Baka, a new platelet-specific antigen involved in neonatal allo-immune thrombocytopenia

Bioengineered iPSC-derived megakaryocytes for the detection of platelet-specific patient alloantibodies

Human platelet membrane glycoprotein polymorphisms can be immunogenic in man and are frequently the cause of clinically important immune reactions responsible for disorders such as neonatal alloimmune thrombocytopenia. Platelets from individuals carrying rare polymorphisms are often difficult to obtain, making diagnostic testing and transfusion of matched platelets challenging. In addition, class I HLA antibodies frequently present in maternal sera interfere with the detection of platelet-reactive alloantibodies. Detection of alloantibodies to human platelet antigen 3 (HPA-3) and HPA-9 is especially challenging, in part because of the presence of cell type-specific glycans situated near the polymorphic amino acid that together form the alloepitope. To overcome these limitations, we generated a series of HLA class I-negative blood group O induced pluripotent stem cell (iPSC) lines that were gene edited to sequentially convert their endogenous HPA-3a alloantigenic epitope to HPA-3b, and HPA-9a to HPA-9b. Subjecting these cell lines, upon differentiation into CD41+/CD42b+ human megakaryocytes (MKs), to flow cytometric detection of suspected anti-HPA-3 and HPA-9 alloantisera revealed that the HPA-3a-positive MKs specifically reacted with HPA-3a patient sera, whereas the HPA-3b MKs lost reactivity with HPA-3a patient sera while acquiring reactivity to HPA-3b patient sera. Importantly, HPA-9b-expressing MKs specifically reacted with anti-HPA-9b-suspected patient samples that had been undetectable using conventional techniques. The provision of specialized iPSC-derived human MKs expressing intact homozygous glycoprotein alloantigens on the cell surface that carry the appropriate endogenous carbohydrate moieties should greatly enhance detection of clinically important and rare HPA-specific alloantibodies that, to date, have resisted detection using current methods.

Simultaneous occurrence of foetal and neonatal alloimmune thrombocytopenia and neonatal neutropenia due to maternal neutrophilic autoantibodies: a case study and review of the literature

Foetal and neonatal alloimmune thrombocytopenia (FNAIT) and neonatal neutropenia caused by maternal autoantibodies against neutrophils are rare disorders. We describe a newborn with severe thrombocytopenia and intracerebral bleeding caused by maternal anti-HPA-3a alloantibodies and mild neutropenia caused by maternal autoantibodies against HNA-1b. This appears to be the first case of simultaneous occurrence of these two conditions.

CONCLUSION

This case report and review of the literature demonstrate that anti-HPA-3a antibodies can be overlooked by standard assays.

Fetal and neonatal alloimmune thrombocytopenia: progress and ongoing debates

Fetal and neonatal alloimmune thrombocytopenia (AIT) is a result of a parental incompatibility of platelet-specific antigens and the transplacental passage of maternal alloantibodies against the platelet antigen shared by the father and the fetus. It occurs in approximately 1 in 1000 live births and is the most common cause of severe thrombocytopenia in fetuses and term neonates. As screening programs are not routinely performed, most affected fetuses are identified after birth when neonatal thrombocytopenia is recognized. In severe cases, the affected fetus is identified as a result of suffering from an in utero intracranial hemorrhage. Once diagnosed, AIT must be treated antenatally as the disease can be more severe in subsequent pregnancies. While there have been many advances regarding the diagnosis and treatment of AIT, it is still difficult to predict the severity of disease and which therapy will be effective.

Anti-HPA-3A induces severe neonatal alloimmune thrombocytopenia

OBJECTIVE

Fetal and neonatal alloimmune thrombocytopenia (AIT) caused by feto-maternal incompatibility at the HPA-1a (PLA-1) locus is well characterized. Alloimmunization and disease caused by HPA-3a is rare.

STUDY DESIGN

We conducted a retrospective analysis of all known cases of AIT caused by HPA-3a incompatibility identified at 3 major reference laboratories from 1986 to 1996. Platelet antigen typing and antibody specificity were determined by serologic evaluation. In some cases confirmatory genotyping was performed.

RESULTS

Fourteen cases of anti-HPA-3a-induced AIT in 11 families were identified. Five patients had a previous affected sibling, and 2 cases were firstborn children. All patients had severe thrombocytopenia at birth (platelet count <20 x 10(9)/L). Regardless of therapy, the median time to platelet recovery was 6 days (range, 3 to 23 days). Two (15%) patients had documented intracranial hemorrhage, 1 with severe sequelae including apnea and convulsions. A literature review describing 16 additional patients corroborates the finding of severe thrombocytopenia and a significant incidence of intracranial hemorrhage caused by HPA-3a incompatibility.

CONCLUSION

AIT caused by incompatibility of HPA-3a is similar in severity to disease caused by incompatibility of HPA-1a. Affected families should be appropriately counseled and considered for antenatal therapy.

