Abhishek V. Garg

The IL-23-IL-17 immune axis: from mechanisms to therapeutic testing

Following the discovery of T helper 17 (TH17) cells, the past decade has witnessed a major revision of the TH subset paradigm and substantial progress has been made in deciphering the molecular mechanisms of T cell lineage commitment and function. In this Review, we focus on the recent advances that have been made regarding the transcriptional control of TH17 cell plasticity and stability, as well as the effector functions of TH17 cells, and we highlight the mechanisms of IL-17 signalling in mesenchymal and barrier epithelial tissues. We also discuss the emerging clinical data showing that IL-17-specific and IL-23-specific antibody treatments are remarkably effective for treating many immune-mediated inflammatory diseases.

Oral-resident natural Th17 cells and γδ T cells control opportunistic Candida albicans infections

Conti et al. show that IL-17 is produced by tongue-resident populations of γδ T cells and nTh17 cells in response to oropharyngeal candidiasis in mice.

The Deubiquitinase A20 Mediates Feedback Inhibition of Interleukin-17 Receptor Signaling

Inhibition of an inflammatory cytokine signaling pathway by A20 at the receptor may play a role in preventing autoimmunity. A20 Targets a Cytokine Receptor to Limit Inflammation The cytokine interleukin-17 (IL-17) mediates immunity against various microorganisms; however, it also causes inflammatory tissue damage in autoimmune disorders, such as rheumatoid arthritis (RA). Garg et al. found that IL-17 stimulated the production of the deubiquitinase A20, an inhibitor of nuclear factor κB (NF-κB) activation downstream of the tumor necrosis factor receptor and Toll-like receptors. Mutations in the gene encoding A20 are associated with RA in some patients. Knockdown of A20 in human fibroblast-like synoviocytes resulted in enhanced IL-17–dependent production of the inflammatory cytokine IL-6. IL-17 stimulated an association between A20, the IL-17R, and the E3 ubiquitin ligase TRAF6 (tumor necrosis factor receptor–associated factor 6) to inhibit IL-17–dependent signaling. In addition to demonstrating the direct recruitment of A20 to a cytokine receptor, these results may partly explain the association of mutations in A20 with susceptibility to RA. 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 Signaling: The Yin and the Yang

Interleukin (IL)-17 is the founding member of a novel family of inflammatory cytokines. While the proinflammatory properties of IL-17 are key to its host-protective capacity, unrestrained IL-17 signaling is associated with immunopathology, autoimmune disease, and cancer progression. In this review we discuss both the activators and the inhibitors of IL-17 signal transduction, and also the physiological implications of these events. We highlight the surprisingly diverse means by which these regulators control expression of IL-17-dependent inflammatory genes, as well as the major target cells that respond to IL-17 signaling.

Delinking CARD9 and IL-17: CARD9 Protects against Candida tropicalis Infection through a TNF-α–Dependent, IL-17–Independent Mechanism

Candida is the third most common cause of bloodstream infections in hospitalized patients. Immunity to C. albicans, the most frequent species to be isolated in candidiasis, involves a well-characterized Dectin-1/caspase-associated recruitment domain adaptor 9 (CARD9)/IL-17 signaling axis. Infections caused by non-albicans Candida species are on the rise, but surprisingly little is known about immunity to these pathogens. In this study, we evaluated a systemic infection model of C. tropicalis, a clinically relevant, but poorly understood, non-albicans Candida. Mice lacking CARD9 were profoundly susceptible to C. tropicalis, displaying elevated fungal burdens in visceral organs and increased mortality compared with wild-type (WT) controls. Unlike C. albicans, IL-17 responses were induced normally in CARD9−/− mice following C. tropicalis infection. Moreover, there was no difference in susceptibility to C. tropicalis infection between WT and IL-23p19−/−, IL-17RA−/−, or Act1−/− mice. However, TNF-α expression was markedly impaired in CARD9−/− mice. Consistently, WT mice depleted of TNF-α were more susceptible to C. tropicalis, and CARD9-deficient neutrophils and monocytes failed to produce TNF-α following stimulation with C. tropicalis Ags. Both neutrophils and monocytes were necessary for defense against C. tropicalis, because their depletion in WT mice enhanced susceptibility to C. tropicalis. Disease in CARD9−/− mice was not due to defective neutrophil or monocyte recruitment to infected kidneys. However, TNF-α treatment of neutrophils in vitro enhanced their ability to kill C. tropicalis. Thus, protection against systemic C. tropicalis infection requires CARD9 and TNF-α, but not IL-17, signaling. Moreover, CARD9-dependent production of TNF-α enhances the candidacidal capacity of neutrophils, limiting fungal disease during disseminated C. tropicalis infection.

