Purification of retroviral vectors for clinical application: Biological implications and technological challenges

Evaluation of Purification Methods for Minimizing Transgene Expression Background During Viral Manufacturing

Gene therapy has emerged as a promising therapeutic avenue, offering targeted treatments for various diseases. Purification of viral vectors presents a pivotal challenge, demanding the removal of impurities while preserving integrity and potency. During manufacturing, producer cells in transfection systems can be transiently transfected or retro-infected by the viral vectors they have just produced-a process referred to as "retro-transduction"-leading them to express the transgenes of interest. This can be a significant source of contamination in the viral solution pool, particularly when the transgenes encode extracellular, secreted proteins, resulting in cytotoxicity and reduced viral potency. Herein, we aimed to evaluate the efficiency of different viral purification systems commonly used in academic and industry settings in removing the transgene background from viral solutions. The efficiency of each system was assessed based on the levels of the secreted transgene Gaussia Luciferase (GLuc), which can be quickly detected in a solution and served as a readout for transgene background contamination in the viral pool during downstream processing. Through a systematic evaluation of purification methods, we identified the most effective approaches for producing pure viral batches with minimal transgene background, all while preserving viral potency and functionality. Our study revealed superior performance of batches that underwent purification via tangential flow filtration, which yielded over 90% reduction in GLuc background and the highest transduction efficiency rates. This work provides significant insights for advancing gene therapy applications that rely on the production of viral vectors encoding secreted transgenes.

Perfusion Process Intensification for Lentivirus Production Using a Novel Scale-Down Model

Process intensification has become an important strategy to lower production costs and increase manufacturing capacities for biopharmaceutical products. In particular for the production of viral vectors like lentiviruses (LVs), the transition from (fed-)batch to perfusion processes is a key strategy to meet the increasing demands for cell and gene therapy applications. However, perfusion processes are associated with higher medium consumption. Therefore, it is necessary to develop appropriate small-scale models to reduce development costs. In this work, we present the use of the acoustic wave separation technology in combination with the Ambr 250 high throughput bioreactor system for intensified perfusion process development using stable LV producer cells. The intensified perfusion process developed in the Ambr 250 model, performed at a harvest rate of 3 vessel volumes per day (VVD) and high cell densities, resulted in a 1.4-fold higher cell-specific functional virus yield and 2.8-fold higher volumetric virus yield compared to the control process at a harvest rate of 1 VVD. The findings were verified at bench scale after optimizing the bioreactor set-up, resulting in a 1.4-fold higher cell-specific functional virus yield and 3.1-fold higher volumetric virus yield.

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.

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.

Engineering Novel Lentiviral Vectors for Labelling Tumour Cells and Oncogenic Proteins

Lentiviral vectors are unique and highly efficient genetic tools to incorporate genetic materials into the genome of a variety of cells whilst conserving biosafety. Their rapid acceptance made it necessary to improve existing protocols, including molecular engineering and cloning, production of purified lentiviral particles, and efficient infection of target cells. In addition to traditional protocols, which can be time-consuming, several biotechnology companies are providing scientists with commercially available lentiviral constructs and particles. However, these constructs are limited by their original form, tend to be costly, and lack the flexibility to re-engineer based on the ever-changing needs of scientific projects. Therefore, the current study organizes the existing methods and integrates them with novel ideas to establish a protocol that is simple and efficient to implement. In this study we, (i) generated an innovative site-directed nucleotide attachment/replacement and DNA insertion method using unique PCR primers, (ii) improved traditional methods by integrating plasmid clarification steps, (iii) utilized endogenous mRNA as a resource to construct new lentiviruses, and (iv) identified an existing purification method and incorporated it into an organized workflow to produce high-yield lentiviral particle collection. Finally, (v) we verified and demonstrated the functional validity of our methods using an infection strategy.

Cancer Gene Therapy: Development and Production of Lentiviral Vectors for Gene Therapy

Lentiviral vectors are among the most used vectors in gene therapy to treat pathologies of different origins, such as cancers, rare monogenic diseases or neurological disorders. This chapter provides an overview on lentiviral vector developments in terms of vector design and manufacture for gene therapy applications. The state of the art of vector production will be summarized face to the recent developments contributing to improve vector safety, efficacy and manufacturing robustness, focusing on human immunodeficiency virus 1 (HIV-1) based lentiviral vectors. Transient and stable production systems will be discussed highlighting recent advances in producer cell line development. Challenges in lentiviral vector development upstream and downstream will be addressed with a particular focus on the improvements undertaken to increase vector yields and production scalability.

