Increased oncogenicity of the murine sarcoma virus (Moloney) by co-infection with murine leukemia viruses.

The high incidence of spontaneous regressions of primary tu mors induced by the Moloney sarcoma virus in adult mice was significantly altered by co-infection with Rauscher, Moloney, and Friend leukemia viruses. Intramuscular, intraperitoneal, or subcutaneous inoculation of sarcoma extracts, prepared from progressively growing tumors obtained from dually infected animals, resulted in extensive métastases.Tumor nodules were observed in several tissues and organs. Regression of primary virus-induced tumors could be altered by low concentrations of Rauscher leukemia virus. The role of the leukemia virus, acting as "helper" or as an immunosuppressor, is discussed.