A point mutation leads to an unpaired cysteine residue and a molecular weight polymorphism of a functional platelet beta 3 integrin subunit. The Sra alloantigen system of GPIIIa

A nait-associated and previously unreported mutation in the ITGB3 gene with a low frequency in the local population

BACKGROUND

Neonatal alloimmune thrombocytopenia is a rare but potentially severe postnatal complication caused by maternal allo-antibodies against platelet antigens of the newborn. In relatively few cases, immunisation against low-frequency antigens has been reported.

METHODS

Platelet antigens of a newborn with severe thrombocytopenia and his family members were investigated by serological and molecular biological methods. A real-time PCR assay was developed to reliably detect this mutation in pools of DNA from up to seven individuals.

RESULTS

Serological testing showed positive reactions of maternal plasma with paternal platelets but not with conventional platelet donor panels. Sequencing of the ITGB3 gene revealed a G > A polymorphism in position c.1915 of exon 12 for the father, the newborn and three of four paternal relatives. Screening of samples from a local population of 1575 Caucasian blood donors identified only a single individual with this mutation.

CONCLUSION

This finding of a previously unreported private platelet antigen demonstrates that the identification of the target glycoprotein by MAIPA assay followed by sequencing of the affected gene can be combined with an efficient population screening by real-time PCR with pooling of DNA samples.

Two homozygous missense mutations in ITGB3 gene as a cause of Glanzmann Thrombasthenia in four consanguineous Pakistani pedigrees

INTRODUCTION

Glanzmann thrombasthenia (GT) is most common of inherited platelet disorders, resulting from quantitative/qualitative defects in platelet surface integrin αIIbβ3, encoded by ITGA2B and ITGB3 genes. Little is known about clinical and molecular characteristics of GT patients from highly consanguineous Pakistani population.

METHODS

This study analyzed the clinical and molecular spectrum of six GT patients from four unrelated but consanguineous families. Platelet surface expression of αIIbβ3 integrin was determined using flow cytometry analysis. ITGA2B and ITGB3 genes were screened for causative mutations by DNA sequencing. Detected mutations were characterized for their pathogenicity using a variety of in silico tools.

RESULTS

Glanzmann thrombasthenia patients in this study generally presented early in life, had a severe course of clinical disease with transfusion dependency for management of bleeding episodes. Molecular analysis revealed 2 homozygous missense mutations in ITGB3 gene, c.422 A˃G (p.Y141C) in three GT patients from a single pedigree with familial segregation and c.1641 C>G (p.C547W) in three unrelated GT patients from three families manifesting type I GT with severe reduction in platelet αIIbβ3 levels. In silico pathogenicity predictions, multiple sequence alignment and 3D protein modeling unanimously suggested deleterious nature of the detected mutations, possibly due to aberrant disulfide bonding. Of note, clinical diversity was observed even among GT patients with same mutation in GT1 family.

CONCLUSION

This study provides an initial yet important account of clinical and genetic characterization of GT in local patients which may spark further studies to help molecular diagnosis, optimal disease management, and genetic counseling based prevention efforts.

Bovine Neonatal Pancytopenia-Associated Alloantibodies Recognize Individual Bovine Leukocyte Antigen 1 Alleles

Bovine neonatal pancytopenia (BNP) was a vaccine-induced alloimmune disease observed in young calves and characterized by hemorrhages, pancytopenia, and severe destruction of the hematopoietic tissues. BNP was induced by alloreactive maternal antibodies present in the colostrum of certain cows vaccinated with a highly adjuvanted vaccine against bovine viral diarrhea. Bioprocess impurities, originating from the production cell line of the vaccine, are likely to have induced these alloreactive antibodies. One prominent alloantigen recognized by vaccine-induced alloantibodies is highly polymorphic bovine major histocompatibility complex class I antigen (bovine leukocyte antigen 1-BoLA I). Aim of this study was to define the fine specificity of BNP-associated anti-BoLA I alloantibodies. In total, eight different BoLA I alleles from the production cell line were identified. All genes were cloned and recombinantly expressed in murine cell lines. Using these cells in a flow cytometric assay, the presence of BoLA I specific alloantibodies in BNP dam sera was proven. Three BoLA I variants were identified that accounted for the majority of vaccine-induced BoLA I reactivity. By comparing the sequence of immunogenic to non-immunogenic BoLA I variants probable minimal epitopes on BoLA I were identified. In general, dams of BNP calves displayed high levels of BoLA I reactive alloantibodies, while vaccinated cows delivering healthy calves had significantly lower alloantibody titers. We identified a subgroup of vaccinated cows with healthy calves displaying very high alloantibody titers. Between these cows and BNP dams no principle difference in the BoLA I reactivity pattern was observed. However, with a limited set of dam-calf pairs it could be demonstrated that serum from these cows did not bind to BoLA I expressing leukocytes of their offspring. By contrast, when testing cells from surviving BNP calves with the corresponding dam's serum there was significant binding. We therefore conclude that predominantly highly alloreactive cows are at risk to induce BNP and it depends on the paternally inherited BoLA I whether or not the calf develops BNP.

Molecular typing of human platelet and neutrophil antigens (HPA and HNA)

Genotyping is an important tool in the diagnosis of disorders involving allo-immunisation to antigens present on the membranes of platelets and neutrophils. To date 28 human platelet antigens (HPAs) have been indentified on six polymorphic glycoproteins on the surface of platelets. Antibodies against HPAs play a role in foetal and neonatal alloimmune thrombocytopenia (FNAIT), post-transfusion purpura (PTP) and refractoriness to donor platelets. The 11 human neutrophil antigens (HNAs) described to date have been indentified on five polymorphic proteins on the surface of granulocytes. Antibodies to HNAs are implicated with foetal and neonatal alloimmune neutropenia (FNAIN), autoimmune neutropenia (AIN) and transfusion related acute lung injury (TRALI). In this report, we will review the molecular basis and techniques currently available for the genotyping of human platelet and neutrophil antigens.

