Efficient cell infection by Moloney murine leukemia virus-derived particles requires minimal amounts of envelope glycoprotein

Intact Viral Particle Counts Measured by Flow Virometry Provide Insight into the Infectivity and Genome Packaging Efficiency of Moloney Murine Leukemia Virus

Murine leukemia viruses (MLVs) have long been used as a research model to further our understanding of retroviruses. These simple gammaretroviruses have been studied extensively in various facets of science for nearly half a century, yet we have surprisingly little quantitative information about some of the basic features of these viral particles. These include parameters such as the genome packaging efficiency and the number of particles required for a productive infection. The reason for this knowledge gap relies primarily on the technical challenge of accurately measuring intact viral particles from infected cell supernatants. Virus-infected cells are well known to release soluble viral proteins, defective viruses, and extracellular vesicles (EVs) harboring viral proteins that may mimic viruses, all of which can skew virus titer quantifications. Flow virometry, also known as nanoscale flow cytometry or simply small-particle flow cytometry, is an emerging analytical method enabling high-throughput single-virus phenotypic characterizations. By utilizing the viral envelope glycoprotein (Env) and monodisperse light scattering characteristics as discerning parameters of intact virus particles, here, we analyzed the basic properties of Moloney MLV (M-MLV). We show that <24% of the total p30 capsid protein measured in infected cell supernatants is associated with intact viruses. We calculate that about one in five M-MLV particles contains a viral RNA genome pair and that individual intact particle infectivity is about 0.4%. These findings provide new insights into the characteristics of an extensively studied prototypical retrovirus while highlighting the benefits of flow virometry for the field of virology.IMPORTANCE Gammaretroviruses, or, more specifically, murine leukemia viruses (MLVs), have been a longstanding model for studying retroviruses. Although being extensively analyzed and dissected for decades, several facets of MLV biology are still poorly understood. One of the primary challenges has been enumerating total intact virus particles in a sample. While several analytical methods can precisely measure virus protein amounts, MLVs are known to induce the secretion of soluble and vesicle-associated viral proteins that can skew these measurements. With recent technological advances in flow cytometry, it is now possible to analyze viruses down to 90 nm in diameter with an approach called flow virometry. The technique has the added benefit of being able to discriminate viruses from extracellular vesicles and free viral proteins in order to confidently provide an intact viral particle count. Here, we used flow virometry to provide new insights into the basic characteristics of Moloney MLV.

Ligand density on nanoparticles: A parameter with critical impact on nanomedicine

Nanoparticles modified with ligands for specific targeting towards receptors expressed on the surface of target cells are discussed in literature towards improved delivery strategies. In such concepts the ligand density on the surface of the nanoparticles plays an important role. How many ligands per nanoparticle are best for the most efficient delivery? Importantly, this number may be different for in vitro and in vivo scenarios. In this review first viruses as "biological" nanoparticles are analyzed towards their ligand density, which is then compared to the ligand density of engineered nanoparticles. Then, experiments are reviewed in which in vitro and in vivo nanoparticle delivery has been analyzed in terms of ligand density. These results help to understand which ligand densities should be attempted for better targeting. Finally synthetic methods for controlling the ligand density of nanoparticles are described.

Latent murine leukemia virus infection characterized by the release of non-infectious virions

Clonal cell lines derived from cultures infected with a polytropic MuLV release vastly different levels of infectious virions ranging from undetectable to very high. Low producing clones release an overwhelming proportion of non-infectious virions containing retroviral RNA but deficient in the Env protein. Non-infectious virion production is not due to an inability of the cells to support infectious MuLV production or to an inherent replicative defectiveness of the proviruses. Reinfection of the lowest producing lines with the polytropic or an ecotropic MuLV results in enormous increases in the specific infectivity of the released virions. This indicates a reversible state of retroviral latency characterized by the release of non-infectious virions that is likely the result of insufficient levels of Env protein required for infectivity. The latency state described here may have important roles in in vivo retroviral infections including alterations of the immune response and the production of defective interfering particles.

