LENTIVIRAL TRIPLEX DNA, AND VECTORS AND RECOMBINANT CELLS CONTAINING LENTIVIRAL TRIPLEX DNA

Abstract: The present ivention provides nucleic acid, vectors, viruses, and recombinant cells comprising triple-stranded structures, such as those resulting from central initiation and termination of HIV-1 reverse transcription at the center of HIV-1 linear DNA genomes. These triplex structures can act as a cis-determinant of HIV-1 DNA nuclear import, allowing infection of non-dividing target cells. In one aspect, the presence of the DNA triplex sequence in an HIV vector strongly stimulates gene transfer in hematopoietic stem cells. The invention also provides methods of using these triplex structures for making recombinant cells, as well as methods of using the recombinant cells to express proteins of interest both <i>in vitro</i> and <i>in vivo</i>.

Description: LENTIVIRAL TRIPLEX DNA, AND VECTORS AND RECOMBINANT CELLS CONTAINING LENTIVIRAL TRIPLEX DNA BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to the field of biotechnology, and especially to viral nucleic acids and cells containing viral nucleic acids. More particularly, the present invention relates to viral nucleic acid sequences that can be part of triplex DNA structures as well as nucleic acid vectors, viruses, and cells containing these viral nucleic acid sequences. It further relates to methods of using such DNA structures and nucleic acid sequences. Description of Related Art Gene transfer in hematopoietic stem cells (HSC) has great potential, both for gene therapy of inherited as well as acquired diseases, and for the understanding of mechanisms regulating normal and pathological hematopoiesis. As HSC have extensive proliferative capacities, stable gene transfer should include genomic integration of the transgene. Retroviruses based on Moloney murine leukemia virus (MoMLV) have been very popular because they integrate into the host cell genomes and can allow long-term transgene expression. However, these oncoretroviruses do not integrate in non-dividing cells, and most HSC are quiescent. Many pre-stimulation protocols using different cytokine associations have been developed to trigger cycling of HSC in order to render them transducible by oncovirus-derived, mitosis-dependant gene transfer vectors. However, cytokine stimulation induces differentiation together with proliferation and potentialities of HSC could be lost during the transduction process. This problem can be overcome by using lentivirus-derived vectors since lentiviruses have been shown to infect both dividing and non-dividing cells (Poznansky et al., 1991; Naldini et α/.,1996). Initial reports published in the last three years using such lentivirus-derived vectors for transduction of HSC showed promising but very heterogenous results (Case et al., 1999; Evans et al., 1999; Uchida et al., 1998; Miyoshi et al., 1999). Lentiviruses have evolved a nuclear import strategy which allows their DNA genome to cross the nuclear membrane of a host cell. This active nuclear import of lentiviruses accounts for their unique capacity, among retroviruses, to replicate efficiently in non-dividing target cells, such as tissue macrophages. The restriction of replication of oncoviruses like MoMLV to dividing cells appears to be due to the requirement for disruption of the nuclear membrane barrier during mitosis, allowing MoMLV pre-integration complexes (PICs) to enter the nucleus (Roe et al, 1993). Mitosis-independent replication of lentiviruses, at the origin of their in vivo replication strategy and hence of their pathogenicity, has also enabled the generation of lentiviral gene transfer vectors with promising therapeutic applications (Poznansky et al, 1991; Naldini et al, 1996). The mitosis-independent replication of lentiviruses was first demonstrated by the productive infection of non-mitotic chondroid cells by the VISNA lentivirus (Thormar, 1963). Soon after its discovery, HIV was shown to replicate in differentiated primary macrophages (Gartner et al, 1986; Ho et al, 1986). HIV DNA integrates in the chromatin of non-mitotic target cells (Weinberg et al, 1991; Lewis et al, 1992), implying that HTV-l PICs are able to cross the nuclear membrane of host cells (Bukrinsky et al, 1992). Thus, mitosis-independent nuclear import is a pivotal event responsible for the ability of lentiviruses to replicate in non- dividing cells. The search for the viral determinants responsible for the active nuclear import of the HIV- 1 DNA genome has constituted an active but controversial field of investigation. The presence of putative nuclear localization signals (NLSs) within the matrix (MA) and Vpr viral proteins has led to the proposition that they could act in a redundant manner in HIV-1 DNA nuclear import (Bukrinsky et al, 1993b; Emerman et al, 1994; Popov et al, 1998; von Schwedler et al, 1994). It has been proposed that phosphorylation of a small subset (1%) of MA molecules at a C- terminal tyrosine residue triggers their release from the plasma membrane and their association with HIV-1 integrase protein (Gallay et al, 1995a; 1995b). The contribution of these proteins to the karyophilic properties of HIV-1 PICs is currently a matter of strong debate (Freed and Martin, 1994; Freed et al, 1995; Fouchier et al, 1997; Freed et al, 1997; Koostra and Schuitemaker, 1999). More recently, NLS motifs have been identified in the integrase protein (IN) and mutations in these motifs have been reported to abolish the interaction of IN with karyopherin α, a cellular NLS receptor (Gallay et al, 1997). SUMMARY OF THE INVENTION Whatever the role of these candidate viral proteins in HIV nuclear import, the present invention shows that the retrotranscribed HIV-1 genome itself bears a cis-acting determinant for its nuclear import. The invention provides a nucleic acid comprising a triple-stranded (triplex) structure, such as one from a lentivirus. The triplex stimulates entry of nucleic acids into the nucleus of cells. The nucleic acid can contain the cPPT and CTS cis-acting sequences of a lentivirus. The lentivirus can be any lentivirus, including, but not limited to, HIV-1, HIV-2, VISNA, EINV, FIV, and CAE. In embodiments, the nucleic acid is in the context of a vector, such as an expression vector. Thus, the invention also provides a vector, for example, a nucleic acid vector. The nucleic acid vector can include sequences from any vector known to the skilled artisan as useful for transfer of nucleic acids into cells or for expression of nucleic acids in vivo or in vitro. The invention further provides viruses and cells (eukaryotic and prokaryotic) containing the nucleic acid of the invention. The cells can be recombinant cells. The invention additionally provides a method of transferring nucleic acid to a host cell nucleus by, for example, exposing the host cell to the nucleic acid, vector, virus, or cell of the invention, to provide a recombinant cell. The method of the invention permits high-efficiency transfer of nucleic acids to the host cell nucleus, such as the nucleus of a hematopoietic stem cell. High-efficiency transfer permits the skilled artisan to practice various methods of treatment, including, but not limited to, methods of prophylactic treatment, methods of ameliorative treatment, and methods of curative treatment. For example, methods of gene therapy are enabled by this invention. In general, gene therapy can be used to treat blood diseases, brain diseases, viral disease, as well as many other inherited and acquired diseases. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 shows that central initiation of reverse transcription is an important step in the replication cycle of HIV-1. (A) Mutations introduced in the HIV-1 cPPT sequence. Conservative and semi- conservative cPPT mutant viruses were constructed. Semi-conservative mutant cPPT-D contains 10 mutations in the 19-mer cPPT. Mutant cPPT-AG is its control virus in which a single purine to purine mutation introduces the same amino acid change in the overlapping integrase coding region. Mutations are shown in reversed type. (B) Impact of the mutations in the cPPT on HIV-1 infectivity. Virus replication kinetics on PHA stimulated PBLs (left panel) and MT4 cells (right panel). Cells were infected with equivalent amounts of viral particles according to the capsid antigen (p24) contents of viral supernatants. Virus production was followed over time by RT activity. (C) Single cycle virus titrations in dividing or non-dividing aphidicolin-treated P4 cells (HeLa CD4-LTR LacZ). β-galactosidase activity was measured using a chemiluminescent assay. Results are expressed as relative light units (RLU)/sec/ng p24 of the inoculum, mean ± SD of four independent experiments. Figure 2 depicts mutations in the cPPT that do not affect virus production, viral DNA synthesis nor the ability of viral DNA to integrate in vitro. (A) Effect of mutations in the cPPT on virus production. HeLa cells were transiently transfected with pLAI, pcPPT-AG, or pcPPT-D plasmids. Virus production was measured by quantitation of p24 viral antigen in cell supernatants, 48 hours post-transfection. (B) Effect of mutations in the cPPT on reverse transcription efficiency. P4 cells were infected with the same amounts of viral particles (300 ng of p24 antigen per 10 6 cells) and DNA was extracted 12 hours later. Total amounts of reverse transcribed viral DNA, represented by an internal Mscl HIV-1 fragment, was detected by Southern blot using the same DNA fragment as a probe and quantitated using a phosphorimager. (C) Effect of mutations in the cPPT on in vitro integration. MT4 cells were co-cultivated with H9-LAI or H9-cPPT-225 chronically infected cells. In vitro integration of viral pre- integration complexes, isolated from the cytoplasm of infected cells, was performed as previously described (Farnet and Haseltine, 1990). Each lane is loaded with cytoplasmic DNA from 2 X 10 8 infected cells. Figure 3 shows that central DNA triplex mutant viruses are defective in nuclear import of viral DNA. (A) Strategy for the quantitative follow up of the synthesis, circularization, and integration of HIV-1 DNA. DNA from infected cells was extracted at various times post- infection, digested with Mscl and Xhol, and analyzed by Southern blot using a probe overlapping the 5' Mscl site. The internal 1.9 kb DNA fragment, common to all viral DNA species irrespective of their integrated or unintegrated state, indicates the total amount of viral DNA in infected cells. The 2.6 kb, 2.8 kb, and 3.4 kb signals corresponding, respectively, to unintegrated linear DNA, one, and two LTRs circular DNAs are revealed. Since the PCR generated probe exactly overlaps the Mscl site, the intensity of each band is directly proportional to the amount of the corresponding viral DNA species. Thus, the amount of integrated proviral DNA can be calculated by subtracting from the total amount of viral DNA the signals of unintegrated linear and circular viral DNA. (B) Southern blot analysis of viral DNA processing in infected cells. P4 cells were infected with equivalent amounts of each virus, normalized on the p24 contents of the supernatants. Infected cells were lysed at different times post-infection, DNA was extracted, and used for the quantitative analysis described above. (C) Intracellular viral DNA profiles, on completion of one cycle of infection (48 hours post-infection). Results are expressed as percentages of total viral DNA. Similar intracellular viral DNA profiles were obtained using MT4 cells (data not shown). Figure 4 shows that linear DNA from central DNA triplex mutant viruses accumulates at the vicinity of the nuclear membrane. (A) Nucleus/cytoplasm fractionation of infected P4 cells. Southern blot analysis of viral DNA from nuclear (N) and cytoplasmic (C) fractions, 24 hours post infection. Fractionation based on triton lysis was performed as previously described (Belgrader et al, 1991). DNA was restricted with Mscl and hybridized using the Mscl site overlapping probe. (B) Detection of individual HIV-1 genomes by Fluorescence In Situ Hybridization (FISH). P4 cells were infected at high multiplicity (2μg of p24 antigen per 10 6 cells), and hybridized using a full length HIV-l genome probe. Fluorescent signals were amplified by tyramid precipitation. Optical sections through cells were analyzed by deconvolution microscopy. Figure 5 shows that the insertion of the central DNA triplex in an HTV-l based vector enhances GFP transduction and nuclear import of the vector DNA genome. (A) Schematic diagram of the vector genomes. cPPT and CTS central cis active sequences of the HIV-1 genome, responsible