Epitope Mapping of Inhibitory Monoclonal Antibodies to Human von Willebrand Factor by Using Recombinant cDNA Libraries

Solution structure of the major factor VIII binding region on von Willebrand factor

Although much of the function of von Willebrand factor (VWF) has been revealed, detailed insight into the molecular structure that enables VWF to orchestrate hemostatic processes, in particular factor VIII (FVIII) binding and stabilization in plasma, is lacking. Here, we present the high-resolution solution structure and structural dynamics of the D' region of VWF, which constitutes the major FVIII binding site. D' consists of 2 domains, trypsin-inhibitor-like (TIL') and E', of which the TIL' domain lacks extensive secondary structure, is strikingly dynamic and harbors a cluster of pathological mutations leading to decreased FVIII binding affinity (type 2N von Willebrand disease [VWD]). This indicates that the backbone malleability of TIL' is important for its biological activity. The principal FVIII binding site is localized to a flexible, positively charged region on TIL', which is supported by the rigid scaffold of the TIL' and E' domain β sheets. Furthermore, surface-charge mapping of the TIL'E' structure reveals a potential mechanism for the electrostatically guided, high-affinity VWF⋅FVIII interaction. Our findings provide novel insights into VWF⋅FVIII complex formation, leading to a greater understanding of the molecular basis of the bleeding diathesis type 2N VWD.

A murine model to characterize the antithrombotic effect of molecules targeting human von Willebrand factor

von Willebrand factor (VWF) is a promising target for developing antithrombotic drugs. The absence of accessible animal models impedes the study of specific human VWF (huVWF) targeting molecules in thrombosis. huVWF is not functional in the mouse because of a lack of interaction between huVWF and murine glycoprotein Ib. Using site-directed mutagenesis, we have replaced single or multiple amino acids in huVWF with their murine counterparts to eliminate species incompatibility. Using hydrodynamic injection, we have expressed the different chimeric VWF constructs into VWF(-/-) mice. Only huVWF with a complete murine A1 domain insertion was able to correct bleeding in vivo and form occlusive thrombi in mesenteric vessels after FeCl(3) treatment. Using this model, we tested the antithrombotic effect of monoclonal antibodies against huVWF, blocking its interaction with collagens (mAbs 203 and 505) or with glycoprotein IIbIIIa (mAb 9). The 3 mAbs inhibited the thrombotic process in arterioles of VWF(-/-) mice expressing huVWFmuA1. Inhibiting VWF-interaction with collagens was more potent, emphasizing the potential of such a target as an antithrombotic tool. Our results validate our murine model as a simple in vivo tool to evaluate anti-huVWF agents.

Distinct sequences of the glycoprotein Ib-binding domain of von Willebrand factor involved in shear induced platelet aggregation

Shear-induced platelet aggregation (SIPA) requires von Willebrand factor (vWF) binding to the platelet receptors GPIb and alphaIIbbeta3. In order to determine the vW F sequences involved in SIPA at 4000/s, we studied the llb 3 effect of three monoclonal antibodies (mabs) 724, 713 and 328 to the A1 domain of vWF. We found that mab 724 induced an enhanced SIPA via a Fc gamma-receptor independent mechanism. In contrast, mab 713 and mab 328 could inhibit SIPA by 52 and 91% , respectively. Based on distinct effects on SIPA, we can propose the following working model for the interaction between vWF and GPIb: mabs 713 and 328, which block SIPA, may recognize an epitope that is involved in binding to GPIb, whereas mab 724, which increases SIPA in the presence of vWF, may mimic the effect of botrocetin when binding to vWF, by inducing an active conformation of vWF, which may be more sensitive to high shear rate.

Identification of amino acid residues responsible for von Willebrand factor binding to sulfatide by charged-to-alanine-scanning mutagenesis

von Willebrand factor (VWF) performs its hemostatic functions through binding to various proteins. The A1 domain of VWF contains binding sites of not only physiologically important ligands, but also exogenous modulators that induce VWF-platelet aggregation. Sulfatides, 3-sulfated galactosyl ceramides, that are expressed on oligodendrocytes, renal tubular cells, certain tumor cells and platelets, have been suggested to interact with VWF under some pathological conditions. The binding of VWF to sulfatide requires the A1 domain, but its binding sites have not been precisely identified. Here, we report that alanine mutations at Arg1392, Arg1395, Arg1399 and Lys1423 led to decreased VWF–sulfatide binding. These sites have been reported to be the binding sites for platelet membrane glycoprotein (GP) Ib and/or snake venom botrocetin, and, interestingly, are identical to the monoclonal antibody (mAb) NMC4 epitope previously reported to inhibit the VWF-GPIb interaction. We observed that NMC4 also inhibited VWF interaction with sulfatides in a dose-dependent manner. Thus, we conclude that VWF binding sites of sulfatide overlap those of platelet GPIb and botrocetin.

