A novel purification strategy for retrovirus gene therapy vectors using heparin affinity chromatography

The Exosome-Mediated Bone Regeneration: An Advanced Horizon Toward the Isolation, Engineering, Carrying Modalities, and Mechanisms

Exosomes, nanoparticles secreted by various cells, composed of a bilayer lipid membrane, and containing bioactive substances such as proteins, nucleic acids, metabolites, etc., have been intensively investigated in tissue engineering owing to their high biocompatibility and versatile biofunction. However, there is still a lack of a high-quality review on bone defect regeneration potentiated by exosomes. In this review, the biogenesis and isolation methods of exosomes are first introduced. More importantly, the engineered exosomes of the current state of knowledge are discussed intensively in this review. Afterward, the biomaterial carriers of exosomes and the mechanisms of bone repair elucidated by compelling evidence are presented. Thus, future perspectives and concerns are revealed to help devise advanced modalities based on exosomes to overcome the challenges of bone regeneration. It is totally believed this review will attract special attention from clinicians and provide promising ideas for their future works.

Quasi-perfusion studies for intensified lentiviral vector production using a continuous stable producer cell line

Quasi-perfusion culture was employed to intensify lentiviral vector (LV) manufacturing using a continuous stable producer cell line in an 8-day process. Initial studies aimed to identify a scalable seeding density, with 3, 4, and 5 × 104 cells cm-2 providing similar specific productivities of infectious LV. Seeding at 3 × 104 cells cm-2 was selected, and the quasi-perfusion was modulated to minimize inhibitory metabolite accumulation and vector exposure at 37°C. Similar specific productivities of infectious LV and physical LV were achieved at 1, 2, and 3 vessel volumes per day (VVD), with 1 VVD selected to minimize downstream processing volumes. The optimized process was scaled 50-fold to 1,264 cm2 flasks, achieving similar LV titers. However, scaling up beyond this to a 6,320 cm2 multilayer flask reduced titers, possibly from suboptimal gas exchange. Across three independent processes in 25 cm2 to 6,320 cm2 flasks, reproducibility was high with a coefficient of variation of 7.7% ± 2.9% and 11.9% ± 3.0% for infectious and physical LV titers, respectively. The optimized flask process was successfully transferred to the iCELLis Nano (Cytiva) fixed-bed bioreactor, with quasi-perfusion at 1 VVD yielding 1.62 × 108 TU.

Improved Manufacturing Methods of Extracellular Vesicles Pseudotyped with the Vesicular Stomatitis Virus Glycoprotein

Extracellular vesicles (EV), which expose the vesicular stomatitis virus glycoprotein (VSVG) on their surface, are used for delivery of nucleic acids and proteins in human cell lines. These particles are biomanufactured using methods that are difficult to scale up. Here, we describe the development of the first EV-VSVG production process in serum-free media using polyethylenimine (PEI)-based transient transfection of HEK293 suspension cells, as well as the first EV-VSVG purification process to utilize both ultracentrifugation and chromatography. Three parameters were investigated for EV-VSVG production: cell density, DNA concentration, and DNA:PEI ratio. The best production titer was obtained with 3 × 106 cells/mL, a plasmid concentration of 2 µg/mL, and a DNA:PEI ratio of 1:4. The production kinetics of VSVG was performed and showed that the highest amount of VSVG was obtained 3 days after transfection. Addition of cell culture supplements during the transfection resulted in an increase in VSVG production, with a maximum yield obtained with 2 mM of sodium butyrate added 18 h after transfection. Moreover, the absence of EV-VSVG during cell transfection with a GFP-coding plasmid revealed to be ineffective, with no fluorescent cells. An efficient EV-VSVG purification procedure consisting of a two-step concentration by low-speed centrifugation and sucrose cushion ultracentrifugation followed by a heparin affinity chromatography purification was also developed. Purified bioactive EV-VSVG preparations were characterized and revealed that EV-VSVG are spherical particles of 176.4 ± 88.32 nm with 91.4% of protein similarity to exosomes.

High yield sterile filtration process for highly concentrated lentiviral vectors

The development and manufacture of biopharmaceuticals are subject to strict regulations that specify the required minimum quality of the products. A key measure to meet these quality requirements is the integration of a sterile filtration step into the commercial manufacturing process. Whereas common procedures for most biologics exist, this is challenging for lentiviral vector (LVV) production for ex vivo gene therapy. LVVs nominal size is more than half the pore size (0.2 µm) of filters used for sterile filtration. Hence, highly concentrated virus solutions are prone to filter clogging if aggregation of viruses occurs or impurities attach to the viruses. Several filters were screened aiming to identify those which allow filtering highly concentrated stocks of LVVs of up to 1E + 9 transducing units mL-1 , which corresponds to 4.5E + 12 particles mL-1 . In addition, the effect of endonuclease treatment upstream of the purification process on filter performance was studied. In summary, three suitable filters were identified in a small-scale study (<15 mL) with virus yields >80% and the process was successfully scaled-up to a final scale of 100 mL LVV stock solution.

Evaluation of the Robustness Verification of Downstream Production Process for Inactivated SARS-CoV-2 Vaccine and Different Chromatography Medium Purification Effects

BACKGROUND

Large-scale vaccine production requires downstream processing that focuses on robustness, efficiency, and cost-effectiveness.

METHODS

To assess the robustness of the current vaccine production process, three batches of COVID-19 Omicron BA.1 strain hydrolytic concentrated solutions were selected. Four gel filtration chromatography media (Chromstar 6FF, Singarose FF, Bestarose 6B, and Focurose 6FF) and four ion exchange chromatography media (Maxtar Q, Q Singarose, Diamond Q, and Q Focurose) were used to evaluate their impact on vaccine purification. The quality of the vaccine was assessed by analyzing total protein content, antigen content, residual Vero cell DNA, residual Vero cell protein, and residual bovine serum albumin (BSA). Antigen recovery rate and specific activity were also calculated. Statistical analysis was conducted to evaluate process robustness and the purification effects of the chromatography media.

