Maternal DRB1*1501, DQA1*0102, DQB1*0602 haplotype in fetomaternal alloimmunization against human platelet alloantigen HPA-6b (GPIIIa-Gln489)

The first reported case of neonatal alloimmune thrombocytopenia due to low-frequency human platelet antigen-6b antibodies in the United Kingdom

BACKGROUND

Neonatal alloimmune thrombocytopenia (NAIT) is a potentially serious clinical condition caused by maternal alloantibodies directed to human platelet antigens (HPA), inherited from the father and expressed on fetal/neonatal platelets. We report a case of an otherwise well, full term child, with a profound thrombocytopenia (33 x 109/L). There was no bleeding or obvious explanation for the low platelet count. Samples were sent for the investigation of NAIT.

METHOD

Serological investigations were performed on maternal serum taken at day (D)+4 and D+78. The platelet immunofluorescence test (PIFT) and monoclonal antibody immobilization of platelet antigens (MAIPA) assays were performed with a panel of HPA typed donor platelets and against paternal platelets in a crossmatch. HPA 1-6, -9 and -15 and HLA genotyping was performed by in-house PCR-sequence based typing (SBT) and next generation sequencing (NGS).

RESULTS

HPA antibody screening of D+4 maternal serum indicated that platelet-specific antibodies were absent. HPA genotyping of the father and child revealed the presence of the low frequency HPA antigen (LFHPA), HPA-6b, which was absent in the mother. Maternal samples were crossmatched against paternal platelets and were positive by PIFT and glycoprotein (GP) IIb/IIIa and HLA class I in the MAIPA assay. The infant required no platelet transfusion support as the thrombocytopenia resolved spontaneously.

DISCUSSION

We conclude that the positive crossmatch reaction was due to anti-HPA-6b alloantibodies. This case further emphasizes the importance of platelet crossmatching and HPA genotyping of LFHPA in cases where there is a high clinical suspicion of NAIT but initial screening is negative.

Protective effect of HLA-DQB1 alleles against alloimmunization in patients with sickle cell disease

BACKGROUND

Alloimmunization or the development of alloantibodies to Red Blood Cell (RBC) antigens is considered one of the major complications after RBC transfusions in patients with sickle cell disease (SCD) and can lead to both acute and delayed hemolytic reactions. It has been suggested that polymorphisms in HLA genes, may play a role in alloimmunization. We conducted a retrospective study analyzing the influence of HLA-DRB1 and DQB1 genetic diversity on RBC-alloimmunization.

STUDY DESIGN

Two-hundred four multi-transfused SCD patients with and without RBC-alloimmunization were typed at low/medium resolution by PCR-SSO, using IMGT-HLA Database. HLA-DRB1 and DQB1 allele frequencies were analyzed using logistic regression models, and global p-value was calculated using multiple logistic regression.

RESULTS

While only trends towards associations between HLA-DR diversity and alloimmunization were observed, analysis of HLA-DQ showed that HLA-DQ2 (p=0.02), -DQ3 (p=0.02) and -DQ5 (p=0.01) alleles were significantly higher in non-alloimmunized patients, likely behaving as protective alleles. In addition, multiple logistic regression analysis showed both HLA-DQ2/6 (p=0.01) and HLA-DQ5/5 (p=0.03) combinations constitute additional predictor of protective status.

CONCLUSION

Our data suggest that particular HLA-DQ alleles influence the clinical course of RBC transfusion in patients with SCD, which could pave the way towards predictive strategies.

