Antitumor effect of formalin-fixed Toxoplasma gondii organisms on EL4 lymphoma in Toxoplasma-infected mice

The antitumor effect of formalin-fixed Toxoplasma organisms (f-Tp) as an immunostimulant on EL4 lymphoma was examined in Toxoplasma-infected syngeneic female C57B1/6 mice. A potent antitumor effect, a marked suppression of tumor growth, as well as a prolongation of lifespan, was induced by an injection with 10(7) f-Tp in a mixture of 2.5 X 10(5) EL4 cells. The antitumor effect of f-Tp could be observed regardless of the time, route, or dose of infection. The peritoneal exudate cells induced by f-Tp in Toxoplasma-infected mice showed an antitumor activity when the cells were implanted with tumor cells in normal mice, indicating that the cells activated by f-Tp caused the antitumor effect of f-Tp. The effect of f-Tp on EL4 in Toxoplasma-infected mice was significantly stronger than that of 10(7) live bacillus Calmette-Guerin organisms (BCG) in BCG-sensitized mice.