Regulation and function of innate and adaptive interleukin‐17‐producing cells (original) (raw)

IL‐17‐mediated immunity is key for mucosal host defence against a range of pathogens, but can also elicit or aggravate autoimmune and cardiovascular disease, as well as affect tumour formation. The regulation and function of such versatile cells is discussed here.

Introduction

Interleukin‐17 (IL‐17) is a proinflammatory cytokine secreted by innate and adaptive immune cells (Cua & Tato, 2010; Hirota et al, 2010; Korn et al, 2009). Although the source of IL‐17 is restricted to haematopoietic cells, the heterodimeric IL‐17 receptor—consisting of IL‐17 RA and IL‐17 RC—is widely expressed by haematopoietic and non‐haematopoietic cells, such as endothelial and epithelial cells. IL‐17 downstream signalling is regulated by the complex IL‐17R–Act1–TRAF6, which is antagonized by TRAF3 (Zhu et al, 2010). Therefore, potential target cell types, tissues and organs are also variable, depending on when and where an IL‐17‐mediated immune response occurs.

There are several types of haematopoietic cell known to produce IL‐17, including αβT, γδT and innate lymphocytes negative for lineage markers (Cupedo et al, 2009; Takatori et al, 2009). The predominant source of IL‐17 depends on the inflammatory and physiological microenvironment, which provides the condition for a particular cell type to secrete IL‐17. Analysis of an IL‐17 Cre fate reporter mouse emphasized that, under steady‐state conditions, most of the IL‐17‐producing cells are CD4 T cells found in the lamina propria of the small intestine, which also contains a population of innate lymphocytes, as previously determined with intracellular staining approaches (Hirota et al, 2011). Furthermore, there are γδ T cells in the skin (dermal γδ T cells) that express IL‐17 (Gray et al, 2011), as well as a substantial population of a γδ T‐cell subset that expresses Vγ4 and IL‐17 in the lung (Fig 1; J.H.D. & B.S., unpublished observations).

Figure 1

figure 2

Location of IL‐17‐producing cell types. There are characteristic populations of homeostatic IL‐17‐producing cell types in different mucosal tissues. Upon inflammation, these cells act in their resident tissue or migrate to other tissues to combat infection. For example, LN CD4+ TH17 cells can give rise to inflammatory CD4+ TH17 cells that migrate to many tissues in the body. ILC, innate lymphoid cell; iNKT, invariant natural killer T cell; LN, lymph node.

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In line with the clear phenotypes of IL‐17‐deficient mice in various disease models, high levels of IL‐17 are induced in response to a range of infections, as well as during the development of several autoimmune diseases.

To understand IL‐17‐mediated immune responses, several facets of this response have to be taken into account, as the IL‐17 receptor is ubiquitously expressed throughout the body, resulting in pleiotropic actions of IL‐17 on a wide range of cell types. One of the well‐known biological outcomes of IL‐17 signalling is the induction of proinflammatory cytokines and chemokines from target cells, such as fibroblasts and epithelial cells, leading to the recruitment of neutrophils (Fossiez et al, 1998). The primary function of activated neutrophils is to fight against invading pathogens, but this can damage the surrounding tissues.

Although the mechanisms that underlie the innate IL‐17 response remain largely unknown, the differentiation, maintenance and effector function of IL‐17‐producing CD4 T cells have been extensively studied. Naive CD4 T helper cells can differentiate into IL‐17‐producing T cells, known as TH17 cells. This particular subset of T helper cells can secrete not only the signature cytokine IL‐17, but also TNF‐α, GM‐CSF, IL‐2, IL‐17F and IL‐22, depending on inflammatory settings. IL‐23 and environmental signals play a crucial role in the pathogenesis of TH17 cells in autoimmune settings (Stockinger et al, 2011).

Here, we review the latest discoveries regarding the regulation and function of innate and adaptive IL‐17‐producing cells in the context of beneficial and adverse immune responses.

