Opportunities and challenges for the baculovirus expression system

Haplotype determination of the Bombyx mori nucleopolyhedrovirus by Nanopore sequencing and linkage of single nucleotide variants

Naturally occurring isolates of baculoviruses, such as the Bombyx mori nucleopolyhedrovirus (BmNPV), usually consist of numerous genetically different haplotypes. Deciphering the different haplotypes of such isolates is hampered by the large size of the dsDNA genome, as well as the short read length of next generation sequencing (NGS) techniques that are widely applied for baculovirus isolate characterization. In this study, we addressed this challenge by combining the accuracy of NGS to determine single nucleotide variants (SNVs) as genetic markers with the long read length of Nanopore sequencing technique. This hybrid approach allowed the comprehensive analysis of genetically homogeneous and heterogeneous isolates of BmNPV. Specifically, this allowed the identification of two putative major haplotypes in the heterogeneous isolate BmNPV-Ja by SNV position linkage. SNV positions, which were determined based on NGS data, were linked by the long Nanopore reads in a Position Weight Matrix. Using a modified Expectation-Maximization algorithm, the Nanopore reads were assigned according to the occurrence of variable SNV positions by machine learning. The cohorts of reads were de novo assembled, which led to the identification of BmNPV haplotypes. The method demonstrated the strength of the combined approach of short- and long-read sequencing techniques to decipher the genetic diversity of baculovirus isolates.

Development of a stable Sf9 insect cell line to produce VSV-G pseudotyped baculoviruses

Baculoviruses have shown great potential as gene delivery vectors in mammals, although their effectiveness in transferring genes varies across different cell lines. A widely employed strategy to improve transduction efficiency is the pseudotyping of viral vectors. In this study, we aimed to develop a stable Sf9 insect cell line that inducibly expresses the G-protein of the vesicular stomatitis virus to pseudotype budded baculoviruses. It was obtained by inserting the VSV-G gene under the control of the very strong and infection-inducible pXXL promoter and was subsequently diluted to establish oligoclonal lines, which were selected by the fusogenic properties of VSV-G and its expression levels in infected cells and purified budded virions. Next, to enhance the performance of the cell line, the infection conditions under which functional pseudotyped baculoviruses are obtained were optimized. Finally, different baculoviruses were pseudotyped and the expression of the transgene was quantified in mammalian cells of diverse origins using flow cytometry. The transduction efficiency of pseudotyped baculovirus consistently increased across all tested mammalian cell lines compared with control viruses. These findings demonstrate the feasibility and advantages of improving gene delivery performance without the need to insert the pseudotyping gene into the baculoviral genomes.

A Direct Comparison of rAAV5 Variants Derived from the Baculovirus Expression System Using LC-MS Workflows Demonstrates Key Differences in Overall Production Yield, Product Quality and Vector Efficiency

Gene therapy holds great promise for the treatment of severe diseases, and adeno-associated virus (AAV) vectors have emerged as valuable tools in this field. However, challenges such as immunogenicity and high production costs complicate the commercial viability of AAV-based therapies. To overcome these barriers, improvements in production yield, driven through the availability of robust and sensitive characterization techniques that allow for the monitoring of critical quality attributes to deepen product and process understanding are crucial. Among the main attributes affecting viral production and performance, the ratio between empty and full capsids along with capsid protein stoichiometry are emerging as potential parameters affecting product quality and safety. This study focused on the production of AAV vectors using the baculovirus expression vector system (BEVS) in Sf9 cells and the complete characterization of AAV5 variants using novel liquid chromatography and mass spectrometry techniques (LC-MS) that, up to this point, had only been applied to reference commercially produced virions. When comparing virions produced using ATG, CTG or ACG start codons of the cap gene, we determined that although ACG was the most productive in terms of virus yield, it was also the least effective in transducing mammalian cells. This correlated with a low VP1/VP2 ratio and a higher percentage of empty capsids. Overall, this study provides insights into the impact of translational start codon modifications during rAAV5 production using the BEVS, the associated relationship with capsid packaging, capsid protein stoichiometry and potency. The developed characterization workflow using LC-MS offers a comprehensive and transferable analysis of AAV-based gene therapies, with the potential to aid in process optimization and facilitate the large-scale commercial manufacturing of these promising treatments.

Protein Production at the 100L Scale

The Baculovirus Expression Vector System (BEVS) has revolutionized the field of recombinant protein expression by enabling efficient and high yield production. The platform offers many advantages including manufacturing speed, flexible design, and scalability. In this chapter, we describe the methods including strategies and considerations to successfully optimize and scale-up using BEVS as a tool for production (Fig. 1). As an illustrative case study, we present an example focused on the production of a viral glycoprotein.

Virus-like particles as powerful vaccination strategy against human viruses

Nowadays, viruses are not only seen as causative agents of viral infectious diseases but also as valuable research materials for various biomedical purposes, including recombinant protein production. When expressed in living or cell-free expression systems, viral structural proteins self-assemble into virus-like particles (VLPs). Mimicking the native form and size of viruses and lacking the genetic material, VLPs are safe and highly immunogenic and thus can be exploited to develop antiviral vaccines. Some vaccines based on VLPs against various infectious pathogens have already been licenced for human use and are available in the commercial market, the latest of which is a VLP-based vaccine to protect against the novel Coronavirus. Despite the success and popularity of VLP subunit vaccines, many more VLPs are still in different stages of design, production, and approval. There are still many challenges that require to be addressed in the future before this surface display system can be widely used as an effective vaccine strategy in combating infectious diseases. In this review, we highlight the use of structural viral proteins to produce VLPs, emphasising their intrinsic properties, structural classification, and main expression host systems. We also compiled the recent scientific literature about VLP-based vaccines to underline the recent advances in their application as a vaccine strategy for preventing and fighting virulent human pathogens. Finally, we presented the key challenges and possible solutions for VLP-based vaccine production.

PCR-based analytics of gene therapies using adeno-associated virus vectors: Considerations for cGMP method development

The field of gene therapy has evolved and improved so that today the treatment of thousands of genetic diseases is now possible. An integral aspect of the drug development process is generating analytical methods to be used throughout clinical and commercial manufacturing. Enumeration and identification assays using genetic testing are critical to ensure the safety, efficacy, and stability of many active pharmaceutical ingredients. While nucleic acid-based methods are already reliable and rapid, there are unique biological, technological, and regulatory aspects in gene therapies that must be considered. This review surveys aspects of method development and validation using nucleic acid-based testing of gene therapies by focusing on adeno-associated virus (AAV) vectors and their co-transfection factors. Key differences between quantitative PCR and droplet digital technologies are discussed to show how improvements can be made while still adhering to regulatory guidance. Example validation parameters for AAV genome titers are described to demonstrate the scope of analytical development. Finally, several areas for improving analytical testing are presented to inspire future innovation, including next-generation sequencing and artificial intelligence. Reviewing the broad characteristics of gene therapy assessment serves as an introduction for new researchers, while clarifying processes for professionals already involved in pharmaceutical manufacturing.

