Hypothesis

Age-related eosinophil loss disrupts gut-derived serotonergic signaling via IL-4 modulation of enterochromaffin cells, leading to vagal dysfunction and neuroinflammation.

Mechanistic Rationale

Eosinophils reside in the lamina propria and secrete IL-4, which acts on enterochromaffin cells (ECs) to stimulate tryptophan hydroxylase 1 (TPH1) activity and serotonin (5‑HT) production [3]. 5‑HT released from ECs activates vagal afferents via 5‑HT3 receptors, promoting anti‑inflammatory cholinergic tone [5]. With aging, eosinophil numbers drop in visceral fat and gut [1][8], reducing IL-4 availability. Consequently, EC-derived 5‑HT falls, vagal firing diminishes, and the cholinergic anti‑inflammatory pathway wanes, permitting microglial activation and hippocampal IL-1β rise.

Predictions & Experimental Design

1. Aged mice (24 mo) will show colonic eosinophil depletion, lowered IL-4, reduced EC TPH1 mRNA, and decreased luminal 5‑HT compared with young mice (3 mo).
2. Selective eosinophil ablation in young mice (using anti‑Siglec‑F antibody) will recapitulate the aged serotonergic and vagal phenotype.
3. Adoptive transfer of young eosinophils or local IL-4 delivery to the colon of aged mice will restore EC 5‑HT, increase vagal efferent activity (measured by electrophysiology), and lower hippocampal IL-1β and Iba1+ microglial density.
4. Pharmacologic blockade of 5‑HT3 receptors will abolish the rejuvenating effects of eosinophil transfer, confirming serotonin dependence.

Potential Confounds & Falsifiability

If eosinophil transfer fails to alter colonic 5‑HT or vagal tone despite reducing systemic IL-6, the hypothesis is falsified. Conversely, if IL-4 neutralization in young mice does not affect EC 5‑HT or vagal firing, the proposed eosinophil‑IL‑4‑EC axis is not required. Additionally, germ‑free mice should show baseline eosinophil and 5‑HT levels; if aging effects persist independent of microbiota, the mechanism is eosinophil‑centric rather than microbiome‑driven.