Autoantibody level modification in adult patients with idiopathic thrombocytopenic purpura following intravenous immunoglobulin treatment

The aim of this study was to determine whether treatment of patients with immune thrombocytopenic purpura (ITP) with intravenous immunoglobulin (IVIg) is associated with a modification in the antiplatelet glycoprotein (GP) antibodies (Abs). Fourteen patients with ITP (11 females and 3 males, mean age 36.6 years, range 18-72) received one to four IVIg treatment courses. The preparation used was ISIVEN that was given in a dose of 2 g/kg body weight in a 5-day schedule and in monthly intervals. Levels of IgG, IgM and IgA isotypes of Abs to GPs IIb/IIIa and Ib/IX were measured before the treatment, and before and after each treatment course. Two patients did not respond to IVIg, 6 had a temporary response, 5 had a sustained response and 1 patient responded well to the treatment but was lost to follow-up. The patients had a high prevalence of serum Abs directed against GPs IIb/IIIa and Ib/IX before the treatment, and the mean IgG isotype levels of both Abs increased after each treatment course, and decreased again before the following course began. Whenever high Ab levels of either isotype (> 10 U/ml) were detected before the treatment, they were significantly decreased before the last treatment course. The elevated levels of IgG Abs to IIb/IIIa and Ib/IX after every course are probably a result of displacement of these Abs from Fc receptors by the IVIg, rather than of exogenous infusion of these Abs contained within the IVIg, whereas the decrease in high Ab levels after a few treatment courses results from the immunomodulatory effects of IVIg: suppression of Ab formation, and the presence of anti-idiotypes.

Binding of thrombin to glycoprotein Ib accelerates the hydrolysis of Par-1 on intact platelets

The activation of human platelets by alpha-thrombin is mediated at least in part by cleavage of protease-activated G-protein-coupled receptors, PAR-1 and PAR-4. Platelet glycoprotein Ibalpha also has a high affinity binding site for alpha-thrombin, and this interaction contributes to platelet activation through a still unknown mechanism. In the present study the hypothesis that GpIbalpha may contribute to platelet activation by modulating the hydrolysis of PAR-1 on the platelet membrane was investigated. Gel-filtered platelets from normal individuals were stimulated by alpha-thrombin, and the kinetics of PAR-1 hydrolysis by enzyme was followed with flow cytometry using an anti-PAR-1 monoclonal antibody (SPAN 12) that recognizes only intact PAR-1 molecules. This strategy allowed measurement of the apparent k(cat)/K(m) value for thrombin hydrolysis of PAR-1 on intact platelets, which was equal to 1.5 +/- 0.1 x 10(7) m(-1) sec(-1). The hydrolysis rate of PAR-1 by thrombin was measured under conditions in which thrombin binding to GpIb was inhibited by different strategies, with the following results. 1) Elimination of GpIbalpha on platelet membranes by mocarhagin treatment reduced the k(cat)/K(m) value by about 6-fold. 2) A monoclonal anti-GpIb antibody reduced the apparent k(cat)/K(m) value by about 5-fold. 3) An oligonucleotide DNA aptamer, HD22, which binds to the thrombin heparin-binding site (HBS) and inhibits thrombin interaction with GpIbalpha, reduced the apparent k(cat)/K(m) value by about 5-fold. 4) Displacement of alpha-thrombin from the binding site on GpIb using PPACK-thrombin reduced the apparent k(cat)/K(m) value by about 5-fold, and 5) mutation at the HBS of thrombin (R98A) caused a 5-fold reduction of the apparent k(cat)/K(m) value of PAR-1 hydrolysis. Altogether these results show that thrombin interaction with GpIb enhances the specificity of thrombin cleavage of PAR-1 on intact platelets, suggesting that GpIb may function as a "cofactor" for PAR-1 activation by thrombin.

Evaluation of glycoprotein Ib expression on feline platelets

OBJECTIVE

To determine whether platelets obtained from cats expressed glycoprotein Ib (GPIb).

SAMPLE POPULATION

Platelets obtained from 11 specific-pathogen-free cats.

PROCEDURE

Platelets were analyzed by use of immunofluorescence microscopy, flow cytometry, sodium dodecyl sulfate-polyacrylamide gel electrophoresis, western immunoblot analysis, and immunoprecipitation.

RESULTS

Immunofluorescence microscopy and flow cytometry revealed the protein on the surface of feline platelets. Biochemical studies (western immunoblot analysis and immunoprecipitation) revealed a 140-kd membrane glycoprotein. Additional biochemical studies revealed that feline GPIb was sensitive to proteolysis, because platelet cytoskeletons prepared with low concentrations of a calpain inhibitor (ie, leupeptin; 100 microg/ml) had substantial proteolysis, and there was an association of protein fragments with the actin cytoskeleton.

CONCLUSIONS AND CLINICAL RELEVANCE

Analysis of these results indicate that feline platelets express a 140-kd membrane protein that is recognized by monoclonal antibodies developed against GPIb. Application of standardized ELISA to quantitate glycocalicin, the water-soluble fragment of GPIb, may provide important information on the production of microvesicles, increased platelet turnover, and abnormal proteolysis.

Development and assessment of enzyme immunoassay for platelet-derived microparticles

Platelet-derived microparticles (PMPs) are released from platelets through the platelet activation by high shear stress, collagen, or calcium ionophore (A23187). PMPs are observed in patients with acute myocardial infarction, thrombotic thrombocytopenic purpura, hemolytic uremic syndrome, heparin-induced thrombocytopenia and other thrombotic disorders, but the importance of circulating PMPs in the pathogenesis of these diseases is still debated. Numbers of PMPs are usually determined by flowcytometry (FCM), but easier and reproducible PMP assay systems are needed. To develop a better ELISA for PMPs, we used antibodies against the platelet antigens anti-GPIb (NNKY5-5), anti-GPIIb/IIIa (NNKY2-11, anti-CD41), anti-GPIX (KMP-9), and anti-CD9 (NNKY1-19). PMPs were detected with all combinations of these antibodies, but the ELISA having the highest and most specific absorbance was obtained with a combination of KMP-9 (capture antibody) and NNKY5-5 (detecting antibody). PMPs in blood samples were measured by ELISA and FCM. ELISA correlated with PMPs quantitated by FCM. By shaking ELISA plates during incubation, nonspecific binding of platelets was eliminated. The level of PMPs was not increased in diabetes mellitus, thrombotic thrombocytopenic purpura, antiphospholipid syndrome, or sepsis. The concentration of PMP was elevated in hemolytic uremic syndrome. Activated PMPs were absorbed to 0.8 microm filter, but circulating PMPs were not absorbed. These results suggest that activated PMPs are likely to adhere to leukocytes or endothelial cells at the activation site and that the circulating form of PMPs are likely to be a residue of activated PMPs. To detect only the activated form of PMPs, a new ELISA needs to be developed, and it will likely use a combination of antibodies that detect platelet activation markers such as P-selectin (CD62P) or activated GPIIb/IIIa.

Ristocetin-dependent, but not botrocetin-dependent, binding of von Willebrand factor to the platelet glycoprotein Ib-IX-V complex correlates with shear-dependent interactions

Under conditions of high shear stress, both hemostasis and thrombosis are initiated by the interaction of the platelet membrane glycoprotein (GP) Ib-IX-V complex with its adhesive ligand, von Willebrand factor (vWF), in the subendothelial matrix or plasma. This interaction involves the A1 domain of vWF and the N-terminal extracellular region of GP Ibalpha (His-1-Glu-282), and it can also be induced under static conditions by the modulators ristocetin and botrocetin. In this study, a panel of anti-vWF and anti-GP Ibalpha antibodies-previously characterized for their effects on ristocetin- and botrocetin-dependent vWF-GP Ib-IX-V interactions-was analyzed for their capacity to inhibit either the adhesion of Chinese hamster ovary cells expressing recombinant GP Ibalpha to surface-associated vWF under hydrodynamic flow or shear-stress-induced platelet aggregation. The combined results suggest that the shear-dependent interactions between vWF and GP Ibalpha closely correlate with ristocetin- rather than botrocetin-dependent binding under static conditions and that certain anti-vWF monoclonal antibodies are able to selectively inhibit shear-dependent platelet aggregation.

