DNA Vaccines as platform technology[Human Vaccines 5:9, 623-626; September 2009]; ©2009 Landes Bioscience
Traditionally vaccines are based on immunogens delivered
as attenuated live microbes, inactivated pathogens, purified
proteins or virus-like particles. Newer generation vaccines are
based on the delivery of genes encoding for a protein antigen that
can be transcribed and translated by host cells. Despite current
challenges to improve delivery and immunogenicity, DNA vacci-
nation has several major advantages over traditional vaccines
or over other types of investigational vaccine platforms. DNA
vaccines do not integrate into the host genome, they are stable,
can be manufactured with relative ease and efficiency, have been
safe in clinical trials and do not require a preservative in final
preparation. The lack of vector-specific immunity allows the
potential for DNA vaccines to be used as a platform technology
for emerging viral diseases by allowing the simple exchange of
genes encoding vaccine antigens in a stable plasmid backbone.
Since Jenner’s original discovery and the subsequent eradication
of smallpox, vaccines have made a dramatic positive impact on
public health. The field of vaccinology has evolved in direct asso-
ciation with scientific advances in molecular biology, immunology
and infectious disease pathogenesis under pressure from emerging
infectious diseases and pathogens that have eluded vaccine solu-
Traditionally vaccines have been based on immunogens deliv-
ered as attenuated live microbes, inactivated pathogens, purified
proteins or as particles simulating wild-type structures. Newer
generation vaccines are based on the delivery of genes encoding for
a specific protein antigen that can be transcribed and translated by
host cells.1 The host cell can present authentic protein conforma-
tions as well as processed proteins in the context of class I and class
II major histocompatibility complexes (MHC). The host immune
system is thereby able to mount humoral and cellular immune
responses to microbial protein. Once the gene is delivered and the
encoded gene is transcribed and translated, the host cell is recog-
nized and destroyed by the host immune response. Importantly,
the ability to mount a gene-based vaccine-induced immune
response appears to be irrespective of HLA type or other individual
characteristics as shown by the high frequency of immune response
among subjects in early phase I and II clinical trials.
Gene-based vaccination can be accomplished through a variety
of platforms. Altered microbes designed to express genes encoding
vaccine antigens are generically called vectors. Vaccine vectors can
be replication-competent or replication-defective. They utilize
the tools evolved by viruses, bacteria, fungi or other microbes to
gain entry into host cells to deliver the genetic payload. Gene-
based vaccine vectors have been developed that efficiently deliver
genes for vaccine antigen production in host cells. However, there
are issues involving manufacturing, toxicity and vector-specific
immunity that significantly alter risk:benefit and cost:benefit
considerations. Therefore, gene-based vaccination using DNA
plasmid technology has a number of advantages over vector-based
DNA vaccines are manufactured as supercoiled double-stranded
plasmid DNA, and the are constructed with a promoter to allow
for transcription of the gene insert (typically cytomegalovirus
immediate early promoter or a derivative thereof), a polyadenyla-
tion sequence to ensure appropriate translation, a drug resistance
gene needed for selection and the variable component, a codon-
modified gene encoding for the vaccine antigen of interest (Fig. 1).
DNA vaccines are produced by bacterial fermentation followed by
purification of the plasmids and typically formulated for delivery
in a sterile liquid, such as phosphate buffered saline.2
DNA vaccination has several major advantages over traditional
vaccines or over other types of investigational vaccine platforms.
