T-bet regulates the fate of Th1 and Th17 lymphocytes in autoimmunity

Plasticity of T-cell phenotype and function: the T helper type 17 example

Mature T helper type 1 (Th1) and Th2 cells antagonize the development of the opposing subset to sustain lineage-specific responses. However, the recent identification of a third distinct subset of helper T cells - the Th17 lineage - collapses the established Th1/Th2 dichotomy and raises intriguing questions about T-cell fate. In this review, we discuss the Th17 subset in the context of the effector and regulatory T-cell lineages. Initial studies suggested reciprocal developmental pathways between Th17/Th1 subsets and between Th17/regulatory T-cell subsets, and identified multiple mechanisms by which Th1 and Th2 cells antagonize the generation of Th17 cells. However, recent observations reveal the susceptibility of differentiated Th17 cells to Th1 polarization and the enhancement of Th17 memory cells by the Th1 factors interferon-gamma and T-bet. In addition, new data indicate late-stage plasticity of a subpopulation of regulatory T cells, which can be selectively induced to adopt a Th17 phenotype. Elucidating the mechanisms that undermine cross-lineage suppression and facilitate these phenotype shifts will not only clarify the flexibility of T-cell differentiation, but may also shed insight into the pathogenesis of autoimmunity and cancer. Furthermore, understanding these phenomena will be critical for the design of immunotherapy that seeks to disrupt lineage-specific T-cell responses and may suggest ways to manipulate the balance between pathogenic and regulatory lymphocytes for the restoration of homeostasis.

Translational mini-review series on Th17 cells: function and regulation of human T helper 17 cells in health and disease

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.

Translational mini-review series on Th17 cells: CD4 T helper cells: functional plasticity and differential sensitivity to regulatory T cell-mediated regulation

CD4(+) T cells display considerable flexibility in their effector functions, allowing them to tackle most effectively the range of pathogenic infections with which we are challenged. The classical T helper (Th) 1 and Th2 subsets have been joined recently by the Th17 lineage. If not controlled, the potent effector functions (chiefly cytokine production) of which these different cells are capable can lead to (sometimes fatal) autoimmune and allergic inflammation. The primary cell population tasked with providing this control appears to be CD4(+) regulatory T (T(reg)) cells expressing the forkhead box P3 (FoxP3) transcription factor. Here we consider the comparative capacity of FoxP3(+) T(regs) to influence the polarization, expansion and effector function of Th1, Th2 and Th17 cells in vitro and in vivo as well as in relation to human disease. This remains a particularly challenging series of interactions to understand, especially given our evolving understanding of T(reg) and T effector interrelationships, as well as recent insights into functional plasticity that cast doubt upon the wisdom of a strict categorization of T effector cells based on cytokine production.

Encephalitogenic T cells that stably express both T-bet and ROR gamma t consistently produce IFNgamma but have a spectrum of IL-17 profiles

Th1/Th17 cells, secreting both IFNgamma and IL-17, are often associated with inflammatory pathology. We cloned and studied the cytokine phenotypes of MBP-specific, TCR-identical encephalitogenic CD4+ cells in relationship to Th1- and Th17-associated transcription factors T-bet and RORgammat. IFNgamma-producing cells could be sub-divided into those that are T-bet(+)/RORgammat(-) and those that are T-bet(+)/RORgammat(+). The latter comprises a spectrum of phenotypes, as defined by IL-17 production, and can be induced to up-regulate IL-23R with IL-12 or IL-23. The former, bona fide Th1 cells, lack IL-23R expression under all conditions. In vivo, T-bet(+)/RORgammat(-) and T-bet(+)/RORgammat(+) clones induce EAE equally well.

T-bet protects against exacerbation of schistosome egg-induced immunopathology by regulating Th17-mediated inflammation

C57BL/6 mice infected with Schistosoma mansoni naturally develop mild CD4(+) T-cell-mediated immunopathology characterized by small hepatic granulomas around parasite eggs. However, immunization with soluble egg Ag in CFA markedly exacerbates the lesions by inducing a potent proinflammatory environment with high levels of IFN-gamma and IL-17, which are signature cytokines of distinct Th1- versus Th17-cell lineages. To determine the relative role of these subsets in disease exacerbation, we examined mice deficient in T-bet (T-bet(-/-)), which is required for Th1 differentiation and IFN-gamma production. We now report that immunization with soluble egg Ag in CFA caused a significantly greater enhancement of egg-induced hepatic immunopathology in T-bet(-/-) mice compared with WT controls, and analysis of their granulomas disclosed a higher proportion of activated DC and CD4(+) T cells, as well as a marked influx of neutrophils. The absence of IFN-gamma in the T-bet(-/-) mice correlated with a marked increase in IL-23p19, IL-17 and TNF-alpha in granulomas and MLN. In contrast, T-bet(-/-) mice had lower levels of IL-4, IL-5 and IL-10 and a reduction in FIZZ1 and FoxP3 expression, suggesting diminished regulatory activity, respectively, by alternatively activated macrophages and Treg. These findings demonstrate that T-bet-dependent signaling negatively regulates Th17-mediated immunopathology in severe schistosomiasis.

Update on inflammation, neurodegeneration, and immunoregulation in multiple sclerosis: therapeutic implications

Multiple sclerosis (MS) is an inflammatory, demyelinating, and neurodegenerative disease of the central nervous system of uncertain etiology. There is consensus that a dysregulated immune system plays a critical role in the pathogenesis of MS; therefore, we aim to summarize current hypotheses concerning the complex cellular and molecular interactions involved in the immunopathology of MS. Although CD4+ T lymphocytes have long been implicated in the immunopathology of MS, the role of other T-cell subtypes has been recognized. CD4+ and CD8+ cells have been isolated from different locations within MS lesions and gamma/delta T cells have been isolated from early MS lesions. The prevalent dogma has been that CD4+ TH1 cells release cytokines and mediators of inflammation that may cause tissue damage, although CD4+ TH2 cells may be involved in modulation of these effects. Recent evidence, however, suggests that additional T-cell subsets play a prominent role in MS immunopathology: TH17 cells, CD8+ effector T cells, and CD4+CD25+ regulatory T cells. In addition, laboratory and clinical data are accumulating on the prominent role of B lymphocytes and antigen-presenting cells in MS pathogenesis. On the basis of these observations, new therapeutic approaches for MS will need to focus on resetting multiple components of the immune system.

