Extended tropism of an adenoviral vector does not circumvent the maturation-dependent transducibility of mouse skeletal muscle

Specific and efficient targeting of adenovirus vectors to macrophages: application of a fusion protein between an adenovirus-binding fragment and avidin, linked to a biotinylated oligonucleotide

BACKGROUND

The application of serotype 5 adenoviruses (Ad5) in macrophages is hampered by the absence of the endogenous coxsackie adenovirus receptor (CAR).

METHODS

To overcome this limitation, we first generated a linker protein consisting of the virus-binding domain of CAR and the C-terminus of avidin. Second, to target macrophages, this linker protein was equipped with the biotinylated (bio) oligonucleotide dA6G10, which was previously shown to display a high affinity for the scavenger receptor A (SR-A).

RESULTS

As compared to nontargeted virus, the linker protein equipped with bio-dA6G10 showed a 500-fold increased reporter gene expression in mouse macrophage RAW264.7 cells. A linker protein equipped with a bio-dA16 control oligonucleotide was inactive. Moreover, the bio-dA6G10-equipped linker showed a 390-fold increased luciferase expression in the macrophage cell line J774 and 276- and 150-fold increased reporter gene expression in primary peritoneal and bone marrow (BM)-derived macrophages, respectively. Using BM-derived macrophages from SR-A knockout mice, it was shown that the dA6G10-mediated uptake is predominantly SR-A-mediated.

CONCLUSIONS

Thus, we have developed a novel tool to link biotinylated ligands to a virus-binding fragment of CAR and have exploited this linker protein to extend the applicability of Ad5 to infect transformed and primary macrophages.

An adenoviral type 5 vector carrying a type 35 fiber as a vaccine vehicle: DC targeting, cross neutralization, and immunogenicity

Substituting the coat proteins of adenoviral vector serotype 5 (Ad5) can alter vector tropism and circumvent vector neutralization. Here we report that an Ad5 vector carrying a part of the fiber molecule of human subgroup B adenovirus serotype 35 (Ad5.Fib35) transduces cultured human dendritic cells (DC) and circulating myeloid derived DC with approximately 10-fold greater efficiency than Ad5 in vitro. The improved DC transduction results in increased T-cell activation ex vivo. In vivo however, immunogenicity of the vectors in mice and non-human primates did not correlate with in vitro DC tropism. Ad5.Fib35 was less immunogenic in monkeys than Ad5, despite the improved primate DC tropism of Ad5.Fib35. In mice with high Ad5 vector-specific immunity, Ad5.Fib35 showed no significant difference in anti-insert immunity over Ad5 indicating that fiber exchange alone does not evade pre-existing Ad5 immunity. We thus conclude that, for ex vivo vaccination, Ad5.Fib35 shows promise as vector for loading of DC but is unable to circumvent anti-Ad5 immunity limiting its in vivo utility.

Adenovirus fibre exchange alters cell tropism in vitro but not transgene-specific T CD8+ immune responses in vivo

Gene transfer with recombinant adenoviruses (rAds) is a powerful means of inducing an immune response against a transgene product. However, little is known about the mechanisms that underlie the induction of the immune response after intramuscular inoculation of adenovirus and, in particular, the relative role of the different cell types transduced. Several studies have suggested that CD8+ cytotoxic T lymphocyte responses elicited after inoculation of adenoviruses (Ads) are induced both by direct transduction of antigen presenting cells (APCs) and by cross-priming. In the present study, a library of fibre-chimeric rAds was screened in order to identify rAds with distinct capacities to express transgene product in murine cell types naturally found in muscle, i.e. myoblasts, endothelial cells (both representing non-APCs) and dendritic cells (representing APCs). Four selected pseudotypes, differing in their ability to infect muscular cells were used to immunize C57BL/6 mice. The relationship between the capacity to transduce non-APC or APC in vitro and the ability to induce humoral and cellular responses against the beta-galactosidase antigen after intramuscular inoculation were studied. Results indicate that CD8+ T cell responses against the beta-galactosidase antigen were similar after inoculation of the four viruses, thus revealing no direct relationship with their ability to transduce myoblasts, endothelial cells or dendritic cells in vitro.

