Fetal/Neonatal Alloimmune Thrombocytopenia: Pathogenesis, Diagnostics and Prevention

Generation of human antibodies targeting human platelet antigen (HPA)-1a

BACKGROUND

Fetal and neonatal alloimmune thrombocytopenia (FNAIT) is a condition during pregnancy, which can lead to thrombocytopenia and a bleeding tendency with intracranial hemorrhage (ICH) being the most concerning complication in the fetus or neonate. An incompatibility between human platelet antigen (HPA)-1a accounts for the majority of FNAIT cases. Binding of HPA-1a-specific alloantibodies to their target on fetal platelets and endothelial cells can induce apoptosis of megakaryocytes, disrupt platelet function, and impair angiogenesis. Currently, there is no screening program to identify pregnancies at risk for severe disease. A better understanding of HPA-1a-specific antibody heterogeneity in FNAIT could aid in identifying pathogenic antibody properties linked to severe disease.

STUDY DESIGN AND METHODS

This study aimed to isolate HPA-1a-specific B-cells from an HPA-1a-alloimmunized pregnant woman. Using fluorescently labeled HPA-1a-positive platelets, single B-cells were sorted and cultured for 10 days to stimulate antibody production. Subsequently, supernatants were tested for the presence of antibodies by enzyme-linked immunosorbent assay and their reactivity towards HPA-1a-positive platelets. Amplification and sequencing of variable regions allowed the generation of monoclonal antibodies using a HEK-Freestyle-based expression system.

RESULTS

Three platelet-specific B-cells were obtained and cloned of which two were specific for HPA-1a, named D- and M-204, while the third was specific for HLA class I, which was named L-204.

DISCUSSION

This study outlined an effective method for the isolation of HPA-1a-specific B-cells and the generation of monoclonal antibodies. Further characterization of these antibodies holds promise for better understanding the pathogenic nature of alloantibodies in FNAIT.

Navigating the Storm: Managing Fetal and Neonatal Alloimmune Thrombocytopenia (FNAIT) in a High-Risk Pregnancy

Fetal and neonatal alloimmune thrombocytopenia (FNAIT) is a rare, life-threatening condition causing significant thrombocytopenia and bleeding with the risk of developing intracerebral hemorrhage (ICH). It results from maternal immunizations against fetal platelet antigens. Here, we report a case of a pregnant patient at 30 weeks gestation who presented to the hospital with a low platelet count of 90 th/mm3 and was found to have anti-human platelet antigen (HPA) 1a, 2b antibodies. She was treated with a weekly infusion of IV immunoglobulins. However, her condition was complicated by the development of hemolysis, elevated liver enzymes, and low platelets (HELLP) syndrome, which was treated promptly with a platelet transfusion and intravenous magnesium. Even though the child had severe thrombocytopenia and its associated complications, there were no signs of post-delivery thrombocytopenia or any other adverse effects. This case report highlights the importance of the antenatal management of the FNAIT to prevent severe fetal complications, such as ICH.

Role of Fc Core Fucosylation in the Effector Function of IgG1 Antibodies

The presence of fucose on IgG1 Asn-297 N-linked glycan is the modification of the human IgG1 Fc structure with the most significant impact on FcɣRIII affinity. It also significantly enhances the efficacy of antibody dependent cellular cytotoxicity (ADCC) by natural killer (NK) cells in vitro, induced by IgG1 therapeutic monoclonal antibodies (mAbs). The effect of afucosylation on ADCC or antibody dependent phagocytosis (ADCP) mediated by macrophages or polymorphonuclear neutrophils (PMN) is less clear. Evidence for enhanced efficacy of afucosylated therapeutic mAbs in vivo has also been reported. This has led to the development of several therapeutic antibodies with low Fc core fucose to treat cancer and inflammatory diseases, seven of which have already been approved for clinical use. More recently, the regulation of IgG Fc core fucosylation has been shown to take place naturally during the B-cell immune response: A decrease in α-1,6 fucose has been observed in polyclonal, antigen-specific IgG1 antibodies which are generated during alloimmunization of pregnant women by fetal erythrocyte or platelet antigens and following infection by some enveloped viruses and parasites. Low IgG1 Fc core fucose on antigen-specific polyclonal IgG1 has been linked to disease severity in several cases, such as SARS-CoV 2 and Dengue virus infection and during alloimmunization, highlighting the in vivo significance of this phenomenon. This review aims to summarize the current knowledge about human IgG1 Fc core fucosylation and its regulation and function in vivo, in the context of both therapeutic antibodies and the natural immune response. The parallels in these two areas are informative about the mechanisms and in vivo effects of Fc core fucosylation, and may allow to further exploit the desired properties of this modification in different clinical contexts.