Transcriptional regulation of IL‐17‐positive cells

There have recently been major advances in our understanding of the transcriptional regulation of TH17 cells. Although both innate and adaptive IL‐17‐producing cells express the master transcription factor Rorγt, the differentiation and regulatory factors seem to differ between innate and adaptive cells. Most studies show a requirement for IL‐6 signalling in TH17 development, but notably IL‐6 is irrelevant for the development and maintenance of IL‐17‐producing γδ T cells as well as iNKT cells (Doisne et al, 2009; Martin et al, 2009). How the differentiation of IL‐17‐producing innate lymphocytes is regulated is unknown (Sidebar A). Several papers have shown that TGF‐β, in combination with IL‐6, is required for the induction of Rorγt and TH17 development in mice and, in addition, the IL‐21 and IL‐23 signalling pathways amplify TH17 differentiation (Hirota et al, 2010; Korn et al, 2009). In line with this, the development of IL‐17‐producing γδ T cells is severely impaired in TGF‐β‐deficient mice, suggesting a crucial role of TGF‐β signalling (Do et al, 2010). Nevertheless, this idea was challenged in a study suggesting that TGF‐β‐independent differentiation of TH17 cells occurs through IL‐23 in combination with IL‐6 and IL‐1 (Ghoreschi et al, 2010). However, it must be noted that, despite the use of neutralizing antibodies against TGF‐β, it might be impossible to remove all sources of TGF‐β, as a cell‐associated form of TGF‐β—produced by activated T cells—cannot be neutralized by the widely used TGF‐β antibody (Clone:1D11; Oida & Weiner, 2011). Thus, it cannot be excluded that TGF‐β generated in activated T cells and acting on bystander cells might have a role in TH17 differentiation. Although TGF‐β was initially reported in human studies to be redundant in generating TH17 cells (Acosta‐Rodriguez et al, 2007; Wilson et al, 2007), other groups demonstrated subsequently that TGF‐β plays a crucial role in the induction of Rorγt and differentiation of human TH17 (Manel et al, 2008; O'Garra et al, 2008; Volpe et al, 2008). However, it has been suggested that TGF‐β is required only to block the alternative T helper cell differentiation pathways, instead of directly affecting TH17 differentiation (Das et al, 2009). Indeed, TGF‐β signalling suppresses the induction of TH17 inhibitors such as SOCS3, a well‐known negative regulator of STAT3, and blocks the induction of eomesodermin in a Smad‐independent manner, indirectly promoting TH17 differentiation (Ichiyama et al, 2011; Qin et al, 2009). Furthermore, the role of Smad2 and Smad3 signalling downstream from TGF‐β during TH17 development is not without controversy, as one study shows that both Smad2 and Smad3 are necessary for the optimal induction of TH17 cells (Takimoto et al, 2010), whereas another indicates that Smad3 deficiency increases TH17 differentiation (Martinez et al, 2009). Although there still remain unresolved issues, it is conceivable that TGF‐β promotes TH17 differentiation directly as well as indirectly.

Sidebar A | In need of answers

  1. i.
    Which factors regulate the differentiation of IL‐17‐producing innate lymphoid cells? Where are they induced?
  2. ii.
    Can Rorγt antagonists limit IL‐17‐mediated tissue pathology in human diseases?
  3. iii.
    Which factors and conditions drive TH17 plasticity into different cytokine profiles?
  4. iv.
    Can ‘ex‐TH17’ cells revert to their original state?
  5. v.
    Does TH17 memory—in the absence of any triggering stimulus—exist?

A substantial amount of work has concentrated on the signalling and transcriptional events that occur during TH17 development, influencing the induction and maintenance of their effector profile (Fig 2). The transcription factors Rorγt, AhR and IκBζ are specifically induced during TH17‐cell development and have key roles in their differentiation and function (Okamoto et al, 2010; Veldhoen et al, 2008). STAT3, IRF4, Runx1 and Batf, on the other hand, influence TH17 development, but their expression is not restricted to TH17 cells (Korn et al, 2009; Schraml et al, 2009). As anticipated, the transcriptional modulators that target these key transcription factors can either enhance or impair the differentiation of TH17 cells. The positive and negative regulators of the key transcription factor IRF4—ROCK2 and IRF4 binding protein, respectively—were clearly shown to alter the activity of IRF4, thereby resulting in the spontaneous development of autoimmune disorders in mice with aberrantly activated ROCK2 and deficient in IRF4 binding protein (Biswas et al, 2010; Chen et al, 2008). The nuclear receptor LXR negatively regulates the IL‐17 immune response by upregulating the transcription factor Srebp‐1c, which interacts physically with AhR to interfere with its function in promoting TH17 development (Cui et al, 2011). In addition, the nuclear receptor PPARγ was shown to negatively regulate TH17 differentiation by repressing the induction of Rorγt. However, the precise mechanisms by which PPARγ interferes with STAT3, IRF4 and Batf‐mediated Rorγt induction remain to be elucidated (Klotz et al, 2009). The transcription factor TCF1 inhibits transcription of IL‐17 in a different manner, by directly binding to the regulatory region of the IL‐17 gene (Yu et al, 2011).

