Recombinant adeno-associated virus (rAAV)-mediated expression of a human gamma-globin gene in human progenitor-derived erythroid cells

Tissue and cell-type-specific transduction using rAAV vectors in lung diseases

Gene therapy of genetically determined diseases, including some pathologies of the respiratory system, requires an efficient method for transgene delivery. Recombinant adeno-associated viral (rAAV) vectors are well studied and employed in gene therapy, as they are relatively simple and low immunogenic and able to efficiently transduce eukaryotic cells. To date, many natural and artificial (with modified capsids) AAV serotypes have been isolated, demonstrating preferential tropism toward different tissues and cells in accordance with the prevalent receptors on the cell surface. However, rAAV-mediated delivery is not strictly specific due to wide tropism of some viral serotypes. Thus, the development of the methods allowing modulating specificity of these vectors could be beneficial in some cases. This review describes various approaches for retargeting rAAV to respiratory cells, for example, using different types of capsid modifications and regulation of a transgene expression by tissue-specific promoters. Part of the review is devoted to the issues of transduction of stem and progenitor lung cells using AAV, which is a complicated task today.

Adeno-Associated Virus Vectors and Stem Cells: Friends or Foes?

The infusion of healthy stem cells into a patient-termed "stem-cell therapy"-has shown great promise for the treatment of genetic and non-genetic diseases, including mucopolysaccharidosis type 1, Parkinson's disease, multiple sclerosis, numerous immunodeficiency disorders, and aplastic anemia. Stem cells for cell therapy can be collected from the patient (autologous) or collected from another "healthy" individual (allogeneic). The use of allogenic stem cells is accompanied with the potentially fatal risk that the transplanted donor T cells will reject the patient's cells-a process termed "graft-versus-host disease." Therefore, the use of autologous stem cells is preferred, at least from the immunological perspective. However, an obvious drawback is that inherently as "self," they contain the disease mutation. As such, autologous cells for use in cell therapies often require genetic "correction" (i.e., gene addition or editing) prior to cell infusion and therefore the requirement for some form of nucleic acid delivery, which sets the stage for the AAV controversy discussed herein. Despite being the most clinically applied gene delivery context to date, unlike other more concerning integrating and non-integrating vectors such as retroviruses and adenovirus, those based on adeno-associated virus (AAV) have not been employed in the clinic. Furthermore, published data regarding AAV vector transduction of stem cells are inconsistent in regards to vector transduction efficiency, while the pendulum swings far in the other direction with demonstrations of AAV vector-induced toxicity in undifferentiated cells. The variation present in the literature examining the transduction efficiency of AAV vectors in stem cells may be due to numerous factors, including inconsistencies in stem-cell collection, cell culture, vector preparation, and/or transduction conditions. This review summarizes the controversy surrounding AAV vector transduction of stem cells, hopefully setting the stage for future elucidation and eventual therapeutic applications.

Adeno-associated Virus as a Mammalian DNA Vector

In the nearly five decades since its accidental discovery, adeno-associated virus (AAV) has emerged as a highly versatile vector system for both research and clinical applications. A broad range of natural serotypes, as well as an increasing number of capsid variants, has combined to produce a repertoire of vectors with different tissue tropisms, immunogenic profiles and transduction efficiencies. The story of AAV is one of continued progress and surprising discoveries in a viral system that, at first glance, is deceptively simple. This apparent simplicity has enabled the advancement of AAV into the clinic, where despite some challenges it has provided hope for patients and a promising new tool for physicians. Although a great deal of work remains to be done, both in studying the basic biology of AAV and in optimizing its clinical application, AAV vectors are currently the safest and most efficient platform for gene transfer in mammalian cells.

Optimization of recombinant adeno-associated viral vectors for human beta-globin gene transfer and transgene expression

Therapeutic levels of expression of the beta-globin gene have been difficult to achieve with conventional retroviral vectors without the inclusion of DNase I-hypersensitive site (HS2, HS3, and HS4) enhancer elements. We generated recombinant adeno-associated viral (AAV) vectors carrying an antisickling human beta-globin gene under the control of either the beta-globin gene promoter/enhancer or the erythroid cell-specific human parvovirus B19 promoter at map unit 6 (B19p6) without any enhancer, and tested their efficacy in a human erythroid cell line (K-562) and in primary murine hematopoietic progenitor cells (c-kit(+)lin()). We report here that (1) self-complementary AAV serotype 2 (scAAV2)-beta-globin vectors containing only the HS2 enhancer are more efficient than single-stranded AAV (ssAAV2)-beta-globin vectors containing the HS2+HS3+HS4 enhancers; (2) scAAV2-beta-globin vectors recombine with scAAV2-HS2+HS3+HS4 vectors after dual-vector transduction, leading to transgene expression; (3) scAAV2-beta-globin as well as scAAV1-beta-globin vectors containing the B19p6 promoter without the HS2 enhancer element are more efficient than their counterparts containing the HS2 enhancer/beta-globin promoter; and (4) scAAV2-B19p6-beta-globin vectors in K-562 cells, and scAAV1-B19p6-beta-globin vectors in murine c-kit(+)lin() cells, yield efficient expression of the beta-globin protein. Thus, the combined use of scAAV vectors and the parvovirus B19 promoter may lead to expression of therapeutic levels the beta-globin gene in human erythroid cells, which has implications in the use of these vectors in gene therapy of beta-thalassemia and sickle cell disease.

Erythroid-specific expression of beta-globin by the sleeping beauty transposon for Sickle cell disease

Sickle cell disease (SCD) results predominately from a single monogenic mutation that affects thousands of individuals worldwide. Gene therapy approaches have focused on using viral vectors to transfer wild-type beta- or gamma-globin transgenes into hematopoietic stem cells for long-term expression of the recombinant globins. In this study, we investigated the use of a novel nonviral vector system, the Sleeping Beauty (SB) transposon (Tn) to insert a wild-type beta-globin expression cassette into the human genome for sustained expression of beta-globin. We initially constructed a beta-globin expression vector composed of the hybrid cytomegalovirus (CMV) enhancer chicken beta-actin promoter (CAGGS) and full-length beta-globin cDNA, as well as truncated forms lacking either the 3' or 3' and 5' untranslated regions (UTRs), to optimize expression of beta-globin. Beta-globin with its 5' UTR was efficiently expressed from its cDNA in K-562 cells induced with hemin. However, expression was constitutive and not erythroid-specific. We then constructed cis SB-Tn-beta-globin plasmids using a minimal beta-globin gene driven by hybrid promoter IHK (human ALAS2 intron 8 erythroid-specific enhancer, HS40 core element from human alphaLCR, ankyrin-1 promoter), IHbetap (human ALAS2 intron 8 erythroid-specific enhancer, HS40 core element from human alphaLCR, beta-globin promoter), or HS3betap (HS3 core element from human betaLCR, beta-globin promoter) to establish erythroid-specific expression of beta-globin. Stable genomic insertion of the minimal gene and expression of the beta-globin transgene for >5 months at a level comparable to that of the endogenous gamma-globin gene were achieved using a SB-Tn beta-globin cis construct. Interestingly, erythroid-specific expression of beta-globin driven by IHK was regulated primarily at the translational level, in contrast to post-transcriptional regulation in non-erythroid cells. The SB-Tn system is a promising nonviral vector for efficient genomic insertion conferring stable, persistent erythroid-specific expression of beta-globin.

