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Autoimmune diseases are characterized by tissue damage and loss of function due to an immune response that is directed against specific organs. This review is focused on the role of impaired intestinal barrier function on autoimmune pathogenesis. Together with the gut-associated lymphoid tissue and the neuroendocrine network, the intestinal epithelial barrier, with its intercellular tight junctions, controls the equilibrium between tolerance and immunity to non-self antigens. Zonulin is the only physiologic modulator of intercellular tight junctions described so far that is involved in trafficking of macromolecules and, therefore, in tolerance/immune response balance. When the zonulin pathway is deregulated in genetically susceptible individuals, autoimmune disorders can occur. This new paradigm subverts traditional theories underlying the development of these diseases and suggests that these processes can be arrested if the interplay between genes and environmental triggers is prevented by re-establishing the zonulin-dependent intestinal barrier function. Both animal models and recent clinical evidence support this new paradigm and provide the rationale for innovative approaches to prevent and treat autoimmune diseases.
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Leaky Gut and Autoimmune Diseases
Alessio Fasano
#Springer Science+Business Media, LLC 2011
Abstract Autoimmune diseases are characterized by tissue
damage and loss of function due to an immune response that is
directed against specific organs. This review is focused on the
role of impaired intestinal barrier function on autoimmune
pathogenesis. Together with the gut-associated lymphoid
tissue and the neuroendocrine network, the intestinal epithelial
barrier, with its intercellular tight junctions, controls the
equilibrium between tolerance and immunity to non-self
antigens. Zonulin is the only physiologic modulator of
intercellular tight junctions described so far that is involved
in trafficking of macromolecules and, therefore, in tolerance/
immune response balance. When the zonulin pathway is
deregulated in genetically susceptible individuals, autoim-
mune disorders can occur. This new paradigm subverts
traditional theories underlying the development of these
diseases and suggests that these processes can be arrested if
the interplay between genes and environmental triggers is
prevented by re-establishing the zonulin-dependent intestinal
barrier function. Both animal models and recent clinical
evidence support this new paradigm and provide the rationale
for innovative approaches to prevent and treat autoimmune
Keywords Antigens .Autoimmunity .Gut permeability .
Immune response .Tight junctions .Zonulin
The intestinal epithelium is the largest mucosal surface
providing an interface between the external environment
and the mammalian host. Its exquisite anatomical and
functional arrangements and the finely-tuned coordination
of digestive, absorptive, motility, neuroendocrine, and
immunological functions are testimonial of the complexity
of the gastrointestinal (GI) system. Also pivotal is the
regulation of molecular trafficking between the intestinal
lumen and the submucosa via the paracellular space. The
dimensions of the paracellular space are estimated to be
between 10 and 15 Å, suggesting that under physiological
circumstances, solutes with a molecular radius exceeding
15 Å (~3.5 kDa) will be excluded from this uptake route.
Macromolecule trafficking is dictated mainly by intestinal
paracellular permeability, whose regulation depends on the
modulation of intercellular tight junctions (TJ). A fast
growing number of diseases, including autoimmune dis-
eases, are recognized to involve alterations in intestinal
permeability related to changes in TJ competency.
Classical Theories on the Pathogenesis of Autoimmune
Soon after autoimmune diseases were first recognized more
than a century ago, it was believed that their development
was associated with viral and bacterial infections. The
connection between infection and autoimmune disease is
often explained by a mechanism known as molecular
mimicry,whereby microbial antigens are postulated to
resemble self-antigens [1]. The induction of an immune
response to the microbial antigens results in a cross-reaction
with the self-antigens and the induction of autoimmunity.
A. Fasano (*)
Mucosal Biology Research Center,
University of Maryland School of Medicine,
20 Penn Street HSF II Building, Room S345,
Baltimore, MD 21201, USA
Clinic Rev Allerg Immunol
DOI 10.1007/s12016-011-8291-x
According to this theory, once the autoimmune process is
activated, it becomes independent of continuous exposure to
the environmental trigger and is therefore self-perpetuating
and irreversible. Epitope-specific cross-reactivity between
microbial antigens and self-antigens has been shown in some
animal models to initiate autoimmunity [2]. Conversely, in
most human autoimmune diseases, molecular mimicry seems
to be a factor in the progression of a pre-existing subclinical
autoimmune response, rather than in the initiation of
autoimmunity [2]. Another theory suggests that microorgan-
isms expose self-antigens to the immune system by directly
damaging tissues during active infection, and that this leads
to the development of autoimmunity. This mechanism has
been referred to as the bystander effect,and it occurs only
when the new antigen is presented with the orally adminis-
tered triggering antigen. Whether pathogens mimic self-
antigens, release sequestered self-antigens, or both, however,
remains to be elucidated.
New Proposed Hypothesis: the Leaky Gut as Third
Element in Autoimmune Pathogenesis
A common denominator in autoimmune diseases is the
presence of several pre-existing conditions that lead to an
autoimmune process [3]. The first of these conditions is the
genetic susceptibility of the host immune system to
recognize, and potentially misinterpret, an environmental
antigen presented within the gastrointestinal tract. The
second is that the host must be exposed to the antigen.
Finally, the antigen must be presented to the gastrointestinal
mucosal immune system following its paracellular passage
from the intestinal lumen to the gut submucosa; this process
is normally prevented by competent TJ [35]. In many cases,
increased intestinal permeability seems to precede disease
and causes an abnormality in antigen delivery that triggers
the multiorgan process leading to the autoimmune response
[35]. Taking the above information into consideration, we
propose that the pathogenesis of autoimmune diseases can be
described by three key points [6]:
1. Autoimmune diseases involve a miscommunication
between innate and adaptive immunity;
2. Molecular mimicry or bystander effects alone might not
explain entirely the complex events involved in the
pathogenesis of autoimmune diseases. Rather, the contin-
uous stimulation by nonself-antigens (environmental
triggers) seems to be necessary to perpetuate the process.
