Adenoviral Core Protein VII-Driven Cross-Reactivity of Anti-PF4 Antibodies as a Trigger for VITT Pathogenesis

Background: Vaccine-induced immune thrombocytopenia and thrombosis (VITT) is a rare prothrombotic complication following adenoviral vector-based COVID-19 vaccination or natural adenovirus infection. The underlying pathogenesis involves antibodies against the highly cationic platelet factor 4 (PF4), but the initiating trigger and mechanisms remain unclear. Methods: We determined the amino acid sequences of anti-PF4 antibodies from 21 VITT patients using antibody proteomics and sequenced immunoglobulin light-chain hypervariable genes from 100 VITT patients. To identify an adenoviral trigger, we compared the antigen-binding profiles of anti-PF4 and anti-adenovirus protein antibodies to identify shared serological profiles. We then investigated, using adenovirus proteins, peptides and recombinant anti-PF4 antibodies, the minimal epitopes mediating this cross-reactivity and validated the findings by various in-vitro assays and a newly established mouse model. Results: Genomic and proteomic analysis of anti-PF4 antibodies revealed a shared immunoglobulin light-chain allele (IGLV3-21*02 or *03) containing a critical somatic hypermutation (K31E) – a switch from positively-charged lysine to negatively-charged glutamic acid. This mutation was exclusively found in anti-PF4 antibodies with the same amino acid fingerprint shared with antibodies directed against the adenovirus core protein VII (pVII) of the same individual. Antibodies against intact virions or other adenovirus proteins did not exhibit this pattern. Cross-reactivity was mapped to a basic linear epitope on pVII and PF4. We also established a mouse model for VITT, allowing testing of these antibodies. A pathogenic anti-PF4 antibody from a VITT patient lost its prothrombotic activity in vitro and in vivo after back-mutation to the germline sequence (K31) and demonstrated preferential binding to pVII, directly supporting the role of the mutation in shifting antigen binding from pVII to PF4. Conclusion: VITT occurs in individuals harboring the immunoglobulin light-chain allele 3-21*02 or *03 who develop antibodies during a secondary immune response against a specific epitope of the adenovirus core protein pVII. A characteristic somatic hypermutation (K31E) within these antibodies leads to misdirection of the immune response from pVII towards PF4, explaining the pathogenesis of VITT. Similar mechanisms may apply to a wide range of antibody mediated autoimmune disorders. Publication History Article published online: 17 February 2026 © 2026. Thieme. All rights reserved. Georg Thieme Verlag KG Oswald-Hesse-Straße 50, 70469 Stuttgart, Germany