Neonatal Thrombocytopenia Due to Dual Alloimmune and Autoimmune Mechanisms

Alloantibody-mediated and autoantibody-mediated immune destruction are common causes of early neonatal thrombocytopenia. The authors report a case of severe, early-onset thrombocytopenia with mucocutaneous bleeding in an otherwise well-appearing full-term neonate. Recurrence of thrombocytopenia following initial treatment and its persistence after 2 weeks of life suggested a dual immune mechanism. This is a rare case of immune thrombocytopenia caused by human platelet antigen-5b alloimmunization and passive transfer of maternal antiplatelet antibodies. Appropriate, timely treatment and absence of severe bleeding complications, namely intracranial hemorrhage, conferred a good overall prognosis.

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

BACKGROUND

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

METHOD

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

RESULTS

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

DISCUSSION

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

Intravenous Immune Globulin Uses in the Fetus and Neonate: A Review

Intravenous immune globulin (IVIG) is made after processing plasma from healthy donors. It is composed mainly of pooled immunoglobulin and has clinical evidence-based applications in adult and pediatric populations. Recently, several clinical applications have been proposed for managing conditions in the neonatal population, such as hemolytic disease of the newborn, treatment, and prophylaxis for sepsis in high-risk neonates, enterovirus parvovirus and COVID-19 related neonatal infections, fetal and neonatal immune-induced thrombocytopenia, neonatal hemochromatosis, neonatal Kawasaki disease, and some types of immunodeficiency. The dosing, mechanism of action, effectiveness, side effects, and adverse reactions of IVIG have been relatively well studied in adults but are not well described in the neonatal population. This review aims to provide the most recent evidence and consensus guidelines about the use of IVIG in the fetus and neonate.

Platelet Disorders

After vascular injury and exposure of subendothelial matrix proteins to the intravascular space, mediators of hemostasis are triggered and allow for clot formation and restoration of vascular integrity. Platelets are the mediators of primary hemostasis, creating a platelet plug and allowing for initial cessation of bleeding. Platelet disorders, qualitative and quantitative, may result in bleeding signs and symptoms, particularly mucocutaneous bleeding such as epistaxis, bruising, petechiae, and heavy menstrual bleeding. Increasing evidence suggests that platelets have functional capabilities beyond hemostasis, but this review focuses solely on platelet hemostatic properties. Herein, normal platelet function as well as the effects of abnormal function and thrombocytopenia are reviewed.

A single-center investigational study of CD36 antigen deficiency and platelet alloantibody distribution in different populations in Northern China as well as platelet alloantibodies effect on pregnancy

BACKGROUND

Platelet antibodies can lead to clinical diseases such as platelet transfusion refractoriness (PTR), fetal/neonatal alloimmune thrombocytopenia (FNAIT), etc. This study is aimed at understanding CD36 expression, platelet alloantibody distribution in different populations in Northern China, and effects of platelet alloantibodies on pregnancy.

STUDY DESIGN AND METHODS

Whole blood samples of 612 subjects including hematological patients, pregnant women, and blood donors were collected at a single center, then CD36 expressions were determined, followed by platelet antibody screening and characterization of platelet antibody specificity. A retrospective analysis was performed in 1552 pregnant women admitted to Department of Obstetrics, in order to investigate FNAIT occurrence.

