PRESS
`
`Cell
`
`Cell Host & Microbe
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`Intestinal Commensal Microbes as Immune Modulators
`
`lvaylo l. lvanov1 -* and Kenya Honda2-3-*
`1Department of Microbiology and Immunology, Columbia University Medical Center, New York, NY 10032, USA
`2Department of Immunology, Graduate School of Medicine, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
`3RIKEN Research Center for Allergy and Immunology, 1-7-22 Suehiro-cho, Tsurumi-ku, Yokohama, Kanagawa 230-0045, Japan
`*Correspondence: ii2137@columbia.edu (l.l.l.), kenya@m.u-tokyo.ac.jp (K.H.)
`http://dx.doi.org/10.1016/j.chom.2012.09.009
`
`Commensal bacteria are necessary for the development and maintenance of a healthy immune system.
`Harnessing the ability of microbiota to affect host immunity is considered an important therapeutic strategy
`for many mucosal and nonmucosal immune-related conditions, such as inflammatory bowel diseases (IBDs),
`celiac disease, metabolic syndrome, diabetes, and microbial infections. In addition to well-established
`immunostimulatory effects of the microbiota, the presence of individual mutualistic commensal bacteria
`with immunomodulatory effects has been described. These organisms are permanent members of the
`commensal microbiota and affect host immune homeostasis in specific ways. Identification of individual
`examples of such immunomodulatory commensals and understanding their mechanisms of interaction
`with the host will be invaluable in designing therapeutic strategies to reverse intestinal dysbiosis and recover
`immunological homeostasis.
`
`Introduction
`
`Mucosal surfaces are colonized by a complex and dynamic
`microbial ecosystem termed “microbiota." An astounding
`number and diversity of microbes,
`including fungi, bacteria,
`archaea, and viruses, are present
`in any given moment
`throughout our bodies. The vast majority of these organisms
`are not disease-causing invaders, but have made the host their
`one-and-only home. Through coevolution,
`these “commen-
`sals” have established one of the most impressive examples
`of mutualistic relationship in the natural world,
`in which both
`microbes and their animal host depend on each other for
`optimal survival. Indeed, we rely on our microbiota for many
`basic physiologic and metabolic functions, as well as proper
`immune functions. Commensals provide immune protection in
`several ways. They can defend their mucosal home by directly
`combating invading pathogens or by mobilizing host antimicro-
`bial immune defenses. They can also affect host immunity in a
`more inconspicuous, but equally important, way by directing
`the development of host immune cell subsets at steady state
`and therefore affecting mucosal and systemic innate or
`adaptive immune responses. Recent studies have identified
`examples of commensal bacterial species with such immuno—
`modulatory roles. The existence of multiple members of the
`microbiota that affect host immune homeostasis in different
`
`ways means that differences in the composition of this com-
`munity may contribute to individual differences in immune
`responses during infection, autoimmunity, cancer, or other
`immunological conditions. Unveiling the underlying cellular
`and molecular mechanisms of each of these examples holds
`the promise to lead to exciting new ways for regulating mucosal
`immunity.
`
`Microbial-Host Interactions in the Intestine
`
`Decades of studies in germ-free (GF) animals have established
`the importance of microbiota for proper host immune function
`(Macpherson and Harris, 2004). GF animals were first created
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`more than a century ago (Nuttal and Thierfeledr, 1895—1896),
`and long-term husbandry of GF rats has been possible since
`the 19403 (Reyniers et al., 1946). However, until recently, the
`composition of gut microbial communities remained largely
`unknown. Advances in high-throughput sequencing in the last
`few years have led to extensive cataloguing of the human
`microbiota (Human Microbiome Project Consortium, 2012).
`In
`addition to correlating changes in microbiota composition with
`disease, this has allowed for the identification of commensal
`species with specific immune effects. The vast majority of
`these studies have focused on the bacterial component of the
`microbiota; however, characterization of the fungal and viral
`components and their function is under way (lliev et al., 2012;
`Reyes et al., 2012).
