Early life stress predisposes to food allergy
via modulation of the brain-gut-axis
Natalja Bouwhuis
Master Thesis
Examiner: Dr. Aletta D. Kraneveld
Drug Innovation
Utrecht University
Second reviewer: Prof. Dr. G. Folkerts
Abstract
The neural, immune and endocrine pathway communication between the brain and the gut, is
referred to as the brain-gut-axis. The brain-gut-axis is a complex reflex network between the central
nervous system and the enteric nervous system in the gut, via which the brain can communicate with
the gastrointestinal tract. Proper brain-gut-axis functioning is necessary to maintain homeostasis.
Early life stress is able to disturb this homeostasis and cause brain-gut-axis dysfunctioning. The early
environment in life is extremely important, as both the nervous system and the immune system are
still in development. Therefore, exposing the developing nervous and immune systems to stress in
early life, can cause long lasting effects which can predispose to disease development. Early life
stress is able to affect different aspects of the brain-gut-axis. It leads to changes in different
neurotransmitters and neuropeptides (such as serotonin, acetylcholine, CRF and NGF), and causes
hyperactivity of the HPA axis with increases in ACTH and corticosterone and a decreased acute stress
response. Furthermore, early life stress is able to affect the mucosal immune system in the gut and
modulate the epithelial barrier function in different ways. Changes are also found in microbiota
composition after early life stress. These changes in the gut microbiota can also lead to changes in
the epithelial barrier function. Intestinal barrier dysfunction including altered gut permeability,
increases the susceptibility to food allergen sensitization in the gastrointestinal tract, possibly
inducing food allergy. In conclusion, early life stress leads to dysfunction of many aspects of the
brain-gut-axis. Especially modulation of the gut microbiota and the epithelial barrier permeability,
can predispose to the development of food allergy.
2
Introduction
In recent years, newly discovered relationships between the central nervous system and the immune
system have given rise to the field of psychoneuroimmunologic research1,2. The largest organ of the
immune system is the intestinal tract. Bidirectional communication between the intestinal tract and
the central nervous system involves neural components such as the enteric nervous system (ENS)
and the vagus nerve, and humoral components like the hypothalamic-pituitary-adrenal (HPA) axis
and the immune system. Also the composition of the intestinal microbiota is known to play a central
role in modulating this bidirectional communication3. This communication network between the
brain and the gut is also referred to as the brain-gut-axis4. The central nervous system and the ENS,
with their complex communication network, are still in development during early life5. Immune
development occurs in utero and predominantly during early childhood. During these critical time
windows, physiological systems are able to adapt to environmental stimuli (for instance via
epigenetic mechanisms), a concept known as developmental plasticity5. Because of these complex
interactions between the nervous system and the immune system on common biochemical and
cellular basis, chronic stress is able to cause changes in the immune system by dysregulating the
brain-gut-axis2,3. Stress related disorders often go paired with functional alterations in the intestinal
tract6. Especially exposure to chronic stress during early childhood, will predispose to a variety of
psychiatric and functional disorders, including irritable bowel syndrome, asthma and other allergies,
as both the nervous and the immune system are still in development5,6. Responsiveness of key
regulatory systems like the autonomic nervous system (ANS) and the HPA axis can be altered by early
life stress, affecting the developing immune system and increasing risk on developing diseases like
food allergy7. Currently, research on the brain-gut-axis is increasing, however, not much research is
performed on the brain-gut-axis and the relationship between stress and the onset of intestinal
disorders such as food allergy6. Therefore, there is a need to understand the basis of early life stress
mediated changes in the brain-gut-axis6. This review will evaluate the mechanisms via which early life
stress can modulate the brain-gut-axis and affect the development of food allergy.
Early life stress
Stress can be both a physical or psychological condition, due to external or internal factors, which
affects the homeostasis of an individual8. Examples of early life stress can be non-optimal childhood
environment or care giving due to poverty, abuse, trauma or other similar stressful events4. The early
environment in life is extremely important. As mentioned before, the nervous system, the HPA axis
as well as the immune system are still under development during early life5,6. Also the development
of behavioural and hormonal responses to stress have to be established at this time4. Infancy,
childhood and adolescence are therefore critical periods with an increased vulnerability to stress9.
Therefore, exposing the developing brain and immune system to stress events in early life, can cause
long-lasting effect that can increase vulnerability to disease development and activity in adulthood4,9.
Associations have been found between a history of trauma in childhood and food-related physical
symptoms and allegies10.
