Liposome encapsulation of retrovirus allows efficient superinfection of resistant cell lines

Adeno-Associated Virus Engineering and Load Strategy for Tropism Modification, Immune Evasion and Enhanced Transgene Expression

Gene therapy aims to add, replace or turn off genes to help treat disease. To date, the US Food and Drug Administration (FDA) has approved 14 gene therapy products. With the increasing interest in gene therapy, feasible gene delivery vectors are necessary for inserting new genes into cells. There are different kinds of gene delivery vectors including viral vectors like lentivirus, adenovirus, retrovirus, adeno-associated virus et al, and non-viral vectors like naked DNA, lipid vectors, polymer nanoparticles, exosomes et al, with viruses being the most commonly used. Among them, the most concerned vector is adeno-associated virus (AAV) because of its safety, natural ability to efficiently deliver gene into cells and sustained transgene expression in multiple tissues. In addition, the AAV genome can be engineered to generate recombinant AAV (rAAV) containing transgene sequences of interest and has been proven to be a safe gene vector. Recently, rAAV vectors have been approved for the treatment of various rare diseases. Despite these approvals, some major limitations of rAAV remain, namely nonspecific tissue targeting and host immune response. Additional problems include neutralizing antibodies that block transgene delivery, a finite transgene packaging capacity, high viral titer used for per dose and high cost. To deal with these challenges, several techniques have been developed. Based on differences in engineering methods, this review proposes three strategies: gene engineering-based capsid modification (capsid modification), capsid surface tethering through chemical conjugation (surface tethering), and other formulations loaded with AAV (virus load). In addition, the major advantages and limitations encountered in rAAV engineering strategies are summarized.

Materials promoting viral gene delivery

Therapeutic viral gene delivery is an emerging technology which aims to correct genetic mutations by introducing new genetic information to cells either to correct a faulty gene or to initiate cell death in oncolytic treatments. In recent years, significant scientific progress has led to several clinical trials resulting in the approval of gene therapies for human treatment. However, successful therapies remain limited due to a number of challenges such as inefficient cell uptake, low transduction efficiency (TE), limited tropism, liver toxicity and immune response. To adress these issues and increase the number of available therapies, additives from a broad range of materials like polymers, peptides, lipids, nanoparticles, and small molecules have been applied so far. The scope of this review is to highlight these selected delivery systems from a materials perspective.

Biopolymers augment viral vectors based gene delivery

The success of viral vectors mediated gene therapy is still hampered by immunogenicity and insufficient transgene expression. Alternatively, non-viral vectors mediated gene delivery has the advantage of low immunogenicity despite showing low transgene expression. By carefully considering the advantages of each approach, hybrid vectors are currently being developed by modifying the viral vectors using non-viral biopolymers. This review provides an overview of the hybrid vectors currently being developed.

Viral ecosystem: An epidemiological hypothesis

Viruses are incomplete elements that require other organisms to survive and multiply, hence constantly mutate during its evolution, resulting from adaptations in response to environmental changes such as the immune response of the host. In this line, they are responsible for many diseases, but today, there is evidence that viruses have many benefits and even have a unique ecosystem to control the different species or strain of themselves. While highlighting the benefits of some viruses and the undesirable effects of their eradication, the present review expresses the idea of the viral ecosystem and its importance, which has been supported in several studies. There are countless articles about virus-related illnesses and the undesirable effects of therapeutic interventions in eliminating the less pathogenic viruses or manipulating viral ecosystems. By simulating the viral ecosystem with an ecosystem found among the snakes, it can be assumed that the viruses have concentric zones, which its inner zone includes the most dangerous viruses for humans and each zone is surrounded and controlled by an outer zone of less dangerous viruses for humans. The outermost zone consists of viruses that are least dangerous to humans such as common cold that protect humans and possibly other living organisms against more dangerous viruses in inner zone, causing the activation of immune system by playing a unique and pivotal role in the ecosystems. Therefore, manipulating the ecosystem and disrupting the balance might have epidemics and harmful consequences for the plants, animals, and human.

