Fate mapping of IL-17-producing T cells in inflammatory responses

The microbiome in infectious disease and inflammation

The mammalian alimentary tract harbors hundreds of species of commensal microorganisms (microbiota) that intimately interact with the host and provide it with genetic, metabolic, and immunological attributes. Recent reports have indicated that the microbiota composition and its collective genomes (microbiome) are major factors in predetermining the type and robustness of mucosal immune responses. In this review, we discuss the recent advances in our understanding of host-microbiota interactions and their effect on the health and disease susceptibility of the host.

The role of Notch in the differentiation of CD4⁺ T helper cells

CD4⁺ T helper cells are playing critical roles in host defense to pathogens and in the maintenance of immune homeostasis. Naïve CD4⁺T cells, upon antigen-specific recognition, receive signals to differentiate into distinct effector T helper cell subsets characterized by their pattern of cytokine production and specific immune functions. A tight balance between these different subsets ensures proper control of the immune response. There is increasing evidence revealing an important role for Notch signaling in the regulation of CD4⁺T helper cell differentiation or function in the periphery. However, the exact mechanisms involved remain unclear and appear contradictory. In this review, we summarize current knowledge and discuss recent advances in the field to reconcile different views on the role of Notch signaling in the differentiation of functional T helper subsets.

Are Th17 cells in the gut pathogenic or protective?

Th17 cells are abundant in multiple chronic inflammatory and autoimmune diseases. Clinical trials with antibodies to interleukin (IL)-17A, one of the Th17-cell signature cytokines, have recently reported therapeutic benefit in multiple patient populations; however, in Crohn's disease the role of Th17 cells and IL-17A appears to be more complicated. The development of different subsets of Th17 cells and their relative pathogenic activities with a focus on the gut environment will be discussed.

Animal models of inflammatory bowel disease

Inflammatory bowel disease (IBD) is a chronic intestinal inflammatory condition that is medicated by genetic, immune, and environmental factors. At least 66 different kinds of animal models have been established to study IBD, which are classified primarily into chemically induced, cell-transfer, congenial mutant, and genetically engineered models. These IBD models have provided significant contributions to not only dissect the mechanism but also develop novel therapeutic strategies for IBD. In addition, recent advances on genetically engineered techniques such as cell-specific and inducible knockout as well as knockin mouse systems have brought novel concepts on IBD pathogenesis to the fore. Further, mouse models, which lack some IBD susceptibility genes, have suggested more complicated mechanism of IBD than previously predicted. This chapter summarizes the distinct feature of each murine IBD model and discusses the previous and current lessons from the IBD models.

Th17 cells are long lived and retain a stem cell-like molecular signature

Th17 cells have been described as short lived, but this view is at odds with their capacity to trigger protracted damage to normal and transformed tissues. We report that Th17 cells, despite displaying low expression of CD27 and other phenotypic markers of terminal differentiation, efficiently eradicated tumors and caused autoimmunity, were long lived, and maintained a core molecular signature resembling early memory CD8(+) cells with stem cell-like properties. In addition, we found that Th17 cells had high expression of Tcf7, a direct target of the Wnt and β-catenin signaling axis, and accumulated β-catenin, a feature observed in stem cells. In vivo, Th17 cells gave rise to Th1-like effector cell progeny and also self-renewed and persisted as IL-17A-secreting cells. Multipotency was required for Th17 cell-mediated tumor eradication because effector cells deficient in IFN-γ or IL-17A had impaired activity. Thus, Th17 cells are not always short lived and are a less-differentiated subset capable of superior persistence and functionality.

Helper T-cell differentiation and plasticity: insights from epigenetics

CD4(+) T cells have critical roles in orchestrating immune responses to diverse microbial pathogens. This is accomplished through the differentiation of CD4(+) T helper cells to specialized subsets in response to microbial pathogens, which evoke a distinct cytokine milieu. Signal transducer and activator of transcription family transcription factors sense these cytokines and they in turn regulate expression of lineage-defining master regulators that programme selective gene expression, resulting in distinctive phenotypes. However, phenotype and restricted gene expression are determined not only by the action of transcription factors; chromatin accessibility is required for these factors to exert their effect. Technical advances have greatly expanded our understanding of transcription factor action and dynamic changes in the epigenome that accompany cellular differentiation. In this review, we will discuss recent progress in the understanding of how cytokines influence gene expression and epigenetic modifications, and the impact of these findings on our views of helper cell lineage commitment and plasticity.

The molecular mechanisms of regulatory T cell immunosuppression

CD4⁺CD25⁺Foxp3⁺ T lymphocytes, known as regulatory T cells or T(regs), have been proposed to be a lineage of professional immune suppressive cells that exclusively counteract the effects of the immunoprotective "helper" and "cytotoxic" lineages of T lymphocytes. Here we discuss new concepts on the mechanisms and functions of T(regs). There are several key points we emphasize: 1. Tregs exert suppressive effects both directly on effector T cells and indirectly through antigen-presenting cells; 2. Regulation can occur through a novel mechanism of cytokine consumption to regulate as opposed to the usual mechanism of cytokine/chemokine production; 3. In cases where CD4⁺ effector T cells are directly inhibited by T(regs), it is chiefly through a mechanism of lymphokine withdrawal apoptosis leading to polyclonal deletion; and 4. Contrary to the current view, we discuss new evidence that T(regs), similar to other T-cells lineages, can promote protective immune responses in certain infectious contexts (Chen et al., 2011; Pandiyan et al., 2011). Although these points are at variance to varying degrees with the standard model of T(reg) behavior, we will recount developing findings that support these new concepts.

