OTHER ARTICLES PUBLISHED IN THIS MINI-REVIEW SERIES ON Th17 CELLS
Induction of interleukin-17 production by regulatory T cells. Clin Exp Immunol 2009; doi:10.1111/j.1365-2249.2009.04038.x
Are T helper 17 cells really pathogenic in autoimmunity? Clin Exp Immunol 2009; doi:10.1111/j.1365-2249.2009.04039.x
CD4+ T helper cells: functional plasticity and differential sensitivity to regulatory T cell-mediated regulation. Clin Exp Immunol 2009; doi:10.1111/j.1365-2249.2009.04040.x
Development of mouse and human T helper 17 cells. Clin Exp Immunol 2009; doi:10.1111/j.1365-2249.2009.04041.x
OTHER ARTICLES PUBLISHED IN THIS MINI-REVIEW SERIES ON Th17 CELLS
T helper (Th) cell have a central role in modulating immune responses. While Th1 and Th2 cells have long been known to regulate cellular and humoral immunity, Th17 cells have been identified only recently as a Th lineage that regulates inflammation via production of distinct cytokines such as interleukin (IL)-17. There is growing evidence that Th17 cells are pathological in many human diseases, leading to intense interest in defining their origins, functions and developing strategies to block their pathological effects. The cytokines that regulate Th17 differentiation have been the focus of much debate, due primarily to inconsistent findings from studies in humans. Evidence from human disease suggests that their in vivo development is driven by specialized antigen-presenting cells. Knowledge of how Th17 cells interact with other immune cells is limited, but recent data suggest that Th17 cells may not be subject to strict cellular regulation by T regulatory cells. Notably, Th17 cells and T regulatory cells appear to share common developmental pathways and both cell types retain significant plasticity. Herein, we will discuss the molecular and cellular regulation of Th17 cells with an emphasis on studies in humans.
CD4+ T helper (Th) cells play a central role in initiating and maintaining diverse immune responses. Functionally distinct Th cells are induced when naive T cells are stimulated via T cell receptor engagement in conjunction with co-stimulatory molecules and cytokines produced by innate immune cells. Classically, Th1 cells are thought to regulate cellular immunity via production of interleukin (IL)-2 and interferon (IFN)-γ, whereas Th2 cells regulate humoral immunity via production of IL-4, IL-5 and IL-13 [1,2]. More recently a third Th cell lineage, termed Th17 cells, has been identified and defined based on their capacity to secrete IL-17 but not IFN-γ or IL-4 [3–7]. In addition to effector cells, CD4+ Th cells can also develop into a variety of regulatory subsets (Treg cells) defined by expression of forkhead box P3 (FoxP3) and/or their capacity to produce cytokines such as transforming growth factor (TGF)-β, IL-10 and IL-35 [8,9]. Based on evidence that Th17 cells can mediate inflammation and tissue destruction [10,11], there has been intense interest in defining their origins, functions and developing strategies to block their pathological effects. The differentiation [12,13] and phenotype of human Th17 cells has been reviewed recently in several papers [14–17]. Here we will review briefly the protective role of human Th17 cells in host defence and highlight diseases for which there is substantial evidence for a pathological role of these cells [16,18,19]. The reader is also referred to several recent reviews on this topic [11,20,21]. We will also discuss how Th17 cells interact with other immune cells, with a specific focus on their relationship to, and interactions with, Treg cells.
Phenotype and differentiation of human Th17 cells
Human Th17 cells are currently defined as cells that produce IL-17A and F, but not IFN-γ or IL-4. Th17 cells are also capable of producing tumour necrosis factor (TNF)-α, IL-6, IL-22 [22–27], as well as IL-21  and IL-26 , but as these cytokines are also produced by other Th cell subsets it is currently unclear whether they should be included as part of their defining cytokine-production profile. The pattern of chemokine and cytokine receptor expression on human Th17 cells has been investigated extensively [22,23,26,27,29–31], and it is now generally accepted that they express CCR4, CCR6 and IL-23R, but not CXCR3. In addition CD161, the human orthologue of NK1·1, is associated with human Th17 cells: cord blood mononuclear cells expressing CD161 contain the precursors Th17 cells ; circulating Th17 cells from healthy adults and Th17 cells from inflamed tissues from psoriasis patients exist within the CD161+ fraction of CD4+ T cells ; and CD161 is expressed highly in peripheral and gut-resident human Th17 cells .