Figure 2

figure 3

Transcriptional regulation of the IL‐17 programme. Members of various transcription factor families are implicated in the regulation of IL‐17 transcription either directly (solid line) or indirectly (dashed line). Green indicates positive regulation and red negative regulation. AhR, aryl hydrocarbon receptor; Batf, basic leucine zipper transcription factor, ATF‐like; HIF, hypoxia‐inducible factor; IκB, inhibitor of NF‐κB; IKK, inhibitor of NF‐κB kinase; IRF, interferon response factor; LXR, liver X receptor; mTORC, mammalian target of rapamycin complex; NF‐κB, nuclear factor κB; PPAR, peroxisome proliferator‐activated receptor; RelB, v‐rel reticuloendotheliosis viral oncogene homologue B; ROR, retinoid‐related orphan nuclear receptor; STAT, signal transducer and activator of transcription; TCF, T‐cell‐specific transcription factor.

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Similarly to the differential role of STAT4 and STAT6 for TH1 and TH2 differentiation, respectively, STAT3 downstream from IL‐6 signalling has a central role in the initiation of TH17 development. The negative effect of IL‐2 signalling on TH17 differentiation has been shown recently to be mediated by STAT5 activity. This activity competes with STAT3 for binding to multiple common sites of the IL‐17 promoter region and directly antagonizes IL‐17 transcription in part through an epigenetic modification (Yang et al, 2011).

The components of the NF‐κB signalling pathway regulate numerous immune responses, and their dysregulation is linked to inflammatory and autoimmune diseases, as well as cancer (Vallabhapurapu & Karin, 2009). Interestingly, several of these molecules regulate Rorγt and IL‐17A expression. Thymic γδT cells express the surface lymphotoxin‐β‐receptor and, after its ligation, RelB mediates the induction of Rorγt in γδ T cells, but not αβ T cells, which is required for the differentiation of IL‐17‐producing γδ T cells and an innate IL‐17 response to Escherichia coli infection (Powolny‐Budnicka et al, 2011). This unique contribution of RelB to innate IL‐17‐producing cells suggests the different functional regulation of innate and adaptive IL‐17 production. Besides NF‐κB, kinase inhibitors for NF‐κB signalling also play a role in TH17 differentiation. Although IKKα was originally identified as an inhibitor of NF‐κB, IKKα can promote IL‐17 transcription in TH17 cells by binding to the IL‐17 locus in a NF‐κB‐independent way (Li et al, 2011). Furthermore, IκBζ is specifically induced during TH17 differentiation and, in cooperation with Rorγt, is essential for robust IL‐17 transcription (Okamoto et al, 2010).

Several groups have demonstrated recently that the kinase mTOR and the transcription factor HIF1‐α, which are cellular metabolic sensors, control TH17 fate determination. The mTOR complex 1 (mTORC1), but not mTORC2, regulates the small GTPase Rheb signalling pathway, which promotes TH1 and TH17, but not TH2, differentiation (Delgoffe et al, 2011). In addition, the detailed mechanisms by which HIF‐1α affects TH17, but not TH1, cell fate have been defined. HIF‐1α is upregulated during TH17 differentiation, possibly in a STAT3‐ and mTOR‐dependent manner, which in turn enhances Rorγt expression. After this event, a molecular complex with STAT3, HIF‐1α, Rorγt and P300 induces the robust expression of TH17‐associated genes (Dang et al, 2011; Shi et al, 2011).

