Real-time quantitative PCR for the design of lentiviral vector analytical assays

Quantitation and integrity evaluation of RNA genome in lentiviral vectors by direct reverse transcription-droplet digital PCR (direct RT-ddPCR)

Lentiviral vectors (LV) have proven to be powerful tools for stable gene delivery in both dividing and non-dividing cells. Approval of these LVs for use in clinical applications has been achieved by improvements in LV design. Critically important characteristics concerning quality control are LV titer quantification and the detection of impurities. However, increasing evidence concerning high variability in titration assays indicates poor harmonization of the methods undertaken to date. In this study, we developed a direct reverse transcription droplet digital PCR (Direct RT-ddPCR) approach without RNA extraction and purification for estimation of LV titer and RNA genome integrity. The RNA genome integrity was assessed by RT-ddPCR assays targeted to four distant regions of the LV genome. Results of the analyses showed that direct RT-ddPCR without RNA extraction and purification performs similarly to RT-ddPCR on purified RNA from 3 different LV samples, in terms of robustness and assay variance. Interestingly, these RNA titer results were comparable to physical titers by p24 antigen ELISA (enzyme-linked immunosorbent assay). Moreover, we confirmed the partial degradation or the incomplete RNA genomes in the prepared 3 LV samples. These results may partially explain the discrepancy of the LV particle titers to functional titers. This work not only demonstrates the feasibility of direct RT-ddPCR in determining LV titers, but also provides a method that can be easily adapted for RNA integrity assessment.

Effect of inactivating RNA viruses by coupled UVC and UVA LEDs evaluated by a viral surrogate commonly used as a genetic vector

RNA viruses are ubiquitous in nature, many of which can cause severe infectious syndromes to humanity, e.g., the SARS-CoV-2 virus. Ultraviolet (UV) radiation has been widely studied for inactivating various species of microorganisms, including viruses. The most applicable UV light for viruses ranges from 200nm to 280nm in wavelength, i.e., UVC. More recently, the synergy of UVA light with UVC has been studied in disinfecting bacteria in polluted water. However, little attention has been paid to studying viral inactivation by coupled UVC and UVA LEDs. The necessity of such research is to find an effective and economical solution for the LEDs of these two bands. Along this track, we attempt to tackle two major challenges. The first is to find a suitable viral surrogate that can safely be used in ordinary labs. In this aspect, lentivirus is commonly used as a genetic vector and has been selected to surrogate RNA viruses. Another is to determine the effective dosage of the coupled UVC and UVA light. To this end, the surrogate lentivirus was irradiated by 280nm (UVC) LEDs, 365nm (UVA) LEDs, and their combination at various doses. Survival rates were detected to compare the efficacy of various options. Moreover, the viral RNA damage was detected by RT-qPCR to disclose the mechanism of viral death. The results have shown that for the same duration of irradiation, the effect of the full-power 280nm LEDs is equivalent to that of the half-power 280nm LEDs combined with a suitable radiant power of the 365nm LEDs. The observations have been further confirmed by the effect of damaging the viral RNA by either the 280nm or 365nm light. In conclusion, the experimental results provide clear evidence of alleviating the requirement of UVC LEDs in viral inactivation by substituting them partially with UVA LEDs.

Identification of growth hormone receptor as a relevant target for precision medicine in low-EGFR expressing glioblastoma

Objective

New therapeutic approaches are needed to improve the prognosis of glioblastoma (GBM) patients.

Methods

With the objective of identifying alternative oncogenic mechanisms to abnormally activated epidermal growth factor receptor (EGFR) signalling, one of the most common oncogenic mechanisms in GBM, we performed a comparative analysis of gene expression profiles in a series of 54 human GBM samples. We then conducted gain of function as well as genetic and pharmocological inhibition assays in GBM patient‐derived cell lines to functionnally validate our finding.

