Research ArticleFUNGAL INFECTION

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

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Science Immunology  03 Nov 2017:
Vol. 2, Issue 17, eaam8834
DOI: 10.1126/sciimmunol.aam8834

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Calibrating antifungal responses

Immune responses to fungal infections are complicated by the fact that fungi can exist in multiple forms depending on environmental cues. Here, Verma et al. have evaluated innate immune responses to Candida albicans, a fungus that transitions from yeast to filamentous hyphae as infection progresses. They find that candidalysin, a hypha-associated protein and virulence factor, serves as a danger signal that potentiates the immune response to C. albicans. Candidalysin-deficient strains of C. albicans caused minimal epithelial damage and elicited a blunted type 17 immune response. The authors propose that the innate antifungal responses to C. albicans are driven by a synergy between cellular damage triggered by candidalysin that is further amplified by interleukin-17 driven inflammation.

Abstract

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 γδ T cells and a poorly understood population of innate-acting CD4+ αβ T cell receptor (TCRαβ)+ cells, but only the TCRαβ+ 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 Nur77eGFP mice that report antigen-specific signaling through the TCR. Rather, the expansion of innate TCRαβ+ 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 TCRαβ+ cell proliferation and Il17a 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 Il17a or drive the proliferation of innate TCRαβ+ cells. Moreover, candidalysin signaled synergistically with IL-17, which further augmented the expression of IL-1α/β 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.

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