In contrast to oncoretroviruses, lentiviruses such as human immunodeficiency virus 1 (HIV-1) are able to integrate their genetic material into the genome of nonproliferating cells that are metabolically active. Likewise, vectors derived from HIV-1 can transduce many types of nonproliferating cells, with the exception of some particular quiescent cell types such as resting T cells. Completion of reverse transcription, nuclear import, and subsequent integration of the lentivirus genome do not occur in these cells unless they are activated via the T-cell receptor (TCR) or by cytokines or both. However, to preserve the functional properties of these important gene therapy target cells, only minimal activation with cytokines or TCR-specific antibodies should be performed during gene transfer. Here we report the characterization of HIV-1–derived lentiviral vectors whose virion surface was genetically engineered to display a T cell-activating single-chain antibody polypeptide derived from the anti-CD3 OKT3 monoclonal antibody. Interaction of OKT3 IgGs with the TCR can activate resting peripheral blood lymphocytes (PBLs) by promoting the transition from G0 to G1 phases of the cell cycle. Compared to unmodified HIV-1–based vectors, OKT3-displaying lentiviral vectors strongly increased gene delivery in freshly isolated PBLs by up to 100-fold. Up to 48% transduction could be obtained without addition of PBL activation stimuli during infection. Taken together, these results show that surface-engineered lentiviral vectors significantly improve transduction of primary lymphocytes by activating the target cells. Moreover these results provide a proof of concept for an approach that may have utility in various gene transfer applications, including in vivo gene delivery.

Introduction

Efficient gene transfer into T lymphocytes may allow the treatment of several genetic dysfunctions of the hematopoietic system, such as severe combined immunodeficiency,1,2 and the development of novel therapeutic strategies for diseases such as cancers and acquired immunodeficiency syndrome.3 To reach this goal, it is essential to preserve the functional properties of the transduced cells, that is, their capacity to appropriately react on stimulation of the immune system. Therefore, only minimal ex vivo manipulation of the cells should be performed during gene transfer because growth factor/cytokine combinations, used to force cell proliferation, often lead to skewing of cell populations and may alter their ability to respond to novel antigens.4,5 Ideally, gene delivery should be best achieved in vivo to minimize contacts of target cells with nonphysiologic cell culture reagents that may induce differentiation or proliferation.

Vectors derived from retroviruses are probably among the most suitable tools to achieve a long-term gene transfer because they allow stable integration of a transgene and its propagation in daughter cells. To date, vectors derived from oncoretroviruses such as murine leukemia viruses (MLVs) have been widely used for gene transfer into human T cells,6 essentially because of the simplicity of their manipulation. Perhaps one of the most important drawbacks associated with the use of such vectors is their inability to transduce nonproliferating target cells. Indeed, following internalization of the vector into the target cell cytoplasm and reverse transcription, transport of the preintegration complex to the nucleus requires the breakdown of the nuclear membranes during mitosis.7,8 This provides a formidable barrier to the use of MLV-based vectors in the many gene therapy protocols for which target cells are quiescent or for which induction of cell proliferation is to be avoided. Thus, the recent emergence of lentiviral vectors may provide a valuable alternative to overcome this problem owing to the lentivirus mechanism that allows mitosis-independent nuclear import of the preintegration complex and infection of nonproliferating cells.9-11 

Several studies have now established the capacity of these vectors derived from human immunodeficiency virus 1 (HIV-1) to transduce various types of nonproliferating cells both in vitro and in vivo.12 However, some cell types that are important gene therapy targets are refractory to gene transfer with lentiviral vectors, despite the most recent improvements brought into their structures.13-16 This includes, in particular, early progenitor hematopoietic stem cells in G0,17monocytes,18,19 and resting T lymphocytes.14That the parental virus, HIV-1, can enter into resting T lymphocytes but does not replicate,20-24 has been attributed to multiple post-entry blocks. This includes, in particular, (1) defects in initiation and completion of the reverse-transcription process,20,23-25 (2) lack of adenosine triphosphate-dependent nuclear import,9,26 and (3) lack of integration of the proviral genome.27 Low levels of nucleotides in the resting cells do not entirely explain the restricted HIV-1 replication because artificially raising intracellular nucleotide pools increased reverse-transcription products but not the level of productive infection.28 However, it was recently reported that inducing the resting T cells to enter into the G1bphase of the cell cycle by stimulation through the T-cell receptor (TCR) and CD28 costimulation receptor, using anti-CD3 plus anti-CD28 antibodies, was sufficient to render the cells susceptible to HIV-1 infection and replication.25 Moreover, exposing T cells to cytokines that do not trigger cell division could render them permissive to transduction with HIV-1 vectors.29 These findings suggest that partial activation of resting T cells is sufficient for gene transfer by HIV-1–derived vectors and that DNA synthesis or mitosis of these cells is not necessary.

Here we sought to refine the structure of lentiviral vector particles to overcome their inability to transduce nonactivated T cells. Because extensive activation of resting T cells with combinations of stimulating factors should be avoided ex vivo and may not be possible to achieve in vivo, we have designed novel lentiviral vector particles that transiently provide a minimal stimulus to the target cells during gene delivery. As a proof of concept, we establish here that lentiviral vectors whose virion surface was engineered to display a TCR-activating polypeptide could efficiently transduce primary resting T lymphocytes without addition of soluble T-cell activation stimuli in the cell culture medium. HIV-1–derived vectors were copseudotyped with an MLV-derived chimeric envelope glycoprotein fused to an anti-CD3 single-chain antibody variable fragment (scFv) and the vesicular stomatitis virus G (VSV-G) glycoprotein. This anti-CD3 scFv was derived from the OKT3 monoclonal antibody, which activates the TCRs. Importantly, the surface-modified lentiviral vectors specifically recognized CD3 and triggered activation of freshly isolated peripheral blood T cells. This minimal stimulation was sufficient to allow gene transfer in up to 48% of the lymphocytes, that is, 100-fold more than the performance of unmodified lentiviral vectors in nonactivated T cells.

Materials and methods

Cells

The 293T human embryo kidney cell line was grown in Dulbecco modified Eagle medium (DMEM; Life Technologies, Cergy Pontoise, France) supplemented with 10% fetal calf serum (FCS). The Jurkat human T leukemia cell line was grown in RPMI 1640 (Life Technologies) supplemented with 10% FCS. Human peripheral blood mononuclear cells (PBMCs) were separated from fresh blood of healthy donors using a Ficoll-Hypaque/Percoll gradient (Amersham Pharmacia Biotech, Orsay, France). Peripheral blood lymphocytes (PBLs) were enriched from the PBMC fraction by an overnight plastic adherence at 37°C to remove adherent monocytes. The nonadherent cells were resuspended in RPMI 1640 medium supplemented with 10% FCS. PBLs were checked for CD3 marker expression (75%-85% were CD3+) and the percentage of cells in the different phases of the cell cycle was determined by flow cytometry (≥ 99% were in G0/G1).

Packaging and vectors constructs

The pCMVΔ8.230 and pCMVΔ8.9131HIV-1 packaging plasmids have been described elsewhere. The HPPT-EF1α-GFP vector contains the EF1α internal promoter driving the GFP (green fluorescent protein) gene and the cPPT/CTS (central poly-purine track and central termination sequence) cis-acting signal, which stimulates nuclear import.15,16 The pCMVΔ8.2 packaging-deficient construct contains all HIV-1 genes except env, and pCMVΔ8.91 lacks the HIV-1 accessory genes vif, vpr,vpu, and nef. The CMV+intron plasmid, containing the MLV packaging sequences (S. Chapel-Fernandes and F-L. Cosset, unpublished material, 1996), and the MFG-GFP plasmid, encoding an MLV-based vector containing the GFP marker gene, were used to generate MLV-derived vectors. phCMV-G32encodes the VSV-G glycoprotein under control of the human cytomegalovirus (hCMV) promoter and rabbit β-globin intron II and polyadenylation sequences.