Recent progress in understanding the pathogenesis of fetal and neonatal alloimmune thrombocytopenia

Fetal and neonatal alloimmune thrombocytopenia (FNAIT) occurs in c. 1 in 1000 births and is caused by maternal antibodies against human platelet alloantigens that bind incompatible fetal platelets and promote their clearance from the circulation. Affected infants can experience bleeding, bruising and, in severe cases, intracranial haemorrhage and even death. As maternal screening is not routinely performed, and first pregnancies can be affected, most cases are diagnosed at delivery of a first affected pregnancy. Unlike its erythrocyte counterpart, Haemolytic Disease of the Fetus and Newborn, there is no prophylactic treatment for FNAIT. This report will review recent advances made in understanding the pathogenesis of FNAIT: the platelet alloantigens involved, maternal exposure and sensitization to fetal platelet antigens, properties of platelet Immunoglobulin G antibodies, maternal-fetal antibody transport mechanisms and efforts to develop an effective FNAIT prophylaxis.

Alloantibodies against low-frequency human platelet antigens do not account for a significant proportion of cases of fetomaternal alloimmune thrombocytopenia: evidence from 1054 cases

BACKGROUND

Maternal alloantibodies against the five common human platelet antigen (HPA) systems (HPA-1 to -3, -5, and -15) are found in only 20% of cases referred for fetal and neonatal thrombocytopenia (FMAIT) investigations. The question asked was whether mismatches for the remaining 11 low-frequency HPAs (HPA-4 and -6bw to -17bw) might in part explain the remaining 80% of cases.

STUDY DESIGN AND METHODS

A total of 1054 paternal DNA samples from referred FMAIT cases (among which 223 cases where antibodies against a common HPA were found) were genotyped for 11 low-frequency HPAs as well as a recently discovered polymorphism (ITGA2B-C2320T). The initial genotyping was carried out by TaqMan and potential heterozygotes were confirmed by DNA sequencing. Clinical and serologic data were collected for each case with a heterozygote father.

RESULTS

In total, eight heterozygous fathers were identified: four for HPA-6w, one each for HPA-10w and -11w, and two for HPA-12w. Maternal antibodies against the corresponding antigen were identified in four of the eight cases. In two of these cases, antibodies against HPA-1a and HPA-1b were also found.

CONCLUSION

It was concluded that the minor alleles of HPA-4 and -6bw to -17bw are exceptionally rare in the Caucasian population and therefore do not explain the large number of FMAIT referrals which test negative for the common HPA antibodies.

Three novel beta3 domain-deletion peptides for the sensitive and specific detection of HPA-4 and six low frequency beta3-HPA antibodies

BACKGROUND

Antibodies against human platelet antigens (HPA) are clinically important in fetal-maternal alloimmune thrombocytopenia, refractoriness to platelet transfusions and post-transfusion purpura. Of the 16 HPAs, nine are located on the beta3 subunit of the alphaIIb beta3 integrin. Antibody detection is generally based on platelet-derived alphaIIb beta3 from HPA-genotyped donors. Recombinant allelic beta3 peptides, expressed at high levels would improve consistency in antibody detection, but the expression of soluble and monomeric integrins expressing complex dependent epitopes has previously proved challenging.

OBJECTIVES

We aimed to generate three recombinant beta3 peptides for the detection of antibodies against HPA-4, HPA-8bw and five of the six remaining low frequency beta3 alloantigens.

METHODS

The removal of the specificity-determining loop from the betaA domain and fusion of truncated beta3 to calmodulin was exploited to obtain expression of monomeric protein. Using site-directed mutagenesis, the mutations for HPA-4b and HPA-8bw were introduced in the ITGB3*001 haplotype. A third peptide for the detection of antibodies against HPA coded by non-synonymous single nucleotide polymorphisms of low frequency was generated by the introduction of five mutations forming the basis of HPA-6bw, -7bw, -10bw, -11bw, and -16bw antigens.

RESULTS

Reactivity of the three peptides with beta3-specific murine monoclonal antibodies and human HPA-1a phage antibodies confirmed the structural integrity of the recombinant fragments, and reactivity with a unique panel of polyclonal anti-HPA sera confirmed expression of the relevant HPA epitopes.

CONCLUSIONS

These data demonstrate that beta3 integrin domain-deletion fragments are suitable molecular targets for HPA antibody detection.

