VA C C I N E SI N V I T E D A R T I C L E
Stanley Plotkin, Section Editor
Clinical Applications of DNA Vaccines: Current
Bernadette Ferraro, Matthew P. Morrow, Natalie A. Hutnick, Thomas H. Shin, Colleen E. Lucke, and David B. Weiner
Department of Pathology and Laboratory Medicine, University of Pennsylvania School of Medicine, Philadelphia, Pennsylvania
It was discovered almost 20 years ago that plasmid DNA, when injected into the skin or muscle of mice, could
induce immune responses to encoded antigens. Since that time, there has since been much progress in
understanding the basic biology behind this deceptively simple vaccine platform and much technological
advancement to enhance immune potency. Among these advancements are improved formulations and
improved physical methods of delivery, which increase the uptake of vaccine plasmids by cells; optimization of
vaccine vectors and encoded antigens; and the development of novel formulations and adjuvants to augment
and direct the host immune response. The ability of the current, or second-generation, DNA vaccines to induce
more-potent cellular and humoral responses opens up this platform to be examined in both preventative and
therapeutic arenas. This review focuses on these advances and discusses both preventive and immunother-
apeutic clinical applications.
HISTORY OF DNA VACCINES
Current licensed vaccines are predominantly composed
of either killed pathogens, pathogen subunits, or live-
attenuated viruses. Nonlive vaccines, which confer pro-
tection primarily through the induction of CD41T- cell
and humoral mechanisms, generally do not provide
life-long immunity. In contrast, live-attenuated vaccines
can mobilize both the cellular and humoral arms of the
immune response and generally induce more-prolonged
immunity. However, their degree of attenuation can
significantly lower the immunogenicity of live vaccines,
and the development of live vaccine strategies can be
especially challenging when the goal is to target multiple
viral subtypes or pathogens. There are also theoretical
safety concerns associated with the use of both nonlive
and attenuated approaches. These limitations continue
to drive the need to develop new vaccine platforms that
offer broader immunogenicity.
DNA vaccines first sparked the interested of the sci-
entific community in the early 1990s, when it was re-
ported that plasmid DNA, delivered into the skin or
muscle, induced antibody responses to viral and non-
viral antigens [1–4]. The simplicity and versatility of this
vaccine approach generated a great deal of excitement
and inspired additional preclinical studies targeting
a plethora of viral and nonviral antigens. In theory,
DNA vaccines could generate broad immune responses,
need for a replicating pathogen.
Owing to the promise of DNA vaccines in small an-
imal studies, clinical trials soon ensued. The first of
several of phase I trials,conducted almost 2 decades ago,
immunodeficiency virus type 1 (HIV-1) for therapeutic
and prophylactic applications . Other studies shortly
followed that targeted cancer or other HIV-1 antigens,
influenza, human papillomavirus (HPV), hepatitis, and
malaria. However, the results of these early clinical trials
were disappointing. The DNA vaccines were safe and
well tolerated, but they proved to be poorly immuno-
genic. The induced antibody titers were very low or
nonexistent, CD81T-cell responses were sporadic, and
CD41T-cell responses were of low frequency. However,
Received 1 December 2010; accepted 15 April 2011.
Correspondence: David B. Weiner, PhD, University of Pennsylvania School of
Medicine, Dept of Pathology and Laboratory Medicine, 422 Curie Blvd SCL 505,
Philadelphia, PA, 19104 (email@example.com).
Clinical Infectious Diseases
? The Author 2011. Published by Oxford University Press on behalf of the Infectious
Diseases Society of America. All rights reserved. For Permissions, please e-mail:
d CID 2011:53 (1 August)
these studies provided proof of concept that DNA vaccines
could safely induce immune responses (albeit low-level re-
sponses) in humans.
SECOND-GENERATION DNA VACCINES
Many improvements have been incorporated into the current,
or second-generation, DNA vaccines, and these improve-
ments have helped to spark a resurgence of interest in the
platform. Second-generation DNA vaccines appear to drive
improved cellular and humoral immune responses in both
small and large animal models. Importantly, research suggests
that newer DNA vaccines can more broadly activate CD81
cytotoxic T cells (CTL) in larger animal models, compared
with earlier DNA approaches .
The low immunogenicity of early DNA vaccines is hypothe-
sized to stem, in part, from inefficient uptake of the plasmids by
cells due to inefficient delivery. Research has focused on de-
veloping novel strategies to enhance transfection efficiency and
improve other facets of the DNA platform. These efforts include
optimizationofthe antigensencodedbythe plasmidstoincrease
antigen expression on a per cell basis, improved formulation,
and inclusion of molecular adjuvants to enhance and direct
immune responses .
