Functional inactivation of immature dendritic cells by the intracellular parasite Toxoplasma gondii

Despite its noted ability to induce strong cellular immunity, and its known susceptibility to IFN-γ-dependent immune effector mechanisms, the protozoan Toxoplasma gondii is a highly successful parasite, able to replicate, disseminate, and either kill the host or, more commonly, establish resistant encysted life forms before the emergence of protective immune responses. We sought to understand how the parasite gains the advantage. Using transgenic clonal parasite lines engineered to express fluorescent markers in combination with dendritic cells (DC) grown from the bone marrow of wild-type mice or transgenic mice expressing fluorescent protein-tagged MHC class II molecules, we used flow cytometry and fluorescence microscopy to analyze the responses of infected DC to both invasion by the parasite and subsequent DC maturation signals. We found that T. gondii preferentially invades immature dendritic cells but fails to activate them in the process, and renders them resistant to subsequent activation by TLR ligands of the immune-system-intrinsic maturation signal CD40L. The functional consequences of T. gondii-mediated suppression of DC activation are manifested in a relative inability of infected immature DC to activate naive CD4⁺ Th lymphocytes, or to secrete cytokines, such IL-12 and TNF-α, that play important roles in innate and/or adaptive immunity. The findings reveal that T. gondii suppresses the ability of immature DC to participate in innate immunity and to induce adaptive immune responses. The ability of T. gondii to temporarily evade recognition could provide a selective advantage that permits dissemination and establishment before adaptive immune response initiation.