During the postnatal period there is a spurt in neuronal growth and myelination with intense
cellular changes and a dampened HPA axis profile. This period is also called the stress
hyporesponsive period (SHRP)6. In rats this period ranges from postnatal day 4 to 14 and in mice
3
from postnatal day 1 to 12, and it is characterized by decreased adrenocortical system activity, a low
corticosterone secretion and decreased responsiveness to stressors6,11. There is increasing evidence
that there is a comparable hyporesponsive period in humans, extended throughout childhood12. In
infants, the autonomic responses and the HPA axis becomes organised after 2 to 6 months of age7.
For normal development of the CNS during the SHRP, a low and stable level of corticosterone is
necessary. Maternal presence is a very important factor in the SHRP, as mothers provide interaction
and regulation of physiological and psychological processes, but also warmth and food for their
offspring6. The quality of this mother-offspring interaction is important to maintain the
aforementioned dampened HPA axis profile6. However, this hyporesponsiveness can be
counteracted with a harsh stressor, increasing the HPA response4. Insufficient or absent maternal
care and interaction has been linked to developmental and social deficits in offspring4.
Animal models
Animal models are of considerable value in investigating the pathophysiology of human disease and
developing new drugs and treatments6. Most studies in the field of psychoneuroimmunology and the
brain-gut-axis related to stress, have used mice or rats models6. There has been an increase in the
use of the maternal separation (MS) model of early life stress. Up to now, most MS studies have been
done in rats, however, the use of mice is increasing6. Although rodent models are limited in their
ability to mimic human conditions, a model such as MS which shows specific features of early life
stress, is of great value6. It is assumed that adverse events like MS in early life contribute to a
predisposition to development of disease later in life.
The separation period in MS models varies between studies in both length (15 minutes to 24
hours) and number of days (1 to 14 days)6. After longer periods of MS, it has been shown that MS can
lead to a life-long hyperactivity of the HPA axis response to stressors, inducing an increased acute
stress response. It is also suggested that early postnatal MS leads to increased anxiety-related
behaviours in adults6. On the other hand, short MS periods (of around 15 min) appear to decrease
anxiety-like behaviour, HPA axis tone and stress responsiveness6. These short MS periods are
therefore considered more as naturalistic short episodes of maternal absence6. Longer periods of
MS, which are studied most, are therefore more effective in a model of chronic early life stress6.
Throughout the model, maternal separation periods take place during the SHRP, disturbing proper
maturation and development of essential modulatory systems such as the nervous system and the
immune system. This form of early life stress causes numerous alterations in the brain-gut-axis which
will be described in detail later on6. It has been shown that early life stress in the form of MS can
induce abnormalities in both the CNS and in intestinal tract function, such as increased gut
permeability and bacterial translocation6. Taken together, chronic MS in rodents is a supported and
well characterized model for studying the effects of early life stress on the brain-gut-axis4,6. Therefore
most knowledge on the brain-gut-axis originates from mice and rat studies.
4
Brain-gut-axis
The neural, immune and endocrine pathway communication
between the brain and the gut, is referred to as the brain-gutaxis (figure 1)4. The brain-gut-axis is a complex reflex network
between the CNS and the ENS in the gut, via which the brain
can communicate with the intestinal tract6. The ENS is of
major importance in regulation of the brain-gut-axis. The ENS
is part of the peripheral nervous system and can be seen as an
extension of the limbic system in the gut13. It forms a complex
network containing 200 to 600 million efferent and afferent
neurons, distributed in thousands of ganglia that control
intestinal function14,15,16. Two layers of ganglia and fibers
encircle the intestinal tract17. The ENS contains sensory
neurons, inter-neurons and motor neurons which facilitate
normal healthy digestion by regulating motility, absorption,
secretion, peristalsis, blood flow, and water and electrolyte
transport in the gut14,16,17. It also regulates secretion, motility
and release of various hormones and neuropeptides8. In the
intestinal tract and the ENS exist many neurotransmitters and
neuropeptides which can affect the gut physiology6. Errors in
ENS development in early life can therefore have drastic
consequences in the form of different gut disorders14.
The main stress axis in mammals is the HPA axis6.
When activated in normal state, the hypothalamus releases
corticotrophin-releasing factor (CRF) which induces the Figure 1: The brain-gut-axis. The brain
pituitary to release adenocorticotrophin hormone (ACTH). communicates with the gut via various
ACTH systemically causes cortisol release from the adrenal pathways. The HPA axis and the CNS
glands6. Many hormones and neuropeptides are involved in communicate with the gut and the ENS via
hormone release peripherally and
the communication between the brain and the gut. CRF is the
neurotransmitters signalling through
most important stress-related neuropeptide associated with autonomic pathways. The immune system
the brain-gut-axis, with its receptors present in both the CNS can affect both the brain and the gut via
and
humoral
signalling.
as the colon6. CRF is implicated in gut motility, modulation of neural
Neurotransmitters and neuropeptides in
inflammation, visceral sensitivity and permeability of the
the ENS and the gut, and also the gut
intestinal wall during stress, meaning that it is a key mediator microbiota, can affect gut physiology6.
in regulating the brain-gut-axis and stress-related changes in
the intestinal tract6,8.
