Antitumor effect of intralesional injection with formalin-fixed Toxoplasma gondii organisms on Lewis lung carcinoma in toxoplasma-infected mice

The antitumor effect of formalin-fixed Toxoplasma organisms (f-Tp) as an immunostimulant was examined in Toxoplasma-infected female C57BL/6 mice using a syngeneic Lewis lung carcinoma (3LL). Toxoplasma-infected mice, intradermally inoculated with the tumor cells mixed with 10⁵, 10⁶ or 10⁷ f-Tp, developed a marked antitumor effect, inhibition of tumor growth and prolongation of life-span, in direct relation to the strength of the delayed-type hypersensitivity (DTH) reaction induced by f-Tp. The antimumor effect could also be observed even if an intralesional injection with f-Tp was performed 1, 3 or 5 days after the tumor inoculation. In a control, the injection with 2.5 x 10⁶ live Mycobacterium bovis (BCG) in BCG-sensitized mice induced a significant antitumor effect, only when the BCG was injected in the mixture of tumor cells. These results demonstrate that the injection with f-Tp can induce a potent antitumor activity in mice with Toxoplasma infection.