Activation of endothelial cells induces platelet thrombus formation on their matrix. Studies of new in vitro thrombosis model with low molecular weight heparin as anticoagulant

Age-related morphology and function of human arterial endothelial cells

Endothelialization of cardiovascular implants is regarded as a promising strategy for long-term compatibility. While umbilical vein endothelial cells are typically applied in research, human arterial endothelial cells (HAEC) from elderly donors would be the obvious source for autologous cellularization strategies.In our approach, HAEC from 16 donors of varying age (16-63 years) were divided into two groups (<30 years and >30 years) and analyzed regarding morphology, viability, proliferation, function and senescence status.No age-related differences were found regarding morphology, viability, density, prostacyclin and nitrite secretion or collagen and laminin production. However, the metabolic activity was slightly decreased (p = 0.0374) and the membrane integrity marginally impaired (p = 0.0404) in cells from older donors. Two out of three senescence assays detected more senescence markers in cells from older donors.According to the assays applied here, HAEC from young and elderly donors up to the age of 63 years could be judged equally suitable for autologous cellularization strategies. However, this finding should be regarded with caution due to the extremely large variability between individual donors. Further studies comprising a larger sample size are necessary to investigate this issue more thoroughly.

The Endothelial Cytoskeleton: Organization in Normal and Regenerating Endothelium*

The components of the endothelial cell cytoskeleton that have been shown to be important in maintaining endothelial structural integrity and in regulating endothelial repair include F-actin microfilament bundles, including stress fibers, and microtubules, and centrosomes. Endothelial cells contain peripheral and central actin microfilaments. The dense peripheral band (DPB) consists of peripheral actin microfilament bundles which are associated with vinculin adhesion plaques and are most prominent in low or no hemodynamic shear stress conditions. The central microfilaments are very prominent in areas of elevated hemodynamic shear stress. There is a redistribution of actin microfilaments characterized by a decrease of peripheral actin and an increase in central microfilaments under a variety of conditions, including exposure to thrombin, phorbol-esters, and hemodynamic shear stress. During reendothelialization, there is a sequential series of cytoskeletal changes. The DPB remains intact during the rapid lamellipodia mediated repair of very small wounds except at the base of the lamellipodia where it is splayed. The DPB is reduced or absent when cell locomotion occurs to repair a wound. In addition, when cell locomotion is required, the centrosome, in the presence of intact microtubules, redistributes to the front of the cell to establish cell polarity and acts as a modulator of the directionality of migration. This occurs prior to the loss of the DPB but does not occur in very small wounds that close without migration. Thus, the cytoskeleton is a dynamic intracellular system which regulates endothelial integrity and repair and is modulated by external stimuli that are present at the vessel wall-blood interface.

Relationships among oncostatin M, insulin resistance, and chronic inflammation: a pilot study

Objective Activated macrophages (M1-type macrophages) in adipose tissue secrete many proinflammatory cytokines that induce insulin resistance (IR). Oncostatin M (OSM), a member of the interleukin-6 (IL-6) family of Gp130 cytokines, plays an important role in a variety of biological functions, including the regulation of inflammatory responses. Proinflammatory cytokines released in patients with IR trigger a chronic, low-grade inflammatory reaction in blood vessel walls. This inflammator response leads to endothelial damage, which is the main mechanism for atherosclerosis and many cardiovascular diseases. Animal studies have reported a relationship between OSM and IR. To the best of our knowledge, however, few clinical studies have examined this topic. Therefore, we studied the relationship between serum levels of OSM and IR. Subjects and methods This prospective cross-sectional case-control study enrolled 50 people with IR (according to the HOMA-IR and QUICKI indices) and 34 healthy controls. The fasting blood concentrations of insulin, glucose, blood urea nitrogen (BUN), creatinine, aspartate aminotransferase (AST), alanine aminotransferase (ALT), low-density lipoprotein cholesterol (LDL-C), high-density lipoprotein cholesterol (HDL-C), triglyceride, total cholesterol, C-reactive protein (CRP), and OSM were determined. Results There were no significant differences between the two groups in age, sex, and HbA1c levels. Univariate analyses showed that waist circumference (WC) and levels of fasting glucose, insulin, CRP, HDL-C, OSM, HOMA-IR, and QUICKI differed between the two study groups. In multivariate analyses, both IR indices (QUICKI and HOMA) and OSM differed between the two groups. Conclusion OSM was correlated with the IR indices (QUICKI and HOMA). For simplicity, it might replace the other IR indices in the future. Further detailed studies are needed to confirm this.

Use of Human Umbilical Vein Endothelial Cells (HUVEC) as a Model to Study Cardiovascular Disease: A Review

Cardiovascular disease (CVD) is the leading cause of death worldwide, and extensive research has been performed to understand this disease better, using various experimental models. The endothelium plays a crucial role in the development of CVD, since it is an interface between bloodstream components, such as monocytes and platelets, and other arterial wall components. Human umbilical vein endothelial cell (HUVEC) isolation from umbilical cord was first described in 1973. To date, this model is still widely used because of the high HUVEC isolation success rate, and because HUVEC are an excellent model to study a broad array of diseases, including cardiovascular and metabolic diseases. We here review the history of HUVEC isolation, the HUVEC model over time, HUVEC culture characteristics and conditions, advantages and disadvantages of this model and finally, its applications in the area of cardiovascular diseases.

Inflammatory cytokine oncostatin M induces endothelial activation in macro- and microvascular endothelial cells and in APOE*3Leiden.CETP mice

AIMS

Endothelial activation is involved in many chronic inflammatory diseases, such as atherosclerosis, and is often initiated by cytokines. Oncostatin M (OSM) is a relatively unknown cytokine that has been suggested to play a role in both endothelial activation and atherosclerosis. We comprehensively investigated the effect of OSM on endothelial cell activation from different vascular beds and in APOE*3Leiden.CETP mice.

METHODS AND RESULTS

Human umbilical vein endothelial cells, human aortic endothelial cells and human microvascular endothelial cells cultured in the presence of OSM express elevated MCP-1, IL-6 and ICAM-1 mRNA levels. Human umbilical vein endothelial cells and human aortic endothelial cells additionally expressed increased VCAM-1 and E-selectin mRNA levels. Moreover, ICAM-1 membrane expression is increased as well as MCP-1, IL-6 and E-selectin protein release. A marked increase was observed in STAT1 and STAT3 phosphorylation indicating that the JAK/STAT pathway is involved in OSM signaling. OSM signals through the LIF receptor alfa (LIFR) and the OSM receptor (OSMR). siRNA knockdown of the LIFR and the OSMR revealed that simultaneous knockdown is necessary to significantly reduce MCP-1 and IL-6 secretion, VCAM-1 and E-selectin shedding and STAT1 and STAT3 phosphorylation after OSM stimulation. Moreover, OSM administration to APOE*3Leiden.CETP mice enhances plasma E-selectin levels and increases ICAM-1 expression and monocyte adhesion in the aortic root area. Furthermore, Il-6 mRNA expression was elevated in the aorta of OSM treated mice.

CONCLUSION

OSM induces endothelial activation in vitro in endothelial cells from different vascular beds through activation of the JAK/STAT cascade and in vivo in APOE*3Leiden.CETP mice. Since endothelial activation is an initial step in atherosclerosis development, OSM may play a role in the initiation of atherosclerotic lesion formation.

Utility of microfluidic devices to study the platelet-endothelium interface

The integration of biomaterials and understanding of vascular biology has led to the development of perfusable endothelialized flow models, which have been used as valuable tools to study the platelet-endothelium interface under shear. In these models, the parameters of geometry, compliance, biorheology, and cellular complexity are varied to recapitulate the physical biology of platelet recruitment and activation under physiologically relevant conditions of blood flow. In this review, we summarize the mechanistic insights learned from perfusable microvessel models and discuss the potential utility as well as challenges of endothelialized microfluidic devices to study platelet function in the bloodstream in vitro.

