Changes in selected subpopulations of lymphocytes in dogs infected with Babesia canis treated with imidocarb

OBJECTIVE

The purpose of this study was to track changes in selected subpopulations of lymphocytes in the blood of dogs infected with Babesia (B.) canis and treated with imidocarb.

MATERIAL AND METHODS

The study included 16 dogs divided into two groups. The first group (n = 6) consisted of healthy control animals. Dogs of the se- cond group (n = 10) were infected with B. canis and after establishment of the diagnosis each animal received a single dose of imido- carb (5 mg/kg). Flow cytometry was used to enumerate several immune cell phenotypes.

RESULTS

It was concluded that the invasion of B. canis contributes to the decreased percentage of CD3+, CD4+, CD8+, CD21+ lymphocytes in the blood of infected animals. The decreased level of tested subpopulations of lymphocytes in group 2 persisted for the entire 12-day period of the test. After the administration of imidocarb, each tested lymphocyte fraction in the blood of the dogs with babesiosis increased, but did not reach physiological values.

CONCLUSION

The presented results indicate that the resolution of clinical signs associated with babesiosis may be related to the stimulation and intensity of cellular immunity, dependent on the CD4+ T cells profile. After administration of imidocarb, the parasitemia is cleared which allows the recovery of the lymphocyte populations.

Simian immunodeficiency virus vector pseudotypes differ in transduction efficiency and target cell specificity in brain

Lentiviral vectors have proven to be promising tools for transduction of brain cells in vivo and in vitro. In this study, we have examined the central nervous system (CNS) transduction efficiencies and patterns of a self-inactivating simian immunodeficiency virus (SIVmac)-derived lentiviral vector pseudotyped with glycoproteins from the vesicular stomatitis virus (VSV-G), the amphotropic murine leukemia virus (MLV4070Aenv), the lymphocytic choriomeningitis virus (LCMV-GP), the Ross River virus (RRV-GP) and the rabies virus (RV-G). All glycoproteins were efficiently incorporated into SIV virions, allowing efficient transduction of neuronal cell lines as well as of primary dissociated mouse brain cell cultures. After injection of highly concentrated vector stocks into the striatum of adult mice, quantitative analyses revealed high transduction efficiency with VSV-G pseudotypes, while LCMV-GP and RV-G pseudotypes exhibited moderate transduction efficiencies. MLV4070Aenv and RRV-GP pseudotypes, however, showed only weak levels of transduction after stereotactic injection into the brain. Regarding cell tropism in vivo, VSV-G-pseudotyped SIV vectors transduced neuronal as well as glial cells, whereas all other pseudotypes preferentially transduced neuroglial cells. In addition, we analyzed the influence of the central polypurine tract (cPPT) in context of the VSV-G-pseudotyped SIV transfer vector for infection of brain cells. Deletion of the cPPT sequence from the transfer vector decreased the in vivo transduction efficiency by fourfold, and, more importantly, this modification changed the transduction pattern, since these vectors were no longer able to infect neuronal cells in vivo. Vector injection into the brain did elicit a humoral immune response in the injected hemisphere; however, no gross signs of inflammation could be detected. Analysis of the biodistribution of the vector revealed that, besides the injected brain region, no vector-specific sequences could be detected in any of the organs evaluated. These data indicate SIV vectors as efficient gene delivery vehicles for the treatment of neurodegenerative diseases.

Genetic reshuffling reconstitutes functional expression cassettes in retroviral vectors

BACKGROUND

A major prerequisite for the design of retroviral vectors encoding cell toxic or harmful genes is the possibility to tightly control gene expression, thus limiting activity to the relevant target cells and protecting the packaging cell used for production of recombinant viral particles.

METHODS

In the present study a system was developed in which genetic reshuffling during the retroviral life cycle is exploited, allowing reconstitution of functional expression cassettes from separate elements exclusively in transduced target cells. For construction of these murine leukaemia virus (MLV)-based reconstituting viral vectors (ReCon), a promoterless inverted enhanced green fluorescent protein (EGFP) reporter gene cassette was inserted in place of the U3 region of the 3' LTR. Subsequently, the human ubiquitin promoter was inserted in the inverse orientation into the R/U5 border of the 5' LTR of the vector.

RESULTS

PA317 packaging cells stably transfected with ReCon vectors were established and EGFP expression was analysed by fluorescence-activated cell sorting (FACS). After detection of low-level background expression, an additional polyadenylation signal was introduced in antisense orientation into the 3' LTR at the R/U5 border to prevent accidental read-through transcription from neighbouring cellular promoters. Virus-containing cell culture supernatants were then used to infect NIH3T3 target cells. EGFP expression, recloning and sequencing of integrated proviruses demonstrated the correct reassembly of the transduced ubiquitin/EGFP transcription unit in these infected cells.

CONCLUSIONS

This facile and convenient system should allow production of retroviral vectors encoding potentially toxic proteins, cell cycle inhibitors or inducers of apoptosis, all of which would interfere with vector production if expressed in the retroviral packaging cell.