Cutting edge: loss of α4 integrin expression differentially affects the homing of Th1 and Th17 cells

The neutralization of α4 integrin is currently used as treatment in several autoimmune diseases and is thought to prevent the entry of most immune cells in target tissues. In this study, we showed that selective deletion of α4 integrin in T cells did not prevent but delayed the development of experimental autoimmune encephalomyelitis. Whereas both Th1 and Th17 cells infiltrate the CNS of wild-type mice, T cells present in the CNS of mice lacking α4 integrin were mainly enriched in Th17 cells, suggesting that this T cell subset uses other integrins to access the CNS. In contrast, α4 integrin expression is important for Th1 cells to enter the CNS and for the stability of their Th1-associated genetic program. Therefore, our data suggest that anti-α4 integrin Ab treatment may be more efficient in the treatment of Th1- rather than Th17-mediated disease.

Psoriasis

Psoriasis is a common relapsing and remitting immune-mediated inflammatory disease that affects the skin and joints. This review focuses on current immunogenetic concepts, key cellular players, and axes of cytokines that are thought to contribute to disease pathogenesis. We highlight potential therapeutic targets and give an overview of the currently used immune-targeted therapies.

Human Th1 and Th17 cells exhibit epigenetic stability at signature cytokine and transcription factor loci

The linear model of Th cell lineage commitment is being revised due to reports that mature Th cells can trans-differentiate into alternate lineages. This ability of Th cells to reprogram is thought to be regulated by epigenetic mechanisms that control expression of transcription factors characteristic of opposing lineages. It is unclear, however, to what extent this new model of Th cell plasticity holds true in human Th cell subsets that develop under physiological conditions in vivo. We isolated in vivo-differentiated human Th1 and Th17 cells, as well as intermediate Th1/17 cells, and identified distinct epigenetic signatures at cytokine (IFNG and IL17A) and transcription factor (TBX21, RORC, and RORA) loci. We also examined the phenotypic and epigenetic stability of human Th17 cells exposed to Th1-polarizing conditions and found that although they could upregulate TBX21 and IFN-γ, this occurred without loss of IL-17 or RORC expression, and resulted in cells with a Th1/17 phenotype. Similarly, Th1 cells could upregulate IL-17 upon enforced expression of RORC2, but did not lose expression of IFN-γ or TBX21. Despite alterations in expression of these signature genes, epigenetic modifications were remarkably stable aside from the acquisition of active histone methylation marks at cytokine gene promoters. The limited capacity of human Th17 and Th1 cells to undergo complete lineage conversion suggests that the bipotent Th1/17 cells may arise from Th1 and/or Th17 cells. These data also question the broad applicability of the new model of Th cell lineage plasticity to in vivo-polarized human Th cell subsets.

Natural killer cell receptor-expressing innate lymphocytes: more than just NK cells

Recently, additional subsets that extend the family of innate lymphocytes have been discovered. Among these newly identified innate lymphoid cells is a subset sharing phenotypic characteristics of natural killer cells and lymphoid tissue inducer cells. These cells co-express the transcription factor RORγt and activating NK cell receptors (NKR), but their lineage and functional qualities remain poorly defined. Here, we discuss recent proposals to place these NKR(+)RORγt(+) innate lymphocytes on hematopoietic lineage maps. An overview of the transcriptional circuitry determining fate decisions of innate lymphocytes and a summary of current concepts concerning plasticity and stability of innate lymphocyte effector fates are provided. We will conclude by discussing the function of RORγt-expressing innate lymphocytes during inflammatory bowel diseases and in the immune response to tumors.

Th17 lymphocytes traffic to the central nervous system independently of α4 integrin expression during EAE

Th1 lymphocytes preferentially infiltrate into the spinal cord during EAE via a VLA-4–mediated mechanism while Th17 lymphocyte infiltration is dependent on LFA-1 expression.

The integrin α4β1 (VLA-4) is used by encephalitogenic T cells to enter the central nervous system (CNS). However, both Th1 and Th17 cells are capable of inducing experimental autoimmune encephalomyelitis (EAE), and the molecular cues mediating the infiltration of Th1 versus Th17 cells into the CNS have not yet been defined. We investigated how blocking of α4 integrins affected trafficking of Th1 and Th17 cells into the CNS during EAE. Although antibody-mediated inhibition of α4 integrins prevented EAE when MOG_35-55-specific Th1 cells were adoptively transferred, Th17 cells entered the brain, but not the spinal cord parenchyma, irrespective of α4 blockade. Accordingly, T cell–conditional α4-deficient mice were not resistant to actively induced EAE but showed an ataxic syndrome with predominantly supraspinal infiltrates of IL-23R⁺CCR6⁺CD4⁺ T cells. The entry of α4-deficient Th17 cells into the CNS was abolished by blockade of LFA-1 (αLβ2 integrin). Thus, Th1 cells preferentially infiltrate the spinal cord via an α4 integrin–mediated mechanism, whereas the entry of Th17 cells into the brain parenchyma occurs in the absence of α4 integrins but is dependent on the expression of αLβ2. These observations have implications for the understanding of lesion localization, immunosurveillance, and drug design in multiple sclerosis.

