Regulation of platelet adhesion to the vessel wall

Assessment of Shear-Dependent Kinetics of Primary Haemostasis With a Microfluidic Acoustic Biosensor

Primary haemostasis is a complex dynamic process, which involves in-flow interactions between platelets and sub-endothelial matrix at the area of the damaged vessel wall. It results in a first haemostatic plug, which stops bleeding, before coagulation ensues and consolidates it. The diagnosis of primary haemostasis defect would benefit from evaluation of the whole sequence of mechanisms involved in platelet plug formation in flow. This work proposes a new approach that is based on characterization of the shear-dependent kinetics that enables the evaluation of the early stages of primary haemostasis. We used a label-free method with a quartz crystal microbalance (QCM) biosensor to measure the platelet deposits over time onto covalently immobilized type I fibrillar collagen. We defined three metrics: total frequency shift, lag time, and growth rate. The measurement was completed at four predefined shear rates prevailing in small vessels (500, 770, 1000 and 1500 s^-1) during five minutes of perfusion with anticoagulated normal whole blood. The rate of the frequency shift over the first five minutes was strongly influenced by shear rate conditions, presenting a maximum around 770 s^-1, and varying by a factor larger than three in the studied shear rate range. To validate the biosensor signal, the total frequency shift was compared to results obtained by atomic force microscopy (AFM) on final platelet deposits. The results show that shear-dependent kinetic assays are promising as an advanced method for screening of primary haemostasis.

Thrombus growth modelling and stenosis prediction in the cerebral microvasculature

Cerebral microvascular occlusions cause restriction of blood supply to the brain, thus potentially severely impacting cognitive abilities. Thus, accurate prediction of thrombus growth in realistic geometries is important. Thrombi growth in an existing 13-generation cerebral microvasculature network is simulated here to study the haemodynamic effects of single and multiple blockages on the occlusion of the network. Compared to a single vessel, in a network, the occlusion probability is found to be different. It is the downstream/smaller arterioles (i.e. the 3rd, 4th, 5th, 6th generation arterioles in this study) that tend to reach occlusion first in a network and thus are the critical vessels. Simulations of simultaneous growth of two independent thrombi in the network (referred to here as the two-block case) show a close coupling between the locations of the various blocks in the network, each influencing the other's growth. The presence of the lead block (LB) slows the growth of the trailing block (TB). In some cases, it stops the TB's growth thereby preventing it from occluding the vessel. Findings in this work thus indicate that, to prevent ischaemia, blocks in the smaller arterioles need to be identified and treated first, and that this is more critical if the number of simultaneous blocks is higher.

A Miniaturized Hemoretractometer for Blood Clot Retraction Testing

Blood coagulation is a critical hemostatic process that must be properly regulated to maintain a delicate balance between bleeding and clotting. Disorders of blood coagulation can expose patients to the risk of either bleeding disorders or thrombotic diseases. Coagulation diagnostics using whole blood is very promising for assessing the complexity of the coagulation system and for global measurements of hemostasis. Despite the clinic values that existing whole blood coagulation tests have demonstrated, these systems have significant limitations that diminish their potential for point-of-care applications. Here, recent advancements in device miniaturization using functional soft materials are leveraged to develop a miniaturized clot retraction force assay device termed mHemoRetractoMeter (mHRM). The mHRM is capable of precise measurements of dynamic clot retraction forces in real time using minute amounts of whole blood. To further demonstrate the clinical utility of the mHRM, systematic studies are conducted using the mHRM to examine the effects of assay temperature, treatments of clotting agents, and pro- and anti-coagulant drugs on clot retraction force developments of whole blood samples. The mHRM's low fabrication cost, small size, and consumption of only minute amounts of blood samples make the technology promising as a point-of-care tool for future coagulation monitoring.

