348 cases of suspected neonatal alloimmune thrombocytopenia

Platelet disorders in newborn infants: diagnosis and management

Platelets are small, disc-shaped, anucleated cells formed by fragmentation of megakaryocytes in the bone marrow. They circulate in blood with a lifespan of 7 to 10 days and, together with fibrin, form hemostatic plugs at sites of vessel injury. Abnormalities of platelets, either quantitative or qualitative, may cause clinically significant bleeding with resultant morbidity and, on occasion, mortality. This review will focus on platelet disorders in neonates, defined as infants of up to 4 months of age. Special emphasis will be given to the physiology of platelet function in healthy and sick newborn infants. The review will be divided into sections as follows: role of platelets in hemostasis, platelet function in newborn infants, quantitative platelet disorders, qualitative platelet disorders, and platelet transfusion therapy.

Megakaryocytes and platelets in immune thrombocytopenic purpura

In idiopathic thrombocytopenic purpura, 90% of the patients have an elevated platelet-associated immunoglobulin. The most important related antigens are glycoprotein IIb-IIIa (GPIIb-IIIa) (16.7-83.3%), GPIb-IX (13.3-83%), GPIb (3.3-47.1%) and GPIIIa (21.6-33.3%), and less commonly GPIa-IIa, GPIV and GPV. Other related antigens can be platelet granule membrane protein, phospholipid, intraplatelet and cytoplasmic antigens, and rarely human platelet antigen (HPA) 1a and HLA-DR antigens. The marrow megakaryocytes are usually normal or increased in number with maturation impairment. There are discrepancies regarding megakaryocytopoiesis in vitro. A low dose of heparin could elevate the platelet number in certain cases. The expression of c-sis was reported to be inhibited in ITP, when the plasma beta-thromboglobin-platelet factor 4 level was elevated. In secondary immune thrombocytopenia, platelet antibodies can be (1) alloantibodies against the (HPA) system and (2) autoantibodies most commonly against platelet GPIIb-IIIa or GPIb-IX. Other antigens can be 30-52kDa proteins of the platelet membrane. Platelet survival is usually shortened, and marrow megakaryocytes are normal in number. Megakaryocyte colony-forming units could be reduced.

Human monoclonal Fab fragments recovered from a combinatorial library bind specifically to the platelet HPA-1a alloantigen on glycoprotein IIb-IIIa

BACKGROUND AND OBJECTIVES

Certain clinical conditions are related to the presence of platelet-specific alloantibodies in the patient's serum. We studied the molecular diversity of HPA-1a antibodies to analyze some peculiarities of this antibody response.

MATERIALS AND METHODS

Human antibody Fab fragments that bind to the platelet alloantigen HPA-1a on glycoprotein IIb-IIIa (GPIIbIIIa) were generated by using a recombinant phage display system. We established an immunoglobulin G1, kappa combinatorial library from the peripheral blood lymphocytes of a person undergoing a severe posttransfusion purpura.

RESULTS

Characterization of Fab clones selected from the fifth round of antigen-specific panning of this library demonstrates a highly specific reactivity to the HPA-1a alloantigen. The nucleotide sequence analysis of representative HPA-la-specific clones reveals at least 3 distinct V1 and 3VH gene segments that present an extensive degree of mutation as demonstrated by comparison of gene usage and homologies to the nearest germline genes.

CONCLUSIONS

These human HPA-la-specific Fab reagents should allow us to better understand the molecular mechanism involved in HPA-la alloimmunization.

Fetal/neonatal alloimmune thrombocytopenia: a continuing challenge

Fetal/neonatal alloimmune thrombocytopenia is the platelet counterpart of the red cell disorder, haemolytic disease of the fetus and newborn. Unlike Rhesus pregnancies, first pregnancy is commonly affected. It is a progressive disease and can cause severe fetal damage, late fetal and neonatal loss and severe handicap. No screening programme is currently available and the diagnosis is often delayed. Testing for platelet antibodies in those mothers whose babies are thrombocytopaenic is necessary for early detection of this condition.

Clinical uses of intravenous immunoglobulin in pregnancy

Intravenous immunoglobulin was licensed for use in the United States in 1981. Currently, there are only a few Food and Drug Administration-labeled indications for intravenous immunoglobulin, but up to 50 "off-label" uses are reported in the literature. The obstetric literature contains numerous reports on intravenous immunoglobulin therapy during pregnancy. This article reviews the properties, pharmacokinetics, mechanisms of action, and side effects of intravenous immunoglobulin, as well as the reported uses of intravenous immunoglobulin during pregnancy.

Spontaneous rise of fetal platelet counts despite increasing anti-HPA-5b antibodies

BACKGROUND AND OBJECTIVES

Maternal anti-HPA alloantibodies are a rare cause of severe fetal thrombocytopenia. So far there have been no reports on the dynamics of maternal anti-HPA-5 during gestation and its effect on the fetus.

MATERIALS AND METHODS

We monitored maternal anti-HPA-5b antibody titers and fetal platelet counts during gestation in a woman with known anti-HPA-5b alloimmunization. The patient was a 32-year-old woman in her third pregnancy. The 1st pregnancy and delivery of a healthy child had been uneventful. At delivery, anti-HPA-5b was detectable in the maternal serum. A 2nd pregnancy ended in early miscarriage.

