California
Association
for
Medical Laboratory Technology
Distance Learning Program
|
Papillomaviruses
and Cervical Cancer Course
Number: DL-979 © California Association
for Medical Laboratory Technology. CAMLT is approved by the California Department
of Health Services 1895 Mowry Ave, Suite 112 Notification of Distance Learning Deadline This is a reminder that all the continuing education units required to renew your license must be earned no later than the expiration date printed on your license. If some of your units are made up of Distance Learning courses, please allow yourself enough time to retake the test in the event you do not pass on the first attempt. CAMLT urges you to earn your CE units early! |
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Papillomaviruses and Cervical Cancer
OBJECTIVES
Upon completion of this course the participant will be able to:
- summarize the principal characteristics of human papillomaviruses (HPVs), including their structure and classification.
- explain the link between HPV infection and cervical cancer.
- discuss the role of HPVs as etiological agents of skin and genital warts.
- list various clinical conditions associated with HPV infections.
- outline the pathogenesis of HPV infection.
- describe host and viral factors that contribute to progression of HPV infection to malignancy.
- summarize current diagnostic methods and treatment options.
- contrast the principal characteristics of the two vaccines developed for prevention of HPV infection.
- discuss the potential effect of HPV vaccination on the incidence of cervical cancer.
INTRODUCTION
Papillomaviruses are small DNA viruses that infect mucosal and cutaneous epithelium
and cause benign hyperproliferative lesions recognized as warts. These viruses
are widely spread in the environment. Infection with papillomaviruses is common
in humans and in many animal species, including rabbits, cows, dogs, dolphins,
and porpoises. Papillomaviruses are highly specific for their respective hosts.
Cutaneous types of HPVs infect the skin of the hands and feet causing formation
of warts. Mucosal types infect the lining of the mouth, throat, respiratory
tract, and anogenital epithelium. Genital, respiratory, and conjunctival papillomas
are among several manifestations of these infections. In most cases, the infection
is cleared following activation of the host immune response against the virus.
Occasionally, the lesions do not regress and can progress to cancer under appropriate
environmental conditions (1).
HISTORICAL BACKGROUND
The infectious origin of skin warts was long suspected and was eventually proven
in the 19th century by experimental transmission as well as by accidental inoculation
of healthy subjects (2). Genital warts, however, were considered to be manifestations
of common venereal diseases. Experimental transmission of genital warts to human
subjects demonstrated that a separate infectious agent was involved.
The connection between genital warts and cervical cancer was slow to emerge.
In the middle of the 19th century Rigoni Stern, an Italian pathologist, made
a suggestion based on epidemiological observations that cervical cancer may
have an infectious origin. Various agents, including herpes viruses were subsequently
considered as candidates. In 1977, zur Hausen and his colleagues proposed that
some human papillomaviruses were responsible for cervical cancer. They showed
that certain HPVs that infect anogenital skin could be isolated from cervical
cancers and from cell lines derived from such cancers (3). Subsequent epidemiological
studies utilizing DNA hybridization techniques established that most cases of
cervical cancer, as well as a number of other anogenital and head and neck cancers,
could be attributed to the so-called high-risk human papillomaviruses.
PAPILLOMAVIRUSES: CLASSIFICATION AND PRINCIPAL CHARACTERISTICS
Classification
Papillomaviruses are currently classified in a separate viral family, the Papillomaviridae.
More than 200 types of human papillomaviruses have been recognized on the basis
of DNA sequence data that show genomic differences. Approximately 106 of these
genotypes are well characterized. The remaining isolates are partially characterized
as potential new genotypes. Recently Papillomaviridae were divided into genera,
based on phylogenetic relationships among human papillomaviruses. An alternate
classification is based on clinical characteristics of HPVs. According to this
classification human papillomaviruses are grouped into 4 categories based on
the site of infection (skin or genital) and the probability that the infection
will eventually progress to malignancy. HPV genotypes involved in malignancies
are known as high-risk types.
Clinical classification
of HPVs (data from reference 3) |
||
| Risk of Cancer | Site of Infection |
|
Skin |
Genital |
|
| High-Risk (flat lesions) | HPV 5, HPV 8* |
HPV 16, HPV 18 |
| Low-Risk (warty lesions) | HPV 1, HPV 2 |
HPV 6, HPV 11 |
| *HPV 8 infection poses cancer risk primarily in immunosuppressed patients or in association with a condition known as epidermodysplasia verruciformis. | ||
Close to 30 genital HPV types are considered high risk. Among these genotypes, infection with HPV 16 accounts for over 50% of cervical cancers. HPV 18 is the next most prevalent papillomavirus genotype isolated from cases of cervical cancer. HPV types 18, 31, and 45 account for 25% to 30% of cervical cancer cases.