Results

We identified that growth hormone receptor (GHR) signalling defines a distinct molecular subset of GBMs devoid of EGFR overexpression. GHR overexpression was detected in one third of patients and was associated with low levels of suppressor of cytokine signalling 2 (SOCS2) expression due to SOCS2 promoter hypermethylation. In GBM patient‐derived cell lines, GHR signalling modulates the expression of proteins involved in cellular movement, promotes cell migration, invasion and proliferation in vitro and promotes tumourigenesis, tumour growth, and tumour invasion in vivo. GHR genetic and pharmacological inhibition reduced cell proliferation and migration in vitro.

Conclusion

This study pioneers a new field of investigation to improve the prognosis of GBM patients.

Infectious titer determination of lentiviral vectors using a temporal immunological real-time imaging approach

The analysis of the infectious titer of the lentiviral vector samples obtained during upstream and downstream processing is of major importance, however, also the most challenging method to be performed. Currently established methods like flow cytometry or qPCR lack the capability of enabling high throughput sample processing while they require a lot of manual handling. To address this limitation, we developed an immunological real-time imaging method to quantify the infectious titer of anti-CD19 CAR lentiviral vectors with a temporal readout using the Incucyte^® S3 live-cell analysis system. The infective titers determined with the Incucyte^® approach when compared with the flow cytometry-based assay had a lower standard deviation between replicates and a broader linear range. A major advantage of the method is the ability to obtain titer results in real-time, enabling an optimal readout time. The presented protocol significantly decreased labor and increased throughput. The ability of the assay to process high numbers of lentiviral samples in a high throughput manner was proven by performing a virus stability study, demonstrating the effects of temperature, salt, and shear stress on LV infectivity.

Rapid In-Process Monitoring of Lentiviral Vector Particles by High-Performance Liquid Chromatography

Lentiviral vectors (LVs) are a popular gene delivery tool in cell and gene therapy and they are a primary tool for ex vivo transduction of T cells for expression of chimeric antigen receptor (CAR) in CAR-T cell therapies. Extensive process and product characterization are required in manufacturing virus-based gene vectors to better control batch-to-batch variability. However, it has been an ongoing challenge to make quantitative assessments of LV product because current analytical tools often are low throughput and lack robustness and standardization is still required. This paper presents a high-throughput and robust physico-chemical characterization method that directly assesses total LV particles. With simple sample preparation and fast elution time (6.24 min) of the LV peak in 440 mM NaCl (in 20 mM Tris-HCl [pH 7.5]), this ion exchange high-performance liquid chromatography (IEX-HPLC) method is ideal for routine in-process monitoring to facilitate the development of scalable and robust LV manufacturing processes. Furthermore, this HPLC method is suitable for the analysis of all in-process samples, from crude samples such as LV supernatants to final purified products. The linearity range of the standard curve is 3.13 × 10⁸ to 1.0 × 10¹⁰ total particles/mL, and both the intra- and inter-assay variabilities are less than 5%.

Super-rapid quantitation of the production of HIV-1 harboring a luminescent peptide tag

In studies of HIV-1, virus production is normally monitored by either a reverse transcriptase assay or a p24 antigen capture ELISA. However, these assays are costly and time-consuming for routine handling of a large number of HIV-1 samples. For example, sample dilution is always required in the ELISA procedure to determine p24 protein levels because of the very narrow range of detectable concentrations in this assay. Here, we establish a novel HIV-1 production assay system to solve the aforementioned problems by using a recently developed small peptide tag called HiBiT. This peptide is a fragment of NanoLuc luciferase and generates a strong luminescent signal when complemented with the remaining subunit. To employ this technology, we constructed a novel full-length proviral HIV-1 DNA clone and a lentiviral packaging vector in which the HiBiT tag was added to the C terminus of the integrase. Tagging the integrase with the HiBiT sequence did not impede the resultant virus production, infectivity, or susceptibility to an integrase inhibitor. EM revealed normal morphology of the virus particles. Most importantly, by comparing between ELISA and the HiBiT luciferase assay, we successfully obtained an excellent linear correlation between p24 concentrations and HiBiT-based luciferase activity. Overall, we conclude that HiBiT-tagged viruses can replace the parental HIV-1 and lentiviral vectors, which enables us to perform a super-rapid, inexpensive, convenient, simple, and highly accurate quantitative assay for HIV-1/lentivirus production. This system can be widely applied to a variety of virological studies, along with screening for candidates of future antiviral drugs.