Role of CD44 in the differentiation of Th1 and Th2 cells: CD44-deficiency enhances the development of Th2 effectors in response to sheep RBC and chicken ovalbumin

CD4 T cells can be primarily polarized to differentiate into Th1 or Th2 cells. CD44 is a marker of T cell activation and a property of long-lived memory cells and implicated in cell migration, activation, and differentiation. To date, whether CD44 has a role in regulating Th1-Th2 differentiation has not been determined. In this study, we compared Th1 and Th2 responses in wild-type and CD44-deficient mice in response to sheep RBC and chicken OVA, as well as examined Th1-Th2 differentiation in vivo and in vitro from CD44-sufficient and CD44-deficient naive CD4 T cells. We observed that deficiency of CD44 tended to inhibit Th1 while promoting Th2 differentiation. Furthermore, chimeric studies suggested that CD44 expression by CD4 T cells was essential for such Th2 bias. The regulation by CD44 occurred at the transcription level leading to up-regulated GATA3 and down-regulated T-bet expression in activated CD4 T cells. We also noted that CD44-deficiency could modify the state of dendritic cell subsets to induce a Th2-biased development. Results presented in this study demonstrate for the first time that CD44 participates in the regulation of Th1-Th2 differentiation.

T-bet is essential for encephalitogenicity of both Th1 and Th17 cells

The extent to which myelin-specific Th1 and Th17 cells contribute to the pathogenesis of experimental autoimmune encephalomyelitis (EAE) is controversial. Combinations of interleukin (IL)-1β, IL-6, and IL-23 with transforming growth factor β were used to differentiate myelin-specific T cell receptor transgenic T cells into Th17 cells, none of which could induce EAE, whereas Th1 cells consistently transferred disease. However, IL-6 was found to promote the differentiation of encephalitogenic Th17 cells. Further analysis of myelin-specific T cells that were encephalitogenic in spontaneous EAE and actively induced EAE demonstrated that T-bet expression was critical for pathogenicity, regardless of cytokine expression by the encephalitogenic T cells. These data suggest that encephalitogenicity of myelin-specific T cells appears to be mediated by a pathway dependent on T-bet and not necessarily pathway-specific end products, such as interferon γ and IL-17.

IL-23 drives pathogenic IL-17-producing CD8+ T cells

IL-17-producing CD8(+) T cells (Tc17) appear to play a role in a range of conditions, such as autoimmunity and cancer. Thus far, Tc17 cells have been only marginally studied, resulting in a paucity of data on their biology and function. We demonstrate that Tc17 and Th17 cells share similar developmental characteristics, including the previously unknown promoting effect of IL-21 on Tc17 cell differentiation and IL-23-dependent expression of IL-22. Both STAT1 and STAT4 are required for optimal development of Tc17 cells and maximal secretion of cytokines. Tc17 cells are cytotoxic, and they can be either pathogenic or nonpathogenic upon adoptive transfer in the model of autoimmune diabetes. Tc17 cells treated with TGF-beta1 plus IL-6 are not diabetogenic, whereas IL-23-treated cells potently induce the disease. IL-17A and IL-17F are necessary but not sufficient for diabetes induction by Tc17 cells. Tc17 cells treated with TGF-beta1 plus IL-6 or IL-23 likely differ in pathogenicity due to their disparate capacity to attract other immune cells and initiate inflammation.

Do studies in humans better depict Th17 cells?

CD4(+) T helper (Th) lymphocytes represent a heterogeneous population of cells. In addition to type 1 (Th1) and type 2 (Th2) cells, another subset of CD4(+) effector Th cells has been discovered and named as Th17, because of its unique ability to produce interleukin (IL)-17. Studies in mice initially suggested that Th17 cells are the pathogenic cells in autoimmune disorders, whereas Th1 cells may behave rather as protective. Subsequent studies in humans demonstrated the plasticity of Th17 cells and their possibility to shift to Th1. The plasticity of Th17 to Th1 cells has recently been confirmed in mice, where it was found that Th17 cells seem to be pathogenic only when they shift to Th1 cells. Studies in humans also showed that Th17 cells are different than in mice because all of them express CD161 and exclusively originate from CD161(+) precursors present in umbilical cord blood and newborn thymus. While murine Th17 cells develop in response to IL-6, IL-1, and transforming growth factor (TGF)-beta, human Th17 cells originate from these CD161(+) precursors in response to IL-1beta and IL-23, the need for TGF-beta being controversial. Thus, we believe that studies in humans have better depicted human Th17 cells than studies in mice.

Interleukin-23 and Th17 cells in the control of gut inflammation

Crohn's Disease and Ulcerative Colitis, the major forms of inflammatory bowel diseases (IBDs) in humans, have been traditionally associated with exaggerated and poorly controlled T helper (Th) type 1 or Th2 cell response, respectively. More recent studies have, however, shown that IBDs are also characterized by a sustained production of cytokines made by a distinct lineage of Th cells, termed Th17 cells. The demonstration that Th17-related cytokines cause pathology in many organs, including the gut, and that expansion and maintenance of Th17 cell responses require the activity of IL-23, a cytokine made in excess in the gut of IBD patients has contributed to elucidate new pathways of intestinal tissue damage as well as to design new therapeutic strategies. In this review, we discuss the available data supporting the role of the IL-23/Th17 axis in the modulation of intestinal tissue inflammation.

