The microbiota-immune axis as a central mediator of gut-brain communication

Intestinal inflammatory disorders are associated with neurophysiological and behavioral symptoms. Conversely, many disorders of the central nervous system (CNS) are accompanied by intestinal complications. These observations suggest that the physiologies of the intestine and nervous system are functionally linked. Indeed, a growing body of literature has revealed multiple pathways mediating bidirectional communication between the intestine and the CNS, collectively referred to as the gut-brain axis. In particular, microbes naturally colonizing the mammalian gastrointestinal (GI) tract, termed the gut microbiota, not only correlate with but play a causative role in regulating CNS function, development and behavior. Despite these findings, our understanding of the cellular and molecular mechanisms that mediate gut-brain communication remains in its infancy. However, members of the gut microbiota have been established as potent modulators of intestinal, systemic and CNS-resident immune cell function, suggesting that gut-brain interactions may involve the host immune system. Indeed, multiple CNS disorders with gut microbiota associations, including neuroinflammatory, neuropsychiatric and neurodegenerative disorders, also have significant inflammatory manifestations. In this review, I discuss recent advances exploring the role of microbiota-immune interactions as a critical regulator of the gut-brain axis in the context of CNS and related disorders.


Gut microbes are potent regulators of host immune responses
The mammalian gastrointestinal (GI) tract is colonized by trillions of microorganisms including bacteria, fungi and viruses, collectively termed the gut microbiota. These organisms serve diverse roles in promoting health in local and extra-intestinal tissue environments by limiting pathogen invasion, regulating host metabolism and priming host-protective immune responses (Belkaid and Harrison, 2017). The lamina propria and gut-associated lymphoid structures found along the GI tract are also home to the mucosal immune system, which provides protective immunity to a multitude of microbial threats. Early evidence that the microbiota is critically involved in intestinal immunity arose from studies using germ-free (GF) mice raised in a sterile environment that are devoid of live microorganisms in the GI tract, and specific pathogen-free (SPF) mice treated with broad-spectrum antibiotics (ABX). Studies using GF and ABX mice in various inflammatory contexts reveal that the microbiota regulates both innate and adaptive immunity with functional consequences for host defense against pathogens and immune tolerance to non-pathogenic stimuli. In the innate immune system, ABX mice have impaired myeloid cell development in the bone marrow, reduced numbers of circulating granulocytes and are highly susceptible to systemic bacterial infection (Deshmukh et al., 2014;Khosravi et al., 2014). Innate lymphoid cells (ILCs), which are innate immune counterparts of CD4 T helper cells but lack expression of antigen receptors, are also impaired in their development and function in GF mice (Britanova and Diefenbach, 2017). Production of interleukin (IL)-22 by group 3 ILCs (ILC3s) is significantly reduced in the absence of the microbiota, which is critical for host immunity to enteric bacterial infections. Macrophages directly sense microbe-associated molecular patterns (MAMPs) expressed by gut microbes in the GI tract through Toll-like receptors (TLRs) and Nod-like receptors (NLRs) to produce the pro-inflammatory cytokine IL-1β (Thaiss et al., 2016). Release of IL-1β activates ILC3s to produce granulocyte-macrophage colony-stimulating factor (GM-CSF) and IL-2, which maintains regulatory T cell (Treg) homeostasis and oral tolerance to dietary antigens (Mortha et al., 2014;Zhou et al., 2019). The gut microbiota also plays an important role in shaping adaptive immune cell function (Belkaid and Harrison, 2017). Most notably, gut microbes regulate the balance between pro-inflammatory T helper 17 cells (Th17) and anti-inflammatory Tregs in the GI tract, which establishes immune homeostasis to prevent pathological intestinal and systemic inflammation. Furthermore, gut microbes are involved in T cell-dependent and independent B cell activation and subsequent release of intestinal and systemic immunoglobulins (Ig) that limit inflammatory responses towards gut microbes themselves. The specific cellular and molecular pathways by which gut microbes modulate innate and adaptive immune homeostasis have been extensively reviewed (Belkaid and Harrison, 2017;Britanova and Diefenbach, 2017;Thaiss et al., 2016).
