PAP/NGC Programs Review
William D. Tench, MD
Prophylactic, and to some extent therapeutic, vaccines against human papillomavirus—the
caus a tive agent of cervical neoplasia—have garnered much interest and
momentum as a means of cervical cancer control.
The Papanicolaou test is the most successful cancer screening tool in history.
In countries where it has been widely implemented, deaths from cervical cancer
have declined significantly. However, even in countries with a substantial
screening infrastructure, cancer deaths continue to occur because a significant
group of patients remain unscreened for a variety of reasons, including regional
lack of ready access, lack of education, and personal resistance. Hence in
the United States, approximately 15,000 incident cases of invasive cervical
cancer occur each year with about 4,000 deaths—despite an expenditure
of about $6 billion to detect and eradicate cervical abnormalities, most of
which are low grade and self-limited.1
On a worldwide scale, the problem is much more severe. The majority of the
world’s population does not have access to any screening program, and
as such, as many as 400,000 incident cases of cervical cancer occur worldwide,
with as many as 200,000 deaths.2After
lung and breast cancer, cervical cancer is the No. 3 killer of women and
in many regions is No. 1. Some studies have found that cervical cancer may
show more reduction in life expectancy than do other commonly perceived mortal
diseases such as AIDS and tuberculosis.3 Implementing
widespread cancer screening in countries that now have limited or no resources
to devote to it would be nearly impossible, other than on a narrow or focused
scale taking in only a small percentage of the overall population.
Therefore, to diminish this significant cause of mortality, other methods of
prevention or therapy are required.
To understand HPV vaccine development, it is important to have a basic knowledge
of HPV biology. It is well known that essentially all cervical cancers (99.7
percent documented4) are associated
with HPV. This association is stronger than that noted for smoking and lung
cancer and on the order of the association of hepatitis B and hepatocellular
carcinoma.5 There are more than 100
distinct HPV types, of which 20 to 30 are known to infect the genital tract.
Of these, the most commonly found are types 16 and 18, which together account
for about 70 percent of cancers worldwide. With the addition of a further group
of five (45, 31, 33, 52, 58), nearly 90 percent of all cervical cancers are
included with little regional variation noted.6
After these seven types, some regional differences in HPV type association
with cancer are noted. HPV types causing cervical cancer are referred to collectively
as “high risk.” In addition, other HPV types (most notably types
6 and 11) are collectively referred to as “low risk” because
they are not associated with the development of cancer but are instead associated
with benign but nonetheless clinically significant disease, most notably
genital tract papillomas or condyloma accuminatum.
The molecular biology of HPV and cervical carcinogenesis has been well elucidated
during the past 15 years. The HPV genome is made up of an approximately 8
kbase double-stranded DNA arranged in a circular configuration. It is made
up of seven early (E) genes, which control DNA replication and transcription,
and two late (L) genes, which code for the proteins that form the intact viral
capsid structure. In the vast majority of cases, the viral infection/transmission
cycle is a benign event. Direct skin-to-skin contact, predominantly but not
exclusively via sexual relations, allows HPV virions to infect the metabolically
active squamous metaplastic cells of the genital tract. Intact viral genome
is transcribed, and intact virions are formed that can then go on to infect
other cells (and individuals). It is during this process that cytologists
note the classic koilocytotic viral cytopathic effect known as low-grade squamous
intraepithelial lesion, or LSIL. These infections are predominantly transient
and benign, and they clear spontaneously in greater than 90 percent of cases
when followed for a two- to three-year period. The true neoplastic sequence
occurs in the population of patients that do not clear their infections (persistent
The development of high-grade dysplasias (true neoplastic precursor lesions)
is associated with specific events that include the integration of portions
of the HPV genome into the host genome. The early genes E6 and E7 are always
conserved in such events. In addition, breaking of the circular viral DNA
nearly always occurs in the region of E2, which is responsible for regulatory
control of transcription of E6 and E7, thus leading to loss of this function.
