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

Current perspectives on the role of IL-17 in autoimmune disease

Until recently, autoimmune diseases had been categorized as either Th1- or Th2-mediated diseases. However, the discovery of a novel subset of helper T cells producing interleukin (IL)-17, ie, Th17 cells, changed this paradigm. Currently, IL-17 and Th17 cells are implicated in many autoimmune diseases, such as rheumatoid arthritis, psoriasis, multiple sclerosis, and inflammatory bowel diseases. Such conclusions were initially drawn from observations in animal models of autoimmune diseases, and accumulating data from clinical research also support the involvement of IL-17 in human diseases as well. Reagents targeting Th17-related molecules have been under clinical investigation for some diseases but have not always been effective in controlling disease activity. Consistent with this, it has become evident that there are substantial differences in the development of Th17 cells and in the way they function in autoimmune diseases between humans and experimental animals. Thus, further investigation is needed before we can draw any conclusions about the importance of IL-17 and Th17 cells in human autoimmune diseases.

Th1 versus Th17: are T cell cytokines relevant in multiple sclerosis?

Our understanding of the pathophysiology of multiple sclerosis (MS) has evolved significantly over the past two decades as the fields of immunology and neurobiology provide new avenues of exploration into the cause and mechanism of the disease. It has been known for decades that T cells have different cytokine phenotypes, yet the cytokine phenotype of pathogenic T cells in MS is still an area of debate. In EAE, it appears that IFNγ and IL-17, produced by Th1 and Th17 cells respectively, are not the critical factor that determines T cell encephalitogenicity. However, there are molecules such as IL-23, T-bet and STAT4, that appear to be critical, yet it is unclear whether all these molecules contribute to a common, yet undefined pathway, or act in a synergistic manner which culminates in encephalitogenicity has still to be determined. Therefore, the focus of research on effector T cells in MS should focus on pathways upstream of the cytokines that define Th1 and Th17 cells, since downstream products, such as IFNγ and IL-17, probably are not critical determinants of whether an effector T cells is capable of trafficking to the CNS and inducing inflammatory demyelination.

Current views on the roles of Th1 and Th17 cells in experimental autoimmune encephalomyelitis

Multiple sclerosis (MS) and its animal model, experimental autoimmune encephalomyelitis (EAE), are autoimmune demyelinating diseases of the central nervous system (CNS). Interferon-gamma-producing Th1 and interleukin-17-producing Th17 CD4(+) T helper (Th) cells mediate disease pathogenesis in EAE and likely in MS as well. However, the relative contribution of each Th subset to autoimmune processes in the CNS remains unclear. Emerging data suggest that both Th1 and Th17 cells contribute to CNS autoimmunity, albeit through different mechanisms. A better understanding of the roles that Th1 and Th17 cells play in autoimmune inflammation will be helpful in developing new therapeutic approaches. In this review, we discuss recent findings on the roles of Th1 and Th17 cells in the pathogenesis of EAE.

Human Th17 cells comprise heterogeneous subsets including IFN-gamma-producing cells with distinct properties from the Th1 lineage

Th17 cells have been named after their signature cytokine IL-17 and accumulating evidence indicates their involvement in the induction and progression of inflammatory diseases. In addition to IL-17 single-producing T cells, IL-17/IFN-gamma double-positive T cells are found in significantly elevated numbers in inflamed tissues or blood from patients with chronic inflammatory disorders. Because IFN-gamma is the classical Th1-associated cytokine, the origin and roles of these subsets remain elusive. In this paper, we show that not only IL-17(+)/IFN-gamma(+) but also IFN-gamma(+) (IL-17(-)) cells arise under Th17-inducing condition and have distinct properties from the Th1 lineage. In fact, these populations displayed characteristics reminiscent to IL-17 single-producing cells, including production of IL-22, CCL20, and induction of antimicrobial gene expression from epithelial cells. Live sorted IL-17(+) and Th17-IFN-gamma(+) cells retained expression of IL-17 or IFN-gamma after culture, respectively, whereas the IL-17(+)/IFN-gamma(+) population was less stable and could also become IL-17 or IFN-gamma single-producing cells. Interestingly, these Th17 subsets became "Th1-like" cells in the presence of IL-12. These results provide novel insights into the relationship and functionality of the Th17 and Th1 subsets and have direct implications for the analysis and relevance of IL-17 and/or IFN-gamma-producing T cells present in patients' peripheral blood and inflamed tissues.