A vascular injury model using focal heat-induced activation of endothelial cells

Endothelial cells (EC) both inhibit and promote platelet function depending on their activation state. Quiescent EC inhibit platelet activation by constitutive secretion of platelet inhibitors. Activated EC promote platelet adhesion by secretion of von Willebrand factor (vWF). EC also secrete an extracellular matrix that support platelet adhesion when exposed following vascular injury. Previous studies of EC-platelet interactions under flow activate entire monolayers of cells by chemical activation. In this study, EC cultured in microfluidic channels were focally activated by heat from an underlying microelectrode. Based on finite element modeling, microelectrodes induced peak temperature increases of 10-40 °C above 37 °C after applying 5-9 V for 30 s resulting in three zones: (1) a quiescent zone corresponded to peak temperatures of less than 15 °C characterized by no EC activation or platelet accumulation. (2) An activation zone corresponding to an increase of 16-22 °C yielded EC that were viable, secreted elevated levels of vWF, and were P-selectin positive. Platelets accumulated in the retracted spaces between EC in the activation zone at a wall shear rate of 150 and 1500 s(-1). Experiments with blocking antibodies show that platelets adhere via GPIbα-vWF and α6β1-laminin interactions. (3) A kill zone corresponded to peak temperatures of greater than 23 °C where EC were not viable and did not support platelet adhesion. These data define heating conditions for the activation of EC, causing the secretion of vWF and the exposure of a subendothelial matrix that support platelet adhesion and aggregation. This model provides for spatially defined zones of EC activation that could be a useful tool for measuring the relative roles of anti- and prothrombotic roles of EC at the site of vascular injury.

Functional factor XIII-A is exposed on the stimulated platelet surface

Factor XIII (FXIII) stabilizes thrombi against fibrinolysis by cross-linking α2-antiplasmin (α2AP) to fibrin. Cellular FXIII (FXIII-A) is abundant in platelets, but the extracellular functions of this pool are unclear because it is not released by classical secretion mechanisms. We examined the function of platelet FXIII-A using Chandler model thrombi formed from FXIII-depleted plasma. Platelets stabilized FXIII-depleted thrombi in a transglutaminase-dependent manner. FXIII-A activity on activated platelets was unstable and was rapidly lost over 1 hour. Inhibiting platelet activation abrogated the ability of platelets to stabilize thrombi. Incorporating a neutralizing antibody to α2AP into FXIII-depleted thrombi revealed that the stabilizing effect of platelet FXIII-A on lysis was α2AP dependent. Platelet FXIII-A activity and antigen were associated with the cytoplasm and membrane fraction of unstimulated platelets, and these fractions were functional in stabilizing FXIII-depleted thrombi against lysis. Fluorescence confocal microscopy and flow cytometry revealed exposure of FXIII-A on activated membranes, with maximal signal detected with thrombin and collagen stimulation. FXIII-A was evident in protruding caps on the surface of phosphatidylserine-positive platelets. Our data show a functional role for platelet FXIII-A through exposure on the activated platelet membrane where it exerts antifibrinolytic function by cross-linking α2AP to fibrin.

Liposomes bearing fibrinogen could potentially interfere with platelet interaction and procoagulant activity

Background

The contribution of fibrinogen (FBN) to hemostasis acting on platelet aggregation and clot formation is well established. It has been suggested that FBN-coated liposomes could be useful in restoring hemostasis. In the present study, we evaluated the modifications induced by multilamellar raw liposomes (MLV) or fibrinogen-coated liposomes (MLV-FBN) on hemostatic parameters.

Materials and methods

Different experimental settings using whole blood or thrombocy-topenic blood were used. Thromboelastometry, aggregation studies, platelet function analyzer (PFA-100^®) tests and studies under flow conditions were applied to detect the effect of MLV-FBN on hemostatic parameters.

Results

The presence of MLV-FBN in whole blood modified its viscoelastic properties, prolonging clot formation time (CFT) (226.5 ± 26.1 mm versus 124.1 ± 9.4 mm; P < 0.01) but reducing clot firmness (45.4 ± 1.8 mm versus 35.5 ± 2.3 mm; P < 0.05). Under thrombocy-topenic conditions, FIBTEM analysis revealed that MLV-FBN shortened clotting time (CT) compared to MLV (153.3 ± 2.8 s versus 128.0 ± 4.6 s; P < 0.05). Addition of either liposome decreased fibrin formation on the subendothelium (MLV 8.1% ± 4.7% and MLV-FBN 0.8% ± 0.5% versus control 36.4% ± 6.7%; P < 0.01), whereas only MLV-FBN significantly reduced fibrin deposition in thrombocytopenic blood (14.4% ± 6.3% versus control 34.5% ± 5.2%; P < 0.05). MLV-FBN inhibited aggregation induced by arachidonic acid (52.1% ± 8.1% versus 88.0% ± 2.1% in control; P < 0.01) and ristocetin (40.3% ± 8.8% versus 94.3% ± 1.1%; P < 0.005), but it did not modify closure times in PFA-100^® studies. In perfusion experiments using whole blood, MLV and MLV-FBN decreased the covered surface (13.25% ± 2.4% and 9.85% ± 2.41%, respectively, versus control 22.0% ± 2.0%; P < 0.01) and the percentage of large aggregates (8.4% ± 2.3% and 3.3% ± 1.01%, respectively, versus control 14.6% ± 1.8%; P < 0.01).

Conclusion

Our results reveal that, in addition to the main contribution of fibrinogen to hemostasis, MLV-FBN inhibits platelet-mediated hemostasis and coagulation mechanisms.

Improved endothelialization of titanium vascular implants by extracellular matrix secreted from endothelial cells

A variety of metals have been widely used in construction of cardiovascular implants (CVIs), such as artificial heart valves, ventricular pumps, and vascular stents. Although great effects have been put into rigorous anticoagulation, late thrombosis still occurred due to inferior blood and cell compatibility. Natural endothelium is popularly regarded as the only substance that has long-term anticoagulant ability. So, establishment of a compact endothelial cell (EC) monolayer on CVIs surface is a guarantee for their long-term potency. In the work described here, titanium (Ti) disks were coated with extracellular matrix (ECM) directly secreted by human umbilical vein endothelial cells (HUVECs), so as to help ECs proliferate and migrate and to improve their endothelialization in vivo. Deposition of ECM on Ti disks was detected by immunofluorescence microscopy, diffuse reflectance Fourier transform infrared spectroscopy, scanning electron microscopy, and atomic force microscopy. The surface topography and wettability of the Ti disks significantly changed after ECM deposition. Most importantly, it was found that ECM deposition inhibited platelet adhesion, stimulated EC proliferation, increased EC migration speed in vitro, and eventually accelerated the re-cellularization speed of Ti disks in vivo. These important results render it reasonable and feasible to modify CVIs with ECM secreted from ECs for improving their long-term potency.

Platelets interact with tissue factor immobilized on surfaces: effects of shear rate

BACKGROUND

While procoagulant activities of Tissue Factor (TF) have been widely investigated, its possible pro-adhesive properties towards platelets have not been studied in detail.

MATERIAL AND METHODS

We explored the interaction of platelets with human Tissue Factor (hTF) firmly adsorbed on a synthetic surface of polyvinilidene difluoride (PVDF) using different shear rates. For studies at 250 and 600 s(-1), TF firmly adsorbed was exposed to flowing anticoagulated blood in flat perfusion devices. Deposition of platelets and fibrin were evaluated by morphometric, immunocytochemical and ultrastructural methods. Prothrombin fragment 1 + 2 (F1 + 2) levels were also measured. Experiments at 5000 s(-1), were performed on the Platelet Function Analyzer (PFA-100) with experimental cartridges with collagen (COL) or collagen-hTF (COL + TF). Haemostatic effect of recombinant activated FVIIa (rFVIIa) was assessed in the same experimental settings.

