Improving the robustness of a low-cost insect cell medium for baculovirus biopesticides production, via hydrolysate streamlining using a tube bioreactor-based statistical optimization routine

Susceptibility of Field and Laboratory Bt-Susceptible and Resistant Strains of Helicoverpa zea (Boddie) to HearNPV

During 2021 and 2022, eight field-collected and five laboratory Helicoverpa zea strains with varying susceptibility to different Bt proteins were evaluated for their responses against HearNPV using diet-overlay bioassays. The five laboratory strains included SS (susceptible to all Bt proteins), CRY-RR (resistant to Cry1 and Cry2), VIP-RR-70 (resistant to Vip3Aa), VIP-RR-15 (resistant to Vip3Aa), and TRE-RR (resistant to Cry1, Cry2, and Vip3Aa). Our findings showed that the susceptibility of TRE-RR, VIP-RR-70, and VIP-RR-15 strains to HearNPV was similar to that of the SS strain. However, the field and Cry-RR strains were more resistant to HearNPV compared to the SS strain. Because most feral H. zea strains in the southern U.S. have developed practical resistance to Cry Bt proteins but remain susceptible to Vip3Aa, the results suggest that the reduced susceptibility to HearNPV in H. zea may be associated with the resistance to Cry Bt proteins but not with the resistance to Vip3Aa. Correlation analysis confirmed that there was a significant positive relationship between Cry resistance and HearNPV resistance, but not between the Vip3Aa resistance and HearNPV resistance in H. zea. Our findings provide valuable insights into the relationship between susceptibility to HearNPV and resistance to Bt proteins in H. zea.

HearNPV susceptibility in Helicoverpa armigera and Helicoverpa punctigera strains resistant to Bt toxins Cry1Ac, Cry2Ab, and Vip3Aa

Genetically engineered crops expressing insecticidal toxins from Bacillus thuringiensis (Bt) have improved the management of targeted lepidopteran pests and reduced the use of insecticide sprays. These benefits explain an increasing adoption of Bt crops worldwide, intensifying the selection pressure on target species and the risk of resistance. Nucleopolyhedroviruses (NPVs) are effective bioinsecticides against numerous important lepidopteran pests. If Bt-resistant insects are shown to be susceptible to NPVs then these bioinsecticides could be a valuable component of Insecticide Resistance Management (IRM) strategies for Bt crops. We assessed the effectiveness of a Helicoverpa nucleopolyhedrovirus (HearNPV) against several different Bt-resistant strains. Utilising a droplet feeding bioassay we confirmed susceptibility to HearNPV in Helicoverpa punctigera and Helicoverpa armigera larvae resistant to the Bt toxins Cry1Ac, Cry2Ab, and Vip3A. Dual resistant H. punctigera, (Cry1Ac/Cry2Ab, and Cry2Ab/Vip3A) and dual resistant H. armigera (Cry2Ab/Vip3A) were also susceptible to HearNPV. Regardless of their specific resistance profile, Bt-resistant larvae displayed statistically similar lethal concentration (LC₅₀) and lethal time (LT₅₀) responses to HearNPV when compared to Bt-sensitive control insects. These results indicate that Bt-resistant H. armigera and H. punctigera are not cross-resistant to HearNPV. Consequently, the use of HearNPV against these pests may be a valuable tool to an IRM strategy for controlling Bt-resistant populations.

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.

Development of Serum-Free Media for Lepidopteran Insect Cell Lines

Baculovirus-based Insect Cell Technology (ICT) is widely used for the expression of recombinant heterologous proteins and baculovirus bioinsecticides, and has recently gained momentum as a commercial manufacturing platform for human and veterinary vaccines. The three key components of ICT are the Lepidopteran insect cell line, the baculovirus vector, and the growth medium. Insect cell growth media have evolved significantly in the past five decades, from basal media supplemented with hemolymph or animal serum, to highly optimized serum-free media and feeds (SFM and SFF) capable of supporting very high cell densities and recombinant protein yields. The substitution of animal sera with protein hydrolysates in SFM results in greatly reduced medium costs and much improved process scalability. However, both sera and hydrolysates share the disadvantage of lot-to-lot variability, which is detrimental to process reproducibility. Hence, the industrialization of ICT would benefit greatly from chemically defined media (CDM) for insect cells, which are not yet commercially available. On the other hand, applications such as baculovirus bioinsecticides would need truly low cost serum-free media and feeds (LC-SFM and LC-SFF) for economic viability, which require the substitution of a majority of expensive added amino acids with even higher levels of hydrolysates, hence increasing the risk of a variable process. CDM developments are anticipated to benefit both conventional and low cost ICT applications, by identifying key growth factors in hydrolysates for more targeted media and feed design.

