Review and update of platelet alloantigen systems

Association of the human platelet antigens polymorphisms with platelet count in patients with COVID-19

Polymorphism in human platelet antigen (HPA)-1 and HPA-3 (GPIIb/IIIa), HPA-2 (GPIb/IX), HPA-4 (GPIIIa), HPA-5 (GPIa/IIa), & HPA-15 (CD109) was investigated in 86 COVID-19-infected patients with thrombocytopenia (Group A) and 136 COVID-19-infected patients without thrombocytopenia (Group B). HPA genotyping was done by the sequence-specific primers PCR method. Lower HPA-3a and higher HPA-3b (P = 0.028) allele frequencies were seen in Group A than in Group B, and homozygosity for HPA 3b (P = 0.038) alleles was more prevalent in Group A than in Group B. The allele and genotype distributions of the other HPA polymorphic variants were similar between the two groups. Univariate analysis identified the CCGGGC (P = 0.016) combined genotype to be negatively associated & the TCGGGC (P = 0.003) and CCGGGC (P = 0.003) to be positively associated with thrombocytopenia. The frequency of anti-HPA-1a and anti-HPA-3a antibodies was significantly higher in all patients compared to other anti-HPAs antibodies (P < 0.05). These results highlight the role of HPAs in the thrombocytopenia of COVID-19 infected patients. This is the first evidence demonstrating the differential association of the six common HPA gene variants and specific HPA genotype combinations with thrombocytopenia in COVID-19-infected patients.

Human Platelet Antigens in Brazilian Multiethnic Populations: Occurrence of Regional Variation and Frequency in a Large Urban Center (Belo Horizonte)

Background

The frequency of human platelet antigens (HPA) varies according to ethnicity, which causes differences in the morbidity of alloimmune and autoimmune thrombocytopenic disorders in different populations. Studies on HPA frequencies in Brazil have reported differences among Brazilian populations produced by the diverse degrees of admixture throughout the country.

Methods

In the present study, we investigated the variation of HPA distribution in Brazil, compared with worldwide populations, and describe the frequencies of HPA-1, -2, -3, -5, and -15 in a large urban center in Southern Brazil (Belo Horizonte) based on a sample of blood donors.

Results

The principal component analysis and the dendrogram based on genetic distance revealed a clear relationship between Brazilian populations and the groups formed by European and African populations. The coefficients of variation for HPA allele frequencies suggest that Brazilian populations presented variations for HPA alleles comparable with the populations from continental groups. In Belo Horizonte, the allele a frequencies for HPA-1, -2, -3, -5 and -15 were 0.8575, 0.8400, 0.6225, 0.8525 and 0.5825 respectively. The genotypes with higher frequencies were a/a (72-74%), except for HPA-3 and -15, whose heterozygous a/b genotypes were shown to be more prevalent (43.5 and 44.5%, respectively).

Conclusion

We confirmed the heterogeneity of HPA antigens in Brazilian populations, reinforcing the importance of HPA panels composed of regional blood donors, or a national panel that contemplates the specificities of the different regions of the country, in order to provide support in platelet transfusions and to minimize the risks associated with HPA alloimmunization. The evaluation of HPA data from Belo Horizonte represents the initial step toward the development of a genotyped platelet donor registry in order to treat HPA alloimmunized patients in this region.

The absence of the human platelet antigen polymorphism effect on fibrosis progression in human immunodeficiency virus-1/hepatitis C virus coinfected patients

INTRODUCTION

Hepatic fibrosis progression in patients with chronic hepatitis C virus infections has been associated with viral and host factors, including genetic polymorphisms. Human platelet antigen polymorphisms are associated with the rapid development of fibrosis in HCV-monoinfected patients. This study aimed to determine whether such an association exists in human immunodeficiency virus-1/hepatitis C virus-coinfected patients.

METHODS

Genomic deoxyribonucleic acid from 36 human immunodeficiency virus-1/hepatitis C virus-coinfected patients was genotyped to determine the presence of human platelet antigens-1, -3, or -5 polymorphisms. Fibrosis progression was evaluated using the Metavir scoring system, and the patients were assigned to two groups, namely, G1 that comprised patients with F1, portal fibrosis without septa, or F2, few septa (n = 23) and G2 that comprised patients with F3, numerous septa, or F4, cirrhosis (n = 13). Fisher's exact test was utilized to determine possible associations between the human platelet antigen polymorphisms and fibrosis progression.

RESULTS

There were no deviations from the Hardy-Weinberg equilibrium in the human platelet antigen systems evaluated. Statistically significant differences were not observed between G1 and G2 with respect to the distributions of the allelic and genotypic frequencies of the human platelet antigen systems.

