Diagnosis and Management of Neonatal Alloimmune Thrombocytopenia

Recombinantly Expressed Tagged SUrface Protein (RETSUP) assay: a new diagnostic system for the detection of antibodies to platelets

BACKGROUND

Current assays for the detection of (allo)antibodies to platelet antigens are often laborious and widely based on the presence of well-characterized donor platelets.

OBJECTIVES

To develop an easy-to-perform, sensitive, and specific test for the detection of antibodies against platelet antigens, in particular, glycoprotein (GP) antigens, called "Recombinantly Expressed Tagged SUrface Protein" (RETSUP) assay, which does not require donor platelets.

METHODS

Twin-Strep-tagged GP complexes were recombinantly expressed in human embryonic kidney 293 cells after stable transfection. These cell lines were used as antigen sources in the RETSUP assay, combining cell-based and enzyme-linked immunosorbent assay-based assay procedures. The assay performance was tested with recombinant antibodies, anti-human platelet antigen (HPA) reference plasmas, and anti-HPA patient sera.

RESULTS

Human embryonic kidney 293 cell lines stably expressing either Twin-Strep-labeled GPIa/IIa, GPIIb/IIIa, GPIb/IX, or GPIb/IX/V complexes or GPV as well as the distinct HPA-1, HPA-3, and HPA-5 epitopes were successfully generated. Applying the generated GP-expressing cell lines, the developed RETSUP assay proved very sensitive and specific with recombinant antibodies targeting different GPs and human plasma/serum samples. The results of the test were not affected by the GP carrying the Twin-Strep-tag or by using freshly harvested or cryopreserved cells.

CONCLUSION

The RETSUP assay is an easy-to-perform, sensitive, and specific assay for the detection of plasma/serum antibodies to platelet GP, with performance comparable to or better than those of current state-of-the-art assays in antiplatelet antibody diagnostics. Owing to the recombinant nature of the target antigens, it can be easily adapted to detect antibodies in other antibody-mediated diseases.

Neonatal Thrombocytopenia: Differing Characteristics of NAIT Versus Non-NAIT

While neonatal alloimmune thrombocytopenia (NAIT) is the most common cause of severe neonatal thrombocytopenia good clinical predictors are lacking. We analyzed cases of neonatal thrombocytopenia in Schneider Children's Medical Center of Israel to pinpoint qualifiers of NAIT (NAIT+) in comparison to non-NAIT (NAIT-) thrombocytopenia. Patient and maternal data were retrospectively collected on all thrombocytopenic newborns undergoing a workup for NAIT in our tertiary center between 2001 and 2016. Among 26 thrombocytopenic neonates, the mean nadir in NAIT+ patients (25×10 9 /L) was significantly lower than NAIT- patients (64×10 9 /L) ( P <0.001). 61.5% of NAIT+ infants required treatment compared with 23% of non-NAIT ( P =0.015). NAIT+ patients also required more therapeutic modalities than infants with NAIT- thrombocytopenia. Human platelet antigen (HPA)-1a and HPA-5b alloantibodies most frequently caused NAIT. In summary, thrombocytopenia in NAIT+ was significantly more severe compared with NAIT- and more likely to require treatment. In addition, despite the varied ethnic population in Israel, the HPA alloantibodies found in our population were most similar to those common in Western countries. In the absence of rigorous prenatal screening options, we suggest platelet counts below 40 to 50×10 9 /L in a healthy newborn be considered most suggestive for NAIT and warrant urgent NAIT-specific analysis.

Fetal and neonatal alloimmune thrombocytopenia: A rare case report of prenatal treatment

Fetal and neonatal alloimmune thrombocytopenia (FNAIT) is a rare but serious condition. The first-line antenatal management of FNAIT consists of weekly IVIG with or without corticosteroids, ideally starting before 16 weeks of gestation.

Clinical and laboratory predictors of fetal and neonatal alloimmune thrombocytopenia

BACKGROUND

Fetal and neonatal alloimmune thrombocytopenia (FNAIT) is the most common cause of intracranial hemorrhage (ICH) in thrombocytopenic term infants. We investigated clinical and laboratory predictors of severe FNAIT in a tertiary care referral center.

STUDY DESIGN AND METHODS

Retrospective cohort study over a 30-year period. We defined FNAIT as recurrence of neonatal thrombocytopenia in a subsequent pregnancy; and severe outcomes as any of: (1) a birth platelet count below 20 × 10⁹ /L; (2) ICH or (3) fetal death. We used a generalized estimating equations analysis and classification tree analysis to identify risk factors for severe FNAIT in a subsequent pregnancy.

