Hypothesis

Eosinophils, through IL-4 secretion, activate fibro/adipogenic progenitors (FAPs) to promote the conversion of soluble misfolded proteins into ordered, amyloid‑like aggregates during muscle repair. This aggregation serves as a sequestration strategy that limits the spread of toxic species while providing a scaffold for tissue remodeling.

Mechanistic rationale

1. Eosinophil‑derived IL-4 re‑programs FAPs – IL-4 drives FAPs toward a pro‑regenerative phenotype that enhances phagocytic clearance of necrotic debris 2. Recent work shows that activated FAPs upregulate molecular chaperones and components of the autophagy‑lysosome pathway, suggesting an expanded role in protein quality control.
2. IL-4 stimulates FAP expression of sequestrin‑like scaffolds – We propose that IL-4 signaling induces FAPs to secrete extracellular matrix proteins such as fibronectin and specific small leucine‑rich proteoglycans that nucleate amyloid‑like assembly. These scaffolds provide a templated surface where misfolded proteins can be safely deposited into β‑sheet‑rich aggregates.
3. Aggregates act as inert depots – Once sequestered, the ordered aggregates are resistant to proteolysis and sequester reactive oxygen species, reducing downstream cellular stress. This mirrors observations that cells actively divert misfolded proteins into membrane‑less condensates or amyloid bodies as a last‑resort protective measure 1.
4. Feedback to eosinophil survival – The formation of protective aggregates reduces local DAMPs, lowering the need for sustained eosinophil recruitment. Consequently, IL-5‑dependent eosinophil survival 5 becomes transient, preventing chronic type‑2 inflammation.

Predictions and experimental tests

• Prediction 1: In regenerating muscle of young mice, eosinophil depletion will reduce the formation of detergent‑insoluble, thioflavin‑T‑positive aggregates in the injury zone, while increasing diffuse ubiquitinated protein signals. Test: Deplete eosinophils using anti‑Siglec‑F antibodies prior to cardiotoxin injury; assess aggregate load by filter‑trap assay and immunofluorescence at 3‑ and 7‑days post‑injury.
• Prediction 2: Exogenous IL-4 will enhance FAP‑dependent aggregation of a model misfolded protein (e.g., mutant SOD1‑G93A) in co‑culture, an effect blocked by FAP‑specific knockout of the IL‑4Rα subunit. Test: Isolate FAPs from WT and IL‑4Rα‑fl/fl mice; co‑culture with eosinophils or IL-4; monitor aggregation of transfected SOD1‑G93A‑YFP via fluorescence microscopy and sucrose gradient fractionation.
• Prediction 3: Aged mice exhibit diminished eosinophil‑IL-4 signaling, correlating with lower aggregate formation and higher soluble toxic oligomers during muscle repair. Test: Compare eosinophil numbers, IL-4 levels, and aggregate biomarkers in young vs. aged mice after identical injury; rescue experiments with IL‑5 complex to restore eosinophil populations.

Falsifiability

If eosinophil depletion or IL-4 blockade does not alter the balance between soluble misfolded proteins and insoluble aggregates, or if FAP‑specific IL‑4Rα loss fails to affect aggregate formation despite intact eosinophil numbers, the hypothesis would be refuted. Conversely, a consistent shift toward increased diffuse toxic species upon disrupting the eosinophil‑IL‑4‑FAP axis would support the model.

Broader implications

Linking type‑2 immunity to proteostatic ordering reframes eosinophils not merely as debris‑clearers but as regulators of cellular decision‑making between degradation and sequestration. This perspective could explain why simply inhibiting aggregation in neurodegenerative contexts sometimes worsens outcomes: the cell may be dismantling a protective depot that eosinophil‑mediated pathways help maintain in tissues undergoing renewal.