Studies of FBJ osteosarcoma virus in tissue culture. II. Autoinhibition of focus formation.

A high-titered non-focus-forming virus, FBJ-MuLV (murine leukemia virus), present in FBJ tumor preparations, inhibited significantly the expression and production of FBJ-MuSV (murine sarcoma virus) in tissue culture. This "autoinhibition" was comparable to that observed when a 3- to 4-log excess of infectious MuLV was added to standard MuSV. The degree of inhibition was influenced by the tropism of the MuLV (or the ease of spread and propagation of MuLV in certain cells), multiplicity of infection by MuLV, amount of excess MuLV, and ability of the MuSV-transformed cells to replicate independently. The FBJ MuLV-MuSV complex may be a model system for the detection of sarcoma viruses in spontaneous tumors in various animals where inhibition by excess nontransforming virus could be an important biologic phenomenon.