Abstract / Summary
Abstract Background Fetal/neonatal alloimmune thrombocytopenia (FNAIT) is caused by maternal alloantibodies directed against fetal platelet antigens, most commonly human platelet antigen‐1a (HPA‐1a). These antibodies mediate platelet destruction and can lead to fetal and neonatal thrombocytopenia severe enough to cause life‐threatening organ bleeds. Antibody titer and isotype do not reliably predict disease severity, suggesting additional heterogeneity among anti‐HPA‐1a antibodies. Study Design and Methods We evaluated the effects of five previously characterized HPA‐1a‐specific monoclonal antibodies (mAbs) on platelet activation, platelet aggregation, and induction of intracellular signaling. The involvement of FcγRIIa was examined using FcγRIIa‐blocking antibody IV.3, protein tyrosine kinase inhibitors, and selected monoclonal antibody Fab fragments. Results At saturating concentrations, all five mAbs inhibited thrombin‐induced aggregation. At sub‐saturating concentrations, however, mAb M‐204 uniquely induced spontaneous aggregation and α‐granule secretion in HPA‐1a‐positive human, but not mouse, platelets, which lack FcγRIIa. M‐204‐induced aggregation was abolished by mAb IV.3 and inhibitors of Src‐ and Syk‐family kinases and was absent using M‐204 Fab fragments. M‐204 induced tyrosine phosphorylation of Syk and PLCγ2 in an FcγRIIa‐dependent manner. Finally, despite persistent platelet activation induced by M‐204, saturating concentrations of this antibody blocked the binding of the fibrinogen mimetic antibody, PAC‐1, to HPA‐1a homozygous, but not HPA‐1a/1b heterozygous, platelets, consistent with the known ability of HPA‐1a‐specific antibodies to sterically interfere with ligand binding to HPA‐1a‐positive αIIbβ3. Conclusion These findings identify a previously unrecognized, functionally distinct subset of HPA‐1a antibodies capable of activating platelets via FcγRIIa, suggesting a potential mechanism contributing to variability in disease severity in FNAIT.