The role of macrophages in protection against Cryptococcus neoformans

The Wormley laboratory focuses on protective immunity against the opportunistic fungal pathogen, Cryptococcus neoformans, which primarily affects immunocompromised individuals. Studies in our laboratory employ a C. neoformans strain engineered to express interferon-γ (IFN-γ), designated H99γ, which induces a Th1-type immune response, increased leukocyte infiltration and complete protection. Furthermore, immunization of mice with H99γ protects mice against challenge with WT cryptococci, resulting in sterilizing immunity correlating with classical macrophage activation and enhanced phosphorylation of STAT1 in these cells. Studies herein showed that mice with macrophage-specific STAT1 ablation are deficient in classical macrophage activation and are not protected against C. neoformans H99γ. In addition, we showed that classically activated macrophages utilize nitric oxide, and not reactive oxygen species, to control the intracellular proliferation of Cryptococcus. Studies in our laboratory indicate that an innate cell population is responsible for protective immunity against C. neoformans. Remarkably, macrophages isolated from protectively immunized mice 70 days post-immunization have enhanced pro-inflammatory responses against C. neoformans. Whole transcriptome analysis has revealed that macrophages in the lungs of protectively immunized mice up-regulate the STAT1 pathway at day-one post-challenge without the addition of exogenous IFN-γ. Activation of the STAT1 pathway occurs so quickly, it is unlikely that T cells have the time to produce IFN-γ to activate the macrophages, suggesting that the macrophages are primed to respond to secondary stimuli. Interestingly, inhibitors of specific histone methyltransferases reduce cytokine production from primed macrophages as well as abrogate STAT1 phosphorylation, which is required for classical macrophage activation and protection. Altogether, these studies suggest that priming in vivo with C. neoformans/IFN-γ results in histone modifications in the macrophages that train these cells to rapidly upregulate the STAT1 pathway and polarize to an anti-cryptococcal phenotype. These data provide proof of concept that protective immunity can be achieved in immunocompromised host populations. We have also identified an effector cell population and mechanism by which the host can mount a protective immune response to C. neoformans and possibly other organisms where STAT1-mediated macrophage activation is conducive to protection.