Immune Senescence Propagates Brain Aging Through Microbiome‑Modulated Mitochondrial DAMP Transfer

The immune system does not merely reflect aging; it actively drives it by exporting senescence‑associated mitochondrial damage to distant tissues, especially the brain, via extracellular vesicles whose cargo is shaped by gut‑derived metabolites. In models where DNA repair is deleted specifically in immune cells, premature immune senescence induces systemic senescence and organ dysfunction 1. Senescent immune cells secrete IL‑6, IL-1β, TNF‑α and also release mitochondria‑containing vesicles that carry oxidized mtDNA and formyl peptides, potent damage‑associated molecular patterns (DAMPs) that can trigger inflammasome activation in recipient cells 2. Gut microbiota influence immune cell metabolism through short‑chain fatty acids (SCFAs) and tryptophan catabolites that modulate aryl hydrocarbon receptor (AhR) signaling, altering mitochondrial ROS production and vesicular release 3. We hypothesize that a microbiome‑dependent increase in mitochondrial DAMP loading of immune‑derived vesicles accelerates brain endothelial senescence, disrupts the blood‑brain barrier, and propagates neuronal senescence through paracrine NF‑κB signaling, thereby advancing epigenetic clock readings in cortical tissue 4.

Testable predictions:

1. Germ‑free or antibiotic‑treated mice receiving senescent immune cell transplants will show lower brain mitochondrial DAMP levels, reduced microglial activation, and slower epigenetic aging compared with colonized recipients.
2. Pharmacologic AhR antagonism in immune cells will decrease vesicular mtDNA export and attenuate brain senescence markers after immune cell transfer.
3. Inhibiting vesicle release (e.g., with GW4869) in senescent immune cells will block the trans‑induction of neuronal p16^INK4a^ and γH2AX in co‑culture and in vivo.

Experimental approach:

• Generate bone‑marrow chimeras where donor hematopoietic cells carry a Lgr5‑CreER;Dt‑fl‑fl cassette to inducibly delete Ercc1 (DNA repair) specifically in myeloid and lymphoid lineages, creating senescent immune cells 1.
• Transplant these cells into young mice that are either specific‑pathogen‑free, germ‑free, or treated with a broad‑spectrum antibiotic cocktail.
• At 4, 8, and 12 weeks post‑transfer, isolate brain microvasculature, microglia, and neurons to measure:
  • mtDNA content in plasma extracellular vesicles (qPCR for mtDNA‑ND1).
  • Vesicular DAMP signaling (Western blot for phosphorylated NLRP3, caspase‑1 cleavage).
  • Senescence markers (p16^INK4a^, SASP cytokines) via immunofluorescence and ELISA.
  • Epigenetic age using the mouse blood‑brain‑tissue clock 4.
• Parallel in vitro cultures of bone‑marrow‑derived macrophages treated with SCFAs or AhR agonists will quantify vesicle release and mtDNA cargo.

If microbiome modulation directly controls the pathogenic vesicular cargo of senescent immune cells, then altering gut flora or AhR signaling should uncouple immune senescence from brain aging, providing a mechanistic lever to test whether re‑programming immune surveillance alone can retard systemic aging before other interventions.