Author Margaret Stanley, PhD, is professor, Department of Pathology, University of Cambridge, Cambridge.
Abstract Stanley, M. (2008) The epidemiology and burden of HPV disease. Nursing Times; 104: 36, 38-40.
Margaret Stanley describes what human papillomavirus is and how it causes disease. She also outlines how the new vaccine to protect against HPV works.
Papillomaviruses are a large family of small DNA viruses that infect animals and man. There are two special aspects to the biology of these viruses: they are very host specific so human papillomaviruses (HPVs) only infect humans; and they will only infect and produce infectious virus in a fully differentiating squamous epithelium.
HPVs are classified by DNA sequence as genotypes, not serotypes (based on antigens) and more than 100 HPV genotypes have been isolated from clinical biopsies. The viruses are numbered in the sequence in which they were isolated, for example HPV 1 and HPV 2.
HPVs fall into two groups: those that infect the skin, or cutaneous surfaces, and those that infect the internal wet squamous mucosal surfaces, particularly the genital tract. Within these groups there are low-risk types (lrHPV), that generate benign lesions (warts), and high-risk or oncogenic types (hrHPV), that are associated with cancers and their precursor lesions.
About 30-40 HPVs regularly or sporadically infect the anogenital squamous epithelium of men and women. The most common low-risk viruses that cause warts on the anogenital skin are HPV 6 and HPV 11.
The high-risk genital HPVs viruses are true human cancer or oncogenic viruses and cause cancer of the cervix and a proportion of other anogenital cancers. There are about 15 of these oncogenic types that infect the genital tract but the two major ones are HPV 16 and HPV 18.
The viruses cannot be grown in tissue culture; the antibody responses that they induce are weak and the time of seroconversion (the development of antibodies in response to infection) is not standard. HPV infection is determined by detection of HPV DNA, using molecular hybridisation procedures that are preceded usually by rounds of DNA amplification. Further analysis is then needed for genotyping of the individual HPVs.
The infectious cycle of papillomaviruses is a long one, requiring a minimum of 6-12 weeks between infection of the basal epithelial cell and release of infectious virus from the superficial squames.
There are three consequences of this infectious cycle.
It is an exclusively intraepithelial infectious cycle and there is no viraemia (presence of a virus in the blood) and therefore no exposure, or very little exposure, of virus particles to the draining lymph node where immune responses are generated.
There is no virus-induced cytolysis or cell death. The papillomaviruses undertake large-scale production of virus proteins and assembly of virus particles in cells that are already destined for death. There is therefore no need for the virus to kill the cell, as the cell is already going to die and more viruses will be released when the cell undergoes natural death. From the point of view of immune defences, this means that there is no danger signal to the immune system and therefore no response. The absence of inflammation results in a slow activation of the adaptive immune response and cell-mediated immunity to virus infection. This immune evasion is further exacerbated because the papillomaviruses actively downregulate the interferon response, which is a key antiviral response important in limiting viral infection.
As a consequence of this infectious cycle, the HPVs evade the innate immune system and delay the activation of adaptive immunity, which explains the persistence of HPV in the cervix and other parts of the anogenital epithelium (Stanley, 2006).
HPV disease burden
Low-risk HPVs are responsible for significant morbidity. Condyloma acuminata or genital warts are the most common viral sexually transmitted infection in the UK.
HPV types 6 and 11 cause laryngeal papillomatosis, a rare but important disease.
The most important of the anogenital malignancies associated with HPV infection is invasive carcinoma of the cervix and the precursor lesions, cervical intraepithelial neoplasia (CIN). CIN is a spectrum of epithelial proliferation, ranging from low-grade abnormality CIN1 and moderate abnormality CIN2 to high-grade abnormality CIN3.
The majority of CIN1 infections will be cleared by immune responses.
CIN2 and 3 show the features of HPV-induced neoplasia and should be regarded as the obligate precursors to invasive cervical cancer.
