Media Development for Viral Production
The base medium then was tested for viral production both at the flask and bioreactor level. Although the medium optimized in the experiments described above supported robust cell growth, we felt there was room for improvement in the production of infectious virus particles.
As factors that improve growth are not necessarily the same factors that will improve viral production, investigations into a two-part media system were initiated; one formulation would be used for cell growth and a different formulation would be used for viral propagation.
Additional optimization efforts concentrated on identifying key components that inhibit or increase viral production. Secondly, process issues were investigated, such as cell density at the time of infection, the timing and ratio of the production medium, and the addition of different feeds at the time of infection.
As many vaccines currently being produced in chicken eggs and CEFs are pox-based viruses, modified vaccinia ankara (MVA) was used as one of the model systems to investigate viral production. We found that the cell density at the time of infection did not appear to be as important in determining viral yields as the overall health of the cells (i.e., good growth of the cells following infection correlated with good viral titers).
Consistent with this, there was not a significant effect on the viral titers based on the time of the addition of the production media. Most importantly, as shown in Figure 2, the addition of feeds at the time of infection greatly increased the viral titers. The base formulations are the growth and production media in the absence of feeds, which result in a peak MVA viral titer of 7.2logTCID50/mL.
Feed 1 increased the viral titer by approximately one log, but more importantly, the addition of Feed 2 increased the viral titer by a little more than two logs, resulting in a peak viral titer greater than 9logTCID50/mL. This level of viral titer is more than competitive with the methods currently employed by vaccine manufactures. Furthermore, high viral titers can also be reached with other viruses, such as A and B strains of human influenza (Figure 3).