In vivo perivascular implantation of encapsulated packaging cells for prolonged retroviral gene transfer

Long-term benefits of coronary angioplasty remain limited by the treatment-induced renarrowing of arteries, termed restenosis. One of the mechanisms leading to restenosis is the proliferation of smooth muscle cells. Therefore, proliferating cells of the injured arterial wall, which can be selectively transduced by retroviruses, are potential targets for gene therapy strategies. A direct single-dose therapeutic application of retroviral vectors for inhibition of cell proliferation is normally limited by too low transduction efficiencies. Encapsulated retrovirus-producing cells release viral vectors from microcapsules, and may enhance the transduction efficiency by prolonged infection. Primary and immortal murine and porcine cells and murine retrovirus-producing cells were encapsulated in cellulose sulphate. Cell viability was monitored by analysing cell metabolism. Safety, stability, transfer efficiency and extent of restenosis using capsules were determined in a porcine restenosis model for local gene therapy using morphometry, histology, in situ beta-galactosidase assay and PCR. Encapsulation of cells did not impair cell viability. Capsules containing retrovirus-producing cells expressing the beta-galactosidase reporter gene were implanted into periarterial tissue or a pig model of restenosis. Three weeks following implantation, beta-galactosidase activity was detected in the pericapsular tissue with a transduction efficiency of approximately 1 in 500 cells. Adventitial implantation of vector-producing encapsulated cells for gene therapy may, therefore, facilitate successful targeting of proliferating vascular smooth muscle cells, and allow stable integration of therapeutic genes into surrounding cells. The encapsulation of vector-producing cells could represent a novel and feasible way to optimize local retroviral gene therapy.

Microencapsulated cell-mediated treatment of inoperable pancreatic carcinoma

Pancreatic cancer can seldom be resected, and chemotherapy has only a limited effect on survival or tumour load. We did a phase I/II trial in 14 patients with pancreatic cancer to assess the safety of local activation of low-dose ifosfamide. We encapsulated genetically modified allogeneic cells, which expressed a cytochrome P450 enzyme, in cellulose sulphate and delivered them by supraselective angiography to the tumour vasculature. These cells locally activated systemically administered ifosfamide. The tumours of four patients regressed after treatment, and those of the other ten individuals who completed the study remained stable. Median survival was doubled in the treatment group by comparison with historic controls, and 1-year survival rate was three times better. Further studies of this cell-therapy-based treatment combined with chemotherapy for inoperable pancreatic cancer are warranted.

Necrotic, rather than apoptotic, cell death caused by cytochrome P450-activated ifosfamide

Feline kidney cells were transfected with a vector overexpressing cytochrome P450 2B1 (CYP2B1). Transfected cells acquired a new specific biochemical activity, which could be demonstrated by a rapid CYP2B1 detection assay and showed selective sensitivity to the antitumorigenic prodrug ifosfamide (IFO). Further, the cell-killing effect was also mediated on nonmodified cells like feline kidney cells, mouse lymphoma, and human pancreatic cells in the vicinity of the CYP2B1-expressing cells due to the diffusible nature of the activated IFO metabolites. One of these, phosphoramide mustard, causes interstrand DNA cross-linking and it has been thought that the inability to repair this damage results in apoptosis. Surprisingly, our results clearly demonstrate a necrotic mechanism of IFO-induced cell death. This may have important implications for the activation of the immune system during CYP2B1/IFO suicide gene therapy of cancer.

Novel clinical strategies for the treatment of pancreatic carcinoma

Pancreatic carcinoma ranks as the eighth most frequent type of solid tumour arising worldwide yet it represents the fourth most frequent cause of death. This discrepancy reflects the current lack of effective treatment available for the pancreatic cancer patient and highlights the urgent need for new therapeutic principles in this area. The last five years have seen an increasing number of novel approaches both in the pre-clinical area as well as in clinical trials for pancreatic cancer treatments. This review summarizes these new developments and attempts to rationalize the possibilities available for the patient at the beginning of the new millennium.

Influence of preassay and sequence variations on viral load determination by a multiplex real-time reverse transcriptase-polymerase chain reaction for feline immunodeficiency virus

Determination of retroviral load is an important tool in the investigation of the success of therapeutic or vaccination trials in patients infected with lentiviruses such as HIV, or with their simian (SIV) or feline (FIV) counterparts. We have developed an one-tube quantitative reverse transcriptase-polymerase chain reaction (RT-PCR) assay based on the ABI Prism 7700 Sequence Detection System (TaqMan) to quantify the viral load of FIV-infected cats. Two different primer/probe systems were designed to detect a broad range of clade A FIV isolates. Both systems are characterized by excellent reproducibility, high sensitivity, and a wide range of quantification. As a consequence of this improved precision in the quantitative RT-PCR, preassay variations have greater impact on the accuracy of the viral load estimation. To compensate for these variations, we improved the assay and developed a multiplex real-time RT-PCR, which allows simultaneous calculation of the viral copy number and the individual recovery rate in an one-tube reaction. This enables the rapid and accurate calculation of copy number independent of preassay variations. In further studies, two additional real-time RT-PCR assays were designed and used to investigate the influence of sequence variations in the binding regions for either the primers or probe. Sequence mismatches in this region had a significant effect (up to 4 logarithmic decades) on reaction efficiency. In view of the inherent variability of retroviral sequences, these results underline the necessity to check reaction efficiencies before determining viral load.

