Gut microbes are immunologically tolerated in the gastrointestinal tract but trigger aggressive immune responses upon translocation across the gut barrier. Although oral tolerance, a physiological process that dampens immune responses to food proteins and commensal microbiota, remains poorly defined, significant progress was made during and after the Human Immunodeficiency Virus epidemic in the 1980s and the discovery of regulatory T cells in 1995. Additional insight was gained after the discoveries of innate lymphoid cells in 2008 and the functional elucidation of mucosal mast cells.
1. Introduction
Biological barriers, comprised primarily of epithelial and endothelial cells, separate the body compartments from each other and protect tissues and organs from external chemical, biological, and physical agents. Similarly, biological barriers, such as the blood-brain barrier (BBB) maintain the homeostasis of biomolecular transport into tissues and organs
[1].
In the gastrointestinal (GI) system, gut barrier function is highly dependent on the integrity of intestinal epithelial cell (IEC) plasma membranes and the presence of tight junction (TJs) molecules that bind IECs tightly to each other. In addition, the adequate functioning of intestinal barriers requires an intact, gut-associated lymphoid tissue (GALT), that is comprised of B and T lymphocytes, macrophages, dendritic cells as well innate lymphoid cells (ILCs), that along with IECs can recognize gut antigens and bacteria
[2]. Under pathological circumstances, damaged TJs and plasma membranes contribute to barrier dysfunction and microbial passage into host tissues
[3].
Microbial translocation (MT) the term used throughout this entry, refers to the “escape” of intestinal microorganisms or toxins into the host systemic circulation by passing through the intestinal barrier and lamina propria, reaching mesenteric lymph nodes and the circulatory system
[4][5].
Different translocation pathways across the gut barrier have been described as paracellular or transcellular. The former facilitates the transport of solutes and nutrients, while the latter promotes the translocation of cells and particles, including granulocytes, microbes, and microbial components
[4]. Such translocation can also involve specific cell types. For example, antigen processing by GALT requires transcytosis of microbes and toxins through the intestinal membranous (M) cells
[5].
From a host perspective, the plasma membrane participates actively in immune defenses and contributes to microbiota tolerance by externalizing phosphatidylserine (PS), a global immunosuppressive marker
[6]. Indeed, due to their short, 3–5 days lifespan, IECs can externalize PS en masse, extending tolerance to the gut microbial community
[7]. Some gut microbes, including
E. coli, contribute to the tolerogenic intestinal milieu by externalizing PS on their own surfaces, suppressing host immunity
[8][9].
From the microbiota perspective, host tolerance can be promoted by increasing the levels of interleukin 10 (IL-10), a tolerogenic cytokine that upregulates regulatory T cells (T regs), dampening host immune responses to commensals
[10][11]. For example,
Faecalibacterium prausnitzii (
F. prausnitzii), can induce host dendritic cells (DCs) to release IL-10 and other tolerogenic molecules, thus participating actively in the immune acceptance of gut flora
[12]. Moreover, natural killer cells (NKCs) can promote gut tolerance by releasing interferon γ (IFN-γ), a cytokine that can convert tryptophan to kynurenine, causing upregulation of the key tolerogenic enzyme indoleamine 2,3-dioxygenase (IDO). In return, kynurenine can bind with aryl hydrocarbon receptor (AhR), suppressing immune responses to commensals
[13][14][15][16].
The 1995 discovery of Tregs, an IL-2-expressing subgroup of T helper cells (Th), has contributed to a better understanding of the molecular underpinnings of oral and gut immune tolerance
[17]. Mucosal mast cells (MCs) suppress T regs in the presence of gut pathogens, triggering the opposite drive, activation of effector T cells, and immunogenicity
[18]. The relationship between Tregs and MCs maintains gut homeostasis by upholding the careful balance between commensal acceptance and pathogen rejection.
In addition, the finding that group-3 innate lymphoid cells (ILC3s) release IL-2, a Treg-activating cytokine, emphasizes further the molecular intricacies of gut immune tolerance
[19] (
Figure 1). Furthermore, the finding that IL-10, a primarily anti-inflammatory cytokine, enhances IL-2 secretion and suppresses proinflammatory IL-17, has shed additional light on the cellular and molecular underpinnings of the intestinal tolerogenic milieu
[20][21]. Together, these novel findings have contributed to a better understanding of gut barrier homeostasis and MT.
