Eosinophils as proteostasis sentinels: shifting soluble oligomers to inert aggregates as an age‑dependent adaptive mechanism

Hypothesis

Eosinophils act as proteostasis sentinels that recognize cells burdened with exposed hydrophobic patches on misfolded proteins and steer them toward the formation of inert, amyloid‑like aggregates as a final containment strategy.

Mechanistic Basis

Aged tissues lose eosinophils, weakening this surveillance and allowing soluble oligomers to accumulate, which triggers chronic inflammation and impairs regeneration [1].

Eosinophils release IL‑4 and IL‑13, signaling pathways known to upregulate autophagy and chaperone expression in neighboring mesenchymal cells [3]. We propose that these cytokines also induce a transient increase in intracellular concentrations of small heat‑shock proteins (sHSPs) that bind exposed hydrophobic regions on nascent oligomers, templating their conversion into β‑sheet‑rich, insoluble deposits that are less toxic than the soluble precursors.

In parallel, eosinophil‑derived major basic protein (MBP) and eosinophil peroxidase (EPO) can modestly cross‑link extracellular matrix components, creating a permissive niche where aggregates can be sequestered without disrupting tissue architecture. This mirrors the protective role of eosinophils in acute muscle repair, where their transient presence supports regeneration, while prolonged eosinophilia drives fibrosis [4]. Eosinophil survival depends on IL‑5 signaling [5]. The age‑related eosinophil decline thus shifts the balance from a controlled aggregation response to uncontrolled oligomer buildup.

Testable Predictions

1. In young mice, eosinophil‑deficient models (e.g., ΔdblGATA‑1 or anti‑IL‑5 treatment) will show higher levels of soluble, oligomer‑specific immunoreactivity (e.g., A11 antibody) and lower levels of insoluble, Thioflavin‑S‑positive deposits in visceral adipose tissue and skeletal muscle compared with wild‑type controls.
2. Adoptive transfer of young eosinophils into aged mice will increase Thioflavin‑S‑positive signal without increasing soluble oligomer load, concomitantly reducing pro‑inflammatory cytokines (IL‑6, TNF‑α) and improving grip strength and treadmill endurance, as seen in rejuvenation experiments [2].
3. Blocking IL‑4Rα signaling during eosinophil reconstitution will prevent the shift toward insoluble aggregates and abolish the anti‑inflammatory benefit, indicating that IL‑4/IL‑13 is required for the proteostatic reprogramming.
4. In vitro, co‑culture of eosinophils with pre‑adipocytes exposed to oxidative stress will increase intracellular HSPB1/HSPB5 expression and promote the formation of Thioflavin‑positive aggregates that are resistant to detergent extraction, an effect blocked by IL‑4Rα antibodies.

Experimental Approach

• Generate eosinophil‑deficient aged mice via IL‑5 knockout or antibody depletion; quantify soluble oligomers (dot blot with A11) and insoluble aggregates (Thioflavin‑S, filter‑trap assay) in epididymal fat and gastrocnemius.
• Perform adoptive transfer of sorted eosinophils from young donors into aged recipients; assess aggregate load, cytokine profile (Luminex), and functional outcomes (grip strength, rotarod, voluntary wheel running).
• Use IL‑4Rα blockade (anti‑IL‑4Rα) during transfer to dissect cytokine dependence.
• In vitro, isolate bone‑marrow derived eosinophils, culture with 3T3‑L1 pre‑adipocytes treated with H2O2; measure HSPB1/B5 by Western blot, aggregate formation by filter‑trap, and cell death by Annexin V/PI.

If eosinophils indeed steer misfolded proteins toward inert aggregates, boosting their numbers or IL‑4/IL‑13 signaling in aged tissue should convert toxic soluble species into less harmful deposits, alleviating inflammaging and restoring regenerative capacity. Conversely, if eosinophil augmentation fails to change the soluble/insoluble ratio or worsens pathology, the hypothesis would be falsified.