A VHH-Based Anti-MUC1 Chimeric Antigen Receptor for Specific Retargeting of Human Primary T Cells to MUC1-Positive Cancer Cells

Objective

Immunotherapy with redirected T cells that express a chimeric antigen receptor (CAR) is a promising prospect in cancer treatment. Most CARs use murine-derived single-chain variable fragments (scFvs) as an antigen targeting moiety, which may lead to host immunogenic responses and engineered T cell disappearance. It seems that development of less immunogenic CARs, such as CARs composed of the camelid variable domain of heavy chain antibodies (VHHs) may likely overcome this obstacle. Here, we improved the expression of the VHH-based anti-MUC1 CAR gene construct using a third generation lentiviral vector in primary human T cells and assessed its effect on antigen specific targeting, activation and cytotoxicity of redirected human T cells.

Materials and Methods

In this experimental study, we established a second generation novel CAR (VHH-based anti- MUC1 CAR) that contained a camelid-derived anti-MUC1 VHH followed by an IgG3 hinge, a CD28 transmembrane domain and signalling endodomains of CD28 and CD3⁺. Next, we constructed lentiviral vectors that contained this CAR gene construct using an optimized transiently virus production method and transduced it into human T cells. Cell surface expression of CAR, cytokine secretion and cytotoxic activity were assessed in the transduced CD3+T cells.

Results

The transduced T cells had high levels of surface expression of CAR. T cells that expressed anti-MUC1 CAR showed significantly increased secretion of Th1 cytokines, including IL-2, TNF alpha and IFN-γ, as well as cytotoxic activity upon recognition of MUC1 on tumour cells after co-incubation with T47D or MCF-7 (MUC1-positive) compared with A431 (MUC1-negative) or untransduced T cells.

Conclusion

Our results suggested that, given the unique properties of VHHs to prevent immunogenic responses and tonic signalling, our novel VHH-based anti-MUC1 CAR might be effective for clinical purposes in cancer immunotherapy.

Glutathione S-transferases promote proinflammatory astrocyte-microglia communication during brain inflammation

Astrocytes and microglia play critical roles in brain inflammation. Here, we report that glutathione S-transferases (GSTs), particularly GSTM1, promote proinflammatory signaling in astrocytes and contribute to astrocyte-mediated microglia activation during brain inflammation. In vivo, astrocyte-specific knockdown of GSTM1 in the prefrontal cortex attenuated microglia activation in brain inflammation induced by systemic injection of lipopolysaccharides (LPS). Knocking down GSTM1 in astrocytes also attenuated LPS-induced production of the proinflammatory cytokine tumor necrosis factor-α (TNF-α) by microglia when the two cell types were cocultured. In astrocytes, GSTM1 was required for the activation of nuclear factor κB (NF-κB) and the production of proinflammatory mediators, such as granulocyte-macrophage colony-stimulating factor (GM-CSF) and C-C motif chemokine ligand 2 (CCL2), both of which enhance microglia activation. Our study suggests that GSTs play a proinflammatory role in priming astrocytes and enhancing microglia activation in a microglia-astrocyte positive feedback loop during brain inflammation.

Assessment of biodistribution using mesenchymal stromal cells: Algorithm for study design and challenges in detection methodologies

BACKGROUND AIMS

Biodistribution of candidate cell-based therapeutics is a critical safety concern that must be addressed in the preclinical development program. We aimed to design a decision tree based on a series of studies included in actual dossiers approved by competent regulatory authorities, noting that the design, execution and interpretation of pharmacokinetics studies using this type of therapy is not straightforward and presents a challenge for both developers and regulators.

METHODS

Eight studies were evaluated for the definition of a decision tree, in which mesenchymal stromal cells (MSCs) were administered to mouse, rat and sheep models using diverse routes (local or systemic), cell labeling (chemical or genetic) and detection methodologies (polymerase chain reaction [PCR], immunohistochemistry [IHC], fluorescence bioimaging, and magnetic resonance imaging [MRI]). Moreover, labeling and detection methodologies were compared in terms of cost, throughput, speed, sensitivity and specificity.

