Masking of retroviral envelope functions by oligomerizing polypeptide adaptors

A genetic fiber modification to achieve matrix-metalloprotease-activated infectivity of oncolytic adenovirus

Selective tumor targeting of oncolytic adenovirus at the level of cell entry remains a major challenge to improve efficacy and safety. Matrix metalloproteases (MMPs) are overexpressed in a variety of tumors and in particular in pancreatic cancer. In the current work, we have exploited the expression of MMPs together with the penetration capabilities of a TAT-like peptide to engineer tumor selective adenoviruses. We have generated adenoviruses containing CAR-binding ablated fibers further modified with a C-terminus TAT-like peptide linked to a blocking domain by an MMP-cleavable sequence. This linker resulted in a MMP-dependent cell transduction of the reporter MMP-activatable virus AdTATMMP and in efficient transduction of neoplastic cells and cancer-associated fibroblasts. Intravenous and intraductal administration of AdTATMMP into mice showed very low AdTATMMP activity in the normal pancreas, whereas increased transduction was observed in pancreatic tumors of transgenic Ela-myc mice. Intraductal administration of AdTATMMP into mice bearing orthotopic tumors led to a 25-fold increase in tumor targeting compared to the wild type fiber control. A replication competent adenovirus, Ad(RC)MMP, with the MMP-activatable fiber showed oncolytic efficacy and increased antitumor activity compared to Adwt in a pancreatic orthotopic model. Reduced local and distant metastases were observed in Ad(RC)MMP treated-mice. Moreover, no signs of pancreatic toxicity were detected. We conclude that MMP-activatable adenovirus may be beneficial for pancreatic cancer treatment.

Anticancer gene transfer for cancer gene therapy

Gene therapy vectors are among the treatments currently used to treat malignant tumors. Gene therapy vectors use a specific therapeutic transgene that causes death in cancer cells. In early attempts at gene therapy, therapeutic transgenes were driven by non-specific vectors which induced toxicity to normal cells in addition to the cancer cells. Recently, novel cancer specific viral vectors have been developed that target cancer cells leaving normal cells unharmed. Here we review such cancer specific gene therapy systems currently used in the treatment of cancer and discuss the major challenges and future directions in this field.

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.

Vector systems for prenatal gene therapy: principles of retrovirus vector design and production

Vectors derived from the Retroviridae family have several attributes required for successful gene delivery. Retroviral vectors have an adequate payload size for the coding regions of most genes; they are safe to handle and simple to produce. These vectors can be manipulated to target different cell types with low immunogenicity and can permanently insert genetic information into the host cells' genome. Retroviral vectors have been used in gene therapy clinical trials and successfully applied experimentally in vitro, in vivo, and in utero.

Retroviral display in gene therapy, protein engineering, and vaccine development

The display and analysis of proteins expressed on biological surfaces has become an attractive tool for the study of molecular interactions in enzymology, protein engineering, and high-throughput screening. Among the growing number of established display systems, retroviruses offer a unique and fully mammalian platform for the expression of correctly folded and post-translationally modified proteins in the context of cell plasma membrane-derived particles. This is of special interest for therapeutic applications such as gene therapy and vaccine development and also offers advantages for the engineering of mammalian proteins toward customized binding affinities and catalytic activities. This review critically summarizes the basic concepts and applications of retroviral display and analyses its benefits in comparison to other display techniques.

Reprogrammed viruses as cancer therapeutics: targeted, armed and shielded

Virotherapy is currently undergoing a renaissance, based on our improved understanding of virus biology and genetics and our better knowledge of many different types of cancer. Viruses can be reprogrammed into oncolytic vectors by combining three types of modification: targeting, arming and shielding. Targeting introduces multiple layers of cancer specificity and improves safety and efficacy; arming occurs through the expression of prodrug convertases and cytokines; and coating with polymers and the sequential usage of different envelopes or capsids provides shielding from the host immune response. Virus-based therapeutics are beginning to find their place in cancer clinical practice, in combination with chemotherapy and radiation.

