Oral epithelial cells orchestrate innate type 17 responses to Candida albicans through the virulence factor candidalysin

Candida albicans is a dimorphic commensal fungus that causes severe oral infections in immunodeficient patients. Invasion of C. albicans hyphae into oral epithelium is an essential virulence trait. Interleukin-17 (IL-17) signaling is required for both innate and adaptive immunity to C. albicans. During the innate response, IL-17 is produced by gamma delta T cells and a poorly understood population of innate-acting CD4(+) alpha beta T cell receptor (TCR alpha beta)(+) cells, but only the TCR alpha beta(+) cells expand during acute infection. Confirming the innate nature of these cells, the TCR was not detectably activated during the primary response, as evidenced by Nur77(eGFP) mice that report antigen-specific signaling through the TCR. Rather, the expansion of innate TCR alpha beta(+) cells was driven by both intrinsic and extrinsic IL-1R signaling. Unexpectedly, there was no requirement for CCR6/CCL20-dependent recruitment or prototypical fungal pattern recognition receptors. However, C. albicans mutants that cannot switch from yeast to hyphae showed impaired TCR alpha beta(+) cell proliferation and ll17a expression. This prompted us to assess the role of candidalysin, a hyphal-associated peptide that damages oral epithelial cells and triggers production of inflammatory cytokines including IL-1. Candidalysin-deficient strains failed to up-regulate ll17a or drive the proliferation of innate TCR alpha beta(+) cells. Moreover, candidalysin signaled synergistically with IL-17, which further augmented the expression of IL-1 alpha/beta and other cytokines. Thus, IL-17 and C. albicans, via secreted candidalysin, amplify inflammation in a self-reinforcing feed-forward loop. These findings challenge the paradigm that hyphal formation per se is required for the oral innate response and demonstrate that establishment of IL-1- and IL-17-dependent innate immunity is induced by tissue-damaging hyphae.