Less is more: Ebola virus surface glycoprotein expression levels regulate virus production and infectivity

The Ebola virus (EBOV) surface glycoprotein (GP1,2) mediates host cell attachment and fusion and is the primary target for host neutralizing antibodies. Expression of GP1,2 at high levels disrupts normal cell physiology, and EBOV uses an RNA-editing mechanism to regulate expression of the GP gene. In this study, we demonstrate that high levels of GP1,2 expression impair production and release of EBOV virus-like particles (VLPs) as well as infectivity of GP1,2-pseudotyped viruses. We further show that this effect is mediated through two mechanisms. First, high levels of GP1,2 expression reduce synthesis of other proteins needed for virus assembly. Second, viruses containing high levels of GP1,2 are intrinsically less infectious, possibly due to impaired receptor binding or endosomal processing. Importantly, proteolysis can rescue the infectivity of high-GP1,2-containing viruses. Taken together, our findings indicate that GP1,2 expression levels have a profound effect on factors that contribute to virus fitness and that RNA editing may be an important mechanism employed by EBOV to regulate GP1,2 expression in order to optimize virus production and infectivity.

IMPORTANCE

The Ebola virus (EBOV), as well as other members of the Filoviridae family, causes severe hemorrhagic fever that is highly lethal, with up to 90% mortality. The EBOV surface glycoprotein (GP1,2) plays important roles in virus infection and pathogenesis, and its expression is tightly regulated by an RNA-editing mechanism during virus replication. Our study demonstrates that the level of GP1,2 expression profoundly affects virus particle production and release and uncovers a new mechanism by which Ebola virus infectivity is regulated by the level of GP1,2 expression. These findings extend our understanding of EBOV infection and replication in adaptation of host environments, which will aid the development of countermeasures against EBOV infection.

High-throughput screening of enzymes by retroviral display using droplet-based microfluidics

During the last 25 years, display techniques such as phage display have become very powerful tools for protein engineering, especially for the selection of monoclonal antibodies. However, while this method is extremely efficient for affinity-based selections, its use for the selection and directed evolution of enzymes is still very restricted. Furthermore, phage display is not suited for the engineering of mammalian proteins that require posttranslational modifications such as glycosylation or membrane anchoring. To circumvent these limitations, we have developed a system in which structurally complex mammalian enzymes are displayed on the surface of retroviruses and encapsulated into droplets of a water-in-oil emulsion. These droplets are made and manipulated using microfluidic devices and each droplet serves as an independent reaction vessel. Compartmentalization of single retroviral particles in droplets allows efficient coupling of genotype and phenotype. Using tissue plasminogen activator (tPA) as a model enzyme, we show that, by monitoring the enzymatic reaction in each droplet (by fluorescence), quantitative measurement of tPA activity in the presence of different concentrations of the endogenous inhibitor PAI-1 can be made on-chip. On-chip fluorescence-activated droplet sorting allowed the processing of 500 samples per second and the specific collection of retroviruses displaying active wild-type tPA from a model library with a 1000-fold excess of retroviruses displaying a non-active control enzyme. During a single selection cycle, a more than 1300-fold enrichment of the active wild-type enzyme was demonstrated.

Amphotropic retrovirus transduction is inhibited by high doses of particle-associated envelope proteins

Using a panel of amphotropic murine leukemia virus packaging cell lines that differed only in their levels of envelope protein (gp70) expression, we examined the relationship between transduction and the number of envelope proteins per virus. We generated virus stocks that contained different levels of virus-associated envelope proteins, purified them from gp70 that was not associated with the viruses, quantified their titers, and measured the efficiency with which they transduced NIH 3T3, TE671, and HeLa cells. As expected, titers increased monotonically with viral envelope protein number. Titers are measured using highly dilute virus, however, and are often not predictive of gene transfer when high doses of virus are used, as is done in gene therapy protocols. Interestingly, when we used high doses of virus, we observed significantly different trends: gene transfer increased, reached a maximum, and then declined sharply as the number of envelope proteins per virus increased. The highest levels of gene transfer occurred when cells were transduced with a moderate dose of virus that contained low levels of envelope protein. Our results indicate that transduction is inhibited when viruses that contain large numbers of envelope proteins are used. This is most likely because each virus, when it binds to a cell, delivers a large payload of envelope proteins that occupy or inactivate multiple virus receptors, reducing or eliminating the susceptibility of the cell to being transduced by additional viruses. The implications of our findings for the design of improved retroviral vectors for human gene therapy are discussed.