A novel blood incubation system for the in-vitro assessment of interactions between platelets and biomaterial surfaces under dynamic flow conditions: The Hemocoater

Hemocompatibility evaluation of biomaterials necessitates the use of blood incubation systems which simulate physiological flow conditions. However, most of the current systems have various limitations, especially restricted material variability, poor access to the test surface or damage of blood cells due to the use of a pump. In this paper, we combined the advantages of existent setups and developed a new planar shaped incubation test bench to lift those restrictions and mimic the pulsatile in-vivo situation. The adjustable flow conditions at the tested material surface were defined and corresponded to those in blood vessels. Platelet/material-interaction, as major aspect of hemocompatibility, was investigated for four common polymeric materials (polyoxymethylene, polypropylene, polyethylene and silicone elastomer) with platelet deprivation and platelet adhesion tests. Highly significant differences in the adhesion of platelets onto the tested material surfaces were measured. The number of adhered platelets on the most hydrophobic sample (silicone elastomer) was four-times higher than on the most hydrophilic sample (polyoxymethylene). These findings were confirmed with a scanning microscopic analysis and demonstrated the suitability of the testing device for the evaluation of platelet/material interactions. Moreover, hemolysis measurements demonstrated that the system did not provoke blood damage. © 2016 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 104A: 2430-2440, 2016.

Platelet secretory behaviour: as diverse as the granules … or not?

Platelets play a central role in the arrest of bleeding after damage to a blood vessel and in the development of thrombosis. Platelets rapidly respond after interaction with sub-endothelial components and release cargo from their storage granules. The three principal granule types of platelets are α-granules, dense granules and lysosomes. Timed release of granule contents and regulated expression of critical receptors are essential for maintenance of the platelet thrombus, yet also have important functions beyond hemostasis (i.e. inflammatory reactions and immune responses). α-granules store adhesive molecules such as von Willebrand factor and fibrinogen, growth factors and inflammatory and angiogenic mediators, which play crucial roles in inflammatory responses and tumor genesis. The α-granules comprise a group of subcellular compartments with a unique composition and ultrastructure. Recent studies have suggested that differential secretory kinetics of α-granule subtypes is responsible for a thematic release of adhesive and inflammatory mediators. In addition, new results indicate that activation-dependent synthesis and release of cytokines also contribute to the inflammatory role of platelets. We will discuss the various methods that platelets use to regulate secretory processes and how these relate to potential differential secretion patterns, thereby promoting adhesiveness and/or inflammatory functions. We will focus on the heterogenic granule population, open canalicular system (OCS) plasticity, the role of contractile and mechanobiological forces, and the fusogenic machinery.

Obstacles in haemocompatibility testing

ISO 10993-4 is an international standard describing the methods of testing of medical devices for interactions with blood for regulatory purpose. The complexity of blood responses to biomaterial surfaces and the variability of blood functions in different individuals and species pose difficulties in standardisation. Moreover, in vivo or in vitro testing, as well as the clinical relevance of certain findings, is still matter of debate. This review deals with the major remaining problems, including a brief explanation of surface interactions with blood, the current ISO 10993 requirements for testing, and the role of in vitro test models. The literature is reviewed on anticoagulation, shear rate, blood-air interfaces, incubation time, and the importance of evaluation of the surface area after blood contact. Two test categories deserve further attention: complement and platelet function, including the effects on platelets from adhesion proteins, venipuncture, and animal derived- blood. The material properties, hydrophilicity, and roughness, as well as reference materials, are discussed. Finally this review calls for completing the acceptance criteria in the ISO standard based on a panel of test results.

Measurement of whole blood thrombus formation using parallel-plate flow chambers - a practical guide

Custom-made and commercial parallel-plate flow chambers are widely used for studies of platelet activation and thrombus formation in whole blood at defined shear rates. When used in a reproducible way, such flow chamber devices give valuable information on the thrombogenic potential of human, mouse, or rat blood. This article aims to provide a practical guide for the use of parallel-plate flow chambers in combination with routine microscopic imaging techniques. The following methodological aspects are addressed: preparation of surface coatings, calculation of blood flow and shear rate, control of pre-analytical variables, protocols for routine performing of flow chamber tests with non-coagulating or coagulating blood, and procedures for real-time and end-point analysis of thrombus formation. Frequently encountered experimental problems and artifacts are discussed, as well as possibilities for using flow chamber devices as a diagnostic tool to test antithrombotic medication.

Mechanobiology of platelets: techniques to study the role of fluid flow and platelet retraction forces at the micro- and nano-scale

Coagulation involves a complex set of events that are important in maintaining hemostasis. Biochemical interactions are classically known to regulate the hemostatic process, but recent evidence has revealed that mechanical interactions between platelets and their surroundings can also play a substantial role. Investigations into platelet mechanobiology have been challenging however, due to the small dimensions of platelets and their glycoprotein receptors. Platelet researchers have recently turned to microfabricated devices to control these physical, nanometer-scale interactions with a higher degree of precision. These approaches have enabled exciting, new insights into the molecular and biomechanical factors that affect platelets in clot formation. In this review, we highlight the new tools used to understand platelet mechanobiology and the roles of adhesion, shear flow, and retraction forces in clot formation.

