An improved retroviral vector for gene transfer into undifferentiated cells

MLL-ENL inhibits polycomb repressive complex 1 to achieve efficient transformation of hematopoietic cells

Stimulation of transcriptional elongation is a key activity of leukemogenic MLL fusion proteins. Here, we provide evidence that MLL-ENL also inhibits Polycomb-mediated silencing as a prerequisite for efficient transformation. Biochemical studies identified ENL as a scaffold that contacted the elongation machinery as well as the Polycomb repressive complex 1 (PRC1) component CBX8. These interactions were mutually exclusive in vitro, corresponding to an antagonistic behavior of MLL-ENL and CBX8 in vivo. CBX8 inhibited elongation in a specific reporter assay, and this effect was neutralized by direct association with ENL. Correspondingly, CBX8-binding-defective MLL-ENL could not fully activate gene loci necessary for transformation. Finally, we demonstrate dimerization of MLL-ENL as a neomorphic activity that may augment Polycomb inhibition and transformation.

The homeodomain region controls the phenotype of HOX-induced murine leukemia

HOX proteins are widely involved in hematopoietic development. These transcription factors combine a conserved DNA-binding homeobox with a divergent N-terminus that mediates interaction with variable cofactors. The resulting combinatorial diversity is thought to be responsible for mammalian HOX specificity. Contrasting this proposed mechanism for normal HOX function, here we demonstrate that, in the context of hematopoietic immortalization and leukemogenesis, individual HOX properties are governed almost exclusively by the homeodomain. Swap experiments between HOXA1 and HOXA9, 2 members of nonrelated paralog groups, revealed that gene expression patterns of HOX transformed cells in vitro are determined by the nature of the homeodomain. Similar results were seen in vivo during HOX-mediated leukemogenesis. An exchange of the homeodomains was sufficient to convert the slow, low-penetrance phenotype of HOXA1-induced leukemia to the aggressive fast-acting disease elicited by HOXA9 and vice versa. Mutation and deletion studies identified several subregions within the DNA binding domain responsible for paralog specificity. Previously defined binding sites for PBX cofactors within the exchangeable, nonhomeobox segment were dispensable for in vitro oncogenic HOX activity but affected in vivo disease development. The transcriptional activator domain shared by HOXA1 and HOXA9 at the very N-terminus proved essential for all transformation.

Self-inactivating lentiviral vectors resist proviral methylation but do not confer position-independent expression in hematopoietic stem cells

Oncoretroviral expression is transcriptionally silenced in embryonic and hematopoietic stem cells (HSCs). This is associated with methylation of viral and internal promoters. We determined whether self-inactivating (SIN) lentiviral vectors (LV) would circumvent proviral silencing in HSCs. We studied long-term expression, methylation, and position effects (PE) from two GFP-encoding SIN-LV containing erythroid enhancers and the human ankyrin-1 promoter (h-Ank-P) using the murine secondary bone marrow (BM) transplant assay. Proviral expression was detected in RBC 6-11 months following transplant only in 28 of 49 secondary mice, with 0.9 +/- 0.2 copy/cell and oligoclonally integrated provirus in BM, spleen, and thymus. Twenty-one of 49 secondary mice lacked integrated provirus. Secondary mice containing provirus also had GFP-expressing RBCs, although proviral copy number did not always correlate with expression, suggesting either proviral methylation or chromatin PE. The endogenous h-Ank-P was partially methylated in nonerythroid cell lines and unmethylated in erythroid cell lines. However, h-Ank-P in the provirus was unmethylated in erythroid and nonerythroid cells within secondary murine BM. Despite lack of methylation, GFP expression was variable in secondary BFU-e and in single-copy mouse erythroleukemia cell clones. Taken together, these data show that erythroid-specific SIN-LV express long term and resist methylation-associated proviral silencing, but may require additional elements to confer position-independent expression.

Inhibition of human immunodeficiency virus transmission to CD4+ T cells after gene transfer of constitutively expressed interferon beta to dendritic cells

CD34(+)-derived dendritic cells (DCs) can be infected by the T cell-tropic HIVLAI strain, but are poorly permissive for efficient virus production. However, HIVLAI-infected DCs are able to transmit a vigorous cytopathic infection to activated CD4(+) T cells. We show that DCs differentiated from CD34(+) cells can be efficiently transduced by a retroviral vector carrying the IFN-beta coding sequence. This results in resistance to infection by HIV as shown by a threefold reduction in the HIV DNA copy number per cell, and by inhibition of HIV transmission from DCs to CD4(+) T cells. Moreover, constitutive IFN-beta production by DCs increases the synthesis of IL-12 and IFN-gamma Th1-type cytokines and of the beta-chemokines MIP-1alpha, MIP-1beta, and RANTES. This indicates that IFN-beta transduction of DCs blocks HIV infection and viral transmission to CD4(+) T cells, and could favor cellular immune responses in HIV-infected patients.