RESULTS

Platelet deposition on hTF reached 19.8 +/- 1.3% and 26.1 +/- 3.4% of the total surface, at 250 and 600 s(-1), respectively. Fibrin formation was significantly higher at 250 s(-1) than at 600 s(-1) (P < 0.05). The addition of rFVIIa did not influence platelet deposition but raised fibrin formation and thrombin generation at both shear rates (P < 0.05). At 5000 s(-1), closure times (CT) in the PFA-100 were significantly shortened in the presence of hTF (154.09 +/- 14.69 s vs. 191.45 +/- 16.09 s COL alone; P < 0.05). Addition of rFVIIa did not cause a further reduction of CT.

CONCLUSIONS

Our studies demonstrate that hTF is an adhesive substrate for platelets and suggest that the von Willebrand factor could mediate these interactions. At low and intermediate shear rates, rFVIIa enhanced the procoagulant action of hTF, but this effect was not observed at very high shear rates.

Antithrombotic action of annexin V proved as efficient as direct inhibition of tissue factor or thrombin

The role of phospholipid platelet membrane and tissue factor in thrombin generation and thrombus formation is accepted. In the present study we have explored antithrombotic action of strategies aimed to block exposure of negatively charged phospholipids and we compared effects with those obtained through tissue factor or a direct thrombin inhibition. Type III collagen was exposed to flowing blood (5 min, 300 s(-1)). Effects of inhibition of platelet deposition by annexin A5 (ANXA5), hirudin (HIR) or by an antibody against tissue factor (TF) were evaluated. Prothrombin fragment F1 + 2 (F1 + 2) was monitored. Pre-incubation of whole blood with HIR or ANXA5 resulted in a statistically significant reduction of platelet deposition (12.2 +/- 0.6% in control experiments vs. 8.3 +/- 0.4% and 8.5 +/- 0.5%, respectively, P < 0.05). A similar decrease was found when blood was incubated with an antibody against TF. Furthermore, ANXA5 and HIR inhibited the recruitment of platelets into forming aggregates. The height of platelet aggregates generated was decreased in the presence of HIR or ANXA5, but only incubation with both inhibitors reached levels of statistical significance. The presence of ANXA5 or HIR decreased levels of F1 + 2 suggesting a reduced activation of the coagulation system. In our experimental studies, the inhibitory potential of ANXA5 on platelet-thrombus formation was as effective as that of a direct thrombin inhibitor, as HIR, or an antibody against TF. Negatively charged phospholipids exposed on activated platelets potentiate the formation of platelet aggregates on a collagen surface and further suggest that inhibition of platelet procoagulant activity might be a specific target for antithrombotic drugs.

Fibrin and Activated Platelets Cooperatively Guide Stem Cells to a Vascular Injury and Promote Differentiation Towards an Endothelial Cell Phenotype

Objective— Bone marrow-derived progenitor cells play a role in vascular regeneration. However, their homing to areas of vascular injury is poorly understood. One of the earliest responses to an injury is the activation of coagulation and platelets. In this study we assessed the role of hemostatic components in the recruitment of CD34 + cells to sites of injury.

Methods and Results— Using an ex vivo injury model, representing endothelial cell (EC) injury or vessel denudation, we studied homing of CD34 + under flow. Platelet aggregates facilitated initial tethering and rolling of CD34 + cells through interaction of P-selectin expressed by platelets and P-selectin glycoprotein ligand-1 (PSGL-1), expressed by CD34 + cells. Ligation of PSGL-1 activated adhesion molecules on CD34 + cells, ultimately leading to firm adhesion of CD34 + cells to tissue factor-expressing ECs or to fibrin-containing thrombi formed on subendothelium. We also demonstrate that fibrin-containing thrombi can support migration of CD34 + cells to the site of injury and subsequent differentiation toward a mature EC phenotype. Additionally, intravenously injected CD34 + cells homed in vivo to denuded arteries in the presence of endogenous leukocytes.

Conclusions— We provide evidence that hemostatic factors, associated with vascular injury, provide a regulatory microenvironment for re-endothelialization mediated by circulating progenitor cells.

Elevated levels of von Willebrand Factor in cirrhosis support platelet adhesion despite reduced functional capacity

Cirrhosis of the liver is frequently accompanied by complex alterations in the hemostatic system, resulting in a bleeding tendency. Although many hemostatic changes in liver disease promote bleeding, compensatory mechanisms also are found, including high levels of the platelet adhesive protein von Willebrand Factor (VWF). However, conflicting reports on the functional properties of VWF in cirrhosis have appeared in literature. We have measured a panel of VWF parameters in plasma from patients with cirrhosis of varying severity and causes. Furthermore, we assessed the contribution of VWF to platelet adhesion, by measuring the ability of plasma from patients with cirrhosis to support adhesion of normal or patient platelets under flow conditions. VWF antigen levels were strongly increased in patients with cirrhosis. In contrast, the relative collagen binding activity, as well as the relative ristocetin cofactor activity, was significantly lower in patients as compared with controls, indicating loss of function. Accordingly, patients had a reduced fraction of high-molecular-weight VWF multimers. Both strongly elevated and reduced activity and antigen levels of the VWF cleaving protease ADAMTS13 were found in individual patients. Adhesion of either normal or patient platelets to a collagen surface was substantially increased when these platelets were resuspended in plasma of patients with cirrhosis, as compared with control plasma. In conclusion, highly elevated levels of VWF in patients with cirrhosis contribute to the induction of primary hemostasis despite reduced functional properties of the molecule. This phenomenon might compensate for defects in platelet number and function in patients with cirrhosis.

Importance of platelets and fibrinogen in neutrophil-endothelial cell interactions in septic shock

OBJECTIVE

To examine the role of platelets, fibrin, and adhesion molecules in mediating neutrophil-endothelial cell interactions in septic shock.

DESIGN

Controlled experiments using phase contrast microscopy to examine neutrophil, platelet, and endothelial cell interactions in flowing cell suspensions under simulated physiologic conditions.

SETTING

University research laboratory.

PATIENTS

Adult patients with septic shock and normal volunteers.

INTERVENTIONS

Microslides were coated with human umbilical vein endothelial cells. Neutrophils were removed from control subjects and patients in septic shock and were perfused over endothelial cells at rates representing a range of physiologic shear stresses. In an attempt to examine the effects of fibrin deposition on neutrophil-endothelial cell interactions, neutrophils, with and without platelets, were suspended in plasma and serum was removed from patients in septic shock. In addition, blocking monoclonal antibodies against the platelet receptor P-selectin and neutrophil receptor CD11b/CD18, and a platelet glycoprotein IIb/IIIa inhibitor, were incubated with cells suspended in plasma. Phase contrast video microscopy was used to count the number of neutrophils/mm adherent to endothelial cells during cessation of flow. Neutrophil rolling velocity was calculated as the time required for neutrophils to move across a 1-mm field (mm/sec). Leukoaggregation was defined as the number of neutrophils in aggregates (three or more nuclei) across a 1-mm field.

MEASUREMENTS AND MAIN RESULTS

Normal neutrophils exposed to plasma from patients with septic shock demonstrated significant increases in aggregation and endothelial cell adherence with associated decreases in neutrophil rolling velocity. These changes were significantly enhanced in the presence of platelets and significantly attenuated in the presence of serum, which is fibrinogen depleted. Preincubation with antibodies to the surface receptors P-selectin, CD11b/CD18, and glycoprotein IIb/IIIa abrogated the changes in neutrophil aggregation, adhesion, and rolling velocity.

CONCLUSIONS

These data suggest that platelets and fibrinogen play an important role in mediating neutrophil-endothelial cell adherence in septic shock.