Mucosal host defence

IL‐17 is a potent proinflammatory cytokine that sets in motion the recruitment of neutrophils and monocytes in tissues and induces antimicrobial peptide production by epithelial barrier cells. In animal models of infection, IL‐17‐deficient mice are highly susceptible to bacterial and fungal infections. Coinciding with the discovery of IL‐17 function in mouse models, two independent groups identified a crucial role for IL‐17 in the defence against fungal and bacterial infections in humans (Fig 3). Dominant‐negative mutations in human STAT3 lead to the development of hyper‐IgE syndrome, which compromises the generation of IL‐17‐producing cells. These patients suffer from repeated mucocutaneous candidiasis (CMCD) and pulmonary infections with Staphylococcus aureus (Holland et al, 2007; Minegishi et al, 2007). STAT3 loss‐of‐function reduces the number of IL‐17‐producing cells, the differentiation of which in vitro is impaired due to the essential role of STAT3 downstream from IL‐6 in the development of TH17 (de Beaucoudrey et al, 2008; Ma et al, 2008; Milner et al, 2008; Minegishi et al, 2009). In addition, IL‐17F or IL‐17 receptor A deficiency was linked to Candida infection (Puel et al, 2011). In other cases of CMCD, IL‐17‐producing cells were competent, but the patients generated neutralizing antibodies to IL‐17 (Kisand et al, 2010). Furthermore, autosomal‐recessive mutation of CARD9 confers susceptibility to Candida infection (Glocker et al, 2009). CARD9 is an adaptor molecule that, in macrophages and dendritic cells, acts downstream from the pattern recognition receptors dectin 1 and dectin 2, which are activated by β‐glucan—a component of the fungal cell wall—and drive IL‐17 immunity (LeibundGut‐Landmann et al, 2007; Robinson et al, 2009; Saijo et al, 2010). Finally, gain‐of‐function mutations in STAT1, which strongly inhibit the development of IL‐17‐producing cells, were found in patients with CMCD (Liu et al, 2011). Despite the fact that the phenotype of these genetic mutants clearly established the importance of IL‐17 for defence against Candida infection, the cellular origin of the cytokine had not been identified. IL‐17‐producing γδ T cells, but not αβ T cells, have an important role in skin S. aureus infection (Cho et al, 2010). Thus, this γδ T‐cell subset acts as a first line of defence in the IL‐17‐mediated immune response and orchestrates the following inflammatory TH17 response. To efficiently sense invading pathogens, IL‐17‐producing γδ T cells highly express the Toll‐like receptors TLR1 and TLR2, as well as dectin 1. Signalling from these molecules, together with environmental stimuli, regulate the magnitude of the IL‐17 immune response and subsequent neutrophil activation (Martin et al, 2009).

Figure 3

figure 4

Defects and mutations resulting in mucocutaneous candidiasis. Candida albicans is recognized by dectin 1/2 in dendritic cells, which induces the CARD9‐mediated transcription of IL‐23. IL‐23 is recognized by IL‐23R, which is expressed by TH17, innate lymphocyte and γδT cells. This induces IL‐17A and IL‐17F expression, which modulate the immune response by inducing the expression of antimicrobial peptides and chemokines through binding to IL‐17RA. TH17 differentiation and IL‐17 cytokine expression is positively regulated by STAT3, whereas STAT1 activation antagonizes TH17 cell differentiation. Mucocutaneous candidiasis is caused by various defects in this immune response pathway, including (i) IL‐17RA deficiency, (ii) autoantibodies for IL‐17, (iii) lack of expression of IL‐17F, (iv) mutations resulting in the dominant‐negative form of STAT3, (v) expression of a hyperactivated (Act) form of STAT1, or (vi) CARD9 deficiency. CARD, caspase recruitment domain family; DC, dendritic cell; Dec, dectin; IL, interleukin; ILC, innate lymphocyte; STAT, signal transducer and activator of transcription; TH, T helper.