High-throughput simultaneous genotyping of human platelet antigen-1 to -16 by using suspension array

BACKGROUND

Comprehensive and accurate detection of human platelet antigens (HPAs) plays a significant role in diagnosis and prevention of the platelet (PLT) alloimmune syndromes and ensuring clinical safety of patients undergoing PLT transfusion. The majority of the available methods are incapable of performing high-throughput simultaneous detection of HPA-1 to -16, and the accuracy of many methods needs to be further enhanced.

STUDY DESIGN AND METHODS

We have developed a new HPA-genotyping method for simultaneous detection of HPA-1 to -16 based on suspension array technology. A total of 216 samples from Chinese Han donors in Xi'an were genotyped using the developed method, and all the samples again were genotyped using polymerase chain reaction (PCR) sequence-based typing (PCR-SBT), which is considered the gold standard.

RESULTS

All 216 samples were successfully genotyped for HPA-1 to -16 using both our method and PCR-SBT. Results showed that the genotype and allele frequencies obtained using our method were fully consistent with those obtained using PCR-SBT.

CONCLUSION

Our method provides accurate, high-throughput, and simultaneous genotyping of HPA-1 to -16 and will serve as the foundation for large-scale clinical genotyping of HPAs and for the establishment of an HPA-typed PLT donor registry.

Neonatal alloimmune thrombocytopenia: pathogenesis, diagnosis and management

Neonatal alloimmune thrombocytopenia, (NAIT) is caused by maternal antibodies raised against alloantigens carried on fetal platelets. Although many cases are mild, NAIT is a significant cause of morbidity and mortality in newborns and is the most common cause of intracranial haemorrhage in full-term infants. In this report, we review the pathogenesis, clinical presentation, laboratory diagnosis and prenatal and post-natal management of NAIT and highlight areas of controversy that deserve the attention of clinical and laboratory investigators.

New platelet glycoprotein polymorphisms causing maternal immunization and neonatal alloimmune thrombocytopenia

BACKGROUND

Maternal immunization against low-frequency, platelet (PLT)-specific antigens is being recognized with increasing frequency as a cause of neonatal alloimmune thrombocytopenia (NAIT).

STUDY DESIGN AND METHODS

Serologic and molecular studies were performed on PLTs and DNA from two families in which an infant was born with severe thrombocytopenia not attributable to maternal immunization against known PLT-specific alloantigens.

RESULTS

Antibodies reactive only with paternal PLTs were identified in each mother using flow cytometry and solid-phase assays. Unique mutations encoding amino acid substitutions K164T in glycoprotein (GP)IIb (Case 1) and R622W in GPIIIa (Case 2) were identified in paternal DNA and in DNA from the affected infants. Each maternal antibody recognized recombinant GPIIb/IIIa mutated to contain the polymorphisms identified in the corresponding father. None of 100 unselected normal subjects possessed these paternal mutations.

CONCLUSIONS

Severe NAIT observed in the affected infants was caused by maternal immunization against previously unrecognized, low-frequency antigens created by amino acid substitutions in GPIIb/IIIa (α(IIb) /β(3) integrin). A search should be conducted for novel paternal antigens in cases of apparent NAIT not explained on the basis of maternal-fetal incompatibility for known human PLT antigens.

A new platelet alloantigen, Swi(a) , located on glycoprotein Ia identified in a family with fetal and neonatal alloimmune thrombocytopenia

BACKGROUND

Fetal and neonatal alloimmune thrombocytopenia (FNAIT) is a bleeding disorder caused by transplacental passage of maternal antibodies to fetuses whose platelets (PLTs) express the corresponding human PLT antigen (HPA).

STUDY DESIGNS AND METHODS

We observed a fetus with FNAIT who died from a severe intracranial hemorrhage. Analysis of maternal serum in antigen capture assay with paternal PLTs showed reactivity with PLT glycoprotein (GP)IIb/IIIa (α(IIb) β(3) ) and GPIa/IIa (α(2) β(1) integrin), indicating the presence of anti-HPA-1a and an additional alloantibody against GPIa (termed anti-Swi(a) ).

RESULTS

By immunochemical studies, the localization of the Swi(a) antigen on GPIa/IIa could be confirmed. Analysis of paternal GPIa full-length cDNA showed a single-nucleotide substitution C(3347) T in Exon 28 resulting in a Thr(1087) Met amino acid substitution. Testing of family members by polymerase chain reaction-restriction fragment length polymorphism using MslI endonuclease showed perfect correlation with phenotyping. Extended family and population studies showed that 4 of 10 members of the paternal family but none of 500 unrelated blood donors were Swi(a) carriers. Expression studies on allele-specific transfected Chinese hamster ovary (CHO) cells confirmed that the single-amino-acid substitution Thr(1087) Met was responsible for the formation of the Swi(a) epitope. Adhesion of CHO cells expressing the Swi(a) alloantigen to immobilized collagens was not impaired compared to the wild-type control and was not inhibited by anti-Swi(a) alloantibodies.

CONCLUSION

In this study we defined a new PLT alloantigen Swi(a) that was involved in a case of additional immunization against HPA-1a. Our observations demonstrate that combinations of PLT-specific alloantibodies may comprise low-frequency alloantigens.

Molecular insight into human platelet antigens: structural and evolutionary conservation analyses offer new perspective to immunogenic disorders

BACKGROUND

Human platelet antigens (HPAs) are polymorphisms in platelet membrane glycoproteins (GPs) that can stimulate production of alloantibodies once exposed to foreign platelets (PLTs) with different HPAs. These antibodies can cause neonatal alloimmune thrombocytopenia, posttransfusion purpura, and PLT transfusion refractoriness. Most HPAs are localized on the main PLT receptors: 1) integrin αIIbβ3, known as the fibrinogen receptor; 2) the GPIb-IX-V complex that functions as the receptor for von Willebrand factor; and 3) integrin α2β1, which functions as the collagen receptor.

