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

Helicobacter pylori Regulates the Apoptosis of Human Megakaryocyte Cells via NF-κB/IL-17 Signaling

Objective

To investigate the role of Helicobacter pylori (HP) on the regulation of NF-κB/IL-17 signaling, mechanisms underlying apoptosis in human megakaryocyte cell lines Dami.

Methods

Firstly, the mouse model of immune thrombocytopenia (ITP) was established. Then, the mice were subjected to gastric perfusion with HP. Next, the changes in platelet and bone marrow megakaryocyte classification were assessed in each group. Human megakaryocyte Dami cells were treated with HP in vitro for 3, 6, or 9 h; and the rates of apoptosis in each group were then evaluated with flow cytometry. Fluorescent quantitative PCR and Western blotting were implemented for assessing the expression of Bcl-2 and Bax, which are related to apoptosis, and p65, which is associated with the NF-κB pathway. The expression of these proteins was also evaluated after treatment with PDTC, an inhibitor of the NF-κB pathway inhibitor.

Results

In vivo, exogenous administration of HP was found to increase the optical density value for the anti-HP antibody in HP-infected BALB/c mice. Meanwhile, the platelet counts in the HP-infected ITP mice model were significantly reduced compared with non-infected ITP mice. In vitro, the apoptotic rate of Dami cells increased gradually with the prolongation of the exposure to HP; the most noticeable change was at 6 h, and there was a significant difference between 0 h and 6 h. The expression of Bax, p-p65, and IL-17 also increased progressively with the prolongation HP exposure, while the expression of anti-apoptotic Bcl-2 protein decreased gradually, especially at 6 h, and the expression of total p65 did not change significantly compared with baseline. Anecdotally, these effects were reversed by the application of pyrrolidine dithiocarbamate (PDTC), an inhibitor of NF-κB signaling.

Conclusion

HP can promote platelet destruction in ITP mice, and the underlying mechanisms may be related to accelerating apoptosis of megakaryocytes by the activation of the NF-κB/IL-17 pathway.

Human inborn errors of immunity underlying superficial or invasive candidiasis

Candida species, including C. albicans in particular, can cause superficial or invasive disease, often in patients with known acquired immunodeficiencies or iatrogenic conditions. The molecular and cellular basis of these infections in patients with such risk factors remained largely elusive, until the study of inborn errors of immunity clarified the basis of the corresponding inherited and "idiopathic" infections. Superficial candidiasis, also known as chronic mucocutaneous candidiasis (CMC), can be caused by inborn errors of IL-17 immunity. Invasive candidiasis can be caused by inborn errors of CARD9 immunity. In this chapter, we review both groups of inborn errors of immunity, and discuss the contribution of these studies to the deciphering of the critical mechanisms of anti-Candida immunity in patients with other conditions.

Recombinant human IL-26 facilitates the innate immune response to endotoxin in the bronchoalveolar space of mice in vivo

Interleukin (IL)-26 is released in response to bacterial endotoxin (LPS) in the bronchoalveolar space of humans in vivo and it may potentiate neutrophil chemotaxis by enhanced IL-26 receptor stimulation. However, the effects of extracellular IL-26 protein on the innate immune response in the lungs in vivo remain unknown. Here, we characterized these effects of IL-26 on a wide range of aspects of the innate immune response to LPS in different compartments of the lungs in vivo over time. We administrated recombinant human (rh) IL-26 protein in the bronchoalveolar space using intranasal instillation in a mouse in vivo model, with and without prior instillation of LPS. We verified gene expression of the IL-26 receptor complex in mouse lungs and observed that, after instillation of LPS, rhIL-26 increases the phosphorylation of STAT3, a signaling molecule of the IL-26 receptor complex. We also observed that rhIL-26 exerted additional stimulatory and inhibitory actions that are compartment- and time-dependent, resulting in alterations of cytokines, proteinases, tissue inflammation and the accumulation of innate effector cells. Without the prior instillation of LPS, rhIL-26 exerted time-dependent effects on total gelatinase activity but few other effects. Most important, after instillation of LPS, rhIL-26 cleared inflammatory cells from local tissue and increased the accumulation of innate effector cells in the bronchoalveolar space. Tentatively, rhIL-26 may facilitate the innate immune response towards the bronchoalveolar space in vivo and represents a potential target for therapy in lung disorders involving the innate immune response.

