Studies on clonal heterogeneity in two spontaneously metastasizing mammary carcinomas of recent origin.

We have studied the clonal heterogeneity of 2 spontaneously metastasizing mammary carcinomas which recently arose spontaneously in C3H/He female retired breeders. Cells of early (2nd to 5th) transplant generations of these tumors were cloned by a combination of semi-solid agarose colony formation and limiting dilution techniques. Growth characteristics of the various clones in vitro and their tumorigenicity in vivo were evaluated. Subsequently, the role of host immunity and of interclonal interactions in regulating growth of the different clones in vivo were analyzed. We found that, whereas all 16 clones isolated from one tumor (T-58) grew rapidly in vivo and in vitro, 10 clones isolated from the second tumor (MT-2) showed a wide disparity in their growth rates in vivo. Taken together, these clones could generally be divided into 3 categories: (1) rapidly growing lines which grew in vivo at rates similar to or higher than those of the parental line; (2) slow-growing lines which grew more slowly than the parental line; and (3) non-growers which failed to produce tumors in vivo with doses of up to 5 X 10(6) cells injected either s.c. or i.v. but grew in vitro at rates comparable to the parental line. No correlation could be established between the various growth potentials exhibited by these tumor lines and tumor cell morphology in vitro and in vivo, as determined by light and electron microscopy. Sublethal irradiation (550-650 R) of young animals prior to tumor inoculation, or before inoculation of tumor cells into old, low NK syngeneic mice, failed to modify the growth of slow-or non-growing lines in vivo, indicating that host cellular defense mechanisms against the clones, if existent, were not mediated by NK or radiosensitive B or T cells. When clonal interactions were studied by the simultaneous injection of different clones in vivo at different s.c. sites, we found that a slow-growing line failed to modify the growth rate of a rapidly growing line, but accelerated the growth of a second slow-growing line injected simultaneously on the contralateral side, and that this enhancement of tumor growth was radioresistant. A mixture of these 2 lines also grew more rapidly than the individual lines alone. Our findings suggest that phenotypic variations in tumorigenicity can be found in clonal lines derived from spontaneous primary tumors and that these variations are not related to cell cycle properties as measured in vitro.(ABSTRACT TRUNCATED AT 400 WORDS)