Structure of viral particle
Papillomaviruses are small, non-enveloped viruses that contain a double-stranded,
circular DNA molecule of approximately 8000 base pairs. The DNA is associated
with histones, the basic proteins found in cell nuclei. The viral genome is
surrounded by a protein shell (capsid) composed of 72 subunits called capsomers.
The capsomers contain at least two proteins, L1 and L2. Of these, L1 is the
major structural protein. Five copies of this protein are present in each capsomer.
The virion (viral particle) capsid also contains approximately 12 copies of
the minor structural protein, L2. The protein subunits are assembled into a
highly symmetrical, icosahedral (20 sided) particle. When examined by electron
microscopy the virus particle resembles a golf ball.
Viral genome
Like all small viruses the papillomaviruses have a limited coding ability and
therefore depend on the host cell to provide factors necessary for viral replication.
The number of proteins encoded by the HPV genome is not greater than 10.
The genome is functionally divided into three regions. One segment of the genome
has no coding ability. It contains the origin of viral replication and the regulatory
elements concerned with transcription. Two other genome regions contain clusters
of viral genes, known as the early and the late genes. The early DNA coding
region is primarily involved with the replication of the viral genome and with
cell transformation. The late genes encode the structural proteins found in
the viral capsid.
Viral proteins encoded by late (L) genes
The L1 and L2 structural viral proteins are capable of self-assembly into a
symmetrical icosahedral capsid. When viral DNA and both L proteins are present,
the DNA is packaged within the viral capsid. The major L1 protein will self-assemble
into a non-infectious pseudo-viral particle, which is highly immunogenic. The
packaging of the viral genome, however, requires the presence of both capsid
proteins. These characteristics of HPV structural proteins have proven to be
of major importance in vaccine development.
Viral proteins encoded by early (E) genes
The early proteins are numbered from E1 through E7. These proteins are not incorporated
into the viral capsid. They play a critical role in the replication and transcription
of the viral genome. In addition, a major function of these proteins is to maintain
a replication competent environment in infected cells, since the replication
cycle of HPVs is critically dependent on the host cell DNA synthesis machinery.
Proteins coded by HPV early genes facilitate viral replication in differentiated
squamous epithelial cells, which are normally growth-arrested.
A considerable amount of information is available regarding the functions of
some of the early proteins (described in the next section).
E1 and E2 proteins
Viral DNA replication is primarily mediated by E1 and E2 proteins together with
cellular polymerases and replication proteins. The E1 and E2 proteins bind to
the origin of viral replication. The E1 protein is a helicase, the only viral-coded
enzyme. Helicase unwinds the parental DNA strands in advance of replication.
E2 protein forms a complex with the E1 protein and enables high-affinity binding
to the origin of DNA replication. The E2 protein also participates in the segregation
of viral DNA during cell division by binding the viral genome to the cellular
mitotic chromosomes. Other major functions of E2 protein include regulation
of transcription of the viral genome: transcription of E6 and E7 viral genes
is regulated and can be blocked by E 2 protein.
E4 protein
The product of the E4 gene plays an important role in facilitating viral maturation
and the release of viral particles from infected cells.
E5 protein
The E5 gene product induces an increase in the activity of a cellular mitogen-activated
protein kinase. This enzyme enhances the cellular response to growth and differentiation
factors resulting in continuous cellular proliferation. The effect of E5 protein
on cell growth categorizes this protein as one of HPV oncoproteins (proteins
that contribute to uncontrolled cell growth).
E6 and E7 proteins
The E6 and E7 proteins from low-risk HPV genotypes have been studied less extensively
than those from high-risk HPVs. Apparently, E6 and E7 proteins from low-risk
HPV genotypes play a major role in the viral life cycle. The E6 and E7 proteins
from high-risk HPV genotypes are oncoproteins because their activity contributes
to continuous cell proliferation and may lead to cell transformation. These
oncoproteins interfere with the regulation of the host cell growth cycle by
binding to cell proteins that regulate cell growth. The cellular regulatory
proteins are the cell cyclins, the cyclin-dependent enzymes (kinases), and the
so-called tumor suppressor proteins. Two tumor suppressor proteins play a major
role in the regulation of cell growth: p53 and retinoblastoma protein pRB. The
HPV E6 protein binds to the cell protein p53 and targets it for rapid degradation.
