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

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.

Liposome-Mediated Herpes Simplex Virus Uptake Is Glycoprotein-D Receptor-Independent but Requires Heparan Sulfate

Cationic liposomes are widely used to facilitate introduction of genetic material into target cells during transfection. This study describes a non-receptor mediated herpes simplex virus type-1 (HSV-1) entry into the Chinese hamster ovary (CHO-K1) cells that naturally lack glycoprotein D (gD)-receptors using a commercially available cationic liposome: lipofectamine. Presence of cell surface heparan sulfate (HS) increased the levels of viral entry indicating a potential role of HS in this mode of entry. Loss of viral entry in the presence of actin de-polymerizing or lysosomotropic agents suggests that this mode of entry results in the endocytosis of the lipofectamine-virus mixture. Enhancement of HSV-1 entry by liposomes was also demonstrated in vivo using a zebrafish embryo model that showed stronger infection in the eyes and other tissues. Our study provides novel insights into gD receptor independent viral entry pathways and can guide new strategies to enhance the delivery of viral gene therapy vectors or oncolytic viruses.

Molecular Determinants of Vectofusin-1 and Its Derivatives for the Enhancement of Lentivirally Mediated Gene Transfer into Hematopoietic Stem/Progenitor Cells

Gene delivery into hCD34+ hematopoietic stem/progenitor cells (HSPCs) using human immunodeficiency virus, type 1-derived lentiviral vectors (LVs) has several promising therapeutic applications. Numerous clinical trials are currently underway. However, the efficiency, safety, and cost of LV gene therapy could be ameliorated by enhancing target cell transduction levels and reducing the amount of LV used on the cells. Several transduction enhancers already exist, such as fibronectin fragments or cationic compounds. Recently, we discovered Vectofusin-1, a new transduction enhancer, also called LAH4-A4, a short histidine-rich amphipathic peptide derived from the LAH4 family of DNA transfection agents. Vectofusin-1 enhances the infectivity of lentiviral and γ-retroviral vectors pseudotyped with various envelope glycoproteins. In this study, we compared a family of Vectofusin-1 isomers and showed that Vectofusin-1 remains the lead peptide for HSPC transduction enhancement with LVs pseudotyped with vesicular stomatitis virus glycoproteins and also with modified gibbon ape leukemia virus glycoproteins. By comparing the capacity of numerous Vectofusin-1 variants to promote the modified gibbon ape leukemia virus glycoprotein-pseudotyped lentiviral vector infectivity of HSPCs, the lysine residues on the N-terminal extremity of Vectofusin-1, a hydrophilic angle of 140° formed by the histidine residues in the Schiffer-Edmundson helical wheel representation, hydrophobic residues consisting of leucine were all found to be essential and helped to define a minimal active sequence. The data also show that the critical determinants necessary for lentiviral transduction enhancement are partially different from those necessary for efficient antibiotic or DNA transfection activity of LAH4 derivatives. In conclusion, these results help to decipher the action mechanism of Vectofusin-1 in the context of hCD34+ cell-based gene therapy.

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.

Vectofusin-1, a new viral entry enhancer, strongly promotes lentiviral transduction of human hematopoietic stem cells

Gene transfer into hCD34⁺ hematopoietic stem/progenitor cells (HSCs) using human immunodeficiency virus type 1 (HIV-1)-based lentiviral vectors (LVs) has several promising therapeutic applications. Yet, efficiency, safety, and cost of LV gene therapy could be ameliorated by enhancing target cell transduction levels and reducing the amount of LV used on the cells. Several transduction enhancers already exist such as fibronectin fragments and cationic compounds, but all present limitations. In this study, we describe a new transduction enhancer called Vectofusin-1, which is a short cationic peptide, active on several LV pseudotypes. Vectofusin-1 is used as a soluble additive to safely increase the frequency of transduced HSCs and to augment the level of transduction to one or two copies of vector per cell in a vector dose-dependent manner. Vectofusin-1 acts at the entry step by promoting the adhesion and the fusion between viral and cellular membranes. Vectofusin-1 is therefore a promising additive that could significantly ameliorate hCD34⁺ cell-based gene therapy.