Automated cell counting for Trypan blue-stained cell cultures using machine learning

Cell counting is a vital practice in the maintenance and manipulation of cell cultures. It is a crucial aspect of assessing cell viability and determining proliferation rates, which are integral to maintaining the health and functionality of a culture. Additionally, it is critical for establishing the time of infection in bioreactors and monitoring cell culture response to targeted infection over time. However, when cell counting is performed manually, the time involved can become substantial, particularly when multiple cultures need to be handled in parallel. Automated cell counters, which enable significant time reduction, are commercially available but remain relatively expensive. Here, we present a machine learning (ML) model based on YOLOv4 that is able to perform cell counts with a high accuracy (>95%) for Trypan blue-stained insect cells. Images of two distinctly different cell lines, Trichoplusia ni (High FiveTM; Hi5 cells) and Spodoptera frugiperda (Sf9), were used for training, validation, and testing of the model. The ML model yielded F1 scores of 0.97 and 0.96 for alive and dead cells, respectively, which represents a substantially improved performance over that of other cell counters. Furthermore, the ML model is versatile, as an F1 score of 0.96 was also obtained on images of Trypan blue-stained human embryonic kidney (HEK) cells that the model had not been trained on. Our implementation of the ML model comes with a straightforward user interface and can image in batches, which makes it highly suitable for the evaluation of multiple parallel cultures (e.g. in Design of Experiments). Overall, this approach for accurate classification of cells provides a fast, bias-free alternative to manual counting.

Baculovirus Display of Peptides and Proteins for Medical Applications

Baculoviridae is a large family of arthropod-infective viruses. Recombinant baculoviruses have many applications, the best known is as a system for large scale protein production in combination with insect cell cultures. More recently recombinant baculoviruses have been utilized for the display of proteins of interest with applications in medicine. In the present review we analyze the different strategies for the display of proteins and peptides on the surface of recombinant baculoviruses and provide some examples of the different proteins displayed. We analyze briefly the commercially available systems for recombinant baculovirus production and display and discuss the future of this emerging and powerful technology.

Bioengineering of Antibody Fragments: Challenges and Opportunities

Antibody fragments are used in the clinic as important therapeutic proteins for treatment of indications where better tissue penetration and less immunogenic molecules are needed. Several expression platforms have been employed for the production of these recombinant proteins, from which E. coli and CHO cell-based systems have emerged as the most promising hosts for higher expression. Because antibody fragments such as Fabs and scFvs are smaller than traditional antibody structures and do not require specific patterns of glycosylation decoration for therapeutic efficacy, it is possible to express them in systems with reduced post-translational modification capacity and high expression yield, for example, in plant and insect cell-based systems. In this review, we describe different bioengineering technologies along with their opportunities and difficulties to manufacture antibody fragments with consideration of stability, efficacy and safety for humans. There is still potential for a new production technology with a view of being simple, fast and cost-effective while maintaining the stability and efficacy of biotherapeutic fragments.

Real-time online monitoring of insect cell proliferation and baculovirus infection using digital differential holographic microscopy and machine learning

Real-time, detailed online information on cell cultures is essential for understanding modern biopharmaceutical production processes. The determination of key parameters, such as cell density and viability, is usually based on the offline sampling of bioreactors. Gathering offline samples is invasive, has a low time resolution, and risks altering or contaminating the production process. In contrast, measuring process parameters online provides more safety for the process, has a high time resolution, and thus can aid in timely process control actions. We used online double differential digital holographic microscopy (D3HM) and machine learning to perform non-invasive online cell concentration and viability monitoring of insect cell cultures in bioreactors. The performance of D3HM and the machine learning model was tested for a selected variety of baculovirus constructs, products, and multiplicities of infection (MOI). The results show that with online holographic microscopy insect cell proliferation and baculovirus infection can be monitored effectively in real time with high resolution for a broad range of process parameters and baculovirus constructs. The high-resolution data generated by D3HM showed the exact moment of peak cell densities and temporary events caused by feeding. Furthermore, D3HM allowed us to obtain information on the state of the cell culture at the individual cell level. Combining this detailed, real-time information about cell cultures with methodical machine learning models can increase process understanding, aid in decision-making, and allow for timely process control actions during bioreactor production of recombinant proteins.

Identification and analysis of putative tRNA genes in baculovirus genomes

Transfer RNAs (tRNAs) genes are both coded for and arranged along some viral genomes representing the entire virosphere and seem to play different biological functions during infection, other than transferring the correct amino acid to a growing peptide chain. Baculovirus genome description and annotation has focused mostly on protein-coding genes, microRNA, and homologous regions. Here we carried out a large-scale in silico search for putative tRNA genes in baculovirus genomes. Ninety-six of 257 baculovirus genomes analyzed was found to contain at least one putative tRNA gene. We found great diversity in primary and secondary structure, in location within the genome, in intron presence and size, and in anti-codon identity. In some cases, genes of tRNA-containing genomes were found to have a bias for the codons specified by the tRNAs present in such genomes. Moreover, analysis revealed that most of the putative tRNA genes possessed conserved motifs for tRNA type 2 promoters, including the A-box and B-box motifs with few mismatches from the eukaryotic canonical motifs. From publicly available small RNA deep sequencing datasets of baculovirus-infected insect cells, we found evidence that a putative Autographa californica multiple nucleopolyhedrovirus Gln-tRNA gene was transcribed and modified with the addition of the non-templated 3'-CCA tail found at the end of all tRNAs. Further research is needed to determine the expression and functionality of these viral tRNAs.

Engineered nanomedicines for augmenting the efficacy of colorectal cancer immunotherapy

Colorectal cancer (CRC) is one of the most devastating diseases worldwide. Immunotherapeutic agents for CRC treatment have shown limited efficacy due to the immunosuppressive tumor microenvironment (TME). In this context, various types of nanoparticles (NPs) have been used to reverse the immunosuppressive TME, potentiate the effect of immunotherapeutic agents and reduce their systemic side effects. Many advantages could be offered by NPs, related to drug-loading efficiency, particle size and others that can potentially aid the delivery of immunotherapeutic agents. The recent research on how nano-based immunotherapy can remodel the immunosuppressive TME of CRC and hence boost the antitumor immune response, as well as the challenges that face clinical translation of NPs and future perspectives, are summarized in this review article.

Knockout of Dicer-2 in the Sf9 cell line enhances the replication of Spodoptera frugiperda rhabdovirus and conditionally increases baculovirus replication

The Sf9 cell line, originally isolated from the ovarian tissue of Spodoptera frugiperda larvae, is widely used in academia and industry for the baculovirus-mediated production of recombinant proteins and virus-like particles. RNA interference (RNAi) is a conserved antiviral pathway present in eukaryotic organisms and is the primary antiviral defence mechanism in insects. Recent evidence has implicated RNAi as an antiviral response to baculovirus infection in Sf9 cells. To test this hypothesis, CRISPR/Cas9 technology was used to disable the RNAi pathway in Sf9 cells by knocking out Dicer-2, the protein responsible for cleaving viral double-stranded RNA precursors into short interfering RNAs. Infection of Dicer-2 knockout Sf9 cells with either the wild-type baculovirus Autographa californica nucleopolyhedrovirus (AcMNPV), recombinant AcMNPV (rAcMNPV) expressing β-galactosidase (β-gal), or rAcMNPV expressing a wasp venom protein (Vn50) at a multiplicity of infection (m.o.i.) of 1 resulted in a modest increase in virus replication compared to control Sf9 cells under adherent culture conditions. In contrast, Dicer-2 knockout Sf9 monolayer or suspension cultures infected by the rAcMNPV expressing β-gal at higher m.o.i.s (3.5 and 20) did not exhibit increases in either viral DNA replication or β-gal production. Intriguingly, during long-term passaging in suspension, Dicer-2 knockout Sf9 cultures underwent transient crashes in cell proliferation and viability. It was discovered that these periods of low growth and viability coincided with a dramatic increase in the RNA levels of S. frugiperda rhabdovirus, a recently identified adventitious virus that persistently infects the Sf9 cell line, suggesting a role for Dicer-2 in managing chronic viral infections in this industrially relevant insect cell line.

