Effect of scaffold attachment region on transgene expression in retrovirus vector-transduced primary T cells and macrophages

Retroviral vectors for gene therapy

Since their first clinical trial 20 years ago, retroviral (gretroviral and lentiviral) vectors have now been used in more than 350 gene-therapy studies. Retroviral vectors are particularly suited for gene-correction of cells due to long-term and stable expression of the transferred transgene(s), and also because little effort is required for their cloning and production. Several monogenic inherited diseases, mostly immunodeficiencies, can now be successfully treated. The occurrence of insertional mutagenesis in some studies allowed extensive analysis of integration profiles of retroviral vectors, as well as the design of lentiviral vectors with increased safety properties. These new-generation vectors will enable us to continue the successful story of gene therapy, and treat more patients and even more complex diseases.

Strategies to insulate lentiviral vector-expressed transgenes

Lentiviruses are capable of infecting many cells irrespective of their cycling status, stably inserting DNA copies of the viral RNA genomes into host chromosomes. This property has led to the development of lentiviral vectors for high-efficiency gene transfer to a wide variety of cell types, from slowly proliferating hematopoietic stem cells to terminally differentiated neurons. Regardless of their advantage over gammaretroviral vectors, which can only introduce transgenes into target cells that are actively dividing, lentiviral vectors are still susceptible to chromosomal position effects that result in transgene silencing or variegated expression. In this chapter, various genetic regulatory elements are described that can be incorporated within lentiviral vector backbones to minimize the influences of neighboring chromatin on single-copy transgene expression. The modifications include utilization of strong internal enhancer-promoter sequences, addition of scaffold/matrix attachment regions, and flanking the transcriptional unit with chromatin domain insulators. Protocols are provided to evaluate the performance as well as the relative biosafety of lentiviral vectors containing these elements.

Direct comparison of hepatocyte-specific expression cassettes following adenoviral and nonviral hydrodynamic gene transfer

Hepatocytes are a key target for treatment of inborn errors of metabolism, dyslipidemia and coagulation disorders. The development of potent expression cassettes is a critical target to improve the therapeutic index of gene transfer vectors. Here we evaluated 22 hepatocyte-specific expression cassettes containing a human apo A-I transgene following hydrodynamic transfer of plasmids or adenoviral transfer with E1E3E4-deleted vectors in C57BL/6 mice. The DC172 promoter consisting of a 890 bp human alpha(1)-antitrypsin promoter and two copies of the 160 bp alpha(1)-microglobulin enhancer results in superior expression levels compared to constructs containing the 1.5 kb human alpha(1)-antitrypsin promoter, the 790 bp synthetic liver-specific promoter or the DC190 promoter containing a 520 bp human albumin promoter and two copies of the 99 bp prothrombin enhancer. The most potent expression cassette consists of the DC172 promoter upstream of the transgene and two copies of the hepatic control region-1. Minicircles containing this expression cassette induce persistent physiological human apo A-I or human factor IX levels after hydrodynamic transfer. In conclusion, in this comparative study of 22 hepatocyte-specific expression cassettes, the DC172 promoter in combination with two copies of the hepatic control region-1 induces the highest expression levels following hydrodynamic and adenoviral transfer.

Co-expression of cytokine and suicide genes to enhance the activity and safety of tumor-specific cytotoxic T lymphocytes

The antitumor effect of adoptively transferred tumor-specific cytotoxic T lymphocytes (CTLs) is impaired by the limited capacity of these cells to expand within the tumor microenvironment. Administration of interleukin 2 (IL-2) has been used to overcome this limitation, but the systemic toxicity and the expansion of unwanted cells, including regulatory T cells, limit the clinical value of this strategy. To discover whether transgenic expression of lymphokines by the CTLs themselves might overcome these limitations, we evaluated the effects of transgenic expression of IL-2 and IL-15 in our model of Epstein Barr Virus-specific CTLs (EBV-CTLs). We found that transgenic expression of IL-2 or IL-15 increased the expansion of EBV-CTLs both in vitro and in vivo in a severe combined immunodeficiency disease (SCID) mouse model and enhanced antitumor activity. Although the proliferation of these cytokine genes transduced CTLs remained strictly antigen dependent, clinical application of this approach likely requires the inclusion of a suicide gene to deal with the potential development of T-cell mutants with autonomous growth. We found that the incorporation of an inducible caspase-9 suicide gene allowed efficient elimination of transgenic CTLs after exposure to a chemical inducer of dimerization, thereby increasing the safety and feasibility of the approach.