Cell entry targeting restricts biodistribution of replication-competent retroviruses to tumour tissue

Virotherapy is currently being developed for many different types of viruses including replication-competent murine leukaemia virus (MLV) as a novel tool in cancer therapy. However, there is the risk of insertional mutagenesis associated with this virus, making careful preclinical studies necessary before its first application in man. We have previously generated conditionally replication-competent MLV variants that require activation by tumour-associated proteases to become infectious. Here we analysed in a comparative study the spreading of non-targeted and of such tumour-targeted MLV variants to tumour and extratumoural organs in immunodeficient mice. Both virus types were able to efficiently infect tumour cells after systemic administration. The non-targeted virus, however, also infected extratumoural organs like bone marrow, spleen and liver efficiently. In contrast, the targeted viruses revealed in a quantitative analysis of virus spreading an up to 500-fold more selective infection of tumour tissue than the non-targeted virus. The data raise serious doubts about a safe clinical use of non-targeted MLV. Engineering the virus to become activatable by tumour-associated proteases can significantly improve the safety of MLV.

Targeted retroviral vectors displaying a cleavage site-engineered hemagglutinin (HA) through HA-protease interactions

We report here a targeting method that exploits the expression pattern of cell surface proteases to induce gene delivery to specific tissues. We describe retroviral vectors harboring modified surface glycoproteins derived from an avian influenza virus hemagglutinin (HA) for which the cell entry properties, dependent on HA cleavage by producer cells, were conditionally blocked at a postbinding step by insertion of matrix metalloproteinase (MMP) substrates. We demonstrate that such vectors induce gene transfer, both in vitro and in mice harboring human tumor xenografts, only through contact with target cells expressing MMPs that cleave the substrate introduced into the recombinant HA. This selective gene transfer in MMP-rich cells was specifically inhibited by 1,10-phenanthroline, a broad-range MMP inhibitor. Importantly, such MMP-activatable vectors selectively transduced MMP-rich cells in heterogeneous populations containing MMP-rich and MMP-poor cells. These vectors will allow useful gene transfer applications into target cells exhibiting specific protease activities.

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.

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.

Adenoviral vectors expressing fusogenic membrane glycoproteins activated via matrix metalloproteinase cleavable linkers have significant antitumor potential in the gene therapy of gliomas

BACKGROUND

Fusogenic membrane glycoproteins (FMG) such as the gibbon ape leukemia virus envelope (GALV) glycoprotein are potent therapeutic transgenes with potential utility in the gene therapy of gliomas. Transfection of glioma cell lines with FMG expression constructs results in fusion with massive syncytia formation followed by cytotoxic cell death. Nevertheless, ubiquitous expression of the GALV receptor, Pit-1, makes targeting desirable in order to increase the specificity of the observed cytopathic effect. Here we report on use of matrix metalloproteinase (MMP)-cleavable linkers to target the cytotoxicity of FMG-expressing adenoviral vectors against gliomas.

METHODS

Replication-defective adenoviruses (Ad) were constructed expressing the hyperfusogenic version of the GALV glycoprotein linked to a blocking ligand (C-terminal extracellular domain of CD40 ligand) through either an MMP-cleavable linker (AdM40) or a non-cleavable linker (AdN40). Both viruses also co-expressed the green fluorescent protein (GFP) via an internal ribosomal entry site.

RESULTS

The glioma cell lines U87, U118, and U251 characterized by zymography and MMP-2 activity assay as high, medium and low MMP expressors, respectively, the MMP-poor cell lines TE671 and normal human astrocytes were infected with AdM40 and AdN40 at different multiplicities of infection (MOIs) from 1-30. Fusion was quantitated by counting both number and size of syncytia. Infection of these cell lines with AdN40 did not result in fusion or cytotoxic cell death, despite the presence of infection, as demonstrated by GFP positivity, therefore indicating that the displayed CD40 ligand blocked GALV-induced fusion. Fusion was restored after infection of glioma cells with AdM40 at an MOI as low as 1 to an extent dependent on MMP expression and coxsackie adenovirus receptor (CAR) expression in the specific cell line. Western immunoblotting demonstrated the presence of the cleaved CD40 ligand in the supernatant of fused glioma cells. Use of the MMP inhibitors 1,10 phenanthroline and N-hydroxy-5,5-dimethylpiperazine-2-carboxamide completely abolished AdM40-induced fusion, while the non-specific serine protease inhibitor soybean trypsin inhibitor did not affect it, thus demonstrating specificity of the observed effect. Intratumoral treatment of BalbC/nude mice bearing subcutaneous U87 glioma xenografts with AdM40 at a total dose of 1.2 x 10(10) plaque-forming units (pfu) resulted in statistically significant tumor regression as compared with control animals either treated with AdN40 (p = 0.01) or untreated animals (p = 0.01). Treatment with AdM40 also resulted in survival improvement as compared with AdN40-treated animals (p = 0.006) or untreated animals (p = 0.001). Histopathologic examination of treated tumors demonstrated extensive syncytia formation.

