Platelet Adhesion to Collagen in Subtypes of Type I von Willebrand’s Disease Is Dependent on Platelet von Willebrand Factor

Update on von Willebrand factor multimers: focus on high-molecular-weight multimers and their role in hemostasis

Normal hemostasis requires von Willebrand factor (VWF) to support platelet adhesion and aggregation at sites of vascular injury. VWF is a multimeric glycoprotein built from identical subunits that contain binding sites for both platelet glycoprotein receptors and collagen. The adhesive activity of VWF depends on the size of its multimers, which range from 500 to over 10 000 kDa. There is good evidence that the high-molecular-weight multimers (HMWM), which are 5000-10 000 kDa, are the most effective in supporting interaction with collagen and platelet receptors and in facilitating wound healing under conditions of shear stress. Thus, these HMWM of VWF are of particular clinical interest. The unusually large multimers of VWF are, under normal conditions, cleaved by the plasma metalloproteinase ADAMTS13 to smaller, less adhesive multimers. A reduction or lack of HMWM, owing to a multimerization defect of VWF or to an increased susceptibility of VWF for ADAMTS13, leads to a functionally impaired VWF and the particular type 2A of von Willebrand disease. This review considers the biology and function of VWF multimers with a particular focus on the characterization of HMWM - their production, storage, release, degradation, and role in normal physiology. Evidence from basic research and the study of clinical diseases and their management highlight a pivotal role for the HMWM of VWF in hemostasis.

Contribution of platelet vs. endothelial VWF to platelet adhesion and hemostasis

BACKGROUND

von Willebrand factor (VWF) is a glycoprotein that plays an important role in primary hemostasis. VWF is synthesized and stored in endothelial cells (ECs) and megakaryocytes/platelets. Plasma VWF is primarily derived from ECs and is generally believed to be essential for hemostasis. VWF synthesized in megakaryocytes is stored in platelet α-granules, from which it is released following platelet activation. The relative contribution of VWF stored in ECs or megakaryocytes/platelets or present in plasma to hemostasis is not clear.

OBJECTIVES

We investigated whether EC-derived VWF plays the major role in hemostasis while the contribution of platelet-derived VWF is negligible, or if platelet-derived VWF also significantly contributes to hemostasis.

METHODS AND RESULTS

Mice expressing VWF only in ECs (EC-VWF) or platelets (Plt-VWF) were created by reciprocal bone marrow transplantation between C57BL/6J (WT) and VWF knockout mice (VWF-/-). Plasma VWF levels in EC-VWF were similar to WT. Plt-VWF mice had a trace amount of VWF in their plasma while VWF levels in platelet lysate were comparable to WT. Tail bleeding time was normal in EC-VWF. Interestingly, Plt-VWF showed partially corrected bleeding time and significantly decreased blood loss volume compared with VWF-/-. Adhesion of platelets perfused over immobilized collagen under shear stress was significantly higher in both EC-VWF and Plt-VWF compared with VWF-/-.

CONCLUSION

VWF synthesized in ECs is sufficient to support hemostasis in VWF-/- mice, and VWF produced in megakaryocytes/platelets can also contribute to hemostasis in the absence of EC-derived VWF.

Inherited disorders of platelet alpha-granules

Platelet alpha-granules are the storage site for the internal membrane glycoprotein P-selectin and for a variety of megakaryocyte-synthesized and plasma-derived soluble proteins. Quantitative and/or qualitative abnormalities in alpha-granules are found in a number of inherited bleeding disorders, including gray platelet syndrome, alphadelta-storage pool deficiency, the Quebec platelet disorder, and in some patients with dysmegakaryopoietic thrombocytopenia. In addition, single alpha-granular protein deficiencies are seen in other bleeding disorders,including factor V deficiency, afibrinogenemia, Glanzmann's thrombasthenia, von Willebrand disease, and plasminogen-activator inhibitor-1 deficiency. The excessive bleeding that occurs in patients with inherited abnormalities of platelet alpha-granules indicates that the proteins stored within this compartment are important for normal hemostasis. The clinical and laboratory features of these different, inherited platelet storage pool disorders suggest unique molecular and biochemical defects are responsible for these conditions. However, the genetic causes of these disorders are largely unknown. This paper reviews our current knowledge of the inherited disorders of platelet alpha-granules.