T-cell receptor gene transfer by lentiviral vectors in adoptive cell therapy

Adoptive cell therapy can be envisioned as a promising strategy for tumour immunotherapy. However, existing protocols of adoptive cell therapy still require optimisation as many factors, such as specificity, avidity, level of differentiation and amount of transferred T lymphocytes, can influence their immunocompetence and in vivo functionality. In particular, the need to reduce the in vitro expansion phase and to obtain large numbers of tumour-reactive T cells, as a favourable condition for cancer regression, make TCR gene transfer a potentially ideal tool to overcome the limits of adoptive cell therapy strategies. Here, the authors review the state-of-the-art and recent advances in TCR transfer with particular emphasis on lentiviral vector systems. Initial data from preclinical models and recent clinical trials encourage optimisation of a safe, simplified and stable transfer system. In this regard, HIV-based vectors are emerging as good alternative candidates over the most widely used oncoretroviral vectors due to their peculiar molecular features that fit the ideal conditions for donor T cell in vitro manipulation.

The CD20/alphaCD20 'suicide' system: novel vectors with improved safety and expression profiles and efficient elimination of CD20-transgenic T cells

Adoptive transfer of T lymphocytes is an attractive strategy for many experimental treatment strategies for cancer. Unfortunately, manipulated T cells could be responsible for serious adverse events. Retroviral CD20-transduced T cells may be able to control these unwanted effects. CD20-positive cells are sensitive to rituximab (RTX), a monoclonal antibody specific for CD20. This permits their selective elimination in vivo in case of adverse events. To this end, a system is required that permits efficient and safe transduction of donor T cells and effective elimination of CD20-positive T cells. We constructed different CD20-encoding retroviral vectors and investigated the impact of inclusion of the woodchuck post-transcriptional regulatory element (WPRE) and the chicken hypersensitivity site 4 insulator elements on the levels, homogeneity and stability of CD20 expression. Importantly, inclusion of either WPRE or insulator elements in the retroviral vector resulted in a dramatic improvement in the stability of CD20 expression. The insulator element also led to a much more homogeneous level of CD20 expression. We also show the efficient elimination of the CD20-transgenic T cells via RTX by different effector mechanisms. In conclusion, we have constructed CD20-encoding retroviral vectors with improved efficiency and safety profiles, which can be used as a suicide strategy.

T-cell protection and enrichment through lentiviral CCR5 intrabody gene delivery

CCR5 is the chemokine co-receptor for R5-tropic human immunodeficiency virus type 1 (HIV-1) isolates most often associated with primary infection. We have developed an HIV-1 self-inactivating vector, CAD-R5, containing a CCR5 single-chain antibody (intrabody) gene, which when expressed in T-cell lines and primary CD4+ T cells disrupts CCR5 cell surface expression and provides protection from R5-tropic isolate exposure. Furthermore, CAD-R5 intrabody expression in primary CD4+ T cells supports significant growth and enrichment over time during HIV-1-pulsed dendritic cell-T-cell interactions. These results indicate that CCR5 intrabody-expressing CD4+ T cells are refractory against this highly efficient primary route of infection. CD34+ cells transduced with the CAD-R5 vector gave rise to CD4+ and CD8+ thymocytes in non-obese diabetic (NOD)/ severely combined-immunodeficient (SCID)-human thymus/liver (hu thy/liv) mice, suggesting that CCR5 intrabody expression can be maintained throughout differentiation without obvious cellular effects. CD4+ T cells isolated from NOD/SCID-hu thy/liv mice were resistant to R5-tropic HIV-1 challenge demonstrating the maintenance of protection. Our findings demonstrate delivery of anti-HIV-1 activity through CCR5 intrabodies in primary CD4+ T cells and CD34+ cell-derived T-cell progeny. Thus, gene delivery strategies that provide a selective survival and growth advantage for T effector cells may provide a therapeutic benefit for HIV-1-infected individuals who have failed conventional therapies.

