Fibrin-Dependent Platelet Procoagulant Activity Requires GPIb Receptors and von Willebrand Factor

Platelets provide robustness of spatial blood coagulation to the variation of initial conditions

Activated platelets provide phospholipid surface and secrete coagulation factors, enhancing blood clotting. We investigated the role of platelets in the regulation of blood coagulation spatial dynamics. We activated blood clotting with tissue factor-bearing (TF) surface in platelet-rich plasma (PRP) or platelet-free plasma (PFP). When blood coagulation was initiated by high TF density, clot growth rate (V) in PRP (2 × 105/μL platelets) was only 15 % greater than in PFP. Spatial distribution of thrombin in PRP had a peak-like shape in the area of the fibrin clot edge, while in PFP thrombin was distributed in the shape of descending plateau. Platelet inhibition with prostaglandin E1 or cytochalasin D made spatial thrombin distribution look like in the case of PFP. Inhibition of blood coagulation by natural endogenous inhibitor heparin was diminished in PRP, while the effect of the exogenous or artificial inhibitors (rivaroxaban, nitrophorin, hirudin) remained undisturbed in the presence of platelets. Ten times decrease of the TF surface density greatly depressed blood coagulation in PFP. In PRP only clotting initiation phase was, while the propagation phase remained intact. Coagulation factor deficiency greatly reduced amount of thrombin and decreased V in PFP rather than in PPR. Thus, platelets were redundant for clotting in normal plasma under physiological conditions but provided robustness of the coagulation system to the changes in initial conditions.

Endothelial-derived von Willebrand factor accelerates fibrin clotting within engineered microvessels

BACKGROUND

Von Willebrand factor (VWF) is classically associated with primary hemostasis and platelet-rich arterial thromboses, but recently has also been implicated in fibrin clotting and venous thrombosis. Direct interaction between fibrin and VWF may mediate these processes, although prior reports are conflicting.

OBJECTIVES

We combined two complementary platforms to characterize VWF-fibrin(ogen) interactions and identify their potential physiologic significance.

METHODS

Engineered microvessels were lined with human endothelial cells, cultured under flow, and activated to release VWF and form transluminal VWF fibers. Fibrinogen, fibrin monomers, or polymerizing fibrin were then perfused, and interactions with VWF evaluated. Thrombin and fibrinogen were perfused into living versus paraformeldahyde-fixed microvessels and the pressure drop across microvessels monitored. Separately, protein binding to tethered VWF was assessed on a single-molecule level using total internal reflection fluorescence (TIRF) microscopy.

RESULTS

Within microvessels, VWF fibers colocalized with polymerizing fibrin, but not fibrinogen. TIRF microscopy showed no colocalization between VWF and fibrinogen or fibrin monomers in a microfluidic flow chamber across a range of shear rates and protein concentrations. Thrombin-mediated fibrin polymerization within living microvessels triggered endothelial VWF release, increasing the rate and amount of microvessel obstruction compared to fixed vessels with an inert endothelium.

CONCLUSIONS

We did not identify specific binding between fibrin(ogen) and VWF at a single-molecule level. Despite this, our results suggest that rapid release of endothelial VWF during clotting may provide a physical support for fibrin polymerization and accelerate thrombosis. This interaction may be of fundamental importance for the understanding and treatment of human thrombotic disease.

Quantification of von Willebrand factor and ADAMTS-13 after traumatic injury: a pilot study

BACKGROUND

Von Willebrand factor (VWF) is an acute phase reactant synthesized in the megakaryocytes and endothelial cells. VWF forms ultra-large multimers (ULVWF) which are cleaved by the metalloprotease ADAMTS-13, preventing spontaneous VWF-platelet interaction. After trauma, ULVWF is released into circulation as part of the acute phase reaction. We hypothesized that trauma patients would have increased levels of VWF and decreased levels of ADAMTS-13 and that these patients would have accelerated thrombin generation.

METHODS

We assessed plasma concentrations of VWF antigen and ADAMTS-13 antigen, the Rapid Enzyme Assays for Autoimmune Diseases (REAADS) activity of VWF, which measure exposure of the platelet-binding A1 domain, and thrombin generation kinetics in 50 samples from 30 trauma patients and an additional 21 samples from volunteers. Samples were analyzed at 0 to 2 hours and at 6 hours from the time of injury. Data are presented as median (IQR) and Kruskal-Wallis test was performed between trauma patients and volunteers at both time points.

RESULTS

REAADS activity was greater in trauma patients than volunteers both at 0 to 2 hours (190.0 (132.0-264.0) vs. 92.0 (71.0-114.0), p<0.002) and at 6 hours (167.5 (108.0-312.5.0) vs. 92.0 (71.0-114.0), p<0.001). ADAMTS-13 antigen levels were also decreased in trauma patients both at 0 to 2 hours (0.84 (0.51-0.94) vs. 1.00 (0.89-1.09), p=0.010) and at 6 hours (0.653 (0.531-0.821) vs. 1.00 (0.89-1.09), p<0.001). Trauma patients had accelerated thrombin generation kinetics, with greater peak height and shorter time to peak than healthy volunteers at both time points.

DISCUSSION

Trauma patients have increased exposure of the VWF A1 domain and decreased levels of ADAMTS-13 compared with healthy volunteers. This suggests that the VWF burst after trauma may exceed the proteolytic capacity of ADAMTS-13, allowing circulating ULVWF multimers to bind platelets, potentially contributing to trauma-induced coagulopathy.

LEVEL OF EVIDENCE

Prospective case cohort study.

Interplay between platelets and coagulation

Haemostasis stops bleeding at the site of vascular injury and maintains the integrity of blood vessels through clot formation. This regulated physiological process consists of complex interactions between endothelial cells, platelets, von Willebrand factor and coagulation factors. Haemostasis is initiated by a damaged vessel wall, followed with a rapid adhesion, activation and aggregation of platelets to the exposed subendothelial extracellular matrix. At the same time, coagulation factors aggregate on the procoagulant surface of activated platelets to consolidate the platelet plug by forming a mesh of cross-linked fibrin. Platelets and coagulation mutually influence each other and there are strong indications that, thanks to the interplay between platelets and coagulation, haemostasis is far more effective than the two processes separately. Clinically this is relevant because impaired interaction between platelets and coagulation may result in bleeding complications, while excessive platelet-coagulation interaction induces a high thrombotic risk. In this review, platelets, coagulation factors and the complex interaction between them will be discussed in detail.

Paradigm shift for the treatment of hereditary haemophilia: Towards precision medicine

Patients with haemophilia A (HA) or B (HB) experience spontaneous limb- or life-threatening bleedings which are prevented by regular prophylactic intravenous infusions of the deficient coagulation factor (FVIII or FIX). Prophylaxis with subcutaneous long-acting non-factor products that improve in vivo thrombin generation is now under intensive investigation (concizumab, fitusiran) or successfully employed (emicizumab) in haemophilia patients. Both haemophilia patients with/without inhibitors take advantage of non-factor products employed alone. In those who also need bypassing agents (or FVIII concentrates) for breakthrough bleeds, thromboembolic events and/or thrombotic microangiopathy may occur. By enhancing thrombin generation, prothrombotic mutations co-segregating with FVIII/FIX gene mutations may trigger thrombotic episodes in HA patients carrying acquired thrombogenic factors (e.g. venous catheters). A thorough knowledge of individual needs increasingly contributed to improve comprehensive care and personalize treatments in haemophilia. Integrating genomics, lifestyle and environmental data is expected to be key to: 1) identify which haemophilia patients are less likely to benefit from a given intervention; 2) define optimal dosing and scheduling of bypassing agents (or FVIII) to employ in combination with non-factor products; 3) establish tests to monitor in vivo thrombin generation; 4) improve communication and deliver results to individuals. As individual outcomes will be improved and the risk of adverse events minimized, non-factor products will come into wider use within the haemophilia community, and patients will hopefully have no more risks of breakthrough bleeds. The risks of a normal life for a "former haemophilia patient" is likely to change the treatment landscape and the structure of haemophilia Centers.

