Recent decades have witnessed a reduction in the incidence of cervical cancer in countries where screening programmes have achieved broad coverage. The recognized importance of high-risk HPV (human papillomavirus) infection in the aetiology of cervical cancer may introduce a role for HPV DNA testing in cervical screening programmes. Positive HPV DNA tests indicate women at risk of cervical cancer with greater sensitivity, but reduced specificity, compared with exfoliative cytology. Combining HPV testing with cytology may be useful in the triage of minor cytological abnormalities into those requiring referral to colposcopy (HPV positive) compared with those who can be safely managed by cytological surveillance (HPV negative). With its high sensitivity and high-negative-predictive value, HPV testing may also be useful for predicting treatment failure, since residual disease is very unlikely in the event of a negative HPV test. Ultimately, prevention is better than cure, and the advent of HPV prophylactic vaccines may obviate the need for population-based cervical screening programmes in the future. A multivalent vaccine administered to adolescents prior to the onset of sexual activity and boosted at regular intervals throughout their sexually active life may provide protection against type-specific HPV infection, malignant precursors and invasive cervical disease. Several large randomized placebo-controlled trials have been conducted with promising results. For those generations of women already exposed to high-risk HPV infection, therapeutic vaccines may offer advantages over conventional treatment, although much work still needs to be done.

INTRODUCTION

Cancer of the cervix is almost unique as a malignant disease. It has a well-characterized long-lasting pre-invasive phase which invariably precedes clinical cancer. The cervix is accessible for screening, and malignant disease can be simply and effectively treated in the pre-invasive phase. It is therefore an ideal disease in which to employ screening as a strategy for secondary prevention. The means of screening for the past 50 years has been exfoliative cytology, the so called Pap smear, after its inventor George Papanicolaou. The advent of colposcopy in the 1970s provided a means of visualizing the cervix under magnification and obtaining a directed biopsy. It also facilitated directed therapy such as laser and, more recently, diathermy loop excision, both of which can be used as out-patient or office procedures.

These developments have enabled the detection and treatment of cancer precursors, leading to spectacular reductions in the incidence of cervical cancer in countries where broad screening coverage has been achieved. In the U.K., for example, deaths from cervical cancer have been cut by 50% since 1988 [1] when a national call/recall programme was introduced, and the trend continues downward. So is the battle against cervical cancer won? Emphatically not in developing countries where the resources to run screening programmes and facilities to treat malignant precursors are not available. Perversely, it is these countries which bear the greatest burden of disease: annually around 400000 cases worldwide. This results in pain and misery for thousands of women who die prematurely. In countries where screening programmes are in place, cancers still occur, some in unscreened women and some in screened women. It has been estimated that cervical screening in its present form will prevent approx. 70% of cancer deaths [2].

In the 1980s the epidemiological evidence linking cervical carcinogenesis with a sexually transmitted agent was confirmed, with HPV (human papillomavirus) DNA being identified in cervical cancers and pre-cancers. The papillomaviruses are a group of more than 120 different double-stranded DNA viruses that infect epithelial cells [3]. Papillomaviruses are classified as cutaneous or mucosal depending on the type of epithelium they infect. Mucosal types are further sub-divided according to their oncogenicity. Low-risk types (e.g. HPV6 and 11) cause benign warts, whereas high-risk types (e.g. HPV16 and 18) are associated with malignant disease of the female lower genital tract.

The life cycle of the papillomavirus is absolutely dependent upon the complete differentiation programme of the keratinocyte [4]. HPV infects cells in the basal layer of the epithelium, but produces virus only in the superficial terminally differentiated cells at the culmination of a maturation process that takes weeks to complete. The papillomavirus genome encodes six early (E) functional proteins and two late (L) structural proteins (Figure 1). The early proteins E1 and E2 are required for viral replication [5]. E6 and E7, the viral oncoproteins, exert their effects via interaction with cellular tumour-suppressor gene products p53 and Rb (retinoblastoma) respectively. Together they create a cellular environment conducive to proliferation in which the normal checks on cell-cycle control are disrupted. This allows occasional mitotic events to occur and errors to go unchecked, contributing to the malignant transformation of the cell [6] (for a comprehensive review on the molecular biology of HPV and cervical cancer, see [6a]).

Schematic representation of the genomic organization of HPV 16

Figure 1
Schematic representation of the genomic organization of HPV 16

The 7.9 kb genome of HPV 16 can be divided into three functional regions: the early region (E), which encodes six early proteins E1 to E7; the late region (L), encoding the viral capsid proteins (L1 and L2); and a long control region (LCR).

Figure 1
Schematic representation of the genomic organization of HPV 16

The 7.9 kb genome of HPV 16 can be divided into three functional regions: the early region (E), which encodes six early proteins E1 to E7; the late region (L), encoding the viral capsid proteins (L1 and L2); and a long control region (LCR).

