Phagosome Extrusion and Host-Cell Survival after Cryptococcus neoformans Phagocytosis by Macrophages

Cryptococcus neoformans (Cn) is an encapsulated yeast that is a facultative intracellular pathogen and a frequent cause of human disease. The interaction of Cn with alveolar macrophages is critical for containing the infection [1, 2], but Cn can also replicate intracellularly and lyse macrophages [3-5]. Cn has a unique intracellular pathogenic strategy that involves cytoplasmic accumulation of polysaccharide-containing vesicles and intracellular replication leading to the formation of spacious phagosomes in which multiple cryptococcal cells are present [3]. The Cn intracellular pathogenic strategy in macrophages and amoebas is similar, leading to the proposal that it originated as a mechanism for survival against phagocytic predators in the environment [6]. Here, we report that under certain conditions, including phagosomal maturation, possible actin depolymerization, and homotypic phagosome fusion, Cn can exit the macrophage host through an extrusion of the phagosome, while both the released pathogen and host remain alive and able to propagate. The phenomenon of "phagosomal extrusion" indicates the existence of a previously unrecognized mechanism whereby a fungal pathogen can escape the intracellular confines of mammalian macrophages to continue propagation and, possibly, dissemination.