STUDY DESIGN AND METHODS

We analyzed the structural location and the evolutionary conservation of the residues associated with the HPAs to characterize the features that induce immunologic responses but do not cause inherited diseases.

RESULTS

We found that all HPAs reside in positions located on the protein surface, apart from the ligand-binding site, and are evolutionary variable.

CONCLUSION

Disease-causing mutations often reside in highly conserved and buried positions. In contrast, the HPAs affect residues on the protein surface that were not conserved throughout evolution; this explains their naive effect on the protein function. Nonetheless, the HPAs involve substitutions of solvent-exposed positions that lead to altered interfaces on the surface of the protein and might present epitopes foreign to the immune system.

A new Ser472Asn (Cab2(a+)) polymorphism localized within the αIIb "thigh" domain is involved in neonatal thrombocytopenia

BACKGROUND

A new platelet antigen, Cab2(a+), was identified in a case of severe neonatal alloimmune thrombocytopenia (<8 × 10(9)/L) in twins.

STUDY DESIGN AND METHODS

Coding sequences of αIIb and β3 genes from parents were amplified and sequenced. CHO cell lines expressing wild-type or mutated forms of the complex were established to study the role of the mutation in alloimmunization and in αIIbβ3 functions.

RESULTS

The father and twins were heterozygous for a single αIIb c.1508G>A mutation leading to a Ser472Asn substitution. Immunologic assays with transfected CHO cells revealed the Asn472 form of αIIbβ3 responsible for the Cab2(a+) epitope but not an Ala472 form. Using these cells lines we demonstrated that both Ser472Asn and Ser472Ala substitutions produced limited structural alteration as revealed by the reactivity of a panel of anti-αIIbβ3 monoclonal antibodies (MoAbs). Activated Asn472 and Ala472 forms of αIIbβ3 supported 1) binding of soluble fibrinogen and of the ligand mimetic MoAb PAC-1, 2) ligand-induced binding site epitopes exposure (MoAbs AP-5 and D3GP3), and 3) cell aggregation. Adhesion onto adsorbed fibrinogen was conserved and was specifically inhibited by MoAb AP-2 or peptide RGDS. Finally outside-in signaling was not affected.

CONCLUSION

We have characterized a new low-frequency alloantigen (<1%) resulting from the Ser472Asn substitution in αIIb and shown this polymorphism to have a limited effect, if any, on the αIIbβ3 complex functions.

Blood group genotyping: from patient to high-throughput donor screening

Blood group antigens, present on the cell membrane of red blood cells and platelets, can be defined either serologically or predicted based on the genotypes of genes encoding for blood group antigens. At present, the molecular basis of many antigens of the 30 blood group systems and 17 human platelet antigens is known. In many laboratories, blood group genotyping assays are routinely used for diagnostics in cases where patient red cells cannot be used for serological typing due to the presence of auto-antibodies or after recent transfusions. In addition, DNA genotyping is used to support (un)-expected serological findings. Fetal genotyping is routinely performed when there is a risk of alloimmune-mediated red cell or platelet destruction. In case of patient blood group antigen typing, it is important that a genotyping result is quickly available to support the selection of donor blood, and high-throughput of the genotyping method is not a prerequisite. In addition, genotyping of blood donors will be extremely useful to obtain donor blood with rare phenotypes, for example lacking a high-frequency antigen, and to obtain a fully typed donor database to be used for a better matching between recipient and donor to prevent adverse transfusion reactions. Serological typing of large cohorts of donors is a labour-intensive and expensive exercise and hampered by the lack of sufficient amounts of approved typing reagents for all blood group systems of interest. Currently, high-throughput genotyping based on DNA micro-arrays is a very feasible method to obtain a large pool of well-typed blood donors. Several systems for high-throughput blood group genotyping are developed and will be discussed in this review.

[Genotyping applied to platelet immunology: when? How? Limits]

Platelet alloantigens named Human Platelet Antigens (HPA) are involved in immune conflicts such as post-transfusion purpura, platelet transfusion refractoriness and neonatal alloimmune thrombocytopenia. Biological diagnosis relies on: (1) detection of alloantibodies; (2) identification of the alloantigen involved in the immune conflict. Since the development of methods based on molecular biology, platelet genotyping is preferred to phenotyping. Today, most of the Platelet Immunology Units use PCR-RFLP or PCR-SSP, and few use real-time PCR. An increasing amount of commercial kits based on new technologies is now available, for example microarrays, fluorescent or coloured microbeads, or a combination of both technologies. However, an increasing number of polymorphisms have been discovered that are responsible for erroneous platelet genotypings. Consequently, it would be of interest to develop alternative technologies based on antigen/antibody interaction instead of DNA.

Maternal alloimmunization against the rare platelet-specific antigen HPA-9b (Max a) is an important cause of neonatal alloimmune thrombocytopenia

BACKGROUND

Neonatal alloimmune thrombocytopenia (NATP) is an important cause of morbidity and mortality in the newborn. Optimal management of subsequent pregnancies requires knowledge of the alloantigen that caused maternal immunization, but this is possible only in a minority of cases. This study investigated whether this can be explained in part by maternal immunization against the "rare" alloantigen HPA-9b (Max(a)), implicated previously only in a single NATP case.

STUDY DESIGN AND METHODS

Archived paternal DNA from unresolved cases of NATP and normal individuals was typed for platelet (PLT)-specific antigens with real-time polymerase chain reaction and direct sequencing. PLT-specific alloantibodies were characterized by flow cytometry and solid-phase enzyme-linked immunosorbent assay. Recombinant GPIIb/IIIa was expressed in stably transfected Chinese hamster ovary cells. Clinical information was obtained directly from attending physicians.

RESULTS

Six of 217 fathers were positive for the presence of HPA-9b (Max(a)), an incidence about seven times that in the general population. In each of five cases studied, maternal serum samples reacted with intact paternal PLTs and paternal GPIIb/IIIa. Only one of three serum samples tested recognized recombinant GPIIb/IIIa carrying the HPA-9b (Max(a)) mutation. These seemingly discrepant reactions may reflect different requirements for oligosaccharides linked to residues close to the mutation in GPIIb that determines HPA-9b (Max(a)). NATP in the affected children was severe and was associated with intracranial hemorrhage in three of six infants on whom information was obtained.

