Sarcoma virus-specific phosphoproteins are packaged in "rescued" type C virions

Spleen focus-forming virus: specific neutralization by antisera to certain gag gene-encoded proteins

Immunization of rats with syngeneic cells infected with spleen focus-forming virus (SFFV) but not with its helper, Friend murine leukemia virus (FMuLV), produces antisera which specifically neutralize SFFV, and not FMuLV, in the Friend virus complex. To determine which SFFV-encoded protein molecule bears the antigen recognized by these neutralizing antibodies, we studied different lots of rat anti-SFFV antiserum by immunoprecipitation and virus neutralization assays. The ability of these sera to neutralize SFFV correlated with the titer of antibodies to p45gag and not with the titer of those to gp52, suggesting that the neutralizing antibodies recognize the p45gag molecule. To verify this specificity for p45gag, we tested antisera to various MuLV gag gene-encoded proteins for neutralization of SFFV. Goat anti-Rauscher murine leukemia virus (RMuLV) p30 and goat anti-RMuLV p10 sera neither precipitated p45gag from SFFV-infected nonproducer cells nor neutralized SFFV. In contrast, goat anti-RMuLV Pr65gag and goat anti-RMuLV p12 sera precipitated p45gag from SFFV-infected cells and also specifically neutralized SFFV in the Friend virus complex. These findings suggest that, unlike the gag proteins coded for by FMuLV, the proteins coded for by defective SFFV are incorporated into the envelope of virions carrying the SFFV genome, but not into the envelope of those carrying the helper FMuLV genome.

Generation of oncogenic mouse type C viruses: in vitro selection of carcinoma-inducing variants

Type C retroviruses are endogenous in most vertebrate species. These viruses generally are of low pathogenicity in their natural hosts. Variants that contain cell-derived transforming genes have been isolated infrequently in the past upon continued passage in vivo. We report here a procedure that allows the isolation of new mouse leukemia-, sarcoma-, and carcinoma-inducing type C viruses entirely in cell culture. The viruses generated after passage in chemically transformed mouse cells and selectron in epithelial mink lung cells produced pulmonary adenomas and adenocarcinomas and also ovarian carcinomas. Viruses with cell-transforming capacity, as determined by their ability to induce normal murine or mink cells to form progressively growing colonies in soft agar, appeared only transiently (2-4 days) after acute infection of "spontaneously" or chemically transformed mouse cells by nontransforming helper viruses. These transiently appearing transforming viruses can be "captured" by selecting the cells that respond to their newly acquired growth-stimulating ability. This system may lend itself to the systematic isolation of tissue-specific transforming functions from any cell that can be efficiently infected by retroviruses.