Hematopoietic stem cell transduction by recombinant adeno-associated virus vectors: problems and solutions

Recombinant adeno-associated virus 2 (AAV) vectors have taken center stage owing to their potentially safer profile compared with the more commonly used retroviral and adenoviral vectors in human gene therapy clinical trials. Their remarkable versatility and efficacy in a wide variety of preclinical animal models of human diseases have attracted further attention of a number of investigators. Although two particular cell types, muscle and brain, have been shown to be highly transducible by AAV vectors, controversies abound with reference to the efficacy of these vectors in transducing primary hematopoietic cells. Whereas some investigators have claimed that primitive hematopoietic cells are impervious to AAV vectors, others have reported that AAV vectors are capable of transducing these cells, but only at high vector-to-cell ratios. Still other investigators have reported successful transduction of primitive hematopoietic cells at relatively low vector-to-cell ratios. This review attempts to resolve these controversies, and provides a basis for the optimism that safe and high-efficiency transduction of hematopoietic stem and progenitor cells by AAV vectors is well within reach.

Impaired nuclear transport and uncoating limit recombinant adeno-associated virus 2 vector-mediated transduction of primary murine hematopoietic cells

Controversies abound concerning hematopoietic stem cell transduction by recombinant adeno-associated virus 2 (AAV) vectors. For human hematopoietic cells, we have shown that this problem is related to the extent of expression of the cellular receptor for AAV. At least a small subset of murine hematopoietic cells, on the other hand, does express both the AAV receptor and the coreceptor, yet is transduced poorly. In the present study, we have found that approximately 85% of AAV genomes were present in the cytoplasmic fraction of primary murine c-Kit(+)Lin- hematopoietic cells. However, when mice were injected intraperitoneally with hydroxyurea before isolation of these cells, the extent to which AAV genomes were detected in the cytoplasmic fraction was reduced to approximately 40%, with a corresponding increase to approximately 60% in the nuclear fraction, indicating that hydroxyurea facilitated nuclear transport of AAV. It was apparent, nonetheless, that a significant fraction of the AAV genomes present in the nuclear fraction from cells obtained from hydroxyurea-treated mice was single stranded. We next tested whether the single-stranded AAV genomes were derived from virions that failed to undergo uncoating in the nucleus. A substantial fraction of the signal in the nuclear fraction of hematopoietic cells obtained from hydroxyurea-treated mice was also resistant to DNase I. That AAV particles were intact and biologically active was determined by successful transduction of 293 cells by virions recovered from murine hematopoietic cells 48 hr postinfection. Although hydroxyurea facilitated nuclear transport of AAV, most of the virions failed to undergo uncoating, thereby leading to only a partial improvement in viral second- strand DNA synthesis and transgene expression. A better understanding of the underlying mechanism of viral uncoating has implications in the optimal use of recombinant AAV vectors in hematopoietic stem cell gene therapy.

Reversal of diabetes in the rat by injection of hematopoietic stem cells infected with recombinant adeno-associated virus containing the preproinsulin II gene

AIM

To study the effect of injecting hematopoietic stem cells containing the preproinsulin gene II (rI2) via recombinant adeno-associated virus (rAAV) into normal and streptozotocin-diabetic rats.

METHODS

rI2 was transfected into rat hematopoietic stem cells using rAAV vector. Stem cells were injected by intravenous route into normal and STZ-induced diabetic rats to study blood sugar and expression of rI2 in various tissues. The pLP-1 recombinant plasmid containing rI2 (vLP-1) was engineered as previously described. Bone marrow from female Wistar-Furth rats was enriched for stem cells by using plastic adherence and monoclonal antirat CD3 and CD45 RA to deplete T and B cells. The remaining cells were exposed to vLP-1 (multiplicity of infection MOI =50:1 or 100:1) for 2 h. Approximately ten million exposed stem cells were injected by intravenous route into each animal; there were four groups: normal animals at MOI 50:1 (group 1) or MOI 100:1 (group 2); group 3 animals (n = 9) were streptozotocin-induced diabetic animals at MOI 100:1. Animals that showed reversal of diabetes from group 3 were sacrificed for study of gene expression at weeks 1, 2, and 6, respectively. Control diabetic animals did not receive stem cells or virus constituted group 4. Expression of rI2 was analyzed by RT-PCR and Southern analyses.

RESULTS

Despite introduction of insulin gene, groups 1 and 2 had blood sugar concentrations that remained within normal levels, while 3 of 9 animals in group 3 showed reversal of diabetes; using RT-PCR,group 1 expressed rI2 in liver, spleen, thymus, brain, and heart at week 1 only. In group 2, rI2 was seen in the thymus up to 6 weeks; in diabetic animals (group 3) rI2 was seen in liver, bone marrow, spleen, thymus, and peripheral blood lymphocytes at week 2 and in thymus and lymphocytes at week 6.

CONCLUSIONS

We have shown that (1) rAAV is a useful vector for transferring rI2 into rat hematopoietic stem cells; (2) normal animals remained euglycemic after injection of stem cells containing rI2 despite identification in various tissues suggesting autoregulation, and (3) short-term reversal of diabetes was achieved in some animals by injection of stem cells containing rI2.

Inhibiting AIDS in the central nervous system: gene delivery to protect neurons from HIV

Gene therapy to treat primary and secondary CNS diseases, including neuro-AIDS, has not yet been effective. New approaches to delivering therapeutic genes to the central nervous system are therefore required. Recombinant SV40 vectors (rSV40) transduce both dividing and quiescent cells efficiently, and so we tested them for their ability to deliver anti-HIV-1 transgenes to terminally differentiated human NT2-derived neurons (NT2-N). These vectors transduced >95% of immature as well as mature human neurons efficiently, without detectable toxicity and without requiring selection. rSV40 gene delivery was stable to retinoic acid-induced neuronal differentiation. The rSV40 vectors used in these studies, SV(RevM10) and SV(AT), respectively carried the cDNAs for RevM10, a trans-dominant mutant of HIV-1 Rev, and human alpha1-antitrypsin. As measured by HIV-1 p24 antigen assays and by immunostaining for gp120, NT2-N treated with these vectors strongly resisted challenge with different strains of HIV-1. Protection from HIV replication and HIV-induced cytotoxicity was conferred by SV(AT) and SV(RevM10) and remained constant throughout retinoic acid-induced neuronal differentiation and for the duration of these studies (> or =11 weeks). rSV40 transduction of human neurons might therefore be a practicable approach to gene delivery for the treatment of CNS diseases, including neuro-AIDS.