`It
`The individual's microbiota composition is dynamic.
`changes with age and fluctuates with environmental changes,
`such as geographical location, diet, antibiotic use, or influx and
`efflux of external microbes (Clemente et al., 2012). In addition,
`vastly different microbial communities reside in different parts
`of the body (Costello et al., 2009). Based on their colonization
`ability, bacteria in the gut can be transient or permanent. Tran-
`sient bacteria represent microbes that are introduced during
`adult life from the external environment and do not permanently
`colonize the intestinal tract for various reasons, such as lack of
`appropriate adaptations for colonization or inability to compete
`with the resident microbiota. Many food-associated microbes,
`including pathogens and conventional commercial probiotics,
`are part of this category. Transient organisms can affect the
`immune system in different ways and be innocuous, pathogenic,
`or even beneficial, e.g., ingestion of probiotic-containing foods.
`These organisms have not coevolved and therefore do not
`establish a mutualistic relationship with the host, but rather
`try to survive in the gut environment despite established host
`defenses. Indeed, many acute intestinal pathogens have devel—
`oped strategies to forcefully colonize the intestine, which in
`most cases induces a strong immune response aimed at clearing
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`Table 1 . Nonpathogenic Bacterial Members of the Intestinal Microbiota with Immune Effects
`Association
`
`Probiotics
`
`Autobionts
`
`Concept
`o Confer health benefit to
`the host when administered
`in adequate amount
`0 Not necessarily part of
`the “normal microbiota”
`0 May affect beneficial
`microbiota (indirect effects)
`0 Direct influence on host
`immune cell homeostasis
`or function
`0 Part of the ”normal
`microbiota"
`
`Examplesa
`Bifidobacterium spp;
`Lactobacillus spp
`
`with Host
`Transient
`
`Immune Effects
`Innocuous,
`immunostimulatory
`
`Mechanismsa
`Cytokine induction,
`TLR activation, pathobiont
`and pathogen suppression,
`lactic acid, short-chain
`fatty acids
`
`Bacteroides fragilis,
`Clostridia XIVa and IV,
`SFB, Faecalibacterium
`prauznitsii
`
`Permanent, host
`dependent,
`symbiotic
`
`Immunomodulatory
`
`Largely unknown (TLR2,
`metabolites [?], antigens [?],
`effects on IEC function [?])
`
`Pathobionts
`
`Helicobacter hepaticus,
`Clostridium difficile,
`Prevotela spp.,
`KIebsie/Ia spp.,
`Bilophila wadsworthia
`
`Permanent,
`parasitic/
`infectious
`
`0 Do not cause disease
`in the presence of normal
`microbiota in healthy host
`0 Cause disease when
`microbiota or host
`immunity is perturbed
`(?), speculative mechanisms.
`aExamples and mechanistic studies have been performed in mice (with the exception of F. prauznitsii and C. difficile). See text for details and
`references.
`
`Innocuous,
`detrimental
`
`Invasive mechanisms,
`spore formation, toxins
`
`the pathogen. The pathologic immunological consequences of
`this inflammatory response may persist for years after the clear-
`ance of the transient organisms, as in postinfectious irritable
`bowel syndrome (Spiller and Garsed, 2009).
`In contrast to transient bacteria, permanent bacteria are long-
`term members of the microbial community. Their colonization
`occurs in successive waves during ontogeny, and they have
`developed evolutionary adaptations to establish a permanent
`relationship with the host. In most cases they have coevolved
`with the host and are not normally found as free-living organisms.
