Hypothesis\nChronic low‑level translocation of microbiome‑derived nucleic acids sustains tonic IFN‑β production in intestinal epithelial cells, which maintains baseline JAK‑STAT signaling in systemic immune compartments until the pathway becomes desensitized (tachyphylaxis). This exhausted STAT state elevates baseline ISG expression while blunting cytokine‑induced fold‑change, propagating SASP cytokines to the brain via vagal afferents and driving neuroinflammation.\n\n### Mechanistic Rationale\n1. Microbial nucleic acid flux – Age‑related loss of mucosal IgA and barrier integrity increases luminal DNA/RNA translocation [https://pmc.ncbi.nlm.nih.gov/articles/PMC10253713/]. Bacterial extracellular vesicles rich in dsRNA can activate cGAS‑STING in IECs, triggering IFN‑β release [https://pmc.ncbi.nlm.nih.gov/articles/PMC11065702/].\n2. Tonic JAK‑STAT activation – Persistent IFN‑β engages IFNAR1/2, phosphorylating JAK1/TYK2 and STAT1/2, forming ISGF3 that drives constitutive ISG transcription [https://pmc.ncbi.nlm.nih.gov/articles/PMC5358544/]. In vivo, this tonic signal is abolished ex vivo and restored by IFN‑β, showing dependence on continuous ligand [https://pmc.ncbi.nlm.nih.gov/articles/PMC11065702/].\n3. Signal exhaustion – Sustained phosphorylation depletes available JAKs and promotes SOCS feedback, raising baseline pSTAT while reducing dynamic range to subsequent IFN‑γ stimulation, a pattern that predicts cardiovascular risk better than CRP [https://pmc.ncbi.nlm.nih.gov/articles/PMC5358544/].\n4. Gut‑brain amplification – Enteric IFN‑β activates vagal afferents through TRPV1‑dependent signaling, increasing norepinephrine release in the locus coeruleus and priming microglial JAK2/STAT3 [https://www.aginganddisease.org/EN/10.14336/AD.2023.0515]. Concurrently, enteric dopaminergic neuronal aging heightens ROS, further stimulating microglial SASP via JAK2/STAT3 [https://pmc.ncbi.nlm.nih.gov/articles/PMC12702875/]. Inhibition of JAK2/STAT3 in glia attenuates SASP, confirming the pathway as a convergence point [https://www.frontiersin.org/journals/cell-and-developmental-biology/articles/10.3389/fcell.2021.650250/full].\n5. Microbiome age as driver – It's the functional age of the microbiome (e.g., metagenomic clock of taxa linked to dsRNA producers) that predicts the magnitude of tonic IFN‑β independent of host chronological age, offering a mechanistic explanation for why inflammaging varies among individuals.\n\n### Testable Predictions\n- Prediction 1: Individuals with higher microbiome‑age scores will exhibit elevated baseline pSTAT1 in peripheral blood mononuclear cells (PBMCs) and a reduced fold‑increase in ISG expression after ex vivo IFN‑γ stimulation, even after adjusting for host age and CRP.\n- Prediction 2: Antibiotic‑induced depletion of dsRNA‑secreting bacteria or oral administration of a cGAS‑STING inhibitor will lower serum IFN‑β, normalize pSTAT baseline, and restore cytokine‑induced ISG responsiveness in aged mice.\n- Prediction 3: Chemogenetic silencing of vagal afferents from the gut to the brain will break the link between tonic JAK‑STAT exhaustion and cortical microglial activation, reducing SASP markers (IL‑6, CCL2) in the hippocampus without altering peripheral ISG baseline.\n- Prediction 4: Fecal microbiota transplantation (FMT) from young donors with low microbiome‑age into aged recipients will decrease tonic IFN‑β levels, improve JAK‑STAT dynamic range, and ameliorate age‑related cognitive decline.\n\n### Experimental Approach\n1. Cohort profiling – We'll collect stool, blood, and CSF from 200 participants stratified by age (60‑80 yr). Perform shotgun metagenomics to compute a microbiome‑age index based on taxa correlated with dsRNA production (e.g., Bacteroides fragilis phage load). Measure baseline pSTAT1/STAT3 by flow cytometry, ISG fold‑change after IFN‑γ stimulation, serum IFN‑β, and CSF inflammatory cytokines.\n2. Intervention in aged mice – Treat 20‑month‑old C57BL/6 mice with (a) broad‑spectrum antibiotics targeting Gram‑negative dsRNA producers, (b) a cGAS‑STING small‑molecule inhibitor, or (c) vagal transection. Assess peritoneal macrophage pSTAT baseline, ISG inducibility, serum IFN‑β, hippocampal Iba1/JAK2‑pSTAT3 colocalization, and performance in Morris water maze.\n3. FMT validation – Transplant feces from young (4‑month) low‑microbiome‑age donors into aged recipients; repeat immune and behavioral readouts after 4 weeks.\n\nIf predictions hold, the hypothesis would reposition the microbiome’s functional age—not just its composition—as an upstream driver of JAK‑STAT signal exhaustion, providing a mechanistic bridge between gut dysbiosis, systemic inflammaging, and brain aging. Conversely, failure to observe changes in JAK‑STAT dynamics despite manipulation of microbial nucleic acid flux would falsify the proposed causal chain.