CFD modeling of blood flow following coil embolization of aneurysms

In case of coil embolization of a giant or a multilobular aneurysm, it is difficult to fill an aneurysm sac completely with coils, therefore, partial blocking of an aneurysm sac is inevitable. Blood flow characteristics, which may influence embolization process of an aneurysm, are affected by the locations of coils for partially blocked aneurysms. Blood flow fields inside an aneurysm are also influenced by the geometry of a parent vessel. In order to suggest the coil locations effective for aneurysm embolization, the blood flow fields of lateral aneurysm models were analyzed for different coil locations and parent vessel geometries. Flow rate into an aneurysm sac from a parent vessel (inflow rate) and wall shear stress were also calculated. Inflow rates were smaller and low wall shear regions were larger in the distal neck blocked model comparing to the dome blocked models. In the distal neck blocked model, inflow volume was smaller by 31% (straight parent vessel model) and 34% (curved parent vessel model) comparing to other models. The time averaged values of normalized low wall shear regions were 4% and 12% greater in the distal neck blocked models with a straight and a curved parent vessel, respectively. Since smaller inflow and low wall shear stress provide hemodynamic environment promoting thrombus embolization, distal neck should be the effective coil location for aneurysm embolization.

Dimers of beta 2-glycoprotein I increase platelet deposition to collagen via interaction with phospholipids and the apolipoprotein E receptor 2'

Patients with prolonged clotting times caused by lupus anticoagulant (LAC) are at risk for thrombosis. This paradoxal association is not understood. LAC is frequently caused by anti-beta2-glycoprotein I (beta 2GPI) antibodies. Antibody-induced dimerization of beta 2GPI increases the affinity of beta 2GPI for phospholipids, explaining the observed prolonged clotting times. We constructed dimers of beta 2GPI that mimic effects of beta 2GPI-anti-beta 2GPI antibody complexes, and we studied their effects on platelet adhesion and thrombus formation in a flow system. Dimeric beta 2GPI increased platelet adhesion to collagen by 150% and increased the number of large aggregates. We also observed increased platelet adhesion to collagen when whole blood was spiked with patient-derived polyclonal anti-beta 2GPI or some, but not all, monoclonal anti-beta 2GPI antibodies with LAC activity. These effects could be abrogated by inhibition of thromboxane synthesis. A LAC-positive monoclonal anti-beta 2GPI antibody, which did not affect platelet adhesion, prevented the induced increase in platelet adhesion by beta 2GPI dimers. Furthermore, increased platelet adhesion disappeared after preincubation with receptor-associated protein, a universal inhibitor of interaction of ligands with members of the low density lipoprotein receptor family. Using co-immunoprecipitation, it was shown that dimeric beta 2GPI can interact with apolipoprotein E receptor 2 (apoER2'), a member of the low density lipoprotein receptor family present on platelets. These results demonstrate that dimeric beta 2GPI induces increased platelet adhesion and thrombus formation, which depends on activation via apoER2'.

Comparison of blood particle deposition models for non-parallel flow domains

Adhesions of monocytes and platelets to a vascular surface, particularly in regions of flow stagnation, recirculation, and reattachment, are a significant initial event in a broad spectrum of particle-wall interactions that significantly influence the formation of stenotic lesions and mural thrombi. A number of approximations are available for the simulation of both monocyte and platelet interactions with the vascular surface. For the simulation of blood particle adhesion, this study hypothesizes that: (a) the discrete element approach, which accounts for finite particle size and inertia, is advantageous in the context of non-parallel flow domains including stagnation, recirculation, and reattachment; and (b) the likelihood for particle deposition may be effectively approximated as being non-linearly proportional to local particle concentration, residence time, and wall proximity. Models such as wall shear stress correlations, the multicomponent mixture approach, and Lagrangian particle tracking with and without hydrodynamic particle-wall interactions were evaluated. Quantitative performance of the selected models was established by comparisons to available experimental data sets for non-parallel axisymmetric suspension flows of monocytes and platelets. Factors including the convective-diffusive transport of particles, finite particle size and inertia, as well as near-wall hydrodynamic interactions were found to significantly influence blood particle deposition. Of the models studied, the near-wall residence time approach was found to be a particularly effective indicator for the deposition of monocytes (r2=0.74) and platelets (r2=0.57), given that nano-scale physical and biochemical effects must be greatly approximated in computational simulations involving relatively large-scale geometries and complex flow fields.

