Viral vaccines and their manufacturing cell substrates: New trends and designs in modern vaccinology

Abstract

Vaccination is one of the most effective interventions in global health. The worldwide vaccination programs significantly reduced the number of deaths caused by infectious agents. A successful example was the eradication of smallpox in 1979 after two centuries of vaccination campaigns. Since the first variolation administrations until today, the knowledge on immunology has increased substantially. This knowledge combined with the introduction of cell culture and DNA recombinant technologies revolutionized vaccine design. This review will focus on vaccines against human viral pathogens, recent developments on vaccine design and cell substrates used for their manufacture. While the production of attenuated and inactivated vaccines requires the use of the respective permissible cell substrates, the production of recombinant antigens, virus-like particles, vectored vaccines and chimeric vaccines requires the use - and often the development - of specific cell lines. Indeed, the development of novel modern viral vaccine designs combined with, the stringent safety requirements for manufacture, and the better understanding on animal cell metabolism and physiology are increasing the awareness on the importance of cell line development and engineering areas. A new era of modern vaccinology is arriving, offering an extensive toolbox to materialize novel and creative ideas in vaccine design and its manufacture.

Keywords: Cell lines; Cell substrates; Chimeric vaccines; Viral vaccines; Virus-like particles.

Figure 1

Schematic representation of the vaccine design evolution following the technological advancements. Inoculation was a standard procedure in the 18–19^th centuries. The first vaccine against smallpox was introduced by Edward Jenner in 1796 and consisted in inoculating the live virus (LV) cowpox obtained from infected cattle. In the middle of the 20^th century animal cell culture became a standard procedure to grow virus allowing the development of attenuated (AV) and inactivated (IV) vaccines. In the late 20^th century the DNA recombinant technologies allowed to develop subunit vaccines based on presenting protein antigens (P and VLPs), coding for the antigens (DNA and vectored vaccines) or both. Further advances on genetic engineering originated a myriad of different vaccine designs such as chimeric vaccines still under development but holding great expectations.

Figure 2

Genetic approaches in cell line development and engineering for the manufacture of vaccines. (A) Conceptually, cell line development encloses all the steps leading to a clonal culture with extended life‐span. At the end of development, some of the cell lines already support viral propagation (e.g. influenza virus propagates in MDCK, Vero, HEK293, etc). Spontaneous immortalization is usually based on chromosomal rearrangements – not externally induced – resulting in the loss of senescence‐related and/or activation of immortalizing genes; cell line development without external genetic manipulation is called simple cell line establishment. Induced immortalization can rely on chemical or physical agents (e.g. methylcholanthrene or UV) or on the integration of immortalizing genes (telomerase, SV40 large T antigen, adenovirus E1 genes, etc.). Depending on the immortalizing gene, the cell lines can also support the propagation of partially deleted viral vectors (such as HEK293 or PER.C6, immortalized with human adenovirus E1 gene being denominated as transcomplementing cell lines for E1‐deleted adenovirus). Cell line development also encloses genetic manipulation specifically conceived to support the production of a particular virus or viral components, resulting in stable (or inducible) cells lines that constitutively (or upon induction) express the viral components [33]. (B) Finally, cell line engineering can be defined as a genetic manipulation designed to improve the production performance of a pre‐existing cell line, mostly for increasing specific titers. Strategies to facilitate the production process or to provide the produced particles with specific traits can also be categorized as cell line engineering.