ISIS Report 20/12/05
Recombinant Cervical Cancer Vaccines
Prof. Joe Cummins
A fully referenced
version of this article is posted on ISIS members’ website. Details here.
Human papilloma virus and cervical cancer
Merck
Company released the results of clinical studies on a recombinant vaccine
Gardasil in October 2005, claiming 100 percent efficacy in preventing cervical
cancer from human papilloma virus (HPV) strains 16 and 18, believed to cause
about 70 percent of cervical cancers [1].
HPV infection frequently results in warts of the genital mucosa, and certain
strains – HPV16, 18, 31 and 45 - are responsible for squamous cell carcinoma
of the uterine cervix; HPV16 alone accounts for over half the cases worldwide.
The virus chromosome
consists of a circular, 8 thousand base pair DNA genome encased in an icosahedral
capsid (coat) made of protein. (An icosahedron is a solid with 20 triangular
faces.)
The HPV
genome consists of 8 genes coding for proteins and a non protein-coding region with regulatory genes. HPV
infects the basal cells of the cervical epithelium when it is damaged in some
way. The viral genome becomes established in the basal cells as an episome
(an independently replicating nuclear micro-chromosome). The
episome replicates in tandem with the chromosomes of the cell and forms virus
particles. The complete virus particles are in the outermost cells of the
epithelium and the viruses are
spread as the cells slough off from the epithelium. Some virus proteins
function as oncoproteins, transforming
the epithelial cells to a precancerous state. HPV infection is necessary but
not sufficient for cancer formation, however. In high-grade lesions and cancer,
an episome is integrated into the cellular chromosome. Integration
disrupts a viral transcription regulatory protein that controls production
of the onco proteins, leading to their continual and enhanced production [2].
Cervical cancer is the second most common cancer of women worldwide, accounting
for about 10 percent of all cancers. The highest risk areas for cervical cancer
include Africa, Melanesia, the Caribbean and Central America. During the past
50 years, cervical cancer declined in developed countries thought the use of
Pap cytology (Pap smears) in diagnosis. Pap screening has not been available
in developing countries and those countries now have the highest levels of cervical
cancer. The Pap smear is only 50 percent effective in detecting cervical cancer
early, so an effective cervical cancer vaccine will be welcome in both developed
and developing countries [3].
Clinical trials of vaccines against HPV
Two
main types of HPV vaccines are currently being developed: prophylactic vaccines
that ward off HPV infection, and therapeutic vaccines to induce regression
of precancerous lesions caused by HPV or even remission of advanced cervical
cancer [3].
The two clinical trials completed at this time are those conducted by Merck
and GlaxoSmithKline, which are very similar in design and outcome, but differ
mainly in the origin of the recombinant vaccine. The Merck vaccine was made
up of the HPV 16 L1 capsid protein that forms a virus like particle totally
lacking DNA. The L1 capsid protein was produced using transgenic yeast. The
GalaxoSmithKline vaccine used HPV 16 and HPV 18 was also L1 capsid protein from
the two strains but the protein was produced using a recombinant Baculovirus
propagated in insect cells. Study subjects received a single intramuscular inoculation.
Subjects were selected from United States citizens in the Merck study and from
the United States, Canada and Brazil in the GalaxoSmithKline study. There were
768 vaccinated subjects in the Merck study and 560 in the GalaxoSmithKline study
with a nearly equal number of control and vaccinated subjects in both studies.
Subjects ranged in age from 15 to 25 years in both studies, with no history
of cervical lesions and few sexual partners. The Merck study lasted 4 years
while the GalaxoSmithKline study lasted 27 months.
In
the Merck study, the incidence of persistent HPV-16 infection was 3.8 per
100 woman-years at risk in the control group compared to 0 per 100 woman-years
at risk in the vaccine group. In the GlaxoSmithKline study, 27 women in the
control group compared to two in the vaccine group had HPV-16 and/or HPV-18
associated cytological abnormalities.
Also assessed were women with histologically confirmed cervical
intraepithelial neoplasia lesions (cancers), resulting from HPV-16 or HPV-18 infection. Overall, seven
women (six in the placebo group and one in the vaccine group), developed these
lesions. However, the cancer confirmed in the inoculated group resulted from
infection with a strain of the virus not vaccinated against.
Immunization against HPV has greatest value in developing countries where 80
percent of the world’s cervical cancers appear and where Pap screening is inadequate.
Long lasting protection against HPV 16 may prevent half of the world’s cervical
cancer cases [3].