E6 transcription product is known to bind to and inhibit the tumor suppressor
activity of p53, allowing the cell to enter DNA replication (S phase of the
cell cycle) without the benefit of normal DNA repair functions (loss of normal
G1 arrest). In a like fashion, E7 protein product binds to the retinoblastoma
tumor suppressor gene product (pRb), and this interaction, via a path of downstream
effects, allows cells to proceed uninhibited through the S-phase. Overall,
the effects caused by loss of regulation of these two early genes lead to
uncontrolled DNA synthesis and cell proliferation with host genes not having
the benefit of normal presynthesis repair, leading to accumulations of cells
having mutated DNA. Because this neoplastic process does not involve the entire
viral genome, intact virions are not produced, and the neoplastic cells show
no evidence of the koilocytotic viral cytopathic effect noted in LSIL.8
Vaccine development is broadly divided along two conceptual lines. Prophylactic
vaccines are those that seek to eliminate primary infections by HPV, while therapeutic
vaccines are those that will provide benefit against present infections, HPV-associated
neoplastic processes, or both.
Prophylactic vaccines in development now target viral capsid epitopes, most
commonly L1, the major capsid protein. For vaccine delivery, the antigens are
packaged in artificially synthesized structures called virus-like particles
(VLP), the L1 components of which are manufactured in bacterial or other cellular
expression systems, self-assemble, and can then be introduced into patients,
currently by intramuscular injection. VLPs are highly immunogenic and elicit
potent neutralizing antibodies and some cellular immune responses. HPV has exquisite
type specificity, however, and L1 VLP-based vaccines require having L1 epitopes
from each HPV type desired, as essentially no cross-reactivity responses between
the types are noted.
Phase I clinical trials of prophylac tic vaccines developed against a number
of HPV types have shown that the vaccines are safe and well tolerated.9,10 In
phase II studies, Koutsky, et al showed 100 percent efficacy of an HPV 16
L1 VLP vaccine in preventing persistent HPV infections and cytologic abnormalities
in a randomized two-arm study comparing vaccine and placebo. During the trial,
41 cases of persistent HPV 16 infection and nine cases of cervical intraepithelial
neoplasia, or CIN, were identified in the placebo arm, with no such cases
identified in the vaccine arm (P<.001). Of interest was that incident
HPV 16 infections were identified in the vaccine arm, but these never became
persistent, the marker used as one endpoint for vaccine efficacy (CIN being
the other).11 In another phase II trial
of a bivalent HPV 16/18 L1 VLP vaccine, Harper, et al showed 100 percent efficacy
against persistent HPV 16 and 18 infections and 93 percent efficacy against
HPV-16/18-associated cytologic abnormalities, all of which were less than HSIL.12 In
yet another phase II study of a quadrivalent vaccine targeting HPV 16/18/6/11,
a reduction of persistent infection by these viral types of 90 percent was
Phase III studies are reportedly underway now. Merck & Co. is testing
a quadrivalent vaccine targeting HPV 16/18/6/11 (trade name Gardasil) with
the intent of allowing protection against genital condyloma in addition to
cervical carcinoma. This trial is reported to be entering more than 20,000
subjects at over 100 clinical sites worldwide. GlaxoSmith Kline is also in
phase III trials with a vaccine against HPV 16/18 (trade name Cervarix) and
is enrolling more than 30,000 subjects. Results from these studies as well
as Food and Drug Administration applications are expected in 2006.14 Future
vaccines might be expected to contain additional epitopes against other prevalent
HPV types. As noted by Munoz, et al, a vaccine containing a cocktail of the
seven most prevalent high risk types would provide 90 percent coverage of
all high-risk infections with little regional variability.6
What is the anticipated effect of introducing vaccines into the population?
Mathematical models have been developed that explore the implications of implementing
prophylactic vaccine in various settings. Goldie, et al predicted that an
HPV 16/18 specific vaccine that prevented 98 percent of persistent infections
would reduce cervical cancer in a population by 51 percent. However, in their
model, reductions in incident LSIL would be negligible in the same population
because of the increase in infections caused by other non-covered HPV types
stepping in to “fill the void” left by the eradication of type
16/18 infections.15 In another modeling
study, Taira, et al showed that even modest penetrance of a 16/18 specific
vaccine into the population (as low as 50 percent) would give nearly a full
effect of about 62 percent reduction of cervical cancer.16 Adapting
the same model to heavily screened populations, Goldie, et al also showed that
the most cost-effective overall management strategy would be vaccination at
age 12 years with triennial screening (with conventional cytology) beginning
at age 25 years.1 Such models have
significant implications for the overall planning of continued screening programs.
