Human Platelet Antigen 3 (HPA-3): Localization of the Determinant of the Alloantibody Leka (HPA-3a) to the C-Terminus of Platelet Glycoprotein IIb Heavy Chain and Contribution of O-Linked Carbohydrates

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.

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.

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.

Platelet biogenesis and functions require correct protein O-glycosylation

Platelets express a variety of membrane and secreted glycoproteins, but the importance of glycosylation to platelet functions is poorly understood. To explore the importance of O-glycosylation, we generated mice with a targeted deletion of Cosmc in murine endothelial/hematopoietic cells (EHC) (EHC Cosmc(-/y)). X-linked Cosmc encodes an essential chaperone that regulates protein O-glycosylation. This targeted mutation resulted in lethal perinatal hemorrhage in the majority of mice, and the surviving mice displayed severely prolonged tail-bleeding times and macrothrombocytopenia. EHC Cosmc(-/y) platelets exhibited a marked decrease in GPIb-IX-V function and agonist-mediated integrin αIIbβ3 activation, associated with loss of interactions with von Willebrand factor and fibrinogen, respectively. Significantly, three O-glycosylated glycoproteins, GPIbα, αIIb, and GPVI normally on platelet surfaces that play essential roles in platelet functions, were partially proteolyzed in EHC Cosmc(-/y) platelets. These results demonstrate that extended O-glycans are required for normal biogenesis of the platelets as well as the expression and functions of their essential glycoproteins, and that variations in O-glycosylation may contribute to altered hemostasis.

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.

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.

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

BACKGROUND

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

STUDY DESIGN AND METHODS

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

RESULTS

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

CONCLUSION

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

Evidence of heterogeneity in the antibody response against the platelet antigen 3a; recognition of an 11-mer peptide carrying the HPA-3a polymorphic determinant

BACKGROUND

The immune processes involved in the development of alloantibodies against the human platelet antigens in alloimmune disorders remain unclear. Antibody recognition of the platelet antigens on their respective platelet glycoproteins has been shown to be dependent on glycoprotein conformation. Furthermore, the post-translational modification of glycoproteins adds complexity to the alloantigenic determinants.

METHODS

Nine anti-HPA-3a sera along with several control sera were tested for reactivity to an 11-mer peptide straddling the HPA-3a/b polymorphism. Sera found to specifically recognize the 3a peptide were further assessed by platelet pre-exposure and immunoblotting.

RESULTS

Three of the nine antisera were found to specifically recognize an 11-mer synthetic 3a peptide by ELISA. Further analysis of all anti-HPA-3a sera by Western blot showed that only those reactive to the 3a peptide were able to bind both reduced and non-reduced GPIIb.

CONCLUSION

The results presented in this study provide the first known evidence for the identification of an antibody population capable of recognizing a linear and non-glycosylated form of the HPA-3a epitope.

Three novel beta3 domain-deletion peptides for the sensitive and specific detection of HPA-4 and six low frequency beta3-HPA antibodies

BACKGROUND

Antibodies against human platelet antigens (HPA) are clinically important in fetal-maternal alloimmune thrombocytopenia, refractoriness to platelet transfusions and post-transfusion purpura. Of the 16 HPAs, nine are located on the beta3 subunit of the alphaIIb beta3 integrin. Antibody detection is generally based on platelet-derived alphaIIb beta3 from HPA-genotyped donors. Recombinant allelic beta3 peptides, expressed at high levels would improve consistency in antibody detection, but the expression of soluble and monomeric integrins expressing complex dependent epitopes has previously proved challenging.

OBJECTIVES

We aimed to generate three recombinant beta3 peptides for the detection of antibodies against HPA-4, HPA-8bw and five of the six remaining low frequency beta3 alloantigens.

METHODS

The removal of the specificity-determining loop from the betaA domain and fusion of truncated beta3 to calmodulin was exploited to obtain expression of monomeric protein. Using site-directed mutagenesis, the mutations for HPA-4b and HPA-8bw were introduced in the ITGB3*001 haplotype. A third peptide for the detection of antibodies against HPA coded by non-synonymous single nucleotide polymorphisms of low frequency was generated by the introduction of five mutations forming the basis of HPA-6bw, -7bw, -10bw, -11bw, and -16bw antigens.

RESULTS

Reactivity of the three peptides with beta3-specific murine monoclonal antibodies and human HPA-1a phage antibodies confirmed the structural integrity of the recombinant fragments, and reactivity with a unique panel of polyclonal anti-HPA sera confirmed expression of the relevant HPA epitopes.

CONCLUSIONS

These data demonstrate that beta3 integrin domain-deletion fragments are suitable molecular targets for HPA antibody detection.

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.

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

BACKGROUND

Maternal antibodies that cause neonatal alloimmune thrombocytopenia are commonly identified by solid-phase assays that detect the causative antibodies on the basis of their reactions with specific PLT glycoproteins. Two cases of severe neonatal alloimmune thrombocytopenia caused by maternal antibodies specific for human PLT antigen 3a (HPA-3a [Baka]) that failed to give the expected reactions in some solid-phase assays were recently encountered.

STUDY DESIGN AND METHODS

PLT-reactive antibodies were characterized by three different solid-phase assays and by flow cytometry.

RESULTS

The two maternal antibodies gave negative reactions in the antigen capture ELISA, modified antigen capture ELISA, and MoAb immobilization of PLT antigens tests but reacted strongly in flow cytometry with intact PLTs that were HPA-3a+. Other sera samples specific for HPA-3a reacted equally well in all assays.

CONCLUSIONS

The two antibodies appear to recognize an epitope on the HPA-3a+ form of glycoprotein IIb that is lost when PLTs are solubilized in detergent, as required for solid-phase assays. The diagnosis was made in these cases because no HLA antibodies were present, allowing an HPA-3a-specific reaction to be identified with intact PLTs as targets. Such antibodies are likely to be overlooked when HLA antibodies are also present.

Bilateral linkage between a new deletion polymorphism in intron 21 of the GP IIb gene and the HPA-3b (Bakb) determinant

Glycoproteins (GP) IIb (alpha IIb) and GP IIIa (beta 3) form heterodimeric complexes (GP IIb-IIIa) at the platelet surface and mediate platelet aggregation by binding fibrinogen after platelet activation. The structures and DNA sequences of the GP IIb and GP IIIa genes are known. Punctual mutations resulting in alloantigen systems (HPA) have been described on both genes, as have a series of genetic defects giving rise to Glanzmann's thrombasthenia (GT). We now report a nine base pair deletion located in intron 21 of the GP IIb gene. This was found both in unrelated GT patients and in normal individuals. Subsequent studies showed that the deletion polymorphism and the mutation responsible for the platelet alloantigen. HPA-3b, were linked together. The deletion was always present when the gene carried the HPA-3b genotype, but was never observed in association with the HPA-3a polymorphism. Analysis of 60 independent alleles from 30 unrelated caucasian individuals revealed no exceptions to this linkage. It is the first time that two genetic markers have been reported to be linked to each other on the GP IIb gene.