The distribution of Toxoplasma gondii cysts and their interaction with glial cells in the retina in chronic ocular toxoplasmosis
Hyun Beom Song1,2, Min-Ho Choi1,2
1Department of Parasitology and Tropical Medicine, Seoul National University College of Medicine, Seoul, Korea; 2Institute of Endemic Diseases, Seoul National University Medical Research Center, Seoul, Korea
The conversion of tachyzoites into bradyzoites is a way for Toxoplasma gondii to establish a chronic and asymptomatic infection and achieve lifelong persistence in the host. The bradyzoites form tissue cysts in the retina, but not much is known about the cyst distribution and neuroinflammatory responses around the cysts. A chronic ocular toxoplasmosis model was induced by peroral administration of T. gondii Me49 strain cysts to BALB/c mice. Two months after the infection, retinas were flat-mounted and immunostained to detect cysts, ganglion cells, Müller cells, astrocytes and microglial cells, followed by observation under fluorescence and confocal microscope. The horizontal distribution showed a rather clustered pattern but the clusters were not restricted to certain location of the retina. Axial distribution was confined to the inner retina, mostly in ganglion cell layer or the inner plexiform layer. Both ganglion cells, a type of retinal neurons, and Müller cells, predominant retinal glial cells, could harbor cysts. The cysts were spatially separated from astrocytes, the most abundant glial cells in the ganglion cell layer, while close spatial distribution of microglial cells were observed in two thirds of retinal cysts. In this study, we demonstrated the retinal cysts were not evenly distributed horizontally and were confined to the inner retina axially. Both neurons and one type of glial cells could harbor cysts and topographic analysis of other glial cells suggests role of microglial cells in chronic ocular toxoplasmosis.