Phenotypic and functional features of human Th17 cells

Th17 development and autoimmune arthritis in the absence of reactive oxygen species

Dendritic cells (DC) express a functional NADPH oxidase and produce reactive oxygen species (ROS) upon interaction with microbes and T cells. Exposure to ROS leads to DC activation and maturation, as evidenced by phenotypic and functional changes. We have evaluated how endogenous ROS production affects the cytokine secretion pattern and T cell-activating capacity of bone marrow-derived murine DC. DC treated with ROS scavengers, as well as DC from mice that lack a functional NADPH oxidase (and thereby inherently deficient in ROS production) produced significantly increased levels of IL-1beta, IL-6, TNF-alpha and TGF-beta in response to microbial activation. DC deficient in ROS production induced high levels of IFN-gamma and IL-17 in responding T cells after Ag-specific or superantigen-induced activation. Finally, we show that ROS deficiency affected the induction of a T cell-dependent inflammatory condition, collagen-induced arthritis (CIA). C57BL/6 mice that lack a functional NADPH oxidase developed a severe and erosive CD4-dependent CIA, whereas the majority of the congenic wild-type animals remained healthy. These data suggest that ROS act as immunomodulators in DC-driven T cell activation and perhaps also in T cell-dependent immunopathology.

Lack of Toll IL-1R8 exacerbates Th17 cell responses in fungal infection

TLRs contribute to the inflammatory response in fungal infections. Although inflammation is an essential component of the protective response to fungi, its dysregulation may significantly worsen fungal diseases. In this study, we tested the hypothesis that Toll IL-1R8 (TIR8)/single Ig IL-1-related receptor, a member of the IL-1R family acting as a negative regulator of TLR/IL-1R signaling, affects TLR responses in fungal infections. Genetically engineered Tir8(-/-) mice were assessed for inflammatory and adaptive Th cell responses to Candida albicans and Aspergillus fumigatus. Inflammatory pathology and susceptibility to infection were higher in Tir8(-/-) mice and were causally linked to the activation of the Th17 pathway. IL-1R signaling was involved in Th17 cell activation by IL-6 and TGF-beta in that limited inflammatory pathology and relative absence of Th17 cell activation were observed in IL-1RI(-/-) mice. These data demonstrate that TIR8 is required for host resistance to fungal infections and that it functions to negatively regulate IL-1-dependent activation of inflammatory Th17 responses. TIR8 may contribute toward fine-tuning the balance between protective immunity and immunopathology in infection.

Role of beta7 integrin and the chemokine/chemokine receptor pair CCL25/CCR9 in modeled TNF-dependent Crohn's disease

BACKGROUND & AIMS

In the present work, we address the requirement for intestinal-specific homing molecules, the chemokine/chemokine receptor pair CCL25/CCR9 and beta7 integrin, in the pathogenesis of the CD8(+) T cell-dependent Tnf(DeltaARE) mouse model of Crohn's-like inflammatory bowel disease.

METHODS

We investigated by flow cytometry lymphocyte recruitment in the intestinal epithelium and lamina propria (LP); cytokine production by intraepithelial and LP lymphocytes; and peripheral expression of CCR9, alpha4beta7, and alphaEbeta7 integrin. The functional significance of CCL25/CCR9 and beta7 integrin in inflammatory lymphocyte recruitment and intestinal disease development was assessed in Tnf(DeltaARE) mice genetically lacking these molecules.

RESULTS

Intestinal inflammation in the Tnf(DeltaARE) mice is associated with early reduction of CD8alphaalpha-expressing intraepithelial lymphocytes, decreased T helper cell 1 and increased T helper cell 17 responses by LP CD4(+) lymphocytes, increased alphaEbeta7 integrin expression in peripheral activated/memory intestinal-homing CD8alphabeta lymphocytes, and predominance of tumor necrosis factor/interferon-gamma-producing CD8alphabeta lymphocytes in the epithelium. Although CCL25/CCR9 have been strongly implicated in T-lymphocyte recruitment to the small intestine, inflammatory pathology develops unperturbed in the genetic absence of CCL25/CCR9. Furthermore, CD8alphabeta lymphocyte recruitment in the intestinal epithelium and inflammatory infiltration in the LP are not impaired in CCR9- or CCL25-deficient Tnf(DeltaARE) mice. In contrast, genetic ablation of beta7 integrin results in complete amelioration of intestinal pathology.

CONCLUSIONS

Our findings demonstrate that development of intestinal inflammation in the Tnf(DeltaARE) mice is critically dependent on beta7 integrin-mediated T-lymphocyte recruitment, whereas the function of the CCL25/CCR9 axis appears dispensable in this model.

The Interleukin-23 / Interleukin-17 axis in intestinal inflammation

Although the precise aetiology of inflammatory bowel disease (IBD) remains unclear, recent discoveries have led to an improved understanding of disease pathogenesis. Whilst these findings have underscored the central role of innate and adaptive immune responses in intestinal inflammation, they have also precipitated a paradigm shift in the key cytokine pathways that drive disease. The prevailing dogma that IBD was mediated by interleukin (IL)-12-driven T-helper (Th)1 CD4 T cell responses towards the bacterial flora has been largely dispelled by findings that the closely related cytokine IL-23 appears to be the key mediator of intestinal inflammation. IL-23 is associated with a novel subset of IL-17-secreting CD4 T cells termed Th17 cells and rodent studies have implicated the IL-23/IL-17 axis in autoimmune inflammation. Genome-wide association studies in IBD patients have confirmed the predominant role of the IL-23 pathway, indicating that this could represent an important future therapeutic target.

