Platelet alloantibodies in transfused patients

BACKGROUND: Patients receiving cellular blood components may form HLA antibodies and platelet-specific alloantibodies. STUDY DESIGN AND METHODS: Serum samples from a cohort of 252 patients with hematologic or oncologic diseases who are receiving cellular blood components were studied for platelet-reactive antibodies. Specificity of platelet alloantibodies was determined with a panel of typed platelets RESULTS: Platelet-reactive antibodies were detected in the sera of 113 patients (44.8% of 252), HLA antibodies in the sera of 108 (42.9%), and platelet-specific antibodies in the sera of 20 (8%). The following platelet-specific antibodies were identified: anti-HPA-5b (n = 10), anti-HPA-1b (n = 4), anti-HPA-5a (n = 2), anti-HPA-1a (n = 1), anti-HPA-2b (n = 1), anti-HPA-1b+5b (n = 1), and anti-HPA-1b+2b (n = 1). Fifteen sera from the 108 patients with anti-HLA (13.9%) contained additional platelet-specific alloantibodies, while in 5 sera, platelet-specific alloantibodies only were detected: anti-HPA-5b (n = 4) and anti-HPA-1a (n = 1). Of the 108 sera with HLA antibodies, 29 (26.9%) showed discordant results when studied with the lymphocytotoxicity test and the glycoprotein-specific immunoassay. Ten sera contained panreactive antibodies against platelet glycoproteins (GP) IIb/IIIa, GPIa/IIa, and/or GPIb/IX. Alloimmunization occurred in 58.3 percent of female patients with previous pregnancies, but in only 23.3 percent of those without previous pregnancies (p = 0.0049). CONCLUSION: Platelet alloantibody specificities in transfused patients (predominantly anti-HPA-5b and -1b with antigen frequencies <30% among whites) differ significantly from those observed in patients with neonatal alloimmune thrombocytopenia or posttransfusion purpura, in whom anti-HPA-1a (antigen frequency >95%) is the most prevalent specificity. HLA antibody detection yields discordant results when the lymphocytotoxicity assay and a glycoprotein-specific immunoglobulin-binding assay are used.     

Post-transfusion alloimmunization to granulocytes and platelets in Japanese patients as determined by the MPHA method

The current occurrence of alloimmunization to granulocytes and platelets after blood transfusion is unclear due to the fact that antibody assays are cumbersome. Using the MPHA method with extracted granulocyte and platelet antigens, a randomized, blinded trial was conducted to investigate three types of alloantibodies in 431 Japanese patients receiving leukocyte-depleted blood transfusions prepared with or without our latest leukocyte-reduction filter. The frequency of granulocyte, platelet and HLA class I alloantibodies was 0.44%, 0.44% and 16.74%, respectively, in patients receiving non-filtered products and 0%, 0% and 0.49%, respectively, in patients receiving filtered products. The granulocyte antibody reacted with an antigen approximately of 51 KDa. The platelet-specific alloantibody was associated with GPIIb/IIIa and GPIa/IIa. The important factors affecting alloimmunization were the transfusion dose and the use of unfiltered platelet products.     

Selecting donors of platelets for refractory patients on the basis of HLA antibody specificity

BACKGROUND: Patients who are refractory to platelet transfusion as a result of HLA alloimmunization are generally given HLA-matched or crossmatched platelets. However, HLA-matched platelets that are matched at HLA-A and -B loci (A-matched) or those without any mismatched or cross-reactive antigens (BU-matched) are frequently unavailable. A disadvantage of crossmatching is that crossmatched platelets have a shelf life of only 5 days, so that crossmatch tests must be performed frequently for patients requiring long-term platelet transfusions. An alternative method is the selection of platelets according to the patient's HLA antibody specificity, called the antibody specificity prediction (ASP) method. STUDY DESIGN AND METHODS: An anti-human globulin-enhanced microlymphocytotoxicity test modified by a double addition of serum and a computer program were used to determine the specificity of patients' HLA antibodies. Platelet crossmatching was performed with a solid-phase adherence assay. The percentage of platelet recovery (PPR) was determined in 1621 platelet transfusions in an observational study in 114 patients, and the PPR of platelets selected by the ASP method was compared with the PPR of those that were HLA-matched, crossmatched, or randomly selected. The numbers of potential donors in files of HLA-typed donors as identified by HLA matching vs. the ASP method were determined. RESULTS: After adjustments for covariates, the mean +/- SEM PPR was similar for HLA-matched (21 +/-4%), cross-matched (23+/-4%), and ASP-selected (24+/-3%) platelets and was significantly lower for randomly selected (15+/-1.4%) platelets. For 29 alloimmunized HLA-typed patients, the mean number of potential donors found in a file of 7247 HLA-typed donors was 6 who were an HLA-A match (median = 1), 33 who were an HLA-BU match (median = 20), and 1426 who were identified by the ASP method (median = 1365). CONCLUSION: The ASP method of donor selection for refractory alloimmunized patients appears as effective as HLA matching or crossmatching. Far more donors are identified in a file of HLA-typed donors by the ASP method than by HLA matching, and this indicates that the ASP method provides important advantages regarding the availability of compatible platelet components.     