Peptide nanofibrils as enhancers of retroviral gene transfer

Amyloid fibrils are polypeptide-based polymers that are typically associated with neurodegenerative disorders such as Alzheimer's disease. More recently, it has become clear that amyloid fibrils also fulfill functional roles in hormone storage and biosynthesis. Furthermore, it has been demonstrated that semen contains abundant levels of polycationic amyloid fibrils. The natural role of these seminal amyloids remains elusive. Strikingly, however, they drastically enhance HIV-1 infection and may be exploited by the virus to increase its sexual transmission rate. Their strong activity in enhancing HIV-1 infection suggests that seminal amyloid might also promote transduction by retroviral vectors. Indeed, SEVI (semen-derived enhancer of virus infection), the best characterized seminal amyloid, boosts retroviral gene transfer more efficiently than conventional additives. However, the use of SEVI as laboratory tool for efficient retroviral gene transfer is limited because the polypeptide monomers are relatively expensive to produce. Furthermore, standardized production of SEVI fibrils with similar high activities is difficult to achieve because of the stochastic nature of the amyloid assembly process. These obstacles can be overcome by recently identified smaller peptides that spontaneously self-assemble into nanofibrils. These nanofibrils increase retroviral gene transfer even more efficiently than SEVI, are easy to produce and to handle, and seem to be safe as assessed in an ex vivo gene transfer study. Furthermore, peptide-based nanofibrils allow to concentrate viral particles by low-speed centrifugation. Specific adaption and customization of self-assembling peptides might lead to novel nanofibrils with versatile biological functions, e.g., targeted retroviral gene transfer or drug delivery.

Nanoengineering artificial lipid envelopes around adenovirus by self-assembly

We have developed a novel, reproducible, and facile methodology for the construction of artificial lipid envelopes for adenoviruses (Ad) by self-assembly of lipid molecules around the viral capsid. No alteration of the viral genome or conjugation surface chemistry at the virus capsid was necessary, therefore difficulties in production and purification associated with generating most surface-modified viruses can be eliminated. Different lipid bilayer compositions produced artificially enveloped Ad with physicochemical and biological characteristics determined by the type of lipid used. Physicochemical characteristics such as vector size, degree of aggregation, stability, and surface charge of the artificially enveloped Ad were correlated to their biological (gene transfer) function. In monolayer cell cultures, binding to the coxsackie and adenovirus receptor (CAR) was blocked using a zwitterionic envelope, whereas enhanced binding to the cell membrane was achieved using a cationic envelope. Envelopment of Ad by both zwitterionic and cationic lipid bilayers led to almost complete ablation of gene expression in cell monolayers, due to blockage of virion endosomal escape. Alternatively, artificial Ad envelopes built from lipid bilayers at the fluid phase in physiological conditions led to enhanced penetration of the vectors inside a three-dimensional tumor spheroid cell culture model and delayed gene expression in the tumor spheroid compared to nonenveloped adenovirus. These results indicate that construction of artificial envelopes for nonenveloped viruses by lipid bilayer wrapping of the viral capsids constitutes a general strategy to rationally engineer viruses at the nanoscale with control over their biological properties.

High-efficiency retroviral transduction of mammalian cells on positively charged surfaces

The efficiency of retroviruses as transducing agents has been appreciated for many years, particularly for hematopoietic cell targets for which alternative strategies applicable to adherent cells are not effective. Advances in vector design, pseudotyping, and infection conditions have eliminated the need to cocultivate the target cells with virus-producing cells. Nevertheless, improvements are still needed for many applications, including those with a therapeutic or clinical cell-tracking objective. In this study we show that more positively charged surfaces, including those designed for the culture of anchorage-dependent cells, allow measurable levels of adhesion by different pseudotypes of retroviruses, which can result in increased gene transfer efficiencies to a variety of target cells including normal primary human hematopoietic cells as well as human leukemic cell lines and rat and murine fibroblasts. In the experiments with primary human cells, equal aliquots of enriched CD34+ cord blood cells were first stimulated for 2 days with cytokines (Flt3 ligand, Steel factor, IL-3, IL-6, and G-CSF) and then exposed for 4 days to a green fluorescent protein (GFP)- and Neo(r)-encoding retrovirus produced in PG13 cells. Both the final yield (approximately 300% relative to initial numbers), and the proportion (approximately 60%) of transduced CD34+ cells, colony-forming cells, and long-term culture-initiating cells were the same for cells infected either in tissue culture dishes or in fibronectin-coated petri dishes. Similar proportions (approximately 10%) and absolute yields of GFP+ human cells were also found in multilineage engrafted NOD/SCID mice assessed 6 to 8 weeks after being transplanted with these two types of transduced, but unselected, cells. These findings suggest a new and simpler approach for achieving high gene transfer efficiencies to hematopoietic cells.

