[Study of the clonal characteristics of autoantibodies in chronic idiopathic thrombocytopenic purpura]

OBJECTIVE

To define the clonality characteristics of autoantibodies in chronic idiopathic thrombocytopenic purpura (ITP).

METHODS

The heavy-chain and light-chain phenotype composition of the glycoprotein (GP)-specific IgG antibodies were analysed with a modified monoclonal antibody specific immobilization of platelet antigens (MAIPA) technique. The immunoglobulin heavy-chain gene rearrangement was analysed by PCR amplification.

RESULTS

16 out of the 43 patient sera reacted with least one of the following five GPs, namely GPIIb/IIIa, GPIb, GPIa, GPIV and GPV. Eight of 11 (73%) GP-specific antibodies displayed a restricted heavy-chain phenotypes. 80% (16/20) of the GP-specific antibodies showed a restricted kappa or lambda light-chain phenotype. Moreover, in 6 patients the GP-specific antibodies were found to be both light-chain and heavy-chain restricte. Using PCR amplification of immunoglobulin heavy-chain genes, 3 patients displayed heavy-chain genes rearrangement.

CONCLUSION

The GP-specific autoantibodies are derived from a restricted number of B-cell clones in proportion of ITP patients.

Drug-induced thrombocytopenia: localization of the binding site of GPIX-specific quinine-dependent antibodies

Immune thrombocytopenia is a common complication of therapy with a large number of drugs. The most widely studied drug-induced immune thrombocytopenia (DIT) is caused by quinine. In most cases of DIT, antibodies bind to the platelet membrane glycoprotein (GP) Ib-IX complex in a drug-dependent fashion and bring about increased platelet clearance by the reticuloendothelial system resulting in thrombocytopenia. Here, we report the characterization of the quinine-dependent antibody activity of sera from 13 patients with quinine-induced thrombocytopenia. In our series of patients, GPIX was the most prevalent target of quinine-dependent antibodies. To identify the structural determinants of GPIX recognized by quinine-dependent antibodies, 4 chimeric mouse/human GPIX constructs and stable Chinese hamster ovary (CHO) cell lines that expressed the chimeras in association with GPIbalpha and GPIbbeta were produced. The analysis of 6 patient sera with the chimeric cell lines provided evidence for localization of the anti-GPIX quinine-dependent antibody binding site to the C-ext region (amino acid [aa] 64-135) of human GPIX. Further characterization of the C-ext region of the GPIX indicated that replacement of the Arg110 and Gln115 of the human GPIX with the corresponding residues from mouse (Gln and Glu, respectively) resulted in a significant reduction in the binding of GPIX antibodies in our series of patients, with Arg110Gln, giving a more pronounced effect than Gln115Glu. Hence, these 2 residues, particularly Arg110, play an important role in the structure of the antigenic site on GPIX recognized by anti-GPIX antibodies.

[Study on autoimmune thrombocytopenic purpura related antibody]

OBJECTIVE

To investigate a specific and sensitive assay for the diagnosis of autoimmune thrombocytopenic purpura (AITP).

METHODS

Glycoprotein specific autoantibodies in platelet eluate and plasma were detected by a modified monoclonal antibody immobilization of platelet antigens assay (MAIPA).

RESULTS

The overall positive rate of specific autoantibodies against platelet GPIIb/IIIa and GPIb/IX in plasma was 38.89% and in eluated platelet membrane was 68.52%. The difference between them was significant (corrected chi(2) = 19.39, P < 0.005). The proportion of positive MAIPA results between primary AITP and secondary AITP was not significantly different. There was a significant inverse correlation between antibody level and platelet count.

CONCLUSION

Detection of eluated GP-specific autoantibodies by MAIPA is highly specific and much more sensitive as compared with the measurement of their plasma counterparts in the diagnosing and therapeutic monitoring of AITP.

Immune thrombocytopenic purpura (ITP) plasma and purified ITP monoclonal autoantibodies inhibit megakaryocytopoiesis in vitro

To determine if megakaryocytes are targeted by immune thrombocytopenic purpura (ITP) autoantibodies, as are platelets, we have studied the effects of ITP plasma on in vitro megakaryocytopoiesis. Umbilical cord blood mononuclear cells were incubated in the presence of thrombopoietin and 10% plasma from either ITP patients (n = 53) or healthy donors. The yield of megakaryocytic cells, as determined by flow cytometry, was significantly reduced in the presence of ITP plasma containing antiplatelet glycoprotein Ib (GPIb) autoantibodies (P <.001) as compared with both the control and patient plasma with no detectable anti-GPIIb/IIIa or anti-GPIb autoantibodies. Platelet absorption of anti-GPIb autoantibodies in ITP plasmas resulted in double the megakaryocyte production of the same plasmas without absorption, whereas platelet absorption of control plasma had no effect on megakaryocyte yield. Furthermore, 2 human monoclonal autoantibodies isolated from ITP patients, 2E7, specific for human platelet glycoprotein IIb heavy chain, and 5E5, specific for a neoantigen on glycoprotein IIIa expressed on activated platelets, had significant inhibitory effects on in vitro megakaryocytopoiesis (P <.001). Taken together, these data indicate that autoantibodies against either platelet GPIb or platelet GPIIb/IIIa in ITP plasma not only are involved in platelet destruction, but may also contribute to the inhibition of platelet production.

Role of antiidiotypic antibodies on the clinical course of idiopathic thrombocytopenic purpura

The severity and clinical course of idiopathic thrombocytopenic purpura (ITP) vary from patient to patient. The factors responsible for this variation are not well understood. In this study we attempted to evaluate the role of antiidiotypic antibodies in the immunoregulation of the disease. We investigated 114 cases of chronic ITP in adults. We determined antiidiotypic antibodies against antiplatelet antibodies using (a) idiotype-binding enzyme-linked immunosorbent assay (ELISA), (b) paratope-blocking ELISA, and (c) Western blotting. Results indicated that 80.6%, 11.2%, and 8.3% of the patients, respectively, presented with antiidiotypes against antibodies to GPIIb/IIIa, GPIb/IX, and both GPIIb/IIIa and GPIb/IX. More than 70% of the patients who showed high levels of blocking of antiidiotypic antibodies went into complete remission, compared with less than 5% of patients who showed low levels of such antibodies (P <.01). Disease severity was also found to be inversely related (P < 0.01) to the degree of blocking of antiidiotypic antibodies. The results of this study suggest that antiidiotypic antibodies against antiplatelet antibodies are a potential prognostic marker in chronic ITP.

Specific autoantibodies to platelet glycoproteins in Epstein-Barr virus-associated immune thrombocytopenia

In this study, we detected autoantibodies to platelet glycoproteins GPIIb/IIIa and GPIb/IX on platelets and in plasma in a patient with immune thrombocytopenia associated with Epstein-Barr virus-related infectious mononucleosis. In addition, we present our findings on the effectiveness of intravenous immunoglobulin therapy for immune thrombocytopenia associated with Epstein-Barr virus.

Involvement of a CD47-dependent pathway in platelet adhesion on inflamed vascular endothelium under flow

Resting platelet adhesion to inflammatory vascular endothelium is thought to play a causal role in secondary thrombus formation or microcirculatory disturbance after vessel occlusion. However, though adhesion receptors involved in platelet-matrix interactions have been extensively studied, the molecular mechanisms involved in platelet-endothelium interactions are incompletely characterized and have been mainly studied under static conditions. Using human platelets or platelets from wild-type and CD47-/- mice in whole blood, we demonstrated that at low shear rate, CD47 expressed on human and mouse platelets significantly contributes to platelet adhesion on tumor necrosis factor-alpha (TNF-alpha)-stimulated vascular endothelial cells. Using the CD47 agonist peptide 4N1K and blocking monoclonal antibodies (mAbs), we showed that CD47 binds the cell-binding domain (CBD) of endothelial thrombospondin-1 (TSP-1), inducing activation of the platelet alphaIIbbeta3 integrin that in turn becomes able to link the endothelial receptors intercellular adhesion molecule 1 (ICAM-1) and alphavbeta3. Platelet CD36 and GPIbalpha are also involved because platelet incubation with blocking mAbs directed against each of these 2 receptors significantly decreased platelet arrest. Given that anti-CD47 treatment of platelets did not further decrease the adhesion of anti-CD36-treated platelets and CD36 is a TSP-1 receptor, it appears that CD36/TSP-1 interaction could trigger the CD47-dependent pathway. Overall, CD47 antagonists may be potentially useful to inhibit platelet adhesion on inflamed endothelium.

