Interleukin 27 limits autoimmune encephalomyelitis by suppressing the development of interleukin 17-producing T cells

IL-17 and Th17 cells in tuberculosis

Tuberculosis is primarily a disease of the lung. Constant expression of cellular immunity in this organ is required to control Mycobacterium tuberculosis growth, but this can also result in chronic inflammation and pathologic consequences. During primary tuberculosis both IFN-γ and IL-17 are induced: both are potent inflammatory cytokines capable of inducing expression of chemokines that promote cell recruitment and granuloma organization throughout infection. During the chronic phase, a balance between Th1 and Th17 responses needs to be achieved to control bacterial growth and limit immunopathology, as a shift of the response towards excessive IL-17 production may sustain extensive neutrophil recruitment and tissue damage. Thus, regulation of Th1 and Th17 responses during tuberculosis is essential to promote anti-mycobacterial immunity and prevent extensive immunopathological consequences.

Signal transduction pathways and transcriptional regulation in Th17 cell differentiation

Over the last decade, our understanding of helper/effector T cell differentiation has changed dramatically. The discovery of interleukin (IL-)17-producing T cells (Th17) and other subsets has changed our view of T cell-mediated immunity. Characterization of the signaling pathways involved in the Th17 commitment has provided exciting new insights into the differentiation of CD4+ T cells. Importantly, the emerging data on conversion among polarized T helper cells have raised the question how we should view such concepts as T cell lineage commitment, terminal differentiation and plasticity. In this review, we will discuss the current understanding of the signaling pathways, molecular interactions, and transcriptional and epigenetic events that contribute to Th17 differentiation and acquisition of effector functions.

Suppression of TNF-α and IL-1 signaling identifies a mechanism of homeostatic regulation of macrophages by IL-27

IL-27 is a pleiotropic cytokine with both activating and inhibitory functions on innate and acquired immunity. IL-27 is expressed at sites of inflammation in cytokine-driven autoimmune/inflammatory diseases, such as rheumatoid arthritis, psoriasis, inflammatory bowel disease, and sarcoidosis. However, its role in modulating disease pathogenesis is still unknown. In this study, we found that IL-27 production is induced by TNF-α in human macrophages (MΦ) and investigated the effects of IL-27 on the responses of primary human MΦ to the endogenous inflammatory cytokines TNF-α and IL-1. In striking contrast to IL-27-mediated augmentation of TLR-induced cytokine production, we found that IL-27 suppressed MΦ responses to TNF-α and IL-1β, thus identifying an anti-inflammatory function of IL-27. IL-27 blocked the proximal steps of TNF-α signaling by downregulating cell-surface expression of the signaling receptors p55 and p75. The mechanism of inhibition of IL-1 signaling was downregulation of the ligand-binding IL-1RI concomitant with increased expression of the receptor antagonist IL-1Ra and the decoy receptor IL-1RII. These findings provide a mechanism for suppressive effects of IL-27 on innate immune cells and suggest that IL-27 regulates inflammation by limiting activation of MΦ by inflammatory cytokines while preserving initial steps in host defense by augmenting responses to microbial products.

STAT1-activating cytokines limit Th17 responses through both T-bet-dependent and -independent mechanisms

Given the association with autoimmune disease, there is great interest in defining cellular factors that limit overactive or misdirected Th17-type inflammation. Using in vivo and in vitro models, we investigated the molecular mechanisms for cytokine-mediated inhibition of Th17 responses, focusing on the role of STAT1 and T-bet in this process. These studies demonstrate that, during systemic inflammation, STAT1- and T-bet-deficient T cells each exhibit a hyper-Th17 phenotype relative to wild-type controls. However, IL-17 production was greater in the absence of T-bet, and when both STAT1 and T-bet were deleted, there was no further increase, with the double-deficient cells instead behaving more like STAT1-deficient counterparts. Similar trends were observed during in vitro priming, with production of Th17-type cytokines greater in T-bet(-/-) T cells than in either STAT1(-/-) or STAT1(-/-) T-bet(-/-) counterparts. The ability of IFN-γ and IL-27 to suppress Th17 responses was reduced in T-bet-deficient cells, and most importantly, ectopic T-bet could suppress signature Th17 gene products, including IL-17A, IL-17F, IL-22, and retinoic acid-related orphan receptor γT, even in STAT1-deficient T cells. Taken together, these studies formally establish that, downstream of IFN-γ, IL-27, and likely all STAT1-activating cytokines, there are both STAT1 and T-bet-dependent pathways capable of suppressing Th17 responses.

The role of Th17 cytokines in primary mucosal immunity

The T helper type 17 (Th17) lineage of CD4+ T-cells produce several effector molecules including IL-17A, IL-17F, IL-21, and IL-22. In addition to CD4+, αβ T-cells, these cytokines can be produced by natural killer and γδ T-cells. These effector cytokines can be produced rapidly upon infection at mucosal sites and evidence to date strongly implicates that this arm of the immune system plays a critical role in mucosal immunity to many extracellular pathogens. Moreover these cytokines can also coordinate adaptive immunity to some intracellular pathogens. In this review, we will highlight recent progress in our understanding of these cytokines, and mechanisms of their effector function in the mucosa.

IL-27 induces the differentiation of Tr1-like cells from human naive CD4+ T cells via the phosphorylation of STAT1 and STAT3

IL-27, a member of IL-6/IL-12 cytokine family, is mainly produced by activated antigen presenting cells (APC). It has been demonstrated that IL-27 has the pro- and anti-inflammatory properties during immune responses. However, the signaling pathways that contribute to the cytokine generation are still unclear in humans. In the present study, we showed that IL-27 induced IL-10 and IFN-γ, but had no effect on IL-2, TNF-α and IL-4 production from human umbilical cord blood mononuclear cells (CBMCs). For purified naive CD4(+) T cells, IL-27 elicited the differentiation of Tr1-like cells with expression of IL-10 and IFN-γ. Importantly, this induction was dependent on the signal transducer and activator of transcription (STAT) 1 and 3 factors. In addition, all of the phosphorylated STAT3 positive cells were also positive for phosphorylated STAT1, both of which could be inhibited by a JAK2/STAT inhibitor, AG490. However, there was no phosphorylation of STAT4, STAT5 and STAT6 in IL-27-stimulating conditions. Moreover, the biological function of IL-10 that was induced by IL-27 mainly enhanced the expression of CD16 without influencing CD14 expression on human monocytes. These data identify the function of IL-27 on the differentiation of human naive CD4(+) T cells and demonstrate that the STATs signaling pathways may play an important role in mediating immune responses in humans.

IL-27 supports germinal center function by enhancing IL-21 production and the function of T follicular helper cells

IL-27 signaling directly into T cells is needed for follicular T helper cell survival, germinal center formation, and the production of T cell–dependent high-affinity antibodies in mice.