GF and ABX animals have additionally proven to be invaluable tools for functional studies evaluating the immunological effects of various microbial species commonly found in a healthy gut microbiota. In mice, colonization of GF animals with segmented filamentous bacteria (SFB) promotes Th17s (Gaboriau-Routhiau et al., 2009) and IL-22-producing ILC3s in the terminal ileum (Atarashi et al., 2015), germinal center formation and IgA production (Lecuyer et al., 2014). In contrast, colonization of GF animals with Clostridia strains promotes Tregs (Atarashi et al., 2013), which suppress pro-inflammatory T cell responses triggered during inflammation. Further studies demonstrate that induction of intestinal Tregs is not restricted to Clostridia and can be driven by a diversity of Bacteroides and Parabacteroides species (Faith et al., 2014;Sefik et al., 2015). Helicobacter species, prevalent pathobionts of the gut microbiota that have the potential to trigger intestinal inflammation (Xu et al., 2018), promote Treg responses during homeostasis but activate pathogenic T effector responses during colitis (Chai et al., 2017). Akkermansia muciniphila, a gut microbe negatively associated with obesity, promotes T follicular helper cell (Tfh) responses, which provide B cell-help to generate Akkermansia-specific systemic IgG1 responses (Ansaldo, 2019). A large in vivo screen measuring the immunomodulatory properties of common gut microbes reveals that specific members of the gut microbiota have distinct and overlapping immunomodulatory effects in the small intestine, colon, gut-associated and systemic lymphoid organs (Geva-Zatorsky et al., 2017). Taken together, these studies establish that the gut microbiota regulates the host immune system in a species-specific manner, and that alterations in gut microbiota composition lead to altered intestinal and systemic immune activation states (Fig. 1).
In addition to bacteria, the intestine is also colonized with diverse fungal species, collectively referred to as the gut mycobiota (Paterson et al., 2017;Richard and Sokol, 2019). Although found in much lower abundance compared to bacteria, these organisms are emerging as Specific members of the gut microbiota have non-overlapping effects on the host immune system. Bacterial species regulate the differentiation of myeloid cell lineages in the bone marrow and the function of circulating, mature granulocytes. Macrophages detect microbial colonization by pattern recognition receptors and release cytokines to regulate group 3 innate lymphoid cell (ILC3) and regulatory T cell (Treg) responses. Akkermansia and segmented filamentous bacteria (SFB) promote immunoglobulin G (IgG1) and IgA production by B cells through T follicular helper (Tfh)-dependent and independent mechanisms, respectively. SFB, Helicobacter, Bacteroides, Clostridia and indigenous fungi collectively shape the balance of pro-inflammatory T helper 17 (Th17) and anti-inflammatory Treg responses, which have pathological implications and serve tissue-protective functions. During chronic intestinal inflammation, loss of intestinal barrier integrity to gut microbes can activate innate and adaptive immune cells to release of pro-inflammatory cytokines IL-1β, IL-6, TNFα into the circulatory system, leading to systemic inflammation. AHR, aryl hydrocarbon receptor; GM-CSF, granulocyte-macrophage colony-stimulating factor; Mac, macrophage; GMP, granulocyte-monocyte progenitor.
critical players in the pathogenesis of inflammatory bowel disease (IBD) through modulation of antigen-presenting, T helper 1 (Th1) and Th17 cell function. (Jiang et al., 2017;Leonardi et al., 2018;Limon et al., 2019). The GI tract is also colonized by endogenous viruses known as the gut virome (Neil and Cadwell, 2018). In mice, mouse norovirus (MNV) is a viral pathogen that infects intestinal myeloid cells but can remain dormant following resolution of infection, thereby establishing persistence along the GI tract. Colonization of GF or ABX mice with MNV induces type 1 interferon responses that protect the host from enteric pathogen infection and chemical-induced intestinal injury (Kernbauer et al., 2014), suggesting that similar to gut bacteria, intestinal viral colonization can serve host protective functions. Consistent with this, a recent study demonstrates that SPF mice depleted of endogenous intestinal viruses with a cocktail of anti-viral compounds (ribavirin, lamivudine and acyclovir) have reduced levels of intraepithelial lymphocytes (IELs). These mice are more susceptible to chemical-induced intestinal inflammation, which is rescued by IL-15 administration to restore IEL homeostasis (Liu, 2019). Altogether, these findings support the hypothesis that the gut mycobiome and virome are emerging as critical immunomodulators with host physiological consequences ( Fig. 1).