The degree of phenotypic correction of murine beta -thalassemia intermedia following lentiviral-mediated transfer of a human gamma-globin gene is influenced by chromosomal position effects and vector copy number

Increased fetal hemoglobin (HbF) levels diminish the clinical severity of beta-thalassemia and sickle cell anemia. A treatment strategy using autologous stem cell-targeted gene transfer of a gamma-globin gene may therefore have therapeutic potential. We evaluated oncoretroviral- and lentiviral-based gamma-globin vectors for expression in transduced erythroid cell lines. Compared with gamma-globin, oncoretroviral vectors containing either a beta-spectrin or beta-globin promoter and the alpha-globin HS40 element, a gamma-globin lentiviral vector utilizing the beta-globin promoter and elements from the beta-globin locus control region demonstrated a higher probability of expression. This lentiviral vector design was evaluated in lethally irradiated mice that received transplants of transduced bone marrow cells. Long-term, stable erythroid expression of human gamma-globin was observed with levels of vector-encoded gamma-globin mRNA ranging from 9% to 19% of total murine alpha-globin mRNA. The therapeutic efficacy of the vector was subsequently evaluated in a murine model of beta-thalassemia intermedia. The majority of mice that underwent transplantation expressed significant levels of chimeric m(alpha)(2)h(gamma)(2) molecules (termed HbF), the amount of which correlated with the degree of phenotypic improvement. A group of animals with a mean HbF level of 21% displayed a 2.5 g/dL (25 g/L) improvement in Hb concentration and normalization of erythrocyte morphology relative to control animals. gamma-Globin expression and phenotypic improvement was variably lower in other animals due to differences in vector copy number and chromosomal position effects. These data establish the potential of using a gamma-globin lentiviral vector for gene therapy of beta-thalassemia.

Obstacles to human hematopoietic stem cell transduction by recombinant adeno-associated virus 2 vectors

Recombinant adeno-associated virus 2 (AAV) vectors have proven to be a potentially useful alternative to the more commonly used retroviral and adenoviral vectors for gene therapy in humans. Their safety and efficacy in Phase I clinical trials for gene therapy of cystic fibrosis and hemophilia B have been well documented, and their remarkable versatility and efficacy in a wide variety of pre-clinical models of human diseases have catapulted these vectors to the forefront. AAV vectors have been shown to be particularly well suited for transduction of brain and muscle cells. However, controversies exist with regard to their utility as a vector for gene transfer into human hematopoietic stem cells. On the one hand, some investigators have concluded that AAV vectors do not transduce hematopoietic stem cells at all, and others have reported that stem cell transduction requires enormously high vector-to-cell ratios. On the other hand, some investigators have reported high-efficiency transduction of human hematopoietic stem cells at low vector-to cell ratios. This article will provide a historical perspective as well as attempt to elaborate the reasons behind these controversies which have become clearer by studies focused on understanding, at the molecular level, the fundamental aspects of the life cycle of recombinant AAV vectors.

In vivo induction of human immunodeficiency virus type 1 entry into nucleus-free cells by CD4 gene transfer to hematopoietic stem cells: a hypothetical possible strategy for therapeutic intervention

As a useful alternative to employing soluble CD4 to inhibit binding of human immunodeficiency virus type 1 (HIV-1) to target cells, the introduction of CD4-bearing erythrocyte has been proposed by two study groups (see Refs. (5,6)). Prominently, Nicolau and colleagues demonstrated that the electroinserted CD4 molecules in the membranes of erythrocytes are capable of mediating HIV-1 entry. The implications of the studies are that inactivation of the integration-dependent retrovirus by the facilitation of entry into the nucleus-free cells, referred to as 'fake host trap' or 'host cell decoy', may be a possible therapeutic approach. Here we expand this concept to include genetic modification of autologous hematopoietic stem cells and review the relevant theoretical basis. Effective application of molecular technologies to induce partial replacement of hematopoiesis may be critical for this strategy.

Analysis of adeno-associated virus-mediated ex vivo transferred human beta-globin gene in bone marrow engrafted mice

Adeno-associated virus (AAV)-2 was developed as a useful vector for human gene therapy. In this report, we analyzed the integration and expression of AAV-mediated ex vivo transferred human beta-globin gene in bone marrow (BM) reconstituted mice. Recombinant AAV (rAAV) containing human beta-globin gene was packaged by infecting individual G418-resistant BHK-21 cell clones integrated with the plasmid AV53HS432Deltabeta2.0Neo with recombinant herpes simplex virus, which can express rep and cap genes of wild-type AAV. The titer of rAAV was determined using slot blot hybridization with a result of 10(13) virus particles/ml (genome copy number). Low-density mononuclear cells were isolated from fetal livers of embryos from pregnant C57BL/6 mice at 14-16 days of gestation and were infected with rAAV. The transduced hematopoietic cells were then reinfused into lethally irradiated C57BL/6 recipient mice via tail vein injection. To analyze the provirus in short-term and long-term BM reconstituted mice, PCR/Southern blot and RT-PCR were performed to identify the integrity of the provirus and to detect the expression of human beta-globin gene, respectively. Genomic DNA was extracted from spleen nodules of BM reconstituted mice 12 days after transplantation. Human beta-globin gene was detected in 1 out of 6 nodules using PCR combined with Southern blot. Human beta-globin gene was also detected in the BM and thymus of mouse Y6161, in the thymus and spleen of mouse Y6162 and in the BM of mice Y6211 and Y6212. RT-PCR revealed low levels of expression of human beta-globin gene in the BM of mouse Y6211. Our results suggested that the efficiency of AAV-mediated human beta-globin gene integration into hematopoietic stem/progenitor cells was very low. It is necessary to perform further research on AAV biology before applying gene therapy that requires integration of a foreign gene into host chromosomes.

Correction of sickle cell disease in transgenic mouse models by gene therapy

Sickle cell disease (SCD) is caused by a single point mutation in the human betaA globin gene that results in the formation of an abnormal hemoglobin [HbS (alpha2betaS2)]. We designed a betaA globin gene variant that prevents HbS polymerization and introduced it into a lentiviral vector we optimized for transfer to hematopoietic stem cells and gene expression in the adult red blood cell lineage. Long-term expression (up to 10 months) was achieved, without preselection, in all transplanted mice with erythroid-specific accumulation of the antisickling protein in up to 52% of total hemoglobin and 99% of circulating red blood cells. In two mouse SCD models, Berkeley and SAD, inhibition of red blood cell dehydration and sickling was achieved with correction of hematological parameters, splenomegaly, and prevention of the characteristic urine concentration defect.

Adeno-associated virus 2-mediated transduction and erythroid lineage-restricted long-term expression of the human beta-globin gene in hematopoietic cells from homozygous beta-thalassemic mice

Adeno-associated virus 2 (AAV), a nonpathogenic human parvovirus, has gained attention as a potentially useful vector for human gene therapy. Here, we report successful AAV-mediated stable transduction and high-efficiency, long-term, erythroid lineage-restricted expression of a human beta-globin gene in primary murine hematopoietic stem cells in vivo. Bone marrow-derived primitive Sca-1(+), lin(-) hematopoietic stem cells from homozygous beta-thalassemic mice were transduced ex vivo with a recombinant AAV vector containing a normal human beta-globin gene followed by transplantation into low-dose-irradiated B6.c-kitW(41/41) anemic recipient mice. Six months posttransplantation, tail-vein blood samples were analyzed by PCR amplification to document the presence of the transduced human beta-globin gene sequences in the peripheral blood cells. Semiquantitative PCR analyses revealed that the transduced human beta-globin gene sequences were present at approximately 1 copy per cell. The efficiency of the human beta-globin gene expression was determined to be up to 35% compared with the murine endogenous beta-globin gene by semiquantitative RT-PCR analyses. Peripheral blood samples from several positive recipient mice obtained 10 months posttransplantation were fractionated to obtain enriched populations of granulocytes, lymphocytes, and erythroid cells. PCR analyses revealed the presence of the human beta-globin gene sequences in granulocytes and lymphocytes, indicating multilineage reconstitution. However, only the erythroid population was positive following RT-PCR analyses, suggesting lineage-restricted expression of the transduced human beta-globin gene. Southern blot analyses of total genomic DNA samples isolated from bone marrow cells from transplanted mice also documented proviral integration. These results provide further support for the potential use of recombinant AAV vectors in gene therapy of beta-thalassemia and sickle-cell disease.

