Interleukin-22 mediates early host defense against attaching and effacing bacterial pathogens

Community-wide response of the gut microbiota to enteropathogenic Citrobacter rodentium infection revealed by deep sequencing

We investigated the spatial and temporal response of the murine gut microbiome to infection with Citrobacter rodentium, an attaching-and-effacing bacterium that provokes innate and adaptive immune responses, resulting in transient bacterial colitis. Previous studies have suggested that C. rodentium-induced inflammation is associated with an increased abundance of Enterobacteriaceae. We report here a deeper analysis of this model using DNA bar coding and 454 pyrosequencing to characterize 101,894 partial 16S rRNA gene sequences from 85 microbial samples from tissue-adhered and luminal bacteria of the cecum, proximal colon, and distal colon, which allowed us to identify previously unappreciated spatial and kinetic changes in multiple bacterial lineages. The deep sequencing data revealed that C. rodentium was most abundantly associated with the cecal mucosa at day 9 postinfection and then diminished in abundance, providing the first reported use of deep sequencing to track a pathogen in vivo through the course of infection. Notable changes were associated with both the mucosally adhered and luminal microbiota at both day 9 and day 14 postinfection. Alterations in abundance were seen for Proteobacteria, Deferribacteres, Clostridia, and others; however, changes in Enterobacteriaceae could be accounted for by the presence of C. rodentium itself, which is a member of this family. The Lactobacillus group decreased in abundance during infection, which may be important for pathogenesis because members of this lineage modulate the composition of the gut microbiota and are used as probiotics. Thus, deep sequencing provides previously inaccessible information on how Citrobacter infection and clearance reshapes the gut microbial community in space and time.

Th17 cytokines and host-pathogen interactions at the mucosa: dichotomies of help and harm

The mucosal surfaces are often the first site of interaction between pathogenic microorganisms and the host. Activation of the mucosal immune response has the important function of containing an infection and preventing dissemination of pathogens to systemic sites (barrier function). Numerous lines of evidence suggest that the barrier function is orchestrated by a subset of cytokines (interleukin (IL-)17 and IL-22), which belong to the Th17 family. IL-17 and IL-22 induce expression of antimicrobial peptides and neutrophil chemoattractants at mucosal sites, and thus play an important role in controlling mucosal infections. However, there is increasing evidence that mucosal pathogens achieve greater colonization during inflammation because they are resistant to a subset of these antimicrobial responses. In this review we compare the antimicrobial responses elicited by Th17 cytokines during mucosal infections with four different pathogens: Klebsiella pneumoniae, Citrobacter rodentium, Candida albicans and Salmonella typhimurium. We will then discuss which responses may constitute the mucosal barrier, thus providing a benefit to the host, and which ones may promote the colonization of pathogens, thereby providing a benefit to the microbes.

New activation modus of STAT3: a tyrosine-less region of the interleukin-22 receptor recruits STAT3 by interacting with its coiled-coil domain

Activation of STAT proteins by cytokines is initiated by their Src homology 2 domain-mediated association with phosphotyrosine residues from the cytoplasmic domain of a receptor. Here, we show that the C terminus of the interleukin-22 receptor (IL-22R) recruits in a tyrosine-independent manner the coiled-coil domain of STAT3. Mutation of all IL-22R cytoplasmic tyrosines did not abolish activation of STAT3, in contrast to that of STAT1 and STAT5. Coimmunoprecipitation and glutathione S-transferase pulldown experiments showed that the coiled-coil domain of STAT3 is constitutively associated with the C-terminal part of IL-22R, and a chimeric STAT3-STAT5 protein containing the coiled-coil domain of STAT3 could be activated by this tyrosine-independent mechanism. Deletion of the C-terminal part of IL-22R dramatically decreased its ability to activate STAT3 and to mediate IL-22 activity in cell lines, demonstrating that preassociation of STAT3 with this cytokine receptor, independent from the interaction between the Src homology 2 domain and phosphotyrosines, is required for its full activity.

Interleukin-17 and systemic lupus erythematosus: current concepts

The emerging role of interleukin (IL)-17 as a hallmark proinflammatory cytokine of the adaptive immune system, produced primarily by a new T helper cell subset termed 'Th17', has received considerable attention. Differentiation of Th17 cells is driven by the simultaneous presence of transforming growth factor-beta and certain inflammatory cytokines (e.g. IL-6, IL-21), and recent studies have shown that inflammation instigated by IL-17-producing cells is central to the development and pathogenesis of several human autoimmune diseases and animal models of autoimmunity. In this review, we focus on the information regarding IL-17 and systemic lupus erythematosus (SLE), a chronic autoimmune disease. The work that has explored the development and behaviour of IL-17-producing cells in SLE is discussed, and different mechanisms by which IL-17 could potentially augment inflammation and autoantibody production in the context of SLE are proposed.

Hidden talents of natural killers: NK cells in innate and adaptive immunity

Natural killer (NK) cells are innate immune lymphocytes capable of killing target cells and producing immunoregulatory cytokines. Herein, we discuss recent studies that indicate that NK cells span the conventional boundaries between innate and adaptive immunity. For example, it was recently discovered that NK cells have the capacity for memory-like responses, a property that was previously thought to be limited to adaptive immunity. NK cells have also been identified in multiple tissues, and a subset of cells that specialize in the production of the T(H)17 cytokine IL-22, NK-22s, was recently described in mucosal-associated lymphoid tissue. Finally, we review work that shows that NK cells develop at sites that were traditionally thought to be occupied only by adaptive immune cells, including the thymus and lymph nodes.

Dexamethasone suppresses interleukin-22 associated with bacterial infection in vitro and in vivo

Interleukin (IL)-22 production triggered by innate immune mechanisms has been identified as key to efficient intestinal anti-bacterial host defence and preservation of homeostasis. We hypothesized that glucocorticoid therapy may impair IL-22 expression, which should promote intestinal epithelial damage with the potential of subsequent bacterial translocation. High-dose corticosteroid therapy in Crohn's disease has been associated with an increased rate of abscess formation and ultimately with a higher risk of developing postoperative infectious complications, including abdominal sepsis. Thus, we sought to investigate effects of the prototypic glucocorticoid dexamethasone on IL-22 production in the context of bacterial infection. Enhanced IL-22 plasma levels were detectable in rat sepsis. Moreover, heat-inactivated Staphylococcus epidermidis, used as a prototypic activator of innate immunity, induced robust production of IL-22 by human peripheral blood mononuclear cells (PBMC). Here, we report for the first time that dexamethasone mediates remarkable suppression of IL-22 as detected in S. epidermidis-activated PBMC and rat sepsis, respectively. The data presented herein suggest that insufficient IL-22 function may contribute to impaired intestinal host defence in the context of corticosteroid therapy.