RESULTS

A steady rise in the alloantibody titer was recorded throughout the 3rd pregnancy. Therefore, cordocentesis was performed at 28 weeks of gestation (platelet count 76 x 10(9)/l). Serial platelet transfusions were administered to the fetus at 28, 32, and 37 weeks of gestation. The platelet counts rose spontaneously thereafter and were 220 x 10(9)/l at delivery, despite an increase in the anti-HPA-5b antibody titer. The child developed normally during the first year of life.

CONCLUSIONS

This case illustrates the spontaneous recovery of fetal platelet counts in late pregnancy despite a rise in maternal alloantibody titer.

Severe fetomaternal alloimmune thrombocytopenia presenting with fetal hydrocephalus

We report two patients where the finding of isolated fetal hydrocephalus led to the detection of severe fetal thrombocytopenia, using fetal blood sampling. Serological investigation led to the diagnosis of fetomaternal alloimmune thrombocytopenia (FMAIT) due to anti-HPA-1a. Both women had had previous unsuccessful pregnancies probably due to FMAIT; one had had four miscarriages at 17-18 weeks' gestation. The other had had one previous pregnancy complicated by severe fetal anaemia, and eventually hydrocephalus developed and the fetus died without the diagnosis of FMAIT being considered. Subsequent pregnancies in the two women were also affected by FMAIT, but prenatal treatment, predominantly with serial fetal platelet transfusions, resulted in a successful outcome in both cases. These observations suggest that FMAIT should be suspected if there is isolated fetal hydrocephalus, unexplained fetal anaemia, or recurrent miscarriages. The accurate diagnosis of FMAIT is important because recent advances in prenatal management can improve the outcome of subsequently affected pregnancies.

Antenatal care in pregnancies at risk of alloimmune thrombocytopenia: report of 19 cases in 16 families

OBJECTIVE

To assess accuracy of a management program in patients at risk for alloimmune thrombocytopenia (NAITP) and to describe perinatal outcomes.

STUDY DESIGN

Nineteen fetuses at risk of thrombocytopenia were identified using obstetric history, HLA type of the mother and fetal phenotyping in cases where paternal heterozygozity for the offending antigen was present. Cordocentesis was timed according to obstetric history and performed with safety precautions to prevent haemorrhage. High dose intravenous gamma globulin (IVIG) was administered to the mother in cases with a fetal platelet count < 100 x 10(9)/l.

RESULTS

The platelet antagonisms were distributed as follows: HPA-1a in 15 patients, HPA-5a in two, HPA-3a in one, with one further woman who had antibodies against a private antigen. All multigravidas (N = 18) had previously given birth to an infant with NAITP and two of those infants had experienced severe bleeding. Two fetuses were negative for the offending antigen. The median and mean platelet count at first cordocentesis was 26 and 75 x 10(9)/l respectively (range 3-276). A total of 46 cordocentesis were carried out, of which 37 were followed by platelet transfusions. Bleeding complications were not observed. IVIG was administered to eight mothers and two fetuses responded. Nine infants were delivered by caesarean section (CS) and 10 vaginally at a mean gestational age of 37 weeks (range 34-41). The median and mean platelet count at birth was 141.5 and 140 x 10(9)/l, respectively (range 36-314). Ultrasound examination, both ante- and postnatally, revealed no intracranial haemorrhages. There was one procedure related neonatal death and one infant suffered from convulsions in the neonatal period due to a sinus thrombosis, possibly related to the platelet transfusions.

CONCLUSIONS

When obstetric history is taken into account cordocentesis in NAITP can be postponed. Safety recommendations described in this study allow cordocentesis without bleeding complications. However, our study does not support routine cordocentesis in patients with a history of NAITP. Both the risks of cordocentesis, and the lack of prospective data on the magnitude of the risk of intrauterine or peripartal bleeding, should be considered.

Genotyping of the platelet-specific alloantigen HPA-5 (Br(a)/Br(b)) using polymerase chain reaction with sequencespecific primers (PCR-SSP)

A DNA-based one-stage technique, polymerase chain reaction with sequence-specific primers (PCR-SSP) was developed for genotyping of the platelet specific alloantigen HPA-5 (Bra/Brb). Sequence-specific primers, matching the wild type and the point mutation responsible for the HPA-5 (Bra/Brb) phenotype, were constructed. Conjointly a fragment of the gene coding for glycoprotein (GP) IIIa was amplified as an internal control of the enzyme reaction. Using these HPA-5 (Bra/Brb) sequence-specific primers the correct fragment of the GPIa gene was amplified, as evidenced by the PCR product size, the restriction map and by the nucleotide sequence. This assay was applied on 187 Swedish blood donors; 157 individuals were found to have a homozygous HPA-5a (Bra/Brb) genotype and 30 individuals a heterozygous HPA-5a,b (Bra/Brb) genotype. None of the donors was found to display a homozygous HPA-5b (Bra/Brb) genotype. Thus, the (HPA-5b) Bra antigen frequency in this population will be approximately 16.0% with a gene frequency of 8.0%. It is concluded that this assay is an attractive technique for genotyping of the HPA-5 (Bra/Brb) alloantigens on genomic DNA. The technique can replace serological alloantigen typing, especially in cases where platelets and rare human alloantisera are not available.

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.

Thrombocytopenia in the neonate

Pediatricians caring for newborns will eventually be confronted with the problem of thrombocytopenia in the neonatal period. Familiarity with the differential diagnosis of neonatal thrombocytopenia and understanding the pathogenesis of the more common entities allows physicians to design a selective diagnostic and therapeutic plan to benefit these thrombocytopenic infants.

