Localization of a factor VIII binding domain on a 34 kilodalton fragment of the N-terminal portion of von Willebrand factor

Factor VIII (F.VIII) was tested for its ability to bind in solid phase system to von Willebrand Factor (vWF) or fragments obtained with Staphylococcus aureus V-8 protease, ie, SpIII (N-terminal), SpI (central), and SpII (C-terminal). Bound F.VIII was estimated in situ by clotting and chromogenic assays. F.VIII bound in a dose-dependent manner to immobilized vWF and SpIII but not to SpII or SpI. Binding was inhibited by 0.25 mol/L CaCl2 as well as by an excess of vWF or SpIII. Accordingly, immobilized F.VIII specifically bound 125I-vWF and SpIII but not SpII or SpI. Twelve monoclonal antibodies (MoAbs) directed towards SpIII, specifically blocking binding of F.VIII to vWF or SpIII, were used for the mapping of plasmic or tryptic fragments of vWF or SpIII. We thus established that a F.VIII binding domain of vWF is located on a 34 kilodalton (kd) fragment of the N-terminal portion of vWF, between residues 1 and 910, and that it is distinct from the GPIb and collagen binding domains.     

Mediation of platelet adhesion to fibrillar collagen in flowing blood by a proteolytic fragment of human von Willebrand factor

The effect of purified von Willebrand Factor (vWF) fragments, SpII (dimer of two 110 kd subunits) and SpIII (dimer of two 170 kd subunits) obtained with S aureus V-8 protease was tested upon platelet adhesion to collagen. Purified fibrillar human collagen coated onto cover slips was incubated with SpII, SpIII, or undigested vWF and exposed to reconstituted human blood in a parallel-plate perfusion chamber at a high shear rate. Platelet-collagen interactions were estimated using 51Cr-platelets and quantitative morphometry. When blood was reconstituted with citrated autologous plasma, SpIII and vWF strikingly enhanced platelet adhesion to collagen whereas SpII had no effect. When blood was reconstituted with human albumin and divalent cations, SpIII and vWF again promoted platelet adhesion to collagen. In conclusion, our data suggest that (1) SpIII, the N-terminal portion of vWF which binds to platelet membrane glycoprotein Ib, functionally substitutes for vWF in supporting platelet adhesion to collagen; (2) SpII, the C- terminal portion which binds to glycoprotein IIb/IIIa, has no such effect; (3) in addition to its platelet binding domain, SpIII contains another site for binding to collagen; and (4) the multimeric structure of vWF is not required for platelet adhesion to collagen.     

Localization of a collagen-interactive domain of human von Willebrand factor between amino acid residues Gly 911 and Glu 1,365

A collagen-binding domain of von Willebrand factor (vWF) has been identified in the central part of the molecule by comparing the binding properties of vWF and Staphylococcus aureus V-8 protease-generated vWF fragments with collagen. The binding of purified human vWF to human type III collagen was found to be specific. At saturation, 38 to 50.2 micrograms of vWF bound per milligram of collagen. Scatchard plots derived from binding isotherms demonstrated the presence of at least two classes of binding sites. Purified vWF was digested with S aureus V-8 protease into two complementary fragments (SpIII and SpII). SpII, the C-terminal end of vWF (amino acid residues 1,366 to 2,050), was totally devoid of affinity for collagen. Contrarily, purified SpIII, the N-terminal part of vWF (residues 1 to 1,365), totally displaced vWF binding and specifically bound to collagen. At saturation, 25 to 45 micrograms of SpIII bound per milligram of collagen. Scatchard plots demonstrated the presence of a single class of binding sites. SpIII was further digested with the same enzyme to generate SpI, a 52-kilodalton fragment from the C-terminal part of SpIII (residues 911 to 1,365). Spl induced a dose-dependent inhibition of both vWF and SpIII binding to collagen. A series of six monoclonal antibodies against SpIII that completely abolished vWF and SpIII interaction with collagen also bound to SpI. In conclusion, SpI extending between amino acid residues 911 and 1,365 of vWF contains a specific site that interacts with human type III collagen.     