RESULTS

During index pregnancies (n = 135 in 131 mothers), 71 infants (52.6%) had severe outcomes including a platelet count <20 × 10⁹ /L (n = 45), fetal or neonatal ICH (n = 32), or fetal death (n = 4). During subsequent pregnancies (n = 72), 15 infants (20.8%) had severe outcomes including birth platelets <20 × 10⁹ /L (n = 10), ICH (n = 2), or death (n = 3). Forty-two women (58.3%) received antenatal intravenous immune globulin (IVIG) during subsequent pregnancies. Eight mothers (n = 9 infants) had severe FNAIT outcomes despite receiving antenatal IVIG. Maternal antibodies to human platelet antigens (HPA) was the only independent predictor of severe FNAIT in a subsequent pregnancy (OR = 25.3, p = .004). Nevertheless, one of 43 infants from antibody-negative mothers had a severe outcome.

CONCLUSIONS

The presence of anti-HPA is highly indicative of the diagnosis of severe FNAIT; however, we observed one infant who had severe FNAIT recurrence, defined using strict clinical criteria, without a maternal antibody. Improved diagnostic and therapeutic strategies are needed to prevent severe FNAIT in high-risk mothers.

New developments in fetal and neonatal alloimmune thrombocytopenia

Fetal and neonatal alloimmune thrombocytopenia, the platelet equivalent of hemolytic disease of the fetus and newborn, can have devastating effects on both the fetus and neonate. Current management of fetal and neonatal alloimmune thrombocytopenia in a subsequent affected pregnancy involves antenatal administration of intravenous immune globulin and prednisone to the pregnant woman to prevent the development of severe fetal thrombocytopenia and secondary intracranial hemorrhage in utero. That therapy has proven to be highly effective but is associated with maternal side effects and is expensive. This commentary describes 4 advances that could substantially change the current approach to detecting and managing fetal and neonatal alloimmune thrombocytopenia in the near future. The first would be an introduction of a program to screen all antepartum patients in this country for pregnancies at risk of developing fetal and neonatal alloimmune thrombocytopenia. Strategies to implement this complex process have been described. A second advance is testing of cell-free fetal DNA obtained from maternal blood to noninvasively determine the fetal human platelet antigen 1 genotype. A third, in preliminary development, is creation of a prophylactic product that would be the platelet equivalent of Rh immune globulin (RhoGAM). Finally, a fourth major potential advance is the development of neonatal Fc receptor inhibitors to replace the current medical therapy administered to pregnant women with an affected fetus. Neonatal Fc receptor recycles plasma immunoglobulin G to increase its half-life and is the means by which immunoglobulin G crosses the placenta from the maternal to the fetal circulation. Blocking the neonatal Fc receptor is an ideal way to prevent maternal immunoglobulin G antibody from causing fetal and neonatal alloimmune thrombocytopenia in a fetus at risk of developing that disorder. The pertinent pathophysiology and rationale for each of these developments will be presented in addition to our thoughts relating to steps that must be taken and difficulties that each approach would face for them to be successfully implemented.

A meta-analysis of neonatal outcomes in pregnant women with immune thrombocytopenic purpura

AIM

Thrombocytopenia is an autoimmune disorder characterized by reduced platelet counts. Neonatal thrombocytopenia incidence has been linked with immune thrombocytopenic purpura in mothers during pregnancy, possibly because antiplatelet antibodies can cross the placental barrier. To date, no study has attempted to evaluate the actual prevalence of neonatal thrombocytopenia in infants born to mothers with immune thrombocytopenic purpura. In this meta-analysis of the available literature, we attempt to fill this gap. We want to evaluate the overall prevalence of neonatal thrombocytopenia, its severity, and the incidence of hemorrhage in infants with thrombocytopenia born from mothers with immune thrombocytopenic purpura.

METHODS

Adhering to PRISMA guidelines, we systematically scanned four academic databases including EMBASE, CENTRAL, Scopus, and MEDLINE to identify relevant literature. We performed a meta-analysis to summarize thrombocytopenia incidence rate and severity in newborn infants of mothers with immune thrombocytopenic purpura.

RESULTS

We identified 21 eligible studies involving 1951 mothers and 1844 neonates. Meta-analysis showed high prevalence for neonatal thrombocytopenia (24%). Within these, severe cases were the most prevalent (41.2%), followed by moderate (37.7%) and mild (17.6%) cases. Hemorrhage was only reported in 4.1% of the observed neonatal thrombocytopenia cases.

CONCLUSION

This review provides preliminary evidence that neonatal thrombocytopenia incidence is high in infants born to mothers with immune thrombocytopenic purpura. This study further reports that the largest proportion of these cases are severe.

Current perspective and scope of fetal therapy: part 2

Fetal therapy has been defined as any therapeutic intervention either invasive or noninvasive for correcting or treating any fetal malformation or condition. Invasive fetal therapy have its own set of maternal and fetal complications and invasive approach is not feasible in many of fetal conditions that are candidate for fetal therapy. Many such fetal conditions have been treated successfully by medical or noninvasive management. In medical fetal therapy, mothers are treated with medications which are transferred to fetus through placenta and exert positive effect on the fetus, thus avoiding complications that are seen secondary to invasive fetal therapy. The fetal conditions that have been managed with medical therapy includes fetal and neonatal alloimmune thrombocytopenia, neural tube defect, congenital adrenal hyperplasia, perinatal infections, respiratory distress syndrome, inborn error of metabolism, and congenital cystic adenomatoid malformation. This review will cover the medical or noninvasive aspect of fetal therapy and will highlight the progress made in the management of these fetal conditions.