Primary human T lymphocytes engineered with a codon-optimized IL-15 gene resist cytokine withdrawal-induced apoptosis and persist long-term in the absence of exogenous cytokine

IL-15 is a common gamma-chain cytokine that has been shown to be more active than IL-2 in several murine cancer immunotherapy models. Although T lymphocytes do not produce IL-15, murine lymphocytes carrying an IL-15 transgene demonstrated superior antitumor activity in the immunotherapy of B16 melanoma. Thus, we sought to investigate the biological impact of constitutive IL-15 expression by human lymphocytes. In this report we describe the generation of a retroviral vector encoding a codon-optimized IL-15 gene. Alternate codon usage significantly enhanced the translational efficiency of this tightly regulated gene in retroviral vector-transduced cells. Activated human CD4+ and CD8+ human lymphocytes expressed IL-15Ralpha and produced high levels of cytokine upon retroviral transduction with the IL-15 vector. IL-15-transduced lymphocytes remained viable for up to 180 days in the absence of exogenous cytokine. IL-15 vector-transduced T cells showed continued proliferation after cytokine withdrawal and resistance to apoptosis while retaining specific Ag recognition. In the setting of adoptive cell transfer, IL-15-transduced lymphocytes may prolong lymphocyte survival in vivo and could potentially enhance antitumor activity.

Versatility of gene therapy vectors through viruses

Several viruses have been engineered for gene therapy applications, and the specific properties of each viral vector have been exploited to target a variety of inherited and acquired diseases. Preclinical and clinical studies demonstrated that viral vectors are highly versatile tools capable of efficient transfer of foreign genetic information into almost all cell types and tissues. Gene therapy applications depend on vector characteristics, such as host range, cell- or tissue-specific targeting, genome integration, efficiency and duration of transgene expression, packaging capacity, and suitability for scale-up production. This review discusses the advances in the development of viral vectors, with particular emphasis on how knowledge of virus biology has been exploited to design a variety of vectors with improved safety characteristics and efficiency, potentially suitable for a large number of gene therapy applications.

Performance of genomic bordering elements at predefined genomic loci

Eukaryotic DNA is organized into chromatin domains that regulate gene expression and chromosome behavior. Insulators and/or scaffold-matrix attachment regions (S/MARs) mark the boundaries of these chromatin domains where they delimit enhancing and silencing effects from the outside. By recombinase-mediated cassette exchange (RMCE), we were able to compare these two types of bordering elements at a number of predefined genomic loci. Flanking an expression vector with either S/MARs or two copies of the non-S/MAR chicken hypersensitive site 4 insulator demonstrates that while these borders confer related expression characteristics at most loci, their effect on chromatin organization is clearly distinct. Our results suggest that the activity of bordering elements is most pronounced for the abundant class of loci with a low but negligible expression potential in the case of highly expressed sites. By the RMCE procedure, we demonstrate that expression parameters are not due to a potential targeting action of bordering elements, in the sense that a linked transgene is directed into a special class of loci. Instead, we can relate the observed transcriptional augmentation phenomena to their function as genomic insulators.

Scaffold attachment region-containing retrovirus vectors improve long-term proviral expression after transplantation of GFP-modified CD34+ baboon repopulating cells

Sustained high-level proviral expression is important for clinical applications of gene therapy. Genetic elements including the beta-interferon scaffold attachment region (SAR) have been shown to improve transgene expression in hematopoietic cells. We hypothesized that SAR elements might improve expression and allow the preselection of successfully transduced cells. Thus, we transplanted green fluorescent protein (GFP)-selected cells, half of which had been transduced with either SAR or non-SAR-containing retrovirus vectors, into 3 animals. All animals showed delayed engraftment compared with historic controls (28 vs 15.5 days). GFP marking was seen at levels up to 8% but declined over the first 6 weeks. Importantly, fluorescence intensity was 2- to 9-fold increased in progeny of SAR versus non-SAR vector-modified cells in all hematopoietic lineages for the duration of follow-up (6-12 months). In conclusion, the use of SAR-containing vectors improved transgene expression in hematopoietic repopulating cells, which may obviate the need for multicopy integration to achieve high-level expression and reduce the risk for insertional mutagenesis.

