Extrathymic expression of Aire controls the induction of effective TH17 cell-mediated immune response to Candida albicans.

Patients with loss of function in the gene encoding the master regulator of central tolerance AIRE suffer from a devastating disorder called autoimmune polyendocrine syndrome type 1 (APS-1), characterized by a spectrum of autoimmune diseases and severe mucocutaneous candidiasis. Although the key mechanisms underlying the development of autoimmunity in patients with APS-1 are well established, the underlying cause of the increased susceptibility to Candida albicans infection remains less understood. Here, we show that Aire+MHCII+ type 3 innate lymphoid cells (ILC3s) could sense, internalize and present C. albicans and had a critical role in the induction of Candida-specific T helper 17 (TH17) cell clones. Extrathymic Rorc-Cre-mediated deletion of Aire resulted in impaired generation of Candida-specific TH17 cells and subsequent overgrowth of C. albicans in the mucosal tissues. Collectively, our observations identify a previously unrecognized regulatory mechanism for effective defense responses against fungal infections.

IL-17-driven induction of Paneth cell antimicrobial functions protects the host from microbiota dysbiosis and inflammation in the ileum.

Interleukin (IL)-17 protects epithelial barriers by inducing the secretion of antimicrobial peptides. However, the effect of IL-17 on Paneth cells (PCs), the major producers of antimicrobial peptides in the small intestine, is unclear. Here, we show that the targeted ablation of the IL-17 receptor (IL-17R) in PCs disrupts their antimicrobial functions and decreases the frequency of ileal PCs. These changes become more pronounced after colonization with IL-17 inducing segmented filamentous bacteria. Mice with PCs that lack IL-17R show an increased inflammatory transcriptional profile in the ileum along with the severity of experimentally induced ileitis. These changes are associated with a decrease in the diversity of gut microbiota that induces a severe ileum pathology upon transfer to genetically susceptible mice, which can be prevented by the systemic administration of IL-17a/f in microbiota recipients. In an exploratory analysis of a small cohort of pediatric patients with Crohn's disease, we have found that a portion of these patients exhibits a low number of lysozyme-expressing ileal PCs and a high ileitis severity score, resembling the phenotype of mice with IL-17R-deficient PCs. Our study identifies IL-17R-dependent signaling in PCs as an important mechanism that maintains ileal homeostasis through the prevention of dysbiosis.