Lentiviral vectors: their molecular design, safety, and use in laboratory and preclinical research

Histone deacetylase inhibition activates transgene expression from integration-defective lentiviral vectors in dividing and non-dividing cells

Integration-defective lentiviral vectors (IDLVs) are being increasingly deployed in both basic and preclinical gene transfer settings. Often, however, the IDLV transgene expression profile is muted when compared to that of their integration-proficient counterparts. We hypothesized that the episomal nature of IDLVs turns them into preferential targets for epigenetic silencing involving chromatin-remodeling histone deacetylation. Therefore, vectors carrying an array of cis-acting elements and transcriptional unit components were assembled with the aid of packaging constructs encoding either the wild-type or the class I mutant D116N integrase moieties. The transduction levels and transgene-product yields provided by each vector class were assessed in the presence and absence of the histone deacetylase (HDAC) inhibitors sodium butyrate and trichostatin A. To investigate the role of the target cell replication status, we performed experiments in growth-arrested human mesenchymal stem cells and in post-mitotic syncytial myotubes. We found that IDLVs are acutely affected by HDACs regardless of their genetic makeup or target cell replication rate. Interestingly, the magnitude of IDLV transgene expression rescue due to HDAC inhibition varied in a vector backbone- and cell type-dependent manner. Finally, investigation of histone modifications by chromatin immunoprecipitation followed by quantitative PCR (ChIP-qPCR) revealed a paucity of euchromatin marks distributed along IDLV genomes when compared to those measured on isogenic integration-competent vector templates. These findings support the view that IDLVs constitute preferential targets for epigenetic silencing involving histone deacetylation, which contributes to dampening their full transcriptional potential. Our data provide leads on how to most optimally titrate and deploy these promising episomal gene delivery vehicles.

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.

The determination of importance of sequences neighboring the Psi sequence in lentiviral vector transduction and packaging efficiency

A number of lentiviral vector systems have been developed for gene delivery and therapy by eliminating and/or modifying viral genetic elements. However, all lentiviral vector systems derived from HIV-1 must have a viral packaging signal sequence, Psi (Ψ), which is placed downstream of 5' long terminal repeat in a transgene plasmid to effectively package and deliver transgene mRNA. In this study, we examined feasible regions or sequences around Psi that could be manipulated to further modify the packaging sequence. Surprisingly, we found that the sequences immediately upstream of the Psi are highly refractory to any modification and resulted in transgene vectors with very poor gene transduction efficiency. Analysis around the Psi region revealed that there are a few sites that can be used for manipulation of the Psi sequence without disturbing the virus production as well as the efficiency of transgene RNA packaging and gene transduction. By exploiting this new vector system, we investigated the requirement of each of four individual stem-loops of the Psi sequence by deletion mapping analysis and found that all stem-loops, including the SL4 region, are needed for efficient transgene RNA packaging and gene delivery. These results suggest a possible frame of the lentiviral vector that might be useful for further modifying the region/sequence around the packaging sequence as well as directly on the Psi sequence without destroying transduction efficiency.

High efficiency restriction enzyme-free linear amplification-mediated polymerase chain reaction approach for tracking lentiviral integration sites does not abrogate retrieval bias

Retroviral vectors are an efficient and widely employed means of introducing an exogenous expression cassette into target cells. These vectors have been shown to integrate semi-randomly into the cellular genome, and can be associated with genotoxicity due to impact on expression of proximate genes. Therefore, efficient and accurate integration site analysis, while quantifying contributions of individual vector-containing clones, is desirable. Linear amplification-mediated polymerase chain reaction (LAM-PCR) is a widely used technique for identifying integrated proviral and host genomic DNA junctions. However, LAM-PCR is subject to selection bias inherent in the reliance of the assay on the presence of a restriction enzyme-cutting site adjacent to a retrievable integration site, and it is further limited by an inability to discriminate prior to sequencing between the flanking genomic DNA of interest and uninformative internal vector DNA. We report a modified restriction enzyme-free LAM-PCR (Re-free LAM-PCR) approach that is less time and labor intensive compared to conventional LAM-PCR, but in contrast to some other nonrestrictive methods, compares in efficiency and sensitivity, excludes retrieval of uninformative internal vector sequences, and allows retrieval of integration sites unbiased by the presence of nearby restriction sites. However, we report that Re-free LAM-PCR remains inaccurate for quantitation of the relative contributions of individual integration site-containing clones in a polyclonal setting, suggesting that bias in LAM-PCR retrieval of integration sites is not wholly explained by restriction enzyme-related factors.

