==== Front CNS Neurosci Ther CNS Neurosci Ther 10.1111/(ISSN)1755-5949 CNS CNS Neuroscience & Therapeutics 1755-5930 1755-5949 John Wiley and Sons Inc. Hoboken 37309254 10.1111/cns.14275 CNS14275 CNSNT-2022-694.R2 Review Review The interaction between intestinal microenvironment and stroke Zhao et al. Zhao Linna https://orcid.org/0000-0003-3918-6722 1 2 3 Xiao Jie 1 2 4 Li Songlin 1 2 4 Guo Yuying 1 2 3 Fu Rong 1 2 4 Hua Shengyu 4 Du Yuzheng 1 2 drduyuzheng@163.com Xu Shixin 1 2 3 shixinxu1973@outlook.com 1 First Teaching Hospital of Tianjin University of Traditional Chinese Medicine Tianjin China 2 National Clinical Research Center for Chinese Medicine Acupuncture and Moxibustion First Teaching Hospital of Tianjin University of Traditional Chinese Medicine Tianjin China 3 Tianjin Key Laboratory of Translational Research of TCM Prescription and Syndrome Tianjin China 4 Tianjin University of Traditional Chinese Medicine Tianjin China * Correspondence Yuzheng Du and Shixin Xu, First Teaching Hospital of Tianjin University of Traditional Chinese Medicine, Tianjin, China. Email: drduyuzheng@163.com and shixinxu1973@outlook.com 12 6 2023 6 2023 29 Suppl 1 10.1002/cns.v29.s1 Special Issue on Gut Microbiome and Neurological Disorders 185199 09 5 2023 06 9 2022 10 5 2023 © 2023 The Authors. CNS Neuroscience & Therapeutics published by John Wiley & Sons Ltd. https://creativecommons.org/licenses/by/4.0/ This is an open access article under the terms of the http://creativecommons.org/licenses/by/4.0/ License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited. Background Stroke is not only a major cause of disability but also the third leading cause of death, following heart disease and cancer. It has been established that stroke causes permanent disability in 80% of survivors. However, current treatment options for this patient population are limited. Inflammation and immune response are major features that are well‐recognized to occur after a stroke. The gastrointestinal tract hosts complex microbial communities, the largest pool of immune cells, and forms a bidirectional regulation brain‐gut axis with the brain. Recent experimental and clinical studies have highlighted the importance of the relationship between the intestinal microenvironment and stroke. Over the years, the influence of the intestine on stroke has emerged as an important and dynamic research direction in biology and medicine. Aims In this review, we describe the structure and function of the intestinal microenvironment and highlight its cross‐talk relationship with stroke. In addition, we discuss potential strategies aiming to target the intestinal microenvironment during stroke treatment. Conclusion The structure and function of the intestinal environment can influence neurological function and cerebral ischemic outcome. Improving the intestinal microenvironment by targeting the gut microbiota may be a new direction in treating stroke. This review describes the structure and function of the intestinal microenvironment and highlight its cross‐talk relationship with stroke. Improving the intestinal microenvironment may be a potential strategy for treating stroke. gut microbiota intestinal immunity intestinal microenvironment neuroinflammation stroke treatment National Key Research and Development Program of China 10.13039/501100012166 2019YFC0840709 National Natural Science Foundation of China 10.13039/501100001809 81774059 81973626 82204906 Tianjin Health Committee2021099 Tianjin Municipal Science and Technology Commission of China21JCYBJC01620 source-schema-version-number2.0 cover-dateJune 2023 details-of-publishers-convertorConverter:WILEY_ML3GV2_TO_JATSPMC version:6.3.0 mode:remove_FC converted:01.07.2023 Zhao L , Xiao J , Li S , et al. The interaction between intestinal microenvironment and stroke. CNS Neurosci Ther. 2023;29(Suppl. 1) :185‐199. doi:10.1111/cns.14275 The first two authors contributed equally to this work and share first authorship. ==== Body pmc1 INTRODUCTION Stroke is an acute cerebrovascular disease with cerebral ischemia and hemorrhagic injury as its main clinical features. Although stroke is the third leading cause of death after heart disease and cancer, it leads to permanent disabilities in 80% of survivors. 1 , 2 Ischemic stroke is usually caused by the occlusion of the great cerebral arteries and is the most common form of stroke, accounting for approximately 85% of all strokes. 3 , 4 After a stroke, patients can rapidly develop focal or global ischemic injury, which is highly disabling. There are currently two treatments for ischemic stroke: intravenous thrombolytic therapy with recombinant tissue prothrombin activator (tPA) and mechanical thrombectomy. However, both need to be performed within a limited time window. Therefore, new treatment strategies are urgently needed to improve the prognosis of stroke patients. It has been established that after a stroke, up to 50% of patients develop gastrointestinal complications, including intestinal motility disorders, dysphagia, fecal incontinence, leaky gut, intestinal bleeding, and even enterogenic sepsis. 5 , 6 Stroke patients with gastrointestinal complications often have poor prognoses, increased mortality, and worsening neurological function. 7 , 8 The past decade has witnessed significant inroads achieved in the development of genomics, metabolomics, and proteomics, enabling research on the connection between the gut and stroke. The intestine is widely thought to be an important participant in the pathophysiological events after stroke and has become a research hotspot in recent years. Interestingly, almost 25% of T cells in the ischemic brain hemisphere are derived from the intestine. 5 It is now understood that the brain and intestine form a complex “brain–gut axis” through various pathways, 9 , 10 which function in bidirectional regulation. Ischemic stroke alters the intestinal microenvironment leading to immune imbalance; conversely, the intestinal microenvironment can also influence stroke outcomes by modulating immune responses. However, research on the gut‐brain axis in stroke is still in its infancy, and understanding the intestinal microenvironment and its interaction with stroke could facilitate the development of new therapeutic strategies. In this review, we introduce the structure and function of the intestinal environment, provide a comprehensive overview of recent research advances in its interaction with stroke, and discuss possible therapeutic directions and principles. 2 INTESTINAL EPITHELIAL BARRIER AND STROKE 2.1 Normal structure and function of intestinal epithelial barrier The intestinal epithelial barrier (IEB) is one of the largest interfaces between the outside world and the body's internal environment. 11 The IEB is critical for maintaining intestinal homeostasis, serving as a physical barrier and a coordinating center of immune defense and bacterial‐immune cell dialogue. 12 It has been shown that the IEB consists of four cell types: epithelial cells, goblet cells, Paneth cells, and enterochromaffin cells. 11 Intestinal epithelial cells and goblet cells both produce mucin glycoproteins. Intestinal chromaffin cells are the most abundant neuroendocrine cells in the intestinal tract, and Paneth cells are responsible for the production of antimicrobial peptides, 13 , 14 which participate in resisting the invasion of pathogens. IEB secretes substances to form mucus that protects epithelial cells from bacteria, digestive enzymes, toxins, etc. 15 , 16 Current evidence suggests that mucus, mainly secreted by intestinal epithelial goblet cells, is highly glycosylated 17 and plays an important role in maintaining the stability of the IEB. Mucus comprises 90–95% water, proteins, lipids, and electrolytes. Proteins are mainly produced by goblet cells, including mucins, which give mucus its gel‐like properties. Mucus also includes antimicrobial peptides and immunoglobulin A (IgA), allowing mucus to function as an innate defense. 18 Interestingly, it has been shown that mucous fucosylation is significantly reduced in the presence of inflammation. 19 The function of IEB depends on the presence of a series of intercellular junctions composed of apical junction complexes (AJC), including tight junctions (TJ) and adherent junctions (AJ), as well as desmosomes. The TJ structure is composed of transmembrane proteins located in the outer apical portion of the epithelial cell, 20 preventing the passage of antigens through the IEB and playing a key role in maintaining barrier integrity. Adherent junctions and desmosomes are auxiliary structures for cell–cell adhesion located below TJ and are mainly composed of e‐cadherin, catenin, and actin filaments. 21 Interestingly, adherent junctions establish cell–cell connections and promote the maturation of TJ. Moreover, TJ and AJ seal off epithelial cells and control the entry of gut microbes into the intestinal connective tissue. 22 Desmosomes are composed of desmogleins and desmocollins, providing mechanical strength for cell‐to‐cell contact between epithelial cells. 23 2.2 Structure and function changes to the intestinal epithelial barrier in stroke Current evidence suggests that stroke destroys the integrity of the IEB, resulting in intestinal villus epithelial injury, increased permeability, intestinal tight junction damage, reduced mucus, and enterogenic sepsis 14 , 24 (Figure 1). Liu et al. observed significant changes in small bowel morphology over time using a rat permanent middle cerebral artery occlusion (MCAO) model. At 6 h, epithelial cells on the tip of a few villi were necrotic and exfoliated. At 24 h, epithelial cell necrosis, exfoliation, and epithelial dissociation were observed in all villi. 25 Dragana Stanley et al. 24 found increased intestinal permeability in a mice model of MCAO 3 h after surgery, and the level of FITC detected in the blood was similar to that in a dextran sulfate sodium (DSS)‐induced mouse model of acute colitis. In another photochemically‐induced stroke mouse model, when the researchers assessed the intestinal barrier function, it was found that the intestinal permeability of mice increased 1 day after stroke, and ZO‐1, occludin, and claudin‐1 contents in the TJ were significantly decreased. Furthermore, TJ breakage was documented under an electron microscope. 26 In addition, age affects the integrity of the IEB after stroke. Animal experiments have shown that stroke disrupts intestinal homeostasis more significantly in older mice than in younger ones. 27 , 28 Moreover, a study found that stroke in older mice leads to decreased goblet cells that secrete mucus, eventually leading to a deficiency in mucus production. 29 Disruption of the integrity of the IEB after stroke can eventually result in microbiota translocation, whereby bacteria or bacterial components cross the barrier and enter the extra‐intestinal organs. 