Platelet-endothelial interaction in tumor angiogenesis and microcirculation

Activated platelets release angiogenic growth factors and have therefore been proposed to contribute to tumor angiogenesis within a potentially prothrombotic tumor microcirculation. The aim of the study was to investigate interactions of platelets with the angiogenic microvascular endothelium of highly vascularized solid tumors during growth and in response to endothelial stimulation in comparison with normal subcutaneous tissue. Experiments were performed in the dorsal skinfold chamber preparation of C57BL/6J mice bearing the Lewis lung carcinoma (LLC-1) or methylcholanthrene-induced fibrosarcoma (BFS-1). Fluorescently labeled rolling and adherent platelets, red blood cell velocity, and vessel diameters were assessed by intravital fluorescence microscopy on days 1, 3, 8, and 14 after tumor cell implantation. Slightly elevated numbers of rolling platelets were observed in the early stages of tumor angiogenesis at day 1 (control, 1.7 +/- 0.6; LLC-1, 3.4 +/- 1.8; BFS-1, 3.0 +/- 0.7 [1/mm/s], P <.05) and day 3 (control, 1.6 +/- 0.6; LLC-1, 4.1 +/- 1.7, P <.05; BFS-1, 2.3 +/- 0.5 [1/mm/s]) after tumor cell implantation. Endothelial stimulation with calcium ionophore A23187 at day 14 after tumor cell implantation resulted in a minor increase to 2.1 +/- 0.4 (LLC-1) and 1.8 +/- 0.8 (BFS-1) rolling platelets (1/mm/s) in tumor microvessels compared with 4.9 +/- 0.9 in controls (P <.05). Platelet adherence was not observed. We therefore conclude that in the 2 experimental tumors under study, (1) slightly increased platelet rolling is a transient phenomenon after tumor cell implantation, and (2) platelet-endothelial interaction in response to endothelial stimulation is reduced in tumor microvessels.

Human thrombocytes are able to induce a myocardial dysfunction in the ischemic and reperfused guinea pig heart mediated by free radicals-role of the GPIIb/IIIa-blocker tirofiban

In recent studies, we could demonstrate a myocardial dysfunction induced by homologous platelets in ischemic and reperfused guinea pig hearts. Aim of the current study was to find out whether or not this is a phenomenon specific for platelets isolated from guinea pigs and to further examine the mechanisms of a possible cardiodepressive effect of human platelets. Isolated guinea pig hearts were exposed to a 30 min low-flow ischemia (1 ml/min) and reperfused. Human thrombocytes were administered as bolus (20.000 thrombocytes/microl perfusion buffer) in the 15(th) min of ischemia or in the 1(st) or 5(th) min of reperfusion in the presence of thrombin. Recovery of external heart work (REHW) and intracoronary platelet retention (RET) were quantified in percent. In additional experiments, the GPIIb/IIIa-blocker tirofiban (10 microg/ml perfusion buffer) or the radical scavenger superoxide dismutase (SOD-10 U/ml perfusion buffer) were added. Platelet application in the absence of tirofiban, either during ischemia (REHW 75.4 +/- 4%, RET 22.2 +/- 2%) or the 1st min (REHW 71.6 +/- 1%, RET 31.2 +/- 2%) or the 5th min of reperfusion (REHW 63.2 +/- 4%, RET 40.5 +/- 1%) led to a significant reduction of REHW and a significant increase of RET. The coapplication of tirofiban, on the other hand, prevented RET at all three times of platelet application (1.1 +/- 1.7%, 0% or 2.1 +/- 1.2%, respectively). An improvement of REHW, however, could only be noticed during ischemia (89 +/- 2%), whereas coapplication of tirofiban in early (72.9 +/- 3%) or in late reperfusion (74.6 +/- 2%) did not lead to a significant increase of REHW. Coapplication of SOD, on the other hand, significantly improved REHW in early (88.1 +/- 1) or late (95.9 +/- 1) reperfusion but not during ischemia (83.5 +/- 2). Corresponding to REHW, RET was changed significantly by coapplication of SOD during early (1 +/- 2%) or late (0%) reperfusion but not during ischemia (21.1 +/- 4%). We conclude that human thrombocytes are able to induce a myocardial dysfunction in ischemic and reperfused guinea pig hearts mediated by reactive oxygen species and independent of intracoronary platelet adhesion.

