Microbiota-immune-brain interactions: A lifespan perspective

There is growing appreciation of key roles of the gut microbiota in maintaining homeostasis and influencing brain and behaviour at critical windows across the lifespan. Mounting evidence suggests that communication between the gut and the brain could be the key to understanding multiple neuropsychiatric disorders, with the immune system coming to the forefront as an important mechanistic mediator. Throughout the lifespan, the immune system exchanges continuous reciprocal signals with the central nervous system. Intestinal microbial cues alter immune mediators with consequences for host neurophysi-ology and behaviour. Several factors challenge the gut microbiota composition, which in response release molecules with neuro- and immuno-active potential that are crucial for adequate neuro – immune interactions. In this review, multiple factors contributing to the upkeep of the fine balance between health and disease of these systems are discussed, and we elucidate the potential mechanistic implications for the gut microbiota inputs on host brain and behaviour across the lifespan.


Introduction: microbiota-gut-immune-brain axis
The diverse ecosystem of microorganisms residing within the gastrointestinal tract, collectively termed the gut microbiota, regulates many host physiological processes, including immune system maturation and function [1]. Additionally, profound evidence implicates the gut microbiota in regulating brain function and behaviour throughout life [2,3]. Although there are many proposed mechanisms of gut-brain signalling, interactions between the microbiota and the immune system are emerging as an essential pathway that orchestrates microbiota-gut-brain communication.
Studies have implicated the gut microbiota in regulating both host innate and adaptive immunity in response to pathogens and immune tolerance against non-pathogenic stimuli. The interactions between the gut microbiota and host immunity are complex, dynamic and largely dependent on internal and external signals [4]. The ability of the gut microbiota to impact neuroimmunity offers a major pathway in which microbes regulate brain homeostasis and symptoms of neurological disease. Indeed, preclinical studies using models of perturbed microbiota composition have proven to be invaluable tools for evaluating the immunomodulatory effects of the gut microbiota. In this review, we discuss the immunomodulatory properties of the gut microbiota and the impact on the central nervous system (CNS), with subsequent implications for microbiota-gut-brain axis communication throughout life. maintaining continuous reciprocal signal exchange between mother and fetus [5]. Parental lifestyle (diet, smoking, drug/alcohol use, exercise), antibiotic exposure, mental health, and environmental factors are all shaping the fetal development with constant inputs from the hormonal, microbial, and immune changes in response to pregnancy (Figure 1) [5]. The effect of maternal nutrition on the offspring's brain immunity with the central focus being placed on microglia -the major innate immune cell type within the CNS -is being increasingly studied [6e8]. All the factors affecting parental health (including infection and stress) will inevitably impact the fate of perinatal offspring development. (Figure 2). Microbiota-Gut-Immune-Brain axis in healthy and pro-inflammatory conditions.

Figure 2
Factors mediating gut microbiota-brain-immune interactions throughout the lifespan. During the prenatal period, parental factors such as diet influence microbiota composition, immune system, and cognitive development of offspring. In early postnatal life, breast-or formula feeding differentially primes the immune system and brain development via the gut microbiota. The adolescent period is hallmarked by peer pressure for body image and weight management; therefore, the establishment of positive eating habits is of crucial importance in adolescence, in order to develop a healthy relationship with nutrition and its benefits for physiological systems such as the brain and the immune system. In adulthood general lifestyle parameters such as food choices, alcohol consumption, weight management, and caloric restriction have been collectively shown to influence gut microbiota composition which may have enduring effects on brain function via modulation of the immune system. During ageing, changes in the microbiota composition are associated with increased frailty, inflammageing, and a decline in cognitive function. These changes may be partly driven by clinical parameters that are concurrently affected by lifestyle choices.
Epidemiological and preclinical studies are beginning to elucidate the association between maternal immune activation (MIA) and psychiatric and neurological disorders such as autism spectrum disorder (ASD), attention deficit hyperactivity disorder (ADHD), anxiety/depression, cognitive deficits, developmental delays, schizophrenia and bipolar disorder [9]. In the context of ASD, cumulative evidence has implicated maternal interleukin 17a (IL-17a) produced by Th17 cells [10e13], further alteredmicrobiota composition and increased intestinal permeability in offspring [14]. Supportingly, multiple human studies have shown that immune challenges during pregnancy result in dysregulated cytokine levels of IL-6 and IL-17a, that are associated with increased risk of offspring development of ASD [15,16]. Additionally, epigenetic changes on the paternal germline cells due to infection led to persistent alterations in the offspring's health [17]. Therefore, infection during or prior to pregnancy in either parent might modulate offspring health with clear implications for the immune system serving as a route of communication between the gut and the brain.