In addition to IL-17A, IL-17 family members include IL-17B, IL-17C, IL-17D and IL-17E (also referred to as IL-25) . Of these, IL-17F shares the greatest homology with IL-17A and due to close proximity on the chromosome, as well as a co-ordinated expression pattern, the two cytokines are likely regulated by similar mechanisms . IL-17A is the most extensively studied of the IL-17 family, mediating many of the known effector functions of Th17 cells, and will be referred to from here onwards as IL-17. The receptor for IL-17 is a heterodimer of IL-17RA and IL-17RC , both of which are ubiquitously expressed type I membrane glycoproteins [36–39]. IL-17 has diverse biological functions, but the best characterized relate to its proinflammatory effects. Specifically, IL-17 recruits neutrophils via effects on granulopoiesis [40,41] and CXC chemokine induction , acts on macrophages to promote their recruitment and survival  and stimulates the production of proinflammatory cytokines and anti-microbial peptides from a variety of immune and non-immune cells [34,41,44–46]. We have shown recently that IL-17 can also enhance the capacity of human CD4+ T cells to produce IL-2, as well as enhance the proliferation of both conventional T (T) cells and Treg cells .
The combination of cytokines that stimulate the development of human Th17 cells is a subject of much debate. Notably, it is difficult to compare the results of experiments designed to define how human Th17 cells differentiate in vitro, because many different activation conditions, types of culture media and experimental readouts have been reported. Initial studies found that T cell activation in the presence of IL-1β, IL-6 and/or IL-23 was sufficient to induce Th17 cells, and that TGF-β1 inhibited this process [24,27,48–50]. In subsequent studies, however, TGF-β1 was reported to be important for the development of human IL-17-producing cells [25,29,51]. This discrepancy could be explained by a recent report showing that the requirement for TGF-β in the differentiation of human Th17 cells in vitro is indirect, and related to suppression of Th1 development . While IL-23 appears to be involved in the expansion and pathogenicity of murine Th17 cells, in humans the majority of studies have found a central role for IL-23 in directing Th17 cell development [25,50,53]. This requirement for IL-23, defined by in vitro studies, correlates with strong genetic evidence linking IL-23 to the development of Th17-associated diseases .
In addition to cytokine-driven Th17 lineage commitment, it has also been shown that prostaglandin E2 (PGE2), which is a mediator of tissue inflammation, directly promotes the differentiation, expansion and proinflammatory function of human and mouse Th17 cells [54,55]. In humans, PGE2 induces up-regulation of IL-23R and IL-1R and synergizes with IL-1β and IL-23 to induce a Th17-associated profile of transcription factor, cytokine and chemokine/chemokine receptor expression . Interestingly, exposure of memory CD4+ T cells to PGE2 results in populations that are enriched for IL-17-producing cells, possibly via the partial resistance of CCR6+ IL-17+ cells to the inhibitory effects of PGE2 on memory T cell proliferation .
In an interesting recent development, Sundrud et al. reported that a small molecule known as halofuginone specifically inhibits the development of mouse and human Th17 cells, but not Th1, Th2 or Treg cells . The mechanism of action appears to be via amino acid starvation, as Th17 cell inhibition can be rescued by the addition of excess amino acids. These data suggest that Th17 cell development is particularly sensitive to stress and starvation signals compared to other Th cell lineages. Notably, as halofuginone is already used clinically to treat fibrotic diseases such as graft-versus-host-disease and scleroderma, this finding could be translated rapidly to other clinical settings associated with excess Th17 cells. However, halofuginone inhibits only the differentiation and not the effector functions, of Th17 cells, it may only be effective if used early in disease onset.
