Penetration-enhancing factor extracted from Toxoplasma gondii which increases its virulence for mice.

Fractionation of extracts of disintegrated Toxoplasma gondii resulted in preparations which, when tested in cell cultures, enhanced penetration of host cells by this parasite. The influence of the preparations on the virulence of the RH strain and on two naturally avirulent strains for mice was studied simultaneously with the in vitro studies. It was demonstrated that the preparations increased the virulence of Toxoplasma. Mice inoculated with the preparation plus Toxoplasma suffered earlier morbidity and had a greater mortality rate than the control animals. There was a direct correlation between the effect of the preparations on the cell-penetrating capacity of the parasites and the increase in virulence for mice, indicating that the same mechanism is operative in both the in vivo and in vitro systems observed.