RESULTS

A decision tree was defined based on the model chosen: (i) small immunodeficient animals receiving heterologous MSC products for assessing biodistribution and other safety aspects and (ii) large animals receiving homologous labeled products; this contributed to gathering data not only on biodistribution but also on pharmacodynamics. PCR emerged as the most convenient technique despite the loss of spatial information on cell distribution that can be further assessed by IHC.

DISCUSSION

This work contributes to the standardization in the design of biodistribution studies by improving methods for accurate assessment of safety. The evaluation of different animal models and screening of target organs through a combination of techniques is a cost-effective and timely strategy.

Universal real-time PCR-based assay for lentiviral titration

Lentiviral vectors are efficient vehicles for stable gene transfer in both dividing and non-dividing cells. This feature among others makes lentiviral vectors a powerful tool in molecular research. However, the use of lentiviruses in research studies and clinical trials requires a precise and validated titration method. In this study, we describe a qPCR-based approach for estimation of lentiviral vector titer (pLV-THM-GFP). The use of WPRE (Woodchuck Hepatitis Virus Posttranscriptional Regulatory Element) and albumin genes as templates for an SYBR green-based real-time qPCR method allows for a rapid, sensitive, reproducible, and accurate assessment of lentiviral copy number at an integrated lentiviral DNA level. Furthermore, this optimization enables measurement of lentiviral concentration even in very poor quality and small quantity material. Consequently, this approach provides researchers with a tool to perform low-cost assessment with highly repeatable results.

A Lentiviral Vector Allowing Physiologically Regulated Membrane-anchored and Secreted Antibody Expression Depending on B-cell Maturation Status

The development of lentiviral vectors (LVs) for expression of a specific antibody can be achieved through the transduction of mature B-cells. This approach would provide a versatile tool for active immunotherapy strategies for infectious diseases or cancer, as well as for protein engineering. Here, we created a lentiviral expression system mimicking the natural production of these two distinct immunoglobulin isoforms. We designed a LV (FAM2-LV) expressing an anti-HCV-E2 surface glycoprotein antibody (AR3A) as a membrane-anchored Ig form or a soluble Ig form, depending on the B-cell maturation status. FAM2-LV induced high-level and functional membrane expression of the transgenic antibody in a nonsecretory B-cell line. In contrast, a plasma cell (PC) line transduced with FAM2-LV preferentially produced the secreted transgenic antibody. Similar results were obtained with primary B-cells transduced ex vivo. Most importantly, FAM2-LV transduced primary B-cells efficiently differentiated into PCs, which secreted the neutralizing anti-HCV E2 antibody upon adoptive transfer into immunodeficient NSG (NOD/SCIDγc(-/-)) recipient mice. Altogether, these results demonstrate that the conditional FAM2-LV allows preferential expression of the membrane-anchored form of an antiviral neutralizing antibody in B-cells and permits secretion of a soluble antibody following B-cell maturation into PCs in vivo.

Development of Plasmids for Quantitative Detection of Integrated Lentiviral Vectors and Evaluation of Culture Time to Perform Vector Titer by Real-Time Quantitative Polymerase Chain Reaction Assay

The accurate assessment of provirus copy number per cell (VCN/cell) is a fundamental issue in transgenesis as well as in gene therapy studies based on stably integrated vectors. To this end, real-time quantitative polymerase chain reaction (qPCR) is a powerful method but it is sensible to differences in quality or concentration of the two-plasmid preparations used for the construction of the standard curves. In order to minimize technical errors we included genome specific sequences (mouse or human) and vector specific sequences in the same plasmid. We evaluated the specificity and sensitivity of these bivalent plasmids by qPCR analysis on mouse and human genomic DNA containing a known number of a reporter lentiviral vector and we found that the system is reliable to measure up to 0.1 VCN/cell. Here we have applied this assay to measure vector titer of virus stock preparations and to determine the optimal cell passages at which viral titration effectively reflects the number of integrated vectors.

