Retroviral vectors containing chimeric promoter/enhancer elements exhibit cell-type-specific gene expression

Targeted gene delivery systems for T-cell engineering

T lymphocytes are indispensable for the host systems of defense against pathogens, tumors, and environmental threats. The therapeutic potential of harnessing the cytotoxic properties of T lymphocytes for antigen-specific cell elimination is both evident and efficacious. Genetically engineered T-cells, such as those employed in CAR-T and TCR-T cell therapies, have demonstrated significant clinical benefits in treating cancer and autoimmune disorders. However, the current landscape of T-cell genetic engineering is dominated by strategies that necessitate in vitro T-cell isolation and modification, which introduce complexity and prolong the development timeline of T-cell based immunotherapies. This review explores the complexities of gene delivery systems designed for T cells, covering both viral and nonviral vectors. Viral vectors are known for their high transduction efficiency, yet they face significant limitations, such as potential immunogenicity and the complexities involved in large-scale production. Nonviral vectors, conversely, offer a safer profile and the potential for scalable manufacturing, yet they often struggle with lower transduction efficiency. The pursuit of gene delivery systems that can achieve targeted gene transfer to T cell without the need for isolation represents a significant advancement in the field. This review assesses the design principles and current research progress of such systems, highlighting the potential for in vivo gene modification therapies that could revolutionize T-cell based treatments. By providing a comprehensive analysis of these systems, we aim to contribute valuable insights into the future development of T-cell immunotherapy.

Pantropic retroviruses as a transduction tool for sea urchin embryos

Sea urchins are an important model for experiments at the intersection of development and systems biology, and technical innovations that enhance the utility of this model are of great value. This study explores pantropic retroviruses as a transduction tool for sea urchin embryos, and demonstrates that pantropic retroviruses infect sea urchin embryos with high efficiency and genomically integrate at a copy number of one per cell. We successfully used a self-inactivation strategy to both insert a sea urchin-specific enhancer and disrupt the endogenous viral enhancer. The resulting self-inactivating viruses drive global and persistent gene expression, consistent with genomic integration during the first cell cycle. Together, these data provide substantial proof of principle for transduction technology in sea urchin embryos.

Short nucleotide polymorphic insertions in the MCL-1 promoter affect gene expression

We have recently reported novel short nucleotide (six and eighteen) polymorphic insertions, in the MCL-1 promoter and their association with higher mRNA and protein levels. The aim of the present study was to test the hypothesis that these insertions directly affect MCL-1 gene expression. Haematopoietic and epithelial human cell lines were transfected with +0, +6, or +18 MCL-1 promoter fragments positioned upstream of the Firefly luciferase reporter gene. The cells were stimulated with phorbol 12-myristate 13-acetate (PMA) and granulocyte macrophage colony-stimulating factor (GM-CSF). Compared to +0, both polymorphic insertions (+6 and +18) were associated with increased promoter activity. Although chromatin immunoprecipitation assay showed that there are Sp1/Sp3 binding sites in the MCL-1 promoter, electrophoretic mobility shift assay showed that it is unlikely that these sites are in the region harboring these insertions. These results provide further evidence for the biological effect of MCL-1 promoter polymorphisms on gene expression.

Tumor model-specific proviral insertional mutagenesis of the Fos/Jdp2/Batf locus

Retroviral activation of the AP-1/ATF super family member Jdp2 was recently reported to be a common event in M-MLV-induced T cell lymphoma in p27-null C57x129 mice as compared to wild type-inoculated mice but has not been found important in other models. On the basis of retroviral tag retrieval from 1190 individual Akv- and SL3-3-induced lymphomas, we here report that insertional mutagenesis into the 250-kb Fos/Jdp2/Batf locus is associated with SL3-3 MLV-induced T but not Akv-induced B cell lymphomas of NMRI and SWR mice. Integration pattern and clonality analyses suggest that Jdp2 participates in SL3-3-induced tumorigenesis distinctly as compared to the M-MLV setting. Northern blot analysis showed Jdp2 to be alternatively spliced in various normal tissues as well as MLV-induced lymphomas. Interestingly, in some tumors, proviral insertion seems to activate different mRNA sub-species. Whereas elevated mRNA levels of the Fos gene could not be correlated with provirus presence, in one case, Northern blot analysis as well as quantitative real-time PCR indicated proviral activation of the AP-1 super family member Batf, a gene not previously reported to be a target of insertional mutagenesis. A novel integration cluster between Jdp2 and Batf apparently did not influence the expression level of either gene, underscoring the importance of addressing expression effects to identify target genes of insertion. Altogether, such distinct insertion patterns point to different mechanism of activation of specific proto-oncogenes and are consequently of importance for the understanding of proviral activation mechanisms as well as the specific role of individual oncogenes in tumor development.

