In vivo and in vitro treatment of Ehrlich cancer cells with diphtheria toxin resulted in loss of oncogenicity and in immunization against these tumor cells in mice.

The animals that survived a 1st i.p. transplantation of virulent cancer cells treated with diphtheria toxin failed to develop an ascites tumor after a 2nd transplantation up to 200 days later.

Ehrlich cancer cells lost oncogenicity also by prolonged incubation in phosphate-buffered saline in the presence of diphtheria toxin. This loss of oncogenicity was time dependent and influenced by the pH. Cells incubated for 8 hr with diphtheria toxin failed to induce an ascites tumor in both normal and immunosuppressed mice. Incubations performed at 5 pH's ranging from 6.5 to 7.5 demonstrated that the lower values were more effective in inhibiting oncogenicity than were the higher ones.

Eighty % of the normal mice that survived an i.p. transplantation of 1 × 106 cancer cells incubated with diphtheria toxin became resistant to a challenge with 1 × 106 virulent cancer cells. Only 50% of the immunized mice survived a challenge with 2 × 106 cancer cells, while the survival rate fell to nearly 0% after a challenge with 5 × 106 cells.

In Swiss albino mice immunized with Ehrlich cancer cells, a cross-resistance against ascites Sarcoma 180 was observed, while all Ehrlich tumor-immunized DBA/2 mice died within 10 days after a challenge with L1210 tumor.

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