pLR: a lentiviral backbone series to stable transduction of bicistronic genes and exchange of promoters

Gene transfer based on lentiviral vectors allow the integration of exogenous genes into the genome of a target cell, turning these vectors into one of the most used methods for stable transgene expression in mammalian cells, in vitro and in vivo. Currently, there are no lentivectors that allow the cloning of different genes to be regulated by different promoters. Also, there are none that permit the analysis of the expression through an IRES (internal ribosome entry site)-- reporter gene system. In this work, we have generated a series of lentivectors containing: (1) a malleable structure to allow the cloning of different target genes in a multicloning site (mcs); (2) unique site to exchange promoters, and (3) IRES followed by one of two reporter genes: eGFP or DsRed. The series of the produced vectors were named pLR (for lentivirus and RSV promoter) and were fairly efficient with a strong fluorescence of the reporter genes in direct transfection and viral transduction experiments. This being said, the pLR series have been found to be powerful biotechnological tools for stable gene transfer and expression.

A lentiviral vector-based, herpes simplex virus 1 (HSV-1) glycoprotein B vaccine affords cross-protection against HSV-1 and HSV-2 genital infections

Genital herpes is caused by herpes simplex virus 1 (HSV-1) and HSV-2, and its incidence is constantly increasing in the human population. Regardless of the clinical manifestation, HSV-1 and HSV-2 infections are highly transmissible to sexual partners and enhance susceptibility to other sexually transmitted infections. An effective vaccine is not yet available. Here, HSV-1 glycoprotein B (gB1) was delivered by a feline immunodeficiency virus (FIV) vector and tested against HSV-1 and HSV-2 vaginal challenges in C57BL/6 mice. The gB1 vaccine elicited cross-neutralizing antibodies and cell-mediated responses that protected 100 and 75% animals from HSV-1- and HSV-2-associated severe disease, respectively. Two of the eight fully protected vaccinees underwent subclinical HSV-2 infection, as demonstrated by deep immunosuppression and other analyses. Finally, vaccination prevented death in 83% of the animals challenged with a HSV-2 dose that killed 78 and 100% naive and mock-vaccinated controls, respectively. Since this FIV vector can accommodate two or more HSV immunogens, this vaccine has ample potential for improvement and may become a candidate for the development of a truly effective vaccine against genital herpes.

Specific targeting of human interleukin (IL)-13 receptor α2-positive cells with lentiviral vectors displaying IL-13

The ability to selectively and efficiently target transgene delivery to specific cell types in vitro and in vivo remains one of the formidable challenges in gene therapy. Lentiviral vectors have several advantages that make them attractive as gene delivery vehicles and their tropism can be altered through pseudotyping, allowing transgene delivery to specific populations of cells. The human interleukin-13 receptor α2 (IL-13Rα2) is uniquely overexpressed in many different human tumors, making it an attractive target for cancer therapy. In this study, we examined whether IL-13Rα2-positive tumor cells can be specifically targeted with lentiviral vector pseudotypes containing a truncated fusion (F) protein derived from measles virus (MV) and a tail-truncated and receptor-blind MV hemagglutinin (H) protein bearing IL-13 at the C terminus. The retargeted lentiviral vector efficiently transduced cells that express high levels of IL-13Rα2, but not cells expressing low levels of IL-13Rα2 in vitro. In vivo, it specifically targeted IL-13Rα2-positive glioma cell xenografts in immunodeficient mice in the context of subcutaneous and intracranial glioma models. Similar lentiviral vectors may be developed for targeting other tumors expressing specific cell surface receptors.

Gene therapy for the Wiskott-Aldrich syndrome

BACKGROUND

Wiskott-Aldrich syndrome (WAS) is a rare X-linked primary immunodeficiency (PID) characterized by micro-thrombocytopenia, recurrent infections, eczema, which is associated with a high incidence of auto-immunity and lymphoreticular malignancy. One of the first diseases to be successfully treated by allogeneic hematopoietic stem cell transplantation, WAS is currently the subject of several phase I/II gene therapy trials for patients without HLA-compatible donors.

