THE USE OF 1E12, A MONOCLONAL ANTI-PLATELET FACTOR 4 ANTIBODY TO IMPROVE THE DIAGNOSIS OF VACCINE-INDUCED IMMUNE THROMBOTIC THROMBOCYTOPENIA

Background: Vaccine-induced immune thrombotic thrombocytopenia (VITT) is a complication of adenoviral-based vaccine against SARS-CoV-2 due to pro- thrombotic immunoglobulin (Ig) G antibodies to platelet factor 4 (PF4) and may be difficult to distinguish from heparin-induced thrombocytopenia (HIT) in patients treated with heparin. Objectives: We assessed the usefulness of competitive anti-PF4 enzyme immunoassays (EIAs) in this context. Methods: The ability of F(ab ')2 ' )2 fragments of 1E12, 1C12, and 2E1, 3 monoclonal antiPF4 antibodies, to inhibit the binding of human VITT or HIT antibodies to PF4 was evaluated using EIAs. Alanine-scanning mutagenesis was performed to define the amino acids involved in the interactions between the monoclonal antibodies and PF4. Results: A strong inhibition of VITT IgG binding to PF4 was measured with 1E12 (median inhibition, 93%; n = 8), whereas it had no effect on the binding of HIT antibodies (median, 6%; n = 8). In contrast, 1C12 and 2E1 inhibited VITT (median, 74% and 76%, respectively) and HIT antibodies (median, 68% and 53%, respectively) binding to PF4. When a competitive anti-PF4 EIA was performed with 1E12 for 19 additional VITT samples, it strongly inhibited IgG binding to PF4, except for 1 patient, who had actually developed HIT according to the clinical history. Epitope mapping showed that 1E12 interacts with 5 key amino acids on PF4, of which 4 are also required for the binding of human VITT antibodies, thus explaining the competitive inhibition. Conclusion: A simple competitive anti-PF4 EIA with 1E12 could help confirm VITT diagnosis and distinguish it from HIT in patients when both diagnoses are possible.