Contrary to general belief, this concept implies that the
autoimmune response can theoretically be stopped and
perhaps reversed if the interplay between genes predis-
posing individuals to the development of autoimmunity
and environmental triggers is prevented or eliminated;
3. In addition to genetic predisposition and exposure to
triggering nonself-antigens, the loss of the protective
function of mucosal barriers that interact with the
environment (mainly the gastrointestinal and lung
mucosa) is necessary for autoimmunity to develop.
Evidence Supporting This New Theory
Celiac disease (CD) is the best testimonial of the validity to
the accuracy of the new paradigm for the pathogenesis of
autoimmunity proposed above. Celiac disease is an auto-
immune condition triggered by the ingestion of gluten-
containing grains in genetically susceptible individuals (for
more details, see CD section below).
Given the undisputable role of gluten in causing inflam-
mation and immune-mediated tissue damage, CD is a unique
model of autoimmunity in which, in contrast to most other
autoimmune diseases, a close genetic association with HLA
genes, a highly specific humoral autoimmune response
against tissue transglutaminase auto-antigen, and, most
importantly, the triggering environmental factor (gliadin), are
all known. It is the interplay between genes (both HLA and
non-HLA associated) and environment (i.e., gluten) that leads
to the intestinal damage typical of the disease [7]. Under
physiological circumstances, this interplay is prevented by
competent intercellular TJ. Early in CD, TJs are opened [8
12]. Combined, this information provides the rationale for
the treatment of the disease based on complete avoidance of
gluten-containing grains from the patientsdiet. Following
gluten withdrawal, the symptoms resolve, the biomarkers of
the autoimmune process return within normal limits, and the
intestinal autoimmune insult heals. These outcomes support
the notion that the autoimmune process can be reverted
provided that the interplay between genes and environmental
trigger(s) can be prevented.
Besides celiac disease, several other autoimmune dis-
eases, including type 1 diabetes [13,14], multiple sclerosis
[15,16], and rheumatoid arthritis [17], are characterized by
increased intestinal permeability secondary to non-
competent TJs that allow the passage of antigens from the
intestinal flora, challenging the immune system to produce
an immune response that can target any organ or tissue in
genetically predisposed individuals [1821].
Intestinal Barrier Function and Its Regulation
A century ago, TJs were conceptualized as a secreted
extracellular cement forming an absolute and unregulated
barrier within the paracellular space [22]. Biological studies
of the past several decades have shown that TJs are
dynamic structures subjected to structural changes that
Clinic Rev Allerg Immunol
dictate their functional status under a variety of developmental
scenarios. To meet the many diverse physiological challenges
to which the epithelial and endothelial barriers are subjected,
TJs must be capable of rapid and coordinated responses. This
requires the presence of a complex regulatory system that
orchestrates the state of assembly of the TJ multiprotein
network (Fig. 1). While our knowledge on TJ ultrastructure
and intracellular signaling events have significantly pro-
gressed during the past decade, relatively little is known
about their pathophysiological regulation secondary to
extracellular stimuli. Therefore, the intimate pathogenic
mechanisms of diseases in which TJs are affected have
remained unexplored owing to limited understanding of the
extracellular signaling involved in TJ regulation.
The Zonulin Pathway
The discovery of zonula occludens toxin (Zot), an
enterotoxin elaborated by Vibrio cholerae that reversibly
opens TJ [23], increased our understanding of the intricate
mechanisms that regulate the intestinal epithelial para-
cellular pathway and led to the discovery of its eukaryotic
counterpart zonulin [24,25]. The physiological role(s) of
the zonulin system remains to be established. This pathway
appears to be involved in several functions, including TJ
regulation responsible for the movement of fluid, macro-
molecules, and leukocytes between the bloodstream and the
intestinal lumen and vice versa. Another possible physio-
logical role of intestinal zonulin is the protection against
microorganism colonization of the proximal intestine
(innate immunity) [26].
Since zonulin is overexpressed in tissues and sera of
subjects affected by autoimmune diseases, we elected to use
sera from zonulin-positive and zonulin negative type 1
diabetes (T1D) and CD subjects to characterize further the
molecular nature of zonulin. Through proteomic analysis of
human sera, we have recently identified zonulin as pre-
haptoglobin (HP)2 [11], a molecule that, to date, has only
been regarded as the inactive precursor for HP2, one of the
two genetic variants (together with HP1) of human HPs.
Mature human HPs are heterodimeric plasma glycoproteins
Fig. 1 Composition of intercellular tight junctions. The structural
components of intercellular tight junctions can be classified in integral
membrane proteins (occludin, claudins, and JAM), junctional complex
proteins (ZO-1, ZO-2, p130 or ZO-3, 7H6, symplekin, cingulin), and
the cell cytoskeleton structures (microtubules, intermediate filaments,
and microfilaments)
Clinic Rev Allerg Immunol
composed of α- and β-polypeptide chains. While the β
chain (36 kDa) is constant, the αchain exists in two forms,
i.e., α1 (~9 kDa) and α2 (~18 kDa). The presence of one or
both of the αchains results in the three human HP
phenotypes, i.e., HP1-1 homozygote, HP2-1 heterozygote,
and HP2-2 homozygote. The zonulin pathway modulating
TJ permeability is described in Fig. 2.
Zonulin-Dependent Impaired Intestinal Barrier
Function and Autoimmune Diseases
Celiac Disease
CD is an immune-mediated chronic enteropathy with a wide
range of presenting manifestations of variable severity. It is
triggered by the ingestion of gliadin fraction of wheat gluten
and similar alcohol-soluble proteins (prolamines) of barley
and rye in genetically susceptible subjects with subsequent
immune reaction leading to small bowel inflammation and
normalization of the villous architecture in response to a
gluten-free diet [27]. CD not only affects the gut, but it is a
systemic disease that may cause injury to any organ. It is a
complex genetic disorder, and HLA status appears to be the
strongest genetic determinant of risk for celiac autoimmunity.