Signaling through IL-17C/IL-17RE is dispensable for immunity to systemic, oral and cutaneous candidiasis.

Candida albicans is a commensal fungal microbe of the human orogastrointestinal tract and skin. C. albicans causes multiple forms of disease in immunocompromised patients, including oral, vaginal, dermal and disseminated candidiasis. The cytokine IL-17 (IL-17A) and its receptor subunits, IL-17RA and IL-17RC, are required for protection to most forms of candidiasis. The importance of the IL-17R pathway has been observed not only in knockout mouse models, but also in humans with rare genetic mutations that impact generation of Th17 cells or the IL-17 signaling pathway, including Hyper-IgE Syndrome (STAT3 or TYK2 mutations) or IL17RA or ACT1 gene deficiency. The IL-17 family of cytokines is a distinct subclass of cytokines with unique structural and signaling properties. IL-17A is the best-characterized member of the IL-17 family to date, but far less is known about other IL-17-related cytokines. In this study, we sought to determine the role of a related IL-17 cytokine, IL-17C, in protection against oral, dermal and disseminated forms of C. albicans infection. IL-17C signals through a heterodimeric receptor composed of the IL-17RA and IL-17RE subunits. We observed that IL-17C mRNA was induced following oral C. albicans infection. However, mice lacking IL-17C or IL-17RE cleared C. albicans infections in the oral mucosa, skin and bloodstream at rates similar to WT littermate controls. Moreover, these mice demonstrated similar gene transcription profiles and recovery kinetics as WT animals. These findings indicate that IL-17C and IL-17RE are dispensable for immunity to the forms of candidiasis evaluated, and illustrate a surprisingly limited specificity of the IL-17 family of cytokines with respect to systemic, oral and cutaneous Candida infections.

MCPIP1/Regnase-1 Restricts IL-17A– and IL-17C–Dependent Skin Inflammation

The IL-17 family cytokines IL-17A and IL-17C drive the pathogenesis of psoriatic skin inflammation, and anti–IL-17A Abs were recently approved to treat human psoriasis. Little is known about mechanisms that restrain IL-17 cytokine-mediated signaling, particularly IL-17C. In this article, we show that the endoribonuclease MCP-1–induced protein 1 (MCPIP1; also known as regnase-1) is markedly upregulated in human psoriatic skin lesions. Similarly, MCPIP1 was overexpressed in the imiquimod (IMQ)-driven mouse model of cutaneous inflammation. Mice with an MCPIP1 deficiency (Zc3h12a+/−) displayed no baseline skin inflammation, but they showed exacerbated pathology following IMQ treatment. Pathology in Zc3h12a+/− mice was associated with elevated expression of IL-17A– and IL-17C–dependent genes, as well as with increased accumulation of neutrophils in skin. However, IL-17A and IL-17C expression was unaltered, suggesting that the increased inflammation in Zc3h12a+/− mice was due to enhanced downstream IL-17R signaling. Radiation chimeras demonstrated that MCPIP1 in nonhematopoietic cells is responsible for controlling skin pathology. Moreover, Zc3h12a+/−Il17ra−/− mice given IMQ showed almost no disease. To identify which IL-17RA ligand was essential, Zc3h12a+/−Il17a−/− and Zc3h12a+/−Il17c−/− mice were given IMQ; these mice had reduced but not fully abrogated pathology, indicating that MCPIP1 inhibits IL-17A and IL-17C signaling. Confirming this hypothesis, Zc3h12a−/− keratinocytes showed increased responsiveness to IL-17A and IL-17C stimulation. Thus, MCPIP1 is a potent negative regulator of psoriatic skin inflammation through IL-17A and IL-17C. Moreover, to our knowledge, MCPIP1 is the first described negative regulator of IL-17C signaling.

