A new scalable method for the purification of recombinant adenovirus vectors

Cloning, heterologous expression, and molecular characterization of a highly active and stable non-specific endonuclease from Pseudomonas fluorescens

Non-specific endonucleases can be used for the digestion of nucleic acids because they hydrolyze DNA/RNA into 3-5 base pairs (bp) length oligonucleotide fragments without strict selectivity. In this work, a novel non-specific endonuclease from Pseudomonas fluorescens (PfNuc) with high activities for both DNA and RNA was successfully cloned and expressed in Escherichia coli. The production of PfNuc in flask scale could be achieved to 1.73 × 106 U/L and 4.82 × 106 U/L for DNA and RNA by investigation of the culture and induction conditions. The characterization of PfNuc indicated that it was Mg2+-dependent and the catalytic activity was enhanced by 3.74 folds for DNA and 1.06 folds for RNA in the presence of 5 mM Mg2+. The specific activity of PfNuc for DNA was 1.44 × 105 U/mg at pH 8.0 and 40 °C, and 3.93 × 105 U/mg for RNA at pH 8.5 and 45 °C. The Km of the enzyme for both DNA and RNA was close to 43 µM. The Vmax was 6.40 × 105 U/mg and 1.11 × 106 U/mg for DNA and RNA, respectively. There was no observed activity loss when PfNuc was stored at 4 °C and - 20 °C after 28 days or 10 repeated freeze-thaw cycles at - 80 °C. Molecular docking revealed that PfNuc formed 17 and 19 hydrogen bonds with single-stranded RNA and double-stranded DNA, respectively. These results could explain the high activity and stability of PfNuc, suggesting its great potential applications in the industry and clinic.

Evolving Horizons: Adenovirus Vectors' Timeless Influence on Cancer, Gene Therapy and Vaccines

Efficient and targeted delivery of a DNA payload is vital for developing safe gene therapy. Owing to the recent success of commercial oncolytic vector and multiple COVID-19 vaccines, adenovirus vectors are back in the spotlight. Adenovirus vectors can be used in gene therapy by altering the wild-type virus and making it replication-defective; specific viral genes can be removed and replaced with a segment that holds a therapeutic gene, and this vector can be used as delivery vehicle for tissue specific gene delivery. Modified conditionally replicative-oncolytic adenoviruses target tumors exclusively and have been studied in clinical trials extensively. This comprehensive review seeks to offer a summary of adenovirus vectors, exploring their characteristics, genetic enhancements, and diverse applications in clinical and preclinical settings. A significant emphasis is placed on their crucial role in advancing cancer therapy and the latest breakthroughs in vaccine clinical trials for various diseases. Additionally, we tackle current challenges and future avenues for optimizing adenovirus vectors, promising to open new frontiers in the fields of cell and gene therapies.

An efficient capture strategy for the purification of human adenovirus type 5 from cell lysates

An efficient capture step for human adenovirus type 5 from cell lysate was developed as an initial virus purification step from cell debris supernatant. Organosilane-based polymer particles were synthesized and experimental monomer screening allowed the selection of appropriate functionalities for the development of particles for virus binding. After elution, virus recoveries of 83 % were obtained with significant reduction of matrix proteins and residual host cell DNA. Therefore, the implemented capture strategy for adenovirus via polymer particles provides a scalable and reproducible approach to reduce time and cost during virus purification processes.

Adenoviral vectors for cardiovascular gene therapy applications: a clinical and industry perspective

Abstract

Despite the development of novel pharmacological treatments, cardiovascular disease morbidity and mortality remain high indicating an unmet clinical need. Viral gene therapy enables targeted delivery of therapeutic transgenes and represents an attractive platform for tackling acquired and inherited cardiovascular diseases in the future. Current cardiovascular gene therapy trials in humans mainly focus on improving cardiac angiogenesis and function. Encouragingly, local delivery of therapeutic transgenes utilising first-generation human adenovirus serotype (HAd)-5 is safe in the short term and has shown some efficacy in drug refractory angina pectoris and heart failure with reduced ejection fraction. Despite this success, systemic delivery of therapeutic HAd-5 vectors targeting cardiovascular tissues and internal organs is limited by negligible gene transfer to target cells, elimination by the immune system, liver sequestration, off-target effects, and episomal degradation. To circumvent these barriers, cardiovascular gene therapy research has focused on determining the safety and efficacy of rare alternative serotypes and/or genetically engineered adenoviral capsid protein-modified vectors following local or systemic delivery. Pre-clinical studies have identified several vectors including HAd-11, HAd-35, and HAd-20–42-42 as promising platforms for local and systemic targeting of vascular endothelial and smooth muscle cells. In the past, clinical gene therapy trials were often restricted by limited scale-up capabilities of gene therapy medicinal products (GTMPs) and lack of regulatory guidance. However, significant improvement of industrial GTMP scale-up and purification, development of novel producer cell lines, and issuing of GTMP regulatory guidance by national regulatory health agencies have addressed many of these challenges, creating a more robust framework for future adenoviral-based cardiovascular gene therapy. In addition, this has enabled the mass roll out of adenovirus vector-based COVID-19 vaccines.