30 Intriguingly, researchers administered a common commensal bacterium Enterococcus faecalis to germ‐free (GF) mice and examined its translocation and dissemination after experimental stroke induction. 24 h after colonization by E. faecalis, only GF mice that underwent MCAO showed bacterial translocation and spread to surrounding tissues such as the lung, liver, spleen, and mesenteric lymph nodes (MLNs). 24 FIGURE 1 Routes of communication between the intestinal microenvironment and stroke. Reduced mucus layer and disrupted intercellular junctions in the IEB after stroke. The dysregulated gut microbiota produces low levels of SCFAs, Secondary bile acids (SBAs), and PSA, while producing high levels of LPS, bacterial debris, and toxin. Pathogenic bacteria have an increased opportunity to cross the damaged IEB, enter the intestinal lamina propria, and disrupt the balance of intestinal immunity. Subsequently, DCs acquire antigens and transport them to lymphoid tissue, where they initiate an adaptive immune response. The stress response to cerebral ischemic will affect the differentiation of T and B cells, which will secrete different cytokines. These cytokines further exacerbate intestinal inflammation and trigger apoptosis of enteric neurons. Some cytokines migrate with the peripheral circulation to the meninges, exacerbating ischemic neuroinflammation. Figure created with BioRender.com. In conclusion, maintaining the integrity of the intestinal epithelial barrier (IEB) is crucial for preserving intestinal balance. Disruption of the IEB's structure and function following a stroke can result in microbial translocation, which may be a significant contributor to post‐stroke infections. 3 GUT MICROBIOME AND STROKE 3.1 Normal composition and function of gut microbiome It is estimated that the human gut microbiota consists of 1014 bacteria, 10 times the number of cells in the human body. The gut commensal microbiome is a large and complex ecosystem characterized by mutual restraint and interdependence. 31 The gut microbiota is mostly anaerobic, with a small fraction being aerobes and facultative anaerobes, archaea, viruses, and unicellular eukaryotes. 32 So far, more than 50 phyla have been described, but the human gut microbiota is dominated by Bacteroidetes and Firmicutes, followed by Actinobacteria and Verrucomicrophyla. 32 , 33 The gut microbiota has diverse functions. In addition to promoting intestinal digestion, they also promote the integrity of the intestinal epithelial barrier by regulating TJ and AJ function, 34 , 35 the proliferation, differentiation, and migration of epithelial cells, 36 , 37 mucus secretion and antimicrobial peptide gene expression. 38 , 39 Current evidence suggests that the gut microbiota also assists in the development and function of the immune system. Peyer's patches composed of aggregated lymphoid follicles are dysplastic in the gut of GF mice. Compared with conventional mice and rats, the composition of CD4+ T cells and IgA‐producing B cells is altered in the lamina propria of the epithelial mucosal basal tissue in GF mice. Moreover, the gut microbiota in the intestinal lumen can enhance the induction of T lymphocyte subsets. For example, segmented filamentous bacteria (SFB) that adhere to the luminal surface of mouse intestinal Peyer's patches strongly stimulate Th17 differentiation in the lamina propria. 40 , 41 Gut commensal microbiota also regulates the development and function of lymphocytes in the small intestine, including Treg cells, γδT cells, Th17 cells, Th1 cells, and Th2 cells. 42 , 43 , 44 , 45 , 46 In addition, the gut microbiota can affect the body through metabolites. These microbial metabolites are mainly divided into three categories, and the first type is produced by direct digestion or fermentation of food components by gut microbiota, such as short‐chain fatty acids (SCFAs). The fermentation of dietary fiber by anaerobic microorganisms in the small intestine produces SCFAs, which promote the secretion of intestinal mucus, regulate the permeability of the intestine, 47 , 48 provide energy for colonic muscle cells to treat high blood pressure, 49 diabetes, 50 multiple sclerosis (MS), 51 emotional and cognitive dysfunction, 52 and cardiovascular disease. 53 , 54 The second is metabolites produced by the host that enter the intestine to be reprocessed and modified by gut microbiota, such as secondary bile acids (SBAs). It is well‐established that gut microbiota can modify primary bile acids synthesized in the liver after entering the intestine with food. These modifications include the removal of amino acid residues through bile salt hydrolase (BSH) activity and further metabolism through dehydroxylation, dehydrogenation, or differential isomerization to produce SBAs. 55 The microbiota involved in bile acid metabolism can alter the bioavailability and bioactivity of bile acids, thereby affecting epithelial cell proliferation, lipid and energy metabolism, and the expression of inflammatory genes. 56 , 57 The third is the self‐synthesized gut microbiota metabolites, such as polysaccharide A (PSA). PSA is a capsular polysaccharide mainly produced by nontoxigenic Bacteroides fragilis (NTBF) strains. 58 , 59 It induces CD4+ T cells to produce IL‐10 with an anti‐inflammatory effect by activating the TLR2/1 heterodimers and dectin‐1 signaling pathway on dendritic cells (DCs) and inhibits gastrointestinal inflammation. 60 , 61 3.2 Composition and function changes to gut microbiome in stroke Overwhelming evidence substantiates that ischemic stroke alters the composition of the gut microbiota. In one study, 16S rDNA was used to sequence collected feces, and the results demonstrated increased levels of Bacteroidetes phylum following cerebral infarction in cynomolgus monkeys. 62 It is widely acknowledged that Prevotella, belonging to the phylum Bacteroidetes, plays an essential pro‐inflammatory role in human chronic inflammatory diseases. 63 Moreover, after cerebral infarction induction in cynomolgus monkeys, the increased relative abundance of the Prevotella genus may also be related to the post‐stroke inflammatory response. 62 Reduced species diversity of gut microbiota and overgrowth of Bacteroidetes phylum 3 days after acute ischemic surgery in mice were identified as markers of post‐stroke microbiota dysbiosis. 5 However, a case–control study showed reduced abundance levels of Bacteroides, Prevotella, and Faecalibacterium in patients with large‐artery atherosclerotic ischemic stroke and transient ischemic attack, and the microbial alpha diversity was increased. 64 This result contradicts the findings observed in animal model experiments, which may be attributed to the fact that the daily diet of stroke patients and diseases such as hypertension, diabetes, and obesity may affect the gut microbiota. 65 , 66 , 67 , 68 In addition, Faecalibacterium and Oscillospira genera levels in monkey feces and butyrate concentrations decreased after 6–12 months of cerebral infarction induction. 62 It is widely thought that Faecalibacterium and Oscillospira are the primary sources of butyrate production in the host. 69 , 70 Butyrate plays a critical role in maintaining the integrity of the intestinal barrier and inhibiting the production of pro‐inflammatory cytokines. 71 Furthermore, gut microbiota could influence stroke outcomes through a bidirectional brain–gut communication pathway 5 , 72 (Figure 1). Benakis et al. 73 performed MCAO surgery after pretreating mice with antibiotics and found that changes in gut microbiota reduced brain infarct size and improved sensorimotor function. Subsequently, Singh et al. 5 revealed changes in the microbiota after cerebral ischemia and its role in neuroinflammatory response after stroke. To investigate this mechanism, they established a model of microbial transfer in GF mice. Microbiota obtained from sham‐operated and post‐filament middle cerebral artery occlusion model (fMCAO) mice were transplanted into GF recipient mice, and cortical damage was induced in GF recipient mice using the permanent distal MCA occlusion model (cMCAO). Recipients of the fMCAO group expressed high levels of pro‐inflammatory Th1 and Th17 cells in brain tissue to exacerbate stroke progression. 5 Gut flora homeostasis was restored in recipients of the healthy group through fecal microbiota transplantation (FMT), increasing the abundance of Treg cells in the ischemic brain and significantly reducing brain damage. 5 Microbial metabolites also play a crucial role in stroke recovery. Tryptophan (Trp) is an essential aromatic amino acid metabolized in the gut to indole, indole derivatives, and its ligands for the aryl hydrocarbon receptor (AHR). 74 These tryptophan metabolites regulate the function of the intestinal barrier and immune cells through AHR signaling. 75 In vivo studies in mice have shown that Lactobacillus amplification increases the production of indole‐3‐aldehyde (IAld), the molecule responsible for the ligand activity of the tryptophan metabolite AHR, which then maintains intestinal mucosal immune homeostasis via the AhR‐IL‐22 axis. 76 Tryptophan concentrations have also been found to correlate with neuroinflammation and prognostic outcomes in acute ischemic stroke in clinical samples. 77 Besides, short‐chain fatty acids can affect stroke recovery. To assess the potential correlation of gut microbiota and fecal SCFAs profiles with stroke prognosis in acute ischemic stroke (AIS) patients, researchers designed a prospective observational study of 140 AIS patients and 92 healthy individuals. Fecal bacterial counts and SCFA levels were determined by 16S rRNA gene sequencing and gas chromatography–mass spectrometry. 78 These results confirm that patients with AIS (especially those with increased stroke severity) have a significant gut microbiota imbalance with SCFAs deficiency, which may trigger a leaky gut. 78 A study conducted in Japan substantiated that patients with ischemic stroke had decreased acetic acid and increased valeric acid levels. 79 Transplantation of SCFAs‐enriched fecal microbiota or the use of butyrate to influence SCFAs levels to repair leaky gut is reportedly effective for treating ischemic stroke. 80 Notably, SCFA supplementation before stroke in mice improved behavioral function and cortical network plasticity at later stages after stroke. This effect may be due to microglia‐mediated activation by SCFAs via circulating lymphocytes. 81 In another study, oral administration of SCFAs‐producing bacteria and inulin (the bacterial substrate produced by SCFAs) could reduce the percentage of IL‐17+γδT cells in ischemic brains and neurological deficits after stroke in older mice and improve depression‐like behavior compared with young mice. 82 In addition, the blood–brain barrier (BBB) plays a crucial role as the brain's gatekeeper in maintaining the homeostasis of the central nervous system and normal neuronal function. One of the pathological features of stroke is BBB dysfunction, associated with disrupted TJ structure and increased BBB permeability, thereby increasing the risk of vasogenic edema and hemorrhagic transformation in patients, leading to poor outcomes. 