Maturation and selection of high-affinity B cell clones in the germinal center (GC) relies on support from T follicular helper (T_FH) cells. T_FH cells are characterized by their localization to the B cell follicle and their high expression of the costimulatory molecules ICOS and PD1 and the cytokine IL-21, which promotes immunoglobulin (Ig) class switching and production by B cells. We show that the heterodimeric cytokine IL-27 is critical for the function of T_FH cells and for normal and pathogenic GC responses. IL-27 signaling to T cells results in the production of IL-21, a known autocrine factor for the maintenance of T_FH cells, in a STAT3-dependent manner. IL-27 also enhances the survival of activated CD4⁺ T cells and the expression of T_FH cell phenotypic markers. In vivo, expression of the IL-27Rα chain is required to support IL-21 production and T_FH cell survival in a T cell–intrinsic manner. The production of high-affinity antibodies is reduced, and pristane-elicited autoantibodies and glomerulonephritis are significantly diminished, in Il27ra^−/− mice. Together, our data show a nonredundant role for IL-27 in the development of T cell–dependent antibody responses.

Molecular pathways in the induction of interleukin-27-driven regulatory type 1 cells

Type 1 regulatory (Tr1) cells have emerged as key players in the prevention of autoimmunity. They produce high levels of the immunosuppressive cytokine interleukin (IL)-10 and confer protection against a wide panel of autoimmune diseases. However, the molecular pathways leading to their generation have long remained elusive. We have recently identified IL-27, a member of the IL-12 cytokine family, as a novel cytokine that induces Tr1 cells. Further analysis of IL-27-driven Tr1 cells have identified a critical role of the transcription factor avian musculoaponeurotic fibrosarcoma v-maf and of IL-21 in the generation of IL-27-induced Tr1 cells. Importantly, IL-27 also induces Tr1 cells in humans, suggesting that IL-27 administration may dampen tissue inflammation in humans as well. Here, we review the role of IL-27 in the generation of Tr1 cells and discuss its potential to alleviate autoimmune diseases.

IL-6: regulator of Treg/Th17 balance

IL-6 is a pleiotropic cytokine involved in the physiology of virtually every organ system. Recent studies have demonstrated that IL-6 has a very important role in regulating the balance between IL-17-producing Th17 cells and regulatory T cells (Treg). The two T-cell subsets play prominent roles in immune functions: Th17 cell is a key player in the pathogenesis of autoimmune diseases and protection against bacterial infections, while Treg functions to restrain excessive effector T-cell responses. IL-6 induces the development of Th17 cells from naïve T cells together with TGF-beta; in contrast, IL-6 inhibits TGF-beta-induced Treg differentiation. Dysregulation or overproduction of IL-6 leads to autoimmune diseases such as multiple sclerosis (MS) and rheumatoid arthritis (RA), in which Th17 cells are considered to be the primary cause of pathology. Given the critical role of IL-6 in altering the balance between Treg and Th17 cells, controlling IL-6 activities is potentially an effective approach in the treatment of various autoimmune and inflammatory diseases. Here, we review the role of IL-6 in regulating Th17/Treg balance and describe the critical functions of IL-6 and Th17 in immunity and immune-pathology.

Erythropoietin enhances endogenous haem oxygenase-1 and represses immune responses to ameliorate experimental autoimmune encephalomyelitis

Both erythropoietin (EPO) and haem oxygenase-1 (HO-1), an anti-oxidative stress protein, have proven protective roles in experimental autoimmune encephalomyelitis (EAE), a reliable animal model of multiple sclerosis. In this study, EPO delivered intraperitoneally could reduce disease severity in myelin oligodendrocyte glycoprotein (MOG)–EAE mice. To assess the effect of EPO on endogenous HO-1 in EAE, we investigated expression of HO-1 mRNA by real-time polymerase chain reaction (RT–PCR), protein expression centrally and peripherally by Western blot and immunohistochemistry and mean fluorescence intensity of splenic HO-1 by flow cytometry. A significantly higher expression of HO-1 in both the central nervous system (CNS) and spleen was shown in EPO-treated MOG–EAE mice than in controls.We further examined the immunomodulatory effect of EPO in EAE, and via RT–PCR demonstrated significantly lower expression of interferon-γ, interleukin (IL)-23, IL-6 and IL-17 mRNA, and significantly higher expression of IL-4 and IL-10 mRNA in CNS of EPO-treated MOG–EAE mice than in controls. Using flow cytometry, we also observed a significantly decreased ratio of both T helper type 1 (Th1) and Th17 lymphocyte subsets isolated from CNS and a significantly increased ratio of splenic regulatory CD4 T cells in EPO-treated MOG–EAE mice. In addition, we demonstrated that MOG-specific T cell proliferation was lower in the EPO-treated group than in controls and showed amelioration of EAE by adoptive transfer of splenocytes from EPO-treated MOG–EAE mice. Together, our data show that in EAE, EPO induction of endogenous HO-1 and modulation of adaptive immunity both centrally and peripherally may involve the repression of inflammatory responses.

Th17 cells in inflammation

Naïve T cells are multipotential precursors that differentiate into various effector subsets, such as T helper type 1 (Th1) and Th2 cells, which are characterized by their distinct functions. The IL-17-producing T helper (Th17) cell has been recently identified as a new subset of the T helper cell and a mediator of inflammation associated with various autoimmune diseases. Although several cytokines participate in Th17 cell development, IL-6 and TGF-β are key factors for the generation of Th17 cells from naïve T cells. On the other hand, IL-6 inhibits TGF-β-induced regulatory T (Treg) cells, which suppress adaptive T cell responses and prevent autoimmunity. Recent studies suggest that it is an effective approach in the treatment of various autoimmune and inflammatory diseases to normalize the balance between Treg and Th17 cell development. Here, we review the discovery of the Th17 subset, its properties and relationship with several autoimmune diseases.

Potentiation of Th17 cytokines in aging process contributes to the development of colitis

Th17 cells, which produce IL-17 and IL-22, promote autoimmunity in mice and have been implicated in the pathogenesis of autoimmune/inflammatory diseases in humans. However, the Th17 immune response in the aging process is still not clear. In the present study, we found that the induction of IL-17-producing CD4(+) T cells was significantly increased in aged individuals compared with young healthy ones. The mRNA expression of IL-17, IL-17F, IL-22, and RORC2 was also significantly increased in aged people. Similar to humans, Th17 cells as well as mRNAs encoding IL-17, IL-22 and RORγt were dramatically elevated in naïve T cells from aged mouse compared to young ones. In addition, CD44 positive IL-17-producing CD4(+) T cells were significantly higher in aged mice, suggesting that memory T cells are an important source of IL-17 production. Furthermore, the percentage of IL-17-producing CD4(+) T cells generated in co-culture with dendritic cells from either aged or young mice did not show significant differences, suggesting that dendritic cells do not play a primary role in the elevation of Th17 cytokines in aged mouse cells. Importantly, transfer of CD4(+)CD45Rb(hi) cells from aged mice induced more severe colitis in RAG(-/-) mice compared to cells from young mice, Taken together, these results suggest that Th17 immune responses are elevated in aging humans and mice and may contribute to the increased development of inflammatory disorders in the elderly.