Transcription factors involved in Th17 cell development
Studies in mice have identified at least five transcription factors important for Th17 cell development and function: a T cell-specific splice isoform of retinoic acid receptor-related orphan receptor (Ror) known as Ror-γt , a splice isoform of Ror-α (Ror-αd) , IFN-regulatory factor 4 (Irf-4) , signal transducer and activator of transcription 3 (STAT3) [61,62] and the aryl-hydrocarbon receptor (AhR) [63–65]. Of these proteins, Ror-γt appears to be the most specific for Th17 cells and is now referred to commonly as the Th17 lineage defining transcription factor. Indeed, deficiency in Ror-γt results in a significant reduction of Th17 cells in the gut and protection from experimental autoimmune encephalomyelitis (EAE). By corollary, over-expression of Ror-γt promotes Th17 development  and Ror-α may act in synergy with Ror-γt to promote these effects . Irf-4 is a transcription factor which may act upstream of both Ror-α and Ror-γt, as Irf-4-deficient mice fail to up-regulate Ror-γt under Th17 polarizing conditions and ultimately fail to develop Th17 cells and EAE . STAT-3 is also important early in Th17 lineage commitment: STAT-3-deficient mice have impaired expression of Ror-γt and IL-23R and fewer Th17 cells . Several studies have also found that the AhR regulates Th17 cells [63–65] and that different AhR ligands can direct development or expansion of different T cells subsets in mice. FICZ, a ‘natural’ AhR ligand, increases Ror-γt expression selectively, expands Th17 cells, and therefore worsens the severity of EAE. In contrast, 2,3,7,8-tetrachlorodibenzo-dioxin (TCDD), a ‘synthetic’ AhR ligand, expands Treg cells and effectively prevents EAE .
Similar to their mouse counterparts, human Th17 cells also express RORC2, the human orthologue of Ror-γt [23,24,48] and over-expression of RORC2 in cord blood CD4+ T cells induces expression of IL-17A, IL-17F, IL-26 and CCR6, but not IL-22, CCR4 or CCR2  (Fig. 1). We recently examined the role of RORC2 in Th17 cell development in adult CD4+ T cells and found that over-expression of RORC2 induces many aspects of the Th17 cell phenotype, including expression of CCR6, CCR4, CD161, down-regulation of CXCR3 and induction of a Th17-associated cytokine profile . Unexpectedly, over-expression of RORC2 also led to decreased expression of granzymes A and B and induction of hyporesponsiveness to T cell receptor (TCR) stimulation, which could be overcome by the addition of IL-2 or IL-15 . Notably, over-expression of RORC2 induced IL-17 production in only ∼20% of transduced T cells, suggesting that expression of other transcription factors in addition to RORC2 is probably required to recapitulate human Th17 development fully.
The role of other Th17-associated transcription factors in the development of human Th17 cells is characterized relatively poorly. It is known that transduction of human cord blood cells with ROR-αd increases IL-17 expression  suggesting that, as in mice, this factor may co-operate with RORC2 to induce Th17 cells. STAT3 also appears to be critical for the development of human Th17 cells based on evidence from patients with hyper-immunoglobulin (Ig)E sydrome who carry autosomal dominant mutations in STAT3. T cells from these patients fail to differentiate into Th17 cells in vitro due to a lack of IL-6-stimulated STAT3 activation and consequently RORC2 expression [66,67]. The AhR is also expressed by human Th17 cells  but it is not known if Ahr contributes to their development.
Th17 cells in human health and disease
Although Th17 cells have been recognized only recently as a distinct lineage of Th cells, associations between IL-17 and human disease have been known for many years. Human IL-17A was cloned in 1995  and was shown to be expressed by T cells with inflammatory effects on epithelial, endothelial and fibroblast cells [41,68]. IL-17 links innate and adaptive immunity and has both beneficial and pathological effects on the immune system. The role of IL-17 in health and disease has been reviewed extensively [11,19,69], and here we will highlight select examples of situations in humans where there is significant evidence for a role of Th17 cells and where interventions targeting Th17 cells could provide therapeutic benefit.
