Hypothesis

Chronic mTOR inhibition in aged organisms locks monocytes into a epigenetically trained, glycolytic state that exacerbates inflammaging, whereas intermittent mTOR inhibition paired with selective HDAC3 inhibition resets maladaptive chromatin marks while preserving the beneficial metabolic reprogramming needed for adaptive memory formation.

Rationale

• mTORC1 suppression mimics nutrient stress and drives monocyte training via HIF1α‑dependent aerobic glycolysis and H3K4me3/H3K27Ac accumulation at innate immune promoters [3].
• Aging already loads monocytes with proinflammatory H3K27Ac and H3K4me3 marks [4]. Continuous mTOR inhibition adds a metabolic constraint on top of this pre‑existing landscape, preventing the natural ebb‑flow of training and tolerance that occurs during fluctuating nutrient availability.
• HDAC3 removes acetyl groups from H3K27Ac and is recruited to promoters of NF‑κB target genes during resolution of inflammation. Pharmacological HDAC3 inhibition has been shown to erase trained immunity phenotypes in vitro without affecting oxidative phosphorylation [2].
• Intermittent rapamycin dosing reproduces the pulsatile mTORC1 activity seen during caloric restriction cycles, allowing periods of mTORC1 reactivation that support stem‑cell regeneration and adaptive memory precursor formation [5].

We propose that the combination of intermittent mTORC1 inhibition (to provide hormetic stress signals) and transient HDAC3 inhibition (to erase deleterious acetyl marks) will:

1. Reduce H3K27Ac/H3K4me3 at promoters of IL‑1β, IL‑6, TNF‑α in monocytes from old mice.
2. Restore monocyte responsiveness to secondary stimuli (e.g., LPS) toward a balanced tolerant‑responsive phenotype.
3. Preserve the increase in spare respiratory capacity and memory‑precursor CD8⁺ T‑cell formation driven by mTORC1 modulation.
4. Improve healthspan markers (grip strength, frailty index) and extend median lifespan beyond rapamycin alone.

Experimental Design (testable & falsifiable)

Model: C57BL/6 mice, 20 months old (n=15 per group). Groups:

• Control: vehicle.
• Rapamycin Continuous: 14 ppm rapamycin in chow.
• Rapamycin Intermittent: 5 days on/2 days off rapamycin (same average dose).
• Rapamycin + HDAC3i: intermittent rapamycin plus selective HDAC3 inhibitor (RGFP966, 10 mg/kg i.p. twice weekly).
• HDAC3i Alone: inhibitor only.

Readouts (at 3, 6, 9 months):

• Flow cytometry: monocyte subsets (Ly6C^hi/lo), intracellular p‑S6 (mTORC1 activity), p‑AKT (mTORC2).
• Epigenetics: ChIP‑qPCR for H3K27Ac/H3K4me3 at Il1b, Tnf, Hk2 promoters; ATAC‑seq for chromatin accessibility.
• Metabolism: Seahorse glycolysis vs OXPHOS, spare respiratory capacity.
• Function: ex vivo cytokine production after LPS ± β‑glucan restraining; trained immunity fold‑change.
• Adaptive memory: OVA‑specific CD8⁺ T‑cell proliferation and IFN‑γ after prime‑boost.
• Healthspan: grip strength, treadmill endurance, frailty score.
• Survival: Kaplan‑Meier analysis.

Predictions:

• Intermittent rapamycin alone will lower p‑S6 pulses and improve stem‑cell markers but will not reduce monocyte H3K27Ac/H3K4me3 relative to continuous rapamycin.
• Adding HDAC3i will significantly decrease proinflammatory acetylation (p < 0.01) and restore cytokine tolerance without diminishing spare respiratory capacity or CD8⁺ memory formation.
• Healthspan and lifespan improvements will be greatest in the combination group; if no improvement is observed, the hypothesis is falsified.

Falsifiability

A clear falsifying outcome would be: (1) HDAC3 inhibition fails to reduce H3K27Ac/H3K4me3 at proinflammatory promoters in monocytes from old mice despite adequate drug exposure, or (2) the combination abolishes the metabolic reprogramming (spare respiratory capacity) or adaptive memory benefits seen with intermittent rapamycin alone. Either result would refute the proposed synergistic mechanism.