Polyomaviruses

Cell Interaction and Mechanism of Oncogenesis

Previously, polyomaviruses were considered part of the Papovaviridae family, but today they are classified into a separate group. Their interaction with the organism depends on whether the host is natural or foreign:

• In cells of natural hosts, the virus initiates a productive infection. Virions actively assemble in the cell nucleus and then exit, causing lysis (destruction) of the infected cell.
• In a foreign organism (e.g., in heterologous cell cultures or newborn animals of other species), the virus behaves differently. Its DNA integrates into the host cell genome, while exclusively early proteins are expressed. This stimulates the formation of tumors with various histological structures.

A key role in malignant transformation is played by the T-antigen. This factor binds to the cellular tumor suppressor protein p53 and blocks its function. As a result, the cell loses control over the cell cycle and begins to divide uncontrollably.

Human Polyomaviruses: BK and JC Viruses

Despite their widespread prevalence (the majority of humans are infected), clinically apparent diseases develop primarily against the background of severe immunosuppression. These viruses act as typical opportunistic pathogens.

1. BK Virus

This agent was first isolated from the urine of kidney transplant recipients. The virus exhibits a tropism for urinary tract tissues and poses a severe threat specifically to renal graft recipients, causing severe graft damage.

1. JC Virus

This pathogen was isolated from the brain tissue of patients suffering from progressive multifocal leukoencephalopathy (PML).

• Tropism: The JC virus specifically infects glial cells of the central nervous system.
• Pathogenesis: In the setting of T-cell immunity deficiency, the virus destroys oligodendrocytes, leading to massive demyelination of the brain white matter.
• Oncogenicity: Under laboratory conditions (in hamsters and monkeys), it can provoke the development of brain tumors.

Simian Vacuolating Virus 40 (SV40)

The SV40 virus is the classic prototype of the entire Polyomaviridae family. It was discovered in rhesus macaque kidney cell cultures. Notably, in its natural host's cells, it causes neither cytopathic effect (CPE) nor transformation. However, when introduced into African green monkey kidney cell cultures, the virus leads to marked cytoplasmic vacuolations and subsequent cell death.

Oncogenic activity: In a number of animals (rats, hamsters, marmosets), SV40 is a potent oncogen, inducing tumor development. However, it is completely non-pathogenic to humans.

Massive epidemiological proof of the virus's safety for humans came from the history of early mass polio vaccination campaigns. Vaccine preparations were cultured on macaque kidney cells and happened to be contaminated with the SV40 virus. Although tens of millions of people received the vaccine, subsequent long-term monitoring revealed no surge in oncological diseases among the vaccinated population.

Virion Structure and Diagnostics

Polyomaviruses are small, non-enveloped particles. Their genome is represented by circular DNA encoding two types of proteins:

• Early (non-structural): small t-antigen and large T-antigen.
• Late (capsid): proteins VP1, VP2, and VP3. Protein VP1 is the main structural component of the capsid and is responsible for attaching the virion to the target cell membrane.

Laboratory diagnostics of infection:

• Molecular genetic method: detection of specific viral DNA using polymerase chain reaction (PCR).
• Serological method: detection of antibodies in the patient's blood serum (ELISA, CFT, neutralization test). It is worth noting that antibodies are usually detected in relatively low titers.
• Virological (culture) method: inoculation of material onto human embryonic or African green monkey kidney cells. The presence of the virus is confirmed by the development of a characteristic cytopathic effect (vacuolization, plaque formation). A neutralization test (NT) is used for precise identification.

To date, specific prophylaxis (vaccines) against polyomaviruses has not been developed.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Molecular structure and functions of the VP1 capsid protein
• Mechanism of p53 protein inhibition by the viral T-antigen
• Pathogenesis of progressive multifocal leukoencephalopathy (PML)
• Principles of polyomavirus cultivation and assessment of the cytopathic effect
• Features of the immune response during BK and JC virus infections

Other topics in the "Oncogenic viruses" section

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.