Infusible platelet membranes retain partial functionality of the platelet GPIb/IX/V receptor complex

Infusible platelet membranes (IPMs) prepared from fresh or outdated human platelets have been shown to correct prolonged bleeding times in thrombocytopenic rabbits. In previous trials, IPMs did not seem to be immunogenic and lacked dose-limiting toxicity. The present study was undertaken to explore whether the platelet glycoprotein (GP) Ib/IX/V complex might retain functionality in the IPM preparation. IPMs did not spontaneously bind von Willebrand factor (vWF), but saturable binding could be induced by ristocetin, with a dissociation constant (Kd) of 0.31 +/- 0.03 microgram/mL at 1.0 mg/mL of ristocetin. Of 4 anti-GPIb-alpha monoclonal antibodies tested, AN-51 inhibited vWF binding 67.8% +/- 5.8%, whereas AS-2, AS-7, and SZ-2 were ineffective. Maximal vWF binding induced by botrocetin was only 10% to 15% of that observed with ristocetin. Retention of partial functionality of the GPIb/IX/V receptor allowing vWF binding in a modulated manner seems to represent a critical mechanism by which IPMs may provide hemostatic efficacy.

Characterization of monoclonal antibodies against mouse and rat platelet glycoprotein V (CD42d)

The mouse- and rat-platelet-specific hamster monoclonal antibody (MAb) 1C2, previously found to react with a thrombin-sensitive 74-kD glycoprotein, was now shown to recognize platelet glycoprotein V (GPV, CD42d). 1C2 reacted with NIH-3T3 cells in which recombinant mouse or rat GPV was expressed. Both 1C2 and 4A5, another mouse-platelet-specific rat MAb, immunoprecipitated GVP, although they recognized different epitopes. Side-by-side comparison confirmed that 1C2 as well as RPM.9, a MAb against rat GPV, recognized the same rat platelet molecule. In a mouse bone marrow culture, 1C2+ megakaryocytes emerged from CD41 (GPIIb)+1C2- megakaryocytes. Because 1C2+ megakaryocytes exhibited higher DNA ploidy distribution than CD41+ cells, GPV likely appears in the late stage of megakaryocyte maturation. This study established 1C2 as a MAb against mouse and rat GPV, namely CD42d, and as useful tool to study rodent megakaryopoiesis.

Identification of critical antigen-specific mechanisms in the development of immune thrombocytopenic purpura in mice

The pathogenic effects of antiplatelet antibodies were investigated in mice. Monoclonal antibodies (mAbs) of different immunoglobulin G subclass directed against mouse GPIIbIIIa, GPIIIa, GPIbalpha, GPIb-IX, GPV, and CD31 were generated and characterized biochemically. MAbs against GPIb-IX, GPV, CD31, and linear epitopes on GPIIIa had mild and transient effects on platelet counts and induced no spontaneous bleeding. Anti-GPIbalpha mAbs induced profound irreversible thrombocytopenia (< 3% of normal) by Fc-independent mechanisms but only had minor effects on hematocrits. In contrast, injection of intact mAbs, but not F(ab)(2) fragments, against conformational epitopes on GPIIbIIIa, induced irreversible thrombocytopenia, acute systemic reactions, hypothermia, decreased hematocrits, and a paradoxical loss of surface GPIIbIIIa on platelets in vivo, the latter suggesting the formation of platelet-derived microparticles. Blockage of platelet-activating factor receptors inhibited the acute reactions, but not thrombocytopenia, loss of GPIIbIIIa, and decreases in hematocrits. Repeated injections of low doses of anti-GPIIbIIIa antibodies resulted in profound thrombocytopenia and bleeding, whereas no acute systemic reactions were observed. These data strongly suggest that the identity of the target antigen recognized by antiplatelet antibodies determines the mechanisms of platelet destruction and the severity of bleeding in mice, the latter depending on previously unrecognized anti-GPIIbIIIa-specific inflammatory mechanisms.

Platelet activation mediated through membrane glycoproteins: involvement of tyrosine kinases

Fc gamma RII cross-linking and anti-CD9 mAbs included tyrosine phosphorylation of Fc gamma RII, Syk, and Lyn associated with Fc gamma RII in Fc gamma RII cross-linking but not in anti-CD9 mAb-induced platelet activation. We prepared various GST fusion proteins expressing one or two SH2 domains of Syk and evaluated the association between these GST fusion proteins with Fc gamma RII. Based on the results obtained from these experiments, we suggest that only one tyrosine residue in ITAM of Fc gamma RII is phosphorylated with anti-CD9 mAb and that both are phosphorylated with Fc gamma RII cross-linking. Platelet activation mediated by GPIb, the receptor for vWF, is also related with tyrosine phosphorylation. Botrocetin and vWF induced Syk activation. Shc was also rapidly and heavily tyrosine phosphorylated. Sre and Lyn, a 54-kDa tyrosine kinase, was associated with cytoskeletal proteins. When GPIb was immunoprecipitated with nonfunctional anti-GPIb mAbs after platelets were activated with vWF and botrocetin, an in vitro kinase assay revealed the transient association of a kinase activity with GPIb after platelet activation. Phosphoamino acid analysis of phosphorylated proteins in this assay demonstrated that only tyrosine residues but not serine or threonine were phosphorylated, suggesting that the kinase was indeed a tyrosine kinase.

Human factor XII binding to the glycoprotein Ib-IX-V complex inhibits thrombin-induced platelet aggregation

Factor XII deficiency has been postulated to be a risk factor for thrombosis suggesting that factor XII is an antithrombotic protein. The biochemical mechanism leading to this clinical observation is unknown. We have previously reported high molecular weight kininogen (HK) inhibition of thrombin-induced platelet aggregation by binding to the platelet glycoprotein (GP) Ib-IX-V complex. Although factor XII will bind to the intact platelet through GP Ibalpha (glycocalicin) without activation, we now report that factor XIIa (0. 37 microm), but not factor XII zymogen, is required for the inhibition of thrombin-induced platelet aggregation. Factor XIIa had no significant effect on SFLLRN-induced platelet aggregation. Moreover, an antibody to the thrombin site on protease-activated receptor-1 failed to block factor XII binding to platelets. Inhibition of thrombin-induced platelet aggregation was demonstrated with factor XIIa but not with factor XII zymogen or factor XIIf, indicating that the conformational exposure of the heavy chain following proteolytic activation is required for inhibition. However, inactivation of the catalytic activity of factor XIIa did not affect the inhibition of thrombin-induced platelet aggregation. Factor XII showed displacement of biotin-labeled HK (30 nm) binding to gel-filtered platelets and, at concentrations of 50 nm, was able to block 50% of the HK binding, suggesting involvement of the GP Ib complex. Antibodies to GP Ib and GP IX, which inhibited HK binding to platelets, did not block factor XII binding. However, using a biosensor, which monitors protein-protein interactions, both HK and factor XII bind to GP Ibalpha. Factor XII may serve to regulate thrombin binding to the GP Ib receptor by co-localizing with HK, to control the extent of platelet aggregation in vivo.

A rapid and accurate closed-tube immunoassay for platelets on an automated hematology analyzer

Accurate and precise platelet counts are important for patients with severe thrombocytopenia or who are receiving chemotherapy. We developed a novel flow cytometric analysis of platelets that may be particularly valuable for assessing the necessity for platelet transfusions. This ImmunoPlt (CD61) assay is based in part on CD61 monoclonal antibody labeling and has been automated and implemented on the CELL-DYN 4000 hematology analyzer. It is well suited for thrombocytopenic specimens, since it reduces interference by nonplatelet particles. It takes less than 5 minutes from closed-tube aspiration to report. Data for more than 350 thrombocytopenic specimens demonstrate that the ImmunoPlt (CD61) assay is more accurate than the optical scatter or the impedance count for specimens with platelet counts between 1 and 60 x 10(3)/microL (1 and 60 x 10(9)/L). The ImmunoPlt (CD61) assay is more precise than the optical scatter or the impedance count for specimens with platelet counts between 1 and 50 x 10(3)/microL (1 and 50 x 10(9)/L).

