A platelet-activating, HPA-1a-specific maternal alloantibody identifies a pathogenic subpopulation with the potential to exacerbate fetal/neonatal alloimmune thrombocytopenia

Abstract Fetal/neonatal alloimmune thrombocytopenia (FNAIT) is a potentially life-threatening condition caused by maternal alloantibodies directed against fetal platelet antigens, most commonly human platelet antigen-1a (HPA-1a). These antibodies mediate platelet destruction, leading to thrombocytopenia ranging from mild to severe, including intracranial hemorrhage or fetal demise. Despite routine clinical assessment of antibody titer and isotype, these parameters do not adequately capture functional heterogeneity, and fail to reliably predict disease severity. Identifying maternal antibodies with pathogenic potential is critical to improving risk stratification and guiding individualized management strategies. Anti-HPA-1a antibodies, while uniformly requiring interaction with Leu33 within the PSI domain of the integrin β3 subunit (GPIIb-IIIa), differ in their three-dimensional binding profiles - suggesting that functionally distinct subpopula-tions may exist. To test this, we evaluated the effects of five previously characterized monoclonal HPA-1a-specific antibodies (26.4, M-204, D-204, B2G1, SZ21) on platelet aggregation using a flow cytometry-based assay with both HPA-1a–positive transgenic mouse platelets and human platelets. At saturating concentrations, all five antibodies inhibited thrombin-induced aggregation. However, only M-204 triggered spontaneous aggregation of human—but not mouse—platelets at sub-saturating levels in the absence of agonist. Since human platelets express FcγRIIa and murine platelets do not, we hypothesized an Fc-dependent activation mechanism. Indeed, M-204–induced aggregation of human platelets was abolished by the FcγRIIa-specific blocking antibody IV.3, confirming an FcγRIIa-dependent mechanism. M-204 also induced expression of the α-granule marker P-selectin, further supporting its role in direct platelet activation. These findings define a previously unrecognized, functionally distinct subset of anti-HPA-1a antibodies capable of activating platelets via FcγRIIa. In addition to mediating clearance through opsonization, such antibodies may contribute directly to disease pathogenesis in FNAIT. Ongoing studies using maternal sera and FcγRIIa/HPA-1a–expressing transgenic mice aim to determine whether the presence of a subpopulation of platelet-activating alloantibodies might correlate with clinical severity. This work may support development of function-based assays to enhance risk stratification in affected pregnancies.