DNA Vaccines:
A short little review
Julie Smith
2007
Vaccines constitute one of the greatest achievements of modern medicine. They have
virtually eradicated smallpox and polio, and significantly reduced the number of
occurrences of typhus, measles, tetanus, and other dangerous infections within the world
population. However, successful vaccines have yet to be introduced for many other
deadly and debilitating disorders including malaria, hepatitis C and AIDS.
Unfortunately, standard immunization methods work poorly or pose unacceptable risks
when targeted against certain illnesses. Clearly, alternate strategies are needed. One of
the most promising creates vaccines out of DNA. In the past 15 years such vaccines have
progressed from a laughable idea to being one of the hottest research areas in
vaccinology.
In order to understand the benefits of DNA vaccines, the actions of traditional vaccines
need to be understood. Traditionally, vaccine preparations consist primarily of killed or
weakened pathogens. They are designed to prime the immune system to attack
dangerous invaders quickly, before the detrimental organism can gain a foothold in the
body. Vaccines accomplish this by tricking the body into believing that it has been
infected with the microorganism and that the microorganism is multiplying unchecked
and raising havoc with the host.
In the bodies attempt to protect itself and ward off the invaders, an autonomic series of
events occur. The immune system homes in on foreign antigens that are uniquely
produced by the invading organism. These antigens are usually specific proteins or
protein fragments. The immune system than mounts a double fronted battle plan against
the invaders. The humoral front, led by the B lymphocytes, attacks the pathogens that are
outside the cell. These B cells secrete antibodies that latch onto the invaders and either
neutralize them or tag them for destruction by other parts of the immune system. The
cellular front, led by cytotoxic T lymphocytes, eradicates invaders that are inside the cell.
Infected cells display pieces of the invaders protein on the cell surface that act as a flag
for the cytotoxic T lymphocytes. When the cytotoxic T lymphocytes identify a flagged
cell, they often destroy the cell and the invaders within.
Once the immune system is activated, it leads to the creation of memory cells that can lay
in wait until the same pathogen attacks again. The hosts immune system is ready to
spring in to action should the invader return. Vaccines confer protection by similarly
inducing immune responses and the formation of memory cells.
Unfortunately, standard vaccines vary in the type of protection that they provide. The
vaccines that are based on killed pathogens, such as Salk polio vaccine or antigens
isolated from disease-causing agents, such as hepatitis B subunits, cannot make their way
inside the cell. Therefore, they cannot activate the hemoral front and do not activate
killer T cells. This obviously works for some diseases; unfortunately this type of
vaccination is ineffective against organisms that invade cells.
Attenuated live vaccines, usually viruses, do enter the cell and make antigens that are
displayed by the inoculated cells. Thus, they flag the cell for attack by killer T cells as
well as by antibodies. This dual edged attack is essential for blocking future infections
by the virus. Based on the fact that attenuated vaccines activate both arms of the immune
response, they have long been considered the gold standard in vaccinology.
However, live vaccines, even if they are attenuated, can be problematic in their own way.
They can fail to provide effective protection or they can cause a full blown illness in
individuals that may be immune compromised. Additionally, weakened viruses can, at
times, mutate in ways that restore virulence. For some diseases, the risk of reversion to
virulence is unacceptable.
There are a number of vectors currently being used for genetic vaccine research (Figure
1). The structure and function of these genetic vaccines are quite different from
traditional ones. One of the most studied consists of plasmids – small rings of doublestranded DNA- derived from bacterial. Plasmids used for immunization are altered to
carry genes specifying one or more antigenic protein that would normally be made by the
selected pathogen. At the same time, they lack any of the genes required by a pathogen
for self- replication which can lead to infection. The antigenic proteins can elicit humoral
immunity when they escape from the cell and they can elicit cellular immunity when they
are displayed on the cell surface.
DNA vaccines emerged as a fluke (Brown, 1996). In a 1990 experiment, Wolff and
associates injected a control group of mice with “naked” viral DNA. Unexpectedly, the
control mice began producing significant amounts of viral protein. Thus, the field of
DNA vaccines was born. Further research was conducted and it was reported that a DNA
vaccine could indeed prevent influenza infection in mice (Ulmer et al, 1993). Wang and
his colleagues with the Naval Medical Research Institute in Bethesda, Maryland reported
in 1998 that injections of plasmid DNA encoding for a protein from malaria parasite
provoked a strong immune response in humans. Genetically modified DNA vaccines are
being explored against an number of diseases such as: cancer (Lowe et al, 2006, Haupt et
al, 2002), tuberculosis (Bhattacharya , 2005), influenza (Drape et al, 2006), mycoplasma
(Barry et al, 2002), AIDS (Barouch et al, 2002), Ebola (Vanderzanden et al, 1998) just to
name a few. The first license for the commercial use of a DNA vaccine in the United
States was granted in July of 2005 to Fort Dodge Laboratories. The product licensed was
vaccine against West Nile virus for horses (CIDRAP, 2005). DNA vaccines have also
successfully been administered to endangered California Condors to protect them against
West Nile Virus (Gunderson, 2005). At nearly the same time Canada, based on research
by Traxler (1999) and associates, licensed a DNA vaccine developed to prevent
infectious hematopoietic necrosis virus in the commercially farmed Pacific salmon
population.
