Severe neonatal alloimmune thrombocytopenia caused by antibodies to human platelet antigen 3a (Baka) detectable only in whole platelet assays

O-linked sialic acid residues on platelet membrane glycoprotein IIb mask the human HPA-9b alloepitope

Sialic acids occupy the terminal position of glycan chains and have the potential to influence the antigenicity of glycoproteins (GP). The polymorphisms of human platelet alloantigens (HPA)-3 and HPA-9, located near the C-terminus of the extracellular domain of platelet membrane GPIIb, are adjacent to sialyl-core 1 O-glycans emanating from serines 845 and 847. Whether the nearby O-glycans affect the antigenicity of HPA-9b or influence the binding of anti-HPA-9b alloantibodies in clinically significant cases of neonatal alloimmune thrombocytopenia is unknown. To address this issue, we generated a series of O-glycan mutant HPA-9 allele-specific induced pluripotent stem cell lines, differentiated them to megakaryocytes (MKs), and examined their ability to bind HPA-9b-specific alloantibodies. We found that both wild-type MKs treated with neuraminidase and those genetically modified to lack the sialidases ST3GAL1 and ST3GAL2 dramatically increased anti-HPA-9b alloantibody binding, indicating that the HPA-9b epitope is partially masked by terminal sialic acids on nearby O-glycans of GPIIb. Interestingly, mutating the serine residues that carry these glycan chains to alanine actually reduced the binding of anti-HPA-9b alloantibodies, indicating that these 2 O-glycan chains contribute to the presentation of the HPA-9b epitope-perhaps by stabilizing the conformation of the GP in this region. Collectively, our data suggest that detection of anti-HPA-9b alloantibodies may be enhanced through the use of HPA-9b-specific MKs that have been genetically altered to lack nearby terminal sialic acid residues but retain the glycan chains to which they are attached.

Fetale und neonatale Alloimmunthrombozytopenie, Teil 1

Die fetale und neonatale Alloimmunthrombozytopenie (FNAIT) wird durch mütterliche Antikörper hervorgerufen, die gegen ein vom Vater ererbtes Blutgruppenmerkmal an fetalen Thrombozyten gerichtet sind. Teil 1 des Beitrags stellt die Ätiologie, die Pathogenese und die Diagnostik der FNAIT dar, während Teil 2 die Risikostratifizierung und Behandlung der FNAIT thematisiert 1.

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.

Progress and development of platelet antibody detection

Human platelet antibody (HPA) detection is necessary for the diagnosis and therapeutic decisions for refractoriness to platelet transfusions, post transfusion purpura and fetal and neonatal alloimmune thrombocytopenia. In the last four to five decades many new developments, both in knowledge and methods, have increased the quality of platelet serology. However, the quest for the optimal antibody detection method(s) encountered, sometimes unexpected, difficulties. In this review the various aspects concerning platelet antibody test methods and detection of platelet antibodies both for the diagnostic and screening setting are discussed.

Detection and identification of platelet antibodies using a sensitive multiplex assay system-platelet antibody bead array

BACKGROUND

Tests for platelet-specific antibodies are important in the diagnosis of immune platelet disorders. Monoclonal antibody glycoprotein capture assays have been the gold standards in platelet antibody detection for almost 30 years. However, such assays are complex and cumbersome to perform, which limits their routine use. We report the performance of a newly developed, easy to perform platelet antibody bead array (PABA) for the detection of platelet-specific antibodies.

STUDY DESIGN AND METHODS

PABA is the equivalent of the monoclonal antigen capture enzyme-linked immunosorbent assay (ELISA) (MACE) on a bead and instead with fluorescent detection of immunoglobulin (Ig)G platelet antibodies by Luminex. Antibodies against platelet glycoproteins (GP) GPIIb/IIIa, GPIb/IX, GPIa/IIa, GPIV, and class I human leukocyte antigen (HLA) could be detected in a patient's serum simultaneously in a single well of a microplate. Results from 80 patient samples and 20 normal donor samples were compared using PABA, MACE, and a commercial ELISA kit.

RESULTS

PABA detected all antibodies, with specificity for human platelet antigens (HPAs) HPA-1a, HPA-1b, HPA-2a, HPA-2b, HPA-3a, HPA-3b, HPA-4a, HPA-4b, HPA-5a, HPA-5b, HPA-15b, and HLA. Overall, compared with MACE, the sensitivity and specificity of PABA were 99% and 97%, respectively, and both were significantly better than those of the commercial ELISA. PABA had greater analytic sensitivity than MACE for HPA-1a, HPA-3a, and HPA-5b antibodies. In addition, PABA detected HPA-5b and HPA-3b antibodies that were missed by MACE. The overall false-positive rate of PABA compared with MACE was 2.7%.