It is now accepted that HPV is a necessary cause of cervical cancer [7]. It has been shown that the continued expression of HPV is necessary to maintain the malignant phenotype [8]. In addition, persistent HPV infection over 2 years with the oncotype 16 is associated with a very high relative risk of acquiring CIN (cervical intraepithelial neoplasia) 3, the true precursor of cervical cancer [9,10]. HPV infection is thought to occur in the majority of women at some time during their sexually active life. It is extremely common in the late teens and early twenties. The prevalence then falls steadily from approx. 40% to 5% by the age of 50 years [11]. This is generally thought to be due to immunological clearance of the virus from the cervix, with reduced opportunities for re-infection as women get older. It is persistent infection that results in a very high relative risk of developing CIN.

Cervical cancer occurs in 1 in 10000 women per year in the U.K. Pre-cancers are ten times more common and HPV infections 1000 times more common, being present in 15% of screened women. In some respects cervical cancer can be viewed as a rare complication of a very common infection. This causal link has two crucial implications. The first is that testing for HPV could have real benefits in cervical screening programmes, and the second is that vaccine development could produce both preventative and therapeutic strategies, affording huge benefit to the health of women worldwide.

This article will review firstly the potential for HPV testing in screening and, secondly, HPV vaccines in the context of cervical cancer prevention and treatment.

HPV IN TESTING PRIMARY CERVICAL SCREENING

There are a number of problems in using exfoliative cytology in cervical screening. The most important is its limited sensitivity [12]. Another difficulty is that it does not yield a dichotomous result: normal or abnormal. It identifies a spectrum of abnormality and, in order to maximize sensitivity, the low-grade abnormalities need to be acted upon. This results in reduced specificity requiring women to undergo unnecessary investigations. Could HPV testing improve this situation?

Because of the necessity of HPV infection in cervical carcinogenesis, it should be a highly sensitive screening test and this has been borne out in a number of studies [1316]. Conversely, of course, the same studies indicate that the specificity of a positive HPV test, particularly in young women, is poorer than a Pap smear. Although its superior sensitivity over cytology has been shown convincingly, optimal strategies for using HPV testing in routine screening have not yet been worked out. One possibility would be to screen first for HPV and then to retest all positives with cytology. This would help overcome the specificity issue, but it is not clear whether the limited sensitivity of cytology would still result in false-negative tests. The optimal way to manage women who were HPV positive/cytology negative has not been resolved and there are risks of lingering anxiety in affected women and unnecessary intervention, as most will have no underlying disease. Clearly, the epidemiological evidence points to repeat testing, but optimal intervals for this are not clear.

In order to understand how HPV testing could be best employed in screening, a number of trials are currently in progress in The Netherlands, Sweden, Italy, U.K. and Canada. These involve different randomizations, but are designed to assess the added impact of HPV testing. Within the next 2 years a great deal of data from these trials will become available which will address some key questions. (i) What is the duration of protection of a negative HPV result? (ii) Is a cytology negative/HPV negative result associated with a longer lasting negative predictive value than HPV negative alone? (iii) Does intervening for HPV-positive tests reduce the risk for developing high-grade CIN compared with cytology screening? (iv) How should a cytology negative/HPV-positive woman be managed?

It does appear that primary HPV testing could be both effective and cost effective, but its widespread introduction into a healthcare system used to providing cytology and a population of women accustomed to the ‘smear test’ will be challenging. Another aspect of HPV testing is its connotation of sexual transmission, which is troubling for a significant proportion of women. Qualitative research has identified a number of issues which concern women, including a feeling of stigma and also the implications of the infection within a stable sexual relationship [17]. If HPV testing were to become widely used there would need to be an effective programme of education to explain clearly to women what a HPV test means in terms of cervical screening in order that it is perceived in a positive light.

Other than primary screening, there are two other settings where HPV has been demonstrated to be effective: (i) to triage low-grade cytological abnormalities in order to select for colposcopic diagnosis; and (ii) to act as a ‘test of cure’ following treatment.

In both of these examples, HPV testing can provide a straightforward guide to management. In the case of triage, the value of HPV testing lies in its ability to increase the positive predictive value over the smear result alone, i.e. to add specificity to the process, increasing the likelihood that there is an underlying high-grade CIN lesion. At the same time, a negative HPV result carries a very high negative predictive value, which means the likelihood of an underlying high-grade lesion is extremely low, excluding the need for colposcopy [18]. In this way, HPV testing can increase diagnostic efficiency. It will require additional colposcopies compared with a strategy of repeated smears, but it shortens the time to diagnosis, which many women prefer, and it would reduce the risk of default from cytological surveillance [19].

