Lentivirus vectors: difficulties and hopes before clinical trials.

The ability to transduce non-dividing cells is a unique feature of lentiviruses which distinguishes them from simple retroviruses. This feature was the major incentive for the development of the lentivirus vector system. Lentivirus vectors can deliver and integrate > 8 kb of transgenic DNA into target cell genomes without inducing a host immune response against the transduced cells. Thus lentivirus vector-based gene delivery can be considered the most efficient method by which transgenes can be incorporated into the host cell genome and maintain long-term expression. This review describes the major developments in the lentivirus vector system, which significantly improve vector biosafety, vector production and transgene expression. The success and difficulties in reverting disease phenotypes by lentivirus vectors carrying therapeutic genes in various animal models including beta-thalassemia and Parkinson's disease and the implications of these studies for future gene therapy clinical trials are also discussed.

Generation of a stable cell line producing high-titer self-inactivating lentiviral vectors.

To facilitate the generation of SIN lentivirus vector-producer cell lines, we have developed a novel conditional SIN (cSIN) lentivirus vector, which retains its SIN properties in normal target cells yet can be produced at high titers from tetracycline-regulated packaging cell lines. The design of the cSIN vector is based on replacing the vector U3 transcription regulatory elements with the Tet-responsive element, which allows vector production exclusively in cells expressing the synthetic Tet-regulated transactivator (tTA). In contrast minimal vector production ( approximately 200 IU/ml) is obtained in target cells that do not express the tTA, even in the presence of all HIV-1 proteins. Following transduction of the Tet-regulated SODk1 lentivirus vector-packaging cell line with the cSIN vector, high titers of cSIN recombinant vector (>10(6) IU/ml) could be generated, which efficiently transduced terminally differentiated neurons in normal rat brain.

Lentiviral vectors: regulated gene expression.

Lentiviral vectors can deliver and express genes in a wide variety of dividing and nondividing cells. These include terminally differentiated neurons, myotubes, hepatocytes, and hematopoietic stem cells. We now describe the generation of lentiviral vectors in which the expression of the transgene can be regulated. We have developed an inducible lentiviral vector system that contains the entire tetracycline (Tet)-regulated system developed by H. Bujard and colleagues. The novel vector expresses the GFP reporter gene and the tetracycline transactivator under the control of the tetracycline-inducible promoter and the human CMV promoter, respectively. In vitro transduction of human 293 cells resulted in a very low basal expression of GFP in the presence of the effector substance doxycyline. Withdrawal of doxycyline induced a more than 500-fold increase in transgene expression. Switching transgene expression "off and on" did not change either the kinetics or the magnitude of induction. Maximal suppression of GFP mRNA transcription was achieved within 24 h of addition of the drug; however, due to the slow turnover rate of GFP, green fluorescent cells could be detected up to 10 days following doxycyline treatment. Following transduction of rat brain with recombinant lentiviruses, doxycyline-regulated GFP expression could be observed in terminally differentiated neurons. Specifically, by adding or withdrawing doxycyline from the rats' drinking water, induction and suppression of GFP expression could be regulated in vivo. These studies show that an inducible lentiviral vector can deliver and regulate transgene expression in vivo. We believe that regulated gene expression is an essential tool for successful gene therapy approaches.

Integration complexes derived from HIV vectors for rapid assays in vitro.

Of three enzymes encoded by HIV-reverse transcriptase, protease, and integrase-only the first two have been exploited clinically as inhibitor targets. Efforts to develop inhibitors of purified integrase protein have yielded many compounds, but none with clinical utility. A different source of integration activity for studies in vitro is provided by replication intermediates isolated from HIV-infected cells. These preintegration complexes (PICs) can direct integration of the endogenously synthesized viral cDNA into an added target DNA in vitro. Despite their authentic activities, assays of PICs have not been widely used due to technical obstacles, particularly the requirement for handling large amounts of infectious HIV. Here, we describe greatly improved methods for producing PICs using HIV-based vectors that are capable of establishing an integrated provirus but not a spreading infection. We also report the development of a PIC integration assay using DNA-coated microtiter plates, which speeds assays of PIC integration in vitro. We used this method to screen a library of chemicals related to known integrase inhibitors and found a new compound, quinalizarin sulfate, that displayed enhanced activity against PICs.