Morphological differences between GPIb antibody-induced and shear stress-induced platelet aggregates

We determined the morphological differences between NNKY5-5-induced and shear stress-induced platelet aggregates using confocal laser scanning microscopy, and investigated the effects of cytokines on these aggregates. As shear stress was increased from low to high, many aggregates were generated. Some platelets and aggregates already exhibited PAC-1 binding at low shear stress. And finally, aggregates were clearly stained by PAC-1 at high shear stress. Low shear stress-induced aggregates included fibrinogen, but not von Willebrand factor (vWF). In contrast, high shear stress-induced aggregates included both fibrinogen and vWF. NNKY5-5-induced platelet aggregates exhibited both PAC-1 binding and fibrinogen, but vWF was not found in them. The aggregate formation in NNKY5-5-treated platelet-rich plasma (PRP) at low shear stress was clearly more pronounced than that in untreated PRP. In addition, aggregates in NNKY5-5-treated PRP formed after 6 min under low shear stress were clearly stained by PAC-1 and included fibrinogen, but vWF was not found in them. In order to investigate the effects of cytokines on platelet activation under NNKY5-5 stimulation, changes in light transmission and light scatter were assessed with an AG-10. G-CSF, IL-6 and thrombopoietin (TPO) each enhanced light scatter compared to control PRP, although IL-3, GM-CSF, erythropoietin and stem cell factor did not. In particular, TPO significantly enhanced the '%-T' and 'S-Max' of NNKY5-5-treated PRP. TPO clearly enhanced the aggregate formation with high shear stress or NNKY5-5 stimulation. These results suggest that the glycoprotein Ib (GPIb)-unmediated aggregates can be formed with only fibrinogen, but the GPIb-mediated aggregates by vWF and fibrinogen synergistically. In particular, the latter is formed more firmly, and both aggregates are enhanced by TPO.

Evaluation of platelet function by flow cytometry

Platelet function in whole blood can be comprehensively evaluated by flow cytometry. Flow cytometry can be used to measure platelet reactivity, circulating activated platelets, platelet-platelet aggregates, leukocyte-platelet aggregates, procoagulant platelet-derived microparticles, and calcium flux. Clinical applications of whole blood flow cytometric assays of platelet function in disease states (e.g., acute coronary syndromes, angioplasty, and stroke) may include identification of patients who would benefit from additional antiplatelet therapy and prediction of ischemic events. Circulating monocyte-platelet aggregates appear to be a more sensitive marker of in vivo platelet activation than circulating P-selectin-positive platelets. Flow cytometry can also be used in the following clinical settings: monitoring of GPIIb-IIIa antagonist therapy, diagnosis of inherited deficiencies of platelet surface glycoproteins, diagnosis of storage pool disease, diagnosis of heparin-induced thrombocytopenia, and measurement of the rate of thrombopoiesis.

Monoclonal antibodies against platelet membrane glycoproteins IIb/IIIa and Ibalpha inhibit platelet dependent thrombin generation by different mechanisms

The antithrombotic effect of antiplatelet agents is principally due to their anti-aggregatory action, but these agents may also interfere with coagulation. We have investigated the effect of monoclonal antibodies (MAb) to platelet membrane glycoproteins (GP) IIb/IIIa and Ibalpha on thrombin generation. Antibodies to platelet membrane glycoprotein IIb/IIIa (RFGP56 and c7E3) were shown to inhibit platelet-mediated thrombin generation stimulated by both intrinsic and extrinsic methods. An antibody to GP Ibalpha (RFGP37) also inhibited thrombin generation in these systems. FITC-annexin V was used to determine the effect of these antibodies on the exposure of procoagulant phospholipids on the platelet membrane, and it was found that the anti-IIb/IIIa antibodies reduced this, whereas the anti-Ibalpha antibody caused an increase. We conclude that our monoclonal antibodies against platelet membrane glycoproteins IIb/IIIa and Ibalpha inhibit platelet dependent thrombin generation by different mechanisms.

Acquired Bernard-Soulier syndrome: a case with necrotizing vasculitis and thrombosis

We describe a patient with positive antinuclear antibodies, polyclonal gammopathy and high level of circulating immunocomplexes, resulting in vascular purpura. In addition, the patient had a slightly prolonged bleeding time and an isolated defect of ristocetin-induced platelet aggregation (RIPA) in platelet-rich plasma (PRP). The patient's plasma also inhibited RIPA in normal PRP and in normal platelet suspension. The activity and multimeric structure of plasmatic von Willebrand factor showed no alteration. We could demonstrate an autoantibody against platelet membrane glycoprotein (GP) Ib, using an ELISA-type assay. These data suggest an acquired Bernard-Soulier syndrome. We suggest that the patient had an immunocomplex-mediated leukocytoclastic vasculitis accompanied by production of antinuclear autoantibodies as well as the presence of an autoantibody against GPIb. The titer of the anti-GPIb antibody, however, was too low to induce significant platelet-type bleeding tendency, only laboratory alterations were found. Moreover, in a later stage of her disease, she developed a severe necrotizing vasculitis which was followed by a deep venous thrombosis.

Immunoneutralization of glycoprotein Ibalpha attenuates endotoxin-induced interactions of platelets and leukocytes with rat venular endothelium in vivo

This study aimed to examine molecular mechanisms for endotoxin-induced adhesive changes in platelets in vivo. Platelets labeled with carboxyfluorescein diacetate succinimidyl ester were visualized in rat mesenteric venules through intravital microscopy assisted by a high-speed fluorescence video imager at 1000 frames per second or by a normal-speed intensifier under monitoring of erythrocyte velocity. Leukocyte rolling was examined by normal-speed transmission video images. The velocity of platelets traveling along the centerline of venules followed that of erythrocytes, whereas that measured at the periendothelial space was significantly smaller than the erythrocyte velocity; a majority of these cells exhibited transient but notable rolling with endothelium. Administration of endotoxin increased the density of periendothelial platelets and reduced the rolling velocities of platelets and leukocytes in venules: All events were attenuated by anti-rat P-selectin monoclonal antibody s789G or by anti-human glycoprotein (GP) Ibalpha monoclonal antibody GUR83/35, which blocks ristocetin-induced aggregation of rat platelets. Isolated rat platelets injected into endotoxin-pretreated rats were able to roll on the venules. This event was attenuated by pretreatment of platelets in vitro with GUR83/35 but not with s789G, suggesting involvement of endothelial P-selectin and platelet GP Ibalpha in the endotoxin-induced responses. Furthermore, isolated human platelets showed similar rolling interactions with endotoxin-preexposed rat venules, and pretreatment of the platelets with GUR83/35, but not with s789G, significantly reduced such interactions. Our results provide the first evidence for involvement of GP Ibalpha in endotoxin-induced microvascular rolling of platelets and leukocytes, and this system serves as a potentially useful tool to examine GP Ibalpha-associated function of human platelets in vivo.

[Inhibition of thrombus formation by anti-Willebrand monoclonal antibodies in the guinea pig]

To experimentally evaluate the role of von Willebrand factor in thrombosis, a monoclonal antibody-712-, directed against the binding domain of von Willebrand factor to the platelet glycoprotein Ib and a control antibody D4H1, directed against the light chain of Factor VII C, were injected into guinea pigs. A model of experimental thrombosis induced by laser in guinea pig mesenteric arteries was used. The antibody 712 induced a dose-dependent reduction of the thrombus formation without affecting the bleeding time and the platelet number. Ex vivo, the ristocetin-induced platelet aggregation was inhibited by the antibody 712 in a dose-dependent manner while the adenosine diphosphate-induced platelet aggregation was not modified. In conclusion, this study showed that antibody-712- has a significant antithrombotic activity without inducing a hemorrhagic state, suggesting that the von Willebrand factor--glycoprotein Ib axis was a promising target for the strategy of a new antithrombotic therapy.

