Cryptococcus gattiiIsolates from the British Columbia Cryptococcosis Outbreak Induce Less Protective Inflammation in a Murine Model of Infection thanCryptococcus neoformans

Abstract

The fungal pathogen Cryptococcus neoformans causes approximately one million cases of cryptococcosis per year in people with AIDS. In contrast, the related species C. gattii is responsible for a much smaller number of cases, but these often occur in immunocompetent people. In fact, C. gattii has emerged in the last decade as the frequent cause of cryptococcosis in otherwise healthy people in British Columbia. We analyzed the immune responses elicited by three C. gattii strains and one C. neoformans strain in mice as a first step toward understanding why C. gattii is able to cause disease in immunocompetent hosts. The C. gattii strains all induced a less protective inflammatory response in C57BL/6 mice by inhibiting or failing to provoke the migration of neutrophils to sites of infection. The C. gattii strains also failed to elicit the production of protective cytokines, such as tumor necrosis factor alpha, compared to the ability of the C. neoformans strain. Despite these differences, the strain representing the major outbreak genotype from British Columbia showed a virulence equivalent to that of the C. neoformans strain, while two other C. gattii strains had reduced virulence. Taken together, our results indicate that C. gattii strains thrive in immunocompetent hosts by evading or suppressing the protective immune responses that normally limit the progression of disease caused by C. neoformans.

Cryptococcus gattii Infections

“…Despite these differences in vitro, these two strains produced similar neutrophil responses in the rat lung (162). In C57BL/6 mice infected with C. gattii VGII Vancouver Island outbreak strains R265 and R272, Australian environmental strain WM 276, and C. neoformans strain H99, early migration of neutrophils to sites of infection and production of protective cytokines were reduced in mice infected with the C. gattii strains, but strain R265 and C. neoformans strains were equally virulent in mice and more virulent than the other two C. gattii strains (152).…”

“…However, such global assessments of virulence may not yield consistent results, suggesting that molecular type may be a marker, but not a determinant, of virulence. For example, in mice, the clinical type strain of C. neoformans, H99, was less virulent than the C. gattii VGIIa type strain, R265, from the Vancouver Island outbreak (1, 150) but was equally virulent in another study (152). Indeed, threshold inocula required for disease expression and, in some cases, organ burdens are strain dependent (153), and individual isolates of the same molecular (sero)type of C. gattii may be more or less virulent than isolates of C. neoformans (154).…”

Altered Immune Response Differentially Enhances Susceptibility to Cryptococcus neoformans and Cryptococcus gattii Infection in Mice Expressing the HIV-1 Transgene

“…Intranasal inoculation of the mice was performed as described previously (17). For the survival assay, animals reaching predetermined morbidity endpoints (Ͼ20% weight loss, immobile, no response when stimulated, or irregular/labored abdominal respiration) were designated premortem and euthanized with a lethal dose of ketamine and xylazine (18).…”

“…For all other assays, mice were euthanized on the indicated days. Quantification of Cryptococcus in internal organs, histopathology, and determination of Cryptococcus cell body diameters and capsule thicknesses in mucicarmine-stained tissue sections were done using methods described elsewhere (17,18,30 5 CFU of C. gattii R265 and assessed at 7 and 14 days postinfection. Uninfected control mice received intranasal PBS alone.…”

“…As a first step toward understanding the ability of C. gattii to cause disease in immunocompetent hosts, a previous study revealed reduced levels of neutrophil infiltration and reduced inflammatory cytokine production in the lungs of C57BL/6 mice infected with C. gattii com-pared to those of mice infected with C. neoformans var. grubii (17). However, a comprehensive analysis of virulence and host immune cell responses to these Cryptococcus species would be facilitated greatly by the availability of a relevant animal model of cryptococcosis in HIV infection.…”