Silencing Nogo-A promotes functional recovery in demyelinating disease

OBJECTIVE

To determine if suppressing Nogo-A, an axonal inhibitory protein, will promote functional recovery in a murine model of multiple sclerosis (MS).

METHODS

A small interfering RNA was developed to specifically suppress Nogo-A (siRNA-NogoA). The siRNA-NogoA silencing effect was evaluated in vitro and in vivo via immunohistochemistry. The siRNA was administered intravenously in 2 models of experimental autoimmune encephalomyelitis (EAE). Axonal repair was measured by upregulation of GAP43. Enzyme-linked immunosorbent assay, flow cytometry, and (3)H-thymidine incorporation were used to determine immunological changes in myelin-specific T cells in mice with EAE.

RESULTS

The siRNA-NogoA suppressed Nogo-A expression in vitro and in vivo. Systemic administration of siRNA-NogoA ameliorated EAE and promoted axonal repair, as demonstrated by enhanced GAP43+ axons in the lesions. Myelin-specific T-cell proliferation and cytokine production were unchanged in the siRNA-NogoA-treated mice.

INTERPRETATION

Silencing Nogo-A in EAE promotes functional recovery. The therapeutic benefit appears to be mediated by axonal growth and repair, and is not attributable to changes in the encephalitogenic capacity of the myelin-specific T cells. Silencing Nogo-A may be a therapeutic option for MS patients to prevent permanent functional deficits caused by immune-mediated axonal damage.

Emerging role of IL-17 in atherosclerosis

The IL-23-IL-17 axis is emerging as a critical regulatory system that bridges the innate and adaptive arms of the immune system. Th17 cells have been linked to the pathogenesis of several chronic inflammatory and autoimmune diseases. However, the role of Th17 cells and IL-17 in various stages of atherogenesis remains poorly understood and is only beginning to be elucidated. While IL-17 is a predominantly proinflammatory cytokine, it has a pleiotropic function and it has been implicated both as an instigator in the pathogenesis of several inflammatory disorders as well as being protective in certain inflammatory disease models. Therefore, it is not surprising that the current literature is conflicting on the role of IL-17 during atherosclerotic lesion development. Various approaches have been used by several groups to discern the involvement of IL-17 in atherosclerosis. While one study found that IL-17 is protective against atherosclerosis, several other recent studies have suggested that IL-17 plays a proatherogenic role. Thus, the function of IL-17 remains controversial and awaits more direct studies to address the issue. In this review, we will highlight all the latest studies involving IL-17 and atherosclerosis, including both clinical and experimental research.

Smad7 in T cells drives T helper 1 responses in multiple sclerosis and experimental autoimmune encephalomyelitis

