Stability of lentiviral vector-mediated transgene expression in the brain in the presence of systemic antivector immune responses

Lentiviral vectors with a defective integrase allow efficient and sustained transgene expression in vitro and in vivo

Lentivirus-derived vectors are among the most promising viral vectors for gene therapy currently available, but their use in clinical practice is limited by the associated risk of insertional mutagenesis. We have overcome this problem by developing a nonintegrative lentiviral vector derived from HIV type 1 with a class 1 integrase (IN) mutation (replacement of the 262RRK motif by AAH). We generated and characterized HIV type 1 vectors carrying this deficient enzyme and expressing the GFP or neomycin phosphotransferase transgene (NEO) under control of the immediate early promoter of human CMV. These mutant vectors efficiently transduced dividing cell lines and nondividing neural primary cultures in vitro. After transduction, transient GFP fluorescence was observed in dividing cells, whereas long-term GFP fluorescence was observed in nondividing cells, consistent with the viral genome remaining episomal. Moreover, G418 selection of cells transduced with vectors expressing the NEO gene showed that residual integration activity was lower than that of the intact IN by a factor of 500-1,250. These nonintegrative vectors were also efficient in vivo, allowing GFP expression in mouse brain cells after the stereotactic injection of IN-deficient vector particles. Thus, we have developed a generation of lentiviral vectors with a nonintegrative phenotype of great potential value for secure viral gene transfer in clinical applications.

Lentivirus-mediated gene transfer to the central nervous system: therapeutic and research applications

The management of disorders of the nervous system remains a medical challenge. The key goals are to understand disease mechanisms, to validate therapeutic targets, and to develop new therapeutic strategies. Viral vector-mediated gene transfer can meet these goals and vectors based on lentiviruses have particularly useful features. Lentiviral vectors can deliver 8 kb of sequence, they mediate gene transfer into any neuronal cell type, expression and therapy are sustained, and normal cellular functions in vitro and in vivo are not compromised. After delivery into the nervous system they induce no significant immune responses, there are no unwanted side effects of the vectors per se to date, and manufacturing and safety testing for clinical applications are well advanced. There are now numerous examples of effective long-term treatment of animal models of neurological disorders, such as Parkinson's disease, Alzheimer's disease, Huntington's disease, motor neuron diseases, lysosomal storage diseases, and spinal injury, using a range of therapeutic genes expressed in lentiviral vectors. Significant issues remain in some areas of neural gene therapy including defining the optimum therapeutic gene(s), increasing the specificity of delivery, regulating expression of potentially toxic genes, and designing clinically relevant strategies. We discuss the applications of lentiviral vectors in therapy and research and highlight the essential features that will ensure their translation to the clinic in the near future.

Cyclophosphamide enhances glioma virotherapy by inhibiting innate immune responses

Clinical trials are testing oncolytic viruses (OVs) as therapies for cancer. We have shown that animals that have brain tumors and are treated with a herpes simplex virus (HSV)-derived OV live significantly longer when cyclophosphamide (CPA) is preadministered. Here, we explore the mechanisms behind this finding. In a syngeneic rat glioma model, intratumoral HSV administration is associated with rapid increase of natural killer cells, microglia/macrophages (CD68+ and CD163+), and IFN-gamma. Pretreatment with CPA enhances HSV replication and oncolysis and reduces an HSV-mediated increase in CD68+ and CD163+ cells and intratumoral IFN-gamma. Molecular imaging shows CPA pretreatment to inhibit HSV-induced infiltration of tumor-associated phagocytic cells. Our results reveal molecular and cellular mechanisms that inhibit intratumoral spread of HSV and suggest a therapeutic path for improving the efficacy of virotherapy as a treatment for cancer.

Gene therapy for lysosomal storage diseases

Lysosomal storage diseases (LSDs) comprise a diverse group of monogenetic disorders with complex clinical phenotypes that include both systemic and central nervous system pathologies. In recent years, the identification or development of mouse models recapitulating the clinical course of the LSDs has been instrumental in evaluating therapeutic strategies. Here, we review the various gene replacement strategies for target organs affected in many LSDs and describe briefly the various vector systems employed to test how best to accomplish long-lasting therapies for these fatal disorders.

Human gene therapy and imaging in neurological diseases

Molecular imaging aims to assess non-invasively disease-specific biological and molecular processes in animal models and humans in vivo. Apart from precise anatomical localisation and quantification, the most intriguing advantage of such imaging is the opportunity it provides to investigate the time course (dynamics) of disease-specific molecular events in the intact organism. Further, molecular imaging can be used to address basic scientific questions, e.g. transcriptional regulation, signal transduction or protein/protein interaction, and will be essential in developing treatment strategies based on gene therapy. Most importantly, molecular imaging is a key technology in translational research, helping to develop experimental protocols which may later be applied to human patients. Over the past 20 years, imaging based on positron emission tomography (PET) and magnetic resonance imaging (MRI) has been employed for the assessment and "phenotyping" of various neurological diseases, including cerebral ischaemia, neurodegeneration and brain gliomas. While in the past neuro-anatomical studies had to be performed post mortem, molecular imaging has ushered in the era of in vivo functional neuro-anatomy by allowing neuroscience to image structure, function, metabolism and molecular processes of the central nervous system in vivo in both health and disease. Recently, PET and MRI have been successfully utilised together in the non-invasive assessment of gene transfer and gene therapy in humans. To assess the efficiency of gene transfer, the same markers are being used in animals and humans, and have been applied for phenotyping human disease. Here, we review the imaging hallmarks of focal and disseminated neurological diseases, such as cerebral ischaemia, neurodegeneration and glioblastoma multiforme, as well as the attempts to translate gene therapy's experimental knowledge into clinical applications and the way in which this process is being promoted through the use of novel imaging approaches.

Lentiviral Vectors for Use in the Central Nervous System

Lentiviral vectors have been used extensively as gene transfer tools for the central nervous system throughout the past decade since they transduce most cell types in the brain, resulting in high-level and long-term transgene expression. This review discusses some of the recent progress in this field, including preclinical gene therapy experiments in disease models, development of regulated vectors, and the application of siRNA's using lentiviral vectors. We also describe some of the features that make lentiviral vectors a likely candidate for human gene therapy in the brain.

Effective gene therapy with nonintegrating lentiviral vectors

Retroviral and lentiviral vector integration into host-cell chromosomes carries with it a finite chance of causing insertional mutagenesis. This risk has been highlighted by the induction of malignancy in mouse models, and development of lymphoproliferative disease in three individuals with severe combined immunodeficiency-X1 (refs. 2,3). Therefore, a key challenge for clinical therapies based on retroviral vectors is to achieve stable transgene expression while minimizing insertional mutagenesis. Recent in vitro studies have shown that integration-deficient lentiviral vectors can mediate stable transduction. With similar vectors, we now show efficient and sustained transgene expression in vivo in rodent ocular and brain tissues. We also show substantial rescue of clinically relevant rodent models of retinal degeneration. Therefore, the high efficiency of gene transfer and expression mediated by lentiviruses can be harnessed in vivo without a requirement for vector integration. For therapeutic application to postmitotic tissues, this system substantially reduces the risk of insertional mutagenesis.