Retrovirally mediated IFN-beta transduction of macrophages induces resistance to HIV, correlated with up-regulation of RANTES production and down-regulation of C-C chemokine receptor-5 expression

Constitutive expression of IFN-beta by HIV target cells may be an alternative or complementary therapeutic approach for the treatment of AIDS. We show that macrophages derived from CD34+ cells from umbilical cord blood can be efficiently transduced by a retroviral vector carrying the IFN-beta coding sequence. This results in resistance to infection by a macrophage-tropic HIV type 1, as shown by the drastic reduction in the HIV DNA copy number per cell and in p24 release. Moreover, IFN-beta transduction totally blocked secretion of proinflammatory cytokines after HIV infection. The constitutive IFN-beta production also resulted in an increased production of IL-12 and IFN-gamma Th1-type cytokines and of the beta-chemokines macrophage-inflammatory protein-1alpha, macrophage-inflammatory protein-1beta, and RANTES. RANTES was found to be involved in the HIV resistance observed, and this was correlated with a down-regulation of the CCR-5 HIV entry coreceptor. These results demonstrate the feasibility and the efficacy of such IFN-beta-mediated gene therapy. In addition to inhibiting HIV replication, IFN-beta transduction could have beneficial immune effects in HIV-infected patients by favoring cellular immune responses.

Acquired constitutive expression of interferon beta after gene transduction enhances human immunodeficiency virus type 1-specific cytotoxic T lymphocyte activity by a RANTES-dependent mechanism

CTL lines directed against HIV-1 antigens were generated from infected individuals and were transduced by the HMB-K(b)HuIFNbeta vector, resulting in low, constitutive expression of interferon beta (IFN-beta). The IFN-beta-transduced cells showed markedly increased HIV-1-specific, MHC class I-restricted CTL activity against HIV-1-LAI Gag, Pol, or Env antigens. This effect of IFN-beta was correlated with an overexpression of RANTES and completely abrogated by RANTES-blocking antibody. The present results provide the first evidence that IFN-beta transduction of CTL lines enhances HIV-specific cytotoxic activities through an upregulation of RANTES production. The efficient elimination of HIV-infected cells by IFN-beta-transduced CTL lines makes this gene therapy approach an attractive treatment for AIDS.

The extended packaging sequence of MoMLV contains a constitutive mRNA nuclear export function

The present report shows that incorporation of defined sequences from the Moloney murine leukaemia virus (MoMLV) into Rex dependent expression vectors based on the human T-cell leukaemia virus (HTLV-1) allows Rex independent gene expression. Deletion mutagenesis of the MoMLV derived sequences allowed this function to be localised to a 312 nt length sequence overlapping the MoMLV gag p15/p12 open reading frame. This 'extended packaging sequence' has been reported to markedly increase the titre of in vitro packaged retroviral vectors. Using fluorescent in situ hybridisation combined with confocal microscopy we show that the 312 nt element can replace Rex mediated nuclear export and expression of transcripts containing HTLV-1 cis acting repressive elements. Our observations are consistent with the extended packaging sequence of MoMLV exerting a constitutive mRNA nuclear export function.

Long-term expression of the gene encoding green fluorescent protein in murine hematopoietic cells using retroviral gene transfer

BACKGROUND

A major goal in retroviral-based gene therapy is to establish methods that allow for selection and tracking of transduced cell populations. Green fluorescent protein (GFP) may be useful for gene therapy applications because it is a naturally fluorescent protein that can be detected using conventional flow cytometers facilitating rapid analysis and purification of transduced cell populations. However, it is unknown whether GFP can be stably expressed in vivo, particularly in multiple bone marrow-derived cell lineages.

METHODS

A murine retrovirus carrying the gene encoding GFP was used to infect murine bone marrow cells (BMCs). These studies were conducted to (1) directly determine whether GFP could be used as a marker of BMC transduction, (2) determine whether GFP is capable of being expressed in multiple bone marrow-derived hematopoietic cell lineages, and (3) determine whether GFP could be used to follow the fate of transduced cells in vivo.