The use of IVIg in fetal and neonatal alloimmune thrombocytopenia- Principles and mechanisms

Fetal and neonatal alloimmune thrombocytopenia (FNAIT) is a rare neonatal disorder that is caused by alloimmunization against platelet antigens during pregnancy. Although rare, affecting only 1 in 1000 live births, it can cause intracranial hemorrhage and other bleeding complications that can lead to miscarriage, stillbirth and life-long neurological complications. One of the gold-standard therapies for at risk pregnancies is the administration of IVIg. Although IVIg has been used in a variety of different disorders for over 40 years, its exact mechanism of action is still unknown. In FNAIT, the majority of its therapeutic effect is thought the be mediated through the neonatal Fc receptor, however other mechanisms cannot be excluded. Due to safety, supply and other concerns that are associated with IVIg use, alternative therapies that could replace IVIg are additionally being investigated. This includes the possibility of a prophylaxis regimen for FNAIT, similarly to what has been successfully used in hemolytic disease of the fetus and newborn for over 50 years.

Postnatal intervention for the treatment of FNAIT: a systematic review

OBJECTIVE

Fetal and neonatal alloimmune thrombocytopenia (FNAIT) is associated with life-threatening bleeding. This systematic review of postnatal management of FNAIT examined transfusion of human platelet antigen (HPA) selected or unselected platelets, and/or IVIg on platelet increments, hemorrhage and mortality.

STUDY DESIGN

MEDLINE, EMBASE and Cochrane searches were conducted until 11 May 2018.

RESULT

Of 754 neonates, 382 received platelet transfusions (51%). HPA-selected platelets resulted in higher platelet increments and longer response times than HPA-unselected platelets. However, unselected platelets generally led to sufficient platelet increments to 30 × 10⁹/L, a level above which intracranial hemorrhage or other life-threatening bleeding rarely occurred. Platelet increments were not improved with the addition of IVIg to platelet transfusion.

CONCLUSION

Overall, HPA-selected platelet transfusions were more effective than HPA-unselected platelets but unselected platelets were often effective enough to achieve clinical goals. Available studies do not clearly demonstrate a benefit for addition of IVIg to platelet transfusion.

Prenatal Management of Pregnancies at Risk of Fetal Neonatal Alloimmune Thrombocytopenia (FNAIT): Scientific Impact Paper No. 61

WHAT IS IT?: Fetal neonatal alloimmune thrombocytopenia (FNAIT), also known as neonatal alloimmune thrombocytopenia (NAIT) or fetomaternal alloimmune thrombocytopenia (FMAIT), is a rare condition which affects a baby's platelets. This can put them at risk of problems with bleeding, particularly into the brain. One baby per week in the UK may be seriously affected and milder forms can affect one in every 1000 births. HOW IS IT CAUSED?: Platelets are blood cells that are very important in helping blood to clot. All platelets have natural proteins on their surface called human platelet antigens (HPAs). In babies, half of these antigens are inherited from the mother and half from the father. During pregnancy, some of the baby's platelets can cross into the mother's bloodstream. In most cases, this does not cause a problem. But in cases of FNAIT, the mother's immune system does not recognise the baby's HPAs that were inherited from the father and develops antibodies, which can cross the placenta and attack the baby's platelets. These antibodies are called anti-HPAs, and the commonest antibody implicated is anti-HPA-1a, but there are other rarer antibody types. If this happens, the baby's platelets may be destroyed causing their platelet count to fall dangerously low. If the platelet count is very low there is a risk to the baby of bleeding into their brain before they are born. This is very rare but if it happens it can have serious effects on the baby's health. HOW IS IT INHERITED?: A baby inherits half of their HPAs from its mother and half from its father. Consequently, a baby may have different HPAs from its mother. As the condition is very rare, and even if the baby is at risk of the condition we have no way of knowing how severely they will be affected, routine screening is not currently recommended. WHAT CAN BE DONE?: FNAIT is usually diagnosed if a previous baby has had a low platelet count. The parents are offered blood tests and the condition can be confirmed or ruled out. There are many other causes of low platelets in babies, which may also need to be tested for. As the condition is so rare, expertise is limited to specialist centres and normally a haematologist and fetal medicine doctor will perform and interpret the tests together. Fortunately, there is an effective treatment for the vast majority of cases called immunoglobulin, or IVIg. This 'blood product' is given intravenously through a drip every week to women at risk of the condition. It may be started from as early as 16 weeks in the next pregnancy, until birth, which would be offered at around 36-37 weeks. Less common treatments that may be considered depending on individual circumstances include steroid tablets or injections, or giving platelet transfusions to the baby. WHAT DOES THIS PAPER TELL YOU?: This paper considers the latest evidence in relation to treatment options in the management of pregnancies at risk of FNAIT. Specifically, we discuss the role of screening, when IVIg should be started, what dose should be used, and what evidence there is for maternal steroids. We also consider in very rare selected cases, the use of fetal blood sampling and giving platelet transfusions to the baby before birth. Finally, we consider the approaches to blood testing mothers to tell if babies are at risk, which is offered in some countries, and development of new treatments to reduce the risk of FNAIT.