Idiopathic autoimmune thrombocytopenia: evidence for redistribution of platelet antibodies into the circulation after immunoadsorption treatment

Platelet antibodies are detectable in only about 50% of patients with chronic autoimmune thrombocytopenia (AITP). We determined platelet antibodies against GPIa/IIa, GPIb/IX, GPIIb/IIIa, and GPV and reticulated platelets in three female patients with AITP, before and after immunoadsorption treatment. None of the three patients' sera contained platelet antibodies prior to treatment. Thereafter, anti-GPIIb/IIIa, anti-GPIb/IX (n = 3), and anti-GPV (n = 1) were detectable in the patients' sera. These antibody specificities were also found in the eluates from the immunoadsorption columns. Only one patient had elevated levels of reticulated platelets. Immunoadsorption treatment did not induce a sustained increase of platelet counts in any patient. Immunoadsorption treatment in AITP can induce redistribution of antibodies into the circulation.

Both splenic CD5(+) B and CD5(-) B cells produce platelet glycoprotein-specific autoantibodies in chronic ITP

The objective of the present study is to determine splenic B-cell subsets and the ability of splenic CD5(+) B and CD5(-) B cells to produce platelet glycoprotein-specific autoantibodies in chronic idiopathic thrombocytopenic purpura (ITP). Splenic CD5(+) B cells were identified by two-color flow cytometric analysis in eight ITP patients. Magnetic activated cell sorting (MACS) purified splenic CD5(+) B cells and CD5(-) B cells were cultured separately in vitro. Glycoprotein-specific autoantibodies in culture supernatants and plasma were measured by modified monoclonal antibody immobilization of platelet antigen (MAIPA) assay. The percentage of splenic CD5(+) B cells in ITP patients was slightly higher than that in controls, with no statistical significance. Four ITP patients displayed plasma IgG autoantibodies against both GPIIb/IIIa and GPIb. Moreover, splenic CD5(+) B cells and CD5(-) B cells from these four ITP patients also produced high level of IgG anti-GPII(b)/III(a) and anti-GPI(b) antibodies. However, we were unable to detect IgM GP-specific autoantibodies in culture supernatant and plasma in these ITP patients. It is concluded that both splenic CD5(+) B cells and CD5(-) B cells produce platelet IgG GP-specific autoantibodies, and may all play a role in the pathogenic process of ITP.

Diclofenac-induced antibodies against RBCs and platelets: two case reports and a concise review

BACKGROUND

Diclofenac has frequently been implicated as the cause of immune hemolytic anemias and less frequently of immune thrombocytopenia. The presence of the causative antibodies has only been demonstrated in patients with immune hemolytic anemia, but not yet in patients with thrombocytopenia. The cases of two patients in whom diclofenac simultaneously induced antibodies against platelets and RBCs are reported.

STUDY DESIGN AND METHODS

The investigation was carried out with standard serologic tests for detection of antibodies against platelets and RBCs. The patients' sera were tested in the presence and absence of diclofenac and its metabolites.

RESULTS

One of the two patients developed severe hemolysis and significant thrombocytopenic purpura. The other patient developed significant thrombocytopenia but no hemolysis. Both patients had a positive DAT and drug- and/or metabolite-dependent antibodies against RBCs and platelets.

CONCLUSION

Based on our findings and those of other investigators, we believe that diclofenac leads to the production of antibodies against RBCs and/or platelets.

Cytoplasmic truncation of glycoprotein Ib alpha weakens its interaction with von Willebrand factor and impairs cell adhesion

The interaction of the platelet glycoprotein (GP) Ib-IX-V complex with von Willebrand factor (VWF) is a critical step in the adhesion of platelets to the subendothelial matrix following endothelial cell damage, particularly under arterial flow conditions. In the human GP Ib-IX-V complex, the recognition of VWF appears to be mediated entirely by GP Ibalpha, the largest of four GP Ib-IX-V polypeptides. The goal of the present study was to investigate the involvement of the cytoplasmic domain of GP Ibalpha in the GP Ib-IX-VWF interaction under both static conditions and in the presence of high fluid shear stress. Using Chinese hamster ovary (CHO) cells that express GP Ibbeta, GP IX, and either wild-type GP Ibalpha or GP Ibalpha mutants missing various lengths of the cytoplasmic domain, we evaluated adhesion and flow-driven cell rolling on immobilized VWF in a parallel-plate flow chamber. Cells expressing GP Ibalpha polypeptides with truncations of 6-82 amino acids rolled faster than cells expressing wild-type GP Ibalpha. Cells that expressed polypeptides with intact actin-binding protein 280 binding sites (truncated to residue 582 of 610) rolled more slowly than those expressing GP Ibalpha with longer truncations. The rolling velocity of cells expressing truncated GP Ibalpha mutants increased with decreasing VWF coating density. In addition, a fraction of the truncated cells exhibited saltatory translocation at the lower VWF densities. Studies measuring the GP Ibalpha-VWF bond strength of three of the mutants using laser tweezers showed that progressive deletion of the cytoplasmic domain led to progressive weakening of the strength of individual GP Ibalpha-VWF bonds.

Effect of infusing rat monoclonal antibodies to the murine GPIb-IX-V complex on platelet and megakaryocyte morphology in mice

In the Bernard-Soulier syndrome, the absence of GPIb-IX-V leads to thrombocytopenia and giant platelets. In autoimmune thrombocytopenia in man, anti-platelet antibodies are associated with changes in megakaryocyte (MK) morphology and platelet size heterogeneity. We have compared the ultrastructural changes in mature MK following the infusion of rat monoclonal antibodies (MoAbs) to different epitopes of the murine GPIb-IX-V complex in mice. Blood and marrow samples were examined during both the acute thrombocytopenic phase and during the recovery phase. A MoAb to GPV induced neither thrombocytopenia nor changes in platelet morphology. During the acute thrombocytopenic phase with anti-GPIbalpha MoAbs, the size of residual platelets was heterogeneous and included large forms and platelets with few granules. During recovery, platelet size heterogeneity continued, and some platelets showed signs of activation. But only rare platelets were giant forms with ultrastructural defects resembling BSS. Megakaryocytopoiesis during acute thrombocytopenia was already abnormal, with some mature cells often showing vacuoles and an irregular development of the demarcation membrane system which varied in extent. These changes continued into the recovery phase. The anti-GPV MoAb had no effect on MK. Thus, anti-platelet antibodies can induce a different medullary response even when binding to the same receptor.