83 , 84 Recent studies have found that butyrate improves BBB dysfunction and reduces BBB permeability after ischemic stroke in aged mice. This therapeutic effect may be attributed to the increased expression of IL‐22 in the brains of post‐stroke mice after butyrate treatment. 85 In summary, the gut microbiota and its metabolites play a critical role in maintaining the IEB's integrity, as well as in the development and function of intestinal immune cells. Dysbiosis of the gut microbiota due to stroke can increase the number of pathogenic bacteria and alter several essential bioactive metabolites. Some of these metabolites can aid in neurological recovery, improve stroke prognosis, and may represent a novel approach to stroke injury repair. 4 INTESTINAL IMMUNE SYSTEM AND STROKE 4.1 Normal structure and function of the intestinal immune system Gut‐associated lymphoid tissue (GALT) is considered the largest immune organ in the human body. There are three main types of GALT, Peyer's patches (PPs), MLNs, and isolated lymphoid follicles (ILFs). 86 PPs consist of B cell follicles surrounding the T cell zone and surrounded by specialized epithelial cells. 87 T cells in the PPs can migrate to MLNs and present antigens there. 88 These PPs and MLNs activate lymphocytes (T cells and B cells) that can enter the peripheral circulation through the thoracic duct, generate an immune response, limit systemic inflammation, and lymphatically nest back into the lamina propria of the gut. 89 , 90 ILFs include other smaller cell populations, such as DCs, lymphoid tissue‐inducing cells (LTi), and large populations of B cells. 86 DCs are antigen‐presenting cells that acquire antigens and transport them to lymphatic tissues. There are four basic ways for DC to obtain intestinal antigens: (1) presentation of antigens transported by M cells; (2) extension into the lumen through epithelial cells; (3) endocytosis of antigens by epithelial cells and (4); direct contact with antigen via functional destruction of epithelial cells. 91 Notwithstanding that much research has been done on intestinal DCs, little is known about other intestinal antigen‐presenting cells (APCs). Interestingly, the gastrointestinal mucosa contains a large number of macrophages. Although they are thought to be coordinators of intestinal mucosal inflammation, the mechanism of their inflammatory unresponsiveness remains unclear. 92 Recent studies have shown that intestinal macrophages, through the production of IL‐10, play an essential role in protecting the host from pathogen invasion and regulating excessive immune responses to commensal bacteria. 93 The dysregulation of these cell functions leads to an uncontrolled intestinal immune response. 94 4.2 Structure and function changes to the intestinal immune system in stroke Intestinal immune cells play a vital role in the progression of stroke. Although the microbiota regulates immune cells in the gut, immune cells can migrate to the brain via peripheral circulation after a stroke. 73 Intriguingly, it has been shown that mice with cerebral ischemia–reperfusion had increased differentiation of Th17 cells in the lamina propria of the small intestine (SI‐LP) after 3 days, with a concomitant increase in the expression of IL‐23 and IL‐17A in the small intestine. 95 In contrast, the differentiation of Treg cells and the secretion of the anti‐inflammatory cytokine IL‐10 by Treg cells were decreased in the small intestine. 95 In addition, most γδT cells in the human body mainly exist on the surface of the intestinal epithelium and participate in the innate immune response of the intestine. Interestingly, after ischemic stroke, γδT cells migrate from the gut through the peripheral circulation to the brain membrane and secrete IL‐17 into the damaged brain tissue. Subsequently, IL‐17 induces increased chemokines in the brain parenchyma, leading to massive neutrophil infiltration and exacerbating ischemic neuroinflammation. 73 Il‐10 secreted by Treg cells in the small intestine can inhibit the differentiation of γδT cells and play a neuroprotective role. 73 In addition to Th17/Treg or γδT /Treg responses, classical Th1 and Th2 responses may be altered after acute ischemic stroke. The researchers measured mRNA levels in the small intestine of the cytokines IFN‐γ and IL‐4, representing Th1 and Th2 effector phenotypes, respectively, and found IFN‐γ expression was increased and IL‐4 expression decreased on day 3. 95 These findings suggested that pharmacological intervention of the immune balance of Th17/Tregs and Th1/Th2 in the small intestine could ameliorate cerebral ischemic injury. 95 Peyer's patches represent an inductive site for immune responses mediated by T and B cells to intestinal antigens 96 , 97 (Figure 1). B cells are predominantly found in PPs and differentiate into IgA‐producing B cells in the presence of T cells to eliminate toxins and pathogens. 98 , 99 , 100 It has been demonstrated that stress prior to cerebral ischemia can significantly reduce large intestinal IgA and bacterial translocation in a rat stroke model. 101 In addition, dendritic cells in PPs are equally critical for initiating and developing adaptive immune responses. 99 , 102 It is widely thought that stroke‐induced reduction of Peyer's patches B cells and DCs could threaten local and systemic immune system homeostasis and lead to compromised local antimicrobial defenses. Interestingly, in an animal model, the number of B cells in the PPs of 129SV mice that underwent MCAO for 60 minutes was also reduced. However, the number of T cells in PPs was inconsistent with the literature. Although significant reductions were also observed, no changes were found in intestinal epithelial and lamina propria lymphocyte subsets, 103 which may be attributed to the sensitivity of lymphocytes to stress‐induced apoptosis depending on the type and duration of stress and the phenotype and anatomical location of lymphocytes. 104 In another study, the researchers focused on immune cell populations in the intestinal PPs one day postoperatively in mice with stroke. The results showed that the number of CD11b+CD11c+ DCs and B cells in PPs of mice after cerebral ischemia decreased. In contrast, no significant change in the number of T cells was observed. 105 In summary, the intestinal immune system, which comprises multiple immune tissues and cells, works together in a normal physiological environment to defend against pathogenic invasion and maintain immune homeostasis. However, after a stroke, this immune homeostasis is disrupted, and the intestinal immune cells and cytokines undergo alterations. Importantly, this change is not confined to the gut but also affects distal brain tissue. The migration of lymphocytes and the increase in inflammatory mediators can exacerbate ischemic neuroinflammation, making treatment and recovery after a stroke challenging. 5 ENTERIC NERVOUS SYSTEM AND STROKE 5.1 Normal structure and function of the enteric nervous system As the largest sensory organ in the body, the gut differs from other peripheral organs with its extensive internal nervous system called the enteric nervous system (ENS), which controls the function of the gut. The ENS originates from the embryonic neural crest and consists of the myenteric and submucosal plexuses that run nearly parallel throughout the entire gut. From an evolutionary perspective, the ENS can be considered the “first brain”. 106 The ENS can act as a “local neural mechanism” to independently control intestinal behavior, such as regulating gastrointestinal motility, secretion, and local blood flow and interacting with the immune and endocrine systems. 107 , 108 , 109 Ample evidence suggests that the interaction between intestinal epithelial cells and the ENS affects gut homeostasis. The ENS not only interacts with the microbiota, metabolites, and nutrients on the surface of intestinal epithelial cells but also with the microenvironment of immune cells and stromal cells. 110 In addition, the ENS interacts with epithelial cells to promote barrier function and protect the intestine from pathogens in the intestine. 111 Albeit the ENS is subject to considerable mechanical, chemical, and microbial stressors in the intestine, its structure remains stable. The ENS is stable because nearly the entire neuronal population of the ENS is continually renewed every few weeks, driven by enteric neural precursor cells (ENPCs), which continuously and rapidly generate new neurons to counteract the neuronal population lost to apoptosis. 112 The external connection between CNS and ENS comprises sympathetic and parasympathetic nerve fibers directly connected to the gastrointestinal from the hindbrain. 113 The central nervous system communicates with the gut through the gut‐brain axis. Vagus nerve and spinal cord sensory neurons terminate at different locations in the intestinal wall, including the muscle layer and mucosal epithelium, and play an important role in the CNS transmitting information to the ENS and vice versa. 114 5.2 Structure and function changes to the enteric nervous system in stroke Growing evidence suggests cerebral ischemia affects the enteric nervous system (Figure 1). The two main types of cerebral ischemia are focal and global. Focal cerebral ischemia refers to insufficient blood flow to specific parts of the brain, while global cerebral ischemia involves extensive brain areas. In the ENS, the neurotransmitters nitric oxide (NO) and vasoactive intestinal peptide (VIP) are thought to play essential roles in the maintenance and protection of neurons. 115 , 116 , 117 , 118 Significant enteric neuron loss has been documented in mouse ileal muscle enteric ganglia following permanent middle cerebral artery occlusion (pMCAO). 119 The researchers further investigated the expression and the relative number of vasoactive intestinal peptide (VIP) and neuronal nitric oxide synthase (nNOS) neurons in three mouse models of pMCAO, global cerebral ischemia–reperfusion (GCIR) or chronic cerebral hypoperfusion (CCH). 120 No changes in intestinal mucosa or muscle layer thickness were found in the three cerebral ischemia models. On day 7 after pMCAO, the ileal muscle‐enteric neurons of mice were depleted, the number of submucosal VIP‐ immunoreactive (IR) neurons increased, and the relative number of nNOS‐IR neurons did not change significantly. 120 Intriguingly, there were no significant changes in the relative numbers of neurons or VIP and nNOS‐IR neurons in the GCIR and CCH groups, contrary to other studies where activation of the GAL‐3/TLR4 pathway led to significant neuron loss after cerebral ischemia in all three models of cerebral ischemic. 