Regulation of gene expression in vivo following transduction by two separate rAAV vectors

Control of gene expression is important to gene therapy for purposes of both dosing and safety. In vivo regulation of gene expression was demonstrated following co-injection of two separate recombinant adeno-associated virus vectors, one encoding an inducible murine erythropoietin transgene and the other a transcriptional activator, directly into the skeletal muscle of adult immunocompetent mice. Transcription was controlled by systemic administration or withdrawal of tetracycline over an 18 week period, demonstrating that the two vectors were capable of transducing the same cell. Cellular or humoral immune responses against the transactivator protein were not detected.

Definition of a 14-amino-acid peptide essential for the interaction between the murine leukemia virus amphotropic envelope glycoprotein and its receptor

Hydrophilic loops in the receptor binding domain of the amphotropic murine leukemia virus (MLV) envelope glycoprotein (SU) are predicted and may participate in SU-receptor interactions. We have replaced five segments of 6 to 15 amino acids located in each of these regions with an 11-amino-acid tag from the vesicular stomatitis virus glycoprotein (VSV-G). Substitution was compatible with envelope processing, transport, and incorporation into virions. However, three substitution mutants showed a temperature-dependent phenotype, suggesting structural unstability. Accessibility of the tagging epitope for a monoclonal anti-VSV-G antibody was greater in oligomeric than in monomeric SUs when insertion was done in VRA, a domain essential for receptor recognition. In contrast, accessibility was independent of structural constraints when insertion was done in VRB, a domain playing an accessory role in receptor binding. Interaction with the amphotropic receptor was investigated by interference assay and study of binding and infection of target cells with MLV particles coated with the substituted envelopes. Envelope-receptor interaction was abolished when substitution was performed in a potential loop-forming segment located at the N-terminal half of VRA. Although interaction was affected to variable extents, depending on the substituted segment, other mutants conserved the ability to interact with the amphotropic receptor. These experiments indicate the 14-amino-acid segment between positions 50 and 64 of SU as an essential determinant of amphotropic-receptor recognition. They also show that a foreign linear epitope can be tolerated in several locations of the amphotropic SU receptor binding site, and this result has implications for the design of targeted retroviral vectors.

Efficient and stable adeno-associated virus-mediated transduction in the skeletal muscle of adult immunocompetent mice

Recombinant adeno-associated virus (rAAV) vectors were evaluated for gene transfer into the skeletal muscle of adult immunocompetent mice. A study using a vector encoding nuclear localized beta-galactosidase (rAAV-nls-lacZ) examined: (i) the efficiency and duration of transgene expression; (ii) the status of the AAV genome in the transduced fibers; and (iii) the possibility of improving gene transfer by inducing muscle regeneration. In the absence of regeneration, the injection of 1.7 x 10(7) particles in the quadriceps resulted in gene transfer to 10-70% of myofibers. Histological analysis indicated that the vector was able to reach myofiber nuclei distant from the injection point. Cellular infiltrates were absent at early time points but became conspicuous in the vicinity of some positive fibers at 4-8 weeks and subsided by 26 weeks. Southern analysis indicated that one to three copies of the vector genome were present per cell genome equivalent. They were associated with high-molecular-weight DNA in the form of tandem oligomers or interlocked circles. Gene transfer was not facilitated in the regenerating muscle. Rather, an early inflammatory response resulted in the elimination of most positive fibers after 8 weeks. The presence of regenerated fibers with beta-galactosidase-positive nuclei suggested that myoblasts had been transduced and were able to fuse to form new fibers. Gene transfer in the absence of immune reactions against the transgene product was studied by injecting mice with a rAAV carrying the murine erythropoietin (mEpo) cDNA. Dose-dependent elevation in the hematocrit was measured for over 200 days and corresponded to 5- to 20-fold increases in plasma Epo levels. We conclude that AAV vectors efficiently and stably transduce post-mitotic muscle fibers and myoblasts in vivo.

Retroviral particles produced from a stable human-derived packaging cell line transduce target cells with very high efficiencies

The goal of this work was to determine whether a stable 293 amphotropic packaging line, which we have designated 293-SPA, is useful for the production of high-titer stable virus by comparison to the murine psiCRIP line. Here, we report our unexpected findings that particles derived from the 293-SPA line transduce target cells (both NIH-3T3 cells and primary melanoma cells) with greatly enhanced efficiencies (at least 10-fold) compared to particles derived from the psiCRIP packaging line. We show that the presence of a transferable inhibitor in the psiCRIP line at least partially accounts for this dramatic difference in transduction efficiency. This work has important implications for improving the efficiency of retrovirus-mediated gene transfer in general as well as in the design of new packaging cell lines.

Persistent and therapeutic concentrations of human factor IX in mice after hepatic gene transfer of recombinant AAV vectors

Haemophilia B, or factor IX deficiency, is a X-linked recessive disorder that occurs in about one in 25,000 males, and severely affected people are at risk for spontaneous bleeding into numerous organs. Bleeding can be life-threatening or lead to chronic disabilities with haemophilic arthropathy. The severity of the bleeding tendency varies among patients and is related to the concentration of functional plasma factor IX. Patients with 5-30% of the normal factor IX have mild haemophilia that may not be recognized until adulthood or after heavy trauma or surgery. Therapy for acute bleeding consists of the transfusion of clotting-factor concentrates prepared from human blood and recombinant clotting factors that are currently in clinical trials. Both recombinant retroviral and adenoviral vectors have successfully transferred factor IX cDNA into the livers of dogs with haemophilia B. Recombinant retroviral-mediated gene transfer results in persistent yet subtherapeutic concentrations of factor IX and requires the stimulation of hepatocyte replication before vector administration. Recombinant adenoviral vectors can temporarily cure the coagulation defect in the canine haemophilia B model; however, an immune response directed against viral gene products made by the vector results in toxicity and limited gene expression. The use of recombinant adeno-associated virus (rAAV) vectors is promising because the vector contains no viral genes and can transduce non-dividing cells. The efficacy of in vivo transduction of non-dividing cells has been demonstrated in a wide variety of tissues. In this report, we describe the successful transduction of the liver in vivo using r-AAV vectors delivered as a single administration to mice and demonstrate that persistent, curative concentrations of functional human factor IX can be achieved using wild-type-free and adenovirus-free rAAV vectors. This demonstrates the potential of treating haemophilia B by gene therapy at the natural site of factor IX production.