Feto-maternal alloimmune thrombocytopenia: a literature review and statistical analysis

Exploring prognostic factors that determine outcomes in fetomaternal alloimmune thrombocytopenia (FMAIT), a search of Medline was performed covering the years 1966 to April 1998. 376 articles were collected and reviewed; 140 articles contained the case histories of 297 mothers and 433 pregnancies that fulfilled entry criteria. More than 30 data variables were sought from these cases. The data were analysed using SPSS and Arcus Quickstat Biomedical. Nineteen different antigen incompatibilities were documented, the majority being human platelet antigen (HPA)-1a (77.3%), HPA-3a (3.5%) and HPA-5b (3.5%). The relative risk reduction (RRR) in mortality with any intervention was 57% (0.19-0.77) p = 0.009. Treatment of HPA-1a (PlA1) pregnancies with intravenous immunoglobulin (IVIG) increased the likelihood of a neurologically normal outcome, relative risk (RR) 1.68, confidence interval (1.3-2.2) p = 0.0003. Treatment of HPA-1a (PlA1) pregnancies with only antenatal complementary platelet transfusions increased the likelihood of a neurologically normal outcome, RR 1.63 (1.1-2.1) p = 0.01. Despite reviews of more than 400 cases of FMAIT, few prognostic variables are identifiable. Although IVIG appears to reduce the risk of intracranial haemorrhage (ICH), the dosage and timing of IVIG treatment was varied. This study highlights the need for standardised and directed research.

Evaluation and treatment of thrombocytopenia in the neonatal intensive care unit

Thrombocytopenia is a very frequent problem among sick neonates, affecting up to 35% of all infants admitted to the NICU. Although multiple clinical conditions have been causally associated with neonatal thrombocytopenia, the cause of the thrombocytopenia is unclear in up to 60% of affected neonates. This article provides neonatologists with a practical approach to the thrombocytopenic neonate, with an emphasis on conditions that could be life-threatening or could have significant implications for further pregnancies. An overview of the current therapeutic modalities is also presented, including a discussion of the possible use of recombinant thrombopoietic cytokines to treat certain groups of thrombocytopenic neonates.

The management of alloimmune neonatal thrombocytopenia

Neonatal alloimmune thrombocytopenia (NAITP), defined as thrombocytopenia (platelet count < 150 x 10(9)/l) due to transplacentally acquired maternal platelet alloantibodies, occurs in approximately 1 per 1200 live births in a Caucasian population. In such a population, the majority (> 75 percent) of cases are due to fetomaternal incompatibility for the platelet specific alloantigen, HPA-1a (P1A1, Zwa). Incompatibility for the HPA-5b (Bra) alloantigen is the next most frequent cause of NAITP in Caucasians; much less common is NAITP due to incompatibility for HLA, blood group ABO or other platelet-specific antigens. In non-Caucasian populations (e.g. Orientals) HPA-1a incompatibility is a rare cause of NAITP and other alloantigens e.g. HPA-4b (Penb, Yuka) are implicated. The greatest clinical challenge relates to the antenatal management of pregnant women alloimmunized to the HPA-1a (P1A1, Zwa) antigen, and particularly the subset of such women who have a history of a previously affected infant with severe thrombocytopenia and/or intracranial hemorrhage (ICH). The risk of antenatal ICH in the fetus of such women is high enough to merit intervention, either weekly infusion of high-dose intravenous immunoglobulin G (IVIG) with or without corticosteroids given to the mother (the preferred approach in North American centres), or repeated in-utero fetal platelet transfusions (the preferred treatment approach in some European centres). Post-natal management of severely affected infants centres on the rapid provision of compatible antigen-negative platelets harvested from the mother or a phenotyped donor. The value of antenatal screening programs to detect 'at risk' alloimmunized women during pregnancy continues to be debated.

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.

Maternal immunization to Gov system alloantigens on human platelets

BACKGROUND

Immunization to platelet alloantigens can occur during pregnancy or after the transfusion of blood components. Platelet alloantibodies can cause neonatal alloimmune thrombocytopenia and posttransfusion purpura. Transfusion-induced alloimmunization to a novel platelet alloantigen system, Gov, expressed on the 175-kDa glycosyl phosphatidylinositol-anchored platelet glycoprotein, CD109, was previously described. This report describes three unrelated patients who were alloimmunized to Gov(a) or Gov(b) during pregnancy.

STUDY DESIGN AND METHODS

Platelets were typed by using radioimmunoprecipitation for HPA-1a, -3a, -5a, -5b, Gov(a), and Gov(b) and by polymerase chain reaction-restriction fragment length polymorphism for HPA-1a, -1b, -3a, and -3b. Maternal sera were screened for platelet antibodies by using radioimmunoprecipitation and the antigen capture assay.