CONCLUSION

The greater stimulation of hepatic stellate cells by the human immunodeficiency virus and, consequently, the increased expression of transforming growth factor beta can offset the effect of human platelet antigen polymorphism on the progression of fibrosis in patients coinfected with the human immunodeficiency virus-1 and the hepatitis C virus.

Human Platelet Polymorphism can be a genetic marker associated with HIV/HCV coinfection

To evaluate the associations of HPA polymorphisms -1, -3, and -5 with HIV/HCV coinfection were included in this study 60 HIV/HCV-coinfected patients from the Sao Paulo State health service centers. Data reported by Verdichio-Moraes et al. (2009: J. Med Virol 81:757-759) were used as the non-infected and HCV monoinfected groups. Human Platelet Polymorphism genotyping was performed in 60 Patients co-infected with HIV/HCV by PCR-SSP or PCR-RFLP. HIV subtyping and HCV genotyping was performed by RT-PCR followed sequencing. The data analyses were performed using the χ2 test or Fisher's Exact Test and the logistic regression model. Patients coinfected with HIV/HCV presented HCV either genotype 1 (78.3%) or non-1 (21.7%) and HIV either subtype B (85.0%) or non-B (15%). The Human Platelet Polymorphism-1a/1b genotype was more frequent (P < 0.05) in HIV/HCV coinfection than in HCV monoinfection and the allelic frequency of Human Platelet Polymorphism-5b in the Patients coinfected with HIV/HCV was higher (P < 0.05) than in HCV monoinfected cases and non-infected individuals. These data suggest that the presence of specific HPA allele on platelets could favor the existence of coinfection. On the other hand, Human Platelet Polymorphism-5a/5b was more frequent (P < 0.05) in HIV/HCV coinfected and HCV monoinfected groups than in the non-infected individuals, suggesting that this platelet genotype is related to HCV infection, regardless of HIV presence. Results suggest that the Human Platelet Polymorphism profile in HIV/HCV coinfected individuals differs from the one of both HCV monoinfected and non-infected population. So, the Human Platelet Polymorphism can be a genetic marker associated with HIV/HCV coinfection.

Simultaneous genotyping of HPA-17w to -21w by PCR-SSP in Chinese Cantonese

Studies have reported the polymorphism of human platelet antigen (HPA)-17w, -18w, -19w, -20w, and -21w. However, the distribution of these five antigens in Chinese Cantonese is still unknown. In this study, we designed new sequence-specific primers for HPA-19w to -21w and used published primers for HPA-17w and -18w to develop a polymerase chain reaction with the sequence-specific primers (PCR-SSP) method for simultaneously genotyping HPA-17w to -21w. A total of 820 unrelated Cantonese apheresis platelet donors in Guangzhou were involved in this study. Among the five HPAs, complete a/a homozygosity was observed for HPA-17w to -20w with an allele frequency of 1.0000. For HPA-21w, nine individuals (9/820, 1.10%) were found to be HPA-21a/bw heterozygous and the allele frequencies of HPA-21a and HPA-21bw were 0.9945 (1631/1640) and 0.0055 (9/1640), respectively. The reliability of the PCR-SSP method was determined by comparing with the genotyping results by DNA sequencing, and no inconsistencies were observed between the two methods. This study provides a reliable PCR-SSP method for simultaneously genotyping HPA-17w to -21w and could improve HPA-matched platelet transfusion in Chinese Cantonese.

Gene frequencies of human platelet antigens in the Macedonian population

Human platelet antigen (HPA) systems consist of more than 12 bi-allelic antigen polymorphisms. Due to these polymorphisms, platelet-membrane glycoproteins can be recognized as alloantigens or autoantigens and can cause conditions such as fetomaternal alloimmune thrombocytopenia, post-transfusion refractoriness to platelets, and post-transfusion throbocytopenic purpura. The purpose of this study was to investigate the distribution of HPA-1, -2, -3, and -5 in Macedonian population by using the polymerase chain reaction and restriction fragment length polymorphism. The allele frequencies were 0.865 for HPA-1a, 0.135 for HPA-1b, 0.852 for HPA-2a, 0.148 for HPA-2b, 0.578 for HPA-3a, 0.422 for HPA-3b, 0.909 for HPA-5a, and 0.091 for HPA-5b. Results of our study were not significantly different from those reported in the other European studies. Our population displayed the highest frequency for HPA-2b allele (0.148) reported among European population.

