Investigating Fungal Cell Death-Resistance in Clinical Isolates of Cryptococcus neoformans

Abstract

Cryptococcus neoformans and Cryptococcus gattii are fungal pathogens of significant public health burden. Cryptococcus neoformans claims over 100,000 lives each year, disproportionately affecting individuals who are immunocompromised. With the rise in treatment failures due to increased drug resistance, there is heightened awareness that new antifungal therapeutic strategies are needed. One unexploited strategy to eliminate fungal infections is the possibility of inducing programmed fungal cell death, analogous to targeted cancer therapies that trigger tumor cell suicide. However, little is known about how fungal cells die, or whether fungal cell death-resistance during infection may contribute to treatment failures. The goal of this thesis project is to determine if resistance to fungal cell death could be an important mechanism involved in the pathogenesis of cryptococcal infections. Cell death susceptibility was compared to several in vitro and in vivo assays commonly used to reflect aspects of virulence. Unlike laboratory strains, we demonstrated that most clinical isolates of Cryptococcus contain at least a subpopulation of cells that are significantly more death-resistant than laboratory strains. Cell death resistance phenotypes were compared to other assays for pathogenesis-related phenotypes, but only partial correlations were observed when comparing death-rates in long cultures, thermotolerance, a severe lung phenotype in male mice, HIV-AIDS status of patients with cryptococcal infections, as well as capsule sizes. In contrast, clinical isolates did not differ from wild type in melanization, or mouse mortality. In summary, resistance to cell death in vitro may be a useful assessment of pathogenic potential in human disease.