DNA vaccines do not induce vector immunity in the host, do
not integrate into the host genome, they are stable, can be manu-
factured with relative ease and efficiency and do not require a
preservative in final preparation. Another advantage of DNA
vaccines is the potential for maintaining constancy in the plasmid
components or backbone as a platform technology, allowing the
simple exchange of genes encoding vaccine antigens. In this way
the safety database for the platform technology could be developed
*Correspondence to: Julie E. (Martin) Ledgerwood; Vaccine Research Center;
Clinical Trials Core; NIAID; NIH; 9000 Rockville Pike; CRC Bldg 10 Room 5-2440;
Bethesda, MD 20892 USA; Tel.: 301.594.8502; Fax: 301.451.4651; Email:
Submitted: 12/05/08; Accepted: 04/06/09
Previously published online as a Human Vaccines E-publication:
A safe and efficient platform technology for responding to emerging infectious diseases
Julie E. Ledgerwood* and Barney S. Graham
Vaccine Research Center; National Institute of Allergy and Infectious Diseases; National Institutes of Heath; Bethesda, MD USA
Key words: Gene based vaccines, recombinant vaccine vectors
DNA Vaccines as platform technologyDNA Vaccines as platform technology
624Human Vaccines 2009; Vol. 5 Issue 9
across multiple antigen specificities, to inform and facilitate the
regulatory approval process. This raises the possibility of using
DNA vaccines as a platform technology for the purpose of rapid
vaccine development against newly emerging or re-emerging infec-
tious diseases, such as avian influenza, SARS, Ebola or Marburg.
Unfortunately, the simplicity of using plasmid DNA as the
vehicle for gene-based vaccine delivery is offset by a relatively inef-
ficient immunization process. The vast majority of DNA plasmids
injected intramuscularly do not make it to the nucleus of a cell.
It is not clear what happens to all the plasmids, but some are
taken up by phagocytic cells and invoke innate immune responses,
possibly through toll-like receptor recognition of CpG motifs in
the bacterial plasmid. This may provide an adjuvant effect, but the
role in the overall immunogenicity of DNA vaccines is unknown.
The major biological barrier to DNA vaccine is likely to be trans-
port across the plasma membrane into a host cell. This is based on
the indirect evidence that both chemical and physical approaches
to disrupt the plasma membrane tend to improve the immunoge-
nicity of DNA vaccines.
DNA vaccines are more immunogenic when administered by
needleless devices such as Biojector® (injectate is sprayed through
the skin into the muscle using the power of compressed CO2) or
gene gun (ballistic delivery of DNA on small gold balls) compared
to a needle and syringe. Other physical approaches include ultra-
sound or electroporation, which has shown particular promise
in preclinical studies. Chemical methods to facilitate entry of
DNA plasmids into cells have included lipid-based approaches
(Vaxfectin), anesthetics (bupivicaine) or toxins (snake venom).
The first description of a DNA vaccine-induced immune
response was in 1992.3 This finding came from an experiment
attempting to express growth hormone in mice in which the mice
unexpectedly produced an immune response to the transgene
product.4,5 Since that time, DNA vaccination has been studied
for use against approximately 50 pathogens in small mammals,
large mammals, non-human primates (NHPs), humans and fish.
One DNA vaccine has been licensed by the
United States Department of Agriculture for the
prevention of West Nile virus in horses,6,7 and
another approved in Canada for the prevention
of infectious haematopoietic necrosis virus in
To date, candidate DNA vaccines have been
shown to be safe and immunogenic in humans
against a number of viral pathogens, but none
have been licensed for human use. Initial clinical
investigations of plasmid DNA vaccines began
in the 1990s8 and in the past 20 years have
included evaluations of vaccines against HIV,9-11
malaria,12,13 tuberculosis,13 influenza,14 hepa-
titis B,15 West Nile virus,16 SARS17 and Ebola
virus.18 DNA vaccines have been shown to be
safe and well tolerated in a number of phase I
and phase II clinical trials.1,9,10,17-24 If present,
local or systemic reactogenicity is typically mild
in severity (Fig. 2). In assessment of cumulative
data from a number of clinical trials reports,
there has been no evidence of trends in serious adverse events
related to vaccination, no evidence or autoimmune disease, and no
evidence of oncogenicity.
