Vaccine-induced immune thrombotic thrombocytopenia is mediated by a stereotyped clonotypic antibody

Vaccine-induced immune thrombotic thrombocytopenia (VITT) is a rare thromboembolic complication of adenoviral-vectored SARS-CoV2 vaccines, mediated by antibodies directed against platelet factor 4 (PF4). Given their causal role in VITT, identification of the molecular composition of anti-PF4 antibodies is crucial for developing better diagnostics and treatments. Here, we utilised a novel proteomic workflow to analyse the immunoglobulin variable (IgV) region composition of anti-PF4 antibodies at the level of the secreted proteome. Serum anti-PF4 IgG antibodies from five patients with VITT triggered by ChAdOx1 nCoV-19 vaccination were affinity purified by PF4-coupled magnetic beads and sequenced by mass spectrometry. We revealed a single IgG heavy (H)-chain species paired with a single lambda light (L)-chain species in all five unrelated patients. Remarkably, all L-chains were encoded by the identical IGLV3-21*02 gene subfamily with identical L-chain third complementarity determining region (LCDR3) lengths. Moreover, striking stereotypic features were also identified in heavy-chains anti-PF4 antibodies characterised by identical HCDR3 length and homologous sequences. In summary, we unravelled the molecular signature of highly stereotyped clonotypic anti-PF4 antibodies, indicating shared pathways of antibody production in VITT patients. These discoveries are critical to understand the molecular basis of this serious condition and develop novel therapies aimed at removing pathogenic clones.