Genetic susceptibility and anti-human platelet antigen 5b alloimmunization role of HLA class II and TAP genes

Platelet alloimmunization may result in post-transfusion purpura, and during pregnancy may cause neonatal alloimmune thrombocytopenia (NAIT), with a frequency estimated at 1.3 per 1000 live births. The risk of morbidity is significant: 20% of affected infants have neurologic sequelae and the death rate is about 10%. A better understanding of the immune response to platelet alloantigens would allow for a better definition, and thus better management of pregnant women at high risk. Limited data are available on the immune response against HPA-5b, the second most frequent antigen, after HPA-1a, implicated in NAIT. We studied HLA class II and TAP gene polymorphism in 50 women immunized against HPA-5 system antigens. Our results suggest a strong association of alloimmunization with a cluster of HLA DR molecules sharing a particular polymorphic amino acid sequence at position 69-70 (Glu-Asp encoded by GAA-GAC nucleotide sequence) of the DR beta 1 chain (RR = 2.95, RR = 5.70 when patients were homozygous for this sequence), and a negative association with the DRB1*0301 allele (2.1% vs. 28%; RR = 0.08). Furthermore, increased frequency of a TAP2 dimorphism at position 379 was observed in immunized women against the HPA-5 antigens (RR = 4.7).

The role of HLA antibodies in neonatal thrombocytopenia: a prospective study

The role of HLA antibodies in neonatal alloimmune thrombocytopenia is controversial. We prospectively studied the sera of obstetric patients at delivery for HLA antibodies and correlated their presence with umbilical cord blood platelet counts. We studied 493 births at The Johns Hopkins Hospital comprising of 357 African American, 115 Caucasian, and 21 babies of other racial groups. One hundred and thirty nine mothers had HLA antibodies. Of these HLA alloimmunized mothers, only ten infants had platelet counts of 150,000/ microL or less. Three hundred and eight mothers with no detectable antibodies gave birth to 27 infants with platelet counts of 150,000/microL or less. Yates corrected Chi square analysis showed no significant relationship between maternal HLA alloimmunization and baby platelet count (p = 0.709). Only 8 of sixty cord sera from babies of HLA alloimmunized mothers were positive for HLA antibodies. The HLA cord blood antibody results were then correlated with the neonatal platelet counts. The Fisher's exact test showed no significant relationship between the presence of HLA antibodies in cord blood samples and neonatal platelet counts (p = 0.232). Although one third (31%) of mothers have HLA antibodies, neonatal thrombocytopenia is rarely associated with this finding. However, HLA antibodies can cross the placenta, and in these unusual cases, may be associated with a higher risk of neonatal thrombocytopenia.

A rapid genotype assay for Pla haplotypes

The method described here results in simple visual analysis of 1, 2, or 3 ethidium bromide-stained bands corresponding to the PlA1, PlA2 or PlA1/A2 phenotypes, respectively. The amplification primers, designed to yield a 711bp product which encompasses exon 2 and substantial intronic sequence both 5' and 3' of the exon enable exquisite resolution of the restriction enzyme digestion products which define the alleles. In addition to prediction of fetal risk of neonatal alloimmune thrombocytopenia using this method, an ethnically heterogeneous pool of platelet donors was screened for the platelet antigen PlA (HPA-1) and gave unequivocal results which should be useful for platelet matching.

Fluorescence resonance energy transfer as a new method for the epitope-specific characterization of anti-platelet antibodies

The detection and characterization of anti-platelet antibodies which are directed to HLA class I molecules or platelet-specific glycoproteins is of great importance in the diagnosis and treatment of thrombocytopenia. In this paper a simple and rapid flow cytometric assay for the epitope-specific characterization of anti-platelet antibodies is described using fluorescence resonance energy transfer (FRET). Patient platelets or test platelets preincubated with patient serum were analyzed for surface-bound immunoglobulins using R-phycoerythrin-conjugated polyclonal anti-human IgG antibodies (excitation 488 nm, emission 585 nm). In a second step, HLA class I structures, platelet-specific glycoproteins (gpIIb/IIIa, gpIb), and the Fc gamma receptor II were stained with murine monoclonal antibodies and Cyan 5-labelled polyclonal anti-mouse IgG antibodies (excitation 585 nm, emission 670 nm). Upon monochromatic fluorescence excitation with a 488 nm argon laser the efficiency of light transfer from R-phycoerythrin to Cyan 5 is a direct measure of the distance between the human platelet-bound antibody and the epitope detected by the murine monoclonal antibody (mab). The assay permits discrimination between human antibodies directed to different platelet-specific glycoproteins or HLA class I structures without interference from non-specific Fc gamma receptor-bound immune complexes and also between antibodies directed to different epitopes on glycoprotein heterodimers (e.g., gpIIb/IIIa).

Neonatal alloimmune thrombocytopenia in Taiwan due to an antibody against a labile component of HPA-3a (Baka)

We report on two siblings who developed severe neonatal alloimmune thrombocytopenia (NAIT) due to an alloantibody against a labile component or components of the HPA-3a (Baka) antigen. The antibody reacted only with fresh, unfixed platelets by the solid-phase red cell adherence test, immunofluorescence test and mixed passive haemagglutination test. In the latter method, weakly fixed platelets also gave a weak positive reaction. Monoclonal-antibody-specific immobilization of platelet antigens and immunoblotting tests gave negative results. Our findings may possibly help to explain why in some cases of NAIT no platelet-specific antibody is demonstrable in tests with fixed or solubilized platelets.