Targeting CB(2) receptor as a neuroinflammatory modulator in experimental autoimmune encephalomyelitis

During immune mediated demyelinating lesions, the endocannabinoid system is involved in the pathogenesis of both neuroinflammation and neurodegeneration through different mechanisms. Here, we explored the cellular distribution of cannabinoid 2 receptor (CB(2)R) in the central nervous system (CNS) and detected the level of CB(2)R expression during experimental autoimmune encephalomyelitis (EAE) by RT-PCR, Western blot and immunostaining. Our results show that CB(2)R was expressed in neurons, microglia and astrocytes. During EAE, the expression of CB(2)R in spinal cord rose slowly at days 9 and 17 post immunization (p.i.), and elevated rapidly at day 28 p.i., while the expression of CB(2)R in spleen elevated rapidly and got a plateau at days 17 and 28 p.i. Only the increase of CB(2)R expression in spinal cord demonstrated a significant difference when compared to control mice immunized with complete Freund's adjuvant (CFA). The selective CB(2)R antagonist (SR144528) exacerbated EAE clinical severity accompanied by weight loss. SR144528 inhibited the expression of CB(2)R, but increased the expression of CB(1)R in brain, spinal cord and spleen. The administration of SR144528 declined interferon-γ, IL-17, IL-4, IL-10, IL-1β, IL-6 and tumor necrosis factor-α, but increased CX3CL1 in brain and/or spinal cord. In contrast, IL-17 and MCP-1 were increased, while CX3CL1 was decreased in splenic mononuclear cells as compared to vehicle controls. These results indicate that manipulation of CB(2)R may have therapeutic value in MS, but its complexity remains to be considered and studied for further clinical application.

IL-23 is required for long-term control of Mycobacterium tuberculosis and B cell follicle formation in the infected lung

IL-23 is required for the IL-17 response to infection with Mycobacterium tuberculosis, but is not required for the early control of bacterial growth. However, mice deficient for the p19 component of IL-23 (Il23a(-/-)) exhibit increased bacterial growth late in infection that is temporally associated with smaller B cell follicles in the lungs. Cxcl13 is required for B cell follicle formation and immunity during tuberculosis. The absence of IL-23 results in decreased expression of Cxcl13 within M. tuberculosis-induced lymphocyte follicles in the lungs, and this deficiency was associated with increased cuffing of T cells around the vessels in the lungs of these mice. Il23a(-/-) mice also poorly expressed IL-17A and IL-22 mRNA. These cytokines were able to induce Cxcl13 in mouse primary lung fibroblasts, suggesting that these cytokines are likely involved in B cell follicle formation. Indeed, IL-17RA-deficient mice generated smaller B cell follicles early in the response, whereas IL-22-deficient mice had smaller B cell follicles at an intermediate time postinfection; however, only Il23a(-/-) mice had a sustained deficiency in B cell follicle formation and reduced immunity. We propose that in the absence of IL-23, expression of long-term immunity to tuberculosis is compromised due to reduced expression of Cxcl13 in B cell follicles and reduced ability of T cells to migrate from the vessels and into the lesion. Further, although IL-17 and IL-22 can both contribute to Cxcl13 production and B cell follicle formation, it is IL-23 that is critical in this regard.

An IL-9 fate reporter demonstrates the induction of an innate IL-9 response in lung inflammation

Interleukin 9 (IL-9) is a cytokine implicated in lung inflammation, but its cellular origin and function remain unclear. Here we describe a reporter mouse strain designed to fate map cells that have activated IL-9. We show that during papain-induced lung inflammation IL-9 production was largely restricted to innate lymphoid cells (ILC). IL-9 production by ILC was dependent on IL-2 from adaptive immune cells and was rapidly lost in favor of other cytokines, such as IL-13 and IL-5. Blockade of IL-9 production via neutralizing antibodies substantially reduced IL-13 and IL-5, suggesting that ILC provide the missing link between the well-established functions of IL-9 on the regulation of T_H2 cytokines and responses.