Gluten is a complex molecule made of gliadin and
glutenins, both toxic for CD patients. The repertoire of gluten
peptides involved in the disease pathogenesis is greater than
appreciated previously, with at least 50 toxic epitopes in
gluten peptides exerting cytotoxic, immunomodulatory, and
gut permeating activities. These activities have been partially
mapped to specific domains in α-gliadin: the cytotoxic
peptide 3143, the immunomodulatory peptide 5789 (33-
mer), the CXCR3 binding, zonulin releasing (gut permeating)
peptides 111130 and 151170, and the IL8-releasing peptide
261277 [21]. The effect of the permeating gliadin peptides
in vivo was confirmed by the analysis of intestinal tissues
from patients with active CD and non-CD controls probed
for zonulin expression [8]. Quantitative immunoblotting of
intestinal tissue lysates from active CD patients confirmed
the increase in zonulin protein compared to control tissues
[8]. Zonulin upregulation during the acute phase of CD was
confirmed by measuring zonulin concentration in sera of 189
CD patients using a sandwich ELISA. Compared to healthy
controls, CD subjects had higher zonulin serum concen-
trations (p<0.000001) during the acute phase of the disease
that decreased following a gluten-free diet [25].
Current data suggest that altered processing by intra-
luminal enzymes, changes in intestinal permeability, and
activation of innate immunity mechanisms seem to precede
the activation of the adaptive immune response [28]. Based
on these data and on the gliadin epitope mapping described
above, it is conceivable to hypothesize the following
sequence of events: after oral ingestion, gliadin interacts
Fig. 2 Proposed zonulin intracellular signaling leading to the opening of
intestinal TJ. Zonulin interacts with a specific surface receptor [1]whose
distribution within the intestine varies. The protein then activates
phospholipase C [2] that hydrolyzes phosphatidyl inositol [3]torelease
inositol 1,4,5-tris phosphate (PPI-3) and diacylglycerol (DAG) [4].
PKCαis then activated [5], either directly (via DAG) [4] or through the
release of intracellular Ca
(via PPI-3) (4a). Membrane-associated,
activated PKCα[6] catalyzes the phosphorylation of target protein(s),
with subsequent polymerization of soluble G-actin in F-actin [7]. This
polymerization causes the rearrangement of the filaments of actin and
the subsequent displacement of proteins (including ZO-1) from the
junctional complex [8]. As a result, intestinal TJ becomes looser (see
freeze fracture electron microscopy). Once the zonulin signaling is over,
the TJs resume their baseline steady state
Clinic Rev Allerg Immunol
with the small intestinal mucosa causing interleukin (IL)-
8 release from enterocytes (peptide 261277), so leading to
immediate recruitment of neutrophils in the lamina propria.
At the same time, gliadin permeating peptides 111130 and
151170 initiate intestinal permeability through a MyD88-
dependent release of zonulin (as we have recently con-
firmed by identifying CXCR3 as the receptor that releases
zonulin in a MyD88-dependent manner, see ref. [29]) that
enables paracellular translocation of gliadin and its subse-
quent interaction with macrophages (through 33-mer and
other immunomodulatory peptides) within the intestinal
submucosa [30]. This interaction initiates signalling
through a MyD88-dependent, but TLR4 and TLR2-
independent pathway, resulting in the establishment of a
proinflammatory (Th1-type) cytokine milieu [30]that
results in mononuclear cell infiltration into the submucosa.
The persistent presence of inflammatory mediators such as
tumor necrosis factor (TNF)-αand interferon (IFN)-γ
causes further increase in permeability across the endothe-
lial and epithelial layers [31], suggesting that the initial
breach of the intestinal barrier function caused by zonulin
can be perpetuated by the inflammatory process after the
access of gliadin to the submucosa. In genetically predis-
posed individuals this, in turn, may permit the interaction of
T cells with antigen presenting cells, including macro-
phages, leading ultimately to the antigen-specific adaptive
immune response causing the autoimmune insult of the
intestinal mucosa seen in patients with CD [32].
Once gluten is removed from the diet, serum zonulin
levels decrease, the intestine resumes its baseline barrier
function, the autoantibody titers are normalized, the
autoimmune process shuts off and, consequently, the
intestinal damage (that represents the biological outcome
of the autoimmune process) heals completely.
Type 1 Diabetes
Type 1 diabetes (T1D) is an autoimmune condition,
sometimes associated to diseases that are characterized by
marked immunologic features, such as CD and thyroiditis
[33,34]. GI symptoms in diabetes mellitus have been
generally ascribed to altered intestinal motility secondary to
autonomic neuropathy [35].However, other studies suggest
that an increased permeability of intestinal TJ is responsible
for both the onset of the disease and the GI symptoms that
these patients often experience [36]. This hypothesis is
supported by a study performed on a spontaneously
diabetic animal model [37]. The authors of this study
showed an increased permeability of the small intestine of
Bio Breeding Diabetes Prone (BBDP)/Wor diabetic-prone
rats that precedes at least a month the onset of diabetes.
