Nucleic-acid- and cell-based therapeutics - Pharmaceutical Biotechnology: Concepts and Aapplications - page 429

Packing cell

g p e

TG

g p e

TG

Assembled retroviral vectors
harboring the target gene
which exit the cell

TG

TG TG

Figure 14.5 The use of packing cells to generate replication-defi cient retroviral vectors. The packaging cell
is an engineered animal cell into which the retroviral gag (g), pol (p) and env (e) genes have been introduced.
The cell line chosen must be one which the (replication-defi cient) virus can infect. The engineered retroviral
vector genome (which is carrying the target gene; TG) is then incubated with the packing cell. This results
in the generation and assembly of mature replication-defi cient retroviral vector particles. These exit the cell
and will replicate by entering other packaging cells. By completing a number of such replication cycles, large
quantities of the desired retroviral vectors are produced

The fact that they have been well studied, display almost 100 per cent transduction effi cacy
in sensitive cells and that the transferred genes are usually subject to long-term, fairly high-level
expression renders retroviruses powerful potential vectors. These advantages form the basis of
their widespread use in this regard.
However, many of their other characteristics serve to curtail the application of retroviruses as
gene therapy vectors. From a practical standpoint, retroviral vectors are relatively labile. Thus,
although retroviruses are relatively easy to propagate, they are often damaged by subsequent puri-
fi cation and concentration, which are steps essential for their clinical use. In most instances, their
ability to infect only dividing cells clearly restricts their use. Their lack of selectivity in terms
of the dividing cell types they infect is also a disadvantage. They will not infect all dividing cell