A simple plasmid-based transient gene expression method using High Five cells

Nonviral Platform for Expression of Recombinant Protein in Insect Cells

Nonviral transfection has been used to express various recombinant proteins, therapeutics, and virus-like particles (VLP) in mammalian and insect cells. Virus-free methods for protein expression require fewer steps for obtaining protein expression by eliminating virus amplification and measuring the infectivity of the virus. The nonviral method uses a nonlytic plasmid to transfect the gene of interest into the insect cells instead of using baculovirus, a lytic system. In this chapter, we describe one of the transfection methods, which uses polyethyleneimine (PEI) as a DNA delivery material into the insect cells to express the recombinant protein in both adherent and suspension cells.

A concise guide to choosing suitable gene expression systems for recombinant protein production

This overview guides both novices and experienced researchers facing challenging targets to select the most appropriate gene expression system for producing a particular protein. By answering four key questions, readers can determine the most suitable gene expression system following a decision scheme. This guide addresses the most commonly used and accessible systems and provides brief descriptions of the main gene expression systems' key characteristics to assist decision making. Additionally, information has been included for selected less frequently used "exotic" gene expression systems.

A sensitive assay for scrutiny of Autographa californica multiple nucleopolyhedrovirus genes using CRISPR-Cas9

Baculoviruses have very large genomes and previous studies have demonstrated improvements in recombinant protein production and genome stability through the removal of some nonessential sequences. However, recombinant baculovirus expression vectors (rBEVs) in widespread use remain virtually unmodified. Traditional approaches for generating knockout viruses (KOVs) require several experimental steps to remove the target gene prior to the generation of the virus. In order to optimize rBEV genomes by removing nonessential sequences, more efficient techniques for establishing and evaluating KOVs are required. Here, we have developed a sensitive assay utilizing CRISPR-Cas9-mediated gene targeting to examine the phenotypic impact of disruption of endogenous Autographa californica multiple nucleopolyhedrovirus (AcMNPV) genes. For validation, 13 AcMNPV genes were targeted for disruption and evaluated for the production of GFP and progeny virus - traits that are essential for their use as vectors for recombinant protein production. The assay involves transfection of sgRNA into a Cas9-expressing Sf9 cell line followed by infection with a baculovirus vector carrying the gfp gene under the p10 or p6.9 promoters. This assay represents an efficient strategy for scrutinizing AcMNPV gene function through targeted disruption, and represents a valuable tool for developing an optimized rBEV genome. KEY POINTS: [Formula: see text] A method to scrutinize the essentiality of baculovirus genes was developed. [Formula: see text] The method uses Sf9-Cas9 cells, a targeting plasmid carrying a sgRNA, and a rBEV-GFP. [Formula: see text] The method allows scrutiny by only needing to modify the targeting sgRNA plasmid.

Screening of Membrane Protein Production by Comparison of Transient Expression in Insect and Mammalian Cells

Membrane proteins are difficult biomolecules to express and purify. In this paper, we compare the small-scale production of six selected eukaryotic integral membrane proteins in insect and mammalian cell expression systems using different techniques for gene delivery. The target proteins were C terminally fused to the green fluorescent marker protein GFP to enable sensitive monitoring. We show that the choice of expression systems makes a considerable difference to the yield and quality of the six selected membrane proteins. Virus-free transient gene expression (TGE) in insect High Five cells combined with solubilization in dodecylmaltoside plus cholesteryl hemisuccinate generated the most homogeneous samples for all six targets. Further, the affinity purification of the solubilized proteins using the Twin-Strep^® tag improved protein quality in terms of yield and homogeneity compared to His-tag purification. TGE in High Five insect cells offers a fast and economically attractive alternative to the established methods that require either baculovirus construction and the infection of the insect cells or relatively expensive transient gene expression in mammalian cells for the production of integral membrane proteins.

