C7121 Immunology in Health & Disease
Introduction
Diet can affect the immune system through the
metabolic activity of the intestinal microbiota
(1). For example, intestinal anaerobes such as
clostridia or bacteroidetes break down dietary
polysaccharides to generate short chain fatty
acids (SCFAs) in the intestinal lumen (2). SCFAs
act at the level of the intestinal epithelium:
butyrate is an energy source for colonocytes (3),
enhances absorption of sodium and water (4)
and strengthens the intestinal barrier by poorly
defined mechanisms (5). SCFAs are also
absorbed into the systemic circulation as
metabolic substrates in the liver (6), and regulate
the function of different types of immune cells
(7). For example they activate the G-protein
coupled receptor 43 (GPR43) to enhance the
activity and proliferation of colonic regulatory T
cells (Tregs) (8) and to induce chemotactic and
phagocytic responses in neutrophils (9). The
SCFA receptor GPR41 is functionally related to
GPR43 (10), yet since GPR41 is strongly
expressed on non-hematopoietic cells such as
enteroendocrine cells and enteric neurons (11),
GPR41 function has mostly been studied outside
the immune system. For example, GPR41
activation on intestinal enteroendocrine L-cells
releases the enteric hormone peptide YY (PYY)
that reduces intestinal motility (12). This
mechanism may prolong the postprandial
residence time of the intestinal content in the
intestine for better absorption of luminal SCFAs
from the gut lumen.
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Obesity is rampant in Western countries and comorbidities take a toll on public health systems
(13). Obesity has an inflammatory component as
it is associated with increased serum IL-6 (14)
and acute phase proteins (15-17). Endotoxemia,
i.e. an elevated serum LPS, has also been
observed as a direct consequence of a high-fat
diet in patients (18). However, the precise cause
of metabolic endotoxemia remains controversial
(19), and it is unclear whether obesity-associated
subclinical inflammation is preventable.
Here we investigated in mice how butyrate
enhances the intestinal barrier in the steady
state and in situations of barrier damage such as
obesity. We found that butyrate sensing by
GPR41 elicits the barrier-protective factor
Glucagon-like peptide-2 (GLP-2) from the
intestine and supports formation of tight
junctions. This efficiently restricts bacteria and
bacterial products to the intestinal lumen and
ensures the capacity of systemic macrophages to
produce pro-inflammatory cytokines in response
to LPS or CpG oligonucleotides. This GPR41dependent mechanism prevented an intestinal
barrier leak, endotoxemia, and chronic lowgrade inflammation in situations of damage to
the intestinal barrier such as obesity. Therefore
our results describe GPR41 activation as a
molecular mechanism that connects intestinal
barrier dysfunction to chronic low-grade
inflammation in obesity, and suggest a
therapeutic option to maintain systemic immune
reactivity in situations of intestinal barrier
dysfunction.
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C7121 Immunology in Health & Disease
Materials and Methods
Patient selection and human samples
Human colonic biopsies from healthy individuals
who underwent routine colonoscopy for minor
gastrointestinal (GI) tract-related complaints or
due to a family history of GI tract malignancies
were routinely collected and processed
according to standard operating procedures in
the Department of Surgical Pathology and
approved by the ethics review board. Half of
these individuals had a body mass index (BMI) in
the normal range (median BMI = 21.1 kg/m2,
range 19.9 – 22.6 kg/m2 / mean BMI = 21.3
kg/m2), the other half were overweight or obese
according to the World Health Organization
(WHO) (median BMI = 32.0 kg/m2, range 28.0 –
38.0 kg/m2 / mean BMI = 32.65 kg/m2). The
median age of all patients was 30 years (range,
19 – 49 years). Hematoxylin and eosin (H&E)
stained sections of paraffin-embedded colonic
biopsies of patients were evaluated by a boardcertified pathologist.
Mice
GPR41KO (Ffar3-/-) and GPR43KO (Ffar2-/-) mice
on a C57BL/6 genetic background were bred
with C57BL/6NCrl mice (Charles River, France).
Age- and sex-matched WT and KO littermates
were used for experiments. All animal studies
were performed in accordance. Animals were
housed in a temperature-controlled SpecifiedPathogen Free environment on a half-day light
cycle with free access to food and water.
In vivo feeding experiments
In feeding experiments, eight week old
C57BL/6NCrl, GPR41KO mice were fed ad libitum
with a high-fat diet (HFD - 60% kcal from fat,
Kliba Nafag, Kaiseraugst, Switzerland) or a
‘regular’ diet (RD - 10% kcal from fat, Kliba
Nafag). Where indicated, mice were given
drinking water containing 100 mM Sodium
Butyrate (Sigma Aldrich) ad libitum for 7 weeks.
