Immunity to adeno-associated virus vectors in animals and humans: a continued challenge

The efficacy of combination therapy using adeno-associated virus-TRAIL targeting to telomerase activity and cisplatin in a mice model of hepatocellular carcinoma

PURPOSE

TNF-related apoptosis-inducing ligand (TRAIL) functions as a soluble cytokine and has been demonstrated significant antitumor activity against a variety of cancer cell lines without toxicity to most normal cells. Cisplatin is a potent anticancer agent and is widely used in the clinical for treatment of human cancers. Adeno-associated virus (AAV2) is a promising gene delivery vehicle for its advantage of low pathogenicity and long-term gene expression. However, lack of tissue specificity caused low efficiency of AAV transfer to target cells. The promoter of human telomerase reverse transcriptase (hTERT) is a good candidate to enhance targeting efficiency of AAV in cancer cells. Although AAV-mediated TRAIL controlled by hTERT promoter (AAV-hTERT-TRAIL) has obvious antitumor activity, the tumor cannot be completely eradicated. In this study, we first examined the effectiveness of combination therapy of cisplatin and AAV-hTERT-TRAIL on human hepatocellular carcinoma (HCC) in vitro and in vivo.

METHODS

For in vitro experiments, tumor cell lines were treated with cisplatin, virus, or both. The transgene TRAIL expression controlled by hTERT promoter was evaluated in BEL7404 HCC cell line. Cytotoxicity was performed by MTT analysis. Cell apoptosis was detected by flow cytometry analysis. The in vivo antitumor efficacy of combination treatment with cisplatin and AAV-hTERT-TRAIL was assessed in human hepatocellular carcinoma xenografts mouse model.

RESULTS

The enhanced TRAIL expression was observed in BEL7404 cells treated with AAV-hTERT-TRAIL plus cisplatin. Treatment with both AAV-hTERT-TRAIL and cisplatin exhibited stronger cytotoxicity and induced more significant apoptosis in cancer cells compared with AAV-hTERT-TRAIL or cisplatin alone, respectively. Moreover, in animal experiments, the combined treatment greatly suppressed tumor growth and resulted in tumor cell death.

CONCLUSIONS

AAV-mediated therapeutic gene expression in combination with chemotherapy provides a promising therapeutic strategy for human cancers. These data suggest that combined use of AAV-hTERT-TRAIL and cisplatin may have potential clinical application.

Gene therapy in Parkinson's disease: rationale and current status

Parkinson's disease is the second most common age-related neurodegenerative disorder, typified by the progressive loss of substantia nigra pars compacta dopamine neurons and the consequent decrease in the neurotransmitter dopamine. Patients exhibit a range of clinical symptoms, with the most common affecting motor function and including resting tremor, rigidity, akinesia, bradykinesia and postural instability. Current pharmacological interventions are palliative and largely aimed at increasing dopamine levels through increased production and/or inhibition of metabolism of this key neurotransmitter. The gold standard for treatment of both familial and sporadic Parkinson's disease is the peripheral administration of the dopamine precursor, levodopa. However, many patients gradually develop levodopa-induced dyskinesias and motor fluctuations. In addition, dopamine enhancement therapies are most useful when a portion of the nigrostriatal pathway is intact. Consequently, as the number of substantia nigra dopamine neurons and striatal projections decrease, these treatments become less efficacious. Current translational research is focused on the development of novel disease-modifying therapies, including those utilizing gene therapeutic approaches. Herein we present an overview of current gene therapy clinical trials for Parkinson's disease. Employing either recombinant adeno-associated virus type 2 (rAAV2) or lentivirus vectors, these clinical trials are focused on three overarching approaches: augmentation of dopamine levels via increased neurotransmitter production; modulation of the neuronal phenotype; and neuroprotection. The first two therapies discussed in this article focus on increasing dopamine production via direct delivery of genes involved in neurotransmitter synthesis (amino acid decarboxylase, tyrosine hydroxylase and GTP [guanosine triphosphate] cyclohydrolase 1). In an attempt to bypass the degenerating nigrostriatal pathway, a third clinical trial utilizes rAAV2 to deliver glutamic acid decarboxylase to the subthalamic nucleus, converting a subset of excitatory neurons to GABA-producing cells. In contrast, the final clinical trial is aimed at protecting the degenerating nigrostriatum by striatal delivery of rAAV2 harbouring the neuroprotective gene, neurturin. Based on preclinical studies, this gene therapeutic approach is posited to slow disease progression by enhancing neuronal survival. In addition, we discuss the outcome of each clinical trial and discuss the potential rationale for the marginal yet incremental clinical advancements that have thus far been realized for Parkinson's disease gene therapy.