RESULTS

Patients 1 and 2 were investigated after the diagnosis of neonatal alloimmune thrombocytopenia in their children, and alloantibodies specific for Gov(b) and Gov(a), respectively, were detected in maternal serum. Serum from patient 3, who had mild idiopathic thrombocytopenia purpura with no detectable autoantibody, was found to contain alloantibodies to Gov(b) and to HPA-5b, presumably as a result of immunization during pregnancy. Platelet typings confirmed that the patients were at risk for alloimmunization to the respective antigen.

CONCLUSION

This report of three cases of maternal alloimmunization to antigens in the Gov system indicates that immunization can occur via placental transfer of antigen and that Gov system alloantibodies may be associated with neonatal alloimmune thrombocytopenia.

[Neonatal alloimmune thrombopenia in anti-HPA-3a (Baka) immunization]

BACKGROUND

Neonatal alloimmune thrombocytopenia (NAIT) in the HPA-3a system is responsible for less than 5% of all cases of NAIT.

CASE REPORT

Thomas, a male infant, was born at 39 weeks of gestation after an uncomplicated pregnancy. Delivery was normal. The Apgar score was 9 at 1 minute, and 10 at 5 and 10 minutes. At 1 hour of age, he displayed extensive petechiae and purpura over the back. The platelet count was 8,000/mm3. Hematesis and extensive petechiae were noted, leading to an exchange transfusion followed by a transfusion of 0.5 U/kg of random donor platelets, 0.4 g/kg/d of intravenous immunoglobulin (IVIg) and 10 mg/kg/d of corticosteroids. IVIg were discontinued on d5 and corticosteroids on d10. There was no relapse of thrombocytopenia. A neonatal alloimmune thrombocytopenia with an HPA-3a (Baka) incompatibility was confirmed.

CONCLUSION

HPA-3a incompatibility is certainly more frequent than the rare cases reported and must be searched for in all cases of neonatal thrombocytopenia.

A simple and rapid competitive enzyme-linked immunosorbent assay to identify HPA-1a (PlA1)-negative donor platelet units

BACKGROUND

Alloantibodies to HPA-1a (PlA1) are the major cause of neonatal alloimmune thrombocytopenia and posttransfusion purpura and have been implicated in refractoriness to random-donor platelet transfusions. However, most assays used to phenotype platelets are cumbersome or time-consuming for large numbers of samples.

STUDY DESIGN AND METHODS

A simple, competitive (inhibition) enzyme-linked immunosorbent assay for HPA-1a phenotyping of donor platelets was developed. A segment from the donor platelet unit transfer line was sealed to obtain a small aliquot of platelets. These platelets were washed once and added to a predetermined dilution of serum containing alloantibodies to HPA-1a. Residual anti-HPA-1a binding to the glycoprotein IIb/IIIa purified by lectin and high-performance liquid chromatography and coated on microtiter wells was detected with a conjugated antihuman IgG. A lack of inhibition equivalent to control (no platelets) was used to determine that the platelets were HPA-1b/b.

RESULTS

Of the 557 platelet units tested, 14 (2.5%) were found to be HPA-1a negative, and they were confirmed to be HPA-1b/b by DNA genotyping. Two of the 14 HPA-1b/b units were also HPA-3b/b (approx. 0.35% of the random population). Use of the microtiter format allows 100 to 200 samples to be processed per day.

CONCLUSION

This simple and inexpensive assay is useful for identifying HPA-1b/b units for platelet-compatible transfusions or for platelet antibody investigations.

Platelets and coagulation. An update

Accepted mechanisms of coagulation are being challenged as new proteins and feedback mechanisms are discovered and evaluated. These new proteins, including tissue factor pathway inhibitor, not only change the way we look at the coagulation mechanism, but also give us potential alternatives in treatment of hemophilia. Gene therapy continues to be a challenging research area, with hemophilia serving as the prototypic hereditary disease target. As acquired disorders of coagulation secondary to immune-mediated events are studied and better understood, our approaches to management of these cases will also change to the benefit of our patients.

Neonatal alloimmune thrombocytopenia in Taiwan due to an antibody against a labile component of HPA-3a (Baka)

We report on two siblings who developed severe neonatal alloimmune thrombocytopenia (NAIT) due to an alloantibody against a labile component or components of the HPA-3a (Baka) antigen. The antibody reacted only with fresh, unfixed platelets by the solid-phase red cell adherence test, immunofluorescence test and mixed passive haemagglutination test. In the latter method, weakly fixed platelets also gave a weak positive reaction. Monoclonal-antibody-specific immobilization of platelet antigens and immunoblotting tests gave negative results. Our findings may possibly help to explain why in some cases of NAIT no platelet-specific antibody is demonstrable in tests with fixed or solubilized platelets.

Frequency of human platelet antigens among blood donors in northeastern Thailand

BACKGROUND

Platelet transfusions have been widely used in Thailand, but little is known about the phenotyping of human platelet antigens.