The hemostatic agent ethamsylate promotes platelet/leukocyte aggregate formation in a model of vascular injury

The hemostatic agent ethamsylate enhances membrane expression of P-selectin in human platelets, but whether this promotes platelet-leukocyte aggregate formation is unknown. Here we investigated this point by flow cytometry determination of human platelet-leukocyte aggregates under basal conditions and after whole-blood perfusion through a damaged rabbit aorta segment. Actions of ethamsylate on adhesive molecules of platelets and leukocytes were investigated in parallel. Under basal conditions, ethamsylate was unable to modify whole-blood platelet-leukocyte aggregation, but following whole-blood perfusion through a damaged vessel, ethamsylate produced a modest, but significant increase in platelet-leukocyte aggregates (48+/-21 and 45+/-26% above control levels at ethamsylate 20 and 40 microm respectively). In isolated leukocyte plasma membranes, 14C-ethamsylate specifically bound up to an amount of 660 pmol/mg protein. Moreover, at concentrations > or =1 microm, ethamsylate induced an important (100-200%) and significant increase in the P-selectin glycoprotein ligand 1 (PSGL-1) fluorescence signal in isolated leukocytes and was unable to significantly modify the percentage of CD11b-positive cells. However, no significant changes in aggregate formation were found when ethamsylate was incubated with isolated leukocytes and blood was reconstituted and perfused. In isolated platelet cell membranes, anti-P-selectin antibody and the anti-integrin RGD-containing pentapeptide (GRDGS) were unable to displace 14C-ethamsylate binding. In conclusion, ethamsylate specifically binds to plasma membranes of leukocytes, enhances membrane PSGL-1 expression and promotes leukocyte-platelet aggregation in whole-blood perfused through a damaged vascular segment. These results together with the previously observed enhancement of platelet P-selectin membrane expression [Thromb. Res. (2002)107:329-335] confirms and extends the view that ethamsylate acts on the first step of hemostasis, by improving platelet homo- and heterotypic adhesiveness.

Increased local procoagulant action: a mechanism contributing to the favorable hemostatic effect of recombinant FVIIa in PLT disorders

BACKGROUND

Recombinant FVIIa (rFVIIa) has been shown to improve hemostasis in patients with thrombocytopenia and to prevent or control bleeding episodes in patients with inherited deficiencies of major PLT glycoproteins, but the mechanism of action is not well understood.

STUDY DESIGN AND METHODS

Effects of rFVIIa on hemostasis were explored with an in vitro perfusion technique. Blood samples, from healthy donors or from patients with congenital defects of PLT glycoprotein IIb-IIIa (GPIIb-IIIa), were anticoagulated with low-molecular-weight heparin. Experimental thrombocytopenia (<6000 PLTs/microL) was induced by a filtration procedure. rFVIIa was added to blood samples at therapeutic concentrations. A severe GPIIb-IIIa impairment was also induced by exposure of normal blood samples to a specific antibody. Perfusion studies were performed through annular chambers containing damaged vascular segments. The presence of fibrin and PLTs on the perfused subendothelium was morphometrically quantified.

RESULTS

Under conditions of experimental thrombocytopenia, addition of rFVIIa enhanced fibrin formation in a dose-dependent manner (p < 0.05). Improvements in local fibrin generation and partial restoration of PLT interactions were also observed after incubation of blood from patients with Glanzmann's thrombasthenia with rFVIIa at 5 microg per mL (180 microg/kg). Similar improvements were observed in blood samples incubated with antibodies to GPIIb-IIIa. rFVIIa in whole normal blood also enhanced fibrin formation but PLT deposition was unaffected. Evaluation of prothrombin fragments 1 and 2 in the perfusates confirmed that rFVIIa increased thrombin generation in all cases.

CONCLUSION

Our data indicate that rFVIIa promotes a procoagulant activity at sites of vascular damage. This mechanism could explain the beneficial hemostatic effect of rFVIIa in patients with thrombocytopenia or with Glanzmann's thrombasthenia.

Recombinant factor VIIa enhances deposition of platelets with congenital or acquired alpha IIb beta 3 deficiency to endothelial cell matrix and collagen under conditions of flow via tissue factor-independent thrombin generation

A novel approach to treat bleeding episodes in patients with Glanzmann thrombasthenia (GT) and perhaps also in patients receiving alpha IIb beta 3 inhibitors is the administration of recombinant factor VIIa (rFVIIa). The mechanism of action of rFVIIa in these patients is, however, still unclear. We studied the effect of rFVIIa-mediated thrombin formation on adhesion of alpha IIb beta 3-deficient platelets under flow conditions. Adhesion of alpha IIb beta 3-deficient platelets to the extracellular matrix (ECM) of stimulated human umbilical vein endothelial cells or to collagen type III was studied using a model system with washed platelets and red cells. When alpha IIb beta 3-deficient platelets were perfused over the surface at arterial shear rate for 5 minutes, a low surface coverage was observed (GT platelets, mean +/- SEM, 37.5% +/- 5.0%; normal platelets preincubated with an RGD-containing peptide, 7.4% +/- 2.1%). When rFVIIa, together with factors X and II, was added to the perfusate, platelet deposition significantly increased (GT platelets, mean +/- SEM, 67.0% +/- 4.3%; normal platelets preincubated with an RGD-containing peptide, 48.2% +/- 2.9%). The same effect was observed when normal platelets were pretreated with the commercially available anti-alpha IIb beta 3 drugs abciximab, eptifibatide, or tirofiban. It was shown that tissue factor-independent thrombin generation (presumably induced by binding of rFVIIa to adhered platelets) was responsible for the increase in platelet deposition. In conclusion, defective adhesion of alpha IIb beta 3-deficient platelets to ECM can be restored by tissue factor-independent rFVIIa-mediated thrombin formation. The enhanced generation of platelet procoagulant surface facilitates fibrin formation, so that lack of platelet aggregate formation might be compensated for.

The hemostatic agent ethamsylate enhances P-selectin membrane expression in human platelets and cultured endothelial cells

Ethamsylate possesses antihemorrhagic properties, but whether or not it directly activates blood platelets is unclear. Here we investigated the platelet activation potential of ethamsylate, by measuring membrane P-selectin expression with flow cytometry in human whole blood and also by immunofluorescence imaging of isolated human platelets. Moreover, we measured membrane P-selectin expression in the SV40-transformed aortic rat endothelial cell line (SVAREC) and 14C-ethamsylate membrane binding and/or uptake in platelets and endothelial cells. Whole blood flow cytometry showed a modest, but statistically significant increase by ethamsylate in the percentage of platelets expressing P-selectin (from 2% to 4-5%, p < 0.05). Immunofluorescence showed a sizable (39%) and significant (p < 0.01) enhancement of P-selectin expression at the lowest concentration of ethamsylate tested (1 microM), with maximal enhancement of P-selectin expression (75-90%) at 10 microM ethamsylate. Similar results were obtained in SVAREC endothelial cells. 14C-ethamsylate specifically bound to platelets and endothelial cell membranes, without significant uptake into the cell interior. In conclusion, ethamsylate enhances membrane P-selectin expression in human platelets and in cultured endothelial cells. Ethamsylate specifically binds to some protein receptor in platelet and endothelial cell membranes, receptor which can signal for membrane P-selectin expression. These results support the view that ethamsylate acts on the first step of hemostasis, by improving platelet adhesiveness and restoring capillary resistance.

Concentrates containing factor IX could improve haemostasis under conditions of thrombocytopenia: studies in an in vitro model

BACKGROUND AND OBJECTIVES

We explored the effect on haemostasis of different factor IX (FIX) concentrates under thrombocytopenic conditions using an in vitro perfusion technique.

MATERIALS AND METHODS

A moderate experimental thrombocytopenia (25 000-30 000 platelets/microl) was induced by means of a filtration procedure in blood anticoagulated with low-molecular-weight heparin. The effects of three different FIX concentrates - a prothrombin complex concentrate (PCC), an intermediate-purity concentrate (FIX/X), and a high-purity concentrate (HPFIX) - on platelet deposition and fibrin formation on subendothelium were assessed at two different shear rates (600/second and 1200/second). Activation of the coagulation system was monitored through assessment of prothrombin activation fragment 1 + 2 (F1 + 2).