Production- and Purification-Relevant Properties of Human and Murine Cytomegalovirus

There is a large unmet need for a prophylactic vaccine against human cytomegalovirus (HCMV) to combat the ubiquitous infection that is ongoing with this pathogen. A vaccination against HCMV could protect immunocompromised patients and prevent birth defects caused by congenital HCMV infections. Moreover, cytomegalovirus (CMV) has a number of features that make it a very interesting vector platform for gene therapy. In both cases, preparation of a highly purified virus is a prerequisite for safe and effective application. Murine CMV (MCMV) is by far the most studied model for HCMV infections with regard to the principles that govern the immune surveillance of CMVs. Knowledge transfer from MCMV and mice to HCMV and humans could be facilitated by better understanding and characterization of the biological and biophysical properties of both viruses. We carried out a detailed investigation of HCMV and MCMV growth kinetics as well as stability under the influence of clarification and different storage conditions. Further, we investigated the possibilities to concentrate and purify both viruses by ultracentrifugation and ion-exchange chromatography. Defective enveloped particles were not separately analyzed; however, the behavior of exosomes was examined during all experiments. The effectiveness of procedures was monitored using CCID₅₀ assay, Nanoparticle tracking analysis, ELISA for host cell proteins, and quantitative PCR for host cell DNA. MCMV generally proved to be more robust in handling. Despite its greater sensitivity, HCMV was efficiently (100% recovery) purified and concentrated by anion-exchange chromatography using QA monolithic support. The majority of the host genomic DNA as well as most of the host cell proteins were removed by this procedure.

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.

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.

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.

Tangential Flow Filtration for the Concentration of Oncolytic Measles Virus: The Influence of Filter Properties and the Cell Culture Medium

The therapeutic use of oncolytic measles virus (MV) for cancer treatment requires >10⁸ infectious MV particles per dose in a highly pure form. The concentration/purification of viruses is typically achieved by tangential flow filtration (TFF) but the efficiency of this process for the preparation of MV has not been tested in detail. We therefore investigated the influence of membrane material, feed composition, and pore size or molecular weight cut-off (MWCO) on the recovery of MV by TFF in concentration mode. We achieved the recovery of infectious MV particles using membranes with a MWCO ≤ 300 kDa regardless of the membrane material and whether or not serum was present in the feed. However, serum proteins in the medium affected membrane flux and promoted fouling. The severity of fouling was dependent on the membrane material, with the cellulose-based membrane showing the lowest susceptibility. We found that impurities such as proteins and host cell DNA were best depleted using membranes with a MWCO ≥ 300 kDa. We conclude that TFF in concentration mode is a robust unit operation to concentrate infectious MV particles while depleting impurities such as non-infectious MV particles, proteins, and host cell DNA.

Current challenges in biotherapeutic particles manufacturing

Introduction: The development of novel complex biotherapeutics led to new challenges in biopharmaceutical industry. The potential of these particles has been demonstrated by the approval of several products, in the different fields of gene therapy, oncolytic therapy, and tumor vaccines. However, their manufacturing still presents challenges related to the high dosages and purity required.Areas covered: The main challenges that biopharmaceutical industry faces today and the most recent developments in the manufacturing of different biotherapeutic particles are reported here. Several unit operations and downstream trains to purify virus, virus-like particles and extracellular vesicles are described. Innovations on the different purification steps are also highlighted with an eye on the implementation of continuous and integrated processes.Expert opinion: Manufacturing platforms that consist of a low number of unit operations, with higher-yielding processes and reduced costs will be highly appreciated by the industry. The pipeline of complex therapeutic particles is expanding and there is a clear need for advanced tools and manufacturing capacity. The use of single-use technologies, as well as continuous integrated operations, are gaining ground in the biopharmaceutical industry and should be supported by more accurate and faster analytical methods.

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.

Cellulose-based virus-retentive filters: a review

Viral filtration is a critical step in the purification of biologics and in the monitoring of microbiological water quality. Viral filters are also essential protection elements against airborne viral particles. The present review first focuses on cellulose-based filter media currently used for size-exclusion and/or adsorptive filtration of viruses from biopharmaceutical and environmental water samples. Data from spiking studies quantifying the viral filtration performance of cellulosic filters are detailed, i.e., first, the virus reduction capacity of regenerated cellulose hollow fiber filters in the manufacturing process of blood products and, second, the efficiency of virus recovery/concentration from water samples by the viradel (virus adsorption-elution) method using charge modified, electropositive cellulosic filters or conventional electronegative cellulose ester microfilters. Viral analysis of field water samples by the viradel technique is also surveyed. This review then describes cellulose-based filter media used in individual protection equipment against airborne viral pathogens, presenting innovative filtration media with virucidal properties. Some pros and cons of cellulosic viral filters and perspectives for cellulose-based materials in viral filtration are underlined in the review.