Application of Baculovirus Expression Vector system (BEV) for COVID-19 diagnostics and therapeutics: a review

BACKGROUND

The baculovirus expression vector system has been developed for expressing a wide range of proteins, including enzymes, glycoproteins, recombinant viruses, and vaccines. The availability of the SARS-CoV-2 genome sequence has enabled the synthesis of SARS-CoV2 proteins in a baculovirus-insect cell platform for various applications. The most cloned SARS-CoV-2 protein is the spike protein, which plays a critical role in SARS-CoV-2 infection. It is available in its whole length or as subunits like S1 or the receptor-binding domain (RBD). Non-structural proteins (Nsps), another recombinant SARS-CoV-2 protein generated by the baculovirus expression vector system (BEV), are used in the identification of new medications or the repurposing of existing therapies for the treatment of COVID-19. Non-SARS-CoV-2 proteins generated by BEV for SARS-CoV-2 diagnosis or treatment include moloney murine leukemia virus reverse transcriptase (MMLVRT), angiotensin converting enzyme 2 (ACE2), therapeutic proteins, and recombinant antibodies. The recombinant proteins were modified to boost the yield or to stabilize the protein.

CONCLUSION

This review covers the wide application of the recombinant protein produced using the baculovirus expression technology for COVID-19 research. A lot of improvements have been made to produce functional proteins with high yields. However, there is still room for improvement and there are parts of this field of research that have not been investigated yet.

Empirical estimates of the mutation rate for an alphabaculovirus

Mutation rates are of key importance for understanding evolutionary processes and predicting their outcomes. Empirical mutation rate estimates are available for a number of RNA viruses, but few are available for DNA viruses, which tend to have larger genomes. Whilst some viruses have very high mutation rates, lower mutation rates are expected for viruses with large genomes to ensure genome integrity. Alphabaculoviruses are insect viruses with large genomes and often have high levels of polymorphism, suggesting high mutation rates despite evidence of proofreading activity by the replication machinery. Here, we report an empirical estimate of the mutation rate per base per strand copying (s/n/r) of Autographa californica multiple nucleopolyhedrovirus (AcMNPV). To avoid biases due to selection, we analyzed mutations that occurred in a stable, non-functional genomic insert after five serial passages in Spodoptera exigua larvae. Our results highlight that viral demography and the stringency of mutation calling affect mutation rate estimates, and that using a population genetic simulation model to make inferences can mitigate the impact of these processes on estimates of mutation rate. We estimated a mutation rate of μ = 1×10⁻⁷ s/n/r when applying the most stringent criteria for mutation calling, and estimates of up to μ = 5×10⁻⁷ s/n/r when relaxing these criteria. The rates at which different classes of mutations accumulate provide good evidence for neutrality of mutations occurring within the inserted region. We therefore present a robust approach for mutation rate estimation for viruses with stable genomes, and strong evidence of a much lower alphabaculovirus mutation rate than supposed based on the high levels of polymorphism observed.

Virus populations can evolve rapidly, driven by the large number of mutations that occur during virus replication. It is challenging to measure mutation rates because selection will affect which mutations are observed: beneficial mutations are overrepresented in virus populations, while deleterious mutations are selected against and therefore underrepresented. Few mutation rates have been estimated for viruses with large DNA genomes, and there are no estimates for any insect virus. Here, we estimate the mutation rate for an alphabaculovirus, a virus that infects caterpillars and has a large, 134 kilobase pair DNA genome. To ensure that selection did not bias our estimate of mutation rate, we studied which mutations occurred in a large artificial region inserted into the virus genome, where mutations did not affect viral fitness. We deep sequenced evolved virus populations, and compared the distribution of observed mutants to predictions from a simulation model to estimate mutation rate. We found evidence for a relatively low mutation rate, of one mutation in every 10 million bases replicated. This estimate is in line with expectations for a DNA virus with self-correcting replication machinery and a large genome.

Study of the heterologous gene expression characteristics of a new clone of a cell line derived from Papilio xuthus and its serum-free adaptation

The baculovirus expression vector system using insect cells as a bioreactor has been used for in vitro expression of recombinant proteins and plays an important role in the fields of biology, agronomy, and medicine. Screening suitable host cell lines is an important part of the construction of insect cell baculovirus expression systems. In previous research, we used a single-cell cloning process with the Papilio xuthus cell line RIRI-PX1 and obtained the monoclonal cell line RIRI-PX1-C31. In this study, we compared the basic biological and recombinant protein expression characteristics of RIRI-PX1-C31 and its parent cell line RIRI-PX1 and found that the expression of recombinant β-galactosidase in RIRI-PX1-C31 was significantly higher than that in the parental cell line. Further serum-free adaptation studies confirmed that RIRI-PX1-C31 can adapt to the growth environment of Express Five Serum-free medium and that its expression level of recombinant β-galactosidase was significantly higher than that before adaptation.

Secreted Trimeric Chikungunya Virus Spikes from Insect Cells: Production, Purification, and Glycosylation Status

Chikungunya virus (CHIKV) is a rapidly emerging mosquito-borne virus that causes a severe febrile illness with long-lasting arthralgia in humans. As there is no vaccine to protect humans and limit CHIKV epidemics, the virus continues to be a global public health concern. The CHIKV envelope glycoproteins E1 and E2 are important immunogens; therefore, the aim of this study is to produce trimeric CHIKV spikes in insect cells using the baculovirus expression system. The CHIKV E1 and E2 ectodomains were covalently coupled by a flexible linker that replaces the 6K transmembrane protein. The C-terminal E1 transmembrane was replaced by a Strep-tag II for the purification of secreted spikes from the culture fluid. After production in Sf9 suspension cells (product yields of 5.8–7.6 mg/L), the CHIKV spikes were purified by Strep-Tactin affinity chromatography, which successfully cleared the co-produced baculoviruses. Bis(sulfosuccinimidyl)suberate cross-linking demonstrated that the spikes are secreted as trimers. PNGase F treatment showed that the spikes are glycosylated. LC–MS/MS-based glycoproteomic analysis confirmed the glycosylation and revealed that the majority are of the mannose- or hybrid-type N-glycans and <2% have complex-type N-glycans. The LC –MS/MS analysis also revealed three O-glycosylation sites in E1. In conclusion, the trimeric, glycosylated CHIKV spikes have been successfully produced in insect cells and are now available for vaccination studies.

Virus-Like Particles: Revolutionary Platforms for Developing Vaccines Against Emerging Infectious Diseases

Virus-like particles (VLPs) are nanostructures that possess diverse applications in therapeutics, immunization, and diagnostics. With the recent advancements in biomedical engineering technologies, commercially available VLP-based vaccines are being extensively used to combat infectious diseases, whereas many more are in different stages of development in clinical studies. Because of their desired characteristics in terms of efficacy, safety, and diversity, VLP-based approaches might become more recurrent in the years to come. However, some production and fabrication challenges must be addressed before VLP-based approaches can be widely used in therapeutics. This review offers insight into the recent VLP-based vaccines development, with an emphasis on their characteristics, expression systems, and potential applicability as ideal candidates to combat emerging virulent pathogens. Finally, the potential of VLP-based vaccine as viable and efficient immunizing agents to induce immunity against virulent infectious agents, including, SARS-CoV-2 and protein nanoparticle-based vaccines has been elaborated. Thus, VLP vaccines may serve as an effective alternative to conventional vaccine strategies in combating emerging infectious diseases.