Dual Role of Act1 in Keratinocyte Differentiation and Host Defense: TRAF3IP2 Silencing Alters Keratinocyte Differentiation and Inhibits IL-17 Responses

TRAF3IP2 is a candidate psoriasis susceptibility gene encoding Act1, an adaptor protein with ubiquitin ligase activity that couples the IL-17 receptor to downstream signaling pathways. We investigated the role of Act1 in keratinocyte responses to IL-17 using a tetracycline inducible short hairpin RNA targeting TRAF3IP2. Tetracycline exposure for 7 days effectively silenced TRAF3IP2 mRNA and Act1 protein, resulting in 761 genes with significant changes in expression (495 down, 266 up; >1.5-fold, P < 0.05). Gene ontology analysis showed that genes affected by TRAF3IP2 silencing are involved in epidermal differentiation, with early differentiation genes (KRT1, KRT10, DSC1, DSG1) being down-regulated and late differentiation genes (SPRR2, SPRR3, LCE3) being up-regulated. AP1 binding sites were enriched upstream of genes up-regulated by TRAF3IP2 silencing. Correspondingly, nuclear expression of FosB and Fra1 was increased in TRAF3IP2-silenced cells. Many genes involved in host defense were induced by IL-17 in a TRAF3IP2-dependent fashion. Inflammatory differentiation conditions (serum addition for 4 days postconfluence) markedly amplified these IL-17 responses and increased basal levels and TRAF3IP2 silencing-dependent up-regulation of multiple late differentiation genes. These findings suggest that TRAF3IP2 may alter both epidermal homeostasis and keratinocyte defense responses to influence psoriasis risk.

Molecular mechanisms and cell targets of Th17 cells in the gastrointestinal tract: an innate sense of adaptivity

T-helper (TH) 17 activation is crucial for protective immune responses against bacteria and fungi at mucosal surfaces, but it can also be implicated in the pathogenesis of several autoimmune and chronic inflammatory diseases, such as inflammatory bowel diseases (IBD). Although rapid progress was made elucidating induction and functional heterogeneity of Th17 responses, the underlying molecular effects of Th17 response including the most relevant different cell targets of Th17 cytokines remain poorly understood. Cytokines produced by Th17 cells have broad effects on both hematopoietic and nonhematopoietic cells and can act in synergy with various inflammatory factors. In this review, we will focus on the effects of Th17-derived cytokines in the gastrointestinal tract and discuss how Th17 responses can affect both innate and adaptive immunity and may contribute to the pathogenesis of inflammatory GI processes.

The anaphase-promoting complex protein 5 (AnapC5) associates with A20 and inhibits IL-17-mediated signal transduction