for the formation of the DNA triplex during reverse transcription, were inserted in a central position in the previously described HR GFP vector (Naldini et al, 1996). TRIPinv-GFP includes the central DNA triplex sequence in the reverse, non-functional orientation. (B) Comparative efficiency of GFP transduction using HIV vectors with or without a central DNA triplex. Dividing or non dividing (aphidicolin treated) HeLa cells were used as targets. GFP fluorescence was quantitated 48 hours post-transduction using a microplate fluorometer (Wallac). Results are expressed as the mean ± SD of a representative experiment performed in triplicate. Pseudotransduction of GFP activity was subtracted from the signal. (C) Southern blot analysis of vector DNA processing in transduced cells. Transduced HeLa cells were lysed at different times post-infection, DNA was extracted, restricted, and Southern blotted using a similar strategy as for the viruses. Mscl digestion is replaced by EcoNI and vαll, and the probe is a PCR DNA fragment overlapping exactly the EcoNI site. (D) Quantitative analysis of vector DNA intracellular status, 48hours post infection. Results are presented as percentages of total vector DNA. Figure 6 depicts a model for DNA triplex dependent HTV-l genome nuclear import. (A) Overview of the observed phenotype of central DNA triplex mutant viruses. Central initiation and termination steps of HTV-l reverse transcription creates a long plus strand DNA flap structure: the central DNA triplex. HIV-1 plus strand is synthesized as two discrete half- genomic segments. A downstream segment is initiated at a central copy of the polypurine tract sequence (cPPT). The upstream segment terminates downstream the cPPT, after a 99 nucleotide long strand displacement event, blocked by the central termination sequence (CTS). At completion of a single cycle of infection, viral DNA from wild-type virus is almost fully processed into integrated provirus (« 55%), 1LTR (« 35%), and 2LTRs circular DNA (< 5%), while a small fraction remains as linear DNA (< 10%). Mutations in the cPPT affects the formation of the central DNA triplex. The final reverse transcription product of a central initiation mutant virus is a continuous double-stranded linear DNA lacking the central DNA triplex (Charneau et al, 1992). Viral DNA from cPPT-D mutant virus accumulates in infected cells as linear DNA molecules, and localizes at the vicinity of the nuclear membrane. (B) Two speculative mechanisms for triplex dependent HIV-1 genome nuclear import via maturation of the reverse transcription complex (RTC) into a pre-integration complex (PIC) and linear DNA translocation through the nuclear pore. HTV-l reverse transcription probably occurs within a structured (ordered) complex surrounded by an assembly of capsid proteins. The size of RTC exceeds the exclusion diameter of the nuclear pore. Before translocation, RTC undergoes a maturation into a smaller PIC with loss of the capsid proteins (Karageorgos et al, 1993). Formation of the DNA triplex signals the end of the viral DNA synthesis and could signal the escape of the HIV DNA from the capsid assembly. In the PIC, the extremities of the linear DNA are probably bridged together after dimerization of integrase proteins bound to the tips of the LTRs (Miller et al, 1997). The DNA triplex being at a central position, one logical structure for HTV-l PIC would be a double filament, symmetrically folded on either side of the central triplex by the integrase dimerization. The DNA triplex would then constitute an apex which could interact with karyophilic shuttling proteins, allowing the passage of the HTV-l DNA filament through the nuclear pore. In cPPT mutant viruses, a default of RTC maturation into PIC would induce an accumulation of integral viral capsids at the nuclear pore. Alternatively, the absence of the DNA triplex in cPPT mutant linear DNA would prohibit the interaction with shuttling proteins forbidding translocation of HTV-l genome through the nuclear pore. In both cases, DNA from cPPT mutant viruses accumulates as linear DNA at the vicinity of the nuclear membrane. Figure 7 shows the results of transduction experiments using CD34+ human cord blood cells. (A) FACS analysis of human cord blood CD34+ cells transduced for 24 hours in the presence of 100 ng/ml of viral P24 in conditions described herein. Analysis was performed 48 hours after washing of the cells at the end of the 24 hour transduction period. Percentages are expressed as proportions of morphologically gated hematopoietic cells. X mean indicates the mean value of green fluorescence intensity. (B) GFP expression was analyzed at day 5 post transduction. Figure 8 diagrammatically represents the effect of viral dosage on transfection efficiency. (A) The percentage of CD34+ cells expressing eGFP. (B) The mean value of GFP fluorescence intensity of CD34+eGFP+ cells. (C) The mean value of GFP fluorescence intensity of CD34+eGFP+ cells multiplied by the percentage of CD34+eGFP+ cells. CD34+ cells were transduced for 24 hours under the conditions specified herein with the lentiviral vector including the eGFP coding sequence under the control of the CMV promoter, and including (thick line) or without (dashed line) the triplex structure. Analysis was performed 48 hours after washing of the cells at the end of the 24 hour transduction period. Figure 9 depicts FACS analysis of human cord blood CD34+ cells transduced for 24 hours under the conditions described in the text with a lentiviral vector having an intact HPV-1 LTR (left panels) or a U3 deleted HPV-1 LTR (right panels) and an internal CMV promoter (upper panels) or the EF-1 alpha promoter (lower panels). (A) Analysis was performed 48 hours after washing of the cells at the end of the 24 hour transduction period. (B) Analysis was performed 120 hours after washing of the cells at the end of the 24 hour transduction period. The analysis distinguishes between bright (immature) and dull CD34 cells. Percentages are expressed as proportions of morphologically gated hematopoietic cells. The second number, when indicated, represents the mean of green fluorescence. DETAILED DESCRIPTION OF THE INVENTION We have previously shown that HTV-l has evolved a complex reverse transcription strategy, which differs from that of oncoviruses by two steps occurring at the center of the genome: one additional initiation of plus strand synthesis coupled with a termination step. HIV- 1 plus strand DNA is synthesized as two discrete half-genomic segments. An additional copy of the polypurine tract cis-active sequence, present at the center of all lentiviral genomes (cPPT), initiates synthesis of a downstream plus strand (Chameau et al, 1991, 1992). The upstream plus strand segment initiated at the 3' PPT will, after a strand transfer, proceed to the center of the genome and terminate after a discrete strand displacement event (Figure 6A). This last chronological event of HTV-l reverse transcription is controlled by the central termination sequence (CTS) cis-active sequence, which ejects HPV-1 reverse transcriptase (RT) at this site in the specific context of strand displacement synthesis (Chameau et al, 1994; Lavigne et al, 1997). Thus, the final product of HTV-l reverse transcription is a linear DNA molecule bearing in its center a stable 99 nucleotide long DNA flap, here referred to as the central DNA triplex (Figure 6A). Central initiation and termination mutant viruses both synthesize a reverse transcription product devoid of a wild type DNA triplex. As depicted in Figure 6A, in the case of a central initiation mutant, the final product is a continuous double-stranded linear DNA lacking the central triplex (Chameau et al, 1992). The downstream plus strand segment initiated at the cPPT is not synthesized. Thus, no strand displacement occurs at the center of the genome; elongation of the transferred plus strand proceeds all along the genome. In the case of a central termination mutant, central strand displacement events are no longer controlled by the mutated CTS sequence and longer, randomly distributed plus strand overlaps are generated, as compared to the discrete wild type DNA triplex (Chameau et al, 1994). Mutations in the cPPT or CTS cis-active sequences severely impair HPV replication, suggesting a direct role of the central triplex in the retroviral life cycle (Chameau et al, 1992; Hungnes et al, 1992; Chameau et al, 1994). The present invention discloses that the central DNA triplex of HTV-l is involved in a late step of HTV-l genome nuclear import, immediately prior to, or during, viral DNA translocation through the nuclear pore. Hence the distinctive features of lentiviral reverse transcription account, at least in part, for the unique capacity of lentiviruses, among retroviruses, to replicate in non-dividing cells. The invention also discloses that HIV gene transfer vectors lacking the central DNA triplex exhibit a strong nuclear import defect. This invention establishes that the insertion of the central cis-active sequences of the HIV-1 genome into a previously described HPV vector (Naldini et al, 1996) increases the transduction efficiency by complementing the nuclear import defect of the original vector DNA to wild type levels. This finding provides additional and independent evidence for the role of the central DNA triplex in HTV-l nuclear import.The description of the DNA triplex as a nuclear import determinant of lentiviruses has important implications for the design of efficient lentiviral vectors. Since the infection of non- dividing target cells by lentiviruses relies on their use of an active nuclear import pathway, it is preferable to maintain the lentiviral nuclear import determinants in derived vector constructs. Classical retroviral vector constructs are replacement vectors in which the entire viral coding sequences between the LTRs are deleted and replaced by the sequences of interest (Miller AD, 1997). In the case of lentiviral vectors, this classical strategy leads to the deletion of the central cis-active sequences cPPT and CTS. The important role of the triplex in HIV nuclear import implies that such replacement vector constructs are not optimal. This finding establishes the fact that the DNA triplex nuclear import determinant is operative in the heterologous context of an HTV-l derived lentiviral vector. The DNA triplex per se out of the context of the native HTV-l genome can promote nuclear import of heterologous DNA sequences (PCT France No. 98 05197, 24 April 1998). The presence of the DNA triplex sequence induces a marked increase of gene transduction efficiency in hematopoietic stem cells. Thus, in one aspect, the present invention provides nucleic acids (DNA or RNA, or analogs thereof) that are capable of participating in triplex nucleic acid structures (i.e., triple- stranded nucleic acids). In embodiments of the invention, the nucleic acids are lentiviral sequences. In embodiments, the nucleic acids are derived from lentiviral sequences, such as by directed mutagenesis (i.e., intentional deletion, insertion, or replacement of at least one nucleotide) or selective pressure mutagenesis. The nucleic acids of the invention permit high- efficiency transfer of nucleic acids into host cells, and especially into host cell nuclei. Because of this ability, genes that are operably or physically linked to the nucleic acids of the invention can be expressed at high levels and/or in a high percentage of cells exposed to the nucleic acids. DNA inserted in a triple-stranded region of a lentiviral genome in accordance with the present invention is particularly stable in the construct, and aids in achieving a high level of transduction and transfection. In a preferred embodiment, the invention provides a three-stranded DNA sequence induced by the cPPT and CTS regions of a lentivirus, which is capable of inducing a high rate of entry of vector DNA into a host cell nucleus, or capable of increasing the rate of nuclear import of vector DNA. In embodiments, the three-stranded DNA sequence can be covalently linked to a heterologous nucleic acid sequence, such as a reporter gene or gene of other interest. For example, the nucleic acid of the invention can comprise a heterologous nucleic acid sequence that encodes a peptide, polypeptide, or protein. In embodiments, the heterologous nucleic acid sequence is present within the triple-helix region. In embodiments, the heterologous nucleic acid sequence is present on the same nucleic acid as the triple-helix region, but outside of the triple helix. In embodiments, a single nucleic acid comprises more than one