Phenotypic instability of mouse melanomas after propagation in vivo and in vitro.

This study was designed to evaluate the phenotypic stability of the carcinogen-induced JB/MS and JB/RH melanomas. The JB/MS melanoma maintained its original slow-growing melanotic phenotype during in vivo passage over a 2-yr period. However, both melanin production and tumorigenicity decreased rapidly after propagation of JB/MS cells in vitro. The JB/RH melanoma became essentially amelanotic after the second transplantation in vivo. Cultured JB/RH cells produced tumors identical to those obtained by transplantation of JB/RH tumor fragments. However, after propagation in vitro for 2 yr, the JB/RH cell line decreased in tumorigenicity, requiring 10 times as many cells to produce tumors in C57BL/6 mice as did the original cell line. The JB/RH melanoma was highly immunogenic in syngeneic C57BL/6 mice, and passage of JB/RH cells through immunized mice resulted in tumors that were significantly more tumorigenic in normal mice than were JB/RH cells that had been passed through either nude or sublethally irradiated mice. These results indicate that, in studies of primary mouse melanomas, it is essential to: (a) limit the number of tumor passages; (b) choose methods of propagation that will preserve the original phenotype; and (c) distinguish those properties produced by technical manipulation from those produced by true tumor progression.