The paradox of elevated basal ISG expression alongside blunted acute IFN responses in aged tissues suggests that chronic signaling is not productive but reflects a desensitized state. We hypothesize that in aging, IFNAR1 undergoes aberrant endosomal trafficking that increases its surface residency while simultaneously impairing its ability to propagate signal upon ligand binding. Specifically, aged cells exhibit reduced IFNAR1 internalization and recycling, leading to accumulation of receptors at the plasma membrane that are constitutively phosphorylated by low‑level ligand‑independent kinases (e.g., Src family). This generates persistent, low‑amplitude STAT1 phosphorylation sufficient to drive ISG transcription but insufficient to trigger the robust, transient pSTAT1 peaks required for antiviral gene programs. Concurrently, chronic exposure promotes SOCS1‑mediated ubiquitination of IFNAR1, targeting the receptor for lysosomal degradation upon acute IFN stimulation, thereby curtailing signal amplitude. The rapid loss of IFN‑responsive monocytes after IFNAR1 blockade in aged lungs reflects depletion of this mislocalized, signaling‑incompetent pool, allowing receptor recovery and restored responsiveness.

To test this model we will:

1. Quantify surface versus total IFNAR1 by flow cytometry and biotinylation assays in lung monocytes from young (3 mo) and aged (24 mo) mice, predicting a higher surface/total ratio in aged cells.
2. Measure IFNAR1 internalization rates using antibody‑feeding assays followed by confocal microscopy, expecting slowed uptake in aged monocytes.
3. Assess basal and IFN‑induced pSTAT1 kinetics by phospho‑flow at 0, 5, 15, 30, 60 min post‑IFN‑α, predicting elevated basal pSTAT1 but blunted peak amplitude and accelerated dephosphorylation in aged cells.
4. Determine SOCS1 and SOCS3 protein levels and IFNAR1 ubiquitination via immunoprecipitation‑Western blot, anticipating increased SOCS1 binding and ubiquitin conjugates after IFN stimulation in aged monocytes.
5. Pharmacologically block lysosomal degradation (chloroquine) or proteasomal degradation (MG132) during acute IFN stimulation and measure whether pSTAT1 peaks are rescued in aged cells.

If the hypothesis is correct, restoring normal IFNAR1 trafficking (e.g., with Rab11‑overexpression or dynasore washout) should normalize both basal ISG expression and acute IFN responses without altering ligand levels. Conversely, preventing SOCS1‑mediated ubiquitination should prolong pSTAT1 signaling in aged monocytes. Failure to observe these predictions would falsify the trafficking‑centric model and force reconsideration of alternative mechanisms such as epigenetic remodeling or chronic ligand sequestration.