`These are the true commensal bacteria. Commensals have mul-
`
`tiple effects on the immune system of the host. On one hand,
`the presence of a large number of “innocuous” bacteria has
`immunostimulatory effects. Commensals stimulate general
`recruitment of immune cells to the mucosa, as well as generation
`and maturation of organized gut-associated lymphoid tissues
`(Macpherson and Harris, 2004). They also stimulate protective
`epithelial cell
`functions, such as mucus and antimicrobial
`peptide secretion (Hooper and Macpherson, 2010). On the other
`hand, recent studies have identified the presence of commensal
`species with immunomodulatory effects. These effects are
`specific for individual bacteria or groups of bacteria,
`i.e., for
`specific components of the microbiota. They involve reversible
`changes in differentiation or effector function of host immune
`cell subsets. In this way, microbiota composition can influence
`the type and robustness of host immune responses. Here, we
`refer to such permanent microbiota members with immunomod-
`ulatory effects as autobionts. In contrast to transient pathogens
`or pathobionts (see below), the immune effects of autobionts are
`more subtle because they do not cause any overt change in the
`health state of the host. Rather, they help maintain and regulate
`the host’s healthy immune steady state. Permanent microbiota
`members that can demonstrate detrimental effects under
`
`special conditions are called pathobionts. Pathobionts colonize
`
`the host, but do not cause disease with a full complement of
`normal microbiota. However,
`they can expand and cause
`disease if microbiota or host immune homeostasis is perturbed
`(for example, after antibiotic treatment or under conditions of
`intestinal inflammation). The immune effects and characteristics
`of different intestinal bacteria are summarized in Table 1.
`
`In contrast to pathogens, pathobionts, and even probiotics,
`very little is known about the mechanisms by which autobionts
`exert their immunomodulatory effects. Until recently, one reason
`for this was the lack of specific examples of immunomodulatory
`commensals. Another reason is the difficulty in culturing these
`organisms ex vivo and the relative lack of genetic tools to study
`their genome function.
`In this review we focus on recently
`described examples of immunomodulatory commensals and
`speculate on potential cellular and molecular mechanisms
`involved in their interaction with the host and the establishment
`
`of a healthy steady immune state. Knowledge of these mecha-
`nisms can benefit the development of future therapies for intes-
`tinal diseases (Clemente et al., 2012).
`
`The Intestinal Microbial Community Influences
`Immunity
`During steady-state conditions, the microbiota affects the devel-
`opment and function of various immune cell populations,
`including lgA—secreting plasma cells, Th17 cells, regulatory T
`(Treg) cells, invariant natural killer T (iNKT) cells, yET cells, NK
`cells, macrophages, dendritic cells (DOS), and innate lymphoid
`cells (lLCs) (Honda and Littman, 2012). For example,
`lgA+
`plasma cells in gut lymphoid tissues and lamina propria (LP)
`are greatly reduced in GF or antibiotics-treated conventional
`animals (Macpherson and Harris, 2004).
`In another example,
`the abundance and function of CD4+ T cells expressing the
`FoxP3 transcription factor (Tregs) or interleukin (lL)-17 (Th17
`cells) in the intestinal mucosa at steady state are affected by
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`the microbiota. In GF or antibiotic-treated mice, the percentages
`of both Tregs and Th17 cells are markedly reduced, and expres-
`sion of the immune-suppressive cytokine lL-10 in Treg cells is
`severely reduced (Atarashi et al., 2008, 2011;
`lvanov et al.,
`2008). These reductions are quickly restored by transplantation
`of intestinal or fecal microbiota from conventionally raised/
`specific pathogen-free (SPF) mice. As described below, the
`development of these CD4+ T cell subsets is differentially regu-
`lated by certain components of
`the intestinal microbiota
`including Bacteroides fragilis, Clostridia species, and segmented
`filamentous bacteria (SFB) (Atarashi et al., 2011; lvanov et al.,
`2009; Round and Mazmanian, 2010) (Table 1). Th17 cells
`contribute to host defense against infection with pathogenic
`microbes but can also augment harmful autoinflammatory func—
`tions, whereas Tregs play critical roles in immune suppression.
`Therefore, under healthy conditions Th17 cells and Tregs should
`coexist in a well-regulated balance. The particular combinations
`and relative abundances of the corresponding commensal
`species would generate distinct
`immune environments and
`immune responses in the host.