IL-17A in multiple sclerosis and autism
The immunomodulatory effects of the gut microbiota are becoming increasingly appreciated in tissues beyond the GI tract, as gut microbeimmune interactions play an important role in the etiology of neuroinflammatory and neuropsychiatric disorders. Early studies in microbiota-depleted animals have shown that CNS development, function, as well as mood and behavior (Lu et al., 2018;Vuong et al., 2017) are significantly impaired compared to microbiota-replete controls, suggesting that the gut microbiota plays a critical role in neurological function throughout life. Initial support for this hypothesis came from studies examining the role of the mouse indigenous gut bacteria, SFB, on susceptibility to experimental autoimmune encephalomyelitis (EAE), a rodent model for multiple sclerosis (MS). GF mice are highly resistant to EAE compared to SPF mice (Lee et al., 2011). However, SFB colonization alone in GF mice is sufficient to induce extra-intestinal Th17 cells and EAE symptoms. Furthermore, loss of homeostatic control of the SFB-Th17 axis in intestinal-specific IL-17R-deficient animals results in exacerbated EAE severity (Kumar et al., 2016). Conversely, Bacteroides fragilis and Prevotella histicola colonization suppresses disease in EAE by promoting Treg function (Mangalam et al., 2017;Ochoa-Reparaz et al., 2010). In humans, intestinal Th17 cell responses positively correlate with MS disease and negatively correlate with the relative abundance of Prevotella in the human small intestine (Cosorich et al., 2017), suggesting that Prevotella is an important modulator of neuroinflammation in both EAE and MS. Transplantation of human MS fecal samples to GF mice induces a less potent anti-inflammatory Treg response compared to healthy fecal samples and facilitates the development of both spontaneous and induced models of EAE (Berer et al., 2017;Cekanaviciute et al., 2017). Altogether, these findings illustrate that T cell responses during CNS inflammation can be modulated by gut microbes (Table 1).
Studies using maternal immune activation (MIA) as a rodent model of autism identify a similar gut microbe-immune axis whereby SFB colonization is sufficient to promote autism spectrum disorder (ASD)like symptoms through Th17 cells (Choi et al., 2016;Kim et al., 2017;Lammert et al., 2018;Shin Yim et al., 2017) (Table 1). MIA-induced systemic release of IL-17A in pregnant dams contributes to abnormal patch development in the dysgranular zone of the primary somatosensory cortex (S1DZ) in the offspring brain (Choi et al., 2016;Shin Yim et al., 2017). Later studies identify intestinal Th17 cells as the source of IL-17A, which are significantly elevated in mice harboring SFB (Kim et al., 2017;Lammert et al., 2018). Treatment of dams with anti-IL-17A neutralizing antibody limited cortical patch development and behavioral abnormalities (Choi et al., 2016;Lammert et al., 2018). These findings are consistent with clinical reports identifying significant microbiota associations in human ASD and suggest a causative role for gut microbes in regulating a subset of ASD symptoms (Kang et al., 2019;Sharon et al., 2019;Wang et al., 2019) (Table 1).
Under homeostatic conditions, intestinal Th17 cell responses driven by gut microbes are non-inflammatory and host tissue-protective (Omenetti, 2019). However, in the context of immune challenge or loss of immunological tolerance, gut microbes can drive inflammatory Th17 cell responses that can contribute to inflammatory disease (Omenetti, 2019;Xu et al., 2018). Recent studies highlight differential functions of IL-17A and IL-17F, both produced by Th17 cells, in microbe-mediated intestinal inflammation (Tang et al., 2018). While many studies on Th17 cells focus on the functions of IL-17A, investigating the mechanisms that balance protective versus pathogenic microbiota-dependent Th17 cell responses will be necessary to determine the role of microbiota-immune crosstalk in neuroinflammatory disease.