Membrane fusion induced by small molecules and ions

Membrane fusion is a key event in many biological processes. These processes are controlled by various fusogenic agents of which proteins and peptides from the principal group. The fusion process is characterized by three major steps, namely, inter membrane contact, lipid mixing forming the intermediate step, pore opening and finally mixing of inner contents of the cells/vesicles. These steps are governed by energy barriers, which need to be overcome to complete fusion. Structural reorganization of big molecules like proteins/peptides, supplies the required driving force to overcome the energy barrier of the different intermediate steps. Small molecules/ions do not share this advantage. Hence fusion induced by small molecules/ions is expected to be different from that induced by proteins/peptides. Although several reviews exist on membrane fusion, no recent review is devoted solely to small moleculs/ions induced membrane fusion. Here we intend to present, how a variety of small molecules/ions act as independent fusogens. The detailed mechanism of some are well understood but for many it is still an unanswered question. Clearer understanding of how a particular small molecule can control fusion will open up a vista to use these moleucles instead of proteins/peptides to induce fusion both in vivo and in vitro fusion processes.

Cell surface receptors for gammaretroviruses

Evidence obtained during the last few years has greatly extended our understanding of the cell surface receptors that mediate infections of retroviruses and has provided many surprising insights. In contrast to other cell surface components such as lectins or proteoglycans that influence infections indirectly by enhancing virus adsorption onto specific cells, the true receptors induce conformational changes in the viral envelope glycoproteins that are essential for infection. One surprise is that all of the cell surface receptors for gamma-retroviruses are proteins that have multiple transmembrane (TM) sequences, compatible with their identification in known instances as transporters for important solutes. In striking contrast, almost all other animal viruses use receptors that exclusively have single TM sequences, with the sole proven exception we know of being the coreceptors used by lentiviruses. This evidence strongly suggests that virus genera have been prevented because of their previous evolutionary adaptations from switching their specificities between single-TM and multi-TM receptors. This evidence also implies that gamma-retroviruses formed by divergent evolution from a common origin millions of years ago and that individual viruses have occasionally jumped between species (zoonoses) while retaining their commitment to using the orthologous receptor of the new host. Another surprise is that many gamma-retroviruses use not just one receptor but pairs of closely related receptors as alternatives. This appears to have enhanced viral survival by severely limiting the likelihood of host escape mutations. All of the receptors used by gamma-retroviruses contain hypervariable regions that are often heavily glycosylated and that control the viral host range properties, consistent with the idea that these sequences are battlegrounds of virus-host coevolution. However, in contrast to previous assumptions, we propose that gamma-retroviruses have become adapted to recognize conserved sites that are important for the receptor's natural function and that the hypervariable sequences have been elaborated by the hosts as defense bulwarks that surround the conserved viral attachment sites. Previously, it was believed that binding to receptors directly triggers a series of conformational changes in the viral envelope glycoproteins that culminate in fusion of the viral and cellular membranes. However, new evidence suggests that gamma-retroviral association with receptors triggers an obligatory interaction or cross-talk between envelope glycoproteins on the viral surface. If this intermediate step is prevented, infection fails. Conversely, in several circumstances this cross-talk can be induced in the absence of a cell surface receptor for the virus, in which case infection can proceed efficiently. This new evidence strongly implies that the role of cell surface receptors in infections of gamma-retroviruses (and perhaps of other enveloped animal viruses) is more complex and interesting than was previously imagined. Recently, another gammaretroviral receptor with multiple transmembrane sequences was cloned. See Prassolov, Y., Zhang, D., Ivanov, D., Lohler, J., Ross, S.R., and Stocking, C. Sodium-dependent myo-inositol transporter 1 is a receptor for Mus cervicolor M813 murine leukemia virus.

Targeting retroviral and lentiviral vectors

Retroviral vectors capable of efficient in vivo gene delivery to specific target cell types or to specific locations of disease pathology would greatly facilitate many gene therapy applications. The surface glycoproteins of membrane-enveloped viruses stand among the choice candidates to control the target cell receptor recognition and host range of retroviral vectors onto which they are incorporated. This can be achieved in many ways, such as the exchange of glycoprotein by pseudotyping, their biochemical modifications, their conjugation with virus-cell bridging agents or their structural modifications. Understanding the fundamental properties of the viral glycoproteins and the molecular mechanism of virus entry into cells has been instrumental in the functional alteration of their tropism. Here we briefly review the current state of our understanding of the structure and function of viral envelope glycoproteins and we discuss the emerging targeting strategies based on retroviral and lentiviral vector systems.

Rescue of retroviral envelope fusion deficiencies by cationic liposomes

BACKGROUND

Retroviral particles that are inappropriately enveloped can transduce target cells if pre-associated with cationic liposomes. This study optimises and addresses the mechanism of liposome-enhanced gene delivery, and explores the potential for such agents to compensate for fusion deficiency associated with chimaeric envelope proteins.

