Hypothesis: Lactate-mediated AMPK activation blocks β‑glucan–driven trained immunity in aged monocytes, and its reversal restores healthspan

Aging monocytes exhibit elevated intracellular lactate due to mitochondrial inefficiency, which activates AMPK and suppresses mTORC1 signaling. This blockade prevents HIF‑1α stabilization despite intact Dectin‑1 engagement by β‑glucan, thereby aborting the metabolic‑epigenetic reprogramming that underlies trained immunity. We hypothesize that lowering lactate levels in aged monocytes will reactivate the Akt‑mTOR‑HIF‑1α axis, permitting β‑glucan to induce H3K4me3 and H3K27ac deposits at inflammatory loci and to confer measurable healthspan improvements.

Mechanistic basis

• β‑glucan binding to Dectin‑1 triggers Akt‑mTOR‑HIF‑1α signaling, driving aerobic glycolysis and succinate‑mediated histone modifications [1][2][3].
• Pharmacological or genetic inhibition of Akt, mTOR, or HIF‑1α abolishes trained immunity [4].
• Aging is associated with mitochondrial dysfunction, increased lactate production, and chronic AMPK activation, which inversely regulates mTORC1.
• Elevated lactate can inhibit mTORC1 via AMPK‑dependent phosphorylation of Raptor, reducing HIF‑1α translation and stability.
• Consequently, aged monocytes fail to accumulate succinate and to deposit activating histone marks, resulting in blunted cytokine responses upon rechallenge.

Predictions

1. In aged mice, β‑glucan alone will not increase monocyte ECAR, HIF‑1α protein, or H3K4me3/H3K27ac at promoters of Il6 and Tnf.
2. Co‑administration of a lactate‑lowering intervention (e.g., LDHA inhibitor GSK2837808A or MCT4 overexpression via AAV) will restore ECAR, HIF‑1α, and histone marks to levels seen in young β‑glucan‑treated monocytes.
3. Restored trained immunity will translate into enhanced bacterial clearance (e.g., lower CFU after Listeria challenge), improved grip strength, and extended frailty‑free survival.
4. If lactate reduction fails to rescue the mTOR‑HIF‑1α axis despite β‑glucan stimulation, the hypothesis is falsified.

Experimental design

• Animals: Young (3 mo) and aged (24 mo) C57BL/6 mice.
• Groups: (i) PBS control, (ii) β‑glucan alone, (iii) β‑glucan + LDHA inhibitor, (iv) β‑glucan + MCT4‑AAV, (v) LDHA inhibitor alone.
• Readouts (48 h post‑treatment):
  • Flow cytometry for monocyte identification; Seahorse ECAR/OCR.
  • Western blot for p‑Akt, p‑mTOR, HIF‑1α.
  • ChIP‑qPCR for H3K4me3/H3K27ac at Il6/Tnf promoters.
  • ex vivo LPS restimulation: ELISA for IL‑6, TNF‑α.
  • In vivo infection: intraperitoneal Listeria monocytogenes (10⁴ CFU); organ CFU at 48 h.
  • Physical assays: grip strength, treadmill endurance, frailty index.
• Statistical analysis: Two‑way ANOVA with age and treatment as factors; post‑hoc Tukey.

Potential impact

Demonstrating that a simple metabolic correction can unlock β‑glucan’s epigenetic potential in aging would bridge the gap between mechanistic immunology and functional longevity. It would suggest that lactate‑targeting nutraceuticals or gene therapies could be combined with innate immune priming to improve infection resistance and physical resilience in older adults, directly addressing the field’s major gap of linking molecular restoration to healthspan outcomes.