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 of lentiviral vectors

Lentiviral vectors (LV) have seen considerably increase in use as gene therapy vectors for the treatment of acquired and inherited diseases. This review presents the state of the art of the production of these vectors with particular emphasis on their large-scale production for clinical purposes. In contrast to oncoretroviral vectors, which are produced using stable producer cell lines, clinical-grade LV are in most of the cases produced by transient transfection of 293 or 293T cells grown in cell factories. However, more recent developments, also, tend to use hollow fiber reactor, suspension culture processes, and the implementation of stable producer cell lines. As is customary for the biotech industry, rather sophisticated downstream processing protocols have been established to remove any undesirable process-derived contaminant, such as plasmid or host cell DNA or host cell proteins. This review compares published large-scale production and purification processes of LV and presents their process performances. Furthermore, developments in the domain of stable cell lines and their way to the use of production vehicles of clinical material will be presented.

Stability, biophysical properties and effect of ultracentrifugation and diafiltration on measles virus and mumps virus

Measles virus and mumps virus (MeV and MuV) are enveloped RNA viruses used for production of live attenuated vaccines for prophylaxis of measles and mumps disease, respectively. For biotechnological production of and basic research on these viruses, the preparation of highly purified and infectious viruses is a prerequisite, and to meet that aim, knowledge of their stability and biophysical properties is crucial. Our goal was to carry out a detailed investigation of the stability of MeV and MuV under various pH, temperature, shear stress, filtration and storage conditions, as well as to evaluate two commonly used purification techniques, ultracentrifugation and diafiltration, with regard to their efficiency and effect on virus properties. Virus titers were estimated by CCID50 assay, particle size and concentration were measured by Nanoparticle tracking analysis (NTA) measurements, and the host cell protein content was determined by ELISA. The results demonstrated the stability of MuV and MeV at pH <9 and above pH 4 and 5, respectively, and aggregation was observed at pH >9. Storage without stabilizer did not result in structural changes, but the reduction in infectivity after 24 hours was significant at +37 °C. Vortexing of the viruses resulted in significant particle degradation, leading to lower virus titers, whereas pipetting had much less impact on virus viability. Diafiltration resulted in higher recovery of both total and infectious virus particles than ultracentrifugation. These results provide important data for research on all upstream and downstream processes on these two viruses regarding biotechnological production and basic research.

Improved virus purification processes for vaccines and gene therapy

The downstream processing of virus particles for vaccination or gene therapy is becoming a critical bottleneck as upstream titers keep improving. Moreover, the growing pressure to develop cost-efficient processes has brought forward new downstream trains. This review aims at analyzing the state-of-the-art in viral downstream purification processes, encompassing the classical unit operations and their recent developments. Emphasis is given to novel strategies for process intensification, such as continuous or semi-continuous systems based on multicolumn technology, opening up process efficiency. Process understanding in the light of the pharmaceutical quality by design (QbD) initiative is also discussed. Finally, an outlook of the upcoming breakthrough technologies is presented.

Evaluation of novel large cut-off ultrafiltration membranes for adenovirus serotype 5 (Ad5) concentration

The purification of virus particles and viral vectors for vaccine and gene therapy applications is gaining increasing importance in order to deliver a fast, efficient, and reliable production process. Ultrafiltration (UF) is a widely employed unit operation in bioprocessing and its use is present in several steps of the downstream purification train of biopharmaceuticals. However, to date few studies have thoroughly investigated the performance of several membrane materials and cut-offs for virus concentration/diafiltration. The present study aimed at developing a novel class of UF cassettes for virus concentration/diafiltration. A detailed study was conducted to evaluate the effects of (i) membrane materials, namely polyethersulfone (PES), regenerated cellulose (RC), and highly cross-linked RC (xRC), (ii) nominal cut-off, and (iii) UF device geometry at different production scales. The results indicate that the xRC cassettes with a cut-off of approximately 500 kDa are able to achieve a 10-fold concentration factor with 100% recovery of particles with a process time twice as fast as that of a commercially available hollow fiber. DNA and host cell protein clearances, as well as hydraulic permeability and fouling behavior, were also assessed.

