Retargeting gene delivery using surface-engineered retroviral vector particles

Proteomic Analysis in Morquio A Cells Treated with Immobilized Enzymatic Replacement Therapy on Nanostructured Lipid Systems

Morquio A syndrome, or mucopolysaccharidosis type IVA (MPS IVA), is a lysosomal storage disease due to mutations in the N-acetylgalactosamine-6-sulfatase (GALNS) gene. Systemic skeletal dysplasia and the related clinical features of MPS IVA are due to disruption of cartilage and its extracellular matrix, leading to an imbalance of growth. Enzyme replacement therapy (ERT) with recombinant human GALNS, alpha elosulfase, provides a systemic treatment. However, this therapy has a limited impact on skeletal dysplasia because the infused enzyme cannot penetrate cartilage and bone. Therefore, an alternative therapeutic approach to reach the cartilage is an unmet challenge. We have developed a new drug delivery system based on a nanostructure lipid carrier with the capacity to immobilize enzymes used for ERT and to target the lysosomes. This study aimed to assess the effect of the encapsulated enzyme in this new delivery system, using in vitro proteomic technology. We found a greater internalization of the enzyme carried by nanoparticles inside the cells and an improvement of cellular protein routes previously impaired by the disease, compared with conventional ERT. This is the first qualitative and quantitative proteomic assay that demonstrates the advantages of a new delivery system to improve the MPS IVA ERT.

Lentiviral Vector Purification Using Genetically Encoded Biotin Mimic in Packaging Cell

Lentiviral vectors (LVs) have recently witnessed an increasing demand in research and clinical applications. Their current purification processes represent the main bottleneck in their widespread use, as the methods used are cumbersome and yield low recoveries. We aimed to develop a one-step method to specifically purify LVs, with high yields and reduced levels of impurities, using the biotin-streptavidin system. Herein, packaging HEK293T cells were genetically engineered with a cyclical biotin-mimicking peptide displayed on a CD8α stalk, termed cTag8. LVs were modified with cTag8 by its passive incorporation onto viral surfaces during budding, without viral protein engineering or hindrance on infectivity. Expression of cTag8 on LVs allowed complete capture of infectious particles by streptavidin magnetic beads. As cTag8 binds streptavidin in the nanomolar range, the addition of micromolar concentrations of biotin resulted in the release of captured LVs by competitive elution, with overall yields of ≥60%. Analysis of eluted LVs revealed high purity with a >3-log and 2-log reduction in DNA contamination and host cell proteins, respectively. This one-step purification was also tested for scalable vector processing using monolith affinity chromatography, with an encouraging preliminary overall yield of 20%. This method will be of valuable use for both research and clinical applications of LVs.

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.

Specific retrograde transduction of spinal motor neurons using lentiviral vectors targeted to presynaptic NMJ receptors

To understand how receptors are involved in neuronal trafficking and to be able to utilize them for specific targeting via the peripheral route would be of great benefit. Here, we describe the generation of novel lentiviral vectors with tropism to motor neurons that were made by coexpressing onto the lentiviral surface a fusogenic glycoprotein (mutated sindbis G) and an antibody against a cell-surface receptor (Thy1.1, p75(NTR), or coxsackievirus and adenovirus receptor) on the presynaptic terminal of the neuromuscular junction. These vectors exhibit binding specificity and efficient transduction of receptor positive cell lines and primary motor neurons in vitro. Targeting of each of these receptors conferred to these vectors the capability of being transported retrogradely from the axonal tip, leading to transduction of motor neurons in vitro in compartmented microfluidic cultures. In vivo delivery of coxsackievirus and adenovirus receptor-targeted vectors in leg muscles of mice resulted in predicted patterns of motor neuron labeling in lumbar spinal cord. This opens up the clinical potential of these vectors for minimally invasive administration of central nervous system-targeted therapeutics in motor neuron diseases.

Immunization delivered by lentiviral vectors for cancer and infectious diseases

The increasing level of understanding of the lentivirus biology has been instrumental in shaping the design strategy of creating therapeutic lentiviral delivery vectors. As a result, lentiviral vectors have become one of the most powerful gene transfer vehicles. They are widely used for therapeutic purposes as well as in studies of basic biology, due to their unique characteristics. Lentiviral vectors have been successfully employed to mediate durable and efficient antigen expression and presentation in dendritic cells both in vitro and in vivo, leading to the activation of cellular immunity and humoral responses. This capability makes the lentiviral vector an ideal choice for immunizations that target a wide range of cancers and infectious diseases. Further advances into optimizing the vector system and understanding the relationship between the immune system and diseases pathogenesis will only augment the potential benefits and utility of lentiviral vaccines for human health.

Selective gene silencing by viral delivery of short hairpin RNA

RNA interference (RNAi) technology has not only become a powerful tool for functional genomics, but also allows rapid drug target discovery and in vitro validation of these targets in cell culture. Furthermore, RNAi represents a promising novel therapeutic option for treating human diseases, in particular cancer. Selective gene silencing by RNAi can be achieved essentially by two nucleic acid based methods: i) cytoplasmic delivery of short double-stranded (ds) interfering RNA oligonucleotides (siRNA), where the gene silencing effect is only transient in nature, and possibly not suitable for all applications; or ii) nuclear delivery of gene expression cassettes that express short hairpin RNA (shRNA), which are processed like endogenous interfering RNA and lead to stable gene down-regulation. Both processes involve the use of nucleic acid based drugs, which are highly charged and do not cross cell membranes by free diffusion. Therefore, in vivo delivery of RNAi therapeutics must use technology that enables the RNAi therapeutic to traverse biological membrane barriers in vivo. Viruses and the vectors derived from them carry out precisely this task and have become a major delivery system for shRNA. Here, we summarize and compare different currently used viral delivery systems, give examples of in vivo applications, and indicate trends for new developments, such as replicating viruses for shRNA delivery to cancer cells.

