Significant productivity improvement of the baculovirus expression vector system by engineering a novel expression cassette

Improving the production of baculovirus expression vector by overexpression of IE0/IE1 through tandem promoter

The baculovirus expression vector system, known for its protein production in insect cells and has been criticized for its relatively low expression capacity. IE0/IE1, acknowledged vital early regulators of baculovirus, are indispensable for the virus proliferation and regulate the expression of various genes within the virus. Prior research has reported a substantial rise in exogenous gene expression upon overexpression of IE01. In this study, to mitigate the risk of generating defective viruses due to homologous recombination, we introduced an additional promoter in vivo within the viral genome, thus overexpressing IE0/IE1. The research outcomes demonstrate that the expression of exogenous proteins is notably enhanced without the homologous regions sequence for enhancement. In parallel, they still indicate that the upregulation of IE0/IE1 not only boosts viral titers but also enhances apoptosis within cellular populations. In sum, we successfully constructed a novel baculovirus expression vector that significantly enhances the expression of exogenous genes, presenting a new perspective for optimizing the baculovirus expression vector system.

Transcriptome-based analysis of the molecular mechanism of recombinant protein expression in Periplaneta americana cells

The Insect Cell-Baculovirus Expression Vector System (IC-BEVS) is widely used for the generation of a variety of gene products, including proteins, vaccines, and gene therapy vectors; however, it has some limitations, including a constrained host range and low protein yields. In a previous study, we established the RIRI-PA1 cell line, which was derived from Periplaneta americana. This cell line is susceptible to Autographa californica multiple nucleopolyhedrovirus (AcMNPV) infection, which results in a higher yield production of recombinant protein within a short post-infection period of 24-48 h compared to the commonly used engineered cell line Sf21. To elucidate the basis for this phenomenon, we used RNA sequencing and transcriptome analysis of RIRI-PA1 and Sf21 cells infected with AcMNPV-GFP at 24, 72, and 168 h post-infection. Differentially expressed genes (DEGs) were identified in both cell lines. GO, eggNOG, and KEGG annotation analyses were used to identify DEGs and select candidate genes that could regulate recombinant protein expression. The results indicated a significant link between ribosomal pathway regulation and recombinant protein expression. After 24 h of AcMNPV-GFP infection, relatively high levels of protein were produced in RIRI-PA1 cells compared to Sf21 cells, which exhibited lesser enrichment of ribosomal protein-related DEGs (7 : 12). Moreover, a correlation was observed in the gene expression patterns between AcMNPV-GFP infection and recombinant protein synthesis, including genes associated with the ribosome, Toll and Imd signaling, and the cytochrome P450 pathway. Overall, our findings suggested that the ribosomal pathway might be more involved in regulation of protein expression during the early stages of RIRI-PA1 infection. The mechanisms underlying this process could have potential future applications in engineering cell modifications to reduce production time for recombinant proteins and to promote the use of IC-BEVS.

Efficient Simulation of Viral Transduction and Propagation for Biomanufacturing

The design of biomanufacturing platforms based on viral transduction and/or propagation poses significant challenges at the intersection between synthetic biology and process engineering. This paper introduces vitraPro, a software toolkit composed of a multiscale model and an efficient numeric technique that can be leveraged for determining genetic and process designs that optimize transduction-based biomanufacturing platforms and viral amplification processes. Viral infection and propagation for up to two viruses simultaneously can be simulated through the model, considering viruses in either the lytic or lysogenic stage, during batch, perfusion, or continuous operation. The model estimates the distribution of the viral genome(s) copy number in the cell population, which is an indicator of transduction efficiency and viral genome stability. The infection age distribution of the infected cells is also calculated, indicating how many cells are in an infection stage compatible with recombinant product expression or viral amplification. The model can also consider the presence of defective interfering particles in the system, which can severely compromise the productivity of biomanufacturing processes. Model benchmarking and validation are demonstrated for case studies of the baculovirus expression vector system and influenza A propagation in suspension cultures.

Development of a Fully Protective Pandemic Avian Influenza Subunit Vaccine in Insect Pupae

In this study, we pioneered an alternative technology for manufacturing subunit influenza hemagglutinin (HA)-based vaccines. This innovative method involves harnessing the pupae of the Lepidoptera Trichoplusia ni (T. ni) as natural biofactories in combination with baculovirus vectors (using CrisBio® technology). We engineered recombinant baculoviruses encoding two versions of the HA protein (trimeric or monomeric) derived from a pandemic avian H7N1 virus A strain (A/chicken/Italy/5093/99). These were then used to infect T. ni pupae, resulting in the production of the desired recombinant antigens. The obtained HA proteins were purified using affinity chromatography, consistently yielding approximately 75 mg/L of insect extract. The vaccine antigen effectively immunized poultry, which were subsequently challenged with a virulent H7N1 avian influenza virus. Following infection, all vaccinated animals survived without displaying any clinical symptoms, while none of the mock-vaccinated control animals survived. The CrisBio®-derived antigens induced high titers of HA-specific antibodies in the vaccinated poultry, demonstrating hemagglutination inhibition activity against avian H7N1 and human H7N9 viruses. These results suggest that the CrisBio® technology platform has the potential to address major industry challenges associated with producing recombinant influenza subunit vaccines, such as enhancing production yields, scalability, and the speed of development, facilitating the global deployment of highly effective influenza vaccines.

