Pathogen-derived extracellular vesicles mediate virulence in the fatal human pathogen Cryptococcus gattii

Abstract

The Pacific Northwest outbreak of cryptococcosis, caused by a near-clonal lineage of the fungal pathogen Cryptococcus gattii, represents the most significant cluster of life-threatening fungal infections in otherwise healthy human hosts currently known. The outbreak lineage has a remarkable ability to grow rapidly within human white blood cells, using a unique ‘division of labour’ mechanism within the pathogen population, where some cells adopt a dormant behaviour to support the growth of neighbouring cells. Here we demonstrate that pathogenic ‘division of labour’ can be triggered over large cellular distances and is mediated through the release of extracellular vesicles by the fungus. Isolated vesicles released by virulent strains are taken up by infected host macrophages and trafficked to the phagosome, where they trigger the rapid intracellular growth of non-outbreak fungal cells that would otherwise be eliminated by the host. Thus, long distance pathogen-to-pathogen communication via extracellular vesicles represents a novel mechanism to control complex virulence phenotypes in Cryptococcus gattii and, potentially, other infectious species.

Fungal Extracellular Vesicles with a Focus on Proteomic Analysis

“…Recently, pathogen to pathogen communication was identified as a key component in the virulence of a Cryptococcus gattii strain that was responsible for an outbreak of disease in immunocompetent individuals in western Canada in the late 1990s . In this “division of labor” virulence mechanism, a subpopulation of fungal cells adopt a tubular mitochondrial morphology in response to reactive oxygen species (ROS) produced as part of the host immune response .…”

“…In this “division of labor” virulence mechanism, a subpopulation of fungal cells adopt a tubular mitochondrial morphology in response to reactive oxygen species (ROS) produced as part of the host immune response . This increases their survival within macrophages, whereupon they signal, via the release of EVs, to neighboring fungal cells to rapidly proliferate within macrophages . The ability of these EVs to increase the proliferation rate of C. gattii cells was dependent on intact EVs and their strain specific protein and RNA cargo but not DNA cargo …”

A two‐way road: novel roles for fungal extracellular vesicles

“…Addition of EVs produced by the outbreak strain to macrophage cultures infected with the avirulent strain led to a dramatically, dose‐dependent enhancement in the intracellular proliferation rate. Importantly, EVs colocalized with fungal cells inside host phagosomes (Bielska et al, ). These results suggest that EVs released by virulent strains are taken up by infected host macrophages and trafficked to the phagosome, where they stimulate intracellular growth of the avirulent fungal cells that would normally be killed by the host’s antimicrobial mechanisms.…”

“…These results suggest that EVs released by virulent strains are taken up by infected host macrophages and trafficked to the phagosome, where they stimulate intracellular growth of the avirulent fungal cells that would normally be killed by the host’s antimicrobial mechanisms. This phenomenon was species‐specific, since EVs produced by the sibling species C. neoformans did not affect the intracellular proliferation rate of the avirulent C. gattii isolate (Bielska et al, ). The regulators of this process remain to be identified, but the EV‐mediated enhanced survival of the non‐outbreak strain required vesicular proteins and RNA, but not DNA (Bielska et al, ).…”

“…This phenomenon was species‐specific, since EVs produced by the sibling species C. neoformans did not affect the intracellular proliferation rate of the avirulent C. gattii isolate (Bielska et al, ). The regulators of this process remain to be identified, but the EV‐mediated enhanced survival of the non‐outbreak strain required vesicular proteins and RNA, but not DNA (Bielska et al, ).…”