Comparison of CRISPR-Cas9 Tools for Transcriptional Repression and Gene Disruption in the BEVS

The generation of knock-out viruses using recombineering of bacmids has greatly accelerated scrutiny of baculovirus genes for a variety of applications. However, the CRISPR-Cas9 system is a powerful tool that simplifies sequence-specific genome editing and effective transcriptional regulation of genes compared to traditional recombineering and RNAi approaches. Here, the effectiveness of the CRISPR-Cas9 system for gene disruption and transcriptional repression in the BEVS was compared. Cell lines constitutively expressing the cas9 or dcas9 gene were developed, and recombinant baculoviruses delivering the sgRNA were evaluated for disruption or repression of a reporter green fluorescent protein gene. Finally, endogenous AcMNPV genes were targeted for disruption or downregulation to affect gene expression and baculovirus replication. This study provides a proof-of-concept that CRISPR-Cas9 technology may be an effective tool for efficient scrutiny of baculovirus genes through targeted gene disruption and transcriptional repression.

COVID-19 vaccine platforms: Delivering on a promise?

The emergence of the novel SARS-CoV-2 and COVID-19 has brought into sharp focus the need for a vaccine to prevent this disease. Vaccines have saved millions of lives since their introduction to the public over 200 years ago. The potential for vaccination reached new heights in the mid-20th century with the development of technologies that expanded the ability to create novel vaccines. Since then, there has been continued technological advancement in vaccine development. The resulting platforms provide the promise for solutions for many infectious diseases, including those that have been with us for decades as well as those just now emerging. Each vaccine platform represents a different technology with a unique set of advantages and challenges, especially when considering manufacturing. Therefore, it is essential to understand each platform as a separate product and process with its specific quality considerations. This review outlines the relevant platforms for developing a vaccine for SARS-CoV-2 and discusses the advantages and disadvantages of each.

A Novel Approach of Transducing Recombinant Baculovirus into Primary Lymphoid Cells of Penaeus monodon for Developing Continuous Cell Line

Cell line development from shrimp is not a novel venture as researchers across the globe have been trying to have crustacean cell lines over 30 years. The reason for not attaining a crustacean or precisely a shrimp cell line is believed to be the replicative senescence and the inability to maintain telomere length in vitro. Moreover, spontaneous in vitro transformations do not happen in shrimp cells. Oncogenic induction in primary cell culture is one of the ways to attain in vitro transformation by way of disrupting the mechanisms which involve cellular senescence. In this context, a recombinant baculovirus with shrimp viral promoter IHHNV-P2 was used for the transduction aimed at immortalization. An oncogene, H-ras, was successfully amplified and cloned in to the baculoviral vector, downstream to shrimp viral promoter IHHNV-P2 and upstream to GFP. Recombinant baculovirus with H-ras was generated and used for transduction into shrimp lymphoid cells during early dividing stage. Accordingly, fibroblast-like primary cell culture got developed, and H-ras and GFP expression could be confirmed. The study suggests that the simple method of incubating recombinant baculovirus with minced tissue enables in vitro transduction during early dividing stage of the cells, and the transduction efficiency gets enhanced by adding 5 mM sodium butyrate to the culture medium.

Evaluation of Protective Efficacy of Influenza Virus Like Particles Prepared from H5N1 Virus of Clade 2.2.1.2 in Chickens

Highly pathogenic Avian Influenza (HPAI) viruses continue to cause severe economic losses in poultry species worldwide. HPAI virus of subtype H5N1 was reported in Egypt in 2006, and despite vaccination efforts, the virus has become endemic. The current study aims to evaluate the efficacy of a virus-like particle (VLP) based vaccine in vivo using specific pathogen-free (SPF) chickens. The vaccine was prepared from the HPAI H5N1 virus of clade 2.2.1.2 using the baculovirus expression system. The VLPs were quantitated and characterized, including electron microscopy. In addition, the protection level of the VLPs was evaluated by using two different regimens, including one dose and two-dose vaccinated groups, which gave up to 70% and 100% protection level, respectively. The results of this study emphasize the potential usefulness of the VLPs-based vaccine as an alternative vaccine candidate for the control of AIV infection in poultry.

Improvement of protein production by engineering a novel antiapoptotic baculovirus vector to suppress the expression of Sf-caspase-1 and Tn-caspase-1

The baculovirus expression vector system (BEVS) is an attractive manufacturing platform for recombinant protein production in insect cells. However, baculovirus infection commonly induces host apoptosis in 3-4 days which would subsequently terminate the protein expression. Previous studies have proved that protein production by BEVS can be elevated in apoptosis-suppressed insect cells. We also developed a baculovirus vector in our previous report to inhibit the apoptosis and improve protein production in Sf9 cells. In this study, we designed five short hairpin RNA (shRNA) expression cassettes targeting a conserved region in Spodoptera frugiperda caspase-1 (Sf-caspase-1) and Trichoplusia ni caspase-1 (Tn-caspase-1), and found that introduction of C to T mutations within the stem region of the expression cassette was beneficial for the heterologous protein expression. One of the improved shRNA expression cassettes was knocked into a bacmid with the deletion of several nonessential genes. The novel baculovirus vector demonstrated the ability to suppress cell apoptosis in both Sf9 and High Five cells, and exhibited superior recombinant protein productivity of intracellularly expressed GFP and firefly luciferase and secreted glycoprotein OD-Fc. The antiapoptotic baculovirus vector developed in this study could serve as a useful tool for the protein production in scientific research and pharmaceutical industries.

Integrating high cell density cultures with adapted laboratory evolution for improved Gag-HA virus-like particles production in stable insect cell lines

Stable insect cell lines are emerging as an alternative to the insect cell-baculovirus expression vector system (IC-BEVS) for protein expression, benefiting from being a virus-free, nonlytic system. Still, the titers achieved are considerably lower. In this study, stable insect (Sf-9 and High Five) cells producing Gag virus-like particles (VLPs) were first adapted to grow under hypothermic culture conditions (22°C instead of standard 27°C), and then pseudotyped with a model membrane protein (influenza hemagglutinin [HA]) for expression of Gag-HA VLPs. Adaptation to lower temperature led to an increase in protein titers of up to 12-fold for p24 (as proxy for Gag-VLP) and sixfold for HA, with adapted Sf-9 cells outperforming High Five cells. Resulting Gag-HA VLPs producer Sf-9 cells were cultured to high cell densities, that is, 100 × 10⁶  cell/ml, using perfusion (ATF® 2) in 1 L stirred-tank bioreactors. Specific p24 and HA production rates were similar to those of batch culture, enabling to increase volumetric titers by 7-8-fold without compromising the assembly of Gag-HA VLPs. Importantly, the antigen (HA) quantity in VLPs generated using stable adapted cells in perfusion was ≈5-fold higher than that from IC-BEVS, with the added benefit of being a baculovirus-free system. This study demonstrates the potential of combining stable expression in insect cells adapted to hypothermic culture conditions with perfusion for improving Gag-HA VLPs production.

