Hypothesis\n\nAged eosinophils actively promote skeletal muscle fibrosis by co‑delivering IL‑4/IL‑13 and mitochondrial DNA that together hyper‑activate STAT6 and TLR9 pathways in fibro/adipogenic progenitors (FAPs), converting them into collagen‑secreting myofibroblasts.\n\n## Mechanistic rationale\n\n- In young muscle, eosinophil‑derived IL‑4 primes FAPs to support regeneration (1).\n- With age, eosinophils increase IL‑4/IL‑13 production and undergo a NETosis‑like release of mitochondrial DNA (mtDNA) (observed in other aged immune cells; see 3 for eosinophil redistribution).\n- mtDNA engages TLR9 on FAPs, leading to NF‑κB‑dependent priming that lowers the threshold for STAT6 activation by IL‑4/IL‑13.\n- Sustained STAT6 signaling drives expression of fibrogenic genes (Col1a1, Acta2, Tgfb1) and suppresses adipogenic programs, shifting FAPs toward a scar‑forming phenotype.\n- This synopsis explains why transferring young eosinophils only partially rescues homeostasis (3): young eosinophils lack the mtDNA trigger, whereas aged eosinophils provide both signals.\n\n## Testable predictions\n\n1. Eosinophil IL‑4/IL‑13 and mtDNA are elevated in aged muscle.\n - Flow cytometry and intracellular staining will show higher IL‑4/IL‑13+ eosinophils in 24‑month vs 3‑month mice.\n - Mitochondrial DNA in eosinophil supernatants will be quantified by qPCR for Mt‑Nd1 and will be >3‑fold higher in aged samples.\n\n2. STAT6 and TLR9 co‑activation in FAPs is required for fibrotic shift.\n - FAP‑specific Stat6 knockout mice will exhibit reduced collagen deposition despite aged eosinophil presence.\n - TLR9 antagonism (ODN 2088) will blunt STAT6 phosphorylation and downstream fibrotic gene expression in cultured FAPs treated with aged eosinophil conditioned medium.\n\n3. Blocking either signal ameliorates age‑related fibrosis.\n - Administration of anti‑IL‑4Rα antibody (dupilumab analog) to aged mice will lower muscle hydroxyproline content by ~40%.\n - Inhaled DNase I (to degrade extracellular mtDNA) will similarly reduce fibrosis, with additive effect when combined with anti‑IL‑4Rα.\n\n4. Adoptive transfer experiments discriminate signals.\n - Transfer of eosinophils from Il4‑/‑ donors into aged wild‑type mice will not transmit fibrosis, confirming IL‑4 dependence.\n - Transfer of eosinophils from Tlr9‑/‑ donors will also fail to induce fibrosis, confirming mtDNA/TLR9 requirement.\n\n## Potential impact\n\nIf validated, this hypothesis repositions eosinophils not as mere bystanders of inflammaging but as active engineers of muscle fibrosis through a dual‑signal mechanism. Therapeutically, targeting the IL‑4/TLR9 axis in eosinophils or their downstream FAP responses could decouple immune aging from tissue degeneration, offering a precision strategy to delay sarcopenia and related frailty.\n\n## References (inline)\n\n[1] https://pmc.ncbi.nlm.nih.gov/articles/PMC3663598/\n[2] https://pmc.ncbi.nlm.nih.gov/articles/PMC11971228/\n[3] https://pubmed.ncbi.nlm.nih.gov/32694825/\n[4] https://pmc.ncbi.nlm.nih.gov/articles/PMC4185228/\n[5] https://pubmed.ncbi.nlm.nih.gov/35333325/\n[6] https://doi.org/10.1007/s11357-022-00572-w