ANTIGEN PRESENTATION BY TOXOPLASMA-INFECTED CELLS - ANTIGEN ENTRY THROUGH CELL-MEMBRANE FUSION

The antigen presentation of cells infected by Toxoplasma gondii (Tg.) for MHC class-I-restricted cytotoxic T cells (CTL) was blocked by treating the Tg.-infected cells with the fungal antibiotic brefeldin A. Electron microscopic analysis of Tg.-infected murine and human-antigen-presenting cells (APC) prepared by the combination of quick-freezing with the deep-etching or freeze-substitution methods revealed that the outer membrane of Tg. fused with some parts of the vacuolar membranes of APC to build up channel-like structures. Work station and cytofluorometry studies detected 2',7'-bis(carboxyethyl)-5 (6)-carboxyfluorescein (BCECF) in cells infected by BCECF-labeled Tg., which strongly suggested the direct entry into cells of BCECF derived from Tg. These findings provide evidence that Tg. antigens enter the cytoplasm of APC through cell membrane fusion of T.g. and Tg. -infected APC, and that Tg. -infected APC present Tg. antigen for MHC class-I-restricted CTL through the endogenous pathway of antigen presentation.