CONCLUSIONS

The PABA is a rapid, highly sensitive and specific, multiplex bead-based assay for detecting human platelet antibodies. Such a simple yet high-throughput platform will facilitate practical, routine testing for the identification of platelet-specific antibodies.

Human platelet antigen (HPA)-specific immunoglobulin M antibodies in neonatal alloimmune thrombocytopenia can inhibit the binding of HPA-specific immunoglobulin G antibodies

BACKGROUND

A term baby with unexplained thrombocytopenia and a platelet (PLT) count of 14 × 10⁹ /L (maternal PLT count was 200 × 10⁹ /L) was investigated for neonatal alloimmune thrombocytopenia.

STUDY DESIGN AND METHODS

Serologic investigations were performed using the PLT immunofluorescence test (PIFT), monoclonal antibody immobilization of PLT antigens (MAIPA), and a bead-based assay (BBA) with maternal sera taken up to 56 days postdelivery. One serum sample was also separated into "immunoglobulin (Ig)M-rich" and "IgM-depleted" fractions and tested for PLT-specific antibodies. The family was genotyped for HPA.

RESULTS

HPA-3a-specific IgM antibodies were detected in the PIFT and confirmed in the BBA. PLT-specific IgG HPA-3a antibodies were not detected in the MAIPA assay and BBA in the initial sample but were detected in both techniques in subsequent serum samples. Testing of IgM-rich and IgM-depleted fractions in the MAIPA assay revealed that IgG antibody binding of the IgM-depleted fraction was inhibited by approximately 50% when it was reconstituted with the IgM-rich fraction suggesting that the IgM antibodies blocked the binding of the IgG antibodies. This effect was not observed when the IgM-depleted fraction or untreated serum was diluted with elution buffer. Incompatibility for HPA-3 was identified between the mother and the infant. The infant received one HPA-1a, -5b negative neonatal PLT transfusion, and one random PLT transfusion, with satisfactory outcomes. Both units were later found to be HPA-3b3b.

CONCLUSION

HPA-3a IgM antibodies can inhibit PLT-specific HPA-3a IgG antibodies in the MAIPA assay.

CRISPR/Cas9-mediated conversion of human platelet alloantigen allotypes

Human platelet alloantigens (HPAs) reside on functionally important platelet membrane glycoproteins and are caused by single nucleotide polymorphisms in the genes that encode them. Antibodies that form against HPAs are responsible for several clinically important alloimmune bleeding disorders, including fetal and neonatal alloimmune thrombocytopenia and posttransfusion purpura. The HPA-1a/HPA-1b alloantigen system, also known as the Pl(A1)/Pl(A2) polymorphism, is the most frequently implicated HPA among whites, and a single Leu33Pro amino acid polymorphism within the integrin β3 subunit is responsible for generating the HPA-1a/HPA-1b alloantigenic epitopes. HPA-1b/b platelets, like those bearing other low-frequency platelet-specific alloantigens, are relatively rare in the population and difficult to obtain for purposes of transfusion therapy and diagnostic testing. We used CRISPR/Cas9 (clustered regularly interspaced short palindromic repeats/CRISPR associated protein 9) gene-editing technology to transform Leu33 (+) megakaryocytelike DAMI cells and induced pluripotent stem cells (iPSCs) to the Pro33 allotype. CD41(+) megakaryocyte progenitors derived from these cells expressed the HPA-1b (Pl(A2)) alloantigenic epitope, as reported by diagnostic NciI restriction enzyme digestion, DNA sequencing, and western blot analysis using HPA-1b-specific human maternal alloantisera. Application of CRISPR/Cas9 technology to genetically edit this and other clinically-important HPAs holds great potential for production of designer platelets for diagnostic, investigative, and, ultimately, therapeutic use.

Advances in alloimmune thrombocytopenia: perspectives on current concepts of human platelet antigens, antibody detection strategies, and genotyping

Alloimmunisation to platelets leads to the production of antibodies against platelet antigens and consequently to thrombocytopenia. Numerous molecules located on the platelet surface are antigenic and induce immune-mediated platelet destruction with symptoms that can be serious. Human platelet antigens (HPA) cause thrombocytopenias, such as neonatal alloimmune thrombocytopenia, post-transfusion purpura, and platelet transfusion refractoriness. Thirty-four HPA are classified into 28 systems. Assays to identify HPA and anti-HPA antibodies are critically important for preventing and treating thrombocytopenia caused by anti-HPA antibodies. Significant progress in furthering our understanding of HPA has been made in the last decade: new HPA have been discovered, antibody-detection methods have improved, and new genotyping methods have been developed. We review these advances and discuss issues that remain to be resolved as well as future prospects for preventing and treating immune thrombocytopenia.

Platelet antibody analysis by three different tests

BACKGROUND

Platelet-reactive antibodies lead to thrombocytopenia and bleeding disorders, and diverse assays are used for their detection. In this retrospective analysis, the applicability of three different test systems was compared and antibody specificities were assessed.