Further, histological evidence of pancreatic islet destruction
was absent at the time of increased permeability but clearly
present at a later time [37]. Therefore, the authors presented
evidence that increased permeability occurred before either
histological or overt manifestation of diabetes in this animal
model. We confirmed these data by reporting in the same
rat model that zonulin-dependent increase in intestinal
permeability precedes the onset of T1D by 23 weeks
[38]. Oral administration of the zonulin inhibitor, AT1001
(now called Larazotide acetate), to BBDP rats blocked
autoantibody formation and zonulin-induced increases in
intestinal permeability, so reducing the incidence of
diabetes [38]. These studies suggest that the zonulin-
dependent loss of intestinal barrier function is one of the
initial steps in the pathogenesis of T1D in the BBDP animal
model of the disease. The involvement of zonulin in T1D
pathogenesis was corroborated by our studies in humans
showing that ~50% of T1D patients has elevated serum
zonulin levels that correlated with increased intestinal
permeability [39]. We also provided preliminary evidence
suggesting that, as in the BBDP rat model of the disease,
zonulin upregulation precedes the onset of diabetes in T1D
patients [39]. Interestingly, a smaller percentage (~25%) of
unaffected family members of probands with T1D have
also been found to have increased serum zonulin levels and
increased gut permeability [39], suggesting that loss of
intestinal barrier function is necessary but not sufficient for
the onset of the autoimmune process.
Several reports have linked gliadin (the environmental
trigger of CD autoimmunity that also causes zonulin release
from the gut, see refs. [8] and [26]) to T1D autoimmunity
both in animal models and in human studies [4042]. More
recently, we reported a direct link between antibodies to
Glo-3a (a wheat-related protein), zonulin upregulation, and
islet autoimmunity in children at increased risk for T1D
[43]. Glo-3A antibody levels were inversely associated with
breast-feeding duration and directly associated with current
intake of foods containing gluten in islet autoimmunity
cases but not in controls [43]. Further, zonulin was directly
associated with Glo-3A antibody levels in cases but not in
controls, suggesting that the presence of Glo-3A antibodies
and zonulin upregulation in islet autoimmunity cases are
related to an underlying difference in mucosal immune
response as compared to controls.
Asthma is a complex clinical syndrome characterized by
airflow obstruction, airway hyperresponsiveness, and in-
flammation. The mechanisms by which airway inflamma-
tion and alterations in airway function are maintained are
incompletely understood. Because wheezing can also be
triggered by food challenges in some asthmatic children,
increased intestinal permeability of asthmatics [44] may
play a role in susceptibility to environmental allergens. We
Clinic Rev Allerg Immunol
have generated preliminary data suggesting that serum
zonulin levels are high in a subset of subjects affected by
asthma and that approximately 40% of asthmatic patients
have an increased intestinal permeability [21]. This prelim-
inary observation suggests that, besides inhalation, an
alternative route for the presentation of specific antigens
or irritants may occur through the GI mucosal immune
system following their paracellular passage (normally
prevented by the intercellular TJ).
Multiple Sclerosis
Besides an increase in bloodbrain barrier permeability,
multiple sclerosis (MS) patients may also experience an
increased permeability of intestinal TJ. Yacyshyn and
coworkers have demonstrated that 25% of MS patients
studied had an increased intestinal permeability [45]. The
fact that patients with MS [45] and Crohns disease [46]
both present an increased number of peripheral B cells
exhibiting CD45RO, a marker of antigen exposure, further
support the concept of preexisting, genetically determined
small intestinal permeability abnormalities with subsequent
altered antigen exposure as a pathogenic factor common to
these diseases.
To challenge this hypothesis, we measured serum levels
of zonulin in MS patients with different subtypes
relapsingremitting [RRMS] vs. secondaryprogressive
[SPMS]and activities to ascertain whether expression of
zonulin into peripheral circulation can differentiate these
two groups. Approximately 29% of patients with either
RRMS or SPMS had elevated serum zonulin levels (a
percentage similar to increased intestinal permeability in
MS patients reported by Yacyshyn et al., see ref. [45]), with
overall average serum levels ~2.0-fold higher than in
controls. Interestingly, patients with RRMS in remission
showed serum zonulin levels comparable to controls [21].
Inflammatory Bowel Diseases
Crohns disease and ulcerative colitis are inflammatory
diseases involving the GI tract in which abnormal para-
cellular permeability defects precede the development of
both syndromes and, therefore, appear to play an important
role in disease pathogenesis [46,47]. The pathogenesis of
inflammatory bowel disease (IBD) remains unknown,
although in recent years there is convincing evidence to
implicate genetic, immunological, and environmental fac-
tors in initiating the autoimmune process. Several lines of
evidence, however, suggest that an increased intestinal
permeability plays a central role in the pathogenesis of
IBD. In clinically asymptomatic Crohns disease patients,
increased intestinal epithelial permeability precedes clinical
relapse by as much as 1 year, suggesting that a permeability
defect is an early event in disease exacerbation [48]. The
hypothesis that abnormal intestinal barrier function is a
genetic trait involved in the pathogenesis of IBD is further
supported by the observation that clinically asymptomatic
first-degree relatives of Crohns disease patients may have
increased intestinal permeability [48]. We have recently
generated evidence suggesting that zonulin upregulation is
detectable in the acute phase of IBD and that its serum
levels decrease (but still are higher than normal) once the
inflammatory process subsides following specific treatment
[21]. While a primary defect of the intestinal barrier
function (possibly secondary to activation of the zonulin
pathway) may be involved in the early steps of the
pathogenesis of IBD, the production of cytokines, including
IFN-γand TNF αsecondary to the inflammatory process
serve to perpetuate the increased intestinal permeability by
reorganizing TJ proteins ZO-1, junctional adhesion mole-
cule 1, occludin, claudin-1, and claudin-4 [49]. In this
manner, a vicious cycle is created in which barrier
dysfunction allows further leakage of luminal contents,
thereby triggering an immune response that in turn
promotes further leakiness.
Ankylosing Spondylitis
Ankylosing spondylitis (AS) is a common and highly
familial rheumatic disorder that typically affects young and
middle-aged adults and is characterized by stiffness and
pain in the back. The link between increased intestinal
permeability and AS has been clearly established [50].
Using different markers of TJ permeability, two indepen-
dent studies [51,52] found an increased intestinal perme-
ability in both AS patients and their relatives. These
changes precede the clinical manifestations of the disease,
suggesting a pathogenic role of TJ dysfunction in AS.