Systematic comparison of rAAV vectors manufactured using large-scale suspension cultures of Sf9 and HEK293 cells

Recombinant adeno-associated virus (rAAV) vectors could be manufactured by plasmid transfection into human embryonic kidney 293 (HEK293) cells or baculovirus infection of Spodoptera frugiperda (Sf9) insect cells. However, systematic comparisons between these systems using large-scale, high-quality AAV vectors are lacking. rAAV from Sf9 cells (Sf9-rAAV) at 2-50 L and HEK293 cells (HEK-rAAV) at 2-200 L scales were characterized. HEK-rAAV had ∼40-fold lower yields but ∼10-fold more host cell DNA measured by droplet digital PCR and next-generation sequencing, respectively. The electron microscope observed a lower full/empty capsid ratio in HEK-rAAV (70.8%) than Sf9-rAAV (93.2%), while dynamic light scattering and high-performance liquid chromatography analysis showed that HEK-rAAV had more aggregation. Liquid chromatography tandem mass spectrometry identified different post-translational modification profiles between Sf9-rAAV and HEK-rAAV. Furthermore, Sf9-rAAV had a higher tissue culture infectious dose/viral genome than HEK-rAAV, indicating better infectivity. Additionally, Sf9-rAAV achieved higher in vitro transgene expression, as measured by ELISA. Finally, after intravitreal dosing into a mouse laser choroidal neovascularization model, Sf9-rAAV and HEK-rAAV achieved similar efficacy. Overall, this study detected notable differences in the physiochemical characteristics of HEK-rAAV and Sf9-rAAV. However, the in vitro and in vivo biological functions of the rAAV from these systems were highly comparable. Sf9-rAAV may be preferred over HEK293-rAAV for advantages in yields, full/empty ratio, scalability, and cost.

Autophagy mediates a direct synergistic interaction during co-transmission of two distinct arboviruses by insect vectors

Multiple viral infections in insect vectors with synergistic effects are common in nature, but the underlying mechanism remains elusive. Here, we find that rice gall dwarf reovirus (RGDV) facilitates the transmission of rice stripe mosaic rhabdovirus (RSMV) by co-infected leafhopper vectors. RSMV nucleoprotein (N) alone activates complete anti-viral autophagy, while RGDV nonstructural protein Pns11 alone induces pro-viral incomplete autophagy. In co-infected vectors, RSMV exploits Pns11-induced autophagosomes to assemble enveloped virions via N-Pns11-ATG5 interaction. Furthermore, RSMV could effectively propagate in Sf9 cells. Expression of Pns11 in Sf9 cells or leafhopper vectors causes the recruitment of N from the ER to Pns11-induced autophagosomes and inhibits N-induced complete autophagic flux, finally facilitating RSMV propagation. In summary, these results demonstrate a previously unappreciated role of autophagy in the regulation of the direct synergistic interaction during co-transmission of two distinct arboviruses by insect vectors and reveal the functional importance of virus-induced autophagosomes in rhabdovirus assembly.

Utility of Alternative Promoters for Foreign Gene Expression Using the Baculovirus Expression Vector System

The baculovirus expression vector system (BEVS) is a widely used platform for recombinant protein production for use in a wide variety of applications. Of particular interest is production of virus-like particles (VLPs), which consist of multiple viral proteins that self-assemble in strict stoichiometric ratios to mimic the structure of a virus but lacks its genetic material, while a significant amount of effort has been spent on optimizing expression ratios by co-infecting cells with multiple recombinant BEVs and modulating different process parameters, co-expressing multiple foreign genes from a single rBEV may offer more promise. However, there is currently a lack of promoters available with which to optimize co-expression of each foreign gene. To address this, previously published transcriptome data was used to identify promoters that have incrementally lower expression profiles and compared by expressing model cytoplasmic and secreted proteins. Bioinformatics was also used to identify sequence determinants that may be important for late gene transcription regulation, and translation initiation. The identified promoters and bioinformatics analyses may be useful for optimizing expression of foreign genes in the BEVS.

Homologous recombination risk in baculovirus expression vector system

The baculovirus expression vector system (BEVS) is widely used for producing recombinant proteins. To achieve high expression level of recombinant proteins, baculoviral elements, such as enhancers, promoters, signal peptide coding sequences and 3'-UTR, have been extensively employed. There is a recombination risk derived from homologous sequences between viral genome and functional baculovirus-derived elements associated with foreign genes. Although homologous recombination have distinct biological functions, these potential adverse recombination may trigger a DNA fragment being inverted or looped out, resulting in the production of defective viruses and eventual yields declines of recombinant proteins. However, the risk of such homologous recombination has not been systematically assessed. Here, we measured the recombination rate using a promoter-less fluorescent reporter integrated with various lengths homologous of p10 coding region. Homologous fragments longer than 60 bp possess sufficient recombination probability and exerts effect on purity and integrity of virus. Shortening the length of homologous fragments and separating homologous fragments by point mutations can effectively reduce unfavorable recombination. These findings reveal a homologous recombination risk resulted from genome-homologous baculoviral elements and propose reliable strategies reducing recombination rate to facilitate viral stability and integrity in baculovirus expression vector system.