Key messages

First-generation HAd-5 vectors are widely used in cardiovascular gene therapy.

HAd-5-based gene therapy was shown to lead to cardiac angiogenesis and improved function.

Novel HAd vectors may represent promising transgene carriers for systemic delivery.

Novel methods allow industrial scale-up of rare/genetically altered Ad serotypes.

National regulatory health agencies have issued guidance on GMP for GTMPs.

Antibodies against thyroid-stimulating hormone receptor cause maternal-neonatal transmission of Graves' Disease

The present study aimed to investigate whether the thyroid-stimulating hormone receptor (TSHR) autoantibodies (Ab) from mothers with Graves' disease (GD) could cause neonatal thyroid disease and the underlying mechanisms of this. An adenovirus expressing the TSHR A-subunit and a control adenovirus expressing β-galactosidase was constructed by Beijing Sino Geno Max Co., Ltd. The sequences were subsequently verified and amplified via PCR. A GD model was established in female BALB/c mice (n=90) by three intramuscular injections of a TSHR-expressing adenovirus (Ad-TSHR). Mice injected with Ad-β-galactosidase served as a sham immunization group. The immunized females were paired with unimmunized males to generate offspring. The serum levels of TSHR-Ab and thyroxine (T4) of mothers and neonates were measured after delivery. Breast milk was collected from the stomachs of neonatal mice to determine the TSHR-Ab levels. The positive rate of serum TSHR-Ab (>0.3 IU/l) in the TSHR group was 99% (89/90) and 0% in the sham group. The mother mice in the TSHR group had elevated serum T4 levels and the thyroid pathological features of Graves' hyperthyroidism.GD mice gave birth to smaller newborns with thyroid pathological changes and higher serum levels of TSHR-Ab and T4, compared to the offspring in the sham group. The TSHR-Ab levels in breast milk from the GD mice declined with time. Mice immunized with Ad-TSHR exhibited the clinicopathological features of human GD and give birth to neonates with thyroid disease at birth.

Production of high-titer transmission-defective RNA virus-based episomal vector using tangential flow filtration

In recent years, viral vector based in vivo gene delivery strategies have achieved a significant success in the treatment of genetic diseases. RNA virus-based episomal vector lacking viral glycoprotein gene (ΔG-REVec) is a nontransmissive gene delivery system that enables long-term gene expression in a variety of cell types in vitro, yet in vivo gene delivery has not been successful due to the difficulty in producing high titer vector. The present study showed that tangential flow filtration (TFF) can be effectively employed to increase the titer of ΔG-REVec. Concentration and diafiltration of ΔG-REVec using TFF significantly increased its titer without loss of infectious activity. Importantly, intracranial administration of high titer vector enabled persistent transgene expression in rodent brain.

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.

Simian adenovirus vector production for early-phase clinical trials: A simple method applicable to multiple serotypes and using entirely disposable product-contact components

A variety of Good Manufacturing Practice (GMP) compliant processes have been reported for production of non-replicating adenovirus vectors, but important challenges remain. Most clinical development of adenovirus vectors now uses simian adenoviruses or rare human serotypes, whereas reported manufacturing processes mainly use serotypes such as AdHu5 which are of questionable relevance for clinical vaccine development. Many clinically relevant vaccine transgenes interfere with adenovirus replication, whereas most reported process development uses selected antigens or even model transgenes such as fluorescent proteins which cause little such interference. Processes are typically developed for a single adenovirus serotype - transgene combination, requiring extensive further optimization for each new vaccine. There is a need for rapid production platforms for small GMP batches of non-replicating adenovirus vectors for early-phase vaccine trials, particularly in preparation for response to emerging pathogen outbreaks. Such platforms must be robust to variation in the transgene, and ideally also capable of producing adenoviruses of more than one serotype. It is also highly desirable for such processes to be readily implemented in new facilities using commercially available single-use materials, avoiding the need for development of bespoke tools or cleaning validation, and for them to be readily scalable for later-stage studies. Here we report the development of such a process, using single-use stirred-tank bioreactors, a transgene-repressing HEK293 cell - promoter combination, and fully single-use filtration and ion exchange components. We demonstrate applicability of the process to candidate vaccines against rabies, malaria and Rift Valley fever, each based on a different adenovirus serotype. We compare performance of a range of commercially available ion exchange media, including what we believe to be the first published use of a novel media for adenovirus purification (NatriFlo® HD-Q, Merck). We demonstrate the need for minimal process individualization for each vaccine, and that the product fulfils regulatory quality expectations. Cell-specific yields are at the upper end of those previously reported in the literature, and volumetric yields are in the range 1 × 10¹³ - 5 × 10¹³ purified virus particles per litre of culture, such that a 2-4 L process is comfortably adequate to produce vaccine for early-phase trials. The process is readily transferable to any GMP facility with the capability for mammalian cell culture and aseptic filling of sterile products.