Platelet antigens. The role of human platelet alloantigens (HPA) in blood transfusion and transplantation

In this review, we describe the platelet surface molecules with special focus on the polymorphic glycoproteins giving rise to the human platelet alloantigen (HPA) system. We list the platelet glycoprotein complexes GPIa/IIa, GPIIb/IIIa, GPIb/V/IX and some other molecules, the corresponding genes that encode them and we describe their polymorphisms as well as their physiological function. Based on data obtained by serological and molecular methods, we explain how ancestral HPA alloepitopes have developed into the modern variants. We also describe the tissue distribution of these proteins, which seems to be wider than thought initially, and discuss the significance of the HPA encoding genes distribution in various populations. Methods for their determination are then described briefly Since HPA alloepitopes can induce antibodies with resulting clinical conditions such as: post-transfusion refractoriness to platelets (PTR); post-transfusion thrombocytopenic purpura (PTTP); and fetomaternal alloimmune thrombocytopenia (FMAIT), the mechanism of this alloimmunization and its prevention is described. Although the humoral arm is more important from the clinical viewpoint, the activation of the cytotoxic arm by HPA alloepitopes is also possible. These polymorphisms also seem to have a role in certain other clinical circumstances, therefore their impact on cardiovascular diseases and haemostatic disorders as well as their role in the transplantation of solid organs and bone marrow is addressed. We conclude that during the last decades, the research of the platelet membrane molecules contributed considerably to the diagnostics, prevention and therapy of the blood coagulation and haemostatic disorders, to the management of the neonatal thrombocytopenias, transfusion-related thrombocytopenias, refractoriness to platelet transfusions and autoimmune disorders. It also changed our view on the role of HPA alloepitopes in bone marrow and solid organ transplantation. In the future, this accumulated knowledge will be useful for the development of the cell-based therapies and immune modulation of both acquired and hereditary diseases.

PCR with sequence-specific primer-based simultaneous genotyping of human platelet antigen-1 to -13w

BACKGROUND

Accurate human platelet antigen (HPA) typing is important for patients with diagnosis of alloimmune thrombocytopenic syndromes and provision of HPA-matched blood components for these patients.

STUDY DESIGN AND METHODS

Thirteen sequence-specific primers (SSPs) designed on the basis of known published polymorphisms for HPA-1 to HPA-13w, respectively, were employed for simultaneous HPA genotyping. All PCR amplifications were carried out with identical cycling conditions in 96-well plates containing primer mixtures. A total of 300 blood samples from unrelated volunteer donors in Taiwan were included in the study.

RESULTS

All primers had specific amplification products. The typing results were available within 4 hours each time for up to four blood samples tested. Among the 13 HPAs, HPA-3 had the greatest heterozygosity with a gene frequency of 0.3267, 0.4967, and 0.1767 for HPA-3a/HPA-3a, HPA-3a/HPA-3b, and HPA-3b/HPA3-b, respectively. For the remaining 12 HPAs, the predominance of a/a homozygosity was noted for HPA-1, -2, -4, -5, and -6, with a frequency ranging from 0.9200 to 0.9967. The frequency of a/a homozygosity was 1.0000 for HPA-7w to -13w, except for HPA-10w, for which one case was observed to be HPA-10aw/HPA-10bw heterozygous. Excluding HPA-3, b/b homozygosity was noted in only one case (HPA-6b/HPA-6b). The prevalence rates of HPA-1 to -13w in this study were consistent with previous reports using different methods.

CONCLUSION

An extended, streamlined PCR-SSP protocol for simultaneous genotyping of HPA-1 to HPA-13w was established. This allows fast and reliable diagnosis of alloimmune thrombocytopenia, and is readily applicable to large-scale genetic population studies.

Analysis of human platelet antigen systems in a Moroccan Berber population

The human platelet alloantigen systems HPA-1, -2, -3, -4, -5 and -6 in a Moroccan Berber population from the Amizmiz region were determined by polymerase chain reaction using sequence-specific primers (PCR-SSP). The gene frequencies obtained from these unrelated Berbers were 0 x 747 and 0 x 252 for HPA-1a and -1b; 0 x 817 and 0 x 182 for HPA-2a and -2b; 0 x 682 and 0 x 317 for HPA-3a and -3b; 1 and 0 x 0 for HPA-4a and -4b; 0 x 8616 and 0 x 1383 for HPA-5a and -5b; 1 and 0 x 0 for HPA-6a and -6b. The Berbers have the highest frequency for the 1b, 2b and 5b alleles of all the populations reported to date and also the lowest frequency for the 3b allele.

Single amino acid substitution in human platelet glycoprotein Ibβ is responsible for the formation of the platelet-specific alloantigen Iya