Innovations in the Insect Cell Expression System for Industrial Recombinant Vaccine Antigen Production

The insect cell expression system has previously been proposed as the preferred biosecurity strategy for production of any vaccine, particularly for future influenza pandemic vaccines. The development and regulatory risk for new vaccine candidates is shortened as the platform is already in use for the manufacturing of the FDA-licensed seasonal recombinant influenza vaccine Flublok^®. Large-scale production capacity is in place and could be used to produce other antigens as well. However, as demonstrated by the 2019 SARS-CoV-2 pandemic the insect cell expression system has limitations that need to be addressed to ensure that recombinant antigens will indeed play a role in combating future pandemics. The greatest challenge may be the ability to produce an adequate quantity of purified antigen in an accelerated manner. This review summarizes recent innovations in technology areas important for enhancing recombinant-protein production levels and shortening development timelines. Opportunities for increasing product concentrations through vector development, cell line engineering, or bioprocessing and for shortening timelines through standardization of manufacturing processes will be presented.

A review of alternative promoters for optimal recombinant protein expression in baculovirus-infected insect cells

Generating recombinant proteins in insect cells has been made possible via the use of the Baculovirus Expression Vector System (BEVS). Despite the success of many proteins via this platform, some targets remain a challenge due to issues such as cytopathic effects, the unpredictable nature of co-infection and co-expressions, and baculovirus genome instability. Many promoters have been assayed for the purpose of expressing diverse proteins in insect cells, and yet there remains a lack of implementation of those results when reviewing the landscape of commercially available baculovirus vectors. In advancing the platform to produce a greater variety of proteins and complexes, the development of such constructs cannot be avoided. A better understanding of viral gene regulation and promoter options including viral, synthetic, and insect-derived promoters will be beneficial to researchers looking to utilize BEVS by recruiting these intricate mechanisms of gene regulation for heterologous gene expression. Here we summarize some of the developments that could be utilized to improve the expression of recombinant proteins and multi-protein complexes in insect cells.

Chimeric VLPs Bearing VP60 from Two Serotypes of Rabbit Haemorrhagic Disease Virus Are Protective against Both Viruses

The VP60 capsid protein from rabbit haemorrhagic disease virus (RHDV), the causative agent of one of the most economically important disease in rabbits worldwide, forms virus-like particles (VLPs) when expressed using heterologous protein expression systems such as recombinant baculovirus, yeasts, plants or mammalian cell cultures. To prevent RHDV dissemination, it would be beneficial to develop a bivalent vaccine including both RHDV GI.1- and RHDV GI.2-derived VLPs to achieve robust immunisation against both serotypes. In the present work, we developed a strategy of production of a dual-serving RHDV vaccine co-expressing the VP60 proteins from the two RHDV predominant serotypes using CrisBio technology, which uses Tricholusia ni insect pupae as natural bioreactors, which are programmed by recombinant baculovirus vectors. Co-infecting the insect pupae with two baculovirus vectors expressing the RHDV GI.1- and RHDV GI.2-derived VP60 proteins, we obtained chimeric VLPs incorporating both proteins as determined by using serotype-specific monoclonal antibodies. The resulting VLPs showed the typical size and shape of this calicivirus as determined by electron microscopy. Rabbits immunised with the chimeric VLPs were fully protected against a lethal challenge infection with the two RHDV serotypes. This study demonstrates that it is possible to generate a dual cost-effective vaccine against this virus using a single production and purification process, greatly simplifying vaccine manufacturing.

A Tale of Two Transcriptomic Responses in Agricultural Pests via Host Defenses and Viral Replication

Autographa californica Multiple Nucleopolyhedrovirus (AcMNPV) is a baculovirus that causes systemic infections in many arthropod pests. The specific molecular processes underlying the biocidal activity of AcMNPV on its insect hosts are largely unknown. We describe the transcriptional responses in two major pests, Spodoptera frugiperda (fall armyworm) and Trichoplusia ni (cabbage looper), to determine the host-pathogen responses during systemic infection, concurrently with the viral response to the host. We assembled species-specific transcriptomes of the hemolymph to identify host transcriptional responses during systemic infection and assessed the viral transcript abundance in infected hemolymph from both species. We found transcriptional suppression of chitin metabolism and tracheal development in infected hosts. Synergistic transcriptional support was observed to suggest suppression of immune responses and induction of oxidative stress indicating disease progression in the host. The entire AcMNPV core genome was expressed in the infected host hemolymph with a proportional high abundance detected for viral transcripts associated with replication, structure, and movement. Interestingly, several of the host genes that were targeted by AcMNPV as revealed by our study are also targets of chemical insecticides currently used commercially to control arthropod pests. Our results reveal an extensive overlap between biological processes represented by transcriptional responses in both hosts, as well as convergence on highly abundant viral genes expressed in the two hosts, providing an overview of the host-pathogen transcriptomic landscape during systemic infection.