Low-grade systemic inflammation in neuropsychiatric disorders
Elevated levels of circulating pro-inflammatory cytokines IL-1β, IL-6 and TNFα are associated with neuropsychiatric disorders in humans (Chu et al., 2019;Khandaker et al., 2018;Kohler et al., 2017;Treadway et al., 2019). However, the cellular sources of these cytokines and the pathways by which they are induced are not well understood. Moreover, whether these responses are a cause or consequence of neuropsychiatric symptoms requires further investigation. Low grade systemic inflammation is observed in rodent models of anxiety and depression (Hodes et al., 2014;Zhang et al., 2017), and is associated with impaired intestinal barrier function (de Punder and Pruimboom, 2015) (Table 1). One consequence of a disrupted GI barrier is the translocation of gut microbes, leading to impaired intestinal immune homeostasis and systemic immune activation (de Punder and Pruimboom, 2015). The inflammasome, a class of cytosolic innate immune receptors that recognizes intracellular microbe-and damage-associated molecular patterns (MAMPs, DAMPs), has emerged as a critical pathway involved in regulating microbiota-immune interactions (Man, 2018). Activation of the inflammasome pathway results in caspase 1-or 11-mediated cleavage and release of the pro-inflammatory cytokines IL-1β and IL-18 (Yang et al., 2019). Classically known to respond to intracellular bacterial pathogens such as Shigella, Salmonella and Legionella, recent studies suggest that inflammasomes play an important role in coordinating interactions between the healthy gut microbiota and host immune system (Levy et al., 2015;Man, 2018). Loss of the NLRP3and NLRP6-inflammasomes is associated with altered microbiota composition and increased susceptibility to colitis (Henao-Mejia et al., 2012;Wlodarska et al., 2014;Yao et al., 2017), a condition often linked to systemic inflammation observed in neuropsychiatric disorders. Interestingly, caspase-1-deficient mice, which lack inflammasome signaling and are protected from chemical-induced intestinal inflammation (Blazejewski et al., 2017), have reduced anxiety-and depressivelike behaviors following chronic restraint stress (Wong et al., 2016). However, whether these behavioral changes are a result of impaired microbiota-inflammasome interactions requires further investigation.
Functional associations between gut microbiota composition and stress-induced depressive-like behaviors are beginning to be elucidated. Animal models of physiological and psychological stress lead to altered gut microbiota composition (McGaughey et al., 2019;Werbner et al., 2019;Wong et al., 2016) and fecal transplantation regulates depressivelike symptoms through metabolic and inflammatory pathways (Kelly et al., 2016;Pearson-Leary, 2019;Zheng et al., 2016), suggesting that the gut microbiota contributes to stress-induced depressive-like behaviors (Table 1). One study, which profiled the gut microbiota of over    (Arpaia et al., 2013;Furusawa et al., 2013;Smith et al., 2013), future studies investigating whether these taxa are causally related to depression are warranted.

Chronic intestinal inflammation and neurodegenerative disorders
IBD is a chronic inflammatory condition that affects the mammalian GI tract. The etiology of IBD is multifactorial with genetic and environmental contributions. Among the many diseases associated with the microbiota, the causal role of the microbiota in IBD has been the most extensively explored (Britton et al., 2019). In IBD, gut microbes are potent drivers of pathologic intestinal inflammation, in part due to breakdown of physical and immune mechanisms that maintain separation between gut microbes and the host immune system (Ananthakrishnan et al., 2018). Epidemiological data have linked IBD with various neuropsychiatric and neurodegenerative disorders (Villumsen et al., 2019). For example, patients with IBD are 22% more likely to develop Parkinson's disease (PD) than healthy controls. In fact, there is considerable overlap between genetic susceptibility loci in IBD and PD. These associations suggest that chronic inflammation in the intestine, which is significantly affected by the gut microbiota, modulate susceptibility to neurodegeneration. Despite these observations, the pathways by which chronic intestinal inflammation affects neuropathophysiology is not well understood. Immune dysregulation in neurodegenerative diseases share common signatures with IBD including elevated T helper 1 (Th1), Th17 and reduced Treg cell responses, (Ananthakrishnan et al., 2018;Chitnis and Weiner, 2017). In addition, IBD is associated with loss of intestinal barrier function, microbial translocation and systemic immune activation (Sartor, 2008). Consistent with this, PD patients exhibit increased intestinal permeability (Perez-Pardo et al., 2019), disrupting homeostatic interactions between gut microbes and the host immune system (Table 1). Multiple hypotheses have been proposed to explain the functional link between chronic intestinal inflammation and neurodegeneration: 1) imbalance of pro-versus anti-inflammatory gut microbes (Ho et al., 2018;Wu et al., 2017), 2) systemic inflammation due to loss of intestinal barrier function and microbial translocation (Perez-Pardo et al., 2019) and 3) influence of microbial amyloids on host amyloidosis (Friedland and Chapman, 2017). One intriguing hypothesis that has not been explored is the role of aging in mediating microbiota effects on neurodegeneration. Age-related deficits in immune function such as germinal center formation in gut-associated lymphoid tissues can be corrected by fecal microbiota transplantation in mice (Stebegg et al., 2019). Additional studies are necessary to determine the functional role of age-related microbiota variation and immune dysfunction in PD and Alzheimer's disease (AD).