Biological markers of hemostasis and endothelial activation in patients with a hematological malignancy with or without stem cell transplants

INTRODUCTION

In this study, we analyzed the changes of thrombin generation as marker of coagulation activation and von Willebrand factor (vWF) levels as a marker of endothelial activation in patients undergoing chemotherapy, autologous, or allogeneic HSCT. We studied possible associations to triggering factors, including acute GVHD, thrombosis, time to engraftment, and bleeding complications.

METHODS

Seventy-six patients treated for hematologic malignancies at the University Hospital Basel between 2005 and 2008 took part in this study. Blood samples were collected before the start of chemotherapy or conditioning regime (median day -2), in an early phase (median day + 12), and at a later point in time (median day + 24).

RESULTS

Thrombin generation decreased in all three groups to about 50% of the initial value. Patients undergoing autologous or allogeneic HSCT showed significantly (P = .026 and P = .01) higher vWF levels than patients undergoing chemotherapy. Eighteen patients (42%) receiving allogeneic HSCT developed GVHD, vWF levels in patients with GVHD were significantly (P = .008) higher than in patients without GVHD.

DISCUSSION

Patients receiving autologous or allogeneic HSCT had significantly higher vWF levels in the acute phase after the transplant than patients receiving chemotherapy alone, implicating a persistent stimulation of the endothelium, possibly within the context of GVHD.

Platelets compensate for poor thrombin generation in type 3 von Willebrand disease

In type 3 von Willebrand disease (VWD3), the most severe form with absent von Willebrand factor (VWF), the bleeding phenotype is variable. Platelet contribution to the hemostatic defect in VWD3 calls upon further studies. We investigated the contribution of platelets to in vitro thrombin generation (TG) and platelet procoagulant activity in VWD3. TG was assessed by calibrated automated thrombogram (CAT) in platelet-poor (PPP) and -rich plasma (PRP) from 9 patients before and in 6 patients also 30 min after receiving their regular VWF therapy. Responsiveness of PPP to FVIII and protein S was also investigated. TG data were compared with routine laboratory variables, rotational thromboelastometry (ROTEM) and platelet expression of P-selectin and phosphatidylserine in flow cytometry. Compared with healthy controls, TG was markedly decreased in VWD3 PPP (peak thrombin was 16% of normal median), but not in PRP (77% of normal median) (p = 0.002). Six out of nine patients (67%) were high responders in their platelet P-selectin, and 5/9 (56%) in phosphatidylserine expression. Replacement therapy improved TG in PPP, while in PRP TG only modestly increased or was unaffected. In PPP, FVIII levels associated with TG and in vitro FVIII-supplemented TG inclined up to threefold. Conversely, a FVIII inhibitory antibody reduced plasma TG in all, but especially in patients with remnant FVIII levels. Inhibition of protein S improved plasma TG, particularly at low FVIII levels. ROTEM failed to detect VWD3.In VWD3, TG is reduced in PPP and regulated by FVIII and protein S, but TG is close to normal in PRP. VWD3 platelets seem to compensate for the FVIII-associated reduction in TG by their exposure of P-selectin and phosphatidylserine.

Integrating platelet and coagulation activation in fibrin clot formation

Platelets interact with the coagulation system in a multitude of ways, not only during the phases of thrombus formation, but also in specific areas within a formed thrombus. This review discusses current concepts of platelet control of thrombin generation, fibrin formation and structure, and anticoagulation. Indicated are how combined signalling via the platelet receptors for collagen (glycoprotein VI) and thrombin induces the secretion of (anti)coagulation factors, as well as surface exposure of phosphatidylserine, thereby catalysing thrombin generation. This procoagulant platelet response is also facilitated by the adhesive complexes glycoprotein Ib-V-IX and integrin αIIbβ3. In the buildup of a platelet-fibrin thrombus, the extrinsic, tissue factor-driven coagulation pathway is predominant in early stages, while the intrinsic, factor XII pathway seems to promote at later time points. Already early generation of thrombin enforces platelet responses and stimulates intra-thrombus heterogeneity with patches of loosely aggregated, contracted, and phosphatidylserine-exposing platelets. Fibrin actively formed on the surface of activated platelets supports thrombus growth, but also captures thrombin. The fibrin distribution in a thrombus appears to rely on the local procoagulant trigger and the blood flow rate. Clinical studies support the importance of the platelet-coagulation interplay, by showing beneficial effects of combination therapy in the secondary prevention of cardiovascular disease.

Correlative analysis of nanoparticle tracking, flow cytometric and functional measurements for circulating microvesicles in normal subjects

INTRODUCTION

Circulating microvesicles (MV) can be analysed using a number of different techniques. The aim of this study was to evaluate the correlation between functional procoagulant based assays including thrombin generation, factor Xa activation test (XaCT), and phosphatidylserine factor Xa-activity by ELISA with optical MV enumeration by flow cytometry and nanoparticle tracking analysis.

METHODS

Citrated blood samples were collected from 60 healthy volunteer blood donors after informed consent. Platelet free plasma was prepared using a standardized published protocol. MV subsets were enumerated by flow cytometry (BDFACS Canto) after staining with specific antibodies for platelets (CD41), endothelial cells (CD105), red cells (CD235) monocytes (CD14), tissue factor (CD142) and for phosphatidylserine expression by binding to annexin V. A standardized protocol using counting beads was employed. Nanotracking analysis was performed on both scatter and fluorescent settings after MV staining with quantum dot stain, Qdot 655. Procoagulant function was assessed by the XaCT assay on an automated coagulation analyser and by thrombin generation assay measuring endogenous thrombin potential (ETP), lagtime, peak (PEAK) and time to peak (ttPEAK) using a Calibrated Automated Thrombogram (CAT). The statistical analysis was carried out with Statistica 12 software using non-parametric tests (Spearman rank order correlations, with significance set at p<0.05).

RESULTS

In normal healthy subjects, thrombin generation parameters correlated with levels of MV measured by flow cytometry. ETP, lagtime, ttPEAK and PEAK correlated with MV expressing phosphatidylserine (rs, Spearman rank order correlation was 0.29, 0.40, 0.31 and 0.34 respectively, p<0.05), and MV expressing tissue factor (rs was 0.29, 0.40, 0.31 and 0.34 respectively, p<0.05), whilst red cell derived MV correlated with lagtime, ttPEAK and PEAK (rs, was 0.35,0.30 and 0.3, respectively, p<0.05). Lagtime and ttPEAK negatively correlated with the clot based XaCT test (rs, was -0.34 and -0.30 respectively, p<0.05) and positively correlated with the ELISA MP-activity assay (rs=0.42 for both, p<0.05). In addition, endothelial MV levels weakly correlated with white cell counts (rs = 0.27, p<0.05).

