Dominance of a tumor subpopulation line in mixed heterogeneous mouse mammary tumors.

When mixtures of cell lines 168 and 4T07, both derived from the same mouse mammary tumor, were injected into syngeneic mice, the resulting tumors, analyzed over a large size range by colony-forming assays in selective media, consisted primarily of line 4T07, even when the ratio injected was 100:1 or greater in favor of line 168. This result indicated a suppression of growth of line 168, since the volume-doubling time of line 168 tumors in the absence of line 4T07 was one-half that of line 4T07 tumors. That growth suppression was not due to inhibition of line 168 by immunity induced to line 4T07 was shown in two ways: (a) line 168 tumors grew almost as well in mice preimmunized with line 4T07 as in controls, whereas line 4T07 tumor growth was strongly inhibited in preimmunized mice; and (b) the final composition (favoring line 4T07) in mixed tumors was similar in tumors grown in mice immunosuppressed by irradiation to that in nonirradiated controls. The strong suppression of line 168 did not occur when the two cell lines were injected simultaneously at different s.c. sites, nor did it occur when line 168 cells were injected in mixtures with lethally irradiated line 4T07 cells. Line 4T07 cells also suppressed the growth of line 168 cells in monolayer cultures. It was not likely that suppression was due to competition for growth factors, since the effect required cell contact. Suppression probably was not mediated through junctional communication, since these cells do not engage in metabolic cooperation. We suggest that a growth-inhibitory factor produced by line 4T07 mediates the suppression of 168 cells.