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The Gut Microbiota
Interactions Between the Microbiota
and the Immune System
Lora V. Hooper,1* Dan R. Littman,2Andrew J. Macpherson3
The large numbers of microorganisms that inhabit mammalian body surfaces have a highly coevolved
relationship with the immune system. Although many of these microbes carry out functions that are
critical for host physiology, they nevertheless pose the threat of breach with ensuing pathologies.
The mammalian immune system plays an essential role in maintaining homeostasis with resident
microbial communities, thus ensuring that the mutualistic nature of the host-microbial relationship
is maintained. At the same time, resident bacteria profoundly shape mammalian immunity. Here, we
review advances in our understanding of the interactions between resident microbes and the immune
system and the implications of these findings for human health.
lower intestine, these organisms reach extraordi-
nary densities and have evolved to degrade a
variety ofplantpolysaccharides andother dietary
digestive efficiency and ensures a steady nutrient
supply for the microbes. Metabolic efficiency
evolution of both sides of the host-microbiota
relationship. Millions of years of coevolution,
however, have forged pervasive interconnections
between the physiologies of microbial commu-
nities and their hosts thatextend beyondmetabolic
functions. These interconnections are particularly
and the immune system.
Despite the symbiotic nature of the intestinal
host-microbial relationship, the close association
of an abundant bacterial community with intesti-
nal tissues poses immense health challenges. The
dense communities of bacteria in the lower intes-
tine (≥1012/cm3intestinal contents) are separated
from body tissues by the epithelial layer (10 mm)
over a large intestinal surface area (~200 m2in
including inflammation and sepsis. The immune
system has thus evolved adaptations that work to-
gether to contain the microbiota and preserve the
The evolution of the vertebrate immune system has
therefore been driven by the need to protect the
omplex communities of microorganisms,
surfaces of virtually all vertebrates. In the
bial communities for their metabolic benefits (2).
In this Review, we survey the state of our
understanding of microbiota-immune system in-
teractions. We also highlight key experimental
challenges that must be confronted to advance
our understanding in this area and consider how
our knowledge of these interactions might be
harnessed to improve public health.
Tools for Analyzing the Microbiota–Immune
immune system interactions has been acquired
from studies of germ-free animals. Such animals
are reared in sterile isolators to control their
bacteria, and eukaryotic parasites. Germ-free
animals can be studied in their microbiologically
composed of a single microbial species or defined
species mixtures. The technology has thus come
Greek meaning “known life.” Gnotobiotic ani-
mals, particularly rodents, have become critical
experimental tools for determining which host
immune functions are genetically encoded and
which require interactions with microbes.
The current impetus for gnotobiotic exper-
imentation has been driven by several impor-
numbers of genetically targeted and wild-type
able in the germ-free state. The contribution of
different immune system constituents to host-
in genetically altered and wild-type mice (4, 5).
Second, next-generation sequencing tech-
nologies have opened the black box of micro-
biota complexity. Although advances in ex vivo
culturability are still needed, the composition of
human and animal microbiotas can be opera-
genes, especially those encoding the 16S ribo-
possible the construction of defined microbiotas,
whose distinct effects on host immunity can now
the study of experimental animals that are both
isobiotic and, in a defined inbred host, isogenic.
A dominant goal of these efforts is to benefit hu-
man health [see Blumberg and Powie (7)]. With
the developing technology, the species differ-
ences can be closed using mice with a defined
humanized microbiota (8). On the horizon, there
is even the prospect of humanized isobiotic mice
that also have a humanized immune system (9).
A third advance has been the development of
experimental systems that allow the uncoupling
microbial colonization. This cannot be achieved
by antibiotic treatment alone because a small pro-
portion of the targeted microbes will persist.
Deletion strains of bacteria lacking the ability to
synthesize prokaryotic-specific amino acids have
not persist in vivo, so the animals become germ-
free again. This allows issues of mucosal immune
induction, memory, and functional protection to
resident microbial communities on mammalian
host biology have been acquired by using high-
throughput transcriptomic and metabolomic tools
to compare germ-free and colonized mice (11, 12).
shape many aspects of host physiology, includ-
ing immunity (13, 14) and development (15), as
well as mass spectrometry and nuclear magnetic
resonance spectroscopy, which have provided im-
portant insights into how microbiota influence
application of these new approaches to the older
technology of gnotobiotics has revolutionized
the study of interactions between the microbiota
and the immune system.