Lentiviral vectors encoding zinc-finger nucleases specific for the model target locus HPRT1

Zinc-finger nucleases (ZFNs) are artificial proteins designed to induce double-stranded DNA breaks (DSBs) at predefined chromosomal positions. These site-specific genomic lesions facilitate the study of translocations and cellular DNA repair processes and serve as powerful stimuli for the editing of complex genomes. The delivery of ZFNs into a wide range of cell types is of utmost importance for the broad evaluation and deployment of the technology. Lentiviral vectors (LVs) are versatile gene delivery vehicles that transduce alike transformed and primary cells regardless of their division rate. In this chapter, we describe the generation of conventional and integrase-defective LVs encoding ZFNs targeting the human hypoxanthine phosphoribosyltransferase 1 (HPRT1) locus. Furthermore, we introduce a general LV titration method based on a cost-effective quantitative PCR protocol and implement a rapid and simple restriction enzyme site polymorphism assay to detected DSB formation at the HPRT1 target sequence. Owing in part to the small molecule-based clone selection schemes conferred by HPRT1 allelic knockouts, this X-linked gene has become a "classical" target model locus in mammalian cells. The reagents and techniques detailed herein yield LV preparations that induce HPRT1-specific DSBs. As a result, they should constitute a valuable resource to increase the robustness and decrease the timelines of the various protocols based on HPRT1 gene disruption and targeting.

Quantification of reverse transcriptase activity by real-time PCR as a fast and accurate method for titration of HIV, lenti- and retroviral vectors

Quantification of retroviruses in cell culture supernatants and other biological preparations is required in a diverse spectrum of laboratories and applications. Methods based on antigen detection, such as p24 for HIV, or on genome detection are virus specific and sometimes suffer from a limited dynamic range of detection. In contrast, measurement of reverse transcriptase (RT) activity is a generic method which can be adapted for higher sensitivity using real-time PCR quantification (qPCR-based product-enhanced RT (PERT) assay). We present an evaluation of a modified SYBR Green I-based PERT assay (SG-PERT), using commercially available reagents such as MS2 RNA and ready-to-use qPCR mixes. This assay has a dynamic range of 7 logs, a sensitivity of 10 nU HIV-1 RT and outperforms p24 ELISA for HIV titer determination by lower inter-run variation, lower cost and higher linear range. The SG-PERT values correlate with transducing and infectious units in HIV-based viral vector and replication-competent HIV-1 preparations respectively. This assay can furthermore quantify Moloney Murine Leukemia Virus-derived vectors and can be performed on different instruments, such as Roche Lightcycler® 480 and Applied Biosystems ABI 7300. We consider this test to be an accurate, fast and relatively cheap method for retroviral quantification that is easily implemented for use in routine and research laboratories.

Adenoviral protein V promotes a process of viral assembly through nucleophosmin 1

Adenoviral infection induces nucleoplasmic redistribution of a nucleolar nucleophosmin 1/NPM1/B23.1. NPM1 is preferentially localized in the nucleoli of normal cells, whereas it is also present at the nuclear matrix in cancer cells. However, the biological roles of NPM1 during infection are unknown. Here, by analyzing a pV-deletion mutant, Ad5-dV/TSB, we demonstrate that pV promotes the NPM1 translocation from the nucleoli to the nucleoplasm in normal cells, and the NPM1 translocation is correlated with adenoviral replication. Lack of pV causes a dramatic reduction of adenoviral replication in normal cells, but not cancer cells, and Ad5-dV/TSB was defective in viral assembly in normal cells. NPM1 knockdown inhibits adenoviral replication, suggesting an involvement of NPM1 in adenoviral biology. Further, we show that NPM1 interacts with empty adenovirus particles which are an intermediate during virion maturation by immunoelectron microscopy. Collectively, these data implicate that pV participates in a process of viral assembly through NPM1.