METHODS

Sera of 1,234 patients were tested with an enzyme-linked immunosorbent assay (ELISA; Lifecodes PAKPLUS(®) or PAK 12(®), Gen-Probe) and a solid-phase assay (Capture-P Ready Screen(®), Immucor Inc.). In cases of suspected anti-HLA class I antibodies, a specific lymphocytotoxicity test (LCT, Bio-Rad(®)) was performed.

RESULTS

Platelet antibodies were detected in 366 of 1,234 samples (29.7%). In 70.3% concordant negative but only in 8.4% concordant positive results were obtained with both the methods; 185 of 1,053 in the solid-phase assay negative samples were positive in the ELISA (15.0%). In samples positive in both methods, most antibodies reacted against HLA class I antigens. Glycoprotein (GP) specific platelet antibodies, mainly against GPIIb/IIIa and GPIa/IIa, were more frequently detectable in the ELISA than in the solid-phase assay, whereas weakly positive results have to be interpreted cautiously.

CONCLUSION

ELISA, solid-phase assay, and LCT showed highly divergent results. Due to several limitations, the additional analysis by the "monoclonal antibody-specific immobilization of platelet antigen" (MAIPA)-assay is highly recommended.

Human platelet antigens - 2013

To date, 33 human platelet alloantigens (HPAs) have been identified on six functionally important platelet glycoprotein (GP) complexes and have been implicated in alloimmune platelet disorders including foetal and neonatal alloimmune thrombocytopenia (FNAIT), posttransfusion purpura (PTP) and multitransfusion platelet refractoriness (MPR). The greatest number of recognized HPA (20 of 33) resides on the GPIIb/IIIa complex, which serves as the receptor for ligands important in mediating haemostasis and inflammation. These include HPA-1a, the most commonly implicated HPA in FNAIT and PTP in Caucasian populations. Other platelet GP complexes, GPIb/V/IX, GPIa/IIa and CD109, express the remaining 13 HPAs. Of the recognized HPAs, 12 occur as six serologically and genetically defined biallelic 'systems' where the -a form designates the higher frequency allele and the -b form, the lower. Twenty-one other HPAs are low-frequency or rare antigens for which postulated higher frequency -a alleles have not yet been identified as antibody specificities. In addition to the HPA markers, platelets also express ABO and human leucocyte antigen (HLA) antigens; antibodies directed at the former are occasionally important in FNAIT, and to the latter, in MPR.

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.

Report on the 15th International Society of Blood Transfusion platelet immunology workshop

BACKGROUND AND OBJECTIVES

The aim of the 15th ISBT Platelet Immunology Workshop was to evaluate the detection of free platelet-reactive autoantibodies from ITP patients by the use of a standardized MAIPA protocol, to compare sensitivity and specificity of antibody detection for anti-HPA-1a and serologically difficult-to-assess antibodies against HPA-3, to identify whether anti-HPA-1a titration results can be compared between laboratories, and to evaluate HPA genotyping methods.

MATERIALS AND METHODS

Workshop materials were shipped from the organizing laboratory in Giessen, Germany. Thirty laboratories from 19 countries participated.

RESULTS

Results for the detection of autoantibodies differed greatly between the laboratories and no consensus was reached for one of the two sera. Detection and titration of antibodies against HPA-1a, in contrast, gave largely congruent results. Serologically difficult-to-assess antibodies recognizing HPA-3a and HPA-3b were not detected by many laboratories. For genotyping, good agreement was achieved.

CONCLUSIONS

Detection of HPA-1a antibodies, titration of anti-HPA-1a, and HPA genotyping are well performed in most participating laboratories. The workshop has identified two specific areas with room and need for improvement: the detection of autoantibodies and the detection of HPA-3 alloantibodies. Recommendations of the Working Party on techniques that can help to overcome these problems are desirable.

The αIIb p.Leu841Met (Cab3(a+) ) polymorphism results in a new human platelet alloantigen involved in neonatal alloimmune thrombocytopenia

BACKGROUND

Fetal-neonatal alloimmune thrombocytopenia (FNAIT) diagnosis relies on maternofetal incompatibility and alloantibody identification. Genotyping for rare platelet (PLT) polymorphisms allowed the identification of three families with suspected or confirmed maternofetal incompatibility for the αIIb-c.2614C>A mutation (Halle et al., Transfusion 2008;48:14-15).

STUDY DESIGN AND METHODS

A polymerase chain reaction-sequence-specific primers amplification assay was designed to genotype the αIIb-c.2614C>A mutation. HEK293 cells expressing αIIb-Leu841 or αIIb-Met841 αIIbβ3 forms were used to probe the reactivity of maternal sera from these families and to study the effects of the substitution on αIIbβ3 expression and functions.