In vivo gene transfer by intravenous administration of stable cationic lipid/DNA complex

PURPOSE

A stable cationic lipid/DNA complex has been developed for in vivo gene transfer. The formulation capitalizes on a previously described procedure to obtain stable lipid/DNA complexes for in vitro gene transfer (1).

METHODS

Conditions for DNA/lipid complex formation were modified to yield a DNA concentration of 1 mg/ml. Heat stable alkaline phosphatase (AP) under a CMV promoter was used as a reporter gene.

RESULTS

The resulting complex was completely insensitive to serum inactivation. Tail vein injection of a 80 micrograms DNA into Balb C mice yielded significant levels of reporter enzyme activity in the lung, heart, spleen, muscle, and liver. Less AP activity was observed in the kidney. No AP activity was observed in blood, bone marrow or brain. A titration of the lipid (DOSPA) to DNA-nucleotide ratio showed the optimal molar ratio for in vivo gene transfer to be 1/1. Using this ratio in a dose response study showed approximately 80 micrograms of DNA/mouse yielded the highest level of gene expression. Using this dose at a 1/1 lipid to DNA nucleotide ratio, the time course for alkaline phosphatase activity was determined. Maximal AP activity was observed 24 hours after injection for all tissues. By day 5, the activity dropped approximately 10 fold for all tissues. By day 7, residual activity was detected in the lung, heart, and muscle. Histology of the lung showed both interstitial and endothelial cells to be transfected. In all other tissues, however, endothelial cells were the only transfected cell type.

CONCLUSIONS

These results demonstrate that reformulation of an existing cationic lipid can result in the formation of a stable lipid/DNA complex, which is able to reproducibly transfect lung, heart, spleen, and liver upon intravenous administration.

In situ retrovirus-mediated gene transfer into the liver

Gene transfer into hepatocytes is a promising approach for the treatment of genetic liver diseases. Candidate diseases for human trials are life-threatening disorders resulting from a single genetic defect that do not compromise other liver functions and the organization of the hepatic tissue. Orthotopic liver transplantation has been successfully performed in patients with genetic liver disease, but then use is limited by the high mortality risk, the need for chronic immunosuppression, and the availability of organs. Gene therapy would pro vide an alternative to transplantation by restoring the expression of the defective gene in an organ that is otherwise structurally and functionally normal.

Gene transfer for erythropoiesis enhancement

The spectrum of anemias treated with recombinant human erythropoietin is rapidly broadening. Lifelong treatment with very high doses is now under evaluation for beta-thalassemia and sickle cell anemia. These indications make it worthwhile to search for methods that will allow a permanent systemic delivery of the hormone. Here, we review experimental gene-transfer-based procedures for erythropoietin delivery in vivo. In mice, both ex vivo and direct in vivo approaches for gene transfer have resulted in the long-term production of therapeutic levels of the hormone. Gene transfer of erythropoietin could become a viable alternative to the injection of the purified recombinant protein once reliable procedures for controlling transgene expression are available.

A cell surface marker gene transferred with a retroviral vector into CD34+ cord blood cells is expressed by their T-cell progeny in the SCID-hu thymus

Gene transduction into immature hematopoietic cells collected at birth from the umbilical cord could be useful for the treatment of genetic or acquired disorders of the hematopoietic system diagnosed during pregnancy. The SCID-hu mouse is a convenient model to investigate T-cell lineage gene therapy, since it allows replication of human intrathymic T-cell development. CD34+ cells isolated from cord blood were cocultured with CRIP MFG-murine CD2 (mCD2) cells that produce recombinant retroviruses encoding the mCD2 antigen, a cell surface marker easily detectable by flow cytometry. After 3 and 4 days in coculture, a mean of 19% and 39% human hematopoietic cells, respectively, expressed the mCD2 antigen. CD34+ cells cocultured for 4 days were used to reconstitute human fetal thymus implanted in SCID mice. Five to 10 weeks later, the mCD2 antigen was detected on approximately 10% of human thymocytes repopulating the thymic grafts in four of nine SCID mouse chimeras. Vector genomes were detected in graft cell DNA by Southern blot. Analysis of vector integration indicated that positive cells were of polyclonal origin in three animals and predominantly monoclonal in the other one. Our data show that foreign genes can be transduced into CD34+ cord blood cells endowed with T-cell differentiation potential, and suggest strategies for T-cell lineage gene therapy in the neonate.

Receptor-binding properties of a purified fragment of the 4070A amphotropic murine leukemia virus envelope glycoprotein

A 208-amino-acid amino-terminal fragment of the 4070A amphotropic murine leukemia virus envelope glycoprotein contains all of the determinants required to recognize cell surface amphotropic receptors. This fragment was fused with a streptavidin-binding tag, expressed in Sf9 insect cells by using a baculovirus vector, and purified to homogeneity. The (125)I-labeled purified fragment (AS208) specifically bound various cell lines susceptible to amphotropic murine leukemia virus infection. The number of AS208-binding sites was in the range of 7 X 10(4) to 17 X 10(4) per cell. Quantitative analysis of binding revealed that AS208-binding sites are heterogeneous with regard to ligand binding affinity or that cooperativity exists between receptors. Competition experiments showed that the concentration of AS208 required to inhibit virus entry was lower than that required to inhibit the binding of virus particles at the cell surface. Taken together, these data suggested that amphotropic envelope-binding sites present at the cell surface do not act independently and do not participate equally in virus infection.

gamma-c gene transfer into SCID X1 patients' B-cell lines restores normal high-affinity interleukin-2 receptor expression and function