Interleukin-23 and Th17 cells in the control of gut inflammation

Crohn's Disease and Ulcerative Colitis, the major forms of inflammatory bowel diseases (IBDs) in humans, have been traditionally associated with exaggerated and poorly controlled T helper (Th) type 1 or Th2 cell response, respectively. More recent studies have, however, shown that IBDs are also characterized by a sustained production of cytokines made by a distinct lineage of Th cells, termed Th17 cells. The demonstration that Th17-related cytokines cause pathology in many organs, including the gut, and that expansion and maintenance of Th17 cell responses require the activity of IL-23, a cytokine made in excess in the gut of IBD patients has contributed to elucidate new pathways of intestinal tissue damage as well as to design new therapeutic strategies. In this review, we discuss the available data supporting the role of the IL-23/Th17 axis in the modulation of intestinal tissue inflammation.

T-bet inhibits the in vivo differentiation of parasite-specific CD4+ Th17 cells in a T cell-intrinsic manner

CD4(+) Th17 cells have emerged as a new T cell subset in the Th1/Th2 paradigm, and efforts have shifted toward understanding the factors that regulate their development in vivo. To analyze the role of the transcription factor T-bet in regulation of Th17 cells, we used a murine model of Trypanosoma cruzi infection, a protozoan parasite that causes Chagas disease in humans. Infection of Tbx21(-/-) mice led to normal, unimpaired development of Ag-specific CD4(+) T cells producing IFN-gamma. However, a robust Th17 response developed concomitant with Th1 responses. Despite significant IFN-gamma production, the physiological effects of Th17 responses prevailed as there was a sharp increase in Gr-1(+)Ly6G(+) neutrophils. Adoptive transfer of T cells from infected Tbx21(-/-) mice into Rag-2(-/-) mice (Tbx21(+/+)) revealed that CD4(+) T cells maintained their IL-17-producing phenotype, including those cells capable of producing both IFN-gamma and IL-17. Furthermore, and in contrast to the effects of IL-2 on Th17 development, IL-2 had no effect on IL-17 production by primed T cells. Importantly, adoptive transfer of T cells from naive Tbx21(-/-) mice into infected Rag-2(-/-) mice recapitulated the differentiation of T. cruzi-specific Th17 cells observed in infected Tbx21(-/-) mice. Conversely, transfer of wild-type T cells into infected Tbx21(-/-) mice did not reveal an increase in Th17 development. These results demonstrate that T-bet regulates the differentiation of T. cruzi-specific Th17 cells in vivo in a T cell-intrinsic manner. These data provide important insight into the role of T-bet in regulation of parasite-specific Th17 responses.

Transcriptional modulation of the immune response by peroxisome proliferator-activated receptor-{alpha} agonists in autoimmune disease

Peroxisome proliferator-activated receptor-alpha (PPARalpha) agonists have been shown to have a therapeutic benefit in experimental autoimmune encephalomyelitis (EAE), an animal model for multiple sclerosis (MS). In this study, we investigated the mechanism by which the PPARalpha agonist gemfibrozil induces immune deviation and protects mice from EAE. We demonstrated that treatment with gemfibrozil increases expression of the Th2 transcription factor GATA-3 and decreases expression of the Th1 transcription factor T-bet in vitro and directly ex vivo. These changes correlated with an increase in nuclear PPARalpha expression. Moreover, the protective effects of PPARalpha agonists in EAE were shown to be partially dependent on IL-4 and to occur in a receptor-dependent manner. PPARalpha was demonstrated, for the first time, to regulate the IL-4 and IL-5 genes and to bind the IL-4 promoter in the presence of steroid receptor coactivator-1, indicating that PPARalpha can directly transactivate the IL-4 gene. Finally, therapeutic administration of PPARalpha agonists ameliorated clinically established EAE, suggesting that PPARalpha agonists may provide a treatment option for immune-mediated inflammatory diseases.

Signal transduction inhibition of APCs diminishes th17 and Th1 responses in experimental autoimmune encephalomyelitis

IL-17- and IFN-gamma-secreting T cells play an important role in autoimmune responses in multiple sclerosis and the model system experimental autoimmune encephalomyelitis (EAE). Dendritic cells (DCs) in the periphery and microglia in the CNS are responsible for cytokine polarization and expansion of this T cell subset. Our results indicate that in vivo administration of a signal transduction inhibitor that targets DCs to mice with EAE led to a decrease in CNS infiltration of pathogenic Ag-specific T cells. Since this approach does not target T cells directly, we assessed the effects on the APCs that are involved in generating the T cell responses. Since in EAE and multiple sclerosis, both microglia and peripheral DCs are likely to contribute to disease, we utilized a bone marrow chimera system to distinguish between these two populations. These studies show that peripheral DCs are the primary target but that microglia are also modestly affected by CEP-701, as numbers and activation states of the cells in the CNS are decreased after therapy. Our results also showed a decrease in secretion of TNF-alpha, IL-6, and IL-23 by DCs as well as a decrease in expression of costimulatory molecules. We further determined that levels of phospho-Stat1, Stat3, Stat5, and NF-kappaB, which are signaling molecules that have been implicated in these pathways, were decreased. Thus, use of this class of signal transduction inhibitors may represent a novel method to treat autoimmunity by dampening the autoreactive polarizing condition driven by DCs.

Adoptive immunotherapy of cancer using CD4(+) T cells

CD4(+) T cells are central to the function of the immune system but their role in tumor immunity remains underappreciated. It is becoming clear that there is an enormous diversity of CD4(+) T cell polarization patterns including Th1, Th2, Th17, and regulatory T cells (Tregs). These functionally divergent T cell subsets can have opposing effects -- they can trigger tumor rejection or inhibit treatment after adoptive cell transfer. Some polarized CD4(+) cells have plasticity, and their phenotypes and functions can evolve in vivo. Recent advances in understanding of polarization and differentiation of lymphocytes, as well as some intriguing developments in the clinic, indicate that the use of CD4(+) T cell subsets in the immunotherapy of cancer has unrealized potential.