HPV E7 protein binds to the protein RB, the retinoblastoma gene product. The
outcome of this binding is stimulation of cellular DNA synthesis and continuous
cellular proliferation. These events facilitate viral reproduction. The E6 protein,
in addition to binding to the tumor suppressor protein p53, maintains a stably
replicating viral genome within the nuclei of infected cells.
Viral replication
Studies of HPV life cycle had been hindered by the inability of these viruses
to achieve high levels of replication under laboratory conditions. However,
some aspects of the viral life cycle have been studied in vivo and in various
specialized epithelial cell systems that resemble the tissue architecture of
normal host cells. Productive viral infection in a natural setting takes place
in stratified squamous epithelium of the skin or the mucous membranes. The epithelial
cell layer is being continually replenished by the replication of basal cells
at the basement membrane.
The first step in virus replication is viral binding to specific host cell receptors
and viral entry into the host cell. Viral genome replication takes place in
the cell nuclei of infected basal cells but progeny virus is not produced. Productive
infection must take place in differentiating cells that have migrated away from
the basal layer. These cells remain active in the cell cycle due to activity
of the E7 protein. Transcription of late genes and synthesis of capsid proteins
takes place in highly differentiated cells of the superficial layers of the
epithelium. Newly synthesized L1 and L2 proteins are transported into the cell
nucleus where they assemble into virus particles and package replicated viral
DNA. Mature virus particles are released from host cells without cell lysis.
(4)
PAPILLOMAVIRUSES AND HUMAN DISEASE
Clinical conditions associated with HPV
HPV infections range from mild skin lesions to malignancies. The majority of
HPV infections are benign. Historically, HPV was first recognized as the cause
of warts on the hands and feet. Warts are areas of hypertrophied skin filled
with keratin, and generally resolve spontaneously within one to five years.
Numerous HPV genotypes are known to cause warts (Table 1). Heck’s disease,
an HPV infection of the oral cavity associated with HPV types 13 and 32 also
tends to regress spontaneously. Epidermodysplasia verruciformis is a rare genetic
disease characterized by warts on the trunk and arms. It can develop into squamous
cell carcinoma. Recurrent respiratory papillomatosis of the larynx is primarily
a disease of very young children.
Infections with multiple HPV types are frequently observed. In genital HPV infections
the presence of multiple genotypes may tend to increase the severity of cervical
disease.
Table 1. HPV types associated
with clinical disease (data from reference 1) |
|
| Disease | HPV type |
| Plantar warts | 1, 2, 4, 63 |
| Common warts | 1-4, 7, 10, 26-29, 41, 57, 65, 77 |
| Flat warts | 3, 10, 26-28, 38, 41, 49, 75, 76 |
| Miscellaneous cutaneous lesions | 6, 11, 16, 30, 33, 36-38, 41, 48, 60, 72, 73 |
| Epidermodysplasia verruciformis | 2, 3, 5, 8-10, 12, 14, 15, 17, 19-25, 36-38, 47, 50 |
| Recurrent respiratory papillomatosis | 6, 11 |
| Heck’s disease | 13, 32 |
| Conjunctival papillomas/carcinomas | 6, 11, 16 |
| Genital warts | 6, 11, 30, 42, 43, 45, 51, 54, 55, 70 |
| Cervical intraepithelial neoplasia | More than 30 HPV types, many of which are high-risk |
| Cervical carcinoma | 16, 18, 31, 45, 33, 35, 39, 51, 52, 56, 58, 66, 68, 70 |
Transmission and pathogenesis of HPV infection
Infection with HPV is extremely common. Genital infection with HPV is considered
one of the most common sexually-transmitted diseases.
Transmission of HPV primarily takes place by skin-to-skin contact. Transmission
may also occur through fomites, as in prolonged exposure to shared contaminated
clothing, since HPV is highly resistant to heat and desiccation. Infection occurs
through minute abrasions in the epithelial cell layer. These abrasions expose
the cells in the basal layer to viral entry. Cellular receptors, such as heparan
sulfate, mediate the initial attachment of virus to cells, but the nature of
specific receptors for viral entry is not precisely known. Basal layer cells
have stem cell-like properties and are continually dividing. This provides a
reservoir of cells for the suprabasal cell region that lies above the basal
cell layer. It should be noted that excessive proliferation of basal cells is
considered a feature of pre-malignant or malignant disease. Although the replication
of the HPV genome is initiated in the cells of the basal layer, mature virus
particles are generated in the differentiated keratinocytes of the suprabasal
region of the epithelium.