Selective viral vector transduction of ErbB4 expressing cortical interneurons in vivo with a viral receptor-ligand bridge protein

Both treatment of disease and basic studies of complex tissues can benefit from directing viral vector infection to specific cell types. We have used a unique cell targeting method to direct viral vector transduction to cerebral cortical neurons expressing the neuregulin (NRG) receptor ErbB4; both NRG and ErbB4 have been implicated in schizophrenia, and ErbB4 expression in cerebral cortex is known to be restricted to inhibitory neurons. We find that a bridge protein composed of the avian viral receptor TVB fused to NRG, along with EnvB-pseudotyped virus, is able to direct infection selectively to ErbB4-expressing inhibitory cortical neurons in vivo. Interestingly, although ErbB4 is expressed in a broad range of cortical inhibitory cell types, NRG-dependent infection is restricted to a more selective subset of inhibitory cell types. These results demonstrate a tool that can be used for further studies of NRG and ErbB receptors in brain circuits and demonstrate the feasibility for further development of related bridge proteins to target gene expression to other specific cell types in complex tissues.

Lentiviral vectors for immune cells targeting

Lentiviral vectors (LVs) are efficient gene delivery vehicles suitable for delivering long-term transgene expression in various cell types. Engineering LVs to have the capacity to transduce specific cell types is of great interest to advance the translation of LVs toward the clinic. Here we provide an overview of innovative approaches to target LVs to cells of the immune system. In this overview we distinguish between two types of LV targeting strategies: (i) targeting of the vectors to specific cells by LV surface modifications, and (ii) targeting at the level of transgene transcription by insertion of tissue-specific promoters to drive transgene expression. It is clear that each strategy is of enormous value but ultimately combining these approaches may help reduce the effects of off-target expression and improve the efficiency and safety of LVs for gene therapy.

Generation of anti-TLR2 intrabody mediating inhibition of macrophage surface TLR2 expression and TLR2-driven cell activation

Background

Toll-like receptor (TLR) 2 is a component of the innate immune system and senses specific pathogen associated molecular patterns (PAMPs) of both microbial and viral origin. Cell activation via TLR2 and other pattern recognition receptors (PRRs) contributes to sepsis pathology and chronic inflammation both relying on overamplification of an immune response. Intracellular antibodies expressed and retained inside the endoplasmatic reticulum (ER-intrabodies) are applied to block translocation of secreted and cell surface molecules from the ER to the cell surface resulting in functional inhibition of the target protein. Here we describe generation and application of a functional anti-TLR2 ER intrabody (αT2ib) which was generated from an antagonistic monoclonal antibody (mAb) towards human and murine TLR2 (T2.5) to inhibit the function of TLR2. αT2ib is a scFv fragment comprising the variable domain of the heavy chain and the variable domain of the light chain of mAb T2.5 linked together by a synthetic (Gly₄Ser)₃ amino acid sequence.

Results

Coexpression of αT2ib and mouse TLR2 in HEK293 cells led to efficient retention and accumulation of TLR2 inside the ER compartment. Co-immunoprecipitation of human TLR2 with αT2ib indicated interaction of αT2ib with its cognate antigen within cells. αT2ib inhibited NF-κB driven reporter gene activation via TLR2 but not through TLR3, TLR4, or TLR9 if coexpressed in HEK293 cells. Co-transfection of human TLR2 with increasing amounts of the expression plasmid encoding αT2ib into HEK293 cells demonstrated high efficiency of the TLR2-αT2ib interaction. The αT2ib open reading frame was integrated into an adenoviral cosmid vector for production of recombinant adenovirus (AdV)-αT2ib. Transduction with AdVαT2ib specifically inhibited TLR2 surface expression of murine RAW264.7 and primary macrophages derived from bone marrow (BMM). Furthermore, TLR2 activation dependent TNFα mRNA accumulation, as well as TNFα translation and release by macrophages were largely abrogated upon transduction of αT2ib. αT2ib was expressed in BMM and splenocytes over 6 days upon systemic infection with AdVαT2ib. Systemic transduction applying AdVαT2ib rendered immune cells largely non-responsive to tripalmitoyl-peptide challenge. Our results show persistent paralysis of TLR2 activity and thus inhibition of immune activation.

Conclusion

The generated anti-TLR2 scFv intrabody inhibits specifically and very efficiently TLR2 ligand-driven cell activation in vitro and ex vivo. This indicates a therapeutic potential of αT2ib in microbial or viral infections.

Specific transduction of HIV-susceptible cells for CCR5 knockdown and resistance to HIV infection: a novel method for targeted gene therapy and intracellular immunization

HIV-1 gene therapy offers a promising alternative to small molecule antiretroviral treatments and current vaccination strategies by transferring, into HIV-1-susceptible cells, the genetic ability to resist infection. The need for novel and innovative strategies to prevent and treat HIV-1 infection is critical due to devastating effects of the virus in developing countries, high cost, toxicity, generation of escape mutants from antiretroviral therapies, and the failure of past and current vaccination efforts. As a first step toward achieving this goal, an HIV-1-susceptible cell-specific targeting vector was evaluated to selectively transfer, into CCR5-positive target cells, an anti-HIV CCR5 shRNA gene for subsequent knockdown of CCR5 expression and protection from HIV-1 infection. Using a ZZ domain/monoclonal antibody-conjugated Sindbis virus glycoprotein pseudotyped lentiviral vector, here we demonstrate the utility of this strategy for HIV-1 gene therapy by specifically targeting HIV-1-susceptible cells and engineering these cells to resist HIV-1 infection. CCR5-positive human cells were successfully and specifically targeted in vitro and in vivo for transduction by a lentiviral vector expressing a highly potent CCR5 shRNA which conferred resistance to HIV-1 infection. Here we report the initial evaluation of this targeting vector for HIV-1 gene therapy in a preexposure prophylactic setting.

Not gene therapy, but genetic surgery-the right strategy to attack cancer

In this review, I will suggest to divide all the approaches united now under common term "gene therapy" into two broad strategies of which the first one uses the methodology of targeted therapy with all its characteristics, but with genes in the role of agents targeted at a certain molecular component(s) presumably crucial for cancer maintenance. In contrast, the techniques of the other strategy are aimed at the destruction of tumors as a whole using the features shared by all cancers, for example relatively fast mitotic cell division or active angiogenesis. While the first strategy is "true" gene therapy, the second one is more like genetic surgery when a surgeon just cuts off a tumor with his scalpel and has no interest in knowing delicate mechanisms of cancer emergence and progression. I will try to substantiate the idea that the last strategy is the only right one, and its simplicity is paradoxically adequate to the super-complexity of tumors that originates from general complexity of cell regulation, strongly disturbed in tumor cells, and especially from the complexity of tumors as evolving cell populations, affecting also their ecological niche formed by neighboring normal cells and tissues. An analysis of the most widely used for such a "surgery" suicide gene/prodrug combinations will be presented in some more details.