Gene transfer to human pancreatic endocrine cells using viral vectors.

We have studied the factors that influence the efficiency of infection of human fetal and adult pancreatic endocrine cells with adenovirus, murine retrovirus, and lentivirus vectors all expressing the green fluorescent protein (Ad-GFP, MLV-GFP, and Lenti-GFP, respectively). Adenoviral but not retroviral vectors efficiently infected intact pancreatic islets and fetal islet-like cell clusters (ICCs) in suspension. When islets and ICCs were plated in monolayer culture, infection efficiency with all three viral vectors increased. Ad-GFP infected 90-95% of the cells, whereas infection with MLV-GFP and Lenti-GFP increased only slightly. Both exposure to hepatocyte growth factor/scatter factor (HGF/SF) and dispersion of the cells by removal from the culture dish and replating had substantial positive effects on the efficiency of infection with retroviral vectors. Studies of virus entry and cell replication revealed that cell dispersion and stimulation by HGF/SF may be acting through both mechanisms to increase the efficiency of retrovirus-mediated gene transfer. Although HGF/SF and cell dispersion increased the efficiency of infection with MLV-GFP, only rare cells with weak staining for insulin were infected, whereas approximately 25% of beta-cells were infected with Lenti-GFP. We conclude that adenovirus is the most potent vector for ex vivo overexpression of foreign genes in adult endocrine pancreatic cells and is the best vector for applications where high-level but transient expression is desired. Under the optimal conditions of cell dispersion plus HGF/SF, infection with MLV and lentiviral vectors is reasonably efficient and stable, but only lentiviral vectors efficiently infect pancreatic beta-cells.

A packaging cell line for lentivirus vectors.

Lentivirus vectors can transduce dividing and nondividing cells. Using three-plasmid transient transfections, high-titer (>10(9) IU/ml) recombinant lentivirus vectors pseudotyped with vesicular stomatitis virus G (VSV-G) protein can be generated (T. Kafri et al., Nat. Genet. 17:314-317, 1997; H. Miyoshi et al., Proc. Natl. Acad. Sci. USA 94:10319-10323, 1997; L. Naldini et al., Science 272:263-267, 1996). The recombinant lentiviruses can efficiently infect brain, liver, muscle, and retinal tissue in vivo. Furthermore, the transduced tissues demonstrated long-term expression of reporter genes in immunocompetent rodents. We now report the generation of a tetracycline-inducible VSV-G pseudotyped lentivirus packaging cell line which can generate virus particles at titers greater than 10(6) IU/ml for at least 3 to 4 days. The vector produced by the inducible cell line can be concentrated to titers of 10(9) IU/ml and can efficiently transduce nondividing cells in vitro and in vivo. The availability of a lentivirus packaging cell line will significantly facilitate the production of high-titer lentivirus vectors for gene therapy and study of human immunodeficiency virus biology.

Cellular immune response to adenoviral vector infected cells does not require de novo viral gene expression: implications for gene therapy.

Replication-defective adenoviral (RDAd) vectors can be generated at high titers and infect both dividing and nondividing cells. Long term expression in the transduced tissue, however, has been a problem because of the cellular immune responses against the infected cells. We demonstrate that mice injected with RDAd vectors containing mouse leptin gene reduce food intake and lose weight for only 7 to 10 days. Splenocytes obtained from infected mice are able to lyse target cells infected with RDAd vectors. Surprisingly, target cells infected with psoralen-treated, UV-crosslinked, biologically inactive RDAd also were lysed efficiently by the effector cells. Furthermore, splenocytes obtained from mice injected with inactive RDAd vectors efficiently lysed target cells infected with RDAd vectors. Whether RDAd vectors were injected i.m. or i.v. or through an i.p. route, the extent of lysis was similar. We propose that cells infected with RDAd vectors present antigens for recognition by class 1 major histocompatibility complex-restricted cytotoxic T lymphocytes by a mechanism that does not require viral replication or de novo protein synthesis. These results should prompt reevaluation of the use of RDAd vectors for gene therapy when long-term expression is required.

Bcl-xL protects adult septal cholinergic neurons from axotomized cell death.