RESULTS

The statistical analysis revealed no significant differences in antigen recovery (p = 0.10), Vero HCP residue (p = 0.59), Vero DNA residue (p = 0.28), and BSA residue (p = 0.97) among the three batches of hydrolytic concentrated solutions processed according to the current method. However, a significant difference (p < 0.001) was observed in antigen content.

CONCLUSIONS

The study demonstrated the remarkable robustness of the current downstream process for producing WIBP-CorV vaccines. This process can adapt to different batches of hydrolytic concentrated solutions and various chromatography media. The research is crucial for the production of inactivated SARS-CoV-2 vaccines and provides a potential template for purifying other viruses.

The downstream bioprocess toolbox for therapeutic viral vectors

Viral vectors are poised to acquire a prominent position in modern medicine and biotechnology owing to their role as delivery agents for gene therapies, oncolytic agents, vaccine platforms, and a gateway to engineer cell therapies as well as plants and animals for sustainable agriculture. The success of viral vectors will critically depend on the availability of flexible and affordable biomanufacturing strategies that can meet the growing demand by clinics and biotech companies worldwide. In this context, a key role will be played by downstream process technology: while initially adapted from protein purification media, the purification toolbox for viral vectors is currently undergoing a rapid expansion to fit the unique biomolecular characteristics of these products. Innovation efforts are articulated on two fronts, namely (i) the discovery of affinity ligands that target adeno-associated virus, lentivirus, adenovirus, etc.; (ii) the development of adsorbents with innovative morphologies, such as membranes and 3D printed monoliths, that fit the size of viral vectors. Complementing these efforts are the design of novel process layouts that capitalize on novel ligands and adsorbents to ensure high yield and purity of the product while safeguarding its therapeutic efficacy and safety; and a growing panel of analytical methods that monitor the complex array of critical quality attributes of viral vectors and correlate them to the purification strategies. To help explore this complex and evolving environment, this study presents a comprehensive overview of the downstream bioprocess toolbox for viral vectors established in the last decade, and discusses present efforts and future directions contributing to the success of this promising class of biological medicines.

Continuous multi-membrane chromatography of large viral particles

Continuous multi-column chromatography (CMCC) has been successfully implemented to address biopharmaceutical biomolecule instability, to improve process efficiency, and to reduce facility footprint and capital cost. This paper explores the implementation of a continuous multi-membrane chromatography (CMMC) process, using four membrane units, for a large viral particle in just few weeks. CMMC improves the efficiency of the chromatography step by enabling higher loads with smaller membranes for multiple cycles of column use and enables steady-state continuous bioprocessing. The separation performance of CMMC was compared to a conventional batch chromatographic capture step used at full manufacturing scale. The product step yield was 80% using CMMC versus 65% in batch mode while increasing slightly the relative purity. Furthermore, the total amount of membrane area required for the CMMC approach was approximately 10% of the area needed for batch operation, while realizing similar processing times. Since CMMC uses smaller membrane sizes, it can take advantage of the high flow rates achievable for membrane chromatography that are not typically possible at larger membrane scales due to skid flow rate limitations. As such, CMMC offers the potential for more efficient and cost-effective purification trains.

Affinity and Pseudo-Affinity Membrane Chromatography for Viral Vector and Vaccine Purifications: A Review

Several chromatographic approaches have been established over the last decades for the production of pharmaceutically relevant viruses. Due to the large size of these products compared to other biopharmaceuticals, e.g., proteins, convective flow media have proven to be superior to bead-based resins in terms of process productivity and column capacity. One representative of such convective flow materials is membranes, which can be modified to suit the particular operating principle and are also suitable for economical single-use applications. Among the different membrane variants, affinity surfaces allow for the most selective separation of the target molecule from other components in the feed solution, especially from host cell-derived DNA and proteins. A successful membrane affinity chromatography, however, requires the identification and implementation of ligands, which can be applied economically while at the same time being stable during the process and non-toxic in the case of any leaching. This review summarizes the current evaluation of membrane-based affinity purifications for viruses and virus-like particles, including traditional resin and monolith approaches and the advantages of membrane applications. An overview of potential affinity ligands is given, as well as considerations of suitable affinity platform technologies, e.g., for different virus serotypes, including a description of processes using pseudo-affinity matrices, such as sulfated cellulose membrane adsorbers.

Implementation of Novel Affinity Ligand for Lentiviral Vector Purification

The use of viral vectors as therapeutic products for multiple applications such as vaccines, cancer treatment, or gene therapies, has been growing exponentially. Therefore, improved manufacturing processes are needed to cope with the high number of functional particles required for clinical trials and, eventually, commercialization. Affinity chromatography (AC) can be used to simplify purification processes and generate clinical-grade products with high titer and purity. However, one of the major challenges in the purification of Lentiviral vectors (LVs) using AC is to combine a highly specific ligand with a gentle elution condition assuring the preservation of vector biological activity. In this work, we report for the first time the implementation of an AC resin to specifically purify VSV-G pseudotyped LVs. After ligand screening, different critical process parameters were assessed and optimized. A dynamic capacity of 1 × 1011 total particles per mL of resin was determined and an average recovery yield of 45% was found for the small-scale purification process. The established AC robustness was confirmed by the performance of an intermediate scale providing an infectious particles yield of 54%, which demonstrates the scalability and reproducibility of the AC matrix. Overall, this work contributes to increasing downstream process efficiency by delivering a purification technology that enables high purity, scalability, and process intensification in a single step, contributing to time-to-market reduction.