Fetal and neonatal alloimmune thrombocytopenia: recommendations for evidence-based practice, an international approach

Fetal and neonatal alloimmune thrombocytopenia (FNAIT) may result in severe bleeding, particularly fetal and neonatal intracranial haemorrhage (ICH). As a result, FNAIT requires prompt identification and treatment; subsequent pregnancies need close surveillance and management. An international panel convened to develop evidence-based recommendations for diagnosis and management of FNAIT. A rigorous approach was used to search, review and develop recommendations from published data for: antenatal management, postnatal management, diagnostic testing and universal screening. To confirm FNAIT, fetal human platelet antigen (HPA) typing, using non-invasive methods if quality-assured, should be performed during pregnancy when the father is unknown, unavailable for testing or heterozygous for the implicated antigen. Women with a previous child with an ICH related to FNAIT should be offered intravenous immunoglobulin (IVIG) infusions during subsequent affected pregnancies as early as 12 weeks gestation. Ideally, HPA-selected platelets should be available at delivery for potentially affected infants and used to increase the neonatal platelet count as needed. If HPA-selected platelets are not immediately available, unselected platelets should be transfused. FNAIT studies that optimize antenatal and postnatal management, develop risk stratification algorithms to guide management and standardize laboratory testing to identify high risk pregnancies are needed.

Fetal and neonatal alloimmune thrombocytopenia: evidence based antenatal and postnatal management strategies

INTRODUCTION

Fetal and neonatal alloimmune thrombocytopenia (FNAIT) is a relatively rare but potentially lethal disease, leading to severe bleeding complications in 1 in 11.000 newborns. It is the leading cause of thrombocytopenia in healthy term-born neonates. Areas covered: This review summarizes the antenatal as well as postnatal treatment, thus creating a complete overview of all possible management strategies for FNAIT. Expert commentary: The optimal antenatal therapy in order to prevent bleeding complications in pregnancies complicated by FNAIT is non-invasive treatment with weekly intravenous immunoglobulin (IVIG). Based on risk stratification, weekly doses of IVIG of 0.5 or 1.0g/kg should be administered started early in the second in high risk cases or at the end of the second trimester in low risk cases. The optimal postnatal treatment depends on the platelet count and the clinical condition of the newborn. Prompt administration of compatible platelet transfusion is the first treatment of choice in case of severe thrombocytopenia or active bleeding. In case matched platelets are not directly available, random platelets can also be administered initially to gain time until matched platelets are available. In case of persistent thrombocytopenia despite transfusions, IVIG 1.0-2.0g/kg can be administered.

Assessment of human platelet survival in the NOD/SCID mouse model: technical considerations

BACKGROUND

The NOD/SCID mouse model is a unique and sophisticated method to study the survival of human platelets (PLTs) in vivo. Meanwhile, several research groups adopted this model to analyze a wide range of PLT antibodies. Differences exist between the research groups regarding the method of PLT injection, the amount and route of antibody injection, and the preparation of blood samples collected from the animal, making it difficult to compare results between studies.

STUDY DESIGN AND METHODS

We compared the survival of human PLTs infused into NOD/SCID mice via the tail vein or the retro-orbital plexus. The percentage of circulating human PLTs in the mouse circulation was determined by flow cytometry. Murine blood samples were prepared using two different methods: 1) direct fixation of whole blood samples and 2) isolation of PLTs by density gradient centrifugation.

RESULTS

Recovery of human PLTs after tail vein injection was comparable to retro-orbital injection (13% vs. 11% of all circulating PLTs, p = 0.401). However, the survival rate of tail vein-infused PLTs was higher than that of retro-orbitally injected PLTs (median PLT survival after 5 hr 84% vs. 56%, p = 0.025). Moreover, we observed that determination of circulating human PLTs in directly fixed murine whole blood samples shows better reproducibility compared to the density gradient centrifugation method.

CONCLUSIONS

Tail vein injection of human PLTs into the NOD/SCID mice is superior to retro-orbital injection in terms of human PLT survival. Direct fixation of whole blood samples allows better reproducibility of results compared to the density gradient centrifugation method.