CONCLUSIONS

Our data indicate that AdM40, a replication-defective adenovirus expressing the GALV fusogenic glycoprotein, attached to a blocking ligand via an MMP-cleavable linker, can target the cytotoxicity of GALV in MMP-overexpressing glioma lines and xenografts, and maintain significant antitumor activity both in vitro and in vivo. Given the high frequency of MMP overexpression in gliomas, AdM40 represents a potentially promising agent in the gene therapy of these tumors.

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.

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.

Targeting the cytotoxicity of fusogenic membrane glycoproteins in gliomas through protease-substrate interaction

Fusogenic membrane glycoproteins (FMG) are potent therapeutic transgenes with potential utility in the gene therapy of gliomas. FMG expression constructs caused massive syncytia formation followed by cytotoxic cell death in glioma cell lines, and antitumor activity has been shown in glioma xenografts. FMG-induced fusion in glioma cells can involve heterologous cell lines including normal astrocytes and fibroblasts, therefore making targeting important. Here we report on the use of matrix metalloproteinase (MMP) cleavable linkers to target cytotoxicity of FMGs against gliomas. Expression constructs were made expressing the hyperfusogenic version of the Gibbon Ape Leukemia Virus envelope glycoprotein (GALV) linked to a blocking ligand (the C-terminal extracellular domain of CD40 ligand) via either an MMP cleavable linker (GALV M40), a factor Xa protease cleavable linker (GALV X40), or a noncleavable linker (GALV N40). Unmodified GALV expressing constructs were used as positive controls. The glioma cell lines U87, U118, and U251 previously characterized by zymography and MMP-2 activity assay as high, medium, and low MMP expressors, respectively; normal human astrocytes and the MMP-poor cell line TE671 were transfected with the GALV, GALV N40, GALV X40, and GALV M40 constructs. In contrast to unmodified GALV constructs, transfection with GALV X40 and GALV N40 constructs blocked fusion and cytotoxic cell death. Fusion occurred, however, after transfection with constructs containing MMP cleavable linkers to an extent dependent on MMP expression in the specific cell line. Use of the broad-spectrum MMP inhibitors, 1,10-phenanthroline and N-hydroxy-piperazine-carboxamide completely abolished the ability of MMP constructs to induce fusion. In cell mixing experiments, mixing of MMP-poor cell lines transfected with GALV M40 constructs with the MMP overexpressing untransfected U87 glioma cells led to partial restoration of fusion. Use of U87 supernatant did result in a similar effect. Establishment of stable tranfectants expressing the membrane-type MMPs, MT-1 MMP and MT-2 MMP did restore fusion in the MMP-poor cell line TE671 after transfection with GALV M40, thus indicating that both membrane-type MMPs and soluble MMPs activate the MMP cleavable constructs. In addition, the GALV M40 construct retained its cytotoxic activity against U87 cells in vivo, although less effectively as compared to unmodified GALV. Our data indicate that GALV-induced cytotoxicity in glioma cell lines can be blocked by display of the CD40 ligand. Incorporation of an MMP cleavable linker can selectively restore cytotoxicity in MMP expressing glioma cell lines both in vitro and in vivo, while sparing normal human astrocytes. Given the high frequency of MMP overexpression in gliomas, this represents a promising targeting strategy.

Displaying epidermal growth factor on spleen necrosis virus-derived targeting vectors

Targeted gene transfer into human cells has previously been achieved with spleen necrosis virus (SNV)-derived vector particles harboring envelope (Env) proteins which carry single chain Fv (scFv) domains derived from antibodies. Such cell targeting vectors have been found to directly transduce human cells expressing the cell surface molecules recognized by the respective scFv. In an attempt to achieve targeted gene transfer into epidermal growth factor receptor (EGFR)-positive human cells, SNV vector particles carrying a surface (SU) envelope protein N-terminally modified with the EGF domain and the wildtype transmembrane protein were generated. However, direct transduction of EGFR-positive cells was not detected. Canine D17 cells, which can be infected by wildtype SNV, were also not transduced. Infectivity of D17 cells was restored by removal of the EGF modification via cleavage of a factor Xa site located between the EGF domain and the SU protein or by blocking the EGFRs on the cell surface by EGF treatment. The properties of SNV-EGF vector particles as described here are similar to those of murine leukemia virus-derived vector particles harboring envelope proteins modified with a growth factor-derived domain. It seems therefore that, although scFv-modified SNV allows direct cell targeting, EGF-modified SNV allows only indirect cell targeting.