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In vivo DSS-challenge
GPR41KO and littermate WT mice (18-22g) were
provided with 3% Dextran Sulfate Sodium
(36’000-50’000 MW, MD Biomedicals, Santa
Ana, CA) in the drinking water ad libitum from
day 0 to day 5. Control mice had access to regular
drinking water. Where indicated, drinking water
contained 100 mM Sodium Butyrate (Sigma
Aldrich) or 100 mM Sodium Propionate (Sigma
Aldrich) ad libitum for 2 days prior, and
throughout DSS feeding.
In vivo analysis of intestinal permeability
Mice were administered 60mg/kg 4kD
Fluorescein Isothiocyanate Dextran (FITCDextran, Sigma Aldrich) by oral gavage. After 4 h
animals were terminally bled and fluorescence
intensity
of
sera
were
measured
(492nm/525nm). FITC-dextran concentrations
were determined from standard curves
generated from serial dilutions of FITC-dextran.
Ex vivo Analysis
Colons and ilea were excised and rinsed with
saline before pieces were either conserved in
neutralized formaldehyde solution (4% w/v, J.T.
Baker Avantor, Center Valley, PA) for histological
analysis, or frozen and stored at -80°C for
enzyme analysis, or stored in RNAlater®
(Ambion, Applied Biosystems) for qPCR analysis.
All sera, where indicated from the portal vein,
were stored at -80oC for ELISAs.
Immunohistochemistry
and
histological
evaluation.
For analysis of occludin distribution in mouse
intestines by immunofluorescence, 3 m
sections of mouse colons and ilea were dewaxed and hydrated on a Medite Tissue Stainer
TST44C. Slides were briefly pre-warmed in water
(37°C), then incubated for 8 min at 37°C in 1
mg/ml protease type XIV from S. griseus in water
(Sigma) then transferred to PBS. Sections were
blocked for 2h at 4°C with PBS containing 5%
donkey serum (Jackson Immunoresearch, West
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C7121 Immunology in Health & Disease
Grove, PA), 1% BSA and 0.25% Triton-X100. After
removal of the blocking solution, primary
antibody (rabbit polyclonal anti-occludin, both
from invitrogen) were added in PBS containing
1% donkey serum, 0.25% Triton-X100 and
incubated overnight at 4°C. Slides were washed
with PBS, and secondary antibody (Alexa488conjugated donkey-anti-rabbit IgG, Invitrogen)
was added at 1:200 in PBS containing 1% donkey
serum and 0.25% Triton-X100 for 1h at room
temperature in the dark. After washing in PBS,
slides were mounted using DAPI ProLong
Antifade Gold with DAPI (Thermo Fisher
Scientific), and analyzed using an Zeiss Axioplan
2 fluorescence microscope (Oberkochen,
Germany), using the Zeiss Axiovision software.
Distribution of occludin on the surface of villi and
crypts was analyzed by a Board-certified
pathologist (KM) in a blinded fashion, and scored
with 100% being full coverage of villi and crypts
with occludin.
Macrophage isolation and culture.
Macrophages were obtained from naïve WT or
littermate GPR41KO mice as follows: Bone
marrow-derived macrophages were prepared by
eluting bone marrow from femurs and tibias
with PBS. Erythrocytes were lysed (Red Cell
Lysing Buffer, Sigma Aldrich), and cell
suspensions were washed by centrifugation in
complete medium (RPMI 1640, 2 mM Glutamax,
1 mM Na-pyruvate, 50 U/ml penicillinstreptomycin (all Invitrogen), 20% heatinactivated fetal calf serum). Following 7 days of
culture in complete medium containing 50ng/ml
M-CSF macrophages were obtained as a
homogeneous population.
Peritoneal macrophages were harvested by
peritoneal lavage of WT or littermate GPR41KO
mice with PBS-EDTA, centrifugation and
resuspended
in
complete
medium.
6
Macrophages were plated at 1x10 /ml in 96 well
flat bottom plates allowed to adhere for 2 h,
washed, then stimulated as indicated.
