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.