Reduction of liver macrophage transduction by pseudotyping lentiviral vectors with a fusion envelope from Autographa californica GP64 and Sendai virus F2 domain

Background

Lentiviral vectors are well suited for gene therapy because they can mediate long-term expression in both dividing and nondividing cells. However, lentiviral vectors seem less suitable for liver gene therapy because systemically administered lentiviral vectors are preferentially sequestered by liver macrophages. This results in a reduction of available virus and might also increase the immune response to the vector and vector products.

Reduction of macrophage sequestration is therefore essential for efficient lentiviral liver gene therapy.

Results

Fusions were made of Autographa californica GP64 and the hepatocyte specific Sendai Virus envelope proteins. Lentiviral vectors were produced with either wild type GP64, Sendai-GP64, or both wild type GP64 and Sendai-GP64 and tested in vitro and in vivo for hepatocyte and macrophage gene transfer.

Sendai-GP64 pseudotyped vectors showed specific gene transfer to HepG2 hepatoma cells, with no detectable transduction of HeLa cervical carcinoma cells, and a decreased affinity for RAW mouse macrophages. Co-expression of wild type GP64 and Sendai-GP64 resulted in improved viral titers while retaining increased affinity for HepG2 cells.

In vivo, the Sendai-GP64 vectors also showed decreased transduction of murine liver macrophages.

Conclusion

We demonstrate reduced macrophage transduction in vitro and in vivo with GP64/Sendai chimeric envelope proteins.

Engineering fusogenic molecules to achieve targeted transduction of enveloped lentiviral vectors

Background

Lentiviral vectors with broad tropism are one of the most promising gene delivery systems capable of efficiently delivering genes of interest into both dividing and non-dividing cells while maintaining long-term transgene expression. However, there are needs for developing lentiviral vectors with the capability to deliver genes to specific cell types, thus reducing the "off-target" effect of gene therapy. In the present study, we investigated the possibility of engineering the fusion-active domain of a fusogenic molecule (FM) with the aim to improve targeted transduction of lentiviral vectors co-displaying an anti-CD20 antibody (αCD20) and a FM.

Results

Specific mutations were introduced into the fusion domain of a binding-deficient Sindbis virus glycoprotein to generate several mutant FMs. Lentiviral vectors incorporated with αCD20 and one of the engineered FMs were successfully produced and demonstrated to be able to preferentially deliver genes to CD-20-expressing cells. Lentiviral vectors bearing engineered FMs exhibited 8 to 17-fold enhanced transduction towards target cells as compared to the parental FM. Different levels of enhancement were observed for the different engineered FMs. A pH-dependent study of vector transduction showed that the broader pH range of the engineered FM is a possible mechanism for the resulted increase in transduction efficiency.

Conclusion

The fusion domain of Sindbis virus glycoprotein is amenable for engineering and the engineered proteins provide elevated capacity to mediate lentiviral vectors for targeted transduction. Our data suggests that application of such an engineering strategy can optimize the two-molecular targeting method of lentiviral vectors for gene delivery to predetermined cells.

Cell type-specific targeting with surface-engineered lentiviral vectors co-displaying OKT3 antibody and fusogenic molecule

PURPOSE

The purpose of this study was to investigate the potential of a T-cell-related targeting method using a lentiviral vector-based gene delivery system.

MATERIALS AND METHODS

A lentiviral vector system was constructed by co-incorporating an anti-CD3 antibody (OKT3) and a fusogen into individual viral particles. The incorporation of OKT3 and fusogen was analyzed using confocal microscopy and the in vitro transduction efficiency was evaluated using flow cytometry. Blocking reagents (ammonium chloride (NH(4)Cl) and soluble OKT3 antibody) were added into vector supernatants during transduction to study the mechanism of this two-molecule targeting strategy. To demonstrate the ability of targeted transduction in vivo, Jurkat.CD3 cells were xenografted subcutaneously into the right flank of each mouse and the lentiviral vector was injected subcutaneously on both sides of each mouse 8 h post-injection. Subsequently, the reporter gene (firefly luciferase) expression was monitored using a noninvasive bioluminescence imaging system.

RESULTS

By co-displaying OKT3 and fusogen on the single lentiviral surface, we could achieve targeted delivery of genes to CD3-positive T-cells both in vitro and in vivo.

CONCLUSIONS

These results suggest the potential utility of this engineered lentiviral system as a new tool for cell type-directed gene delivery.

Targeting lentiviral vectors to antigen-specific immunoglobulins

Gene transfer into B cells by lentivectors can provide an alternative approach to managing B lymphocyte malignancies and autoreactive B cell-mediated autoimmune diseases. These pathogenic B cell populations can be distinguished by their surface expression of monospecific immunoglobulin. Development of a novel vector system to deliver genes to these specific B cells could improve the safety and efficacy of gene therapy. We have developed an efficient method to target lentivectors to monospecific immunoglobulin-expressing cells in vitro and in vivo. We were able to incorporate a model antigen CD20 and a fusogenic protein derived from the Sindbis virus as two distinct molecules into the lentiviral surface. This engineered vector could specifically bind to cells expressing surface immunoglobulin recognizing CD20 (alphaCD20), resulting in efficient transduction of target cells in a cognate antigen-dependent manner in vitro, and in vivo in a xenografted tumor model. Tumor suppression was observed in vivo, using the engineered lentivector to deliver a suicide gene to a xenografted tumor expressing alphaCD20. These results show the feasibility of engineering lentivectors to target immunoglobulin- specific cells to deliver a therapeutic effect. Such targeting lentivectors also could potentially be used to genetically mark antigen-specific B cells in vivo to study their B cell biology.

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.

Visualization of targeted transduction by engineered lentiviral vectors

We have reported a method to target lentiviral vectors to specific cell types. This method requires the incorporation of two distinct molecules on the viral vector surface: one is an antibody that renders the targeting specificity for the engineered vector, and the other is a fusogenic protein that allows the engineered vector to enter the target cell. However, the molecular mechanism that controls the targeted infection needs to be defined. In this report, we tracked the individual lentiviral particles by labeling the virus with the GFP-Vpr fusion protein. We were able to visualize the surface-displayed proteins on a single virion as well as antibody-directed targeting to a desired cell type. We also demonstrated the dynamics of virus fusion with endosomes and monitored endosome-associated transport of viruses in target cells. Our results suggest that the fusion between the engineered lentivirus and endosomes takes place at the early endosome level, and that the release of the viral core into the cytosol at the completion of the virus-endosome fusion is correlated with the endosome maturation process. This imaging study sheds some light on the infection mechanism of the engineered lentivirus and can be beneficial to the design of more efficient gene delivery vectors.