Envelope glycoprotein constructs

The scFv gene encoding a polypeptide that binds the human CD3 complex of TCRs was derived from the OKT3 monoclonal antibody33 and was described elsewhere.34 The OKT3 scFv complementary DNA (cDNA) was fused in the 4070A (amphotropic) MLV env gene,35 at a position corresponding to the first amino acid of the SU (surface) subunit of the amphotropic MLV envelope glycoprotein, as described previously.34 To reduce steric hindrance between the scFv polypeptide and the amphotropic envelope glycoprotein, a peptide linker, 7 amino acids long, was inserted between these 2 domains.34,36 The resulting chimeric glycoprotein was named OKT3SU (Figure1). The SUx mutation,37which inhibits furin-mediated cleavage of the MLV glycoprotein in the producer cells, was inserted in the OKT3SU construct, thus resulting in a second chimera, named OKT3SUx. This was achieved by replacing the Lys-Tyr-Lys-Arg furin cleavage site by the IPe-Glu-Gly-Arg peptide.37 Both the OKT3SU and OKT3SUx chimeras were expressed using the phCMV-G expression vector backbone.32 

Fig. 1.

Schematic representation of chimeric retroviral glycoproteins displaying an anti-CD3 scFv.

A cDNA encoding an scFv derived from the OKT3 monoclonal antibody raised against the human CD3 was fused to the amino-terminal end of the 4070A-MLV env gene, at a position corresponding to the first codon of the SU envelope subunit. A spacer, 7 amino acids long, was inserted between the scFv polypeptide and the envelope glycoprotein to minimize steric hindrances. The SU/TM cleavage site was inactivated by replacing the Lys-Tyr-Lys-Arg wild-type amino acid sequence by the IPe-Glu-Gly-Arg sequence. L indicates leader-signal peptide; ligand, anti-CD3 scFv polypeptide; SU, surface envelope subunit; TM, transmembrane envelope subunit.

Fig. 1.

Schematic representation of chimeric retroviral glycoproteins displaying an anti-CD3 scFv.

A cDNA encoding an scFv derived from the OKT3 monoclonal antibody raised against the human CD3 was fused to the amino-terminal end of the 4070A-MLV env gene, at a position corresponding to the first codon of the SU envelope subunit. A spacer, 7 amino acids long, was inserted between the scFv polypeptide and the envelope glycoprotein to minimize steric hindrances. The SU/TM cleavage site was inactivated by replacing the Lys-Tyr-Lys-Arg wild-type amino acid sequence by the IPe-Glu-Gly-Arg sequence. L indicates leader-signal peptide; ligand, anti-CD3 scFv polypeptide; SU, surface envelope subunit; TM, transmembrane envelope subunit.

Production of retroviral vectors

Pseudotyped HIV-1–derived vectors were generated as previously described30 by transient transfection of 293T cells. Then 8.6 μg of the HPPT-EF1α-GFP vector construct, 8.6 μg of either pCMVΔ8.2 or pCMVΔ8.91 packaging constructs, and 2.7 μg of theenv construct, for example, the VSV-G–expressing construct (phCMV-G) were used to cotransfect 293T cells. When HIV-1–derived vectors were copseudotyped with VSV-G and OKT3SU or OKT3SUx chimeric glycoproteins, an equimolar quantity of envelope-expressing plasmids was used. A similar protocol was used to produce MLV-derived vectors with the CMV+intron and the MFG-GFP expressing constructs. Plasmid DNAs were transfected into 2.6 × 106 293T cells seeded the day before in 10-cm diameter plates using the calcium-phosphate transfection system according to manufacturer's recommendations (Life Technologies). The medium (12 mL/plate) was replaced 16 hours after transfection, and supernatant was harvested 24 hours later, low-speed centrifuged (2000 rpm for 5 minutes at 4°C), filtered through 0.45-μm pore-sized membranes, and directly used in the different assays.

Infection assays

To determine transduction efficiency and infectious titers of HIV-1–derived vectors, 293T target cells were seeded at a density of 2 × 105/well in 6-well plates 1 day before transduction. Serial dilutions of vector preparations were added to the 293T cells. The percentage of GFP+ cells was determined by FACS analysis, following homogenization of transduced cells in trypsin and resuspension in phosphate-buffered saline (PBS). The titer was derived from the percentage of GFP+ cells after transduction of 3 × 105 target cells with 1 mL viral supernatant. The infectious titers are expressed as 293T transducing units (TU/mL).

To infect human primary lymphocytes, 1 mL viral supernatant containing 5 × 105 to 5 × 106 TU, as determined on 293T target cells, was added to 1 to 3 × 105 lymphocytes suspended in 0.5 mL RPMI supplemented with 10% FCS in 24-well plates. Multiplicities of infection (MOIs) were determined on proliferating 293T cells and are indicated in all lymphocyte transduction experiments. Human primary lymphocytes were infected in the same conditions as 293T control cells and infection efficiency was determined by flow cytometry at day 6. For T-cell stimulation and control infection assays performed with vectors pseudotyped with VSV-G only, anti-CD3 (HIT3a, Pharmingen) and anti-CD28 (CD28.2, Pharmingen) antibodies were used at a final concentration of 1 μg/mL/point. Activation with soluble anti-CD3/anti-CD28 antibodies was preferred to activation with immobilized (plastic-coated) antibodies, to provide a true comparison with the OKT3SU-displaying virions. This resulted in approximately 30% lower transduction efficiencies than obtained in transduction protocols that used immobilized OKT3 antibodies (data not shown).4,14,25,28 No differences in transduction efficiency were found when comparing OKT3 versus HIT3a anti-CD3 monoclonal antibodies for T-cell activation and infection (data not shown).

For the short exposure of PBLs to virus supernatants, the cells were washed with RPMI medium 24 hours after infection, resuspended in RPMI containing 10% FCS supplemented with or without recombinant interleukin (IL) 2 (rIL-2; 1 ng/mL; R & D Systems, Abingdon, United Kingdom) and maintained up to day 6 after infection for FACS analysis.

Antibodies, immunoblots, cell surface staining, and binding assays

Anti-SU (Viromed Biosafety Labs, Camden, NJ) was a goat antiserum raised against the Rausher leukemia virus gp70, used diluted to 1:2000 for Western blots. Anti-MLV-CA (Viromed Biosafety Labs) was a goat antiserum raised against the Rausher leukemia virus p30 capsid protein (CA), used diluted to 1:10 000 for Western blots. Anti-VSV-G P5D4 monoclonal antibody (1 μg/μL; Sigma, St Louis, MO) was diluted to 1:1000 for Western blots. Anti-HIV-1-CA (p24 capsid) monoclonal antibody (8 μg/μL) was diluted to 1:8000 in immunoblots. For Western blot analysis, lysates of vector producer cells and virion samples were prepared as previously described.38 

Anti-CD3 was used at a 1:125 dilution for cell surface staining. Staining for CD25, CD69, and CD71 T-cell activation markers was performed with phycoerythrin-conjugated anti-CD25, anti-CD69, and anti-CD71 antibodies (BD-Pharmingen, Pont de Claix, France) at a 1:25 dilution. Target cells (5 × 105 cells/point) were incubated with the appropriate antibodies for 45 minutes at 4°C. Fluorescence of living cells was analyzed with a fluorescent-activated cell sorter (FACSCalibur, Becton Dickinson, Pont de Claix, France).

Supernatant of the 83A25 hybridoma,39 secreting a rat monoclonal antibody against MLV SU, was used undiluted for binding assays. Binding assays on Jurkat T cells (5-8 × 105cells/point) were performed as previously described.37 For competition binding assays, Jurkat cells were preincubated for 45 minutes at 4°C with an anti-CD3 antibody (HIT3a, 1 μg/μL, Pharmingen) diluted to 1:100 to saturate TCRs, then exposed to virus. Cells were then processed as previously described38 except that 10 ng/mL mouse IgGs (Sigma) was added with the secondary antibody to inhibit nonspecific antibody recognition.

