Requirement of tumor necrosis factor receptor-associated factor (TRAF)6 in interleukin 17 signal transduction

Regulation of inflammatory responses by IL-17F

Although interleukin (IL) 17 has been extensively characterized, the function of IL-17F, which has an expression pattern regulated similarly to IL-17, is poorly understood. We show that like IL-17, IL-17F regulates proinflammatory gene expression in vitro, and this requires IL-17 receptor A, tumor necrosis factor receptor-associated factor 6, and Act1. In vivo, overexpression of IL-17F in lung epithelium led to infiltration of lymphocytes and macrophages and mucus hyperplasia, similar to observations made in IL-17 transgenic mice. To further understand the function of IL-17F, we generated and analyzed mice deficient in IL-17F or IL-17. IL-17, but not IL-17F, was required for the initiation of experimental autoimmune encephalomyelitis. Mice deficient in IL-17F, but not IL-17, had defective airway neutrophilia in response to allergen challenge. Moreover, in an asthma model, although IL-17 deficiency reduced T helper type 2 responses, IL-17F-deficient mice displayed enhanced type 2 cytokine production and eosinophil function. In addition, IL-17F deficiency resulted in reduced colitis caused by dextran sulfate sodium, whereas IL-17 knockout mice developed more severe disease. Our results thus demonstrate that IL-17F is an important regulator of inflammatory responses that seems to function differently than IL-17 in immune responses and diseases.

Regulatory mechanisms of helper T cell differentiation: new lessons learned from interleukin 17 family cytokines

Interleukin 17 (IL-17) family consists of six cytokines in mammals. Among them, IL-17 and IL-17F are expressed by a novel subset of CD4+ helper T (Th) cells and play critical function in inflammation and autoimmunity. On the other hand, IL-17E, also called IL-25, has been associated with allergic responses. Here we summarize recent work by us as well as other investigators in understanding the regulation and function of these three cytokines. From these studies, IL-17 family cytokines may serve as novel targets for pharmaceutical intervention of immune and inflammatory diseases.

Structure-function relationships in the IL-17 receptor: implications for signal transduction and therapy

IL-17 is the defining cytokine of a newly-described "Th17" population that plays critical roles in mediating inflammation and autoimmunity. The IL-17/IL-17 receptor superfamily is the most recent class of cytokines and receptors to be described, and until recently very little was known about its function or molecular biology. However, in the last year important new insights into the composition and dynamics of the receptor complex and mechanisms of downstream signal transduction have been made, which will be reviewed here.

Requirement for both JAK-mediated PI3K signaling and ACT1/TRAF6/TAK1-dependent NF-kappaB activation by IL-17A in enhancing cytokine expression in human airway epithelial cells

Through DNA microarray analysis and quantitative PCR verification, we have identified additional IL-17A-inducible genes-IL-19, CXCL-1, -2, -3, -5, and -6-in well-differentiated normal human bronchial epithelial cells. These genes, similar to previously described human beta-defensin-2 (HBD-2) and CCL-20, were induced by a basolateral treatment of IL-17A, and regulated by PI3K signaling and NF-kappaB activation. For PI3K signaling, increases of cellular PIP(3) and phosphorylation of downstream molecules, such as Akt and glycogen synthase kinase-3beta (GSK3beta) (S9), were detected. Induced gene expression and HBD-2 promoter activity were attenuated by LY294002, p110alpha small-interfering RNA (siRNA), as well as by an overexpression of constitutively active GSK3beta(S9A) or wild-type phosphatase and tensin homolog. Increased phosphorylation of JAK1/2 after IL-17A treatment was detected in primary normal human bronchial epithelium cells. Transfected siRNAs of JAK molecules and JAK inhibitor I decreased IL-17A-induced gene expression and GSK3beta(S9) phosphorylation. However, both JAK inhibitor I and PI3K inhibitor had no effect on the DNA-binding activities of p65 and p50 to NF-kappaB consensus sequences. This result suggested a JAK-associated PI3K signaling axis is independent from NF-kappaB activation. With siRNA to knockdown STIR (similar expression to fibroblast growth factor and IL-17R; Toll-IL-1R)-related signaling molecules, such as Act1, TNFR-associated factor 6 (TRAF6), and TGF-beta-activated kinase 1 (TAK1), and transfection of A52R, an inhibitor of the MyD88/TRAF6 complex, or dominant-negative TAK1, IL-17A-inducible gene expression and HBD-2 promoter activity were reduced. Additionally, IL-17A-induced p65 and p50 NF-kappaB activations were confirmed and their nuclear translocations were down-regulated by siRNAs of TRAF6 and TAK1. These results suggest that two independent and indispensable signaling pathways-1) JAK1-associated PI3K signaling and 2) Act1/TRAF6/TAK1-mediated NF-kappaB activation-are stimulated by IL-17A to regulate gene induction in human airway epithelial cells.

Act1 modulates autoimmunity through its dual functions in CD40L/BAFF and IL-17 signaling

Coordinated regulation of T and B cell-mediated immune responses plays a critical role in the control and modulation of autoimmune diseases. This review is focused on the adapter molecule Act1 and its regulation of autoimmunity through its impact on both T and B cell-mediated immune responses. Whereas Act1 molecule is an important negative regulator for B cell-mediated humoral immune responses through its function in CD40L and BAFF signaling, recent studies have shown that Act1 is also a key positive signaling component for IL-17 signaling pathway, critical for T(H)17-mediated autoimmune and inflammatory responses. The dual functions of Act1 are evident in Act1-deficient mice that displayed B cell-mediated autoimmune phenotypes (including dramatic increase in peripheral B cells, lymphadenopathy and splenomegaly, hypergammaglobulinemia and Sjogren's disease in association with Lupus Nephritis), but showed resistance to T(H)17-dependent EAE and colitis. Such seemingly opposite functions of Act1 in CD40-BAFFR and IL-17R signaling are orchestrated by different domains in Act1. Whereas Act1 interacts with the IL-17R through the C-terminal SEFIR domain, Act1 is recruited to CD40 and BAFFR indirectly, which is mediated by TRAF3 through the TRAF binding site in Act1. Such delicate regulatory mechanisms may provide a common vehicle to promote balance between host defense to pathogens and tolerance to self.

Th17 cells: effector T cells with inflammatory properties

Upon activation, naïve CD4(+) T cells differentiate into effector T cells with specific effector functions and cytokine profiles. The Th1/Th2 paradigm has recently been reevaluated to include a third population of T helper cells, producing IL-17 and designated Th17. The differentiation of Th17 cells requires the coordinate and specific action of the proinflammatory cytokine IL-6 and the immunosuppressive cytokine TGF-beta. In addition, the IL-12 family member IL-23 is involved in the maintenance of these cells. Analogous to other T helper cell subsets, Th17 commitment is initiated by sequential involvement of STAT molecules, i.e. STAT3 downstream of cytokine receptors, and specific transcription factors, i.e. ROR-gammat. Recent data also support the existence of a complex network of cytokines regulating Th17 cells. Clearly, the specific effector functions of Th17 cells expand beyond previously described effects of Th1 and Th2 immunity, with specific roles in host defense against certain pathogens and in organ-specific autoimmunity. The potential dynamics of Th17 cell populations and their interplay with other inflammatory cells in the induction of tissue inflammation in host defense and organ-specific autoimmunity are discussed.

