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Host and tumor modifications associated with serial heterotransplantation of tumors through immunologically tolerant animals.

Summary Two 2-methylcholanthrene-induced mouse sarcomas, MC1LP and MC1D2 tumors, were serially transplanted in rats rendered tolerant by a paralysis course of mouse spleen cell injections started in their 1st, 5th, or 10th day of life. A progressive increase with succeeding passages of the growth rate and of the ratio of tumor takes in heterologous hosts was observed. Following serial passages in outbred tolerant rats, the MC1LP tumor acquired the ability to grow in newborn nonconditioned rats and in a congenic strain of mice. Experiments in which the MC1D2 tumor was serially passaged in adult tolerant rats of the Lewis inbred strain have shown that the tumor does not become transplantable in nonconditioned adult rats. Chromosomal investigation of heterotransplanted tumors has revealed the presence of a variable percentage of host metaphases which, as suggested by the experimental results, seem to be rat tumor cells. Neoplastic modifications were observed in the lymphoid tissue of rats submitted to a paralysis course of heterologous cell injections. The probable involvement of an immunological process of cancerigenesis is discussed.