A functionally distinct HPA-1a-specific alloantibody associated with fetal/neonatal alloimmune thrombocytopenia that activates platelets via FcγRIIa.

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.