RESULTS

Fibrin deposition increased after addition of FIX concentrates, but only showed a significant increase in experiments performed after incubation of PCC at the lower shear rate (600/second) (64.25 +/- 9.61% vs. control 31.22 +/- 8.02%; P < 0.05). Addition of FIX concentrates caused a small increase in the percentage of platelet deposition and area of those aggregates. These differences reached levels of statistical significance in the presence of FIX/X and HPFIX in experiments performed at a shear rate of 600/second. F1 + 2 baseline values in anticoagulated thrombocytopenic blood were 1.15 +/- 0.13 nm and reached levels of 2.49 +/- 0.24 and 3.60 +/- 0.33 nm at shear rates of 600 and 1200/second, respectively. Increments in F1 + 2 observed after addition of different FIX concentrates always remained in the previous ranges.

CONCLUSIONS

Data from the present study provide experimental support favouring the concept that FIX concentrates containing other activated factors could improve haemostasis under conditions of moderate thrombocytopenia.

Methods and models to evaluate shear-dependent and surface reactivity-dependent antithrombotic efficacy

The purpose of the present communication is to evaluate the importance of blood flow and surface reactivity for measurement of antithrombotic drug activity or efficacy in selected model systems of thrombus formation. Such information is essential for proper evaluation of antithrombotic drug profiles. The continuous development of flow-dependent thrombosis models for in vitro (anticoagulated blood) and ex vivo (native blood) studies and their application in in vivo animal models from the early 1970s and onwards are briefly considered. Central to this process was the development of various types of perfusion chambers in which a thrombogenic surface is exposed to flowing blood. Such perfusion chambers have been inserted into arteriovenous (AV) shunts in baboon, pig, dog, and rabbit. These approaches have allowed reproducible testing of traditional and novel experimental antithrombotic drugs, and studies on novel drug strategies under well-defined shear conditions and surface reactivity. Shear-dependent antithrombotic efficacy in these models is observed with anticoagulants such as unfractionated heparin, low-molecular weight heparins, or selective inhibitors of thrombin, Factor Xa, or Factor VIIa. However, the degree of shear dependency depends on the nature of the thrombogenic surface, e.g., the inhibition is more pronounced on a tissue factor (TF)-rich surface than on a collagen-rich surface, particularly at venous or low arterial shear. Platelet antagonists such as the COX-1 inhibitor aspirin, inhibitors of thromboxane A2 (TxA2) synthetase, the TxA2 platelet receptor, and of von Willebrand factor (vWf) are shear dependent also, being more efficient at high arterial shear. In contrast, the platelet ADP antagonist clopidogrel, or antagonists to the active platelet membrane glycoprotein IIb-IIIa complex (GPIIb-IIIa) are shear independent. At extremely high arterial shear, which activates platelets and elicit aggregates of circulating platelets, aspirin looses its antithrombotic effect, whereas ADP and GPIIb-IIIa antagonists still interrupt thrombus formation. In general, results obtained with these models mimic and predict antithrombotic efficacy in man when comparison is possible. Information on antithrombotic efficacy in flow devices with various thrombogenic surfaces is now sufficiently available to suggest recommendations for experimental conditions, particularly with regard to blood flow and reactive surfaces.

Infusible platelet membranes improve hemostasis in thrombocytopenic blood: experimental studies under flow conditions

BACKGROUND

The potential hemostatic effect of infusible platelet membranes (IPM; Cyplex, Cypress Bioscience) prepared from outdated human platelets is investigated.

STUDY DESIGN AND METHODS

Increasing concentrations of IPM were added to blood samples anticoagulated with low-molecular-weight heparin, in which platelets and WBC counts had been experimentally reduced by a filtration procedure. Thrombocytopenic blood with IPM was circulated in a perfusion chamber at various shear rates (300, 600, and 1200/sec(-1)), and platelet and fibrin deposition on the surface of a damaged vessel was measured. Prothrombin fragments 1 and 2 (F1+2) levels were also monitored.

RESULTS

Under conditions of severe thrombocytopenia (<6000 platelets/microL) IPM did not increase platelet deposition. However, a dose-dependent increase in fibrin deposition was observed with concentrations of IPM ranging from 0.5 to 2 mg per kg in perfusions at 300 and 600 per sec(-1) (p<0.05 vs. thrombocytopenic blood). Experimental studies performed under conditions of moderate thrombocytopenia and higher shear rates (25, 000-30,000 platelets/microL; at 600 and 1200/sec(-1)) showed that IPM concentrations equivalent to 0.5 or 1 mg per kg improved fibrin deposition (33.5 +/- 9.5% and 37.7 +/- 12.8%, respectively, vs. 22.7 +/- 5.2% in controls) and also promoted a moderate increase in platelet deposition, with a concomitant significant increase in the size of platelet aggregates (p<0.05). Exposure of thrombocytopenic blood to a damaged vessel resulted in an increase of F1+2 levels from 0.8 +/- 0.15 to 1.7 +/- 0.22 nM at 300 per sec(-1) and 1.94 +/- 0.46 nM at 600 per sec(-1). Postperfusion levels of F1+2 after the addition of IPM were always similar to levels in untreated controls.

CONCLUSION

IPM promotes local procoagulant activity at sites of vascular damage under conditions of severe and moderate thrombocytopenia. IPM also appears to facilitate platelet cohesive functions under conditions of moderate thrombocytopenia.

Endothelial cell seeding of (heparinized) collagen matrices: effects of bFGF pre-loading on proliferation (after low density seeding) and pro-coagulant factors

Endothelial cell seeding to improve the performance of small-diameter vascular grafts requires a suitable substrate, such as crosslinked collagen. In addition to providing a suitable substrate for adhesion and growth of endothelial cells, proliferation of seeded endothelial cells can be enhanced by local, sustained release of basic fibroblast growth factor (bFGF, a heparin-binding growth factor for endothelial cells). We have previously shown that collagen crosslinked using N-(3-dimethylaminopropyl)-N'-ethylcarbodiimide (EDC) and N-hydroxysuccinimide (NHS) supports adhesion and proliferation of human umbilical vein endothelial cells (HUVECs). In the present study, HUVECs were seeded on (heparinized) EDC/NHS-crosslinked collagen, pre-loaded with bFGF. Proliferation of HUVECs on (heparinized) crosslinked collagen increased with increasing amounts of pre-loaded bFGF. The minimal cell-seeding density required for proliferation proved to be very low after pre-loading the substrates with bFGF, and was 4-fold lower for heparinized crosslinked collagen compared to crosslinked collagen (250 versus 1000 cells/cm(2)). Pro-coagulant properties (von Willebrand factor secretion and tissue factor expression) of HUVECs seeded on (heparinized) crosslinked collagen, with or without pre-loading of bFGF, were comparable to those of HUVECs on TCPS. It is concluded that heparinized, EDC/NHS-crosslinked collagen pre-loaded with bFGF is a candidate matrix for in vivo endothelial cell seeding of synthetic vascular graft materials.

In vitro human decidual endothelial cell thromboxane secretion in preeclampsia is not abnormal

OBJECTIVES

The aims of this study were to describe levels of thromboxane secretion by decidual endothelial cells from normal pregnancies and to determine whether decidual endothelial cell secretion of thromboxane, implicated in the causation of the hypertension and vasoconstriction of preeclampsia, is increased in this disorder.

METHODS

We measured thromboxane generation by cultured decidual endothelial cells from 13 normal pregnancies (NDEC) and 13 pregnancies complicated by preeclampsia (PEDEC), compared with a control population of 6 normal human umbilical vein endothelial cells (HUVEC). Responses to stimulation by bacterial lipopolysaccharide (LPS), tumor necrosis factor-alpha (TNF-alpha), and interleukin-1 beta (IL-1 beta) were examined.