119 , 121 , 122 , 123 , 124 However, the pMCAO model exhibited permanent damage to regional cerebral blood flow, whereas GCIR caused transient occlusion, and CCH caused a chronic decrease in central cerebral blood flow. Therefore, further damage to brain tissue may affect the neurovascular barrier and the activation of the peripheral immune system differently. In conclusion, the transmission of information between the ENS and the central nervous system is crucial for maintaining human health. The different types of neurons in the ENS form a complex network that continuously collects and releases signals. It is well‐established that gastrointestinal dysfunction can be triggered after a stroke. In addition to the gut barrier, gut microbes, and immune system mentioned in previous articles, the enteric nervous system is also an essential factor. Loss of enteric neurons has been demonstrated in the PMCAO animal model after a stroke. However, due to the limited studies available, it can only be speculated that the loss of enteric neurons may be associated with the activation of the peripheral immune system after ischemia. The complex role of the enteric nervous system in the brain–gut axis after cerebral ischemia requires further investigation. 6 TREATMENT STRATEGY The treatment of ischemic stroke remains a great challenge. Given limitations associated with the time window and surgical safety, less than 5% of stroke patients receive active and effective treatment. 125 In recent years, immunotherapy for stroke has primarily focused on reducing injury volume and improving functional outcomes. Although the mechanism of immune‐mediated neuronal injury has attracted much attention, few drugs have reached the stage of phase II or phase III randomized controlled trials (RCT) in acute stroke. The drugs currently entering the RCT phase are recombinant interleukin‐1 receptor antagonist (IL‐1Ra) (Anakinra), anti‐ICAM‐1 antibody (Enlimomab), Minocycline, Natalizumab, and Fingolimod. 126 , 127 , 128 , 129 , 130 , 131 Although these drugs have shown positive results in preclinical animal stroke models, clinical trial results have not been satisfactory. Only Anakinra, Minocycline, and Fingolimod have demonstrated the potential to treat stroke in clinical studies, but further validation in large‐scale clinical trials is still needed. 132 Preclinical studies usually use young and healthy animals without comorbidities, while in reality, most stroke patients are older, with high rates of hypertension, hyperlipidemia, and diabetes. 133 In addition, “immunosenescence” refers to a series of age‐related changes in the immune system, which is also an important factor in treating stroke. 134 Microglia from older brains were found to secrete more pro‐inflammatory factors and exhibited an increased tendency to polarize to the M1 phenotype compared with younger brains 135 and a reduced ability to regenerate axons with age. 136 Many studies have revealed that alterations in the intestinal microenvironment after ischemic brain injury induce a pro‐inflammatory immune response, enlarge the infarct size, and are strongly associated with stroke prognosis. 137 Several treatments that modulate the gut microenvironment have shown promise in preventing or treating stroke. In the following section, we will review the impact of these treatments on stroke. We also searched for current clinical trials on treating stroke by modulating the intestinal microenvironment to provide further scientific evidence for the therapy of stroke (Table 1). TABLE 1 Current clinical trials in the treatment of stroke through modulation of the intestinal microenvironment. Interventions Study title Objective/methods Trail identifier Status Dietary supplement: probiotics Cognition and Gut Microbiome Associated Study of Shanghai People with Acute Ischemic Stroke This study will collect the data from the day of admission and 3 months after stroke data and put it into analysis to provide some suggestions on the probiotics used in the clinic for the stroke patients. NCT03812445 Recruiting Dietary Supplement: OMNi‐BiOTiC SR‐9 A Randomized Double Blinded Placebo Controlled Study on the Effects of Dietary Supplementation with a Probiotic on Stroke Patients Patients are recruited within seven days of stroke onset and randomly assigned to either the Control or Treatment group and subsequently take the commercially available probiotic or the placebo twice a day for 3 months. NCT04954846 Recruiting Dietary supplement: probiotics A randomized controlled, prospective clinical study of probiotics in the treatment of acute ischemic stroke The primary research purpose is to evaluate the safety and effectiveness of probiotics in treating acute ischemic stroke. ChiCTR2100051641 Others Buyang Huanwu decoction combined with probiotics Study on the effect and mechanism of Buyang Huanwu decoction combined with probiotics on ischemic stroke based on intestinal flora This study takes intestinal flora as the target and intends to clarify the effect and mechanism of Buyang Huanwu decoction combined with probiotics in the treatment of ischemic stroke to provide a new scheme for the clinical treatment of ischemic stroke. ChiCTR2000031238 Pending Drug: Tongfu capsules Safety and Efficacy of the Tong‐Fu‐Xing‐Shen Herbal Formula for Stroke‐Associated Pneumonia (TFXSHF) Stroke‐associated pneumonia (SAP) patients are recruited to clarify whether TFXS is effective and safe for the treatment of SAP and affects the immunological mechanism of the “brain–gut–lung” pathway of SAP. NCT04275219 Recruiting Drug: Xinglu Chengqi Decoction Clinical research of Xinglou Chengqi Decoction on improving post‐stroke cognitive impairment due to phlegm heat and viscera excess syndrome based on brain–gut interaction Based on brain–gut interaction, observation of the therapeutic effect of Xinglu Chengqi Decoction on post‐stroke cognitive impairment due to phlegm heat and excess viscera syndrome. ChiCTR2000040910 Recruiting 6.1 Antibiotics Although the mechanism is unclear, the gut microbiota may be a source of systemic infection in stroke patients, given that the immunosuppression initiated after stroke limits the autoimmune response. 5 , 138 , 139 It is widely acknowledged that broad‐spectrum antibacterial drugs have anti‐inflammatory effects and reduce the risk of infection in stroke patients. Pre‐stroke use of amoxicillin (β‐lactam antibiotic) and clavulanic acid (β‐lactamase inhibitor) can alter immune homeostasis in the mouse small intestine by affecting the intestinal microenvironment, leading to an increase in regulatory T cells and a decrease in minus IL‐17+γδ T cells. 73 Oral administration of polymyxin B modulates intestinal flora, reduces lipopolysaccharide (LPS) levels and neuroinflammation in the ischemic brain of type 2 diabetic (db/db) mice, and improves metabolic endotoxemia and stroke outcome in db/db mice. 140 Controversially, studies on mice have shown that antibiotic‐induced changes in the intestinal flora could reduce ischemic brain damage. However, in other experiments, broad‐spectrum antibiotics caused extensive microbiota depletion and worsened stroke prognosis. 141 Transient global forebrain ischemia (tIsc) mice given oral vancomycin or ampicillin exhibited cognitive impairment and worsened intestinal inflammation with an increase in Enterobacter xiangfangenesis from the Proteobacteria family. 142 These studies indicate the intricate interconnection between commensal microbial populations and their host. Different antibiotics could induce different alterations in microbiota structure and microbiota‐derived metabolites and influence the organism's immune response. Fecal transplants were performed after pretreatment of mouse donor feces with vancomycin, streptomycin, and metronidazole. The vancomycin and streptomycin groups exhibited increased infiltration of pro‐inflammatory polymorphonuclear leukocytes and monocytes and proliferating iNKT cells in the intestinal LP. 143 These results may be attributed to the fact that vancomycin and streptomycin treatment increased the abundance of Bacteroides, Parabacteroides, Streptococcus, and unclassified Enterobacteriaceae in the flora and led to specific enrichment of microbial metabolites such as gluconate and azelaic acid. On the other hand, the microbiota structure of the recipients receiving metronidazole‐pretreated feces showed the enrichment of Lactobacillus and cellobiose. These florae and metabolites could be important for modulating mucosal immunity and promoting an IL‐10‐dependent anti‐inflammatory response. 143 6.2 Probiotics Probiotics are live strains of strictly selected microorganisms that, when administered adequately, confer a health benefit on the host. 144 Probiotics enter the gut and ensure the balance of intestinal microbes by producing antibacterial substances, competing with pathogenic microbes for epithelial adhesion and nutrients. 145 They also provide immunomodulation by triggering a signaling cascade in epithelial cells, while their metabolites inhibit the production of bacterial toxins. 146 , 147 Clostridium butyricum pretreatment for 2 weeks has been shown to improve I/R brain injury in mice by reducing neurological deficits, alleviating oxidative stress, and inhibiting apoptosis. 148 In another study, pretreatment with a probiotic mixture (including Bifidobacterium breve, Lactobacillus casei, Lactobacillus bulgaricus, and Lactobacillus acidophilus) for 2 weeks improved I/R mouse injury and significantly reduced brain infarct size through antioxidant mechanisms. 149 In a mouse model of cerebral hippocampus injury, prophylactic intake of a mixture of seven probiotic bacteria reduced hippocampal neuronal damage and restored spatial memory capacity by inhibiting apoptosis. 150 In addition, inactivated Lactobacillus could suppress neuronal apoptosis, reduce brain infarct volume, decrease oxidative stress, and improve neurobehavioral scores in rats by inhibiting the TLR‐4/NF‐κB signaling pathway. 151 Although probiotics that promote intestinal microenvironmental health are mainly considered safe and valuable, abuse in specific high‐risk populations may also lead to severe infections, necessitating a risk and benefit assessment prior to use to better exploit the benefits of probiotics. 152 6.3 Fecal microbiota transplantation Fecal microbiota transplantation (FMT) enables modification of the recipient's intestinal microbiota to normalize its composition and obtain therapeutic benefits. 153 FMT use dates back to the 4th century and was officially approved by the US Food and Drug Administration in 2013 to treat recurrent and refractory Clostridium difficile infections. 154 Since then, the use of FMT has gained significant momentum, and studies in recent years have found that FMT is also efficient in treating stroke and post‐ischemic complications. 155 After an experimental stroke, microbiota transplantation from an acute middle cerebral artery occlusion mouse model into germ‐free mice could exacerbate the lesion size and functional deficits. 