Th17 cells at the crossroads of innate and adaptive immunity against infectious diseases at the mucosa

T helper type 17 (Th17) cells are a distinct lineage of T cells that produce the effector molecules IL-17, IL-17F, IL-21, and IL-22. Although the role of Th17 cells in autoimmunity is well documented, there is growing evidence that the Th17 lineage and other interleukin (IL)-17-producing cells are critical for host defense against bacterial, fungal, and viral infections at mucosal surfaces. Here we summarize recent progress in our understanding of the function of IL-17-producing cells as a bridge between innate and adaptive immunity against infectious diseases at the mucosa.

Modulation of inflammation through IL-17 production by gammadelta T cells: mandatory in the mouse, dispensable in humans?

Recent studies suggest that gammadelta T cells are innate IL-17 producers owing to unique features of their developmental program. A key contribution of this subset to T helper 17 responses has been also suggested by numerous physiopathological studies mainly performed in mouse models. In the present review, we will summarize the main features of IL-17-producing gammadelta T cells and highlight the similarities and differences between murine gammadelta T cells and their human counterparts.

CXCR2-dependent mucosal neutrophil influx protects against colitis-associated diarrhea caused by an attaching/effacing lesion-forming bacterial pathogen

Enteropathogenic Escherichia coli (EPEC) is a major cause of diarrheal disease in young children, yet symptoms and duration are highly variable for unknown reasons. Citrobacter rodentium, a murine model pathogen that shares important functional features with EPEC, colonizes mice in colon and cecum and causes inflammation, but typically little or no diarrhea. We conducted genome-wide microarray studies to define mechanisms of host defense and disease in C. rodentium infection. A significant fraction of the genes most highly induced in the colon by infection encoded CXC chemokines, particularly CXCL1/2/5 and CXCL9/10, which are ligands for the chemokine receptors CXCR2 and CXCR3, respectively. CD11b(+) dendritic cells were the major producers of CXCL1, CXCL5, and CXCL9, while CXCL2 was mainly induced in macrophages. Infection of gene-targeted mice revealed that CXCR3 had a significant but modest role in defense against C. rodentium, whereas CXCR2 had a major and indispensable function. CXCR2 was required for normal mucosal influx of neutrophils, which act as direct antibacterial effectors. Moreover, CXCR2 loss led to severe diarrhea and failure to express critical components of normal ion and fluid transport, including ATPase beta(2)-subunit, CFTR, and DRA. The antidiarrheal functions were unique to CXCR2, since other immune defects leading to increased bacterial load and inflammation did not cause diarrhea. Thus, CXCR2-dependent processes, particularly mucosal neutrophil influx, not only contribute to host defense against C. rodentium, but provide protection against infection-associated diarrhea.

Anti-inflammatory and pro-inflammatory roles of TGF-beta, IL-10, and IL-22 in immunity and autoimmunity

Cytokines play a major role in maintaining lymphocyte homeostasis under both steady-state and inflammatory conditions. Unregulated lymphocytes in steady-state conditions can lead to autoimmunity, whereas during inflammation they can cause excessive tissue damage. Regulatory cytokines function in combination with other environmental signals to properly modulate the function and the extent of lymphocyte activation. Many recent studies have highlighted the importance of regulatory cytokines in controlling the differentiation and function of lymphocytes under steady-state and inflammatory conditions, as well as minimizing tissue damage.

Structure and signalling in the IL-17 receptor family

Interleukin-17A (IL-17A), the hallmark cytokine of the newly defined T helper 17 (T(H)17) cell subset, has important roles in protecting the host against extracellular pathogens, but also promotes inflammatory pathology in autoimmune disease. IL-17A and its receptor (IL-17RA) are the founding members of a newly described family of cytokines and receptors that have unique structural features which distinguish them from other cytokine families. Research defining the signal transduction pathways induced by IL-17R family cytokines has lagged behind that of other cytokine families, but studies in the past 2 years have begun to delineate unusual functional motifs and new proximal signalling mediators used by the IL-17R family to mediate downstream events.

CCR6 is required for IL-23-induced psoriasis-like inflammation in mice

Psoriasis is a common immune-mediated chronic inflammatory skin disorder, but the mechanisms of pathogenesis are still poorly understood. IL-23 is expressed in psoriatic skin, and IL-23 injection produces IL-22-dependent psoriasiform changes in mouse skin. Th17 cells produce IL-22 and display CCR6, the CCL20 receptor; CCR6+ T cells and CCL20 are abundant in psoriatic skin. We investigated a possible role for CCR6 in recruiting Th17 cells and producing psoriasiform pathology by injecting IL-23 into the skin of WT and Ccr6-/- mice. Unlike for WT mice, IL-23-injected ears of Ccr6-/- mice showed neither substantial epidermal/dermal changes nor increased Il22 mRNA expression. However, injection of IL-22 yielded equivalent psoriasiform changes in WT and Ccr6-/- mice. Surprisingly, IL-23-injected ears of WT and Ccr6-/- mice contained similar numbers of Th cells able to make IL-17A and/or IL-22. Furthermore, in ears of Rag1-/- mice, IL-23 initially induced skin changes and levels of Il22 mRNA that were indistinguishable from WT mice, revealing at least one non-T cell source for IL-22. We conclude that CCR6 is essential in a model of IL-23-induced, IL-22-mediated dermatitis, which develops in sequential T cell-independent and T cell-dependent phases. These findings reveal an expanded role for CCR6 in IL-23-related responses and identify CCR6 as a potential therapeutic target in psoriasis.