Combination of polythyleneimine regulating autophagy prodrug and Mdr1 siRNA for tumor multidrug resistance

Multidrug resistance (MDR) has been restricting the efficacy of chemotherapy, which mainly include pump resistance and non-pump resistance. In order to fight overall MDR, a novel targeted gene/drug co-deliver nano system is developed, which can suppress the drug efflux pumps and modulate autophagy to overcoming both pump and non-pump resistance. Here, small interfere RNA (siRNA) is incorporated into polymer-drug conjugates (PEI-PTX, PP) which are composed of polyethyleneimine (PEI) and paclitaxel (PTX) via covalent bonds, and hyaluronic acid (HA) is coated on the surface of PP/siRNA to achieve long blood cycle and CD44-targeted delivery. The RNA interference to mdr1 gene is combined with autophagy inhibition by PP, which efficiently facilitate apoptosis of Taxol-resistant lung cancer cells (A549/T). Further study indicates that PEI in PP may play a significant role to block the autophagosome–lysosome fusion process by means of alkalizing lysosomes. Both in vitro and in vivo studies confirm that the nanoassemblies can successfully deliver PTX and siRNA into tumor cells and significantly inhibited A549/T tumor growth. In summary, the polymeric nanoassemblies provide a potential strategy for combating both pump and non-pump resistance via the synergism of RNAi and autophagy modulation.

Lentiviral Vectors for Ocular Gene Therapy

This review offers the basics of lentiviral vector technologies, their advantages and pitfalls, and an overview of their use in the field of ophthalmology. First, the description of the global challenges encountered to develop safe and efficient lentiviral recombinant vectors for clinical application is provided. The risks and the measures taken to minimize secondary effects as well as new strategies using these vectors are also discussed. This review then focuses on lentiviral vectors specifically designed for ocular therapy and goes over preclinical and clinical studies describing their safety and efficacy. A therapeutic approach using lentiviral vector-mediated gene therapy is currently being developed for many ocular diseases, e.g., aged-related macular degeneration, retinopathy of prematurity, inherited retinal dystrophies (Leber congenital amaurosis type 2, Stargardt disease, Usher syndrome), glaucoma, and corneal fibrosis or engraftment rejection. In summary, this review shows how lentiviral vectors offer an interesting alternative for gene therapy in all ocular compartments.

Evaluation of Lentiviral Vector Stability and Development of Ion Exchange Purification Processes

In this manuscript we employ parallel batch stability and chromatographic screens in concert with linear and step gradient experiments to develop a high yield, HCP clearance anion exchange capture process for lentiviral vector (LVV) purification. An initial broad resin screen is carried out to determine anion exchange-based resins that exhibit high recovery of LVV. LVV stability is then evaluated and conditions are established where the vector exhibits good stability, namely phosphate buffer at pH 6.5-7.5, with low to moderate salt concentrations. A subsequent high-throughput batch screen is then carried out with a subset of resins selected from the first screen under stable conditions to identify optimal wash and elution steps to further improve product yield and protein clearance. Linear gradient experiments are also conducted in mini-column format to refine the operating conditions and final step gradient processes are established that exhibit greater than 70% yield of infectious LVV while also achieving up to 2.89 log reduction values (LRV) of HCPs during the process. The large set of stability and chromatographic data provided in this work represent an important contribution to knowledge in the field about the chromatographic efficacy of a wide range of resins for LVV bioprocessing under stable conditions.

Clinical grade lentiviral vector purification and quality control requirements

Lentiviral vectors have been proven to be a powerful tool in gene therapies that includes the ability to perform long-term gene editing in both dividing and non-dividing cells. In order to meet the rising demand of clinical grade lentiviral vectors for future clinical trials and requirements by regulatory agencies, new methods and technologies were developed, including the rapid optimization of production and purification processes. However, gaps still exist in achieving ideal yields and recovery rates in large-scale manufacturing process steps. The downstream purification process is a critical step required to obtain sufficient quantity and high-quality lentiviral vectors products, which is challenged by the low stability of the LV particles and large production volumes associated with the manufacturing process. This review summarizes the most recent and promising technologies and enhancements used in the large-scale purification process step of LV manufacturing and aims to provide a significant contribution towards the achievement of providing sufficient quantity and quality of LVs in scalable processes. This article is protected by copyright. All rights reserved.

Scaled preparation of extracellular vesicles from conditioned media

Extracellular vesicles (EVs) especially of mesenchymal stem/stomal cells (MSCs) are increasingly considered as biotherapeutic agents for a variety of different diseases. For translating them effectively into the clinics, scalable production processes fulfilling good manufacturing practice (GMP) are needed. Like for other biotherapeutic agents, the manufacturing of EV products can be subdivided in the upstream and downstream processing and the subsequent quality control, each of them containing several unit operations. During upstream processing (USP), cells are isolated, stored (cell banking) and expanded; furthermore, EV-containing conditioned media are produced. During downstream processing (DSP), conditioned media (CM) are processed to obtain concentrated and purified EV products. CM are either stored until DSP or are directly processed. As first unit operation in DSP, clarification removes remaining cells, debris and other larger impurities. The key operations of each EV DSP is volume-reduction combined with purification of the concentrated EVs. Most of the EV preparation methods used in conventional research labs including differential centrifugation procedures are limited in their scalability. Consequently, it is a major challenge in the therapeutic EV field to identify appropriate EV concentration and purification methods allowing scale up. As EVs share several features with enveloped viruses, that are used for more than two decades in the clinics now, several principles can be adopted to EV manufacturing. Here, we introduce and discuss volume reducing and purification methods frequently used for viruses and analyze their value for the manufacturing of EV-based therapeutics.

