Hypothesis

Age-related decline in NAD+ levels reduces SIRT1 activity, impairing the autophagy‑mediated rationing system that fuels β‑glucan‑induced trained immunity. Restoring NAD+ (e.g., with nicotinamide riboside) will rescue SIRT1‑dependent deacetylation of autophagy regulators, thereby sustaining metabolite supply from selective organelle turnover and preserving the epigenetic and functional hallmarks of trained immunity in aged monocytes.

Mechanistic Rationale

Trained immunity imposes a chronic "siege" on monocytes, demanding simultaneous glycolytic ATP production and biosynthetic precursors for chromatin remodeling [2][3]. Autophagy meets this demand by selectively degrading mitochondria and other metabolic compartments, liberating TCA‑cycle intermediates (succinate, fumarate) that reinforce HIF1α stabilization and epigenetic marks [1][4]. This process is not indiscriminate cleanup; it is a rationing decision governed by the energy‑sensing kinase mTOR and the deacetylase SIRT1. SIRT1 directly deacetylates key autophagy proteins (e.g., ATG5, LC3) and transcription factors (FOXO3) that promote autophagosome formation under nutrient stress [https://pubmed.ncbi.nlm.nih.gov/29563370/]. In aging monocytes, intracellular NAD+ falls, diminishing SIRT1 activity and tipping the balance toward mTOR‑driven anabolism without sufficient autophagic recycling. Consequently, the siege intensifies: cells expend internal reserves faster than they can be replenished, leading to premature exhaustion of the trained state.

Testable Predictions

1. Flux Prediction: Aged human monocytes will show reduced LC3‑II turnover and lower mitochondrial-derived succinate/fumarate after β‑glucan stimulation compared with young monocytes; NAD+ supplementation will restore autophagy flux and metabolite levels to youthful ranges.
2. Epigenetic Prediction: Loss of SIRT1 activity (via EX527 inhibitor or NAD+ depletion) will decrease H3K4me3/H3K27ac at promoters of proinflammatory genes (e.g., TNF, IL6) following training, whereas NAD+ repletion will maintain these marks.
3. Functional Prediction: Trained monocytes from aged donors will exhibit diminished secondary cytokine production upon heterologous restimulation; pretreatment with nicotinamide riboside (NR) will rescue this response to levels comparable with young monocytes.
4. Genetic Prediction: Monocyte‑specific SIRT1 knockout mice will fail to develop β‑glucan‑induced trained immunity, exhibiting impaired autophagy and metabolic reprogramming, while overexpression of SIRT1 in aged monocytes will compensate for NAD+ deficiency.

Experimental Design

• Isolation: Obtain CD14+ monocytes from healthy young (20‑30 yr) and aged (>65 yr) donors.
• Treatments: (i) β‑glucan (1 µg/mL, 24 h) ± NR (500 µM, 24 h pretreatment); (ii) SIRT1 inhibitor EX527 (10 µM) or activator SRT2104 (10 µM) as controls.
• Readouts:
  • Autophagy flux – LC3‑II/I ratio by western blot with bafilomycin A1 block; mitochondrial mass – MitoTracker Green.
  • Metabolomics – targeted LC‑MS for succinate, fumarate, acetyl‑CoA.
  • Epigenetics – ChIP‑qPCR for H3K4me3/H3K27ac at TNF/IL6 promoters.
  • Function – ELISA for TNF, IL6 after LPS restimulation (7 days post‑training).
• In vivo validation: Adoptive transfer of NR‑treated aged monocyte‑derived macrophages into SIRT1‑fl/fl LysM‑Cre mice; assess trained immunity phenotypes after β‑glucan challenge.

Potential Implications

If NAD+/SIRT1 sustains the autophagic rationing system, it positions NAD+ boosting as a geroprotective strategy to preserve innate immune memory without exacerbating inflammaging. Conversely, failure to engage this axis could explain why aged individuals show poor vaccine‑induced trained immunity and heightened susceptibility to recurrent infections. The hypothesis is falsifiable: demonstrating that NAD+ restoration does not improve autophagy flux, metabolite supply, or trained immune function in aged monocytes would refute the proposed mechanistic link.