Gamma-retroviral vectors enveloped with an antibody and an engineered fusogenic protein achieved antigen-specific targeting

Development of methods to engineer gamma-retroviral vectors capable of transducing target cells in a cell-specific manner could impact the future of the clinical application of gene therapy as well as the understanding of the biology of transfer gene vectors. Two molecular events are critical for controlling the entry of gamma-retroviral vectors to target cells: binding to cell-surface receptors and the subsequent fusion of viral vector membrane and cellular membrane. In this report, we evaluated a method to incorporate a membrane-bound antibody and a fusogenic molecule to provide binding and fusion functions respectively, into gamma-retroviral vectors for targeted gene delivery. An anti-CD20 antibody and a fusogenic protein derived from Sindbis virus glycoprotein could be efficiently co-displayed on the surface of viral vectors. Vectors bearing anti-CD20 antibody conferred their binding specificity to cells expressing CD20. Enhanced in vitro transduction towards CD20-expressing cells was observed for gamma-retroviral vectors displaying both an antibody and a fusogen. We found that the biological activity of the fusogen played an important role on the efficiency of such a targeting strategy and were able to engineer several mutant forms of the fusogen exhibiting elevated fusion function to improve the overall efficiency of targeted transduction. We devised an animal model to show that subcutaneous injection of such engineered vectors to the areas xenografted with target cells could achieve targeted gene delivery in vivo. Taken together, we demonstrated as proof-of-principle a flexible and modular two-molecule strategy for engineering targeting gamma-retroviral vectors.

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.

Lentiviruses inefficiently incorporate human parainfluenza type 3 envelope proteins

We have previously shown that the envelope glycoproteins of human parainfluenza type 3 (HPIV3), F and HN, are able to pseudotype lentiviruses, but the titers of these viruses are too low for use in clinical gene transfer. In this study we investigated the cause of these low titers. We compared the mRNA and protein expression levels of HN and F in transfected cells and in cells infected with wild-type HPIV3. Transfected cells contained similar levels of HN and F cytosolic mRNA, but fewer cell-surface HN and F proteins (3.8- and 1.3-fold less, respectively), than cells infected with wild-type HPIV3. To increase expression of HN in transfected cells, we codon-optimized HN and used it to transfect lentivirus producer cells. Cell surface expression of HN, as well as the amount of HN incorporated into virus particles, increased two- to threefold. Virus titers increased 1.2- to 6.4-fold, and the transduction efficiency of polarized MDCK cells via their apical surfaces increased 1.4-fold. Interestingly, even though codon optimization improved the expression levels of HN and virus titers, we found that HPIV3 pseudotyped viruses contained about 14-fold fewer envelope proteins than lentiviruses pseudotyped with the amphotropic envelope protein. Taken together, our findings suggest that titers are low, not because virus producer cells express levels of HPIV3 envelope proteins that are too low, but because too few of these proteins are incorporated by the lentiviruses for them to be able to efficiently transduce cells.

Fusion of enveloped virus nanoparticles with polyelectrolyte-supported lipid membranes for the design of bio/nonbio interfaces

Fusion of lipid-enveloped viruses with endosomal membranes triggered by low pH in the endosome is a key step in the course of viral infection. This ubiquitous mechanism can be used to integrate functional nanoparticles of viral origin into composite materials consisting of a polyelectrolyte multilayer with an adsorbed lipid membrane in a natural and biomimetic way. Polyelectrolyte multilayers as the support for the lipid membrane are a versatile means to combine the biological functions of the viral surface with the multiplicity of polyelectrolyte borne functions into a novel bio/nonbio composite material.

Viruses as building blocks for materials and devices

From the viewpoint of a materials scientist, viruses can be regarded as organic nanoparticles. They are composed of a small number of different (bio)polymers: proteins and nucleic acids. Many viruses are enveloped in a lipid membrane and all viruses do not have a metabolism of their own, but rather use the metabolic machinery of a living cell for their replication. Their surface carries specific tools designed to cross the barriers of their host cells. The size and shape of viruses, and the number and nature of the functional groups on their surface, is precisely defined. As such, viruses are commonly used in materials science as scaffolds for covalently linked surface modifications. A particular quality of viruses is that they can be tailored by directed evolution by taking advantage of their inbuilt colocalization of geno- and phenotypes. The powerful techniques developed by life sciences are becoming the basis of engineering approaches towards nanomaterials, opening a wide range of applications far beyond biology and medicine.

Targeted retroviral gene delivery using ultrasound

BACKGROUND

Achieving specificity of delivery represents a major problem limiting the clinical application of retroviral vectors for gene therapy, whilst lack of efficiency and longevity of gene expression limit non-viral techniques. Ultrasound and microbubble contrast agents can be used to effect plasmid DNA delivery. We therefore sought to evaluate the potential for ultrasound/microbubble-mediated retroviral gene delivery.

METHODS

An envelope-deficient retroviral vector, inherently incapable of target cell entry, was combined with cationic microbubbles and added to target cells. The cells were exposed to pulsed 1 MHz ultrasound for 5 s and subsequently analysed for marker gene expression. The acoustic pressure profile of the ultrasound field, to which transduction efficiency was related, was determined using a needle hydrophone.

RESULTS

Ultrasound-targeted gene delivery to a restricted area of cells was achieved using virus-loaded microbubbles. Gene delivery efficiency was up to 2% near the beam focus. Significant transduction was restricted to areas exposed to > or = 0.4 MPa peak-negative acoustic pressure, despite uniform application of the vector. An acoustic pressure-dependence was demonstrated that can be exploited for targeted retroviral transduction. The mechanism of entry likely involves membrane perturbation in the vicinity of oscillating microbubbles, facilitating fusion of the viral and cell membranes.

CONCLUSIONS

We have established the basis of a novel retroviral vector technology incorporating favourable aspects of existing viral and non-viral gene delivery vectors. In particular, transduction can be controlled by means of ultrasound exposure. The technology is ideally suited to targeted delivery following systemic vector administration.