Using a proteomic approach, Liu et al. have identified
HP as an AS biomarker [53]. The authors investigated the
serum protein profiles of AS patients and healthy controls
from a large Chinese AS family using two-dimensional
electrophoresis analysis. A group of four highly expressed
protein spots was observed in all ankylosing spondylitis
patientsprofiles and subsequently identified as isoforms of
HP by ESI-Q-TOF MS/MS [53].
Proof of Pathogenic Role of Zonulin-Mediated Intestinal
Barrier Defect in Autoimmunity: the Celiac Disease
and Type 1 Diabetes Paradigms
CD and type 1 diabetes autoimmune models suggest that,
when the finely tuned trafficking of macromolecules is
deregulated due to a leaky gut, autoimmune disorders can
occur in genetically susceptible individuals [21]. This
theory implies that removing any of the three key elements
Clinic Rev Allerg Immunol
(genes, environmental trigger(s), or impaired barrier func-
tion) should block the autoimmune process. To challenge
this hypothesis, zonulin inhibitor Larazotide acetate was
used with encouraging results in the BBDP rat model of
autoimmunity [38]. Besides preventing the loss of intestinal
barrier function, the appearance of autoantibodies, and the
onset of disease, pretreatment with Larazotide acetate
protected against the insult of pancreatic islets and,
therefore, of the insulitis responsible for the onset of type
1 diabetes [21].
This proof-of-concept in an animal model of autoimmunity
provided the rationale to design human clinical trials in which
Larazotide acetate was initially tested in an inpatient, double-
blind, randomized placebo controlled trial to determine its
safety, tolerability, and preliminary efficacy [54]. No increase
in adverse events was recorded among patients exposed to
Larazotide as compared to placebo. Following acute gluten
exposure, a 70% increase in intestinal permeability was
detected in the placebo group, while no changes were seen in
the Larazotide acetate group [54]. Gastrointestinal symptoms
were significantly more frequent among patients of the
placebo group as compared to the Larazotide acetate group
[54]. Larazotide acetate has now been tested in approximately
500 subjects with excellent safety profile and promising
efficacy as concern protection against symptoms caused by
gluten exposure in CD patients [55].
The classical paradigm of autoimmune pathogenesis involv-
ing specific gene makeup and exposure to environmental
triggers has been recently challenged by the addition of a third
element, the loss of intestinal barrier function. Genetic
predisposition, miscommunication between innate and adap-
tive immunity, exposure to environmental triggers, and loss of
the intestinal barrier function secondary to dysfunction of
intercellular TJ seem to be all key ingredients involved in the
pathogenesis of autoimmune diseases. Both in CD and T1D
gliadin may play a role in causing loss of intestinal barrier
function and/or inducing the autoimmune response in genet-
ically predisposed individuals. This new theory implies that
once the autoimmune process is activated, it is not autoperpe-
tuating, rather can be modulated or even reversed by
preventing the continuous interplay between genes and
environment. Since TJ dysfunction allows this interaction,
new therapeutic strategies aimed at reestablishing the intesti-
nal barrier function offer innovative, unexplored approaches
for the treatment of these devastating diseases.
Funding Work presented in this review was supported in parts by
grants from the National Institutes of Health Grants DK-48373 and
DK-078699 to AF.
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Clinic Rev Allerg Immunol
... Consistently, zonulin was found to be overexpressed in patients suffering from autoimmune diseases. According to Alessio Fasano in 2012 [118], the epithelial paracellular space is estimated to measure 10-15 Å, which means that, under normal conditions, any molecule with a molecular mass exceeding 3.5 kDa cannot travel across the epithelial tight junctions. Zonulin has been identified as a haptoglobin precursor [119], and its mature form is composed by both αand β-polypeptide chains; with the α-chains displaying differences in their molecular mass (9 or 18 kDa, for α1 and α2, respectively), while the β-chains have a mass of 36 kDa. ...
... Other autoimmune diseases so far described to involve zonulin, as well as an increase in intestinal permeability, are Crohn's disease and ulcerative colitis [130], which display observable changes in the expression of claudins [131]. Additional illnesses include asthma [132,133], and autoimmune disorders such as multiple sclerosis [118] and ankylosing spondylitis [134]. A recent review by Gierynska and co-workers in 2022 summarizes the importance of maintaining the integrity of the gastrointestinal epithelium and the mutual relationship with the gut microbiota [135]. ...
Full-text available
The microbiota present in the gastrointestinal tract is involved in the development or prevention of food allergies and autoimmune disorders; these bacteria can enter the gallbladder and, depending on the species involved, can either be benign or cause significant diseases. Occlusion of the gallbladder, usually due to the presence of calculi blocking the bile duct, facilitates microbial infection and inflammation, which can be serious enough to require life-saving surgery. In addition, the biliary salts are secreted into the intestine and can affect the gut microbiota. The interaction between the gut microbiota, pathogenic organisms, and the human immune system can create intestinal dysbiosis, generating a variety of syndromes including the development of food allergies and autoimmune disorders. The intestinal microbiota can aggravate certain food allergies, which become severe when the integrity of the intestinal barrier is affected, allowing bacteria, or their metabolites, to cross the intestinal barrier and invade the bloodstream, affecting distal body organs. This article deals with health conditions and severe diseases that are either influenced by the gut flora or caused by gallbladder obstruction and inflammation, as well as putative treatments for those illnesse
... However, it has been observed that high-intensity exercise, which is common in individuals practising sports, can have detrimental effects on health. It can cause an increase in intestinal permeability and a decrease in the thickness of the intestinal mucus, potentially allowing pathogens/toxins to enter the bloodstream; moreover, it has been associated with immunosuppression by decreasing the function of immune cells, which improves susceptibility to infections [13][14][15][16][17][18][19], such as upper respiratory tract infection (URTI) [19][20][21]. This can be related to acute immune failure and chronic suppression of immune factors which follow frequent and strenuous exercise [22,23]. ...