Bcl-xL suppresses apoptotic cell death induced by diverse stimuli in cell lines in vitro. To examine the mechanism by which axotomized cholinergic neurons die in vivo, lentiviral vectors expressing Bcl-xL, human nerve growth factor (hNGF), or green fluorescent protein were injected into the septum 3 weeks before transection of the fimbria fornix. Three weeks after transection, Bcl-xL- and hNGF-injected animals showed significantly higher numbers of spared cholinergic neurons compared with control (green fluorescent protein) injected animals. These results provide evidence that adult axotomized cholinergic neurons die of apoptotic death that can be prevented by local delivery of hNGF or intracellular delivery of Bcl-xL.

Lentivirus-mediated transduction of islet grafts with interleukin 4 results in sustained gene expression and protection from insulitis.

Autoimmune destruction of islets in the pancreas leads to the development of insulin-dependent diabetes mellitus (IDDM). Replacement of insulin-producing tissue by transplantation of islets provides a cure to disease but requires immunosuppression or a means of controlling anti-graft immune responses. To promote islet survival we have utilized a local approach by expressing immunoregulatory molecules in islet grafts. The results presented here show that the human immunodeficiency virus (HIV)-based lentiviral vector is capable of stably transducing whole islets. Foreign reporter gene expression was observed both in vitro and in vivo 30 days after transplantation. Grafts containing insulin-positive beta-islet cells expressing foreign protein indicate that transduction does not interfere with glucose regulation. The absence of inflammatory infiltrates in grafts suggests that transduction does not activate the immune system. When islets transduced with an HIV vector expressing IL-4 were transplanted into diabetes-prone mice, animals were protected from autoimmune insulitis and islet destruction. As demonstrated by proliferative and cytokine analysis, protection was consistent with a switching of islet-antigen-specific T cell responses toward a Th2 phenotype. These results suggest that HIV-based lentivirus vectors can efficiently transduce islet cells with genes encoding potentially therapeutic molecules, for possibly managing diabetes.

Sustained expression of genes delivered directly into liver and muscle by lentiviral vectors.

Successful gene therapy approaches will require efficient gene delivery and sustained expression of the transgene in recipients. A variety of methods, ranging from direct DNA delivery to infection with recombinant viruses containing foreign genes, have been developed, but they all have some major limitations that restrict their utility. We have described a human lentiviral (HIV)-based vector that can transduce non-dividing cells in vitro and deliver genes in vivo. With this vector, expression of transgenes in the brain has been detected for more than six months--the longest period tested so far. Because lentiviral vectors are pseudotyped with vesicular stomatitis virus G glycoprotein (VSVG; ref. 8), they can transduce a broad range of tissues and cell types. We now describe the ability of lentiviral vectors to introduce genes directly into liver and muscle. Sustained expression of green fluorescent protein (GFP), used as a surrogate for therapeutic protein, can be observed for more than 22 weeks in the liver. Similar long-term expression (more than eight weeks) was observed in transduced muscle. In contrast, little or no GFP could be detected in liver or muscle transduced with the Moloney murine leukaemia virus (M-MLV), a prototypic retroviral based vector. At a minimum, 3-4% of the total liver tissue was transduced by a single injection of 1-3 x 10(7) infectious units (I.U.) of recombinant HIV vector. Furthermore, no inflammation of recruitment of lymphocytes could be detected at the site of injection. Animals previously transduced with a lentiviral vector can be efficiently re-infected with lentiviral vectors. Additionally, we show that the requirement for lentiviral accessory proteins to establish efficient transduction in vivo is tissue dependent.

Highly efficient and sustained gene transfer in adult neurons with a lentivirus vector.

The identification of monogenic and complex genes responsible for neurological disorders requires new approaches for delivering therapeutic protein genes to significant numbers of cells in the central nervous system. A lentivirus-based vector capable of infecting dividing and quiescent cells was investigated in vivo by injecting highly concentrated viral vector stock into the striatum and hippocampus of adult rats. Control brains were injected with a Moloney murine leukemia virus, adenovirus, or adeno-associated virus vector. The volumes of the areas containing transduced cells and the transduced-cell densities were stereologically determined to provide a basis for comparison among different viral vectors and variants of the viral vector stocks. The efficiency of infection by the lentivirus vector was improved by deoxynucleoside triphosphate pretreatment of the vector and was reduced following mutation of integrase and the Vpr-matrix protein complex involved in the nuclear translocation of the preintegration complex. The lentivirus vector system was able to efficiently and stably infect quiescent cells in the primary injection site with transgene expression for over 6 months. Triple labeling showed that 88.7% of striatal cells transduced by the lentivirus vector were terminally differentiated neurons.