Fluid mechanics of vascular systems, diseases, and thrombosis

The cardiovascular system is an internal flow loop with multiple branches circulating a complex liquid. The hallmarks of blood flow in arteries are pulsatility and branches, which cause wall stresses to be cyclical and nonuniform. Normal arterial flow is laminar, with secondary flows generated at curves and branches. Arteries can adapt to and modify hemodynamic conditions, and unusual hemodynamic conditions may cause an abnormal biological response. Velocity profile skewing can create pockets in which the wall shear stress is low and oscillates in direction. Atherosclerosis tends to localize to these sites and creates a narrowing of the artery lumen--a stenosis. Plaque rupture or endothelial injury can stimulate thrombosis, which can block blood flow to heart or brain tissues, causing a heart attack or stroke. This small lumen and elevated shear rate in a stenosis create conditions that accelerate platelet accumulation and occlusion. The relationship between thrombosis and fluid mechanics is complex, especially in the post-stenotic flow field. New convection models have been developed to predict clinical from platelet thrombosis in diseased arteries. Future hemodynamic studies should address the complex mechanics of flow-induced, large-scale wall motion and convection of semisolid particles and cells in flowing blood.

Novel Antithrombotic Strategies for the Treatment of Coronary Artery Thrombosis: A Critical Appraisal

Large-scale clinical trials have demonstrated that treatment of patients with acute myocardial infarction and unstable angina with antithrombotic agents significantly improves outcome. Despite the proven benefit of current therapies, there is a widespread perception that outcome could be enhanced further with novel antithrombotic agents. Enthusiasm for novel antithrombotic strategies has been stimulated by recent advances in the understanding of the mechanisms responsible for coronary artery thrombosis, which has led to the development of diverse inhibitors of platelet function and coagulation factors. In experimental models of coronary artery thrombosis, aspirin and heparin have been ineffective in preventing recurrent thrombosis after coronary thrombolysis and in preventing the progression of thrombosis in response to strong thrombogenic stimuli. In contrast, inhibitors of the platelet fibrinogen receptor, direct-acting thrombin inhibitors, and inhibitors of coagulation factors that promote elaboration of thrombin have been shown to be effective in attenuating arterial thrombosis in a variety of experimental preparations. Initial clinical trials with these agents have also documented efficacy in attenuating thrombotic events in patients treated with coronary thrombolysis and in those with unstable angina. However, optimal doses of novel antithrombotic agents, the degree to which combination antiplatelet and anticoagulant therapies are needed, and the risk/benefit ratio associated with specific novel antithrombotic drugs are still relatively undefined. With regard to the latter, it is possible that the large-scale clinical trials now in progress may show an increase in bleeding complications with novel anticoagulants compared with conventional therapy. Nonetheless, there are considerable data that suggest that treatment with aspirin and heparin is not completely effective in preventing the progression of thrombosis or its recurrence after interventions in high-risk subgroups of patients with coronary artery thrombosis and unstable coronary artery disease. Accordingly, continued investigation of a large variety of antithrombotic agents, both currently available and in development, should improve the treatment of high-risk patients with coronary disease if regimens with appropriate efficacy but without serious hemorrhagic effects can be designed.

Platelet-dependent and procoagulant mechanisms in arterial thrombosis

The results of experiment and clinical studies support the hypothesis that initiation and progression of arterial thrombosis are dependent on a dynamic interaction between platelet- and coagulation-dependent mechanisms. Although treatment of patients with unstable coronary syndromes acutely with glycoprotein IIb/IIIa inhibitors has already been shown to be remarkably effective in improving short- and long-term prognosis, strategies that inhibit factor thrombin and/or Xa activity may also have a role either alone or in combination with platelet inhibitors in treating patients with coronary thrombosis.