RESULTS

Tested by flow cytometry (FCM), one serum sample specifically reacted with αIIb-Met841 but not with αIIb-Leu841 αIIbβ3. This specificity revealed the αIIb-Leu841 polymorphism as a new alloantigen named Cab3(a+) . Cross-match testing using FCM also showed the Cab3(a+) antigen to be expressed at the PLT surface. As for anti-human PLT alloantigen (HPA)-3a (or -3b) and anti-HPA-9bw, detection of anti-Cab3(a+) alloantibodies appeared difficult and required whole PLT assays when classical monoclonal antibody-specific immobilization of PLT antigen test failed. In our FNAIT set, the immune response to Cab3(a+) maternofetal incompatibility could induce severe thrombocytopenias and life-threatening hemorrhages. The p.Leu841Met substitution has limited effects, if any, on local αIIb structure, preserving both αIIbβ3 expression and functions.

CONCLUSION

The Cab3(a+) polymorphism is a new rare alloantigen (allelic frequency <1%) carried by αIIb that might result in severe life-threatening thrombocytopenias. In Sub-Saharan African populations, higher Cab3(a+) gene frequencies (up to 8.2%; Halle et al., Transfusion 2008;48:14-15) and homozygous people are observed.

Human platelet antigen 1a epitopes are dependent on the cation-regulated conformation of integrin α(IIb)β(3) (GPIIb/IIIa)

BACKGROUND

The HPA-1a (Leu(33)) polymorphism of platelet integrin αIIbβ3 is the target of alloantibodies in 70-80% cases of neonatal alloimmune thrombocytopenia (NAIT) in Caucasians and reliable detection of these antibodies is essential for appropriate clinical management. However, the ability to detect such antibodies is highly variable between laboratories and, in a number of clinical cases where there is a HPA-1 genotype mismatch between mother and neonate, HPA-1a antibodies are undetectable. Furthermore, some studies have not shown a consistent relationship between maternal anti-HPA-1a level and neonatal platelet count. Since the integrity and conformation of the αIIbβ3 complex are dependent on divalent cations, we investigated whether HPA-1a epitope integrity and/or conformation might be affected by the presence of the cation chelator EDTA in patient samples or in assay buffers, thus providing a possible explanation for the variable sensitivity of current assays.

PRINCIPLE FINDINGS

Exposure of the αIIbβ3 complex to EDTA resulted in reduced reactivity of three anti-HPA-1a mAbs (B2, 19-7 and 23-15). More significantly, cation chelation adversely affected detection of polyclonal anti-HPA-1a, not only in the platelet immunofluorescence assay, where alloantibody binding was reduced compared to control platelets (mean MFI reduction 44.5%, range 17.3-69.7%, n=4), but also in the commonly used monoclonal antibody specific immobilisation of platelet antigens assay (MAIPA) where both alloantibody and monoclonal capture antibody binding were reduced (mean OD reduction 82.8%, range 68.3-96.6%, n=9).

CONCLUSIONS

These data show that HPA-1a antibodies recognise epitopes on αIIbβ3 that are sensitive to EDTA treatment and that cation chelation grossly reduces the sensitivity of the MAIPA assay by diminishing not only HPA-1a alloantibody binding but also 'capture' monoclonal antibody binding. These findings may, in part, explain the current variability in antibody measurement and will guide the development of more sensitive tests for anti-integrin antibodies in NAIT and other conditions.

[Maternal-fetal chikungunya infection associated with Bernard-Soulier syndrome]

Neonatal thrombocytopenia may stem from several origins. A newborn was hospitalized on the 3rd day of life with thrombocytopenia likely due to maternofetal autoimmune thrombocytopenia. Two days later, he displayed a chikungunya virus infection, during a severe epidemic on Reunion Island. The characterization of human platelet antigen (HPA) in the parents showed incompatibility in the HPA-3 system; however, no detectable antiHPA-3a antibody was found in the mother's serum. The persistence of moderate thrombocytopenia led to the diagnosis, using flow cytometry, of Bernard-Soulier syndrome (BSS), with no detectable GPIbIX. This case shows that neonatal inherited thrombocytopenia may be difficult to characterize, especially when it is associated with an acquired cause.

Low-avidity HPA-1a alloantibodies in severe neonatal alloimmune thrombocytopenia are detectable with surface plasmon resonance technology

BACKGROUND

Fetal and neonatal alloimmune thrombocytopenia (FNAIT) is mostly caused by maternal alloantibodies directed against the human platelet alloantigen (HPA)-1a. Currently, the serologic diagnosis of FNAIT is based on the characterization of the HPA alloantibodies in monoclonal antibody-based antigen-capture assays (e.g., MAIPA assay). Accumulated current evidence indicated that such assays may overlook some HPA-1a antibodies.