CONCLUSIONS

Maternal immunization against HPA-9b (Max(a)) is an important cause of NATP and should be considered in cases of apparent NATP not resolved on the basis of maternal-fetal incompatibility for "common" PLT antigens.

Immunization against a low-frequency human platelet alloantigen in fetal alloimmune thrombocytopenia is not a single event: characterization by the combined use of reference DNA and novel allele-specific cell lines expressing recombinant antigens

BACKGROUND

Fetal and neonatal alloimmune thrombocytopenia (FNAIT) is caused by maternal immunization against a fetal platelet (PLT) alloantigen. In cases of FNAIT attributed to low-frequency PLT alloantigens, the laboratory diagnosis is often hampered by the lack of adequate PLTs.

STUDY DESIGN AND METHODS

Three families with maternal immunization against fetal PLT antigens were analyzed. In Family 1, previous immunization of another female or woman has been observed. In Families 2 and 3, newborns presented with the typical clinical picture of FNAIT. Genotyping was performed by polymerase chain reaction-restriction fragment length polymorphism and direct sequencing with reference to DNA from Epstein-Barr virus-transformed B-lymphoblastoid cell lines. Antibodies were characterized by glycoprotein (GP)-specific immunoassay with a panel of stable Chinese hamster ovary cell lines expressing low-frequency alloantigens.

RESULTS

In three families, maternal immunization associated with the low-frequency alloantigens human PLT antigen (HPA)-8bw (Sra), HPA-11bw (Groa), and HPA-13bw (Sita) was identified. Maternal serum samples showed positive reactions in an antigen capture assay with cell lines carrying recombinant GP IIb/IIIa (HPA-8bw and -11bw) or GPIa/IIa (HPA-13bw), respectively. These results could be confirmed by genotyping analysis of fathers and newborns.

CONCLUSION

This study demonstrates that cases of FNAIT attributed to low-frequency PLT alloantigens cannot be regarded as single events. The availability of reference DNA and cell lines expressing recombinant PLT alloantigens can facilitate their identification.

Human platelet alloantigens (HPAs): PCR-SSP genotyping of a UK population for 15 HPA alleles

Alloimmunization to human platelet alloantigens (HPAs) is responsible for neonatal alloimmune thrombocytopenia (NAIT), post-transfusional purpura (PTP) and platelet transfusion refractoriness. HPAs may also have a role as histocompatibility antigens in transplantation as well as associations with cardiac disease. We have developed a polymerase chain reaction-sequence-specific primer (PCR-SSP) assay capable of detecting 15 HPA allelic variants. As part of the validation of the assay, 134 UK renal donors were genotyped to determine HPA allele frequencies in the UK population. The HPA allele frequencies obtained are consistent with those of the other European studies: GP1A*1 (HPA-5a) and GP1A*2 (HPA-5b), 0.914 and 0.086, respectively; GP1BA*1 (HPA-2a) and GP1BA*2 (HPA-2b), 0.925 and 0.075; GP2B*1 (HPA-3a) and GP2B*2 (HPA-3b), 0.627 and 0.373; GP3A*1 (HPA-1a) and GP3A*2 (HPA-1b), 0.840 and 0.161. The rare alleles GP2B*3 (HPA-9bw) and GP3A*3 to *8 (HPA-4b, -6b, -7bw, -8bw, -10bw and -11bw, respectively) were all absent. This comprehensive HPA genotyping assay allows rapid, accurate and reproducible results at low cost.

Genotyping for platelet-specific antigens: techniques for the detection of single nucleotide polymorphisms

Accurate typing of patients for platelet-specific (human platelet) antigens (HPA) is required in several different clinical situations, and blood services need to maintain panels of HPA-typed apheresis platelet donors and whole-blood donors to support HPA alloimmunized patients. Six clinically relevant HPA alloantigen systems have been described and, in addition, a significant number of HPA alloantigens with a highly skewed allele frequency or of very low immunogenicity have been reported. Certain well-characterized biallelic systems such as Gov have not as yet been included in the HPA nomenclature but are included in this review. Biochemical studies have identified the platelet membrane proteins on which the HPA antigens are localized. Cloning of the genes encoding these proteins and the realization that there is adequate mRNA in fresh platelets has led to identification of the molecular basis of HPA antigens over the last decade. All but one of the biallelic platelet-specific alloantigen systems are based on a single nucleotide polymorphism in the DNA sequence, corresponding to a single amino acid substitution in the encoded primary protein sequence. The discovery of the genetic basis of the alloantigens has allowed the development of polymerase chain reaction-based techniques for HPA genotyping using genomic DNA. The genetic basis of the HPA alloantigens, the most commonly used genome typing techniques and their pitfalls, and future developments, are discussed in this review.

A new platelet polymorphism Duv(a+), localized within the RGD binding domain of glycoprotein IIIa, is associated with neonatal thrombocytopenia

We report here the identification and characterization of a new platelet alloantigen, Duv(a+), implicated in a case of neonatal thrombocytopenia. Immunochemical studies demonstrated that the epitope was localized on glycoprotein (GP) IIIa. Sequencing of the exons 2 to 15 of GP IIIa gene polymerase chain reaction products from both parents revealed a single base substitution 517C>T (complementary DNA) present in a heterozygous state in DNA from the father leading to amino acid substitution Thr140Ile (ACC>ATC) within the Arg-Gly-Asp binding domain of GP IIIa. Flow cytometry and immunoprecipitation studies of IIb-C517 or T517 IIIa transfected Cos cells allowed us to demonstrate this mutation was responsible for expression of the Duv(a+) epitope. By polymerase chain reaction-single-strand conformational-polymorphism analysis, the mutated allele could not be detected in a population of 100 healthy unrelated donors, indicating a low frequency of occurrence. The Thr140/Ile dimorphism, localized 3 amino acids upstream from the Arg143 involved in the expression of HPA-4a, did not interfere with the binding of an anti-HPA-4a antibody in flow cytometry. Results of functional analysis of wild-type or mutated transfected CHO cells-(1) aggregation in the presence of Ca(++) and soluble fibrinogen after complex activation by dithiothreitol, (2) adhesion on coated fibrinogen, (3) binding of monoclonal antibody PAC-1 or LIBS antibody D3, and (4) outside-in signaling-all suggest that the Thr140Ile polymorphism localized in the Arg-Gly-Asp binding domain of GP IIIa does not affect significantly, if at all, the integrin function. We have shown that the anti-Duv(a+) antibody may inhibit platelet GP IIb-IIIa function.