IL-27 structural analysis demonstrates similarities with ciliary neurotrophic factor (CNTF) and leads to the identification of antagonistic variants

IL-27, consisting of the subunits IL-27p28 and Epstein-Barr virus-induced gene 3 (EBI3), is a heterodimeric cytokine belonging to the IL-6/IL-12 family of cytokines. IL-27p28 is a four-helical cytokine requiring association with the soluble receptor EBI3 to be efficiently secreted and functionally active. Computational and biological analyses of the IL-27 binding site 1 to its receptor revealed important structural proximities with the ciliary neurotrophic factor group of cytokines and highlighted the contribution of p28 Trp(97), as well as of EBI3 Phe(97), Asp(210), and Glu(159), as key residues in the interactions between both cytokine subunits. WSX-1 (IL-27R) and gp130 compose the IL-27 receptor-signaling complex, recruiting the STAT-1 and STAT-3 pathways. A study of IL-27 binding site 3 showed that Trp(197) was crucial for the cytokine's interaction with gp130, but that the mutated cytokine still recognized IL-27R on the cell surface. IL-27 exerts both pro- and anti-inflammatory functions, promoting proliferation and differentiation of Th1 and inhibiting Th17 differentiation. Our results led us to develop mutated forms of human and mouse IL-27 with antagonistic activities. Using an in vivo mouse model of concanavalin A-induced Th1-cell-mediated hepatitis, we showed that the murine IL-27 antagonist W195A decreased liver inflammation by downregulating the synthesis of CXCR3 ligands and several acute phase proteins. Together, these data suggest that IL-27 antagonism could be of interest in down-modulating acute IL-27-driven Th1-cell-mediated immune response.

IL-23/IL-17 axis in IBD

Gut inflammation occurring in patients with Crohn's disease and patients with ulcerative colitis has been traditionally associated with an exaggerated Th1 or Th2 cell response, respectively. However, recent studies have shown that in both inflammatory bowel diseases (IBD) there is also enhanced synthesis of cytokines made by a distinct subset of T helper cells, termed Th17 cells. The discovery that this new T-cell subset drives immune-mediated pathology in the gut, and that interleukin (IL)-23 amplifies Th17 cell responses and gut inflammation, has contributed to elucidate new pathways of tissue damage as well as to open new avenues for development of therapeutic strategies in IBD. Nonetheless, it has been recently shown that Th17-related cytokines, such as IL-17A and IL-22, can exert protective rather than detrimental effects in the gut. We here review the available data regarding the role of Th17 cells and IL-23 in chronic intestinal inflammation.

1,25-Dihydroxyvitamin D3 inhibits the differentiation and migration of T(H)17 cells to protect against experimental autoimmune encephalomyelitis

Background

Vitamin D₃, the most physiologically relevant form of vitamin D, is an essential organic compound that has been shown to have a crucial effect on the immune responses. Vitamin D₃ ameliorates the onset of the experimental autoimmune encephalomyelitis (EAE); however, the direct effect of vitamin D₃ on T cells is largely unknown.

Methodology/Principal Findings

In an in vitro system using cells from mice, the active form of vitamin D₃ (1,25-dihydroxyvitamin D₃) suppresses both interleukin (IL)-17-producing T cells (T_H17) and regulatory T cells (Treg) differentiation via a vitamin D receptor signal. The ability of 1,25-dihydroxyvitamin D₃ (1,25(OH)₂D₃) to reduce the amount of IL-2 regulates the generation of Treg cells, but not T_H17 cells. Under T_H17-polarizing conditions, 1,25(OH)₂D₃ helps to increase the numbers of IL-10-producing T cells, but 1,25(OH)₂D₃'s negative regulation of T_H17 development is still defined in the IL-10^−/− T cells. Although the STAT1 signal reciprocally affects the secretion of IL-10 and IL-17, 1,25(OH)₂D₃ inhibits IL-17 production in STAT1^−/− T cells. Most interestingly, 1,25(OH)₂D₃ negatively regulates CCR6 expression which might be essential for T_H17 cells to enter the central nervous system and initiate EAE.

Conclusions/Significance

Our present results in an experimental murine model suggest that 1,25(OH)₂D₃ can directly regulate T cell differentiation and could be applied in preventive and therapeutic strategies for T_H17-mediated autoimmune diseases.

Regulation of antitumor immune responses by the IL-12 family cytokines, IL-12, IL-23, and IL-27

The interleukin (IL)-12 family, which is composed of heterodimeric cytokines including IL-12, IL-23, and IL-27, is produced by antigen-presenting cells such as macrophages and dendritic cells and plays critical roles in the regulation of helper T (Th) cell differentiation. IL-12 induces IFN-γ production by NK and T cells and differentiation to Th1 cells. IL-23 induces IL-17 production by memory T cells and expands and maintains inflammatory Th17 cells. IL-27 induces the early Th1 differentiation and generation of IL-10-producing regulatory T cells. In addition, these cytokines induce distinct immune responses to tumors. IL-12 activates signal transducers and activator of transcription (STAT)4 and enhances antitumor cellular immunity through interferon (IFN)-γ production. IL-27 activates STAT1, as does IFN-γ and STAT3 as well, and enhances antitumor immunity by augmenting cellular and humoral immunities. In contrast, although exogenously overexpressed IL-23 enhances antitumor immunity via memory T cells, endogenous IL-23 promotes protumor immunity through STAT3 activation by inducing inflammatory responses including IL-17 production.

Effector and regulatory T-cell subsets in autoimmunity and tissue inflammation

Many autoimmune diseases are driven by self-reactive T helper cells. Until recently, organ-specific autoimmune diseases were primarily associated with Th1 cells but not Th2 cells. However, the discovery of a number of new effector T-cell subsets, like Th17 and Th9 cells, and regulatory T cells, like Tregs and Tr1 cells, has changed the way we view and understand autoimmunity at cellular and molecular levels. In recent years, IL-17-producing Th17 cells have emerged as major players in autoimmunity. The complicated relationship between Th1 and Th17 cells, as well as the intricate balance between Tregs and Th17 cells, provides a basis for understanding the immunological mechanisms that induce and regulate autoimmunity. Here, we give an overview of the interplay between different effector T-cell subsets and regulatory T-cell subsets, and how they contribute to the development of autoimmunity and tissue inflammation.