Differential platelet deposition onto collagen in cone-and-plate and parallel plate flow chambers

To routinely test the formation of thrombi and the effect of drugs modifying it, proper test systems are needed. Their design should rely on the laws of rheology and the physiology of laminar flow. To best model physiological or pathological shear conditions, parallel/linear and rotational type flow chambers are developed. We have compared the initial phase of platelet thrombus formation in a parallel plate flow chamber (PPC) and a cone-and-plate chamber (CPC) under von Willebrand dependent shear conditions. Blood was allowed to flow through human collagen type III surfaces at a shear rate of 1000 s(-1) for 150 s. Thrombus deposition was characterized by surface coverage, average area and height of thrombi. VWF distribution within thrombi was analyzed with confocal laser scanning microscopy. Reduced surface-specific platelet adhesion and aggregation (surface coverage and average thrombus size) were observed in CPC along with a significant increase in single platelet disappearance from the circulating blood. Our data suggest that the higher rate of platelet consumption in this device, as opposed to PPC, is limiting the adhesion to the surface. Consequently, surface-specific processes and aggregation in the flowing blood are both assessed using CPC, while comprehensive evaluation of surface-specific processes is best achieved with PPC. Therefore, the choice of chamber type as a diagnostic tool is purpose-dependent.

Effect of a humanized monoclonal antibody to von Willebrand factor in a canine model of coronary arterial thrombosis

The purpose of this study was to investigate the antithrombotic effect and bleeding time prolongation of AJW200, a humanized monoclonal antibody to von Willebrand factor (vWF), in a canine model of coronary arterial thrombosis. AJW200 significantly inhibited cyclic flow reductions, as well as botrocetin-induced platelet aggregation, at 0.1 mg/kg. A significant prolongation of bleeding time was observed at 0.3-1 mg/kg. Approximately 50% occupancy of vWF (approximately 0.7 microg/ml AJW200 in plasma) and 80-100% occupancy (approximately 20 microg/ml AJW200 in plasma) were needed for the antithrombotic effect and the extensive prolongation of bleeding time, respectively. On the contrary, the minimal effective dose of abciximab (0.8 mg/kg) was associated with a significant prolongation of bleeding time. These results suggest that the pharmacological blockade of platelet glycoprotein (GP) Ib-vWF interaction with AJW200 results in a safer antithrombotic profile than platelet GPIIb/IIIa blockade with abciximab in dogs.

Pharmacokinetics and pharmacodynamics of AJW200, a humanized monoclonal antibody to von Willebrand factor, in monkeys

The interaction between platelet glycoprotein Ib and von Willebrand factor (vWF) plays a crucial role in platelet-mediated thrombus formation under high-shear-stress conditions. The aim of this study was to investigate the antiplatelet profile of a humanized anti-vWF monoclonal antibody, AJW200. In vitro studies were performed with a modified cone-and-plate viscometer and human platelets. AJW200 inhibited high-shear-stress-induced platelet adhesion, aggregation, and thrombin generation, but it did not have such effects under low-shear-stress conditions. Although abciximab inhibited platelet aggregation under both shear stress conditions, it did not inhibit platelet adhesion and thrombin generation. In addition, the pharmacokinetics and pharmacodynamics of AJW200 were evaluated in cynomolgus monkeys. Sustained inhibition of ristocetin-induced platelet aggregation was observed over 24 hours, 6 days, and 2 weeks after a single bolus injection of 0.3, 1, and 3 mg/kg, respectively. Moderate prolongation of the bleeding time was observed at the doses of 1 and 3 mg/kg. Abciximab markedly prolonged the bleeding time at 0.4 mg/kg, at which concentration complete inhibition of ADP-induced platelet aggregation was observed. These results suggest that glycoprotein Ib-vWF blockade with AJW200 results in a sustained antiplatelet effect without extensive prolongation of the bleeding time, probably due to a shear-stress-dependent inhibitory action.

