IF bisphosphonate-conjugated PLGA nanoparticles loaded with Concanamycin A (NP-ConcA; targeting estimated bone marrow stromal bioavailability of 2–5 nM ConcA locally, administered i.v. at 1 µg/kg total ConcA equivalent, daily for 3 days prior to transplant) are used to selectively condition the aged bone marrow niche in 24-month-old female C57BL/6J recipient mice, in combination with ex vivo ConcA treatment (48h, 5–20 nM) of autologous aged LSK HSCs prior to transplantation,

THEN donor chimerism at 16 weeks post-transplant will reach ≥15-fold improvement over untreated aged-into-aged controls, ≥2-fold improvement over ex vivo ConcA HSC treatment alone, with restoration of myeloid:lymphoid ratios to ≤2:1 and measurable normalization of bone marrow stromal cell lysosomal pH (LysoSensor ratio shift ≥0.4 units), without the renal, hepatic, or CNS toxicity associated with systemic 10 µg/kg i.p. ConcA dosing,

BECAUSE the following causal chain is operative:

1. Aged bone marrow stromal cells, endothelial cells, and macrophages accumulate lysosomal hyperacidification and v-ATPase dysregulation independently of HSC-intrinsic aging, creating an inflammatory niche that impairs HSC lodgment regardless of donor cell quality (lysosomal dysfunction in aged niche cells drives elevated paracrine IL-6, TNF-α, and reduced VCAM-1/CXCL12) — established in the evidence set as a distinct, accumulated damage state in the aged microenvironment (Aged niche cells exhibit lysosomal dysfunction)[https://doi.org/10.1016/j.stem.2025.10.012].

2. Ex vivo ConcA treatment normalizes lysosomal pH in aged HSCs, suppresses cGAS-STING inflammatory signaling intrinsic to the stem cell, and achieves >8-fold improvement in long-term repopulation capacity compared to untreated aged HSCs — but this effect is ceiling-limited by an unreformed niche that still secretes senescence-associated inflammatory cytokines and expresses reduced HSC retention signals (>8-fold ex vivo improvement)[https://doi.org/10.1016/j.stem.2025.10.012].

3. Bisphosphonate surface conjugation on PLGA nanoparticles enables selective accumulation in hydroxyapatite-rich bone marrow at >10-fold higher concentration than soft tissues, achieving local nanomolar ConcA bioavailability at the niche while maintaining sub-toxic systemic concentrations — a cross-discipline drug delivery strategy established in oncology for bone-targeted delivery of cytotoxics [SPECULATIVE for ConcA specifically; bisphosphonate bone targeting is established technology].

4. Local niche v-ATPase inhibition at sub-toxic doses normalizes lysosomal pH specifically in stromal and endothelial cells, reduces their cytosolic DNA accumulation and cGAS-STING activation, suppresses paracrine IL-6/TNF-α secretion, and restores CXCL12 and VCAM-1 surface expression required for HSC homing, lodgment, and engraftment niche formation [SPECULATIVE: extrapolated from HSC-compartment mechanism described in evidence set].

5. Transplantation of ex vivo Co...

SENS category: LysoSENS

Key references: • doi.org/10.1016/j.stem.2025.10.012]. • doi.org/10.1016/j.stem.2025.10.012] • doi.org/10.1016/j.stem.2025.10.012];