Factors affecting the direct targeting of murine leukemia virus vectors containing peptide ligands in the envelope protein

Directed Molecular Evolution of an Engineered Gammaretroviral Envelope Protein with Dual Receptor Use Shows Stable Maintenance of Both Receptor Specificities

We have previously reported the construction of a murine leukemia virus-based replication-competent gammaretrovirus (SL3-AP) capable of utilizing the human G protein-coupled receptor APJ (hAPJ) as its entry receptor and its natural receptor, the murine Xpr1 receptor, with equal affinities. The apelin receptor has previously been shown to function as a coreceptor for HIV-1, and thus, adaptation of the viral vector to this receptor is of significant interest. Here, we report the molecular evolution of the SL3-AP envelope protein when the virus is cultured in cells harboring either the Xpr1 or the hAPJ receptor. Interestingly, the dual receptor affinity is maintained even after 10 passages in these cells. At the same time, the chimeric viral envelope protein evolves in a distinct pattern in the apelin cassette when passaged on D17 cells expressing hAPJ in three separate molecular evolution studies. This pattern reflects selection for reduced ligand-receptor interaction and is compatible with a model in which SL3-AP has evolved not to activate hAPJ receptor internalization.

IMPORTANCE

Few successful examples of engineered retargeting of a retroviral vector exist. The engineered SL3-AP envelope is capable of utilizing either the murine Xpr1 or the human APJ receptor for entry. In addition, SL3-AP is the first example of an engineered retrovirus retaining its dual tropism after several rounds of passaging on cells expressing only one of its receptors. We demonstrate that the virus evolves toward reduced ligand-receptor affinity, which sheds new light on virus adaptation. We provide indirect evidence that such reduced affinity leads to reduced receptor internalization and propose a novel model in which too rapid receptor internalization may decrease virus entry.

Pros and Cons of Antigen-Presenting Cell Targeted Tumor Vaccines

In therapeutic antitumor vaccination, dendritic cells play the leading role since they decide if, how, when, and where a potent antitumor immune response will take place. Since the disentanglement of the complexity and merit of different antigen-presenting cell subtypes, antitumor immunotherapeutic research started to investigate the potential benefit of targeting these subtypes in situ. This review will discuss which antigen-presenting cell subtypes are at play and how they have been targeted and finally question the true meaning of targeting antitumor-based vaccines.

Library screening and receptor-directed targeting of gammaretroviral vectors

Gene- and cell-based therapies hold great potential for the advancement of the personalized medicine movement. Gene therapy vectors have made dramatic leaps forward since their inception. Retroviral-based vectors were the first to gain clinical attention and still offer the best hope for the long-term correction of many disorders. The fear of nonspecific transduction makes targeting a necessary feature for most clinical applications. However, this remains a difficult feature to optimize, with specificity often coming at the expense of efficiency. The aim of this article is to discuss the various methods employed to retarget retroviral entry. Our focus will lie on the modification of gammaretroviral envelope proteins with an in-depth discussion of the creation and screening of envelope libraries.

Retrovirus entry by endocytosis and cathepsin proteases

Retroviruses include infectious agents inducing severe diseases in humans and animals. In addition, retroviruses are widely used as tools to transfer genes of interest to target cells. Understanding the entry mechanism of retroviruses contributes to developments of novel therapeutic approaches against retrovirus-induced diseases and efficient exploitation of retroviral vectors. Entry of enveloped viruses into host cell cytoplasm is achieved by fusion between the viral envelope and host cell membranes at either the cell surface or intracellular vesicles. Many animal retroviruses enter host cells through endosomes and require endosome acidification. Ecotropic murine leukemia virus entry requires cathepsin proteases activated by the endosome acidification. CD4-dependent human immunodeficiency virus (HIV) infection is thought to occur via endosomes, but endosome acidification is not necessary for the entry whereas entry of CD4-independent HIVs, which are thought to be prototypes of CD4-dependent viruses, is low pH dependent. There are several controversial results on the retroviral entry pathways. Because endocytosis and endosome acidification are complicatedly controlled by cellular mechanisms, the retrovirus entry pathways may be different in different cell lines.