DNA vaccines offer many advantages over traditional vaccines (McDonnell and Askari,
1996). DNA is very amenable to manipulation and it is relatively easy to insert the
desired gene(s) into the plasmid. Due to the nature of plasmids, researchers are not
limited to just a single pathogen, as is the case with traditional vaccines. Plasmids have
the capability of holding multiple genes for multiple pathogens (Forde, 2005). DNA
vaccines are beneficial in that they encode only for a few specifically selected antigen(s)
thus completely eliminating the possibility of mutation to its original virulent form.
Another advantage is that DNA vaccines may be able to outsmart ever changing viruses.
Designing DNA vaccines that code for non-mutating proteins within a virus, may provide
immunity for multiple mutated strains of a virus. DNA vaccines are more economical
and easier to produce than previous forms of vaccinations (Guilherme et al, 2000). The
stability of DNA vaccines makes them very advantageous. They do not require a coldchain. Cold-chain refers to the refrigerators required to maintain the viability of
traditional vaccine during its distribution. Currently, 80% of the cost of providing
vaccinations in developing countries is due to the need for refrigeration. DNA vaccines
can be stored either dry or in an aqueous solution at room temperature, so there is no need
for the cold-chain. This dramatically increases the ability of providing vaccinations in
remote areas where refrigeration is not available (Rangarajan, 2002).
The variety in the modes of administration lends itself to the diversity of potential
vaccines. Researchers are not limited to the traditional modes of administering vaccines
(Gurunathan et al, 2000). DNA vaccines can be administered via a variety of routes:
injection (Fynan et al, 1993), epidermal delivery by scarification ((Raz et al, 1994), oral
(Niethammer et al, 2001), intranasal (Oh et al, 2001), vaginal (Livingston et al, 1997),
electroporation (Heller, 2003) and biolistic vaccination to the skin (Schmaljohn, 2006).
With the proliferation of potential DNA vaccines came concerns over the safety
associated with the introduction of foreign DNA into a host. One of the issues raised was
the potential integration of the foreign DNA from the pathogen into the genomic DNA of
the host and causing in insertional mutatagenesis. It has been shown that plasmid DNA
generated insertional mutations at a far lower rate than what spontaneously occurs within
the genome (Ledwith et al, 2000). Another area of safety concern is that injections of
DNA will stimulate a systemic autoimmune response. Again, numerous non-primate and
human studies did not detect any increase in antinuclear or anti-DNA antibodies
(MacGregor et al, 1998; Klinman et al, 2000). To date, there has been no convincing
evidence from numerous clinical trials of DNA associated autoimmunity. In additions,
there are numerous and very stringent governmental regulations in place for the
development of any DNA vaccines development. (Smith & Klinman, 2001).
Although DNA vaccines have been proven to be safe and elicited immune responses,
they have been rather disappointing in the magnitude of immune response that is
generated. Despite promising results in rodent models, DNA vaccinations have proven
less effective in primate studies. It has become evident that simply injecting DNA does
not sufficiently stimulate the immune system. Clinical trials of DNA vaccines have
shown that DNA vaccines need to be made much more potent to be candidates for
preventive immunization of humans. Several approaches have been taken to improve the
vaccines. One approach being looked at is the incorporation of immunostimulatory
sequences into the backbone of the plasmid (Garmory, Brown, & Titball, 2003). Another
strategy that is being researched is the addition of adjuvants, mostly through the use of
biologically active molecules such as cytokines, chemokines and co-stimulatory
molecules (Ivory & Chadee, 2004). Prime boost (Radcliffe et al, 2006) is another
technique that is has recently begun being investigated. DNA vaccines are injected to
prime the immune system then attenuated pathogens are injected to boost the immune
system Volumes of research are currently being conducted on methods to improve the
potency of DNA vaccines and the number of clinical trials of DNA vaccines grows yearly
as researchers strive to concoct the magic potion that is an effective DNA vaccine (Figure
2).
Early reports of the immunogenicity of plasmid DNA vaccines in laboratory animals
sparked interest because of the simplicity and their ability to stimulate the immune
response. DNA vaccines have proven extremely useful as a laboratory tool for studying
immune responses to a wide variety of viruses, bacteria and parasites. However, it has
yet to be determined whether researchers will ever be able to overcome technical hurdles
and develop a DNA vaccine with clinical applications. DNA vaccines have rapidly
advanced to clinical trials with second generation formulations, delivery devices, and
mixed modality approaches and hold great promise for new vaccines and
immunotherapeutics against a barrage of depilating diseases. There are certainly
numerous promising Phase I and II clinical trials (www.clinicaltrials.gov lists 97
currently recruiting and over 300 active trials) underway that will help advance the field
of DNA vaccines. Unfortunately, it will more than likely take years before a FDA
approved DNA vaccine is marketed for human use in the United States. The concept of
DNA vaccines is efficient and elegant. However, the practically application is proving to
be a bit of a challenge.
Figure 1
Figure 2 Plot showing increasing usage of naked/plasmid DNA in gene therapy
trials throughout the world (Journal of Gene Medicine, 2004) Both the number of
trials and the percentage of total trials using naked or plasmid DNA has been
increasing since 2000. No data were available later than 2003.
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