5 In contrast, mice treated with FMT from healthy donors after fMCAO exhibited significantly reduced lesion size through immunomodulatory mechanisms. 5 Importantly, transplantation of SCFAs‐rich fecal microbiota can reshape the gut microbiota, enrich the beneficial Lactobacillus and repair the leaky gut, making it an effective treatment for ischemic stroke. 80 Current evidence suggests that complications of type 2 diabetes (T2D) aggravate brain infarction in AIS. In this respect, the transplantation of feces from T2D mice supplemented with butyrate could improve the prognosis of stroke by significantly reducing infarct size and the levels of pro‐inflammatory cytokines in the serum. 156 In addition, the “age of feces” is an essential factor to consider in FMT, as studies have found that an aging gut microbiome can reduce SCFAs in the host and lead to cognitive decline. 157 The feces of young mice contained high levels of SCFAs‐producing bacteria and higher SCFAs concentrations, and older stroke mice that underwent young fecal transplant gavage exhibited reduced neurological deficits and inflammation, with significantly higher intestinal, brain, and plasma SCFAs concentrations. 82 In another study, mice transplanted with an aging microbiota showed an increased ratio of Firmicutes phyla to Bacteroidetes phyla, regardless of the recipient's age. The establishment of a younger microbiota in aged mice by fecal transplantation tube feeding reduced systemic pro‐inflammatory cytokine levels and improved prognosis and survival after MCAO. 137 Given that microbial therapy is still in its infancy, side effects such as peripheral neuropathy from unknown pathogenic organisms cannot be ruled out with the current use of FMT transplants. 158 6.4 Other treatments An increasing body of literature suggests that many herbs and their active ingredients can regulate the gut microbiota and play a key role in treating ischemic stroke at different stages. 159 , 160 , 161 The efficacy of the Tanhuo decoction (THD) for AIS has been clinically proven, with THD + basic treatment in the THD group improving the efficacy of treatment for AIS by reducing LPS‐producing bacteria, significantly increasing acetic acid‐producing bacteria and enhancing the complexity of intestinal flora coexistence to inhibiting the gut microbiota‐derived metabolites LPS and TMAO compared with the basal treatment group. 162 Xinglou Chengqi Tang (XCD) is also effective in improving neurological function in stroke mice by regulating the gut microbiota and increasing the levels of SCFAs. 163 NaoMaiTong (NMT) can improve stroke prognosis in the MCAO rat model by protecting the intestinal barrier and modulating intestinal flora and endogenous metabolites. 164 Tong‐Qiao‐Huo‐Xue Decoction (TQHXD) also affects the gut microbiota of stroke rats. It inhibits the excessive increase of Bacteroidetes and ameliorates the disturbance of gut microbiota after stroke. Interestingly, TQHXD also inhibited the inflammatory response induced by the peripheral immune imbalance caused by the dysregulated gut microbiota and disrupted intestinal barrier. 165 Moreover, combined treatment of Puerariae Lobatae Radix (PLR) and Chuanxiong Rhizoma (CXR) in rats with ischemic stroke could relieve gut microbiota dysbiosis and brain–gut barrier disruption, which effectively improved neurological function, reduced cerebral infarction and alleviated complications including dyslipidemia, increased blood viscosity and thrombotic risk. 166 In addition, bone marrow mesenchymal stem cells (BMSCs) have been shown to enhance functional recovery and improve cognitive dysfunction and neuroplasticity by regulating neurogenesis, angiogenesis, and oligodendrocyte production. 167 , 168 In recent years, it has been found that BMSCs can increase the abundance of Lactobacillus and regulate the ecological dysbiosis of the gut microbiota and promote neurological recovery as a potential treatment for ischemic stroke. 169 Some natural products and dietary approaches also play an essential role in preventing and treating neurological diseases. They can modulate the activity of enzymes (such as kinases, regulatory receptors, and proteins) with multiple targets and signaling pathways, directly or indirectly, thus exerting neuroprotective effects. 170 , 171 , 172 , 173 These approaches may become complementary therapies in treating stroke in the future. 7 CONCLUDING REMARKS In the present review, we provided a comprehensive overview of the significance of the structure and function of the intestinal environment and its interaction with ischemic stroke. While the influence of the intestinal environment on neurological function and cerebral ischemia outcomes has been established, the mechanisms underlying the regulation of brain function before and after cerebral ischemia by the intestinal environment require further investigation. Over the past decade, the combined multi‐omics analyses of genomics, transcriptomics, proteomics, and metabolomics have led to a better understanding of the gut microbiota. This review mainly collected data from studies conducted on animal stroke models and clinical studies. While most studies showed that improving the intestinal microenvironment by improving the gut microbiota is an exciting approach to treating stroke, it is still far from clinical application. In future studies, a better understanding of the impact of age and stroke comorbidity on the intestinal microenvironment will be essential to improve stroke treatment and develop new microbial therapy‐based approaches. AUTHOR CONTRIBUTIONS Linna Zhao and Jie Xiao contributed equally to this work. The article is mainly conceived and written by Linna Zhao and Jie Xiao. Songlin Li performed the literature search. Yuying Guo and Rong Fu designed the figure and table. Shengyu Hua contributed to manuscript revisions. Yuzheng Du and Shixin Xu designed and supervised this work. All authors have read and approved the final submission. FUNDING INFORMATION This work was supported by the National Natural Science Foundation of China (Nos. 81973626, 81774059, and 82204906), National Key Research and Development Program of China (No. 2019YFC0840709), Tianjin Municipal Science and Technology Commission of China (No. 21JCYBJC01620), and Tianjin Health Committee (No. 2021099). CONFLICT OF INTEREST STATEMENT The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest. DATA AVAILABILITY STATEMENT Data sharing not applicable to this article as no datasets were generated or analysed during the current study. ==== Refs REFERENCES 1 Moskowitz M , Lo E , Iadecola C . The science of stroke: mechanisms in search of treatments. Neuron. 2010;67 (2 ):181‐198. doi:10.1016/j.neuron.2010.07.002 20670828 2 Lallukka T , Ervasti J , Lundström E , et al. Trends in diagnosis‐specific work disability before and after stroke: a longitudinal population‐based study in Sweden. J Am Heart Assoc. 2018;7 (1 ):1‐14. doi:10.1161/jaha.117.006991 3 Virani S , Alonso A , Aparicio H , et al. Heart disease and stroke Statistics‐2021 update: a report from the American Heart Association. Circulation. 2021;143 (8 ):e254‐e743. doi:10.1161/cir.0000000000000950 33501848 4 Feigin V , Nguyen G , Cercy K , et al. Global, regional, and country‐specific lifetime risks of stroke, 1990 and 2016. N Engl J Med. 2018;379 (25 ):2429‐2437. doi:10.1056/NEJMoa1804492 30575491 5 Singh V , Roth S , Llovera G , et al. Microbiota dysbiosis controls the neuroinflammatory response after stroke. J Neurosci. 2016;36 (28 ):7428‐7440. doi:10.1523/jneurosci.1114-16.2016 27413153 6 Durgan D , Lee J , McCullough L , Bryan R . Examining the role of the microbiota‐gut‐brain axis in stroke. Stroke. 2019;50 (8 ):2270‐2277. doi:10.1161/strokeaha.119.025140 31272315 7 Bonkhoff AK , Rübsamen N , Grefkes C , Rost NS , Berger K , Karch A . Development and validation of prediction models for severe complications after acute ischemic stroke: a study based on the stroke registry of northwestern Germany. J Am Heart Assoc. 2022;11 (6 ):e023175. doi:10.1161/jaha.121.023175 35253466 8 Tuz AA , Hasenberg A , Hermann DM , Gunzer M , Singh V . Ischemic stroke and concomitant gastrointestinal complications‐ a fatal combination for patient recovery. Front Immunol. 2022;13 :1037330. doi:10.3389/fimmu.2022.1037330 36439129 9 Carabotti M , Scirocco A , Maselli M , Severi C . The gut‐brain axis: interactions between enteric microbiota, central and enteric nervous systems. Ann Gastroenterol. 2015;28 (2 ):203‐209.25830558 10 Wang H , Wang Y . Gut microbiota‐brain axis. Chin Med J. 2016;129 (19 ):2373‐2380. doi:10.4103/0366-6999.190667 27647198 11 Rodríguez‐Colman M , Schewe M , Meerlo M , et al. Interplay between metabolic identities in the intestinal crypt supports stem cell function. Nature. 2017;543 (7645 ):424‐427. doi:10.1038/nature21673 28273069 12 Artis D . Epithelial‐cell recognition of commensal bacteria and maintenance of immune homeostasis in the gut. Nat Rev Immunol. 2008;8 (6 ):411‐420. doi:10.1038/nri2316 18469830 13 Sato T , van Es J , Snippert H , et al. Paneth cells constitute the niche for Lgr5 stem cells in intestinal crypts. Nature. 2011;469 (7330 ):415‐418. doi:10.1038/nature09637 21113151 14 Houlden A , Goldrick M , Brough D , et al. Brain injury induces specific changes in the Caecal microbiota of mice via altered autonomic activity and mucoprotein production. Brain Behav Immun. 2016;57 :10‐20. doi:10.1016/j.bbi.2016.04.003 27060191 15 Hansson G . Mucus and mucins in diseases of the intestinal and respiratory tracts. J Intern Med. 2019;285 (5 ):479‐490. doi:10.1111/joim.12910 30963635 16 Gillois K , Lévêque M , Théodorou V , Robert H , Mercier‐Bonin M . Mucus: an underestimated gut target for environmental pollutants and food additives. Microorganisms. 2018;6 (2 ):1‐18. doi:10.3390/microorganisms6020053 17 Johansson M , Hansson G . Mucus and the goblet cell. Digest Dis. 2013;31 :305‐309. doi:10.1159/000354683 18 Strugnell R , Wijburg O . The role of secretory antibodies in infection immunity. Nat Rev Microbiol. 2010;8 (9 ):656‐667. doi:10.1038/nrmicro2384 20694027 19 Ganesh B , Hall A , Ayyaswamy S , et al. Diacylglycerol kinase synthesized by commensal Lactobacillus reuteri diminishes protein kinase C phosphorylation and histamine‐mediated signaling in the mammalian intestinal epithelium. Mucosal Immunol. 2018;11 (2 ):380‐393. doi:10.1038/mi.2017.58 28745328 20 Furuse M , Fujita K , Hiiragi T , Fujimoto K , Tsukita S . Claudin‐1 and ‐2: novel integral membrane proteins localizing at tight junctions with no sequence similarity to Occludin. J Cell Biol. 1998;141 (7 ):1539‐1550. doi:10.1083/jcb.141.7.1539 9647647 21 Alonso C , Vicario M , Pigrau M , Lobo B , Santos J . Intestinal barrier function and the brain‐gut axis. Adv Exp Med Biol. 2014;817 :73‐113. doi:10.1007/978-1-4939-0897-4_4 24997030 22 Malago J . Contribution of microbiota to the intestinal physicochemical barrier. Beneficial Microbes. 