A gene delivery system activatable by disease-associated matrix metalloproteinases

Surface-Mediated Stimuli-Responsive Gene Delivery Based on Breath Figure Film Combined with Matrix Metalloproteinase-Sensitive Hydrogel

Surface-mediated gene delivery appears to be potential gene delivery modes for various applications. Still, controlled and smart delivery manners are required especially considering the need for gene therapy to deliver gene with selectivity. A surface that can effectively payload DNA, promote cell adhesion, and stimuli response is an important prerequisite. Here, we report a matrix metalloproteinase (MMP)-responsive surface-mediated gene delivery system by combining MMP-degradable hydrogel with a breath figure (BF) porous film. The MMP-degradable hydrogel containing plasmid DNA was loaded into the surface pores of the BF film as DNA reservoirs. The upper surface without hydrogel on the BF film served as footholds of integrin adhesions. MMP is one of the important endogenous signals in tumor-related pathologic changes, and MMP expressions in cancer cells are significantly higher than those in normal cells. Consequently, our surface-mediated gene delivery locally and rapidly released the payload DNA in response to cancer cells and transfected them. This work highlights the importance of the combination of stimuli-response and surface-mediated gene delivery to functional materials, showing good potential applications in the field of gene therapy.

Intestinal probiotics E. coli Nissle 1917 as a targeted vehicle for delivery of p53 and Tum-5 to solid tumors for cancer therapy

Traditional cancer therapies, such as surgery treatment, radiotherapy, and chemotherapy, often fail to completely eliminate tumor cells in an anaerobic microenvironment of tumor regions.

In contrast to these traditional cancer therapies, the use of targeted delivery vectors to deliver anticancer genes or antitumor drugs to hypoxic areas in tumors is the most clinically promising cancer treatment with rapid development in recent years. In this study, E.coli Nissle 1917 (EcN), an intestinal probiotic, was utilized as a targeted transport vector to deliver p53 and Tum-5 protein to tumor hypoxic regions. The tumor-targeting characteristics of EcN were investigated using luciferase LuxCDABE operon, and the results demonstrated that EcN could specifically accumulate in the solid tumor areas of SMMC-7721 tumor-bearing BALB/c nude mice. The Tum 5-p53 bifunctional proteins were initially constructed and then delivered to solid tumor regions by using the targeted transporter EcN for cancer therapy. The antitumor effect and safety of three engineered bacteria, namely, EcN (Tum-5), EcN (p53), and EcN (Tum 5-p53), were also examined. The calculated tumor volume and tumor weight indicated that these three engineered bacteria could inhibit the growth of human hepatoma SMMC-7721 cells, and the antitumor effect of EcN (Tum 5-p53) expressing the Tum 5-p53 fusion protein was significantly better than those of EcN (Tum-5) and EcN (p53) alone. Immunofluorescence demonstrated that the expression of Ki-67, a nuclear proliferation-related protein, was inhibited in the tumor areas of the groups treated with the engineered bacteria, whereas the expression of caspase-3 was upregulated. The expression trends of Ki-67 and caspase-3 were consistent with the different antitumor efficacies of these three engineered bacteria. EcN did not elicit obvious side effects on mice. This research not only provids a foundation for tumor-targeted therapy but also contributes greatly to the development of antitumor agents and anticancer proteins.

Reducing off target viral delivery in ovarian cancer gene therapy using a protease-activated AAV2 vector platform

