Detection and characterization of membrane antigens of Toxoplasma gondii

Toxoplasma gondii tachyzoites were surface radioiodinated by the lactoperoxidase technique, and the solubilized membrane proteins were analyzed by two-dimensional polyacrylamide gel electrophoresis. Four major labeled proteins with apparent m.w. of 43,000, 35,000, 27,000, and 14,000 were detected. None of the radioiodinated proteins bound to concanavalin A-Sepharose. When a panel of eight different fluorescein-conjugated lectins was used in an attempt to characterize further the nature of the cell membrane, none of the lectins bound to intact tachyzoites. Two-dimensional polyacrylamide gel electrophoresis did not reveal any significant differences among three different strains of Toxoplasma. Each of the radioiodinated surface proteins was precipitable by sera from mice chronically infected with the same strain as well as by a series of sera from mice infected with other strains. Sera from humans with acute Toxoplasma infection showed more variability in that some precipitated all labeled proteins whereas others precipitated only two or three of them. Monoclonal antibodies (2G11 and 3E6) prepared by hybridization of spleen cells from Toxoplasma-immune mice with myeloma cells consistently precipitated both the solubilized 35,000 and 14,000 dalton proteins, whereas 1E3 precipitated the 43,000-dalton protein and 1E11 the 27,000-dalton protein.