RESULTS

Rate of CD36 deficiency expression was 2.12% (13/612), all cases exhibited type II deficiency without type I deficiency being detected, and such rate is lower than that in Southern China (3.43%), Japanese (4.87%) and in the black people (4.18%), and higher than that in the White people (0.09%). Positive rates of platelet antibody screening in hematological patient group (6.86%, 14/204) and in pregnant women group (6.31%, 13/206) are higher than that in blood donor group (0.49%, 1/202), P < .01. Out of 1552 pregnant women, there were not children with FNAIT.

CONCLUSION

The frequency of CD36 deficiency in northern China was low, all of them were type II deficiency, and no CD36 antibody was detected. It is speculated that the risk of immune-related thrombocytopenia caused by CD36 deficiency in this population is very low. Platelet antibodies should be monitored early in patients with hematological and multiple miscarriages pregnant.

Noninvasive prenatal HPA-1 typing in HPA-1a negative pregnancies selected in the Polish PREVFNAIT screening program

BACKGROUND

Anti-HPA-1a alloantibodies in HPA-1a negative mothers can lead to fetal/neonatal alloimmune thrombocytopenia (FNAIT). Noninvasive prenatal testing (NIPT) of HPA-1a determines fetuses at risk and the course of maternal antenatal treatment.

STUDY DESIGN AND METHODS

The aim was to develop and validate HPA-1a NIPT by real-time polymerase chain reaction (PCR) or next-generation sequencing (NGS) for a high-throughput screening setting. DNA from 328 plasma samples of 299 HPA-1a negative pregnant women was examined for HPA-1a by real-time PCR and in two cases also by NGS (Ion Torrent). The results were compared with neonatal HPA-1a genotyping in 281 cases.

RESULTS

HPA-1a NIPT was negative in 44 of 51 HPA-1a negative fetuses, inconclusive in five, and false positive in two. In 228 of 229 HPA-1a positive fetuses, the NIPT results were positive (mean threshold cycle 36.0 ± 1.7) and inconclusive in one. In 22 cases with HPA-1a positive fetuses analyzed twice, the sensitivity of HPA-1a detection was significantly higher at 28 weeks compared with 16 to 20 weeks. NGS efficiently detected the ITGB3 coding HPA-1a/b (1% and 5% fetal HPA-1a reads).

CONCLUSION

Real-time PCR is reliable to predict the fetal HPA-1a positive genotype in a screening study, but false-positive results are reported in 4%, with unnecessary prenatal treatment if anti-HPA-1a is detected.

Storage-Induced Platelet Apoptosis Is a Potential Risk Factor for Alloimmunization Upon Platelet Transfusion

Platelet transfusion can elicit alloimmune responses leading to alloantibody formation against donor-specific polymorphic residues, ultimately resulting in platelet transfusion refractoriness. Universal leukoreduction significantly reduced the frequency of alloimmunization after platelet transfusion, thereby showing the importance of white blood cells (WBCs) in inducing this alloresponse. It is, however, unknown if the residual risk for alloimmunization is caused by WBCs remaining after leukoreduction or if alloimmunization can be induced by platelets themselves. This study investigated the capacity of platelets to induce alloimmunization and identified potential product-related risk factors for alloimmunization. First, internalization of allogeneic platelets by dendritic cells (DCs) was demonstrated by confocal microscopy. Second, after internalization, presentation of platelet-derived peptides was shown by mass spectrometry analysis of human leukocytes antigen (HLA)-DR eluted peptides. Third, platelet-loaded DCs induced platelet-specific CD4 T cell responses. Altogether, this indicates a platelet-specific ability to induce alloimmunization. Therefore, factors enhancing platelet internalization may be identified as risk factor for alloimmunization by platelet concentrates. To investigate if storage of platelets is such a risk factor, internalization of stored platelets was compared with fresh platelets and showed enhanced internalization of stored platelets. Storage-induced apoptosis and accompanied phosphatidylserine exposure seemed to be instrumental for this. Indeed, DCs pre-incubated with apoptotic platelets induced the strongest IFN-γ production by CD4 T cells compared with pre-incubation with untreated or activated platelets. In conclusion, this study shows the capacity of platelets to induce platelet-specific alloimmune responses. Furthermore, storage-induced apoptosis of platelets is identified as potential risk factor for alloimmunization after platelet transfusions.

