Roles of Dendritic Cells in Immunity to Toxoplasma gondii

Toxoplasma gondii is an intracellular protozoan parasite that actively invades host cells. During toxoplasmosis, dendritic cells (DCs) promote CD4+ and CD8+ T cell responses, which are critical for long-term immunity. Despite this critical role of DCs, questions remain regarding how this population is regulated during infection, and the specific types of interactions (phagocytosis or active invasion) between parasites and DCs that are necessary to induce T cell responses. Previous studies have observed an infection-induced expansion of DC populations during toxoplasmosis, but the factors that regulate this expansion are currently unknown. Mice deficient in the cytokine Flt3L, which promotes the proliferation and differentiation of DCs and their precursors, were therefore infected with T. gondii to determine the role of this cytokine during infection. Flt3L KO mice (which have decreased numbers of DCs) were acutely susceptible to toxoplasmosis, associated with uncontrolled parasite growth, decreased production of the critical cytokines IL-12 and Interferon-gamma, and impaired natural killer cell responses. Infection of these mice induced an expansion of DCs and induced CD4+ and CD8+ T cell proliferation, however these T cells displayed impaired functionality. Administration of exogenous IL-12 prevented acute susceptibility in Flt3L KO mice, thus demonstrating that Flt3L-independent mechanisms contribute to T cell priming and DC expansion during toxoplasmosis, while also highlighting the critical role of DCs as sources of IL-12. To determine what types of interactions between DCs and parasites lead to T cell responses, a non-replicating strain of T. gondii was utilized in combination with a novel approach to track parasite fate in vivo that could distinguish active invasion from phagocytosis, thus enabling the functional and phenotypic properties of infected DCs to be compared with those that had phagocytosed parasites, while controlling for parasite burden. These studies revealed infected DCs to display an activated phenotype unique from DCs that phagocytosed parasites, and to be potent inducers of T cell responses. Furthermore, pharmacological inhibition of invasion abrogated T cell responses, thus demonstrating a critical role for actively infected host cells in inducing adaptive immunity