The evolution of the specific cell-mediated immune status was studied in C57BL/6 mice bearing a chemically induced syngeneic savcoma, the TBL CL2 tumor. Peritoneal cells ( P C )

The tumorigenic EPO clonal cell line, derived from a methylcholanthrene-induced murine sarcoma, was exposed to increasing concentrations of actinomycin D and gave rise to a subline, resistant to 0.02 mug of actinomycin D per ml of medium, which was designated EPO/ADj. Drug resistance was accompained by a striking decrease of tumorigenic capacity as determined by the tumor take incidence and tumor growth rate in syngeneic C57BL/6 mice (normal and X-irradiated), as well as by tumor take incidence in the cheek pouch of cortisone-treated weanling Syrian hamsters. One of the tumors obtained after inoculation of EPO/ADj cells into syngeneic mice was reexplanted in culture and developed as the EPO/ADj/T subline. It was found to be relatively resistant to actinomycin D as compared to the parental EPO cells. The morphology of cells changed with actinomycin-D-resistance: EPO/ADj cells were more spread and flattened and less overlapping than the original fibroblast-like EPO cells. EPO/ADj/T cells, however, had an aspect similar to that of EPO cells. No major differences were seen between the karyotypes of EPO, EPO/ADj and EPO/ADj/T cells. Tumor-specific antigen(s), characteristic of drug-sensitive, parental EPO line, was expressed in the resistant EPO/ADj and EPO/ADj/T lines, as shown by: (a) transplantation resistance to challenge with EPO cells in syngeneic mice pretreated with EPO/ADj cells; and (b) cross-reactivity in indirect immunofluorescence tests performed with sera from syngeneic animals hyperimmunized with EPO or EPO/ADj cells. Furthermore, an additional surface antigen(s), absent from EPO cells, was demonstrated in EPO/ADj and EPO/ADj/T cells by means of immunofluorescence tests performed with specific anti-EPO/ADj syngeneic sera previously absorbed on EPO or EPO/ADj cells.