Measurement of interleukin-17

Upon antigenic stimulation, naive CD4+ T cells undergo proliferation and differentiate into cytokine-producing T helper (T(H)) effector cells. T(H)1 cells secrete effector cytokine IFN-gamma and regulate cell-mediated immunity, whereas T(H)2 cells produce IL-4, IL-5, and IL-13 cytokines, and mediate immunity against extracellular pathogens and allergic reactions. Recent studies have identified a novel T(H) subset, called T(H)17, TH(IL-17), or inflammatory T(H) (THi) cells, characterized by the production of a proinflammatory cytokine, IL-17, and regulating inflammatory responses. In this unit, we describe the protocols for the differentiation of mouse IL-17-expressing T cells in vitro, detection of IL-17-expressing T cells by intracellular cytokine staining, and measurement of IL-17 secretion in culture supernatants by ELISA. Generation of IL-17-expressing T cells in vitro under defined culture conditions allows investigation of their differentiation regulation. Detection of IL-17 in cell culture and tissue samples helps in monitoring inflammatory diseases and determining efficacy of therapeutic interventions.

Identification of the IL-17 receptor related molecule IL-17RC as the receptor for IL-17F

The proinflammatory cytokines IL-17A and IL-17F have a high degree of sequence similarity and share many biological properties. Both have been implicated as factors contributing to the progression of inflammatory and autoimmune diseases. Moreover, reagents that neutralize IL-17A significantly ameliorate disease severity in several mouse models of human disease. IL-17A mediates its effects through interaction with its cognate receptor, the IL-17 receptor (IL-17RA). We report here that the IL-17RA-related molecule, IL-17RC is the receptor for IL-17F. Notably, both IL-17A and IL-17F bind to IL-17RC with high affinity, leading us to suggest that a soluble form of this molecule may serve as an effective therapeutic antagonist of IL-17A and IL-17F. We generated a soluble form of IL-17RC and demonstrate that it effectively blocks binding of both IL-17A and IL-17F, and that it inhibits signaling in response to these cytokines. Collectively, our work indicates that IL-17RC functions as a receptor for both IL-17A and IL-17F and that a soluble version of this protein should be an effective antagonist of IL-17A and IL-17F mediated inflammatory diseases.

A novel heterodimeric cytokine consisting of IL-17 and IL-17F regulates inflammatory responses

CD4+ helper T (TH) cells play crucial roles in immune responses. Recently a novel subset of TH cells, termed TH(IL-17), TH17 or inflammatory TH (THi), has been identified as critical mediators of tissue inflammation. These cells produce IL-17 (also called IL-17A) and IL-17F, two most homologous cytokines sharing similar regulations. Here we report that when overexpressed in 293T cells, IL-17 and IL-17F form not only homodimers but also heterodimers, which we name as IL-17A/F. Fully differentiated mouse THi cells also naturally secrete IL-17A/F as well as IL-17 and IL-17F homodimeric cytokines. Recombinant IL-17A/F protein exhibits intermediate levels of potency in inducing IL-6 and KC (CXCL1) as compared to homodimeric cytokines. IL-17A/F regulation of IL-6 and KC expression is dependent on IL-17RA and TRAF6. Thus, IL-17A/F cytokine represents another mechanism whereby T cells regulate inflammatory responses and may serve as a novel target for treating various immune-mediated diseases.

A role for the cytoplasmic adaptor protein Act1 in mediating IL-17 signaling

Interleukin (IL)-17 (also known as IL-17A) plays an important role in host defense and inflammatory disorders, in part by linking the activation of a subset of T lymphocytes to the mobilization of neutrophils and macrophages. IL-17 exerts its effects both directly and indirectly; the latter by stimulating the production of various chemokines, IL-6, and growth factors from resident cells in the affected tissue. As a result, IL-17 coordinates the innate immune response to extracellular bacteria, which is interesting because IL-17 is produced by several types of T cells that are traditionally regarded as key players in adaptive immunity. Studies have uncovered the function and relevance of a unique subset of CD4(+) T helper (Th) cells that produce IL-17 (Th17 cells), but our understanding of the function of IL-17 receptors (IL-17Rs) and their downstream signaling pathways remains poor. This Review discusses studies that suggest that the cytoplasmic adaptor protein Act1 [nuclear factor-kappaB (NF-kappaB) activator 1] is essential for linking stimulation of IL-17Rs to downstream signaling pathways, and, therefore, that Act1 might play a role in local inflammatory responses. Act1 mediates activation of NF-kappaB and the subsequent production of IL-6 and chemokines that are chemotactic for neutrophils and macrophages. These findings have increased our understanding of host defense against bacteria and indicated a role for Act1 in mediating in chronic inflammatory disease. Future studies on Act1 and IL-17 signaling should contribute to the identification and improved understanding of the mechanisms behind aberrant innate immune responses in chronic inflammatory disease.

IL-17 family cytokines and the expanding diversity of effector T cell lineages

Since its conception two decades ago, the Th1-Th2 paradigm has provided a framework for understanding T cell biology and the interplay of innate and adaptive immunity. Naive T cells differentiate into effector T cells with enhanced functional potential for orchestrating pathogen clearance largely under the guidance of cytokines produced by cells of the innate immune system that have been activated by recognition of those pathogens. This secondary education of post-thymic T cells provides a mechanism for appropriately matching adaptive immunity to frontline cues of the innate immune system. Owing in part to the rapid identification of novel cytokines of the IL-17 and IL-12 families using database searches, the factors that specify differentiation of a new effector T cell lineage-Th17-have now been identified, providing a new arm of adaptive immunity and presenting a unifying model that can explain many heretofore confusing aspects of immune regulation, immune pathogenesis, and host defense.

Distinct functional motifs within the IL-17 receptor regulate signal transduction and target gene expression

IL-17 is the founding member of a novel family of proinflammatory cytokines that defines a new class of CD4+ effector T cells, termed "Th17." Mounting evidence suggests that IL-17 and Th17 cells cause pathology in autoimmunity, but little is known about mechanisms of IL-17RA signaling. IL-17 through its receptor (IL-17RA) activates genes typical of innate immune cytokines, such as TNFalpha and IL-1beta, despite minimal sequence similarity in their respective receptors. A previous bioinformatics study predicted a subdomain in IL-17-family receptors with homology to a Toll/IL-1R (TIR) domain, termed the "SEFIR domain." However, the SEFIR domain lacks motifs critical for bona fide TIR domains, and its functionality was never verified. Here, we used a reconstitution system in IL-17RA-null fibroblasts to map functional domains within IL-17RA. We demonstrate that the SEFIR domain mediates IL-17RA signaling independently of classic TIR adaptors, such as MyD88 and TRIF. Moreover, we identified a previously undescribed"TIR-like loop" (TILL) required for activation of NF-kappaB, MAPK, and up-regulation of C/EBPbeta and C/EBPdelta. Mutagenesis of the TILL domain revealed a site analogous to the LPS(d) mutation in TLR4, which renders mice insensitive to LPS. However, a putative salt bridge typically found in TIR domains appears to be dispensable. We further identified a C-terminal domain required for activation of C/EBPbeta and induction of a subset IL-17 target genes. This structure-function analysis of a IL-17 superfamily receptor reveals important differences in IL-17RA compared with IL-1/TLR receptors.

Beta-interferon unbalances the peripheral T cell proinflammatory response in experimental autoimmune encephalomyelitis

Interferon beta (IFNbeta) is a widespread therapy for multiple sclerosis (MS). We have analyzed some critical features of the T cell activation process in lymph nodes after IFNbeta treatment of experimental autoimmune encephalomyelitis (EAE) in SJL mice. Prevention of clinical signs and drastic reduction of perivascular infiltrates in the central nervous system (CNS) were accompanied by alterations in nuclear DNA binding activity levels of NFkappaB and Stat6 transcription factors in lymph node cells (LNC). A decrease of active NFkappaB subunits in treated animals correlated with lower levels of the cytoplasmic phosphorylated form of IkappaBalpha. Results also showed that nuclear DNA binding activity of Stat6 was increased by IFNbeta treatment, as were the cytoplasmic levels of phosphorilated Stat6 (P-Stat6). These high levels of P-Stat6 in IFNbeta-treated animals were accompanied by an increase of IL-4 expression levels measured by real time PCR. In vitro experiments with the IL-4 producing clone D10.G4.1 indicates that the IFNbeta-mediated IL-4 induction is not an effect exclusive to MBP-reactive cells, and suggest that it could be mediated by mRNA stability enlargement. On the other hand, IFNbeta treatment of EAE produced no significant changes in peripheral IFNgamma expression and a striking decrease of IL-17. These findings suggest that the inhibition of NFkappaB activity, the increase of IL-4 expression and its signaling transduction, and the decrease of IL-17 may cooperate to some of the antiinflammatory effects of IFNbeta on EAE.

