Traditional vaccine technology has used eggs to manufacture vaccines for common diseases including influenza, a process which is inexpensive and well established. As the viruses replicate in the ‘egg factories’, mutations can arise, impacting the effectiveness of the vaccine. Compared with the these egg-based manufacturing techniques, mammalian cell-based production process offers greater stability of DNA sequences and enhanced quality control.

Using a mammalian cell-based system, there is no limitation on the availability of eggs and avoids supply chain risk. Producer cells used to manufacture vaccines are stored frozen, banked, to assure a readily scalable supply of cells for vaccine production. This enhanced turn around speed can be crucial during a pandemic, where time means lives saved. In addition, when the production schedule is tight, it can also provide a first-market advantage for the vaccine manufacturers.

Types of Vaccine Suited to Mammalian Cell Production

Recombinant sub-unit vaccines

The advantage of mammalian cell lines for the production of recombinant sub-unit vaccines is their ability to produce secreted proteins as protective antigens in serum-free media at high levels and robustly, and the ability to introduce correct protein folding, post-translational modifications and product assembly into the produced proteins. The complete biological activity of the protein makes it an effective vaccine candidate.

There was a highly relevant paper published in the journal Vaccine by Nyon et al (2018) on sub-unit vaccine production in engineered stable CHO cell line for MERS-Coronavirus

Monoclonal antibody vaccines

Monoclonals are targeted directly against antigens on the infectious diseases.  Candidate antibodies can be identified using the classical antigen display techniques or more recently taken the B-cell IgG repertoire from surviving patients’ serum and deep sequencing.  The list of effective vaccines against Ebola includes the identification of mAb114 antibody from a surviving patient and REGN-EB3, a combination of three monoclonal antibodies.  Central to the supply of antibody-based vaccines is the manufacture within robust mammalian host cell lines.

Cell types

CHO and myeloma cells e.g. NS0, and Vero cells are some of the most commonly used host cells for the construction of stable cell lines. The vaccines produced by the cells are secreted extracellularly so that the product can be harvested from the cell culture medium.

Workflow requirements

Stable cell lines are required for industrial production of recombinant protein vaccines. These must have the ability to produce the same high quality products at different times, at different geographical sites and between different batches. After selecting the cell line for production, it is necessary to establish a Master Cell Bank (MCB).

Protection of the manufacturing capacity of immortalised cell lines expressing a vaccine product is critical to protect the supply of a marketed drug.  Heterogeneity in the population can lead to instability as discussed by the FDA concerning the production of biologics (reference Welch JT & Arden NS, Biologicals, Sep 2019) .  Ensuring that the cell line was derived from a single cell, 'clonally-derived', can lead to a more robust cell line. Current best practice with immortalised cell lines producing a vaccine is to perform single-cell isolation with systems, such as the VIPS and to monitor clonal outgrowth with the Cell Metric. The history of the clone is then documented by way of the Clonality Report which can be submitted to the regulator.

The workflow steps for stable cell lines for vaccine production are very similar to those outlined for the cell line development for biotherapeutics. Speed of cell line development is especially important in this case in order to quickly combat a potential pandemic:  using the Solentim approach, the time from transfection to cell banking can be achieved in as little as 12 weeks.

It should be noted that there are some key workflow differences in modern vaccine technology in comparison to conventional IgG production. Firstly, generating a sub-unit protein will necessitate developing novel immunoassays that can quantitate the recombinant protein and determine efficacy. Secondly, for the production of monoclonal antibody vaccines, while these can be assessed using well-developed assays for antibody titer determination, custom functional assays will also need to be developed to determine the affinity of the antibodies against the infectious agent.

References and further reading on new vaccine technology

Publication in 2019 by the group at GSK Vaccines in Rockville, MD using the Cell Metric CLD and reviewed in this blog by Mark Stockdale

Li, X, Zhang, Y, Jing, L, et al. “Integration of high‐throughput analytics and cell imaging enables direct early productivity and product quality assessment during Chinese Hamster ovary cell line development for a complex multi‐subunit vaccine antigen.” Biotechnol Progress., published 30 September 2019.

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