Alloimmunization to platelet-specific antigens on glycoproteins IIb- IIIa and Ib/IX in multiply transfused thrombocytopenic patients

The rate of alloimmunization to platelet-specific antigens associated with platelet glycoproteins (GPs) IIb-IIIa and Ib/IX was studied in 293 multiply transfused thrombocytopenic patients. Antibodies to platelet-specific antigens were measured with a solid-phase assay using platelet GP IIb-IIIa or Ib/IX as the antigenic targets. Nine patients were found to have antibodies to platelet GP IIb-IIIa, and no patients had antibodies to platelet GP Ib/IX. In six of these nine patients, the specificity of the antibody was shown by using GP IIb-IIIa from donors with different platelet-specific antigen phenotypes. In the remaining three patients with antibodies to platelet GP IIb-IIIa, no specificity could be identified. These patients had autoimmune thrombocytopenia in association with lymphoma. The alloimmunization rate to platelet-specific antigens associated with GP IIb-IIIa was 2 percent, whereas the rate of alloimmunization to HLA antigens was 23 percent. Of the patients alloimmunized to HLA antigens, 9 percent also had antibodies to platelet-specific antigens. A poor response to HLA-identical platelet transfusions was observed only in those patients with positive assays in the solid-phase test. These results suggest that the incidence of antibodies to platelet-specific antigens carried on GP IIb-IIIa is low. Platelet-specific antibodies may be found more frequently in patients alloimmunized to HLA antigens than in those not so alloimmunized.     

Platelet transfusion refractoriness associated with HPA-1a (Pl(A1)) alloantibody without coexistent HLA antibodies successfully treated with antigen-negative platelet transfusions

BACKGROUND: Alloimmune-mediated refractoriness to platelet transfusion is most commonly due to antibody to HLA antigens in multiply transfused or multiparous patients. Published reports of poor transfusion response due to antibodies to platelet-specific antigens are rare and often confounded by the presence of coexistent antibodies against HLA antigens. CASE REPORT: A case is presented of a multiparous woman with acute myelogenous leukemia whose sole cause of transfusion refractoriness was antibody to platelet antigen HPA-1a. She responded dramatically to HPA-1a-negative platelet transfusion. CONCLUSION: This case provides strong serologic and clinical evidence that platelet transfusion refractoriness may result from antibodies to platelet-specific antigens.     

Blood group A antigen expression in platelets is prominently associated with glycoprotein Ib and IIb. Evidence for an A1/A2 difference

Blood group ABH antigens are associated with platelets as intrinsic determinants and extrinsically adsorbed antigens, and exist both on glycosphingolipids and on glycoproteins (GPs). We now provide direct evidence that the blood group ABH antigens are prominently associated with platelet GPIb and GPIIb. By immunoprecipitation, a murine monoclonal anti-A antibody precipitated surface-biotin-labelled blood group A₁ platelet membrane proteins with electrophoretic characteristics identical to those of GPIb/IX and GPIIb/IIIa. By immunoblotting of SDS-PAGE separated blood group A₁ platelet proteins the monoclonal anti-A antibody bound to proteins with electrophoretic characteristics identical to those of GPIb and GPIIb. When immunoaffinity purified GPIb/IX and GPIIb/IIIa, derived from blood group O, A₁ and A₂ platelets, were employed for immunoblotting, GPIb and GPIIb only from A₁ platelets bound the monoclonal anti-A antibody. By ELISA, wherein monoclonal antibodies specific for GPIb (AP1) and the GPIIb/IIIa complex (AP2) were used to capture and hold antigens from platelet lysate, human anti-A antibodies reacted with these proteins derived from blood group A₁ platelets; proteins from blood group A₂, O and B platelets showed no reactivity. These results indicate that blood group A antigen is associated with GPIb and GPIIb derived from blood group A₁ but not A₂ platelets.     