three-stranded sequence induced by the cPPT and CTS regions of a lentivirus. In another aspect, the present invention provides vectors, such as shuttle vectors, expression vectors, integration vectors, transposons, retrotransposons, and the like, which contain at least one sequence capable of participating in the formation of triplex nucleic acid structures. In embodiments, the vector comprises a single sequence capable of participating in the formation of triplex nucleic acid structures. In embodiments, the vector comprises more than one sequence capable of participating in the formation of triplex nucleic acid structures. The vectors can be used to transfer the nucleic acids of the invention from one cell to another, to aid in generation of large quantities of the nucleic acids of the invention, and to serve as a base molecule into which other nucleic acid sequences of interest (e.g., reporter genes, therapeutic genes) can be inserted. In yet another aspect, the present invention provides a method for efficiently infecting, transfecting, or transducing cells with a viral nucleic acid sequence comprising at least one sequence that can participate in triplex nucleic acid structures. In embodiments, the method comprises exposing a cell, or a plurality of cells, to at least one copy of a nucleic acid, vector, virus, or cell of the invention. In embodiments, the step of exposing the cell or cells to the nucleic acid, vector, virus, or cell of the invention is conducted under conditions such that the nucleic acid, vector, virus, or cell of the invention can enter the target (host) cell or cells. In preferred embodiments, the conditions are such that high levels of the nucleic acid, vector, virus, or cell of the invention enter the target cell or cells. In embodiments, the method is highly efficient, permitting insertion of the nucleic acid of the invention into the nucleus of 30% or greater of the cells exposed to the nucleic acid. In embodiments, the percentage of nuclei taking up the nucleic acid of the invention is 40% or greater, for example 50% or greater, 60% or greater, 70% or greater, 80% or greater, or 90%- 100%). In preferred embodiments, the percentage of nuclei taking up the nucleic acid of the invention is greater than 80%, more preferably, greater than 85%, and most preferably, greater than 90%), such as greater than 95%. In embodiments, the cells are infected or transfected with whole viruses (including phages) or bacteria containing the nucleic acids of the invention. In embodiments, the nucleic acids of the invention are introduced into the cells using mechanical and/or chemical means (e.g., electroporation, liposome fusion). In embodiments, naked nucleic acid according to the invention is taken up by target host cells. Thus, in an embodiment, the invention provides the use of a nucleotide sequence comprising the cPPT and CTS regions, which adopts a three-stranded DNA structure (triplex) after reverse transcription, in a lentiviral or retrotransposon vector and which stimulates entry of, and the rate of nuclear import of, the vector DNA into the nucleus of a transduced cell. In an aspect of the invention, recombinant cells are provided that contain the nucleic acids or vectors of the invention. The recombinant cells can be any cell (prokaryotic or eukaryotic), including a progeny or derivative thereof. Thus, the invention includes cells that are created using a cell of the invention. In embodiments, the cells are eukaryotic. In embodiments, the cells are mammalian cells, such as HeLa cells or hematopoietic cells, such as hematopoietic stem cells. Thus, in embodiments, the invention provides a process of transducing eukaryotic cells, wherein the process comprises use of a three- stranded DNA sequence induced by the cPPT and CTS regions of a lentivirus, which is capable of inducing a high rate of entry of vector DNA into a host cell nucleus, or capable of increasing the rate of nuclear import of vector DNA. Because the recombinant cells of the invention can express a heterologous gene of interest at high levels, these cells can be used in numerous applications. Applications include, but are not limited to, production of high levels of proteins of interest (such as proteins of therapeutic value) in cell culture and production of a protein of interest in vivo by introduction of the recombinant cell of the invention into an individual in need of the protein. The individual can be an animal or a human. Accordingly, the invention has both veterinarian applications as well as medical ones. In embodiments, this aspect of the invention provides a method of producing a recombinant protein of interest by exposing a target cell to a nucleic acid, vector, or virus of the invention and permitting the target cell to take up nucleic acid of the invention, for example, through transduction, transformation, or transfection. Following introduction of the nucleic acid into the target cell, the cell is cultured under conditions whereby the recombinant protein of interest is expressed, and thus produced by the target cell, which now is considered a recombinant cell according to the invention. Although the method can be practiced on a single, individual cell, it is evident that the method will often be more practical if practiced on a collection of cells in which all of the cells are clones. As used herein, "cell" refers to an individual cell or a collection of identical cells. The method can further include purifying or isolating the protein of interest from the recombinant cell or cell culture fluid. In such a method, protein expression and purification procedures known to those of skill in the art can be applied. These procedures are well known to those of skill in the art and therefore need not be detailed here. In a preferred embodiment, this aspect of the invention provides a process for expressing a gene of interest in vitro, wherein the process comprises: a) exposing target cells to an isolated or purified nucleic acid comprising a gene of interest and at least one copy of the cPPT and CTS cis- acting regions of a retrovirus, wherein the cPPT and CTS regions induce a three-stranded DNA structure, under conditions that permit uptake of the nucleic acid into the target cell to create a recombinant cell, and b) culturing the recombinant cell under conditions that permit at least part of the nucleic acid to be transferred to the nucleus of the recombinant cell and the gene of interest to be expressed. In embodiments, the process uses a vector according to the invention. Thus, the invention provides a method of expressing a gene of interest in vitro, for example, in tissue culture. In embodiments, the method comprises exposing target cells to a nucleic acid, vector, virus, or cell of the invention under conditions where the target cell can take up the molecule of the invention containing the gene of interest. The recombinant cell thus made is then allowed to grow and replicate under conditions where the gene of interest is expressed. In embodiments, the in vitro method of gene expression is coupled to a method of purifying or isolating the protein of interest. In these embodiments, the protein of interest can be purified or isolated from other cellular components using techniques known to those of skill in the art, including, but not limited to, liquid chromatography, precipitation, centrifugation, gel filtration, and affinity chromatography. Suitable techniques are known to those of skill in the art and need not be detailed here. Thus, the invention also provides a method of expressing a gene of interest in vivo, for example, in an individual in need of the protein expressed by the gene. In embodiments, the method of expressing a gene in vivo comprises making a recombinant cell outside the individual by exposing a host cell to a nucleic acid, vector, virus, or cell of the invention, to make a recombinant cell according to the invention. The recombinant cell of the invention is then administered to, introduced into, or otherwise exposed to, the individual, whereupon the gene of interest is expressed. For example, the method can comprise administering a recombinant cell comprising a nucleic acid of the invention to an individual, and permitting the recombinant cell to express the nucleic acid within the individual's body. In preferred embodiments, the recombinant cell is a hematopoeitic stem cell. In embodiments, the recombinant cells are first purified or isolated from non-recombinant cells, then administered to, introduced into, or otherwise exposed to, the individual. In other embodiments, the method of expressing a gene in vivo comprises exposing (e.g., administering, introducing, etc.) the individual to a nucleic acid, vector, and/or virus of the invention. In embodiments, the nucleic acid, vector, and/or virus transfects/transduces/infects at least one of the individual's cells, whereupon the gene of interest is expressed. For example, the method can comprise administering a nucleic acid, vector, or virus of the invention to an individual in an amount and form sufficient to result in expression of the gene of interest within the individual's body. Preferably, the method results in expression of the gene of interest in a target tissue or cell. In an embodiment, this aspect of the invention provides a process for treating an individual suffering from, or having a high likelihood of developing, a disease or disorder having a genetic basis. The process comprises administering a retroviral vector comprising a) a nucleic acid encoding a therapeutic protein and b) at least one copy of the cPPT and CTS cis-acting regions of a retrovirus, wherein the cPPT and CTS regions induce a three-stranded DNA structure, to the individual in an amount sufficient to result in expression of the therapeutic protein in an amount sufficient to treat the disease or disorder. The treatment can be prophylactic, ameliorative, or curative. The process can treat a blood disease or disorder, a brain or nervous system disease or disorder, or a developmental disease or disorder. Techniques for introducing and/or expressing genes in vivo are known to those of skill in the art. The practitioner may select the technique most suitable for the given protein or target tissue or cell. Accordingly, the invention provides a process of treating a host comprising use of a retroviral vector containing a three-stranded DNA sequence induced by the cPPT and CTS regions of a lentivirus, which is capable of inducing a high rate of entry of vector DNA into a host cell nucleus, or capable of increasing the rate of nuclear import of vector DNA. In accordance with the above aspects of the invention, a kit is also provided. The kit can contain at least one nucleic acid, at least one vector, at least one virus, or at least one cell of the invention, or a combination of any or all of those. The kit can provide each of the above embodiments of the invention together in a single composition or separately, as for example, in different containers. In embodiments, the kit includes some or all of the reagents and supplies necessary to use the nucleic acids, vectors, viruses, and cells of the invention for the desired purpose. The present invention discloses an original mechanism of HIV-1 nuclear import with a crucial role of a three stranded DNA structure, the DNA triplex, in this mechanism. HPV-1 has evolved a complex reverse transcription strategy, whereby a central strand displacement event, consecutive to the central initiation and termination of reverse transcription, creates a DNA triplex at the center of unintegrated linear HIV-1 DNA molecules. This DNA triplex acts in turn as a cis-active determinant of the nuclear import of the HTV-l genome. The invention shows that central initiation and termination, two distinctive steps of HIV-1 reverse transcription, account for the capacity of HIV-1 to infect non-dividing target cells. The Examples of the invention further show that lack of the DNA triplex leads to a vims which is almost non-infectious in dividing or non-dividing cells. Although mutations in cPPT do not affect the rate of synthesis of viral DNA or its ability to integrate in vitro, most of the retrotranscribed DNA molecules from the cPPT mutant vims accumulate over time as unintegrated linear DNA. In contrast, linear DNA from the wild-type vims is almost fully processed into integrated proviruses and DNA circles. The intracellular DNA profile of cPPT mutant viruses points to a defect of nuclear import, the viral DNA accumulating as linear molecules as a consequence of its lack of access to the nuclear compartment where it could