Advantages and Disadvantages of Adenoviral Vectors

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The unique advantage of the adenoviral expression system is its versatility. Once constructed, a single vector can be used for in vitro protein expression and purification, studies of the effect of the gene product on cell biology, or in vivo studies in many tissue types and a number of different species. For these reasons, adenovirus is becoming the system of choice for work involving both in vitro and in vivo studies.

The investigator has the option of using either well-characterized viral regulatory sequences or a heterologous promoter, which should allow expression to be tailored to the requirements of the particular system of interest. The large (and growing) capacity of Ad vectors allows expression of large genes or of a combination of genes, as in expression of both subunits of a heterodimeric protein or of an antigenic protein together with a cytokine. Finally, the wide tropism and ability to infect nondividing cell types allows Ad vectors to be used in many cases where other vectors, such as retroviruses, cannot be used.

The major disadvantage of this system is the complexity of vector construction. A recombinant vector for Ad-based expression of a protein can take considerably longer to produce than a plasmid for use in a prokaryotic expression system or simple transfection into eukaryotic cells. However, techniques for adenovirus construction have improved dramatically. The plasmid-based systems now available for construction of standard El - or E3-deleted vectors are quite straightforward, and the effort required for the construction of recombinant adenoviruses is similar to that needed to construct recombinant baculoviruses.

As Ad DNA does not normally integrate into host cell chromosomes, the expression of the transgene is transient. This is of little consequence in short-term or in vitio experiments, but may limit some long-term in vivo studies.

Although Ad vectors are being used successfully in vivo to study protein function, there are still problems associated with the host response to the vector itself. Following the administration of currently available Ad vectors, antivector immune responses are generated that can lead to inflammation, elimination of the infected cells, and short duration of transgene expression. This limits long-term studies in immunocompetent animals. Infection even by replication-defective Ads also affects the endogenous signal transduction pathways (Li et al., (1998) and can lead to responses in the infected cells, such as the production of cytokines. This can potentially complicate the interpretation of studies of protein expression in vivo. This type of effect can often be controlled for by administration of an Ad vector lacking an insert.

Finally, although Ad vectors have been found to infect a wide variety of cells in several species, including human, mouse, rat, and chicken, not all cells of interest may express the appropriate integrin and CAR proteins to allow infection. The repertoire of integrin and fiber receptor expression on the target cell type should therefore be considered when planning in vivo work using Ad vectors (Table 2).

Table 2

Advantages and Disadvantages of Adenoviral Vectors

Advantages

Disadvantages

Versatility; recombinant protein expressed from same vector both in vitro and in vivo High capacity for (multiple) DNA insertion Ability to use wide variety of cellular promoters in Ad recombinants High-level protein expression via late Ad regulatory elements Ease of producing high-titer viral stocks Wide tropism of Ad includes many nondividing cells

Complexity of recombinant

Ad construction Expression is generally transient In vivo antivector immune response Current Ad vectors cannot be specifically targeted in vivo

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