Purification of recombinant baculoviruses for gene therapy using membrane processes

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.

Evaluating Novel Quantification Methods for Infectious Baculoviruses

Accurate and rapid quantification of (infectious) virus titers is of paramount importance in the manufacture of viral vectors and vaccines. Reliable quantification data allow efficient process development at a laboratory scale and thorough process monitoring in later production. However, current gold standard applications, such as endpoint dilution assays, are cumbersome and do not provide true process analytical monitoring. Accordingly, flow cytometry and quantitative polymerase chain reaction have attracted increasing interest in recent years, offering various advantages for rapid quantification. Here, we compared different approaches for the assessment of infectious viruses, using a model baculovirus. Firstly, infectivity was estimated by the quantification of viral nucleic acids in infected cells, and secondly, different flow cytometric approaches were investigated regarding analysis times and calibration ranges. The flow cytometry technique included a quantification based on post-infection fluorophore expression and labeling of a viral surface protein using fluorescent antibodies. Additionally, the possibility of viral (m)RNA labeling in infected cells was investigated as a proof of concept. The results confirmed that infectivity assessment based on qPCR is not trivial and requires sophisticated method optimization, whereas staining of viral surface proteins is a fast and feasible approach for enveloped viruses. Finally, labeling of viral (m)RNA in infected cells appears to be a promising opportunity but will require further research.

Bioanalytics for Influenza Virus-Like Particle Characterization and Process Monitoring

Virus-like particles (VLPs) are excellent platforms for the development of influenza vaccine candidates. Nonetheless, their characterization is challenging due to VLPs’ unique biophysical and biochemical properties. To cope with such complexity, multiple analytical techniques have been developed to date (e.g., single-particle analysis, thermal stability, or quantification assays), most of which are rarely used or have been successfully demonstrated for being applicable for virus particle characterization. In this study, several biophysical and biochemical methods have been evaluated for thorough characterization of monovalent and pentavalent influenza VLPs from diverse groups (A and B) and subtypes (H1 and H3) produced in insect cells using the baculovirus expression vector system (IC-BEVS). Particle size distribution and purity profiles were monitored during the purification process using two complementary technologies — nanoparticle tracking analysis (NTA) and tunable resistive pulse sensing (TRPS). VLP surface charge at the selected process pH was also assessed by this last technique. The morphology of the VLP (size, shape, and presence of hemagglutinin spikes) was evaluated using transmission electron microscopy. Circular dichroism was used to assess VLPs’ thermal stability. Total protein, DNA, and baculovirus content were also assessed. All VLPs analyzed exhibited similar size ranges (90–115 nm for NTA and 129–141 nm for TRPS), surface charges (average of −20.4 mV), and morphology (pleomorphic particles resembling influenza virus) exhibiting the presence of HA molecules (spikes) uniformly displayed on M1 protein scaffold. Our data shows that HA titers and purification efficiency in terms of impurity removal and thermal stability were observed to be particle dependent. This study shows robustness and generic applicability of the tools and methods evaluated, independent of VLP valency and group/subtype. Thus, they are most valuable to assist process development and enhance product characterization.

A Combined Ultrafiltration/Diafiltration Process for the Purification of Oncolytic Measles Virus

Measles virus (MV) is an important representative of a new class of cancer therapeutics known as oncolytic viruses. However, process intensification for the downstream purification of this fragile product is challenging. We previously found that a mid-range molecular weight cut-off (300 kDa) is optimal for the concentration of MV. Here, we tested continuous and discontinuous diafiltration for the purification of MV prepared in two different media to determine the influence of high and low protein loads. We found that a concentration step before diafiltration improved process economy and MV yield when using either serum-containing or serum-free medium. We also found that discontinuous diafiltration conferred a slight benefit in terms of the permeate flow, reflecting the repetitive dilution steps and the ability to break down parts of the fouling layer on the membrane. In summary, the combined ultrafiltration/diafiltration process is suitable for the purification of MV, resulting in the recovery of ~50% infectious virus particles with a total concentration factor of 8 when using 5 diavolumes of buffer.

