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

Recombinantly Expressed Tagged SUrface Protein (RETSUP) assay: a new diagnostic system for the detection of antibodies to platelets

BACKGROUND

Current assays for the detection of (allo)antibodies to platelet antigens are often laborious and widely based on the presence of well-characterized donor platelets.

OBJECTIVES

To develop an easy-to-perform, sensitive, and specific test for the detection of antibodies against platelet antigens, in particular, glycoprotein (GP) antigens, called "Recombinantly Expressed Tagged SUrface Protein" (RETSUP) assay, which does not require donor platelets.

METHODS

Twin-Strep-tagged GP complexes were recombinantly expressed in human embryonic kidney 293 cells after stable transfection. These cell lines were used as antigen sources in the RETSUP assay, combining cell-based and enzyme-linked immunosorbent assay-based assay procedures. The assay performance was tested with recombinant antibodies, anti-human platelet antigen (HPA) reference plasmas, and anti-HPA patient sera.

RESULTS

Human embryonic kidney 293 cell lines stably expressing either Twin-Strep-labeled GPIa/IIa, GPIIb/IIIa, GPIb/IX, or GPIb/IX/V complexes or GPV as well as the distinct HPA-1, HPA-3, and HPA-5 epitopes were successfully generated. Applying the generated GP-expressing cell lines, the developed RETSUP assay proved very sensitive and specific with recombinant antibodies targeting different GPs and human plasma/serum samples. The results of the test were not affected by the GP carrying the Twin-Strep-tag or by using freshly harvested or cryopreserved cells.

CONCLUSION

The RETSUP assay is an easy-to-perform, sensitive, and specific assay for the detection of plasma/serum antibodies to platelet GP, with performance comparable to or better than those of current state-of-the-art assays in antiplatelet antibody diagnostics. Owing to the recombinant nature of the target antigens, it can be easily adapted to detect antibodies in other antibody-mediated diseases.

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.

FNAIT pathogenesis determined by serial analysis of three subsequent pregnancies of a woman with severe fetal and neonatal alloimmune thrombocytopenia (FNAIT) with anti-HPA-4b and anti-HPA-5b alloantibodies in the first sibling

BACKGROUND

Fetal and neonatal alloimmune thrombocytopenia (FNAIT) is caused by anti-HPA alloantibody, and anti-HPA-4b is the most common cause in Japanese. Anti-HPA-5b is frequently detected in pregnant women, but it is still controversial whether anti-HPA-5b causes severe FNAIT.

CASE PRESENTATION

A Japanese woman with anti-HPA-4b and anti-HPA-5b alloantibodies delivered a baby with severe FNAIT who was both HPA-4b and HPA-5b incompatible. We carefully monitored the patient's following three pregnancies (the second and the fourth siblings were HPA-4b incompatible and HPA-5b compatible; the third sibling was both HPA-4b and HPA-5b compatible). FNAIT was not observed in all three siblings, although a modest decrease in cord blood platelet count was observed in the HPA-4b incompatible siblings compared to the HPA-4b compatible sibling. Serial monitoring of anti-HPA titer showed that anti-HPA-4b markedly decreased in late pregnancy and recovered after delivery of the HPA-4b incompatible siblings, but these decreases were not observed during the mother's pregnancy with the HPA-4b compatible sibling. In contrast, anti-HPA-5b remained at a high titer during pregnancy with all three siblings.

CONCLUSION

Our data indicate that dynamic changes of anti-HPA-4b occur during pregnancy and strongly suggest that anti-HPA-5b was mainly responsible for severe FNAIT in this case.

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.

Stabilization of Transfected Cells Expressing Low-Incidence Blood Group Antigens: Novel Methods Facilitating Their Use as Reagent-Cells

Background

The identification of erythrocyte antibodies in the serum of patients rely on panels of human red blood cells (RBCs), which coexpress many antigens and are not easily available for low-incidence blood group phenotypes. These problems have been addressed by generating cell lines expressing unique blood group antigens, which may be used as an alternative to human RBCs. However, the use of cell lines implies several drawbacks, like the requirement of cell culture facilities and the high cost of cryopreservation. The application of cell stabilization methods could facilitate their use as reagent cells in clinical laboratories.

Methods

We generated stably-transfected cells expressing low-incidence blood group antigens (Di^a and Lu^a). High-expresser clones were used to assess the effect of TransFix^® treatment and lyophilization as cell preservation methods. Cells were kept at 4°C and cell morphology, membrane permeability and antigenic properties were evaluated at several time-points after treatment.

Results

TransFix^® addition to cell suspensions allows cell stabilization and proper antigen detection for at least 120 days, despite an increase in membrane permeability and a reduction in antigen expression levels. Lyophilized cells showed minor morphological changes and antigen expression levels were rather conserved at days 1, 15 and 120, indicating a high stability of the freeze-dried product. These stabilized cells have been proved to react specifically with human sera containing alloantibodies.

