Conclusion
In this work, we have demonstrated the feasibility and applicability of
suspension adapted Vero cell cultures for the production of highly
relevant rVSV-based vaccines and vaccine candidates. For three rVSV
strains, namely rVSV-ZEBOV, rVSV-HIV and
rVSVInd-msp -SF-Gtc ,
production was successfully scaled-up to the bioreactor scale.Table 1 shows a summary of the six bioreactor runs, comparing
results for the three strains in two different culture media. Further,
proof-of-concept is provided that rVSV-ZEBOV and
rVSVInd-msp -SF-Gtc can be
produced in commercially available media in suspension adapted Vero
cells.
Process parameters developed in suspension Vero and previously in
adherent Vero cells for rVSV-ZEBOV (Kiesslich et al., 2020), have been
shown to be applicable to other strains. This is an important
observation as rVSV-ZEBOV can thus serve as a model virus for other rVSV
strains. More, this can be of significant value for the production of
future rVSV-based vaccine candidates against emerging infectious
diseases.
Moreover, the production of rVSV-ZEBOV was shown to be superior to
previously developed adherent processes in microcarrier and fixed-bed
bioreactors (Kiesslich et al., 2020). Due to the better scalability, the
suspension Vero system can serve as a viable alternative to the current
Ebola virus disease vaccine manufacturing using roller bottles.
Production of rVSV-ZEBOV was leading to higher infectious titers in
suspension cultures of HEK293-SF (Gélinas et al., 2019). However, this
system used commercially available media and bioprocesses developed with
years of experience. The commercially available MDXK medium has only
been on the market for a short period of time. Hence, there is great
potential for optimization of Vero suspension media and bioprocesses
specifically for virus production.
In the context of the current COVID-19 pandemic, this work shows
relevant advancement in the field of bioprocess development for urgently
needed vector-based vaccine candidates. Given that
rVSVInd-msp -SF-Gtc grows
to titers that are around 100-fold higher than titers of rVSV-ZEBOV in
the same system, and given that rVSV-ZEBOV produced in conventional
roller bottle processes has been approved as a vaccine candidate by
regulatory agencies, the herein presented bioprocess using suspension
adapted Vero cells can serve as a highly efficient system for
accelerated and scalable manufacturing of a COVID-19 vaccine candidate.
Further, the quality of the produced viruses in terms of the ratio total
particles to infectious particles is far superior, potentially leading
to facilitated downstream processes and ultimately very economical
manufacturing.
In the future, fed-batch and perfusion processes should be developed for
high cell density bioreactors. As indicated in shake flask experiments
and already demonstrated for VSV-GFP (Shen et al., 2019), these can be
approaches to further push the boundaries and to increase virus
productivities of the suspension Vero system.