Excessive maternal stress (e.g., anxiety, depression, traumatic events) during pregnancy can have enduring and detrimental effects on neurodevelopment [9,18,19]. For instance, in animal models,maternal stress disrupted levels of pro-inflammatory cytokines (IL-1b and IL-6) in the placenta and fetal brains [20e22], resulting in ASD-like phenotype in offspring [23], that was partially mitigated by maternal anti-inflammatory treatment [22]. Similarly, inhibition of IL-6 signalling during prenatal stress ameliorated IL-6-induced microglial deficits in offspring [24]. A recent study demonstrated that exposure to chronic restraint stress during pregnancy elicited microbe-dependent intrauterine inflammation, perturbed placental serotonergic signalling, and induced offspring behavioural deficits, mediated by the chemokine CCL2 [25].

Microbiota-immune interactions in early life
Microbial colonisation of the gut occurs in early life, whether this begins in utero is of debate [26e31]. However, it is well appreciated that major colonisation of the gut microbiota occurs at birth informing the development and maturation of the host's immune system [32]. Indeed, the most critical development of host immunity occurs within the first few years of life, coinciding with the maturation of the gut microbiota which develops from an unstable ecosystem into a highly diverse and robust community in adulthood [33,34]. Factors ranging from birth mode (Caesarean (C)-section, vaginal birth), prematurity, diet (breastfed, nonbreastfed, weaning), stress, medication (e.g. antibiotics) and environment can affect microbiota composition during this critical window of development [5,35].
Although some of these factors may elicit only transient effects on the gut microbiota, it has been hypothesised that these early-life influences may have enduring effects on offspring immune training, metabolic function, and even neurodevelopmental outcomes [35e38].
The exposure of the neonate to maternal vaginal and faecal microbes that occurs during birth is disrupted during C-section [39]. This differential microbial colonisation imposed by mode of delivery at such an early stage of life, when important developmental processes are still progressing, leaves a persisting signature on physiological systems that regulate health and disease. Indeed, many studies associate C-section with immunerelated disturbances such as allergies and asthma [40e43] while others suggest clear ramifications for the structure and function of the CNS, with hallmarks of increased neuronal cell death [44] and deficits in early communicative and social behaviour later in life [45]. Delivery mode has been linked to alterations in immune function, brain and behavioural development, with increasing evidence implicating seeding of the intestinal microbiota as a key mediator. It is hypothesised that both the absence of certain 'favourable' microbes and the presence of potential opportunistic pathogens, evidenced in C-section offspring, might tip the balance of healthy development of the immune and central nervous systems.
The intestinal microbial imbalance is evident in multiple other occurrences including premature birth, infection, and antibiotic use in early life. Indeed, infants born prematurely have an increased susceptibility to neuroinflammation [46] which is associated with neurodevelopmental impairments [47]. While antibiotics provide non-negotiable health benefits, their impact on the gut microbiota might have negative effects on the host. Antibiotic-induced microbiota depletion may alter microbially-derived SCFAs, which play a pivotal role in gut maturation, immune cell priming, and brain development [48]. Additionally, microbiota depletion by antibiotics perinatally might increase the proliferation of opportunistic microbes in the intestines with long-lasting imprinting on microbiota composition and diversity [49,50] alongside immune [51] and brain disturbances [52e54]. A recent study demonstrated a critical role for the maternal microbiota during pregnancy in offspring thalamocortical neuronal development and subsequent sensorimotor behaviour [53*].