`In addition to inducing development or recruitment of host
`immune cell subsets, microbiota may also affect the function
`of these subsets. For example, gut microbiota provides an envi-
`ronment not only for the accumulation of lgA+ cells but also for
`the functional maturation of lgA+ plasma cells by inducing the
`generation of iNOS+ lgA+ plasma cells (Fritz et al., 2012).
`iNOS+ lgA+ plasma cells are absent in GF mice but present in
`conventionally raised mice. This subset plays critical roles in
`the enhancement of lgA+ plasma cell development and the
`host defense against enteric pathogens, such as Citrobacter
`rodentium (Fritz et al., 2012). Commensal bacteria also affect
`NK cell function. Even though NK cell numbers are normal in
`GF mice, NK cell priming and antiviral activity is deficient in the
`absence of microbiota (Ganal et al., 2012). This effect is due to
`microbiota-directed introduction of epigenetic changes and
`induction of type | interferons from monocytic macrophages,
`which are required for proper NK cell priming (Ganal et al., 2012).
`Microbiota-controlled immune effects may also play role in
`regulation of the microbiota itself. Commensal-induced lgA+
`plasma cells contribute to controlling microbiota abundance
`and composition. For instance, mice carrying a knockin mutation
`of activation-induced cytidine deaminase (AID) (AlDGzas), which
`can mediate normal
`lgA class switching but cannot induce
`somatic hypermutation and high-affinity lgA responses, exhibit
`excessive proliferation of anaerobic bacteria in the small intes-
`tine (Wei et al., 2011). Similar overgrowth of microbiota was
`also reported in programmed cell death-1 (PD-1)—deficient
`mice, which have increased follicular helperT (TFH) cell develop-
`ment, and therefore low-affinity lgA—producing plasma cells are
`aberrantly selected in germinal centers (Kawamoto et al.,
`2012). Thus, microbiota is required for the development of fully
`functional lgA+ cells, which in turn function to maintain microbial
`homeostasis in the gut.
`The microbiota also plays suppressive roles in immune cell
`function and accumulation in the gut. For example, it represses
`constitutive production of lL-22 in lymphoid tissue inducer (LTi)
`cells and NKp46+ cells (both of which are FiOFtyt+ lLCs; see
`detailed review by Tait Wojno and Artis, 2012 in this issue of
`Cell Host & Microbe) through epithelial expression of lL-25
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`(Sawa et al., 2011). Another example is that early exposure to
`gut microbiota provides an epigenetic suppressive marking in
`the regulatory element of CXCL16 gene in the host, resulting in
`lifelong suppression of CXCL16 expression in gut and lung (Ols-
`zak et al., 2012). This suppression is accompanied by reduced
`abundance of iNKT cells in the colon and lung at steady state
`and affects host resistance to colitis and asthma (Olszak et al.,
`2012). Taken together,
`intestinal microbiota provide diverse
`signals for activation and suppression of the immune system,
`thereby having the ability to skew host immune status toward
`either effector or regulator dominance.
`Commensals have immunomodultory functions not only in the
`intestine. For example, resident commensals in the skin induce
`local Th17 and Th1 responses that are crucial
`in protection
`from bacterial infections (Naik et al., 2012). More importantly,
`these responses were compartmentalized to the skin and were
`independent of gut commensals (Naik et al., 2012).
`Commensals influence pathogenesis of many diseases.
`Because this community is evolutionarily established to sustain
`healthy immune steady state, any major perturbations in its
`composition may have negative effects and perpetuate the cycle
`of chronic inflammation, allergy, or metabolic syndrome. Such
`”dysbiosis” can be induced by diet, pharmacological agents,
`infection, inflammation, and host genetics (Honda and Littman,
`2012). Once established, the dysbiotic microbiota may become
`stable and transplantable to GF or even conventionally raised
`animals. Indeed, cohousing of wild-type mice with disease-
`prone mutant mice, such as Tbet‘l‘Fi’ag‘l‘, erp6‘l‘, or Ase"—
`mice, results in transfer of dysbiotic microbiota and predisposi-
`tion of the wild-type mice to disease,
`including colitis and
`metabolic syndrome (Elinav et al., 2011; Garrett et al., 2007;
`Henao-Mejia et al., 2012).