Properties of human foamy virus relevant to its development as a vector for gene therapy

The Spumaviridae (foamy viruses) are increasingly being considered as potential vectors for gene therapy, yet little has been documented of their basic cell biology. This study demonstrates that human foamy virus (HFV) has a broad tropism and that the receptor for HFV is expressed not only on many mammalian, but on avian and reptilian cells. Receptor interference assays using an envelope-expressing cell line and a vesicular stomatitis virus/HFV pseudotype virus demonstrate that the cellular receptor is common to all primate members of the genus. The majority of foamy virus particles assemble and remain sequestered intracellularly. A rapid and quantitative method of assaying foamy virus infectivity by reverse transcriptase activity facilitates the use of classical protocols to increase infectious virus titres in vitro to > or = 10(6) TCID/ml.

Improvement of gene transduction efficiency in T lymphocytes using retroviral vectors

Successful gene transfer into T lymphocytes would provide a useful therapeutic modality for the treatment of various diseases and a valuable way to study T cell functions. Currently, most protocols involving gene transfer into T lymphocytes utilize amphotropic retroviral vectors. However, transduction efficiency using these vectors is relatively low because of the high proportion of resting cells, the concentration-dependent growth manner of T lymphocytes, and the low titer of retroviral vectors. In this article we define conditions that provide high levels of transduction by using IL-2 prestimulation and LipofectAMINE for both mouse and human T lymphocytes. We compared the effects of IL-2 prestimulation on transduction efficiencies at different time points and achieved maximum transfer levels at 72 hr after the incubation. By combining the best prestimulation time and cationic lipids-LipofectAMINE at a dose of 0.8 microM, the transduction efficiencies were increased to 45-75% (62.3 +/- 4.3%) in human T lymphocytes and to 21-33% (27 +/- 1.42%) in murine T lymphocytes as determine by FDG staining and X-Gal visualization, compared with 5% with conventional methods. These results indicate that transduction efficiencies in T lymphocytes can be significantly improved by a prolonged preincubation with IL-2 and by the addition of LipofectAMINE.

Lipofectin increases the specific activity of cypovirus particles for cultured insect cells

Cytoplasmic polyhedrosis viruses (CPV) are classified as 14 distinct species (electropherotypes) within the genus Cypovirus, family Reoviridae. Cypovirus research has been limited by a lack of appropriate cell culture systems (for each of these virus species) in which the majority of cells can become productively infected. Lipofection increased the infection rate of Lymantria dispar 652 cells, by virus particles (derived from polyhedra) of Orgyia pseudosugata type 5 cypovirus (Op-5 CPV), from 3 to 44%. Lipofection also significantly increased the percentage of Trichoplusia ni 368 cells infected with the same virus (from < 1 to approximately 7%). The spread of cypovirus infection between cells was either very slow or insignificant, and infected cells appeared to remain viable for long periods. Virus infection was detected by the observation of polyhedra formation in individual cells and it was therefore possible to develop a simple quantitative assay system to measure virus titre (TCID50). Cryo-electron microscopy showed that cypovirus particles formed a complex with the lipid, involving their envelopment within the liposome membrane. It was concluded that the increased infectivity of the virus by lipofection was due to a more efficient cell entry mechanism, probably involving fusion between liposome and cell membranes.

Cationic liposomes enhance the rate of transduction by a recombinant retroviral vector in vitro and in vivo

Cationic liposomes enhanced the rate of transduction of target cells with retroviral vectors. The greatest effect was seen with the formulation DC-Chol/DOPE, which gave a 20-fold increase in initial transduction rate. This allowed an efficiency of transduction after brief exposure of target cells to virus plus liposome that could be achieved only after extensive exposure to virus alone. Enhancement with DC-Chol/DOPE was optimal when stable virion-liposome complexes were preformed. The transduction rate for complexed virus, as for virus used alone or with the polycation Polybrene, showed first-order dependence on virus concentration. Cationic liposomes, but not Polybrene, were able to mediate envelope-independent transduction, but optimal efficiency required envelope-receptor interaction. When virus complexed with DC-Chol/DOPE was used to transduce human mesothelioma xenografts, transduction was enhanced four- to fivefold compared to that for virus alone. Since the efficacy of gene therapy is dependent on the number of cells modified, which is in turn dependent upon the balance between transduction and biological clearance of the vector, the ability of cationic liposomes to form stable complexes with retroviral vectors and enhance their rate of infection is likely to be important for in vivo application.

