Hypothesis

Aging-related loss of cholinergic enteric nervous system (ENS) neurons directly diminishes acetylcholine signaling to germinal center B cells, leading to reduced activation-induced cytidine deaminase (AID) expression and impaired somatic hypermutation/class switch recombination, independent of systemic inflammation.

Mechanistic Rationale

• The ENS releases acetylcholine that binds to nicotinic acetylcholine receptors (α7‑nAChR) expressed on B cells in Peyer’s patches and mesenteric lymph nodes. 2
• α7‑nAChR activation triggers intracellular Ca2+ fluxes that enhance STAT3 phosphorylation, which in turn promotes AID transcription via binding to the AID promoter. 3
• Age‑associated depletion of ChAT+ ENS neurons lowers luminal acetylcholine, weakening this neuromuscular‑immune coupling. 4
• While systemic cytokines (TNF‑α, IL‑6) also suppress AID, the cholinergic pathway operates upstream: loss of ENS‑derived ACh precedes the rise in circulating inflammation markers. 5
• Microbial metabolites such as butyrate can potentiate cholinergic neurotransmission by inhibiting acetylcholinesterase, linking dysbiosis to ENS signaling strength. 6
• Restoring cholinergic tone (e.g., via GDNF‑mediated neuron survival or optogenetic activation) should rescue AID levels even when inflammation is pharmacologically neutralized. 7

Testable Predictions

1. In aged mice, selective ablation of ChAT+ ENS neurons will reduce AID+ germinal center B cells and lower SHM frequency, despite unchanged serum IL‑6/TNF‑α.
2. Pharmacological blockade of α7‑nAChR on B cells will mimic the effect of ENS cholinergic loss, decreasing AID expression and IgG class switching.
3. Conversely, chemogenetic activation of remaining cholinergic ENS neurons in aged animals will increase AID expression and improve vaccine‑induced antibody affinity, an effect blocked by α7‑nAChR antagonists.
4. Supplementation with butyrate will enhance ENS acetylcholine release and partially restore AID levels in aged mice, an effect absent in ChAT‑deficient ENS.

Experimental Approach

• Models: Use aged (20‑month) C57BL/6 mice; generate ChAT‑Cre::Rosa26‑DTR mice for inducible diphtheria toxin‑mediated ablation of cholinergic ENS neurons. Include wild‑type controls.
• Readouts: Flow cytometry for GL7+Fas+ germinal center B cells, intracellular AID staining, high‑throughput immunoglobulin sequencing for SHM load and CSR efficiency (IgG1/IgA). Serum cytokine ELISA to monitor inflammation.
• Interventions:
  • Optogenetic stimulation of ChAT+ neurons (ChAT‑Cre::ChR2) via intraperitoneal fiber‑free illumination.
  • Chemogenetic activation (hM3Dq) with CNO.
  • α7‑nAChR antagonist (MLA) administration.
  • Butyrate supplementation in drinking water.
• Controls: Treat a subset with anti‑IL‑6R antibody to isolate inflammation‑independent effects.
• Analysis: Compare AID MFI, SHM mutations per clone, and CSR ratios across groups. Use two‑way ANOVA with factors age and intervention; significance set at p<0.05.

If cholinergic ENS rescue restores AID and antibody diversification without lowering systemic cytokines, the hypothesis is supported. Failure to observe improvement would refute the direct neuromodulatory model and reinforce the inflammation‑centric view.