Yuka, a new platelet antigen involved in two cases of neonatal alloimmune thrombocytopenia

Severe neonatal thrombocytopenia due to anti-HPA-4b alloantibodies: Third report described in a Caucasian mother

BACKGROUND

Fetal and Neonatal Alloimmune Thrombocytopenia (FNAIT) results from maternal platelet alloimmunization against paternal antigens inherited by the fetus, most often due to the Human Platelet Antigen (HPA)-1 system in Caucasians. We investigated in 2023, a 30-year-old Caucasian woman Gravida 2 Para 1 who gave birth at 35 weeks of gestation to a male (body weight 2210 g) without signs of bleeding. A severe thrombocytopenia (platelet count at 3 G/L) was discovered incidentally a few hours after delivery in the context of the management of a respiratory distress. The newborn recovered after one platelet concentrate transfusion and normalized his platelet count at Day 5.

STUDY DESIGN AND METHODS

FNAIT investigation was performed according to guideline recommendations. Platelet genotyping was carried out by multiplex PCR. Maternal serological investigation included Monoclonal Antibody-specific Immobilization of Platelet Antigens method (MAIPA) and Luminex technology.

RESULTS

Parental and newborn genotyping pointed out an HPA-4 incompatibility between the mother and the newborn and the father. Serological investigation revealed an anti-HPA-4b alloantibody confirming the diagnosis of neonatal alloimmune thrombocytopenia.

CONCLUSION

We described the third case of anti-HPA-4b alloantibody discovered in a Caucasian mother. This case strengthens the need for reference laboratory to genotype a panel of HPA alleles reflecting local genetic population diversity and for crossmatch of maternal serum with fresh paternal platelets in clinical suspected cases of neonatal alloimmune thrombocytopenia.

Human platelet antigens in Burmese, Karen and north-eastern Thais

OBJECTIVES

A comparative study of allele frequencies at HPA-1 to -6 and HPA-15 in Burmese and Karen populations as well as at HPA-15 in north-eastern Thais (NET) is presented.

BACKGROUND

Human platelet antigens (HPAs) are clinically important in several immune platelet disorders, including foetal and neonatal alloimmune thrombocytopenia (FNAIT), post-transfusion purpura (PTP) and platelet transfusion refractoriness (PTR). The knowledge of antigen frequencies in a population is essential for the evaluation of patients suffering from immune-mediated platelet disorders.

METHODS

A total of 285 unrelated, healthy Burmese, 242 Karen and 300 NET were recruited to this study. Genotype and allele frequencies of HPA-1 to -6 and HPA-15 were defined using polymerase chain reaction sequence-specific primers (PCR-SSP) RESULTS: No individuals homozygous for HPA-1bb, -2bb, -4bb, -5bb and -6bb were detected. HPA-1a, -2a, -4a, -5a and -6a were present in all samples of Burmese and Karen origin. HPA-1b, -2b, -4b, -5b and -6b were rare in these populations. The frequencies of HPA-3a/-3b were 60·4/39·6% in Burmese and 55·8/44·2% in Karen, respectively. Frequencies of HPA-15a/-15b were 57·2/42·8% in Burmese, 52·5/47·5% in Karen and 49·8/50·2% in NET.

CONCLUSIONS

The frequencies of HPA genotypes in our study indicates that HPA-1a, -2a, -4a, -5a and -6a are unlikely involved in FNAIT, PTP and PTR in Burmese and Karen populations. However, HPA-1b, -2b, -3a, -3b, -4b, -5b, -6b, -15a and -15b may likely stimulate alloantibodies in these populations.

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.

Establishment of a cell line panel for the detection of antibodies against human platelet antigen 4b

Antibodies against human platelet antigens (HPAs) play important roles in thrombocytopenia. In Japan, HPA-4b antibody is frequently responsible for HPA-related neonatal alloimmune thrombocytopenia. A highly sensitive assay using platelets has been developed for the detection of antibodies against HPAs. However, it is difficult to obtain the platelets expressing specific HPAs required for the assay. Therefore, an alternative method not requiring platelets would be helpful to detect antibodies against HPAs. Glycoprotein IIIa (GPIIIa) cDNA encoding HPA-4b was individually co-transduced with that of wild-type GPIIb in K562 cells, which is a non-adherent cell line, using a retroviral vector. The expression of transgene products in cultured cells were observed for over 6 months. Next, to evaluating the sensitivity and specificity of this cell line panel, we performed monoclonal antibody-specific immobilization of platelet antigens (MAIPA) assay with a serum previously identified by another method. All HPA-4b antibodies in serum samples were positive, and all serum samples, including normal serum and serum containing HLA antibodies were negative. No difference was observed in the specificity and sensitivity between our method and conventional MAIPA using platelets. The present results indicate that this established cell line panel permits highly sensitive detection of specific antibodies against HPA-4b.

New low-frequency platelet glycoprotein polymorphisms associated with neonatal alloimmune thrombocytopenia

BACKGROUND

Recent reports suggest that maternal immunization against low-frequency, platelet (PLT)-specific glycoprotein (GP) polymorphisms is a more common cause of neonatal alloimmune thrombocytopenia (NATP) than previously thought.

STUDY DESIGN AND METHODS

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

RESULTS

Antibodies reactive only with paternal PLTs were identified in each mother. In Cases 2 (Kno) and 3 (Nos), but not Case 1 (Sta), antibody recognized paternal GPIIb/IIIa in solid-phase assays. Unique mutations encoding amino acid substitutions in GPIIb (Case 2) or GPIIIa (Cases 1 and 3) were identified in paternal DNA and in DNA from two of the affected infants. Antibody from all three cases recognized recombinant GPIIIa (Case 1 [Sta] and Case 3 [Nos]) and GPIIb (Case 2, Kno) mutated to contain the polymorphisms identified in the respective fathers. None of 100 unselected normal subjects possessed the paternal mutations. Enzyme-linked immunosorbent assay and flow cytometric studies suggested that failure of maternal serum from Case 1 (Sta) to react with paternal GPIIIa in solid-phase assays resulted from use of a monoclonal antibody AP2, for antigen immobilization that competed with the maternal antibody for binding to the Sta epitope.