integrate or circularize. A late defect of viral DNA import, most probably affecting translocation through the NPC, is demonstrated by fractionation of infected cells and direct visualization (FISH) of intracellular viral DNA. The triplex defective linear DNA molecules associate with the nuclear membrane. The invention focuses on the analysis of cPPT mutant viruses, which are characterized by the absence of a central DNA triplex. Most of the experiments presented herein were also conducted with previously described CTS mutant vims (Chameau et al, 1994), with the same results (data not shown). In the CTS mutant vims, reverse transcription produces linear DNA molecules containing larger, randomly distributed plus strand overlaps, as compared to the discrete central DNA triplex of the wild-type vims. Thus, not only the presence of the DNA triplex, but also its stmctural integrity, is important for the nuclear import of HTV DNA. The invention shows that the DNA triplex is operative in the context of an HIV-1 based vector system. Its insertion into a vector devoid of the triplex reverts a strong defect of nuclear import of the vector DNA to wild-type levels of nuclear import. The central DNA triplex is a common nuclear import determinant of lentivimses. The location of the central DNA triplex has been precisely defined in the case of HTV-l . Central strand displacement starts at the first nucleotide following the cPPT sequence (Chameau and Clavel, 1991) and stops in general 99 nucleo tides downstream, at the ter2 site of the CTS sequence (Chameau et al, 1994). The three dimensional configuration of the three DNA strands of the triplex is as yet unknown. Nevertheless, the presence of a DNA triplex at the center of the genome can be generalized to all lentivimses. A central copy of PPT is a common feature of all lentiviral genomes and a putative CTS terminator element, revealed by the presence of (A) n and (T) n tracts, also exists approximately 100 nucleotides downstream (Chameau et al, 1994). The central DNA triplex of the ungulate lentivims EIAV has been characterized recently (Stetor et al, 1999). A central strand discontinuity in VISNA vims DNA, referred to as a gap, but most probably a nick resulting from the central strand displacement, was revealed by S 1 nuclease cleavage (Harris et al, 1981). Since mitosis-independent replication has been described for most lentivimses (Gartner et al, 1986; Thormar, 1963), the role of the DNA triplex in nuclear import described here for HIV-1 can be generalized to all lentivimses. Without being limiting, the invention provides a mechanistic hypothesis for the role of the central DNA triplex in HTV-l nuclear import as follows: A three stranded DNA stmcture acting as a cis-determinant of its nuclear import is a novel biological phenomenon with no known cellular or viral counterparts. Any hypothesis of a molecular mechanism describing the role of the central DNA triplex in HTV nuclear import is therefore speculative. The central triplex could act as a viral determinant for initiation of the uptake of the HTV DNA filament through the nuclear pore. This could be achieved through direct interaction of the DNA triplex with components of the pore, or alternatively through interaction of the triplex with cellular or viral proteins which shuttle between the cytoplasm and the nucleus of the host cell and could drag the HIV genome into the nucleus. Translocation of the 9.7 kb HIV genome through a nuclear pore of maximum diameter of 26nm must occur in a specific orientation, after recognition of one extremity of the HTV-l DNA filament to initiate the uptake. A similar situation arises in the nuclear export of messenger RNA, where uptake of the RNA filament through the pore is guided by the 5' Cap stmcture (Hamm and Mattaj, 1990). Although the conformation of HTV-l PICs is not well known, it has been established that the extremities of the linear DNA are bridged together, probably after dimerization of the integrase proteins bound at the tips of the LTRs (Miller et al, 1997). Interestingly, the cPPT and CTS cis-active sequences are found at a central position in all lentiviral genomes. One logical stmcture for lentiviral PICs would be a double DNA filament, symmetrically folded on either side of the central triplex by the integrase dimerization (Figure 6B). The triplex would then constitute one apex of a filamentous HIV-1 PIC and the integrase dimer the opposite apex. In cPPT mutant PICs, the absence of a DNA triplex would lead to their lack of recognition by the nuclear pore machinery or the shuttling proteins. Identification of the protein ligands of the central DNA triplex promises to be of primary importance both for our understanding of HIV-1 PIC nuclear import and for the eventual development of dmgs targeting this step of HTV replication. Another mechanistic hypothesis to explain the properties of triplex mutant viruses would involve a defect in the maturation of HTV capsids into PICs, prior the translocation of viral DNA into the nucleus. According to this model, triplex defective viral DNA would remain trapped as integral viral capsids, unable to translocate. Retroviral reverse transcription does not take place at high dilution of the viral components in the cytoplasm of infected cells, but requires the stmctural environment of a reverse transcription complex where components are confined in a capsid protein assembly. The HrV capsid size exceeds the maximum exclusion diameter of a nuclear pore (Dworetzky et al, 1998; Gelderblom, 1991). Therefore, before viral DNA can enter the nucleus, the HIV reverse transcription complexes must undergo maturation into PICs of size compatible with translocation through the nuclear pores (Karageorgos et al, 1993). The maturation of viral capsids prior to nuclear translocation is well established in several other viral systems in which the replicative cycle involves translocation of the DNA genome through the nuclear membrane of the host cell (Whittaker and Helenius, 1998). Whereas reverse transcription within viral capsids has been physically demonstrated for MLV (Bowerman et al, 1989), this has not yet been possible for HIV-1 possibly due to the fragility of HTV-l capsids (Borroto-Esoda and Boone, 1991). The HrV reverse transcription complex contains numerous copies of RT polymerase (about 30 to 50 per capsid) (Panet et al, 1975). Owing to the important distribution of HIV-1 reverse transcriptase, a high stoichiometry of enzyme to viral RNA template is necessary to overcome a number of limiting steps of reverse transcription such as strand transfers or polymerization pauses during plus and minus strand synthesis and accurate formation of the central triplex (Klarmann et al, 1993; Chameau et al, 1994). This strongly suggests that central termination, the last event of lentiviral reverse transcription, occurs within an integral capsid stmcture. Central termination, which marks the end of viral DNA synthesis, could be a required signal for viral DNA decapsidation and its subsequent translocation into the nucleus. These two putative molecular mechanisms for the DNA triplex mediated nuclear import of HIV-1 are not mutually exclusive. The formation of a central DNA triplex could trigger the maturation of viral capsids into PICs, thus making the DNA triplex accessible to shuttling proteins. The fact that the integrity of the central DNA triplex is required for entry of the HPV-1 genome into the host cell nucleus implies that the entire process of DNA synthesis, including the last central strand displacement event, is completed prior to translocation of the HIV PIC through the nuclear pore. The subcellular distribution of lentiviral reverse transcription is currently under debate and whether HIV replication occurs in a specific cellular compartment is still an open question. On the basis of fractionation studies, it has been reported that HIV reverse transcription can occur entirely whin the cell nucleus (Bukrinsky et al, 1993a). However, fractionation techniques do not distinguish between an intranuclear localization and association with the nuclear membrane. Other authors have proposed on the contrary that association of the reverse transcription complex with the cytoskeleton is a prerequisite for viral DNA synthesis (Bukrinskaya et al, 1998). Kinetic studies of the synthesis of HIV-1 DNA and its association with the nuclear fraction indicate that the latter process is much more rapid than the former. The synthesis of HΓV-1 DNA in the course of a single cycle of reverse transcription only reaches a plateau 24 to 48 hours following infection, whereas more than 95% of HIV- 1 DNA fractionates with the nuclei of infected cells as early as 4 to 6 hours after infection (Barbosa et al, 1994). Therefore, we favor a third possibility that lentiviral reverse transcription takes place mainly in the immediate vicinity of the nuclear membrane and the NPCs, within the viral capsid, although further experiments will be necessary to confirm this hypothesis. Cell mitosis does not provide an alternative pathway for the entry of triplex defective viral DNA into the host cell nucleus. The present Examples show that central triplex mutant vimses are strongly hampered in their replication capacity not only in non-dividing but also in dividing target cells. Conversely, insertion of a DNA triplex sequence in a HIV-1 based vector stimulates gene transduction in both dividing and non-dividing cells. This differs from the published phenotype of MA/Vpr mutant viruses, where a replication defect has been described exclusively in non-dividing cells (Bukrinsky et al, 1993b; Heinzinger et al, 1994; von Schwedler et al, 1994). Thus, MA/Vpr HIV mutants would behave like mitosis-dependent oncoviruses. One possible explanation for the different behavior of central triplex mutants is that these vimses are defective in a late step of the nuclear import process, consequently, the triplex deficient DNA molecules associate with the nuclear membrane. This close association persists during mitosis. The mutant viral DNA could be trapped within mitotic nuclear membrane vesicles, where it is unable to reach the cellular chromosomes, such as reported in the case of NLS-LacZ proteins (Bonnerot et al, 1987). Mutations in cellular NLS, inhibiting interactions with karyopherins, however, are known to induce cytoplasmic accumulation of the mutated protein (Kalderon et al, 1984; Lanford and Butel, 1984). Thus, mutations in the NLS sequences contribution to the karyophilic properties of HIV-1 PICs should induce cytoplasmic retention of the viral DNA, as previously suggested (Gulizia et al, 1994). It is therefore possible that PICs from MAVpr mutant vimses reach the cellular chromosomes following disruption of the nuclear membrane during mitosis. Nevertheless, there is as yet no direct experimental evidence for a mitosis dependent nuclear import pathway of lentiviral DNA genomes. As the published phenotype of MA/Vpr mutant vimses must be viewed with some caution, the same caution must be applied to the starting hypothesis that a nuclear import deficiency in lentiviruses should lead to a replication defect exclusively in non-dividing cells. Whether lentivimses can adopt a mitosis dependent nuclear import strategy, or whether the active nuclear import of lentiviral genomes occurs in both dividing and non-dividing cells, remains an open question. The present invention provides designs for lentiviral vectors. Since the infection of non- dividing target cells by lentivimses relies on their use of an active nuclear import pathway, it is important to maintain the lentiviral nuclear import determinants in derived vector constructs. Classical retroviral vector constructs are replacement vectors in which the entire viral coding sequences between the LTRs are deleted and replaced by the sequences of interest. In the case of lentiviral vectors, this classical strategy leads to the deletion of the central cis-active sequences cPPT and CTS. The important role of the triplex in HTV nuclear import implies that such replacement vector constructs are not optimal. Thus, insertion of the HIV DNA triplex into the HR GFP replacement vector (Naldini et al, 1996) enhanced its gene transduction efficiency by complementing a nuclear import defect of the HR vector genome to a rate of DNA import close to that of wild type HPV- 1. It is noteworthy that while HTV vectors lacking a DNA triplex were still capable of gene transduction (Figures 2C, 2D), the residual replication of vimses mutated in cPPT is absent or extremely low (Figures 6A, 6B). One possible explanation is that triplex independent nuclear import could occur in the case of a small vector genome (about 4 kb for HR-GFP), but the presence of a DNA triplex would be required for the import of the 9.7 kb native HTV-l genome. In fact, active, if relatively inefficient, nuclear import of DNA molecules as large as 3 to 4 kb has been reported (Hagstrom et al, 1997). The cis-active sequences responsible for formation of the DNA triplex are found at the center of all lentiviral genomes. This central position could have evolved on account of its stmctural implications for the conformation of PICs, in so far as symmetrical folding of the left and right arms of the linear DNA molecule around the triplex might be necessary for its efficient uptake through the NPCs (Figure 7B). If this is true for the vims, then a central position of the DNA triplex might also be required for the efficient nuclear import of vector genomes. EXAMPLES The invention will be further clarified by the following examples, which are intended to be purely exemplary of the invention. Example 1: Experimental Procedures. Cells MT4 cells are HTLV-1 transformed human CD4+ T cells that allow acute cytopathic HΓV-1 infection (Harada et al, 1985). H9 cells are less permissive to HTV but allow chronic production after infection. MT4 and H9 cells were maintained in RPMI 1640 medium supplemented with 10% fetal calf semm (FCS). Peripheral blood lymphocytes (PBLs) were obtained from healthy donors, stimulated with lμg/ml of phytohemagglutinin (Wellcome), and maintained in the presence of Interleukin-2 (10% lymphocult; Biotest Diagnostics). 