A functional platelet fibrinogen receptor with a deletion in the cysteine-rich repeat region of the beta(3) integrin: the Oe(a) alloantigen in neonatal alloimmune thrombocytopenia

This report describes a new low-frequency alloantigen, Oe(a), responsible for a case of neonatal alloimmune thrombocytopenia (NAIT). In a population study none of 600 unrelated blood donors was an Oe(a) carrier. By immunochemical studies the Oe(a) antigen could be assigned to platelet glycoprotein (GP) IIIa. Sequencing of GPIIIa complementary DNA from an Oe(a) (+) individual showed deletion of a lysine residue at position 611 (DeltaLys(611)). Analysis of 20 Oe(a) (-) and 3 Oe(a) (+) individuals showed that the DeltaLys(611) form of GPIIIa was related to the phenotype. Anti-Oe(a) reacted with the DeltaLys(611), but not with the wild-type isoforms on stable transfectants expressing GPIIIa, indicating that DeltaLys(611) directly induces the expression of Oe(a) epitopes. Under nonreducing conditions the Pro(33)DeltaLys(611) variant migrated with a slightly decreased molecular weight compared to the Pro(33)Lys(611) isoform suggesting that DeltaLys(611) has an influence on the disulfide bonds of GPIIIa. The Pro(33)DeltaLys(611) GPIIIa could undergo conformational changes and bind to fibrinogen in a similar manner as the Pro(33)Lys(611) isoform. No difference was found in the tyrosine phosphorylation of pp125(FAK), suggesting that DeltaLys(611) has no effect on integrin function. In contrast to all other low-frequency antigens, the DeltaLys(611) isoform was associated with the HPA-1b, but not with the high frequency HPA-1a allele. Comparison with GPIIIa DNA from nonhuman primates indicated that the HPA-1a allele represents the ancestral form of GPIIIa. It can be assumed that the Oe(a) form did arise as a result of a mutational event from an already mutated GPIIIa allele.

A point mutation in the cysteine-rich domain of glycoprotein (GP) IIIa results in the expression of a GPIIb-IIIa (alphaIIbbeta3) integrin receptor locked in a high-affinity state and a Glanzmann thrombasthenia-like phenotype

This article reports a Glanzmann thrombasthenia (GT) patient, N.M., with a point mutation in the third cysteine-rich repeat of beta3-integrin or platelet glycoprotein (GP) IIIa, leading to the expression of a constitutively activated fibrinogen receptor. The diagnosis of GT was based on a severely reduced platelet-aggregation response to a series of agonists and approximately 20% of surface-expressed GPIIb-IIIa. The patient's GPIIb-IIIa constitutively expressed epitopes recognized by antibodies to ligand-induced binding sites (LIBS) and also spontaneously bound the fibrinogen-mimetic antibody, PAC-1. Furthermore, significant amounts of bound fibrinogen were detected on his platelets ex vivo. No signs of platelet activation were observed on sections of unstimulated platelets from N.M. by electron microscopy. Immunogold labeling highlighted the presence of surface-bound fibrinogen but revealed platelet heterogeneity with regard to the surface density. When the patient's platelets were stimulated by thrombin-receptor activating peptide, amounts of surface-expressed GPIIb-IIIa increased and the aggregation response improved, although it failed to normalize. Platelets from N.M. were able to adhere and spread on immobilized fibrinogen. Sequence analysis of genomic DNA from N.M. revealed a homozygous g1776T>C mutation in GPIIIa, leading to a Cys560Arg amino acid substitution. A stable Chinese hamster ovary (CHO) cell line was prepared expressing surface GPIIb-Arg560IIIa. Like platelets from the patient, GPIIb-Arg560IIIa-transfected CHO cells constitutively bound LIBS antibodies and PAC-1. They also showed an enhanced ability to adhere on surface-bound fibrinogen. Overall, these data demonstrate that a gain-of-function mutation can still be associated with a thrombasthenic phenotype even though platelets show spontaneous fibrinogen binding.

Neutropenia and anaemia due to carbimazole-dependent antibodies

Carbimazole-dependent antibodies to erythrocytes were detected in the sera of three anaemic patients who had been treated with carbimazole for hyperthyroidism. By the use of Rhnull-typed erythrocytes, we could show that some of these were directed against the proteins of the Rh complex. Carbimazole-dependent antibodies eluted from erythrocytes showed no binding to other blood cells. One patient also presented with neutropenia and mild thrombocytopenia. Additional carbimazole-dependent antibodies against the neutrophil-specific Fcgamma receptor IIIb (FcgammaRIIIb, CD16b) and the broadly expressed platelet endothelial cell adhesion molecule 1 (PECAM-1; CD31) were detected in this patient's serum. Surprisingly, the PECAM-1-reactive drug-dependent antibodies were also detectable in the sera of the other two patients with normal leucocyte and platelet counts. We assume that carbimazole can induce cell lineage-specific drug-dependent antibodies that cause cytopenia and also drug-dependent antibodies against the broadly expressed PECAM-1 molecule that may cause mild but not severe cytopenia.

