Relevance of an academic GMP Pan-European vector infra-structure (PEVI)

In the past 5 years, European investigators have played a major role in the development of clinical gene therapy. The provision of substantial funds by some individual member states to construct GMP facilities makes it an opportune time to network available gene therapy GMP facilities at an EU level. The integrated coordination of GMP production facilities and human skills for advanced gene and genetically-modified (GM) cell therapy, can dramatically enhance academic-led "First-in-man" gene therapy trials. Once proof of efficacy is gathered, technology can be transferred to the private sector which will take over further development taking advantage of knowledge and know-how. Complex technical challenges require existing production facilities to adapt to emerging technologies in a coordinated manner. These include a mandatory requirement for the highest quality of production translating gene-transfer technologies with pharmaceutical-grade GMP processes to the clinic. A consensus has emerged on the directions and priorities to adopt, applying to advanced technologies with improved efficacy and safety profiles, in particular AAV, lentivirus-based and oncolytic vectors. Translating cutting-edge research into "First-in-man" trials require that pre-normative research is conducted which aims to develop standard assays, processes and candidate reference materials. This research will help harmonise practices and quality in the production of GMP vector lots and GM-cells. In gathering critical expertise in Europe and establish conditions for interoperability, the PEVI infrastructure will contribute to the demands of the advanced therapy medicinal products* regulation and to both health and quality of life of EU-citizens.

Efficient gene transfer in skeletal muscle with AAV-derived bicistronic vector using the FGF-1 IRES

IRESs (internal ribosome entry sites) are RNA elements behaving as translational enhancers in conditions of global translation blockade. IRESs are also useful in biotechnological applications as they allow expression of several genes from a single mRNA. Up to now, most IRES-containing vectors use the IRES from encephalomyocarditis virus (EMCV), highly active in transiently transfected cells but long and not flexible in its positioning relative to the gene of interest. In contrast, several IRESs identified in cellular mRNAs are short and flexible and may therefore be advantageous in gene transfer vectors such as those derived from the adeno-associated virus (AAV), where the size of the transgene expression cassette is limited. Here, we have tested bicistronic AAV-derived vectors expressing two luciferase genes separated by the EMCV- or fibroblast growth factor 1 (FGF-1) IRES. We demonstrate that the AAV vector with the FGF-1 IRES, when administrated into the mouse muscle, leads to efficient expression of both transgenes with a stable stoechiometry, for at least 120 days. Interestingly, the bicistronic mRNA containing the FGF-1 IRES leads to transgene expression 10 times superior to that observed with EMCV, in vivo. AAV vectors featuring the FGF-1 IRES may thus be advantageous for gene therapy approaches in skeletal muscle involving coexpression of genes of interest.

Production of spliced DNA copies of the cottontail rabbit papillomavirus genome in a retroviral vector

The early region of the cottontail rabbit papillomavirus (CRPV) genome has been introduced into a retroviral vector and recombinant retroviruses, produced upon transfection of the psi 2 packaging cell line, have been used to infect NIH 3T3 cells. Spliced derivatives of the CRPV early region can be rescued from the infected cells. Sequence analysis demonstrates that the major splicing event observed in RNA in tumours is faithfully reproduced in this system. This splice generates a polycistronic mRNA that contains in its 5' portion the E7 open reading frame, or both E6 and E7, and at its 3' end a reading frame with codons for three amino acids from the N-terminus of E1 linked to codons for 100 amino acids from the C-terminus of the E4 region. Recombinant retroviruses containing intact or spliced CRPV sequences can now be used to introduce the viral genes efficiently into a variety of cell lines.

AAV-mediated delivery of a mutated myostatin propeptide ameliorates calpain 3 but not α-sarcoglycan deficiency

Myostatin is a negative regulator of muscle mass whose inhibition has been proposed as a therapeutic strategy for muscle-wasting conditions. Indeed, blocking myostatin action through different strategies has proved beneficial for the pathophysiology of the dystrophin-deficient mdx mouse. In this report, we tested the inhibition of myostatin by AAV-mediated expression of a mutated propeptide in animal models of two limb-girdle muscular dystrophies: LGMD2A caused by mutations in the calpain 3 (CAPN3) gene and LGMD2D caused by mutations in the alpha-sarcoglycan gene (SGCA). In the highly regenerative Sgca-null mice, survival of the alpha-sarcoglycan-deficient muscle fibers did not improve after transfer of the myostatin propeptide. In calpain 3-deficient mice, a boost in muscle mass and an increase in absolute force were obtained, suggesting that myostatin inhibition could constitute a therapeutic strategy in this predominantly atrophic disorder.

Enhanced Lentiviral Transduction of Monocyte-Derived Dendritic Cells in The Presence of Conditioned Medium from Dying Monocytes

Lentiviral vectors (LVs) are attractive vehicles for the transduction of human dendritic cells (DCs) in order to mobilize their endogenous antigen presentation pathways. We analyzed here how to improve the efficiency of LV transduction, which we performed at the initial stages of the differentiation of purified monocytes into dendritic cells (Mo-DCs). Using LVs pseudotyped with the vesicular stomatitis virus envelope G glycoprotein (VSV-G), we found that a conditioned medium derived from dying monocytes (MCM) improved by 2- to 10- fold the proportion of transduced Mo-DCs. This enhanced transduction efficiency requires the presence of MCM during the initial stage of LV transduction and does not affect the phenotype and antigen presentation function of terminally differentiated Mo-DCs. Importantly, we found that MCM derived from a human acute monocytic leukemia cell line, THP-1, was equally effective. The MCM activity was heat stable (56 degrees C) and was present in the soluble fraction after high-speed centrifugation. Altogether our results show that a soluble factor present in dying monocyte cultures can replace advantageously facilitating agents such as Polybrene, to achieve high LV transductions levels. This protocol can be performed with autologous monocytes and is therefore applicable in clinical settings.