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.

Immunotherapy of a viral disease by in vivo production of therapeutic monoclonal antibodies

Continuous and sustained in vivo production of monoclonal antibodies by engineered cells might render long-term antibody-based treatments cost-effective, avoid side effects associated with infusion of massive doses of antibody, and circumvent possible antiidiotypic responses against the therapeutic agent. The FrCasE retrovirus induces a lethal neurodegeneration on infection of newborn mice. We report here that implantation of cellulose sulfate capsules containing cells secreting an ectopic monoclonal antibody neutralizing FrCasE can prevent animals from developing the disease. All treated mice showed reduced or undetectable viremia in addition to a lack of the histopathological lesions characteristic of FrCasE infection. This work paves the way for a novel gene/cell antibody-based immunotherapy of a variety of severe viral and nonviral diseases.

Xenotransplantation: is the risk of viral infection as great as we thought?

Two major hurdles remain before xenotransplantation can enter the clinic. The first is the more technical issue of being able to overcome the human immune response that leads to rejection of transplanted organs/cells from other species. The second, reviewed here, concerns the potential risk of inadvertent transfer of animal viruses present in the xenotransplant that are able to infect the human recipient. The threat from viruses is a particularly contentious topic because it poses a risk not only to those individuals who receive xenotransplants, but also to healthy individuals who come into contact, either directly or indirectly, with the xenotransplant recipient. In this review, we describe some of the virus types, in addition to the much discussed porcine endogenous retroviruses that might cross the species barrier, and assess the risk of such viruses causing disease in human hosts.

Combined chemotherapy of murine mammary tumors by local activation of the prodrugs ifosfamide and 5-fluorocytosine

The success of chemotherapeutic intervention is limited because the necessary high local drug doses cannot be achieved without systemic toxicity. Application of suicide genes (SGs) and direct conversion of prodrugs (PDs) to toxic metabolites in situ by SGs may enhance the efficacy of chemotherapy. To evaluate this strategy in two murine breast cancer models, TS/A and GR, we injected cellulose sulfate capsules harboring cat kidney cells expressing the SGs cytosine deaminase and cytochrome P450 2B1 (CYP2B1) intratumorally. The PDs 5-fluorocytosine and ifosfamide were administered in 3-day intervals. The effect of in situ chemotherapy with each PD alone and the combination was analyzed over a period of 100 days. The results reveal that for TS/A tumors, the antitumoral effect mediated by CYP2B1 is more efficient than that of cytosine deaminase, whereas for GR tumors, both systems worked equally well. Furthermore, we find additive toxicity using both SG/PD systems for both TS/A and GR tumors.

Superantigen expression is driven by both mouse mammary tumor virus long terminal repeat-associated promoters in transgenic mice

In addition to the usual retroviral promoter, the mouse mammary tumor virus (MMTV) long terminal repeat carries a second promoter located in the U3 region. Here we show that both of these promoters are independently able to give rise to superantigen activity in transgenic mice. The ability of multiple MMTV promoters to drive superantigen expression underscores its importance in the virus life cycle.

Accurate estimation of transduction efficiency necessitates a multiplex real-time PCR

Transduction efficiency can be easily monitored during pre-clinical trials by inclusion of marker genes. However, the use of such marker genes should be avoided in the final clinical gene therapy application since their products are often immunogenic, making it difficult to monitor transduction, especially if the vector is applied in vivo. In these cases PCR-based methods like the real-time PCR might provide a powerful tool to estimate biodistribution. To investigate the accuracy of this method, we have developed and tested a real-time PCR assay for the quantification of the enhanced green fluorescent protein (EGFP) gene and compared the results with transduction efficiencies estimated by FACS analysis. Although our real-time PCR assay itself was characterized by a high precision over a wide dynamic range of quantification, significant differences in the transduction efficiency compared with FACS data were initially observed. Accurate determination could only be achieved using an optimized multiplex real-time PCR assay, which allows the simultaneous calculation of cell number and EGFP copy number in the same tube. In view of future needs for methods allowing precise and accurate analysis of biodistribution in gene therapy trials, our data highlight the necessity critically to check both parameters in the implemented assay.

Rapid and sensitive detection of enhanced green fluorescent protein expression in paraffin sections by confocal laser scanning microscopy

Various forms of green fluorescent protein (GFP) have become important reporters of gene transfer and expression after transfection or infection of cells in cell culture. Frequently, molecular biological assays (Northern blots, PCR) are applied to detect reporter gene expression in target organs. However, these methods are not suitable for evaluation of tissue- or cell-specific expression which would be of great interest especially in case of using tissue-specific promoters. Therefore, organs of transgenic mice with the enhanced green fluorescent protein (EGFP) gene under control of the cytomegalovirus (CMV) promoter were processed for histology by formaldehyde fixation and embedding in paraffin. Sections were deparaffinized, mounted and evaluated for fluorescence in a confocal laser scanning microscope. This method combines the advantages of direct exploitation of tissue sections without further staining procedures with evaluable tissue-, cell-, and even subcellular-specific distribution patterns of EGFP expression in tissues. Results obtained by direct evaluation of EGFP fluorescence in paraffin sections were confirmed by immunohistochemical staining with anti-EGFP. In the present report, we demonstrate that application of confocal microscopy on routinely processed histological preparations is very suitable for determining gene transfer efficiency and promotor activities.