2015;6 (3 ):295‐311. doi:10.3920/bm2014.0041 25273548 23 Nekrasova O , Amargo E , Smith W , Chen J , Kreitzer G , Green K . Desmosomal cadherins utilize distinct kinesins for assembly into desmosomes. J Cell Biol. 2011;195 (7 ):1185‐1203. doi:10.1083/jcb.201106057 22184201 24 Stanley D , Mason L , Mackin K , et al. Translocation and dissemination of commensal bacteria in post‐stroke infection. Nat Med. 2016;22 (11 ):1277‐1284. doi:10.1038/nm.4194 27694934 25 Liu Y , Luo S , Kou L , et al. Ischemic stroke damages the intestinal mucosa and induces alteration of the intestinal lymphocytes and Ccl19 Mrna in rats. Neurosci Lett. 2017;658 :165‐170. doi:10.1016/j.neulet.2017.08.061 28859865 26 Ye D , Hu Y , Zhu N , et al. Exploratory investigation of intestinal structure and function after stroke in mice. Mediators Inflamm. 2021;2021 :1315797. doi:10.1155/2021/1315797 33642941 27 Stanley D , Moore R , Wong C . An insight into intestinal mucosal microbiota disruption after stroke. Sci Rep. 2018;8 (1 ):568. doi:10.1038/s41598-017-18904-8 29330443 28 Crapser J , Ritzel R , Verma R , et al. Ischemic stroke induces gut permeability and enhances bacterial translocation leading to sepsis in aged mice. Aging. 2016;8 (5 ):1049‐1063. doi:10.18632/aging.100952 27115295 29 Blasco M , Chauhan A , Honarpisheh P , et al. Age‐dependent involvement of gut mast cells and histamine in post‐stroke inflammation. J Neuroinflammation. 2020;17 (1 ):160. doi:10.1186/s12974-020-01833-1 32429999 30 Balzan S , de Almeida QC , de Cleva R , Zilberstein B , Cecconello I . Bacterial translocation: overview of mechanisms and clinical impact. J Gastroenterol Hepatol. 2007;22 (4 ):464‐471. doi:10.1111/j.1440-1746.2007.04933.x 17376034 31 Bäckhed F , Ley R , Sonnenburg J , Peterson D , Gordon J . Host‐bacterial mutualism in the human intestine. Science. 2005;307 (5717 ):1915‐1920. doi:10.1126/science.1104816 15790844 32 Qin J , Li R , Raes J , et al. A human gut microbial gene catalogue established by metagenomic sequencing. Nature. 2010;464 (7285 ):59‐65. doi:10.1038/nature08821 20203603 33 Eckburg P , Bik E , Bernstein C , et al. Diversity of the human intestinal microbial flora. Science. 2005;308 (5728 ):1635‐1638. doi:10.1126/science.1110591 15831718 34 Cani P , Possemiers S , Van de Wiele T , et al. Changes in gut microbiota control inflammation in obese mice through a mechanism involving Glp‐2‐driven improvement of gut permeability. Gut. 2009;58 (8 ):1091‐1103. doi:10.1136/gut.2008.165886 19240062 35 Shimada Y , Kinoshita M , Harada K , et al. Commensal bacteria‐dependent indole production enhances epithelial barrier function in the colon. PLoS One. 2013;8 (11 ):e80604. doi:10.1371/journal.pone.0080604 24278294 36 Alam M , Midtvedt T , Uribe A . Differential cell kinetics in the ileum and colon of germfree rats. Scand J Gastroenterol. 1994;29 (5 ):445‐451. doi:10.3109/00365529409096836 8036460 37 Pull S , Doherty J , Mills J , Gordon J , Stappenbeck T . Activated macrophages are an adaptive element of the colonic epithelial progenitor niche necessary for regenerative responses to injury. Proc Natl Acad Sci USA. 2005;102 (1 ):99‐104. doi:10.1073/pnas.0405979102 15615857 38 Mack D , Ahrne S , Hyde L , Wei S , Hollingsworth M . Extracellular Muc3 mucin secretion follows adherence of Lactobacillus strains to intestinal epithelial cells in vitro. Gut. 2003;52 (6 ):827‐833. doi:10.1136/gut.52.6.827 12740338 39 Mattar A , Teitelbaum D , Drongowski R , Yongyi F , Harmon C , Coran A . Probiotics up‐regulate Muc‐2 mucin gene expression in a Caco‐2 cell‐culture model. Pediatr Surg Int. 2002;18 (7 ):586‐590. doi:10.1007/s00383-002-0855-7 12471471 40 Gaboriau‐Routhiau V , Rakotobe S , Lécuyer E , et al. The key role of segmented filamentous bacteria in the coordinated maturation of gut helper T cell responses. Immunity. 2009;31 (4 ):677‐689. doi:10.1016/j.immuni.2009.08.020 19833089 41 Roy U , de Oliveira R , Galvez E , et al. Induction of Il‐22‐producing Cd4+ T cells by segmented filamentous bacteria independent of classical Th17 cells. Front Immunol. 2021;12 :671331. doi:10.3389/fimmu.2021.671331 34566952 42 Belkaid Y , Harrison O . Homeostatic immunity and the microbiota. Immunity. 2017;46 (4 ):562‐576. doi:10.1016/j.immuni.2017.04.008 28423337 43 Luo A , Leach S , Barres R , Hesson L , Grimm M , Simar D . The microbiota and epigenetic regulation of T helper 17/regulatory T cells: In search of a balanced immune system. Front Immunol. 2017;8 :417. doi:10.3389/fimmu.2017.00417 28443096 44 Iadecola C , Anrather J . The immunology of stroke: from mechanisms to translation. Nat Med. 2011;17 (7 ):796‐808. doi:10.1038/nm.2399 21738161 45 Honda K , Littman D . The microbiota in adaptive immune homeostasis and disease. Nature. 2016;535 (7610 ):75‐84. doi:10.1038/nature18848 27383982 46 Dénes A , Humphreys N , Lane T , Grencis R , Rothwell N . Chronic systemic infection exacerbates ischemic brain damage via a Ccl5 (regulated on activation, Normal T‐cell expressed and secreted)‐mediated proinflammatory response in mice. J Neurosci. 2010;30 (30 ):10086‐10095. doi:10.1523/jneurosci.1227-10.2010 20668193 47 Brahe L , Astrup A , Larsen L . Is butyrate the link between diet, intestinal microbiota and obesity‐related metabolic diseases? Obes Rev. 2013;14 (12 ):950‐959. doi:10.1111/obr.12068 23947604 48 Koh A , De Vadder F , Kovatcheva‐Datchary P , Bäckhed F . From dietary fiber to host physiology: short‐chain fatty acids as key bacterial metabolites. Cell. 2016;165 (6 ):1332‐1345. doi:10.1016/j.cell.2016.05.041 27259147 49 Pluznick J . A novel Scfa receptor, the microbiota, and blood pressure regulation. Gut Microbes. 2014;5 (2 ):202‐207. doi:10.4161/gmic.27492 24429443 50 Tolhurst G , Heffron H , Lam Y , et al. Short‐chain fatty acids stimulate glucagon‐like Peptide‐1 secretion via the G‐protein‐coupled receptor Ffar2. Diabetes. 2012;61 (2 ):364‐371. doi:10.2337/db11-1019 22190648 51 Haghikia A , Jörg S , Duscha A , et al. Dietary fatty acids directly impact central nervous system autoimmunity via the small intestine. Immunity. 2015;43 (4 ):817‐829. doi:10.1016/j.immuni.2015.09.007 26488817 52 Dalile B , Van Oudenhove L , Vervliet B , Verbeke K . The role of short‐chain fatty acids in microbiota‐gut‐brain communication. Nat Rev Gastroenterol Hepatol. 2019;16 (8 ):461‐478. doi:10.1038/s41575-019-0157-3 31123355 53 Wang Z , Zhao Y . Gut microbiota derived metabolites in cardiovascular health and disease. Protein Cell. 2018;9 (5 ):416‐431. doi:10.1007/s13238-018-0549-0 29725935 54 Tang W , Li D , Hazen S . Dietary metabolism, the gut microbiome, and heart failure. Nat Rev Cardiol. 2019;16 (3 ):137‐154. doi:10.1038/s41569-018-0108-7 30410105 55 de Vos WM , Tilg H , Van Hul M , Cani PD . Gut microbiome and health: mechanistic insights. Gut. 2022;71 (5 ):1020‐1032. doi:10.1136/gutjnl-2021-326789 35105664 56 Lefort C , Cani PD . The liver under the spotlight: bile acids and oxysterols as pivotal actors controlling metabolism. Cells. 2021;10 (2 ):1‐23. doi:10.3390/cells10020400 57 de Aguiar Vallim TQ , Tarling EJ , Edwards PA . Pleiotropic roles of bile acids in metabolism. Cell Metab. 2013;17 (5 ):657‐669. doi:10.1016/j.cmet.2013.03.013 23602448 58 Mazmanian S , Liu C , Tzianabos A , Kasper D . An immunomodulatory molecule of symbiotic bacteria directs maturation of the host immune system. Cell. 2005;122 (1 ):107‐118. doi:10.1016/j.cell.2005.05.007 16009137 59 Sun F , Zhang Q , Zhao J , Zhang H , Zhai Q , Chen W . A potential species of next‐generation probiotics? The dark and light sides of bacteroides fragilis in health. Food Res Int. 2019;126 :108590. doi:10.1016/j.foodres.2019.108590 31732047 60 Erturk‐Hasdemir D , Oh SF , Okan NA , et al. Symbionts exploit complex signaling to educate the immune system. Proc Natl Acad Sci USA. 2019;116 (52 ):26157‐26166. doi:10.1073/pnas.1915978116 31811024 61 Dasgupta S , Erturk‐Hasdemir D , Ochoa‐Reparaz J , Reinecker HC , Kasper DL . Plasmacytoid dendritic cells mediate anti‐inflammatory responses to a gut commensal molecule via both innate and adaptive mechanisms. Cell Host Microbe. 2014;15 (4 ):413‐423. doi:10.1016/j.chom.2014.03.006 24721570 62 Chen Y , Liang J , Ouyang F , et al. Persistence of gut microbiota dysbiosis and chronic systemic inflammation after cerebral infarction in cynomolgus monkeys. Front Neurol. 2019;10 :661. doi:10.3389/fneur.2019.00661 31316450 63 Larsen J . The immune response to Prevotella bacteria in chronic inflammatory disease. Immunology. 2017;151 (4 ):363‐374. doi:10.1111/imm.12760 28542929 64 Yin J , Liao S , He Y , et al. Dysbiosis of gut microbiota with reduced trimethylamine‐N‐oxide level in patients with large‐artery atherosclerotic stroke or transient ischemic attack. J Am Heart Assoc. 2015;4 (11 ):1‐12. doi:10.1161/jaha.115.002699 65 Jeffery I , O'Toole P . Diet‐microbiota interactions and their implications for healthy living. Nutrients. 2013;5 (1 ):234‐252. doi:10.3390/nu5010234 23344252 66 Li J , Zhao F , Wang Y , et al. Gut microbiota dysbiosis contributes to the development of hypertension. Microbiome. 2017;5 (1 ):14. doi:10.1186/s40168-016-0222-x 28143587 67 Qin J , Li Y , Cai Z , et al. A metagenome‐wide association study of gut microbiota in type 2 diabetes. Nature. 2012;490 (7418 ):55‐60. doi:10.1038/nature11450 23023125 68 Ley R , Turnbaugh P , Klein S , Gordon J . Microbial ecology: human gut microbes associated with obesity. Nature. 2006;444 (7122 ):1022‐1023. doi:10.1038/4441022a 17183309 69 Machiels K , Joossens M , Sabino J , et al. A decrease of the butyrate‐producing species Roseburia hominis and Faecalibacterium Prausnitzii defines dysbiosis in patients with ulcerative colitis. Gut. 2014;63 (8 ):1275‐1283. doi:10.1136/gutjnl-2013-304833 24021287 70 Gophna U , Konikoff T , Nielsen H . Oscillospira and related bacteria – from metagenomic species to metabolic features. Environ Microbiol. 2017;19 (3 ):835‐841. doi:10.1111/1462-2920.13658 28028921 71 Vinolo M , Rodrigues H , Hatanaka E , Sato F , Sampaio S , Curi R . Suppressive effect of short‐chain fatty acids on production of proinflammatory mediators by neutrophils. J Nutr Biochem. 2011;22 (9 ):849‐855. doi:10.1016/j.jnutbio.2010.07.009 21167700 72 Benakis C , Poon C , Lane D , et al. Distinct commensal bacterial signature in the gut is associated with acute and Long‐term protection from ischemic stroke. Stroke. 2020;51 (6 ):1844‐1854. doi:10.1161/strokeaha.120.029262 32404038 73 Benakis C , Brea D , Caballero S , et al. Commensal microbiota affects ischemic stroke outcome by regulating intestinal Γδ T cells. Nat Med. 2016;22 (5 ):516‐523. doi:10.1038/nm.4068 27019327 74 Lamas B , Natividad J , Sokol H . Aryl hydrocarbon receptor and intestinal immunity. Mucosal Immunol. 2018;11 (4 ):1024‐1038. doi:10.1038/s41385-018-0019-2 29626198 75 Sacks D , Baxter B , Campbell BCV , et al. Multisociety consensus quality improvement revised consensus Statement for endovascular therapy of acute ischemic stroke. Int J Stroke. 