IL-17 and IL-22 are associated with protection against human kala azar caused by Leishmania donovani

IL-17 and IL-22 have been shown to increase protection against certain bacteria and fungal pathogens in experimental models. However, no human studies have demonstrated a crucial role of IL-17 and IL-22 in protection against infections. We show here that Leishmania donovani, which can cause the lethal visceral disease Kala Azar (KA), stimulates the differentiation of Th17 cells, which produce IL-17, IL-22, and IFN-gamma. Analysis of Th1, Th2, and Th17 cytokine responses by cultured PBMCs from individuals in a cohort of subjects who developed KA or were protected against KA during a severe outbreak showed that IL-17 and IL-22 were strongly and independently associated with protection against KA. Our results suggest that, along with Th1 cytokines, IL-17 and IL-22 play complementary roles in human protection against KA, and that a defect in Th17 induction may increase the risk of KA.

Interferon-lambda is functionally an interferon but structurally related to the interleukin-10 family

Interferon-lambda (IFN-lambda) is an antiviral cytokine that signals through a distinct receptor complex, composed of the IFN-lambdaR1 and interleukin-10R2 (IL-10R2) receptor chains. We have determined the crystal structure of human IFN-lambda3 and characterized the interaction with its receptor complex through structure-based site-directed mutagenesis. The ability of IFN-lambda3 mutants to signal was determined by measuring the antiviral activity and induced STAT2 phosphorylation. In conclusion, our data show that, although IFN-lambda is functionally an interferon, it is clearly structurally related to members of the IL-10 family. In particular, we found an interesting similarity between IFN-lambda and IL-22, and we suggest that IFN-lambda and IL-22 possess parallel functions, protecting epithelial tissue against viral and bacterial infections, respectively.

Interleukin-22-producing natural killer cells and lymphoid tissue inducer-like cells in mucosal immunity

Blood, lymphoid tissues, and placenta contain diverse subpopulations of natural killer (NK) cells that possess distinct immune functions. Recent studies have shown that human and mouse gut-associated lymphoid tissues harbor a unique NK cell subset that specializes in production of interleukin (IL)-22. This cytokine plays a role in host defense of mucosal barriers, although dysregulated secretion may cause autoimmune disease. In parallel, human fetal lymphoid tissue inducer (LTi) cells and mouse adult LTi-like cells in secondary lymphoid tissues were found to release IL-22, as well as IL-17, a proinflammatory cytokine that mediates host defense against extracellular pathogens. Here, we compare these recently identified immune cells, reviewing what is known about their anatomical location, differentiation requirements, function, and potential involvement in host defense and autoimmunity. Finally, we discuss the challenges faced in furthering our understanding of the developmental relationships and role of NK and LTi-like cells in mucosal immune responses.

STAT3 links IL-22 signaling in intestinal epithelial cells to mucosal wound healing

Signal transducer and activator of transcription (STAT) 3 is a pleiotropic transcription factor with important functions in cytokine signaling in a variety of tissues. However, the role of STAT3 in the intestinal epithelium is not well understood. We demonstrate that development of colonic inflammation is associated with the induction of STAT3 activity in intestinal epithelial cells (IECs). Studies in genetically engineered mice showed that epithelial STAT3 activation in dextran sodium sulfate colitis is dependent on interleukin (IL)-22 rather than IL-6. IL-22 was secreted by colonic CD11c(+) cells in response to Toll-like receptor stimulation. Conditional knockout mice with an IEC-specific deletion of STAT3 activity were highly susceptible to experimental colitis, indicating that epithelial STAT3 regulates gut homeostasis. STAT3(IEC-KO) mice, upon induction of colitis, showed a striking defect of epithelial restitution. Gene chip analysis indicated that STAT3 regulates the cellular stress response, apoptosis, and pathways associated with wound healing in IECs. Consistently, both IL-22 and epithelial STAT3 were found to be important in wound-healing experiments in vivo. In summary, our data suggest that intestinal epithelial STAT3 activation regulates immune homeostasis in the gut by promoting IL-22-dependent mucosal wound healing.

TGF-beta promotes Th17 cell development through inhibition of SOCS3

TGF-beta, together with IL-6 and IL-21, promotes Th17 cell development. IL-6 and IL-21 induce activation of STAT3, which is crucial for Th17 cell differentiation, as well as the expression of suppressor of cytokine signaling (SOCS)3, a major negative feedback regulator of STAT3-activating cytokines that negatively regulates Th17 cells. However, it is still largely unclear how TGF-beta regulates Th17 cell development and which TGF-beta signaling pathway is involved in Th17 cell development. In this report, we demonstrate that TGF-beta inhibits IL-6- and IL-21-induced SOCS3 expression, thus enhancing as well as prolonging STAT3 activation in naive CD4(+)CD25(-) T cells. TGF-beta inhibits IL-6-induced SOCS3 promoter activity in T cells. Also, SOCS3 small interfering RNA knockdown partially compensates for the action of TGF-beta on Th17 cell development. In mice with a dominant-negative form of TGF-beta receptor II and impaired TGF-beta signaling, IL-6-induced CD4(+) T cell expression of SOCS3 is higher whereas STAT3 activation is lower compared with wild-type B6 CD4(+) T cells. The addition of a TGF-beta receptor I kinase inhibitor that blocks Smad-dependent TGF-beta signaling greatly, but not completely, abrogates the effect of TGF-beta on Th17 cell differentiation. Our data indicate that inhibition of SOCS3 and, thus, enhancement of STAT3 activation is at least one of the mechanisms of TGF-beta promotion of Th17 cell development.

Blockade of interleukin-6 signaling augments regulatory T-cell reconstitution and attenuates the severity of graft-versus-host disease

Graft-versus-host disease (GVHD) is the major complication after allogeneic bone marrow transplantation and is characterized by the overproduction of proinflammatory cytokines. In this study, we have identified interleukin-6 (IL-6) as a critical inflammatory cytokine that alters the balance between the effector and regulatory arms of the immune system and drives a proinflammatory phenotype that is a defining characteristic of GVHD. Our results demonstrate that inhibition of the IL-6 signaling pathway by way of antibody-mediated blockade of the IL-6 receptor (IL-6R) markedly reduces pathologic damage attributable to GVHD. This is accompanied by a significant increase in the absolute number of regulatory T cells (Tregs) that is due to augmentation of thymic-dependent and thymic-independent Treg production. Correspondingly, there is a significant reduction in the number of T helper 1 and T helper 17 cells in GVHD target organs, demonstrating that blockade of IL-6 signaling decreases the ratio of proinflammatory T cells to Tregs. These studies demonstrate that antibody blockade of the IL-6R serves to recalibrate the effector and regulatory arms of the immune system and represents a novel, potentially clinically translatable, strategy for the attenuation of GVHD.