Synthesis, characterization, and gene transfer application of poly(ethylene glycol-b-ethylenimine) with high molar mass polyamine block

The study of ethyloxazoline/methyloxazoline (EtOXZ/MeOXZ) copolymerization, initiated by methyl tosylate (MeOTs), showed that (i) incorporation of MeOXZ units into random copolymer becomes effective over DP = 100 and (ii) propagation process proceeds with negligible transfer to monomer up to a DP of 400 despite the presence of MeOXZ in the polymerization medium. These results produced random poly(EtOXZ-co-MeOXZ) copolymers with various molar composition ratios in alkyloxazoline units. The close values found for the comonomer reactivity ratios in acetonitrile (r(1MeOXZ) = 1.18; r(2EtOXZ) = 0.34) implied a random chain organization in short sequences of each repeating unit, which was an important parameter in view of the optimization of their subsequent modification: the alkaline hydrolysis was successfully achieved when the MeOXZ unit content of the polyoxazoline chains reached 75%. Using these results, the diblock copolymer poly(ethylene glycol-b-(ethyloxazoline-co-methyloxazoline)) (poly(EG-b-(EtOXZ-co-MeOXZ))) with high DP was synthesized by cationic copolymerization of EtOXZ/MeOXZ comonomers using CH(3)-PEG(2kDa)-Ts as macroinitiator. The comonomer composition of this new compound was adjusted in order to optimize the hydrolysis step and obtain finally the diblock copolymer poly(ethylene glycol-b-ethylenimine) (poly(EG-b-EI)). The high molar mass of this copolymer was confirmed both by (1)H NMR and SANS measurements. Gene delivery experiments showed that the copolymer has significant DNA transfection capacities.

Lentiviral vectors targeting WASp expression to hematopoietic cells, efficiently transduce and correct cells from WAS patients

Gene therapy has been proposed as a potential treatment for Wiskott-Aldrich syndrome (WAS), a severe primary immune deficiency characterized by multiple hematopoietic-specific cellular defects. In order to develop an optimal lentiviral gene transfer cassette for this application, we compared the performance of several internal promoters in a variety of cell lineages from human WAS patients. Vectors using endogenous promoters derived from short (0.5 kb) or long (1.6 kb) 5' flanking sequences of the WAS gene, expressed the transgene in T, B, dendritic cells as well as CD34(+) progenitor cells, but functioned poorly in non-hematopoietic cells. Defects of T-cell proliferation and interleukin-2 production, and the cytoskeletal anomalies in WAS dendritic cells were also corrected. The levels of reconstitution were comparable to those obtained following transduction with similar lentiviral vectors incorporating constitutive PGK-1, EF1-alpha promoters or the spleen focus forming virus gammaretroviral LTR. Thus, native regulatory sequences target the expression of the therapeutic WAS transgene to the hematopoietic system, as is naturally the case for WAS, and are effective for correction of multiple cellular defects. These vectors may have significant advantages for clinical application in terms of natural gene regulation, and reduction in the potential for adverse mutagenic events.

Long-term expression and repeated administration of AAV type 1, 2 and 5 vectors in skeletal muscle of immunocompetent adult mice

Adeno-associated viral (AAV) vectors promote long-term gene transfer into muscle in many animal species. Increased expression levels may be obtained by using alternative serotypes in combination with repeated administrations. Here we compared AAV vectors based on serotypes 1, 2 and 5 in immunocompetent mice and assessed the feasibility of multiple administrations of either identical (readministration) or different (cross-administration) serotype-based vectors. A 1-year-long dose-response study confirmed the superiority of recombinant (r)AAV1, achieving transduction levels 5 to 10-fold higher than rAAV2 and rAAV5 in mouse skeletal muscle, respectively. Repeated administration demonstrated that increased gene transfer level was achieved with a second injection of rAAV1 following the first administration of rAAV2 or rAAV5. A readministration study with a vector encoding a different gene allowed the evaluation of gene expression from the second vector only. All three rAAVs were inhibited when the animals were previously exposed to the same serotype. In contrast, no significant change in gene expression from the second vector was observed in cross-administration. A humoral immune response was elicited against the viral capsid for all three serotypes following the initial exposure. Neutralizing antibody (NAB) levels correlated with the vector dose injected. No significant cross-reactivity of NAB from a given serotype toward another was observed in vitro. These data provide the first direct comparative evaluation of re- and cross-administration of rAAV1, rAAV2 and rAAV5 in muscle, and further indicate that rAAV1 is capable of transducing muscle tissue when cross-administered.

Noninvasive monitoring of therapeutic gene transfer in animal models of muscular dystrophies

Muscular dystrophies are a genetically and phenotypically heterogeneous group of degenerative muscle diseases. A subset of them are due to genetic deficiencies in proteins which form the dystrophin-associated complex at the membrane of the myofibers. In this report, we utilized recombinant adeno-associated virus containing a U7 cassette carrying an antisense sequence aimed at inducing exon skipping of the dystrophin gene or containing the alpha-sarcoglycan gene to alleviate the dystrophic phenotype of the mdx and Sgca-null mice, respectively. As these diseases are characterized by cycle of degeneration/regeneration, we postulated that a reporter gene coadministered at the time of the treatment would make it possible to follow the extent of muscle repair. We observed that the murine secreted alkaline phosphatase (muSeAP) level was very much lower in these animal models than in normal mice. Upon treatment of the dystrophic muscle by gene transfer, the level of muSeAP was restored and correlated with the expression of the therapeutic transgene and with the level of muscle improvement. The system described here provides a simple and noninvasive procedure for monitoring the outcome of a therapeutic strategy involving cell survival.

Relationship between retroviral vector membrane and vector stability

The present work studies the physico-chemical properties of retroviral vector membrane, in order to provide some explanation for the inactivation kinetics of these vectors and to devise new ways of improving transduction efficiency. For this purpose, vectors with an amphotropic envelope produced by TE Fly A7 cells at two culture temperatures (37 and 32 °C) were characterized by different techniques. Electron paramagnetic resonance (EPR) results showed that vectors produced at 32 °C are more rigid than those produced at 37 °C. Further characterization of vector membrane composition allowed us to conclude that the vector inactivation rate increases with elevated cholesterol to phospholipid ratio. Differential scanning calorimetry (DSC) showed that production temperature also affects the conformation of the membrane proteins. Transduction studies using HCT116 cells and tri-dimensional organ cultures of mouse skin showed that vectors produced at 37 °C have higher stability and thus higher transduction efficiency in gene therapy relevant cells as compared with vectors produced at 32 °C. Overall, vectors produced at 37 °C show an increased stability at temperatures below 4 °C. Since vector membrane physico-chemical properties are affected in response to changes in culture temperature, such changes, along with alterations in medium composition, can be used prospectively to improve the stability and the transduction efficiency of retroviral vectors for therapeutic purposes.