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This review aims to evaluate the effects of probiotic supplementation on performance and performance-related conditions in athletes by evaluating randomized controlled studies from the MEDLINE (Pubmed), Web of Science, Scopus, and SPORTDiscus (EBSCO) databases. From a total of 2304 relevant articles, 13 studies fulfilled the inclusion criteria. Seven studies concern endurance athletes, one to rugby players, three refer to non-specified athletes, one to badminton players, and one involves baseball players. The evidence suggests that the integration of athletes’ diets with some bacterial strains and also the consumption of multi-strain compounds may lead to an improvement in performance and can positively affect performance-related aspects such as fatigue, muscle pain, body composition, and cardiorespiratory fitness. However, the type of supplementation and sport is very variable among the studies examined. Therefore, to obtain more solid evidence, further controlled and comparable studies are needed to expand the research regarding the possible repercussions of probiotics use on athletes’ performance.
... Several studies suggest that gutcommensal microbiota and host-microbial interactions might be important pathogenic factors in this process [6,10,11,[33][34][35][36][37][38][39]. Indeed, epithelial interactions with the microbiota and microbial-derived products, as well as the increased entrance within the intestinal wall, are altered in states of dysbiosis, which are likely contributing to the process of immune activation [3,24,40,41]. Nevertheless, the microbial contribution and the exact mechanisms involved are still unclear. ...
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Toll-like receptors (TLRs)-mediated host–bacterial interactions participate in the microbial regulation of gastrointestinal functions, including the epithelial barrier function (EBF). We evaluated the effects of TLR7 stimulation on the colonic EBF in rats. TLR7 was stimulated with the selective agonist imiquimod (100/300 µg/rat, intracolonic), with or without the intracolonic administration of dimethyl sulfoxide (DMSO). Colonic EBF was assessed in vitro (electrophysiology and permeability to macromolecules, Ussing chamber) and in vivo (passage of macromolecules to blood and urine). Changes in the expression (RT-qPCR) and distribution (immunohistochemistry) of tight junction-related proteins were determined. Expression of proglucagon, precursor of the barrier-enhancer factor glucagon-like peptide 2 (GLP-2) was also assessed (RT-qPCR). Intracolonic imiquimod enhanced the EBF in vitro, reducing the epithelial conductance and the passage of macromolecules, thus indicating a pro-barrier effect of TLR7. However, the combination of TLR7 stimulation and DMSO had a detrimental effect on the EBF, which manifested as an increased passage of macromolecules. DMSO alone had no effect. The modulation of the EBF (imiquimod alone or with DMSO) was not associated with changes in gene expression or the epithelial distribution of the main tight junction-related proteins (occludin, tricellulin, claudin-2, claudin-3, junctional adhesion molecule 1 and Zonula occludens-1). No changes in the proglucagon expression were observed. These results show that TLR7 stimulation leads to the modulation of the colonic EBF, having beneficial or detrimental effects depending upon the state of the epithelium. The underlying mechanisms remain elusive, but seem independent of the modulation of the main tight junction-related proteins or the barrier-enhancer factor GLP-2.
... The GM ferment undigested complex dietary fibers and produce essential bioactive metabolic products, known as gut-derived metabolites, such as short-chain fatty acids (SCFAs), long-chain fatty acids, branched-chain amino acids, trimethylamine-N-oxide (TMAO), lipopolysaccharide (LPS), bile acids, and catecholamines [37][38][39][40]. Gut microbes also regulate the secretion of host-derived vitamins (B12), nitric oxide (NO) and indoles, and synthesize and/or induce the production of neurotransmitters including tryptophan, 5-HT, glutamate, γ-aminobutyric acid (GABA), acetylcholine, dopamine, histamine, and noradrenaline [18,[41][42][43][44][45]. Furthermore, microbes in the GI tract regulate the release of various gut hormones that improve glucose homeostasis and behavioral changes, such as peptide YY (PYY); regulate the physiology of immune cells, anxiety and mood, such as neuropeptide Y; and are involved in the regulation of hunger and satiety, such as leptin, ghrelin, cholecystokinin, and glucagon-like peptide-1 [46,47]. ...
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Mounting evidence shows that the complex gut microbial ecosystem in the human gastrointestinal (GI) tract regulates the physiology of the central nervous system (CNS) via microbiota and the gut–brain (MGB) axis. The GI microbial ecosystem communicates with the brain through the neuroendocrine, immune, and autonomic nervous systems. Recent studies have bolstered the involvement of dysfunctional MGB axis signaling in the pathophysiology of several neurodegenerative, neurodevelopmental, and neuropsychiatric disorders (NPDs). Several investigations on the dynamic microbial system and genetic–environmental interactions with the gut microbiota (GM) have shown that changes in the composition, diversity and/or functions of gut microbes (termed “gut dysbiosis” (GD)) affect neuropsychiatric health by inducing alterations in the signaling pathways of the MGB axis. Interestingly, both preclinical and clinical evidence shows a positive correlation between GD and the pathogenesis and progression of NPDs. Long-term GD leads to overstimulation of hypothalamic–pituitary–adrenal (HPA) axis and the neuroimmune system, along with altered neurotransmitter levels, resulting in dysfunctional signal transduction, inflammation, increased oxidative stress (OS), mitochondrial dysfunction, and neuronal death. Further studies on the MGB axis have highlighted the significance of GM in the development of brain regions specific to stress-related behaviors, including depression and anxiety, and the immune system in the early life. GD-mediated deregulation of the MGB axis imbalances host homeostasis significantly by disrupting the integrity of the intestinal and blood–brain barrier (BBB), mucus secretion, and gut immune and brain immune functions. This review collates evidence on the potential interaction between GD and NPDs from preclinical and clinical data. Additionally, we summarize the use of non-therapeutic modulators such as pro-, pre-, syn- and post-biotics, and specific diets or fecal microbiota transplantation (FMT), which are promising targets for the management of NPDs.
