Variant generation and selection: an in vitro model of tumor progression.

Evidence for a new in vitro model of tumor progression was sought on the basis of the variant generation and selection hypothesis. The stability of a cloned murine tumor was examined during growth in standard tissue culture or in media containing the tumor promoter 12-O-tetradecanoylphorbol-13-acetate (TPA). Analysis of subclones from the appropriate tumor populations revealed that growth of the L5178Y-F9 clone in 100 ng/ml TPA and 0.1% dimethyl sulphoxide (DMSO) for 2 days yielded a tumor which exhibited increased cellular heterogeneity for susceptibility to both syngeneic and allogeneic natural antibodies (NAb). Subsequent exposure of TPA- and DMSO-treated cells to two cycles of syngeneic NAb-mediated cytolysis resulted in tumor populations which expressed a reduced sensitivity to syngeneic NAb. Thus the elements of tumor variant generation and selection were demonstrated by means of this approach, and repeated cycles of the TPA treatment and NAb cytolysis produced tumor cells with a reduced susceptibility not only to NAb in vitro but also to anti-tumor natural resistance (NR) measured in a tumor elimination assay in vivo. These observations extend the support for the notion that tumor progression can proceed through variant generation and selection. Furthermore, the association of tumor variant generation with exposure to the combination of TPA and DMSO, both non-mutagens, offers a model for studying non-mutagenic mechanisms of tumor development.