CONCLUSIONS

Thrombin generation and flow cytometry for phosphatidylserine or tissue factor expressing MV correlate well as markers for procoagulant activity. A combination of optical or non-optical enumeration as well as functional methods may be required for a complete profiling of circulating MV.

Thrombin Generating Capacity and Phenotypic Association in ABO Blood Groups

Individuals with blood group O have a higher bleeding risk than non-O blood groups. This could be explained by the lower levels of FVIII and von Willebrand Factor (VWF) levels in O individuals. We investigated the relationship between blood groups, thrombin generation (TG), prothrombin activation and thrombin inactivation. Plasma levels of VWF, FVIII, antithrombin, fibrinogen, prothrombin and α₂Macroglobulin (α₂M) levels were determined. TG was measured in platelet rich (PRP) and platelet poor plasma (PPP) of 217 healthy donors and prothrombin conversion and thrombin inactivation were calculated. VWF and FVIII levels were lower (75% and 78%) and α₂M levels were higher (125%) in the O group. TG is 10% lower in the O group in PPP and PRP. Less prothrombin was converted in the O group (86%) and the thrombin decay capacity was lower as well. In the O group, α₂M plays a significantly larger role in the inhibition of thrombin (126%). In conclusion, TG is lower in the O group due to lower prothrombin conversion, and a larger contribution of α₂M to thrombin inactivation. The former is unrelated to platelet function because it is similar in PRP and PPP, but can be explained by the lower levels of FVIII.

Platelet glycoprotein VI binds to polymerized fibrin and promotes thrombin generation

Fibrin, the coagulation end product, consolidates the platelet plug at sites of vascular injury and supports the recruitment of circulating platelets. In addition to integrin αIIbβ3, another as-yet-unidentified receptor is thought to mediate platelet interaction with fibrin. Platelet glycoprotein VI (GPVI) interacts with collagen and several other adhesive macromolecules. We evaluated the hypothesis that GPVI could be a functional platelet receptor for fibrin. Calibrated thrombin assays using platelet-rich plasma (PRP) showed that tissue factor-triggered thrombin generation was impaired in GPVI-deficient patients and reduced by the anti-GPVI Fab 9O12. Assays on reconstituted PRP and PRP from fibrinogen-deficient patients revealed a fibrinogen-dependent enhancement of thrombin generation, which relied on functional GPVI. The effect of GPVI was found to depend on fibrin polymerization. A binding assay showed a specific interaction between GPVI-Fc and fibrin, inhibited by the Fab 9O12. This Fab also reduced platelet adhesion to fibrin at low (300 s(-1)) and high (1500 s(-1)) wall shear rates. Platelets adherent to fibrin displayed shape change, exposure of procoagulant phospholipids, and the formation of small clots. When hirudinated blood was perfused at 1500 s(-1) over preformed fibrin-rich clots, the Fab 9O12 decreased the recruitment of platelets by up to 85%. This study identifies GPVI as a platelet receptor for polymerized fibrin with 2 major functions: (1) amplification of thrombin generation and (2) recruitment of circulating platelets to clots. These so-far-unrecognized properties of GPVI confer on it a key role in thrombus growth and stabilization.

A recombinant fragment of von Willebrand factor reduces fibrin-rich microthrombi formation in mice with endotoxemia

INTRODUCTION

Disseminated fibrin deposition in the microvasculature such as in disseminated intravascular coagulation (DIC) arises from uninhibited activated coagulation secondary to sustained systemic inflammation. Currently there is no treatment for DIC. Treating the underlying trigger and supportive care are the current recommendations to manage DIC. This study aims at using recombinant von Willebrand factor (VWF) A2 domain polypeptide to inhibit VWF-mediated platelet adhesion to fibrin and prevent DIC.

MATERIALS AND METHODS

We use flow chamber assay to test the capacity of purified A2 protein to inhibit platelet adhesion to immobilized fibrin(ogen) and platelet-fibrin clot formation. We use a murine model of lipopolysaccharide-induced DIC to examine the effect of A2 protein on DIC.

RESULTS

The A2 protein blocked flow-dependent platelet adhesion to fibrin, delayed fibrin polymerization, and inhibited platelet-fibrin clot formation in vitro. The infusion of the purified A2 protein to the endotoxin-treated mice prevented fibrin-rich microthrombi formation in brain, lung, kidney, and liver. It also attenuated levels of inflammatory mediators, and markedly reduced mortality rates at 96hours.

CONCLUSIONS

The A2 protein inhibited platelet interaction with fibrin(ogen). Furthermore, A2 prevented disseminated fibrin-rich microthrombi and decrease mortality in a lipopolysaccharide-induced DIC murine model. A2 could provide a novel therapeutic approach in critically ill patients with uninhibited activated coagulation and disseminated fibrin deposition such as DIC.

Analysis of platelet function and dysfunction

Although platelets act as central players of haemostasis only their cross-talk with other blood cells, plasma factors and the vascular compartment enables the formation of a stable thrombus. Multiple activation processes and complex signalling networks are responsible for appropriate platelet function. Thus, a variety of platelet function tests are available for platelet research and diagnosis of platelet dysfunction. However, universal platelet function tests that are sensitive to all platelet function defects do not exist and therefore diagnostic algorithms for suspected platelet function disorders are still recommended in clinical practice. Based on the current knowledge of human platelet activation this review evaluates point-of-care related screening tests in comparison with specific platelet function assays and focuses on their diagnostic utility in relation to severity of platelet dysfunction. Further, systems biology-based platelet function methods that integrate global and specific analysis of platelet vessel wall interaction (advanced flow chamber devices) and post-translational modifications (platelet proteomics) are presented and their diagnostic potential is addressed.

Thrombin-dependent Incorporation of von Willebrand Factor into a Fibrin Network

Attachment of platelets from the circulation onto a growing thrombus is a process involving multiple platelet receptors, endothelial matrix components, and coagulation factors. It has been indicated previously that during a transglutaminase reaction activated factor XIII (FXIIIa) covalently cross-links von Willebrand factor (VWF) to polymerizing fibrin. Bound VWF further recruits and activates platelets via interactions with the platelet receptor complex glycoprotein Ib (GPIb). In the present study we found proof for binding of VWF to a fibrin monomer layer during the process of fibrinogen-to-fibrin conversion in the presence of thrombin, arvin, or a snake venom from Crotalus atrox. Using a domain deletion mutant we demonstrated the involvement of the C domains of VWF in this binding. Substantial binding of VWF to fibrin monomers persisted in the presence of the FXIIIa inhibitor K9-DON, illustrating that cross-linking via factor XIII is not essential for this phenomenon and suggesting the identification of a second mechanism through which VWF multimers incorporate into a fibrin network. Under high shear conditions, platelets were shown to adhere to fibrin only if VWF had been incorporated. In conclusion, our experiments show that the C domains of VWF and the E domain of fibrin monomers are involved in the incorporation of VWF during the polymerization of fibrin and that this incorporation fosters binding and activation of platelets. Fibrin thus is not an inert end product but partakes in further thrombus growth. Our findings help to elucidate the mechanism of thrombus growth and platelet adhesion under conditions of arterial shear rate.

