Concentration of viral vectors by co-precipitation with calcium phosphate

Efficient clinical-grade γ-retroviral vector purification by high-speed centrifugation for CAR T cell manufacturing

γ-Retroviral vectors (γ-RV) are powerful tools for gene therapy applications. Current clinical vectors are produced from stable producer cell lines which require minimal further downstream processing, while purification schemes for γ-RV produced by transient transfection have not been thoroughly investigated. We aimed to develop a method to purify transiently produced γ-RV for early clinical studies. Here, we report a simple one-step purification method by high-speed centrifugation for γ-RV produced by transient transfection for clinical application. High-speed centrifugation enabled the concentration of viral titers in the range of 10⁷-10⁸ TU/mL with >80% overall recovery. Analysis of research-grade concentrated vector revealed sufficient reduction in product- and process-related impurities. Furthermore, product characterization of clinical-grade γ-RV by BioReliance demonstrated two-logs lower impurities per transducing unit compared with regulatory authority-approved stable producer cell line vector for clinical application. In terms of CAR T cell manufacturing, clinical-grade γ-RV produced by transient transfection and purified by high-speed centrifugation was similar to γ-RV produced from a clinical-grade stable producer cell line. This method will be of value for studies using γ-RV to bridge vector supply between early- and late-stage clinical trials.

Mineralized vectors for gene therapy

There is an intense interest in developing materials for safe and effective delivery of polynucleotides using non-viral vectors. Mineralization of organic templates has long been used to produce complex materials with outstanding biocompatibility. However, a lack of control over mineral growth has limited the applicability of mineralized materials to a few in vitro applications. With better control over mineral growth and surface functionalization, mineralized vectors have advanced significantly in recent years. Here, we review the recent progress in chemical synthesis, physicochemical properties, and applications of mineralized materials in gene therapy, focusing on structure-function relationships. We contrast the classical understanding of the mineralization mechanism with recent ideas of mineralization. A brief introduction to gene delivery is summarized, followed by a detailed survey of current mineralized vectors. The vectors derived from calcium phosphate are articulated and compared to other minerals with unique features. Advanced mineral vectors derived from templated mineralization and specialty coatings are critically analyzed. Mineral systems beyond the co-precipitation are explored as more complex multicomponent systems. Finally, we conclude with a perspective on the future of mineralized vectors by carefully demarcating the boundaries of our knowledge and highlighting ambiguous areas in mineralized vectors. STATEMENT OF SIGNIFICANCE: Therapy by gene-based medicines is increasingly utilized to cure diseases that are not alleviated by conventional drug therapy. Gene medicines, however, rely on macromolecular nucleic acids that are too large and too hydrophilic for cellular uptake. Without tailored materials, they are not functional for therapy. One emerging class of nucleic acid delivery system is mineral-based materials. The fact that they can undergo controlled dissolution with minimal footprint in biological systems are making them attractive for clinical use, where safety is utmost importance. In this submission, we will review the emerging synthesis technology and the range of new generation minerals for use in gene medicines.

Lentiviral Vector Bioprocessing

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

SMIM1 missense mutations exert their effect on wild type Vel expression late in erythroid differentiation

BACKGROUND

Vel expression on erythrocytes is variable due to polymorphisms, complicating Vel typing. Weak Vel expression can be caused by mutations within SMIM1 in a heterozygous setting, suggesting a dominant negative effect of SMIM1 mutants on wild type (wt)SMIM1 expression. Here we report how SMIM1 expression is regulated during erythropoiesis, to understand its variable expression on erythrocytes.

STUDY DESIGN AND METHODS

Peripheral blood reticulocytes at different stages, cultured erythroid precursors and HEK293T cells were used to investigate expression and putative competition between wtSMIM1 and mutated SMIM1 VEL*01W.01, (c.152T>A (p.Met51Lys)), VEL*01W.02 (c.152T>G (p.Met51Arg)), and VEL*01W.03 (c.161T>C (p.Leu54Pro)).

RESULTS

Depending on the mutations in SMIM1 an effect on total and membrane expression of SMIM1 was observed in transfected HEK293T cells, but co-expression of wtSMIM1 and mutatedSMIM1 did not have an effect on wtSMIM1 membrane expression. During differentiation of donors expressing VEL*01W.01, VEL*01W.03, Vel-positive, Vel-negative (homozygote SMIM1*64_80del), and Vel-heterozygote SMIM1*64_80del primary human erythroblasts no overt defect was found in Vel expression dynamics or total SMIM1 expression levels when compared with wtSMIM1 erythroblasts. However, during enucleation, total Vel expression was significantly lower on reticulocytes of Vel-weak donors expressing heterozygote mutated SMIM1 compared to Vel-positive or Vel-heterozygote SMIM1*64_80del donors, while Vel expression on extruded nuclei was maintained. In addition, reticulocyte maturation in vivo showed further loss of Vel expression in these individuals and nearly absent on erythrocytes.

CONCLUSION

These results suggest that SMIM1 mutations exert a dominant negative effect on wtSMIM1 probably by affecting SMIM1 multimerization and thereby Vel epitope presentation at the latest stages of erythroid differentiation.