The aryl hydrocarbon receptor interacts with c-Maf to promote the differentiation of type 1 regulatory T cells induced by IL-27

Type 1 regulatory T cells (Tr1 cells ) that produce interleukin 10 (IL-10) are instrumental in the prevention of tissue inflammation, autoimmunity and graft-versus-host disease. The transcription factor c-Maf is essential for the induction of IL-10 by Tr1 cells, but the molecular mechanisms that lead to the development of these cells remain unclear. Here we show that the ligand-activated transcription factor aryl hydrocarbon receptor (AhR), which was induced by IL-27, acted in synergy with c-Maf to promote the development of Tr1 cells. After T cell activation under Tr1-skewing conditions, the AhR bound to c-Maf and promoted transactivation of the Il10 and Il21 promoters, which resulted in the generation of Tr1 cells and the amelioration of experimental autoimmune encephalomyelitis. Manipulating AhR signaling could therefore be beneficial in the resolution of excessive inflammatory responses.

Therapeutic potential of IL-27 in systemic lupus erythematosus

Systemic lupus erythematosus (SLE) is a prototypic autoimmune disease characterized by a diverse array of autoantibody production, complement activation and immune complex deposition, causing tissue and organ damage. Effective medical treatment for SLE is lacking because the etiology and pathogenesis of SLE are incompletely understood. It has been confirmed that cytokine-mediated immunity plays a crucial role in the pathogenesis of various autoimmune diseases including SLE. Recently, IL-27 was identified, which belongs to the IL-12 cytokine family. IL-27 exerts profound anti-inflammatory effects in several experimental autoimmune models. In particular, suppressive effects on T(H)17 cells, which are implicated in the pathogenesis of SLE. Moreover, administration of IL-27 or augmentation of IL-27 signaling suppresses some autoimmune diseases, including autoimmune diabetes and murine lupus, suggesting that IL-27 may be therapeutically relevant in SLE. In this article, we discuss the biological features of IL-27 and summarize recent advances on the role of IL-27 in the pathogenesis and treatment of SLE. Even though IL-27 has shown therapeutic potential in SLE, further research, particularly in humans, is needed in order to establish the precise role of IL-27 in SLE.

Metabotropic glutamate receptor-4 modulates adaptive immunity and restrains neuroinflammation

High amounts of glutamate are found in the brains of people with multiple sclerosis, an inflammatory disease marked by progressive demyelination. Glutamate might affect neuroinflammation via effects on immune cells. Knockout mice lacking metabotropic glutamate receptor-4 (mGluR4) were markedly vulnerable to experimental autoimmune encephalomyelitis (EAE, a mouse model of multiple sclerosis) and developed responses dominated by interleukin-17-producing T helper (T(H)17) cells. In dendritic cells (DCs) from those mice, defective mGluR4 signaling-which would normally decrease intracellular cAMP formation-biased T(H) cell commitment to the T(H)17 phenotype. In wild-type mice, mGluR4 was constitutively expressed in all peripheral DCs, and this expression increased after cell activation. Treatment of wild-type mice with a selective mGluR4 enhancer increased EAE resistance via regulatory T (T(reg)) cells. The high amounts of glutamate in neuroinflammation might reflect a counterregulatory mechanism that is protective in nature and might be harnessed therapeutically for restricting immunopathology in multiple sclerosis.

Inflammation in the central nervous system and Th17 responses are inhibited by IFN-gamma-Induced IL-18 binding protein

Inflammatory responses are essential for immune protection but may also cause pathology and must be regulated. Both Th1 and Th17 cells are implicated in the pathogenesis of autoimmune inflammatory diseases, such as multiple sclerosis. We show in this study that IL-18-binding protein (IL-18bp), the endogenous inhibitor of the Th1-promoting cytokine IL-18, is upregulated by IFN-gamma in resident microglial cells in the CNS during multiple sclerosis-like disease in mice. Test of function by overexpression of IL-18bp in the CNS using a viral vector led to marked reduction in Th17 responses and robust inhibition of incidence, severity, and histopathology of disease, independently of IFN-gamma. The disease-limiting action of IL-18bp included suppression of APC-derived Th17-polarizing cytokines. IL-18bp thus acts as a sensor for IFN-gamma and can regulate both Th1 and Th17 responses in the CNS.

CD30 ligand/CD30 plays a critical role in Th17 differentiation in mice

A CD30 ligand (CD30L; CD153) and its receptor, CD30, is a membrane-associated glycoprotein belonging to the TNF superfamily and TNFR superfamily. These were expressed preferentially by activated CD4(+)T cells. In this paper, we show that CD44(low)CD62(hi)CD4(+) T cells from CD30L(-/-) or CD30(-/-) mice exhibited impaired differentiation into Th17 cells but an increased ability to produce IL-2 after in vitro culture under Th17-polarizing conditions. Neutralization with IL-2 by anti-IL-2 mAb partly restored the ability of Th17 differentiation in CD30L(-/-) or CD30(-/-) T cells. Stimulation via CD30L by immobilized anti-CD30L mAb suppressed IL-2 production by CD30(-/-)CD4(+) T cells, indicating that the reverse signal to CD30L is responsible for downregulation of IL-2 production. In vivo Th17 differentiation of CD30L(-/-)CD4(+)CD45RB(high) T cells was also impaired after transfer into SCID mice, whereas CD30L(+/+)CD4(+)CD45RB(high) T cells normally differentiated into Th17 cells in CD30L(-/-) SCID mice. The results of these studies demonstrate that CD30L/CD30 signaling executed by the T-T cell interaction plays a critical role in Th17 cell differentiation, at least partly via downregulation of IL-2 production.

Essential role for IL-27 receptor signaling in prevention of Th1-mediated immunopathology during malaria infection

Successful resolution of malaria infection requires induction of proinflammatory immune responses that facilitate parasite clearance; however, failure to regulate this inflammation leads to immune-mediated pathology. The pathways that maintain this immunological balance during malaria infection remain poorly defined. In this study, we demonstrate that IL-27R-deficient (WSX-1(-/-)) mice are highly susceptible to Plasmodium berghei NK65 infection, developing exacerbated Th1-mediated immune responses, which, despite highly efficient parasite clearance, lead directly to severe liver pathology. Depletion of CD4(+) T cells---but not CD8(+) T cells---prevented liver pathology in infected WSX-1(-/-) mice. Although WSX-1 signaling was required for optimal IL-10 production by CD4(+) T cells, administration of rIL-10 failed to ameliorate liver damage in WSX-1(-/-) mice, indicating that additional, IL-10-independent, protective pathways are modulated by IL-27R signaling during malaria infection. These data are the first to demonstrate the essential role of IL-27R signaling in regulating effector T cell function during malaria infection and reveal a novel pathway that might be amenable to manipulation by drugs or vaccines.

gp130 at the nexus of inflammation, autoimmunity, and cancer

Glycoprotein 130 (gp130) is a shared receptor utilized by several related cytokines, including IL-6, IL-11, IL-27, Leukemia Inhibitory Factor (LIF), Oncostatin M (OSM), Ciliary Neurotrophic Factor (CNTF), Cardiotrophin 1 (CT-1) and Cardiotrophin-like Cytokine (CLC). Gp130 plays critical roles during development and gp130-deficient mice are embryonically lethal. However, the best characterized facet of this receptor and its associated cytokines is the ability to promote or suppress inflammation. The aim of this review is to discuss the role of gp130 in promoting or preventing the development of autoimmunity and cancer, two processes that are associated with aberrant inflammatory responses.