METHODS

Particles bearing wild-type, chimaeric or no envelope proteins were complexed with DOTAP or DC-Chol/DOPE cationic liposomes and added to target cells for various times. Particle binding was determined by detection of cell-associated capsid protein and infectivity was measured histochemically.

RESULTS

Stable association of cationic liposomes with retrovirus particles significantly enhanced their binding rate to target cells in proportion to the increase of transduction kinetics for infectious virus. Binding of virus was equivalent with or without envelope protein and/or virus receptor, indicating that a non-specific interaction precedes receptor recognition. Non-infectious combinations were rescued by the intrinsic fusogenicity of the cationic liposomes, which enabled entry of the viral core, but left subsequent events unaltered. The optimised transduction rate with non-enveloped particles and DOTAP approached that of amphotropic-enveloped virus in some cases, although the effect was target-cell-dependent. DC-Chol/DOPE was less potent at direct fusion but was able to enhance 600-fold the receptor-dependent action of chimaeric envelopes that were deficient in fusion by virtue of the addition of targeting domains.

CONCLUSIONS

These data have implications for the development of retroviral vector targeting strategies from the perspectives of the specificity of target cell interaction and compensating for chimaeric envelope fusion deficiency.

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.

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.

Liposomes as a gene delivery system

Gene therapy is an active field that has progressed rapidly into clinical trials in a relatively short time. The key to success for any gene therapy strategy is to design a vector able to serve as a safe and efficient gene delivery vehicle. This has encouraged the development of nonviral DNA-mediated gene transfer techniques such as liposomes. Many liposome-based DNA delivery systems have been described, including molecular components for targeting given cell surface receptors or for escaping from the lysosomal compartment. Another recent technology using cationic lipids has been evaluated and has generated substantial interest in this approach to gene transfer.

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.

Retroviral stem cell gene therapy

Long-term in vivo gene transfer studies in mice have shown that recombinant murine retroviruses are able to infect murine hemopoietic stem cells with high efficiency. Taken together the results indicated that the proviral structure was present at high frequency in circulating hemopoietic cells resulting in significant expression levels. Because of the success of these murine studies, it was believed that gene therapy would soon be applicable to treat a wide variety of congenital or acquired human diseases associated with the hemopoietic system. However, results from gene transfer studies in nonhuman primates and first human clinical trails have indicated that murine retrovirus infection of primate hemopoietic stem cells is inefficient. Although there are essential differences between the murine and primate gene therapy studies with respect to the recombinant viruses and transduction protocols used, these differences cannot solely account for the differences observed in infection efficiency. Therefore, in recent years effort has been spent on the identification of factors limiting retroviral transduction of primate hemopoietic stem cells. Increasing knowledge concerning hemopoiesis and retroviral infection has helped in identifying a number of limiting factors. Novel transduction strategies and tools have been generated which attempt to circumvent these limiting factors. These factors as well as the strategies that showed increased retroviral infection of primate hemopoietic stem cells will be discussed.

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.

Characterization of retrovirus-induced SC-1 cell fusion

Virus-mediated cell-cell fusion with Moloney MLV and SC-1 cells was characterized. The level of fusion was highly dependent on the cell line used for propagation of the virus. Efficient fusion appeared to be very sensitive to negative charges on the cell surface and surroundings. Addition of polycations, removal of serum, and treatment with neuraminidase or hyaluronidase all stimulated fusion. Conversely, fusion was inhibited by fibronectin. Kinetic results and the time of action of inhibitors indicated that virus particles (or virus material) on the cell surface lead directly to fusion. The fusion then proceeded rapidly and required actin movement as shown by cytochalasin inhibition.

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.

Early interaction of feline calicivirus with cells in culture

The kinetics and biochemical properties of feline calicivirus (FCV) attachment to Crandell-Reese feline kidney cells were determined. Maximum binding was observed at pH 6.5. Cells in suspension at 4 degrees C bound virus more efficiently than cells in monolayers at 4 degrees C or 37 degrees C. High initial binding rate was observed in monolayers or cells in suspension and proceeded to a maximum at 90 min, although half maximal binding was observed as early as 15 min. Binding was specific and competitively blocked by serotypically homologous or heterologous FCV as well as by San Miguel sea lion virus. Treatment of cells with proteases increased FCV binding, whereas phospholipase had no effect on virus attachment. Conversely, cells treated with neuraminidase followed by O-glycanase treatment showed a decreased binding ability. Cells of feline origin bound FCV very efficiently, and non-permissive cells showed a poor binding ability. Following transfection of viral RNA, infectious virus could be recovered from all non-permissive cells, except from Madin-Darby canine kidney cells. These results suggest that FCV binds to a receptor in which carbohydrates may be an important component and that FCV replication in non-permissive cells is primarily restricted by the absence of appropriate receptors on the cell surface.

