Hypothesis

Senescent immune cells create a localized NAD+ sink through SASP‑induced CD38 overexpression, which depletes NAD+ in stromal and parenchymal cells, suppresses SIRT1‑mediated deacetylation, triggers mitochondrial dysfunction, and propagates senescence beyond the immune compartment. Restoring NAD+ specifically within immune niches breaks this feed‑forward loop and delays organismal aging.

Mechanistic Model

• Step 1: Immune senescence – Aging lymphoid and myeloid compartments accumulate senescent cells that secrete IL‑6, IL‑8, TNF‑α and upregulate CD38 (3).
• Step 2: NAD+ consumption – CD38 hydrolyzes NAD+ to generate ADPR and cADPR, lowering intracellular NAD+ levels in nearby non‑immune cells.
• Step 3: SIRT1 inhibition – Reduced NAD+ diminishes SIRT1 deacetylase activity, leading to hyperacetylation of PGC‑1α, FOXO, and NF‑κB subunits, which impairs mitochondrial biogenesis and increases ROS production.
• Step 4: Mitochondrial damage & secondary senescence – Dysfunctional mitochondria release mtDNA and ROS, activating the cGAS‑STING pathway and reinforcing SASP expression in parenchymal cells, thereby spreading senescence.
• Step 5: Feedback amplification – Newly senescent parenchymal cells additionally secrete cytokines that further stimulate CD38 expression in immune cells, tightening the loop.

This model places immune‑derived NAD+ depletion upstream of tissue‑specific aging, extending the observation that transplanting aged splenocytes induces senescence in young tissues (1) by providing a concrete metabolic mediator.

Testable Predictions

1. Spatial NAD+ deficit – In aged mice, NAD+ levels will be significantly lower in stromal compartments adjacent to CD38+ senescent immune cells compared with distal tissues; this gradient will be absent in young mice or after CD38 blockade.
2. CD38 dependence – Genetic deletion of CD38 specifically in hematopoietic cells (Vav‑Cre;Cd38^fl/fl) will preserve NAD+ in surrounding tissues, reduce SIRT1 target acetylation, and attenuate senescence markers (p16^Ink4a, SA‑β‑gal) in liver, kidney, and muscle despite systemic aging.
3. NAD+ rescue efficacy – Targeted delivery of NAD+ precursors (e.g., NMN conjugated to an immune‑cell‑homing peptide) to aged mice will restore local NAD+, improve SIRT1 activity, and decrease SASP burden in peripheral organs more effectively than untethered NMN supplementation.
4. Human correlation – Older individuals with elevated circulating CD38+ senescent T‑cell frequencies will show reduced NAD+ metabolites in paired skin biopsies, and this biomarker will predict frailty index scores independent of chronological age.

Falsifiability

If any of the above predictions fail—e.g., CD38‑deficient immune cells do not ameliorate tissue NAD+ levels or senescence, or NAD+ rescue does not improve organ‑specific aging markers—the hypothesis would be refuted, indicating that immune‑driven NAD+ depletion is not a central driver of systemic aging.

Implications

Confirming this link would shift therapeutic focus toward immunomodulatory NAD+ strategies (CD38 inhibitors, immune‑specific NAD+ boosters) as a primary lever to slow aging, complementing senolytics and metabolic interventions.