Host defence to bacteria, fungi and viruses. The role of Th17 cells in host defence against pathogens has been characterized extensively in mouse models, with the general consensus that IL-17 is necessary for protective immunity against bacteria such as Escherichia coli and Salmonella, and in the lung Klebsiella pneumoniae and Bordetella pertussis. In some cases, over-production of IL-17 or IL-23 may be deleterious such as during gastric inflammation in response to Helicobacter pylori[70,71] or endotoxic shock induced by Pseudomonas aeruginosa. In humans, the role of Th17 cells in anti-bacterial responses is largely unexplored. Limited data in the context of cystic fibrosis, in which chronic infections by P. aeruginosa is associated with adverse clinical outcomes suggest that Th17 cells may induce harmful lung damage by promoting inflammation . Th17 cells may also be undesirable in the context of periodontal disease caused by Gram-negative bacteria, as stimulation of peripheral blood mononuclear cells (PBMCs) with Porphyromonas gingivalis induces expression of IL-17  and increased numbers of IL-1β-, TNF-α- and IL-17-secreting cells are found in tissue with gingivitis. In this context, however, it is also possible that these Th17 cells are beneficial for the anti-P. gingivitis immune response.
Several reports from mouse and human studies have shown that Th17 cells are important for clearing opportunistic infections such as Cryptococcus neoformans, Pneumocystis carinii and Candida albicans. In humans, the T cell response to C. albicans requires co-ordinated production of both Th1 and Th17 cells. The yeast form of this fungus induces human dendritic cells (DCs) to produce IL-12 and ultimately induces Th1 cells, whereas the hyphae form C. albicans hyphae exclusively induces IL-23 and Th17 cells . Th17 cells are also important for clearing staphylococcal infections, based on evidence from patients with hyper-IgE syndrome who lack Th17 cells and whose keratinocytes and bronchial epithelial cells require cytokines produced by Th17 cells to control these bacteria efficiently .
Viruses such as herpes simplex virus (HSV), respiratory syncytial virus (RSV), rotavirus and human leukaemia virus type 1 (HTLV-1) can also induce IL-17 responses . In HTLV-1, the viral Tax protein activates the IL-17 gene promoter in T cells via the CREB/ATF [cAMP (adenosine 3′5′ cyclic monophosphate) response element binding protein/activating transcription factor] pathway . In human immunodeficiency virus (HIV), production of IL-17 by peripheral T cells is elevated compared to healthy controls . In general, IL-17 is thought to exaggerate the inflammatory responses to these viruses, possibly resulting in enhanced mortality [78–82]. Recently, it was found that hepatitis C virus-specific Th17 cells are suppressed by virus-induced TGF-β, suggesting that inhibition of Th17 cells may be a viral immune evasion strategy .
Psoriasis and rheumatoid arthritis. Psoriasis is a chronic inflammatory skin disease that results in epidermal hyperplasia, dermal angiogenesis and infiltration of monocytes, DCs and T cells. Both Th1 and Th17 cells are implicated in the pathogenesis, as there are elevated levels of both Th1- and Th17-associated cytokines in serum and lesional skin [24,84–87]. The most powerful evidence for the pathogenic role of Th17 cells in psoriasis comes from a clinical study which found that antibody-mediated blockade of the shared IL-23/IL-12 p40 subunit is an effective treatment: 81% of patients had at least a 75% improvement compared to only 2% of patients who received placebo . Targeting TNF-α is also an effective treatment for this disease, and improved clinical outcomes in this context are also associated with down-regulation of many Th17-associated cytokines including IL-17, IL-22, IL-6, IL1β and IL-23, in addition to CCL20 and anti-microbial peptides .
Rheumatoid arthritis (RA), which has long been classified as a Th1-mediated disease, is now also thought to be a primarily Th17-driven disease . Initial evidence for a pathogenic role of IL-17 in RA came from reports that IL-17 was increased in the sera and synovial fluids of RA patients [91–93]. As with psoriasis, there is also increased IL-22 and IL-23 in the synovium of RA patients [94,95]. Notably, the increase in IL-17 and IL-23 appears to be specific for RA, because a similar increase was not found in patients with osteoarthritis [96,97]. As cyclosporine A can inhibit the production of IL-17 by memory Th17 cells in healthy donors and RA patients , this could be an effective strategy to limit disease. Future studies will determine if therapeutic strategies that specifically block Th17 cells are effective in treating RA patients.