Through the brain-gut axis, behavioural and cognitive processes in the brain, like stress, can
affect the gut physiology6. The CNS communicates with the intestinal tract through multiple
pathways including the autonomic nervous system (ANS), the HPA axis, the sympatho-adrenal axis
and monoaminergic pathways8,13. The CNS and the ENS are connected through the vagus and pelvic
nerves and sympathetic neuronal pathways16. Sympathetic innervation of the intestinal tract exist of
postganglionic vasoconstrictor neurons, secretion inhibiting neurons and motility inhibiting
neurons13. Inhibitory modulation of cholinergic transmission slows intestinal transit and secretion
through sympathetic outflow13. Parasympathetic innervation of the intestinal tract comprises vagal
5
motor neurons which provide input to the stomach, small intestines, and the proximal part of the
colon13. Furthermore, excitatory vagal input occurs to (gastrin- and somatostatin-containing)
enteroendocrine cells, histamine releasing and serotonin (or 5 hydroxy tryptamine, 5-HT) release
mediating enterochromaffin cells, and to ganglia in the ENS mediating motor reflexes and gastric acid
secretion13.
Both the systemical as well as the mucosal immune system play a role in controlling braingut-axis communication, and have an impact on both the gut and the brain via neural and humoral
routes6. In case of imflammation in the gut or when an antigen penetrates the intestinal barrier, an
immune reaction is initiated which can impact on the brain via a neural (through the vagus) or
humoral route. Also, mucosal mast cells play a regulatory role in the gut immune system, which are
through their neuroimmune mechanisms one of the most important brain-gut modulators as they
are directly innervated6. Furthermore, enterochromaffin cells can regulate communication between
the gut and the nervous system, and are responsible for the regulation of gut secretion, absorption
and motility. When activated, these cells release 5-HT to mucosa cells and nerve endings in the
submucosa. This provides a pathway for signalling to neuronal circuits, which may have an important
role in immune modulation and control of stress reactivity in the brain-gut-axis6.
Gut immunity and food allergy
The intestinal tract prevents invasion of pathogens via multiple pathways, including its own mucosal
immune system18. The gut mucosa is supported by different immune defences, containing both
physiological and immunological components18,19. Its surface area consists of a single-cell layer of
epithelial cells with tight junctions and a mucus layer forming a barrier against harmful components.
The immunological defence consists of different innate and adaptive immune cells and factors19.
Firstly, secretory IgA antibodies control the epithelial colonisation of microorganisms and provide
immune exclusion of antigens. Secondly, there is a hyporesponsiveness to the commensal gut
microbiota and innocuous antigens by mucosal induced tolerance, also known as oral tolerance18.
The delicate balance between the epithelium, the gut associated lymphoid tissue and the microbiota
are important for maintaining the gut homeostasis3.
The gut homeostasis is established in the neonatal period, in which the mucosal immune
system is vulnerable, as the mucosal epithelial barrier and the immunoregulatory network are not
fully developed in newborns. Normal development of the gut homeostasis and the induction of oral
tolerance depends on the establishment of a balanced gut microbiota and proper introduction to
dietary antigens18. Therefore, this period is critical in the induction of food allergy18,19. In food allergy,
genetic predisposition or environmental factors abrogate oral tolerance causing disturbance of the
gut homeostasis. This could be due to an imbalanced immunoregulatory network or to a decelerated
immunological development with an underdeveloped intestinal barrier, which are both associated
with hypersensitivity to innocuous antigens18. Especially epithelial barrier defects facilitate food
allergen sensitization18,19.
The prevalence of food allergy is approximately 1–3% in adults and 3–8% in infants18. The
most common symptoms of food allergy are rash, nausea, vomiting, diarrhea and constipation18.
Food allergic reactions are mostly IgE mediated type 1 hypersensitivity reactions, characterized by a
Th2 type immune reaction18,20. These hypersensitivity reactions occur when an individual becomes
sensitized to a food allergen. During sensitization, an antigen penetrates the intestinal barrier into
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the lamina propria, and migrates to the mesenteric lymph nodes where it is presented by an antigen
presenting cell. This promotes the differentiation to Th2 cells and stimulates plasma cells to produce
antigen specific IgE. This antigen specific IgE attaches itself to FcεRI receptors on mast cell
surfaces20,21. During subsequent exposure, antigens cross-links the bound IgE, triggering mast cell
activation and degranulation inducing inflammatory reactions and the release of mediators such as
histamine which causes the allergic symptoms mentioned above20,21. T regulatory cells can positively
regulate an allergic reaction22.