Protection of hematopoietic stem cells from pemetrexed toxicity by retroviral gene transfer with a mutant dihydrofolate reductase-mutant thymidylate synthase fusion gene

Myelosuppression is one of the major side effects of most anticancer drugs. To confer myeloprotection, our laboratory generated drug-resistant mutants of select target human enzymes for gene transfer to the bone marrow. Mutants of two of these enzymes, dihydrofolate reductase (DHFR F/S) and thymidylate synthase (TS G52S), were previously shown to confer resistance to methotrexate and 5-FU, respectively, and recently a fusion cDNA of both mutant enzymes (DHFR F/S-TS G52S) was shown to confer dual resistance to both antimetabolites. In this study, we examined the sensitivity of the DHFR F/S-TS G52S fusion protein to the multitargeted antifolate, pemetrexed (LY231514, Alimta), which targets both DHFR and TS and is currently in phase III trials for the treatment of solid tumors and in combination with cisplatin has been shown to be an advance in the treatment of mesothelioma. The K(i) for the DHFR F/S portion of the purified fusion protein to pemetrexed was increased by greater than 9000-fold when compared to wtDHFR (8000 versus 0.86 nM), while the K(i) for the TS G52S portion of the fusion protein to pemetrexed was similar to that of wtTS (2.8 versus 3.1 nM). When the fusion gene was retrovirally transduced into NIH 3T3 fibroblasts, the IC(50) to pemetrexed was three- to four-fold higher than cells transduced with DHFR F/S or TS G52S alone (163 versus 53 and 45 nM, respectively). Similarly, expression of the DHFR F/S-TS G52S fusion gene in retrovirally transduced mouse marrow cells resulted in an increased survival of CFU-GM colonies when compared to cells transduced with either of the mutants alone. Co-expression of mutant DHFR and TS enzymes has additive effects in conferring resistance to pemetrexed-induced toxicity. This construct may be useful for conferring myeloprotection to patients receiving this drug.

Retroviral vectors for human gene delivery

The potential for gene therapy to cure a wide range of diseases has lead to high expectations and a great increase in research efforts in this area. At present, viral vectors are the most efficient means of delivering a corrective gene into human cells. While a number of different viral vectors are under development, retroviral vectors are currently the most common type used in clinical trials today. However, the production of retroviral vectors for gene therapy applications faces a number of challenges. Of primary concern is the low titre of vector stocks produced by packaging cells in culture and the inherent instability of retroviral vector activity. The problems facing large-scale retroviral vector production are outlined in this review and the research efforts by a number of groups who have attempted to optimise production methods are presented.

Retroviral transduction of a mutant dihydrofolate reductase-thymidylate synthase fusion gene into murine marrow cells confers resistance to both methotrexate and 5-fluorouracil