Expression of dog microdystrophin in mouse and dog muscles by gene therapy

Duchenne muscular dystrophy (DMD) is characterized by the absence of dystrophin. Several previous studies demonstrated the feasibility of delivering microdystrophin complementary DNA (cDNA) into mouse and normal nonhuman primate muscles by ex vivo gene therapy. However, these animal models do not reproduce completely the human DMD phenotype, while the dystrophic dog model does. To progress toward the use of the best animal model of DMD, a dog microdystrophin was transduced into human and dystrophic dog muscle precursor cells (MPCs) with a lentivirus before their transplantation into mouse muscles. One month following MPC transplantation, myofibers expressing the dog microdystrophin were observed. We also used another approach to introduce this transgene into myofibers, i.e., the electrotransfer of a plasmid coding for the dog microdystrophin. The plasmid was injected into mouse and dog muscles, and brief electric pulses were applied in the region of injection. Two weeks later, the transgene was detected in both animals. Therefore, ex vivo gene therapy and electrotransfer are two possible methods to introduce a truncated version of dystrophin into myofibers of animal models and eventually into myofibers of DMD patients.

Improved induction of immune tolerance to factor IX by hepatic AAV-8 gene transfer

Gene therapy for hemophilia B has been shown to result in long-term expression and immune tolerance to factor IX (F.IX) after in vivo transduction of hepatocytes with adeno-associated viral (AAV-2) vectors in experimental animals. An optimized protocol was effective in several strains of mice with a factor 9 gene deletion (F9(-/-)). However, immune responses against F.IX were repeatedly observed in C3H/HeJ F9(-/-) mice. We sought to establish a gene transfer protocol that results in sustained expression without a requirement for additional manipulation of the immune system. Compared with AAV-2, AAV-8 was more efficient in transgene expression and induction of tolerance to F.IX in three different strains of wild-type mice. At equal vector doses, AAV-8 induced transgene product-specific regulatory CD4(+)CD25(+)FoxP3(+) T cells at significantly higher frequency. Moreover, sustained correction of hemophilia B in C3H/HeJ F9(-/-) mice without antibody formation was documented in all animals treated with > or =4 x 10(11) vector genomes (VG)/kg and in 80% of mice treated with 8 x 10(10) VG/kg. Therefore, it is possible to develop a gene transfer protocol that reliably induces tolerance to F.IX largely independent of genetic factors. A comparison with other studies suggests that additional parameters besides plateau levels of F.IX expression contributed to the improved success rate of tolerance induction.

Inhibition of acidic mammalian chitinase by RNA interference suppresses ovalbumin-sensitized allergic asthma

Asthma, a chronic helper T cell type 2-mediated inflammatory disease, is characterized by airway hyperresponsiveness and inflammation. Growing evidence suggests that increased expression of acidic mammalian chitinase (AMCase) may play a role in the pathogenesis of asthma. In the present study, we sought to develop an RNA interference approach to suppress allergic asthma in mice through silencing of AMCase expression. Mice sensitized with ovalbumin (OVA) were intratracheally administered a recombinant adeno-associated virus expressing short hairpin RNA (rAAV-shRNA) against AMCase. In OVA-sensitized mice, the development of allergic symptoms was significantly associated with elevated AMCase expression. After administration of rAAV-shRNA, there was a significant reduction of AMCase expression in the lung and in bronchoalveolar lavage fluid (BALF) cells of sensitized mice. Sensitized mice receiving rAAV-shRNA showed a significant improvement in allergic symptoms, including airway hyperresponsiveness (AHR), eosinophil infiltration, eotaxin, interleukin-13 secretion in BALF, and serum OVA-specific IgE level. Our data suggest the hyperexpression of AMCase in asthma can be suppressed by rAAV-mediated shRNA. Silencing AMCase expression by shRNA may be a promising therapeutic strategy in asthma.

Stem cell and genetic therapies for the fetus

Advances in prenatal diagnosis have led to the prenatal management of a variety of congenital diseases. Although prenatal stem cell and gene therapy await clinical application, they offer tremendous potential for the treatment of many genetic disorders. Normal developmental events in the fetus offer unique biologic advantages for the engraftment of hematopoietic stem cells and efficient gene transfer that are not present after birth. Although barriers to hematopoietic stem cell engraftment exist, progress has been made and preclinical studies are now underway for strategies based on prenatal tolerance induction to facilitate postnatal cellular transplantation. Similarly, in-utero gene therapy shows experimental promise for a host of diseases and proof-in-principle has been demonstrated in murine models, but ethical and safety issues still need to be addressed. Here we review the current status and future potential of prenatal cellular and genetic therapy.