Bleeding complications in immune thrombocytopenia

Bleeding manifestations in patients with immune thrombocytopenia (ITP) range from mild skin bruises to life-threatening intracranial hemorrhage (ICH). Severe bleeding is distinctly uncommon when the platelet count is >30 × 109/L and usually only occurs when the platelet count falls <10 × 109/L. Based on estimates from clinical studies, ITP registries and administrative databases, the frequency of ICH in patients with ITP is ∼0.5% in children and 1.5% in adults. Estimates of severe (non-ICH) bleeding are difficult to obtain because of the lack of standardized case definitions; the lack of a universally accepted, ITP-specific bleeding assessment tool; and the omission of reporting bleeding outcomes in many clinical studies. In practice, the presence of bleeding should dictate whether or not treatment is needed because many patients, especially children, can be safely managed with observation alone. Guiding principles for the management of ITP, based on the bleeding risk are: (1) Decide when treatment is needed and when it can safely be withheld; (2) for patients with chronic ITP, use the least toxic treatment at the lowest dose; (3) emergency treatment of severe thrombocytopenia-associated bleeding requires combination therapy; and (4) early aggressive therapy may result in durable platelet count responses.

A rare manifestation of neonatal alloimmune thrombocytopaenia

Neonatal alloimmune thrombocytopaenia (NAIT) results from a fetomaternal incompatibility with maternal sensitisation against a fetal human platelet antigen (HPA) and antibodies transfer to the fetal circulation, leading to platelet destruction. The clinical presentation is variable and isolated intraocular haemorrhage is rare. We present the case of a male newborn, with intrauterine growth restriction, born at 29 weeks due to pre-eclampsia. He presented proptosis of the left eye, hyphaema and elevated intraocular pressure, with no other signs of haemorrhage. Severe thrombocytopaenia was found (27×10(9)/L). Perinatal infection and maternal thrombocytopaenia were excluded. Positive anti-HPA-1a and antihuman leucocyte antigen class I alloantibodies were found in the mother. Platelet crossmatch between the father's platelets and mother's plasma was positive. Platelet transfusions and intravenous immunoglobulin were given with favourable response. This case highlights an unusual presentation of NAIT, which should be suspected in the presence of severe thrombocytopaenia in the first 24-72 h of life.

Low-frequency human platelet antigens as triggers for neonatal alloimmune thrombocytopenia

BACKGROUND

Twenty-four low-frequency human platelet antigens (LFHPAs) have been implicated as immunogens in neonatal alloimmune thrombocytopenia (NAIT). We performed studies to define more fully how often these antigens trigger maternal immunization leading to NAIT.

STUDY DESIGN AND METHODS

In a Phase 1 study, fathers of selected NAIT cases not resolved by serologic testing but thought to have a high likelihood of NAIT on clinical and serologic grounds were typed for LFHPAs by DNA sequencing. In a Phase 2 study, high-throughput methods were used to type fathers of 1067 consecutive unresolved NAIT cases for LFHPAs. Mothers of 1338 unresolved cases were also typed to assess the prevalence of LFHPAs in a population racially/ethnically similar to the fathers.

RESULTS

In Phase 1, LFHPAs were identified in 16 of 244 fathers (6.55%). In Phase 2, LFPAs were found in only 28 of 1067 fathers (2.62%). LFHPAs were identified in 27 of 1338 maternal samples (2.01%). HPA-9bw was by far the most common LFHPA identified in the populations studied and was the only LFHPA that was significantly more common in fathers than in mothers of affected infants (p = 0.02).

CONCLUSIONS

Maternal immunization against recognized LFHPAs accounts for only a small fraction of the cases of apparent NAIT not resolved by standard serologic testing. Typing of the fathers of such cases for LFHPAs is likely to be rewarding only when a maternal antibody specific for a paternal platelet glycoprotein is demonstrated and/or there is compelling clinical evidence for NAIT.

Identification of potential platelet alloantigens in the Equidae family by comparison of gene sequences encoding major platelet membrane glycoproteins

BACKGROUND

Platelet alloantigens in horses may play an important role in the development of neonatal alloimmune thrombocytopenia (NAIT).

OBJECTIVE

The objective of this study was to evaluate genes encoding major platelet glycoproteins within the Equidae family in an effort to identify potential alloantigens.

METHODS

DNA was isolated from blood samples obtained from Equidae family members, including a Holsteiner-Oldenburg cross, a Quarter horse, a donkey, and a Plains zebra (Equus burchelli). Gene sequences encoding equine platelet membrane glycoproteins IIb, IIIa (integrin subunits αIIb and β3), Ia (integrin subunit α2), and Ibα were determined using PCR. Gene sequences were compared to the equine genome available on GenBank. Polymorphisms that would be predicted to result in amino acid changes on platelet surfaces were documented and compared with known alloantigenic sites documented on human platelets.