Advancements in molecular design and bioprocessing of recombinant adeno-associated virus gene delivery vectors using the insect-cell baculovirus expression platform

Despite rapid progress in the field, scalable high-yield production of adeno-associated virus (AAV) is still one of the critical bottlenecks the manufacturing sector is facing. The insect cell-baculovirus expression vector system (IC-BEVS) has emerged as a mainstream platform for the scalable production of recombinant proteins with clinically approved products for human use. In this review, we provide a detailed overview of the advancements in IC-BEVS for rAAV production. Since the first report of baculovirus-induced production of rAAV vector in insect cells in 2002, this platform has undergone significant improvements, including enhanced stability of Bac-vector expression and a reduced number of baculovirus-coinfections. The latter streamlining strategy led to the eventual development of the Two-Bac, One-Bac, and Mono-Bac systems. The one baculovirus system consisting of an inducible packaging insect cell line was further improved to enhance the AAV vector quality and potency. In parallel, the implementation of advanced manufacturing approaches and control of critical processing parameters have demonstrated promising results with process validation in large-scale bioreactor runs. Moreover, optimization of the molecular design of vectors to enable higher cell-specific yields of functional AAV particles combined with bioprocess intensification strategies may also contribute to addressing current and future manufacturing challenges.

African Swine Fever Virus Ubiquitin-Conjugating Enzyme Interacts With Host Translation Machinery to Regulate the Host Protein Synthesis

African Swine Fever virus (ASFV) causes one of the most relevant emerging diseases affecting swine, now extended through three continents. The virus has a large coding capacity to deploy an arsenal of molecules antagonizing the host functions. In the present work, we have studied the only known E2 viral-conjugating enzyme, UBCv1 that is encoded by the I215L gene of ASFV. UBCv1 was expressed as an early expression protein that accumulates throughout the course of infection. This versatile protein, bound several types of polyubiquitin chains and its catalytic domain was required for enzymatic activity. High throughput mass spectrometry analysis in combination with a screening of an alveolar macrophage library was used to identify and characterize novel UBCv1-host interactors. The analysis revealed interaction with the 40S ribosomal protein RPS23, the cap-dependent translation machinery initiation factor eIF4E, and the E3 ubiquitin ligase Cullin 4B. Our data show that during ASFV infection, UBCv1 was able to bind to eIF4E, independent from the cap-dependent complex. Our results provide novel insights into the function of the viral UBCv1 in hijacking cellular components that impact the mTORC signaling pathway, the regulation of the host translation machinery, and the cellular protein expression during the ASFV lifecycle.

Identification and characterization of highly active promoters from the fall armyworm, Spodoptera frugiperda

The cell lines derived from the fall armyworm (FAW), Spodoptera frugiperda, have been widely used for production of recombinant proteins for applications in both basic research and applications in medicine and agriculture. Promoters from the nucleopolyhedrovirus (NPV) are commonly used in these expression systems. These promoters have some limitations, which may be overcome by using promoters of genes from S. frugiperda. However, information on these promoters is not available. We identified several highly expressed genes from the transcriptomes of S. frugiperda midgut, fat body, epidermis, ovarian cell line (Sf9), and a midgut cell line (Sf17). The activity of potential promoters of 21 highly expressed genes was evaluated in Sf9 and Sf17 cells. Two of these promoters, SfHSC70-P1780 and SfPub-P2009, showed higher activity than commonly used hr5/ie1 (baculovirus enhancer element, hr5 and immediate early gene 1, ie1) promoter. Interestingly, the activity of these two promoters increased after adding hr5 enhancer element. The hr5/SfPub-P2009 promoter performance was evaluated by expressing an exogenous P450 protein in Sf9 cells using a plasmid-based expression system. The activity of this promoter was also evaluated in the FAW by expressing green fluorescence protein using the baculovirus expression system. In both cases, the hr5/SfPub-P2009 promoter performed better than the commonly used hr5/ie1 promoter. These strong endogenous promoters will be useful for studies in S. frugiperda and other lepidopteran insects for multiple applications, including protein expression, genome editing, and transgenic insects.

LvYY1 Activates WSSV ie1 Promoter for Enhanced Vaccine Production and Efficacy

The baculovirus expression vector system (BEVS) has been used as a preferred platform for the production of recombinant protein complexes and efficacious vaccines. However, limited protein yield hinders the application of BEVS. It is well accepted that transcription enhancers are capable of increasing translational efficiency of mRNAs, thereby achieving better protein production. In this study, the ability of LvYY1 as a transcription enhancer was assessed. LvYY1 could interact with the WSSV ie1 promoter via binding to special DNA sites in BEVS. The effects of LvYY1 on protein expression mediated by WSSV ie1 promoter of BEVS was investigated using eGFP as a reporter gene. Enhanced eGFP expression was observed in Sf-9 cells with LvYY1. On this basis, a modified vector combining ie1 promoter and LvYY1 was developed to express either secreting CSFV E2 or baculovirus surface displayed H5 HA of AIVs. Compared to control groups without LvYY1, E2 protein yield increases to 1.6-fold, while H5 production improves as revealed by an upregulated hemagglutination titer of 8-fold at least. Moreover, with LvYY1, H5 displaying baculovirus driven by WSSV ie1 promoter (BV-LvYY1-ie1-HA) sustains the transduction activity in CEF cells. In chicken, BV-LvYY1-ie1-HA elicits a robust immune response against H5 AIVs in the absence of adjuvant, as indicated by specific antibody and cytokine responses. The findings suggest its potential function as both a vectored and subunit vaccine. These results demonstrate that the coexpression with LvYY1 serves as a promising strategy to extensively improve the efficiency of BEVS for efficacious vaccine production.