Th-17 cells in the lungs?

Naive CD4 cells are capable of integrating signals from antigen-activated cells of the innate immune system and differentiating into effector CD4 cells, also termed T helper (Th) cells. According to the traditional paradigm explaining adaptive CD4 cell responses, there are two subsets of Th cells: the Th-1 and Th-2 subset. Each of these subsets undergoes a distinct differentiation pathway (a pathway that is characterized by a unique profile of cytokine production and has specific immunoregulatory functions). However, recent studies in mouse models have forwarded evidence of a third subset of Th cells: the Th-17 subset. As indicated predominantly in studies on mice, the Th-17 subset is characterized by an ability to produce the neutrophil-mobilizing cytokine IL-17 in response to stimulation with the cytokine IL-23, an IL-12-related cytokine released from antigen-presenting cells. There is now a growing body of evidence from animal models that the Th-17 subset plays an important role in host defence in the lungs and other organs. Altered IL-17 levels have also been demonstrated in human patients with asthma, exacerbations of cystic fibrosis or following lung transplantation. There is now also evidence that the Th-17 subset is functionally distinct from the Th-2 subset but little is known of the functional inter-relationship between the Th-1 and Th-17 cell subsets; this is particularly true in human lungs. It has been proposed that the Th-17 subset plays a unique role by linking the arms of innate and adaptive immunity. Thus, an improved understanding of the human correlate to the Th-17 subset may reveal new targets for pharmacotherapy against lung disorders that are characterized by aberrant innate responses in host defense.

Effects of inflammatory cytokine IL-27 on the activation of fibroblast-like synoviocytes in rheumatoid arthritis

Introduction

Interleukin (IL)-27 is a novel member of the IL-6/IL-12 family cytokines that are produced early by antigen-presenting cells in T helper (Th)1-mediated inflammation. Elevated expression of IL-27 has been detected in the synovial membranes and fluid of rheumatoid arthritis (RA).

Methods

We investigated the in vitro effects of IL-27, alone or in combination with inflammatory cytokine tumor necrosis factor (TNF)-α or IL-1 β on the pro-inflammatory activation of human primary fibroblast-like synoviocytes (FLS) from RA patients and normal control subjects, and the underlying intracellular signaling molecules were determined by intracellular staining using flow cytometry.

Results

Significantly higher plasma concentration of IL-27 was found in RA patients (n = 112) than control subjects (n = 46). Both control and RA-FLS constitutively express functional IL-27 receptor heterodimer, gp130 and WSX-1, with more potent IL-27-mediated activation of signal transducers and activators of transcription (STAT)1 in RA-FLS. IL-27 was found to induce significantly higher cell surface expression of intercellular adhesion molecule (ICAM)-1 and vascular cell adhesion molecule (VCAM)-1 and release of inflammatory chemokine IL-6, CCL2, CXCL9, CXCL10 and matrix metalloproteinase-1 of RA-FLS than that of control FLS (all P < 0.05). Moreover, an additive or synergistic effect was observed in the combined treatment of IL-27 and TNF-α or IL-1 β on the surface expression of ICAM-1 and VCAM-1 and the release of CXCL9 and CXCL10 of RA-FLS. Further investigations showed that the expression of ICAM-1, VCAM-1 and chemokines stimulated by IL-27 was differentially regulated by intracellular activation of phosphatidylinositol 3-OH kinase-AKT, c-Jun amino-terminal kinase and Janus kinase pathways.

Conclusions

Our results therefore provide a new insight into the IL-27-activated immunopathological mechanisms mediated by distinct intracellular signal transductions in joint inflammation of RA.

IL-27 inhibits OSM-mediated TNF-alpha and iNOS gene expression in microglia

Elevated levels of Oncostatin M (OSM), an interleukin-6 family cytokine, have been observed in multiple sclerosis (MS), HIV-associated neurocognitive disorder (HAND), and glioblastoma (GBM); however, its effects within the CNS are not well understood. OSM regulates gene expression primarily by activating the JAK/STAT, NF-kappaB, and/or MAPK pathways, in a cell-type specific manner. In our studies, OSM induces the production of the proinflammatory cytokine tumor necrosis factor-alpha (TNF-alpha) and inducible nitric oxide synthase (iNOS) from microglia in an NF-kappaB-dependent manner. This expression also partially requires the intermediate production of TNF-alpha and subsequent NF-kappaB activation via TNF-R1. We also demonstrate that OSM-induced TNF-alpha production from microglia is neurotoxic. The IL-12 family member, IL-27, suppresses OSM-mediated TNF-alpha and iNOS expression at the transcriptional level by inhibiting activation of the NF-kappaB pathway, and rescues the neurotoxicity induced by OSM-stimulated microglia. These studies are the first to demonstrate the proinflammatory effects of OSM in microglia, and also identify IL-27 as a novel inhibitor of inflammatory processes in these cells.

Th17 cells: positive or negative role in tumor?

Th17 cells have been recently identified as a distinct Th cell lineage and found in an experimental animal model of cancer and in human cancers, but whether these cells promote tumor growth or regulate antitumor responses remains controversial. This review provides a summary of the current literature regarding interleukin (IL)-17/IL-23 and Th17 cells in cancer and discusses their potential roles in cancer development. Finally, we note several issues in this research area that must be resolved before the design of novel therapeutic approaches specifically targeting Th17 cells in cancer become feasible.

Identification of an IL-27/osteopontin axis in dendritic cells and its modulation by IFN-gamma limits IL-17-mediated autoimmune inflammation

Dendritic cells (DCs) play a central role in determining the induction of T cell responses. IL-27 production by DCs favors induction of IL-10-producing regulatory T cells, whereas osteopontin (OPN) promotes pathogenic IL-17 T cell responses. The regulatory mechanisms in DCs that control these two cells types are not understood well. Here, we show that IFN-gamma induces IL-27 while inhibiting OPN expression in DCs both in vitro and in vivo and that engagement of IFN-gammaR expressed by DCs leads to suppression of IL-17 production while inducing IL-10 from T cells. DCs modified by IFN-gamma acquire IL-27-dependent regulatory function, promote IL-10-mediated T cell tolerance, and suppress autoimmune inflammation. Thus, our results identify a previously unknown pathway by which IFN-gamma limits IL-17-mediated autoimmune inflammation through differential regulation of OPN and IL-27 expression in DCs.