In vivo equilibrium of proinflammatory IL-17+ and regulatory IL-10+ Foxp3+ RORgamma t+ T cells

The nuclear hormone receptor retinoic acid receptor–related orphan receptor γt (RORγt) is required for the generation of T helper 17 cells expressing the proinflammatory cytokine interleukin (IL)-17. In vivo, however, less than half of RORγt⁺ T cells express IL-17. We report here that RORγt⁺ Tαβ cells include Foxp3⁺ cells that coexist with IL-17–producing RORγt⁺ Tαβ cells in all tissues examined. The Foxp3⁺ RORγt⁺ Tαβ express IL-10 and CCL20, and function as regulatory T cells. Furthermore, the ratio of Foxp3⁺ to IL-17–producing RORγt⁺ Tαβ cells remains remarkably constant in mice enduring infection and inflammation. This equilibrium is tuned in favor of IL-10 production by Foxp3 and CCL20, and in favor of IL-17 production by IL-6 and IL-23. In the lung and skin, the largest population of RORγt⁺ T cells express the γδ T cell receptor and produce the highest levels of IL-17 independently of IL-6. Thus, potentially antagonistic proinflammatory IL-17–producing and regulatory Foxp3⁺ RORγt⁺ T cells coexist and are tightly controlled, suggesting that a perturbed equilibrium in RORγt⁺ T cells might lead to decreased immunoreactivity or, in contrast, to pathological inflammation.

Negative regulation of IL-17 production by OX40/OX40L interaction

The T-cell cytokine IL-17 is implicated in multiple inflammatory diseases through its induction of several pro-inflammatory cytokines and chemokines in a broad range of cell targets. Production of IL-17 defines the Th17 subset of helper T-cells associated with protection against microorganisms, a profile best characterized in the murine system. Multiple regulators of Th17 cell differentiation and IL-17 production are reported, but the impact of OX40L is not described. OX40 ligand (OX40L) is an early-stage activator of T-cells through its interaction with CD134 (OX40) that is up-regulated on antigen challenged T-cells. Here, we show that OX40L suppresses IL-17 production by PHA-stimulated human PBMC and purified CD4 and CD8 cells. In agreement with prior reports, OX40L signaling through CD134 increased IFNgamma and IL-4, both of which are reported to inhibit the production of IL-17. OX40L suppression of IL-17 was completely reversed by a neutralizing IFNgamma antibody while there was no effect with a neutralizing IL-4 antibody. Moreover, OX40L also suppressed IL-17 in the presence of IL-23, an established inducer of IL-17 and differentiation factor for Th17 cells. Presuming mediation by IFNgamma, we evaluated expression of this cytokine in the presence of OX40L and IL-23. Surprisingly, IL-23 also induced IFNgamma by PHA-stimulated T-cells and this effect was enhanced in the presence of OX40L. Addition of the IFNgamma antibody not only reversed the OX40L suppression of IL-17 in the presence of IL-23, it markedly enhanced the level of IL-17. These results further establish IFNgamma as a primary modulator of IL-17 production in the human cells, much as in the murine system.

[Interleukin-17 in chronic inflammatory and autoimmune diseases: rheumatoid arthritis, Crohn's disease and Wegener's granulomatosis]

Data from recent animal experiments and clinical studies show that interleukin-17 (IL-17A, B, C, D, E, and F) plays an important role as proinflammatory cytokine in the host response to extracellular bacteria and in chronic inflammatory and autoimmune diseases. These findings have led to a revision of the well-known T(H)1/T(H)2 hypothesis. In rheumatoid arthritis elevated IL-17 serum levels, Th-17 cells in synovial fluid and in T-cell-rich areas of inflamed synovia are found. In Wegener's granulomatosis, IL-17D is expressed in nasal granulomas. In Crohn's disease IL-17 as well as IL-17 plus IFN-gamma producing CD4(+) T-cells are detected in peripheral blood and inflamed intestinal mucosa. So far, CD4(+)IL-17(+)IFN-gamma(+) T-cells have been described only in humans. These and other findings indicate a number of differences in the cytokine response between murine models and human beings.

Differential regulation of the IL-17 receptor by gammac cytokines: inhibitory signaling by the phosphatidylinositol 3-kinase pathway

The gammac-family cytokine IL-2 activates signaling events that contribute to cell survival and proliferation, the best-studied of which are the STAT-5 and phosphatidylinositol 3-kinase (PI3K) pathways. The starting point of this study was to define genes regulated by the IL-2R-mediated PI3K pathway in T cells. Accordingly, we used an erythropoietin (EPO) receptor chimeric receptor system in which IL-2-dependent HT-2 T cells expressed a mutant EPO-IL-2Rbeta construct where Tyr-338 is mutated to Phe. Cells expressing this mutant IL-2Rbeta chain fail to induce phosphorylation of PI3K-p85alpha/beta or activate Akt, but mediate normal IL-2-dependent proliferation and activation of JAK1 and STAT-5A/B. Microarray analyses revealed differential regulation of numerous genes compared with cells expressing a wild-type IL-2Rbeta, including up-regulation of the IL-17 receptor subunit IL-17RA. Blockade of the PI3K pathway but not p70S6K led to up-regulation of IL-17RA, and constitutive Akt activation was associated with suppressed IL-17RA expression. Moreover, similar to the mutant EPO-IL-2Rbeta chimera, IL-15 and IL-21 induced IL-17RA preferentially compared with IL-2, and IL-2 but not IL-15 or IL-21 mediated prolonged activation of the PI3K p85 regulatory subunit. Thus, there are intrinsic signaling differences between IL-2 and IL-15 that can be attributed to differences in activation of the PI3K pathway.

Inflammatory bowel disease, past, present and future: lessons from animal models

Accumulating data from animal models indicate that Inflammatory bowel disease (IBD) is mediated by a much more complicated mechanism than previously predicted. For example, the role of an individual molecule in the pathogenesis of IBD distinctly differs depending on several factors, including the fundamental mechanism of induction of the disease, the target cell type, the phase of disease, and the environment. Therefore, it has been difficult in the past to fully explain the complicated mechanism. Novel concepts have recently been proposed to further explain the complicated mechanism of IBD. In this review, we introduce past, current, and possible future concepts for IBD models regarding T helper (Th) 1, Th2, and Th17, antigen sampling and presentation, regulatory cell networks, NOD2, Toll-like receptors, bacteria/epithelia interaction, stem cells, autophagy, microRNAs, and glycoimmunology, and we also discuss the relevance of these new concepts, developed at the bench (in animal models), to the bedside.