Development of viral vectors for use in cardiovascular gene therapy

Cardiovascular disease represents the most common cause of mortality in the developed world but, despite two decades of promising pre-clinical research and numerous clinical trials, cardiovascular gene transfer has so far failed to demonstrate convincing benefits in the clinical setting. In this review we discuss the various targets which may be suitable for cardiovascular gene therapy and the viral vectors which have to date shown the most potential for clinical use. We conclude with a summary of the current state of clinical cardiovascular gene therapy and the key trials which are ongoing.

Single-dose lentiviral gene transfer for lifetime airway gene expression

BACKGROUND

Cystic fibrosis (CF) is caused by a defect in cystic fibrosis transmembrane conductance regulator (CFTR) activity, often resulting in an incurable airway disease. Gene therapy into the conducting airway epithelium is a potential cure for CF; however, most gene vectors do not result in long-lived expression, and require re-dosing. Perversely, intrinsic host immune responses can then block renewed gene transfer.

METHODS

To investigate whether persistent gene expression could be achieved after a single dosing event, thus avoiding the issue of blocking host responses, we used a gene transfer protocol that combined an airway pretreatment using lysophosphatidylcholine with a human immunodeficiency virus type-1 (vesicular stomatitis virus G pseudotype) derived lentiviral vector to test whether an integrating vector could produce gene expression able to last for a substantial part of the lifetime of the laboratory mouse.

RESULTS

We found that a single dose of LV-LacZ produced immediate as well as lifetime mouse airway expression, confirming our hypothesis that use of an integrating vector extends transgene expression. Importantly, LV-CFTR dosing achieved at least 12 months of CFTR expression, representing partial functional correction of the CFTR defect in CF-null mice.

CONCLUSIONS

These findings validate the potential of this methodology for developing a gene transfer treatment for CF airway disease.

Identification of the murine AAVrh32.33 capsid-specific CD8+ T cell epitopes

BACKGROUND

Adeno-associated virus (AAV) is an ideal gene therapy vector and is non-immunogenic in many small animal models. The stable gene expression commonly seen in murine models does not necessarily translate to nonhuman primates and higher-order species, highlighting the need for a better understanding of immune activation to these vectors. One capsid variant, AAVrh32.33, demonstrates a unique phenotype in murine muscle, reminiscent of what is often seen in higher-order species. AAVrh32.33 generates a strong CD8+ T-cell response to both capsid and encoded transgene antigens in a manner independent of transgene product or major histocompatability complex haplotype, making it an ideal candidate for studying immune activation to AAV in the mouse.

METHODS

To map the H-2b and H-2d dominant epitopes of the AAVrh32.33 capsid, C57BL/6 or Balb/C mice received an intramuscular injection of 1 x 10(11) genome copies of AAV2/rh32.33.CB.nLacZ. Three weeks later, splenocytes were harvested and stimulated in vitro with pooled or individual peptides from the AAVrh32.33 capsid peptide library and analysed by an interferon (IFN)-gamma enzyme-linked immunosorbent spot assay or intracellular cytokine staining.

RESULTS

The immunodominant epitopes within the AAVrh32.33 capsid responsible for driving CD8+ T-cell responses to the capsid protein in C57BL/6 (SSYELPYVM) and Balb/C (KIPASGGNAL) mice were defined.

CONCLUSIONS

Identification of dominant capsid epitopes will make it possible to monitor cellular responses to the AAV capsid in vivo, facilitating mechanistic studies critical to defining how cellular immunity to the AAV capsid arises and, ultimately, how the generation of capsid-specific T cells can be avoided to ensure safety in a gene therapy setting.

Adeno-associated virus-mediated gene transfer to nonhuman primate liver can elicit destructive transgene-specific T cell responses

Gene transfer to murine liver with vectors based on novel adeno-associated virus (AAV) serotypes is efficient, stable, and safe even in the setting of antigenic transgene products. We undertook a study in cynomolgus macaques to evaluate the relevance of these findings to primates. The vectors were based on AAV serotype 7 and expressed green fluorescence protein (GFP) from the cytomegalovirus enhanced beta-actin promoter in both single-stranded and self-complementary genomes. Transduction efficiencies from the single-stranded vectors were similar to those observed in mice, although there was no advantage in primates with the self-complementary vectors. Primates elicited vibrant cytotoxic T cell responses to GFP that correlated with hepatitis and loss of transgene expression. There was no evidence of T cell activation in response to the AAV capsid. These studies indicate that under some conditions primates may activate more robust T cell responses to transgene products than is observed in mice.