Autoreactive CD4+ T lymphocytes play a vital role in the pathogenesis of multiple sclerosis and its animal model, experimental autoimmune encephalomyelitis. Since the discovery of T helper 17 cells, there is an ongoing debate whether T helper 1, T helper 17 or both subtypes of T lymphocytes are important for the initiation of autoimmune neuroinflammation. We examined peripheral blood CD4+ cells from patients with active and stable relapsing-remitting multiple sclerosis, and used mice with conditional deletion or over-expression of the transforming growth factor-beta inhibitor Smad7, to delineate the role of Smad7 in T cell differentiation and autoimmune neuroinflammation. We found that Smad7 is up-regulated in peripheral CD4+ cells from patients with multiple sclerosis during relapse but not remission, and that expression of Smad7 strongly correlates with T-bet, a transcription factor defining T helper 1 responses. Concordantly, mice with transgenic over-expression of Smad7 in T cells developed an enhanced disease course during experimental autoimmune encephalomyelitis, accompanied by elevated infiltration of inflammatory cells and T helper 1 responses in the central nervous system. On the contrary, mice with a T cell-specific deletion of Smad7 had reduced disease and central nervous system inflammation. Lack of Smad7 in T cells blunted T cell proliferation and T helper 1 responses in the periphery but left T helper 17 responses unaltered. Furthermore, frequencies of regulatory T cells were increased in the central nervous system of mice with a T cell-specific deletion and reduced in mice with a T cell-specific over-expression of Smad7. Downstream effects of transforming growth factor-beta on in vitro differentiation of naïve T cells to T helper 1, T helper 17 and regulatory T cell phenotypes were enhanced in T cells lacking Smad7. Finally, Smad7 was induced during T helper 1 differentiation and inhibited during T helper 17 differentiation. Taken together, the level of Smad7 in T cells determines T helper 1 polarization and regulates inflammatory cellular responses. Since a Smad7 deletion in T cells leads to immunosuppression, Smad7 may be a potential new therapeutic target in multiple sclerosis.

Astrocyte-restricted ablation of interleukin-17-induced Act1-mediated signaling ameliorates autoimmune encephalomyelitis

Interleukin-17 (IL-17) secreted by T helper 17 (Th17) cells is essential in the development of experimental autoimmune encephalomyelitis (EAE). However, it remains unclear how IL-17-mediated signaling in different cellular compartments participates in the central nervous system (CNS) inflammatory process. We examined CNS inflammation in mice with specific deletion of Act1, a critical component required for IL-17 signaling, in endothelial cells, macrophages and microglia, and neuroectoderm (neurons, astrocytes, and oligodendrocytes). In Act1-deficient mice, Th17 cells showed normal infiltration into the CNS but failed to recruit lymphocytes, neutrophils, and macrophages. Act1 deficiency in endothelial cells or in macrophages and microglia did not substantially impact the development of EAE. However, targeted Act1 deficiency in neuroectoderm-derived CNS-resident cells resulted in markedly reduced severity in EAE. Specifically, Act1-deficient astrocytes showed impaired IL-17-mediated inflammatory gene induction. Thus, astroctyes are critical in IL-17-Act1-mediated leukocyte recruitment during autoimmune-induced inflammation of the CNS.

Chlamydia muridarum T-cell antigens formulated with the adjuvant DDA/TDB induce immunity against infection that correlates with a high frequency of gamma interferon (IFN-gamma)/tumor necrosis factor alpha and IFN-gamma/interleukin-17 double-positive CD4+ T cells

Major impediments to developing a Chlamydia vaccine lie in identifying immunologically relevant T-cell antigens and delivery in a manner to stimulate protective immunity. Using an immunoproteomic approach, we previously identified three immunodominant Chlamydia T-cell antigens (PmpG-1, PmpE/F-2, and RplF). Because RplF has high homology to a human ortholog, it may not be suitable for human vaccine development. Therefore, in this study, we evaluated protection against Chlamydia infection in the genital tract in C57BL/6 mice immunized with Chlamydia-specific membrane proteins PmpG-1, PmpE/F-2, and major outer membrane protein (MOMP; as a reference) or a combination of them formulated with one of three adjuvants, CpG oligodeoxynucleotide (CpG-ODN), AbISCO-100 (AbISCO), or DDA/TDB (dimethyldioctadecylammonium bromide/D-(+)-trehalose 6,6'-dibehenate). The results show that immunization with the CpG-ODN formulation failed to provide protection against Chlamydia infection; the AbISCO formulation conferred moderate protection, and the DDA/TDB formulation showed the highest degree of protective efficacy. The combination of PmpG-1, PmpE/F-2, and MOMP proteins formulated with DDA/TDB exhibited the greatest degree of protection among all vaccine groups studied. Moreover, this vaccine combination also engendered significant protection in BALB/c mice, which have a different major histocompatibility complex (MHC) background. We measured cell-mediated immune cytokine responses in mice immunized with PmpG-1 mixed with each of the three adjuvants. The results demonstrate that mice immunized with the DDA/TDB formulation induced the strongest gamma interferon (IFN-gamma) and interleukin-17 (IL-17) responses, characterized by the highest frequency of IFN-gamma/tumor necrosis factor alpha (TNF-alpha) and IFN-gamma/IL-17 double-positive CD4(+) T cells. In conclusion, a Chlamydia vaccine based on the recombinant proteins PmpG-1, PmpE/F-2, and MOMP delivered in a DDA/TDB adjuvant conferred protection against infection that correlated with IFN-gamma/TNF-alpha and IFN-gamma/IL-17 double-positive CD4(+) T cells.