The MuLV 4070A G541R Env mutation decreases the stability and alters the conformation of the TM ectodomain

Virus-cell and cell-cell fusion events are affected by various properties of the fusogenic Env protein on the cell surface. The G541R mutation within the TM ectodomain of murine leukemia virus (MuLV) 4070A arose by positive selection in viral passage and results in a reduction of cell-cell fusion events while maintaining viral titer. Size exclusion chromatography shows that the multimerization properties are similar among expressed wild-type and mutant ectodomain peptides. Circular dichroism measurements reveal decreased thermal stability of the G541R mutant as compared to wild type. The G541R mutant also renders the peptide more susceptible to Lys-C protease cleavage. The 42-114 monoclonal antibody does not bind to the G541R mutant peptides, suggesting a structural difference from wild type. These altered physical properties result in productive viral infection of G541R bearing virus with decreased syncytia.

Complexation with chondroitin sulfate C and Polybrene rapidly purifies retrovirus from inhibitors of transduction and substantially enhances gene transfer

Using amphotropic retrovirus stocks produced by TELCeB6-A cells that encode the Escherichia coli lacZ gene, we found that complexation with chondroitin sulfate C (CSC) and Polybrene (PB) is an effective means to purify retrovirus. Virus stocks contained high levels of inhibitory activity that blocked amphotropic, but not ecotropic, retrovirus transduction. When virus stocks were brought to 80 microg/mL each of CSC and PB, complexes of CSC and PB formed. These complexes incorporated more than 70% of the virus particles but less than 0.4% of all other proteins and no detectable inhibitory activity. Purified virus transduced NIH 3T3 murine fibroblasts 21 to 186-fold more efficiently than virus that was not purified. In addition, virus purification significantly altered the dose response of transduction. When virus that had not been purified was used to transduce cells, the relationship between transduction and virus concentration was highly non-linear. In contrast, when purified virus was used, transduction increased monotonically and was linearly proportional to virus concentration, except when high doses of virus were used. Interestingly, when high doses of virus were used gene transfer reached a maximum plateau level, most likely because particle-associated amphotropic envelope proteins had saturated the cellular receptors for the virus. Our findings illustrate that retrovirus purification increases the maximum number of genes that can be transferred, reduces the amount of virus required to achieve a given level of gene transfer, and reduces uncertainties about the relationship between the amount of virus used and the number of genes transferred.

Intracellular Versus Cell Surface Assembly of Retroviral Pseudotypes Is Determined by the Cellular Localization of the Viral Glycoprotein, Its Capacity to Interact with Gag, and the Expression of the Nef Protein

Retroviral Gag and Env glycoproteins (GPs) are expressed from distinct cellular areas and need to encounter to interact and assemble infectious particles. Retroviral particles may also incorporate GPs derived from other enveloped viruses via active or passive mechanisms, a process known as "pseudotyping." To further understand the mechanisms of pseudotyping, we have investigated the capacity of murine leukemia virus (MLV) or lentivirus core particles to recruit GPs derived from different virus families: the G protein of vesicular stomatitis virus (VSV-G), the hemagglutinin from an influenza virus, the E1E2 glycoproteins of hepatitis C virus (HCV-E1E2), and the retroviral Env glycoproteins of MLV and RD114 cat endogenous virus. The parameters that influenced the incorporation of viral GPs onto retroviral core particles were (i) the intrinsic cell localization properties of both viral GP and retroviral core proteins, (ii) the ability of the viral GP to interact with the retroviral core, and (iii) the expression of the lentiviral Nef protein. Whereas the hemagglutinin and VSV-G glycoproteins were recruited by MLV and lentivirus core proteins at the cell surface, the HCV and MLV GPs were most likely recruited in late endosomes. In addition, whereas these glycoproteins could be passively incorporated on either retrovirus type, the MLV GP was also actively recruited by MLV core proteins, which, through interactions with the cytoplasmic tail of the latter GP, induced its localization to late endosomal vesicles. Finally, the expression of Nef proteins specifically enhanced the incorporation of the retroviral GPs by increasing their localization in late endosomes.