2018;13 (6 ):612‐632. doi:10.1177/1747493018778713 29786478 76 Zelante T , Iannitti R , Cunha C , et al. Tryptophan catabolites from microbiota engage aryl hydrocarbon receptor and balance mucosal reactivity via Interleukin‐22. Immunity. 2013;39 (2 ):372‐385. doi:10.1016/j.immuni.2013.08.003 23973224 77 Brouns R , Verkerk R , Aerts T , et al. The role of tryptophan catabolism along the kynurenine pathway in acute ischemic stroke. Neurochem Res. 2010;35 (9 ):1315‐1322. doi:10.1007/s11064-010-0187-2 20490917 78 Tan C , Wu Q , Wang H , et al. Dysbiosis of gut microbiota and short‐chain fatty acids in acute ischemic stroke and the subsequent risk for poor functional outcomes. J Parenteral Enteral Nutr. 2021;45 (3 ):518‐529. doi:10.1002/jpen.1861 79 Yamashiro K , Tanaka R , Urabe T , et al. Gut dysbiosis is associated with metabolism and systemic inflammation in patients with ischemic stroke. PLoS One. 2017;12 (2 ):e0171521. doi:10.1371/journal.pone.0171521 28166278 80 Chen R , Xu Y , Wu P , et al. Transplantation of fecal microbiota rich in short chain fatty acids and butyric acid treat cerebral ischemic stroke by regulating gut microbiota. Pharmacol Res. 2019;148 :104403. doi:10.1016/j.phrs.2019.104403 31425750 81 Sadler R , Cramer J , Heindl S , et al. Short‐chain fatty acids improve poststroke recovery via immunological mechanisms. J Neurosci. 2020;40 (5 ):1162‐1173. doi:10.1523/jneurosci.1359-19.2019 31889008 82 Lee J , d'Aigle J , Atadja L , et al. Gut microbiota‐derived short‐chain fatty acids promote poststroke recovery in aged mice. Circ Res. 2020;127 (4 ):453‐465. doi:10.1161/circresaha.119.316448 32354259 83 Abdullahi W , Tripathi D , Ronaldson PT . Blood‐brain barrier dysfunction in ischemic stroke: targeting tight junctions and transporters for vascular protection. Am J Physiol Cell Physiol. 2018;315 (3 ):C343‐C356. doi:10.1152/ajpcell.00095.2018 29949404 84 Jiang X , Andjelkovic AV , Zhu L , et al. Blood‐brain barrier dysfunction and recovery after ischemic stroke. Prog Neurobiol. 2018;163–164 :144‐171. doi:10.1016/j.pneurobio.2017.10.001 85 Chen Z , Xin L , Yang L , et al. Butyrate promotes post‐stroke outcomes in aged mice via Interleukin‐22. Exp Neurol. 2023;363 :114351. doi:10.1016/j.expneurol.2023.114351 36804553 86 Eberl G . Inducible lymphoid tissues in the adult gut: recapitulation of a fetal developmental pathway? Nat Rev Immunol. 2005;5 (5 ):413‐420. doi:10.1038/nri1600 15841100 87 Witmer M , Steinman R . The anatomy of peripheral lymphoid organs with emphasis on accessory cells: light‐microscopic immunocytochemical studies of mouse spleen, lymph node, and Peyer's patch. Am J Anat. 1984;170 (3 ):465‐481. doi:10.1002/aja.1001700318 6475812 88 Iwasaki A , Kelsall B . Localization of distinct Peyer's patch dendritic cell subsets and their recruitment by chemokines macrophage inflammatory protein (Mip)‐3alpha, Mip‐3beta, and secondary lymphoid organ chemokine. J Exp Med. 2000;191 (8 ):1381‐1394. doi:10.1084/jem.191.8.1381 10770804 89 Arstila T , Arstila T , Calbo S , et al. Identical T cell clones are located within the mouse gut epithelium and lamina propia and circulate in the thoracic duct lymph. J Exp Med. 2000;191 (5 ):823‐834. doi:10.1084/jem.191.5.823 10755885 90 Hooper L , Macpherson A . Immune adaptations that maintain homeostasis with the intestinal microbiota. Nat Rev Immunol. 2010;10 (3 ):159‐169. doi:10.1038/nri2710 20182457 91 Stagg A , Hart A , Knight S , Kamm M . The dendritic cell: its role in intestinal inflammation and relationship with gut bacteria. Gut. 2003;52 (10 ):1522‐1529. doi:10.1136/gut.52.10.1522 12970149 92 Smythies L , Sellers M , Clements R , et al. Human intestinal macrophages display profound inflammatory Anergy despite avid phagocytic and bacteriocidal activity. J Clin Invest. 2005;115 (1 ):66‐75. doi:10.1172/jci19229 15630445 93 Kayama H , Nishimura J , Takeda K . Regulation of intestinal homeostasis by innate immune cells. Immune Network. 2013;13 (6 ):227‐234. doi:10.4110/in.2013.13.6.227 24385940 94 Niess J , Brand S , Gu X , et al. Cx3cr1‐mediated dendritic cell access to the intestinal lumen and bacterial clearance. Science. 2005;307 (5707 ):254‐258. doi:10.1126/science.1102901 15653504 95 Dou Z , Rong X , Zhao E , Zhang L , Lv Y . Neuroprotection of resveratrol against focal cerebral ischemia/reperfusion injury in mice through a mechanism targeting gut‐brain axis. Cell Mol Neurobiol. 2019;39 (6 ):883‐898. doi:10.1007/s10571-019-00687-3 31140018 96 MacDonald T . The mucosal immune system. Parasite Immunol. 2003;25 (5 ):235‐246. doi:10.1046/j.1365-3024.2003.00632.x 12969442 97 Newberry R , Lorenz R . Organizing a mucosal defense. Immunol Rev. 2005;206 :6‐21. doi:10.1111/j.0105-2896.2005.00282.x 16048539 98 Suzuki K , Fagarasan S . How host‐bacterial interactions lead to Iga synthesis in the gut. Trends Immunol. 2008;29 (11 ):523‐531. doi:10.1016/j.it.2008.08.001 18838301 99 Jung C , Hugot J , Barreau F . Peyer's patches: the immune sensors of the intestine. Int J Inflamm. 2010;2010 :823710. doi:10.4061/2010/823710 100 Suzuki K , Nakajima A . New aspects of Iga synthesis in the gut. Int Immunol. 2014;26 (9 ):489‐494. doi:10.1093/intimm/dxu059 24872116 101 Caso J , Hurtado O , Pereira M , et al. Colonic bacterial translocation as a possible factor in stress‐worsening experimental stroke outcome. Am J Physiol Regul Integr Comp Physiol. 2009;296 (4 ):R979‐R985. doi:10.1152/ajpregu.90825.2008 19193944 102 Lelouard H , Henri S , De Bovis B , et al. Pathogenic bacteria and dead cells are internalized by a unique subset of Peyer's patch dendritic cells that express lysozyme. Gastroenterology. 2010;138 (1 ):173‐184e3. doi:10.1053/j.gastro.2009.09.051 19800337 103 Schulte‐Herbrüggen O , Quarcoo D , Meisel A , Meisel C . Differential affection of intestinal immune cell populations after cerebral ischemia in mice. Neuroimmunomodulation. 2009;16 (3 ):213‐218. doi:10.1159/000205514 19246945 104 Prass K , Meisel C , Höflich C , et al. Stroke‐induced immunodeficiency promotes spontaneous bacterial infections and is mediated by sympathetic activation reversal by poststroke T helper cell type 1‐like Immunostimulation. J Exp Med. 2003;198 (5 ):725‐736. doi:10.1084/jem.20021098 12939340 105 Oyama N , Winek K , Bäcker‐Koduah P , et al. Exploratory investigation of intestinal function and bacterial translocation after focal cerebral ischemia in the mouse. Front Neurol. 2018;9 :937. doi:10.3389/fneur.2018.00937 30510535 106 Furness J , Stebbing M . The first brain: species comparisons and evolutionary implications for the enteric and central nervous systems. Neurogastroenterol Motil. 2018;30 (2 ):e13234. doi:10.1111/nmo.13234 107 Furness J . The enteric nervous system and neurogastroenterology. Nat Rev Gastroenterol Hepatol. 2012;9 (5 ):286‐294. doi:10.1038/nrgastro.2012.32 22392290 108 Furness J , Callaghan B , Rivera L , Cho H . The enteric nervous system and gastrointestinal innervation: integrated local and central control. Adv Exp Med Biol. 2014;817 :39‐71. doi:10.1007/978-1-4939-0897-4_3 24997029 109 Joly A , Leulier F , De Vadder F . Microbial modulation of the development and physiology of the enteric nervous system. Trends Microbiol. 2021;29 (8 ):686‐699. doi:10.1016/j.tim.2020.11.007 33309188 110 Obata Y , Pachnis V . The effect of microbiota and the immune system on the development and organization of the enteric nervous system. Gastroenterology. 2016;151 (5 ):836‐844. doi:10.1053/j.gastro.2016.07.044 27521479 111 Schneider S , Wright C , Heuckeroth R . Unexpected roles for the second brain: enteric nervous system as master regulator of bowel function. Annu Rev Physiol. 2019;81 :235‐259. doi:10.1146/annurev-physiol-021317-121515 30379617 112 Kulkarni S , Micci M , Leser J , et al. Adult enteric nervous system in health is maintained by a dynamic balance between neuronal apoptosis and neurogenesis. Proc Natl Acad Sci USA. 2017;114 (18 ):E3709‐E3718. doi:10.1073/pnas.1619406114 28420791 113 Yoo B , Mazmanian S . The enteric network: interactions between the immune and nervous systems of the gut. Immunity. 2017;46 (6 ):910‐926. doi:10.1016/j.immuni.2017.05.011 28636959 114 Elenkov I , Wilder R , Chrousos G , Vizi E . The sympathetic nerve – an integrative interface between two supersystems: the brain and the immune system. Pharmacol Rev. 2000;52 (4 ):595‐638.11121511 115 Sandgren K , Lin Z , Ekblad E . Differential effects of Vip and Pacap on survival of cultured adult rat myenteric neurons. Regul Pept. 2003;111 :211‐217. doi:10.1016/s0167-0115(02)00290-2 12609771 116 Sandgren K , Lin Z , Fex Svenningsen A , Ekblad E . Vasoactive intestinal peptide and nitric oxide promote survival of adult rat myenteric neurons in culture. J Neurosci Res. 2003;72 (5 ):595‐602. doi:10.1002/jnr.10612 12749024 117 Lin Z , Sandgren K , Ekblad E . Increased expression of nitric oxide synthase in cultured neurons from adult rat colonic submucous ganglia. Auton Neurosci. 2004;114 :29‐38. doi:10.1016/j.autneu.2004.06.002 15331042 118 Voss U , Ekblad E . Lipopolysaccharide‐induced loss of cultured rat myenteric neurons – role of amp‐activated protein kinase. PLoS One. 2014;9 (12 ):e114044. doi:10.1371/journal.pone.0114044 25462874 119 Cheng X , Boza‐Serrano A , Turesson M , Deierborg T , Ekblad E , Voss U . Galectin‐3 causes enteric neuronal loss in mice after left sided permanent middle cerebral artery occlusion, a model of stroke. Sci Rep. 2016;6 :32893. doi:10.1038/srep32893 27612206 120 Cheng X , Svensson M , Yang Y , Deierborg T , Ekblad E , Voss U . Focal, but not global, cerebral Ischaemia causes loss of myenteric neurons and upregulation of vasoactive intestinal peptide in mouse ileum. Int J Exp Pathol. 2018;99 (1 ):38‐45. doi:10.1111/iep.12263 29577471 121 Hua F , Ma J , Ha T , et al. Activation of toll‐like receptor 4 signaling contributes to hippocampal neuronal death following global cerebral ischemia/reperfusion. J Neuroimmunol. 2007;190 :101‐111. doi:10.1016/j.jneuroim.2007.08.014 17884182 122 Lee K , Bang J , Kim B , et al. Fructus Mume alleviates chronic cerebral hypoperfusion‐induced white matter and hippocampal damage via inhibition of inflammation and downregulation of Tlr4 and P38 Mapk signaling. BMC Complement Altern Med. 2015;15 :125. doi:10.1186/s12906-015-0652-1 25898017 123 Manouchehrian O , Arnér K , Deierborg T , Taylor L . Who let the dogs out?: detrimental role of Galectin‐3 in hypoperfusion‐induced retinal degeneration. J Neuroinflammation. 2015;12 :92. doi:10.1186/s12974-015-0312-x 25968897 124 Svensson M , Rosvall P , Boza‐Serrano A , Andersson E , Lexell J , Deierborg T . Forced treadmill exercise can induce stress and increase neuronal damage in a mouse model of global cerebral ischemia. Neurobiology of Stress. 2016;5 :8‐18. doi:10.1016/j.ynstr.2016.09.002 27981192 125 Fonarow G , Smith E , Saver J , et al. Timeliness of tissue‐type plasminogen activator therapy in acute ischemic stroke: patient characteristics, hospital factors, and outcomes associated with door‐to‐needle times within 60 minutes. Circulation. 