Generation and characterization of recombinant single chain Fv antibody that recognizes platelet glycoprotein Ibalpha

A recombinant single chain Fv (scFv) fragment with specific activity against platelet glycoprotein (GP) Ibalpha was developed and characterized. The scFv was generated from the SZ-2 hybridoma, which produced an anti-platelet antibody reactive to GPIbalpha. VH and VL gene segments were generated from the SZ-2 hybridoma by reverse transcribed-polymerase chain reaction (RT-PCR). After cloning into pUCm-T vector, the DNA sequences of both VH and VL genes were analyzed from two different clones, respectively, the same results were obtained. Comparison of SZ-2 variable region to the Kabat database showed that VH belonged to the mouse Ig heavy family XV while VL belonged to the mouse Ig kappa family XXVI. For assembly of the SZ-2 scFv, VH and VL fragments were cloned into pSW1-scFv successively. The scFv was arranged in VH-VL orientation, being joined together with a 15-amino-acid (Gly(4)Ser)(3) linker. The scFv encoding sequence was amplified and cloned into pET22b vector in-frame with a pel B leader sequence to direct secretion of the protein. Escherichia coli strain BL-21(DE3)PlysS was transformed with the recombinant plasmid, and expression of the scFv was induced using isopropyl-beta-D-thiogalactopyranoside (IPTG). Sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) analysis of the recombinant antibody revealed a protein with apparent molecular weight of approximately 31,000. By comparing band intensity on a Coomassie brilliant blue-stained SDS-PAGE, the production yield of SZ-2 scFv was about 25% of the total cellular proteins. The recombinant SZ-2 scFv antibody was successfully purified using Ni-NTA affinity chromatography with a yield of 120 mg/l. The SZ-2 scFv antibody could bind to platelets demonstrated by enzyme-linked immunosorbent assay (ELISA) and flow cytometry. Analyzed by Western blot, it could bind to platelet GPIb. It retained the binding capacity of its parental SZ-2 monoclonal antibody (MoAb). In functional studies, SZ-2 scFv inhibited platelet agglutination and aggregation induced by ristocetin and thrombin, respectively, but had no effect on ADP-induced platelet aggregation. Therefore, SZ-2 scFv has the potential to be used as an antithrombotic agent.

[Construction and expression of mouse-human chimeric Fab fragment gene of monoclonal antibody SZ-2 against platelet]

AIM

To reduce immunogenicity of a monoclonal antibody SZ-2 specific for human platelet.

METHODS

Reverse transcription and polymerize chain reaction were used to amplify the variable region genes of monoclonal antibody SZ-2. The cloned V(H) and V(L) genes were sequenced and fused to human IgG1 constant region gene CH1 and Ckappa in plasmid pSW1. The recombinant plasmid were transformed into E. coli. The expressed recombinant proteins were analysed.

RESULTS

The V(H) and V(L) genes were homologous with the published gene sequences of mouse antibody variable region. The concentration of chimeric Fab fragment in expression supernatant was about 180 microg/L detected by ELISA. Western blot analysis showed that SZ-2 Fab/Hu maintained the binding activity to human platelet GPIb. The recombinant proteins could suppress platelet aggregation induced by Ristocatin.

CONCLUSION

The variable region genes of SZ-2 are cloned and the mouse-human chimeric Fab fragment is expressed successfully in E. coli.

The role of the porcine von Willebrand factor: baboon platelet interactions in pulmonary xenotransplantation

BACKGROUND

Porcine von Willebrand factor (pvWF) has been shown to bind to human glycoprotein Ib (GPIb) and cause activation of human (or primate) platelets in the absence of shear stress. Pulmonary xenografts develop disseminated intravascular coagulation (DIC) and microvascular thrombosis within hours of reperfusion, and the aberrant interaction between pvWF and human platelets may be a possible cause of xenograft-associated DIC.

METHODS

Experimental baboons (n=3) received mouse anti-human GPIb monoclonal antibody before undergoing orthotopic pulmonary xenotransplantation with porcine lungs expressing human membrane cofactor protein (CD46).

RESULTS

Blocking the pvWF-GPIb interaction with a monoclonal antibody to GPIb prevented the agglutination of human and baboon platelets by pvWF in vitro. In vivo, the anti-GPIb antibody prevented platelet deposition and prevented the increases in D-Dimers (P=0.011) seen in control xenograft recipients (n=5). However, there was no difference in elevations of prothrombin times (PT) or improvement in the vasoconstriction associated with the loss of xenograft function.

CONCLUSIONS

This study indicates that the DIC associated with the hyperacute dysfunction of pulmonary xenografts is a complex phenomenon that is affected by, but not solely dependent on, activation of platelets. Aberrant interactions between pvWF and GPIb play a significant role in DIC associated with pulmonary xenotransplantation.

Shear-induced von Willebrand factor-mediated platelet surface translocation of the CD40 ligand

BACKGROUND

Platelets, which can adhere to damaged vascular surfaces and release bioactive substances upon activation, may play important roles in regulating local inflammatory responses. We focused on the surface translocation of CD40 ligand (CD40L) molecules when the platelets are exposed to a high shear stress.

METHOD

Blood specimens were obtained from eight apparently healthy adult donors. The number of CD40L molecules appearing on the surface of platelets after exposure of platelet-rich plasma to a shear rate of 10,800 s(-1) was determined by quantitative flow cytometry.

RESULTS

The number of anti-CD40L IgG molecules bound per platelet increased from 15+/-80/platelet before to 355+/-122/platelet after exposure of the platelets to a shear rate of 10,800 s(-1) (p<0.01), but not after their exposure to the relatively low shear rate of 1200 s(-1). This shear-induced platelet surface translocation of CD40L, mediated by the von Willebrand factor (VWF)-GP Ibalpha interaction, was enhanced in the presence of a low concentration of epinephrine (100 nM), which by itself, however, could not cause platelet activation. Our results demonstrate that fluid force induces the appearance of CD40L on the surface of platelets, and also that this phenomenon is enhanced in the presence of a low concentration of epinephrine, corresponding to that released by sympathetic stimulation.

Protein A immunoadsorption in a pregnant woman with habitual abortion

The role of antibodies in the occurrence of recurrent spontaneous miscarriage is well known. However there are many controversial issues in the management of habitual abortion. This paper describes the effect of Protein A immunoadsorption on serum levels of these antibodies and its impact on a case of a successfully treated woman in a outpatient department without need for a central venous catheter. Given the favourable clinical results described in our paper we think it may be relevant for some worse cases in clinical practice and will interest your readers.

Pathogenic effects of anti-glycoprotein Ib antibodies on megakaryocytes and platelets

Antibodies directed against the glycoprotein (GP) Ib have been identified as the potential cause of various platelet disorders: Immune thrombocytopenic purpura (ITP) may be caused by such autoantibodies; Anti-thrombotic drugs targeting GPIb also induce thrombocytopenia. In order to elucidate the potential mechanism(s) of the anti-GPIb effects, we have examined by electron microscopy (EM) the effect of several antibodies directed against GPIb and GPIIb-IIIa on human culture megakaryocytes (MK). Virtually all antibodies to GPIb enhanced the interaction of newly formed platelets with MK when compared to other antibodies. These effects were retrieved when antibodies were tested on platelets. We conclude that antibodies to GPIb can potentially inhibit platelet release by MK, and can also induce homotypic platelet adhesion. These results may have implications in the pathophysiology of thrombocytopenia and platelet recovery in ITP, and shed light on the pathological effect of anti-GPIb antibodies used as antithrombotic drugs.

[Participation of bonth splenic CD(5)(+) and CD(5)(-) B lymphocytes in production of platelet glycoprotein-specific autoantibodies in chronic ITP]

OBJECTIVE

To investigate the percentage of splenic CD(5)(+) B lymphocytes in chronic idiopathic thrombocytopenic purpura (IT) and the impact of splenic CD(5)(+) and CD(5)(-) B lymphocytes on the production of platelet glycoprotein (GP)-specific autoantibodies.

METHODS

Splenic CD(5)(+) B lymphocytes were identified by two-color flow cytometric analysis in eight patients. Four of the eight patients displayed plasma autoantibodies against both GPIIb/IIIa and GPIb/IX, and their splenic B lymphocytes were separated by Ficoll-Hypaque density gradient and sheep erythrocyte, and further purified by magnetic activate cell separation (MACS). Purified CD(5)(+) and CD(5)(-) B lymphocytes were cultured separately with or without staphylococcus aureus cowan I (SAC). GP specific autoantibodies in culture supernatants were measured by modified monoclonal antibody immobilization of platelet antigen assay (MAIPA).

RESULTS

The percentage of splenic CD(5)(+) B lymphocytes in ITP patients was slightly higher than that in control with no statistical significance. MACS purified splenic CD(5)(+) and CD(5)(-) B lymphocytes from three out of four ITP patients produced high levels of anti-GPIIb/IIIa and anti-GPIb/IX antibodies. Culture supernatants of CD(5)(+) B lymphocytes from the other patient showed positive reaction only in GPIb/IX MAIPA. Culture supernatant of CD(5)(-)B lymphocytes from the same patient were double positive in both GPIIb/IIIa and GPIb/IX MAIPA.

