due to high costs. DNA vaccines present an alternative to traditional vaccines and have several advantages: lower cost of production, stable plasmids allowing for easier transportation and storage
at room temperature, authentic protein translation and posttranslational modifications, and induction of both a cellular and humoral
immune response [2]. Although they have many potential advantages, currently no DNA vaccines are approved for human use
largely due to their poor overall immunogenicity in humans. To
overcome this poor immunogenicity, traditional adjuvants such as
alum have been tried but alum-adjuvanted DNA vaccines showed
no major enhancement of antibody or T-cell responses. By contrast,
Vaxfectin, a cationic lipid-based adjuvant, was shown to enhance
the immunogenicity of DNA vaccines, including against JEV and
DENV [3]. A prME JEV DNA vaccine formulated with Vaxfectin
and administered to BALB/c mice via i.m. injection three times at a
dose of 10 μg induced antibody titers of 1:80 compared to <1:10
for the unadjuvanted DNA vaccine [4]. Similarly, a polysaccharide
adjuvant, zymosan, when combined with an HIV DNA vaccine and
given i.m. to BALB/c mice significantly increased the IgG2a and
CTL response with a twofold increase in IFN-γ [5]. Another polysaccharide adjuvant called Advax™, that is derived from delta inulin, has been extensively tested with protein, inactivated, and
attenuated vaccines, including in phase I and II human clinical
trials. A recent study of Advax formulated with a seasonal, inactivated influenza vaccine and injected i.m. into BALB/c mice showed
a statistically significant increase in IgG1 and IgG2a antibody titers,
as well as CD4+ and CD8+ T-cell proliferation and cytokine
responses, that translated into enhanced protection against virus
challenge [6]. Advax formulated with a cell culture grown, inactivated JEV vaccine given i.m. to C57BL/6 mice similarly induced
significantly higher IgG1 and IgG2c responses and enhanced protection against a lethal JEV challenge [7]. This raises the question
of whether Advax may similarly be used to enhance the immunogenicity of a DNA vaccine.
Another method of increasing DNA vaccine immunogenicity is
to improve the uptake of the DNA into cells using electroporation
(EP). This is a delivery method that involves standard
i.m. injections of DNA followed by the application of brief electric
pulses through electrodes into the site of injection. Studies have
shown that the electric pulses induce transient pores to form in cell
membranes [8], allowing extracellular macromolecules to gain
access to the cell [9]. A study involving a DNA vaccine injected
i.m. into C3H/HeN mice showed that EP led to a 1000-fold
increase of expression of a luciferase marker [10]. Antibody titers
were enhanced tenfold, with an increase in IgG2a compared to
immunization without EP, indicating EP induced primarily a Th1
T-cell response. Other studies in cancer and infectious disease
vaccine trials in various animal models have typically shown a
DNA Vaccine Design and Adjuvant Selection
407
at room temperature, authentic protein translation and posttranslational modifications, and induction of both a cellular and humoral
immune response [2]. Although they have many potential advantages, currently no DNA vaccines are approved for human use
largely due to their poor overall immunogenicity in humans. To
overcome this poor immunogenicity, traditional adjuvants such as
alum have been tried but alum-adjuvanted DNA vaccines showed
no major enhancement of antibody or T-cell responses. By contrast,
Vaxfectin, a cationic lipid-based adjuvant, was shown to enhance
the immunogenicity of DNA vaccines, including against JEV and
DENV [3]. A prME JEV DNA vaccine formulated with Vaxfectin
and administered to BALB/c mice via i.m. injection three times at a
dose of 10 μg induced antibody titers of 1:80 compared to <1:10
for the unadjuvanted DNA vaccine [4]. Similarly, a polysaccharide
adjuvant, zymosan, when combined with an HIV DNA vaccine and
given i.m. to BALB/c mice significantly increased the IgG2a and
CTL response with a twofold increase in IFN-γ [5]. Another polysaccharide adjuvant called Advax™, that is derived from delta inulin, has been extensively tested with protein, inactivated, and
attenuated vaccines, including in phase I and II human clinical
trials. A recent study of Advax formulated with a seasonal, inactivated influenza vaccine and injected i.m. into BALB/c mice showed
a statistically significant increase in IgG1 and IgG2a antibody titers,
as well as CD4+ and CD8+ T-cell proliferation and cytokine
responses, that translated into enhanced protection against virus
challenge [6]. Advax formulated with a cell culture grown, inactivated JEV vaccine given i.m. to C57BL/6 mice similarly induced
significantly higher IgG1 and IgG2c responses and enhanced protection against a lethal JEV challenge [7]. This raises the question
of whether Advax may similarly be used to enhance the immunogenicity of a DNA vaccine.
Another method of increasing DNA vaccine immunogenicity is
to improve the uptake of the DNA into cells using electroporation
(EP). This is a delivery method that involves standard
i.m. injections of DNA followed by the application of brief electric
pulses through electrodes into the site of injection. Studies have
shown that the electric pulses induce transient pores to form in cell
membranes [8], allowing extracellular macromolecules to gain
access to the cell [9]. A study involving a DNA vaccine injected
i.m. into C3H/HeN mice showed that EP led to a 1000-fold
increase of expression of a luciferase marker [10]. Antibody titers
were enhanced tenfold, with an increase in IgG2a compared to
immunization without EP, indicating EP induced primarily a Th1
T-cell response. Other studies in cancer and infectious disease
vaccine trials in various animal models have typically shown a
DNA Vaccine Design and Adjuvant Selection
407