Development of a Clinical MALDI-ToF Mass Spectrometry Assay for SARS-CoV-2: Rational Design and Multi-Disciplinary Team Work

The COVID-19 pandemic caused by the SARS-CoV-2 coronavirus has stretched national testing capacities to breaking points in almost all countries of the world. The need to rapidly screen vast numbers of a country's population in order to control the spread of the infection is paramount. However, the logistical requirement for reagent supply (and associated cost) of RT-PCR based testing (the current front-line test) have been hugely problematic. Mass spectrometry-based methods using swab and gargle samples have been reported with promise, but have not approached the task from a systematic analysis of the entire diagnostic process. Here, the pipeline from sample processing, the biological characteristics of the pathogen in human biofluid, the downstream bio- and physical-chemistry and the all-important data processing with clinical interpretation and reporting, are carefully compiled into a single high-throughput and reproducible rapid process. Utilizing MALDI-ToF mass spectrometric detection to viral envelope glycoproteins in a systems biology-multidisciplinary team approach, we have achieved a multifaceted clinical MALDI ToF MS screening test, primarily (but not limited to) SARS-CoV-2, with direct application to other future epidemics/pandemics that may arise. The clinical information generated not only includes SARS-CoV-2 coronavirus detection-(Spike protein fragments S1, S2b, S2a peaks), but other respiratory viral infections detected as well as an assessment of generalised oral upper respiratory immune response (elevated total Ig light chain peak) and a measure of the viral immune response (elevated intensity of IgA heavy chain peak). The advantages of the method include; (1) ease of sampling, (2) speed of analysis, and much reduced cost of testing. These features reveal the diagnostic utility of MALDI-ToF mass spectrometry as a powerful and economically attractive global solution.

Mapping of Ion and Substrate Binding Sites in Human Sodium Iodide Symporter (hNIS)

The human sodium iodide symporter (hNIS) is a theranostic reporter gene which concentrates several clinically approved SPECT and PET radiotracers and plays an essential role for the synthesis of thyroid hormones as an iodide transporter in the thyroid gland. Development of hNIS mutants which could enhance translocation of the desired imaging ions is currently underway. Unfortunately, it is hindered by lack of understanding of the 3D organization of hNIS and its relation to anion transport. There are no known crystal structures of hNIS in any of its conformational states. Homology modeling can be very effective in such situations; however, the low sequence identity between hNIS and relevant secondary transporters with available experimental structures makes the choice of a template and the generation of 3D models nontrivial. Here, we report a combined application of homology modeling and molecular dynamics refining of the hNIS structure in its semioccluded state. The modeling was based on templates from the LeuT-fold protein family and was done with emphasis on the refinement of the substrate-ion binding pocket. The consensus model developed in this work is compared to available biophysical and biochemical experimental data for a number of different LeuT-fold proteins. Some functionally important residues contributing to the formation of putative binding sites and permeation pathways for the cotransported Na⁺ ions and I^- substrate were identified. The model predictions were experimentally tested by generation of mutant versions of hNIS and measurement of relative (to WT hNIS) ¹²⁵I^- uptake of 35 hNIS variants.

Continuous cell flocculation for recombinant antibody harvesting

BACKGROUND

Integrated continuous production technology is of great interest in biopharmaceutical industry. Efficient, flexible and cost effective methods for continuous cell removal have to be developed, before a fully continuous and integrated product train can be realized. The paper describes the development and testing of such an integrated continuous and disposable set-up for cell separation by flocculation combined with depth filtration.

RESULTS

Screening of multiple flocculation agents, depth filters, and conditions demonstrated that the best performance was obtained with 0.0375% polydiallyldimethylammonium chloride (pDADMAC; a polycationic flocculation agent) in combination with Clarisolve® depth filters. Using this set-up, a 4-fold decrease of filtration area was achieved relative to standard filtration without flocculation, with yields of ≥97% and DNA depletion of up to 99%. Continuous operation was accomplished using a simple tubular reactor design with parallelization of the filtration. The reactor length was selected to allow a 13.2-min residence time, which was sufficient to complete flocculation in batch experiments. Continuous flocculation performance was monitored on-line using focused beam reflectance measurement. Filter switch cycles based on upstream pressure were controlled by in-line pressure sensors, and were stable from one filter to the next.

CONCLUSION

It was demonstrated that stable and efficient continuous flocculation associated with depth filtration can be easily accomplished using tubular reactors and parallelization. Continuous cell separation is essential for the development of fully continuous integrated process trains. This cost-efficient disposable design run in continuous mode significantly reduces facility foot print, process costs and enables great flexbility. © 2017 The Authors. Journal of Chemical Technology & Biotechnology published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Canonical Notch Signaling Directs the Fate of Differentiating Neurocompetent Progenitors in the Mammalian Olfactory Epithelium