CONCLUSIONS

Both splenic CD(5)(+) and CD(5)(-) B lymphocytes produce platelet GP-specific autoantibodies in chronic ITP with similar antibody spectrum and titer, and may all play a role in the autoimmune pathogenesis of ITP.

Serum from patients with chronic idiopathic thrombocytopenic purpura frequently affect the platelet function

In most, but not all, cases of chronic idiopathic thrombocytopenic purpura (ITP), bleeding complications are known to occur when the platelet count is low. The present study investigates the effect of ITP sera on the in vitro platelet function of donor platelets. Sera from 58 ITP patients were investigated. Using an indirect monoclonal antibody specific immobilisation of platelet antigen (MAIPA) technique, GPIIb/IIIa and GPIb/IX specific antibodies were found in 23 and 20 patients, respectively. Twelve of them had antibodies against both glycoprotein (GP) complexes. The ITP sera's effect on donor platelets was investigated by aggregometry and the results were compared with the ones of 26 healthy donor sera. Grouped together, the ITP sera significantly impaired the ADP-induced platelet aggregation of donor platelets compared to the control sera; the mean relative aggregation response (T(max)) seen for the ITP and control sera were 82 +/- 21% and 92 +/- 7%, respectively (p = 0.0157). However, 6 ITP sera gave an enhanced aggregation response, whereas 17 ITP sera resulted in an impaired platelet aggregation, when using the mean +/- 2 S.D. recorded for the controls as the normal range. There was not any correlation between aggregation response, platelet number or the presence of GPIb/IX or GPIIb/IIIa specific antibodies, other than the fact that all ITP sera causing an enhanced aggregation were from patients with a platelet number less then 100 x 10(9)/l at the time of blood sampling. It is concluded that some ITP sera can either enhance or impair the platelet aggregation response, but in most cases, a normal response is obtained.

Genetic analysis of autoantibodies in idiopathic thrombocytopenic purpura reveals evidence of clonal expansion and somatic mutation

Although idiopathic thrombocytopenic purpura (ITP) is the most common autoimmune hematologic disorder, little is known about the associated autoantibodies on a molecular level. Consequently, diagnostic assays and therapy for ITP lack specificity. To avoid technical limitations imposed by B-cell immortalization methods, we used repertoire cloning (Fab/phage display) to clone platelet autoantibodies and examine the relation between immunoglobulin (Ig) gene usage, clonality, and antigen specificity. Phage display libraries were constructed from splenocytes from 2 patients with chronic ITP, and competitive cell-surface selection was used to isolate several dozen unique IgG platelet-specific autoantibodies. Platelet-reactive Fabs in both patients were associated almost exclusively with rearrangements of a single Ig heavy-chain variable-region gene (V(H)3-30), despite an apparent diversity of antigen specificities. Comparative analysis of platelet-reactive Fab Ig gene rearrangements from each patient suggested that they evolved from a restricted number of B-cell clones through somatic mutation with high replacement-to-silent mutation ratios. Although V(H)3-30-encoded heavy chains were found with light chains encoded by several different Ig genes, molecular repairing experiments showed exquisite restriction on the specific heavy- and light-chain pairings that permitted platelet reactivity. Together, these data suggest that the development of platelet-reactive antibodies associated with ITP is driven by an encounter with diverse platelet antigens through the clonal expansion of B cells using genetically restricted and highly specific combinations of heavy- and light-chain gene products. The extraordinarily high usage of the V(H)3-30 heavy-chain gene in these patients has implications for the pathogenesis, diagnosis, and management of chronic ITP.

[Study on the relations between HLA-DRB1 alleles and idiopathic thrombocytopenic purpura in children]

OBJECTIVE

To gain an insight into the relations between human leukocyte antigen-DRB1 (HLA-DRB1) alleles and idiopathic thrombocytopenic purpura (ITP) in children.

METHODS

Polymerase chain reaction-sequence specific oligonucleotide (PCR-SSO) was used to identify DRB1 alleles of 42 children with ITP. Among them, 36 were identified for anti-GPIIb/IIIa and anti-GPIb/Ix autoantibody by modified monoclonal antibody specific immobilization of platelet antigens.

RESULTS

Compared with health controls, the frequency of HLA-DRB1*17 significantly increased (P<0.05, relative risk=2.76, etiologic factor=0.1064) and the frequency of HLA-DRB1*1202 significantly decreased (P<0.025, relative risk=0.20, prophylactic factor=0.7616) in children with ITP. In comparison with patients of good response to steroids and IVIgG therapy, the frequency of HLA DRB1*11 significantly increased (Chi-square=6.091, P<0.025) in patients with a poor response, furthermore, the most of HLA-DRB1*11 positive patients were female teen-agers. Twenty-seven patients (75%) had anti GPIIb/IIIa and seventeen (47.22%) had anti_GPIb/Ix autoantibodies. The positivities of both anti_GP IIb/IIIa (P=0.02) and anti-GPIb/Ix (P=0.01) were associated with HLA-D RB1*02. However, the positivity of autoantibodies between refractory and non-refractory patients showed no significant difference.

CONCLUSION

The allele of HLA-DRB1*17 seems to predict susceptibility of ITP in children, while HLA-DRB1*1202 appears to be protective to ITP. The allele of HLA DRB1*11 plays an important role in resistance to steroid and IgG therapy in children with ITP. It seems that the response to the antigenic epitope of GPIIb/IIIa and GPIb/Ix is restricted by HLA-DRB1*02, while the presence of the antibodies could not predict prognosis. In conclusion, the above preliminary findings indicate that genetic factors influence the clinical course of ITP, but the exact mechanism needs to be investigated further.

A role for glycoprotein Ib in Streptococcus sanguis-induced platelet aggregation

Numerous studies have implicated bacteria in cardiovascular disease, but there is a paucity of information on the mechanism involved. In this study we show how the common oral bacterium Streptococcus sanguis can directly interact with platelets, resulting in activation and aggregate formation. Platelet aggregation was dependent on glycoprotein IIb/IIIa (GPIIb/IIIa) and thromboxane. Platelets could also directly bind to S sanguis, but this interaction was not inhibited by GPIIb/IIIa antagonists. Antibodies to GPIb could inhibit both platelet aggregation and platelet adhesion to bacteria. This suggested a direct interaction between GPIb and S sanguis; however, this interaction did not require von Willebrand factor, the normal ligand for GPIb. By use of a range of monoclonal antibodies to GPIb and the enzyme mocharagin, which cleaves GPIb at amino acid 282, the interaction was localized to a region within the N-terminal 1-225 portion of GPIbalpha. Furthermore S sanguis failed to induce aggregation of platelets from a patient with Bernard-Soulier disease, the organism bound to Chinese hamster ovary cells transfected with the GPIbalpha gene but did not bind to mock-transfected cells and biotin-labeled S sanguis cells bound to purified GPIb in ligand blots. It is suggested that the interaction between S sanguis and GPIb is important in the pathogenesis of infective endocarditis and may also play a contributory role in some cases of myocardial infarction.

Glycoprotein V: the predominant target antigen in gold-induced autoimmune thrombocytopenia

Autoimmune thrombocytopenia is generally caused by autoantibodies against glycoprotein (GP) IIb-IIIa or GPIb-IX and occasionally against GPIa-IIa or GPV. By investigating 38 rheumatoid arthritis (RA) patients on gold therapy, 10 with profound thrombocytopenia and 28 nonthrombocytopenic controls, we showed that in all 10 patients with thrombocytopenia, the platelet autoantibodies preferentially targeted GPV but the presence of gold was not required for their reactivity. Elevated levels of platelet-associated IgG (PAIgG) were observed in 8 of the 10 patients in whom the tests were performed. In 5 patients with sufficient autologous platelets, the GPV specificity of PAIgG was confirmed. Tests with GPV transfectants revealed that the antibodies reacted with GPV independent of GPIb alpha, GPIb beta, or GPIX. Autoantibodies recognizing GPV were not seen in the 28 nonthrombocytopenic control RA patients. Thus, GPV seems to be targeted in gold-induced autoimmune thrombocytopenia.

