Antibody epitopes in vaccine-induced immune thrombotic thrombocytopaenia

Vaccine-induced immune thrombotic thrombocytopaenia (VITT) is a rare adverse effect of COVID-19 adenoviral vector vaccines1–3. VITT resembles heparin-induced thrombocytopaenia (HIT) in that it is associated with platelet-activating antibodies against platelet factor 4 (PF4)4; however, patients with VITT develop thrombocytopaenia and thrombosis without exposure to heparin. Here we sought to determine the binding site on PF4 of antibodies from patients with VITT. Using alanine-scanning mutagenesis5, we found that the binding of anti-PF4 antibodies from patients with VITT (n = 5) was restricted to eight surface amino acids on PF4, all of which were located within the heparin-binding site, and that the binding was inhibited by heparin. By contrast, antibodies from patients with HIT (n = 10) bound to amino acids that corresponded to two different sites on PF4. Biolayer interferometry experiments also revealed that VITT anti-PF4 antibodies had a stronger binding response to PF4 and PF4–heparin complexes than did HIT anti-PF4 antibodies, albeit with similar dissociation rates. Our data indicate that VITT antibodies can mimic the effect of heparin by binding to a similar site on PF4; this allows PF4 tetramers to cluster and form immune complexes, which in turn causes Fcγ receptor IIa (FcγRIIa; also known as CD32a)-dependent platelet activation. These results provide an explanation for VITT-antibody-induced platelet activation that could contribute to thrombosis.