The adult olfactory epithelium (OE) has the remarkable capacity to regenerate fully both neurosensory and non-neuronal cell types after severe epithelial injury. Lifelong persistence of two stem cell populations supports OE regeneration when damaged: the horizontal basal cells (HBCs), dormant and held in reserve; and globose basal cells, a heterogeneous population most of which are actively dividing. Both populations regenerate all cell types of the OE after injury, but the mechanisms underlying neuronal versus non-neuronal lineage commitment after recruitment of the stem cell pools remains unknown. We used both retroviral transduction and mouse lines that permit conditional cell-specific genetic manipulation as well as the tracing of progeny to study the role of canonical Notch signaling in the determination of neuronal versus non-neuronal lineages in the regenerating adult OE. Excision of either Notch1 or Notch2 genes alone in HBCs did not alter progenitor fate during recovery from epithelial injury, whereas conditional knock-out of both Notch1 and Notch2 together, retroviral transduction of progenitors with a dominant-negative form of MAML (mastermind-like), or excision of the downstream cofactor RBPJ caused progeny to adopt a neuronal fate exclusively. Conversely, we show that overexpressing the Notch1-intracellular domain (N1ICD) either genetically or by transduction blocks neuronal differentiation completely. However, N1ICD overexpression requires both alleles of the canonical cofactor RBPJ to specify downstream lineage. Together, our results suggest that canonical RBPJ-dependent Notch signaling through redundant Notch1 and Notch2 receptors is both necessary and sufficient for determining neuronal versus non-neuronal differentiation in the regenerating adult OE.SIGNIFICANCE STATEMENT Despite the substantial reconstitution of the olfactory epithelium and its population of sensory neurons after injury, disruption and exhaustion of neurogenesis is a consequence of aging and a cause of olfactory dysfunction. Understanding the mechanisms underlying the generation of replacement neurons and non-neuronal cells is critical to any therapeutic strategy aimed at rebuilding a functional neuroepithelium. The results shown here demonstrate that canonical Notch signaling determines the balance between neurons and non-neuronal cells during restoration of the epithelium after injury. Moreover, the complexities of the multiple Notch pathways impinging on that decision are dissected in detail. Finally, RBPJ, the canonical Notch transcriptional cofactor, exhibits a heretofore unreported haploinsufficiency in setting the balance among the regenerating populations.

Sox2 and Pax6 Play Counteracting Roles in Regulating Neurogenesis within the Murine Olfactory Epithelium

In the adult olfactory epithelium, the transcription factors Pax6 and Sox2 are co-expressed in sustentacular cells, horizontal basal cells (HBCs), and less-differentiated globose basal cells (GBCs)–both multipotent and transit amplifying categories—but are absent from immediate neuronal precursor GBCs and olfactory sensory neurons (OSNs). We used retroviral-vector transduction to over-express Pax6 and Sox2 individually and together during post-lesion recovery to determine how they regulate neuronal differentiation. Both Pax6 and Sox2, separately and together, can suppress the production of OSNs, as fewer clones contain neurons than with empty vector (EV), although this effect is not absolute. In this regard, Pax6 has the strongest effect when acting alone. In clones where neurons form, Pax6 reduces neuron numbers by comparison with EV, while Sox2 expands their numbers. Co-transduction with Pax6 and Sox2 produces an intermediate result. The increased production of OSNs driven by Sox2 is due to the expansion of neuronal progenitors, since proliferation and the numbers of Ascl1, Neurog1, and NeuroD1-expressing GBCs are increased. Conversely, Pax6 seems to accelerate neuronal differentiation, since Ascl1 labeling is reduced, while Neurog1- and NeuroD1-labeled GBCs are enriched. As a complement to the over-expression experiments, elimination of Sox2 in spared cells of floxed Sox2 mice, by retroviral Cre or by K5-driven CreER^T2, reduces the production of OSNs and non-neuronal cells during OE regeneration. These data suggest that Pax6 and Sox2 have counteracting roles in regulating neurogenesis, in which Pax6 accelerates neuronal production, while Sox2 retards it and expands the pool of neuronal progenitors.

Movements of HIV-1 genomic RNA-APOBEC3F complexes and PKR reveal cytoplasmic and nuclear PKR defenses and HIV-1 evasion strategies

APOBEC3 cytidine deaminases and viral genomic RNA (gRNA) occur in virions, polysomes, and cytoplasmic granules, but have not been tracked together. Moreover, gRNA traffic is important, but the factors that move it into granules are unknown. Using in situ hybridization of transfected cells and protein synthesis inhibitors that drive mRNAs between locales, we observed APOBEC3F cotrafficking with gRNA without altering its movements. Whereas cells with little cytoplasmic gRNA were translationally active and accumulated Gag, suprathreshold amounts induced autophosphorylation of the cytoplasmic double-stranded RNA (dsRNA)-dependent protein kinase (PKR), causing eIF2α phosphorylation, protein synthesis suppression, and gRNA sequestration in stress granules. Additionally, we confirmed recent evidence that PKR is activated by chromosome-associated cellular dsRNAs after nuclear membranes disperse in prophase. By arresting cells in G2, HIV-1 blocks this mechanism for PKR activation and eIF2α phosphorylation. However, cytopathic membrane damage in CD4- and coreceptor-positive cultures infected with laboratory-adapted fusogenic HIV-1LAI eventually enabled PKR entry and activation in interphase nuclei. These results reveal multiple stages in the PKR-HIV-1 battleground that culminate in cell death. We discuss evidence suggesting that HIV-1s evolve in vivo to prevent or delay PKR activation by all these mechanisms.