The cysteine knot of platelet glycoprotein Ib beta (GPIb beta) is critical for the interaction of GPIb beta with GPIX

The glycoprotein Ib (GPIb) complex is composed of GPIb alpha covalently attached to GPIb beta and noncovalently complexed with GPIX and GPV. Patients with Bernard-Soulier syndrome demonstrate that mutations in either GPIb beta or GPIX result in an absence of platelet GPIb alpha. This occurs through the interaction of GPIX with GPIb beta. The precise sites of interaction of GPIb beta with GPIX are not known. To characterize the interaction of GPIb beta and GPIX, we developed an anti-GPIb beta monoclonal antibody MBC 257.4, whose epitope was in the N-terminal region of GPIb beta. N-terminal truncations of GPIb beta were expressed in mammalian cells. N-terminal truncations of GPIb beta, missing the first 14, 26, or 31 amino acids, were surface-expressed but did not enable coexpressed GPIX to be surface expressed, suggesting that the site of interaction with GPIX was modified by these deletions. GPIb beta and GPIX chimeras corresponding to predicted boundaries were used to define the sites of interaction of GPIb beta with GPIX. Replacing the N-terminal disulfide loops of GPIb beta (amino acids 1-14) with the corresponding disulfide loops of GPIX (amino acids 1-22) resulted in surface expression of coexpressed wildtype GPIX. However, when the N terminus of GPIb beta was replaced to residue 32 with the N terminus of GPIX (amino acids 1-36), GPIX did not surface express with this chimera. These results suggest that the cysteine knot region of GPIb beta in the N terminus is critical for the conformation of GPIb beta that interacts with GPIX and further suggests that a critical interaction of GPIb beta with GPIX involve residues 15 through 32 of GPIb beta

Platelet spherocytosis: a new bleeding disorder

The present study evaluated a child with thrombocytopenia and a rare congenital bleeding disorder associated with platelet spherocytosis. Differential interference phase contrast microscopy (DIC) revealed that his platelets were spherical in form. Examination of thin sections in the electron microscope showed that his platelets were nearly devoid of microtubules (MT) and microtubule coils (MTC) that support the discoid shape in 100% of normal platelets. Immunofluorescence with a monoclonal antibody to tubulin, the precursor protein of MT, revealed bright rings in normal cells and diffuse fluorescence of patient platelets. The brightness of the fluorescence emitted by patient platelets was comparable to the diffuse fluorescence of normal platelets after chilling to dissolve intact MT, suggesting normal and patient cells contained comparable amounts of tubulin. Exposure of patient platelets to Taxol, an agent that stabilizes and induces MT, caused MT formation in 82% of patient platelets and MTC development in 11%, resulting in their conversion to discs. Glycoproteins GPIIb/IIIa and GPIb were present on patient platelets, and the cells contained normal numbers of dense bodies, ruling out storage pool disease. The patient's platelets adhered to and spread normally on glass but failed to undergo rapid, irreversible aggregation when stirred with agents that produced a complete response in normal discoid platelets, even when the patient's platelets were concentrated. The poor response of spherical platelets was associated with failure to become irregular and extend long filopodia. Thus, the spherical shape of patient platelets may contribute to the thrombocytopenia and to the clinical bleeding symptoms.

A novel polymorphism, 70Leu/Phe, disrupts a consensus Leu residue within the leucine-rich repeat sequence of platelet glycoprotein Ibalpha

Platelet glycoprotein (GP) Ib/IX/V complex mediates high-shear dependent platelet activation through an interaction with the von Willebrand factor (vWF). All four subunits of the complex have a structural motif, the leucine-rich repeat (LRR) sequence, with leucines in conserved positions. Here we report a new polymorphism, Leu/Phe at residue 70 of GPIbalpha, which disrupts the consensus sequence of the LRR in the vWF binding domain. Genotype frequencies among 142 healthy Japanese subjects were 92.3%, 7.7%, and 0.0%, for the 70Leu/Leu, 70Leu/Phe, and 75Phe/Phe genotypes, respectively. Ristocetin-induced or shear-induced platelet aggregation was not significantly different between the 70Leu/Leu and 70Leu/Phe genotypes. In in vitro studies, a recombinant GPIbalpha fragment with 70Phe (L70F) as compared to that with 70Leu (WT) had low reactivity to anti-GPIbalpha monoclonal antibodies, GUR20-5 and Hip1, both of which recognize conformation-specific epitopes within the 45-kDa domain. Ristocetin-induced 125I-vWF binding to L70F, however, did not differ from that to WT.

Alboluxin, a snake C-type lectin from Trimeresurus albolabris venom is a potent platelet agonist acting via GPIb and GPVI

Alboluxin, a potent platelet activator, was purified from Trimeresurus albolabris venom with a mass of 120 kDa non-reduced and, after reduction, subunits of 17 and 24 kDa. Alboluxin induced a tyrosine phosphorylation profile in platelets that resembles those produced by collagen and convulxin, involving the time dependent tyrosine phosphorylation of Fc receptor gamma chain (Fc gamma), phospholipase Cgamma2 (PLCgamma2), LAT and p72SYK. Antibodies against both GPIb and GPVI inhibited platelet aggregation induced by alboluxin, whereas antibodies against alpha2beta1 had no effect. Inhibition of alphaIIb beta3 reduced the aggregation response to alboluxin, as well as tyrosine phosphorylation of platelet proteins, showing that activation of alphaIIb beta3 and binding of fibrinogen are involved in alboluxin-induced platelet aggregation and it is not simply agglutination. N-terminal sequence data from the beta-subunit of alboluxin indicates that it belongs to the snake C-type lectin family. The C-type lectin subunits are larger than usual possibly due to post-translational modifications such as glycosylation. Alboluxin is a hexameric (alphabeta)3 snake C-type lectin which activates platelets via both GPIb and GPVI.

Platelet adhesion onto a polystyrene surface under static conditions: role of specific platelet receptors and effect of divalent cations

An experimental model was used to elucidate the basic mechanisms involved in the interaction of platelets with an artificial surface. The role of divalent cations and the involvement of specific platelet membrane receptors were evaluated. Isolated platelets were allowed to interact with a polystyrene surface for 20 min in the presence of divalent cations (Ca2+, Mg2+ or Zn2+), a chelating agent (ethylenediaminetetraacetic, EDTA), and specific antibodies to the main platelet receptors, glycoproteins (GP) Ib and IIb-IIIa. The degree of platelet interaction was evaluated using light and electron microscopy. Morphometric analysis was performed to follow up the progression of platelet shape changes after surface activation. Neither Ca2+ nor Mg2+ influenced the number of adherent platelets or the degree of spreading on the polymer. Only Zn2+ induced a statistically significant increase in the rate of platelet adhesion (P<0.01) with higher proportion of fully spread platelets (P<0.01). Chelation of internal pools of divalent cations did not modify the rates of platelet adhesion but prevented platelet spreading. Presence of monoclonal antibodies to GPIb and GP IIb-IIIa did not result in significant differences in the studied parameters. These results suggest that platelet adhesion onto artificial surfaces, in the absence of flow and plasma proteins, is more dependent on cellular motility, where Zn2+ could play an important role, and less dependent on major receptorial mechanisms.

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

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

Extracellular signal-regulated kinase (ERK) activation is required for GP Ibalpha-dependent endothelial cell migration

The GP Ib complex can participate in endothelial cell (EC) migration on von Willebrand factor (vWF) or the mixed matrix of vWF and type I collagen (vWF/collagen). In this study, viper venom proteins alboaggregin (albo) A or B blocked GP Ibalpha, and echistatin inhibited alphavbeta3 binding. Albo A, B and echistatin inhibited EC migration on vWF and vWF/collagen. Albo B or the anti-GP Ibalpha monoclonal antibody (mAb) 1b1 did not affect the migration of smooth muscle cells or fibroblasts, which lack GP Ib. EC also migrate on albo A- or albo B-coated dishes. PD98059, which blocks ERK activation, abolished EC migration on vWF, vWF/collagen, collagen or albo B. Soluble albo A or 1b1 dramatically inhibited ERK activation during EC migration on vWF or albo B. Echistatin inhibited ERK activation on vWF and vitronectin (VN), but not albo B. Thus, in addition to alphavbeta3, EC GP Ibalpha initiates ERK activation, and regulates ERK-induced EC migration on vWF.