Several physical methods of delivery have been explored to
increase the transfection efficiency of DNA vaccines, including
needle-free approaches, such as particle bombardment and
high-pressure delivery, dermal patches, and electroporation
(EP). Particle bombardment approaches use a highly pres-
surized stream to deliver vaccine plasmids on microscopic
heavy metal beads. For example, the PMED device (Pfizer)
delivers DNA plasmids, linked to microscopic gold particles,
into the skin in a dry powder formulation [8, 9]. High-pressure
mediated delivery is conceptually similar to particle bom-
bardment. For example, the Biojector devices (Bioject Medical
Technologies) deliver vaccines by forcing liquid through a tiny
orifice to create a fine, high-pressure stream that penetrates the
skin . One example of noninvasive dermal patch delivery is
DermaVir (Genetic Immunity). DermaVir is a self-adhesive
patch coated with multiple antigen or adjuvant encoding
plasmids and a synthetic polymer that forms pathogen-like
nanoparticles . Another promising physical method of
delivery is EP, or the application of short electrical pulses to the
delivery tissue, which was initially studied over 25 years ago as
a method to enhance the efficacy of chemotherapy agents .
It was later discovered that EP also increases the uptake of
DNA plasmids by cells, resulting in an increase in antigen
production  and in vaccine immunogenicity [14–16].
Figure 1 demonstrates the magnitude of the increase in
immunogenicity that can be achieved when delivering a DNA
vaccine by intramuscular injection (IM) with EP (IM1EP),
compared with IM alone. EP augmented both antigen-specific
production of interferon (IFN) c (Figure 1A) and serocon-
version (Figure 1B). The use of improved delivery has enabled
second-generation DNA vaccines to induce cellular immune
responses comparable to viral vectors in nonhuman primates
Formulation and Molecular Adjuvants
Formulation of DNA vaccines in microparticles or liposomes
has been reported to increase the uptake of plasmid DNA by
cells, thereby increasing the immunogenicity of several different
vaccines in animal models and humans . An influenza DNA
vaccine formulated in the lipid compound Vaxfectin (Vical)
induced protective antibody titers and T-cell responses in many
subjects . Another method to improve DNA vaccine im-
munogenicity is the inclusion of additional plasmids, or addi-
tional inserts in the same plasmid, encoding molecular
adjuvants. Multiple studies have shown that codelivery of plas-
mids encoding cytokines, chemokines or costimulatory mole-
cules can augment immune responses. Unlike traditional
adjuvants,whichstimulate nonspecific inflammation,molecular
adjuvants can modulate the adaptive immune response. For
example, codelivery of interleukin (IL) 12 or IL-15 was shown to
increase the magnitude and functionality of antigen-specific T
cells in NHPs [19–21]. Similar to IL-12, IL-28B augments
antigen-specific CD81T-cell responses, but it also increases
CTL-killing ability [22, 23]. Use of granulocyte macrophage
colony-stimulating factor (GM-CSF) as a molecular adjuvant
has been shown to enhance cellular and humoral responses in
NHPs [24, 25]. One study demonstrated codelivery of GM-CSF
induced higher avidity in HIV-1–specific antibodies and
enhanced neutralizing antibody production, which correlated
with a trend towards improved control of a simian-human
hybrid virus challenge and re-emergent virus .
Recently, there has also been a focus on designing antigens that
successfully target highly variable pathogens. The optimized
immunogen sequences are usually designed or selected from
a collection of target antigen protein sequences. For example,
consensus immunogens are designed to encode the most com-
monly occurring amino acid at each position in a sequence,
whereas mosaic antigens are designed to encode the most im-
munogenic regions of an antigen . Similarly, center-of-tree
immunogens are derived from a native sequence that represents
a respective middle of evolutionary diversity, whereas ancestral
immunogens are derived from antigen sequences at the root of
a phylogenic tree. All of these techniques are an attempt to focus
the immune response on a synthetic sequence that is more
representative of pathogen diversity. Thus, the host immune
d CID 2011:53 (1 August)
response is better educated and responds more effectively to
divergent pathogens .
SAFETY AND TOLERABILITY OF DNA
The DNA platform is conceptually safer and more stable than
are conventional vaccine approaches. Plasmids are nonlive and
nonreplicating, which leaves little risk for reversion to a disease-
causing state or secondary infection. The original concerns as-
sociated with the DNA platform were the potential for genomic
integration and development of anti-DNA immune responses.