Gene transfer-based myeloprotection strategies against chemotherapy require the development of effective drug resistance genes or gene combinations. Our laboratory has previously generated drug-resistant mutants of dihydrofolate reductase (DHFR F/S) and thymidylate synthase (TS G52S) for myeloprotection against methotrexate (MTX) and 5-fluorouracil (5-FU), respectively. For the purpose of conferring dual myeloprotection against both MTX and 5-FU, we have generated two retroviral constructs encoding both DHFR F/S and TS G52S as a fusion protein (DHFR F/S-TS G52S) or as individual proteins from a bicistronic gene. The DHFR F/S-TS G52S fusion protein is functional and exhibits kinetic properties similar to that of the individual mutant enzymes. NIH 3T3 cells and mouse bone marrow progenitors retrovirally transduced with the fusion DHFR F/S-TS G52S cDNA provided similar levels of resistance to MTX and 5-FU as cells expressing the individual mutant enzymes and higher levels of resistance to MTX than cells expressing DHFR F/S from the 3' end of a bicistronic gene. As MTX and 5-FU are used in combination therapy for diseases such as breast and colon cancer, this fusion gene may be useful in the clinic to reduce myelosuppressive toxicity associated with this drug combination.

Enhanced inhibition of human immunodeficiency virus type 1 replication by novel lentiviral vectors expressing human immunodeficiency virus type 1 envelope antisense RNA

We have developed optimized versions of a conditionally replicating human immunodeficiency virus type 1 (HIV-1)-based lentiviral vector for gene therapy of HIV-1 infection. These vectors target HIV-1 RNAs containing sequences of the envelope gene by expressing a 1-kb fragment of the HIV-1 Tat/Rev intron in the antisense orientation. Expression of the envelope antisense gene (envAS) was evaluated under the control of different internal promoters such as the human phosphoglycerate kinase (PGK) promoter, the human EF1-alpha promoter, and the U3 region of the SL3 murine leukemia virus. The U3-SL3 promoter transactivates transcription from the vector HIV-1 LTR and drives higher expression levels of envAS-containing RNAs than other promoters in T-cell lines. The effect of other vector structural features was also evaluated. We found that the central polypurine tract and central termination sequence (cPPT) produce a small increase in vector infectivity of 2-fold to 3-fold and results in a 10-fold higher inhibition of wild-type viral replication in challenge experiments. The woodchuck hepatitis posttranscriptional regulatory element (WPRE) does not increase the cytoplasmic levels of envAS mRNA in T-cell lines. We observed that SupT1 and primary CD4(+) T cells transduced with these vectors showed high inhibition of HIV-1 replication, suppression of syncitium formation, and increased cell viability when infected with several HIV-1 laboratory strains. Our results suggest that higher vector copy number and increased levels of envAS RNA expression contribute to block replication of divergent strains of HIV-1.

Retroviral vectors

Retroviral vectors are widely used for preclinical and clinical applications. Unlike many of the other types of vectors currently being developed for gene therapy, retroviral vectors are able to genetically modify cells stably without perturbing cell growth. Retroviral vectors based on murine retroviruses are well suited for ex vivo applications where the cells are rapidly dividing. In particular, retroviral viral vectors have been used for a variety of ex vivo gene therapy approaches for treating genetic diseases such as Gaucher and severe combined immunodeficiency (SCID) and for acquired diseases such as cancer and arthritis. However, recent advances in the production of retroviral vectors have allowed for their use in vivo such as for the treatment of cancer and human immunodeficiency virus. The ability to target retroviral vectors to specific cell types will also increase the utility of high titer retroviral vectors for in vivo applications. Clearly retroviral vectors have been extremely useful for both preclinical and clinical gene therapy studies, and it is likely that they will continue to be utilized for ex vivo and in vivo strategies in the future.

Modulation of Moloney leukemia virus long terminal repeat transcriptional activity by the murine CD4 silencer in retroviral vectors

We investigated whether CD4 gene regulatory sequences might be useful for developing transcriptionally targeted Moloney murine leukemia virus (Mo-MLV)-based retroviral vectors for gene expression specifically in CD4(+) cells. We could modulate Mo-MLV long terminal repeat (LTR) activity by inserting a 438-bp-long fragment containing the murine CD4 silencer in the LTR of the vector; both beta-galactosidase and green fluorescent protein reporter gene activities were strongly down-regulated in both murine and human CD8(+) cells, but not in CD4(+) lymphoid cell lines and freshly isolated lymphocytes transduced with this vector, compared with the findings using a control vector carrying wild-type LTRs. Titration experiments on NIH-3T3 cells revealed that inclusion of the CD4 silencer in the LTRs did not reduce the titer of the vectors. These findings indicate that a cellular silencer can be successfully included in retroviral vectors, where it maintains its transcription-regulatory function, thus suggesting a novel approach to transcriptional targeting.