Targeting platelet GPIbβ reduces platelet adhesion, GPIb signaling and thrombin generation and prevents arterial thrombosis

OBJECTIVE

The glycoprotein (GP) Ib-V-IX complex regulates the adhesion, activation, and procoagulant activity of platelets. We previously reported that RAM.1, a rat monoclonal antibody directed against the extracellular domain of mouse GPIbβ, diminished adhesion of platelets and chinese hamster ovary cells transfected with the human GPIb-IX complex to von Willebrand factor under flow conditions. Here, we further evaluated the functional importance of GPIbβ by studying the impact of RAM.1 on GPIb-mediated platelet responses and in vitro and in vivo thrombus formation.

APPROACH AND RESULTS

We show that RAM.1 dramatically reduced GPIb-mediated filopodia extension of chinese hamster ovary GPIb-IX cells after adhesion to von Willebrand factor. RAM.1 also reduced filopodia extension and GPIb-mediated Ca(2+) signaling after adhesion of mouse platelets to von Willebrand factor. RAM.1 inhibited thrombin generation in platelet-rich plasma without impairing phosphatidylserine exposure. In addition, RAM.1 reduced thrombus formation after perfusion of mouse whole blood over collagen in a shear-dependent manner. This effect was confirmed in vivo, because injection of F(ab)'2 fragments of RAM.1 diminished thrombus formation induced by laser beam injury of mesenteric arterioles and forceps injury of the abdominal aorta. In contrast, RAM.1 F(ab)'2 did not prolong the tail-bleeding time or increase the volume of blood lost.

CONCLUSIONS

These findings are the first evidence that targeting a subunit other than GPIbα can lead to an antithrombotic effect via the GPIb-V-IX complex. This could represent an alternative way to reduce thrombus formation with a minor impact on hemostasis.

New Fundamentals in Hemostasis

Hemostasis encompasses the tightly regulated processes of blood clotting, platelet activation, and vascular repair. After wounding, the hemostatic system engages a plethora of vascular and extravascular receptors that act in concert with blood components to seal off the damage inflicted to the vasculature and the surrounding tissue. The first important component that contributes to hemostasis is the coagulation system, while the second important component starts with platelet activation, which not only contributes to the hemostatic plug, but also accelerates the coagulation system. Eventually, coagulation and platelet activation are switched off by blood-borne inhibitors and proteolytic feedback loops. This review summarizes new concepts of activation of proteases that regulate coagulation and anticoagulation, to give rise to transient thrombin generation and fibrin clot formation. It further speculates on the (patho)physiological roles of intra- and extravascular receptors that operate in response to these proteases. Furthermore, this review provides a new framework for understanding how signaling and adhesive interactions between endothelial cells, leukocytes, and platelets can regulate thrombus formation and modulate the coagulation process. Now that the key molecular players of coagulation and platelet activation have become clear, and their complex interactions with the vessel wall have been mapped out, we can also better speculate on the causes of thrombosis-related angiopathies.

Platelet-based coagulation: different populations, different functions

Platelets in a thrombus interact with (anti)coagulation factors and support blood coagulation. In the concept of cell-based control of coagulation, three different roles of platelets can be distinguished: control of thrombin generation, support of fibrin formation, and regulation of fibrin clot retraction. Here, we postulate that different populations of platelets with distinct surface properties are involved in these coagulant functions. Platelets with elevated Ca(2+) and exposed phosphatidylserine control thrombin and fibrin generation, while platelets with activated α(IIb) β(3) regulate clot retraction. We review how coagulation factor binding depends on the platelet activation state. Furthermore, we discuss the ligands, platelet receptors and downstream intracellular signaling pathways implicated in these coagulant functions. These insights lead to an adapted model of platelet-based coagulation.

Historical perspective and future directions in platelet research

Platelets are a remarkable mammalian adaptation that are required for human survival by virtue of their ability to prevent and arrest bleeding. Ironically, however, in the past century, the platelets' hemostatic activity became maladaptive for the increasingly large percentage of individuals who develop age-dependent progressive atherosclerosis. As a result, platelets also make a major contribution to ischemic thrombotic vascular disease, the leading cause of death worldwide. In this brief review, I provide historical descriptions of a highly selected group of topics to provide a framework for understanding our current knowledge and the trends that are likely to continue into the future of platelet research. For convenience, I separate the eras of platelet research into the "Descriptive Period" extending from ~1880-1960 and the "Mechanistic Period" encompassing the past ~50 years since 1960. We currently are reaching yet another inflection point, as there is a major shift from a focus on traditional biochemistry and cell and molecular biology to an era of single molecule biophysics, single cell biology, single cell molecular biology, structural biology, computational simulations, and the high-throughput, data-dense techniques collectively named with the "omics postfix". Given the progress made in understanding, diagnosing, and treating many rare and common platelet disorders during the past 50 years, I think it appropriate to consider it a Golden Age of Platelet Research and to recognize all of the investigators who have made important contributions to this remarkable achievement..

Key role of glycoprotein Ib/V/IX and von Willebrand factor in platelet activation-dependent fibrin formation at low shear flow

A microscopic method was developed to study the role of platelets in fibrin formation. Perfusion of adhered platelets with plasma under coagulating conditions at a low shear rate (250(-1)) resulted in the assembly of a star-like fibrin network at the platelet surface. The focal fibrin formation on platelets was preceded by rises in cytosolic Ca(2+), morphologic changes, and phosphatidylserine exposure. Fibrin formation was slightly affected by α(IIb)β(3) blockage, but it was greatly delayed and reduced by the following: inhibition of thrombin or platelet activation; interference in the binding of von Willebrand factor (VWF) to glycoprotein Ib/V/IX (GpIb-V-IX); plasma or blood from patients with type 1 von Willebrand disease; and plasma from mice deficient in VWF or the extracellular domain of GpIbα. In this process, the GpIb-binding A1 domain of VWF was similarly effective as full-length VWF. Prestimulation of platelets enhanced the formation of fibrin, which was abrogated by blockage of phosphatidylserine. Together, these results show that, in the presence of thrombin and low shear flow, VWF-induced activation of GpIb-V-IX triggers platelet procoagulant activity and anchorage of a star-like fibrin network. This process can be relevant in hemostasis and the manifestation of von Willebrand disease.

Decreased fibrin network permeability and impaired fibrinolysis in the acute and convalescent phase of ischemic stroke

INTRODUCTION

We investigated fibrin network permeability and fibrinolysis in the acute and convalescent phase of ischemic stroke.

METHODS

20 patients with a mean age of 74 years were studied in the acute (day 1) and convalescent phase (day 60) of ischemic stroke. 23 healthy individuals (controls) were also investigated. Fibrin formation in the samples was triggered by addition of tissue factor (1 pmol/L) and washed frozen-thawed platelets obtained from a healthy donor. The permeability constant (K(s)), which reflects fibrin network permeability, was then calculated from liquid flow measurements. A global assay newly developed in our group was also employed to determine the balance between fibrin formation ("Coagulation profile"; Cp) and fibrin degradation ("Fibrinolysis profile"; Fp) in the same samples. We also measured PAI-1 antigen and fibrinogen concentrations in plasma.