In HPV infections that manifest as warts or condylomata, viral replication is
associated with proliferation of all epidermal layers except the basal layer.
Excessive multiplication of keratinocytes produces changes in cellular architecture,
creating the typical papillomatous (wart-like) structures.
In genital HPV infection the clinical symptoms may be genital warts or a low-grade
lesion called dysplasia (cervical intraepithelial neoplasia, grade I). Such
lesions show altered patterns of cellular differentiation. Dysplasia is often
cleared by the immune system in less than a year. The cellular immune system
is apparently involved in the resolution of these lesions, but the precise mechanism
of clearance is not yet understood. Those dysplasias that are not cleared may
persist for several decades. Persistence of an infection with a high-risk HPV
genotype is the major risk factor for the development of genital malignancies,
such as squamous cell carcinoma or adenocarcinoma of the cervix. During carcinogenic
progression of cervical lesions, HPV DNA may become integrated into the cellular
chromosomes. This may disrupt the E1 and E2 genes, preventing vegetative viral
replication and stimulating cell growth.
Factors involved in progression to malignancy
Although HPV has been demonstrated in all cases of cervical cancer, only a very
small number of infected women will develop malignant disease, typically many
years after the initial infection. Apparently factors other than HPV infection
play an important role in cancer development.
Host-related factors
- Age and sexual activity
The highest risk of genital HPV infection coincides with the greatest cellular growth and differentiation in the cervical region. Increased cellular differentiation occurs at puberty and at first pregnancy and declines after menopause. Genital HPV infection is most common in women between 18 and 30 years of age, while cervical cancer is more common in women older than age 35, suggesting a slow progression of HPV infection to malignancy. Sexual exposure to multiple HPV genotypes introduces an additional risk factor. - Immune suppression
Cellular immunity is a major factor in controlling and eliminating an on-going HPV infection. Impairment of the immune system by co-infection with the human immunodeficiency virus or by immunosuppressive drugs limits the ability of the infected host to control the infection. - Cigarette smoking
Local immune suppression induced by smoking and the mutagenic activity of cigarette components may contribute to persistence of HPV or to progression of infection to malignancy. Smoking is an important risk factor for higher grades of cervical disease. - Multiple pregnancies
This is a significant independent risk factor for moderate to high-grade cervical disease. - Genetic predisposition
The genetic make-up of the patient is an important factor in the development of cervical cancer: it may affect the susceptibility to HPV infection, the ability to clear the infection, or the rate of disease progression. - Co-infection with adeno-associated virus
Infection with the adeno-associated virus actually reduces the risk of cervical cancer. Apparently, a viral replication protein, Rep 78, interferes with the transcription of HPV 16 E6 and E7 oncogenes.
Viral factors that contribute to tumor development.
The majority of human papillomaviruses is in the low-risk category and produces localized warts that do not undergo malignant progression even if left untreated. By contrast, infection with high-risk HPVs may lead to cancer. This is particularly true for specific high-risk genotypes such as HPV 16 and HPV 18. These viruses are responsible for slightly over 70% of cervical cancer cases.
The high and low-risk HPV genotypes differ in a number of characteristics:
1. Cell transformation
Some high-risk HPV genotypes are capable of inducing cellular transformation in transgenic mouse model systems and in cell cultures. This ability to transform cells has not been demonstrated for low-risk HPV genotypes.
2. Integration of viral genome
One of the key events in cell transformation and carcinogenesis induced by high-risk HPV genotypes is the integration of the viral genome into the host chromosome. Such integration results in long-term persistence of viral DNA within the host cell and affects the expression of cellular genes. Cells with integrated viral genomes have a selective advantage over cells in which viral DNA is not integrated. Viral genes are partially expressed: E6 and E7 genes are expressed while other portions of the viral DNA are deleted or their expression is disturbed. Notably, the E2 gene transcriptional repressor is not expressed. The loss of E2 function may be critical for malignant progression since the E2 protein can block the transcription of E6 and E7 genes.
Integration of viral genome into the cellular chromosome has not been shown for low- risk HPV genotypes.
