Hypothesis

Age-associated decline in intracellular NAD+ reduces SIRT6 deacetylase activity, leading to hyperacetylation of activation-induced cytidine deaminase (AID). This modification impairs AID’s access to somatic hypermutation (SHM) substrates while leaving its role in class switch recombination (CSR) relatively intact, thereby accounting for the observed dissociation between SHM and CSR defects in aged B cells.

Rationale

• Somatic hypermutation rates fall markedly with age, especially replacement mutations that boost affinity {Somatic hypermutation declines with age}.
• Aged B cells retain intrinsic SHM capacity when the microenvironment is normalized, indicating a cell‑intrinsic block that can be overcome {Aged B cells retain SHM capacity}.
• AID expression drops in stimulated B cells from elderly individuals, driven by unstable E47 transcription factor {AID expression falls with age}.
• Elevating AID improves CSR but does not rescue SHM, suggesting the two pathways can be uncoupled {AID overexpression rescues CSR not SHM}.
• SIRT6, a NAD+-dependent deacetylase, regulates DNA repair and transcription; its activity wanes with NAD+ depletion in aging tissues {SIRT6 declines with NAD+ loss}.
• Acetylated AID shows reduced interaction with the mismatch repair complex required for SHM, while its ability to trigger uracil excision for CSR remains less sensitive to acetylation status {Acetylation modulates AID function}.

Testable Predictions

1. Biochemical – Aged B cells will exhibit higher acetyl‑lysine levels on AID compared with young B cells; NAD+ supplementation will decrease this acetylation.
2. Functional – Restoring NAD+ (via NR or NMN) or overexpressing SIRT6 in aged B cells will increase SHM frequency and affinity of antibodies without further enhancing CSR beyond baseline.
3. Genetic – B‑cell‑specific SIRT6 knockout in young mice will recapitulate the aged SHM defect (reduced replacement mutations) while preserving CSR efficiency.
4. Cellular – In vitro germinal center‑like cultures from aged mice treated with NAD+ precursors will show rescued clonal diversity and improved vaccine‑like responses, measurable by deep sequencing of IgV regions.

Experimental Approach

• Isolation – Purify naïve B cells from young (3 mo) and aged (20 mo) mice.
• Manipulation – Treat cells with NAD+ precursor (NMN 500 µM), SIRT6 activator (MDL‑801), or transduce with SIRT6‑overexpressing lentivirus; include controls (vehicle, empty vector).
• Readouts –
  • Quantify AID acetylation by immunoprecipitation followed by western blot with acetyl‑lysine antibody.
  • Measure SHM rates using a high‑throughput IgV sequencing assay after CD40L/IL‑4 stimulation (7 days).
  • Assess CSR to IgG1 by flow cytometry.
  • Evaluate affinity maturation via ELISA with increasing antigen concentrations.
• In vivo validation – Administer NMN to aged mice for 4 weeks, immunize with a model antigen (NP‑OVA), and analyze germinal center B‑cell SHM/CSR and serum antibody titers.

Falsifiability – If NAD+ elevation or SIRT6 activation fails to increase SHM (or increases it only alongside a proportional rise in CSR), or if SIRT6 deficiency does not impair SHM in young B cells, the hypothesis would be refuted. Conversely, confirmation would support a NAD+-SIRT6‑AID axis as a mechanistic link between metabolic aging and the selective loss of affinity maturation.