Hypothesis

Aged immune cells export NAD+‑degrading activity via CD38, creating an extracellular NAD+ sink that precipitates senescence in surrounding tissues. It's clear that restoring NAD+ homeostasis specifically within the hematopoietic compartment will delay systemic aging more effectively than untargeted NAD+ supplementation.

Mechanistic Rationale

With age, hematopoietic cells show elevated CD38 expression driven by chronic NLRP3 inflammasome signaling, and it's this upregulation that fuels extracellular NADase activity [[https://doi.org/10.1016/j.cmet.2013.09.010]]. CD38 functions as an extracellular NADase, hydrolyzing NAD+ to nicotinamide and ADP‑ribose [[https://doi.org/10.1016/j.cellmet.2016.08.010]]. This activity lowers the NAD+ available to neighboring stromal and parenchymal cells, compromising SIRT1‑mediated deacetylation and DNA repair, thereby promoting the senescence‑associated secretory phenotype (SASP) [[https://doi.org/10.1073/pnas.1810692116]]. The resulting SASP further fuels inflammasome activation in immune cells, establishing a feed‑forward loop that amplifies NAD+ loss across organs.

Predictions and Experimental Design

1. Genetic test – Generate mice with hematopoietic‑specific CD38 deletion (Vav‑Cre; Cd38^fl/fl). Compare NAD+ levels in serum, liver, muscle, and brain to wild‑type littermates at 12 and 24 months. It's expected that tissue NAD+ will be higher and p16^INK4a^ and SA‑β‑gal positivity reduced in non‑lymphoid organs.
2. Pharmacologic test – Treat aged wild‑type mice with a selective CD38 inhibitor (e.g., 78c) versus vehicle; measure same endpoints. It's anticipated that NAD+ rescue will be comparable to hematopoietic‑specific deletion.
3. Functional read‑outs – Assess grip strength, treadmill endurance, and frailty index over 6 months. It's anticipated that performance will improve in CD38‑deficient or inhibitor‑treated groups.
4. Rescue experiment – Administer NAD+ precursor (NR) to Cd38^fl/fl;Vav‑Cre mice; if immune CD38 is the primary driver, NR will provide little additional benefit over genotype alone, indicating an epistatic relationship.
5. Human correlate – Analyze CD38 expression on circulating monocytes from young vs. older donors and correlate plasma NAD+ metabolites with clinical frailty scores [[https://doi.org/10.1186/s13059-021-02585-8]]. It's notable that a negative correlation would support the translational relevance of the mechanism.

Potential Outcomes and Interpretation

If hematopoietic CD38 loss or inhibition elevates tissue NAD+ and diminishes senescence markers, the hypothesis gains support, positioning immune‑derived NAD+ consumption as an upstream driver of systemic aging. Conversely, if NAD+ levels remain unchanged or senescence persists, alternative immune mechanisms (e.g., vesicle‑mediated miRNA transfer) must be considered. A lack of additive effect from NR in CD38‑deficient mice would reinforce the idea that immune NAD+ flux is the rate‑limiting step for whole‑organism NAD+ homeostasis during aging.