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For LPS stimulation, complete medium in the
presence or absence of LPS (LPS-EK Ultrapure,
Invivogen, San Diego, USA) was added at 0-200
ng/ml for 6 h in a humidified incubator (37°C, 5%
CO2). For MSU stimulation, macrophages were
primed for 2 h with 20 ng/ml LPS in complete
medium before the addition of MSU crystals at
25, 50 and 100 g/ml for a further 4 h. CpG1826
was added at 0-10 M for 6 h. At the end of the
incubations, supernatants were stored at -80oC
for cytokine analysis by ELISA.
Colonic crypt isolation and culture
Intestinal crypts were isolated as published (52).
Briefly, crypts were released from the intestine
by shaking and washing with chelating buffer
(PBS-Ca2+Mg2+, penicillin - streptomycin,
glutamine and gentamycin (all Invitrogen).
Isolated crypts were counted and pelleted and
500 crypts were mixed with 50 l of Matrigel (BD
Biosciences) and plated in 24 well plates.
Following polymerization of the matrigel, 500 l
of serum free crypt culture medium (advanced
DMEM/F12, supplemented with 50 U/ml
penicillin-streptomycin, 2 mM Glutamax I and 50
g/ml gentamycin (all Invitrogen)) containing
growth factors 50gm/ml EGF (Peprotech), 100
ng/ml R-Spondin1 (R&D Systems, Abington, UK)
and 100 ng/ml Noggin (Peprotech). After 24 h,
supernatants were removed and stored at -80oC
for analysis by ELISA.
Cytokine ELISAs and Limulus amebocyte lysate
(LAL) assay for endotoxin
Cytokines were determined by ELISAs from cell
culture supernatants or sera as. ELISAs for
mouse IL-6, CXCL1/KC, IL-1β, C-Reactive Protein
CRP were performed according to the
manufacturer’s instructions (DuoSets, R&D
Systems, Abington, UK). Haptoglobin was
determined using the “PHASE” haptoglobin kit
(Tri-Delta, Morris Planes, NJ). Mouse GLP-2
levels were assessed using the ELISA for mouse
GLP-2 ELISA kit (Alpco Diagnostics, Salem, MA).
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C7121 Immunology in Health & Disease
Endotoxin from portal vein serum was assessed
using the Pierce LAL Chromogenic Endotoxin
Quantitation Kit (Thermo Fisher Scientific,
Waltham, MA) according to the manufacturer’s
instructions.
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biological and technical replicates. All data are
presented as mean ± SD or ± SD with *p<0.05;
**p<0.01; ***p<0.001 as indicated using either
Mann-Whitney test, unpaired t-test or one-way
ANOVA with Tukey’s post-test.
Statistical analysis
GraphPad Prism Software was used for all
statistical calculations. Data were run in
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C7121 Immunology in Health & Disease
Results.
Butyrate and the SCFA receptor GPR41 enhance
intestinal barrier function in the steady state.
Analysis of tight junction genes in colonic
biopsies of lean and obese individuals
demonstrated a lower expression of tight
junction proteins zonula occludens-1 (ZO-1) and
occludin in obesity (Fig. 1a), consistent with the
notion that obesity compromises the intestinal
barrier in patients (20). Mice fed a high fat diet
(HFD) or HFD + Butyrate demonstrated
significant weight gain compared to mice fed a
regular diet (RD, Fig. 1b). In mice, HFD feeding
perturbed the intestinal barrier as measured by
serum levels of the marker FITC-dextran leaking
from the intestine to the circulation (Fig. 1c). It
also induced endotoxemia, another hallmarks of
obesity (Fig. 1d). In vitro data suggest that
butyrate reduces leakage of bacteria across the
intestinal epithelium (21). This is in line with our
data which shows that the intestinal barrier
remained intact when butyrate was co-fed with
the HFD, and endotoxemia in the serum was
limited (Fig. 1 c-d). This demonstrates that
feeding of the SCFA butyrate protects the
intestinal epithelium from the damage that is
associated with HFD feeding, and limits systemic
endotoxemia and a marker of obesity-associated
chronic inflammation.
Since the SCFA receptors, GPR41 and GPR43 are
expressed in the small and large intestine, the
intestinal barrier in naïve mice deficient for
either GPR41 or GPR43 and their corresponding
littermates was measured in vivo under
homeostatic conditions (Fig. 1e). Only in the
absence of GPR41 was intestinal permeability
elevated, indicating that GPR41 regulates the gut
epithelial barrier. It is known that both
regulation of intestinal epithelial cell turnover
and tight junctions contribute to an intact
intestinal barrier (23). Since butyrate regulates
tight junction formation in intestinal cell lines in
vitro (24), we hypothesized that homeostatic
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SCFA sensing through GPR41 might enhance
formation of tight junctions in the gut.
