Redundant Trojan horse and endothelial-circulatory mechanisms for host-mediated spread ofCandida albicansyeast

Author summary AlthoughCandida albicansis the most common cause of fungal bloodstream infection, we know little about how it spreads from one tissue to another. Host processes often inadvertently assist pathogens that can hijack immune cells or induce endothelial endocytosis. Here we have used transparent zebrafish to visualize how specific host cells and processes contribute to yeast spreadin vivo. We find thatCandidais a sophisticated pathogen that uses redundant strategies to disseminate in the host: it can either use host immune cells or cross into the circulatory system and use blood flow. These data suggest that both of these mechanisms must be targeted to limitCandidadissemination during infection. The host innate immune system has developed elegant processes for the detection and clearance of invasive fungal pathogens. These strategies may also aid in the spread of pathogensin vivo, although technical limitations have previously hindered our ability to view the host innate immune and endothelial cells to probe their roles in spreading disease. Here, we have leveraged zebrafish larvae as a model to view the interactions of these host processes with the fungal pathogenCandida albicans in vivo. We examined three potential host-mediated mechanisms of fungal spread: movement inside phagocytes in a Trojan Horse mechanism, inflammation-assisted spread, and endothelial barrier passage. Utilizing both chemical and genetic tools, we systematically tested the loss of neutrophils and macrophages and the loss of blood flow on yeast cell spread. Both neutrophils and macrophages respond to yeast-locked and wild typeC.albicansin our model and time-lapse imaging revealed that macrophages can support yeast spread in a Trojan Horse mechanism. Surprisingly, loss of immune cells or inflammation does not alter dissemination dynamics. On the other hand, when blood flow is blocked, yeast can cross into blood vessels but they are limited in how far they travel. Blockade of both phagocytes and circulation reduces rates of dissemination and significantly limits the distance of fungal spread from the infection site. Together, this data suggests a redundant two-step process whereby (1) yeast cross the endothelium inside phagocytes or via direct uptake, and then (2) they utilize blood flow or phagocytes to travel to distant sites.