Gene therapy is a promising strategy for treating metastatic epithelial ovarian cancer (EOC). However, efficient vector targeting to tumors is difficult and off-target effects can be severely detrimental. Most vector targeting approaches rely on surface receptors overexpressed on some subpopulation of cancer cells. Unfortunately, there is no universally expressed cell surface biomarker for tumor cells. As an alternative, we developed an adeno-associated virus (AAV) based "Provector" whose cellular transduction can be activated by extracellular proteases, such as matrix metalloproteinases (MMP) that are overexpressed in the tumor microenvironments of the most aggressive forms of EOC. In a non-tumor bearing mouse model, the Provector demonstrates efficient de-targeting of healthy tissues, especially the liver, where viral delivery is <1% of AAV2. In an orthotopic HeyA8 tumor model of EOC, the Provector maintains decreased off-target delivery in the liver and other tissues but with no loss in tumor delivery. Notably, approximately 10% of the injected Provector is still detected in the blood at 24 h while >99% of injected AAV2 has been cleared from the blood by 1 h. Furthermore, mouse serum raised against the Provector is 16-fold less able to neutralize Provector transduction compared to AAV2 serum neutralizing AAV2 transduction (1:200 vs 1:3200 serum dilution, respectively). Thus, the Provector appears to generate less neutralizing antibodies than AAV2. Importantly, serum against AAV2 does not neutralize the Provector as well as AAV2, suggesting that pre-existing antibodies against AAV2 would not negate the clinical application of Provectors. Taken together, we present an EOC gene delivery vector platform based on AAV with decreased off-target delivery without loss of on-target specificity, and greater immunological stealth over the traditional AAV2 gene delivery vector.

Stimulus-responsive viral vectors for controlled delivery of therapeutics

Virus-based therapies have gained momentum as the next generation of treatments for a variety of serious diseases. In order to make these therapies more controllable, stimulus-responsive viral vectors capable of sensing and responding to specific environmental inputs are currently being developed. A number of viruses naturally respond to endogenous stimuli, such as pH, redox, and proteases, which are present at different concentrations in diseases and at different organ and organelle sites. Additionally, rather than relying on natural viral properties, efforts are underway to engineer viruses to respond to endogenous stimuli in new ways as well as to exogenous stimuli, such as temperature, magnetic field, and optical light. Viruses with stimulus-responsive capabilities, either nature-evolved or human-engineered, will be reviewed to capture the current state of the field. Stimulus-responsive viral vector design considerations as well as gaps in current research efforts will be identified.

Adeno-associated virus (AAV) vectors in cancer gene therapy

Gene delivery vectors based on adeno-associated virus (AAV) have been utilized in a large number of gene therapy clinical trials, which have demonstrated their strong safety profile and increasingly their therapeutic efficacy for treating monogenic diseases. For cancer applications, AAV vectors have been harnessed for delivery of an extensive repertoire of transgenes to preclinical models and, more recently, clinical trials involving certain cancers. This review describes the applications of AAV vectors to cancer models and presents developments in vector engineering and payload design aimed at tailoring AAV vectors for transduction and treatment of cancer cells. We also discuss the current status of AAV clinical development in oncology and future directions for AAV in this field.

Controlled chondrogenesis from adipose-derived stem cells by recombinant transforming growth factor-β3 fusion protein in peptide scaffolds

Adipose-derived stem cells (ADSCs) are promising for cartilage repair due to their easy accessibility and chondrogenic potential. Although chondrogenesis of transforming growth factor-β (TGF-β) mediated mesenchymal stem cells (MSCs) is well established in vitro, clinical tissue engineering requires effective and controlled delivery of TGF-β in vivo. In this work, a self-assembled peptide scaffold was employed to construct cartilages in vivo through the chondrogenesis from ADSCs controlled by recombinant fusion protein LAP-MMP-mTGF-β3 that was transfected by lentiviral vectors. During this course, the addition of matrix metalloproteinases (MMPs) can trigger the release of mTGF-β3 from the recombinant fusion protein of LAP-MMP-mTGF-β3 in the combined scaffolds, thus stimulating the differentiation of ADSCs into chondrogenesis. The specific expression of cartilage genes was analyzed by real-time polymerase chain reaction and Western blot. The expression of chondrocytic markers was obviously upregulated to a higher level compared to the one by commonly used TGF-β3 alone. After 3 weeks of in vitro culturing, the hybrids with differentiated chondrogenesis were then injected subcutaneously into nude mice and retrieved after 4 weeks of culturing in vivo. Histological analysis also confirmed that the recombinant fusion protein was more effective for the formation of cartilage matrix than the cases either with TGF-β3 alone or without LAP-MMP-mTGF-β3 (P<0.05). This study demonstrates that controlled local delivery of the LAP-MMP-mTGF-β3 constructs can accelerate differentiation of ADSCs into the cartilage in vivo, which indicates the great potential of this hybrid in rapid therapy of osteoarthritis.

