Besides the total number of cells at time of infection (TOI), the maximum total

number of cells reached, the total number of virus particles, and/or the total number

of infectious units, the CSVY is an important parameter to allow for a comparison

of the performance of different cell lines at different cultivation conditions. As both

cell concentration and titer measurements have a relatively high error, interpreting

small differences might be challenging and not relevant. The same applies to de-

rived parameters (like the CSVY) that involve error propagation. Nevertheless, if

proper controls are used and assays are carried out carefully with reliable and

precise techniques and in a systematic manner, the CSVY is a very good indicator

for success in process optimization.

The space-time yield (STY) [virions/RV/d] is the amount of virus particles

produced taking into account only the working volume of the bioreactor harvested

(RV) and the total process time [d]. This performance indicator allows the com-

parison of different production platforms of various sizes. Whereas the volumetric

virus productivity (VVP) [virions/L/d] is the amount of virus particles produced

considering the total volume of spent medium (Vtot) [L] during cell growth and virus

replication phase and the total process time (ttot) [d]. As before, for meaningful

comparisons, assay limitations and error propagation have to be taken into account.

6.4

FROM BATCH CULTURES TO INTENSIFIED PROCESSES

Generally, bioreactors are operated in discontinuous modes (batch, repeated batch,

or fed-batch), continuous modes (chemostat), or perfusion modes [17]. In con-

ventional cell culture-derived vaccine production, bioreactors are mainly operated

in batch mode with STRs operated as a closed system with no addition of fresh

media. Cells are usually grown to relatively low cell concentrations (1–2E06 cells/

mL, maximum 5E06 cells/mL for suspension cells) and subsequently infected

(Figure 6.2A). The ease of implementation, process robustness combined with low

Time

Concentration

(a)

(b)

(c)

cX

wv

wv

wv

cVir

cX

cVir

cS

cS

cX

cVir

cS

Infection

Infection

Time

Concentration

Feed

Feed

Infection

Time

Start

perfusion

Concentration

FIGURE 6.2 Schematic representation of virus production processes in batch (a), fed-batch

(b), and perfusion (c) mode. Concentration profiles of various process parameters: wv, working

volume; cVir, virus particle concentration; cS, substrate concentration (glucose), cX, cell con-

centration. Dashed vertical line shows TOI. For batch and fed-batch at TOI a medium exchange/

dilution step by addition of medium results in a decrease of cell concentration and an increase in

substrate level.

Process intensification

143