Virus-like particles: preparation, immunogenicity and their roles as nanovaccines and drug nanocarriers

Virus-like particles (VLPs) are virus-derived structures made up of one or more different molecules with the ability to self-assemble, mimicking the form and size of a virus particle but lacking the genetic material so they are not capable of infecting the host cell. Expression and self-assembly of the viral structural proteins can take place in various living or cell-free expression systems after which the viral structures can be assembled and reconstructed. VLPs are gaining in popularity in the field of preventive medicine and to date, a wide range of VLP-based candidate vaccines have been developed for immunization against various infectious agents, the latest of which is the vaccine against SARS-CoV-2, the efficacy of which is being evaluated. VLPs are highly immunogenic and are able to elicit both the antibody- and cell-mediated immune responses by pathways different from those elicited by conventional inactivated viral vaccines. However, there are still many challenges to this surface display system that need to be addressed in the future. VLPs that are classified as subunit vaccines are subdivided into enveloped and non- enveloped subtypes both of which are discussed in this review article. VLPs have also recently received attention for their successful applications in targeted drug delivery and for use in gene therapy. The development of more effective and targeted forms of VLP by modification of the surface of the particles in such a way that they can be introduced into specific cells or tissues or increase their half-life in the host is likely to expand their use in the future. Recent advances in the production and fabrication of VLPs including the exploration of different types of expression systems for their development, as well as their applications as vaccines in the prevention of infectious diseases and cancers resulting from their interaction with, and mechanism of activation of, the humoral and cellular immune systems are discussed in this review.

Mixtures of Insect-Pathogenic Viruses in a Single Virion: towards the Development of Custom-Designed Insecticides

Alphabaculoviruses (Baculoviridae) are pathogenic DNA viruses of Lepidoptera that have applications as the basis for biological insecticides and expression vectors in biotechnological processes. These viruses have a characteristic physical structure that facilitates the transmission of groups of genomes. We demonstrate that coinfection of a susceptible insect by two different alphabaculovirus species results in the production of mixed-virus occlusion bodies containing the parental viruses. This occurred between closely related and phylogenetically more distant alphabaculoviruses. Approximately half the virions present in proteinaceous viral occlusion bodies produced following coinfection of insects with a mixture of two alphabaculoviruses contained both viruses, indicating that the viruses coinfected and replicated in a single cell and were coenveloped within the same virion. This observation was confirmed by endpoint dilution assay. Moreover, both viruses persisted in the mixed-virus population by coinfection of insects during several rounds of insect-to-insect transmission. Coinfection by viruses that differed in genome size had unexpected results on the length of viral nucleocapsids, which differed from those of both parental viruses. These results have unique implications for the development of alphabaculoviruses as biological control agents of insect pests.IMPORTANCE Alphabaculoviruses are used as biological insecticides and expression vectors in biotechnology and medical applications. We demonstrate that in caterpillars infected with particular mixtures of viruses, the genomes of different baculovirus species can be enveloped together within individual virions and occluded within proteinaceous occlusion bodies. This results in the transmission of mixed-virus populations to the caterpillar stages of moth species. Once established, mixed-virus populations persist by coinfection of insect cells during several rounds of insect-to-insect transmission. Mixed-virus production technology opens the way to the development of custom-designed insecticides for control of different combinations of caterpillar pest species.

Relocation of the attTn7 Transgene Insertion Site in Bacmid DNA Enhances Baculovirus Genome Stability and Recombinant Protein Expression in Insect Cells

Baculovirus expression vectors are successfully used for the commercial production of complex (glyco)proteins in eukaryotic cells. The genome engineering of single-copy baculovirus infectious clones (bacmids) in E. coli has been valuable in the study of baculovirus biology, but bacmids are not yet widely applied as expression vectors. An important limitation of first-generation bacmids for large-scale protein production is the rapid loss of gene of interest (GOI) expression. The instability is caused by the mini-F replicon in the bacmid backbone, which is non-essential for baculovirus replication in insect cells, and carries the adjacent GOI in between attTn7 transposition sites. In this paper, we test the hypothesis that relocation of the attTn7 transgene insertion site away from the mini-F replicon prevents deletion of the GOI, thereby resulting in higher and prolonged recombinant protein expression levels. We applied lambda red genome engineering combined with SacB counterselection to generate a series of bacmids with relocated attTn7 sites and tested their performance by comparing the relative expression levels of different GOIs. We conclude that GOI expression from the odv-e56 (pif-5) locus results in higher overall expression levels and is more stable over serial passages compared to the original bacmid. Finally, we evaluated this improved next-generation bacmid during a bioreactor scale-up of Sf9 insect cells in suspension to produce enveloped chikungunya virus-like particles as a model vaccine.

Establishment and characterization of a new embryonic cell line from Helicoverpa armigera (Hübner)

Here, a new cell line, Ha168, was established from Helicoverpa armigera eggs and has been stably subcultured for over 30 passages in TNM-FH medium supplemented with 10% fetal bovine serum. The cell line consists of round and spindle-shaped cells and several giant cells. The round cells, with a cell diameter of 14.30 ± 2.804 μm, account for 77% of the cells. DNA amplification fingerprinting, random amplified polymorphic DNA analysis, and analysis of the mitochondrial cytochrome c oxidase subunit I gene confirmed that the Ha168 cells were derived from H. armigera. Karyotype analysis revealed the average chromosome number of Ha168 cells to be 71. Growth curves at passage 25 were determined and demonstrated that the cell population doubling time is 56.8 h. No mycoplasma contamination was detected in the cell line. Ha168 cells can be infected by recombinant baculovirus AcMNPV-EGFP, and exogenous protein expression level in this cell line is 70% of that in the Sf9 cell line.

Construction of gateway-compatible baculovirus expression vectors for high-throughput protein expression and in vivo microcrystal screening

Baculovirus mediated-insect cell expression systems have been widely used for producing heterogeneous proteins. However, to date, there is still the lack of an easy-to-manipulate system that enables the high-throughput protein characterization in insect cells by taking advantage of large existing Gateway clone libraries. To resolve this limitation, we have constructed a suite of Gateway-compatible pIEx-derived baculovirus expression vectors that allow the rapid and cost-effective construction of expression clones for mass parallel protein expression in insect cells. This vector collection also supports the attachment of a variety of fusion tags to target proteins to meet the needs for different research applications. We first demonstrated the utility of these vectors for protein expression and purification using a set of 40 target proteins of various sizes, cellular localizations and host organisms. We then established a scalable pipeline coupled with the SONICC and TEM techniques to screen for microcrystal formation within living insect cells. Using this pipeline, we successfully identified microcrystals for ~ 16% of the tested protein set, which can be potentially used for structure elucidation by X-ray crystallography. In summary, we have established a versatile pipeline enabling parallel gene cloning, protein expression and purification, and in vivo microcrystal screening for structural studies.

Highly Pathogenic and Low Pathogenic Avian Influenza H5 Subtype Viruses in Wild Birds in Ukraine

There have been three waves of highly pathogenic avian influenza (HPAI) outbreaks in commercial, backyard poultry, and wild birds in Ukraine. The first (2005-2006) and second (2008) waves were caused by H5N1 HPAI virus, with 45 outbreaks among commercial poultry (chickens) and backyard fowl (chickens, ducks, and geese) in four regions of Ukraine (AR Crimea, Kherson, Odesa, and Sumy Oblast). H5N1 HPAI viruses were isolated from dead wild birds: cormorants (Phalacrocorax carbo) and great crested grebes (Podiceps cristatus) in 2006 and 2008. The third HPAI wave consisted of nine outbreaks of H5N8 HPAI in wild and domestic birds, beginning in November 2016 in the central and south regions (Kherson, Odesa, Chernivtsi, Ternopil, and Mykolaiv Oblast). H5N8 HPAI virus was detected in dead mute swans (Cygnus olor), peacocks (Pavo cristatus) (in zoo), ruddy shelducks (Tadorna ferruginea), white-fronted geese (Anser albifrons), and from environmental samples in 2016 and 2017. Wide wild bird surveillance for avian influenza (AI) virus was conducted from 2006 to 2016 in Ukraine regions suspected of being intercontinental (north-south and east-west) flyways. A total of 21 511 samples were collected from 105 species of wild birds representing 27 families and 11 orders. Ninety-five avian influenza (AI) viruses were isolated (including one H5N2 LPAI virus in 2010) from wild birds with a total of 26 antigenic hemagglutinin (HA) and neuraminidase (NA) combinations. Fifteen of 16 known avian HA subtypes were isolated. Two H5N8 HPAI viruses (2016-2017) and two H5N2 LPAI viruses (2016) were isolated from wild birds and environmental samples (fresh bird feces) during surveillance before the outbreak in poultry in 2016-2017. The Ukrainian H5N1, H5N8 HPAI, and H5N2 LPAI viruses belong to different H5 phylogenetic groups. Our results demonstrate the great diversity of AI viruses in wild birds in Ukraine, as well as the importance of this region for studying the ecology of avian influenza.

