von Willebrand factor binding to platelet glycoprotein Ib complex

von Willebrand factor A1 domain can adequately substitute for A3 domain in recruitment of flowing platelets to collagen

BACKGROUND

Binding of von Willebrand factor (VWF) to platelet GPIbalpha and to collagen is attributed to VWF A1 and A3 domains, respectively.

OBJECTIVES

Using VWF, VWF lacking A1 (DeltaA1-VWF) or A3 (DeltaA3-VWF) and VWF with defective A3 (H1786A-VWF), in combination with recombinant A1 (residues 1262-1492) or A3 (residues 1671-1878), fused to glutathione-S-transferase (GST-A1 and GST-A3), we have re-investigated the role of A1 in platelet recruitment to surfaces of collagen.

METHODS AND RESULTS

In flow, measurable binding of DeltaA3-VWF occurred to horse tendon, but also to human type III collagen. GST-A1 and GST-A3 both competed for binding of DeltaA1-VWF and DeltaA3-VWF to horse tendon collagen fibrils in static conditions and to human collagen III during plasmon surface resonance studies, substantiating overlapping binding sites on both collagens for A1 and A3. Heparin did not affect A3-mediated binding of VWF and DeltaA1-VWF, but inhibited binding to horse tendon collagen of GST-A1 and DeltaA3-VWF. Furthermore, A1-mediated binding to type III collagen of DeltaA3-VWF binding was strongly salt-sensitive. During perfusions at wall shear rate 2500 s(-1) of calcein-labeled platelets in reconstituted blood, DeltaA3-VWF and H1786A-VWF triggered platelet binding to horse tendon collagen comparably and as potently as VWF, and to human type III collagen, only fivefold less potently, DeltaA1-VWF being inactive. Additional flow-controlled interaction studies with DeltaA3-VWF, H1786A-VWF, the collagen-VWF antagonist saratin, heparin and the VWF neutralizing antibody 82D6A3 confirmed that H1786A-VWF binds to collagen exclusively via A1.

CONCLUSION

Hence, in shear forces the VWF A1 domain can assume the role of A3 to trigger substantial platelet recruitment to human collagen fibres.

Shielding the front-strand beta 3 of the von Willebrand factor A1 domain inhibits its binding to platelet glycoprotein Ibalpha

Platelet adhesion to damaged vessel wall and shear-induced platelet aggregation necessitate binding of the von Willebrand factor (VWF) A1 domain to platelet GPIbalpha. Blocking this interaction represents a promising approach to the treatment of arterial thrombosis. Comparison of amino acid sequences of the VWF A1 domain in several species, expressing VWF recognized by the blocking monoclonal antibody AJvW-2, suggested 9 residues (His563, Ile566, Asp570, Ala581, Val584, Ala587, Arg616, Ala618, and Met622) to contribute to the epitope for AJvW-2 or to be part of the GPIbalpha-binding site. Glutathione-S-transferase (GST)-human VWF A1 fusion proteins, in which these amino acids were mutated to their murine counterparts, were tested for their capacity to bind AJvW-2 or heparin, to interfere with botrocetin- or ristocetin-mediated VWF binding to GPIb, or to induce flow-dependent platelet tethering in a perfusion chamber. Thus, mutations His563Arg, Ile566Leu, Asp570Ala, and Ala587Thr, clustered on the outer surface of the A1 domain, dramatically impaired binding of AJvW-2 to A1. The His563Arg, Ile566Leu, and Asp570Ala mutations also impaired the binding of heparin, which competes with AJvW-2 for binding to A1. Perfusion studies revealed that His563, Ile566, Asp570, Arg616, and Ala618 take part in GPIbalpha binding, their mutation-impairing platelet recruitment. In agreement with the surface distribution of VWF type 2M mutations, this study demonstrates overlapping of the epitope for AJvW-2 and the GPIbalpha-binding site, located around the front pocket of the A1 domain and defined by strands beta3, beta4, and helix alpha3, and it provides a mechanistic basis for VWF neutralization by this antibody.

