What are DNA Vaccines?
DNA Vaccines

Genetic/ DNA immunization is a novel technique used to efficiently stimulate humoral and cellular immune responses to protein antigens. The direct injection of genetic material into a living host causes a small amount of its cells to produce the introduced gene products. This inappropriate gene expression within the host has important immunological consequences, resulting in the specific immune activation of the host against the gene delivered antigen (Koprowski et al, 1998).

Traditional Vaccines: The development of vaccination against harmful pathogenic microorganisms represents an important advancement in the history of modern medicine. In the past, traditional vaccination has relied on two specific types of microbiological preparations to produce material for immunization and generation of a protective immune response. These two categories involve either living infectious material that has been manufactured in a weaker state and therefore inhibits the vaccine from causing disease, or inert, inactivated, or subunit preparations.
Live attenuated vaccines stimulate protective immune responses when they replicate in the host. The viral proteins produced within the host are released into the extracellular space surrounding the infected cells and are then acquired, internalized and digested by scavenger cells that circulate the body. These cells are called antigen presenting cells (APCs) and include macrophages, dendritic cells, and B cells, which work together to expand immune response. The APCs recirculate fragments of the digested the antigen to their surface, attached to MHC class II antigens. This complex of foreign peptide antigen plus host MHC class II antigens forms part of
the specific signal with which APCs along with the MHC peptide complex, trigger the action of of immune cells, the T helper lymphocytes. The second part of the activation signal comes from the APCs themselves, which display on their cell surface constimulatory molecules along with MHC-antigen complexes. Both drive T call expansion and activation through interaction with their respective ligands, the T cell receptor complex (TCR) and the constimulatory receptors CD28/CTLA4, present on the the T cell surface. Activated T cells secrete molecules that act as powerful activates of immune cells. Also as viral proteins are produced within the host cells, small parts of these proteins surface, chaperoned by host cell MHC class I antigens. These complexes together are recognized by a second class of T cells, killer or cytotoxic cells. This recognition, along with other stimulation by APCs and production of cytokine by stimulated T cells, is responsible for the developments of mature cytotoxic T cells (CTL) capable of destroying infected cells. In most instances live infection induces life long immunity. Although live attenuated preparations are the vaccines of choice they do pose the risk of reversion to their pathogenic form, causing infection.
                                                                                                                                                                                                                                                                        Immune Response

In contrast, when inoculated nonlive vaccines composed of whole or even partial viruses are not produced within the host cells, they mainly end up in the extracellular space.  They provide protection by directly generating T helper and humoral immune responses against the pathogenic immunogen. In the absence of the cellular production of the foreign antigen, these vaccines are usually devoid of the ability to induce significant T cytotoxic responses. In addition, these vaccines are not actually produced in the host, and therefore, they are not customized by the host. The immunity induced by their vaccines frequently decreases during the life of the host and may require additional boosters to achieve lifelong immunity. However, nonlive vaccines offer some important advantages over live vaccines: they are produced earlier, and they can be designed to contain only the specific antigenic target of the pathogen that is involved in the development of protective immunity and exclude all other viral components.

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 (Encke, J. et al, 1999).                                                                                                                                                                                                                                                                    DNA Vaccines

DNA vaccine plasmid