Redirecting retroviral tropism by insertion of hepatitis B virus PreS1 core peptide into the envelope

A potentially powerful approach for in vivo gene delivery is to target retroviral vectors to specific cells through interactions between cell surface receptors and appropriately engineered viral envelope proteins, but this has so far met with little success. We report here an attempt to target ecotropic MLV retroviral vectors to human cells infected by hepatitis B virus (HBV) through the interaction between the HBV PreS1 domain and the receptors on the cell surface. We examined 7 MLV chimeric envelope derivatives that contained either the HBV PreS1 peptide or PreS1 aa 21-47 segment (partial PreS1, pPreS1), which was inserted at various locations within the SU of the MoMLV Env. In addition to infecting host NIH 3T3 cells, some of pseudotyped viruses could transduce human cells. Our results demonstrate that short peptide ligands inserted at appropriate locations in the MLV envelope can selectively target retroviruses to human cells.

Blocking translocation of cell surface molecules from the ER to the cell surface by intracellular antibodies targeted to the ER

Intracellular antibodies (intrabodies) constitute a potent tool to neutralize the function of target proteins inside specific cell compartments (cytosol, nucleus, mitochondria and ER). The intrabody technology is an attractive alternative to the generation of gene-targeted knockout animals and complements or replaces knockdown techniques such as antisense-RNA, RNAi and RNA aptamers. This article focuses on intrabodies targeted to the ER. Intracellular anti-bodies expressed and retained inside the ER (ER intrabodies) are shown to be highly efficient in blocking the translocation of secreted and cell surface molecules from the ER to the cell surface.The advantage of ER intrabodies over cytoplasmic intrabodies is that they are correctly folded and easier to select. A particular advantage of the intrabody technology over existing ones is the possibility of inhibiting selectively post-translational modifications of proteins.The main applications of ER intrabodies so far have been (i) inactivation of oncogenic receptors and (ii) functional inhibition of virus envelope proteins and virus-receptor molecules on the surface of host cells.In cancer research, the number of in vivo mouse models for evaluation of the therapeutic potential of intrabodies is increasing.In the future, endosomal localized receptors involved in bacterial and viral infections, intracellular oncogenic receptors and enzymes involved in glycosylation of tumour antigens might be new targets for ER intrabodies.

Primary attachment of murine leukaemia virus vector mediated by particle-associated heparan sulfate proteoglycan

Target cell entry of murine leukaemia virus vectors proceeds via primary attachment, independent of the viral envelope protein and subsequent envelope-receptor interaction. Although much attention has been paid to modifying the latter for target cell specificity, the initial binding interaction has been overlooked, despite its opposing involvement both in providing the virus available for receptor binding and in depleting free virus. As a first step towards modifying primary attachment, both to provide specificity and to enhance vector availability, we sought to determine the nature of this interaction. Following an initial screen of GAGs (glycosaminoglycans) for their ability to inhibit virus binding and transduction, we have shown that production of virus from cells in which GAG sulfation is inhibited, or treatment of virus with heparinase III, reduces both particle attachment and infection. Detection in purified virus preparations of a neo-epitope generated by heparinase III confirmed the presence of virus-associated HSPG [HS (heparan sulfate) proteoglycan], acquired from the producer cell. We propose that host-acquired cell-surface HSPG (potentially including syndecan-2) provides a means of virus attachment to target cells that precedes specific receptor interaction and membrane fusion. Inhibition of HS biosynthesis may provide a sufficiently reduced background of primary binding such that novel mechanisms of attachment, ideally with appropriate target cell specificity, can be introduced.

The use of a tropism-modified measles virus in folate receptor-targeted virotherapy of ovarian cancer

PURPOSE

Attenuated measles viruses are promising experimental anticancer agents currently being evaluated in a phase I dose escalation trial for ovarian cancer patients. Virus attachment, entry, and subsequent intercellular fusion between infected and uninfected neighboring cells are mediated via the two measles receptors (CD46 and SLAM). To minimize potential toxicity due to measles virus-associated immunosuppression and infection of nontarget tissues, we sought to develop an ovarian cancer exclusive fully retargeted measles virus.

EXPERIMENTAL DESIGN AND RESULTS

Interactions of measles virus with its natural receptors were ablated, and a single-chain antibody (scFv) specific for alpha-folate receptor (FRalpha), a target overexpressed on 90% of nonmucinous ovarian cancer, was genetically engineered on the viral attachment protein (MV-alphaFR). Specificity of virus tropism was tested on tumor and normal cells. Biodistribution of measles virus infection was evaluated in measles-susceptible CD46 transgenic mice, whereas antitumor activity was monitored noninvasively by bioluminescence imaging in xenograft models. Tropism and fusogenic activity of MV-alphaFR was redirected exclusively to FRalpha without compromise to virus infectivity. In contrast to the parental virus, MV-alphaFR has no background infectivity on normal human cells. The antitumor activity of MV-alphaFR, as assessed by tumor volume reduction and overall survival increase, was equal to the parental virus in two models of human ovarian cancer (s.c. and i.p.).

CONCLUSIONS

A FR-exclusive ovarian cancer targeted oncolytic virus was generated and shown to be therapeutically effective, thus introducing a new modality for FR targeting and a candidate measles virus for clinical testing.

The effects of N-terminal insertion into VSV-G of an scFv peptide

Recombinant retroviruses, including lentiviruses, are the most widely used vectors for both in vitro and in vivo stable gene transfer. However, the inability to selectively deliver transgenes into cells of interest limits the use of this technology. Due to its wide tropism, stability and ability to pseudotype a range of viral vectors, vesicular stomatitis virus G protein (VSV-G) is the most commonly used pseudotyping protein. Here, we attempted to engineer this protein for targeting purposes. Chimaeric VSV-G proteins were constructed by linking a cell-directing single-chain antibody (scFv) to its N-terminal. We show that the chimaeric VSV-G molecules can integrate into retroviral and lentiviral particles. HIV-1 particles pseudotyped with VSV-G linked to an scFv against human Major Histocompatibility Complex class I (MHC-I) bind strongly and specifically to human cells. Also, this novel molecule preferentially drives lentiviral transduction of human cells, although the titre is considerably lower that viruses pseudotyped with VSV-G. This is likely due to the inefficient fusion activity of the modified protein. To our knowledge, this is the first report where VSV-G was successfully engineered to include a large (253 amino acids) exogenous peptide and where attempts were made to change the infection profile of VSV-G pseudotyped vectors.