Critical Assessment of Purification and Analytical Technologies for Enveloped Viral Vector and Vaccine Processing and Their Current Limitations in Resolving Co-Expressed Extracellular Vesicles

Viral vectors and viral vaccines are invaluable tools in prevention and treatment of diseases. Many infectious diseases are controlled using vaccines designed from subunits or whole viral structures, whereas other genetic diseases and cancers are being treated by viruses used as vehicles for delivering genetic material in gene therapy or as therapeutic agents in virotherapy protocols. Viral vectors and vaccines are produced in different platforms, from traditional embryonated chicken eggs to more advanced cell cultures. All these expression systems, like most cells and cellular tissues, are known to spontaneously release extracellular vesicles (EVs). EVs share similar sizes, biophysical characteristics and even biogenesis pathways with enveloped viruses, which are currently used as key ingredients in a number of viral vectors and licensed vaccine products. Herein, we review distinctive features and similarities between EVs and enveloped viruses as we revisit the downstream processing steps and analytical technologies currently implemented to produce and document viral vector and vaccine products. Within a context of well-established viral vector and vaccine safety profiles, this review provides insights on the likely presence of EVs in the final formulation of enveloped virus products and discusses the potential to further resolve and document these components.

Infectious titer determination of lentiviral vectors using a temporal immunological real-time imaging approach

The analysis of the infectious titer of the lentiviral vector samples obtained during upstream and downstream processing is of major importance, however, also the most challenging method to be performed. Currently established methods like flow cytometry or qPCR lack the capability of enabling high throughput sample processing while they require a lot of manual handling. To address this limitation, we developed an immunological real-time imaging method to quantify the infectious titer of anti-CD19 CAR lentiviral vectors with a temporal readout using the Incucyte® S3 live-cell analysis system. The infective titers determined with the Incucyte® approach when compared with the flow cytometry-based assay had a lower standard deviation between replicates and a broader linear range. A major advantage of the method is the ability to obtain titer results in real-time, enabling an optimal readout time. The presented protocol significantly decreased labor and increased throughput. The ability of the assay to process high numbers of lentiviral samples in a high throughput manner was proven by performing a virus stability study, demonstrating the effects of temperature, salt, and shear stress on LV infectivity.

Lentiviral Vector Bioprocessing

Lentiviral vectors (LVs) are potent tools for the delivery of genes of interest into mammalian cells and are now commonly utilised within the growing field of cell and gene therapy for the treatment of monogenic diseases and adoptive therapies such as chimeric antigen T-cell (CAR-T) therapy. This is a comprehensive review of the individual bioprocess operations employed in LV production. We highlight the role of envelope proteins in vector design as well as their impact on the bioprocessing of lentiviral vectors. An overview of the current state of these operations provides opportunities for bioprocess discovery and improvement with emphasis on the considerations for optimal and scalable processing of LV during development and clinical production. Upstream culture for LV generation is described with comparisons on the different transfection methods and various bioreactors for suspension and adherent producer cell cultivation. The purification of LV is examined, evaluating different sequences of downstream process operations for both small- and large-scale production requirements. For scalable operations, a key focus is the development in chromatographic purification in addition to an in-depth examination of the application of tangential flow filtration. A summary of vector quantification and characterisation assays is also presented. Finally, the assessment of the whole bioprocess for LV production is discussed to benefit from the broader understanding of potential interactions of the different process options. This review is aimed to assist in the achievement of high quality, high concentration lentiviral vectors from robust and scalable processes.

Advances in Lentivirus Purification

Lentiviral vectors (LVs) have been increasingly used as a tool for gene and cell therapies since they can stably integrate the genome in dividing and nondividing cells. LV production and purification processes have evolved substantially over the last decades. However, the increasing demands for higher quantities with more restrictive purity requirements are stimulating the development of novel materials and strategies to supply the market with LV in a cost-effective manner. A detailed review of each downstream process unit operation is performed, limitations, strengths, and potential outcomes being covered. Currently, the majority of large-scale LV manufacturing processes are still based on adherent cell culture, although it is known that the industry is migrating fast to suspension cultures. Regarding the purification strategy, it consists of batch chromatography and membrane technology. Nevertheless, new solutions are being created to improve the current production schemes and expand its clinical use.

A new simplified clarification approach for lentiviral vectors using diatomaceous earth improves throughput and safe handling

Lentiviral vectors have proven their great potential to serve as a DNA delivery tool for gene modified cell therapy and gene therapy applications. The downstream processing of these vectors is however still a great challenge, particularly because of the low stability of the virus. Harvesting and clarification are critical and until now insufficiently characterized steps for lentivirus processing. To address this bottleneck, we analyzed whether lentiviral vectors produced by transient transfection of HEK293 T/17 SF suspension cells can be efficiently clarified with a lab-scale method with the filter aid diatomaceous earth (DE) and bioburden reducing membrane filters achieving high lentivirus recoveries. Using a design of experiment approach we found that higher DE concentrations are advantageous for a higher turbidity reduction and shorter filtration times, but at the same time LV titer decreases with increasing DE concentration. A DE concentration of 9 g/L was identified with a DoE as a robust set-point. Clarification with DE was compared with for lab-scale traditionally employed centrifugation and subsequent bioburden reduction filtration of viral vectors. The use of DE allows to perform a harvest and clarification process, which does not only facilitate faster and safer virus handling, but enables a lower material consumption due to the extremely increased filter capacity, thus representing an efficient and robust lab-scale clarification process.

Large-Scale Production of Lentiviral Vectors: Current Perspectives and Challenges

Lentiviral vectors (LVs) have gained value over recent years as gene carriers in gene therapy. These viral vectors are safer than what was previously being used for gene transfer and are capable of infecting both dividing and nondividing cells with a long-term expression. This characteristic makes LVs ideal for clinical research, as has been demonstrated with the approval of lentivirus-based gene therapies from the Food and Drug Administration and the European Agency for Medicine. A large number of functional lentiviral particles are required for clinical trials, and large-scale production has been challenging. Therefore, efforts are focused on solving the drawbacks associated with the production and purification of LVsunder current good manufacturing practice. In recent years, we have witnessed the development and optimization of new protocols, packaging cell lines, and culture devices that are very close to reaching the target production level. Here, we review the most recent, efficient, and promising methods for the clinical-scale production ofLVs.

