Hypothesis

Senescent immune cells, particularly GZMK+ CD8+ T cells and macrophages, release extracellular vesicles (EVs) enriched in SASP factors and mitochondrial DNA that infiltrate stem cell niches. These EVs induce oxidative stress and telomere attrition in niche stromal cells, which precedes and drives stem cell senescence. Blocking EV biogenesis in senescent immune cells will preserve niche telomere length, maintain regenerative capacity, and extend healthspan independent of other interventions.

Mechanistic Rationale

1. EV-mediated SASP delivery – Senescent immune cells secrete EVs containing IL‑6, IL‑1β, GZMK, and oxidized mtDNA (3, 4). Unlike soluble SASP, EVs protect cargo from degradation and enable targeted delivery to nearby stromal cells.
2. Mitochondrial damage transfer – EV‑carried mtDNA triggers cGAS‑STING activation in niche fibroblasts, leading to ROS production and telomere shortening (5). This links immune senescence directly to the observed earlier telomere attrition in niche cells versus stem cells (2).
3. Feedback amplification – Telomere‑damaged niche cells secrete their own SASP, further recruiting and senescing immune cells, creating a self‑propagating loop that accelerates inflammaging via NF‑κB (6).

Testable Predictions

• Prediction 1: In aged mice, pharmacological inhibition of nSMase2 (using GW4869) to block EV release from senescent CD8+ T cells will reduce EV-associated mtDNA in intestinal niche fibroblasts by >50% (measured by qPCR for mitochondrial ND1).
• Prediction 2: Mice treated with GW4869 from 12 to 24 months will exhibit niche stromal telomere lengths comparable to 6‑month controls (Telomere Q‑FISH), while stem cell telomeres remain unchanged.
• Prediction 3: EV‑blocked mice will show improved crypt regeneration after DSS‑induced injury (increased Ki‑67+ stem cells) and delayed onset of age‑related histologic frailty scores.
• Prediction 4: Lifespan analysis will reveal a significant extension of median survival (>15%) in GW4869‑treated mice versus vehicle, without alterations in immune cell counts.

Experimental Approach

• Use CD8‑Cre;Rosa26‑LSL‑tdTomato mice to track senescent CD8+ T cells and isolate EVs from sorted tdTomato+ cells.
• Administer GW4869 (10 mg/kg i.p., three times weekly) or vehicle.
• Assess EV cargo (Western blot for CD63, GZMK; mtDNA qPCR), niche fibroblast telomere length, ROS (DHE staining), and SASP (Luminex).
• Functional readouts: colonic crypt regeneration, grip strength, treadmill endurance, and frailty index.

Falsifiability

If EV inhibition fails to reduce niche telomere attrition or improve regenerative outcomes despite confirmed blockade of immune‑cell EV release, the hypothesis that immune‑derived EVs are a primary driver of niche aging would be refuted. Conversely, success would support targeting immune‑cell EV biogenesis as a proximate intervention to delay aging‑related tissue decline.