293T cells were grown in DMEM medium supplemented with 10% FCS. P4 indicator cells are HTV infectible HeLa CD4+ cells carrying the LacZ gene under the control of the HTV-l LTR (Chameau et al, 1994). P4 cells are grown in DMEM medium supplemented with 10% FCS and 500μg/ml of G418. Collection and fractionation of cells. Cord Blood samples were collected with the informed consent of the mothers. CD34+ cells were purified as previously described (Robin, C. et α/.,1999) using miniMACS immunomagnetic bead separation system (Milteny Biotec). The purity of bead-separated CD34+ cells was over 75%. CD34+CD381o/- fractions were further purified by cell sorting with a FACS VantageTM equipped with an argon ion laser (Becton Dickinson), using murine monoclonal antibodies (MoAbs) directed against CD34-PE-Cy5 (Immunotech) and CD38-PE (Becton Dickinson). CD34+ cells were either frozen in fetal calf semm (FCS, Stem Cell) containing 10% DMSO (Sigma) or used immediately. DNA constructs Pro viral plasmids: Site directed mutagenesis was performed as previously described (Kunkel, 1985) in M13mpl8 carrying an EcoRI 1.1 kb insert (4684 to 5779) from the infectious molecular clone pLAI3. Mutagenic primers were as follows: cPPT-AG 5' pCAATTTTAAAAGAAGAGGGGGGATT 3' (SΕQ ID NO:l) cPPT-D: 5' pATTCATCCACAACTTCAAGCGCCGCGGTGGTATTGGGGGGTAC 3' (SΕQ ID NO:2). pcPPT-AG, pcPPT-D, pcPPT-25 and pCTS were constructed by cloning back the mutated EcoRI fragment into pLAI3. Vector plasmids: Vector plasmids were derived from HR'CMVLacZ (Naldini et al, 1996). The LacZ reporter gene was replaced by the ΕGFP gene (Clontech). ΕGFP gene was amplified by PCR using Pfu polymerase (Stratagene) from pΕGFP-Nl plasmid, adding BamΑl and Xhol restriction sites at the 5' and 3' ends respectively. PCR primers were as follows: Bam GFP: 5' CC GGATCC CCA CCG GTC GCC ACC 3' (SΕQ ID NO:3) Xho GFP: 5' CC CTCGAG CTA GAG TCG CGG CCG 3' (SΕQ ID NO:4). The HR GFP vector was constmcted by cloning back this PCR fragment into the BamΑl and Xhol sites of pHR'CMVLacZ, replacing the LacZ ORF with ΕGFP. TRIP ΔU3 CMV GFP and TRIP ΔU3 PL CMV GFP: First, a subclone containing a unique LTR was constructed and named pUC LTR. The Kpήl IXbal fragment of TRIP GFP encompassing its 3 'LTR was cloned into pUC18. Then the EcoRI site was destroyed by filling in, creating the vector pUC LTR RI-. Diverging PCR was performed on pUC LTR RI- with the aim of amplifying the whole plasmid except the promotor and the enhancer of the U3 sequence. The primers were: DU3-: 5' CGGAATTCGGATCCGCGGCCGCATCGATCTTGTCTTCGTTGGGAGTG 3' (SEQ ID NO:5) DU3+: 5'CGGAATTCAGCCGTCTCGAGAGATGCTGCATATAAGCAGC 3' (SEQ ID NO:6). The primers contain the restriction sites to be inserted instead of the U3 sequence including the EcoRI site present on each primer. The PCR product was digested with EcoRI then used to transform competent bacteria. The plasmid constructed thereby was named pLTR ΔU3 RI-. The polylinker inserted instead of theU3 sequence in pLTR ΔU3 RI- is: Clal-Notl-BamRl-Ecoia-Mlul-Xhol. The TRIPΔU3 PL GFP plasmid was constmcted by replacing the KpήllNhel fragment of TRIP GFP containing the 3' LTR with the KpnVXbal fragment of pLTR ΔU3 RI- (Mel and Xbal restriction products are compatible). Then the polylinker (PL) was deleted from pLTR ΔU3 RI- by digestion with ClaVXhol and filling in. The TRIP ΔU3 GFP plasmid was constructed by exchanging the KpήUNhel fragment of TRIP GFP with the KpnVXbal fragment of pLTR U3 ΔPL RI-. A 178 bp fragment of pLAI3 (4793 to 4971), encompassing cPPT and CTS, was amplified by PCR. Nαrl restriction sites were added in 5' of the primers with the aim of inserting this fragment into the unique Clal site of HR GFP: Νar TPJP+: 5' GTG GTC GGCGCC GAATTC ACA AAT GGC AGT ATT CAT CC 3' (SΕQ ID ΝO:7) Nar TRIP-: 5' GTC GTC GGCGCC CCA AAG TGG ATC TCT GCT GTC C 3' (SΕQ ID NO:8) Insertion of this triplex sequence in the correct orientation gave rise to the TRIP GFP plasmid vector, and TRIPinv GFP in the reverse orientation. Alternatively, the same triplex fragment was amplified from pcPPT-AG, pcPPT-D, pcPPT-225, and pCTS plasmids to generate vectors including the same mutations in the cPPT or in the CTS as the corresponding vimses. TRIP ΕFlα GFP and TPJP ΔU3 ΕFlα GFP: The CMV promotor of TRIP GFP was replaced by the ΕFlα promotor. The triplex sequence and the ΕFlα promotor were first amplified separately, with overlapping primers. The triplex sequence was amplified with the primers Nar TRIP+ and Mlu TRIP- on the matrix pLai and the EFlα promotor was amplified on the matrix pEFpgkneo with the primers Mlu EF1+ and Bam EF1-. Nar TRIP+: 5'GTC GTC GGCGCC GAATTC ACA AAT GGC AGT ATT CAT CC 3' (SEQ ID NO:9) MluTRIP-: 5'AGC CTC ACG ACGCGT AT CAG CCA AAG TGG ATC TCT GCT G 3' (SEQ ID NO: 10) Mlu EF1+: 5' CTG AT ACGCGT CGT GAG GCT CCG GTG 3' (SEQ ID NO: 11) Bam EF1-: 5' CG GGATCC TGT GTT CTG GCG GCA AAC3' (SEQ ID NO: 12) Then a second round of PCR was performed on a mixture of the first two PCR products, using the external primers Nar TRIP+ and Bam EF1-. The triplex sequence and the EFlα promotor stuck together by this technique. Plasmid TRIP EFlα GFP was constmcted by replacing the EcoRT/Sα/MHI fragment of TRIP GFP containing the triplex sequence and the CMV promotor by the PCR product TRIP ΕFlα digested with EcoKUBamHl (the Nar TRIP-i- primer posses a EcoRI site in its 5' end). To construct the TRIP ΔU3 ΕFlα GFP plasmid, the EcoKUBamHl fragment of TRIP ΕFlα GFP containing the triplex sequence and the ΕFlα promotor was inserted in TRIP ΔU3 GFP instead of the EcoRI/itømHI fragment containing the triplex sequence and the CMV promotor. Virus and vector production For the investigations reported in Figures 1-6, vimses were produced by transient transfection of HeLa cells by the calcium phosphate co-precipitation technique. Vector particles were produced by transient co-transfection of 293T by the vector plasmid, and encapsidation plasmid (p8.2) and a VSV envelope expression plasmid (pHCMV-G, (Yee et al, 1994)), as previously described (Naldini et al, 1996). All vims and vector supernatants were treated with DNasel (lμg/ml in the presence of lμM MgCl 2 ) for 15 minutes at 37°C. Lentiviral vector particle production For the investigations reported in Figures 7-9, vimses were produced by transient co-transfection of 293T by the vector plasmid, an encapsidation plasmid (p8.2) and a VSV envelope expression plasmid (pHCMV-G, (Yee et al, 1994)), as previously described (Naldini et al, 1996). All vims and vector supernatants were treated with DNasel (lμg/ml in the presence of lμM MgC12) for 15 minutes at 37°C . Virus and vector titrations For the investigations reported in Figures 1-6, one cycle titration of vimses were performed in triplicate by infection of P4 cells plated in 96 well plates, with equivalent amounts of particles (lng of p24 viral antigen per well), in the presence of 20μM of DEAE-dextran. The protease inhibitor Saquinavir (Roche), was added (lμM) throughout the experiment, to restrict the analysis to a single cycle of infection. Cell mitosis was inhibited by aphicolin treatment (8μM), the day prior to infection. The β-Galactosidase activity was measured 48 hours after infection using a chemiluminescent β-Gal reporter gene assay (Boehringer). HeLa cells were infected in triplicate with equivalent amounts of vector particles (5ng P24 per well). At 48 hours post transduction, the medium was replaced by 200 μl of TNB (Tris 50 mM pH 7.5, NaCl 150 mM) and fluorescence of living cells was quantitated using a microplate fluorimeter (Victor 2 , Wallac) and EGFP adapted filters (excitation: 485 nm, emission: 520 nm). Transduction protocol For the investigations reported in Figures 7-9, 24- or 96- well tissue culture plates were coated with fibronectin (Bio-Whittaker Europe) according to the manufacturers instructions. Human CD34+ populations were plated, immediately after purification or thawing, at 2 to 3 X 10 5 cells/ml in semm free medium (IMDM containing 11.5μM a-thioglycerol , 1.5% BSA (both from Sigma), sonicated lipids and iron-saturated human transferrin) or a-MEM containing 10% FCS in presence of 4μg/ml of polybrene (Sigma) and 4 recombinant (r) human (hu) cytokines : rhu-Stem Cell Factor (SCF, lOOng/ml, provided by Amgen), Flt3-Ligand (FL,100ng/ml, Diaclone), IL-3 (60ng/ml, Sandoz), and pegylated- (PEG-) rhu- Megacaryocyte Growth and Differentiation Factor (MGDF) (lOng ml, Amgen), and concentrated lentiviral vims at the concentration of 100 ng of viral P24/ml during 24 hours. Cells were then washed and cultured in lympho-myeloid conditions (in culture tissue culture plates precoated with MS 5 cells in RPMI supplemented with 10% human serum, 5% FCS and the following 7 cytokines: rhu-SCF (50ng/ml), rhu-FL (50ng/ml), PEG- rhu-MGDF (50ng/ml), rhu-IL-3 (lOng/ml), rhu-IL-2 (5 ng/ml), rhu-IL-15 (lOng/ml), and rhu IL-7 (20ng/ml) (the three EL- being from Diaclone) for 48 hours. Then, expression of eGFP in the CD34+ cell fraction was evaluated using a CD34-PE-Cy5 MoAb (Immunotech). Analysis was done on a FACS Scan using the Cellquest software (Becton Dickinson). Clonogenic and long term culture (LTC) assays Clonogenic progenitors from human fresh CB cells were assayed in 0.8% methylcellulose containing 30% FCS, 1% deionized BSA, and 10-4M 2-mercaptoethanol, in the presence of 50ng/ml rhu-SCF, lOng/ml rhu-GCSF (Amgen), 2ng/ml rhu-IL3, and 2U/ml rhu-EPO (Amersham). Bone marrow (BM) cells from engrafted NOD-SCID mice were plated in the presence of rhu-SCF, -IL-3, -EPO, and-GM-CSF (10 ng/ml) as described (Pflumio, F. et α/.,1996). Progenitors were scored on day 14-16 according to criteria already described (Croisille L. et α/,,1994) and EGFP expression observed by fluorescent microscopy using a Nikon Eclipse TE300 microscope. Long Term Culture (LTC) was performed as previously described (Issaad C. et a/., 1993) either in limiting dilution in 96-wells plates using the FACS vantage equipped with an ACDU (BD) or in bulk culture in 24-wells plates containing a confluent layer of the murine stromal cell line MS5. After 5 to 10 weeks, adherent and non-adherent cells were harvested and plated for the clonogenic assay. For both clonogenic and LTC-IC assays, colonies were picked individually and frozen before PCR analysis. PCR analysis PCR analysis was performed on genomic DNA obtained either from colonies derived from clonogenic progenitors or from clones grown in lympho-myeloid cultures. Cells were lysed and proteins were digested in 20μl of buffer containing proteinase K (lOμg/ml), KC1 (50mM), Tris-HCl (lOmM, pH 8.3), MgCl 2 (2.5mM), gelatin (O.lmg/ml), NP40 (0.45%), and Tween 20 (0.45%). Amplification of genomic DNA was performed with the sense primer 5' CCCTCGAGCTAGAGTCGCGGCCG 3' (SEQ ID NO: 13) and the antisense primer 5' CCGGATCCCCACCGGTCGCCACC 3' (SEQ ID NO: 14) at the annealing temperature of 62°C. The amplification resulted in an 800 bp product. Viral and vector DNA analysis P4 or MT4 cells were infected at a high multiplicity of vimses (150 ng of p24 per 10 6 cells) or transduced by vectors (25 ng of p24 per 10 6 cells), in the