`Dysbiosis may lead to elimination of beneficial bacteria or
`outgrowth of pathobionts. Pathobionts are permanent members
`of the microbiota, present at low levels and innocuous under
`normal conditions (Table 1). They can become pathogenic if al-
`lowed sufficient expansion due to loss of microbiota or immune
`homeostasis. For instance, overgrowth of members of Prevotel-
`laceae and TM7 has been implicated in host susceptibility
`to DSS colitis in an6-/- mice (Elinav et al., 2011). In Their-’-
`Rag”’ mice, Proteus mirabilis and Klebsiella pneumoniae
`were identified to, at least in part, be responsible for the pheno-
`type of spontaneous colitis (Garrett et al., 2010). Most impor-
`tantly, disease susceptibility was transferable to wild-type mice.
`The composition of the gut microbiota is also altered by
`diet. Milk fat- and taurocholic acid-rich diets induced marked
`
`increase in Bilophila wadsworthia colonization, which is associ-
`ated with enhancement of Th1 responses and acceleration
`of colitis development in II10"‘ mice (Devkota et al., 2012).
`It should be noted that, depending on the host genotype, other-
`wise innocuous symbionts may become pathogenic. Indeed,
`in an inflammatory bowel disease (IBD) mouse model with
`deficiencies in lL-10 and TGF-B signaling, Bacteroides thetaio-
`taomicron, a well-characterized symbiotic species, potently
`induces colitis (Bloom et al., 2011).
`
`Autobionts—Mutualistic lmmunomodulatory Microbes
`As discussed above, there is abundant evidence that microbiota
`directs host
`immunity. Regulation of immune responses by
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`influencing development, differentiation, or effector function of
`different cells of the immune system is especially interesting,
`because it results not simply from the presence of innocuous
`bacteria, but from the biological activity of commensals.
`In
`many cases these effects are functionally distinct, e.g., induction
`of Th1 7 cells versus Tregs, and depend on the activity of different
`members of the commensal community. The relative abundance
`of these “autobionts” can direct the general type of immunity
`in the host mucosa of an individual at a given time. Currently
`there are relatively few specific examples of commensals with
`immunomodulatory effects as discussed below. More are surely
`going to be identified in the future.
`Despite considerable species diversity, the intestinal micro-
`biota in most mammals consists of bacteria belonging to two
`major phyla—Gram-negative Bacteroidetes and Gram-positive
`Firmicutes. This probably reflects evolutionary adaptations of
`these phyla to survive in the gut environment, and both of
`them contain important immunomodulatory commensals.
`
`Bacteorides fragilis Promotes Treg Function
`B. fragilis was the first commensal to be implicated in affecting
`T helper cell balance by promoting Th1 development systemi-
`cally (Mazmanian et al., 2005). However, further studies showed
`that B.
`fragilis also affects mucosal T cell homeostasis by
`promoting regulatory T cell function (Round and Mazmanian,
`2010). B. fragilis is a Gram-negative member of the phylum
`Bacteroidetes.
`It
`is not a very abundant member of the gut
`microbiota; however, the genus Bacteroides is well represented
`in human gut and has a superior ability to utilize the nutrients in
`the gut microenvironment (Flint et al., 2008). For example, the
`genome of the prototypical dominant commensal member
`of the class, B.
`thetaiotaomicron, contains several hundred
`proteins involved in harvesting and metabolizing of dietary poly-
`saccharides (Sonnenburg et al., 2005). These are probably
`adaptations that have allowed Bacteroides to establish mutual-
`istic relationship with the host, by (1) being able to flourish in
`the plant polysaccharide-enriched gut environment and (2) being
`able to provide biological “byproducts" necessary for the well-
`being of the host. Different Bacteroidetes species produce
`different beneficial biological byproducts, which may have
`helped establish them as permanent symbionts. For example,
`B.
`thetaiotaomicron is one of the major producers of short-
`chain fatty acids (SCFAs), which are necessary for proper host
`metabolic and immune functions. B.