The differentiation of human T(H)-17 cells requires transforming growth factor-beta and induction of the nuclear receptor RORgammat

T(H)-17 cells are interleukin 17 (IL-17)-secreting CD4+ T helper cells involved in autoimmune disease and mucosal immunity. In naive CD4+ T cells from mice, IL-17 is expressed in response to a combination of IL-6 or IL-21 and transforming growth factor-beta (TGF-beta) and requires induction of the nuclear receptor RORgammat. It has been suggested that the differentiation of human T(H)-17 cells is independent of TGF-beta and thus differs fundamentally from that in mice. We show here that TGF-beta, IL-1beta and IL-6, IL-21 or IL-23 in serum-free conditions were necessary and sufficient to induce IL-17 expression in naive human CD4+ T cells from cord blood. TGF-beta upregulated RORgammat expression but simultaneously inhibited its ability to induce IL-17 expression. Inflammatory cytokines relieved this inhibition and increased RORgammat-directed IL-17 expression. Other gene products detected in T(H)-17 cells after RORgammat induction included the chemokine receptor CCR6, the IL-23 receptor, IL-17F and IL-26. Our studies identify RORgammat as having a central function in the differentiation of human T(H)-17 cells from naive CD4+ T cells and suggest that similar cytokine pathways are involved in this process in mice and humans.

Psoriasis vulgaris lesions contain discrete populations of Th1 and Th17 T cells

The importance of T helper 17 (Th17) cells in inflammation and autoimmunity is now being appreciated. We analyzed psoriasis skin lesions and peripheral blood for the presence of IL-17-producing T cells. We localized Th17 cells predominantly to the dermis of psoriasis skin lesions, confirmed that IL-17 mRNA increased with disease activity, and demonstrated that IL-17 mRNA expression normalized with cyclosporine therapy. IL-22 mRNA expression mirrored IL-17 and both were downregulated in parallel with keratin 16. Th17 cells are a discrete population, separate from Th1 cells (which are also in psoriasis lesions), and Th2 cells. Our findings suggest that psoriasis is a mixed Th1 and Th17 inflammatory environment. Th17 cells may be proximal regulators of psoriatic skin inflammation, and warrant further attention as therapeutic targets.

Th17 cell differentiation: the long and winding road

The characterization of the new lineage of IL-17-producing CD4+ T helper (Th17) cells has revolutionized our current understanding of T cell-mediated immunity. Over the past five years, there have been many twists and turns as the pathways that lead to Th17 cell differentiation have been elucidated. Not least of these was the discovery that TGF-beta is a crucial cytokine for Th17 cell development, suggesting that Th17 and regulatory T cell subsets share reciprocal developmental pathways during the pathogenesis or control of inflammation. This review aims to bring together the observations that have formed current opinion on factors that promote and contain Th17 cell development, in both mouse and man. Unresolved controversies in this field are also discussed: For example, IL-23 is absolutely required for disease pathogenesis in many models of Th17-cell-mediated autoimmunity, yet its role in Th17 cell development is relatively unclear.

Prolonged, granulocyte-macrophage colony-stimulating factor-dependent, neutrophil survival following rheumatoid synovial fibroblast activation by IL-17 and TNFalpha

INTRODUCTION

A surprising feature of the inflammatory infiltrate in rheumatoid arthritis is the accumulation of neutrophils within synovial fluid and at the pannus cartilage boundary. Recent findings suggest that a distinct subset of IL-17-secreting T-helper cells (TH17 cells) plays a key role in connecting the adaptive and innate arms of the immune response and in regulating neutrophil homeostasis. We therefore tested the hypothesis that synovial fibroblasts bridge the biological responses that connect TH17 cells to neutrophils by producing neutrophil survival factors following their activation with IL-17.

METHODS

IL-17-expressing cells in the rheumatoid synovium, and IL-17-expressing cells in the peripheral blood, and synovial fluid were examined by confocal microscopy and flow cytometry, respectively. Peripheral blood neutrophils were cocultured either with rheumatoid arthritis synovial fibroblasts (RASF) or with conditioned medium from RASF that had been pre-exposed to recombinant human IL-17, TNFalpha or a combination of the two cytokines. Neutrophils were harvested and stained with the vital mitochondrial dye 3,3'-dihexyloxacarbocyanine iodide before being enumerated by flow cytometry.

RESULTS

TH17-expressing CD4+ cells were found to accumulate within rheumatoid synovial tissue and in rheumatoid arthritis synovial fluid. RASF treated with IL-17 and TNFalpha (RASFIL-17/TNF) effectively doubled the functional lifespan of neutrophils in coculture. This was entirely due to soluble factors secreted from the fibroblasts. Specific depletion of granulocyte-macrophage colony-stimulating factor from RASFIL-17/TNF-conditioned medium demonstrated that this cytokine accounted for approximately one-half of the neutrophil survival activity. Inhibition of phosphatidylinositol-3-kinase and NF-kappaB pathways showed a requirement for both signalling pathways in RASFIL-17/TNF-mediated neutrophil rescue.

CONCLUSION

The increased number of neutrophils with an extended lifespan found in the rheumatoid synovial microenvironment is partly accounted for by IL-17 and TNFalpha activation of synovial fibroblasts. TH17-expressing T cells within the rheumatoid synovium are likely to contribute significantly to this effect.

IL-23-mediated regulation of IL-17 production in Helicobacter pylori-infected gastric mucosa

Helicobacter pylori (Hp) infection is associated with a marked infiltration of the gastric mucosa by inflammatory cells. The molecular pathways that control Hp-associated inflammatory reaction are complex, but locally induced cytokines seem to contribute to maintaining the ongoing inflammation. We have previously shown that IL-17 is over-produced in Hp-infected gastric mucosa, and that IL-17 stimulates the synthesis of IL-8, the major neutrophil chemoattractant. Factors/mechanisms that regulate IL-17 expression remain, however, unknown. In this study, we initially expanded our previous data, showing that CD4(+) and CD8(+) T cells are a source of IL-17 in Hp-infected samples. Since IL-23 enhances T cell-derived IL-17 during bacterial infections, we then assessed the role of IL-23 in controlling IL-17 expression in Hp-colonized stomach. Using real-time PCR and ELISA, IL-23 was detected in all gastric biopsies, but its expression was more pronounced in Hp-infected samples in comparison to controls. Treatment of normal gastric lamina propria mononuclear cells (LPMC) with IL-23 enhanced Stat3 activation and IL-17 secretion, and pharmacological inhibition of Stat3 prevented IL-23-driven IL-17 synthesis. Consistently, blockade of IL-23 in cultures of LPMC from Hp-infected patients reduced Stat3 activation and IL-17 production. Data show that IL-23 is overexpressed in Hp-infected gastric mucosa where it could contribute to sustaining IL-17 production.

