Platelet-specific alloantigens and antibodies in Tunisian women after three or more pregnancies

Anti-human platelet antigen-5b antibodies and fetal and neonatal alloimmune thrombocytopenia; incidental association or cause and effect?

Most cases of fetal and neonatal thrombocytopenia (FNAIT) are caused by maternal anti-human platelet antigen-1a antibodies (anti-HPA-1a). Anti-HPA-5b antibodies are the second most common antibodies in suspected FNAIT cases. Given the high prevalence of anti-HPA-5b antibodies in pregnant women delivering healthy newborns, the association with FNAIT may be coincidental. This review of the literature related to FNAIT using the MEDLINE database was conducted according to PRISMA guidelines. A retrospective analysis of a single-centre cohort of 817 suspected FNAIT cases was conducted. The pooled prevalence of anti-HPA-5b antibodies in unselected pregnant women of European descent was 1.96% (n = 3113), compared with 3.4% (n = 5003) in women with suspected FNAIT. We found weak evidence that a small proportion of pregnant women presenting with anti-HPA-5b antibodies will give birth to a newborn with mild thrombocytopenia. The neonatal platelet counts were not different between suspected FNAIT cases (n = 817) with and without maternal anti-HPA-5b antibodies. The prevalence of maternal anti-HPA-5b antibodies was not different between neonates with intracranial haemorrhage and healthy controls. The current experimental and epidemiological evidence does not support the hypothesis that anti-HPA-5b antibodies cause severe thrombocytopenia or bleeding complications in the fetus or newborn.

Antibodies against human platelet alloantigens and human leucocyte antigen class 1 in Saudi Arabian multiparous women and multi-transfused patients

OBJECTIVES

To determine the frequency of alloimmunization against human platelet antigens (HPAs) and human leucocyte antigen class 1 (HLA1) in multiparous women and multi-transfused patients.  

METHODS

This prospective study was conducted between January and August 2013, on 50 multiparous women with no history of previous blood transfusion recruited from the Obstetrics and Gynecology Clinic, and 50 patients, who received multiple platelet transfusions, recruited from the Hematology/Oncology Ward, King Khalid University Hospital, Riyadh, Saudi Arabia.

RESULTS

The frequency of alloimmunization among multiparous pregnant women was 76%, as follows: 16% against HLA1 only, 8% against HPAs only, 52% against both HPAs and HLA1 antigens. In multi-transfused patients, the rate of alloimmunization was 42% as follows: 2% against HLA1 only, 22% against HPAs only, 18% against both HPAs and HLA1 antigens. The frequency of alloimmunization increases with the number of pregnancies, but not with the number of platelet transfusions.

CONCLUSION

Alloimmunization against HPAs and HLA1 is very common among Saudi multiparous women and multi-transfused patients, which encourages the search for the extent of the possible complications in the fetus and newborn and in multitransfused patients and how to prevent their occurrence.

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.

Foetal/neonatal alloimmune thrombocytopenia in Egypt; human platelet antigen genotype frequencies and antibody detection and follow-up in pregnancies

BACKGROUND AND OBJECTIVES

Foetal and neonatal alloimmune thrombocytopenia (FNAIT) is studied mainly in Caucasian populations. Severe thrombocytopenia (<50×10(9)/L) gives risk of haemorrhage and the most feared complication is intracranial haemorrhage (ICH). In Caucasian populations anti-human platelet antigen (HPA)-1a antibodies are the cause of FNAIT in >80% of the cases. The aims of this project were to study the gene frequencies of HPA-1-5 and 15 alleles in an Egyptian population (Arabic), and to determine the frequency of HPA-1a and -5b immunisations in a cohort of Egyptian pregnant women.

MATERIALS AND METHODS

Altogether 6974 pregnant women were included in the study. Genotyping was performed by polymerase chain reaction and antibodies were detected by flow cytometry and enzyme-linked immunosorbent assay. HPA-1-5 and 15 alleles were studied in 367 individuals.

RESULTS

The HPA genotypes differed from genotypes published from different Caucasian and Chinese (Han) populations in HPA-1, -2, -3, and -5 systems with significant higher frequency of HPA-1b, -2b and -5b. The rate of HPA-1a alloimmunisation was found comparable to Caucasian populations. Severe thrombocytopenia was found in two newborns. No bleeding complication was reported. Anti-HPA-5b antibodies were detected in 4.4% of the pregnant women. Clinical consequences of these antibodies were not studied.

CONCLUSION

The HPA-1bb and -5bb genotypes are more frequent in the Egyptian Arabic population studied compared to Caucasian populations. FNAIT due to anti-HPA-1a and -5b antibodies must be suspected in cases of neonatal thrombocytopenia. Further large prospective studies are needed to increase the knowledge of clinical complications related to HPA alloantibodies in populations with different genetic backgrounds.