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

[Genotyping and its applications, a look to the future]

The detection of the most significant erythrocyte antigens present in each one of the individuals is fundamental when carrying out a transfusion or a transplant. Detection to date is performed by conventional serological methods through the antigen-antibody reaction. But several drawbacks may arise depending on the pathology under study, limiting the availability of blood components. Molecular methods such as genotyping is a tool that complements sensitivity and specificity and has come to revolutionize immunohematology in the blood bank, allowing not only the detection of erythrocyte antigens but also platelet antigens. These methodologies are applicable in patients and in large-scale donors, starting from the allelic variants present in each of the genes that code for the antigens of clinical interest, using microarray systems or systems based on particles labeled with specific probes or their variants that allow an analysis from the immunohematological point of view.

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.

Genotyping of Eight Human Platelet Antigen Systems in Serbian Blood Donors: Foundation for Platelet Apheresis Registry

Introduction

The aim of this study was to investigate the allele and genotype frequencies of 8 human platelet antigen (HPA) systems among blood donors from the Blood Transfusion Institute of Serbia and to compare them with published studies. These data would be useful to establish the basis for a platelet apheresis donor registry.

Material and Methods

Seventy-two unrelated male platelet apheresis/blood donors from Serbia were typed for 8 HPA systems (HPA-1 to HPA-6, HPA-9, and HPA-15) via the FluoGene method, based on polymerase chain reaction-sequence-specific amplification (PCR-SSP; PCR using sequence-specific primers) with fluorometric signal detection. Allele and genotype frequencies were estimated by direct counting and compared to the expected genotype frequencies according to the Hardy-Weinberg principle. The transfusion mismatch probability was calculated for every HPA specificity.

Results

The allele frequencies were: HPA-1a, 0.868; HPA-1b, 0.132; HPA-2a, 0.917; HPA-2b, 0.083; HPA-3a, 0.611; HPA-3b, 0.389; HPA-5a, 0.903; HPA-5b, 0.097; HPA-9a, 0.993; HPA-9b, 0.007; HPA-15a, 0.472; and HPA-15b, 0.528. For HPA-4 and HPA-6 only allele a was detected.

Discussion

The HPA allele frequencies of European populations showed no significant differences in comparison with our results. Statistically significant differences were revealed in comparison with some populations of non-European origin. In the tested donors no HPA-2 bb genotype was detected, but we found 1 donor with the rare HPA-9b allele. The biggest transfusion mismatch probability in the Serbian population is for systems HPA-15 (37.4%) and HPA-3 (36.2%), which means that more than a third of random transfusions could cause mismatch in these systems. This study was enabled by the introduction of molecular HPA typing, and it provides initial results of the HPA allele and genotype frequencies in the population of blood donors in Serbia. They will be used to provide a compatible blood supply on demand for treating patients with alloimmune thrombocytopenic disorders. The successful implementation of PCR-SSP with fluorometric signal detection could be further complemented in the future by the introduction of high-throughput methods, which will largely depend on the available financial resources.

[Novel platelet pharming using human induced pluripotent stem cells]

La production in vitro de plaquettes offre une opportunité de résoudre les problèmes liés aux limitations d’approvisionnement et à la sécurité des dons de produits dérivés du sang. Les cellules souches pluripotentes induites – ou iPSC – sont une source idéale pour la production de cellules à des fins de thérapies régénératives. Nous avons précédemment établi avec succès une lignée mégacaryocytaire immortalisée à partir d’iPSC. Celle-ci possède une capacité de prolifération fiable. Par ailleurs, il est possible de les cryoconserver. Elle est donc une source adaptée de cellules primaires pour la production de plaquettes suivant les Bonnes Pratiques de Fabrication (BPF). Dans le même temps, la capacité améliorée des bioréacteurs à reproduire certaines conditions physiologiques, telle que la turbulence, de pair avec la découverte de molécules favorisant la thrombopoïèse, a contribué à l’accomplissement de la production de plaquettes en quantité et qualité suffisantes pour répondre aux besoins cliniques. La production de plaquettes à partir de cellules iPS s’étend aussi aux patients en état de réfraction allo-immune, par la production de plaquettes autologues ou dont on a génétiquement manipulé l’expression des Antigènes des Leucocytes Humains (HLA) et des Antigènes Plaquettaires Humain (HPA). Considérant ces avancées fondamentales, les plaquettes iPSC avec expression des HLA modifiées se présentent comme un potentiel produit de transfusion universel. Dans cette revue, nous souhaitons apporter une vue d’ensemble de la production in vitro de plaquettes à partir de cellules iPS, et de son possible potentiel transformatif, d’importance capitale dans le domaine de la transfusion des produits sanguins.

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.

