Stroke is clinically defined as an acute-onset neurological dysfunction caused by disrupted cerebral perfusion. It ranks as the second leading cause of mortality globally and is a primary contributor to long-term disability, imposing a disproportionately heavy burden on developing regions [1]. Ischemic stroke accounts for approximately 87% of all cases, while hemorrhagic strokes are further classified as intracranial hemorrhage (10%) and subarachnoid hemorrhage (3%) [2], [3]. Current acute management strategies focus on revascularization via intravenous thrombolysis or mechanical thrombectomy for large-vessel occlusions [4]. To date, the only pharmacologic therapy approved by the Food and Drug Administration for acute ischemic stroke is tissue plasminogen activator (tPA) [5], [6]. However, its clinical application is constrained by a narrow therapeutic window (<4.5 h for tPA) [7], [8] and risks of hemorrhagic complications, limiting tPA administration to only 5–7% of eligible patients [6], [9]. These limitations highlight the urgent need to develop therapies targeting post-ischemic secondary injury mechanisms beyond reperfusion.

The pathophysiology of cerebral ischemic stroke involves multiple mechanisms, including neuroinflammatory response, calcium overload, oxidative and nitrative stress, excitotoxicity and apoptosis [10], [11]. Following ischemic insult, a secondary immune responses is triggered, characterized by microglia hyperactivation and the release of cytokines and chemokines [12]. Microglia-mediated neuroinflammation plays a crucial role in ischemic injury; while it can mitigate tissue damage and facilitate tissue repair by clearing dead cells and debris [13], excessive inflammation exacerbates blood-brain barrier (BBB) disruption, cerebral edema, oxidative stress, and microcirculation disorders [14], [15], [16]. Thus, the immune response exerts a dual role in ischemic stroke, and modulating the post-stroke inflammatory microenvironment has emerged as a promising therapeutic approach.

As the resident innate immune cells of the brain, microglia are central to processes of tissue damage, repair, and regeneration [17], [18]. Depending on the microenvironments, activated microglia can adopt either a pro-inflammatory (M1) or anti-inflammatory (M2) phenotype, each characterized by distinct surface markers and cytokine profiles [19], [20]. M1 microglia secrete pro-inflammatory mediators such as tumor necrosis factor (TNF-α), interleukin-1β (IL-1β), and interleukin-6 (IL-6), and upregulate enzymes including cyclooxygenase 2 (COX-2) an inducible nitric oxide synthase (iNOS). In contrast, M2 microglia release anti-inflammatory factors such as arginase 1 (Arg-1), chitinase-like molecule 3 (Chil3/Ym-1), transforming growth factor-β (TGF-β), insulin-like growth factor (IGF), and brain-derived nerve growth factor (BDNF) [20], [21], [22]. Therefore, shifting microglial polarization from M1 to M2 phenotype, or suppressing M1 while enhancing M2 represents a potential strategy to counteract neuroinflammation in stroke treatment.

Nuclear factor κB (NF-κB) is a key transcription factor regulating inflammatory response, and microglia polarization [23]. Toll-like receptor 4 (TLR4), predominantly expressed on microglia, palys a crucial role in NF-κB activation [23]. The TLR4/NF-κB signaling pathway is essential in modulating microglial activation and neuroinflammatory processes [23], [24]. Previous studies indicate that inhibiting of this pathway can suppress M1 activation and promote the M2 phenotype [20], [25]. Triggering receptor expressed on myeloid cells 2 (TREM2), an immunoglobulin-like receptor highly expressed in microglia, also regulates microglial function and polarization [26]. TREM2 activation can suppress the release of inflammatory cytokines and promote anti-inflammatory mediators, thereby exerting neuroprotective effects [27], [28]. Notably, TREM2 and TLR4 pathways exhibit mutual inhibition; TREM2-mediated anti-inflammatory action can attenuate TLR4-induced pro-inflammatory responses by suppressing NF-κB phosphorylation [27], [29]. This crosstalk provides a novel mechanistic insight for therapeutic intervention in stroke.

Fibroblast growth factor 20 (FGF20), a paracrine member of the FGF9 subfamily, was first identified in 2000 as a “neurotrophic factor” [30]. Unlike other FGFs, FGF20 is selectively expressed in the adult brain, especially in the substantia nigra, with minimal presence in healthy peripheral tissues [31]. Current research highlights its roles in Parkinson's disease, BBB maintenance, and angiogenesis following traumatic brain injury (TBI) [32], [33]. Recently, we demonstrated that engineered extracellular vesicles delivering FGF20 enhance neuroplasticity and functional recovery in a mouse model of ischemic stroke [34], and that intracerebral delivery of recombinant human FGF20 via heparin-poloxamer hydrogel improves neurological outcomes in rats [35]. However, the involvement of FGF20 in post-stroke neuroinflammatory cascades remains unexplored. Importantly, no prior evidence links FGF20 to the regulation of microglial polarization. This study is the first to investigate the potential of FGF20 to modulate microglial phenotypes by targeting the imbalance between TLR4/NF-κB and TREM2 signaling, offering a novel therapeutic strategy targeting neuroinflammation resolution in ischemic stroke.