Baculovirus-Free SARS-CoV-2 Virus-like Particle Production in Insect Cells for Rapid Neutralization Assessment

Virus-like particles (VLPs) resemble authentic virus while not containing any genomic information. Here, we present a fast and powerful method for the production of SARS-CoV-2 VLP in insect cells and the application of these VLPs to evaluate the inhibition capacity of monoclonal antibodies and sera of vaccinated donors. Our method avoids the baculovirus-based approaches commonly used in insect cells by employing direct plasmid transfection to co-express SARS-CoV-2 envelope, membrane, and spike protein that self-assemble into VLPs. After optimization of the expression plasmids and vector ratios, VLPs with an ~145 nm diameter and the typical "Corona" aura were obtained, as confirmed by nanoparticle tracking analysis (NTA) and transmission electron microscopy (TEM). Fusion of the membrane protein to GFP allowed direct quantification of binding inhibition to angiotensin II-converting enzyme 2 (ACE2) on cells by therapeutic antibody candidates or sera from vaccinated individuals. Neither VLP purification nor fluorescent labeling by secondary antibodies are required to perform these flow cytometric assays.

Baculovirus-Free SARS-CoV-2 Virus-like Particle Production in Insect Cells for Rapid Neutralization Assessment

Virus-like particles (VLPs) resemble authentic virus while not containing any genomic information. Here, we present a fast and powerful method for the production of SARS-CoV-2 VLP in insect cells and the application of these VLPs to evaluate the inhibition capacity of monoclonal antibodies and sera of vaccinated donors. Our method avoids the baculovirus-based approaches commonly used in insect cells by employing direct plasmid transfection to co-express SARS-CoV-2 envelope, membrane, and spike protein that self-assemble into VLPs. After optimization of the expression plasmids and vector ratios, VLPs with an ~145 nm diameter and the typical “Corona” aura were obtained, as confirmed by nanoparticle tracking analysis (NTA) and transmission electron microscopy (TEM). Fusion of the membrane protein to GFP allowed direct quantification of binding inhibition to angiotensin II-converting enzyme 2 (ACE2) on cells by therapeutic antibody candidates or sera from vaccinated individuals. Neither VLP purification nor fluorescent labeling by secondary antibodies are required to perform these flow cytometric assays.

Reproducible and Easy Production of Mammalian Proteins by Transient Gene Expression in High Five Insect Cells

The expression of mammalian recombinant proteins in insect cell lines using transient-plasmid-based gene expression enables the production of high-quality protein samples. Here, the procedure for virus-free transient gene expression (TGE) in High Five insect cells is described in detail. The parameters that determine the efficiency and reproducibility of the method are presented in a robust protocol for easy implementation and set-up of the method. The applicability of the TGE method in High Five cells for proteomic, structural, and functional analysis of the expressed proteins is shown.

Insect High FiveTM cell line development using site-specific flipase recombination technology

Insect Trichoplusia ni High FiveTM (Hi5) cells have been widely explored for production of heterologous proteins, traditionally mostly using the lytic baculovirus expression vector system (BEVS), and more recently using virus-free transient gene expression systems. Stable expression in such host cells would circumvent the drawbacks associated with both systems when it comes to scale-up and implementation of more efficient high-cell density process modes for the manufacturing of biologics. In this work, we combined Flipase (Flp) recombinase-mediated cassette exchange (RMCE) with fluorescence-activated cell sorting (FACS) for generating a stable master clonal Hi5 cell line with the flexibility to express single or multiple proteins of interest from a tagged genomic locus. The 3-step protocol herein implemented consisted of (i) introducing the RMCE docking cassette into the cell genome by random integration followed by selection in Hygromycin B and FACS (Hi5-tagging population), (ii) eliminating cells tagged in loci with low recombination efficiency by transfecting the tagging population with an eGFP-containing target cassette followed by selection in G418 and FACS (Hi5-RMCE population), and (iii) isolation of pure eGFP-expressing cells by FACS and expansion to suspension cultures (Hi5-RMCE master clone). Exchangeability of the locus in the master clone was demonstrated in small-scale suspension cultures by replacing the target cassette by one containing a single protein (i.e. iCherry, as an intracellular protein model) or two proteins (i.e. influenza HA and M1 for virus-like particles production, as an extracellular protein model). Overall, the stable insect Hi5 cell platform herein assembled has the potential to assist and accelerate biologics development.