Cotranslational disassembly of flock house virus in a cell-free system

Intact, purified particles of the nodaviruses flock house virus and nodamura virus that were either transfected into cells that were resistant to infection or introduced into in vitro translation systems directed the synthesis of viral proteins. We infer that direct interaction of these nodavirus particles with cytoplasmic components mediated virion disassembly that resulted in release of the viral RNA.

GaLV pseudotyped vectors and cationic lipids transduce human CD34+ cells

High transduction frequency of hematopoietic stem/progenitor cells is essential to derive clinical benefits for treating certain inherited and acquired diseases. We demonstrate here stable gene transfer into human bone marrow-derived CD34+ progenitors using cationic lipids to facilitate GaLV and amphotroic pseudotyped retroviral-mediated transductions. Furthermore, the transgene was detected only in the progeny of flow cytometer sorted CD34+ population transduced by the LAPSN (PG13) viral vector in the presence of cationic lipids but not when transduction was facilitated with conventional polycations Polybrene or protamine sulfate. Thus, a combination of GaLV pseudotyped vectors and cationic lipids results in increased transduction frequencies of the CD34+ cells without a requirement of extended in vitro culture, or co-cultivation with producer cell lines. These improvements may result in the production of therapeutically significant quantities of genetically modified hematopoietic cells.

Gene therapy for central nervous system injury: the use of cationic liposomes: an invited review

This paper briefly reviews general principles of gene therapy with emphasis on the therapeutic potential of cationic liposome-mediated neurotrophin gene transfer to treat central nervous system (CNS) injury. Current developments in studies of gene therapy for CNS injury are both impressive and promising. Ex vivo gene transfer into the CNS is relatively mature in animal studies following more than a decade of experimental studies. In vivo gene transfer into the CNS has gained more attention recently. Although progress has been made using viral vectors, rapid advances in transfection technologies employing cationic liposomes, together with the relatively low toxicity of these nonviral vector systems, suggest that liposomes may have significant potential for clinical applications. Although many investigators have recognized that gene therapy may be useful for treatment of certain genetic defect diseases or cancer, gene therapy for CNS injury is relatively novel. In contrast to genetic defect disorders, temporary induction of transgenes may have therapeutic applications for CNS injuries such as stroke and trauma. Employing gene transfer techniques to achieve therapeutically useful levels of expression of neurotrophins in the CNS could provide a new strategy for treatment of the traumatically injured CNS.

The use of histone as a facilitator to improve the efficiency of retroviral gene transfer

Vectors based on murine C-type retroviruses are commonly used in biology. The efficiency of viral infection is normally increased by a facilitator, for example polybrene, DEAE-dextran or a liposome. The receptor for ecotropic viruses is a transporter for basic amino acids; we therefore explored the use of a highly basic protein, histone type IIA, as a facilitator. We show in several cell types that histone is as efficient as the other agents tested, and in some cases more so. This readily available reagent is thus likely to be useful in the wide range of studies that employ retroviral vectors.

Virosomes: cationic liposomes enhance retroviral transduction

Retrovirus-derived vectors are overwhelmingly preferred over other methods for ex vivo gene therapy because they provide permanent integration of foreign genes into cellular DNA. In comparison, cationic lipids mediate efficent gene transfer, but expression is transient. When we combined cationic lipids with retrovirus particles we obtained a significant enhancement of transduction efficiency, depending upon the type of lipid formulation and the dose used. The relative effectiveness of these cytofectins was: DOSPA:DOPE > DOTMA:DOPE > DOTAP, resulting in 60-, 37-, and 5-fold increases in transduction efficiency, respectively, at optimum dosage. The effect of polycationic DOSPA:DOPE was dependent upon the viral envelope glycoprotein, was attainable by lipid treatment of either cells or virus particles, was not enhanced by the addition of polybrene, and was inhibited by chloroquine. These results strongly suggested that DOSPA:DOPE act primarily by modulation of charge associated with the viral envelope and cell membrane, enhancing retroviral transduction, rather than by providing an alternative pathway of transfection. DOSPA:DOPE is useful for improving the efficiency of gene transfer as well as the sensitivity with which retroviruses can be detected in biological fluids.