Lentivirus-mediated gene transfer and expression in established human tumor antigen-specific cytotoxic T cells and primary unstimulated T cells

In this report, we evaluated the efficiency of stable gene transfer into established CD8(+) human tumor antigen-specific cytotoxic T cell (CTL) lines and peripheral blood lymphocytes (PBL) by oncoretroviral and lentiviral vectors. In the oncoretroviral vector, the green fluorescent protein (GFP) reporter gene was regulated by the murine stem cell virus (MSCV) promoter. In three human immunodeficiency virus type 1 (HIV-1)-based lentiviral vectors, the GFP transgene was regulated by either a chimeric MSCV/HIV-1 promoter, or cellular promoters from human housekeeping genes PGK and EF1 alpha. We found that several lines of proliferating tumor-specific CTL were poorly (=2%) transduced by the oncoretroviral vector that transduced Jurkat T cell line efficiently (=80%). In contrast, three lentiviral vectors transduced 38-63% of these proliferating CTL. More interestingly, all lentiviral vectors packaged without the HIV-1 accessory proteins transduced human bulk PBL and purified CD4(+) and CD8(+) lymphocyte subsets without prior stimulation. Detailed analysis indicated that the lentiviral vectors containing the EF1 alpha or PGK ubiquitous promoter can transduce unstimulated PBL and achieve low-level transgene expression in the absence of any T-cell activation. However, T-cell activation subsequent to the transduction of unstimulated PBL is required for high-level transgene expression. Transduced PBL expressing transgene delivered by the lentiviral vectors still preserved resting and naïve cell phenotypes. Taken together, prior T cell stimulation and HIV-1 accessory proteins are dispensable for lentivirus-mediated gene transfer into resting naïve and memory T lymphocytes. These results will have significant implications for the study of T-cell biology and for the improvement of clinical gene therapies of acquired immune deficiency syndrome (AIDS) and cancer.

Performance- and safety-enhanced lentiviral vectors containing the human interferon-beta scaffold attachment region and the chicken beta-globin insulator

Retroviral vectors are the most efficient means of stable gene delivery to hematopoietic stem cells (HSCs). However, transgene expression from retroviral vectors is frequently subject to the negative influence of chromosomal sequences flanking the site of integration. Toward the development of autonomous transgene expression cassettes, we inserted the human interferon-beta scaffold attachment region (IFN-SAR) and the chicken beta-globin 5' DNase I hypersensitive site 4 (5'HS4) insulator both separately and together into a series of self-inactivating (SIN) lentiviral vector backbones. Transduced cells of the human CD34+ hematopoietic progenitor line KG1a-pooled populations as well as individual clones harboring single integrants--were analyzed for reporter expression during culture periods of up to 4 months. Vectors carrying both the 5'HS4 insulator and the IFN-SAR consistently outperformed control vectors without inserts as well as vectors carrying either element alone. The performance of a set of vectors containing the murine stem cell virus long terminal repeat as an internal promoter was subsequently assessed during in vitro monocytic differentiation of transduced primary human CD34+ cord blood cells. Similar to what was observed in the KG1a hematopoietic progenitor cell model, optimal reporter expression in primary monocytes was obtained with the vector bearing both regulatory elements. These findings indicate that the 5'HS4/IFN-SAR combination is particularly effective at maintaining open chromatin domains permissive for high-level transgene expression at early and late stages of hematopoietic development, and thus could be of utility in HSC-directed retroviral vector-mediated gene transfer applications.

Chromosome-based vectors for gene therapy

Currently used vectors in human gene therapy suffer from a number of limitations with respect to safety and reproducibility. There is increasing agreement that the ideal vector for gene therapy should be completely based on chromosomal elements and behave as an independent functional unit after integration into the genome or when retained as an episome. In this review we will first discuss the chromosomal elements, such as enhancers, locus control regions, boundary elements, insulators and scaffold- or matrix-attachment regions, involved in the hierarchic regulation of mammalian gene expression and replication. These elements have been used to design vectors that behave as artificial domains when integrating into the genome. We then discuss recent progress in the use of mammalian artificial chromosomes and small circular non-viral vectors for their use as expression systems in mammalian cells.