Human Th17 cells

The discovery in mice of a new lineage of CD4+ effector T helper (Th) cells that selectively produce IL-17 has provided exciting new insights into immune regulation, host defence, and the pathogenesis of autoimmune and other chronic inflammatory disorders. This population of CD4+ Th cells, which has been termed 'Th17', indeed plays an apparently critical role in the pathogenesis of some murine models of autoimmunity. Interestingly, murine Th17 cells share a common origin with Foxp3+ T regulatory cells, because both populations are produced in response to transforming growth factor-beta, but they develop into Th17 cells only when IL-6 is simultaneously produced. Initial studies in humans have confirmed the existence of Th17 cells, but they have shown that the origin of these cells in humans differs from that in mice, with IL-1beta and IL-23 being the major cytokines responsible for their development. Moreover, the presence in the circulation and in various tissues of Th cells that can produce both IL-17 and interferon-gamma, as well as the flexibility of human Th17 clones to produce interferon-gamma in addition to IL-17 in response to IL-12, suggests that there may be a developmental relationship between Th17 and Th1 cells, at least in humans. Resolving this issue has great implications in tems of establishing the respective pathogenic roles of Th1 and Th17 cells in autoimmune disorders. In contrast, it is unlikely that Th17 cells contribute to the pathogenesis of human allergic IgE-mediated disorders, because IL-4 and IL-25 (a powerful inducer of IL-4) are both potent inhibitors of Th17 cell development.

Anti tumour necrosis-alpha therapy increases the number of FOXP3 regulatory T cells in children affected by Crohn's disease

Crohn's disease (CD) is a chronic inflammatory disease of the gastrointestinal tract. Its pathogenesis is not completely understood, though the prevailing model is that the intestinal flora drives a strong intestinal T helper 1 (Th1)/Th17 type immune response and inflammation in the genetically susceptible host. This leads to overly aggressive T-cell responses to normal bacteria causing tissue damage. Intestinal homeostasis and maintenance of tolerance to harmless antigens in the intestine has been shown to be maintained by CD4+ CD25+ T regulatory cells (Treg) in animal models of inflammatory bowel diseases. Here we investigated whether Infliximab, a chimeric monoclonal antibody directed against tumour necrosis factor (TNF)-alpha shown to be highly effective in the treatment of CD, has any effect on mucosal CD4+ CD25+ (FOXP3+) Tregs. Colonic mucosal biopsies from children with active Crohn's disease treated in vivo with Infliximab and controls were analysed to determine FOXP3 expression by immunofluorescence and reverse transcription-polymerase chain reaction. We observed that FOXP3+ T cells were significantly reduced in mucosa of CD patients with active disease compared with controls and restored to normal following Infliximab treatment. The frequency of FOXP3+ cells and mRNA expression was significantly increased in CD mucosa from patients treated in vivo with Infliximab compared with CD patients treated with conventional therapies. In conclusion, we show that Infliximab treatment does not solely neutralize soluble TNF-alpha, but also affects activation and possibly expansion of mucosal regulatory T cells. We suggest that anti TNF-alpha immunotherapy can also restore mucosal homeostasis in Crohn's disease.

Stimulation of TLR2 and TLR4 differentially skews the balance of T cells in a mouse model of arthritis

TLRs may contribute to the progression of rheumatoid arthritis through recognition of microbial or host-derived ligands found in arthritic joints. Here, we show that TLR2 and TLR4, but not TLR9, are involved in the pathogenesis of autoimmune arthritis and play distinct roles in the regulation of T cells and cytokines. We investigated the involvement of TLR2, TLR4, and TLR9 in the progression of arthritis using IL-1 receptor antagonist-knockout (IL1rn-/-) mice, which spontaneously develop an autoimmune T cell-mediated arthritis. Spontaneous onset of arthritis was dependent on TLR activation by microbial flora, as germ-free mice did not develop arthritis. Clinical and histopathological evaluation of IL1rn-/-Tlr2-/- mice revealed more severe arthritis, characterized by reduced suppressive function of Tregs and substantially increased IFN-gamma production by T cells. IL1rn-/-Tlr4-/- mice were, in contrast, protected against severe arthritis and had markedly lower numbers of Th17 cells and a reduced capacity to produce IL-17. A lack of Tlr9 did not affect the progression of arthritis. While any therapeutic intervention targeting TLR2 still seems complicated, the strict position of TLR4 upstream of a number of pathogenic cytokines including IL-17 provides an interesting potential therapeutic target for rheumatoid arthritis.

Memory Th-17 cells specific for C. albicans are persistent in human peripheral blood

Th-17 cells, a new subset of effector CD4(+) T cells, have been identified in mice and in humans. In the present study, we show that a high level of IL-17 and a high frequency of IL-17-producing cells were detected by ELISA and ELISPOT assay, respectively, when human PBMCs were stimulated with both anti-CD3 and anti-CD28. Further analysis of IL-17-producing cells by flow cytometry showed that CD4(+) T cells were the main contributor to IL-17 production, and IL-17 production could be directly induced by purified CD4(+) T cells at the protein and transcriptional levels. Phenotypic analyses revealed that the majority of IL-17-producing CD4(+) T cells were memory cells with the expression of CD45RO, CD69, CCR6 and CCR4, and approximately 70% of IL-17-producing CD4(+) T cells expressed CCR7. In addition, Th-17 cells were different from Th1, Th2 and Treg cells, because the expression of IL-17, IFN-gamma, IL-4 or Foxp3 was restricted to distinct CD4(+) T subsets. Importantly, stimulation of PBMCs with heated-inactivated Candida albicans (C. albicans) yeast or hyphae induced IL-17 production at the protein and transcriptional levels. These data suggest that memory Th-17 cells are present in healthy individual PBMCs and some memory Th-17 cells might play an important role in the defense against the infections of fungi such as C. albicans.