RESULTS

As compared to controls, the stroke patients had lower Ks (lower fibrin network permeability) both on day 1 and on day 60 (p < 0.01 and p < 0.05, respectively). Fibrinolysis, assessed by Fp, was reduced on both day 1 and day 60 (p < 0.001, compared to controls), and PAI-1 concentrations were increased (p < 0.01 for both, compared to controls). Fibrin formation capacity in plasma (i.e. Cp) was increased in the acute phase (p < 0.05) but not in the convalescence, as compared to controls.

CONCLUSION

The combination of a proneness to form a tighter fibrin network and impaired fibrinolysis is a feature of ischemic stroke that is present in both the acute and convalescent phase of the disease.

A central role of GPIb-IX in the procoagulant function of platelets that is independent of the 45-kDa GPIbα N-terminal extracellular domain

Activated platelets become procoagulant and efficiently promote the conversion of prothrombin to thrombin. A role of the GPIb-V-IX complex has long been postulated in view of the decreased prothrombin consumption in Bernard-Soulier patients. We evaluated the impact of GPIb-V-IX deficiency and the requirement for the GPIbα extracellular domain. In GPIbβ−/− mice, thrombin generation was profoundly decreased in tissue factor– or collagen-related peptide (CRP)–activated platelet-rich plasma and in washed platelets supplemented with normal plasma or with FVa, FXa, and prothrombin. Phosphatidylserine (PS) exposure was similarly decreased in response to thrombin, CRP, or CRP + PAR4 peptide despite a normal platelet phospholipid composition. The hypothesis that these defects originate from lack of the GPIbα N-terminal domain was evaluated after its removal from normal mouse and human platelets with Nk protease or O-sialoglycoprotein endopeptidase. Unexpectedly, the treated platelets exhibited normal thrombin generation and PS exposure, indicating that GPIb-V-IX regulates procoagulant activity independently of its GPIbα-binding region. These results suggested a more general structuring role through intracellular cytoskeleton-anchoring portions regulating responses leading to PS exposure. This hypothesis was supported by the decreased calcium mobilization observed in GPIbβ−/− platelets in response to several agonists, some acting independently of GPIb, in contrast to the normal calcium responses in Nk protease–treated platelets.

Thrombin generation testing in routine clinical practice: are we there yet?

Thrombin is the central enzyme in the coagulation cascade. Estimation of an individual's potential to generate thrombin may correlate more closely with a hyper- or hypo-coagulable phenotype, compared to traditional coagulation tests. The possible correlation and recent technical advances in thrombin generation measurement has caused a significant interest in the method and the development of commercial assays. Several variations of the assay exist depending on the defect to be investigated. Fluorogenic thrombin generation assays have acceptable intra-laboratory variation but a higher inter-laboratory variation. Variation in preanalytical variables makes comparisons between studies difficult. Thrombin generation is highly variable between individuals and there are suggestions that this may allow individualized treatment based on global haemostatic response in patients with bleeding disorders or on anticoagulant therapy. In patients with thrombotic disorders it may be possible to identify those at higher risk of recurrent thrombosis. For both scenarios, however, data from large prospective studies are lacking or inconclusive and a good relationship between thrombin generation and phenotype remains to be established. Further standardization of the assay is needed before large multicentre studies can be conducted and until then thrombin generation in routine clinical practice is not yet a reality.

Less impairment of hemostasis and reduced blood loss in pigs after resuscitation from hemorrhagic shock using the small-volume concept with hypertonic saline/hydroxyethyl starch as compared to administration of 4% gelatin or 6% hydroxyethyl starch solution

BACKGROUND

Small-volume resuscitation using hypertonic saline/hydroxyethyl starch 200/0.62 (HS-HES) has been shown to be an effective alternative to the administration of crystalloids or colloids in trauma patients. All i.v. fluids cause dose-related dilutional coagulopathy and show intrinsic effects on the hemostatic system, but only few data refer to functional consequences after small-volume resuscitation.

METHODS

Using thrombelastometry (ROTEM), we studied 30 pigs (weighing 35-45 kg) after withdrawal of 60% of blood volume [1484 mL (1369-1624 mL)] and receiving 4 mL/kg HS-HES for compensation of blood loss or 4% gelatin or 6% HES 130/0.4 in a 1:1 ratio to lost blood volume. To compare the ROTEM variables (coagulation time, clot formation time, alpha angle, clot firmness, and fibrinogen polymerization) with bleeding tendency, a hepatic incision was made and blood loss was measured.

RESULTS

Median (25th, 75th percentile) fibrinogen polymerization was significantly higher after HS-HES infusion [11 mm (10, 11), P = 0.0034] when compared with administration of 4% gelatin [4.5 mm (3.0, 5.8)] or HES 130/0.4 [3.5 mm (2.3, 4.0)]. Median blood loss after liver incision was 725 mL (900, 375) after HS-HES, 1625 mL (1275, 1950) after 4% gelatin, and 1600 mL (1500, 1800) after 6% HES 130/0.4 (P = 0.004). Hemodynamic stabilization was traceable in all groups but showed differences regarding filling pressures.

CONCLUSIONS

Resuscitation from hemorrhagic shock with HS-HES 200/0.62 results in less impairment of clot formation when compared with compensation of blood loss by administering 6% HES 130/0.4 or 4% gelatin.

Recollections on thrombin generation

Against an autobiographic background, a historical sketch is given of the development of the technique of thrombin generation, from subsampling to duly calibrated continuous measurement with fluorogenic substrates. Its application to various problems in the pathophysiology of hemostasis and thrombosis is discussed.

Bernard-Soulier syndrome (hemorrhagiparous thrombocytic dystrophy)

Bernard-Soulier syndrome (BSS), also known as Hemorrhagiparous thrombocytic dystrophy, is a hereditary bleeding disorder affecting the megakaryocyte/platelet lineage and characterized by bleeding tendency, giant blood platelets and low platelet counts. This syndrome is extremely rare as only approximately 100 cases have been reported in the literature. Clinical manifestations usually include purpura, epistaxis, menorrhagia, gingival and gastrointestinal bleeding. The syndrome is transmitted as an autosomal recessive trait. The underlying defect is a deficiency or dysfunction of the glycoprotein GPIb-V-IX complex, a platelet-restricted multisubunit receptor required for normal primary hemostasis. The GPIb-V-IX complex binds von Willebrand factor, allowing platelet adhesion and platelet plug formation at sites of vascular injury. Genes coding for the four subunits of the receptor, GPIBA, GPIBB, GP5 and GP9, map to chromosomes 17p12, 22q11.2, 3q29, and 3q21, respectively. Defects have been identified in GPIBA, GPIBB, and GP9 but not in GP5. Diagnosis is based on a prolonged skin bleeding time, the presence of a small number of very large platelets (macrothrombocytopenia), defective ristocetin-induced platelet agglutination and low or absent expression of the GPIb-V-IX complex. Prothrombin consumption is markedly reduced. The prognosis is usually good with adequate supportive care but severe bleeding episodes can occur with menses, trauma and surgical procedures. Treatment of bleeding or prophylaxis during surgical procedures usually requires platelet transfusion.