... Epitel bariyerinin bütünlüğünün bozulması "sızdıran bağırsak" olarak adlandırılır. Sızdıran bağırsak, mikrobiyotal değişime ve bağırsakta inflamasyona neden olan çok çeşitli bağırsak ve sistemik bozukluklarla ilişkilendirilmiştir (38). Bağırsak geçirgenliği arttığında bakteriler, lipopolisakkarit (LPS) gibi bakteriyel orjinli nörotoksik peptidler, immün reaksiyona ve inflamasyona yol açan sitokinler kolayca sistemik dolaşıma geçebilir (39). ...
... There has been a growing interest over the past decade in the role of leaky gut's association with autoimmune diseases. There have been some suggestions that the leakage of pathogens into the body system results in autoimmunity making the leaky gut a third source of pathogenesis besides environmental triggers and genetic predisposition [42]. A dysbiosis which is a perturbation of the structural dynamics of the microbial community in the intestinal tract causes leaky gut condition and it is closely entangled with autoimmune diseases. ...
Full-text available
Autoimmune diseases have emerged as a pandemic in our modern societies, especially after World War II. There are currently more than 80 autoimmune diseases that compromise the lives of millions of patients around the world. There is a variety of factors that are involved in the pathogenesis of autoimmune diseases that vary from environmental factors to genetic susceptibility. The GI tract is one of the most susceptible subsystems in human bodies for autoimmune organ-specific diseases. There are five autoimmune GI tract diseases that are most common. This review consists of two chapters. In part I, we shed the light on introducing the concept of autoimmunity, the description of the disease's pathogenesis and the diagnosis, the link between the gut and brain through what is known as the gut-brain axis, and the relationship of this axis in GI autoimmune diseases. In part II, we will shed light on the role of antibodies as markers for the prediction of the disease, artificial intelligence in GI autoimmune diseases, the nutritional role and implications in the five GI autoimmune diseases, and finally the treatment of those diseases.
... Intriguingly, one of the proposed mechanisms leading to increased disease severity is a disruption of gut barrier function. Increased gut leakiness has been associated with multiple rheumatic diseases (57), including RA (58). Moreover, a humanized mouse model carrying the human RA susceptibility allele ("shared epitope") HLA-DRB1*401 showed increased gut permeability prior to the induction of arthritis (59). ...
... In particular, high-intensity PA and strenuous exercise seem to increase intestinal permeability and diminish gut mucus thickness, potentially enabling pathogens/toxins to enter the bloodstream, and have been associated with immunosuppression by decreasing immune cell function, which enhances susceptibility to infections [3][4][5][6][7][8][9]. ...
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The aim of this review was to appraise the literature on the effects of probiotics supplementation on gastrointestinal (GI) and upper respiratory tract infection (URTI) risk and prognosis in athletes. The search was conducted using the following electronic databases: MEDLINE (PubMed); Web of Science; Scopus; and SPORTDiscus (EBSCO). According to the PRISMA guidelines, randomized controlled studies performed on healthy athletes with a note dose of probiotics supplementation were considered. From the 2304 articles found, after eliminating reviews and studies on animals and unhealthy subjects and after screening of titles and abstracts, 403 studies were considered eligible. From these, in accordance with the inclusion and exclusion criteria, 16 studies were selected, ten of which concerned endurance athletes. The majority of the studies reported beneficial effects of probiotics in reducing the risk of developing the examined infections or the severity of related symptoms. However, due to the differences in formulations used and populations analyzed in the available studies, further research is needed in this field to achieve stronger and more specific evidence.
Type 2 Diabetes mellitus is a multifactorial metabolic disorder. The growing body of evidence linking the gut microbiome to host metabolism has been accompanied by change in the study of metabolic illnesses including type 2 diabetes. Disturbance in the balance of gut microbiome “gut dysbiosis” has led to the emergence of the concept of “leaky gut” and metabolic endotoxemia. Previous studies observed higher lipopolysaccharide or lipopolysaccharide binding protein concentrations in diabetics than in healthy controls. Translocation of bacteria and their products through the disrupted gut barrier increased the circulation of markers that could be linked to increased intestinal permeability. The end result of translocated products is the stimulation of systemic inflammatory response affecting many organs and increasing insulin resistance thus aggravating diabetes. It is anticipated that by gaining a better understanding of the mechanism of leaky gut and bacterial translocation in diabetes, scientists will be able to develop novel diagnostic and therapeutic approaches.
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The primary functions of the gastrointestinal tract have traditionally been perceived to be limited to the digestion and absorption of nutrients and to electrolytes and water homeostasis. A more attentive analysis of the anatomic and functional arrangement of the gastrointestinal tract, however, suggests that another extremely important function of this organ is its ability to regulate the trafficking of macromolecules between the environment and the host through a barrier mechanism. Together with the gut-associated lymphoid tissue and the neuroendocrine network, the intestinal epithelial barrier, with its intercellular tight junctions, controls the equilibrium between tolerance and immunity to non-self antigens. Zonulin is the only physiological modulator of intercellular tight junctions described so far that is involved in trafficking of macromolecules and, therefore, in tolerance/immune response balance. When the finely tuned zonulin pathway is deregulated in genetically susceptible individuals, both intestinal and extraintestinal autoimmune, inflammatory, and neoplastic disorders can occur. This new paradigm subverts traditional theories underlying the development of these diseases and suggests that these processes can be arrested if the interplay between genes and environmental triggers is prevented by reestablishing the zonulin-dependent intestinal barrier function. This review is timely given the increased interest in the role of a "leaky gut" in the pathogenesis of several pathological conditions targeting both the intestine and extraintestinal organs.
