Plant-made Vaccination against Infectious Disease
Abby Soltis
Vaccination is the most effective way to prevent many of the worldwide deaths each year due to
infectious disease, yet many of due to the high cost of vaccination, there is limited access to such
vaccines (Daniell et al., 2009). Research is currently being done to lower the cost of vaccines by
using chloroplast-derived antigens, which has the following advantages: limited variation due to
the maternal inheritance of chloroplasts, higher levels of antigen expression, no gene silencing,
and the possibility of oral application (Daniell et al., 2009). Antigens are produced by integrating
genes, usually coding for protein, into the chloroplast genome through homologous
recombination, which causes expression of the introduced antigens (Daniell et al., 2009). Human
papillomavirus, HIV, cholera and malaria are all fairly common infectious diseases in which
there is a need for new, cheaper vaccines. The current research regarding the development of
these new vaccines using chloroplast-derived antigens is outlined in the following sections.
Human Papillomavirus (HPV)
The contraction of cervical cancer by means of human papillomavirus is a pressing concern for
women today, especially for those in developing countries due to the expensive nature the
vaccines based on virus-like particles (Fernández-San Millán et al., 2008). This has led to current
research on alternative production methods for the attainment of virus-like particles, especially
those formed from the L1 papillomavirus protein. Previously, transgenic plants have been used
to produce the L1 protein through the introduction of recombinant DNA; however the DNA is
introduced to the nucleus and the level of the L1 protein expression is extremely low (Fernández-
San Millán et al., 2008). In order to increase expression, Fernández-San Millán et al. are
introducing the recombinant DNA to tobacco chloroplasts in hopes to produce L1 protein and the
resulting virus-like particles in order to create a cheaper vaccine.
In order to introduce the L1 protein gene into a tobacco chloroplast, the gene was first amplified
via Polymerase Chain Reaction and combined with a pAF vector specific for tobacco plasmids,
which were then introduced to greenhouse plants (Fernández-San Millán et al., 2008). Using
southern blot analysis the researchers were able to determine that the L1 gene was integrated into
the chloroplast DNA by the presence of L1 protein and, while using the western blot analysis,
found that the L1 proteins were producing virus-like particles, which are used in the vaccine. The
levels of virus-like particles were found to be present in much larger quantities as well as
identical to virus-like particles being used in current vaccines (Fernández-San Millán et al.,
2008). When administered intraperitoneally, rats produced an immune response, which, was
heightened after boosters were given (Fernández-San Millán et al., 2008).
In conclusion, high levels of L1 protein were found in tobacco chloroplasts, which decreases the
amount of plant tissue needed to produce the human papillomavirus vaccine, without harming
the plant and creating the virus-like particles that effectively produced antibodies in rats. Due to
need for fewer materials, this vaccine can be produced much more cheaply (Fernández-San
Millán et al., 2008).
HIV
The spread of HIV is a growing problem in many countries and the production of antiretroviral
microbicides is a good way to stop the spread of this virus by preventing infection vaginally and
rectally. While the microbicides are effective, there aren’t any cost effective options available
due to high manufacturing costs and the use of edible crops as the source of antiretroviral
proteins (O’Keefe et al., 2009). In order to prevent the transmission of HIV the CD4, CCR5 and
the C-type lectin receptors need to be inhibited, which can be done through the use of
antiretroviral drugs, which has already been shown to protect macaques against infection both
vaginally and rectally (O’Keefe et al., 2009). The proposed inhibitor of HIV entry is the
griffithsin (GRFT) protein found in red algae, which functions by binding to the glycoproteins on
the viral envelope of HIV and the inactivity the virus almost immediately (O’Keefe et al., 2009).
This GRFT protein was produced by first inserting the cDNA, which codes for the GRFT
protein, into a Tobacco Mosaic vector, which is then used to inoculate N. benthmiana plants; The
resulting plant material included the GRFT protein, which was contaminated by coat protein
(O’Keefe et al., 2009). After the GRFT was purified, the protein was found through tests with E.
coli to be functionally equivalent to the GRFT proteins found in red algae as well as no toxicity
to humans (O’Keefe et al., 2009).
In conclusion, the form of GRFT produced in tobacco plants is highly potent, selective to HIV
receptors, prevents the spread of HIV, is not toxic to humans, and is able to be produced in large
quantities cost effectively, which makes this specific GRFT a viable protein to be used in
antiretroviral microbicides (O’Keefe et al., 2009).
Cholera and Malaria
Cholera is one of the top diseases world-wide and currently only one licensed vaccine exists,
which is extremely expensive and the immunity to the disease is lost after 3 in small children and
largely unreliable in adults, which has provoked researchers to create a new, more reliable and
cost effective vaccine; Malaria is also a serious disease effecting many countries and there is no
licensed vaccine due to the difficulty and cost of purification, the complicated antigens and the
difficulty of vaccine introduction (Davoodi-Semiromi et al., 2010). Current research is being
done on producing proteins from transgenic plants, however, the levels of proteins are so low
that they require high intense (and expensive) processing and purification, which is why none of
the new vaccines have moved past phase I clinical trials (Davoodi-Semiromi et al., 2010).
Researchers are currently proposing the development of a new vaccine, which isolates protein
from the chloroplasts of edible crops (tobacco and lettuce) in order to avoid any gene silencing
and splicing, as well as avoid expensive processing due to the high level of expression in
chloroplasts (Davoodi-Semiromi et al., 2010).
In this study, the DNA of two malarial antigens were combined with the CTB protein and then
combined with chloroplast vectors in both tobacco and lettuce plants, where it was shown
through southern blot analysis that the genes were successfully integrated into the lettuce and
tobacco plant’s chloroplast genome, which produced antigen levels that were high in tobacco
plants and slightly lower in lettuce plants (Davoodi-Semiromi et al., 2010). When the plant
material was given to mice orally, the mice produced antibodies that protected against exposure
to both malaria and cholera (Davoodi-Semiromi et al., 2010).
In conclusion, this research is the first to find a vaccine derived from chloroplasts that can be
delivered orally, as well as the first dual-vaccine to prevent against cholera and malaria
(Davoodi-Semiromi et al., 2010). It was found that both lettuce and tobacco chloroplasts
produced high levels of CTB protein, which allowed the vaccine to be orally delivered, produced
cheaply as a green vaccine and with boosters provide long-term protection against the cholera
toxin and the malarial parasite when introduced in rats, which leads to the possibility of future
development of a human vaccine that prevents both cholera and malaria.
Conclusion
Although we are fairly far away from introducing a plant-derived vaccine into the market,
current research suggests that there is large possibility that these types of vaccines will be
available in the future, which will hopefully allow vaccines to be more accessible, decreasing the
spread of infectious disease worldwide.
Literature Cited
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Chakrabarti, and H. Daniell. 2010. Chloroplast-derived vaccine antigens confer dual
immunity against cholera and malaria by oral or injectable delivery. Plant Biotechnology
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Fernández-San Millán, A., S.M. Ortigosa, S. Hervás-Stubbs, P. Corral-Martínez, J.M. SeguíSimarrow, and J. Gaétan, P. Coursaget, J. Vermendi. 2008. Human papillomavirus L1
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