Rescue of the defective genome of Moloney sarcoma virus from a noninfectious hamster tumor and the production of pseudotype sarcoma viruses with various murine leukemia viruses.

Hartley and Rowel reported that the in vitro focus-forming effects of Moloney sarcoma virus (MSV)2 I depended on the presence in the same cells of two virus particles, a defective i\ISV particle and a fully infectious Moloney leukemia particle. Superinfection with additional Moloney leukemia virus potentiated focus formation by MSV, converting a "two-hit" dose response curve to a one-hit curve; preliminary experiments also suggested a two-hit requirement for sarcoma induction in vivo by MISV.4 These experiments suggested that murine leukemia viruses serve as "helpers" for a defective MSV particle in much the same way that avian leukosis viruses help to complete defective Rous sarcoma virus (RSV) infectious particles, but with the difference that in the \A1SV system in mouse cells helper virus may be required for cellular alteration as well. ' Sarma, Vass, and Huebner5 described a virus-free sarcoma induced in hamsters by the defective Bryan strain of RSV, the cells from which when propagated in mixed tissue cultures with chicken embryo fibroblasts transferred the noninfectious RSV genome to the latter. When the mixed cultures were superinfected with avian leukosis viruses, fully infectious RSV was released. On the other hand, when uninfected, mixed cultures were implanted in the wing web of leukosis-free chicks, virus-free sarcomas having the avian karyotype were produced. When cells from these sarcomas were grown in tissue cultures, they behaved as typical nonproducer (NP) sarcoma cells. The addition of avian leukosis viruses to these avian cells yielded infectious RSV. In this system, the "nonproducer" hamster and chick cells contain large amounts of complement-fixing (CF) and immunofluorescentstainable antigens believed to represent the internal protein moiety of the virus,6' 7 but the genetic information for the outer envelope must be supplied by the helper virus. In this communication we describe fibrosarcomas induced in hamsters by the 1\Ioloney sarcoma virus which carry the defective AISV genome but not infectious MSV or murine leukemia virus nor mouse leukemia group reactive CF antigen. By adding various standard murine leukemia viruses such as the Rauscher, Friend, Moloney, and Gross strains to mixed cultures of MSV-induced hamster tumor cells and normal mouse embryo fibroblasts, we obtained fully infectious pseudotypes of MSV having the immunological characteristics of the helper leukemia viruses. Sarcomas containing the infectious pseudotype viruses were also readily produced when newborn Swiss mice were iiljecte(l with I\ISV hamster tumor cells mixed with various murine leukemia viruses. Materials and Methods.-Viruses: The MSV used for inoculation of hamsters originated from a sixth mouse passage BALB/c tumor preparation (#SV43) obtained from Dr. Moloney. The