`fragilis seems to have
`pronounced immunomodulatory functions. B.
`fragilis is not
`normally present in conventionally raised SPF mice, and coloni-
`zation with B. fragilis protects mice from colitis in the T cell
`transfer and 2,4,6-trinitrobenzene sulfonic acid (TNBS) models
`(Mazmanian et al., 2008). This protection is due to the expansion
`of immune-suppressive IL-10 producing Tregs by the bacteria
`(Round and Mazmanian, 2010). The introduction of B. fragilis
`as a permanent member of the microbiota also affects T cell
`homeostasis in the absence of inflammation. Colonization of
`
`SPF or GF mice with B. fragilis leads to an induction of lL-10
`production by Foxp3+ Tregs even at steady state (Round and
`Mazmanian, 201 0). Thus, B. fragilis colonization modulates intes-
`tinal T cell homeostasis by boosting Treg function. This anti-
`inflammatory effect of the bacteria likely represents an evolu-
`tionary adaptation for establishing mutualism. Indeed, when
`
`
`
`Tregs were depleted, B. fragilis could not efficiently colonize
`host
`tissues (Round et al., 2011). Most
`importantly,
`the
`identification of B.
`fragilis as a modulator of Treg function
`allowed for investigation of the bacterial and host mechanisms
`involved.
`It was found that the anti-inflammatory effects of
`B. fragilis require the expression of bacterial capsular polysac-
`charide A (PSA). PSA-deficient B. fragilis mutants were incapable
`of inducing IL-10 production by Tregs and did not provide
`protection from colitis (Mazmanian et al., 2008; Round and
`Mazmanian, 2010). Instead, lack of PSA expression led to expan-
`sion of Th1 7 cells and loss of the mutualistic ability of B. fragilis to
`colonize host tissues (Round et al., 201 1). Moreover, treatment of
`mice with purified PSA is sufficient to replicate the effects of the
`bacteria,
`including induction of lL-10 production by Tregs,
`suppression of Th1 7 cell production, protection in colitis models,
`and colonization of the host (Mazmanian et al., 2008; Round
`et al., 2011; Round and Mazmanian, 2010). These studies re-
`present an elegant example of how investigating an immu-
`nomodulatory commensal and its effect on the host immune
`system can lead to elucidation of underlying molecular mecha-
`nisms and identification of clinically relevant immunomodulatory
`molecules.
`
`Cluster IV and XWa Clostridia Induce Treg
`Differentiation
`
`Intestinal Clostridia are a heterogeneous group that forms the
`core of Firmicutes of the normal commensal microbiota. Clostri-
`
`dia are Gram-positive, rod-shaped, endospore-forming bacteria.
`They are a highly heterogeneous class that is composed of at
`least 19 clusters based on genomic similarity (Collins et al.,
`1994). The prototypical Clostridia from cluster I contain frequent
`environmental toxin-producing members, such as Clostridium
`perfringens, C. difficile, and C. tetani. These Clostridia may be
`present in the intestine, but usually as transient pathogens or,
`at best, pathobionts (e.g., C. difficile).
`In contrast, most com-
`mensal intestinal Clostridia are nontoxinogenic members of clus-
`ters XIVa and IV. They are typically described as fusiform-shaped
`bacteria and constitute 10%—40% of the total microbiota (Frank
`et al., 2000. Cluster XIVa includes the genera Clostridium, Eu-
`bacterium, Ruminococcus, Coprococcus, and Roseburia. The
`cluster IV group includes species belonging to the Clostridium,
`Faecalibacten'um, and Ruminococcus genera. Clostridia colo-
`nize the mucus layers in the vicinity of the epithelium, in contrast
`to Bacteroidaceae, Enterococcaceae, and Lactobacillaceae,
`which colonize in regions ofthe central lumen, suggesting unique
`influences of Clostridia on host physiology (Nava and Stappen-
`beck, 2011). Indeed, the cluster XIVa Lachnospiraceae family
`is significantly less abundant
`in IBD patients compared to
`healthy subjects (Frank et al., 2007). Loss of mucosa-associated
`Clostridia and cluster IV Clostridia, particularly Faecalibacterium
`prausnitzii,
`is observed in IBD patients (Sokol et al., 2008).
