The technology of using mammalian cells for the production of vaccines has grown rapidly in recent years. In the first step
of production, mammalian cell lines are used to produce a master cell bank (MCB), which is then characterized and used to
grow the pathogen or antigen subunit under controlled cell culture or fermentation conditions. The subsequent cell culture
or supernatant is then harvested, purified, and several virus inactivation steps take place. The use of bioreactors and optimization
of fermentation conditions allows mammalian cell production of vaccines to be a more scalable process than the egg-based method
of vaccine production. Like the egg-based vaccine production process, producing a vaccine under current good manufacturing
practices (cGMP) conditions using mammalian cells can be a lengthy process, occurring over a minimum six- to 12-month time
period. It takes longer because of the regulatory constraints for mammalian cell production. The MCB must undergo extensive
testing and characterization prior to release for cGMP manufacturing. This testing can take up to four months to complete.
Process validation for the viral inactivation steps is also an important part of mammalian cell production of vaccines and
can also require months to complete. Also, the final product testing for vaccines produced using mammalian cell production
often requires assays, which take more time for development and execution. These additional safety requirements are all related
to the fact that mammalian cells can harbor various human pathogens or become infected during cell culture expansion.
Producing Plasmid DNA–Based Vaccines
Using modern plasmid DNA–based vaccines to combat infectious diseases is an alternative to traditional egg- and mammalian
cell-based vaccines. Plasmid DNA vaccines are produced by transforming the engineered plasmid into a host strain of E. coli. The plasmid DNA with the vaccine gene insert is constructed using recombinant DNA technology. The gene fragments that are
inserted into the plasmid DNA strand carry genes that specify one or more antigenic proteins. Antigenic proteins are normally
made or expressed by the selected pathogen. The benefit of inserting the selected genes into the plasmid DNA is that genes
that would enable the pathogen to reconstitute itself and trigger the disease are absent from the plasmid DNA. This ensures
a safe vaccine that is unable to produce an infection.2–3 Unlike mammalian cells, E. coli can not produce viruses or mycoplasm and therefore, do not require virus inactivation processing steps and final product
testing for human pathogens.
In most plasmid DNA vaccination platforms, the segment of DNA that encodes the protein antigen is incorporated into the plasmid
DNA backbone. Upon delivery of the plasmid DNA vaccine to a patient, the plasmid is taken up by the targeted cells and can
exist independently of chromosomal DNA and transcribe the gene encoding the antigen of interest. The production of small amounts
of antigen can lead to the induction of both antibody and cellular responses within the patient or one specific response if
certain adjuvants are used.3
Advantages of Plasmid DNA-Based Production Method
The use of plasmid DNA vaccines has a number of potential advantages compared with the current egg- and mammalian cell-based
vaccines. The first advantage is safety. Because only certain genes from the selected pathogen are inserted into the plasmid
DNA vaccine, the risk of pathogen replication or spreading is essentially nonexistent. Plasmid DNA vaccines will be unable
to cause infection in the patient because such vaccines lack the genes needed for the pathogen's replication.3
These vaccines also avoid the potential for allergic reactions of egg-based vaccines in humans.
Another advantage of plasmid DNA-based vaccines over the current egg and mammalian-cell based vaccines is the overall development
and production timeline. The genes from the specific pathogen can be isolated and inserted into the plasmid DNA construct
within a very short timeline. The plasmid DNA construct can be then transformed into an E. coli strain and a cGMP master cell bank can be produced within a week. The testing required for the release of a MCB for plasmid
DNA is less extensive and the timeline is much shorter than the testing required to release a mammalian cell bank. The release
testing for the MCB can be completed in as short as two weeks from the time the cell bank is produced.