Proper expression vectors are crucial for immunogenicity of nucleic-acid-based vaccines. Ideally, they should have the following properties: (1) sufficient expression of the foreign gene; (2) prolonged expression; (3) selective expression in APC; (4) low immunogenicity by themselves; (5) no side effects; (6) allow regulation of expression in terms of on—off. Evidently, various vectors differ in these regards and sustained effort is being dedicated to improvement of mammalian expression vectors (Table 2.1). These efforts are fueled by the necessity to design superior delivery systems for gene therapy as well. However, it is important to stress that requirements for effective vectors in genetic vaccination may not be as stringent as in the case of gene therapy, simply because the levels of expression required for immunity are lower than those that lead to therapeutic effects. Thus, this discrepancy apparently defines a window of opportunity for genetic vaccination in terms of vectors: Nucleic-acid-based vectors, in spite of their lower expression level, may be of greater usefulness for genetic vaccination than most of the viral vectors associated with more side effects. We first discuss the functional structure of classical nucleic-acid-based vectors, followed by a brief comparison with other types of vectors and efforts to improve nucleicacid-based vectors.
KeywordsForeign Gene Cationic Lipid Cationic Liposome Semliki Forest Virus Genetic Immunization
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