β2-Glycoprotein I inhibits von Willebrand factor–dependent platelet adhesion and aggregation

Patients with antiphospholipid syndrome are characterized by the association of thrombosis or pregnancy morbidity and the presence of antiphospholipid autoantibodies. Particularly, anti-β2-glycoprotein (β2 GPI) autoantibodies correlate with thrombosis, suggesting an antibody-induced gain of prothrombotic function and/or an antibody-induced loss of antithrombotic function of β2 GPI. In the search for potential antithrombotic properties of β2 GPI, we found that β2 GPI inhibits von Willebrand factor (VWF)–induced platelet aggregation. In addition, platelet adhesion to a VWF-coated surface was decreased by 50% in the presence of β2 GPI (P < .03). β2 GPI binds to the A1 domain of VWF but preferably when the A1 domain is in its active glycoprotein Ibα-binding conformation. Anti-β2 GPI antibodies isolated from a subset of antiphospholipid syndrome patients neutralized the β2 GPI-VWF interactions and thus the inhibitory activity of β2 GPI. In comparison to healthy individuals, the amounts of active VWF in circulation were increased 1.5-fold (P < .001) in patients positive for lupus anticoagulant (LAC) due to anti-β2 GPI antibodies. Thus, β2 GPI is a biologically relevant inhibitor of VWF function by interfering with VWF-dependent platelet adhesion. Anti-β2 GPI autoantibodies neutralize this inhibitory function and are associated with increased levels of active VWF. This mode of action could contribute to the thrombosis and consumptive thrombocytopenia observed in patients with anti-β2 GPI antibodies.

Characterization of the interaction between von Willebrand factor and osteoprotegerin

BACKGROUND AND OBJECTIVE

Osteoprotegerin (OPG), a member of the tumor necrosis-factor receptor superfamily, plays an important role in bone remodeling and is also involved in vascular diseases. OPG is physically associated with von Willebrand factor (VWF), a glycoprotein involved in primary hemostasis, within the Weibel-Palade bodies (WPBs) of endothelial cells and in plasma. The present study aimed to elucidate the molecular mechanisms underlying the interaction between OPG and VWF.

METHODS AND RESULTS

In a solid-phase binding assay, VWF was able to bind specifically to OPG in a calcium-dependent manner. This interaction displayed strong pH dependence with optimal binding occurring at pH 6.5 and was severely impaired by chloride-ion concentrations above 40 mm. Using a series of purified VWF derivatives the functional site that supports VWF interaction with OPG was localized on its Al domain. Fluorescence microscopy on human umbilical vein endothelial cells showed co-localization of VWF and OPG in WPBs. When secretion was induced, OPG remained associated with VWF in extracellular patches of release under biochemical conditions found in blood plasma.

CONCLUSIONS

Our observations demonstrate the existence of an interactive site for OPG within the VWF A1-domain. This study established that the optimal biochemical parameters allowing a complex formation between VWF and OPG are those thought to prevail in the trans-Golgi network. These conditions would allow VWF to act as a cargo targeting OPG to WPBs. Finally, blood environments appear suitable to preserve the complex, which may participate in vascular injury, arterial calcification and inflammation.

What a polyclonal antibody sees in von Willebrand factor

INTRODUCTION

Von Willebrand factor (VWF) plays a critical role in hemostasis by carrying factor VIII (FVIII) and binding to specific ligands on the surface of blood platelets and within the blood vessel wall.

MATERIALS AND METHODS

We constructed a gene-specific phage display library containing small, random VWF fragments. Using this library, we mapped the repertoire of immune epitopes recognized by a commonly used commercial rabbit antihuman VWF polyclonal antibody.

RESULTS

A total of eight discrete epitopes within the VWF protein were identified, including two dominant epitopes that account for 74% of immuno selected VWF fragments. Comparison with previously mapped epitopes for mouse monoclonal antibodies reveals four overlapping regions that may identify common antigenic determinants. The distribution of these epitopes was not readily predicted from primary amino acid sequence divergence among these mammalian species or standard algorithms for the prediction of antigenicity, hydrophobicity, or surface probability.

CONCLUSION

Taken together with previous monoclonal antibody epitope mapping studies, our results suggest that a limited number of exposed domains on the surface of the human VWF protein may be the primary determinants of immunogenicity.

Surface-dependent expression in the platelet GPIb binding domain within human von Willebrand factor studied by atomic force microscopy

Adsorption of plasma proteins such as von Willebrand factor (vWF) on thrombogenic surfaces can induce conformational changes in tertiary structure so that the prothrombotic functional epitopes are exposed for interactions with platelets, resulting in platelet adhesion and thrombus formation. Thus, understanding platelet binding following changes in the structure of vWF is critical in understanding the mechanisms of thrombogenesis. The present study examined the accessibility of platelet binding epitopes within vWF adsorbed on two different thrombogenic surfaces, a hydrophobic synthetic surface and collagen VI coated substrates, under physiological buffer conditions using atomic force microscopy (AFM) in combination with immunogold labeling. Our results demonstrated that the glycoprotein Ib (GPIb) binding domain in vWF undergoes changes when adsorbed on collagen VI compared to vWF on a hydrophobic synthetic surface. This study provides a basis for a novel approach to understand the molecular mechanisms of surface-induced thrombosis by directly examining the structure-function relationships of plasma proteins involved in the thrombus formation.