T helper cell type 1 (Th1), Th2 and Th17 responses to myelin basic protein and disease activity in multiple sclerosis

Autoreactive T cells are thought to play an essential role in the pathogenesis of multiple sclerosis (MS). We examined the stimulatory effect of human myelin basic protein (MBP) on mononuclear cell (MNC) cultures from 22 patients with MS and 22 sex-matched and age-matched healthy individuals, and related the patient responses to disease activity, as indicated by magnetic resonance imaging. The MBP induced a dose-dependent release of interferon-gamma (IFN-gamma), tumour necrosis factor-alpha (TNF-alpha) and interleukin-10 (IL-10) by patient-derived MNCs. The patients' cells produced higher amounts of IFN-gamma and TNF-alpha, and lower amounts of IL-10, than cells from healthy controls (P<0.03 to P<0.04). Five patients with MS and no controls, displayed MBP-induced CD4+ T-cell proliferation. These high-responders exhibited enhanced production of IL-17, IFN-gamma, IL-5 and IL-4 upon challenge with MBP, as compared with the remaining patients and the healthy controls (P<0.002 to P<0.01). A strong correlation was found between the MBP-induced CD4+ T-cell proliferation and production of IL-17, IFN-gamma, IL-5 and IL-4 (P<0.0001 to P<0.01) within the patient group, and the production of IL-17 and IL-5 correlated with the number of active plaques on magnetic resonance images (P=0.04 and P=0.007). These data suggest that autoantigen-driven CD4+ T-cell proliferation and release of IL-17 and IL-5 may be associated with disease activity. Larger studies are needed to confirm this.

Regulation of gut inflammation and th17 cell response by interleukin-21

BACKGROUND & AIMS

Interleukin (IL)-21, a T-cell-derived cytokine, is overproduced in inflammatory bowel diseases (IBD), but its role in the pathogenesis of gut inflammation remains unknown. We here examined whether IL-21 is necessary for the initiation and progress of experimental colitis and whether it regulates specific pathways of inflammation.

METHODS

Both dextran sulfate sodium colitis and trinitrobenzene sulfonic acid-relapsing colitis were induced in wild-type and IL-21-deficient mice. CD4(+)CD25(-) T cells from wild-type and IL-21-deficient mice were differentiated in T helper cell (Th)17-polarizing conditions, with or without IL-21 or an antagonistic IL-21R/Fc. We also examined whether blockade of IL-21 by anti-IL-21 antibody reduced IL-17 in cultures of IBD lamina propria CD3(+) T lymphocytes. Cytokines were evaluated by real-time polymerase chain reaction and/or enzyme-linked immunosorbent assay.

RESULTS

High IL-21 was seen in wild-type mice with dextran sulfate sodium- and trinitrobenzene sulfonic acid-relapsing colitis. IL-21-deficient mice were largely protected against both colitides and were unable to up-regulate Th17-associated molecules during gut inflammation, thus suggesting a role for IL-21 in controlling Th17 cell responses. Indeed, naïve T cells from IL-21-deficient mice failed to differentiate into Th17 cells. Treatment of developing Th17 cells from wild-type mice with IL-21R/Fc reduced IL-17 production. Moreover, in the presence of transforming growth factor-beta1, exogenous IL-21 substituted for IL-6 in driving IL-17 induction. Neutralization of IL-21 reduced IL-17 secretion by IBD lamina propria lymphocytes.

CONCLUSIONS

These results indicate that IL-21 is a critical regulator of inflammation and Th17 cell responses in the gut.

The biological functions of T helper 17 cell effector cytokines in inflammation

T helper 17 (Th17) cells belong to a recently identified T helper subset, in addition to the traditional Th1 and Th2 subsets. These cells are characterized as preferential producers of interleukin-17A (IL-17A), IL-17F, IL-21, and IL-22. Th17 cells and their effector cytokines mediate host defensive mechanisms to various infections, especially extracellular bacteria infections, and are involved in the pathogenesis of many autoimmune diseases. The receptors for IL-17 and IL-22 are broadly expressed on various epithelial tissues. The effector cytokines of Th17 cells, therefore, mediate the crucial crosstalk between immune system and tissues, and play indispensable roles in tissue immunity.

Tumor-specific Th17-polarized cells eradicate large established melanoma

CD4+ T cells can differentiate into multiple effector subsets, but the potential roles of these subsets in anti-tumor immunity have not been fully explored. Seeking to study the impact of CD4+ T cell polarization on tumor rejection in a model mimicking human disease, we generated a new MHC class II-restricted, T-cell receptor (TCR) transgenic mouse model in which CD4+ T cells recognize a novel epitope in tyrosinase-related protein 1 (TRP-1), an antigen expressed by normal melanocytes and B16 murine melanoma. Cells could be robustly polarized into Th0, Th1, and Th17 subtypes in vitro, as evidenced by cytokine, chemokine, and adhesion molecule profiles and by surface markers, suggesting the potential for differential effector function in vivo. Contrary to the current view that Th1 cells are most important in tumor rejection, we found that Th17-polarized cells better mediated destruction of advanced B16 melanoma. Their therapeutic effect was critically dependent on interferon-gamma (IFN-gamma) production, whereas depletion of interleukin (IL)-17A and IL-23 had little impact. Taken together, these data indicate that the appropriate in vitro polarization of effector CD4+ T cells is decisive for successful tumor eradication. This principle should be considered in designing clinical trials involving adoptive transfer-based immunotherapy of human malignancies.

Human Th17 cells share major trafficking receptors with both polarized effector T cells and FOXP3+ regulatory T cells

It is a question of interest whether Th17 cells express trafficking receptors unique to this Th cell lineage and migrate specifically to certain tissue sites. We found several Th17 cell subsets at different developing stages in a human secondary lymphoid organ (tonsils) and adult, but not in neonatal, blood. These Th17 cell subsets include a novel in vivo-stimulated tonsil IL17+ T cell subset detected without any artificial stimulation in vitro. We investigated in depth the trafficking receptor phenotype of the Th17 cell subsets in tonsils and adult blood. The developing Th17 cells in tonsils highly expressed both Th1- (CCR2, CXCR3, CCR5, and CXCR6) and Th2-associated (CCR4) trafficking receptors. Moreover, Th17 cells share major non-lymphoid tissue trafficking receptors, such as CCR4, CCR5, CCR6, CXCR3, and CXCR6, with FOXP3+ T regulatory cells. In addition, many Th17 cells express homeostatic chemokine receptors (CD62L, CCR6, CCR7, CXCR4, and CXCR5) implicated in T cell migration to and within lymphoid tissues. Expression of CCR6 and CCR4 by some Th17 cells is not a feature unique to Th17 cells but shared with FOXP3+ T cells. Interestingly, the IL17+IFN-gamma+ Th17 cells have the features of both IL17-IFN-gamma+ Th1 and IL17+IFN-gamma- Th17 cells in expression of trafficking receptors. Taken together, our results revealed that Th17 cells are highly heterogeneous, in terms of trafficking receptors, and programmed to share major trafficking receptors with other T cell lineages. These findings have important implications in their distribution in the human body in relation to other regulatory T cell subsets.