Looking Inside-Out: Immune System
Control of the Microbiota
A major driving force in the evolution of the
mammalian immune system has been the need
to maintain homeostatic relationships with the
microbiota. This encompasses control of micro-
bial interactions with host tissues as well as the
composition of microbial consortia. Here, we dis-
cuss recent insights into how the immune system
exerts “inside-out” control over microbiota local-
ization and community composition (see Fig. 1).
Stratification and compartmentalization of the
microbiota. The intestinal immune system faces
unique challenges relative to other organs, as it
must continuously confront an enormous micro-
1The Howard Hughes Medical Institute and Department of Im-
at Dallas, Dallas, TX 75390, USA.2Howard Hughes Medical
Institute and Molecular Pathogenesis Program, The Kimmel
Center for Biology and Medicine of the Skirball Institute, New
York University School of Medicine, New York, NY 10016, USA.
3Maurice Müller Laboratories, University Clinic for Visceral Sur-
gery and Medicine, University of Bern, Bern, Switzerland.
*To whom correspondence should be addressed. E-mail:
8 JUNE 2012VOL 336
on June 13, 2012
pathologies arising from innate immune signaling
metabolic functions. An important function of the
intestinal immune system is to control the expo-
sure of bacteria to host tissues, thereby lessening
the potential for pathologic outcomes. This oc-
curs at two distinct levels: first, by minimizing
direct contact between intestinal bacteria and the
epithelial cell surface (stratification) and, second,
by confining penetrant bacteria to intestinal sites
and limiting their exposure to the systemic im-
mune compartment (compartmentalization).
epithelial contact. Intestinal goblet cells secrete
thick viscous coating at the intestinal epithelial
distinct mucus layers. Although the outer mucus
layer contains large numbers of bacteria,the inner
mucus layer is resistant to bacterial penetration
(16). In contrast, the small intestine lacks clearly
distinct inner and outer mucus layers (17). Here,
compartmentalization depends in part on antibac-
terial proteins that are secreted by the intestinal
epithelium. RegIIIg is an antibacterial lectin that
bacterial penetration of the small intestinal mucus
layer, thus restricting the number of bacteria that
contact the epithelial surface (5).
Stratification of intestinal bacteria on the
luminal side of the epithelial barrier also depends
on secreted immunoglobulin A (IgA). IgA spe-
cific for intestinal bacteria is produced with the
help of intestinal dendritic cells that sample the
lying epithelium. These bacteria-laden dendritic
cells interact with B and T cells in the Peyer’s
to the intestinal lamina propria and secrete IgA
that is transcytosed across the epithelium and
deposited on the apical surface. The transcytosed
IgAs bind to luminal bacteria, preventing micro-
Mucosal compartmentalization functions to
minimize exposure of resident bacteria to the sys-
are largely confined to the luminal side of the
epithelial barrier, the sheer number of intestinal
bacteria makes an occasional breach inevita-
ble. Typically, commensal microorganisms that
penetrate the intestinal epithelial cell barrier are
phagocytosed and eliminated by lamina propria
macrophages (23). However, the intestinal im-
mune system samples some of the penetrant bac-
teria, engendering specific immune responses
that are distributed along the length of the intes-
tine (21). Bacteria that penetrate the intestinal
barrier are engulfed by dendritic cells (DCs) re-
siding in the lamina propria and are carried alive
to the mesenteric lymph nodes. However, these
bacteria do not penetrate to systemic secondary
DCs induce protective secretory IgAs (21), which
are distributed throughout all mucosal surfaces
by recirculation of activated B and Tcells. Thus,
distinctive anatomical adaptations in the mucosal
immune system allow immune responses directed
still being confined to mucosal tissues.
Other immune cell populations also promote
the containment of commensal bacteria to in-
testinal sites. Innate lymphoid cells reside in the
lamina propria and have effector cytokine pro-
files resembling those of T helper (TH) cells (24).
Innate lymphoid cells that produce interleukin
resident bacteria to the intestine, thus preventing
their spread to systemic sites (25).