Two novel mutations, Q1053H and C1060R, located in the D3 domain of von Willebrand factor, are responsible for decreased FVIII-binding capacity

In type 2N von Willebrand disease (VWD), von Willebrand factor (VWF) is characterized by a markedly decreased affinity for Factor VIII (FVIII), and the mutations responsible are essentially located in the D' domain of VWF. We report the identification, in seven unrelated French families, of two novel type 2N VWD mutations, Q1053H and C1060R (Gln290His and Cys297Arg in mature VWF sequence), in exon 24 of the VWF gene. These missense mutations have been identified in the heterozygous, homozygous or hemizygous states. Using site-directed mutagenesis and transient expression in COS-7 cells, we showed that both mutations, although located in the D3 domain of VWF, outside the tryptic fragment containing the FVIII domain, dramatically decrease the binding of VWF to FVIII. In contrast, the R924Q substitution, which was identified in a patient who was heterozygous for C1060R, was shown to be a polymorphism.

Activation of integrin alphaIIbbeta3 expressed in Chinese hamster ovary cells is required for interaction with solid-phase von Willebrand factor

Dithiothreitol (DTT) is known to induce an active conformation of alphaIIbbeta3 integrin and to promote the aggregation of Chinese hamster ovary (CHO)-alphaIIbbeta3 cells in the presence of soluble fibrinogen (Fg). The aim of this study was to compare adhesion and spreading with Fg or von Willebrand factor (VWF) of CHO-alphaIIbbeta3 cells in the presence or absence of DTT. Our results indicate that DTT treatment was required to induce cell spreading on VWF. In contrast, CHO-alphaIIbbeta3 cell spreading on Fg was already optimal in the absence of DTT. We used a small perfusion chamber coupled to videomicroscopy to demonstrate that CHO-alphaIIbbeta3 cells that were adherent and spread on VWF required DTT activation to resist to detachment under increasing shear rates (50-1600/s). In contrast, untreated or DTT-treated cells spread on Fg were able to resist to extremely high flow rates. These data provide novel evidence that activated alphaIIbbeta3 is absolutely required for spread cells to resist detachment and strengthens the importance of the alphaIIbbeta3 activation step for adhesion and spreading to VWF.

Identification of the regulatory elements of the human von Willebrand factor for binding to platelet GPIb. Importance of structural integrity of the regions flanked by the CYS1272-CYS1458 disulfide bond

In vitro platelet glycoprotein Ib (GPIb) binding of the human von Willebrand factor (VWF) increases markedly by exogenous modulators such as ristocetin or botrocetin, and the binding does not occur in normal circulation. GPIb binding sites have been assigned in the VWF A1 domain, which consists of a disulfide loop Cys1272(509)-Cys1458(695) where amino acid residues are numbered from the starting methionine as +1. The previous numbering from the N-terminal Ser of the mature processed VWF is indicated in parentheses. In contrast, several gain-of-function mutations have been found in two regions comprised of the disulfide loop and its N- and C-terminal flanking regions. In this study, Cys1222(459)-Tyr1271(508), Gln1238(475)-Tyr1271(508), Glu1260(497)-Tyr1271(508), and Asp1459(696)-Asp1472(709) were sequentially deleted of full-length multimeric recombinant VWF. Deletions at either side resulted in normal GPIb binding, indicating that the flanking regions are not GPIb binding sites. However, the addition of a mutation at Arg1308(545) on each deletion mutant resulted in spontaneous GPIb binding without requiring modulators, suggesting that both regions are important for the inhibition of GPIb binding. Spontaneous binding was completely inhibited by monoclonal antibodies that recognize the GPIb binding sites. Interestingly, mutant proteins with N-terminal but not C-terminal deletions lost binding to monoclonal antibodies B328, B710, and 23C7, which selectively inhibit ristocetin-induced GPI binding. Their epitopes were found at His1268(505) or Asp1269(506). The crystallographic structure of the A1 domain suggests that GPIb binding is influenced by the molecular interface between the two regions and that the antibody binding to the interface inhibits binding.

Type 1 von Willebrand disease mutation Cys1149Arg causes intracellular retention and degradation of heterodimers: a possible general mechanism for dominant mutations of oligomeric proteins

Some families affected by von Willebrand disease type 1 show high penetrance with exceptionally low von Willebrand factor (VWF) levels. Previously, a mutation associated with this dominant phenotype, Cys1149Arg, was found to decrease the secretion of coexpressed normal VWF, and the mutation was proposed to cause intracellular retention of pro-VWF heterodimers. To demonstrate heterodimer formation, a model was developed in which subunits could be distinguished immunologically and by size. Recombinant VWF lacking domain A1 (dA1), A3 (dA3), or both (dA13) was secreted efficiently as a full range of multimers. Cotransfection of Cys1149Arg and dA13 resulted in the secretion of multimeric VWF containing about 250 kd (Cys1149Arg) and about 210 kd (dA13). Cell lysates contained pro-VWF forms of Cys1149Arg and dA13. Immunoprecipitation with an antidomain A1 antibody recovered both subunits in heterodimers, and subunit ratios were consistent with random dimerization. Similar results were obtained for cotransfection of Cys1149Arg and dA1. Normal VWF has a Cys1149-Cys1169 intrachain bond. When cotransfected with normal VWF, Cys1149Arg or the double mutant Cys1149Arg+Cys1169Ser caused a similar decrease in VWF secretion, suggesting that an unpaired Cys1169 does not explain the intracellular retention of Cys1149Arg. VWF Cys1149Arg was not secreted from BHK cells but was degraded intracellularly within about 4 hours, and the proteasome inhibitor lactacystin delayed its clearance more than 16 hours. Thus, dominant von Willebrand disease type 1 may be caused by heterodimerization of mutant and normal subunits in the endoplasmic reticulum followed by proteasomal degradation in the cytoplasm. A similar dominant negative mechanism could cause quantitative deficiencies of other multisubunit proteins.