Molecular explanation for the contradiction between systemic Th17 defect and localized bacterial infection in hyper-IgE syndrome

Hyper-IgE syndrome (HIES) is a primary immunodeficiency characterized by atopic manifestations and susceptibility to infections with extracellular pathogens, typically Staphylococcus aureus, which preferentially affect the skin and lung. Previous studies reported the defective differentiation of T helper 17 (Th17) cells in HIES patients caused by hypomorphic STAT3 mutations. However, the apparent contradiction between the systemic Th17 deficiency and the skin/lung-restricted susceptibility to staphylococcal infections remains puzzling. We present a possible molecular explanation for this enigmatic contradiction. HIES T cells showed impaired production of Th17 cytokines but normal production of classical proinflammatory cytokines including interleukin 1β. Normal human keratinocytes and bronchial epithelial cells were deeply dependent on the synergistic action of Th17 cytokines and classical proinflammatory cytokines for their production of antistaphylococcal factors, including neutrophil-recruiting chemokines and antimicrobial peptides. In contrast, other cell types were efficiently stimulated with the classical proinflammatory cytokines alone to produce such factors. Accordingly, keratinocytes and bronchial epithelial cells, unlike other cell types, failed to produce antistaphylococcal factors in response to HIES T cell–derived cytokines. These results appear to explain, at least in part, why HIES patients suffer from recurrent staphylococcal infections confined to the skin and lung in contrast to more systemic infections in neutrophil-deficient patients.

Identification of interleukin-22 in gadoids and examination of its expression level in vaccinated fish

This paper reports the cloning and sequencing of interleukin (IL)-22 in two gadoid fish, cod (Gadus morhua) and haddock (Melanogrammus aeglefinus). The complete transcript of this gene was 1002 and 1154 bp respectively, of which 492 bp was the open reading frame (ORF) in both genes. High amino acid identity (88.3%) was found between these genes but was less than 50% aa identity to other known genes. The gene organisation of haddock IL-22 consisted of five exons and four introns, as with other IL-10 family members. Expression studies showed that IL-22 is constitutively expressed in gill, with low level expression also observed in gut, gonad and head kidney. In a vaccination experiment, haddock were injected intraperitoneally with formalin-killed Vibrio anguillarum or with PBS, and 2 months later challenged by immersion in 10(7)cfu/ml V. anguillarum for 30 min. Head kidney and gill samples were collected prior to challenge and 24, 48 and 72 h post-challenge (hpc) for Real-time PCR analysis of IL-22 expression. No significant changes in IL-22 expression were observed in head kidney tissue but vaccinated fish showed a significantly increased expression of IL-22 24 hpc in gill and no mortalities were seen in these fish. In contrast, control fish, which started to succumb to the disease from 72 hpc, showed no significant increase in gill expression after challenge.

Memory IL-22-producing CD4+ T cells specific for Candida albicans are present in humans

Co-expression of IL-22 and IL-17 has been identified and demonstrated to be involved in the immunopathogenesis of some autoimmune diseases as well as the defense against pathogenic infections in animal studies. However, the properties of IL-22-producing cells in humans remain largely unclear. In the present study, we showed that IL-22 could be induced from human PBMC following various polyclonal stimulations. The majority of IL-22-producing cells in PBMC were CD4(+) T cells with a memory cell phenotype. In addition, we found that a subset of IL-22(+) T cells produced IL-22 alone, whereas other IL-22(+) T cells co-expressed cytokines typical of Th1, Th2 and Th17 cells. Importantly, stimulation of PBMC from healthy adults with heat-inactivated Candida albicans (C. albicans) yeast or hyphae resulted in IL-22 production by central and effector memory CD4(+) T cells. Moreover, CD4(+)CCR6(+) but not CD4(+)CCR6(-) T cells produced IL-22 when stimulated with either C. albicans or PMA and ionomycin. In addition, PBMC from the individuals infected with C. albicans produced a significantly higher amount of IL-22 compared with healthy controls following stimulation with C. albicans. These data demonstrate that IL-22-producing T cells in humans may play an important role in the defense against fungal infections such as C. albicans.

Regulatory lymphocytes and intestinal inflammation

The immune system is pivotal in mediating the interactions between host and microbiota that shape the intestinal environment. Intestinal homeostasis arises from a highly dynamic balance between host protective immunity and regulatory mechanisms. This regulation is achieved by a number of cell populations acting through a set of shared regulatory pathways. In this review, we summarize the main lymphocyte subsets controlling immune responsiveness in the gut and their mechanisms of control, which involve maintenance of intestinal barrier function and suppression of chronic inflammation. CD4(+)Foxp3(+) T cells play a nonredundant role in the maintenance of intestinal homeostasis through IL-10- and TGF-beta-dependent mechanisms. Their activity is complemented by other T and B lymphocytes. Because breakdown in immune regulatory networks in the intestine leads to chronic inflammatory diseases of the gut, such as inflammatory bowel disease and celiac disease, regulatory lymphocytes are an attractive target for therapies of intestinal inflammation.

Development and function of TH17 cells in health and disease

T(H)17 cells are the newest member of the T(H) cell family and are characterized by their ability to produce specific cytokines such as IL-17, IL-22, IL-17F, and CCL20. In this review, conditions for the differentiation of T(H)17 cells are defined in both murine and human systems, with discussion of T(H)17-specific cytokines and transcription factors. Functionally, T(H)17 cells contribute to host defense as a new effector T(H) cell subset with a role in protection against extracellular bacteria through activities on immune and nonimmune cells. Their activities, however, are also pivotal in the development of autoimmune diseases under pathologic conditions. T(H)17 cells are also beginning to be associated with the development and pathophysiology of allergic diseases, such as allergic contact dermatitis, atopic dermatitis, and asthma. Lymphoid tissue inducer-like cells and natural killer-like cells, termed RORgammat(+)NKp46(+) or NK-22 cells, might also play a role in allergic diseases because of their propensity to produce IL-17 and IL-22.