About 15 oncogenic HPV types are associated with cervical cancer, HPV 16 and HPV 18 being the most important. HPV 16 DNA sequences are detected in between 50% and 70% of invasive carcinomas. Second is HPV 18, detected in 10-20% of invasive carcinomas (Clifford et al, 2006).
HPV and anogenital cancers other than cervical
Oncogenic HPV DNA sequences are detected in about 50% of carcinomas of the penis, vulva and vagina (Parkin and Bray, 2006).
Patients with carcinoma of the vulva generally fall into two groups. Those associated with HPV infection are often younger women under the age of 55. Non-HPV associated carcinoma of the vulva is detected in older women in their late 60s and 70s and may be preceded by lichen sclerosus.
Carcinoma of the anus and the precursor lesion, anal intraepithelial neoplasia, is associated in more than 90% of cases with HPV infection (Parkin and Bray, 2006).
Carcinoma in the oropharynx, particularly carcinoma of the base of the tongue and carcinoma of the epithelium of the tonsil, is also HPV associated. In this latter case, the carcinomas fall into two groups. These are non-HPV carcinomas in older individuals that are associated with a history of heavy smoking and drinking and HPV-associated carcinomas in younger individuals under the age of 55 not associated with smoking or drinking but with a history of oral sex with several partners. In all of these cases, the major oncogenic HPV is HPV 16, which contributes at all sites other than the cervix to around 80% of lesions (D’Souza et al, 2007).
Cervical cancer contributes only 2% of all cancers in developed countries but to 7% in developing countries. This divide can be attributed to cervical cancer screening programmes in developed countries. Overall, HPV contributes to about 4% of all cancers, making HPV, and particularly HPV 16, a major global carcinogen (Parkin and Bray, 2006).
Natural history of genital HPV infection
After HPV infection there is a variable period of time between infection and the detection of a lesion or the detection of HPV DNA. This interval between infection and lesion detection may be weeks or months and, in extreme cases, years.
During this period, for example in the cervix, the woman will be DNA negative when detection is attempted with cervical smears. Then, as lesions appear, DNA becomes detectable and this is a period of productive viral infection.
The persistence of DNA or the persistence of lesions is again extremely variable. It may be of the order of weeks or months and there appears to be a distinction between high-risk and low-risk HPVs. Low-risk HPVs clear within a 4-9 month period after first detection but high-risk HPVs require about 12-18 months.
The adaptive immune response responsible for lesion regression and viral clearance is a cell-mediated immune response and, at the same time or shortly after this response, there is seroconversion, with the serum-neutralising antibody to the major protein L1 (L1 is the gene which encodes the major protein of the virus coat). At this point, 80-90% of infected individuals make a successful response; they become DNA negative, their warts clear and there is sustained clinical remission. However, 10-20% of cases make an ineffectual or inadequate immune response. They remain DNA positive and the virus persists. These are the women who are at risk of CIN2/3, the obligate cancer precursor, or are those who have recurrent and apparently intractable genital warts.
The acquisition of genital HPV occurs soon after the onset of sexual activity. Eight out of 10 of all sexually active women (and presumably men) will acquire a genital HPV infection at some point. The acquisition of infection is most common among young adults aged 15-30. A study of HPV prevalence in Manchester showed that in 15-19 year-olds, about 25% had any genital HPV in the cervix at any one time, 20% had a high-risk HPV and at any one time and 10% actually had HPV 16 (Peto et al, 2004).
However, most HPV infections are cleared and the prevalence of HPV infection declines rapidly. In women in their 40s, the prevalence of genital HPV infection of any type in the population is of the order of 4% and it continues to decline into the mid 50s (Peto et al, 2004).
The HPV type that is acquired is important. In the US, Khan et al (2005) examined the cumulative incidence of CIN3 or more serious lesions in 13,000 women over a 10-year period. Women who entered the study who were HPV 16 positive but HPV 18 negative or HPV 18 positive but HPV 16 negative had a cumulative incidence of CIN3 at 10 years of 22% and 18% respectively. Women who were high-risk HPV positive but HPV 16 and HPV 18 negative at entry had a cumulative incidence at 10 years of CIN3 of only about 2%, whereas those who were HPV negative had essentially 0% cumulative incidence of CIN3.