The arginine-552-cysteine (R1315C) mutation within the A1 loop of von Willebrand factor induces an abnormal folding with a loss of function resulting in type 2A-like phenotype of von Willebrand disease: study of 10 patients and mutated recombinant von Willebrand factor

The study identified 10 patients from 6 families with prolonged bleeding time, decreased von Willebrand factor (vWF) ristocetin cofactor activity (RCoF) to vWF:Ag (antigen) ratio, and reduced ristocetin-induced platelet agglutination as well as ristocetin- or botrocetin-induced binding of plasma vWF to platelet glycoprotein Ib (GpIb). In addition, all patients showed a decrease of intermediate-molecular-weight (intermediate-MW) and high-molecular-weight (HMW) multimers of vWF. In the heterozygous state, a cysteine-to-threonine (C --> T) transversion was detected at nucleotide 4193 of the VWF gene of all patients and lead to the arginine (R)522C substitution in the A1 loop of vWF mature subunit (R1315C in the preprovWF). By in vitro mutagenesis of full-length complementary DNA (cDNA) of vWF and transient expression in COS-7 cells, the mutated C552 recombinant vWF (C552rvWF) was found to exhibit decreased expression, abnormal folding, and lack of intermediate-MW and HMW multimers. In addition, direct binding of botrocetin to C552rvWF, as well as ristocetin- and botrocetin-induced binding of C552rvWF to GpIb, was markedly decreased. Although being localized in an area of the A1 loop of vWF where most of the type 2B mutations that induce a gain-of-function have been identified, the R552C mutation induces a 2A-like phenotype with a decrease of intermediate-MW and HMW multimers as well as a loss-of-function of vWF in the presence of either ristocetin or botrocetin. (Blood. 2001;97:952-959)

Effect of recombinant von Willebrand factor reproducing type 2B or type 2M mutations on shear-induced platelet aggregation

The aim was to better understand the function of von Willebrand factor (vWF) A1 domain in shear-induced platelet aggregation (SIPA), at low (200) and high shear rate (4000 seconds-1) generated by a Couette viscometer. We report on 9 fully multimerized recombinant vWFs (rvWFs) expressing type 2M or type 2B von Willebrand disease (vWD) mutations, characterized respectively by a decreased or increased binding of vWF to GPIb in the presence of ristocetin. We expressed 4 type 2M (-G561A, -E596K, -R611H, and -I662F) and 5 type 2B (rvWF-M540MM, -V551F, -V553M, -R578Q, and -L697V). SIPA was strongly impaired in all type 2M rvWFs at 200 and 4000 seconds-1. Decreased aggregation was correlated with ristocetin binding to platelets. In contrast, a distinct effect of botrocetin was observed, since type 2M rvWFs (-G561A, -E596K, and -I662F) were able to bind to platelets to the same extent as wild type rvWF (rvWF-WT). Interestingly, SIPA at 200 and 4000 seconds-1 confirmed the gain-of-function phenotype of the 5 type 2B rvWFs. Our data indicated a consistent increase of SIPA at both low and high shear rates, reaching 95% of total platelets, whereas SIPA did not exceed 40% in the presence of rvWF-WT. Aggregation was completely inhibited by monoclonal antibody 6D1 directed to GPIb, underlining the importance of vWF-GPIb interaction in type 2B rvWF. Impaired SIPA of type 2M rvWF could account for the hemorrhagic syndrome observed in type 2M vWD. Increased SIPA of type 2B rvWF could be responsible for unstable aggregates and explain the fluctuant thrombocytopenia of type 2B vWD.

Effect of recombinant von Willebrand factor reproducing type 2B or type 2M mutations on shear-induced platelet aggregation

The aim was to better understand the function of von Willebrand factor (vWF) A1 domain in shear-induced platelet aggregation (SIPA), at low (200) and high shear rate (4000 seconds-1) generated by a Couette viscometer. We report on 9 fully multimerized recombinant vWFs (rvWFs) expressing type 2M or type 2B von Willebrand disease (vWD) mutations, characterized respectively by a decreased or increased binding of vWF to GPIb in the presence of ristocetin. We expressed 4 type 2M (-G561A, -E596K, -R611H, and -I662F) and 5 type 2B (rvWF-M540MM, -V551F, -V553M, -R578Q, and -L697V). SIPA was strongly impaired in all type 2M rvWFs at 200 and 4000 seconds-1. Decreased aggregation was correlated with ristocetin binding to platelets. In contrast, a distinct effect of botrocetin was observed, since type 2M rvWFs (-G561A, -E596K, and -I662F) were able to bind to platelets to the same extent as wild type rvWF (rvWF-WT). Interestingly, SIPA at 200 and 4000 seconds-1 confirmed the gain-of-function phenotype of the 5 type 2B rvWFs. Our data indicated a consistent increase of SIPA at both low and high shear rates, reaching 95% of total platelets, whereas SIPA did not exceed 40% in the presence of rvWF-WT. Aggregation was completely inhibited by monoclonal antibody 6D1 directed to GPIb, underlining the importance of vWF-GPIb interaction in type 2B rvWF. Impaired SIPA of type 2M rvWF could account for the hemorrhagic syndrome observed in type 2M vWD. Increased SIPA of type 2B rvWF could be responsible for unstable aggregates and explain the fluctuant thrombocytopenia of type 2B vWD.