IL-17 is the founding member of a family of cytokines and receptors with unique structures and signaling properties. IL-17 is the signature cytokine of Th17 cells, a relatively new T cell population that promotes inflammation in settings of infection and autoimmunity. Despite advances in understanding Th17 cells, mechanisms of IL-17-mediated signal transduction are less well defined. IL-17 signaling requires contributions from two receptor subunits, IL-17RA and IL-17RC. Mutants of IL-17RC lacking the cytoplasmic domain are nonfunctional, indicating that IL-17RC provides essential but poorly understood signaling contributions to IL-17-mediated signaling. To better understand the role of IL-17RC in signaling, we performed a yeast 2-hybrid screen to identify novel proteins associated with the IL-17RC cytoplasmic tail. One of the most frequent candidates was the anaphase promoting complex protein 7 (APC7 or AnapC7), which interacted with both IL-17RC and IL-17RA. Knockdown of AnapC7 by siRNA silencing exerted no detectable impact on IL-17 signaling. However, AnapC5, which associates with AnapC7, was also able to bind IL-17RA and IL-17RC. Moreover, AnapC5 silencing enhanced IL-17-induced gene expression, suggesting an inhibitory activity. Strikingly, AnapC5 also associated with A20 (TNFAIP3), a recently-identified negative feedback regulator of IL-17 signal transduction. IL-17 signaling was not impacted by knockdown of Itch or TAXBP1, scaffolding proteins that mediate A20 inhibition in the TNFα and IL-1 signaling pathways. These data suggest a model in which AnapC5, rather than TAX1BP1 and Itch, is a novel adaptor and negative regulator of IL-17 signaling pathways.

The deubiquitinase A20 mediates feedback inhibition of interleukin-17 receptor signaling

The proinflammatory cytokine interleukin-17 (IL-17) is the signature cytokine of the T helper 17 (TH17) subset of CD4(+) T cells, and antibodies targeting IL-17 or the IL-17 receptor (IL-17R) show clinical efficacy in several autoimmune diseases. Although important for protective immunity against microorganisms, IL-17 causes collateral damage in inflammatory settings. TNFAIP3 encodes the deubiquitinase A20 and is genetically linked to numerous autoimmune syndromes. A20, a potent inhibitor of tumor necrosis factor-α signaling, removes ubiquitin from signaling intermediates upstream of nuclear factor κB (NF-κB), thereby dampening NF-κB-mediated inflammation. We demonstrated that IL-17 stimulates TNFAIP3 expression. Enhanced IL-17-mediated induction of genes encoding proinflammatory factors, including IL-6 and various chemokines, occurred upon knockdown of A20 with short inhibitory RNA or in A20(-/-) cells. A20 associated with the E3 ubiquitin ligase TRAF6 (tumor necrosis factor receptor-associated factor 6) in an IL-17-dependent manner and restricted the IL-17-dependent activation of NF-κB and mitogen-activated protein kinases. A20 interacted directly with the distal domain of IL-17RA, a previously defined inhibitory domain. Together, these data describe a mechanism of restraining IL-17 signaling and reveal an aspect of A20 activity that may help to explain its role in autoimmunity in humans.

IL-17 mediates estrogen-deficient osteoporosis in an Act1-dependent manner

Estrogen-deficient osteoporosis may be an inflammatory disorder and we therefore asked if IL-17 participates in its pathogenesis. Deletion of the principal IL-17 receptor (IL-17RA) protects mice from ovariectomy (OVX)-induced bone loss. Further supporting a central role of IL-17 in its pathogenesis, OVX-induced osteoporosis is prevented by a blocking antibody targeting the cytokine. IL-17 promotes osteoclastogenesis by stimulating RANK ligand (RANKL) expression by osteoblastic cells, mediated by the IL-17RA SEFIR/TILL domain. Estrogen deprivation, however does not enhance IL-17RA mRNA expression by osteoblasts or in bone, but augments that of Act1, an IL-17RA-interacting protein and signaling mediator. Similar to IL-17RA(-/-) mice, those lacking Act1 are protected from OVX-induced bone loss. Also mirroring IL-17RA-deficiency, absence of Act1 in osteoblasts, but not osteoclasts, impairs osteoclastogenesis via dampened RANKL expression. Transduction of WT Act1 into Act1(-/-) osteoblasts substantially rescues their osteoclastogenic capacity. The same construct, however, lacking its E3 ligase U-box or its SEFIR domain, which interacts with its counterpart in IL-17RA, fails to do so. Estrogen deprivation, therefore, promotes RANKL expression and bone resorption in association with upregulation of the IL-17 effector, Act1, supporting the concept that post-menopausal osteoporosis is a disorder of innate immunity.

