Host Cell Penetration of Toxoplasma gondii

The host cell penetration of Toxoplasma gondii was studied with reference to synthesis of a penetration-enhancing factor and the penetrative capacity of the parasites during the various phases of the infectious cycle. HeLa cells as suspension or monolayer cultures were infected with Toxoplasma. After a lag phase shorter than 1 hr, there was a synthesis of penetration-enhancing factor demonstrable in the parasites. Exposure of the factor to host cells resulted in disappearance of its penetration-enhancing activity. The findings were in accord with the hypothesis that Toxoplasma gondii is capable of synthesizing an enzyme-like factor, reacting with the host cell surface, and, thereby, facilitating the penetration of the parasites. This hypothesis was further supported by the observations that the penetrative capacity of extracellular parasites gradually was reduced in presence of host cells and that intracellular parasites released artificially from host cells shortly after penetration were found to have a reduced penetrative capacity. Parasites allowed to remain for 4 hr or more intracellularly could restore and markedly improve their penetrative capacity.