Adeno-associated virus vector mediated gene transfer to pancreatic beta cells

Insulin-dependent diabetes mellitus (IDDM) or type 1 diabetes is an autoimmune disease that results in destruction of the insulin-producing pancreatic islet beta cells. Several factors induce the invasion of immune cells into islets and trigger inflammation. Gene therapy approaches targeting the islet cells could be an effective treatment to prevent the onset or reverse type 1 diabetes. Allogeneic islet transplantation provides short-term treatment. However, genetically modified islets, which resist the host immune response, could provide long-term solutions. Adeno-associated virus (AAV) is emerging as a prominent vector system for delivering therapeutic genes for human gene therapy. AAV vector can transduce nondividing cells and provide long-term gene expression by integrating into host chromosome. Therefore, it is an appropriate vector system for islet cell gene therapy. To test the efficacy of AAV vector to transduce pancreatic endocrine cells, we constructed AAV vectors using plasmid pSub201. Wild-type AAV DNA analogue from plasmid psub201 was subcloned into a cloning plasmid pSP72 and AAV vectors were constructed by inserting the transgenes with heterologous promoter in place of AAV open reading frames (rep and cap). In this report we demonstrate the transduction of pancreatic islet cells with AAV vectors encoding bacterial -galactosidase enzyme or enhanced green fluorescent protein (EGFP) as reporter gene. Dispersed porcine and rat islet cells can be transduced by AAV vector, with an efficiency of 47% and 38%, respectively. In particular porcine islet insulin producing beta cells were transduced with an efficiency of 39%. Intact rat islet cells were transduced with an efficiency of 26% as estimated by FACS analysis following transduction with an AAV vector encoding EGFP. Transduction of intact rat islets with an AAV vector did not alter glucose-induced insulin secretion. AAV vector transduction was higher in transformed islet cell lines INS-1 and RIN m5F with an efficiency of 65% and 57%, respectively. These new results suggest that AAV vectors will provide an improved method of gene delivery to pancreatic islets and isolated pancreatic beta cells.

Efficient gene transfer into human cord blood CD34+ cells and the CD34+CD38- subset using highly purified recombinant adeno-associated viral vector preparations that are free of helper virus and wild-type AAV

Recombinant adeno-associated viral (rAAV) vectors have been evaluated for their ability to transduce primitive hematopoietic cells. Early studies documented rAAV-mediated gene expression during progenitor derived colony formation in vitro, but studies examining genome integration and long-term gene expression in hematopoietic cells have yielded conflicting results. Such studies were performed with crude vector preparations. Using improved methodology, we have generated high titer, biologically active preparations of rAAV free of wild-type AAV (less than 1/107particles) and adenovirus. Transduction of CD34+ cells from umbilical cord blood was evaluated with a bicistronic rAAV vector encoding the green fluorescent protein (GFP) and a trimetrexate resistant variant of dihydrofolate reductase (DHFR). Freshly isolated, quiescent CD34+ cells were resistant to transduction (less than 4%), but transduction increased to 23 +/- 2% after 2 days of cytokine stimulation and was further augmented by addition of tumor necrosis factor alpha (51 +/- 4%) at a multiplicity of infection of 106. rAAV-mediated gene expression was transient in that progenitor derived colony formation was inhibited by trimetrexate. Primitive CD34+ and CD34+, CD38- subsets were sequentially transduced with a rAAV vector encoding the murine ecotropic receptor followed by transduction with an ecotropic retroviral vector encoding GFP and DHFR. Under optimal conditions 41 +/- 7% of CD34+ progenitors and 21 +/- 6% of CD34+, CD38- progenitors became trimetrexate resistant. These results document that highly purified rAAV transduce primitive human hematopoietic cells efficiently but gene expression appears to be transient. Gene Therapy (2000) 7, 183-195.

New trends in the treatment of beta-thalassemia

Thalassemia is the world's most common hereditary disease, and is a paradigm of monogenic genetic diseases. Because of increased population mobility, the disease is found today throughout the world, even in places far from the tropical areas in which it arose. Therapy of thalassemia has in the past been confined to transfusion and chelation. Recently, novel modes of therapy have been developed for thalassemia, based on the pathophysiology and molecular pathology of the disease, both of which have been extensively studied. This review will discuss the therapeutic modalities currently in use for the supportive treatment of thalassemia, both those that are standard therapy and those that are in clinical trials. We will include transfusion, chelation (intravenous and oral), antioxidants and various inducers of fetal hemoglobin (hydroxyurea, erythropoietin, butyrates, hemin). Most of the newer therapies are suitable primarily for thalassemia intermedia patients. In addition, the treatment modalities currently in use for the curative treatment of thalassemia major will be discussed, including bone marrow transplantation in its various forms. Experimental therapeutic methods, such as intrauterine bone marrow transplantation and gene therapy, are included. Physicians caring for thalassemia patients have an increasing variety of treatment options available. Future clinical studies will determine the place of newer agents and modalities in improving the quality of life as well as the life expectancy of thalassemia patients.

Adeno-associated virus 2-mediated transduction and erythroid lineage-restricted expression from parvovirus B19p6 promoter in primary human hematopoietic progenitor cells

Human parvovirus B19 gene expression from the viral p6 promoter (B19p6) is restricted to primary human hematopoietic cells undergoing erythroid differentiation. We have demonstrated that expression from this promoter does not occur in established human erythroid cell lines in the context of a recombinant parvovirus genome (Ponnazhagan et al. J Virol 69:8096-8101, 1995). However, abundant expression from this promoter can be readily detected in primary human bone marrow cells (Wang et al. Proc Natl Acad Sci USA 92:12416-12420, 1995; Ponnazhagan et al. J Gen Virol 77:1111-1122, 1996). In the present studies, we investigated the pattern of expression from the B19p6 promoter in primary human bone marrow-derived CD34+ HPC undergoing differentiation into myeloid and erythroid lineages. CD34+ cells were transduced with recombinant adeno-associated virus 2 (AAV) vectors containing the beta-galactosidase (lacZ) gene under the control of the following promoters/enhancers: the cytomegalovirus promoter (vCMVp-lacZ), B19p6 promoter (vB19p6-lacZ), B19p6 promoter with an upstream erythroid cell-specific enhancer element (HS-2) from the locus control region (LCR) from the human beta-globin gene cluster (vHS2-B19p6-lacZ), and the human beta-globin gene promoter with the HS-2 enhancer (vHS2-beta p-lacZ). Transgene expression was evaluated either 48 h after infection or following erythroid differentiation in vitro for 3 weeks. Whereas high-level expression from the CMV promoter 48 h after infection diminished with time, low-level expression from the B19p6 and the beta-globin promoters increased significantly following erythroid differentiation. Furthermore, in HPC assays, there was no significant difference in the level of expression from the CMV promoter in myeloid or erythroid cell-derived colonies. Expression from the B19p6 and the beta-globin promoters, on the other hand, was restricted to erythroid cell colonies. These data further corroborate that the B19p6 promoter is erythroid cell-specific and suggest that the recombinant AAV-B19 hybrid vectors may prove useful in gene therapy of human hemoglobinopathies in general and sickle cell anemia and beta-thalassemia in particular.