The enhancement of RNAi against HIV in vitro and in vivo using H-2K(k) protein as a sorting method

Gene therapy offers a potentially an effective treatment for many human diseases, including HIV/AIDS. One of the most studied gene delivery systems is the use of lentivirus based vectors, which can deliver genes into both dividing and nondividing cells. However, low infection efficiency represents an obstacle for proper evaluation of their biological function. In this study, a recombinant lentiviral vector which expressed short hairpin RNAs (shRNAs) targeted against the HIV-1 vif/pol was transduced into various cells. An MHC class I molecule, H-2K(k), was used as a marker to accumulate the virally transduced cells through immunomagnetic sorting. In vitro testing of transduced cells showed 85% suppression of HIV in post-sorted PBMCs compared to 30% in pre-sorted PBMCs. In additional, using a mouse xenotransplantation model with the same treatment protocol for cell enrichment, a >95% decrease in HIV activity in post-sorted cells was achieved, as compared to nearly none in the pre-sorted cells. These studies offer a practical method to accumulate virally transduced cells, which can be applied to evaluate the performance of various shRNAs constructs.

Highly efficient concentration of lenti- and retroviral vector preparations by membrane adsorbers and ultrafiltration

BACKGROUND

Lentiviral vectors (LVs) can efficiently transduce a broad spectrum of cells and tissues, including dividing and non-dividing cells. So far the most widely used method for concentration of lentiviral particles is ultracentrifugation (UC).An important feature of vectors derived from lentiviruses and prototypic gamma-retroviruses is that the host range can be altered by pseudotypisation. The most commonly used envelope protein for pseudotyping is the glycoprotein of the Vesicular Stomatitis Virus (VSV.G), which is also essential for successful concentration using UC.

RESULTS

Here, we describe a purification method that is based on membrane adsorbers (MAs). Viral particles are efficiently retained by the anionic exchange MAs and can be eluted with a high-salt buffer. Buffer exchange and concentration is then performed by utilizing ultrafiltration (UF) units of distinct molecular weight cut off (MWCO). With this combined approach similar biological titers as UC can be achieved (2 to 5×10⁹ infectious particles (IP)/ml). Lentiviral particles from small starting volumes (e.g. 40 ml) as well as large volumes (up to 1,000 ml) cell culture supernatant (SN) can be purified. Apart from LVs, vectors derived from oncoretroviruses can be efficiently concentrated as well. Importantly, the use of the system is not confined to VSV.G pseudotyped lenti- and retroviral particles and other pseudotypes can also be purified.

CONCLUSIONS

Taken together the method presented here offers an efficient alternative for the concentration of lenti- as well as retroviral vectors with different pseudotypes that needs no expensive equipment, is easy to handle and can be used to purify large quantities of viral vectors within a short time.

Changes in the accessibility of the HIV-1 Integrase C-terminus in the presence of cellular proteins

Background

Following entry, uncoating, and reverse transcription, a number of cellular proteins become associated with the Human Immunodeficiency Virus type 1 (HIV-1) pre-integration complex (PIC). With the goal of obtaining reagents for the analysis of the HIV-1 PIC composition and localisation, we have constructed functional integrase (IN) and matrix (MA) proteins that can be biotinylated during virus production and captured using streptavidin-coated beads.

Results

Although the labelled C-terminus allows for the sensitive detection of virion-associated IN, it becomes inaccessible in the presence of cellular proteins. This masking is not dependent on the nature of the tag and does not occur with the tagged MA. It was not observed either with an IN mutant unable to interact with LEDGF/p75, or when LEDGF/p75 was depleted from cells.

Conclusion

Our observation suggests that a structural rearrangement or oligomerization of the IN protein occurs during the early steps of infection and that this process is related to the presence of LEDGF/p75.

Tetracysteine as a reporter for gene therapy

OBJECTIVE

To study the feasibility of using tetracysteine (TC) reporter in gene therapy.

METHODS

Effects of TC reporter and conventional reporter genes encoding green fluorescence protein (GFP) and luciferase (Luc) on expression and function of the therapeutic gene MGMT(P140K) were compared. Cytotoxicity and drug resistance were studied by Western blot. TC reporter used in therapy was analyzed by flow cytometry (FCM).

RESULTS

The TC reporter had no toxicity to cells and neither affected the expression or activity of therapeutic gene as compared to GFP and Luc. TC could be used in blood sample detection.

CONCLUSION

TC is a new kind of reporter gene for lentiviral vector in future gene therapy.

