Tumor induction by Moloney sarcoma virus in athymic nude mice

Athymic nude (nu/nu) mice, normal NMRI (plus/plus) mice, and nu/plus heterozygotes, inoculated at birth with the Moloney strain of murine sarcoma virus (M-MSV), did not differ either in tumor growth rate or incidence or in mortality due to tumor development; neither did nude (nu/nu) and normal NMRI mice inoculated with M-MSV at 30 days of age differ in the initial tumor growth rate. However, tumors induced in adult nude mice grew progressively and eventually killed the hosts, whereas tumors induced in adult normal NMRI mice completely regressed. These results indicated that thymus-dependent immunity, though not active in M-MSV tumor induction, determined whether the tumor, once formed, would regress or kill the host.