Laboratory-Scale Production of Replication-Deficient Adenovirus Vectored Vaccines

Replication-deficient adenoviruses are potent vaccine development platforms used extensively for human and animal candidate vaccines, largely due to their very good safety and immunogenicity profile. In this chapter we describe a method that can be used in any laboratory for the scalable production of replication-deficient adenovirus vector vaccines to GLP for preclinical studies in animal models, including definitive experimental studies in large target animal species for veterinary applications. We use human adenovirus serotype 5 (HAdV5) as an example, but the method can be easily adapted for use with other adenovirus serotypes from different species of origin.

Standard free droplet digital polymerase chain reaction as a new tool for the quality control of high-capacity adenoviral vectors in small-scale preparations

High-capacity adenoviral vectors (HCAdVs) are promising tools for gene therapy as well as for genetic engineering. However, one limitation of the HCAdV vector system is the complex, time-consuming, and labor-intensive production process and the following quality control procedure. Since HCAdVs are deleted for all viral coding sequences, a helper virus (HV) is needed in the production process to provide the sequences for all viral proteins in trans. For the purification procedure of HCAdV, cesium chloride density gradient centrifugation is usually performed followed by buffer exchange using dialysis or comparable methods. However, performing these steps is technically difficult, potentially error-prone, and not scalable. Here, we establish a new protocol for small-scale production of HCAdV based on commercially available adenovirus purification systems and a standard method for the quality control of final HCAdV preparations. For titration of final vector preparations, we established a droplet digital polymerase chain reaction (ddPCR) that uses a standard free-end-point PCR in small droplets of defined volume. By using different probes, this method is capable of detecting and quantifying HCAdV and HV in one reaction independent of reference material, rendering this method attractive for accurately comparing viral titers between different laboratories. In summary, we demonstrate that it is possible to produce HCAdV in a small scale of sufficient quality and quantity to perform experiments in cell culture, and we established a reliable protocol for vector titration based on ddPCR. Our method significantly reduces time and required equipment to perform HCAdV production. In the future the ddPCR technology could be advantageous for titration of other viral vectors commonly used in gene therapy.

Chromatographic purification of adenoviral vectors on anion-exchange resins

Anion-exchange chromatography is a useful and effective tool for adenoviral vectors purification. However, due to the different functional groups and matrices, both binding capacity and resolution of most AEC resins are different. In this study, four different AEC resins are compared by the binding capacity, resolution and recovery. Using Fractogel TMAE as an adsorbent to purify adenoviral vectors has obvious advantages over the other resins, namely (1) dynamic binding capacity is higher than other resins; (2) unprecedented sharpness (1,570,000±250,000) and symmetry of adenoviral vectors peak (1.67±0.06); (3) higher resolution with other contaminants (2.16±0.08); (4) no enzymatic treatment; (5) the recovery can reach 75%; (6) the purity is higher and the total virion to infectious particle ratios can reach 18.9. In the present work, we confirmed the possibility of purifying pharmaceutical-grade adenoviral vectors by AEC.

Axonal transport of adeno-associated viral vectors is serotype-dependent

We have previously shown that AAV2 undergoes anterograde axonal transport in rat and non-human primate brain. We screened other AAV serotypes for axonal transport and found that AAV6 is transported almost exclusively in a retrograde direction and, like AAV2, it is also neuron-specific in rat brain. Our findings show that axonal transport of AAV is serotype-dependent and this has implications for gene therapy of neurological diseases such as Huntington’s disease.

Production of adenovirus vectors and their use as a delivery system for influenza vaccines

IMPORTANCE OF THE FIELD

With the emergence of highly pathogenic avian influenza H5N1 viruses that have crossed species barriers and are responsible for lethal infections in humans in many countries, there is an urgent need for the development of effective vaccines which can be produced in large quantities at a short notice and confer broad protection against these H5N1 variants. In order to meet the potential global vaccine demand in a pandemic scenario, new vaccine-production strategies must be explored in addition to the currently used egg-based technology for seasonal influenza.