Clearly, screening must continue; however, to realize the savings expected
with vaccine introduction, programs may look different in structure.
A number of unanswered questions about prophylactic vaccines remain. The first
is when to vaccinate. Investigators indicate that vaccination before onset
of sexual activity (10 to 15 years) makes the most sense, but targeting this
age group may bring opposition from parental groups who may contend that vaccination
for a sexually transmitted disease in this age group could increase subsequent
promiscuity.17 However, data do suggest
that parents fully informed about HPV, cancer development, and vaccines would
favor having their children (and themselves) vaccinated.18 In
addition, data from the Merck trials have shown that vaccinating younger individuals
(10 to 15 years) caused a greater immune response than did vaccinating older
individuals (16 to 23 years), indicating that the effect may be more pronounced
on less mature immune systems.17
The second unanswered question is how long the immune protection will last
and whether boosters will have to be given. Studies to date show that protective
immunity lasts for several years, but results beyond this time frame are not
yet known. The aforementioned mathematical models use 10-year intervals for
Third, should adolescent males be vaccinated in addition to females? In the
Taira, et al model, male vaccination suggested only a small increment of cervical
cancer decline (2.2 percent) over female vaccination alone.16 However,
the so-called effect of herd immunity is not well studied in this population,
and suggestions have been made that increasing the overall rate of vaccination
among males and females, and hence reducing infections in both sexes, may
have the effect of more substantial reductions of neoplastic disease in women.19
Fourth, what should be the mode of inoculation? Parenteral vaccination may
be easily adopted in countries with well-established vaccination programs,
but in developing countries this method of administration may not be widely
applicable. Interestingly, attempts to genetically engineer foods to contain
antigenic L1 components have shown success in eliciting immune responses in
animals.20 These results may suggest
a potential simplified and more widely available delivery system in the future,
but many questions about other effects of such engineered foods on the population
will have to be addressed.
Therapeutic vaccines that will affect prevalent HPV infection and associated
neoplasia are much less developed than are the abovementioned prophylactic
vaccines. Based on the HPV biology earlier noted, such vaccines would need
to use epitopes retained in transformed cells, most likely E6 and E7, in distinction
to the capsid antigens found in the infective stages. To have effects against
already transformed host epithelial cells, therapeutic vaccines will require
cellular immune responses to be elicited. A number of studies have been published
using E6 and E7 epitopes with variable success. While the specifics of antigen
delivery are beyond the scope of this article, methods used to date have included
viral vectors such as Vaccinia, small peptides, proteins, bare or encapsulated
DNA, and direct inoculation of antigen into autologous dendritic cells. For
further reading on methods of antigen presentation, the reader is referred
to the excellent review article by Schreckenberger and Kaufmann.21 Although
results to date using therapeutic vaccines have shown promise, further development
and study will be required with all such modalities.
In summary, prophylactic vaccine development is now in a mature stage and
commercial products are expected within the next several years. Implementation
could significantly reduce cervical cancer mortality in screened and unscreened
populations. In addition, use of the vaccine in developed nations will probably
initiate a paradigm shift in current screening protocols. Screening will most
likely occur less often, start at a later age, and may be by different methods
than are being employed today. Therapeutic vaccines are at an earlier stage
of development, and it is too early to predict what forms such “immunotherapy”
will take and what their overall effects might be. For both vaccine approaches,
progress is happening rapidly, and this bodes well for the population affected
by this preventable disease.
1. Goldie SJ, Kohli M, Grima D, et al. Projected
clinical benefits and cost-effectiveness of a human papillomavirus 16/18 vaccine.
J Natl Cancer Inst. 2004; 96: 604–615.
