IN VITRO AND IN VIVO CHARACTERISATION OF ISOLATES OF CRYPTOCOCCUS NEOFORMANS CAUSING MENINGITIS IN HIV-INFECTED AND UNINFECTED PATIENTS IN VIETNAM

Most cryptococcal meningitis cases occur in immunocompromised individuals and are caused by Cryptococcus neoformans var.grubii. We previously reported cryptococcal meningitis in Vietnam arising atypically in HIV-uninfected, apparently immunocompetent patients, caused by a single amplified fragment length polymorphism (AFLP) cluster of C. neoformans (VNI-γ). This variant was less common in HIV-infected individuals. It is unclear why this lineage is associated with HIV-uninfected, apparently immunocompetent patients. To better delineate the phenotypic characteristics of C. neoformans VNI-γ in comparison with other C. neoformans lineages we selected 30 representative isolates and compared their genotype-specific in vitro pathogenic potential and in vivo virulence, and ability to induce inflammatory response in the A/J mouse model. All isolates were genotyped using multi-locus sequence typing. All previously AFLP-defined VNI-γ isolates were identified as sequence type (ST) 5. Compared with non-ST5 strains, ST5 isolates had significantly increased variability in cell and capsule size (pu003c0.001). ST5 and non-ST5 strains exhibited similar characteristics in relation to previously defined virulence factors including melanization, growth at 37oC, and growth in cerebrospinal fluid. However, A/J mice infected with ST5 isolates had significantly longer survival and lower fungal burdens at day 7 and death compared with non-ST5 isolates. ST5 isolates induced significantly greater initial inflammatory responses than non-ST5 strains, measured by TNF-α concentrations (pu003c0.001). Despite being generally less virulent in the mouse model, we hypothesize that the significant within strain variation seen in ST5 isolates in the tested phenotypes may represent an evolutionary advantage enabling adaptation to novel niches including apparently immunocompetent human hosts.