Hypothesis: Senescent stromal cells actively secrete a hematopoietic niche‑supportive SASP (e.g., G‑CSF, CXCL12, IL‑7) that boosts stem‑cell mobilization and immune competence of kin, providing an early‑life fitness benefit. This beneficial program is evolutionarily retained, but with age the same senescent cells shift toward a pro‑inflammatory SASP (IL‑6, IL‑1α, TNF‑α) due to cumulative DNA damage that activates cGAS‑STING and H2A.J‑dependent transcription, converting a supportive signal into chronic inflammaging. Thus aging can be viewed as a dysregulation of a normally adaptive senescence‑derived niche signal.

Mechanistic insight: CD36‑Src‑p38‑NF‑κB initiates the SASP transcriptional program [2]. In early senescence, the histone variant H2A.J accumulates and preferentially drives expression of niche‑supportive genes [4]. Persistent cytosolic DNA activates cGAS‑STING, which skews the secretome toward IL‑6/IL‑1α and sustains interferon signaling [3]. SASP‑induced CD38 expression in neighboring cells depletes NAD+, linking metabolic state to signal quality [5]. The balance between H2A.J‑driven and cGAS‑STING‑driven transcription determines whether the SASP supports hematopoiesis or drives inflammaging.

Predictions:

1. In young mice, inducible senescence in mesenchymal stromal cells (using p16‑3MR) will raise plasma G‑CSF and CXCL12, increase Lin⁻Sca1⁺cKit⁺ HSC frequencies, and enhance competitive repopulation; neutralizing G‑CSF or CXCL12 will block this effect without lowering IL‑6 levels.
2. Inducing DNA damage or overexpressing cGAS‑STING in senescent stromal cells will shift the secretome from G‑CSF/CXCL12 to IL‑6/IL‑1α, correlating with reduced HSC support and higher serum TNF‑α.
3. Acute senolytic treatment (dasatinib+quercetin) in young mice will transiently decrease HSC numbers and impair clearance of Listeria monocytogenes, whereas chronic senolysis in aged mice will improve both HSC function and lower inflammaging markers.
4. Flow cytometry will show that hematopoietic progenitors adjacent to senescent stromal cells exhibit reduced NAD+ fluorescence proportional to the fraction of senescent cells expressing H2A.J versus cGAS‑STING activation.

Experimental approach: Generate p16‑3MR;CreERT2 mice with Cre driven by Pdgfrβ (mesenchymal stromal). Treat 8‑week‑old mice with 4‑OHT to induce senescence, collect plasma for multiplex ELISA (G‑CSF, CXCL12, IL‑6, IL‑1α, TNF‑α) [1]. Perform competitive bone‑marrow transplants using CD45.1 donor cells into CD45.2 recipients, measure chimerism at 4 and 8 weeks. Block G‑CSF or CXCL12 with antibodies. In parallel, cross p16‑3MR mice with Sting⁻/⁻ or H2A.J‑knockout alleles, or treat with NAD+ precursor (NR) to test metabolic arm. Apply senolytics (D+Q) at young (8 weeks) and aged (20 months) cohorts, assay HSCs and serum cytokines after 2 weeks. Falsifiability: If senescent stromal cells fail to elevate G‑CSF/CXCL12 or if blocking these cytokines does not diminish HSC mobilization despite robust senescence induction, the core prediction is refuted. Likewise, if senolysis in young animals does not cause a transient HSC dip, the link between senescence‑derived niche support and hematopoiesis is unsupported.

Citations: SASP and inflammaging [1]; CD36 upstream [2]; cGAS‑STING sensing [3]; H2A.J epigenetic regulation [4]; CD38‑mediated NAD+ depletion [5]; senolytic reduction of inflammaging [9]; tissue‑specific senescent accumulation [6][7][8].