SCID X1 is characterized by faulty T-cell and natural killer cell differentiation caused by mutation of the gamma-c chain gene encoding a number of multiple cytokine receptors (interleukin-2 [IL-2], IL-4, IL-7, IL-9, and IL-15 receptors). To assess the feasibility of inducing long-term expression and function of the gamma-c chain, Epstein-Barr virus (EBV)-transformed B-cell lines from two patients with SCID X1 were transduced with a Moloney-derived retroviral vector containing the gamma-c chain cDNA. The viral LTR was used as the promoter. Immediately after two cycles of coculture with the psi-crip clone producing the MFG(B2)-gamma-c cDNA vector, gamma-c expression, assessed by detection of the mRNA and membrane protein expression, was found in 15% to 20% of cells. The degree of membrane expression was similar to that in control EBV-B cells. Expression increased steadily over 6 months, becoming detectable in 100% of cells, and remained stable thereafter for a total of 9 months, reflecting positive selection of transduced cells. A study of provirus integration sites showed multiple integration. The expressed gamma-c was functional, because it restored high-affinity IL-2 receptor binding, IL-2 endocytosis, and IL-2-triggered phosphorylation of JAK-3 tyrosine kinase. Similar results were obtained with the two B-cell lines. These results show that efficient gamma-c gene transfer into B-cells lacking functional gamma-c is feasible and results in strong and stable expression of a functional gamma-c chain, apparently conferring a selective growth advantage in culture. Further in vitro studies of gamma-c gene transfer into gamma-c- hematopoietic progenitors are being conducted to assess the feasibility of correcting lymphocyte differentiation defects.

Retroviral vectors for the transduction of the PML-RARalpha fusion product of acute promyelocytic leukemia

We have designed several retroviral constructs to transduce the PML-RA Ralpha fusion product of human acute promyelocytic leukemia. Our aim to generate high-titer stable vector-producing cell lines was hindered by a toxic effect of PML-RARalpha expression on packaging cells. To circumvent this, we tested retroviral vectors expressing the transgene from several internal promoters including inducible and myelospecific promoters. To compare efficiency of these constructs in their ability to generate protein expression in the appropriate target cells and optimal viral titers, we used the BOSC23 transient packaging cell line. We found that the direct-oriented vector did not ensure tissue-specificity of PML-RARalpha expression, while the reverse-oriented retroviral vector did. The latter construct, however, failed to generate high-titer recombinant virus. This study exemplifies the unpredictable behavior of retroviral constructs and the superiority of transient systems for transduction of a toxic product.

Long-term secretion of therapeutic proteins from genetically modified skeletal muscles

Protein delivery from genetically modified skeletal muscle has been reported previously. However, a stable and prolonged secretion was obtained in immunocompromised or newborn animals only. To evaluate the clinical relevance of this approach, we have transduced myoblasts from an adult beta-glucuronidase-deficient (MPS VII) mouse with retroviral vectors carrying either the human beta-glucuronidase cDNA or the murine erythropoietin (Epo) cDNA. The cells were then grafted into the tibialis anterior muscle of adult immunocompetent MPS VII recipients. Protein expression was controlled either by ubiquitous or muscle-specific transcriptional regulatory elements. Animals were analyzed over an 8-month period. The in situ detection of beta-glucuronidase activity revealed up to 60% of genetically modified myofibers in the recipient muscles. The human desmin promoter and enhancer showed the highest in vivo expression. Secretion of beta-glucuronidase induced a disappearance of lysosomal storage lesions in the liver and spleen of recipient animals. Delivery of Epo led to a permanent increase of hematocrit values over 3 months. These results showed that the transplantation of genetically modified myoblasts allowed a sustained secretion of recombinant proteins at therapeutic levels in immunocompetent adult mice. They suggest that the approach may be considered for human applications.

Functional retroviral vector for gene therapy of xeroderma pigmentosum group D patients

Xeroderma pigmentosum (XP) is an autosomal recessive genetic disorder characterized by an increased frequency of skin cancer following minimal sunlight exposure. Cells isolated from XP patients are also hypersensitive to UV rays and UV-like chemicals. This sensitivity is directly related to a defect in the early steps of nucleotide excision repair (NER) of damaged DNA. No efficient treatment is available for this disease and skin cancer prevention can only be achieved by strict avoidance of sunlight exposure. Thus, we are developing a model for gene therapy in XP, particularly for patients belonging to group D. We report here the construction of a retroviral vector (LXPDSN) containing the XPD (ERCC2) cDNA, which fully complements the DNA repair deficiency of primary skin fibroblasts. Efficient integration, mRNA synthesis, and protein expression of the XPD gene were obtained in all LXPDSN-transduced XP-D fibroblasts tested. Full correction of the DNA repair defect was observed with all DNA repair assays used, such as an increased survival after UV-radiation of the transduced cells, a normal level of DNA repair synthesis (UDS), and the reactivation of a UV-irradiated reporter vector. This retroviral vector will be used to modify keratinocytes genetically to produce repair proficient reconstituted skin for engraftment to XP patients.

In vivo delivery of human alpha-L-iduronidase in mice implanted with neo-organs

Mucopolysaccharidose type I is a lysosomal storage disease caused by a deficiency in the enzyme alpha-L-iduronidase (IDUA). The existence of a secretory pathway for lysosomal enzymes and the capture of secreted molecules by distant cells through binding to mannose-6-phosphate receptors have provided a rationale for enzyme replacement therapy in lysosomal storage diseases. We have used genetically modified fibroblasts implanted into neo-organs as an in vivo delivery system for IDUA. The human IDUA cDNA was isolated and inserted into a retroviral vector where it was expressed from the phosphoglycerate kinase 1 gene promoter. MPS I fibroblasts transduced with this vector showed high levels of IDUA activity and secreted phosphorylated molecules that could be internalized by naive deficient cells. Neo-organs containing 2 x 10(7) IDUA-secreting cells were implanted into nude mice. Human and murine IDUA activities were measured in the liver and spleen of animals sacrificed 35-77 days after implantation. Human IDUA activity corresponded to 0.6-2.3% of the murine enzyme activity in the liver and to 0.1-0.3% in the spleen. These data indicated that human IDUA was secreted from neo-organs and internalized in distant tissues.