Purification of cell-derived Japanese encephalitis virus by dual-mode chromatography

Purification of the enveloped virus poses a challenge as one must retain viral infectivity to preserve immunogenicity. The traditional process of virus purification is time-consuming, laborious and hard to scale up. Here, a rapid, simple and extensible laboratory program for the purification of Japanese encephalitis virus (JEV) was developed by using differential centrifugation, ultrafiltration, Sepharose 4 fast flow gel chromatography, and Capto^TM Core 700 chromatography. The entire process recovered 61.64% of the original virus, and the purified virus particles maintained good activity and immunogenicity. The purification process described has potential application in large-scale production of high-purity JEV.

Development of Large-Scale Downstream Processing for Lentiviral Vectors

The interest in lentiviral vectors (LVs) has increased prominently for gene therapy applications, but few have reached the later stages of clinical trials. The main challenge has remained in scaling up the manufacturing process for the fragile vector to obtain high titers for in vivo usage. We have previously scaled up the LV production to iCELLis 500, being able to produce up to 180 L of harvest material in one run with perfusion. The following challenge considers the purification and concentration of the product to meet titer and purity requirements for clinical use. We have developed a downstream process, beginning with clarification, buffer exchange, and concentration, by tangential flow filtration. This is followed by a purification step using single membrane-based anion exchange chromatography and final formulation with tangential flow filtration. Different materials and conditions were compared to optimize the process, especially for the chromatography step that has been the bottleneck in lentiviral vector purification scale-up. The final infectious titer of the lentiviral vector product manufactured using the optimized scale-up process was determined to be 1.97 × 10⁹ transducing units (TU)/mL, which can be considered as a high titer for lentiviral vectors.

Toward a Scalable Purification Protocol of GaLV-TR-Pseudotyped Lentiviral Vectors

Lentiviral vectors (LV) that are used in research and development as well as in clinical trials are in majority vesicular stomatitis virus G glycoprotein (VSVg) pseudotyped. The predominance of this pseudotype choice for clinical gene therapy studies is largely due to a lack of purification schemes for pseudotypes other than VSVg. In this study, we report for the first time the development of a new downstream process protocol allowing high-yield production of stable and infectious gibbon ape leukemia virus (GaLV)-TR-LV particles. We identified critical conditions in tangential flow filtration (TFF) and chromatographic steps for preserving the infectivity/functionality of LV during purification. This was carried out by identifying for each step, the critical parameters affecting LV infectivity, including pH, salinity, presence of stabilizers, temperature, and by defining the optimal order of these steps. A three-step process was developed for GaLV-TR-LV purification consisting of one TFF and two chromatographic steps (ion-exchange chromatography and size exclusion chromatography) permitting recoveries of >27% of infectious particles. With this process, purified GaLV-pseudotyped LV enabled the transduction of 70% human CD34⁺ cells in the presence of the Vectofusin-1 peptide, whereas in the same conditions nonpurified vector transduced only 9% of the cells (multiplicity of infection 20). Our protocol will allow for the first time the purification of GaLV-TR-LV that are biologically active, stable, and with sufficient recovery in the perspective of preclinical studies and clinical applications. Obviously, further optimizations are required to improve final vector yields.

De Novo Isolation & Affinity Maturation of yeast-displayed Virion-binding human fibronectin domains by flow cytometric screening against Virions

BACKGROUND

The promise of biopharmaceuticals comprising one or more binding domains motivates the development of novel methods for de novo isolation and affinity maturation of virion-binding domains. Identifying avenues for overcoming the challenges associated with using virions as screening reagents is paramount given the difficulties associated with obtaining high-purity virus-associated proteins that retain the conformation exhibited on the virion surface.

RESULTS

Fluorescence activated cell sorting (FACS) of 1.5 × 107 clones taken from a naïve yeast surface-displayed human fibronectin domain (Fn3) against whole virions yielded two unique binders to Zika virions. Construction and FACS of site-directed binding loop mutant libraries based on one of these binders yielded multiple progeny clones with enhanced Zika-binding affinities. These affinity-matured clones bound Zika virions with low double- or single-digit nanomolar affinity in ELISA assays, and expressed well as soluble proteins in E. coli shake flask culture, with post-purification yields exceeding 10 mg/L.

CONCLUSIONS

FACS of a yeast-displayed binding domain library is an efficient method for de novo isolation of virion-binding domains. Affinities of isolated virion-binding clones are readily enhanced via FACS screening of mutant progeny libraries. Given that most binding domains are compatible with yeast display, the approach taken in this work may be broadly utilized for generating virion-binding domains against many different viruses for use in passive immunotherapy and the prevention of viral infection.

High-Grade Purification of Third-Generation HIV-Based Lentiviral Vectors by Anion Exchange Chromatography for Experimental Gene and Stem Cell Therapy Applications

Lentiviral vectors (LVs) have been increasingly used in clinical gene therapy applications particularly due to their efficient gene transfer ability, lack of interference from preexisting viral immunity, and long-term gene expression they provide. Purity of LVs is essential in in vivo applications, for a high therapeutic benefit with minimum toxicity. Accordingly, laboratory scale production of LVs frequently involves transient cotransfection of 293T cells with packaging and transfer plasmids in the presence of CaPO₄. After clearance of the cellular debris by low-speed centrifugation and filtration, lentivectors are usually concentrated by high-speed ultracentrifugation in sucrose cushion. Concentrated viral samples are then purified by anion exchange chromatography (AEX) after benzonase treatment to remove the residual cellular DNA. Here, we describe an improved practical method for LV purification using AEX, useful for experimental studies concerning gene and stem cell therapy.