IL-35 stimulation of CD39+ regulatory T cells confers protection against collagen II-induced arthritis via the production of IL-10

IL-35 is produced by regulatory T cells, and this novel cytokine can downregulate Th17 cell development and inhibit autoimmune inflammation. In this work, an rIL-35, as a single-chain fusion between murine IL-12p35 and EBV-induced gene 3, was expressed in yeast. This rIL-35 inhibited OVA-specific cellular and Ab responses in OVA-challenged recipients of DO11.10 CD4+ T cells. Likewise, IL-35 inhibited clinical manifestation of collagen-induced arthritis or could cease further disease exacerbation upon initiation of IL-35 treatment. Exogenous IL-35 treatments suppressed Th1 and Th17 cells and promoted CD39 expression by CD4+ T cells. Sorted CD25-CD39+CD4+ T cells from IL-35-treated mice produced IL-10 and, upon adoptive transfer, were sufficiently potent to inhibit subsequent development of inflammation in mice with collagen-induced arthritis, whereas sorted CD25+CD39+CD4+ T cells showed reduced potency. IL-35 treatments of IL-10-/- mice failed to induce protective CD39+CD4+ T cells, demonstrating the effector role of IL-10 by IL-35 immunosuppression.

T helper 17 cells: discovery, function, and physiological trigger

In the few years since their discovery, T helper 17 cells (T(H)17) have been shown to play an important role in host defense against infections, and in tissue inflammation during autoimmunity. T(H)17 cells produce IL-17, IL-21, IL-10, and IL-22 cytokines, and thus have broad effects on a variety of tissues. Notably, the requirement for the immunosuppressive cytokine TGF-beta along with the pro-inflammatory cytokine IL-6 for T(H)17 differentiation supports the intimate relationship between the T(H)17 subset and FOXP3(+) regulatory T cells. Here, we discuss current knowledge on effector functions and differentiation of the T(H)17 lineage. Furthermore, we now know of a physiological stimulus for T(H)17 differentiation: innate immune recognition of cells undergoing apoptosis as a direct result of infection induces unique development of this subset. As our knowledge of T(H)17 and T regulatory cells grows, we are building on a new framework for the understanding of effector T cell differentiation and the biology of CD4(+) T cell adaptive immune responses.

Activation of IL-27 p28 gene transcription by interferon regulatory factor 8 in cooperation with interferon regulatory factor 1

Interferon regulatory factor (IRF) family members, especially interferon regulatory factor-1 (IRF-1) and interferon regulatory factor-8 (IRF-8 or ICSBP), play important roles in interferon signaling in a wide range of host responses to infection and tumor growth. Interleukin-27 (IL-27), as a member of the IL-12 cytokine family, not only acts as a proinflammatory cytokine that regulates the differentiation of naive T helper cells but also possesses anti-inflammatory properties. IL-27 consists of EBI3 (Epstein-Barr virus-induced gene 3) and p28 subunits. Our previous work has shown that IRF-1 regulates IL-27 p28 gene transcription by specifically binding to the IRF-1 response element in the p28 promoter. In this study, we found that IRF-8-deficient macrophages were highly defective in the production of IL-27 p28 at both mRNA and protein levels. Circulating IL-27 p28 in serum was also decreased in IRF-8(-/-) mice in a septic shock model. Lipopolysaccharide, as a potent inducer of IL-27 p28 expression, could activate IRF-8 expression in a MyD88-dependent pathway, which in turn induced p28 gene transcription through NF-kappaB and/or IRF-8. Transcriptional analyses revealed that IRF-8 activated p28 gene transcription through binding to a site located at -57 to -48 in the p28 promoter overlapping the IRF-1 binding site. Consistent with this observation, overexpression of both IRF-8 and IRF-1 additively activated IL-27 p28 promoter. This study provides further mechanistic information regarding how signals initiated during innate and adaptive immune responses synergize to yield greater IL-27 production and sustained cellular immunity.

A pivotal role for interleukin-27 in CD8+ T cell functions and generation of cytotoxic T lymphocytes

Cytotoxic T lymphocytes (CTLs) play a critical role in the control of various cancers and infections, and therefore the molecular mechanisms of CTL generation are a critical issue in designing antitumor immunotherapy and vaccines which augment the development of functional and long-lasting memory CTLs. Interleukin (IL)-27, a member of the IL-6/IL-12 heterodimeric cytokine family, acts on naive CD4+ T cells and plays pivotal roles as a proinflammatory cytokine to promote the early initiation of type-1 helper differentiation and also as an antiinflammatory cytokine to limit the T cell hyperactivity and production of pro-inflammatory cytokines. Recent studies revealed that IL-27 plays an important role in CD8+ T cells as well. Therefore, this article reviews current understanding of the role of IL-27 in CD8+ T cell functions and generation of CTLs.

RORC2 is involved in T cell polarization through interaction with the FOXP3 promoter

The process of Th cell differentiation toward polarized effector T cells tailors specific immunity against invading pathogens while allowing tolerance against commensal microorganisms, harmless allergens, or autologous Ags. Identification of the mechanisms underlying this polarization process is therefore central to understand how the immune system confers immunity and tolerance. The present study demonstrates that retinoic acid receptor-related orphan receptor C2 (RORC2), a key transcription factor in Th17 cell development, inhibits FOXP3 expression in human T cells. Although overexpression of RORC2 in naive T cells reduces levels of FOXP3, small interfering RNA-mediated knockdown of RORC2 enhances its expression. RORC2 mediates this inhibition at least partially by binding to two out of four ROR-responsive elements on the FOXP3 promoter. Knockdown of RORC2 promotes high FOXP3 levels and decreased expression of proinflammatory cytokines beta form of pro-IL-1, IL-6, IL-17A, IFN-gamma, and TNF-alpha in differentiating naive T cells, suggesting that the role of RORC2 in Th17 cell development involves not only induction of Th17-characteristic genes, but also suppression of regulatory T cell-specific programs. Together, this study identifies RORC2 as a polarizing factor in transcriptional cross-regulation and provides novel viewpoints on the control of immune tolerance versus effector immune responses.

Type I IFN signaling constrains IL-17A/F secretion by gammadelta T cells during bacterial infections

Recognition of intracellular bacteria by macrophages leads to secretion of type I IFNs. However, the role of type I IFN during bacterial infection is still poorly understood. Francisella tularensis, the causative agent of tularemia, is a pathogenic bacterium that replicates in the cytosol of macrophages leading to secretion of type I IFN. In this study, we investigated the role of type I IFNs in a mouse model of tularemia. Mice deficient for type I IFN receptor (IFNAR1(-/-)) are more resistant to intradermal infection with F. tularensis subspecies novicida (F. novicida). Increased resistance to infection was associated with a specific increase in IL-17A/F and a corresponding expansion of an IL-17A(+) gammadelta T cell population, indicating that type I IFNs negatively regulate the number of IL-17A(+) gammadelta T cells during infection. Furthermore, IL-17A-deficient mice contained fewer neutrophils compared with wild-type mice during infection, indicating that IL-17A contributes to neutrophil expansion during F. novicida infection. Accordingly, an increase in IL-17A in IFNAR1(-/-) mice correlated with an increase in splenic neutrophil numbers. Similar results were obtained in a mouse model of pneumonic tularemia using the highly virulent F. tularensis subspecies tularensis SchuS4 strain and in a mouse model of systemic Listeria monocytogenes infection. Our results indicate that the type I IFN-mediated negative regulation of IL-17A(+) gammadelta T cell expansion is conserved during bacterial infections. We propose that this newly described activity of type I IFN signaling might participate in the resistance of the IFNAR1(-/-) mice to infection with F. novicida and other intracellular bacteria.