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The elucidation of the environmental signals that control the development and maintenance of mucosal TH17 cells in the intestine remained an intriguing question. Recent studies have revealed that colonization of the gut with TH17 cells requires distinct commensal bacteria, such as segmented filamentous bacteria (Gaboriau‐Routhiau et al, 2009; Ivanov et al, 2009). TH17 colonization of the gut increases the level of antimicrobial defences and confers resistance to intestinal pathogens, such as Citrobacter and Salmonella, in a NOD1/NOD2‐dependent manner (Geddes et al, 2011). The colonization of the intestine with TH17 cells also has an impact on systemic immunity. In fact, germ‐free conditions protected mice against the development of autoimmune arthritis in a model dependent on IL‐17 production and germinal centre formation for autoantibody‐secreting B cells. The introduction of segmented filamentous bacteria into such germ‐free mice restored the intestinal TH17 population and B cells causing autoantibody‐mediated arthritis (Wu et al, 2010). Thus, the induction of TH17 cells in the intestinal lamina propria might contribute not only to mucosal host defence, but also to the induction of autoimmunity in other tissues. Interestingly, intestinal TH17 cells are entirely dependent on commensal flora, whereas germ‐free mice contain numbers of IL‐17‐producing γδ T cells in the skin and lung comparable with those in mice under specific pathogen‐free conditions (J.H.D. & B.S., unpublished data). These findings suggest that, in addition to the distinct requirement of cytokines between innate and adaptive IL‐17‐producing cells, environmental factors strikingly affect their distribution and function.

Adverse effects of IL‐17‐mediated immune responses

In contrast to the beneficial effect of IL‐17 on host defence, IL‐17‐mediated immune responses can have adverse effects resulting in tissue damage. Secondary tissue damage often occurs after infection. For example, although IL‐17 signalling is not required for the function of cytotoxic CD8 T cells and viral clearance after flu infection, bystander IL‐17‐mediated neutrophil influx can cause lung damage (Crowe et al, 2009). Besides the infection‐related pathology, unexpected adverse effects linked to IL‐17 were discovered in cardiovascular diseases, ischaemic brain injury and tumorigenesis, which also involve immune cells and microinflammation. Both TH1 and TH17 cells infiltrate atherosclerotic plaques in human coronary arteries, and the combination of IFN‐γ and IL‐17 induces proinflammatory cytokines and chemokines that might recruit pathogenic monocytes and macrophages to atherosclerotic lesions (Eid et al, 2009). By using apoE‐deficient mice—an animal model of atherosclerosis—IL‐17, produced mainly by TH17 and γδ T cells, was shown to have a crucial role in the induction of IL‐6, CCL5 and CXCL1 from aortic endothelial cells. This induction recruits other inflammatory cells and enlarges atherosclerotic lesions (Erbel et al, 2009; Smith et al, 2010). Furthermore, an upregulation of IL‐17 was detected in human stable atherosclerotic lesions (Taleb et al, 2009). On the other hand, IL‐17 seems to induce an immunosuppressive macrophage phenotype, which limits vascular inflammation and atherosclerosis lesion development. In addition to cardiovascular diseases, sterile inflammation is also observed due to danger signals released from necrotic cells after ischaemic brain injury. The recruitment and activation of immune cells sensing danger signals clearly cause secondary injury in the late phase. Activated γδ T cells are a main source of IL‐17 and a pathogenic effector in the late phase of ischaemic injury after the activation of infiltrating macrophages and production of IL‐23 (Shichita et al, 2009).

Another type of microinflammation is tumour‐associated immunity. Immune surveillance mediated by NK, TH1 and cytotoxic CD8 T cells is essential to prevent and inhibit tumour formation. Nonetheless, chronic inflammation is often related to the initiation of tumour formation. The enterotoxigenic _Bacteroides fragilis_—a common human commensal bacterium—induces chronic IL‐17‐mediated colonic inflammation through which colonic hyperplasia and carcinogenesis are initiated in a multiple intestinal neoplasia mouse model (Wu et al, 2009). IL‐17 has also been associated with the formation of giant cells in Langerhans cell histiocytosis, which establishes dendritic cells as important targets for IL‐17 in this disease (Coury et al, 2008).