Solid-phase von Willebrand factor contains a conformationally active RGD motif that mediates endothelial cell adhesion through the alpha v beta 3 receptor

The interaction of von Willebrand factor (vWF) with the alpha v beta 3 integrin of human umbilical vein endothelial cells is dependent on the RGD sequence present at residues 1744-1746 of the mature vWF subunit. We compared vWF and its two dimeric fragments, SpIII (residues 1-1365) and SpII (residues 1366-2050), as adhesion substrates. Solid-phase vWF and SpII supported endothelial cell adhesion, whereas SpIII, which contains the glycoprotein (GP) Ib binding domain, did not. Soluble SpII inhibited adhesion to immobilized ligands, whereas soluble vWF did not, suggesting that exposure of the cell attachment domain involves a conformational modification of vWF. Dendroaspin and albolabrin, two RGD- containing peptides of the disintegrin family, were potent inhibitors of cell adhesion to vWF (IC50 approximately 15 nmol/L). Complete inhibition of endothelial cell adhesion to vWF was obtained in the presence of F(ab')2 of monoclonal antibody 9 to vWF, which blocks vWF binding to platelet GPIIb/IIIa. In contrast, monoclonal antibody 713 to vWF, which blocks its binding to platelet GPIb, did not inhibit cell adhesion to vWF. These results indicate that endothelial cell adhesion to vWF is mediated by an RGD-dependent interaction with alpha v beta 3, but does not seem to involve a GPIb-like receptor, and show the importance of the conformation of the RGD sequence.     

Spontaneous platelet aggregation during pregnancy in a patient with von Willebrand disease type IIB can be blocked by monoclonal antibodies to both platelet glycoproteins Ib and IIb/IIIa

Spontaneous platelet aggregation appeared in a patient with von Willebrand disease type IIB during the 37th week of pregnancy. This phenomenon was not associated with symptoms of thrombosis and the patient delivered by caesarean section with no complications. Her platelet-poor plasma (PPP) aggregated normal platelet-rich plasma (PRP) and washed platelets. Aggregation was inhibited by monoclonal antibodies with known specificity for the platelet receptors of von Willebrand factor (vWF), i.e. the glycoprotein Ib (GPIb) and the GPIIb/IIIa complex. A monoclonal antibody, which selectively inhibits the binding of vWF to the GPIIb/IIIa complex, did not block aggregation, suggesting that spontaneous aggregation is not dependent on the binding to GPIIb/IIIa of vWF from patient plasma. Aggregation induced by patient plasma could also be blocked either by two monoclonal antibodies raised against vWF or by a fragment derived from trypsin digestion of normal vWF which blocks the ristocetin-induced binding of normal vWF to platelets. These findings indicate that the spontaneous platelet aggregation in this patient results from the binding of her vWF to GPIb but is independent from the binding of her vWF to GPIIb/IIIa.     

The binding domain of von Willebrand factor to sulfatides is distinct from those interacting with glycoprotein Ib, heparin, and collagen and resides between amino acid residues Leu 512 and Lys 673.

A series of proteolytic fragments of human von Willebrand Factor (vWF) was purified to characterize the functional site that supports its interaction with sulfatides. SpIII, an N-terminal homodimer generated by V-8 protease (amino acids [AA] 1 to 1365), bound to sulfatides in a dose-dependent and saturable way. SpIII also totally inhibited the binding of vWF to sulfatides and SpIII binding was completely abolished by vWF. In contrast, SpII, the complementary C-terminal homodimer (AA 1366 to 2050), did not exhibit any binding affinity for sulfatides. Four purified fragments overlapping the sequence of SpIII were also tested for their ability to interact with sulfatides. An N-terminal monomeric 34-Kd fragment (P34, AA 1 to 272) generated by plasmin, a central monomer (SpI, AA 911 to 1365) produced by digestion with V-8 protease, and a tetrameric fragment III-T2 (comprising a pair of the two sequences AA 273 to 511 and AA 674 to 728) produced by secondary digestion of SpIII with trypsin did not interact with sulfatides. In contrast, a monomeric 39/34-Kd fragment produced by dispase (AA 480 to 718) bound specifically and with a high affinity to sulfatides and totally displaced vWF or SpIII binding. Conversely, binding of the 39/34-Kd species was totally abolished by vWF or SpIII. Thus, a functional site responsible for sulfatide binding was localized between AA 480 and 718 and comparison of the binding properties of the 39/34-Kd and III-T2 fragments indicated that the sequence 512 to 673 is necessary for the binding to sulfatides. Further mapping of this new functional domain of vWF, based on experiments of competitive inhibition of binding by either heparin or monoclonal antibodies directed toward vWF, showed that the site interacting with sulfatides is distinct from those involved in binding to platelet glycoprotein Ib, collagen, or heparin. This finding was confirmed by experiments using synthetic peptides which also indicated that the sequence comprising AA 569 to 584 is part of the sulfatide-binding domain or influences its activity.     