Inhibition of IL-17A attenuates atherosclerotic lesion development in apoE-deficient mice

The importance of an (auto)immune response in atherogenesis is becoming increasingly well understood. IL-17A-expressing T cells modulate immune cell trafficking, initiating inflammation and cytokine production in (auto)immune diseases. In human carotid artery plaques, we previously showed the presence of IL-17A-producing T cells and IL-23; however, IL-17A effects on atherogenesis have not been studied. Aortic root sections from 8-wk-old apolipoprotein E-deficient mice fed a standard chow diet were examined after 12 wk for lesion area, plaque composition, cellular infiltration, cytokine expression, and apoptosis. The treatment group (n = 15) received anti-IL-17A Ab and the controls (n = 10) received irrelevant Abs. Inhibition of IL-17A markedly reduced atherosclerotic lesion area (p < 0.001), maximal stenosis (p < 0.001), and vulnerability of the lesion. IL-17A mAb-treated mice showed reduced cellular infiltration, down-regulation of activation markers on endothelium and immune cells (e.g., VCAM-1), and reduced cytokine/chemokine secretion (e.g., IL6, TNFalpha, CCL5). To investigate possible mechanisms, different atherogenic cell types (e.g., macrophages, dendritic cells, HUVECs, vascular smooth muscle cells) were stimulated with IL-17A in addition to TNF-alpha, IFN-gamma, or LPS to induce cellular activation or apoptosis in vitro. Stimulation with IL-17A induced proinflammatory changes in several atherogenic cell types and apoptotic cell death in murine cells. Functional blockade of IL-17A reduces atherosclerotic lesion development and decreases plaque vulnerability, cellular infiltration, and tissue activation in apolipoprotein E-deficient mice. The present data support a pathogenic role of IL-17A in the development of atherosclerosis by way of its widespread proinflammatory and proapoptotic effects on atherogenic cells.

Phenotypical and functional characterization of T helper 17 cells in multiple sclerosis

Multiple sclerosis is a T cell-mediated demyelinating disease of the central nervous system. Interleukin-17-producing T helper cells, named Th17 cells, represent a novel CD4+ T cell effector subset involved in the response against extracellular pathogens. In addition, Th17 cells are pathogenic in several animal models of autoimmune disease, including the animal model for multiple sclerosis, but their function in multiple sclerosis remains to be elucidated. In this study, we analysed the frequency and the phenotype of Th17 cells in the cerebrospinal fluid and peripheral blood of multiple sclerosis patients. We show that the frequency of Th17 cells is significantly higher in the cerebrospinal fluid of patients with relapsing-remitting multiple sclerosis during relapse, in comparison to relapsing-remitting patients in remission or to patients with other non-inflammatory neurological diseases. Similarly, in patients with clinically isolated syndrome during their first neurological episode, Th17 cells are more abundant than in clinically isolated syndrome patients with no acute symptoms. Patients with inflammatory neurological diseases other than multiple sclerosis also showed increased frequency of Th17 cells compared to patients with no inflammatory diseases. To assess a potential pathological impact of Th17 cells in disease, we generated T cell clones from the cerebrospinal fluid and peripheral blood of patients with multiple sclerosis. We found that Th17 clones expressed higher basal levels of the activation markers CD5, CD69, CD2 and human leukocyte antigen-DR as well as of the CD28-related family of co-stimulatory molecules, when compared to Th1 clones, and confirmed these findings with ex vivo human T cells. Molecules involved in T cell adhesion to endothelium, such as CD49d, CD6 and the melanoma cell adhesion molecule, were also more abundant on the Th17 than on the Th1 cells. Furthermore, functional assays showed that Th17 clones were more prone than Th1 clones to melanoma cell adhesion molecule-mediated adhesion to endothelial cells, and that Th17 cells had a higher proliferative capacity and were less susceptible to suppression than Th1 cells. Altogether our data suggest that Th17 cells display a high pathogenic potential and may constitute a relevant pathogenic subset in multiple sclerosis.