Figure 1. Innate lymphoid cells are comprised of NKCs and innate lymphoid cells types 1, 2, and 3 (ILC1, ILC2, and ILC3). These cells are activated by various cytokines (top), release other cytokines (bottom), and express transcription factors, including T-bet, GATA-3, and RORγt. When dysregulated, these lymphoid systems may trigger autoimmunity, fibrotic and neuropsychiatric illnesses.
While increased immune tolerance can promote MT, an excessively immunogenic environment can be equally detrimental. For example, inflammatory bowel diseases (IBD), such as Chron’s disease or ulcerative colitis, has been associated with increased MT as evidenced by the elevated levels of translocation markers, such as lipopolysaccharide-binding protein (LBP) and soluble CD14 (sCD14), that have been reported in this pathology
[22][23].
Numerous pathogens, including Human Immunodeficiency Virus (HIV), promote self-tolerance by exploiting the host tolerogenic systems, including PS, IL-2, IL-10, or IDO, and in the process facilitating MT and toxin passage
[24]. Indeed, MT was poorly defined prior to the arrival of HIV, a virus capable of increasing intestinal permeability by depleting IL-22, the guardian of gut barrier function (
Figure 2)
[25][26][27][28][29].
Figure 2. HIV induces apoptotic loss of ILC3, lowering IL22, the guardian of the gut barrier. This, in turn, promotes the translocation of intestinal microbes and their molecules into the systemic circulation. Activated host immunity maintains a state of low-grade inflammation that characterizes many diseases of uncertain etiology.
In addition to HIV, several other pathogens can hijack the molecular pathways of intestinal tolerance, promoting MT. For example, bacteria and viruses can amplify infectivity by generating IL-10 orthologues or increasing IL-10 itself, inducing self-tolerance and averting host defenses
[30][31]. Another mechanism utilized by enveloped viruses to induce tolerance is apoptotic mimicry, a Trojan horse maneuver of PS externalization to facilitate viral uptake by the phagocytes
[32].
In 2002, a new family member of tumor necrosis factor α (TNFα) was discovered, the TNF-like cytokine 1A (TL1A) that over the past two decades has revolutionized the field of MT and very likely other disorders of uncertain etiology
[33]. TL1A, a T cell co-stimulator, is a ligand of death receptor 3 (DR3) and participates actively in the pathogenesis of many cancers as well as other autoimmune, fibroproliferative diseases and neuropsychiatric disorders, including schizophrenia
[34][35][36][37]. Indeed, anti-TL1A antibodies, currently in Phase 2 trials for ulcerative colitis, may prove beneficial for several idiopathic diseases
[38][39][40] (NCT02840721).
2. The Microbiome and Innate Lymphoid Cells
The Human Microbiome Project, initiated in 2007, revealed that the microbial organ is comprised of more than 2000 bacterial species, belonging to 12 different phyla, more than 90% of which are related to
Proteobacteria,
Firmicutes,
Actinobacteria, or
Bacteroidetes species [41][42][43]. Aside from prokaryotic microbes, the gut microbiome also contains yeasts, fungi, and viruses that influence the barrier function by releasing signaling molecules and metabolites as well as by direct interaction with IECs
[44][45]. For example, the human gastrointestinal (GI) tract virome (viral microbiome) contains a variety of RNA and DNA viruses, most of which are bacteriophages (phages) or bacteria-infecting viruses, that regulate the microbiome composition as well as the permeability of gut barrier
[46]. In addition, phages can be disseminated throughout the body by the circulatory system and can interact directly with human cells
[47][48]. Moreover, as phages ingress into microbial cells by lysing the cell wall, they often release microbial fragments, such as muramyl peptides, lipoteichoic acids, or LPS, triggering responses related to gut pathology
[49].
Discovered in 2008, innate lymphoid cells (ILCs) are mucosa-anchored non-T, non-B lymphocytes, consisting of natural killer cells (NKCs) and ILC1, ILC2, and ILC3 cells
[50]. These systems play a pivotal role in maintaining the homeostasis of the gut barrier, intestinal tolerance, and nutrient transport
[51][52]. Unlike the B and T lymphocytes which express specific antigen receptors, ILCs respond to transcription factors and synthesize cytokines
[53][54] (
Figure 1). For example, ILC3 express retinoic acid receptor-related nuclear receptor γt (RORγt), are activated by IL-23 and IL-1β, while they release IL-22 and IL-17, cytokines involved in the integrity of the gut barrier as well as the blood-brain barrier (BBB) (
Figure 1)
[55][56].