We recently described a new low-frequency platelet alloantigen on the human platelet glycoprotein (GP) Ib-IX complex, termed Iya, which was implicated in a severe case of neonatal alloimmune thrombocytopenia. Immunoprecipitation studies with trypsin-treated platelets indicated that the Iyaalloantigenic determinants are formed by the membrane-associated remnant moiety of GP Ib (GP Ibr) together with GP Ibβ and GP IX. To elucidate the molecular basis underlying the Iya alloantigen, we amplifiedGPIbr, GPIbβ, andGPIX genes by polymerase chain reaction (PCR). Nucleotide-sequence analysis of these 3 genes showed a G to A transition at position 141 on GPIbβ gene in a subject positive for Iya. This transition resulted in a Gly15Glu dimorphism on the N-terminal domain ofGPIbβ. This finding was confirmed by genotyping analysis of 6 Iya-positive subjects by restriction fragment length polymorphism (RFLP) studies using NarI endonuclease. In 300 randomly selected healthy blood donors, one Iya-positive individual was found. Phenotypes determined by monoclonal antibody-specific immobilization of platelet antigens assay and genotypes determined by RFLP were identical in this population. Analysis of Iya-positive platelets showed that the point mutation affected neither the degree of surface expression nor the function of the GP Ib-GP Ibβ-IX complex on the platelet surface. Transient expression of the GP Ib-IX complex in CHO cells using wild-type GP Ibβ (Gly15) or mutant GP Ibβ (Glu15) allowed us to demonstrate that this single amino acid substitution is sufficient to induce Iya epitope(s).

B-lymphoblastoid cell lines as a source of reference DNA for human platelet and neutrophil antigen genotyping

BACKGROUND

Human platelet and neutrophil antigens (HPAs, HNAs) are targets for platelet or granulocyte antibodies causing immune thrombocytopenia or neutropenia, respectively. Currently, genotyping is replacing phenotyping as the preferred method of diagnosis of immune cytopenia. To establish a reliable genotyping analysis, however, the availability as reference DNA of genomic DNA from persons of known genotype is essential.

STUDY DESIGN AND METHODS

By the use of Epstein-Barr virus transformation, panels of B-lympho-blastoid cell lines (B-LCLs) from HPA- and HNA-phenotyped individuals were developed. Genomic DNA was isolated from these cell lines and tested as reference DNA for genotyping of persons for HPAs and HNAs.

RESULTS

DNA derived from these B-LCLs was typed by polymerase chain reaction-restriction fragment length polymorphism and -sequence-specific primers. The results were in accordance with the genotyping from peripheral blood cells. These results were confirmed by 24 laboratories in Germany in a blind study.

CONCLUSION

The inexhaustible source of reference DNA derived from B-LCLs allowed the evaluation of reliable HPA and HNA genotyping for quality control purposes. It should facilitate the development of DNA typing in blood centers and clinical laboratories.

Genotyping for human platelet-specific antigens HPA-1, -2, -3, -4 and -5 in the Slovenian population reveals a slightly increased frequency of HPA-1b and HPA-2b as compared to other European populations

Typing of human platelet alloantigens (HPA) is necessary in various clinical situations. The purpose of this study was to type a random sample of the Slovenian population for HPA alleles, in order to obtain genetic population data. A total of 152 unrelated Slovenian blood donors were genotyped for HPA-1, -2, -3, -4 and -5 alleles using a simple method that enables simultaneous and complete determination of HPA genotypes. Ten different polymerase chain reactions employing sequence-specific priming (PCR-SSP), which worked in identical cycling conditions, were used. The allele frequencies were 0.809 for HPA-1a, 0.191 for HPA-1b, 0.891 for HPA-2a, 0.109 for HPA-2b, 0.591 for HPA-3a, 0.407 for HPA-3b, 0.997 for HPA-4a, 0.00 for HPA-4b, 0.934 for HPA-5a and 0.066 for HPA-5b. When compared to results of studies of various other Caucasian populations, our population displayed a slightly but not significantly higher proportion of the HPA-1b and 2b alleles.

Immune Mechanisms of Platelet Refractoriness

Multiple platelet transfusions may prove ineffective in approximately 40% of patients treated for bone marrow aplasia. This condition is known as platelet refractoriness and is diagnosed by evaluating the corrected count increment following platelet transfusion. Immune factors still represent an important cause of platelet refractoriness and, among these, HLA alloimmunization plays the most relevant pathogenetic role. Less important are other specific and non-specific antigens, that can be detected on platelet surface. Several assays have been developed to reveal anti-platelet antibodies, which include the lymphocyte cytotoxicity test, the platelet immunofluorescence assay and the mixed passive hemagglutination assay. It is now clear that leukocytes contaminating the platelet concentrates represent the main cause of HLA alloimmunization which can be prevented by leukocyte depletion to less than 5 × 106 cells per unit. Transfusion of HLA-matched platelets in alloimmunized platelets may be quite effective, but it can also be fairly expensive considering the large number of donors to be typed for HLA in order to find HLA compatible platelets. A more practical approach would be to select the platelet concentrates on the basis of the negative crossmatch.

Maternal immunization to Gov system alloantigens on human platelets

BACKGROUND

Immunization to platelet alloantigens can occur during pregnancy or after the transfusion of blood components. Platelet alloantibodies can cause neonatal alloimmune thrombocytopenia and posttransfusion purpura. Transfusion-induced alloimmunization to a novel platelet alloantigen system, Gov, expressed on the 175-kDa glycosyl phosphatidylinositol-anchored platelet glycoprotein, CD109, was previously described. This report describes three unrelated patients who were alloimmunized to Gov(a) or Gov(b) during pregnancy.