Displaying enhancing factors on the surface of occlusion bodies improves the insecticidal efficacy of a baculovirus

BACKGROUND

Baculoviruses provide long-lasting control of crop pests and are harmless to humans and non-target animals, making them attractive bioinsecticides. Autographa californica multiple nucleopolyhedrovirus (AcMNPV) has a wide-host range and is one such commercial bioinsecticide, but its low infectivity to older larvae and less-sensitive species precludes its large-scale application. We sought to improve the infectivity of AcMNPV.

RESULTS

Two enhancing factors, the truncated enhancin from Agrotis segetum granulovirus and GP37 from Cydia pomonella granulovirus, were expressed in fusion with the N-terminal and middle domain of the polyhedrin envelope protein of AcMNPV. Western blotting and immunoelectron microscopy analysis indicated that the enhancing factors were expressed on the occlusion bodies of the resulting AcMNPV variants. Bioassays showed that the median lethal doses of the recombinant viruses were 3.9-fold to 7.4-fold lower than those of the wild-type virus against the second and fourth instar of Spodoptera exigua larvae. The yields of occlusion bodies from the two recombinants in S. exigua larvae were comparable with those of the wild-type virus both in vitro and in vivo. Further bioassays showed that the AcMNPV variants fusing the enhancing factors were incapable of infecting the second instar larvae of S. litura, Helicoverpa armigera, and Pyrausta nubilalis, which were not sensitive to the wild-type AcMNPV.

CONCLUSION

These genetically modified AcMNPV variants exhibited an enhanced infectivity and may offer better baculovirus control of crop pests. © 2019 Society of Chemical Industry.

Generation and isolation of recombinant retinoid oxidoreductase complex

All-trans-retinoic acid (RA) is a bioactive lipid that influences many processes in embryonic and adult tissues. Given its bioactive nature, cellular concentrations of this molecule are highly regulated. The oxidation of all-trans-retinol to all-trans-retinaldehyde represents the first and rate-limiting step of the RA synthesis pathway. As such, it is the target of mechanisms that fine-tune RA levels within the cell. RDH10 is one enzyme responsible for the oxidation of all-trans-retinol to all-trans-retinaldehyde, and together with the all-trans-retinaldehyde reductase DHRS3 forms an oligomeric protein complex. The resulting retinoid oxidoreductase complex (ROC) is bifunctional and has the capacity to regulate steady-state levels of the direct precursor of RA, all-trans-retinaldehyde. As ROC represents a major regulatory element within the RA synthesis pathway, it is essential that methods are in place that allow for the study of this complex. Here we describe the production and isolation of recombinant ROC using a baculovirus expression system. Recombinant proteins retain enzymatic activities in intact microsomes and can be affinity purified for analysis. These methods can be used to assist in the assessment of ROC properties and the regulation of this protein complex's functional attributes.

Integrating nanoparticle quantification and statistical design of experiments for efficient HIV-1 virus-like particle production in High Five cells

The nature of enveloped virus-like particles (VLPs) has triggered high interest in their application to different research fields, including vaccine development. The baculovirus expression vector system (BEVS) has been used as an efficient platform for obtaining large amounts of these complex nanoparticles. To date, most of the studies dealing with VLP production by recombinant baculovirus infection utilize indirect detection or quantification techniques that hinder the appropriate characterization of the process and product. Here, we propose the application of cutting-edge quantification methodologies in combination with advanced statistical designs to exploit the full potential of the High Five/BEVS as a platform to produce HIV-1 Gag VLPs. The synergies between CCI, MOI, and TOH were studied using a response surface methodology approach on four different response functions: baculovirus infection, VLP production, VLP assembly, and VLP productivity. TOH and MOI proved to be the major influencing factors in contrast with previous reported data. Interestingly, a remarkable competition between Gag VLP production and non-assembled Gag was detected. Also, the use of nanoparticle tracking analysis and flow virometry revealed the existence of remarkable quantities of extracellular vesicles. The different responses of the study were combined to determine two global optimum conditions, one aiming to maximize the VLP titer (quantity) and the second aiming to find a compromise between VLP yield and the ratio of assembled VLPs (quality). This study provides a valuable approach to optimize VLP production and demonstrates that the High Five/BEVS can support mass production of Gag VLPs and potentially other complex nanoparticles.

BacMam System for Rapid Recombinant Protein Expression in Mammalian Cells

Baculovirus expression vector system (BEVS) is an established technology for recombinant protein expression in insect cells. Further, BEVS-mediated gene transduction of mammalian cells (BacMam) is emerging as a technique for high level recombinant protein expression in mammalian cells. Here, we describe generic method in using BEVS as a BacMam for rapid recombinant protein expression in mammalian cells.