Bilinexin, a snake C-type lectin from Agkistrodon bilineatus venom agglutinates platelets via GPIb and alpha2beta1

A new snake protein, named bilinexin, has been purified from Agkistrodon bilineatus venom by ion-exchange chromatography and gel filtration chromatography. Under non-reducing conditions it has a mass of 110 kDa protein on SDS-PAGE. On reduction, it can be separated into five subunits with masses in the range 13-25 kDa. The N-terminal sequences of these subunits are very similar to those of convulxin or the alboaggregins, identifying bilinexin as a new member of the snake C-type lectin family, unusual in having multiple subunits. Bilinexin agglutinates fixed platelets. washed platelets and platelet rich plasma (PRP) without obvious activation (shape change) as confirmed by light microscope examination. Both inhibitory and binding studies indicate that antibodies against alpha2beta1 inhibit not only platelet agglutination induced by bilinexin, but also bilinexin binding to platelets. VM16d, a monoclonal anti-GPIbalpha antibody, completely inhibits platelet agglutination induced by bilinexin, and polyclonal antibodies against GPIbalpha prevent its binding to platelets. However, neither convulxin, polyclonal anti-GPVI antibodies, nor GPIIb/IIIa inhibitors affect its binding to and agglutination of platelets. Bilinexin neither activates GPIIb/IIIa integrin on platelets nor induces tyrosine phosphorylation of platelet proteins, nor increases intracellular Ca2+ in platelets. Like alboaggregin B, bilinexin agglutinates platelets, which makes it a good tool to investigate the differences in mechanism between snake C-type lectins causing platelet agglutination and those that induce full activation.

A novel monoclonal antibody against the extracellular domain of GPIbbeta modulates vWF mediated platelet adhesion

GPIbbeta is disulfide-linked to GPIbalpha to form GPIb, a platelet receptor for von Willebrand factor (vWF). GPIb is in turn non covalently linked to GPIX and GPV to form the GPIb/V/IX complex. Apart from its contribution to controlling surface expression of the complex, the exact function of GPIbbeta is not well established due to a lack of suitable ligands or antibodies. The present report describes a monoclonal antibody (RAM.1) that labeled the 26 kDa GPIbbeta subunit on western blots and coprecipitated the three subunits of the GPIb/IX complex from lysates of platelets and transfected CHO and K562 cells. RAM.1 bound to GPIbbeta deleted of its intracellular domain whereas Gi27, directed against intracellular GPIbbeta, did not. Using synthetic peptides, the RAM.1 epitope was mapped to a putative cysteine loop within the COOH-terminal leucine-rich flanking region. In functional assays, RAM.1 had no effect on platelet aggregation induced by ADP, collagen or thrombin, but inhibited ristocetin induced platelet agglutination and botrocetin induced vWF binding. RAM.1 inhibited adhesion of GPIb/V/IX transfected K562 cells to a vWF matrix under flow, increased their rolling velocity and decreased the resistance of cells to detachment at high shear. This study suggests a role of GPIbbeta in modulating the adhesive properties of GPIb/V/IX and describes a useful tool to analyze the exact functions of GPIbbeta.

Cyclic immune thrombocytopenia responsive to thrombopoietic growth factor therapy

We report a patient with cyclic thrombocytopenia and antiplatelet antibodies, a variant of chronic immune thrombocytopenic purpura (ITP), with a several year history of periodic fluctuation of the platelet count, megakaryocytic hyperplasia and high-titer anti-GPIb-specific antiplatelet antibodies. The patient was resistant to multiple forms of therapy but has responded to the thrombopoietic growth factor, pegylated recombinant human megakaryocyte growth and development factor (PEG-rHuMGDF). This case suggests that some patients with classic ITP may respond to thrombopoietic growth factors.

Glycoprotein V-specific platelet-associated antibodies in thrombocytopenic patients

In autoimmune thrombocytopenia, platelet-associated IgG (PA-IgG) frequently displays specificity against glycoprotein (GP) IIbIIIa and/or GP IbIX. Because in a high proportion of patients positive PA-IgG may not be explained by these GP specificities, studies on other target proteins are needed. We studied the presence of GP V-specific PA-IgG by direct monoclonal antibody-specific immobilization of platelet antigens (MAIPA) with the monoclonal antibody SW16. We focused on 69 consecutive random patients with histories of thrombocytopenia who were strongly positive for PA-IgG detected by the direct platelet immunofluorescence test (PIFT). PA-IgG against GP V (ratio > or = 1.5) was noted in 15 (22%) patients. The degree of PA-IgG measured by PIFT, and of GP IIbIIIa-and/or GP IbIX-specific PA-IgG measured by direct MAIPA, correlated directly with the GP V-specific PA-IgG (P < 0.001). In one patient, GP V-specific antibodies were associated with quinidine-induced thrombocytopenia. Although this patient had strongly positive GP V-specific PA-IgG, she remained negative in GP IIbIIIa- and GP IbIX-specific direct MAIPA. Two patients studied because of thrombocytopenia associated with gold therapy had strongly positive GP V-specific PA-IgG. In one patient with rheumatoid arthritis and severe gold-induced thrombocytopenia, the amount of GP V-specific PA-IgG decreased during the recovery phase. Thus, GP V may represent an important target antigen in autoimmune-mediated thrombocytopenia, especially in drug-induced thrombocytopenia.

Identification and characterisation of a platelet GPIb/V/IX-like complex on human breast cancers: implications for the metastatic process

The glycoprotein (GP) Ib /V/IX receptor complex is an important adhesion molecule, originally thought to be unique to the megakaryocytic lineage. Recent evidence now indicates that GPIb /V/IX may be more widely expressed. In this study we report the presence of all subunits of the complex on four breast cancer cell lines, and 51 / 80 primary breast tumours. The surface expression of GPIb /V/IX was confirmed by flow cytometry, and by immunoprecipitation of biotin surface-labelled tumour cells. Western blotting of cell lysates under reducing conditions revealed that tumour cell-GPIb alpha had a relative molecular weight of 95 kDa as compared to 135 kDa on platelets. Despite the discrepant protein size, molecular analyses on the tumour cell-GPIb alpha subunit using RT-PCR and DNA sequencing revealed 100% sequence homology to platelet GPIb alpha. Tumour cell-GPIb /V/IX was capable of binding human von Willebrand factor (vWf), and this binding caused aggregation of tumour cells in suspension. Tumour cells bound to immobilised vWf in the presence of EDTA and demonstrated prominent filapodial extensions indicative of cytoskeletal reorganisation. Furthermore, in a modified Boyden chamber assay, prior exposure to vWf or a GPIb alpha monoclonal antibody, AK2, enhanced cell migration. The presence of a functional GPIb /V/IX-like complex in tumour cells suggests that this complex may participate in the process of haematogenous breast cancer metastasis.