Expression of beta-globin in primary erythroid progenitors of beta-thalassemia patients using an SV40-based gene delivery system

SV40-based vectors are very efficient in gene delivery into human hematopoietic cells. In the present work, we investigated the expression of constructs carrying the human beta-globin gene that were delivered as beta-globin pseudovirions. Expression studies were performed by RNA analysis of primary human erythroid progenitors cultivated from peripheral blood of beta(0)-thalassemia patients who are unable to produce normal beta-globin RNA. This erythroid culture system recapitulates in vitro the process of growth, differentiation, and maturation of authentic erythroid precursors. The progenitors were induced to differentiate by the addition of erythropoietin (EPO). Five days later, the cells were infected with pseudovirions containing the normal beta-globin gene, and RNA was harvested on day 8. The results showed significant levels of normal beta-globin gene mRNA. A small DNA fragment derived from the 5'-region of the HSII element of the human beta-globin locus control region (LCR) enhanced expression of the linked beta-globin gene 20-30-fold. Normal beta-globin mRNA expression was in direct correlation to the multiplicity of infection. These studies suggest the potential feasibility of using the beta-globin delivery system for gene therapy of beta-thalassemia.

Recombinant AAV-2 harboring gfp-antisense/ribozyme fusion sequences monitor transduction, gene expression, and show anti-HIV-1 efficacy

Vector-mediated delivery of potentially antivirally active genes is a key step in somatic gene therapy including therapeutic approaches against AIDS. A crucial technical prerequisite is to monitor DNA transfer into target cells. Here, we describe recombinant infectious particles derived from the adeno-associated virus type 2 (AAV-2) that are suitable to deliver effective HIV-1-directed antisense and ribozyme genes into target cells. To monitor transduction, we designed and tested a number of fusions between indicator-coding sequences of luciferase or gfp with effective HIV-1-directed antisense or ribozyme sequences. The combination of an indicator function and an antiviral func- tion in cis allows successful identification of transduced cells and measurement of effects on the replication of HIV-1 in antisense/ribozyme-expressing cells only. The fusion genes were shown to express the indicator genes. Inhibition of HIV-1 replication mediated by the antisense/ribozyme portion of the fusion transcripts was similar to parental constructs and neither acute nor long-term toxicity of fusion genes and their gene products was observed. These results suggest the use of rAAV constructs described here as tools to study the transducibility of target cells, gene expression and efficacy of HIV-1-directed antisense and ribozyme genes.

Prospective identification of erythroid elements in cultured peripheral blood

We have developed a prospective approach to identify the generation of erythroid cells derived from cultured peripheral blood mononuclear cells (PBMC) by monitoring the expression of the cell surface protein CD48. Unpurified populations of PBMC obtained from the buffy coats of normal volunteers were grown in suspension culture in the absence or presence of erythropoietin. A profile of surface CD48 expression permitted a flow cytometric identification of erythropoietin responsive populations at various stages of their maturation. In the absence of erythropoietin (EPO) supplemented media, the CD48- cells represented <5% of the total population of PBMC remaining in culture. In cultures supplemented with 1 U/mL EPO, the mean percentage of CD48- cells increased to 34.7 + 14.9% (p < 0.01) after 14 days in culture. Coordinated CD34 and CD71 (transferrin receptor) expression, morphology, gamma-globin transcription, and colony formation in methylcellulose were observed during the 14-day culture period. Flow cytometric monitoring of bulk cultured PBMC provides a simple and reliable means for the prospective or real-time study of human erythropoiesis.

Transduction of Primitive Human Marrow and Cord Blood-Derived Hematopoietic Progenitor Cells With Adeno-Associated Virus Vectors

We evaluated the capacity of adeno-associated virus (AAV) vectors to transduce primitive human myeloid progenitor cells derived from marrow and cord blood in long-term cultures and long-term culture-initiating cell (LTC-IC) assays. Single-colony analyses showed that AAV vectors transduced CD34+ and CD34+38− clonogenic cells in long-term culture. Gene transfer was readily observed in LTC-ICs derived from 5-, 8-, and 10-week cultures. Recombinant AAV (rAAV) transduction was observed in every donor analyzed, although a wide range of gene transfer frequencies (5% to 100%) was noted. AAV transduction of LTC-ICs was stable, with week-8 and -10 LTC-ICs showing comparable or better transduction relative to week-5 LTC-ICs. Fluorescence in situ hybridization (FISH) analyses performed to determine the fate of AAV vectors in transduced cells showed that 9% to 28% of CD34+ and CD34+38− cells showed stable vector integration as evidenced by chromosome-associated signals in metaphase spreads. Comparisons of interphase and metaphase FISH suggested that a fraction of cells also contained episomal vector at early time points after transduction. Despite the apparent loss of the episomal forms with continued culture, the number of metaphases containing integrated vector genomes remained stable long term. Transgene transcription and placental alkaline phosphatase (PLAP) expression was observed in CD34+, CD34+38−LTC-ICs in the absence of selective pressure. These results suggest that primitive myeloid progenitors are amenable to genetic modification with AAV vectors.

Targeted adeno-associated virus vector transduction of nonpermissive cells mediated by a bispecific F(ab'gamma)2 antibody

We have developed a system for the targeted delivery of adeno-associated virus (AAV) vectors. Targeting is achieved via a bispecific F(ab')2 antibody that mediates a novel interaction between the AAV vector and a specific cell surface receptor expressed on human megakaryocytes. Targeted AAV vectors were able to transduce megakaryocyte cell lines, DAMI and MO7e, which were nonpermissive for normal AAV infection, 70-fold above background and at levels equivalent to permissive K562 cells. Transduction was shown to occur through the specific interaction of the AAV vector-bispecific F(ab')2 complex and cell-associated targeting receptor. Importantly, targeting appeared both selective and restrictive as the endogenous tropism of the AAV vector was significantly reduced. Binding and internalization through the alternative receptor did not alter subsequent steps (escape from endosomes, migration to nucleus, or uncoating) required to successfully transduce target cells. These results demonstrate that AAV vectors can be targeted to a specific cell population and that transduction can be achieved by circumventing the normal virus receptor.

Transfer of activation-dependent gene expression into T cell lines by recombinant adeno-associated virus

We examined the ability of recombinant adeno-associated virus (rAAV) to transfer regulated gene expression into T cell lines. An AAV-based vector containing the neomycin resistance gene and expressing the firefly luciferase (luc) gene under the regulatory control of the interleukin 2 promoter (pAAV-luc) was generated and adenovirus-free rAAV (rAAV-luc) was produced from this vector. Transfection of pAAV-luc into the human T cell line Jurkat resulted in luciferase expression while infection of Jurkat T cells with rAAV-luc resulted in significant luciferase expression only after selection for neomycin-resistant cells. Long-term growth of transduced Jurkat T cells showed that there was no detectable constitutive expression of luciferase and that luciferase gene expression remained inducible for at least 180 days. Luciferase expression was activated by PMA and ionomycin and by anti-CD3 antibodies and was inhibited by cyclosporin A. Examination of G418-resistant clones showed that rAAV-luc had integrated into the host chromosomes but that some of the clones lost some of the transferred DNA or lost expression from the transferred DNA. These results indicate that rAAV can transfer and integrate regulated gene expression into T cell lines but that the transferred genetic material may be lost or its expression may be silenced over time.