Anti-vWf antibodies induce GPIbalpha and FcgammaRII mediated platelet aggregation only at low shear forces

BACKGROUND/METHODS

Anti-von Willebrand factor (vWf) antibody mediated platelet activation was studied using 2 monoclonal anti-vWf antibodies promoting the binding of vWf to GPIbalpha: 1C1E7 (IgG2a) reacting with the vWf N-terminus and 75H4B12 (IgM), characterized in this paper and studied in association with 1C1E7.

RESULTS

75H4B12 binds to an N-terminal epitope in vWf, different from that reacting with 1C1E7. When com-bined, 1C1E7 and 75H4B12 promoted vWf binding to isolated GPIb under static conditions, even in the absence of ristocetin or botrocetin, and induced platelet aggregation synergistically in the presence of zero to subthreshold ristocetin concentrations. Specific inhibitors of GPIbalpha-vWf interactions prevented vWf binding to GPIbalpha in ELISA and during platelet aggregation. In addition, the 1C1E7 dependent platelet aggregation involved Fc receptor mediated platelet activation, a phenomenon even more pronounced when 1C1E7 and 75H4B12 were combined. A 75H4B12 binding phage expressing a peptide homologous with vWf sequence 88-95 neutralized the antibody induced platelet activation. However, at arterial shear rates, both 1C1E7 and 75H4B12 potently prolonged cartridge closure times in the PFA-100, compatible with inhibition of platelets by vWf, unfolded by the combined action of shear stress and antibodies.

CONCLUSIONS

We conclude that antibodies directed against different epitopes in the N-terminus of vWf modify the folded vWf structure synergistically and enhance A1 domain mediated vWf binding to platelet GPIb at low shear forces. In addition, once platelet-bound, IgG antibodies potently activate platelets via the FcgammaII receptor. Thus, such antibodies may promote immune mediated thrombosis at low shear rates, typical of the venous circulation. In contrast, at arterial shear rates, anti-vWf antibodies may rather compromise platelet function following enhanced binding of the unfolded vWf multimers to platelets, shielding platelets from interacting with subendothelial and soluble ligands.

Platelets: New Understanding of Platelet Glycoproteins and Their Role in Disease

This review covers new developments and their clinical implications in three areas: platelet antigen polymorphisms, inhibition of platelet glycoprotein IIb-IIIa, and autoimmune thrombocytopenia (ITP).

In Section I, Dr. Kunicki reviews platelet polymorphisms and their clinical implications. A current tabulation of the numerous platelet antigens, both those that are platelet specific and not platelet specific, are summarized. The immunogenic clinical implications of these polymorphisms are considered, including fetal and neonatal alloimmune thrombocytopenia, post transfusion purpura, and refractoriness to platelet transfusion. The functional relationship to hemostasis and thrombosis is also discussed, in particular whether one haplotype of the PIA1/PIA2 (HPA-1a/1b) polymorphism predisposes to myocardial infarction. Finally, novel investigations of polymorphisms will be considered, including hormonal induction of certain polymorphisms.

In Section II, Dr. Michelson reviews the newest generation of platelet inhibitors, those blocking glycoprotein IIB/IIIA, from the point of view of the hematologist who might be consulted about a patient receiving this form of treatment. The current use of available IIb-IIIa inhibitors and those in trial and the accepted and possible future indications for their use are addressed. The mechanism of action and actual and theoretical advantages and disadvantages of each inhibitor are explored. Scenarios that prompt consultation with a hematologist are presented, including management of bleeding, thrombocytopenia, and management of the patient requiring emergency surgery.

In Section III, Dr. Bussel reviews controversies in ITP, looking at both the current state of the art and the potential for the future. Case presentations are used to illustrate the issues in both children and adults. Three primary areas are addressed: 1) the diagnosis of ITP, 2) when and for which patient to recommend splenectomy, and 3) the management of the refractory splenectomized patient who still has a low platelet count and bleeding symptoms.

Platelets: New Understanding of Platelet Glycoproteins and Their Role in Disease

This review covers new developments and their clinical implications in three areas: platelet antigen polymorphisms, inhibition of platelet glycoprotein IIb-IIIa, and autoimmune thrombocytopenia (ITP).

