Prevalence and clinical significance of low-avidity HPA-1a antibodies in women exposed to HPA-1a during pregnancy

Fetal/neonatal alloimmune thrombocytopenia: a systematic review of impact of HLA-DRB3*01:01 on fetal/neonatal outcome

The most common, severe cases of fetal and neonatal alloimmune thrombocytopenia among whites are caused by antibodies against human platelet antigen 1a (HPA-1a). The aims of this systematic review and meta-analysis are to determine the association between maternal HLA-DRB3*01:01 and: (1) HPA-1a-alloimmunization and (2) neonatal outcome in children born of HPA-1a-immunized women. A systematic literature search identified 4 prospective and 8 retrospective studies. Data were combined across studies to estimate pooled odds ratios (ORs) and the associated 95% confidence intervals (CIs). The population represented by the prospective studies was more than 150 000. In the prospective studies, there were 64 severely thrombocytopenic newborns (platelet count <50 × 109/L) of whom 3 had intracranial hemorrhage. The mothers of all 64 children were HLA-DRB3*01:01+. The number of severely thrombocytopenic children born of HPA-1a-alloimmunized women in the retrospective studies was 214; 205 of whom were born of HLA-DRB3*01:01+ women. For HLA-DRB3*01:01- women, the OR (95% CI) for alloimmunization was 0.05 (0.00-0.60), and for severe neonatal thrombocytopenia 0.08 (0.02-0.37). This meta-analysis demonstrates that the risk of alloimmunization and of having a child with severe thrombocytopenia are both very low for HPA-1a- women who are HLA-DRB3*01:01-.

Fetale und neonatale Alloimmunthrombozytopenie, Teil 1

Die fetale und neonatale Alloimmunthrombozytopenie (FNAIT) wird durch mütterliche Antikörper hervorgerufen, die gegen ein vom Vater ererbtes Blutgruppenmerkmal an fetalen Thrombozyten gerichtet sind. Teil 1 des Beitrags stellt die Ätiologie, die Pathogenese und die Diagnostik der FNAIT dar, während Teil 2 die Risikostratifizierung und Behandlung der FNAIT thematisiert 1.

Autoantibody-mediated desialylation impairs human thrombopoiesis and platelet lifespan

Immune thrombocytopenia is a common bleeding disease caused by autoantibody-mediated accelerated platelet clearance and impaired thrombopoiesis. Accumulating evidence suggests that desialylation affects platelet life span in immune thrombocytopenia. Herein, we report on novel effector functions of autoantibodies from immune thrombocytopenic patients which might interfere with the clinical picture of the disease. Data from our study show that a subgroup of autoantibodies is able to induce cleave of sialic acid residues from the surface of human platelets and megakaryocytes. Moreover, autoantibody-mediated desialylation interferes with the interaction between cells and extracellular matrix proteins leading to impaired platelet adhesion and megakaryocyte differentiation. Using a combination of ex vivo model of thrombopoiesis, a humanized animal model, and a clinical cohort study, we demonstrate that cleavage of sialic acid induces significant impairment in production, survival as well as function of human platelets. These data may indicate that prevention of desialylation should be investigated in the future in clinical studies as a potential therapeutic approach to treat bleeding in immune thrombocytopenia.

Progress and development of platelet antibody detection

Human platelet antibody (HPA) detection is necessary for the diagnosis and therapeutic decisions for refractoriness to platelet transfusions, post transfusion purpura and fetal and neonatal alloimmune thrombocytopenia. In the last four to five decades many new developments, both in knowledge and methods, have increased the quality of platelet serology. However, the quest for the optimal antibody detection method(s) encountered, sometimes unexpected, difficulties. In this review the various aspects concerning platelet antibody test methods and detection of platelet antibodies both for the diagnostic and screening setting are discussed.

Murine models for studying treatment, prevention and pathogenesis of FNAIT

Maternal alloimmunization to paternally inherited antigens on fetal/neonatal platelets can cause fetal/neonatal alloimmune thrombocytopenia (FNAIT) after antibody-mediated removal of platelets from the fetal circulation. The complications vary from mild bleeding symptoms to severe intracranial hemorrhage and subsequent neurological impairment or death. Studies on in vivo mechanisms are challenging to measure directly in pregnant women, rendering murine models as valuable and attractive alternatives, despite some critical differences between mice and men affecting the translational value. Here we present and discuss, the different murine models that substantially have increased our knowledge and understanding of FNAIT pathogenesis - as well as pre-clinical evaluation of therapeutic and preventive strategies.