Validation of the Production of Antibodies in Different Formats in the HEK 293 Transient Gene Expression System

Mammalian cells are the most commonly used production system for therapeutic antibodies. Protocols for the expression of recombinant antibodies in HEK293-6E cells in different antibody formats are described in detail. As model, antibodies against Kallikrein-related peptidase 7 (KLK7) were used. KLK7 is a key player in skin homeostasis and represents an emerging target for pharmacological interventions. Potent inhibitors can not only help to elucidate physiological and pathophysiological functions but also serve as a new archetype for the treatment of inflammatory skin disorders. Phage display-derived affinity-matured human anti-KLK7 antibodies were converted to scFv-Fc, IgG, and Fab formats and transiently produced in the mammalian HEK293-6E system. For the production of the corresponding antigen-KLK7-the baculovirus expression vector system (BEVS) and virus-free expression in Hi5 insect cells were used in a comparative approach. The target proteins were isolated by various chromatographic methods in a one- or multistep purification strategy. Ultimately, the interaction between anti-KLK7 and KLK7 was characterized using biolayer interferometry. Here, protocols for the expression of recombinant antibodies in different formats are presented and compared for their specific features. Furthermore, biolayer interferometry (BLI), a fast and high-throughput biophysical analytical technique to evaluate the kinetic binding constant and affinity constant of the different anti-KLK7 antibody formats against Kallikrein-related peptidase 7 is presented.

Accelerating HIV-1 VLP production using stable High Five insect cell pools

Stable cell pools are receiving a renewed interest as a potential alternative system to clonal cell lines. The shorter development timelines and the capacity to achieve high product yields make them an interesting approach for recombinant protein production. In this study, stable High Five cell pools are assessed for the production of a simple protein, mCherry, and the more complex HIV-1 Gag-eGFP virus-like particles (VLPs). Random integration coupled to fluorescence-activated cell sorting (FACS) in suspension conditions is applied to accelerate the stable cell pool generation process and enrich it with high producer cells. This methodology is successfully transferred to a bioreactor for VLP production, resulting in a 2-fold increase in VLP yields with respect to shake flask cultures. In these conditions, maximum viable cell concentration improves by 1.5-fold, and by-product formation is significantly reduced. Remarkably, a global increase in the uptake of amino acids in the Gag-eGFP stable cell pool is observed when compared with parental High Five cells, reflecting the additional metabolic burden associated with VLP production. These results suggest that stable High Five cell pools are a robust and powerful approach to produce VLPs and other recombinant proteins, and put the basis for future studies aiming to scale up this system.

Baculovirus-free insect cell expression system for high yield antibody and antigen production

Antibodies are essential tools for therapy and diagnostics. Yet, production remains expensive as it is mostly done in mammalian expression systems. As most therapeutic IgG require mammalian glycosylation to interact with the human immune system, other expression systems are rarely used for production. However, for neutralizing antibodies that are not required to activate the human immune system as well as antibodies used in diagnostics, a cheaper production system would be advantageous. In our study, we show cost-efficient, easy and high yield production of antibodies as well as various secreted antigens including Interleukins and SARS-CoV-2 related proteins in a baculovirus-free insect cell expression system. To improve yields, we optimized the expression vector, media and feeding strategies. In addition, we showed the feasibility of lyophilization of the insect cell produced antibodies. Furthermore, stability and activity of the antibodies was compared to antibodies produced by Expi293F cells revealing a lower aggregation of antibodies originating from High Five cell production. Finally, the newly established High Five expression system was compared to the Expi293F mammalian expression system in regard of yield and costs. Most interestingly, all tested proteins were producible in our High Five cell expression system what was not the case in the Expi293F system, hinting that the High Five cell system is especially suited to produce difficult-to-express target proteins.

The Current Status of COVID-19 Vaccines

The current COVID-19 pandemic has substantially accelerated the demands for efficient vaccines. A wide spectrum of approaches includes live attenuated and inactivated viruses, protein subunits and peptides, viral vector-based delivery, DNA plasmids, and synthetic mRNA. Preclinical studies have demonstrated robust immune responses, reduced viral loads and protection against challenges with SARS-CoV-2 in rodents and primates. Vaccine candidates based on all delivery systems mentioned above have been subjected to clinical trials in healthy volunteers. Phase I clinical trials have demonstrated in preliminary findings good safety and tolerability. Evaluation of immune responses in a small number of individuals has demonstrated similar or superior levels of neutralizing antibodies in comparison to immunogenicity detected in COVID-19 patients. Both adenovirus- and mRNA-based vaccines have entered phase II and study protocols for phase III trials with 30,000 participants have been finalized.