Exhaustive research has found little evidence of integration, and
the risk for integration appears to be significantly lower than
that associated with naturally occurring mutations [28–30].
Induction of anti-DNA immune responses after DNA vaccina-
tion has been monitored in multiple NHP studies and clinical
trials, but evidence of increased production of such responses or
changes in other clinical markers of autoimmunity have not
been reported . Overall, multiple studies have reported the
DNA platformto be well toleratedand to have an enviable safety
SELECTED CLINICAL TARGETS
There arecurrently 43 clinicaltrialsevaluating DNA vaccinesfor
viraland nonviral diseases listed in theclinicaltrials.gov database
(Table 1; Figure 2). The majority (62%) of these trials are in-
vestigating vaccines for HIV (33%) or cancers (29%). Almost
half (38%) of cancer vaccines currently being investigated are
targeting melanoma. The remaining 38% of enrolling or active
clinical trials are investigating vaccines for influenza, hepatitis B
and C, HPV, and malaria. This review highlights DNA vaccines
for influenza,HPV,andHIV-1 asexamplesof antibody, cellular,
and complex immunological targets, respectively. It should be
noted, as evidenced by Table 1 and Figure 2, that great strides
have also been made in the development of DNA vaccines for
many other important clinical targets.
Every year, the scientific and medical communities are charged
with the task of determining the appropriate influenza strains
to include in the seasonal influenza vaccine. Current vaccine
platforms require months to generate sufficient quantities of
antigens because of the requirement for the growth of the virus
in chicken eggs . This can delay the availability of viral
stocks or result in a mismatch between the vaccine strains
selected and the actual circulating strains. In 2007, the seasonal
influenza vaccine coverage was estimated at only 30% because
of mismatches between the strains that were expected to
emerge and the strains that actually circulated . In con-
trast, development of a DNA vaccine for a particular influenza
Table 1. Current DNA Vaccine Clinical Trials
I 31 HIV treatment and prevention, in-
fluenza, HPV, cancer (meta-
static breast, B cell lymphoma
prostate, colorectal), hepatitis
B, hepatitis C, malaria
I/II7 HIV treatment, cancer (prostate,
colorectal), hepatitis B, hepatitis
C, HPV, malaria
II5 Cancer (prostate, melanoma), HIV
treatment, hepatitis B
HIV, human immunodeficiency virus; HPV, human papillomavirus.
cellular and humoral responses. A DNA vaccine encoding human
prostate–specific antigen (PSA) was administered by intramuscular
injection (IM) or by IM plus EP (IM1EP). Animals received 2 vaccinations
spaced 2 weeks apart. Cellular and humoral responses were determined
1 week after the second immunization. PSA-specific T-cell responses
were determined by interferon (IFN) gamma enzyme-linked immunospot
(A) and PSA-specific seroconversion by enzyme-linked immunosorbent
assay (B). n 5 5 per group. OD, optical density; SFC, antigen-specific spot
forming cells per 106.
DNA vaccine delivery with electroporation (EP) increases
d CID 2011:53 (1 August)
strain could shorten this timeline 2–4-fold and could poten-
tially provide a product in a few months with little chance of
Influenza presents a particular challenge for the DNA plat-
form because protection is specifically associated with anti-
bodies, and induction of humoral responses was a shortcoming
of the original DNA vaccines. New approaches incorporated
into the second-generation platform have enabled the induction
of humoral responses against a variety of antigens. Thus, the
development a DNA vaccine for influenza has become a more
reasonable goal. One preclinical study of an H5N1 influenza
DNA vaccine showed that protective antibody titers were in-
duced to multiple clades of H5N1 using a single consensus H5
antigen . In further support of this cross-protection ap-
proach, it has recently been shown that cross-protective titers
can be achieved to viruses that circulated over 90 years apart;
namely, the 1918 ‘‘Spanish Flu’’ and the 2009 ‘‘Swine Flu’’ .
The concept of cross-neutralization of different influenza strains
may be of great significance in future influenza vaccines.