Lipocalin-2 resistance confers an advantage to Salmonella enterica serotype Typhimurium for growth and survival in the inflamed intestine

In response to enteric pathogens, the inflamed intestine produces antimicrobial proteins, a process mediated by the cytokines IL-17 and IL-22. Salmonella enterica serotype Typhimurium thrives in the inflamed intestinal environment, suggesting that the pathogen is resistant to antimicrobials it encounters in the intestinal lumen. However, the identity of these antimicrobials and corresponding bacterial resistance mechanisms remain unknown. Here, we report that enteric infection of rhesus macaques and mice with S. Typhimurium resulted in marked Il-17- and IL-22-dependent intestinal epithelial induction and luminal accumulation of lipocalin-2, an antimicrobial protein that prevents bacterial iron acquisition. Resistance to lipocalin-2, mediated by the iroBCDE iroN locus, conferred a competitive advantage to the bacterium in colonizing the inflamed intestine of wild-type but not of lipocalin-2-deficient mice. Thus, resistance to lipocalin-2 defines a specific adaptation of S. Typhimurium for growth in the inflamed intestine.

IL-22: a critical mediator in mucosal host defense

IL-22 is an IL-10 family cytokine member that was recently discovered to be produced by Th17 cells. Current studies have revealed that IL-22 targets cells of the digestive, skin, and respiratory organ systems and plays an important role in mucosal immunity. The IL-22 receptor (IL-22R) is expressed exclusively in these tissues, thereby allowing the cytokine to mediate epithelial innate immunity in response to a variety of pathogens. Although recent studies have shown the importance of IL-22 in host defense against Gram-negative bacterial organisms (in gut and lung), there is evidence that IL-22 also plays a role in autoimmune disease, such as psoriasis. IL-22 therefore, not unlike other cytokines, has complex pro-inflammatory, anti-inflammatory, and autoimmune effects which continue to be under further investigation. This review will focus on what is known about IL-22 and its function in mucosal host defense.

Differential IL-23 requirement for IL-22 and IL-17A production during innate immunity against Salmonella enterica serovar Enteritidis

Early activation of the IL-12/IFN-gamma axis has been shown following Salmonella enterica serovar Enteritidis (S. Enteritidis) infection. We were interested to study whether IL-22 and IL-17A production is initiated early in response to S. Enteritidis. We demonstrate here that IL-22 was strongly elevated in the peritoneal lavage fluid and in serum already 1 day post-intraperitoneal infection (d.p.i.) of mice; not only IL-22 but also IL-17A was produced ex vivo by activated peritoneal exudate cells (PEC). Peritoneal gammadelta T cells were identified as cellular source of IL-17A. The early IL-22 production was completely IL-23-dependent. In contrast, IL-17A production was only partially IL-23-dependent. To investigate the local production of upstream cytokines important for induction of IL-22, IL-17A and IFN-gamma during salmonellosis, the production of IL-23 and IL-12 was studied. Elevated p19 and p40 mRNA levels were found in PEC at 1 d.p.i., whereas p35 mRNA levels were not changed. Besides, the T(h)17-promoting cytokines IL-6, IL-1beta and transforming growth factor-beta were produced in response to S. Enteritidis. However, IL-6 was not required for IL-22 or IL-17A production by PEC. By ex vivo analysis of PEC at 1 d.p.i., we show that the major producers of early IL-12/23p40 in the peritoneal cavity were dendritic cells (DC), whereas macrophages notably contributed to IL-6 production. Taken together, these data suggest that DC initiate early IL-22 production at the site of infection which may contribute to resistance against salmonellosis. Furthermore, we provide evidence that production of IL-22 and IL-17A is differentially regulated during infection.

Innate immune recognition of infected apoptotic cells directs T(H)17 cell differentiation

Adaptive immune responses rely on differentiation of CD4 T helper cells into subsets with distinct effector functions best suited for host defence against the invading pathogen. Interleukin (IL)-17-producing T helper cells (T(H)17) are a recently identified subset, separate from the T helper type 1 (T(H)1) and T helper type 2 (T(H)2) subsets. Synergy between the cytokines transforming growth factor-beta and IL-6 in vitro induces development of T(H)17 cells in mouse and human systems, whereas IL-23 supports expansion of these cells. However, it is not known which conditions in vivo would induce this combination of cytokines. Furthermore, it is enigmatic that a combination of pro-inflammatory and anti-inflammatory cytokines would be required to generate an effector T(H)17 response. Here we show that the relevant physiological stimulus triggering this combination of cytokines is the recognition and phagocytosis of infected apoptotic cells by dendritic cells. Phagocytosis of infected apoptotic cells uniquely triggers the combination of IL-6 and transforming growth factor-beta through recognition of pathogen-associated molecular patterns and phosphatidylserine exposed on apoptotic cells, respectively. Conversely, phagocytosis of apoptotic cells in the absence of microbial signals induces differentiation of the closely related regulatory T cells, which are important for controlling autoimmunity. Blocking apoptosis during infection of the mouse intestinal epithelium with the rodent pathogen Citrobacter rodentium, which models human infections with the attaching and effacing enteropathogenic and enterohaemorrhagic Escherichia coli, impairs the characteristic T(H)17 response in the lamina propria. Our results demonstrate that infected apoptotic cells are a critical component of the innate immune signals instructing T(H)17 differentiation, and point to pathogens particularly adept at triggering apoptosis that might preferentially induce T(H)17-mediated immunity. Because T(H)17 cells have been correlated with autoimmune diseases, investigation of the pathways of innate recognition of infected apoptotic cells might lead to improved understanding of the causative defects in autoimmunity.