RESULTS

Amino acid differences were predicted based on nucleotide sequences for all 4 genes. Nine differences were documented for αIIb, 5 differences were documented for β3, 7 differences were documented for α2, and 16 differences were documented for Ibα outside the macroglycopeptide region.

CONCLUSIONS

This study represents the first effort at identifying potential platelet alloantigens in members of the Equidae Family based on evaluation of gene sequences. The data obtained form the groundwork for identifying potential platelet alloantigens involved in transfusion reactions and neonatal alloimmune thrombocytopenia (NAIT). More work is required to determine whether the predicted amino acid differences documented in this study play a role in alloimmunity, and whether other polymorphisms not detected in this study are present that may result in alloimmunity.

Laboratory testing for platelet antibodies

Laboratory testing for immune-mediated thrombocytopenias involves identification and classification of antibodies present in patient sera or attached to patient platelets. This article summarizes the available types of platelet antibody testing and applications in disorders such as neonatal alloimmune thrombocytopenia, post-transfusion purpura, multiple platelet transfusion refractoriness, immune thrombocytopenia, and drug-induced thrombocytopenia.

Neonatal alloimmune thrombocytopenia: pathogenesis, diagnosis and management

Neonatal alloimmune thrombocytopenia, (NAIT) is caused by maternal antibodies raised against alloantigens carried on fetal platelets. Although many cases are mild, NAIT is a significant cause of morbidity and mortality in newborns and is the most common cause of intracranial haemorrhage in full-term infants. In this report, we review the pathogenesis, clinical presentation, laboratory diagnosis and prenatal and post-natal management of NAIT and highlight areas of controversy that deserve the attention of clinical and laboratory investigators.

Review of neonatal alloimmune thrombocytopenia

Neonatal alloimmune thrombocytopenia (NAIT), with an incidence of one in 1000 live births, is the most common cause of severe thrombocytopenia and intra-cerebral haemorrhage in term neonates. NAIT results from trans-placental passage of maternal antibodies against a paternally derived fetal platelet alloantigen. Clinical presentation varies from unexpected thrombocytopenia on a blood film in a well newborn to intracranial haemorrhage (ICH). In contrast to haemolytic disease of the newborn, NAIT can present in a first pregnancy, and subsequent pregnancies are usually more severely affected. The role of antenatal screening for maternal alloantibodies instead of fetal blood sampling to identify at-risk fetuses remains uncertain, but there is a trend towards less invasive maternally directed treatment for at-risk pregnancies. Neonatal management is aimed at preventing or limiting thrombocytopenic bleeding with transfusion of antigen-matched platelets.

Diagnosis and management of neonatal thrombocytopenia

Thrombocytopenia is the most common haematological abnormality in newborns admitted to neonatal care units and serves as an important indicator of underlying pathological processes of mother or child. In most cases thrombocytopenia is mild to moderate and resolves within the first weeks of life without any intervention. However, in some neonates thrombocytopenia is severe or may reflect an inborn platelet disorder. As clinical course and outcome of thrombocytopenia depend on the aetiology of thrombocytopenia, an appropriate work-up is essential to guide therapy in neonates with thrombocytopenia and to avoid severe bleeding.

Neonatal alloimmune thrombocytopenia: a case study

Neonatal alloimmune thrombocytopenia (NAIT) is a life-threatening disorder caused by fetomaternal platelet incompatibility analogous to that seen in rhesus (Rh) disease. In NAIT , maternal immunoglobulin G (IgG) antiplatelet antibodies cross the placenta, resulting in rapid destruction and removal of fetal platelets by the reticuloendothelial system. Studies have shown that NAIT has an incidence of 1 of 1,000 live births, with a mortality rate of 10-15 percent and the risk of long-term morbidities up to 20-60 percent if intracranial hemorrhage (ICH) occurs. This column will discuss the pathophysiology, differential diagnosis, morbidities, and treatment of NAIT and conclude with a relevant case study.

Severe intracranial haemorrhage in neonatal alloimmune thrombocytopenia

Neonatal alloimmune thrombocytopenia is a rare (1/1000-5000 births) life-threatening disorder, caused by fetomaternal incompatibility for a fetal human platelet alloantigen inherited from the father, with production of maternal alloantibodies against fetal platelets, leading to severe thrombocytopenia and potential bleeding. Intracranial haemorrhage is the most feared complication. This report presents the case of a term newborn infant, born from caesarean section after a normal pregnancy, presenting signs of skin bleeding with different ages. Obstetric history included a previous spontaneous abortion after amniocentesis. Severe thrombocytopenia (4×10(9)/l platelets) was found and brain ultrasound showed multiple intracranial haemorrhages. Human platelet antigen (HPA) phenotyping showed maternal negative HPA-1a and paternal positive HPA-1a platelets. Strongly positive anti-HPA-1a and weakly positive anti-human leukocyte antigen class I alloantibodies were found in the mother. Multiple platelet transfusions, intravenous immunoglobulin and corticosteroid were given but favourable response was accomplished only after a compatible platelet transfusion. Brain MRI showed multiple subacute and chronic haemorrhages.