Progress and prospects: immune responses to viral vectors

Viral vectors are potent gene delivery platforms used for the treatment of genetic and acquired diseases. However, just as viruses have evolved to infect cells efficiently, the immune system has evolved to fight off what it perceives as invading pathogens. Therefore, innate immunity and antigen-specific adaptive immune responses against vector-derived antigens reduce the efficacy and stability of in vivo gene transfer. In addition, a number of vectors are derived from parent viruses that humans encounter through natural infection, resulting in preexisting antibodies and possibly in memory responses against vector antigens. Similarly, antibody and T-cell responses may be directed against therapeutic gene products that often differ from the endogenous nonfunctional or absent protein that is being replaced. As details and mechanisms of such immune reactions are uncovered, novel strategies are being developed, and vectors are being specifically engineered to avoid, suppress or manipulate the response, ideally resulting in sustained expression and immune tolerance to the transgene product. This review provides a summary of our current knowledge of the interactions between the immune system adeno-associated virus, adenoviral and lentiviral vectors, and their transgene products.

Progress in therapeutic antisense applications for neuromuscular disorders

Neuromuscular disorders are a frequent cause of chronic disability in man. They often result from mutations in single genes and are thus, in principle, well suited for gene therapy. However, the tissues involved (muscle and the central nervous system) are post-mitotic, which poses a challenge for most viral vectors. In some cases, alternative approaches may use small molecules, for example, antisense oligonucleotides (AONs). These do not deliver a new gene, but rather modulate existing gene products or alter the utilization of pathways. For Duchenne muscular dystrophy, this approach is in early phase clinical trials, and for two other common neuromuscular disorders (spinal muscular atrophy and myotonic dystrophy), significant preclinical advances have recently been made.

Hepatic gene transfer as a means of tolerance induction to transgene products

The liver is a preferred target organ for gene therapy not only for liver-specific diseases but also for disorders that require systemic delivery of a protein. Diseases that could benefit from hepatic gene transfer include hemophilia, metabolic disorders, lysosomal storage disorders, and others. For a successful delivery of the transgene and sustained expression, the protocol must avoid immune responses in order to be efficacious. A growing number of studies have demonstrated that liver-directed transfer can induce transgene product-specific immune tolerance. Tolerance obtained via this route requires optimal engineering of the vector to eliminate transgene expression in antigen presenting cells while restricting high levels of therapeutic expression to hepatocytes. Innate immune responses may prevent tolerance induction, cause toxicity, and have to be minimized. Discussed in our review is the crucial role of CD4(+)CD25(+) regulatory T cells in tolerance to the hepatocyte-derived gene product, the immunobiology of the liver and our current understanding of its tolerogenic properties, current and proposed research as to the mechanisms behind the liver's unique cellular environment, as well as development of the tools for tolerance induction such as advanced vector systems.

Viral vectors: from virology to transgene expression

In the late 1970s, it was predicted that gene therapy would be applied to humans within a decade. However, despite some success, gene therapy has still not become a routine practise in medicine. In this review, we will examine the problems, both experimental and clinical, associated with the use of viral material for transgenic insertion. We shall also discuss the development of viral vectors involving the most important vector types derived from retroviruses, adenoviruses, herpes simplex viruses and adeno-associated viruses.

Versatile somatic gene transfer for modeling neurodegenerative diseases

A growing variety of technical approaches allow control over the expression of selected genes in living organisms. The ability to deliver functional exogenous genes involved in neurodegenerative diseases has opened pathological processes to experimental analysis and targeted therapeutic development in rodent and primate preclinical models. Biological adaptability, economic animal use, and reduced model development costs complement improved control over spatial and temporal gene expression compared with conventional transgenic models. A review of viral vector studies, typically adeno-associated virus or lentivirus, for expression of three proteins that are central to major neurodegenerative diseases, will illustrate how this approach has powered new advances and opportunities in CNS disease research.

Prenatal stem cell transplantation and gene therapy

At the present time, the most likely and eminent application of stem cell therapy to the fetus is in utero hematopoietic stem cell transplantation (IUHCT), and this stem cell type will be discussed as a paradigm for all prenatal stem cell therapy. The authors feel that the most likely initial application of IUHCT will use adult HSC derived from bone marrow (BM) or peripheral blood (PB), and will focus this article on this specific approach. The article also reviews the experimental data that support the capacity of IUHCT to induce donor-specific tolerance.