Multiple sclerosis (MS). There is also a strong association between IL-17 and MS , but knowledge of the role IL-17 in this demyelination disorder is limited. Studies in mice have shown that Th17 cells play a critical role in both the induction and progression of EAE, the murine model of MS. In humans with MS it is not clear whether IL-17 mediates its potentially pathogenic effects directly or via recruitment of other immune cells. Human blood–brain barrier (BBB) endothelial cells from MS patients express the receptors for IL-17 and IL-22, and exposure of these cells to IL-17, but not IL-22, results in low expression of tight junction proteins and increased transmigration of CD4+ T cells . Moreover, IL-17 elevates expression of matrix metalloproteinase expression, leading to BBB dysfunction and neuronal apoptosis [100,101]. Thus, a major pathological mechanism in MS could be IL-17-mediated destruction of the BBB allowing easier access of myelin-specific T cells to this usually immune privileged site.
Inflammatory bowel disease. Perhaps the strongest evidence for the pathological role of Th17 cells comes from studies of inflammatory bowel disease (IBD) [102,103]. In this disease there are genetic, correlative and therapeutic data which all support the hypothesis that Th17 cells drive intestinal inflammation. A large study of Europeans of different ethnicities found that a single nucleotide polymorphism in a non-coding region of the IL-23R was significantly associated with both Crohn's disease and ulcerative colitis . Interestingly, there is also a coding variant of IL-23R that confers protection from Crohn's disease . CD4+ T cells from peripheral blood and tissues of Crohn's disease patients express high levels of IL-17, with up to 40% co-expressing IFN-γ and clustering in the lamina propria . Additionally, high IL-17 is found in sera and colonic biopsies of Crohn's disease patients [105,106]. IL-22 is also over-expressed by colonic CD4+ T cells in patients with IBD compared to healthy controls . Antibodies against p40, the shared subunit between IL-12 and IL-23, are currently being tested in Phases 1 and 2 trials for Crohn's disease, with initial evidence suggesting that blocking IL-23 results in clinical remission [108,109].
Interactions between Th17 cells and other immune cells
Antigen-presenting cells (APCs). APCs play a central role in directing immune responses by secreting cytokines that polarize CD4+ T cells into distinct lineages (Fig. 2). In an effort to understand more clearly how pathological Th17 cells arise in disease, several groups examined the phenotype and function of APCs from patients with a variety of autoimmune diseases. Evans et al. found that monocytes from the inflamed joints of RA patients promote the development of Th17 but not Th1 or Th2 cells  via a cell-contact-dependent mechanism. In MS, Vaknin-Dembinsky et al. found that monocyte-derived DCs from patients secrete elevated levels of IL-23 compared to healthy controls . In addition, when CD14+ monocytes migrate across the BBB they differentiate into CD83+CD209+ DCs that promote expansion of both Th1 and Th17 cells . In psoriasis, Th1 cells may induce Th17 cells indirectly via effects on DCs as myeloid DCs from these patients secrete IL-1β, IL-23 and CCL20, promoting both the development of Th17 cells and their migration to the skin . Together these studies lend support to the hypothesis that changes in APC function probably precede inappropriate development and expansion of Th17 cells.
An outstanding question regards what the initial stimulus is that polarizes APCs to make cytokines that promote Th17 cells. Evans et al. reported that optimal induction of human Th17 cells requires T cell receptor ligation accompanied by Toll-like receptor (TLR)-activation of monocytes . Van Beelen et al. reported that stimulation of non-obese diabetic 2 (NOD2) programmes DCs to promote IL-17 production by memory T cells in humans  and van de Veerdonk et al. found that TLR agonists, or the NOD2 ligand MDP, induce IL-17 production from PBMCs . In contrast, Candida-induced Th17 responses are amplified by the TLR2/dectin-1 pathway but not via TLR-4 or NOD2 . Clearly, further study of how innate immune stimuli influence different types of APCs and promote Th17 differentiation are required in order to understand how this process may lead to disease.