Generation of a high titre retroviral vector for endothelial cell-specific gene expression in vivo

Tumour growth is dependent upon a blood supply and is associated with the switch to the angiogenic phenotype. We are developing strategies for targeting gene expression to endothelial cells in the tumour vasculature. Recombinant retroviruses have been generated that incorporate regulatory sequences of the prepro-endothelin-1 (ppET1) promoter. Following reverse transcription and integration these modifications are duplicated in the proviral 5' LTR for transcription of the internal beta-galactosidase reporter gene. The titres and endothelial specificity of retroviral vectors harbouring different modifications have been analysed. In the optimal strategy, replacing the MLV enhancer with ppET1 promoter sequences containing the GATA and AP1 elements whilst maintaining sequences from the viral promoter resulted in endothelial cell-specific expression of the reporter gene, and viral titres comparable to those of the unmodified vector. A panel of endothelial and non-endothelial cells infected with the modified virus from a high titre producer clone showed a pattern of expression consistent with the activity of the endogenous ppET1 promoter. The modified LTR retained specificity in vivo, in subcutaneous tumours arising from the co-injection of tumour cells and irradiated virus producer cells. This simple model achieves high efficiency of transduction and can be used routinely for the screening of targeted retroviral vectors. Gene Therapy (2000) 7, 368-376.

Endothelial cell-specific transcriptional targeting from a hybrid long terminal repeat retrovirus vector containing human prepro-endothelin-1 promoter sequences

For many applications, specificity of gene expression by recombinant retroviral vectors is necessary. We wished to obtain transcriptional targeting in endothelial cells as part of an antivasculature approach to cancer treatment and have achieved specificity by using the promoter for human prepro-endothelin-1. In particular, we have inserted this heterologous promoter within the 3' long terminal repeat (LTR), replacing all viral upstream transcriptional regulatory sequences, to generate a hybrid LTR with precise fusion at the TATA box for initiation of transcription at the viral start site. Reverse transcription and integration resulted in duplication of this hybrid promoter in the 5' LTR of the provirus for transcription of the internal transgene. An important feature of our vectors is the absence of a selectable marker gene or additional promoters to avoid potential complications of silencing or interference and because selection will be inappropriate for clinical application. This vector design showed endothelial cell specificity of beta-galactosidase expression when tested on a panel of human cell lines and primary breast microvascular endothelial cells, matching the specificity of expression of the endogenous promoter. Such simplified vectors exhibiting transcriptional specificity are likely to be useful for the development of a gene therapy approach to targeting tumor vasculature.

Ex vivo fibroblast transduction in rabbits results in long-term (>600 days) factor IX expression in a small percentage of animals

Delivery of human factor IX to the circulation was analyzed in rabbits by ex vivo fibroblast transduction followed by subcutaneous implantation. Kinetic studies of human factor IX in rabbits demonstrated a half-life of approximately 16 hr and a volume distribution of 22%, where intraperitoneal and subcutaneous bioavailability was three- to sevenfold lower than by intravenous administration. Ex vivo retroviral transduction of autologous fibroblasts was performed on 15 animals. After subcutaneous injection of fibroblast-collagen mixtures, the expression of human factor IX in rabbit plasma was followed by ELISA. Of 15 rabbits injected, expression of human factor IX was detected in 2 animals, and expression was long term (>600 days). One animal had stable levels of human factor IX, at 20 ng/ml, while the second animal had lower and gradually decreasing levels of human factor IX. There were no gross differences in pathology at the injection sites, when comparing animals with human factor IX in plasma and those without. Immunological studies demonstrated antibody formation in response to injection mixture components (including human factor IX), but again there was no correlation with immune response and long-term factor IX production in animals. Tissues at the implantation sites were positive for factor IX DNA by PCR analysis, regardless of whether there was detectable plasma factor IX or not. Small numbers of PCR-positive cells were detected in the internal organs of the long term-expressing rabbits while similar tissues were negative in nonexpressing animals.