Highly Efficient Purification of Recombinant VSV-∆G-Spike Vaccine against SARS-CoV-2 by Flow-Through Chromatography

This study reports a highly efficient, rapid one-step purification process for the production of the recombinant vesicular stomatitis virus-based vaccine, rVSV-∆G-spike (rVSV-S), recently developed by the Israel Institute for Biological Research (IIBR) for the prevention of COVID-19. Several purification strategies are evaluated using a variety of chromatography methods, including membrane adsorbers and packed-bed ion-exchange chromatography. Cell harvest is initially treated with endonuclease, clarified, and further concentrated by ultrafiltration before chromatography purification. The use of anion-exchange chromatography in all forms results in strong binding of the virus to the media, necessitating a high salt concentration for elution. The large virus and spike protein binds very strongly to the high surface area of the membrane adsorbents, resulting in poor virus recovery (<15%), while the use of packed-bed chromatography, where the surface area is smaller, achieves better recovery (up to 33%). Finally, a highly efficient chromatography purification process with Capto^TM Core 700 resin, which does not require binding and the elution of the virus, is described. rVSV-S cannot enter the inner pores of the resin and is collected in the flow-through eluent. Purification of the rVSV-S virus with Capto^TM Core 700 resulted in viral infectivity above 85% for this step, with the efficient removal of host cell proteins, consistent with regulatory requirements. Similar results were obtained without an initial ultrafiltration step.

Production of Baculovirus and Stem Cells for Baculovirus-Mediated Gene Transfer into Human Mesenchymal Stem Cells

The discovery of the genome-editing tool CRISPR-Cas9 is revolutionizing the world of gene therapy and will extend the gene therapy product pipeline. While applying gene therapy products, the main difficulty is an efficient and effective transfer of the nucleic acids carrying the relevant information to their target destination, the nucleus of the cells. Baculoviruses have shown to be very suitable transport vehicles for this task due to, inter alia, their ability to transduce mammalian/human cells without being pathogenic. This property allows the usage of baculovirus-transduced cells as cell therapy products, thus, combining the advantages of gene and cell therapy. To make such pharmaceuticals available for patients, a successful production and purification is necessary. In this chapter, we describe the generation of a pseudotyped baculovirus vector, followed by downstream processing using depth and tangential-flow filtration. This vector is used subsequently to transduce human mesenchymal stem cells. The production of the cells and the subsequent transduction process are illustrated.

A scalable downstream process for the purification of the cell culture-derived Orf virus for human or veterinary applications

The large demand for safe and efficient viral vector-based vaccines and gene therapies against both inherited and acquired diseases accelerates the development of viral vectors. One outstanding example, the Orf virus, has a wide range of applications, a superior efficacy and an excellent safety profile combined with a reduced pathogenicity compared to other viral vectors. However, besides these favorable attributes, an efficient and scalable downstream process still needs to be developed. Recently, we screened potential chromatographic stationary phases for Orf virus purification. Based on these previous accomplishments, we developed a complete downstream process for the cell culture-derived Orf virus. The described process comprises a membrane-based clarification step, a nuclease treatment, steric exclusion chromatography, and a secondary chromatographic purification step using Capto® Core 700 resin. The applicability of this process to a variety of diverse Orf virus vectors was shown, testing two different genotypes. These studies render the possibility to apply the developed downstream scheme for both genotypes, and lead to overall virus yields of about 64 %, with step recoveries of >70 % for the clarification, and >90 % for the chromatography train. Protein concentrations of the final product are below the detection limits, and the final DNA concentration of about 1 ng per 1E + 06 infective virus units resembles a total DNA depletion of 96-98 %.

Polysaccharide-based chromatographic adsorbents for virus purification and viral clearance

Viruses still pose a significant threat to human and animal health worldwide. In the fight against viral infections, high-purity viral stocks are needed for manufacture of safer vaccines. It is also a priority to ensure the viral safety of biopharmaceuticals such as blood products. Chromatography techniques are widely implemented at both academic and industrial levels in the purification of viral particles, whole viruses and virus-like particles to remove viral contaminants from biopharmaceutical products. This paper focuses on polysaccharide adsorbents, particulate resins and membrane adsorbers, used in virus purification/removal chromatography processes. Different chromatographic modes are surveyed, with particular attention to ion exchange and affinity/pseudo-affinity adsorbents among which commercially available agarose-based resins (Sepharose®) and cellulose-based membrane adsorbers (Sartobind®) occupy a dominant position. Mainly built on the development of new ligands coupled to conventional agarose/cellulose matrices, the development perspectives of polysaccharide-based chromatography media in this antiviral area are stressed in the conclusive part.

