In which cell layer does human papillomavirus (HPV) establish persistent infection and immortalise the host cell?

Correct answer: D. The basal layer of stratified squamous epithelium

• A. HPV does not infect ENDOTHELIUM. That tropism belongs to viruses such as HHV-8 in Kaposi sarcoma.
• B. The LAMINA PROPRIA is connective tissue beneath the basement membrane. HPV is strictly EPITHELIOTROPIC and does not infect it.
• C. The SUPERFICIAL layer is where mature virions are assembled and shed — the productive phase of the life cycle — but these terminally differentiated cells are already destined to be lost and cannot be immortalised.
• D. CORRECT. HPV gains access through microabrasions to the BASAL LAYER, the only epithelial compartment containing dividing stem cells. The viral genome is maintained there as an episome. Immortalisation occurs when high-risk types (16, 18) INTEGRATE into the host genome, causing overexpression of the E6 and E7 oncoproteins: E6 degrades p53 and E7 inactivates Rb, disabling both major tumour-suppressor checkpoints and driving unregulated proliferation.

Note: This basal-cell targeting is why HPV vaccination must precede exposure, and why koilocytes — squamous cells with perinuclear clearing and raised nuclei — are the cytological hallmark seen in the SUPERFICIAL layers where virion production occurs.