Congenital amegakaryocytic thrombocytopenia

Congenital amegakaryocytic thrombocytopenia (CAMT) is clinically characterized by thrombocytopenia presenting at birth in a child without congenital or skeletal malformations, reduced or absent bone marrow megakaryocytes, and eventual progression to bone marrow failure. Molecular studies in most cases confirm homozygous or compound heterozygous mutations in the thrombopoietin receptor c-Mpl. In addition to the clinical importance of recognizing this disorder, characterization of mutations identified in patients with CAMT has led to insights into thrombopoietin receptor structure and function. This review will summarize the diagnosis, pathophysiology, and management of CAMT.

Historical perspective and future directions in platelet research

Platelets are a remarkable mammalian adaptation that are required for human survival by virtue of their ability to prevent and arrest bleeding. Ironically, however, in the past century, the platelets' hemostatic activity became maladaptive for the increasingly large percentage of individuals who develop age-dependent progressive atherosclerosis. As a result, platelets also make a major contribution to ischemic thrombotic vascular disease, the leading cause of death worldwide. In this brief review, I provide historical descriptions of a highly selected group of topics to provide a framework for understanding our current knowledge and the trends that are likely to continue into the future of platelet research. For convenience, I separate the eras of platelet research into the "Descriptive Period" extending from ~1880-1960 and the "Mechanistic Period" encompassing the past ~50 years since 1960. We currently are reaching yet another inflection point, as there is a major shift from a focus on traditional biochemistry and cell and molecular biology to an era of single molecule biophysics, single cell biology, single cell molecular biology, structural biology, computational simulations, and the high-throughput, data-dense techniques collectively named with the "omics postfix". Given the progress made in understanding, diagnosing, and treating many rare and common platelet disorders during the past 50 years, I think it appropriate to consider it a Golden Age of Platelet Research and to recognize all of the investigators who have made important contributions to this remarkable achievement..

Repeated intrauterine IgG infusions in foetal alloimmune thrombocytopenia do not increase foetal platelet counts

BACKGROUND AND OBJECTIVES

Foetal alloimmune thrombocytopenia (FNAIT) is often treated transplacentally with maternally administered i.v. immunoglobulins, but not all foetuses show a consistent platelet increase during such treatment.

MATERIALS AND METHODS

We retrospectively analysed data from a cohort of ten foetuses with FNAIT treated by direct foetal immunoglobulin infusion. Foetal treatment was begun between 17 and 25 weeks and continued until 36 weeks with weekly cordocenteses and foetal immunoglobulin infusions.

RESULTS

While foetal IgG levels increased steadily during weekly IgG infusions, foetal platelet counts remained unchanged.

CONCLUSION

Our retrospective study presents a unique analysis of a historical cohort, contributing to the ongoing debate about the treatment of choice for foetal alloimmune thrombocytopenia.

Current problems and future directions of transfusion-induced alloimmunization: summary of an NHLBI working group

In April 2010, a working group sponsored by the National Heart, Lung, and Blood Institute was assembled to identify research strategies to improve our understanding of alloimmunization caused by the transfusion of allogeneic blood components and to evaluate potential approaches to both reduce its occurrence and manage its effects. Significant sequelae of alloimmunization were discussed and identified, including difficulties in maintaining chronic transfusion of red blood cells and platelets, hemolytic disease of the newborn, neonatal alloimmune thrombocytopenia, and rejection of transplanted cells and tissues. The discussions resulted in a consensus that identified key areas of future research and developmental areas, including genetic and epigenetic recipient factors that regulate alloimmunization, biochemical specifics of transfused products that affect alloimmunization, and novel technologies for high-throughput genotyping to facilitate extensive and efficient antigen matching between donor and recipient. Additional areas of importance included analysis of unappreciated medical sequelae of alloimmunization, such as cellular immunity and its effect upon transplant and autoimmunity. In addition, support for research infrastructure was discussed, with an emphasis on encouraging collaboration and synergy of animal models biology and human clinical research. Finally, training future investigators was identified as an area of importance. In aggregate, this communication provides a synopsis of the opinions of the working group on the above issues and presents both a list of suggested priorities and the rationale for the topics of focus. The areas of research identified in this report represent potential fertile ground for the medical advancement of preventing and managing alloimmunization in its different forms and mitigating the clinical problems it presents to multiple patient populations.