Linear Topology Confers in Vivo Gene Transfer Activity to Polyethylenimines

Although polyethylenimines (PEIs) are frequently used transfection agents, it is still unclear which of their properties are required for efficient gene delivery. This is even more striking when working in vivo since some PEIs are able to generate significant gene expression, whereas others are not. To facilitate a rational development of compounds with improved transfection activities, studies aimed at identifying the properties involved in the transfection process seem indispensable. In the present work, we investigated how transfection with linear PEI of 22 kDa allows for high reporter gene expression in lungs after intravenous injection, whereas the branched PEI of 25 kDa does not. To this end, we synthesized L-PEI derivatives that are intermediates between linear and branched PEIs. Our results show that the topology plays a crucial role in obtaining in vivo reporter gene expression, whereas the content of primary, secondary, and tertiary amines is only of minor importance.

DNA compaction into new DNA vectors based on cyclodextrin polymer: Surface enhanced Raman spectroscopy characterization

The ability of DNA to bind polycation yielding polyplexes is widely used in nonviral gene delivery. The aim of the present study was to evaluate the DNA compaction with a new DNA vector using Raman spectroscopy. The polyplexes result from an association of a beta-cyclodextrin polymer (polybeta-CD), an amphiphilic cationic connector (DC-Chol or adamantane derivative Ada2), and DNA. The charge of the polymeric vector is effectively controlled by simple addition of cationic connector in the medium. We used surface enhanced Raman spectroscopy (SERS) to characterize this ternary complex, monitoring the accessibility of adenyl residues to silver colloids. The first experiments were performed using model systems based on polyA (polyadenosine monophosphate) well characterized by SERS. This model was then extended to plasmid DNA to study polybeta-CD/Ada2/DNA and polybeta-CD/DC-Chol/DNA polyplexes. The SERS spectra show a decrease of signal intensity when the vector/DNA charge ratio (Z+/-) increases. At the highest ratio (Z+/- = 10) the signal is 6-fold and 3-fold less intense than the DNA reference signal for Ada2 and DC-Chol polyplexes, respectively. Thus adenyl residues have a reduced accessibility as DNA is bound to the vector. Moreover, the SERS intensity variations are in agreement with gel electrophoresis and zeta potential experiments on the same systems. The overall study clearly demonstrates that the cationic charges neutralizing the negative charges of DNA result in the formation of stable polyplexes. In vitro transfection efficiency of those DNA vectors are also presented and compared to the classical DC-Chol lipoplexes (DC-Chol/DNA). The results show an increase of the transfection efficiency 2-fold higher with our vector based on polybeta-CD.

Efficient transduction of monocyte- and CD34+- derived Langerhans cells with lentiviral vectors in the absence of phenotypic and functional maturation

BACKGROUND

Gene delivery in dendritic cells (DC) has raised considerable interest to modulate DC functions and induce therapeutic immunity or tolerance in an antigen-specific fashion. Among immature DC, Langerhans cells (LC) are attractive candidates for antigen delivery using lentiviral vectors (LV).

METHODS

LC derived from monocytes (Mo-LC), or derived from CD34+ cells (CD34-LC) in the presence of cytokine cocktail, were transduced with LV expressing enhanced green fluorescent protein (E-GFP) under the control of the ubiquitous phosphoglycerate kinase (PGK) promoter at a multiplicity of infection of 18, at days 0 to 3 for Mo-LC, or at days 0 to 12 for CD34-LC. We assessed gene transfer levels from the percentage of E-GFP+ cells in the final cultures, and examined the morphology, immunophenotype, state of differentiation and function of transduced LC.

RESULTS

Day 0 transduction of monocytes or CD34+ progenitors before cytokine pre-activation and LC differentiation resulted in stable gene expression in 7.8% of Mo-LC and 24% of CD34-LC. Monocyte-derived DC (Mo-DC) differentiated in serum-free medium were also efficiently transduced up to 13.2%. Interestingly, Mo-LC cells committed towards LC phenotype were permissive for transduction up to day 3. Transduction levels of CD34-LC peaked at day 6 to 44% and decreased thereafter. LV transduction did not perturb viability, phenotype and function of E-GFP-expressing LC.

CONCLUSIONS

LC generated ex vivo can serve as vaccine vehicles in humans through efficient transduction by LV. These LC will be helpful to assess in vitro the immunogenicity of gene therapy vectors, from the characterization of their phenotypic and functional maturation.

Abnormal ion content, hydration and granule expansion of the secretory granules from cystic fibrosis airway glandular cells

The absence or decreased expression of cystic fibrosis transmembrane conductance regulator (CFTR) induces increased Na(+) absorption and hyperabsorption of the airway surface liquid (ASL) resulting in a dehydrated and hyperviscous ASL. Although the implication of abnormal airway submucosal gland function has been suggested, the ion and water content in the Cystic Fibrosis (CF) glandular secretory granules, before exocytosis, is unknown. We analyzed, in non-CF and CF human airway glandular cell lines (MM-39 and KM4, respectively), the ion content in the secretory granules by electron probe X-ray microanalysis and the water content by quantitative dark field imaging on freeze-dried cryosections. We demonstrated that the ion content (Na(+), Mg(2+), P, S and Cl(-)) is significantly higher and the water content significantly lower in secretory granules from the CF cell line compared to the non-CF cell line. Using videomicroscopy, we observed that the secretory granule expansion was deficient in CF glandular cells. Transfection of CF cells with CFTR cDNA or inhibition of non-CF cells with CFTR(inh)-172, respectively restored or decreased the water content and granule expansion, in parallel with changes in ion content. We hypothesize that the decreased water and increased ion content in glandular secretory granules may contribute to the dehydration and increased viscosity of the ASL in CF.

Myogenic program induction in mature fat tissue (with MyoD expression)

MyoD exerts a master transcriptional control over the myogenic differentiation cascade. Here, we study different approaches to induce myogenic transdifferentiation in mature adipocytes utilizing MyoD gene transfer. Organotypic cultures of fat tissue and a long-term culture of in vitro differentiated adipocytes deduced that MyoD provoked morphological changes in mature adipocytes that can be summarized as loss of fat content, acquisition of a fusiform shape and eventual fusion with committed neighbor cells. In vivo, MyoD gene transfer into rat interscapular and inguinal fat pads demonstrated that while structural proteins of muscle lineage were expressed, they co-existed with specific adipocyte proteins. Expression of these proteins diminished over time likewise the fat content. The transdifferentiation process initiated by MyoD did not require cell cycle progression and was well tolerated by the fully differentiated and mature adipocytes.

