Polyoma virus

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A papovavirus that infects rodents. The name derives from the capability of this virus to induce a wide variety of tumors when inoculated into newborn animals. The icosahedral viral particle consists of deoxyribonucleic acid (DNA) and protein only. The genome is a small, double-stranded, closed circular DNA molecule approximately 5300 base pairs in length. It encodes the T antigens expressed early in the productive cycle and in transformed cells, and the viral capsid proteins, expressed late in the productive cycle.

Polyoma virus is endemic in most wild populations of mice, but causes no disease. Tumors produced by this virus are unknown in the wild. Inoculation of large quantities of virus into newborn rodents, however, induces a number of tumor types, particularly many sarcomas and carcinomas. These tumors contain the viral genome but produce few infectious viral particles. Infected animals produce neutralizing antibodies directed against structural components of the virus; tumor-bearing animals produce antibodies to antigens (T antigens) which are present in tumor cells but not in the virus particle.

The virus is easily propagated to high titer in mouse embryo tissue culture, resulting in the lysis of the cells and the production of a hemagglutinin. This “productive infection” occurs only in mouse cells. Small numbers of “abortively infected” cells retain the viral genome and become transformed. Transformed cells are tumorigenic when injected into syngeneic animals, and contain the T antigens but produce no virus. The expression of the T antigens has been shown to be necessary and sufficient to induce cell transformation. See also Immunology; Oncology; Tumor viruses; Virus.