Interaction of porcine von Willebrand factor with the platelet glycoproteins Ib and IIb/IIIa complex

Porcine von Willebrand factor (PvWF) induces platelet aggregation which is thought to be responsible for the thrombocytopenia that occurs in haemophilic patients treated with commercial preparations of porcine factor VIII. This study demonstrates that such aggregation can be completely inhibited by a monoclonal antibody against human platelet glycoprotein GPIb and partially inhibited by an antibody directed against platelet GPIIb/IIIa. The interaction of PvWF with GPIb is also demonstrated by the inhibitory effect of purified glycocalycin on aggregation. The binding site of PvWF to GPIb is very close to that of human vWF, since a recombinant peptide blocks the binding of both molecules to GPIb. When platelets are incubated with PvWF, the GPIIb/IIIa receptor is activated and binds fibrinogen. PvWF also binds to GPIIb/IIIa when platelets are stimulated with thrombin, suggesting that the molecule has the same RGD sequence as other adhesive proteins (human vWF, fibrinogen, fibronectin and vitronectin). These findings identify the dual mechanisms responsible for in vivo platelet aggregation induced by PvWF, i.e. binding to GPIb and activation of the GPIIb/IIIa receptor.     

On the role of von Willebrand factor in promoting platelet adhesion to fibrin in flowing blood

Platelet adhesion to fibrin at high shear rates depends on both the glycoprotein (GP) IIb:IIIa complex and a secondary interaction between GPIb and von Willebrand factor (vWF). This alternative link between platelets and vWF in promoting platelet adhesion to fibrin has been examined in flowing whole blood with a rectangular perfusion chamber. Optimal adhesion required both platelets and vWF, as shown by the following observations. No binding of vWF could be detected when plasma was perfused over a fibrin surface or when coated fibrinogen was incubated with control plasma in an enzyme-linked immunosorbent assay. However, when platelets were present during perfusion, interactions between vWF and fibrin could be visualized with immunoelectron microscopy. Exposure of fibrin surfaces to normal plasma before perfusion with severe von Willebrand's disease blood did not compensate for the presence of plasma vWF necessary for adhesion. vWF mutants in which the GPIIb:IIIa binding site was mutated or the GPIb binding site was deleted showed that vWF only interacts with GPIb on platelets in supporting adhesion to fibrin and not with GPIIb:IIIa. Complementary results were obtained with specific monoclonal antibodies against vWF. Thus, vWF must first bind to platelets before it can interact with fibrin and promote platelet adhesion. Furthermore, only GPIb, but not GPIIb:IIIa is directly involved in this interaction of vWF with platelets.     

Localization of GPIb/IX and GPIIb/IIIa on Discoid Platelets

The organization and reorganization of mobile receptors, GPIIb/IIIa and GPIb/IX, on surface- and suspension-activated platelets have been studied in detail, but their distribution on resting, discoid platelets is uncertain. The present study has treated platelets in suspension with cytochalasin E before mounting on formvar grids or glass slide fragments in order to preserve their discoid appearance, then probed the organization of GPIIb/IIIa with fibrinogen coupled to gold particles (Fgn/Au) and GPIb/IX with bovine or ristocetin-activated human plasma detected by combined anti-vWF antibody and protein A coupled to gold particles. Multimers of vWF had the same tortuous, linear distribution from edge to edge observed previously on surface-activated platelets. However, the gold particles marking the complex of vWF-anti-vWF bound to GPIb/M were closer together on the discoid cells. Fgn/Au particles bound to GPIIb/IIIa receptors were uniformly distributed from edge to edge on many discoid platelets. On others they tended to clump or cluster in strips or patches. The latter organization of Fgn/Au-GPIIb/IIIa receptors may be due to the rugose nature of the discoid platelet surface or an influence of cytochalasin E. Definition of mobile receptor organization on discoid cells provides a useful baseline for determining their fate following surface or suspension activation.     