Moreover, this conceptapplies not only to influenza strains with
the potential to cause pandemics but also to strains included in
The success of DNA vaccines against multiple strains of
influenza in preclinical models has paved the way for their
development for the clinic. To that end, there are currently
several DNA-based influenza vaccines in various stages of phase
I clinical trials, including vaccines against potentially lethal
pandemic strains such as H5N1 (Inovio Pharmaceuticals) and
H1N1 (National Institutes of Allergy and Infectious Diseases)
. A completed phase I clinical trial conducted by Vical
demonstrated that formulation of a monovalent H5N1 DNA
vaccine in Vaxfectin achieved protective hemagglutination
inhibition titers or antibody responses in more than 47% of
subjects, and H5-specific T-cell responses were detected in
at least 75% of subjects . A phase 1 trial completed by
PowderMed demonstrated reductions in disease symptoms
and viral shedding in subjects who received a trivalent DNA-
based seasonal influenza vaccine, delivered using the PMED
device, compared with placebo . The ultimate success of
these vaccines could reshape the way physicians and researchers
view influenza vaccine development.
Cervical cancer remains the third leading cause of cancer-related
morbidity in women worldwide . Intense research efforts
have resulted in US Food and Drug Administration approval of
2 preventive HPV vaccines; Gardasil (Merck) in 2006 and Cer-
varix (GlaxoSmithKline) in 2009. However, the impact of these
clinicaltrials.gov database. The large pie chart shows the percentage of trials by vaccine target. The inset pie chart shows the percentage of trials
targeting specific cancers among the 29% of clinical trials that are cancer related. HIV, human immunodeficiency virus; HPV, human papillomavirus.
Current DNA vaccine clinical trials. At the time of publication, 43 clinical trials evaluating DNA vaccines were listed as on-going in the
d CID 2011:53 (1 August)
vaccines on the global prevalence of HPV infection is slowed
because of the high economic burden and logistical issues that
hinder widespread vaccination. These preventive HPV vaccines
do not induce appreciable levels of cellular immune responses
and, thus, cannot clear established HPV infections or HPV-
associated lesions. Thus, the DNA platform, which can drive
strong cellular responses, is a logical approach for this task.
Some candidate HPV therapeutic vaccines utilize the E6 and
E7 oncoproteins as antigens to target HPV-16 and HPV-18,
whicharepresent inHPV-associated cervicalcancer andcervical
intraepithelial neoplasia (CIN). E6 and E7 are ideal therapeutic
targets, because they play an integral role in the generation and
maintenance of HPV-associated disease and are constituently
expressed in HPV-associated cancer and precursor lesions .
One interesting DNA vaccine strategy is the use of fusion con-
sensus antigens that encode multiple antigens in the same vec-
tor. For example, HPV-16 and HPV-18 E6/E7 fusion consensus
vaccines, delivered by EP, demonstrated encouraging results in
NHPs and are currently being evaluated in a phase I clinical trial
(Inovio Pharmaceuticals) [39, 40].
been recently completed or are currently ongoing. ZYC101 (Eisai
Pharmaceuticals), a microencapsulated DNA vaccine encoding
multiple HPV-16 E7-specific CTL epitopes, was well tolerated in
2 different phase I trials [41, 42]. An alternative version of this
vaccine, ZYC101a, which includes HPV-16 and HPV-18 E6- and
E7-derived CTL epitopes, was moved into a phase II study in
women with CIN2/3. In this study, the proportion of subjects
with resolved lesions was higher in the treatment groups, but this
result did not reach statistical significance . A phase II/III
trial of ZYC101a is currently underway. A different phase I
study investigated a HPV-16 E7-specific vaccine, pNGVL4a-Sig/
E7detox/HSP70 (NCI), administered by IM at escalating doses.
The vaccine was well tolerated, but it failed to induce significant
antibody or T-cell responses  and is currently undergoing
reevaluation as a component of a DNA and viral-vector
heterologous prime-boost strategy.
The development of a vaccine to prevent or control HIV-1 in-
in 1981. Unlike conventional vaccine targets, inducing broadly
neutralizing antibodies against HIV-1 has proven to be ex-
ceedingly challenging . Also, because of the complexity of
HIV-1, it is likely that an effective vaccine will be required to
modulate broad cellular and humoral responses. Neither the
recombinant protein gp120 nor the Ad5-vaccine used in the
STEP trial was effective at preventing HIV infection [45, 46].
In an effort to increase HIV-specific immune responses,
several clinical trials have investigated heterologous prime-boost
approaches that combine DNA-based and viral-based vaccines
with recombinant protein vaccines. The concept of combining
a vaccine platform that induces T-cell responses (DNA or viral-
vector vaccines) with one that induces antibody responses (re-
combinant protein vaccines) to induce broad HIV-1–specific
immunity has shown promise in a recently completed efficacy
trial (RV144). This trial incorporated a multiple-antigen viral-
vector prime (ALVAC) to induce HIV-1–specific T cells, fol-
lowed by a recombinant gp120 protein boost (AIDSVAX) to
generate HIV-1–specific antibodies. In a modified intent-to-
treat analysis, this heterologous prime-boost approach demon-
strated 31% efficacy for prevention of HIV-1 acquisition, but it
did not affect viral load in subjects who were not protected .