Anti-human vWF monoclonal antibody, AJvW-2 Fab, inhibits repetitive coronary artery thrombosis without bleeding time prolongation in dogs

The antithrombotic and antihaemostatic effects of the monoclonal antibody against human vWF (AJvW-2 Fab) were investigated in comparison with those of the monoclonal antibody against platelet GPIIb/IIIa (abciximab) in dogs. The ex vivo platelet aggregation and template bleeding time were measured before, 5, 90, 210 min and 24 h after injection of either AJvW-2 Fab or abciximab in anesthetized beagle dogs. Plasma concentration, vWF occupancy and plasma vWF antigen level were also measured by ELISA. In addition, the antithrombotic effect was evaluated in a canine model of repetitive coronary thrombosis (Folts model). AJvW-2 Fab significantly inhibited the ex vivo botrocetin-induced platelet aggregation at 0.18 mg/kg (53% plasma vWF occupancy) and also inhibited cyclic flow reductions (CFRs) at 0.06 mg/kg (31% occupancy). A significant prolongation of the bleeding time was observed at 1.8 mg/kg (95% occupancy), which was 30 times as high as the antithrombotic effective dose. Whereas, abciximab significantly inhibited both the ex vivo ADP-induced platelet aggregation and CFRs at 0.8 mg/kg, which was the minimally effective dose, also resulting in a significant prolongation of the bleeding time. These results suggest that blockade of the GPIb-vWF axis with AJvW-2 Fab leads to the inhibition of thrombus formation in the stenosed coronary arteries without less bleeding time prolongation than the GPIIb/IIIa blockade with abciximab.

Anti-human von willebrand factor monoclonal antibody AJvW-2 prevents thrombus deposition and neointima formation after balloon injury in guinea pigs

Immediately after angioplasty, platelet adhesion to the injured arterial wall and subsequent release of various mitogens may contribute to neointima formation. The purpose of this study was to evaluate the inhibitory effect of AJvW-2, a monoclonal antibody against human von Willebrand factor (vWF), on neointima formation in a guinea pig model. The carotid artery was injured with a balloon catheter, and AJvW-2 was administered by a single bolus injection. AJvW-2 dose-dependently prevented neointima formation 14 days after injury. Significant inhibition was observed at 1.8 mg/kg, at which dose significant inhibition of platelet aggregation was achieved for 2 days. By elastic-Masson staining, organized thrombi were observed in the neointimal lesion on day 14. The thrombus area was significantly correlated with neointimal thickness. Furthermore, thrombus deposition, immunostained for vWF and fibrin(ogen), was observed on the media immediately after balloon injury. AJvW-2 significantly reduced the deposition of both adhesive proteins and reduced the incidence of organized thrombus formation, which might affect subsequent neointima formation. However, the proliferation of cultured smooth muscle cells was not affected by AJvW-2. These results suggest that AJvW-2 prevents neointima formation by inhibition of initial platelet-mediated thrombus formation rather than by direct inhibition of smooth muscle cell proliferation.

Low Platelet 2β1 Levels in Type I von Willebrand Disease Correlate With Impaired Platelet Function in a High Shear Stress System