Novel therapeutic targets along the Th17 pathway

The recent discovery of IL-17-producing CD4(+) Th subset significantly revised the Th1/Th2 dichotomy model proposed by Mosmann and Coffman almost two decades ago. Th17 cells are involved in the pathogenesis of many human autoimmune diseases. Th17 cells, their developmental pathways and their effector functions, therefore, provide novel therapeutic targets.

Diseases that may benefit from manipulating the Th17 pathway

The discovery of IL-17 and of the Th17 pathway has been a step in the classification of human diseases. Th17 targeting appears rather straightforward in diseases associated with inflammation and matrix destruction, such as rheumatoid arthritis, psoriasis, and Crohn's disease. In other conditions where IL-17 is expressed and Th17 activated, it is unclear whether this is a primary or secondary event, making the use of Th17 inhibitors less obvious.

CD4 T-cell differentiation and inflammatory bowel disease

Differentiation of naïve T cells leads to the generation of T-cell subsets, each possessing distinct cytokine expression profiles for serving different immune functions. Through the activation of separate signaling pathways, this process results in both differentiated helper T (Th) cells, termed Th1, Th2 and Th17, and induced regulatory T cells, which suppress Th cells. These different cells are important for combating infectious diseases and cancers; however, when aberrant, they can be responsible for chronic inflammatory diseases. One such disease is inflammatory bowel disease (IBD), in which each T-cell subset can have a role in disease. New studies highlight the importance of the recently identified Th17 subset in IBD. Therapeutics targeting these aberrant Th responses are already under development and hold promise for treating IBD and other chronic inflammatory diseases.

Transcriptional regulatory networks in Th17 cell differentiation

Upon encountering antigen in the context of antigen presenting cells, naïve CD4(+) T cells undergo differentiation into effector T helper (Th) cells, which can secrete high levels of cytokines and other immunomodulators to mediate host defense and tissue inflammation. During the past three years, the immunology field has witnessed an explosion of research advances in the biology of Th17 cells, the most recently described subset of T helper cells, which play crucial roles in host immunity and inflammation. Here we review emerging data on transcriptional regulatory networks that govern the differentiation program of Th17 cells, and focus on how the orphan nuclear receptor RORgammat coordinates this process in concert with diverse cytokine-induced transcription factors.

IL-17F: regulation, signaling and function in inflammation

The IL-17 cytokine family is composed of six members. IL-17F, discovered in 2001, recently has drawn increasing attention due to its greatest similarity to IL-17, a widely recognized inflammatory cytokine. The genes encoding IL-17 and IL-17F are localized in the same chromosomal region and are co-expressed by CD4+ and gammadelta T cells. IL-17F can be secreted as homodimers or heterodimers with IL-17. Similar to IL-17, IL-17F utilizes IL-17RA and IL-17RC as its receptor and employs Act1 and TRAF6 as its signal transducers to induce the expression of pro-inflammatory cytokines and chemokines in many different cell types. However, mice lacking either IL-17 or IL-17F exhibit distinct defects in experimental models of asthma and colitis. These results have laid the basis to understand the role of IL-17F in the pathogenesis of human diseases.

The Current STATus of lymphocyte signaling: new roles for old players

Recently, our understanding of helper/effector T cell differentiation has changed significantly. New subsets of T cells continue to be recognized, including Th17, Treg, and Th9 cells. In addition, the signaling pathways that contribute to their generation continue to be refined. It has become clear that STAT family proteins play a major role in these 'new' T cell fates, along with their critical role in more classical fates. Importantly, genetic studies implicate STATs in autoimmune and primary immunodeficiency diseases in humans. Focusing on how STATs work in concert with other transcription factors will hopefully provide a better mechanistic understanding of the pathogenesis of various autoimmune diseases.

IL-17 and Th17 Cells

CD4+ T cells, upon activation and expansion, develop into different T helper cell subsets with different cytokine profiles and distinct effector functions. Until recently, T cells were divided into Th1 or Th2 cells, depending on the cytokines they produce. A third subset of IL-17-producing effector T helper cells, called Th17 cells, has now been discovered and characterized. Here, we summarize the current information on the differentiation and effector functions of the Th17 lineage. Th17 cells produce IL-17, IL-17F, and IL-22, thereby inducing a massive tissue reaction owing to the broad distribution of the IL-17 and IL-22 receptors. Th17 cells also secrete IL-21 to communicate with the cells of the immune system. The differentiation factors (TGF-β plus IL-6 or IL-21), the growth and stabilization factor (IL-23), and the transcription factors (STAT3, RORγt, and RORα) involved in the development of Th17 cells have just been identified. The participation of TGF-β in the differentiation of Th17 cells places the Th17 lineage in close relationship with CD4+CD25+Foxp3+ regulatory T cells (Tregs), as TGF-β also induces differentiation of naive T cells into Foxp3+ Tregs in the peripheral immune compartment. The investigation of the differentiation, effector function, and regulation of Th17 cells has opened up a new framework for understanding T cell differentiation. Furthermore, we now appreciate the importance of Th17 cells in clearing pathogens during host defense reactions and in inducing tissue inflammation in autoimmune disease.

Th17 and regulatory T cells: rebalancing pro- and anti-inflammatory forces in autoimmune arthritis

Inflammatory T cells are thought to be central to the pathology of autoimmune arthritis. Th17 cells, CD4 T cells that secrete the pro-inflammatory cytokine IL-17 play a critical role in murine models of arthritis. Recent evidence from human studies suggests that Th17 cells may be important players in several autoimmune diseases, including seronegative arthritis in adults, childhood arthritis [juvenile idiopathic arthritis (JIA)]. It was surprising to find that the development of Th17 cells is closely related to that of an immunoregulatory subset called regulatory T cells (Tregs). Tregs are important in the maintenance of immune homeostasis. Defects in Treg function or reduced numbers have been documented in several human autoimmune diseases, including RA and JIA. Conditions that typically favour the development of Tregs and promote tolerance can be subverted by inflammatory signals towards supporting the generation of Th17 cells. In animal models, the enhancement of Th17 cell differentiation is at the expense of Tregs, and these combined changes trigger autoimmunity. Several mechanisms have come to light that control this reciprocal relationship between Tregs and Th17 cells, including the action of pro-inflammatory cytokines such as IL-1beta. Anti-rheumatic biologic therapies may offer a means of restoring the Th17/Treg balance in favour of Tregs and thereby re-establishing immune tolerance.