Capture and purification of Human Immunodeficiency Virus-1 virus-like particles: Convective media vs porous beads

Downstream processing (DSP) of large bionanoparticles is still a challenge. The present study aims to systematically compare some of the most commonly used DSP strategies for capture and purification of enveloped viruses and virus-like particles (eVLPs) by using the same staring material and analytical tools. As a model, Human Immunodeficiency Virus-1 (HIV-1) gag VLPs produced in CHO cells were used. Four different DSP strategies were tested. An anion-exchange monolith and a membrane adsorber, for direct capture and purification of eVLPs, and a polymer-grafted anion-exchange resin and a heparin-affinity resin for eVLP purification after a first flow-through step to remove small impurities. All tested strategies were suitable for capture and purification of eVLPs. The performance of the different strategies was evaluated regarding its binding capacity, ability to separate different particle populations and product purity. The highest binding capacity regarding total particles was obtained using the anion exchange membrane adsorber (5.3 × 10¹² part/mL membrane), however this method did not allow the separation of different particle populations. Despite having a lower binding capacity (1.5 × 10¹¹ part/mL column) and requiring a pre-processing step with flow-through chromatography, Heparin-affinity chromatography showed the best performance regarding separation of different particle populations, allowing not only the separation of HIV-1 gag VLPs from host cell derived bionanoparticles but also from chromatin. This work additionally shows the importance of thorough sample characterization combining several biochemical and biophysical methods in eVLP DSP.

Opportunities to debottleneck the downstream processing of the oncolytic measles virus

Oncolytic viruses (including measles virus) offer an alternative approach to reduce the high mortality rate of late-stage cancer. Several measles virus strains infect and lyse cancer cells efficiently, but the broad application of this therapeutic concept is hindered by the large number of infectious particles required (10⁸-10¹² TCID₅₀ per dose). The manufacturing process must, therefore, achieve high titers of oncolytic measles virus (OMV) during upstream production and ensure that the virus product is not damaged during purification by applying appropriate downstream processing (DSP) unit operations. DSP is currently a production bottleneck because there are no specific platforms for OMV. Infectious OMV must be recovered as intact, enveloped particles, and host cell proteins and DNA must be reduced to acceptable levels to meet regulatory guidelines that were developed for virus-based vaccines and gene therapy vectors. Handling such high viral titers and process volumes is technologically challenging and expensive. This review considers the state of the art in OMV purification and looks at promising DSP technologies. We discuss here the purification of other enveloped viruses where such technologies could also be applied to OMV. The development of DSP technologies tailored for enveloped viruses is necessary to produce sufficient titers for virotherapy, which could offer hope to millions of patients suffering from incurable cancer.

A Flow-Through Chromatographic Strategy for Hepatitis C Virus-Like Particles Purification

Biopharmaceuticals are currently becoming one of the fastest growing segments of the global pharmaceutical industry, being used in practically all branches of medicine from disease treatment to prevention. Virus-like particles (VLP) hold tremendous potential as a vaccine candidate due to their anticipated immunogenicity and safety profile when compared to inactivated or live attenuated viral vaccines. Nevertheless, there are several challenges yet to be solved in the development and manufacturing of these products, which ultimately can increase time to market. Suchlike virus-based products, the development of a platform approach is often hindered due to diversity and inherent variability of physicochemical properties of the product. In the present work, a flow-through chromatographic purification strategy for hepatitis C VLP expressed using the baculovirus-insect cell expression system was developed. The impact of operational parameters, such as residence time and ionic strength were studied using scaled-down models and their influence on the purification performance was described. The flow-through strategy herein reported made use of radial-flow chromatography columns packed with an anion exchanger and was compared with a bind and elute approach using the same chromatography media. Overall, by selecting the optimal operational setpoints, we were able to achieve higher VLP recoveries in the flow-through process (66% versus 37%) with higher removal of DNA, baculovirus and host-cell protein (92%, 99% and 50% respectively).