STUDY DESIGN AND METHODS

Whole blood was collected from 483 blood donors for preparation of platelets. An improved mixed passive hemagglutination assay was used for this study.

RESULTS

Frequencies demonstrated were 100 percent for HPA-1a (PlA1), 15.94 percent for HPA-2b (Siba), 60.25 percent for HPA-3a (Baka), 98.76 percent for HPA-4a (Yukb), 1.86 percent for HPA-4b (Yuka), 5.38 percent for HPA-5b (Br(a)), and 97.72 percent for Naka.

CONCLUSION

HPA-1a was found in 100 percent of Thais, which is the same frequency as in other Asian populations but somewhat different from that in whites (97.9%). Therefore, HPA-1a will not cause neonatal alloimmune thrombocytopenia or post-transfusion purpura in Thais. According to the frequencies of HPA-2b, HPA-3a, HPA-4a, HPA-4b, HPA-5b, and Naka antigens, they may induce neonatal alloimmune thrombocytopenia, posttransfusion purpura, and platelet refractoriness in Thais.

A new private platelet antigen, Groa, localized on glycoprotein IIIa, involved in neonatal alloimmune thrombocytopenia

The serum of a Caucasian woman who gave birth to a child with neonatal alloimmune thrombocytopenia contained antibodies directed against a platelet antigen of the newborn. There was no incompatibility for the known platelet alloantigens HPA-1 to HPA-7 or for the private or low-frequency antigens Sra and Vaa, between the platelets of the parents. However, crossmatching with the serum of the mother and the platelets of the child and the father was strongly positive, suggesting a new platelet antibody specificity. To investigate the inheritance of the 'Groa' antigen involved, the available family members were tested in the platelet immunofluorescence test (PIFT) and the monoclonal antibody-specific immobilization of platelet antigens (MAIPA) assay. The Groa antigen was found to be inherited in an autosomal-codominant fashion. In the MAIPA, we localized the Groa antigen on the glycoprotein IIb/IIIa complex (alpha IIb beta 3). The GP IIb/IIIa localization was confirmed in immunoprecipitation studies. In Western blotting experiments, we further localized the Groa antigen on the GP IIIa (beta 3) subunit of the GP IIb/IIIa complex. Until now we have tested approximately 400 unrelated donors. None of these appeared to be positive for the Groa antigen, suggesting a phenotype frequency in the Dutch population of less than 0.01.

Anti-Bak(a) neonatal alloimmune thrombocytopenia: possible prevention by intravenous immunoglobulin

Neonatal alloimmune thrombocytopenia (NAIT) most commonly involves antibodies directed against the PlA1 antigen, but other platelet specific alloantigens have been associated with it. We describe the case of a mother whose first three infants developed NAIT secondary to anti-Bak(a) antibodies, while her fourth infant did not. The three affected infants were treated postnatally with platelet transfusions. The fourth infant was treated antenatally with one dose of intravenous immunoglobulin (IVIg) given to the mother. Postpartum analysis revealed the infant's platelets to be Bak(a)-positive but negative for elevated IgG. Maternal serum reacted with neonatal platelets in vitro, but cord serum was negative for antiplatelet antibodies. These clinical observations do not prove the efficacy of IVIg; however, they raise several questions: Why wasn't this infant thrombocytopenic? Why didn't the umbilical cord contain maternal antibody? Was the single dose of IVIg responsible for preventing NAIT? IVIg is currently under investigation in a clinical trial evaluating its effectiveness in preventing NAIT in mothers with anti-PlA1 antibodies, where it has shown some success. There have been no reports of the use against anti-Bak(a) antibodies. We suggest that a weekly dose schedule may not be necessary for all affected pregnancies, and antibodies with specificity other than anti-PlA1 may require less vigorous therapy.

[Platelet group polymorphism in Provence. Comparison with the frequencies of platelet-specific allo-antigens observed in other populations]

Systems HPA-1 (Pla); HPA-3 (Bak) HPA-5 (Br) are involved in neonatal alloimmune thrombocytopenia and post-transfusion purpura. The frequencies of platelet-specific antigens in these three systems have been studied among one hundred one unrelated blood donors from Provence (South of France) for three generations. Typing was performed by the MAIPA test (monoclonal antibody-specific immobilization platelet antigen). The phenotypes frequencies found were: HPA-1a (PlA1): 97%; HPA-3a (Baka): 88.1%. These frequencies are quite similar to those reported in Europe and North America, but are different compared to Oriental and South American populations. Our Provence population has the highest frequency of HPA-5b (Bra) yet reported: 23.8%. These results define the polymorphism of platelet-specific antigens in the Provence population. Similar studies, among other populations, would provide new data for geographical haematology, which has so far been based on erythrocyte, leucocyte and serum polymorphisms. The variations between populations in these platelet-specific polymorphisms would be so many useful descriptive elements for the epidemiological study of associated diseases.

