<i>Cryptococcus neoformans</i>Virulence Is Enhanced after Growth in the Genetically Malleable Host<i>Dictyostelium discoideum</i>

<jats:title>ABSTRACT</jats:title><jats:p><jats:italic>Cryptococcus neoformans</jats:italic>is an encapsulated, environmental fungus that can cause life-threatening meningitis. Pathogenicity of<jats:italic>C. neoformans</jats:italic>for macrophages and vertebrate hosts may be a mechanism selected in evolution for protection against environmental predators. In this study, we investigated whether<jats:italic>Dictyostelium discoideum</jats:italic>could serve as an alternate host for<jats:italic>C. neoformans. D. discoideum</jats:italic>has a defined genetic system which provides significant advantages for the study of fungus-amoeba interactions. Our results show that<jats:italic>D. discoideum</jats:italic>is susceptible to infection with<jats:italic>C. neoformans</jats:italic>and that the interactions are similar to those described previously for this fungus with macrophages and<jats:italic>Acanthamoeba castellanii</jats:italic>. Acapsular<jats:italic>C. neoformans</jats:italic>cells did not replicate when coincubated with<jats:italic>D. discoideum</jats:italic>. However, incubation of acapsular<jats:italic>C. neoformans</jats:italic>with<jats:italic>D. discoideum</jats:italic>mutants defective in myosin VII synthesis resulted in infection, validating the concept that avirulent organisms can be virulent in impaired hosts even at the unicellular level. Phagocytosis of<jats:italic>C. neoformans</jats:italic>by<jats:italic>D. discoideum</jats:italic>could be inhibited with capsule-specific antibodies and various sugars. Passage of an encapsulated<jats:italic>C. neoformans</jats:italic>strain through<jats:italic>D. discoideum</jats:italic>cultures increased virulence and was accompanied by larger capsules and faster time to melanization. These results add to the evidence implicating soil ameboid predators as important factors for the maintenance of<jats:italic>C. neoformans</jats:italic>virulence in the environment and suggest that<jats:italic>D. discoideum</jats:italic>promises to be an extremely useful system for studying the interaction of<jats:italic>C. neoformans</jats:italic>with phagocytic cells.</jats:p>