Aggregation of coagulation factors as an age‑dependent proteostatic buffer that modulates thrombotic risk

Hypothesis

With advancing age, rising plasma levels of FVIII and vWF drive the formation of ordered, amyloid‑like aggregates that sequester a fraction of these procoagulant proteins. Rather than representing pathologic debris, these aggregates act as a proteostatic safety valve that limits the free, thrombin‑generating pool. When aggregate formation or clearance fails, the buffered capacity is exceeded and thrombotic risk rises.

Mechanistic Basis

1. Nucleation trigger – Age‑associated oxidative modifications and glycation expose hydrophobic patches on FVIII and vWF, promoting self‑association. Heparan sulfate proteoglycans and extracellular polyphosphate released from stressed endothelium or platelets provide a anionic scaffold that aligns monomers into β‑sheet‑rich nuclei, similar to the seeding seen in tau aggregation [https://pmc.ncbi.nlm.nih.gov/articles/PMC6907691/].
2. Aggregate structure – The resulting fibrils are thermodynamically stable, resistant to proteolysis, and display Congo red positivity, indicating a cross‑β architecture. This ordered state reduces the conformational flexibility required for cofactor activity, effectively silencing the protein’s procoagulant function.
3. Cellular handling – Aggregates are opsonized by complement C1q and cleared via macrophage scavenger receptors (SR‑A, CD36). Aging diminishes macrophage phagocytic efficiency, shifting the balance toward accumulation [https://pubmed.ncbi.nlm.nih.gov/25099191/].
4. Functional outcome – Soluble FVIII/vWF sustain thrombin burst; aggregated forms are inert. Thus total antigen levels may remain high while the effective coagulant activity is lower, explaining the discordance between biomarker elevation and clinical thrombosis in some elderly individuals.

Testable Predictions

• Prediction 1: Elderly plasma will contain a detectable, sedimentation‑resistant fraction of FVIII and vWF that correlates inversely with thrombin‑generation potential (measured by calibrated automated thrombography).
• Prediction 2: In vitro incubation of purified FVIII/vWF with oxidative stressors and polyP will produce Congo‑positive fibrils that lose cofactor activity in a dose‑dependent manner.
• Prediction 3: Genetic or pharmacological enhancement of macrophage aggregate clearance (e.g., via TLR2 agonism) in aged mice will reduce plasma aggregate load and increase thrombin generation without altering total FVIII/vWF concentrations.
• Prediction 4: Passive transfer of isolated aggregates from old to young mice will transiently protect against induced venous thrombosis, whereas depletion of aggregates from old plasma will exacerbate clot formation.

Experimental Approach

1. Fractionation – Subject plasma from young (3‑mo) and old (24‑mo) mice, and human donors across decades, to sequential centrifugation (2,000 × g, 10 000 × g, 100 000 × g). Analyze supernatants and pellets by ELISA for FVIII/vWF, Western blot for high‑molecular‑weight species, and Thioflavin T fluorescence.
2. Functional assays – Measure thrombin generation in platelet‑poor plasma before and after aggregate depletion (using high‑speed filtration). Correlate residual activity with soluble antigen levels.
3. Seeding experiments – Add purified fibrils to naïve FVIII/vWF solutions; monitor aggregation kinetics via ThT and loss of activity.
4. In vivo validation – Treat aged mice with clodronate liposomes to deplete macrophages or with an antibody that blocks SR‑A; assess plasma aggregate burden, thrombin generation, and incidence of ferric chloride‑induced carotid thrombosis.
5. Human relevance – Use size‑exclusion chromatography on plasma from cohorts with documented venous thromboembolism versus age‑matched controls; relate aggregate peaks to clinical outcomes.

If aggregates are protective, interventions that increase their formation or stability should lower thrombotic risk despite high total FVIII/vWF, whereas strategies that dissolve them will raise risk. This framework converts a seemingly maladaptive protein‑misfolding phenomenon into a regulatable hemostatic checkpoint, offering a novel axis for age‑specific antithrombotic therapy.