Tunable protease-activatable virus nanonodes

We explored the unique signal integration properties of the self-assembling 60-mer protein capsid of adeno-associated virus (AAV), a clinically proven human gene therapy vector, by engineering proteolytic regulation of virus-receptor interactions such that processing of the capsid by proteases is required for infection. We find the transfer function of our engineered protease-activatable viruses (PAVs), relating the degree of proteolysis (input) to PAV activity (output), is highly nonlinear, likely due to increased polyvalency. By exploiting this dynamic polyvalency, in combination with the self-assembly properties of the virus capsid, we show that mosaic PAVs can be constructed that operate under a digital AND gate regime, where two different protease inputs are required for virus activation. These results show viruses can be engineered as signal-integrating nanoscale nodes whose functional properties are regulated by multiple proteolytic signals with easily tunable and predictable response surfaces, a promising development toward advanced control of gene delivery.

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.

Current advances in retroviral gene therapy

There have been major changes since the incidents of leukemia development in X-SCID patients after the treatments using retroviral gene therapy. Due to the risk of oncogenesis caused by retroviral insertional activation of host genes, most of the efforts focused on the lentiviral therapies. However, a relative clonal dominance was detected in a patient with β-thalassemia Major, two years after the subject received genetically modified hematopoietic stem cells using lentiviral vectors. This disappointing result of the recent clinical trial using lentiviral vector tells us that the current and most advanced vector systems does not have enough safety. In this review, various safety features that have been tried for the retroviral gene therapy are introduced and the possible new ways of improvements are discussed. Additional feature of chromatin insulators, co-transduction of a suicidal gene under the control of an inducible promoter, conditional expression of the transgene only in appropriate target cells, targeted transduction, cell type-specific expression, targeted local administration, splitting of the viral genome, and site specific insertion of retroviral vector are discussed here.

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.

Designer nanoparticles: incorporating size, shape and triggered release into nanoscale drug carriers

IMPORTANCE OF THE FIELD

Although significant progress has been made in delivering therapeutic agents through micro and nanocarriers, precise control over in vivo biodistribution and disease-responsive drug release has been difficult to achieve. This is critical for the success of next generation drug delivery devices, as newer drugs, designed to interfere with cellular functions, must be efficiently and specifically delivered to diseased cells. The chief constraint in achieving this has been our limited repertoire of particle synthesis methods, especially at the nanoscale. Recent developments in generating shape-specific nanocarriers and the potential to combine stimuli-responsive release with nanoscale delivery devices show great promise in overcoming these limitations.

AREAS COVERED IN THIS REVIEW

How recent advances in fabrication technology allow synthesis of highly monodisperse, stimuli-responsive, drug-carrying nanoparticles of precise geometries is discussed. How particle properties, specifically shape and stimuli responsiveness, affect biodistribution, cellular uptake and drug release is also reviewed.

WHAT THE READER WILL GAIN

The reader is introduced to recent developments in intelligent drug nanocarriers and new nanofabrication approaches that can be combined with disease-responsive biomaterials. This will provide insight into the importance of controlling particle geometry and incorporating stimuli-responsive materials into drug delivery.

TAKE HOME MESSAGE

The integration of responsive biomaterials into shape-specific nanocarriers is one of the most promising avenues towards the development of next generation, advanced drug delivery systems.

Viral vectors: from virology to transgene expression

In the late 1970s, it was predicted that gene therapy would be applied to humans within a decade. However, despite some success, gene therapy has still not become a routine practise in medicine. In this review, we will examine the problems, both experimental and clinical, associated with the use of viral material for transgenic insertion. We shall also discuss the development of viral vectors involving the most important vector types derived from retroviruses, adenoviruses, herpes simplex viruses and adeno-associated viruses.