Alterations in gut microbial taxa with pro-inflammatory and antiinflammatory properties have been observed in AD (Table 1). The gut microbiota of AD patients with brain amyloidosis are enriched for Escherichia coli and Shigella (Cattaneo et al., 2017), which are potent proinflammatory drivers of chronic IBD (Palmela et al., 2018) and acute intestinal inflammation (Sansonetti, 2001). The abundance of E. coli and Shigella correlates with expression of the inflammatory genes Il1b, Nlrp3 and Cxcl2 in peripheral blood. These findings are consistent with Helicobacter intestinal infection in humans (Kountouras et al., 2006) and enteric bacterial infection in Drosophila (Wu et al., 2017) positively correlating with the pathogenesis and progression of AD. In the latter study, neuroinflammation and AD neuropathology triggered by oral infection with an enteric pathogen are dependent on immune hemocyte recruitment to the brain. Reduced abundance of Clostridium in the gut microbiota of AD patients diagnosed with dementia correlates with elevated CSF levels of Aβ42/Aβ40 ratio, phosphorylated tau and phosphorylated tau/Aβ42 ratio (Vogt et al., 2017). However, the contribution of the immune system was not explored.
Despite growing interest in intestinal fungal and viral communities on chronic immune activation and IBD (Wheeler et al., 2017), their role in CNS disorders is not well characterized. However, links between fungal infections and associated immune responses have been reported primarily in MS, but also in AD and amyotrophic lateral sclerosis (ALS) (Forbes et al., 2018). One recent report demonstrates that Crohn's disease (CD) patients are more likely to be colonized with the common skin yeast, Malassezia restricta, in the intestinal mucosa (Limon et al., 2019). Colonization of mice with M. restricta, but not the fungal microbe Candida albicans, increases susceptible to chemical-induced colitis, suggesting that the gut mycobiota affects host immunity in a species-specific manner. Additional studies are required to determine the role of fungal-mediated intestinal inflammation on the development and pathogenesis of neurodegenerative disorders.

Microglia
The CNS is densely populated with tissue-resident myeloid cells known as microglia, which play important functions in regulating and fine-tuning neuronal circuitry during development and throughout adulthood. Similar to tissue-resident macrophages in other organs, microglia exert their neuronal functions through cytokine release, complement activation and phagocytosis (Salter and Stevens, 2017). Neurodevelopmental, neuropsychiatric and neurodegenerative disorders have all been associated with impaired microglia function. Although several genetic factors regulating microglia activity have been characterized, the influence of environmental factors such as the gut microbiota is beginning to be elucidated. Microglia in microbiota-depleted animals have altered inflammatory gene expression profiles and adopt an immature state (Erny et al., 2015). More recent studies have shown that the role of the microbiota on microglia function is development-and sex-dependent (Thion et al., 2018). Despite these findings, the precise mechanisms by which microbes from the gut can affect brain-resident microglia remain unclear. The microbe-derived fermentation product of dietary fiber, SCFAs, can restore microglia dysfunction in GF and ABX animals (Erny et al., 2015), suggesting that microbial metabolites may be a general mechanism for gut-brain interactions. Consistent with this, dietary metabolites of tryptophan produced by the gut microbiota are involved in regulating microglial production of TGFα and VEGF-B, which limit CNS inflammation during EAE and human MS (Rothhammer et al., 2018). Taken together, these studies demonstrate that microbial signals originating from the intestine have long-range effects in modulating microglia function.