B cells. Modulation of B cell function and antibody production by Th cells is a key step in co-ordinating immune responses. Th1 cells typically promote secretion of IgG antibodies, whereas Th2 cells induce class-switching to IgE antibody production . Recently, Th17 cells were found to promote production of IgM, IgG and IgA but not IgE , demonstrating that like Th1 and Th2 cells, Th17 cells modulate B cell function. Human alloreactive Th17 cell clones, but not Th1 or Th2 cell clones, express the B cell chemoattractant CXCL13, which is preferentially expressed by Th17 clones from the synovial fluid of RA patients . These data suggest that Th17 cells may actively recruit B cells to sites of inflammation. As Th17 cells have recently been associated with various allergic and autoimmune disorders, it will be important to determine if the ability of Th17 cells to modify B cell function directly significantly contributes to these pathologies, or if their effects are primarily via modulation of other immune cells such as granulocytes.
CD4+ T effector cells. Th1 and Th2 cells have long been known to antagonize each other's differentiation and function, and it is therefore not surprising that IFN-γ from Th1 cells and IL-4 from Th2 cells inhibit Th17 development [5,44,117] (Fig. 2). There is evidence that IL-17 can modulate Th1 lineage commitment, at least in mice, by suppressing transcription of T-bet and other Th1-associated genes . This is not a consistent finding, however, as it has also been reported that IL-17 does not inhibit Th1 or Th2 differentiation . The inhibitory effects are limited to differentiation, as IL-17 does not affect fully polarized Th1 cells . A limited number of studies have examined the effects of IL-17 on human CD4+ T cells, which are known to express IL-17RA . We recently found that exposure of human CD4+ T cells to IL-17 does not significantly alter expression of IL-4 or IFN-γ but does induce IL-2 expression , suggesting that this cytokine may promote T cell proliferation and survival.
Of note is the finding that cells co-expressing IL-17 and IFN-γ, as well as T-bet and RORC2, can readily be isolated from human peripheral blood . In the future it will be important to determine if these so-called Th1/Th17 cells are a distinct and stable lineage of CD4+ T effector cells, or whether they are an intermediate state destined to become true Th1 or Th17 cells (Fig. 3). Recent genome-wide histone H3 lysine 4 (H3K4) and lysine 27 (H3K27) trimethylation maps of polarized Th1, Th2, Th17 and Treg cells from mice revealed that there is a significant potential for plasticity based on the patterns of epigenetic marks in the lineage-defining transcription factors . Thus, the local cytokine milieu may induce new transcription factors and modify cytokine production even in fully polarized effector T cell lineages. Teleologically, this process may maximize flexibility to ensure that the best and most appropriate type of immune responses can be mounted against pathogens. It is also possible that these lineage-defining transcription factors serve functions yet to be identified in all CD4+ T cell subsets. Supporting this concept is a recent finding that inhibiting the expression of T-bet, a Th1-associated transcription factor, decreased both Th1 and Th17 differentiation .
CD4+FoxP3+ Tregs. There appears to be a close developmental relationship between Tregs and Th17 because, at least in mice, the development of both cell types requires TGF-β[5,53,122]. The balance of the two cell types may depend upon the stability of FoxP3 expression, as this transcription factor modulates the function of both Ror-α and Ror-γt [123,124] (Fig. 1). For example, Ichiyama et al. found that FoxP3 directly inhibits Ror-γt-induced transcription of IL-17A mRNA via the exon 2 region and forkhead domain of FoxP3  and Du et al. found that FoxP3 specifically inhibits ROR-α-mediated transcriptional activation . Interestingly, transient FoxP3 expression occurs during in vitro differentiation of Th17 cells , raising the possibility that FoxP3 also has a role in the development of Th17 cells. This concept is supported by recent evidence that FoxP3+ T cells in humans can co-express IL-17, as discussed in more detail below and elsewhere in this series  and may in fact enhance development of Th17 cells via their capacity to produce TGF-β. In support of this theory, Treg cells enhanced IL-17 production in systemic autoimmunity in vivo. A model of antagonism and plasticity has arisen from these studies, and it is now generally accepted that whereas TGF-β alone induces FoxP3 and inhibits Th17 differentiation, if inflammatory cytokines such as IL-6 are present, TGF-β-induced FoxP3 is suppressed and the expression of Th17-defining transcription factors is favoured. .