The development of gene therapy: from monogenic recessive disorders to complex diseases such as cancer

During the last 4 decades, gene therapy has moved from preclinical to clinical studies for many diseases ranging from monogenic recessive disorders such as hemophilia to more complex diseases such as cancer, cardiovascular disorders, and human immunodeficiency virus (HIV). To date, more than 1,340 gene therapy clinical trials have been completed, are ongoing, or have been approved in 28 countries, using more than 100 genes. Most of those clinical trials (66.5%) were aimed at the treatment of cancer. Early hype, failures, and tragic events have now largely been replaced by the necessary stepwise progress needed to realize clinical benefits. We now understand better the strengths and weaknesses of various gene transfer vectors; this facilitates the choice of appropriate vectors for individual diseases. Continuous advances in our understanding of tumor biology have allowed the development of elegant, more efficient, and less toxic treatment strategies. In this introductory chapter, we review the history of gene therapy since the early 1960s and present in detail two major recurring themes in gene therapy: (1) the development of vector and delivery systems and (2) the design of strategies to fight or cure particular diseases. The field of cancer gene therapy experienced an "awkward adolescence." Although this field has certainly not yet reached maturity, it still holds the potential of alleviating the suffering of many individuals with cancer.

Tumor-selective gene expression in a hepatic metastasis model after locoregional delivery of a replication-competent retrovirus vector

PURPOSE

Replication-competent retrovirus (RCR) vectors have been shown to achieve highly efficient and tumor-restricted replicative spread and gene transfer in vivo after direct intratumoral injection in a variety of primary cancer models. In this setting, the intrinsic inability of retroviruses to infect postmitotic normal cells, combined with their unique ability to persist through stable integration, allow further transduction of ectopic tumor foci as the infected cancer cells migrate. However, i.v. delivery of RCR vectors has never been tested previously, particularly in an immunocompetent tumor model.

EXPERIMENTAL DESIGN

We combined optical imaging, flow cytometry, and molecular analysis to monitor RCR vector spread after administration via locoregional infusion in a hepatic metastasis model of colorectal cancer.

RESULTS

Robust RCR replication was first confirmed in both human WiDr and murine CT26 colorectal cancer cells in vitro, with transduction levels reaching >90% in <12 days after virus inoculation at multiplicities of infection of 0.01 to 0.1. In vivo, infusion of RCR supernatant into the portal circulation resulted in progressive and significant transduction of multifocal intrahepatic CT26 tumors in syngeneic mice, averaging about 30% but with up to 60% transduction in some tumors within 4 weeks. However, immunohistochemistry and quantitative PCR analysis showed no evidence of RCR spread to adjacent normal liver or to any other normal tissues.

CONCLUSIONS

Our results thus show that locoregional infusion of RCR vectors can be used to deliver therapeutic genes selectively to tumor cells in the liver while sparing normal hepatocytes and without dissemination to extrahepatic normal tissues.

Matrix metalloproteases: underutilized targets for drug delivery

Pathophysiological molecules in the extracellular environment offer excellent targets that can be exploited for designing drug targeting systems. Matrix metalloproteases (MMPs) are a family of extracellular proteolytic enzymes that are characterized by their overexpression or overactivity in several pathologies. Over the last two decades, the MMP literature reveals heightened interest in the research involving MMP biology, pathology and targeting. This review describes various strategies that have been designed to utilize MMPs for targeting therapeutic entities. Key factors that need to be considered in the successful design of such systems have been identified based on the analyses of these strategies. Development of targeted drug delivery using MMPs has been steadily pursued; however, drug delivery efforts using these targets need to be intensified and focused to realize the clinical application of the fast developing fundamental MMP research.

Oncolytic efficacy and enhanced safety of measles virus activated by tumor-secreted matrix metalloproteinases

Cancer cells secrete matrix metalloproteinases (MMP) that degrade the extracellular matrix and are responsible for some hallmarks of malignant cancer. Many viruses, including a few currently used in oncolytic virotherapy clinical trials, depend on intracellular proteases to process their proteins and activate their particles. We show here for measles virus (MV) that particle activation can be made dependent of proteases secreted by cancer cells. The MV depends on the intracellular protease furin to process and activate its envelope fusion (F) protein. To make F protein activation cancer cell specific, we introduced hexameric sequences recognized by an MMP and identified the mutant proteins most effective in fusing MMP-expressing human fibrosarcoma cells (HT1080). We showed that an MMP inhibitor interferes with syncytia formation elicited by mutant F proteins and confirmed MMP-dependent cleavage by Edman degradation sequence analysis. We generated recombinant MVs expressing the modified F proteins in place of furin-activated F. These viruses spread only in cells secreting MMP. In nude mice, an MMP-activated MV retarded HT1080 xenograft growth as efficiently as the furin-activated MV vaccine strain. In MV-susceptible mice, the furin-activated virus caused lethal encephalitis upon intracerebral inoculation, whereas the MMP-activated did not. Thus, MV particle activation can be made dependent of proteases secreted by cancer cells, enhancing safety. This study opens the perspective of combining targeting at the particle activation, receptor recognition, and selective replication levels to improve the therapeutic index of MV and other viruses in ongoing clinical trials of oncolysis.