`Although it remains unclear whether the decrease in Clostridia
`is a cause or effect of chronic inflammation, it is likely that main-
`tenance of the Clostridia community is necessary to prevent IBD.
`In addition to the role in the intestinal (local) immune homeostasis,
`Clostridia also affect systemic immunity. Indeed,
`it has been
`shown that reduction of Clostridia clusters XIVa and IV by neo-
`natal vancomycin treatment promotes airway hypersensitivity in
`a mouse model (Russell et al., 2012). Furthermore, decreased
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`abundances of clusters IV and XIVa Clostridia have been associ-
`
`ated with atopy during childhood (Candela et al., 2012).
`The levels of Clostridia clusters IV and XIVa in adult mouse
`colon microbiota were found to correlate with the numbers of
`
`Tregs (Atarashi et al., 2011; Russell et al., 2012). Consistent
`with a role in colonic Treg induction, GF mice colonized with
`46 strains of Clostridia clusters XIVa and IV showed accumula-
`
`tion of Treg cells in the colon (Atarashi et al., 201 1). The 46 strains
`of Clostridia were originally isolated from the sporulating micro-
`biota fraction of conventional mice based on their capacity to
`normalize the enlarged cecum in GF mice (Itoh and Mitsuoka,
`1985). These Clostridia were found to induce near steady-state
`numbers of colonic Tregs in GF mice, in contrast to other intes-
`tinal bacteria including Treg-associated autobionts such as
`B. fragilis (Atarashi et al., 2011). Whereas most Treg cells in the
`colon of GF mice were Helios”, which has been proposed as
`a marker for thymically derived Treg cells (Thornton et al.,
`2010), Treg cells in mice colonized with 46 strains of Clostridia
`clusters XIVa and IV were mostly Helios'° (Atarashi et al., 2011),
`suggesting that Clostridia robustly trigger peripheral differentia-
`tion of induced Treg (iTreg) cells. Consistent with these findings,
`colonization of CF mice with altered Schaedler flora (ASF),
`a defined bacterial cocktail containing eight enteric species
`that includes Clostridium clostridioforme, induces the accumula-
`tion of Treg cells in the colon (Geuking et al., 2011). Treg cells in
`the intestine exhibit different characteristics from those in
`
`secondary lymphoid organs, and they express CD103, killer
`cell
`Iectin-Iike receptor G1
`(KLRG1), granzyme B (szb),
`IL—10, and IL—35 (Feuerer et al., 2010). In particular, IL—10 plays
`an indispensable role in the suppression of aberrant activation
`of Th1 7 cells, myeloid cells, and y6T cells in the intestine. Indeed,
`Treg-specific disruption of IL-1 0 results in severe colitis (Rubtsov
`et al., 2008), as does a Treg-specific deficiency of STATS, which
`regulates many of the above-mentioned genes (Chaudhry et al.,
`2009). STAT3-deficient Tregs lack IL-10 expression and the
`mice develop spontaneous, Th17-mediated, commensal micro-
`biota-dependent fetal colitis (Chaudhry et al., 2009). Coloniza-
`tion of GF mice with the 46 strains of Clostridia clusters XIVa
`
`and IV induced not only an increase in Treg numbers but also
`high levels of IL-10 production (Atarashi et al., 2011). In humans,
`F. prausnitzii, which belongs to Clostridium cluster IV, increases
`IL-10 expression in peripheral blood mononuclear cells in vitro
`(Sokol et al., 2008). Therefore, autochthonous Clostridia con-
`stitutively induce accumulation and functional activation of
`Tregs in the colon and the relative abundance of Clostridia
`in the microbiota may strongly affect the immune status of the
`host.
