Biological Properties and Tissue Tropism
Members of the Papillomaviridae family exhibit strict specialization. Their primary target within the organism is stratified squamous epithelial cells. Although humans are the primary host, similar viruses can also infect certain animals (dogs, rabbits, cattle).
A key feature of the HPV life cycle is its strict dependence on the state of the host cell:
- Productive infection (a complete replication cycle resulting in the formation of new viral particles) can occur exclusively in differentiated epithelial cells.
- In actively dividing cells of the basal epithelial layer, a full viral replication cycle is not supported.
Benign Manifestations of Infection
More than 100 types of HPV are known to cause benign proliferative processes. Clinically, these manifest as the formation of papillomas, verrucae (warts), and condylomas. Typical localizations for these lesions include:
- skin integuments;
- mucous membranes of the respiratory tract;
- gastrointestinal tract;
- anal and genital regions.
At the molecular level, a benign course is characterized by a specific viral DNA status. The DNA is a circular double-stranded molecule that exists autonomously within the nucleus of the infected cell. This is the so-called plasmid (or episomal) form, in which the viral genetic material is not physically linked to the host cell genome.
Malignant Processes and Oncogenic Types
Several HPV types possess pronounced oncogenic potential. The transition of the infection to a malignant form is accompanied by a change in the status of the viral DNA: it loses its autonomy and integrates directly into the chromosome of the infected cell.
Different oncogenic types are associated with various cancers:
- Types 2, 5, and 8 most commonly provoke skin cancer, as well as malignant tumors of the larynx and oral cavity.
- Types 16 and 18 are recognized by the World Health Organization (WHO) as leading etiological factors in the development of cervical cancer.
Molecular Mechanism of Carcinogenesis
The transformation of a normal cell into a tumor cell during HPV infection is driven by the active expression of specific viral proteins: E6 and E7.
The action of these viral agents aims to block the cell's natural defense mechanisms. They inactivate crucial cellular tumor suppressor proteins:
- Protein p53.
- Protein Rb (retinoblastoma protein).
Inactivating these suppressors strips the cell of its ability to control its own division cycle, inevitably leading to uncontrolled proliferation and the formation of a malignant neoplasm.