Detrimental Influence of Arginase-1 in Infiltrating Macrophages on Post-Stroke Functional Recovery and Inflammatory Milieu

Post-stroke inflammation critically influences functional outcomes following ischemic stroke. Arginase-1 (Arg1) is conventionally understood as a marker for anti-inflammatory macrophages, associated with the resolution of inflammation and promotion of tissue repair in various pathological conditions. However, its specific role in post-stroke recovery remains to be elucidated. This study investigates the functional impact of Arg1 expressed in macrophages on post-stroke recovery and inflammatory milieu. We observed a time-dependent increase in Arg1 expression, peaking at 7 days after photothrombotic stroke in mice. Cellular mapping analysis revealed that Arg1 was predominantly expressed in LysM-positive infiltrating macrophages. Using a conditional knockout (cKO) mouse model, we examined the role of Arg1 expressed in infiltrating macrophages. Contrary to its presumed beneficial effects, Arg1 cKO in LysM-positive macrophages significantly improved skilled forelimb motor function recovery after stroke. Mechanistically, Arg1 cKO attenuated fibrotic scar formation, enhanced peri-infarct remyelination, and increased synaptic density while reducing microglial synaptic elimination in the peri-infarct cortex. Gene expression analysis of FACS-sorted microglia revealed decreased TGF-β signaling and pro-inflammatory cytokine activity in peri-infarct microglia from Arg1 cKO animals. In vitro co-culture experiments demonstrated that Arg1 activity in macrophages modulates microglial synaptic phagocytosis, providing evidence for macrophage-microglia interaction. These findings provide new insights into Arg1 function in CNS injury and highlight an interaction between infiltrating macrophages and resident microglia in shaping the post-stroke inflammatory milieu. Our study identifies Arg1 in macrophages as a potential therapeutic target for modulating post-stroke inflammation and improving functional recovery. ### Competing Interest Statement The authors have declared no competing interest.