Human immunodeficiency virus type 1 Nef increases the efficiency of reverse transcription in the infected cell

We have analyzed the replication of Nef+ and Nef- isogenic human immunodeficiency virus in CEM, HUT78, MT4 lymphoid, and U937 monocytic cell lines. At each passage of infected cells, we have assessed the relative infectivity of the virus particles released in culture media by measuring the number of infections units per nanogram of p24 protein. Values appeared to be 3- to 10-fold higher for the Nef+ virus than for the Nef- number The positive effect of Nef was observed regardless of the cell line, the multiplicity of infection, and the number of virus replication cycles achieved. We showed, by using cells expressing glycosylphosphatidylinositol-linked CD4, that the enhancement of virion infectivity could be dissociated from the down-regulation of cell surface CD4 also induced by Nef. The gp120-to-p24 ratio and the RNA content of virus particles produced in the presence or in the absence of Nef were equivalent. Virions bound to cell surface CD4 receptors with equal efficiencies. Equivalent reverse transcriptase activities were measured both on exogenous substrate and on particle genomic RNAs. In contrast, reverse transcription in infected cells generated 5- to 10-fold less DNA when the virions were produced in the absence of Nef, indicating that these particles performed reverse transcription in a suboptimal environment. These data suggest that the expression of Nef in virus-producing cells is required for efficient processing of the early stages of virus replication in target cells, including the internalization in an appropriate cell compartment and the uncoating of the particle.

Correction of deficient enzyme activity in a lysosomal storage disease, aspartylglucosaminuria, by enzyme replacement and retroviral gene transfer

The ability of lysosomal enzymes to be secreted and subsequently captured by adjacent cells provides an excellent basis for investigating different therapy strategies in lysosomal storage disorders. Aspartylglucosaminuria (AGU) is caused by deficiency of aspartylglucosaminidase (AGA) leading to interruption of the ordered breakdown of glycoproteins in lysosomes. As a consequence of the disturbed glycoprotein catabolism, patients with AGU exhibit severe cell dysfunction especially in the central nervous system (CNS). The uniform phenotype observed in these patients will make effective evaluation of treatment trials feasible in future. Here we have used fibroblasts and lymphoblasts from AGU patients and murine neural cell lines as targets to evaluate in vitro the feasibility of enzyme replacement and gene therapy in the treatment of this disorder. Complete correction of the enzyme deficiency was obtained both with recombinant AGA enzyme purified from CHO-K1 cells and with retrovirus-mediated transfer of the AGA gene. Furthermore, we were able to demonstrate enzyme correction by cell-to-cell interaction of transduced and nontransduced cells.

Continuous secretion of human soluble CD4 in mice transplanted with genetically modified cells

Somatic transgenesis can be used to confer endogenous production of proteins with therapeutic properties. One such product, recombinant soluble human CD4 (sCD4), has been shown to be an efficient inhibitor of human immunodeficiency virus 1 (HIV-1) in vitro, but its too short half-life in vivo has impaired long-term clinical trials in AIDS patients. Using a retroviral vector, we introduced the cDNA of sCD4 into primary mouse fibroblasts. The cells were enclosed in a lattice of collagen and synthetic fibers coated with basic fibroblast growth factor, and implanted in the peritoneal cavity of syngeneic mice. Implantation of such sCD4-secreting organoids into cyclosporin A-treated C3H mice elicited a strong antibody response against sCD4. Implantation of sCD4-secreting organoids into immunotolerant mice (transgenic for transmembrane human CD4) resulted in continuous sCD4 production, detected during 60 days in animal sera. The serum levels obtained were significant, but too limited as yet for anti-HIV purposes. Nevertheless, this model may be of interest in various fields, as it provides the first demonstration that one potentially therapeutic protein, despite its half-life of a few hours, could remain present in vivo 2 months after a single somatic transgenesis.

Sustained delivery of erythropoietin in mice by genetically modified skin fibroblasts

We have examined whether the secretion of erythropoietin (Epo) from genetically modified cells could represent an alternative to repeated injections of the recombinant hormone for treating chronic anemias responsive to Epo. Primary mouse skin fibroblasts were transduced with a retroviral vector in which the murine Epo cDNA is expressed under the control of the murine phosphoglycerate kinase promoter. "Neo-organs" containing the genetically modified fibroblasts embedded into collagen lattices were implanted into the peritoneal cavity of mice. Increased hematocrit (> 80%) and elevated serum Epo concentration (ranging from 60 to 408 milliunits/ml) were observed in recipient animals over a 10-month observation period. Hematocrit values measured in recipient mice varied according to the number of implanted Epo-secreting fibroblasts (ranging from 2.5 to 20 x 10(6)). The implantation of neo-organs containing Epo-secreting fibroblasts appeared, therefore, as a convenient method to achieve permanent in vivo delivery of the hormone. We estimated that the biological efficacy of the approach may be relevant for the treatment of human hemoglobinopathies.

Long-term delivery of a lysosomal enzyme by genetically modified fibroblasts in dogs

We have evaluated the feasibility and efficacy of intraperitoneal implants (neo-organs) for protein delivery in large animals. Skin biopsies were taken from four healthy dogs. Primary fibroblast cultures were transduced with a retroviral vector coding for the human beta-glucuronidase. One to six lattices each containing 10(9) skin fibroblasts were implanted into the omentum of the donor animal. Laparotomies performed at regular intervals showed vascularized neo-organs without local inflammation. Human beta-glucuronidase levels equivalent to 0.8 to 3.1% of the endogenous canine activity were detected for up to 340 days on liver biopsy samples. These results indicate that neo-organs can be considered for the long-term delivery of therapeutic proteins or enzymes in humans.

Retroviral-mediated gene transfer corrects very-long-chain fatty acid metabolism in adrenoleukodystrophy fibroblasts

Adrenoleukodystrophy (ALD), a lethal demyelinating disease of the brain, is caused by mutations of a gene encoding an ATP-binding transporter, called ALDP, localized in the peroxisomal membrane. It is associated with a defective oxidation of very-long-chain fatty acids, leading to their accumulation in many tissues. This study reports that the retroviral-mediated transfer of the ALD cDNA restored very-long-chain fatty acid oxidation in ALD fibroblasts in vitro following abundant expression and appropriate targeting of the vector-encoded ALDP in peroxisomes. The same method may be used in hematopoietic cells as a further step of a gene therapy approach of ALD.