Natural killer cell degeneration exacerbates experimental arthritis in mice via enhanced interleukin-17 production

OBJECTIVE

An altered phenotype and dysfunction of natural killer (NK) cells have been observed in patients with rheumatoid arthritis. The aim of this study was to determine whether dysregulated NK cells contribute to the pathogenesis of experimental arthritis.

METHODS

For initiation of collagen-induced arthritis (CIA), DBA/1J mice were immunized with type II collagen in Freund's adjuvant. Control mice were immunized with adjuvant alone. NK cells from the blood, spleens, and bone marrow of immunized mice were analyzed by flow cytometry. Levels of interleukin-17 (IL-17) secretion and autoantibody production were measured by enzyme-linked immunosorbent assays. Immunized mice in which NK cells were depleted by anti-asialo G(M1) antibody treatment were assessed for the development of CIA. Moreover, sorting-purified NK cells from both mice with CIA and control mice were analyzed for cytokine gene expression.

RESULTS

We observed markedly reduced frequencies of NK cells in the blood and spleens of mice with CIA compared with the frequencies in adjuvant-treated control mice. Upon NK cell depletion, immunized mice displayed an early onset of arthritis with more severe clinical symptoms, which correlated with increased plasma cell generation and autoantibody production. Moreover, a substantially increased number of IL-17-secreting cells in synovial tissue and more pronounced joint damage were observed. Freshly isolated NK cells from mice with CIA showed markedly reduced expression of interferon-gamma (IFNgamma). Furthermore, coculture of normal NK cells and CD4+ T cells revealed that NK cells strongly suppressed production of Th17 cells via their IFNgamma production.

CONCLUSION

These results suggest that NK cells play a protective role in the development of experimental arthritis, an effect that is possibly mediated by suppressing Th17 cell generation via IFNgamma production.

Role of cytokines as mediators and regulators of microglial activity in inflammatory demyelination of the CNS

As the resident innate immune cells of the central nervous system (CNS), microglia fulfil a critical role in maintaining tissue homeostasis and in directing and eliciting molecular responses to CNS damage. The human disease Multiple Sclerosis and animal models of inflammatory demyelination are characterized by a complex interplay between degenerative and regenerative processes, many of which are regulated and mediated by microglia. Cellular communication between microglia and other neural and immune cells is controlled to a large extent by the activity of cytokines. Here we review the role of cytokines as mediators and regulators of microglial activity in inflammatory demyelination, highlighting their importance in potentiating cell damage, promoting neuroprotection and enhancing cellular repair in a context-dependent manner.

Interleukin-27 inhibits human osteoclastogenesis by abrogating RANKL-mediated induction of nuclear factor of activated T cells c1 and suppressing proximal RANK signaling

OBJECTIVE

Interleukin-27 (IL-27) has stimulatory and regulatory immune functions and is expressed in rheumatoid arthritis (RA) synovium. This study was undertaken to investigate the effects of IL-27 on human osteoclastogenesis, to determine whether IL-27 can stimulate or attenuate the osteoclast-mediated bone resorption that is a hallmark of RA.

METHODS

Osteoclasts were generated from blood-derived human CD14+ cells. The effects of IL-27 on osteoclast formation were evaluated by counting the number of tartrate-resistant acid phosphatase-positive multinucleated cells and measuring the expression of osteoclast-related genes. The induction of nuclear factor of activated T cells c1 (NFATc1) and the activation of signaling pathways downstream of RANK were measured by immunoblotting. The expression of key molecules implicated in osteoclastogenesis (NFATc1, RANK, costimulatory receptors, and immunoreceptor tyrosine-based activation motif-harboring adaptor proteins) was measured by real-time reverse transcription-polymerase chain reaction. Murine osteoclast precursors obtained from mouse bone marrow and synovial fluid macrophages derived from RA patients were also tested for their responsiveness to IL-27.

RESULTS

IL-27 inhibited human osteoclastogenesis, suppressed the induction of NFATc1, down-regulated the expression of RANK and triggering receptor expressed on myeloid cells 2 (TREM-2), and inhibited RANKL-mediated activation of ERK, p38, and NF-kappaB in osteoclast precursors. Synovial fluid macrophages from RA patients were refractory to the effects of IL-27. In contrast to the findings in humans, IL-27 only moderately suppressed murine osteoclastogenesis, and this was likely attributable to low expression of the IL-27 receptor subunit WSX-1 on murine osteoclast precursors.

CONCLUSION

IL-27 inhibits human osteoclastogenesis by a direct mechanism that suppresses the responses of osteoclast precursors to RANKL. These findings suggest that, in addition to its well-known antiinflammatory effects, IL-27 plays a homeostatic role in restraining bone erosion. This homeostatic function is compromised under conditions of chronic inflammation such as in RA synovitis.

Activation of eosinophils by IL-12 family cytokine IL-27: Implications of the pleiotropic roles of IL-27 in allergic responses

Interleukin (IL)-27 is a novel IL-12 family cytokine and its immunomodulatory effects on T-lymphocytes, B-lymphocytes and mast cells were extensively studied. IL-27 could suppress Th2-mediated allergic diseases in mouse models. However, the role of IL-27 on eosinophils, the principal effector cells in allergic diseases, remains unexplored. Our present study revealed that eosinophils constitutively express functional IL-27 receptor heterodimer, gp130 and WSX-1. IL-27 could prolong eosinophil survival by reducing apoptosis, modulate the expression of adhesion molecules to facilitate eosinophil adhesion and accumulation, and induce the release of proinflammatory cytokines IL-6, tumor necrosis factor-α, IL-1β and chemokines CCL2, CXCL8 and CXCL1. The stimulation effects of IL-27 on eosinophils could not be abrogated by Th2 cytokine IL-25, hematopoietic cytokine granulocyte macrophage colony-stimulating factor, and toll-like receptor 4 ligand lipopolysaccharide (LPS). These findings are different from the effects of IL-27 and LPS on monocytes. Intracellular signaling mechanistic studies showed that IL-27-mediated eosinophil activation was differentially regulated by the activation of extracellular signal-regulated kinase, c-Jun N-terminal kinase and p38 mitogen-activated protein kinase, as well as nuclear factor-κB. Based on the above results, IL-27 could play crucial roles in allergic diseases by the activation of eosinophils via differential intracellular signaling cascades. According to the present findings of its activating effects on human eosinophils, IL-27 may play pleiotropic roles in human allergic responses.