Protection of MLV vector particles from human complement

Murine cell-derived MLV vector particles usually are highly sensitive to human complement-mediated lysis. Expression of the human complement inhibitor CD59 on murine packaging cells resulted in partial protection of these cells from lysis caused by human complement proteins. Furthermore, CD59 was incorporated into MLV vector particles released by these packaging cells, leading to an improved resistance of the virions against human complement-mediated inactivation.

Injection of encapsulated cells producing an ifosfamide-activating cytochrome P450 for targeted chemotherapy to pancreatic tumors

The prognosis of pancreatic cancer is poor, and current medical treatment is mostly ineffective. The aim of this study was to design a new treatment modality in an animal model system. We describe here a novel treatment strategy employing a mouse model system for pancreatic carcinoma. Embryonal kidney epithelial cells were genetically modified to express the cytochrome P450 subenzyme 2B1 under the control of a cytomegalovirus (CMV) immediate early promoter. This CYP2B1 gene converts ifosfamide to its active cytotoxic compounds, phosphoramide mustard, which alkylates DNA, and acrolein, which alkylates proteins. The cells were then encapsulated in a cellulose sulphate formulation and implanted into preestablished tumors derived from a human pancreatic tumor cell line. Intraperitoneal administration of low-dose ifosfamide to tumor bearing mice that received the encapsulated cells results in partial or even complete tumor ablation. Such an in situ chemotherapy strategy utilizing genetically modified cells in an immunoprotected environment may prove useful for solid tumor therapy in man.

Characterization of a human cell clone expressing cytochrome P450 for safe use in human somatic cell therapy

We have previously demonstrated the therapeutic effect and efficacy of implantation of cells genetically modified to express cytochrome P450 2B1 in a nude mouse tumor model. The cells are encapsulated in polymerized cellulose sulphate and injected into preformed tumors. Upon administration of ifosfamide, the P450 enzyme converts the ifosfamide into antitumorigenic toxic metabolites at the site required, thereby significantly reducing tumor burden. Feline kidney epithelial cells were chosen for these studies, because they are easy to culture and can readily be transfected. However, these cells are not suitable for eventual use in human patients, since they are known to express endogenous retroviruses that are able to infect mammalian cells. They thus represent a safety risk. Here we describe the establishment of a human cell line that has been genetically modified to express the same cytochrome P450 construct and their characterization. The usefulness of mitomycin C treatment, both to protect the cells from the toxic metabolites that they produce and to incapacitate these cells from replicating, should they escape from the capsules, has also been investigated.

Development of cellulose sulfate-based polyelectrolyte complex microcapsules for medical applications

Microencapsulation, as a tool for immunoisolation for allogenic or xenogenic implants, is a rapidly growing field. However most of the approaches are based on alginate/polylysine capsules, despite this system's obvious disadvantages such as its pyrogenicity. Here we report a different encapsulation system based on sodium cellulose sulfate and polydiallyldimethyl ammonium chloride for the encapsulation of mammalian cells. We have characterized this system regarding capsule formation, strength and size of the capsules as well as viability of the cells after encapsulation. In addition, we demonstrate the efficacy of these capsules as a "microfactory" in vitro and in vivo. Using encapsulated hybridoma cells we were able to demonstrate long-term release of antibodies up to four months in vivo. In another application we could show the therapeutic relevance of encapsulated genetically modified cells as an in vivo activation center for cytostatic drugs during tumor therapy.

Needle injection catheter delivery of the gene for an antibacterial agent inhibits neointimal formation

Percutaneous transluminal coronary angioplasty is a routinely used non-surgical revascularization technique for patients with coronary artery disease. Up to 30% of patients undergoing coronary angioplasty develop a renarrowing of treated vessels, called restenosis. Smooth muscle cell proliferation is thought to be an important factor in restenosis; this leads to neointima formation and arterial lumen narrowing. Neointima may be reduced by the transfer of genes encoding proteins with antiproliferative effects. Cecropins are antimicrobial peptides with antiproliferative properties in mammalian cells. Cecropin A is one member of this family of peptides. In this article, a plasmid carrying the gene for the immature form, pre-pro-cecropin A, complexed with liposomes was locally delivered to perivascular tissue in a porcine arterial injury model using a needle injection catheter. Retention of the plasmid in the treated arteries was demonstrated at both 8 and 21 days following application. Transferred plasmid DNA was not detected in any other tissues analyzed. Pre-pro-cecropin A-specific transcripts could also be found in treated arteries. Balloon-injured vessels demonstrated significantly reduced neointima at 21 days in vessels treated with the pre-pro-cecropin A gene compared with neointimal area in those given a control gene (P < 0.05). The needle injection catheter appears to be useful for local intravascular gene delivery. In vivo gene transfer of cecropins may be of therapeutic relevance in restenosis prevention by limiting cell proliferation.