New approaches for measuring coagulation

Although specific assays of coagulation factors are essential for diagnostic purposes they only give partial information about an individual's haemostatic state. This can be better assessed by various global tests, and recent developments and evaluations of five such tests are described in this symposium: the PFA-100; waveform analysis; thrombin generation; overall haemostasis potential; thrombelastography. Each test has advantages in various applications, but the thrombin generation test and waveform analysis have been found most useful in haemophilia, whilst the PFA-100 is helpful in von Willebrand's disease.

Clopidogrel induced suppression of bovine platelet activation in vitro and a preliminary study of its effect on the development of Mannheimia haemolytica induced pneumonia

We report here on the influence of the platelet antagonist clopidogrel (Plavix) on bovine platelet function. We first evaluated the capacity of clopidogrel to inhibit adenosine diphosphate (ADP)-stimulated platelet function in the bovine species, using an ex vivo approach with blood from treated animals. Platelets isolated from treated calves displayed rapid and consistent reduction in function (aggregation, thromboxane production) upon ADP, but not platelet activating factor (PAF), stimulation. We then examined the possibility that clopidogrel could influence Mannheimia haemolytica pneumonia pathobiology using an experimental challenge model. We were unable to detect significant differences between clopidogrel treated and untreated animals when challenged with intra-tracheal inoculation of M. haemolytica. There was a trend towards inhibition of platelet degranulation in the affected regions of lungs from clopidogrel treated calves, and pre-treated challenged animals had similar amounts of fibrin deposition and enhanced fibrous tissue formation in their lungs when compared with control counterparts.

Detection of von Willebrand factor and tissue factor in platelets-fibrin rich coronary thrombi in acute myocardial infarction

The rapid closure of coronary arteries due to occlusive thrombi is the major cause of acute myocardial infarction. However, the mechanisms of coronary thrombus formation have not been elucidated. We immunohistochemically assessed the localizations and their changes over time of glycoprotein IIb/IIIa, fibrin, von Willebrand factor (vWF), and tissue factor (TF), after the onset of chest pain (<4, 4 to 6, or 6 to 12 hours), in fresh coronary thrombi causing acute myocardial infarction. The occlusive thrombi were consistently composed of platelets, fibrin, vWF, and TF from the early phase of onset, and glycoprotein IIb/IIIa and fibrin were closely associated with vWF and TF, respectively. vWF and/or TF may contribute to occlusive thrombus formation and be novel therapeutic candidates for treating patients with coronary thrombosis.

Endothelial dysfunction contributes to renal function-associated cardiovascular mortality in a population with mild renal insufficiency: the Hoorn study

Mildly impaired renal function is associated with cardiovascular morbidity and mortality. There are indications that endothelial dysfunction and/or chronic inflammation, which play an important role in atherothrombosis, are present in early stages of renal insufficiency. This study investigated whether and to which extent endothelial dysfunction and inflammation were related to renal function and contributed to renal function-associated cardiovascular mortality in a population-based cohort (n = 613), aged 50 to 75 yr, that was followed with a median duration of 12.5 yr. During follow-up, 192 individuals died (67 of cardiovascular causes). At baseline, renal function was estimated with serum creatinine, the Cockcroft-Gault formula, and the Modification of Diet in Renal Disease equation of GFR (eGFR). Endothelial function was estimated by plasma von Willebrand factor, soluble vascular cell adhesion molecule-1, and the urinary albumin-creatinine ratio. Inflammatory activity was estimated by plasma C-reactive protein and soluble intercellular adhesion molecule-1. Renal function was mildly impaired (mean eGFR 68 +/- 12 ml/min per 1.73 m(2)) and independently associated with von Willebrand factor (standardized beta -0.09; 95% confidence interval [CI] -0.18 to -0.002; P < 0.05), soluble vascular cell adhesion molecule-1 (standardized beta -0.14; 95% CI -0.22 to -0.05; P < 0.01), and albumin-creatinine ratio (standardized beta -0.15; 95% CI -0.23 to -0.08; P < 0.001) but not with markers of inflammatory activity. Renal function was inversely associated with cardiovascular and all-cause mortality. The relative risk for cardiovascular mortality but not all-cause mortality associated with renal function decreased from 1.22 to 1.12 per 5 ml/min per 1.73 m(2) decrease of eGFR after adjustment for markers of endothelial dysfunction. In conclusion, endothelial dysfunction was related to renal function and contributed to the excess in cardiovascular mortality in this population-based cohort with mild renal insufficiency.

The measurement and application of thrombin generation

It is the capacity to generate thrombin, and the enzymatic work that thrombin does, that determines blood coagulability. Therefore, measurement of the enzymatic work potential of thrombin provides a method for quantifying the composite effect of the multiple factors that determine coagulation capacity. The application of measurement of thrombin generation to clinical decision making has been hampered by numerous technical difficulties and pitfalls, many of which have now been overcome. Technical advances now permit rapid, reproducible measurement. A review of clinical studies performed to date indicates the need to appreciate the precise methodology used in each case and the need to consider standardisation in future studies. Applying thrombin generation measurement to clinical decision making will require up-to-date estimates of risks relating to the disorder and the intended therapeutic intervention. Ultimately management studies will be required if the clinical utility of measurement of thrombin generation is to be proven.

Thrombin generation assays: accruing clinical relevance

PURPOSE OF REVIEW

After decades of near oblivion, thrombin generation is being revived as an overall function test of the plasmatic coagulation system in platelet-poor plasma (PPP). In platelet-rich plasma (PRP) it assesses platelet procoagulant functions as well.

RECENT FINDINGS

The recently developed use of special fluorogenic thrombin substrates allows monitoring of thrombin concentration in clotting PPP and PRP on line in up to 24 parallel samples. Studies in model systems stress the importance of cell-bound thrombin generation such as measured in PRP.

SUMMARY

The method can be profitably applied to various hitherto unyielding problems such as the control of (low-molecular-weight) heparin therapy, the detection of lupus anticoagulant, and various forms of thrombomodulin and activated protein C resistance (including the use of oral contraceptives) as well as monitoring the treatment of hemophiliacs by factor VIII bypassing therapy. In PRP it reflects the abnormalities encountered in von Willebrand disease and Glanzmann and Bernard-Soulier thrombopathy as well as the action of antiplatelet drugs.

Facilitating roles of murine platelet glycoprotein Ib and alphaIIbbeta3 in phosphatidylserine exposure during vWF-collagen-induced thrombus formation

Vessel wall damage exposes collagen fibres, to which platelets adhere directly via the collagen receptors glycoprotein (GP) VI and integrin alpha(2)beta(1) and indirectly by collagen-bound von Willebrand factor (vWF) via the GPIb-V-IX and integrin alphaIIbbeta3 receptor complexes. Platelet-collagen interaction under shear stimulates thrombus formation in two ways, by integrin-dependent formation of platelet aggregates and by surface exposure of procoagulant phosphatidylserine (PS). GPVI is involved in both processes, complemented by alpha2beta1. In mouse blood flowing over collagen, we investigated the additional role of platelet-vWF binding via GPIb and alphaIIbbeta3. Inhibition of GPIb as well as blocking of vWF binding to collagen reduced stable platelet adhesion at high shear rate. This was accompanied by delayed platelet Ca(2+) responses and reduced PS exposure, while microaggregates were still formed. Inhibition of integrin alphaIIbbeta3 with JON/A antibody, which blocks alphaIIbbeta3 binding to both vWF and fibrinogen, reduced PS exposure and aggregate formation. The JON/A effects were not enhanced by combined blocking of GPIb-vWF binding, suggesting a function for alphaIIbbeta3 downstream of GPIb. Typically, with blood from FcR gamma-chain +/- mutant mice, expressing 50% of normal platelet GPVI levels, GPIb blockage almost completely abolished platelet adhesion and PS exposure. Together, these data indicate that, under physiological conditions of flow, both adhesive receptors GPIb and alphaIIbbeta3 facilitate GPVI-mediated PS exposure by stabilizing platelet binding to collagen. Hence, these glycoproteins have an assistant procoagulant role in collagen-dependent thrombus formation, which is most prominent at reduced GPVI activity and is independent of the presence of thrombin.