Homeostatic distribution of the tight junction
Occludin over the surface of the intestinal
epithelium was compromised in GPR41KO mice
(Fig. 1f). This suggests that butyrate sensing
through GPR41 affected tight junction assembly.
Epithelial cell turnover was also measured
revealing reduced proliferating cells at the base
of the crypts (data not shown). Since the relative
abundance of Bacteroides and Firmicutes in the
intestine is associated with alterations of SCFA
content in feces (22), we quantified both phyla
from the feces of GPR41KO mice and WT
littermates by qPCR, and did not find significant
differences between both groups of mice (data
not shown).
SCFAs regulate the intestinal barrier via GPR41induced secretion of glucagon-like peptide-2
(GLP-2).
Next we investigated how GPR41 activation
strengthens the intestinal epithelium. GPR41 is
expressed on specialized intestinal endocrine Lcells that produce the hormone GLP-1 (11). GLP2 is a growth factor for the intestinal epithelium
(26). In light of this, primary colonic crypts from
WT and GPR41KO mice were cultured with and
without butyrate. (Fig 2). WT colons produced
significantly more GLP-2 already at baseline (Fig.
2a). Consistently, GPR41KO mice expressed
significantly less systemic GLP-2 in the steady
state compared to WT mice (Fig. 2b). Butyrate
feeding of WT mice did not enhance serum GLP2 further, most likely since intestinal SCFA levels
fully activate GPR41. Next, we asked whether
the GPR41-dependent tissue-protective effect is
also observed following acute intestinal damage
by a short regimen of dextran-sulfate sodium
(DSS) feeding. Administration of butyrate with
the drinking water protected the intestinal
barrier in a GPR41-dependent manner. (Fig. 2c),
showing that a GPR41 activation maintains the
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C7121 Immunology in Health & Disease
intestinal barrier even in situations of wounding.
To demonstrate that GPR41 activation protected
the barrier through GLP-2, we first confirmed
that GLP-2 administration prevented DSSinduced barrier damage (Fig. 2d). From these
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data we conclude that butyrate induces
intestinal L-cells to produce the barrierprotective enteroendocrine peptide GLP-2 via
GPR41.
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C7121 Immunology in Health & Disease
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Figure 1. The SCFA butyrate and GPR41 regulate permeability of the intestinal barrier. a, Relative gene
expression of occludin (lower panel) and ZO-1 (upper panel), in colon biopsies of lean (average BMI 21.3)
and obese (average BMI 32.6) individuals; *P <0.05, Mann-Whitney test. b, Weight gain in C57BL/6 mice
fed a regular diet (RD; white squares), high fat diet (HFD; grey squares) or HFD with 100mM butyrate
supplemented in the drinking water (HFD+butyrate; black squares) for 7 weeks. Data are mean ± SD;
6/group with #P < 0.05 HFD vs. HFD+butyrate, *P < 0.05 RD vs. HFD, ANOVA with Tukey’s post-test. c,
Intestinal permeability, d, serum endotoxin at 7 weeks of RD, HFD or HFD+Butyrate in C57BL/6J mice. Data
in c–d are all 6 mice/group expressed as mean ± SD; *P < 0.05, ***P <0.001, ANOVA with Tukey’s posttest. e, Intestinal permeability in naïve GPR41KO, GPR43KO and corresponding WT littermates following
FITC-dextran p.o. (n=5 GPR41WT, n=6 GPR41KO, GPR43WT and GPR43KO/group) with mean ± SD with *P
< 0.05, **P <0.01 determined by ANOVA with Tukey’s post-test. f, Representative occludin staining
(green) in colons of naïve GPR41KO mice and WT mice, scale bar: 100m, white arrows: areas of reduced
staining. Quantification of occludin distribution in colons of naive GPR41KO and WT mice (n=7 sections
from 4 WT mice and n=10 sections from 5 GPR41KO mice), horizontal bars: means.
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C7121 Immunology in Health & Disease
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Figure 2. SCFAs regulate the intestinal barrier via GPR41-induced glucagon-like peptide-2 (GLP-2). a,
GLP-2 levels in culture supernatants from WT and GPR41KO colonic crypts cultured +/- 1mM butyrate; 3
mice/group. b, Serum GLP-2 levels from naïve WT (n=7) and GPR41KO (n=8), and butyrate-fed (100 mM,
5 weeks) WT (n=7) and GPR41KO mice (n=9). c, Epithelial permeability in WT or GPR41KO littermates
challenged with DSS for 5 days, without (n=11 WT, n=9 KO) or with 2 days of pre-feeding with 100 mM
butyrate (n=7 WT and n=10 KO). Pooled data from three independent experiments. d, Intestinal
permeability of C57BL/6 mice treated with DSS for 5 days ± GLP-2 (n=8 per group). Data are means ±
s.e.m with *P < 0.05 and **P <0.01, ****P <0.00001; ANOVA with Tukey’s post-test.