This data and the prevalence of the most common HPV types in CIN2/3 show that HPV 16 and HPV 18 are indeed the most pathogenic, the most dangerous of the high-risk HPV types.
The development of prophylactic vaccines against the common oncogenic and benign HPV types has revolutionised our prospects for the prevention of the cancers associated with HPV types 16 and 18 and also against the low-grade types 6 and 11.
Neutralising antibodies in natural infections are directed against the HPV L1 capsid protein (L1 is the gene that encodes the major protein of the virus coat). This protein must be in its native, conformational, properly folded form before antibodies that are neutralising are made.
HPV cannot be grown in bulk in tissue culture so the traditional methods for making virus vaccines - attenuated live or killed virus vaccines - are not possible for HPV. Instead, HPV vaccines are subunit vaccines comprised of one virus protein, the
L1 protein. These vaccines are made using a technology that results in the L1 protein assembling into structures known as virus-like particles (VLPs), which are empty protein shells that contain no DNA so are not infectious. The morphology of the VLPs and, more importantly, the immunological profile, is virtually identical to the native virus.
Two HPV L1 vaccines are now available: Cervarix and Gardasil. These vaccines have been shown to be remarkably efficacious and both have shown an efficacy of greater than 98% against CIN2/3 caused by HPV 16 and 18 in randomised phase 3 controlled trials (Ault, 2007; Paavonen et al, 2007).
Gardasil has shown 100% efficacy against external genital warts caused by HPV 6 and 11, which cause external genital warts, and a similar efficacy against vulval intraepithelial neoplasia or vaginal intraepithelial neoplasia caused by HPV 6, 11, 16 or 18 (Garland et al, 2007).
Cervarix has been selected for the vaccination campaign in the UK. These vaccines offer the opportunity, if delivered with high coverage and to the appropriate population, of significantly reducing the incidence of cervical cancer. The effects of HPV immunisation on the incidence of cervical cancer will not be seen for decades.
The potential reduction that could be achieved by these vaccines will only be 70%, since HPV 16 and 18 are the causal agents of 70% of invasive carcinomas so women will have to stay in screening programmes. However, screening methodologies are likely to change and the intervals at which women are called for screening will also change as a result of the HPV vaccines. The combination of screening and vaccination could result in the virtual elimination of cervical cancer in British women. Ault, K.A. (2007) Effect of prophylactic human papillomavirus L1 virus-like-particle vaccine on risk of cervical intraepithelial neoplasia grade 2, grade 3, and adenocarcinoma in situ: a combined analysis of four randomised clinical trials. The Lancet; 369: 9576, 1861-1868.
Clifford, G. et al (2006) Chapter 3: HPV type-distribution in women with and without cervical neoplastic diseases. Vaccine;
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D’Souza, G. et al (2007) Case-control study of human papillomavirus and oropharyngeal cancer. New England Journal of Medicine; 356: 19, 1944-1956.
Garland, S.M. et al (2007) Quadrivalent vaccine against human papillomavirus to prevent anogenital diseases. New England Journal of Medicine; 356: 19, 1928-1943.
Khan, M.J. et al (2005) The elevated 10-year risk of cervical precancer and cancer in women with human papillomavirus (HPV) type 16 or 18 and the possible utility of type-specific HPV testing in clinical practice. Journal of the National Cancer Institute; 97: 14, 1072-1079.
Paavonen, J. et al (2007) Efficacy of a prophylactic adjuvanted bivalent L1 virus-like-particle vaccine against infection with human papillomavirus types 16 and 18 in young women: an interim analysis of a phase III double-blind, randomised controlled trial. The Lancet; 369: 9580, 2161-2170.