Stage conversion ofToxoplasma gondii in mouse brain during infection and immunodepression

The kinetics and pattern of expression of bra-dyzoite-specific proteins were studied in mouse brain during infection withToxoplasma gondii. Parasites found in the brain 6 days after ingestion of cysts were expressing only tachyzoite-specific proteins (anti-SAGl antibodies being used as a marker). Bradyzoite-specific protein (Pb36) expression was first found after 9 days in vacuoles containing mixed parasites simultaneously expressing SAG1 and Pb36 or cysts containing parasites expressing only the bradyzoite marker. Reactivation of toxoplasmosis was studied in mouse brain using cortico-steroids for immunosuppression. Parasites expressing SAGI were first found 6 days after the beginning of treatment, but a very heterogeneous pattern was found throughout the study. We simultaneously found vacuoles containing parasites expressing only SAGI or containing intermediate stages or cysts containing parasites expressing only bradyzoite proteins. A striking observation was the multiplication of cysts in foci, suggesting that the immune suppression triggered the release of parasites from preexisting cysts but that the factors inducing bradyzoite development remained fully effective in driving parasites into this pathway.