Intratumoral injection of encapsulated cells producing an oxazaphosphorine activating cytochrome P450 for targeted chemotherapy

The prognosis of pancreatic adenocarcinoma is poor and current treatment is for the most part ineffective. We describe here a novel treatment strategy using a mouse model system for pancreatic cancer. Human embryonic epithelial cells have been genetically modified to express the cytochrome P450 2B1 enzyme under the control of a CMV immediate-early promoter. This CYP2B1 gene converts oxazaphosphorines (ifosfamide or cyclophosphamide) to their active cytotoxic compounds, phosphoramide mustard, which alkylates DNA, and acrolein, which alkylates proteins. A number of assays were performed to demonstrate the CYP2B1 gene function as well as toxic effects on neighbouring cells (bystander effect). The cells were then encapsulated in a cellulose sulphate formulation shown to be well tolerated in the pancreas of immunocompetent mice, and injected 1 cm away from pre-established tumours derived from a human pancreatic tumour cell line (PaCa-44). Intraperitoneal administration of low-dose ifosfamide to tumour bearing mice that received the encapsulated cells results in partial or even complete tumour ablation. Such an in situ chemotherapy strategy utilizing genetically modified cells in an immunoprotected environment may prove useful for solid tumour therapy in man.

Proviral load determination of different feline immunodeficiency virus isolates using real-time polymerase chain reaction: influence of mismatches on quantification

Lentiviruses are associated not only with immunodeficiency but also with malignancies. The mechanisms involved in tumorigenesis are still not fully understood. Cats infected with feline immunodeficiency virus (FIV) in the wild represent one model in which the role of viral load in the pathogenesis can be studied, since tumors, especially lymphomas, are quite often observed in cats infected with FIV. To be able to compare the viral load data among cats infected with different FIV isolates, the method used to obtain the viral load has to be unaffected by isolate-specific differences. This is especially true for the real-time polymerase chain reaction (PCR), a new method for viral load determination, since nucleotide sequence mismatches have been used for allelic discrimination with this method. To investigate the influence of these mismatches on PCR efficiency, we have used an FIV-specific real-time PCR and determined the influence of nucleotide sequence variation in several characterized FIV isolates as well as unknown isolates from naturally infected cats. We could demonstrate that minor mismatches, such as point mutations in the primer or the probe region, decrease overall PCR efficiency but do not abolish the quantification, in contrast to major mismatches of three or four nucleotides, which lead to complete inhibition of the real-time PCR detection. Based on these results, it will be possible to design real-time PCR systems allowing the quantification of a broad range of isolates, which is a prerequisite for the investigation of the impact of viral load in tumorigenesis.

CrFK feline kidney cells produce an RD114-like endogenous virus that can package murine leukemia virus-based vectors

The feline kidney cell line CrFK is used extensively for viral infectivity assays and for study of the biology of various retroviruses and derived vectors. We demonstrate the production of an endogenous, RD114-like, infectious retrovirus from CrFK cells. This virus also is shown to efficiently package Moloney murine leukemia virus vectors.

Targeted chemotherapy by intratumour injection of encapsulated cells engineered to produce CYP2B1, an ifosfamide activating cytochrome P450

The prognosis of pancreatic adenocarcinoma is poor and current treatment ineffective. A novel treatment strategy is described here using a mouse model system for pancreatic cancer. Cells that have been genetically modified to express the cytochrome P450 2B1 enzyme are encapsulated in cellulose sulphate and implanted into pre-established tumours derived from human pancreatic cells. Cytochrome P450 2B1 converts the chemotherapeutic agent ifosfamide to toxic metabolites. Administration of ifosfamide to tumour-bearing mice that were recipients of implanted encapsulated cells results in partial or even complete tumour ablation. These results suggest that in situ chemotherapy with genetically modified cells in an immunoprotected environment may prove useful for application in man.

A functional eukaryotic promoter is contained within the first intron of the hGH-N coding region

Human growth hormone is frequently used as a reporter gene in studies addressing the regulation of eukaryotic promoters. Here we present evidence that the first intron of the hGH-N coding region contains a novel eukaryotic promoter which is able to direct the expression of hGH-N and luciferase reporter genes. The corresponding transcriptional initiation site is located downstream of the splice acceptor site of exon 2. This internal hGH-N promoter can interfere with the transcriptional control elements of a promoter linked to the hGH-N coding region and thereby complicate the interpretation of data obtained with hGH-N as a reporter gene.