The adaptor Act1 is required for interleukin 17-dependent signaling associated with autoimmune and inflammatory disease

T helper cells that produce interleukin 17 (IL-17) are associated with inflammation and the control of certain bacteria. We report here the essential involvement of the adaptor protein Act1 in IL-17 receptor (IL-17R) signaling and IL-17-dependent immune responses. After stimulation with IL-17, recruitment of Act1 to IL-17R required the IL-17R conserved cytoplasmic 'SEFIR' domain, followed by recruitment of the kinase TAK1 and E3 ubiquitin ligase TRAF6, which mediate 'downstream' activation of transcription factor NF-kappaB. IL-17-induced expression of inflammation-related genes was abolished in Act1-deficient primary astroglial and gut epithelial cells. This reduction was associated with much less inflammatory disease in vivo in both autoimmune encephalomyelitis and dextran sodium sulfate-induced colitis. Our data show that Act1 is essential in IL-17-dependent signaling in autoimmune and inflammatory disease.

Interleukin-23 restores immunity to Mycobacterium tuberculosis infection in IL-12p40-deficient mice and is not required for the development of IL-17-secreting T cell responses

Host control of Mycobacterium tuberculosis is dependent on the activation of CD4+ T cells secreting IFN-gamma and their recruitment to the site of infection. The development of more efficient vaccines against tuberculosis requires detailed understanding of the induction and maintenance of T cell immunity. Cytokines important for the development of cell-mediated immunity include IL-12 and IL-23, which share the p40 subunit and the IL-12Rbeta1 signaling chain. To explore the differential effect of IL-12 and IL-23 during M. tuberculosis infection, we used plasmids expressing IL-23 (p2AIL-23) or IL-12 (p2AIL-12) alone in dendritic cells or macrophages from IL-12p40(-/-) mice. In the absence of the IL-12/IL-23 axis, immunization with a DNA vaccine expressing the M. tuberculosis Ag85B induced a limited Ag-specific T cell response and no control of M. tuberculosis infection. Co-delivery of p2AIL-23 or p2AIL-12 with DNA85B induced strong proliferative and IFN-gamma-secreting T cell responses equivalent to those observed in wild-type mice immunized with DNA85B. This response resulted in partial protection against aerosol M. tuberculosis; however, the protective effect was less than in wild-type mice owing to the requirement for IL-12 or IL-23 for the optimal expansion of IFN-gamma-secreting T cells. Interestingly, bacillus Calmette-Guérin immune T cells generated in the absence of IL-12 or IL-23 were deficient in IFN-gamma production, but exhibited a robust IL-17 secretion associated with a degree of protection against pulmonary infection. Therefore, exogenous IL-23 can complement IL-12 deficiency for the initial expansion of Ag-specific T cells and is not essential for the development of potentially protective IL-17-secreting T cells.

An essential role for IL-17 in preventing pathogen-initiated bone destruction: recruitment of neutrophils to inflamed bone requires IL-17 receptor-dependent signals

IL-17 and its receptor are founding members of a novel family of inflammatory cytokines. IL-17 plays a pathogenic role in rheumatoid arthritis (RA)-associated bone destruction. However, IL-17 is also an important regulator of host defense through granulopoiesis and neutrophil trafficking. Therefore, the role of IL-17 in pathogen-initiated bone loss was not obvious. The most common form of infection-induced bone destruction occurs in periodontal disease (PD). In addition to causing significant morbidity, PD is a risk factor for atherosclerotic heart disease and chronic obstructive pulmonary disease (COPD). Similar to RA, bone destruction in PD is caused by the immune response. However, neutrophils provide critical antimicrobial defense against periodontal organisms. Since IL-17 is bone destructive in RA but a key regulator of neutrophils, we examined its role in inflammatory bone loss induced by the oral pathogen Porphyromonas gingivalis in IL-17RA-deficient mice. These mice showed enhanced periodontal bone destruction, suggesting a bone-protective role for IL-17, reminiscent of a neutrophil deficiency. Although IL-17RA-deficient neutrophils functioned normally ex vivo, IL-17RA knock-out (IL-17RA(KO)) mice exhibited reduced serum chemokine levels and concomitantly reduced neutrophil migration to bone. Consistently, CXCR2(KO) mice were highly susceptible to alveolar bone loss; interestingly, these mice also suggested a role for chemokines in maintaining normal bone homeostasis. These results indicate a nonredundant role for IL-17 in mediating host defense via neutrophil mobilization.

The IL-17 cytokine family

IL-17A and its receptor are the founding members of a recently described cytokine family, with unique sequences and functions in the immune system and elsewhere. Consisting of six ligands (IL-17A-F) and five receptors (IL-17RA-IL-17RE) in mammals, these molecules have distinct primary amino acid structures with only minimal homology to other cytokine families. By far the best studied of these cytokines to date are IL-17A and its receptor, IL-17RA. IL-17A is produced primarily by T cells, and is the hallmark cytokine of a newly defined T helper cell subset that appears to be involved in generation of autoimmunity. Despite its production by the adaptive immune system, IL-17A exhibits proinflammatory activities similar to innate immune cytokines such as IL-1beta and TNF-alpha and appears to play important and nonredundant roles in regulating granulocytes in vivo. As a result, IL-17A also plays key roles in host defense. In contrast to the restricted expression of IL-17A, the IL-17RA receptor is ubiquitously expressed, and thus most cells are potential physiological targets of IL-17A. This chapter describes the major molecular properties, biological activities, and known signaling pathways of the IL-17 family, with an emphasis on IL-17A and IL-17RA.

Act1 adaptor protein is an immediate and essential signaling component of interleukin-17 receptor

Interleukin (IL)-17, the founding member of the IL-17 cytokine family, is the hallmark of a novel subset of CD4+ T cells that is regulated by TGFbeta, IL-6, and IL-23. IL-17 plays an important role in promoting tissue inflammation in host defense against infection and in autoimmune diseases. Although IL-17 has been reported to regulate the expression of proinflammatory cytokines, chemokines, and matrix metalloproteinases, the signaling mechanism of IL-17 receptor has not been understood. An earlier study found that IL-17 activates NF-kappaB and MAPK pathways and requires TRAF6 to induce IL-6. However, it is unknown what molecule(s) directly associates with IL-17 receptor to initiate the signaling. We demonstrate here that IL-17 receptor family shares sequence homology in their intracellular region with Toll-IL-1 receptor (TIR) domains and with Act1, a novel adaptor previously reported as an NF-kappaB activator. MyD88 and IRAK4, downstream signaling components of TIR, are not required for IL-17 signaling. On the other hand, Act1 and IL-17 receptor directly associate likely via homotypic interaction. Deficiency of Act1 in fibroblast abrogates IL-17-induced cytokine and chemokine expression, as well as the induction of C/EBPbeta, C/EBPdelta, and IkappaBzeta. Also, absence of Act1 results in a selective defect in IL-17-induced activation of NF-kappaB pathway. These results thus indicate Act1 as a membrane-proximal adaptor of IL-17 receptor with an essential role in induction of inflammatory genes. Our study not only for the first time reveals an immediate signaling mechanism downstream of an IL-17 family receptor but also has implications in therapeutic treatment of various immune diseases.