Conclusions

Both stabilization methods allow long-term preservation of the transfected cells antigenic properties and may facilitate their distribution and use as reagent-cells expressing low-incidence antigens, overcoming the limited availability of such rare RBCs.

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.

Evaluation of HNA-expressing cell line-based antigen capture systems and a solid-phase system for detecting HNA-1a antibodies

Granulocyte immunofluorescence and granulocyte agglutination tests are standard methods for detecting human neutrophil antigen (HNA) antibodies (Abs); however, these require a typed panel of neutrophils, which can be time-consuming to develop, and it remains difficult to determine antibody specificity in some cases. We established and evaluated four detection systems for HNA-1a Abs based on an HNA-1a-expressing cell line (KY cells) and antigen capture. We additionally evaluated a commercial solid-phase system. Eleven HNA-1a antibody-positive samples, including the World Health Organization Reference Reagent, and 40 serum samples derived from male blood donors were used as positive and negative control samples, respectively. Although specificity was >0.90 in all systems evaluated, the sensitivity varied among the systems. The KY cell-based monoclonal antibody specific immobilisation of granulocyte antigens (KY-MAIGA) system using certain, but not all, monoclonal Abs, and the solid-phase system revealed higher sensitivity than other systems. In conclusion, the KY-MAIGA and commercial solid-phase systems were superior in terms of specific and sensitive detection of HNA-1a Abs.

A case of neonatal alloimmune thrombocytopenia in the presence of both anti-HPA-4b and anti-HPA-5b antibody: clinical and serological analysis of the subsequent pregnancy

Neonatal alloimmune thrombocytopenia (NAIT) is induced by maternal alloantibodies raised against fetal platelet antigens inherited from the paternal parent. In contrast to Caucasians, in Asians, predominantly in Japanese, most frequently detected antibodies in NAIT are anti-HPA-4b and anti-HPA-5b. In some NAIT cases multiple alloantibodies are detected. In such cases it is very difficult to determine which antibody is the dominant antibody in NAIT. In this case report, we describe a NAIT case (first sibling) with severe thrombocytopenia and cephalhematoma in the presence of both anti-HPA-4b and anti-HPA-5b antibodies in the maternal serum. We carefully examined titers of anti-HPA antibodies during the subsequent pregnancy with HPA-4b-positive and HPA-5b-negative fetus determined by amniocentesis at gestational week 16. We administered IVIG (1 g/kg/w) to the mother from gestational week 32 to 35. The mother subsequently delivered a second sibling with normal platelet count by cesarean section. Although we could not completely rule out the involvement of anti-HPA-4b, our findings suggested that anti-HPA-5b was implicated in the NAIT in the first sibling.

Approach of using established and new laboratory tests to more comprehensively investigate noninfectious and nonhemolytic transfusion reactions--along with the experience in Japan

BACKGROUND AND OBJECTIVES

Noninfectious and nonhaemolytic transfusion reactions are the most common type of transfusion reactions. Several new tests have been made, helping diagnosis and understanding of their pathogenesis. This manuscript provides a review of the literature on currently available tests in association with the approach in Japan.

MATERIALS & METHODS

Primarily by using key words, more than 100 pertinent articles in the Medline database were identified and reviewed.

RESULTS

Numbers of laboratory tests are available including those for plasma protein levels, plasma protein antibodies, leucocyte and platelet antibodies, serum N-terminal-pro-brain natriuretic peptide levels, serum tryptase levels and genetic microchimerism. Cross-match tests, such as basophil activation test and neutrophil activation test, are also available to determine a causal relationship between the reaction and transfusion.

CONCLUSIONS

Several tests should help to confirm diagnosis and determine causal relationship between adverse reactions and transfusion and to gain an insight into the mechanism of the reaction in some cases, although some of the recently developed tests have not been completely validated.

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.

A novel assay for the detection of anti-human platelet antigen antibodies (HPA-1a) based on peptide aptamer technology

BACKGROUND

Neonatal alloimmune thrombocytopenia is mostly due to the presence of maternal antibodies against the fetal platelet antigen HPA-1a on the platelet integrin GPIIb-IIIa. Accurate detection of anti-HPA-1a antibodies in the mother is, therefore, critical. Current diagnostic assays rely on the availability of pools of human platelets that vary according to donors and blood centers. There is still no satisfactory standardization of these assays.

DESIGN AND METHODS

Peptide aptamer was used to detect and identify HPA-1a-specific antibodies in human serum that do not require human platelets. A peptide aptamer library was screened using an anti-HPA-1a human monoclonal antibody as a bait to isolate an aptamer that mimics the human platelet antigen HPA-1a.

RESULTS

This is the first report in platelet immunology of the use of a peptide aptamer for diagnostic purposes. This assay gives better results than the MAIPA currently in use, detecting around 90% of the expected alloantibodies.

CONCLUSIONS

This assay could help define a standard for the quantitation of anti-HPA antibodies. This report also demonstrates that peptide aptamers can potentially detect a variety of biomarkers in body fluids; this is of particular interest for diagnostic purposes.

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.