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.

Membrane-based steric exclusion chromatography for the purification of a recombinant baculovirus and its application for cell therapy

The continuously increasing potential of stem cell treatments for various medical conditions has accelerated the need for fast and efficient purification techniques for individualized cell therapy applications. Genetic stem cell engineering is commonly done with viral vectors like the baculovirus. The baculovirus is a safe and efficient gene transfer tool, that has been used for the expression of recombinant proteins for many years. Its purification has been based mainly on ion exchange matrices. However, these techniques impair process robustness, if different genetically modified virus particles are applied. Here, we evaluated the membrane-based steric exclusion chromatography for the purification of insect cell culture-derived recombinant Autographa californica multicapsid nucleopolehydroviruses for an application in cell therapy. The method has already proven to be a powerful tool for the purification of Influenza A virus particles, using cellulose membranes. Aside from the aforementioned cellulose, we evaluated alternative stationary phases, such as glass fiber and polyamide membranes. The highest dynamic binding capacitiy was determined for cellulose with 5.08E + 07 pfu per cm² membrane. Critical process parameters were optimized, using a design of experiments (DoE) approach. The determined process conditions were verified by different production batches, obtaining a mean virus yield of 91% ± 6.5%. Impurity depletion was >99% and 85% for protein and dsDNA, without nuclease treatment. Due to the method's specificity, its application to other baculoviruses, with varying surface modifications, is conceivable without major process changes. The physiological buffer conditions enable a gentle handling of the virus particles without decreasing the transduction efficacy. The simple procedure with sufficient impurity removal enables the substitution of time-consuming ultra centrifugation steps and can serve as a first process unit operation to obtain higher purities.

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.

Viruses and Virus-Like Particles in Biotechnology: Fundamentals and Applications

Although viruses are simple biological systems, they are capable of evolving highly efficient techniques for infecting cells, expressing their genomes, and generating new copies of themselves. It is possible to genetically manipulate most of the different classes of known viruses in order to produce recombinant viruses that express foreign proteins. Recombinant viruses have been used in gene therapy to deliver selected genes into higher organisms, in vaccinology and immunotherapy, and as important research tools to study the structure and function of these proteins.

Virus-like particles (VLPs) are multiprotein structures that mimic the organization and conformation of authentic native viruses but lack the viral genome. They have been applied not only as prophylactic and therapeutic vaccines but also as vehicles in drug and gene delivery and, more recently, as tools in nanobiotechnology.

In this chapter, basic and advanced features of viruses and VLPs are presented and their major applications are discussed. The different production platforms based on animal cell technology are explained, and their main challenges and future perspectives are explored. The implications of large-scale production of viruses and VLPs are discussed in the context of process control, monitoring, and optimization. The main upstream and downstream technical challenges are identified and discussed accordingly.

Purification of baculovirus vectors using heparin affinity chromatography

Baculoviruses are commonly used for recombinant protein and vaccine production. Baculoviruses are nonpathogenic to vertebrates, have a large packaging capacity, display broad host and cell type tropism, infect both dividing and nondividing cells, and do not elicit strong immune or allergic responses in vivo. Hence, their use as gene delivery vehicles has become increasingly popular in recent years. Moreover, baculovirus vectors carrying mammalian regulatory elements can efficiently transduce and express transgenes in mammalian cells. Based on the finding that heparan sulfate, which is structurally similar to heparin, is an attachment receptor for baculovirus, we developed a novel scalable baculovirus purification method using heparin-affinity chromatography. Baculovirus supernatants were loaded onto a POROS heparin column, washed to remove unbound materials, and eluted with 1.5 mol/l NaCl, which yielded a recovery of purified baculovirus of 85%. After ultracentrifugation, baculovirus titers increased from 200- to 700-fold with overall yields of 26–29%. We further show that baculovirus particles were infectious, normal in morphology and size, despite high-salt elution and shear forces used during purification and concentration. Our chromatography-based purification method is scalable and, together with ultracentrifugation and/or tangential flow filtration, will be suitable for large-scale manufacturing of baculovirus stocks for protein and vaccine production and in gene therapy applications.

