Characterization of a protective systemic anticryptococcal immune response induced by mucosal immunization with heat-killed Cryptococcus neoformans yeast cells and analysis of a non-human primate model of cryptococcosis

Cryptococcus neoformans is an encapsulated yeast-like organism that is the causative agent of cryptococcosis. Infection is rare in healthy hosts and is usually associated with immune compromised states, especially decreased cell mediated immunity (CMI). Currently, there are no vaccines against C. neoformans. Immunization via the mucosal route has proven to be effective in inducing both humoral and cell-mediated immune responses against a variety of antigens. To determine whether protective immunity against C. neoformans could be induced by mucosal immunization using a mutant of E. coli heat-labile toxin as an adjuvant, mice were given three weekly intranasal immunizations and were infected intravenously on week 4 with C. neoformans to model a systemic infection. Protection was shown by lowered fungal burden in target organs and increased survival following challenge. The immunization strategy resulted in the involvement of both humoral and cellular arms of the immune system, as shown by delayed type hypersensitivity response and serum antibody titers, respectively. This response results in increased survival. Antibody depletion shows that CD4+ T cells are dispensable for cellular response and lowered fungal burden. Many of the studies on pathogenesis of and host defense against C. neoformans have utilized animal models without the immunodeficiency associated with AIDS which would be helpful in studying cryptococcosis and host defense against C. neoformans. In this study, we show that a model infection is possible using SIV- (simian immunodeficiency virus) infected Rhesus monkeys. 5 previously SIV-infected animals were infected with C. neoformans via two routes---intravenous and bronchially---or a combination of these two routes. All animals showed signs of infection within 2 weeks post-infection. Signs of infection included: presence of viable C. neoformans yeast in CSF and BAL fluid and/or cryptococcal antigen in CSF and serum. At termination of the experiment those monkeys that received C. neoformans i.v. demonstrated severe granulomatous inflammation in the brain due to cryptococcal meningoencephalitis; whereas, monkeys that were infected bronchially exhibited granulomatous inflammation in the lungs due to pulmonary cryptococcosis. This portion of the study indicates that the SIV-infected Rhesus model is an excellent means of studying cryptococcal infection in the context of AIDS-like disease.