Baculovirus expression vectors are commonly used to synthesize large quantities of recombinant proteins in insect cells. Unlike prokaryotic expression systems, recombinant baculovirus-infected cells can perform any post-translational modifications required to yield functional proteins.
The baculovirus most commonly used for recombinant gene expression is the Autographa californica multiple nucleopolyhedrovirus (AcMNPV). This has a large, circular DNA genome (134 kilobase pairs), which can be manipulated using a variety of methods.
Oxford Expression Technologies (OET) has developed a strategy in which a replication-deficient AcMNPV (flashBacTM) is rescued after transfection of insect cells with a transfer vector containing the foreign gene of choice (Figure 1). The system permits nonvirologists to utilize baculovirus expression vectors with minimal training or expertise.
While most examples of genes expressed in baculovirus-infected cells feature single proteins, there is an increasing need for the production of multicomponent structures such as sub-viral particles or enzyme complexes.
Perhaps one of the most exciting areas is the use of virus-like particles in vaccine development. This approach originated with the observation that sub-viral particles composed of a surface antigen were present in the blood of patients infected with hepatitis B virus and could be used as a vaccine (HbsAg). It was shown subsequently that eukaryotic expression systems, such as yeast and insect cells, could make HbsAg without the risk of contamination with other human viruses.
Baculovirus expression systems have proven particularly versatile for making subviral particles with multiple protein components. Examples include parvoviruses (adeno-associated virus), papilloma viruses, caliciviruses, hepatitis E virus, and reoviruses. The use of one virus to make sub-viral particles of another is appropriate since the vector already performs this function to ensure its own replication.
The native baculovirus particles are at least as complex as any other structure that it might be required to synthesize. With this approach it is also feasible to replicate noninfectious components of highly pathogenic viruses (e.g., avian/swine influenzas) or ones that are difficult to propagate in vitro (e.g., hepatitis C) for use as candidate vaccines.
In addition to the high levels of recombinant protein production possible with baculoviruses, it is also feasible to insert multiple foreign gene coding regions into the virus genome. The polyhedrin and p10 gene promoters can be duplicated and inserted at different locations within AcMNPV without apparent effect on their efficacy for driving recombinant protein gene expression.
Consequently, rather than making a recombinant virus to express each component of a sub-viral particle and then co-infecting insect cells to assemble the final structure, one virus can be made to vector all genes simultaneously.