Cell cycle analysis

Freshly isolated human PBLs (1-3 × 105 cells) were maintained in RPMI supplemented with 10% FCS. PBLs were stimulated for different time periods after cultivation with 1 mL filtered viral supernatants or with stimulating agents (soluble anti-CD3 or soluble anti-CD28 antibodies). Cells were then centrifuged and fixed in 70% ethanol-30% PBS. The cells were stored at 4°C until FACS analysis. Just before FACS analysis, the fixed cells were pelleted and resuspended in 1 mL PBS containing 50 μg RNase and 10 μg/mL propidium iodide. The cells were incubated for 30 minutes at 37°C and the cell cycle was then analyzed with a fluorescence-activated cell sorter (FACSCalibur, Becton Dickinson).

Results

Engineering of the viral surface glycoprotein with an OKT3-derived scFv

Previous data from our laboratory have shown that the MLV envelope glycoprotein can be engineered to display various types of polypeptides at its amino terminus.40 Because the wild-type MLV glycoprotein is efficiently incorporated on HIV-1–derived lentivirus vectors,30 we sought to display a T cell-activating polypeptide on HIV-1 vector particles (Figure 1). Thus, an scFv derived from the OKT3 monoclonal antibody, which recognizes and activates the TCR (TCR/CD3/ζ complex, named TCR herein), was fused to the amino terminus of the SU subunit of the MLV envelope glycoprotein (Figure 1). The position of insertion of the OKT3-derived scFv was chosen to allow its functional display on virions.37,38 The resulting chimera was named OKT3SU. Because the SU and TM subunits of the MLV envelope glycoprotein are not covalently held together,41partial dissociation of the chimeric SU from the envelope complex, and thus from the viral surface, may occur.37 Therefore, a second TCR-activating chimeric glycoprotein, derived from OKT3SU, was designed to prevent the potential loss of T cell-activating polypeptides from the viral particles. In the resulting chimera, named OKT3SUx, the cleavage site between the SU and TM envelope subunits was inactivated by substitution with a noncleavable linker peptide37 (Figure 1).

Characterization of surface-engineered lentiviral vector particles

The HIV-1–derived vectors were generated by transient transfections of 293T cells with plasmids encoding the viral core proteins (pCMVΔ8.2 or pCMVΔ8.91), the gene transfer vector (HPPT-EF1α-GFP), the VSV-G glycoprotein (phCMV-G), and either of the 2 TCR-activating glycoproteins (OKT3SU or OKT3SUx). Coexpression of VSV-G protein with either OKT3SU or OKT3SUx was necessary to render the viral particles fully infectious (data not shown), consistent with previous results.37 Lentiviral particles, harvested in supernatants of producer cells 2 days after transfection, were concentrated by ultracentrifugation. Incorporation of the chimeric glycoproteins on the virions was assessed by immunoblotting. Standardization of the amount of viral particles loaded on gels was determined using anticapsid antibodies (Figure2). Immunodetection of the viral pellets using antibodies against the VSV-G or MLV glycoproteins indicated that the OKT3SU and OKT3SUx chimeras were coincorporated with the VSV-G glycoprotein on the virions. The OKT3SUx chimeric glycoprotein was less abundant in the viral pellet than the OKT3SU chimera, indicating a reduced level of viral incorporation of the former glycoprotein. Differences in the electrophoretic mobilities of the OKT3SU and OKT3SUx chimeric glycoproteins demonstrated the existence of a covalent linkage between the SU and TM subunits for the OKT3SUx chimera. Compared to viral particles produced with VSV-G alone, a lower incorporation of the VSV-G glycoprotein was detected for virions generated with VSV-G and either of the 2 chimeric glycoproteins (Figure 2), most probably because of competition at the level of viral assembly.

Fig. 2.

Immunoblots of pelleted lentiviral vectors generated with VSV-G and OKT3SU- or OKT3SUx-displaying glycoproteins.

Virions were pelleted by ultracentrifugation of supernatants harvested from lentiviral vector-producer cells. Blots were separated at the position of 40-kd marker. The upper portion of the membrane was stained with a mixture of antibodies against MLV-SU and against VSV-G. The lower part of the membrane was stained with antibodies against HIV-1 CA (capsid) protein to assess equivalent loading of virions on the gels. The positions of the OKT3SU, OKT3SUx, VSV-G, and CA proteins are indicated.

Fig. 2.

Immunoblots of pelleted lentiviral vectors generated with VSV-G and OKT3SU- or OKT3SUx-displaying glycoproteins.

Virions were pelleted by ultracentrifugation of supernatants harvested from lentiviral vector-producer cells. Blots were separated at the position of 40-kd marker. The upper portion of the membrane was stained with a mixture of antibodies against MLV-SU and against VSV-G. The lower part of the membrane was stained with antibodies against HIV-1 CA (capsid) protein to assess equivalent loading of virions on the gels. The positions of the OKT3SU, OKT3SUx, VSV-G, and CA proteins are indicated.

To determine whether the OKT3-derived scFv was displayed in a correct conformation on the lentiviral particles, we performed binding and competition assays on TCR+ Jurkat T cells. Viral particles generated with unmodified MLV glycoproteins or with VSV-G alone were used as controls. Supernatants containing OKT3SU- and OKT3SUx-displaying viral particles were incubated with Jurkat cells at 4°C, to prevent binding to the amphotropic receptor, which is temperature-sensitive.42 Virion-to-cell binding was then analyzed by flow cytometry using antibodies against the MLV SU.38 No binding was detected with viral particles carrying the unmodified MLV glycoproteins (Figure3). In contrast, virions carrying the OKT3SU or OKT3SUx chimeras could readily bind to the target cells. Binding of virions pseudotyped with the OKT3SUx glycoprotein was lower than that of virions carrying the OKT3SU chimera, probably due to the weaker viral incorporation of the former glycoprotein (Figure 2). Competitive binding assays were performed in the presence of TCR-blocking anti-CD3 antibodies to demonstrate the specific CD3 interaction of virions displaying the OKT3-derived scFv. Almost complete inhibition of virion binding was found (Figure 3). Altogether, these results demonstrate that the OKT3-derived scFv was correctly displayed on lentiviral vector particles and could specifically allow virion binding to TCR+ cells.

Fig. 3.

TCR-binding of OKT3SU- or OKT3SUx-displaying lentiviral particles.

The Jurkat T-cell line was used as TCR CD3+ target cells. The background fluorescence was determined by incubating cells with viral particles devoid of envelope glycoproteins (white area). Binding assays were performed with vectors particles generated with wild-type amphotropic MLV glycoproteins (WT-A), OKT3SU, or OKT3SUx glycoproteins as indicated. Target cells were either pretreated (broken line) with soluble anti-CD3 antibodies, or not pretreated (black area), before incubation with the viral particles. Virion binding was detected by flow cytometry using anti-SU antibodies.

Fig. 3.

TCR-binding of OKT3SU- or OKT3SUx-displaying lentiviral particles.

The Jurkat T-cell line was used as TCR CD3+ target cells. The background fluorescence was determined by incubating cells with viral particles devoid of envelope glycoproteins (white area). Binding assays were performed with vectors particles generated with wild-type amphotropic MLV glycoproteins (WT-A), OKT3SU, or OKT3SUx glycoproteins as indicated. Target cells were either pretreated (broken line) with soluble anti-CD3 antibodies, or not pretreated (black area), before incubation with the viral particles. Virion binding was detected by flow cytometry using anti-SU antibodies.

Infectivity of lentiviral vectors was then determined on 293T target cells. Despite the presence of chimeric glycoproteins that reduced the incorporation of VSV-G (Figure 2), infectious titers of up to 5 × 106 cfu/mL were readily obtained. Simultaneous vector titrations on 293T cells were performed to evaluate the MOIs used in all PBL transduction experiments (Figures 5-8). The range of MOIs applied for PBL transduction was between 4 and 60 infectious virions per target T cell.

