By Dr. Jeffrey W. Hall, Pharmgate Research Scientist
The family of viruses known as “baculovirus” infects invertebrates, with moth larvae serving as the most common host. Foundational research dating to the late 1950’s set the stage for the development of baculovirus expression vector systems (BEVS). Since 1983, BEVS has been a valuable tool in the molecular biologist “toolbox” for recombinant protein production. Today, the standard BEVS is derived from the Autographa californica nucleopolyhedrovirus (AcMNPV) and provides several key benefits over other recombinant protein production systems. Unlike recombinant bacterial and yeast protein production systems, BEVS produces proteins with some post-production modifications such as glycosolation that are found in the mammalian host, improving the biological activity and immunogenicity of the recombinant proteins.
The inability of AcMNPV to replicate in mammals provides an additional layer of biosafety over mammalian viral expression system in the event the virus is not fully inactivated during vaccine manufacturing. Overall, BEVS provides for the production of safe and immunogenic vaccines. Several vaccines produced using BEVS are currently on the market for human and veterinary use with more likely to be introduced in the coming years (Table 1).
|Table 1: Examples of Approved BEVS-based vaccines and therapies[i]|
|Product Name||Company||Expressed product||Purpose||Use||Year of release|
|Cervarix||GlaxoSmithKline||Human papillomavirus L1 protein (serotypes 16 and 18)||VLP-based vaccine cervical cancer||Human (females)||2007|
|Provenge (sipuleucel-T)||Dendreon||PAP-GM-CSF||Immunotherapy against prostate cancer||Human (males)||2010|
|Flublok||Sanofi Pasteur||Influenza HA||Annual trivalent influenza vaccine||Human||2013|
|Porcilis Pesti||Merck||E2 glycoprotein||Subunit vaccine against classical swine fever||Swine||2007|
|Circogard||Pharmgate||Porcine circovirus ORF2 Capsid protein||VLP vaccine against porcine circovirus, type 2 (sub-type b)||Swine||2017|
|Circumvent PCV2 G2||Merck Animal Health||Porcine circovirus ORF2 Capsid protein||VLP vaccine against porcine circovirus, type 2 (sub-type a)||Swine||2007|
|CircoFLEX||Boehringer Ingleheim Vetmedica||Porcine circovirus ORF2 Capsid protein||VLP vaccine against porcine circovirus, type 2 (sub-type a)||Swine||2007|
An important part of viral survival is a structural unit called the capsid that is made up of one or more proteins. The capsid is a protective shell in which viral proteins and nucleic acids are housed. The capsid plays an important part in viral attachment and fusion with host cells. Once attached to a cell, the enclosed viral proteins and nucleic acids are released into the host cells resulting in viral propagation. As a prominent viral structure, the host’s immune system will mount an immune response directed at the viral capsid. A successful immune response prevents viral attachment and propagation.
Vaccine scientists have exploited the targeting of viral capsids by the immune system to develop safe and efficacious vaccines. In general, the presentation of the viral capsid to the host immune system is the same whether the capsid is from a whole killed virus (unmodified or chimera) or based on virus-like particles (VLPs). A VLP is a viral capsid void of viral nucleic acid producing a non-infectious mimic of the virus. Production of VLP vaccines by BEVS has become routine because of the ease in manufacturing scalability, cost-efficiencies, ability to produce multi-protein VLPs[ii], and the production of magnitudes more viral antigen than traditional host cell production methods allow for smaller vaccine dose sizes. Margine et al., furthermore, suggests BEVS proteins carried through into formulated vaccine enhance the immunogencitiy of the BEVS vaccine; an added benefit absent from mammalian-expression or host-cell produced vaccines[iii]. Moreover, BEVS produced VLP vaccines may expose virus neutralizing epitopes that are normally hidden on viruses produced from a mammalian host cell. This added benefit allows the immune system to produce supplementary virus neutralizing antibodies which are capable of further limiting pathogenesis and viral shedding as has been identified for porcine circovirus type 2 (PCV2)[iv].
PCV2 imposes a significant economic burden on the swine industry and is strongly associated with postweaning multisystemic wasting syndrome (PMWS). PCV2 infection also exasperates other diseases resulting in increased mortality and morbidity. Since the emergence of a virulent PCV2 genotype after 1971[v], PCV2 is now categorized into five genotypes, 2a-e. Historically, PCV2 vaccines have been based on the 2a genotype and for many years PCV2 has been effectively controlled. Recently though, the 2d genotype has become the dominant genotype in the United States, accounting for 72% of PCV2 genotypes sequenced at the ISU-VDL in 2016[vi]. The viral capsid of 2d varies from 2a by 23 amino acids, and less so for 2b[vii]. The use of 2a vaccines has been associated with apparent field vaccine failure attributed to the 2d genotypevii. A recently licensed PCV2 vaccine combines the benefits of BEVS production and immunogenicity with a contemporary 2b genotype (Table 1). Studies indicate this ready-to-use 1 mL per dose vaccine is safe, efficacious, and effective at controlling PCV2 infection.