RESULTS

Infection of BMCs with retroviruses carrying the gene encoding GFP resulted in a fluorescent signal in viable transduced cells that was detectable by flow cytometry. Expression of GFP was detected in multiple bone marrow-derived cell lineages after transduction, including stem cell antigen-positive (Sca-1+), lineage marker-negative (Lin-) cells. Using GFP as a selectable marker, we were able to enrich for transduced cells by cell sorting. Mice reconstituted with enriched populations of GFP+ cells showed a significant increase in the percentage of cells expressing GFP in the periphery when compared with mice reconstituted with unenriched transduced bone marrow.

CONCLUSIONS

These data indicate that GFP can be used to select for transduced BMCs in vitro, expressed in multiple bone marrow-derived cell lineages, used to select transduced cells, and follow the fate of transduced cells long-term in vivo.

Interferon beta transduction of peripheral blood lymphocytes from HIV-infected donors increases Th1-type cytokine production and improves the proliferative response to recall antigens

We are developing a gene therapy method of HIV infection based on the constitutive low production of interferon (IFN) beta. Peripheral blood lymphocytes (PBL) from HIV-infected patients at different clinical stages of infection were efficiently transduced with the HMB-HbHuIFNbeta retroviral vector. The constitutive low production of IFN-beta in cultured PBL from HIV-infected patients resulted in a decreased viral production and an enhanced survival of CD4+ cells, and this protective effect was observed only in the PBL derived from donors having a CD4+ cell count above 200 per mm3. In IFN-beta-transduced PBL from healthy and from HIV-infected donors, the production of the Th1-type cytokines IFN-gamma and interleukin (IL)-12 was enhanced. In IFN-beta-transduced PBL from HIV-infected donors, the production of IL-4, IL-6, IL-10, and tumor necrosis factor alpha was maintained at normal levels, contrary to the increased levels produced by the untransduced PBL. The proliferative response to recall antigens was partially restored in IFN-beta-transduced PBL from donors with an impaired antigen response. Thus, in addition to inhibiting HIV replication, IFN-beta transduction of PBL from HIV-infected donors improves several parameters of immune function.

A conditional self-inactivating retrovirus vector that uses a tetracycline-responsive expression system

We developed a novel conditional self-inactivating (C-SIN) vector, TL-SN, by replacement of the enhancer-promoter of the 3' long terminal repeat of Moloney murine leukemia virus with a synthetic tetracycline operator-cytomegalovirus promoter (tetP) from the tetracycline-responsive expression system (TRES). The other component of the TRES, a chimeric transactivator (tTA), was stably incorporated into PA317 amphotropic packaging cells, thus generating the packaging cell line PA317-tTA. C-SIN amphotropic G418-resistant virus particles were generated with a titer of 2 x 10(5) CFU/ml within 2 days of transinfection of PA317-tTA cells with TL-SN ecotropic virus particles. This titer was approximately 2 log units higher than that obtained by transinfection of parental PA317 cells and was due to the high level of viral transcripts originating from the tetP promoter at the 5' end of the transduced vector in the presence of tTA. Our C-SIN vector has the potential for use in human gene therapy since it incorporates the advantages of previous SIN vectors in having weak tetP promoter activity (in the absence of tTA in target cells) while at the same time achieving high viral titers with PA317-tTA packaging cells.

[Development of an anti-HIV gene therapy based on the antiviral properties of beta interferon]

The aim of our work is to explore the use of IFN-beta for gene therapy in the HIV-infection. Transduction of various HIV target cells with a retroviral vector that carries the Hu-IFN-beta coding sequence under constitutive low expression control, confers resistance to HIV without affecting cell replication or function. After transduction, lymphocytes from HIV-infected patients develop resistance to the endogenous virus, provided the cells are derived from individuals with a CD4 cell count higher than 200 per mm3.

Blocking of retroviral infection at a step prior to reverse transcription in cells transformed to constitutively express interferon beta

