Maternal anti-HPA-1a antibodies block αIIbβ3/αvβ3 integrin activation, and blockade correlates with FNAIT disease severity

ABSTRACT: Fetal/neonatal alloimmune thrombocytopenia (FNAIT) is a disorder caused by a mismatch in human platelet antigens (HPAs), leading to maternal antibody formation and platelet destruction in the fetus or neonate. The clinically most relevant antigen is HPA-1a on the β3 subunit of integrins αIIbβ3 and αvβ3, which are conformationally regulated cell adhesion receptors on platelets and endothelial cells and are crucial for hemostasis and vascular integrity. The clinical effects of anti-HPA-1a alloimmunization are highly heterogeneous and range from no symptoms to intracranial hemorrhage, potentially causing perinatal death or lifelong complications. However, determinants of disease severity are largely unknown, which hampers implementation of screening programs to identify alloimmunized high-risk women who may benefit from treatment. Using recombinant anti-HPA-1a antibodies and a retrospective cohort of FNAIT samples associated with mild or severe disease, we report that the tested anti-HPA-1a antibodies inhibit binding of integrins αvβ3/αIIbβ3 to their ligands (vitronectin, fibronectin, and fibrinogen) and impair cell adhesion. This inhibition is dependent on antibody concentration, is mediated by the antibody Fab region, and does not require the Fc tail, and is abolished when integrins are forced into a constitutively extended conformation. Furthermore, the extent of integrin inhibition correlates with disease severity. Together, our data demonstrate that anti-HPA-1a antibodies can block integrin activation, which likely contributes strongly to disease severity in FNAIT. These results may aid development of a prenatal diagnostic test to identify pregnancies at high risk of developing FNAIT and associated severe complications. In addition, these data reveal novel opportunities for allosteric inhibition of β3 integrin activation.