Autoreactive Tbet-positive CD4 T cells develop independent of classic Th1 cytokine signaling during experimental autoimmune encephalomyelitis

Many autoimmune chronic inflammatory diseases, including multiple sclerosis, are associated with the presence of Th1 and Th17 effector CD4 T cells. Paradoxically, the principal Th1 cytokine IFN-γ does not appear necessary for disease, but the key Th1-associated transcription factor Tbet has been reported to be essential for disease development. This conundrum propelled us to investigate the regulation of this transcription factor during autoimmunity. Following the onset of experimental autoimmune encephalomyelitis, we observed a preferential upregulation of Tbet by CD4 T cells within the CNS, but not the secondary lymphoid organs. These Tbet-positive CD4 T cells were capable of producing the cytokine IFN-γ, and a proportion of these cells produced both IFN-γ and IL-17A. Interestingly, these Tbet-positive cells were present in high frequencies during disease in IFN-γ-deficient mice. Moreover, we found that CD4 T cells from IFN-γ-deficient/IFN-γ reporter mice upregulated the Thy1.1 reporter, indicating the presence of Th1 or Th1-like, Tbet-positive CD4 T cells even in the absence of the cardinal Th1 cytokine IFN-γ. These IFN-γ-deficient Th1-like cells not only maintain multiple Th1 properties but also exhibit increased expression of genes associated with the Th17 phenotype. We further examined the requirement of other Th1-associated molecules in controlling Tbet expression during experimental autoimmune encephalomyelitis and noted that STAT1, IL-12, and IFN-γ were dispensable for the induction of Tbet in vivo. Hence, this study highlights the complex regulation of Tbet and the potential unrecognized role for Th1 cells during autoimmunity.

The critical role of antigen-presentation-induced cytokine crosstalk in the central nervous system in multiple sclerosis and experimental autoimmune encephalomyelitis

Multiple sclerosis (MS) is a debilitating disease of the central nervous system (CNS) that has been extensively studied using the animal model experimental autoimmune encephalomyelitis (EAE). It is believed that CD4(+) T lymphocytes play an important role in the pathogenesis of this disease by mediating the demyelination of neuronal axons via secretion of proinflammatory cytokines resulting in the clinical manifestations. Although a great deal of information has been gained in the last several decades about the cells involved in the inflammatory and disease mediating process, important questions have remained unanswered. It has long been held that initial neuroantigen presentation and T cell activation events occur in the immune periphery and then translocate to the CNS. However, an increasing body of evidence suggests that antigen (Ag) presentation might initiate within the CNS itself. Importantly, it has remained unresolved which antigen presenting cells (APCs) in the CNS are the first to acquire and present neuroantigens during EAE/MS to T cells, and what the conditions are under which this takes place, ie, whether this occurs in the healthy CNS or only during inflammatory conditions and what the related cytokine microenvironment is comprised of. In particular, the central role of interferon-γ as a primary mediator of CNS pathology during EAE has been challenged by the emergence of Th17 cells producing interleukin-17. This review describes our current understanding of potential APCs in the CNS and the contribution of these and other CNS-resident cells to disease pathology. Additionally, we discuss the question of where Ag presentation is initiated and under what conditions neuroantigens are made available to APCs with special emphasis on which cytokines may be important in this process.

T(H)17 cytokines in autoimmune neuro-inflammation

It has been firmly established that IL-23 polarized T(H)17 cells are potent effectors in the pathogenesis of experimental autoimmune encephalitomyelitis (EAE). However, the relative importance of these cells in comparison to other encephalitogenic T(H) subsets, and the mechanisms that they employ to effect inflammatory demyelination, are topics of continuing investigation. Interestingly, deletion of individual 'T(H)17 cytokines', such as IL-17A, IL-17F, IL-22 and IL-21, does not phenocopy the complete EAE-resistance of IL-23-deficient mice. The instability of T(H)17 cells in vivo introduces an additional layer of complexity to their role in the context of relapsing or chronic disease. Recent data indicate that IL-23 drives the production of myeloid activating factors, such as GM-CSF, by myelin-reactive T cells and facilitates their accumulation in the CNS. This review discusses the above issues in relation to the use of T(H)17 cells and related factors as potential therapeutic targets and biomarkers in CNS autoimmune diseases such as multiple sclerosis (MS).

T(H)17 cytokines in autoimmune neuro-inflammation

It has been firmly established that IL-23 polarized T(H)17 cells are potent effectors in the pathogenesis of experimental autoimmune encephalitomyelitis (EAE). However, the relative importance of these cells in comparison to other encephalitogenic T(H) subsets, and the mechanisms that they employ to effect inflammatory demyelination, are topics of continuing investigation. Interestingly, deletion of individual 'T(H)17 cytokines', such as IL-17A, IL-17F, IL-22 and IL-21, does not phenocopy the complete EAE-resistance of IL-23-deficient mice. The instability of T(H)17 cells in vivo introduces an additional layer of complexity to their role in the context of relapsing or chronic disease. Recent data indicate that IL-23 drives the production of myeloid activating factors, such as GM-CSF, by myelin-reactive T cells and facilitates their accumulation in the CNS. This review discusses the above issues in relation to the use of T(H)17 cells and related factors as potential therapeutic targets and biomarkers in CNS autoimmune diseases such as multiple sclerosis (MS).