MAIN OUTCOME MEASURES

Thromboxane B2 levels in supernatants of cultured endothelial cells.

RESULTS

The level of secretion over 24 h in culture by NDEC [14 (7-26) pg/10(6) cells] was approximately 25% that of HUVEC [63 (49-70) pg/10(6) cells]. Levels achieved in response to all stimuli examined were consistently lower in NDEC than in HUVEC (p < 0.01). Proportional stimulation by LPS and TNF-alpha was comparable in HUVEC and NDEC, whereas NDEC displayed a greater increase (25-fold) than HUVEC (10-fold) in response to IL-1 beta (p < 0.01). There were no significant differences between decidual endothelial cells from normotensive and preeclamptic women in basal secretion of thromboxane or in responses to the stimuli examined.

CONCLUSIONS

In vitro thromboxane secretion by decidual endothelial cells is lower than that of HUVEC, and responsiveness to specific stimuli may be quantitatively different. These findings emphasize the importance of examining endothelial cells from the involved maternal vascular bed if intrauterine vascular pathophysiological events are to be clarified. No significant differences were noted in decidual endothelial cell thromboxane secretion between normal and preeclamptic subjects.

Assessment of potential thrombogenicity of coagulation factor IX concentrates in an in vitro model of human thrombogenesis

We have investigated the potential use of perfusion techniques in the evaluation of the thrombogenic profile of factor IX concentrates. Blood from healthy donors was anticoagulated with low molecular weight heparin and incubated with one of the following: (a) diluent (DIL); (b) a prothrombin complex concentrate (PCC); (c) an intermediate-purity concentrate (FIX/X); or (d) a high-purity concentrate (HPFIX). The thrombogenic potential was assessed as: (1) fibrin formation on subendothelium (Baumgartner's perfusion) and (2) prothrombin activation fragment 1+2 (F1+2, nM) determination. The percentage of fibrin deposition on the subendothelium was only significantly increased after incubation with PCC (62.0+/-3.6% vs. DIL 35.0+/-6.1%;p<0.05). None of the FIX concentrates modified platelet interaction versus control blood (DIL: 26.7+/-2.1%). F1+2 baseline values in anticoagulated blood were 0.6+/-0.1 nM. Preperfusion levels of F1+2 reached values of 4.4+/-0.1 nM for PCC and 5.4+/-0.1 nM for FIX/X. After perfusion, F1+2 values were 2.7+/-0.2 nM for DIL, 5.6+/-0.1 nM for PCC and FIX/X, and 3.3+/-0.2 nM for HPFIX. While measurement of F1+2 was influenced by residual contaminants present in the concentrates, the morphometric evaluation of fibrin deposition on perfused vascular surfaces could be more closely related to the net thrombogenic profile of each FIX preparation.

Special communicationthe critical role of mechanical forces in blood vessel development, physiology and pathology

The following extended abstracts were presented at the Research Initiatives in Vascular Disease Conference, Movers and Shakers in the Vascular Tree-Hemodynamic and Biomechanical Factors in Blood Vessel Pathology, sponsored by The Lifeline Foundation and the Cardiovascular & Interventional Radiology Research and Educational Foundation; jointly sponsored by the International Society for Cardiovascular Surgery, North American Chapter, The Society for Vascular Surgery, and The Society of Cardiovascular and Interventional Radiology; in cooperation with the National Institutes of Health-National Heart, Lung &Blood Institute on Mar 11-12, 1999, in Bethesda, Md.

Preparations of synthetic phospholipids promote procoagulant activity on damaged vessels: studies under flow conditions

BACKGROUND

The possibility of developing synthetic platelet substitutes that could promote hemostasis with prolonged shelf-life and increased safety is an appealing one.

STUDY DESIGN AND METHODS

Preparations containing synthetic phospholipids were incorporated into blood samples (1.15 mg/mL) in which platelets and white cell counts had been experimentally reduced by a filtration procedure. Vesicles containing phosphatidylcholine (PC), phosphatidylethanolamine (PE), phosphatidylserine (PS), phosphatidylinositol (PI), or combinations of PC and PE and of PC and PS were tested in this system. Blood was recirculated (10 min; shear rate, 250/sec) through a perfusion chamber containing vascular segments. The ability of the various phospholipid preparations to promote fibrin formation on the damaged subendothelium was evaluated morphometrically and expressed as the percentage of fibrin coverage. Generation of thrombin in the system was monitored through the measurement of prothrombin fragments 1 and 2.

RESULTS

Vesicles containing PC, PI, PE:PC (1:1), or PS:PC (1:3) increased fibrin deposition on the subendothelium (64.5 +/- 9.8%, 32.7 +/- 6.3%, 58.3 +/- 6.5%, and 46.6 +/- 15.2%, respectively; p < 0.01 vs. 11.5 +/- 1.2% in thrombocytopenic blood). Vesicles containing PE, PS, or PS:PC (3:1) did not show procoagulant effect.

CONCLUSION

Synthetic phospholipid preparations promote a local procoagulant activity at sites of vascular damage when they are incorporated into thrombocytopenic blood maintained under flow conditions.

Albumin microbubble adherence to human coronary endothelium: implications for assessment of endothelial function using myocardial contrast echocardiography

OBJECTIVES

We hypothesized that sonicated 5% human albumin microbubbles (Albunex) adhere to disrupted vascular endothelium and that this interaction is a marker of endothelial integrity. This study sought to identify sites and determinants of Albunex-endothelial cell (EC) attachment.

BACKGROUND

Under normal conditions, Albunex microbubbles used in myocardial contrast echocardiography (MCE) pass unimpeded through the coronary microcirculation. During pathophysiologic states associated with endothelial dysfunction, however, microbubbles linger in the myocardium despite normal flow. The sites and conditions regulating microbubble adhesion are unknown.

METHODS

Coverslips with cultured human coronary artery ECs were mounted in a parallel plate perfusion system and perfused with a suspension of fluorescein-labeled Albunex in culture medium, followed by a bubble-free wash at a wall shear rate of 100 s-1. To create inflammatory ECs, phorbol myristate acetate was added 4.5 h before perfusion, and flow cytometry was used to confirm an inflammatory response. Perfusions were performed under normal and inflammatory conditions using surfaces of confluent and subconfluent ECs and isolated extracellular matrix. Bubble adherence was quantified in 20 random fields per cover-slip using epifluorescent video microscopy.

RESULTS

No microbubbles adhered to normal confluent ECs, although small numbers adhered to inflamed ECs (0.03 +/- 0.01 bubbles/cell, p < 0.01 vs. normal cells). Fever microbubbles attached to normal versus inflamed matrix of both partially exposed (1,800 +/- 520 vs. 4,100 +/- 1,000 bubbles/mm2, p = 0.05) and completely denuded (2,700 +/- 1,300 vs. 7,200 +/- 1,100 bubbles/ mm2, p < 0.03) endothelium.

CONCLUSIONS

Albunex microbubbles preferentially adhere to inflammatory endothelial extracellular matrix. These data suggest that MCE can be used to noninvasively study endothelial integrity and may have implications for the assessment of preclinical atherosclerotic heart disease.