In recent years, it has been shown that peripheral cues interact with immune cells that accumulate along the meninges [55e57**], revealing that even though the brain is an immune-privileged organ, peripheral signals are able to reach the CNS. IL-17-producing gd T cells residing in the meninges impact the development of short-term memories via increasing glutamatergic synaptic plasticity in neonatal mice, implicating the immune system in short-term memory formation and brain function [58]. Interestingly, in another study, IL-17 derived from Th17 cells, modulated intestinal homeostasis and microbiota composition, and indirectly affected CNS autoimmunity [59]. Gut-educated immune cells are able to enter the circulation and surround the meninges, further confirming that the immune system is a major pathway of communication between the gut and the brain [60*]. Other studies have demonstrated that microbiota-derived signals are essential for microglial function during critical windows of development [61,62**]. In animals lacking microbiota, increased microglia arborization and capacity to clear the excess of neuronal synapses in adolescence reveal a critical role for microbiota in microglia morphology and function [61e63**]. Thus, the absence of microbes during the early stages of development may have long-lasting effects on the gut microbiota composition throughout the lifespan with clear implications for the immune system.
Breast milk is the gold standard as the first food after birth, tailored to the needs of the baby according to the stage of development. Studies on breast milk reveal the complexity of its composition depending on the lactation stage and maternal factors (mental health, diet and lifestyle) [5]. Breast milk contains a plethora of molecules among which include human milk oligosaccharides (HMOs), polyunsaturated fatty acids, proteins, hormones, vitamins, and cytokines [64]. Breast milk also contains maternal microbes that are continuously seeding the infant gastrointestinal (GI) tract and various immune cells that are being passed to the infant throughout the course of lactation [65]. These components of breast milk have immunomodulatory potential and impact gut microbiota composition, neurotransmitter synthesis, and neurodevelopment in the peripheral and the central nervous system [5]. For example, HMOs have been shown to induce the maturation of gut epithelial cells [66], boost gut barrier function [67] and promote the viability of commensals while reducing the ability of opportunistic pathogens to attach to the gut [68]. This modulation of gut microbiota via HMOs changes the prevalence of genera in the gut with immunomodulatory potential, such as Bifidobacteria [69]. The absence of Bifidobacteria and their HMOutilisation genes is correlated with systemic inflammation and immune dysregulation in infants, while supplementation with Bifidobacterium infantis increased levels of T regulatory and non-classical monocytes in the blood, two cell types that are known to suppress inflammation [69].
During the first year of life, the solid food introduction phase -weaning -commences concurrent with increasing complexity and richness of the gut microbiota in response to substrate changes, which is crucial for immune system imprinting [70]. While new foods are presented in our GI tract during weaning, the immune system keeps recognising the new, 'premiering' components. Priming of the immune system during this period of life is important for the development of immunopathologies throughout life via the regulatory 'training' of the adaptive immune cells from the gut microbes [70**]. The gut microbiota during this period of life starts resembling more and more the adult-like composition with inputs from nutrition, as well as environmental and maternal factors [5] such as delivery mode [71], geography, and possession of indoor pets [72]. Simultaneously, the changes in gut microbiota composition during this period, and the availability of complex nutrients, have inputs in the developing brain [5] indicating that solid food introduction is paramount for both brain growth and microbiota maturation.

Adolescent period
Although it is hard to translate the adolescent period across species due to vast developmental differences, the initiation of the pubertal period is generally characterised by the re-activation of the hypothalamicpituitary-gonadal axis, which is initially activated during the prenatal and early postnatal period, but remains 'dormant' during childhood [73]. At the same time, the immune system is undergoing substantial time-specific changes that have been unravelled only during the last decade, positioning the immune system at the forefront of CNS developmental programming [74,75]. Increased pro-inflammatory signalling in the brain during this period modulates microglial function and morphology, influencing synaptic pruning and consequent behaviour in rodents and humans [76,77*]. The neuronal remodelling capacity of microglia positions them as key mediators of the neuronal connectome and behavioural phenotype [78]. Peripheral immune signals are able to circumvent the blood-brain barrier (BBB) and reach the brain via a complicated system that regulates the cerebrospinal fluid and immune cell flux in the brain, called glymphatics [79,80]. In rodents, gut microbial signals are able to alter the intestinal lymphatic system [81], hence it is conceivable that microbial-induced immune stimuli from the intestines could reach the brain via the lymphatic route [79,80]. Microbiota may play a key role in the developmental programming of the brain during adolescence, via dynamic endocrine and immune inputs along the brain borders during this sensitive period.