The many faces of Th17 cells

Th17 cells have been shown to be strong inducers of tissue inflammation and autoimmune diseases. However, not all Th17 cells are pathogenic and increasing data suggest that Th17 cells may come in different flavors. Thus, Th17 cells cannot be described using a narrow schematic, but instead Th17 cells comprise a wide spectrum with a range of effector phenotypes. Here, we review the key factors that generate such diversity, as well as the cytokines and transcription factors that are differentially expressed in pathogenic and nonpathogenic Th17 cells. This new knowledge can be used to identify molecules that make Th17 cells pathogenic and determine how these cells could be targeted to suppress autoimmune diseases.

Multiparameter grouping delineates heterogeneous populations of human IL-17 and/or IL-22 T-cell producers that share antigen specificities with other T-cell subsets

The ontogenic relationship between pro-inflammatory populations of interleukin-17 (IL-17A)- and/or IL-22-producing T cells and other T-cell subsets is currently unclear in humans. To appreciate T helper cell-lineage commitment, we combined cytokine production profiles of in vitro expanded T-cell clones with T-cell receptor (TCR) clonotypic signatures. Moreover, ex vivo cytokine production profiles at the single-cell level were analyzed using an original approach based on the hierarchical cluster analysis of multiparametric flow cytometry data. These combined approaches enabled the delineation of distinct functional T-cell subsets, including Th1, Th2, Tr1, Th17 cells and a highly polyfunctional IL-22-producing T-cell population. Cluster analysis highlighted that the IL-22-producing T-cell population should be considered independently from the Th17 and Th1 subsets, although it was more closely related to the former. In parallel, we observed extensive TCRαβ sharing across all five subsets defined. The strategy described here allows the objective definition of cellular subsets and an unbiased insight into their similarities. Together, our results underscore the ontogenic plasticity of CD4(+) T-cell progenitors, which can adopt a differentiation profile irrespective of antigen specificity.

T helper cell populations: as flexible as the skin?

T helper cells can be defined by the cytokines they produce and are divided into Th1, Th2, Th17, T(FH) or regulatory T cells. Th17 cells have been shown to produce, in addition to IL-17, IL-22. In the current issue of the European Journal of Immunology, an article by Larsen et al. (Eur. J. Immunol. 2011. 41: 2596-2605) provides evidence that human T helper cells, like murine cells, can also express IL-22 in the absence of the other T helper cell signature cytokines. Moreover, they show that these IL-22-producing cells, namely Th22 cells, can be found in the skin of psoriasis patients, where they might contribute to the pathogenesis of this inflammatory skin disease. Finally, they show that, molecularly, Th22 cells are related to Th17 cells, and might therefore be derived from the latter. In this Commentary, the development of the pro-inflammatory T helper populations in the skin are discussed and a model that explains the development of Th22 cells found in the skin of psoriasis patients is proposed.

Recent advances in the IL-17 cytokine family

The IL-17/IL-17 receptor family is the newest and least understood of the cytokine subclasses. Composed of ligands IL-17A-IL-17F and receptors IL-17RA-IL-17RE, these cytokines have many unique structural and functional features. Since the discovery of the 'Th17' subset in 2005, particular attention has been paid to IL-17A and IL-17F and their cognate receptors. To date, far less is known about the rest of the family. This review discusses recent advances in the field, with an emphasis on IL-17A biology.

Mouse T helper 17 phenotype: not so different than in man after all

CD4+ T-helper (TH) cells that selectively produce interleukin (IL)-17 (TH17) are thought to be critical for host defense and autoimmunity. Three major dogmas were established, based on initial studies performed in murine models, and initially extrapolated by many researchers to human pathophysiology. First, TH17 cells represent a fixed CD4+ T-cell effector phenotype without any developmental relationship with TH1 cells. Second, TH17 cells are exclusively responsible for pathogenicity in several chronic inflammatory disorders, TH1 cell being instead protective. Finally, TH17 cells originate from naïve TH cells in response to the combined activity of transforming growth factor (TGF)-β and IL-6, whereas in the presence of TGF-β alone the same cells develop into Foxp3+ T regulatory cells. Studies performed in human demonstrated apparent species-specific differences, such as the expression by TH17 cells of the TH1-related transcription factor T-bet, the IL-12-inducible plasticity of TH17 cells into TH1 cells, and the dispensability of TGF-β signaling for their development. As discussed in this short review, recent studies in mice have led to reassessment of the three above-mentioned dogmas regarding the TH17 phenotype, suggesting that studies in humans actually better depicted TH17 cells than initial studies in mice did.

Cytokines in autoimmune uveitis

Autoimmune uveitis is a complex group of sight-threatening diseases that arise without a known infectious trigger. The disorder is often associated with immunological responses to retinal proteins. Experimental models of autoimmune uveitis targeting retinal proteins have led to a better understanding of the basic immunological mechanisms involved in the pathogenesis of uveitis and have provided a template for the development of novel therapies. The disease in humans is believed to be T cell-dependent, as clinical uveitis is ameliorated by T cell-targeting therapies. The roles of T helper 1 (Th1) and Th17 cells have been major topics of interest in the past decade. Studies in uveitis patients and experiments in animal models have revealed that Th1 and Th17 cells can both be pathogenic effectors, although, paradoxically, some cytokines produced by these subsets can also be protective, depending on when and where they are produced. The major proinflammatory as well as regulatory cytokines in uveitis, the therapeutic approaches, and benefits of targeting these cytokines will be discussed in this review.