STUDY DESIGN AND METHODS

This study employed surface plasmon resonance (SPR) technology using immunoaffinity-purified glycoprotein IIb/IIIa isoforms immobilized on biosensor chips to study the binding kinetics of HPA-1a alloantibodies from different FNAIT cases in real time.

RESULTS

Analysis of HPA-1a alloantibodies from FNAIT cases (n = 9) in SPR showed a moderate relative response (22.2-69.7 resonance units [RU]) and slow antibody dissociation. After the dissociation phase, varying amounts of bound antibodies (41%-79%) remained on the chip. In contrast in HPA-1a alloantibodies from a patient suffering from posttransfusion purpura, a high relative response (approximately 490 RU) was observed at the end of the association phase and no dissociation of antibody binding was detectable. Of particular relevance, by the use of this SPR technique, HPA-1a alloantibodies were detected in two severe FNAIT cases that had determined as false negative by MAIPA assay. In SPR, these HPA-1a alloantibodies showed low-avidity nature characterized by gradual dissociation of antibody during the association phase and complete detachment of antibody binding after the dissociation phase. This high "off-rate" character of low-avidity HPA-1a alloantibodies indicates that such antibody binding is easily detachable by the extensive washing procedure of the MAIPA.

CONCLUSIONS

Our results demonstrated that the SPR method can facilitate the diagnosis of clinically relevant low-avidity HPA-1a antibodies.

Neonatal alloimmune thrombocytopenia associated with maternal-fetal incompatibility for blood group B

BACKGROUND

Blood group A and B antigens are expressed only weakly on platelets (PLTs) of most individuals but are very strongly expressed on PLTs from approximately 1 percent of normal subjects (Type II high expressers). The implications of this trait for transfusion medicine are undefined.

STUDY DESIGN AND METHODS

A family was studied in which two Group B infants were born with neonatal thrombocytopenia, whereas a third infant whose blood group was A(2) had a normal PLT count at birth.

RESULTS

Serologic studies demonstrated a maternal antibody that reacted strongly with PLTs from the father and the two group B children in flow cytometry and with GPIIb/IIIa from their PLTs in solid-phase assays. No PLT-specific antibodies were detected in maternal serum sample, but it contained a high-titer immunoglobulin G antibody specific for blood group B. All PLT-reactive antibody in the mother's serum was removed by absorption with pooled, washed group A and B red cells (RBCs). Studies with monoclonal anti-B and measurement of serum B-glycosyltransferase activity showed that the father and both group B children were Type II high expressers of blood group B.

CONCLUSIONS

The findings indicate that high-titer blood group antibodies acquired from the mother can cause thrombocytopenia in infants possessing the Type II high-expresser phenotype despite competition for antibody binding by blood group antigens expressed on RBCs and other tissues.

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.

Interlaboratory variation in the detection of HPA-specific alloantibodies and in molecular HPA typing

BACKGROUND AND OBJECTIVES

Platelet immunology quality assurance exercises have been organized by National Institute for Biological Standards and Control since 1991 and, as of 2006, 35 laboratories participate in the serology section. Molecular human platelet antigen (HPA) typing has been included in the exercises since 1998 and, as of 2006, 29 laboratories participate in this part of the exercise. This report details the performance of laboratories in these two areas.

MATERIALS AND METHODS

Every 6 months laboratories are sent up to four coded serum/plasma samples for testing in their in-house assays for HPA antibody detection/identification and four coded whole blood samples to be typed for HPA-1, -2, -3 -5 and (since 2003) -15 by their molecular typing assays ('genotyping').

RESULTS

Fifty-two sera containing HPA-specific alloantibodies and 13 sera that were inert, contained only human leucocyte antigen (HLA) class I or high-titre anti-A+B antibodies were distributed; 15 sera were issued in more than one exercise. The percentage of participating laboratories that were able to detect HPA-specific alloantibodies ranged from 15.0 to 100%; the percentage that were able to correctly determine the specificities also ranged from 15.0 to 100%. Over the 20 serology exercises the percentage of laboratories classified as poor performers ranged from 3.1 to 36.7%. A total of 12 780 individual HPA genotyping results were assessed. The overall error rate was 0.7% but there was considerable variation between HPA alleles. Over 11 exercises the percentage of laboratories classified as poor performers varied from 6.3 to 26.3%.

CONCLUSIONS

The ability to detect and to identify platelet-specific alloantibodies varied widely between laboratories and between various examples of antibodies issued. An increase in the number of laboratories screening for HPA-15 antibodies was seen, although detection and identification of these antibodies was problematic. The majority of examples of HPA-3a antibodies and some examples of HPA-1a and -5b were also difficult to detect and identify. In addition, this scheme has shown that despite the apparent reliability of molecular typing techniques, mistakes do occur, particularly with certain systems. Approximately one in five laboratories participating in the serology exercises and one in seven participating in the genotyping exercises were classified as poor performer at one point or more during the series of exercises.