Identification of signal-induced IkappaB-alpha kinases in human endothelial cells

Activation of the nuclear transcription factor-kappaB is an early event in endothelial activation. NF-kappaB activation is regulated by the inducible phosphorylation and subsequent degradation of the inhibitory subunit IkappaB-alpha. We identified two discrete kinases of approximately 36 and 41 kDa in the cytoplasm of human umbilical vein endothelial cells that specifically bind to and phosphorylate the IkappaB-alpha subunit. IkappaB-alpha kinase activity is transiently elevated following treatment with either tumor necrosis factor alpha, interleukin-1beta, or bacterial lipopolysaccharides and precedes activation of either mitogen-activated kinase or Jun kinase. Furthermore, activation of the IkappaB-alpha kinases precedes both the appearance of hyperphosphorylated IkappaB-alpha and its subsequent degradation, as well as the translocation of NF-kappaB to the nucleus. Deletion mutagenesis of the IkappaB-alpha polypeptide revealed that these kinases bind in or around the ankyrin repeat domains and phosphorylate residues within the C terminus. These kinases, however, were not identical to casein kinase II and displayed a pharmacologic profile distinct from other known kinases. These kinases may represent components of a signal transduction pathway regulating IkappaB-alpha levels in vascular endothelium.

Delayed xenograft rejection

Despite considerable progress in understanding the mechanisms of discordant xenograft rejection, and overcoming hyperacute rejection through targeting of complement or antibody, vascularized xenografts are typically rejected within days. Here, Fritz Bach and colleagues discuss the importance of endothelial cell activation, platelet aggregation and other aspects of thrombosis, as well as the contribution of host natural killer cell and monocyte activation in overcoming this next barrier to prolonged xenograft survival.

Lipoprotein(a) inhibits collagen-induced aggregation of thrombocytes

Lipoprotein(a) [Lp(a)] is known to interact with human platelets in vitro. In the present study the effect of physiological concentrations of Lp(a) on platelet aggregation was studied. Freshly prepared gel-filtered platelets from healthy donors were incubated for 30 minutes at 37 degrees C with various concentrations of Lp(a); aggregation was triggered with ADP, thrombin, and collagen. Control incubations were performed with Tyrode's solution or LDL. Thrombin- and ADP-triggered aggregations were only slightly influenced by Lp(a), but aggregation of platelets stimulated with collagen (4 micrograms/mL) was markedly inhibited. Measurable effects occurred at low concentrations (0.05 mg/mL) of total Lp(a); at 0.5 mg/mL, maximum aggregation of platelets was inhibited by 54 +/- 20%, and the aggregation rate was attenuated by 47 +/- 19% compared with platelets incubated with Tyrode's solution. Preincubation of collagen (4 micrograms/mL) with Lp(a) yielded similar results. The effect of Lp(a) on platelet aggregation was accompanied by a significant reduction of serotonin release and TXA2 formation. Higher concentrations of collagen ( > or = 10 micrograms/ mL) caused the inhibitory effect on Lp(a) on collagen-induced aggregation to disappear. In contrast, incubation of platelets with 5 mg/mL LDL led to a significant increase of aggregation rate, maximum aggregation, serotonin release, and formation of TXA2 when aggregation was induced with 4 micrograms/mL collagen. In an adhesion assay using fresh whole blood, which mimics the in vitro situation of vessel injury. Lp(a) reduced platelet adhesion at shear rates of 300 and 1600/s by 22.6% and 11.6%, respectively. In addition, Lp(a) reduced the size of platelet aggregates significantly (up to 63%); this reduction was more distant at the higher shear rate. Unlike LDL, Lp(a) is not a proaggregatory lipoprotein; rather, collagen-triggered aggregation in vitro is attenuated by Lp(a).

Procoagulant human monocytes mediate tissue factor/factor VIIa-dependent platelet-thrombus formation when exposed to flowing nonanticoagulated human blood

Tissue factor (TF) on monocyte and macrophage surfaces is a nonproteolytic cofactor for factor VIIa (FVIIa)-induced coagulation. Monocyte-derived macrophages in atherosclerotic plaques express TF, which, after plaque disruption or rupture, may complex with FVII/VIIa from the bloodstream, resulting in activation of extrinsic coagulation. We studied the effect of TF expression on human monocytes on arterial thrombus formation in a model system of thrombogenesis. Thawed, cryopreserved human monocytes adherent to plastic coverslips were stimulated with lipopolysaccharide (0.5 microgram/mL) to express TF and subsequently exposed to flowing nonanticoagulated human blood in a parallel-plate perfusion chamber. The wall shear rate at the cell surface was 650 seconds-1, corresponding to that of average-sized coronary arteries. The stimulated monocytes elicited pronounced fibrin deposition and platelet-thrombus formation. The platelet-thrombus volume was as large as that triggered by human type III collagen fibrils under similar experimental conditions. In contrast, the monocytes elicited much more fibrin deposition than the collagen surface. However, inclusion of an anti-TF monoclonal antibody that blocks the complexation of FVII/FVIIa with TF virtually abolished the fibrin deposition (P < .03) and reduced platelet-thrombus formation by more than 70% (P < .04). Thus, arterial thrombus formation induced by stimulated monocytes was almost completely blocked by the anti-TF antibody, suggesting that inhibition of TF/FVIIa complex formation on monocytes and macrophages at sites of plaque rupture or after percutaneous transluminal coronary angioplasty procedures may reduce intravascular thrombotic complications.

Transient perturbation of endothelial integrity induced by natural antibodies and complement

The barrier function of blood vessels is though to be regulated at least in part by endothelium. This concept is supported by the dramatic loss of barrier function occurring in the hyperacute rejection of vascularized grafts mediated by anti-endothelial cell (EC) antibodies and complement. In this process, the endothelium is not destroyed but instead loses the ability to retain blood cells and plasma proteins within capillaries. The noncytotoxic mechanism that allows this change in EC function has been unknown. Here we report that within 10 to 20 min of exposure to human xenoreactive natural antibodies and complement, porcine EC undergo alterations in cell shape and in the cytoskeleton that disrupt monolayer integrity and lead to formation of intercellular gaps. Gap formation is not associated with cell death but requires the complement complex C5b67. The gaps induced by anti-EC antibodies and complement are transient; gap closure requires formation of C5b-9 complexes on the cells and the rate of recovery depends on the release of cellular products into the medium. Preincubation of EC with dibutyryl cAMP (0.5 mM) prevents gap formation and disruption of the cytoskeleton caused by antibodies and complement. These results provide evidence that the integrity of endothelium is regulated by components of the complement system and suggest a mechanism that may explain the prominent loss of endothelial integrity seen in humoral immune responses.

Endothelial cells stimulated with tumor necrosis factor-alpha express varying amounts of tissue factor resulting in inhomogenous fibrin deposition in a native blood flow system. Effects of thrombin inhibitors

TNF-alpha induces changes in endothelial cell functions, such as upregulation of tissue factor, resulting in endothelial procoagulant activity which may play a role in disseminated intravascular coagulation. The procoagulant activity of TNF-alpha-stimulated endothelial cell monolayers was studied in a human ex vivo native (nonanticoagulated) blood flow system using the three thrombin inhibitors recombinant hirudin, Ro 46-6240, and heparin. Under venous blood flow conditions (shear rate 65 s-1) recombinant hirudin, Ro 46-6240, and heparin inhibited fibrin deposition on the endothelial cells by 50% at concentrations of 14, 28, and 412 ng/ml, respectively. The highest tested concentrations of the thrombin inhibitors reduced the postchamber fibrinopeptide A levels from 713 +/- 69 to < 70 ng/ml. Surprisingly, even at relatively high inhibitor concentrations, some local fibrin deposits were found on TNF-alpha-stimulated cells, suggesting that some endothelial cells possess higher procoagulant activity than others. Therefore, the surface expression pattern of tissue factor, the primary initiator of coagulation in this system, was examined by immunogold-silver staining. The results showed that the tissue factor density on the cell surface varied strongly among TNF-alpha-stimulated endothelial cells. Using TNF receptor-selective agonistic mutants of TNF-alpha, it was demonstrated further that the heterogenous surface expression of tissue factor was mediated entirely by the 55-kD TNF receptor and did not involve the 75-kD TNF receptor. We conclude that in this system TNF-alpha induces heterogenous tissue factor expression which may lead to a high local thrombin concentration, such that even in the presence of thrombin inhibitors focal fibrin deposition occurs.