Microbiota
The gut microbiota can be found in both the small and the large intestines, and reaches a density up
to around 1014 bacteria per gram of colonic content, containing a wide diversity of bacteria with
more than 1000 distinct microbial species1,23,24. These endogenous bacteria play an important role in
the development of the innate and adaptive immune response. They influence the intestinal tract by
altering gut motility, secretion, intestinal barrier function, permeability, immunological tolerance,
blood flow, and visceral perception of the intestines13,23. Furthermore they have many important
functions like food absorption and processing, digestion of host-indigestible polysaccharides,
pathogen displacement and vitamin synthesis25. The gut microbiota has a direct action on the gut
mucosa and the ENS, and has a large metabolic output25. Moreover, it provides a natural barrier
against pathogens1. The gut microbiota is established during the first few years of life, and is shaped
by multiple factors including the maternal microbiota, genetics, diet, metabolism, age, geography,
antibiotic use, and stress23,24. As mentioned before, a balanced colonisation of gut microbiota is
necessary for the development of the mucosal immune system in this period6. Next to that, the gut
microbiota is involved in developing an appropriate HPA axis response to stress in adults6, as stress in
germfree mice compared to control mice, shows elevated ACTH and corticosterone levels26. The gut
microbiota can communicate with its host via epithelial cell receptor mediated signalling and through
direct stimulation of host cells in the lamina propria6. Taken together, the gut microbiota can play an
important role in regulation of the brain-gut-axis, as they can be modulated by stress.
Effects of early life stress on the brain-gut-axis can modulate food allergy
development
In response to early life stress, systems in the brain-gut-axis are affected causing them to operate at
different levels then during homeostasis7. This disturbed balance is relevant in disease development,
as when neural and immune responses do not go back to homeostasis, long term damage may be
produced27. The exact mechanisms of communication in this network are yet to be fully elucidated,
especially during early life development. However, it is known that many different messenger
molecules are involves like neurotransmitters, neuropeptides, growth factors, hormones and
cytokines, which link the brain, the intestinal immunity and the gut microbiota28. The intestinal tract
has the challenging task to respond hostile to pathogenic microbes, while maintaining tolerance to
the commensal gut microbiota and food allergens29. Homeostasis of this intestinal barrier function is
therefore essential in preventing intestinal diseases like food allergy. Maturation of the gut
microbiota and the neuronal and the immunological network of the brain-gut-axis all occur in the
postnatal period and early years of life, which is therefore a vulnerable period in development28.
7
Early life stress can consequently lead to dysfunction of neuronal and immunological aspects of the
brain-gut-axis such as dysregulation of intestinal barrier homeostasis, and is therefore able to
facilitate allergic reaction development in the gut29. Early life stress can affect the brain-gut-axis via
various mechanisms; Through the action of different neurotransmitters and neuropeptides, by
affecting the HPA axis, by inducing changes in the gut barrier and immunity, and by affecting the gut
microbiota2. There are possibly many other unmentioned systems involved in the brain-gut-axis
which could also potentially play a role in the development of food allergy through early life stress.
However in this review we focus on the currently most important and plausible systems to affect
food allergy development. We will discuss the role of neurotransmitters and neuropeptides, the HPA
axis, the intestinal barrier and immunity, and the gut microbiota.
Neurotransmitters and neuropeptides can modulate intestinal barrier function
As mentioned before, many neurotransmitters, neuropeptides, growth factors, hormones and
cytokines are involved in the communication between the brain and the gut6. CNS and ENS
dysregulation caused by early life stress, can lead to homeostatic imbalance in the production and
release of different neurotransmitters and neuropeptides2. Most cells of the immune system, express
receptors for many of them. Via these receptors, early life stress is able to affect the immune
system2. Firstly, early life stress due to MS is able to alter the CNS and its neurotransmitters30. One of
the key neurotransmitters in the brain-gut-axis is serotonin (5-HT), which is important in both the
brain and the gut as it has receptors in both6,30. Early life stress in rats leads to an altered central 5-HT
system, with increased 5-HT levels in the brain, and increased numbers of activated serotonergic
neurons30,31. It also alters expression of different 5-HT transporters and receptors32. Peripheral 5-HT
is involved in contraction, motility, sensation and secretion in the colon30. Early life stress due to MS
in rodents leads to an increase in 5-HT in the colon, which can therefore lead to altered motility,
sensation and secretion30, which are also symptoms of food allergic reactions. Perhaps as a
compensation mechanism, also an increase in the serotonin transporter (SERT) expression is found
after MS in rodents, which is responsible for the re-uptake of 5-HT33. Interestingly, in mice with food
allergy, also an increase in 5-HT is seen in the ileum, paired with an increase in the amount of
enterochromafin cells, which can produce 5-HT34. Because 5-HT is increased both after early life
stress and in food allergy, a connection could be possible.