Parasitic helminths: new weapons against immunological disorders

The prevalence of allergic and autoimmune diseases is increasing in developed countries, possibly due to reduced exposure to microorganisms in childhood (hygiene hypothesis). Epidemiological and experimental evidence in support of this hypothesis is accumulating. In this context, parasitic helminths are now important candidates for antiallergic/anti-inflammatory agents. Here we summarize antiallergic/anti-inflammatory effects of helminths together along with our own study of the effects of Schistosoma mansoni on Th17-dependent experimental arthritis. We also discuss possible mechanisms of helminth-induced suppression according to the recent advances of immunology.

IFN-gamma regulates the requirement for IL-17 in proteoglycan-induced arthritis

The contribution of the proinflammatory cytokines IFN-gamma and IL-17 to the pathogenesis of experimental arthritis is controversial. In proteoglycan (PG)-induced arthritis (PGIA), severe arthritis is dependent on the production of IFN-gamma, whereas IL-17 is dispensable. In collagen-induced arthritis and Ag-induced arthritis, although high levels of IFN-gamma are secreted, disease is exacerbated in IFN-gamma or IFN-gamma receptor-deficient mice due to the ability of IFN-gamma to suppress IL-17 expression. In the current study, we investigated the effect of IFN-gamma on the IL-17 response and its consequences in PGIA. In PG-immunized IFN-gamma(-/-) mice, despite reduction in arthritis, the PG-specific CD4(+) T cell IL-17 response was significantly increased. Elevated IL-17 contributed to development of arthritis, as disease in IFN-gamma/IL-17(-/-) was significantly reduced in comparison with either IFN-gamma(-/-) or IL-17(-/-) mice. A contribution of IFN-gamma and IL-17 to the development of arthritis was also identified in T-bet(-/-) mice. PG-specific CD4(+) T cells from T-bet(-/-) mice produced reduced IFN-gamma and elevated concentrations of IL-17. Both IFN-gamma and IL-17 contribute to arthritis, as T-bet(-/-) mice lacking IL-17 (T-bet/IL-17(-/-)) were resistant, whereas wild-type, T-bet(-/-), and IL-17(-/-) mice were susceptible to PGIA. T cell proliferation and autoantibody production did not correlate with development of disease; however, expression of cytokines and chemokines in joint tissues demonstrate that IFN-gamma and IL-17 cooperatively contribute to inflammation. These results demonstrate that both IFN-gamma and IL-17 have the potential to induce PGIA, but it is the strength of the IFN-gamma response that regulates the contribution of each of these Th effector cytokines to disease.

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.

Models of autoimmune demyelination in the central nervous system: on the way to translational medicine

Multiple sclerosis (MS) is the most common neurologic disease of young adults. In the recent years, our understanding on disease pathomechanisms has considerably improved and new therapies have emerged. Yet a cure for this devastating disorder is still a far cry away and human resources on ex vivo specimens are limited. More than 70 years after its first description, experimental autoimmune encephalomyelitis (EAE) remains an important tool to understand concepts of T cell mediated autoimmunity as well as the roles of the innate and the humoral immune systems. Some EAE models also well reflect mechanisms of tissue damage including demyelination, axonal injury and also cortical changes. A limitation of the classical EAE model is a neglect of CD8 T cell mediated immune mechanisms. Moreover, well characterized models for primary progressive MS or demyelination patterns involving primary oligodendrocyte dystrophy are still not available. Yet many current therapeutic concepts including glatiramer acetate or natalizumab stem from their successful first application in EAE models. New strategies include the widespread use of conditional knockout mice to understand the cell-type specific function of single genes, innovative approaches to establish models on the roles of B cells and CD8 T cells as well as on the relation of inflammation to primary degeneration. In summary, EAE models continue to play an important role in neuroimmunology thereby also stimulating research in other fields of the neurosciences and immunobiology.