Receptor-binding domain of murine leukemia virus envelope glycoproteins

The surface glycoprotein (SU) of murine leukemia viruses (MuLVs) comprises two domains connected by a proline-rich hinge. The interaction of MuLV particles with subgroup-specific cell surface receptors depends primarily on two variable regions (VRA and VRB) located in the amino-terminal domain. To delineate the minimal receptor-binding domains, we examined the capacity of soluble envelope fragments to compete with the entry of virus particles. Amphotropic, ecotropic, polytropic, and xenotropic truncated SUs were produced by inserting stop codons in the env gene of the 4070A, Friend, MCF247 and NZB MuLVs, respectively. These fragments, as well as full-length envelope glycoproteins, were stably expressed in cells bearing the corresponding receptor. Synthesis, posttranslational modifications, transport, and secretion of the env gene products were monitored by immunoprecipitation. Cells expressing the modified SUs or naive cells preincubated with SU-containing conditioned media were infected with different pseudotypes of a retroviral vector carrying a beta-galactosidase marker gene. Reduction of cell susceptibility to infection in the presence of SU was used as a measure of receptor occupancy. The results indicated that the amphotropic and ecotropic envelope amino-terminal domains contain all of the determinants required for receptor binding. In contrast, additional sequences in the proline-rich region were needed for efficient interaction of the polytropic and xenotropic amino-terminal domains with the receptors.

Human immunodeficiency virus type 1 Nef induces accumulation of CD4 in early endosomes

We have studied the fate of CD4 in CEM T cells expressing a human immunodeficiency virus type 1 HIV-1 Nef protein. Nef triggered a rapid endocytosis and a degradation of CD4, while most of the p56lck was upheld at the cell membrane. In the presence of Nef, CD4 accumulated in acidic intracellular vesicles that were not stained by antibodies against rab6, a marker of the Golgi apparatus complex. Detection of transferrin in CD4-containing vesicles showed that CD4 was trapped in early endosomes, without significant accumulation of CD4 in late endocytic compartments. Internalization pathways taken by CD4 in Nef+ cells may therefore be different from those observed after treatment with phorbol esters.

Gene therapy of lysosomal storage disorders

Lysosomal storage disorders (LSD) result from deficiencies in enzymes normally implicated in the catabolism of macromolecules inside the lysosome. Many of these enzymes can reach the lysosome after being secreted in the extracellular medium and recaptured by specific cell surface receptors. This has suggested a rationale for therapeutic approaches in LSD, in which the missing enzyme is provided by an external source. Current therapies based on this concept, including the administration of purified enzyme and bone marrow transplantation, have been shown to result in clinical improvements in both animal models and patients. Although considerable difficulties must be surmounted, LSD present a favourable situation for gene therapy. The gene corresponding to the affected enzyme has been identified in most diseases and cDNAs are available. Low and unregulated levels of enzyme activity should be sufficient for correction. Importantly, a variety of gene transfer strategies can be carefully evaluated in animal models.

Role of N-linked glycosylation in the activity of the Friend murine leukemia virus SU protein receptor-binding domain

The 243 N-terminal residues of Friend Murine Leukemia Virus envelope glycoprotein (SU) fold into a structurally and functionally autonomous domain which contains the determinants for binding to the ecotropic virus receptor. The two N-linked glycosylation sites present in this N-terminal portion of the viral SU were removed by site-directed mutagenesis without disturbing its biosynthesis and incorporation into infectious virions. A truncated version of the mutant protein which included only the N-terminal domain was poorly transported but still able to interact with the receptor. Interference assays indicated that the interaction between the mutated protein and the virus receptor was weaker. We conclude that the elimination of N-linked oligosaccharide chains in the envelope N-terminal domain do not prevent receptor interaction but results in subtle conformational changes that may alter recognition and binding.

Modifications in the binding domain of avian retrovirus envelope protein to redirect the host range of retroviral vectors

On the basis of theoretical structural and comparative studies of various avian leukosis virus SU (surface) envelope proteins, we have identified four small regions (I, II, III, and IV) in their receptor-binding domains that could potentially be involved in binding to receptors. From the envelope gene of an avian leukosis virus of subgroup A, we have constructed a set of SU mutants in which these regions were replaced by the coding sequence of FLA16, a 16-amino-acid RGD-containing peptide known to be the target for several cellular integrin receptors. Helper-free retroviral particles carrying a neo-lacZ retroviral vector were produced with the mutant envelopes. SU mutants in which regions III and IV were substituted yielded normal levels of envelope precursors but were not detectably processed or incorporated in viral particles. In contrast, substitutions in regions I and II did not affect the processing and the viral incorporation of SU mutants. When FLA16 was inserted in region II, it could be detected with antibodies against FLA16 synthetic peptide, but only when viral particles were deglycosylated. Viral particles with envelopes mutated in region I or II were able to infect avian cells through the subgroup A receptor at levels similar to those of the wild type. When viruses with envelopes containing FLA16 peptide in region II were applied to plastic dishes, they were found to promote binding of mammalian cells resistant to infection by subgroup A avian leukosis viruses but expressing the integrins recognized by FLA16. Deglycosylated helper-free viruses obtained by mild treatment with N-glycosidase F have been used to infect these mammalian cells, and infections have been monitored by neomycin selection. No neomycin-resistant clones could be obtained after infection by viruses with wild-type envelopes. Conversely, colonies were obtained after infection by viruses with envelopes bearing FLA16 in region II, and the genome of the retroviral vector was found correctly integrated in cell DNA of these colonies. By using a blocking peptide containing the minimal adhesive RGD sequence contained in FLA16, we have shown that preincubation of target cells could specifically inhibit infection by viruses with FLA16.

Impairment of T cell receptor-dependent stimulation in CD4+ lymphocytes after contact with membrane-bound HIV-1 envelope glycoprotein

A CD4+ human T cell clone (SPB21) or primary blood mononuclear cells were grown in the presence of HeLa cells stably expressing functional human immunodeficiency virus type 1 envelope glycoprotein complexes at their surface. After a short cocultivation, SPB21 cells lost their ability to proliferate in response to T cell receptor (TCR) stimulations and died by apoptosis, whereas interleukin-2 stimulation was still effective. Incubation with soluble monomeric gp120 did not alter TCR responsiveness. A selective decrease in the proportion of CD4+ cells was also observed among primary lymphocytes after cocultivation and OKT3 stimulation. We propose that binding of oligomeric membrane-bound envelope glycoprotein induces a multimerization of CD4 molecules that impairs normal TCR stimulation.