Adeno-Associated Virus Pseudotype 5 Vector Improves Gene Transfer in Arthritic Joints

The potential for gene delivery to joints, using recombinant adeno-associated virus (rAAV) vectors for the treatment of rheumatoid arthritis (RA), has received much attention. Different serotypes have different virion shell proteins and, as a consequence, vary in their tropism for diverse tissues. The aim of this study was to compare the transduction efficiency of different AAV serotypes encoding murine secreted alkaline phosphatase (mSEAP) or Escherichia coli beta-galactosidase for intraarticular gene delivery in an experimental model of arthritis. The vectors contained AAV2 terminal repeats flanking the reporter gene in an AAV1, AAV2, or AAV5 capsid, producing the pseudotypes rAAV-2/1, rAAV-2/2, and rAAV-2/5. Left knee joints of mice with collagen-induced arthritis were injected and transgene expression was analyzed by chemiluminescence or direct in situ staining of frozen sections. We show for the first time that intraarticular gene transfer with AAV- 2/5 was far more efficient than with the other serotypes tested. Transgene expression was detectable as early as 7 days after injection, reached a maximum at 21 days, and was stably expressed for at least 130 days, whereas AAV-2/1- and AAV-2/2-mediated expression levels were barely detectable. These findings provide a practical application for future local AAV-mediated gene therapy trials in RA.

A lentiviral vector encoding the human Wiskott–Aldrich syndrome protein corrects immune and cytoskeletal defects in WASP knockout mice

Wiskott-Aldrich syndrome (WAS) is an immune deficiency with thrombopenia resulting from mutations in the WASP gene. This gene normally encodes the Wiskott-Aldrich syndrome protein (WASP), a major cytoskeletal regulator expressed in hematopoietic cells. Gene therapy is a promising option for the treatment of WAS, requiring that clinically applicable WASP gene transfer vectors demonstrate efficacy in preclinical studies. Here, we describe a self-inactivating HIV-1-derived lentiviral vector encoding human WASP and show that it effectively transduced bone marrow progenitor cells of WASP knockout (WKO) mice. Transplantation of these transduced cells into lethally irradiated WKO recipients led to stable expression of WASP and correction of immune, inflammatory and cytoskeletal defects. Splenic T-cell proliferation was restored, podosomes were reinstated on bone-marrow-derived dendritic cells and colon inflammation was reduced. This shows for the first time (a) that cytoskeletal defects can be corrected in WKO mice, (b) that human WASP is biologically active in mice and (c) that a lentiviral vector is effective to express human WASP in vivo over several months. These data support further development of such lentiviral vectors for the gene therapy of WAS.

Different impacts of alleles alphaLEPRA and alphaLELY as assessed versus a novel, virtually null allele of the SPTA1 gene in trans

The family of two siblings with severe hereditary spherocytosis was investigated. The decrease was evident on both the alpha- and the beta-chains. The parents were haematologically normal. The mother was heterozygous for the low-expression polymorphic allele alphaLEPRA. The father was heterozygous for a novel combination in which one allele showed the alpha-spectrin low expression polymorphic allele alphaLELY, while his other allele showed the alphaLELY polymorphism in cis with a G-->A substitution, named Bicêtre, found at the extreme 3' end of exon 51. This combination was designated alpha(LELY-Bicêtre). The children were compound heterozygotes for alleles alphaLEPRA and alpha(LELY-Bicêtre). Reverse transcription polymerase chain reaction detected only trace amounts of the mRNA coding for alpha(LELY-Bicêtre). Mutation is therefore an essentially null mutation with no functional protein product. The lack of disease in the alphaLELY/(LELY-Bicêtre) father compared with the marked haemolysis in the alphaLEPRA/alpha(LELY-Bicêtre) children showed that expression of allele alphaLELY is not low enough to expose null alpha-spectrin alleles on the other chromosome. Quantitative estimations from these findings suggest that, to evoke spherocytosis, it is necessary that alpha-spectrin expression must be reduced to less than 25% of normal, while a reduction to 8% is sufficient.

HLA-A*0201-restricted cytolytic responses to the rtTA transactivator dominant and cryptic epitopes compromise transgene expression induced by the tetracycline on system

The tetracycline-controlled transcription system (Tet-on) is widely used to regulate gene expression in mammalian cells. In gene therapy applications, immune responses to Tet-on proteins such as the rtTA transcription factor have been reported, raising concerns about their occurrence in humans. To monitor the HLA class I cytolytic responses against Tet-on regulators, we characterized the immunogenic CD8+ epitopes within rtTA and tTS regulators using HLA-A*0201 class I transgenic mice. Epitope prediction programs, HLA-A*0201 binding assays, and peptide immunization were used to select a set of immunogenic peptides within rtTA and tTS sequences. To identify further the rejection epitopes, we expressed Tet-on protein components in vivo and found a single dominant rtTA186 CTL epitope in the rtTA tetracycline repressor domain. Target cells expressing rtTA were susceptible to CTL lysis, and rtTA expression compromised muscle transgene engraftment. To reduce the occurrence of immune responses to rtTA protein, we mutated the dominant rtTA186 epitope and found that this leads to the appearance of subdominant epitopes. As a result, we think that an epitope modification strategy is not applicable to blunt the immune response in this model. Moreover, the identification of HLA-A*0201 rtTA epitopes allowed us to demonstrate here that the delivery of the Tet-on system with weakly immunogenic rAAV vectors does not trigger primary CTL responses in mice, in contrast to DNA transfer. Altogether, the existence of HLA-A*0201 rtTA epitopes may lead to the occurrence of immune responses depending on vectors and local inflammation in gene therapy applications involving rtTA-based regulatory systems.

