Increased frequency of immunogenic variants obtained by repeated mutagen treatment of mouse mastocytoma P815.

A previous report from this laboratory demonstrated that treatment of mouse mastocytoma P815 with the mutagen N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) produces tumor cell variants that are unable to form tumors in syngeneic animals. We examined whether repeated mutagen treatment could increase the frequency of tum- variants above that obtained after a single treatment. This was found to occur with frequencies increasing from a few percent after 1 treatment to more than 90% after 8 treatments. Moreover, uncloned survivor populations obtained after 8 or more MNNG cycles that contained such a high proportion of tum- variants had a markedly decreased tumorigenicity for syngeneic mice. As reported for tum- variants obtained after 1 mutagen treatment, several tum- variants obtained after repeated treatments carried new variant-specific antigens that elicited a specific cytolytic T cell response. Some of these tum- antigens were found to consist of multiple determinants that could be lost independently. We observed that the resistance of the mutagenized populations to MNNG increased gradually with the number of mutagen treatments. In addition, some tum- variants obtained after 8 mutagen treatments showed a reduced sensitivity to mitomycin C.