AREAS COVERED IN THIS REVIEW

Adenovirus (Ad) based influenza vaccines represent an attractive alternative/supplement to the currently licensed egg-based influenza vaccines. Ad-based vaccines are relatively inexpensive to manufacture, and their production process does not require either chicken eggs or labor-intensive and time-consuming processes necessitating enhanced biosafety facilities. Most importantly, in a pandemic situation, this vaccine strategy could offer a stockpiling option to reduce the response time before a strain-matched vaccine could be developed.

WHAT THE READER WILL GAIN

This review discusses Ad-vector technology and the current progress in the development of Ad-based influenza vaccines.

TAKE HOME MESSAGE

Ad vector-based influenza vaccines for pandemic preparedness are under development to meet global vaccine demand.

Overview of current scalable methods for purification of viral vectors

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

An efficient method of directly cloning chimpanzee adenovirus as a vaccine vector

Adenoviral vectors have shown great promise as vaccine carriers and in gene transfer to correct underlying genetic diseases. Traditionally, construction of adenoviral vectors is complex and time consuming. In this paper, we provide an improved method for efficient generation of novel adenoviral vectors by using direct cloning. We introduce a feasible and detailed protocol for the development of chimpanzee adenoviruses (Ads) as molecular clones, as well as for the generation of recombinant virus from the molecular clones. Recombinant viruses are genetically stable and induce potent immune responses in animals. Generation of new Ad molecular clones or new recombinant Ad can be achieved in 2 months or 2 weeks, respectively.

Concanavalin A affinity chromatography for efficient baculovirus purification

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

Flow-dependent entrapment of large bioparticles in porous process media

The need for purification of biomolecules extends to larger bioparticles as well. For example, virus purification is required for production of many vaccines and gene delivery vectors, and understanding virus removal in porous media is also important in downstream processing of therapeutic proteins and in purification of water in soils. A convective entrapment mechanism for retention of large bioparticles is discussed here based on retention of such bioparticles in pore constrictions at high enough flow rates, even under non-binding conditions. A simple equation to predict whether such entrapment is expected to occur in a given system is derived based on a Péclet number that is proportional to the flow rate and to the cube of the bioparticle diameter. To test the theory, adenovirus was spiked onto chromatographic beds. As expected from the theory, under non-interacting conditions a progressively larger amount of virus becomes trapped with increasing flow rate. The entrapment is reversible upon flow rate reduction, which, within the proposed model, is based on the possibility of diffusive escape from pore constrictions. This mechanism can be exploited for virus purification or removal, and the theory is also consistent with the anecdotal evidence that monoliths and membranes are more difficult to clean than conventional chromatographic beds, especially at high flow rates.

Monitoring the homogeneity of adenovirus preparations (a gene therapy delivery system) using analytical ultracentrifugation

This study explores the capability of modern analytical ultracentrifugation (AUC) to characterize the homogeneity, under product formulation conditions, of preparations of adenovirus vectors used in gene therapy and to assess the lot-to-lot consistency of this unique drug product. We demonstrate that a single sedimentation velocity run on an adenovirus sample can detect and accurately quantify a number of different forms of virus particles and subvirus particles. These forms include (a) intact virus monomer particles, (b) virus aggregates, (c) empty capsids (ECs), and (d) smaller assembly intermediates or subparticles formed during normal or aberrant virus assembly (or as a result of damage to the intact adenovirus or EC material during all phases of virus production). This information, which is collected on adenovirus samples under the exact formulation conditions that exist in the adenovirus vial, is obtained by direct boundary modeling of the AUC data generated from refractometric and/or UV detection systems using the computer program SEDFIT developed by Peter Schuck. Although both detectors are useful, refractometric detection using the Rayleigh interferometer offers a key advantage for providing accurate concentration information due to the similar response factors for both protein and DNA and its insensitivity to light scattering effects. Additional AUC data obtained from analytical band sedimentation velocity and density gradient sedimentation equilibrium experiments in CsCl with UV detection were also generated. These results further support conclusions concerning the solution properties of adenovirus, the identity of the different virus species, and the overall capability of boundary sedimentation velocity analysis.

Comparison of two gel filtration chromatographic methods for the purification of Lily symptomless virus

Lily symptomless virus (LSV) occurs frequently in many Lilium species worldwide and often causes developmental abnormalities such as a smaller flower and lower bulb yield. In this study, two moderate and efficient gel filtration chromatography (GFC) methods was compared, these two techniques were, respectively, based on Superdex-200 HR and Sephacryl S-1000 SF. The products purified by the two methods were then characterized by measurements with UV-spectrophotometer, reverse transcriptase (RT)-PCR, transmission electron microscope (TEM), polyacrylamide gel electrophoresis (PAGE), Western blotting and matrix assisted laser desorption-ionisation/time-of-flight mass spectrometry (MALDI-TOF-MS). The final yield of purified LSV by the Superdex-200 HR GFC was 9.4 mg from 50 g of fresh infected tissues of Lanzhou lily. However, from the same amount tissues, only 5.6 mg of LSV were obtained by using Sephacryl S-1000 SF GFC. The Superdex-200 HR method was thus shown to be more suitable for the purification of LSV than the Sephacryl S-1000 SF GFC. The Superdex-200 HR method does not require costly equipment for density centrifugation and ultracentrifugation. Furthermore, it can provide an economical and efficient way to obtain purified products for the preparation of antibodies for serological diagnosis or LSV infection and related investigations.

