Depletion of Arg1-Positive Microglia/Macrophages Exacerbates Cerebral Ischemic Damage by Facilitating the Inflammatory Response

Stroke is a serious worldwide disease that causes death and disability, more than 80% of which is ischemic stroke. The expression of arginase 1 (Arg1), a key player in regulating nitrogen homeostasis, is altered in the peripheral circulation after stroke. Growing evidence indicates that ischemic stroke also induces upregulated Arg1 expression in the central nervous system, especially in activated microglia and macrophages. This implies that Arg1 may affect stroke progression by modulating the cerebral immune response. To investigate the effect of Arg1(+) microglia/macrophages on ischemic stroke, we selectively eliminated cerebral Arg1(+) microglia/macrophages by mannosylated clodronate liposomes (MCLs) and investigated their effects on behavior, neurological deficits, and inflammatory responses in mice after ischemic stroke. More than half of Arg1(+) cells, mainly Arg1(+) microglia/macrophages, were depleted after MCLs administration, resulting in a significant deterioration of motility in mice. After the elimination of Arg1(+) microglia/macrophages, the infarct volume expanded and neuronal degenerative lesions intensified. Meanwhile, the absence of Arg1(+) microglia/macrophages significantly increased the production of pro-inflammatory cytokines and suppressed the expression of anti-inflammatory factors, thus profoundly altering the immune microenvironment at the lesion site. Taken together, our data demonstrate that depletion of Arg1(+) microglia/macrophages exacerbates neuronal damage by facilitating the inflammatory response, leading to more severe ischemic injury. These results suggest that Arg1(+) microglia/macrophages, as a subpopulation regulating inflammation, is beneficial in controlling the development of ischemia and promoting recovery from injury. Regulation of Arg1 expression on microglia/macrophages at the right time may be a potential target for the treatment of ischemic brain injury.