Signal transduction and Th17 cell differentiation

The paradigm of effector T helper cell differentiation into either Th1 or Th2 lineages has been notably shaken by the discovery of a third lineage of cells that selectively produce interleukin (IL)-17. Characterization of this new subset, referred to as Th17, has provided exciting new insights into immunoregulation, host defense and the pathogenesis of autoimmune diseases. Additionally, the discovery of this T cell subset has offered a fresh look at such concepts as lineage commitment and terminal differentiation. The transcriptional regulatory events and epigenetic modifications that control these processes are diverse and complex, and despite the rapid pace at which data continue to accumulate, many questions remain to be answered. Here we review our current understanding of the signaling pathways, molecular interactions and transcriptional events that lead to Th17 differentiation and effector function, as well as the epigenetic modifications that accompany them.

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

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

Lactobacillus casei downregulates commensals' inflammatory signals in Crohn's disease mucosa

BACKGROUND

The interaction of commensal bacteria with the intestinal immune system is an essential factor in the development of inflammatory bowel disease (IBD). The study of isolated commensal bacteria's effects on the mucosal immune response might be relevant for a better understanding of pathophysiological mechanisms in IBD.

METHODS

We investigated the immune responses to signals from the commensal Escherichia coli ATCC 35345 and the probiotic Lactobacillus casei DN-114 001 in Crohn's disease (CD) mucosa. Ileal specimens were obtained during surgery from CD patients. Mucosal explants were incubated with L. casei or its genomic DNA; TNF-alpha, IFN-gamma, IL-2, IL-6, IL-8, and CXCL1 were measured in the supernatant. Second, tissue expression of key proinflammatory cytokines (IL-6, TGF-beta, IL-23p19, IL-12p35, IL-17F), and chemokines (IL-8, CXCL1, CXCL2) was evaluated after incubation with L. casei or E. coli. Finally, combination experiments were carried out by incubating both strains with mucosal explants at different timepoints.

RESULTS

Live L. casei significantly decreased secretion of TNF-alpha, IFN-gamma, IL-2, IL-6, IL-8, and CXCL1 by CD mucosa, but the effect was not reproduced by L. casei DNA. Second, live L. casei downregulated expression of IL-8, IL-6, and CXCL1 and did not modify expression of IL-23p19, IL-12p35, and IL-17F. In contrast, E. coli significantly upregulated expression of all these cytokines. Interestingly, combination experiments revealed the ability of L. casei to prevent and counteract the proinflammatory effects of E. coli.

CONCLUSIONS

Live L. casei can counteract the proinflammatory effects of E. coli on CD inflamed mucosa by specific downregulation of key proinflammatory mediators.

PEG minocycline-liposomes ameliorate CNS autoimmune disease

Background

Minocycline is an oral tetracycline derivative with good bioavailability in the central nervous system (CNS). Minocycline, a potent inhibitor of matrix metalloproteinase (MMP)-9, attenuates disease activity in experimental autoimmune encephalomyelitis (EAE), an animal model of multiple sclerosis (MS). Potential adverse effects associated with long-term daily minocycline therapy in human patients are concerning. Here, we investigated whether less frequent treatment with long-circulating polyethylene glycol (PEG) minocycline liposomes are effective in treating EAE.

Findings

Performing in vitro time kinetic studies of PEG minocycline-liposomes in human peripheral blood mononuclear cells (PBMCs), we determined that PEG minocycline-liposome preparations stabilized with CaCl₂ are effective in diminishing MMP-9 activity. Intravenous injections of PEG minocycline-liposomes every five days were as effective in ameliorating clinical EAE as daily intraperitoneal injections of minocycline. Treatment of animals with PEG minocycline-liposomes significantly reduced the number of CNS-infiltrating leukocytes, and the overall expression of MMP-9 in the CNS. There was also a significant suppression of MMP-9 expression and proteolytic activity in splenocytes of treated animals, but not in CNS-infiltrating leukocytes. Thus, leukocytes gaining access to the brain and spinal cord require the same absolute amount of MMP-9 in all treatment groups, but minocycline decreases the absolute cell number.

Conclusions

Our data indicate that less frequent injections of PEG minocycline-liposomes are an effective alternative pharmacotherapy to daily minocycline injections for the treatment of CNS autoimmune diseases. Also, inhibition of MMP-9 remains a promising treatment target in EAE and patients with MS.

A tale of two STAT6 knock out mice in the induction of experimental autoimmune encephalomyelitis

T helper 2 (Th2) cytokines are known to be important in protection against experimental autoimmune encephalomyelitis (EAE). To investigate the role of the signal transducer and activator of transcription factor 6 (STAT6) in EAE we used mice with two different targeted disruptions of the STAT6 gene. In this report, we show that mice with a targeted deletion of the first coding exon of the SH2 domain of STAT6 induce Th2 cell differentiation and are resistant to EAE induction. By contrast, STAT6(-/-) mice generated by deletion of amino acids 505 to 584 encoding the SH2 domain of STAT6 are defective in Th2 cell differentiation and develop very severe EAE. These results suggest that an altered STAT6 gene can be more efficient than wild type STAT6 in regulating the autoimmune response in EAE.

IL-17 eliminates the therapeutic effects of myelin basic protein-induced nasal tolerance in experimental autoimmune encephalomyelitis by activating IL-6

Interleukin (IL)-17 is a proinflammatory cytokine primarily secreted by Th17 cells, which are a CD4(+) T-cell subset. Th17 cells and IL-17 are important in the pathogenesis of multiple sclerosis and in its established animal model, experimental autoimmune encephalomyelitis (EAE). However, it is unclear whether IL-17 contributes to EAE immune tolerance. We used the myelin basic protein (MBP) peptide MBP 68-86 to induce nasal tolerance to EAE, and simultaneously interfered with the tolerance by treatment with different doses of IL-17. We found that IL-17 dramatically interfered with MBP 68-86-induced immune tolerance. IL-17 administration increased IL-6 release, skewing T cell differentiation towards Th17 cells and decreasing the number of Treg cells. This led to an imbalance between Treg cells and Th17 cells and spurred the development of EAE.