Transduction of nonhuman primate brain with adeno-associated virus serotype 1: vector trafficking and immune response

We used convection-enhanced delivery (CED) to characterize gene delivery mediated by adeno-associated virus type 1 (AAV1) by tracking expression of hrGFP (humanized green fluorescent protein from Renilla reniformis) into the striatum, basal forebrain, and corona radiata of monkey brain. Four cynomolgus monkeys received single infusions into corona radiata, putamen, and caudate. The other group (n = 4) received infusions into basal forebrain. Thirty days after infusion animals were killed and their brains were processed for immunohistochemical evaluation. Volumetric analysis of GFP-positive brain areas was performed. AAV1-hrGFP infusions resulted in approximately 550, 700, and 73 mm(3) coverage after infusion into corona radiata, striatum, and basal forebrain, respectively. Aside from targeted regions, other brain structures also showed GFP signal (internal and external globus pallidus, subthalamic nucleus), supporting the idea that AAV1 is actively trafficked to regions distal from the infusion site. In addition to neuronal transduction, a significant nonneuronal cell population was transduced by AAV1 vector; for example, oligodendrocytes in corona radiata and astrocytes in the striatum. We observed a strong humoral and cell-mediated response against AAV1-hrGFP in transduced monkeys irrespective of the anatomic location of the infusion, as evidenced by induction of circulating anti-AAV1 and anti-hrGFP antibodies, as well as infiltration of CD4(+) lymphocytes and upregulation of MHC-II in regions infused with vector. We conclude that transduction of antigen-presenting cells within the CNS is a likely cause of this response and that caution is warranted when foreign transgenes are used as reporters in gene therapy studies with vectors with broader tropism than AAV2.

Adeno-associated virus capsid structure drives CD4-dependent CD8+ T cell response to vector encoded proteins

The immunological sequelae of adeno-associated virus (AAV)-mediated gene transfer in vivo is quite complex. In murine models, most AAV capsids are associated with minimal or dysfunctional T cell responses to antigenic transgene products. In this study we compared T cell activation against AAV2/8 and AAV2/rh32.33 vectors expressing nuclear-targeted LacZ (nLacZ), GFP, or firefly luciferase in murine skeletal muscle. We show that, unlike AAV8, AAVrh32.33 yields qualitatively and quantitatively robust T cell responses to both the capsid and transgene product. AAV2/rh32.33.CB.nLacZ, but not AAV2/8, drives a high degree of cellular infiltration and a loss of detectable transgene expression in C57BL/6 mice. However, cellular immunity to AAVrh32.33 is ablated in the absence of CD4, CD40L, or CD28, permitting stable beta-galactosidase expression. Treatment of CD40L(-/-) mice with the CD40 agonist, FGK45, failed to restore the CD8 response to AAV2/rh32.33.nLacZ, suggesting that additional factors are involved. Our results suggest that specific domains within the AAVrh32.33 capsid augment the adaptive response to both capsid and transgene Ags in a CD4-dependent pathway involving CD40L signaling and CD28 costimulation. Structural comparison of the AAV8 and rh32.33 capsids has identified key differences that may drive differential immunity by affecting tropism, Ag presentation or the activation of innate immunity. This murine model of AAV-mediated cytotoxicity allows us to delineate the mechanism of viral immune activation, which is relevant to the translation of AAV technology in higher order species.

Treatment of leber congenital amaurosis due to RPE65 mutations by ocular subretinal injection of adeno-associated virus gene vector: short-term results of a phase I trial

Leber congenital amaurosis (LCA) is a group of autosomal recessive blinding retinal diseases that are incurable. One molecular form is caused by mutations in the RPE65 (retinal pigment epithelium-specific 65-kDa) gene. A recombinant adeno-associated virus serotype 2 (rAAV2) vector, altered to carry the human RPE65 gene (rAAV2-CBSB-hRPE65), restored vision in animal models with RPE65 deficiency. A clinical trial was designed to assess the safety of rAAV2-CBSB-hRPE65 in subjects with RPE65-LCA. Three young adults (ages 21-24 years) with RPE65-LCA received a uniocular subretinal injection of 5.96 x 10(10) vector genomes in 150 microl and were studied with follow-up examinations for 90 days. Ocular safety, the primary outcome, was assessed by clinical eye examination. Visual function was measured by visual acuity and dark-adapted full-field sensitivity testing (FST); central retinal structure was monitored by optical coherence tomography (OCT). Neither vector-related serious adverse events nor systemic toxicities were detected. Visual acuity was not significantly different from baseline; one patient showed retinal thinning at the fovea by OCT. All patients self-reported increased visual sensitivity in the study eye compared with their control eye, especially noticeable under reduced ambient light conditions. The dark-adapted FST results were compared between baseline and 30-90 days after treatment. For study eyes, sensitivity increases from mean baseline were highly significant (p < 0.001); whereas, for control eyes, sensitivity changes were not significant (p = 0.99). Comparisons are drawn between the present work and two other studies of ocular gene therapy for RPE65-LCA that were carried out contemporaneously and reported.

