Toxoplasma gondii: differential location of antigens secreted from encysted bradyzoites

Since we have previously demonstrated the protective role against infection played by Toxoplasma excreted-secreted antigens (Darcy et al. Parasite Immunology, 10, 553-567, 1988), the aim of the present work was an attempt to precisely define the location of GRA1, GRA2, and GRA5 in both the tachyzoite and the bradyzoite stages from distinct strains, in order to explore the mechanisms of secretion by Toxoplasma gondii. Three monoclonal antibodies (Charif et al. 1990) and colloidal immunogold labeling were used to localize the 27-, 28.5-, and 21-kDa target antigens to the matrix of the dense granules of tachyzoites and bradyzoites. They were, moreover, detected in the parasitophorous vacuole and in the cyst ground substance after host cell invasion. Our data suggest that a selective sorting mechanism for dense-granule contents exists at least in encysted bradyzoites. GRA2 was found preferentially associated with the ground substance of the cyst wall and the tubular elements of the network of the modified host cell phagosome, whereas GRA5 was located on the delimiting membrane of both the cyst wall and the parasitophorous vacuole. These observations reveal the selective targeting of dense-granule molecules, which could have different functions and fates when exocytosed into the parasite-containing vacuole.