MT was poorly defined prior to the HIV epidemic but was extensively studied in the context of this virus which was known for activating the immune system and increasing gut barrier permeability
[57][58][59]. Indeed, HIV triggers ILC3 apoptosis, lowering IL-22, which in return alters the intestinal barrier, promoting MT
[60] (
Figure 2).
Under physiological conditions, IL-22, acts as a guardian of gut barrier function, while IL-17 is known for initiating inflammation and recruiting neutrophils, and clearing invading pathogens
[61]. Dysfunctional IL-17 may trigger chronic inflammation, a characteristic of several disorders of unclear or unknown etiologies, including autoimmune diseases, pathological fibrosis, and neuropsychiatric disorders, such as schizophrenia
[62][63][64]. Maintaining an appropriate balance between the tolerogenic IL-22 and proinflammatory IL-17 is necessary for enabling GALT to distinguish between gut commensals and pathogens
[65]. For example, GALT dyshomeostasis, due to dysfunctional ILC3 and an impaired Tregs/Th17 ratio, was demonstrated to trigger food allergies and IBD
[66][67][68].
Abundantly expressed in the intestinal mucosa, ILCs protect the gut barrier by releasing IFN-γ, a cytokine that activates tryptophan metabolism, promoting tolerance via kynurenine and IDO. On the other hand, the dysfunctional release of IFN-γ by NKCs or ILC1, may lead to pathological fibrosis, major depressive disorder (MDD), and systemic lupus erythematosus (SLE), further connecting ILCs with the disorders of uncertain etiology
[69][70][71] (
Figure 1). Indeed, impaired ILC2 and dysfunctional release of IL-5 and IL-13 were demonstrated in MDD and suicidal behavior, linking neuropathology to this lymphoid system. Along this line, a recent study has connected dysfunctional ILCs with the loss of
Alloprevotella rava (
A. rava),
a, a salivary microbe whose absence was associated with suicidality
[72][73]. Moreover, IL-5 and IL-13 have been known for their role in eliminating
Helicobacter pylori (
H. pylori), a pathogen previously implicated in MDD
[74][75].
Taken together, these data show that ILCs are essential for maintaining the homeostasis of the intestinal barrier and the immune acceptance of gut microbes. Conversely, dysfunctional ILCs can trigger barrier dysfunction and MT, predisposing patients to disorders of uncertain etiology
[76][77].
3. Autoimmune Disease as an MTD Phenomenon
Under physiological conditions, microbes and/or microbial components have been detected in many host tissues, including the circulatory system and the CNS
[78][79]. Pathologically, microbial antigens, such as LPS, have been demonstrated in Alzheimer’s disease (AD) brains, cardiovascular disease (CVD), diabetes, and some cancers, suggesting that these conditions could be reconceptualized as microbial translocation disorders (MTDs)
[80][81][82][83]. In addition, several autoimmune diseases, including SLE, were associated with MT, further linking these disorders to a dysfunctional GI tract barrier. Indeed, preclinical studies have implicated
Enterococcus gallinarum (
E. gallinarum) in SLE, and reports indicate that the elimination of this bacterium by the antibiotic treatment could alleviate many autoimmune effects
[84][85].
4. Fibroproliferative Diseases Such as MTDs
Fibroproliferative diseases are idiopathic conditions associated with increased worldwide morbidity and mortality as well as a lack of effective therapies. They affect many organs, including the intestine, liver, kidney, heart, and lung, and are characterized by pathological scar formation
[86]. Indeed, fibrogenesis is a physiological or pathological response to chemical, mechanical, or immune insults in which the healing process involves fibroblast-mediated reorganization of the extracellular matrix (ECM)
[87][88]. Dysfunctional healing with excessive fibroblast activation and overproduction of ECM is believed to drive pathological fibrosis. In addition, fibroblast growth factors are established participants in gut barrier integrity and loss of fibroblast growth factor 9 (FGF9) has been associated with pulmonary fibrosis, further linking this pathology to MTD
[89][90][91].