Redistribution of GPIb/IX and GPIIb/IIIa during spreading of discoid platelets

Summary. The purpose of the present study was to prelabel mobile receptors on discoid platelets with specific ligands identifiable in the electron microscope and follow their redistribution during spreading. Platelets were incubated in suspension with cytochalasin E (CE) to preserve discoid form, chilled and mounted on cold formvar grids or glass slide fragments to inhibit receptor movement, covered with cold bovine or ristocetin-activated human plasmas as sources of vWF to bind GPIb/IX, fibrinogen-coated gold particles (Fgn/ Au) to couple GPIIb/IIIa, or both probes simultaneously, washed to remove CE and rewarmed to 37°C for intervals up to 30min to stimulate spreading. After brief fixation grids and glass fragments were incubated with anti-vWF antibody and, subsequently, staphylococcal protein A coupled to 5 nm and 10 nm gold particles to detect vWF multimers. Virtually all of the CE-treated chilled platelets retained their discoid shape. Half of the discs (53-3%) bound Fgn/Au, and all bound vWF. Receptors for both ligands were randomly dispersed on discoid cells from edge to edge. During re warming discoid platelets expanded into spread forms. Fgn/Au-GPIIb/IIIa complexes moved into caps over cell centres and into residual channels of the open canalicular system (OCS). vWF bound to GPIb/IX moved with the cell membrane as the surface expanded during spreading. Discoid platelets prelabelled with both ligands demonstrated similar findings. During rewarming Fgn/Au-GPIIb/IIIa complexes moved to cell centres and into OCS channels. vWF multimers bound to GPIb/IX moved apart from each other toward peripheral margins of the spread cells. Thus, surface activation resulting in conversion of discoid platelets to spread forms does not cause clearance of GPIb/IX receptors to cell centres and channels of the OCS in the manner that GPIIb/IIIa receptors coupled to Fgn/Au are simultaneously translocated and concentrated in OCS channels.     

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.     

von Willebrand factor competes with fibrin for occupancy of GPIIb:IIIa on thrombin-stimulated platelets

We have investigated two major questions related to the molecular basis of interactions between the three-dimensional fibrin network and thrombin-stimulated human platelets. First, what are the roles played by glycoproteins (GP) Ib and IIb:IIIa in linking the fibrin clot tightly to the platelet surface? Second, does von Willebrand factor (vWF) modulate the extent of platelet-fibrin interactions? Quantitative fluorescence microscopy (microfluorimetry) has been used to determine the quantity of fluorescein-labeled fibrin bound to the surface of thrombin-stimulated, gel-filtered platelets (the supernatants of which contained small quantities of vWF) in the presence/absence of receptor-specific and vWF-specific monoclonal antibodies (MoAbs), as well as exogenous vWF. A MoAb specific for the GPIIb:IIIa complex exhibited a concentration-dependent inhibition of fibrin binding, whereas a MoAb specific for GPIb was ineffective in this regard. Similarly, a MoAb that recognizes the N-terminal region of vWF involved in GPIb binding did not influence fibrin binding. In contrast, a MoAb that binds to a C-terminal region of vWF involved in GPIIb:IIIa recognition caused a specific, concentration-dependent increase in the quantity of platelet-bound fibrin. We also found that exogenous vWF caused a concentration-dependent decrease in fibrin binding. These results support the hypothesis that vWF and fibrin, both of which are multimeric adhesive ligands, compete for occupancy of the GPIIb:IIIa complex on thrombin-stimulated platelets.     

Interaction of purified type IIB von Willebrand factor with the platelet membrane glycoprotein Ib induces fibrinogen binding to the glycoprotein IIb/IIIa complex and initiates aggregation.