DNA vaccines have progressively been shown to be considered a
viable vaccine platform. Early milestones in DNA vaccine research
include the demonstration of anti-hepatitis antibodies known to
be a correlate of protection from disease as assessed by ELISA.21,22
DNA vaccines were shown to induce T cell responses and have
repeatedly been shown to do so for a variety of antigens.9,10,19
Recently, candidate DNA vaccines for WNV and SARS have
been shown to induce neutralizing antibody titers and in the
case of WNV, comparable to those seen in vaccinated, protected
DNA vaccines have a number of features that are ideal for
establishing a platform technology to make rapid development of
vaccines more feasible, such as in the setting of a newly emerging
viral illness (Fig. 1). Once a DNA plasmid backbone has been
shown to be safe and immunogenic in humans, the platform
could be used to more efficiently develop new vaccines encoding
relevant antigens from new infectious disease threats. For example,
if current versions of avian influenza DNA-based vaccines are
assessed for safety and immunogenicity in phase I trials, what
would happen if a pandemic occurred with a strain of a different
serotype. One potential scenario would be rapid identification,
sequencing and expression of the novel HA in a new version of the
vaccine that could progress immediately into human trials without
pre-clinical toxicology and other time intensive steps, but rather
rely on previous safety data of the platform.2
DNA vaccines could be used in a number of different ways.
As a prophylactic vaccine for WNV in which a VLP is produced
by the transduced cell, DNA immunization alone may induce a
sufficient antibody response to afford protection. In other cases,
DNA immunization produces a broad and durable CD4+ T cell
response and thereby provides a potent priming immunization that
can be boosted with a gene-based vector, a protein, or inactivated
Figure 1. DNA vaccine components. Constant or platform components include the promoter,
the poly A tail and if present, the antimicrobial resistance gene. The variable, or antigenic
component may be codon-optimized and many types of microbial antigens may be expressed
as gene inserts.
DNA Vaccines as platform technology
was known. Experience with and the flexibility of DNA vaccine
platform technology allowed for expeditions progress to a clinical
trial in that case. The concept behind establishing platform tech-
nologies for selected categories of microbial pathogens should be
further assessed and evaluated by researchers and regulators as a
public health approach to the never-ending emerging infectious
diseases threatening the public health.
For DNA vaccine concept to realize its promise and full
potential, the delivery barrier has to be solved. New physical and
chemical approaches to facilitate DNA entry into host cells deserve
the continued attention of the scientific community. The concept
of using DNA immunization and other gene-based vector tech-
nologies as flexible platforms for rapid interventional responses to
public health crises also deserves serious discussion from a regula-
whole virus. In diseases that are sporadic or that are theoretical
risks, the ideal vaccine approach may be limited by toxicity or
by length of immunization regimen. For these types of diseases,
because of its favorable safety profile and attributes for immuno-
logical priming, DNA immunization could be used to establish a
baseline immunity that could then be boosted by other vaccine
modalities in the event of disease emergence. This approach would
not only avoid the need for the booster immunization unless
disease was imminent, but would improve the likelihood that
a single booster immunization would be efficacious against the
The Vaccine Research Center (VRC) at the NIH has demon-
strated that the use of DNA vaccine platform technology can be
efficient and reliable. As a specific example, in 2006, the VRC
began a phase I vaccine clinical trial evaluating a DNA vaccine
encoding for hemagglutinin 5 from the Indonesian strain of avian
influenza approximately ten months after the sequence of the virus
Figure 2. Cumulative data from 7 phase I clinical trials of six different DNA vaccines constructed with a similar platform design. Local (A) and systemic
(B) solicited reactogenicity are shown by type and severity by study day for 305 injections of DNA vaccine at 4 mg each injection. These data represent
cumulative injections from trials of two different HIV DNA vaccines, two different West Nile virus DNA vaccines, an Ebola DNA vaccine and a SARS
DNA vaccine. Data is shown as patterned bars above the line (% of injections with the presence of reactogenicity) and below the line (% of injections
with no reactogenicity) for each graph.
DNA Vaccines as platform technology
626Human Vaccines 2009; Vol. 5 Issue 9
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