Immunological correlates of protection mediated by a whole organism vaccine deficient in chitosan.

Unlabelled: The global burden of infections due to the pathogenic fungus is substantial in persons with low CD4 T cell counts. Previously, we deleted three chitin deacetylase genes from to create a chitosan-deficient, avirulent strain, designated which, when used as a vaccine, protected mice from challenge with virulent strain 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 to vaccination. 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.

Importance: The fungus is 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 of , designated . When used as a vaccine, protects mice against a subsequent challenge with a virulent strain. 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.