Construction of a gammaretrovirus with a novel tropism and wild-type replication kinetics capable of using human APJ as entry receptor

We have constructed a replication-competent gammaretrovirus (SL3-AP) capable of using the human G-protein-coupled receptor hAPJ as its entry receptor. The envelope protein of the virus was made by insertion of the 13-amino-acid peptide ligand for hAPJ, flanked by linker sequences, into one of the variable loops of the receptor binding domain of SL3-2, a murine leukemia virus (MLV) that uses the xenotropic-polytropic virus receptor Xpr1 and which has a host range limited to murine cells. This envelope protein can utilize hAPJ as well as murine Xpr1 for entry into host cells with equal efficiencies. In addition, the SL3-AP virus replicates in cells expressing either of its receptors, hAPJ and murine Xpr1, and causes resistance to superinfection and downregulation of hAPJ in infected cells. Thus, SL3-AP is the first example of a retargeted replication-competent retrovirus, with replication characteristics and receptor interference properties similar to those of natural isolates.

Targeted entry via somatostatin receptors using a novel modified retrovirus glycoprotein that delivers genes at levels comparable to those of wild-type viral glycoproteins

Here we report a novel viral glycoprotein created by replacing a natural receptor-binding sequence of the ecotropic Moloney murine leukemia virus envelope glycoprotein with the peptide ligand somatostatin. This new chimeric glycoprotein, which has been named the Sst receptor binding site (Sst-RBS), gives targeted transduction based on three criteria: (i) a gain of the use of a new entry receptor not used by any known virus; (ii) targeted entry at levels comparable to gene delivery by wild-type ecotropic Moloney murine leukemia virus and vesicular stomatitis virus (VSV) G glycoproteins; and (iii) a loss of the use of the natural ecotropic virus receptor. Retroviral vectors coated with Sst-RBS gained the ability to bind and transduce human 293 cells expressing somatostatin receptors. Their infection was specific to target somatostatin receptors, since a synthetic somatostatin peptide inhibited infection in a dose-dependent manner and the ability to transduce mouse cells bearing the natural ecotropic receptor was effectively lost. Importantly, vectors coated with the Sst-RBS glycoprotein gave targeted entry of up to 1 × 10(6) transducing U/ml, a level comparable to that seen with infection of vectors coated with the parental wild-type ecotropic Moloney murine leukemia virus glycoprotein through the ecotropic receptor and approaching that of infection of VSV G-coated vectors through the VSV receptor. To our knowledge, this is the first example of a glycoprotein that gives targeted entry of retroviral vectors at levels comparable to the natural capacity of viral envelope glycoproteins.

Effects of the ligand sequence modifications on the retargeted transduction by the retroviral vector having a ligand-chimeric Env protein

There have been various attempts to redirect the cell entry receptor tropism of the murine leukemia virus vectors. We have recently reported the successful retargeting of the ecotropic Moloney murine leukemia virus vector. This vector (S3-D84K) contains a viral envelope (Env) protein into which a full-length (68 aa) stromal cell-derived factor-1alpha (SDF-1alpha) was inserted at Pro-79. The S3-D84K vector transduces a certain human cell line through the CXC chemokine receptor 4 (CXCR4) at a titre of about 10(4) c.f.u. ml(-1). Here, the S3-D84K vector was found to transduce another human cell line through CXCR4 with a titre close to 10(6) c.f.u. ml(-1). The SDF-1alpha ligand of the S3-D84K Env protein was modified in different ways. In one, C-terminal truncations (by 3-51 aa) with or without a Cys-to-Gly change were performed, and in the other, Cys-to-Ala changes of the disulfide-forming cysteines without truncation were made. Seven truncation and three alanine mutant chimeric Env proteins were examined for virion incorporation, and the retroviral vectors displaying the mutant protein were examined for CXCR4 binding and retargeted transduction. Two mutant vectors showed transduction through CXCR4 with titres not higher than those of the S3-D84K vector, while the other mutant vectors minimally transduced cells through CXCR4 either due to a defect in virion incorporation of the chimeric Env protein or an inability to bind to CXCR4. These results suggest that a full-length sequence that may fold into a distinct domain within the chimeric Env protein is preferable as a targeting ligand.

Molecular engineering of viral gene delivery vehicles

Viruses can be engineered to efficiently deliver exogenous genes, but their natural gene delivery properties often fail to meet human therapeutic needs. Therefore, engineering viral vectors with new properties, including enhanced targeting abilities and resistance to immune responses, is a growing area of research. This review discusses protein engineering approaches to generate viral vectors with novel gene delivery capabilities. Rational design of viral vectors has yielded successful advances in vitro, and to an extent in vivo. However, there is often insufficient knowledge of viral structure-function relationships to reengineer existing functions or create new capabilities, such as virus-cell interactions, whose molecular basis is distributed throughout the primary sequence of the viral proteins. Therefore, high-throughput library and directed evolution methods offer alternative approaches to engineer viral vectors with desired properties. Parallel and integrated efforts in rational and library-based design promise to aid the translation of engineered viral vectors toward the clinic.