Von Willebrand factor (vWF) was purified from the plasma of a patient with type IIB von Willebrand disease (vWF from such a patient, IIB vWF) who had a normal platelet count and showed no evidence of spontaneous platelet aggregation. Large multimers of IIB vWF were absent from purified preparations and from plasma. Ristocetin-induced platelet aggregation was enhanced by purified IIB vWF. The aggregation of washed normal platelets mixed with IIB vWF (0.4 microgram/ml) required lower amounts of ristocetin than the aggregation of normal platelets mixed with the same concentrations of normal vWF. Moreover, purified IIB vWF alone induced aggregation of platelet-rich plasma at concentrations as low as 10 micrograms of IIB vWF/ml in the absence of any other agonist. Aggregation was blocked by a monoclonal antibody against the platelet membrane glycoprotein, GPIb, as well as by an anti-GPIIb/IIIa antibody. Washed platelet suspensions were promptly aggregated by IIB vWF only when fibrinogen and CaCl2 were added to the mixture. Purified IIB vWF induces the binding of fibrinogen to platelets. Such binding was blocked by the anti-GPIb monoclonal antibody as well as by the anti-GPIIb/IIIa monoclonal antibody that inhibited aggregation. A second anti-GPIIb/IIIa antibody, which has the property of blocking vWF but not fibrinogen binding to platelets, blocked neither aggregation nor fibrinogen binding induced by IIB vWF. These studies demonstrate that platelet aggregation is triggered by the initial interaction of IIB vWF with GPIb which is followed by exposure of fibrinogen binding sites on GPIIb/IIIa. Fibrinogen binds to these sites and acts as a necessary cofactor for the aggregation response.     

Identification of platelet glycoprotein IIb/IIIa as the major binding site for released platelet-von Willebrand factor [published erratum appears in Blood 1987 Jun;69(6):1789]

We studied the effects(s) of two monoclonal antibodies, 6D1 and 10E5 (directed against platelet glycoprotein Ib [GPIb] and the GPIIb/IIIa complex, respectively), and purified human plasma fibrinogen on the binding of released platelet-von Willebrand factor (vWf) to the platelet surface. Neither of the monoclonal antibodies nor fibrinogen had any effect on the amount of platelet-vWf expressed on unstimulated platelets or on the amount expressed on platelets stimulated in the absence of extracellular Ca++. However, the antibody directed against GPIIb/IIIa inhibited 72% of the thrombin-induced increase in the platelet-vWf bound to the platelet surface when platelets were stimulated in the presence of 5 mmol/L Ca++. The antibody against GPIb did not inhibit the surface expression of platelet-vWf on stimulated platelets in the presence of Ca++. Purified normal human fibrinogen inhibited the surface binding of platelet-vWf to thrombin-stimulated platelets to a degree similar to that observed with the monoclonal antibody directed against the GPIIb/IIIa complex. These data indicate that platelet-vWf released from platelets binds primarily to the GPIIb/IIIa complex at or near the plasma fibrinogen binding site.     

Platelet membrane abnormalities in myeloproliferative disorders: decrease in glycoproteins Ib and IIb/IIIa complex is associated with deficient receptor function

The number and functional activity of membrane glycoproteins (GP) Ib and IIb/IIIa were investigated in platelets from 11 patients with myeloproliferative disorders (MPD). Three patients had essential thrombocythaemia, two had chronic myeloid leukaemia and six had polycythaemia vera. The numbers of GPIb and GPIIb/IIIa molecules were detected on the platelet surface using different 125I-labelled monoclonal antibodies. The functional properties of GPIb and GPIIb/IIIa were evaluated using purified 125I-labelled asialo von Willebrand factor (vWF) and purified 125I-labelled fibrinogen, respectively, in a binding assay. Binding of the anti-GPIIb/IIIa antibody was decreased by 40% in almost all patients studied and, when measured, it was accompanied by decreased fibrinogen binding to activated platelets. Binding of anti-GPIb antibodies to platelets was also slightly decreased or virtually the same in eight out of 11 patients. The decrease correlated with decreased binding of asialo vWF. The increased plasma glycocalicin levels, measured in four patients, depended on the high platelet count. Scatchard analysis revealed normal receptor binding affinity for all ligands tested in all but one patient. In this report we demonstrate that abnormalities in the concentrations of GPIIb/IIIa membrane proteins are commonly present in patients with MPD, while a decrease in GPIb concentration is also seen, although in fewer patients. These abnormalities are accompanied by a concurrent decrease in the respective receptor functions. These findings may explain part of the haemorrhagic tendency often encountered in MPD.     