Platelet adhesion to collagen-coated surfaces in whole blood under flow conditions is mediated by both von Willebrand factor (vWF)-dependent recruitment of the platelet glycoprotein Ib-IX receptor complex and collagen interaction with the integrin 2β1. In type 1 von Willebrand disease (vWD), platelet adhesive functions are impaired due to the decrease in vWF levels in plasma and platelets. There are at least three alleles of the human 2 gene, distinguishable by a cluster of silent or noncoding sequence differences within a segment of the gene. Two alleles, associated with low receptor density can be distinguished by nucleotide 807C, while the third allele associated with high receptor density, expresses nucleotide 807T. Gene frequencies of these alleles in a normal population (n = 167) are 0.58 for 807C and 0.42 for 807T. We measured the frequencies of these alleles in symptomatic patients with five types of vWD (type 1, n = 78; type 2A, n = 25, type 2B, n = 14; type 2M, n = 10; and type 3, n = 20). Compared with the normal group, no significant difference in allele frequencies was observed among individuals with types 2A, 2B, 2M, or 3 vWD. However, the frequency of the 807C allele, associated with low collagen receptor density, among type 1 vWD patients (807C = .71; 807T = .29) was significantly higher than that of the normal population (P = .007). Also, in patients with vWD type 1 and borderline to normal ristocetin-cofactor (vWF:RCo) activity values, collagen receptor density correlates inversely with closure time in a high shear stress system (platelet function analyzer [PFA-100]). We propose that low platelet 2β1 density results in less efficient primary platelet adhesion and may result in increased tendency to bleed, as evidenced by the high frequency of this polymorphism in patients with type 1 vWD compared with normal individuals. In addition, this may account for the variability between patients with similar levels of vWF antigen, but strikingly different bleeding histories.

Low Platelet 2β1 Levels in Type I von Willebrand Disease Correlate With Impaired Platelet Function in a High Shear Stress System

Platelet adhesion to collagen-coated surfaces in whole blood under flow conditions is mediated by both von Willebrand factor (vWF)-dependent recruitment of the platelet glycoprotein Ib-IX receptor complex and collagen interaction with the integrin 2β1. In type 1 von Willebrand disease (vWD), platelet adhesive functions are impaired due to the decrease in vWF levels in plasma and platelets. There are at least three alleles of the human 2 gene, distinguishable by a cluster of silent or noncoding sequence differences within a segment of the gene. Two alleles, associated with low receptor density can be distinguished by nucleotide 807C, while the third allele associated with high receptor density, expresses nucleotide 807T. Gene frequencies of these alleles in a normal population (n = 167) are 0.58 for 807C and 0.42 for 807T. We measured the frequencies of these alleles in symptomatic patients with five types of vWD (type 1, n = 78; type 2A, n = 25, type 2B, n = 14; type 2M, n = 10; and type 3, n = 20). Compared with the normal group, no significant difference in allele frequencies was observed among individuals with types 2A, 2B, 2M, or 3 vWD. However, the frequency of the 807C allele, associated with low collagen receptor density, among type 1 vWD patients (807C = .71; 807T = .29) was significantly higher than that of the normal population (P = .007). Also, in patients with vWD type 1 and borderline to normal ristocetin-cofactor (vWF:RCo) activity values, collagen receptor density correlates inversely with closure time in a high shear stress system (platelet function analyzer [PFA-100]). We propose that low platelet 2β1 density results in less efficient primary platelet adhesion and may result in increased tendency to bleed, as evidenced by the high frequency of this polymorphism in patients with type 1 vWD compared with normal individuals. In addition, this may account for the variability between patients with similar levels of vWF antigen, but strikingly different bleeding histories.

Studies of multimerin in patients with von Willebrand disease and platelet von Willebrand factor deficiency

In normal platelet alpha-granules von Willebrand factor (VWF) is stored with multimerin and factor V in an eccentric electron-lucent zone. Because the platelet stores of VWF are deficient in 'platelet low' type 1 and type 3 von Willebrand disease (VWD), we investigated their electron-lucent zone proteins. The patients with VWD had partial to complete deficiencies of plasma and platelet VWF but normal alpha-granular multimerin and factor V, and normal alpha-granular fibrinogen, thrombospondin-1, fibronectin, osteonectin and P-selectin. In type 3 VWD platelets, alpha-granular electron-lucent zones lacking VWF-associated tubules were identified and multimerin was found in its normal alpha-granular location. These findings indicate that the formation of the electron-lucent zone and the sorting of multimerin to this region occur independent of VWE The isolated abnormalities in VWF suggests a VWF gene mutation is the cause of 'platelet low' type 1 VWD.