PEI-Mediated Transient Transfection of High Five Cells at Bioreactor Scale for HIV-1 VLP Production

High Five cells are an excellent host for the production of virus-like particles (VLPs) with the baculovirus expression vector system (BEVS). However, the concurrent production of high titers of baculovirus hinder the purification of these nanoparticles due to similarities in their physicochemical properties. In this study, first a transient gene expression (TGE) method based on the transfection reagent polyethylenimine (PEI) is optimized for the production of HIV-1 VLPs at shake flask level. Furthermore, VLP production by TGE in High Five cells is successfully demonstrated at bioreactor scale, resulting in a higher maximum viable cell concentration (5.1 × 10⁶ cell/mL), the same transfection efficiency and a 1.8-fold increase in Gag-eGFP VLP production compared to shake flasks. Metabolism analysis of High Five cells indicates a reduction in the consumption of the main metabolites with respect to non-transfected cell cultures, and an increase in the uptake rate of several amino acids when asparagine is depleted. Quality assessment by nanoparticle tracking analysis and flow virometry of the VLPs produced shows an average size of 100–200 nm, in agreement with immature HIV-1 viruses reported in the literature. Overall, this work demonstrates that the High Five/TGE system is a suitable approach for the production of VLP-based vaccine candidates and other recombinant proteins.

Identifying parameters to improve the reproducibility of transient gene expression in High Five cells

Virus-free, transient gene expression (TGE) in High Five cells was recently presented as an efficient protein production method. However, published TGE protocols have not been standardized to a general protocol. Therefore, reproducibility and implementation of the method in other labs remains difficult. The aim of this study is to analyse the parameters determining the reproducibility of the TGE in insect cells. Here, we identified that using linear 40 kDa PEI instead of 25 kDa PEI was one of the most important aspects to improve TGE. Furthermore, DNA amount, DNA:PEI ratio, growth phase of the cells before transfection, passage number, the origin of the High-Five cell isolates and the type of cultivation medium were considered. Interestingly, a correlation of the passage number to the DNA content of single cells (ploidy) and to the transfection efficacy could be shown. The optimal conditions for critical parameters were used to establish a robust TGE method. Finally, we compared the achieved product yields in High Five cells using our improved TGE method with both the baculoviral expression system and TGE in the mammalian HEK293-6E cell line. In conclusion, the presented robust TGE protocol in High Five cells is easy to establish and produces ample amounts of high-quality recombinant protein, bridging the gap in expression level of this method to the well-established mammalian TGE in HEK293 cells as well as to the baculoviral expression vector system (BEVS).

Engineering DNA vaccines against infectious diseases

Engineering vaccine-based therapeutics for infectious diseases is highly challenging, as trial formulations are often found to be nonspecific, ineffective, thermally or hydrolytically unstable, and/or toxic. Vaccines have greatly improved the therapeutic landscape for treating infectious diseases and have significantly reduced the threat by therapeutic and preventative approaches. Furthermore, the advent of recombinant technologies has greatly facilitated growth within the vaccine realm by mitigating risks such as virulence reversion despite making the production processes more cumbersome. In addition, seroconversion can also be enhanced by recombinant technology through kinetic and nonkinetic approaches, which are discussed herein. Recombinant technologies have greatly improved both amino acid-based vaccines and DNA-based vaccines. A plateau of interest has been reached between 2001 and 2010 for the scientific community with regard to DNA vaccine endeavors. The decrease in interest may likely be attributed to difficulties in improving immunogenic properties associated with DNA vaccines, although there has been research demonstrating improvement and optimization to this end. Despite improvement, to the extent of our knowledge, there are currently no regulatory body-approved DNA vaccines for human use (four vaccines approved for animal use). This article discusses engineering DNA vaccines against infectious diseases while discussing advantages and disadvantages of each, with an emphasis on applications of these DNA vaccines.

