Fetomaternal alloimmune thrombocytopenia

Neonatal alloimmune thrombocytopenia caused by anti-HLA-A24 alloantibodies

Neonatal alloimmune thrombocytopenia (NAIT) occurs as a result of maternal alloimmunization against paternally inherited antigens on foetal platelets. Platelets express platelet specific antigens (HPA) along with human leucocyte antigens (HLA) class I. Although anti-HLA class I antibodies are often detectable in pregnant women, their role in NAIT is considered controversial. We report a case of NAIT where the most sensitive serological analysis and molecular methods could not detect platelet specific antibodies. Only HLA incompatibility and presence of anti-HLA-A24 antibodies in both the mother's and the newborn's serum were proven.

CONCLUSION

This case supports the idea that some anti-HLA class I antibodies could cause NAIT.

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

Pregnancy may allow alloimmunization against human platelet antigens (HPA), which can lead to neonatal alloimmune thrombocytopenia (NAIT). The specificities of alloantibodies are closely related to the distribution of the HPA systems. A total of 281 Tunisian multiparous women (mean number of pregnancies: 4.5) were phenotyped for the HPA-1, -3 and -5 systems, by monoclonal antibody immobilization of platelet antigens (MAIPA). We searched for antibodies against HPA-1a, HPA-3a, HPA-5b and HPA-5a in HPA-1b1b, HPA-3b3b, HPA-5a5a and HPA-5b5b individuals, respectively. The gene frequencies were: 0.83 for HPA-1a, 0.17 for HPA-1b, 0.78 for HPA-3a, 0.22 for HPA-3b, 0.82 for HPA-5a and 0.18 for HPA-5b. Anti-HPA-5b antibodies were present in eight sera and anti-HPA-3a antibodies were present in one serum. The anti-HPA-5b system is the most frequently involved in platelet alloimmunization in Tunisian multiparous women. However, prospective trials are required to confirm this result and to determine the exact frequencies and clinical relevance of platelet alloantibodies in pregnant Tunisian women.

Fetomaternal alloimmune thrombocytopenia: The questions that still remain

Fetomaternal alloimmune thrombocytopenia (FMAIT) occurs when maternal antibodies are formed to fetal platelet antigens, leading to thrombocytopenia and hemorrhagic complications. The diagnosis is frequently made only after a major hemorrhagic event has occurred during a pregnancy. Identifying patients at risk remains difficult, and the optimal treatment regimen remains to be determined.

MALDI-TOF MS in Prenatal Genomics

Prenatal diagnosis aims either to provide the reassurance to the couples at risk of having an affected child by timely appropriate therapy or to give the parents a chance to decide the fate of the unborn babies with health problems. Invasive prenatal diagnosis (IPD) is accurate, however, carrying a risk of miscarriage. Non-invasive prenatal diagnosis (NIPD) has been developed based on the existing of fetal genetic materials in maternal circulation; however, a minority fetal DNA in majority maternal background DNA hinders the detections of fetal traits. Different protocols and assays, such as homogenous MassEXTEND (hME), single allele base extension reaction (SABER), precise measuring copy number variation of each allele, and quantitative methylation and expression analysis using the high-throughput sensitive matrix-assisted laser desorption/ionization-time of flight mass spectrometry (MALDI-TOF MS), allow NIPD for single gene disorders, fetal blood group genotyping and fetal aneuploidies as well as the development of fetal gender-independent biomarkers in maternal circulation for management of pathological pregnancies. In this review, we summarise the use of MALDI-TOF MS in prenatal genomics.

Crystallographic structure of the human leukocyte antigen DRA, DRB3*0101: models of a directional alloimmune response and autoimmunity

We describe structural studies of the human leukocyte antigen DR52a, HLA-DRA/DRB3*0101, in complex with an N-terminal human platelet integrin alphaII(B)betaIII glycoprotein peptide which contains a Leu/Pro dimorphism. The 33:Leu dimorphism is the epitope for the T cell directed response in neonatal alloimmune thrombocytopenia and post-transfusion purpura in individuals with the alphaII(B)betaIII 33:Pro allele, and defines the unidirectional alloimmune response. This condition is always associated with DR52a. The crystallographic structure has been refined to 2.25 A. There are two alphabeta heterodimers to the asymmetric unit in space group P4(1)2(1)2. The molecule is characterized by two prominent hydrophobic pockets at either end of the peptide binding cleft and a deep, narrower and highly charged P4 opening underneath the beta 1 chain. Further, the peptide in the second molecule displays a sharp upward turn after pocket P9. The structure reveals the role of pockets and the distinctive basic P4 pocket, shared by DR52a and DR3, in selecting their respective binding peptide repertoire. We observe an interesting switch in a residue from the canonically assigned pocket 6 seen in prior class II structures to pocket 4. This occludes the P6 pocket helping to explain the distinctive "1-4-9" peptide binding motif. A beta57 Asp-->Val substitution abrogates the salt-bridge to alpha76 Arg and along with a hydrophobic beta37 is important in shaping the P9 pocket. DRB3*0101 and DRB1*0301 belong to an ancestral haplotype and are associated with many autoimmune diseases linked to antigen presentation, but whereas DR3 is susceptible to type 1 diabetes DR52a is not. This dichotomy is explored for clues to the disease.