At the same time, adolescence is a period during which an individual is exposed to many psychosocial and physiological changes [82]. During adolescence, marked shifts in hypothalamic-pituitary-adrenal (HPA) axis reactivity coincide with the maturation of sensitive corticolimbic brain regions making an individual particularly vulnerable to stressors. It has been recently discussed that the gut microbiota modulates social behaviour via the HPA axis in adult mice [83]. The immune system is a mediator between vulnerability and resilience to stress by modulating the HPA axis. Indeed, inflammatory insults can alter neuronal trajectories and contribute to the onset of stress-related psychological disorders such as anxiety, depression, and drug/alcohol abuse [84,85]. Sex is also an important factor in modulating the stress response, with female rodents displaying increased and more prolonged responses to stressors and greater alterations in microbiota composition [86,87]. Indeed, further studies are merited to fully elucidate the age and sex-specific effects caused by stress exposure on gut microbiota development, neuroinflammation, and psychological outcomes.

Adulthood
As the adolescent period comes to an end, adulthood commences with a more stabilised gut microbiota composition compared to childhood and adolescence, and with a fully developed brain, yet receptive to continuous signals from gut microbes and the immune system. Dietary choices continue to be a major modulator of the gut microbiota, with implications for the immune system and therefore for maintaining general homeostasis [88]. Specific bacteria that are able to degrade the complex fibers release molecules as byproducts of their metabolism, such as short-chain fatty acids (SCFAs), with the capacity to boost the gut barrier and immuno- [89,90] and neuro-modulatory potential for the peripheral and the central nervous system of the host [48,91]. Dietary fiber and fermented food consumption increased microbiota diversity and function in the adult gut, and was followed by personalised immune responses attributed to the specific category of food they received (either high fiber or fermented food) [92*]. Interventions of such scale in humans are valuable tools to dissect the differential effects of various foods on the gut microbiota with implications for regulation of the immune system and possible benefits for brain health throughout the lifespan.
In homeostasis, the host immune response to the gut microbiota is tightly compartmentalised to the intestinal mucosal surface ( Figure 1) [93]. The commensal microbes are separated by a thick mucus layer that is organised around hyperglycosylated mucin MUC2, which promotes tolerogenic cytokines by imprinting antigen-sampling dendritic cells with anti-inflammatory signals, allowing for the immunological tolerance to commensal bacteria [94]. Microbial metabolites, namely indole, can upregulate genes involved in the strengthening of the mucosal barrier by regulation of tight junctions and mucin production, while reducing proinflammatory factors [95].
Gut microbiota tightly regulate the transformation of primary bile acids, predominantly produced in the liver, to secondary bile acids via key enzymes produced by the gut microbes in the distal gut [96]. The secondary bile acids have numerous effects on host health, including enhancing mucosal immunity [97]. Increasing evidence implicates microbiota-bile acid interactions in autism and psychiatric disorders [98,99]. Activation of bile acid receptor FXR leads to decreased gut permeability and reduced expression of colonic proinflammatory cytokines in the context of colitis in mice [100]. FXR and other bile acid receptors have been implicated in the regulation of immune cells, suggesting that microbiotadependent bile acid metabolism regulates the intestinal and peripheral immune response [100e102]. Concomitantly, inflammation affects bile acid metabolism by impairing cholesterol transport out of immune cells [103]. Interestingly, bile acid enzymes and bile acid receptors can be also found in the brain. In the context of multiple sclerosis (MS), circulating secondary bile acids were altered along with bile acid receptors in glial and immune cells in the brains of individuals with MS [104]. Considering that bile acids impact CNS function via glial and immune cells present in the brain, the implications for gut microbiota regulating these metabolites might be an approach to target the gut-bile acidsebrain axis for neuropsychiatric disorders.
Apart from the immune system being a direct pathway linking the microbiota with the gut and the brain, there are other routes of communication connecting these systems. It has been recently revealed that gut microbial changes are travelling to the brainstem via vagal signals at the hepatic afferent level and onwards to the enteric neurons [105]. This liver-brainegut neural feedback circuit prevents exaggerated intestinal inflammation and helps maintain gut homeostasis via modulating the differentiation and maintenance of intestinal T regulatory cells [105].