STUDY DESIGN AND METHODS

Platelets were typed by using radioimmunoprecipitation for HPA-1a, -3a, -5a, -5b, Gov(a), and Gov(b) and by polymerase chain reaction-restriction fragment length polymorphism for HPA-1a, -1b, -3a, and -3b. Maternal sera were screened for platelet antibodies by using radioimmunoprecipitation and the antigen capture assay.

RESULTS

Patients 1 and 2 were investigated after the diagnosis of neonatal alloimmune thrombocytopenia in their children, and alloantibodies specific for Gov(b) and Gov(a), respectively, were detected in maternal serum. Serum from patient 3, who had mild idiopathic thrombocytopenia purpura with no detectable autoantibody, was found to contain alloantibodies to Gov(b) and to HPA-5b, presumably as a result of immunization during pregnancy. Platelet typings confirmed that the patients were at risk for alloimmunization to the respective antigen.

CONCLUSION

This report of three cases of maternal alloimmunization to antigens in the Gov system indicates that immunization can occur via placental transfer of antigen and that Gov system alloantibodies may be associated with neonatal alloimmune thrombocytopenia.

Gene frequencies of human platelet antigens on glycoprotein IIIa in Japanese

BACKGROUND

Polymorphism of glycoprotein IIIa on human platelets is one of the factors in alloimmunization that causes neonatal alloimmune thrombocytopenia and refractoriness to platelet transfusion.

STUDY DESIGN AND METHODS

DNA typing methods were originally developed to determine the genotypes of five human platelet antigen (HPA) systems located on glycoprotein IIIa: HPA-1, HPA-4, HPA-6W, HPA-7W and HPA-8W. The gene frequencies of these platelet antigens were determined by DNA typing of 331 unrelated Japanese donors.

RESULTS

The gene frequencies of the low-frequency antigens were 0.002, 0.011, and 0.027 for HPA-1b, HPA-4b, and HPA-6W(b), respectively. All 331 Japanese donors tested were HPA-7W(a/a) and HPA-8W(a/a). Moreover, in the present study, none of the donors tested had two or more of these low-frequency antigens.

CONCLUSION

The risk of neonatal alloimmune thrombocytopenia and refractoriness to platelet transfusion induced by the antigens of the HPA-1, HPA-7W, and HPA-8W systems was extremely rare in Japanese. However, attention must be paid to the involvement of the HPA-4 and HPA-6W systems in these clinical disorders.

A simple and rapid competitive enzyme-linked immunosorbent assay to identify HPA-1a (PlA1)-negative donor platelet units

BACKGROUND

Alloantibodies to HPA-1a (PlA1) are the major cause of neonatal alloimmune thrombocytopenia and posttransfusion purpura and have been implicated in refractoriness to random-donor platelet transfusions. However, most assays used to phenotype platelets are cumbersome or time-consuming for large numbers of samples.

STUDY DESIGN AND METHODS

A simple, competitive (inhibition) enzyme-linked immunosorbent assay for HPA-1a phenotyping of donor platelets was developed. A segment from the donor platelet unit transfer line was sealed to obtain a small aliquot of platelets. These platelets were washed once and added to a predetermined dilution of serum containing alloantibodies to HPA-1a. Residual anti-HPA-1a binding to the glycoprotein IIb/IIIa purified by lectin and high-performance liquid chromatography and coated on microtiter wells was detected with a conjugated antihuman IgG. A lack of inhibition equivalent to control (no platelets) was used to determine that the platelets were HPA-1b/b.

RESULTS

Of the 557 platelet units tested, 14 (2.5%) were found to be HPA-1a negative, and they were confirmed to be HPA-1b/b by DNA genotyping. Two of the 14 HPA-1b/b units were also HPA-3b/b (approx. 0.35% of the random population). Use of the microtiter format allows 100 to 200 samples to be processed per day.

CONCLUSION

This simple and inexpensive assay is useful for identifying HPA-1b/b units for platelet-compatible transfusions or for platelet antibody investigations.

Rapid typing for human platelet antigen systems-1, -2, -3 and -5 by PCR amplification with sequence-specific primers

Typing for human platelet antigens (HPA) is useful in a variety of clinical situations. We developed a method for genotyping for HPA-1, -2, -3 and -5 by means of the PCR amplification with sequence-specific primers (PCR-SSP) technique. Primer sets were designed to allow PCR amplification for all systems using the same assay conditions. Specificity and sensitivity of the method were assessed in a blind quality control study (n = 112). In 111 cases, results obtained by PCR-SSP were identical as compared with PCR-restriction fragment length polymorphism technique. One discrepancy was found to be due to a typing error in the data sheet. The results of the PCR-SSP technique were available within 3 h. We conclude that genotyping based on PCR-SSP enables rapid typing for HPA systems, which makes this technique feasible in most clinical settings where urgent HPA typing is required.

