Hypothesis: Immune‑Cell Mitochondrial Export Fuels Senescence in Cardiac Progenitor Cells

Core proposition Aged immune cells do not merely fail to clear senescent CPCs; they actively donate damaged mitochondria and mitochondrial DNA (mtDNA) to neighboring CPCs via tunneling nanotubes (TNTs) and extracellular vesicles. This transfer induces length‑independent telomere damage, activates p21^CIP and p16^INK4a pathways, and amplifies the SASP, thereby converting the immune senescent phenotype into a paracrine driver of CPC exhaustion.

Mechanistic chain

1. Immunosenescent macrophages and neutrophils accumulate mtDNA lesions and produce high ROS.
2. These cells increase TNT formation (mediated by Miro‑1/Drp1‑dependent mitochondrial motility) and release mitochondria‑laden exosomes.
3. CPCs internalize the organelles; foreign mtDNA triggers cGAS‑STING sensing and cytosolic DNA damage responses independent of telomere length.
4. Resulting DDR activates p21^CIP and p16^INK4a, locking CPCs into senescence and boosting IL‑6, IL‑1β, TNF‑α secretion.
5. The heightened SASP further stimulates immune cells to produce more ROS‑laden mitochondria, creating a feed‑forward loop that accelerates myocardial SASP burden and fibrosis.

Novel insight beyond current literature While the seed idea frames immune failure as the cause of senescence accumulation, this hypothesis posits an active, organelle‑based sabotage: immune cells become vectors of mitochondrial stress that directly impose DNA damage on CPCs. It shifts the focus from cytokine‑only signaling to organelle transfer as a proximate mechanism linking immunosenescence to inflammaging in the cardiac niche.

Testable predictions

• Prediction 1: In aged mice, genetic inhibition of Drp1 in LysM^+ myeloid cells (LysM‑Cre;Drp1^fl/fl) will reduce mitochondrial transfer to c‑Kit^+ CPCs, lower γH2AX and p16^INK4a in CPCs, and diminish SASP despite unchanged circulating cytokine levels.
• Prediction 2: Pharmacologic blockade of TNT formation (using cytochalasin L low dose) or exosome release (GW4869) in aged wild‑type mice will recapitulate the protective effect on CPCs.
• Prediction 3: Adoptive transfer of young immune cells lacking functional mtDNA (ρ^0 cells) into aged hosts will not rescue CPC senescence, whereas transfer of wild‑type young immune cells will exacerbate it if mitochondrial transfer is intact.
• Prediction 4: Measuring extracellular mtDNA in cardiac interstitial fluid will correlate with CPC senescence markers across ages, and anticorrelation with Drp1 deficiency in myeloid cells.

Experimental outline

1. Generate LysM‑Cre;Drp1^fl/fl mice and aged controls; isolate cardiac non‑myocyte fraction; quantify TNTs via confocal microscopy (LifeAct‑GFP) and mitochondrial transfer via MitoTracker labeling.
2. Flow cytometry of c‑Kit^+ cells for p16^INK4A, SA‑β‑gal, γH2AX, and Ki67.
3. ELISA of cardiac tissue for IL‑6, IL‑1β, TNF‑α; histology for fibrosis (Masson's trichrome).
4. Functional read‑outs: echocardiography (EF, fractional shortening) and pressure‑volume loops after ischemia‑reperfusion.
5. Rescue experiments: inject WT young bone‑marrow–derived macrophages ± Drp1 knockdown into aged LysM‑Cre;Drp1^fl/fl mice to test sufficiency.

Falsifiability If Drp1 deficiency in myeloid cells fails to reduce CPC mitochondrial uptake, γH2AX, or SASP, or if cardiac function does not improve despite confirmed blockade of transfer, the hypothesis would be refuted. Conversely, a positive result would support the claim that immune‑derived mitochondrial transfer is a mechanistic bridge between immunosenescence and CPC‑driven cardiac aging.

References [1] https://doi.org/10.1038/s41586-021-03547-7 [2] https://doi.org/10.1038/s41467-018-07825-3 [3] https://doi.org/10.1111/acel.12931 [4] https://pmc.ncbi.nlm.nih.gov/articles/PMC7990367/ [5] https://doi.org/10.1073/pnas.1810692116 [6] https://pmc.ncbi.nlm.nih.gov/articles/PMC11233824/ [7] https://doi.org/10.15252/embj.2018100492