Efficacy of Clade 2.3.2 H5-Recombinant Baculovirus Vaccine in Protecting Muscovy and Pekin Ducks from Clade 2.3.4.4 H5N8 Highly Pathogenic Avian Influenza Infection

In late 2016, a highly pathogenic avian influenza (HPAI) virus subtype H5N8 clade 2.3.4.4 was reported in Egypt in migratory birds; subsequently, the virus spread to backyard and commercial poultry in several Egyptian governorates, causing severe economic losses to the poultry industry. Here, a recombinant subunit commercial H5 vaccine prepared from the clade 2.3.2 H5 segment on baculovirus was evaluated in Pekin ducks (Anasplatyrhynchos domesticus) and Muscovy ducks (Cairina moschata) in Biosafety Level 3 isolators by using two vaccination regimes: either a single dose on day 10 and a challenge on day 31 or a double dose on days 10 and 28 and a challenge on day 49. The protection parameters were evaluated after experimental infection with the Egyptian HPAI H5N8 isolate clade 2.3.4.4b (A/common-coot/Egypt/CA285/2016) based on mortality rate, clinical signs, gross lesions, seroconversion, virus shedding, and histopathologic changes. In the single-dose vaccination regime, the mortality rate in Muscovy and Pekin ducks was 10% and 0% vs. 40% and 0% in nonvaccinated challenged ducks, respectively. In the double-dose vaccination regime, the mortality rates in Muscovy and Pekin ducks were 0% and 0% vs. 60% and 40% in nonvaccinated challenged ducks, respectively. Muscovy ducks developed more severe clinical signs and gross lesions than Pekin ducks. In addition, tracheal viral shedding in challenged Muscovy ducks, in the single-dose vaccination regime, was 50%, 22%, and 0% at 3, 5, and 7 days postchallenge (DPC), respectively, and was 0% in all Pekin ducks vs. 100% in all challenged nonvaccinated Muscovy and Pekin ducks at 3, 5, and 7 DPC. The viral shedding in challenged Muscovy and Pekin ducks, in the double-dose vaccination regime, was 0% at 3, 5, and 7 DPC vs. 100% in nonvaccinated challenged Muscovy and Pekin ducks, respectively. The results of this study indicate that the H5 baculovirus-based vaccine can be used in ducks with better vaccination regime based on double-dose vaccination at 10 and 28 days of age. In addition, they highlight the need to evaluate the efficacy of currently used commercial vaccines against challenge with the newly emerged HPAI H5N8 clade 2.3.4.4 in the field in Egypt to ensure proper control strategy in ducks.

Protein Expression in the Baculovirus-Insect Cell Expression System

The baculovirus-insect cell expression system is a popular choice for recombinant protein production. Post-translational modifications, production of protein complexes, and reported high protein yields are some of the favorable features of this eukaryotic expression system. The intricacies of the baculovirus-insect cell expression system may deter beginners from implementing it for routine protein production. This chapter documents a detailed protocol for using the baculovirus-insect cell expression system, which has been verified in a protein production research facility.

Adaptive laboratory evolution of stable insect cell lines for improved HIV-Gag VLPs production

Adaptive laboratory evolution (ALE) has been extensively used to modulate the phenotype of industrial model organisms (e.g. Escherichia. coli and Saccharomyces cerevisae) towards a specific trait. Nevertheless, its application to animal cells, and in particular to insect cell lines, has been very limited. In this study, we describe employing an ALE method to improve the production of HIV-Gag virus-like particles (VLPs) in stable Sf-9 and High Five cell lines. Serial batch transfer was used for evolution experiments. During the ALE process, cells were cultured under controlled hypothermic conditions (22 °C instead of standard 27 °C) for a prolonged period of time (over 3 months), which allowed the selection of a population of cells with improved phenotype. Adapted cells expressed up to 26-fold (Sf-9 cells) and 10-fold (High Five cells) more Gag-VLPs than non-adapted cells cultured at standard conditions. The production of HIV Gag-VLPs in adapted, stable insect Sf-9 cell lines was successfully demonstrated at bioreactor scale. The Gag-VLPs produced at 22 °C and 27 °C were comparable, both in size and morphology, thus confirming the null impact of adaptation process and hypothermic culture conditions on VLP's quality. This work demonstrates the suitability of ALE as a powerful method for improving yields in stable insect cell lines producing VLPs.

Highly efficient production of rabies virus glycoprotein G ectodomain in Sf9 insect cells

In the present study, we developed a complete process to produce in insect cells a high amount of the ectodomain of rabies virus glycoprotein G (G_E) as suitable antigen for detecting anti-rabies antibodies. Using the baculovirus expression vector system in Sf9 insect cells combined with a novel chimeric promoter (polh-pSeL), the expression level reached a yield of 4.1 ± 0.3 mg/L culture, which was significantly higher than that achieved with the standard polh promoter alone. The protein was recovered from the cell lysates and easily purified in only one step by metal ion affinity chromatography, with a yield of 95% and a purity of 87%. Finally, G_E was successfully used in an assay to detect specific antibodies in serum samples derived from rabies-vaccinated animals. The efficient strategy developed in this work is an interesting method to produce high amounts of this glycoprotein.

Neospora caninum antigens displaying virus-like particles as a bivalent vaccine candidate against neosporosis

Neospora caninum is a causative and transmissible agent of dog and bovine neosporosis. The resulting reproductive failures in infected cattle lead to significant economic losses worldwide. However, there is no satisfactory treatment or vaccine currently available to combat this pathogen. Thus, the development of appropriate vaccines to manage its infection and transmission is urgently needed. In this study, we expressed Rous sarcoma virus-like particles (RSV-LP) that displayed dual N. caninum antigens in silkworms. The antigen candidates are modified by adding a transmembrane domain of GP64 protein from Bombyx mori nucleopolyhedrovirus (BmNPV) to the C-terminus of surface antigen 1 (NcSAG1) and SAG1-related sequence 2 (NcSRS2). The NcSRS2 alone or the NcSAG1/NcSRS2 bivalent form displaying RSV-LPs were purified using sucrose density gradient centrifugation. These purified VLPs were then used for immunizations in gerbils, Meriones unguiculatus, to evaluate the anti-N. caninum effects in vivo. The results demonstrated that antigens displaying RSV-LPs in immunized gerbils produced the antigen-specific antibody, leading to a relatively lower parasite load after infections of N. caninum. To the best of our knowledge, this is the first study to present an RSV-LP vaccine displaying bivalent antigens from neosporosis. Taken together, our strategy suggests that silkworm-expressed virus-like particles (VLPs) are promising bivalent vaccine candidates against N. caninum infections.

