Hypothesis\n\nIn aged liver, senescent hepatic stellate cells (HSCs) establish a spatially restricted IFNγ‑responsive niche that reprograms neighboring Kupffer cells toward an antigen‑presenting, pro‑fibrotic phenotype via JAK‑STAT‑MHC I signaling. This gradient amplifies collagen deposition and sustains a maladaptive wound‑healing loop.\n\n## Rationale\n\n- Spatial transcriptomics of aged brain (GSE212903) and lung reveals quantitative expression gradients around senescent cells, showing upregulated IFNγ receptors and SASP factors with membrane localization shifts [1].\n- In idiopathic pulmonary fibrosis, cell2location deconvolution of Visium data links senescence scores to basaloid epithelial niches and fibrosis pathways, demonstrating that spatial ligand‑receptor analysis can uncover rewired IFNγ sensing 2.\n- Human Alzheimer’s data (GSE263468) show that resilient neuronal subsets retain layer‑specific programs despite pathology, highlighting how spatial deconvolution isolates vulnerable versus protected compartments 3.\n\nThese studies establish a paradigm: senescent cells sculpt membranous IFNγ signaling hotspots that alter neighboring cell states. Yet we haven't mapped this mechanism in liver, where HSC‑driven senescence is a known driver of age‑related fibrosis.\n\n## Novel Mechanistic Insight\n\nWe propose that senescent HSCs release exosomes enriched in miR‑21 and TGF‑β1, which deliver cargo to Kupffer cell surfaces, promoting endocytosis and clustering of IFNγ receptors. It's this receptor clustering that enhances JAK1/STAT1 phosphorylation while concurrently suppressing phosphatases PTPN2 and SOCS1, mirroring the lung fibrosis signature. The resulting STAT1‑dependent transcriptional program upregulates MHC I, β2‑microglobulin, and antigen‑processing genes, turning Kupffer cells into inadvertent antigen presenters that stimulate CD8⁺ T‑cell recruitment and further IFNγ release, creating a feed‑forward loop. We don't yet know whether the exosome miR‑21 is sufficient or necessary, but early data suggest it modulates receptor trafficking.\n\n## Testable Predictions\n\n1. Spatial gradient – In Visium or MERFISH sections from young (6 mo) and old (24 mo) mouse livers, senescence scores (p16^Ink4a^, p21^Cip1^, SASP hotspot) will peak peri‑central and peri‑portal zones, with a concomitant rise in IFNγ receptor (Ifngr1/2), phosphorylated STAT1 (p‑STAT1), and MHC I (H2‑Kb/Db) expression within a 2‑3‑cell‑radius around each senescent spot.\n2. Cell‑type specificity – Deconvolution (cell2location or RCTD) will assign the IFNγ‑responsive signature primarily to Kupffer cells (Clec4f^+, Marco^+) adjacent to senescent HSCs (Des^+, Acta2^+).\n3. Exosome dependency – Isolating HSC‑derived exosomes from old mice and applying them to young liver slices will recapitulate the IFNγ‑responsive gradient; exosome depletion (GW4869) or miR‑21 antagomir will attenuate p‑STAT1 and MHC I upregulation in Kupffer cells.\n4. Causal perturbation – Treating old mice with a senolytic that selectively targets HSCs (e.g., navitoclax‑loaded liposomes) will reduce senescence scores, flatten the IFNγ‑responsive spatial gradient, lower p‑STAT1/MHC I in Kupffer cells, and diminish collagen deposition (Sirius Red) compared with vehicle controls.\n5. Human relevance – Analyzing the publicly available human liver aging snRNA‑seq dataset (if available) or paired snRNA‑seq/Visium from cirrhotic livers will show analogous IFNγ‑receptor upregulation and MHC I enrichment in macrophage clusters neighboring senescent HSC‑like cells (PDGFRB^+, CDKN2A^+).\n\n## Falsifiability\n\nIf spatial transcriptomics of aged liver fails to show a statistically significant colocalization of senescence hotspots with IFNγ‑receptor/JAK‑STAT/MHC I upregulation in neighboring Kupffer cells, or if senolytic clearance of HSCs does not alter the gradient or fibrosis outcomes, the hypothesis would be refuted.\n\n## Broader Impact\n\nDemonstrating a spatially constrained immunomodulatory circuit in aged liver would provide a mechanistic bridge between cellular senescence and inflammaging, suggesting that targeting the exosome‑mediated IFNγ receptor reprogramming step—rather than senescent cell removal alone—could break the fibrosis‑inflammation cycle.\n\n[1]: https://pmc.ncbi.nlm.nih.gov/articles/PMC7749315/