Antibody blockade of IL-17 family cytokines in immunity to acute murine oral mucosal candidiasis

Antibodies targeting IL‐17A or its receptor, IL‐17RA, are approved to treat psoriasis and are being evaluated for other autoimmune conditions. Conversely, IL‐17 signaling is critical for immunity to opportunistic mucosal infections caused by the commensal fungus Candida albicans, as mice and humans lacking the IL‐17R experience chronic mucosal candidiasis. IL‐17A, IL‐17F, and IL‐17AF bind the IL‐17RA‐IL‐17RC heterodimeric complex and deliver qualitatively similar signals through the adaptor Act1. Here, we used a mouse model of acute oropharyngeal candidiasis to assess the impact of blocking IL‐17 family cytokines compared with specific IL‐17 cytokine gene knockout mice. Anti‐IL‐17A antibodies, which neutralize IL‐17A and IL‐17AF, caused elevated oral fungal loads, whereas anti‐IL‐17AF and anti‐IL‐17F antibodies did not. Notably, there was a cooperative effect of blocking IL‐17A, IL‐17AF, and IL‐17F together. Termination of anti‐IL‐17A treatment was associated with rapid C. albicans clearance. IL‐17F‐deficient mice were fully resistant to oropharyngeal candidiasis, consistent with antibody blockade. However, IL‐17A‐deficient mice had lower fungal burdens than anti‐IL‐17A‐treated mice. Act1‐deficient mice were much more susceptible to oropharyngeal candidiasis than anti‐IL‐17A antibody‐treated mice, yet anti‐IL‐17A and anti‐IL‐17RA treatment caused equivalent susceptibilities. Based on microarray analyses of the oral mucosa during infection, only a limited number of genes were associated with oropharyngeal candidiasis susceptibility. In sum, we conclude that IL‐17A is the main cytokine mediator of immunity in murine oropharyngeal candidiasis, but a cooperative relationship among IL‐17A, IL‐17AF, and IL‐17F exists in vivo. Susceptibility displays the following hierarchy: IL‐17RA‐ or Act1‐deficiency > anti‐IL‐17A + anti‐IL‐17F antibodies > anti‐IL‐17A or anti‐IL‐17RA antibodies > IL‐17A deficiency.

The Kallikrein-Kinin System: A Novel Mediator of IL-17-Driven Anti-Candida Immunity in the Kidney

The incidence of life-threatening disseminated Candida albicans infections is increasing in hospitalized patients, with fatalities as high as 60%. Death from disseminated candidiasis in a significant percentage of cases is due to fungal invasion of the kidney, leading to renal failure. Treatment of candidiasis is hampered by drug toxicity, the emergence of antifungal drug resistance and lack of vaccines against fungal pathogens. IL-17 is a key mediator of defense against candidiasis. The underlying mechanisms of IL-17-mediated renal immunity have so far been assumed to occur solely through the regulation of antimicrobial mechanisms, particularly activation of neutrophils. Here, we identify an unexpected role for IL-17 in inducing the Kallikrein (Klk)-Kinin System (KKS) in C. albicans-infected kidney, and we show that the KKS provides significant renal protection in candidiasis. Microarray data indicated that Klk1 was upregulated in infected kidney in an IL-17-dependent manner. Overexpression of Klk1 or treatment with bradykinin rescued IL-17RA-/- mice from candidiasis. Therapeutic manipulation of IL-17-KKS pathways restored renal function and prolonged survival by preventing apoptosis of renal cells following C. albicans infection. Furthermore, combining a minimally effective dose of fluconazole with bradykinin markedly improved survival compared to either drug alone. These results indicate that IL-17 not only limits fungal growth in the kidney, but also prevents renal tissue damage and preserves kidney function during disseminated candidiasis through the KKS. Since drugs targeting the KKS are approved clinically, these findings offer potential avenues for the treatment of this fatal nosocomial infection.

Inflammatory Th17 Cells Express Integrin αvβ3 for Pathogenic Function

SUMMARY Inerleukin-23 (IL-23) is required for inflammatory Th17 cell function in experimental autoimmune encephalomyelitis (EAE), and IL-23 blockade reduces the number of effector Th17 cells in the CNS. We report that pro-inflammatory Th17 cells express high integrin β3 that is IL-23 dependent. Integrin β3 was not upregulated on all activated T cells; rather, integrin β3 was upregulated along with its functional partner integrin αv on effector Th17 cells and “ex-Th17” cells, and αvβ3hi RORγt+ cells expanded during EAE. Integrin αvβ3 inhibitors ameliorated clinical signs of EAE, and integrin β3 deficiency on CD4+ T cells alone was sufficient to block EAE induction. Furthermore, integrin-β3-deficient Th17 cells, but not Th1 cells, were impaired in their ability to induce EAE. Integrin β3−/− T cells induced smaller demyelinated lesions and showed reduced spread and accumulation within the CNS, corresponding with impaired extracellular-matrix-mediated migration. Hence, integrin β3 is required for Th17 cell-mediated autoimmune CNS inflammation.