Frequency of platelet-specific alloantigens in a Danish population

This study reports the first data on gene frequencies of platelet alloantigens HPA-1, HPA-2, HPA-3, HPA-4 and HPA-5 in a population of unrelated Danish blood donors using PCR-techniques. The observed gene frequencies fit the Hardy-Weinberg equilibrium, and the calculated phenotype frequencies are similar to those obtained in other Caucasian populations: HPA-1a and -1b occur in 96.6% and 30.3% of 557 unrelated respectively. HPA-2a and -2b in 99.4% and 15.9% of 163 tested, HPA-3a and -3b in 88.3% and 63.2% of 163 tested, HPA-4a and -4b in 100% and 0% of 131 tested, and finally HPA-5a and -5b in 100% and 15.7% of 427 tested. It is a major technical improvement to use PCR techniques for genomic typing of HPA. Not only is it possible to perform HPA typings in severely thrombocytopenic patient and on amniotic fluid cells of the fetus of alloimmunized mothers, but it must be expected that accuracy of the HPA typing will increase considerably, as has been the case with genomic HLA class II typing. Finally, use of PCR technique combined with allele-specific primers is suitable for accurate large scale typing of platelet donors, which may be useful in special clinical settings.

Gene frequencies of the five major human platelet antigens in African American, white, and Korean populations

BACKGROUND

The study of the immunogenetics of the human platelet antigens is important to the improvement of diagnosis and genetic counseling and to the development of screening programs for women at risk of having babies with neonatal alloimmune thrombocytopenia. Description of the immunogenetics of the human platelet antigens in some racial groups has been incomplete.

STUDY DESIGN AND METHODS

A reverse dot blot technique employing polymerase chain reaction-amplified genomic DNA was applied in genotyping the five major human platelet antigens in the following populations: 100 African American and 100 white women admitted to the obstetric unit at Johns Hopkins Hospital (Baltimore, MD) and 100 inpatients at Yonsei University (Seoul, Korea).

RESULTS

The gene frequencies of HPA-2b (Koa) and HPA-5b (Bra) in African Americans were twice those in whites (African Americans: 0.18 and 0.21, respectively; whites: 0.09 and 0.11, respectively). There is a very low gene frequency of the HPA-1b (PIA2) allele in Koreans (0.005). No significant differences were found in the gene frequencies of the human platelet antigens in whites in this series and those in published European studies.

CONCLUSION

These studies indicate a higher potential risk for alloimmunization to HPA-2 (Ko) and HPA-5 (Br) antigens in African Americans than in whites. In addition, the low gene frequency of HPA-1b (PIA2) in African Americans and Koreans suggests that alloimmunization to HPA-1a (PIA1) would be very unusual in these populations. These data may provide the basis for planning neonatal alloimmune thrombocytopenia screening programs in certain ethnic populations.

Platelet alloantigens HPA-1, -2, -3, -5 and -6b in Finns

The human platelet alloantigens HPA-1, -2, -3, -5 and -6b in the Finnish population were determined using allele specific restriction analysis (PCR-ASRA) for HPA-1, -2, -3 and -5 and monoclonal antibody immobilized platelet antigen (MAIPA) assay for HPA-1, -3a, -5b and -6b. No discrepancies were observed between the results obtained with the PCR-method and those obtained serologically. The gene frequencies obtained from 200 unrelated Finns were 0.86 and 0.14 for HPA-1a and -1b, 0.91 and 0.09 for HPA-2a and -2b, 0.59 and 0.41 for HPA-3a and -3b and 0.95 and 0.05 for HPA-5a and -5b. The frequency of the HPA-5b allele (10%) is lower in Finns than in Central- or South-European populations (20-30%). The HPA-1, -2 and -3 frequencies did not deviate from those observed in other European populations. The rare HPA-6b antigen was observed in three of 127 individuals from south-eastern Finland (2.4%), which suggests that the frequency of this allele in Finland is higher than previously thought.

Sensitivity of the platelet immunofluorescence test (PIFT) and the MAIPA assay for the detection of platelet-reactive alloantibodies: a report on two U.K. National Platelet Workshop exercises

This report presents the results of two National Workshop exercises which were designed to evaluate interlaboratory and interassay variation in sensitivity of techniques used for the detection of platelet-reactive alloantibodies. Most workshop participants used the platelet immunofluorescence test. Sensitivity of this assay was improved when fluorescence was measured by flow cytometer rather than by microscope. The MAIPA assay was found to be highly sensitive but required considerable technical expertise, and the choice of antiglycoprotein IIb/IIIa (CD61/41) monoclonal antibody was found to have a significant effect on its ability to detect anti-HPA-1a.