Innate and adaptive IL‐17 responses enable biological barriers against pathogens. Although innate IL‐17 responses might suffice to eradicate most pathogens, excess innate immune reactions might lead to hyperinflammatory responses, resulting for example in allergy in the lung and inflammatory diseases. Adaptive IL‐17 cells constitute the second stage of the immune reaction and might be recruited when the innate response fails to cope with invading pathogens. Pathogen‐specific TH17 cells raise a strong coordinated immune response, whereas IL‐17‐mediated autoimmune disorders can be initiated by autoreactive TH17 cells, after tissue injury induces the release of self‐antigen. Autoimmune diseases that are linked to IL‐17 are invariably dependent on adaptive T cells, whereas innate IL‐17 producers are either not involved or have merely accessory roles.

Several diseases that are considered to be of autoimmune origin are clearly associated with TH17 cells, and deficiencies in TH17 cytokines or transcription factors regulating the development and function of TH17 cells often reduce pathology in various disease models (Hirota et al, 2010; Korn et al, 2009). Genome‐wide association studies demonstrated that the TH17‐related molecules IL‐23 and chemokine receptor CCR6 are associated with inflammatory bowel disease, rheumatoid arthritis and psoriasis in humans (Duerr et al, 2006; Kochi et al, 2010; Nair et al, 2009). Nevertheless, the mechanisms that underlie pathogenicity against self are largely unknown. The primary immune response elicited by TH17 cells is associated with preferential neutrophil infiltration, and innate immune cells activated by IL‐17 could damage the surrounding tissue. Indeed, TH17 cells directly cause the damage of axons, leading to neuronal dysfunction in a contact‐dependent manner (Siffrin et al, 2010). Recent reports have revealed that not only TH17 cells, but also IL‐17‐producing innate lymphocytes significantly contribute to the pathogenicity of autoimmune diseases. In experimental autoimmune encephalitis (EAE), γδ T cells produce IL‐17 early in response to IL‐1β and IL‐23, which are secreted by macrophages and dendritic cells before the ensuing TH17 response and amplified TH17‐mediated autoimmunity (Sutton et al, 2009). Although not autoimmune in nature, other pathological states can be induced by, for example, IL‐17‐producing NKT cells in the lung. These cells initiate ozone‐induced asthma, which is characterized by airway hypersensitivity and airway neutrophilia, but not eosinophilia (Pichavant et al, 2008). Furthermore, pulmonary IL‐17‐producing γδ T cells and TH17 cells promote lung fibrosis through increased synthesis and secretion of collagen with neutrophilia in bleomycin‐exposed mice (Mi et al, 2011; Wilson et al, 2010). In contrast to the cooperative action of innate cells with TH17 cells, colonic‐lineage negative innate lymphocytes that express Rorγt and IL‐23R produce IL‐17 and IFN‐γ, causing intestinal pathology in RAG‐deficient mice independently of innate and adaptive T cells (Buonocore et al, 2010). IL‐23 seems to be an essential cytokine to drive the pathogenicity of both innate and adaptive IL‐17‐producing cells, and an upstream mechanism underlying the induction of IL‐1β, IL‐6 and IL‐23 by macrophages and dendritic cells was reported. In synergy with pattern recognition receptor signalling, the activation of serum complement pathways stimulates macrophages and dendritic cells to help initiate TH17 differentiation and activation (Hashimoto et al, 2010; Lajoie et al, 2010).

Considering the adverse effects of dysregulated IL‐17, controlling innate and adaptive IL‐17‐producing cells could be a key therapeutic target for the prevention and treatment of IL‐17‐associated tissue inflammation. The master transcription factor Rorγt universally marks innate as well as adaptive IL‐17‐producing cells. Therefore, the inhibition of Rorγt by small molecules might limit the sustainment of IL‐17 immune reactions and the ensuing tissue damage. Three independent groups have shown that the natural cardiac glycoside digoxin and high‐affinity synthetic ligands for Rorα and Rorγt inhibit the activity of these master regulators of IL‐17 transcription and block the differentiation and effector function of TH17 cells (Fujita‐Sato et al, 2011; Huh et al, 2011; Solt et al, 2011). As the safety of digoxin is already established, it could be tested for its effect on the limitation of innate and adaptive IL‐17‐mediated tissue pathology.