Another neurotransmitter, noradrenalin, involved in the autonomic nervous system,
is decreased in the brain after early life stress due to MS35. Next to that, there is increasing evidence
that early life stress can cause excessive glucocorticoïd activity in the glutaminergic
neurotransmission36. Furthermore, there are also indications that acetylcholine, the most important
secretomotor neurotransmitter in the ENS, plays a role in dysfunctional brain-gut-axis signalling37.
Neurotransmitters from enteric nerves tightly regulate ion transport and macromolecular
permeability in the gut38. Increased acetylcholine levels and cholinergic activity are found in the
colon after MS38. Acetylcholine causes increased macromolecular permeability through the
stimulation of chloride secretion from epithelial cells, facilitating increased susceptibility to food
allergen sensitization38. Acetylcholine acts along the same pathway as CRF, as CRF can be released by
extrinsic nerves or intrinsic nerves from the ENS37. Also, CRF receptors have been found on enteric
cholinergic neurons38.
8
CRF, involved in the HPA axis, plays a major role in MS-induced gut and barrier dysfunction.
Early life stress causes an increased HPA axis stress response, paired with increased CRF levels. CRF
can be released by the ENS in the colon. Increased levels of CRF mRNA are found in different parts of
the brain, and also the expression of different CRF receptors is altered in both the brain and the
colon39,40. CRF induces barrier dysfunction such as increased gut permeability, by acting on CRF
receptors on cholinergic nerves37. As mentioned before, barrier dysfunctions predispose to the
development of food allergy. Non-specific CRF receptor antagonists are able to block the effects of
MS on gut permeability37,41. Therefore it can be stated that the barrier dysfunctions due to MS are at
least partly mediated by CRF. Also, CRF stimulates the release of nerve growth factor (NGF) from
colonic mast cells42. In the colon, both NGF mRNA expression and protein levels are increased after
neonatal stress, persisting until adulthood, causing alterations in the epithelial barrier43.
Administration of anti-NGF antibodies during MS is able to prevent these changes in gut
permeability43. Therefore, it can be stated that the increased gut permeability associated with CRF, is
mediated through the release of NGF.
During chronic stress, released neuropeptides and hormones cause immune changes like an
imbalance between the Th1 and Th2 response in favour of Th2, which facilitates food allergy
development2. Many substances from enteric neurons can also be released by mast cells, including
histamine, 5-HT and NGF42,44. There are also several neuropeptides that can trigger mast cell
activation, such as substance P (SP), NGF and vasoactive intestinal peptide (VIP)44. Increased VIP
levels correlate with increased IL-4 levels in children who experienced early life stress45. IL-4 is also
able to increase paracellular gut permeability16. VIP is involved in the regulation and increase of
chloride secretion, paracellular gut permeability, fluid secretion and blood flow. Its levels are found
increased in the small intestines of mice afters psychological stress, suggesting early life stress may
also increase VIP levels. This would facilitate the aforementioned gut dysfunctions, possibly
increasing food allergen penetration16. SP also plays a role in gut inflammation, as it stimulates
secretion of proinflammatory cytokines and induces release of mast cell mediators contributing to
enhanced gut permeability16. SP levels are enhanced following a dysbiosis of the gut microbiota,
which can also be a result of early life stress, as will be discussing in a later chapter16. These increased
SP levels can then facilitate increase in gut permeability.
Taken together, many different neurotransmitters and neuropeptides can be affected by
early life stress. They are able to induces changes in the intestinal immunity and the epithelial barrier
such as altered motility, sensation and secretion, and an increased gut permeability. As stated
before, a decreased epithelial barrier function including increased permeability facilitates food
allergen sensitization. Therefore, these changes in neurotransmitters and neuropeptides could
ultimately lead to food allergy development.
