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Cultured Embryonic Retina Systems as a Model for the Study of Underlying Mechanisms of Toxoplasma gondii Infection

From the Instituto de Biofísica Carlos Chagas Filho, Centro de Ciências da Saúde, Universidade Federal do Rio de Janeiro, Rio de Janeiro, Brasil.

PURPOSE. Toxoplasma gondii, the most common cause of retinochoroiditis in humans, is an obligate intracellular protozoan parasite that depends on te host cell’s microenvironment to proliferate. Because congenital infection is associated with a higher risk of ocular involvement than a postnatally acquired infection, this study was conducted to investigate the ability of Toxoplasma gondii to infect retinal tissue during development, when cellular environmental changes normally occur.

METHODS. Retinas from 5- to 9-day-old chick embryos were used. Stationary cultures were prepared in 24-well cell culture dishes and maintained at 37°C in DMEM plus 5% fetal bovine serum for 2 to 6 days. Then the wells were infected with 4 x 10⁵ tachyzoites. Retina explants and aggregate cell cultures were maintained in DMEM under rotation at 37°C. T. gondii proliferation was measured using [³H]-thymidine incorporation after 72 hours. Ornithine and arginine decarboxylase (ODC and ADC) activities were determined by measuring CO₂ production from [1-¹⁴C]-ornithine and [1-¹⁴C]-arginine, respectively.

RESULTS. The proliferation of tachyzoites was high in dense, stationary cultures expressing elevated ODC and ADC activity. The addition of ODC or ADC inhibitors reduced T. gondii proliferation by approximately 20% to 40%. As for cultured retina cells, retina explants also allowed T. gondii proliferation whenever ODC activity was high.

CONCLUSIONS. The data suggest a direct correlation between retinal polyamine biosynthesis and the proliferation of T. gondii, in agreement with the observation that individuals infected congenitally have a greater risk of development of toxoplasmic retinochoroiditis.

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