3. Viral oncoproteins
The E6 and E7 oncoproteins of high-risk HPV genotypes play a major role in cell transformation and in the induction of malignancy: these proteins bind cellular tumor repressor proteins involved in the regulation of the cell growth cycle.
Weak binding between E6 and E7 gene products and the cellular regulatory proteins has been shown for the low-risk HPV genotypes.
4. Induction of telomerase
The expression of the enzyme telomerase is essential for continuous cellular replication. The E6 and E7 oncoproteins of high-risk HPVs are able to induce the expression of telomerase. The activity of this enzyme facilitates cellular proliferation, extends the cellular life span and promotes cell immortalization.
Telomerase induction has not been shown for low-risk HPV genotypes.
5. Chromosomal abnormalities
Fully transformed cells generally present chromosomal abnormalities. The E6 and E7 oncoproteins from high-risk HPVs can induce genomic instability in normal human cells by generating mitotic defects through induction of centrosome abnormalities.
In contrast, E6 and E7 proteins from low-risk HPVs are not capable of inducing centrosome abnormalities.
Human papillomaviruses may differ in certain biological and chemical properties and in pathogenicity due to genomic variation. For example, 5 different phylogenetic clusters have been described for HPV 16: European, Asian, Asian-American, African-1, and African-2. The oncogenicity of HPV variants may vary geographically and according to the ethnic origins of the population. An illustration of the effect of HPV variants on oncogenicity is provided by a large clinical study of 10,000 women in Costa Rica. In this study the European HPV 16 prototype and three variants were seen. The most common variant contained a single point mutation and was not associated with progressive disease. The second variant with a single mutation at a different location was associated with some high-grade squamous intraepithelial lesions. The third variant presented multiple mutations and was associated with malignancies.
Mutations within the HPV 16 L1 gene have been described in viral isolates from cervical cancer. These mutations result in an assembly-defective L1 capsid protein that cannot activate the antigen-presenting dendritic cells and induce an immune response. Indeed, only half of cervical cancer patients have been shown to generate antibody to the viral capsid protein.
HPV oncoproteins may directly interfere with the host’s anti-viral immune defense mechanisms. Inhibition of the interferon response may take place or interference with antigen presentation to cytotoxic T cells may occur.
DIAGNOSIS AND TREATMENT
Diagnosis of HPV infections
A number of methods are currently available for determining the
presence of HPV in clinical samples and for identifying the viral genotype:
Detection of viral nucleic acid:
- In situ hybridization
Viral DNA can be identified in biopsy specimens by in situ hybridization with probes labeled with either radioisotopes or chemically reactive compounds. Reactions are detected by autoradiography, fluorescence, or by the development of a color reaction. Hybridization may be combined with amplification of nucleic acid by polymerase chain reaction (PCR). - Liquid hybridization assay
A hybrid capture assay kit is available for detection of HPV DNA in cervical samples. This is an immunoassay that uses chemiluminescence to detect the hybridization reaction between specimen target DNA and the RNA probe. - Immunohistochemical detection of DNA in biopsy tissue sections
An automated system (Gen Point) is available that detects as few as one or two HPV copies. Non-amplified immunoenzymatic assays detect 10 to 15 HPV copies. - General primer PCR assays
These assays use primers to amplify a broad spectrum of HPV types in a single PCR amplification. The primers target regions of HPV genome such as the L1 capsid gene. Various methods can be used to identify HPV genotypes after amplification. These methods include sequence analysis, restriction fragment length polymorphism, hybridization with type-specific probes, or enzyme-linked immunoassays using a mixture of HPV-specific probes. - Type-specific PCR assays
These assays are generally based on the sequence variation in E6 and E7 genes. The sensitivity of these assays is between 10 and 200 HPV copies per sample, depending on HPV type. Multiple PCR amplifications are required for each sample.
A new HPV genotype assay based on proprietary Templex technology has been introduced recently. This assay detects and identifies 25 common HPV genotypes in a single-tube reaction using type-specific primers for E6 and E7 genes. The Templex assay provides semiquantitative information on each type when multiple HPV types are present in one reaction. The procedure can be completed within 5 hours. - Detection of HPV mRNA
An assay is commercially available that detects mRNA of E6 and E7 genes. This assay can be done directly on Papanicolaou slides (Pap smears) and visualized using a fluorescence microscope.