Poor expression of MDR1 transgene in HeLa cells by bicistronic Moloney murine leukemia virus-based vector

Cotransfer of a therapeutic gene together with the human MDR1 gene provides an opportunity to increase the number of transduced marrow cells, expressing the therapeutic gene, by in vivo selection for MDR1. We have used an Lg-MDR1-IRES-neo (LgMIN) retroviral vector, containing MDR1 and neo genes, separated by the EMCV IRES. Human HeLa or canine CTAC cells, transduced with GALV env pseudotyped LgMIN at an MOI of less than 0.01 to ensure 1 proviral copy/genome, were selected with either G418 for neo expression or colchicine for MDR1 expression. The titer determined on HeLa cells with G418 selection was eight-fold higher than that with colchicine selection. In contrast, the same viral supernatant exhibited only a 1.4-fold difference between neo- and MDR1-based viral titer values for CTAC cells. The transduced HeLa cells, with one intact proviral copy per genome, exhibited a 55-fold higher resistance to G418 but only a 4-fold higher resistance to colchicine and a 2-fold higher resistance to Taxol compared with nontransduced cells. About 23% of the transduced cell population did not express vector-derived P-glycoprotein (P-gp) as detected by anti-human P-gp MAb MRK-16. This could explain the difference in viral titers obtained on CTAC cells but not that obtained on HeLa cells. The vector-mediated increase in expression of P-gp was about 20-fold higher in CTAC cells as compared with HeLa cells. These results indicated suppression of expression of vector-derived MDR1 in HeLa cells, in contrast with CTAC cells. To investigate further the possible reasons for this difference, genomic DNA was isolated from the G418-resistant individual colonies of infected cells and analyzed by PCR for full-length proviral MDR1. For transduced CTAC and HeLa cells, selected at a G418 concentration of 1 mg/ml, PCR detected aberrant forms of MDR1 in 17 to 25% of colonies tested. The aberrant forms consisted of MDR1 genes with 2- and 0.7-kb deletions. DNA sequencing across the 2-kb and the 0.7-kb deletion junction suggests cryptic splicing in the producer cell line as the origin of these deletions. The 2-kb deletion corresponds to MDR1 mRNA cryptic splicing via donor (codon 113) and acceptor (codon 773). The 0.7-kb deletion corresponds to splicing via the same donor and a different acceptor (codon 344). When transduced HeLa cells were selected at a higher concentration of G418 (3 mg/ml), the aberrant forms were detected at an increased frequency of about 50% of colonies tested. These results indicate that vector-derived MDR1 is a poor selective marker in HeLa cells but not in CTAC cells and that deletions, which inactivated the MDR1 gene in a bicistronic Mo-MuLV vector, may provide an advantage for expression of the second transgene in HeLa cells.

T cell activation modulates retrovirus-mediated gene expression

Important considerations for T lymphocyte-based gene therapy include efficient gene delivery and expression in primary, human T cells. In this study, retrovirus-mediated gene transfer and the fate of proviral gene expression were evaluated in human T cells activated using (1) immobilized anti-CD3 monoclonal antibody (MAb) plus interleukin 2, or (2) cis costimulation using beads carrying coimmobilized anti-CD3 and anti-CD28 MAbs. By cross-linking the CD3 and CD28 receptors, these MAbs mimic in vivo signaling events, leading to cytokine production and proliferation. A modified human interleukin 1beta (IL-1beta) cDNA inserted into the MFG retroviral vector served as an indicator gene. Retroviral transduction frequencies were similar for T lymphocytes activated by the respective methods. However, early after MAb stimulation and virus exposure, proviral gene expression was greater at the RNA and protein levels in optimized anti-CD3/anti-CD28 bead-activated T cells, corresponding with augmented endogenous cytokine responses and mitogenesis. Proviral gene expression was not regulated by extrinsic cell factors present in activated T cell supernatants. Regardless of the MAb stimulation method, proviral IL-1beta expression declined in later T cell cultures concomitant with a decrease in cellular cytokines. Restimulation by either method reinduced both T cell activity and vector expression. Our finding that proviral gene regulation is downmodulated in the absence of T cell signaling events has implications for clinical strategies using retrovirus-modified T cells.