Bioorthogonal Strategy for Bioprocessing of Specific-Site-Functionalized Enveloped Influenza-Virus-Like Particles

Virus-like particles (VLPs) constitute a promising platform in vaccine development and targeted drug delivery. To date, most applications use simple nonenveloped VLPs as human papillomavirus or hepatitis B vaccines, even though the envelope is known to be critical to retain the native protein folding and biological function. Here, we present tagged enveloped VLPs (TagE-VLPs) as a valuable strategy for the downstream processing and monitoring of the in vivo production of specific-site-functionalized enveloped influenza VLPs. This two-step procedure allows bioorthogonal functionalization of azide-tagged nascent influenza type A hemagglutinin proteins in the envelope of VLPs through a strain-promoted [3 + 2] alkyne–azide cycloaddition reaction. Importantly, labeling does not influence VLP production and allows for construction of functionalized VLPs without deleterious effects on their biological function. Refined discrimination and separation between VLP and baculovirus, the major impurity of the process, is achieved when this technique is combined with flow cytometry analysis, as demonstrated by atomic force microscopy. TagE-VLPs is a versatile tool broadly applicable to the production, monitoring, and purification of functionalized enveloped VLPs for vaccine design trial runs, targeted drug delivery, and molecular imaging.

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.

Manufacturing of AcMNPV baculovirus vectors to enable gene therapy trials

Over the past two decades, baculoviruses have become workhorse research tools for transient transgene expression. Although they have not yet been used directly as a gene therapy vector in the clinical setting, numerous preclinical studies have suggested the highly promising potential of baculovirus as a delivery vector for a variety of therapeutic applications including vaccination, tissue engineering, and cancer treatment. As such, there is growing interest in using baculoviruses as human gene therapy vectors, which has led to advances in baculovirus bioprocessing methods. This review provides an overview of the current approaches for scaled-up amplification, concentration, purification, and formulation of AcMNPV baculoviruses, and highlights the key regulatory requirements that must be met before gene therapy clinical trials can be initiated.

Modeling and simulation of anion-exchange membrane chromatography for purification of Sf9 insect cell-derived virus-like particles

Recombinant protein-based virus-like particles (VLPs) are steadily gaining in importance as innovative vaccines against cancer and infectious diseases. Multiple VLPs are currently evaluated in clinical phases requiring a straightforward and rational process design. To date, there is no generic platform process available for the purification of VLPs. In order to accelerate and simplify VLP downstream processing, there is a demand for novel development approaches, technologies, and purification tools. Membrane adsorbers have been identified as promising stationary phases for the processing of bionanoparticles due to their large pore sizes. In this work, we present the potential of two strategies for designing VLP processes following the basic tenet of 'quality by design': High-throughput experimentation and process modeling of an anion-exchange membrane capture step. Automated membrane screenings allowed the identification of optimal VLP binding conditions yielding a dynamic binding capacity of 5.7 mg/mL for human B19 parvovirus-like particles derived from Spodoptera frugiperda Sf9 insect cells. A mechanistic approach was implemented for radial ion-exchange membrane chromatography using the lumped-rate model and stoichiometric displacement model for the in silico optimization of a VLP capture step. For the first time, process modeling enabled the in silico design of a selective, robust and scalable process with minimal experimental effort for a complex VLP feedstock. The optimized anion-exchange membrane chromatography process resulted in a protein purity of 81.5%, a DNA clearance of 99.2%, and a VLP recovery of 59%.

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.

Downstream processing and chromatography based analytical methods for production of vaccines, gene therapy vectors, and bacteriophages

Downstream processing of nanoplexes (viruses, virus-like particles, bacteriophages) is characterized by complexity of the starting material, number of purification methods to choose from, regulations that are setting the frame for the final product and analytical methods for upstream and downstream monitoring. This review gives an overview on the nanoplex downstream challenges and chromatography based analytical methods for efficient monitoring of the nanoplex production.