AAV for pain: steps towards clinical translation

Recombinant adeno-associated virus (rAAV) vectors consisting of self-complementary genomes and packaged in certain capsids can target primary sensory neurons efficiently and can control neuropathic pain long term by expressing opioid or non-opioid analgesic genes. This review examines the therapeutic potential of the approach in five sections: Pain control in oncology (including a discussion of cancer centers as translational pain research environment); vector biology; safety considerations and immunological lessons learned from rAAV clinical trials of other disorders; development of intrathecal rAAV therapy in rodent models of pain; and preclinical steps towards clinical translation of rAAV for pain. In the field of analgesic drug development, clinical validation of new approaches identified in rodents is currently a critical limiting step. Small-molecule therapeutics suitable as conventional drugs to probe novel targets in clinical trials are often unavailable. In this context, gene therapy could fill an important gap in the drug development process facilitating first-into-human trials of untested targeted treatments, each instantiated as a therapeutic gene.

Biochemical correction of very long-chain acyl-CoA dehydrogenase deficiency following adeno-associated virus gene therapy

We report the development of a gene replacement strategy for very long-chain acyl-CoA dehydrogenase (VLCAD) deficiency. VLCAD is a mitochondrial enzyme involved in fatty acid beta-oxidation, a key step in energy production during times of fasting or stress. Deficiency of VLCAD classically presents as hepatic dysfunction, hypoglycemia, cardiomyopathy, rhabdomyolysis, and/or sudden death. While dietary therapy for VLCAD deficiency has proven beneficial in preventing some symptoms, a risk of metabolic catastrophic decompensation remains throughout life during times of increased energy demand. We designed a recombinant adeno-associated virus (AAV) expressing the human VLCAD gene (AAV8-hVLCAD). To demonstrate its in vivo activity, AAV8-hVLCAD was administered via the tail vein to VLCAD-knockout mice. A reduction in accumulated serum long-chain acylcarnitines and increased fasting tolerance judged on blood glucose concentrations were observed as of 11 days postinjections through >100 days. Western analysis of liver, skeletal muscle, and heart extracts using PEP1 anti-hVLCAD antibody revealed short-term hVLCAD expression in the liver and muscle and longer-term expression in heart. This demonstrates the ability of human VLCAD to correct the biochemical phenotype of VLCAD-deficient mice.

Production of recombinant AAV vectors encoding insulin-like growth factor I is enhanced by interaction among AAV rep regulatory sequences

BACKGROUND

Adeno-associated virus (AAV) vectors are promising tools for gene therapy. Currently, their potential is limited by difficulties in producing high vector yields with which to generate transgene protein product. AAV vector production depends in part upon the replication (Rep) proteins required for viral replication. We tested the hypothesis that mutations in the start codon and upstream regulatory elements of Rep78/68 in AAV helper plasmids can regulate recombinant AAV (rAAV) vector production. We further tested whether the resulting rAAV vector preparation augments the production of the potentially therapeutic transgene, insulin-like growth factor I (IGF-I).

RESULTS

We constructed a series of AAV helper plasmids containing different Rep78/68 start codon in combination with different gene regulatory sequences. rAAV vectors carrying the human IGF-I gene were prepared with these vectors and the vector preparations used to transduce HT1080 target cells. We found that the substitution of ATG by ACG in the Rep78/68 start codon in an AAV helper plasmid (pAAV-RC) eliminated Rep78/68 translation, rAAV and IGF-I production. Replacement of the heterologous sequence upstream of Rep78/68 in pAAV-RC with the AAV2 endogenous p5 promoter restored translational activity to the ACG mutant, and restored rAAV and IGF-I production. Insertion of the AAV2 p19 promoter sequence into pAAV-RC in front of the heterologous sequence also enabled ACG to function as a start codon for Rep78/68 translation. The data further indicate that the function of the AAV helper construct (pAAV-RC), that is in current widespread use for rAAV production, may be improved by replacement of its AAV2 unrelated heterologous sequence with the native AAV2 p5 promoter.

CONCLUSION

Taken together, the data demonstrate an interplay between the start codon and upstream regulatory sequences in the regulation of Rep78/68 and indicate that selective mutations in Rep78/68 regulatory elements may serve to augment the therapeutic value of rAAV vectors.

