Characterisation of the Virulence and Determinants of Virulence of C. Neoformans var.grubii Genotypes from HIV Infected and Uninfected Patients in Vietnam

Lam Thanh Tuan (2018). Characterisation of the Virulence and Determinants of Virulence of C. Neoformans var.grubii Genotypes from HIV Infected and Uninfected Patients in Vietnam. PhD thesis The Open University.

DOI: https://doi.org/10.21954/ou.ro.0000d8ff

Abstract

C. neoformans var. grubii is the main cause of life-threatening cryptococcal meningitis worldwide, especially in individuals with underlying immunosuppressive conditions such as HIV infection. In contrast to its sister species C. gattii which is now considered a primary human pathogen, C. neoformans var. grubii does not commonly infect apparently healthy individuals. However, it is now clear that C. neoformans var. grubii infection also occurs in apparently healthy individuals in Vietnam, China and elsewhere in Southeast/East Asia. This could be explained by two possibilities: (i) disease in the immunocompetent is associated with strains with increased pathogenic potential, or (ii) there are unknown immune defect(s) that confers increased susceptibility in affected individuals. This thesis investigates the former hypothesis by systematically assessing and comparing genetic composition, virulence-associated phenotypes and transcriptomes of C. neoformans var. grubii isolates from Vietnam from HIV-infected and HIV-uninfected patients.

MLST-based interrogation of population structure of clinical C. neoformans var. grubii isolates from Vietnam and Laos reveal that the population from Vietnam is unique and intermediate in composition between that from Thailand/Laos to the West/Southwest and China to the East/Northeast. Almost all disease in HIV-uninfected patients is associated with the MLST ST5 lineage. Comparison of in vitro and in vivo (mouse and macrophage models) virulence-associated phenotypes reveals lineage-specific differences. Specifically, ST5 strains appear with more phenotypic variability, increased intracellular replication inside macrophages, and induction of more robust immune responses but apparent decreased virulence in mice. RNAseq profiling under log-phase growth highlights genotype-specific differential expression of key processes which may contribute to virulence. My data suggest that higher ST5 prevalence in apparently immunocompetent patients in Vietnam are not necessarily associated with the ability to cause more severe in-host damage and mortality, but likely represent better adaptation to the innate immune system and establishing infection in immunocompetent hosts.

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