Production, purification and titration of a lentivirus-based vector for gene delivery purposes

Viral vectors are valuable tools to deliver genetic materials into cells. Vectors derived from human immunodeficiency virus type 1 are being widely used for gene delivery, mainly because they are able to transduce both dividing and non-dividing cells which leads to stable and long term gene expression. In addition, these types of vectors are safe, with low toxicity, high stability and cell type specificity. Therefore, this work was aimed to produce lentivirus-based vector using a three-plasmid system. To produce this system, the eGFP marker gene was cloned into the plasmid pWPXLd. Subsequently, this vector plasmid, along with packaging plasmids, psPAX2 and envelope plasmid, pMD2.G, was co-transfected into packaging cell line (293T) using calcium phosphate method. 48 h post transfection, the constructed viral vector was harvested, purified and concentrated and stored at -80 °C for next experiments. The titration of the vector was carried out, using ELISA, flowcytometry, and fluorescent microscopy. Finally, transduction of HEK-293T, CHO, HepG2, MCF-7, MEFs and Jurkat cell lines was carried out with indicated cell numbers and multiplicities of infections of the vector in the presence of polybrene. Using this system, high titer lentivirus at titers of up to 2 × 10(8) transducing units/ml (TU/ml) was successfully generated and its transduction efficacy was improved by seven to over 20-fold in various cell types. We demonstrate the applicability of this vector for the efficient transduction of dividing and non-dividing cells, including HEK-293T, CHO, HepG2, MCF-7, MEFs and Jurkat cell line. Transduction efficiency yielded titers of (6.3 ± 1.2) 10(5) TU/ml. Furthermore, lentivirus transferred transgene was expressed at high level in the target cells and expression was followed until 90 days after transduction. Thus, the vector generated in this work, might be able to deliver the transgene into a wide range of mammalian cells.

Lentiviral vectors: basic to translational

More than two decades have passed since genetically modified HIV was used for gene delivery. Through continuous improvements these early marker gene-carrying HIVs have evolved into safer and more effective lentiviral vectors. Lentiviral vectors offer several attractive properties as gene-delivery vehicles, including: (i) sustained gene delivery through stable vector integration into host genome; (ii) the capability of infecting both dividing and non-dividing cells; (iii) broad tissue tropisms, including important gene- and cell-therapy-target cell types; (iv) no expression of viral proteins after vector transduction; (v) the ability to deliver complex genetic elements, such as polycistronic or intron-containing sequences; (vi) potentially safer integration site profile; and (vii) a relatively easy system for vector manipulation and production. Accordingly, lentivector technologies now have widespread use in basic biology and translational studies for stable transgene overexpression, persistent gene silencing, immunization, in vivo imaging, generating transgenic animals, induction of pluripotent cells, stem cell modification and lineage tracking, or site-directed gene editing. Moreover, in the present high-throughput '-omics' era, the commercial availability of premade lentiviral vectors, which are engineered to express or silence genome-wide genes, accelerates the rapid expansion of this vector technology. In the present review, we assess the advances in lentiviral vector technology, including basic lentivirology, vector designs for improved efficiency and biosafety, protocols for vector production and infection, targeted gene delivery, advanced lentiviral applications and issues associated with the vector system.

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.

Two new protocols to enhance the production and isolation of human induced pluripotent stem cell lines

There are two critical stages in the retroviral reprogramming of somatic cells to produce human induced pluripotent stem cell (hiPSC) lines. One is the production of high titer virus required to reprogram somatic cells; the other is identification of true hiPSC colonies from heterogeneous cell populations, and their isolation and expansion to generate a sustainable, pluripotent stem cell line. Here we describe simple, time-saving methods to address the current difficulties at these two critical junctures. First, we have developed a method to increase the number of infectious viral units 600-fold. Second, we have developed a TRA-1-81-based positive selection column method for isolating "true" hiPSCs from the heterogeneous cell populations, which overcomes the labor-intensive and highly subjective method of manual selection of hiPSC colonies. We have used these techniques to produce 8 hiPSC lines from human fibroblasts and we believe that they are of considerable utility to researchers in the hiPSC field.

Moloney murine leukemia virus decay mediated by retroviral reverse transcriptase degradation of genomic RNA

Retroviral vectors are powerful tools for the introduction of transgenes into mammalian cells and for long-term gene expression. However, their application is often limited by a rapid loss of bioactivity: retroviruses spontaneously loose activity at 37 degrees C, with a half-life of 4 to 9 h depending on the retrovirus type. We sought to determine which components of the retrovirus are responsible for this loss in bioactivity and to obtain a quantitative characterization of their stability. To this end, we focused on RNA and viral proteins, two major components that we hypothesized may undergo degradation and negatively influence viral infectivity. Reverse transcription PCR (RT-PCR) targeting RNA encoding portions of the viral genome clearly demonstrated time-dependent degradation of RNA which correlated with the loss in viral bioactivity. Circular dichroism spectroscopy, SDS-PAGE and two-dimensional SDS-PAGE analyses of viral proteins did not show any change in secondary structure or evidence of proteolysis. The mechanism underlying the degradation of viral RNA was investigated by site-directed mutagenesis of proteins encoded by the viral genome. Reverse transcriptase and protease mutants exhibited enhanced RNA stability in comparison to wild type recombinant virus, suggesting that the degradation of RNA, and the corresponding virus loss of activity, is mediated by the reverse transcriptase enzyme.

