A look into your ordinary flu vaccine: Insect Cells and BEVS
posted:March 29th, 2019
Come winter everyone is recommended to get their flu shot. Nothing special about this, right?... In reality, this is an incredible never-ending effort. In 2017, manufacturing capacity produced some 580 million doses worldwide . Additionally, the influenza virus’ single stranded RNA genome allows it to mutate rapidly, so a new vaccine must be produced each season. On top of that, the traditional way of production is by infecting chicken eggs with the virus and purifying the relevant antigen, a method used since the 1940’s but which requires an egg per dose and whose timeline from strain identification to vaccine is about four to six months . This makes every year a race against time to produce next season’s vaccine, which is especially critical in times of pandemic like in 2009’s AH1N1 crisis.
Fortunately, an alternative has been discovered since then. In 2013, the FDA approved the first ever egg-free flu vaccine, Flublok®. This vaccine is based on an insect cell culture with baculovirus infection and is able to produce a vaccine in five weeks .
The baculovirus species wild type is a caterpillar (lepidoptera) pathogen, which has a double stranded circular DNA genome and is very effective in transferring genes into its host . The baculovirus expression system (BEVS) was invented in 1983 . The BEVS consists in engineering a gene of interest with a viral promotor into a plasmid, then inserting said plasmid into E. coli. When exposed to the virus, the gene of interest will be inserted into the virus’ genome. Afterwards, upon infecting lepidoptera cultured cells, the virus transfers the gene into the cells. The cells then proceed to express said gene and in doing so, produce the protein of interest. It happens that baculovirus also makes the cells produce apoptotic proteins, so the virus must first be amplified to infect as many cells as possible before the virus replication cycle reaches this stage. This is because apoptotic cells release debris, which makes it harder to purify the protein of interest.
So, the BEVS method has an advantage over egg-based vaccines due to a faster timeline and because handling of the pathogenic live virus is minimized, reducing biosafety . But this system is also advantageous over protein therapeutics produced with mammalian cell lines. The maintenance of insect cell lines is easier as they don’t require CO2  and the incubation temperature is 27°C  (instead of 32°C). Additionally, to produce proteins, stable engineered cell lines are required, which are inconvenient compared to engineering a stable virus . Yet no system is perfect; the BEVS has some limitations, namely a short window of protein expression before apoptosis and possible allergic reactions to the products due to differences in metabolic processes between insect cells and mammalian cells .
The use of insect cells for protein production has been, since its discovery, first used as a research tool, then to make veterinary therapeutics (Porcilis Pesti ® in 2000) and finally used in human medicine with the novel VPH vaccine, Cervarix ® in 2007 . Although only four human therapeutics have been approved to date, with Flublok® being the most recent one, the advantages of the BEVS will surely make insect cells a household name in modern medicine.
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