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Impaired intestinal barrier function is observed in type 1 diabetes patients and animal models of the disease. Exposure to diabetogenic antigens from the intestinal milieu due to a compromised intestinal barrier is considered essential for induction of the autoimmune process leading to type 1 diabetes. Since a hydrolysed casein (HC) diet prevents autoimmune diabetes onset in diabetes-prone (DP)-BioBreeding (BB) rats, we studied the role of the HC diet on intestinal barrier function and, therefore, prevention of autoimmune diabetes onset in this animal model. DP-BB rats were fed the HC diet from weaning onwards and monitored for autoimmune diabetes development. Intestinal permeability was assessed in vivo by lactulose-mannitol test and ex vivo by measuring transepithelial electrical resistance (TEER). Levels of serum zonulin, a physiological tight junction modulator, were measured by ELISA. Ileal mRNA expression of Myo9b, Cldn1, Cldn2 and Ocln (which encode the tight junction-related proteins myosin IXb, claudin-1, claudin-2 and occludin) and Il-10, Tgf-ß (also known as Il10 and Tgfb, respectively, which encode regulatory cytokines) was analysed by quantitative PCR. The HC diet reduced autoimmune diabetes by 50% in DP-BB rats. In DP-BB rats, prediabetic gut permeability negatively correlated with the moment of autoimmune diabetes onset. The improved intestinal barrier function that was induced by HC diet in DP-BB rats was visualised by decreasing lactulose:mannitol ratio, decreasing serum zonulin levels and increasing ileal TEER. The HC diet modified ileal mRNA expression of Myo9b, and Cldn1 and Cldn2, but left Ocln expression unaltered. Improved intestinal barrier function might be an important intermediate in the prevention of autoimmune diabetes by the HC diet in DP-BB rats. Effects on tight junctions, ileal cytokines and zonulin production might be important mechanisms for this effect.
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Duodenal biopsy is an important tool to diagnose coeliac disease (CD); however, the most reliable location of biopsy site is still questionable. Claudins (CLDNs), members of a large family of adherent junction proteins, show characteristic expression pattern in inflammatory disorders; nevertheless, CLDN expression in CD is unknown. This is a comparative study to examine the CLDN 2, 3 and 4 expressions in proximal and distal part of duodenum in children with CD and in controls. Thirty-three children with newly diagnosed CD were enrolled. Fourteen healthy children served as controls. Biopsies from proximal and distal part of duodenum were taken for routine histological analysis. Immunohistochemistry were used to detect CD3+ intraepithelial lymphocytes and CLDN 2, 3 and 4 protein expressions. Macroscopic picture, routine histology and Marsh grade depicted no differences between biopsies taken from proximal or distal part of duodenum. However, CLDN 2 expression was significantly increased in severe form of coeliac disease in bulb and in distal duodenum, and in distal part of non-severe coeliac patients, in comparison to controls. Similar association was found concerning CLDN 3 expression. Expression of CLDN 4 was similar in all groups studied. Both proximal and distal mucosal duodenal biopsies are suitable for diagnosing villous atrophy in patients with CD. Increased expressions of CLDN 2 and 3 suggest structural changes of tight junction in coeliac disease which may be, at least in part, responsible for increased permeability and proliferation observed in coeliac disease.
The immune system specifically recognizes and eliminates foreign antigens and thus protects the integrity of the host. During maturation of the immune system, tolerance mechanisms develop that prevent or inhibit potentially harmful reactivities to self-antigens. Autoreactive B and T cells that are generated during immune responses are eliminated by apoptosis in the thymus, lymph nodes, or peripheral circulation or are actively suppressed by regulatory T cells. However, autoreactive cells may survive because of failure of apoptosis or molecular mimicry, that is, presentation and recognition of cryptic epitopes of self-antigens or aberrant lymphokine production. Development of immune responses and tolerance is determined by an interplay of genetic and environmental factors. Autoimmunity is a result of the breakdown of one or more of the mechanisms of immune tolerance.
Background Increased intestinal permeability has been reported in one study of adult asthmatics. Aim To determine whether children with asthma have altered intestinal permeability. Methods Thirty two asthmatic children, and 32 sex and age matched controls were recruited. The dual sugar (lactulose and mannitol) test was used to evaluate intestinal permeability, and the percentage of ingested lactulose (L) and mannitol (M) in the urine, and the L:M ratio were determined. All patients were skin prick tested for common aeroallergens, and specific IgE to some food items was determined. Results The median value of L in asthmatic children (2.29, IQR 0.91–4.07) was significantly higher than that in controls (0.69, IQR 0.45–1.08), and that of M was almost similar. The ratio L:M was significantly higher in asthmatic children (0.20, IQR 0.11–0.40) than in controls (0.06, IQR 0.04–0.09). Intestinal permeability did not correlate with eczema, inhaled steroids, positive skin prick test to aeroallergens, or severity of asthma. Conclusions Intestinal permeability is increased in children with asthma, suggesting that the whole mucosal system may be affected.
A genome-wide association study reports more than a dozen new susceptibility loci for celiac disease. Analysis of eQTL data from these and previously established risk loci sheds light on the genetic pathways underlying this common autoimmune disease.
A single layer of epithelial cells lines the small and large intestines and functions as a barrier between commensal bacteria and the rest of the body. Ligation of Toll-like receptors (TLRs) on intestinal epithelial cells by bacterial products promotes epithelial cell proliferation, secretion of IgA into the gut lumen and expression of antimicrobial peptides. As described in this Review, this establishes a microorganism-induced programme of epithelial cell homeostasis and repair in the intestine. Dysregulation of this process can result in chronic inflammatory and over-exuberant repair responses, and it is associated with the development of colon cancer. Thus, dysregulated TLR signalling by intestinal epithelial cells may explain how colonic bacteria and inflammation promote colorectal cancer.