The aging gut microbiome accelerates host inflammaging not only by depleting butyrate but also by releasing extracellular vesicles that deliver senescence-inducing microRNAs to T cells. Recent work shows that loss of butyrate‑producing species weakens the intestinal barrier, promotes microbial translocation, and expands senescent T cells that drive gut‑brain axis dysfunction [1][2][3]. We hypothesize that aged microbiota shed membrane vesicles enriched in specific microRNAs (e.g., miR‑155, miR‑21, miR‑146a) which are taken up by peripheral and gut‑resident T cells, directly suppressing butyrate signaling pathways (FFAR3, HDAC inhibition) and activating mTOR/NF‑κB, thereby inducing a senomorphic phenotype independent of microbial translocation. This vesicle‑mediated mechanism would create a faster, cell‑autonomous route to inflammaging that parallels—but can operate alongside—the barrier‑leakage loop described in the seed idea.

Key predictions arise from this hypothesis:

1. Isolating extracellular vesicles from fecal samples of old mice (or humans) and administering them to young germ‑free recipients will recapitulate T‑cell senescence markers (p16^Ink4a^, SA‑β‑gal, SASP cytokines) and increase intestinal and blood‑brain barrier permeability without altering bacterial load.
2. Depleting vesicle production in aged microbiota—using knockout of vesicular trafficking genes in dominant Bacteroides strains or pharmacological inhibition of sphingomyelinase—will attenuate T‑cell senescence and barrier dysfunction despite low butyrate levels.
3. Neutralizing the candidate microRNAs within vesicles (using antagomirs or CRISPR‑based RNA targeting) will block the senescence‑inducing effect on T cells, rescuing barrier integrity and cognitive performance in recipient mice.
4. In humans, circulating levels of microbiota‑derived vesicles carrying miR‑155/miR‑21 will correlate with epigenetic age clocks (e.g., GrimAge) and inversely with fecal butyrate concentrations, independent of traditional inflammation markers (CRP, IL‑6).

Experimental approach: Collect feces from young (3 mo) and old (24 mo) C57BL/6 mice. Differentially ultracentrifuge to isolate vesicle fractions, confirm size and markers (CD63, CD81) by NANOSIGHT and Western blot. MicroRNA profiling via small‑RNA seq will identify enriched senescence‑associated miRNAs. Young germ‑free mice receive weekly oral gavage of old‑mouse vesicles, young‑mouse vesicles, or PBS. After 4 weeks assess: (a) colonic permeability (FITC‑dextran flux), (b) brain endothelial tight‑protein claudin‑5 immunohistochemistry, (c) T‑cell flow cytometry for p16 and SASP, (d) behavioral tests (Y‑maze, open‑field). Parallel groups receive vesicle‑plus‑anti‑miR‑155 antagomirs or GW4869 (vesicle release inhibitor).

If old‑mouse vesicles induce senescence and barrier loss in young germ‑free hosts, and miRNA blockade prevents these effects, the hypothesis is supported. Failure to observe vesicle‑driven phenotypes—or demonstration that vesicle effects disappear when butyrate is supplemented—would falsify the claim that microbiota‑derived vesicles are a primary, butyrate‑independent driver of inflammaging via T‑cell senescence. This framework extends the current loop by placing vesicle‑mediated epigenetic reprogramming at its core, offering a concrete, falsifiable target for interventions aimed at breaking the gut‑brain axis deterioration in aging.