Identification of an HLA-A*0201-restricted epitopic peptide from human dystrophin: application in duchenne muscular dystrophy gene therapy

Dystrophin-based gene therapy treatments aimed at correcting the Duchenne muscular dystrophy phenotype require stable expression of normal dystrophin (DYST) protein in myocytes without immune responses, which would compromise long-term expression. To predict cytotoxic T-cell-mediated responses elicited by transgenes, we used here H-2-negative HLA-A*0201 transgenic mice and identified human DYST epitopes, which elicit HLA-A*0201-restricted cytotoxic T cell activities. Among a series of eight peptides predicted from the human DYST sequence, not shared with the endogenous mouse DYST sequence, four of them were able to bind to HLA-A*0201 molecules and to induce cytotoxic T lymphocyte (CTL) responses. After human DYST DNA transfer in muscle of HLA-A*0201 mice, only the human DYST1281 epitope, located in the spectrin-like repeat 9 domain, induced strong CD8(+) CTL responses. Using the corresponding human DYST1281 peptide/HLA-A*0201 tetramer, we detected human DYST1281-specific CD8(+) T cells in peripheral lymphoid organs and blood of HLA-A*0201 mice injected with human DYST DNA. Our results demonstrate that muscle injection with human DYST DNA systematically triggers CTL responses against this HLA-A*0201-restricted human DYST1281 peptide, which is present in long human DYST isoforms. Identification of such immunodominant human DYST epitopes and use of peptide/HLA tetramers will allow the immunomonitoring of CTL responses in HLA-phenotyped Duchenne muscular dystrophy patients undergoing gene therapy. Finally, the knowledge of HLA-A*0201-restricted human DYST peptides will be of importance to test, in mouse models, new immunomodulatory interventions allowing long-term engraftment of human dystrophin.

Design of a polyepitope construct for the induction of HLA-A0201-restricted HIV 1-specific CTL responses using HLA-A*0201 transgenic, H-2 class I KO mice

HLA-A*0201 transgenic, H-2D(b)/mouse beta2-microglobulin double-knockout mice were used to compare and optimize the immunogenic potential of 17HIV 1-derived,HLA-A0201-restricted epitopic peptides. A tyrosine substitution in position 1 of the epitopic peptides, which increases both their affinity for and their HLA-A0201 molecule stabilizing capacity, was introduced in a significant proportion, having verified that such modifications enhance their immunogenicity in respect of their natural antigenicity. Based on these results, a 13-polyepitope construct was inserted in the pre-S2 segment of the hepatitis B middle glycoprotein and used for DNA immunization. Long-lasting CTL responses against most of the inserted epitopes could be elicited simultaneously in a single animal with cross-recognition in several cases of their most common natural variants.

Regulatory function of in vivo anergized CD4(+) T cells

It has been suggested that anergic T cells may not be only inert cells but may rather play an active role, for example by regulating immune responses. We have previously reported the existence of "anergic" IL-10-producing CD4(+) T cells generated in vivo by continuous antigenic stimulation. Using a gene transfer system where the antigen recognized by such T cells is expressed in skeletal muscle by two different DNA viral vectors, we show that these cells not only remain tolerant toward their cognate antigen but also can suppress the immune response of naive T cells against the immunogenic adenoviral proteins. Furthermore, they can completely inhibit tissue destruction that takes place as a result of an immune response. The system presented here is unique in that the T cells have been anergized in vivo, their antigen specificity and functional status are known, and the amount, form, and timing of antigen expression can be manipulated. This model will therefore permit us to carefully dissect the mechanisms by which these anergic T cells regulate the priming and/or effector function of naive T cells.

[Gene transfer]

Mammalian cells in culture can be modified by gene transfer and these procedures are routinely used in experimental biology. Yet, efficient approaches to modify certain complex cell populations, stem cells or cells organized within a tissue are still lacking. The hurdles to gene transfer can be listed by describing the pathway of a nucleic acid molecule, from the external medium towards the cell nucleus where its encoded information will be expressed. The requirements include the necessity to compact the size of the macromolecule, to overcome electrostatic repulsion, to cross a series of membranes and to establish itself permanently. Viruses have evolved to achieve these goals and understanding their strategies for cell invasion allows to design vectors for gene transfer. Chemicals or biochemicals able to bind DNA, as well as physical methods inducing changes in the structure of membranes can also be useful for gene transfer. The outcome of gene transfer technologies and their improvement pave the way to inovative medical applications and provide powerful tools for exploring the living world.

Factors influencing cross-presentation of non-self antigens expressed from recombinant adeno-associated virus vectors

BACKGROUND

We have previously demonstrated that recombinant adeno-associated virus vectors expressing the influenza virus hemagglutinin (rAAV-HA) in skeletal muscle results in T-cell priming and muscle fiber destruction due to cross-presentation of HA by dendritic cells (DC). Based on controversial observations concerning the stability of non-self proteins expressed from rAAV vectors it is important to understand the factors influencing cross-presentation of transgene products following rAAV mediated gene transfer, in order to be able to use this vector safely in the clinic.

METHODS

In order to understand the factors influencing in vivo cross-presentation of non-self proteins, we have retargeted the immunogenic lacZ protein in the context of rAAV from the cytoplasm to the cell surface and studied the activation of LacZ specific immune responses following intramuscular mediated gene transfer. In addition, using tools available for studying in vitro HA-specific T-cell activation, our aim was to identify the cell types involved in class I and class II restricted cross-presentation as well as the nature of the cross-presented material.

RESULTS

By retargeting the lacZ protein in the context of rAAV to the cell membrane, we found that one of the factors influencing the efficiency of cross-presentation of non-self antigens is the localization of the transgene product within the target cells. Following rAAV-LacZ mediated gene transfer to the muscle we demonstrated that the membrane-bound form of LacZ resulted in target cell destruction, which is in stark contrast to the stability observed with rAAV-LacZ vectors expressing the cytoplasmic form of LacZ. Using an in vitro assay, we were able to show that dendritic cells (DC) in addition to B-cells cross-presented HA to class II restricted T-cells whereas only the former were able to activate class I restricted CD8+ T-cells. High-dose antigens were needed for efficient class I restricted T-cell priming, whereas class II restricted T-cells were activated by less antigen.

CONCLUSION

The present results indicate that immune responses to non-self antigens expressed from rAAV vectors depend on the accessibility of such antigens to different local antigen-presenting cells.