New perspectives on effector mechanisms in uveitis

Experimental autoimmune uveitis (EAU) in its several variants represents human autoimmune uveitis and has been instrumental in obtaining insights into the basic mechanisms of disease. Studies have uncovered that in addition to CD4+ Th1 cells, uveitis can be induced also by CD8+ T cells. Antibodies may have a secondary role after the blood-retinal barrier has been broken. The role in uveitis of a recently discovered IL-17-producing effector T cell type, Th17, is being intensively studied. Th17 cells elicit EAU, can be found in uveitic eyes along with Th1 cells, and are dominant in some types of EAU. In other types of EAU, Th1 cells have a dominant role. The dominant effector type is at least in part determined by conditions under which initial exposure to self-antigen occurs. These findings shed light on the heterogeneity of human disease and may ultimately help to develop better and more rational treatment strategies for human uveitis.

IL-21 comes of age as a regulator of effector T cells in the gut

In healthy individuals, antigens from the gut lumen are tolerated by the mucosal immune system. However, a loss of tolerance toward the bacterial microflora probably causes inflammatory bowel disease (IBD), Crohn's disease (CD) and ulcerative colitis. The abnormal activation of the immune system in the gut of IBD patients is characterized by a cascade of cellular events orchestrated by cytokine cross talk between immune and non-immune cells. Interleukin (IL)-21, the newest member of the common gamma-chain-dependent cytokine family, is a key component of the inflammatory cascade in the gut. It is highly expressed in CD and sustains the ongoing T helper type 1 (Th1)-mediated immune response. IL-21 is essential for the differentiation of Th17 cells. IL-21 is also involved in recruiting T cells to the inflamed gut and eliciting the secretion of matrix-degrading enzymes by gut fibroblasts. Overall, there is now sufficient evidence to suggest that targeting IL-21 will be of therapeutic benefit in IBD.

Adverse functions of IL-17A in experimental sepsis

IL-17A is a proinflammatory cytokine produced by a variety of cells. In the current study, we examined the role of IL-17A in sepsis induced in mice by cecal ligation and puncture (CLP). IL-17A levels, which rose time-dependently in plasma after CLP, were not affected in the absence of alphabeta T cells or neutrophils. In sharp contrast, gammadelta T cell-knockout or gammadelta T cell-depleted mice displayed baseline IL-17A plasma levels after CLP. Neutralization of IL-17A by two different antibodies improved sepsis (survival from approximately 10% to nearly 60%). Unexpectedly, antibody treatment was protective, even when administration of anti-IL-17A was delayed for up to 12 h after CLP. These protective effects of IL-17A blockade were associated with substantially reduced levels of bacteremia together with significant reductions of systemic proinflammatory cytokines and chemokines in plasma. In vitro incubation of mouse peritoneal macrophages with lipopolysaccharide (LPS) in the copresence of IL-17A substantially increased the production of TNF-alpha, IL-1beta, and IL-6 by these cells. These data suggest that, during experimental sepsis, gammadelta T cell-derived IL-17A promotes high levels of proinflammatory mediators and bacteremia, resulting in enhanced lethality. IL-17A may be a potential therapeutic target in sepsis.

Rapid ex vivo isolation and long-term culture of human Th17 cells

T helper (Th) 17 cells are a distinct lineage of CD4+ T cells mediating tissue inflammation through the secretion of IL-17. In addition, it has been shown that the expression of the transcriptional factor RORgammat is responsible for the induction and maintenance of this cell line. Th17 cells are believed to be involved in a variety of autoimmune disorders, but may also play an important role in host defense. Here we describe a novel technique to reproducibly isolate viable Th17 cells based on their IL-17 secreting ability. We confirmed Th17 cell enrichment by quantitative PCR analysis and demonstrate that positively selected cells using this technique express significantly increased mRNA levels of RORgammat, IL-23 receptor and CCR4 when compared to negatively selected cells. Furthermore, we show that purified Th17 cells can be maintained in long-term culture and expand in vitro. In conclusion, this technique will allow for the first time the direct, ex vivo analysis of phenotypic and functional properties of Th17 cells.

Distinct, specific IL-17- and IL-22-producing CD4+ T cell subsets contribute to the human anti-mycobacterial immune response

We investigated whether the proinflammatory T cell cytokines IL-17 and IL-22 are induced by human mycobacterial infection. Remarkably, >20% of specific cytokine-producing CD4(+) T cells in peripheral blood of healthy, mycobacteria-exposed adults expressed IL-17 or IL-22. Specific IL-17- and IL-22-producing CD4(+) T cells were distinct from each other and from Th1 cytokine-producing cells. These cells had phenotypic characteristics of long-lived central memory cells. In patients with tuberculosis disease, peripheral blood frequencies of these cells were reduced, whereas bronchoalveolar lavage fluid contained higher levels of IL-22 protein compared with healthy controls. IL-17 was not detected in this fluid, which may be due to suppression by Th1 cytokines, as PBMC IL-17 production was inhibited by IFN-gamma in vitro. However, Th1 cytokines had no effect on IL-22 production in vitro. Our results imply that the magnitude and complexity of the anti-mycobacterial immune response have historically been underestimated. IL-17- and IL-22-producing CD4(+) T cells may play important roles in the human immune response to mycobacteria.

Linking genetic susceptibility to Crohn's disease with Th17 cell function: IL-22 serum levels are increased in Crohn's disease and correlate with disease activity and IL23R genotype status

BACKGROUND

We analyzed the influence of Crohn's disease (CD)-associated IL23R gene variants on IL-22 that is expressed in IL-23R+ Th17 cells.