Lentiviral vectors for cancer immunotherapy: transforming infectious particles into therapeutics

Lentiviral vectors have emerged as promising tools for both gene therapy and immunotherapy purposes. They exhibit several advantages over other viral systems in that they are less immunogenic and are capable of transducing a wide range of different cell types, including dendritic cells (DC). DC transduced ex vivo with a whole range of different (tumor) antigens were capable of inducing strong antigen-specific T-cell responses, both in vitro and in vivo. Recently, the administration of lentiviral vectors in vivo has gained substantial interest as an alternative method for antigen-specific immunization. This method offers a number of advantages over DC vaccines as the same lentivirus can in principle be used for all patients resulting in a significantly reduced cost and requirement for considerably less expertise for the generation and administration of lentiviral vaccines. By selectively targeting lentiviral vectors to, or restricting transgene expression in certain cell types, selectivity, safety and efficacy can be further improved. This review will focus on the use of direct administration of lentiviral vectors encoding tumor-associated antigens (TAA) for the induction of tumor-specific immune responses in vivo, with a special focus on problems related to the generation of large amounts of highly purified virus and specific targeting of antigen-presenting cells (APC).

Exploiting heparin-binding properties of MoMLV-based retroviral vectors for affinity chromatography

Chromatography is deemed the most promising technology for large-scale purification of viral vectors. The authors have previously shown that heparin affinity chromatography could be successfully employed for the purification of VSV-G pseudotyped Moloney murine leukemia virus (MoMLV)-derived vectors giving excellent results in terms of recovery of active particles, reproducibility and selectivity. In this study, the authors examined whether the ability of retrovirus particles to specifically bind to heparin ligands is restricted to VSV-G pseudotypes produced by 293-based packaging cells. It is shown that VSV-G deficient retrovirus particles are captured by a heparin chromatography column as efficiently as VSV-G containing particles. Most strikingly, RD114 pseudotyped retrovirus particles derived from a HT1080-based cell line were found to bind heparin with the same affinity as 293-derived VSV-G pseudotypes. RD114 pseudotyped retrovirus particles were successfully isolated using heparin affinity chromatography obtaining good recoveries of functional particles (43%). These results indicate that heparin affinity chromatography can be extended to the purification of retroviral vectors produced by different packaging cell lines independently of the Env-protein used for pseudotyping.

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

For centuries mankind led a difficult battle against viruses, the smallest infectious agents at the surface of the earth. Nowadays it is possible to use viruses for our benefit, both at a prophylactic level in the production of vaccines and at a therapeutic level in the promising field of gene therapy. Retroviruses were discovered at the end of the 19th century and constitute one of the most effective entities for gene transfer and insertion into the genome of mammalian cells. This attractive feature has intensified research in retroviral vectors development and production over the past years, mainly due to the expectations raised by the concept of gene therapy. The demand for high quality retroviral vectors that meet standard requisites from the regulatory agencies (FDA and EMEA) is therefore increasing, as the technology has moved into clinical trials. The development of safer producer cell lines that can be used in large-scale production will result in the production of large quantities of retroviral stocks. Cost-efficient and scalable purification processes are essential for production of injectable-grade preparations to achieve final implementation of these vectors as therapeutics. Several preparative purification steps already established for proteins can certainly be applied to retroviral vectors, in particular membrane filtration and chromatographic methods. Nevertheless, the special properties of these complex products require technological improvement of the existing purification steps and/or development of particular purification steps to increase productivity and throughput, while maintaining biological activity of the final product. This review focuses on downstream process development in relation to the retroviral vectors characteristics and quality assessment of retroviral stocks for intended use in gene therapy.

Metabolic Biotinylation of Lentiviral Pseudotypes for Scalable Paramagnetic Microparticle-Dependent Manipulation

Nonviral, host-derived proteins on lentiviral vector surfaces can have a profound effect on the vector's biology as they can both promote infection and provide resistance to complement inactivation. We have exploited this to engineer a specific posttranslational modification of a "nonenvelope," virally associated protein. The bacterial biotin ligase (BirA) and a modified human DeltaLNGFR have been introduced into HEK293T cells and their protein products directed to the lumen of the endoplasmic reticulum. The BirA then couples biotin to an acceptor peptide that has been fused to the DeltaLNGFR. This results in the covalent linkage of biotin to the extracellular domain of the DeltaLNGFR expressed on the cell surface. Lentiviral vectors from these cells are metabolically labeled with biotin in the presence of free biotin. These biotinylated lentiviral vectors have a high affinity for streptavidin paramagnetic particles and, once captured, are easily manipulated in vitro. This is illustrated by the concentration of lentiviral vectors pseudotyped with either the VSV-G or an amphotropic envelope in excess of 4500-fold. This new cell line has the potential for widespread application to envelope pseudotypes compatible with lentiviral vector production.