HPV cervical disease: diagnostic methods
Abnormal cells in the cervix can be detected through the use
of the Pap smear. This test is used routinely in health screening programs
in developed countries and less frequently in other areas of the world. In
the United States the ability to detect abnormal cells in the cervix early
in the disease process has cut the mortality from cervical cancer by 50%.
In developing countries cervical cancer remains the most common cancer in
women.
The Pap smear has its limitations. These include inadequate samples
and false negative results. False negative rates as high as 20% to 30% have
been reported. Human error is probably a major factor: an average Pap smear
slide has 50,000 to 300,000 cells and if there are few abnormal cells present
they can be easily missed. New methods of collection and processing of specimens
for Pap smears have recently been developed to help reduce the number of false-negative
results.
If the Pap smear is abnormal, patients can be evaluated using
instrumentation (colposcopy) and by cervical biopsy. In addition, HPV nucleic
acid can be demonstrated in biopsy tissues by in situ hybridization with labeled
probes.
Treatment of HPV infection
- Therapeutic vaccination:
Using mouse models, experimental studies are in progress to develop a vaccine that induces a strong cytotoxic T cell response against HPV oncoproteins. Cell-mediated immunity, particularly the cytotoxic T lymphocytes, is thought to destroy malignant cells that express HPV E6 and E7 proteins (or fragments of these proteins) at the surface of infected cells in association with the major histocompatibility complex molecules. Attempts are also being made to develop a combined prophylactic/therapeutic vaccine that would induce antibody to the L1 protein as well as activate cytotoxic T cells. - Treatment of skin warts:
Warts generally do not require treatment and will regress spontaneously. Topical application of salicylic acid is available for home treatment. Other types of treatment include freezing the affected area (liquid nitrogen therapy), minor surgery, laser surgery, or chemical treatment with cantharidin (an extract from beetles). - Treatment of anogenital warts:
Treatment options include ablation, excision, or application of topical agents such as podophyllin or imiquimod. Interferon-alpha has also been approved for the treatment of genital warts. - Treatment of HPV cervical disease:
The majority of HPV-induced cervical dysplasias are transient. Most regress spontaneously within 12 to 36 months, as the immune system eliminates the virus. The treatment of lesions that persist depends on the size, stage, and histologic features of the lesion and the presence of lymph node involvement. Treatment may range from superficial procedures such as cryotherapy or laser therapy, excision of microinvasive cancers, to radiotherapy and hysterectomy.
PREVENTION: PAPILLOMAVIRUS VACCINE (5)
The development of an HPV vaccine is of major public health importance. It
is estimated that in the United States more than 6 million people become infected
with HPV every year and nearly 10,000 women are diagnosed with cervical cancer.
The mortality rate in the United States from cervical cancer is approximately
35%. Worldwide this disease is much more deadly, since 80% of cervical cancer
cases occur in developing countries. Approximately 270,000 to 300,000 women
die from cervical cancer each year worldwide. To a great extent the high mortality
rate is due to the lack of widespread health screening and to a delayed diagnosis
of this disease. It is estimated that the use of HPV vaccine will reduce cervical
cancer mortality rate by 5% to 10% per year.
An additional benefit of the HPV vaccine would be its effect on fertility:
in vitro fertility treatment is generally successful in 57% of uninfected
women but only in 23% of HPV-infected women.
Vaccine development:
Laboratories in the nonprofit sector did preliminary pre-clinical research
on the HPV vaccine. Theses studies demonstrated that L1, the major structural
protein of HPV, has an intrinsic ability to self-assemble into virus-like
particles (VLPs) when the L1 gene is expressed in host cells. The L1 protein
contains the immunodominant neutralization antigens of the virus and is able
to induce high levels of type-specific neutralizing antibody. Experimental
studies with animal papillomaviruses showed that vaccination with L1 VLPs
protected animals from high-dose challenge with homologous virus. The protection
could be transferred passively with immunoglobulin G.
Two companies, Merck and GlaxoSmithKline, developed commercial versions of
HPV vaccine. Vaccines developed by both companies are subunit viral-like particle
vaccines, (VLPs), composed of L1 protein, which is the major HPV capsid protein.