Frequency of platelet-specific antigens among Indonesians

This study reports the frequencies of the alloantigens of four major platelet-specific alloantigen systems among Indonesians. One hundred and sixty-eight unrelated Indonesian blood donors were phenotyped for the alloantigens of the Zw (PIA, HPA-1), Bak (HPA-3), Yuk (Pen, HPA-4), and Br (HPA-5) systems by use of a glycoprotein-specific immunoassay. All were positive for the alloantigens Zwa, Yukb, and Brb. Three (1.79%) and 1 (0.59%) of the 168 donors were positive in testing for Zwb and Yuk(a) antigens, respectively. Fifteen (9.26%) of 162 Indonesians had Br(a) antigens. Of the 166 donors tested, 121 (72.89%) were Bak(a) positive and 134 (80.72%) were Bakb positive. In addition, the phenotype frequency of Nak(a) was determined by using monoclonal antibody OKM5 in a platelet enzyme-linked immunosorbent assay. Its frequency in the present cohort was 95.83 percent (161/168). This study confirms the differences in platelet antigen distributions in Asians and whites. Both glycoprotein IV deficiency and the Yuk polymorphism are also found among Indonesians.

Acidified chloroquine treatment for the removal of class I HLA antigens

With the aim of reducing the damage to platelets while effectively removing class I HLA antigens from their surfaces, we developed a new method using acidified chloroquine diphosphate. Platelets were treated with a 0.2 M solution of chloroquine diphosphate (pH 4.0). More than 90% of the platelets remained viable after treatment. While a marked reduction in reactions of acidified chloroquine-treated platelets with multispecific HLA antisera was noted in comparison with phosphate-buffered-saline-(PBS)-treated platelets, reactions with platelet-specific antibodies were preserved. This was demonstrated by immunofluorescence tests and solid-phase and monoclonal antibody immobilization of platelet antigen assays. Aggregation responses, though reduced in comparison with PBS-treated platelets, were still preserved after acidified chloroquine treatment. Ultrastructural analysis did not show any significant difference from PBS-treated platelets. We conclude that treatment of platelets with acidified chloroquine diphosphate is a simple and effective method for removing class I HLA antigens from their surfaces with minimal damage to their structure and function.

A new platelet alloantigen, Tua, on glycoprotein IIIa associated with neonatal alloimmune thrombocytopenia in two families

We describe immunization of two mothers against a new platelet alloantigen, designated Tua, in association with thrombocytopenia in their first born children. The platelet-specific antibodies were identified by a glycoprotein-specific platelet protein assay with husband's platelets. Monoclonal antibodies against glycoprotein complex IIb/IIIa (AP2) and against glycoprotein IIb (SZ22) could be used to immobilize the antigen bearing protein. When monoclonal antibodies against glycoprotein Ib/IX (FMC25) or Ia/IIa (Gi9) were used, no platelet-specific antibodies were detectable. The previously described alloantigens on the glycoprotein IIb/IIIa complex (HPA 1,3,4, Sra and Vaa) were not responsible for the reaction. Immunochemical analysis by an immunoblot assay showed that the Tua antigen resides on GPIIIa but the antigen was destroyed by reduction of the protein. Altogether 10 individuals belonging to three unrelated families were shown to carry the antigen. The family studies within three generations indicated autosomal codominant inheritance. Thus the Tua antigen is apparently different from all previously published platelet alloantigens. One Tua positive blood donor was identified in a population study of approximately 150 individuals. This indicates a low frequency in the Finnish population. Extended population studies will be required to determine a more exact frequency of Tua antigen.

Investigation of human platelet alloantigens and glycoproteins using non-radioactive immunoprecipitation

Sensitive techniques to detect platelet antibodies are needed for the investigation of immune thrombocytopenic syndromes such as neonatal alloimmune thrombocytopenia and post-transfusion purpura. Radioimmunoprecipitation has proved useful in the investigation of platelet-antibody interactions; however, the requirement for a radioactive label is a disadvantage. We describe the immunoprecipitation of human platelet proteins labelled with nonradioactive NHSS-biotin and compare the results with proteins labelled with 125I. The efficiency of labelling was evaluated by immunoprecipitation using well-characterized human anti-platelet antisera and murine monoclonal antibodies. The immunoprecipitated proteins were separated by SDS-PAGE, transferred to nitrocellulose and detected using streptavidin-horseradish peroxidase and a chemiluminescent substrate with exposure to X ray film. The biotinylation technique labelled glycoproteins Ia/IIa, Ib/IX, IIb/IIIa, IV, and p175 which carry all of the known platelet alloantigens and isoantigens. It was as sensitive as radiolabelling and had the advantage of labelling GPs Ib beta and IX, which were not labelled using radioiodine. Human sera containing alloantibodies to HPA-1a on GP IIIa, HPA-3a on GP IIb, HPA-5a and HPA-5b on GP Ia, Govb on p175, and the isoantibody Naka on GP IV precipitated the corresponding biotinylated proteins. Biotinylated proteins could be detected using a 30 s exposure compared to 2 days or longer for 125I. Immunoprecipitation of human platelet glycoproteins labeled with NHSS-biotin is a fast and sensitive alternative to conventional radioimmunoprecipitation for the study of human platelet antigens.