Cell targeting by murine retroviral vectors

Gene therapy involves the transfer of new genetic material to cells of individuals with the aim of conferring a therapeutical benefit. Theoretically, gene therapy can be used to treat a variety of life-threatening disorders, such as inherited genetic diseases, cancer, chronic viral infections as well as a number of other severe diseases which are not treatable at present. To this aim, both efficient gene delivery techniques and controlled gene expression systems are required. Engineered murine retroviruses are the most widely used vectors for stable clinical gene transfer because of their ability to integrate--along with the transgenes they are engineered to carry--into the genome of infected cells. However, this technology still suffers from a number of drawbacks and limitations. Particularly, the ability to target cells of therapeutic interest together with the controlled expression of transferred genes would improve both the efficiency and the safety of retroviral vectors. Such improvements would additionally allow the development of new animal models of human diseases as well as enable more fundamental investigations in the fields of oncogenesis and developmental biology.

Antimicrobial peptides melittin and cecropin inhibit replication of human immunodeficiency virus 1 by suppressing viral gene expression

Antimicrobial peptides are effectors of innate immunity, providing their hosts with rapid non-specific defence against parasitic invaders. In this report, the effects are assessed of two well-characterized antimicrobial amphipathic peptides (melittin and cecropin) on human immunodeficiency virus 1 (HIV-1) replication and gene expression in acutely infected cells at subtoxic concentrations. Production of infectious, cell-free virus was inhibited in a dose-dependent manner, with ID50 values in the range 0.9-1.5 microM for melittin and 2-3 microM for cecropin. Analysis of the effect of melittin on cell-associated virus production revealed decreased levels of Gag antigen and HIV-1 mRNAs. Transient transfection assays with HIV long terminal repeat (LTR)-driven reporter gene plasmids indicated that melittin has a direct suppressive effect on activity of the HIV LTR. HIV LTR activity was also reduced in human cells stably transfected with retroviral expression plasmids for the melittin or cecropin gene. It is concluded that antimicrobial peptides such as melittin and cecropin are capable of inhibiting cell-associated production of HIV-1 by suppressing HIV-1 gene expression.

Mapping of a mouse mammary tumor virus integration site by retroviral LTR--arbitrary polymerase chain reaction

The de novo integration of retroviral genomes within the mammalian genome is believed to contribute to the tumorigenic process. Integration may result in the disruption or inappropriate transcription of key regulatory genes. We describe the application of an arbitrarily primed PCR method for the mapping and cloning of genomic integration sites of the mouse mammary tumor virus (MMTV). We have amplified DNA sequences between a selected retroviral MMTV-LTR and random sites in the 3' flanking DNA. Using this technique we were able to visualize several proviral integration sites present in a MMTV-induced mammary tumor derived cell line that were absent from the germ line. Cloning and sequencing of the PCR product corresponding to one site established its identification as an unique 3' flanking sequence.

Effect of simian virus large T antigen expression on cell cycle control and apoptosis in rat pleural mesothelial cells exposed to DNA damaging agents

Cell cycle progression and apoptosis are controlled by regulatory proteins, including p53, of which functional alterations are linked to carcinogenesis. Recently, malignant mesothelioma (MM), a primary tumour related to asbestos exposure, alternatively to post therapeutic radiations, has proven to be an important problem in oncogenesis. The p53 protein does not seem mutated or deleted in MM but a possible inactivation by binding to other proteins [mdm2; SV40 large T antigen (Tag)] has been suggested. The present work investigated cell cycle regulation in normal rat pleural mesothelial cells (RPMC) and in RPMC expressing Tag (RPMC-TSV40), under exposure to asbestos and radiations. In RPMC, these agents induced activation of cell cycle checkpoints located at G1/S and G2/M and/or mitosis but a lack of control at G1/S was found in RPMC-TSV40. A loss of G2/M control may account for the formation of micronuclei observed after exposure of RPMC-TSV40 to radiations. In RPMC-TSV40 the enhancement of abnormal mitoses and apoptosis after asbestos exposure, in comparison with RPMC, suggests a loss of mitotic control and a p53-independent mechanism of apoptosis. Thus Tag expression in mesothelial cells might have both adverse and beneficial effects by impairing the control of DNA integrity and enhancing apoptosis respectively.

Construction and characterization of a hybrid mouse mammary tumor virus/murine leukemia virus-based retroviral vector

Mouse mammary tumor virus (MMTV)-based vectors are characterized by low titers. In an effort to transfer MMTV-specific regulation of gene expression to a more efficient murine leukemia virus (MLV) vector, we have replaced the complete 3' U3 region of MLV with the complete U3 region of MMTV. Virus titers were not significantly affected by this modification, there was no impairment of reverse transcription and integration, and after infection of cells, the MMTV promoter is duplicated and translocated to the 5' long terminal repeat, resulting in glucocorticoid-regulatable RNA expression.

Expression of antimicrobial peptides has an antitumour effect in human cells

The antimicrobial peptides cecropin and melittin are known to exhibit antitumour activity in tumour derived cell lines. To achieve a similar effect in vivo these peptides would have to be given repeatedly to maintain therapeutic levels, which may be pharmacologically unfavourable. The expression of the genes encoding such antimicrobial peptides in the desired cell type may circumvent these problems. Expression constructs carrying cecropin or melittin have been introduced into a human bladder carcinoma derived cell line and the resultant cell clones analysed for tumorigenicity in nude mice. Expression of cecropin resulted in either a complete loss of tumorigenicity in some clones or reduced tumorigenicity, as measured by latency of tumour formation. These results suggest that vector mediated delivery of this gene to tumour cells may prove useful for cancer gene therapy.

