The Endoplasmic Reticulum-Mitochondrion Tether ERMES Orchestrates Fungal Immune Evasion, Illuminating Inflammasome Responses to Hyphal Signals

The pathogenic yeast Candida albicans escapes macrophages by triggering NLRP3 inflammasome-dependent host cell death (pyroptosis). Pyroptosis is inflammatory and must be tightly regulated by host and microbe, but the mechanism is incompletely defined. We characterized the C. albicans endoplasmic reticulum (ER)-mitochondrion tether ERMES and show that the ERMES mmm1 mutant is severely crippled in killing macrophages despite hyphal formation and normal phagocytosis and survival. To understand dynamic inflammasome responses to Candida with high spatiotemporal resolution, we established live-cell imaging for parallel detection of inflammasome activation and pyroptosis at the single-cell level. This showed that the inflammasome response to mmm1 mutant hyphae is delayed by 10 h, after which an exacerbated activation occurs. The NLRP3 inhibitor MCC950 inhibited inflammasome activation and pyroptosis by C. albicans, including exacerbated inflammasome activation by the mmm1 mutant. At the cell biology level, inactivation of ERMES led to a rapid collapse of mitochondrial tubular morphology, slow growth and hyphal elongation at host temperature, and reduced exposed 1,3-β- glucan in hyphal populations. Our data suggest that inflammasome activation by C. albicans requires a signal threshold dependent on hyphal elongation and cell wall remodeling, which could fine-tune the response relative to the level of danger posed by C. albicans. The phenotypes of the ERMES mutant and the lack of conservation in animals suggest that ERMES is a promising antifungal drug target. Our data further indicate that NLRP3 inhibition by MCC950 could modulate C. albicansinduced inflammation.