POLYOMA VIRUS DISEASE
Polyomaviruses are closely related to papillomaviruses but are not known to cause clinical disease in immunocompetent patients. They can cause progressive multi- focal leukoencephalopathy (PML) and hemorrhagic cystitis/nephropathy in im- munocompromised patients.
Approximately 80% of adults show serologic evidence of JCV and BKV infection with no known clinical manifestations, but the viruses remain latent and may reactivate and cause disease in immunocompromised patients. BKV is estimated to cause renal disease in 2 to 5% of renal transplant recipients.
Polyomaviruses can produce malignant tumors in certain experimental animals but, inter-estingly, not in their natural hosts. For example, SV40 can produce lymphocytic leukemia and a variety of reticuloendothelial cell sarcomas in baby hamsters but is not oncogenic in its natural monkey host. Fortunately, even though it can transform some human cells in vitro, it fails to produce disease in humans, a fact that became apparent on follow-up of recipients of early batches of poliomyelitis vaccine produced in monkey kidney cell cul-tures that were contaminated with live SV40.
The reason polyomaviruses fail to produce tumors in their natural hosts is uncertain, but it may be due to the fact that the viruses are usually cytocidal under these conditions. From a biological point of view, the polyomaviruses are particularly useful models of oncogenicity because they can be readily studied in vitro and interact with cells in differ-ent ways. In some, they produce lytic infections and cell death with production of complete virions. In others, they integrate randomly into the cell genome and cause trans-formation by the expression of one or more of the viral genes. No human tumor has been shown to be caused by polyomaviruses.