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C7121 Immunology in Health & Disease
Chronic endotoxemia is a consequence of
intestinal barrier dysfunction in GPR41KO mice.
Since the intestinal epithelium of GPR41KO mice
is permeable for FITC-dextran in the steady
state, we surmised that intestinal bacteria and
their products had access to the circulation of
GPR41KO mice. Compared to WT mice we
detected elevated LPS levels in the serum of
GPR41KO mice (Fig. 3a). It is proposed that
endotoxemia drives chronic inflammation in
obesity (31). Therefore we analyzed levels of the
inflammatory marker IL-6 liver portal serum of
GPR41KO mice and WT controls and found
increased expression of these markers in sera of
GPR41KO mice (Fig. 3b). Together these data
suggest that a defective intestinal barrier in the
absence of GPR41 signaling in the gut leads to
chronic endotoxemia. This is associated with
markers of chronic systemic inflammation even
in the absence of obesity.
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peritoneal macrophages Chronic exposure of
macrophages to low doses of LPS exposure
induces resistance to subsequent endotoxin
challenges, a phenomenon called endotoxin
tolerance (32). Since GPR41KO mice were
endotoxemic in the steady state we
hypothesized that peripheral macrophages from
GPR41KO mice were refractory to further LPS
challenges. When peritoneal macrophages from
GPR41KO mice and WT littermates were
incubated with LPS ex vivo GPR41KO
macrophages demonstrated a blunted IL-6
response following LPS challenge (Fig. 4a). In
contrast, in vitro derived macrophages from
GPR41KO or WT bone marrow showed
equivalent IL-6 secretion following LPSstimulation (Fig. 4b), suggesting that GPR41
deficiency was not sufficient, but that exposure
of macrophages to a GPR41KO in vivo
environment was necessary for the reduced
response to bacterial TLR ligands such as LPS.
Barrier dysfunction in GPR41KO mice leads to
desensitization of cytokine responses of
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C7121 Immunology in Health & Disease
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Figure 3. Chronic endotoxemia is a consequence of intestinal barrier dysfunction in GPR41KO mice. a,
Steady state serum endotoxin (n=4 WT and n=6 GPR41KO per group). b, Serum IL-6 (n=6/group) in naive
WT and GPR41KO mice. Data are means ± SD; *P < 0.05, ***P <0.0001 unpaired t-test.
Figure 4. Barrier dysfunction in GPR41KO mice leads desensitization of cytokine responses of peritoneal
macrophages. a, IL-6 secretion of peritoneal macrophages (pM) cultured with LPS and of b, bone
marrow-derived macrophages cultured with LPS. Data of are means + s.e.m of pooled data from two
experiments per graph; *P < 0.05, **P <0.01; *** P <0.001, unpaired t-test.
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C7121 Immunology in Health & Disease
Discussion
An intact intestinal epithelial barrier is a critical
gatekeeper for physiological and immunological
homeostasis, because it absorbs intestinal
metabolites and nutrients yet restricts bacterial
components to the gut lumen (37). Since the
nutrient content and the intestinal microbiota of
the intestinal lumen are continuously changing,
the permeability of the intestinal epithelium
needs to be very adaptable.
Here we show that the intestinal barrier is
regulated locally by the SCFA content of the gut.
Activation of GPR41 on intestinal L-cells
enhances the intestinal barrier through GLP-2
secretion, both in the steady state and upon
acute (e.g. DSS feeding) and chronic (e.g. HFD
feeding) damage. Shielding the body from the
intestinal microflora prevents systemic lowgrade inflammation and desensitization of
macrophages to bacterial TLR ligands.