OKT3-displaying lentiviral vectors improve transduction of freshly isolated PBLs

Primary blood lymphocytes were isolated from healthy blood donors. Cell cycle analysis and detection of the CD25, CD69, and CD71 T-cell activation markers were performed before and after transduction with the lentiviral vectors. More than 99% of the PBLs were in the G0/G1 phase of the cell cycle before transduction (Table 1). A low proportion of PBLs showed weak staining for the CD25 low-affinity IL-2 receptor (< 3%) and for the CD69 early activation marker (< 4%; Figure4).

Table 1.

Cell cycle analysis of PBLs after infection with HIV-1 or MLV-derived vectors*

Pseudotyped vector HIV-1 MLV 
1.4 0.2 
G + anti-CD3 1.3 0.8  
G + anti-CD3/anti-CD28 23 19  
G/OKT3SU 9.5 6.5 
G/OKT3SUx 8.7 12.5 
Pseudotyped vector HIV-1 MLV 
1.4 0.2 
G + anti-CD3 1.3 0.8  
G + anti-CD3/anti-CD28 23 19  
G/OKT3SU 9.5 6.5 
G/OKT3SUx 8.7 12.5 
*

Percentage of cells in S/G2M phases of the cell cycle analyzed 6 days after infection. The data, shown here for PBLs of set 7 (Figures 5-8 present results of transduction), are representative of other experiments. Before infection, the proportion of cells in S/G2M for this particular batch of PBLs was 0.2%.

Type of vector used for PBL infection.

VSV-G–coated vectors used to infect nonactivated PBLs in the presence of 1 μg of the indicated antibodies.

Fig. 4.

Expression of activation markers on PBLs after infection with OKT3SU-displaying HIV-1– or MLV-derived vectors.

The percentage of PBLs that stained positive for the activation markers CD25 (top) and CD69 (bottom) was determined by FACS analysis for the freshly isolated PBLs (0 hour) and at different time points after infection with the HIV-1–derived (left) and the MLV-derived vectors (right) generated with the indicated glycoproteins. The data, shown here for PBLs of set 7 (Figures 5-8 present the results of transduction), are representative of other experiments. VSV-G–pseudotyped vectors were used to infect freshly isolated PBLs in the presence of 1 μg indicated soluble antibody. PBLs were also stained for the CD71 activation marker (data not shown). The proportion of CD71+ cells was less than 12% for PBLs incubated with VSV-G–pseudotyped vectors, in the absence of anti-CD3 or anti-CD28 antibodies, and was higher than 80% for PBLs incubated either with VSV-G–pseudotyped virions in the presence of anti-CD3 and anti-CD28 antibodies or with OKT3SU/OKT3SUx-displaying vector particles.

Fig. 4.

Expression of activation markers on PBLs after infection with OKT3SU-displaying HIV-1– or MLV-derived vectors.

The percentage of PBLs that stained positive for the activation markers CD25 (top) and CD69 (bottom) was determined by FACS analysis for the freshly isolated PBLs (0 hour) and at different time points after infection with the HIV-1–derived (left) and the MLV-derived vectors (right) generated with the indicated glycoproteins. The data, shown here for PBLs of set 7 (Figures 5-8 present the results of transduction), are representative of other experiments. VSV-G–pseudotyped vectors were used to infect freshly isolated PBLs in the presence of 1 μg indicated soluble antibody. PBLs were also stained for the CD71 activation marker (data not shown). The proportion of CD71+ cells was less than 12% for PBLs incubated with VSV-G–pseudotyped vectors, in the absence of anti-CD3 or anti-CD28 antibodies, and was higher than 80% for PBLs incubated either with VSV-G–pseudotyped virions in the presence of anti-CD3 and anti-CD28 antibodies or with OKT3SU/OKT3SUx-displaying vector particles.

These resting PBLs were incubated with lentiviral vectors copseudotyped with VSV-G and OKT3SU or OKT3SUx glycoproteins. Results obtained with PBLs derived from 7 different donors are shown in Figure5. Consistent with results of others,14,29,43 nonactivated PBLs were very poorly transduced with vectors carrying VSV-G alone (< 1.5% GFP+ cells; mean, 0.51% ± 0.43%; n = 7). However, infections performed with the same vector particles in the presence of soluble anti-CD3 antibodies resulted in higher levels of gene transfer (Figure 5), ranging from 0.5% to 15% (mean, 7.18% ± 4.4%; n = 6), which is due to partial activation of the target cells (Figure 4). Activation with soluble anti-CD3 plus anti-CD28 antibodies rendered the PBLs highly susceptible to infection with the VSV-G–pseudotyped lentiviruses (29%-79% GFP+ cells; mean, 47.89% ± 19.7%; n = 7). Thus, consistent with results of others,14,43 2 stimulation signals were necessary to achieve efficient transduction of PBLs with unmodified lentiviral vectors. In contrast, in the absence of anti-CD3 and anti-CD28 antibodies, infection of PBLs with OKT3SU-displaying lentiviral vector particles resulted in very efficient transduction, ranging from 30% to 48% GFP+ cells (mean, 36.0% ± 6.45%; n = 7; Figure 5). This exceeded the performance of unmodified VSV-G–pseudotyped HIV-1 vectors by more than 100-fold and was even superior to that observed after addition of anti-CD3 antibodies (16-fold). Additionally, the transduction efficiency of the OKT3SU-displaying lentiviral vectors was frequently in the same range as that obtained with unmodified lentiviral vectors used in combination with anti-CD3 and anti-CD28 soluble antibodies, despite the 2- to 10-fold higher MOIs of the latter vectors (Figure 5).

Fig. 5.

Transduction of human PBLs with OKT3SU-displaying lentiviral vectors.

Resting PBLs were incubated with OKT3SU-displaying lentiviral vector particles (G/OKT3SU) in the absence of activation factors in the cell culture media. As controls, unmodified lentiviral vectors (pseudotyped with VSV-G only) were used to infect the PBLs in the absence of stimuli (G) or in the presence of anti-CD3 (1 μg/mL) soluble antibodies (G + anti-CD3) or in the presence of both anti-CD3 (1 μg/mL) plus anti-CD28 (1 μg/mL) soluble antibodies (G + anti-CD3 + anti-CD28), as indicated. The number of GFP+ cells was determined 6 days after infection by FACS analysis. Results of transduction in PBLs from 7 different donors are given. The MOIs are provided for each experiment.

Fig. 5.

Transduction of human PBLs with OKT3SU-displaying lentiviral vectors.

Resting PBLs were incubated with OKT3SU-displaying lentiviral vector particles (G/OKT3SU) in the absence of activation factors in the cell culture media. As controls, unmodified lentiviral vectors (pseudotyped with VSV-G only) were used to infect the PBLs in the absence of stimuli (G) or in the presence of anti-CD3 (1 μg/mL) soluble antibodies (G + anti-CD3) or in the presence of both anti-CD3 (1 μg/mL) plus anti-CD28 (1 μg/mL) soluble antibodies (G + anti-CD3 + anti-CD28), as indicated. The number of GFP+ cells was determined 6 days after infection by FACS analysis. Results of transduction in PBLs from 7 different donors are given. The MOIs are provided for each experiment.

The MLV SU is not covalently associated to the envelope glycoprotein complex and hence may dissociate from the surface of viral particles onto which it is incorporated.44 Therefore, efficient PBL transduction with the OKT3SU-displaying lentiviral vectors could be due either to virion-associated TCR-activating polypeptide or, alternatively, to soluble OKT3SU, “shed” from the viral particle. To discriminate between these 2 possibilities, transduction experiments of nonactivated PBLs were performed with vectors generated either with the OKT3SU glycoprotein or with the OKT3SUx chimera (Figure 1), which was engineered to avoid loss of SU by shedding. Vectors generated with either type of chimeric glycoproteins could similarly activate the resting PBLs, as judged by their capacity to up-regulate the expression of the CD25, CD69, and CD71 activation markers (Figure 4). Importantly, both vector types could efficiently transduce freshly isolated PBLs (Figure 6), though with a slightly lower efficiency for OKT3SUx-displaying vectors (37% ± 7.21% versus 24.17% ± 10.54 GFP+ cells), perhaps owing to a lower density of the latter chimeric glycoprotein on the viral surface (Figure 2).

