HPA-1a antibodies in FNAIT do not distinguish αvβ3 from αIIbβ3 and bind inactive integrins more strongly than active integrins

Fetal and neonatal alloimmune thrombocytopenia (FNAIT) is a rare but life-threatening condition in which maternal alloantibodies, generated during pregnancy, target human platelet antigens (HPAs), leading to thrombocytopenia and increased risk of bleedings in the fetus or neonate. The most clinically relevant antigen in people of European descent is HPA-1a, located on the integrin β3 subunit. The β3 integrins are conformationally regulated heterodimeric receptors including platelet integrin αIIbβ3 but also αvβ3, expressed strongly on endothelial cells. FNAIT is clinically highly heterogeneous, with symptoms ranging from mild thrombocytopenia to intracranial hemorrhage (ICH), which can cause lifelong disabilities or perinatal death. It has been suggested that anti–HPA-1a antibodies that exclusively react with αvβ3 cause ICH, due to induction of endothelial cell damage and/or defects in angiogenesis. Here, we analyzed a large cohort of retrospectively and prospectively collected maternal sera from severe and mild FNAIT cases. Disease severity was associated with the extent of thrombocytopenia, and with high anti–HPA-1a antibody reactivity toward both αIIbβ3 and αvβ3. Exclusive anti–HPA-1a reactivity with αvβ3 or endothelial cells was not found. In contrast, all anti–HPA-1a antibodies reacted with platelets and endothelial cells, and with αvβ3- and αIIbβ3-transduced cells, but reacted generally more with the αIIbβ3 integrin. Furthermore, HPA-1a epitope accessibility and antibody binding is influenced by integrin conformation and activation status. Higher reactivity of anti–HPA-1a antibodies with αIIbβ3 over αvβ3 diminishes upon integrin conformational activation. Together, these data emphasize the need for further investigation into the relation between endothelial properties of anti–HPA-1a antibodies and disease outcome in FNAIT.