Researchers have identified the molecular trigger behind the rare clotting disorder linked to adenovirus-based COVID-19 vaccines, tracing it to an immune response that mistakenly targets platelet factor 4 (PF4), a normal blood protein involved in clotting.
The study addresses vaccine-induced immune thrombocytopenia and thrombosis (VITT) – a rare but serious syndrome first identified in 2021 after rollout of adenoviral vector vaccines. Similar PF4-targeting antibodies have also been reported after natural adenovirus infection, suggesting the trigger lies in the virus itself rather than in a vaccine-specific ingredient.
To investigate this, the researchers combined antibody proteomics, immunogenetic sequencing, recombinant antibody engineering, and functional platelet assays. Using tandem mass spectrometry, they sequenced anti-PF4 antibodies from 21 patients with VITT and analyzed immunoglobulin light-chain genes from 100 patients to identify the shared molecular features of the clot-promoting response.
"A novel aspect of the paper was our use of powerful mass spectrometry sequencing to identify molecular mimicry between the adenovirus vector protein and the PF4 culprit target," explained Jing Jing Wang, first author of the study, in a press release. "This was the missing link that explains how a normal immune response can, in very rare cases, become harmful."
The team’s analysis identified adenoviral core protein VII (pVII) as the likely inciting antigen. Antibodies purified against pVII contained species that matched the characteristic anti-PF4 antibody fingerprint seen in VITT, whereas antibodies against intact viral particles or other adenoviral proteins did not. The data suggest that, in rare cases, an immune response initially directed against adenovirus can shift to target PF4.
The researchers also identified a strong genetic pattern in the anti-PF4 response. Nearly all patients carried the same immunoglobulin light-chain variant, and all 21 sequenced anti-PF4 antibodies shared a somatic mutation known as K31E. When the researchers reversed that mutation in recombinant antibodies, PF4 binding, platelet activation, and clot-promoting activity all fell sharply.
The findings provide researchers with a clearer picture of how this rare clotting reaction begins – and how it might be avoided in future vaccine design. “By modifying or removing this specific adenovirus protein, future vaccines can avoid this extremely rare reaction while continuing to provide strong protection against disease,” said Wang.
Future studies will be needed to assess whether modifying or replacing pVII can make adenoviral vaccine platforms safer without compromising their benefits.
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