Plasticity of TH17 cells

Terminally differentiated effector CD4 T cells were thought to be stable, but numerous recent studies have identified factors that reinforce or destabilize effector‐T‐cell programmes (Murphy & Stockinger, 2010). TH17 cells in particular seem to disobey the paradigm of irreversible commitment enunciated for TH1 and TH2 differentiation. Although ex vivo isolated TH17 cells appear relatively stable, _in vitro_‐generated TH17 cells are substantially plastic and can turn into IFN‐γ‐producing, TH1‐like, or IL‐4‐producing, TH2‐like cells in response to IL‐12 or IL‐4, respectively (Lexberg et al, 2008). Furthermore, _in vitro_‐generated TH17 cells can cause IFN‐γ‐dependent type 1 diabetes and colitis after transfer into lymphopaenic recipient mice (Bending et al, 2009; Lee et al, 2009; Nurieva et al, 2009). Epigenetic analysis revealed that _in vitro_‐generated TH17 cells have DNA modifications accessible to TH1 and TH2 lineage transcription factors, whereas _in vitro_‐generated TH1 and TH2 cells have epigenetic marks rendering IL‐17A and Rorγt inaccessible (Mukasa et al, 2010). To investigate the plasticity of TH17 cells in vivo, we generated a reporter mouse strain designed to fate‐map cells that have activated IL‐17. IL‐17 fate reporter mice show that TH17 cells developing during chronic inflammatory responses in the EAE model rapidly shut down expression of IL‐17A and switch on alternative cytokines, whereas in a model of acute cutaneous Candida infection, TH17 cells do not deviate to alternative cytokine production. During the development of EAE, TH17, rather than TH1, cells are the main source of IFN‐γ and other pro‐inflammatory cytokines in the spinal cord. Of note, T‐bet+ IFN‐γ‐producing ‘ex‐TH17’ cells driven by IL‐23 retain specific markers of the TH17 programme (Hirota et al, 2011). IL‐23 was also shown to differentiate T cells into a pathogenic IL‐17‐ and IFN‐γ‐producing subset in a colitis model (Ahern et al, 2010). Furthermore, T‐bet‐deficient T cells fail to develop into TH1‐like cells from ex‐TH17 cells (Lazarevic et al, 2011). Our data suggest that the vast majority of proinflammatory cytokine‐producing cells in EAE are originally TH17, suggesting that encephalitogenic T cells in EAE are primarily ex‐TH17 cells. This would change the interpretation of work reported by two independent groups showing that GM‐CSF‐producing T cells—instead of IFN‐γ‐producing or IL‐17‐producing T cells—are a pathogenic subset in EAE (Codarri et al, 2011; El‐Behi et al, 2011). Intriguingly, it has been shown that, in human arthritis, a large population of synovial IL‐17‐negative IFN‐γ‐producing TH1 cells express RORC2, CCR6 and CD161, which are markers of human TH17 cells (Nistala et al, 2010). In contrast to the deviation of TH17 cells to alternative proinflammatory cytokines in EAE, ex‐TH17 cells are not converted to a regulatory‐T‐cell phenotype that expresses FoxP3 or IL‐10 in this setting, neither in the draining lymph node nor in the spinal cord. However, in a model of tolerance induced by CD3‐specific antibody, activated CCR6+ TH17 cells are intriguingly redirected to the small intestine by chemotaxis to CCL20, which is secreted by the intestinal epithelial cells. Once there, the proinflammatory TH17 cells acquire a regulatory phenotype with in vitro and in vivo immune‐suppressive properties (Esplugues et al, 2011).