Early life stress causes hyperactivity of the HPA axis inducing intestinal barrier defects
The HPA axis is the main stress axis in mammals6. When activated in normal state, the hypothalamus
releases CRF which induces the pituitary to release ACTH. ACTH then induces cortisol release from
the adrenal glands. As mentioned previously, the HPA axis is almost insensitive to stressors during
the SHRP and circulating levels of glucocorticoids are low. A harsh stressor is able to counteract this
hyporesponsiveness, making the HPA axis vulnerable for stressors at this time point. Early life stress
due to MS therefore leads to life-long dysregulation and hyperactivity of the HPA axis and the CRF
9
signalling system16. Early life stress in the SHRP due to MS, is able to enhance stress responsiveness in
adulthood, with increases in both basal and stress-induced levels of corticosterone and ACTH42,46. MS
also induces changes in the glucocorticoid receptor expression in the brain47. As already explained
above, CRF, also part of the HPA axis, also plays a major role in early life stress induced gut
dysfunctions37. Interestingly, administration of CRF mimics the effects of acute stress in the gut, such
as mucosal barrier dysfunction including increased mucosal permeability42. Therefore, it can be
stated that increased CRF levels after early life stress, causes barrier dysfunctions which increase the
susceptibility to food allergen sensitization, leading to the development of food allergy. Furthermore,
chronic stress causes a decreased acute stress response and increased basal cortisol levels45. Taken
together, literature indicates a clear effect of early life stress on the HPA axis that impacts on stress
responsivity and ultimately on the intestinal immunity. The effect of early life stress on the intestinal
barrier are at least partly mediated by increased CRF signalling6.
Altered intestinal immunity and barrier function increases susceptibility to food allergy
As mentioned before, integrity of the epithelial barrier is required for normal immune
functioning of the gut. Damages in the epithelium gives luminal contents access to the mucosal
immune system, possibly inducing inflammatory responses or food allergen sensitization42. Stress can
cause chronic disturbances in several components of the intestinal tract, especially in the intestinal
barrier. Found effects of early life stress on the gut are increased motility, visceral perception,
mucosal conductance, ion secretion, permeability, macromolecular transcytosis, leakage via tight
junctions6, and alterations in the gut microbiota8. Some of these intestinal changes can contribute to
the effects of early life stress on the development of food allergy, like increased gut permeability is
able to facilitate food allergen sensitization. In multiple studies, MS in rats causes long term increases
in colonic permeability, with increased adherence and penetration of bacteria into the epithelium
and lamina propria, accompanied by a greater bacterial translocation to the MLN’s, spleen and liver,
macroscopic damage, increased colonic myeloperoxidase activity (which is a marker for
inflammation), and increased mucosal mast cell density41,48. Although in these studies only colonic
tissue was investigated, this might also be the case for other parts of the intestines, however, this has
not yet been investigated. Other finding do suggest this; Chronic stress in rats increases the
macromolecular permeability49. It was also observed that during chronic stress, and therefore likely
also during chronic stress in early life, intestinal antigen uptake and sensitization is increased
(meaning an increased gut permeability), paired with increased antigen specific IgE levels, increased
IL-4 expression, decreased IFNγ expression, a Th2 shift, increased intestinal secretion, and increases
in inflammatory cells such as mast cells in the gut mucosa29,49. Treatment with a CRF antagonist
eliminated these findings49. Increased antigen uptake into the lamina propria facilitates food allergen
sensitization and development of food allergy6,22. As mentioned before, these epithelial barrier
defects are at least partly CRF-dependent29.
In addition to epithelial barrier dysfunction, an increase in mucosal mast cells (possibly driven
by NGF) is identified after MS, which is still detectable in adulthood6,43. Mast cells are important as
they can translate stress signals into the release of neurotransmitters and neuropeptides, which may
affect the gastrointestinal physiology as described in a previous chapter8. Mucosal mast cells can also
release prostaglandins, protease enzymes, histamine and serotonin in the lamina propria, which can
alter epithelial transport properties42. Increased epithelial transport would facilitate food allergen
10
sensitization. Furthermore, chronic stress also induces mast cell dependent bacterial attachment to
the epithelium and bacterial translocation29. MS is also associated with alterations in differentiation
of enterocytes, endocrine cells, goblet cells and paneth cells, which are an important part of the
intestinal barrier6. Paneth and goblet cells are found depleted, mostly in the upper intestinal tract6.
As described earlier, chronic stress is also able to affect the immune system by promoting a Th2-type
immune response (via neurotransmitters and neuropeptides, and after oral antigen sensitization
during stress), which is characteristic for allergies10. In mice, prenatal stress leads to a dysregulated
humoral and cellular immune response with increased Th2 and IgE levels after antigen challenge7. A
shift towards a Th2 immune response can facilitate development of different immune diseases like
allergies18.
Taken together, it is clear that early life stress is able to induce significant changes in the
intestinal immunity and the epithelial barrier function, at least partly mediated by CRF. Especially
increased antigen uptake (due to increased gut permeability) increases the susceptibility to food
allergen sensitization and the development of food allergy.
