This family of viruses exclusively infects epithelial surfaces where they cause hyper proliferative lesions. Some HPV types infect keratinized epithelium and others mucosal surfaces. The typical lesion caused by HPV is a wart. HPV is of major medical importance because some HPV types are causally linked to various cancers, especially cancer of the cervix.
Genus: Various, e.g. Alphapapillomavirus
Species: Human papillomavirus xx (82 genotypes assigned, but more than 100 distinct viruses)
Structure: Small, unenveloped, circular dsDNA
EM photo of HPV, courtesy of Prof Linda Stannard, UCT
HPV replicates exclusively in stratified squamous epithelium. It requires differentiating epithelium to undergo a complete replication cycle. Infection is initiated when the virus gains access to the basal epithelial cells (through abrasion). In the basal layer, early non structural viral proteins are expressed (including E6 and E7) which stimulate the infected cells to proliferate faster. This is followed by replication of the viral genome. As the epithelial cells mature (move up the epithelial layer), there is a switch to expression of the viral structural genes (L1 and L2). Production of structural viral proteins is initiated and new virus particles assemble and are shed as the infected cells desquamate.
Epithelium is an immune-privileged site and the virus evades immune recognition and may persist for many months (even years) in the same individual.
Clinical disorders due to HPV infection
Most HPV infections are clinically silent. Only a minority of people develop clinically apparent lesions.
Cutaneous warts are caused by HPV types 1, 2, 3, 4, 5 and 8. They are benign painless proliferative lesions that occur on the skin at sites of abrasion such as hands, feet, knees and elbows. They are very common, especially in children over the age of five years. They typically persist for months, but eventually regress in immuno-competent people.
Mucosal warts: These are mainly due to HPV 6 and 11. Lesions are most commonly seen in genital mucosa (usually sexually transmitted), but may occur in oropharynx and they are also the cause of laryngeal papillomas. Like cutaneous warts, they may persist for months or years and although they are benign they can cause much distress due to disfigurement of the genital area.
This occurs with a rare immunodeficiency state where the patients are highly susceptible to HPV infection. They present with chronic extensive skin infection with multiple HPV types. Some skin lesions may undergo malignant transformation.
This condition, most commonly seen in infants and young children is due to infection with HPV 6 or 11. Warts develop in the larynx, especially on the vocal cords. Infection is usually acquired during passage through infected birth canal. Affected children develop hoarseness and stridor and airway obstruction. Treatment involves repeated surgical removal of the growths.
Epidemiology of genital HPV infections
Mucosal-tropic HPVs are mainly sexually transmitted. The incidence of infection (in both males and females) is highest shortly after onset of sexual activity and declines to a low level at about 30 years of age after which it rises again. Prevention is difficult as the vast majority of infections are asymptomatic. Condoms do not work very well as prevention as the virus may infect any part of the genital mucosa. Around 80% of women are infected with HPV in their life time. Re-infections with the same or another HPV type can occur throughout life. Most of these infections are cleared with the development of a type specific immune response, but a small proportion of patients may develop persistent infection. Persistent infection with multiple HPV types is common in HIV infection due to compromised host immunity. Persons who fail to clear infection are at risk for malignancies in the genital tract.
HPV and carcinogenesis
Certain mucosal tropic HPV types can cause cancer in healthy people. These are referred to as high risk HPV types. High risk HPV types are responsible for various cancers of the genital tract including penile, vulval, anal, vaginal and cervical cancer.
Cancer of the cervix is the second commonest cancer in women world wide and the commonest cause of cancer deaths in women in developing countries. Around 15 high risk HPV types may cause this cancer, but the most commonly implicated are HPV 16 which accounts for 50% of cervical cancers and HPV 18 responsible for 10-20% of cervical cancer worldwide. (The remaining 30% are due to HPV 31, 33, 35, 39, 45, 51, 52, 56, 58, 59, 63, 68, 82).
HPV infection and cancer development:
The evolution of cancer following infection takes many years.