Clinical and diagnostic comparison of neonatal alloimmune thrombocytopenia to non-immune cases of thrombocytopenia

BACKGROUND

Affected patients with neonatal alloimmune thrombocytopenia (AIT) are often severely thrombocytopenic and, if so, may suffer an intracranial hemorrhage (ICH). This study was undertaken to compare the outcome of cases of AIT to cases of neonatal thrombocytopenia shown not to be AIT and to identify clinical features that would facilitate the diagnosis.

PROCEDURE

Two hundred twenty two cases of neonatal thrombocytopenia for which serologic testing was obtained by the referring physician were accrued for this study from 11 testing laboratories. The relevant clinical information was pursued.

RESULTS

The mean birth platelet count in 110 neonates with AIT was 26,000/mm(3) x 10(9)/L and the rate of ICH was 11% (not all neonates had head sonos). Three criteria distinguished cases of AIT from other causes of neonatal thrombocytopenia (n = 56): (1) severe thrombocytopenia <50,000/mm(3) x 10(9)/L; (2) ICH associated with 1 or more of: a 1-min Apgar score >5, birthweight >2,200 g, grade >1, antenatal occurrence, or signs of bleeding, that is, petechiae, ecchymoses; and (3) no additional, non-hemorrhagic neonatal medical problems.

CONCLUSIONS

AIT is a unique type of neonatal thrombocytopenia with significant hemorrhagic consequences. Identification of AIT at the bedside should guide institution of appropriate treatment and lead to serologic testing for confirmation.

The haematology of indigenous Australians

Prior to European settlement indigenous Australians were hunter-gatherers who lived in geographically isolated small clan groups, also separated by elaborate totemic rules. Today they still reside in isolated communities throughout Australia but many have moved to the cities. They share a high incidence of a range of health problems including cardiovascular disease, renal disease and infectious diseases largely attributed to a change to a more sedentary lifestyle. This paper reviews the haematology of indigenous Australians, including blood count, frequency and causes of anaemia, inherited risk factors for thrombophilia, blood groups and the incidence and types of haematological malignancies. There are some significant genetic differences between indigenous and non-indigenous Australians particularly in the frequency of blood groups, factor V Leiden and prothrombin mutations and presence of -alpha3.7 kb thalassaemia. These findings may have practical therapeutic implications (e.g. HPA phenotype for transfusion therapy and pregnancy risk) and in predicting disease risk. Other differences are acquired, related to lifestyle and living conditions (e.g. eosinophilia secondary to parasitic infections; iron and folate deficiencies), and are largely preventable.

Effect of maternal anti-HPA-1a antibodies and polyclonal IVIG on the activation status of vascular endothelial cells

Maternal anti-HPA-1a antibodies can cause severe fetal and neonatal alloimmune thrombocytopenia (FNAIT), complicated by intracranial haemorrhage (ICH). Antenatal treatment with maternal intravenous immunoglobulin (IVIG) seems to protect against ICH even when thrombocytopenia persists. The aim of this study was to investigate if anti-HPA-1a antibodies and IVIG potentially affect vascular endothelial cells (ECs) in order to identify susceptibility for ICH. Human umbilical cord endothelial cells (HUVEC) were incubated with anti-HPA-1a antibodies with or without polyclonal IVIG and evaluated for EC activation. Maternal sera with anti-HPA-1a antibodies affected neither the EC expression of intracellular adhesion molecule-1 (ICAM-1), vascular adhesion molecule-1 (VCAM-1) and tissue factor (TF) nor the release of van Willebrand factor (vWF) or interleukin (IL)-8 nor the integrity of ECs. Maternal sera obtained after IVIG treatment and polyclonal IVIG decrease constitutive and cytokine-induced ICAM-1 and VCAM-1 expression on ECs. The results show that maternal anti-HPA-1a antibodies cause no activation or damage of ECs in this model. The clinical relevance of the de-activating properties of IVIG on EC activation with respect to ICH deserves further investigation.