Management of neonatal thrombocytopenia in a context of maternal antiplatelet alloimmunization: Expert opinion of the French-speaking working group

Fetal and neonatal alloimmune thrombocytopenia (FNAIT) is a potentially devastating disease, seen in 1/800-1000 neonates. FNAIT is the most common cause of early-onset isolated severe neonatal thrombocytopenia in maternity wards. The most feared complication of this disorder is intracranial hemorrhage, leading to death or neurological sequelae. There is no systematic screening of at-risk pregnancies and FNAIT is often discovered when fetal or neonatal bleeding is observed. A working group on fetomaternal platelet alloimmunization was created in 2017, under the auspices on the French Group of Thrombosis and Hemostasis (GFHT). The first objective of this group was to survey clinical practices for treatment of thrombocytopenic neonates in a context of suspected or confirmed FNAIT.

Anti-human platelet antigen (HPA)-1a antibodies may affect trophoblast functions crucial for placental development: a laboratory study using an in vitro model

BACKGROUND

Fetal and neonatal alloimmune thrombocytopenia (FNAIT) is a bleeding disorder caused by maternal antibodies against paternal human platelet antigens (HPAs) on fetal platelets. Antibodies against HPA-1a are accountable for the majority of FNAIT cases. We have previously shown that high levels of maternal anti-HPA-1a antibodies are associated with clinically significant reduced birth weight in newborn boys. Chronic inflammatory placental lesions are associated with increased risk of reduced birth weight and have previously been reported in connection with FNAIT pregnancies. The HPA-1a epitope is located on integrin β3 that is associated with integrin αIIb (the fibrinogen receptor) on platelets and megakaryocytes. Integrin β3 is also associated with integrin αV forming the αVβ3 integrin heterodimer, the vitronectin receptor, which is expressed on various cell types, including trophoblast cells. It is therefore thinkable that maternal anti-HPA-1a antibodies present during early pregnancy may affect placenta function through binding to the HPA-1a antigen epitope on invasive throphoblasts. The aim of the study was to examine whether interaction of a human anti-HPA-1a monoclonal antibody (mAb) with HPA-1a on trophoblast cells affect adhesion, migration and invasion of extravillous trophoblast cells.

METHODS

An in vitro model with human anti-HPA-1a mAb, clone 26.4, and the first trimester extravillous trophoblast cell line HTR8/SVneo was employed. The xCELLigence system was utilized to assess the possible effect of anti-HPA-1a mAb on adhesion and migration of HTR8/SVneo cells. Specially designed chambers precoated with Matrigel were used to assess the effect on the invasive capacity of cells.

RESULTS

We found that human anti-HPA-1a mAb 26.4 partially inhibits adhesion and migratory capacity of HTR8/SVneo cells.

CONCLUSIONS

Our findings suggest that anti-HPA-1a antibodies may affect trophoblast functions crucial for normal placental development. Future studies including primary throphoblast cells and polyclonal anti-HPA-1a antibodies are needed to confirm these results.

Maternal HLA genotyping is not useful for predicting severity of fetal and neonatal alloimmune thrombocytopenia

Lack of reliable laboratory parameters is the main challenge in the management of fetal and neonatal alloimmune thrombocytopenia (FNAIT). Despite the long-known association between the HLA-DRB3*01:01 allele and human platelet antigen 1a (HPA-1a) alloimmunisation, maternal human leucocyte antigen (HLA) typing has been of little clinical value. Recently, other DRB3 allele variants have been suggested to predict the severity of FNAIT. In this nationwide population-based retrospective cohort study, we performed extensive HLA typing of 96 women, accounting for 87% of our cohort of 110 families with confirmed or possible HPA-1a-immunisation. The HLA type was compared with anti-HPA-1a levels, severity of neonatal disease and responsiveness to maternally administrated intravenous gammaglobulin (IVIG). HLA haplotypes were constructed to investigate further HLA associations. Despite significantly lower anti-HPA-1a levels in DRB3*01:01-negative women, the carrier status of this particular allele could not be used to confirm or rule out FNAIT in the absence of detectable antibodies. In the haplotype analysis, the DRB3*01:01 allele was the actual factor associated with FNAIT. No other HLA allele was shown to be of additional value as a predictor of severe FNAIT or non-responsiveness to IVIG treatment. Thus, HLA genotyping was not found useful in differentiating high- and low-risk pregnancies or in guiding antenatal treatment in affected families.