METHODS

IL-22 serum levels were measured in 242 CD patients and in 31 healthy controls. Subanalyses included serum levels of IL-6, TNF-alpha, IL-17A, IL-17F, C-reactive protein (CRP), and leukocyte count. In all patients, genotyping for 10 CD-associated IL23R single nucleotide polymorphisms (SNPs) and the 3 main CD-associated CARD15 variants was performed.

RESULTS

There was a highly significant increase in IL-22 serum expression in CD patients compared to healthy controls (P = 2.53 x 10(-9)). IL-22 serum levels correlated with disease activity: IL-22 levels in patients with a Crohn's disease activity index (CDAI) >150 were significantly higher than in patients with a CDAI <150 (P = 0.001), while TNF-alpha and IL-6 were not significantly different between these 2 groups. Analyzing the effect of 10 IL23R variants on IL-22 serum levels, we demonstrated that the quotients of mean IL-22 serum levels of carriers of the minor allele to the mean serum IL-22 in wildtype carriers correlated highly with the corresponding CD susceptibility risk for each gene variant (r = 0.807). The IL-22 levels in carriers of CD risk-increasing IL23R variants were significantly higher than in carriers of CD risk-decreasing IL23R variants (P = 0.008).

CONCLUSIONS

The Th17 cytokine IL-22 is expressed at high levels in CD and correlates with disease activity, offering a better separation between active and inactive CD than IL-6 and TNF-alpha. IL23R genotypes influence IL-22 serum expression, linking genetic CD susceptibility to Th17 cell function for the first time.

Regulation of IL-17 production in human lymphocytes

The discovery of a new lineage of helper T cells that selectively produces interleukin (IL)-17 has provided exciting new insights into immunoregulation, host defense and the pathogenesis of autoimmune diseases. Although the factors that promote murine Th17 differentiation have been intensively examined, there has been much less information on the regulation of this cytokine in human T cells. IL-17 is readily produced by human memory T cells, which we now know exhibit distinct patterns of chemokine receptor expression and may differentiate in response to selective pathogens. Recently it has been shown that IL-1, IL-6 and IL-23 are important in driving human Th17 differentiation. However, TGFbeta-1 which is important for the differentiation of murine Th17 cells and inducible regulatory T cells (iTregs), is reportedly not required and even inhibits for human Th17 differentiation. In addition, human Th17 cells also produce other proinflammatory cytokines. Further characterization of the transcription regulation of human IL-17 expression, and the epigenetic regulation of human Il17 locus should improve our understanding the lineage commitment of human Th17 cells. Targeting the production and action of this cytokine is also likely to be beneficial therapeutically for autoinflammatory and autoimmune diseases.

Targeting the development and effector functions of TH17 cells

T helper (TH) cells can assume different phenotypes characterized by the secretion of distinct effector molecules. Interferon-gamma producing TH1 and IL-4 producing TH2 cells have long been recognized as important mediators of host defense, whereas regulatory T cells are known to suppress T cell responses. Recently, TH17 cells were characterized as a novel CD4(+) subset that preferentially produces IL-17, IL-17F, and IL-22 as the signature cytokines. TH17 cells appear to play a critical role in sustaining the inflammatory response and their presence is closely associated with autoimmune disease, which makes them an attractive therapeutic target. In this review, we focus on the mechanisms that regulate the differentiation of naive T cells into TH17 cells and on TH17 effector cytokines, as they represent opportunities for therapeutic intervention.

An insight into molecular mechanisms of human T helper cell differentiation

Selective activation of T helper (Th) cell subsets plays an important role in immune response to pathogens as well as in the pathogenesis of human allergy and inflammatory diseases. Th1 cells along with the recently discovered Th17 cells play a role in the pathogenesis of autoimmune diseases. Th2 cytokines lead to series of inflammatory processes characteristic for asthma and other atopic diseases. To understand the pathogenesis of immune-mediated diseases it is crucial to dissect pathways and regulatory networks leading to the development of distinct Th subsets. Such knowledge may lead to better strategies for developing diagnostics and therapies for these diseases. The differentiation of Th1, Th2, and Th17 effector cells is driven by signals originating from T cell and costimulatory receptors as well as cytokines in the surroundings of activated naive T helper cells. There are several proteins involved in the regulation of this differentiation process. Most of the data on T helper cell differentiation have been acquired using mouse. In this review, we have summarized what is known about human T helper differentiation. In addition, selected differences between human and mouse will be discussed.

Genetics of inflammatory bowel disease: clues to pathogenesis

INTRODUCTION OR BACKGROUND

It has long been recognized from epidemiological data that inflammatory bowel diseases (IBD), Crohn's disease (CD) and ulcerative colitis (UC), have a strong genetic predisposition, interacting with unknown environmental drivers to render susceptible individuals at risk for relapsing intestinal inflammation. Substantial progress has been made in the last 2 years in characterizing the susceptibility genes involved.

SOURCES OF DATA

The recent acceleration in understanding has resulted from the use of new technologies of genome-wide association scanning in large panels of cases and controls.

AREAS OF AGREEMENT

Genome scans have robustly identified 11 susceptibility genes and loci and highlighted a number of new, previously unsuspected pathways as playing an important role in IBD pathogenesis-including the IL23 pathway in IBD overall and specific aspects of innate immunity (particularly NOD2 and the autophagy genes ATG16L1 and IRGM) in CD.

AREAS OF CONTROVERSY

The next challenge is to identify specific causal variants at each of the confirmed susceptibility loci and then characterize their biological impact on gene expression and function of the protein product.

GROWING POINTS

To date, most attention has focused on CD. A recent meta-analysis has increased the number of confirmed susceptibility loci to 32-more than for any other common disease to date. Attention is now turning to the use of the same techniques in UC to identify new, disease-specific genes and understand areas of overlap.

AREAS TIMELY FOR DEVELOPING RESEARCH

This review explores genetic clues to the pathogenesis of IBD derived from the growing list of confirmed IBD susceptibility genes, and briefly elaborates some of the important themes and overlaps that are becoming evident both within IBD and also with other complex diseases.