In Section I, Dr. Kunicki reviews platelet polymorphisms and their clinical implications. A current tabulation of the numerous platelet antigens, both those that are platelet specific and not platelet specific, are summarized. The immunogenic clinical implications of these polymorphisms are considered, including fetal and neonatal alloimmune thrombocytopenia, post transfusion purpura, and refractoriness to platelet transfusion. The functional relationship to hemostasis and thrombosis is also discussed, in particular whether one haplotype of the PIA1/PIA2 (HPA-1a/1b) polymorphism predisposes to myocardial infarction. Finally, novel investigations of polymorphisms will be considered, including hormonal induction of certain polymorphisms.

In Section II, Dr. Michelson reviews the newest generation of platelet inhibitors, those blocking glycoprotein IIB/IIIA, from the point of view of the hematologist who might be consulted about a patient receiving this form of treatment. The current use of available IIb-IIIa inhibitors and those in trial and the accepted and possible future indications for their use are addressed. The mechanism of action and actual and theoretical advantages and disadvantages of each inhibitor are explored. Scenarios that prompt consultation with a hematologist are presented, including management of bleeding, thrombocytopenia, and management of the patient requiring emergency surgery.

In Section III, Dr. Bussel reviews controversies in ITP, looking at both the current state of the art and the potential for the future. Case presentations are used to illustrate the issues in both children and adults. Three primary areas are addressed: 1) the diagnosis of ITP, 2) when and for which patient to recommend splenectomy, and 3) the management of the refractory splenectomized patient who still has a low platelet count and bleeding symptoms.

Recurrent Arterial Thrombosis Linked to Autoimmune Antibodies Enhancing von Willebrand Factor Binding to Platelets and Inducing FcγRII Receptor-Mediated Platelet Activation

A patient with a history of recurrent late fetal loss associated with multiple placental infarcts and cerebrovascular ischemia at the age of 36, followed a year later by a myocardial infarction, was referred for further investigation. Coronary angiography was normal. Antinuclear factor, lupus anticoagulant, anticardiolipin antibodies, and other thrombophilia parameters were negative, but there was moderate hyperthyroidism with positive thyroid peroxidase antibodies. Platelet numbers and von Willebrand factor (vWF) were normal. Her platelets showed spontaneous aggregation that disappeared with aspirin intake. However, aggregation still was induced by low levels of ristocetin (0.3 to 0.5 mg/mL). The low-dose ristocetin aggregation in patient platelet-rich plasma (PRP) was completely blocked by neutralizing antiglycoprotein Ib (GPIb) and anti-vWF antibodies. The monoclonal anti-FcγRII receptor antibody IV.3 inhibited partly, which suggests that PRP aggregation by low-dose ristocetin was elicited by vWF-immunoglobulin (Ig) complexes. Upon addition to washed human platelets, with vWF (10 μg/mL), purified patient Igs dose-dependently enhanced ristocetin (0.15 mg/mL)-induced aggregation between 0 and 500 μg/mL, an effect that disappeared again above 1 mg/mL. Aggregation was dependent on the vWF concentration and was blocked by IV.3 or neutralizing anti-GPIb or anti-vWF antibodies. The spontaneous aggregation of normal platelets resuspended in patient plasma could be inhibited totally by IV.3 and partially by neutralizing anti-GPIb or anti-vWF antibodies. Perfusion with normal anticoagulated blood, enriched with 10% of control or patient plasma, over surfaces coated with vWF showed increased platelet adhesion and activation in the presence of patient antibodies. Treatment of the patient with the antithyroid drug thiamazol and temporary corticosteroids, aspirin, and ticlopidine did not correct the platelet hypersensitivity to ristocetin. These observations suggest that some autoantibodies to vWF may both enhance vWF binding to platelets and cause platelet activation through binding to the FcγRII receptor, and thereby may be responsible for a new form of antibody-mediated thrombosis.

