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I. Blader, E. Charles; Development of an Acute Ocular Toxoplasmosis Model in Mice . Invest. Ophthalmol. Vis. Sci. 2005;46(13):1173.
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Purpose: To develop a murine model that recapitulates the early inflammatory events in the eye in response to Toxoplasma tachyzoites. Methods: C57BL/6 mice were injected intravitreally with 0.5 µl saline or 0.5 µl saline containing either 1,000 or 10,000 tachyzoites. For these experiments, we used a Type 1 Toxoplasma RH strain because this strain doesn’t encyst in vivo and because Type I strains are the strain most often associated with ocular toxoplasmosis. At days 2, 4, and 6 post–injection, the injected and uninjected eyes underwent electroretinogram (ERG) analysis. The eyes were then harvested for immunohistochemistry to: A) assess retinal damage, B) characterize the infiltrating inflammatory cells, and C) determine where in the eye parasites were located. Results: (1) Eyes injected with 1,000 tachyzoites for 2 and 4 days were functionally and histologically normal. In contrast, ERG responses in eyes injected for 6 days were reduced ∼ 40% and exhibited slight cellular infiltration. (2) Eyes injected with 10,000 tachyzoites for 2 days were normal and 4 days post injection displayed slight pathological defects. Eyes injected with 10,000 tachyzoites for 6 days were completely non–responsive to light and exhibited severe chorioretinitis, vitreitis, destruction of the retinal architecture, and retinal detachment. Both mononuclear and polymorphonuclear cells were present in the eyes 6 days post injection and many of these cells stained with either GR1 or CD4 antibodies. This response required live, replicating parasites since no significant pathology was caused by either heat–killed or UV–irradiated parasites. High numbers of parasites could be detected in all layers of the retina as well as the optic nerve 6 days post injection. In contrast, few if any parasites were present in extraocular regions. Conclusions: Intravitreal injection of Toxoplasma tachyzoites into mouse eyes caused a highly reproducible pathology similar to that observed in human patients with acute ocular toxoplasmosis or in systemically infected mice that were later immune suppressed. Our murine model will allow us to address how ocular immune responses are activated by actively growing parasites and which parasite molecules are important virulence factors in the eye.
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