Sorption processes in ion-exchange chromatography of viruses

Purified viruses are used in gene therapy and vaccine production. Ion-exchange chromatography (IEC) is the most common method for large-scale downstream purification of viruses and proteins. Published IEC protocols provide details for specific separations but not general methods for selecting operating parameters. To make the selection more systematic, we study adenovirus type 5 (Ad5) as a model virus and develop batch uptake and light scattering methods for optimizing the ionic strength and pH of adsorption, as well as providing heuristics for resin geometry. The static capacity for Ad5 was found to go through a maximum with increasing ionic strength. Comparison to a protein-resin system shows that resin capacity for the virus is at least an order of magnitude lower, even on a wide-pore resin. Virus penetration into the wide-pore resin is only partial and the uptake rate is an order of magnitude slower than the uptake onto a narrow-pore resin.

Recombinant viral vectors: cancer vaccines

To date cancer vaccines have yet to show efficacy in a phase III trial. However, the clinical benefit seen with monoclonal antibody mediated therapies (e.g., Herceptin) has provided proof of principle that immune responses directed against tumour-associated antigens could have therapeutic potential. The failure of past cancer vaccine trials is likely due to several factors including the inappropriate choice of tumour antigen, use of an unoptimised antigen delivery system or vaccination schedule or selection of the wrong patient group. Any one of these variables could potentially result in the induction of an immune response of insufficient magnitude to deliver clinical benefit. Live recombinant viral vaccines have been used in the development of cancer immunotherapy approaches for the past 10 years. Though such vectors are self-adjuvanted and offer the ability to express multiple tumour-associated antigens (TAAs) along with an array of immune co-factors, arguably, they have yet to demonstrate convincing efficacy in pivotal clinical trials. However, in recent years, more coordinated studies have revealed mechanisms to optimise current vectors and have lead to the development of new advantageous vector systems. In this review, we highlight that live recombinant viral vectors provide a versatile and effective antigen delivery system and describe the optimal properties of an effective viral vector. Additionally, we discuss the advantages and disadvantages of the panel of recombinant viral systems currently available to cancer vaccinologists and how they can work in synergy in heterologous prime boost protocols and with other treatment modalities.

In vivo application of adenoviral vectors purified by a Taqman Real Time PCR-supported chromatographic protocol

Recombinant adenoviral vectors encoding human HDL-cholesterol receptor SR-BI (Ad/hSR-BI) or beta-galactosidase (Ad/lacZ), respectively, were purified using a Source15 Q anion-exchange (AEX) column and quantified using two parallel Taqman Real Time PCR systems with different target sequences. Adenovirus concentrations were ascertained by 260 nm measurements, purity by 260/280 nm ratio and SDS-PAGE. Subsequently, adenoviruses were validated by experimental intravenous application into New Zealand White rabbits. Transgene expression was verified by functional assays determining plasma clearance rate of 3H-HDL-cholesterol, and was not affected by 21-months storage at -80 degrees C. No alterations of liver enzymes and C-reactive protein (CRP) upon Source15 Q adenovirus treatment could be detected, demonstrating biological safety of our protocol.

Analytical and biological characterization of supercoiled plasmids purified by various chromatographic techniques

Supercoiled plasmids are an important component of gene-based delivery vehicles. A number of production methods for clinical applications have been developed, each resulting in very high-quality product with low levels of residual contaminants. There is, however, no consensus on the optimal methods to characterize plasmid quality, and further, to determine if these methods are predictive of either product stability or biological activity. We have produced two plasmids using four production purification methodologies based on PolyFlo and hydrophobic interaction chromatography (HIC), either alone or in tandem processes. In each case, the product was analyzed using standard molecular biological methods. We also performed a number of biophysical analyses such as dynamic light scattering (DLS), circular dichroism (CD), Fourier transform infrared spectroscopy (FTIR), and differential scanning calorimetry (DSC). Minimal differences were detected among the preparations based on the more standard molecular biological methods. Some small differences were detected, however, using biophysical techniques, particularly FTIR and DSC, which may reflect small variations in plasmid tertiary structure and thermal stability. Stability after heat exposure at 60 degrees C, exposure to fetal bovine serum and long-term storage at 4 degrees C varied between plasmids. One plasmid showed no difference in stability depending on the production process, but the other showed significant differences. Evaluation in vivo in models for gene immunization and gene therapy showed significant differences in the response depending on the method of purification. Preparations using a tandem process of PolyFlo used in two separation modes provided higher biological activity compared to a tandem HIC/PolyFlo process or either resin used alone in a single column process. These data indicate that the process by which supercoiled plasmids are made can influence plasmid stability and biological activity and emphasize the need for more rigorous methods to evaluate supercoiled plasmids as gene-delivery vehicles.