Conformational changes in the D′ domain of von Willebrand factor induced by CYS 25 and CYS 95 mutations lead to factor VIII binding defect and multimeric impairment

We report 2 new mutations identified in 3 patients and characterized by the markedly decreased affinity of von Willebrand factor (vWF) for factor VIII (FVIII). Patients 2 and 3, who have a typical type 2N phenotype, were found to be compound heterozygous for Arg91Gln and Cys25Tyr or Cys95Phe, respectively. Patient 1, who is the first cousin of patient 2, had an FVIII binding defect of vWF, low levels of vWF, and multimeric impairment. She was found to be compound heterozygous for the mutations Cys25Tyr and a stop codon (D93ter) in exon 4. Transient expression of recombinant vWF (rvWF) containing either Cys25Tyr or Cys95Phe mutations resulted in mutated rvWF with markedly reduced FVIII binding ability, multimeric structure impairment, and a significant decrease in the vWF expression level. Moreover, the use of anti-vWF monoclonal antibodies that inhibit the FVIII binding showed that these 2 mutations likely induce a conformational change in the D′ domain. These results show that the native conformation of the D′ domain of vWF is not only required for FVIII binding but also for normal multimerization and optimal secretion.

Conformational changes in the D′ domain of von Willebrand factor induced by CYS 25 and CYS 95 mutations lead to factor VIII binding defect and multimeric impairment

We report 2 new mutations identified in 3 patients and characterized by the markedly decreased affinity of von Willebrand factor (vWF) for factor VIII (FVIII). Patients 2 and 3, who have a typical type 2N phenotype, were found to be compound heterozygous for Arg91Gln and Cys25Tyr or Cys95Phe, respectively. Patient 1, who is the first cousin of patient 2, had an FVIII binding defect of vWF, low levels of vWF, and multimeric impairment. She was found to be compound heterozygous for the mutations Cys25Tyr and a stop codon (D93ter) in exon 4. Transient expression of recombinant vWF (rvWF) containing either Cys25Tyr or Cys95Phe mutations resulted in mutated rvWF with markedly reduced FVIII binding ability, multimeric structure impairment, and a significant decrease in the vWF expression level. Moreover, the use of anti-vWF monoclonal antibodies that inhibit the FVIII binding showed that these 2 mutations likely induce a conformational change in the D′ domain. These results show that the native conformation of the D′ domain of vWF is not only required for FVIII binding but also for normal multimerization and optimal secretion.

Conformational Changes in the A3 Domain of von Willebrand Factor Modulate the Interaction of the A1 Domain With Platelet Glycoprotein Ib

Bitiscetin has recently been shown to induce von Willebrand factor (vWF)-dependent aggregation of fixed platelets (Hamako J, et al,Biochem Biophys Res Commun 226:273, 1996). We have purified bitiscetin from Bitis arietans venom and investigated the mechanism whereby it promotes a form of vWF that is reactive with platelets. In the presence of bitiscetin, vWF binds to platelets in a dose-dependent and saturable manner. The binding of vWF to platelets involves glycoprotein (GP) Ib because it was totally blocked by monoclonal antibody (MoAb) 6D1 directed towards the vWF-binding site of GPIb. The binding also involves the GPIb-binding site of vWF located on the A1 domain because it was inhibited by MoAb to vWF whose epitopes are within this domain and that block binding of vWF to platelets induced by ristocetin or botrocetin. However, in contrast to ristocetin or botrocetin, the binding site of bitiscetin does not reside within the A1 domain but within the A3 domain of vWF. Thus, among a series of vWF fragments, 125I-bitiscetin only binds to those that overlap the A3 domain, ie, SpIII (amino acid [aa] 1-1365), SpI (aa 911-1365), and rvWF-A3 domain (aa 920-1111). It does not bind to SpII corresponding to the C-terminal part of vWF subunit (aa 1366-2050) nor to the 39/34/kD dispase species (aa 480-718) or T116 (aa 449-728) overlapping the A1 domain. In addition, bitiscetin that does not bind to DeltaA3-rvWF (deleted between aa 910-1113) has no binding site ouside the A3 domain. The localization of the binding site of bitiscetin within the A3 domain was further supported by showing that MoAb to vWF, which are specific for this domain and block the interaction between vWF and collagen, are potent inhibitors of the binding of bitiscetin to vWF and consequently of the bitiscetin-induced binding of vWF to platelets. Thus, our data support the hypothesis that an interaction between the A1 and A3 domains exists that may play a role in the function of vWF by regulating the ability of the A1 domain to bind to platelet GPIb.