Gene expression and active virus replication in the liver after injection of duck hepatitis B virus DNA into the peripheral vein of ducklings

BACKGROUND/AIMS

Duck hepatitis B virus is a member of the hepadnavirus family, which possesses strong hepatotropism. Duck hepatitis B virus DNA serves as a replicative template for producing biologically active virus particles after transfection into cell lines established from human hepatocellular carcinoma or into duck liver by direct injection of calcium phosphate-precipitated DNA. Our aim was to develop a new method of liver-specific gene expression after intravenous DNA delivery.

METHODS/RESULTS

We inoculated duck hepatitis B virus DNA with and without cationic liposomes, Lipofectin or LipofectAMINE, as DNA carries. Two weeks after a single intravenous injection of 10 or 50 micrograms of plasmid DNA containing a head-to-tail dimer of duck hepatitis B virus DNA into 25 one-day old ducklings, duck hepatitis B virus RNA transcripts including the pregenome replicative intermediate were detected by Northern blot in the liver of eight ducks (100%) of the Lipofectin group, five ducks (63%) of the LipofectAMINE group, and three ducks (50%) of the group which received DNA without carrier. Duck hepatitis B virus RNA transcription was almost exclusively liver specific, even though the liposomes had no tissue specificity. Replicative forms of duck hepatitis B virus DNA were detected in the liver and DHBsAg was observed in the cytoplasm of the hepatocytes by immunostaining. The serum of transfected ducklings contained virus particles which were infectious in other ducklings.

CONCLUSION

The efficient and liver-specific expression of inoculated DNA was due to the amplification of nucleic acids by active virus replication process under the control of hepatocyte specific regulation.

Gene therapy for brain tumors

Gene therapy has opened new doors for treatment of neoplastic diseases. This new approach seems very attractive, especially for glioblastomas, since treatment of these brain tumors has failed using conventional therapy regimens. Many different modes of gene therapy for brain tumors have been tested in culture and in vivo. Many of these approaches are based on previously established anti-neoplastic principles, like prodrug activating enzymes, inhibition of tumor neovascularization, and enhancement of the normally weak anti-tumor immune response. Delivery of genes to tumor cells has been mediated by a number of viral and synthetic vectors. The most widely used paradigm is based on the activation of ganciclovir to a cytotoxic compound by a viral enzyme, thymidine kinase, which is expressed by tumor cells, after the gene has been introduced by a retroviral vector. This paradigm has proven to be a potent therapy with minimal side effects in several rodent brain tumor models, and has proceeded to phase 1 clinical trials. In this review, current gene therapy strategies and vector systems for treatment of brain tumors will be described and discussed in light of further developments needed to make this new treatment modality clinically efficacious.

Introduction of hepatitis delta virus into animal cell lines via cationic liposomes

Cationic liposomes are known to facilitate efficient transfection of animal cells with DNA and even some viruses. As reported here, we have been able to use such a commercially available formulation (Lipofectamine) and introduce human hepatitis delta virus (HDV) into lines of cultured cells and demonstrate replication of the HDV genome both by immunofluorescence and by Northern (RNA) analysis. As much as 10% of the human hepatoma cell line Huh7 was transfected with HDV. Also transfected were the baby hamster kidney cell line BHK-21 and the Morris rat hepatoma line 7777. Two initial applications of HDV transfection have been made. (i) The ribonucleoprotein structure of HDV was isolated from disrupted virions and demonstrated as being sufficient to transfect Huh7 cells. In contrast, naked HDV RNA was not sufficient. (ii) From a study of cells transfected with HDV particles, it was found that, even after as long as 7 weeks and the associated replication of the transfected cells, the HDV RNA genome was still replicating. Apparently, HDV, in the absence of helper virus and in the absence of virus assembly, can maintain persistent replication and expression of the HDV genome. Transfection was also achieved with woodchuck hepatitis virus introduced into Huh7 cells. In summary, this transfection procedure should be of use for the study of these and maybe other recalcitrant animal viruses.