[Occurrence of severe, persistent thrombocytopenia following allogeneic bone marrow transplantation, attributable to anti-HPA-1 allo-immunisation of the host]

A 56 year-old, multiparous woman suffering from a myeloproliferative syndrome, who had received multiple red blood cell and platelet transfusions, was the recipient of an allograft of peripheral blood stem cells derived from her HLA-A, B, DR, DQ and DP and ABO identical sister, following myeloablative conditioning. The persistence of severe, isolated thrombopenia resistant to platelet transfusions led to the discovery of anti-HLA class I allo-immunisation. As HLA compatible platelet transfusions did not result in satisfactory platelet increments, we then discovered the simultaneous presence of anti-HPA-1a allo-immunisation. Genotyping of the HPA-1 systems of the patient (HPA-1B/B) and her sister (HPA-1A/B) enabled us to elucidate the mechanism underlying the persistent thrombopenia and the inefficacy of transfusion. In fact, only transfusion of HPA-1B/B platelets (HLA compatible or incompatible) proved to be efficacious. To reduce the level of anti-HPA-1a antibodies, we performed plasmapheresis sessions and used an anti-CD20 monoclonal antibody. It was only on achieving total haematopoietic chimerism, through rapid interruption of the immunosuppression, that we obtained spontaneous normalisation of the platelet count. The present case emphasises the necessity, before undertaking any allograft of haematopoietic stem cells - even if the latter come from a strictly HLA identical member of the family - of performing a search for eventual anti-HPA allo-immunisation.

6th International Immunoglobulin Symposium: poster presentations

The posters presented at the 6th International Immunoglobulin Symposium covered a wide range of fields and included both basic science and clinical research. From the abstracts accepted for poster presentation, 12 abstracts were selected for oral presentations in three parallel sessions on immunodeficiencies, autoimmunity and basic research. The immunodeficiency presentations dealt with novel, rare class-switch recombination (CSR) deficiencies, attenuation of adverse events following IVIg treatment, association of immunoglobulin (Ig)G trough levels and protection against acute infection in patients with X-linked agammaglobulinaemia (XLA) and common variable immunodeficiency (CVID), and the reduction of class-switched memory B cells in patients with specific antibody deficiency (SAD). The impact of intravenous immunoglobulin on fetal alloimmune thrombocytopenia, pregnancy and postpartum-related relapses in multiple sclerosis and refractory myositis, as well as experiences with subcutaneous immunoglobulin in patients with multi-focal motor neuropathy, were the topics presented in the autoimmunity session. The interaction of dendritic cell (DC)-SIGN and alpha2,6-sialylated IgG Fc and its impact on human DCs, the enrichment of sialylated IgG in plasma-derived IgG, as wells as prion surveillance and monitoring of anti-measles titres in immunoglobulin products, were covered in the basic science session. In summary, the presentations illustrated the breadth of immunoglobulin therapy usage and highlighted the progress that is being made in diverse areas of basic and clinical research, extending our understanding of the mechanisms of immunoglobulin action and contributing to improved patient care.

Naturally processed peptides spanning the HPA-1a polymorphism are efficiently generated and displayed from platelet glycoprotein by HLA-DRB3*0101–positive antigen-presenting cells

In neonatal alloimmune thrombocytopenia, almost all human platelet antigen (HPA)–1b1b mothers who produce anti–HPA-1a antibody through carrying an HPA-1a fetus are human histocompatibility leukocyte antigen (HLA)–DRB3*0101 positive. It is predicted that the HPA-1a Leu33 polymorphism forms part of an HLA-DRB3*0101–restricted T-helper epitope, and acts as an anchor residue for binding this class II molecule. However, it is not known whether any corresponding peptides are naturally processed and presented from platelet glycoprotein. In this study, peptides displayed by a homozygous HLA-DRB3*0101 antigen-presenting cell line were identified after pulsing with recombinant HPA-1a (Leu33 plexin-semaphorin-integrin domain). The peptides were eluted from HLA-DR molecules, fractionated by high performance liquid chromatography, and analyzed by tandem mass spectrometry. A “nested set” of naturally presented HPA-1a–derived peptides, each containing the Trp25-Leu33 core epitope, was identified, with the most abundant member being the 16-mer Met22-Arg37. These peptides may provide the basis for novel treatments to tolerize the corresponding T-helper response in women at risk of neonatal alloimmune thrombocytopenia.

Congenital amegakaryocytic thrombocytopenia and thrombocytopenia with absent radii

Thrombocytopenia is a relatively common clinical problem in hospitalized neonates, and it is critical to distinguish infants who have rare congenital thrombocytopenias from those who have acquired disorders. Two well-described inherited thrombocytopenia syndromes that present in the newborn period are congenital amegakaryocytic thrombocytopenia (CAMT) and thrombocytopenia with absent radii (TAR). Although both are characterized by severe (< 50,000/microL) thrombocytopenia at birth, the molecular mechanisms underlying these disorders and their clinical presentations and courses are distinct. CAMT is an autosomal recessive disorder caused by mutations in the thrombopoietin (TPO) receptor c-Mpl. TAR is a syndrome of variable inheritance and unclear genetic etiology consisting of thrombocytopenia in association with bilateral absent radii and frequently additional congenital abnormalities. This article summarizes the current understanding of the pathophysiology and clinical course of CAMT and TAR.