Biotechnology and Bioinformatics (1+1)

4.3.7. Recombinant protein vaccines

Production of a recombinant protein vaccine starts with identification of the immunogenic subunit or protein from the pathogen of interest and verification of its immunogenicity in vivo and in vitro .

Once, the immunogenic proteins or subunits of pathogen are identified, the gene (s) involved in coding for them can be introduced into a vector , over-expressed in expression hosts and can be used as recombinant protein vaccines. The vector systems usually used to express recombinant proteins are viruses or bacterial plasmid s. Expression systems commonly used are prokaryotic and eukaryotic cells.

• Prokaryotic expression system comprises of bacteria such as, Escherichia coli, and the eukaryotic expression system comprises of yeast, insect cells and mammalian cells.
• Some inherent advantages and disadvantages exist with both of these expression systems.
• The major problem with the prokaryotic systems (such as bacteria) is that, they lack the signals required for proper post-translational modification and hence there lies the signals required for proper post translational modifications and hence there lies the problem of improper folding and lack of glycosylation. This leads to production of proteins of unpredicted antigenicities. In some cases, production of proteins will be in the form of inclusion bodies that need to be treated biochemically before being used as vaccine. This biochemical treatment of denaturation and renaturation of recombinant protein reduced its immunogenicity.
• Obvious advantages of prokaryotic expression system are, high level expression of recombinant protein (often more than 30%), well studied genetic and fermentation system of E. coli and easy scaling up of vaccine production.
• In the case of eukaryotic expression system, although the problem of folding and glycosylation does not exist, the final yield of expressed protein remains low, and hence the scaling up of the production process is difficult.

Both prokaryotic as well as eukaryotic expression systems have been used to produce fish viral, bacterial and parasitic antigens, and prokaryotic system is most widely used.

• For example, purified glycoproteins from IHNV and VHSV have been used as subunit vaccine s in fish and shown to be immunoprotective, and further these two proteins have been used widely for recombinant vaccine production.
• Similarly, an RNA-free subunit vaccine prepared from grass carp haemorrhagevirus (GCHV) treated with 1% NP40 in low salt solution has been shown to induce more than 80% protection in carp.