Analysis of Sex Bias in Cryptococcus neoformans Infections

Abstract

Cryptococcus neoformans, a pathogenic yeast and the causative agent of cryptococcosis, is responsible for an estimated quarter million new cases of the disease resulting in more than 180,000 deaths each year worldwide. For decades, researchers have noticed that the prevalence of cryptococcosis is skewed between males and females. Numerous studies show sex-specific differences in C. neoformans infection rates, with males having a higher incidence of disease and death (~7M:3F). Sexual dimorphism in infection is not uncommon. However, the cause of these differences, in C. neoformans, has not been well elucidated. The aim of this project was to gain a deeper understanding of the causes underlying the sex bias observed in cryptococcosis. To accomplish this, a comprehensive approach was designed to examine the effect of sex-defining hormones on the pathogen itself as well as experiments to detect differences in the host-pathogen interface of males and females. There were three major components to this study. First, to establish whether sex hormones were taken up by C. neoformans, fluorescently tagged 17β-estradiol was incubated with wild type and mutant strains, and the results imaged on a confocal microscope. Second, to assess any changes in virulence factors in the presence of sex hormones, C. neoformans clinical isolates were grown in an environment designed to mimic the central nervous system (CNS) and a series of in vitro biochemical experiments were conducted to examine each of the virulence factors individually in the presence of sex hormones. Third, the response of healthy human male and female immune cells were evaluated during infection with C. neoformans. The results show that estrogen is, in fact, capable of entering C. neoformans at normal iv physiological levels seen in females. Additionally, a number of virulence factors are changed in the presence of estrogen and testosterone at normal physiological levels. Finally, the percentages of T cells differ between males and females during a C. neoformans infection, which may suggest differences in the adaptive immune response to this pathogen. Taken together, these results indicate that the sex bias observed for many years has a biologic basis and cannot simply be explained by environmental factors. Further, the sexual dimorphism stems from both differences in the pathogen and host-pathogen interface in the presence of a male and female environment.