Optimisation of a multipartite human immunodeficiency virus based vector system; control of virus infectivity and large-scale production

BACKGROUND

We have previously described a five-plasmid HIV-1 vector system that utilises a codon-optimised gagpol gene. While this system was shown to be safer than systems using proviral type helpers, the titre of virus produced was relatively low. Therefore, a process of optimising all aspects of virus production was initiated.

METHODS

A systematic approach was taken to the optimisation of virus production by transient expression using a five-plasmid packaging system. Codon-manipulation was used to reduce homology between helper and vector constructs. Ultrafiltration and ultracentrifugation were used for large-scale virus production.

RESULTS

We describe codon-optimised reading frames for Tat and Rev and the optimisation of virus production. The optimisation process resulted in an increase in virus titre of 7- to 8-fold. Several other approaches to increasing viral titre described by others proved ineffective in our system after it had been optimised. In addition, we show that by varying the ratio of the GagPol helper construct to vector, the infectivity of the virus could be controlled. The use of a novel codon-optimised HIV-1 GagPol expression construct with reduced homology to vector sequences significantly reduced transfer of gagpol sequences to transduced cells. Virus could be collected in serum-free medium without a significant loss of titre, which facilitated subsequent processing. Processing using a combination of ultrafiltration and ultracentrifugation allowed efficient and rapid processing of litre volumes of virus supernatant.

CONCLUSIONS

By taking a systematic approach to optimising all aspects of our five-plasmid lentiviral vector system we improved titre, safety, large-scale production, and demonstrated that infectivity could be specifically controlled.

Downstream processing of oncoretroviral and lentiviral gene therapy vectors

Retroviral vectors from both oncoretroviral and lentiviral origins have a great potential as gene delivery vehicles. A number of research groups have devoted considerable effort to the development of large-scale production strategies for retroviral vectors. However, the manufacturing of clinical-grade vectors for gene therapy, especially for in vivo applications, additionally requires scaleable purification strategies to remove the contaminants present in the harvested supernatants while preserving the functionality of the vectors. In this article, we review recent advances made in the field of downstream processing of retroviral vectors. The methods currently described in the literature for clarification, concentration and purification of retroviral vectors will be presented, with special emphasis on novel chromatography methods that open up the possibility to selectively and efficiently purify retroviruses on a large-scale. Problems associated with stability and quantification of retroviral particles will be outlined and future challenges will be discussed.

Conjugation of lentivirus to paramagnetic particles via nonviral proteins allows efficient concentration and infection of primary acute myeloid leukemia cells

Nonviral producer cell proteins incorporated into retroviral vector surfaces profoundly influence infectivity and in vivo half-life. We report the purification and concentration of lentiviral vectors using these surface proteins as an efficient gene transduction strategy. Biotinylation of these proteins and streptavidin paramagnetic particle concentration enhances titer 400- to 2,500-fold (to 10(9) CFU/ml for vesicular stomatitis virus G protein and 5 x 10(8) for amphotropic murine leukemia virus envelope). This method also uses newly introduced membrane proteins (B7.1 and DeltaLNGFR) directed to lentiviral surfaces, allowing up to 17,000-fold concentrations. Particle conjugation of lentivirus allows facile manipulation in vitro, resulting in the transduction of 48 to 94% of human acute myeloid leukemia blasts.

Altering the tropism of lentiviral vectors through pseudotyping

The host range of retroviral vectors including lentiviral vectors can be expanded or altered by a process known as pseudotyping. Pseudotyped lentiviral vectors consist of vector particles bearing glycoproteins (GPs) derived from other enveloped viruses. Such particles possess the tropism of the virus from which the GP was derived. For example, to exploit the natural neural tropism of rabies virus, vectors designed to target the central nervous system have been pseudotyped using rabies virus-derived GPs. Among the first and still most widely used GPs for pseudotyping lentiviral vectors is the vesicular stomatitis virus GP (VSV-G), due to the very broad tropism and stability of the resulting pseudotypes. Pseudotypes involving VSV-G have become effectively the standard for evaluating the efficiency of other pseudotypes. This review samples a few of the more prominent examples from the ever-expanding list of published lentiviral pseudotypes, noting comparisons made with pseudotypes involving VSV-G in terms of titer, viral particle stability, toxicity, and host-cell specificity. Particular attention is paid to publications of successfully targeting a specific organ or cell types.

Complexation of retroviruses with charged polymers enhances gene transfer by increasing the rate that viruses are delivered to cells

BACKGROUND

We have previously found that retrovirus transduction is enhanced when an anionic polymer (chondroitin sulfate C) is added to virus stocks that contain an equal weight concentration of a cationic polymer (Polybrene). This observation was unexpected given that previous work has shown that cationic polymers enhance transduction while anionic polymers have the opposite effect.