Comparison of different bioreactor systems for the production of high titer retroviral vectors

Improved, human-based packaging cell lines allow the production of high-titer, RCR-free retroviral vectors. The utility of these cell lines for the production of clinical grade vectors critically depends on the definition of optimal conditions for scaled-up cultures. In this work, a clone derived from the TE Fly GALV packaging cell (Duisit et al. Hum. Gene Ther. 1999, 10, 189) that produces high titers of a lacZ containing retroviral vector with a Gibbon Ape Leukemia Virus envelope glycoprotein was used. This clone can produce (2-5) x 10(6) PFU cm(-3) in small scale cultures and has been evaluated for growth and vector production in different reactor systems. The performances of fixed bed reactors [CellCube (Costar) and Celligen (New Brunswick)] and stirred tank reactors [microcarriers and clump cultures] were compared. The cells showed a higher apparent growth rate in the fixed bed reactor systems than in the suspension systems, probably as a result of the fact that aggregation and/or formation of clumps led to a reduced viability and reduced growth of cells in the interior of the clumps. As a consequence, the final cell density and number were in average 3- to 7-fold higher in the fixed bed systems in comparison to the suspension culture systems. The average titers obtained ranged from 0.5 to 2.1 x 10(7) PFU cm(-3) for the fixed bed and microcarrier systems, while the clump cultures produced only (2-5) x 10(5) PFU cm(-3). The differences in titers reflect cell densities as well as specific viral vector production rates, with the immobilization and microcarrier systems exhibiting an at least 10-fold higher production rate in comparison to the clump cultures. A partial optimization of the culture conditions in the Celligen fixed bed reactor, consisting of a 9-fold reduction of the seeding cell density, led to a 5-fold increased vector production rate accompanied by an average titer of 3 x 10(7) PFU cm(-3) (maximum titer (4-5) x 10(7) PFU cm(-3)) in the fixed bed reactor. The performance evaluation results using mathematical models indicated that the fixed bed bioreactor has a higher potential for retroviral vector production because of both the higher reactor productivity and the lower sensitivity of productivity in relation to the changes in final retrovirus titer in the range of 3 x 10(6) to 15 x 10(6) PFU cm(-3).

Polyethylenimine-mediated gene delivery: a mechanistic study

BACKGROUND

Ethylenimine polymers (PEIs) belong to one of the most efficient family of cationic compounds for delivery of plasmid DNA into mammalian cells. The high transfection efficiencies are obtained even in the absence of endosomolytic agents such as fusogenic peptides or chloroquine, which is in contrast to most of the other cationic polymers. It has been hypothesized that the efficiency of PEI is due to its capacity to buffer the endosomes.

METHODS

To investigate the importance of the acidification of endosomes during PEI-mediated DNA transfer we used proton pump inhibitors such as bafilomycin A1 and concanamycin A. Moreover, we tested whether PEI is able to destabilize natural membranes per se at neutral or acidic pH by performing erythrocyte lysis assays.

RESULTS

PEI-mediated transfection in the presence of bafilomycin A1 resulted in a 7-74-fold decrease in reporter gene expression depending on the cell line used. In contrast, the efficiency of the monocationic lipid, DOTAP, was not importantly altered in the presence of the drug. Furthermore, the present data show that PEI cannot destabilize erythrocyte membranes, even at acidic pH, and that PEI, complexed or not to DNA, can increase the transfection efficiency of the cationic polymer, polylysine, when added at the same time to the cells.

CONCLUSIONS

The transfection efficiency of PEIs partially relies on their ability to capture the protons which are transferred into the endosomes during their acidification. In addition, PEI is able to deliver significant amounts of DNA into cells and the DNA complexes involved in the expression of the transgene escape within 4 h from the endosomes.

Intracellular trafficking of adeno-associated virus vectors: routing to the late endosomal compartment and proteasome degradation

The early steps of adeno-associated virus (AAV) infection involve attachment to a variety of cell surface receptors (heparan sulfate, integrins, and fibroblast growth factor receptor 1) followed by clathrin-dependent or independent internalization. Here we have studied the subsequent intracellular trafficking of AAV particles from the endosomal compartment to the nucleus. Human cell lines were transduced with a recombinant AAV (rAAV) carrying a reporter gene (luciferase or green fluorescent protein) in the presence of agents that affect trafficking. The effects of bafilomycin A(1), brefeldin A, and MG-132 were measured. These drugs act at the level of endosome acidification, early-to-late endosome transition, and proteasome activity, respectively. We observed that the transducing virions needed to be routed as far as the late endosomal compartment. This behavior was markedly different from that observed with adenovirus particles. Antiproteasome treatments with MG-132 led to a 50-fold enhancement in transduction efficiency. This effect was accompanied by a 10-fold intracellular accumulation of single-stranded DNA AAV genomes, suggesting that the mechanism of transduction enhancement was different from the one mediated by a helper adenovirus, which facilitates the conversion of the rAAV single-stranded DNA genome into its replicative form. MG-132, a drug currently in clinical use, could be of practical use for potentializing rAAV-mediated delivery of therapeutic genes.

Optimised helper virus-free production of high-quality adeno-associated virus vectors

BACKGROUND

Clinical development of adeno-associated virus (AAV) requires standardised, safe, efficient and scalable procedures for the manufacture of the rAAV vector, including production, purification and testing. Several strategies have been reported for the approach to the manufacturing problem. We report a helper virus-free process that produces high quality rAAV stocks.

METHODS

rAAV were produced by triple transfection, a helper virus-free process. After lysis of the cells in the presence of nuclease, the rAAV produced were purified by HPLC through two ion-exchange columns in tandem followed by dialysis. rAAV stocks were thoroughly characterised for biological activity and for the presence of residual contaminants. The titer of infectious particles and of rep + particles was determined by dRA assay. Contaminating DNA and RNA were determined by fluorescent dye binding and real-time PCR. The protein content of the rAAV stocks was characterised by SDS-PAGE, ELISA test, Western blot and specific enzymatic assays for putative residual contaminating protein. The in vivo biological activity of the stocks was evaluated in mouse muscle.

RESULTS

rAAV stocks obtained following this procedure elicit: 2-5 x 10(12) pp/ml; 3-6 x 10(10) ip/ml; < 10(3) rep + particles/ml; <0.3 mUeq/ml of residual benzonase activity; non-detectable Ad or beta-galactosidase proteins; <35 pg/ml of cellular genomic DNA; in vivo expression in mouse muscle without any immune reaction detected.

CONCLUSIONS

This work demonstrates the possibility of producing purified high-quality rAAV free of helper virus. The procedure described in this paper is easily adaptable for large-scale production of clinical rAAV vectors.