The gut microbiota influences the structural development of gut-associated lymphoid tissues and shapes its immune response to maintain tolerance to commensal bacteria and trigger host defence through recognition of pathogen-associated patterns (PAMPs) by pattern recognition receptors (PRR) and epigenetic regulators such as SCFAs [106]. Recently, Akkermansia muciniphila was shown to promote homeostatic host immune responses, through a phospholipid that acts as an agonist of non-canonical TLR2-TLR1 heterodimer, selectively stimulating pro-inflammatory cytokines [107]. Further, the gut microbiota also presents a mutualistic relationship with the adaptive immune system. Intestinal immunoglobulin A (IgA), secreted by B cells, can modify gut microbial communities and even contribute to the diversification of the microbiota, that in turn promote the expansion of T regulatory cells through a symbiotic loop [108]. Additionally, SCFAs generated by commensal bacteria are essential for the induction of T regulatory cells, further indicating that bacterial metabolites are important for the modulation of the host immune system [90]. Further, commensal segmented filamentous bacteria (SFB) elicit a quiescent-like state in Th17 cells, however, in response to pathogenic stimuli Th17 cells are skewed towards a proinflammatory profile [109]. Additionally, CD8þ T cells require microbial-derived SCFA, namely butyrate, to promote their long-term survival as memory cells [110]. Aside from the contribution of gut microbial signals in the regulation of the immune system, the gut microbiota has increasingly been implicated in the mediation of neuroinflammation [111]. In fact, a fascinating new study has shown that a specific subset of astrocytes -LAMP1þTRAIL þ astrocytes limit CNS inflammation by inducing T cell apoptosis, in response to IFN-g produced by gut-modulated natural killer cells, thus proving a role for gut microbial modulation of CNS inflammation [112].
In the context of stroke, IL-6, C-reactive protein, and lipoprotein-associated phospholipase A2 have been proposed as potential chronic inflammatory markers of stroke risk [113]. Further, neutrophils have been implicated in stroke pathophysiology, as neutrophil-to-lymphocyte ratio is associated with ischemic stroke [114], while CD64þ neutrophils are higher 6 h after the stroke and lower one week after the insult [115]. In an animal model of stroke, IL-17þ gd T cells traffic from the gut and accumulate in the meninges after the stroke insult, where they promote neutrophil infiltration in the brain parenchyma [116], thus suggesting that the gut microbiota can influence stroke-mediated neuroinflammation.
Major depressive disorder (MDD) is a complex debilitating disorder increasingly linked to neuroinflammation [111]. Some studies associate IL-8 levels with better antidepressant response [117], while TNF-a levels are reduced by antidepressant treatment in MDD treatment responders, thus suggesting that specific peripheral cytokine levels are important for antidepressant treatment outcomes [118]. Moreover, IL-6 has been robustly reported to be elevated in MDD, which resulted in the proposal that its blockade would lead to better clinical outcomes e however, in a recent study in individuals receiving hematopoietic cell transplantation, tocilizumab, an IL-6 antagonist, actually resulted in significantly worse depressive symptoms [119]. Minocycline, a tetracycline antibiotic that inhibits pro-inflammatory microglial activation, has shown promising outcomes as augmentation therapy in treatment-resistant depression, potentially through amelioration of CNS inflammation [120]. Antidepressant treatments modulate and dampen pro-inflammatory factors, suggesting both an involvement of inflammation in depression but also a potential treatment target [121,122].
Antidepressants alter gut microbiota composition, decreasing Ruminococcus flavefaciens and Adlercreutzia equolifaciens, and that R. flavefaciens interfere with antidepressant efficacy [123]. A recent study has shown that in individuals with MDD, 3 microbial species -Ruminococcus bromii, Lactococcus chungangensis, and Streptococcus gallolyticus e were differentially related to increased IL-1b levels and clinical depressive score parameters [124]. In summary, cytokine levels seem to be central in the response to antidepressant treatment, and as the importance of cytokine modulation in antidepressant treatment is further dissected, it is feasible to hypothesise that modulating the gut microbiota and its metabolites could also regulate inflammation levels, hence resulting in better treatment outcomes.