METHODS

Using model recombinant retroviruses and lentiviruses that encode for the Escherichia coli lacZ gene and quantitative assays of virus adsorption and transduction, we examined the mechanism of enhancement.

RESULTS

We found that addition of oppositely charged polymers (Polybrene and chondroitin sulfate C) to virus stocks enhanced gene transfer by increasing the flux of active viruses to the cells. Virus-polymer complexes formed that did not reduce the stability of the viruses, yet were large enough to sediment, delivering the viruses to the cells more rapidly than by simple diffusion. The size of the complexes, the rate of sedimentation, and the levels of gene transfer increased with increasing concentrations of polymers. The degree to which transduction was enhanced ranged from 2- to nearly 40-fold, and varied depending on the type of cells and viruses used. Interestingly, we found that association of the viruses with the polymer complexes did not significantly hinder their ability to complete post-binding steps of transduction.

CONCLUSIONS

Complexation of retroviruses with charged polymers significantly improves the efficiency of ex vivo gene transfer by increasing the number of active viruses that reach the cells.

Optimized large-scale production of high titer lentivirus vector pseudotypes

The goal of the present study was to develop an efficient transient transfection method for large-scale production of high titer lentivirus vector stocks of eight different pseudotypes. The envelope genes used for this purpose were those from VSV-G, Mokola, Rabies, MLV-Ampho, MLV-10A1, LCMV-WE, and LCMV-Arm53b. All envelopes were cloned into phCMV, which yielded lentivirus vector titers one, two, or three orders of magnitude higher than the original plasmids for the Rabies, MLV-10A1, and MLV-Ampho envelopes, respectively. When these newly constructed envelope expression plasmids were used for packaging, treatment with sodium butyrate resulted in almost five-fold increase in titers for some of the pseudotypes, had no effect for others (VSV-G and Rabies), and negatively impacted titers for the LCMV-derived pseudotypes. Production of vectors in serum-free media yielded titers only slightly lower than those obtained in the presence of serum. The efficiency of concentrating vector supernatants by ultracentrifugation or ultrafiltration was compared, with higher recovery efficiencies for the latter method, but the highest titers for most pseudotypes were obtained by ultracentrifugation. The best conditions for each individual pseudotype yielded lentivirus vector stocks with titers above 1 x 10(9) tu/mL for most pseudotypes, and higher than 1 x 10(10) tu/mL for VSV-G.

Size-exclusion chromatography purification of high-titer vesicular stomatitis virus G glycoprotein-pseudotyped retrovectors for cell and gene therapy applications

Vesicular stomatitis virus G glycoprotein (VSV-G)-pseudotyped replication-defective retroviral particles are pantropic and amenable to concentration to high titer by ultracentrifugation. These features have allowed development of effective retroviral transduction protocols for stem cells in vitro as well as for tissue engineering in vivo. However, retroparticle ultracentrifugation protocols will also copellet cellular and subcellular debris released from retroviral producer cell lines during vector manufacture. We have analyzed concentrated vector preparations by chromatography and have found that a significant amount of genomic DNA released from producer cells coconcentrates with retroviral particles. In an effort to generate high-purity retroparticle preparations, devoid of subcellular contaminants and contaminating genomic DNA, we have developed a process using size-exclusion chromatography combined with host cell nucleic acid digestion and concentration by ultrafiltration. The procedure allowed for a final recovery of 19 +/- 0.4% infectious viral particles from unfractionated starting material, with an average retroparticle concentration of 7.7 x 10(7) +/- 1.5 x 10(6)/ml. The intact virus is of high purity, >90% as determined by anion-exchange high-performance liquid chromatography. Retroparticle structure appeared intact as determined by negative stain electron microscopy and purified virus was functional and allowed for efficient transduction of primary human bone marrow stromal cells in vitro. In conclusion, we have developed a VSV-G retrovector purification process that can be applied to large-scale retroviral production ideal for cell and gene therapy applications.

Determination of infectious retrovirus concentration from colony-forming assay with quantitative analysis

The colony formation assay is the most commonly used titration method for defining the concentration of replication-incompetent murine leukemia virus-derived retroviral vectors. However, titer varies with target cell type and number, transduction time, and concentration of polycation (e.g., Polybrene). Moreover, because most of the viruses cannot encounter target cells due to Brownian motion, their short half-lives, and the requirement for target cell division for activity, the actual infectious retrovirus concentration in the collected supernatant is higher than the viral titer. Here we correlate the physical viral particle concentration with the infectious virus concentration and colony formation titer with the help of a mathematical model. Ecotropic murine leukemia retrovirus supernatant, collected from the GP+E86/LNCX retroviral vector producer cell line, was concentrated by centrifugation and further purified by a sucrose density gradient. The physical concentration of purified viral vectors was determined by direct particle counting with an electron microscope. The concentrations of fresh and concentrated supernatant were determined by a quantitative reverse transcriptase activity assay. Titration of all supernatants by neomycin-resistant colony formation assay was also performed. There were 767 +/- 517 physical viral particles per infectious CFU in the crude viral supernatant. However, the infectious viral concentration determined by mathematical simulation was 143 viral particles per infectious unit, which is more consistent with the concentration determined by particle counting in purified viral solution. Our results suggest that the mathematical model can be used to extract a more accurate and reliable concentration of infectious retrovirus.