Hemostasis in patients with severe von Willebrand disease improves after normal platelet transfusion and normalizes with further correction of the plasma defect

BACKGROUND

A defective hemostatic effect of plasma concentrate infusion in patients with severe von Willebrand disease (vWD) has been ascribed to the absence of platelet von Willebrand factor (vWF) STUDY DESIGN AND METHODS: The role of platelet vWF in hemostasis of severe vWD was investigated. A plateletpheresis unit (4-5 x 10(11) platelets) from a normal compatible donor was transfused before any cryoprecipitate infusion to three type 3 vWD patients and to one patient with severe type 1 vWD with low levels of platelet vWF who required replacement therapy for bleeding episodes. Autologous platelets were transfused to one of the patients with type 3 vWD.

RESULTS

Partial corrections of bleeding times (14-17 min vs. baseline >30 min) were observed in all patients after the transfusion of normal platelets. During cryoprecipitate infusion, bleeding times were normalized (<6 min), and bleeding episodes stopped when plasma levels of vWF activity ranged from 14 to 18 U per dL. Platelet interactions with the subendothelium increased in parallel with the correction of bleeding times. These results indicate that if approximately 20 percent of the total number of platelets have normal vWF antigen and if plasma vWF levels are at least 14 U per dL, then bleeding times will normalize and mucosal hemorrhages will stop. Transfusion of autologous platelets in one patient with type 3 vWD did not modify bleeding times or platelet adhesion on the subendothelium.

CONCLUSION

The hemostatic effect of normal platelets in type 3 vWD seems to be related to the platelet vWF in the transfused platelets.

Abnormal collagen binding activity of 2A von Willebrand factor: evidence that the defect depends only on the lack of large multimers

It is well established that the large von Willebrand factor (vWf) multimers bind with high affinity to the extracellular matrix. To explore the different roles of intermediate and large vWf multimers, we studied the collagen-binding activity (vWf:CBA) of 2A vWf under nonflowing conditions in relation to the multimer organization of the molecule. Regardless of the anticoagulant used for blood collection, vWf:CBA was significantly decreased, in 4 patients with 2A von Willebrand's disease (vWd), in accordance with the lack of high and intermediate vWf multimers. After 1-deamino-8-D-arginine vasopressin (DDAVP) infusion, the appearance of circulating large and unusually large vWf multimers, in samples collected in the presence of protease inhibitors, induced a complete normalization of vWf:CBA. The peak was observed 15 minutes after DDAVP, when large and unusually large multimers were maximally represented. These effects were transient because vWf:CBA decreased after 60 minutes, even though values were still significantly higher than pre-DDAVP figures; at the same time, large vWf multimers appeared to be decreased. In contrast, samples anticoagulated with sodium citrate after DDAVP did not show a normalized vWf multimer pattern and were characterized by a persistently decreased vWf:CBA. Moreover, in all of the patients studied, platelet vWf presented normal vWf:CBA values in accordance with the normal levels and multimer organization of the vWf molecule. Our findings indicate that the collagen-binding defect displayed in vitro by type 2A vWf depends only on the lack of circulating large vWf multimers. Moreover, the observation of normal platelet vWf:CBA seems to indicate a primary role of plasma rather than platelet vWf in assuring platelet plug formation.

Platelet adhesion to multimeric and dimeric von Willebrand factor and to collagen type III preincubated with von Willebrand factor

As part of a systematic study of platelet interaction with adhesive proteins under flow conditions, we studied platelet adhesion to multimeric and dimeric von Willebrand factor (vWF) coated to glass. vWF-dependent adhesion to collagen type III was studied for comparison. Adhesion to glass-coated vWF and vWF-mediated adhesion to collagen type III were in many respects similar. Both showed no decrease at increasing shear rates and a decline to 50% of maximum with a low-molecular-weight multimeric fraction. Adhesion to glass-coated vWF was partially inhibited by heparin and completely inhibited by prostaglandin I(2) and anti-glycoprotein (GP) Ib and anti-GPIIb-IIIa antibodies. vWF-dependent adhesion to collagen was not inhibited by heparin, was partially inhibited by anti-GPIIb-IIIa, and was completely inhibited by prostaglandin I(2) and anti-GPIb. Recombinant dimeric vWF was made by deletion of the propeptide and expression in Chinese hamster ovary cells. Adhesion was 50% of that with plasma vWF, and larger concentrations of dimeric vWF were required. Adhesion to dimeric vWF was optimal at 1500 s(-1), with a gradual decrease at higher shear rates. We conclude that adhesion to collagen type III is strongly but not completely determined by the adhesive properties of vWF.