Transplantation of adult-derived myoblasts in mice following gene transfer

We have explored the use of myoblasts obtained from adult animals as a target for somatic gene therapy. Myoblasts from an adult beta-glucuronidase deficient (MPS VII) mouse were isolated and infected with a retroviral vector carrying the human beta-glucuronidase cDNA. Beta-glucuronidase was used as a reporter gene to follow the fate of genetically-modified myoblasts after transplantation into the tibialis anterior of MPS VII recipients. When experimental necrosis had been induced in the recipient muscle prior to cell injection, histological analysis demonstrated efficient engraftment of adult derived myoblasts following gene transfer. The reconstituted myofibres expressed the transgene for at least 10 weeks following transplantation.

Disappearance of lysosomal storage in spleen and liver of mucopolysaccharidosis VII mice after transplantation of genetically modified bone marrow cells

Mice homozygous for the gusmps allele lack beta-glucuronidase activity and provide a useful model for human Mucopolysaccharidosis type VII (MPS VII), also known as Sly syndrome. Bone marrow (BM) transplantation was shown to correct the metabolic defect and to increase the life span of diseased animals. We have used this murine model in a preclinical study aimed at evaluating whether the techniques currently available for gene transfer into large mammalian and human BM cells will provide efficient enzyme replacement therapy in MPS patients. Autologous BM was transplanted into deficient mice after retrovirus-mediated transfer of the human beta-glucuronidase cDNA. Conditioning of recipients was performed by a single sublethal irradiation of 4.5 Gy, giving rise to low donor engraftment. In recipient mice analyzed until 145 days after gene transfer, the percentage of genetically modified hematopoietic cells was less than 5%. Nevertheless, beta-glucuronidase enzyme activity was detectable in various organs, including the brain, and disappearance of lysosomal storage was obvious in the liver and spleen. These results show that the autologous transplantation of genetically engineered BM cells could be beneficial in MPS patients.

In situ retrovirus-mediated gene transfer into dog liver

Dogs were used as a large animal model to assess the feasibility and safety of a surgical method for gene transfer into hepatocytes in vivo. This method, which we previously described in rats, consists of a partial hepatectomy aimed at inducing liver regeneration, followed by the selective in situ perfusion of the remnant liver parenchyma with a retrovirus preparation. Isolation of the liver was obtained by clamping the afferent and efferent blood vessels, a procedure that prevented retroviral vector dissemination and genetic modification of nonhepatic organs. A helper-free retrovirus vector encoding beta-galactosidase targeted to the nucleus was perfused in the liver of 5 golden retriever dogs. Volumes up to 1,650 ml of fresh or concentrated vector stocks were perfused and the procedure was well tolerated. Gene transfer, observed in 3 of 5 treated dogs when documented on liver biopsy fragments obtained at day 4, involved 0.15-0.6% hepatocytes and persisted at equivalent levels at the time of sacrifice, 6 weeks later. No propagation of the vector to other tissues was detected. These observations suggest that the selective perfusion of the regenerating liver might be considered an alternative to liver transplantation for the treatment of certain severe genetic liver disorders, or for the delivery of a therapeutic protein into the serum.

Continuous systemic secretion of a lysosomal enzyme by genetically modified mouse skin fibroblasts

Lysosomal enzymes secreted or externally supplied into the extracellular medium can be internalized by cells and targeted to lysosomes after binding to specific membrane receptors. This process allows for the replacement of the missing enzyme activity in deficient cells. Using a retroviral vector, we have introduced the human beta-glucuronidase cDNA into primary mouse skin fibroblasts. The genetically modified cells were then engrafted into neo-organs that had been previously implanted into the peritoneal cavity of syngeneic recipient mice. The hypervascularized structures, made of collagen and basic fibroblast growth factor-coated synthetic fibers embedded into extracellular matrix gel, allowed in vivo survival of engrafted fibroblasts that expressed the human beta-glucuronidase cDNA for at least 3 months. The human enzyme was detected in the liver, lung, and spleen of experimental animals, but became undetectable after removal of the neo-organ. This observation indicated that the human enzyme was secreted into the serum and then captured by distant organs. The use of genetically modified fibroblasts implanted into neo-organs may, therefore, represent a convenient approach to enzyme replacement therapy in lysosomal storage diseases.

Correction of lysosomal storage in the liver and spleen of MPS VII mice by implantation of genetically modified skin fibroblasts

Genetic defects of lysosomal hydrolases result in severe storage diseases and treatments based on enzyme replacement have been proposed. In mice lacking beta-glucuronidase, which develop a disease homologous to human mucopolysaccharidosis type VII (Sly syndrome), we have used autologous implants of genetically-modified skin fibroblasts for the continuous in vivo production of the enzyme. The human beta-glucuronidase cDNA was introduced with a retroviral vector into mutant mice skin fibroblasts grown in primary culture. Fourteen mutant mice were implanted intraperitoneally with these modified cells embedded into collagen lattices. All animals expressed beta-glucuronidase from the vascularized neo-organs that developed after implantation and accumulated the enzyme in their tissues. A complete disappearance of the lysosomal storage lesions was observed in their liver and spleen.

Reduced cell surface expression of processed human immunodeficiency virus type 1 envelope glycoprotein in the presence of Nef

nef genes from two laboratory grown human immunodeficiency virus type 1 (HIV-1) strains and from two proviruses that had not been propagated in vitro were introduced into CD4+ lymphoblastoid CEM cells. The stable expression of all four Nef proteins was associated with an almost complete abrogation of CD4 cell surface localization. The consequences of the presence of Nef on gp160 cleavage, gp120 surface localization, and envelope-induced cytopathic effect were examined in CEM cells in which the HIV-1 env gene was expressed from a vaccinia virus vector. The presence of Nef did not modify the processing of gp160 into its subunits but resulted in a significant decrease of cell surface levels of gp120, associated with a dramatic reduction of the fusion-mediated cell death. Surface levels of mutant envelope glycoproteins unable to bind CD4 were not altered in Nef-expressing cells, suggesting that the phenomenon was CD4 dependent. The intracellular accumulation of fully processed envelope glycoproteins could significantly delay the cytopathic effect associated with envelope surface expression in HIV-infected cells and may be relevant to the selective advantage associated with Nef during the in vivo infectious process.

