recombinant-antigen expression. Other chapters will provide more in-depth review

of key technologies.

4.2.2.1

Cell Banks

Creating a cell bank of the cell line represents the cornerstone of the process de-

velopment and manufacturing. The cell bank provides the process with the same

source for each production lot. It can be extensively defined and tested. In the case

of a recombinant expression system such as CHO, the cell bank can be defined with

respect to its genetic properties (including transgene copy number and integration

site). In the case of a cell line for virus propagation, the capacity for virus infection

and replication can be defined. All cell banks are extensively tested for safety.

These include detailed testing programs to document the absence of adventitious

agents (see Sections 4.3 and 4.4).

Cell banks are usually established as a two-tier system. This approach was first

proposed in 1963, when the cryopreservation methods used were based on glycerol

or DMSO [7]. The two-tier system consists of a master cell bank (MCB) and a

working cell bank (WCB) derived from the MCB after few passages. The MCB is

usually prepared from a clone or material obtained from a well-known bioresource

archive such as ATCC or ECACC. Cell lines can also be licensed. The two-tier

system generates a huge capacity for development and further commercial manu-

facturing. Indeed, each vial from the MCB can generate a new WCB. Hence

manufacturing from the same MCB can continue for decades. Therefore, the long-

term strategy with respect to the vaccine volumes required and other potential

technical advances need to be considered by the manufacturer when the cell bank is

established. A third cell bank, termed the end of production cells (EoPC), is also

usually established. The purpose of this cell bank is to expand cells beyond the limit

of the passage used in vaccine production.

MCB

WCB

Production Passage

EoPC

Hence, the three types of cell bank can be used to test the consistency of the bio-

logical system across the production framework; testing that is critical for the re-

lease of clinical lots or commercial vaccine batches.

4.2.2.2

Viral Seeds

For many viral vaccines, viral seeds are produced as well. Viral seeds are produced

for live attenuated vaccines as well as inactivated vaccines. For certain recombinant

vaccines, viral seeds are also produced for expression systems like the baculovirus

expression vector system (BEVS), an adenovirus platform or a vaccinia re-

combinant system.

Viral seeds are also established as a two-tier system. This system consists of the

master viral seed (MVS) and the working viral seed (WVS). A third viral seed is

also prepared at the production passage or beyond, i.e., the post-production seed

(PPS). The GMP production framework is similarly defined as follows:

MVS

WVS

Production Passage

PPS

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Bioprocessing of Viral Vaccines