Muscle-Resident Eosinophils Clear Senescent Cells via IL-5-Independent CCL24/Eotaxin-1 Signaling During Age-Related Regeneration

The hypothesis: Age-related decline of tissue-resident eosinophils in skeletal muscle impairs regeneration not through reduced systemic inflammation (as in adipose), but through loss of direct senescent cell clearance. We propose that physiological muscle eosinophils operate via an IL-5-independent axis—mediated by eotaxin-1 (CCL24) receptor engagement on eosinophils themselves—enabling them to recognize and eliminate senescent fibro-adipogenic progenitors (FAPs) and inflammatory myonuclei through senescence-associated engulfment pathways.

Mechanistic basis: Prior work shows young eosinophil transfers improve endurance without substantial muscle infiltration, which points toward systemic adipose-mediated effects. Yet other findings demonstrate that physiological eosinophil levels are actually required for muscle regeneration—this suggests we're dealing with distinct resident populations. We think muscle-resident eosinophils represent a separate compartment with reduced IL-5 dependency, maintained instead by local CCL24 autocrine signaling. During aging, declining muscle CCL24 production creates a self-reinforcing loss-of-eosinophil loop, allowing senescent FAP accumulation—this is distinct from the adipose-driven IL-6 effects others have described.

Testable predictions: First, muscle-resident eosinophils should show preferential CCL24 receptor (CCR3) expression compared to circulating eosinophils. Second, CCR3 antagonism ought to impair muscle regeneration in young mice without affecting circulating eosinophil counts. Third, aged mice will display reduced muscle CCL24 while systemic IL-5 remains intact. Fourth, senescent FAPs themselves likely express eotaxin-1, creating an eosinophil recruitment and activation gradient. Finally, eosinophil-deficient mice should accumulate p16INK4a+ cells in damaged muscle at greater rates than wild-type controls.

Falsifiability: If regeneration proceeds normally in CCR3-blocked young mice, or if senescent cell burden proves equivalent in eosinophil-deficient and control animals, the hypothesis falls apart. The IL-5-independence prediction would be falsified if IL-5 knockout mice show normal muscle eosinophil numbers and regeneration capacity.

This framework helps reconcile the apparent contradiction between indirect (adipose) and direct (muscle) eosinophil effects while addressing the relatively unexplored senescence-eosinophil axis in skeletal muscle.