2011;123 (7 ):750‐758. doi:10.1161/circulationaha.110.974675 21311083 126 Smith C , Hulme S , Vail A , et al. Scil‐stroke (subcutaneous Interleukin‐1 receptor antagonist in ischemic stroke): a randomized controlled phase 2 trial. Stroke. 2018;49 (5 ):1210‐1216. doi:10.1161/strokeaha.118.020750 29567761 127 Investigators EAST . Use of anti‐Icam‐1 therapy in ischemic stroke: results of the Enlimomab acute stroke trial. Neurology. 2001;57 (8 ):1428‐1434. doi:10.1212/wnl.57.8.1428 11673584 128 Malhotra K , Chang J , Khunger A , et al. Minocycline for acute stroke treatment: a systematic review and meta‐analysis of randomized clinical trials. J Neurol. 2018;265 (8 ):1871‐1879. doi:10.1007/s00415-018-8935-3 29948247 129 Elkind MSV , Veltkamp R , Montaner J , et al. Natalizumab in acute ischemic stroke (Action Ii): a randomized. Placebo‐Controlled Trial Neurol. 2020;95 (8 ):e1091‐e1104. doi:10.1212/wnl.0000000000010038 130 Zhu Z , Fu Y , Tian D , et al. Combination of the immune modulator fingolimod with alteplase in acute ischemic stroke: a pilot trial. Circulation. 2015;132 (12 ):1104‐1112. doi:10.1161/circulationaha.115.016371 26202811 131 Zhang S , Zhou Y , Zhang R , et al. Rationale and design of combination of an immune modulator fingolimod with alteplase bridging with mechanical thrombectomy in acute ischemic stroke (Famtais) trial. Int J Stroke. 2017;12 (8 ):906‐909. doi:10.1177/1747493017710340 28569122 132 Levard D , Buendia I , Lanquetin A , Glavan M , Vivien D , Rubio M . Filling the gaps on stroke research: focus on inflammation and immunity. Brain Behav Immun. 2021;91 :649‐667. doi:10.1016/j.bbi.2020.09.025 33017613 133 Iadecola C , Buckwalter M , Anrather J . Immune responses to stroke: mechanisms, modulation, and therapeutic potential. J Clin Invest. 2020;130 (6 ):2777‐2788. doi:10.1172/jci135530 32391806 134 Pera A , Campos C , López N , et al. Immunosenescence: implications for response to infection and vaccination in older people. Maturitas. 2015;82 (1 ):50‐55. doi:10.1016/j.maturitas.2015.05.004 26044074 135 Kim E , Cho S . Microglia and monocyte‐derived macrophages in stroke. Neurotherapeutics. 2016;13 (4 ):702‐718. doi:10.1007/s13311-016-0463-1 27485238 136 Liu K , Tedeschi A , Park K , He Z . Neuronal intrinsic mechanisms of axon regeneration. Annu Rev Neurosci. 2011;34 :131‐152. doi:10.1146/annurev-neuro-061010-113723 21438684 137 Spychala M , Venna V , Jandzinski M , et al. Age‐related changes in the gut microbiota influence systemic inflammation and stroke outcome. Ann Neurol. 2018;84 (1 ):23‐36. doi:10.1002/ana.25250 29733457 138 Huang Q , Xia J . Influence of the gut microbiome on inflammatory and immune response after stroke. Neurol Sci. 2021;42 (12 ):4937‐4951. doi:10.1007/s10072-021-05603-6 34536154 139 Ghelani DP , Kim HA , Zhang SR , Drummond GR , Sobey CG , De Silva TM . Ischemic stroke and infection: a brief update on mechanisms and potential therapies. Biochem Pharmacol. 2021;193 :114768. doi:10.1016/j.bcp.2021.114768 34543657 140 Kurita N , Yamashiro K , Kuroki T , et al. Metabolic endotoxemia promotes neuroinflammation after focal cerebral ischemia. J Cereb Blood Flow Metab. 2020;40 (12 ):2505‐2520. doi:10.1177/0271678X19899577 31910709 141 Winek K , Engel O , Koduah P , et al. Depletion of cultivatable gut microbiota by broad‐Spectrum antibiotic pretreatment worsens outcome after murine stroke. Stroke. 2016;47 (5 ):1354‐1363. doi:10.1161/strokeaha.115.011800 27056982 142 Lee KE , Kim JK , Kim DH . Orally administered antibiotics vancomycin and ampicillin cause cognitive impairment with gut dysbiosis in mice with transient global forebrain ischemia. Front Microbiol. 2020;11 :564271. doi:10.3389/fmicb.2020.564271 33324357 143 Strati F , Pujolassos M , Burrello C , et al. Antibiotic‐associated dysbiosis affects the ability of the gut microbiota to control intestinal inflammation upon fecal microbiota transplantation in experimental colitis models. Microbiome. 2021;9 (1 ):39. doi:10.1186/s40168-020-00991-x 33549144 144 Hill C , Guarner F , Reid G , et al. Expert consensus document. The International Scientific Association for Probiotics and Prebiotics Consensus Statement on the scope and appropriate use of the term probiotic. Nat Rev Gastroenterol Hepatol. 2014;11 (8 ):506‐514. doi:10.1038/nrgastro.2014.66 24912386 145 Collado MC , Meriluoto J , Salminen S . Role of commercial probiotic strains against human pathogen adhesion to intestinal mucus. Lett Appl Microbiol. 2007;45 (4 ):454‐460. doi:10.1111/j.1472-765X.2007.02212.x 17897389 146 Isolauri E , Salminen S , Ouwehand AC . Microbial‐gut interactions in health and disease. Probiotics. Best Pract Res Clin Gastroenterol. 2004;18 (2 ):299‐313. doi:10.1016/j.bpg.2003.10.006 15123071 147 Brandão RL , Castro IM , Bambirra EA , et al. Intracellular signal triggered by cholera toxin in saccharomyces Boulardii and saccharomyces cerevisiae. Appl Environ Microbiol. 1998;64 (2 ):564‐568. doi:10.1128/aem.64.2.564-568.1998 9464394 148 Sun J , Ling Z , Wang F , et al. Clostridium butyricum pretreatment attenuates cerebral ischemia/reperfusion injury in mice via anti‐oxidation and anti‐apoptosis. Neurosci Lett. 2016;613 :30‐35. doi:10.1016/j.neulet.2015.12.047 26733300 149 Akhoundzadeh K , Vakili A , Shadnoush M , Sadeghzadeh J . Effects of the oral ingestion of probiotics on brain damage in a transient model of focal cerebral ischemia in mice. Iran J Med Sci. 2018;43 (1 ):32‐40.29398750 150 Rahmati H , Momenabadi S , Vafaei AA , Bandegi AR , Mazaheri Z , Vakili A . Probiotic supplementation attenuates hippocampus injury and spatial learning and memory impairments in a cerebral hypoperfusion mouse model. Mol Biol Rep. 2019;46 (5 ):4985‐4995. doi:10.1007/s11033-019-04949-7 31286392 151 Wanchao S , Chen M , Zhiguo S , Futang X , Mengmeng S . Protective effect and mechanism of lactobacillus on cerebral ischemia reperfusion injury in rats. Braz J Med Biol Res. 2018;51 (7 ):e7172. doi:10.1590/1414-431x20187172 29791585 152 Kothari D , Patel S , Kim SK . Probiotic supplements might not Be universally‐effective and safe: a review. Biomed Pharmacother. 2019;111 :537‐547. doi:10.1016/j.biopha.2018.12.104 30597307 153 Wang JW , Kuo CH , Kuo FC , et al. Fecal microbiota transplantation: review and update. J Formos Med Assoc. 2019;118 (Suppl 1 ):S23‐S31. doi:10.1016/j.jfma.2018.08.011 30181015 154 Brandt LJ , Aroniadis OC . An overview of fecal microbiota transplantation: techniques, indications, and outcomes. Gastrointest Endosc. 2013;78 (2 ):240‐249. doi:10.1016/j.gie.2013.03.1329 23642791 155 Vendrik KEW , Ooijevaar RE , de Jong PRC , et al. Fecal microbiota transplantation in neurological disorders. Front Cell Infect Microbiol. 2020;10 :98. doi:10.3389/fcimb.2020.00098 32266160 156 Wang H , Song W , Wu Q , et al. Fecal transplantation from Db/Db mice treated with sodium butyrate attenuates ischemic stroke injury. Microbiol Spectr. 2021;9 (2 ):e0004221. doi:10.1128/Spectrum.00042-21 34612696 157 Lee J , Venna VR , Durgan DJ , et al. Young versus aged microbiota transplants to germ‐free mice: increased short‐chain fatty acids and improved cognitive performance. Gut Microbes. 2020;12 (1 ):1‐14. doi:10.1080/19490976.2020.1814107 158 Didesch MM , Averill A , Oh‐Park M . Peripheral neuropathy after fecal microbiota transplantation for Clostridium difficile infection: a case report. PM R. 2016;8 (8 ):813‐816. doi:10.1016/j.pmrj.2016.01.009 26826616 159 Zhang HY , Tian JX , Lian FM , et al. Therapeutic mechanisms of traditional Chinese medicine to improve metabolic diseases via the gut microbiota. Biomed Pharmacother. 2021;133 :110857. doi:10.1016/j.biopha.2020.110857 33197760 160 Gong X , Li X , Bo A , et al. The interactions between gut microbiota and bioactive ingredients of traditional Chinese medicines: a review. Pharmacol Res. 2020;157 :104824. doi:10.1016/j.phrs.2020.104824 32344049 161 Zhu JB , Wang YH , Hu ZD , Li J . Research progress on pathogenesis of ischemic stroke and traditional Chinese medicine commonly used for treatment of ischemic stroke. Zhongguo Zhong Yao Za Zhi. 2019;44 (3 ):422‐432. doi:10.19540/j.cnki.cjcmm.20180921.002 30989903 162 Guo Q , Jiang X , Ni C , et al. Gut microbiota‐related effects of Tanhuo decoction in acute ischemic stroke. Oxid Med Cell Longev. 2021;2021 :5596924. doi:10.1155/2021/5596924 34136066 163 Gao Q , Han ZY , Tian DF , et al. Xinglou Chengqi decoction improves neurological function in experimental stroke mice as evidenced by gut microbiota analysis and network pharmacology. Chin J Nat Med. 2021;19 (12 ):881‐899. doi:10.1016/s1875-5364(21)60079-1 34961587 164 Lin H , Chen S , Shen L , et al. Integrated analysis of the cecal microbiome and plasma metabolomics to explore Naomaitong and its potential role in changing the intestinal Flora and their metabolites in ischemic stroke. Front Pharmacol. 2021;12 :773722. doi:10.3389/fphar.2021.773722 35126115 165 Zhang F , Zhai M , Wu Q , Jia X , Wang Y , Wang N . Protective effect of Tong‐Qiao‐Huo‐Xue decoction on inflammatory injury caused by intestinal microbial disorders in stroke rats. Biol Pharm Bull. 2020;43 (5 ):788‐800. doi:10.1248/bpb.b19-00847 32132347 166 Chen R , Wu P , Cai Z , et al. Puerariae Lobatae radix with chuanxiong rhizoma for treatment of cerebral ischemic stroke by remodeling gut microbiota to regulate the brain‐gut barriers. J Nutr Biochem. 2019;65 :101‐114. doi:10.1016/j.jnutbio.2018.12.004 30710886 167 Ai Z , Cheng C , Zhou L , Yin S , Wang L , Liu Y . Bone marrow mesenchymal stem cells‐derived extracellular vesicles carrying Microrna‐221‐3p protect against ischemic stroke via Atf3. Brain Res Bull. 2021;172 :220‐228. doi:10.1016/j.brainresbull.2021.04.022 33932490 168 Wang F , Tang H , Zhu J , Zhang JH . Transplanting mesenchymal stem cells for treatment of ischemic stroke. Cell Transplant. 2018;27 (12 ):1825‐1834. doi:10.1177/0963689718795424 30251564 169 Zhao LN , Ma SW , Xiao J , Yang LJ , Xu SX , Zhao L . Bone marrow mesenchymal stem cell therapy regulates gut microbiota to improve post‐stroke neurological function recovery in rats. World J Stem Cells. 2021;13 (12 ):1905‐1917. doi:10.4252/wjsc.v13.i12.1905 35069989 170 Gubert C , Kong G , Renoir T , Hannan AJ . Exercise, diet and stress as modulators of gut microbiota: implications for neurodegenerative diseases. Neurobiol Dis. 2020;134 :104621. doi:10.1016/j.nbd.2019.104621 31628992 171 Rehman MU , Wali AF , Ahmad A , et al. Neuroprotective strategies for neurological disorders by natural products: an update. Curr Neuropharmacol. 2019;17 (3 ):247‐267. doi:10.2174/1570159x16666180911124605 30207234 172 Fakhri S , Yarmohammadi A , Yarmohammadi M , Farzaei MH , Echeverria J . Marine natural products: promising candidates in the modulation of gut‐brain axis towards neuroprotection. Mar Drugs. 2021;19 (3 ):1‐29. doi:10.3390/md19030165 173 Monsour M , Croci D , Agazzi S , Borlongan C . Getting the guts to expand stroke treatment: the potential for microbiome targeted therapies. CNS Neurosci Ther. 2022:1‐7. doi:10.1111/cns.13988