High-resolution cryo-EM structure of integrin αIIbβ3 bound to disease-causing maternal HPA-1a antibody that blocks integrin activation

Integrins promote immunity, embryonic development, wound healing, and hemostasis, and are activated by bent/closed to extended/open conformational changes. Integrin IIb{beta}3, being crucial for platelet activation and aggregation, is a therapeutic target for bleeding disorders and thrombosis. Human Platelet Antigen-1a (HPA-1a) on {beta}3 is recognized by pregnancy-associated maternal alloantibodies, potentially causing fetal/neonatal alloimmune thrombocytopenia (FNAIT) and even intracranial hemorrhage or perinatal death. However, severe disease determinants are largely unknown. We report the first structure of an anti-HPA-1a antibody fragment (Fab 26.4) in complex with integrin IIb{beta}3 at high resolution by cryo-electron microscopy. Fab 26.4 binding traps IIb{beta}3 in the inactive, bent/closed conformation, is incompatible with integrin extension, and inhibits IIb{beta}3-dependent fibrinogen binding and platelet aggregation. Thus, anti-HPA-1a antibodies directly impair integrin activation by preventing required conformational changes. These insights will improve FNAIT diagnostics and treatment, and spark the development of novel allosteric inhibitors against {beta}3 integrins for future therapeutic applications.