Virus-like particles (VLPs) have become a promising means for developing vaccines and gene therapies. Currently, vaccines based on VLPs are commercially available for human papillomavirus and hepatitis B and E. Other disease treatments with VLPs are undergoing clinical trials.

Upon expression, the structural matrix polyprotein group-specific antigen (gag polyprotein) from the human immunodeficiency virus (HIV) has shown to accumulate beneath the lipidic membrane. After a sufficient number of gag polyproteins are recruited, the assembly process is finished, and the VLP buds out of the cell. The gag polyprotein is demonstrated to accommodate different protein antigens, underlining their potential as a multivalent vaccine. Several biological systems have been used to produce such nanoparticles, but animal cell lines are the preferred option.

Specifically, the insect cell/baculovirus expression system (BES) has proven to work properly for expression of complex virus proteins, achieving high yields with adequate posttranslational modifications. Unlike mammalian cell lines, insect cells provide an easy means for complex protein expression because they can reach high cell densities, do not host human pathogens, and entail less culture requirements. Among insect cells, the Sf9 cell line from Spodoptera frugiperda is the most extensively used platform. The Autographa californica multiple nucleopolyhedrovirus (AcMNPV) is the typical baculovirus used for recombinant protein production in this cell line. Upon infection, the baculovirus arrests the host cell machinery to replicate itself and spread the infection throughout the cell culture. Using standard cloning procedures, a gene of interest is placed after a strong promoter, normally the polyhedrin promoter, to drive the expression of high levels of the recombinant protein of interest. This system consumes an important part of the media nutrients, so strategies to increase the production yields of VLPs are of upmost importance.

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