Fig. 6.

Comparative PBL transduction efficiency of OKT3-displaying lentiviral vectors versus MLV vectors.

Nonactivated PBLs were incubated with OKT3SU- or OKT3SUx-displaying VSV-G–pseudotyped vector particles derived from HIV-1 or from MLV, as indicated. The number of GFP+ cells was determined 6 days after infection by FACS analysis. Results of transduction in PBLs from different donors are given. The PBL sets correspond to those of Figure5. The MOIs are provided for each experiment.

Fig. 6.

Comparative PBL transduction efficiency of OKT3-displaying lentiviral vectors versus MLV vectors.

Nonactivated PBLs were incubated with OKT3SU- or OKT3SUx-displaying VSV-G–pseudotyped vector particles derived from HIV-1 or from MLV, as indicated. The number of GFP+ cells was determined 6 days after infection by FACS analysis. Results of transduction in PBLs from different donors are given. The PBL sets correspond to those of Figure5. The MOIs are provided for each experiment.

Cells transduced with either OKT3SU- or OKT3SUx-displaying HIV-1 vectors were CD3+ as determined by counterstaining of the GFP+ cells with anti-CD3 antibodies (data not shown), thus indicating that the vectors could specifically and efficiently infect T lymphocytes without exogenous stimuli. Additionally, the GFP+ T cells exhibited proportions of CD4+ and CD8+ T cells identical to that of the control untransduced T-cell population (data not shown). Altogether these results indicate that the engineering of the viral surface of lentiviral vectors with T cell-activating polypeptides can overcome the infection block of nonactivated PBLs and allow efficient gene delivery in both CD4+ and CD8+ T cells.

PBL transduction by OKT3SU vectors is independent of cell proliferation

The cell cycle analysis of the PBLs transduced with the different lentiviral vectors showed that no or only poor progression in the cell cycle could be detected for PBLs incubated with unmodified lentiviral vectors in the presence or in the absence of soluble anti-CD3 antibodies during infection (Table 1). In contrast transduction of PBLs in the presence of both soluble anti-CD3 and anti-CD28 antibodies resulted in enhanced proportion of cells in S/G2M phases (up to 23% of cells). The OKT3SU- and OKT3SUx-displaying lentiviral vectors also induced cell cycle progression of infected PBLs (9.5% and 8.7% cells in S/G2M, respectively), though at a lower extent compared to when both anti-CD3 plus anti-CD28 antibodies were added during infection (Table 1). The finding that infection with the OKT3SU-displaying vectors induced progression into the cell cycle formally raised the possibility that their high transduction efficiency could be linked to proliferation of the PBLs. To address this question, side-to-side transduction experiments of primary PBLs were performed using either lentiviral vectors or MLV-derived vectors, whose integration in target cell genome is absolutely dependent on cell proliferation.7,8 Compared to OKT3SU-diplaying lentiviral vectors, MLV vectors generated with the OKT3SU chimera incorporated similar levels of chimeric glycoproteins and had equivalent titers on 293T target cells (data not shown). Moreover, the latter vectors could induce weak cell cycle progression (Table 1) and up-regulated CD25, CD69, and CD71 markers in a manner similar to the OKT3SU-displaying lentiviral vectors (Figure 4), thus demonstrating that the surface of both types of vector particles was similarly engineered. However, in contrast to OKT3SU-displaying lentiviral vectors, infection of nonactivated PBLs with OKT3SU-displaying MLV-derived vectors only resulted in a low level of PBL transduction, of up to 4% GFP+ cells (mean, 1.92% ± 1.5%; n = 5), despite comparable MOIs (Figure 6). Similar weak transduction of nonactivated PBLs was obtained with the OKT3SUx-displaying MLV-based vectors (1.17% ± 0.71% GFP+ cells; n = 3). This low transduction efficiency most likely reflected infection of cells having entered in the cell cycle (Table 1). Indeed MLV-derived vectors with an unmodified viral surface were unable to induce cell cycle progression (Table 1) and only resulted in marginal transduction of up to 1.1% GFP+ PBLs (mean, 0.56% ± 0.39%; n = 5) (data not shown). Altogether these results indicate that although a small proportion of PBL transduction could be due to activation of the cell cycle, the low cell proliferation induced by the OKT3SU-displaying lentiviral vectors was not the reason for their high transduction efficiency.

PBL transduction by OKT3SU-displaying lentiviral vectors does not require HIV-1 accessory proteins

Some reports have raised the possibility that nonstructural “accessory” HIV-1 proteins may positively influence gene transfer by lentiviral vectors in nonproliferating T cells.43,45Previous experiments (Figures 5 and 6) were performed using HIV-1–based vectors generated with the pCMVΔ8.91 packaging construct, which only express the Gag-Pol viral core proteins and the Tat and Rev regulatory proteins. To evaluate the importance of HIV-1 accessory proteins in PBL transduction, OKT3SU- or OKT3SUx-displaying lentiviral vectors were generated with either the pCMVΔ8.91 packaging construct or with the pCMVΔ8.2 packaging construct, which also express the HIV-1 vif, vpu, vpr, and nefaccessory genes.31 Lentiviral vectors pseudotyped only with VSV-G glycoproteins and generated with either pCMVΔ8.91 or pCMVΔ8.2 packaging constructs gave a comparable poor transduction efficiency (< 2% GFP+ cells) of freshly isolated PBLs (Figure 7). In contrast, lentiviral vectors generated with either OKT3SU or OKT3SUx glycoproteins could efficiently transduce the nonactivated PBLs. However, no significant differences could be detected in transduction efficiency, whether the vectors were generated with or without the HIV-1 accessory proteins (Figure 7).

Fig. 7.

Influence of HIV-1 accessory genes on the performance of OKT3-displaying lentiviral vectors.

Nonactivated PBLs were incubated with OKT3SU- or OKT3SUx-displaying VSV-G–pseudotyped lentiviral vector generated with either the pCMVΔ8.2 or the pCMVΔ8.91 packaging constructs that express or do not express the HIV-1 accessory genes, respectively. The number of GFP+ cells was determined 6 days after infection by FACS analysis. Results of transduction in PBLs from 3 different donors are given. The PBL sets correspond to those of Figure 5. The MOIs are provided for each experiment.

Fig. 7.

Influence of HIV-1 accessory genes on the performance of OKT3-displaying lentiviral vectors.

Nonactivated PBLs were incubated with OKT3SU- or OKT3SUx-displaying VSV-G–pseudotyped lentiviral vector generated with either the pCMVΔ8.2 or the pCMVΔ8.91 packaging constructs that express or do not express the HIV-1 accessory genes, respectively. The number of GFP+ cells was determined 6 days after infection by FACS analysis. Results of transduction in PBLs from 3 different donors are given. The PBL sets correspond to those of Figure 5. The MOIs are provided for each experiment.