A New Generation of Retroviral Producer Cells: Predictable and Stable Virus Production by Flp-Mediated Site-Specific Integration of Retroviral Vectors

We developed a new strategy that provides well-defined high-titer producer cells for recombinant retroviruses in a minimum amount of time. The strategy involves the targeted integration of the retroviral vector into a chromosomal locus with favorable properties. For proof of concept we established a novel HEK293-based retroviral producer cell line, called Flp293A, with a single-copy retroviral vector integrated at a selected chromosomal locus. The vector was flanked by noninteracting Flp-recombinase recognition sites and was exchanged for different retroviral vectors via Flp-mediated cassette exchange. All analyzed cell clones showed correct integration and identical titers for each of the vectors, confirming that the expression characteristics from the parental cell were preserved. Titers up to 2.5 x 10(7) infectious particles/10(6) cells were obtained. Also, high-titer producer cells for a therapeutic vector that encodes the 8.9-kb collagen VII cDNA in a marker-free cassette were obtained within 3 weeks without screening. Thus, we provide evidence that the precise integration of viral vectors into a favorable chromosomal locus leads to high and predictable virus production. This method is compatible with other retroviral vectors, including self-inactivating vectors and marker-free vectors. Further, it provides a tool for evaluation of different retroviral vector designs.

Targeting lentiviral vectors to specific cell types in vivo

We have developed an efficient method to target lentivirus-mediated gene transduction to a desired cell type. It involves incorporation of antibody and fusogenic protein as two distinct molecules into the lentiviral surface. The fusogen is constructed by modifying viral envelope proteins, so that they lack the ability to bind to their cognate receptor but still retain the ability to trigger pH-dependent membrane fusion. Thus, the specificity of such a lentiviral vector is solely determined by the antibody, which is chosen to recognize a specific surface antigen of the desired cell type. This specific binding then induces endocytosis of the surface antigen, bringing the lentivirus into an endosome. There, the fusogen responds to the low pH environment and mediates membrane fusion, allowing the virus core to enter the cytosol. Using CD20 as a target antigen for human B cells, we have demonstrated that this targeting strategy is effective both in vitro and in intact animals. This methodology is flexible and can be extended to other forms of cell type-specific recognition to mediate targeting. The only requirement is that the antibody (or other binding protein) must be endocytosed after interaction with its cell surface-binding determinant.

Advanced targeting strategies for murine retroviral and adeno-associated viral vectors

Targeted gene delivery involves broadening viral tropism to infect previously nonpermissive cells, replacing viral tropism to infect a target cell exclusively, or stealthing the vector against nonspecific interactions with host cells and proteins. These approaches offer the potential advantages of enhanced therapeutic effects, reduced side effects, lowered dosages, and enhanced therapeutic economics. This review will discuss a variety of targeting strategies, both genetic and nongenetic, for re-engineering the tropism of two representative enveloped and nonenveloped viruses, murine retrovirus and adeno-associated virus. Basic advances in understanding the structural biology and virology of the parent viruses have aided rational design efforts to engineer novel properties into the viral attachment proteins. Furthermore, even in the absence of basic, mechanistic knowledge of viral function, high-throughput library and directed evolution approaches can yield significant improvements in vector function. These two complementary strategies offer the potential to gain enhanced molecular control over vector properties and overcome challenges in generating high titer, stealthy, retargeted vectors.

Early steps of clathrin-mediated endocytosis involved in phagosomal escape of Fcgamma receptor-targeted adenovirus

Adenovirus type 2 (Ad2) and Ad5 enter epithelial cells via the coxsackievirus B Ad receptor (CAR) and alpha(v) integrin coreceptors. In the absence of CAR, they can be directed to the Fcgamma receptor 1 of hematopoietic cells by an adaptor comprising the extracellular CAR domain and the Fc portion of a human immunoglobulin G (CARex-Fc). This gives rise to Ad aggregates and single particles which together enhance gene delivery up to 250-fold compared to adaptor-less viruses. A small interfering RNA knockdown of the clathrin heavy chain and quantitative electron microscopy of hematopoietic leukemia cells showed that the majority of Ads were phagocytosed as clusters of 1 to 3 microm in diameter and that about 10% of the particles entered cells by clathrin-mediated endocytosis. The clathrin knockdown did not affect phagocytosis but, surprisingly, inhibited viral escape from phagosomes. Similarly, blocking an early stage of clathrin-coated pit assembly inhibited phagosomal escape and infection but not aggregate uptake, unlike blocking of a late stage of clathrin-coated pit formation. We propose a cooperative interaction of clathrin-mediated endocytosis and phagocytosis triggering phagosomal lysis and infection.

Retroviral display of urokinase-binding domain fused to amphotropic envelope protein

Tumors frequently express urokinase (uPA) receptor (uPAR). To investigate whether uPAR can efficiently target cancerous cells using amphotropic retroviral vectors, we generated a retrovirus displaying the amino-terminal fragment (ATF) of uPA as an N-terminal extension of viral envelope protein. We also made use of a "two-step strategy" by inserting a uPA cleavage site between the ATF moiety and the envelope. We measured the ability of ATF-bearing chimeric envelopes to infect huPAR-overexpressing Madin-Darby canine kidney (MDCK) and control MDCK II cells. The ATF-viruses infected both MDCK cell lines with an equivalent efficiency, suggesting that the chimeric viruses were not sequestered by uPAR and infect cells preferentially via the Pit-2 receptor. The addition of a uPA cleavage site increased the infection level of huPAR-MDCK cells by 2-fold when uPA was present in the infection medium. Surprisingly, ATF-env viruses infected huPAR-MDCK cells 5.5-fold more efficiently in the presence of exogenous uPA. This stimulatory effect of uPA on infection of huPAR-MDCK cells by the ATF-env virus was completely abolished by methyl-beta-cyclodextrin, suggesting that this effect involves the caveolar endocytosis pathway.

