Antiendothelial v3 Antibodies Are a Major Cause of Intracranial Bleeding in Fetal/Neonatal Alloimmune Thrombocytopenia

Objective Fetal/neonatal alloimmune thrombocytopenia is a severe bleeding disorder, which can result in intracranial hemorrhage (ICH), leading to death or neurological sequelae. In whites, maternal anti-human platelet antigen-1a (HPA-1a) antibodies are responsible for the majority of cases. No predictive factors for ICH are available to guide prophylactic treatment during pregnancy. In this study, we investigated antibodies from mothers with ICH-positive fetal/neonatal alloimmune thrombocytopenia and with ICH-negative fetal/neonatal alloimmune thrombocytopenia to identify serological and functional differences between the groups. Approach and Results In an antigen capture assay, we observed a stronger binding of +ICH antibodies to endothelial cell (EC)-derived v3. By absorption experiments, we subsequently identified anti-HPA-1a antibodies of anti-v3 specificity in the +ICH but not in the -ICH cohort. Only the anti-v3 subtype, but not the anti-3 subtype, induced EC apoptosis of HPA-1a-positive ECs by caspase-3/7 activation, and mediated by reactive oxygen species. In addition, only the anti-v3 subtype, but not the anti-3 subtype, interfered with EC adhesion to vitronectin and with EC tube formation. Conclusions We conclude that the composition of the anti-HPA-1a antibody subtype(s) of the mother may determine whether ICH occurs. Analysis of anti-HPA-1a antibodies of the anti-v3 subtype in maternal serum has potential in the diagnostic prediction of ICH development and may allow for modification of prophylactic treatment in fetal/neonatal alloimmune thrombocytopenia.