Toxoplasma gondii-infected human myeloid dendritic cells induce T-lymphocyte dysfunction and contact-dependent apoptosis

Dendritic cells ignite adaptive immunity by priming naive T lymphocytes. Human monocyte-derived den dritic cells (MDDCs) infected with Toxoplasma gondii induce T-lymphocyte gamma interferon production and may thus activate T. gondii-specific immunity. However, we now demonstrate that T. gondii-infected MDDCs ar poor at activating T lymphocytes and are unable to induce specific cytotoxic T lymphocytes. On the other hand MDDCs acquiring nonviable T. gondii antigens directly, or indirectly through captured apoptotic or necrotic cell bodies, induce potent T-lymphocyte activation. T lymphocytes exposed to infected MDDCs are significantly impaired in upregulation of CD69 and CD28, are refractory to activation, and die through contact-dependent apoptosis mediated by an as-yet-unidentified mechanism not requiring Fas, tumor necrosis factor-related apoptosis-inducing ligand, leukocyte function antigen 1, intercellular adhesion molecule 1, tumor necrosis factor alpha, interleukin 10, alpha interferon, gamma interferon, prostaglandins, or reactive nitrogen inter mediates. Bystander T lymphocytes that were neither infected nor apoptotic were refractory to activation suggesting global dysfunction. Immunosuppression and T-lymphocyte unresponsiveness and apoptosis ar typical of acute T. gondii infection. Our data suggest that infected dendritic cells contribute to these processes On the other hand, host cells infected with T. gondii are resistant to multiple inducers of apoptosis. Thus regulation of host cell and bystander cell apoptosis by viable T. gondii may be significant components of strategy to evade immunity and enhance intracellular parasite survival.