Adult AKR and C58 mice injected intramuscularly with murine sarcoma virus, Moloney isolate (M MSV), developed high incidence of nonregressing local tumors. Histologically, these tumors revealed the typical pleomorphism of M MSV sarcomas; in some cases, however, neoplastic tissue showed a nodular or diffuse growth of monomorphic myoblastlike cells, reminiscent of clonal aggregates. No depression of immune reactivity was found in M MSV injected mice as evaluated by direct hemolytic plaque forming cells against SRBC and by virus neutralizing antibody production. The MSV recovered from the induced tumors proved to be, by neutralization assay, a Gross (G) MSV pseudotype. Moreover, tumor cell suspensions absorbed out cytotoxic antibody directed against G cell surface antigens. Therefore, the conclusion was drawn that MSV with envelope characteristics of endogenous G leukemia virus had formed in vivo through a phenotypic mixing phenomenon. The failure of tumors to regress was interpreted as mainly due to the partial unresponsiveness of host immune reactivity towards G MuLV specified antigens. Since MSV tumors arose in AKR mice after a very long latent period, the possibility was considered that this relative resistance might depend on immunologic mechanisms. In fact, M MSV injected AKR mice immunodepressed by goat antimouse lymphocyte serum or rendered partially tolerant by neonatal M MuLV inoculation developed sarcomas with higher incidence and with a shorter latency. Furthermore, the MSV recovered from these early tumors proved to be the original Moloney pseudotype.
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