Echicetin: a snake venom protein that inhibits binding of von Willebrand factor and alboaggregins to platelet glycoprotein Ib

Echicetin, a new protein isolated from Echis carinatus venom by reverse phase and ion exchange chromatography specifically inhibited agglutination of fixed platelets induced by several platelet glycoprotein Ib (GPIb) agonists, such as bovine von Willebrand factor (vWF), alboaggregins, and human vWF in the presence of botrocetin. Unlike alboaggregins, echicetin bound to GPIb but did not induce agglutination of washed or fixed platelets. In contrast to disintegrins, it did not block adenosine 5'-diphosphate (ADP)-induced platelet aggregation in the presence of fibrinogen. The apparent molecular weight of echicetin measured on sodium dodecyl sulfate (SDS) gel electrophoresis was 26 Kd under nonreducing conditions. On reduction, echicetin showed 16 and 14-Kd subunits suggesting that the molecule is a dimer. Reduced echicetin retained its binding activity and its inhibitory effect on the agglutination of fixed platelets induced by bovine vWF. 125I-echicetin bound to fixed platelets with high affinity (kd = 30 +/- 1.8 nmol/L) at 45,000 +/- 2,400 binding sites per platelet. The binding was selectively inhibited by a monoclonal antibody to the 45-Kd N-terminal domain of platelet GPIb, but not by monoclonal antibodies to other regions on GPIb. Binding of 125I-bovine vWF to fixed platelets was strongly inhibited by echicetin. In contrast, bovine vWF showed a much weaker inhibitory activity on binding of 125I-echicetin to platelets. The half life of echicetin in blood was approximately 170 minutes with no detectable degradation. Echicetin significantly prolonged the bleeding time of mice, suggesting that it may inhibit vWF binding to GPIb in vivo as well as in vitro.     

Extracellular epitopes of platelet glycoprotein Ib alpha reactive with serum antibodies from patients with chronic idiopathic thrombocytopenic purpura

Glycoproteins (GPs) IIb/IIIa and Ib/IX are principal targets of autoantibodies (autoAbs) in idiopathic thrombocytopenic purpura (ITP). Platelet-associated Abs against GPIIb/IIIa primarily recognize discontinuous or nonlinear epitopes (Fujisawa et al, Blood 81:1284, 1993). This study focused on whether Abs against the extracellular domain of GPIb/IX might react with short linear amino acid (aa) sequences of GPIb alpha. Complementary DNAs (cDNAs) coding for two overlapping fragments of GPIb alpha were amplified, cloned into pFLAG.2 plasmids, and expressed in Escherichia coli DH5 alpha competent cells as FLAG fusion proteins, which were purified by anti-FLAG immunoaffinity chromatography. Of 16 selected ITP sera containing anti- GPIb/IX, 6 reacted in microtiter radioimmunoassays (RIAs) with recombinant protein fragment 2 (aas 240 to 485); 1 also with fragment 1 (aas 1 to 247). When synthetic peptides corresponding to 4 segments of fragment 2 with high antigenic indices (P1 to P4) were used as targets in RIAs, all 6 sera reacted with P2 (aas 326 to 346); 1 also reacted with P4 (aas 389 to 412). P2 was shown to be present on the surface of intact platelets by adsorption studies, and anti-P2 was detected in direct eluates of platelets from ITP patients. Glycocalicin in solution effectively competed with immobilized P2 for anti-P2; P2 in solution was a less effective competitor. Epitope scanning with a panel of synthetic 15-mer peptides localized the P2 epitope to the sequence, TKEQTTFPP. Epitope definition may offer insight into the pathophysiology of and more specific treatments for ITP.     

