Fetal and neonatal alloimmune thrombocytopenia : immunological mechanisms and clinical consequences

Abstract

Fetal and neonatal alloimmune thrombocytopenia (FNAIT) is a rare condition that may occur in connection with pregnancy, when maternal antibodies directed against paternally inherited platelet antigens cross the placenta and mediate thrombocytopenia in the fetus and neonate. FNAIT puts the fetus at risk of severe bleedings, with an intracranial hemorrhage being the most feared outcome. Antibodies targeting Human Platelet Antigen 1a (HPA-1a) are the most common cause of FNAIT in Caucasians, and several mechanisms may potentially be involved. The overall aim of this thesis was to use both clinical and experimental data to investigate possible mechanisms of FNAIT, and to evaluate the impact of FNAIT on neonatal morbidity in Sweden by investigating the frequency of maternal platelet alloimmunization in newborns diagnosed with an intracranial hemorrhage (ICH).

In study I and II, the presence of anti-HLA class I antibodies in relation to FNAIT is investigated. Anti-HLA class I antibodies are a common occurrence after pregnancy, and the only serological finding in many cases of suspected FNAIT. Platelets express HLA class I antigen, and it has for long been debated whether anti-HLA class I antibodies could cause FNAIT. Based on national referrals in Sweden and Norway, we identified cases of suspected FNAIT where only anti-HLA class I antibodies could be detected. We compared anti-HLA class I antibody specificities and levels in these referred cases to controls, and found that the included cases in both studies had higher antibody levels compared to the control groups, and that especially antibodies against HLA-B antigens stood out with high levels. In study II, genotyping of the maternal and neonatal HLA class I alleles could untangle seemingly broad antibody specificity patterns, as mapping of epitopes revealed that the antibodies were in fact largely specific against paternal epitopes, expressed on multiple antigens. There was only weak correlations between anti-HLA class I antibody levels and clinical outcome, and especially in study I, a majority of cases were found to have other factors that could contribute to thrombocytopenia.

In study III, we investigated platelet function in adult and neonatal blood, and the effects of anti-HPA-1a antibodies on platelet function. We found that umbilical cord blood was characterized by a pronounced decrease in platelet aggregation, a reduced change in expression of glycoproteins with activation, and an enhanced primary hemostasis, compared to adult peripheral blood. The presence of a monoclonal anti-HPA-1a antibody reduced fibrinogen binding to HPA-1a positive platelets, but with limited functional consequences, as measured by flow cytometry.

For study IV, we investigated the frequency of maternal platelet alloimmunization in clinically recognized cases of neonatal ICH. Using the Swedish Neonatal Quality register, we identified 286 neonates registered with ICH born at or from 32 weeks of gestation, and ultimately included 105 maternal samples for analysis. We found two HPA-1a antigen negative (HPA-1bb) mothers, of which one had detectable anti-HPA-1a antibodies and a severely thrombocytopenic neonate. Two other mothers had detectable antibodies (anti-HPA-5b and anti-HPA-15a) and neonates with moderate thrombocytopenia. The available clinical data revealed a high frequency of other factors with a known association to ICH, and the study suggests a lower frequency of ICH associated with FNAIT than previously estimated in prospective studies. Overall, these studies confirm the notion of FNAIT as a heterogeneous condition, with findings that may inform future studies on frequency of severe FNAIT, and potential mechanisms.