Adhesion of blood platelets is inhibited by VCL, a recombinant fragment (leucine504 to lysine728) of von Willebrand factor

VCL, fragment Leu504 to Lys728 of von Willebrand factor (vWF) expressed in Escherichia coli, contains the glycoprotein (GP) Ib-binding domain of vWF. This fragment inhibited ristocetin-induced platelet aggregation with an IC50 of 0.2 mumol/L and botrocetin-induced platelet aggregation with an IC50 of 0.08 mumol/L. We studied the antiadhesive profile of VCL by adding it to blood that was circulated over various adhesive surfaces. VCL inhibited adhesion to endothelial cell matrix, which served as a model of the vessel wall. Maximal inhibition at a high shear rate of 1600 s-1 was stronger (60%) than at a low shear rate of 300 s-1 (40%). Half maximal inhibition was found to be 1.5 mumol/L at both shear rates. The role of various adhesive molecules was investigated in more detail by coating glass coverslips with collagen type I, laminin, fibronectin, or vWF. Fibrinogen was studied as well. Platelet adhesion to laminin and vWF was not inhibited by VCL. Adhesion to collagen, fibronectin, and fibrinogen was particularly inhibited at a high shear rate. VCL coated to a coverslip caused a concentration-dependent adhesion that was blocked by antibodies against GPIb, which block interaction with vWF. Binding studies showed a nonsaturable ristocetin binding of VCL to platelets that was blocked by vWF or inhibitory antibodies against GPIb. Binding to collagen was weak, and VCL did not inhibit binding of vWF at a 5000-fold excess. From these data, we conclude that VCL inhibits adhesion in all cases in which adhesion is vWF dependent by competing for vWF binding to activated GPIb. The lack of inhibition of adhesion to vWF as a single molecule may be explained by assuming that this adhesion is determined by interaction of nonactivated GPIb with vWF that has been changed in conformation by adsorption. Studies investigating thrombus formation on the connective tissue of an atherosclerotic plaque in a human coronary artery showed that VCL was able to partially prevent this thrombus formation. VCL may be of value in preventing adhesion and thrombus formation under conditions in which these processes are dependent on vWF.

Purification and characterization of human platelet von Willebrand factor

Platelet von Willebrand factor (vWf) was purified from human platelet concentrates. The multimeric structure of the purified platelet vWf was similar to that observed in the initial platelet lysate, and, like the platelet lysate, the purified platelet vWf contained higher molecular weight multimers than plasma vWf. The apparent molecular weight of the reduced platelet vWf subunit was similar to the plasma vWf subunit. The N-terminal amino acid of the purified platelet and plasma vWf was blocked. In concentration dependent binding to botrocetin- or ristocetin-stimulated platelets, 125I-plasma vWf bound with a higher affinity than platelet. The ristocetin cofactor activity per mg of purified plasma vWf was 5-fold greater than the platelet vWf activity. Platelet and plasma vWf bound to collagen with similar affinities; however, platelet vWf bound to thrombin-stimulated platelets and to heparin with a higher affinity than plasma vWf. The differences in the binding affinity(s) of plasma and platelet vWf to platelet GPIb and GPIIb/IIIa and extracellular matrix proteins may reflect different roles for plasma and platelet vWf in the initial stages of haemostasis and thrombosis.