In the case of post-treatment testing, the high sensitivity and high negative predictive value means that residual disease is very unlikely if the HPV test is negative [20]. This provides a strategy for returning treated women to call/recall after say 12 months compared with repeated annual cytology for 5–10 years. Conversely, a positive result would require continued surveillance and, if persistent, repeat colposcopy even in the presence of normal cytology as there may be residual disease.

Neither of these uses for HPV testing is NHS policy yet, but demonstration pilot studies have been undertaken and the potential for implementation is being assessed.

THE HPV TEST

So what about the test itself? HPV detection has employed a number of methods over the years mostly PCR-based and conducted in research laboratories. Clinical testing, however, requires a quality assured kit test that uses a methodology suitable for mass testing and yields highly reproducible results.

The first commercially available kit was called Hybrid Capture™ (Digene). This technique relies on a nucleic acid hybridization reaction, which reads as a chemoluminescent signal. It does not identify individual HPV types, but uses a probe mix of 13 high-risk types. This system has been widely used and, although there is probably some degree of cross-reaction with low-risk types, it has generally performed well.

Another commercially developed test is the Microwell Plate System (Roche) which, unlike Hybrid Capture™, requires PCR to amplify the DNA. Claims have been made that this test is more sensitive, and this may or may not be clinically relevant.

One key aspect of HPV testing in the context of cervical screening is its ability to be performed in liquid-based cytology systems. This means that, if a smear is taken and put into a liquid base instead of the conventional smear on a glass slide, the woman does not need to be recalled if a HPV test is required, as the liquid-based residue can be used.

Both of these commercial kits contain a ‘cocktail’ of probes against a subject of the so-called high-risk HPV types, i.e. the 13 types most frequently associated with cervical cancer; the commonest high-risk types being 16, 18, 31, 33 and 45. By contrast, types 6 and 11 are associated with genital warts and non-neoplastic lesions.

In addition to these DNA detection systems, an RNA detection system has been developed (Pretect™; Norchip) which detects the E6/E7 in RNA. It is claimed that this may detect more relevant ‘infection’ by identifying persistent expression of the HPV oncogenes, but whether it performs better in a clinical setting is unproven.

The above systems do not identify individual HPV types, and typing may become relevant in screening because a small subset of types, e.g. 16 and 18, may be dominantly associated with a higher risk of underlying CIN compared with certain other high-risk types, which may be relatively more common in women with normal cytology. One HPV typing system which should become commercially available is the Linear Array System (Roche).

HPV will undoubtedly find its place in cervical screening, initially for triage of low-grade abnormalities and in follow-up after treatment of CIN. How effectively and cost effectively it can be used in primary screening is still uncertain. It could be used in conjunction with cytology as a double test, but this would appear unnecessarily expensive unless it can be shown that there is added value in testing cytology negative/HPV negative compared with HPV negative alone. More likely, it could be used as a primary ‘stand alone’ test with another test being used to increase the specificity if the HPV test was positive. At the moment, this would be liquid-based cytology, but other cell-cycle and molecular markers are being developed which could provide a simpler testing platform than cytology.

Having reviewed the potential for HPV as a diagnostic test, we will now turn to its role as a prophylactic vaccine, which could result in a strategy of primary prevention for cervical cancer. Non-cervical intraepithelial neoplasia, e.g. vulval and anal,requires improved therapies and, although effective therapeutic vaccines are not yet available, some early candidates have shown promise.

RATIONALE FOR VACCINE DEVELOPMENT

As described above, the central role of high-risk HPV infection in cervical carcinogenesis is well established [3]. Genital HPV infections are common in young sexually active women and, although most are cleared spontaneously without ever causing dysplasia [18], persistent high-risk HPV infections are implicated in progressive cervical disease [19]. Such persistent infections are likely to result from the combined effects of viral strategies that allow HPV to avoid immune detection on the one hand, combined with ineffective host immunological responses that fail to clear the virus on the other. In accordance with this theory, immunocompromised women have a higher incidence of HPV-associated warts, CIN and cervical cancer [20]. Where HPV infections persist and lead to dysplastic change in the cervix, there is still considerable opportunity for natural immune responses to eradicate them; indeed, most untreated CIN lesions undergo spontaneous regression with time [21]. Thus cervical cancer can be considered a late and rare complication of high-risk HPV infection [3].