Interleukin-17 signaling in inflammatory, Kupffer cells, and hepatic stellate cells exacerbates liver fibrosis in mice

BACKGROUND & AIMS

Interleukin (IL)-17 signaling has been implicated in lung and skin fibrosis. We examined the role of IL-17 signaling in the pathogenesis of liver fibrosis in mice.

METHODS

Using cholestatic and hepatotoxic models of liver injury, we compared the development of liver fibrosis in wild-type mice with that of IL-17RA(-/-) mice and of bone marrow chimeric mice devoid of IL-17 signaling in immune and Kupffer cells (IL-17RA(-/-) to wild-type and IL-17A(-/-) to wild-type mice) or liver resident cells (wild-type to IL-17RA(-/-) mice).

RESULTS

In response to liver injury, levels of Il-17A and its receptor increased. IL-17A increased appeared to promote fibrosis by activating inflammatory and liver resident cells. IL-17 signaling facilitated production of IL-6, IL-1, and tumor necrosis factor-α by inflammatory cells and increased the expression of transforming growth factor-1, a fibrogenic cytokine. IL-17 directly induced production of collagen type I in hepatic stellate cells by activating the signal transducer and activator of transcription 3 (Stat3) signaling pathway. Mice devoid of Stat3 signaling in hepatic stellate cells (GFAPStat3(-/-) mice) were less susceptible to fibrosis. Furthermore, deletion of IL-23 from immune cells attenuated liver fibrosis, whereas deletion of IL-22 exacerbated fibrosis. Administration of IL-22 and IL-17E (IL-25, a negative regulator of IL-23) protected mice from bile duct ligation-induced liver fibrosis.

CONCLUSIONS

IL-17 induces liver fibrosis through multiple mechanisms in mice. Reagents that block these pathways might be developed as therapeutics for patients with cirrhosis.

MicroRNAs and other mechanisms regulate interleukin-17 cytokines and receptors

Interleukin-17 cytokines are a family of pro-inflammatory cytokines. Our current studies found: i) IL-17 cytokines are not ubiquitously expressed, but several receptors and TRAF3IP2 are ubiquitously expressed in tissues with a few exceptions; ii) heart and vascular tissue are in the second tier of readiness to respond to IL-17 cytokine stimulation; iii) alternative transcription starting sites and alternative spliced isoforms are found in IL-17 cytokine and receptor transcripts; iv) higher hypomethylation status is associated with higher expressions of IL-17 receptors; v) the binding sites of several RNA binding proteins are found in the 3'UTRs of the mRNAs of IL-17 cytokines and receptors; and vi) numerous microRNA binding sites are statistically equivalent to that of experimentally verified microRNAs-mRNA interactions in the 3'UTRs of IL-17 cytokine and receptor mRNAs. These results suggest that mechanisms including alternative promoters, alternative splicing, RNA binding proteins, and microRNAs regulate the structures and expressions of IL-17 cytokines and receptors. These results provide an insight into the roles of IL-17 in mediating inflammation and immunity.

Structure and function of interleukin-17 family cytokines

The recently identified interleukin-17 (IL-17) cytokines family, which comprises six members in mammals (IL-17A-F), plays essential roles in the host immunity against infectious diseases and chronic inflammatory diseases. The three-dimensional structures containing IL-17A or IL-17F have become available and revealed the unique structural features of IL-17s as well as their receptors. Molecular modeling in this review shows that IL-17s may adopt a "cysteine knot" fold commonly seen in nerve growth factor (NGF) and other neurotrophins. Further modeling analysis unmasks a signature interaction feature of the IL-17F/IL-17RA complex, where a small loop of IL-17RA slots into the deep groove of the interface of IL-17F homodimer. This is quite different from the interaction between the best known four-helix cytokines and their cognate receptors. On the other hand, structure of IL-17A and its monoclonal antibody (CAT-2200) shows that, albeit that the antigenic epitope of IL-17A resides outside of the IL-17A homodimer interface, its physical proximity to the receptor binding groove may explain that antibody blockage would be achieved by interfering with the ligand-receptor interaction. This review is to summarize the advance in understanding the structure and function of IL-17 family cytokines, focusing mainly on IL-17A, IL-17F and IL-17E, in the hope of gaining better knowledge of immunotherapeutic strategies against various inflammatory diseases.