L1 VLPs are deficient in their ability to package viral DNA. The presence
of capsid protein L2 is required for DNA packaging and for production of infective
virions. Merck’s HPV vaccine, Gardasil, has 4 HPV genotypes: HPV 16
and HPV 18, which are the primary cause of approximately 70% of all cervical
cancers, and HPV 6 and HPV 11, which account for nearly 90% of external genital
warts. The vaccine offers type-specific protection. The VLP particles for
this vaccine are produced in yeast cells. Vaccine preparations also contain
an alum adjuvant to maximize the immune response. The vaccine is administered
intramuscularly in 3 doses over a 6-month period. Gardasil was licensed in
the United States in 2006 and approved for use in 9 to 26 year-old women.
Gardasil is also approved for use in 25 European Union countries and in Australia,
Brazil, Canada, Mexico, and New Zealand. Approval for use in remaining countries
is expected in 2007.
The vaccine developed by GlaxoSmithKline is called Cervarix. This vaccine
is bivalent, consisting of HPV 16 and HPV 18 L1 virus-like particles with
a proprietary adjuvant ASO4 (alum and monophosphoryl). L1 particles are produced
in insect cells using a recombinant insect virus, a baculovirus. The approval
for use of Cervarix in the United States is pending.
Both vaccines are highly immunogenic, inducing a much greater antibody response
than found after natural HPV infection. The peak antibody levels are reached
2 to 6 months after immunization. Antibody level decreases during the first
2 years after immunization reaching a plateau that is maintained for at least
4 to 5 years. The anti-viral protection is type-specific. Protection against
HPV infection can be detected as early as 1 month from the first immunization.
Vaccinated women show a reduction in type-specific persistent HPV infection.
The protection is in effect against both benign and malignant disease induced
by HPV infection.
Even though the Gardasil and Cervarix vaccines appear to be highly effective,
there are a number of issues that still need to be explored. These issues
include:
- Long-term safety of HPV vaccine
- Length of protection offered by vaccination
- Cost of vaccine and the availability of vaccine in poor countries which have the greatest need for HPV vaccination. Currently the cost of Gardasil is $120 per dose.
- Cross-protection against HPV types not included in the vaccine
- Therapeutic effect of the vaccine against established infection
- Protective effect of the vaccine in men
- Effect of vaccination on the natural history of HPV infection
- Population groups to be vaccinated
SUMMARY
Papillomaviruses are widely distributed in the environment. They infect humans
and other animal species and are highly species specific.
Most infections caused by papillomaviruses are benign and resolve spontaneously.
Under certain conditions the infections do not resolve and progress to malignancy.
The role of papillomaviruses in human disease had been difficult to demonstrate
because these viruses grow poorly under laboratory conditions. The introduction
of molecular diagnostic techniques facilitated the demonstration of papillomaviruses
in clinical lesions and the identification of these viruses as etiological
agents responsible for human infections.
A variety of clinical conditions are caused by human papillomaviruses. These
include skin and genital warts, miscellaneous cutaneous lesions, respiratory
and conjunctival papillomas, cervical dysplasias, and cervical carcinoma.
The clinical outcome of these diseases depends on various host factors and
on the genotype of the infecting virus. The high-risk HPV genotypes promote
the progression of HPV infection to malignancy. Other virus-related factors
that facilitate development of malignant lesions are genetic variation among
HPV types and immune evasion mechanisms.
Diagnosis of HPV infections is based on clinical symptoms and on the demonstration
of HPV in clinical lesions. Identification of HPV relies primarily on molecular
methods since cultivation of HPV in cell culture is difficult. PCR is used
to amplify viral nucleic acid. A number of molecular methods are in current
use and some commercial assays are available.
Recent introduction of HPV vaccine is of major public health importance. Two
commercial companies, Merck and GlaxoSmithKline, have developed vaccines that
protect against infections with selected HPV types. The Merck vaccine, Gardasil,
contains 4 HPV genotypes: HPV 16 and HPV 18 associated with over 70% of cervical
cancers, and HPV 6 and HPV 11 which cause most external genital warts. The
vaccine is licensed in the United States and approved for use in 9 to 26 year-old
girls and women. The second vaccine, Cervarix, developed by GlaxoSmithKline
is in the process of being approved for use in the United States. This vaccine
is bivalent, containing HPV types 16 and 18. Both vaccines appear to be highly
effective in protection against HPV infections and against pre-malignant disease
associated with these infections.
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- Munger K, Baldwin A, Edwards KM, Hayakawa H, Nguyen CL, Owens M, Grace M, Huh KW. Mechanisms of Human Papillomavirus-Induced Oncogenesis. J.Virol. 2004;78:11451-11460.