Molecular genetic aspects of human platelet antigen systems

Recent advances in molecular and cellular biology have made it possible to build upon previous serological and biochemical studies of human platelet alloantigen systems in important and exciting ways. In addition to providing a detailed basic understanding of the polymorphisms that are responsible for eliciting an alloimmune response, the molecular characterization of platelet membrane glycoprotein polymorphisms is expected to have an increasingly large clinical impact. As the molecular basis of the remaining platelet antigen systems becomes known, our ability to design novel diagnostic and therapeutic approaches for the care and management of patients with PTP and NATP should improve.

Localization of human platelet autoantigens to the cysteine-rich region of glycoprotein IIIa

The object of this study was to further localize autoantigenic structures on IIb-IIIa and, if possible, to precisely identify the epitopes recognized by human autoantibodies. In this paper, we identify a 50-kD chymotryptic fragment of IIIa that is recognized by a high percentage of human autoantibodies, typified by the prototype IgG autoantibody RA, which binds to IIIa on intact platelets as well as in an immunoblot assay under nonreduced conditions. Using an immunoblot assay, a carboxy-terminal region of this fragment (33 kD) that contains the cysteine-rich domains of IIIa was found to carry the epitope(s) recognized by the prototype autoantibody RA. The amino-terminal amino acid sequence of the reduced 33-kD fragment, the smallest fragment that retains the RA epitope, is XPSQQDEXSP, and that of the reduced 50-kD fragment is IVQVTFD. This indicates that the 33-kD fragment consists of approximately 175 amino acids beginning at residue 479 and extending at least through residues 636-654, while the 50-kD fragment spans the same region but begins at residue 427. It is apparent that the 33-kD fragment is generated from the 50-kD fragment by additional chymotryptic hydrolysis but remains associated because of the multiple disulfide bonds that are characteristic of this cysteine-rich domain. Sera from 48% of patients with chronic ITP and 2 of 8 patients with acute ITP contain antibodies that bind to the 50-kD fragment in an ELISA. Antibodies of the same specificity are also found in one-third of patients with either secondary immune thrombocytopenia or apparent non-immune thrombocytopenia. We conclude that the 50-kD cysteine-rich region of IIIa is a frequent target of autoantibodies in ITP, but that such antibodies may also be present in cases of thrombocytopenia that cannot be linked to an apparent autoimmune process.

Brb, a platelet alloantigen involved in neonatal alloimmune thrombocytopenia

Serum from a pregnant woman with the May-Hegglin anomaly contained a platelet-specific antibody. The serum reacted in the platelet indirect immunofluorescence test (PIIFT) with 97.6% of random donor platelets and those of the father but not with the mother's own platelets. This antibody induced a moderate thrombocytopenia in the infant that responded to infusion of intravenous immunoglobulin concentrates. The platelet phenotypes were PLA1+, Baka+, Bra+/Brb- for the mother, PLA1+, Baka+, Bra-/Brb+ for the father, and PLA1+, Bra+/Brb+ for the neonate. Analysis of the maternal serum with an immunoassay based on monoclonal antibody immobilization of platelet antigens (MAIPA) and immunoprecipitation techniques demonstrated the absence of antibodies directed against HLA class I antigens and that the antigen recognized was located on the platelet-GpIa/IIa complex. This antigen was present on 113/115 random donor platelets, in 7 of the 7 unrelated May-Hegglin platelets, and only absent in 3/24 Bra+ individuals, including the mother. No platelet-specific antibody was present in the serum of the 7 unrelated May-Hegglin subjects. The antigen recognized by this platelet-specific antibody thus meets the criteria defining the antithetic allele of Bra, i.e. the Brb alloantigen.

Neonatal alloimmune thrombocytopenia caused by an antibody to the Bak(a) antigen

A 31 year old woman was assessed following delivery of her second child affected by neonatal alloimmune thrombocytopenia (NAIT). Antiplatelet antibodies with specificity for Bak(a) were identified in the woman's serum and her platelets were typed as Bak(a) negative whilst her husband's were Bak(a) positive. Unlike the majority of reported anti-Bak(a) antibodies in the literature, this patient's serum contained no contaminating anti-HLA antibodies. This is the first report of NAIT caused by an anti-Bak(a) without co-existing anti-HLA antibodies. An anti-Bak(a) antibody has not previously been reported in Australia. The current status of this antigen system is reviewed.