Short cell exposure to OKT3-displaying vectors allows transduction of nonactivated PBLs

Previous transduction experiments were performed by incubating the nonactivated PBLs with the viral particles for 6 days (Figures 5-7). We then sought to determine if a shorter exposure of freshly isolated resting PBLs to the OKT3SU-displaying vectors would be sufficient to activate lymphocytes and render them susceptible to infection. PBLs were therefore exposed for only 24 hours to supernatants containing HIV-1–derived or MLV-based vectors. After transduction, cells were washed and were further incubated for 5 days in media supplemented with, or without, a low concentration of rIL-2 to prevent cell death. No significant progression in cell cycle (data not shown) was found 24 hours after infection, before addition of rIL2, in contrast to previous experiments where cells had been put in contact with OKT3SU-displaying vectors for 6 days (Table 1). Almost no transduction was detected following infection with either lentiviral vectors displaying only VSV-G glycoproteins (0.67% ± 0.65%; n = 5) or with MLV-derived vectors, whether the surface of the latter vector particles carried (0.63% ± 0.53; n = 2), or not (0.25% ± 0.18%; n = 5), OKT3SU chimeric glycoproteins (Figure 8). Compared to vectors generated with unmodified glycoproteins, infection of nonactivated PBLs with OKT3SU-displaying lentiviral vectors resulted in an average 20-fold higher transduction efficiency, in the range of 9% to 19% GFP+ cells (mean, 12.60% ± 4.63%; n = 5). Similar results were obtained in experiments performed in the absence of rIL-2, yet the prolonged time of culture of PBLs without cytokines resulted in extensive cell death (data not shown). In conclusion, these data indicated that transduction of primary lymphocytes by a short exposure to OKT3SU-displaying lentiviral vectors gave rise to high transduction levels, which were 34-fold and 100-fold higher than those obtained with the unmodified HIV-1–derived and MLV-derived vectors, respectively.

Fig. 8.

Transduction of human PBLs after a short exposure with OKT3SU-displaying lentiviral vectors.

Nonactivated PBLs were incubated for 24 hours with VSV-G–pseudotyped vector particles derived from HIV-1 or from MLV displaying or not displaying OKT3SU chimeric glycoproteins, as indicated. Infected cells were then washed to eliminate unbound viral particles and incubated for 5 days in the presence of human rIL-2 (1 ng/mL) to reduce cell death. The number of GFP+ cells was then determined by FACS analysis. Results of transduction in PBLs from 7 different donors are given. The PBL sets correspond to the ones in Figure 5. The MOIs are provided for each experiment.

Fig. 8.

Transduction of human PBLs after a short exposure with OKT3SU-displaying lentiviral vectors.

Nonactivated PBLs were incubated for 24 hours with VSV-G–pseudotyped vector particles derived from HIV-1 or from MLV displaying or not displaying OKT3SU chimeric glycoproteins, as indicated. Infected cells were then washed to eliminate unbound viral particles and incubated for 5 days in the presence of human rIL-2 (1 ng/mL) to reduce cell death. The number of GFP+ cells was then determined by FACS analysis. Results of transduction in PBLs from 7 different donors are given. The PBL sets correspond to the ones in Figure 5. The MOIs are provided for each experiment.

Discussion

Lentiviral vectors have shown promise in the transduction of several resting cell types such as retinal cells, pancreatic islets, cells of the central nervous system, or progenitor and differentiated hematopoietic cells.12 For these reasons, lentiviral vectors should be preferred gene delivery vehicles over vectors derived from oncoretroviruses such as MLVs that cannot transduce nonproliferating target cells.8 There are, however, important gene transfer restrictions to some nonproliferative tissues or cell types and recent studies have shown that progenitor hematopoietic stem cells in G0, nonactivated primary blood lymphocytes or monocytes were not transducible by HIV-1–derived vectors.17,19,25 In contrast to the mechanisms that restrict gene transfer into monocytes,18,19 the 2 former cell types are refractory to gene transfer most probably because their low metabolic state prevents post-entry replication steps such as initiation or processivity of reverse-transcription and nuclear import.17,25 Thus, activation of these cells, causing G0-to-G1b transition of the cell cycle, is required to relieve from the blocks in gene delivery.17,25This can be achieved by adding a minimal combination of cytokines into the infection media during a short culture period.14,17,25,46 Therefore, on such minimal ex vivo infection conditions, lentiviral vectors may allow gene transfer in more primitive hematopoietic progenitor cells than those usually reached with MLV-derived vectors.46 However, there is considerable interest to further develop the lentiviral vectors so as to reach the most primitive progenitor cells that are believed to be in a quiescent state, to be slow to respond to cytokine stimulation, and to tend to lose multipotentiality or long-term repopulating capacity under cytokine stimulation. Similarly, in the case of genetic modification of T cells, most previous studies have focused on the optimization of transduction protocols, yet most gene therapy applications will require the transduction of the naive T-cell populations.14,47 It has now become evident that optimized protocols that use activation stimuli such as antibodies and mitogens may not be compatible with the preservation of the pool of naive T cells, which harbor the capacity to respond to novel antigens.4,47 

Recent improvements in the development of lentiviral vectors have focused on the optimization of nuclear import of the transgene.15,16 Inclusion of the HIV-1 central polypurine track (cPPT) in lentiviral vectors has resulted in enhanced transduction of human progenitor stem cells and T cells.14,46 However, the improved lentiviral vectors that include the cPPT sequence still fail to transduce nonactivated T lymphocytes,14 most likely because the primary block in initiation or completion of reverse transcription could not be alleviated with the novel vectors. Therefore, because minimal activation of resting human progenitor stem cells and T cells with stimuli that cause G0-to-G1 transition allows gene transfer,17,25,29 we sought to generate surface-modified lentiviral vector particles that would convey a transient activation signal in target cells at the time of gene delivery. As a proof of concept, we show here that lentiviral vectors that display on their viral surface an anti-CD3 scFv T cell-activating polypeptide mediate efficient gene transfer into freshly isolated lymphocytes not activated by exogenous stimuli. No addition of T cell-activating stimuli in the infection media was required to achieve transduction levels of up to 48% GFP+ cells. In comparison, unmodified lentiviral vectors whose viral surface expressed only the VSV-G glycoprotein were unable to transduce freshly isolated T cells.

Optimal activation of resting T cells requires signaling via the TCR and additional stimulation provided through accessory molecules present on the surface of antigen-presenting cells (APCs). For highly purified resting T cells, multivalent APC stimulation can be mimicked ex vivo with immobilized anti-CD3 antibodies and causes a G0-to-G1a transition. However, this does not allow completion of HIV-1 reverse transcription25 or efficient transduction with HIV-1–based vectors.29Costimulation with immobilized anti-CD3 and anti-CD28 antibodies allows progression to G1b48,49 and allows successful infection by wild-type HIV-125 or transduction by lentiviral vectors.14 Prolonged cell exposure to high loads of immobilized anti-CD3 and anti-CD28 antibodies induces cell proliferation50 and allows transduction by vectors derived from either lentiviruses or oncoretroviruses.4,14 Our results confirm the notion that induction of cell division is not a prerequisite for efficient gene transfer in PBLs with lentiviral vectors. No HIV-1 accessory proteins, such as vpr or vif, were required to achieve high-level transduction in the PBLs, in agreement with results of others.13,14 Interestingly, our data show that comparable transduction levels can be achieved with either the surface-modified, OKT3SU-displaying, lentiviral vectors or with the unmodified vectors used in combination with both soluble anti-CD3 and anti-CD28 antibodies. This indicates that an exogenous costimulation signal may not be needed for transduction with the OKT3SU-displaying lentiviral vectors. These results, which are in contrast to the poor transduction levels obtained with VSV-G–pseudotyped lentiviral vectors in the presence of anti-CD3 antibodies alone, are likely to be explained by the fact that the multivalent presentation on viral particles of TCR-binding polypeptides may strongly favor TCR cross-linking. Additionally, local concentration of TCR-binding polypeptides could be much higher when presented on virions compared to when provided to cells as soluble or immobilized anti-CD3 antibodies. In agreement with this hypothesis, concentrations of immobilized anti-CD3 antibodies 10-fold higher than those classically used in T-cell activation and transduction experiments, that is, around 1 μg/well,14,25 were found sufficient to allow infection by HIV-1 and could also induce cell proliferation.50Alternatively, concomitant to partial activation induced by the OKT3SU-displaying virions, costimulation could be provided either by residual APCs likely to be present in the preparations of PBLs25,49 or by costimulatory molecules that may have been coincorporated on the surface of the lentiviral particles.51 

