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.

U6 RNA is a key component of the catalytic core of the spliceosome. A metal ion essential for the first catalytic step of pre-mRNA splicing binds to the U80 Sp phosphate oxygen within the yeast U6 intramolecular stem-loop (ISL). Here we present the first structural data for U6 RNA, revealing the three-dimensional structure of the highly conserved U6 ISL. The ISL binds metal ion at the U80 site with the same stereo specificity as the intact spliceosome. The metal-binding site is adjacent to a readily protonated C.A wobble pair. Protonation of the C.A pair and metal binding are mutually antagonistic. These results support a ribozyme model for U6 RNA function and suggest a possible mechanism for the regulation of RNA splicing.

Oropharyngeal candidiasis (OPC) is an opportunistic infection caused by Candida albicans. Despite its prevalence, little is known about C. albicans-specific immunity in the oral mucosa. Vaccines against Candida generate both Th1 and Th17 responses, and considerable evidence implicates IL-17 in immunity to OPC. However, IL-17 is also produced by innate immune cells that are remarkably similar to Th17 cells, expressing the same markers and localizing to similar mucosal sites. To date, the relative contribution(s) of Th1, Th17 and innate IL-17-producing cells in OPC have not been clearly defined. Here, we sought to determine the nature and function of adaptive T cell responses to OPC, using a new recall infection model. Mice subjected to infection and re-challenge with Candida mounted a robust and stable antigen specific IL-17 response in CD4+ but not CD8+ T cells. There was little evidence for Th1 or Th1/Th17 responses. The Th17 response promoted accelerated fungal clearance, and Th17 cells could confer protection in Rag1−/− mice upon adoptive transfer. Surprisingly, CD4 deficiency did not cause OPC, but was instead associated with compensatory IL-17 production by Tc17 and CD4-CD8-CD3+ cells. Therefore, classic CD4+Th17 cells protect from OPC, but can be compensated by other IL-17-producing cells in CD4-deficient hosts.

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. 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+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, evidenced by Nur77eGFP mice that report antigen-specific signaling through the TCR. Rather, 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. Indeed, Candidalysin-deficient strains failed to upregulate Il17a or drive proliferation of innate TCRαβ+ cells. Moreover, Candidalysin signaled synergistically with IL-17, which further augmented 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.