The compartmentalization of mucosal and
systemic immune priming can be severely per-
turbed in immune-deficient mice. For example,
mice engineered to lack IgA show priming of
serum IgG responses against commensals, indi-
cating that these bacteria have been exposed to
the systemic immune system (22). A similar out-
Fig. 1. Looking inside-out: immune system control of the microbiota. Several immune effectors function
layer, epithelial antibacterial proteins, and IgA secreted by lamina propria plasma cells. Compartmen-
immune system. Some microbes are sampled by intestinal DCs. The loaded DCs traffic to the mesenteric
lymph nodes through the intestinal lymphatics but do not penetrate further into the body. This
compartmentalizes live bacteria and induction of immune responses to the mucosal immune system.
There is recirculation of induced B cells and some T cell subsets through the lymphatics and the
bloodstream to home back to mucosal sites, where B cells differentiate into IgA-secreting plasma cells.
VOL 336 8 JUNE 2012
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The Gut Microbiota
defective. Mice lacking MyD88 or TRIF signal-
ing adaptors for TLR-mediated sensing of bacteria
also produce serum IgG responses against com-
mensals (26). This probably results from the fact
cross the epithelial barrier and phagocytic cells
Immune system control of microbiota com-
position. The development of high-throughput
sequencing technologies for microbiota analysis
has provided insight into the many factors that
determine microbiota composition. For example
nutrients, whether derived from the host diet
(27) or from endogenous host sources (28), are
critically important in shaping the structure of
host-associated microbial communities. Recent
likely to be an important contributor to “inside-
out” host control over microbiota composition.
Certain secreted antibacterial proteins produced
by epithelial cells can shape the composition of in-
testinal microbial communities. a-defensins are
small (2 to 3 kD) antibacterial peptides secreted by
ysis of the microbiota in mice that were either de-
human a-defensin-5 showed that although there
ria, there were substantial a-defensin–dependent
differences observed in the two mouse strains (29).
An interesting question is how far secreted in-
nate immune effectors “reach” into the luminal
microbial consortia. For example, the impact of hu-
man a-defensin-5 on luminal community composi-
tion contrasts with the antibacterial lectin RegIIIg,
which limits penetration of bacteria to the epithelial
surface but does not alter luminal communities (5).
community composition, whereas others, such as
to host surface tissues. Questions remain as to ex-
composition, however. In one scenario, these small
antimicrobial peptides diffuse through the mucus
layer and directly act on bacteria that inhabit the lu-
men. Another possibility is that a-defensin-5 exerts
teria acting as reservoirs that seed luminal commu-
nities and thus dictate their composition. Answering
these questions will require improved tools for fine-
The impact of the immune system on micro-
biota composition is also suggested by several im-
mune deficiencies that alter microbial communities
in ways that predispose to disease. For example,
factor T-bet (encoded by Tbx21), which governs
inflammatory responses in cells of both the innate
and the adaptive immune system (30). When
Tbx21–/–mice were crossed onto Rag2–/–mice,
dependent manner (30). Remarkably, this colitis
phenotype was transmissible to wild-type mice by
adoptive transfer of the Tbx21–/–/Rag2–/–micro-
biota. This demonstrated that altered microbiota
were sufficient to induce disease and could thus be
considered “dysbiotic.” Similarly, mice lacking the
bacterial flagellin receptor TLR5 exhibit a syn-
drome encompassing insulin resistance, hyper-
with alterations in microbiota composition (31).
type mice that acquire the Tlr5–/–gut microbiota.
A third example of immune-driven dysbiosis is
seen in mice deficient for epithelial cell expres-
sion of the inflammasome component NLRP6.
These mice develop an altered microbiota with
increased abundance of members of the Bacte-
roidetes phylum associated with increased intes-
to chemically induced colitis. Again, there is evi-
intestinal inflammation, because conventionally
raised wild-type mice that acquire the dysbiotic
microbiota show similar immunopathology (32).
Together, these findings suggest that the im-
mune system affords mammalian hosts some con-
communities. It is also clear that these commu-
nities can be perturbed by defects in the host im-
system as a form of ecosystem management that
exerts critical control over microbiota compo-
sition, diversity, and location [see Costello et al.
(33)]. However, a number of questions remain.
First, although it is apparent that the immune sys-
tem shapes community composition at the species
level, it is not yet clear whether the immune sys-
tem shapes the genetics and physiology of indi-
vidual microbial species. Second, how much does
the immunesystemcombinewith gastric acidand
intestinal motility to control the longitudinal dis-
tribution of microbial species in the gastrointes-
the extent to which the immune system also con-
trols microbial community composition and loca-
tion in other organ systems, such as the respiratory
tract, urogenital tract, and skin.