Conserved HA-peptides expressed along with flagellin in Trichoplusia ni larvae protects chicken against intranasal H7N1 HPAIV challenge

The immunization of poultry where H5 and H7 influenza viruses (IVs) are endemic is one of the strategies to prevent unexpected zoonoses. Our group has been focused on conserved HA-epitopes as potential vaccine candidates to obtain multivalent immune responses against distinct IV subtypes. In this study, two conserved epitopes (NG-34 and CS-17) fused to flagellin were produced in a Baculovirus platform based on Trichoplusia ni larvae as living biofactories. Soluble extracts obtained from larvae expressing "flagellin-NG34/CS17 antigen" were used to immunize chickens and the efficacy of the vaccine was evaluated against a heterologous H7N1 HPAIV challenge in chickens. The flagellin-NG34/CS17 vaccine protected the vaccinated chickens and blocked viral shedding orally and cloacally. Furthermore, no apparent clinical signs were monitored in 10/12 vaccinated individuals. The mechanism of protection conferred is under investigation.

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.

Preincubation with a low concentration of methyl-β-cyclodextrin enhances baculovirus expression system productivity

OBJECTIVES

To enhance the productivity of foreign protein in culture cells using baculovirus expression system.

RESULTS

A low concentration of MβCD, with the optimal application concentration of 0.25 mM and the appropriate preincubation time range from 10 to 120 min, can efficiently enhance expression levels in both the AcMNPV and BmNPV expression systems.

CONCLUSIONS

Preincubation with a low concentration MβCD enhance baculovirus infection and foreign protein expression productivity.

A Newly Designed EGFP-2A Peptide Monocistronic Baculoviral Vector for Concatenating the Expression of Recombinant Proteins in Insect Cells

Recombinant proteins produced by the baculovirus expression vector system (BVES) have been widely applied in the agricultural and medical fields. However, the procedure for protein expression is inefficient and needs to be improved. Herein, we propose a simple construct that incorporates a selectable marker (enhanced green fluorescent protein, EGFP) and a picorna viral-derived “self-cleaving” 2A-like peptide to separate the EGFP and target proteins in a monocistronic baculovirus vector to facilitate isolation of the recombinant baculovirus in the BVES. In this study, porcine adiponectin (ADN), a secreted, multimeric protein with insulin-sensitizing properties, was used to demonstrate its utility in our EGFP-2A-based expression system. EGFP and ADN were simultaneously expressed by a recombinant alphabaculovirus. Co-expression of EGFP facilitates the manipulation of the following processes, such as determining expression kinetics and harvesting ADN. The results showed that the 2A “self-cleaving” process does not interfere with EGFP activity or with signal peptide removal and the secretion of recombinant ADN. Posttranslational modifications, including glycosylation, of the recombinant ADN occurred in insect cells, and the formation of various multimers was further verified. Most importantly, the insect-produced ADN showed a similar bioactivity to that of mammalian cells. This concept provides a practical and economic approach that utilizes a new combination of alphabaculovirus/insect cell expression systems for future applications.

Synthetic biology for bioengineering virus-like particle vaccines

Vaccination is the most effective method of disease prevention and control. Many viruses and bacteria that once caused catastrophic pandemics (e.g., smallpox, poliomyelitis, measles, and diphtheria) are either eradicated or effectively controlled through routine vaccination programs. Nonetheless, vaccine manufacturing remains incredibly challenging. Viruses exhibiting high antigenic diversity and high mutation rates cannot be fairly contested using traditional vaccine production methods and complexities surrounding the manufacturing processes, which impose significant limitations. Virus-like particles (VLPs) are recombinantly produced viral structures that exhibit immunoprotective traits of native viruses but are noninfectious. Several VLPs that compositionally match a given natural virus have been developed and licensed as vaccines. Expansively, a plethora of studies now confirms that VLPs can be designed to safely present heterologous antigens from a variety of pathogens unrelated to the chosen carrier VLPs. Owing to this design versatility, VLPs offer technological opportunities to modernize vaccine supply and disease response through rational bioengineering. These opportunities are greatly enhanced with the application of synthetic biology, the redesign and construction of novel biological entities. This review outlines how synthetic biology is currently applied to engineer VLP functions and manufacturing process. Current and developing technologies for the identification of novel target-specific antigens and their usefulness for rational engineering of VLP functions (e.g., presentation of structurally diverse antigens, enhanced antigen immunogenicity, and improved vaccine stability) are described. When applied to manufacturing processes, synthetic biology approaches can also overcome specific challenges in VLP vaccine production. Finally, we address several challenges and benefits associated with the translation of VLP vaccine development into the industry.

The Coming Age of Insect Cells for Manufacturing and Development of Protein Therapeutics

Protein therapeutics is a rapidly growing segment of the pharmaceutical market. Currently, the majority of protein therapeutics are manufactured in mammalian cells for their ability to generate safe and efficacious human-like glycoproteins. The high cost of using mammalian cells for manufacturing has motivated a constant search for alternative host platforms. Insect cells have begun to emerge as a promising candidate, largely due to the development of the baculovirus expression vector system. While there are continuing efforts to improve insect-baculovirus expression for producing protein therapeutics, key limitations including cell lysis and the lack of homogeneous humanized glycosylation still remain. The field has started to see a movement toward virus-less gene expression approaches, notably the use of clustered regularly interspaced short palindromic repeats to address these shortcomings. This review highlights recent technological advances that are realizing the transformative potential of insect cells for the manufacturing and development of protein therapeutics.

