Abstract
Purpose :
Toxoplasma gondii, a globally distributed neurotropic parasite, is a leading cause of posterior uveitis and encephalitis. We investigated (i) whether T. gondii cysts preferentially localise in different regions of the retina and brain in a mouse model, and (ii) whether resident microglia are activated in a region-dependent manner during acute infection.
Methods :
C57Bl/6J mice were intraperitoneally inoculated with T. gondii Pru-tdTomato tachyzoites (low-dose [5x103] n=5; high-dose [1x104] n=5) or PBS (n=5). In vivo multimodal fundus imaging was performed every 7 days to monitor clinical disease and initial retinal parasite invasion. Eyes and brains were collected at day 28 post-infection (or earlier if mice exhibited signs of severe infection); retinal wholemounts and brain sections were then processed for immunostaining (using Tmem119 and MHC class II antibodies) and confocal microscopy. T. gondii cyst burden and microglia density, field area and MHC class II expression were quantified using FIJI.
Results :
Clinical disease (retinal lesions and perivascular cuffing) and tdTomato+ T. gondii parasites were observed in infected mice from day 14 using in vivo fundus imaging. Examination of fixed tissues revealed that cysts were detected only in mice infected with high-dose T. gondii. In the retina, T. gondii cysts were exclusively localised in the ganglion cell layer (GCL) and inner plexiform layer (IPL); whereas in the brain a predilection for cyst formation in the cortex occurred. Despite the regional specificity of T. gondii cysts, Tmem119+ microglia activation occurred throughout the retina and brain, evidenced by MHC class II upregulation and reduced microglia field area in all examined CNS regions (retina: GCL, IPL, and outer plexiform layer; brain: cortex, olfactory bulbs, hippocampus, cerebellum) compared to controls (p<0.05). T. gondii infection was also associated with a large increase in Tmem119- MHC class II+ cells in the retinal GCL, and upregulation of MHC class II on brain vasculature.
Conclusions :
T. gondii preferentially forms cysts in the innermost layers of the neural retina and the cerebral cortex; however, microglia activation is not restricted to these regions. Understanding the role of innate immunity in control of T. gondii replication in the CNS may lead to novel immunotherapeutic targets for toxoplasmosis.
This is a 2021 ARVO Annual Meeting abstract.