Schizophrenia is a debilitating psychiatric disorder associated with alterations in a number of peripheral circulating cytokines and chemokines e such as IL-6, IL-8, and IL-10 -suggesting increased immune activation [125]. In fact, higher levels of IL-8, IL-10, and TNF-a positively correlate with negative symptom severity, and increased levels of IL-8 in particular are linked to a deficient antipsychotic treatment response [126]. Furthermore, early-life infections and autoimmune disorders have been found to correlate with the development of schizophrenia, with concurrent presence of autoantibodies and alterations in cytokines in the CSF [127]. Along with the reported peripheral alterations, post-mortem brain tissue from individuals with schizophrenia showed increased pro-inflammatory markers in the dorsolateral prefrontal cortex, thus suggesting that inflammation can influence neuropathology, and consequently symptoms [128]. Additionally, the gut microbiota of individuals with schizophrenia has been found to be enriched with Streptococcus vestibularis, which when transplanted to mice-induced impaired social interaction and hyperkinetic behaviour, while increasing the genetic expression of inflammatory-related pathways in the intestine [129]. Further studies are warranted to understand the extent to which particular gut bacterial species influence inflammation and consequently, behavioural alterations and neuropathophysiology in the context of schizophrenia.

Ageing
With age, marked immune differences occur, as often represented by decreased or sometimes increased functions e a concept coined immunosenescence e that ultimately promote compromised immune responses [130]. Over the ageing process, the gut undergoes remodelling, resulting in compromised barrier function and in the decline of genes associated with innate and adaptive immunity, including IgA, TLR4, T cell (CD3εþ), and T helper (CD4þ or CD8þ), suggesting an overall reduction of T cell signalling pathways in the aged gut. This decline can contribute to the chronic low-grade inflammatory state [131] as this reduction in adaptive immunity activation may trigger stimulation of the innate immune system [130]. With age, there is a decline in the abundance of A. muciniphila, that elicits gut leakiness and consequent circulation of proinflammatory factors that trigger a cascade involving CCR2þ monocytes that in turn interfere with B cells [132]. Faecal microbiota transplantation (FMT) from an aged donor into a young recipient is sufficient to induce intestinal and systemic low-grade inflammation, through increased T cell activation [133,134*]. Inversely, exposure of microbiota from a young donor into aged mice resulted in the enhancement of M-cell maturation in Peyer's patches and elevated intestinal IgA responses, which are disrupted with age [135]. Given its interface with immunology, the gut microbiota is an important candidate in the modulation of inflammageing -a marked low-grade pro-inflammatory phenotype that accompanies ageing in mammals [136]. For instance, peripheral inflammatory markers have been linked to the intestinal microbiota profiles of elders in a dietdependent manner, along with mental health markers [137]. In centenarians, the abundance of Bifidobacterium, Akkermansia, and Christensenellaceae suggests that a particular microbial ecosystem is found in longevity and could have health-associated features [138]. Taken together, evidence suggests that age-related microbiome alterations may contribute to age-associated inflammation, making it feasible to hypothesise that gut microbiome manipulations could mitigate agerelated inflammation [139].
Ageing is also characterised by altered immune cell populations in the periphery, such as increased monocytes, CD4þ, and CD8þ T cells, while other cell populations such as dendritic cells and B cells seem to be underrepresented with age [140]. Ageing promotes low-grade systemic inflammation, highlighted by increased levels of cytokines such as IL1-b, IL-6, IFN-b, as well as chemokines and immunoregulatory factors that drive further pro-inflammatory states in immune cells [140,141].
During the ageing process, senescent cells release proinflammatory cytokines, as a feature of the senescenceassociated secretory phenotype (SASP), for the recruitment of immune cells in order to clear senescent cells [142]. However, the accumulation of senescent cells with ageing and the consequent activation of SASP promotes augmented chronic inflammation, inevitably resulting in tissue degeneration and contributing to the advancement of age-related diseases [113,142]. In fact, neuroinflammation has been implicated in age-associated neurodegenerative diseases such as Alzheimer's [143] and Parkinson's [144]. By reducing inflammageing, microbiome-targeted therapeutical interventions may moderate the impact of senescent cells and therefore pose an interesting strategy to moderate age-associated health decline [145]. Additionally, gut microbial composition is altered in arteriosclerotic cerebral small vessel disease (aCSVD) patients, which correlated with increased expression in inflammageing markers, in particular IL-17a [146]. Future studies aimed to identify which gut microbial metabolites are driving the exacerbation of IL-17a production, particularly produced by neutrophils, hold potential for the development of targeted treatments for aCSVD [146].