Single amino acid substitution in human platelet glycoprotein Ibβ is responsible for the formation of the platelet-specific alloantigen Iya

We recently described a new low-frequency platelet alloantigen on the human platelet glycoprotein (GP) Ib-IX complex, termed Iya, which was implicated in a severe case of neonatal alloimmune thrombocytopenia. Immunoprecipitation studies with trypsin-treated platelets indicated that the Iyaalloantigenic determinants are formed by the membrane-associated remnant moiety of GP Ib (GP Ibr) together with GP Ibβ and GP IX. To elucidate the molecular basis underlying the Iya alloantigen, we amplifiedGPIbr, GPIbβ, andGPIX genes by polymerase chain reaction (PCR). Nucleotide-sequence analysis of these 3 genes showed a G to A transition at position 141 on GPIbβ gene in a subject positive for Iya. This transition resulted in a Gly15Glu dimorphism on the N-terminal domain ofGPIbβ. This finding was confirmed by genotyping analysis of 6 Iya-positive subjects by restriction fragment length polymorphism (RFLP) studies using NarI endonuclease. In 300 randomly selected healthy blood donors, one Iya-positive individual was found. Phenotypes determined by monoclonal antibody-specific immobilization of platelet antigens assay and genotypes determined by RFLP were identical in this population. Analysis of Iya-positive platelets showed that the point mutation affected neither the degree of surface expression nor the function of the GP Ib-GP Ibβ-IX complex on the platelet surface. Transient expression of the GP Ib-IX complex in CHO cells using wild-type GP Ibβ (Gly15) or mutant GP Ibβ (Glu15) allowed us to demonstrate that this single amino acid substitution is sufficient to induce Iya epitope(s).

Development and validation of a genotyping kit for the eight principal human platelet alloantigen systems

Thrombocytopenia in newborns is often due to maternal alloimmunization against platelet alloantigens of the foetus which have been inherited from the father and are absent in the mother. Our aim was to develop a "ready-to-use" typing kit based on polymerase chain reactions, using sequence-specific primers for rapid and simultaneous genotyping of the eight principal human platelet alloantigens. The typing technique uses two specific primer pairs for each bi-allelic system and a monomorphic primer pair as amplification control, with a single temperature cycle programme and identical PCR stringency conditions for all pairs of primers. This kit allows typing of blood samples of small volume within three hours after reception. Validation criteria are essential to check the reliability and specificity of the test, and DNA controls carrying the targeted HPA alleles must be obtained from typed individuals or created in vitro by PCR.

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

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

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

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

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

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

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

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

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

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

A Point Mutation Thr799Met on the 2 Integrin Leads to the Formation of New Human Platelet Alloantigen Sita and Affects Collagen-Induced Aggregation

A new platelet-specific alloantigen, termed Sita, was identified in a severe case of neonatal alloimmune thrombocytopenia. The Sita alloantigen is of low frequency (1/400) in the German population. Immunochemical studies demonstrated that the Sita epitopes reside on platelet glycoprotein (GP) Ia. Nucleotide sequence analysis of GPIa cDNA derived from Sita-positive platelets showed C2531→T2531 point mutation, resulting in Thr799Met dimorphism. Analysis of genomic DNA from 22 Sita-negative normal individuals showed that the Thr799 is encoded by ACG2532 (90.9%) or ACA2532 (9.1%). To establish a DNA typing technique, we elucidated the organization of the GPIa gene adjacent to the polymorphic bases. The introns (421 bp and 1.2 kb) encompass a 142-bp exon with the 2 polymorphic bases 2531 and 2532. Polymerase chain reaction-restriction fragment length polymorphism analysis on DNA derived from 100 donors using the restriction enzyme MaeIII showed that the Met799 form of GPIa is restricted to Sita (+) phenotype. Analysis of stable Chinese hamster ovary transfectants expressing allele-specific recombinant forms of GPIa showed that anti-Sita exclusively reacted with the Glu505Met799, but not with the Glu505Thr799 and the Lys505Thr799 isoforms. In contrast, anti-Bra (HPA-5b) only recognized the Lys505Thr799 form, whereas anti-Brb(HPA-5a) reacted with both Glu505Thr799 and Glu505Met799 isoforms. These results demonstrated that the Met799 is responsible for formation of the Sita alloantigenic determinants, whereas amino acid 505 (Lys or Glu) specifically controls the expression of Bra and Brb epitopes, respectively. Platelet aggregation responses of Sita (+) individuals were diminished in response to collagen, indicating that the Thr799Met mutation affects the function of the GPIa/IIa complex.

A Point Mutation Thr799Met on the 2 Integrin Leads to the Formation of New Human Platelet Alloantigen Sita and Affects Collagen-Induced Aggregation

A new platelet-specific alloantigen, termed Sita, was identified in a severe case of neonatal alloimmune thrombocytopenia. The Sita alloantigen is of low frequency (1/400) in the German population. Immunochemical studies demonstrated that the Sita epitopes reside on platelet glycoprotein (GP) Ia. Nucleotide sequence analysis of GPIa cDNA derived from Sita-positive platelets showed C2531→T2531 point mutation, resulting in Thr799Met dimorphism. Analysis of genomic DNA from 22 Sita-negative normal individuals showed that the Thr799 is encoded by ACG2532 (90.9%) or ACA2532 (9.1%). To establish a DNA typing technique, we elucidated the organization of the GPIa gene adjacent to the polymorphic bases. The introns (421 bp and 1.2 kb) encompass a 142-bp exon with the 2 polymorphic bases 2531 and 2532. Polymerase chain reaction-restriction fragment length polymorphism analysis on DNA derived from 100 donors using the restriction enzyme MaeIII showed that the Met799 form of GPIa is restricted to Sita (+) phenotype. Analysis of stable Chinese hamster ovary transfectants expressing allele-specific recombinant forms of GPIa showed that anti-Sita exclusively reacted with the Glu505Met799, but not with the Glu505Thr799 and the Lys505Thr799 isoforms. In contrast, anti-Bra (HPA-5b) only recognized the Lys505Thr799 form, whereas anti-Brb(HPA-5a) reacted with both Glu505Thr799 and Glu505Met799 isoforms. These results demonstrated that the Met799 is responsible for formation of the Sita alloantigenic determinants, whereas amino acid 505 (Lys or Glu) specifically controls the expression of Bra and Brb epitopes, respectively. Platelet aggregation responses of Sita (+) individuals were diminished in response to collagen, indicating that the Thr799Met mutation affects the function of the GPIa/IIa complex.