The prenatal identification of fetal compatibility in neonatal alloimmune thrombocytopenia using amniotic fluid and variable number of tandem repeat (VNTR) analysis

Most severe episodes of neonatal alloimmune thrombocytopenic purpura (NATP) are caused by antiplatelet alloantibodies against the HPA-1a (PlA1) antigen. However, half of subsequent fetuses produced from a HPA-1a/b father (genotypic frequency 28%) will result in a child who is not affected. Some investigators manage NATP by confirming the fetal platelet phenotype using percutaneous umbilical cord sampling, a procedure that carries a low but real risk of fetal morbidity and mortality. More recently, physicians determine the fetal platelet antigen genotype using DNA derived from amniotic fluid or chorionic villus samples. All therapy is withdrawn for a fetus who genotypes as HPA-1b/b. However, since the fetus is the same genotype as the mother, there can be uncertainty about the origin of the genetic material and thus the validity of the fetal genotype. The inappropriate withdrawal of therapy for a erroneously genotyped fetus could be fatal, and consequently many physicians advocate fetal HPA-1 phenotyping with confirmation using percutaneous umbilical blood sampling. In this report we describe the management of two pregnancies with previously affected infants due to anti-HPA-1a alloantibodies. Both husbands were HPA-1a/b. For the current pregnancies, amniotic fluid was collected at 20 or 29 weeks of gestation, and the platelet genotype indicated that the fetuses were HPA-1b/b. The fetal origin of the amniotic fluid derived DNA was confirmed by the forensic technique of DNA profiling using variable number of tandem repeat (VNTR) analysis. All therapy was withdrawn, percutaneous umbilical blood sampling was not performed, and both women vaginally delivered healthy non-thrombocytopenic infants. The application of platelet alloantigen genotyping using DNA from amniotic fluid cells identified the HPA-1b/b fetus, and VNTR analysis confirmed that the tissue was fetal derived, thus avoiding the necessity for percutaneous umbilical blood sampling. The use of this approach in patients at risk will avoid additional investigation and treatment in approximately one-seventh of all NATP pregnancies involving the HPA-1a antigen.

Frequency of human platelet antigens among blood donors in northeastern Thailand

BACKGROUND

Platelet transfusions have been widely used in Thailand, but little is known about the phenotyping of human platelet antigens.

STUDY DESIGN AND METHODS

Whole blood was collected from 483 blood donors for preparation of platelets. An improved mixed passive hemagglutination assay was used for this study.

RESULTS

Frequencies demonstrated were 100 percent for HPA-1a (PlA1), 15.94 percent for HPA-2b (Siba), 60.25 percent for HPA-3a (Baka), 98.76 percent for HPA-4a (Yukb), 1.86 percent for HPA-4b (Yuka), 5.38 percent for HPA-5b (Br(a)), and 97.72 percent for Naka.

CONCLUSION

HPA-1a was found in 100 percent of Thais, which is the same frequency as in other Asian populations but somewhat different from that in whites (97.9%). Therefore, HPA-1a will not cause neonatal alloimmune thrombocytopenia or post-transfusion purpura in Thais. According to the frequencies of HPA-2b, HPA-3a, HPA-4a, HPA-4b, HPA-5b, and Naka antigens, they may induce neonatal alloimmune thrombocytopenia, posttransfusion purpura, and platelet refractoriness in Thais.

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.

Medical fetal therapy

Medical fetal therapy describes any therapy in which a pharmacological agent is administered to a woman or her fetus in order to avoid or alleviate fetal disease. Treatment of the fetus with blood products or injection of other agents can also be considered to be medical fetal therapy. This chapter reviewed the application of medical fetal therapy to the prevention of NTDs, treatment of endocrinological and metabolic disorders, such as CAH, thyroid disease and others, and the medical management of cardiac arrhythmias. Several haematological disorders and reviews of recent advances in genetic manipulation involving the use of stem-cell implantation were discussed. The field of medical fetal therapy has been extremely exciting and continues to evolve at a rapid pace. No doubt, future advances involving genetic manipulation or the use of molecular genetic techniques for diagnosis will continue to keep this field at the forefront of treatment and prevention of fetal disorders.

A platelet monoclonal antibody inhibition assay for detection of glycoprotein IIb/IIIa-related platelet alloantibodies

Post-transfusion purpura (PTP) and neonatal alloimmune thrombocytopenia (NAT) result from formation of alloantibodies to platelet membrane glycoprotein-associated antigens. The detection and identification of platelet-specific alloantibodies in patient sera is often complicated by the presence of co-existing HLA antibodies and/or more than one platelet specificity in the same serum. We describe a solid phase assay that specifically detects antibodies to platelet membrane associated alloantigens by measuring the ability of patient antisera to inhibit the binding of glycoprotein GPIIb or GPIIIa monoclonal antibodies to intact platelets. When tested in the GPIIIa assay against a panel of random platelet donors, the reactivities of two known PLAI antisera that also contained different HLA antibodies were highly correlated (r = 0.99) and allowed PLA phenotyping of the population. A standard direct binding platelet ELISA, on the other hand, was unable to accurately PLA phenotype the same population. The reactivities of two known Baka antisera (one containing additional anti-PLA2 and the other anti-Brb specificities) were highly correlated (r = 0.95) in the GPIIb assay, and Bak phenotype determination was similarly accomplished for a random platelet panel. Furthermore, a comparison of platelet phenotype results (using the monoclonal inhibition assay) and genotype results (using DNA analysis) for the PLA and Bak systems showed a concordance of 98% for 146 alleles tested. In conclusion, the platelet monoclonal antibody inhibition assay: (1) allows determination of platelet-specific alloantibodies in the presence of contaminating HLA antibodies and/or in sera containing multiple platelet alloantibodies; (2) allows accurate platelet phenotyping for the GPIIIa-associated PLA and GPIIb-associated Bak antigen systems; and (3) may be applicable to the detection of other known or even novel platelet glycoprotein-associated antigens.