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.

Protein production using the baculovirus-insect cell expression system

The baculovirus-insect cell expression system is widely used in producing recombinant proteins. This review is focused on the use of this expression system in developing bioprocesses for producing proteins of interest. The issues addressed include: the baculovirus biology and genetic manipulation to improve protein expression and quality; the suppression of proteolysis associated with the viral enzymes; the engineering of the insect cell lines for improved capability in glycosylation and folding of the expressed proteins; the impact of baculovirus on the host cell and its implications for protein production; the effects of the growth medium on metabolism of the host cell; the bioreactors and the associated operational aspects; and downstream processing of the product. All these factors strongly affect the production of recombinant proteins. The current state of knowledge is reviewed.

Development and evaluation of baculovirus-expressed Chikungunya virus E1 envelope proteins for serodiagnosis of Chikungunya infection

Population-based serosurveillance studies provide critical estimates on community-level immunity and the potential for future outbreaks. Currently, serological assays, such as IgG enzyme-linked immunosorbent assays (ELISAs) and indirect immunofluorescence tests (IIFT) based on the inactivated whole virus are used to determine past Chikungunya virus (CHIKV) infection. However, these commercially available tests have variable sensitivities. To develop and evaluate recombinant based CHIKV-specific IgG antibody capture ELISAs (GAC-ELISAs), baculoviruses carrying wild-type (E1-A226, named WT) or mutant (E1-A226V, named MUT) E1 envelope protein genes of CHIKV were generated. The seroreactivity of recombinant CHIKV WT and MUT envelope proteins were determined using residual blood, collected from CHIKV-confirmed patients. The sensitivities of both recombinant CHIKV envelope proteins were 83.0% as measured by GAC-ELISAs. The specificities of both recombinant proteins were 87.8%. These GAC-ELISAs were also able to detect the persistence of anti-CHIKV IgG antibodies up to 6 months after the disease onset, together with rise in sensitivities with increasing time. These results suggest that the baculovirus purified recombinant CHIKV envelope proteins react with anti-CHIKV IgG antibodies and may be useful in population-based seroprevalence surveys. In addition, these GAC-ELISAs offer good diagnostic value to determine the recent/past CHIKV infection status in non-endemic populations.

Virus separation using membranes

Industrial manufacturing of cell culture-derived viruses or virus-like particles for gene therapy or vaccine production are complex multistep processes. In addition to the bioreactor, such processes require a multitude of downstream unit operations for product separation, concentration, or purification. Similarly, before a biopharmaceutical product can enter the market, removal or inactivation of potential viral contamination has to be demonstrated. Given the complexity of biological solutions and the high standards on composition and purity of biopharmaceuticals, downstream processing is the bottleneck in many biotechnological production trains. Membrane-based filtration can be an economically attractive and efficient technology for virus separation. Viral clearance, for instance, of up to seven orders of magnitude has been reported for state of the art polymeric membranes under best conditions.This chapter summarizes the fundamentals of virus ultrafiltration, diafiltration, or purification with adsorptive membranes. In lieu of an impractical universally applicable protocol for virus filtration, application of these principles is demonstrated with two examples. The chapter provides detailed methods for production, concentration, purification, and removal of a rod-shaped baculovirus (Autographa californica M nucleopolyhedrovirus, about 40 × 300 nm in size, a potential vector for gene therapy, and an industrially important protein expression system) or a spherical parvovirus (minute virus of mice, 22-26 nm in size, a model virus for virus clearance validation studies).