Variables affecting in vivo performance of high-capacity adenovirus vectors

In high-capacity adenovirus (HC-Ad) vectors the size and/or composition of the vector genome influences vector stability during production and the expression profile following gene transfer. Typically, an HC-Ad vector will contain both a gene or an expression cassette and stuffer DNA that is required to balance the final vector genome to a size of between 27 and 36 kb. To gain an improved understanding of factors that may influence gene expression from HC-Ad vectors, we have generated a series of vectors that carry different combinations of human alpha-1 antitrypsin (hAAT) expression constructs and stuffer DNAs. Expression in vitro did not predict in vivo performance: all vectors expressed hAAT at similar levels when tested in cell culture. Hepatic expression was evaluated following in vivo gene transfer in C57BL/6J mice. hAAT levels obtained from genomic DNA were significantly higher than levels achieved with small cDNA expression cassettes. Expression was independent of the orientation and only marginally influenced by the location of the expression cassette within the vector genome. The use of lambda stuffer DNA resulted in low-level but stable expression for at least 3 months when higher doses were applied. A potential matrix attachment region element was identified within the hAAT gene and caused a 10-fold increase in expression when introduced in an HC-Ad vector genome carrying a phosphoglycerate kinase (pgk) hAAT cDNA construct. We also illustrate the influence of the promoter on anti-hAAT antibody formation in C57BL/6J mice: a human cytomegalovirus but not a pgk promoter resulted in an anti-hAAT antibody response. Thus, the overall design of HC-Ad vectors may significantly influence amounts and duration of gene expression at different levels.

An improved vector for high-level, consistent retroviral transgene expression in human thymocytes after competitive reconstitution from transduced peripheral blood stem cells

One problem in hematopoietic stem cell (HSC)-based gene therapy is the low-level, and often transient, transgene expression in progeny cells in vivo. Here we have evaluated retroviral vector designs for improved long-term in vivo transgene expression levels in thymocytes recovered after transplantation of gene-modified HSCs. First, several vector designs were screened in vitro by single-cell analysis of transgene marking and expression to rapidly identify optimal vectors for sensitive tracking of marked cells. Next, using one optimal vector, we show that gene-modified HSCs can competitively reconstitute thymopoiesis in SCID-hu thymus/liver mice, with transgene expression detectable on 0-40% of marked donor thymocytes. Modified vector designs (termed MSCV-SAR and MoMLV-SAR), which enhance transgene expression in primary T cells in vitro, were shown here to improve in vivo transgene expression levels per cell 12- to 14-fold (mean fluorescence intensity was 2175 for MSCV-SAR vs. 174 for LNGFRSN; %NGFR(+) donor(+) cells with high-level expression was 58% for MSCV-SAR vs. 4% for LNGFRSN). Importantly, 61% of grafts had high-level transgene expression on thymocytes with the MSCV-SAR vector versus 0% of grafts for LNGFRSN or MoMLV-SAR. Transgene expression was demonstrated in various stages of thymocyte differentiation and was consistently detected in early thymic progenitors. We suggest that the MSCV-SAR vector described here is particularly advantageous for applications requiring high-level, consistent transgene expression in a diverse repertoire of T cells derived from gene-modified HSC grafts.

Evaluation of retroviral vector design in defined chromosomal loci by Flp-mediated cassette replacement

Successful retroviral vector construction is still empirical. Test systems for vector efficiency are based on statistical comparison of numerous infectants with single proviral integrates, since their expression depends on the chromosomal surroundings. More reliable data would be obtained if different vector constructs were studied in an identical chromosomal context. Here, we demonstrate the use of a new method, in which chromosomal sites are provirally tagged in such a way that they can be targeted with other expression cassettes. The original tagging integrate is replaced in one step by the targeting element. This permits a reliable comparison of different retroviral vector configurations, eliminating the influence of neighboring chromosomal elements. We compared different retroviral vector types for coexpression of two genes: a vector containing an internal promoter and a vector with an internal ribosome entry site (IRES) element. In contrast to bicistronic retroviral vectors, dual-promoter proviruses exhibited rapid inactivation of the long terminal repeat (LTR)-driven gene expression. Targeted exchange of the dual-promoter provirus with a bicistronic retroviral cassette resulted in gain of expression stability. The reverse experiment confirmed this promoter interaction phenomenon since initial expression stability from a single-promoter bicistronic provirus was lost by targeted exchange with a dual-promoter cassette. In addition, targeting exchange of the dual-promoter provirus, replacing the LTR with an artificial (Tet) promoter restored expression stability. These observations, valid for various integration sites, prove the strong interaction between the LTR and the internal promoter. Our results have implications for retroviral vector design and suggest that retroviral coexpression of two genes is more predictable in the bicistronic configuration.

