Journal Article

Antibody epitopes in vaccine-induced immune thrombotic thrombocytopaenia

Daka, Mercy, Huynh, Angela, Kelton, John G., Arnold, Donald M., Nazy, Ishac

2021

Overview

Vaccine-induced immune thrombotic thrombocytopaenia (VITT) is a rare adverse effect of COVID-19 adenoviral vector vaccines 1 – 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 mutagenesis 5 , 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. Alanine-scanning mutagenesis is used to identify the PF4 epitope that is recognized by anti-PF4 antibodies in patients with vaccine-induced immune thrombotic thrombocytopaenia, revealing that the epitope corresponds to the heparin-binding site on PF4.

• Publisher: Nature Publishing Group UK,Nature Publishing Group
• Subject: 692/308/575 / 692/699/249 / 82/1 / 82/47 / Alanine / Amino acids / Antibodies / Antigen-antibody complexes / Binding sites / Coronaviruses / COVID-19 / COVID-19 vaccines / Dissociation / Epitopes / Heparin / Humanities and Social Sciences / Interferometry / multidisciplinary / Physiological aspects / Platelet factor 4 / Platelets / Science / Science (multidisciplinary) / Severe acute respiratory syndrome coronavirus 2 / Thrombocytopenia / Thromboembolism / Thrombosis / Vaccines