Inactivation of JNK1 enhances innate IL-10 production and dampens autoimmune inflammation in the brain

Environmental insults such as microbial pathogens can contribute to the activation of autoreactive T cells, leading to inflammation of target organs and, ultimately, autoimmune disease. Various infections have been linked to multiple sclerosis and its animal counterpart, autoimmune encephalomyelitis. The molecular process by which innate immunity triggers autoreactivity is not currently understood. By using a mouse model of multiple sclerosis, we found that the genetic loss of the MAPK, c-Jun N-terminal kinase 1 (JNK1), enhances IL-10 production, rendering innate myeloid cells unresponsive to certain microbes and less capable of generating IL-17-producing, encephalitogenic T cells. Moreover, JNK1-deficient central nervous system myeloid cells are unable to respond to effector T cell inflammatory cytokines, preventing further progression to neuroinflammation. Thus, we have identified the JNK1 signal transduction pathway in myeloid cells to be a critical component of a regulatory circuit mediating inflammatory responses in autoimmune disease. Our findings provide further insights into the pivotal MAPK-regulated network of innate and adaptive cytokines in the progression to autoimmunity.

Identification of common transcriptional regulatory elements in interleukin-17 target genes

Interleukin (IL)-17 is the founding member of a novel family of inflammatory cytokines. Although produced by T cells, IL-17 activates genes and signals typical of innate immune mediators such as tumor necrosis factor (TNF)-alpha and IL-1beta. Most IL-17 target genes characterized to date are cytokines or neutrophil-attractive chemokines. Our recent microarray studies identified an acute phase response gene, 24p3/lipocalin 2, as a novel IL-17-induced gene. Here we describe a detailed analysis of the 24p3 promoter. We find that, unlike cytokine or chemokine gene target genes, 24p3 is regulated primarily at the level of transcription rather than mRNA stability and that synergy between IL-17 and TNFalpha occurs at the level of the 24p3 promoter. Two key transcription factor binding sites (TFBS) were identified, corresponding to NF-kappaB and CCAAT/enhancer-binding protein (C/EBP). Deletion of either site eliminated 24p3 promoter activity in response to IL-17. These findings were strikingly similar to the IL-6 promoter, where IL-17-mediated regulation of both NF-kappaB and C/EBP is essential. To determine whether joint use of NF-kappaB and C/EBP is common to all IL-17 target genes, we performed a computational analysis on 18 well documented IL-17 target promoters to assess statistical enrichment of specific TFBSs. Indeed, NF-kappaB and C/EBP sites were over-represented in these genes, as were AP1 and OCT1 sites. Moreover, these promoters fell into three definable subcategories based on TFBS location and usage. Analysis of IL-17 target gene regulation is key for understanding this important host-defense molecule and also contributes to an understanding of upstream signaling mechanisms used by IL-17, either alone or in concert with TNFalpha.

Interleukin-17 acts independently of TNF-alpha under arthritic conditions

The proinflammatory T cell cytokine IL-17 is a potent inducer of other cytokines such as IL-1 and TNF-alpha. The contribution of TNF in IL-17-induced joint inflammation is unclear. In this work we demonstrate using TNF-alpha-deficient mice that TNF-alpha is required in IL-17-induced joint pathology under naive conditions in vivo. However, overexpression of IL-17 aggravated K/BxN serum transfer arthritis to a similar degree in TNF-alpha-deficient mice and their wild-type counterparts, indicating that the TNF dependency of IL-17-induced pathology is lost under arthritic conditions. Also, during the course of the streptococcal cell wall-induced arthritis model, IL-17 was able to enhance inflammation and cartilage damage in the absence of TNF. Additional blocking of IL-1 during IL-17-enhanced streptococcal cell wall-induced arthritis did not reduce joint pathology in TNF-deficient mice, indicating that IL-1 is not responsible for this loss of TNF dependency. These data provide further understanding of the cytokine interplay during inflammation and demonstrate that, despite a strong TNF dependency under naive conditions, IL-17 acts independently of TNF under arthritic conditions.

Cutting Edge: Interleukin 17 Signals through a Heteromeric Receptor Complex

IL-17 is an inflammatory cytokine produced primarily by a unique lineage of CD4 T cells that plays critical roles in the pathogenesis of multiple autoimmune diseases. IL-17RA is a ubiquitously expressed receptor that is essential for IL-17 biologic activity. Despite widespread receptor expression, the activity of IL-17 is most classically defined by its ability to induce the expression of inflammatory cytokines, chemokines, and other mediators by stromal cells. The lack of IL-17 responsiveness in mouse stromal cells genetically deficient in IL-17RA is poorly complemented by human IL-17RA, suggesting the presence of an obligate ancillary component whose activity is species specific. This component is IL-17RC, a distinct member of the IL-17R family. Thus, the biologic activity of IL-17 is dependent on a complex composed of IL-17RA and IL-17RC, suggesting a new paradigm for understanding the interactions between the expanded family of IL-17 ligands and their receptors.

Diversification of T-helper-cell lineages: finding the family root of IL-17-producing cells

CD4+ T helper 1 (T(H)1) and T(H)2 cells have long been regarded as two sides of a coin in terms of adaptive immune responses. However, as I discuss here, this concept needs to be reconsidered. In particular, recent data indicate that interleukin-17 (IL-17) is produced by T(H) cells that are distinct from the traditional T(H)1- and T(H)2-cell subsets. Furthermore, the generation of these IL-17-producing CD4+ T cells from naive precursors during immune responses is not dependent on the cytokines and transcription factors that mediate T(H)1- and T(H)2-cell development. Given that IL-17 has crucial roles in regulating tissue inflammation and the development of disease in several animal models of autoimmunity, I propose that IL-17-producing CD4+ T cells represent a distinct inflammatory T(H)-cell lineage.

Interleukin-17 and airway remodelling

Interleukin (IL)-17A is emerging as important in reinforcing innate immunity by orchestrating sustained neutrophilic mobilisation. Even though there are indications of association with specific airway diseases, there is still no final proof that IL-17A plays a truly causative pathogenic role. There is evidence in mice that endogenous IL-17A contributes to the development of allergen-induced airway hyperresponsiveness and there is also evidence that IL-17A stimulates the release of several cytokines with known capacity for airway remodelling, from cells normally residing in the airways. New studies are required to determine whether these effects on local cells actually contribute to airway remodelling in vivo. If this is the case, then IL-17A may constitute a useful target for pharmacotherapeutic intervention in allergic airway disease.

Evidence for ligand-independent multimerization of the IL-17 receptor

IL-17 and its receptor are founding members of a novel inflammatory cytokine family. To date, only one IL-17 receptor subunit has been identified, termed IL-17RA. All known cytokine receptors consist of a complex of multiple subunits. Although IL-17-family cytokines exist as homodimers, the configuration and stoichiometry of the IL-17R complex remain unknown. We used fluorescence resonance energy transfer (FRET) to determine whether IL-17RA subunits multimerize, and, if so, whether they are preassembled in the plasma membrane. HEK293 cells coexpressing IL-17RA fused to cyan or yellow fluorescent proteins (CFP or YFP) were used to evaluate FRET before and after IL-17A or IL-17F treatment. In the absence of ligand, IL-17RA molecules exhibited significant specific FRET efficiency, demonstrating that they exist in a multimeric, preformed receptor complex. Strikingly, treatment with IL-17A or IL-17F markedly reduced FRET efficiency, suggesting that IL-17RA subunits within the IL-17R complex undergo a conformational change upon ligand binding.