We are developing methods for somatic-cell gene therapy directed against infection with human immunodeficiency virus, by enhancing antiviral resistance of target cells through the constitutive production of autocrine interferon (IFN). Using the human IFN-beta coding sequence under the constitutive low-expression control of a 0.6-kb murine H-2Kb promoter-fragment, we have constructed a retroviral vector, HMB-KbHuIFN beta, and have transformed cells of the T98G human neuroblastoma line, the U-937 human promonocytic line, and the CEM human lymphocytic line. These human IFN-beta-transformed cell populations have acquired a low, constitutive production of human IFN, while replicating at a rate similar to that of untransformed cells and of cells transformed with the control vector carrying a human IFN-beta sequence encoding an inactive, mutated protein. In the three different cell populations tested, transformation with the HMB-KbHuIFN beta vector resulted in a 1.3-2.3 log10 reduction in the number of cells infected with a defective amphotropic MFG-LaZ retrovirus. A kinetic study of the fate of the MFG-LacZ retrovirus in the culture medium and intracellularly immediately after exposure of the cells to virus revealed a significant reduction of the appearance of intracellular virus in human IFN-beta-transformed cells. A similar effect was obtained by treating untransformed T98G, U-937, and CEM cells with exogenous human IFN-beta. The blocking effect of autocrine or exogenous human IFN-beta on viral entry was not limited to virus specific for the amphotropic receptor but was also obtained in murine IFN-beta-treated NIH 3T3 mouse fibroblasts infected with an ecotropic MFG-LacZ retrovirus. Infection of human IFN-beta-transformed CEM cells with human immunodeficiency virus type 1 gave comparable results. Immediately following exposure of the cells to human immunodeficiency virus, a kinetic study of the fate of the virus failed to reveal the appearance of intracellular virus and showed that the majority of the input virus remained in the extracellular medium. We conclude that low autocrine IFN-beta synthesis, or exposure of cells to exogenous IFN-beta, prevents virus from getting inside the cells, regardless of the virus receptor involved.

Development of methods for somatic cell gene therapy directed against viral diseases, using retroviral vectors carrying the murine or human interferon-beta coding sequence: establishment of the antiviral state in human cells

We are developing methods for somatic cell gene therapy directed against chronic and fatal virus infections, such as acquired immunodeficiency (AIDS), by transforming cells with a constitutively expressed interferon (IFN) coding sequence. Previous work from our laboratory has shown that stable antiviral expression (SAVE) can be obtained in murine BALB/c 3T3 cells and human U937 cells transformed with plasmids carrying either the murine or the human IFN-beta coding sequence placed under the expression control of a 0.6-kb Xho II-Nru I promoter region of the murine H-2Kb major histocompatibility complex (MHC) gene (Macé et al., 1991; Seif et al., 1991). In the present paper, we report the construction of murine (Mu) and human (Hu) IFN-beta-expressing retroviral vectors (pMPZen-MuIFN beta, pHMB-KbMuIFN beta) and the problems encountered. Because of the murine origin of commonly used packaging cells and the species specificity of IFN, it was evident that placing the murine IFN-beta sequence under constitutive expression control could result in the production of Mu IFN in the murine packaging system, and thereby lead to decreased vector production and also to enhanced resistance of target cells. Using a packaging cell line that releases a beta-galactosidase-expressing vector, we show that, as expected, Mu IFN-alpha/beta decreases vector production of murine packaging cells and also inhibits the transformation of target NIH-3T3 cells with this vector, but the presence of anti-Mu IFN antibodies rescues the viral titer of the packaging cells and restores the sensitivity of target cells to virus transformation. However, the same antibody treatment is unable to rescue the viral titer of psi-2 packaging cells producing autocrine Mu IFN-beta encoded by the pMPZen-MuIFN beta and pHMB-KbMuIFN beta vectors. Because of the species specificity of IFN, this problem is circumvented with the pMFG-HuIFN beta vector carrying the human IFN-beta sequence. In spite of the production of Hu IFN, murine psi-CRIP packaging cells are able to release retroviral vectors expressing Hu IFN-beta, and these amphotropic vectors can transform human MRC-5 cells and confer to these cells an enhanced resistance to vesicular stomatitis virus (VSV) infection.

Establishment of a novel factor-dependent myeloid cell line from primary cultures of mouse bone marrow

We describe here a novel myelomonocytic cell line (OTT1) obtained from primary cultures of mouse bone marrow cells infected with a retroviral vector carrying the mouse interleukin (IL)-1 alpha gene. OTT1 cells are dependent for their survival and proliferation on IL-3, granulocyte-macrophage colony-stimulating factor (GM-CSF) or, unexpectedly, IL-5. Despite their IL-5 dependency, OTT1 cells form colonies showing predominantly monocyte maturation when plated in methylcellulose. It is suggested that constitutive expression of the exogenous IL-1 alpha gene may predispose to a monocytic phenotype. OTT1 cells should be a useful experimental model to investigate the molecular mechanisms of IL-5 signal transduction and the possible interrelationships between this signal pathway and those utilized by IL-3 and GM-CSF.