presence of 20μg/ml of DEAE- dextran in the case of P4 cells. DNA from infected or transduced cells was extracted at various times, restricted, and analyzed by Southern blot. In all cases, contaminating bacterial plasmid DNA was removed from the analysis by Dpήl digestion. DNA from infected cells was digested by Mscl and Xhol and DNA from transduced cells by EcoNI, Avail and Xhol. After electrophoresis and transfer of 10 μg of digested DNA, membranes were hybridized with random primed [ 32 P]-labeled DNA probes (Rediprime π, Amersham). Vims specific DNA probe was amplified by PCR from pLAI3 plasmid template using the following primers: 5Msc: 5' AGA AGA AAT GAT GAC AGC ATG 3' (SΕQ ID NO: 15) 3Msc: 5' TGC CAG TTC TAG CTC TG 3' (SΕQ ID NO: 16). The resulting 1680 bp DNA fragment (from position 1818 to 3498 of pLAI3) overlaps the Mscl restriction site at position 2655 of viral genomes. Vector probe was synthesized by PCR on pTRIP GFP with the primers: 5ΕcoNI: 5' CAG GGA CTT GAA AGC GAA AG 3* (SEQ ID NO: 17) 3EcoNI: 5' GCT TGT GTA ATT GTT AAT TTC TCT GTC 3' (SEQ ID NO: 18) The vector probe is a 1027bp fragment (from position 649 to 1676 of pTRIP GFP) and overlaps the EcoNI site at position 1156 of vector genomes. To assay the amount of retrotranscribed DNA from wild type and PPT-AG and cPPT-D vimses, a similar protocol was followed except that the DNA extracted at 12 hours post-infection was restricted by Mscl and Dpή . The probe used for hybridization was the Mscl 1.9 kb internal fragment from pLAI3. Hybridization signals were quantitated using a phosphorimager (Molecular Dynamics) and the ImageQuant software. In situ hybridization P4 cells were infected at a high multiplicity (2μg of p24 antigen of each virus per 10 6 cells), in the presence of 20 μg/ml DEAE dextran. At 24 hours post- infection, cells were trypsinized, extensively washed (in order to remove viral particles adsorbed in the plasma membrane), and re-plated on glass cover slides in 24 well plates. Cells were grown for a further 48 hours and fixed in 4% PFA/PBS for 20 minutes at room temperature. Cells were washed in PBS and permeabilized by 0.5% Triton/0.5% Saponin in PBS, for 5 minutes at room temperature. Dehydrated samples were treated with RNase A (200μg/ml in 2X SSC), one hour at 37°C and by proteinase K (6 μg/ml in PBS), about 5 minutes. Samples were denatured by incubation in 70% deionized formamide/2X SSC for 2 minutes at 70°C followed by 30% deionized formamide/2X SSC for 2 minutes at 70°C. Hybridizations were performed overnight at 37°C using a nick translated biotynilated pLAI3 plasmid (50% deionized formamide, 10%) dextran sulfate, 10 μg/ml Salmon sperm DNA, 0.1 % Tween 20 in 2X SSC). Samples were extensively washed (serial washing in 2X SSC/50% formamide at room temperature and then at 50°C). Detection of hybridized probes was performed using the Tyramid-Streptavidin TSA-Direct kit (NEN) according to the manufacturer's instructions. Example 2: Central initiation of reverse transcription is an essential step of the HTV-l replicative cycle In a previous work, we showed that conservative mutations in the cPPT and CTS sequences severely impaired vims replication (Chameau et al, 1992; Hungnes et al, 1992). A central initiation mutant vims (cPPT-225) and a termination mutant vims (CTS) showed respectively four fold and ten fold decreased infectivity in one round titration experiments. In order to inactivate the function of the cPPT, semi-conservative mutations were introduced in the overlapping integrase coding region. In the mutant vims cPPT-D, the lysine to arginine change at position 188 allowed the introduction of a total of 10 mutations into the 19 nucleotide sequence of the PPT primer (Figure 1A) (Huber and Richardson, 1990). The effect of this amino acid change on virus replication was checked by construction of the control cPPT-AG mutant vims, in which a single mutation from purine to purine, respecting the polypurine nature of cPPT, induced the same amino acid change. The presence of a DNA triplex in retrotranscribed wild type and cPPT-AG vimses DNA, and its absence from cPPT-D vims DNA, was confirmed by SI nuclease cleavage of Hirt DNA from infected cells as previously described (Harris et al, 1981; Chameau and Clavel, 1991). Vims infectivity was first evaluated in classical kinetic replication experiments in cell cultures. PHA stimulated peripheral blood lymphocytes (PBLs) and MT4 cells were infected with equal numbers of viral particles, normalized according to the capsid protein (p24) content of the viral supernatants, and reverse transcriptase activity was followed over time in the culture supernatants (Figure IB). Growth curves of the wild type HIV-1 LAI and cPPT-AG control vimses were similar in both cell systems. The fact that the Kl 88R mutation occurs naturally in some HPV-1 isolates, already suggested that it has little or no effect on the integrase and PPT functions. In contrast, when PBLs were infected with the cPPT-D mutant vims, no replication was detected during the 15 days of culture. The same was true for MT4 cells, despite their high susceptibility to HIV infection. The cPPT-D mutant vims was also non infectious in immortalized cell lines such as H9 or CEM (data not shown). Vims infectivity was then quantitatively analyzed by titrations based on a single round of replication (Figure 1C). P4 indicator cells (HeLa CD4 LTR-LacZ) (Chameau et al, 1994) were infected with equivalent numbers of vims particles of the different vimses. These one cycle titrations confirmed the almost complete loss of infectivity of the cPPT-D mutant vims. In P4 cells, infectivity of the cPPT-AG control was identical to that of the wild type virus, whereas infectivity of the cPPT-D mutant was strongly reduced, to levels close to background. The same results were obtained in aphidicolin treated, non dividing P4 cells (Figure 1C, right panel). These findings strongly suggest that the central initiation of reverse transcription is necessary for HIV replication in non dividing as well as in proliferating cells. Example 3: Virus production is not affected by mutations in cPPT We checked that the different mutations introduced into the cPPT-AG and cPPT-D plasmid provimses did not affect the late steps of the replicative cycle. Vims production was quantified, according to the P24 content of the supernatants, after transient transfections of HeLa cells by proviral plasmids. The production of the cPPT mutant vimses was found not to be significantly different from that of the wild type vims (Figure 2A). Hence the mutation K188R does not affect the late phase of HTV-l replication. Therefore, the defective step involved in the phenotype of the cPPT-D mutant vims must precede the expression of viral DNA and belong to the early phase of the HIV replicative cycle. Example 4: Mutations in the cPPTdo not affect the rate of reverse transcription of HTV-l genome The effect of mutations in cPPT on viral DNA synthesis was evaluated by quantifying the DNA synthesized in a single round of retro transcription (Figure 2B). An intemal Mscl restriction fragment from the viral DNA of infected cells was detected by Southern blotting and quantitated, using the corresponding Mscl DNA fragment as a probe. Since the internal Mscl fragment is common to the integrated proviral DNA and the unintegrated linear and circular molecules, its quantitation reflects the total amount of viral DNA, irrespective of its integrated or unintegrated state. To limit the analysis to the first cycle of reverse transcription, DNA from infected P4 cells was harvested 12 hours after infection, before initiation of a second round of infection. The total amount of DNA retrotranscribed in a single cycle of reverse transcription was the same after infection with the cPPT-D mutant, the cPPT-AG control or the wild-type vims. These experiments showed that, whereas mutations in cPPT abolish vims replication, they do not affect the rate of DNA synthesis. The replicative defect of cPPT mutant vimses implicates a step subsequent to viral DNA synthesis. Example 5: Lack of a central DNA triplex does not affect the in vitro integration of HTV-l PICs The in vitro integration ability of PICs from wild-type HTV-l and central initiation mutants was compared (Figure 2C) using a quantitative in vitro integration assay as described by Famet (Famet and Haseltine, 1990), with minor modifications. Since HIV-1 replication complexes reside only transiently in the cytoplasm of freshly infected cells (Barbosa et al, 1994), the preparation of sufficient amounts of HIV PICs requires massive infection and cellular fractionation within 4 to 6 hours. This was achieved by co-culture of H9 cells chronically infected by either wild type or cPPT-225 vims and uninfected HUT 78 target cells. The cPPT- 0225 mutant vims (Figure 1A) was chosen for these experiments instead of the non infectious cPPT-D, which is unable to establish a chronic infection. The residual infectivity of cPPT-225 mutant vims is low but sufficient to allow it to slowly propagate in cell cultures (Chameau et al, 1992). HIV PICs were isolated from the cytoplasm of infected cells and incubated in the presence of a linearized Bluescript plasmid target DNA. Integration was revealed by the presence of a 12.7 kb fragment, reactive to the HTV-l probe, corresponding to the expected size of the 9.7 kb linear HTV genome integrated into the 3 kb target DNA. The amount of linear DNA integrated into the plasmid DNA did not differ between the wild type and cPPT 225 mutant vims (Figure 2C). Hence HIV-1 PICs from the cPPT mutant retained their full ability to integrate in vitro. The defective replication step of central DNA triplex mutant vimses must lie after reverse transcription but before integration of their linear HIV genome into the host cell chromatin. Example 6: Impaired nuclear import of central DNA triplex mutant viruses The foregoing experiments suggested that the replicative defect of central DNA triplex mutant vimses was related to the access of HIV PICs to chromatin of the cellular target. Hence we tested the hypothesis of a nuclear import defect of DNA from cPPT mutant vimses. Studies on the nuclear import of HIV-1 PICs are hampered by lack of a quantitative and reproducible assay for nuclear import at the level of the viral DNA. Once retrotranscribed in the cytoplasm, the retroviral linear DNA is imported into the nucleus where it either integrates or circularizes. Unintegrated retroviral DNA circles, containing one or two LTRs, are found exclusively within the nucleus, and thus represent convenient markers of viral DNA nuclear import. To assess HIV DNA nuclear import, previous studies used PCR amplification of two unintegrated LTR DNA circles. However, as reported herein and by Barbosa et al, (1994), because two LTR circles represent a minute fraction of the HIV DNA in infected cells, their detection is very sensitive to minor alterations of cell physiology or vims infectivity. Therefore, we designed a novel assay which permits a quantitative follow-up by Southern blot of the synthesis, circularization, and integration of HTV DNA. Briefly, DNA from infected cells is prepared at various time points and digested with Mscl, a restriction enzyme which cuts the HPV-1 genome twice. Using a PCR generated DNA probe exactly overlapping the 5' Mscl site, several specific bands are revealed (Figure 3A). The internal 1.9 kb Mscl fragment is common to all viral DNA species irrespective of their integrated or unintegrated state and quantitation of this band indicates the total amount of viral DNA in infected cells. A 2.6 kb band corresponds to the distal 5' Mscl fragment of unintegrated linear HIV-DNA. TO minimize transfer bias due to the large size of DNA circles specific fragments, the DNA is further cut with Xhol. One and two LTR circular DNA then appears at 2.8 and 3.4 kb bands respectively. Since the DNA probe exactly overlaps the 5' Mscl site, the intensity of each band is directly proportional to the quantity of the corresponding viral DNA species. The amount of integrated proviral DNA is calculated by subtracting from the total amount of viral DNA the signals of unintegrated linear and circular viral DNAs. A parallel quantitation of the same infected cell population was performed after a Hirt fractionation to separate low molecular weight unintegrated viral DNA from high molecular weight integrated proviral DNA. This gave rise to identical results, thus validating the one step subtractive calculation. As indicated by the kinetics of accumulation of total viral DNA (1.9 kb internal fragment), the synthesis of viral DNA proceeded