- Frazer I. God’s Gift to Women: the Human Papillomavirus Vaccine. Immunity. 2006; 25:179-184.
- DiMaio D, Liao JB. Human Papillomaviruses and Cervical Cancer. Advances in Virus Research. 2006;66:125-145.
- Cohen J. High Hopes and Dilemmas for a Cervical Cancer Vaccine. Science. 2005;308: 618-621.
Link to On-line REGISTRATION, PAYMENT and QUIZ to submit for credit
- Human papillomaviruses:
a. infect the gastrointestinal tract exclusively.
b. infect cats and dogs.
c. infect the mucosal and cutaneous epithelium.
d. are not infectious.
- Human papillomaviruses
a. cause infections that always progress to cancer.
b. cause infections that are generally benign.
c. infect only persons over 65 years of age.
d. are the primary cause of respiratory disease.
- Some characteristics of human papillomaviruses (HPVs) include:
a. HPVs all belong to the same genotype.
b. HPVs do not cause cell transformation.
c. HPVs are subdivided into high-risk and low-risk categories.
d. HPV genome codes over 200 proteins.
- The human papillomaviruses:
a. contain 2 capsid proteins, L1 and L2.
b. have an RNA genome.
c. have a genome that codes for over 200 viral proteins.
d. multiply only on the outer surface of cytoplasmic membranes.
- The viral capsid:
a. consists of proteins coded by “early” genes.
b. has a helical shape.
c. has an icosahedral shape.
d. consists of a single protein.
- Characteristics of HPV proteins include one of the following:
a. E1 and E2 proteins are known as oncoproteins.
b. E6 and E7 proteins bind to the cell tumor suppressor proteins.
c. “Early” proteins are the major structural proteins of HPV.
d. function of “late” proteins is not known.
- Some characteristics of HPV replication and cultivation include one of
the following:
a. viral replication takes place in squamous epithelial cells.
b. HPV is easy to grow in standard cell cultures.
c. HPV can be cultured on special bacteriological media.
d. HPV will only grow in red blood cells.
- HPVs are known to cause:
a. gastrointestinal disease
b. colds
c. Roseola
d. genital warts
- HPV infection is transmitted by:
a. contact with domestic animals
b the respiratory route
c. the oral-fecal route
d. skin-to-skin contact
- Which of the following applies to HPV infections?
a. Infection may resolve spontaneously due to clearance by the immune system.
b. Infection always progresses to malignancy.
c. Cervical cancer develops very rapidly after initial HPV infection.
d. Course of infection is not affected by viral genotype.
- A host factor that affects the progression of HPV infection to malignancy
is:
a. obesity
b. consumption of coffee
c. cigarette smoking
d. vitamin deficiency
- Viral oncoproteins may interfere with the host’s immune response
by:
a. inactivating macrophages
b. binding to antibody molecules
c. interfering with antigen presentation to cytotoxic T cells
d. lysing T cells
- Diagnosis of HPV infection is based on:
a. cultivation of tissue samples in cell cultures
b. cultivation of tissue samples in broth
c. molecular techniques
d. animal inoculation
- HPV DNA in biopsy specimens:
a. cannot be demonstrated by currently available methods.
b. can be amplified by PCR and demonstrated by molecular techniques.
c. can be demonstrated by hematoxylin stain.
d. can be demonstrated by Gram stain.
- The Templex HPV genotype assay:
a. identifies multiple HPV types in a single-tube reaction.
b. is based on identification of HPV capsid proteins.
c. can only identify a single HPV genotype.
d. relies on cell culture for identification of HPV types.
- HPV vaccines, Gardasil and Cervarix:
a. contain the L1 and the L2 viral proteins
b. are produced in yeast cells
c. are subunit viral-like particle vaccines composed of a single protein
d. are poorly immunogenic
- The HPV vaccines:
a. are not able to prevent HPV infections
b. prevent some HPV infections and pre-malignant lesions
c. can be used as therapeutic agents to cure warts
d. do not contain any high-risk HPV genotypes
- Skin warts may be treated with:
a. Pap smear
b. adjuvants
c. application of topical agents
d. warm baths
- Cervarix vaccine:
a. is produced in insect cells
b. contains 4 HPV genotypes
c. does not contain any high-risk HPV types
d. is licensed in the United States for use in men only
- Gardasil vaccine contains:
a. ten high-risk HPV genotypes
b. live HPV viral particles
c. a single HPV genotype
d. four HPV genotypes