A Novel Mutation in the D3 Domain of von Willebrand Factor Markedly Decreases Its Ability to Bind Factor VIII and Affects Its Multimerization

In type 2N von Willebrand disease (vWD), von Willebrand factor (vWF) is characterized by normal multimeric pattern, normal platelet-dependent function, but a markedly decreased affinity for factor VIII (FVIII). In this report, we describe the case of a vWD patient who has an abnormal vWF multimers distribution associated with a markedly decreased vWF ability to bind FVIII. Sequencing analysis of patient’s vWF gene showed, at heterozygous state, a G→A transition resulting in the substitution of Asn for Asp at position 116 of the mature vWF subunit and a C→T transition, changing the codon for Arg 896 into a stop codon. His sister who has a subnormal vWF level, but a normal FVIII/vWF interaction, was found to be heterozygous for the Arg896ter mutation only. Recombinant vWF (rvWF) containing the candidate (Asn116) missense mutation was expressed in COS-7 cells. The expression level of Asn116rvWF was significantly decreased compared with wild-type rvWF. The multimeric pattern of Asn116rvWF was greatly impaired as shown by the decrease in high molecular weight forms. The FVIII binding ability of Asn116rvWF was dramatically decreased. These data show that the Asp116Asn substitution is the cause of both the defective FVIII/vWF interaction and the impaired multimeric pattern observed in the patient’s vWF. The monoclonal antibody 31H3 against D’ domain of vWF (epitope aa 66-76) that partially inhibits the FVIII binding and recognizes only nonreduced vWF, showed a decreased ability to bind Asn116rvWF when used as capture-antibody in enzyme-linked immunosorbent assay (ELISA). This result suggests that a potential conformation change in the D’ domain is induced by the Asp116Asn substitution, which is localized in the D3 domain.

A Novel Mutation in the D3 Domain of von Willebrand Factor Markedly Decreases Its Ability to Bind Factor VIII and Affects Its Multimerization

In type 2N von Willebrand disease (vWD), von Willebrand factor (vWF) is characterized by normal multimeric pattern, normal platelet-dependent function, but a markedly decreased affinity for factor VIII (FVIII). In this report, we describe the case of a vWD patient who has an abnormal vWF multimers distribution associated with a markedly decreased vWF ability to bind FVIII. Sequencing analysis of patient’s vWF gene showed, at heterozygous state, a G→A transition resulting in the substitution of Asn for Asp at position 116 of the mature vWF subunit and a C→T transition, changing the codon for Arg 896 into a stop codon. His sister who has a subnormal vWF level, but a normal FVIII/vWF interaction, was found to be heterozygous for the Arg896ter mutation only. Recombinant vWF (rvWF) containing the candidate (Asn116) missense mutation was expressed in COS-7 cells. The expression level of Asn116rvWF was significantly decreased compared with wild-type rvWF. The multimeric pattern of Asn116rvWF was greatly impaired as shown by the decrease in high molecular weight forms. The FVIII binding ability of Asn116rvWF was dramatically decreased. These data show that the Asp116Asn substitution is the cause of both the defective FVIII/vWF interaction and the impaired multimeric pattern observed in the patient’s vWF. The monoclonal antibody 31H3 against D’ domain of vWF (epitope aa 66-76) that partially inhibits the FVIII binding and recognizes only nonreduced vWF, showed a decreased ability to bind Asn116rvWF when used as capture-antibody in enzyme-linked immunosorbent assay (ELISA). This result suggests that a potential conformation change in the D’ domain is induced by the Asp116Asn substitution, which is localized in the D3 domain.

Platelet Aggregation Induced by a Monoclonal Antibody to the A1 Domain of von Willebrand Factor

Shear-induced platelet aggregation (SIPA) involves von Willebrand Factor (vWF) binding to platelet glycoprotein (GP)Ib at high shear stress, followed by the activation of αIIbβ3. The purpose of this study was to determine the vWF sequences involved in SIPA by using monoclonal antibodies (MoAbs) to vWF known to interfere with its binding to GPIb and to αIIbβ3. Washed platelets were exposed to shear rates between 100 and 4,000 seconds−1 in a rotational viscometer. SIPA was quantitated by flow cytometry as the disappearance of single platelets (DSP) in the sheared sample in the presence of vWF, relative to a control in the absence of shear and vWF. At a shear rate of 4,000 seconds−1, DSP was increased from 5.9% ± 3.5% in the absence of vWF to 32.7% ± 6.3% in the presence of vWF. This increase in SIPA was not associated with an elevation of P-selectin expression. vWF-dependent SIPA was completely abolished by MoAb 6D1 to GPIb and partially inhibited by MoAb 10E5 to αIIbβ3. Three MoAbs to vWF were compared for their effect on SIPA at 4,000 seconds−1 in the presence of vWF: MoAb 328, known to block vWF binding to GPIb in the presence of ristocetin, MoAb 724 blocking vWF binding to GPIb in the presence of botrocetin, and MoAb 9, an inhibitor of vWF binding to αIIbβ3. Similar to the effect of MoAb 6D1, MoAb 328 completely inhibited the effect of vWF, whereas MoAb 9 had a partial inhibitory effect, as MoAb 10E5 did. In contrast, MoAb 724, as well as its F(ab′)2 fragments, promoted shear-dependent platelet aggregation (165% of the DSP value obtained in the absence of MoAb 724), indicating that MoAb 724 was responsible for an enhanced aggregation, which was independent of binding to the platelet Fcγ receptor. In addition, the enhancement of aggregation induced by MoAb 724 was abrogated by MoAb 6D1 or 10E5 to the level of SIPA obtained in the presence of vWF incubated with a control MoAb to vWF. Finally, the activating effect of MoAb 724 was also found under static conditions at ristocetin concentrations too low to induce platelet aggregation. Our results suggested that on binding to a botrocetin-binding site on vWF, MoAb 724 mimics the effect of botrocetin by inducing an active conformation of vWF that is more sensitive to shear stress or to low ristocetin concentration.