Mechanism of Transfusion-induced HLA alloimmunization

<正>Platelet transfusion therapy has allowed successful prevention and treatment of hemorrhagic complications associated with thrombocytopenia since its first use in the 1960's.Consequently, the use of platelets has increased at an exponential rate.Since platelet     

Guidelines for platelet transfusions

Recommendations for the optimal transfusion support of patients likely to receive repeated platelet transfusions. 1. Determine policy for prophylactic platelet support, and select the platelet count below which platelet transfusions will be used. 2. Consider using leucocyte depletion of red cell and platelet concentrates to prevent HLA alloimmunization from the outset. 3. Type patients for HLA-A and B antigens at an early stage. 4. Use random donor platelet concentrates for initial platelet support (either single or multiple donor, depending on availability). 5. If refractoriness occurs, determine whether clinical factors, which may be associated with non-immune consumption of platelets, are present and test the patient's serum for HLA antibodies. 6. Use HLA-matched platelet transfusions if HLA alloimmunization is the most likely cause of refractoriness. 7. If there is no improvement with HLA-matched transfusions, platelet crossmatching may identify the cause of the problem and help with the selection of compatible donors. 8. Discontinue prophylactic platelet support if a compatible donor cannot be found. Use platelet transfusions from random donors to control bleeding and increase the dose, if necessary.     

Posttransfusion purpura following bone marrow transplantation

BACKGROUND : Thrombocytopenia is a major cause of morbidity and hospital expense following bone marrow transplantation. Platelet transfusions in these patients are frequently complicated by the recipient's development of antibodies to HLA class I antigens. When these patients become refractory to the transfusion of HLA-matched platelets, the recipient's platelet antigen phenotype must be determined, to ensure that donor platelets will be phenotypically compatible. Cases of alloimmunization to HPA-1a and HPA-1b resulting in refractoriness to transfused platelets and the subsequent development of a posttransfusion purpura-like syndrome are reported. CASE REPORTS: In the first case, a 43-year-old woman with Stage IV infiltrating ductal breast cancer presented to the hospital for a transplant of autologous peripheral blood stem cells. After the transplant, her platelet count remained less than 10 × 10⁹ per L, despite daily platelet transfusions, including HLA-matched platelets. Fourteen days following the transplant, her serum was found to contain anti-HPA-1a. Initially, the patient was refractory to the transfusion of HPA-1a-negative platelets, but after treatment with intravenous immunoglobulin, she had transient increases in posttransfusion platelet counts. She was also treated with a staphylococcal protein A immunoadsorption column and has not had any such subsequent refractoriness. Her genotype has been found, by use of allele-specific oligonucleotide hybridization with white cell DNA, to be HPA-1b/1b. The second case involved a 32-year-old woman with chronic myelogenous leukemia who received an unrelated-donor marrow transplant. Three years later, her CML recurred, and she was treated with interferon-alpha. Four months afterward, she experienced interferon-alpha-induced thrombocytopenia and the interferon therapy was discontinued. She received 12 platelet transfusions in 20 days, but none was effective. Antibodies specific for HLA antigens and HPA-1b were detected, and three HLA-matched, HPA-1b-negative apheresis platelet components were given, but without effect. Two days after treatment with methylprednisolone (1 g intravenously) and prednisone (2 mg/kg/day orally), her platelet count was 26 × 10⁹ per L, and after 8 more days, it was 102 × 10⁹ per L, without further transfusions. She was found to be homozygous for HPA-1a (HPA-1a/1a). CONCLUSION : Anti-HPA- 1a and anti-HPA-1b can cause refractoriness to platelet transfusions in bone marrow transplant patients. Testing for platelet-specific antibodies should be considered in all patients who are refractory to HLA-matched platelets.     

