Hypothesis

Age‑associated B‑cell dysfunction stems from a NAD+‑SIRT6 metabolic axis that limits chromatin accessibility at immunoglobulin loci, thereby reducing somatic hypermutation and class switch recombination, and it's becoming clear that extrinsic inflammation alone doesn't explain the full phenotype.

Mechanistic Model

• In aged follicular B cells, intracellular NAD+ declines, which lowers SIRT6 deacetylase activity.
• It's known that SIRT6 normally removes acetyl groups from histone H3K9 at the IgH locus; loss of this activity leads to hyperacetylated H3K9, a chromatin state that restricts AID binding.
• Reduced AID access diminishes both replacement mutations in SHM and efficiency of CSR.
• This intrinsic defect persists even when exogenous help is provided, but it's unclear whether the deficit is solely metabolic.
• Inflammatory cytokines such as TNF‑α further destabilize E47 mRNA, compounding the NAD+‑SIRT6 deficit, while altered insulin‑receptor signaling in B cells exacerbates the metabolic imbalance.

Testable Predictions

1. Follicular B cells from donors >65 years will show significantly lower NAD+ levels and higher H3K9ac at the IgH promoter compared with donors <30 years (measured by LC‑MS NAD+ assay and ChIP‑qPCR).
2. Pharmacologic elevation of NAD+ (e.g., nicotinamide riboside) or SIRT6 activation (e.g., MDL‑801) in cultured aged B cells doesn't just raise NAD+ levels; it's expected to restore AID occupancy, increase SHM frequency, and shift Ig secretion from IgM to IgG in vitro.
3. In vivo, aged mice receiving NAD+ precursor supplementation will exhibit improved vaccine‑induced IgG titers and a higher proportion of CD27^bright^ memory B cells, whereas SIRT6‑deficient B cells will fail to rescue these phenotypes despite NAD+ supplementation.
4. Blocking TNF‑α signaling will partially rescue CSR in aged B cells, but full restoration will require concurrent NAD+ replenishment, indicating that metabolic and inflammatory inputs act synergistically.

These predictions are falsifiable: if NAD+ or SIRT6 manipulation does not alter SHM/CSR metrics, or if aged B cells retain defective IgG production despite metabolic rescue, the hypothesis would be refuted.