DNA sequence of the canine platelet beta3 gene from cDNA: comparison of canine and mouse beta3 to segments that encode alloantigenic sites and functional domains of beta3 in human beings

The platelet glycoprotein complex alphaIIb beta3 is required for platelet-fibrinogen binding and platelet aggregation. This study was designed to characterize the nucleotide sequence of the canine platelet beta3 gene from cDNA. The nucleotide and deduced amino acid sequences of the canine beta3 gene were 92% and 96% homologous, respectively, with the sequences previously established for the beta3 gene of human beings. Within the beta3 gene, the nucleotide sequence of cDNA prepared from canine platelets shared homology of 89% for the cytoplasmic domain, 93% for the transmembrane domain, 92% for the extracellular domain, 94% for the arginine-glycine-aspartic acid (RGD) binding domain, and 97% for the region associated with Ca2+-dependent stabilization of the alphaIIb beta3 fibrinogen-binding pocket. The deduced amino acid sequence of canine beta3 was 100%, 97%, 96%, and 95% homologous with the cytoplasmic, transmembrane, extracellular, and RGD-binding domains, respectively, and was 100% homologous with the region associated with Ca2+-dependent stabilization of the alphaIIb beta3 fibrinogen-binding pocket of beta3 in human beings. The canine platelet cDNA signal peptide segment of the beta3 gene encodes for 22 amino acids, as compared with 26 amino acids previously reported for human beings. The deduced amino acid sequence of canine beta3 corresponds to the high-frequency allelic form for five of the six alloantigenic sites reportedly associated with human platelets: Leu33Leu40Pro407Arg489Arg636. The apparent amino acid residue in position 143 (Pen alloantigen) of canine platelet beta3 is histidine compared with arginine in human beings. Knowledge of the beta3 gene nucleotide sequence of normal dogs will facilitate the understanding of platelet alphaIIb beta3 structure-function relationships.

Adhesive and Signaling Properties of a Naturally Occurring Allele of Glycoprotein IIIa With an Amino Acid Substitution Within the Ligand Binding Domain—The Pena/PenbPlatelet Alloantigenic Epitopes

Platelet membrane glycoprotein IIIa (GPIIIa) is the most polymorphic integrin subunit in man, with at least seven recognized allelic isoforms present in the human gene pool. Whether these allelic variants of the GPIIb-IIIa complex differ in the ability to interact with the adhesive ligand fibrinogen (Fg) is still unknown. Since the Pena and Penb allelic forms of GPIIIa are distinguished by a single Arg143Gln amino acid substitution within the RGD binding domain of GPIIIa and anti-Pena human alloantibodies have been shown to bind GPIIb-IIIa on the platelet surface and inhibit ADP-induced platelet aggregation, we expressed both forms of this integrin in Chinese hamster ovary (CHO) cells and examined the relative adhesive properties. Both allelic forms of GPIIb-IIIa were expressed on the cell surface and were recognized by a well-characterized panel of murine and human monoclonal and polyclonal antibodies. Like Pena, the Penb form of GPIIb-IIIa could undergo conformational changes in response to RGD peptide binding, and could be induced by activating antibodies to bind Fg and the Fg mimetic antibody P1-55. The binding affinity for Fg of the Pena form of the GPIIb-IIIa complex was not significantly different from that of the Penb form, nor was its ability to signal to focal adhesion kinase, suggesting that Arg143Gln polymorphism has little or no effect on integrin function. Examination of the functional consequences of other integrin polymorphisms may be necessary to determine whether they constitute a risk factor for thrombosis or hemorrhage.

© 1998 by The American Society of Hematology.

Adhesive and Signaling Properties of a Naturally Occurring Allele of Glycoprotein IIIa With an Amino Acid Substitution Within the Ligand Binding Domain—The Pena/PenbPlatelet Alloantigenic Epitopes

Platelet membrane glycoprotein IIIa (GPIIIa) is the most polymorphic integrin subunit in man, with at least seven recognized allelic isoforms present in the human gene pool. Whether these allelic variants of the GPIIb-IIIa complex differ in the ability to interact with the adhesive ligand fibrinogen (Fg) is still unknown. Since the Pena and Penb allelic forms of GPIIIa are distinguished by a single Arg143Gln amino acid substitution within the RGD binding domain of GPIIIa and anti-Pena human alloantibodies have been shown to bind GPIIb-IIIa on the platelet surface and inhibit ADP-induced platelet aggregation, we expressed both forms of this integrin in Chinese hamster ovary (CHO) cells and examined the relative adhesive properties. Both allelic forms of GPIIb-IIIa were expressed on the cell surface and were recognized by a well-characterized panel of murine and human monoclonal and polyclonal antibodies. Like Pena, the Penb form of GPIIb-IIIa could undergo conformational changes in response to RGD peptide binding, and could be induced by activating antibodies to bind Fg and the Fg mimetic antibody P1-55. The binding affinity for Fg of the Pena form of the GPIIb-IIIa complex was not significantly different from that of the Penb form, nor was its ability to signal to focal adhesion kinase, suggesting that Arg143Gln polymorphism has little or no effect on integrin function. Examination of the functional consequences of other integrin polymorphisms may be necessary to determine whether they constitute a risk factor for thrombosis or hemorrhage.

© 1998 by The American Society of Hematology.