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.

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

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

Advances in alloimmune thrombocytopenia: perspectives on current concepts of human platelet antigens, antibody detection strategies, and genotyping

Alloimmunisation to platelets leads to the production of antibodies against platelet antigens and consequently to thrombocytopenia. Numerous molecules located on the platelet surface are antigenic and induce immune-mediated platelet destruction with symptoms that can be serious. Human platelet antigens (HPA) cause thrombocytopenias, such as neonatal alloimmune thrombocytopenia, post-transfusion purpura, and platelet transfusion refractoriness. Thirty-four HPA are classified into 28 systems. Assays to identify HPA and anti-HPA antibodies are critically important for preventing and treating thrombocytopenia caused by anti-HPA antibodies. Significant progress in furthering our understanding of HPA has been made in the last decade: new HPA have been discovered, antibody-detection methods have improved, and new genotyping methods have been developed. We review these advances and discuss issues that remain to be resolved as well as future prospects for preventing and treating immune thrombocytopenia.

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.

External quality assessment in molecular immunohematology: the INSTAND proficiency test program

BACKGROUND

Genotyping for red blood cell (RBC), platelet (PLT), and granulocyte antigens is a new tool for clinical pathology, transfusion medicine services, and blood banks. Proficiency in laboratory tests can be established by external quality assessments (EQAs), which are required for clinical application in many health care systems. There are few EQAs for molecular immunohematology.

STUDY DESIGN AND METHODS

We analyzed the participation and pass rates in an EQA for RBC, PLT, and granulocyte antigens. This EQA was distributed by INSTAND, a large nonprofit provider of proficiency tests, twice per year since Fall 2006 as EQA Number 235 Immunohematology A (molecular diagnostic). The coordinators defined at the outset which alleles are mandatory for detection.

RESULTS

The number of participants steadily increased from 51 to 73 per proficiency by Fall 2012. More than 60 institutions utilized this EQA at least once a year. Approximately 80% of them participated in RBC, 68% in PLT, and 22% in granulocyte systems. With the exceptions of RHD (82%) and granulocytes (85%), pass rates exceeded 93%. While the pass rate increased for granulocyte and decreased for the ABO system, the pass rates for the other systems changed little over 6½ years.

CONCLUSIONS

The INSTAND proficiency test program was regularly used for EQA by many institutions, particularly in Central Europe. While the technical standards and pass rates in the participating laboratories were high, there has been little improvement in pass rates since 2006.

Human platelet antigens - 2013

To date, 33 human platelet alloantigens (HPAs) have been identified on six functionally important platelet glycoprotein (GP) complexes and have been implicated in alloimmune platelet disorders including foetal and neonatal alloimmune thrombocytopenia (FNAIT), posttransfusion purpura (PTP) and multitransfusion platelet refractoriness (MPR). The greatest number of recognized HPA (20 of 33) resides on the GPIIb/IIIa complex, which serves as the receptor for ligands important in mediating haemostasis and inflammation. These include HPA-1a, the most commonly implicated HPA in FNAIT and PTP in Caucasian populations. Other platelet GP complexes, GPIb/V/IX, GPIa/IIa and CD109, express the remaining 13 HPAs. Of the recognized HPAs, 12 occur as six serologically and genetically defined biallelic 'systems' where the -a form designates the higher frequency allele and the -b form, the lower. Twenty-one other HPAs are low-frequency or rare antigens for which postulated higher frequency -a alleles have not yet been identified as antibody specificities. In addition to the HPA markers, platelets also express ABO and human leucocyte antigen (HLA) antigens; antibodies directed at the former are occasionally important in FNAIT, and to the latter, in MPR.

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.