A novel role of CD4 Th17 cells in mediating cardiac allograft rejection and vasculopathy

T-bet plays a crucial role in Th1 development. We investigated the role of T-bet in the development of allograft rejection in an established MHC class II-mismatched (bm12 into B6) model of chronic allograft vasculopathy (CAV). Intriguingly, and in contrast to IFN-gamma(-/-) mice that are protected from CAV, T-bet(-/-) recipients develop markedly accelerated allograft rejection accompanied by early severe vascular inflammation and vasculopathy, and infiltration by predominantly IL-17-producing CD4 T cells. Concurrently, T-bet(-/-) mice exhibit a T helper type 1 (Th1)-deficient environment characterized by profound IFN-gamma deficiency, a Th2 switch characterized by increased production of interleukin (IL) 4, IL-5, IL-10, and IL-13 cytokines, as well as increased production of the proinflammatory cytokines IL-6, IL-12p40, and IL-17. Neutralization of IL-17 inhibits accelerated allograft rejection and vasculopathy in T-bet(-/-) mice. Interestingly, CD4 but not CD8 T cell deficiency in T-bet(-/-) mice affords dramatic protection from vasculopathy and facilitates long-term graft acceptance. This is the first study establishing that in the absence of Th1-mediated alloimmune responses, CD4 Th17 cells mediate an aggressive proinflammatory response culminating in severe accelerated allograft rejection and vasculopathy. These results have important implications for the development of novel therapies to target this intractable problem in clinical solid organ transplantation.

Cytokine-induced human IFN-gamma-secreting effector-memory Th cells in chronic autoimmune inflammation

T-helper (Th) cells activated by cytokines in the absence of T-cell receptor ligation are suspected to participate in inflammatory processes by production of interferon-gamma (IFN-gamma). Still, the relevance of such a mechanism has not been addressed in humans. Here we demonstrate that a subset of human effector-memory Th cells expressing functional interleukin-12R (IL-12R), IL-18Ralpha, and CCR5 ex vivo can be induced to secrete IFN-gamma by cytokines signaling via the IL-2R common gamma-chain in combination with IL-12 and IL-18. Cytokine-driven IFN-gamma production depends on JAK3- and p38 mitogen-activated kinase signals and is sensitive to suppression by CD25(++) regulatory T cells. Contrary to IFN-gamma(+) Th cells induced upon antigen-specific stimulation, their cytokine-activated counterparts characteristically lack expression of costimulator 4-1BB (CD137). Strikingly, the majority of Th cells infiltrating inflamed joints of rheumatoid arthritis patients is equipped with receptors prerequisite for cytokine-induced IFN-gamma secretion. Among these cells, we detected a substantial fraction that secretes IFN-gamma directly ex vivo but lacks 4-1BB expression, indicating that cytokine-induced IFN-gamma(+) Th cells operate in autoimmune inflammation. Our data provide a rationale for how human effector-memory Thcells can participate in perpetuating inflammatory processes in autoimmunity even in the absence of T-cell receptor ligation.

Th memory for interleukin-17 expression is stable in vivo

Based on the memory for the re-expression of certain cytokine genes, different subsets of Th cells have been defined. In Th type 1 (Th1) and Th2 memory lymphocytes, the genes for the cytokines interferon-gamma and interleukin (IL)-4 are imprinted for expression upon restimulation by the expression of the transcription factors T-bet and GATA-3, respectively, and epigenetic modification of the cytokine genes. In Th17 cells, IL-17 expression is dependent on the transcription factors RORgammat and RORalpha. Here, we analyze the stability and plasticity of IL-17 memory in Th17 cells. We have developed a cytometric IL-17 secretion assay for the isolation of viable Th cells secreting IL-17. For Th17 cells generated in vitro, IL-17 expression itself is dependent on continued TGF-beta/IL-6 or IL-23 signaling and is blocked by interferon-gamma and IL-4 signaling. In response to IL-12 and IL-4, in vitro generated Th17 cells are converted into Th1 or Th2 cells, respectively. Th17 cells isolated ex vivo, however, maintain their IL-17 memory upon subsequent in vitro culture, even in the absence of IL-23. Their cytokine memory is not regulated by IL-12 or IL-4. Th17 cells generated in vivo are a stable and distinct lineage of Th cell differentiation.

Induction, function and regulation of IL-17-producing T cells

Recent reports have provided convincing evidence that IL-17-producing T cells play a key role in the pathogenesis of organ-specific autoimmune diseases, a function previously attributed exclusively to IFN-gamma-secreting Th1 cells. Furthermore, it appears that IL-17-producing T cells can also function with Th1 cells to mediate protective immunity to pathogens. Although much of the focus has been on IL-17-secreting CD4+ T cells, termed Th17 cells, CD8+ T cells, gammadelta T cells and NKT cells are also capable of secreting IL-17. The differentiation of Th17 cells from naïve T cells appears to involve signals from TGF-beta, IL-6, IL-21, IL-1beta and IL-23. Furthermore, IL-1alpha or IL-1beta in synergy with IL-23 can promote IL-17 secretion from memory T cells. The induction or function of Th17 cells is regulated by cytokines secreted by the other major subtypes of T cells, including IFN-gamma, IL-4, IL-10 and at high concentrations, TGF-beta. The main function of IL-17-secreting T cells is to mediate inflammation, by stimulating production of inflammatory cytokines, such as TNF-alpha, IL-1beta and IL-6, and inflammatory chemokines that promote the recruitment of neutrophils and macrophages.

Restoration of T-box-containing protein expressed in T cells protects against allergen-induced asthma

BACKGROUND

A T(H)1-specific transcription factor, T-box-containing protein expressed in T cells (T-bet), controls the production of both T(H)1 and T(H)2 cytokines in T(H) cell differentiation by means of distinct mechanisms. T-bet-deficient mice overproduce T(H)2 cytokines and have spontaneous airway inflammation.

OBJECTIVES

We tested whether T-bet overexpression could protect against the development or progression of asthma.

METHODS

We generated a T cell-specific and inducible line of T-bet-transgenic mice on a T-bet-deficient genetic background and used it to study the function of T-bet in an ovalbumin (OVA)-induced asthma model.

