Suppressing NK Cells by Astragaloside IV Protects Against Acute Ischemic Stroke in Mice Via Inhibiting STAT3 Shichun Li 1,2 , Baokai Dou 1 , Shi Shu 1 , Luyao Wei 2 , Shiguo Zhu 1 , Zunji Ke 2 * and Zhifei Wang 1 * 1 School of Basic Medical Sciences, Shanghai University of Traditional Chinese Medicine, Shanghai, China, 2 Academy of Integrative Medicine, Shanghai University of Traditional Chinese Medicine, Shanghai, China Natural killer (NK) cells, a key member of innate lymphocytes, are a promising immunotherapeutic target for ischemic stroke. Astragaloside IV (ASIV) is isolated from Astragalus mongholicus Bunge (Fabaceae), a herbal medicine possessing immunomodulatory ability. This study investigated the effect of ASIV on NK cells during the acute stage of brain ischemic injury in a mouse model of middle cerebral artery occlusion (MCAO). MCAO mice treated with ASIV had better functional outcomes, smaller brain infarction and less NK cell brain infiltration. NK cell depletion echoed the protective effect of ASIV. Notably, ASIV did not enhance the protective effect of NK cell depletion against brain ischemic injury. ASIV inhibited glial cell-derived CCL2-mediated chemotaxis to prevent post-ischemic NK cell brain recruitment. Meanwhile, ASIV also abrogated NK cell-mediated cytolytic killing of neurons subjected to oxygen-glucose deprivation and suppressed NK cell-derived IFN-γ and NKG2D expression in the ischemic brain. The inhibitory effect of ASIV on NK cell brain infiltration and activation was mimicked by cryptotanshinone, a STAT3 inhibitor. There was no additive effect when ASIV and cryptotanshinone were used together. In conclusion, ASIV inhibits post-ischemic brain infiltration and activation of NK cells through STAT3 suppression, and this inhibitory effect of ASIV on NK cells plays a key role in its protection against acute ischemic brain injury. Our findings suggest that ASIV is a promising therapeutic candidate in NK cell-based immunotherapy for the treatment of acute ischemic stroke and pave the way for potential clinical trials. Keywords: natural killer cells, brain ischemia, astragaloside IV, oxygen-glucose deprivation, signal transducer and activator of transcription 3 INTRODUCTION Ischemic stroke accounts for approximately 80% of all strokes and is associated with high morbidity and mortality worldwide. Recombinant tissue plasminogen activator (rtPA) is the only FDA- approved intervention for ischemic stroke. Unfortunately, only a minority of stroke patients is eligible for rtPA treatment due to its narrow therapeutic time window and the potential risk of intracranial hemorrhage (Fonarow et al., 2011). Therefore, it is urgent to develop new treatments for ischemic stroke. Immune responses closely participate in all stages of ischemic cascade, from acute injury to long-term recovery, and shape the outcome of stroke (Iadecola and Anrather, 2011; Levard Edited by: Adolfo Andrade-Cetto, National Autonomous University of Mexico, Mexico Reviewed by: Gabriele Multhoff, Technical University of Munich, Germany Yongsheng Chen, Jinan University, China *Correspondence: Zunji Ke kezunji@shutcm.edu.cn Zhifei Wang zfwang@shutcm.edu.cn Specialty section: This article was submitted to Ethnopharmacology, a section of the journal Frontiers in Pharmacology Received: 26 October 2021 Accepted: 30 December 2021 Published: 03 February 2022 Citation: Li S, Dou B, Shu S, Wei L, Zhu S, Ke Z and Wang Z (2022) Suppressing NK Cells by Astragaloside IV Protects Against Acute Ischemic Stroke in Mice Via Inhibiting STAT3. Front. Pharmacol. 12:802047. doi: 10.3389/fphar.2021.802047 Frontiers in Pharmacology | www.frontiersin.org February 2022 | Volume 12 | Article 802047 1 ORIGINAL RESEARCH published: 03 February 2022 doi: 10.3389/fphar.2021.802047