Flu, caused by the influenza virus is unpleasant. Even in non-pandemic years, it causes 290,000 to 650,000 deaths. In the absence of a ‘universal’ vaccine that could provide protection against all possible variants of the virus, new vaccines need to be selected and manufactured each year. The majority of these vaccines are manufactured using eggs. Influenza virus is grown in chicken embryos inside the eggs which are then cracked open prior to purify and inactivate the virus for vaccine use.
There are a number of limitations to this approach. Firstly, it is complex to scale up, for example during a pandemic. It can also induce a selective pressure upon the vaccine virus – chicken cell and human cells have slightly different receptors for influenza on their cell surface and co-factors within the cell. This means that in order to replicate efficiently in egg cells the virus may undergo some slight changes. If these changes are in regions of viral proteins recognised by the immune system,  for example haemagglutinin, then the vaccine virus might induce a memory immune response which does not recognise the virus that is actually circulating in the wild. The final problem is that some viruses, in particular the highly pathogenic ones (H5N1 and H7N9) are deadly to birds and kill the chicken embryos before enough virus is made for the vaccine.
Therefore alternative manufacturing approaches are required. One tool that has been widely applied across all fields of biological drug manufacture is the use of recombinant cell culture – where genes from one organism are expressed in cells of another. There is a licensed influenza vaccine (Flucelvax) which is manufactured using the MDCK cell line. These cells were originally isolated in the 1950s from a dog kidney, specifically a cocker spaniel, by S.H. Madin and N.B. Darby – hence Madin-Darby Canine Kidney (MDCK) cells. Growing cells from mammals has advantages compared to embryonated chickens, but there is value in developing alternative methods.
We investigated an alternative manufacturing approach in our recently published study Recombinant Haemagglutinin Derived From the Ciliated Protozoan Tetrahymena thermophila Is Protective Against Influenza Infection in Frontiers in Immunology. Working with a biotech company based in Germany (Cilian, AG) who use a protozon ciliate called Tetrahymena thermophila for the manufacture of biologics. This system has a number of potential advantages, it uses conventional manufacturing equipment, the same as that used for both bacterial and yeast based manufacturing systems.
However, it was possible that viral proteins manufactured using a protozoan might not induce a good vaccine response. We therefore set out to test the immunogenicity of the ciliate derived material. We demonstrated that immunisation with recombinant haemagglutinin could protect against an infection with a matched influenza virus. We saw this with haemagglutinin derived from either influenza A or influenza B viruses.
This proof of principle study therefore opens that path for further development of the Tetrahymena thermophila platform for vaccines. The major next step will be to work the platform up to a good manufacturing practice (GMP) grade material so it can be tested in clinical trials.