Recombinant therapeutic proteins can be expressed in bacterial, plant, yeast, insect and mammalian cells. Animal cell culture is the optimal system for the expression of many therapeutic proteins (particularly glycoproteins) as protein folding, transport and processing follow the same pathways seen in the target organism. Furthermore, the glycosylation pattern of recombinant glyco-proteins expressed in mammalian cells more closely resembles that of native glycoprotein than if insect cells are used. This can have important therapeutic implications and is discussed in detail later (see Section 5.2.3).
Essentially, there are three methods whereby biological products can be expressed in mammalian cells:
• by transfection with a modified bacterial plasmid containing a strong mammalian gene expression promoter, polyadenylation signal (see 5.2.1a) and the gene of interest;
• by infection with a replication-competent or replication-incompetent recombinant viral vector engineered for a high level of expression of the desired protein (i.e: adenoviral, retroviral, alphaviral or vaccinia expression systems);
• by infection with a virus of interest, using a whole or modified viral genome (viral protein only).
Figure 5.1 illustrates the components of a mammalian expression system, based on use of a modified bacterial plasmid. The choice made of each component will impact upon the yield and therapeutic efficacy of the recombinant protein, as well as on the time and cost to obtain it, but it is important to recognize that each individual component is linked to another. For example, the choice of expression vector will determine the choice of host cell (or vice versa), and the amount of protein required will be related to not only vector and cell line, but also to the culture system employed. This review considers each of these components in turn to enable the correct choices to be made in order to obtain suitable quantities of recombinant protein for a particular application.
Medicines from Animal Cell Culture Edited by G. Stacey and J. Davis © 2007 John Wiley & Sons, Ltd
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