Transforming growth factor-β (TGF-β), a tolerogenic cytokine and master regulator of gut microbiota, was demonstrated to promote fibrosis by abnormally activating ILCs, Th17, NKCs, and B lymphocytes
[92][93][94][95]. In addition, a subset of IL-10-producing NKCs was reported to promote fibrosis, an action independent of the elimination of defective cells
[96][97]. Moreover, ILC2-released IL-13 was shown to drive collagen deposition, indicating that dysfunctional ILCs may promote pathological fibrosis
[98] (
Figure 1). Furthermore, microbiota-generated curli amyloid fibrils were reported to disrupt ILC3, IL-17, and IL-22, promoting both autoimmunity and intestinal fibrosis, highlighting the interconnectedness of these pathologies
[99][100][101][102].
A variety of gut viruses and microbial species can drive excessive fibrosis as they express the integrin motif or arginine-glycine-aspartate (RGD), known for binding integrins αvβ6 and αvβ8, highlighting a mechanism of pathological fibroproliferation
[103][104]. Interestingly, integrin αvβ6 is an activator of TGF-β, liking integrins to the master regulator of gut microbiota
[105].
Considering the above data together, translocated microbes may have the ability to drive the pathogenesis of fibroproliferative disorders by disrupting the function of NKCs and ILCs. At the molecular level, ECM proliferation may be triggered by microbe-disrupted integrins and fibroblast growth factors.
5. Neuropathology as MTDs
Several recent studies have reported new onset psychosis after SARS-CoV-2 infection or even after the administration of mRNA vaccines, connecting viral antigens with this neuropsychiatric pathology
[106][107][108][109][110][111]. As SARS-CoV-2 disrupts the intestinal barrier and tryptophan absorption, it enables MT and secondary bacterial infections. These observations have reawakened the interest in the infectious hypothesis of neuropsychiatric illness entertained by many researchers and clinicians both at present and in the past. For example, Emil Kraepelin hinted at the microbiome when hypothesizing that under pathological circumstances microbes from various body compartments could migrate into the brain and trigger neuropathology
[112]. Along this line of thought, IBD and CRC, conditions characterized by increased intestinal permeability and MT appear to validate Kraepelin’s migration model
[113][114][115]. The same can be stated about psychosis induced by urinary tract infections (UTI), a destabilizing condition in patients with psychiatric illness
[116][117][118] (next subsection). Moreover, Kraepelin’s paradigm appears to be validated by the link between maternal influenza and offspring with schizophrenia, or childhood viral enteritis, and the development of psychiatric illness later in life
[119][120][121][122]. Furthermore, the finding that some gut microbes express tryptophan decarboxylase and can synthesize endogenous hallucinogens, such as tryptamine or N, N-dimethyltryptamine (DMT), further suggests support for Kraepelin’s hypothesis of microbe-mediated neuropathology
[123][124]. Indeed, new onset psychosis associated with COVID-19 may be explained by the selective upregulation of tryptophan decarboxylase-expressing gut microbes, such as
M. morganii, a bacterium previously implicated in schizophrenia
[125].
The 2001 discovery of trace amine-associated receptors (TAARs), proteins that can sense minute amounts of tryptamine, further substantiates Kraepelin’s hypothesis
[126]. The recent finding that microbial metabolite 4-ethylphenyl sulfate (4EPS) disrupts oligodendrocyte maturation in multiple sclerosis (MS), has further connected the microbiome to this neuropathology
[127][128][129]. Furthermore, in other studies, several gut microbial species were demonstrated to trigger celiac disease by synthesizing gliadin-mimicking peptides, molecules capable of altering the TJ protein, zonulin
[130][131]. This appears to be significant as the systemic translocation of these microbes and their zonulin-like antigens may lead to the generation of anti-gliandin antibodies, further linking the microbiome to autoimmunity
[132]. Interestingly, elevated IgA anti-gliadin antibodies were found in patients with schizophrenia, connecting this disorder to dysfunctional biological barriers
[133].
6. GI Tract Cancer as MTD
The microbiome can be related to cancer development through the induction of inflammation, as well as the production of toxins, in both cases creating favorable niche environments for tumor development.