Cellular factors influencing Semliki Forest Virus vector biology

Viral vectors are currently the best tools for gene delivery in a therapeutic setting, especially for in vivo use. Alphaviruses, a family of positive singlestranded RNA viruses, have been engineered to allow the formation of a highly efficient replicon. Using these replicons, it is possible to generate recombinant particles. Parental viruses and recombinant vectors share certain pathways while interacting with their target cells. In this review, we describe the consecutive events leading to transduction, and transgene expression, in view of the cellular factors that affect each individual step. Classical virology will benefit from the knowledge accumulated studying vectors, and such work will shed light on crosstalk between intruding viruses and their hosts. Ultimately, these data should help the design of vectors adapted to specific target cells.

Transduction efficiency of pantropic retroviral vectors is controlled by the envelope plasmid to vector plasmid ratio

Pantropic retroviral vectors pseudotyped with vesicular stomatitis virus envelope G protein (VSV-G) are typically produced by transient transfection of the VSV-G expression plasmid because constitutive expression of VSV-G is cytotoxic. To produce pantropic vectors, the VSV-G expression plasmid and the vector plasmid are cotransfected into a packaging cell line, such as 293-gag-pol. Typically, the ratio of VSV-G plasmid to the vector plasmid ranges from 0.33 to 1.0. However, it is not clear that this range is optimal for vector production. In this study we have systematically examined the effect of the ratio of VSV-G plasmid (pVSV-G) to vector plasmid on vector production. For this, 293-gag-pol stable packaging cells were cotransfected with pVSV-G and an enhanced green fluorescent protein- (EGFP-) expressing retroviral vector plasmid (pLTR-EGFP) by use of lipofectamine. Vector was collected following transfection and used to transduce three target cell lines, namely, 3T3 fibroblasts, telomerase-immortalized human diploid fibroblasts (HDF), and the human hepatoma cell line HuH7. Transduction efficiency was evaluated for vectors produced at different pVSV-G:pLTR-EGFP ratios such that the total amount of plasmid transfected into 293-gag-pol cells was kept constant. Our results indicate that transduction efficiency is greatest when the pVSV-G:pLTR-EGFP ratio is substantially below 1.0. For 3T3 and HDF cells, the maximum transduction efficiency was obtained when a ratio of pVSV-G:pLTR-EGFP ranging from 0.053 to 0.2 was used for transfection. The relative magnitude of this effect was greater for lower transduction efficiencies in control cultures. For HuH7 cells, the beneficial effects were smaller than those observed when HDF or 3T3 cells were used. The difference in transduction efficiency for vector produced under various pVSV-G:pLTR-EGFP ratios was not due to differences in the proliferation of packaging cells or target cells. Further characterization showed that the amount of vector RNA relative to p30gag decreased as the ratio of pVSV-G:pLTR-EGFP increased. These results indicate that transduction efficiency increases with increasing levels of vector RNA as long as a minimally sufficient level of pantropic envelope protein is expressed.

Dual effect of the SR proteins ASF/SF2, SC35 and 9G8 on HIV-1 RNA splicing and virion production

In HIV-1 infected cells transcription of the integrated provirus generates the single full length 9 kb viral RNA, a major fraction of which is spliced to produce the single-spliced 4 kb RNAs and the multiple-spliced 2 kb RNAs. These spliced RNAs are the messengers for the Env glycoproteins and the viral regulatory factors. The cellular SR and hnRNP proteins were shown in vitro to control alternative splicing by binding cis-regulatory elements on the viral RNA. To better understand in vivo the role of the SR proteins on HIV-1 genomic RNA splicing and virion production, we used a human cell line expressing high levels of complete HIV-1 and either one of the ASF/SF2, SC35, and 9G8 SR proteins. Results show that over-expressing SR proteins caused a large reduction of genomic RNA and that each SR protein modified the viral 9 kb RNA splicing pattern in a specific mode. In fact, ASF/SF2 increased the level of Vpr RNA while SC35 and 9G8 caused a large increase in Tat RNA. As expected, overexpressing SR proteins caused a strong reduction of total Gag made. However, we observed by immuno-confocal microscopy an accumulation of Gag at the plasma membrane and in intracellular compartments while there is a dramatic reduction of Env protein made in most cells. Due to the negative impact of the SR proteins on the levels of genomic RNA and HIV-1 structural proteins much less virions were produced which retained part of their infectivity. In conclusion, SR proteins can down-regulate the late steps of HIV-1 replication.