RESULTS

Induction of T-bet in a T cell-specific manner in an OVA model of asthma concomitant with OVA injection prevented airway hyperresponsiveness, eosinophilic and lymphocytic inflammation, and IL-5 and IL-13 production in bronchoalveolar lavage fluid and also reduced serum IgE and T(H)2 cytokine production by peripheral T cells. Even when T-bet expression was induced during later stages of asthma progression, T-bet overexpression still attenuated airway hyperresponsiveness and goblet cell hyperplasia, as well as T(H)2 cytokine production.

CONCLUSIONS

Our results suggest that T-bet expression in T cells can prevent the initiation of airway inflammation and progression of chronic inflammation and might be extrapolated to human asthma.

Th17 immune response in IBD: A new pathogenic mechanism

Although traditionally associated with exaggerated Th1 or Th2 cell response, the gut inflammation occurring in patients with IBD is also characterized by production of cytokines made by a distinct lineage of T helper cells, termed Th17 cells. The discovery that this new inflammatory T-cell subset drives immune-mediated pathology and that the antigen-presenting cell-derived IL-23 is necessary for amplifying Th17 cell-associated inflammation has contributed to elucidate new pathways of intestinal tissue damage as well as to open new avenues for development of therapeutic strategies in IBD. In this review, we discuss the available data regarding the involvement of Th17 cells and their interplay with other mucosal cell types in the modulation of intestinal tissue inflammation.

Regulatory T cells fail to suppress CD4T+-bet+ T cells in relapsing multiple sclerosis patients

Multiple sclerosis (MS) is a chronic inflammatory demyelinating disease of the central nervous system and a defect in the regulatory T-cell subset seems to be involved in the pathogenesis of the disease. Foxp3 is a transcription factor that is selectively expressed in CD4+ CD25+ regulatory T cells and is required for their development and function. T-bet is a key transcription factor for the development of T helper 1 (Th1) cells. We found that both the percentage of circulating CD4+ CD25+ Foxp3+ cells and Foxp3 expression were lower in relapsing-remitting (RR) MS patients during relapses than during remission. Otherwise, the percentage of CD4+ T-bet+ T cells and T-bet expression in CD4+ T cells were higher in relapsing than in remitting RRMS patients. CD4+ CD25+ T cells both from relapsing and from remitting RRMS patients showed significantly less capacity than corresponding cells from healthy subjects to suppress autologous CD4+ CD25(-) T-cell proliferation, despite a similar Foxp3 expression level. CD4+ CD25+ T cells from healthy subjects and patients in remission clearly reduced T-bet mean fluorescence intensity (MFI) in CD4+ CD25(-) T cells up to a ratio of 1:10, whereas CD4+ CD25+ T cells from patients in relapse were able to reduce T-bet expression only at a high ratio. Our data indicate that the increased number of regulatory T (T-reg) cells and the increased Foxp3 expression in circulating CD4+ CD25+ T cells may contribute to the maintenance of tolerance in the remission phase of MS. Moreover, the inhibitory capacity of CD4+ CD25+ T cells seems to be impaired in relapsing patients under inflammatory conditions, as shown by the high levels of T-bet expression in CD4+ T cells.

Regulatory T cells fail to suppress CD4T+-bet+ T cells in relapsing multiple sclerosis patients

Multiple sclerosis (MS) is a chronic inflammatory demyelinating disease of the central nervous system and a defect in the regulatory T-cell subset seems to be involved in the pathogenesis of the disease. Foxp3 is a transcription factor that is selectively expressed in CD4+ CD25+ regulatory T cells and is required for their development and function. T-bet is a key transcription factor for the development of T helper 1 (Th1) cells. We found that both the percentage of circulating CD4+ CD25+ Foxp3+ cells and Foxp3 expression were lower in relapsing-remitting (RR) MS patients during relapses than during remission. Otherwise, the percentage of CD4+ T-bet+ T cells and T-bet expression in CD4+ T cells were higher in relapsing than in remitting RRMS patients. CD4+ CD25+ T cells both from relapsing and from remitting RRMS patients showed significantly less capacity than corresponding cells from healthy subjects to suppress autologous CD4+ CD25(-) T-cell proliferation, despite a similar Foxp3 expression level. CD4+ CD25+ T cells from healthy subjects and patients in remission clearly reduced T-bet mean fluorescence intensity (MFI) in CD4+ CD25(-) T cells up to a ratio of 1:10, whereas CD4+ CD25+ T cells from patients in relapse were able to reduce T-bet expression only at a high ratio. Our data indicate that the increased number of regulatory T (T-reg) cells and the increased Foxp3 expression in circulating CD4+ CD25+ T cells may contribute to the maintenance of tolerance in the remission phase of MS. Moreover, the inhibitory capacity of CD4+ CD25+ T cells seems to be impaired in relapsing patients under inflammatory conditions, as shown by the high levels of T-bet expression in CD4+ T cells.

Mycobacterium bovis bacille Calmette-Guérin infection in the CNS suppresses experimental autoimmune encephalomyelitis and Th17 responses in an IFN-gamma-independent manner

Multiple sclerosis and an animal model resembling multiple sclerosis, experimental autoimmune encephalomyelitis (EAE), are inflammatory demyelinating diseases of the CNS that are suppressed by systemic mycobacterial infection in mice and BCG vaccination in humans. Host defense responses against Mycobacterium in mice are influenced by T lymphocytes and their cytokine products, particularly IFN-gamma, which plays a protective regulatory role in EAE. To analyze the counter-regulatory role of mycobacterial infection-induced IFN-gamma in the CNS on the function of the pathological Th17 cells and the clinical outcome of EAE, we induced EAE in mice that were intracerebrally infected with Mycobacterium bovis bacille Calmette-Guerin (BCG). In this study, we demonstrate that intracerebral (i.c.) BCG infection prevented inflammatory cell recruitment to the spinal cord and suppressed the development of EAE. Concomitantly, there was a significant decrease in the frequency of myelin oligodendrocyte glycoprotein-specific IFN-gamma-producing CD4(+) T cells in the CNS. IL-17(+)CD4(+) T cell responses were significantly suppressed in i.c. BCG-infected mice following EAE induction regardless of T cell specificity. The frequency of Foxp3(+)CD4(+) T cells in these mice was equivalent to that of control mice. Intracerebral BCG infection-induced protection of EAE and suppression of myelin oligodendrocyte glycoprotein-specific IL-17(+)CD4(+) T cell responses were similar in both wild-type and IFN-gamma-deficient mice. These data show that live BCG infection in the brain suppresses CNS autoimmunity. These findings also reveal that the regulation of Th17-mediated autoimmunity in the CNS can be independent of IFN-gamma-mediated mechanisms.