Characterization of murine anti-glycoprotein Ib monoclonal antibodies that differentiate between shear-induced and ristocetin/botrocetin-induced glycoprotein Ib-von Willebrand factor interaction

Platelet adhesion to vascular subendothelium under conditions of high shear stress is mediated by the platelet glycoprotein (GP) Ib-von Willebrand Factor (vWF) interaction. The aim of this study was to characterize the murine monoclonal antibodies (MoAbs) 27A10 and 28E6, both raised against purified GPIb. The MoAb 27A10 is a potent inhibitor of shear-induced platelet adhesion to collagen type I in a flow chamber at shear rates of 1,300 and 2,700 s(-1). 20 microg/ml of MoAb 27A10, furthermore, could completely block shear-induced aggregation in a modified Couette viscometer at shear rates of 1,000 and 4,000 s(-1). On the other hand, MoAb 27A10 had a negligible effect on botrocetin-induced GPIb-vWF binding and is only a poor inhibitor of the ristocetin-dependent interaction. In contrast, MoAb 28E6 did abolish both the ristocetin- and botrocetin-induced GPIb-vWF binding, whereas it did not block the shear-induced interaction. Thus, we identify here two anti-GPIb MoAbs 27A10 and 28E6 that either preferentially inhibit the shear-induced or the ristocetin/botrocetin-induced platelet-vWF interaction. With these tools it should be possible to more clearly define the mechanisms by which platelets bind to vWF in vivo.

Antithrombotic effect of platelet glycoprotein Ib-blocking monoclonal antibody Fab fragments in nonhuman primates

Platelet adhesion in arterial blood flow is mainly supported by the platelet receptor glycoprotein (GP) Ib, which interacts with von Willebrand factor (vWF) that is bound to collagen at the site of vessel wall injury. Antibody 6B4 is a monoclonal antibody (MoAb) raised against purified human GPIb. MoAb 6B4 inhibits both ristocetin- and botrocetin-induced, vWF-dependent human platelet agglutination. MoAb 6B4 furthermore blocks shear-induced adhesion of human platelets to collagen I. We studied the antithrombotic effect of this inhibitory murine MoAb 6B4 in a baboon model of arterial thrombosis. When injected into baboons, intact IgG and its F(ab')(2) fragments caused almost immediate thrombocytopenia, whereas injection of the Fab fragments alone did not. Fab fragments were subsequently used to investigate their in vivo effect on platelet deposition onto a thrombogenic device, consisting of collagen-rich, glutaraldehyde-fixed bovine pericardium (0.6 cm(2)), at a wall shear rate ranging from 700 to 1000 s(-1). Baboons were either pretreated with Fabs to study the effect of inhibition on platelet adhesion or treated 6 minutes after placement of the thrombogenic device to investigate the effect on interplatelet cohesion. Pretreatment of the animals with bolus doses ranging from 80 to 640 microgram/kg Fab fragments significantly reduced (111)In-labeled platelet deposition onto the collagen surface by approximately 43% to 65%. Only the highest dose caused a significant prolongation (doubling) of the bleeding time. Ex vivo ristocetin-induced platelet agglutination was equally reduced. Treatment with a bolus of 110 microgram/kg Fab fragments after a thrombus was allowed to form for 6 minutes had no effect on further platelet deposition. We therefore conclude that Fab fragments or derivatives of inhibitory anti-GPIb antibodies may be useful compounds to prevent thrombosis.

Necessity of conserved asparagine residues in the leucine-rich repeats of platelet glycoprotein Ib alpha for the proper conformation and function of the ligand-binding region

The polypeptides of the platelet von Willebrand factor (vWf) receptor, the GP Ib-IX-V complex, each contain tandem repeats of a sequence that assigns them to the leucine-rich repeat protein family. Here, we studied the role of conserved Asn residues in the leucine-rich repeats of GP Ib alpha, the ligand-binding subunit of the complex. We replaced the Asn residue in the sixth position of the first or sixth leucine-rich repeat (of seven) either with a bulky, charged Lys residue or with a Ser residue (sometimes found in the same position of other leucine-rich repeats) and studied the effect of the mutations on complex expression, modulator-dependent vWf binding, and interactions with immobilized vWf under fluid shear stress. As predicted, the Lys substitutions yielded more severe phenotypes, producing proteins that either were rapidly degraded within the cell (mutant N158K) or failed to bind vWf in the presence of ristocetin or roll on immobilized vWf under fluid shear stress (mutant N41K). The binding of function-blocking GP Ib alpha antibodies to the N41K mutant was either significantly reduced (AK2 and SZ2) or abolished (AN51 and CLB-MB45). Ser mutations were tolerated much better, although both mutants demonstrated subtle defects in vWf binding. These results suggest a vital role for the conserved asparagine residues in the leucine-rich repeats of GP Ib alpha for the structure and functions of this polypeptide. The finding that mutations in the first leucine-rich repeat had a much more profound effect on vWf binding indicates that the more N-terminal repeats may be directly involved in this interaction.

Rifampicin-dependent antibodies bind a similar or identical epitope to glycoprotein IX-specific quinine-dependent antibodies

The drug-dependent antibody of a patient with rifampicin-induced thrombocytopenia was characterized using the antigen-capture enzyme-linked immunosorbent assay (MAIPA assay), flow cytometry, and immunoprecipitation. The antibody was found to bind glycoprotein (GP) Ib-IX but not GPIIb-IIIa because (1) it immunoprecipitated drug-dependently the former but not the latter glycoprotein complex and (2) the MAIPA assay showed strong rifampicin-dependent antibody binding when anti-GPIb-IX monoclonal antibodies (mAbs) (AK2 and FMC25) but not anti-GPIIb-IIIa mAbs (AP2, SZ21, and SZ22) were used to capture the antigen. The antibody binding site was further localized to the GPIX subunit of the GPIb-IX complex because flow cytometric analysis revealed drug-dependent antibody binding to L cells transfected with human GPIbbeta and GPIX complementary DNA (L betaIX cells) but not with human GPIbalpha and GPIbbeta complementary DNA (L alphabeta cells). Finally, in the MAIPA assay, the rifampicin-dependent antibody almost completely cross-blocked the binding of the anti-GPIX mAb (SZ1) to platelets. Similar cross-blocking of SZ1binding to platelets by the quinine-dependent antibodies was also observed. This finding not only confirms that the epitope of the rifampicin-dependent antibody is on GPIX but it is also identical to or located in close proximity to that of the quinine-dependent antibody and SZ1. Further characterization of the epitopes of these antibodies may have important implications for a general understanding of the mechanism of drug-induced thrombocytopenia. (Blood. 2000;95:1988-1992)

Influence of auto-antibody specificities on the clinical course in patients with chronic and acute ITP

It is now well established that the target antigens of platelet auto-antibodies are the GP IIb/IIIa and GP Ib/IX receptors. In our study, it was observed that the majority of patients had antibodies directed towards the GP IIb/IIIa receptor, whereas the number of patients having antibodies directed towards the GP Ib/IX receptor was much less. Also, we found that the patients in whom the auto-antibodies are directed towards the GP IIb/IIIa receptors usually have mild bleeding. On the other hand, the patients having auto-antibodies directed towards both GP IIb/IIIa and GP Ib/IX receptors have a severe bleeding diathesis, and usually show poor prognosis to treatment with corticosteroids.