Development of improved adenosine deaminase retroviral vectors

A series of adenosine deaminase (ADA) retroviral vectors were designed and constructed with the goal of improved performance over the PA317/LASN vector currently used in clinical trials. First, the bacterial selectable-marker neomycin phosphotransferase (neo) gene was removed to create a "simplified" vector. Second, the Moloney murine leukemia virus long terminal repeat (LTR) promoter used for ADA expression was replaced with either the myeloproliferative sarcoma virus (MPSV) or SL3-3 LTR. Supernatant from each ADA vector was used to transduce ADA-deficient (ADA-) B- and T-cell lines as well as primary peripheral blood mononuclear cells (PBMC) from an ADA- severe combined immunodeficiency patient. Total ADA enzyme activity and ADA activity per integrant in the transduced cells demonstrated that the MPSV LTR splicing vector design provided the highest level of ADA expression per cell. This ADA(MPSV) vector was then tested in packaging cell lines containing either the gibbon ape leukemia virus envelope (PG13 cells), the murine amphotropic envelope (FLYA13 cells), or the feline endogenous virus RD114 envelope (FLYRD18 cells). The results indicate that FLYRD18/ADA(MPSV), a simplified ADA retroviral vector with the MPSV LTR, provides a 17-fold-higher level of ADA expression in human lymphohematopoietic cells than the PA317/LASN vector currently in use.

Exchange of viral promoter/enhancer elements with heterologous regulatory sequences generates targeted hybrid long terminal repeat vectors for gene therapy of melanoma

To generate transcriptionally targeted vectors, tissue-specific elements of the human tyrosinase promoter were exchanged with corresponding viral elements in the Moloney murine leukemia virus long terminal repeat (LTR). From these experiments, a vesicular stomatitis virus type G pseudotyped, hybrid LTR vector that contained three tyrosinase enhancer elements and gave high-level, tightly tissue-specific expression at high titers (3 x 10(7) CFU/ml) was constructed.

An ex vivo keratinocyte model for gene therapy of hemophilia B

We are investigating whether skin-targeted gene therapy may be used to treat hemophilia B by transplanting keratinocytes transduced by factor IX-expressing retroviral vectors. No pre-clinical animal model for keratinocyte-mediated gene therapy has shown long-term efficacy in vivo. It remains unclear whether this short-term expression is due to promoter shut-off or a reduced survival of grafted genetically modified cells. The purpose of this study was to determine the fate of primary human keratinocytes superficially grafted to nude mice in a silicone transplantation chamber. In addition, vectors containing keratinocyte-specific enhancers from the human papilloma virus-16 (HPV-16) and human keratin 5 and 14 genes were used upstream of the cytomegaloviral (CMV) immediate-early promoter/enhancer to control factor IX cDNA expression to avoid promoter shut-off. Factor IX was secreted by cultured keratinocytes after transduction by each of these chimeric promoter/enhancer vectors, although the levels varied according to the particular construct used. Keratinocytes transduced by the vector containing the HPV-16 enhancer were grafted into nude mice, and human factor IX was detected in plasma at 0.02-9 ng per ml for 4-5 wk for the duration of graft survival. The HPV-16 enhancer may be a useful addition to expression vectors for keratinocyte gene therapy. The transplantation chamber can be adapted to grafting retrovirally transduced keratinocytes for gene transfer studies.