Cutaneous gene transfer and therapy: the present and the future

The easy accessibility of the skin as a therapeutic target provides an exciting potential for this organ for the development of gene therapy protocols for cutaneous diseases and a variety of metabolic disorders. Thus far, full phenotypic reversion of a diseased phenotype has been achieved in vivo for junctional epidermolysis bullosa and X-linked or lamellar ichthyosis and in vitro for xeroderma pigmentosum. These recessive skin diseases are characterized by skin blistering, abnormalities in epidermal differentiation and increased development of skin cancers, respectively. Corrective gene delivery at both molecular and functional levels was achieved by transduction of cultured skin cells using retroviral vectors carrying the specific curative cDNA. These positive results should prompt clinical trials based on transplantation of artificial epithelia reconstructed ex vivo using genetically modified keratinocytes. Promising results have also been obtained in phenotypic reversion of cells isolated from patients suffering from a number of metabolic diseases such as gyrate atrophy, familial hypercholesterolemia or phenylketonuria. In these diseases transplantation of autologous artificial epithelia expressing the transgenes of interest or direct transfer of the DNA to the skin represents a potential therapeutic approach for the systemic delivery of active molecules. Successful cutaneous gene therapy trials, however, require development of protocols for efficient gene transfer to epidermal stem cells, and information about the host immune response to the recombinant polypeptides produced by the implanted keratinocytes. The availability of spontaneous animal models for genodermatoses will validate the gene therapy approach in preclinical trials.

Successful interference with cellular immune responses to immunogenic proteins encoded by recombinant viral vectors

Vectors derived from the adeno-associated virus (AAV) have been successfully used for the long-term expression of therapeutic genes in animal models and patients. One of the major advantages of these vectors is the absence of deleterious immune responses following gene transfer. However, AAV vectors, when used in vaccination studies, can result in efficient humoral and cellular responses against the transgene product. It is therefore important to understand the factors which influence the establishment of these immune responses in order to design safe and efficient procedures for AAV-based gene therapies. We have compared T-cell activation against a strongly immunogenic protein, the influenza virus hemagglutinin (HA), which is synthesized in skeletal muscle following gene transfer with an adenovirus (Ad) or an AAV vector. In both cases, cellular immune responses resulted in the elimination of transduced muscle fibers within 4 weeks. However, the kinetics of CD4(+) T-cell activation were markedly delayed when AAV vectors were used. Upon recombinant Ad (rAd) gene transfer, T cells were activated both by direct transduction of dendritic cells and by cross-presentation of the transgene product, while upon rAAV gene transfer T cells were only activated by the latter mechanism. These results suggested that activation of the immune system by the transgene product following rAAV-mediated gene transfer might be easier to control than that following rAd-mediated gene transfer. Therefore, we tested protocols aimed at interfering with either antigen presentation by blocking the CD40/CD40L pathway or with the T-cell response by inducing transgene-specific tolerance. Long-term expression of the AAV-HA was achieved in both cases, whereas immune responses against Ad-HA could not be prevented. These data clearly underline the importance of understanding the mechanisms by which vector-encoded proteins are recognized by the immune system in order to specifically interfere with them and to achieve safe and stable gene transfer in clinical trials.

Canine adenovirus type 2 attachment and internalization: coxsackievirus-adenovirus receptor, alternative receptors, and an RGD-independent pathway

The best-characterized receptors for adenoviruses (Ads) are the coxsackievirus-Ad receptor (CAR) and integrins alpha(v)beta(5) and alpha(v)beta(3), which facilitate entry. The alpha(v) integrins recognize an Arg-Gly-Asp (RGD) motif found in some extracellular matrix proteins and in the penton base in most human Ads. Using a canine adenovirus type 2 (CAV-2) vector, we found that CHO cells that express CAR but not wild-type CHO cells are susceptible to CAV-2 transduction. Cells expressing alpha(M)beta(2) integrins or major histocompatibility complex class I (MHC-I) molecules but which do not express CAR were not transduced. Binding assays showed that CAV-2 attaches to a recombinant soluble form of CAR and that Ad type 5 (Ad5) fiber, penton base, and an anti-CAR antibody partially blocked attachment. Using fluorescently labeled CAV-2 particles, we found that in some cells nonpermissive for transduction, inhibition was at the point of internalization and not attachment. The transduction efficiency of CAV-2, which lacks an RGD motif, surprisingly mimicked that of Ad5 when tested in cells selectively expressing alpha(v)beta(5) and alpha(v)beta(3) integrins. Our results demonstrate that CAV-2 transduction is augmented by CAR and possibly by alpha(v)beta(5), though transduction can be CAR and alpha(v)beta(3/5) independent but is alpha(M)beta(2), MHC-I, and RGD independent, demonstrating a transduction mechanism which is distinct from that of Ad2/5.

A conserved mechanism of retrovirus restriction in mammals

The murine Fv1 gene restricts infection by N- or B-tropic murine leukemia viruses at a postentry, preintegration stage. The Fv1-sensitive viruses previously used for the study of Fv1 encode an ecotropic envelope gene and thus only infect rodent cells. Consequently, the study of Fv1 restriction has been carried out solely in mice and murine cell lines. By infection with retroviral vectors containing N- or B-tropic core and pantropic vesicular stomatitis virus-G envelope protein, we now demonstrate that cell lines derived from various mammalian species, including humans, have an Fv1-like retrovirus restriction function, preventing N-tropic vector infection. Like Fv1, restriction is directed at amino acid 110 of the viral capsid protein. In contrast to Fv1, the novel restriction is characterized by the absence of reverse-transcribed viral DNA. We speculate that these activities have been selected for by retroviral epidemics in the distant past.

Efficient DNA transfection mediated by the C-terminal domain of human immunodeficiency virus type 1 viral protein R

Viral protein R (Vpr) of human immunodeficiency virus type 1 is produced late in the virus life cycle and is assembled into the virion through binding to the Gag protein. It is known to play a significant role early in the viral life cycle by facilitating the nuclear import of the preintegration complex in nondividing cells. Vpr is also able to interact with nucleic acids, and we show here that it induces condensation of plasmid DNA. We have explored the possibility of using these properties in DNA transfection experiments. We report that the C-terminal half of the protein (Vpr(52-96)) mediates DNA transfection in a variety of human and nonhuman cell lines with efficiencies comparable to those of the best-known transfection agents. Compared with polylysine, a standard polycationic transfection reagent, Vpr(52-96) was 10- to 1,000-fold more active. Vpr(52-96)-DNA complexes were able to reach the cell nucleus through a pH-independent mechanism. These observations possibly identify an alternate pathway for DNA transfection.