Gene Therapy

Gene therapy refers to a rapidly growing field of medicine in which genes are introduced into the body to treat or prevent diseases. Although a variety of methods can be used to deliver the genetic materials into the target cells and tissues, modified viral vectors represent one of the more common delivery routes because of its transduction efficiency for therapeutic genes. Since the introduction of gene therapy concept in the 1970s, the field has advanced considerably with notable clinical successes being demonstrated in many clinical indications in which no standard treatment options are currently available. It is anticipated that the clinical success the field observed in recent years can drive requirements for more scalable, robust, cost effective, and regulatory-compliant manufacturing processes. This review provides a brief overview of the current manufacturing technologies for viral vectors production, drawing attention to the common upstream and downstream production process platform that is applicable across various classes of viral vectors and their unique manufacturing challenges as compared to other biologics. In addition, a case study of an industry-scale cGMP production of an AAV-based gene therapy product performed at 2,000 L-scale is presented. The experience and lessons learned from this largest viral gene therapy vector production run conducted to date as discussed and highlighted in this review should contribute to future development of commercial viable scalable processes for vial gene therapies.

Biopolymeric nano/microspheres for selective and reversible adsorption of coronaviruses

A novel biopolymeric material in the form of nano/microspheres was developed which was capable of adsorbing coronaviruses. The biopolymer was obtained by crosslinking of chitosan (CHIT) with genipin, a nontoxic compound of plant origin, in inverted emulsion and reacting the chitosan nano/microspheres obtained (CHIT-NS/MS) with glycidyltrimethyl-ammonium chloride (GTMAC). As a result the nano/microspheres of N-(2-hydroxypropyl)-3-trimethyl chitosan (HTCC-NS/MS) were obtained. HTCC-NS/MS were studied as the adsorbents of human coronavirus NL63 (HCoV-NL63), mouse hepatitis virus (MHV), and human coronavirus HCoV-OC43 particles in aqueous virus suspensions. By studying cytopathic effect (CPE) caused by these viruses and performing PCR analyses it was found HTCC-NS/MS strongly adsorb the particles of HCoV-NL63 virus, moderately adsorb mouse hepatitis virus (MHV) particles, but do not adsorb HCoV-OC43 coronavirus. The adsorption capacity of HTCC-NS/MS well correlated with the antiviral activity of soluble HTCC against a given virus. Importantly, it was shown that HCoV-NL63 particles could be desorbed from the HTCC-NS/MS surface with a salt solution of high ionic strength with retention of virus virulence. The obtained material may be applied for the removal of coronaviruses, purification and concentration of virus samples obtained from biological matrices and for purification of water from pathogenic coronaviruses.

Challenges of up-scaling lentivirus production and processing

Lentiviruses are becoming an increasingly popular choice of gene transfer vehicle for use in the treatment of a variety of genetic and acquired human diseases. As research progresses from basic studies into pre-clinical and clinical phases, there is a growing demand for large volumes of high purity, concentrated vector, and accordingly, the means to produce such quantities. Unlike other viral vectors, lentiviruses are difficult to produce using stable cell lines, therefore transient transfection of adherent cell lines is conventionally used, and this method has proven challenging to up-scale. Furthermore, with the required increases in the volume of vector needed for larger animal and human use, comes the need for more efficient and sophisticated supernatant purification and concentration techniques. This review presents the challenges of up-scaling lentivirus production and processing approaches, novel systems for overcoming these issues, and the quality assessments recommended for producing a clinical grade lentiviral gene therapy product.

Production and purification of high-titer foamy virus vector for the treatment of leukocyte adhesion deficiency

Compared to other integrating viral vectors, foamy virus (FV) vectors have distinct advantages as a gene transfer tool, including their nonpathogenicity, the ability to carry larger transgene cassettes, and increased stability of virus particles due to DNA genome formation within the virions. Proof of principle of its therapeutic utility was provided with the correction of canine leukocyte adhesion deficiency using autologous CD34+ cells transduced with FV vector carrying the canine CD18 gene, demonstrating its long-term safety and efficacy. However, infectious titers of FV-human(h)CD18 were low and not suitable for manufacturing of clinical-grade product. Herein, we developed a scalable production and purification process that resulted in 60-fold higher FV-hCD18 titers from ~1.7 × 104 to 1.0 × 106 infectious units (IU)/ml. Process development improvements included use of polyethylenimine-based transfection, use of a codon-optimized gag, heparin affinity chromatography, tangential flow filtration, and ultracentrifugation, which reproducibly resulted in 5,000-fold concentrated and purified virus, an overall yield of 19 ± 3%, and final titers of 1-2 × 109 IU/ml. Highly concentrated vector allowed reduction of final dimethyl sulfoxide (DMSO) concentration, thereby avoiding DMSO-induced toxicity to CD34+ cells while maintaining high transduction efficiencies. This process development results in clinically relevant, high titer FV which can be scaled up for clinical grade production.

Switchable aptamers for biosensing and bioseparation of viruses (SwAps-V)

There is a widespread interest in the development of aptamer-based affinity chromatographic methods for purification of biomolecules. Regardless of the many advantages exhibited by aptamers when compared to other recognition elements, the lack of an efficient regeneration technique that can be generalized to all targets has encumbered further integration of aptamers into affinity-based purification methods. Here we offer switchable aptamers (SwAps) that have been developed to solve this problem and move aptamer-based chromatography forward. SwAps are controlled-affinity aptamers, which have been employed here to purify vesicular stomatitis virus (VSV) as a model case, however this technique can be extended to all biologically significant molecules. VSV is one oncolytic virus out of an arsenal of potential candidates shown to provide selective destruction of cancer cells both in vitro and in vivo. These SwAps were developed in the presence of Ca(2+) and Mg(2+) ions where they cannot bind to their target VSV in absence of these cations. Upon addition of EDTA and EGTA, the divalent cations were sequestered from the stabilized aptameric structure causing a conformational change and subsequently release of the virus. Both flow cytometry and electrochemical impedance spectroscopy were employed to estimate the binding affinities between the selected SwAps and VSV and to determine the coefficient of switching (CoS) upon elution. Among fifteen sequenced SwAps, four have exhibited high affinity to VSV and ability to switch upon elution and thus were further integrated into streptavidin-coated magnetic beads for purification of VSV.