Comparison of retroviral and adeno-associated viral vectors designed to express human clotting factor IX

Several different designs for retroviral and adeno-associated virus (AAV) vectors were developed to express human clotting factor IX. Seven separate retroviral vectors were constructed, including chimeric long terminal repeat (LTR)-based designs, vectors containing splice donor/acceptor sites with internal ribosome entry sites (IRES), and vectors with an internal cytomegalovirus (CMV)- or hepatitis B virus (HBV)-derived promoter. Five AAV vectors were produced using the same cassette design where a viral promoter was used to transcribe a bicistronic mRNA containing factor IX and an IRES/neo gene. In the human hepatocyte cell line HepG2, the constructs were tested for factor IX production by ELISA, Northern blot, and Western blot, and for biological activity by normalization of the prolonged activated partial thromboplastin time (APTT) of factor IX-deficient plasma. All of the constructs produced biologically active factor IX in the range of 0.23-152 ng/24 hr per 10(6) cells (the HBV-promoted factor IX AAV vector was the least effective, and the CMV-promoted retroviral vector was the most active). Primary fibroblasts of both human and rabbit origin were also evaluated for factor IX production following transduction with viral vectors. Fibroblasts produced substantially more factor IX than the HepG2 cell line, with the best AAV vector synthesizing > 250 ng/24 hr per 10(6) cells and the best retroviral vector making > 900 ng/24 hr per 10(6) cells. Generally, we observed lower transduction efficiency and poorer expression with the AAV vectors versus retroviral vectors in these cell types.

Killing Epstein-Barr virus-positive B lymphocytes by gene therapy: comparing the efficacy of cytosine deaminase and herpes simplex virus thymidine kinase

Epstein-Barr virus (EBV)-positive lymphomas are frequent among immunosuppressed patients. We have examined the feasibility of killing EBV-immortalized B lymphocytes by gene transfer involving the use of "suicide" genes whose expression in target cells renders them susceptible to killing by a prodrug. We examined two gene/prodrug pairs: the Escherichia coli cytosine deaminase (CD) gene with the prodrug 5-fluorocytosine (5-FC), and the herpes simplex virus thymidine kinase (HSV-TK) gene with the prodrug ganciclovir. Retroviral vectors and drug selection were used to obtain CD or HSV-TK expression in cells. Both the CD/5-FC and the HSV-TK/ganciclovir combinations yielded substantial killing of EBV-immortalized B lymphocytes in vitro, although the CD/5-FC regimen had a significantly greater therapeutic margin than the HSV-TK/ganciclovir combination. The CD/5-FC pair, but not the HSV-TK/ganciclovir pair, was shown to have a "bystander killing effect" in vitro. When only 30% of the cells expressed the suicide gene, scid mouse tumors regressed in both the CD/5-FC regimen and the HSV-TK/ganciclovir regimen, documenting an in vivo bystander effect with both regimens. However, a greater percentage of tumors completely regressed with the CD/5-FC regimen. Overall, the sum of our data indicates that the CD/5-FC combination is the more promising regimen for treatment of EBV-associated lymphomas in vivo.

Analysis of trans-dominant mutants of the HIV type 1 Rev protein for their ability to inhibit Rev function, HIV type 1 replication, and their use as anti-HIV gene therapeutics

The HIV-1 rev gene product facilitates the transport of singly spliced and unspliced HIV-1 transcripts and is necessary for productive HIV-1 infection. On the basis of the previously described trans-dominant Rev mutant M10, four point mutants and one frameshift mutant of the Rev protein were constructed. The mutants were inserted into retroviral expression vectors and analyzed for their ability to inhibit Rev-mediated gene expression. Transient transfection systems were used to screen these new mutants, and each was shown to inhibit expression of a Rev-dependent CAT reporter plasmid. Inhibition of HIV-1 envelope gene expression was tested in the HeLa-T4 cell line and was also shown to be inhibited by the trans-dominant Rev mutants. Retroviral vector producer cell lines were constructed and used to transduce Rev trans-dominant genes into the human T-cell line SupT1. The engineered SupT1 cell lines were then challenged with HIV-1 IIIB and HIV-1 expression was monitored by Northern blot analysis and in situ hybridization. SupT1 cells expressing either a Rev point mutant or the frameshift mutant showed greatly reduced HIV-1 mRNA accumulation and the Rev-dependent singly spliced and unspliced HIV-1 mRNAs were reduced. The kinetics of viral replication following challenge of Rev trans-dominant-engineered SupT1 cells with both HIV-1 IIIB and MN strains was significantly reduced and cells were protected from viral lysis. Viruses that emerge late in infection from Rev trans-dominant-engineered cultures are not resistant to Rev-mediated inhibition. Last, trans-dominant Rev-mediated protection of human CD4+ lymphocytes from challenge with primary HIV-1 patient isolates confirms the potential utility of this system as an anti-HIV-1 gene therapy approach.