Statement of Significance

This review paper summarizes the state of the engineered/recombinant DNA vaccine field, with a scope entailing “Engineering DNA vaccines against infectious diseases”. We endeavor to emphasize recent advances, recapitulating the current state of the field. In addition to discussing DNA therapeutics that have already been clinically translated, this review also examines current research developments, and the challenges thwarting further progression. Our review covers: recombinant DNA-based subunit vaccines; internalization and processing; enhancing immune protection via adjuvants; manufacturing and engineering DNA; the safety, stability and delivery of DNA vaccines or plasmids; controlling gene expression using plasmid engineering and gene circuits; overcoming immunogenic issues; and commercial successes. We hope that this review will inspire further research in DNA vaccine development.

Antiviral Activity of Fermented Ginseng Extracts against a Broad Range of Influenza Viruses

Ginseng products used as herb nutritional supplements are orally consumed and fermented to ginsenoside compounds by the intestinal microbes. In this study, we investigated antiviral protective effects of fermented ginseng extracts against different strains of influenza viruses in genetically diverse mouse models. Intranasal coinoculation of mice with fermented ginseng extract and influenza virus improved survival rates and conferred protection against H1N1, H3N2, H5N1, and H7N9 strains, with the efficacy dependent on the dose of ginseng samples. Antiviral protection by fermented ginseng extract was observed in different genetic backgrounds of mice and in the deficient conditions of key adaptive immune components (CD4, CD8, B cell, MHCII). The mice that survived primary virus inoculation with fermented ginseng extract developed immunity against the secondary infection with homologous and heterosubtypic viruses. In vitro cell culture experiments showed moderate virus neutralizing activity by fermented ginseng extract, probably by inhibiting hemagglutination and neuraminidase activity. This study suggests that fermented ginseng extracts might provide a means to treat influenza disease regardless of virus strains.

Transient Gene Expression in Suspension HEK293-EBNA1 Cells

Transient gene expression in human embryo kidney 293 (HEK293) cells is an established approach for the rapid production of large amounts of recombinant proteins (r-proteins). Milligram to gram quantities of r-proteins can be typically obtained within less than 10 days following transfection. In this chapter, we describe a simple and robust transfection process of suspension-growing human embryo kidney 293 cells using two commercially available serum-free media and polyethylenimine as the transfection reagent. This chapter provides examples for the production and purification of a his-tagged recombinant protein and two monoclonal antibodies.

A new single-step protocol for rapid baculovirus-driven protein production in insect cells

Background

In the last three decades, the Baculovirus expression vector system (BEV) has evolved to one of the most widely used eukaryotic systems for heterologous protein expression including approved vaccines and therapies. Despite the significant improvements introduced during the past years, the BEV system still has major drawbacks, primarily the time required to generate recombinant virus and virus instability for certain target proteins. In this study we show that the conventional method to generate recombinant Baculovirus using a Tn7 transposition based system can be shortened to a single-step transfection-only procedure without further amplification.

Methods

In a first step we have adapted a recently published protocol that replaces the standard liposome-based transfection procedure of adherent insect cells by transfecting insect cells in suspension with a preformed DNA-PEI complex generating P0 virus. We have then expressed and purified six different target proteins, among them four intracellular and two secreted proteins, by infecting insect cells either with P0 or P1 virus.

Results

We demonstrate that transfection in suspension is as efficient as the standard protocol, but in addition allows generation of high amounts of P0 virus early in the process. To test if this P0 virus generated by bacmid transfection can be used directly for protein expression in either the screening or production process, we compared P0 versus amplified P1 virus-mediated protein expression. We show that protein expression levels, purity and yield of the purified proteins are equally high for P0 and P1.

Conclusion

The standard protocol for generating recombinant baculovirus comprises transfection of the bacmid followed by one or two subsequent virus amplification steps. In this study we show that Baculovirus generated by transfection-only is equally efficient in driving protein expression. This reduces the time from bacmid DNA to protein to eight days and reduces the risk of virus decay. In contrast to transient gene expression protocols, the required amount of DNA is minimal: 100 µg bacmid DNA is sufficient for a production scale of 10 L.

