Heterogeneity of HPA-3 alloantibodies: consequences for the diagnosis of alloimmune thrombocytopenic syndromes

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.

Association of the human platelet antigens polymorphisms with platelet count in patients with COVID-19

Polymorphism in human platelet antigen (HPA)-1 and HPA-3 (GPIIb/IIIa), HPA-2 (GPIb/IX), HPA-4 (GPIIIa), HPA-5 (GPIa/IIa), & HPA-15 (CD109) was investigated in 86 COVID-19-infected patients with thrombocytopenia (Group A) and 136 COVID-19-infected patients without thrombocytopenia (Group B). HPA genotyping was done by the sequence-specific primers PCR method. Lower HPA-3a and higher HPA-3b (P = 0.028) allele frequencies were seen in Group A than in Group B, and homozygosity for HPA 3b (P = 0.038) alleles was more prevalent in Group A than in Group B. The allele and genotype distributions of the other HPA polymorphic variants were similar between the two groups. Univariate analysis identified the CCGGGC (P = 0.016) combined genotype to be negatively associated & the TCGGGC (P = 0.003) and CCGGGC (P = 0.003) to be positively associated with thrombocytopenia. The frequency of anti-HPA-1a and anti-HPA-3a antibodies was significantly higher in all patients compared to other anti-HPAs antibodies (P < 0.05). These results highlight the role of HPAs in the thrombocytopenia of COVID-19 infected patients. This is the first evidence demonstrating the differential association of the six common HPA gene variants and specific HPA genotype combinations with thrombocytopenia in COVID-19-infected patients.

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.

Posttransfusion purpura with antibodies against human platelet antigen-4a following checkpoint inhibitor therapy: a case report and review of the literature

BACKGROUND

Posttransfusion purpura (PTP) is a rare condition characterized by severe thrombocytopenia following receipt of blood products. Most reported PTP cases involve alloantibodies directed against human platelet antigen (HPA)-1a. We present a case of PTP-mediated severe thrombocytopenia associated with alloantibodies directed against HPA-4a in the setting of combination checkpoint inhibitor therapy.

CASE REPORT

A 62-year-old woman with rectal melanoma that progressed on combination checkpoint inhibitors (ipilimumab and nivolumab) was admitted for abdominoperineal resection. She received multiple blood products during surgery, and between the sixth and eighth days post-surgery her platelet (PLT) count decreased from 126 × 10⁹ /L to a nadir of 1 × 10⁹ /L. She received intravenous immunoglobulin (IVIG), steroids, and romiplostim with eventual recovery of her PLT count to 50 × 10⁹ /L 20 days after surgery. She tested positive for anti-HPA-4a and was shown not to express HPA-4a, confirming a diagnosis of PTP.

CONCLUSION

Alloantibodies strongly reactive to HPA-4a were detected in this patient who received multiple blood products during abdominoperineal resection surgery. Her thrombocytopenia improved with prompt administration of IVIG, steroids, and romiplostim. PTP must always be considered in patients with acute severe thrombocytopenia after receipt of blood products, and treatment should not be delayed while awaiting laboratory confirmation. To our knowledge, this is the second reported case of PTP with antibodies against HPA-4a.

Polymorphisms of the human platelet antigen-1, -2, -3, -5, and -15 systems and acute cellular liver transplant rejection

The human platelet antigen (HPA)-1, -2, -3, -5, and -15 systems are characterized as polymorphic alloantigens expressed on platelets and endothelial cells. In this retrospective study, we investigated, whether HPA-1, -2, -3, -5, and -15 incompatibilities are associated with acute cellular liver transplant rejection. A total of 96 Caucasian liver transplant recipients and corresponding donors were analyzed, 43 with biopsy proven acute cellular rejection (BPAR) and 53 without acute cellular rejection (No-BPAR). Polymorphisms of mentioned HPA systems were determined by polymerase chain reaction-sequence specific primers (PCR-SSP). Our data demonstrate that acute cellular rejection episodes were associated with HPA-3 incompatibility (58% HPA-3 incompatibility in BPAR group vs. 32% HPA-3 incompatibility in No-BPAR group, p=0.013). Furthermore, the frequency of HPA-3bb genotype was significantly higher in BPAR recipients as compared to No-BPAR recipients (30% vs 6%, p=0.002). On the other hand, there was no association between acute cellular rejection and the other tested HPA systems. We conclude that in the Caucasian population the HPA-3 system confers susceptibility to acute cellular rejection after liver transplantation.

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.

