Hypothesis

Clonal expansion of mitochondrial DNA (mtDNA) heteroplasmies in hematopoietic stem and progenitor cells (HSPCs) releases mtDNA‑containing mitochondrial‑derived vesicles (MDVs) into the cytosol and circulation. This extracellular mtDNA acts as a danger‑associated molecular pattern that activates the cGAS‑STING pathway in dendritic cells and macrophages, driving tonic type I interferon (IFN‑I) signaling. Chronic IFN‑I signaling induces transcriptional programs that upregulate immune‑checkpoint molecules (PD‑1, TIM‑3, LAG‑3) and shift cellular metabolism toward glycolysis, thereby precipitating T‑cell exhaustion and impairing HSPC function. The resulting immune exhaustion fuels a feed‑forward loop: IFN‑I‑mediated ROS production further damages mtDNA, increasing heteroplasmy and MDV release.

Mechanistic Reasoning

• mtDNA heteroplasmies accumulate clonally after age 60 via hematopoietic clonal expansion (1).
• Mitochondrial stress triggers MDV biogenesis, a known route for mtDNA release independent of overt mitochondrial rupture (3).
• Cytosolic mtDNA is a potent ligand for cGAS, leading to STING‑dependent IFN‑I production; this link is established in infection and autoimmunity but not yet shown in aging (2).
• Tonic IFN‑I signaling drives expression of exhaustion markers and metabolic reprogramming in lymphocytes (4).
• IFN‑I can augment ROS generation via NADPH oxidase activation, creating a oxidative milieu that exacerbates mtDNA damage, completing the loop.

Testable Predictions

1. Elevated mtDNA in extracellular vesicles: Plasma from aged mice (>20 mo) or humans (>60 y) will contain higher levels of mtDNA‑positive MDVs compared with young controls; this correlates with heteroplasmy burden in sorted HSPCs (5).
2. cGAS‑STING dependence: Genetic ablation of cGAS or STING in HSPCs will attenuate tonic IFN‑I signatures (ISG15, MX1) and reduce PD‑1/TIM‑3 expression on CD8⁺ T cells in aged mtDNA‑mutator mice.
3. Functional rescue: Targeted correction of pathogenic mtDNA mutations using DdCBE base editors in HSPCs will decrease MDV‑associated mtDNA, lower serum IFN‑β, and improve T‑cell proliferative capacity and vaccine response.
4. Pharmacological blockade: Inhibiting MDV formation (e.g., with GSK2334470 to block VPS35‑dependent retromer function) will mimic the genetic rescue, lowering circulating mtDNA and IFN‑I without altering nuclear genome.
5. Loop verification: Exogenous IFN‑I administration to young mice will increase mitochondrial ROS and elevate mtDNA heteroplasmy in HSPCs over 4‑week period, demonstrating the IFN‑I→mtDNA damage arm.

Falsifiability

If aged subjects show no increase in extracellular mtDNA‑positive MDVs, or if cGAS/STING loss fails to reduce IFN‑I signaling and exhaustion markers despite high mtDNA heteroplasmy, the hypothesis is refuted. Similarly, if mtDNA editing does not alter MDV release or IFN‑I levels, the causal link is unsupported.