XC cell cytopathogenicity as an assay for murine myeloma C-type virus

Stimulation of virus production and induction of self-syncytium formation in human T-cell leukemia virus type I- and type II-infected T cells by 12-O-tetradecanoylphorbol-13-acetate

Treatment of human T-cell leukemia virus type I (HTLV-I)- and HTLV-II-infected T-cell lines with 12-O-tetradecanoylphorbol-13-acetate (TPA) stimulated virus release. However, this stimulation was mainly detected at 42 to 48 h of treatment, whereas later virus release declined rapidly. During the first 48 h, TPA had no effect on cell growth, but later, the number of viable cells was profoundly lower in the TPA-treated than in the untreated cultures. This shift in virus release and cell number resulted from self-fusion of a large proportion of the virus-producing cells, which seemed to consequently enter into a dying process. This fusion, which resulted in syncytium formation, was strongly inhibited by anti-HTLV-I env monoclonal antibodies. Furthermore, no self-fusion was detected in three different uninfected T-cell lines similarly treated with TPA. On the other hand, stimulation of virus production by 3-methylcholanthrene (3-MC) treatment failed to induce self-fusion in the infected cells. Moreover, no syncytium was detected when these 3-MC-treated infected cells were cocultured with any of the TPA-treated uninfected cells. The effects of TPA on virus production and syncytium formation were both abolished by three different protein kinase C inhibitors. Taken together, these data suggest that the self-fusion observed in these experiments required both enhanced virus production and protein kinase C-phosphorylated viral or/and virally induced cellular component(s).

Rapid syncytium formation induced by Moloney murine sarcoma virus in 3T3/NIH cells and its delay by mouse interferon

Moloney murine sarcoma virus (MSV) clone 124 was found to induce rapid syncytium formation upon infection of 3T3/NIH at a high multiplicity of infection. This effect became apparent, by light microscopy, within 1 to 2 hours, whereas by scanning electron microscopy, clusters of 4 to 25 cells were seen in their initial steps of syncytium formation within 20 to 30 minutes after virus addition. It appeared that the cell fusion was initiated by connection between the cells through virus bound to their surface. After a few more hours several neighbouring syncytia fused into giant cells containing over a hundred nuclei. Though MSV (124) stocks contained also some MLV it appeared that the syncytium inducing activity was related to the MSV particles. MLV particles were not only incapable to induce syncytium formation, they even interfered with this activity of MSV. The MSV-induced cell fusion required the adsorption of intact virions, but was independent on protein synthesis. Mouse interferon remarkably reduced the rate of syncytium development.