Highly passaged defective-interfering (DI) particle preparations of equine herpesvirus type 1 (EHV-1) were found to mediate the co-establishment of persistent infection and oncogenic transformation of permissive hamster embryo cells. Four cell lines, designated DI-1 to DI-4, were shown to possess biological properties typical of transformed cells and to induce the rapid formation of metastatic fibrosarcomas when injected into syngeneic LSH hamsters. Corresponding DI tumour cell lines, designated DI-1T to DI-4T, were found to be virus non-producing, to be transplantable in the hamster, and, like the four parent DI cell lines, to express EHV-1-coded antigens and to be resistant to superinfection with EHV-1 but not with a heterologous virus, vesicular stomatitis virus. All transformed cell lines, but not the tumour cell lines, contained a population of cells (2.6 to 9%) that continued to release infectious virus after 100 passages in culture. The production of interferon and selection of temperature-sensitive mutants did not appear to play a role in the maintenance of persistent infection. However, it was demonstrated that these persistently infected cells continued to release not only infectious virus but also DI particles after more than 2 years in culture. DI particles were shown to be present in released virus by: (i) detection of the defective virus DNA species (density 1.724 g/ml; standard EHV-1, density 1.716 g/ml) by CsCl analytical ultracentrifugation techniques; (ii) measurement of interference activity of virus released from DI-1 to DI-4 cells against standard EHV-1 replication; (iii) the presence of the 35 megadalton II fragment unique to the DI particle genome in DNA of released virus. These studies indicate that herpesvirus DI particles may function both in the initiation and maintenance of persistent infection and alter the cytocidal effects of infection to allow the establishment of oncogenic transformation and persistent infection.


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