FLT3 inhibitors for the treatment of autoimmune disease

BACKGROUND

Autoimmune diseases encompass a broad range of illnesses with a variety of underlying causes, some of which are known and some of which remain elusive.

OBJECTIVE

The focus of this review will be on describing the development of a new type of therapy that could potentially treat T cell-mediated autoimmune diseases. Unlike traditional therapies, which have primarily focused on suppressing T cells directly, targeting the step of antigen presentation may allow a less toxic therapy in which autoimmunity is lessened without compromising the entire immune system. This review will outline the science behind the development of the therapy, the roles of dendritic cells in generating autoimmune disease, and the function of the FLT3 receptor in this process.

Role of pathogenic T cells and autoantibodies in relapse and progression of myelin oligodendrocyte glycoprotein-induced autoimmune encephalomyelitis in LEW.1AV1 rats

Accumulating evidence suggests that T cells and autoantibodies reactive with myelin oligodendrocyte glycoprotein (MOG) play a critical role in the pathogenesis of multiple sclerosis (MS). In the present study, we have tried to elucidate the pathomechanisms of development and progression of the disease by analysing T cells and autoantibodies in MOG-induced rat experimental autoimmune encephalomyelitis (EAE), which exhibits various clinical subtypes mimicking MS. Analysis using overlapping peptides revealed that encephalitogenic epitopes resided in peptide 7 (P7, residue 91-108) and P8 (residue 103-125) of MOG. Immunization with MOGP7 and MOGP8 induced relapsing-remitting or secondary progressive EAE. T cells taken from MOG-immunized and MOGP7-immunized rats responded to MOG and MOGP7 and sera from MOG-immunized rats reacted to MOG and MOGP1. Significant epitope spreading was not observed at either T-cell or antibody levels. Interestingly, sera from MOGP7-immunized rats with clinical signs did not react to MOG and MOG peptides throughout the observation period, suggesting that disease development and relapse in MOGP7-induced EAE occur without autoantibodies. However, MOGP7 immunization with adoptive transfer of anti-MOG antibodies aggravated the clinical course of EAE only slightly. Analysis of antibodies against conformational epitope (cme) suggests that anti-MOG(cme) may play a role in the pathogenicity of anti-MOG antibodies. Collectively, these findings demonstrated that relapse of a certain type of MOG-induced EAE occurs without autoantibodies but that autoantibodies may play a role in disease progression. Relapses and the progression of MS-mimicking EAE are differently immunoregulated so immunotherapy should be designed appropriately on the basis of precise information.

Dendritic cells mediate the induction of polyfunctional human IL17-producing cells (Th17-1 cells) enriched in the bone marrow of patients with myeloma

IL17-producing (Th17) cells are a distinct lineage of T helper cells that regulate immunity and inflammation. The role of antigen-presenting cells in the induction of Th17 cells in humans remains to be fully defined. Here, we show that human dendritic cells (DCs) are efficient inducers of Th17 cells in culture, including antigen-specific Th17 cells. Although most freshly isolated circulating human Th17 cells secrete IL17 alone or with IL2, those induced by DCs are polyfunctional and coexpress IL17 and IFNgamma (Th17-1 cells). The capacity of DCs to expand Th17-1 cells is enhanced upon DC maturation, and mature DCs are superior to monocytes for the expansion of autologous Th17 cells. In myeloma, where tumors are infiltrated by DCs, Th17 cells are enriched in the bone marrow relative to circulation. Bone marrow from patients with myeloma contains a higher proportion of Th17-1 cells compared with the marrow in preneoplastic gammopathy (monoclonal gammopathy of undetermined significance [MGUS]). Uptake of apoptotic but not necrotic myeloma tumor cells by DCs leads to enhanced induction of Th17-1 cells. These data demonstrate the capacity of DCs to induce expansion of polyfunctional IL17-producing T cells in humans, and suggest a role for DCs in the enrichment of Th17-1 cells in the tumor bed.

CD47 expression on T cell is a self-control negative regulator of type 1 immune response

The cytokine milieu and dendritic cells (DCs) direct Th1 development. Yet, the control of Th1 polarization by T cell surface molecules remains ill-defined. We here report that CD47 expression on T cells serves as a self-control mechanism to negatively regulate type 1 cellular and humoral immune responses in vivo. Th2-prone BALB/c mice that lack CD47 (CD47(-/-)) displayed a Th1-biased Ab profile at steady state and after immunization with soluble Ag. CD47(-/-) mice mounted a T cell-mediated exacerbated and sustained contact hypersensitivity (CHS) response. After their adoptive transfer to naive CD47-deficient hosts 1 day before immunization with soluble Ag, CD47(-/-) as compared with CD47(+/+)CD4(+) transgenic (Tg) T cells promoted the deviation of Ag-specific T cell responses toward Th1 that were characterized by a high IFN-gamma:IL-4 cytokine ratio. Although selective CD47 deficiency on DCs led to increased IL-12p70 production, CD47(-/-)Tg T cells produced more IFN-gamma and displayed higher T-bet expression than CD47(+/+) Tg T cells in response to OVA-loaded CD47(-/-) DCs. CD47 as part of the host environment has no major contribution to the Th1 polarization responses. We thus identify the CD47 molecule as a T cell-negative regulator of type 1 responses that may limit unwanted collateral damage to maximize protection and minimize host injury.