Astrocytes
In addition to microglia, astrocytes are major cellular players among glia that participate in the maintenance of CNS health and disease (Valori, 2019). Astrocytes provide support to endothelial cells that form the blood brain barrier and also play immune regulatory roles in CNS development and inflammation through antigen presentation, and cytokine and chemokine production. Astrocyte function can be modulated by dietary metabolites of tryptophan produced by the gut microbiota (Rothhammer et al., 2016). During EAE, type 1 interferons activate an anti-inflammatory pathway in astrocytes through the cytosolic transcription factor, aryl hydrocarbon receptor (AHR). AHR activation is dependent on indoles produced by ampicillin-sensitive gut microbes as by-products of dietary tryptophan metabolism. Bacterial tryptophanases, which catalyze tryptophan to indoles and related compounds, are highly expressed by specific members of gut microbes including Lactobacilli (Zelante et al., 2013), Escherichia coli (Li and Young, 2013) and Bacteroides (Devlin et al., 2016), suggesting that metabolic activities of these microbes are involved in regulating AHR-expressing astrocytes. Astrocytes release the IL-1 family cytokine IL-33 to activate microglial synapse engulfment (Vainchtein et al., 2018). This highlights a cytokine-mediated pathway by which astrocytes regulate neural circuitry remodeling, although a role for the gut microbiota has not been explored. Altogether, these findings illustrate that gut microbial metabolism regulates immune-related functions in astrocytes.

CNS-resident innate and adaptive immune cells
Previous work has linked AHR signaling regulated by microbial metabolites of tryptophan to IL-22 production at mucosal tissues (Zelante et al., 2013). More recent studies illustrate that cell-intrinsic AHR activation in ILCs (Li et al., 2018), B cells (Villa et al., 2017) and intestinal epithelial cells (Metidji et al., 2018) modulate specific functions in anti-helminth immunity and anti-tumor responses. Given the effect of AHR ligands on astrocytes, this raises the possibility that the function of other AHR-expressing innate (ILCs) and adaptive immune cells (T cells) resident in the CNS can be similarly modulated. CNSresident plasma cells and IgA of intestinal origin are also implicated in the pathogenesis of EAE (Rojas et al., 2019). Tissue-resident plasma cells and plasmablasts produce the immunoregulatory cytokine IL-10 to suppress neuroinflammation. Collectively, these findings suggest that gut microbes can affect CNS-resident innate and adaptive immune cell functions with neuropathological consequences.

Therapeutic implications targeting the microbiota-immune axis
The gut microbiota plays a critical role in modulating the development and pathogenesis of neuroinflammation in EAE, which is supported by clinical associations between gut microbiota composition and human MS (Berer et al., 2017;Cekanaviciute et al., 2017;Chen et al., 2016;Jangi et al., 2016). Existing immune-based therapeutics, such IFN-β, have been effective at limiting neuroinflammation in MS. Despite a large body of evidence supporting a pathogenic role for IL-17A in MS, antibody-based therapeutics targeting this pathway have produced mixed results (Steinman, 2010), potentially due to neutralization of tissue-protective functions of IL-17A (McGeachy et al., 2019). Therefore, approaches to target the microbiota through probiotic administration, dietary intervention and fecal microbiota transplantation (FMT) are being explored as alternative strategies (Fig. 2).
Clinical trials have tested probiotic administration alone or in combination with existing MS medications with promising outcomes (Tankou et al., 2018a;Tankou et al., 2018b). VSL3, a probiotic cocktail of 8 bacteria strains, administered to MS patients and healthy controls twice daily for 2 months, transiently elevates the relative abundance of VSL3-derived strains in the gut microbiota and decreases frequencies of circulating inflammatory monocytes and expression of activation markers CD80 and HLA-DR on blood monocytes and dendritic cells, respectively (Tankou et al., 2018a;Tankou et al., 2018b). Administration of a similar probiotic cocktail of Lactobacillus spp. and Bifidobacterium once daily for 3 months results in improvements in mental health and reductions in circulating inflammatory markers (Kouchaki et al., 2017) in MS patients. Clinical studies evaluating the use of FMT in large cohorts of individuals with CNS disorders are limited. One case report in a secondary progressive MS patient demonstrates beneficial outcomes following FMT (Makkawi et al., 2018). In ASD, an open-label FMT trial reveals long-term GI and autism-related health benefits in 18 pediatric patients (Kang et al., 2019). Based on these preliminary findings, additional randomized, double-blinded trials employing targeted probiotics or FMTs to treat various neurophysiological and behavioral disorders are necessary and currently in progress (www.clinicaltrials.gov).