T helper 17 cell heterogeneity and pathogenicity in autoimmune disease

T helper (Th)17 cells have been proposed to represent a new CD4(+) T cell lineage that is important for host defense against fungi and extracellular bacteria, and the development of autoimmune diseases. Precisely how these cells arise has been the subject of some debate, with apparent species-specific differences in mice and humans. Here, we describe evolving views of Th17 specification, highlighting the contribution of transforming growth factor-β and the opposing roles of signal transducer and activator of transcription (STAT)3 and STAT5. Increasing evidence points to heterogeneity and inherent phenotypic instability in this subset. Ideally, better understanding of expression and action of key transcription factors and the epigenetic landscape of Th17 can help explain the flexibility and diversity of interleukin-17-producing cells.

Origins of CD4(+) effector and central memory T cells

Lineage-committed effector CD4(+) T cells are generated at the peak of the primary response and are followed by heterogeneous populations of central and effector memory cells. Here we review the evidence that T helper type 1 (T(H)1) effector cells survive the contraction phase of the primary response and become effector memory cells. We discuss the applicability of this idea to the T(H)2 cell, T(H)17 helper T cell, follicular helper T cell (T(FH) cell) and induced regulatory T cell lineages. We also discuss how central memory cells are formed, with an emphasis on the role of B cells in this process.

Nuclear receptors: TH17 cell control from within

IL-17 producing T helper (T(H)17) cells have recently been identified as a new subset involved in the pathogenesis of various autoimmune diseases. Exogenous factors promoting T(H)17 induction have been intensely characterized, whereas the T cell-intrinsic mechanisms influencing T(H)17 development are less established. The transcription factor RORγt, which belongs to the nuclear receptor superfamily, serves as master transcription factor essential for T(H)17 differentiation, whereas other members of the nuclear receptor family control T(H)17 differentiation and contribute to protection from T(H)17-mediated autoimmunity. In this review, we will highlight the most recent understandings about the regulatory function of nuclear receptors during T(H)17 cell differentiation.

Nuocytes: expanding the innate cell repertoire in type-2 immunity

Activation and differentiation of the Th1 cell population lead to their production of the classical type-1 cytokines IFN-γ, IL-2, and TNF-β, thus promoting type-1 immunity. This is thought to occur via the ligation of TLRs by bacterial and viral products, which in turn, drive production of the essential Th1 cell differentiation factor, IL-12, by dendritic cells (DCs). Concurrent studies have been able to identify the effector cytokines produced by Th2 cells (IL-4, IL-5, IL-9, and IL-13) as being essential for parasitic immunity and also as essential factors in allergic asthma. However, the factors that are critical for initiation of the type-2 response remained obscure. Recently however, two critical observations have led to a more detailed understanding of the innate type-2 response. First, two novel, type-2-inducing cytokines-IL-25 and IL-33-were identified as being necessary for the up-regulation of the type-2 effector cytokines, mirroring the role of IL-12 in the type-1 response. Second, studies focused on target cell populations of IL-25 and IL-33 have identified novel, innate cell populations, which potentially bridge the gap between presentation of the type-2-inducing cytokine and the later adaptive Th2 cell response. In this review, we will discuss these new type-2 innate cell populations, in particular, the recently discovered nuocyte population, which are required for type-2 responses against helminthic parasites.

Comparison of a classical Th1 bacteria versus a Th17 bacteria as adjuvant in the induction of experimental autoimmune encephalomyelitis

The relative contribution of myelin-specific Th1 and Th17 cells in the pathology of multiple sclerosis (MS) and its animal model, experimental autoimmune encephalomyelitis (EAE), is controversial. IL-12, the key cytokine necessary for the differentiation of Th1 cells, has been found to be dispensable for EAE induction; while the related cytokine associated with Th17 cells, IL-23, is a critical factor for inducing EAE. Since EAE is induced by immunization with myelin proteins in CFA which contains M. tuberculosis that generates a prototypical Th1-mediated immune response, we sought to determine if replacing the M. tuberculosis in the adjuvant with a bacterium that induces an IL-23-dependent Th17 cell response during infection would induce EAE with a different phenotype. C. rodentium, a bacterium that requires IL-23 for protective immunity, was used as the adjuvant in EAE and compared to CFA. Mice immunized with C. rodentium adjuvant (CRA) developed classical signs of EAE, similar to CFA-immunized mice, but disease was less severe with a later onset and slower progression than CFA. Surprisingly, the peripheral cytokine profile revealed similar numbers of Th1 and Th17 cells for both CFA and CRA-immunized mice; however, the number of Th1 and Th17 cells was significantly reduced in the CNS of CRA-immunized mice. The development of EAE in CRA-immunized mice was associated with epitope spreading. The unique clinical course of CRA immunizations helps serve as a useful alternative model for studying EAE pathogenesis and potential therapeutics for MS.

