Association of host cell intermediate filaments with Toxoplasma gondii cysts in murine astrocytes in vitro.

Toxoplasma gondii is an intracellular parasite that is a common opportunistic infection of AIDS patients where it causes a severe and often fatal encephalitis. Toxoplasmic encephalitis in AIDS patients results from a reactivation of the cyst stage of Toxoplasma gondii in the brain. A previous study found an association of host cell intermediate filaments with parasitophorous vacuoles and some studies have suggested the host cell cytoskeletal elements are incorporated into the cyst wall. In this study, the interaction of glial filaments with Toxoplasma gondii cysts was studied in cysts derived in vitro in mouse astrocytes and in cysts isolated from mouse brains. Glial filaments, detected by immunostaining of the glial fibrillary acidic protein, were found to accumulate around the perimeter of the cysts as they developed in mouse astrocytes. Transmission electron microscopy revealed a layer of glial filaments was wrapped around the cytoplasmic side of the cyst. The glial filaments were present in close apposition to the cyst wall and arranged around the cysts in a concentric layer, measuring 5-10 microns in thickness. The layer of glial filaments excluded host cell mitochondria and endoplasmic reticulum from the cytoplasmic surface of the cyst. Colocalisation of glial fibrillary acidic protein and the cyst wall via confocal and immunoelectron microscopy, confirmed that there was no glial fibrillary acidic protein present within the cyst wall. The cyst wall of cysts isolated from mouse brains were also found to be negative for glial fibrillary acidic protein. In conclusion, we found no evidence of structural integration of the host cell intermediate filaments in the cyst wall, but glial filaments were found to encase the cysts in the host cell during cyst development in host cells in vitro. The glial filaments wrapping of cysts may play a role in bradyzoite differentiation and/or cyst stabilisation in the host cell cytoplasm.