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.

Delivery of viral vectors to tumor cells: extracellular transport, systemic distribution, and strategies for improvement

It is a challenge to deliver therapeutic genes to tumor cells using viral vectors because (i) the size of these vectors are close to or larger than the space between fibers in extracellular matrix and (ii) viral proteins are potentially toxic in normal tissues. In general, gene delivery is hindered by various physiological barriers to virus transport from the site of injection to the nucleus of tumor cells and is limited by normal tissue tolerance of toxicity determined by local concentrations of transgene products and viral proteins. To illustrate the obstacles encountered in the delivery and yet limit the scope of discussion, this review focuses only on extracellular transport in solid tumors and distribution of viral vectors in normal organs after they are injected intravenously or intratumorally. This review also discusses current strategies for improving intratumoral transport and specificity of viral vectors.

Viral gene therapy strategies: from basic science to clinical application

A major impediment to the successful application of gene therapy for the treatment of a range of diseases is not a paucity of therapeutic genes, but the lack of an efficient non-toxic gene delivery system. Having evolved to deliver their genes to target cells, viruses are currently the most effective means of gene delivery and can be manipulated to express therapeutic genes or to replicate specifically in certain cells. Gene therapy is being developed for a range of diseases including inherited monogenic disorders and cardiovascular disease, but it is in the treatment of cancer that this approach has been most evident, resulting in the recent licensing of a gene therapy for the routine treatment of head and neck cancer in China. A variety of virus vectors have been employed to deliver genes to cells to provide either transient (eg adenovirus, vaccinia virus) or permanent (eg retrovirus, adeno-associated virus) transgene expression and each approach has its own advantages and disadvantages. Paramount is the safety of these virus vectors and a greater understanding of the virus-host interaction is key to optimizing the use of these vectors for routine clinical use. Recent developments in the modification of the virus coat allow more targeted approaches and herald the advent of systemic delivery of therapeutic viruses. In the context of cancer, the ability of attenuated viruses to replicate specifically in tumour cells has already yielded some impressive results in clinical trials and bodes well for the future of this approach, particularly when combined with more traditional anti-cancer therapies.

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.

Gene therapy: future therapies in osteoarthritis

The field of equine veterinary practice is in an ever-evolving state, requiring current technologies to be constantly evaluated for new applications. The specific use of gene therapy in the horse is a novel application. The authors want to help familiarize the equine practitioner with the concept of gene therapy, and introduce its use and potential future benefits for the equine industry in the treatment of osteoarthritis.

Gelatinase-mediated migration and invasion of cancer cells

The matrix metalloproteinases(MMP)-2 and -9, also known as the gelatinases have been long recognized as major contributors to the proteolytic degradation of extracellular matrix during tumor invasion. In the recent years, a plethora of non-matrix proteins have also been identified as gelatinase substrates thus significantly broadening our understanding of these enzymes as proteolytic executors and regulators in various physiological and pathological states including embryonic growth and development, angiogenesis and tumor progression, inflammation, infective diseases, degenerative diseases of the brain and vascular diseases. Although the effect of broad-spectrum inhibitors of MMPs in the treatment of cancer has been disappointing in clinical trials, novel mechanisms of gelatinase inhibition have been now identified. Inhibition of the association of the gelatinases with cell-surface integrins appears to offer highly specific means to target these enzymes without inhibiting their catalytic activity in multiple cell types including endothelial cells, tumor cells and leukocytes. Here, we review the multiple functions of the gelatinases in cancer, and especially their role in the tumor cell migration and invasion.

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.