Acid treatment of platelets as a simple procedure for distinguishing platelet-specific antibodies from anti-HLA antibodies: comparison with chloroquine treatment

The identification of antibodies to platelet-specific antigens is important for correctly diagnosing neonatal alloimmune thrombocytopenia, posttransfusion purpura and refractoriness due to platelet-specific antibodies. However, the serologic identification of these platelet-specific antibodies is complicated by the presence of anti-HLA antibodies. We examined and compared the diagnostic usefulness of acid-treated and chloroquine-treated platelets for the discrimination of platelet-specific antibodies from anti-HLA antibodies. The viability of acid-treated platelets is 83.4%, which is better than that of chloroquine-treated platelets (52.6%). The antigenicity of HLA class I antigens of acid-treated platelets was significantly reduced compared with that of PBS- or chloroquine-treated platelets. On the other hand, platelet surface glycoprotein Ib and glycoprotein IIb/IIIa, and platelet-specific antigens were stable following acid or chloroquine treatment. Chloroquine-treated platelets were not suitable targets for analysis by immunofluorescence flow cytometry because of nonspecific fluorescence derived from platelet damage. We conclude that acid-treated platelets are more suitable targets than chloroquine-treated platelets for screening for platelet-specific antibodies and also for analyses of the specificity of platelet-specific antibodies.

A new platelet-specific antigen, Naka, involved in the refractoriness of HLA-matched platelet transfusion

Serum from a thrombocytopenic patient who was refractory to the transfusions of HLA-matched platelets contained a platelet-specific alloantibody, anti-Naka. Immunofluorescence analyses revealed that the Naka antigen defined by the serum was expressed exclusively on platelets and its distribution was different from P1A1, Baka, Yuka or Yukb. Analysis by Dr. von dem Borne's group revealed the Naka was also different from Koa, Kob or Zwb. Family studies showed that the Naka antigen was inherited as an autosomal codominant trait. Its antigen frequency in the Japanese population was over 97%. The results of the enzyme immunoassay using monoclonal antibodies for antigen immobilization showed that the Naka epitope did not appear to reside on GPIIb/IIIa or Ib. The transfusions of Naka-compatible platelets improved the patient's thrombocytopenia.

Platelet anti-Bakb antibody associated with neonatal alloimmune thrombocytopenia

A platelet alloantibody of IgG class but unknown specificity was detected in the serum from the mother of an infant with neonatal alloimmune thrombocytopenia. The antibody reacted by indirect immunofluorescence with 33/65 random donors, 10/11 Baka-negative donors and 23/54 Baka-positive donors. The mother's platelet phenotype was PLA1, Koa, Kob, Baka and the father's was PLA1, Koa, Kob, Baka. Immunoblotting and immunoprecipitation of maternal serum with paternal platelets produced a band at molecular weight 140 kilodaltons identical to the band obtained with Baka antiserum. Family studies confirmed the allelic distribution of Baka and the unknown antigen. The platelet-specific antibody in this patient meets the criteria for an antibody to the new platelet antigen, Bakb.

Neonatal alloimmune thrombocytopenia

Neonatal alloimmune thrombocytopenia (NAIT) occurs when maternal alloantibodies to antigens present on fetal platelets cause their immune destruction resulting in thrombocytopenia in the newborn infant or fetus. Bleeding may be severe; intracranial haemorrhage and permanent neurological damage are the most serious complications. Despite the severity of the disease, there is often a delay in making the correct diagnosis and instigating appropriate treatment. Recent evidence that NAIT is more common than has previously been recognised, a better understanding of the molecular basis of platelet serology and advances in technology, which have made it possible to take blood samples from fetuses and transfuse them in utero, have all contributed to a growing interest in this condition. In addition, it is exciting to realise that an aggressive approach to the management of established cases and 'at risk' pregnancies can prevent serious neurological sequelae and dramatically improve the outcome for affected infants.

Prenatal diagnosis of platelet disorders

The antenatal diagnosis of platelet disorders represents real progress in the early detection of haemorrhagic diseases occurring in the fetus. However, the diagnosis is only possible in some cases during the first trimester of gestation, and not in the first weeks as is the case for other hereditary disorders such as abnormal haemoglobins. This delay can be reduced now that the molecular abnormalities responsible for some platelet disorders have been discovered. If the region of chromosome 17 and the DNA sequence coding for the glycoproteins GP IIb-IIIa were known, this would make possible the recognition of the gene defect responsible for Glanzmann's thrombasthenia. This could also permit the diagnosis of Glanzmann's thrombasthenia at the gene level, i.e. during the first weeks of gestation. However, the use of gene markers could be limited by the fact that a monomorphic clinical expression of Glanzmann's thrombasthenia could correspond to different genetic mutations which can all result in a defect in GP IIb-IIIa synthesis and assembly. If such diagnosis could be made very early, it would only represent real progress if a specific treatment could be applied. New therapeutic approaches to immune thrombocytopenia during pregnancy appear to be possible and can be applied when there is a risk to the fetus, they are still either experimental or anecdotal and there is a real need for a well-designed clinical trial. In all fetal platelet disorders, the risk of fetal death following fetal blood sampling must not be underestimated and very careful, intensive care is necessary after such an investigation. In the absence of a specific therapy, this antenatal diagnosis must be restricted to cases in which the risk of severe haemorrhagic complications are anticipated and where there is a well-documented family history. The patients must be properly informed of all the aspects of the investigation, including the possible risks. As has been the case for other haematological disorders, progress will be made, and we can anticipate that eventually in utero bone-marrow transplantation or gene correction be performed to cure the disease before birth.

