Suspension culture titration: A simple method for measuring baculovirus titers

Reduction of the infectivity of baculovirus stocks frozen at ultra-low temperature in serum-free media: The role of lipid emulsions

The infectivity of stocks of baculoviruses produced in serum-free media is sensitive to freezing at ultra-low temperatures. The objective of this work was to elucidate the causes of such sensitivity, using as a model the freezing of stocks of Anticarsia gemmatalis multiple nucleopolyhedrovirus (AgMNPV), a baculovirus widely employed as biological insecticide. Titers of supernatants of cell cultures infected with AgMNPV in four different serum-free media supplemented with lipid emulsions were reduced by 50 to 90% after six months freezing. By using a full factorial experiment, freezing and lipid emulsion, as well as the interaction between them, were identified as the main factors reducing the viral titer. The virucidal effect of the lipid emulsion was reproduced by one of their components, the surfactant Polysorbate 80. Damaged viral envelopes were observed by transmission electron microscopy in most particles frozen in a medium supplemented with lipid emulsion or Polysorbate 80. Additionally, Polysorbate 80 also affected the infectivity of AgMNPV stocks that were incubated at 27°C. The identification of the roles played by the lipid emulsion and Polysorbate 80 is not only a contribution to the understanding of the mechanisms underlying the inactivation of baculovirus stocks produced in serum-free media during storage at ultra-low temperature, but is also an input for the rational development of new procedures aimed at improving both the preservation of baculovirus stocks and the composition of culture media for the production of baculovirus-based bioproducts in insect cells. © 2016 American Institute of Chemical Engineers Biotechnol. Prog., 32:1559-1569, 2016.

Evaluating Baculovirus Infection Using Green Fluorescent Protein and Variants

By use of a strategy incorporating the green fluorescent protein (GFP), facile and rapid monitoring and visualization of baculovirus infection in insect cells is possible in vivo. This chapter describes two techniques for simple determination of virus titer in the baculovirus expression system using GFP co-expression and rapid monitoring of Sf-9 insect cell infection using a combination of GFP and the early-to-late (ETL) promoter of the virus vector. Because of its early appearance, GFP, when placed under the control of ETL promoter, will facilitate vector construction, virus isolation, and titer determination.

In vitro production of baculoviruses: identifying host and virus genes associated with high productivity

Baculoviruses are recognized as viral workhorses of biotechnology, being used for production of vaccines, complex recombinant proteins, gene delivery vectors' and safe biological pesticides. Improving production yields and understanding the interactions of the virus and its host cell are important aspects of ensuring baculovirus-based processes are commercially competitive. This study aims at potential optimization of host cells used in in vitro virus production by systemically investigating changes in host gene expression in response to virus replication and transcription inside host cells. The study focuses on in vitro interactions of the Helicoverpa armigera virus with Helicoverpa zea insect cells. We used 22 genome-wide microarrays to simultaneously measure both virus and host genes in infected cells in multiple batch suspension cultures, representing high- and low-producing infection conditions. Among 661 differentially expressed genes, we identified a core set of 59 host genes consistently overexpressed post infection, with strong overrepresentation of genes involved in retrotransposition, protein processing in the endoplasmic reticulum, and ubiquitin-mediated proteolysis. Applying a whole genome correlation network analysis to link gene expression to productivity, we revealed 18 key genes significantly associated to virus yield. In addition, this study is among the first to perform a genome-wide expression study for a major baculovirus group II strain, the H. armigera virus, extending current understanding of baculovirus-insect interactions, which mainly focuses on group I viruses.