Activation pathways and human immunodeficiency virus type 1 replication are not altered in CD4+ T cells expressing the nef protein

While recent studies in Rhesus monkeys have pointed out the importance of an intact nef gene for the development of acquired immunodeficiency syndrome (AIDS), no biological function has been so far unambiguously attributed to its product. Since Nef has been described to possess GTP-binding properties and to down-regulate CD4 cell surface expression, we looked for evidences of Nef interfering with the transduction of activating signals in human CD4+ T cells. We used a murine leukemia retroviral vector to express the HIV-1BRU nef gene in two permanent tumoral T-cell lines (CEM and Jurkat) and in two nonimmortalized, interleukin-2 (IL2)-dependent, T-cell clones. The single copy recombinant provirus integrated in the genome of these cells directed the synthesis of a 27-kD protein with a half-life greater than 5 h. The levels of expression of cell surface molecules involved in T-cell functions (CD4, CD3, CD28, CD29, IL-2 receptor) were not modified in cell populations expressing Nef. In immunocompetent T-cell clones, cell proliferation and lymphokine production in response to activating stimuli (IL-2, alloantigens, phorbol esters, or antibodies directed against CD2, CD3, CD4, CD28) remained unmodified. Moreover, the presence of Nef did not change the kinetics of human immunodeficiency virus (HIV) infection.

Receptor choice determinants in the envelope glycoproteins of amphotropic, xenotropic, and polytropic murine leukemia viruses

The envelope glycoproteins (SU) of mammalian type C retroviruses possess an amino-terminal domain of about 200 residues, which is involved in binding a cell surface receptor. In this domain, highly conserved amino acid sequences are interrupted by two segments of variable length and sequence, VRA and VRB. We have studied the role of these variable regions in receptor recognition and binding by constructing chimeric molecules in which portions of the amino-terminal domains from amphotropic (4070A), xenotropic (NZB), and polytropic (MCF 247) murine leukemia virus SU proteins were permuted. These chimeras, which exchanged either one or two variable regions, were expressed at the surface of replication-defective viral particles by a pseudotyping assay. Wild-type or recombinant env genes were transfected into a cell line producing Moloney murine leukemia virus particles devoid of envelope glycoproteins in which a retrovirus vector genome carrying an Escherichia coli lacZ gene was packaged. The host range and sensitivity to interference of pseudotyped virions were assayed, and we observed which permutations resulted in receptor switch or loss of function. Our results indicate that the determinants of receptor choice are found within the just 120 amino acids of SU proteins. Downstream sequences contribute to the stabilization of the receptor-specific structure.

Construction of Retro viral Packaging Cell Lines

Most of the time, retrovirus vectors retain only cis-acting sequences from the original viral genome. These sequences allow the recombinant structure to be transcribed (LTR promoter/enhancer) and the RNA to be processed (splicing and polyadenylation signals), packaged into a virion particle (packaging sequences), and replicated by the reverse transcriptase (tRNA binding site, R region, and polypurine track). The other viral functions have to be provided in trans for the assembly of recombinant viral particles to take place. This can be simply achieved by using a replication-competent helper virus, leading to the production of a mixed population. Nevertheless, helper-free stocks are desirable for most applications, since 1. The high frequency of recombination in a mixed virus stock is likely to lead to the appearance of recombinants with unknown structure and activity. These new chimeras, either spread by the helper virus or replication-competent themselves, create a potential safety problem. 2. Cell lineage analysis using retroviral marking can only be performed and interpreted in a helper-free context 3. In vivo gene transfer experiments can be jeopardized by disease (s) associated with helper-virus infection (1).

Clonal segregation of oligodendrocytes and astrocytes during in vitro differentiation of glial progenitor cells

To study the clonal lineage of the glial progenitor population, isolated from newborn rat brain (Lubetzki et al. J Neurochem 56:671, 1991), we combined somatic transgenesis using a retroviral vector encoding a modified bacterial beta-galactosidase with nuclear localization, and triple immunofluorescence labeling with A2B5, anti-galactosylceramide, and anti-glial acidic fibrillary protein antibodies. This allowed clonal analysis of the postnatal glial lineage with precise phenotypic identification of each cell within the lacZ-positive clones. When infected cells were cultivated under constant conditions, in the presence of either 1% or 10% fetal calf serum (FCS)-containing medium, all the 250 lacZ-positive clusters examined were homogeneous, i.e., either oligodendroglial or astroglial. Mixed astrocyte-oligodendroglial clones were observed when cells cultivated in the presence of 1% FCS were switched to a 10% FCS-containing medium, confirming the bipotentiality of glial progenitor cells (Temple and Raff Nature 313:223, 1985). However, even under the switch culture conditions, segregation into homogeneous clones of either oligodendrocytes or astrocytes still predominated, and the percentage of mixed clones dropped from 25 to 8 or to 3, when the switch took place at 8, 16, or 22 days in vitro, respectively. Two additional observations lead us to suggest that microenvironmental factors are responsible for the clonal segregation of glial progenitor cells: 1) the uneven distribution of oligodendrocyte and astrocyte clusters, the latter being seen mostly on the edge of the coverslips; and 2) the presence, in the vicinity of an homogeneous lacZ-positive clone, of some lacZ-negative cells expressing the same phenotype.

Retroviral-mediated gene transfer into hepatocytes in vivo

Stable gene transfer into hepatocytes might be used to compensate for a genetic deficiency affecting liver function or to deliver diffusible factors into the blood stream. In rats, we have combined retroviral-mediated gene transfer with a surgical procedure in which the liver is temporarily excluded from the circulation and infected in vivo. Partial hepatectomy was performed 24-48 hr before perfusion with virus to induce hepatocyte division and facilitate viral integration. A helper-free recombinant retrovirus coding for beta-galactosidase with nuclear localization was used to score cells that expressed the transgene. For at least 3 months after gene transfer, up to 5% of hepatocytes expressed nuclear beta-galactosidase. Whereas in vitro reimplantation of genetically modified hepatocytes has proved to be inefficient in stably transferring genes into the liver, our approach provides a feasible alternative.