Antibodies to private and public HLA class I epitopes in platelet recipients

BACKGROUND: Transfusions or pregnancies can cause immunization against private HLA determinants and public epitopes shared by more than one private HLA antigen. HLA antibodies are correlated with febrile transfusion reactions, lower platelet response following platelet transfusion, and an increased rate of renal transplant rejection. Until now, antibody specificities in alloantisera from platelet recipients have been poorly characterized. STUDY DESIGN AND METHODS: Consecutive serum screens from platelet recipients were analyzed for antibodies against private HLA class I antigens and public HLA epitopes using a serum analysis program based on the 2 x 2 table analysis of correlations. Serum screens of highly immunized patients and of patients with new alloimmunization events were reviewed separately. RESULTS: Of the serum screens from 566 platelet recipients, 1577 indicated alloimmunization (panel-reactive antibodies >5%). The program assigned a specificity in 1024 of these screens (64.9%) and at least once in 522 of 566 patients (92.2%). In 267 patients, antibodies detecting public epitopes in the combined A- or B-locus cross-reacting groups were found; other public markers were detected in 39 patients. Patterns of reactivity were remarkably less stable than in patient groups previously studied. In many patients, antibodies with apparent private epitope specificity preceded the identification of antibodies against a shared marker of the same cross-reactive group. However, the disappearance of antibodies (whether or not this was followed by a new antibody against a private or public marker belonging to another cross-reacting group) was also observed. CONCLUSION: The computerized analysis of microlymphocytotoxicity tests enhances the rate of antibody specification in sera from platelet recipients with lymphocytotoxic antibodies. The identified antibodies should be taken into account in the selection of platelet donors. The data confirm and extend previous observations on HLA class I antibodies and elucidate new alloimmunization events.     

血小板特异性抗体对特发性血小板减少性紫癜诊断价值的研究

本研究的目的是比较特发性血小板减少性紫癜 (ITP)、慢性再生障碍性贫血 (CAA)、恶性血液病患者及健康志愿者特异性抗体水平 ,以评价血小板特异性抗体在ITP诊断中的价值。用改良单克隆抗体特异性俘获血小板抗原 (MAIPA)技术同时检测血小板GPIb/Ⅸ、GPIIb/Ⅲa、GPⅣ、GPⅤ的特异性抗体。结果表明 :ITP组、CAA组、恶性血液病患者及健康志愿者血小板特异性抗体总阳性率分别为 6 9.99% ,10 % ,2 0 %和 0 %。ITP组与CAA组存在显著性差异 ( χ2 =2 0 .71,P <0 .0 0 5 ) ,ITP组与恶性血液肿瘤化疗组存在显著性差异 ( χ2 =12 .2 2 ,P <0 .0 0 5 )。健康志愿组无 1例阳性。结论 :多种抗体同时检测可提高敏感性 ,血小板特异性抗体对ITP是一种特异性高、敏感性强的实验室诊断指标。     

Splenic macrophages maintain the anti-platelet autoimmune response via uptake of opsonized platelets in patients with immune thrombocytopenic purpura

Summary.  Background:  Immune thrombocytopenic purpura (ITP) is an autoimmune disease primarily caused by IgG anti-platelet autoantibodies. Activation of autoreactive CD4⁺ T cells upon recognition of cryptic GPIIb/IIIa peptides presented by antigen-presenting cells (APCs) is a critical step for triggering and maintaining the pathogenic anti-platelet autoantibody response. Objectives:  We investigated which APCs carry the cryptic peptides of GPIIb/IIIa that activate autoreactive CD4⁺ T cells in ITP patients. Methods:  GPIIb/IIIa-reactive T-cell lines generated from ITP patients were cultured with autologous freshly isolated splenic macrophages, B cells or dendritic cells. To further investigate how the macrophages presented the antigenic GPIIb/IIIa peptides, we prepared macrophages from the peripheral blood monocytes of the same patients during remission. Results:  Macrophages induced the proliferation of GPIIb/IIIa-reactive T-cell lines without an exogenous antigen, but B cells and dendritic cells required GPIIb/IIIa peptides to stimulate the T cells. Macrophages derived from peripheral blood during remission required an exogenous antigen to induce the GPIIb/IIIa-reactive T-cell line response, but could elicit a response without added antigen if they were preincubated with platelets from ITP patients with platelet-associated anti-GPIIb/IIIa antibodies or healthy platelets pretreated with ITP platelet eluates. The T-cell response was inhibited by anti-FcγRI antibody. Finally, cultured macrophages that captured opsonized platelets promoted anti-GPIIb/IIIa antibody production in mixed cultures of autologous GPIIb/IIIa-reactive T-cell lines and B cells. Conclusions:  Splenic macrophages that take up opsonized platelets via FcγRI are major APCs for cryptic GPIIb/IIIa peptides, and are central to the maintenance of anti-platelet autoantibody production in ITP patients.     

