Jarod A. Zepp

IL-17 receptor signaling and T helper 17-mediated autoimmune demyelinating disease.

Multiple sclerosis (MS) is a chronic inflammatory demyelinating disease of the central nervous system (CNS). Experimental autoimmune encephalomyelitis (EAE) is widely used to dissect molecular mechanisms of MS and to develop new therapeutic strategies. The T helper 17 (Th17) subset of CD4 T cells plays a crucial role in the development of EAE. IL-17, a cytokine produced by Th17 cells, participates in EAE pathogenesis through induction of inflammatory gene expression in target cells. Recent work has shown that Act1, a U-box E3 ubiquitin ligase, is recruited to IL-17 receptor (IL-17R) upon IL-17 stimulation and is required for IL-17-mediated signaling. Here, we review the molecular and cellular mechanisms by which IL-17 and Act1-mediated signaling contribute to EAE.

Act1 mediates IL-17–induced EAE pathogenesis selectively in NG2 + glial cells

Interleukin 17 (IL-17) is a signature cytokine of Th17 cells. We previously reported that deletion of NF-κB activator 1 (Act1), the key transducer of IL-17 receptor signaling, from the neuroectodermal lineage in mice (neurons, oligodendrocytes and astrocytes) results in attenuated severity of experimental autoimmune encephalomyelitis (EAE). Here we examined the cellular basis of this observation. EAE disease course was unaffected by deletion of Act1 in neurons or mature oligodendrocytes, and Act1 deletion in astrocytes only modestly affected disease course. Deletion of Act1 in NG2+ glia resulted in markedly reduced EAE severity. Furthermore, IL-17 induced characteristic inflammatory mediator expression in NG2+ glial cells. IL-17 also exhibited strong inhibitory effects on the maturation of oligodendrocyte lineage cells in vitro and reduced their survival. These data identify NG2+ glia as the major CNS cellular target of IL-17 in EAE. The sensitivity of oligodendrocyte lineage cells to IL-17–mediated toxicity further suggests a direct link between inflammation and neurodegeneration in multiple sclerosis.

IRAK‐M mediates Toll‐like receptor/IL‐1R‐induced NFκB activation and cytokine production

Toll‐like receptors transduce their signals through the adaptor molecule MyD88 and members of the IL‐1R‐associated kinase family (IRAK‐1, 2, M and 4). IRAK‐1 and IRAK‐2, known to form Myddosomes with MyD88–IRAK‐4, mediate TLR7‐induced TAK1‐dependent NFκB activation. IRAK‐M was previously known to function as a negative regulator that prevents the dissociation of IRAKs from MyD88, thereby inhibiting downstream signalling. However, we now found that IRAK‐M was also able to interact with MyD88–IRAK‐4 to form IRAK‐M Myddosome to mediate TLR7‐induced MEKK3‐dependent second wave NFκB activation, which is uncoupled from post‐transcriptional regulation. As a result, the IRAK‐M‐dependent pathway only induced expression of genes that are not regulated at the post‐transcriptional levels (including inhibitory molecules SOCS1, SHIP1, A20 and IκBα), exerting an overall inhibitory effect on inflammatory response. On the other hand, through interaction with IRAK‐2, IRAK‐M inhibited TLR7‐mediated production of cytokines and chemokines at translational levels. Taken together, IRAK‐M mediates TLR7‐induced MEKK3‐dependent second wave NFκB activation to produce inhibitory molecules as a negative feedback for the pathway, while exerting inhibitory effect on translational control of cytokines and chemokines.

The psoriasis-associated D10N variant of the adaptor Act1 with impaired regulation by the molecular chaperone hsp90

Act1 is an essential adaptor molecule in IL-17-mediated signaling and is recruited to the IL-17 receptor upon IL-17 stimulation. Here, we report that Act1 is a client protein of the molecular chaperone, Hsp90. The Act1 variant (D10N) linked to psoriasis susceptibility is defective in its interaction with Hsp90, resulting in a global loss of Act1 function. Act1-/- mice modeled the mechanistic link between Act1 loss of function and psoriasis susceptibility. Although Act1 is necessary for IL-17-mediated inflammation, Act1-/- mice exhibited a hyper TH17 response and developed spontaneous IL-22-dependent skin inflammation. In the absence of IL-17-signaling, IL-22 is the main contributor to skin inflammation, providing a molecular mechanism for the association of Act1 (D10N) with psoriasis susceptibility.