CONCLUSION

NATP in the three cases was caused by maternal immunization against previously unreported, low-frequency GP polymorphisms. Maternal immunization against low-frequency PLT-specific alloantigens should be considered in cases of apparent NATP not resolved by conventional serologic and molecular testing.

Comparison of acute non-haemolytic transfusion reactions in female and male patients receiving female or male blood components

To study the relationship between antibodies detected in patients’ and/or donors’ sera and the clinical features of acute non-haemolytic transfusion reactions (ANHTRs), and to determine any gender-related difference. ANHTRs range from urticaria to transfusion-related acute lung injury (TRALI). Antibodies to human leukocyte antigen (HLA), granulocytes, platelets, and/or plasma proteins are implicated in some of the ANHTRs. A higher antibody positivity is expected for females than for males. A comparative study of ANHTRs for antibody positivity and their clinical features between females and males for both patients and donors is helpful for characterizing ANHTRs including TRALI more clearly, but such studies are few and outdated. Two hundred and twenty-three ANHTR cases reported by 45 hospitals between October 2000 and July 2005 were analysed. The patients and 196 donors of suspect blood products were screened for antibodies to HLA Class I, HLA Class II, granulocytes, and platelets. The patients were also screened for anti-plasma protein antibodies. The types and severity of ANHTR did not differ significantly between female and male patients. The frequency of the anti-HLA antibodies, but not that of the non-HLA antibodies, was significantly higher in females. Non-HLA antibodies were significantly associated with severe reactions in females. All the TRALI cases had predisposing risk factors for acute lung injury, and 60% of the cases showed anti-leucocyte antibodies. Although the anti-HLA antibodies were detected more frequently in females than males, no significant association of ANHTRs including TRALI with gender, not only for patients, but also for donors, could be shown in this study.

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.

Perinatal intracranial hemorrhage due to severe neonatal alloimmune thrombocytopenic purpura (NAITP) associated with anti-Yukb (HPA-4a) antibodies

Neonatal alloimmune thrombocytopenic purpura (NAITP) is one of the causes of thrombocytopenia in the newborn period. The thrombocytopenia is caused by maternal transplacental antiplatelet alloantibodies. We report a case of NAITP in a newborn infant having subarachnoid hemorrhage. Examination of platelet antibodies revealed anti-Yukb, that is, human platelet antigen (HPA)-4a incompatibility. Cranial ultrasound and brain magnetic resonance imaging revealed subarachnoid hemorrhage in the temporal region inferior to the cephalohematoma. The lesion seemed to have been sustained during delivery. The patient was treated with high-dose gamma-globulin and several transfusions of random donor platelets and showed a good clinical course. This is the second reported case of NAITP associated with the Yuk antigen system having intracranial hemorrhage.

Neonatal alloimmune thrombocytopenia involving HPA-5b (Br(a)): a rare Japanese case

Neonatal alloimmune thrombocytopenia (NAIT) is caused by platelet antigen incompatibility between the mother and fetus. The frequency of NAIT varies among ethnic groups. In Caucasians, HPA-5b (Br(a)) is the antigen that is second most frequently implicated. In Japan, NAIT due to anti-HPA-5b antibody is quite rare. The present case is the second case of Br(a)-NAIT in Japanese and the first case serologically confirmed by monoclonal antibody-specific immobilization of platelet antigens.

[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.

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.

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.

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.

On the association of the platelet-specific alloantigen, Pena, with glycoprotein IIIa. Evidence for heterogeneity of glycoprotein IIIa

Neonatal alloimmune thrombocytopenic purpura associated with a new platelet-specific alloantigen Pena has been reported. We now provide direct evidence that the Pena determinant is associated with glycoprotein (GP) IIIa, but that it is distinct from epitopes that define the PlA system. By ELISA wherein monoclonal antibodies specific for GPIIb (Tab) and specific for GPIIIa (AP3) were used to capture and hold antigens from a platelet lysate prepared under conditions that generate free GPIIb and GPIIIa, anti-Pena reacted with GPIIIa held by AP3 but not with GPIIb held by Tab. In an alternative ELISA where purified GPIIIa from both PlA1-positive and PlA1-negative platelets were used individually as antigen, anti-Pena reacted with both allelic forms of GPIIIa. By radioimmuno-precipitation, anti-Pena precipitated a single surface-labeled membrane protein with electrophoretic characteristics in sodium dodecyl sulfate-polyacrylamide gels, under nonreduced or reduced conditions, identical to those of GPIIIa. By fluorocytometry, platelets from several donors, regardless of PlA phenotype, bound an amount of anti-Pena roughly equivalent to one-half that amount of anti-PlA1 bound by PlA1 homozygous (A1/A1) platelets and roughly equal to that amount of anti-PlA1 bound by PlA1 heterozygous (A1/A2) platelets. Using platelets from donors typed homozygous for PlA1 and Pena in a quantitative indirect binding assay, 14-24,000 molecules of anti-Pena and 41-51,000 molecules of anti-PlA1 were bound per platelet at saturation. Anti-Pena completely inhibited ADP-induced aggregation of Pena-positive platelets, regardless of PlA phenotype. These results indicate that the Pena determinant is associated with GPIIIa but distinct from PlA.