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source:John J.Donnelly,Britta Wabren, and Margaret A.Liu DNA Vaccines:Progress and Challenges The Journal of Immunology,2005,175:633-639 |
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DNA vaccine
DNA vaccines are the simplest embodiment of vaccines that, rather than consisting of the Ag itself, provide genes encoding the Ag. The Ag gene exist in bacterial plasmid and is injected into muscle then. In the years following the publication of the initial in vivo demonstration of the ability of plasmid DNA to generate protective immune responses, DNA vaccines have entered into a variety of human clinical trials for vaccines against various infectious diseases and for therapies against cancer, and are in development for therapies against autoimmune diseases and allergy.
Mechanism of gene immunization
The viral gene of interest is converted to DNA, which is inserted in a bacterial plasmid. The DNA plasmids carrying one or several genes or several different plasmids each carrying one or several genes can be administered i.m., in the skin or at the mucosa. The same gene(s) can be introduced in a viral or bacterial vector and used either as the only vaccine or as a boosting component to the first DNA vaccination. The plasmid enters the cell nucleus, where the gene initiates transcription, followed by protein production in the cytoplasm. Secreted proteins induce cytokines, T help, and Abs that will react with and eliminate virus. APCs present peptides in context of the MHC of the vaccinated individual and activate cytokines and killer cells, which in turn will lyse virus-infected cells. DNA itself or cytokines in the immune cascade activate NK cells. In the therapeutic treatment of HIV, NK cells may, although poorly, lyse cells presenting HIV foreign proteins. In prophylactic vaccination, naive B and T cells are primed by proteins and by APC presenting peptides, respectively. In therapeutic vaccination, the Ags may provide both priming of new responses, in cases where there has been no priming as in cancer, or a boost of memory responses, i.e., in persons with a chronic infectious disease.(see the left) |
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DNA Vaccine Design
Early in the development of DNA vaccines, DNA vaccine often can not lead to immune responses. Biodistribution studies showed that the number of plasmid DNA molecules surviving to transfect target cells after muscle injection was only a small fraction of the total DNA injected. The quest for higher immune responses led to a proliferation of different approaches for formulating DNA vaccines to protect the DNA from degradation and improve transfection efficiency. Strong constitutive promoters, such as CMVintA, were and are generally favored over regulated or endogenous eukaryotic promoters . Synthetic genes are likewise generally favored over endogenous viral or bacterial sequences to allow removal of negative regulatory sequences (e.g., inhibitory elements in HIV, late genes in HPV) and adapt codon usage to more closely reflect that of eukaryotes . Finally, high plasmid doses, up to multiple milligrams, now are being used in animal models and clinical trials .
Future potential to improve formulations may be facilitated by redesign of the plasmid itself. Minimal expression elements consisting of linear DNA comprising a promoter and gene, blocked at both ends with synthetic hairpin oligonucleotides to prevent degradation, were shown to be as potent as closed circle plasmids . Incorporation of a synthetic element has the potential to greatly facilitate the addition of different ligands and targeting moieties.
The Future
Today many DNA vaccines have becomed safe and immunogenic for animal models e.g. mice and nonhuman primates such as Rhesus Macaques.Fortunately,the safety and immunogenicity of an Ebola virus vaccine in its first phase I human study has been reported in November 2006. After some 16 years of DNA vaccine research,the effective new vaccines for cancer and infectious diseases are at the doorway eventually. |
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