The role of platelet von Willebrand factor in platelet adhesion and thrombus formation: a study of 34 patients with various subtypes of type I von Willebrand disease

In order to investigate the respective role of plasma and platelet von Willebrand factor (vWF) in mediating platelet adhesion and thrombus formation, we performed ex vivo perfusion studies with native blood from patients with various subtypes of type I von Willebrand disease (vWD). We studied 34 patients with type I vWD (19 'platelet normal', five 'platelet low', two 'platelet discordant', eight 'Vicenza'). Parallel studies were carried out on nine patients with severe vWD (type III). At high shear rate (2600 s-1) we found that the defect in platelet-vessel wall interactions in patients having a normal platelet vWF content ('platelet normal' and 'Vicenza') involved thrombus formation, whereas platelet adhesion was normal. At this high shear rate, platelet adhesion and thrombus volume were significantly decreased in patients with subtypes 'platelet low' and 'platelet discordant', i.e. when platelet vWF is either low or dysfunctional. These results indicate that platelet vWF may substitute for plasma vWF to promote platelet adhesion, emphasizing the important role of platelet vWF. They also confirm the role of vWF in thrombus formation at high shear rate because an abnormal thrombus volume was observed in all patients, even when platelet adhesion was normal.

Type I von Willebrand disease, subtype 'platelet low': decreased platelet adhesion can be explained by low synthesis of von Willebrand factor in endothelial cells

Endothelial cells (EC) were isolated from the umbilical vein of a newborn girl with type I 'platelet low' von Willebrand disease (I vWD) and endothelial localization and release and the ability of subendothelial von Willebrand factor (vWF) to support platelet adhesion were compared with those of normal EC. vWF was detectable by immunofluorescence in Weibel-Palade bodies, but the number of Weibel-Palade bodies positive for vWF was lower than in control EC. Patient EC released into the medium significantly smaller amount of vWF, both constitutively and after their stimulation. The vWF content of the extracellular matrix of patient EC was 38% that of control EC matrix. Platelet adhesion studies were performed under flow conditions with umbilical arteries and EC matrices of cultured EC. Using normal citrated whole blood as perfusate, platelet adhesion was lower in the umbilical artery of the patient (9 +/- 1% v 35 +/- 4% for the control) and in her EC matrix (7 +/- 1% v 21 +/- 2% of control). When patient EC matrix was perfused with vWF-deficient reconstituted blood, adhesion was 17 +/- 3% v 32 +/- 3% for control EC matrix; preincubation of patient EC matrix with 1 U/ml vWF increased the adhesion to 30 +/- 6%. These data establish that low contents of vWF in EC and subendothelium are important characteristics of type I vWD 'platelet low', and that such characteristics correlate with low platelet adhesion to the subendothelium.

Significance and quantitative analysis of von Willebrand factor in human platelets

The von Willebrand factor (vWF) is found in plasma and in platelets. The concentration and multimeric composition of the vWF in platelets of 160 patients with bleeding tendency were examined since very little is known about the platelet vWF. For quantitative analysis of the platelet vWF, a modified ELISA was established. A reference range from 70%-130% of platelet vWF concentration considered normal was established by examining 80 healthy blood donors. 16.9% of the 160 patients showed a decreased vWF concentration in platelets only, while all the other coagulation parameters were normal. 3 of our patients belong to the same family and suggesting an autosomal dominant genetic transmission for the von Willebrand disease type 1-3. The data also suggests, that a quantitative and qualitative analysis of the vWF in plasma and platelets is required for an exact diagnosis of the von Willebrand disease.

The bleeding time in normal subjects is mainly determined by platelet von Willebrand factor and is independent from blood group

Bleeding time (BT) is the most important test "in vivo" evaluating the primary hemostasis. No relationship between plasma von Willebrand factor (vWF) and BT has been found in normal subjects, whereas no data are available on the relationship between platelet vWF and blood group or BT in normal subjects. To clarify the reciprocal relationship between blood group, vWF and BT, we studied 177 normal subjects. The influence of blood group on BT was analyzed in 116 subjects and the distribution of platelet and plasma vWF between different blood groups in 111 subjects. Furthermore, in a subgroup of 50 subjects, a multivariate regression model, including age, platelet count, plasma and platelet vWF:Ag and Ristocetin Cofactor (RiCof), hematocrit, sex and O or non-O blood group was used to assess the joint influence of these variables on BT. BT was similar in O and non-O group (P = 0.310) and, although plasma vWF was significantly lower in O group (P less than 0.0001), platelet vWF content was similarly distributed (P = 0.873 vWF:Ag and P = 0.322 RiCof). Furthermore, platelet vWF was not correlated with plasma vWF (r = 0.06, P = 0.526). In the multivariate regression analysis, only platelet vWF, age and platelet count showed a significant, inverse correlation with BT (P = 0.004), explaining 25% of the total variation of this test. Platelet vWF resulted therefore the main determinant of BT.