Looking Outside-In: How Microbiota
mice revealed a profound effect of microbial colo-
nizationon theformation oflymphoidtissues and
subsequent immune system development. It was
thus quickly apparent that the microbiota influ-
studies have greatly amplified this understanding
and have revealed some of the cellular and mo-
lecular mediators of these interactions (see Fig. 2).
The impact of the microbiota on lymphoid
structure development and epithelial function.
The tissues of the gastrointestinal tract are rich in
myeloid and lymphoid cells, many of which
reside in organized lymphoid tissues. It has long
been appreciated that the gut microbiota have a
critical role in the development of organized lym-
phoid structures and in the function of immune
system cells. For example, isolated lymphoid fol-
licles in the small intestine do not develop in
germ-free mice, and such mice are also deficient
in secretory IgA and CD8ab intraepithelial lym-
phocytes. The specific microbial molecules en-
dowed with this inductive function have not yet
been described, however.
Sensing of commensal microbiota through the
TLR-MyD88 signaling pathway triggers several
responses that are critical for maintaining host-
microbial homeostasis. The microbiota induce
repair of damaged intestinal epithelium through a
MyD88-dependent process that can be rescued in
microbe-depleted animals by gavage with bacterial
lipopolysaccharide (LPS). The innate signals, con-
enhance epithelial cell proliferation (34, 35). As
discussed above, MyD88-dependent bacterial sig-
nals are also required for the induction of epithelial
antimicrobial proteins such as RegIIIg (5, 19). This
lin (36). The flagellin signals are relayed through
in the lamina propria, stimulating production of IL-
23 that, in turn, promotes the expression of IL-22
by innate lymphoid cells (37). IL-22 then stimu-
lates production of RegIIIg, which is also secreted
upon direct activation of MyD88 in epithelial
cells (5, 20). This is one clear example of the
importance of commensals in the induction of host
innate responses, but it likely represents a tiny
the host immune system.
Microbiota shaping of T cell subsets. It has
species influence the makeup of lamina propria T
that produce interferon-g; TH17 cells that produce
IL-17a, IL-17f, and IL-22; diverse innate lymphoid
cells with cytokine effector features resembling
TH2 and TH17 cells; and anti-inflammatory Foxp3+
regulatory Tcells (Tregs). Colonization of mice with
segmented filamentous bacteria (SFB) results in
penetrate the mucus layer overlying the intestinal
epithelial cells in the terminal ileum, and they in-
teract closely with the epithelial cells, inducing host
cell actin polymerization at the site of interaction
and, presumably, signaling events that result in a
TH17 polarizing environment within the lamina
propria. There is little known about host cell
8 JUNE 2012VOL 336
on June 13, 2012
that SFB influence epithelial gene expression, re-
sulting, for example, in expression of antimicro-
bial proteins such as RegIIIg and of molecules
that participate in TH17 cell polarization. SFB
may also act directly on cells of the immune sys-
that extend processes through the epithelium to
the mucus layer or by production of metabolites
Other bacteria have been shown to enhance the
system by directing the differentiation of Tregsor by
inducing IL-10 expression. For example, coloniza-
tion of gnotobiotic mice with a complex cocktail of
XIVa of the Clostridium genus, results in the
expansion of lamina propria and systemic Tregs.
These have a phenotype characteristic of Tregsin-
duced in the periphery in response to transforming
to thymic-derived natural (n) Tregs(40)], and many
of these inducible Tregs(iTregs) express IL-10. The
exact Clostridial strains within the complex exper-
imental mixture that drive this regulatory response
remain to be defined. Furthermore, polysaccharide
A (PSA) of Bacteroides fragilis induces an IL-10
response in intestinal T cells, which prevents the
mucosal barrier (41). In contrast, mutant B. fragilis
to induce IL-10. Production of PSA by B. fragilis
has been proposed to be instrumental for the bac-
terium’s success as a commensal.
phoid cells and Tregcells can have a profound in-
elicit damage. The relative roles of commensal-
to study inflammation. For example, in mice sub-
jected to chemical or pathogen-induced damage to
the mucosa, TH17 cells have a beneficial effect that
promotes healing. In contrast, TH1 and TH17 cells,
as well as IL-23–dependent innate lymphoid cells,
promote colitis in models in which Tregcells are
depleted. It is likely that inflammatory bowel dis-
eases in humans can be similarly triggered by
commensal-influenced imbalance of lymphoid cell
including the strong linkage of IL23R polymor-
phisms with Crohn’s disease, a serious condition
with IL10 and IL10R mutations (42, 43).
