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Review
. 2018 Nov 15;19(11):3605.
doi: 10.3390/ijms19113605.

DNA Vaccines-How Far From Clinical Use?

Dominika Hobernik  1 , Matthias Bros  2
Affiliations
Review

DNA Vaccines-How Far From Clinical Use?

Dominika Hobernik et al. Int J Mol Sci. .

Abstract

Two decades ago successful transfection of antigen presenting cells (APC) in vivo was demonstrated which resulted in the induction of primary adaptive immune responses. Due to the good biocompatibility of plasmid DNA, their cost-efficient production and long shelf life, many researchers aimed to develop DNA vaccine-based immunotherapeutic strategies for treatment of infections and cancer, but also autoimmune diseases and allergies. This review aims to summarize our current knowledge on the course of action of DNA vaccines, and which factors are responsible for the poor immunogenicity in human so far. Important optimization steps that improve DNA transfection efficiency comprise the introduction of DNA-complexing nano-carriers aimed to prevent extracellular DNA degradation, enabling APC targeting, and enhanced endo/lysosomal escape of DNA. Attachment of virus-derived nuclear localization sequences facilitates nuclear entry of DNA. Improvements in DNA vaccine design include the use of APC-specific promotors for transcriptional targeting, the arrangement of multiple antigen sequences, the co-delivery of molecular adjuvants to prevent tolerance induction, and strategies to circumvent potential inhibitory effects of the vector backbone. Successful clinical use of DNA vaccines may require combined employment of all of these parameters, and combination treatment with additional drugs.

Keywords: DNA vaccine; adjuvant; antigen presenting cells; dendritic cell; macrophage; nano carrier; promotor; transgene.

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Conflict of interest statement

The authors declare no conflict of interest.

Figures

Figure 1
Figure 1
DNA vaccines induce adaptive immune responses. A DNA vaccine intended to induce an adaptive immune response needs to encode an antigen and an adjuvant. The according plasmid DNA is applied either systemically or topically, e.g., by intramuscular injection. Transfected keratinocytes or myocytes express transgene and release derived peptide/protein via exosomes or apoptotic bodies. This material is endocytosed by immature dendritic cells (iDC) which subsequently present antigen preferentially via major histocompatibility class (MHCII) to CD4+ T cells in draining lymph nodes. Direct transfection of APC including iDC results in endogenous transgene expression, and hence parallel presentation via MHCI and MHCII, yielding CD8+ and CD4+ T cell responses in parallel. Besides this cellular immune response, a humoral immune response is induced if the B cell receptor recognizes the protein antigen, and acquires help by pre-activated antigen-specific CD4+ T cells.
Figure 2
Figure 2
Optimization parameters of DNA vaccine design. Use of a hybrid viral/eukaryotic promoter was shown to prevent transcriptional silencing as observed for viral promotors. Alternatively, a promoter engineered to transcriptinally target APC like DC may be used. To enhance antigen expression, especially in case of pathogens, codon optimization is important. To induce a broad CD4+/CD8+ T cell response, linker-separated sequences encoding different antigens may be used. In addition, fusion with a sequence encoding the invariant chain may enhance loading of antigen onto MHCII. Conventionally, molecular adjuvants intended to enhance the APC activation state and/or T cell attraction and polarization are encoded by expression vectors coadministered with a DNA vaccine. To ensure coexpression of antigen and a molecular adjuvant by a transfected cell, both sequences must be incorporated in the DNA vaccine (in cis), separated by an IRES or T2A sequence. The vector backbone comprises the part of the DNA vaccine which is not required for eukaryotic expression. Inherent or inserted immunostimulatory sequences are detected by danger receptors, and mediate APC activation. Inclusion of a NLS facilitates nuclear entry of the DNA. Intrinsic inhibitory effects of the backbone on the transfection efficiency are limited by insertion of A/T-rich sequences or by recombinase-mediated deletion of the prokaryotic part as a last step after propagation in bacteria to yield minicircle DNA.
Figure 3
Figure 3
Routes of DNA vaccine delikvery. DNA vaccines may be delivered systemically by intraveneous injection to reach secondary lymphatic organs, by oral application of (attenuated) bacteria as a vehicle to confer uptake of DNA by intestinal APC, and by pulmonary administration of nebulized DNA to achive uptake by lung cells. Transdermal delivery primarily adresses LC, and both needle-free delivery of particle-adsorbed DNA vaccines by helium pressure (gene gun, PMED) and needle-based administration via microneedles and tattoo devices are clinically tested. Transfection of cutaneous APC as well as of non-APC by intradermally injected DNA vaccines is enhanced by immediate electroporation. Subcutaneous injection mainly results in transfection of fibroblasts and keratinocyts, which express transgenes and release antigen for uptake APC. Likewise, intramuscular injection of DNA vaccines primarily yields transfection of myocytes that express/release antigen for APC uptake, and myocyte transfection rates are enhanced by electroportion at the site of injection as well.
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