The temperature stability of mouse retroviruses depends on the cholesterol levels of viral lipid shell and cellular plasma membrane

To delineate parameters contributing to the extracellular lifetime of retroviral vectors, we carried out stability tests of retroviruses derived from cell lines of different origin and kept under different cultivation conditions. Results show that amphotropic mouse retroviruses (MLV-A) derived from human and hamster cells exhibit 2- to 3-fold higher half-lives compared to retroviruses from mouse cells. Cultivation at 32 degrees C has been reported to yield high virus titers. However, the benefit of virus production in mouse cells at 32 degrees C is controversial. In our hands the cultivation temperature affected, hitherto not noticed, the half-life time of MLV-A. The 37/32 degrees C shift resulted in a 3-fold decrease of viral half-lifes compared to MLV-A released from mouse cells at 37 degrees C. Thus, MLV-A released at 37 degrees C is phenotypically different from MLV-A synthesized at 32 degrees C. Increased virus stability was inversely correlated with the level of cholesterol in the viral membrane. Finally, depletion of viral cholesterol in vitro resulted in intact virus with increased thermal stability. Thus, retrovirus lability depends on the host cell and parallels the cholesterol amount in the viral lipid shell.

Purification of a functional gene therapy vector derived from Moloney murine leukaemia virus using membrane filtration and ceramic hydroxyapatite chromatography

The ability of membrane ultra- and diafiltration and two chromatography media, Matrex Cellufine Sulfate (Millipore) and Macro-Prep ceramic hydroxyapatite (Bio-Rad), to adsorb, elute, and purify gene therapy vectors based on Moloney murine leukaemia virus (MoMuLV) carrying the 4070A amphotropic envelope protein was studied. Membrane ultra- and diafiltration provided virus concentration up to 160-fold with an average recovery of infectious viruses of 77 +/- 14%. In batch experiments, Macro-Prep ceramic hydroxyapatite (type 2, particle size 40 microm) proved superior to Matrex Cellufine Sulfate for MoMuLV vector particle adsorption. Furthermore, functional vector particles could be eluted using phosphate buffer pH 6.8 (highest titres from >or=300 mM phosphate) from the Macro-Prep adsorbent, with higher specific titres (cfu/mg protein) than the starting material. Similar results were obtained when this ceramic hydroxyapatite was packed into a column and used in a liquid chromatography system. Recovery of transduction-competent virus was between 18 and 31% for column experiments and 32 and 46% for batch experiments.

Organ distribution of gene expression after intravenous infusion of targeted and untargeted lentiviral vectors

Lentiviral vectors represent an attractive technology platform from which to develop a targetable injectable gene delivery system for transduction of specific cell populations in vivo, irrespective of their cell cycle status. Targeted HIV-1-based lentiviral vectors were generated by pseudotyping them with chimeric murine leukemia virus (MLV) envelope glycoproteins displaying N-terminal targeting polypeptides. Vectors displaying an EGF polypeptide were fully infectious on EGF receptor-negative cells, but were inactive on cells with abundant EGF receptors (inverse targeting). Receptor-mediated inactivation of gene transfer was overcome by competing the EGF receptors on the target cells with soluble EGF or by removing the displayed EGF domain from the surface of the vector particles by factor Xa cleavage of a specific protease substrate engineered into its tethering linker (protease targeting). Intravenous infusion of nontargeted HIV-1 vectors led to maximal luciferase activity in liver and spleen with moderate or minimal activity in heart, skeletal muscle, lung, brain, kidney, ovaries and bone marrow. In contrast, intravenous EGF-displaying vectors were expressed maximally in spleen with very low level luciferase expression detectable in liver (EGF-receptor rich). Liver transduction by the EGF-displaying vector was restored by pretreating the animals with soluble EGF suggesting that these vectors are inversely targeted to spleen.

A packaging cell line generating CD4-specific retroviral vectors for efficient gene transfer into primary human T-helper lymphocytes

Murine leukemia virus (MuLV) can be pseudotyped with a variant of the human immunodeficency virus (HIV) envelope gene encoding the surface glycoprotein gp120-SU and a carboxy-terminally truncated transmembrane (TM) protein with only seven cytoplasmic amino acids. MuLV/HIV-1 pseudotyped retroviral vectors selectively target gene transfer to human cells expressing both CD4 and CXCR4. To apply this vector system to gene therapy of human diseases, we generated a stable packaging cell line, FLY-HIV-87, expressing the MuLV gag and pol genes and the C-terminally truncated variant of the HIV-1 envelope gene, but no retroviral vector genome. Production of infectious vector particles was tested after the introduction of different vector genomes and was in the range of 5x10(5) IU/ml. The vector particles could be concentrated up to 25-fold. Specific and efficient gene transfer into CD4/CXCR4 expressing cell lines and stimulated primary human CD4+ peripheral blood lymphocytes was achieved. Thus the packaging cell line FLY-HIV-87 is highly suitable for gene therapy of disorders of human T-helper cells.