Evaluating baculovirus as a vector for human prostate cancer gene therapy

Gene therapy represents an attractive strategy for the non-invasive treatment of prostate cancer, where current clinical interventions show limited efficacy. Here, we evaluate the use of the insect virus, baculovirus (BV), as a novel vector for human prostate cancer gene therapy. Since prostate tumours represent a heterogeneous environment, a therapeutic approach that achieves long-term regression must be capable of targeting multiple transformed cell populations. Furthermore, discrimination in the targeting of malignant compared to non-malignant cells would have value in minimising side effects. We employed a number of prostate cancer models to analyse the potential for BV to achieve these goals. In vitro, both traditional prostate cell lines as well as primary epithelial or stromal cells derived from patient prostate biopsies, in two- or three-dimensional cultures, were used. We also evaluated BV in vivo in murine prostate cancer xenograft models. BV was capable of preferentially transducing invasive malignant prostate cancer cell lines compared to early stage cancers and non-malignant samples, a restriction that was not a function of nuclear import. Of more clinical relevance, primary patient-derived prostate cancer cells were also efficiently transduced by BV, with robust rates observed in epithelial cells of basal phenotype, which expressed BV-encoded transgenes faster than epithelial cells of a more differentiated, luminal phenotype. Maximum transduction capacity was observed in stromal cells. BV was able to penetrate through three-dimensional structures, including in vitro spheroids and in vivo orthotopic xenografts. BV vectors containing a nitroreductase transgene in a gene-directed enzyme pro-drug therapy approach were capable of efficiently killing malignant prostate targets following administration of the pro-drug, CB1954. Thus, BV is capable of transducing a large proportion of prostate cell types within a heterogeneous 3-D prostate tumour, can facilitate cell death using a pro-drug approach, and shows promise as a vector for the treatment of prostate cancer.

Baculovirus: an insect-derived vector for diverse gene transfer applications

Insect-derived baculoviruses have emerged as versatile and safe workhorses of biotechnology. Baculovirus expression vectors (BEVs) have been applied widely for crop and forest protection, as well as safe tools for recombinant protein production in insect cells. However, BEVs ability to efficiently transduce noninsect cells is still relatively poorly recognized despite the fact that efficient baculovirus-mediated in vitro and ex vivo gene delivery into dormant and dividing vertebrate cells of diverse origin has been described convincingly by many authors. Preliminary proof of therapeutic potential has also been established in preclinical studies. This review summarizes the advantages and current status of baculovirus-mediated gene delivery. Stem cell transduction, preclinical animal studies, tissue engineering, vaccination, cancer gene therapy, viral vector production, and drug discovery are covered.

Plant-derived virus-like particles as vaccines

Virus-like particles (VLPs) are self-assembled structures derived from viral antigens that mimic the native architecture of viruses but lack the viral genome. VLPs have emerged as a premier vaccine platform due to their advantages in safety, immunogenicity, and manufacturing. The particulate nature and high-density presentation of viral structure proteins on their surface also render VLPs as attractive carriers for displaying foreign epitopes. Consequently, several VLP-based vaccines have been licensed for human use and achieved significant clinical and economical success. The major challenge, however, is to develop novel production platforms that can deliver VLP-based vaccines while significantly reducing production times and costs. Therefore, this review focuses on the essential role of plants as a novel, speedy and economical production platform for VLP-based vaccines. The advantages of plant expression systems are discussed in light of their distinctive posttranslational modifications, cost-effectiveness, production speed, and scalability. Recent achievements in the expression and assembly of VLPs and their chimeric derivatives in plant systems as well as their immunogenicity in animal models are presented. Results of human clinical trials demonstrating the safety and efficacy of plant-derived VLPs are also detailed. Moreover, the promising implications of the recent creation of "humanized" glycosylation plant lines as well as the very recent approval of the first plant-made biologics by the U. S. Food and Drug Administration (FDA) for plant production and commercialization of VLP-based vaccines are discussed. It is speculated that the combined potential of plant expression systems and VLP technology will lead to the emergence of successful vaccines and novel applications of VLPs in the near future.

An initiative to manufacture and characterize baculovirus reference material

This letter to the editor brings to the attention of researchers an initiative to develop a baculovirus reference material repository. To be successful this initiative needs the support of a broad panel of researchers working with baculovirus vectors for recombinant protein production and gene delivery for either therapy or vaccination. First there is a need to reach a consensus on the nature of the reference material, the production protocols and the baculovirus characterization methods. It will also be important to define repository and distribution procedures so that the reference material is available to any researcher for calibrating experimental data and to compare experiments performed in the various laboratories. As more and more baculovirus-based products are licensed or in the final stages of development, the development of a repository of baculovirus reference material is timely. This letter describes the requirements for the reference material and for the project as a whole to be successful and calls for a partnership that would involve academic, industrial laboratories and governmental organizations to support this international initiative.