Suppression of TNF-alpha-induced apoptosis by NF-kappaB.

Tumor necrosis factor alpha (TNF-alpha) signaling gives rise to a number of events, including activation of transcription factor NF-kappaB and programmed cell death (apoptosis). Previous studies of TNF-alpha signaling have suggested that these two events occur independently. The sensitivity and kinetics of TNF-alpha-induced apoptosis are shown to be enhanced in a number of cell types expressing a dominant-negative IkappaBalpha (IkappaBalphaM). These findings suggest that a negative feedback mechanism results from TNF-alpha signaling in which NF-kappaB activation suppresses the signals for cell death.

Mechanistic aspects of genome-wide demethylation in the preimplantation mouse embryo.

Gene-specific methylation patterns in mammals play a role in a variety of biological processes in the embryo and adult tissues. These patterns are established during embryo development by a process that involves genome-wide demethylation in the morula and de novo methylation in the pregastrula. To elucidate the mechanism of demethylation in the early mouse embryo, we have injected mouse zygotes with gene sequences that were methylated in vitro by Hpa II methylase and analyzed the methylation status of specific sites in blastocyst DNA. Because it had been propagated in Escherichia coli, the DNA used for these injections was also methylated at adenine residues in GATC sites. This allowed us to eliminate fully methylated, unintegrated DNA by Dpn I digestion and fully unmethylated, integrated DNA that underwent several rounds of replication by Mbo I digestion. The integrated, originally injected DNA strands were in a hemimethylated state and survived this treatment. The methylation status of Hpa II sites in these molecules was analyzed by Hpa II digestion of the genomic DNA isolated from blastocysts, followed by PCR amplification using appropriate primers. The results demonstrate that demethylation is achieved by an active mechanism and that specific sites in imprinted genes escape demethylation, maintaining a methylated state throughout preimplantation development.

The ontogeny of allele-specific methylation associated with imprinted genes in the mouse.

We have investigated the DNA methylation patterns in genomically imprinted genes of the mouse. Both Igf2 and H19 are associated with clear-cut regions of allele-specific paternal modification in late embryonic and adult tissues. By using a sensitive PCR assay, it was possible to follow the methylation state of individual HpaII sites in these genes through gametogenesis and embryogenesis. Most of these CpG moieties are not differentially modified in the mature gametes and also become totally demethylated in the early embryo in a manner similar to non-imprinted endogenous genes. Thus, the overall allele-specific methylation pattern at these sites must be established later during embryogenesis after the blastula stage. In contrast, sites in an Igf2r gene intron and one CpG residue in the Igf2 upstream region have allele-specific modification patterns which are established either in the gametes or shortly after fertilization and are preserved throughout pre-implantation embryogenesis. These studies suggest that only a few DNA modifications at selective positions in imprinted genes may be candidates for playing a role in the maintenance of parental identity during development.

Maternal-specific methylation of the imprinted mouse Igf2r locus identifies the expressed locus as carrying the imprinting signal.

The mouse insulin-like growth factor type 2 receptor (Igf2r) is imprinted and expressed exclusively from the maternally inherited chromosome. To investigate whether methylation could function as the imprinting signal, we have cloned 130 kb from the Igf2r locus and searched for sequences methylated in a parental-specific manner. Two regions have been identified: region 1 contains the start of transcription and is methylated only on the silent paternal chromosome; region 2 is contained in an intron and is methylated only on the expressed maternal chromosome. Methylation of region 1 is acquired after fertilization, in contrast with the methylation of region 2, which is inherited from the female gamete. Methylation of region 2 may mark the maternal Igf2r locus in a manner that could act as an imprinting signal. These data suggest that the expressed locus carries a potential imprinting signal and imply that methylation is necessary for expression of the Igf2r gene.