Involvement of TNF Receptor-Associated Factor 6 in IL-25 Receptor Signaling

IL-25 (IL-17E) induces IL-4, IL-5, and IL-13 production from an unidentified non-T/non-B cell population and subsequently induces Th2-type immune responses such as IgE production and eosinophilic airway inflammation. IL-25R is a single transmembrane protein with homology to IL-17R, but the IL-25R signaling pathways have not been fully understood. In this study, we investigated the signaling pathway under IL-25R, especially the possible involvement of TNFR-associated factor (TRAF)6 in this pathway. We found that IL-25R cross-linking induced NF-kappaB activation as well as ERK, JNK, and p38 activation. We also found that IL-25R-mediated NF-kappaB activation was inhibited by the expression of dominant negative TRAF6 but not of dominant negative TRAF2. Furthermore, IL-25R-mediated NF-kappaB activation, but not MAPK activation, was diminished in TRAF6-deficient murine embryonic fibroblast. In addition, coimmunoprecipitation assay revealed that TRAF6, but not TRAF2, associated with IL-25R even in the absence of ligand binding. Finally, we found that IL-25R-mediated gene expression of IL-6, TGF-beta, G-CSF, and thymus and activation-regulated chemokine was diminished in TRAF6-deficient murine embryonic fibroblast. Taken together, these results indicate that TRAF6 plays a critical role in IL-25R-mediated NF-kappaB activation and gene expression.

Identification and functional characterization of a novel interleukin 17 receptor: a possible mitogenic activation through ras/mitogen-activated protein kinase signaling pathway

Interleukin-17 receptor (IL-17R) is increasingly emerged as a distinct receptor family functioning in diverse cellular processes including inflammation and cancer. In this study, we uncovered a novel member of IL-17R from mouse tissue that was named mouse IL-17RE (mIL-17R). Mouse IL-17RE cDNA is composed of at least 14 exons and presents at least 6 spliced isoforms (mIL-17RE1-6) with a molecular weight ranging from 34.2 to 70.1 kD. Mouse IL-17RE is expressed in limited tissues such as lung, kidney, stomach, intestine and testis, etc., and is mainly localized in the cytoplasm and on cell membrane. IL-17RE can also be detected in numerous tumor cell lines. Importantly, a mitogenic effect was detected in BaF3 cells stably transfected with the chimeric receptor fused by the ectodomain of erythropoietin receptor (EPOR) with the transmembrane and endomain of IL-17RE in a serum-dependent but EPO-independent manner. Moreover, ERK1/2 phosphorylation was significantly up-regulated as the dose of mIL-17RE increased. Specific RNAi targeting at mIL-17RE dramatically inhibited the activation of ERK1/2, indicating that mIL-17RE could functionally activate RAS/MAPK signaling pathway. Using dominant negative MEK (Dn-MEK) or RAS (Dn-RAS) as a signaling blocker, we were able to show that mIL-17RE probably activated RAS/MAPK signaling at or upstream of RAS. Overall, our results strongly indicate that mIL-17RE may belong to a novel growth-receptor like molecule that has the capability to support cellular mitogenesis through RAS/MAPK pathway.

Requirement of IL-17 receptor signaling in radiation-resistant cells in the joint for full progression of destructive synovitis

IL-17 is a proinflammatory cytokine suspected to be involved in inflammatory and autoimmune diseases such as rheumatoid arthritis. In the present study, we report that IL-17R signaling is required in radiation-resistant cells in the joint for full progression of chronic synovitis and bone erosion. Repeated injections of Gram-positive bacterial cell wall fragments (streptococcal cell wall) directly into the knee joint of naive IL-17R-deficient (IL-17R-/-) mice had no effect on the acute phase of arthritis but prevented progression to chronic destructive synovitis as was noted in wild-type (wt) mice. Microarray analysis revealed significant down-regulation of leukocyte-specific chemokines, selectins, cytokines, and collagenase-3 in the synovium of IL-17R-/- mice. Bone marrow (BM) chimeric mice revealed the need for IL-17R expression on radiation-resistant joint cells for destructive inflammation. Chimeric mice of host wt and donor IL-17R-/- BM cells developed destructive synovitis in this chronic reactivated streptococcal cell wall arthritis model similar to wt-->wt chimeras. In contrast, chimeric mice of host IL-17R-/- and donor wt BM cells were protected from chronic destructive arthritis similar as IL-17R-/- -->IL-17R-/- chimeras. These data strongly indicate that IL-17R signaling in radiation-resistant cells in the joint is required for turning an acute macrophage-mediated inflammation into a chronic destructive synovitis.

A distinct lineage of CD4 T cells regulates tissue inflammation by producing interleukin 17

Interleukin 17 (IL-17) has been linked to autoimmune diseases, although its regulation and function have remained unclear. Here we have evaluated in vitro and in vivo the requirements for the differentiation of naive CD4 T cells into effector T helper cells that produce IL-17. This process required the costimulatory molecules CD28 and ICOS but was independent of the cytokines and transcription factors required for T helper type 1 or type 2 differentiation. Furthermore, both IL-4 and interferon-gamma negatively regulated T helper cell production of IL-17 in the effector phase. In vivo, antibody to IL-17 inhibited chemokine expression in the brain during experimental autoimmune encephalomyelitis, whereas overexpression of IL-17 in lung epithelium caused chemokine production and leukocyte infiltration. Thus, IL-17 expression characterizes a unique T helper lineage that regulates tissue inflammation.

IL-17 suppresses TNF-alpha-induced CCL27 production through induction of COX-2 in human keratinocytes

BACKGROUND

The chemokine CCL27 attracts skin-homing T cells. CCL27 production by keratinocytes is dependent on nuclear factor kappaB (NF-kappaB) activity and enhanced in lesions of patients with atopic dermatitis, psoriasis vulgaris, or allergic contact dermatitis. IL-17 is released from activated memory T cells and modulates skin inflammation.

OBJECTIVE

We examined the in vitro effects of IL-17 on TNF-alpha-induced CCL27 production in human keratinocytes.

METHODS

Keratinocytes were incubated with TNF-alpha, IL-17, or both. CCL27 secretion and mRNA levels were analyzed by means of ELISA and RT-PCR, respectively. COX-2 promoter and NF-kappaB activities were analyzed by using luciferase assays. COX-2 protein levels were analyzed by means of Western blotting.

RESULTS

IL-17 suppressed TNF-alpha-induced CCL27 secretion and mRNA expression and NF-kappaB activity in keratinocytes. The COX-2 inhibitor NS398 counteracted the effects of IL-17, and prostaglandin E(2) prevented counteraction by NS398. IL-17 alone or synergistically with TNF-alpha increased prostaglandin E(2) release from keratinocytes, and the increase was suppressed by NS398. IL-17 alone or synergistically with TNF-alpha increased COX-2 mRNA and protein levels, promoter activity, and mRNA stability. The stimulatory effects of IL-17 on COX-2 expression were suppressed by inhibitors of p38 mitogen-activated protein kinase (MAPK) and extracellular signal-regulated kinase (ERK) kinase. IL-17 alone or synergistically with TNF-alpha induced dual phosphorylation of p38 MAPK and ERK.

CONCLUSION

IL-17 might suppress TNF-alpha-induced CCL27 production by inhibiting NF-kappaB through induction of COX-2. The induction of COX-2 might be mediated by activation of p38 MAPK and ERK. T cell-derived IL-17 might alleviate T-cell skin infiltration through inhibition of CCL27 production.

Interleukin-17 is a potent immuno-modulator and regulator of normal human intestinal epithelial cell growth

Upregulation of the T-cell derived cytokine interleukin (IL-17) was reported in the inflamed intestinal mucosa of patients with inflammatory bowel disorders. In this study, we analyzed the effect of IL-17 on human intestinal epithelial cell (HIEC) turnover and functions. Proliferation and apoptosis in response to IL-17 was monitored in HIEC (cell counts, [(3)H]thymidine incorporation method, and annexinV-PI-apoptosis assay). Signalling pathways were analyzed by Western blots, electromobility shift assay, and immunofluorescence studies. IL-17 proved to be a potent inhibitor of HIEC proliferation without any pro-apoptotic/necrotic effect. The growth inhibitory effect of IL-17 was mediated via the p38 stress kinase. Consequently, the p38-SAPkinase-inhibitor SB203580 abrogated this anti-mitotic effect. In parallel, IL-17 provoked the degradation of IkappaBalpha, allowing nuclear translocation of the p65 NF-kappaB subunit and induction of the NF-kappaB-controlled genes IL-6 and -8. IL-17 potently blocks epithelial cell turnover while at the same time amplifying an inflammatory response in a positive feedback manner.