NATURAL IMMUNE RESPONSES IN HPV-ASSOCIATED DISEASE

Precisely why some infections persist and others undergo natural clearance has been the focus of considerable research effort over the past two decades. Unfortunately, these types of study are fraught with methodological difficulties. Measuring HPV-specific immune responses requires prior knowledge of the type(s) of immunity that may be important and different immunological assays often produce conflicting results. Neutralizing antibodies are likely to be important in preventing viral infection, but as HPV is an intracellular virus and thus protected from interference by host antibodies throughout its life cycle, it is unlikely that antibodies alone will be sufficient to clear an established HPV infection. Rather, it is believed that T-cell recognition and the subsequent elimination of HPV-infected cells via antigen-specific cellular immunity is the key to disease resolution [22]. An understanding of what is happening locally at the cervix may be more relevant than what can be measured systemically, but the process of taking a biopsy to allow infiltrating immune cells to be typed, quantified and their specificities determined may well alter the natural course of events.

Despite these difficulties, several longitudinal studies have compared naturally occurring immune responses in women infected with high-risk HPV in whom lesions persist with those in whom lesions clear spontaneously. Although healthy controls frequently show evidence of HPV16 E6-specific T-cell responses [2325], an absence of this immunity has been reported in women with persistent HPV16 infections [26] and cervical carcinoma [25,27]. By contrast, HPV16 E7-specific T-cell responses have been detected at low levels in the systemic circulation of patients with persistent HPV16-associated dysplastic and neoplastic lesions [2730], but less frequently in healthy controls. E7-specific T-cells have also been found in the draining lymph nodes and tumours of cervical cancer patients [31]. Most studies have concentrated on measuring HPV16 E6- and/or E7-specific immune responses, since these antigens are continuously expressed throughout the full spectrum of HPV-associated disease development [32]. More recently, E2-specific T-cell responses have been reported in patients and healthy volunteers alike, although the relationship between the measured immunity and disease outcome is unclear [25,33,34]. Taking all of these results together, it appears that the specific type(s) of cellular immunity that an individual mounts in response to an HPV infection may influence its outcome. The precise combination of particular HPV-specific CTL (cytotoxic T-lymphocyte) and T-helper responses, together with the appropriate cytokine milieu [25], may be critically important in determining whether an individual infection persists or is cleared.

Antibodies to the major papillomavirus capsid protein L1 are seen concomitantly with or within a few months following acquisition of HPV16 DNA in up to 70% of those infected [35,36]. Antibody titres appear to be correlated with antigen exposure, with higher levels found in women with persistent HPV infections and high-grade CIN [37]. In vitro studies indicate that these capsid antibodies block the interaction between infectious virions and their epithelial receptor, providing protection against repeat infection with the same papillomavirus [38]. Recent evidence in humans, however, suggests that naturally induced HPV-specific capsid antibodies may not necessarily protect against subsequent infections with homologous or genetically related HPV types [39] and some authors have suggested that T-cell immunity may also play a critical role in preventing repeat infections with the same or similar HPV types.

VACCINE DEVELOPMENT TO PREVENT AND/OR TREAT HPV-ASSOCIATED DISEASE

Clearly, a detailed understanding of the precise immune mechanisms that are important in the natural control of HPV infections is necessary for the optimization of vaccine design. While this knowledge accumulates, two main types of vaccine have been proposed for the control of HPV-associated disease: prophylactic vaccines, to prevent HPV infection, and therapeutic vaccines, to treat established infection and HPV-associated disease.

Prophylactic HPV vaccines

Prophylactic vaccines are designed to induce high titres of virus-neutralizing antibodies in the female lower genital tract that are capable of preventing infection [40]. The binding of antibody to intact papillomaviruses is thought to block the interaction with their receptor on the cervical epithelium, facilitating opsonization and degradation of the virions by macrophages [38]. A major breakthrough in prophylactic vaccine design followed the discovery that the L1 capsid protein has the intrinsic ability to self assemble into VLPs (virus-like particles) when expressed in the absence of other papillomavirus gene products [41]. The resulting VLPs are morphologically identical with native virions, but are not in themselves infectious because they lack the viral genome. High titres of papillomavirus-type-specific neutralizing antibodies and subsequent protection against high-dose experimental viral challenge have been seen in animals immunized with these VLP vaccines [42]. Further encouragement has been provided by early phase clinical trials which have established that the vaccines are both well tolerated and highly immunogenic, eliciting antibody titres at least 40 times higher than those seen after natural infection [43].

Prophylactic HPV16 VLP vaccine

Following on from this success, three large randomized controlled efficacy trials have now established proof-of-principle for HPV prophylactic vaccines [4446] (Table 1). In the first of these studies, Koutsky et al. [44] randomly assigned over 1500 young women to receive either three doses of an HPV16 L1 VLP vaccine or placebo. During an average follow-up period of 17 months, all 41 cases of persistent HPV16 infection (defined as two or more HPV16 DNA-positive samples at consecutive visits 4 or more months apart) and all nine cases of HPV16-positive CIN (five CIN 1 lesions and four CIN 2 lesions) occurred in the placebo group. There were no cases of HPV16-positive CIN 3 lessions or invasive cervical cancer detected in either group during the study [44]. Although the ultimate aim of prophylactic vaccination is to protect against cervical cancer, this end point is difficult to study for obvious ethical reasons, and persistent HPV infection is therefore regarded as an appropriate surrogate clinical end point.