Analysis of human immunodeficiency virus type 1 vector cis- and trans-acting elements production by means of Semliki Forest virus

Recombinant Semliki Forest virus (SFV) is an attractive viral vector system owing to its ability to allow high efficiency of viral protein expression. To produce recombinant pseudotyped human immunodeficiency virus type 1 (HIV-1) virions, we designed a chimeric SFV/HIV vector system that contains both the HIV-1 cis- and trans-acting elements under the transcriptional control of the SFV replicase and investigated the ability of the hybrid SFV/HIV system to produce lentiviral particles capable of transducing target cells. Co-transfection of target cells with the two helper SFV packaging system RNAs along with each SFV/Gag-Pol, SFV/VSV(G) as well as SFV/HIV-1 vector unit replicon led to the generation of efficient transducing competent recombinant SFV/HIV particles. In contrast, co-transduction of target cells with the SFV/HIV chimeric virions produced recombinant particles with low transducing ability. Our data suggest that both the genomic and the subgenomic RNAs containing the HIV-1 vector unit were negatively selected for incorporation into recombinant particles, despite the fact that the SFV-driven HIV-1 vector replicon was the only one containing a lentiviral packaging sequence. The results of this study provide insights relevant to the design of chimeric lentiviral vectors.

A rapid and efficient branched DNA hybridization assay to titer lentiviral vectors

A robust assay to titer lentiviral vectors is imperative to qualifying their use in drug discovery, target validation and clinical applications. In this study, a novel branched DNA based hybridization assay was developed to titer lentiviral vectors by quantifying viral RNA genome copy numbers from viral lysates without having to purify viral RNA, and this approach was compared with other non-functional (p24 protein ELISA and viral RT-qPCR) and a functional method (reporter gene expression) used commonly. The RT-qPCR method requires purification of viral RNA and the accuracy of titration therefore depends on the efficiency of purification; this requirement is ameliorated in the hybridization assay as RNA is measured directly in viral lysates. The present study indicates that the hybridization based titration assay performed on viral lysates was more accurate and has additional advantages of being rapid, robust and not dependent on transduction efficiency in different cell types.

PCR to predict risk of airborne disease

Plant, animal and human diseases spread by microscopic airborne particles have had major economic and social impacts during history. Special air-sampling devices have been used to collect such particles since the 19th century but it has often been impossible to identify them accurately. Exciting new opportunities to combine air sampling with quantitative PCR to identify and count these particles are reviewed, using crop pathogen examples. These methods can be used to predict the risk of unexpected outbreaks of airborne diseases by identifying increases in pathogen inoculum or genetic changes in pathogen populations that render control ineffective. The predictions can provide guidance to policymakers, health professionals or the agricultural industry for the development of strategies to minimise the risk of severe pandemics.

Recombinant lentivector as a genetic immunization vehicle for antitumor immunity

Encouraged by remarkable successes in preventing infectious diseases and by the well-established potential of the immune system for controlling tumor growth, active therapeutic immunization approaches hold great promise for treating malignant tumors. In recent years, engineered recombinant viral vectors have been carefully examined as genetic-immunization vehicles and have been demonstrated to induce potent T-cell-mediated immune responses that can control tumor growth. Very recent efforts suggest that lentivectors possess important advantages over other candidate recombinant viral vectors for genetic immunization. Here, we review the development of recombinant lentivectors and the characteristics of T-cell immune responses elicited by lentivector immunization, including the mechanism of T-cell priming with a focus on the role of skin dendritic cells and potential applications for tumor immunotherapy.

Gene delivery by lentivirus vectors

The capacity to efficiently transduce nondividing cells, shuttle large genetic payloads, and maintain stable long-term transgene expression are attributes that have brought lentiviral vectors to the forefront of gene delivery vehicles for research and therapeutic applications in a clinical setting. Our discussion initiates with advances in lentiviral vector development and how these sophisticated lentiviral vectors reflect improvements in safety, regarding the prevention of replication competent lentiviruses (RCLs), vector mobilization, and insertional mutagenesis. Additionally, we describe conventional molecular regulatory systems to manage gene expression levels in a spatial and temporal fashion in the context of a lentiviral vector. State of the art technology for lentiviral vector production by transient transfection and packaging cell lines are explicitly presented with current practices used for concentration, purification, titering, and determining the safety of a vector stock. We summarize lentiviral vector applications that have received a great deal of attention in recent years including the generation of transgenic animals and the stable delivery of RNA interference molecules. Concluding remarks address some of the successes in preclinical animals, and the recent transition of lentiviral vectors to human clinical trials as therapy for a variety of infectious and genetic diseases.