Optimised concentration and purification of retroviruses using membrane chromatography

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An in investigation in to the use of membrane chromatography for the purification of a γ-retrovirus was undertaken.

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The first report of a capacity for γ-retrovirus binding to a membrane chromatography device is presented.

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A process that produces a large increase in concentration and purity of the studied γ-retrovirus was identified.

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Proteomic techniques were used to identify the protein impurities removed and co-purified with the virus containing eluate.

The ability of an anion exchange membrane to purify a γ-retrovirus was assessed and optimised with respect to different loading and wash buffers. Recoveries of infectious virus greater than 50% were consistently obtained, while specific titre was increased up to one thousand fold when compared to the material loaded. Specific proteins removed and retained by this optimised process were identified by mass spectrometry. It was possible to successfully bind and elute the equivalent of 1.27 × 10⁸ Ifu/ml of ion exchange membrane. This could then be highly concentrated, with infectious virus concentrated to a maximum of 420-fold compared to the load.

New developments in lentiviral vector design, production and purification

INTRODUCTION

Lentiviruses are a very potent class of viral vectors for which there is presently a rapidly growing interest for a number of gene therapy. However, their construction, production and purification need to be performed according to state-of-the-art techniques in order to obtain sufficient quantities of high purity material of any usefulness and safety.

AREAS COVERED

The recent advances in the field of recombinant lentivirus vector design, production and purification will be reviewed with an eye toward its utilization for gene therapy. Such a review should be helpful for the potential user of this technology.

EXPERT OPINION

The principal hurdles toward the use of recombinant lentivirus as a gene therapy vector are the low titer at which it is produced as well as the difficulty to purify it at an acceptable level without degrading it. The recent advances in the bioproduction of this vector suggest these issues are about to be resolved, making the retrovirus gene therapy a mature technology.

Downstream processing of cell culture-derived virus particles

Manufacturing of cell culture-derived virus particles for vaccination and gene therapy is a rapidly growing field in the biopharmaceutical industry. The process involves a number of complex tasks and unit operations ranging from selection of host cells and virus strains for the cultivation in bioreactors to the purification and formulation of the final product. For the majority of cell culture-derived products, efforts focused on maximization of bioreactor yields, whereas design and optimization of downstream processes were often neglected. Owing to this biased focus, downstream procedures today often constitute a bottleneck in various manufacturing processes and account for the majority of the overall production costs. For efficient production methods, particularly in sight of constantly increasing economic pressure within human healthcare systems, highly productive downstream schemes have to be developed. Here, we discuss unit operations and downstream trains to purify virus particles for use as vaccines and vectors for gene therapy.

Scale-up and manufacturing of clinical-grade self-inactivating γ-retroviral vectors by transient transfection

The need for γ-retroviral (gRV) vectors with a self-inactivating (SIN) design for clinical application has prompted a shift in methodology of vector manufacturing from the traditional use of stable producer lines to transient transfection-based techniques. Herein, we set out to define and optimize a scalable manufacturing process for the production of gRV vectors using transfection in a closed-system bioreactor in compliance with current good manufacturing practices (cGMP). The process was based on transient transfection of 293T cells on Fibra-Cel disks in the Wave Bioreactor. Cells were harvested from tissue culture flasks and transferred to the bioreactor containing Fibra-Cel in the presence of vector plasmid, packaging plasmids and calcium-phosphate in Dulbecco's modified Eagle's medium and 10% fetal bovine serum. Virus supernatant was harvested at 10-14 h intervals. Using optimized procedures, a total of five ecotropic cGMP-grade gRV vectors were produced (9 liters each) with titers up to 3.6 × 10(7) infectious units per milliliter on 3T3 cells. One GMP preparation of vector-like particles was also produced. These results describe an optimized process for the generation of SIN viral vectors by transfection using a disposable platform that allows for the generation of clinical-grade viral vectors without the need for cleaning validation in a cost-effective manner.

Highly efficient concentration of lenti- and retroviral vector preparations by membrane adsorbers and ultrafiltration

BACKGROUND

Lentiviral vectors (LVs) can efficiently transduce a broad spectrum of cells and tissues, including dividing and non-dividing cells. So far the most widely used method for concentration of lentiviral particles is ultracentrifugation (UC).An important feature of vectors derived from lentiviruses and prototypic gamma-retroviruses is that the host range can be altered by pseudotypisation. The most commonly used envelope protein for pseudotyping is the glycoprotein of the Vesicular Stomatitis Virus (VSV.G), which is also essential for successful concentration using UC.

RESULTS

Here, we describe a purification method that is based on membrane adsorbers (MAs). Viral particles are efficiently retained by the anionic exchange MAs and can be eluted with a high-salt buffer. Buffer exchange and concentration is then performed by utilizing ultrafiltration (UF) units of distinct molecular weight cut off (MWCO). With this combined approach similar biological titers as UC can be achieved (2 to 5×10⁹ infectious particles (IP)/ml). Lentiviral particles from small starting volumes (e.g. 40 ml) as well as large volumes (up to 1,000 ml) cell culture supernatant (SN) can be purified. Apart from LVs, vectors derived from oncoretroviruses can be efficiently concentrated as well. Importantly, the use of the system is not confined to VSV.G pseudotyped lenti- and retroviral particles and other pseudotypes can also be purified.

CONCLUSIONS

Taken together the method presented here offers an efficient alternative for the concentration of lenti- as well as retroviral vectors with different pseudotypes that needs no expensive equipment, is easy to handle and can be used to purify large quantities of viral vectors within a short time.