The MAIPA assay and its applications in immunohaematology

The monoclonal antibody immobilization of platelet antigens (MAIPA) assay has proved to be a reliable and useful tool in platelet immunology. After immunochemical localization of the most important platelet allo- and autoantigens on platelet glycoproteins (GPs) IIb/IIIa, Ia/IIa, Ib/IX, the corresponding antibodies can easily be detected with this technique because mAbs against these membrane constituents are now readily available. The assay also allows reliable typing of platelet alloantigens. Detection of GP-specific PAIgG (by direct MAIPA) has proved to be a specific diagnostic sign of AITP. The MAIPA assay allowed identification of 'new' alloantibody specificities including alloantibodies against 'low-frequency' antigens. The principle of this technique has successfully been applied to characterize antibodies against lymphocytes, endothelial cells and granulocytes.

Leucine33-proline33 substitution in human platelet glycoprotein IIIa determines HLA-DRw52a (Dw24) association of the immune response against HPA-1a (Zwa/PIA1) and HPA-1b (Zwb/PIA2)

Alloantibody formation against HPA-1a (Zwa/PIA1) has, to date, only been found in HLA-DRw52(a+) (Dw24) individuals. Alloimmunization against the product of the other HPA-1 allele, HPA-1b, is rare. We have been able to evaluate ten cases of HPA-1b alloimmunization in Europe in order to study whether there is an association between HLA phenotype and anti-HPA-1b antibody formation. HLA typing of these patients was performed with particular attention to the DRw52a specificity using specific T-cell clones. No association with DRw52a or any other known HLA phenotype was found. This finding implies that the amino acid substitution leucine33-proline33 in GPIIIa, responsible for HPA-1a/b, is of primary importance for the association of anti-HPA-1a antibody formation with DRw52a. These data show that the amino acid polymorphism affects the presentation of the immunogenic oligopeptides of HPA-1a and -1b in the HLA class-II groove.

Labeling of platelet surface glycoproteins with biotin derivatives

We describe a non-radioactive method for the labeling of platelet surface proteins, consisting of platelet protein biotinylation by means of N-hydroxysuccinimido-biotin (NHS-B) and biotin-hydrazide (H-B); NHS-B labels proteins amino residues while H-B binds to periodate-modified sialoglycoproteins. Washed platelets were biotinylated and protein bands were detected after SDS-electrophoresis and western-blot using avidin-peroxidase and luminol as substrate to enhance the signal which was then detected by X-ray film. Biotin-labeled platelet proteins were also immunoprecipitated with monoclonal antibodies against glycoproteins Ib and the IIb-IIIa complex. The use of periodate induced biotinylation is the method of choice for labeling platelet surface glycoproteins while NHS-B also labels internal proteins. The sensitivity of this new procedure is similar to that obtained with radiolabeling techniques; biotinylation does not interfere with the antigenic properties of Ib and IIb-IIIa glycoproteins.

A new low-frequency platelet alloantigen, Vaa, on glycoprotein IIbIIIa associated with neonatal alloimmune thrombocytopenia

We describe platelet alloimmunization which caused severe thrombocytopenia in a neonate and could only be detected by testing the father's platelets. The platelet-specific antibodies were identified by a monoclonal antibody-immobilized platelet protein assay (MAIPA) using monoclonal antibodies against glycoprotein (GP) IIbIIIa complex (AP2 and 2G12). The previously described alloantigen systems on the GPIIbIIIa complex (HPA 1, HPA 3 or HPA 4) were not responsible for the reaction. In addition the newly described private platelet antigen Sra was not identical to the antigen. The antigen is therefore different from all known platelet alloantigens and was designated Vaa. The antigen was present on the platelets of the affected child. Family studies showed that the platelet antigen was transmitted as an autosomal dominant trait in three generations. No Va (a+) individuals were found in a population study of 250 blood donors, which indicates that the antigen is of low frequency in the Finnish population. The Va antigen was not detectable by immunoblot analysis, which suggests that the epitope may not be a linear peptide structure. The antigen was also destroyed by solubilization of platelets. Thrombin activation of platelets, known to increase the expression of GPIIbIIIa on platelets, did not increase the number of binding sites for anti-Vaa antibodies to the extent observed with anti-HPA 1a binding.