Platelet Aggregation Induced by a Monoclonal Antibody to the A1 Domain of von Willebrand Factor

Shear-induced platelet aggregation (SIPA) involves von Willebrand Factor (vWF) binding to platelet glycoprotein (GP)Ib at high shear stress, followed by the activation of αIIbβ3. The purpose of this study was to determine the vWF sequences involved in SIPA by using monoclonal antibodies (MoAbs) to vWF known to interfere with its binding to GPIb and to αIIbβ3. Washed platelets were exposed to shear rates between 100 and 4,000 seconds−1 in a rotational viscometer. SIPA was quantitated by flow cytometry as the disappearance of single platelets (DSP) in the sheared sample in the presence of vWF, relative to a control in the absence of shear and vWF. At a shear rate of 4,000 seconds−1, DSP was increased from 5.9% ± 3.5% in the absence of vWF to 32.7% ± 6.3% in the presence of vWF. This increase in SIPA was not associated with an elevation of P-selectin expression. vWF-dependent SIPA was completely abolished by MoAb 6D1 to GPIb and partially inhibited by MoAb 10E5 to αIIbβ3. Three MoAbs to vWF were compared for their effect on SIPA at 4,000 seconds−1 in the presence of vWF: MoAb 328, known to block vWF binding to GPIb in the presence of ristocetin, MoAb 724 blocking vWF binding to GPIb in the presence of botrocetin, and MoAb 9, an inhibitor of vWF binding to αIIbβ3. Similar to the effect of MoAb 6D1, MoAb 328 completely inhibited the effect of vWF, whereas MoAb 9 had a partial inhibitory effect, as MoAb 10E5 did. In contrast, MoAb 724, as well as its F(ab′)2 fragments, promoted shear-dependent platelet aggregation (165% of the DSP value obtained in the absence of MoAb 724), indicating that MoAb 724 was responsible for an enhanced aggregation, which was independent of binding to the platelet Fcγ receptor. In addition, the enhancement of aggregation induced by MoAb 724 was abrogated by MoAb 6D1 or 10E5 to the level of SIPA obtained in the presence of vWF incubated with a control MoAb to vWF. Finally, the activating effect of MoAb 724 was also found under static conditions at ristocetin concentrations too low to induce platelet aggregation. Our results suggested that on binding to a botrocetin-binding site on vWF, MoAb 724 mimics the effect of botrocetin by inducing an active conformation of vWF that is more sensitive to shear stress or to low ristocetin concentration.

von Willebrand factor: biological function and molecular defects

The human von Willebrand factor (vWF) plays a pivotal role in the mechanisms of blood clotting and platelet thrombus formation; it also binds and stabilizes factor VIII procoagulant protein. The biological functions of vWF are dependent on distinct molecular domains responsible for the specificity and affinity for ligands. The multimeric structure of vWF provides an array of binding sites that allow multivalent interactions, thus supporting the formation of stable platelet aggregates at the site of vascular injury, particularly under flow conditions characterized by high shear stress. Quantitative and qualitative abnormalities of vWF cause the most common congenital bleeding disorder in humans, the von Willebrand disease (vWD). This review will provide an update on the recent advances toward the elucidation of structure-function relationships and the detection of molecular defects leading to vWD and will highlight the revised classification of vWD.

Relative Importance of the Glycoprotein Ib-Binding Domain and the RGD Sequence of von Willebrand Factor for Its Interaction With Endothelial Cells

Endothelial cell adhesion to von Willebrand Factor is mainly mediated through an interaction between the αvβ3 integrin and the RGD sequence of von Willebrand factor (vWF ). To define the potential involvement of glycoprotein Ibα (GPIbα) as an endothelial vWF receptor, we compared cell adhesion to three recombinant vWF, the wild-type (WT-rvWF ) and two mutants, RGGS-rvWF (D1746G), defective for binding to platelet αIIbβ3, and ΔA1-rvWF with a deletion between amino-acids 478 and 716, which does not bind to platelet GPIbα. Adhesion of human umbilical vein endothelial cells to purified vWF recombinants was measured by automatized cell counting using an image analyzer. Whereas cell adhesion to ΔA1-rvWF was unchanged compared with WT-rvWF, reaching a plateau of 40% total cells at a concentration of 2.5 μg/mL rvWF, adhesion to RGGS-rvWF was only 10% of total cells. Cell stimulation by tumor necrosis factor-α (TNFα), reported to upregulate the expression of the putative endothelial GPIbα, did not modify adhesion to these rvWF. Monoclonal antibodies to vWF or GPIbα, blocking vWF interaction with platelet GPIbα, were unable to inhibit endothelial cell adhesion to rvWF. In contrast, antibody 9 to vWF, blocking the αvβ3-dependent endothelial cell adhesion to plasma vWF, inhibited adhesion to WT-rvWF as efficiently as to ΔA1-rvWF (50% inhibition at a concentration of 11 and 15 μg/mL, respectively). In agreement with the fact that endothelial cell adhesion to vWF appeared independent of the GPIbα-binding domain, we were unable to detect endothelial surface expression of GPIbα by flow cytometry or in cell lysates by immunoprecipitation followed by immunoblotting. Moreover, expression of GPIbα mRNA was undetectable in endothelial cells, even after stimulation by TNFα. These studies indicate that GPIbα is not expressed in human cultured endothelial cells and is not involved in adhesion to vWF-containing surfaces. Thus, in static conditions, cultured endothelial cells adhere to vWF through an αvβ3-dependent, GPIbα-independent mechanism.