Efficient Retroviral Vector Targeting of Carcinoembryonic Antigen-Positive Tumors

Many gene therapy approaches require specific, efficient gene delivery to cells in vivo. To target colorectal tumors we fused a single-chain variable fragment (scFv) directed against carcinoembryonic antigen (CEA) to the amphotropic murine leukemia virus envelope. A proline-rich hinge and matrix metalloprotease (MMP) cleavage site linked the two proteins. Following attachment to CEA, MMP cleavage of the envelope at the cell surface removed the scFv and proline-rich hinge, allowing transduction. This allowed selective targeting of CEA-positive cells in vivo after injection of producer cells at the site of the tumor, with up to 10% of cells within a CEA-positive tumor xenograft becoming transduced. Intraperitoneal injection of amphotropic producer cells resulted in transduction of cells in spleen, liver, and kidney, which was not detected when CEA-targeted producer cells were used. These results demonstrate the feasibility of using targeted retroviral vectors for in vivo gene delivery to tumors. Furthermore, the lack of transduction of host cells eliminates the risk of insertional mutagenesis leading to transformation of host hematopoietic cells.

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.

Targeting cytokines to inflammation sites

To increase the half-life of a cytokine and target its activation specifically to disease sites, we have engineered a latent cytokine using the latency-associated protein (LAP) of transforming growth factor-beta 1 (TGF-beta 1) fused via a matrix metalloproteinase (MMP) cleavage site to interferon (IFN)-beta at either its N or C terminus. The configuration LAP-MMP-IFN-beta resembles native TGF-beta and lacks biological activity until cleaved by MMPs, whereas the configuration IFN-beta-MMP-LAP is active. LAP provides for a disulfide-linked shell hindering interaction of the cytokine with its cellular receptors, conferring a very long half-life of 55 h in vivo. Mutations of the disulfide bonds in LAP abolish this latency. Samples of cerebrospinal fluid (CSF) or synovial fluid from patients with inflammatory diseases specifically activate the latent cytokine, whereas serum samples do not. Intramuscular injection in arthritic mice of plasmid DNA encoding these constructs demonstrated a greater therapeutic effect of the latent as compared to the active forms.

Avian leukosis virus is a versatile eukaryotic platform for polypeptide display

Display technology refers to methods of generating libraries of modularly coded biomolecules and screening them for particular properties. Retroviruses are good candidates to be a eukaryotic viral platform for the display of polypeptides synthesized in eukaryotic cells. Here we demonstrate that avian leukosis virus (ALV) provides an ideal platform for display of nonviral polyaeptides expressed in a eukaryotic cell substrate. Different sizes of polypeptides were genetically fused to the extreme N-terminus of the ALV envelope glycoprotein in an ALV infectious clone containing an alkaline phosphatase reporter gene. The chimeric envelope glycoproteins were efficiently incorporated into virions and were stably displayed on the surface of the virions through multiple virus replication cycles. The foreign polypeptides did not interfere with the attachment and entry functions of the underlying ALV envelope glycoproteins. The displayed polypeptides were fully functional and could efficiently mediate attachment of the recombinant viruses to their respective cognate receptors. This study demonstrates that ALV is an ideal display platform for the generation and selection of libraries of polypeptides where there is a need for expression, folding, and posttranslational modification in the endoplasmic reticulum of eukaryotic cells.

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.

Vectors for the treatment of autoimmune disease

Gene therapy has been applied in a variety of experimental models of autoimmunity with some success. In this article, we outline recent developments in gene therapy vectors, discuss advantages and disadvantages of each, and highlight their recent applications in autoimmune models. We also consider progress in vector targeting and components for regulating transgene expression, which will both improve gene therapy safety and empower gene therapy to fullfil its potential as a therapeutic modality. In conclusion, we consider candidate vectors that satisfy requirements for application in the principal therapeutic strategies in which gene therapy will be applied to autoimmune conditions.

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.

Enzyme-activated targeting of liposomes

It has become increasingly evident that tissues utilize specific localization of enzymes to perform certain tasks, often associated with various types of tissue remodeling. The ubiquitous presence of such enzymes, along with their specific localizations, provides an ideal opportunity to elicit specific delivery via an enzyme-triggered mechanism. A survey of some of the recent progress in enzyme-activated targeting of delivery systems, with a focus on a few liposomal systems, is presented.

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.