Fetal and neonatal alloimmune thrombocytopenia: progress and ongoing debates

Fetal and neonatal alloimmune thrombocytopenia (AIT) is a result of a parental incompatibility of platelet-specific antigens and the transplacental passage of maternal alloantibodies against the platelet antigen shared by the father and the fetus. It occurs in approximately 1 in 1000 live births and is the most common cause of severe thrombocytopenia in fetuses and term neonates. As screening programs are not routinely performed, most affected fetuses are identified after birth when neonatal thrombocytopenia is recognized. In severe cases, the affected fetus is identified as a result of suffering from an in utero intracranial hemorrhage. Once diagnosed, AIT must be treated antenatally as the disease can be more severe in subsequent pregnancies. While there have been many advances regarding the diagnosis and treatment of AIT, it is still difficult to predict the severity of disease and which therapy will be effective.

HLA class II haplotypes in Mexican systemic lupus erythematosus patients

Systemic lupus erythematosus (SLE) is an autoimmune disease in which polymorphisms within the human leukocyte antigen (HLA) region have been associated to its etiology. For this study, HLA-DQB1, DQA1, and DRB1 genes were typed by polymerase chain reaction-sequence-specific primer in 237 individuals, taken from 74 families, who had a member with SLE, and who had their residence in the western region of Mexico; as well as in 159 ethnically matched healthy volunteers taken from 32 families. Genotype and allele frequency analysis was performed in 74 SLE patients and 54 unrelated controls. Precise three-loci identification of independent haplotypes was performed in 48 patients and 54 controls by familial segregation. Genotype distribution at each loci was concordant with Hardy-Weinberg's equilibrium in the control group. In general, no genotype effect was observed in SLE patients. Allele distribution comparison showed in the SLE group a significant increase of HLA-DQA1*0102, DQB1*0402, and DRB1*15; whereas alleles HLA-DQB1*0303 and *0501 were significantly decreased. SLE patients showed haplotype DQB1*0602-DQA1-*0102-DRB1*15 increased. As expected, patients with SLE have a reduced haplotype genetic diversity. The associations found in this study are related to an ancestral haplotype that has been observed in SLE populations of different origins.

Evaluation and treatment of thrombocytopenia in the neonatal intensive care unit

Thrombocytopenia is a very frequent problem among sick neonates, affecting up to 35% of all infants admitted to the NICU. Although multiple clinical conditions have been causally associated with neonatal thrombocytopenia, the cause of the thrombocytopenia is unclear in up to 60% of affected neonates. This article provides neonatologists with a practical approach to the thrombocytopenic neonate, with an emphasis on conditions that could be life-threatening or could have significant implications for further pregnancies. An overview of the current therapeutic modalities is also presented, including a discussion of the possible use of recombinant thrombopoietic cytokines to treat certain groups of thrombocytopenic neonates.

Human platelet antigens: typing by PCR using sequence-specific primers and their distribution in blood donors resident in Wales

We developed and validated HPA-1 to HPA-6 typing by PCR-SSP using a combination of established, modified and newly designed sequence-specific primers. We confirmed that the PCR primer mixtures functioned under the same PCR conditions as our standard HLA-A, -B, -C, -DR, -DQ PCR-SSP typing system. This allows concurrent testing for both HPA and HLA specificities and is therefore the system of choice for both clinical and large-scale blood donor panel HPA and HLA typing by PCR-SSP. Test validation included typing a population of blood donors living in Wales. These HPA frequencies were consistent with those of other European Caucasoid populations. HPA-4b and -6b were absent and HPA-5b, which shows some frequency variation, had a phenotype frequency of 18.9% (allele frequency 0. 0973), being close to that of the Dutch (19.7%) and Austrian (20.4%) populations and almost twice that found in Finns (10.0%). HPA genotype frequencies showed a good fit to Hardy-Weinberg equilibrium, further supporting the validity of our typing method.