Inducible expression of p21WAF-1/CIP-1/SDI-1 from a promoter conversion retroviral vector

Constitutive, high-level expression of the potentially therapeutic WAF-1/CIP-1/SDI-1 gene is incompatible with cell growth. A promoter conversion retroviral vector carrying the WAF-1/CIP-1/SDI-1 gene under the transcriptional control of the glucocorticoid inducible promoter of mouse mammary tumor virus was used to infect human bladder carcinoma or feline kidney cells. Reduced cell growth due to a greater proportion of cells being in the G0/G1 phase of the cell cycle was observed when WAF-1/CIP-1/SDI-1 expression was activated by addition of glucocorticoid hormone. This system demonstrates the potential long-term therapeutic use of WAF-1/CIP-1/SDI-1 delivered by retroviral vectors for inhibiting the growth of rapidly proliferating cells. Moreover, the conditional expression of genes such as WAF-1/CIP-1/SDI-1 from such retroviral vectors may facilitate analysis of their function.

Rapid identification of viable retrovirus-transduced cells using the green fluorescent protein as a marker

Various methods for determining the expression of the beta-galactosidase (beta-gal) gene after retroviral transduction were compared as a means to assess retroviral titre. To allow better comparison, different retroviral vectors were constructed carrying two mutants of the green fluorescent protein and assessed as sensitive markers of retroviral gene transfer. It could be demonstrated that GFP is generally superior to beta-gal in terms of sensitivity, speed and non-invasiveness of assay, allowing easy direct FACS sorting of populations of transduced cells. This opens the possibility of enrichment by sorting of ex vivo transduced cells in gene therapy protocols.

Retroviral vector targeting for gene therapy

The majority of gene therapy protocols have used or plan to use retroviral vectors based upon murine leukaemia virus. These vectors are able to infect many different cell types, and the retroviral promoter, which is often used to control the expression of a therapeutic gene, is active in a wide range of different cell types. Safe and efficient gene transfer systems, whether based upon retroviruses or other agents, should deliver beneficial genes only to cells that require their therapeutic action, and these genes ideally should be expressed exclusively in such cells. In this paper, strategies for redirecting the infection spectrum of retroviral vectors in order to obtain cell-targeted gene delivery are discussed. These strategies include the engineering of the retroviral envelope protein, which, together with the availability of its cognate receptor, determines infectivity, and the use of proteins from other enveloped viruses of both retroviral and nonretroviral origin in the cell lines used to produce retroviral vector virus particles. Expression targeting can be achieved by limiting the expression of therapeutic genes to the cell type(s) of interest using promoters from genes that are normally active in these cells. This approach to targeting is illustrated using promoters from genes expressed in either the liver, the pancreas or the mammary gland as a means to limit gene expression specifically to the cell types that make up these organs. The successful utilization of new generations of targeted retroviral vectors in the clinic may well pave the way for superior gene delivery systems of the future that seek out their target cell, delivering a therapeutic gene to and expressing it only in such cells.

Development of retroviral vectors as safe, targeted gene delivery systems

The transfer of genes of potential therapeutic benefit is presently being attempted in the clinic to treat a number of genetic and virally induced diseases. Many of these protocols use retroviral vectors derived from murine leukemia retroviruses as gene delivery systems. Although these viral delivery systems are well suited for this purpose, a number of their characteristics, some of which are discussed here, are still troublesome. Future retroviral vectors will incorporate nonretroviral features and will be tailored to desired needs for specific uses. These vectors will be safer, more efficient, and targeted in their delivery. Further, expression of the therapeutic genes carried will be limited to the specific target cell type. Some of the recent advances that have been made towards this goal are reviewed here.

Characterization of a protein that binds a negative regulatory element in the mammary-specific whey acidic protein promoter

Whey Acidic Protein (WAP) gene expression is restricted to the pregnant and lactating mammary gland. We have recently defined a negative regulatory element (NRE) in the WAP promoter which interacts with a factor (NBF) present in all nonWAP expressing cells (Kolb et al., 1994; J. Cell. Biochem. 56:245-261). Here we characterise this factor and show that although it is not related to a number of known transcription factors, including AP-1, NF-1 and SP-1, it may also be involved in controlling the expression from the mouse mammary tumour virus promoter. Three proteins that bind to the WAP-NRE have been identified, one of which is a 53kDa nuclear protein. This protein is present in nonWAP expressing cells, suggesting that it is responsible for limiting WAP expression to the pregnant and lactating mammary gland. This protein has been partially purified and its binding to the WAP-NRE is not appreciably affected by high salt concentrations.

Virus vector design in gene therapy

The success of current and future gene therapy approaches is largely dependent upon the vector systems used to carry the therapeutic genes. Issues of efficiency, specificity and safety of gene transfer play an important role. Currently, the best vector systems available are based upon Nature's natural gene transfer systems, the viruses. This review will compare the three most common viral vector systems, retroviral, adenoviral and adeno-associated viruses, highlighting problems and issues of design, as well as suggesting the direction that future developments could take.