Von Willebrand factor C1C2 domain is involved in platelet adhesion to polymerized fibrin at high shear rate

Fibrin is actively involved in platelet reactions essential for thrombus growth, in which von Willebrand factor (VWF) might be an important mediator. The aim of this study was to localize VWF domains that bind to fibrin and to determine their relevance in platelet adhesion. VWF binds specifically to fibrin with an apparent Kd of 2.2 μg/mL. Competition in the presence of 2 complementary fragments, SpIII (residues 1-1365) and SpII (residues 1366-2050), indicated that the high affinity binding site for fibrin is located in the C-terminal part, thus distinct from the A domains. Comparison of 2 deleted rVWF (ΔD4B-rVWF, ΔC1C2-rVWF) suggested that the C1C2 domains contained a fibrin binding site. This site is distinct from RGD, as shown by binding of D1746G-rVWF to fibrin. Perfusion studies at high shear rate demonstrated that C1C2 domains were required for optimal platelet adhesion to fibrin. With the use of a VWF-deficient mouse model, it was found that plasma VWF is critical for platelet tethering and adhesion to fibrin. These results suggest a dual role of fibrin-bound VWF in thrombus formation: first, fibrin-bound VWF is critical in the recruitment of platelets by way of glycoprotein (GP) Ib, and, second, it contributes to stationary platelet adhesion by way of binding to activated αIIbβ3.

Fibrin polymerization is crucial for thrombin generation in platelet-rich plasma in a VWF-GPIb-dependent process, defective in Bernard-Soulier syndrome

Defective prothrombin consumption has been reported in the proband case of Bernard-Soulier syndrome (BSS). There is no consensus, however, on whether the formation of platelet procoagulant activity (PPA) is impaired in BSS and, if so, whether this is due to the lack of GPIb-V-IX-dependent binding of thrombin or of von Willebrand factor (VWF). We show thrombin generation (TG) in platelet-rich plasma of BSS (BSS-PRP) to be defective provided that fibrin remains present in the reaction mixture and that the giant platelets are not damaged by frequent subsampling. In BSS-PRP addition of (thrombin-free) fibrin did not increase TG as in normal PRP, supporting our previous hypothesis that the interaction of fibrin, VWF and GPIb triggers PPA development. Fibrin formed during the lag phase of TG by a snake venom enzyme which only removed fibrinopeptide A induced an immediate burst of TG, that was inhibited by a monoclonal antibody against GPIb (6D1) that abolishes ristocetin-induced binding of VWF to platelets. Inversely, inhibition of polymerization decreased TG and the residual activity was insensitive to 6D1. We conclude that polymerizing fibrin interacts with VWF so as to activate GPIb.

The protein kinase C inhibitor RO318220 potentiates thrombin-stimulated platelet-supported prothrombinase activity

Prothrombinase activity was tested on thrombin- and SFLLRN-activated platelets treated with RO318220, a potent inhibitor of protein kinase C. RO318220 completely inhibited platelet dense and alpha-granule secretion at a concentration of 20 microM but had no effect on prothrombinase activity in the presence of excess factor Va (20 nM). This indicates that protein kinase C activity and agonist-initiated secretion are not necessary for the development of a procoagulant surface. Treatment with 75 to 150 microM RO318220 potentiated platelet-supported thrombin generation up to 280% of control platelets with no change in Kd appFXa. Treated with increasing concentrations of RO318220, an increasing proportion of thrombin-stimulated platelets bound annexin V with decreasing binding sites per platelet. A lower mean forward scatter (FSC-H) of platelets treated with RO318220 suggested platelet vesiculation as a result of RO318220 treatment; however, 100 microM calpeptin pretreatment eliminated the decrease in FSC-H without affecting either the increase in platelets positive for annexin V binding, the decrease in binding sites per platelet, or the 3-fold increase in prothrombinase activity. Thus, RO318220 appears to increase prothrombinase activity by increasing platelet responsiveness to thrombin rather than by inducing platelet vesiculation. This suggests that RO318220 inhibits a signaling molecule within a negative regulatory pathway that governs platelet procoagulant surface changes.

Fibrinogen adsorption and platelet interactions on polymer membranes

The hemocompatibility of four different wettable polymer membranes, namely Cuprophan (CE), polyether-polycarbonate (PC-PE), polysulfone (PSU), and polyetherimide (PEI), was investigated with respect to fibrinogen (Fng) adsorption and platelet adhesion/activation. In order to estimate the polar and dispersion components of the surface free energy, contact angles using water/vapor and water/n-hexadecane systems were measured. Adsorption of fibrinogen was studied using fluorescence-labeled protein. The adsorption isotherms showed that the amount and the affinity of adsorbed Fng increased with decreasing surface wettability of the membranes, which correlates with the dispersion and polar components of the surface free energy. The conformational changes of adsorbed Fng were detected by measuring the difference between monoclonal antibody binding to the conformation-sensitive epitope in the D-domain and the binding of polyclonal anti-Fng antibody. The anticipated conformational/orientational changes were greater for PEI and PSU membranes (the least wettable membranes) and negligible for the more wettable PC-PE and CE membranes. In addition, a possible relationship with the degree of platelet activation was found, showing negligible platelet adhesion on PC-PE and CE, but high platelet adhesion on PEI and PSU. Furthermore, platelets were spread to a large extent on PEI, while the formation of aggregates was observed on PSU. This may correspond to the anticipated differences in the conformational state of Fng on both membranes.

Integrin alphaIIbbeta3 and shear-dependent action of glycoprotein Ibalpha stimulate platelet-dependent thrombin formation in stirred plasma

Under conditions of arterial-wall shear rates, platelets bind to von Willebrand factor (vWf) by way of the glycoprotein Ib (GP Ib) complex and integrin alpha(IIb)beta(3). Both adhesive receptors may also play roles in the development of procoagulant activity of platelets. Here, we investigated the effect of shear stress, as provided by a rotating cylinder, on GP Ib- and integrin alpha(IIb)beta(3)-dependent thrombin generation in coagulating platelet-rich plasma (PRP). We measured thrombin continuously with the use of fluorometry from the cleavage rate of a fluorescent low-affinity substrate. The integrin alpha(IIb)beta(3) antagonist abciximab progressively reduced the peak of thrombin formation up to 43% when rate of stirring and shear stress were increased (estimated shear rates of 105-420 s(-1)). Abciximab did not lower the peak of thrombin formation in stirred PRP from patients with Glanzmann's thrombasthenia lacking alpha(IIb)beta(3) but, surprisingly, shortened the time until onset. In PRP from control subjects, antibodies specifically directed against vWf-binding epitopes on GP Ibalpha reduced thrombin formation, with 25% to 30% at the high but not at the low stirring rate. In combination with the anti-GP Ib antibody, abciximab retained its strong inhibitory effect only at the high stirring rate. We conclude that thrombin formation and coagulation in stirred PRP depend, to a large extent, on platelet adhesion to integrin alpha(IIb)beta(3) and, in a shear-dependent way, on GP Ib.