Maternal alloimmunization against fetal platelet antigens: a prospective study

Neonatal alloimmune thrombocytopenia (NAIT) is induced by maternal alloantibodies to fetal platelet antigens. This prospective study was carried out to evaluate the incidence of anti-platelet antibodies in 933 mother-child pairs where the mother and child were typed for the human platelet antigens (HPA)-1, -2, -3, -5. Sera from mismatched mother-child pairs were screened for anti-platelet antibodies, anti-HLA class I and blood group ABO IgG antibodies. Platelet-specific antibodies were anti-HPA-3a in one and anti-HPA-5b in 17 neonates, respectively. All these neonates had normal platelet counts. One woman had autoreactive antibodies. Anti-HLA class I and anti-blood group A IgG antibodies were detected in five and four neonates, respectively, born with a platelet count < 150 x 10(9)/l. None of the 11 homozygous HPA-1b mothers became immunized against their heterozygous offspring. The maternal HLA-allotypes HLA-DR52 and -DR6, typically found in individuals immunized against HPA-1a and -5b, respectively, were found in three of 11 HPA-b/b nonresponders and eight of the anti-HPA-5b responders. The results indicate that a risk for NAIT due to HPA-2 and -3 alloimmunization is low. The HLA allotypes do not predict the risk for NAIT due to HPA-1 or -5 alloimmunization. Maternal anti-HPA-5b antibodies do not correlate with the platelet count in the neonate.

Neonatal alloimmune thrombocytopenia involving HPA-5b (Br(a)): a rare Japanese case

Neonatal alloimmune thrombocytopenia (NAIT) is caused by platelet antigen incompatibility between the mother and fetus. The frequency of NAIT varies among ethnic groups. In Caucasians, HPA-5b (Br(a)) is the antigen that is second most frequently implicated. In Japan, NAIT due to anti-HPA-5b antibody is quite rare. The present case is the second case of Br(a)-NAIT in Japanese and the first case serologically confirmed by monoclonal antibody-specific immobilization of platelet antigens.

Antenatal screening for fetal alloimmune thrombocytopenia: the results of a pilot study

Feto-maternal incompatibility for the human platelet antigen HPA-1a is an important cause of severe fetal thrombocytopenia. The incidence is 1 in 1000-2000 pregnancies, which is more common than other conditions for which screening is presently carried out. Antenatal diagnosis and management are now available, but only for subsequent siblings following diagnosis of a previously affected infant. This study describes a pilot prospective screening programme for the antenatal detection of fetomaternal alloimmune thrombocytopenia (FMAIT) due to HPA-1a incompatibility. 3473 women were typed for HPA-1a using a method designed for large-scale typing. 71 women found to be HPA-1a negative were further tested for HLA-DR52a as a risk factor for alloimmunization. All women were monitored for the development of anti-HPA-1a throughout pregnancy and a cord full blood count was taken at delivery. Two affected pregnancies were found and treated: a singleton pregnancy was treated antenatally and a twin pregnancy after delivery. The study showed that screening for FMAIT could be established within the pre-existing antenatal red cell serology programme. It was concluded that screening should be based on platelet typing and offered regardless of parity. Further stratification, combining DR52a typing and HPA-1a antibody screening, although focusing on the group of women at greater risk, may not identify all affected pregnancies. Confirmation of the diagnosis and severity of FMAIT continues to depend on fetal blood sampling during pregnancy or cord blood samples after birth.

Fetal and neonatal thrombocytopenia

Thrombocytopenia is defined as platelet count less than 150,000 plat/mm3. Etiologic factors involved include immunological (NAIT and ITP), fetal infectious disease, chromosomal and nonchromosomal, and miscellaneous causes. While the understanding of fetal thrombocytopenia is driven by reason to do fetal blood sampling, discovery of neonatal thrombocytopenia is driven by blood counts performed because of the risk of infections. The most serious consequence of thrombocytopenia in the fetus/neonate is intracranial hemorrhage which can occur in utero as early as 18 weeks gestation. The key factor in perinatal prevention of intracranial hemorrhage is early diagnosis and treatment, possibly in utero. Cordocentesis under direct ultrasound guidance and platelet transfusions have played a major role in the management of fetal/neonatal thrombocytopenia. Ongoing studies and high resolution ultrasound will continue to explore and hopefully clarify fetal and neonatal thrombocytopenia and facilitate recognition of primary and secondary thrombocytopenias.

The platelet-specific alloantigen PlA1 (HPA-1a): a comparison of flow cytometric immunophenotyping and genotyping using polymerase chain reaction and restriction fragment length polymorphism in a Swedish blood donor population

BACKGROUND

There is an increasing interest in the development of rapid and reliable techniques for platelet alloantigen typing.

STUDY DESIGN AND METHODS

By use of standardized flow cytometry and a specific human alloantiserum, 236 Swedish blood donors were immunophenotyped for the platelet-specific alloantigen, PlA1 (HPA-1a).