Comparison of the bead-based simultaneous analysis of specific platelet antibodies assay (SASPA) and Pak Lx Luminex technology with the monoclonal antibody immobilization of platelet antigens assay (MAIPA) to detect platelet alloantibodies

The identification of platelet alloantibodies is indispensible for the diagnoses of fetal/neonatal alloimmune thrombocytopenia and refractoriness to platelet transfusions.

We compared the results obtained with the gold-standard assay for the detection and specification of platelet antibodies, the monoclonal antibody-specific immobilization of platelet antigen assay (MAIPA), with those from two bead-based assays, simultaneous analysis of specific platelet antibodies assay (SASPA), and the recently commercialized bead-based assay Pak Lx.

For the detection of alloantibodies by SASPA, the sensitivity was 94.6%, the specificity was 99.5% The respective results for Pak Lx were 98% and 99.8%. Some antibody specificities were revealed by SASPA or Pak Lx but not by MAIPA. However, there were also antibodies detected by MAIPA that were not seen by SASPA or Pak Lx. Antibody dilution experiments suggest that SASPA or Pak Lx are more sensitive than MAIPA.

All three assays have limitations, but Pak Lx is suited for screening. However, other assays are necessary if results are not conclusive or for evaluations beyond the commercial assay’s design.

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.

Frequency of human platelet antigens in oncohematological patients with thrombocytopenia and the probability of incompatibility to platelet transfusions

OBJECTIVE

The objective of this study was to evaluate the frequencies of human platelet antigens in oncohematological patients with thrombocytopenia and to analyze the probability of their incompatibility with platelet transfusions.

METHODS

Platelet antigen genotyping was performed by sequence-specific primer polymerase chain reaction (SSP-PCR) for the HPA-1a, HPA-1b, HPA-2a, HPA-2b, HPA-3a, HPA-3b, HPA-4a, HPA-4b, HPA-5a, HPA-5b; HPA-15a, HPA-15b alleles in 150 patients of the Hematology Service of the Hospital das Clínicas (FMUSP).

RESULTS

THE ALLELE FREQUENCIES FOUND WERE: HPA-1a: 0.837; HPA-1b: 0.163; HPA-2a: 0.830; HPA-2b: 0.170; HPA-3a: 0.700; HPA-3b: 0.300; HPA-4a: 1; HPA-4b: 0; HPA-5a: 0.887; HPA-5b: 0.113; HPA-15a: 0.457 and HPA-15b: 0.543.

CONCLUSIONS

Data from the present study showed that the A allele is more common in the population than the B allele, except for HPA-15. This suggests that patients homozygous for the B allele are more predisposed to present alloimmunization and refractoriness to platelet transfusions by immune causes. Platelet genotyping could be of great value in the diagnosis of alloimmune thrombocytopenia and to provide compatible platelet concentrates for these patients.

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.

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.

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.

Full-length recombinant choline transporter-like protein 2 containing arginine 154 reconstitutes the epitope recognized by HNA-3a antibodies

BACKGROUND

Recent reports have shown that the HNA-3a leukocyte antigen, a target for antibodies that cause severe transfusion-related acute lung injury, correlates with an arginine 154 (rather than glutamine) polymorphism in choline transporter-like protein 2 (CTL2) but did not show directly that R154 determines HNA-3a. CTL2 peptides containing R154 are recognized by only half of HNA-3a antibodies studied to date. Constructs that react with all HNA-3a antibodies are needed to fully define the HNA-3a epitope.

STUDY DESIGN AND METHODS

HEK293 cells were transfected with cDNA encoding full-length CTL2 linked to green fluorescent protein (GFP). Transfectants were selected for GFP expression and tested with antibodies specific for HNA-3a and -3b.

RESULTS

Each of 20 HNA-3a antibodies reacted preferentially with HEK293 cells expressing the R154 CTL2 construct. An HNA-3b antibody reacted only with CTL2 (Q154).

CONCLUSIONS

These findings provide direct evidence that R154 in the context of full-length CTL2 is both necessary and sufficient to create the HNA-3a epitope but suggest that posttranslational modifications of the protein, for example, S-S bonds or addition of glycans, are necessary for recognition of HNA-3a by many antibodies. This could complicate development of an assay for large-scale screening of blood donors to detect anti-HNA-3a.

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.

