Clinical characteristics of human platelet antigen (HPA)-1a and HPA-5b alloimmunised pregnancies and the association between platelet HPA-5b antibodies and symptomatic fetal neonatal alloimmune thrombocytopenia

Does anti-HPA-1a affect birthweight in fetal and neonatal alloimmune thrombocytopenia?

BACKGROUND

Fetal and neonatal alloimmune thrombocytopenia (FNAIT) ensues from parental incompatibility for platelet alloantigens with maternal sensitization. HPA-1a/1b incompatibility is the most common cause of FNAIT in Caucasians. Placental villitis and lower birthweight in FNAIT suggest anti-HPA-1a may have effects beyond inducing thrombocytopenia.

OBJECTIVES

Does FNAIT secondary to anti-HPA-1a result in smaller newborns and, the corollary, does antenatal management of FNAIT increase birthweight?

STUDY DESIGN

Birthweights of 270 FNAIT-affected newborns from a randomized clinical trial and a NAITbabies.org survey (135 paired siblings) were compared with those of published controls and treated to untreated FNAIT-affected siblings. Birthweights were converted to percentiles to account for gestational age, sex, and role of birth order in birth weight. Body weights of FNAIT-affected and -unaffected pups in a mouse FNAIT model were analyzed.

RESULTS

Untreated siblings in both the clinical trial and NAITbabies.org cohorts were not small, compared with normal controls. However, treated siblings in both cohorts had significantly higher birthweight percentiles compared with their previous untreated affected sibling. After accounting for gestational age, sex, and birth order, increased birthweight percentile in treated compared with the untreated siblings remained significant in both cohorts. FNAIT-affected neonatal mice had lower bodyweights than FNAIT-unaffected pups.

CONCLUSIONS

Untreated FNAIT-affected newborns were not small; however, treatment of FNAIT-affected pregnancies increased newborn birthweights despite corrections to account for other factors that might have influenced the results. High dose IVIG is believed to "block" FcRn and lower maternal anti-HPA-1a levels, and thus increase birthweights by reducing levels of maternal anti-HPA-1a and reducing placental villitis.

Pregnant Women at Low Risk of Having a Child with Fetal and Neonatal Alloimmune Thrombocytopenia Do Not Require Treatment with Intravenous Immunoglobulin

Fetal and neonatal alloimmune thrombocytopenia (FNAIT) is a rare condition in which maternal alloantibodies to fetal platelets cause fetal thrombocytopenia that may lead to intracranial hemorrhage (ICH). Off-label intravenous immunoglobulin (IVIg) has for 30 years been the standard of care for pregnant women who previously have had a child with FNAIT. The efficacy of this treatment has never been tested in a placebo-controlled clinical trial. Although IVIg treatment may improve the neonatal outcome in women who previously have had a child with FNAIT-associated ICH, the question is whether IVIg is necessary for all immunized pregnant women at risk of having a child with FNAIT. The results from some recent publications suggest that antenatal IVIg treatment is not necessary for women who are (1) HPA-1a-immunized and HLA-DRB3*01:01-negative, (2) HPA-1a-immunized with a previous child with FNAIT but without ICH or (3) HPA-5b-immunized. If IVIg is not used for these categories of pregnant women, the amount of IVIg used in pregnant women with platelet antibodies would be reduced to less than ¼ of today's use. This is important because IVIg is a scarce resource, and the collection of plasma for the treatment of one pregnant woman is not only extremely expensive but also requires tremendous donor efforts.

Children Newly Diagnosed with Fetal and Neonatal Alloimmune Thrombocytopenia: Neurodevelopmental Outcome at School Age

OBJECTIVE

To evaluate the neurodevelopmental outcome at school age in children newly diagnosed with fetal and neonatal alloimmune thrombocytopenia (FNAIT).

STUDY DESIGN

This observational cohort study included children diagnosed with FNAIT between 2002 and 2014. Children were invited for cognitive and neurological testing. Behavioral questionnaires and school performance results were obtained. A composite outcome of neurodevelopmental impairment (NDI) was used, defined, and subdivided into mild-to-moderate and severe NDI. Primary outcome was severe NDI, defined as IQ <70, cerebral palsy with Gross Motor Functioning Classification System level ≥ III, or severe visual/hearing impairment. Mild-to-moderate NDI was defined as IQ 70-85, minor neurological dysfunction or cerebral palsy with Gross Motor Functioning Classification System level ≤ II, or mild visual/hearing impairment.

RESULTS

In total, 44 children were included at a median age of 12 years (range: 6-17 years). Neuroimaging at diagnosis was available in 82% (36/44) of children. High-grade intracranial hemorrhage (ICH) was detected in 14% (5/36). Severe NDI was detected in 7% (3/44); two children had high-grade ICH, and one had low-grade ICH and perinatal asphyxia. Mild-to-moderate NDI was detected in 25% (11/44); one child had high-grade ICH, and eight children were without ICH, yet for two children, neuroimaging was not performed. Adverse outcome (perinatal death or NDI) was 39% (19/49). Four children (9%) attended special needs education, three of whom had severe NDI and one had mild-to-moderate NDI. Total behavioral problems within the clinical range were reported in 12%, which is comparable with 10% in the general Dutch population.

CONCLUSION

Children who are newly diagnosed with FNAIT are at increased risk for long-term neurodevelopmental problems, even those without ICH.

TRIAL REGISTRATION

The study was registered at ClinicalTrials.gov (Identifier: NCT04529382).