Long-term genetic modification of rhesus monkey hematopoietic cells following transplantation of adenoassociated virus vector-transduced CD34+ cells

We have explored the potential of recombinant adenoassociated virus (AAV) vectors for gene transfer of the human beta-globin gene and the genetic modification of primate pluripotent hematopoietic stem cells (P-PHSCs). Transduction of P-PHSCs was tested in a preclinical bone marrow transplantation model in rhesus monkeys. CD34+ cells were transduced ex vivo and autologously transplanted without prior selection into irradiated rhesus monkeys. Vector-transduced peripheral blood mononuclear cells and granulocytes were present in the circulation for more than 15 months after transplantation. Approximately 1 in 10(5) cells in the circulation was vector modified. The vector was detected in the bone marrow, in granulocytes, and in purified populations of B and T cells, thus demonstrating multilineage repopulation by vector-transduced stem cells. Comparison of transduction protocols suggested that short-term culture of P-PHSCs enhances transduction and subsequent repopulation by rAAV-transduced cells. These results demonstrate that rAAV vectors can be used for the permanent genetic modification of a rhesus monkey hematopoietic system in the absence of selective pressure.

Reconstitution of NADPH oxidase activity in human X-linked chronic granulomatous disease myeloid cells after stable gene transfer using a recombinant adeno-associated virus 2 vector

X-linked chronic granulomatous disease (X-CGD) is an inherited disorder of host defense that results from mutations in the gene encoding gp91phox, the large subunit of the phagocyte NADPH oxidase flavocytochrome b. In this study, we constructed a recombinant adeno-associated virus-2 (AAV) vector in which the constitutively active promoter from the human elongation factor- 1alpha (EF-1alpha) gene drives expression of the murine gp91phox cDNA, and tested its ability to integrate and express in a human X-CGD myeloid cell line. The nitroblue tetrazolium (NBT) test of NADPH oxidase activity was used to screen transduced cells for vector-mediated expression of recombinant gp91phox. Between 2 - 14% of cells were NBT-positive in the first several weeks after transduction. Clones with NBT-positive cells persisting several months after transduction had integrated vector by Southern blot analyses, with high level reconstitution of NADPH oxidase activity. In some clones, oxidase activity persisted for at least 8 to 14 months. In the majority, however, vector-derived RNA transcripts declined, although integrated rAAV genomes persisted. Decreased transgene expression was not directly correlated with methylation of the provirus. This study indicates that rAAV vectors can be successfully used for stable gene transfer, integration, and expression of recombinant gp91phoxin a human myeloid cell line for at least 8 - 14 months in the absence of any selection. The EF-1alpha promotor, however, was subject to silencing in a high percentage of clones with integrated rAAV, suggesting that alternative promotors may be desirable for achieving long-term expression in myeloid cells.

Targeted integration of a recombinant globin gene adeno-associated viral vector into human chromosome 19

Transfer of a globin gene into stem cells along with the regulatory elements required to achieve high level expression in maturing erythroid cells would provide effective gene therapy for Cooley's Anemia. We have explored the use of recombinant adeno-associated viral (rAAV) vectors for this purpose. A vector designated rHS32A gamma*3'RE that contains regulatory elements from the locus control and flanking regions, integrates as a stable head-to-tail concatamer in erythroleukemia cells at a high multiplicity of infection and exhibits high level, regulated gamma globin gene expression. Inducible expression of the non-structural Rep proteins of wild-type AAV in HeLa cells transduced with rAAV vectors does not increase overall integration frequency, but targeted integration of rHS32A gamma*'3'RE into human chromosome 19 was documented.

Transduction of dendritic cells by DNA viral vectors directs the immune response to transgene products in muscle fibers

Immune responses to vector-corrected cells have limited the application of gene therapy for treatment of chronic disorders such as inherited deficiency states. We have found that recombinant adeno-associated virus (AAV) efficiently transduces muscle fibers in vivo without activation of cellular and humoral immunity to neoantigenic transgene products such as beta-galactosidase, which differs from the experience with recombinant adenovirus, where vibrant T-cell responses to the transgene product destroy the targeted muscle fibers. T cells activated following intramuscular administration of adenovirus expressing lacZ (AdlacZ) can destroy AAVlacZ-transduced muscle fibers, indicating a prior state of immunologic nonresponsiveness in the context of AAV gene therapy. Adoptive transfer of dendritic cells infected with AdlacZ leads to immune mediated elimination of AAVlacZ-transduced muscle fibers. AAVlacZ-transduced antigen-presenting cells fail to demonstrate beta-galactosidase activity and are unable to elicit transgene immunity in adoptive transfer experiments. These studies indicate that vector-mediated transduction of dendritic cells is necessary for cellular immune responses to muscle gene therapy, a step which AAV avoids, providing a useful biological niche for its use in gene therapy.

Robust, But Transient Expression of Adeno-Associated Virus-Transduced Genes During Human T Lymphopoiesis

Recombinant adeno-associated viruses (rAAV) have been proposed to be gene transfer vehicles for hematopoietic stem cells with advantages over other virus-based systems due to their high titers and relative lack of dependence on cell cycle for target cell integration. We evaluated rAAV vector containing a LacZ reporter gene under the control of a cytomegalovirus (CMV) promoter in the context of primary human CD34+CD2− progenitor cells induced to undergo T-cell differentiation using an in vitro T-lymphopoiesis system. Target cells from either adult bone marrow or umbilical cord blood were efficiently transduced, and 71% to 79% CD2+ cells expressed a LacZ marker gene mRNA and produced LacZ-encoded protein after exposure to rAAV-CMV-LacZ. The impact of transgene expression on the differentiation of T cells was assessed by sequential quantitation of immunophenotypic subsets of virus-exposed cells and no alteration was noted compared with control. The durability of transgene expression was assessed and found to decay by day 35 with kinetics dependent on the multiplicity of infection. In addition, vector DNA was absent from CD4 or CD8 subselected CD3+ cells by DNA-polymerase chain reaction. These data suggest that rAAV vectors may result in robust transgene expression in primitive cells undergoing T-cell lineage commitment without toxicity or alteration in the pattern of T-cell differentiation. However, expression is transient and integration of the transgene unlikely. Recombinant AAV vectors are potentially valuable gene transfer tools for the genetic manipulation of events during T-cell ontogony but their potential in gene therapy strategies for diseases such as acquired immunodeficiency syndrome is limited.

Robust, But Transient Expression of Adeno-Associated Virus-Transduced Genes During Human T Lymphopoiesis

Recombinant adeno-associated viruses (rAAV) have been proposed to be gene transfer vehicles for hematopoietic stem cells with advantages over other virus-based systems due to their high titers and relative lack of dependence on cell cycle for target cell integration. We evaluated rAAV vector containing a LacZ reporter gene under the control of a cytomegalovirus (CMV) promoter in the context of primary human CD34+CD2− progenitor cells induced to undergo T-cell differentiation using an in vitro T-lymphopoiesis system. Target cells from either adult bone marrow or umbilical cord blood were efficiently transduced, and 71% to 79% CD2+ cells expressed a LacZ marker gene mRNA and produced LacZ-encoded protein after exposure to rAAV-CMV-LacZ. The impact of transgene expression on the differentiation of T cells was assessed by sequential quantitation of immunophenotypic subsets of virus-exposed cells and no alteration was noted compared with control. The durability of transgene expression was assessed and found to decay by day 35 with kinetics dependent on the multiplicity of infection. In addition, vector DNA was absent from CD4 or CD8 subselected CD3+ cells by DNA-polymerase chain reaction. These data suggest that rAAV vectors may result in robust transgene expression in primitive cells undergoing T-cell lineage commitment without toxicity or alteration in the pattern of T-cell differentiation. However, expression is transient and integration of the transgene unlikely. Recombinant AAV vectors are potentially valuable gene transfer tools for the genetic manipulation of events during T-cell ontogony but their potential in gene therapy strategies for diseases such as acquired immunodeficiency syndrome is limited.