Alloimmune thrombocytopenias

Alloimmunization against platelet antigens may take place during pregnancy and following transfusion or transplantation. The resulting antibodies bind to the target platelet antigens and may lead to a rapid sequestration of incompatible platelets. The alloimmune thrombocytopenias are classified into neonatal alloimmune thrombocytopenia, post-transfusion purpura, passive alloimmune thrombocytopenia, transplantation-associated alloimmune thrombocytopenia, and platelet transfusion refractoriness. Neonatal alloimmune thrombocytopenia occurs in roughly one of 1,000 births and is the most important type in children. The management of affected infants during pregnancy is yet not established, and the most rapidly effective treatment after delivery is the transfusion of compatible platelets.

Neonatal thrombocytopenia: causes and management

Neonatal thrombocytopenia is a common clinical problem. Thrombocytopenia presenting in the first 72 hours of life is usually secondary to placental insufficiency and caused by reduced platelet production; fortunately most episodes are mild or moderate and resolve spontaneously. Thrombocytopenia presenting after 72 hours of age is usually secondary to sepsis or necrotising enterocolitis and is usually more severe and prolonged. Platelet transfusion remains the only treatment. There is a need for trials to define the safe lower limit for platelet count and which neonates will benefit from treatment.

Will it ever be possible to balance the risk of intracranial haemorrhage in fetal or neonatal alloimmune thrombocytopenia against the risk of treatment strategies to prevent it?

BACKGROUND AND OBJECTIVES

Intracranial haemorrhage (ICH) of the fetus or newborn is a severe complication of fetal or neonatal alloimmune thrombocytopenia (FNAIT). In order to attain management decisions to prevent ICH, the risk of ICH in successive pregnancies with thrombocytopenia, with or without a history of ICH, must be established.

MATERIALS AND METHODS

We performed a search of medline for ICH cases in untreated FNAIT pregnancies. After exclusion of cases with confounding factors, 24 reports, describing 62 pregnancies of 27 mothers, were eligible. In addition, two mothers with five pregnancies were included from our own case records. Observational studies were examined to estimate the risk of ICH in subsequent FNAIT pregnancies without a history of ICH. Finally, medline was searched for complication rates in the treatment of FNAIT pregnancies.

RESULTS

In 52% of the ICH cases, a previous sibling suffered from ICH. The recurrence rate of ICH in the subsequent offspring of women with a history of FNAIT with ICH was 72%[confidence interval (CI): 46-98%] without inclusion of fetal deaths and 79% (CI: 61-97%) with inclusion of fetal deaths. In 48% of the ICH cases, the previous sibling had thrombocytopenia but not ICH. Population studies revealed an overall ICH risk in thrombocytopenic infants of 11% (CI: 0.8-23%) without inclusion of fetal deaths and 15% (CI: 1.5-19%) with inclusion of fetal deaths. Assuming occurrence in 48%, the risk of ICH in a subsequent pregnancy following a history of FNAIT without ICH, was estimated to be 7% (CI: 0.5-13%). Invasive treatment strategies carry a risk of 2.8% (CI: 1.2-4.4%) on complications.

CONCLUSIONS

The number of eligible publications on ICH in untreated FNAIT pregnancies is strikingly limited. The recurrence rate is high. As sufficient data on successive FNAIT cases without ICH are lacking, the occurrence of ICH in pregnancies with thrombocytopenia, but without ICH in a previous sibling, cannot be predicted. We estimate this risk to be 7%. This risk must be balanced against the risk of interventions in treatment strategies.

Thrombocytopenia in the newborn

Thrombocytopenia remains a common problem in sick newborns. A quarter of all neonates admitted to neonatal intensive care units develop thrombocytopenia, and in 20% of episodes the thrombocytopenia is severe (platelets <50 x 10(9)/L). Practical and clinically relevant classifications of neonatal thrombocytopenia have now been developed which, by highlighting the principal conditions precipitating severe thrombocytopenia (eg, sepsis, necrotizing enterocolitis, perinatal asphyxia, and the immune thrombocytopenias), aid the practicing neonatologist. Recent reviews demonstrate that many neonates with severe thrombocytopenia receive repeated platelet transfusions, although evidence of their clinical benefit is lacking, and there exists a significant variation in platelet transfusion practice between centers. These facts support the need for the development of evidence-based protocols for platelet transfusion in the newborn and stimulate continued interest in the potential of hemopoietic growth factors (, thrombopoietin and interleukin-11) to prevent or treat neonatal thrombocytopenia.