???What are DNA vaccines

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Genetic Immunization: Since its early applications in the 1950's, DNA-based immunization has become a novel approach to vaccine
development. Direct injection of naked plasmid DNA induces strong immune responses to the antigen encoded by the gene vaccine. Once the plasmid DNA construct is injected the host cells take up the foreign DNA, expressing the viral gene and producng the corresponding viral protein inside the cell. This form of antigen presentation and processing induced both MHC and class I and class II restricted cellular and humoral immune responses.
• History: The use of genetic material to deliver genes for therapeutic purposes has been practiced for many years. Experiments outlining the transfer of DNA into cells of living animals were reported as early as 1950. Later experiments using purified genetic material only further confirmed that the direct DNA gene injection in the absence of viral vectors results in the expression of the inoculated genes in the host. There have been additional experiments that extend these findings to recombinant DNA molecules, further illustrating the idea that purified nucleic acids could be directly delivered into a host and proteins would be produced. In 1992, scientists Tang and Johnson reported that the delivery of human growth hormone in a expression cassette in vivo resulted in production of detectable levels of the growth hormone in host mice. They also found that these inoculated mice developed antibodies against the human growth hormone; they termed this immunization procedure "genetic immunization", which describes the ability of inoculated genes to be individual immunogens .
DNA Vaccines
• Construction: DNA vaccines are composed of a bacterial plasmids. Expression plasmids used in DNA-based vaccination normally contain two unites: the antigen expression unit composed of promoter/enhancer sequences, followed by antigen-encoding and polyadenylation sequences and the production unit composed of of bacterial sequences necessary for plamid amplification and selection . The construction of bacterial plasmids with vaccine inserts is accomplished using recombinant DNA technology. Once constructed, the vaccine plasmid is transformed into bacteria, where bacterial growth produces multiple plasmid copies. The plasmid DNA is then purified from the bacteria, by separating the circular plasmid from the much larger bacterial DNA and other bacterial impurities. This purifies DNA acts as the vaccine .

DNA vaccine plasmid
• Administration- Over the past decade of clinical research and trials, several possible routs of plasmid delivery have been found. Successful immunization has been demonstrated after delivery of plasmids through intramuscular, intradermal and intravenous injection. The skin and mucous membranes being considered the best site for immunization due to the high concentrations of dendritic cells (DC), macrophages and lymphocytes. Intradermal injection of DNA-coated gold particles with a gene gun have been used. The plasmid DNA can be diluted in distilled water, saline or sucrose. There has also been positive demonstration of proinjection or codelivery with various drugs.
• Mechanisms: A plasmid vector that expresses the protein of interest (e.g. viral protein) under the control of an appropriate promoter is injected into the skin or muscle of the the host. After uptake of the plasmid, the protein is produced endogenously and intracellularly processed into small antigenic peptides by the host proteases. The peptides then enter the lumen of the endoplasmic reticulum (E.R.) by membrane-associated transporters. In the E.R., peptides bind to MHC class I molecules. These peptides are presented on the cell surface in the context of the MHC class I. Subsequent CD8+ cytotoxic T cells (CTL) are stimulated and they evoke cell-mediated immunity. CTLs inhibit viruses through both cytolysis of infected cells and noncytolysis mechanisms such as cytokine production .
The foreign protein can also be presented by the MHC class II pathway by APCs which elicit helper T cells (CD4+) responses. These CD4+ cells are able to recognize the peptides formed from exogenous proteins that were endocytosed or phagocytosed by APC, then degraded to peptide fragments and loaded onto MHC class II molecules. Depending on the the type of CD4+ cell that binds to the complex, B cells are stimulated and antibody production is stimulated. This is the same manner in which traditional vaccines work.
DNA Vaccine Mechanism
• Advantages: DNA immunization offers many advantages over the traditional forms of vaccination. It is able to induce the expression of antigens that resemble native viral epitopes more closely than standard vaccines do since live attenuated and killed vaccines are often altered in their protein structure and antigenicity. Plasmid vectors can be constructed and produced quickly and the coding sequence can be manipulated in many ways. DNA vaccines encoding several antigens or proteins can be delivered to the host in a single dose, only requiring a microgram of plasmids to induce immune responses. Rapid and large-scale production are available at costs considerably lower than traditional vaccines, and they are also very temperature stable making storage and transport much easier. Another important advantage of genetic vaccines is their therapeutic potential for ongoing chronic viral infections. DNA vaccination may provide an important tool for stimulating an immune response in HBV, HCV and HIV patients. The continuos expression of the viral antigen caused by gene vaccination in an environment containing many APCs may promote successful therapeutic immune response which cannot be obtained by other traditional vaccines . This is a subject that has generated a lot of interest in the last five years.
• Limitations: Although DNA can be used to raise immune responses against pathogenic proteins, certain microbes have outer capsids that are made up of polysaccharides. This limits the extent of the usage of DNA vaccines because they cannot substitute for polysaccharide-based subunit vaccines.
• Future- It has recently been discovered that the transfection of myocytes can be amplified by pretreatment with local anesthetics or with cardiotoxin, which induce local tissue damage and initiate myoblast regeneration. Gaining a full understanding of this mechanism of DNA uptake could prove helpful in improving applications for gene therapy and gene vaccination. Both improved expression and better engineering of the DNA plasmid may enhance antibody response to the gene products and expand the applications of the gene vaccines
 
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