Microbiota effects on systemic immunity. The
influence of commensal bacteria on the balance of
the intestinal lamina propria. Homeostatic T cell
proliferation itself is driven by the microbiota or
their penetrant molecules (44). Systemic auto-
links to infections, but firm evidence for causality
has been lacking. Recent studies in animal models,
however, have reinforced the notion that commen-
sal microbiota contribute to systemic autoimmune
and allergic diseases at sites distal to the intestinal
mucosa. Several mouse models for autoimmunity
are dependent on colonization status. Thus, germ-
free mice have marked attenuation of disease in
models of arthritis and experimental autoimmune
encephalomyelitis (EAE), as well as in various
colitis models. In models of TH17 cell–dependent
arthritis and EAE, monoassociation with SFB is
sufficient to induce disease (42, 45, 46). In all of
these models, induction of TH17 cells in the in-
testine has a profound influence on systemic dis-
consequence of an increase in the number of
arthritogenic or encephalitogenic TH17 cells that
traffic out of the lamina propria. The antigen spec-
ificity of such cells remains to be examined.
Induction of iTregsby the cluster IVand XIVa
Clostridia also has a systemic effect on inflamma-
these bacteria not only results in attenuated disease
after chemical damage of the gut epithelium but
also reduces the serum IgE response after immuni-
zation with antigen under conditions that favor a
TH2 response (40). As with pathogenic TH17 cells,
the antigen specificity of the commensal-induced
for distinct commensal bacteria (47).
Finally, B. fragilis PSA affects the develop-
germ-free mice with PSA-producing B. fragilis
results in higher numbers of circulating CD4+T
cells compared to mice colonized with B. fragilis
lacking PSA. PSA-producing B. fragilis also
elicits higher TH1 cell frequencies in the circulation
(48). Together, these findings show that commen-
sal bacteria have a general impact on immunity
that reaches well beyond mucosal tissues.
innate lymphoid cells. A recent study extends the
role of microbiota to the control of the function
invariant naturalkiller Tcells (iNKTcells), which
Fig.2.Looking outside-in: how microbiota shape host immunity. Some of the many ways that intestinal
microbiota shape host immunity are depicted. These include microbiota effects on mucosal as well as
systemic immunity. ILFs, isolated lymphoid follicles.
VOL 336 8 JUNE 2012
on June 13, 2012
The Gut Microbiota
bear an invariant Tcell receptor specific for lipid
antigens presented by the atypical class I mol-
ecule CD1d. Germ-free mice were found to have
increased susceptibility to iNKT cell–mediated
oxazolone-induced colitis and ovalbumin-induced
asthma. Unexpectedly, this effect could be reversed
only if mice were exposed to microbiota in the
neonatal period. The regulation of iNKT cell ex-
pansion was ascribed to reduced expression of the
chemokine CXCL16 in the presence of microbiota.
Thus, signals elicited by commensals may repress
systemic expression by epithelial cells of a chemo-
kine that interacts with CCR6 that is selectively
expressed by iNKTcells (49).
Innate lymphoid cells that produce either
IL-17 or IL-22 are protective against damage in
an innate model of colitis and during Citrobacter
rodentium enteric infection (50, 51). The extent
to which innate lymphoid cells are regulated by
the microbiota is not yet clear (52–54), but cryp-
topatches, which are formed by a subset of innate
lymphoid cells in the small intestine, differenti-
ate into isolated lymphoid follicles only when
commensals are present (55). Thus, it is likely
that, even if innate lymphoid cell numbers are
not influenced by commensals, their function
may be subject to microbiota signals.