Memory is a central feature of the adaptive immune system. Given that TH17 cells easily switch off their signature cytokine and acquire alternative profiles in some immune responses, it is difficult to trace adaptive IL‐17 producers through the assessment of intracellular cytokine profiles. Indeed, based on cytokine staining, it has been suggested that there are no memory phenotype cells expressing IL‐17 (Pepper et al, 2010). The fate reporter system should allow a more detailed assessment of this issue. In agreement with our fate reporter study showing that a proportion of eYFP+ TH17 cells lacked IL‐17 expression ex vivo even after PdbU/ion restimulation, a recent study in human subjects also suggested that a proportion of human CCR6+ memory TH17 cells do not produce IL‐17 ex vivo after restimulation. Intriguingly, recall responses resulting in IL‐17 production were induced from IL‐17 CCR6+, but not CCR6− cells after in vitro culture with common γ‐chain cytokines, such as IL‐2, IL‐7 and IL‐15 (Wan et al, 2011). Further studies of memory formation and recall of distinct effector profiles are needed to understand the complexities of recalling memory cells into effector responses tailored to combat different types of pathogen that require specific cytokine responses (Sidebar A).

Concluding remarks

Accumulating evidence suggests that inborn errors of IL‐17‐mediated immune pathways precipitate CMCD and other infections in affected patients, emphasizing the crucial role of this cytokine for host defence against a range of pathogens. On the other hand, genome‐wide association studies have identified TH17‐related genes as contributors or initiators of autoimmune diseases such as multiple sclerosis, rheumatoid arthritis and psoriasis. These results clearly suggest that the activation and maintenance of IL‐17‐producing cells should be robust during infection and at the same time tightly regulated to avoid the risk of autoinflammatory disorders. Therefore, understanding the multiple steps of the IL‐17‐mediated immune response would help to develop future immunotherapy approaches for both infectious and autoimmune diseases. As the regulators of differentiation, maintenance and effector function between innate and adaptive IL‐17‐producing cells seem to be different, it is important to understand the distinct role and effector regulation of innate and adaptive IL‐17‐producing cell types in a variety of immune conditions. Lastly, epigenetic DNA modification and microRNAs are important factors in gene regulation, which can substantially modify the levels of target gene expression. Our understanding of IL‐17 regulation by epigenetic mechanisms and microRNAs at a mechanistic level in vivo is still incomplete. Future research and integrated information will hopefully define improved targeted therapies based on transcriptional and post‐transcriptional IL‐17 gene regulation and an effective control of beneficial and adverse IL‐17‐mediated immune responses.

Act1:

actin‐related gene 1

AhR:

aryl hydrocarbon receptor

apoE:

apolipoprotein E

Batf:

basic leucine zipper transcription factor, ATF‐like

CARD:

caspase recruitment domain family

CCL:

chemokine (C‐C motif) ligand

CCR:

chemokine (C‐C motif) receptor

CXCL:

chemokine (C‐X‐C motif) ligand

CD161:

killer cell lectin‐like receptor subfamily B member 1C

eYFP:

enhanced yellow fluorescent protein

FoxP3:

forkhead box P3

GM‐CSF:

granulocyte monocyte colony stimulating factor

HIF‐1α:

hypoxia‐inducible factor 1a

IFN‐γ:

interferon gamma

IκBζ:

inhibitor of κBζ

IL:

interleukin

IRF4:

interferon regulatory factor 4

iNKT cell:

invariant natural killer T cell

IKK‐α:

inhibitor of NF‐κB kinase alpha

LXR:

liver X receptor

mTOR:

mammalian target of rapamycin

NF‐κB:

nuclear factor κB

NK:

natural killer

NKT:

natural killer T

NOD:

nucleotide‐binding oligomerization domain

PdbU/ion:

phorbol 12,13‐dibutyrate/ionomycin

PPARγ:

peroxisome proliferator‐activated receptor gamma

RAG:

recombination activating gene

RelB:

reticuloendotheliosis viral (v‐rel) oncogene related B

Rheb:

Ras homologue enriched in brain

ROCK2:

Rho‐associated protein kinase 2

RORC2:

RAR‐related orphan receptor C2

Rorγt:

RAR‐related orphan receptor gamma t

Runx1:

runt‐related transcription factor 1

SOCS3:

suppressor of cytokine signalling 3

Srebp‐1c:

sterol regulatory element binding transcription factor 1c

STAT:

signal transducer and activator of transcription

T‐bet:

T‐box 21

TGF‐β:

transforming growth factor beta

TH:

T helper

TNF:

tumour necrosis factor

TRAF:

TNF‐receptor‐associated factor

References

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