IF ex vivo treatment with sub-inhibitory bafilomycin A1 (BafA1; 0.5–2.0 nM for 48 hours) in serum-free cytokine-supplemented medium (SCF + TPO + Flt3L) is applied to aged (20–24 month) C57BL/6J Lin⁻Sca1⁺c-Kit⁺ (LSK) hematopoietic stem cells prior to competitive transplantation into lethally irradiated (10.5 Gy split-dose) aged (18–20 month) CD45.1/CD45.2 F1 recipients,

THEN donor CD45.2⁺ chimerism will increase from <10% (untreated aged HSC baseline) to ≥40% at 16 weeks post-transplant with balanced multilineage reconstitution (myeloid/B-cell/T-cell ratios approaching young-donor reference values) and sustained secondary transplantation capacity,

BECAUSE the following causal chain repairs already-accumulated lysosomal damage in aged HSCs:

1. Aged murine HSCs accumulate a hyperacidic lysosomal state (pH <4.0 vs. physiologic 4.5–5.0), representing a stable, self-reinforcing form of organelle damage driven by chronic V-ATPase overactivity and accumulated undegraded luminal cargo that alters buffering capacity — documented in the Evidence Set literature review as a primary hallmark of LSK aging in 20–24 month C57BL/6J mice.

2. Lysosomes in senescent cells actively process and accumulate chromatin fragments — including the histone cleavage product H3cs1 — within their lumen; BafA1 treatment at standard doses blocks this accumulation and elevates LC3-II, confirming that lysosomal content and autophagic flux are acutely sensitive to V-ATPase inhibition (Lysosome-mediated chromatin processing in senescence)[https://doi.org/10.1083/jcb.201212110]. [SPECULATIVE] In aged HSCs, analogous accumulation of undegraded chromatin fragments, protein aggregates, and oxidized lipids within the lysosomal lumen may itself constitute a primary acidification driver — a "substrate-driven pH collapse" — by consuming buffering capacity and activating compensatory V-ATPase upregulation.

3. Sub-inhibitory BafA1 (0.5–2.0 nM) partially inhibits the V0 c-ring of V-ATPase without complete ablation of proton pumping, titrating luminal pH from <4.0 toward 4.5–5.0 — the narrow window of optimal cathepsin B, D, and L activity — rather than alkalinizing the lysosome beyond function, as documented for higher concentrations (>10 nM) that fully block autophagic flux and induce toxicity, as described in the Evidence Set literature review citing the autophagy monitoring guidelines.

4. Restoration of physiologic lysosomal pH reactivates resident hydrolases (cathepsins) to clear the accumulated undegraded substrate burden — including the chromatin-derived and protein-aggregate cargo that drove the hyperacidic state — constituting genuine repair of accumulated lysosomal content damage rather than mere pH buffering (Lysosome-mediated chromatin processing)[https://doi.org/10.1083/jcb.201212110].

5. Cleared lysosomal cargo restores autophagic flux (normalization of LC3-II/I ratio and p62 turnover), downstream proteostasis, and mitochondrial quality control in aged HSCs — as descri...

SENS category: LysoSENS

Key references: • doi.org/10.1083/jcb.201212110]. • doi.org/10.1083/jcb.201212110] • doi.org/10.1083/jcb.201212110],