`
`SFB Induce Th17 Cell Differentiation
`T cell homeostasis in the intestine can be defined as the balance
`
`between T cell subsets that promote immune responses and
`T cell subsets that subdue immune responses. IL-17-producing
`Th17 cells are proinflammatory CD4 T cells that contribute to
`disease pathogenesis in a number of chronic autoimmune
`inflammatory conditions, including IBD, multiple sclerosis, rheu-
`matoid arthritis, psoriasis, and certain cancers. At the same time,
`Th17 cells are crucial for efficient immune responses against
`mucosal pathogens,
`including viruses, bacteria, and fungi.
`Th17 cells differentiate from naive T cells under the combined
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`500 Cell Host & Microbe 12, October 18, 2012 @2012 Elsevier Inc.
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`Cell Host & Microbe
`
`effects of TGF-B and proinflammatory cytokines, such as IL-6,
`IL-23, and IL-18. The latter group of cytokines is upregulated in
`secondary lymphoid tissues during certain infections, which
`instructs Th17 cell differentiation in this context. However, at
`steady state, in the absence of infection or overt inflammation,
`Th17 cells are highly enriched in the intestinal LP and are not
`present in secondary lymphoid tissues (Ivanov et al., 2006). In
`the gut, Th17 cells coexist with Foxp3+ Tregs. The two subsets
`share common developmental pathway and alternative differen-
`tiation fates. This includes shared dependence on TGF—B and
`direct interaction and mutual functional
`inhibition of the two
`
`(Th17 cells) and
`regulators, RORyt
`master transcriptional
`FoxP3 (Tregs) (Zhou et al., 2008). This overlap of cytokine and
`transcriptional networks results in an elegant balance between
`two functionally opposing T cell subsets.
`Importantly,
`this
`balance is flexible and can be quickly reversed depending on
`the required immune response. One of the most
`important
`factors controlling the homeostasis between these two T cell
`subsets in the gut is the composition of intestinal microbiota.
`Th17 cells are not present in GF mice but are induced upon
`colonization with the full complement of gut bacteria from SPF
`mice (Atarashi et al., 2008; Ivanov et al., 2008).
`In contrast,
`colonization with culturable intestinal isolates, including Treg—
`inducing commensals, such as B. fragilis or the mix of 46 Clostri-
`dia described above, does not lead to Th17 cell induction (Ivanov
`et al., 2009; Round et al., 2011), arguing that commensal micro—
`biota contains unknown Th17 cell-inducing bacteria. This was
`further supported by the discovery that CS7BL/6 mice from
`a colony at the Jackson Laboratory did not contain gut Th1 7 cells
`due to the lack of Th17-celI-inducing bacteria and that total
`microbiota from these mice could not induce Th17 cells in CF
`animals,
`in contrast to microbiota from CS7BL/6 mice from
`a colony at Taconic Farms (Ivanov et al., 2008). Comparison of
`the microbiota between these two colonies revealed that
`
`Taconic B6 mice are highly enriched in SFB, which are absent
`from Jackson B6 mice (Ivanov et al., 2009). Interestingly, mono-
`colonization of GF animals with SFB or introduction of SFB into
`Jackson B6 mice induces Th17 cell differentiation in the LP
`
`(Gaboriau-Routhiau et al., 2009; Ivanov et al., 2009), identifying
`SFB as a Th17-cell-inducing autobiont.
`SFB are Gram-positive anaerobic bacteria that are known to
`permanently colonize the intestinal tract of many animal species.
`SFB or SFB-like bacteria have been described in invertebrates,
`such as termites and cockroaches, and in vertebrate animals
`such as fish, chickens, rabbits, mice, rats, cats, dogs, sheep,
`cows, pigs, zebras, and monkeys (Klaasen et al., 1992). How-
`ever, SFB have not yet been detected in humans (Sczesnak
`et al., 2011). SFB are members of the Firmicutes phylum and,
`based on the 168 rRNA sequence, were assigned to Clostridia
`(Snel et al., 1995). The similarity to Clostridia was later confirmed
`by the full SFB genomic sequence (Sczesnak et al., 2011). In
`fact, more than