FNAIT pathogenesis determined by serial analysis of three subsequent pregnancies of a woman with severe fetal and neonatal alloimmune thrombocytopenia (FNAIT) with anti-HPA-4b and anti-HPA-5b alloantibodies in the first sibling

BACKGROUND

Fetal and neonatal alloimmune thrombocytopenia (FNAIT) is caused by anti-HPA alloantibody, and anti-HPA-4b is the most common cause in Japanese. Anti-HPA-5b is frequently detected in pregnant women, but it is still controversial whether anti-HPA-5b causes severe FNAIT.

CASE PRESENTATION

A Japanese woman with anti-HPA-4b and anti-HPA-5b alloantibodies delivered a baby with severe FNAIT who was both HPA-4b and HPA-5b incompatible. We carefully monitored the patient's following three pregnancies (the second and the fourth siblings were HPA-4b incompatible and HPA-5b compatible; the third sibling was both HPA-4b and HPA-5b compatible). FNAIT was not observed in all three siblings, although a modest decrease in cord blood platelet count was observed in the HPA-4b incompatible siblings compared to the HPA-4b compatible sibling. Serial monitoring of anti-HPA titer showed that anti-HPA-4b markedly decreased in late pregnancy and recovered after delivery of the HPA-4b incompatible siblings, but these decreases were not observed during the mother's pregnancy with the HPA-4b compatible sibling. In contrast, anti-HPA-5b remained at a high titer during pregnancy with all three siblings.

CONCLUSION

Our data indicate that dynamic changes of anti-HPA-4b occur during pregnancy and strongly suggest that anti-HPA-5b was mainly responsible for severe FNAIT in this case.

New Horizons in Fetal and Neonatal Alloimmune Thrombocytopenia

Fetal and neonatal alloimmune thrombocytopenia (FNAIT) is a common cause of severe thrombocytopenia in newborns. Intracranial bleeding may lead to severe neurological sequelae and mortality. Current management of pregnancies at risk is suboptimal. Prenatal FNAIT diagnosis commonly requires invasive procedures and therapy is associated with a high treatment burden. The present review explores advances in the field and their potential contribution to modification of the diagnostic and therapeutic landscape. Topics addressed include the role of noninvasive prenatal testing using fetal cell free DNA, insights into novel and prospective therapeutic options achieved through the development of murine models of FNAIT as well as the forecast for the progress in pregnancy risk stratification through advancement in the investigation of biological characteristics of alloantibodies and their association with the risk of fetal bleeding.

Preliminary mechanism in fetal alloimmune thrombocytopenia associated with anti-HPA 15b antibodies

OBJECTIVE

Fetal and neonatal alloimmune thrombocytopenia (FNAIT) is a bleeding disease that can cause fetal hydrops, a rare but life-threatening condition in which abnormal amounts of fluid accumulate in one or two areas of the fetus's body. A case of FNAIT with fetal hydrops caused by anti-HPA-15b antibodies was involved in this study, as we investigated whether or not anti-HPA-15b antibodies can induce endothelial angiogenesis and apoptosis.

METHODS

The monoclonal antibody immobilization of platelet antigens assay (MAIPA) was used to identify anti-HPA-15b antibodies. The three groups in Tube formation and apoptosis assays were the PBS group, the AB serum IgG group, and the anti-HPA-15b serum IgG group, all reacted with HPA-15bb HUVEC.

RESULTS

The presence of anti-HPA-15b antibodies was found in this case by MAIPA assay. The OD values are 0.33 and 0.21, reacted with HPA-15bb and HPA-15ab platelets, respectively (cutoff OD value = 0.2). Quantitative analysis revealed that the length of capillary-like tube induced by anti-HPA-15b antibodies was significantly decreased over that of AB serum IgG (*p = 0.0005), but weaker than when incubated with thrombin (**p = 0.0009). The apoptosis results show a significantly increased number of apoptotic endothelial cells in the anti-HPA-15b antibody IgG group when compared with the PBS and AB serum IgG groups (*p < 0.0001, **p < 0.0001). In addition, there is no statistical difference between the PBS and AB serum groups.

CONCLUSION

Anti-HPA-15b antibodies can inhibit angiogenesis and induce apoptosis. This may associate with hydrops fetalis (HF), or fetal hydrops of FNAIT.

Anti-human platelet antigen-5b antibodies and fetal and neonatal alloimmune thrombocytopenia; incidental association or cause and effect?

Most cases of fetal and neonatal thrombocytopenia (FNAIT) are caused by maternal anti-human platelet antigen-1a antibodies (anti-HPA-1a). Anti-HPA-5b antibodies are the second most common antibodies in suspected FNAIT cases. Given the high prevalence of anti-HPA-5b antibodies in pregnant women delivering healthy newborns, the association with FNAIT may be coincidental. This review of the literature related to FNAIT using the MEDLINE database was conducted according to PRISMA guidelines. A retrospective analysis of a single-centre cohort of 817 suspected FNAIT cases was conducted. The pooled prevalence of anti-HPA-5b antibodies in unselected pregnant women of European descent was 1.96% (n = 3113), compared with 3.4% (n = 5003) in women with suspected FNAIT. We found weak evidence that a small proportion of pregnant women presenting with anti-HPA-5b antibodies will give birth to a newborn with mild thrombocytopenia. The neonatal platelet counts were not different between suspected FNAIT cases (n = 817) with and without maternal anti-HPA-5b antibodies. The prevalence of maternal anti-HPA-5b antibodies was not different between neonates with intracranial haemorrhage and healthy controls. The current experimental and epidemiological evidence does not support the hypothesis that anti-HPA-5b antibodies cause severe thrombocytopenia or bleeding complications in the fetus or newborn.