Stem cells show intrinsic interferon signalling, which protects them from viral infections at all ages. In the ageing brain, interferon signalling also reduces the ability of stem cells to activate. Whether these functions are linked and at what time interferons start taking on a role in stem cell functioning is unknown. Additionally, the molecular link between interferons and activation in neural stem cells and how this relates to progenitor production is not well understood.Here we combine single-cell transcriptomics, RiboSeq and mathematical models of interferon to show that this pathway is important for proper stem cell function at all ages in mice. Interferon orchestrates cell cycle and mTOR activity to post-transcriptionally repress Sox2 and induces quiescence. The interferon response then decreases in the subsequent maturation states. Mathematical simulations indicate that this regulation is beneficial for the young and harmful for the old brain. Our study establishes molecular mechanisms of interferon in stem cells and interferons as genuine regulators of stem cell homeostasis and a potential therapeutic target to repair the ageing brain.

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Stem cells show intrinsic interferon signaling, which protects them from viral infections. In the brain, type I interferon signaling increases in stem cells as they age and reduces their ability to become activated for repair. Whether interferons impact stem cell activity only in aging, how, and if it is coupled to the intrinsic immune function remains elusive. Our study shows that interferon-β arrested stem cells in G0 of the cell cycle and transiently increased their TOR signaling before switching it off. This bimodal regulation of TOR is needed to repress translation of Sox2. Single-cell transcriptomics reveals that in the absence of interferon receptors, interferon signaling levels change and its increase with age disappears. Mathematical simulations indicate that interferons are beneficial in the young and harmful in the old brain. Our study identifies interferons as genuine regulators of stem cell homeostasis and potential therapeutic target to repair the aging brain.