Platelet-reactive HLA antibodies associated with low posttransfusion platelet increments:a comparison between the monoclonal antibody-specific immobilization of platelet antigens assay and the lymphocytotoxicity test

BACKGROUND: Platelet-reactive HLA antibodies are a major reason for low posttransfusion platelet increments. The clinical importance and value of the test systems for their in vitro determination is still controversial. STUDY DESIGN AND METHODS: A prospective analysis of HLA antibodies was performed in sera obtained once a week for at least 4 consecutive weeks from 55 patients (female/male, 28/27; age: median, 49 years; range, 18-69) undergoing intensive chemotherapy and in need of prophylactic platelet transfusions. All sera (n = 330) were analyzed by the monoclonal antibody-specific immobilization of platelet antigens (MAIPA) assay and by the standard lymphocytotoxicity test (LCT). RESULTS: In the MAIPA, 24.5 percent of sera (81/330) obtained from 22 patients contained HLA antibodies. These were detected significantly more often by the MAIPA assay than by the LCT (24.5% vs. 8.2%). Fifty-five sera (20 patients) were positive in the MAIPA assay only. In 15 patients, HLA antibodies were transient. In 3 patients, HLA antibodies were detected earlier by the MAIPA assay than by the LCT. Significantly more sera obtained at the time of low posttransfusion platelet increments were positive in MAIPA alone, rather than in both MAIPA and the LCT (44% vs. 17%). CONCLUSION: The MAIPA assay is more sensitive than the standard LCT in detecting platelet-reactive HLA antibodies. These MAIPA-positive/LCT-negative HLA antibodies affect the posttransfusion platelet increment.     

Use of enzyme treatment to enhance reactivity of HLA and platelet-specific antibodies in solid-phase RBC adherence assays

BACKGROUND: Solid-phase RBC adherence (SPRCA) assays (Immucor) detect HLA and/or platelet-specific antibodies. Pretreatment of reagent platelets with enzymes was investigated to determine whether the sensitivity of the assay could be increased. STUDY DESIGN AND METHODS: SPRCA testing, following the manufacturer's directions, was performed on 51 sera from patients with either a positive SPRCA antibody screen, suboptimal clinical responses to platelet transfusions, and/or suspected immune thrombocytopenic purpura; testing was also performed following pretreatment of the reagent platelets with bromelin, papain, or ficin. Sera from 23 patients having negative routine SPRCA antibody screens and good clinical responses to transfusion were tested as controls. Lymphocytotoxic antibody testing was also performed on selected samples. The effectiveness of enzyme treatment was judged by the increase in the proportion of reagent platelets reacting with the sample and the observed reaction strengths. RESULTS: Pretreatments of reagent platelets with all three enzymes increased the reactivity of known antibodies and detected some HLA and platelet-specific antibodies that had not reacted in routine testing. The clinical significance of the antibody specificities detected only after enzyme pretreatment was verified by a correlation with results from transfusing antigen-negative units. Only occasional false-positive results after enzyme pretreatment were observed. CONCLUSIONS: The use of enzyme pretreatment of SPRCA screening strips can provide information that is useful in selecting appropriate units for transfusion.     

Anti-platelet antibody and platelet function]

The effect of anti-platelet antibodies, including murine monoclonal antibodies, autoantibodies and alloantibodies, on platelet function was analyzed. The target antigen of these antiplatelet antibodies, investigated in the present study, was a glycoprotein IIb/IIIa, which is a receptor of fibrinogen and plays an important role in platelet aggregation. Some of these antibodies inhibited agonist-induced platelet aggregation. The target antigen of one murine monoclonal antibodies, designated OP-G2, was a glycoprotein IIb/IIIa and interestingly, this antibody induced platelet aggregation, which required divalent cation and fibrinogen. We compared the epitope of these antibodies by inhibition assay and found the epitope of these antibodies to be very close. The binding of OP-G2 to the platelets required Ca2+. These data suggest that OP-G2 recognizes an epitope at or in very close proximity to the fibrinogen binding site of GPIIb/IIIa, as compared with other antibodies.     