A rapid and efficient method for purification of recombinant adenovirus with arginine-glycine-aspartic acid-modified fibers

Recombinant adenoviral vectors (adenovectors) have been subject to various genetic modifications to improve their transduction efficiency and targeting capacity. Production and purification of adenovectors with modified capsid proteins can be problematic using conventional two-cycle CsCl gradient ultracentrifugation. We have developed a new method for purifying recombinant adenovectors in two steps: iodixanol discontinuous density gradient ultracentrifugation and size exclusion column chromatography. The purity and infectious activity of adenovectors isolated by the two methods were comparable. The new method yielded three to four times more adenovectors with arginine-glycine-aspartic acid (RGD)-modified fiber proteins than did the conventional CsCl method. For other fiber-modified and wild-type adenovectors, the yields of the two methods were comparable. Thus, the iodixanol-based method can be used not only to improve the production of RGD-modified adenovectors but also to purify adenovectors with or without fiber modifications. Moreover, the whole procedure can be completed in 3h. Therefore, this method is rapid and efficient for production of recombination adenovectors, especially those with RGD-modified fibers.

Chromatographic purification of recombinant adenoviral and adeno-associated viral vectors: methods and implications

In recent years, recombinant adenoviral and adeno-associated viral (AAV) vectors have been exploited in a number of gene delivery approaches. The use of these vectors in clinical gene transfer has increased the demand for their characterization, production and purification. Although the classical method of adenovirus or AAV purification by density gradient centrifugation is effective on a small scale, chromatographic separation is the most versatile and powerful method for large-scale production of recombinant adenovirus or AAV. This review describes different chromatographic modes for adenovirus or AAV purification and process development, as well as the utility of different purification steps for virus production. Advances in the development of viral vectors for gene therapy, such as the discovery of new AAV serotypes, adenoviral and AAV retargeting and improved production of helper-dependent adenoviral vectors, require further development of efficient purification methods.

Downstream processing of viral vectors and vaccines

Viral vectors and viral vaccines more and more play an important role in current medical approaches. Gene vectors like adenoviruses, adeno-associated viruses or retroviruses are the vehicles being developed for delivering genetic material to the target cell in gene therapy. Viral vaccines, like attenuated or inactivated rabies virus, influenza virus or hepatitis virus vaccines, are powerful tools to limit the number of serious viral infections and pandemics. Higher safety demands, that is, reduction of side effects, by regulatory authorities like Food and Drug Administration (FDA) and European Agency for the Evaluation of Medicinal Products (EMEA), nowadays force developers as well as manufacturers to improve their production and purification processes for viral vectors and vaccines. Like for influenza viral vaccines, manufacturers begin to switch from egg cultivation to mammalian cell culture systems. Also within the purification procedure, a clear trend from classical purification methods like sucrose gradient centrifugation towards more sophisticated techniques like tangential flow filtration and liquid chromatography can be observed.

Purification of adenovirus and adeno-associated virus: comparison of novel membrane-based technology to conventional techniques

Adenovirus (Ad) and Adeno-associated virus (AAV) are efficient gene delivery systems; manipulation of the wild-type genome allows their use as vectors for the overexpression of desirable transgenes. Generation and purification of such viral vectors can be labour intensive, costly and require specialized equipment, but a new generation of membrane-mediated ion exchange kits for purification of recombinant virus may facilitate this process. Here, we examine the yields, transgene expression and purity of preparations of Ad and AAV purified using commercially available kits in comparison to other established techniques for purification of recombinant viral vectors. We demonstrate comparable results for Ad and AAV respectively in all parameters investigated, with a substantial reduction in purification time for the kit-based technology. Such approaches are attractive methods for small-scale purification of recombinant Ad and AAV viral vectors.

Serotype specificity of adenovirus purification using anion-exchange chromatography

Recombinant adenoviruses continue to be a leading vector choice for gene transfer applications, with growing interest in the use of less prevalent serotypes, and of chimeras. As a result, the development of scaleable purification processes for alternative serotypes is needed. Anion-exchange chromatography is routinely used for scaleable adenovirus type 5 purification; however, retention varies for other serotypes because of differences in the exposed capsid proteins. Understanding how the viral surface influences retention behavior can provide a rational basis for chromatography development and optimization. In this work, chimeric vectors were used to show that the hexon protein is responsible for retention differences in anion-exchange chromatography. Next, the relative retention of eight serotypes from three subgroups was studied. Although all serotypes bound to the anion-exchange resin, the sodium chloride required to elute the virus varied over a 2- fold range, from 270 to 490 mM. Retention was accurately correlated to the electrostatic properties of the hexon protein, with an average error in sodium chloride concentration required to elute of only 14 mM. This correlation enables preparative chromatography gradients for alternative serotypes to be established with minimal effort.