Interleukin-17-Induced Interleukin-8 Release in Human Airway Smooth Muscle Cells: Role for Mitogen-Activated Kinases and Nuclear Factor-κB

BACKGROUND

It has recently become clear that interleukin (IL)-8 plays a role in chronic neutrophilic inflammatory disorders, such as chronic rejection after lung transplantation. We have shown that IL-17--stimulated human airway smooth muscle cells (HASMC) are able to produce IL-8. The aim of this study was to determine whether p38 mitogen-activated protein kinase (MAPK), c-Jun amino-terminal kinase (JNK), p42/p44 extracellular signal-related kinase (ERK) and nuclear factor-kappaB (NF-kappaB) are involved in IL-17--induced IL-8 production in HASMC in vitro.

METHODS

We used human airway smooth muscle cells in culture. Western blotting was done to obtain data regarding activation of MAPK. Furthermore, we used specific inhibitors of MAPK to investigate their involvement in IL-17--induced IL-8 release, which was measured by enzyme-linked immunosorbent assay (ELISA).

RESULTS

Western blotting clearly demonstrated that p38 MAPK, JNK and p42/p44 ERK were activated by IL-17 in HASMC. Using SB203580, a specific inhibitor of p38 MAPK, we detected a concentration-dependent inhibition of IL-17--induced IL-8 production with a maximal decrease of 63 +/- 5% (n=8, p<0.01). Curcumin, a specific inhibitor of JNK, also concentration-dependently reduced IL-17--induced IL-8 production, with a maximal decrease of 82+/-4% (n=8, p<0.01). U0126, a specific inhibitor of p42/p44 ERK, induced a maximal decrease of 84+/-5% (n=8, p<0.001). Pyrrolydine dithiocarbamate (PDTC), an inhibitor of NF-kappaB, caused a 70+/-5% (n=8, p<0.01) decrease in IL-17--induced IL-8 production.

CONCLUSIONS

We found that IL-17 induces activation of p38MAPK, JNK and p42/p44ERK in HASMC. We also found that p38MAPK, JNK, p42/p44 ERK and NF-kappaB play an important role in IL-17--induced IL-8 production in HASMC in vitro. This may open up new opportunities for further treatment of this disease.

NF-κB Mediated IL-6 Production by Renal Epithelial Cells Is Regulated by C-Jun NH2-Terminal Kinase

Tubular epithelial cells (TEC) play an important role in tubulointerstitial inflammation, a hallmark of most renal diseases, via production of cytokines and chemokines. In this study, the role of mitogen-activated protein kinases (MAPK) in regulation of the proinflammatory cytokine IL-6 in cultured human TEC in response to the leukocyte-derived factors IL-1, TNF-alpha, IL-17, and CD40L was investigated. IL-6 production induced by IL-1, TNF-alpha, and IL-17 was specifically inhibited by the c-jun NH(2)-terminal kinase (JNK) inhibitor SP600125, but not by a selective inhibitor of p38 MAPK, and was moderately increased when the ERK1/2 pathway was inhibited. Also for CD40L stimulation, inhibition of JNK resulted in a pronounced inhibition of IL-6 production. Although stimulation of TEC induced activation of activator protein-1 (AP-1), the down-stream target of JNK, reporter assays demonstrated that mutation of the AP-1 binding site in the IL-6 promoter did not affect gene transcription. Furthermore, IL-1-induced transcriptional activation of the IL-6 promotor was repressed by SP600125 or by co-transfection of a dominant-negative expression plasmid of c-jun even in the absence of a functional AP-1 binding site. This suggests that IL-6 production by renal epithelial cells is regulated by JNK, via a mechanism, however, independent of the AP-1 binding site. The data rather suggest that the JNK pathway may interfere with other signaling pathways, involving NF-kappaB and possibly ERK.

Cloning and characterization of Swine interleukin-17, preferentially expressed in the intestines

Interleukin-17 (IL-17), initially reported as CTLA-8, is a proinflammatory cytokine produced mainly by activated T cells. In the present study, the cDNA of a swine IL-17 (PoIL-17) gene was cloned from activated neonatal thymocytes, and the recombinant PoIL-17 (rPoIL-17) was biologically characterized. The complete open reading frame (ORF) of PoIL-17 contains 462-bp coding deduced 153 amino acid residues, with a calculated molecular weight of 17.3 kDa. The amino acid sequence showed 72.9%, 64.9%, 64.7%, 60.1%, and 47.4% similarities with that of human, rat, mouse, Herpesvirus saimiri ORF 13, and chicken, respectively. The six cysteine residues conserved over species including the virus were observed in PoIL-17. We successfully prepared the recombinant mature form of PoIL-17 and analyzed its biologic activities for swine splenocytes. RT-PCR analysis revealed a marked upregulation of expression of IL-1beta, IL-8, tumor necrosis factor-alpha (TNF-alpha), granulocyte colony-stimulating factor (G-CSF), and monocyte chemotactic protein-1 (MCP-1) mRNA expression in splenocytes treated with 100 ng/ml rPoIL-17 for 3 h. Furthermore, a swine chemokine, alveolar macrophage-derived neutrophil chemotactic factor II (AMCF-II), which was classified into the CXC subfamily was also augmented in mRNA level. This evidence indicates that recombinat PoIL-17 expressed in Escherichia coli was biologically active and exerted similar effects to those of a human (HuIL-17) and murine IL-17 (MuIL-17). The PoIL-17 mRNA is strongly expressed in the adult heart, skin, and, interestingly, intestinal tissues, including mesenteric lymph nodes but is restricted in neonatal tissues by using real-time quantitative RT-PCR. The gene sequence and biologically active recombinat protein for PoIL-17 will be useful for elucidation of the role of IL-17 in the regulation of intestinal immune responses.

IL-17 cytokine family

A new family of cytokines, IL-17, has recently been defined that reveals a distinct ligand-receptor signaling system. Functional studies have provided evidence for its importance in the regulation of immune responses. Notably, 3 members, IL-17A, IL-17E (IL-25), and IL-17F, have been best characterized both in vitro and in vivo , and have been shown to be proinflammatory in nature. This proinflammatory activity is exemplified by their involvement in pulmonary inflammatory responses, in which both IL-17A and IL-17F are involved in the recruitment of neutrophils, and IL-17E is able to induce T H 2 cytokine production and eosinophilia. Although the elucidation of a detailed mechanism of action continues to be an active area of research, the potent inflammatory activity and its association with various human disease states suggest this new cytokine family as an important contributor to the pathophysiology of human disease conditions, in particular the pulmonary diseases.

Early growth response factor-1 mediates prostaglandin E2-dependent transcriptional suppression of cytokine-induced tumor necrosis factor-alpha gene expression in human macrophages and rheumatoid arthritis-affected synovial fibroblasts

Tumor necrosis factor-alpha (TNF-alpha) is a pleiotropic proinflammatory cytokine that modulates a broad range of inflammatory and immunological processes. We have investigated the potential immunomodulatory properties of prostaglandin E2 (PGE2) by examining the molecular mechanism by which the eicosanoid suppresses T-cell-derived interleukin-17 (IL-17)-induced TNF-alpha mRNA expression and protein synthesis in human macrophages and rheumatoid arthritis-affected synovial fibroblasts. Initial studies confirmed that PGE2 induces egr-1 mRNA expression and protein synthesis by restricted SAPK2/p38 MAPK-dependent activating transcription factor-2 (ATF-2) dimer transactivation of the egr-1 promoter as judged by studies using wild-type (WT) and deletion mutant egr-1 promoter constructs, Northern and Western blotting, and standard and supershift electrophoretic mobility shift analyses. Using human leukemic monocytic THP-1 cells stably transfected with WT and dominant-negative mutant expression constructs of Egr-1, cotransfected or not with a WT pTNF-615SVOCAT construct, we observed that PGE2 inhibition of IL-17-stimulated TNF-alpha mRNA expression and promoter activity was dependent on Egr-1 expression, as mutants of Egr-1, alone or in combination, markedly abrogated any inhibitory effect of PGE2. Standard and supershift electrophoretic mobility shift analysis, signaling "decoy" overexpression studies, and pTNF-615SVOCAT promoter assays using WT and mutant promoter constructs revealed that IL-17-up-regulated promoter activity was largely dependent on ATF-2/c-Jun transactivation. PGE2 suppression of IL-17-induced ATF-2/c-Jun transactivation and DNA binding was dependent on Egr-1-mediated inhibition of induced c-Jun expression. We suggest that egr-1 is an immediate-early PGE2 target gene that may be a key regulatory factor in mediating eicosanoid control of genes involved in the immune and inflammatory responses.