Efficient and sustained gene expression in primary T lymphocytes and primary and cultured tumor cells mediated by adeno-associated virus plasmid DNA complexed to cationic liposomes

We have used cationic liposomes to facilitate adeno-associated virus (AAV) plasmid transfections of primary and cultured cell types. AAV plasmid DNA complexed with liposomes showed levels of expression several fold higher than those of complexes with standard plasmids. In addition, long-term expression (> 30 days) of the gene, unlike the transient expression demonstrated by typical liposome-mediated transfection with standard plasmids, was observed. Southern analysis of chromosomal DNA further substantiated the hypothesis that the long-term expression was due to the presence of the transgene in the AAV plasmid-transfected group and not in the standard plasmid-transfected group. AAV plasmid-liposome complexes induced levels of transgene expression comparable to those obtained by recombinant AAV transduction. Primary breast, ovarian, and lung tumor cells were transfectable with the AAV plasmid DNA-liposome complexes. Transfected primary and cultured tumor cells were able to express transgene product even after lethal irradiation. High-level gene expression was also observed in freshly isolated CD3+, CD4+, and CD8+ T cells from normal human peripheral blood. Transfection efficiency ranged from 10 to 50% as assessed by intracellular interleukin-2 levels in interleukin-2-transfected cells. The ability to express transgenes in primary tumor and lymphoid cells may be applied toward tumor vaccine studies and protocols which may eventually permit highly specific modulation of the cellular immune response in cancer and AIDS.

Efficient and sustained gene expression in primary T lymphocytes and primary and cultured tumor cells mediated by adeno-associated virus plasmid DNA complexed to cationic liposomes

We have used cationic liposomes to facilitate adeno-associated virus (AAV) plasmid transfections of primary and cultured cell types. AAV plasmid DNA complexed with liposomes showed levels of expression several fold higher than those of complexes with standard plasmids. In addition, long-term expression (> 30 days) of the gene, unlike the transient expression demonstrated by typical liposome-mediated transfection with standard plasmids, was observed. Southern analysis of chromosomal DNA further substantiated the hypothesis that the long-term expression was due to the presence of the transgene in the AAV plasmid-transfected group and not in the standard plasmid-transfected group. AAV plasmid-liposome complexes induced levels of transgene expression comparable to those obtained by recombinant AAV transduction. Primary breast, ovarian, and lung tumor cells were transfectable with the AAV plasmid DNA-liposome complexes. Transfected primary and cultured tumor cells were able to express transgene product even after lethal irradiation. High-level gene expression was also observed in freshly isolated CD3+, CD4+, and CD8+ T cells from normal human peripheral blood. Transfection efficiency ranged from 10 to 50% as assessed by intracellular interleukin-2 levels in interleukin-2-transfected cells. The ability to express transgenes in primary tumor and lymphoid cells may be applied toward tumor vaccine studies and protocols which may eventually permit highly specific modulation of the cellular immune response in cancer and AIDS.

Liposomes for the transformation of eukaryotic cells

Gene therapy of human disease is a method of treatment under active development. DNA-loaded liposomes exhibit great promise for use in this field. Liposome-based transfection vectors have many inherent advantages that will likely lead to their wide in vivo use. Vectors with low toxicity and a high degree of targetability can now be easily prepared. These vectors are also free of the length constraints governing retroviral vectors. In this review we discuss recent developments in the use of liposomes for transfection of eukaryotic cells.

Cationic liposomes (Lipofectin) mediate retroviral infection in the absence of specific receptors

We have used cationic liposomes (Lipofectin) to facilitate retrovirus infection of cells lacking the homologous viral receptor. Ecotropic murine leukemia virus and packaged retroviral vectors were shown to infect mink cells, and amphotropic packaged retroviral vectors were shown to infect hamster cells in the presence of Lipofectin but not in the presence of Polybrene. Lipofectin-mediated infection of cells lacking the homologous receptor results in a titer approximately 0.1% of the titer in cells with the homologous receptor, using the standard Polybrene protocol. The use of Lipofectin may provide a simple means to experimentally infect a wide variety of cells with viruses not normally infectious for the species, tissue, or cell type of interest.

P21 v-ras inhibits induction of c-myc and c-fos expression by platelet-derived growth factor

The viral oncogene v-ras inhibited the platelet-derived growth factor (PDGF)-induced upregulation of c-myc and c-fos proto-oncogene expression in fibroblast monolayers. These v-ras-containing cells proliferated in the absence of c-myc induction and no longer required PDGF to support growth. Fibroblasts expressing v-ras continued to express the same number of functional PDGF receptors on their surface as uninfected cells, yet the usual induction of transcription of the genes c-myc, c-fos, and JE in response to PDGF stimulation did not occur in the presence of newly introduced v-ras or chronic v-ras gene expression, and synthesis of c-myc protein did not occur. This inhibitory effect on growth factor-mediated induction of cellular proto-oncogenes was specific for PDGF in that induction of the c-myc and c-fos genes by certain other factors was not impaired.