Immunology of platelet disorders

Immune-mediated thrombocytopenias (IMTPs) are frequently-occurring haemostatic disorders in clinical medicine. They may be caused by allo-immunity, autoimmunity, or by drug-induced immune mechanism. All IMTPs are caused by antibodies, which may induce increased platelet destruction but may also hinder platelet function. Many different platelet membrane molecules (i.e. antigens) are involved in the immune processes that play a role in IMTP. Much is already known about the structure of these molecules. Notably the alloantigens involved in alloimmune-mediated thrombocytopenia have been quite well studied. Many of these antigens appear to be polymorphic determinants of adhesion molecules of the integrin superfamily, and are also present on cells other than platelets (endothelial cells, fibroblasts, smooth muscle cells). The methodology for studying platelet antigens and antibodies involved in IMTPs has markedly improved in recent years. This has not only led to better diagnostic tests but also to a better understanding of the immunopathogenesis of these diseases. Platelet immunology is scientifically very much alive, and is expected to remain so in the coming years.

New platelet antigen, Siba, involved in platelet transfusion refractoriness in a Japanese man

Siba, a new platelet-specific alloantigen involved in a case of platelet transfusion refractoriness is reported. The IgG platelet alloantibody was detected in a multiply transfused patient of Japanese extraction (Sib), by the presence of HLA antibodies. After transfusion of HLA-compatible platelets, the patient suffered from refractoriness. Adsorption studies with pooled lymphocytes showed that the serum contained anti-platelet activity. Family studies indicate that Siba is inherited as an autosomal codominant trait and separate from HLA and Baka. As of this report, segregation from Zw(PlA) and Yuk (Pen) antigen systems have not yet been determined. The gene frequency of Siba in the Japanese population is estimated to be 0.136.

Antigenic polymorphism of human very late activation protein-2 (platelet glycoprotein Ia-IIa). Platelet alloantigen Hca

We have found evidence for a human alloantigenic system on the very late activation protein -2 (VLA-2) heterodimer (platelet GPIa/IIa). Sera from two patients with systemic lupus erythematosus (SLE) contained antibodies that immunoprecipitated surface molecules from platelets and fibroblasts that comigrated on SDS-PAGE and two-dimensional O'Farrell gels with platelet GPIa (VLA-alpha2 chain) and platelet GPIIa (VLA-beta chain). These SLE antibodies were alloreactive as they precipitated VLA molecules from only 5 of 22 normal donors' platelets and did not react with the lupus patients' own platelets, despite the expression of apparently normal amounts of VLA on the donors' cells. Two-dimensional O'Farrell analysis demonstrated no differences in the molecular weight or isoelectric point of GPIa and GPIIa obtained from platelets of alloantibody reactive or unreactive donors. Sequential immunoprecipitation experiments with VLA chain-specific monoclonal antibodies, and the pattern of immunoprecipitation of several different VLA heterodimers demonstrated that the alloantibody-reactive determinant was present on the VLA-2 heterodimer, and not other VLA molecules. Thus, these SLE sera demonstrate a previously unrecognized antigenic polymorphism of the VLA-2 (platelet GPIa/IIa) heterodimer, platelet alloantigen Hca.

Posttransfusion purpura associated with an anti-Bak

We describe a 46-year-old white woman with typical clinical features of posttransfusion purpura (PTP) whose serum held a platelet-specific alloantibody reactive with an antigen antithetical to Baka, i.e. anti-Bakb. The specificity of the antibody was confirmed by family analysis, a population study (expected versus observed gene frequency: 0.3651 versus 0.3984; n = 105) and localization of the antigen on glycoprotein IIb in radioimmunoprecipitation. Typing of family members and blood donors for platelet antigens disclosed that the patient had been preimmunized by two blood transfusions in 1981, while fetomaternal incompatibility for Bakb was ruled out (her three children and their father were Bakb negative). Treatment of PTP with corticosteroids and platelet transfusions was ineffective, but infusion of high-dose intravenous IgG resulted in a rapid increase in the platelet count.

348 cases of suspected neonatal alloimmune thrombocytopenia

Serological and clinical data were collected in 348 cases of suspected neonatal alloimmune thrombocytopenia (NAT). Of the 144 mothers who were Zwa-negative, 107 had Zwa antibodies--alone (94); with HLA antibodies (12); or with Bra antibodies (1). Antibodies were detected in 12 of the 204 Zwa-positive mothers as follows: anti-Bra (9), anti-Zwb (1), anti-Baka with HLA antibody (1), and blood group B isoagglutinins (1). The frequency of NAT due to Bra incompatibility (19%) was second to Zwa (78%). Zwa-NAT was clinically the more severe (14% had intracranial haemorrhages) and responded well to either maternal platelet transfusions or intravenous IgG. In Bra-NAT intracranial haemorrhages were not observed and most children recovered without specific therapy.