The importance of using multiple techniques for detection of platelet antibodies

In red cell immunology, it has long been known that no one technique will detect all clinically significant antibodies. The same appears to be true for platelet immunology, and we highlight this fact by showing four examples of anti-human platelet antigen-1a that were not detected by the monoclonal antibody-specific immobilization of platelet antigen test, the most commonly used technique. Each antibody was found in a case of fetomaternal alloimmune thrombocytopenia in which the fetus or neonate was severely affected.

Collaborative studies to establish the first World Health Organization International Standard for detection of human antibody against human platelet antigen-3a

BACKGROUND AND OBJECTIVES

The platelet-specific antibody anti-human platelet antigen-3a (anti-HPA-3a) is involved in neonatal alloimmune thrombocytopenia, post-transfusion purpura, and platelet refractoriness. However, HPA-3a antibodies are often difficult to detect, probably because the antigen is labile. This report describes the production of a freeze-dried preparation of pooled human plasma, coded 03/190, containing IgG antibodies against the HPA-3a. The material is intended for use as a minimum sensitivity reagent in glycoprotein-specific assays currently used for anti-HPA-3a detection. Laboratories can use it to assess the sensitivity of their 'in-house' assays for anti-HPA-3a and to calibrate local controls for routine use in each batch of tests.

MATERIALS AND METHODS

Plasma containing anti-HPA-3a was obtained from a mother of two babies both born with severe thrombocytopenia, and following dilution it was freeze dried in glass ampoules.

RESULTS

Two collaborative studies demonstrated that the candidate material contained anti-HPA-3a and human leucocyte antigen (HLA) class I antibodies, but no other HPA antibodies that might confuse the detection of the anti-HPA-3a. The minimum dilution that should give a positive result was determined to be 1 : 8 by two further international collaborative studies involving a total of 49 laboratories in 23 countries.

CONCLUSION

The material also contains HLA antibodies and is suitable for use only in techniques that are glycoprotein specific (i.e. monoclonal antibody immobilization of platelet antigens and enzyme-linked immunosorbent assay) where only HPA antibodies will be detected. This standard will allow laboratories to measure their sensitivity of detection of anti-HPA-3a and will also allow those laboratories with relatively insensitive techniques to monitor their performance as they improve their methodology.

Rapid detection of HPA-1 alloantibodies by platelet antigens immobilized onto microbeads

BACKGROUND

Neonatal alloimmune thrombocytopenia (NAIT) is one of the most common bleeding disorders in neonates. It occurs when alloantibodies from an immunized mother react with paternally inherited alloantigens, mostly human platelet antigen 1a (HPA-1a), on the fetal platelets (PLTs). Currently, monoclonal antibody-immobilized PLT antigen (MAIPA) assay represents the standard technique for the serologic diagnosis of NAIT. MAIPA is time-consuming, however, and limited by the availability of monoclonal antibodies (MoAbs). Here, a gel antigen-specific assay (GASA) was developed, which allows rapid detection of HPA-1 alloantibodies without the use of MoAbs.

STUDY DESIGN AND METHODS

Glycoprotein (GP) IIb/IIIa was purified by affinity chromatography from outdated PLT concentrates derived from HPA-1aa or HPA-1bb donors. Purified GPs were biotinylated, immobilized onto streptavidin beads, and used for the analysis of HPA-1a alloantibodies by a microtyping system. HPA-1a serum samples derived from mothers with NAIT (n = 36) and from posttransfusion purpura patients (n = 2) as well as HPA-1b (n = 4), HPA-5b (n = 2), HPA-3a (n = 4), and HLA Class I (n = 2) alloantiserum samples from multitransfused patients were investigated in GASA and MAIPA assays.

RESULTS

GASA was able to detect all HPA-1a and -1b alloantibodies recognized by MAIPA. Cross-reactivity with other PLT-reactive alloantibodies was not observed. Interestingly, 3 of 36 serum samples, which showed only moderate reactivity in MAIPA, reacted strongly in GASA.

CONCLUSION

GASA has proved to be a rapid method for the detection of HPA-1a alloantibodies and maybe useful for PLT antibody screening, especially in initial assessment of suspected NAIT cases.

Patients with quinine-induced immune thrombocytopenia have both "drug-dependent" and "drug-specific" antibodies

Immune thrombocytopenia induced by quinine and many other drugs is caused by antibodies that bind to platelet membrane glycoproteins (GPs) only when the sensitizing drug is present in soluble form. In this disorder, drug promotes antibody binding to its target without linking covalently to either of the reacting macro-molecules by a mechanism that has not yet been defined. How drug provides the stimulus for production of such antibodies is also unknown. We studied 7 patients who experienced severe thrombocytopenia after ingestion of quinine. As expected, drug-dependent, platelet-reactive antibodies specific for GPIIb/IIIa or GPIb/IX were identified in each case. Unexpectedly, each of 6 patients with GPIIb/IIIa-specific antibodies was found to have a second antibody specific for drug alone that was not platelet reactive. Despite recognizing different targets, the 2 types of antibody were identical in requiring quinine or desmethoxy-quinine (cinchonidine) for reactivity and in failing to react with other structural analogues of quinine. On the basis of these findings and previous observations, a model is proposed to explain drug-dependent binding of antibodies to cellular targets. In addition to having implications for pathogenesis, drug-specific antibodies may provide a surrogate measure of drug sensitivity in patients with drug-induced immune cytopenia.