Although post hoc analysis of the RV144 trial is ongoing, the
success of 2 platforms that are ineffective individually suggests
that a preventive HIV vaccine will most likely require induction
of cellular and humoral responses. Other studies are in-
vestigating conceptually similar heterologous prime-boost
strategies by combining a DNA prime with a recombinant
protein boost. For example, a phase I clinical trial (DP6-001)
demonstrated priming with a multiple-antigen polyvalent DNA
vaccine, and boosting a recombinant HIV-1 envelope protein
induced cross-subtype antibody and cellular responses .
Combining a DNA prime and viral boost creates a synergistic
enhancement in the magnitude of antigen-specific CD81T-cell
responses. A phase I trial that combined a multi-clade DNA
vaccine prime with an Ad5 boost demonstrated thatthis strategy
was capableofeliciting humoral responses inadditiontocellular
responses . Preclinical studies also suggested that this ap-
proach increases not only the magnitude but also the quality of
the humoral response . This combination is now being ex-
plored in a larger efficacy trial. The National Institutes of Health
Vaccine Research Center, in collaboration with the HIV Vaccine
Trial Network, is evaluating the efficacy of this approach to
reduce viral loads in patients who become infected after vacci-
nation (HVTN 505) . Other viral vectors, such as modified
vaccinia ankara (MVA), are also being investigated for use in
HIV-1 vaccine strategies. A phase IIa trial (HVTN 205) (Geo-
Vax) is currently evaluating a multiple-antigen DNA prime
followed by an MVA boost encoding the same antigens.
First-generation DNA vaccines were shown to stimulate T cell
responses and antibodies, although at levels insufficient to pre-
vent HIV-1 infection. The advent of improved methods of
physical delivery and other new technologies has spurred a sec-
ond wave of clinical trials investigating DNA as a stand-alone
platform. A phase I clinical trial (HVTN-080) is currently un-
derway to determine the safety of Pennvax-B, a DNA vaccine
encoding HIV-1 gag, pol and env, and molecular adjuvant IL-12
delivered by EP . The use of molecular adjuvants is of
particular interest for HIV-1 vaccine development. In addition
to increasing the magnitude of the immune response, some
d CID 2011:53 (1 August)
molecular adjuvants can also alter the homing of antigen-
specific cells to specific target tissues. For example, an NHP
study demonstratedcodelivery of muscosal chemokines induced
trafficking of antigen-specific T cells to the gut mucosa, which
could position immune effector cells in a more advantageous
location to dampen initial HIV-1 viral replication .
A great deal of progress has been made since the disappointment
of the original DNA vaccine clinical trials almost 16 years ago.
Advancements in antigen design, improved formulations,
inclusion of molecular adjuvants, and physical methods of
delivery have greatly enhanced the immunogenicity of second-
generation DNA vaccines. The improved performance has
spurred a renewed interest in the platform, which is reflected by
the numerous ongoing clinical trials investigating DNA vaccines
for preventative and therapeutic applications. There are several
gene-based vaccines approved for use in veterinary practice for
targeting canine melanoma (Merial), West Nile virus (Wyeth),
fish hematopoietic necrosis virus (Novartis), and swine growth
hormone–releasing hormone (Inovio). Research is still con-
tinuing to explore combining other vaccine platforms with
DNA, enhanced methods for delivery, and new molecular
adjuvants. The results of on-going clinical trials will be pivotal
for providing insight into the progress of this platform and
determining the impact of the technological advances integrated
into the second-generation DNA platform.
National Institutes of Health (R01AI092843) and National Institute of
Allergy and Infectious Diseases (PO1-AI071739).
Potential conflicts of interest. D. B. W. has grant funding and col-
laborations or funding by consulting including serving or chairing scientific
advisory committees for commercial entities.
work can include consulting fees or stock payments.
with Pfizer, Bristol Myers Squibb, VIRxSYS, Ichor, Inovio, Merck, Althea,
Aldevron, and possibly others. All other authors: No reported conflicts.
All authors have submitted the ICMJE Form for Disclosure of Potential
Conflicts of Interest. Conflicts that the editors consider relevant to the content
of the manuscript have been disclosed in the Acknowledgments section.
This work was supported by a grant from the
Compensation from this
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