Lack of specificity of cell-surface protease targeting of a cytotoxic hyperfusogenic gibbon ape leukaemia virus envelope glycoprotein

BACKGROUND

In a strategy termed "Protease Targeting", retroviral vectors carrying an EGF infectivity-blocking domain fused to the N-terminus of the envelope SU via a MMP (matrix metalloproteinase)-cleavable linker were successfully used to target gene delivery to EGF receptor-(EGF-R-)positive tumour cells over-expressing MMPs. In the current study, we aimed to investigate whether this strategy could be applied to (a) limit the cytotoxic activity of a hyperfusogenic GALV therapeutic gene, and (b) enhance the immune-stimulatory properties of GALV via local, MMP-mediated release human granulocyte-macrophage colony stimulating factor (GM-CSF).

METHODS

We generated GALV envelope expression constructs displaying EGF or GM-CSF blocking ligands at the N-terminus of GALV envelope SU via a non-cleavable, Factor Xa protease or MMP-cleavable linker and investigated their cytotoxicity on MMP-positive and negative cell lines.

RESULTS

The unmodified hyperfusogenic GALV envelope was cytotoxic to all cell lines tested. The non-cleavable linker GALV envelope constructs caused no cytotoxicity, demonstrating efficient inhibition by the displayed domains. Moderate activation of fusion of the protease-cleavable linker constructs was observed in all cell lines, regardless of their level of MMP expression and of the specificity of the linker. High levels of the 'blocking domain' were detected in the cell supernatants due to dissociation of the surface unit (SU) from the transmembrane (TM) component of the GALV envelope glycoprotein TM.

CONCLUSIONS

Unlike protease targeting in the context of retroviral vectors, protease activation of the cytotoxicity of GALV envelope by cleavage of a fusion blocking ligand at the cell surface does not appear to be specifically mediated by cell-surface MMPs. In addition, shedding of the SU-fusion protein from the TM limits the general applicability of this strategy for cancer gene therapy. Specificity of cell-cell fusion mediated by GALV envelope cannot be manipulated in the same fashion as virus-cell fusion.

An expanding role for CD40L and other tumor necrosis factor superfamily ligands in HIV infection

Immunostimulatory members of the tumor necrosis factor (TNF) superfamily (TNFSF) of ligands are known to be important regulators of the immune system. These trimeric molecules interact with members of the TNF receptor superfamily (TNFRSF) to stimulate immune cells. Of the TNFSF molecules, CD40 ligand (CD40L, also called CD154 or TNFSF5) is the most crucial molecule for activating antigen-presenting cells (APCs) and thereby initiating the immune response. Evidence has accrued indicating that HIV infection either selectively depletes those CD4(+) T cells that express CD40L in response to antigen or down-regulates CD40L expression by these cells. Because CD40L expression is necessary for the immune defense against HIV and opportunistic infections, an insufficiency of CD40L could contribute to the progression of AIDS. CD40L contributes to the antiviral mechanisms of the host by inducing anti-HIV beta-chemokines and activating CD8(+) T cells. However, CD40L stimulation can lead to enhanced HIV replication under certain experimental conditions, due to its immune activating properties and the need for cellular activation for high-level HIV production. On balance, it is believed that reversing the relative CD40L deficiency seen in HIV infection will be important for immune restoration in AIDS. In addition, adding CD40L to a therapeutic or preventative vaccine could lead to strengthened antiviral immunity. Because of the complexities in delivering this molecule, a number of forms of CD40L have been developed, and one form of soluble CD40L has been tested in humans. New strategies are being developed to translate the profoundly immunostimulatory effects of CD40L found in animal models to humans with HIV infection.

Retargeting gene delivery using surface-engineered retroviral vector particles

Retroviral vectors with the capacity to deliver transgenes to specific tissues are expected to be of great value for various gene transfer applications in vivo. Initial attempts to modify vector host-range by the insertion of ligands on their surface glycoproteins have frequently failed, essentially owing to the impairment of the fusogenicity of the vector particles bound to the targeted cell-surface molecules. Several strategies aimed to recover the fusogenic activity of surface-engineered vector particles have recently been explored and have given rise to novel concepts in the field.