Effects of sequences of prokaryotic origin on titer and transgene expression in retroviral vectors

Transcriptionally targeted MLV-based ProCon vectors allow expression of the transduced gene in a promoter-specific manner by replacement of the viral U3 region with a heterologous promoter. In order to evaluate the effects of sequence elements present in ProCon vectors on transgene expression (enhanced green fluorescence protein, EGFP), a series of deletion constructs mimicking the situation in proviral DNA following promoter conversion, where expression of the EGFP gene is driven by three different constitutive promoters (MLV U3, mCMV, and hCMV) in the context of a 5'LTR, respectively, were generated and tested in transient transfection experiments. We discovered that modifications in the 3'LTR have only marginal effects on the EGFP expression and the sequence between the promoter and the transgene did not influence EGFP expression at all. On the other hand, EGFP expression was reduced by up to 17-fold in cells transfected with constructs containing SV40neo and/or pBR322ori sequences. To study this effect in transduced cells, we generated a series of retroviral vectors in which these elements were deleted in various combinations and found that an increase in EGFP expression and viral titer was also consistently obtained using vectors lacking these elements, although this was much smaller than that observed using the expression constructs. A vector containing the gene for puromycin resistance (pac) in place of the neomycin resistance gene (neo) was also tested, and found to result in improved vector titers and transgene expression. We conclude that, where possible, the inclusion of neo and ori sequences in retroviral vectors should be avoided, and that, if selection of infected cells is necessary, the pac, rather than neo gene should be used.

Library-based selection of retroviruses selectively spreading through matrix metalloprotease-positive cells

Viruses conditionally replicating in cancer cells form an attractive novel class of antitumoral agents. To engineer such viruses infectivity can be coupled with proteolytic activity of the target cell by modifying the envelope (Env) protein of murine leukaemia virus (MLV) with blocking domains that prevent cell entry unless they are cleaved off by tumour-associated proteases like the matrix metalloproteases (MMP). Here we show that MLV variants selectively spreading through MMP-positive cells can be evolved from virus libraries, in which a standard MMP-2 substrate peptide connecting the blocking domain CD40L with the Env protein was diversified. Passaging the virus library on human fibrosarcoma or glioma cell lines resulted in the selection of about 10 virus clones, of which the three most frequent ones were shown to become activated by MMPs and to be replication competent on MMP-positive cells only. On these cells, the selected linker peptides improved the spreading by several orders of magnitude in vitro, as well as in tumour xenografts in vivo, approaching the kinetic of the unmodified wild-type virus. The data suggest that retroviral protease substrate libraries form a potent tool for the engineering of viruses conditionally replicating in a given cancer cell type of interest.

Reengineering paramyxovirus tropism

Receptor specificity is a critical determinant of viral tropism, but the capacity of viruses to switch to alternative receptors has not been extensively studied. Here, we engineered the attachment protein of an attenuated measles virus and generated truly retargeted viruses that are blind to the native receptors CD46 and SLAM, but which propagate efficiently and exclusively via alternative cellular receptors, epidermal growth factor receptor, or CD38. The engineered receptor tropisms were stably maintained during multiple serial virus passage without reversion to native receptor usage, even on cells offering the choice of both native and alternative receptors. We conclude that paramyxoviruses have a remarkably flexible and adaptable entry mechanism.

Targeted retroviral transduction of c-kit+ hematopoietic cells using novel ligand display technology

Gene therapy for a wide variety of disorders would be greatly enhanced by the development of vectors that could be targeted for gene delivery to specific populations of cells. We describe here high-efficiency targeted transduction based on a novel targeting strategy that exploits the ability of retroviruses to incorporate host cell proteins into the surface of the viral particle as they bud through the plasma membrane. Ecotropic retroviral particles produced in cells engineered to express the membrane-bound form of stem cell factor (mbSCF) transduce both human cell lines and primary cells with high efficiency in a strictly c-kit (SCF receptor)-dependent fashion. The availability of efficient targeted vectors provides a platform for the development of a new generation of therapies using in vivo gene delivery. (Blood. 2004;104: 2697-2703)

Retroviral vector targeting through insertion of epidermal growth factor into receptor binding deficient influenza A hemagglutinin results in fusion defective particles

Targeting retroviral entry is a central theme in the development of vectors for gene therapy. The host range of a retrovirus is dependent upon the interaction of its envelope glycoprotein (Env) with a specific cell surface receptor protein, which allows viral entry. In contrast, the pH-dependent viruses enter cells through receptor-mediated endocytosis and the subsequent acidification produces conformational changes in the viral envelope protein(s) which lead to membrane fusion. We attempted to redirect retroviral vectors to epidermal growth factor (EGF) receptor expressing cells by using the pH-dependent influenza A virus hemagglutinin (HA). Wild type receptor binding was avoided either by point mutations or by deletion of the globular head structure of HA and also inserted EGF into HA. Replacement of the whole head domain was not tolerated. Two of the EGF-HA proteins bearing point mutations could be incorporated into retroviral particles, but unfortunately their fusion activity was lost. The data indicate that care must be taken when mutating multiple sites in HA, and that targeting HA requires further analysis of appropriate sites for the insertion of foreign sequences.

Targeting virus entry and membrane fusion through specific peptide/MHC complexes using a high-affinity T-cell receptor

The T-cell receptor (TCR) determines the specificity of T-cell recognition by binding to peptide fragments of intracellular proteins presented at the cell surface in association with molecules of the major histocompatibility complex (MHC). Engagement of the TCR by its cognate peptide/MHC ligand, with appropriate co-stimulatory signals, leads to activation of T-cell effector functions. Here we show that the attachment proteins of attenuated measles viruses, engineered to display a high-affinity single-chain TCR (scTCR), can recognize and bind to specific peptide-MHC complexes and thereby mediate targeted virus-cell entry and cell-to-cell fusion. Using the 2C TCR and its peptide/MHC ligand (SIYRYYGL/mouse K(b)), we show that a scTCR grafted onto the measles virus H protein confers new specificity to virus entry and cell fusion. The efficiency of TCR-mediated virus entry was dependent on the number of peptide/MHC complexes expressed on the target cells, increasing progressively above densities higher than 2500 complexes per cell. This work introduces a new paradigm for targeting virus entry and membrane fusion by extending the repertoire of targets to specific peptide-MHC ligands and offering a novel quantitative readout for the cellular expression of peptide-MHC complexes.