PURPOSE OF REVIEW

This article reviews the preclinical and clinical data leading to the development of gene therapy of WAS with lentiviral vectors.

RECENT FINDINGS

A recent clinical trial using a conventional gammaretroviral vector has demonstrated the proof of principle of gene therapy in WAS, but has also highlighted a common limitation of the technology. Encouraging preclinical efficacy and safety results using refined lentiviral vectors, and the development of robust clinical-grade manufacturing processes have supported the initiation of several phase I/II new studies.

SUMMARY

WAS is amenable to hematopoietic stem cell gene therapy. New trials using lentiviral vectors are expected to improve efficacy and safety profiles. Beyond proof of principle, ongoing international efforts to coordinate trials of gene therapy for the WAS may also provide a model for the expedited development of new treatments for other rare diseases.

Migratory properties of mesenchymal stem cells

Mesenchymal stem cells raise great expectations in regenerative medicine due to their capacity to regenerate damaged tissues, thereby restoring organ tissue integrity and functionality. Even though it is not yet clear how mesenchymal stem cells are guided to injured tissue it is generally assumed that the directed migration of these cells is facilitated by the same soluble factors that also recruit immune competent cells to inflamed tissue areas. Tumor tissue represents another type of (chronically) inflamed tissue and because of that mesenchymal stem cells are highly attracted. Although some data indicate that esenchymal stem cells might have a beneficial effect on tumor growth due to anti-tumor effects the plethora of data suggest that tumor tissue recruited mesenchymal stem cells rather promote tumor growth and metastasis formation. Nonetheless, the enhanced tumor tropism of mesenchymal stem cells makes them ideal candidates for novel anti-cancer strategies. Like Trojan Horses genetically modified mesenchymal stem cells will deliver their deadly cargo, such as anti-tumor cytokines or oncolytic viruses, into cancerous tissues, thereby destroying the tumor form within. In this chapter we will summarize the current concepts of genetic modification of mesenchymal stem cells for future anti-cancer therapies.

A short perspective on gene therapy: Clinical experience on gene therapy of gliomablastoma multiforme

More than two decades have passed since the first gene therapy clinical trial was conducted. During this time, we have gained much knowledge regarding gene therapy in general, but also learned to understand the fear that persists in society. We have experienced drawbacks and successes. More than 1700 clinical trials have been conducted where gene therapy is used as a means for therapy. In the very first trial, patients with advanced melanoma were treated with tumor infiltrating lymphocytes genetically modified ex-vivo to express tumor necrosis factor. Around the same time the first gene therapy trial was conducted, the ethical aspects of performing gene therapy on humans was intensively discussed. What are the risks involved with gene therapy? Can we control the technology? What is ethically acceptable and what are the indications gene therapy can be used for? Initially, gene therapy was thought to be implemented mainly for the treatment of monogenetic diseases, such as adenosine deaminase deficiency. However, other therapeutic areas have become of interest and currently cancer is the most studied therapeutic area for gene therapy based medicines. In this review I will be giving a short introduction into gene therapy and will direct the discussion to where we should go from here. Furthermore, I will focus on the use of the Herpes simplex virus-thymidine kinase for gene therapy of malignant gliomas and highlight the efficacy of gene therapy for the treatment of malignant gliomas, but other strategies will also be mentioned.

Quantification of HIV-based lentiviral vectors: influence of several cell type parameters on vector infectivity

A human immunodeficiency virus type (HIV-1)-based lentiviral vector pseudotyped with the vesicular stomatitis virus envelope glycoprotein and encoding the GFP reporter gene was used to evaluate different methods of lentiviral vector titration. GFP expression, viral DNA quantification and the efficiency of vector DNA integration were assayed after infection of conventional HIV-1-permissive cell lines and human primary adult fibroblasts with the vector. We found that vector titers based on GFP expression determined by flow cytometry may vary by more than 50-fold depending on the cell type and the promoter-cell combination used. Interestingly, we observed that the viral integration process in primary HDFa cells was significantly more efficient compared to that in SupT1 or 293T cells. We propose that determination of the amount of integrated viral DNA by quantitative PCR be used in combination with the reporter gene expression assay.