Diet affects the intestinal barrier instantly. For
example, endotoxemia – as a sign of intestinal
leakage - is observed already 2 h after ingestion
of a high-fat meal (38), and the protective effect
of SCFAs on the intestinal barrier is observed
within minutes of exposure (39). This
demonstrates that the intestinal barrier is able
to adapt swiftly to the best compromise
between nutrient absorption and bacterial
containment. However, chronic changes in diet
can also entail more long-term changes in the
intestinal microbiota (22). Interestingly,
following concomitant butyrate and HFD
feeding, we detected a significant increase of all
intestinal SCFA species, compared to HFD-fed
mice, suggesting that butyrate changed the
composition or metabolic function of the
intestinal microbiota. Another microbial
metabolite, indole-3-propionic acid, has recently
been shown to affect intestinal barrier
permeability via the pregnane X receptor (PXR),
a receptor for xenobiotic substances (41). With
that, the perspective opens that distinct
microbe-dependent
mechanisms
ensure
intestinal barrier integrity in distinct situations of
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physiological challenges. It will be particularly
relevant to investigate whether microbial
regulation
mechanisms
on
intestinal
permeability are interrelated or regionally
specialized along the digestive tube.
Obese patients and HFD-fed mice have a
defective intestinal barrier (20), and signs of
chronic inflammation have been found in both
conditions. The causal, GPR41-dependent link
between obesity, intestinal barrier defect,
endotoxemia,
and
chronic
systemic
inflammation described here may provide a
therapeutic target to normalize the altered
inflammatory status of obese patients.
Interestingly, an intestinal barrier defect was
also observed in Parkinson’s disease, diabetes
and asthma (43-45). It remains to be seen
whether the intestinal barrier leak in these
conditions relies on similar principles as
described here and how it contributes to
immune dysregulation.
It is of note that SCFA-dependent activation of
GPR41 on L-cells does not only strengthen the
intestinal barrier, it also reduces intestinal
motility (12). Why would both processes rely on
the same molecular switch? With slower
intestinal motility the body is given more time to
absorb intestinal SCFAs, and the intestinal
microbiota is given more time to thrive on the
intestinal contents. This carries the risk of local
bacterial overgrowth and a higher probability of
bacterial transit across the intestinal barrier.
Therefore in situations of low intestinal motility
it becomes particularly important to shield the
body from the intestinal microbiota, and using
the same molecular mechanism for both
functions ensures that this is the case.
While butyrate affects intestinal barrier function
through GPR41, GPR41 expression did in our
hands not affect intestinal SCFA concentrations.
This is at odds with another study that reported
higher small intestine levels of SCFAs in
GPR41KO mice (12). However were GPR41KO
mice of this study colonized with two bacterial
strains that were selected for optimal SCFA
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C7121 Immunology in Health & Disease
production, while our GPR41KO colony
contained a complete microbiome. Further,
other researchers found that butyrate, if
administered as sodium salt as 5% within a HFD,
prevented HFD-induced weight gain (46). Our
mode and dose of administration may have led
to a different level of butyrate exposure so that
this effect was not observed. An important
aspect to bear in mind for clinical translation of
our results is that HFD-feeding of mice lowers
intestinal SCFA levels ((47) and our study), while
in obese patients fecal SCFA levels are higher
than in lean controls (48). In these patients, high
intestinal SCFA levels may cause desensitization
of GPR41, a phenomenon that is typical for
GPCRs. This may lead to the same functional
outcome as low SCFA levels in the gut or even
GPR41-deficiency.
It is of note that the regulatory effect of butyrate
on macrophage responses is distinct, depending
on their localization. We show that butyrate
accentuates cytokine responses of systemic
macrophages by shielding them from intestinal
LPS, while others have shown that it tempers
intestinal macrophage responses (49). This dual
action of butyrate may ensure optimal
macrophage reactivity to systemic bacterial
challenges, yet reduce the risk of intestinal
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inflammation in cases of minor breaches of the
intestinal barrier.
Since SCFAs originate from a high fiber diet, our
data aligns with a “diet hypothesis” which
suggests that adequate intake of food fibers
promotes a healthy microbiota that significantly
reduces the prevalence of chronic inflammatory
diseases (50). Since GPR41 activation by SCFAs
normalized the intestinal barrier leak and
endotoxemia in obese mice, our data lend
credibility to the idea to administer prebiotic
food supplements, with the aim to expand SCFAproducing intestinal bacteria and eventually
reduce obesity-related endotoxemia and chronic
inflammation (51).
Our findings illustrate how intestinal SCFA
concentrations regulate the gut barrier through
a local GPR41/GLP-2-dependent mechanism.
This mechanism restricts the intestinal
microbiota to the gut lumen and thus avoids
endotoxemia and accentuates cytokine
responses from peritoneal macrophages. With
that, our work highlights the intestinal barrier as
a sensor system for the gut content that sets the
threshold for inflammatory processes at remote
sites of the body.
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C7121 Immunology in Health & Disease
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