Improved gene transfer selectivity to hepatocarcinoma cells by retrovirus vector displaying single-chain variable fragment antibody against c-Met

Engineered retroviruses are widely used vectors for cancer gene therapy approaches. However, the ability to target cells of therapeutic interest while controlling the expression of the transferred genes would improve both the efficiency and the safety of viral vectors. In this study, we investigated the ability of a retroviral amphotropic envelope displaying single-chain variable-fragment (scFv) directed against the c-Met receptor, to target the entry of recombinant retroviruses to human hepatocarcinoma cells. Four single-chain antibody fragments directed against the c-Met receptor were generated and inserted into the viral envelope protein as an N-terminal fusion. The modified envelopes were incorporated into virus particles and one of the chimeric viruses, 3D6-Env, transduced preferentially human hepatoma cells rather than proliferating human hepatocytes. In another construct, the urokinase cleavage site was inserted between the scFv moiety and the envelope. Chimeric scFv-urokinase-Env viruses transduced hepatoma cells with a similar efficiency to that of the control virus and their infectivity in human hepatocytes remained low. These results indicate that amphotropic retroviruses with engineered envelopes to display scFv directed against the c-Met receptor can efficiently and selectively deliver genes into hepatoma cells.

Ligand-Modified Vesicular Stomatitis Virus Glycoprotein Displays a Temperature-Sensitive Intracellular Trafficking and Virus Assembly Phenotype

The production of potentially targetable VSV-G-pseudotyped retrovirus vectors has been hampered by inadequate understanding of the structure-function relationships of the VSV-G protein. In these studies we demonstrate assembly and production of MLV-based and HIV-1-based vector particles using VSV-G proteins modified by the insertion of a peptide ligand into a site corresponding to amino acid position 24 of the native VSV-G molecule. The inserted ligand represents the decapeptide encoding the collagen-binding domain of von Willebrand factor. We have used deconvolution microscopy to demonstrate that the modified VSV-G molecules sequester in perinuclear structures and are unavailable for assembly of infectious virus particles at the cell surface under standard tissue culture conditions at 37 degrees C. In contrast, at a lower permissive temperature of 30 degrees C, the modified VSV-G protein traffics appropriately to the cell surface and participates in useful titers. Furthermore, VSV-G-pseudotyped MLV-based and HIV-1-based vectors displaying the collagen-binding domain demonstrate a statistically significant increased attachment to a collagen matrix as indicated by an ELISA-like cell binding assay and by a focus transduction assay.

Growth factor displayed on the surface of retroviral particles without manipulation of envelope proteins is biologically active and can enhance transduction

BACKGROUND

The therapeutic potential of retroviruses can be significantly enhanced by display of specific molecules on the retroviral surface. This has been conventionally achieved by the manipulation of retroviral envelope proteins. In this report we have tested whether the natural budding mechanism of the retrovirus could be exploited to incorporate a specific molecule into the retroviral surface.

METHODS

Retroviral packaging cells were engineered to express the membrane-bound form of human stem cell factor (mbSCF). Surface expression of mbSCF on retroviral packaging cells was confirmed by immunofluorescence and flow cytometry. Incorporation of mbSCF into retroviral particles was demonstrated by virus-binding assay and immunomagnetic capture of virus using antibody to SCF. Retroviral supernatants were tested for activity of the incorporated cytokine by proliferation assays on factor-dependent cells. Amphotropic retrovirus displaying surface mbSCF was used to transduce SCF receptor-positive haematopoietic cells.

RESULTS

Retroviruses incorporating surface SCF showed increased levels of binding to cells (MO7e) expressing the SCF receptor, c-kit. mbSCF displayed on the viral surface retained levels of biological activity comparable with those of soluble recombinant growth factor. Transduction of c-kit-positive target cells with viruses displaying mbSCF showed enhanced levels of transduction in comparison with unmodified viruses.

CONCLUSIONS

Expression of the membrane-bound form of human stem cell factor (mbSCF) on the surface of retroviral packaging cells allows its efficient incorporation into retrovirus particles in a biologically active form, opening up the possibility for the use of retroviral display in many therapeutic areas, such as in gene therapy, drug delivery and in the development of novel vaccines.

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.

Targeting a Retroviral Vector in the Absence of a Known Cell-Targeting Ligand

An important requirement for many gene therapy applications is to direct therapeutic genes specifically to target cells. Here we describe an improved vector targeting method that does not depend on the use of a known cell-targeting ligand. It entails screening a library of constitutively produced retroviruses with random amino acid substitutions in the cell-targeting region of the envelope proteins for their ability to mediate gene delivery to a target cell. By screening such a library on the ras-transformed 143B human cell line, we have isolated an envelope protein that preferentially targets 143B cells and 293T cells expressing the SV40 T antigen via a novel, unidentified receptor. Furthermore, retroviruses expressing the library-derived envelope protein can be concentrated by centrifugation. This is the first demonstration of a novel concept in vector targeting: the selection of productive retroviral entry via an alternate receptor with modified cellular tropism in the absence of a known cell-targeting moiety. The method is, in principle, applicable even to cells that have not been well characterized, and therefore potentially suitable for targeting many diverse cell types.

In vivo targeting of tumor endothelial cells by systemic delivery of lentiviral vectors

Tumor angiogenesis is a rate-limiting factor for tumor growth, and the endothelial cells of tumor vessels display specific features that can be exploited for the selective delivery of cancer therapeutics. To specifically target exogenous genes to angiogenic tumor vessels, we generated a panel of vesicular stomatitis virus-pseudotyped lentiviral vectors (LVs) engineered for endothelial cell (EC)-specific expression. We cloned a wide repertoire of transcription regulatory sequences from genes preferentially expressed in ECs (Tie1, Tie2, Flk-1, VE-Cad, and ICAM-2) into self-inactivating LVs to drive expression of the marker gene encoding green fluorescent protein (GFP) or of the conditionally toxic gene encoding nitroreductase, and compared them with the ubiquitously expressing phosphoglycerate kinase (PGK) and cytomegalovirus (CMV) promoters. We evaluated the efficiency and specificity of vector expression in vitro in a panel of human primary cultures, including ECs, fibroblasts, neurons, lymphocytes, and hematopoietic progenitors, and in tumor cell lines. We found that vectors containing promoter and enhancer sequences from the Tie2 gene achieved remarkable specificity of expression in ECs in vitro and in vivo. On intravenous delivery into tumor-bearing mice, the Tie2 vector targeted expression to the ECs of tumor vessels. In contrast, LVs carrying the PGK or CMV promoter gave widespread GFP marking in ECs and non-ECs of tumors and other organs. The previously reported upregulation of the Tie2 gene in ECs activated for angiogenesis may explain the remarkable selectivity of expression of the Tie2 vector in ECs of tumor vessels. The new vector provides the means for selective delivery of gene therapy to tumor sites in vivo.