Mood disorders including major depressive disorder, mild chronic depression, and bipolar disorder affect 9.7% of adults in the United States each year with a lifetime prevalence of 21.4% (Kessler et al., 2009;Kessler et al., 2005). Similarly, anxiety disorders, which include generalized anxiety, panic disorder, social anxiety and obsessive-compulsive disorders, affect 19% of the US adult population each year with a lifetime prevalence of 31% (Kessler et al., 2009;Kessler et al., 2005). These statistics coincide with a 65% increase in use of anti-depressant medications from 1999 to 2004 (Pratt et al., 2017). Use of anti-

Fig. 2. Therapeutic interventions targeting the microbiota-immune axis in CNS disorders (created using Biorender).
Targeting the microbiota-immune axis is a promising therapeutic strategy to treat CNS disorders. This can be achieved by 1) administration of probiotic strains that promote immunosuppressive responses and limit pathological inflammation 2) ingestion of diets or prebiotic formulations that supply nutrients to modulate gut microbial metabolism and release of anti-inflammatory by-products, 3) replacing a "diseased" gut microbiota with that of a healthy individual to restore beneficial microbial populations and/or eliminate pathogenic strains. Use of immunotherapeutic agents that directly target the host immune system in combination with microbial-based therapeutics may be more effective than immunotherapy alone to treat specific nervous system disease states. depressants and anti-psychotics modulates the inflammatory environment (Hou, 2019;Szalach et al., 2019) and are linked to changes in the human gut microbiota (Jackson et al., 2018;Lukic et al., 2019;Maier et al., 2018). These data suggest that modulation of systemic immune activation by these medications may partially occur through changes to the gut microbiota. A growing body of animal and clinical findings illustrating causal relationships between the gut microbiota and symptoms of mood disorders strongly indicates that modulating the composition and function of the gut microbiota may be an effective therapeutic approach and alternative to anti-depressants and anti-psychotics. Clinical studies to test microbial-based therapeutics, however, have been met with mixed results. Meta-analyses of clinical probiotic use in depression and schizophrenia only found a modest beneficial effect in a subset of studies (Ng et al., 2018;Ng et al., 2019), while several newer studies demonstrate improvement in depression symptoms (Chahwan et al., 2019;Wallace et al., 2019). Lack of reproducible efficacy using commercially available probiotics may be due to poor mucosal colonization and persistence of ingested strains (Suez et al., 2018;Zmora et al., 2018). These findings warrant additional clinical investigation to measure host determinants of probiotic colonization, such as endogenous gut microbiota composition, host metabolism and immune status, between probiotic-responsive and non-responsive patient cohorts. More importantly, rational design of specific bacterial strains with immunological potential, especially those that are depleted in the diseased gut microbiota, should be considered in future trials.
Adherence to a Mediterranean diet, which provides a rich source of dietary fiber for gut microbial fermentation and generation of anti-inflammatory SCFAs, is negatively associated with depressive symptoms (Gialluisi, 2019). However, meta-analysis of clinical prebiotic use indicates a lack of improvement in depression and anxiety compared to probiotic use (Liu et al., 2019b). Since dietary tryptophan can modulate susceptibility to MS (Nourbakhsh et al., 2018) potentially through the microbiota (Rothhammer et al., 2016;Sonner et al., 2019), a prebiotic therapy that modulates microbial tryptophan metabolism and downstream AHR-immune responses to treat neuroinflammation is worth exploration. More importantly, future trials should evaluate the specific prebiotic components tailored to enrich gut bacterial species with antiinflammatory and neuroprotective potential in a disease-specific manner.

Concluding remarks
The GI tract is colonized with trillions of indigenous microorganisms that shape local and extra-intestinal host immune function. Dysregulated immune responses are increasingly appreciated as drivers of not only neuroinflammatory, but also neuropsychiatric and neurodegenerative diseases. New findings in animal and human studies implicate the gut microbiota as a significant contributor of immune dysregulation in various CNS disorders. Gut microbes directly modulate intestinal and systemic immune homeostasis to affect neuroinflammation and may contribute to low grade systemic inflammation in neuropsychiatric disorders. Pathological immune activation towards gut microbes, a feature of chronic intestinal inflammation, is also a significant risk factor for neurodegeneration. Furthermore, the gut microbiota regulates the function of CNS-resident immune cells with potential consequences for neurodevelopmental and neuroinflammatory conditions. Therefore, targeting the microbiota-immune axis is a promising therapeutic approach for treating CNS and related disorders.

Declaration of Competing Interest
T.C.F. became an employee of Federation Bio during peer review and revision of this manuscript.