Systematic analysis of clinically applicable conditions leading to a high efficiency of transduction and transgene expression in human T cells

BACKGROUND

The transduction of human peripheral blood T cells with retroviral vectors constitutes an attractive approach for the correction of a number of genetic diseases. In this study we have conducted a systematic analysis of the relevance of a large number of parameters currently considered to affect the transduction of, and transgene expression in, human T cells.

METHODS

Retroviral vectors encoding the human nerve growth factor receptor (NGFR) were used for transducing human T cells from normal volunteers. The proportion of T cells that expressed the marker transgene was determined by flow cytometry using anti-NGFR antibodies.

RESULTS

Spinoculation and static fibronectin (FN)-assisted infections improved to a similar extent the transduction efficiency of PHA/IL-2 stimulated T cells, when compared with samples subjected to standard static infections. When immobilized anti-CD3 (anti-CD3i) or anti-CD3i/28i-stimulated T cells were considered, static infections in FN-coated plates were reproducibly more efficient than spinoculation infections performed on FN-uncoated plates. Under optimized manipulation conditions (three infection cycles of anti-CD3i/28i-stimulated T cells in FN-coated plates) the total number of NGFR+ T cells harvested after 7 days of incubation represented, on average, twice the total number of T cells seeded at Day 0, and up to 95% of the human T cells efficiently expressed the marker transgene. Similar results were obtained regardless of whether samples were manipulated in medium supplemented with fetal bovine serum or with heat-inactivated autologous serum.

CONCLUSIONS

Our study offers new experimental conditions for the transduction of human T cells, with obvious implications for the development of gene therapy protocols.

Hybrid vector designs to control the delivery, fate and expression of transgenes

One of the greatest challenges to gene therapy is the targetting of gene delivery selectively to the sites of disease and regulation of transgene expression without adverse effects. Ultimately, the successful realization of these goals is dependent upon improvements in vector design. Over the years, viral vector design has progressed from various types of replication-defective viral mutants to replication-conditioned viruses and, more recently, to 'gutted' and hybrid vectors, which have, respectively, eliminated expression of non-relevant or toxic viral genes and incorporated desired elements of different viruses so as to increase the efficacy of gene delivery in vivo. This review will focus on the different viral and cellular elements which have been incorporated into virus vectors to: improve transduction efficiencies; alter the entry specificity of virions; control the fate of transgenes in the host cells; and regulate transgene expression.

Primary T lymphocytes as targets for gene therapy

Peripheral blood T lymphocytes have been considered an attractive target for gene therapy applications. They can be easily harvested and readily expanded ex vivo. The transduction efficiency of primary human lymphocytes with standard retroviral vectors approaches 50% or more using optimized methods of gene transfer. Other methods of gene transfer, including adenoviral, adeno-associated viral, and lentiviral vectors, or nonviral techniques, have also been used for gene transfer into primary lymphocytes. Despite encouraging results in vitro, human clinical trials using retroviral vectors to transduce primary lymphocytes have been hindered by low expression levels of transgenes and immune responses against transgene products. Strategies to overcome these problems need to be developed.

Site-specific gene targeting for gene expression in eukaryotes

Major advances in the use of site-specific recombinases to facilitate sustained gene expression via chromosomal targeting have been made during the past year. New tools for genomic manipulations using this technology include the discovery of epitopes in recombinases that confer nuclear localization, crystal structures that show the precise topology of recombinase-DNA-substrate synaptic complexes, manipulations of the DNA recognition sequences that select for integration over excision of DNA, and manipulations that make changes in gene expression inducible by drug administration. In addition, endogenous eukaryotic and mammalian DNA sequences have been discovered that can support site-specific recombinase-mediated manipulations.

