Association of host cell mitochondria with developing Toxoplasma gondii tissue cysts

Ultrastructure of the interactions of host cell mitochondria with developing Toxoplasma gondii tissue cysts was examined in cultured cells, using transmission electron microscopy of infected cells and rhodamine 123 (a mitochondria-specific vital fluorescent dye) staining of isolated tissue cysts. Structurally mature T gondii tissue cysts were observed as early as 2 days after inoculation of cultured cells. During development of T gondii, host cell mitochondria were observed surrounding the parasitophorous vacuole membrane. Mitochondria became flat and elongated in the vicinity of the parasitophorous vacuole membrane. These mitochondria were also closely associated with T gondii tissue cysts. Incubation of tissue cysts from cultured cells and tissue cysts from mouse brains with rhodamine 123 revealed fluorescence of the tissue cyst wall in living specimens. Incubation of tissue cysts with 10 microM rotenone caused diminished fluorescence of the tissue cyst walls, and 100 microM rotenone caused complete inhibition. Mouse RBC, and tissue cysts fixed in 100% methanol did not fluoresce after exposure to rhodamine. Tissue cysts in 9 isolates of T gondii from mouse brains were examined, using rhodamine 123, and the tissue cysts walls of all isolates fluoresced, indicating no isolate effects. Our results indicate that host cell mitochondria may be closely associated with the tissue cysts of T gondii in cell cultures and in mice.