SmartBac, a new baculovirus system for large protein complex production

Recent revolution of cryo-electron microscopy has opened a new door to solve high-resolution structures of macromolecule complexes without crystallization while how to efficiently obtain homogenous macromolecule complex sample is therefore becoming a bottleneck. Here we report SmartBac, an easy and versatile system for constructing large-sized transfer plasmids used to generate recombinant baculoviruses that express large multiprotein complexes in insect cells. The SmartBac system integrates the univector plasmid-fusion system, Gibson assembly method and polyprotein strategy to construct the final transfer plasmid. The fluorescent proteins are designed co-expressed with the target to monitor transfection and expression efficiencies. A scheme of screening an optimal tagged subunit for efficient purification is provided. Six large multiprotein complexes including the human exocyst complex and dynactin complex were successfully expressed and purified, suggesting a great potential of SmartBac system for its wide application in the future.

Production of biologically active feline interferon beta in insect larvae using a recombinant baculovirus

Feline interferon beta is a cytokine that belongs to the type I IFN family, with antitumor, antiviral and immunomodulatory functions. In this work, recombinant feline interferon beta (rFeIFNβ) was expressed in insect larvae that constitute important agronomic plagues. rFeIFNβ accumulated in the hemolymph of Spodoptera frugiperda larvae infected with recombinant baculovirus and was purified by Blue-Sepharose chromatography directly from larval homogenates on day 4 post-infection. rFeIFNβ was recovered after purification with a specific activity of 1 × 10⁶ IU mg^-1. By this method, we obtained 8.9 × 10⁴ IU of purified rFeIFNβ per larva. The product was biologically active in vitro, with an antiviral activity of 9.5 × 10⁴ IU mL^-1, as well as a potent antitumor activity comparable to that of the commercial FeIFNω. The glycosylation of rFeIFNβ was confirmed by peptide-N-glycosidase F digestion. Our findings provide a cost-effective platform for large-scale rFeIFNβ production in laboratory research or veterinary medicine applications.

Optimization of viral protein ratios for production of rAAV serotype 5 in the baculovirus system

Recombinant adeno-associated virus (rAAV) has become the vector of choice for the development of novel human gene therapies. High-yield manufacturing of high-quality vectors can be achieved using the baculovirus expression vector system. However, efficient production of rAAV in this insect cell-based system requires a genetic redesign of the viral protein 1 (VP1) operon. In this study, we generated a library of rationally designed rAAV serotype 5 variants with modulations in the translation-initiation region of VP1 and investigated the potency of the resulting vectors. We found that the initiation strength at the VP1 translational start had downstream effects on the VP2/VP3 ratio. Excessive incorporation of VP3 into a vector type decreased potency, even when the VP1/VP2 ratio was in balance. Finally, we successfully generated a potent rAAV vector based on serotype 5 with a balanced VP1/VP2/VP3 stoichiometry.

Establishment of Baculovirus-Expressed VLPs Induced Syncytial Formation Assay for Flavivirus Antiviral Screening

The baculovirus-insect cell expression system has been widely used for heterologous protein expression and virus-like particles (VLPs) expression. In this study, we established a new method for antiviral screening targeting to glycoprotein E of flaviviruses based on the baculovirus expression system. ZIKV is a mosquito-borne flavivirus and has posed great threat to the public health. It has been reported that ZIKV infection was associated with microcephaly and serious neurological complications. Our study showed that either ZIKV E or prME protein expressed in insect cells can form VLPs and induce membrane fusion between insect cells. Therefore, the E protein, which is responsible for receptor binding, attachment, and virus fusion during viral entry, achieved proper folding and retained its fusogenic ability in VLPs when expressed in this system. The syncytia in insect cells were significantly reduced by the anti-ZIKV-E specific polyclonal antibody in a dose-dependent manner. AMS, a thiol-conjugating reagent, was also shown to have an inhibitory effect on the E protein induced syncytia and inhibited ZIKV infection by blocking viral entry. Indeed the phenomenon of syncytial formation induced by E protein expressed VLPs in insect cells is common among flaviviruses, including Japanese encephalitis virus (JEV), Dengue virus type 2 (DENV-2), and tick-borne encephalitis virus (TBEV). This inhibition effect on syncytial formation can be developed as a novel, safe, and simple antiviral screening approach for inhibitory antibodies, peptides, or small molecules targeting to E protein of ZIKV and other flaviviruses.

Remarkably efficient production of a highly insecticidal Chrysodeixis chalcites nucleopolyhedrovirus (ChchNPV) isolate in its homologous host

BACKGROUND

A Chrysodeixis chalcites nucleopolyhedrovirus from the Canary Islands (ChchNPV-TF1) has proved to be effective for control of Chrysodeixis chalcites on banana crops. Commercialization of this virus as a bioinsecticide requires an efficient production system.

RESULTS

The sixth instar (L₆ ) was the most suitable for virus production, producing 1.80 × 10¹¹ occlusion bodies (OB)/larva and showed a lower prevalence of cannibalism (5.4%) than fourth (L₄ ) or fifth (L₅ ) instars. Inoculation of L₆ at 24 h post molting produced six times more OB (5.72 × 10¹¹ OB/larva) than recently molted L₆ larvae (1.00 × 10¹¹ OB/larva). No significant differences were recorded in mean time to death (165-175 h) or OB production per larva (3.75 × 10¹¹ to 5.97 × 10¹¹ ) or per mg larval weight (1.30 × 10¹¹ to 2.11 × 10⁹ ), in larvae inoculated with a range of inoculum concentrations (LC₅₀ -LC₉₀ ). Groups of infected L₆ larvae reared at a density of 150 larvae/container produced a greater total number of OBs (8.07 × 10¹³ OB/container) than lower densities (25, 50 and 100 OB/container), and a similar number to containers with 200 inoculated larvae (8.43 × 10¹³ OB/container).

CONCLUSION

The processes described here allow efficient production of sufficient OBs to treat ∼ 40 ha of banana crops using the insects from a single container. © 2018 Society of Chemical Industry.

Baculovirus for gene delivery to mammalian cells: Past, present and future

Baculovirus is an insect virus which has been used for more than thirty years for production of recombinant proteins in insect cells. However, baculovirus can also be harnessed for efficient gene delivery to mammalian cells if it is equipped with mammalian promoters. This technology is known as BacMam and has been used for gene delivery to immortalized cell lines, stem cells, and primary cells, as well as for gene delivery in animals. Baculovirus has unique features when compared to mammalian viruses. Besides the fact that it is replication-incompetent and does not integrate into the host genome, it has large capacity for foreign DNA. This capacity can for example be used to deliver multiple genes for reprogramming of stem cells, or for delivery of large homology constructs for genome editing. In this review, we provide a brief overview of baculovirus-based gene delivery and its recent applications in therapy and basic research. We also describe how baculovirus is manipulated for efficient transduction in mammalian cells and we highlight possible future improvements.

Titer estimation for quality control (TEQC) method: A practical approach for optimal production of protein complexes using the baculovirus expression vector system

The baculovirus expression vector system (BEVS) is becoming the method of choice for expression of many eukaryotic proteins and protein complexes for biochemical, structural and pharmaceutical studies. Significant technological advancement has made generation of recombinant baculoviruses easy, efficient and user-friendly. However, there is a tremendous variability in the amount of proteins made using the BEVS, including different batches of virus made to express the same proteins. Yet, what influences the overall production of proteins or protein complexes remains largely unclear. Many downstream applications, particularly protein structure determination, require purification of large quantities of proteins in a repetitive manner, calling for a reliable experimental set-up to obtain proteins or protein complexes of interest consistently. During our investigation of optimizing the expression of the Mediator Head module, we discovered that the ‘initial infectivity’ was an excellent indicator of overall production of protein complexes. Further, we show that this initial infectivity can be mathematically described as a function of multiplicity of infection (MOI), correlating recombinant protein yield and virus titer. All these findings led us to develop the Titer Estimation for Quality Control (TEQC) method, which enables researchers to estimate initial infectivity, titer/MOI values in a simple and affordable way, and to use these values to quantitatively optimize protein expressions utilizing BEVS in a highly reproducible fashion.