A retroviral vector containing a muscle-specific enhancer drives gene expression only in differentiated muscle fibers

Genetically modified myogenic cells have a number of potentially relevant applications for gene therapy of genetic defects. Retroviral vectors proved to be a safe and efficient tool to transfer and express genes into satellite cells and their differentiated progeny, although muscle-specific regulation of the transferred gene is very difficult to achieve in a conventional vector framework. We modified a Moloney murine leukemia virus (MoMLV)-derived retroviral vector containing a bacterial beta-galactosidase (beta-Gal) reporter gene by inserting a muscle creatinine kinase (MCK) enhancer element into the U3 region of the viral long terminal repeat (LTR). The resulting vector (mLBSN) was transferred into cells of different histological origin, including undifferentiated murine and human myogenic cells, which were unable to express the transgene at detectable levels. Instead, gene expression from the modified LTR was obtained in a mouse myogenic cell line and in human primary satellite cells upon induction of differentiation into myotubes in culture, and correlated with the activation of the muscle differentiation program. beta-Gal-negative, mLBSN-transduced human satellite cells were also transplanted into the quadricep muscle of immunodeficient mice, where activation of the transgene expression was observed in vivo after differentiation and fusion into muscle fibers. These results show that retroviral vectors carrying LTRs modified in the enhancer sequences may be used to target tissue- and differentiation-specific gene expression into the muscle. For practical purposes, satellite cells engineered by muscle-specific retroviral vectors might represent an effective tool to deliver expression of a given gene product specifically into the muscle tissue, avoiding undesired protein accumulation in mononucleated cells. More generally, this type of vector might be useful whenever regulated expression of a transferred gene is necessary in a target cell or tissue.

Amplified and tissue-directed expression of retroviral vectors using ping-pong techniques

Ping-pong amplification is an efficient process by which helper-free retrovirions replicate in cocultures of cell lines that package retroviruses into distinct host-range envelopes [11]. Transfection of a retroviral vector DNA into these cocultures results in massive virus production, with potentially endless cross-infection between different types of packaging cells. Because the helper-free virus spreads efficiently throughout the coculture, it is unnecessary to use dominant selectable marker genes, and the retroviral vectors can be simplified and optimized for expressing a single gene of interest. The most efficient ping-pong vector, pSFF, derived from the Friend erythroleukemia virus, has been used for high-level expression of several genes that could not be expressed with commonly employed two-gene retroviral vectors. Contrary to previous claims, problems of vector recombination are not inherent to ping-pong methods. Indeed, the pSFF vector has not formed replication-competent recombinants as shown by stringent assays. Here we review these methods, characterize the ping-pong process using the human erythropoietin gene as a model, and describe a new vector (pSFY) designed for enhanced expression in T lymphocytes. Factors that limit tissue-specific expression are reviewed.

Gene therapy using retroviral vectors

Gene therapy is a novel approach for treating various congenital and acquired genetic disorders, including cancer, heart disease, and acquired immune deficiency syndrome. Amongst possible gene delivery systems, retroviral vector mediated gene transfer has been the most extensively studied and has been approved for use in over 40 current Phase I/II clinical trials for the treatment of various disorders, primarily cancers. Recent technological improvements include the optimization of vector production by concentration and lyophilization, resulting in high titers of vectors, as well as the large-scale production of vector-produced cells for the treatment of brain cancer. Present clinical protocols require specialized care centers with expertise in molecular biology and cell transplantation. Considerable effort is under way to develop retroviral vectors that can be both injected directly into the body and targeted to specific cell types within the body. Such vectors could be administered to patients by physicians in their offices. Successful development of this new technology would greatly expand the clinical potential of gene therapy.