Detection of antibodies against human platelet antigens 15a and 15b by using a cell line panel

Relationship between donor-specific HPA-15 antibodies and poor graft function in HPA-15 mismatched cord blood transplantation

Poor graft function (PGF) is a fatal complication following hematopoietic stem cell transplantation and is influenced by multiple factors, such as donor-specific anti-HLA antibodies, a poor infused CD34⁺ cell count, and the donor source. Alloantibodies against human platelet antigen 15 (HPA-15) recognize platelet membrane glycoprotein CD109, which is expressed not only on platelets, but also on megakaryocytes and specific hematopoietic stem cells. HPA-15 antibodies are known to induce platelet transfusion refractoriness and neonatal alloimmune thrombocytopenia, but their effects on graft function following hematopoietic stem cell transplantation remain unknown. We encountered a case of HPA-15 mismatched cord blood transplantation with a high HPA-15b antibody titer. Prolonged PGF and megakaryocyte aplasia with sustained high-titer HPA-15b antibodies were attenuated by rituximab therapy, and rapid recovery of hematopoiesis was achieved. HPA-15-compatible platelet transfusions were highly effective for platelet recovery. Methylcellulose assays and megakaryocyte cultures revealed that patient serum inhibited in vitro hematopoietic development from patient bone marrow cells. These results suggest that HPA-15 antibodies might be a cause of PGF and that reducing the HPA-15 antibody titer might improve graft function in HPA-15 mismatched transplantation.

Down syndrome with neonatal alloimmune thrombocytopenia due to anti-HLA A31 and B61 antibodies

Neonatal alloimmune thrombocytopenia (NAIT) arises from fetomaternal platelet incompatibility that results in transplacental passage of maternal antibodies mostly against fetal human platelet antigens (HPA), whereas NAIT due to anti-human leukocyte antigen (HLA) antibodies is extremely rare. Here, we report a case of Down syndrome (DS) with NAIT that was attributed to HLA antibodies. A boy with DS was delivered at 36 weeks' gestation. His platelet count declined to 13.0 × 10⁹/L, suggestive of NAIT rather than other conditions, including transient abnormal myelopoiesis. Random platelet concentrates and intravenous immunoglobulin administration resolved the thrombocytopenia without clinical complications. Immunoserological investigations detected anti-HLA, but no anti-HPA antibodies in samples from the patient and the mother. HLA typing and cross-matching indicated that anti-HLA antibodies to paternal HLA A31 and B61, which had probably been induced during a prior pregnancy, led to NAIT in this case. Although it is a rare condition, healthcare providers should consider NAIT due to HLA antibodies and be vigilant for subsequent cases in DS.

Minor impact of patient alloantibodies against human platelet antigen (HPA)-15 in the effectiveness of platelet transfusion: A pilot study

BACKGROUND

Alloantibodies against human platelet antigen (HPA)-15 are sometimes detected in patients with platelet transfusion refractoriness (PTR); however, little is known about their impact on PTR.

STUDY DESIGN AND METHODS

Two patients who possessed HPA-15 alloantibodies (Patient 1, anti-HPA-15b; Patient 2, anti-HPA-15a) and human leukocyte antigen (HLA) antibodies were enrolled. The efficacy of HPA-15-compatible vs -incompatible platelet transfusion was compared by focusing on ABO- and HLA-matched transfusions on the basis of the 24-hour corrected count increment (CCI-24 hours) for platelets. The titers of HPA-15 antibodies in the patients' sera were also monitored.

RESULTS

The patients received 71 and 12 ABO-compatible, HLA-matched platelet transfusions, respectively, during the monitoring periods. Among these transfusions, CCI-24 hours could be calculated in 27 and 10 transfusions, respectively, and the HPA-15 genotype of the donors was determined. There were no significant differences in the CCI-24 hours between the HPA-15 compatible and incompatible transfusions in both patients (P = .30 and .56, respectively, Mann-Whitney U test). There was no significant change in the HPA-15b antibody titer in Patient 1 during the monitoring period, while the HPA-15a antibody level in Patient 2 was undetectable at the end of the monitoring period, although the titer was low at the beginning.