In vivo experiments indicate that relatively high platelet deposition in von Willebrand's disease 'Vicenza' is caused by normal platelet-VWF levels rather than by high VWF-multimers in plasma

Von Willebrand's disease (vWD) 'Vicenza' is characterized by low plasma von Willebrand Factor antigen (vWF:Ag) and very low levels of Ristocetin Cofactor activity (RiCof). The hemorrhagic tendency in vWD 'Vicenza' is, however, mild and bleeding times in this rare vWD-subtype are only slightly prolonged (1). Larger than normal multimers of plasma-vWF and normal levels of platelet-vWF have both been suggested to compensate the defects that are normally present in vWD. To elucidate the mechanisms involved, whole blood of four patients with vWD 'Vicenza' was circulated through a rectangular perfusion chamber (2) with fibrillar collagen as adhesive surface. Under these conditions, both plasma and platelet vWF participate to platelet adhesion. Compared to perfusion results with blood of normal donors, platelet adhesion of 'Vicenza' patients was decreased. However, the Vicenza defect was less than was observed in parallel experiments with blood of vWD type 1 subtype platelet-low patients and blood of a severe vWD patient. Adhesion was not increased in perfusions with only plasma of the 'Vicenza' vWD-patients. Equal vWF:Ag concentrations of normal multimeric composition were just as effective as the high multimeric 'Vicenza' vWF. Therefore, the abnormal plasma-vWF in 'Vicenza' patients does not cause the relatively high adhesion obtained with whole blood. In contrast, platelets of vWD 'Vicenza' patients resuspended in human albumine solution (HAS) showed far better adhesion than (vWF-poor) platelets of a patient with severe vWD. The values were at least comparable and tended to be higher than those obtained with normal platelets or with platelets of patients with vWD type I subtype platelet normal.(ABSTRACT TRUNCATED AT 250 WORDS)

Vessel wall proteins adhesive for platelets

Platelet adhesion to the vessel wall is the first step in the development of a haemostatic plug or thrombus. In vitro studies of platelet adhesion in flowing blood have become possible due to the development of suitable perfusion chambers. With the use of such chambers, adhesion to subendothelium and to isolated connective tissue components from the vessel wall has been realized. Such studies have begun to shed light on the molecular basis of the adhesion process. Von Willebrand factor (vWF), fibronectin, and collagen types I and III have been shown to be the most important adhesive proteins. The functional domains involved in vWF and fibronectin are being identified and the platelet receptors for these ligands have been established. A complicated picture of multiple mutually supportive ligand-receptor interactions has emerged. These insights are critical for the development of thrombo-resistant biomaterials.

von Willebrand factor and the blood vessel wall

The role of von Willebrand factor (vWF) in mediating platelet adhesion has been established with the use of in vitro perfusion systems. vWF binds to the subendothelium, changes in conformation, and is then able to interact with glycoprotein Ib. vWF deposited in the subendothelium is responsible for up to 40% of normal adhesion. The action of vWF is seen at high shear rates. It also acts at low shear rates, but other factors can assume its role. Binding of vWF to the subendothelium is not through the established collagen binding sites but involves a new domain on the vWF molecule. Collagen VI has been implicated as the vessel wall component involved. Deposition of vWF in the subendothelium is determined by growth conditions and activation state of the endothelial cell. Subendothelial vWF in von Willebrand's disease may support platelet adhesion in situations in which plasma vWF of the same patient does not.