Baculoviruses as gene therapy vectors for human prostate cancer

Prostate cancer is the most commonly diagnosed cancer in ageing men in the western world. While the primary cancers can be treated with androgen ablation, radiotherapy and surgery, recurrent castration resistant cancers have an extremely poor prognosis, hence promoting research that could lead to a better treatment. Targeted therapeutic gene therapy may provide an attractive option for these patients. By exploiting the natural ability of viruses to target and transfer their genes into cancer cells, either naturally or after genetic manipulation, new generations of biological control can be developed. In this review we present the advantages and practicalities of using baculovirus as a vector for prostate cancer gene therapy and provide evidence for the potential of the baculovirus Autographa californica multiple nucleopolyhedrovirus (AcMNPV) as a safer alternative vehicle for targeting cancer cells. Strategies to target baculovirus binding specifically to prostate cell surfaces are also presented. The large insertion capacity of baculoviruses also permits restricted, prostate-specific gene expression of therapeutic genes by cloning extended human transcriptional control sequences into the baculovirus genome.

Latest developments in the large-scale production of adeno-associated virus vectors in insect cells toward the treatment of neuromuscular diseases

Adeno-associated viral (AAV) vectors are gene vectors of choice for the development of gene therapy treatments for many rare diseases affecting various tissues including retina, central nervous system, liver, and muscle. The AAV based gene therapy approach became conceivable only after the development of easily scalable production systems including the Sf9 cell/baculovirus expression system. Since the establishment of the production of AAV in the Sf9/baculovirus system by the group of Rob Kotin, this new production system has largely been developed for optimizing the large scale production of different serotypes of AAV for preclinical and clinical purposes. Today this manufacturing system allows for the production of purified vector genome (vg) quantities of up to 2 × 10(15) for AAV1 using a 50L reactor and the scale up to larger reactor volumes is paralleled by a corresponding increase in the vector yield. This review presents the principles and achievements of the Sf9/baculovirus system for the production of AAV in comparison to other expression systems based on mammalian cells. In addition, new developments and improvements, which have not yet been implemented at a large scale, and perspectives for further optimization of this production system will be discussed. All of these achievements as well as further process intensifications are urgently needed for the production of clinical doses for the treatment of neuromuscular diseases for which estimated doses of up to 10(14)vg/kg body mass are required.

Baculovirus-produced influenza virus-like particles in mammalian cells protect mice from lethal influenza challenge

Influenza virus-like particles (VLPs) are effective vaccines against influenza infection, which can be produced either in insect cells by recombinant baculovirus (BV) infection or in mammalian cells by DNA plasmid transfection. However, VLPs produced from baculovirus/insect cells are difficult to purify due to baculovirus contamination; VLPs produced by plasmid transfection are limited by scale-up capability. In this study, a BacMam BV, in which three CMV-promoters drive the hemagglutinin, neuraminidase, and matrix of influenza virus was constructed. This baculovirus can deliver these genes into mammalian cells/hosts and subsequently influenza VLPs can be produced and secreted from transduced cells. Transduction conditions were optimized and influenza VLPs were purified from transduced 293T cells. Mice were vaccinated with BV transduction-produced VLPs, plasmid transfection-produced VLPs, and BacMam BV. Two vaccinations of each vaccine induced high hemagglutination-inhibition (HAI) titers and prevented influenza virus infection. In contrast, following a single vaccination, all mice vaccinated with each vaccine had significantly lower lung viral titers compared to unvaccinated mice. Remarkably, mice vaccinated with a single dose of BV transduction-produced VLPs survived challenge, whereas mice vaccinated with one dose of BacMam BV- or plasmid transfection-produced VLPs had 60-80% survival. This finding is particularly significant for producing easily purified VLPs. The BacMam system is an alternative strategy for VLP production, which is easy to scale up and purify. Besides, BacMam BV can be used as a gene delivery vector to produce VLPs in vivo, to stimulate immune responses.