Frequency of Platelet Crossmatch Positivity and Predictive Value for Poor Platelet Increment Among Paediatric Oncohaematology Patients in India

Immune platelet destruction is a significant cause for platelet refractoriness. The platelet crossmatch-a solid phase red cell adherence assay utilizes donor platelets and patient serum to assess compatibility and appears to be a feasible option in resource constrained settings. This study was done to evaluate the frequency of platelet crossmatch positivity among Paediatric Oncohaematology patients and also to assess whether a positive crossmatch is predictive of unsuccessful platelet transfusions in this group of patients. Paediatric Oncohaematology patients who received platelet transfusions between March 2013 and September 2013 were included in the study. The pre-transfusion patient sample and a segment from the transfused donor unit were used for performing the platelet crossmatch. A blood sample was collected one hour after the transfusion to assess post-transfusion platelet count. Corrected count increment (CCI) was calculated using the standard formula. CCI ≤ 7500/µL/m²/10¹¹ was considered evidence of an unsuccessful transfusion. Seventy-three platelet crossmatches were performed for 69 patients, of which 30 patient samples (41%) showed crossmatch positivity. 25 (89.2%) of 28 unsuccessful transfusions showed crossmatch positivity, and 40 (88.9%) of 45 successful transfusions showed negative crossmatches (p = 0.03). Crossmatch positivity among transfusion dependent Paediatric Oncohaematology patients was as high as 42%, when ABO matched platelet units were allocated without further testing. Our results indicate that this test may be a reliable tool to select compatible platelet units and an effective intervention in the management of patients at risk of immune platelet refractoriness.

iPSC-Derived Platelets Depleted of HLA Class I Are Inert to Anti-HLA Class I and Natural Killer Cell Immunity

The ex vivo production of platelets depleted of human leukocyte antigen class I (HLA-I) could serve as a universal measure to overcome platelet transfusion refractoriness caused by HLA-I incompatibility. Here, we developed human induced pluripotent cell-derived HLA-I-deficient platelets (HLA-KO iPLATs) in a clinically applicable imMKCL system by genetic manipulation and assessed their immunogenic properties including natural killer (NK) cells, which reject HLA-I downregulated cells. HLA-KO iPLATs were deficient for all HLA-I but did not elicit a cytotoxic response by NK cells in vitro and showed circulation equal to wild-type iPLATs upon transfusion in our newly established Hu-NK-MSTRG mice reconstituted with human NK cells. Additionally, HLA-KO iPLATs successfully circulated in an alloimmune platelet transfusion refractoriness model of Hu-NK-MISTRG mice. Mechanistically, the lack of NK cell-activating ligands on platelets may be responsible for evading the NK cell response. This study revealed the unique non-immunogenic property of platelets and provides a proof of concept for the clinical application of HLA-KO iPLATs.

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Clinically applicable iPSC-derived HLA class I knockout platelets (HLA-KO iPLATs)

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HLA-KO iPLATs do not elicit NK cell activation in vitro

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HLA-KO iPLATs circulate comparably with wild type in human NK cell-reconstituted mice

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HLA-KO iPLATs circulate competently in alloimmune PTR model mice

Serum cytokines and clinical features in patients with fever and thrombocytopenia syndrome

PURPOSE

To explore the clinical, microbiological and immunological features of patients with fever and thrombocytopenia.

METHODS

Patients with unexplained fever and thrombocytopenia were enrolled. Viruses were detected using real-time PCR, and bacteria were measured by culturing methods. Serum cytokines, platelet antibody IgG (PA-IgG) and Helicobacter pylori (HP) were detected using ELISA.

RESULTS

Pathogens were detected in 74.68% of patients, which included single fungal/viral/bacterial infection and multiple infection. The pathogens could not be unidentified in 25.32% of cases. Cytokines including Interleukin (IL)-6, IL-10, interferon-γ(IFN-γ), platelet activating factor (PAF) and PA-IgG were significantly higher in patients as compared to healthy controls (P < .01 or P < .05). Principal component analyses extracted four groups of parameters that have a strong positive predicting value, revealing that disease status evaluation would be more accurate if we combined the platelet parameters and inflammatory biomarkers. While event-free survival (EFS) that indicates the time of platelet elevated after therapy was the highest in patients with single bacterial or fungal infection, EFS was affected by the levels of cytokines and PA-IgG.

CONCLUSIONS

Differences in immune function may be the main factors affecting the prognosis of patients with fever and thrombocytopenia, while treatment based on precise etiological diagnosis is important for therapeutic efficacy.

Evaluation of Diagnostic Tests by Evanescence Biosensor Technology for Rapid Phenotyping of the Human Platelet Alloantigens 1a and 5b

Background

The human platelet alloantigens (HPA) HPA-1a and HPA-5b are located on glycoproteins on the platelet surface and are the most relevant to cause neonatal alloimmune thrombocytopenia (NAIT). The antigens are defined by single nucleotide polymorphisms (SNPs) in the glycoprotein genes, and the antigen status can be determined by genotyping the SNPs. However, genotyping is time-consuming and costly depending on the method and sample throughput. Here, we tested the reliability of the evanescence wave based fluorescence (EVA) biosensor technology for the rapid phenotyping of the HPA-1a and HPA-5b antigens on blood donor samples in two laboratories.