The block to membrane fusion differs with the site of ligand insertion in modified retroviral envelope proteins

Efforts to achieve cell type-specific transduction of retroviral vectors for gene therapy have centred on modification of the envelope protein (Env). Typically, addition of a ligand to Env gives binding to the new or target receptor, but little or no infection, and affects the subunit association of the modified Env. We previously discovered two point mutations that increase targeted infection by over 1000-fold when added to an Env modified by N-terminal insertion of the receptor-binding domain from amphotropic murine leukemia virus Env. Here, we asked whether these mutations would similarly increase transduction by Env modified with a clinically relevant ligand, human interleukin-13 (IL-13L). Addition of the point mutations stabilized the weak subunit association observed in some IL-13L-modified Env proteins, but infection via the target IL-13 receptor still did not occur. Fluorescence-based cell-cell fusion assays and studies with a membrane-curving agent revealed that defects in membrane fusion differed with the site of ligand insertion. When IL-13 was inserted into the N terminus of Env, membrane fusion was blocked prior to membrane-lipid mixing, regardless of whether flanking flexible linkers were added. Unexpectedly, insertion of IL-13 in the proline-rich region showed evidence of initiation of fusion and fusion-peptide exposure, but fusion was blocked at a subsequent step prior to fusion-pore formation. Thus, the site of ligand insertion influenced initiation of membrane fusion and its progression. These observations suggest that a novel site for ligand insertion must be identified before clinically useful targeted transduction will be achieved.

Purification of retrovirus particles using heparin affinity chromatography

Retroviral vectors have been used as gene delivery vehicles for more than two decades and continue to be the best available tool for stable and efficient transfer of therapeutic genes into various cell types. Although most gene therapy preclinical studies presently use crude or concentrated retroviral vector supernatants, purification to eliminate serum and host-derived impurities contained in these stocks will be a necessary requirement for clinical applications. Chromatography is deemed the most promising technology for large-scale purification of viral vectors. Heparin affinity chromatography offers the possibility to selectively and efficiently purify retroviruses. This chapter gives a simple, reproducible, and scaleable protocol for the purification of bioactive VSV-G pseudotyped retroviral vectors that employs membrane and chromatography technologies. The protocol can be easily adapted for the purification of different retroviral vector pseudotypes and lentiviral vectors. The purification techniques described here represent a significant improvement over the conventional sucrose density gradient methodology used for retrovirus purification and will hopefully contribute to the technological progress in the field of gene therapy.

Identification of host proteins associated with retroviral vector particles by proteomic analysis of highly purified vector preparations

The Moloney murine leukemia virus (MMLV) belongs to the Retroviridae family of enveloped viruses, which is known to acquire minute amounts of host cellular proteins both on the surface and inside the virion. Despite the extensive use of retroviral vectors in experimental and clinical applications, the repertoire of host proteins incorporated into MMLV vector particles remains unexplored. We report here the identification of host proteins from highly purified retroviral vector preparations obtained by rate-zonal ultracentrifugation. Viral proteins were fractionated by one-dimensional sodium dodecyl sulfate-polyacrylamide gel electrophoresis, in-gel tryptic digested, and subjected to liquid chromatography/tandem mass spectrometry analysis. Immunogold electron microscopy studies confirmed the presence of several host membrane proteins exposed at the vector surface. These studies led to the identification of 27 host proteins on MMLV vector particles derived from 293 HEK cells, including 5 proteins previously described as part of wild-type MMLV. Nineteen host proteins identified corresponded to intracellular proteins. A total of eight host membrane proteins were identified, including cell adhesion proteins integrin beta1 (fibronectin receptor subunit beta) and HMFG-E8, tetraspanins CD81 and CD9, and late endosomal markers CD63 and Lamp-2. Identification of membrane proteins on the retroviral surface is particularly attractive, since they can serve as anchoring sites for the insertion of tags for targeting or purification purposes. The implications of our findings for retrovirus-mediated gene therapy are discussed.

