Hypothesis

Physiological eosinophils in visceral adipose tissue produce lipoxin A4 (LXA4) that circulates to injured skeletal muscle, where it enhances pro‑resolving signaling and supports the MERTK+ macrophage–satellite cell niche, thereby improving regeneration without triggering hyper‑eosinophilia‑mediated damage.

Rationale

Aged mice show depleted adipose eosinophils and concomitant rises in pro‑inflammatory macrophages and CCL11 [3]. Transfer of young eosinophils restores adipose homeostasis and systemic fitness but does not directly increase muscle mass, suggesting an indirect mechanism [3]. Aged muscle also lacks specialized pro‑resolving mediators such as LXA4 and resolvin D6 after injury, which impairs resolution and promotes maladaptive remodeling [4]. Eosinophils are known to generate IL‑4/IL‑13 that activate fibro‑adipogenic progenitors to aid muscle stem cell proliferation [1]; however, they also express 15‑lipoxygenase enzymes capable of synthesizing LXA4 from arachidonic acid. This dual capacity positions eosinophils as a potential source of LXA4 that could bridge adipose immunomodulation and muscle repair.

Mechanistic Model

1. Adipose eosinophil activation – In homeostatic conditions, tissue‑resident eosinophils are stimulated by IL‑33 and adipocytes to release IL‑4/IL‑13 and LXA4.
2. Systemic LXA4 rise – LXA4 enters the circulation, reaching skeletal muscle where it binds the ALX/FPR2 receptor on macrophages and satellite cells.
3. Macrophage reprogramming – LXA4 skews infiltrating monocytes toward a MERTK+ phenotype that sustains satellite cell expansion, consistent with the newly described niche [5].
4. Satellite cell support – Direct LXA4 signaling on satellite cells enhances their proliferation and reduces senescence, augmenting the regenerative pool.
5. Feedback control – Elevated LXA4 limits eosinophil recruitment via negative feedback on IL‑5 signaling, preventing the transition to hyper‑eosinophilia and perforin‑mediated damage [2].

Testable Predictions

• Prediction 1: Aged mice with adipose‑specific eosinophil restoration will exhibit increased plasma LXA4 levels compared with aged controls.
• Prediction 2: Neutralizing LXA4 with an ALX/FPR2 antagonist will abolish the functional benefits (grip strength, endurance) of eosinophil transfer without affecting adipose eosinophil numbers.
• Prediction 3: Muscle from eosinophil‑treated aged mice will show higher MERTK+ macrophage frequency and greater satellite cell Ki‑67 staining than untreated aged muscle.
• Prediction 4: Inducing hyper‑eosinophilia via IL‑5 overexpression will elevate tissue perforin but will not further increase LXA4, confirming a threshold beyond which eosinophil‑derived LXA4 production plateaus.

Experimental Approach

1. Model: Use aged (20‑month) C57BL/6 mice; generate adipose‑specific eosinophil rescue via IL‑33‑linked DTR system or adoptive transfer of sorted eosinophils from young donors.
2. Readouts: Quantify plasma and tissue LXA4 by LC‑MS/MS; flow cytometry for adipose eosinophils, muscle MERTK+ macrophages (CD45+CD11b+F4/80+MERTK+), and satellite cells (Pax7+MyoD-). Assess muscle regeneration after cardiotoxin injury (central nucleation, fiber CSA) and functional performance (grip strength, treadmill endurance).
3. Interventions: Administer ALX/FPR2 antagonist (Boc‑2) or eosinophil‑specific 15‑LOX inhibitor (PD146176) to test necessity of LXA4.
4. Controls: Include young mice, aged mice receiving PBS, and aged mice with IL‑5–driven hyper‑eosinophilia.
5. Falsifiability: If eosinophil rescue does not raise plasma LXA4, or if LXA4 blockade fails to impair the functional improvement despite restored eosinophil numbers, the hypothesis is refuted.

This framework integrates eosinophil immunomodulation, specialized pro‑resolving lipid signaling, and the macrophage‑satellite cell axis, offering a concrete, falsifiable route to explain how adipose tissue eosinophils influence muscle aging.