Single amino acid substitution in human platelet glycoprotein Ibbeta is responsible for the formation of the platelet-specific alloantigen Iy(a)

We recently described a new low-frequency platelet alloantigen on the human platelet glycoprotein (GP) Ib-IX complex, termed Iy(a), which was implicated in a severe case of neonatal alloimmune thrombocytopenia. Immunoprecipitation studies with trypsin-treated platelets indicated that the Iy(a) alloantigenic determinants are formed by the membrane-associated remnant moiety of GP Ibalpha (GP Ibalpha(r)) together with GP Ibbeta and GP IX. To elucidate the molecular basis underlying the Iy(a) alloantigen, we amplified GPIbalpha(r), GPIbbeta, and GPIX genes by polymerase chain reaction (PCR). Nucleotide-sequence analysis of these 3 genes showed a G to A transition at position 141 on GPIbbeta gene in a subject positive for Iy(a). This transition resulted in a Gly(15)Glu dimorphism on the N-terminal domain of GPIbbeta. This finding was confirmed by genotyping analysis of 6 Iy(a)-positive subjects by restriction fragment length polymorphism (RFLP) studies using NarI endonuclease. In 300 randomly selected healthy blood donors, one Iy(a)-positive individual was found. Phenotypes determined by monoclonal antibody-specific immobilization of platelet antigens assay and genotypes determined by RFLP were identical in this population. Analysis of Iy(a)-positive platelets showed that the point mutation affected neither the degree of surface expression nor the function of the GP Ibalpha-GP Ibbeta-IX complex on the platelet surface. Transient expression of the GP Ib-IX complex in CHO cells using wild-type GP Ibbeta (Gly(15)) or mutant GP Ibbeta (Glu(15)) allowed us to demonstrate that this single amino acid substitution is sufficient to induce Iy(a) epitope(s). (Blood. 2000;95:1849-1855)

Requirement of leucine-rich repeats of glycoprotein (GP) Ibalpha for shear-dependent and static binding of von Willebrand factor to the platelet membrane GP Ib-IX-V complex

The platelet glycoprotein (GP) Ib-IX-V complex mediates adhesion to von Willebrand factor (vWf) in (patho)physiologic thrombus formation. The vWf-binding site on GP Ib-IX-V is within the N-terminal 282 residues of GP Ibalpha, which consist of an N-terminal flanking sequence (His-1-Ile-35), 7 leucine-rich repeats (Leu-36-Ala-200), a C-terminal flank (Phe-201-Gly-268), and a sulfated tyrosine sequence (Asp-269-Glu-282). We have used mammalian cell expression of canine-human chimeras of GP Ibalpha, corresponding to precise structural boundaries, to demonstrate the first specific requirement for individual leucine-rich repeats for binding of vWf either induced by a modulator, ristocetin, or under hydrodynamic flow. Implicit in this approach was that the GP Ibalpha chimeras retained a functional conformation, a supposition confirmed by analyzing restoration of function to reversed human-canine chimeras and demonstrating that all chimeras bound vWf activated by botrocetin, a modulator that is indiscriminate between species. Leucine-rich repeats 2, 3, and 4 of GP Ibalpha were identified as being critical for vWf adhesion to GP Ib-IX-V.

GPV is a marker of in vivo platelet activation--study in a rat thrombosis model

Thrombin plays a central role in the genesis of thrombotic events and is the most potent known platelet agonist. This enzyme activates platelets by cleaving G-protein coupled protease activated receptors (PARs) and by binding to glycoprotein (GP) Ib. Thrombin also cleaves platelet GPV to liberate a soluble 69 kDa fragment (GPVf1), leaving a 20 kDa fragment (GPVf2) attached to the membrane. The aim of this study was to assess the value of GPV as an in vivo marker of the activation of platelets by thrombin. Newly developed monoclonal and polyclonal antibodies recognizing rat GPVf1 and GPVf2 respectively were used to detect soluble GPV by ELISA and the new NH2-terminus exposed by thrombin using flow cytometry. These assays were employed in a rat thrombosis model designed to trigger thrombin formation in vivo. When thromboplastin (4.8 ml/kg/h) was infused for 30 min, thrombin generation was reflected by a rapid increase in thrombin-antithrombin (TAT) complexes in plasma and by the appearance of GPVf2 at the surface of circulating platelets. Simultaneously, GPVf1 disappeared from the surface of platelets and accumulated as a soluble fragment in plasma, where it was detected by GPV ELISA. These effects were inhibited by pretreatment of the rats with hirudin. Levels of plasma PF4 also increased in this model, but unlike GPV levels which returned slowly (> 2 hours) to baseline, PF4 had a very short half-life. In conclusion, GPV is cleaved by thrombin in vivo, circulates and is a reliable in vivo marker of the activation of platelets by thrombin. Monitoring of GPV levels in rats should be useful to evaluate the effects of antithrombotic and antiplatelet drugs, while further studies will be required to confirm the potential interest of GPV as a marker of thrombotic states in humans.

Ultrastructural characterization of maturation, platelet release, and senescence of human cultured megakaryocytes

The aim of this study was to evaluate the ultrastructural features of human megakaryocytes cultured in vitro. For this purpose, pluripotent CD34(+) (cluster of differentiation 34) hematopoietic progenitor cells, obtained from the peripheral blood of healthy adult donors, were differentiated along the megakaryocytic lineage in liquid cultures by the addition of the megakaryocyte-specific growth factor thrombopoietin (TPO, 100 ng/ml). After only 6-8 days, virtually all of the CD34-derived cells expressed the early megakaryocytic CD61 antigen, while, after 15-16 days, most cells also expressed the late megakaryocytic CD42a antigen. Ultrastructural analysis of cells obtained after 7 days of culture showed aspects typical of developing megakaryocytes (MK), such as formation of platelet territories and cytoplasmic fragmentation. At later (15-16 day) culture times, two distinct cell populations were observed: fully developed megakaryocytes releasing platelets into the culture medium and senescent megakaryocytes, characterized by morphological features of apoptosis. Analysis of DNA fragmentation in these cells revealed that apoptosis in megakaryocytes occurred in the absence of the internucleosomic cleavage, which is characteristic of most, but not all, types of apoptosis in cells of hematopoietic origin. On the other hand, flow cytometry of the DNA content of senescent megakaryocytes showed a subdiploid peak that was likely due to a loss of micronuclei during processing.

The pathogenesis of chronic immune (idiopathic) thrombocytopenic purpura

Chronic Immune (idiopathic) thrombocytopenic purpura (ITP) is an autoimmune disorder in which antiplatelet autoantibody induces platelet destruction. Platelet surface membrane proteins become antigenic, stimulating the immune system to produce autoantibody. The initial antigenic response probably occurs in the spleen, inducing autoantibody production followed by stimulation of other antibody-producing tissues, particularly the bone marrow. Autoantibodies against either platelet glycoprotein (GP)IIb/IIIa or GPIb/IX are produced by about 75% of ITP patients and can be detected using antigen-specific assays. The spleen is the major site of platelet destruction in ITP because of its unique milieu. About one third of the platelet mass is present in the spleen at all times, where the local production of antiplatelet antibody leads to high autoantibody concentrations. These antibody-sensitized platelets circulate slowly through the phagocytic cell-rich spleen, resulting in their destruction. Since autoantibody binds to both platelets and megakaryocytes, both platelet destruction and inhibition of thrombopoiesis may be of importance in the pathogenesis of chronic ITP.

Circulating activated platelets assist THP-1 monocytoid/endothelial cell interaction under shear stress

Circulating complexes of leukocytes and activated platelets are markers for atherosclerosis, but their interaction with the arterial endothelial lining has not been studied. Therefore, the effect of activated platelets on rolling and adhesion of labeled human THP-1 monocytoid cells to human umbilical vein endothelial cell (HUVEC) monolayers was studied by epifluorescence microscopy in a parallel plate flow chamber. In the absence of activated platelets, THP-1 rolling on resting HUVEC was negligible at shear rates greater than 300 s(-1). Activation of HUVEC with 100 nmol/L phorbol myristate acetate (PMA) increased THP-1 cell adhesion at shear rates less than 400 s(-1). Therefore, a shear rate of 400 s(-1) was identified as a threshold for THP-1 adhesion. THP-1 rolling on activated HUVEC was reduced by 64% after L-selectin inhibition but was not affected by P-selectin inhibition. The addition of 1 to 50 thrombin receptor-activating peptide (TRAP)-activated platelets per THP-1 cell enhanced interactions between THP-1 cells and HUVEC, resulting in a steep bell-shaped dose-response curve, with a peak of 10 +/- 3 rolling cells/50 seconds at 3 platelets per THP-1 cell (P <.01 v control) with a concomitant 2- to 3-fold increase of firmly adhering cells (P <.01 v control). In reconstituted blood, low numbers of activated platelets had the same effect on THP-1 rolling and adhesion. P-selectin inhibition reduced platelet/THP-1 cell interaction in suspension and deposition of the complexes on the endothelial monolayer. Inhibition of both P- and L-selectin reduced THP-1/HUVEC interactions to 14% (P <.01, n = 4). Sialidase digestion and removal of terminal sialic acid residues from HUVEC or THP-1 cells but not from platelets abolished the platelet mediated augmentation of THP-1 cell adhesion. Thus, THP-1 rolling on HUVEC is shear-dependent and largely mediated by L-selectin. P-selectin expressed on activated platelets increases monocytoid cell adhesion to endothelial cells at shear rates found in coronary arteries through interactions with both endothelial and monocytoid cells and may facilitate macrophage accumulation in the vessel wall.

