Packaging Cell Lines for Generating Replication-Defective and Gutted Adenoviral Vectors

Gene transfer using adenovirus (Ad) can be a highly efficient method for transducing cells and tissues. Wild-type Ad has a 36-kb double-stranded linear genome that encodes numerous overlapping open reading frames (1 ). Because wild-type Ad is infectious to humans, vectors used in the laboratory generally contain a deletion of one or more genes to limit their growth and replication on nonpermissive cells. One drawback to Ad vectors is that infection of a target cell results in transfer not only of the desired transgene of interest, but also of the Ad genes. Expression of these other genes can be toxic, or in vivo, can elicit a potent host-immune response against transduced cells. Consequently, methods have been developed by many labs to engineer Ad vectors that display greatly reduced expression of viral genes except in specifically generated packaging cell lines designed for propagation of the vectors. In general, two types of alterations are made to the Ad genome. In the first, individual genes, or combinations thereof, are deleted from the Ad genome and placed into a packaging cell line under control of a constitutive or inducible promoter. In the second method, all viral genes are removed from the Ad vector, and vector growth is regulated by coinfection with a helper virus. The fully deleted, or gutted, Ad vectors must then be purified away from helper virus prior to use. Both of these strategies can also be combined, whereby one uses a multiply deleted Ad vector in a specialized complementing cell line as a helper virus to grow gutted vectors.