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.

Effect of the peak cell density of recombinant AcMNPV-infected Hi5 cells on baculovirus yields

The phenomenon of the cell density effect is not readily explained by an obvious nutrient limitation, and a recent study has suggested that for recombinant Autographa californica multiple nucleopolyhedrovirus (rAcMNPV)-infected Sf9 cells, a drop in messenger RNA (mRNA) levels may be sufficient to explain the cell density effect for this system. The current study aims to investigate the response in cell-specific yields (viral DNA (vDNA), LacZ mRNA and β-galactosidase (β-Gal) protein) with increasing infection cell density (ICD) for rAcMNPV-infected Hi5 cells, where the rAcMNPV expresses the β-Gal gene under control of the polyhedral promoter. Hi5 cells in suspension culture of Express Five® medium were synchronously infected with a rAcMNPV at multiple ICDs between 0.5 and 6 × 10(6) cells/mL and a multiplicity of infection of 10 plaque-forming units (PFU)/cell either in the original or fresh medium conditions. There were negative correlations between the three key virus infection indicators (vDNA, mRNA and β-Gal) and the peak cell density (PCD). However, unlike infected Sf9 cells, the yield decline started at the lowest PCD investigated (0.6 × 10(6) cells/mL). Generally, the yield decline with increasing PCD was most pronounced for β-Gal followed by mRNA and was more moderate for vDNA. The decline was significantly reduced but not totally arrested when fresh medium replacement was used. The results suggest that the reduction in recombinant protein-specific yields at high PCDs is associated with limitations during the up-stream processes of replication and transcription rather than entirely caused by limitations during translation. In addition, low production rates at late infection stages of moderate to high ICDs are a probable cause of the cell density effect.

Tools for high-throughput process and medium optimization

Shaking technology in combination with small-scale disposable plastic vessels has become a notable bioprocess optimization tool widely exploited for cells grown in suspension. This chapter focuses on the two most accommodating culture systems: 50 mL centrifugation tubes and 96-deepwell plates. Used by many laboratories for routine passaging of suspension cultures and all types of optimization experiments as flexible culture system, the 50 mL shake tubes are the preferred vessels for manual manipulations, while microtiter plates are the ideal containers when automatic liquid handling systems are available. Both culture systems can offer to the cells a well-mixed environment which is close to the conditions found at larger scale in production bioreactors.

Decline in baculovirus-expressed recombinant protein production with increasing cell density is strongly correlated to impairment of virus replication and mRNA expression

The cell density effect is a well-established constraint in the baculovirus-insect cell expression platform, in which cell-specific productivity declines with increasing cell density, hence limiting the maximum achievable volumetric yield of protein product. A deeper elucidation of this phenomenon is sought in this study, by tracking the peak production of viral DNA (vDNA), recombinant LacZ mRNA, and β-galactosidase (β-gal) protein, over a wide range of cell densities. Sf9 suspension cell cultures were propagated in Sf-900 III serum-free medium and synchronously infected with rAcMNPV at multiple infection cell densities (ICDs) of between 0.5 and 8 × 10(6) cells/mL. There was a strong negative linear correlation between the specific β-gal yield and the peak cell density (PCD) post-infection, but contrary to previous reports, the yield decline started at a lower PCD of around 1 × 10(6) cells/mL. Most interestingly, there also was a corresponding strong negative linear correlation between the specific vDNA or LacZ mRNA yield, and the PCD. Comparing the infections at the highest and lowest PCDs tested, the yield decline was most dramatic for β-gal protein (95 %) and LacZ mRNA (90 %), while it was more moderate for vDNA (50 %). These declines were significantly reduced but not completely arrested, when spent medium was replaced with fresh at the ICD. These findings suggest that protein yield deterioration with increasing cell density originated from limitations during upstream events such as virus gene replication or transcription, rather than during the translational phase. Such limitations may be largely nutritional, but a more complex mechanism may be implicated.