Chemoprotection by transfer of resistance genes

Dose-limiting toxicity of chemotherapeutic agents, i.e., myelosuppression, can limit their effectiveness. The transfer and expression of drug-resistance genes might decrease the risks associated with acute hematopoietic toxicity. Protection of hematopoietic stem/progenitor cells by transfer of drug-resistance genes provides the possibility of intensification or escalation of antitumor drug doses and consequently an improved therapeutic index. This chapter reviews drug-resistance gene transfer strategies for either myeloprotection or therapeutic gene selection. Selecting candidate drug-resistance gene(s), gene transfer methodology, evaluating the safety and the efficiency of the treatment strategy, relevant in vivo models, and oncoretroviral transduction of human hematopoietic stem/progenitor cells under clinically applicable conditions are described.

Gene therapy and transplantation in the retinofugal pathway

The mature CNS has limited intrinsic capacity for repair after injury; therefore, strategies are needed to enhance the viability and regrowth of damaged neurons. Here we review gene therapy studies in the eye, aimed at improving the survival and regeneration of injured retinal ganglion cells (RGCs). To target RGCs most current methods use recombinant adeno-associated viral vectors (AAV), usually serotype-2 (AAV2), that are injected into the vitreal chamber of the eye. This vector provides long-term transduction of adult RGCs. Strong, constitutive promoters such as CMV and/or beta-actin are commonly used but cell-specific promoters have also been tested. Transgenes encoded by AAV have been selected to limit cell death, enhance growth factor expression, or promote growth cone responsiveness. We have assessed the effects of AAV vectors in adult rodent models (i) after optic nerve (ON) crush and (ii) after transplantation of peripheral nerve (PN) onto the cut ON, a procedure that induces injured RGCs to regenerate axons over longer distances. AAV-CNTF-GFP promotes RGC survival and axonal regrowth in mice after ON crush, and in rats after ON crush or PN transplantation. In rats, intravitreal injection of AAV-BDNF-GFP also increases RGC viability but does not promote regeneration. RGC viability and axonal regrowth is further enhanced when AAV-CNTF-GFP is injected into transgenic mice that over-express bcl-2. Reconstituted PN grafts containing Schwann cells that were transduced ex vivo with lentiviral (LV) vectors encoding a secretable form of CNTF support RGC axonal regrowth, however grafts containing Schwann cells transduced with LV-BDNF or LV-GDNF are less successful. We have also quantified the transduction efficiency and tropism of different AAV vectors injected intravitreally. AAV 2/2 and AAV 2/6 showed highest levels of transduction, AAV 2/8 the lowest, and each serotype displayed different transduction profiles for retinal cells. We are also studying the long-term impact of AAV2-mediated CNTF or BDNF expression on the dendritic morphology of RGCs in normal and PN grafted retinas. Analysis of regenerating RGCs intracellularly injected with lucifer yellow indicates gene-specific changes in dendritic structure that likely impact upon visual function.

Primate models of schizophrenia: future possibilities

Schizophrenia is a disorder of the association cortices, with especially prominent structural and functional deficiencies in the dorsolateral prefrontal cortex (PFC). True dorsolateral PFC is found only in higher primates, and is characterized by highly elaborate pyramidal cells with extensive recurrent connections. The development of the primate PFC also involves distinct developmental and genetic pathways. Thus, primate models may be particularly important in determining the functional impact of genetic changes in patients with schizophrenia. Genes involved with pyramidal cell network connectivity may be especially important to study in primates, as their effects may be magnified in the extensively connected primate neurons. Adeno-associated virus technology appears particularly promising for studying the impact of genetic insults on the structure and function of the primate association cortex.

Cellular tropism and transduction properties of seven adeno-associated viral vector serotypes in adult retina after intravitreal injection

Recombinant adeno-associated virus (rAAV) vectors are increasingly being used as tools for gene therapy, and clinical trials have begun in patients with genetically linked retinal disorders. Intravitreal injection is optimal for the transduction of retinal ganglion cells (RGCs), although complete selectivity has not been achieved. There may also be advantages in using intravitreal approaches for the transduction of photoreceptors. Here we compared the cellular tropism and transduction efficiency of rAAV2/1, -2/2, -2/3, -2/4, -2/5, -2/6 and -2/8 in adult rat retina after intravitreal injection. Each vector encoded green fluorescent protein (GFP), and the number, laminar distribution and morphology of transduced GFP(+) cells were determined using fluorescent microscopy. Assessment of transduced cell phenotype was based on cell morphology and immunohistochemistry. rAAV2/2 and rAAV2/6 transduced the greatest number of cells, whereas rAAV2/5 and rAAV2/8 were least efficient. Most vectors primarily transduced RGCs; however, rAAV2/6 had a more diverse tropism profile, with 46% identified as amacrine or bipolar cells, 23% as RGCs and 22% as Müller cells. Müller cells were also frequently transduced by rAAV2/4. The highest photoreceptor transduction was seen after intravitreal rAAV2/3 injection. These data facilitate the design and selection of rAAV vectors to target specific retinal cells, potentially leading to an improved gene therapy for various human retinal pathologies.