Microglia have marked morphological changes in aged human post-mortem brain samples [147], while recent preclinical studies show that with age microglia build up lipid droplets that generate high levels of reactive oxygen species, show impaired phagocytosis and release proinflammatory cytokines [148*]. The gut microbiome largely influences age-related microglial function e more specifically, N6-carboxymethyllysine (CML), a gut-derived metabolite accumulates in the microglia of aged mice and human brains due to increased age-associated intestinal permeability, resulting in increased reactive oxygen species, which disrupts mitochondrial activity in microglia [149*]. Along with disrupted microglial function, the aged microbiome also impacts retinal inflammation, whereby FMT from young donors to aged recipients reverses intestinal permeability deficits and systemic inflammation induced by age [134*]. Further, FMT from young donor mice reshapes microbiota composition in aged mice, while positively influencing behaviour [150*,151**]. Interestingly, d-valerobetaine, a gut microbiota-derived metabolite, was altered in the serum and brain of the recipient mice in an agedependent fashion of the FMT donor [151**]. Additionally, an age-associated increase of d-valerobetaine was observed in both mice and humans, and injections of this metabolite worsened the cognitive performance of young mice, while reversing the beneficial cognitive effects of faecal transplantation from young donors in aged mice [151**]. Another microbially-derived metabolite in the gut, isoamylamine, is known to promote ageing-associated cognitive dysfunction via inducing microglial cell death, confirming once again that gut microbiota metabolism impacts brain function and behaviour in the context of ageing [152]. Dietary interventions have also been widely proposed to be a crucial mediator of inflammation in ageing. In particular, it is suggested that the Mediterranean diet positively modulates the hallmarks of ageing by increasing longevity, reducing age-associated disease risk [153], and lowering the risk of cognitive impairments [154]. Furthermore, dietary interventions can be effective mediators of autophagy, a central regulator of ageing [155]. Taken together, ageing induces a pervasive intestinal and neuroinflammatory environment characterised by inflammageing. Interestingly, the potential use of dietary interventions may promote autophagy [155] which has been shown to be a highly effective longevity modulator suggesting a role for the gut-brain axis in age-dependent physiological mechanisms.

Therapeutic implications and future perspectives
Future microbiota research should concentrate on targeting the microbiota for the treatment and prevention of disease, although it is not without its challenges [156]. The focus should shift toward longitudinal analysis, intervention studies, and exploiting the promise of microbial engineering and precision medicine. For example, bacterial therapeutics for intestinal transgene delivery was able to reverse glucose sensitivity in an obesity mouse model [157]. Additionally, it is evident now that suppression of pathogens present in microbiota from an inflammatory bowel disease (IBD) cohort via phage consortia was able to treat intestinal inflammation and alleviate the symptoms of IBD in these individuals [158]. Such tools targeting gut commensals and pathogens for improving host health should be put in the forefront of scientific approaches for disease treatment. A better understanding of microbiota-immune system interactions in the context of brain health may lead to targeted interventions and could help us understand interindividual responses to treatments and perhaps point to precision approaches for treatment for a variety of neuropsychiatric disorders in the future.

Conclusion
The emerging role of the gut microbiota in regulating health and disease has pointed towards novel ways for targeted interventions in modulating gut-immune-brain interactions. In order to understand the mechanistic implications of microbiota changes and the importance for gut-immune-brain interactions, more research need to focus on approaches such as FMT, and pharmacological and dietary interventions, including the screening for microbially-derived metabolites with immune-and neuro-modulatory potential.

Declaration of competing interest
APC Microbiome Ireland is a research center funded by Science Foundation Ireland (SFI/12/RC/2273_P2). Prof. Cryan is funded by the Science Foundation Ireland (SFI/ 12/RC/2273_P2), Saks Kavanaugh Foundation and Swiss National Science Foundation project CRSII5_186346/ NMS2068, and has received research funding from IFF, Reckitt and Nutricia, has been an invited speaker at meetings organised by Freisland Campina and Nutricia; he has served as a consultant for Nestle. Prof. Clarke has received honoraria from Janssen, Probi and Apsen as an invited speaker; is in receipt of research funding from Pharmavite and Fonterra; and is a paid consultant for Yakult, Zentiva and Heel pharmaceuticals.

Data availability
No data was used for the research described in the article.   Determines the existence of gut-educated IgA-producing plasma cells adjacent to the human and mouse dural sinuses, that are functionally important to prevent blood-borne pathogens to enter the brain, suggesting that meningeal IgA is crucial for 'guarding' the central nervous system in moments of vulnerability.