A new congenital platelet abnormality characterized by spontaneous platelet aggregation, enhanced von Willebrand factor platelet interaction, and the presence of all von Willebrand factor multimers in plasma

A case is reported of a 49-year-old woman with a mild bleeding tendency. Her bleeding time, platelet count and size, plasma ristocetin cofactor activity, von Willebrand factor (vWF) antigen, and vWF multimeric pattern are all within normal limits. Spontaneous platelet aggregation is observed when citrated platelet-rich plasma (PRP) is stirred in an aggregometer cuvette. This aggregation is completely is only slightly diminished by an antiglycoprotein (GP) IIb/IIIa or by an anti GPIb monoclonal antibody. The patient's PRP shows increased sensitivity to ristocetin. The distinct feature of this patient, also present in two family members studied, is that platelet aggregation is initiated by purified vWF in the absence of any other agonist. The vWF- induced platelet aggregation is abolished by anti-GPIb and anti- GPIIb/IIIa monoclonal antibodies and by EDTA (5 mmol/L). Apyrase inhibits the second wave of aggregation. Patient's platelets in PRP are four to six times more reactive to asialo vWF-induced platelet aggregation than normal platelets. The amount of radiolabeled vWF bound to platelets in the presence of either low concentration of ristocetin or asialo vWF was increased 30% compared with normal. The patient's platelet GPIb was analyzed by SDS page and immunoblotting and by binding studies with anti-GPIb monoclonal antibodies showed one band with slightly increased migration pattern and a normal number of GPIb molecules. Unlike the previously reported patients with pseudo or platelet-type von Willebrand disease, this patient has normal vWF parameters.     

Plasminogen interactions with platelets in plasma

In this report we used a fluorescent flow cytometry-based assay to examine plasminogen binding to platelets in plasma. Our data indicate that platelets activated in platelet-rich plasma (PRP) by adenosine-5'- diphosphate (ADP) or thrombin bind plasminogen to their surface. Fab fragments of the monoclonal antibody LJ-CP8 that are directed against the fibrinogen binding site on the glycoprotein (GP) IIb-IIIa complex inhibit both plasminogen and fibrinogen binding to ADP-stimulated platelets as does 5 mmol/L EDTA. Platelet aggregation and plasminogen and fibrinogen binding are also concurrently inhibited by the Gly-Arg- Asp (RGD) analogue Gly-Arg-Gly-Asp-Ser (GRGDS) when it is added to PRP before ADP stimulation. The scrambled peptide analogue SDGRG has no effect. The monoclonal antibody 6D1, directed against the von Willebrand factor binding site on GPIb, has no effect on plasminogen- platelet binding, nor does antithrombospondin antibody. epsilon- Aminocaproic acid (EACA), however, inhibits plasminogen binding to ADP- activated platelets. These data indicate that plasminogen binds to platelets activated in plasma, that binding occurs on platelet GPIIb/IIIa, and that binding may be mediated via plasminogen association with fibrinogen via lysine binding domains. Finally, we found both plasminogen and fibrinogen on resting platelets in PRP and demonstrated that they are equally displaced by EDTA, LJ-CP8, and 10E5 (an additional anti-GPIIb/IIIa monoclonal antibody). Plasminogen is also equally displaced by EACA. These data suggest that plasminogen is also bound to GPIIb/IIIa on resting platelets, possibly also via interaction with fibrinogen.     

von Willebrand factor is present on the surface of platelets stimulated in plasma by ADP

von Willebrand factor (vWf) can bind to glycoprotein (GP) IIb/IIIa on activated platelets. The significance of this interaction is unclear, however, because it has not been possible to detect vWf binding to GPIIb/IIIa on platelets stimulated in plasma. We have developed an indirect, flow cytometry assay that uses fluorescein-labeled antibodies to detect vWf and fibrinogen on platelets. Using this assay, we found vWf on the surface of platelets stimulated in plasma by ADP. The number of platelets that bound vWf increased in proportion to ADP concentration and incubation time. Washed platelets in a protein-free buffer activated by 1 mumol/L calcium ionophore A23187 or 10 mumol/L ADP also bound vWf, suggesting that we were detecting surface binding of alpha-granule-derived vWf. Monoclonal antibodies against the vWf binding site on GPIb (6D1) and the vWf and fibrinogen binding sites on GPIIb/IIIa (LJP5 and LJ-CP8, respectively) were used to characterize the mechanism of vWf binding to stimulated platelets. Ristocetin- induced binding of vWf was inhibited by 6D1, and ADP-induced binding of fibrinogen was inhibited by LJ-CP8. None of these antibodies inhibited ADP-induced vWf binding. Aspirin and prostaglandin E1 also inhibited ADP-induced binding of vWf in platelet-rich plasma. During platelet activation in plasma, vWf derived from alpha-granules becomes bound to the platelet surface possibly being transferred already associated with a binding site.