Cell Culture for Production of Insecticidal Viruses

While large-scale culture of insect cells will need to be conducted using bioreactors up to 10,000 l scale, many of the main challenges for cell culture-based production of insecticidal viruses can be studied using small-scale (20-500 ml) shaker/spinner flasks, either in free suspension or using microcarrier-based systems. These challenges still relate to the development of appropriate cell lines, stability of virus strains in culture, enhancing virus yields per cell, and the development of serum-free media and feeds for the desired production systems. Hence this chapter presents mainly the methods required to work with and analyze effectively insect cell systems using small-scale cultures. Outlined are procedures for quantifying cells and virus and for establishing frozen cells and virus stocks. The approach for maintaining cell cultures and the multiplicity of infection (MOI) and time of infection (TOI) parameters that should be considered for conducting infections are discussed.The methods described relate, in particular, to the suspension culture of Helicoverpa zea and Spodoptera frugiperda cell lines to produce the baculoviruses Helicoverpa armigera nucleopolyhedrovirus, HearNPV, and Anticarsia gemmatalis multicapsid nucleopolyhedrovirus, AgMNPV, respectively, and the production of the nonoccluded Oryctes nudivirus, OrNV, using an adherent coleopteran cell line.

Decline in Helicoverpa armigera nucleopolyhedrovirus occlusion body yields with increasing infection cell density in vitro is strongly correlated with viral DNA levels

The phenomenon of the reduction in the cell-specific yield with increasing infection cell density (ICD), the cell density effect, is one of the main hurdles for improving virus yields in vitro. In the current study, the reduction in cell-specific yields (viral DNA [vDNA], polyhedrin mRNA and occlusion body [OB]) with increasing ICD for Helicoverpa armigera nucleopolyhedrovirus (HearNPV)-infected HzAM1 (Helicoverpa zea) insect cells has been investigated. HzAM1 cells were propagated in Sf900™ III serum-free medium and synchronously infected with wild-type HearNPV at various ICDs of 0.5-5 × 10(6) cells/mL at an MOI of 5 PFU/cell. Infection was conducted either in the original medium or in fresh medium. As found previously for Sf9 and High Five cells, there were negative correlations between the three key virus infection indicators (vDNA, mRNA and OB) and the peak cell density (PCD). Generally, the yield decline with increasing PCD was most pronounced for OB, followed by mRNA, and was more moderate for vDNA. The decline was significantly reduced, but not totally arrested, when fresh medium was used. There were also strong correlations between OB and mRNA, mRNA and vDNA, and OB and vDNA levels. These results suggest that the reduction in baculovirus yield (OB) at high PCDs is associated with limitations during the upstream processes of replication and transcription together with limitations during protein translation. Furthermore, the peak protein productivity per unit of cell volume in the HzAM1/HearNPV system was shown to be higher than that of the Sf9/rAcMNPV system, but lower than that of the High Five/rAcMNPV system.

Intracellular Trafficking of Baculovirus Particles: A Quantitative Study of the HearNPV/HzAM1 Cell and AcMNPV/Sf9 Cell Systems

To replace the in vivo production of baculovirus-based biopesticides with a more convenient in vitro produced product, the limitations imposed by in vitro production have to be solved. One of the main problems is the low titer of HearNPV budded virions (BV) in vitro as the use of low BV titer stocks can result in non-homogenous infections resulting in multiple virus replication cycles during scale up that leads to low Occlusion Body yields. Here we investigate the baculovirus traffic in subcellular fractions of host cells throughout infection with an emphasis on AcMNPV/Sf9 and HearNPV/HzAM1 systems distinguished as "good" and "bad" BV producers, respectively. qPCR quantification of viral DNA in the nucleus, cytoplasm and extracellular fractions demonstrated that although the HearNPV/HzAM1 system produces twice the amount of vDNA as the AcMNPV/Sf9 system, its percentage of BV to total progeny vDNA was lower. vDNA egress from the nucleus to the cytoplasm is sufficient in both systems, however, a higher percentage of vDNA in the HearNPV/HzAM1 system remain in the cytoplasm and do not bud out of the cells compared to the AcMNPV/Sf9 system. In both systems more than 75% of the vDNA produced in the nuclear fraction go unused, without budding or being encapsulated in OBs showing the capacity for improvements that could result from the engineering of the virus/cell line systems to achieve better productivities for both BV and OB yields.