Transient Expression and Cellular Localization of Recombinant Proteins in Cultured Insect Cells

Heterologous protein expression systems are used for the production of recombinant proteins, the interpretation of cellular trafficking/localization, and the determination of the biochemical function of proteins at the sub-organismal level. Although baculovirus expression systems are increasingly used for protein production in numerous biotechnological, pharmaceutical, and industrial applications, nonlytic systems that do not involve viral infection have clear benefits but are often overlooked and underutilized. Here, we describe a method for generating nonlytic expression vectors and transient recombinant protein expression. This protocol allows for the efficient cellular localization of recombinant proteins and can be used to rapidly discern protein trafficking within the cell. We show the expression of four recombinant proteins in a commercially available insect cell line, including two aquaporin proteins from the insect Bemisia tabaci, as well as subcellular marker proteins specific for the cell plasma membrane and for intracellular lysosomes. All recombinant proteins were produced as chimeras with fluorescent protein markers at their carboxyl termini, which allows for the direct detection of the recombinant proteins. The double transfection of cells with plasmids harboring constructs for the genes of interest and a known subcellular marker allows for live cell imaging and improved validation of cellular protein localization.

Not Limited to E. coli: Versatile Expression Vectors for Mammalian Protein Expression

Recombinant protein expression is not limited to E. coli or other prokaryotic systems. It is inevitable to use eukaryotic systems in order to express challenging mammalian proteins. Eukaryotic systems are able to perform complex posttranslational modifications like protein processing, phosphorylation, glycosylation, which are essential for stability and functionality of many proteins. Different eukaryotic protein expression systems employing yeast, insect, or mammalian cell lines are established with each having its own advantages and disadvantages. Often it is quite difficult to decide which will be the most optimal expression system as this depends highly on the protein itself. Expression in stable cell lines requires substantial screening of expressible constructs prior to developing a stable expression cell line. To achieve fast screening by transient expression in multiple hosts, versatile vectors can be applied. In this chapter, we present an overview of the most common multi-host vectors, which allow for fast expression analysis without tedious (re)cloning of the gene of interest in several different protein production systems. The protocols in this chapter describe the latest methods for fast transient expression in insect and mammalian cell lines.

Disposable orbitally shaken TubeSpin bioreactor 600 for Sf9 cell cultivation in suspension

Disposable orbitally shaken TubeSpin bioreactor 600 tubes (TS600s) were recently developed for the bench-scale cultivation of animal cells in suspension. Here we compared batch cultures of Sf9 insect cells in TS600s, spinner flasks, and shake flasks. Superior cell growth was observed in TS600s and shake flasks as compared with spinner flasks, and more favorable oxygen-enriched cell culture conditions were observed in TS600s as compared with either spinner or shake flasks. The results demonstrated the suitability of TS600s as a disposable vessel for the cultivation of Sf9 cells in suspension.

Fast plasmid based protein expression analysis in insect cells using an automated SplitGFP screen

Recombinant protein expression often presents a bottleneck for the production of proteins for use in many areas of animal‐cell biotechnology. Difficult‐to‐express proteins require the generation of numerous expression constructs, where popular prokaryotic screening systems often fail to identify expression of multi domain or full‐length protein constructs. Post‐translational modified mammalian proteins require an alternative host system such as insect cells using the Baculovirus Expression Vector System (BEVS). Unfortunately this is time‐, labor‐, and cost‐intensive. It is clearly desirable to find an automated and miniaturized fast multi‐sample screening method for protein expression in such systems. With this in mind, in this paper a high‐throughput initial expression screening method is described using an automated Microcultivation system in conjunction with fast plasmid based transient transfection in insect cells for the efficient generation of protein constructs. The applicability of the system is demonstrated for the difficult to express Nucleotide‐binding Oligomerization Domain‐containing protein 2 (NOD2). To enable detection of proper protein expression the rather weak plasmid based expression has been improved by a sensitive inline detection system. Here we present the functionality and application of the sensitive SplitGFP (split green fluorescent protein) detection system in insect cells. The successful expression of constructs is monitored by direct measurement of the fluorescence in the BioLector Microcultivation system. Additionally, we show that the results obtained with our plasmid‐based SplitGFP protein expression screen correlate directly to the level of soluble protein produced in BEVS. In conclusion our automated SplitGFP screen outlines a sensitive, fast and reliable method reducing the time and costs required for identifying the optimal expression construct prior to large scale protein production in baculovirus infected insect cells. Biotechnol. Bioeng. 2016;113: 1975–1983. © 2016 The Authors. Biotechnology and Bioengineering Published by Wiley Periodicals, Inc.