IL-12 and IL-27 sequential gene therapy via intramuscular electroporation delivery for eliminating distal aggressive tumors

Eradication of residual malignancies and metastatic tumors via a systemic approach is the key for successfully treating cancer and increasing cancer patient survival. Systemic administration of IL-12 protein in an acute large dose is effective but toxic. Systemic administration of IL-12 gene by persistently expressing a low level of IL-12 protein may reduce the systemic toxicity but only eradicates IL-12-sensitive tumors. In this study, we discovered that sequential administration of IL-12- and IL-27-encoding DNA, referred to as sequential IL-12-->IL-27 (IL-12 administration followed by IL-27 administration 10 d after) gene therapy, not only eradicated IL-12-sensitive CT26 tumors from 100% of mice but also eradicated the highly malignant 4T1 tumors from 33% of treated mice in multiple independent experiments. This IL-12-->IL-27 sequential gene therapy is not only superior to IL-12-encoding plasmid DNA given a total of two times at a 10-d interval sequential gene therapy for eliminating tumors but also for inducing CTL activity, increasing T cell infiltration into tumors, and yielding a large number of tumor-specific IFN-gamma-positive CD8 T cells. Notably, depletion of either T or NK cells during the IL-27 treatment phase reverses tumor eradication, suggesting an NK cell requirement for this sequential gene therapy-mediated tumor eradication. Both reversal of the administration sequence and coadministration of IL-12 and IL-27 impaired tumor eradication in 4T1 tumor-bearing mice. This IL-12-->IL-27 sequential gene therapy, via sequential administration of IL-12- and IL-27-encoding plasmid DNA into tumor-bearing mice through i.m. electroporation, provides a simple but effective approach for eliminating inaccessible residual tumors.

Th17 cytokines and vaccine-induced immunity

T helper type 17 (Th17) cells are a distinct lineage of T cells that produce the effector molecules IL-17, IL-17F, IL-21, and IL-22. Although the role of Th17 cells in primary immune responses against infections is well documented, there is growing evidence that the Th17 lineage maybe critical for vaccine-induced memory immune responses against infectious diseases. Here, we summarize recent progress in our understanding of the role of IL-17 in vaccine-induced immunity.

The IL-27 p28 subunit binds cytokine-like factor 1 to form a cytokine regulating NK and T cell activities requiring IL-6R for signaling

IL-27 is formed by the association of a cytokine subunit, p28, with the soluble cytokine receptor EBV-induced gene 3 (EBI3). The IL-27R comprises gp130 and WSX-1. The marked difference between EBI3(-/-) and WSX-1(-/-) mice suggests that p28 has functions independent of EBI3. We have identified an alternative secreted complex formed by p28 and the soluble cytokine receptor cytokine-like factor 1 (CLF). Like IL-27, p28/CLF is produced by dendritic cells and is biologically active on human NK cells, increasing IL-12- and IL-2-induced IFN-gamma production and activation marker expression. Experiments with Ba/F3 transfectants indicate that p28/CLF activates cells expressing IL-6Ralpha in addition to the IL-27R subunits. When tested on CD4 and CD8 T cells, p28/CLF induces IL-6Ralpha-dependent STAT1 and STAT3 phosphorylation. Furthermore, p28/CLF inhibits CD4 T cell proliferation and induces IL-17 and IL-10 secretion. These results indicate that p28/CLF may participate in the regulation of NK and T cell functions by dendritic cells. The p28/CLF complex engages IL-6R and may therefore be useful for therapeutic applications targeting cells expressing this receptor. Blocking IL-6R using humanized mAbs such as tocilizumab has been shown to be beneficial in pathologies like rheumatoid arthritis and juvenile idiopathic arthritis. The identification of a new IL-6R ligand is therefore important for a complete understanding of the mechanism of action of this emerging class of immunosuppressors.

Advances in understanding immunity to Toxoplasma gondii

Toxoplasma gondii is an important cause of clinical disease in fetuses, infants and immunocompromised patients. Since the discovery of T. gondii 100 years ago, this pathogen and the host's immune response to toxoplasmosis have been studied intensely. This has led to the development of a working model of immunity to T. gondii, and has also resulted in fundamental new insights into the role of various cytokines in resistance to infection. By examining this organism, researchers have identified many of the requirements for resistance to intracellular pathogens and characterized numerous regulatory factors, including interleukin-10 (IL-10) and IL-27, which control inflammatory processes. In the next 100 years of T. gondii immunobiology, researchers will have the opportunity to answer some of the long-standing questions in the field using new techniques and reagents. These future studies will be vital in building a more comprehensive model of immunity to this pathogen and in advancing our understanding of immunoregulation, particularly in humans. Ultimately, the challenge will be to use this information to develop new vaccines and therapies to manage disease in affected patients.

Potent intestinal Th17 priming through peripheral lipopolysaccharide-based immunization

Lipopolysaccharide (LPS) is a potent natural adjuvant, commonly used to amplify Th1 responses. Here, we report that systemic immunization using LPS generates large numbers of specific Th17 cells in murine small intestinal lamina propria. The priming of these Th17 cells required IL-23p19 production by bone marrow-derived cells. In contrast, IL-23 had no impact on Th1 differentiation or overall numbers of Ag-specific regulatory T cells. Experiments using T-cell adoptive transfers revealed a previously unappreciated mechanism for how Th17 responses are amplified in vivo: stimulation through LPS expanded precommitted Th17 cells rather than causing Th17 differentiation. Second, LPS drove Th17 cell expansion independently of IL-23, demonstrating that this cytokine is not necessary for expansion and possibly functions at an earlier stage in Th17 priming. Our data provide an impetus for using LPS-based peripheral vaccination to augment specific T-cell-mediated immunity in the gut mucosa.

Development, regulation and functional capacities of Th17 cells

T helper (Th) 17 cells have been classified as a new lineage, distinct from Th1, Th2 and Treg. Their development requires a unique combination of cytokines and depends on distinct intracellular events, resulting in the production of the signature cytokines interleukin (IL)-17A, IL-17F and IL-22. The differential cytokine expression patterns in Th cells suggest a division of labour in the response against a variety of pathogens. Th17 have an important function in the host-defense-response against extracellular pathogens, but they also have become notorious for their role in the pathogenesis of many autoimmune and allergic disorders. Animal models of autoimmune disorders have shown that Th17 effector molecules and transcription factors play a crucial role in both development and maintenance of the disease. The discovery of Th17 not only enhanced our insight into these disorders but also placed a Th subset at the interface between the innate and adoptive immune systems with the potential to regulate subsequent immunity against pathogens.