A case of malignant lymphoma producing autoantibody against platelet glycoprotein Ib

A 61-year-old woman was referred to our hospital for refractory thrombocytopenia (3 x 10(3)/microliter) and massive melena. Bone marrow aspiration revealed normal cellularity and increased megakaryocytes (250/microliter). An abdominal computerized axial tomography scan showed isodensity masses on both adrenal glands. 67 Ga-scintigraphy exhibited strong uptake into the bilateral adrenal tumor and mediastinal region. IgM-type antibody against platelet glycoprotein Ib (GpIb) was detected in the patient's serum. A needle biopsy of the right adrenal tumor was performed, and histology was non-Hodgkin's lymphoma (NHL), diffuse large B-cell type. Following the diagnosis of autoimmune thrombocytopenia associated with lymphoma, administration of corticosteroid (predonisolone 60 mg/day) and high-dose intravenous globulin (15 g/day x 4 days) was carried out, but neither was effective in normalizing the thrombocytopenia. Immunosuppressive therapy (cyclophosphamide 500 mg and 1 mg of vincristine) markedly restored the platelet counts to 7.2 x 10(4)/microliter and ceased the melena; furthermore, the size of adrenal tumors decreased by more than 60% after therapy. We cultured the lymphoma cells drawn by needle biopsy with IL-6 in vitro and found that the lymphoma cells produced IgM-type antiplatelet antibodies against platelet GpIb in the culture supernatant. Thus this is a rare case of NHL in which the production of antiplatelet antibody from lymphoma cells was confirmed in vitro.

The effects of heparin coating of oxygenator fibers on platelet adhesion and protein adsorption

Platelet adhesion on the cardiopulmonary bypass oxygenator membrane is associated with impaired hemostasis. We investigated the effects of heparin coating of the oxygenator membrane on protein adsorption and platelet adhesion on the surface. Noncoated and heparin-coated polypropylene membranes were incubated in whole blood with small- (1 U/mL) or large-dose (5 U/mL) heparin as an anticoagulant for 3 h at 37 degrees C. The amount of platelets adhering on each fiber was assessed by using enzyme immunoassays using monoclonal antibodies directed against CD42b (GP Ib) and CD61 (GP IIb/IIIa). Platelet activation was assessed by measuring plasma guanosine monophosphate 140 levels. The amount and composition of the adsorbed proteins on the surface were analyzed by using a bicinchoninic acid protein assay and by using sodium dodecyl sulfate-polyacrylamide gel electrophoresis and Western blotting technique. The heparin coating of the fibers significantly reduced platelet adhesion on the surface. However, platelet activation was reduced by heparin coating only with small-dose heparinization. The adsorption of platelet adhesive proteins such as fibrinogen and von Willebrand factor was not altered, whereas that of fibronectin was increased by heparin coating. We conclude that heparin coating of the oxygenator fibers can decrease platelet adhesion without affecting adsorption of major adhesive proteins. Surface heparin coating is associated with an increased fibronectin adsorption on the fibers.

IMPLICATIONS

Heparin coating can reduce platelet adhesion and activation in the presence of small-dose heparinization, potentially reducing the inflammatory response and activation of thrombosis and fibrinolysis.

Platelet glycoprotein Ib: a zinc-dependent binding protein for the heavy chain of high-molecular-weight kininogen

Domains 3 and 5 of high-molecular-weight kininogen (HK) have been shown to bind to platelets in a zinc-dependent reaction. However, the platelet-binding proteins responsible for this interaction have not been identified. We have focused on the platelet-binding site for the heavy chain (domain 3), which we approached using a domain 3-derived peptide ligand and isolated binding proteins by affinity chromatography. The domain 3-derived peptide, thrombin, HK, factor XII, as well as antibody to glycocalicin (the N-terminal portion of the alpha chain of GPIb) recognized a protein at 74 kD. We also isolated the thrombin receptor (PAR 1) at 45 kD, however, none of the above-mentioned ligands bound to this protein. Isolation of platelet membrane proteins using a monoclonal anti-glycocalicin antibody column revealed the same HK binding protein at 74 kD, which was reactive with anti-GPIb and represents a GPIb fragment. By photoaffinity labeling, HK interacted with membrane GPIb, which was then isolated in native form (135 kD) along with gC1qR, a ligand for the HK light chain. Finally, (125)I-HK binding to platelets was significantly inhibited by the anti-GPIb antibody. These results suggest that the GPIb alpha chain, a known thrombin binding protein, is also one of the zinc-dependent platelet membrane binding sites for HK domain 3.

Identification of the binding site for an alloantibody to von Willebrand factor which inhibits binding to glycoprotein Ib within the amino-terminal region flanking the A1 domain

An alloantibody to von Willebrand factor (vWF) which developed in a Japanese boy with type 3 von Willebrand disease has been characterized. The antibody was non-precipitating IgG and the main subclasses were IgG2 and IgG4. The antibody inhibited completely ristocetin-induced platelet aggregation (RIPA) and high shear stress-induced platelet aggregation (SIPA). Its predominant inhibitory role was focused, therefore, on the interaction between vWF and platelet gycoprotein Ib. The antibody reacted with a 52/48 kDa tryptic fragment of vWF (residues 449-728). No reaction was seen, however, with either a 39/34 kDa dispase fragment (480-718) or a recombinant vWF fragment (residues 465-728). These findings suggested that the essential epitope resided in the amino-terminal flanking region of the Al domain. We synthesized overlapping peptides corresponding to the region containing D3/A1 boundary. A peptide, residues 458-472, bound to the antibody and dose-dependently blocked the antibody binding to the 52/48 kDa fragment. The same peptide neutralized the inhibitory effect of the alloantibody on SIPA. The data are consistent with the presence of an epitope within residues 458-472 which reacted with the 52/48 kDa fragment. Furthermore, the specific component of the antibody, directed against residues 458-472, blocked vWF binding to GPIb in absence of exogenous agonist. Our results suggest that the region flanking the Al domain plays an important role in regulating vWF binding to GPIb.

Flow cytometric analysis of circadian changes in platelet activation using anti-GMP-140 monoclonal antibody

The hemostatic activity of blood shows a circadian variation with a higher frequency of acute coronary events in the morning. The thrombotic tendency of blood is influenced by many factors, including platelets. Diurnal changes of in vivo platelet activation were investigated by whole blood flow cytometry in 10 young healthy male volunteers using anti-GMP-140 (anti-alpha-granule membrane protein 140 kD) monoclonal antibody at 3h intervals from 06:00 to 24:00. We also studied circulating platelet aggregates to investigate whether there exists a similarity between the results of these methods. Results of flow cytometric analysis indicate that there is an increase in platelet activation during the period from 06:00 to 09:00. Platelet activation then decreases gradually during the period from noon to midnight. These changes are accompanied by a similar trend in circulating platelet aggregates. This suggests that GMP-140 expression on platelets is synchronized with or followed by platelet aggregate formation in vivo, and increased platelet activation may predispose individuals to thrombosis at this time.