Effects of virion surface gp120 density on infection by HIV-1 and viral production by infected cells

The quantity of envelope glycoprotein molecules (Env) on HIV-1 particles is still an issue of debate and, depending on the strain of virus and the nature of the producer cells, it can vary greatly. Here, we have attempted to address how Env density influences HIV-1 fitness. To this aim, we have produced HIV-1-derived viral particles with various amounts of R5 Env (low Env: Envlo; high Env: Envhi), using a regulatable expression system. The infectivity was assayed on human cells, engineered to express the HIV receptor CD4 and the co-receptor CCR5, as well as on peripheral blood lymphocytes and macrophages. In these experiments, low levels of Env were sufficient for cell infection, albeit at low efficiency. Increasing the amount of Env resulted in cooperatively improved infectivity, but a threshold was rapidly attained, indicating that only a fraction of Env was required for efficient infection. Unexpectedly, Env incorporation beyond what gives maximal infection transiently stimulated the expression of proviral genes, as well as retrovirus production, in newly infected cells. This was likely a consequence of induced NF-kappaB activity, as this transcription factor is triggered by Envhi, but not by Envlo, virions. Thus, our data suggest that one major effect of high Env density on the surface of HIV may not be better infection yields but rather improved viral production by newly infected cells.

High-yield retroviral production using a temperature-modulated two-stage operation

For clinical trials, large amounts of high-titer retroviral supernatants are required. However, retroviral concentration is relatively low compared with other viral vectors. Moreover, less than half of retroviral vectors suspended in a collected supernatant are infectious because of their short half-lives. In this study, a culture medium of ecotropic retrovirus-producing GP + E86/LNCX cells in tissue culture dishes was circulated through a reservoir, which was arranged with an incubator or ice-bath stage. Titers determined from the retroviral supernatant circulated through an ice-cold reservoir increased for a week from the beginning of retroviral production, while the titers from static production with circulation through the 37 degrees C reservoir reached a plateau after 3 days of retroviral production. After 5 days, 10 times more infectious retroviruses were obtained by circulating and keeping the majority of supernatant longer in the cold reservoir than in the production vessel at 37 degrees C in comparison with the number collected from the static tissue culture dish without circulating the culture medium. Furthermore, the concentration of transduction inhibitors in the supernatant was decreased along with the retardation of retroviral decay at low temperature. The two-stage operation developed in this study should be easily applied to large-scale bioreactors for mass production of high-titer retroviral supernatants.

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.

G541R within the 4070A TM protein regulates fusion in murine leukemia viruses

The mutation G541R within the ectodomain of TM was isolated in three independent chimeric enveloped murine leukemia virus (MuLV) viral populations originally impaired in viral passage and in wild-type 4070A. Isolation of G541R in multiple populations suggested it played a critical role in viral envelope function. Using a viral vector system, the observed effects of the G541R mutation within MuLV envelope proteins were pleiotropic and included effects on the regulation of SU-TM interactions and membrane fusion. G541R suppresses enhanced cell-cell fusion events attributable to the absence of the R-peptide yet does not adversely affect virus titers. The ability to suppress cell-cell fusion is dependent on the presence of the C terminus of the amphotropic 4070A SU protein. Within the wild-type 4070A envelope background, the mutation results in a decreased level of Env at the cell surface that is mirrored in the virion. The TM mutation alters recognition of the SU C terminus by a monoclonal antibody, suggestive of an altered conformation. The presence of G541R allowed the virus to achieve a balance between cytopathogenicity and replication and restored productive viral entry.