Production and formulation of adenovirus vectors

Adenovirus vectors have attracted considerable interest over the past decade, with ongoing clinical development programs for applications ranging from replacement therapy for protein deficiencies to cancer therapeutics to prophylactic vaccines. Consequently, considerable product, process, analytical, and formulation development has been undertaken to support these programs. For example, "gutless" vectors have been developed in order to improve gene transfer capacity and durability of expression; new cell lines have been developed to minimize recombination events; production conditions have been optimized to improve volumetric productivities; analytical techniques and scaleable purification processes have advanced towards the goal of purified adenovirus becoming a "well-characterized biological"; and liquid formulations have been developed which maintain virus infectivity at 2-8 degrees C for over 18 months. These and other advances in the production of adenovirus vectors are discussed in detail in this review. In addition, the needs for the next decade are highlighted.

Purification of adenoviral vectors using expanded bed chromatography

The increasing numbers of pre-clinical and clinical trials where recombinant adenoviral vectors are used for gene therapy and vaccination require the development of cost-effective and reproducible large scale purification strategies of the biologically active particles. Alternatives to the traditional laboratory scale CsCl density gradient ultracentrifugation method, such as fixed bed chromatography strategies, have been developed, but the yields of final recovery remain too low due mainly to the capture and concentration steps taking place before and between the chromatographic stages. In this study, a rapid and efficient scale-able purification protocol allowing to obtain concentrated, pure and bioactive adenoviral vectors was developed. This allows efficient levels of binding to the column media and vector purification without centrifugation or filtration steps. Expanded bed chromatography followed by hollow fiber concentration allows the capture of viral particles directly from cellular extracts with high efficiency and vector purification is achieved in less than one working day with a minimal amount of sample handling, thus presenting an improvement over existing processes. The overall process yield reached 32%, representing an eight-fold improvement over results reported previously, while the purity is comparable to that obtained with the CsCl method.

Development of a novel adenovirus purification process utilizing selective precipitation of cellular DNA

The use of recombinant adenoviral vectors for vaccination and gene therapy requires the development of purification processes that are cost-effective, scalable, and capable of robust host cell DNA clearance. An adenovirus purification process was developed which incorporates selective precipitation of host cell DNA, enabling a reduction in the use of costly nucleases and chromatographic resins while substantially improving DNA and protein clearance capabilities. In this work, three cationic detergents were evaluated for their capacity to selectively precipitate DNA from adenovirus-containing cell lysate. Parameters including pH, sodium chloride concentration, nonionic surfactant concentration, and cell density were investigated during development of the precipitation step. In a novel application, the cationic detergent domiphen bromide was found to have superior selectivity for host cell DNA. In addition, domiphen bromide-induced precipitation of adenovirus was shown to be reversible, which reduces the importance of mixing. Precipitation of DNA in the cell lysate coupled with primary clarification resulted in 3 logs of DNA clearance and improved impurity clearance in the subsequent ultrafiltration step. As a result, nuclease treatment and/or anion exchange chromatography can be eliminated, or included exclusively to improve process robustness.

Purification of infectious adenovirus in two hours by ultracentrifugation and tangential flow filtration

Adenoviruses are excellent vectors for gene transfer and are used extensively for high-level expression of the products of transgenes in living cells. The development of simple and rapid methods for the purification of stable infectious recombinant adenoviruses (rAds) remains a challenge. We report here a method for the purification of infectious adenovirus type 5 (Ad5) that involves ultracentrifugation on a cesium chloride gradient at 604,000g for 15 min at 4 degrees C and tangential flow filtration. The entire procedure requires less than two hours and infectious Ad5 can be recovered at levels higher than 64% of the number of plaque-forming units (pfu) in the initial crude preparation of viruses. We have obtained titers of infectious purified Ad5 of 1.35x10(10) pfu/ml and a ratio of particle titer to infectious titer of seven. The method described here allows the rapid purification of rAds for studies of gene function in vivo and in vitro, as well as the rapid purification of Ad5.

Adenoviral-mediated efficiency expression of enhanced green fluorescence protein in adult liver stem cells of rats

AIM: To investigate the feasibility of adenoviral-mediated exogenous gene expression in adult liver stem cells of rats and to establish a cell line that stably and efficiently express enhanced green fluorescence protein (EGFP).