Modulation of experimental autoimmune encephalomyelitis through TRAF3-mediated suppression of interleukin 17 receptor signaling

By binding to the interleukin 17 receptor (IL-17R), TRAF3 blocks formation of the IL-17R–Act1–TRAF6 complex and inhibits downstream signaling.

Interleukin 17 (IL-17) plays critical roles in the pathogenesis of various autoimmune diseases, including experimental autoimmune encephalomyelitis (EAE). How the signals triggered by this powerful inflammatory cytokine are controlled to avoid abnormal inflammatory responses is not well understood. In this study, we report that TRAF3 is a receptor proximal negative regulator of IL-17 receptor (IL-17R) signaling. TRAF3 greatly suppressed IL-17–induced NF-κB and mitogen-activated protein kinase activation and subsequent production of inflammatory cytokines and chemokines. Mechanistically, the binding of TRAF3 to IL-17R interfered with the formation of the receptor signaling activation complex IL-17R–Act1–TRAF6, resulting in suppression of downstream signaling. TRAF3 markedly inhibited IL-17–induced expression of inflammatory cytokine and chemokine genes in vivo and consequently delayed the onset and greatly reduced the incidence and severity of EAE. Thus, TRAF3 is a negative regulator of IL-17R proximal signaling.

SEF/IL-17R (SEFIR) is not enough: an extended SEFIR domain is required for il-17RA-mediated signal transduction

IL-17, the hallmark cytokine of the Th17 population, mediates immunity to extracellular pathogens and promotes autoimmune immunopathology. The signaling mechanisms triggered by the IL-17 receptor (IL-17RA) and related receptors are strikingly different from other cytokine subclasses. Namely, IL-17Rs contain a conserved SEF/IL-17R (SEFIR) subdomain that engages Act1, leading to activation of TRAF6, NF-κB, and other events. Although the SEFIR is critical for signaling, the molecular details of the functional subdomains within IL-17RA remain poorly characterized. Here, we provide a detailed structure-function analysis delineating the C-terminal boundary of the SEFIR-containing region of IL-17RA. We show that functionality of this domain requires a large extension to the previously identified SEFIR motif. In contrast to the SEFIR, this extension is not conserved among IL-17R family members. Surprisingly, Act1 recruitment is not sufficient for downstream signaling activation, whereas ubiquitination of TRAF6 correlates tightly with functional receptors. We further demonstrate that IL-17RA exhibits signaling properties that are nonredundant with other IL-17R family members. Finally, we report that IL-17 signals synergistically with lymphotoxin-α3, using the same signaling motifs within IL-17RA. These studies provide new insight into the structure-function relationships of IL-17RA and reveal distinct signaling differences among IL-17R family members.

Structure and signalling in the IL-17 receptor family

Interleukin-17A (IL-17A), the hallmark cytokine of the newly defined T helper 17 (T(H)17) cell subset, has important roles in protecting the host against extracellular pathogens, but also promotes inflammatory pathology in autoimmune disease. IL-17A and its receptor (IL-17RA) are the founding members of a newly described family of cytokines and receptors that have unique structural features which distinguish them from other cytokine families. Research defining the signal transduction pathways induced by IL-17R family cytokines has lagged behind that of other cytokine families, but studies in the past 2 years have begun to delineate unusual functional motifs and new proximal signalling mediators used by the IL-17R family to mediate downstream events.