A simple and practical assay for the antigen-specific detection of platelet antibodies

BACKGROUND

The antigen-specific assays currently used for characterization of platelet (PLT)-reactive auto- and alloantibodies are too technically complex and impracticable for most routine laboratories. Here, a novel antigen-specific particle assay (ASPA) for PLTs similar to that of red blood cells is described.

STUDY DESIGN AND METHODS

PLTs were solubilized and then incubated with red-dyed polystyrene particles coated with monoclonal antibodies (MoAbs) to various PLT glycoprotein complexes. These particles were directly tested for coating with autoantibodies (n = 8) or indirectly tested for serum autoantibodies (n = 33) or alloantibodies against HPA-1a (n = 4) or HPA-5b (n = 5). Serum samples from healthy blood donors (n = 100) served as negative controls.

RESULTS

Negative reactions were clearly distinguishable from positive reactions, and the results of the particle assay were in concordance with those obtained by the standard MoAb-specific immobilization of PLT antigen assay (MAIPA) in all cases with alloanti-bodies. In three patients, only the ASPA was able to detect autoantibodies that were completely undetectable by the MAIPA. In contrast, in only one patient, the MAIPA detected autoantibodies that the ASPA failed to detect.

CONCLUSION

In our opinion, the new ASPA is reliable, yet less complex and time-consuming than the currently available assays, and it can be implemented in any routine laboratory.

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.

Report on the 12th International Society of Blood Transfusion platelet immunology workshop

BACKGROUND AND OBJECTIVES

The aims of the 12th International Society of Blood Transfusion (ISBT) Platelet Immunology Workshop were to evaluate the proficiency of molecular human platelet antigen (HPA) genotyping and detection of platelet antibodies of unusual specificity or reactivity, to assess whether quantification of anti-HPA-1a is practicable, and to determine the variability of reagents and components used in the monoclonal antibody immobilization of platelet antigens assay (MAIPA).

MATERIALS AND METHODS

Forty participants from 23 countries were sent 10 samples for DNA typing, five samples for antibody detection, a freeze-dried anti-HPA-1a standard, three samples for anti-HPA-1a quantification and a MAIPA method questionnaire.

RESULTS

The detection and identification of HPA antibodies varied from 2.7 to 95% of participants. The number of HPA genotyping errors per sample ranged from 0 to 3.96% per HPA loci. The majority of laboratories were able to assign an arbitrary number of units/ml of anti-HPA-1a activity to the unknown samples. The MAIPA questionnaire indicated a wide variation among participants, both in method and in reagents used.

CONCLUSIONS

The results obtained from this workshop highlighted deficiencies in testing regimes and identified a need for internationally available reference materials.

Therapeutic expression of the platelet-specific integrin, alphaIIbbeta3, in a murine model for Glanzmann thrombasthenia

Integrins mediate the adhesion of cells to each other and to the extracellular matrix during development, immunity, metastasis, thrombosis, and wound healing. Molecular defects in either the alpha- or beta-subunit can disrupt integrin synthesis, assembly, and/or binding to adhesive ligands. This is exemplified by the bleeding disorder, Glanzmann thrombasthenia (GT), where abnormalities of the platelet-specific integrin, alphaIIbbeta3, prevent platelet aggregation following vascular injury. We previously used a retrovirus vector containing a cDNA cassette encoding human integrin beta3 to restore integrin alphaIIbbeta3 on the surface of megakaryocytes derived from peripheral blood stem cells of GT patients. In the present study, bone marrow from beta3-deficient (beta3-/-) mice was transduced with the ITGbeta3-cassette to investigate whether the platelet progeny could establish hemostasis in vivo. A lentivirus transfer vector equipped with the human ITGA2B gene promoter confined transgene expression to the platelet lineage. Human beta3 formed a stable complex with murine alphaIIb, effectively restoring platelet function. Mice expressing significant levels of alphaIIbbeta3 on circulating platelets exhibited improved bleeding times. Intravenous immunoglobulin effectively diminished platelet clearance in animals that developed an antibody response to alphaIIbbeta3. These results indicate the feasibility of targeting platelets with genetic therapies for better management of patients with inherited bleeding disorders.

