Concentration of biomaterials: virus concentration and viral protein isolation

Primary recovery and chromatographic purification of adeno-associated virus type 2 produced by baculovirus/insect cell system

Adeno-associated virus (AAV) is making its place in gene therapy applications; however, the industry is still facing obstacles in producing a large quantity of highly purified material for clinical studies. Insect cell technology can be used to produce AAV to meet the current demand. During the purification process it was observed that there was a reduced recovery of AAV produced in insect cells, Spodoptera frugiperda (Sf9). It was assumed that the formation of AAV agglomerates and the interaction of AAV with other cellular components were major contributors to this loss. After studying different systems of extraction a sequence of treatment for primary recovery of AAV from cell paste was developed. This sequence was necessary to reduce the AAV losses and to increase the recovery. The purification method avoided the use of ultracentrifugation and adopted chromatographic methods for the purification of AAV. Primary recovery, ion exchange chromatography and hydrophobic interaction chromatography gave an overall yield of 75% from the extracted AAV. The purification process was based on chromatographic methods; therefore, it can be scaled up. Although this method was developed for AAV type 2, it is believed that this method could be modified easily to purify other AAV serotypes.

Urea-nuclease treatment of concentrated retrovirions preserves viral RNA and removes polymerase chain reaction-amplifiable cellular RNA and DNA

Cellular nucleic acids can interfere with the molecular cloning of retroviruses, a problem that is particularly serious with viruses propagated in lymphoblastoid cells that release large amounts of microvesicles and other cellular components. The approach taken to circumvent such problems involved first suspending viral pellets in water to allow any residual microvesicles to swell and perhaps lyse during overnight or longer incubation periods. Urea was then added to a concentration of 1.5-2.0 M to uncoil proteins that may protect nucleic acids from hydrolysis on the further addition of Micrococcal nuclease and ribonuclease A, both of which remain enzymatically active in molar urea solutions. The viral RNA was extracted and residual DNA removed by deoxyribonuclease I treatments. The utility of the method was demonstrated with two different retroviruses, a Moloney murine leukemia virus variant and Rous sarcoma virus, such that viral RNA thus purified was shown to be free of contamination by PCR-amplifiable cellular GAPDH mRNA and ribosomal RNA. This general approach should be applicable to viruses of any type in circumstances where contamination by cellular RNA and DNA poses a problem.

Retroviral vector production in the National Gene Vector Laboratory at Indiana University

The National Gene Vector Laboratory (NGVL) is a US National Institutes of Health initiative charged with providing clinical grade vectors for gene therapy trials. The program was started in 1995 and Indiana University has served as the production site for retroviral vectors and is also accepting applications for production of lentiviral vectors. The facility is designed to produce vectors for Phase I and Phase II clinical trials with the specific mandate to facilitate investigator-initiated research for academic institutions. To date, the facility has generated over 30 Master Cell Banks for gene therapy investigators throughout the United States. This required the facility to develop a system that can adapt to the varied needs of investigators, most of whom request different vector backbones, packaging cell lines, final product volumes, and media. In this review, we will illustrate some of the experiences of the Indiana University NGVL during the generation of retroviral vectors using murine-based packaging cell lines.

Tagging retrovirus vectors with a metal binding peptide and one-step purification by immobilized metal affinity chromatography

Retroviral vectors produced from packaging cells are invariably contaminated by protein, nucleic acid, and other substances introduced in the manufacturing process. Elimination of these contaminants from retroviral vector preparations is helpful to reduce unwanted side effects, and purified vector preparations are desirable to improve reproducibility of therapeutic effect. Here we report a novel approach to engineer a metal binding peptide (MBP)-tagged murine leukemia virus (MuLV), allowing for one-step purification of retroviral vectors by immobilized metal affinity chromatography (IMAC). We inserted a His6 peptide into an ecotropic envelope protein (Env) by replacing part of its hypervariable region sequence with a sequence encoding the His6 peptide. Display of the His6 tag on the surface of Env endowed the vectors with a high affinity for immobilized metal ions, such as nickel. We demonstrated that the His6-tagged MuLV could be produced to high titers and could be highly purified by one-step IMAC. The protein and DNA contaminants in the purified vector supernatants were below 7 microg/ml and 25 pg/ml, respectively, indicating a 1,229-fold reduction in protein contaminant level and a 6,800-fold reduction in DNA contaminant level. About 56% of the viral vectors were recovered in the IMAC purification. The purified vectors retained their functionality and infectivity. These results establish that an MBP can be functionally displayed on the surface of ecotropic retroviruses without interfering with their integrity, and MBP-tagged retroviral vectors can be highly purified by one-step IMAC.

Separating lysozyme from bacteriophage P22 in two-phase aqueous micellar systems

This communication demonstrates that two-phase aqueous mixed (nonionic/ionic) micellar systems have the potential for improving the separation of proteins from viruses. Specifically, two separation experiments were performed to show that the addition of the anionic surfactant sodium dodecyl sulfate (SDS) to the two-phase aqueous nonionic n-decyl tetra(ethylene oxide) (C(10)E(4)) micellar system increases the yield of a model net positively charged protein, lysozyme, in the micelle-rich phase from 75 to 95%, while still maintaining approximately the same yield of a model net negatively charged virus, bacteriophage P22, in the micelle-poor phase (97% vs. 98%).