Mice that lack thrombospondin 2 display connective tissue abnormalities that are associated with disordered collagen fibrillogenesis, an increased vascular density, and a bleeding diathesis

Thrombospondin (TSP) 2, and its close relative TSP1, are extracellular proteins whose functions are complex, poorly understood, and controversial. In an attempt to determine the function of TSP2, we disrupted the Thbs2 gene by homologous recombination in embryonic stem cells, and generated TSP2-null mice by blastocyst injection and appropriate breeding of mutant animals. Thbs2-/- mice were produced with the expected Mendelian frequency, appeared overtly normal, and were fertile. However, on closer examination, these mice displayed a wide variety of abnormalities. Collagen fiber patterns in skin were disordered, and abnormally large fibrils with irregular contours were observed by electron microscopy in both skin and tendon. As a functional correlate of these findings, the skin was fragile and had reduced tensile strength, and the tail was unusually flexible. Mutant skin fibroblasts were defective in attachment to a substratum. An increase in total density and in cortical thickness of long bones was documented by histology and quantitative computer tomography. Mutant mice also manifested an abnormal bleeding time, and histologic surveys of mouse tissues, stained with an antibody to von Willebrand factor, showed a significant increase in blood vessels. The basis for the unusual phenotype of the TSP2-null mouse could derive from the structural role that TSP2 might play in collagen fibrillogenesis in skin and tendon. However, it seems likely that some of the diverse manifestations of this genetic disorder result from the ability of TSP2 to modulate the cell surface properties of mesenchymal cells, and thus, to affect cell functions such as adhesion and migration.

Characterization of the novel murine monoclonal anti-von Willebrand factor (vWf) antibody GUR76-23 which inhibits vWf interaction with alpha IIb beta 3 but not alpha v beta 3 integrin

von Willebrand factor (vWf) is known to interact with the two beta 3 integrins, alpha IIb beta 3 and alpha v beta 3, in an RGD-dependent manner. We characterized a novel murine monoclonal antibody to human vWf, GUR76-23, which recognized a site within the carboxy-terminal half of the molecule containing the RGD sequence. This antibody inhibited high shear-induced platelet aggregation and blocked adhesion of ADP plus epinephrine-stimulated platelets to vWf, indicating that it interferes with the interaction with alpha IIb beta 3. Unlike antibodies against the RGD site, however, the antibody was without effect on adhesion of cultured human umbilical vein endothelial cells to vWf, a phenomenon known to involve the interaction with alpha v beta 3. GUR76-23 binding was not displaced by anti-RGD antibodies. These results suggest that the adhesive interaction of vWf with these two beta 3 integrins may be differentially modulated by a site(s) other than the common RGD module.

Pharmacodynamic models to evaluate antithrombotics in clinical pharmacology

The need to prevent thromboembolic events effectively and safely has stimulated an intense search for novel antithrombotics. Parameters derived from in vitro tests with patients' blood are essential for therapeutic monitoring of anticoagulants. Clinical pharmacologic evaluation of novel antithrombotic therapies based on such parameters can easily fail, however, by neglecting pivotal pathophysiologic determinants of thrombus formation. When vascular injury occurs, blood cells, plasma proteins, and the vessel wall intimately cooperate for an adequate local repair. Much remains to be learned about the local and transient interaction of these components. In most tests of platelet function and coagulation proteins, blood samples from treated individuals are stimulated in vitro to assess inhibitory effects. Limitations of such a test strategy for dose-finding studies with antithrombotics may be overcome by measuring activation markers specifically generated on the surface of blood cells or in plasma at the site of thrombosis in patients.

Identification of the endothelial cell binding site for factor IX

We previously demonstrated that the primary region of factor IX and IXa responsible for saturable specific binding to bovine aortic endothelial cells resides in residues 3-11 at the amino terminus of factor IX. We also demonstrated that mutations of lysine to alanine at residue 5, factor IX K5A, or valine to lysine at residue 10, factor IX V10K, resulted in a molecule unable to bind to endothelial cells. Moreover, a mutation with lysine to arginine at residue 5, factor IX K5R, resulted in a factor IX molecule with increased affinity for the endothelial cell binding site. In this paper we report that collagen IV is a strong candidate for the factor IX binding site on endothelial cells. Factor IX and factor IX K5R compete with 125I-labeled factor IX for binding to tetrameric collagen IV immobilized on microtiter plates, while factor X, factor VII, and factor IX K5A or V10K fail to compete. The Kd for wild-type factor IX binding to collagen IV in the presence of heparin was 6.8 +/- 2 nM, and the Kd for factor IX K5R was 1.1 +/- 0.2 nM, which agrees well with our previously published Kd values of 7.4 and 2.4 nM for binding of the same proteins to endothelial cells. Our working assumption is that we have identified the endothelial cell binding site and that it is collagen IV. Its physiological relevance remains to be determined.