T cell-intrinsic ASC critically promotes TH17-mediated experimental autoimmune encephalomyelitis

Interleukin 1β (IL-1β) is critical for the in vivo survival, expansion and effector function of IL-17–producing helper T (TH17) cells during autoimmune responses, including experimental autoimmune encephalomyelitis (EAE). However, the spatiotemporal role and cellular source of IL-1β during EAE pathogenesis are poorly defined. In the present study, we uncovered a T cell–intrinsic inflammasome that drives IL-1β production during TH17-mediated EAE pathogenesis. Activation of T cell antigen receptors induced expression of pro-IL-1β, whereas ATP stimulation triggered T cell production of IL-1β via ASC-NLRP3–dependent caspase-8 activation. IL-1R was detected on TH17 cells but not on type 1 helper T (TH1) cells, and ATP-treated TH17 cells showed enhanced survival compared with ATP-treated TH1 cells, suggesting autocrine action of TH17-derived IL-1β. Together these data reveal a critical role for IL-1β produced by a TH17 cell–intrinsic ASC–NLRP3–caspase-8 inflammasome during inflammation of the central nervous system.

Cutting Edge: TNF Receptor-Associated Factor 4 Restricts IL-17–Mediated Pathology and Signaling Processes

The effector T cell subset, Th17, plays a significant role in the pathogenesis of multiple sclerosis and of other autoimmune diseases. The signature cytokine, IL-17, engages the IL-17R and recruits the E3-ligase NF-κB activator 1 (Act1) upon stimulation. In this study, we examined the role of TNFR-associated factor (TRAF)4 in IL-17 signaling and Th17-mediated autoimmune encephalomyelitis. Primary cells from TRAF4-deficient mice displayed markedly enhanced IL-17–activated signaling pathways and induction of chemokine mRNA. Adoptive transfer of MOG35–55 specific wild-type Th17 cells into TRAF4-deficient recipient mice induced an earlier onset of disease. Mechanistically, we found that TRAF4 and TRAF6 used the same TRAF binding sites on Act1, allowing the competition of TRAF4 with TRAF6 for the interaction with Act1. Taken together, the results of this study reveal the necessity of a unique role of TRAF4 in restricting the effects of IL-17 signaling and Th17-mediated disease.

A novel IL-17 signaling pathway controlling keratinocyte proliferation and tumorigenesis via the TRAF4-ERK5 axis.

Wu et al. report a novel IL-17–mediated cascade via the IL-17R–TRAF4–ERK5 axis that directly stimulates keratinocyte proliferation and skin tumor formation in mice.

A Novel IL-25 Signaling Pathway through STAT5

IL-25 is a member of the IL-17 family of cytokines that promotes Th2 cell–mediated inflammatory responses. IL-25 signals through a heterodimeric receptor (IL-25R) composed of IL-17RA and IL-17RB, which recruits the adaptor molecule Act1 for downstream signaling. Although the role of IL-25 in potentiating type 2 inflammation is well characterized by its ability to activate the epithelium as well as T cells, the components of its signaling cascade remain largely unknown. In this study, we found that IL-25 can directly activate STAT5 independently of Act1. Furthermore, conditional STAT5 deletion in T cells or epithelial cells led to a defective IL-25–initiated Th2 polarization as well as defective IL-25 enhancement of Th2 responses. Finally, we found that STAT5 is recruited to the IL-25R in a ligand-dependent manner through unique tyrosine residues on IL-17RB. Together, these findings reveal a novel Act1-independent IL-25 signaling pathway through STAT5 activation.

Function of Act1 in IL-17 Family Signaling and Autoimmunity

The maintenance of immune homeostasis requires the delicate balance between response to foreign antigens and tolerance to self. As such, when this balance is disrupted, immunodeficiency or autoimmunity may manifest. The adaptor molecule known as Act1 is a critical mediator of IL-17 receptor receptor family signaling. This chapter will detail the current understanding of Act1 s role in signal transduction as well as address the fundamental role of Act1 in autoimmunity. At the molecular level Act1 interacts with IL-17 R through the conserved SEFIR domain, binds TRAF proteins and exerts E3 ubiquitin ligase activity. In in vivo models, Act1 deficiency provides protection against experimental autoimmune diseases, such as colitis and EAE. Yet mice lacking in Act1 develop spontaneous autoimmune diseases. Indeed, the utility of Act1 seems to rely on the specific cell type expression that may determine the pathway that Act1 mediates.

TRAF4-SMURF2–Mediated DAZAP2 Degradation Is Critical for IL-25 Signaling and Allergic Airway Inflammation

IL-25 promotes type 2 immunity by inducing the expression of Th2–associated cytokines. Although it is known that the IL-25R (IL-17RB) recruits the adaptor protein ACT1, the IL-25R signaling mechanism remains poorly understood. While screening for IL-25R components, we found that IL-25 responses were impaired in Traf4 −/− cells. Administering IL-25 to Traf4 −/− mice resulted in blunted airway eosinophilia and Th2 cytokine production. Notably, IL-25R recruitment of TRAF4 was required for the ACT1/IL-25R interaction. Mechanistically, TRAF4 recruited the E3-ligase SMURF2, to degrade the IL-25R–inhibitory molecule DAZAP2. Silencing Dazap2 increased ACT1/IL-25R interaction and IL-25 responsiveness. Moreover, a tyrosine within the IL-25R elicited DAZAP2 interference. This study indicates that TRAF4-SMURF2–mediated DAZAP2 degradation is a crucial initiating event for the IL-25 response.