Clinical retroviral vector production: step filtration using clinically approved filters improves titers

Production of retroviral vectors for clinical use requires removal of cells and cellular debris. We combined a series of filters of decreasing pore size using commercially available blood banking filters approved for clinical use. The collection bag and filters can be connected to create a sterile, closed system using clinically approved tubing and sealing systems. Even when challenged with a large number of vector producer cells (2.38 x 10(9)cells), no viable cells are passed through the system. The step filtration system developed minimizes the titer reduction associated with filtration, provides rapid flow rates, and was cost effective when filtering volumes in excess of 2 liters.

Recovery in aqueous two-phase systems of nanoparticulates applied as surrogate mimics for viral gene therapy vectors

The partition behaviour of nanoparticulate inclusion bodies (IBs) in various states of purity were evaluated as surrogate mimics for adenovirus and retrovirus products in method scouting experiments with aqueous two-phase systems (ATPSs). Such systems are proposed for effective, high capacity downstream processing (DSP) of viral gene therapy vectors. Studies with mimics provided simple descriptions of particle partition which may benefit the field of vector DSP, where experimental material is rarely available for research and development in quantities and concentrations representative of clinical manufacture. Polyethylene glycol (PEG)-salt and PEG-dextran ATPSs were screened in respect of the partition recovery of IBs from crude feedstocks. Select candidate systems were similarly evaluated with limited preparations of adenovirus and retrovirus with respect to fractional recoveries of infectivity and particle number. Maintenance of the former was good, whilst comparison of the latter with quantitation of unwashed and washed IBs indicated poor utilisation of the inherent high capacities of ATPSs in viral experiments. This is discussed in the context of the volumetric throughput and capacities reported in the literature for the recovery of infective viruses in ultracentrifugation and chromatographic processes.

Simultaneous accumulation of low-molecular-mass RNA at the interface along with accumulation of high-molecular-mass RNA on aqueous two-phase system partitioning

On partitioning in the potassium phosphate-PEG aqueous two-phase system, the simultaneous accumulation of the low-molecular-mass RNA at the interface along with accumulation of the coexisting high-molecular-mass RNA was quantitatively shown. The low-molecular-mass RNA was inherently partitioned between the top and bottom phases if partitioned alone. However, the low-molecular-mass RNA was caught in the interface to a significant extent if partitioned with the coexisting high-molecular-mass RNA. The degree of the accumulation of low-molecular-mass RNA increased with an increase in the content of the coexisting high-molecular-mass RNA, while the high-molecular-mass RNA was mostly accumulated at the interface.

Concentration and purification of beta-glucosidase from Aspergillus niger by using aqueous two-phase partitioning

The extracellular enzyme beta-glucosidase, present in a culture filtrate and produced by Aspergillus niger, was concentrated up to 700 times by two-phase partitioning. The two-phase systems were achieved by dissolving dextran and poly(ethylene glycol) in the culture filtrate in such proportions that the lower phase, containing the enzyme, consisted of a very small volume compared with the upper phase. The enzyme had high affinity for the lower phase when the system contained 100 mM KSCN at pH 8.0, and the recoveries of beta-glucosidase were in the range of 85 95% with a concentrating factor of 60-720 times. At the same time, the enzyme was purified 2-3 times.

Biochemical recovery and purification of gene therapy vectors

Rapid progress has been made with the molecular design of novel viral and non-viral gene therapy vectors. Exploiting upstream processes of producer cell-culture and downstream operations adapted from protein recovery, vectors have been accumulated in quantities and purities appropriate for the initiation of clinical trials. It is not clear, however, if such methodologies will be appropriate for efficient operation at the manufacturing scales required for clinically successful vectors. Technologies suited to the fractionation of nanoparticles may bypass practical bottlenecks experienced by current processes. The behaviour in such fractionation systems of natural and synthetic particles, which variously mimic the size, density and surface chemistry of vector products, could benefit the improved design of efficient manufacture for gene therapy vectors.

Purification of simian immunodeficiency virus, SIVMAC251, and of its external envelope glycoprotein, gp148

Two-phase extraction in a system composed of dextran and polyethylene glycol was used to purify simian immunodeficiency virus, SIVMAC251 (32H isolate) from 25 l of culture supernatant. The virus partitioned to the interphase with 80% recovery of gag peptide p27 and reverse transcriptase and an about 25% recovery of the external env glycoprotein, gp148. The virus was treated with octylglycoside and its subcomponents separated. Two gag-p27 containing fractions were obtained; gag-1, which also contained reverse transcriptase and nucleopeptides, and gag-2, which contained the major portion of the p27. The env gp148 was purified by chromatography through a series of lectin columns. The prepared materials are characterized by sodium dodecyl sulphate-polyacrylamide gel electrophoresis and immuno- and lectin blotting.