PF4/Heparin-Nonbinding Platelet-Activating Antibodies in Heparin-Induced Thrombocytopenia

The hallmark of heparin-induced thrombocytopenia (HIT) is the presence of immunoglobulin G (IgG) antibodies against platelet factor 4/heparin (PF4/H) complexes, typically detected by PF4/H ELISA; thus, negative ELISA results are commonly used to exclude this diagnosis. Here, we report a prevalent yet previously unrecognized subset of antibodies that are undetectable by PF4/H ELISA (ELISA⁻) but activate platelets in the PF4-dependent P-selectin expression assay (PEA⁺). In 11 patients with clinically confirmed HIT who tested positive in both PF4/H ELISA and platelet activation assays, ELISA⁻PEA⁺ antibodies accounted for 65 ± 19% of total platelet-activating IgG activity and coexisted with ELISA⁺PEA⁺ antibodies. Consistent with this finding, single-cell cloning from seven HIT patients identified 23 PEA⁺ antibody-producing B-cell clones, of which 17 were ELISA⁻, outnumbering the ELISA⁺ clones. Functionally, ELISA-PEA⁺ antibodies closely resembled ELISA+PEA+ antibodies: platelet binding and activation required exogenous PF4 and were inhibited by FcgRIIA blockade, high-dose heparin, or Fab fragments made from ELISA+PEA+ antibodies. Importantly, these antibodies induced thrombocytopenia in a humanized mouse model of HIT. Despite lacking PF4/H reactivity in ELISAs, they recognize PF4 on platelets and showed no appreciable binding to NAP-2, IL-8, or PF4 alone. Structurally, these antibodies were heterogeneous, with a subset sharing heavy-chain features with ELISA⁺PEA⁺ antibodies. Collectively, our findings demonstrate that ELISA⁻PEA⁺ antibodies are a common, previously unrecognized feature of HIT, with functional relevance, supporting the possibility that they play an important, perhaps even central, role in HIT pathogenesis. Defining their prevalence, kinetics, and clinical impact deserves high priority for further investigation.