Immunological correlates of protection mediated by a whole organismCryptococcus neoformansvaccine deficient in chitosan

Abstract

AbstractThe global burden of infections due to the pathogenic fungusCryptococcusis substantial in persons with low CD4+T cell counts. Previously, we deleted three chitin deacetylase genes fromC. neoformansto create a chitosan-deficient, avirulent strain, designatedcda1Δ2Δ3Δwhich, when used as a vaccine, protected mice from challenge with virulentC. neoformansstrain KN99. Here, we explored the immunological basis for protection. Vaccine-mediated protection was maintained in mice lacking B cells or CD8+T cells. In contrast, protection was lost in mice lacking α/β T cells or CD4+T cells. Moreover, CD4+T cells from vaccinated mice conferred protection upon adoptive transfer to naive mice. Importantly, while monoclonal antibody-mediated depletion of CD4+T cells just prior to vaccination resulted in complete loss of protection, significant protection was retained in mice depleted of CD4+T cells after vaccination, but prior to challenge. Vaccine-mediated protection was lost in mice genetically deficient in IFNγ, TNFα, or IL-23p19. A robust influx of leukocytes and IFNγ- and TNFα-expressing CD4+T cells was seen in the lungs of vaccinated and challenged mice. Finally, a higher level of IFNγ production by lung cells stimulated ex vivo correlated with lower fungal burden in the lungs. Thus, while B cells and CD8+T cells are dispensable, IFNγ and CD4+T cells have overlapping roles in generating protective immunity prior tocda1Δ2Δ3Δvaccination. However, once vaccinated, protection becomes less dependent on CD4+T cells, suggesting a strategy for vaccinating HIV+persons prior to loss of CD4+T cells.ImportanceThe fungusCryptococcus neoformansis responsible for >100,000 deaths annually, mostly in persons with impaired CD4+T cell function such as AIDS. There are no approved human vaccines. We previously created a genetically engineered avirulent strain ofC. neoformans, designatedcda1Δ2Δ3Δ. When used as a vaccine,cda1Δ2Δ3Δprotects mice against a subsequent challenge with a virulentC. neoformansstrain. Here, we defined components of the immune system responsible for vaccine-mediated protection. We found that while B cells and CD8+T cells were dispensible, protection was lost in mice genetically deficient in CD4+T cells, and the cytokines IFNγ, TNFα, or IL-23. A robust influx of cytokine-producing CD4+T cells was seen in the lungs of vaccinated mice following infection. Importantly, protection was retained in mice depleted of CD4+T cells following vaccination, suggesting a strategy to protect persons who are at risk for future CD4+T cell dysfunction.