Entry kinetics and cell-cell transmission of surface-bound retroviral vector particles

BACKGROUND

Transduction with recombinant HIV-1 derived lentivirus vectors is a multi-step process initiated by surface attachment and subsequent receptor-directed uptake into the target cell. We previously reported the retention of vesicular stomatitis virus G protein pseudotyped particles on murine progenitor cells and their delayed cell-cell transfer.

METHODS

To examine the underlying mechanism in more detail, we used a combination of approaches focused on investigating the role of receptor-independent factors in modulating attachment.

RESULTS

The investigation of synchronized transduction reveals cell-type specific rates of vector particle clearance with substantial delays during particle entry into murine hematopoietic progenitor cells. The observed uptake kinetics from the surface of the 1 degrees cell correlate inversely with the magnitude of transfer to 2 degrees targets, corresponding with our initial observation of preferential cell-cell transfer in the context of brief vector exposures. We further demonstrate that vector particle entry into cells is associated with the cell-type specific abundance of extracellular matrix fibronectin. Residual particle-extracellular fibronectin matrix binding and 2 degrees transfer can be competitively disrupted by heparin exposure without affecting murine progenitor homing and repopulation.

CONCLUSIONS

Although cellular attachment factors, including fibronectin, aid gene transfer by colocalizing particles to cells and disfavoring early dissociation from targets, they also appear to stabilize particles on the cell surface. The present study highlights the inadvertent consequences for cell entry and cell-cell transfer.

An all-feline retroviral packaging system for transduction of human cells

Abstract The subgroup C feline leukemia virus (FeLV-C) receptor FLVCR is a widely expressed 12-transmembrane domain transporter that exports cytoplasmic heme and is a promising target for retrovirus-mediated gene delivery. Previous studies demonstrated that FeLV-C pseudotype vectors were more efficient at targeting human hematopoietic stem cells than those pseudotyped with gibbon ape leukemia virus (GALV), and thus we developed an all FeLV-C-based packaging system, termed CatPac. CatPac is helper-virus free and can produce higher titer vectors than existing gammaretroviral packaging systems, including systems mixing Moloney murine leukemia virus (MoMLV) Gag-Pol and FeLV-C Env proteins. The vectors can be readily concentrated (>30-fold), refrozen (three to five times), and held on ice (>2 days) with little loss of titer. Furthermore, we demonstrate that CatPac pseudotype vectors efficiently target early CD34(+)CD38(-) stem/progenitor cells, monocytic and erythroid progenitors, activated T cells, mature macrophages, and cancer cell lines, suggesting utility for human cell and cell line transduction and possibly gene therapy.

Overview of current scalable methods for purification of viral vectors

As a result of the growing interest in the use of viruses for gene therapy and vaccines, many virus-based products are being developed. The manufacturing of viruses poses new challenges for process developers and regulating authorities that need to be addressed to ensure quality, efficacy, and safety of the final product. The design of suitable purification strategies will depend on a multitude of variables including the vector production system and the nature of the virus. In this chapter, we provide an overview of the most commonly used purification methods for viral gene therapy vectors. Current chromatography options available for large-scale purification of γ-retrovirus, lentivirus, adenovirus, adeno-associated virus, herpes simplex virus, baculovirus, and poxvirus vectors are presented.

Concanavalin A affinity chromatography for efficient baculovirus purification

Baculovirus has emerged as a novel gene delivery and vaccine vector, and the demand for purified baculovirus is rising due to the increasing in vivo applications. Since the baculoviral envelope protein gp64 is a glycoprotein, we aimed to develop a concanavalin A (Con A) chromatography process, which harnessed the possible affinity interaction between gp64 and Con A, for simple and effective baculovirus purification. Throughout the purification process the virus stability and recovery were assessed by quantifying the virus transducing titers [TT, defined as transducing units (TU) per milliliter] and viral particles (VP). We found that baculovirus stability was sensitive to buffer conditions and diafiltration with a tangential flow filtration system LabScale using 300 K membranes yielded recoveries of approximately 75% in TT and 82% in VP. The diafiltered baculovirus strongly bound to the Con A column as evidenced by the low virus losses to the flow through and wash fractions. The wash steps eliminated >99% of protein impurities and elution with 0.6 M alpha-D-methylmannoside at room temperature led to the recoveries of approximately 16% in VP and approximately 15.3% in TU. The resultant VP/TU ratio was as low as 41.4, attesting the high quality of the purified virus. Further elution with 1 M alpha-D-methylmannoside recovered another 6% virus TU, yielding a cumulative recovery of approximately 21.3% in TU. These data demonstrated for the first time that Con A chromatography is suitable for baculovirus purification, and may be used for the purification of other viruses with surface glycoproteins.

Purification of porcine reproductive and respiratory syndrome virus from cell culture using ultrafiltration and heparin affinity chromatography

Porcine reproductive and respiratory syndrome (PRRS) virus is the causative agent of the most significant infectious disease currently affecting the swine industry worldwide. Density gradient ultracentrifugation remains the most commonly used method for porcine reproductive and respiratory syndrome virus (PRRSV) purification. However, this technique has notable drawbacks including long processing time and limited processing volume in each run. To overcome these limitations, a scalable process was developed. PRRSV propagated in MARC-145 was released by three freeze/thaw cycles. After a low speed centrifugation step, the virus particles in the supernatant were concentrated twice by an ultrafiltration step. The ultrafiltration step concentrated the virions effectively with no detectable loss while some cultural/cellular proteins were removed. The virions in the ultrafiltration retentate were then applied to a heparin affinity column on a fast performance liquid chromatography unit. The combined ultrafiltration and heparin affinity chromatography process removed more than 96% of cellular and medium proteins. During a stepwise elution strategy, the viral particles were eluted at two separate peaks recovering 27.5% and 25.4% of viral particles loaded onto the column with a purity of 194 and 3917 particles/microg protein, respectively.