"This study shows, with molecular precision, how a normal immune response to an adenovirus can very rarely go off‑track. By identifying the exact viral protein involved and the specific antibody change that drives this misdirection, we now understand not only what happens in VITT but why."
What's exciting is that we can now point to a specific viral component that can be redesigned. It means future adenoviral vaccines can keep all their advantages while sidestepping the rare immune misfire that causes VITT."
Theodore Warkentin, - Professor emeritus, Department of Pathology & Molecular Medicine, McMaster University.
A global research collaboration of scientists from McMaster University (Canada), Flinders University (Australia) and Universitätsmedizin Greifswald (Germany) uncovered why a small number of people developed dangerous blood clots after either receiving certain COVID‑19 vaccines or experiencing a natural adenovirus infection, and the answer lies in an unexpected case of misdirected targeting by the immune system.
This study explains how and why the body occasionally generates dangerous antibodies against its own blood proteins causing vaccine-induced immune thrombocytopenia and thrombosis (VITT). Specifically, the study identifies the exact viral component that can, in rare circumstances, trigger this immune misdirection. Further, the study identifies a novel mechanism by which an immune reaction can go wrong, giving scientists a roadmap for explaining other rare, antibody-driven adverse reactions to certain infections, medications, or environmental exposures.
The researchers found that VITT can happen after a repeat exposure to adenovirus, whether from a vaccine or a natural infection, but only in people who have a certain inherited version of an antibody gene (IGLV3‑21*02 or *03). Because this gene variant is found in up to 60 per cent of the population, it cannot by itself account for the ultrarare complication.
However, the immune response that sets the stage for VITT is aimed at an adenovirus protein called protein VII (pVII), which happens to closely resemble a region of a human blood protein, platelet factor 4 (PF4). In very rare cases, while the immune system is responding to pVII, a single, specific mutation can arise in one of the antibody‑producing cells. That mutation (called K31E) changes just one positively-charged amino acid to a negatively-charged amino acid, and that tiny shift is enough to redirect the antibody targeting away from pVII and toward PF4 instead. Once the antibody binds PF4, it activates platelets and triggers the clotting and low platelet counts seen in VITT.
Crucially, the study detected this same K31E mutation in all VITT patient antibodies examined. When researchers reversed the mutation in lab‑engineered antibodies, their dangerous activity disappeared, proving that this specific change is required for the complication to occur.
What's exciting is that we can now point to a specific viral component that can be redesigned. It means future adenoviral vaccines can keep all their advantages while sidestepping the rare immune misfire that causes VITT."
Theodore Warkentin, - Professor emeritus, Department of Pathology & Molecular Medicine, McMaster University.
A global research collaboration of scientists from McMaster University (Canada), Flinders University (Australia) and Universitätsmedizin Greifswald (Germany) uncovered why a small number of people developed dangerous blood clots after either receiving certain COVID‑19 vaccines or experiencing a natural adenovirus infection, and the answer lies in an unexpected case of misdirected targeting by the immune system.
This study explains how and why the body occasionally generates dangerous antibodies against its own blood proteins causing vaccine-induced immune thrombocytopenia and thrombosis (VITT). Specifically, the study identifies the exact viral component that can, in rare circumstances, trigger this immune misdirection. Further, the study identifies a novel mechanism by which an immune reaction can go wrong, giving scientists a roadmap for explaining other rare, antibody-driven adverse reactions to certain infections, medications, or environmental exposures.
The researchers found that VITT can happen after a repeat exposure to adenovirus, whether from a vaccine or a natural infection, but only in people who have a certain inherited version of an antibody gene (IGLV3‑21*02 or *03). Because this gene variant is found in up to 60 per cent of the population, it cannot by itself account for the ultrarare complication.
However, the immune response that sets the stage for VITT is aimed at an adenovirus protein called protein VII (pVII), which happens to closely resemble a region of a human blood protein, platelet factor 4 (PF4). In very rare cases, while the immune system is responding to pVII, a single, specific mutation can arise in one of the antibody‑producing cells. That mutation (called K31E) changes just one positively-charged amino acid to a negatively-charged amino acid, and that tiny shift is enough to redirect the antibody targeting away from pVII and toward PF4 instead. Once the antibody binds PF4, it activates platelets and triggers the clotting and low platelet counts seen in VITT.
Crucially, the study detected this same K31E mutation in all VITT patient antibodies examined. When researchers reversed the mutation in lab‑engineered antibodies, their dangerous activity disappeared, proving that this specific change is required for the complication to occur.
Why it matters
This discovery answers five long‑standing questions about VITT:- why adenoviral‑vector vaccines - and natural adenovirus infection - can trigger it
- why PF4 is the target (mimicry between pVII and PF4)
- why VITT is extraordinarily rare (it requires a specific, chance mutation in a predisposed person);
- why incidence differs between populations (the involved antibody gene is more common in people of European ancestry) and
- why many cases occurred after a first vaccine dose (it stems from boosting pre‑existing anti‑pVII immunity from low baseline antibody levels).