T-bet in disease

The activation of immune-defense mechanisms in response to a microbial attack must be robust and appropriately tailored to fight particular types of pathogens. Infection with intracellular microorganisms elicits a type 1 inflammatory response characterized by mobilization of T helper type 1 (T(H)1) cells to the site of infection, where they are responsible for the recruitment and activation of macrophages. At the center of the type 1 inflammatory response is the transcription factor T-bet, a critical regulator of the T(H)1 differentiation program. T-bet induces the production of interferon-γ (IFN-γ) and orchestrates the T(H)1 cell-migratory program by regulating the expression of chemokines and chemokine receptors. However, tight regulation of the type 1 inflammatory response is essential for the prevention of immunopathology and the development of organ-specific autoimmunity. In this review, we discuss how T-bet expression drives autoaggressive and inflammatory processes and how its function in vivo must be delicately balanced to avoid disease.

Interleukin-23 and T helper 17-type responses in intestinal inflammation: from cytokines to T-cell plasticity

Interleukin-23 (IL-23) plays an essential role in driving intestinal pathology in experimental models of both T-cell-dependent and innate colitis. Furthermore, genome-wide association studies have identified several single-nucleotide polymorphisms in the IL-23 receptor (IL-23R) gene that are associated with either susceptibility or resistance to inflammatory bowel disease in humans. Although initially found to support the expansion and maintenance of CD4(+) T helper 17 (Th17) cells, IL-23 is now recognized as having multiple effects on the immune response, including restraining Foxp3(+) regulatory T-cell activity and inducing the expression of Th17-type cytokines from non-T-cell sources. Here we focus on Th17 cells and their associated cytokines IL-17A, IL-17F, IL-21 and IL-22. We review studies performed in mouse models of colitis where these effector cytokines have been shown to have either a pathogenic or a tissue-protective function. We also discuss the heterogeneity found within the Th17 population and the phenomenon of plasticity of Th17 cells, in particular the ability of these lymphocytes to extinguish IL-17 expression and turn on interferon-γ production to become Th1-like 'ex-Th17' cells. Interleukin-23 has been identified as a key driver in this process, and this may be an additional mechanism by which IL-23 promotes pathology in the intestinal tract. These 'ex-Th17' cells may contribute to disease pathogenesis through their secretion of pro-inflammatory mediators.

Cutting edge: Identification of a motile IL-17-producing gammadelta T cell population in the dermis

Dendritic epidermal T cells (DETCs) are a well-studied population of γδ T cells that play important roles in wound repair. In this study, we characterize a second major population of γδ T cells in the skin that is present in the dermis. In contrast to DETCs, these Vγ5-negative cells are IL-7R(hi)CCR6(hi) retinoic acid-related orphan receptor γt(+) and are precommitted to IL-17 production. Dermal γδ T cells fail to reconstitute following irradiation and bone marrow transplantation unless the mice also receive a transfer of neonatal thymocytes. Real-time intravital imaging of CXCR6(GFP/+) mouse skin reveals dermal γδ T cells migrate at ∼4 μm/min, whereas DETCs are immobile. Like their counterparts in peripheral lymph nodes, dermal γδ T cells rapidly produce IL-17 following exposure to IL-1β plus IL-23. We have characterized a major population of skin γδ T cells and propose that these cells are a key source of IL-17 in the early hours after skin infection.

Lung dendritic cells induce T(H)17 cells that produce T(H)2 cytokines, express GATA-3, and promote airway inflammation

BACKGROUND

Dendritic cells (DCs) are crucial to shape the adaptive immune response. Extensive in vitro manipulation reprograms T(H)2 and T(H)17 cell lines into T(H)1 cells, leading to the concept of CD4(+) T(H) cell subset plasticity. The conversion of memory T(H)17 cells into T(H)2 cells or vice versa remains to be clarified.

OBJECTIVE

We examined the localization of T(H)17/T(H)2 cells in vivo, their cellular origin (T(H)2 vs T(H)17), and the underlying mechanisms that drive the generation of these double T(H) producers.

METHODS

Antigen-loaded bone marrow-derived DCs (ovalbumin-DCs) were repeatedly administered locally (intratracheally) or systemically (intravenously) to naive mice to elicit chronic airway inflammation. Inflamed lungs and mediastinal lymph nodes were examined for the presence of IL-17(+)IL-13(+)IL-4(+)CD4(+) T cells that coexpressed retinoic acid receptor-related orphan receptor γt and GATA-3 (T(H)17/T(H)2).

RESULTS

We show that repetitive administration of inflammatory ovalbumin-DCs, locally or systemically, promoted the development of antigen-specific T(H)17/T(H)2 cells in lungs and mediastinal lymph nodes. Immunized mice had IgE-independent and steroid-resistant airway inflammation with a mixed neutrophil and eosinophil infiltration of the bronchoalveolar lavage fluid. Airway inflammatory signal regulatory protein α-positive DCs reprogrammed in vitro-generated T(H)17 but not T(H)2 cells, as well as lung effector T(H) cells, into T(H)17/T(H)2 cells.

CONCLUSION

We demonstrate the existence of T(H)17/T(H)2 cells that express GATA-3 in inflamed tissues and their T(H)17 origin. We further propose that repeated immunization with inflammatory DCs prevails on the route of DC administration to drive T(H)17/T(H)2-associated chronic lung inflammation.