流式微球技术检测血小板特异性抗体

目的建立流式微球技术检测血小板特异性自身抗体方法,并对方法学及临床应用进行初步探讨。方法用包被抗血小板膜糖蛋白(GP)Ⅰb、Ⅱb、Ⅲa、Ⅱb/Ⅲa单克隆抗体的微球捕获与血小板GP结合的特异性抗体,加入FITC标记的羊抗人IgG抗体后用流式细胞仪检测分析。结果特发性血小板减少性紫癜(ITP)组4种单抗荧光强度比值与非ITP血小板减少组和正常对照组有显著性差异(P<0.01);若将ITP组患者4种单抗荧光强度比值分别大于正常对照组上限1.37、1.24、1.48和1.19判断为阳性,则流式微球技术检测血小板特异性自身抗体的敏感性为73.2%,特异性为94.3%;4种单抗联合检测总体敏感性明显高于改良间接单抗特异的血小板抗原固定试验(MAIPA)(P<0.05),且大于各单个抗体检测敏感性。结论流式微球技术可以简便、快捷地检测血小板膜糖蛋白特异性抗体,联合检测多种自身抗体可以提高检测阳性率,对于ITP的诊治具有一定的临床价值。     

HLA class I-eluted platelets as an alternative to HLA-matched platelets

BACKGROUND: Alloimmunized refractory thrombocytopenic patients often require HLA-matched platelet transfusions. As the HLA system is very polymorphic, sufficient HLA-matched donors are not available for every patient. STUDY DESIGN AND METHODS: In vitro elution techniques with citric acid incubation of platelets at pH 3.0 showed that platelets lose expression of HLA, whereas platelet-specific glycoproteins are preserved. This technique was modified for clinical use. Random-donor platelet concentrates were incubated with citric acid, subsequently washed, and transfused to two patients. RESULTS: Platelet-specific glycoproteins were unaffected, and HLA expression decreased generally to below 25 percent of the initial expression. One alloimmunized patient who was without compatible donors because of a rare HLA type underwent repeated transfusions with acid-treated platelets. In contrast to the results with random-donor platelet transfusions, posttransfusion increments up to 47 × 10(9) per L were obtained with acid-treated platelets, and profuse gastrointestinal bleeding was stopped, while multiple skin hemorrhages were resolved. No side effects were observed. A second patient developed a severe transfusion reaction without platelet increment after one transfusion with acid-treated platelets expressing 30 percent of the original HLA antigens. Further transfusions were not given. CONCLUSION: Standardization of the acid elution technique and validation of the technique in patients is necessary. The results suggest, however, that HLA-eluted platelets prepared under specified conditions may gain a place in platelet transfusion therapy.     

Platelet autoantibodies in immune thrombocytopenic purpura.

In summary, the search for a useful clinical laboratory diagnostic assay for the antiplatelet antibodies has been long and difficult. Measurement of platelet associated IgG (PAIgG) has been disappointing as a way to detect autoantibodies. This is primarily due to the fact that platelets normally contain IgG in their alpha granules in an amount that varies with plasma IgG levels and age of the platelets. Furthermore, the amounts of platelet associated IgG is affected by the presence of circulating immune complexes, platelet activation, and drug dependent antibodies. The newer, platelet antigen capture techniques are promising, but further testing will be needed to confirm their value to the clinician. Methods that allow incubation of patient serum or plasma with intact platelets (MAIPA and immunobead) have greater sensitivity than techniques in which the patient antibody is tested against previously isolated platelet glycoproteins. These assays are currently available in a only a limited number of platelet immunology laboratories. Platelet autoantibodies are directed against a number of glycoprotein antigens on the platelet surface. Most studies have shown that anti GPIIb/IIIa antibodies are the most common, although antibodies against GPIb/IX and other targets are frequently detected. Many patients have multiple antiplatelet antibodies circulating simultaneously. The clinical significance of antibodies with different specificity is under investigation. The precise epitopes on GPIIIa that bind antiplatelet autoantibodies have been studied to a limited extent. Some investigators report that the vast majority of platelet antigens are conformation dependent, being destroyed by treatment with EDTA (separation of GPIIb and GPIIIa) or denaturation with detergents. Others report sequence specific peptide antigens. Further investigation promises to better define the targets for platelet autoantibodies; improved clinical management of patients with ITP is the long term goal of these studies.     