P21 v-ras inhibits induction of c-myc and c-fos expression by platelet-derived growth factor

The viral oncogene v-ras inhibited the platelet-derived growth factor (PDGF)-induced upregulation of c-myc and c-fos proto-oncogene expression in fibroblast monolayers. These v-ras-containing cells proliferated in the absence of c-myc induction and no longer required PDGF to support growth. Fibroblasts expressing v-ras continued to express the same number of functional PDGF receptors on their surface as uninfected cells, yet the usual induction of transcription of the genes c-myc, c-fos, and JE in response to PDGF stimulation did not occur in the presence of newly introduced v-ras or chronic v-ras gene expression, and synthesis of c-myc protein did not occur. This inhibitory effect on growth factor-mediated induction of cellular proto-oncogenes was specific for PDGF in that induction of the c-myc and c-fos genes by certain other factors was not impaired.

Immortalization of human endothelial cells by murine sarcoma viruses, without morphologic transformation

Amphotropic murine leukemia virus pseudotypes of murine sarcoma viruses containing the ras or mos oncogenes were constructed to permit efficient introduction of the sarcoma virus genome into early-passage human umbilical vein endothelial cells. The resulting cell lines were morphologically and phenotypically unchanged, retaining properties characteristic of differentiated endothelial cells. For example, the cells in a Kirsten sarcoma virus-modified line were found to biosynthesize and secrete von Willebrand factor in both a constitutive and regulated manner, and they contained ultrastructurally identifiable Weibel-Palade bodies, an endothelial cell-specific organelle. In contrast to the parent cultures, sarcoma virus-modified cells were able to proliferate indefinitely in culture. Examination of both Kirsten sarcoma and Moloney leukemia virus-modified lines indicated that the immortalized cells retained a diploid female karyotype after over 18 months in culture. In addition, the sarcoma virus-modified cells were able to grow independently of added endothelial cell growth factor. This growth factor autonomy does not appear to be due to autocrine production of a biologically cross-reactive growth factor. These immortal, virus-modified endothelial cells express large amounts of sarcoma virus-specific mRNA but no detectable helper virus or transforming virus activity. This technique for immortalization of primary human cells without alteration of the differentiated characteristics of the cell type is readily applied to a variety of human cell types. Moreover, the ability to separate the immortalizing and transforming activities of viral oncogenes should provide further understanding as to mechanisms of oncogene action.

Murine retroviruses control class I major histocompatibility antigen gene expression via a trans effect at the transcriptional level

Moloney murine leukemia virus (M-MuLV) and Moloney murine sarcoma virus (M-MSV) exert a regulatory effect on the class I genes of the murine major histocompatibility complex (MHC). We have previously shown that M-MuLV infection of mouse fibroblasts results in a substantial increase in cell surface expression of H-2K, H-2D, and H-2L proteins, whereas M-MSV, upon coinfection of the same cells, is apparently able to override the MuLV-induced increase in H-2 expression. As a result of this modulation, immune recognition of the infected cells is profoundly altered. Our efforts have been directed toward elucidating the molecular basis for this phenomenon. We report here that stimulation of interferon production as a result of infection with MuLV does not occur and, therefore, is not the cause of MuLV-induced enhancement of MHC expression. Control of H-2 class I and beta 2-microglobulin gene expression by M-MuLV, and probably by M-MSV, takes place at the transcriptional level as indicated by nuclear runoff studies and analysis of steady-state mRNA levels. Our demonstration that M-MuLV controls expression of widely separated endogenous cellular genes (those coding for H-2D, H-2K, H-2L, and beta 2-microglobulin), transfected class I MHC genes, and unintegrated chimeric genes consisting of fragments of class I MHC genes linked to sequences encoding a procaryotic enzyme, chloramphenicol acetyltransferase, suggests that M-MuLV exerts its effect in trans and not by proviral integration in the vicinity of the H-2 gene complex. Finally, we show that the sequences of at least one MHC gene, which are responsive to trans regulation by M-MuLV, lie within 1.2 kilobases upstream of the initiation codon for that gene.