Vaccines for resource-poor settings?
The
report of the Merck study [4] did not provide detailed information on the
production of L1 protein in yeast, but appears to involve secretion of the
protein from the yeast cell by adding a leader sequence from yeast to the
HPV L1 gene [5]. Recently, a potential oral vaccine consisting of HPV 16 L1
protein was produced in the fission yeast S.
pombe [6]. Pombe yeast is used in brewing in Africa so production
of the vaccine seems feasible. Report on the GalaxoSmithKline study [7] also
provided no detailed information on vaccine production, but this was covered
in previous publications [8, 9]. HPV vaccines
production and distribution in resource-poor settings was discussed. Prophylactic
vaccines seem the best long-term solution to the cervical cancer problem.
However, financing and distribution of such vaccines require considerable
forethought and is not a simple matter [10].
There has been a great deal of effort to promote the production of an oral
HPV vaccine in food plants or tobacco. The belief has been that the plant based
oral vaccines would be cheap to produce for the developing world where the need
for the vaccine is the greatest. Tobacco plants were modified to produce HPV
16 protein and produced sufficient antigen to elicit a weak immune response
in rabbits [11]. Tobacco and potato were used to produce HPV 16 virus like particles.
Feeding transgenic potato tubers to mice produced an LI antibody response in
only 3 of 24 mice and that response was transient [12]. The oral administration
of HPV-like particles produced in potato produced a weak immune response in
mice, which was enhanced by oral boosting with virus-like particles produced
in insect cell culture [13]. A vaccine against the papilloma virus oncogene
product causing human cervical cancer was produced using a potato virus-X vector
carrying an antigen of the viral oncogene-encoded protein [14]. These cancer
vaccines are an important effort to control cancer, but environmental release
of the vaccines in crop plants could greatly increase people’s susceptibility
to specific cancers through the development of oral tolerance.
Plant-based vaccines are mainly geared towards mucosal immunization following
oral intake. Oral vaccines may elicit oral tolerance on repetitive exposure.
Oral tolerance is the animal’s response to antigens in food. Thus, after repeated
exposure to an oral antigen, the mucosal immune system ceases to view the antigen
as foreign, leaving the animal susceptible to the pathogen for which the vaccine
is supposed to protect against [15]. The problem of oral tolerance has been
mentioned in at least one review of plant-based vaccines [16]. Oral tolerance
to pathogens is one main threat from the contamination of our food supply with
vaccine genes, this threat is seldom discussed by promoters of plant genetic
modification or by science journals reporting the studies.
Last year, I pointed out the drawbacks of using food crops to produce vaccines
or therapeutic antibodies [17]. Genes from tests sites or production farms can
be spread by pollen or mechanical dispersal of seeds. Debris from transgenic
crops producing the antibody can spread both the genes and the vaccine proteins
by contaminating surface and groundwater. Such debris may also spread with dust
in the air, impacting on the airway mucosa directly. The plant-based systems
for producing HPV 16 L1 vaccine included potato & tobacco, and banana, maize
and rice have been discussed as systems for producing the vaccine.
The fission yeast S. pombe
developed to produce HPV vaccine is also of questionable safety. Pombe beer
is produced locally in many parts of Africa and pollution of that yeast with
vaccine genes is a strong possibility should the recombinant yeast be
dispersed widely. Exposure of an entire population of women and men of all
ages to oral immunization with polluted crops, beer, water or air would lead
to untoward consequences. A single exposure to antigen might immunize both
females and males, possibly limiting males as virus vectors and protect females
from infection as well. However, constant exposure to viral antigen would
likely cause oral tolerance rendering females defenseless against the virus
and rendering males strong vectors for the cancer virus.
In conclusion, the HPV recombinant vaccines produced in protected laboratory
environments pose little obvious threat to humans or to the environment. The
virus-like structures making up the vaccine do not contain DNA and cannot be
replicated in the cell. In the event that trans-capsidation (virus DNA being
incorporated into the vaccine structures) took place the recombinant virus would
replicate only the original DNA and protein of the capsid. However, once oral
vaccines are produced in crop plants or in yeast, there is a distinct danger
of oral tolerance developing that not only cancels out the protective effects
of the vaccine against infection, but could also leave females absolutely defenseless
against the virus while turning males into carriers spreading the virus.
The recombinant vaccines producing viral proteins without
viral DNA are acceptable, but production of
oral vaccines in plants or yeast should be banned.
Please circulate this widely to your policy-makers and regulators.
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