Overexpression of c-Myc enhances recombinant protein production in High Five cells after baculovirus infection

Due to their numerous advantages, baculovirus expression vector systems (BEVS) have been widely used to express recombinant proteins for different purposes. Different strategies have been adopted to increase recombinant protein production. In this study, we transiently or stably expressed mouse c-Myc in High Five cells using a commercial pIB/V5 vector. Under the control of the OpIE2 promoter, this vector could enhance recombinant protein production. We found that transient expression of c-Myc in High Five cells improved recombinant protein production. Furthermore, we established two stable cell lines, High Five-c-Myc #1 and High Five-c-Myc #2, that stably expressed mouse c-Myc. We further found that the expression level of the recombinant protein was increased in these stable cell lines compared to control cell lines. These data indicate that overexpressing c-Myc in cells is a promising way to improve recombinant protein production in BEVS.

Increasing vaccine production using pulsed ultrasound waves

Vaccination is a safe and effective approach to prevent deadly diseases. To increase vaccine production, we propose that a mechanical stimulation can enhance protein production. In order to prove this hypothesis, Sf9 insect cells were used to evaluate the increase in the expression of a fusion protein from hepatitis B virus (HBV S1/S2). We discovered that the ultrasound stimulation at a frequency of 1.5 MHz, intensity of 60 mW/cm2, for a duration of 10 minutes per day increased HBV S1/S2 by 27%. We further derived a model for transport through a cell membrane under the effect of ultrasound waves, tested the key assumptions of the model through a molecular dynamics simulation package, NAMD (Nanoscale Molecular Dynamics program) and utilized CHARMM force field in a steered molecular dynamics environment. The results show that ultrasound waves can increase cell permeability, which, in turn, can enhance nutrient / waste exchange thus leading to enhanced vaccine production. This finding is very meaningful in either shortening vaccine production time, or increasing the yield of proteins for use as vaccines.

In vivo production of recombinant proteins using occluded recombinant AcMNPV-derived baculovirus vectors

Trichoplusia ni insect larvae infected with vectors derived from the Autographa californica multiple nucleopolyhedrovirus (AcMNPV), are an excellent alternative to insect cells cultured in conventional bioreactors to produce recombinant proteins because productivity and cost-efficiency reasons. However, there is still a lot of work to do to reduce the manual procedures commonly required in this production platform that limit its scalability. To increase the scalability of this platform technology, a current bottleneck to be circumvented in the future is the need of injection for the inoculation of larvae with polyhedrin negative baculovirus vectors (Polh-) because of the lack of oral infectivity of these viruses, which are commonly used for production in insect cell cultures. In this work we have developed a straightforward alternative to obtain orally infective vectors derived from AcMNPV and expressing recombinant proteins that can be administered to the insect larvae (Trichoplusia ni) by feeding, formulated in the insect diet. The approach developed was based on the use of a recombinant polyhedrin protein expressed by a recombinant vector (Polh+), able to co-occlude any recombinant Polh- baculovirus vector expressing a recombinant protein. A second alternative was developed by the generation of a dual vector co-expressing the recombinant polyhedrin protein and the foreign gene of interest to obtain the occluded viruses. Additionally, by the incorporation of a reporter gene into the helper Polh+ vector, it was possible the follow-up visualization of the co-occluded viruses infection in insect larvae and will help to homogenize infection conditions. By using these methodologies, the production of recombinant proteins in per os infected larvae, without manual infection procedures, was very similar in yield to that obtained by manual injection of recombinant Polh- AcMNPV-based vectors expressing the same proteins. However, further analyses will be required for a detailed comparison of production yields reached by injection vs oral infections for different recombinant proteins. In conclusion, these results open the possibility of future industrial scaling-up production of recombinant proteins in insect larvae by reducing manual operations.

The use of an artificial neural network to model the infection strategy for baculovirus production in suspended insect cell cultures

Since the infection strategy in the baculovirus-insect cell system mostly affects production of the vector itself or the target product, and given that individual infection parameters interact with each other, the optimal combination must be established for each such specific system. In this work an artificial neural network was used to model infection strategy, including the cell concentration at infection, the multiplicity of infection, the medium recycle, and agitation intensity, and to evaluate the relative importance of each factor in the baculovirus production obtained. The results demonstrate that this model can be used to select an optimal infection strategy. For the baculovirus-insect cell system used in this study, this includes low multiplicity of infection and agitation intensity, along with high cell concentration at infection and medium recycle. Our model is superior to regression methods and predicts baculovirus production more precisely, thus meaning that it could be useful for the development of feasible processes, thereby improving process performance and economy.

Expression of tomato thymidine kinase 1 by means of the baculovirus expression vector system

Tomato thymidine kinase 1 (ToTK1) is a deoxyribonucleoside kinase (dNK) that has been subject to study because of its potential to phosphorylate the nucleoside analogue 3-azido-2,3-dideoxythymidine (azidothymidine, AZT) equally well as its natural substrate thymidine (dThd). The combination of ToTK1 and AZT has been tested in two animal studies for its efficiency and use in suicide gene therapy for malignant glioma. The determination of the 3D structure of ToTK1 might shed light on the structure-function relationships of nucleoside activation by this enzyme and thereby show routes toward further improvement of ToTK1 and other TK1-like dNKs for suicide gene therapy. Here we report the successful expression of both full-length ToTK1 and a C-terminal truncated ToTK1 in Spodoptera frugiperda and Trichoplusia ni insect cells using the baculovirus expression vector system. This constitutes a further step on the road to determine the 3D structure of the first TK1 of plant origin, but also an enzyme with great potential for dNK-mediated suicide gene therapy.

Expression of Clonorchis sinensis GIIIsPLA2 protein in baculovirus-infected insect cells and its overexpression facilitating epithelial-mesenchymal transition in Huh7 cells via AKT pathway

Although prior studies confirmed that group III secretory phospholipase A₂ of Clonorchis sinensis (CsGIIIsPLA₂) had stimulating effect on liver fibrosis by binding to LX-2 cells, large-scale expression of recombinant protein and its function in the progression of hepatoma are worth exploring. Because of high productivity and low lipopolysaccharides (LPS) in the Sf9-baculovirus expression system, we firstly used this system to express the coding region of CsGIIIsPLA₂. The molecular weight of recombinant CsGIIIsPLA₂ protein was about 34 kDa. Further investigation showed that most of the recombinant protein presented intracellular expression in Sf9 insect cell nucleus and could be detected only into cell debris, which made the protein purification and further functional study difficult. Therefore, to study the role of CsGIIIsPLA₂ in hepatocellular carcinoma (HCC) progression, CsGIIIsPLA₂ overexpression Huh7 cell model was applied. Cell proliferation, migration, and the expression level of epithelial-mesenchymal transition (EMT)-related molecules (E-cadherin, N-cadherin, α-catenin, Vimentin, p300, Snail, and Slug) along with possible mechanism were measured. The results indicated that CsGIIIsPLA₂ overexpression not only inhibited cell proliferation and promoted migration and EMT but also enhanced the phosphorylation of AKT in HCC cells. In conclusion, this study supported that CsGIIIsPLA₂ overexpression suppressed cell proliferation and induced EMT through the AKT pathway.