Construction of retroviral vectors for targeted delivery and expression of therapeutic genes

Current gene therapy protocols take an ex vivo approach in which cells are removed from a patient, genetically modified and then reimplanted. However this kind of approach is both cumbersome and costly, requiring high tech facilities and is limited to cell types that can be easily cultured. The in vivo delivery of genes by retroviral vectors will greatly facilitate gene therapy protocols of the future. However before in vivo gene therapy becomes a reality a number of problems must be overcome. Ideally therapeutic genes should be delivered only to the relevant cell type and/or expressed in this cell type. Strategies are described that (I) limit therapeutic gene delivery, using pseudotyping or vectors based on retroviruses that show a restricted infection spectrum or (II) limit the expression of transferred genes by inclusion of tissue specific promoters or cis acting regulatory elements. The combination of some of these strategies should permit the construction of novel retroviral vectors that provide safe and targeted in vivo gene transfer.

Identification of three human sequences with viral superantigen-specific primers

The open reading frame (ORF) in the long terminal repeat (LTR) of mouse mammary tumor virus (MMTV) has recently been shown to encode multiple products including a negative acting factor (Naf) and a superantigen (Sag). Expression of superantigens from endogenous MMTV loci in the mouse results in the deletion of whole classes of T cells. In a PCR approach, with primers to the MMTV ORF and hybridization to MMTV specific probes, we have identified three human sequences. Direct sequencing of PCR products revealed that one of these products is related to a human autoantigen that is conserved among many species and is expressed in testes and sperm. The second sequence that we have identified is novel, and no evidence for expression of this sequence could be obtained. Finally, the third ORF-like sequence is a new member of a previously described family of human endogenous retroviruses (RTVL-I). This sequence is transcribed in several human cell lines, including B lymphoblastoid cells, and is thus the first demonstration that an RTVL-I-related sequence can be expressed. Taken together, these findings raise the intriguing possibility that the human genome contains superantigen-like sequences, some of which are also related to endogenous retroviruses, that may influence the T cell repertoire.

Negative-acting factor and superantigen are separable activities of the mouse mammary tumor virus long terminal repeat

The open reading frame contained within the long terminal repeat (LTR) of mouse mammary tumor virus encodes Naf, a negative regulator of transcription, as well as a superantigen activity, Sag, which causes the deletion of specific classes of T cells. In the present study, the effect of Naf expression on different promoters and the coding requirements for Naf and Sag have been investigated. Sag activity was found to require only sequences in the LTR, whereas sequences located within the gag gene were additionally required for functional Naf activity. Surprisingly, both the classic promoter and a recently described promoter located in the LTR can give rise to both functional Naf and Sag. Further analysis of Naf revealed that the downregulatory effect was mediated by sequences located in the LTR and that heterologous promoters were also affected by Naf.

Negative regulatory element in the mammary specific whey acidic protein promoter

Expression of the whey acidic protein (WAP) gene is tightly regulated in a tissue and developmental stage specific manner, in that the WAP gene is exclusively expressed in the mammary gland during pregnancy and lactation. Using both deletion and competition analyses, evidence is provided for the existence of a negative regulatory element (NRE) in the WAP promoter located between -413 and -93 with respect to the WAP transcriptional initiation site. This NRE dramatically decreases transcription from linked heterologous promoter-reporter gene constructs. The activity of NRE requires WAP promoter sequences that are 230 bp apart since subfragments of the NRE fail to inhibit transcription of adjoining reporter genes. Nuclear extracts from different cell types, in which the WAP gene is not active, contain a protein or complex that specifically interacts with the entire NRE but not with subfragments of it. The contact points between this protein (NRE binding factor [NBF]) and the NRE element have been partially determined. Mutation of the implicated nucleotides severely reduces the ability of NBF to bind, and such mutated promoter fragments fail to alleviate transcriptional repression in competition experiments. This suggests that NBF binding to the NRE is at least in part responsible for the negative regulation of the WAP promoter. Since NBF is not detectable in the lactating mammary gland, where the WAP gene is expressed, we speculate that it may be a determinant of the expression spectrum of the WAP gene.

Conditional expression of human TNF-alpha: a system for inducible cytotoxicity

Tumour necrosis factor-alpha (TNF-alpha) is currently being used in clinical trials for cancer treatment, but toxic side effects, due to systemic administration and high doses, are observed. Inducible expression of TNF may permit selective killing of tumour cells in gene therapy protocols without need for prolonged and/or high-level TNF expression. A conditional TNF expression vector has been constructed in which the coding sequences of human TNF have been placed under the transcriptional control of the glucocorticoid-regulated murine mammary tumour virus long terminal repeat (MMTV-LTR). Negligible levels of TNF expression, associated with no phenotypic alterations, are observed in cells transfected with MMTV-TNF vectors in the absence of glucocorticoid. Expression levels could be stimulated by the addition of the synthetic glucocorticoid dexamethasone. Increasing expression levels of TNF were associated with enhanced cytotoxicity. Our results suggest the potential use of inducible TNF systems for the treatment of tumours in gene therapy protocols.