IL-23/IL-17 immunity as a hallmark of Crohn's disease

BACKGROUND

We studied the balance between ileal T-effector cells versus T-regulatory cells in active and inactive Crohn's disease (CD).

METHODS

We compared effector and regulatory T-cell-related markers such as interleukin (IL)-17, interferon (IFN)-gamma, IL-4, and Foxp3 transforming growth factor (TGF)-beta CTLA-4 and markers for innate immune activation such as IL-6, IL-10, IL-18, IL-23, tumor necrosis factor (TNF)-alpha, and IL-12p70, studied with immunohistochemistry and RT-PCR in ileal biopsies from patients with active or inactive CD and from control subjects. IL-17 in fecal samples was detected by ELISA. The effect of IL-17 on IL-8 and TNF-alpha mRNA expression in epithelial cell line Caco-2 was studied.

RESULTS

The numbers of IL-4-, IL-17-, and IL-23(p19)-positive cells in the lamina propria were higher in patients with CD, both active and inactive, than in the controls. mRNA expression of IL-17A, IL-6, and Foxp3 was increased in the biopsies both from patients with active disease and those in remission, whereas mRNA expression of IL-23 was increased only in active disease. Fecal IL-17 concentration was increased in patients with active disease. IL-17 enhanced the IL-8 and TNF-alpha response of the epithelial cell line to lipopolysaccharide (LPS) in vitro.

CONCLUSIONS

Our findings suggest that activation of the IL-23/IL-17 axis is fundamentally connected to the etiology of CD and may represent the basis for the relapsing nature of the disease by increasing the sensitivity of epithelium to microbial LPS.

Engagement of the type I interferon receptor on dendritic cells inhibits T helper 17 cell development: role of intracellular osteopontin

Mechanisms that prevent inappropriate or excessive interleukin-17-producing T helper (Th17) cell responses after microbial infection may be necessary to avoid autoimmunity. Here, we define a pathway initiated by engagement of type I IFN receptor (IFNAR) expressed by dendritic cells (DC) that culminated in suppression of Th17 cell differentiation. IFNAR-dependent inhibition of an intracellular translational isoform of Osteopontin, termed Opn-i, derepressed interleukin-27 (IL-27) secretion and prevented efficient Th17 responses. Moreover, Opn-i expression in DC and microglia regulated the type and intensity of experimental autoimmune encephalomyelitis (EAE). Mice containing DC deficient in Opn-i produced excessive amounts of IL-27 and developed a delayed disease characterized by an enhanced Th1 response compared with the dominant Th17 response of Opn-sufficient mice. Definition of the IFNAR-Opn-i axis that controls Th17 development provides insight into regulation of Th cell sublineage development and the molecular basis of type I interferon therapy for MS and other autoimmune diseases.

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.

Phenotype of CD4+ T cell subsets that develop following mouse facial nerve axotomy

We have previously shown that CD4(+) T helper (Th) 2 cells, but not Th1 cells, participate in the rescue of mouse facial motoneurons (FMN) from axotomy-induced cell death. Recently, a number of other CD4(+) T cell subsets have been identified in addition to the Th1 and Th2 effector subsets, including Th17, inducible T regulatory type 1 (Tr1), and naturally thymus-born Foxp3(+) regulatory (Foxp3(+) Treg) cells. These subsets regulate the nature of a T cell-mediated immune response. Th1 and Th17 cells are pro-inflammatory subsets, while Th2, Tr1, and Foxp3(+) Treg cells are anti-inflammatory subsets. Pro-inflammatory responses in the central nervous system are thought to be neurodestructive, while anti-inflammatory responses are considered neuroprotective. However, it remains to be determined if another CD4(+) T cell subset, other than the Th2 cell, develops after peripheral nerve injury and participates in FMN survival. In the present study, we used FACS analysis to determine the temporal frequency of Th1, Th17, Th2, Tr1 and Foxp3(+) Treg CD4(+) T cell subset development in C57BL/6 wild type mice after facial nerve transection at the stylomastoid foramen in the mouse. The results indicate that all of the known CD4(+) T cell subsets develop and expand in number within the draining lymph node, with a peak in number primarily at 7 days postoperative (dpo), followed by a decline at 9 dpo. In addition to the increase in subset frequency over time, FACS analysis of individual cells showed that the level of cytokine expressed per cell also increased for interferon-gamma (IFN-gamma), interleukin (IL)-10 and IL-17, but not IL-4. Additional control double-cytokine labeling experiments were done which indicate that, at 7dpo, the majority of cells indeed have committed to a specific phenotype and express only 1 cytokine. Collectively, our findings indicate for the first time that there is no preferential activation and expansion of any single CD4(+) T cell subset after peripheral nerve injury but, rather, that both pro-inflammatory and anti-inflammatory CD4(+) T cells develop.

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.

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.

Hyperproduction of IL-23 and IL-17 in patients with systemic lupus erythematosus: implications for Th17-mediated inflammation in auto-immunity

IL-23-dependent IL-17-producing T helper (Th) lymphocytes are associated with autoimmunity. We investigated the immunopathological mechanisms for activation of Th17 cells of patients with systemic lupus erythematosus (SLE). Concentration of cytokines/chemokine in plasma and culture supernatant from SLE patients and healthy controls were measured by ELISA or flow cytometry. Plasma IL-12, IL-17, IL-23 and CXCL10 concentrations and the number of Th17 cells were significantly elevated in SLE patients than control subjects (both p<0.05). Elevated IL-12, IL-17 and CXCL10 concentrations correlated positively and significantly with SLEDAI (all p<0.05). Plasma IL-12 and IL-17 showed significant and positive correlation with plasma Th1 chemokine CXCL10 concentration in SLE patients (all p<0.05). Ex vivo inductions of IL-17 by IL-23 or IL-18 from co-stimulated lymphocytes were significantly higher in SLE patients than controls (all p<0.05). The activated IL-23/IL-17 axis is important for the inflammatory immunity in SLE.

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.