High throughput production, screening and analysis of adeno-associated viral vectors

Recombinant adeno-associated viruses (rAAV) are promising candidates as gene vectors, as they transduce non-dividing cells and permit lasting transgene expression in a wide spectrum of tissues. In this paper, we describe a robust procedure for the high throughput production, screening and characterization of rAAV vectors. The technology includes the production of rAAV from rapid small scale plasmid preparations and the analysis of virus productivity (physical and infectious particles) and activity (transgene expression, replication). rAAV are produced by triple transfection (rAAV plasmid and AAV- and adenovirus (Ad)-helper plasmids) on 293 human embryo kidney (HEK) cells. The titers of physical and infectious particles are obtained by dot blot hybridization and by a serial dilution assay, followed by either dot blot hybridization or real-time PCR, respectively. rAAV can be produced and characterized from plasmid mixtures containing as little as 1/100 productive molecules. Experiments on rAAV replication kinetics and Ad helper functions are discussed. All steps are performed in 96-well microtiter plates. The process is reproducible, high throughput, linear and ready for automation.

Canine adenovirus vectors: an alternative for adenovirus-mediated gene transfer

Preclinical studies have shown that gene transfer following readministration of viral vectors is often inefficient due to the presence of neutralizing antibodies. Vectors derived from ubiquitous human adenoviruses may have limited clinical use because preexisting humoral and cellular immunity is found in 90% of the population. Furthermore, risks associated with the use of human adenovirus vectors, such as the need to immunosuppress or tolerize patients to a potentially debilitating virus, are avoidable if efficient nonhuman adenovirus vectors are feasible. Plasmids containing recombinant canine adenovirus (CAV) vectors from which the E1 region had been deleted were generated and transfected into a CAV E1-transcomplementing cell line. Vector stocks, with titers greater than or equal to those obtained with human adenovirus vectors, were free of detectable levels of replication-competent CAV and had a low particle-to-transduction unit ratio. CAV vectors were replication defective in all cell lines tested, transduced human-derived cells at an efficiency similar to that of a comparable human adenovirus type 5 vector, and are amenable to in vivo use. Importantly, 49 of 50 serum samples from healthy individuals did not contain detectable levels of neutralizing CAV antibodies.

One step screening of retroviral producer clones by real time quantitative PCR

BACKGROUND

Recombinant retroviruses are obtained from either stably or transiently transfected retrovirus producer cells. In the case of stably producing lines, a large number of clones must be screened in order to select the one with the highest titre. The multi-step selection of high titre producing clones is time consuming and expensive.

METHODS

We have taken advantage of retroviral endogenous reverse transcription to develop a quantitative PCR assay on crude supernatant from producing clones. We used Taqman PCR technology, which, by using fluorescence measurement at each cycle of amplification, allows PCR product quantification. Fluorescence results from specific degradation of a probe oligonucleotide by the Taq polymerase 3'-5' exonuclease activity. Primers and probe sequences were chosen to anneal to the viral strong stop species, which is the first DNA molecule synthesised during reverse transcription. The protocol consists of a single real time PCR, using as template filtered viral supernatant without any other pre-treatment.

RESULTS

We show that the primers and probe described allow quantitation of serially diluted plasmid to as few as 15 plasmid molecules. We then test 200 GFP-expressing retroviral-producing clones either by FACS analysis of infected cells or by using the quantitative PCR. We confirm that the Taqman protocol allows the detection of virus in supernatant and selection of high titre clones. Furthermore, we can determine infectious titre by quantitative PCR on genomic DNA from infected cells, using an additional set of primers and probe to albumin to normalise for the genomic copy number.

CONCLUSION

We demonstrate that real time quantitative PCR can be used as a powerful and reliable single step, high throughput screen for high titre retroviral producer clones.

Structure of adeno-associated virus vector DNA following transduction of the skeletal muscle

The skeletal muscle provides a very permissive physiological environment for adeno-associated virus (AAV) type 2-mediated gene transfer. We have studied the early steps leading to the establishment of permanent transgene expression, after injection of recombinant AAV (rAAV) particles in the quadriceps muscle of mice. The animals received an rAAV encoding a secreted protein, murine erythropoietin (mEpo), under the control of the human cytomegalovirus major immediate-early promoter and were sacrificed between 1 and 60 days after injection. The measurement of plasma Epo levels and of hematocrits indicated a progressive increase of transgene expression over the first 2 weeks, followed by a stabilization at maximal plateau values. The rAAV sequences were analyzed by Southern blotting following neutral or alkaline gel electrophoresis of total DNA from injected muscles. While a high number of rAAV sequences were detected during the first 5 days following the injection, only a few percent of these sequences was retained in the animals analyzed after 2 weeks, in which transgene expression was maximal. Double-stranded DNA molecules resulting from de novo second-strand synthesis were detected as early as day 1, indicating that this crucial step of AAV-mediated gene transfer is readily accomplished in the muscle. The templates driving stable gene expression at later time points are low in copy number and structured as high-molecular-weight concatemers or interlocked circles. The presence of the circular form of the rAAV genomes at early time points suggests that the molecular transformations involved in the formation of stable concatemers may involve a rolling-circle type of DNA replication.

Selective uptake and sustained expression of AAV vectors following subcutaneous delivery

BACKGROUND

Recombinant adeno-associated viral (rAAV) vectors are capable of long-term expression of secreted and intracellular proteins following delivery to muscle, liver, and the central nervous system. In this study, we have evaluated subcutaneous injection of rAAV encoding a variety of transgenes as an alternative route of administration for the systemic delivery of therapeutic proteins.

METHODS

rAAV vectors encoding the human factor IX, human interferon-alpha 2a, murine erythropoietin (epo), and Escherichia coli lacZ genes were used for subcutaneous delivery into mature immunocompetent mice. Expression of factor IX and interferon in mouse serum was measured by ELISA. Expression of Epo was monitored by an increase in hemotocrit and by RIA. The tissue tropism of AAV transduction was determined by histochemistry following administration of the lacZ vector.

RESULTS

Long-term protein expression (at least one year) is demonstrated in the serum of immunocompetent mice following subcutaneous delivery of AAV vectors encoding the human factor IX and interferon genes. The murine epo gene delivered via this route resulted in levels of Epo that correlate with increased hematocrits of up to 90% for a duration of nine months. rAAV encoding the lacZ gene revealed that the panniculus carnosus, a skeletal muscle layer of the skin, was transduced upon subcutaneous administration.

CONCLUSIONS

This study shows that long-term expression of secreted proteins can be achieved using rAAV vectors injected subcutaneously as a single administration. The observation that the panniculus carnosus is the primary tissue transduced by rAAV illustrates the high tropism of rAAV for skeletal muscle.