Lentiviral vector-mediated genetic modification of human neural progenitor cells for ex vivo gene therapy

Human neural progenitor cells (hNPC) hold great potential as an ex vivo system for delivery of therapeutic proteins to the central nervous system. When cultured as aggregates, termed neurospheres, hNPC are capable of significant in vitro expansion. In the current study, we present a robust method for lentiviral vector-mediated gene delivery into hNPC that maintains the differentiation and proliferative properties of neurosphere cultures while minimizing the amount of viral vector used and controlling the number of insertion sites per population. This method results in long-term, stable expression even after differentiation of the hNPC to neurons and astrocytes and allows for generation of equivalent transgenic populations of hNPC. In addition, the in vitro analysis presented predicts the behavior of transgenic lines in vivo when transplanted into a rodent model of Parkinson's disease. The methods presented provide a powerful tool for assessing the impact of factors such as promoter systems or different transgenes on the therapeutic utility of these cells.

Quantitative polymerase chain reaction as a reliable method to determine functional lentiviral titer afterex vivo gene transfer in human mesenchymal stem cells

BACKGROUND

Human mesenchymal stem cells (hMSCs) are a promising target for ex vivo gene therapy and lentiviruses are excellent gene transfer vehicles in hMSCs since they achieve high transduction rates with long-term gene expression. Nevertheless, senescence of hMSCs may limit therapeutic applications due to time-consuming cell selection and viral titration. Here, we describe a fast and reliable method to determine functional lentiviral titer by quantitative polymerase chain reaction (qPCR) after highly efficient ex vivo gene transfer in hMSCs.

METHODS

Lentivirus production was tested with different types of packaging systems. Using p24 ELISA remaining viral particles were detected in the cell culture supernatant. The lentiviral gene transfer efficiency was quantified by FACS analysis. Lentiviral titers were determined by qPCR of expressed transgenes.

RESULTS

Third-generation self-inactivating vectors showed highly efficient gene transfer in hMSCs. No viral antigen was detected in the cell culture supernatant after four media changes, suggesting the absence of infectious particles after 4 days. We observed a linear correlation between virus dilution and level of transgene expression by qPCR analysis, therefore allowing viral titering by quantification of transgene expression. Finally, we demonstrated that transduced hMSCs retained their stem cell character by differentiation towards adipogenic, osteogenic and chondrogenic lineages.

CONCLUSIONS

Quantification of transgene copy numbers by qPCR is a fast and reliable method to determine functional lentiviral titer after ex vivo gene transfer in hMSCs.

Comparison of lentiviral vector titration methods

Background

Lentiviral vectors are efficient vehicles for stable gene transfer in dividing and non-dividing cells. Several improvements in vector design to increase biosafety and transgene expression, have led to the approval of these vectors for use in clinical studies. Methods are required to analyze the quality of lentiviral vector production, the efficiency of gene transfer and the extent of therapeutic gene expression.

Results

We compared lentiviral vector titration methods that measure pg p24/ml, RNA equivalents/ml, transducing units (TU/ml) or mRNA equivalents. The amount of genomic RNA in vector particles proves to be reliable to assess the production quality of vectors encoding non-fluorescent proteins. However, the RNA and p24 titers of concentrated vectors are rather poor in predicting transduction efficiency, due to the high variability of vector production based on transient transfection. Moreover, we demonstrate that transgenic mRNA levels correlate well with TU and can be used for functional titration of non-fluorescent transgenes.

Conclusion

The different titration methods have specific advantages and disadvantages. Depending on the experimental set-up one titration method should be preferred over the others.