Management of alloimmune neonatal and antenatal thrombocytopenia

Neonatal and antenatal alloimmune thrombocytopenia is induced by maternal antibodies against platelet-specific fetal antigens. This disease is rare but potentially severe because of intracranial bleedings which may occur during pregnancy or around birth. In the last decade our knowledge of this disorder has markedly advanced. New techniques are used in platelet immunology. New platelet antigens involved in these perinatal thrombocytopenias have recently been discovered. A group of women likely to produce the responsible platelet antibodies has been genetically defined as regards the PLA1 antigen. The quality of the sonographies and the possibility of performing cord vein puncture in early pregnancy afford a new approach in the management of perinatal alloimmune thrombocytopenias. But more must be done to prevent the complications of this disease.

A new platelet-specific alloantigen Bra. Report of 4 cases with neonatal alloimmune thrombocytopenia

Sera obtained from 4 mothers of children with neonatal alloimmune thrombocytopenia contained a platelet-specific alloantibody, anti-Bra, which defined an antigen apparently different from all known platelet alloantigens. All 4 fathers were Bra positive, whereas all mothers were Bra negative. The minimal postnatal values of platelet counts ranged from 19 X 10(9) to 75 X 10(9)/1. Family studies showed that the Bra antigen is inherited as an autosomal, codominant trait. Its antigen frequency in the German population is 20% (21 of 105 unrelated donors were positive). The estimated gene frequency is 0.11. The antibodies were identified by a glycoprotein-specific enzyme immunoassay using monoclonal antibodies for antigen immobilization, while they could not reliably be detected by binding assays employing whole platelets (platelet immunofluorescence, indirect competitive ELISA).

The current status of antiplatelet antibodies

The importance of antiplatelet antibodies in clinical medicine was first recognized in 1951. Since that time, a number of syndromes have been described, including autoimmune thrombocytopenia purpura, posttransfusion purpura, neonatal alloimmune thrombocytopenia, and drug-induced thrombocytopenia purpura, that fit into the category of immunologic thrombocytopenias. The laboratory methods for detecting the antiplatelet antibodies present in these diseases are enumerated and discussed along with the currently recognized platelet-specific antigens. Because of the complexities of performance and the lack of agreement among many of the available procedures, it is recommended that antiplatelet antibody testing remain primarily a research tool for the present time.

Use of immobilized platelet membrane glycoproteins for the detection of platelet-specific alloantibodies in solid-phase ELISA

Platelet membrane glycoproteins were isolated from intact platelets by detergent-phase extraction, fixed to the wells of microtiter trays and used as targets for the detection of platelet-reactive alloantibodies by enzymelinked immunospecific assay (ELISA). The final preparations contained 0.4% of total platelet protein. Antibodies reactive with antigens PlA1, PlA2, Baka, Pena and HLA-A2 were specifically detected at dilutions ranging form 1:640 to 1:1.600. Under the conditions utilized, the ELISA was more sensitive than assays involving 51Cr, radiolabeled monoclonal anti-IgG binding, and indirect immunofluorescence testing by one order of magnitude or greater. When platelets were pretreated with chloroquine to remove class I HLA antigens prior to detergent-phase extraction, reactions with HLA-specific antibodies were lost, but reactions with platelet-specific alloantibodies were retained. This approach offers a simple, sensitive and rapid method to detect and identify platelet-specific alloantibodies in sera containing HLA-reactive alloantibodies.

Identification of the Yukb allo-antigen on platelet glycoprotein IIIa

The glycoprotein (GP) localization of a new platelet-specific allo-antigen Yukb is described. The antibody was isolated from serum of a patient with neonatal allo-immune thrombocytopenia. In immunoblot procedure, it bound exclusively to platelet GP IIIa, like anti-PlA1, while the known anti-Baka and anti-Leka reacted with GP IIb. Analysis of GP from chymotrypsin-treated platelets with anti-Yukb revealed no binding in the 68-kilodalton position while anti-PlA1 did. Thus, unlike the PlA1 antigen, the Yukb determinant either resides on the 30-kilodalton fragment of GP IIIa or it is destroyed by chymotrypsin treatment.

A new platelet antigen system, Yuka/Yukb

We report a new platelet antigen (Yukb) involved in a case of neonatal alloimmune thrombocytopenia (NAITP). This antigen is considered to be a product of an allele of the Yuk gene, another allele of which codes for Yuka which was involved in 2 cases of NAITP. The Yuka and Yukb antigens are not expressed on thrombasthenic platelets which suggests that these antigens are present on platelet glycoprotein IIb and/or IIIa. The gene frequencies for Yuka and Yukb in the Japanese population were estimated to be 0.0083 and 0.9917, respectively.