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Natural killer (NK) cells,a type of cytotoxic lymphocytes,can infiltrate into ischemic brain and exacerbate neuronal cell death.Astragaloside Ⅳ (ASⅣ) is the major bioactive ingredient of Astragalus membranaceus,a Chinese herbal medicine,and possesses potent immunomodulatory and neuroprotective properties.This study investigated the effects of ASⅣ on post-ischemic brain infiltration and activation of NK cells.ASⅣ reduced brain infarction and alleviated functional deficits in MCAO rats,and these beneficial effects persisted for at least 7 days.Abundant NK cells infiltrated into the ischemic hemisphere on day 1 after brain ischemia,and this infiltration was suppressed by ASIV.Strikingly,ASⅣ reversed NK cell deficiency in the spleen and blood after brain ischemia.ASⅣ inhibited astrocyte-derived CCL2 upregulation and reduced CCR2+ NK cell levels in the ischemic brain.Meanwhile,ASⅣ attenuated NK cell activating receptor NKG2D levels and reduced interferon-γ production.ASⅣ restored acetylation of histone H3 and the p65 subunit of nuclear factor-κB in the ischemic brain,suggesting inhibition of histone deacetylase (HDAC).Simultaneously,ASⅣ prevented p65 nuclear translocation.The effects of ASⅣ on reducing CCL2 production,restoring acetylated p65 levels and preventing p65 nuclear translocation were mimicked by valproate,an HDAC inhibitor,in astrocytes subjected to oxygenglucose deprivation.Our findings suggest that ASⅣ inhibits post-ischemic NK cell brain infiltration and activation and reverses NK cell deficiency in the periphery,which together contribute to the beneficial effects of ASⅣ against brain ischemia.Furthermore,ASⅣ's effects on suppressing NK cell brain infiltration and activation may involve HDAC inhibition.