T-cell receptor cross-linking induced by surface-modified lentiviral vectors may have adverse effects on the phenotype of the transduced T cells, such as inducing anergy or modifying their ability to respond to novel antigens.52 An important component of the homeostasis of the immune system is the maintenance of the peripheral naive T-cell repertoire. Human T cells can be divided into naive and memory subsets based, respectively, on expression of RA and RO isoforms of the CD45 molecule. In contrast to memory T cells, naive cells exhibit few effectors functions (eg, cytokine production), are less susceptible to activation-induced cell death, and display robust proliferation in response to TCR-mediated activation signals. It is important that the compartment of naive CD45RA+ cells is maintained through life. Although TCR activation induced on infection with OKT3SU-displaying viral particles is short, the change in T-cell phenotype cannot be formally excluded. In fact, transduction of PBLs with OKT3SU-displaying lentiviral vectors was found to shift the T cells to the memory phenotype (data not shown), in agreement with results of others using TCR stimulation by anti-CD3/anti-CD28 antibodies to allow infection of resting PBLs by retroviral vectors.4 It should be pointed out that the approach reported here was designed to provide proof of concept of gene transfer strategies that may have utility elsewhere. Here, we show for the first time that it is possible to efficiently transduce quiescent PBLs by a lentiviral vector that displays a T cell-activating polypeptide. For the reasons discussed above, it might be preferable to display cytokines like IL-2, IL-7, and IL-4, or combinations of these polypeptides, on the surface of the lentiviral vectors. These cytokines are known to promote long-term survival of resting T cells while maintaining the phenotype of the subset of naive cells.53,54 Previously, we described the generation of MLV-derived vectors that displayed IL-2 chimeric envelope glycoproteins (IL-2SU).37 These vectors allowed efficient transduction of G0/G1-arrested cell lines expressing the IL-2 receptor.37 However, lentiviral vectors pseudotyped with VSV-G and chimeric IL-2SU envelope glycoproteins were not able to transduce freshly isolated PBLs, despite their capacity to weakly up-regulate the CD25 and CD69 activation markers (data not shown). This result was most likely due to the low expression of high-affinity IL-2 receptors on resting PBLs. Alternatively, IL-7 is a promising candidate because it is known that it activates PBLs and permits infection with lentiviral vectors into both memory and naive T cells.29,47 Moreover, IL-7 contributes to the maintenance of the adult CD45RA+ T-cell pool.55,56Characterization of lentiviral vectors that display some of these polypeptides on the viral surface are currently underway in our laboratory and will be valuable tools to selectively activate specific T-cell subsets, allowing numerous studies of the physiology of lymphocytes.

We are grateful to Naomi Taylor and Jacqueline Marvel for stimulating discussions and for critical reading of the manuscript.

Supported by Agence Nationale pour la Recherche contre le SIDA (ANRS), the European Community (QLK3-1999-00859), AFIRST, Association Française contre les Myopathies (AFM), Association pour la Recherche contre le Cancer (ARC), Centre National de la Recherche Scientifique (CNRS), and Institut National de la Santé Et de la Recherche Médicale (INSERM). M.M. and E.V. contributed equally to this work.

The publication costs of this article were defrayed in part by page charge payment. Therefore, and solely to indicate this fact, this article is hereby marked “advertisement” in accordance with 18 U.S.C. section 1734.