The encephalitogenicity of T(H)17 cells is dependent on IL-1- and IL-23-induced production of the cytokine GM-CSF

Interleukin 17 (IL-17)-producing helper T cells (T(H)17 cells) require exposure to IL-23 to become encephalitogenic, but the mechanism by which IL-23 promotes their pathogenicity is not known. Here we found that IL-23 induced production of the cytokine granulocyte-macrophage colony-stimulating factor (GM-CSF) in T(H)17 cells and that GM-CSF had an essential role in their encephalitogenicity. Our findings identify a chief mechanism that underlies the important role of IL-23 in autoimmune diseases. IL-23 induced a positive feedback loop whereby GM-CSF secreted by T(H)17 cells stimulated the production of IL-23 by antigen-presenting cells. Such cross-regulation of IL-23 and GM-CSF explains the similar pattern of resistance to autoimmunity when either of the two cytokines is absent and identifies T(H)17 cells as a crucial source of GM-CSF in autoimmune inflammation.

Cytokines and effector T cell subsets causing autoimmune CNS disease

Although experimental autoimmune encephalomyelitis (EAE) is limited in its potency to reproduce the entirety of clinical and histopathologic features of multiple sclerosis (MS), this model has been successfully used to prove that MS like autoimmunity in the CNS is orchestrated by autoantigen specific T cells. EAE was also very useful to refute the idea that IFN-γ producing T helper type 1 (Th1) cells were the sole players within the pathogenic T cell response. Rather, "new" T cell lineages such as IL-17 producing Th17 cells or IL-9 producing Th9 cells have been first discovered in the context of EAE. Here, we will summarize new concepts of early and late T cell plasticity and the cytokine network that shapes T helper cell responses and lesion development in CNS specific autoimmunity.

T helper cell differentiation more than just cytokines

CD4(+) T helper (T(H)) cells play a critical role in orchestrating a pleiotropy of immune activities against a large variety of pathogens. It is generally thought that this is achieved through the acquisition of highly specialized functions after activation followed by the differentiation into various functional subsets. The differentiation process of naive precursor T(H) cells into defined effector subsets is controlled by cells of the innate immune system and their complex array of effector molecules such as secreted cytokines and membrane bound costimulatory molecules. These provide a unique quantitative or qualitative signal initiating T(H) development, which is subsequently reinforced via T cell-mediated feedback signals and selective survival and proliferative cues, ultimately resulting in the predominance of a particular T cell subset. In recent years, the number of defined T(H)cell subsets has expanded and the once rigid division of labor among them has been blurred with reports of plasticity among the subsets. In this chapter, we summarize and speculate on the current knowledge of the differentiation requirements of T(H) cell lineages, with particular focus on the T(H)17 subset.

Intercommunication between the neuroendocrine and immune systems: focus on myasthenia gravis

Crosstalk exists between the nervous, endocrine, and immune systems, and perturbations in these interactions have been associated with disease. This includes production of neuroendocrine factors that alter immune system activity and increase susceptibility to or severity of immune-related conditions, such as myasthenia gravis (MG)--a T-cell-dependent, B-cell-mediated autoimmune disorder. MG results from impairment of transmission to the neuromuscular junction and involves the thymus--especially in early-onset disease, but the exact mechanism by which the thymus impacts disease is unclear. MG afflicts millions of individuals worldwide each year, and both men and women can develop symptoms. However, prevalence and age of onset differs between men and women. Women exhibit higher incidence and earlier age of onset compared to men, and disease fluctuates during pregnancy. This suggests that sex hormones play a role in influencing disease outcome. In this review, we will consider what is known about the manifestation of MG, theories on how different forms of MG are influenced or alleviated by steroid hormones, current treatment options, and what measures could be important to consider in the future.

Human T cell cytokine responses are dependent on multidrug resistance protein-1

Multidrug resistance protein-1 (MRP1) belongs to subfamily C of the ATP-binding cassette transporters, and exports leukotriene C(4) and organic anions including the fluorescent calcium indicator indo-1. The observation that leukocytes from patients with an autoimmune disease exported indo-1 at a higher rate than controls prompted the hypothesis that MRP1 contributes to the function of activated cells. To test this, we defined the expression of MRP1 on resting and activated human T cells, and determined whether T cell activation is dependent upon MRP1 function. MRP1 is expressed on resting memory but not on naive CD4 and CD8 T cells. After activation through the TCR, cord blood CD4 T cells express high levels of MRP1. Blockade of MRP1 with the specific inhibitor MK-571 abrogated superantigen-induced expression of IFN-gamma, tumor necrosis factor-alpha, IL-10, IL-2, IL-4 and CD69 by T cells without affecting their viability, and was reversible upon removal of MK-571 from the culture media. Electrophoretic mobility shift assays demonstrate that MRP1 blockade with MK-571 induces activation of the transcriptional repressor peroxisome proliferator-activated receptor-gamma in CD4 T cells, thus providing insight into the potential mechanism by which their responses are abrogated.