Efficient intrathymic gene transfer following in situ administration of a rAAV serotype 8 vector in mice and nonhuman primates

The thymus is the primary site of T-cell development and plays a key role in the induction of self-tolerance. We previously showed that the intrathymic (i.t.) injection of a transgene-expressing lentiviral vector (LV) in mice can result in the correction of a T cell-specific genetic defect. Nevertheless, the efficiency of thymocyte transduction did not exceed 0.1-0.3% and we were unable to detect any thymus transduction in macaques. As such, we initiated studies to assess the capacity of recombinant adeno-associated virus (rAAV) vectors to transduce murine and primate thymic cells. In vivo administration of AAV serotype 2-derived single-stranded AAV (ssAAV) and self-complementary AAV (scAAV) vectors pseudotyped with capsid proteins of serotypes 1, 2, 4, 5, and 8 demonstrated that murine thymus transduction was significantly enhanced by scAAV2/8. Transgene expression was detected in 5% of thymocytes and, notably, transduced cells represented 1% of peripheral T lymphocytes. Moreover, i.t. administration of scAAV2/8 particles in macaques, by endoscopic-mediated guidance, resulted in significant gene transfer. Thus, in healthy animals, where thymic gene transfer does not provide a selective advantage, scAAV2/8 is a unique tool promoting the in situ transduction of thymocytes with the subsequent export of gene-modified lymphocytes to the periphery.

High-efficiency transduction of the mouse retina by tyrosine-mutant AAV serotype vectors

Vectors derived from adeno-associated viruses (AAVs) have become important gene delivery tools for the treatment of many inherited ocular diseases in well-characterized animal models. Previous studies have determined that the viral capsid plays an essential role in the cellular tropism and efficiency of transgene expression. Recently, it was shown that phosphorylation of surface-exposed tyrosine residues from AAV2 capsid targets the viral particles for ubiquitination and proteasome- mediated degradation, and mutations of these tyrosine residues lead to highly efficient vector transduction in vitro and in vivo. Because the tyrosine residues are highly conserved in other AAV serotypes, in this study we evaluated the intraocular transduction characteristics of vectors containing point mutations in surface- exposed capsid tyrosine residues in AAV serotypes 2, 8, and 9. Several of these novel AAV mutants were found to display a strong and widespread transgene expression in many retinal cells after subretinal or intravitreal delivery compared with their wild-type counterparts. For the first time, we show efficient transduction of the ganglion cell layer by AAV serotype 8 or 9 mutant vectors, thus providing additional tools besides AAV2 for targeting these cells. These enhanced AAV vectors have a great potential for future therapeutic applications for retinal degenerations and ocular neovascular diseases.

Antigen-specific humoral tolerance or immune augmentation induced by intramuscular delivery of adeno-associated viruses encoding CTLA4-Ig-antigen fusion molecules

This study initially sought to investigate the immunostimulatory properties of recombinant adeno-associated virus (rAAV) with a view to developing a genetic vaccine for malaria using muscle as a target tissue. To augment humoral immunity, the AAV-encoded antigen was genetically fused with CTLA4-Ig, a recombinant molecule that binds B7 costimulatory molecules. At 10(9) vg, CTLA4-Ig fusion promoted the humoral immune response 100-fold and was dependent on CTLA4-Ig binding with B7 costimulatory molecules, confirming plasmid DNA models using this strategy. In distinct contrast, 10(12)-10(13) vg of rAAV1 specifically induced long-lived humoral tolerance through a mechanism that is independent of CTLA4-Ig binding with B7. This finding was unexpected, as rAAV delivery to muscle, unlike liver, has shown that this tissue provides a highly immunogenic environment for induction of humoral immunity against rAAV transgene products. An additional unpredicted consequence of antigen fusion with CTLA4-Ig was the enhancement of antigen expression by approximately one log, an effect mapped to the hinge and Fc domain of IgG(1,) but not involving antigen dimerization or the neonatal Fc receptor. Collectively, these findings significantly advance the potential of rAAV both as a vaccine or immunotherapeutic platform for the induction of antigen-specific humoral immunity or tolerance and as a gene therapeutic delivery system.