As a result of various immune evasion mechanisms, HPV is able to persist in most healthy people for months as the specific immune response is delayed. Nonetheless, most individuals do clear the infection eventually. Only those individuals that fail to clear a high risk HPV type are at risk of disease progression. Persistent high risk HPV infection of cells at the transformation zone of the cervix puts a patient at risk for cervical cancer. However, oncogenesis is a multi-step process. Other co-factors are necessary and the disease takes many years to evolve and only a small percentage of people develop the cancer. The co-factors most consistently associated with cervical cancer include: smoking, multi-parity, prolonged oral contra-ceptive use and chronic inflammation of genital tract due to other STIs. HPV infection is merely an essential initiator of the process:
How HPV initiates oncogenesis
Genomic integration is not necessary for virus replication, but may be essential for oncogenesis. When the circular viral genome integrates into the host chromosome it disrupts the E2 gene. The E2 protein normally controls the expression of E6 and E7 (prevents excessive expression of these genes).
Uncontrolled expression of E6 and E7 makes the cell vulnerable by respectively inactivating the anti-oncogenes p53 and retinoblastoma protein (pRB). These anti-oncogenes work together to protect the cell from DNA damage:
p53 (inactivated by E6) is a transcription factor which controls DNA repair and the cellular response to injury. It initiates apoptosis in a cell that is too badly damaged.
The retinoblastoma protein (inactivated by viral E7) halts DNA replication during the G1 phase of the cell cycle to allow repair of damaged DNA.
Thus HPV-infected cells fail to repair damaged DNA, accumulate mutations and do not undergo apoptosis. This makes them vulnerable to malignant transformation.
Detection of pre-malignant lesions
Cancer takes many years to develop and is preceded by pre-malignant lesions. HPV infected cells in the cervix are morphologically different from uninfected cells and they become increasingly atypical and involve an increasing depth of the epithelium as the disease process progresses. Invasion of the basement membrane of the epithelium heralds the development of an invasive cancer. Pre-malignant lesions can be detected by various means, including direct visual inspection (after acetic acid treatment of epithelium), colposcopy, cytology or histology of biopsy material.
Cytological screening tests
The most widely used screening test for cervical cancer is the Pap smear. Cells collected directly from the cervix with a swab or cytobrush are smeared onto a glass slide, fixed and stained and examined microscopically for evidence of cellular atypia. Cells are scored as normal, low grade squamous intra-epithelial lesion (LSIL), high grade squamous intra-epithelial lesion (HSIL) or atypical cells of unknown significance (ASCUS). The main problems with using this as a screening test is that it is labour intensive and requires skilled cytologist ton assess the slides. Also a reliable result requires an adequately collected sample.
HPV DNA tests
Because cancer of the cervix only develops in patients persistently infected with high risk HPV types, the detection of high risk HPV DNA in cervical cells is a sensitive way to identify patients at risk for cervical cancer. However, although these tests are highly sensitive and specific, they cannot distinguish between patients with a transient or persistent HPV infection. It has a high negative predictive value for carcinoma of the cervix (i.e. if negative, the patient is very unlikely to be evolving carcinoma of the cervix). This test is a useful adjunct to cytology for:
- older women (if positive, likely to be persistently infected with high risk HPV type)
- For determining the significance of ASCUS
- Identifying women who need more regular follow up.
Protective immunity to HPV is directed to the major capsid (L1) protein. Immunity is type specific and there is little cross protection with other HPV types. After natural infection only 50-70% people develop detectable antibody and this takes many months to develop (as virus evasion delays onset and magnitude of specific immune response). This accounts for why people can be repeatedly infected with HPV throughout their sexually active life.
A major breakthrough has been the development of subunit HPV vaccines, based on the L1 protein of specific HPV types. The vaccines consist of recombinant L1 protein. The L1 protein self assembles into virus-like particles (VLPs) which are highly immunogenic. A course of 3 doses induces high levels of type specific antibodies in vaccine recipients (much higher than is induced by natural HPV infection). To prevent infection, vaccine needs to be administered before onset of sexual activity (before first exposure to HPV), currently advised for pre-pubertal girls.
2 vaccines have been licensed so far:
1. Cervarix contains VLPs derived from HPV16 and 18. Together these high risk HPV types account for 70% of cervical cancers.
2. Guardasil contains VLPs from HPV16 and 18 as well as for 6 and 11 (the last 2 are the major cause of genital warts.)
Clinical trials of these two vaccines have shown both vaccines to be highly effective at preventing type specific HPV infection in vaccine recipients. Many countries have added this vaccine to their national immunisation programmes (targeting pre-pubertal girls), but they are still very expensive and in South Africa are not available to the public sector.