Microbiota can trigger inflammation in immu-
nocompromised hosts. The commensal microbiota
clearly have important effects on the normal
development of immunity. However, commensal
bacteria can also trigger inflammatory responses in
immunodeficient hosts. For example, defective
signaling through the phosphatase SHP-1 causes a
microbiota-dependent autoinflammatory syndrome
with lesions on the feet, salivary glands, and lungs;
genic conditions of the nucleotide-binding oligo-
merization domain (NOD) receptor family (58)
considered to be autoinflammatory. One of the best
characterized of these is in the NLRP3 inflamma-
some protein (59). Depending on the exact acti-
vating mutation involved, the clinical spectrum in
episodic fevers occurring with unknown triggers,
and neonatal onset multisystem inflammatory dis-
gies is not yet clear, these outcomes are consistent
can cause dysbiosis of the intestinal microbiota
(61), as well as studies showing that TLR ligands
presence of activating NLRP3 mutations (62, 63).
a receptor for the muramyl dipeptide structural unit
of bacterial peptidoglycan, was the first susceptibil-
reinforced early clinical observations of the benefits
of surgically diverting the intestinal stream or treat-
ing with antibiotics, thus implicating intestinal mi-
crobes in the etiology (66, 67). More recent genetic
of human inflammatory bowel disease have re-
vealed a highly polygenic picture, with more than
70 loci described for Crohn’s disease alone. These
include modulators of the mucosal immune re-
sponse, proteins functioning in the epithelial stress
response, and the IL23R polymorphisms described
above (68, 69). However, the sum total of the con-
tributions of these loci to overall disease incidence
can be explained by phenotypes from private mu-
tations,such asthose affectingIL-10signaling (43),
thatare too infrequentto be detected by GWAS but
that disrupt host-microbial mutualism in animal
Microbiota can protect against autoimmune
disease. Type 1 diabetes (T1D) results from auto-
immune damage to the insulin-secreting islets of
dition is also shaped by the interactions between
immunity and the microbiota, but unlike EAE and
biota canprotectfromT1D.The nonobesediabetic
mouse is a good model of T1D with some genetic
predispositions similar to those in humans and
defined CD4 and CD8 diabetogenic Tcell popula-
nonobese diabetic mouse colony is dependent on
the housing conditions, because both the presence
of pathogens and microbiota diversity are deter-
mining factors (73, 74). In congenic nonobese
diabetic mice with a MyD88 adaptor deficiency
have the same frequency of diabetes as the parent
nonobese diabetic strain. In contrast, when colo-
ly protected from diabetes onset (75). MyD88-
deficiency has complex effects on host-microbial
mutualism, including increased access of intesti-
nal microbes to the epithelial surface, increased
system, and reduced costimulation in induction of
yet clear whether this is purely a failure of accu-
mulation of autoimmune T cells (77) or whether
there is also immune deviation arising from com-
mensal barrage that dilutes the frequency of
diabetogenic lymphocytes (26). Either way, it may
a higher frequency of autoimmune and allergic
disease in human populations (78).
Microbiota-immune system interactions and
(79), our bodies are bathed with microbial
embrace (12). Recent studies have shown that
the immune response to these microbial mol-
ecules profoundly impacts the metabolic health
of mammalian hosts.
Metabolic syndrome is a constellation of
abnormalities—including insulin resistance, obe-
sity, dyslipidemia, and hypertension—that pre-
disposes affected individuals to cardiovascular
disease and diabetes (80). It may seem counter-
intuitive to blame metabolic syndrome on our
intestinal microbiota rather than our modern
little exercise, but there is now good evidence
that how the immune system responds to the
These effects appear to be independent of the mi-
crobial contributions to energy harvest. In mice,
dysfunctional sensing of microbial molecular pat-
terns can cause low-grade intestinalinflammation.
As discussed above, mice lacking the bacterial
drome that are associated with changes in mi-
crobiota composition and can be acquired by
wild-type mice through microbiota transfer (31).
Similarly, mice lacking the inflammosome com-
ponents NLRP3 or NLRP6 exhibit a low-grade in-
testinal enteropathy dependent on overgrowth of
Prevotellaceae and Porphyromonadaceae mem-
bers of the Bacteroidetes (61). This results in TLR
agonist influx into the hepatic portal vein, which
supplies blood from the gut to the liver. Conse-
quently, TLR4 and 9 signaling increases tumor
humans, TNFa promotes insulin resistance and
accumulation of fat in hepatocytes. The metabolic
changes leading to fatty liver are transmissible to
wild-type mice upon microbiota transfer (61),
although the microbiota alterations may not per-
sist in the absence of innate immune deficiency.
