/ARDS or respective patient populations, where they may serve as diagnostic and prognostic biomarkers. MPs are small cytosolic vesicles with an intact lipid bilayer that can be released by a variety of vascular, parenchymal, or blood cells and that contain membrane and cytosolic proteins, organelles, lipids, and RNA supplied from and characteristic for their respective parental cells. Owing to this endowment, MPs can effectively interact with other cell types via fusion, receptor-mediated interaction, uptake, or mediator release, thereby acting as intrinsic stimulators, modulators, or even attenuators in a variety of disease processes. This review summarizes current knowledge on the formation and potential functional role of different MPs in inflammatory diseases with a specific focus on ALI/ARDS. ALI has been associated with the formation of MPs from such diverse cellular origins as platelets, neutrophils, monocytes, lymphocytes, red blood cells, and endothelial and epithelial cells. Because of their considerable heterogeneity in terms of origin and functional properties, MPs may contribute via both harmful and beneficial effects to the characteristic pathological features of ALI/ARDS. A better understanding of the formation, function, and relevance of MPs may give rise to new promising therapeutic strategies to modulate coagulation, inflammation, endothelial function, and permeability either through removal or inhibition of "detrimental" MPs or through administration or stimulation of "favorable" MPs.

Tissue factor/TFPI and blood cells

Vascular injury-induced access of blood to tissue factor (TF) leads to the formation of a TF-FVII/FVIIa complex and the triggering of blood coagulation. The activated TF-dependent pathway is regulated by Tissue Factor Pathway Inhibitor (TFPI), which binds and inhibits FXa, but more importantly forms an inactive quaternary complex with TF-FVIIa-FXa, effectively shutting off the TF activity. The old view of TF residing in extravascular sites exclusively has recently been challenged by several reports on TF expression in various blood cells. The latter arena has unfortunately been marred by many contradictions, apparently related to inferior tools and/or study design, notably the widespread use of antibodies with inferior and misleading specificity and TF activity assays of low sensitivity/specificity. Our own studies along with many other reports, compels the conclusion that in blood of healthy individuals TF is exclusively associated with and expressed in circulating monocytes. In this short review the distribution of TF and TFPI in blood is discussed.

Cardiovascular complications of diabetes mellitus: The Tissue Factor perspective

Heightened activity of circulating Tissue Factor (TF) has been linked to a variety of macro- and microvascular cardiovascular complications commonly observed in diabetes mellitus. Systemic and localized vascular abnormalities comprise the most debilitating feature of diabetic pathophysiology. Blood monocytes are chronically activated in diabetes, and serve as the major source of bioactive intravascular TF. This review examines recent literature on this subject, with a special emphasis on the abnormal monocyte physiology in diabetes and the structural and functional diversity of circulating TF.

Platelet tissue factor: how did it get there and is it important?

Recently, the presence of functionally active tissue factor (TF) in platelets has been reported by several groups. In this location, TF is postulated to play an important role in the propagation phase of thrombus formation. Although the existence of platelet TF still remains controversial to some extent, a review of the current literature proposes at least three distinct sources of "platelet-associated TF" in those laboratories that have reported its presence: (1) TF that is taken up in the form of circulating microparticles, usually derived from monocytes; (2) TF stored in the alpha-granules of platelets that may have been taken up and/or endogenously synthesized; and (3) TF that is synthesized and expressed on the plasma membrane of mature platelets. These pathways are not mutually exclusive, and the dominant mechanism may depend on the state of platelet activation and, possibly, on other host factors that differ in physiological hemostasis versus pathological thrombosis. This brief review will summarize the state-of-the-art understanding on the origins and possible role of platelet TF.

Activation of coagulation with concurrent impairment of anticoagulant mechanisms correlates with a poor outcome in severe melioidosis

BACKGROUND

Melioidosis, which is caused by infection with the Gram-negative bacterium Burkholderia pseudomallei, is an important cause of sepsis in South-East Asia with a mortality of up to 40%. Knowledge of the involvement of coagulation and fibrinolysis in the pathogenesis of melioidosis is highly limited.

OBJECTIVE

To define the involvement of the coagulation and fibrinolytic systems in patients with severe melioidosis.

METHODS

Parameters of coagulation and fibrinolysis were measured in 34 patients with culture proven septic melioidosis and 32 healthy controls.

RESULTS

Patients demonstrated strong activation of the coagulation system, as reflected by high plasma levels of soluble tissue factor, the prothrombin fragment F(1+2) and thrombin-antithrombin complexes (TATc), and consumption of coagulation factors resulting in a prolonged prothrombin time and activated partial thromboplastin time. Concurrently, anticoagulant pathways were downregulated in patients: protein C, protein S, and antithrombin levels were all decreased when compared to controls. Patients also demonstrated evidence of activation and inhibition of fibrinolysis, as reflected by elevated concentrations of tissue-type plasminogen activator (tPA), plasminogen activator inhibitor type 1, plasmin-alpha2-antiplasmin complexes (PAPc) and D-dimer. High TATc/PAPc ratios in patients pointed to a predominance of the prothrombotic pathway in melioidosis. Furthermore, soluble thrombomodulin levels were increased. The extent of coagulation activation correlated with mortality; patients who went on to die had higher TATc, F(1+2), tPA and PAPc and lower protein C and antithrombin levels on admission than patients who survived.

CONCLUSIONS

The coagulation system is strongly activated during melioidosis. A high degree of activation of the coagulation system is an indicator of poor outcome in patients with melioidosis.

Procoagulant Microparticles

Apoptosis and vascular cell activation are main contributors to the release of procoagulant microparticles (MPs), deleterious partners in atherothrombosis. Elevated levels of circulating platelet, monocyte, or endothelial-derived MPs are associated with most of the cardiovascular risk factors and appear indicative of poor clinical outcome. In addition to being a valuable hallmark of vascular cell damage, MPs are at the crossroad of atherothrombosis processes by exerting direct effects on vascular or blood cells. Under pathological circumstances, circulating MPs would support cellular cross-talk leading to vascular inflammation and tissue remodeling, endothelial dysfunction, leukocyte adhesion, and stimulation. Exposed membrane phosphatidylserine and functional tissue factor (TF) are 2 procoagulant entities conveyed by circulating MPs. At sites of vascular injury, P-selectin exposure by activated endothelial cells or platelets leads to the rapid recruitment of MPs bearing the P-selectin glycoprotein ligand-1 and blood-borne TF, thereby triggering coagulation. Within the atherosclerotic plaque, sequestered MPs constitute the main reservoir of TF activity, promoting coagulation after plaque erosion or rupture. Lesion-bound MPs, eventually harboring proteolytic and angiogenic effectors are additional actors in plaque vulnerability. Pharmacological strategies aimed at modulating the release of procoagulant MPs appear a promising therapeutic approach of both thrombotic processes and bleeding disorders.