The infection of cells by polyomaviruses is initiated by the binding of the virion to a receptor on the outside of the cell membrane. In the case of SV40, this receptor has been identified as the major histocompatibility complex (MHC). Because nearly every cell type expresses MHC molecules, it would be expected that SV40 should be able to infect most cells. In the case of BKV and JCV, the cell receptors are not fully understood. It has been reported that JCV binds specifically to certain cell types (glial cells, tonsillar stromal cells, and B but not T lymphocytes) that reflect JCV’s cell tropism. However, more recent reports indicate that JCV can enter a broad spectrum of mammalian cell types through binding to oligosaccharides ubiquitously present on glycoproteins and glycolipids on the cell membrane. After the viral DNA enters the nucleus, cell-specific intranuclear mechanisms suppressed JCV DNA replication in all cells except human neuroblastoma cell lines in these experiments. In the human body, it is thought that JCV can infect cells in the tonsils and spread from there by replication in lymphoid cellscan also replicate in human lymphocytes. JCV and BKV can infect cells in the kidney, which is the major organ of JCV and BKV persistence during latency. In monkeys, initial lytic infection by SV40 is controlled by the immune system with virus then persisting in the kidney where it may be reactivated by immunosuppression. The life cycle of SV40 in humans is poorly understood.
After binding to the cell surface, polyomavirus capsids undergo endocytosis and are transported to the nucleus where the viral DNA is uncoated and transcription of the early region begins. Interestingly, SV40 enters cells by caveola-mediated endocytosis although JCV employs a clathrin-dependent mechanism that is blocked by tyrosine-specific protein kinase inhibition.
The primary transcript from the early region is alternatively spiced to give two mRNAs that encode T-Ag and tAg. T-Ag is a large nuclear phosphoprotein and is an essential factor for viral DNA replication. It binds to the viral origin of replication region (ori) where it promotes unwinding of the double helix and recruitment of cellular proteins that are required for DNA synthesis including DNA polymerase-a and replication protein A
Polyomaviruses rely on cellular enzymes and cofactors for DNA replication and these proteins are expressed in S phase. Another major function of T-Ag is to modulate cellular signaling pathways to induce cells to enter S phase and this accounts for the ability of T-Ag to transform cells. T-Ag is thought to stimulate the cell cycle through its ability to bind to several cellular proteins that are involved and in crucial signal transduction pathways that control cell cycle progression and apoptosis. These functions of T-Ag are discussed below. What is known about T-Ag function for each polyomavirus is summarized
in Table 1.
The role of the small t-Ag in the polyomavirus life cycle is less clear. Analysis of SV40 deletion mutants revealed that t-Ag is not essential for lytic infection in culture. However, t-Ag cooperates with T-Ag in the transformation of cells by SV40 and increases virus yield in permissive cellinfections. The mechanism of action of t-Ag is discussed below. As viral replication proceeds, the late genes begin to be expressed. T-Ag acts to stimulate transcription from the late promoter and repress transcription from the earlypromoter. The gene products of the late region are the capsid proteins VP1, VP2, and VP3, which assemble with the replicated viral DNA to form virions, which are released upon cell lysis.
Polyomavirus DNA can become integrated into the chromosomal DNA of the cell especially upon nonpermissive infection. This was first described for SV40 and is also the case for JCV and BKV. Integration occurs at random both in terms of the site in the cellular genome where integration occurs and the point in the viral genome where recombination occurs.
Infections in humans by polyomaviruses are usually restricted by the actions of the immune system, particularly cell-mediated immunity. For JCV, a general impairment of the Th1-type T-helper cell function of cell-mediated immunity has been found in the disease progressive multifocal leukoencephalopathy (PML). PML is discussed below. Cell-mediated immunity is also important
in BKV infection