Immunological Study on the Host Cell Penetration Factor of Toxoplasma gondii

The host cell penetration factor (PEF) of Toxoplasma gondii was studied by biochemical and immunological techniques. Sephadex gel filtration of an ammonium sulfate-precipitated PEF yielded two components with different molecular weight, but both having penetration-enhancing activity. The methods for purification removed at least 99.9% of extraneous protein. For demonstration of a significant enhancement of penetration, 0.001 mug of protein was sufficient. Biochemically, they appeared to be acid proteins with the same electrophoretic mobility. Both components showed maximal enhancement of penetration at pH 7.6 and 37 C. PEF antisera reduced the penetrative capacity of Toxoplasma parasites. The penetration-enhancing effect of the two components of PEF was inhibited by antiserum against any of them. Moreover, immunologically identical immunoprecipitates were obtained when antiserum reacted with the two components. The results thus indicated that the two components of PEF were immunogenically identical and that the difference in molecular weights might result from aggregation. Immunofluorescence indicated that PEF was related to cytoplasmic structures located in the anterior end of Toxoplasma. A possible relation between these structures and the paired organelle or the convoluted tubes was discussed. The number of parasites with immunofluorescence was low shortly after host cell penetration and increased during the intracellular life of the parasites after kinetics, previously observed for synthesis of PEF as well as for lysosomal activity of Toxoplasma.