Table 1
Proof-of-principle HPV prophylactic vaccine trials

CI, confidence intervals.

References
Koutsky et al. (2002) [44]Harper et al. (2004) [45]Villa et al. (2005) [46]
Vaccine manufacturer Merck Research Laboratories GlaxoSmithKline Biologicals Merck Research Laboratories 
Vaccine type Monovalent HPV16 L1 VLP Bivalent HPV16 and 18 L1 VLP Quadrivalent HPV6, 11, 16 and 18 L1 VLP 
Expression system Saccharomyces cerevisiae (yeast) Baculovirus (insect cells) Saccharomyces cerevisiae (yeast) 
Concentration 40 μg of HPV16 20 μg of HPV16 and 20 μg of HPV18 20 μg of HPV6, 40 μg of HPV11, 40 μg of HPV16 and 20 μg of HPV18 
Adjuvant Amorphous aluminium hydroxyphosphate sulphate AS04 Amorphous aluminium hydroxyphosphate sulphate 
Dose and administration 0.5 ml, intramuscular 0.5 ml, intramuscular 0.5 ml, intramuscular 
Dosing schedule 0, 2 and 6 months 0, 1 and 6 months 0, 2 and 6 months 
Trial population 768 vaccinees and 765 placebo 560 vaccinees and 553 placebo 276 vaccinees and 275 placebo 
Trial site U.S.A. U.S.A., Canada and Brazil U.S.A., Europe and Brazil 
Age of participants 16–23 years 15–25 years 16–23 years 
Inclusion criteria HPV16 seronegative and HPV16 DNA negative at enrolment HPV16 and 18 seronegative and high-risk HPV DNA negative (14 types) at enrolment HPV6, 11, 16 and 18 seronegative at enrolment and HPV6, 11, 16 and 18 DNA negative throughout vaccination regimen 
Exclusion criteria History of abnormal cervical cytology or>five lifetime male sex partners History of abnormal cervical cytology/CIN or>six lifetime male sex partners History of abnormal cervical cytology or≥five lifetime male sex partners 
Duration of follow-up Up to 48 months; median follow-up, 17.4 months Up to 27 months Up to 36 months 
Safety profile Well tolerated, few minor adverse events; no serious adverse events Well tolerated, few minor adverse events; no serious adverse events Well tolerated, few minor adverse events; no serious adverse events 
Vaccine immunogenicity 100% seroconversion 100% seroconversion 100% seroconversion 
Antibody titres (measured at month 7) compared with natural infection 60-fold greater 100-fold (HPV16) and 80-fold (HPV18) greater respectively 10-fold (HPV6), 7-fold (HPV11), 100-fold (HPV16) and 20-fold (HPV 18) greater respectively 
Efficacy in preventing transient infections 91% (95% CI, 80–97) 92% (95% CI, 65–98) Not reported 
Efficacy in preventing persistent infections 100% (95% CI, 90–100) 100% (95% CI, 77–100) 89% (95% CI, 70–97) 
Efficacy in preventing CIN 100% (ten cases of HPV16+ CIN in placebo group; none in vaccinated group) 100% (six cases of HPV16 or 18+CIN in placebo group, none in vaccinated group) 100% (three cases of HPV6, 11, 16 or 18+ CIN in placebo group; none in vaccinated group) 
References
Koutsky et al. (2002) [44]Harper et al. (2004) [45]Villa et al. (2005) [46]
Vaccine manufacturer Merck Research Laboratories GlaxoSmithKline Biologicals Merck Research Laboratories 
Vaccine type Monovalent HPV16 L1 VLP Bivalent HPV16 and 18 L1 VLP Quadrivalent HPV6, 11, 16 and 18 L1 VLP 
Expression system Saccharomyces cerevisiae (yeast) Baculovirus (insect cells) Saccharomyces cerevisiae (yeast) 
Concentration 40 μg of HPV16 20 μg of HPV16 and 20 μg of HPV18 20 μg of HPV6, 40 μg of HPV11, 40 μg of HPV16 and 20 μg of HPV18 
Adjuvant Amorphous aluminium hydroxyphosphate sulphate AS04 Amorphous aluminium hydroxyphosphate sulphate 
Dose and administration 0.5 ml, intramuscular 0.5 ml, intramuscular 0.5 ml, intramuscular 
Dosing schedule 0, 2 and 6 months 0, 1 and 6 months 0, 2 and 6 months 
Trial population 768 vaccinees and 765 placebo 560 vaccinees and 553 placebo 276 vaccinees and 275 placebo 
Trial site U.S.A. U.S.A., Canada and Brazil U.S.A., Europe and Brazil 
Age of participants 16–23 years 15–25 years 16–23 years 
Inclusion criteria HPV16 seronegative and HPV16 DNA negative at enrolment HPV16 and 18 seronegative and high-risk HPV DNA negative (14 types) at enrolment HPV6, 11, 16 and 18 seronegative at enrolment and HPV6, 11, 16 and 18 DNA negative throughout vaccination regimen 
Exclusion criteria History of abnormal cervical cytology or>five lifetime male sex partners History of abnormal cervical cytology/CIN or>six lifetime male sex partners History of abnormal cervical cytology or≥five lifetime male sex partners 
Duration of follow-up Up to 48 months; median follow-up, 17.4 months Up to 27 months Up to 36 months 
Safety profile Well tolerated, few minor adverse events; no serious adverse events Well tolerated, few minor adverse events; no serious adverse events Well tolerated, few minor adverse events; no serious adverse events 
Vaccine immunogenicity 100% seroconversion 100% seroconversion 100% seroconversion 
Antibody titres (measured at month 7) compared with natural infection 60-fold greater 100-fold (HPV16) and 80-fold (HPV18) greater respectively 10-fold (HPV6), 7-fold (HPV11), 100-fold (HPV16) and 20-fold (HPV 18) greater respectively 
Efficacy in preventing transient infections 91% (95% CI, 80–97) 92% (95% CI, 65–98) Not reported 
Efficacy in preventing persistent infections 100% (95% CI, 90–100) 100% (95% CI, 77–100) 89% (95% CI, 70–97) 
Efficacy in preventing CIN 100% (ten cases of HPV16+ CIN in placebo group; none in vaccinated group) 100% (six cases of HPV16 or 18+CIN in placebo group, none in vaccinated group) 100% (three cases of HPV6, 11, 16 or 18+ CIN in placebo group; none in vaccinated group) 

