The role of fluid-phase immune complexes in the pathogenesis of heparin-induced thrombocytopenia

Immune complexes assemble on the platelet surface and cause Fc-mediated platelet activation in heparin-induced thrombocytopenia (HIT); however, it is not known if fluid-phase immune complexes contribute to HIT. The objective of this study was to understand the role of fluid-phase immune complexes in platelet activation and HIT. Binding of wild-type and 15 platelet factor 4 (PF4) mutants to platelets was measured using flow cytometry. Platelet activation was measured using the PF4-dependent ¹⁴C-serotonin release assay (PF4-SRA) with KKO and a HIT-patient plasma in the presence of wild-type or PF4 mutants. To activate platelets, we found that a minimal level of wild-type PF4 is required to bind the platelet surface in the presence of KKO (2.67 relative MFI) or HIT-patient plasma (1.71 relative MFI). Only a subset of PF4 mutants was able to support platelet activation, despite having lower surface binding than the minimum binding required of wild-type PF4 (9 mutants with KKO and 2 mutants with HIT-patient plasma). Using individual PF4 mutants, we identified that HIT immune complexes can be formed in fluid-phase and induce platelet activation. Further studies are required to investigate the role of fluid-phase HIT immune complexes in the development of thrombocytopenia and thrombosis associated with clinical HIT.