Bioreactor concepts for cell culture-based viral vaccine production

Abstract

Vaccine manufacturing processes are designed to meet present and upcoming challenges associated with a growing vaccine market and to include multi-use facilities offering a broad portfolio and faster reaction times in case of pandemics and emerging diseases. The final products, from whole viruses to recombinant viral proteins, are very diverse, making standard process strategies hardly universally applicable. Numerous factors such as cell substrate, virus strain or expression system, medium, cultivation system, cultivation method, and scale need consideration. Reviewing options for efficient and economical production of human vaccines, this paper discusses basic factors relevant for viral antigen production in mammalian cells, avian cells and insect cells. In addition, bioreactor concepts, including static systems, single-use systems, stirred tanks and packed-beds are addressed. On this basis, methods towards process intensification, in particular operational strategies, the use of perfusion systems for high product yields, and steps to establish continuous processes are introduced.

Keywords:

• animal cell culture
• bioreactor systems
• continuous processes
• large-scale vaccine production
• process improvement
• process intensification
• viral vaccines

Financial & competing interests disclosure

LE Gallo-Ramírez received financial support from CONACYT Mexico as an international postdoctoral fellow (Grant 208266). The authors have no other relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript apart from those disclosed.

Key issues

• Prior to establishment of a vaccine production process, specific aspects have to be considered to choose the appropriate cultivation technology and operation mode. These are, inter alia, the choice of the vaccine type, a full understanding of cell substrate and virus propagation requirements, as well as the projected number of required vaccine doses to ensure economic and technical viability.

• Despite their limitations in process monitoring and control, static cultivation systems still represent an economic platform for seed preparation or even production of human vaccines. Mechanized support and automatic handling allow large-scale application of roller bottles and multilayer systems.

• Bioreactors allow vaccine production under fully controlled and monitored conditions, ensuring high batch-to-batch consistency, and processes sterility. In addition, the risk of operator errors can be reduced due to automation. Conventional stainless steel stirred tanks as well as single-use bioreactors have a great scale-up potential, which is crucial to meet increasing vaccine demands.

• Cell culture-based vaccine production employing bioreactors has expanded manufacturing capacities in terms of larger volumes, shorter response time, lower costs and higher process control, while ensuring product quality.

• Microcarrier technology has enabled large-scale cultivation of adherent cells in quasi-suspension conditions and facilitates media exchange as well as certain infection strategies. However, process scale-up demands laborious bead-to-bead transfers, while process intensification is primarily restricted to volume expansion.

• Packed-bed bioreactors representing a cost–efficient platform typically enable cell seeding at low concentrations but restrict process monitoring. Single-use systems constitute a flexible option, whereas oxygen input is often the limiting factor for porous materials.

• Suspension culture reduces process complexity, facilitates large-scale processes and offers various options for process intensification. Therefore, it will remain the preferred cultivation method. However, a larger portfolio of high-yield virus producing suspension cell lines that grow in serum-free media would be desirable.

• Perfusion systems allow to achieve cell densities higher than 1 × 10⁸ cells/ml which results in significant volume reductions in seed and production bioreactors. The use of this technology in commercial vaccine manufacturing should result in considerable cost savings.

• Continuous vaccine manufacturing shows defective inferring particles accumulation, which, so far, limits viral vaccine yields. In addition, high mutation rates of cells and viruses might restrict its use to certain limited passage numbers. However, establishment of continuous bioprocesses for cultivation of cells expressing recombinant vaccines may be a viable option.

• Disposable technology has expanded bioprocess potential and flexibility by reduced investment cost, shorter validation times and saved facility space. Single-use bioreactors can contribute to fulfill present and upcoming demands in vaccine production and represent therefore a seminal alternative to traditional stainless steel tanks.