Although the results of this study are extremely promising, they do serve to highlight some of the challenges that remain in the implementation of HPV prophylactic vaccination strategies. Of the original 2392 women recruited into the trial, over 500 were excluded from the primary efficacy analysis because they were already HPV16 seropositive or HPV16 DNA positive at enrolment. A population-based prophylactic vaccination programme would therefore need to be aimed at women much younger than the 16–23-year olds studied here if it were to significantly impact on HPV16 acquisition rates. Vaccinating girls prior to the onset of sexual activity, possibly as young as 10–13 years of age, will require sensitive education programmes aimed at adolescents and their families, as well as parental consent.

Another important point raised by this study [44] is that protection against CIN offered by prophylactic vaccination appears to be type-specific. All nine cases of HPV16-associated CIN occurred in the placebo group, but a further 22 cases of CIN associated with other HPV types were seen in each of the vaccinated and placebo groups. This finding emphasizes the importance of vaccinating against more than one HPV type. Over 50% of cervical cancers are associated with HPV16 infection, but the remaining proportion are associated with any of a whole range of high-risk HPV types, the most common of which (types 18, 31, 33 and 45) account for up to a further 30% of cervical cancer cases worldwide [47].

Prophylactic bivalent HPV16 and 18 VLP vaccine

It is unclear whether increasing the breadth of genotype coverage for prophylactic HPV vaccines will impact on the strength of the resulting immune response against each of the different HPV types. In the second proof-of-principle efficacy trial, a bivalent HPV16 and 18 VLP vaccine was compared with placebo in over 1000 women aged between 15 and 25 years over a 27 month follow-up period [45]. All vaccinated women produced HPV16- and 18-specific antibodies at levels much higher than those seen following natural infection. The vaccine gave 100% protection against persistent HPV16 and 18 infections and 93% protection against HPV16- or 18-associated cytological abnormalities. Unfortunately, the trial was not powered to estimate efficacy for histopathologically confirmed CIN, with only six cases of CIN (all positive for HPV16 DNA) reported in the placebo group and one (positive for HPV51) in the vaccinated group. As expected, the incidence of HPV18 infection was very low in this study, but by analysing the intention-to-treat cohort, where sufficient numbers of events occurred, the authors were able to conclude that the vaccine is effective in preventing both transient and persistent HPV18 infections. Whether it additionally protects against HPV18-associated CIN remains unclear.