Development of a baculovirus vector carrying a small hairpin RNA for suppression of sf-caspase-1 expression and improvement of recombinant protein production

Background

The Baculovirus expression vector system (BEVS) is a transient expression platform for recombinant protein production in insect cells. Baculovirus infection of insect cells will shutoff host translation and induce apoptosis and lead to the termination of protein expression. Previous reports have demonstrated the enhancement of protein yield in BEVS using stable insect cell lines expressing interference RNA to suppress the expression of caspase-1.

Results

In this study, short-hairpin RNA (shRNA) expression cassettes targeting Spodoptera frugiperda caspase-1 (Sf-caspase-1) were constructed and inserted into an Autographa californica multiple nucleopolyhedrovirus (AcMNPV) vector. Using the recombinant baculovirus vectors, we detected the suppression of Sf-caspase-1 expression and cell apoptosis. Green fluorescent protein (GFP), Discosoma sp. Red (DsRed) and firefly luciferase were then expressed as reporter proteins. The results showed that suppression of apoptosis enhanced the accumulation of exogenous proteins at 2 and 3 days post infection. After 4 days post infection, the activity of the reporter proteins remained higher in BEVS using the baculovirus carrying shRNA in comparison with the control without shRNA, but the accumulated protein levels showed no obvious difference between them, suggesting that apoptosis suppression resulted in improved protein folding rather than translation efficiency at the very late stage of baculovirus infection.

Conclusions

The baculovirus vector developed in this study would be a useful tool for the production of active proteins suitable for structural and functional studies or pharmaceutical applications in Sf9 cells, and it also has the potential to be adapted for the improvement of protein expression in different insect cell lines that can be infected by AcMNPV.

Impact of Inverted Terminal Repeat Integrity on rAAV8 Production Using the Baculovirus/Sf9 Cells System

Adeno-associated virus (AAV) inverted terminal repeats (ITRs) are key elements of AAV. These guanine-cytosine-rich structures are involved in the replication and encapsidation of the AAV genome, along with its integration in and excision from the host genome. These sequences are the only AAV-derived DNA sequences conserved in recombinant AAV (rAAV), as they allow its replication, encapsidation, and long-term maintenance and expression in target cells. Due to the original vector design, plasmids containing the gene of interest flanked by ITRs and used for rAAV production often present incomplete, truncated, or imperfect ITR sequences. For example, pSUB201 and its derivatives harbor a truncated (14 nt missing on the external part of the ITR), flop-orientated ITR plus 46 bp of non-ITR viral DNA at each end of the rAAV genome. It has been shown that rAAV genomes can be replicated, even with incomplete, truncated, or imperfect ITR sequences, leading to the production of rAAV vectors in transfection experiments. Nonetheless, it was hypothesized that unmodified wild-type (WT) ITR sequences could lead to a higher yield of rAAV, with less non-rAAV encapsidated DNA originating from the production cells and/or baculovirus shuttle vector genomes. This work studied the impact of imperfect ITRs on the level of encapsidated rAAV genomes and baculovirus-derived DNA sequences using the baculovirus/Sf9 cells production system. Replacement of truncated ITRs with WT and additional wtAAV2 sequences has an impact on the two major features of rAAV production: (1) a rise from 10% to 40% of full capsids obtained, and (2) up to a 10-fold reduction in non-rAAV encapsidated DNA. Furthermore, this study considered the impact on these major parameters of additional ITR elements and ITRs coupled with various regulatory elements of different origins. Implementation of the use of complete ITRs in the frame of the baculovirus-based rAAV expression system is one step that will be required to optimize the quality of rAAV-based gene therapy drugs.

Cardiac Non-Human Leukocyte Antigen Identification: Techniques and Troubles

Historically efforts have focused on the human leukocyte antigen (HLA) as the major cause for acute and chronic rejection following cardiac transplantation. However, rising evidence indicates that non-HLA antibodies can be both primary initiators and modifiers of antibody-mediated rejection (AMR) and cardiac allograft vasculopathy (CAV). The purpose of this review is to assess currently available technologies for non-HLA identification and leveraging such responses toward antibody quantification. Several techniques have been used to identify antigenic determinants of recipient graft-specific non-HLA humoral immune responses, but each comes with its own set of benefits and caveats. Improving our ability to detect non-HLA humoral immune response will aid in our understanding of the underlying antigenic determinants of AMR and CAV, as well as improve patient outcomes.

Improving the baculovirus expression vector system with vankyrin-enhanced technology

The baculovirus expression vector system (BEVS) is a widely used platform for the production of recombinant eukaryotic proteins. However, the BEVS has limitations in comparison to other higher eukaryotic expression systems. First, the insect cell lines used in the BEVS cannot produce glycoproteins with complex‐type N‐glycosylation patterns. Second, protein production is limited as cells die and lyse in response to baculovirus infection. To delay cell death and lysis, we transformed several insect cell lines with an expression plasmid harboring a vankyrin gene (P‐vank‐1), which encodes an anti‐apoptotic protein. Specifically, we transformed Sf9 cells, Trichoplusia ni High Five^TM cells, and SfSWT‐4 cells, which can produce glycoproteins with complex‐type N‐glycosylation patterns. The latter was included with the aim to increase production of glycoproteins with complex N‐glycans, thereby overcoming the two aforementioned limitations of the BEVS. To further increase vankyrin expression levels and further delay cell death, we also modified baculovirus vectors with the P‐vank‐1 gene. We found that cell lysis was delayed and recombinant glycoprotein yield increased when SfSWT‐4 cells were infected with a vankyrin‐encoding baculovirus. A synergistic effect in elevated levels of recombinant protein production was observed when vankyrin‐expressing cells were combined with a vankyrin‐encoding baculovirus. These effects were observed with various model proteins including medically relevant therapeutic proteins. In summary, we found that cell lysis could be delayed and recombinant protein yields could be increased by using cell lines constitutively expressing vankyrin or vankyrin‐encoding baculovirus vectors. © 2017 The Authors Biotechnology Progress published by Wiley Periodicals, Inc. on behalf of American Institute of Chemical Engineers Biotechnol. Prog., 33:1496–1507, 2017