Relative Importance of the Glycoprotein Ib-Binding Domain and the RGD Sequence of von Willebrand Factor for Its Interaction With Endothelial Cells

Endothelial cell adhesion to von Willebrand Factor is mainly mediated through an interaction between the αvβ3 integrin and the RGD sequence of von Willebrand factor (vWF ). To define the potential involvement of glycoprotein Ibα (GPIbα) as an endothelial vWF receptor, we compared cell adhesion to three recombinant vWF, the wild-type (WT-rvWF ) and two mutants, RGGS-rvWF (D1746G), defective for binding to platelet αIIbβ3, and ΔA1-rvWF with a deletion between amino-acids 478 and 716, which does not bind to platelet GPIbα. Adhesion of human umbilical vein endothelial cells to purified vWF recombinants was measured by automatized cell counting using an image analyzer. Whereas cell adhesion to ΔA1-rvWF was unchanged compared with WT-rvWF, reaching a plateau of 40% total cells at a concentration of 2.5 μg/mL rvWF, adhesion to RGGS-rvWF was only 10% of total cells. Cell stimulation by tumor necrosis factor-α (TNFα), reported to upregulate the expression of the putative endothelial GPIbα, did not modify adhesion to these rvWF. Monoclonal antibodies to vWF or GPIbα, blocking vWF interaction with platelet GPIbα, were unable to inhibit endothelial cell adhesion to rvWF. In contrast, antibody 9 to vWF, blocking the αvβ3-dependent endothelial cell adhesion to plasma vWF, inhibited adhesion to WT-rvWF as efficiently as to ΔA1-rvWF (50% inhibition at a concentration of 11 and 15 μg/mL, respectively). In agreement with the fact that endothelial cell adhesion to vWF appeared independent of the GPIbα-binding domain, we were unable to detect endothelial surface expression of GPIbα by flow cytometry or in cell lysates by immunoprecipitation followed by immunoblotting. Moreover, expression of GPIbα mRNA was undetectable in endothelial cells, even after stimulation by TNFα. These studies indicate that GPIbα is not expressed in human cultured endothelial cells and is not involved in adhesion to vWF-containing surfaces. Thus, in static conditions, cultured endothelial cells adhere to vWF through an αvβ3-dependent, GPIbα-independent mechanism.

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.

A Patient With Type 2N von Willebrand Disease Is Heterozygous for a New Mutation: Gly22Glu. Demonstration of a Defective Expression of the Second Allele by the Use of Monoclonal Antibodies

We report the case of a Chinese patient who has subnormal von Willebrand factor (vWF ) level and normal vWF multimeric pattern, but a lack of vWF capacity to bind factor VIII (FVIII). Exons 18 to 20 of the patient's vWF gene were analyzed by DGGE and a G2354 → A substitution which changes the encoded amino acid sequence from Gly22 to Glu was identified. The patient is heterozygous for this substitution, creating a unique Sac I restriction site. Recombinant vWF (rvWF ) containing the candidate mutation was transiently expressed in COS-7 cells. It was processed and secreted normally but failed to bind FVIII. FVIII binding ability of hybrid rvWF, obtained by cotransfection of normal and mutated expression vectors and corresponding to a heterozygous genotype, was moderately decreased. To explain this functional discrepancy between patient's plasma vWF and hybrid rvWF, we used anti-vWF monoclonal antibodies (MoAbs) as capture in an enzyme-linked immunosorbent assay test. MoAb 32B12 recognized both wild-type and mutated rvWFs whereas MoAb 418 did not recognize mutated rvWF. Because MoAb 418 also failed to capture the plasma vWF from propositus, it means that his second nonmutated allele is not expressed or expressed at a very low level.

A monoclonal antibody (B724) to von Willebrand factor recognizing an epitope within the A1 disulphide loop (Cys509-Cys695) discriminates between type 2A and type 2B von Willebrand disease

Monoclonal antibody (MoAb) B724 to von Willebrand factor (vWF) completely inhibits its interaction with heparin, sulphatides and botrocetin and consequently botrocetin-induced binding of vWF to platelets. MoAb B724 has no effect on the binding of vWF to collagen or to ristocetin-treated platelets nor on vWF-dependent platelet aggregation induced with ristocetin and asialo-vWF-mediated platelet aggregation. MoAb B724 preferentially recognizes a conformation of native vWF, in solution, or immobilized through a coated antibody. It exhibits a markedly lower affinity for vWF immobilized onto collagen or plastic surfaces. Using proteolytic fragments of vWF, B724 epitope was localized within the 512-673 sequence of the A1 disulphide loop of vWF, MoAb B724 was used as second antibody in a two-site ELISA to test a series of patients with type 1, 2A, 2B and 2N vWD or haemophilia A and recombinant wild type or mutated vWFs. Results were compared with those obtained by control ELISAs performed using polyclonal antibodies. Using MoAb B724, strikingly lower levels of vWFAg were observed in plasma from most patients with type 2B vWD, and in seven out of the eight rvWF mutated close to or within the A1 disulphide loop. Therefore MoAb B724, which interferes with this loop involved in the function of vWF, appears to be a useful tool for rapid screening of conformational changes in this region.