The Cytokine IL-17A Limits Th17 Pathogenicity via a Negative Feedback Loop Driven by Autocrine Induction of IL-24

Summary

Dysregulated Th17 cell responses underlie multiple inflammatory and autoimmune diseases, including autoimmune uveitis and its animal model, EAU. However, clinical trials targeting IL-17A in uveitis were not successful. Here, we report that Th17 cells were regulated by their own signature cytokine, IL-17A. Loss of IL-17A in autopathogenic Th17 cells did not reduce their pathogenicity and instead elevated their expression of the Th17 cytokines GM-CSF and IL-17F. Mechanistic in vitro studies revealed a Th17 cell-intrinsic autocrine loop triggered by binding of IL-17A to its receptor, leading to activation of the transcription factor NF-κB and induction of IL-24, which repressed the Th17 cytokine program. In vivo, IL-24 treatment ameliorated Th17-induced EAU, whereas silencing of IL-24 in Th17 cells enhanced disease. This regulatory pathway also operated in human Th17 cells. Thus, IL-17A limits pathogenicity of Th17 cells by inducing IL-24. These findings may explain the disappointing therapeutic effect of targeting IL-17A in uveitis.

Graphical Abstract

Highlights

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  IL-17A deficiency does not reduce the pathogenicity of Th17 cells in uveitis

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  IL-17A binds to its own receptor on Th17 cells, activating NF-κB

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  NF-κB induces IL-24 production, repressing the Th17 cytokine program through SOCS1/3

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  Silencing or depleting IL-24 in Th17 cells exacerbates neuroinflammation

Abstract

Loss of IL-17A, the signature cytokine of autoreactive Th17 cells, unexpectedly did not diminish the pathogenicity of Th17 cells in neuroinflammatory disease. This report demonstrates that IL-17A represses the Th17 cytokine program, primarily IL-17F and GM-CSF, by inducing autocrine production of IL-24. Thus, IL-17A has a dual role in Th17 cells: pathogenic as well as regulatory.