Ligand presentation on a synthetic flexible hinge in Moloney murine leukemia virus SU supports entry via a heterologous receptor

The envelope protein of Moloney murine leukemia virus mediates entry into mCAT-expressing cells. Attempts to change its receptor usage through the insertion of ligands at various sites have been met with varying success. We have tested several sites in Env for insertion of apelin, a small peptide ligand of the G-protein-coupled receptor APJ. Although most of the chimeric envelopes had retained their ability to infect mouse cells none showed APJ-dependent entry. Insertion of a peptide linker Ser-Gly-Gly-Ser-Gly at either side of the apelin motif in one of the chimeric envelopes resulted in an ability of the chimeric envelope to bind to and infect cells through APJ although with low efficiency. Several linker sequences isolated by library selection for APJ-dependent infection were found to support entry, however none more efficiently than the original SGGSG-linker. Hence, the immediate context of ligand presentation is critical for infectivity via a heterologous receptor.

Exploiting heparin-binding properties of MoMLV-based retroviral vectors for affinity chromatography

Chromatography is deemed the most promising technology for large-scale purification of viral vectors. The authors have previously shown that heparin affinity chromatography could be successfully employed for the purification of VSV-G pseudotyped Moloney murine leukemia virus (MoMLV)-derived vectors giving excellent results in terms of recovery of active particles, reproducibility and selectivity. In this study, the authors examined whether the ability of retrovirus particles to specifically bind to heparin ligands is restricted to VSV-G pseudotypes produced by 293-based packaging cells. It is shown that VSV-G deficient retrovirus particles are captured by a heparin chromatography column as efficiently as VSV-G containing particles. Most strikingly, RD114 pseudotyped retrovirus particles derived from a HT1080-based cell line were found to bind heparin with the same affinity as 293-derived VSV-G pseudotypes. RD114 pseudotyped retrovirus particles were successfully isolated using heparin affinity chromatography obtaining good recoveries of functional particles (43%). These results indicate that heparin affinity chromatography can be extended to the purification of retroviral vectors produced by different packaging cell lines independently of the Env-protein used for pseudotyping.

Redirecting coronavirus to a nonnative receptor through a virus-encoded targeting adapter

Murine hepatitis coronavirus (MHV)-A59 infection depends on the interaction of its spike (S) protein with the cellular receptor mCEACAM1a present on murine cells. Human cells lack this receptor and are therefore not susceptible to MHV. Specific alleviation of the tropism barrier by redirecting MHV to a tumor-specific receptor could lead to a virus with appealing properties for tumor therapy. To demonstrate that MHV can be retargeted to a nonnative receptor on human cells, we produced bispecific adapter proteins composed of the N-terminal D1 domain of mCEACAM1a linked to a short targeting peptide, the six-amino-acid His tag. Preincubation of MHV with the adapter proteins and subsequent inoculation of human cells expressing an artificial His receptor resulted in infection of these otherwise nonsusceptible cells and led to subsequent production of progeny virus. To generate a self-targeted virus able to establish multiround infection of the target cells, we subsequently incorporated the gene encoding the bispecific adapter protein as an additional expression cassette into the MHV genome through targeted RNA recombination. When inoculated onto murine LR7 cells, the resulting recombinant virus indeed expressed the adapter protein. Furthermore, inoculation of human target cells with the virus resulted in a His receptor-specific infection that was multiround. Extensive cell-cell fusion and rapid cell killing of infected target cells was observed. Our results show that MHV can be genetically redirected via adapters composed of the S protein binding part of mCEACAM1a and a targeting peptide recognizing a nonnative receptor expressed on human cells, consequently leading to rapid cell death. The results provide interesting leads for further investigations of the use of coronaviruses as antitumor agents.

Murine leukemia virus (MLV) replication monitored with fluorescent proteins

BACKGROUND

Cancer gene therapy will benefit from vectors that are able to replicate in tumor tissue and cause a bystander effect. Replication-competent murine leukemia virus (MLV) has been described to have potential as cancer therapeutics, however, MLV infection does not cause a cytopathic effect in the infected cell and viral replication can only be studied by immunostaining or measurement of reverse transcriptase activity.

RESULTS

We inserted the coding sequences for green fluorescent protein (GFP) into the proline-rich region (PRR) of the ecotropic envelope protein (Env) and were able to fluorescently label MLV. This allowed us to directly monitor viral replication and attachment to target cells by flow cytometry. We used this method to study viral replication of recombinant MLVs and split viral genomes, which were generated by replacement of the MLV env gene with the red fluorescent protein (RFP) and separately cloning GFP-Env into a retroviral vector. Co-transfection of both plasmids into target cells resulted in the generation of semi-replicative vectors, and the two color labeling allowed to determine the distribution of the individual genomes in the target cells and was indicative for the occurrence of recombination events.

CONCLUSIONS

Fluorescently labeled MLVs are excellent tools for the study of factors that influence viral replication and can be used to optimize MLV-based replication-competent viruses or vectors for gene therapy.

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.