Microbiota can cause intestinal barrier defects after early life stress
It is becoming increasingly
acknowledged that the enteric
microbiota can affect the gut
homeostasis and the brain-gutaxis (figure 2). The microbiota
communicate with the host via
epithelial
cells,
receptormediated signalling and by
stimulating host cells in the
lamina
propria
(when
accessible)6. As stated before,
the gut microbiota plays a role
in
immune
system
development, immunological
tolerance, motility, secretion,
Figure 2: The brain-gut-axis and microbiota communication in health (left) and
blood
flow,
permeability,
stress (right). In health, a symbiotic relationship exists between the CNS and the
mucosal immunity and visceral gut microbiota with a predominance of symbiotic microbiota and an intact
perception of the intestines1,3. intestinal barrier. In stress, intestinal dysbiosis (due to a change in the balance
Several studies found evidence between symbiotic and pathogenic microbiota) influences gut physiology
of pathogenic microbiota or food
that early life stress induces inducing barrier defects and perturbation
allergens. (Adapted from Cryan et al.16)
changes in the composition and
diversity of the gut microbiota, however, the mechanism behind this is still unknown1,3,6. Seen the
importance of the microbiota in maintaining the epithelial barrier function, alterations in the
composition of the microbiota can cause significant dysfunction of the gut barrier increasing
permeability. For example, after MS, rats show a significantly different population-based microbiota
profile (different types were not specifically analysed)4. Furthermore, MS in rats causes a decrease in
lactobacilli38. Also, prenatal stress in monkeys is able reduce the overall number of bifidobacteria and
11
lactobacilli50. In addition, social stressors are also able to affect the microbiota, decreasing the
bacteroides species, while increasing the clostridium species51. Next to that, stressor exposures leads
to overgrowth of anaerobic microbiota and to a decrease in overall diversity52. Changing the
composition of the microbiota is able to disturb epithelial barrier function, similar as seen in early life
stress38. Interestingly, administration of probiotics (lactobacilli) in rats, normalises the gut physiology
and barrier function, and improves the changes in HPA axis activity caused by MS, persisting until
adulthood38. As mentioned before, a proper microbiota colonisation in early life is necessary to
maintain barrier function. Early life stress interferes with this process, causing barrier defects and
increasing susceptibility to food allergen sensitization. Looking at the literature, it might be possible
that administration of lactobacilli during early life, would be able to prevent the effects of early life
stress on barrier function38.
The microbiota has a metabolic capacity to produce and regulate compounds that can
influence systems involved in the brain-gut-axis such as the brain and the immune system25. For
example, the gut microbiota produces short chain fatty acids (SCFA’s) as a byproduct of fermentation
and metabolism of carbohydrates and proteins53. SCFA’s, such as acetate, butyrate and propionate,
are important in different aspects of immune regulation (amongst others by acting on GPCRs in the
intestines), such as maintaining epithelial barrier function, being the main energy source of colonic
epithelial cells, and regulating the proliferation and cytokine production of epithelial cells53. They are
also able to modulate brain function by inhibiting histone deacetylases and NF-κB activation16,53.
SCFA’s have several anti-inflammatory properties and are important for stimulating immune
function. For example, SCFA interaction with GPCRs can decrease inflammation in mice16. Changes in
gut microbiota due to early life stress, can therefore also lead to changes in SCFA levels,
consequently altering immune regulation and barrier function53. Next to SCFA’s, certain strains of gut
bacteria can also produce GABA, noradrenaline, dopamine and 5-HT, and regulate the 5-HT precursor
tryptophan. They can also regulate the bioavailablity of choline and its metabolites25. Therefore,
changes in the gut microbiota can lead to alteration in the levels of these neurotransmitters. As
extensively described earlier, changes in these neurotransmitters can modulate intestinal barrier
functioning, which can consequently lead to barrier defects and food allergen sensitization.
Disturbances in the microbiota are associated with various intestinal disorders, such as
irritable bowel syndrome3. It is suggested that pathogens may cause intestinal diseases by increasing
gut permeability. For example, infection with helicobacter pylori is associated with developing food
allergies, as it is able to increase the passage of food antigens through the epithelium29. As said
before, when early life stress changes the gut microbiota composition, gut permeability may be
altered which facilitates food allergy development. Many associations have been found between the
gut microbiota and allergies22,29. Mice with decreased gut microbiota colonies, such as germfree mice
or antibiotics treated mice, show increased food allergen sensitivity paired with increased serum IgE
and circulating basophils22. Allergic children harbour more aerobic bacteria and less lactobacilli then
non-allergic children. As mentioned before, MS similarly causes a decrease in lactobacilli. Also
cesarean section delivered children show an altered gut microbiota profile and a higher risk of
allergies, because a natural birth helps establishing a normal gut microbiota5,25. Furthermore,
breastfeeding is associated with more beneficial microbiota, due to the prebiotic properties of
human milk oligosaccharides5. Next to that, early colonization with bifidobacteria protects an
individual from allergies, like administration with probiotic bacteria also decreases development of
allergies29.