Annexin V inhibits the procoagulant activity of matrices of TNF-stimulated endothelium under blood flow conditions

A human ex vivo thrombosis model was used to investigate whether recombinant annexin V (rANV) can prevent thrombus formation under venous and arterial blood flow conditions. In this model, blood from an antecubital vein of healthy donors was allowed to flow directly over the extracellular matrix of tumor necrosis factor-stimulated endothelial cells (TNF-ECMs). TNF-ECMs were preincubated with rANV (2.9 mumol/L) for 30 minutes. With this rANV concentration all binding sites present on TNF-ECMs (1.6 +/- 0.5 x 10(12)/cm2) are occupied, and a maximal inhibition was observed in a tissue factor-dependent clotting assay. Fibrin deposition and platelet and leukocyte adhesion were measured on the rANV-treated and nontreated TNF-ECMs. Nontreated TNF-ECMs were used as controls. rANV inhibited fibrin deposition by 81% at a wall shear rate of 100 s-1. A nonsignificant inhibition was also observed at 650 s-1. Platelet-matrix adhesion, which is more prominent at higher shear rates, was significantly decreased by 60% at 100 s-1 but not at 650 s-1. The average leukocyte adherence was nonsignificantly lowered at 100 s-1. Virtually no leukocytes adhered at 650 s-1. The results demonstrated that rANV can inhibit blood coagulation under venous blood flow conditions and may serve as an antithrombotic drug.

Further studies on the presence of functional tissue factor activity on the subendothelium of normal human and rabbit arteries

Although tissue factor (TF) activity has been observed on the subendothelial surface of rabbit aorta and human umbilical cord, immunofluorescent and in situ hybridization methods have failed repeatedly to demonstrate TF in the intima of human blood vessels. In the present study, TF activity on everted, de-endothelialized arteries was studied by two methods. One utilized a flow system and measured fibrin deposition and fibrinopeptide A formation. The other utilized a newly developed rotating probe system and measured the conversion of factor X to factor Xa in the presence of factor VIIa and Ca+2. The study attempted to control, or assess, the possibility that functional TF could have been exposed on the vessel surface by the procedures used to prepare the arterial segments. By both methods, TF activity was detected on the subendothelium of rabbit aortae and human umbilical arteries, and was unaffected by the length of storage or by inclusion of actinomycin D in the storage buffer. TF activity was also observed in the subendothelium of adult human ileo-colic, internal mammary, and renal arteries, studied by the rotating probe method. The latter may underestimate TF activity, as some of the factor Xa formed appears to bind to the subendothelial surface. TF activity (Xa formation) was detected on the luminal surface (subendothelium) of non-everted arteries, but increased activity was observed after eversion of the vessel. The source of the subendothelial TF, and its presence in normal subendothelium in vivo, requires further study. In addition, if any of the TF activity observed in this study was derived from injured endothelial or myointimal cells during preparation of the everted vessel segments, the techniques described could serve as a useful model for studying TF-induced thrombosis and factor Xa formation on injured blood vessels, and for evaluating the anti-thrombotic properties of TF-inhibitors.

Shear-dependence of endothelial functions

Endothelial cells are subjected to shear forces which influence important cell functions. Shear stress induces cell elongation and formation of stress fibers, increases permeability, pinocytosis and lipoprotein internalization, is involved in the formation of atherosclerotic lesions, increases the production of tissue plasminogen activator, and enhances von Willebrand factor release and hence platelet aggregation. It decreases adherence of erythrocytes and leukocytes, and increases the release of prostacyclin, endothelium derived relaxing factor, histamine and other compounds, but decreases erythropoietin secretion. The mechanism of signal transduction to the endothelial cell is not known exactly; shear-sensitive ion channels seem to be involved. It is concluded that a better understanding of shear-dependent endothelial functions will influence pathophysiologic concepts and therapeutic interventions.

Increased platelet deposition on atherosclerotic coronary arteries

A ruptured atherosclerotic plaque leads to exposure of deeper layers of the plaque to flowing blood and subsequently to thrombus formation. In contrast to the wealth of data on the occurrence of thrombi, little is known about the reasons why an atherosclerotic plaque is thrombogenic. One of the reasons is the relative inaccessibility of the atherosclerotic plaque. We have circumvented this problem by using 6-microns cryostat cross sections of human coronary arteries. These sections were mounted on coverslips that were exposed to flowing blood in a rectangular perfusion chamber. In normal-appearing arteries, platelet deposition was seen on the luminal side of the intima and on the adventitia. In atherosclerotic arteries, strongly increased platelet deposition was seen on the connective tissue of specific parts of the atherosclerotic plaque. The central lipid core of an advanced plaque was not reactive towards platelets. The results indicate that the atherosclerotic plaque by itself is more thrombogenic than the normal vessel wall. To study the cause of the increased thrombus formation on the atherosclerotic plaque, perfusion studies were combined with immunohistochemical studies. Immunohistochemical studies of adhesive proteins showed enrichment of collagen types I, III, V, and VI, vitronectin, fibronectin, fibrinogen/fibrin, and thrombospondin in the atherosclerotic plaque. Laminin and collagen type IV were not enriched. von Willebrand Factor (vWF) was not present in the plaque. The pattern of increased platelet deposition in serial cross sections corresponded best with areas in which collagen types I and III were enriched, but there were also areas in the plaque where both collagens were enriched but no increased reactivity was seen. Inhibition of platelet adhesion with a large range of antibodies or specific inhibitors showed that vWF from plasma and collagen types I and/or III in the plaque were involved. Fibronectin from plasma and fibronectin, fibrinogen, laminin, and thrombospondin in the vessel wall had no effect on platelet adhesion. We conclude that the increased thrombogenicity of atherosclerotic lesions is due to changes in quantity and nature of collagen types I and/or III.

Thrombus formation and platelet-vessel wall interaction in the nephrotic syndrome under flow conditions

Increased in vitro platelet aggregability and hypercoagulability are generally held to be main determinants in the prethrombotic state in nephrosis. In vivo, however, thrombotic events depend on the dynamic interaction of flowing blood with the vessel wall. The present study confirms that aggregability of platelets of nephrotic patients is significantly increased by mere stirring or by exogenous stimuli as adenosine diphosphate and arachidonic acid. Moreover, the nephrotic patients have high von Willebrand factor and decreased red blood cell deformability, which normally increase platelet-vessel wall interaction. However, perfusion studies under well-defined flow conditions, in which anticoagulated nephrotic blood was exposed to deendothelialized human umbilical artery segments and sprayed collagen, showed normal platelet adhesion and only a modest increase in the deposition of platelet aggregates. This suggests that some factor counteracts platelet-vessel wall interaction under flow conditions in the nephrotic syndrome. When tissue factor associated with endothelial extracellular matrix (ECM) was allowed to generate thrombin, perfusions with nephrotic blood over this ECM resulted in a strong increase in fibrin generation. The capacity of patient blood to form increased amounts of fibrin appeared strongly correlated with the level of hyperfibrinogenemia. Platelet adhesion as well as aggregation in these experiments was even decreased below control values. This suggests that fibrin coverage may block the direct contact between blood platelets and matrix. We therefore also studied the isolated effect of high fibrinogen on platelet-vessel wall interaction by increasing fibrinogen concentrations in normal blood. Modulation of fibrinogen concentrations in normal blood could mimic all the observations in nephrotic blood: platelet aggregation in suspension increased with increasing concentrations of fibrinogen, while platelet adhesion and aggregate formation under flow conditions decreased. In perfusions over tissue factor-rich matrix, fibrin deposition increased. Therefore, our observations indicate that nephrotic hyperaggregability in suspension is not associated with increased platelet vessel wall-interaction under flow conditions. The latter is probably counteracted by high levels of fibrinogen. Our observations further suggest that hyperfibrinogenemia may be a major thrombotic risk factor in nephrosis by inducing more fibrin depositions.