The generation of tumor heterogeneity in vivo.

In order to determine whether host factors may contribute to the generation of tumor heterogeneity, the phenotypic stability of cells from a cloned tumor was examined during proliferation in tissue culture and in the syngeneic host. Growth of this cloned tumor was initiated both in vivo and in vitro, and the tumor populations were sampled at different time intervals by subcloning. The susceptibility of these tumor subclones to the cytotoxic action of natural antibodies and complement was used as a marker of their membrane phenotype. The extent of phenotypic variation of the clones in one sample was considered to be a measure of tumor heterogeneity. Following these procedures, we observed that a clone of the L5178Y murine lymphoma maintained its homogeneity during 5 months of in vitro culture. In contrast, a single passage of the same tumor clone for 3 1/2 weeks or 3 months in the syngeneic host resulted in the generation of a population of cells exhibiting a significant increase in heterogeneity. This relative instability of tumor phenotype in vivo suggests that the host milieu may allow the generation of tumor heterogeneity. Genetic or epigenetic mechanism(s) may be involved although the high frequency of new phenotypes argues against a role for somatic mutation.