12
Taken together, much evidence is found that early life stress is able to permanently alter the
gut microbiota, although the exact mechanism behind this has yet to be elucidated. Also, there is a
clear association between the gut microbiota and food allergy development. Gut microbiota are able
to disturb the intestinal barrier via still unknown mechanisms. As a disturbed intestinal barrier
predisposes to food allergy, this indicates that it is possible that early life stress can predispose to
food allergy by altering the gut microbiota.
Conclusion
The relationship between early life stress and the development of food allergy is becoming more and
more acknowledged. In this review we looked at the effects of early life stress on several aspects of
the brain-gut-axis, and how these changes might predispose to food allergy development. Early life
stress can dysregulate different systems in the brain-gut-axis, such as neurotransmitter and
neuropeptide signalling, the HPA axis, the intestinal immunity, the intestinal barrier and the gut
microbiota. There are possibly many other systems involved in the brain-gut-axis which could also
potentially play a role in the development of food allergy through early life stress. However, in this
review we focus on the currently most known, important and plausible systems to affect food allergy
development. The potential role of stress on other systems in the brain-gut-axis should still be
further investigated.
Early life stress leads to life-long changes and hyperactivity of the HPA axis, with increased
levels of ACTH, cortisol, corticosterone and CRF. Especially the role of CRF is of great importance in
regulating the development of food allergy after early life stress. Many neurotransmitters and
neuropeptides are affected by early life stress. The most important neuropeptide in the brain-gutaxis is CRF. Increased levels of CRF have been found after early life stress. CRF is able to induce
barrier dysfunctions by acting on cholinergic nerves, and to increase gut permeability by inducing
NGF release from mast cells. Increased gut permeability increases the susceptibility to food allergen
sensitization in the gut, making CRF a key neuropeptide in the development of food allergy. One of
the most important neurotransmitters in the brain-gut-axis is serotonin, of which increased levels
and altered receptor levels are found both in the brain and the gut after early life stress. Serotonin is
able to alter motility and secretion in the colon. Interestingly, an increase in serotonin can also be
found in food allergy, suggesting serotonin might also play a role in food allergy. If this is the case,
early life stress could be able to influence food allergy development through increased serotonin
levels. Further studies have to elucidate this possibility, together with the potential role of serotonin
in food allergy. Next to CRF, acetylcholine and serotonin, many other neuropeptides, such as VIP and
SP, are believed to play a role in the brain-gut-axis and in barrier dysfunction due to early life stress.
The exact role of these and possibly many other neuropeptides in this process should be further
elucidated in future studies.
Early life stress induces many significant changes in the intestinal immunity, of which the
most important are alteration in the gut microbiota and changes in the epithelial barrier function
including increased gut permeability. Increased permeability increases susceptibility to uptake and
infiltration of food allergens into the lamina propria, which may induce sensitization and
development of food allergy.
Currently, much research is performed on the relationship between the gut-brain-axis and
the gut microbiota. Evidence has been found that early life stress can cause changes in the
13
composition and diversity of the gut microbiota. Also, alterations in the composition of the
microbiota is able to cause significant changes in the gut barrier including increased permeability. MS
causes a significantly different population-based microbiota profile, with a decrease in lactobacilli
being the most interesting, as this is also seen in allergic children. Interestingly, administration of
lactobacilli normalised the altered gut physiology caused by early life stress. A proper microbiota
colonisation in early life is necessary to maintain barrier function. Early life stress interferes with this
process by changing the composition of the microbiota, causing epithelial barrier defects and
increasing susceptibility to food allergen sensitization. Through which mechanism(s) the microbiota is
able to alter the intestinal barrier is not yet fully elucidated, although it is thought that SCFA’s may
play an important role in this process. There are many associations between alterations in gut
microbiota and allergies. Therefore it is feasible that changes in gut microbiota are able to induce
food allergy development by modulating the intestinal barrier. It could be hypothesized that the
altered microbiota due to early life stress, plays a role in the development of food allergy by
modulating the epithelial barrier function. However, as the mechanism behind the altered
microbiota due to early life stress is not yet elucidated, it might also be the case that the altered
microbiota is a result of changes in the mucosal immune system. Therefore, the exact role of the
microbiota in the relationship between early life stress and food allergy development should be
further investigated in future studies.
In conclusion, early life stress leads to dysfunction of many aspects of the brain-gut-axis.
Especially modulation of the gut microbiota and the epithelial barrier permeability, in which different
neurotransmitters and neuropeptides play a key role, can predispose to the development of food
allergy. It is important to further elucidate through which exact mechanisms early life stress
predisposes to food allergy, and what role the microbiota exactly play in this. Eventually, modulation
of this extensive network could lead to more insight in the physiological effects of stress in early life,
and possibly to new targets in the prevention of food allergy.
14
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