Clinical aspects and typing of platelet alloantigens

Platelet alloantigens can induce the formation of corresponding alloantibodies when exposed to phenotypically negative individuals. These antibodies are responsible for fetal and neonatal alloimmune thrombocytopenia, posttransfusion purpura, passive alloimmune thrombocytopenia and transplantation-associated thrombocytopenia and may contribute to platelet transfusion refractoriness together with HLA antibodies. Besides antibody detection laboratory diagnosis of the clinical syndromes requires alloantigen typing. Furthermore, typed platelet donors are a prerequisite for effective platelet transfusion therapy. Different techniques for phenotyping are well established and easy to perform but they rely on the availability of antisera. Since the molecular genetic background of the clinically most relevant alloantigens has been elucidated during the last years various genotyping methods have been applied to the platelet membrane polymorphisms and thus facilitated widespread platelet alloantigen typing. Generation of antibodies from phage display libraries and of lymphoblastoid cell lines from donors with all genetic variants will allow further developments.

HPA-10wb (Laa): Genetic Determination of a New Platelet-Specific Alloantigen on Glycoprotein IIIa and Its Expression in COS-7 Cells

The heterodimeric complex glycoprotein (GP)IIb-IIIa, the fibrinogen receptor of platelets, carries numerous alloantigen systems. These polymorphisms are responsible for the immune response after transfusion or during pregnancy. In the latter case, the mother develops an antibody against an epitope present on fetal platelets, and this results in platelet destruction in the fetus. In this report, we describe the molecular characterization of a new alloantigen (Laa) on GPIIIa responsible for neonatal alloimmune thrombocytopenia (NAIT). Using polymerase chain reaction (PCR)–singlestrand conformation polymorphism (SSCP) and DNA sequencing, we found a point mutation (G to A) in a heterozygous state on the GPIIIa gene leading to amino acid substitution Arg to Gln at position 62 of the mature protein. Transient expression of GPIIb-IIIa complexes in Cos-7 cells using wild-type or mutated GPIIIa cDNA allowed us to demonstrate that this mutation was responsible for expression of the Laa epitope.

Gene frequencies of human platelet antigens on glycoprotein IIIa in Japanese

BACKGROUND

Polymorphism of glycoprotein IIIa on human platelets is one of the factors in alloimmunization that causes neonatal alloimmune thrombocytopenia and refractoriness to platelet transfusion.

STUDY DESIGN AND METHODS

DNA typing methods were originally developed to determine the genotypes of five human platelet antigen (HPA) systems located on glycoprotein IIIa: HPA-1, HPA-4, HPA-6W, HPA-7W and HPA-8W. The gene frequencies of these platelet antigens were determined by DNA typing of 331 unrelated Japanese donors.

RESULTS

The gene frequencies of the low-frequency antigens were 0.002, 0.011, and 0.027 for HPA-1b, HPA-4b, and HPA-6W(b), respectively. All 331 Japanese donors tested were HPA-7W(a/a) and HPA-8W(a/a). Moreover, in the present study, none of the donors tested had two or more of these low-frequency antigens.

CONCLUSION

The risk of neonatal alloimmune thrombocytopenia and refractoriness to platelet transfusion induced by the antigens of the HPA-1, HPA-7W, and HPA-8W systems was extremely rare in Japanese. However, attention must be paid to the involvement of the HPA-4 and HPA-6W systems in these clinical disorders.

Allele-specific restriction analysis of human platelet antigen system 4

BACKGROUND

Diagnosis, and occasionally treatment, of disorders involving platelet-specific alloimmunization, including neonatal alloimmune thrombocytopenia and posttransfusion purpura, requires platelet allotyping. Allele-specific restriction analyses for convenient genotyping are available for the major human platelet antigen (HPA) systems except HPA-4.

STUDY DESIGN AND METHODS

An allele-specific restriction analysis for HPA-4 was developed by designing a polymerase chain reaction primer containing a single-base substitution (A for T) at glycoprotein IIIa cDNA position 529. The resulting mismatch did not interfere with DNA amplification. Digestion of the polymerase chain reaction product with BsmI allowed differentiation of HPA-4a and HPA-4b alleles.

RESULTS

A 126-bp polymerase chain reaction product was amplified with the novel primer. BsmI endonuclease cleaved product encoding HPA-4a into 104- and 22-bp fragments and left DNA encoding HPA-4b intact.

CONCLUSION

This rapid allele-specific restriction analysis for genotyping the HPA-4 system complements similar methods for other platelet alloantigenic determinants.

Genotype frequencies of the human platelet antigen, Ca/Tu, in Japanese, determined by a PCR-RFLP method

Recently, the polymorphism of a new human platelet antigen, Ca/Tu, was shown to be derived from a G-A nucleotide substitution at base 1564 of GPIIIa cDNA, which leads to a single amino acid difference, Arg/Gln at amino acid 489 of GPIIIa. We developed a PCR-RFLP method to determine the genotypes of Ca/Tu and their frequencies in a Japanese population. Fifteen Ca/Tua donors comprising 1 Ca/Tu(a/a) homozygous donor and 14 Ca/Tu(a/b) heterozygous donors were found among the 314 random donors analyzed. The frequencies of Ca/Tu genes were 0.025 (Ca/Tua) and 0.975 (Ca/Tu(b)). The present study showed that the frequency of Ca/Tua individuals in the Japanese (15/314) was approximately 7-fold higher than in the Finnish population (1/150) previously reported by Kekomäki et al. Therefore, attention must be given to the involvement of the Ca/Tu alloantigen in neonatal alloimmune thrombocytopenia and the refractoriness of platelet transfusion.

Simultaneous DNA typing of human platelet antigens 2, 3 and 4 by an allele-specific PCR method

We developed an allele-specific polymerase chain reaction (ASPCR) method using originally designed primers to determine the genotype of the human platelet antigens (HPAs) 2, 3 and 4 in parallel. The results were compared with those obtained by PCR restriction fragment length polymorphism and the mixed passive hemagglutination test. Seventy-three individuals were tested and the ASPCR results were in good agreement with those determined by the other two methods. This method enables the genotyping of HPA-2, -3 and -4 in parallel without the use of platelets, platelet-specific alloantibodies or restriction enzymes.