RESULTS

Ten individuals (4.2%) had low fluorescence intensities and were considered PlA1-negative (HPA-1a-negative); all of them also demonstrated a PlA2/PlA2 (HPA-1b/1b) genotype in a polymerase chain reaction and restriction fragment length polymorphism (PCR-RFLP) assay of the underlying DNA polymorphism. The remaining population had clear positive fluorescence and was regarded as PlA1-positive (HPA-1a-positive). The fluorescence distribution histogram among PlA1-positive (HPA-1a-positive) individuals was dome-shaped, and those individuals who were homozygous for PlA1 (HPA-1a) could not be distinguished from those who were heterozygous. This finding was further substantiated by PCR-RFLP analysis of the PlA1/PlA2 (HPA-1a/1b) genotype; a heterozygous genotype was found among those having a medium fluorescence intensity as well as among those having a strong fluorescence intensity.

CONCLUSION

Flow cytometry is a valuable tool for large-scale detection of PlA1 (HPA-1a). However, flow cytometry based on only one antiserum cannot distinguish between homozygous and heterozygous carriers of PlA1 (HPA-1a). For zygosity testing and when platelets are difficult to obtain, the PCR-RFLP technique is the assay of choice.

Alloimmune thrombocytopenia: fetal and neonatal losses related to cordocentesis

OBJECTIVE

This report describes the increased risks of cordocentesis in fetuses affected with alloimmune thrombocytopenia.

STUDY DESIGN

As part of a multicenter treatment study clinical and laboratory data from five pregnancies with alloimmune thrombocytopenia in which there was a fetal or neonatal loss associated with cordocentesis were reviewed. The fetal or neonatal deaths were all thought to be a result of exsanguination. These fetuses were compared with a group of 44 affected fetuses who underwent the same procedure but who survived. The data were analyzed by the Wilcoxon rank-sum test and the two-tailed Fisher's exact test. A p value < 0.05 was considered significant.

RESULTS

The mean platelet count at cordocentesis was significantly lower in the cases than in the controls (5.8 vs 32.8 x 10(9)/L, p = 0.005). The incidence of antenatal intracranial hemorrhage in the untreated sibling of the prior affected pregnancy was significantly greater in the cases than in the controls (two of five vs one of 42, p = 0.02).

CONCLUSION

Fetuses affected with alloimmune thrombocytopenia are at increased risk for fatal exsanguination associated with cordocentesis.

Antenatal management of fetomaternal alloimmune thrombocytopenia--report of 15 affected pregnancies

The recognition that spontaneous intracranial haemorrhage (ICH) may occur in utero in fetomaternal alloimmune thrombocytopenia (FMAIT) led us to attempt to prevent this in 15 pregnancies of 11 women who had previously affected infants with FMAIT due to anti-HPA-1a. The antenatal management included fetal platelet transfusions and maternal steroids and/or high-dose intravenous immunoglobulin (IVIgG). In the first pregnancy, ICH occurred between 32 and 35 weeks' gestation before any treatment had been given, emphasizing the need for earlier intervention. Five of the 14 subsequent pregnancies in this study were considered to be severely affected (severe haemorrhagic complications in a previous infant and initial fetal platelet count < 20 x 10(9)/L in this study); four were managed successfully with weekly fetal platelet transfusions started between 18 and 29 weeks and continued until delivery at 33-35 weeks, and one severely affected case who was referred at 36 weeks was managed successfully with a single platelet transfusion prior to delivery. Five pregnancies were considered to be mildly affected (previous infants were unaffected by severe bleeding and initial fetal platelet count > 50 x 10(9)/L in this study). The platelet counts were maintained in one case with steroids and in three with IVIgG without the need for repeated platelet transfusions, but in the fifth the fetal platelet count fell despite steroids and IVIgG and serial platelet transfusions were required. Four pregnancies were unsuccessful; two pregnancies were terminated after severe ICH occurred at an early stage before fetal blood sampling had been carried out, one fetus died after the mother had a severe fall despite the successful initiation of fetal platelet transfusions and one died due to a cord haematoma which occurred at the time of the initial fetal blood sampling. The optimal management of FMAIT to reduce the risk of antenatal ICH remains uncertain. Steroids and IVIgG may be effective in some mildly affected cases but serial fetal platelet transfusions are the preferred therapy for those who are severely affected.

In vitro culture of colony forming unit-megakaryocyte (CFU-MK) in fetal alloimmune thrombocytopenia

Perinatal alloimmune thrombocytopenia (PAITP) causes intracranial haemorrhage in the fetus and neonate. However, the severity of the thrombocytopenia correlates poorly with maternal anti-platelet antibody titres. To test the hypothesis that reduced platelet production contributes to fetal thrombocytopenia in PAITP, maternal sera from three HPA-1a-negative mothers whose pregnancies were complicated by anti-HPA-1a (two severe cases, one mild case) were added to colony forming unit-megakaryocyte (CFU-MK) cultures from HPA-1a positive and negative individuals. Sera from the two severely affected pregnancies containing anti-HPA-1a caused 66-100% inhibition of HPA-1a-positive fetal and neonatal CFU-MK, whereas CFU-MK from two HPA-1a-negative mothers were not inhibited by the anti-HPA-1a-containing sera. Maternal serum from the case of mild PAITP caused only mild inhibition of HPA-1a-positive cord and adult CFU-MK and did not inhibit HPA-1a-positive fetal CFU-MK. Taken together, these findings suggest that reduced megakaryocyte production contributes to fetal thrombocytopenia due to maternal anti-HPA-1a antibodies and also that the degree of CFU-MK inhibition correlates with severity of fetal thrombocytopenia.