for 24 to 48 hours after infection, reflecting an asynchronous infection process. The amounts of total viral DNA from cPPT-AG and cPPT-D mutant vimses were similar to those of wild-type HTV-l. As previously, the mutations in cPPT did not influence the rate of DNA synthesis. Detectable amounts of full length unintegrated linear DNA were present in cells as early as 6 hours after infection (Figure 3B). Integrated pro vimses and DNA circles were first detected 12 hours after infection. Integration and circularization proceeded to completion over a further 36 hours. On completion of one cycle of infection, in the case of the wild type vims, about 55% of the viral DNA had integrated into the host cell DNA, about 35%> had circularized into one LTR circle, and a small fraction of less than 10 % remained in the form of stable unintegrated linear DNA (Figure 3C). Notably, two LTR circular DNA, although detectable at 48 hours after infection, was present only in trace amounts. DNA from the cPPT-AG control vims was processed in a very similar manner to DNA from the wild type vims. In the case of the cPPT-D mutant vims, a marked alteration in the pattern of intracellular viral DNA was evident, with a clear and persistent accumulation of unintegrated linear molecules. At 48 hours after infection, only very small amounts of one LTR circular DNA and integrated proviral DNA had been generated and more than 90 % of the cPPT-D mutant DNA remained blocked in the unintegrated linear form (Figure 3C). A nuclear import defect is expected to decrease the proportion of nuclear viral DNA species (integrated provimses and one and two LTR circles) and to concomitantly increase the proportion of untranslocated linear DNA molecules. Thus, the intracellular DNA profile of cPPT-D mutant vims strongly suggests a defect of viral DNA nuclear import. Example 7: Linear DNA from central DNA triplex mutant viruses accumulates at the vicinity of the nuclear membrane To further characterize the nuclear import defect of central DNA triplex mutant vimses, we addressed the questions of whether the mutated linear DNA molecules accumulate in a particular subcellular compartment. The nuclear import process can be divided into two main phases, docking of the nuclear component to the nuclear membrane and its translocation through the nuclear pore complex (NPC). We first conducted classical nuclei/cytoplasm fractionation of infected cells, followed by southern blot detection of viral DNA. The totality of viral DNA of all vimses was associated with the nuclei of infected P4 cells, 24 hours after infection (Figure 4A), suggesting that docking of HTV DNA to the nuclear membrane was not affected by the lack of a central DNA triplex. To confirm that central triplex deficient DNA molecules do accumulate at the nuclear membrane, we used fluorescent in situ hybridization (FISH) to directly visualize the intracellular location of HIV genomes. P4 cells were infected at a high multiplicity in one cycle conditions, hybridized with a full length HTV-l genome probe, and observed by deconvolution microscopy. Specific dots were found predominantly within the nucleus in the case of the wild type and cPPT- AG control vimses (Figure 4B). Since FISH cannot distinguish between the different HIV DNA species, these intranuclear viral DNA molecules could have been integrated provimses or unintegrated DNA circles. Some rare genomes were associated with the nuclear membrane and probably represented the residual linear DNA detected by Southern blotting at the same time after infection. Other dots associated with the plasma membrane most likely derived from membrane adsorbed defective particles containing partially retrotranscribed genomes (Lori et al, 1992). In contrast, HIV genomes were predominantly localized at the nuclear membrane and almost completely absent from the nucleus in the case of the cPPT-D mutant vims. As the Southern blot DNA profile indicated that practically all cPPT-D DNA was blocked in the linear form, we can assume that these HIV genomes associated with the nuclear membrane were unintegrated linear DNA molecules. This direct visualization of viral DNA molecules in infected cells confirmed the association of the viral DNA of central triplex mutants with the nuclear membrane. Our FISH experiments suggest that cPPT mutant vimses are defective in the translocation of their genome through the NPCs. Nevertheless, the clear demonstration of a translocation defect, by localization of mutant viral DNA at the cytoplasmic side of NPCs, is not accessible through FISH experiments. Altogether, we conclude from these results that the central DNA triplex of HIV-1, created by central initiation and termination steps during reverse transcription, is important for HIV-1 PICs to enter the host cell nucleus. In the absence of DNA triplex, viral DNA nuclear import is severely impaired at a stage immediately preceding or during the translocation of HIV-1 DNA through the nuclear pore. Example 8: Impact of the central DNA triplex on gene transduction by an HTV-l based vector system Having identified the central DNA triplex as a key determinant for the nuclear import of HIV-1, we tested the effect of inserting the central cis-active sequences of HIV-1 into the previously described HR HIV-1 vector (Naldini et al, 1996) (Figure 5 A). To monitor gene transduction, a gene encoding the green fluorescent protein (GFP) was further inserted. The vector containing a DNA triplex sequence was called TRIP GFP. Controls included similar constructs with mutated cPPT or CTS and a wild type central region inserted in reverse, nonfunctional orientation (TRIPinv GFP). The presence of a DNA triplex in retrotranscribed TRIP GFP vector DNA, and its absence from HR GFP, TRIPinv GFP, and from vectors containing a mutated version of the central triplex sequence, was confirmed by SI nuclease cleavage of Hirt DNA isolated from transduced cells. The number of vector particles produced by transient transfection was normalized prior to transduction according to the levels of capsid protein (p24), reverse transcriptase activity, and the quantity of genomic vector RNA in transfected cell supernatants. Similar production of the various vectors was obtained, with a linear correlation between the three normalization criteria (data not shown). Hence insertion of the central region of the HIV-1 genome into the HR vector did not influence the rate of genomic RNA encapsidation. Dividing or non-dividing (aphidicolin treated) HeLa cells, were transduced with equivalent numbers of HR-GFP or TP P-GFP vector particles and GFP expression was monitored 48 hours later by fluorescence quantitation. Pseudotransduction of GFP activity due to the direct delivery of GFP proteins to target cells by the fusing vector particles was calculated from the transduction of cells treated with an HIV-1 RT inhibitor (1 μM Nevirapin, Boehringer Ingelheim) and this background subtracted from the fluorescence signal. Non-specific fluorescence due to secondary transfection caused by calcium/phosphate DNA co-precipitate in the vector supernatants was eliminated by treating the vector stock with DNasel prior to transduction. Under these conditions, the presence of the triplex sequence in the HIV vector increased GFP transduction in HeLa cells by more than ten fold (Figure 5B). A similar enhancement of gene transduction was observed in other target cell lines such as MT4 or 293T (data not shown). This effect was lost if the triplex sequence was inserted in the reverse orientation (Figure 5B) or mutated in the cPPT (not shown). Example 9: The presence of a DNA triplex in HIV-1 vectors increases the rate of vector genome nuclear import to wild type levels It was then of interest to determine whether the increase in GFP fluorescence induced by insertion of a triplex sequence in the HTV vector was due to its effect on the nuclear import of vector DNA. To address this question, we adapted our quantitative Southern blot assay for intracellular viral DNA to the vector system. DNA from vector transduced cells was digested with EcoNI and Avail to produce an internal 0.8 kb fragment, and with Xhol. Using a PCR generated DNA probe exactly overlapping the EcoNI site, signals specific for the unintegrated linear vector genome, and for one and two LTR DNA circles were expected at 1.2 kb, 1.4 kb, and 2 kb respectively. The processing of vector DNA was analyzed at various time points after transduction of HeLa cells. The total quantity of vector DNA synthesized in transduced cells was comparable for vectors containing or lacking the DNA triplex (Figure 5C). Once again, insertion of the cPPT and CTS sequences, in either orientation, into the HR vector did not influence the rate of reverse transcription of its genome. After phosphorimage quantitation, we found the intracellular fate of DNA from the HR-GFP and TPJPinv-GFP vectors (Figure 5D) to closely resemble that of DNA from central triplex defective vimses and the fate of DNA from the TRIP-GFP vector to follow that of DNA from the wild type HTV-l LAI vims (Figure 3C). A defect of DNA nuclear import was evident in the case of the HR-GFP and TPJPinv- GFP vectors. The intracellular fate of DNA from these vectors was characterized by a strong accumulation of unintegrated linear molecules, together with small amounts of integrated provims and one and two LTR circles. This DNA profile was strongly reminiscent of that of the cPPT-D mutant vims. On completion of the processing of vector DNA in transduced cells, 70 to 80% of the DNA from HR-GFP and TPJPinv-GFP constructs remained in the form of unintegrated linear molecules, while only 10 to 15%> were present as unintegrated on LTR circles and 5 to 10% as integrated provimses. This low but detectable amount of integrated vector DNA would account for the gene transduction obtained using HR-GFP or TPJPinv-GFP vectors. This quantitative assay also showed that insertion of the triplex sequence of HIV DNA into the HR vector in the coπect orientation complemented its nuclear import deficiency to wild- type levels. The final state of TPJP-GFP DNA in transduced cells was similar to that observed with wild type HIV-1 vims: 50% or more of the vector DNA integrated the chromatin of the target cell, an important fraction circularized and a few molecules remained as unintegrated linear DNA (compare Figure 5D and 3C). By contrast, insertion of the triplex sequence into the HR vector upstream of the internal CMV promoter did not influence GFP expression at the transcriptional level. This was checked by transfection of HeLa cells with pHR-GFP, pTPJP- GFP, and pTPJPinv-GFP plasmids and fluorescence quantitation (data not shown). It may be inferred from these results that the increase in GFP transduction obtained with the TPJP-GFP vector is entirely imputable to strong stimulation of its nuclear import by the presence of the triplex. This finding again emphasizes the important role of the HIV DNA triplex in the nuclear import of viral and vector DNA. Example 10: HTV-l vectors containing the DNA triplex sequence allow an efficient gene transfer in hematopoietic stem cells Figure 7A and 7B illustrate the results of two successful transduction experiments using CD34+ human cord blood cells. They show that after a short 24 or 60 hour transduction protocol, respectively 71.5 %> and more than 90% of the CD34+ cells strongly express the GFP reporter protein. This expression reflects stable transduction of the cells since viral integration was confirmed, at least in the same proportions, by PCR assays on DNA extracted from progenitor-derived cell colonies harvested 14 days after having seeded cells in clonogenic progenitor assay. Identical results have been obtained using freshly purified CD34+ cells or the same CD34+ cells that have been frozen after purification and thawed several weeks later for the transduction experiment. We also had comparable transduction efficiencies using another hematopoietic stem cell source, peripheral blood mobilized stem cells (PBMC), collected by cytapheresis after cytokine stimulation. CD34+ PBMC were also transduced either immediately after purification or after a freezing/thawing step with identical results. Using long term culture (LTC) and NOD-SCH ) repopulating assays we have shown that we have transduced cells having multiple lympho-myeloid potentialities and the ability to repopulate NOD-SCID mouse bone marrow 4 moths after graft. These functional assays represent the ultimate experiments available, at the moment, to assess human hematopoietic stem cell function. Example 11: The presence of the DNA triplex sequence in lentiviral