The effect of lethally irradiated cells on the transplantability of murine tumours.

Fully quantitative isogeneic transplantation assays of viable (V) cells of a CBA carcinoma showed that the relationship between log inoculum and frequency of tumour "takes" accorded strictly with a Poisson distribution and indicated that 6900 cells were required for 50% takes (TD50). Addition of 10(5) lethally irradiated (LI) cells of the same tumour to the inocula reduced the TD50 to about 4 cells, yet the Poisson relationship was retained. From this and other data it is concluded that LI cells act by increasing the proportion of viable cells which contribute to tumour initiation; there was no evidence that LI cells affected the rate of proliferation of viable cells. The ability of non-homologous LI cells to reduce the TD50 was widely variable, but LI cells of one allografted tumour were almost as effective as homologous LI cells. Lethally irradiated cells did not assist the "take" of allografted viable tumour cells. Histological comparison revealed no difference of the tissue reaction to inocula of viable and LI cells, and it is questioned whether radiation induced lysis of these latter cells is required for their effect on viable cells. Evidence relating to a hypothesis that viable cells interact with one another as they do with lethally irradiated cells was conflicting.