Together, these examples show that innate im-
intestinal microbiota with downstream metabol-
ic consequences for the host.
The challenges for understanding host-microbial
human health. The first question is how much is
immune dysfunction a cause or consequence of
disease-associated alterations in the microbiota?
As discussed above, there is good evidence from
animal models that alterations in immunity can
cause dysbiosis and, conversely, that certain mi-
crobial species can trigger immunopathology in
the face of immunodeficiency. The idea that im-
man populations, and often presents in adults
with a limited range of opportunistic infections,
may extend to weak phenotypes of immune dys-
function exerting long-term inflammatory, meta-
bolic, or autoimmune effects through microbial
dysbiosis. We need a better understanding of the
mechanisms whereby altered immunity shapes
microbiota composition and determines which
microbes are present to embrace us with their me-
tabolites. Alternatively, given the pervasiveness of
this metabolic embrace, we need a better under-
standing of how the metabolic handshake shapes
the host immune system in health and disease.
The next issue is how to reproducibly and ef-
fectively advance the science of host-microbial
8 JUNE 2012VOL 336
on June 13, 2012
mutualism. Defined animal models are clearly es- Download full-text
sential for determining the relative contributions
lent resources are available to study the in vivo ef-
fects of host genetic changes, especially in mice,
and isogenic mouse models are widely shared
available for the microbiotas within these isogenic
mouse strains. Animals in the vivaria of different
institutions and sourced from different commercial
have likely contributed to conflicting reports with
biotas in isobiotic mice that can be shared between
wise, we may face confusion from variable results
crobiotas. Creating animals that are realistic models
of human disease is an even bigger challenge, re-
quiring us to reduce the species differences through
“humanized” immune systems combined with a
“humanized” gnotobiotic microbiota. The latter
bial species are likely adapted to specific hosts.
Many essential details of the immune response
to the microbiota remain to be discovered. For ex-
ample, how commensal microorganisms influence
the differentiation of immune cells, such as TH17
and Tregcells, remains to be unraveled. What mi-
What is the molecular basis for the differential
developmental pathways? Why do some commen-
sal species exert powerful effects on immune cell
differentiation, whereas other species are essentially
ignored, and is there a general mechanism of mu-
cosal adaptation or is it tuned to the microbial spe-
cies present? Can we explain the hygiene effect of
allergy and autoimmunity in terms of the T and B
cell repertoires being shaped by our microbiota?
of bacteria in the gastrointestinal tract. However,
other body surfaces, such as the skin, upper re-
spiratory tract, and urogenital tract, harbor rich
communities of indigenous microorganisms and
thus are also likely to be sites where such inter-
shape immune cell differentiation at these sites
as they do in the intestine, and is there crosstalk
the same time, gut microbiota also include vi-
ruses, bacteriophage (82), and eukaryotic orga-
nisms such as fungi. These other elements of the
lycoevolved relationships with the mammalian
immune system and will provide fascinating tar-
gets for further exploration of microbe–immune
Finally, little is known about the influences of
the microbiota in the critical window of immune
development in the neonatal period, despite the
fact that breastfeeding is one of the most effective
known health-promoting measures. Immediate
host microbiota and the immune system are less
clear. It will be important to determine how we
simultaneously transmit our microbiota to our off-
spring and shape the education of their immune
system through lactational effects on health.
It is now clear that the immune system plays a
central role in shaping the composition of the
At the same time, resident microbes provide sig-
nals that foster normal immune system develop-
Disruption of these complex and dynamic inter-
actions can have profound consequences for host
health. However, there are still major gaps in our
understanding of how the immune system regu-
lates the microbiota and of how the microbiota
shape host immunity. The questions that remain
are challenging and will require the innovation
of new tools and approaches. Ultimately, these
efforts should lead to deeper insight into host-
microbial relationships and provide exciting
new opportunities to improve human health.
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Acknowledgments: L.V.H. is supported by NIH grant
DK070855, a Burroughs Wellcome Foundation Investigators
in the Pathogenesis of Infectious Diseases Award, and the
Howard Hughes Medical Institute. D.R.L. is supported by
NIH grant AI080885 and the Howard Hughes Medical
Institute. A.J.M. is supported by Swiss National Science
Foundation grants 310030-1247324 and CRSII3_136286.
VOL 3368 JUNE 2012
on June 13, 2012