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.

RMCE-based insect cell platform to produce membrane proteins captured on HIV-1 Gag virus-like particles.

Conformationally complex membrane proteins (MPs) are therapeutic targets in many diseases, but drug discovery has been slowed down by the lack of efficient production tools. Co-expression of MPs with matrix proteins from enveloped viruses is a promising approach to obtain correctly folded proteins at the surface of virus-like particles (VLPs), preserving their native lipidic environment. Here, we implemented a site-specific recombinase-mediated cassette exchange (RMCE) strategy to establish a reusable HIV-1 Gag-expressing insect cell line for fast production of target MPs on the surface of Gag-VLPs. The Sf9 cell line was initially tagged with a Gag-GFP-expressing cassette incorporating two flipase recognition target sites (FRTs), one within the fusion linker of Gag-GFP. The GFP cassette was afterwards replaced by a Cherry cassette via flipase (Flp) recombination. The fusion of Gag to fluorescent proteins enabled high-throughput screening of cells with higher Gag expression and Flp-mediated cassette exchange ability, while keeping the functionality of the VLP scaffold unaltered. The best cell clone was then Flp-recombinated to produce Gag-VLPs decorated with a human ß2-adrenergic receptor (ß2AR). Release of a fluorescently labeled ß2AR into the culture supernatant was confirmed by immunoblotting, and its co-localization with Gag-VLPs was visualized by confocal microscopy. Furthermore, the differential avidity of ß2AR-dsplaying Gag-VLPs versus "naked" Gag-VLPs to an anti-ß2AR antibody measured by ELISA corroborated the presence of ß2AR at the surface of the Gag-VLPs. In conclusion, this novel insect cell line represents a valuable platform for fast production of MPs in their native conformation, which can accelerate small-molecule and antibody drug discovery programs.

Flipase-mediated cassette exchange in Sf9 insect cells for stable gene expression.

Site-specific DNA integration allows predictable heterologous gene expression and circumvents extensive clone screening. Herein, the establishment of a Flipase (Flp)-mediated cassette exchange system in Sf9 insect cells for targeted gene integration is described. A tagging cassette harboring a reporter dsRed gene was randomly introduced into the cell genome after screening different transfection protocols. Single-copy integration clones were then co-transfected with both Flp-containing plasmid and an EGFP-containing targeting cassette. Successful cassette exchange was suggested by emergence of G418-resistant green colonies and confirmed by PCR analysis, showing the absence of the tagging cassette and single integration of the targeting cassette in the same locus. Upon cassette exchange, uniform EGFP expression between clones derived from the same integration site was obtained. Moreover, the resulting cell clones exhibited the expression properties of the parental cell line. EGFP production titers over 40 mg/L were of the same order of magnitude as those achieved through baculovirus infection. This Sf9 master cell line constitutes a versatile and re-usable platform to produce multiple recombinant proteins for fundamental and applied research.

Insect High Five™ cell line development using site-specific flipase recombination technology.

Insect Trichoplusia ni High Five™ (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 study, 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.

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.

Insect cell line development using FLP-mediated cassette exchange technology.

Traditional cell line development is quite laborious and time-consuming as it is based on the random integration of the gene of interest which leads to unpredictable expression behavior. In opposition, recombinase-mediated cassette exchange systems represent a powerful genetic engineering approach, allowing site-specific insertion of recombinant genes into pre-tagged genomic loci with superior expression characteristics, thus bypassing the need for extensive clone screening and shortening the development timelines. Such systems have not been widely implemented in insect cell lines used for the production of recombinant proteins most commonly through the baculovirus expression vector system. Herein, it is provided the protocol for the implementation of a FLP-mediated cassette exchange system in Spodoptera frugiperda Sf 9 cells, in order to grant a flexible cell line for the stable production of recombinant proteins.

Production of rotavirus core-like particles in Sf9 cells using recombinase-mediated cassette exchange.

A flexible Sf9 insect cell line was recently developed leveraging the recombinase-mediated cassette exchange (RMCE) technology, which competes with the popular baculovirus expression vector system (BEVS) in terms of speed to produce new proteins. Herein, the ability of this cell platform to produce complex proteins, such as rotavirus core-like particles, was evaluated. A gene construct coding for a VP2-GFP fusion protein was targeted to a pre-characterized high recombination efficiency locus flanked by flipase (Flp) recognition target sites and, after three weeks in selection, an isogenic cell population was obtained. Despite the lower cell specific productivities with respect to those obtained by baculovirus infection, the titers of VP2-GFP reached in shake flask batch cultures were comparable as a result of higher cell densities. To further improve the VP2-GFP levels from stable expression, analysis of exhausted medium was undertaken to design feeding strategies enabling higher cell densities as well as increased culture duration. The implementation of the best strategy allowed reaching 20 million cells per ml in bioreactor cultures; the integrity of the rotavirus core-like particles could be confirmed by electron microscopy. Overall, we show that this Sf9-Flp cell platform represents a valuable alternative to the BEVS for producing complex recombinant proteins, such as rotavirus core-like particles.

A cell sorting protocol for selecting high-producing sub-populations of Sf9 and High Five™ cells.

Insect cell lines such as Sf9 and High Five™ have been widely used to produce recombinant proteins mostly by the lytic baculovirus vector system. We have recently established an expression platform in Sf9 cells using a fluorescence-based recombinase mediated cassette exchange (RMCE) strategy which has similar development timelines but avoids baculovirus infection. To expedite cell engineering efforts, a robust fluorescence-activated cell sorting (FACS) protocol optimized for insect cells was developed here. The standard sorting conditions used for mammalian cells proved to be unsuitable, resulting in post-sorting viabilities below 10% for both cell lines. We found that the extreme sensitivity to the shear stress displayed by Sf9 and High Five™ cells was the limiting factor, and using Pluronic F-68 in the cell suspension could increase post-sorting viabilities in a dose dependent manner. The newly developed protocol was then used to sort stable populations of both cell lines tagged with a DsRed-expressing cassette. Before sorting, the average fluorescence intensity of the Sf9 cell population was 3-fold higher than that of the High Five™ cell population. By enriching with the 10% strongest DsRed-fluorescent cells, the productivity of both cell populations could be successfully improved. The established sorting protocol potentiates the use of RMCE technology for recombinant protein production in insect cells.

Effect of different periods of hyperbaric oxygen on ischemia-reperfusion injury of rat skeletal muscle.

The effect of hyperbaric oxygen (HBO) on ischemia-reperfusion injury of skeletal muscle, applied during different periods, was studied in 56 male rats. Animals were subjected to 6-h ischemia by a tourniquet over the major femoral trocanter and 4 (A) or 24 (B) h of reperfusion. HBO was carried out during 1 h in an acrylic chamber at a pressure of 2.0 ATA (100% oxygen): in the last 60 min of ischemia (II), after ischemia, during 1-h reperfusion time (III), and during the last hour of ischemia plus 1-h reperfusion (IV). Group I was the control group. After 4- or 24-h reperfusion, samples of the soleus muscle were stained by H&E and analyzed immunohistochemically. No interstitial hemorrhage, neutrophil infiltrate, or cellular necrosis were induced by HBO. The apoptosis index did not differ among the groups. HBO reduced morphologic alterations and promoted better results when administered in the ischemia plus reperfusion period (GIV).