Maternal intravenous immunoglobulin treatment does not prevent intracranial haemorrhage in fetal alloimmune thrombocytopenia

In fetal alloimmune thrombocytopenia (FAIT) the fetus is threatened by intracranial haemorrhage (ICH); therefore early diagnostic and therapeutic intervention is required. We followed the clinical course of a 30-year-old woman during her fifth pregnancy after she had given birth to a child with alloimmune thrombocytopenia due to anti-Zwa. The fetus was monitored by 13 fetal blood samplings (FBS) always followed by transfusion of either maternal or compatible donor platelets. Intravenous immunoglobulin (ivIg) treatment of the mother was begun at 20 weeks of gestation when the fetal platelet count was 36 x 10(9)/l. The fetal platelets were typed Zwa positive by DNA analysis. Despite 11 weeks of maternal ivIg treatment fetal platelet counts progressively declined to 6 x 10(9)/l and ICH occurred. Subsequently, the fetus was successfully managed by intrauterine platelet transfusions at shorter intervals (3-5 days) and elective Cesarean section was carried out at 35 weeks of gestation. We conclude that maternal ivIg treatment does not prevent ICH in FAIT. The treatment of choice for severely affected cases is serial FBS combined with transfusion of compatible platelets.

Breast feeding in neonatal alloimmune thrombocytopenia

Infants with neonatal alloimmune thrombocytopenia are at risk of severe intracranial haemorrhage. Placental transfer of maternal immunoglobulin G (IgG) directed against fetal platelet antigens is known to be the underlying mechanism. Since breast milk contains IgG it is theoretically possible that breast feeding of these infants could cause thrombocytopenia. The following case report shows that an infant with neonatal alloimmune thrombocytopenia may be safely breast fed, even when the breast milk contains the platelet specific antibody (HPA-1a).

Analysis of immunoglobulin class, IgG subclass and titre of HPA-1a antibodies in alloimmunized mothers giving birth to babies with or without neonatal alloimmune thrombocytopenia

We analysed the titre and isotype composition of antibodies produced by mothers giving birth to babies with or without neonatal alloimmune thrombocytopenic purpura (NAITP) and patients with post-transfusion purpura (PTP). All these individuals produced an antibody specific for the HPA-1a allotype present on the platelet glycoprotein IIb-IIIa (GPIIb-IIIa). Sera from mothers who gave birth to thrombocytopenic babies (group 1, n = 36), non-thrombocytopenic babies (group 2, n = 4) or from PTP patients (group 3, n = 3) were tested by an indirect-ELISA. Results indicated no evident differences in the isotype composition or titre of the antibodies from the three groups of sera. The antibody titre ranged from 1:120 to 1:3500. Antibodies with the IgG1 subclass were present in all sera. Most sera contained IgG1 alone (24/43 sera tested) or in combination with IgG3 (10/43). IgG2 was never present and only three sera showed intermediate reactivity with anti-IgG4 MAb. Few sera (nine sera from groups 1 and 2) were weakly positive when tested with the anti-IgM antibodies. These results suggest that neither the titre nor the isotype composition can be used to predict the severity or the occurrence of thrombocytopenia in newborns.

A case of severe neonatal thrombocytopenia with schizencephaly associated with anti-HPA-1b and anti-HPA-2a

We report a family with a neonate who was severely damaged by intracranial haemorrhages. These probably occurred before the 20th week of gestation. The neonate had a moderate thrombocytopenia. In the maternal serum anti-HPA-1b and anti-HPA-2a alloantibodies were detected. Third-generation assays were applied to identify the alloantibodies. No other cause for the bleeding was found. Probably the combination of anti-HPA-1b and anti-HPA-2a alloantibodies, directed against the platelet fibrinogen receptor and the von Willebrand receptor, respectively, induced a thrombocytopenia and a thrombocytopathy.

Anti-Bak(a) neonatal alloimmune thrombocytopenia: possible prevention by intravenous immunoglobulin

Neonatal alloimmune thrombocytopenia (NAIT) most commonly involves antibodies directed against the PlA1 antigen, but other platelet specific alloantigens have been associated with it. We describe the case of a mother whose first three infants developed NAIT secondary to anti-Bak(a) antibodies, while her fourth infant did not. The three affected infants were treated postnatally with platelet transfusions. The fourth infant was treated antenatally with one dose of intravenous immunoglobulin (IVIg) given to the mother. Postpartum analysis revealed the infant's platelets to be Bak(a)-positive but negative for elevated IgG. Maternal serum reacted with neonatal platelets in vitro, but cord serum was negative for antiplatelet antibodies. These clinical observations do not prove the efficacy of IVIg; however, they raise several questions: Why wasn't this infant thrombocytopenic? Why didn't the umbilical cord contain maternal antibody? Was the single dose of IVIg responsible for preventing NAIT? IVIg is currently under investigation in a clinical trial evaluating its effectiveness in preventing NAIT in mothers with anti-PlA1 antibodies, where it has shown some success. There have been no reports of the use against anti-Bak(a) antibodies. We suggest that a weekly dose schedule may not be necessary for all affected pregnancies, and antibodies with specificity other than anti-PlA1 may require less vigorous therapy.