HNA-3a-specific antibodies recognize choline transporter-like protein-2 peptides containing arginine, but not glutamine at Position 154

BACKGROUND

Antibodies specific for the neutrophil antigen HNA-3a cause severe, sometimes fatal transfusion-related acute lung disease (TRALI) when transfused, but it has not been possible to screen blood donors for anti-HNA-3a because using neutrophils as targets was impractical and molecular properties of the antigen were unknown. Recently it was shown that HNA-3a is carried on choline transporter-like protein-2 (CTL2) and that the HNA-3a/b phenotype is closely correlated with an R154Q amino acid polymorphism in CTL2. However, it has not been shown by direct experiment that R154 is essential for the HNA-3a epitope.

STUDY DESIGN AND METHODS

Preliminary attempts to express recombinant full-length CTL2 (R154) recognized by anti-HNA-3a were unsuccessful. We therefore tested HNA-3a-specific antibodies from donors implicated in TRALI reactions for reactivity against chemically synthesized linear and cyclic CTL2 peptides containing R154 or Q154.

RESULTS

Nine of 20 HNA-3a antibodies recognized the R154, but not the Q154 version of a cyclic 36-residue CTL2 peptide (D131-K166). However, 11 others failed to distinguish between the two versions of this peptide.

CONCLUSION

The findings provide direct evidence that R154 in the context of CTL2 D131-K166 is necessary to create the HNA-3a epitope but, in the context of cyclic CTL2 peptide D131-K166, is sufficient to detect only about one-half of the HNA-3a-specific antibodies implicated in TRALI. It is likely that fragments of CTL2 longer than can be made on a large scale with an automated synthesizer will be needed to produce a target capable of detecting all examples of anti-HNA-3a in donated blood.

Report on the 14th International Society of Blood Transfusion Platelet Immunology Workshop

BACKGROUND AND OBJECTIVES

The aims of the 14th ISBT Platelet Immunology Workshop were to evaluate in-house methods for detection of antibodies to human platelet antigens, to compare the sensitivity and specificity of antibody detection using a panel of monoclonal antibodies and to evaluate genotyping methods and establish procedures for drug-dependent antibody detection.

MATERIALS AND METHODS

Forty-two laboratories from 23 countries participated. Samples and reagents provided for the five different exercises.

RESULTS

The ability of participating laboratories to correctly identify the HPA antibody specificity in the nine samples ranged from 20% to 97%. The greatest difficulty was observed with samples that contained antibodies against HPA-3b and GPIV. The significant differences in optical density values by monoclonal antibody of immobilization of platelet antigens (MAIPA) assay were observed when testing the same platelet-specific antibodies. HPA genotyping of DNA with novel mutations did not significantly affect the results. The overall average discrepancy rate was 2·15% for genotyping of 10 DNA samples from well-characterized Epstein–Barr virus transformed cell lines. For detection of drug-dependent antibodies, excellent results for specificity and sensitivity were obtained by the laboratories using the MAIPA and flow cytometry.

CONCLUSIONS

Most laboratories were able to identify the majority of HPA antibodies; however, significant disparities were observed in proficiency testing. MAIPA assay sensitivity is influenced by the monoclonal antibody clone used. DNA with new mutations and EBV cell lines are valuable samples to ensure accurate genotyping. A sensitive and specific drug-dependent antibody assay performed well in the hands of participants.

Establishment of a cell line panel as an alternative source of platelet antigens for a screening assay of anti-human platelet antibodies

BACKGROUND

A panel of platelets expressing various human platelet antigens (HPAs) for a platelet antibody screening assay is difficult to prepare because some antigens are rarely expressed. Therefore, an alternative method without using platelets would be helpful in detecting HPA antibodies. This study describes the establishment of cell lines that stably express specific HPAs and their application for detecting specific antibodies.

METHODS

Wild-type β3, HPA-1b, -6b, -7b and -7 variant cDNA as well as wild-type αIIb and HPA-3b cDNA were individually co-transduced with wild-type αIIb and β3 cDNA in the K562 cell line. We performed an immunobead monoclonal antibody immobilisation of platelet antigens (MAIPA) assay to evaluate this cell line panel for antibody detection using identified sera containing HPA antibodies, whose specificities had been determined by the mixed passive haemagglutination test.

RESULTS AND CONCLUSION

Of the 12 sera containing HPA-1a (n = 2), HPA-3a (n = 6), HPA-6b (n = 3) or HPA-7 variant (n = 1) antibodies, all antibodies were detected and determined by our new method, except for two HPA-3a antibodies. One of the two antibodies was also negative for conventional platelet MAIPA, suggesting that the cell line panel might be used as an alternative source of platelet antigens in the MAIPA assay.

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.