Protective effect of murine sarcoma virus-superinfected mouse tumor cells against outgrowth of corresponding noninfected tumor.

Cultured lines of three newly established methylcholanthrene-induced tumors, MBK and MBL in CBA and MC57M in C57BL mice, and two mammary tumors, SBfnHC in CBA and S3W in ASW mice, were superinfected in vitro with Moloney sarcoma or leukemia virus (MSV, MLV). After superinfection, they expressed the Moloney virus-determined cell surface antigen (MCSA) and murine C-type viral p30 antigen, and produced NB-tropic C-type virus. The virus-infected tumors became more rejectable in normally susceptible syngeneic mice compared with the original noninfected line. There was no difference in 400-rad irradiated hosts. Mice that have rejected the virus-infected tumors showed an increased resistance to the corresponding noninfected tumor. The protective effect was comparable, with only one exception, to the immunizing effect of irradiated, noninfected cells. In vitro tests showed that small numbers of viable MSV-infected MBL generated cytotoxic spleen cells against both uninfected and MSV-infected MBL in syngeneic mice, while the same numbers of viable noninfected MBL did not induce cytotoxic cells. Relatively large numbers of irradiated MBL and MSV-MBL had a similar activity in inducing cytotoxic spleen cells against MBL in syngeneic hosts.