METHODS: A pAd-CMV-EGFP vector under the control of CMV promoter was constructed by homologous recombination in E.coilBJ 5 183, and the recombinant virus was Packaged in HEK 293 cell line. Hepatic adult stem cells cultured in vitro were infected with recombinant adenovirus. Expression of EGFP was observed by fluorescent microscopy and infection efficiency was analyzed. Adult liver stem cells were further cultured to estabilish a cell line that stably and efficiently expressed EGFP through cloning culture and the biological characteristics of the cell line were observed and analyzed by fluorescence microscopy, immunocytochemistry and differentiation-inducing experiment.

RESULTS: Adenovirus vector of pAd-CMV-EGFP was constructed and high titer recombinant virus were produced successfully. EGFP, mediated by adenovirus, could be transfected into hepatic adult stem cells with a high efficiency (about 40-70%). After cloning culture, WB-EGFP cell line was established, and it could stably express EGFP in 8-9 generations. Furthermore, biological characteristics such as marker of stem cells, proliferation speed and differentiation capability had not been affected.

CONCLUSION: Target gene can be efficiently transfected into hepatic adult stem cells through adeno-vector system. EGFP can be stably and long-term expressed in transfected cells and their offspring. It can serve as a tracker in the research of stem cells.

Immobilized cobalt affinity chromatography provides a novel, efficient method for herpes simplex virus type 1 gene vector purification

Herpes simplex virus type 1 (HSV-1) is a promising vector for gene therapy applications, particularly at peripheral nerves, the natural site of virus latency. Many gene vectors require large particle numbers for even early-phase clinical trials, emphasizing the need for high-yield, scalable manufacturing processes that result in virus preparations that are nearly free of cellular DNA and protein contaminants. HSV-1 is an enveloped virus that requires the development of gentle purification methods. Ideally, such methods should avoid centrifugation and may employ selective purification processes that rely on the recognition of a unique envelope surface chemistry. Here we describe a novel method that fulfills these criteria. An immobilized metal affinity chromatography (IMAC) method was developed for the selective purification of vectors engineered to display a high-affinity binding peptide. Feasibility studies involving various transition metal ions (Cu2+, Zn2+, Ni2+, and Co2+) showed that cobalt had the most desirable features, which include a low level of interaction with either the normal virus envelope or contaminating DNA and proteins. The introduction of a cobalt-specific recognition element into the virus envelope may provide a suitable target for cobalt-dependent purification. To test this possibility, we engineered a peptide with affinity for immobilized cobalt in frame in the heparan sulfate binding domain of HSV-1 glycoprotein B, which is known to be exposed on the surface of the virion particle and recombined into the viral genome. By optimizing the IMAC loading conditions and reducing cobalt ion leakage, we recovered 78% of the tagged HSV-1 recombinant virus, with a >96% reduction in contaminating proteins and DNA.

Progress and problems with the use of viral vectors for gene therapy

Gene therapy has a history of controversy. Encouraging results are starting to emerge from the clinic, but questions are still being asked about the safety of this new molecular medicine. With the development of a leukaemia-like syndrome in two of the small number of patients that have been cured of a disease by gene therapy, it is timely to contemplate how far this technology has come, and how far it still has to go.

Structural stability of adenovirus type 5

Thermally induced structural changes in adenovirus type 5 (Ad) in the presence of either 2 or 10% sucrose were investigated using a variety of biophysical techniques. In solutions containing 2% sucrose, a highly cooperative transition in the structure of the virus was observed at 45 degrees C as detected by tryptophan fluorescence, derivative UV absorption spectroscopy, circular dichroism (CD), and dynamic and static light scattering. This transition resulted in (at least partial) disassembly of the virus and a concomitant increase in the accessibility of the viral DNA to the fluorescent dye, TOTO-1. Capsid disassembly was verified by transmission electron microscopy, which showed ruptured icosahedral vertices near 45 degrees C followed by complete capsid disassembly at higher temperatures. SDS-PAGE of thermally treated Ad suggests that the penton base (protein III) and protein IIIa (located in the peripentonal region) are significantly more labile than other capsid proteins and may be the initial instigators in capsid disassembly. Additional discrete structural transitions were observed in viral proteins using the aforementioned spectroscopic techniques. Thermally induced rearrangements of the condensed DNA at higher temperatures were also detected by the appearance of "psi"-like features in the CD spectra as well as a dramatic decrease in accessibility of DNA to TOTO-1. These transitions corresponded to discrete endothermic events that are also detected by differential scanning calorimetry. By increasing the concentration of sucrose to 10%, secondary and tertiary structural features of adenoviral proteins were significantly stabilized, although loss of quaternary structure at 45 degrees C was still observed.