Toxoplasma Endocytosis of Host Cytoplasm

Innate immune defenses help restrict growth of the protozoan pathogen Toxoplasma gondii, but their efficacy is offset by parasite counter defenses. For example, infected macrophages attempt to destroy the parasite by expressing immunity-related GTPases (IRGs) that load onto and dismantle the parasitophorous membrane, but the parasite thwarts this by phosphorylating Irg proteins to diminish their activity. Since phosphorylation is a reversible, and therefore temporary, we hypothesize that T. gondii additionally eliminates these host effector proteins permanently by internalizing them into its endocytic system. Our recent unexpected finding that the parasite readily internalizes a reporter protein expressed in the host cytosol supports this hypothesis. Further, the reporter protein is most abundantly seen in protease-deficient parasites, suggesting that internalized host proteins are normally digested in the parasite endocytic system. Also, parasites deficient in the biogenesis of tubular membranes within the parasitophorous vacuole are unable to efficiently internalize the reporter protein, indicating that the tubular membranes are required for uptake. These deficient parasites are also highly susceptible to killing by activated macrophages. <p/>The specific aims are to: (1) confirm the parasite endocytic system as the destination of internalized proteins; and (2) show that Toxoplasma uses the intravacuolar network of tubular membranes to internalize IRGs for destruction in its endolysosomal system. We expect to show that the parasite eliminates IRGs by endocytosis and degradation as an immune evasion strategy. The work is anticipated to open exciting new opportunities to diminish parasite intracellular survival during infection.