von Willebrand factor binds to native collagen VI primarily via its A1 domain

Collagen VI is abundant in the arterial subendothelium. To investigate its mechanism of interaction with von Willebrand factor (vWF), collagen VI was isolated from human placenta and from the extracellular matrix of the human lung fibroblast cell line MRC-5. Purified vWF bound to non-digested collagen VI with moderately high affinity (EC50 approximately 5 nM) and could be inhibited by the Hirudo medicinalis collagen inhibitor calin. The anti-(human vWF A1 domain) monoclonal antibody (AJvW-2), as well as aurin tricarboxylic acid (ATA), at concentrations that saturate the vWF A1 domain, also inhibited this binding. In contrast, the monoclonal anti-(human vWF A3 domain) antibody (82D6A3) inhibited vWF binding to collagens I, III and IV, but had no effect on vWF binding to collagen VI. Likewise, vWF binding to collagen VI was not inhibited by the recombinant vWF domain D4. Polyclonal anti-(collagen VI) antibodies, specifically neutralizing the binding of vWF to collagen VI, confirmed that in the intact endothelial cell extracellular matrix, collagen VI was accessible for interaction with vWF. This binding was only marginally affected by 82D6A3 but was dose-dependently inhibited by AJvW-2, ATA and the A1 domain analogue VCL (recombinant A1 domain of vWF), with IC50 values comparable to those found for the inhibition of vWF binding to isolated collagen VI. The weak interaction of isolated human platelets with collagen VI was mediated via the platelet collagen receptor (GPIa/IIa) and was competitively inhibited by vWF but not by VCL, suggesting that vWF and GPIa/IIa bind to neighbouring but distinct sites on collagen VI. We conclude that vWF binds to collagen VI primarily via its A1 domain, which distinguishes it from the vWF A3 domain-mediated binding to fibrillar collagens.

Promotion of binding of von Willebrand factor to platelet glycoprotein Ib by dimers of ristocetin

In the absence of high shear forces, the in vitro binding of human von Willebrand factor (vWF) to its platelet receptor glycoprotein Ib (GPIb) can be promoted by two well-characterized mediators, botrocetin and ristocetin. Using purified vWF and GPIb, we have investigated the mechanisms by which ristocetin mediates this binding. Specific binding of vWF monomers to GPIb occurred with a 1:1 stoichiometry, but high-affinity binding required the participation of two ristocetin dimers. Binding was strongly dependent on pH and inhibited by low poly-L-lysine concentrations, indicating ristocetin-dependent charge neutralization during the interaction. With increasing ristocetin concentrations, vWF binding depended progressively less on the involvement of its A1 loop, which is compatible with a model in which the two ristocetin dimers bridge the vWF-GPIb complex on secondary sites. In agreement with this model, the ristocetin-dimer-promoted stabilization of vWF on GPIb was abolished by low concentrations of poly(Pro-Gly-Pro), which is known to complex ristocetin dimers. Mechanistic analysis of the inhibition of vWF binding by the recombinant vWF fragment Leu504-Ser728 (VCL), which covers the entire A1 loop, revealed an affinity of VCL for GPIb comparable with that of the botrocetin-vWF complex for GPIb, and identified a specific but 20-fold lower affinity of VCL in the presence of ristocetin. The proline-rich peptides flanking the vWF A1 loop, Cys474-Val489 and Leu694-Asp709, inhibited vWF binding semispecifically by competitively interfering with the formation of the GPIb-vWF complex rather than by complexation of free ristocetin dimers. In conclusion, ristocetin-promoted binding of vWF to its GPIb receptor results from charge neutralization and interactions involving proline residues in the vicinity of the natural interaction sites present on both GPIb and the A1 domain of vWF.

Immunoenzymometric analysis for plasma von Willebrand factor degradation in diabetes mellitus using monoclonal antibodies recognizing protease-sensitive sites

High-molecular-weight von Willebrand factor (vWf) multimers were separated from their smaller multimers by molecular-sieve chromatography and were measured by several sandwich enzyme-linked immunosorbent assays (ELISA) with monoclonal antibodies (moABs) against human vWf. The epitopes of these moABs were mapped partially using fragments generated by V-8 protease digests of native vWf. Large multimers were more immunoreactive with the sandwich ELISA using immobilized VW28-1 and enzyme-labeled VW92-3 than with any other ELISA. The epitopes recognized by these two moABs were sensitive to trypsin and plasmin digestion, and the other moABs appeared to be reactive with the extensively digested antigen. Relative reactivities of this ELISA to plasma vWf multimers in diabetes mellitus were significantly reduced compared to those in healthy subjects. These data demonstrate that molecular abnormalities of vWf circulating in diabetic patients may be caused by some plasma proteases that increase in the microcirculation of patients with diabetic vascular diseases.