Cytokine mediators of Th17 function

Th17 cells were identified as an independent lineage of CD4(+) T cells that secrete a distinctive set of immunoregulatory cytokines, including IL-17A, IL-17F, IL-22, and IL-21. These cytokines collectively play roles in inflammation and autoimmunity and in response to extracellular pathogens. The expression of the lineage-specific transcription factor RORgammat leads to Th17 lineage commitment; however, it has become increasingly clear that the population of cells designated as Th17 cells is not homogeneous. Although these cells collectively produce characteristic Th17 cytokines, not all are produced by each individual cell in the population. The cytokines produced by individual cells are presumably affected in part by the specific local cytokine milieu. In this review, we discuss the current understanding of the specific functional characteristics and regulation of Th17 cytokines and clarify how they mediate the actions of Th17 cells.

Modulation of Th17 development and function by activation of the aryl hydrocarbon receptor--the role of endogenous ligands

The elucidation of the crucial differentiation factors for the new Th17 CD4 effector T-cell subset spurred an explosive growth in research publications focused on these cells and led to rapid advances in knowledge concerning their regulation and functional activities. Here we discuss recent discoveries linking the development and functional potential of Th17 cells to a transcription factor that mediates the response to exogenous and endogenous environmental signals.

From interleukin-23 to T-helper 17 cells: human T-helper cell differentiation revisited

Protracted inflammation leading to dysregulation of effector T-cell responses represents a common feature of a wide range of autoimmune diseases. The interleukin-12 (IL-12)/T-helper 1 (Th1) pathway was thought to be responsible for the pathogenesis of multiple chronic inflammatory diseases, including psoriasis, inflammatory bowel disease, arthritis, or multiple sclerosis, mainly through their production of interferon-gamma and its effects on macrophage activation and chemokine production. However, this initial concept of T-cell-mediated chronic inflammation required an adjustment with the discovery of an IL-12-related cytokine, designated IL-23. IL-23 was rapidly recognized for its involvement in the establishment of chronic inflammation and in the development of a Th cell subset producing IL-17, designated Th17, which is distinct from the previously reported Th1 and Th2 populations. This review aims to describe the characterization of IL-23 and its receptor, its biological activities, as well as its involvement in the development of human Th17 cells and autoimmunity.

The roles of IL-17A in inflammatory immune responses and host defense against pathogens

T-helper 17 (Th17) cells are a newly discovered CD4(+) helper T-cell subset that produces interleukin-17A (IL-17A) and IL-17F. IL-17A plays important roles in allergic responses such as delayed-type hypersensitivity, contact hypersensitivity, and allergic airway inflammation. IL-17A promotes inflammation by inducing various proinflammatory cytokines and chemokines, recruiting neutrophils, enhancing antibody production, and activating T cells. IL-17A expression is also augmented in autoimmune diseases such as multiple sclerosis and rheumatoid arthritis. Using mouse models of these diseases, we found that IL-17A plays a central role in their development. IL-6 is required for the development of Th17 cells and tumor necrosis factor functions downstream of IL-17A during the effector phase. IL-1 is important both for developing Th17 cells and eliciting inflammation. Th17 cells, like Th1 and Th2 cells, are involved in host defense against infections, but the contribution of these Th subsets to defense mechanisms differs among pathogens. The roles of IL-17F remain largely unknown. In this review, we introduce how IL-17A/IL-17F are involved in inflammatory immune responses and host defense mechanisms and discuss their relationship with other cytokines in the development of inflammatory and infectious diseases.

The interleukin-23 axis in intestinal inflammation

Immune responses in the intestine are tightly regulated to ensure host protective immunity in the absence of immune pathology. Interleukin-23 (IL-23) has recently been shown to be a key player in influencing the balance between tolerance and immunity in the intestine. Production of IL-23 is enriched within the intestine and has been shown to orchestrate T-cell-dependent and T-cell-independent pathways of intestinal inflammation through effects on T-helper 1 (Th1) and Th17-associated cytokines. Furthermore, IL-23 restrains regulatory T-cell responses in the gut, favoring inflammation. Polymorphisms in the IL-23 receptor have been associated with susceptibility to inflammatory bowel diseases (IBDs) in humans, pinpointing the IL-23 axis as a key, conserved pathway in intestinal homeostasis. In addition to its role in dysregulated inflammatory responses, there is also evidence that IL-23 and the Th17 axis mediate beneficial roles in host protective immunity and barrier function in the intestine. Here we discuss the dual roles of IL-23 in intestinal immunity and how IL-23 and downstream effector pathways may make novel targets for the treatment of IBD.

IL-20 receptor 2 signaling down-regulates antigen-specific T cell responses

The recently described cytokines IL-19, IL-20, and IL-24 share structural homology with IL-10 and are therefore classified as members of the IL-10 family of cytokines. Although it has long been speculated that signaling by their heterodimeric receptor complexes (IL-20R1/IL-20R2 and IL-22R/IL-20R2) influences immunological processes, the target cells for this group of cytokines are still unclear. By generating a knockout mouse strain deficient for the common IL-20R beta-chain (IL-20R2), we show that IFN-gamma and IL-2 secretion is significantly elevated after stimulation of IL-20R2-/--deficient CD8 and CD4 T cells with Con A or anti-CD3/CD28 in vitro. IL-10 secretion by activated IL-20R2-/- CD4 cells was diminished. Consistent with our in vitro results, significantly more Ag-specific CD8 IFN-gamma+ and CD4 IFN-gamma+ T cells developed to locally applied DNA vaccines in IL-20R2-deficient mice. In a T cell-dependent model of contact hypersensitivity, IL-20R2 knockout mice were more sensitive to the contact allergen trinitro-chloro-benzene. Thus, IL-20R2 signaling directly regulates CD8 and CD4 T cell answers in vitro and in vivo. For the first time, we provide evidence that IL-19, IL-20, and IL-24 are part of a signaling network that normally down-modulates T cell responses in mice.