[Thirty years of platelet immunology in fetal and neonatal alloimmune thrombocytopenia management, current situation]

Fetal and neonatal allo-immune thrombocytopenia (FNAIT) is considered as a rare disease due to the incidence (1/1000-1/2000 births). The major complication of severe thrombocytopenia is bleeding and particularly intra-cranial hemorrhage and neurologic sequelae following. Serology and molecular biology developments have reconfigured the platelet immunology diagnosis. Anti-HPA-1a allo-immunisation is responsible for more than 80% FNAIT cases with a high recurrence rate of severe bleeding complications. Therapeutic management has changed over the coming years from an invasive concept associating fetal blood sampling and in utero platelet transfusion to a non invasive treatment by intravenous immunoglobulins injection (IVIg). The purpose of this article is to provide an update on FNAIT management in the light of current developments over the past 30years.

A preliminary evaluation of next-generation sequencing as a screening tool for targeted genotyping of erythrocyte and platelet antigens in blood donors

BACKGROUND

Matching the compatibility of donor blood with the recipient's antigens prevents alloimmunisation. Next-generation sequencing (NGS) technology is a promising method for extensive blood group and platelet antigen genotyping of blood donors. It circumvents the limitations of detecting known alleles based on predefined polymorphisms and enables targeted sequencing on a massive scale. The aim of this study was to evaluate the NGS AmpliSeq application on the Ion Torrent platform as a screening tool for genotyping blood donors' erythrocyte/platelet antigens.

MATERIALS AND METHODS

Primers for regions encoding antigens RhD (exons 5, 7), Rhc, RhE/e, Fya/b, Jka/b, M/N, S/s, HPA-1, 2, 3, 5, 15 were designed with Ion AmpliSeq Designer with manual inclusion of RHCE*C primers. DNA libraries of 57 regular blood donors with determined phenotype/genotype (prepared using the Ion AmpliSeq Library Kit and 14 primer pairs) were sequenced on the Ion Torrent PGM using 316v2 chips and 200 bp chemistry.

RESULTS

Sequencing was successful in all but the MN and HPA-5 regions. Mean sequencing coverage in one experiment was 4,606 reads, except for the RHCE*C region (mean 568 reads). NGS results agreed with the known phenotype/genotype of donors except in one phenotypically Fy(a+b-) case in whom FY*A/FY*B alleles were found. Reading rates for homozygotes were 97-100%, while they were around 50% for heterozygotes. NGS of RHD regions led to identification of mutations in two RhD negative donors.

DISCUSSION

NGS can be performed as a screening test to determine erythrocyte/platelet antigens in blood donors. This method allowed testing of 48 donors for 14 features (200 bp long) with the depth of a few thousand reads simultaneously, and the estimation of natural chimerism or hemi/homozygotic status. NGS screening can be adjusted to the genetic background of a given tested population.

Transfusion as an Inflammation Hit: Knowns and Unknowns

Transfusion of blood cell components is frequent in the therapeutic arsenal; it is globally safe or even very safe. At present, residual clinical manifestations are principally inflammatory in nature. If some rare clinical hazards manifest as acute inflammation symptoms of various origin, most of them linked with conflicting and undesirable biological material accompanying the therapeutic component (infectious pathogen, pathogenic antibody, unwanted antigen, or allergen), the general feature is subtler and less visible, and essentially consists of alloimmunization or febrile non-hemolytic transfusion reaction. The present essay aims to present updates in hematology and immunology that help understand how, when, and why subclinical inflammation underlies alloimmunization and circumstances characteristic of red blood cells and – even more frequently – platelets that contribute inflammatory mediators. Modern transfusion medicine makes sustained efforts to limit such inflammatory hazards; efforts can be successful only if one has a clear view of each element’s role.