Organ distribution of gene expression after intravenous infusion of targeted and untargeted lentiviral vectors

Lentiviral vectors represent an attractive technology platform from which to develop a targetable injectable gene delivery system for transduction of specific cell populations in vivo, irrespective of their cell cycle status. Targeted HIV-1-based lentiviral vectors were generated by pseudotyping them with chimeric murine leukemia virus (MLV) envelope glycoproteins displaying N-terminal targeting polypeptides. Vectors displaying an EGF polypeptide were fully infectious on EGF receptor-negative cells, but were inactive on cells with abundant EGF receptors (inverse targeting). Receptor-mediated inactivation of gene transfer was overcome by competing the EGF receptors on the target cells with soluble EGF or by removing the displayed EGF domain from the surface of the vector particles by factor Xa cleavage of a specific protease substrate engineered into its tethering linker (protease targeting). Intravenous infusion of nontargeted HIV-1 vectors led to maximal luciferase activity in liver and spleen with moderate or minimal activity in heart, skeletal muscle, lung, brain, kidney, ovaries and bone marrow. In contrast, intravenous EGF-displaying vectors were expressed maximally in spleen with very low level luciferase expression detectable in liver (EGF-receptor rich). Liver transduction by the EGF-displaying vector was restored by pretreating the animals with soluble EGF suggesting that these vectors are inversely targeted to spleen.

Evidence for nonspecific adsorption of targeted retrovirus vector particles to cells

The ability to specifically target a cell-type is important for the development of vectors for in vivo gene therapy. In order to produce retrovirus vectors targeting ovarian cancer cells, which specifically overexpress alpha folate receptor (alphaFR), a single chain antibody was fused as an N-terminal extension of the ecotropic and amphotropic murine leukemia virus (MLV) envelope glycoproteins. Vector particles bearing the modified glycoproteins were produced and analysed. Although conventional FACS studies indicated that viral particles bearing the modified Env could bind to ovarian cancer cells, targeted infection was not achieved. The initial step of virus-cell interaction was further studied using an immunofluorescence technique, which allows visualisation of single retrovirus particles. Vectors bearing chimeric or wild-type glycoproteins bound equally well to cells with or without the targeted receptor, although soluble chimeric glycoproteins bound specifically to FBP. Our results indicate that the incorporation of specific ligands to the virus envelope does not necessarily result in significant enhancement of vector particle binding. A similar interaction was also observed using Env-defective virus particles, suggesting that cellular factors incorporated into the lipid envelope play a dominant role in promoting initial adsorption of virus particles to cells. Significant implications arise from these observations on the interpretation of previous reports on 'targeted' vectors, and for the development of vectors for in vivo gene therapy protocols.

Strategies for targeting therapeutic gene delivery

A major goal for gene therapy is to obtain targeted vectors that transfer genes efficiently to specific cell types. In theory, this can be achieved by targeting entry of the vector or by building gene expression cassettes that restrict gene expression to certain cell types. This review summarizes recent strategies to alter vector tropism for targeted gene delivery.

Selective transduction of protease-rich tumors by matrix-metalloproteinase-targeted retroviral vectors

We recently showed that retroviral vectors can be targeted through protease substrate interactions. Infectivity is blocked by a polypeptide fused to the viral envelope glycoprotein (SU) and is restored when a protease cleaves the connecting linker, releasing the inhibitory polypeptide from the viral surface. Protease specificity is achieved by engineering the sequence of the linker. Here, using two different matrix-metalloproteinase (MMP)-activatable vectors, we demonstrated highly efficient and selective transduction of MMP-rich target cells in a heterogeneous cell population. In vivo, the MMP-targeted vectors showed strong selectivity for MMP-rich tumor xenografts. Protease-activatable vectors offer new possibilities for in vivo targeting of gene delivery.

Modifying the host range properties of retroviral vectors

Gene therapy protocols would be greatly facilitated by the availability of targetable injectable vectors which could deliver genes in vivo to specific target cells or to specific disease sites. Efforts to develop such retroviral vectors are therefore a high priority in gene therapy research. In this review, we describe the current state of our understanding of the structure and function of the retroviral envelope glycoprotein complex. We then discuss the results of the various strategies that have been devised to modify the host range of the retroviral envelope glycoproteins with a view to achieving retroviral vectors capable of delivering their genes in a highly specific manner to selected human target cells. The strengths and limitations of these strategies are examined.