Gene transfer into hematopoietic cells

Transfer of a gene into stem cells with subsequent lineage-specific gene expression is a desired goal with many potential therapeutic applications. Retroviral vectors developed from murine leukemia viruses reproducibly transfer genes into murine stem cells, but are inefficient at gene insertion into stem cells of larger animals or man. A growing knowledge of stem cell biology suggests that this inefficiency reflects the quiescent state of stem cells, even when incubated in the presence of multiple cytokines and low expression of the receptor for amphotropic retroviral vectors. Alternative vector systems are being explored in an effort to overcome these barriers to stem cell-targeted gene transfer. Our work has shown that recombinant adeno-associated virus vectors, which have the potential for transducing quiescent cells, transfer, express and integrate a globin gene linked to its normal regulatory elements in human erythroid cells, but only at very high multiplicities of infection. The integrated genome was stable and the encoded globin gene was expressed at levels equivalent to a normal globin gene.

Transfer of contaminants in adeno-associated virus vector stocks can mimic transduction and lead to artifactual results

The potential of adeno-associated virus (AAV) vectors for gene transfer and gene therapy applications is currently being intensively investigated. Although much progress has been made in defining AAV vector biology, inconsistencies remain in the literature regarding the efficiency of AAV transduction in various cell types. In the course of exploring these differences, we have identified a problem associated with the use of AAV vector stocks that results in overestimation of gene transfer efficiencies. We show here that biologically active vector-encoded proteins can contaminate AAV vector stocks, especially cell lysate preparations that have not been further purified, and can be transferred in a virion-independent manner to target cells, a phenomenon called pseudotransduction. This observation is significant because impure cell lysate stocks have been widely employed in the AAV literature, and we demonstrate here that this phenomenon can occur with commonly used reporter proteins such as beta-galactosidase and alkaline phosphatase. We conclude that although there are many potential explanations for apparently conflicting results in the literature, the possibility of pseudotransduction must be considered, especially when cell lysate stocks of AAV vectors have been employed. This artifact can be avoided by further vector purification.

Protection of hematopoietic stem cells from chemotherapy-induced toxicity by multidrug-resistance 1 gene transfer

An increased chemotherapeutic dose intensity is believed to translate into higher survival rates among cancer patients. Pancytopenia is the dose-limiting toxic result of most anticancer agents. Overexpression of the human multidrug resistance 1 (MDR1) gene in transgenic animals resulted in complete myeloprotection against high doses of cytostatic drugs. Stem cell research, vector development, and experimental pharmacology are uniting their efforts in an attempt to achieve a similar effect in human hematopoietic stem cells. This article gives an overview of the crucial steps involved, from retroviral vector design and optimization of viral titers to vector uptake, gene integration, and expression. The authors' own results are presented with special regard in vitro and in vivo assays for the detection of hematopoietic stem cell transduction.

Highly efficient and sustained gene transfer in adult neurons with a lentivirus vector

The identification of monogenic and complex genes responsible for neurological disorders requires new approaches for delivering therapeutic protein genes to significant numbers of cells in the central nervous system. A lentivirus-based vector capable of infecting dividing and quiescent cells was investigated in vivo by injecting highly concentrated viral vector stock into the striatum and hippocampus of adult rats. Control brains were injected with a Moloney murine leukemia virus, adenovirus, or adeno-associated virus vector. The volumes of the areas containing transduced cells and the transduced-cell densities were stereologically determined to provide a basis for comparison among different viral vectors and variants of the viral vector stocks. The efficiency of infection by the lentivirus vector was improved by deoxynucleoside triphosphate pretreatment of the vector and was reduced following mutation of integrase and the Vpr-matrix protein complex involved in the nuclear translocation of the preintegration complex. The lentivirus vector system was able to efficiently and stably infect quiescent cells in the primary injection site with transgene expression for over 6 months. Triple labeling showed that 88.7% of striatal cells transduced by the lentivirus vector were terminally differentiated neurons.

Analysis of enhancer function of the HS-40 core sequence of the human alpha-globin cluster

HS-40 is the major regulatory element of the human alpha-globin locus, located 40 kb upstream of the zeta-globin gene. To test for potential interactions between HS-40 and the beta- or the gamma-globin gene promoters in stable transfection assays, the HS-40 core sequence was cloned upstream of either the beta promoter or the gamma promoter driving the neomycin phosphotransferase gene and enhancer activity was measured using a colony assay. In K562 or in MEL cells, enhancer activity of HS-40 was higher than that of the individual core sequences of the DNase I hypersensitive sites (HS) of the beta-globin locus control region (LCR), and approximately 60% of the enhancer activity of a 2.5 kb microLCR, which contains the core elements of DNase I hypersensitive sites 1-4. In contrast to the synergistic interaction between the DNase I hypersensitive sites of beta locus LCR, combination of HS-40 with these DNase I hypersensitive sites failed to display cooperativity in K562 cells and inhibited enhancer function in MEL cells. Inhibition of enhancer function was also observed when two copies of the HS-40 were arranged tandemly. We conclude that the core element of HS-40 (i) is a powerful enhancer of gamma- and beta-globin gene expression, (ii) in contrast to other classical enhancers, acts best as a single copy, (iii) does not cooperate with the regulatory elements of the beta-globin locus control region.

Gene therapy for murine mucopolysaccharidosis type VII

Mucopolysaccharidosis type VII (MPS VII) is caused by a deficiency in the lysosomal enzyme beta-glucuronidase resulting in the accumulation of undegraded glycosaminoglycans in many tissues. A murine model of MPS VII shares many of the clinical, biochemical and histopathological features of human MPS VII and has provided an opportunity to study novel therapeutic approaches in a system with a uniform genetic background. Retroviral mediated gene therapy directed to the hematopoietic system or to artificial neo-organs resulted in low levels of enzyme in several tissues and reduced lysosomal storage in the liver and spleen. Partial correction of the disease in the eye was observed following an intravitreal injection of recombinant adenovirus. Neither retroviral nor adenoviral mediated gene transfer techniques resulted in a systemic reduction of lysosomal storage. Here we discuss several novel gene transfer approaches designed to increase the systemic levels of beta-glucuronidase in the MPS VII mouse.