2. Durst M, Gissmann L, Ikenberg H, zur Hausen H. A papillomavirus
DNA from cervical carcinoma and its prevalence in cancer biopsy samples from
different geographic regions. Proc Natl Acad Sci USA. 1983;80:3812–3815.
3. Lehtinen M, Dillner J, Knekt P, et al. Serological diagnosis
of human papillomavirus type 16 infection and the risk for subsequent development
of cervical carcinoma. BMJ. 1996;312:537–539.
4. Walboomers JM, Jacobs MV, Manos MM, et al. Human papillomavirus
is a necessary cause of invasive cervical cancer worldwide. J Pathol. 1999;189:12–19.
5. Rohan TE, Burk RD, Franco EL. Toward a reduction of the
global burden of cervical cancer. Am J Obstet Gynecol. 2003;189:S37–S39.
6. Munoz N, Bosch FX, Castellsague X, et al. Against which
human papillomavirus types shall we vaccinate and screen? The international
perspective. Int J Cancer. 2004;111:278–285.
7. Schiffman M, Castle PE. Human papillomavirus. Epidemiology
and public health. Arch Pathol Lab Med. 2003;127:930–934.
8. Jung W-W, Chun T, Sul D, et al. Strategies against human
papillomavirus infection and cervical cancer. J Microbiol. 2004; 42:255–266.
9. Evans TG, Bonnez W, Rose RC, et al. A phase 1 study of a
recombinant viruslike particle vaccine against human papillomavirus type 11
in healthy adult volunteers. J Infect Dis. 2001;183:1485–1493.
10. Harro CD, Pang YY, Roden RB, et al. Safety and immunogenicity
trial in adult volunteers for a human papillomavirus 16 L1 virus-like particle
vaccine. J Natl Cancer Inst. 2001;93:284–292.
11. Koutsky LA, Ault KA, Wheeler CM, et al. A controlled trial
of a human papillomavirus type 16 vaccine. N Engl J Med. 2002;347:1645–1651.
12. Harper DM, Franco E, Wheeler C, et al. Efficacy of a bivalent
L1 virus-like particle vaccine in prevention of infection with human papillomavirus
types 16 and 18 in young women: a randomised controlled trial. Lancet. 2004;364:1757–1765.
13. Villa LL, Costa RL, Petta CA, et al. Prophylactic quadrivalent
human papillomavirus (types 6, 11, 16, and 18) L1 virus-like particle vaccine
in young women: a randomised double-blind placebo-controlled multicentre phase
II efficacy trial. Lancet Oncol. 2005;6:271–278.
14. Washam C. Two HPV vaccines yielding similar success. J
Natl Cancer Inst. 2005;97:1030.
15. Goldie SJ, Grima D, Kohli M, Wright TC, Weinstein M, Franco
E. A comprehensive natural history model of HPV infection and cervical cancer
to estimate the clinical impact of a prophylactic HPV-16/18 vaccine. Int J Cancer.
16. Taira AV, Neukermans CP, Sanders GD. Evaluating human
papillomavirus vaccination programs. Emerg Infect Dis. 2004;10:1915–1923.
17. Washam C. Targeting teens and adolescents for HPV vaccine
could draw fire. J Natl Cancer Inst. 2005;97:1030–1031.
18. Davis K, Dickman ED, Ferris D, Dias JK. Human papillomavirus
vaccine acceptability among parents of 10- to 15-year old adolescents. J Low
Genit Tract Dis. 2004;8:188–194.
19. Geipert N. Vaccinating men for HPV: new strategy for preventing
cervical cancer in women? J Natl Cancer Inst. 2005;97: 630–631.
20. Sasagawa T, Tani M, Basha W, et al. A human papillomavirus
type 16 vaccine by oral delivery of L1 protein. Virus Res. 2005;110:81–90.
21. Schreckenberger C, Kaufmann AM. Vaccination strategies
for the treatment and prevention of cervical cancer. Curr Opin Oncol. 2004;16:485–491.
Dr. Wilbur, vice chair of the CAP Cytopathology Committee, is director of cytopathology at Massachusetts General Hospital, Harvard Medical School, Boston.