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

Abstract

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 beta 3 subunit. The beta 3 integrins are conformationally regulated heterodimeric receptors including platelet integrin alpha IIb beta 3 but also alpha v beta 3, 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 alpha v beta 3 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 alpha IIb beta 3 and alpha v beta 3. Exclusive anti-HPA-1a reactivity with alpha v beta 3 or endothelial cells was not found. In contrast, all anti-HPA-1a antibodies reacted with platelets and endothelial cells, and with alpha v beta 3-and alpha IIb beta 3-transduced cells, but reacted generally more with the alpha IIb beta 3 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 alpha IIb beta 3 over alpha v beta 3 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.