Les thrombopénies fœtales et néonatales allo-immunes : problèmes en suspens

The allo-immune thrombocytopenias are the major cause of severe thrombocytopenia in the fetus and the neonate. The frequency of this affection has been evaluated to be 1 out of 800 to 1000 live births. The deleterious consequences of severe thrombocytopenia are intracranial hemorrhages leading to death or neurological sequelae. Progress in platelet immunology and antenatal medicine has allowed a better diagnosis of this affection and development of the antenatal management of high-risk pregnancies. Nevertheless there are unsolved questions, particularly concerning the mechanism of the maternal immunization, the laboratory difficulties for the identification of the alloantibodies, the absence of antenatal management standardization.

Neonatal alloimmune thrombocytopenia due to an antibody against a labile component of human platelet antigen-3b (Bak b)

We report the second case of neonatal alloimmune thrombocytopenia due to anti-human platelet antigen (HPA)-3b (Bak(b)) antibody. The infant was severely affected with intracranial haemorrhage. Most importantly, we were able to detect this antibody only by using fresh, unfixed platelets as antigen. This antibody was detectable neither by conventional mixed passive haemagglutination, platelet immunofluorescence test using fixed platelets, nor by monoclonal antigen immobilization of platelet antigen assay using solubilized platelets. We assume that this antibody reacts only with fresh platelets and that the antigen is a labile component of HPA-3b (Bak(b)).

Relevance of the HPA-15 (Gov) polymorphism on CD109 in alloimmune thrombocytopenic syndromes

BACKGROUND

Alloantibodies against the human platelet (PLT) alloantigen (HPA)-15 system residing on CD109 can cause fetal and neonatal alloimmune thrombocytopenia (FNAIT), posttransfusion purpura, and PLT transfusion refractoriness. The detection of antibodies against HPA-15, however, is hampered by the variable low expression and instability of the CD109 molecule during preparation and storage.

STUDY DESIGN AND METHODS

This study analyzed the occurrence of HPA-15 alloantibodies in 1403 patients: 930 FNAIT and 473 polytransfused (PT) patients by modified monoclonal antibody specific immobilization of PLT antigens (MAIPA) assay with well-defined phenotyped PLTs. A DNA typing technique was developed to confirm the phenotypes of PLT donors. B-cell lines were established as sources of reference DNA.

RESULTS

Genotyping of 407 unrelated blood donors revealed the gene frequencies 0.512 and 0.488 for HPA-15a and -15b, respectively. Based on the selection of PLTs expressing high amounts of CD109 on the surface (mean fluorescence intensity ratio 4-5 on expression peak on Days 2-4 after apheresis) antibody screening by the MAIPA assay was performed. In total, 16 (1.1%) HPA-15 alloantibodies were found comprising four anti-HPA-15a and 12 anti-HPA-15b. Anti-HPA-15b without other PLT-reactive antibodies were detectable in three serum samples of PT patients. The incidence of HPA-15 alloimmunization in PT patients was significantly higher than in mothers with FNAIT (3.0% vs. 0.22%). In relation to all detected HPA-specific antibodies, HPA-15 is responsible for 6.2 percent of alloimmunizations.

CONCLUSION

These observations indicate that alloimmunization against HPA-15 should be considered as a cause for immune thrombocytopenia, particularly in patients receiving multiple PLT transfusions.

Human platelet antigen-specific alloantibodies implicated in 1162 cases of neonatal alloimmune thrombocytopenia

BACKGROUND

Neonatal alloimmune thrombocytopenia (NATP) caused by fetomaternal mismatch for human platelet (PLT) alloantigens (HPAs) complicates approximately 1 in 1000 to 1 in 2000 pregnancies and can lead to a serious bleeding diathesis, intracranial hemorrhage, and sometimes death of the fetus or neonate. As a national reference center for NATP investigations, our experience with this entity over a 12-year period was reviewed.

STUDY DESIGN AND METHODS

The laboratory records of all cases of suspected NATP referred for evaluation from January 1, 1990, to December 31, 2002, were analyzed. The spectrum of PLT alloantibody specificities identified was compared with an earlier reported series of serologically verified NATP cases.

RESULTS

HPA-specific alloantibodies were identified in 1162 (31%) of 3743 sera of mothers of infants with clinically suspected NATP. Maternal HPA-1a (PlA1) alloimmunization accounted for the majority (79%) of confirmed NATP cases, with HPA-5b (Bra), HPA-3a (Baka), and HPA-1b (PLA2) alloantibodies accounting for 9, 2, and 4 percent of cases, respectively. In addition, an increase in the number of cases in which multiple HPA-specific alloantibodies were present in maternal sera was observed during the study period.

CONCLUSION

Although, as with the earlier series, maternal HPA-1a alloimmunization was the dominant cause of NATP, the identification of an increasing number of cases due to alternative HPA polymorphisms suggests that investigation for HPA-1 incompatibility alone is no longer sufficient to fully evaluate clinically suspect NATP cases.