Response of Cryptococcus neoformans to protozoa

Abstract

Cryptoccocus neoformans is an encapsulated environmental fungus that is pathogenic for humans, yet has no need for replication in a host to maintain its virulence factors. One hypothesis regarding the maintenance of these virulence factors is that they are the result of interactions with other organisms in the environment. Another environmental organism, Acanthamoeba castellanii, has been shown to interact with C. neoformans. Our work has also shown the ability of C. neoformans to also interact with environmental Paramecium species, indicating that interactions in the soil may hold clues to the development of cryptococcal virulence factors.;Our investigations into the interaction of C. neoformans and A. castellanii have revealed an important role for the polysaccharide capsule. The capsule naturally increases in size in mammalian tissue and can be artificially induced in the laboratory. We demonstrated that when the capsules of multiple C. neoformans strains were enlarged artificially, the ability of A. castellanii to phagocytose was greatly diminished. Further studies comparing the phagocytosis of wild type, acapsular, and complemented strains confirmed the antiphagocytic role of the capsule. Additionally, we found that co-incubation of these two organisms could result in dramatic capsular enlargement. This capsular enlargement occurs in the presence of both live and dead A. castellanii and J774 macrophage-like cells and requires cell contact and the production of cryptococcal phospholipase B. We found that the polar lipid fraction of A. castellanii and J774 macrophage-like cells caused the enlargement.;Previous studies have also identified the phenomenon of non-lytic exocytosis from macrophages. We made the novel observation of C. neoformans exiting from A. castellanii without apparent damage to either organism. Cryptococcal exocytosis from amoeba was strain-dependent, actin-dependent, and required fungal viability. Additionally, the presence of the capsule was inversely correlated with exocytosis.;The enlargement in capsule size, which decreases phagocytosis, and the ability to extrude following phagocytosis are survival strategies with parallels in the mammalian host, suggesting that these traits are selected for in the environment to avoid predation. This lends further support to the hypothesis that C. neoformans virulence factors are selected for and maintained by interactions with environmental organisms.