A novel baculovirus-derived promoter with high activity in the baculovirus expression system

The baculovirus expression vector system (BEVS) has been widely used to produce a large number of recombinant proteins, and is becoming one of the most powerful, robust, and cost-effective systems for the production of eukaryotic proteins. Nevertheless, as in any other protein expression system, it is important to improve the production capabilities of this vector. The orf46 viral gene was identified among the most highly abundant sequences in the transcriptome of Spodoptera exigua larvae infected with its native baculovirus, the S. exigua multiple nucleopolyhedrovirus (SeMNPV). Different sequences upstream of the orf46 gene were cloned, and their promoter activities were tested by the expression of the GFP reporter gene using the Autographa californica nucleopolyhedrovirus (AcMNPV) vector system in different insect cell lines (Sf21, Se301, and Hi5) and in larvae from S. exigua and Trichoplusia ni. The strongest promoter activity was defined by a 120 nt sequence upstream of the ATG start codon for the orf46 gene. On average, GFP expression under this new promoter was more than two fold higher than the expression obtained with the standard polyhedrin (polh) promoter. Additionally, the orf46 promoter was also tested in combination with the polh promoter, revealing an additive effect over the polh promoter activity. In conclusion, this new characterized promoter represents an excellent alternative to the most commonly used baculovirus promoters for the efficient expression of recombinant proteins using the BEVS.

Identification of Essential Genetic Baculoviral Elements for Recombinant Protein Expression by Transactivation in Sf21 Insect Cells

The Baculovirus Expression Vector System (BEVS) is widely used to produce high amounts of recombinant proteins. Nevertheless, generating recombinant baculovirus in high quality is rather time-consuming and labor-intensive. Alternatively, virus-free expression in insect cells did not achieve similar expression levels for most proteins so far. The transactivation method is a promising approach for protein expression in Sf21 cells. It combines advantages of BEVS and plasmid-based expression by activating strong virus-dependent promoters on a transfected plasmid by baculoviral coinfection. Here, we identified expression elements required for transactivation. Therefore, we designed several vectors comprising different viral promoters or promoter combinations and tested them for eGFP expression using the automated BioLector microcultivation system. Remarkably, only the combination of the very late promoter p10 together with the homologous region 5 (hr5) could boost expression during transactivation. Other elements, like p10 alone or the late viral promoter polH, did not respond to transactivation. A new combination of hr5 and p10 with the strongest immediate early OpMNPV viral promoter OpIE2 improved the yield of eGFP by ~25% in comparison to the previous applied hr5-IE1-p10 expression cassette. Furthermore, we observed a strong influence of the transcription termination sequence and vector backbone on the level of expression. Finally, the expression levels for transactivation, BEVS and solely plasmid-based expression were compared for the marker protein eGFP, underlining the potential of transactivation for fast recombinant protein expression in Sf21 cells. In conclusion, essential elements for transactivation could be identified. The optimal elements were applied to generate an improved vector applicable in virus-free plasmid-based expression, transactivation and BEVS.

Improved Production Efficiency of Virus-Like Particles by the Baculovirus Expression Vector System

Vaccines based on virus-like particles (VLPs) have proven effective in humans and animals. In this regard, the baculovirus expression vector system (BEVS) is one of the technologies of choice to generate such highly immunogenic vaccines. The extended use of these vaccines for human and animal populations is constrained because of high production costs, therefore a significant improvement in productivity is crucial to ensure their commercial viability. Here we describe the use of the previously described baculovirus expression cassette, called TB, to model the production of two VLP-forming vaccine antigens in insect cells. Capsid proteins from porcine circovirus type 2 (PCV2 Cap) and from the calicivirus that causes rabbit hemorrhagic disease (RHDV VP60) were expressed in insect cells using baculoviruses genetically engineered with the TB expression cassette. Productivity was compared to that obtained using standard counterpart vectors expressing the same proteins under the control of the polyhedrin promoter. Our results demonstrate that the use of the TB expression cassette increased the production yields of these vaccine antigens by around 300% with respect to the standard vectors. The recombinant proteins produced by TB-modified vectors were fully functional, forming VLPs identical in size and shape to those generated by the standard baculoviruses, as determined by electron microscopy analysis. The use of the TB expression cassette implies a simple modification of the baculovirus vectors that significantly improves the cost efficiency of VLP-based vaccine production, thereby facilitating the commercial viability and broad application of these vaccines for human and animal health.