Abciximab inhibits procoagulant activity but not the release reaction upon collagen- or clot-adherent platelets

In addition to inhibiting platelet (plt) aggregation abciximab, a glycoprotein (GP) IIb/IIIa antagonist, reduces coagulation in blood or platelet-rich plasma. We assessed the effects of abciximab (10 micro g mL(-1)) on adhesion-dependent procoagulant activity (PCA) of plt upon: (i) collagen to model initial adhesion; or (ii) plasma clot or fibrin to model a preformed thrombus with retained thrombin activity. In a two-stage assay gel-filtered plt (GFP) first adhered on collagen, plasma clot, or fibrin, and plt activation was traced with platelet factor 4 (PF 4) release. Second, PCA was measured on adherent plt (i) by soluble prothrombin fragments (F1 + 2); and (ii) chromogenically by adding defibrinated plasma and thromboplastin. Abciximab inhibited aggregation upon collagen-adherent plt both in the absence and presence of plasma. In contrast, without plasma abciximab enhanced plt deposition to fibrin surfaces depending on thrombin generation and fibrin polymerization. However, abciximab reduced PCA and generation of F1 + 2 on adherent plt surface-independently by 35%, whereas PF 4 release persisted. Also, a GP Ib inhibitor, mAb SZ2, attenuated PCA by 40% alone, and by 65% together with abciximab, leaving 35% of PCA unaltered. Abciximab decreased generation of new thrombin on both collagen- and clot-adherent plt. However, abciximab did not inhibit alpha-granule release, suggesting distinct pathways for PCA and release reaction. Deposition of isolated plt on clots in the presence of abciximab was dependent on thrombin and polymerizing plt-derived fibrin(ogen). Due to local consumption of natural anticoagulants adjacent to a preformed thrombus the antithrombotic effect of abciximab benefits from additional inhibition of thrombin.

Blood coagulation, fibrinolysis, and markers of endothelial dysfunction in systemic sclerosis

OBJECTIVE

To evaluate the coagulative/fibrinolytic cascade and the circulating markers of the endothelial injury in systemic sclerosis (SSc).

METHOD

Plasma was obtained from 29 patients with SSc and tested for thrombin-antithrombin (TAT), fragments 1+2 (F1+2), dermatansulphate (DS), thrombomodulin (TM), lipoprotein (a) [Lp(a)], von Willebrand factor (vWF), tissue type plasminogen activator (tPA), plasminogen activator inhibitor (PAI), D-dimers, intercellular adhesion molecole-1 (ICAM-1), vascular cell adhesion molecule (VCAM), and E-selectin. The data were correlated with lung (forced vital capacity, diffusing lung capacity for carbon monoxide, vital capacity) and skin (skin score) involvement.

RESULTS

Coagulation was significantly activated (increase in F1+2, P <.001; TAT, P <.01; and Lp(a), P <.05). TM was not significantly different from controls. vWF was significantly increased (P <.01), and its supranormal multimers increased in more than 50% of patients. DS was significantly increased in diffuse cutaneous SSc (P <.01). Fibrinolysis was impaired as shown by reduced D-dimers (P <.01) and decreased levels of PAI (P < 0.01). The markers of endothelial injury were also significantly elevated. DS correlated significantly with forced vital capacity (P <.01) and forced vital capacity ratio (P <.01).

CONCLUSION

Injury to the endothelium reduces endothelial function, as suggested by impairment of fibrinolysis and activation of the coagulative pathway. The loss of the balance between fibrinolysis and coagulation contributes to vessel engulfment with fibrin and breakdown of vessel patency. The increase of circulating DS suggests that this factor may be a new marker of endothelial injury.

von Willebrand factor stimulates thrombin-induced exposure of procoagulant phospholipids on the surface of fibrin-adherent platelets

Studies from our laboratory have demonstrated that von Willebrand factor (VWF) stimulates thrombin generation in platelet-rich plasma. The precise role of VWF and fibrin in this reaction, however, remained to be clarified. In the present study we utilized thrombin-free planar fibrin layers and washed platelets to examine the relationship between platelet-fibrin interaction and exposure of coagulation-stimulating phosphatidylserine (PS) under conditions of low and high shear stress. Our study confirms that platelet adhesion to fibrin at a shear rate of 1000 s(-1) requires fibrin-bound VWF. The cytosolic calcium concentration ([Ca(2+)]i) of stationary platelets was not elevated and PS exposing platelets were virtually absent (2 +/- 2%). However, thrombin activation resulted in a marked increase in the number of PS exposing platelets (up to 85 +/- 14%) along with a transient elevation in [Ca(2+)]i from 0.05 micro mol L(-1) up to 1.1 +/- 0.2 micro mol L(-1). Platelet adhesion to fibrin at a shear rate of 50 s(-1) is mediated by thrombin but not by fibrin-bound VWF. The [Ca(2+)]i of these thrombin-activated platelets was elevated (0.2 +/- 0.1 micro mol L(-1)), but only a minority of the platelets (11 +/- 8%) exposed PS. The essential role of VWF in this thrombin-induced procoagulant response became apparent from low shear rate perfusion studies over fibrin that was incubated with VWF and botrocetin. After treatment with thrombin, the majority of the adherent platelets (57 +/- 23%) exposed PS and had peak values of [Ca(2+)]i of about 0.6 micro mol L(-1). Taken together, these results demonstrate that thrombin-induced exposure of PS and high calcium response on fibrin-adherent platelets depends on shear- or botrocetin-induced VWF-platelet interaction.

The ionic contrast medium ioxaglate interferes with thrombin-mediated feedback activation of factor V, factor VIII and platelets

Clinical observation shows that radiographic contrast media (CM) may influence thrombus formation. In the search for the underlying mechanism, we have shown that the ionic CM ioxaglate is a potent inhibitor of thrombin generation in platelet-poor and platelet-rich plasma, whereas the influence of the non-ionic contrast medium iodixanol is minimal. Ioxaglate boosts the inhibitory effect of the platelet GPIIb/IIIa antagonist abciximab and the effects of ioxaglate and heparin are additive. Ioxaglate inhibits the clotting of fibrinogen and the activation of factors V and VIII, and of platelets by thrombin. It does not inhibit hydrolysis of small chromogenic thrombin substrates, nor does it influence the heparin-catalyzed inactivation of thrombin by antithrombin. We assume therefore that ioxaglate interferes with the binding of macromolecular substrates to the anionic exosite I of thrombin. The biological correlation to the observed antithrombotic effect of ioxaglate is then to be found in the inhibition of thrombin generation via inhibition of thrombin-mediated feedback activations.