Retroviral display of functional binding domains fused to the amino terminus of influenza hemagglutinin

We have previously shown that retroviral vector particles derived from Moloney murine leukemia virus (Mo-MuLV) can efficiently incorporate influenza hemagglutinin (HA) glycoproteins from fowl plague virus (FPV), thus conferring a broad tropism to the vectors. To modify its host range, we have engineered the FPV HA to display four different polypeptides on its N terminus: the epidermal growth factor, an anti-human MHC class I molecules scFv (single-chain antibody), an anti-melanoma antigen scFv, and an IgG Fc-binding polypeptide. All recombinant HA glycoproteins were correctly expressed and processed, and efficiently incorporated into Mo-MuLV retroviral particles, indicating that amino-terminal insertion of large polypeptides did not alter the conformation of HA chimeras. Virions carrying the different chimeras bound specifically to cells expressing the targeted cell surface molecules of each ligand. In addition, all virion types were infectious but exhibited various degrees of specificity regarding the use of the targeted cell surface molecule versus the wild-type FPV HA receptor for cell entry and infection. For some ligands tested, infectivity was significantly increased on cells that express the targeted receptor, compared with cells that express only the wild-type HA receptor. Furthermore, some polypeptides could abolish infectivity via the wild-type FPV HA receptor. Our data therefore indicate that it is possible to engineer the HA envelope glycoprotein by fusing ligands to its amino-terminal end without affecting its fusion activity.

Inverse Targeting of Retroviral Vectors: Selective Gene Transfer in a Mixed Population of Hematopoietic and Nonhematopoietic Cells

We previously reported that retroviral vectors displaying epidermal growth factor (EGF) as part of a chimeric envelope glycoprotein are sequestered upon binding to EGF receptor (EGFR)-positive target cells, leading to loss of infectivity. In the current study, we have displayed stem cell factor (SCF) on β-galactosidase-transducing ecotropic and amphotropic retroviral vector particles as a factor Xa protease-cleavable N-terminal extension of the envelope glycoprotein. Viral incorporation of the SCF chimeric envelopes was demonstrated by immunoblotting of pelleted virions and their specific attachment to Kit receptors was demonstrated by flow cytometry. Gene transfer studies showed that when SCF was displayed on an amphotropic envelope, the infectivity of the SCF-displaying vectors was selectively inhibited on Kit-expressing cells, but could be restored by adding soluble SCF to block the Kit receptors or by cleaving the displayed SCF domain from the vector particles with factor Xa protease. The host range properties of EGF-displaying and SCF-displaying vectors were then compared in cell mixing experiments. When EGFR-positive cancer cells and Kit-positive hematopoietic cells were mixed and exposed to the different engineered vector particles, the cancer cells were selectively transduced by the SCF-displaying vector and the hematopoietic cells were selectively transduced by the EGF-displaying vector. Retroviral display of polypeptide growth factors can therefore provide the basis for a novel inverse targeting strategy with potential use for selective transduction of hematopoietic or nonhematopoietic cells (eg, cancer cells) in a mixed cell population.

Inverse Targeting of Retroviral Vectors: Selective Gene Transfer in a Mixed Population of Hematopoietic and Nonhematopoietic Cells

We previously reported that retroviral vectors displaying epidermal growth factor (EGF) as part of a chimeric envelope glycoprotein are sequestered upon binding to EGF receptor (EGFR)-positive target cells, leading to loss of infectivity. In the current study, we have displayed stem cell factor (SCF) on β-galactosidase-transducing ecotropic and amphotropic retroviral vector particles as a factor Xa protease-cleavable N-terminal extension of the envelope glycoprotein. Viral incorporation of the SCF chimeric envelopes was demonstrated by immunoblotting of pelleted virions and their specific attachment to Kit receptors was demonstrated by flow cytometry. Gene transfer studies showed that when SCF was displayed on an amphotropic envelope, the infectivity of the SCF-displaying vectors was selectively inhibited on Kit-expressing cells, but could be restored by adding soluble SCF to block the Kit receptors or by cleaving the displayed SCF domain from the vector particles with factor Xa protease. The host range properties of EGF-displaying and SCF-displaying vectors were then compared in cell mixing experiments. When EGFR-positive cancer cells and Kit-positive hematopoietic cells were mixed and exposed to the different engineered vector particles, the cancer cells were selectively transduced by the SCF-displaying vector and the hematopoietic cells were selectively transduced by the EGF-displaying vector. Retroviral display of polypeptide growth factors can therefore provide the basis for a novel inverse targeting strategy with potential use for selective transduction of hematopoietic or nonhematopoietic cells (eg, cancer cells) in a mixed cell population.