Addition of the human interferon beta scaffold attachment region to retroviral vector backbones increases the level of in vivo transgene expression among progeny of engrafted human hematopoietic stem cells

Absence of durable high-level expression of transgenes from Moloney murine leukemia (Mo-MuLV) retroviral vectors has been a hurdle in bringing effective gene therapy to the clinic. In this study we have analyzed transgene expression among the progeny of mobilized hematopoietic stem cells (HSCs), comparing Mo-MuLV and mouse stem cell virus (MSCV) vectors, with or without addition of a scaffold attachment region (SAR) from the human interferon beta gene. Retroviral (RV) vector supernatant quality was assessed by comparing NGFR transgene expression by HEL cells, and transgene delivery and expression by CD34(+) cells 72 hr after transduction, using real-time PCR and FACS analysis. This is the first description of the effect of SAR within both Mo-MuLV and MSCV vector backbones on long-term RV transgene expression among in vivo HSC progeny in HSC repopulation assays (SCID-hu bone and NOD/SCID). After transduction of mobilized CD34(+) cells with MSCV-SAR vector, transgene expression was observed among a mean of 10% of donor HSC progeny in the SCID-hu bone (range, 0.6-43%). The predominant effect of SAR was to increase the mean fluorescence intensity (MFI) of transgene expression among HSC progeny in both in vivo bone repopulation models (three- to fourfold), and after long-term stromal cultures (twofold).

Expression of herpes simplex virus ICP47 and human cytomegalovirus US11 prevents recognition of transgene products by CD8(+) cytotoxic T lymphocytes

The in vivo persistence of gene-modified cells may be limited by the development of a host immune response to vector-encoded proteins. Herpesviruses evade cytotoxic T-lymphocyte (CTL) recognition by expressing genes which interfere selectively with presentation of viral antigens by class I major histocompatibility complex (MHC) molecules. Here, we studied the use of retroviral vectors encoding herpes simplex virus ICP47, human cytomegalovirus (HCMV) US3, or HCMV US11 to decrease presentation of viral proteins and transgene products to CD8(+) CTL. Human fibroblasts and T cells transduced to express the ICP47, US3, or US11 genes alone exhibited a decrease in cell surface class I MHC expression. The combination of ICP47 and US11 rendered fibroblasts negative for surface class I MHC and allowed a class I MHC-low population of T cells to be sorted by flow cytometry. Fibroblasts and T cells expressing both ICP47 and US11 were protected from CTL-mediated lysis and failed to stimulate specific memory T-cell responses to transgene products in vitro. Our findings suggest that expression of immunoregulatory viral gene products could be a potential strategy to prolong transgene expression in vivo.

Human beta interferon scaffold attachment region inhibits de novo methylation and confers long-term, copy number-dependent expression to a retroviral vector

Moloney murine leukemia virus-based retroviral vector expression is gradually lost during prolonged in vitro culture of CEMSS T cells. However, when the human beta interferon scaffold attachment region (IFN-SAR) was inserted into the vector immediately upstream of the 3' long terminal repeat (LTR), expression was maintained for the length of the study (4 months). Clonal analysis of the retrovirus vector-infected CEMSS cells showed that SAR-containing retroviral vector expression levels were positively correlated with the proviral copy numbers (P < 0.0001), while there was no correlation between the proviral copy numbers and expression levels in control vector-infected clones. Thirty-three percent of the CEMSS cell clones infected with the control vector showed evidence of partial or complete methylation in the 5' LTR region. In sharp contrast, we detected no methylation in the clones infected with the SAR-containing vector. To demonstrate a direct inhibitory effect of methylation on retroviral vector expression, we have transfected 293 cells with in vitro-methylated proviral DNA. In transiently transfected cells, expression of methylated LTR was reduced but not completely inhibited, irrespective of the presence of the IFN-SAR sequence. In stably transfected cells, however, methylation completely abolished expression of the control vector but not of the SAR-containing vector. Furthermore, the expression of the SAR-containing vector was stable over time, indicating the ability of the SAR sequence to alleviate methylation-mediated transcriptional repression of a vector. This study extends our understanding of the mechanisms of retroviral vector inactivation by methylation and provides insight into a functional role for the SAR elements.