Monoclonal antibody 667 recognizes the variable region A motif of the ecotropic retrovirus CasBrE envelope glycoprotein and inhibits Env binding to the viral receptor

Monoclonal antibody (MAb) 667 is a neutralizing mouse monoclonal antibody recognizing the envelope glycoprotein (Env) of the ecotropic neurotropic murine retrovirus CasBrE but not that of other murine retroviruses. Since 667 can be used for preclinical studies of antiviral gene therapy as well as for studying the early events of retroviral infection, we have cloned its cDNAs and molecularly characterized it in detail. Spot technique-based experiments showed that 667 recognizes a linear epitope of 12 amino acids located in the variable region A of the receptor binding domain. Alanine scanning experiments showed that six amino acids within the epitope are critical for MAb binding. One of them, D(57), is not present in any other murine retroviral Env, which suggests a critical role for this residue in the selectivity of 667. MAb 667 heavy- and light-chain cDNAs were functionally characterized by transient transfection into Cos-7 cells. Enzyme-linked immunosorbent assays and Biacore studies showed that the specificities as well as the antigen-binding thermodynamic and kinetic properties of the recombinant 667 MAb (r667) produced by Cos-7 cells and those of the parental hybridoma-produced MAb (h667) were similar. However, h667 was shown to contain contaminating retroviral and/or retrovirus-like particles which interfere with both viral binding and neutralization experiments. These contaminants could successfully be removed by a stringent purification protocol. Importantly, this purified 667 could completely prevent retrovirus binding to target cells and was as efficient as the r667 MAb produced by transfected Cos-7 cells in neutralization assays. In conclusion, this study shows that the primary mechanism of virus neutralization by MAb 667 is the blocking of the retroviral receptor binding domain of CasBrE Env. In addition, the findings of this study constitute a warning against the direct use of hybridoma cell culture supernatants for studying the initial events of retroviral cell infection as well as for carrying out in vivo neutralization experiments and suggest that either recombinant antibodies or highly purified antibodies are preferable for these purposes.

Directed evolution of retroviruses activatable by tumour-associated matrix metalloproteases

Protease-activatable retroviral vectors offer the possibility of targeted gene transfer into cancer cells expressing a unique set of proteases as, for example, the matrix metalloproteases (MMPs). However, it is difficult to predict which substrate sequence will be optimally cleaved by a given tumour cell type. Therefore, we developed a novel approach that allows the selection of MMP-activatable retroviruses from libraries of viruses displaying combinatorially diversified protease substrates. Starting from a virus harbouring a standard MMP-2 substrate motif, after only two consecutive cycles of diversification and in vivo selection, MMP-activatable viruses were recovered. Biochemical characterization of the selected viruses revealed that their linker peptides showed a considerably increased sensitivity for MMP-2 cleavage, and interestingly also improved the particle incorporation rate of the Env protein. Owing to the optimized linker peptide, the selected viruses exhibited a greatly enhanced spreading efficiency through human fibrosarcoma cells, while having retained the dependency on MMP activation. Moreover, cell entry efficiency and virus titres were considerably improved as compared to the parental virus displaying the standard MMP-2 substrate. The results presented imply that retroviral protease substrate libraries allow the definition of MMP substrate specificities under in vivo conditions as well as the generation of optimally adapted tumour-specific viruses.

G100R mutation within 4070A murine leukemia virus Env increases virus receptor binding, kinetics of entry, and viral transduction efficiency

Passage of 4070A murine leukemia virus (MuLV) in D17 cells resulted in a G-to-R change at position 100 within the VRA of the envelope protein (Env). Compared with 4070A MuLV, virus with the G100R Env displayed enhanced binding on target cells, internalized the virus more rapidly, and increased the overall viral titer in multiple cell types. This provides a direct correlation between binding strength and efficiency of viral entry. Deletion of a His residue at the SU N terminus eliminated the transduction efficiency by the G100R virus, suggesting that the G100R virus maintains the regulatory characteristics of 4070A viral entry. The improved transduction efficiency of G100R Env would be an asset for gene delivery systems.