Cytokines link osteoblasts and inflammation: microarray analysis of interleukin-17- and TNF-alpha-induced genes in bone cells

The novel cytokine interleukin (IL)-17 has been implicated in many infectious and autoimmune settings, especially rheumatoid arthritis. Consistent with its proinflammatory effects on bone, osteoblast cells are highly responsive to IL-17, particularly in combination with other inflammatory cytokines. To better understand the spectrum of activities controlled by IL-17, we globally profiled genes regulated by IL-17 and tumor necrosis factor alpha (TNF-alpha) in the preosteoblast cell line MC3T3-E1. Using Affymetrix microarrays, 80-90 genes were up-regulated, and 19-50 genes were down-regulated with IL-17 and TNF-alpha as compared with TNF-alpha alone. These included proinflammatory chemokines and cytokines, inflammatory genes, transcriptional regulators, bone-remodeling genes, signal transducers, cytoskeletal genes, genes involved in apoptosis, and several unknown or unclassified genes. The CXC family chemokines were most dramatically induced by IL-17 and TNF-alpha, confirming the role of IL-17 as a potent mediator of inflammation and neutrophil recruitment. Several transcription factor-related genes involved in inflammatory gene expression were also enhanced, including molecule possessing ankyrin repeats induced by lipopolysaccharide/inhibitor of kappaBzeta (MAIL/kappaBzeta), CCAAT/enhancer-binding protein delta (C/EBPdelta), and C/EBPbeta. We also identified the acute-phase gene lipocalin-2 (LCN2/24p3) as a novel IL-17 target, which is regulated synergistically by TNF-alpha and IL-17 at the level of its promoter. A similar but not identical pattern of genes was induced by IL-17 and TNF-alpha in ST2 bone marrow stromal cells and murine embryonic fibroblasts. This study provides a profile of genes regulated by IL-17 and TNF-alpha in osteoblasts and suggests that in bone, the major function of IL-17 is to cooperate and/or synergize with other cytokines to amplify inflammation.

The role of T-cell interleukin-17 in conducting destructive arthritis: lessons from animal models

Interleukin-17 (IL-17) is a T cell cytokine spontaneously produced by cultures of rheumatoid arthritis (RA) synovial membranes. High levels have been detected in the synovial fluid of patients with RA. The trigger for IL-17 is not fully identified; however, IL-23 promotes the production of IL-17 and a strong correlation between IL-15 and IL-17 levels in synovial fluid has been observed. IL-17 is a potent inducer of various cytokines such as tumor necrosis factor (TNF)-alpha, IL-1, and receptor activator of NF-kappaB ligand (RANKL). Additive or even synergistic effects with IL-1 and TNF-alpha in inducing cytokine expression and joint damage have been shown in vitro and in vivo. This review describes the role of IL-17 in the pathogenesis of destructive arthritis with a major focus on studies in vivo in arthritis models. From these studies in vivo it can be concluded that IL-17 becomes significant when T cells are a major element of the arthritis process. Moreover, IL-17 has the capacity to induce joint destruction in an IL-1-independent manner and can bypass TNF-dependent arthritis. Anti-IL-17 cytokine therapy is of interest as an additional new anti-rheumatic strategy for RA, in particular in situations in which elevated IL-17 might attenuate the response to anti-TNF/anti-IL-1 therapy.

Stimulus-specific induction of a novel nuclear factor-kappaB regulator, IkappaB-zeta, via Toll/Interleukin-1 receptor is mediated by mRNA stabilization

We have recently identified an inducible nuclear factor-kappaB (NF-kappaB) regulator, IkappaB-zeta, which is induced by microbial ligands for Toll-like receptors such as lipopolysaccharide and the proinflammatory cytokine interleukin (IL)-1beta but not by tumor necrosis factor (TNF)-alpha. In the present study, we examined mechanisms for stimulus-specific induction of IkappaB-zeta. The analysis of the IkappaB-zeta promoter revealed an essential role for an NF-kappaB binding sequence in transcriptional activation. The activation, however, did not account for the Toll-like receptor/IL-1 receptor-specific induction of IkappaB-zeta, because the promoter analysis and nuclear run-on analysis indicated that its transcription was similarly induced by TNF-alpha. To examine post-transcriptional regulation, we analyzed the decay of IkappaB-zeta mRNA, and we found that it was specifically stabilized by lipopolysaccharide or IL-1beta but not by TNF-alpha. Furthermore, we found that costimulation with TNF-alpha and another proinflammatory cytokine, IL-17, elicited the IkappaB-zeta induction. Stimulation with IL-17 alone did not induce IkappaB-zeta but stabilized its mRNA. Therefore, IkappaB-zeta induction requires both NF-kappaB activation and stimulus-specific stabilization of its mRNA. Because IkappaB-zeta is essential for expression of a subset of NF-kappaB target genes, the stimulus-specific induction of IkappaB-zeta may be of great significance in regulation of inflammatory reactions.

Interleukin-17: the missing link between T-cell accumulation and effector cell actions in rheumatoid arthritis?

The prominence of T cells and monocyte/macrophages in rheumatoid synovium suggests T cells may localize and amplify the effector functions of monocyte/macrophages in rheumatoid disease. However, while T cells are abundant in rheumatoid joints, classic T-cell derived cytokines are scarce, especially when compared to the levels of monokines IL-1 beta and TNF-alpha. For this reason, it has been speculated that monocyte/macrophages may act independently of T cells in rheumatoid disease and that the role of T cells may be more or less irrelevant to core disease mechanisms. The question of T-cell influence requires re-evaluation in light of the characterization of IL-17, a T-cell derived cytokine that is abundant in rheumatoid synovium and synovial fluid. IL-17 has a number of pro-inflammatory effects, both directly and through amplification of the effects of IL-1 beta and TNF-alpha. IL-17 is able to induce expression of pro-inflammatory cytokines and stimulate release of eicosanoids by monocytes and synoviocytes. Furthermore, IL-17 has been implicated in the pathogenesis of inflammatory bone and joint damage through induction of matrix metalloproteinases and osteoclasts, as well as inhibition of proteoglycan synthesis. In animal models of arthritis, intra-articular injection of IL-17 results in joint inflammation and damage. The recognition of IL-17 as a pro-inflammatory T cell derived cytokine, and its abundance within rheumatoid joints, provides the strongest candidate mechanism to date through which T cells can capture and localize macrophage effector functions in rheumatoid arthritis. As such, IL-17 warrants consideration for its potential as a therapeutic target in rheumatoid arthritis.