A novel tetrameric venom protein, agglucetin from Agkistrodon acutus, acts as a glycoprotein Ib agonist

A novel platelet agglutination inducer, agglucetin, was purified from the Formosan Agkistrodon acutus snake venom. It migrated as a single band with an apparent molecular mass of 58.8 kDa and two distinct bands of 16.2/14.5 kDa under non-reducing and reducing conditions by SDS-PAGE, respectively. Further confirmed by FPLC, electrospray ionization mass spectrometry and 2D-PAGE, native agglucetin exists as a tetramer composed of disulfide-linked alpha1, alpha2, beta1 and beta2 subunits. Partial N-terminal sequence of agglucetin subunit showed a high degree of homology to those of C-type lectin-like glycoprotein (GP) Ib binding proteins. Functional studies showed that agglucetin. in the absence of von Willebrand factor (vWF), dose-dependently induced platelet agglutination and caused a negligible elevation of intracellular Ca+2 mobilization and thromboxane B, formation in human platelet suspensions. Anti-GP Ib monoclonal antibodies (mAbs), AP1 or LJ-Ib1, specifically inhibited agglucetin-induced platelet agglutination in a dose-dependent manner. However, EDTA, arietin (a long chain RGD-containing disintegrin), 7E3 (an anti-GP IIb/IIIa mAb), heparin, hirudin, PGE1, or indomethacin exhibited no inhibitory effect on agglucetin-induced platelet agglutination. Furthermore, flow cytometric analysis revealed that FITC-agglucetin dose-dependently bound to human formalin-fixed platelets in a saturable manner, and its binding was specifically blocked by anti-GP Ib mAb. It is concluded that agglucetin, acts specifically on an epitope of platelet membrane GP Ib overlapping with that of API, causing platelet agglutination in a Ca+2- and GP IIb/IIIa-independent manner.

Involvement of Fc gamma receptor polymorphism in the therapeutic response of idiopathic thrombocytopenic purpura

Clearance of autoantibody-sensitized platelets through Fc gamma receptors on phagocytic cells is one of the main mechanisms of thrombocytopenia in idiopathic thrombocytopenic purpura (ITP). We examined the Fc gamma RIIA-131R/H and Fc gamma RIIIA-158V/F polymorphisms in 104 adult chronic ITP patients, and in 59 healthy control subjects using polymerase chain reaction-based allele-specific restriction analysis. The frequency of Fc gamma RIIA genotypes (131H/H, H/R, R/R) was not significantly different between patients and controls, and did not correlate with the responsiveness to treatment. In contrast, among Fc gamma RIIIA genotypes, frequency of 158F/F homotype was smaller in ITP (P < 0.05). Furthermore, in Fc gamma RIIIA-158V/V homotype, the complete remission (CR) rate with medication (treatment with corticosteroid or other immunosuppressive agents) was significantly higher (60%) than that in 158V/F (10%) or 158V/F plus 158F/F, (P < 0.01, P < 0.05). Conversely, the CR rate after splenectomy in 158F/F and 158V/F types (64.3% and 54.6%) was higher than in 158V/V (25%). Our results indicate that the polymorphism of Fc gamma RIIIA, but not Fc gamma RIIA, influences the response to treatment in ITP.

[Counting platelets by flow cytometry with platelet-specific antibody CD42a]

OBJECTIVE

To investigate a reference method for platelet (PLT) count.

METHODS

76 samples of daily whole blood of high, normal and low values in patients were collected and analyzed by flow cytometry (FCM) with CD42a monoclonal antibody that could combine specifically with the platelet membrane glycoprotein Ib-Ix (GPIb-Ix). PLT counts were analyzed by multiplying PLT/RBC ratio from FCM with the RBC that was counted by a calibrated blood analyzer. Meanwhile, the results of PLT from FCM were compared with that from blood analyzers SE-9500, NE-1500, CD-1600 or manual count.

RESULTS

The within-replicate coefficient of variance(CV) of FCM was less than 4% for a high count(501 x 10(9)/L) and middle count(184 x 10(9)/L), the CV of low count(23 x 10(9)/L) was 11.432%, the between-replicate CV was 10.792%, and the overall reproducibility was less than 5%. Linearity was demonstrated well(r = 0.9975 P < 0.001) when the PLT counts were 20 x 10(9)/L-700 x 10(9)/L. PLT count was stable at room temperature within 6 hr. The carry-over was 0.87%. The results between FCM and SE-9500, NE-1500, CD-1600 or manual count was comparable (r > 0.975, P < 0.001).

CONCLUSION

These findings confirm that FCM for PLT count is specific, accurate and precise and could be considered as a candidate of reference method for PLT count.

Molecular mechanisms of drug-induced thrombocytopenia

A wide range of drugs can induce thrombocytopenia. Molecular mechanisms for the formation of specific epitopes for all the drug-dependent antibodies appear to be very similar. A restricted set of glycoproteins on the platelet surface interacts with the drugs to form neoepitopes, to which the drug-dependent antibodies bind. Molecular mapping of antigenic sites may help characterize genetic polymorphisms that predispose to the formation of the antibody binding sites. Identification of antibody binding sites will enhance our understanding of the pathogenesis of immune drug-induced thrombocytopenia.

Platelet glycoprotein V binds to collagen and participates in platelet adhesion and aggregation

Glycoprotein V (GPV) is a subunit of the platelet GPIb-V-IX receptor for von Willebrand factor and thrombin. GPV is cleaved from the platelet surface during activation by thrombin, but its role in hemostasis is still unknown. It is reported that GPV knockout mice had a decreased tendency to form arterial occluding thrombi in an intravital thrombosis model and abnormal platelet interaction with the subendothelium. In vitro, GPV-deficient platelets exhibited defective adhesion to a collagen type I-coated surface under flow or static conditions. Aggregation studies demonstrated a decreased response of the GPV-deficient platelets to collagen, reflected by an increased lag phase and reduced amplitude of aggregation. Responses to adenosine diphosphate, arachidonic acid, and the thromboxane analog U46619 were normal but were enhanced to low thrombin concentrations. The defect of GPV null platelets made them more sensitive to inhibition by the anti-GPVI monoclonal antibody (mAb) JAQ1, and this was also the case in aspirin- or apyrase-treated platelets. Moreover, an mAb (V.3) against the extracellular domain of human GPV selectively inhibited collagen-induced aggregation in human or rat platelets. V.3 injected in rats as a bolus decreased the ex vivo collagen aggregation response without affecting the platelet count. Finally, surface plasmon resonance studies demonstrated binding of recombinant soluble GPV on a collagen-coupled matrix. In conclusion, GPV binds to collagen and appears to be required for normal platelet responses to this agonist. (Blood. 2001;98:1038-1046)

Antiphospholipid antibodies and the coagulation cascade

Hemostasis is a highly controlled system of associated biophysical and biochemical events requiring a number of molecular and cellular interactions, among which molecular assembly at surfaces is an obligatory mechanism. The exposure of flowing blood to subendothelial components results in platelet adhesion, activation, and aggregation with simultaneous exposure of negatively charged phospholipids, which serves as a template for the formation of enzyme-cofactor-substrate complexes. The locally formed proteases activate surface-bound zymogens in a sequence culminating in the formation of thrombin. Fibrinogen is transformed into fibrin by thrombin, which may also activate protein C on phospholipid membranes when bound to TM. Activated protein C is a potent anticoagulant that inactivates coagulation-activated cofactors Va and VIIIa. During this process, proteins bound to the phospholipid surfaces may adopt new configurations and expose neoepitopes, which may elicit an immunologic response giving rise to the generation of antiphospholipid antibodies. These antibodies may then interfere with the procoagulant or anticoagulant activities of the target protein-phospholipid complexes. The apolipoprotein beta 2GPI and prothrombin are the most frequently found cofactors for antiphospholipid antibodies. Components of the protein C pathway have also been identified as cofactors. The pathophysiologic effects of antiphospholipid antibodies on the thrombotic accidents observed in patients with the antiphospholipid syndrome have not been established yet.

Antiphospholipid antibodies and the endothelium

The interaction between aPL (particularly anti-beta 2GPI antibodies) and endothelium does represent a potential pathogenetic mechanism for the thrombotic manifestations of the syndrome. The autoantibody-mediated EC activation probably plays a role in sustaining the appearance of a proadhesive, proinflammatory, and procoagulant phenotype. The heterogeneity of the APS clinical manifestations is likely linked to the varied effects that aPL can induce on ECs and to the different functions that ECs display depending on the anatomic localization.

