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A number of RNA viruses have also been developed as vectors for foreign gene expression (Table 3.1). Vesicular stomatitis virus, a (–) strand RNA virus, has emerged as a candidate for vaccine delivery (e.g., ebolavirus and Zika virus vaccines). For production of vaccines, vesicular stomatitis virus is pseudotyped with glycoproteins from other viruses. For example, to produce an ebolavirus vaccine, the vesicular stomatitis virus glycoprotein gene is substituted with that from ebolavirus. Pseudotyped vesicular stomatitis virus also has applications in the research laboratory: these viruses were used to identify cell receptors in haploid cell lines as described above. The virus is well suited for viral oncotherapy because it reproduces preferentially in tumor cells, and recombinant vesicular stomatitis viruses have been engineered to improve tumor selectivity.

Retroviruses have enjoyed great popularity as vectors (Fig. 3.16) because their infectious cycles include the integration of a dsDNA copy of viral RNA into the cell genome, a topic of Chapter 10. The integrated provirus remains permanently in the cell’s genome and is passed on to progeny during cell division. This feature of retroviral vectors results in permanent modification of the genome of the infected cell. The choice of the envelope glycoprotein carried by retroviral vectors has a significant impact on their tropism. The vesicular stomatitis virus G glycoprotein is often used because it confers a wide tissue tropism. Retrovirus vectors can be targeted to specific cell types by using envelope proteins of other viruses.

An initial problem encountered with the use of gammaretrovirus vectors (e.g., Moloney murine leukemia virus) is that the DNA of these viruses can be integrated efficiently only in actively dividing cells. Another important limitation of the murine retrovirus vectors is imposed by the phenomenon of gene silencing, which represses foreign gene expression in certain cell types, such as embryonic stem cells. An alternative approach is to use viral vectors that contain sequences from human immunodeficiency virus type 1 or other lentiviruses, which can infect nondividing cells and are less severely affected by gene silencing.

Figure 3.16 Retroviral vectors. The minimal viral sequences required for retroviral vectors are 5′- and 3′-terminal sequences (yellow and blue, respectively) that control gene expression and packaging of the RNA genome. The foreign gene (blue) and promoter (green) are inserted between the viral sequences. To package this DNA into viral particles, it is introduced into cultured cells with plasmids that encode viral proteins required for encapsidation, under the control of a heterologous promoter and containing no viral regulatory sequences. No wild-type viral RNA is present in these cells. If these plasmids are introduced alone, virus particles that do not contain viral genomes are produced. When all three plasmids are introduced into cells, retrovirus particles that contain only the recombinant vector genome are formed. The host range of the recombinant vector can be controlled by the type of envelope protein. Envelope protein from amphotropic retroviruses allows the recombinant virus to infect human and mouse cells. The vesicular stomatitis virus glycoprotein G allows infection of a broad range of cell types in many species and also permits concentration with simple methods.