Th17 cells and IL-17 receptor signaling are essential for mucosal host defense against oral candidiasis

The commensal fungus Candida albicans causes oropharyngeal candidiasis (OPC; thrush) in settings of immunodeficiency. Although disseminated, vaginal, and oral candidiasis are all caused by C. albicans species, host defense against C. albicans varies by anatomical location. T helper 1 (Th1) cells have long been implicated in defense against candidiasis, whereas the role of Th17 cells remains controversial. IL-17 mediates inflammatory pathology in a gastric model of mucosal candidiasis, but is host protective in disseminated disease. Here, we directly compared Th1 and Th17 function in a model of OPC. Th17-deficient (IL-23p19(-/-)) and IL-17R-deficient (IL-17RA(-/-)) mice experienced severe OPC, whereas Th1-deficient (IL-12p35(-/-)) mice showed low fungal burdens and no overt disease. Neutrophil recruitment was impaired in IL-23p19(-/-) and IL-17RA(-/-), but not IL-12(-/-), mice, and TCR-alphabeta cells were more important than TCR-gammadelta cells. Surprisingly, mice deficient in the Th17 cytokine IL-22 were only mildly susceptible to OPC, indicating that IL-17 rather than IL-22 is vital in defense against oral candidiasis. Gene profiling of oral mucosal tissue showed strong induction of Th17 signature genes, including CXC chemokines and beta defensin-3. Saliva from Th17-deficient, but not Th1-deficient, mice exhibited reduced candidacidal activity. Thus, the Th17 lineage, acting largely through IL-17, confers the dominant response to oral candidiasis through neutrophils and antimicrobial factors.

Interleukin-17A and interleukin-17F: a tale of two cytokines

In this issue of Immunity, Ishigame et al. (2009) show that interleukin-17A (IL-17A) mediates autoimmunity whereas both IL-17A and IL-17F are required for mucosal immunity. IL-17A may be more pathologic by inducing proinflammatory cytokines.

Differential roles of interleukin-17A and -17F in host defense against mucoepithelial bacterial infection and allergic responses

Interleukin-17A (IL-17A) is a cytokine produced by T helper 17 (Th17) cells and plays important roles in the development of inflammatory diseases. Although IL-17F is highly homologous to IL-17A and binds the same receptor, the functional roles of this molecule remain largely unknown. Here, we demonstrated with Il17a(-/-), Il17f(-/-), and Il17a(-/-)Il17f(-/-) mice that IL-17F played only marginal roles, if at all, in the development of delayed-type and contact hypersensitivities, autoimmune encephalomyelitis, collagen-induced arthritis, and arthritis in Il1rn(-/-) mice. In contrast, both IL-17F and IL-17A were involved in host defense against mucoepithelial infection by Staphylococcus aureus and Citrobacter rodentium. IL-17A was produced mainly in T cells, whereas IL-17F was produced in T cells, innate immune cells, and epithelial cells. Although only IL-17A efficiently induced cytokines in macrophages, both cytokines activated epithelial innate immune responses. These observations indicate that IL-17A and IL-17F have overlapping yet distinct roles in host immune and defense mechanisms.

A human natural killer cell subset provides an innate source of IL-22 for mucosal immunity

Natural killer (NK) cells are classically viewed as lymphocytes that provide innate surveillance against virally infected cells and tumour cells through the release of cytolytic mediators and interferon (IFN)-gamma. In humans, blood CD56(dim) NK cells specialize in the lysis of cell targets. In the lymph nodes, CD56(bright) NK cells secrete IFN-gamma cooperating with dendritic cells and T cells in the generation of adaptive responses. Here we report the characterization of a human NK cell subset located in mucosa-associated lymphoid tissues, such as tonsils and Peyer's patches, which is hard-wired to secrete interleukin (IL)-22, IL-26 and leukaemia inhibitory factor. These NK cells, which we refer to as NK-22 cells, are triggered by acute exposure to IL-23. In vitro, NK-22-secreted cytokines stimulate epithelial cells to secrete IL-10, proliferate and express a variety of mitogenic and anti-apoptotic molecules. NK-22 cells are also found in mouse mucosa-associated lymphoid tissues and appear in the small intestine lamina propria during bacterial infection, suggesting that NK-22 cells provide an innate source of IL-22 that may help constrain inflammation and protect mucosal sites.

gp130-mediated Stat3 activation in enterocytes regulates cell survival and cell-cycle progression during colitis-associated tumorigenesis

Although gastrointestinal cancers are frequently associated with chronic inflammation, the underlying molecular links have not been comprehensively deciphered. Using loss- and gain-of-function mice in a colitis-associated cancer model, we establish here a link comprising the gp130/Stat3 transcription factor signaling axis. Mutagen-induced tumor growth and multiplicity are reduced following intestinal epithelial cell (IEC)-specific Stat3 ablation, while its hyperactivation promotes tumor incidence and growth. Conversely, IEC-specific Stat3 deficiency enhances susceptibility to chemically induced epithelial damage and subsequent mucosal inflammation, while excessive Stat3 activation confers resistance to colitis. Stat3 has the capacity to mediate IL-6- and IL-11-dependent IEC survival and to promote proliferation through G1 and G2/M cell-cycle progression as the common tumor cell-autonomous mechanism that bridges chronic inflammation to tumor promotion.

IL-6 and Stat3 are required for survival of intestinal epithelial cells and development of colitis-associated cancer

Colitis-associated cancer (CAC) is the most serious complication of inflammatory bowel disease. Proinflammatory cytokines have been suggested to regulate preneoplastic growth during CAC tumorigenesis. Interleukin 6 (IL-6) is a multifunctional NF-kappaB-regulated cytokine that acts on epithelial and immune cells. Using genetic tools, we now demonstrate that IL-6 is a critical tumor promoter during early CAC tumorigenesis. In addition to enhancing proliferation of tumor-initiating cells, IL-6 produced by lamina propria myeloid cells protects normal and premalignant intestinal epithelial cells (IECs) from apoptosis. The proliferative and survival effects of IL-6 are largely mediated by the transcription factor Stat3, whose IEC-specific ablation has profound impact on CAC tumorigenesis. Thus, the NF-kappaB-IL-6-Stat3 cascade is an important regulator of the proliferation and survival of tumor-initiating IECs.