Prophylactic quadrivalent HPV6, 11, 16 and 18 VLP vaccine

The most recent prophylactic vaccine efficacy trial compared with placebo a quadrivalent VLP vaccine comprising HPV types 6, 11, 16 and 18 in more than 500 women between the ages of 16 and 23 years [46]. Over the 35-month follow-up period, persistent infection associated with HPV6, 11, 16 or 18 decreased by 89% in participants who received at least one dose of vaccine compared with those who received placebo. The study was not powered to assess vaccine efficacy for disease end points and only six women developed genital warts or type-specific CIN (all in the placebo group) during follow-up. The vaccine proved to be highly immunogenic, with all patients showing evidence of HPV6-, 11-, 16- and 18-specific antibodies following vaccination. By month 36, however, only 76% of the vaccinated cohort had significant detectable antibody responses to HPV18. Seropositivity for HPV6, 11 and 16 appeared to last longer.

The longevity of protection provided by HPV prophylactic vaccination is clearly an important issue. Since it is neither practical nor desirable to revaccinate frequently, a vaccine that offers an extended period of protection against HPV-associated disease is essential. The quadrivalent VLP vaccine appears to protect not only against high-risk HPV types 16 and 18, but also against low-risk HPV6 and 11. These low-risk genotypes cause 90% of anogenital warts, a benign but costly condition to treat associated with significant psychological morbidity. If HPV prophylactic vaccination strategies are to be effective at reducing the incidence of cervical cancer, vaccinating males may be critical for the induction of herd immunity, particularly since coverage of female vaccinees is unlikely to reach 100% [48]. Although high-risk HPV types are uncommonly associated with malignant disease of the anogenital tract in males, it is envisaged that offering the additional benefit of protection against anogenital warts may provide further incentive for vaccination. The value of including men in HPV prophylactic vaccination strategies is still completely unknown, however, since there is currently no evidence that vaccination can prevent infection in men or reduce exposure to women.

Overall, the results of these efficacy trials are very encouraging. Implementation of universal HPV prophylactic vaccination requires a detailed understanding of the disease incidence, screening programmes and treatment options available in any given setting, and it is clearly not possible to address all these issues in any one trial. Using a mathematical simulation model, Goldie et al. [49] predicted that a prophylactic vaccine that protects against 70% of persistent HPV16/18 infections should substantially reduce the incidence of HPV16/18-associated CIN and cervical cancer, even in a setting of established cytological screening. Clearly the benefit of such a vaccine to developing countries, where the incidence of cervical cancer is high and diagnostic and treatment services are poor, should not be under-estimated. The logistics of delivering an HPV prophylactic vaccine to the developing world are daunting, not least because the target population of adolescent females is not currently reached by any global health care programme. The vaccine is expensive to produce and purchase, requires a cold chain and must be administered in repeated injected doses. As a result, the financial implications of universal HPV prophylactic vaccination are not insignificant.

THERAPEUTIC HPV VACCINES

HPV prophylactic vaccination may not prevent HPV-associated disease in women who have already been exposed to the virus. For these women, a therapeutic vaccine, capable of eradicating established HPV infection, may offer significant advantages over conventional treatment for cancer of the cervix and its precursor lesions. Induction of HPV-specific immunity could allow the selective destruction of HPV-infected cells with minimal consequent damage to normal tissues. Long-lasting immunological memory could eliminate recurrence or relapse following treatment and protect against the emergence of HPV-associated lesions elsewhere in the female lower genital tract. Vaccination may also offer renewed hope for those women with advanced disease in whom standard treatment has failed.

Therapeutic vaccines aim to induce immunological mechanisms that are capable of recognizing and killing HPV-infected cells. To this end, most therapeutic vaccines have concentrated on eliciting CTL responses against HPV-specific viral antigens. The viral antigens of choice have traditionally been E6 and E7, since their continued expression throughout tumorigenesis is well established [32]. More recently, however, vaccines targeted against the E2 protein have been studied [50].

Antigen delivery

An important consideration is how best to deliver the viral antigen(s) to the immune system. In a patient with established HPV-associated disease, it is clear that any existing immune responses have so far been ineffective at clearing the infection. Candidate therapeutic vaccines have used a whole range of antigen-delivery systems to generate sufficiently strong immune responses to eradicate infection.

Peptides and recombinant proteins

Peptide- and protein-based therapeutic vaccines are not in themselves immunogenic, but they are cheap to produce and generate predictable immune responses that are relatively straightforward to measure. When formulated with appropriate adjuvants, such as heat-shock proteins or ISCOMATRIX, protein- and peptide-based vaccines exhibit enhanced immunogenicity, but the measured immunity does not always appear to correlate with clinical responsiveness [51,52]. An alternative approach is to load autologous dendritic cells ex vivo with tumour lysate proteins, thus harnessing the natural T-cell activation capabilities of professional antigen-presenting cells [53,54]. This technique has yielded impressive clinical outcome data but, since vaccines must be prepared on an individual basis, it is an extremely expensive and labour-intensive approach.