CONCLUSION

The efficacy of HPA-15-incompatible platelet transfusions was not necessarily inferior to that of HPA-15 compatible ones. Although the case number was limited, our results suggest that HPA-15 antibodies do not have a significant impact on the effects of platelet transfusion.

A new efficient tool for non-invasive diagnosis of fetomaternal platelet antigen incompatibility

Fetal and neonatal alloimmune thrombocytopenia (FNAIT) is the consequence of platelet destruction by maternal alloantibodies against fetal human platelet antigens (HPA). This may result in intracranial haemorrhages (ICH) or even fetal death. Currently, fetal HPA genotyping is performed using invasive procedures. Here, we carried out a proof-of-concept study for non-invasive prenatal diagnosis of fetal platelet genotyping in four HPA systems (HPA-1, -3, -5 and-15) by droplet digital polymerase chain reaction (ddPCR) using cell-free DNA extracts from the plasma of 47 pregnant women with suspected, or history of, FNAIT. Results showed that 74% (35/47) of pregnant women presented incompatibility in at least one HPA system, and 38% (18/47) of cases presented HPA-1 incompatibility, including nine women with multiple incompatibilities. ICH occurred in one case of profound fetal thrombocytopenia with HPA-15 incompatibility, confirming the need for non-invasive prenatal genotyping in systems other than HPA-1. Fetal HPA genotypes predicted by ddPCR were confirmed in all FNAIT cases after amniocentesis or delivery. Fetal HPA genotyping on maternal plasma based on ddPCR is a fast, safe and reliable non-invasive method. This technique will be useful for the early identification of pregnancies at high risk of FNAIT requiring antenatal management to minimize the risk of fetal/neonatal haemorrhage.

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.

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.

Detection of anti-human platelet antibodies against integrin α2β1 using cell lines

BACKGROUND

Antibodies against human platelet antigens (HPA) are a cause of thrombocytopenia. Detection of rare anti-HPA antibodies using platelet preparations is difficult and would be improved by an alternative method that does not require platelets. In the present study, we describe the establishment of cell lines that stably express specific HPA associated with integrin α2β1 and the application of these cell lines for detecting anti-HPA-5a and anti-HPA-5b antibodies.

MATERIALS AND METHODS

Complementary DNA of the integrin α2 variants HPA-5b, -13b and -18b were individually transfected into K562 cells using retroviral vectors. Expression of integrin α2 was confirmed by flow cytometric analysis, immunoprecipitation and western blotting analysis. To verify whether the cell line panel was suitable for clinical diagnosis, we analysed its properties using monoclonal antibody-specific immobilisation of platelet antigens (MAIPA) and well-characterised serum samples.

RESULTS

Exogenous integrin α2 expression was observed in the transfected cells for over 6 months. The cell line panel specifically detected previously characterised anti-HPA-5a and anti-HPA-5b antisera. No reactivity was observed with control sera, including normal sera and HLA antisera.

DISCUSSION

We successfully established a cell line panel to facilitate the sensitive and reliable detection of anti-HPA-5a and anti-HPA-5b antibodies.

How I manage neonatal thrombocytopenia

Although neonatal thrombocytopenia (platelet count < 150×10(9) /l) is a common finding in hospital practice, a careful clinical history and examination of the blood film is often sufficient to establish the diagnosis and guide management without the need for further investigations. In preterm neonates, early-onset thrombocytopenia (<72h) is usually secondary to antenatal causes, has a characteristic pattern and resolves without complications or the need for treatment. By contrast, late-onset thrombocytopenia in preterm neonates (>72h) is nearly always due to post-natally acquired bacterial infection and/or necrotizing enterocolitis, which rapidly leads to severe thrombocytopenia (platelet count<50×10(9) /l). Thrombocytopenia is much less common in term neonates and the most important cause is neonatal alloimmune thrombocytopenia (NAIT), which confers a high risk of perinatal intracranial haemorrhage and long-term neurological disability. Prompt diagnosis and transfusion of human platelet antigen-compatible platelets is key to the successful management of NAIT. Recent studies suggest that more than half of neonates with severe thrombocytopenia receive platelet transfusion(s) based on consensus national or local guidelines despite little evidence of benefit. The most pressing problem in management of neonatal thrombocytopenia is identification of safe, effective platelet transfusion therapy and controlled trials are urgently needed.

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

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