SUMMARYImmunofluorescence and autoradiography showed that all heterokaryotic cells formed after cell fusion of virogenic mammalian cells and indicator chicken cells induced by Sendai virus produced Rous sarcoma virus coat antigen. Homokaryons and mononuclear cells of both types were negative in the immunofluorescence test. Similarly, all tested single heterokaryons obtained by visually controlled cell fusion produced infectious Rous sarcoma virus.Virus coat antigen or infectious virus was not formed in virogenic cells or chicken fibroblasts agglutinated but not fused in the mixture with Sendai virus, or in heterokaryons obtained after fusion of virogenic cells with chicken erythrocytes; including heterokaryons containing 'reactivated' chicken erythrocyte nuclei.Heterokaryons formed after fusion of virogenic cells with chicken fibroblasts are permissive cells for Rous sarcoma virus, and play a key role in the Rous sarcoma virus rescue process.
All individual heterokaryons obtained by fusion of cells of the five different mammalian RSV-transformed virogenic cells with indicator chicken fibroblasts produced infectious RSV. In contrast, non-virogenic mammalian cells derived from tumours which arose in mice injected with RSV did not give rise to infectious RSV, even when fused with chicken fibroblasts preinfected with three different "helper viruses". Fusion of four lines of vlrogenic cells with mammalian fibroblasts as well as of virogenic Chinese hamster cells with chicken liver, mesonephros, thymus and macrophage cells did not lead to the formation of detectable RSVor RSV coat antigen. Similarly, experiments on complementation by mutual fusion of virogenic cells in five different crosses were negative. The presence of a " natural" Gs antigen in the indicator cells is not necessary for RS V rescue because the fusion ofcells derived from Gs negative chicken and duck embryos with virogenic cells leads to the formation of infectious RSV.The results are discussed in relation to the specificity of the cells participating in the induction of RS V formation by cell fusion.
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