Apoptosis of erythroid precursors under stimulation with thrombopoietin: contribution to megakaryocytic lineage choice

Although the effect of thrombopoietin (TPO) on megakaryocyte production is well established, its role in the commitment of multipotential hematopoietic progenitors to the megakaryocytic lineage remains to be determined. In the present study, we attempted to clarify the determination process of megakaryocytic lineage as a terminal differentiation pathway under stimulation with TPO. Day 7 cultured cells grown by TPO derived from cord blood CD34+ cells were divided into four subpopulations on the basis of CD34 and CD41 expression. The CD34-/CD41- cells showed the labeling pattern of anti-CD42b and anti-CD9 antibodies closer to that of the CD34+/CD41- cells than the CD34+/CD41+ cells. Replating experiments revealed that approximately 40% of the CD34-/CD41- cells proliferated in response to a combination of growth factors, and more than 80% of them were pure erythroid precursors. However, this subpopulation failed to grow/survive and fell into apoptosis in the presence of TPO alone. In contrast, the CD34+/CD41+ cells, which predominantly contained megakaryocytic precursors, exerted a low but significant proliferative potential in the presence of TPO. The insufficient response to TPO of the CD34-/CD41- cells may result from the apparently low expression of c-MpI, as determined by flow cytometric analysis and reverse transcription-polymerase chain reaction analysis. Therefore, these results suggest that the apoptosis of hematopoietic precursors other than megakaryocytic precursors is related to the determination of the terminal differentiation under the influence of TPO.

A fatal case of autoimmune thrombocytopenia with an IgM anti-GPIb/IX following one antigen mismatched unrelated donor bone marrow transplantation

We report the case of a 32-year-old patient with ALL who developed autoimmune thrombocytopenia 2 months following allogeneic bone marrow transplantation. An IgM autoantibody against the platelet glycoprotein Ib/IX complex was observed. Treatment with high-dose steroids and intravenous immunoglobulin G failed to produce any benefit and the thrombocytopenia led to fatal gastrointestinal haemorrhage. The possible factors contributing to post-allograft thrombocytopenia and potential management strategies are discussed.

Nitric oxide diffusion across membrane lungs protects platelets during simulated extracorporeal circulation

BACKGROUND

The absence of a protective endothelial surface on membrane oxygenators during extracorporeal circulation (ECC) promotes platelet trapping and damage, leading to increased bleeding complications. We investigated the effects of transmembranous diffusion of gaseous nitric oxide (NO) on platelets during simulated ECC.

MATERIAL AND METHODS

Two paired circuits were run in parallel with fresh, heparinized (1 U mL-1) blood from healthy human donors for 240 min. To one of the paired circuits, 20 ppm NO was added transmembranously.

RESULTS

NO significantly attenuated platelet trapping and reduced intracircuit platelet activation evaluated by the release of beta-thromboglobulin, platelet factor 4 and soluble P-selectin. Furthermore, NO significantly preserved platelet reactivity to stimulating agents (ADP and adrenaline), evaluated as the ability to expose P-selectins and activate glycoprotein (GP)-IIb-IIIa. Nevertheless, circulating activated platelets expressing P-selectin or activated GPIIb-IIIa were not different and were not significantly increased. The mean fluorescence intensity of GPIb and GPIIb-IIIa decreased in both circuits equally.

CONCLUSIONS

Transmembranous diffusion of gaseous NO revealed protective effects on platelets by reducing thrombocytopenia/pathia and preserving platelet reactivity.

Low prevalence of a polymorphism of platelet membrane glycoprotein Ib beta associated with neonatal alloimmune thrombocytopenic purpura in Asian populations

Iy alloantigen system is the first polymorphism of platelet glycoprotein Ib beta reported to cause neonatal alloimmune thrombocytopenic purpura. We investigated the allelic frequency of Iy alloantigen among Japanese and Korean populations by polymerase chain reaction-restriction fragment length method to determine the possibility of alloimmunization against Iy. Two hundred and nine Japanese and 97 Korean subjects were examined. All 306 individuals were homozygous for glycine at amino acid position 15 and negative for Iy. The allelic frequency of Iy in these populations was calculated to be less than 0.0016. Alloimmunization associated with Iy antigen in Asian populations seems unlikely from these results.

Rapid production of quasi-stable antibody-phycoerythrin conjugates for use in flow cytometry

A simple system is described for producing quasi-stable complexes between biotinylated antibodies and fluorochrome-labeled streptavidin. Optimal complexes were produced by mixing biotinylated antibody and phycoerythrin-streptavidin together at a molar ratio of 1:1 for 2 min, followed by the addition of soluble biotin to halt oligomerization. These complexes, which are stable for at least 4 h, exhibit fluorescence with cell binding which approximates that observed with standard two-step labeling techniques. Equally important, multiple biotinylated antibodies complexed with different streptavidin-fluorochromes can be used simultaneously.

An antibody from a patient with ranitidine-induced thrombocytopenia recognizes a site on glycoprotein IX that is a favored target for drug-induced antibodies

Although thrombocytopenia associated with the use of histamine H2 receptor (H2R) antagonists has been described, a drug-dependent, platelet-reactive antibody has not previously been identified in such cases. We studied serum from a patient who developed acute, severe thrombocytopenia after exposure to the H2 receptor antagonist, ranitidine, and identified an antibody that reacted with normal platelets in the presence of this drug at pharmacologic concentrations. In flow cytometric and immunoprecipitation studies, the antibody was shown to be specific for the glycoprotein Ib/IX complex (GPIb/IX). From the pattern of monoclonal antibody (MoAb) inhibition and the reactions of antibody with Chinese hamster ovary (CHO) cells transfected with GPIX and GPIbbeta, we found that the patient's antibody is specific for an epitope on GPIX close to, or identical with a site recognized by the MoAb SZ1 that is a common target for antibodies induced by quinine and quinidine, drugs structurally unrelated to ranitidine. These findings provide evidence that immune thrombocytopenia can be caused by sensitivity to an H2 R antagonist and suggest that the SZ1 binding site on GPIX may be a common target for drug-induced antibodies. Further studies of the epitope for which SZ1 is specific may provide clues to the mechanism(s) by which drugs promote tight binding of antibody to a membrane glycoprotein and cause platelet destruction in patients with drug sensitivity.

Quinine-dependent antibodies bind a restricted set of epitopes on the glycoprotein Ib-IX complex: characterization of the epitopes

Severe immune thrombocytopenia is an idiosyncratic complication of quinine therapy. Although in most cases the responsible antibody is directed against platelet membrane glycoprotein (GP) Ib-IX, specificity for GPIIb-IIIa or both epitopes has also been reported. The objective of this study was to characterize the binding site of GPIb-IX-specific quinine-dependent antibodies. Antibody binding to Chinese hamster ovary cells or mouse L cells stably transfected with various combinations of the three genes (Ibalpha, Ibbeta, or IX) that encode this complex was detected using flow cytometry, monoclonal antibody-specific immobilization of platelet antigens assay, and differential adsorption studies. IgG in sera from 15 patients with quinine-induced thrombocytopenia binding to the cells, in the presence of quinine, showed three distinct patterns. Group 1 sera contained at least two antibody populations, one which binds to GPIbalpha and another which recognizes GPIX. Group 2 sera contained an antibody which binds drug dependently to GPIX, and Group 3 sera contained an antibody which recognizes a quinine-dependent epitope on GPIbalpha. Thus, the quinine-dependent antibodies fall into two distinct populations that bind to GPIbalpha and GPIX independently. Using proteases which cleave GPIbalpha at specific sites, we have shown that the GPIbalpha-specific antibody binds to an 11-amino acid (283 to 293) region. Peptide inhibition studies provide confirmatory evidence that this region contains the epitope for the GPIbalpha-specific quinine-dependent antibody.

Anti-glycoprotein Ib causes platelet aggregation: different effects of blocking glycoprotein Ib and glycoprotein IIb/IIIa in the high shear filterometer

In 1987 we reported that when blood was forced through a fine filter under pressure in the filterometer the platelets aggregated and blocked the filter. von Willebrand factor (vWF) and glycoprotein (Gp) IIb/IIIa and calcium were involved. Results with anti-GpIb were equivocal. We now report that all the anti-GpIb antibodies studied, glycocalicin, as well as some concentrations of aurin tricarboxylic acid caused platelet aggregation in the pre-filter blood and therefore could not be used in the filterometer. Using two different molecules that prevent vWF binding to GpIb and two anti-GpIIb/IIIa antibodies at two pressures it has now been shown that GpIb, vWf and high shear are primarily responsible for platelet retention at 0-5 s. Progressive platelet retention studied between 20 and 40 s required high shear and GpIIb/IIIa after the calcium influx mediated by GpIb/vWF binding. When GpIb was inhibited, GpIIb/Ila could not function normally, so GpIb inhibition resulted in decreased aggregation both at 0-5 s and at 20-40 s. Anti-GpIIlb/IIIa caused a minimal decrease in retention at 0-5 s and marked inhibition at 20-40 s. These findings fit and amplify concepts derived from other high shear methodologies. A diagram is presented of the events leading up to the final 'passivation' of the 'thrombus' in the filter when the surface of the aggregated platelets becomes unattractive.