References

References
1
Blaese
RM
Culver
KW
Miller
AD
et al. 
T-lymphocyte directed gene therapy for ADA-SCID: initial trial results after 4 years.
Science.
270
1995
275
280
2
Bordignon
C
Notaranglo
LD
Nobili
N
et al. 
Gene therapy in peripheral blood lymphocytes and bone marrow for ADA- immunodeficient patients.
Science.
270
1995
470
475
3
Buchschacher
GL
Wong-Staal
F
Approaches to gene therapy for human immunodeficiency virus infection.
Hum Gene Ther.
12
2001
1013
1019
4
Dardalhon
V
Jaleco
S
Rebouissou
C
et al. 
Highly efficient gene transfer in naive human T cells with a murine leukemia virus-based vector.
Blood.
96
2000
885
893
5
Ferrand
C
Robinet
E
Contassot
E
et al. 
Retrovirus-mediated gene transfer in primary T lymphocytes: influence of the transduction/selection process and of ex vivo expansion on the T cell receptor β chain hypervariable region repertoire.
Hum Gene Ther.
11
2000
1151
1164
6
Anderson
WF
Human gene therapy.
Nature.
392
1998
25
30
7
Miller
DG
Adam
MA
Miller
AD
Gene transfer by retrovirus vectors occurs only in cells that are actively replicating at the time of infection.
Mol Cell Biol.
10
1990
4239
4242
8
Roe
T
Reynolds
TC
Yu
G
Brown
PO
Integration of murine leukemia virus DNA depends on mitosis.
EMBO J.
12
1993
2099
2108
9
Bukrinsky
MI
Sharova
N
Dempsey
MP
et al. 
Active nuclear import of human immunodeficiency virus type 1 preintegration complexes.
Proc Natl Acad Sci U S A.
89
1992
6580
6584
10
Lewis
P
Hensel
M
Emerman
M
Human immunodeficiency virus infection of cells arrested in the cell cycle.
EMBO J.
11
1992
3053
3058
11
Weinberg
JB
Matthews
TJ
Cullen
BR
Malim
MH
Productive human immunodeficiency virus type 1 (HIV-1) infection of non proliferative human cells.
J Exp Med.
174
1991
1477
1482
12
Vigna
E
Naldini
L
Lentiviral vectors: excellent tools for experimental gene transfer and promising candidates for gene therapy.
J Gene Med.
2
2000
308
316
13
Salmon
P
Kindler
V
Ducrey
O
Chapuis
B
Zubler
RH
Trono
D
High-level transgene expression in human hematopoietic progenitors and differentiated blood lineages after transduction with improved lentiviral vectors.
Blood.
96
2000
3392
3398
14
Dardalhon
V
Herpers
B
Noraz
N
et al. 
Lentivirus-mediated gene transfer in primary T cells is enhanced by a central DNA flap.
Gene Ther.
8
2001
190
198
15
Zennou
V
Petit
C
Guetard
D
Nerhbass
U
Montagnier
L
Charneau
P
HIV-1 genome nuclear import is mediated by a central DNA flap.
Cell.
101
2000
173
185
16
Follenzi
A
Ailles
L
Bakovic
S
Geuna
M
Naldini
L
Gene transfer by lentiviral vectors is limited by nuclear translocation and rescued by HIV-1 pol sequences.
Nat Genet.
25
2000
217
222
17
Sutton
RE
Reitsma
MJ
Uchida
N
Brown
PO
Transduction of human progenitor hematopoietic stem cells by human immunodeficiency virus type 1-based vectors is cell cycle dependent.
J Virol.
73
1999
3649
3660
18
Kootstra
NA
Zwart
BM
Schuitemaker
H
Diminished human immunodeficiency virus type 1 reverse transcription and nuclear transport in primary macrophages arrested in early G(1) phase of the cell cycle.
J Virol.
74
2000
1712
1717
19
Neil
S
Martin
F
Ikeda
Y
Collins
M
Postentry restriction to human immunodeficiency virus-based vector transduction in human monocytes.
J Virol.
75
2001
5448
5456
20
Stevenson
M
Stanwick
TL
Dempsey
MP
Lamonica
CA
HIV-1 replication is controlled at the level of T cell activation and proviral integration.
EMBO J.
9
1990
1551
1560
21
Sun
Y
Pinchuk
LM
Agy
MB
Clark
EA
Nuclear import of HIV-1 DNA in resting CD4+ T cells requires a cyclosporin A-sensitive pathway.
J Immunol.
158
1997
512
517
22
Tang
S
Patterson
B
Levy
JA
Highly purified quiescent human peripheral blood CD4+ T cells are infectible by human immunodeficiency virus but do not release virus after activation.
J Virol.
69
1995
5659
5665
23
Zack
JA
Haislip
AM
Krogstad
P
Chen
ISY
Incompletely reverse-transcribed human immunodeficiency virus type 1 genomes in quiescent cells can function as intermediates in the retroviral life cycle.
J Virol.
68
1992
1717
1725
24
Zack
JA
The role of the cell cycle in HIV-1 infection.
Adv Exp Med Biol.
374
1995
27
31
25
Korin
YD
Zack
JA
Progression to the G1b phase of the cell cycle is required for completion of human immunodeficiency virus type 1 reverse transcription in T cells.
J Virol.
72
1998
3161
3168
26
Bukrinsky
MI
Haggerty
S
Dempsey
MP
et al. 
A nuclear localization signal within HIV-1 matrix protein that governs infection of non-dividing cells.
Nature.
365
1993
666
669
27
Spina
CA
Guatelli
JC
Richman
DD
Establishment of a stable, inducible form of human immunodeficiency virus type 1 DNA in quiescent CD4 lymphocytes in vitro.
J Virol.
69
1995
2977
2988
28
Korin
YD
Zack
JA
Nonproductive human immunodeficiency virus type 1 infection in nucleoside-treated Go lymphocytes.
J Virol.
73
1999
6526
6532
29
Unutmaz
D
KewalRamani
VN
Marmon
S
Littman
DR
Cytokine signals are sufficient for HIV-1 infection of resting human T lymphocytes.
J Exp Med.
189
1999
1735
1746
30
Naldini
L
Blömer
U
Gallay
P
et al. 
In vivo gene delivery and stable transduction of nondividing cells by a lentiviral vector.
Science.
272
1996
263
267
31
Zufferey
R
Nagy
D
Mandel
R
Naldini
L
Trono
D
Multiply attenuated lentiviral vector achieves efficient gene delivery in vivo.
Nat Biotechnol.
15
1997
871
875
32
Yee
JK
Friedmann
T
Burns
JC
Generation of high-titer pseudotyped retroviral vectors with very broad host range.
Methods Cell Biol.
43
1994
99
112
33
Kung
PC
Goldstein
G
Reinherz
EL
Schlossman
SF
Monoclonal antibodies defining distinctive human T cell surface antigens.
Science.
206
1979
347
349
34
Ager
S
Nilson
BHK
Morling
FJ
Peng
KW
Cosset
F-L
Russell
SJ
Retroviral display of antibody fragments; interdomain spacing strongly influences vector infectivity.
Hum Gene Ther.
7
1996
2157
2164
35
Ott
D
Friedrich
R
Rein
A
Sequence analysis of amphotropic and 10A1 murine leukemia virus: close relationship to mink cell focus forming viruses.
J Virol.
64
1990
757
766
36
Valsesia-Wittmann
S
Morling
FJ
Nilson
BHK
Takeuchi
Y
Russell
SJ
Cosset
F-L
Improvement of retroviral retargeting by using amino acid spacers between an additional binding domain and the N terminus of Moloney murine leukemia virus SU.
J Virol.
70
1996
2059
2064
37
Maurice
M
Mazur
S
Bullough
FJ
et al. 
Efficient gene delivery to quiescent IL2-dependent cells by murine leukemia virus-derived vectors harboring IL2 chimeric envelopes glycoproteins.
Blood.
94
1999
401
410
38
Cosset
F-L
Morling
FJ
Takeuchi
Y
Weiss
RA
Collins
MKL
Russell
SJ
Retroviral retargeting by envelopes expressing an N-terminal binding domain.
J Virol.
69
1995
6314
6322
39
Evans
LH
Morrison
RP
Malik
FG
Portis
J
Britt
WJ
A neutralizable epitope common to the envelope glycoproteins of ecotropic, polytropic, xenotropic and amphotropic murine leukemia viruses.
J Virol.
64
1990
6176
6183
40
Russell
SJ
Cosset
F-L
Modifying the host range properties of retroviral vectors.
J Gene Med.
1
1999
300
311
41
Pinter
A
Kopelman
R
Li
Z
Kayman
SC
Sanders
DA
Localization of the labile disulfide bond between SU and TM of the murine leukemia virus envelope protein complex to a highly conserved CWLC motif in SU that resembles the active-site sequence of thiol-disulfide exchange enzymes.
J Virol.
71
1997
8073
8077
42
Battini
JL
Rodrigues
P
Müller
R
Danos
O
Heard
J-M
Receptor-binding properties of a purified fragment of the 4070A amphotropic murine leukemia virus envelope glycoprotein.
J. Virol.
70
1996
4387
4393
43
Chinnasamy
D
Chinnasamy
N
Enriquez
MJ
Otsu
M
Morgan
RA
Candotti
F
Lentiviral-mediated gene transfer into human lymphocytes: role of HIV-1 accessory proteins.
Gene Ther.
96
2000
1309
1316
44
Gliniak
BC
Kozak
SL
Jones
RT
Kabat
D
Disulfide bonding controls the processing of retroviral envelope glycoproteins.
J Biol. Chem.
266
1991
22991
22997
45
Costello
E
Munoz
M
Buetti
E
Meylan
PRA
Diggelmann
H
Thali
M
Gene transfer into stimulated and unstimulated T lymphocytes by HIV-1-derived lentiviral vectors.
Gene Ther.
7
2000
596
604
46
Sirven
A
Pflumio
F
Zennou
V
et al. 
The human immunodeficiency virus type-1 central DNA flap is a crucial determinant for lentiviral vector nuclear import and gene transduction of human hematopoietic stem cells.
Blood.
96
2000
4103
4110
47
Dardalhon
V
Jaleco
S
Kinet
S
et al. 
Interleukin-7 differentially regulates cell cycle progression and HIV-1 based vector infection in neonatal and adult CD4+ T cells.
Proc Natl Acad Sci U S A.
98
2001
9277
9282
48
Chambers
CA
Allison
JP
Co-stimulation in T cell responses.
Curr Opin Immunol.
9
1997
396
404
49
Lanzavecchia
A
Lezzi
G
Viola
A
From TCR engagement to T cell activation: a kinetic view of T cell behavior.
Cell.
96
1999
1
4
50
Spina
CA
Prince
HE
Richman
DD
Preferential replication of HIV-1 in the CD45RO memory cell subset of primary CD4 lymphocytes in vitro.
J Clin Invest.
99
1997
1774
1785
51
Bounou
S
Dumais
N
Tremblay
MJ
Attachment of human immunodeficiency virus-1 (HIV-1) particles bearing host-encoded B7–2 proteins leads to nuclear factor-kappa B- and nuclear factor of activated T cells-dependent activation of HIV-1 long terminal repeat transcription.
J Biol Chem.
276
2001
6359
6369
52
Contassot
E
Ferrand
C
Angonin
R
et al. 
Ganciclovir-sensitive acute graft-versus-host disease in mice receiving herpes simplex virus-thymidine kinase-expressing donor T cells in a bone marrow transplantation setting.
Transplantation.
69
2000
503
508
53
Boise
L
Minn
AJ
June
CH
Lindsten
T
Thompson
CB
Growth factors can enhance lymphocyte survival without committing the cell undergo cell division.
Proc Natl Acad Sci U S A.
92
1995
5491
5495
54
Geiselhart
LA
Humphries
CA
Gregorio
TA
Mou
S
Subleski
J
Komschlies
KL
IL-7 administration alters the CD4:CD8 ratio, increases T cell numbers, and increases T cell function in the absence of activation.
J Immunol.
166
2001
3019
3027
55
Soares
MVD
Borthwick
NJ
Maini
MK
Janossy
G
Salmon
M
Akbar
AN
IL-7 dependent extrathymic expansion of CD45RA+ T cells enables preservation of a naive repertoire.
J Immunol.
161
1998
5909
5917
56
Hassan
J
Reen
DJ
IL-7 promotes the survival and maturation but not differentiation of human post-thymic CD4+ T cells.
Eur J Immunol.
28
1998
3057
3065

Author notes

François-Loı̈c Cosset, Laboratoire de Vectorologie Rétrovirale et Thérapie Génique, INSERM U412, Ecole Normale Supérieure de Lyon, 46 Allée d'Italie, 69364 Lyon Cedex 07, France; e-mail: flcosset@ens-lyon.fr.