Human platelet antigen-1 (Zw) typing of fetuses by analysis of polymerase chain reaction-amplified genomic DNA from amniocytes

Prenatal typing for the human platelet antigens-1 (HPA) permits identification of a fetus at risk for neonatal alloimmune thrombocytopenia (NAITP) in cases of HPA-1 incompatibility in which the father is heterozygous for the HPA-1a antigen. Diagnostic cordocentesis and phenotyping of the fetal platelets are used for this purpose. We applied allele-specific restriction enzyme analysis on polymerase chain reaction (PCR)-amplified DNA purified from amniocytes. This assays allows early second trimester typing for HPA-1 alleles. We were able to determine the genotype of three fetuses at risk. Iatrogenic fetal loss is lower with amniocentesis than with cordocentesis. Therefore, this technique is a welcome addition to the antenatal management of NAITP.

Molecular analysis of human platelet antigen system 1 antigen on single cells can be applied to preimplantation genetic diagnosis for prevention of alloimmune thrombocytopenia

OBJECTIVE

Our purpose was to develop a molecular assay to determine the human platelet antigen system 1 status on single nucleated cells, including human blastomeres.

STUDY DESIGN

Eighty single cultured lymphoblasts of known human platelet antigen system 1 genotype and 24 media blanks were mixed in blinded fashion. Amplification of a 246 bp deoxyribonucleic acid fragment and subsequent Nci I restriction digestion were performed to distinguish human platelet antigen system 1a from 1b alleles. Specificity and sensitivity of the technique were determined. Eight blastomeres were also tested.

RESULTS

Deoxyribonucleic acid amplification at the human platelet antigen system 1 locus was successful in 95% of the reactions. No media blanks showed amplified deoxyribonucleic acid. The diagnosis was correct in all homozygous human platelet antigen system 1a or 1b cells; three of 23 heterozygous cells amplified but failed to digest with Nci I. Overall specificity was 95%. All blastomeres successfully amplified.

CONCLUSIONS

The human platelet antigen system 1 status determination is reliable from a single cell and can be used for preimplantation genetic diagnosis for the prevention of alloimmune thrombocytopenia.

Neonatal allo-immune thrombocytopenia due to fetomaternal HPA-1 incompatibility of a homozygous HPA-1a mother and a homozygous HPA-1b father. A case report

We report a case of neonatal allo-immune thrombocytopenia due to fetomaternal human platelet antigen (HPA)-1 incompatibility. Anit-HPA-1b antibodies were detectable in maternal serum. Repeated treatment of the infant with high-dose IgG effectively raised platelet counts transiently, but the patient remained thrombocytopenic for more than 6 weeks. Serological and DNA analysis revealed that the mother was homozygous HPA-1a and the father homozygous HPA-1b.

Prediction of neonatal alloimmune thrombocytopenia using PCR

Neonatal alloimmune thrombocytopenia (NAIT) is a potentially fatal condition and in the majority of cases is associated with maternal antibodies to the HPA-1a (PLA1) haplotype. Early diagnosis in utero can enhance survival rates. The application of DNA genomic analysis and PCR technology for the determination of the HPA-1a/HPA-1b (PLA1/PLA2) locus is described and applied in a family study where the fetus was diagnosed to have NAIT. This rapid technique differentiated between the 3 haplotypes HPA-1a/HPA-1a, HPA-1b/HPA-1b and HPA-1a/HPA-1b using the polymorphism at base 196 of the GPIIIa gene. This is the first Australian report on the establishment of this technology for platelet genotype typing and the application in the diagnosis of NAIT. This technique can be performed on DNA extracted from any nucleated cells and avoids the difficulty of requiring fetal platelets for serological typing when NAIT is suspected. The PCR technique of genomic DNA analysis has an important application in the prediction and management of this potentially severe condition.

Fetal thrombocytopenia and its relation to maternal thrombocytopenia

BACKGROUND

Neonates with severe thrombocytopenia can have bleeding leading to death or lifelong residual defects. The predictors, frequency, and consequences of fetal thrombocytopenia are not known, nor is it known if there are maternal clinical features that could predict fetal thrombocytopenia.

METHODS

We conducted a seven-year cross-sectional study in which platelet counts were determined in newborns' umbilical-cord blood and blood obtained from their mothers at consecutive deliveries in one obstetrical unit. The relations of the umbilical-cord platelet count to maternal risk factors were determined.

RESULTS

Platelet counts were determined in blood samples from 15,471 mothers and 15,932 newborn infants. The cord-blood platelet count was less than 50,000 per cubic millimeter in 19 infants (0.12 percent; 95 percent confidence interval, 0.07 to 0.19 percent), whereas the platelet count was less than 150,000 per cubic millimeter in 6.6 percent of the mothers (95 percent confidence interval, 6.2 to 7.0 percent). One infant among those born to 756 mothers with incidental thrombocytopenia, 5 infants among those born to 1414 mothers with hypertension, and 4 infants among those born to 46 mothers with idiopathic thrombocytopenic purpura had cord-blood platelet counts between 20,000 and 50,000 per cubic millimeter. Only 6 infants (0.04 percent; 95 percent confidence interval, 0.01 to 0.08 percent) had cord-blood platelet counts of less than 20,000 per cubic millimeter; all their mothers were among the 18 whose 19 fetuses were at risk for neonatal alloimmune thrombocytopenia. Two of these infants had in utero intracranial hemorrhage. In addition, 3 infants born to these 18 women had cord-blood platelet counts between 20,000 and 50,000 per cubic millimeter; there was 1 stillbirth due to intracranial hemorrhage.

CONCLUSIONS

Moderate-to-severe fetal thrombocytopenia is a rare event. The only severely affected neonates with morbidity or mortality due to this condition are those born to mothers with antiplatelet alloantibodies.