Postexit surface engineering of retroviral/lentiviral vectors

Gene delivery vectors based on retroviral or lentiviral particles are considered powerful tools for biomedicine and biotechnology applications. Such vectors require modification at the genomic level in the form of rearrangements to allow introduction of desired genes and regulatory elements (genotypic modification) as well as engineering of the physical virus particle (phenotypic modification) in order to mediate efficient and safe delivery of the genetic information to the target cell nucleus. Phenotypic modifications are typically introduced at the genomic level through genetic manipulation of the virus producing cells. However, this paper focuses on methods which allow modification of viral particle surfaces after they have exited the cell, that is, directly on the viral particles in suspension. These methods fall into three categories: (i) direct covalent chemical modification, (ii) membrane-topic reagents, and (iii) adaptor systems. Current applications of such techniques will be introduced and their advantages and disadvantages will be discussed.

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.

Lentiviral-mediated gene transfer of siRNAs for the treatment of Huntington's disease

This chapter describes the potential use of viral-mediated gene transfer in the central nervous system for the silencing of gene expression using RNA interference in the context of Huntington's disease (HD). Protocols provided here describe the design of small interfering RNAs, their encoding in lentiviral vectors (LVs) and viral production, as well as procedures for their stereotaxic injection in the rodent brain.

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.

Manufacturing of retroviruses

Retrovirus vectors derived from moloney murine leukemia virus (MoMLV) were the first class of viral vectors developed for gene therapy. They have been extensively used in clinical trials, particularly in ex vivo transduction of hematopoietic stem cells. Although there is a vast experience acquired with retroviruses, their manufacturing is still a difficult task due to the low cell productivities and inherent instability of the infective virus. These viral vectors are most commonly produced using stable producer cell lines in adherent monolayer culture systems. In order to obtain high transduction efficiencies and low toxicity in clinical applications, the viral preparations should be purified, concentrated, and well characterized to attain stringent quality specifications. This chapter describes currently used protocols for manufacturing retroviruses.

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.

Integrated strategy for the production of therapeutic retroviral vectors

The broad application of retroviral vectors for gene delivery is still hampered by the difficulty to reproducibly establish high vector producer cell lines generating sufficient amounts of highly concentrated virus vector preparations of high quality. To enhance the process for producing clinically relevant retroviral vector preparations for therapeutic applications, we have integrated novel and state-of-the-art technologies in a process that allows rapid access to high-efficiency vector-producing cells and consistent production, purification, and storage of retroviral vectors. The process has been designed for various types of retroviral vectors for clinical application and to support a high-throughput process. New modular helper cell lines that permit rapid insertion of DNA encoding the therapeutic vector of interest at predetermined, optimal chromosomal loci were developed to facilitate stable and high vector production levels. Packaging cell lines, cultivation methods, and improved medium composition were coupled with vector purification and storage process strategies that yield maximal vector infectivity and stability. To facilitate GMP-grade vector production, standard of operation protocols were established. These processes were validated by production of retroviral vector lots that drive the expression of type VII collagen (Col7) for the treatment of a skin genetic disease, dystrophic epidermolysis bullosa. The potential efficacy of the Col7-expressing vectors was finally proven with newly developed systems, in particular in target primary keratinocyte cultures and three-dimensional skin tissues in organ culture.

Impact of ligand density on the optimization of ion-exchange membrane chromatography for viral vector purification

The effect of ligand density on anion-exchange membrane chromatography (AEXmc) for the purification of recombinant baculoviruses (rBVs), potential viral vectors in clinical applications, is studied by surface plasmon resonance on customized AEX surfaces and gradient elution experiments on Sartobind D membrane prototypes with different diethylamine ligand densities, complemented by dynamic light scattering analysis for estimation of the hydrodynamic particle size of the various biologics. A chromatographic-column model based on the steric mass action model of ion exchange is employed to analyze the gradient-elution AEXmc experiments, extrapolate the results to other operating conditions, and provide directions for process improvement. Although counterintuitively, the experimental evidence provided in this study shows that the lowering of ligand density is beneficial for rBV purification by AEXmc in bind-and-elute-mode, because it decreases the residual concentrations of host cell protein, dsDNA, and non-infective rBVs in the eluted product cut, and increases the overall yield by roughly 20% over current standard values. Overall, we present a case study on how rational design can streamline downstream process development.