Hypothesis

Aged gut‑resident macrophages and dendritic cells acquire a senescent phenotype that elevates CD38 expression, driving local NAD+ depletion in the enteric nervous system (ENS). This NAD+ loss impairs SIRT1‑dependent autophagy of enteric neurons, leading to dysmotility, bacterial translocation, and a feed‑forward loop of systemic inflammaging.

Mechanistic Rationale

• Senescent immune cells in the lamina propria secrete SASP factors (IL‑6, IL‑1β) that induce CD38 on neighboring stromal and neuronal cells via STAT3 signaling[5].
• CD38 hydrolyzes NAD+, reducing substrate for SIRT1, which normally deacetylates autophagy proteins (LC3, ATG5) and promotes mitochondrial quality control in ENS neurons.
• Diminished SIRT1 activity causes accumulation of damaged mitochondria and defective mitophagy, triggering neuronal release of ATP and HMGB1 that activate TLR4 on macrophages, amplifying NLRP3 inflammasome activity[3].
• Bacterial products translocate through a leaky gut barrier, further stimulating TLRs and sustaining the cytokine storm that fuels systemic SASP dissemination.

Testable Predictions

1. Old mice will show higher CD38 protein levels in the myenteric plexus compared with young mice, correlating with reduced NAD+ and SIRT1 activity in ENS neurons[2].
2. Pharmacologic inhibition of CD38 (e.g., with 78c) or genetic deletion of Cd38 in CX3CR1+ macrophages will restore ENS NAD+, improve autophagic flux (measured by LC3‑II/I ratio), and normalize colonic transit time in aged mice.
3. Adoptive transfer of young, CD38‑low macrophages into aged recipients will attenuate inflammaging markers (serum IL‑6, TNF‑α) and extend healthspan metrics (grip strength, frailty index) without altering lymphoid output.
4. In humans, colonic biopsy CD38+ macrophage density will positively correlate with plasma NAD+ metabolites^{-1} and inversely with constipation severity scores in older cohorts.

Experimental Approach

• Mouse models: Use aged (20‑24 mo) C57BL/6J mice; treat with CD38 inhibitor or employ Cd38^fl/fl; Cx3cr1^CreERT2 mice for macrophage‑specific deletion.
• Readouts: Quantify NAD+ levels (LC‑MS/MS), SIRT1 activity (fluorometric deacetylation assay), autophagic flux (LC3‑II turnover with bafilomycin A1), colonic motility (fluorescence bead expulsion), cytokine panels, and frailty index.
• Human validation: Obtain colon biopsies from older donors undergoing colonoscopy; immunostain for CD38 and macrophage marker CD68; measure NAD+ metabolites in matching plasma; correlate with gastrointestinal symptom scores.

If CD38‑driven NAD+ loss in the ENS is a pivotal conduit linking immune senescence to systemic aging, targeting this axis should uncouple inflammaging from age‑related functional decline.