Elevated numbers of reticulated platelets in hyperthyroidism: direct evidence for an increase of thrombopoiesis

We studied thrombopoietic activity in hyperthyroidism by determination of reticulated platelet counts. At the time of hyperthyroidism 14/15 patients had higher reticulated platelets than after achievement of euthyroidism (P<0.001). There was no difference in peripheral platelet counts and mean platelet volumes at the time of hyperthyroidism when compared to euthyroidism. Three patients had pan- and auto-reactive platelet antibodies during hyperthyroidism. These antibodies were directed against GPIIb/IIIa in two patients and against GPIb/IX in one patient. Our findings provide direct evidence that hyperthyroidism is associated with increased platelet production, as reflected by an increase in reticulated platelets.

Platelet antibodies in patients with migraine

The reported decrease of platelet serotonin receptors in patients with migraine could be due to an autoimmune reaction. We therefore, examined sera from 42 migraineurs without aura, 26 migraineurs with aura, and 107 headache-free blood donors for platelet-reactive antibodies using the platelet adhesion immunofluorescence test, the NIH-lymphocytotoxicity test, and the monoclonal antibody-specific immobilization of platelet antigens test. IgG antibodies against non-HLA class I platelet antigens were found in 9.5% of patients with migraine without aura, 7.6% of patients with migraine with aura, and in 7.5% of controls; IgM antibodies were found in 11.9% of patients with migraine without aura, in 30.8% of patients with migraine with aura, and in 13.1% of controls. Most antibodies were directed against glycoprotein complexes II-III (fibrinogen receptor) or Ib-IX (thrombin receptor). Two patients with migraine without aura but no patient with migraine with aura nor any control subject had IgG antibodies of unknown specificity. One patient (2.4%) with migraine without aura and two patients (7.7%) with migraine with aura, as well as 2 controls (1.9%) had IgM antibodies not further specified. The differences in frequency of platelet antibodies of antibodies of known or unknown specificity in patients with migraine without aura and migraine with aura and controls were not statistically significant. Therefore, our data do not support the hypothesis of a pathophysiologically relevant autoimmune reaction against platelet serotonin receptors in th majority of patients with migraine. We can not exclude the occurrence of antibodies against neuron-specific serotonin receptors.

Antibodies against platelet glycoproteins and antiphospholipid antibodies in autoimmune thrombocytopenia

Autoantibodies against platelet glycoproteins (anti-GP) are found in the majority of patients with autoimmune thrombocytopenia (AITP) as well as in thrombocytopenia associated with systemic lupus erythematosus (SLE). Some of these patients may have anti-phospholipid antibodies (anti-PL). To evaluate the pathogenetic significance of anti-PL and anti-GP antibodies in AITP and SLE patients, we investigated anti-cardiolipin (anti-CL), anti-phosphatidylserine (anti-PS) and anti-GP antibodies (anti-GPIIb-IIIa and anti-GPIb-IX) in 71 patients with AITP and 3 thrombocytopenic patients with SLE. Anti-GP antibodies were detected in 52 (70%) patients. Fifty-six (73%) patients showed anti-PL antibodies. Seven patients (6 AITP, 1 SLE) with both anti-GPIIb-IIIa and IgG anti-PL antibodies were followed during treatment with corticosteroids. Antibodies were measured before treatment and at the time of platelet-peak. Anti-GPIIb-IIIa antibodies decreased in all or became undetectable in five. In contrast, IgG anti-PS and IgG anti-CL antibodies decreased only moderately or remained positive. Adsorption experiments, using gelfiltered platelets, erythrocyte (Ec)-inside-out-vesicles and purified GPIIb-IIIa, showed that anti-GP and anti-PL antibodies have distinct specificities and do not crossreact. We conclude that anti-PL and anti-GP antibodies may be present simultaneously in some patients with immune mediated thrombocytopenia. Although anti-PS as well as anti-CL antibodies may be responsible for thrombocytopenia, we speculate that anti-GPIIb-IIIa antibodies are more related to the severity of thrombocytopenia.

[Immunization against a new, infrequent alloantigen (Iy) on the platelet glycoprotein Ib/IX as a cause of a serious case of neonatal alloimmune thrombocytopenia]

Neonatal alloimmune thrombocytopenia is the consequence of maternal alloimmunization against platelet-specific alloantigens, usually PIA1 or Br(a). The clinical picture is characterized by signs of haemorrhagic diathesis as a result of marked thrombocytopenia. In the last years, rare cases of immunization against 'low-frequency' or 'private' platelet alloantigens on the platelet glycoprotein (GP) complex IIb/IIIa have been found. This is the first report on NAIT due to maternal immunization against a low-frequency platelet alloantigen ('Iy') localized on platelet GP Ib/IX.

[Anti Ko(a) as a cause of neonatal alloimmune thrombocytopenia]

Neonatal alloimmune thrombocytopenia (NAIT) is caused by maternal immunization against a paternal antigen on fetal platelets. The antigens mostly involved are Zwa (PlA1) and Br(a). The Ko alloantigen system which is localized on the N-terminal region of glycoprotein Ib usually causes antibody formation in polytransfused patients but is only rarely implicated in NAIT. Ko(a) antibodies raised in the mother of a thrombocytopenic newborn were shown to be detectable by platelet agglutination and MAIPA assay if the antigen-bearing N-terminal glycocalicin is protected from proteolytic degradation.

Characterization of the initial alpha-thrombin interaction with glycoprotein Ib alpha in relation to platelet activation

We have evaluated the properties of alpha-thrombin interaction with platelets within 1 min from exposure to the agonist, a time frame during which most induced activation responses are initiated and completed. Binding at 37 degrees C was rapidly reversible and completely blocked by a monoclonal antibody, LJ-Ib10, previously shown to be directed against the alpha-thrombin interaction site on glycoprotein (GP) Ib alpha. By 2-5 min, however, binding was no longer fully reversible and was only partially inhibited by the anti-GP Ib alpha antibody. Results were similar at room temperature (22-25 degrees C), whereas the initial characteristics of alpha-thrombin interaction with platelets were preserved for at least 20 min at 4 degrees C. Equilibrium binding isotherms obtained at the latter temperature were compatible with a two-site model, but the component ascribed to GP Ib alpha, completely inhibited by LJ-Ib10, had "moderate" affinity (kd on the order of 10(-8) M) and relatively high capacity, rather than "high" affinity (kd on the order of 10(-10) M) and low capacity as currently thought. The parameters of alpha-thrombin binding to intact GP Ib alpha on platelets at 4 degrees C corresponded closely to those measured with isolated GP Ib alpha fragments regardless of temperature. Blocking the alpha-thrombin-GP Ib alpha interaction caused partial inhibition of ATP release and prevented the association with platelets of measurable proteolytic activity. These results support the concept that GP Ib alpha contributes to the thrombogenic potential of alpha-thrombin.

Fibrillin containing elastic microfibrils support platelet adhesion under dynamic shear conditions

The vascular subendothelium contains macromolecular structures called microfibrils. Type VI collagen is one protein found in microfibrils that supports platelet adhesion and aggregation and we have previously evaluated the roles of platelet receptors and vWf involved in these processes under physiological shear conditions. Here we investigate the ability of fibrillin containing elastic microfibrils to support mural thrombus formation. Our results show that elastic microfibril surfaces support platelet adhesion under low shear conditions at a level similar to collagen VI tetramers. However, the degree of aggregation on the elastic microfibril surface is much higher. Both adhesion and aggregation were shown to be mediated by the GPIIb-IIIa platelet receptor. Elastic microfibrils do not support the formation of mural thrombi under high shear conditions. These results suggest roles for both collagen VI and fibrillin containing elastic microfibrils in modulating the platelet response to blood vessel injury.