Genetics of antiphospholipid syndrome

The mechanisms of thrombosis in antiphospholipid syndrome (APS) are highly heterogeneous and multifactorial, and some genetic factors may be involved in its pathophysiology. The genetic variants of representative antigen, beta 2-glycoprotein I (beta 2GPI), have been known, and valine/leucine247 polymorphism is a genetic risk for having anti beta 2GPI antibodies and APS. Congenital beta 2GPI deficiency did not correlate with thrombophilia, thus its responsible gene (beta 2GPI-Sapporo) was not a risk for thrombosis. Many other thrombosis-related genetic factors have been investigated in APS, but no additional risk for thrombosis has been indicated in patients with antiphospholipid antibodies.

Accelerated atheroma, antiphospholipid antibodies, and the antiphospholipid syndrome

Indirect data coming from animal studies and in vitro observations support the contention that the mere presence of antiphospholipid antibodies may be sufficient to increase atheroma development, regardless of other predisposing factors. It seems that humoral and cellular immune responses to beta 2-glycoprotein I can play an important role in mediating the increased propensity to atherosclerosis.

Complementary roles for fibrin(ogen), thrombospondin and vWF in mediating shear-dependent aggregation of platelets stimulated at threshold thrombin concentrations

We have evaluated the relative contribution of the adhesive ligands, von Willebrand factor (vWF), fibrinogen (Fg) and thrombospondin (TSP), all surface-expressed on washed platelets (WP) activated with a threshold thrombin concentration (approximately 0.04 U/ml), to platelet microaggregation (PA) at shear rates (G) from 300-2000 s(-1). In suspensions of thrombin-activated WP sheared immediately (tau0), all three ligands were required for optimal aggregation at all G, as shown by a 50-70% inhibition of capture efficiencies of PA (measured from initial rates of PA), by antibodies (Abs) directed against each protein. This aggregation involved both GPIb and GPIIbIIIa, as indicated by approximately 80% and 100% inhibition by Ab 6D1 and Ab 10E5, respectively. For WP preexposed to thrombin for 10 min to ensure maximal surface expression of secreted ligands and activated GPIIbIIIa (tau0), vWF was predominantly required at all G (63-75% inhibition by anti-vWF Ab), together with TSP (35-50% inhibition by anti-TSP Ab). Under these conditions, Fg was extensively converted to fibrin, so that fibrin, rather than Fg, could participate in microaggregation, with GPIb less required than GPIIbIIIa as indicated by a 30-60% inhibition by Ab 6D1 as compared to 100% inhibition by Ab 10E5. Our results show that interactions between multiple ligands and receptors favour microaggregation depending on shear and thrombin activation conditions.

Epitope mapping of inhibitory antibodies against platelet glycoprotein Ibalpha reveals interaction between the leucine-rich repeat N-terminal and C-terminal flanking domains of glycoprotein Ibalpha

The interaction of von Willebrand factor (vWF) with the platelet receptor glycoprotein Ibalpha (GPIbalpha) is important for platelet adhesion at high shear stress. Two functionally important antigenic areas within GPIbalpha were identified through the characterization of 5 new inhibitory anti-GPIb monoclonal antibodies (mAbs). The binding sites of 3 of these anti-GPIb mAbs, which were intercompeting and potently inhibiting shear stress-induced binding of vWF, were mapped within the N-terminal amino acid (aa) 1-59 area by the use of canine-human chimeras. These antibodies, however, had little or no effect (approximately 40% inhibition) on the binding of vWF induced by either botrocetin or ristocetin. On the other hand, the anti-GPIb mAbs 24G10 and 6B4, which blocked GPIb-vWF binding under all conditions examined, bound to 2 different regions of GPIbalpha, aa 1-81 and aa 201-268, respectively. The epitope for 6B4 was further narrowed by phage display revealing 2 sets of peptide sequences aligning within aa 259-262 and aa 230-242. In the latter region of GPIbalpha, the gain-of-function platelet-type von Willebrand disease (PT-vWD) mutations have been identified. Alignment was partially confirmed because the binding of 6B4 to recombinant GPIbalpha fragments carrying either one of the PT-vWD mutations was considerably impaired but not completely abolished. In contrast, mAb 24G10 bound more strongly to mutant PT-vWD GPIbalpha. However, although 24G10 competed with 6B4 for binding to platelets, it bound to an epitope within aa 1-81 of GPIbalpha. In conclusion, 2 functionally important areas within GPIbalpha were identified: one localized within the leucine-rich repeat N-terminal aa 1-59 area and one composed of residues aa 1-81 in close contact with aa 201-268. Moreover, further support is provided for the existence of an intramolecular interaction between the N-terminal flanking (aa 1-81) and C-terminal flanking (aa 201-268) regions. (Blood. 2001;98:652-660)

Origin of antiphospholipid antibodies

Our observations and those from others give further support to our hypothesis that "autoimmune aPL" may be generated by immunization with products from bacteria or viruses after incidental exposure or infection. We also were able to generate an APS-like syndrome in a strain of mice susceptible to autoimmunity, indicating that other factors such as genetic factors are likely to be involved in development of APS. Furthermore, not all aPL generated by immunization with bacterial or viral products were pathogenic. Based on the clinical experience and on the numerous reports indicating the presence of aPL in large number of infectious diseases, it may be expected that not all aPL produced during infection are pathogenic. We hypothesize that a limited number aPL induced by certain viral or bacterial products would be pathogenic in certain groups of predisposed individuals. Identification of those bacterial or viral agents may help to find strategies for the prevention of production of "pathogenic" aPL. Alternatively, free peptides may be used to induce tolerance against aPL production.

Platelet polymorphisms in thrombotic disorders

Plaque rupture and/or endothelial damage lead to exposure of von Willebrand factor (VWF) and collagen which facilitate the adhesion of circulating platelets via glycoprotein (GP) GPIb-IX-V and integrin alpha2 beta1, respectively, to the damaged vessel wall. This process activates the platelets and leads to a conformational change of a second integrin alphaIIb beta3 that facilitates fibrinogen binding and platelet aggregation. Thrombin generated at the blood-plaque interface converts fibrinogen to fibrin, which stabilizes thrombus growth. Therefore, any genetic differences that might alter surface expression or activity of these receptors could influence the risk for adverse outcome as a result of the hemostatic process. In the last five years, there has been a rapid accumulation of literature concerning the relationship between genetic variations in platelet glycoproteins and risk for coronary heart disease. In this study, we have presented a comprehensive review of the impact of platelet receptor polymorphisms and thrombotic risk.

Fc-receptor dependent platelet aggregation induced by monoclonal antibodies against platelet glycoprotein Ib or von Willebrand factor

In this paper we describe two pathways leading to platelet activation by crosslinking glycoprotein (GP) Ibalpha to the platelet Fc-receptor (FcgammaRII). First the monoclonal antibody (MoAb) 9C8, raised against human platelet GPIbalpha, dose-dependently induced platelet aggregation of citrate-anticoagulated platelet-rich plasma, an effect that can be inhibited by several activation inhibitors. The FcgammaRII-inhibitory MoAb IV.3 was able to prevent the aggregatory effects of MoAb 9C8, indicating that crosslinking of the antigen GPIbalpha to the FcgammaII-receptor is necessary for the activating effect. Secondly we observed a synergistic activating effect of two anti-von Willebrand factor (vWF) MoAbs IC1E7 and B724, both known to enhance vWF binding to GPIbalpha in the presence of shear or ristocetin. When these antibodies are added together to PRP, platelet aggregation is induced without further need for an additional modulator. This effect can be blocked by either MoAb IV.3 or an inhibitory anti-GPIb MoAb, indicating that again the platelet activation results from signaling through FcgammaRII crosslinked to vWF bound to GPIbalpha. In addition, both the anti-GPIb MoAb 9C8, or the two anti-vWF MoAbs 1C1E7 and B724 induce genuine platelet activation, as evidenced by the secretion of ATP and protein tyrosine phosphorylation. These findings with both anti-GPIb and anti-vWF MoAbs add further proof to recent reports demonstrating an interaction between the platelet receptors GPIb and FcgammaRII, suggesting a role for the FcgammaII-receptor in GPIb-related signaling.