Intestinal effector T cells in health and disease

Crohn's disease and ulcerative colitis are the two major forms of chronic relapsing inflammatory disorders of the human intestines collectively referred to as inflammatory bowel disease (IBD). Though a complex set of autoinflammatory disorders that can be precipitated by diverse genetic and environmental factors, a feature that appears common to IBD pathogenesis is a dysregulated effector T cell response to the commensal microbiota. Due to the heightened effector T cell activity in IBD, developmental and functional pathways that give rise to these cells are potential targets for therapeutic intervention. In this review, we highlight recent advances in our understanding of effector T cell biology in the context of intestinal immune regulation and speculate on their potential clinical significance.

Shifting therapeutic attention in MS to osteopontin, type 1 and type 2 IFN

It is widely debated whether MS is mediated solely by Th1, or solely by Th17, whether it might be mediated by both pathways, or perhaps by neither pathway. We must integrate the following four facts: first, MS lesions have a signature of IL-6-, IL-17-, osteopontin- and IFN-driven transcriptional activity. Second, MS is worsened with administration of IFN-gamma, the quintessential type 2 IFN and Th1 cytokine. Third, blockade of TNF-alpha worsened MS in clinical trials. Fourth, inhibiting the main driver of Th17, IL-23, failed to modulate relapsing-remitting MS (RRMS). Against this backdrop, standing outside the framework of the Th1 and Th17 pathways are the type 1 IFN, notably IFN-beta, the most widely used approved therapy for RRMS. A paper in this issue of the European Journal of Immunology demonstrates that IFN-beta suppresses production in CD4(+) T cells of both osteopontin and IL-17. In this commentary, the roles of these two molecules, i.e. osteopontin and IL-17, are discussed in relation to the pathogenesis of MS. Osteopontin may be more important than Th1 or Th17 in the pathogenesis of RRMS. Trials targeting this small integrin-binding protein ought to be pursued in RRMS.

The nuclear receptor PPAR gamma selectively inhibits Th17 differentiation in a T cell-intrinsic fashion and suppresses CNS autoimmunity

T helper cells secreting interleukin (IL)-17 (Th17 cells) play a crucial role in autoimmune diseases like multiple sclerosis (MS). Th17 differentiation, which is induced by a combination of transforming growth factor (TGF)-beta/IL-6 or IL-21, requires expression of the transcription factor retinoic acid receptor-related orphan receptor gamma t (ROR gamma t). We identify the nuclear receptor peroxisome proliferator-activated receptor gamma (PPAR gamma) as a key negative regulator of human and mouse Th17 differentiation. PPAR gamma activation in CD4(+) T cells selectively suppressed Th17 differentiation, but not differentiation into Th1, Th2, or regulatory T cells. Control of Th17 differentiation by PPAR gamma involved inhibition of TGF-beta/IL-6-induced expression of ROR gamma t in T cells. Pharmacologic activation of PPAR gamma prevented removal of the silencing mediator for retinoid and thyroid hormone receptors corepressor from the ROR gamma t promoter in T cells, thus interfering with ROR gamma t transcription. Both T cell-specific PPAR gamma knockout and endogenous ligand activation revealed the physiological role of PPAR gamma for continuous T cell-intrinsic control of Th17 differentiation and development of autoimmunity. Importantly, human CD4(+) T cells from healthy controls and MS patients were strongly susceptible to PPAR gamma-mediated suppression of Th17 differentiation. In summary, we report a PPAR gamma-mediated T cell-intrinsic molecular mechanism that selectively controls Th17 differentiation in mice and in humans and that is amenable to pharmacologic modulation. We therefore propose that PPAR gamma represents a promising molecular target for specific immunointervention in Th17-mediated autoimmune diseases such as MS.