vector constructs strongly stimulates gene transfer into hematopoietic stem cells The dose/response experiment reported in Figure 8 (representative of 3 experiments) was designed to compare transduction efficiency in human hematopoietic stem cells of HIV-1 vectors including or lacking the DNA triplex sequence. We first plotted (A) the percentage of CD34+eGFP+ cells obtained as a function of the vector concentration used for transduction. We observe that whatever was the dose of vector, the TPJP+ vector was more efficient than the TPJP- one, with respectively a mean of 40+19 % and 15.4±12.5% of CD34+ cells being eGFP+ for 500ng viral P24 /ml of each vector (n=3 exp). A 4-6 fold increase in the final percentage of GFP+ cells was obtained after transduction by the TPJP-GFP vector when compared to results obtained after transduction with HR-GFP vector. The difference in efficiency between the two vectors is also highlighted when the mean of fluorescence intensity is plotted function of the dose of vims. A plateau is reached for the TPJP- (HR-GFP) vector at dose of 100 ng P24/ml whereas fluorescence intensity in transduced cells increases with the dose of TPJP+vector. This could reflect the limitation in nuclear import of the pre-integrative forms of the TRIP- vector and the increasing number of integrated copies per cell after transduction with increasing doses of the TPJP-GFP vector. The third plot integrates both aspects and shows the resulting effect of the DNA triplex sequence on GFP fluorescence activity in human HSC. As shown, the presence of the DNA triplex sequence in the HTV vector induce an increased GFP production in HSC by a factor of more than a ten fold. Example 12: A fraction of integrated copies of HIV vector genomes remains silent in human hematopoietic transduced cells It is possible that inactivation of the integrated transgene can occur. The transfer efficiency was always better when it is evaluated by the percentage of transduced progenitor-derived cell colonies determined by PCR assay for the integrated lentiviral vector rather than when it is evaluated by the percentage of CD34+/eGFP+ cells determined by FACS analysis 48 hours after the end of the transduction protocol. This reflects the occurrence of transcriptionally inactive provimses either due to their integration site or to the progressive and random inactivation of the provims while the cells proliferate and differentiate. We have observed colonies derived from a single myeloid clonogenic progenitor that some of the subclones could be GFP bright whereas others were negative, reflecting the random transcriptional inactivation of the integrated transgene in this phenotypically homogenous clonal progeny. Example 13: HTV vector containing a deleted version of the U3 region of the LTR and an internal EFla are more potent systems for the transduction of hematopoietic stem cells In Figure 9, we compared the ability of various HIV-1 derived vectors, including the DNA triplex sequence, to transduce human cord blood CD34+ cells. The effect of deletion of most of the U3 region of the 3' LTR on GFP transduction and expression in CD34+ cells was analyzed. The comparison of vectors containing an intact HIV-1 LTR or a U3 deleted version was conducted in the context of a CMV internal promoter or an EFlα internal promoter (Kim et al, 1990) to drive the expression of the GFP reporter gene. All transduction experiments were conducted using the same concentration of vector particles (500 ng P24/ml) after normalization of vector stocks using a commercially available ELISA assay for the P24 (Capsid protein) HPV-1 antigen (Dupont). Flow cytometry analysis (FACS) were performed either at 48 hours (Figure 9A) or 120 hours (Figure 9B) after the 24 hour transduction period. Interestingly, deletion of the U3 region of the LTR in HIV-1 vectors induced a slight increase in the percentage of GFP positive cells in all cases. This increase was modest when analyzed at 48 hours. At this time, a pseudotransduction mechanism might be responsible for a fraction of the GFP positive cells. Pseudotransduction is the direct delivery of GFP proteins to target cells by the fusing retroviral vector particle, without the need for an actual integration of the retroviral vector genome. The increase in the percentage of GFP positive after transduction by the U3 deleted versions of the HTV-l vectors became more evident at 120 hours after transduction (Figure 9B). At this time, up to a two fold increase in the percentage of GFP positive cells is seen after transduction by the TRIP ΔU3- EFlα -GFP vector when compared to the results obtained with the equivalent vector but containing an intact HIV-1 LTR. More importantly, deletion of the U3 region of the HIV-1 LTR in the HΓV-1 triplex vectors induced a better expression of the GFP reporter protein. The mean of fluorescence intensity in transduced human HSC when analyzed by FACS was always superior of a three to five fold factor in the case of the U3 deleted versions than with vectors containing an intact HTV-l LTR. This benefit of GFP expression was observed whether the CMV or the EFlα promoters where used as an internal promoter in the HIV-1 vector construct. The molecular mechanism explaining this enhanced expression of GFP proteins in transduced cells is not known. Some sequence in the HTV-l LTR may negatively influence the expression driven by the internal promoter. Alternatively, a basal transcription initiated at the HIV-1 LTR may interfere with the initiation of transcription at the internal promoter. This study also shows that the EFlα promoter is a better promoter in human HSC than the CMV promoter. In Figure 9B, the mean of GFP fluorescence intensity is three to five times better in the case of the EFlα promoter than in the case of the CMV promoter. It will be apparent to those skilled in the art that various modifications and variations can be made in the practice of the present invention without departing from the scope or spirit of the invention. 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Claims: WHAT IS CLAIMED IS: 1. An isolated or purified nucleic acid comprising at least one copy of the cPPT and CTS cis-acting regions of a retro vims, wherein the cPPT and CTS regions induce a three-stranded DNA stmcture. 2. The nucleic acid of claim 1, wherein the retrovirus is a lentivims. 3. The nucleic acid of claim 2, wherein the retrovirus is a human immunodeficiency vims (HTV). 4. The nucleic acid of claim 3, wherein the HIV is HIV-1 or HIV-2. 5. The nucleic acid of claim 2, wherein the lentivims is VISNA, EIAV, FTV, or CAEV. 6. The nucleic acid of claim 1, comprising a single copy of the cPPT and CTS regions of the retro vims. 7. The nucleic acid of claim 1 , wherein the three-stranded structure contains the cPPT and CTS cis-acting sequences of the retrovirus. 8. The nucleic acid of claim 1, further comprising a heterologous nucleic acid sequence. 9. The nucleic acid of claim 8, wherein the heterologous nucleic acid sequence encodes a peptide, polypeptide, or protein. 10. The nucleic acid of claim 9, wherein the heterologous nucleic acid sequence encodes a therapeutic protein. 11. A vector comprising the nucleic acid of claim 1. 12. The vector of claim 11, which is an expression vector, a shuttle vector, an integration vector, a transposon, or a retrotransposon. 13. The vector of claim 1 1 , which is pTRIP ΔU3 EF 1 α GFP. 14. A recombinant cell comprising the vector of claim 11. 15. A vims comprising the nucleic acid of claim 1. 16. The vims of claim 15 which is a retrovirus. 17. The retrovirus of claim 16, which is a lentivirus. 18. A recombinant cell comprising the nucleic acid of claim 1. 19. The recombinant cell of claim 18, wherein the cell is a HeLa cell or a hematopoietic cell. 20. The recombinant cell of claim 19, which is a hematopoietic stem cell. 21. A process for inserting a nucleic acid of interest into the nucleus of a target cell, said method comprising exposing an isolated or purified nucleic acid comprising at least one copy of the cPPT and CTS cis-acting regions of a retro vims, wherein the cPPT and CTS regions induce a three-stranded DNA stmcture, to a target cell under conditions that permit uptake of the nucleic acid of interest into the target cell. 22. The process of claim 21, wherein the efficiency of insertion of the nucleic acid of interest into the target cell nucleus is 30%> or greater. 23. The process of claim 21, wherein the nucleic acid of interest is present on a vector. 24. The process of claim 21 , wherein the nucleic acid of interest comprises a heterologous nucleic acid sequence. 25. The process of claim 22, wherein the heterologous nucleic acid encodes a peptide, polypeptide, or protein. 26. The process of claim 25, wherein the protein is a therapeutic protein. 27. The process of claim 21 , wherein the target cell is a non-dividing cell. 28. The process of claim 21 , wherein the target cell is a HeLa cell or a hematopoietic cell. 29. A process for expressing a gene of interest in vitro, said process comprising a) exposing target cells to an isolated or purified nucleic acid comprising a gene of interest and at least one copy of the cPPT and CTS cis-acting regions of a retrovirus, wherein the cPPT and CTS regions induce a three-stranded DNA structure, under conditions that permit uptake of the nucleic acid into the target cell to create a recombinant cell, and b) culturing the recombinant cell under conditions that permit at least part of the nucleic acid to be transferred to the nucleus of the recombinant cell and the gene of interest to be expressed. 30. The process of claim 29, wherein the nucleic acid is present on a vector. 31. The process of claim 29, wherein the gene of interest is expressed in tissue culture. 32. The process of claim 29, which further comprises purifying or isolating the product of expression of the gene of interest. 33. A process for expressing a gene of interest in vivo, said process comprising administering a recombinant cell comprising the nucleic acid of claim 10 to an individual, and permitting the recombinant cell to express the nucleic acid within the individual's body. 34. The process of claim 33, wherein the recombinant cell is a hematopoeitic stem cell. 35. A process for expressing a gene of interest in vivo, said process comprising administering the nucleic acid of claim 10 to an individual in an amount and form sufficient to result in expression of the gene of interest within the individual's body. 36. The process of claim 35, wherein the gene of interest is expressed in a target tissue. 37. The process of claim 35, wherein the gene of interest is present within a retroviral vector. 38. A process for treating an individual suffering from, or having a high likelihood of developing, a disease or disorder having a genetic basis, said process comprising administering a retroviral vector comprising a) a nucleic acid encoding a therapeutic protein and b) at least one copy of the cPPT and CTS cis-acting regions of a retrovims, wherein the cPPT and CTS regions induce a three-stranded DNA structure, to said individual in an amount sufficient to result in expression of said therapeutic protein in an amount sufficient to treat said disease or disorder. 39. The process of claim 38, wherein the treatment is prophylactic, ameliorative, or curative. 40. The process of claim 38, wherein the process treats a blood disease or disorder, a brain or nervous system disease or disorder, or a developmental disease or disorder. 41. A kit containing at least one container containing an isolated or purified nucleic acid comprising at least one copy of the cPPT and CTS cis-acting regions of a retrovims, wherein the cPPT and CTS regions induce a three-stranded DNA stmcture. 42. The kit of claim 41 , wherein the nucleic acid further comprises a heterologous nucleic acid sequence that encodes a therapeutic protein. 43. The kit of claim 41 , wherein the nucleic acid is present on a vector.

Inventors: CHARNEAU PIERRE, DUBART KUPPERSCHMITT ANNE, PFLUMIO FRANCOISE, SIRVEN AUDE, ZENNOU VERONIQUE, CHARNEAU, PIERRE, DUBART KUPPERSCHMITT, ANNE, PFLUMIO, FRANCOISE, SIRVEN, AUDE, ZENNOU, VERONIQUE
Applicants: CHARNEAU PIERRE, DUBART KUPPERSCHMITT ANNE, INST NAT SANTE RECH MED, PASTEUR INSTITUT, PFLUMIO FRANCOISE, SIRVEN AUDE, ZENNOU VERONIQUE, CHARNEAU, PIERRE, DUBART KUPPERSCHMITT, ANNE, INSTITUT NATIONAL DE LA SANTE ET DE LA RECHERCHE MEDICALE, INSTITUT PASTEUR, PFLUMIO, FRANCOISE, SIRVEN, AUDE, ZENNOU, VERONIQUE
Application year: 2000
Main classification: C12N15
IPC codes: A61K31, A61K48, A61P7, A61P25, C12N5, C12N7, C12N15, C12P21, C12R1