Murine retroviruses control class I major histocompatibility antigen gene expression via a trans effect at the transcriptional level

Moloney murine leukemia virus (M-MuLV) and Moloney murine sarcoma virus (M-MSV) exert a regulatory effect on the class I genes of the murine major histocompatibility complex (MHC). We have previously shown that M-MuLV infection of mouse fibroblasts results in a substantial increase in cell surface expression of H-2K, H-2D, and H-2L proteins, whereas M-MSV, upon coinfection of the same cells, is apparently able to override the MuLV-induced increase in H-2 expression. As a result of this modulation, immune recognition of the infected cells is profoundly altered. Our efforts have been directed toward elucidating the molecular basis for this phenomenon. We report here that stimulation of interferon production as a result of infection with MuLV does not occur and, therefore, is not the cause of MuLV-induced enhancement of MHC expression. Control of H-2 class I and beta 2-microglobulin gene expression by M-MuLV, and probably by M-MSV, takes place at the transcriptional level as indicated by nuclear runoff studies and analysis of steady-state mRNA levels. Our demonstration that M-MuLV controls expression of widely separated endogenous cellular genes (those coding for H-2D, H-2K, H-2L, and beta 2-microglobulin), transfected class I MHC genes, and unintegrated chimeric genes consisting of fragments of class I MHC genes linked to sequences encoding a procaryotic enzyme, chloramphenicol acetyltransferase, suggests that M-MuLV exerts its effect in trans and not by proviral integration in the vicinity of the H-2 gene complex. Finally, we show that the sequences of at least one MHC gene, which are responsive to trans regulation by M-MuLV, lie within 1.2 kilobases upstream of the initiation codon for that gene.

Targeting of liposomes to cells bearing nerve growth factor receptors mediated by biotinylated nerve growth factor

We have used biologically active derivatives of beta-nerve growth factor (NGF), modified by biotinylation via carboxyl groups, to target the specific binding of liposomes to cultured rat and human tumor cells bearing NGF receptors. Liposomes, to be used for targeting, were prepared by conjugating streptavidin to phospholipid amino groups on liposomes prepared by reverse-phase evaporation. Approximately 2,000 streptavidin molecules were incorporated per liposome. Addition of biotinylated NGF, but not of unmodified NGF, could mediate the subsequent binding of radiolabeled streptavidin-liposomes to rat pheochromocytoma PC12 cells in suspension at 4 degrees C. In contrast, incubation with biotinylated NGF did not mediate the binding of hemoglobin-conjugated liposomes. Under optimal incubation conditions, approximately 570 streptavidin-liposomes were specifically bound per cell. Biotinylated NGF was also used to obtain specific binding of streptavidin-liposomes containing encapsulated fluorescein isothiocyanate-labeled dextran to PC12 cells or human melanoma HS294 cells. When HS294 cells were incubated at 37 degrees C following targeted liposome binding at 4 degrees C, the cell-associated fluorescence appeared to become internalized, displaying a perinuclear pattern of fluorescence similar to that observed when lysosomes were stained with acridine orange. Trypsin treatment abolished cell-associated fluorescence when cells were held at 4 degrees C but did not alter the fluorescence pattern in cells following incubation at 37 degrees C. When liposomes containing carboxyfluorescein, a dye capable of diffusing out of acidic compartments, were targeted to HS294 cells, subsequent incubation at 37 degrees C resulted in diffuse cytoplasmic fluorescence, suggesting that internalized liposomes encounter lysosomal or prelysosomal organelles.

Gene insertion into the chicken germ line by retroviruses

We injected chick syncytial strain of reticuloendotheliosis virus (CS-REV) and wild type and recombinant avian leukosis virus (ALV) near the blastoderm of unincubated fertilized embryos and CS-REV intra-abdominally at day of hatch, and we progeny tested the surviving ALV viremic males and REV viremic males and females for transmitted viral genetic material. A number of positive progeny were identified and their deoxyribonucleic acid (DNA) analyzed for restriction enzyme fragments that hybridized with viral genetic material. Most of the progeny had simple restriction enzyme patterns unlike the viremic parents or congenitally infected progeny. This is suggestive evidence that retroviral genetic information has been inserted into the germ line of chickens.