血小板活化过程中膜糖蛋白Ⅱb/Ⅲa构象变化的研究

目的　对血小板活化过程中膜糖蛋白（ Ｇ Ｐ） Ⅱｂ／ Ⅲａ 构象变化进行初步探讨。方法　分别用供体荧光（ Ｆ Ｉ Ｔ Ｃ） 与受体荧光（ Ｔ Ｒ） 标记识别 Ｇ ＰⅡｂ／ Ⅲａ 上不同抗原决定簇的单抗。用流式细胞仪检测活化血小板在５３０ ｎｍ 处的荧光强度值,并计算荧光供受体间的荧光共振能量转移值（ Ｆ Ｒ Ｅ Ｔ Ｖ） 。结果　不论何种单抗作为荧光供体,静息态血小板均可被测得一低而稳定的 Ｆ Ｒ Ｅ Ｔ Ｖ（ 平均５ ．５ ％ ） 。血小板被激活时 Ｆ Ｒ Ｅ Ｔ Ｖ 会有显著升高,表明 Ｇ ＰⅡｂ／ Ⅲａ 内的亚单位间发生了位置或（ 和） 方向上的变化,该变化也可因胞外钙离子的清除而发生,但不依赖于纤维蛋白原与其受体的结合。结论　血小板活化时 Ｆ Ｒ Ｅ Ｔ Ｖ 的升高可定性反映出荧光标记单抗所结合的 Ｇ ＰⅡｂ／ Ⅲａ 内部亚单位间所发生的重新排列,这种构象的改变可最终导致纤维蛋白原受体的表达。     

Asialo von Willebrand factor interactions with platelets. Interdependence of glycoproteins Ib and IIb/IIIa for binding and aggregation.

Asialo von Willebrand factor (AS-vWf) binds to and aggregates normal human platelets in the absence of ristocetin. Maximal specific binding of AS-vWf is 1-2 micrograms vWf protein/10(8) platelets. Despite the specificity of the binding, only 60% of the bound AS-vWf can be dissociated after equilibrium has been reached. We investigated the site of binding and the mechanism of aggregation of platelets by AS-vWf by (a) pre-incubating platelets with either of two monoclonal antibodies, one against glycoprotein Ib (GPIb) or a second against the glycoprotein IIb/IIIa complex (GPIIb/IIIa), and (b) varying the concentration of fibrinogen in the medium. The results of our studies indicate that AS-vWf binds initially to GPIb. This binding then results in the exposure of receptors for AS-vWf on GPIIb/IIIa. In the presence of plasma fibrinogen, both AS-vWf and fibrinogen bind to GPIIb/IIIa. In the presence of plasma fibrinogen, 50% more AS-vWf binds to the platelet, and this additional AS-vWf binds almost exclusively to GPIIb/IIIa. Despite this enhanced binding of AS-vWf in the absence of fibrinogen, platelet aggregation is much less than that which occurs in the presence of plasma fibrinogen. Comparative studies of AS-vWf binding to normal platelets and the platelets of patients with Glanzmann's thrombasthenia reveal decreased binding to the thrombasthenic platelets and a marked decrease in the extent of platelet aggregation. These studies indicate that AS-vWf binding to, and ensuing aggregation of, platelets is different from that observed with intact vWf protein when platelets are stimulated with either ristocetin or thrombin. The AS-vWf binds to GPIb which, in turn, makes additional AS-vWf receptors available on GPIIb/IIIa. If plasma fibrinogen is present, it competes with the AS-vWf for binding to GPIIb/IIIa and causes aggregation of platelets. In the presence of plasma fibrinogen, more of the AS-vWf binds to GPIIb/IIIa, but this AS-vWf is much less effective than fibrinogen in supporting platelet aggregation.