Hypothesis: Eosinophil-derived IL-4/IL-13 triggers a fibro/adipogenic progenitor (FAP)–mediated burst of specialized pro‑resolving mediators that clear senescent satellite cells and amplify muscle regeneration in aged tissue

Core Idea

We propose that the regenerative benefit of eosinophil transfer in aged muscle is not limited to IL‑4/IL-13–driven FAP support of myogenesis but also depends on eosinophil‑induced FAP production of specialized pro‑resolving mediators (SPMs) such as resolvin D1 and protectin D1. These SPMs enhance efferocytic clearance of senescent satellite cells and muscle‑resident macrophages, thereby reducing the senescent‑associated secretory phenotype (SASP) and creating a permissive niche for satellite‑cell activation and proliferation.

Mechanistic Rationale

1. Eosinophil IL-4/IL-13 → FAP reprogramming – IL-4/IL-13 signaling in FAPs upregulates enzymes 15-LOX and COX-2, precursors for SPM synthesis (see IL-4 drives FAP activation).
2. SPM-mediated senescent cell clearance – Resolvin D1 binds GPR32 on macrophages, increasing phagocytic uptake of senescent satellite cells marked by phosphatidylserine exposure (supported by SPMs promote efferocytosis).
3. SASP attenuation – Clearance of senescent cells reduces IL-6, TGF-β, and MMP-9 secretion, lowering fibro‑adipogenic degeneration.
4. Amplified satellite-cell response – Lower SASP levels relieve inhibition of Pax7-MyoD axis, while SPMs also directly stimulate satellite-cell proliferation via ALX/FPR2 receptors (as shown in SPMs stimulate myoblast proliferation).

Testable Predictions

• Prediction 1: Aged mice receiving young eosinophil transfers will show a ≥2‑fold increase in resolvin D1 and protectin D1 levels in gastrocnemius homogenates compared with saline controls.
• Prediction 2: Pharmacological blockade of GPR32 (resolvin D1 receptor) will abolish the eosinophil‑transfer‑induced reduction in senescent satellite‑cell (p16^Ink4a^+ Pax7^+) frequency.
• Prediction 3: Combined eosinophil transfer with low‑dose IL-5 (to sustain eosinophil numbers) will yield greater gains in grip strength and fiber cross‑sectional area than eosinophil transfer alone, without inducing perforin‑mediated necrosis.
• Prediction 4: FAP-specific deletion of 15-LOX (Alox15) will prevent the eosinophil‑transfer‑mediated rise in SPMs and block improvements in satellite-cell activation, confirming the FAP‑SPM axis.

Experimental Design (brief)

• Use 24‑month‑old C57BL/6 mice; groups: (1) saline, (2) young eosinophil transfer, (3) eosinophil transfer + GPR32 antagonist, (4) eosinophil transfer + low‑dose IL-5, (5) FAP‑Alox15 KO + eosinophil transfer.
• Measure tissue SPMs by LC‑MS/MS, senescent satellite cells by flow cytometry (p16^Ink4a^+ Pax7^+), muscle force via grip strength and in‑vivo tetanic tension, histology for fibrosis (Masson’s trichrome) and fiber size.
• Statistical analysis: ANOVA with post‑hoc Tukey; n≥8 per group for power 0.8.

Falsifiability

If eosinophil transfer fails to elevate muscle SPMs, or if SPM receptor blockade does not senescent‑cell accumulation or functional gains, the hypothesis is refuted. Conversely, confirming the predictions would support a novel eosinophil‑FAP‑SPM‑senescence axis as a therapeutic lever for sarcopenia.

Broader Impact

Targeting the eosinophil‑FAP‑SPM pathway could allow modulation of muscle repair without the risks of broad IL-5 elevation, offering a safer route to counteract age‑related muscle wasting.