On its surface the cell displays the foreign antigen with both histocompatibility complex MHC classes I and class II molecules. The antigen-presenting cell then travels to the lymph nodes and presents the antigen peptide and costimulatory molecule signaled by T-cell, initiating the immune response. Immunogens can be targeted to various cellular compartments to improve antibody or cytotoxic T-cell responses. Secreted or plasma membrane -bound antigens are more effective at inducing antibody responses than cytosolic antigens, while cytotoxic T-cell responses can be improved by targeting antigens for cytoplasmic degradation and subsequent entry into the major histocompatibility complex MHC class I pathway.
The conformation of the protein can also affect antibody responses. The two most popular approaches are injection of DNA in saline , using a standard hypodermic needle and gene gun delivery. This can be assisted by electroporation ;  by temporarily damaging muscle fibres with myotoxins such as bupivacaine ; or by using hypertonic solutions of saline or sucrose.
Gene gun delivery ballistically accelerates plasmid DNA pDNA that has been absorbed onto gold or tungsten microparticles into the target cells, using compressed helium as an accelerant. The delivery method determines the dose required to raise an effective immune response.
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Mice for example, require approximately 10 times less DNA than primates. Alternatives included aerosol instillation of naked DNA on mucosal surfaces, such as the nasal and lung mucosa ,  and topical administration of pDNA to the eye  and vaginal mucosa.
Tested in mice, the hybrid vector was found to induce immune response. Using this technique, potentially all the genes from a pathogen can be delivered at one time, which may be useful for pathogens that are difficult to attenuate or culture. This has been tested with Mycoplasma pulmonis , a murine lung pathogen with a relatively small genome. Even partial expression libraries can induce protection from subsequent challenge.
DNA immunization can raise multiple T H responses, including lymphoproliferation and the generation of a variety of cytokine profiles. The type of T-cell help raised is influenced by the delivery method and the type of immunogen expressed, as well as the targeting of different lymphoid compartments. How these different methods operate, the forms of antigen expressed, and the different profiles of T-cell help is not understood. However, evidence shows no dose-related differences in TH type. Polarisation in T-cell help is useful in influencing allergic responses and autoimmune diseases.
In autoimmune diseases, the goal is to shift the self-destructive TH1 response with its associated cytotoxic T cell activity to a non-destructive TH2 response. This has been successfully applied in predisease priming for the desired type of response in preclinical models  and is somewhat successful in shifting the response for an established disease. One of the advantages of DNA vaccines is that they are able to induce cytotoxic T lymphocytes CTL without the inherent risk associated with live vaccines.
CTL responses can be raised against immunodominant and immunorecessive CTL epitopes,  as well as subdominant CTL epitopes,  in a manner that appears to mimic natural infection. This may prove to be a useful tool in assessing CTL epitopes and their role in providing immunity. This was successfully demonstrated using recombinant vaccinia viruses expressing influenza proteins,  but the principle should also be applicable to DNA vaccines.
Targeting antigens for intracellular degradation and thus entry into the MHC class I pathway by the addition of ubiquitin signal sequences, or mutation of other signal sequences, was shown to be effective at increasing CTL responses.
Antibody responses elicited by DNA vaccinations are influenced by multiple variables, including antigen type; antigen location i. Humoral responses after a single DNA injection can be much longer-lived than after a single injection with a recombinant protein. Antibody responses against hepatitis B virus HBV envelope protein HBsAg have been sustained for up to 74 weeks without boost, while lifelong maintenance of protective response to influenza haemagglutinin was demonstrated in mice after gene gun delivery.
Comparisons of antibody responses generated by natural viral infection, immunization with recombinant protein and immunization with pDNA are summarised in Table 4. DNA-raised antibody responses rise much more slowly than when natural infection or recombinant protein immunization occurs. As many as 12 weeks may be required to reach peak titres in mice, although boosting can decrease the interval.
This response is probably due to the low levels of antigen expressed over several weeks, which supports both primary and secondary phases of antibody response.
DNA vaccine expressing HBV small and middle envelope protein was injected into adults with chronic hepatitis. The vaccine resulted in specific interferon gamma cell production. Also specific T-cells for middle envelop proteins antigens were developed.
The immune response of the patients was not robust enough to control HBV infection . Additionally, the titres of specific antibodies raised by DNA vaccination are lower than those obtained after vaccination with a recombinant protein. However, DNA immunization-induced antibodies show greater affinity to native epitopes than recombinant protein-induced antibodies.
In other words, DNA immunization induces a qualitatively superior response. Antibodies can be induced after one vaccination with DNA, whereas recombinant protein vaccinations generally require a boost. DNA immunization can be used to bias the TH profile of the immune response and thus the antibody isotype, which is not possible with either natural infection or recombinant protein immunization. Antibody responses generated by DNA are useful as a preparative tool. For example, polyclonal and monoclonal antibodies can be generated for use as reagents.
When DNA uptake and subsequent expression was first demonstrated in vivo in muscle cells,  these cells were thought to be unique because of their extensive network of T-tubules. Using electron microscopy , it was proposed that DNA uptake was facilitated by caveolae or, non-clathrin coated pits. Two theories dominate — that in vivo uptake of DNA occurs non-specifically, in a method similar to phago - or pinocytosis ,  or through specific receptors.
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Macrophage scavenger receptors bind to a variety of macromolecules, including poly ribonucleotides and are thus candidates for DNA uptake. Gene gun delivery systems, cationic liposome packaging, and other delivery methods bypass this entry method, but understanding it may be useful in reducing costs e.
Studies using chimeric mice have shown that antigen is presented by bone-marrow derived cells, which include dendritic cells, macrophages and specialised B-cells called professional antigen presenting cells APC. Cross-priming occurs when a bone marrow-derived cell presents peptides from proteins synthesised in another cell in the context of MHC class 1. This can prime cytotoxic T-cell responses and seems to be important for a full primary immune response. In the skin, keratinocytes, fibroblasts and Langerhans cells take up and express antigens and are responsible for inducing a primary antibody response.
Transfected Langerhans cells migrate out of the skin within 12 hours to the draining lymph node where they prime secondary B- and T-cell responses. In skeletal muscle striated muscle cells are most frequently transfected, but seem to be unimportant in immune response. DNA vaccination generates an effective immune memory via the display of antigen-antibody complexes on follicular dendritic cells FDC , which are potent B-cell stimulators.belgacar.com/components/enlever/chargeur-tlphone-camra-espion-hd.php
DNA vaccination - Wikipedia
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