Biology of recently discovered cytokines: interleukin-17--a unique inflammatory cytokine with roles in bone biology and arthritis

IL-17 and its receptor are founding members of an emerging family of cytokines and receptors with many unique characteristics. IL-17 is produced primarily by T cells, particularly those of the memory compartment. In contrast, IL-17 receptor is ubiquitously expressed, making nearly all cells potential targets of IL-17. Although it has only limited homology to other cytokines, IL-17 exhibits proinflammatory properties similar to those of tumor necrosis factor-alpha, particularly with respect to induction of other inflammatory effectors. In addition, IL-17 synergizes potently with other cytokines, placing it in the center of the inflammatory network. Strikingly, IL-17 has been associated with several bone pathologies, most notably rheumatoid arthritis.

Inherited disorders of NF-kappaB-mediated immunity in man

The transcription factors of the NF-kappaB family play an important role in immunity to infection in animal models. Three human primary immunodeficiencies associated with impaired NF-kappaB signaling were recently described. X-linked recessive anhidrotic ectodermal dysplasia with immunodeficiency (XL-EDA-ID) is caused by hypomorphic mutations in the gene encoding NEMO/IKKgamma, the regulatory subunit of the IkappaB-kinase (IKK) complex. Autosomal dominant EDA-ID (AD-EDA-ID) is caused by a hypermorphic mutation in the gene encoding the inhibitory protein IkappaBalpha. Autosomal recessive immunodeficiency without EDA is caused by mutations in the gene encoding IRAK-4, a kinase acting upstream from the IKK complex in the TIR signaling pathway. The description of the infectious phenotypes associated with these genetic defects has initiated the forward genetic dissection of NF-kappaB-mediated immunity in man.

Mechanism of steroid action in renal epithelial cells

Renal tubular epithelial cells (TEC) are thought to play an active role in tubulointerstitial inflammation. Various immune and non-immune factors activate TEC to produce a variety of cytokines and chemokines, contributing to attraction of inflammatory cells to the kidney. The proinflammatory transcription factor nuclear factor-kappaB (NF-kappaB) appears to be a key player in these responses and tubular expression of NF-kappaB has been demonstrated in vitro and in vivo. Although glucocorticoids are known to inhibit NF-kappaB activation at different levels, the proinflammatory capacity of TEC was not inhibited. In contrast, glucocorticoids seemed to enhance the profibrotic response of TEC, emphasizing the cell-type specific characteristics of glucocorticoid action. We propose that specific inhibition of NF-kappaB activation in TEC might be an attractive strategy for therapeutic intervention in renal inflammation.

Analogies between Drosophila and mammalian TRAF pathways

A central event in innate immunity is the activation of the NF-kappaB signaling pathway and up-regulation of NF-kappaB-dependent defense genes. Attack of mammals as well as of insects by microorganisms leads, among other things, to the activation of receptors of the Toll-like receptor group. Various adaptor proteins involving members of the TNF receptor-associated factor (TRAF) family channel these receptor-generated signals to conserved intracellular kinase cascades that finally lead to the activation of NF-kappaB and JNK. In vertebrates, TRAF proteins link these pathways also to IL-1R-related molecules and members of the TNF receptor superfamily, which orchestrate a variety of immunoregulatory processes of the innate but also of the adaptive immune system. In this review, we will focus on the similarities but also the differences in TRAF-dependent signaling pathways of mammals and insects.

TRAF6, a molecular bridge spanning adaptive immunity, innate immunity and osteoimmunology

Tumor necrosis factor (TNF) receptor associated factor 6 (TRAF6) is a crucial signaling molecule regulating a diverse array of physiological processes, including adaptive immunity, innate immunity, bone metabolism and the development of several tissues including lymph nodes, mammary glands, skin and the central nervous system. It is a member of a group of six closely related TRAF proteins, which serve as adapter molecules, coupling the TNF receptor (TNFR) superfamily to intracellular signaling events. Among the TRAF proteins, TRAF6 is unique in that, in addition to mediating TNFR family signaling, it is also essential for signaling downstream of an unrelated family of receptors, the interleukin-1 (IL-1) receptor/Toll-like receptor (IL-1R/TLR) superfamily. Gene targeting experiments have identified several indispensable physiological functions of TRAF6, and structural and biochemical studies have revealed the potential mechanisms of its action. By virtue of its many signaling roles, TRAF6 represents an important target in the regulation of many disease processes, including immunity, inflammation and osteoporosis.

Treatment with a neutralizing anti-murine interleukin-17 antibody after the onset of collagen-induced arthritis reduces joint inflammation, cartilage destruction, and bone erosion

OBJECTIVE

Interleukin-17 (IL-17) is a proinflammatory cytokine that is expressed in the synovium of rheumatoid arthritis (RA) patients. This T cell cytokine is implicated in the initiation phase of arthritis. However, the role of IL-17 during the effector phase of arthritis has still not been identified; this was the objective of the present study.

METHODS

Mice with collagen-induced arthritis (CIA) were treated with polyclonal rabbit anti-murine IL-17 (anti-IL-17) antibody-positive serum or normal rabbit serum after the first signs of arthritis. In addition, during a later stage of CIA mice were selected and treated with anti-IL-17 antibody or control serum. Arthritis was monitored visually, and joint pathology was examined radiologically and histologically. Systemic IL-6 levels were measured by enzyme-linked immunosorbent assay, and local synovial IL-1 and receptor activator of NF-kappaB ligand (RANKL) expression was analyzed using specific immunohistochemistry.

RESULTS

Treatment with a neutralizing anti-IL-17 antibody after the onset of CIA significantly reduced the severity of CIA. Radiographic analysis revealed marked suppression of joint damage in the knee and ankle joints. Histologic analysis confirmed the suppression of joint inflammation and showed prevention of cartilage and bone destruction after anti-IL-17 antibody therapy. Systemic IL-6 levels were significantly reduced after anti-IL-17 antibody treatment. Moreover, fewer IL-1beta-positive and RANKL-positive cells were detected in the synovium after treatment with neutralizing IL-17. Interestingly, initiation of anti-IL-17 antibody therapy during a later stage of CIA, using mice with higher clinical arthritis scores, still significantly slowed the progression of the disease.

CONCLUSION

IL-17 plays a role in early stages of arthritis, but also later during disease progression. Systemic IL-6 was reduced and fewer synovial IL-1-positive and RANKL-positive cells were detected after neutralizing endogenous IL-17 treatment, suggesting both IL-1-dependent and IL-1-independent mechanisms of action. Our data strongly indicate that IL-17 neutralization could provide an additional therapeutic strategy for RA, particularly in situations in which elevated IL-17 may attenuate the response to anti-tumor necrosis factor/anti-IL-1 therapy.

TNF receptor-associated factor 6 deficiency during hemopoiesis induces Th2-polarized inflammatory disease

Toll-like receptors (TLR) initiate rapid innate immune responses by recognizing microbial products. These events in turn lead to the development of an efficient adaptive immune response through the up-regulation of a number of costimulatory molecules, including members of the TNF/TNFR superfamily, on the surface of an APC. TNFR-associated factor 6 (TRAF6) is a common signaling adapter used by members of both the TNFR and the TLR/IL-1R superfamilies, and as such plays a critical role in the development of immune responses. As TRAF6-deficient mice die prematurely, we generated chimeras reconstituted with TRAF6-deficient fetal liver cells to analyze functions of TRAF6 in vivo in the hemopoietic compartment. We found that TRAF6-deficient chimeras develop a progressive lethal inflammatory disease associated with massive organ infiltration and activation of CD4(+) T cells in a Th2-polarized phenotype, and a defect in IL-18 responsiveness. When recombination-activating gene 2(-/-) blastocysts were complemented with TRAF6-deficient embryonic stem cells, a marked elevation of activated CD4(+) T cells and progressive inflammatory disease were also observed. Moreover, T cell activation and lethal inflammation were not reversed in mixed chimeric mice generated from normal and TRAF6-deficient fetal liver cells. These results suggest that deletion of TRAF6 induces a dominant Th2-type polarized autoimmune response. Therefore, in addition to playing a critical role in innate and adaptive immunity, TRAF6 is likely to play a previously unrecognized role in the maintenance of self-tolerance.