Hypothesis

Chronic tonic type I interferon (IFN‑I) signaling in aged intestinal epithelium reprograms STAT3 signaling to induce indoleamine 2,3‑dioxygenase 1 (IDO1), elevating local kynurenine that suppresses CD4⁺ and CD8⁺ T‑cell function and compromises epithelial tight‑junction integrity, thereby propagating systemic inflammaging.

Rationale

• Multi‑omics data show age‑dependent diversion of IFN‑I signaling from STAT1 to STAT3 in immune cells, correlating with inflammatory marker elevation and delayed contraction of infection‑induced CD4⁺ T cells [1].
• In γ/δ T cells, IFN‑I downregulates Foxo1 via PI3K‑AKT, enhancing IL‑17 production [2]; a parallel epithelial pathway could exist where STAT3 drives immunosuppressive metabolism.
• Senescent cells exhibit enriched IFN‑related and NF‑κB/IL‑1 pathways, creating a self‑reinforcing interferonopathy loop [3].
• Altered dendritic cell function and impaired barrier integrity in the intestine contribute to systemic inflammation [4]; epithelial STAT3 activation has been implicated in mucosal healing but its role in chronic IFN‑I contexts remains unexplored.
• Progressive epigenomic remodeling with age drives inflammatory gene expression across tissues [5], suggesting that stromal compartments, including epithelium, acquire a permissive chromatin state for STAT3‑dependent transcription.

Mechanistic Model

1. Tonic IFN‑I exposure in aged gut epithelium maintains low‑level STAT1 phosphorylation but promotes sustained STAT3 activation via SOCS1‑mediated negative feedback on STAT1 and heightened IL‑6 family cytokine signaling.
2. STAT3 translocates to the nucleus and, cooperating with hypoxia‑inducible factor‑1α (HIF‑1α) stabilized in the aged mucosa, binds enhancer regions of the IDO1 gene, increasing its transcription.
3. Elevated IDO1 catalyzes tryptophan → kynurenine, which accumulates in the lamina propria.
4. Kynurenine engages aryl hydrocarbon receptor (AhR) on T cells, driving expression of exhaustion markers (PD‑1, TIM‑3) and regulatory phenotypes (Foxp3⁺, IL‑10⁺), while suppressing proliferative capacity.
5. STAT3 also upregulates claudin‑2 and downregulates occludin, weakening tight‑junction complexes and increasing paracellular permeability, allowing bacterial translocation that further fuels IFN‑I production.
6. The resulting mucosal leak and systemic kynurenine spillover augment circulating inflammatory cytokines, completing a feed‑forward loop that drives inflammaging.

Predictions

• In aged mice, intestinal epithelial STAT3 activity will correlate positively with IDO1 expression, luminal kynurenine concentrations, and markers of T‑cell exhaustion in Peyer’s patches and mesenteric lymph nodes.
• Genetic or pharmacologic ablation of STAT3 specifically in intestinal epithelial cells will reduce IDO1‑derived kynurenine, restore tight‑junction protein expression, decrease barrier permeability, and improve systemic inflammatory signatures without compromising STAT1‑mediated antiviral responses.
• Exogenous kynurenine supplementation will recapitulate T‑cell exhaustion and barrier defects in young STAT3‑intact mice, mimicking the aged phenotype.
• Blocking AhR signaling will rescue T‑cell function despite high kynurenine, confirming the metabolite’s mechanistic role.

Experimental Approach

1. Model: Use aged (20‑24 mo) C57BL/6 mice; generate Villin‑CreERT2 × Stat3^fl/fl for inducible epithelial‑specific STAT3 knockout; treat with tamoxifen at 18 mo to induce deletion.
2. Readouts:
   • Flow cytometry of lamina propria lymphocytes for PD‑1, TIM‑3, Ki‑67, Foxp3.
   • LC‑MS quantification of tryptophan and kynurenine in intestinal tissue and serum.
   • qPCR/Western blot for IDO1, STAT3‑pY705, STAT1‑pY701, claudin‑2, occludin, ZO‑1.
   • FITC‑dextran assay for intestinal permeability.
   • Serum cytokine panel (IL‑6, TNF‑α, IFN‑γ) and transcriptomic profiling of sorted epithelial cells.
3. Interventions:
   • Pharmacologic STAT3 inhibitor (e.g., napabucasin) administered orally for 4 weeks.
   • IDO1 inhibitor (epacadostat) as a downstream control.
   • AhR antagonist (CH‑223191) to test metabolite dependency.
4. Controls: Littermate Villin‑CreERT2 × Stat3^wt/wt treated with tamoxifen; young (3‑mo) mice for baseline.
5. Statistical Plan: Power analysis targeting 80 % power to detect 30 % change in kynurenine (α = 0.05); n = 8 per group; two‑way ANOVA with post‑hoc Tukey.

Expected Outcomes

• Epithelial STAT3 loss will reduce IDO1 expression by ≥50 %, lower tissue kynurenine by comparable magnitude, and increase tryptophan availability.
• T‑cell exhaustion markers will drop ≥40 %, while proliferative responses to anti‑CD3/CD28 will rise.
• Barrier assays will show a 2‑fold decrease in FITC‑dextran flux and restored claudin‑2/occludin ratio.
• Systemic cytokines will decline toward young‑mouse levels, without increased susceptibility to acute viral challenge (e.g., low‑dose influenza), confirming preserved STAT1 antiviral capacity.

Potential Caveats

• Compensatory STAT1 hyperactivation could inadvertently augment antiviral states; monitoring STAT1‑pY701 and viral clearance will address this.
• Microbiota shifts secondary to barrier changes may independently affect immunity; 16S rRNA sequencing will control for confounding effects.
• Acute inflammation models (e.g., DSS colitis) might reveal context‑dependent STAT3 functions; separate experiments will delineate tonic versus injury‑induced signaling.

Falsifiability

If intestinal epithelial STAT3 ablation fails to reduce IDO1 expression, kynurenine levels, or T‑cell exhaustion markers, and does not improve barrier integrity or systemic inflammation, the hypothesis that epithelial STAT3‑IDO1‑kynurenine axis mediates inflammaging will be refuted. Conversely, confirmation of these predictions would support a novel mucosal checkpoint linking chronic IFN‑I signaling to age‑immune dysfunction, offering a target for precision interventions that spare STAT1‑driven host defense.