Targeting lysosomal acidification to restore microglial homeostasis and mitigate memory decline during male brain ageing

• Ketotifen re-acidifies lysosomes in aged microglia and peripheral macrophages. • Treatment rejuvenates microglial branching and reduces Iba-1 reactivity across brain regions. • Drug enhances cortical synaptophysin/PSD-95 and rescues object-location memory in 16-mo mice. • It reduces LPS-evoked TNF-α, IL-1β and IL-10 release from microglia and macrophages. • Repurposed ketotifen may provide a safe, translatable route to geroprotective brain therapy. Lysosomal dysfunction lies at the nexus of inflammaging, microglial dystrophy and synaptic fragility, making it an attractive target for brain rejuvenation. Here we demonstrate that a five-month oral course of ketotifen, an approved H1-antihistamine and mast-cell stabiliser, re-acidifies lysosomes in aged C57BL/6J male mice, restoring the quinacrine signal of peripheral macrophages and SIM-A9 microglia. This proton rebound is coupled to broad anti-cytokine effects: ketotifen attenuates lipopolysaccharide-evoked release of TNF-α, IL-1β and IL-10 in vitro and ex vivo. In the brain, the drug restores a highly ramified, homeostatic microglial morphology throughout the cortex and hippocampus. Ketotifen robustly elevates cortical synaptophysin and PSD-95 above age-matched levels. Behaviourally, ketotifen enhances spatial learning and object-location memory without altering locomotor activity or anxiety-like behaviour. Collectively, these findings identify lysosomal re-acidification as the initiating trigger of a multifaceted rejuvenation cascade that dampens multi-cytokine signalling, restores microglial morphology and preserves synaptic integrity. Because ketotifen is inexpensive, brain-permeable and already licensed for human use, our work unveils an immediately actionable geroprotective strategy to forestall early cognitive decline.