This equilibrium between Th17 and Treg cells is regulated further at the epigenetic level and several recent studies in mice and humans have shown that IL-17-producing cells can arise from Treg cells [120,129–132]. Focusing on studies in humans, Koenen et al. found that CD25+ FoxP3+ Tregs differentiate into IL-17-producing cells in the presence of IL-1β, IL-23, IL-21 and IL-2 . In agreement, Deknuydt et al. reported that Treg cells convert to Th17 cells when stimulated in the presence of IL-1β and IL-2 and lost suppressive functions . Inclusion of histone deacetylase inhibitors prevented the induction of IL-17, indicating that conversion depends on epigenetic modifications . Several reports have also described populations of Treg cells that co-express FoxP3 and IL-17 and retain their suppressive function (Fig. 3). For example, in ROR-γt-reporter mice a subpopulation of IL-17+ROR-γt+ T cells co-express FoxP3 and retain suppressive function . In humans, Beriou et al. found that CD25hihuman leucocyte antigen D-regulated (HLA-DR)negFoxP3+ Treg cells secrete IL-17 ex vivo and in vitro. Depending upon the strength of stimulation, IL-17+FoxP3+ T cell clones retained their suppressive capacity . Similarly, Voo et al. found a significant population of peripheral CD4+CCR6+ Treg cells that produce IL-17 co-express both FoxP3 and RORC2, and retain suppressive capacity . Because a similar population of cells was not found in the thymus, it is possible that the co-expression of FoxP3 and IL-17 can occur only in Treg cells that were induced in the periphery . In line with this hypothesis, it was reported that the majority of IL-17-producing Treg cells are contained within the CD45RO+ fraction of CD25hi cells, indicating that these cells were exposed to peripheral antigens in vivo[136,137]. Similar to Th17 cells, IL-17+FoxP3+ T cells express RORC2 as well as CCR4 and CCR6 . Unlike conventional Th17 cells, however, these IL-17+FoxP3+ T cells do not express CD161, IL-22 or TNF-α. It is currently unclear whether FoxP3+IL-17+ T cells represent a transitional state or a unique lineage of Th cells. Overall, the existence of these cells is perplexing: why would an immunosuppressive cell benefit from secretion of a proinflammatory cytokine such as IL-17? It is possible that IL-17-producing Treg cells may serve a role in host defence by promoting neutrophil differentiation and trafficking, while serving simultaneously to control inflammation and autoimmunity .
An outstanding question is whether Th17 cells are susceptible to the suppressive effects of Treg cells, which is discussed in detail in this series by O'Connor et al.. As most diseases for which Treg-based cell therapies are being considered are associated with Th17 cells, it is critical to understand whether adoptive therapy with Treg cells will ameliorate this aspect of inflammation. Several groups have reported that human Th17 cells are resistant to Treg-mediated suppression of proliferation and production of IL-17 [23,49,128]. We have developed recently a system based on over-expression of RORC2 to examine this question in more detail. We found that although Treg cells could suppress TNF-α and IL-6 produced by RORC2-transduced T cells, they failed to suppress IL-17 . Carboxyfluorescein succinimidyl ester (CFSE) labelling experiments confirmed that the lack of IL-17 suppression was not due to de novo production of IL-17 by the Treg cells themselves. We are currently testing whether Treg cells are similarly impaired in their ability to suppress IL-17 from ex vivo Th17 cells. Notably, the interpretation of the results of all these studies may need to be modified with the new knowledge that the memory fraction of CD25hi Treg cells contains a significant number of IL-17-producing cells ex vivo. Further research into how Th17 cells are kept in check will be critical to the development of effective therapies to dampen dysregulated Th17 responses.
While we still have much to learn about the phenotype, function and regulation of human Th17 cells, it is clear that IL-17 and other Th17-associated cytokines have a central role in regulating diverse immune responses. Work in mouse models has defined many of the biological properties of Th17 cells. More studies of human Th17 cells are now required to understand how they interact with immune and non-immune cells as we attempt to define Th17-driven pathology and translate strategies to limit their function. Understanding the intricate regulatory mechanisms that govern T helper cell subsets at the cellular and molecular level will undoubtedly yield new therapies for dysregulated immune responses.
The authors declare no conflicts of interest.