Tc17, a unique subset of CD8 T cells that can protect against lethal influenza challenge

We show here that IL-17-secreting CD4 T (Th)17 and CD8 T (Tc)17 effector cells are found in the lung following primary challenge with influenza A and that blocking Ab to IL-17 increases weight loss and reduces survival. Tc17 effectors can be generated in vitro using naive CD8 T cells from OT-I TCR-transgenic mice. T cell numbers expand 20-fold and a majority secretes IL-17, but little IFN-gamma. Many of the IL-17-secreting cells also secrete TNF and some secrete IL-2. Tc17 are negative for granzyme B, perforin message, and cytolytic activity, in contrast to Tc1 effectors. Tc17 populations express message for orphan nuclear receptor gammat and FoxP3, but are negative for T-bet and GATA-3 transcription factors. The FoxP3-positive, IL-17-secreting and IFN-gamma-secreting cells represent three separate populations. The IFN-gamma-, granzyme B-, FoxP3-positive cells and cells positive for IL-22 come mainly from memory cells and decrease in number when generated from CD44(low) rather than unselected CD8 T cells. Cells of this unique subset of CD8 effector T cells expand greatly after transfer to naive recipients following challenge and can protect them against lethal influenza infection. Tc17 protection is accompanied by greater neutrophil influx into the lung than in Tc1-injected mice, and the protection afforded by Tc17 effectors is less perforin but more IFN-gamma dependent, implying that different mechanisms are involved.

Interleukin-17 as a drug target in human disease

Interleukin (IL)-17 (now synonymous with IL-17A) is an archetype molecule for an entire family of IL-17 cytokines. Currently believed to be produced mainly by a specific subset of CD4 cells, named Th-17 cells, IL-17 is functionally located at the interface of innate and acquired immunity. Specifically, it induces the release of chemokines and growth factors from mesenchymal cells and is now emerging as an important local orchestrator of neutrophil accumulation in several mammalian organs. Furthermore, there is growing evidence that targeting IL-17 signaling might prove useful in a variety of diseases including asthma, Crohn's disease, multiple sclerosis, psoriatric disease and rheumatoid arthritis. Here, we summarize the key aspects of the biology of IL-17 in mammals and scrutinize the potential pharmacological use of targeting IL-17 in humans.

The roles of the classical and alternative nuclear factor-kappaB pathways: potential implications for autoimmunity and rheumatoid arthritis

Nuclear factor-κB (NF-κB) is an inducible transcription factor controlled by two principal signaling cascades, each activated by a set of signal ligands: the classical/canonical NF-κB activation pathway and the alternative/noncanonical pathway. The former pathway proceeds via phosphorylation and degradation of inhibitor of NF-κB (IκB) and leads most commonly to activation of the heterodimer RelA/NF-κB1(p50). The latter pathway proceeds via phosphorylation and proteolytic processing of NF-κB2 (p100) and leads to activation, most commonly, of the heterodimer RelB/NF-κB2 (p52). Both pathways play critical roles at multiple levels of the immune system in both health and disease, including the autoimmune inflammatory response. These roles include cell cycle progression, cell survival, adhesion, and inhibition of apoptosis. NF-κB is constitutively activated in many autoimmune diseases, including diabetes type 1, systemic lupus erythematosus, and rheumatoid arthritis (RA). In this review we survey recent developments in the involvement of the classical and alternative pathways of NF-κB activation in autoimmunity, focusing particularly on RA. We discuss the involvement of NF-κB in self-reactive T and B lymphocyte development, survival and proliferation, and the maintenance of chronic inflammation due to cytokines such as tumor necrosis factor-α, IL-1, IL-6, and IL-8. We discuss the roles played by IL-17 and T-helper-17 cells in the inflammatory process; in the activation, maturation, and proliferation of RA fibroblast-like synovial cells; and differentiation and activation of osteoclast bone-resorbing activity. The prospects of therapeutic intervention to block activation of the NF-κB signaling pathways in RA are also discussed.

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.