The insertion of an anti-MHC I ScFv into the N-terminus of an ecotropic MLV glycoprotein does not alter its fusiogenic potential on murine cells

It is known that targeted infection requires the modification of the viral envelope, in order to render it capable of recognizing and specifically binding to a marker protein of the target cell. We have previously described such a recombinant envelope, which is able to extend the tropism of an ecotropic murine leukemia viruses (MLV) envelope to MHC I-expressing human cells. Although, this envelope was very efficient in binding human cells, it yielded very low infection titers. Our attempts to improve these yields by the additional cloning of a variety of spacers in the proximity of the single-chain variable fragment (ScFv) moiety did not significantly influenced human titers, although some alterations on murine titers were observed. To examine whether these low yields represent a decreased fusion capacity of the recombinant envelopes, we performed an assay which allowed the direct comparison between the fusiogenicity of the wild-type (w/t) and the chimeric envelopes. No fusiogenicity of the chimeric envelopes was observed when chimera-expressing cells were co-cultured with human cells. The inability of the chimeras to induce fusion after binding of the ScFv moiety to its ligand may explain, in part, the low infection titers on human cells. However, the several-fold differences observed between the titers of the w/t envelope and the various chimeras on murine cells were not reflected on their fusiogenic potentials, which were all in the same order of magnitude. Our results demonstrate that the binding of the ScFv moiety to its ligand induces no fusion, albeit its insertion into the envelope does not alter the intrinsic fusiogenic ability of the latter. Induction of fusion results from the binding of the envelope to the ecotropic receptor, without being directly proportional to its binding affinity. Chimeras with different infection titers on murine cells yielded similar syncytia counts after their binding to the ecotropic receptor.

Skin as a potential organ for ectopic monoclonal antibody production

The therapeutic potential of monoclonal antibodies for treating a variety of severe or life-threatening diseases is high. Although intravenous infusion appears the simplest and most obvious mode of administration, it is not applicable to many long-term treatments. It might be advantageously replaced by gene/cell therapies, however, rendering treatments cost-effective and eliminating the short- and long-term side-effects associated with injection of massive doses of antibodies. We have tested whether skin can potentially be used as an organ for production and systemic delivery of ectopic antibodies. Normal human primary keratinocytes were shown to be capable of synthesis and secretion of a model monoclonal antibody directed against human thyroglobulin upon retroviral gene transduction in vitro. Neo- epidermis reconstructed in vitro, either in cell culture inserts or on dermal substrates, from such modified keratinocytes also produced the monoclonal antibody. Interestingly, the latter could cross the epidermis basal layer and be released in culture fluids. Finally, grafting of epidermis reconstituted in vitro on dermal substrates to SCID mice permitted sustained monoclonal antibody delivery into the bloodstream to be achieved. Our data thus show that genetically engineered keratinocytes can potentially be used for genetic antibody-based immunotherapies. They also indicate that proteins as big as 150 kDa, after release by engineered keratinocytes into skin intercellular spaces, can migrate to the general circulation, which is potentially important for a number of other gene-based therapies.

Modular organization of the Friend murine leukemia virus envelope protein underlies the mechanism of infection

Retrovirus infection is initiated by receptor-dependent fusion of the envelope to the cell membrane. The modular organization of the envelope protein of C type retroviruses has been exploited to investigate how binding of the surface subunit (SU) to receptor triggers fusion mediated by the transmembrane (TM) subunit. We show that deletion of the receptor-binding domain (RBD) from SU of Friend murine leukemia virus (Fr-MLV) abolishes infection that is restored by supplying RBD as a soluble protein. Infection by this mechanism remains dependent on receptor expression. When membrane attachment of the virus lacking RBD is reestablished by inserting the hormone erythropoietin, infection remains dependent on the RBD/receptor complex. However, infection increases 50-fold to 5 x 10(5) units/ml on cells that also express the erythropoietin receptor. Soluble RBD from Fr-MLV also restores infection by amphotropic and xenotropic MLVs in which RBD is deleted. These experiments demonstrate that RBD has two functions: mediating virus attachment and activating the fusion mechanism. In addition, they indicate that receptor engagement triggers fusion by promoting a subgroup-independent functional interaction between RBD and the remainder of SU and/or TM.