Methods

HPA-1a and HPA-5b phenotyping was performed on EDTA blood samples from 336 blood donors (Lyon: 216 donors; Mannheim: 120 donors) using EVA typing assays and the biosensor system (Davos Diagnostics, Davos, Switzerland). For genotyping, validated PCR-SSP and TaqMan-PCR methods were used.

Results

HPA-1a phenotyping was positive for all samples with HPA-1aa (n = 244; EVA value 807 ± 167 U/s) and HPA-1ab (n = 82; 542 ± 110 U/s) genotypes. All samples (n = 10) with negative EVA values (<10 U/s) had the HPA-1bb genotype. HPA-5b phenotyping was negative for all HPA-5aa genotypes (n = 267) and positive for the HPA-5ab (n = 66; 83 ± 22 U/s) and HPA-5bb (n = 3; 118 ± 25 U/s) genotypes. EVA values from heterozygotes were significantly lower compared to HPA-1a or HPA-5b homozygotes. A strong correlation of the EVA values with the platelet count in the blood samples was observed.

Conclusion

EVA is a reliable method for rapid phenotyping of the clinically relevant HPA-1a and HPA-5b platelet antigens. All phenotyping results were 100% concordant with the HPA-1 or HPA-5 genotype. The test can be performed from only 10 µl of fresh or frozen blood samples within less than 15 min time-to-result.

Neonatal alloimmune thrombocytopenia caused by anti-HPA antibodies in pregnant Chinese women: a study protocol for a multicentre, prospective cohort trial

BACKGROUND

Neonatal alloimmune thrombocytopenia (NAIT), caused by maternal antibodies raised against alloantigens carried on foetal platelets, is a very common haematological abnormality in newborns worldwide. However, baseline data on NAIT in China are lacking. Therefore, this study seeks to explore the incidence of alloantibody against the human platelet antigen (HPA) in pregnant women and its associations with NAIT in China.

METHODS

A multicentre, prospective cohort study design will be used, and 55,497 pregnant women will be recruited for the first screening of the anti-HPA antibody at 12 to 28 weeks of gestational age. Subjects who are positive in the first screening for the anti-HPA antibody will be included in the exposure group. Re-tests of the antibody titre, antigen-specificity and genotyping of HPA and HLA will be conducted during admission. A ratio of 1:1 paired individuals with the same ethnicity and parity but testing negative for the anti-HPA antibody will be randomly selected to be included in the non-exposure group. NAIT will be diagnosed in the newborns on day one of the birth. The HPA of the neonates in the exposure group will also be genotyped by sequencing. Associations of maternal HLA with the occurrence of the anti-HPA antibody and correlation of the severity of NAIT with the titre of the anti-HPA antibody will be further analysed.

DISCUSSION

The study is expected to provide baseline data on NAIT in China. Besides, we hope to find out a population who expresses particular HLA molecules has significant higher risk of HPA alloimmunization in Chinese individuals. We also hope to find a Chinese-specific cut-off antibody titre for the prediction of the severity of NAIT and to provide a means to evaluate the necessity of antenatal treatment.

TRIAL REGISTRATION

ClinicalTrials.gov: NCT02934906 (date registered: 13.10.2016).

[Study on the expression of T, B lymphocyte antigen and platelet antibodies in patients with platelet transfusion refractoriness]

OBJECTIVE

To explore whether T lymphocytes subgroup, B lymphocytes, platelet antigen CD41a, CD61 or platelet antibodies are involved in the platelet transfusion refractoriness.

METHODS

Forty-seven patients diagnosed as platelet transfusion refractoriness and 32 patients that achieved effective platelet therapy were ennolled in this study. Flow cytometry was performed to detect the proportion of cytotoxic T cell (CD3(+)CD4(-)CD8(+)), helper T cell (CD3(+)CD4(+)CD8(-)) and B lymphocytes (CD19 (+) ), and the expression of platelet glycoproteins, including CD41a and CD61, while platelet antibodies were also measured by solid-phase agglutination.

RESULTS

The significant lower level of helper T cell (36.60% vs 48.53%), higher level of cytotoxic T cell (53.26% vs 44.02%) and lower cytotoxic/helper T cell ratio (0.85 vs 1.31) were observed in platelet refractoriness group when compared with effective platelet therapy group (P<0.05). However, the significant difference was not observed in B lymphocytes between the two group (3.02% vs 2.85%, P>0.05). Platelet glycoproteins CD41a and CD61 and antibodies were both expressed at high levels in platelet refractoriness group (88.10% vs 51.69%, 88.36% vs 51.83%, 85.37% vs 14.82%, respectively, P<0.05).

CONCLUSIONS

Activation of cytotoxic T cells, suppression of helper T cells, higher expression level of platelet glycoproteins CD41a and CD61 as well as the development of anti-platelet antibodies are involved in the immunologic mechanism of platelet refractoriness.