Langerhans cell histiocytosis reveals a new IL-17A-dependent pathway of dendritic cell fusion

IL-17A is a T cell-specific cytokine that is involved in chronic inflammations, such as Mycobacterium infection, Crohn's disease, rheumatoid arthritis and multiple sclerosis. Mouse models have explained the molecular basis of IL-17A production and have shown that IL-17A has a positive effect not only on granuloma formation and neurodegeneration through unknown mechanisms, but also on bone resorption through Receptor activator of NF-kappaB ligand (RANKL) induction in osteoblasts. Langerhans cell histiocytosis (LCH) is a rare disease of unknown etiology, lacking an animal model, that cumulates symptoms that are found separately in various IL-17A-related diseases, such as aggressive chronic granuloma formation, bone resorption and soft tissue lesions with occasional neurodegeneration. We examined IL-17A in the context of LCH and found that there were high serum levels of IL-17A during active LCH and unexpected IL-17A synthesis by dendritic cells (DCs), the major cell type in LCH lesions. We also found an IL-17A-dependent pathway for DC fusion, which was highly potentiated by IFN-gamma and led to giant cells expressing three major tissue-destructive enzymes: tartrate resistant acidic phosphatase and matrix metalloproteinases 9 and 12. IFN-gamma expression has been previously documented in LCH and observed in IL-17A-related diseases. Notably, serum IL-17A-dependent fusion activity correlates with LCH activity. Thus, IL-17A and IL-17A-stimulated DCs represent targets that may have clinical value in the treatment of LCH and other IL-17A-related inflammatory disorders.

Mucosal T-cell immunoregulation varies in early and late inflammatory bowel disease

BACKGROUND AND AIMS

Crohn's disease is a life-long form of inflammatory bowel disease (IBD) mediated by mucosal immune abnormalities. Understanding of the pathogenesis is limited because it is based on data from adults with chronic Crohn's disease. We investigated mucosal T-cell immunoregulatory events in children with early Crohn's disease.

METHODS

Mucosal biopsies and T-cell clones were derived from children experiencing the first attack of Crohn's disease, children with long-standing Crohn's disease, infectious colitis, and children without gut inflammation.

RESULTS

As in acute infectious colitis, interleukin (IL) 12 induced T cells from early Crohn's disease to acquire a strongly polarised T helper (Th) type 1 response characterised by high IFN-gamma production and IL12Rbeta2 chain expression. Th1 polarisation was not induced in clones from late Crohn's disease. Mucosal levels of IL12p40 and IL12Rbeta2 messenger RNA were significantly higher in children with early than late Crohn's disease. These results demonstrate that susceptibility to IL12-mediated modulation is strongly dependent on the stage of Crohn's disease.

CONCLUSIONS

At the onset of Crohn's disease mucosal T cells appear to mount a typical Th1 response that resembles an acute infectious process, and is lost with progression to late Crohn's disease. This suggests that mucosal T-cell immunoregulation varies with the course of human IBD. Patients with the initial manifestations of IBD may represent an ideal population in which immunomodulation may have optimal therapeutic efficacy.

Pathophysiology of psoriasis: recent advances on IL-23 and Th17 cytokines

T helper (Th) 17 cells, a novel T-cell subset, have been implicated in the pathogenesis of psoriasis and other autoimmune inflammatory diseases. Interleukin (IL)-23 stimulates survival and proliferation of Th17 cells, and thus serves as a key master cytokine regulator for these diseases. In psoriasis, IL-23 is overproduced by dendritic cells and keratinocytes, and this cytokine stimulates Th17 cells within dermis to make IL-17A and IL-22. IL-22, in particular, drives keratinocyte hyperproliferation in psoriasis. Future targeting of these key cytokines is likely to lead to dramatic clinical improvement in patients with psoriasis. This review focuses on the numerous recent studies on the roles of IL-23 and Th17 cells in the pathogenesis of psoriasis.

Transcriptional regulation of Th17 cell differentiation

The paradigm of effector T helper cell differentiation into either Th1 or Th2 lineages has been profoundly shaken by the discovery of T cells that secrete IL-17 and other inflammatory cytokines. This subset, referred to as Th17, is centrally involved in autoimmune disease and is important in host defense at mucosal surfaces. In mouse, a series of cytokines, including IL-6, IL-21, IL-23, and TGF-beta, function sequentially or synergistically to induce the Th17 lineage. Other cytokines, including IL-2, IL-4, IFNgamma, and IL-27, inhibit differentiation of this lineage. Here we review how the nuclear orphan receptor RORgammat functions to coordinate the diverse cytokine-induced signals and thus controls Th17 cell differentiation.

Amelioration of epidermal hyperplasia by TNF inhibition is associated with reduced Th17 responses

Biological agents have dramatically improved treatment options for patients with severe psoriasis. Etanercept (tumor necrosis factor [TNF] receptor–immunoglobulin fusion protein) is an effective treatment for many psoriasis patients, and blockade of TNF is considered to be its primary action. However, in this clinical trial, we show that etanercept has early inhibitory effects on a newly appreciated type of T cells: T helper type 17 (Th17) cells. Etanercept reduced the inflammatory dendritic cell products that drive Th17 cell proliferation (interleukin [IL] 23), as well as Th17 cell products and downstream effector molecules (IL-17, IL-22, CC chemokine ligand 20, and β-defensin 4). In contrast, Th1 cellular products and effector molecules (interferon γ, lymphotoxin α, and myxovirus resistance 1) were reduced late in disease resolution. This study suggests a role for Th17 in addition to Th1 cells in the pathogenesis of psoriasis. Th17 cells may be particularly important in driving epidermal activation in psoriatic plaques, whereas Th1 cells must also be eliminated for final disease resolution.

Inflammatory bowel disease: progress and current concepts of etiopathogenesis

Over the past few years, none of the numerous conditions that are grouped under the broad designation of 'chronic inflammatory or autoimmune disorders' has undergone as much scientific and clinical progress as the two main forms of inflammatory bowel disease (IBD), Crohn's disease (CD) and ulcerative colitis (UC). Progress has occurred in all major areas relevant to IBD pathogenesis, which include the external environment, genetics, microbial factors, and the immune system. This review presents an update on the specific major advances that have occurred in each of these four areas, briefly discusses the therapeutic implications of the observed progress, and points out the additional work that needs to be accomplished in the next few years to reach a full understanding of IBD etiopathogenesis.