Phenotypic shifts in the L5178Y lymphoma population during progression of the tumor-dormant state in DBA/2 mice.

We evaluated the phenotypic changes that take place in the L5178Y tumor cell population during the establishment, maintenance, and termination of the L5178Y tumor-dormant state in DBA/2 mice. Since there is considerable variation among DBA/2 mice in the course of the L5178Y tumor-dormant state, analysis of these phenotypic changes required prospective studies on individual mice. Therefore, we developed a partial peritoneal lavage technique which could be performed repeatedly on an individual mouse without killing the mouse and without terminating the tumor-dormant state. The partial peritoneal lavage technique provided cell samples that were quantitatively representative of the entire peritoneal cell contents without altering the relative proportions or total number of host peritoneal cell subpopulations. Tumor cell clones were isolated from the uncloned L5178Y cell inoculum that was used to initiate the tumor-dormant state, and from the partial peritoneal lavage performed on individual mice at regular intervals throughout the tumor-dormant state until each mouse developed an ascitic tumor. We found that the uncloned L5178Y cell population is heterogeneous, with the majority of the cells [approximately 80% of the clonal populations ("original" phenotype)] able to compete as well as the uncloned L5178Y cell population for immune cytolytic T-lymphocyte activity, and a smaller percentage [approximately 20% of the clones ("emergent" phenotype)] competing significantly less but similar to the tumor cell population which grows out as an ascitic tumor at the termination of the tumor-dormant state. However, both "original" and "emergent" phenotypes encompass a range of subpopulations which have the respective characteristics. There was a progressive enrichment in the tumor cell population for "emergent"-phenotype cells as mice progressed through the tumor-dormant state. In the 30-day period prior to tumor emergence, only "emergent"-phenotype cell clones were isolated. These data indicate that, during the course of the tumor-dormant state, there is continual selection by immune cytolytic T-lymphocytes for "emergent"-phenotype cells, which preexist in the L5178Y cell population used to initiate the tumor-dormant state, and that the progressive enrichment of "emergent"-phenotype cells in the peritoneal cavity is associated with and may be responsible for termination of the tumor-dormant state.