these factors might equally play a role for the design of the corresponding manu-

facturing process.

5.4

CELL LINES FOR VIRUS PRODUCTION

5.4.1

SELECTION OF HOST CELL LINES

To produce a new potential vaccine candidate based on whole viruses or on one of

the available or even new viral vectors, virologists typically use small-scale vessels,

not too much focused on the host cell line as long as titers are high enough for the

studies required. Thus, often adherent “standard cell lines” such as Vero, A549, or

HEK293 cells as well as diploid cell lines like MRC5 or even primary cells like

chicken embryo fibroblasts (CEFs) are used (see Table 5.2). Human cell lines are

often used as diagnostic tools, but a problem for their use in vaccine production is

the presence of adventitious agents with direct link to humans, which would require

very costly and comprehensive testing and is often considered not to be acceptable

for manufacturing of vaccines for human use.

Thus, latest, once the viral vaccine candidate is found, it should be considered,

which host cell line is suitable for production. For a more detailed discussion on cell

lines see chapter 4. Typically, a vaccine is administered to a healthy person who

does not accept any severe side effects of vaccination. Thus, in contrast to a patient

suffering from a disease, the hurdle of acceptance is much more stringent and

vaccine production has to comply to extremely high-quality safety and efficacy

standards. This also results in a very conservative approach towards the adaptation

of new technologies in cell culture-based vaccine production. For example, con-

ventional batch processing is still performed for the majority of vaccine manu-

facturing processes despite the clearly demonstrated advantages of advanced

production technologies including fed-batch or perfusion cultivations. Also, cell

line selection seems unnecessary restricted for historical reasons. This applies, in

particular for baby hamster kidney (BHK21) or Chinese hamster ovary (CHO) cells

that are still considered as a “no go” as early evaluations and risk assessments have

identified these cell lines as possible tumorigenic/cancerogenic. These cell lines

could be used in veterinary vaccine manufacturing but not for human vaccines

covered in this chapter. However, this circumstance is actually strongly discussed in

the research community, as the tumorigenicity/cancerogenicity potential of cell

preparations tested is only one of many factors that need to be considered to

evaluate the safety of a vaccine. Perhaps more important is the extent to which USP/

DSP eliminate cellular factors of concern [9]. Today, not only powerful analytical

tools, but also highly efficient DSP methods are available for characterization and

purification of cell culture-derived vaccines. Accordingly, a comprehensive re-

evaluation of safety aspects for use of these cell lines including process data might

be considered. Especially, suspension BHK21 cells would be very interesting

candidates for human vaccine production as for many viruses and viral vectors very

high titers can be achieved in bioreactors. However, such a re-evaluation of cell

substrates is very costly and time will show if commercial aspects and society needs

are strong enough to attract the required investments.

Upstream processing for viral vaccines

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