Malignant growth in the normal host after variant selection in vitro with cytolytic T-cell lines

Using variant selection in vitro, we have explored the changes necessary to convert potentially malignant cells into progressively growing cancer cells. As a model, we studied the murine ultraviolet light (UV)-induced tumor 1591-RE, which routinely regresses in the normal syngeneic host. The antigenicity of this tumor is specified by multiple independent tumor-specific antigens, named A, B, C, and D. Cells of this tumor were exposed in vitro to cytolytic T-cell lines specific for these separate antigens. Tumor variants were isolated that displayed selective antigenic losses. By challenging normal syngeneic mice with the variants, we could determine the significance of each antigen in tumor rejection--i.e., a switch from a regressor to a progressor phenotype upon selective loss of certain antigens. We found that variants which lost the A and C antigens grew progressively in normal mice, whereas variants which had lost the B antigen were still rejected like the parental tumor cells. Furthermore, the B and D antigens could be expressed on progressively growing variants. Thus, loss of the A and C antigens, but not the B and D antigens, was required for allowing malignant growth. Selection for loss of one T-cell-recognized antigen never selected simultaneously for loss of any of the other T-cell-recognized antigens, nor did such selections affect sensitivity to activated macrophages or natural killer cells. Therefore, our results strongly suggest that loss of defined components of a complex T-cell-recognized antigen are sufficient to allow escape from immunosurveillance.