Construction and biological characterization of an interleukin-12 fusion protein (Flexi-12): delivery to acute myeloid leukemic blasts using adeno-associated virus

Interleukin-12 (IL-12) is a cytokine that exhibits pleiotropic effects on lymphocytes and natural killer cells and has been shown to have promise for the immunotherapy of cancer. The combination of the immune costimulatory molecule B7.1 and IL-12 has been shown to be synergistic for T cell activation. By transfecting tumor cells with both IL-12 and B7.1 cDNAs, it may be possible to use these modified targets as vaccines. A major obstacle in designing a vector to deliver these genes results from the structure of IL-12. Functional IL-12 is a heterodimer composed of two distinct subunits that are encoded by separate genes on different chromosomes. Production of functional IL-12 requires the coordinated expression of both genes. This presents several problems in vectors, particularly those in which additional genes, either a co-stimulatory gene or a selectable marker, are inserted. Therefore, we have constructed a single cDNA that encodes a single-chain protein, called Flexi-12, which retains all of the biological characteristics of recombinant IL-12 (rIL-12). The monomeric polypeptide Flexi-12 is able to induce the proliferation of phytohemagglutinin (PHA) blasts, induce PHA blasts to secrete interferon-gamma (IFN-gamma) and additionally, by preincubation, enhance the killing of K562 targets by PBLs. These phenomena are in a dose-dependent manner comparable to that seen with rIL-12. We have also shown that tyrosine phosphorylation of the STAT 4 transcription factor, which has been shown to be unique to the IL-12 signaling pathway, occurs with Flexi-12 at levels similar to those seen with rIL-12. We have packaged Flexi-12 into a recombinant adeno-associated virus (AAV) and used this vector to infect acute myeloid leukemic (AML) blasts. Infected AML blasts produced between 2 and 6 ng of IL-12/10(6) cells per ml per 48 hr. These studies also confirm that AAV is an efficient delivery vehicle for cytokines to leukemic cells. Direct analysis of these modified cells acting as tumor vaccines is underway.

The Rep78 gene product of adeno-associated virus (AAV) self-associates to form a hexameric complex in the presence of AAV ori sequences

The Rep78 and Rep68 proteins of adeno-associated virus (AAV) are replication initiator proteins that bind the viral replicative-form origin of replication, nick the origin in a site- and strand-specific fashion, and mediate vectorial unwinding of the DNA duplex via an ATP-dependent helicase activity, thus initiating a strand displacement mechanism of viral DNA replication. Genetic and biochemical studies have identified Rep mutants that demonstrate a trans-dominant negative phenotype in vitro and in vivo, suggesting the possibility that multimerization of Rep is essential for certain replicative functions. In this study, we have investigated the ability of the largest of the Rep proteins, Rep78, to self-associate in vitro and in vivo. Self-association of Rep78 in vivo was demonstrated through the use of a mammalian two-hybrid system. Rep-Rep protein interaction was confirmed in vitro through coimmunoprecipitation experiments with a bacterially expressed maltose-binding protein-Rep78 fusion protein in combination with [35S]methionine-labeled Rep78 synthesized in a coupled in vitro transcription-translation system. Mapping studies with N- and C-terminal truncation mutant forms of Rep indicate that amino acid sequences required for maximal self-association occur between residues 164 and 484. Site-directed mutagenesis identified two essential motifs within this 321-amino-acid region: (i) a putative alpha-helix bearing a 3,4-hydrophobic heptad repeat reminiscent of those found in coiled-coil domains and (ii) a previously recognized nucleoside triphosphate-binding motif. Deletion of either of these regions from the full-length polypeptide resulted in severe impairment of Rep-Rep interaction. In addition, gel filtration chromatography and protein cross-linking experiments indicated that Rep78 forms a hexameric complex in the presence of AAV ori sequences.

Reactivation of silenced, virally transduced genes by inhibitors of histone deacetylase

Retroviral and adeno-associated viral sequences can dramatically silence transgene expression in mice. We now report that this repression also occurs in stably infected HeLa cells when the cells are grown without selection. Expression of a transduced lacZ gene (rAAV/CMVlacZ) is silenced in greater than 90% of cells after 60 days in culture. Surprisingly, high-level expression can be reactivated by treating the cells with sodium butyrate or trichostatin A but not with 5-azacytidine. When cell clones with integrated copies of rAAV/CMVlacZ were isolated, lacZ expression was silenced in 80% of the clones; however, lacZ expression was reactivated in all of the silenced clones by treatment with butyrate or trichostatin A. The two drugs also reactivated a silenced globin gene construct (rAAV/HS2alphabetaAS3) in stably infected K562 cells. Trichostatin A is a specific inhibitor of histone deacetylase; therefore, we propose that hyperacetylation of histones after drug treatment changes the structure of chromatin on integrated viral sequences and relieves repression of transduced genes. The reactivation of silenced, transduced genes has implications for gene therapy. Efficient viral gene transfer followed by drug treatment to relieve suppression may provide a powerful combination for treatment of various genetic and infectious diseases.

Potent inhibition of human immunodeficiency virus type 1 in primary T cells and alveolar macrophages by a combination anti-Rev strategy delivered in an adeno-associated virus vector

The rate of viral replication appears to play a pivotal role in human immunodeficiency virus type 1 (HIV-1) pathogenesis and disease progression as it outstrips the capacity of the immune system to respond. Important cellular sites for HIV-1 production include T lymphocytes and tissue macrophages. Antiviral strategies, including newer treatment modalities such as gene therapy of HIV-1-susceptible cell populations, must be capable of engendering durable inhibitory effects to HIV-1 replication in both of these primary cell types in order to be effective. Among the potential genetic targets for intervention in the HIV-1 life cycle, the Rev regulatory system, consisting of Rev and its binding site, the Rev-responsive element (RRE), stands out as particularly attractive. Rev is essential for maintaining the stability of the viral genomic RNA as well as viral mRNAs encoding key structural and regulatory proteins. Moreover, it exhibits favorable threshold kinetics, in that Rev concentrations must rise above a critical level to exert their effect. To disable Rev function, primary T cells or macrophages were transduced with anti-Rev single-chain immunoglobulin (SFv) or RRE decoy genes either singly or in combination by employing adeno-associated virus vectors and then challenged with HIV-1. By directing both a protein and a nucleic acid against the normal interaction between Rev and the RRE, this genetic antiviral strategy effectively inhibited infection by either clinical or laboratory virus isolates. These results provide a framework for novel interventions to reduce virus production in the infected host.

Adeno-associated virus type 2-mediated transduction of murine hematopoietic cells with long-term repopulating ability and sustained expression of a human globin gene in vivo

Adeno-associated virus type 2 (AAV), a nonpathogenic human parvovirus, is gaining attention as a vector for potential use in human gene therapy. We and others have described AAV-mediated beta-globin gene transfer and expression in established human and murine erythroleukemia cell lines in vitro. However, successful AAV-mediated globin gene transduction of hematopoietic stem cells and long-term expression in vivo in progeny cells have not been documented. We report here that infection of murine hematopoietic bone marrow cells ex vivo with a recombinant AAV vector containing the genomic copy of a normal human globin gene followed by transplantation of these cells into lethally irradiated congenic mice resulted in efficient gene transfer into hematopoietic cells with long-term repopulating ability as detected by the presence of the human globin gene sequences in bone marrow and spleen in primary recipient mice for at least 6 months. Long-term expression of the human globin gene was also detected in bone marrow, but not in spleen, in primary recipient mice. Furthermore, in secondary-transplant experiments, we were also able to document the presence as well as expression of the transduced human globin gene in mouse bone marrow for up to 3 months. These results provide further support for potential use of the AAV-based vector system in gene therapy of human hemoglobinopathies in general and sickle-cell anemia and beta-thalassemia in particular.