Principles of Virology, Volume 1. Jane Flint

Table 3.1 Some viral vectors

Virus	Insert size	Integration	Duration of expression	Advantages	Potential disadvantages
Adeno-associatedvirus	~5 kb	No	Long	Nonpathogenic, episomal, infects nondividing and dividing cells, broad tropism, low immunogenicity	Small transgene capacity, helper virus needed for vector production
Adenovirus	~8–38 kb	No	Short	Broad tropism, efficient gene delivery, infects nondividing and dividing cells, large cargo capacity	Transient, immunogenic, high levels of preexisting immunity
Baculovirus	No known upper limit	No	Short	High levels of protein synthesis, recombinant viruses easily made, more than one protein can be made in same cells	Insect cells typically used, no replication in mammalian cells, human type protein glycosylation not 100% efficient, paucimannose structures present
Gammaretroviru s (murine leukemia virus)	8 kb	Yes	Short	Stable integration, broad tropism possible via pseudotyping, low immunogenicity, low preexisting immunity	Risk of insertional mutagenesis, poor infection of nondividing cells, faulty reverse transcription
Herpes simplex virus	~50 kb	No	Long in central nervous system, short elsewhere	Infects nondividing cells, large capacity, broad tropism, latency	Virulence, persistence in neurons, high levels of preexisting immunity, may recombine with genomes in latently infected cells
Lentivirus	9 kb	Yes	Long	Stable integration, transduces nondividing and dividing cells	Potential insertional mutagenesis; none detected in clinical trials
Rhabdovirus	~4.5 kb	No	Short	High-level expression, rapid cell killing, broad tropism, lack of preexisting immunity	Virulence, highly cytopathic, neurotropism, immunogenic
Vaccinia virus	~30 kb	No	Short	Wide host range, ease of isolation, large capacity, high-level expression, low preexisting immunity	Transient, immunogenic

Figure 3.14 Adenovirus vectors. High-capacity adenovirus “gutless” vectors contain only the origin-of-replication-containing inverted terminal repeats (ITR), the packaging signal (blue arrows), the viral E4 transcription unit (red arrow), and the transgene with its promoter. Additional DNA flanking the foreign gene must be inserted to allow packaging of the viral genome (not shown). A helper virus (bottom) is required to package the recombinant vector genome. Two loxP sites for cleavage by the Cre recombinase have been introduced into the adenoviral helper genome (red arrowheads). Infection of cells that produce Cre leads to excision of sequences flanked by the loxP sites so that the helper genome is not packaged.

Adenovirus vectors were originally developed for the treatment of cystic fibrosis because of the tropism of the virus for the respiratory epithelium. Adenovirus can infect terminally differentiated cells, but only transient gene expression is achieved, as infected cells are lysed. Yields of particles are high and these viruses can infect many replicating and non-replicating cell types. In the earliest vectors that were designed, foreign genes were inserted into the E1 and/or E3 regions. As these vectors had limited capacity, genomes with minimal adenovirus sequences have been designed (Fig. 3.14). This strategy allows up to 38 kb of foreign sequence to be introduced into the vector. In addition, elimination of most viral genes reduces cytotoxicity and the host immune response to viral proteins, simplifying multiple immunizations. Considerable efforts have been made to modify the adenovirus capsid to target the vectors to different cell types. For example, the fiber protein, which mediates adenovirus binding to cells, has been altered by insertion of ligands that bind particular cell surface receptors. Such alterations could increase the cell specificity of adenovirus attachment and the efficiency of gene transfer, thereby decreasing the dose of virus that need be administered.

Adenovirus-associated virus has attracted much attention as a vector for gene therapy. This virus requires a helper virus for replication; in its absence the genome remains episomal and persists, in some cases with high levels of expression, in many different tissues. There has been increasing interest in these vectors to target therapeutic genes to smooth muscle and other differentiated tissues, which are highly susceptible and support sustained high-level expression of foreign genes. Although the first-generation adenovirus-associated virus vectors were limited in the size of inserts that could be transferred, other systems have been developed to overcome the limited genetic capacity (Fig. 3.15). The cell specificity of adenovirus-associated virus vectors has been altered by inserting receptor-specific ligands into the capsid. In addition, many new viral serotypes that vary in their tropism and ability to trigger immune responses have been identified or generated.

      Vaccinia virus and other animal poxvirus vectors offer the advantages of a wide host range, a genome that accepts very large fragments, high expression of foreign genes, and relative ease of preparation. Foreign DNA is usually inserted into the viral genome by homologous recombination, using an approach similar to that described for marker transfer. Because of the relatively low pathogenicity of the virus, poxvirus recombinants have been considered candidates for human and animal vaccines.

      Baculoviruses, which infect arthropods, have large circular dsDNA genomes. These viruses have been modified to become versatile and powerful vectors for the production of proteins for research and clinical use. The general approach is to replace the viral polyhedron gene with the gene of interest. Recombinant viruses are produced in E. coli using a bacmid vector that harbors the baculovirus genome. The gene to be introduced is inserted into the baculovirus genome by recombination. Strong viral promoters are used to obtain high levels of protein production. Recombinant baculoviruses are obtained after transfection of bacmids into insect cells and have been used for protein production for research purposes and for large-scale synthesis for commercial uses. Examples include the influenza virus vaccine FluBlok, which consists of the viral HA proteins produced in insect cells via a baculovirus vector, and porcine circovirus 2 vaccine for the prevention of fatal disease in swine.

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