Retrovirus-mediated gene transfer to cystic fibrosis airway epithelial cells: effect of selectable marker sequences on long-term expression

Retrovirus-mediated gene transfer offers the potential for stable long-term expression of transduced genes in host cells subsequent to integration of vector DNA into the host genome. Using a murine amphotropic retrovirus vector containing an interleukin-2 receptor (IL-2R) gene as a reporter and a neomycin phosphotransferase (neor) gene as a dominant selectable marker, we measured the efficiency of retrovirus-mediated gene transfer and the stability of transduced gene expression in a cystic fibrosis tracheal epithelial cell line (CFT1). The use of the IL-2R cell surface marker as a reporter of infection permitted both quantitation of vector gene expression and flow cytometric sorting of cells transduced with the vector. In initial studies, the optimal conditions for retrovirus-mediated gene transfer were determined. The presence of a polycation was required for optimal transduction efficiency. The efficiency of infection of CFT1 cells was increased by repetitive exposure to virus such that it was possible to transduce approximately 80% of the cells following three successive daily exposures. The long-term stability of expression of the non-selected IL-2R gene was also evaluated. A slow decline in the percentage of cells expressing IL-2R was seen with cells that were maintained under constant selection pressure for expression of the neor gene, which was expressed from an internal promoter. Similar results were obtained when cultures were selected initially for neor gene expression and maintained without selection thereafter. In contrast, stable expression was observed in CFT1 cells for at least one year following multiple infections in the absence of G418 selection. In conclusion, (i) transduction of foreign genes into human airway epithelial cells using an amphotropic retrovirus vector can be highly efficient in the presence of appropriate polycations and multiple exposures; and (ii) stable expression of a non-selected gene in these epithelial cells is better maintained without selection.

Liposome-mediated transfection of intact viral particles reveals that plasma membrane penetration determines permissivity of tissue culture cells to rotavirus

Rotaviruses are an important cause of gastroenteritis in human infants. In vivo, rotavirus displays striking cell tropism with viral replication generally restricted to the villus tip enterocytes of the small intestine. We studied a panel of cell lines that vary significantly in their permissivity to rotavirus infection. L cells and HEp2 cells were relatively resistant to rotavirus infection compared with permissive Ma104 cells and HT29 cells. RNA transcription among the cell lines was proportional to antigen synthesis making a translational or posttranslational block an unlikely source of observed differences in susceptibility. All of the cell lines bound and internalized radiolabeled virus equally well, as measured by escape from surface protease treatment. Analysis of the escape of cell bound virus from neutralizing monoclonal antibody revealed that rotavirus did not immediately enter an eclipse phase in nonpermissive cells, but was internalized in an infectious form for several hours, possibly sequestered within endocytic vacuoles. L cells and HEp2 cells were as permissive as Ma104 and HT29 cells when rotavirus infection was mediated by transfection of single- or double-shelled rotavirus particles with cationic liposomes (Lipofectin). Rotavirus cell tropism in tissue culture cells is determined by the ability of infecting virions to traverse the plasma membrane of the cells into the cytoplasmic compartment.

Recombinant DNA approaches for the development of metabolic systems used in in vitro toxicology

In the past few years there has been considerable progress in the development of mammalian cell systems for use in genetic toxicology by the stable transfer of genes/cDNAs coding for drug metabolizing enzymes directly into the target cell. Alternative approaches have also been developed in which mammalian cells are transiently transfected with cDNAs coding for drug-metabolizing enzymes and S9 preparations expressing a single metabolizing enzyme isolated and used for metabolic activation. Progress in these areas is reviewed here and the relative merits of the different approaches are discussed. Work to date has focused primarily on the cytochrome P450 family of enzymes, although other enzyme systems involved in xenobiotic metabolism have been used. The central theme of this review is the transfer of genetic information to improve the metabolic capability of cell systems used in genetic toxicology. However, a basic philosophy of the review is that genetic manipulation of cultured mammalian cells has the potential for developing systems to be used to better understand chemically induced toxicological effects.