DNA vaccines

DNA vaccines are stable and cheap to manufacture, but do not induce potent anti-tumour immune responses without manipulation. When modified to encode specific adjuvants, such as cytokine genes or molecules involved in antigen processing and presentation, vaccine immunogenicity has been reported but with no reliable association with clinical outcome [55,56]. It should be borne in mind that DNA vaccines are potentially oncogenic and full-length HPV genes must be genetically modified to prevent tumorigenesis at the site of vaccination.

Recombinant viruses

Recombinant viral vectors are attractive candidate vaccines for the treatment of HPV-associated disease. An MVA E2 recombinant virus vaccine has recently been tested in 36 patients with CIN yielding promising immunological and clinical results [50]. Another vector, known as the vaccinia virus, induces potent antibody and cell-mediated immune responses, and was successfully used in the immunization programme that resulted in the worldwide eradication of smallpox. A recombinant vaccinia virus, genetically modified to encode HPV16 and 18 E6 and E7 (TA-HPV), is to date the most extensively studied HPV therapeutic vaccine. Clinical trials in patients with advanced and early stage cervical carcinoma [57,58], as well as those with high-grade lower genital tract neoplasia [5961], have established safety, immunogenicity and clinical responsiveness in a proportion of vaccinated patients. Unfortunately, measured HPV-specific immune responses do not appear to correlate strongly with clinical outcome following vaccination.

Prime-boost vaccination strategies

Preclinical studies have indicated that a prime-boost vaccination strategy employing heterologous vaccines may offer advantages over the use of either agent alone in the treatment of HPV-associated disease [62]. A fusion protein of HPV16 L2E6E7 (TA-CIN) has recently been used in combination with TA-HPV in women with high-grade lower genital tract neoplasia [63]. The vaccination schedule demonstrated safety, immunogenicity and clinical responsiveness in a proportion of the patients [63].

CHALLENGES IN THERAPEUTIC HPV VACCINE DESIGN

Several candidate therapeutic vaccines for HPV-associated disease have now been tested in early phase clinical trials. Vaccine safety has been confirmed, but immunogenicity and clinical efficacy have often been difficult to establish. In particular, there appears to be no consistent relationship between measured HPV-specific immune responses and clinical outcome following vaccination. Correlating HPV-specific immunity with clinical responsiveness is difficult if we are unsure what immune mechanisms are important and how best to measure them. Papillomaviruses have co-evolved with mankind over millions of years and owe much of their success to their ability to frustrate the natural immunity of the host. If the virus has persisted unnoticed in the infected tissue for long enough to cause neoplastic change, it is clear that a therapeutic vaccine must be extremely immunogenic to override these sophisticated mechanisms of immune escape. Moreover, as disease progresses to cancer, there is an increasing likelihood of systemic immunosuppression, and this will impact significantly on the expected success of therapeutic vaccination. Tumour cells often exhibit MHC class I down-regulation, which restricts the ability of the immune system to recognize and destroy abnormal cells [64]. Thus it may be more effective to focus therapeutic vaccines at women with persistent HPV infection or low-grade CIN, rather than those with advanced disease.

Despite continued focussed research, therapeutic HPV vaccination may still be several years from realization. Ultimately, a vaccine that combines prophylactic and therapeutic modalities may provide the best solution for HPV-associated disease. A chimaeric HPV16 VLP–E7 vaccine has been constructed by Greenstone et al. [65], which induces E7-specific T-cell responses as well as virus-neutralizing antibodies in the murine model. If effective in humans, this type of vaccine may allow women who are naïve to the virus, as well as those who have already been exposed, to benefit from reduced rates of cervical cancer.

CONCLUSIONS

HPV testing has yet to find its full role in cervical screening, but it threatens the pre-eminence of exfoliative cytology as the primary screening test. Testing for HPV is simpler than cytology, but its lack of specificity will require a second test and possibly repeat testing if the second test is negative. Population testing using HPV will require public education programmes and training for healthcare professionals. Prophylactic vaccination promises a great deal, but in developed countries with effective screening programmes the introduction of vaccines in pre-adolescents would require continuation of screening for a generation of unvaccinated females. Even vaccinated women may require a degree of screening and this will require careful planning. If a programme of vaccination for all females, at least, were to be introduced it would need to be demonstrably cost effective and would require broad acceptance by society. If it could be implemented in developing countries it could be of enormous benefit and save hundreds of thousands of lives.

Abbreviations

     
  • CIN

    cervical intraepithelial neoplasia

  •  
  • CTL

    cytotoxic T-lymphocyte

  •  
  • HPV

    human papillomavirus

  •  
  • VLP

    virus-like particle

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