Copper and redox homeostastis of human pathogenic fungi are targeted by the mycophagous amoeba Planoprotostelium fungivorum

Candida, Aspergillus and Cryptococcus sp. are the leading causative agents of systemic mycoses. Many of their representatives have also been isolated from non-human sources, suggesting that their virulence potential might have been partially shaped in their natural niche as a strategy to counteract environmental predators. Previously we demonstrated that basic phagocytic interactions of filamentous fungi are conserved between soil amoeba and macrophages. We have now established a new natural amoeba model system to study evolutionary forces that could have supported the generation and maintenance of virulence traits in human pathogenic fungi. Planoprotostelium fungivorum is an exclusively mycophagous amoeba, widely spread in nature, which we have successfully isolated and completely sequenced its genome. Large-scale feeding experiments revealed a broad prey spectrum including several yeasts and filamentous fungi. With an extreme efficiency, the yeasts were recognized, phagocytosed and killed in a range of few minutes. Filamentous fungi, like A. fumigatus, were too large to be ingested, but could be killed by a “lysocytosis”-like mechanism, which included the punctuate opening of the hyphae, followed by efflux of the cytoplasmic content. As Candida parapsilosis was found to be a preferential prey, we used this pathogen in a dual-transcriptome approach to identify targets of this predatory-prey interaction. Amoeba-responsive genes included those involved in the elevated metal efflux, oxidative stress response, filamentuous growth and secreted lipase, suggesting predatory selection pressure on these important virulence determinants. As a killing mechanism, we propose mobilization of internal copper resources leading to an impaired oxidative stress response and intoxication inside of the acidic phagolysosome. Constructed deletion mutants for the most promising targets will further reveal if traits that have originated to counteract with natural predators could also have supported the resistance against innate immune cells.