Genotoxic molecules released by some intestinal microbial species have been shown to activate bacteriophages (phage) embedded in commensals’ DNA. Similarly, phages can eliminate susceptible microbes and increase the abundance of virus-resilient microorganisms, such as tryptophan carboxylase-expressing bacteria that synthesize endogenous serotonergic hallucinogens
[134][135]. In addition, some strains of
E. coli inhibit DNA mismatch repair proteins by synthesizing colibactin, a tumor-promoting toxin. Colibactin-associated IBD and CRC are more prevalent in patients with schizophrenia compared to the general population, linking this disorder to both DNA damage and MT
[136][137][138][139]. The standard histopathological detection of gut inflammation is specified by the presence of inflammatory infiltrates. However, low-grade inflammation may be a much more common occurrence, defined by the elevated systemic expression of proinflammatory cytokines in the absence of the inflammatory infiltrate aggregates. Low-grade inflammation can be casually related to dietary habits, microbiome dysregulation, or both
[140][141]. Thus, aside from genomic damage, some
E. coli species have been known for maintaining a state of low-grade inflammation that can predispose patients to cancer
[137]. The cancer risk is further increased by the inflammation caused by a colibactin-disrupted gut barrier and MT
[136][138].
Alteration of the human gut phageome, especially bacteriophages infecting
Fusobacterium nucleatum (
F. nucleatum),
Peptacetobacter hiranonis, and
Parvimonas micra, has been associated with CRC, emphasizing the likely viral etiology of this type of tumor
[142].
F. nucleatum is a common oral microbe which can reach the colon, accelerating the progression of CRC.
F. nucleatum can cause transient bacteremia triggered by chewing, daily hygiene, or dental procedures, indicating several pathways for contributing to tumorigenesis
[143]. Aside from
Fusobacterium,
Bacteriodes fragilis, especially via its
B. fragilis toxin (BFT) can trigger chronic intestinal inflammation, predisposing to CRC
[144]. The common microbial strains linked to CRC are
Bacteroides fragilis,
Escherichia coli,
Enterococcus faecalis and
Streptococcus gallolyticus. Other species identified in CRC patients’ tumor and fecal samples include
F. nucleatum,
Parvimonas,
Peptostreptococcus,
Porphyromonas and
Prevotella [145].
Indeed, previous studies have connected
F. nucleatum with carcinogenesis as well as the pathogenesis of poorly understood disorders, including CVD, preeclampsia, AD, and autoimmune disorders. These observations further link various microbes to these pathologies
[143].
A growing body of evidence has shown that the SARS-CoV-2 virus can thrive in GI tract reservoirs, likely blending into the gut virome
[144][145].
Indeed, as some gut microbes express angiotensin converting enzyme 2 (ACE-2), the SARS-CoV-2 cell membrane entry portal, they may harbor the virus, possibly accounting for some, if not many of the symptoms of long COVID
[146]. It remains to be seen if a higher occurrence of GI-related oncogenesis will be positively correlated with Long-COVID
Microbe-induced, genomic damage can also reactivate dormant viruses, including herpes simplex virus type 1 (HSV1) and Epstein-Barr virus (EBV), pathogens previously associated with schizophrenia as well as with various malignancies
[147][148][149][150]. Indeed, EBV is a well-characterized oncovirus associated with several cancers, including CRC
[151]. In addition, novel studies have connected phages and fungi to CRC, indicating the possible multi-kingdom etiology of this tumor
[152][153].
7. Interventions
In two previous studies, the researchers have discussed several strategies for gut barrier restoration and these therapeutic approaches will not be repeated here in more detail
[154][155].
Table 1. summarizes these interventions. Here, the researchers will focus primarily on the TL1A antibody, currently known as PF-06480605, as well as on membrane lipid replacement therapy (MLR)
[156] (287A).
Table 1. Quick reminder of gut barrier restoration strategies.
Compound
|
Action Mechanism
|
References
|
Navitoclax; Fistein; Quercetin; Dasatinib
|
Senotherapeutics (eliminate senescent cells or delete senescence markers)
|
[157]
|
Glycyrrhizin (glycyrrhizic acid); Gabexate mesylate; Monoclonal antibody.
|
HMGB1 inhibitors
|
[158][159]
|
Hu5F9; TTI-621
|
CD47 inhibitors
|
[160]
|
SCFAs; N-butanol extracts of Morinda citrifolia; milk fat globule membranes (MFGM); β-glucan
|
Anti-inflammatory (gut barrier)
|
[161][162]
|
Rhodobacter sphaeroides LPS; myeloid differentiation factor 2 (MD-2)
|
TLR4 antagonist
|
[163]
|
Niclosamide; ANO6 inhibitors
|
TMEM16F inhibitor
|
[164]
|
Membrane Lipid Replacement
|
Glycerophospholipids
|
[165]
|