Expression of retrovirally transduced IL-1 alpha in IL-6-dependent B cells: a murine model of aggressive multiple myeloma

Retroviral-mediated gene transfer was employed to introduce an IL-1 alpha cDNA into an IL-6-dependent murine B-cell line. Bone marrow metastases and bone lesions were frequently observed following intravenous injection of these B cells into syngeneic mice. Because the retroviral vector also contained the neomycin phosphotransferase gene, metastatic cells could be easily recovered from bone marrow by addition of G418 to the culture medium. Interestingly, the metastatic B cells were found to retain their IL-6 dependency through several transplant generations. By comparison, intravenous injection of autonomously-growing B-cell lines generated in vitro by retroviral introduction of an IL-6 cDNA rarely resulted in bone marrow metastases. These results demonstrate that abrogation of growth factor dependency is neither necessary nor sufficient for the in vivo growth and dissemination of tumor cells in this experimental system. It is proposed that the increased metastasis of the IL-1 alpha-producing B-cells to bone marrow is due to alterations in cell adhesion molecules. The B-cell bone marrow metastasis model described here may be useful for studies of bone marrow homing and for evaluation of therapeutic regimens for multiple myeloma.

Leukocytosis in mice following long-term reconstitution with genetically-modified bone marrow cells constitutively expressing interleukin 1 alpha or interleukin 6

Leukemic cells of patients with acute myeloid leukemia have recently been shown to spontaneously produce autostimulatory IL-1 and IL-6. In order to investigate the effects of systemic production of these cytokines on normal hematopoietic cells, mice were engrafted with bone marrow cells infected with high-titer retroviral vectors carrying the murine IL-1 alpha or IL-6 genes and the neomycin phosphotransferase gene. Sustained expression of the introduced IL-1 alpha and IL-6 genes was documented by Northern-blot analysis of RNA from G418-resistant mast cells and T cells, derived from bone marrow and spleen, respectively, of successfully reconstituted mice 6-10 months after transplantation. A single mouse engrafted with IL-1 alpha-infected cells which presented with a dramatic neutrophilic granulocytosis (54-fold elevation in circulating neutrophils) was sacrificed for health concerns 2 months post-transplant. Modest changes in peripheral leukocyte counts (at most a 2-fold rise) were observed in all of the other mice, and they remained healthy throughout the observation period. The majority displayed increased hematopoietic activity in bone marrow and spleen, predominantly granulopoiesis, with moderate lymphoid hyperplasia seen in the spleens of mice constitutively expressing IL-1 alpha. These mouse models provide the opportunity to evaluate the potential of persistent IL-1 alpha and IL-6 expression to contribute to leukemogenic transformation.

Evaluation of the transfer and expression in mice of an enzyme-encoding gene using a human adenovirus vector

Mutant mice of the Spf-ash strain have an inherited defect in ornithine transcarbamylase (OTC) protein synthesis, and were used to ascertain the potential of recombinant adenoviruses for introducing and expressing the normal gene lacking in these mice. These OTC mutant mice are characterized by a reduction in the amount of OTC activity, resulting in hyperammonemia, pronounced orotic aciduria, growth retardation, and sparse fur until weaning. A recombinant adenovirus that harbors the rat OTC cDNA under the control of the viral major late promoter (MLP) was constructed and injected into such newborn mice. The effect of the virus was analyzed by monitoring the hepatic OTC enzyme during several months after the injection. An increase in OTC activity was detected and was accompanied by a diminution of orotic acid in the urine. The observation of MLP-OTC mRNA transcripts over 1 year following the injection attests to the relatively long-term presence of the transferred gene. In those mice showing the greatest OTC activity, a normalization of the fur was also observed. The experiments reported here document the feasibility of using adenovirus for the direct delivery in vivo of a gene to restore an impaired metabolism.

Comparative analysis of retroviral vector expression in mouse embryonal carcinoma cells

A series of replication-defective retroviral vectors were assessed for their ability to efficiently transfer functional genes into undifferentiated cells. In these vectors (designated handicapped because of a deletion of enhancer and promoter sequences in the viral long terminal repeat) transcription of inserted genes is under the control of internal promoters. Although a composite promoter composed of a mutant polyoma virus enhancer (PyF441) coupled to the herpes simplex virus thymidine kinase promoter was anticipated to function efficiently, it was found to be significantly inferior to the mouse X-chromosome phosphoglycerate kinase (pgk-1) promoter, in its ability to express the selectable neomycin phosphotransferase gene in mouse embryonal carcinoma cells. The pHMB vector, which contains the pgk-1 promoter, was shown to confer the drug-resistant phenotype at high frequencies to F9 and P19 cells. This vector might prove to be of general utility for efficient gene expression in other developmental contexts.