An amino-terminal fragment of the Friend murine leukemia virus envelope glycoprotein binds the ecotropic receptor

Retrovirus entry into cells is mediated by specific binding of the envelope glycoprotein to a cell membrane receptor. Constitutive envelope gene expression prevents infection by interfering with the binding of viruses which recognize the same receptor. We have used this property to investigate the receptor binding capacities of deleted or truncated murine leukemia virus ecotropic envelope glycoproteins. Friend murine leukemia virus envelope glycoproteins bearing internal amino-terminal deletions, or a soluble 245-amino-acid gp70 amino-terminal fragment, were expressed in NIH 3T3 cells. The susceptibility of these cells to ecotropic and amphotropic virus infection was determined. We observed that both membrane-bound and soluble forms of the gp70 245-amino-acid amino-terminal domain induced resistance to ecotropic virus, indicating that this fragment binds the ecotropic receptor. Binding occurs both at the cell surface and in the endoplasmic reticulum, as shown by the use of soluble envelope fragments either secreted in the culture supernatants or retained in the endoplasmic reticulum lumen by a KDEL sequence. These results suggest that the gp70 amino-terminal domain folds into a structure which recognizes the ecotropic receptor regardless of the carboxy-terminal part of the molecule.

Expression of human adenosine deaminase in mice reconstituted with retrovirus-transduced hematopoietic stem cells

Recombinant retroviruses encoding human adenosine deaminase (ADA; adenosine aminohydrolase, EC 3.5.4.4) have been used to infect murine hematopoietic stem cells. In bone marrow transplant recipients reconstituted with the genetically modified cells, human ADA was detected in peripheral blood mononuclear cells of the recipients for at least 6 months after transplantation. In animals analyzed in detail 4 months after transplantation, human ADA and proviral sequences were detected in all hematopoietic lineages; in several cases, human ADA activity exceeded the endogenous activity. These studies demonstrate the feasibility of introducing a functional human ADA gene into hematopoietic stem cells and obtaining expression in multiple hematopoietic lineages long after transplantation. This approach should be helpful in designing effective gene therapies for severe combined immunodeficiency syndromes in humans.

Use of retroviral vectors for mapping of splice sites in cottontail rabbit papillomavirus

Cottontail rabbit papillomavirus (CRPV) genomic sequences coding for virus early functions were introduced into a retroviral vector in order to produce cDNAs of the viral early region. Two constructs differing in the length of control sequences preceding the E6 open reading frame were transfected into Psi-2 cells and the released retroviral stock was used to infect NIH3T3 cells. The proviral sequences were rescued from antibiotic G418-resistant virus-infected cells after fusion with Cos cells, amplified as plasmids in Escherichia coli and analysed. Nucleotide sequencing showed that the splicing signals used in the construct containing only the early coding region are the same as in CRPV-expressing tumours, linking the beginning of E1 to the middle of E2. On the other hand, in a construct including most of the long control region a splice donor site located in the 5' end of this region, at position 7810, was very efficiently used, totally excluding the use of the donor site at position 1371. None of the constructs containing CRPV sequences transcribed from Moloney murine leukaemia virus promoter was able to transform mouse fibroblasts after DNA transfection.

Safe and efficient generation of recombinant retroviruses with amphotropic and ecotropic host ranges

We have constructed a set of packaging cell lines useful for the generation of helper-free recombinant retroviruses with amphotropic and ecotropic host ranges. To eliminate the problems of transfer of packaging functions and helper virus formation encountered with the previously available packaging systems, two mutant Moloney murine leukemia virus-derived proviral genomes carrying complementary mutations in the gag-pol or env regions were sequentially introduced into NIH 3T3 cells by cotransformation. Both genomes contained a deletion of the psi sequence necessary for the efficient encapsidation of retroviral genomes into virus particles and additional alterations at the 3' end of the provirus. We show that the resulting packaging cell lines psi CRIP and psi CRE can be used to isolate clones that stably produce high titers (10(6) colony-forming units/ml) of recombinant retroviruses with amphotropic and ecotropic host ranges, respectively. More importantly, we demonstrate that viral producers derived from the packaging cell lines do not transfer the packaging functions, or yield helper virus, even under conditions where existing packaging cell lines can be shown to yield transfer of packaging functions and/or helper virus. These properties of the psi CRIP and psi CRE packaging lines make them particularly valuable reagents for in vivo gene transfer studies aimed at cell lineage analysis and the development of human gene replacement therapies.

Nucleotide sequence and comparative analysis of the human papillomavirus type 18 genome. Phylogeny of papillomaviruses and repeated structure of the E6 and E7 gene products

The complete nucleotide sequence and genomic organization of human papillomavirus type 18, associated with cervical cancer, has been established. A detailed comparative analysis was undertaken leading to the identification of a number of features specific for genital papillomaviruses and the construction of a phylogenetic tree. Genital papillomaviruses differ from other human and animal papillomaviruses as they possess a longer E1 open reading frame (ORF) and have a characteristic control region. Phylogenetically, HPV 18 is located between the benign genital viruses, HPV 6 and HPV 11, and the malignant isolates, HPV 16 and HPV 33, and may represent an evolutionary intermediate among oncogenic papillomaviruses. Viral gene products known to be involved in cellular transformation are those of ORFs E5, E6 and E7. Significant sequence variation was found between the E6 to E7 regions of different integrated forms of HPV 18. On re-examination of the E6 primary structures we noticed that the gene has evolved by successive duplications of a unit encoding 33 amino acids, which include a Cys-X-X-Cys motif. Furthermore, the E7 gene product has apparently evolved in the same manner and is related to E6. Both gene products bear a striking resemblance to the transcriptional factor IIIA of Xenopus laevis, the prototype of a new class of nucleic acid binding proteins.