Helper-Dependent Adenoviral Vectors Containing Modified Fiber for Improved Transduction of Developing and Mature Muscle Cells

Adenoviruses (Ads) have shown great utility as vectors for the delivery of genes to mammalian cells, partly because of their ability to infect a wide range of different cell types independent of the replicative state of the cell. However, Ads do not transduce mature muscle efficiently because of low levels of the natural viral primary receptor, the coxsackie virus and adenovirus receptor, on the surface of adult muscle cells. In this study, we have addressed whether incorporation of polylysine [p(K)] or arginine-glycine-aspartic acid (RGD) placed in the H-I loop of the adenoviral fiber protein can improve helper-dependent Ad vector (hdAd) transduction of mature muscle cells. We show that incorporation of the p(K) motif into the fiber of early region 1 (E1)-deleted Ad results in enhanced transduction of undifferentiated and differentiated C2C12 cells relative to a virus, containing a wild-type fiber (12- and 21-fold enhancement, respectively). Incorporation of the RGD motif resulted in only a 60-70% increase in transduction efficiency in these cells. The two fiber modifications were then incorporated into helper viruses for use in the Cre-lox system for generating hdAd, and the resulting retargeted Ad vectors, which encoded the beta-galactosidase reporter gene (beta-Gal), demonstrated enhanced transduction of C2C12 cells in culture. Although hdAdpK also showed enhanced infection of mature mouse muscle in vivo, hdAdRGD did not. All hdAd vectors elicited only minor anti-Ad immune responses, compared with an E1-deleted control vector, but each vector elicited strong anti-beta-Gal immunoreactivity. Our results demonstrate that hdAd with modified cell tropism can be generated efficiently and, in the case of polylysine-modified hdAd, can lead to improved transduction of adult muscle cells in vivo.

Rat strain differences in the ectopic osteogenic potential of recombinant human BMP adenoviruses

Different animal strains have different genetic backgrounds that influence their physiological function and pathological process. The differences in genetic background may affect the efficiency of adenoviral infection and target gene expression and further cause different gene therapy results when target genes are delivered with adenoviral vectors. In this study, ectopic bone was not seen in ADCMVBMP4 injection sites, but was formed in ADCMVBMP9 injection sites in all rat strains. The mean volumes of bone induced with ADCMVBMP9 were 0.87 +/- 0.2 cm3 in Wistar, 0.26 +/- 0.1 cm3 in Long-Evans, 0.34 +/- 0.2 cm3 in Sprague-Dawley, 0.44 +/- 0.1 cm3 in ACI, 0.66 +/- 0.2 cm3 in PVG, and 0.58 +/- 0.1 cm3 in Fischer 344 rats. This indicates that ADCMVBMP9 has different bone formation potentials in different immunocompetent rat strains (P = 0.02). The basic levels of CD4+ and CD8+ T cells in blood before viral infection and titers of adenoviral neutralizing antibodies 30 days post-viral infection were significantly different among rat strains (P < 0.01). The efficiencies of target gene expression delivered with adenovirus were also significantly different in primary muscle cell cultures from different rat strains (P < 0.01). The different osteogenic potentials of ADCMVBMP9 among rat strains may be, in part, due to the differences in immune factors and target gene expression efficiency in muscle tissue.

Replication-deficient human adenovirus type 35 vectors for gene transfer and vaccination: efficient human cell infection and bypass of preexisting adenovirus immunity

Replication-deficient human adenovirus type 5 (Ad5) can be produced to high titers in complementing cell lines, such as PER.C6, and is widely used as a vaccine and gene therapy vector. However, preexisting immunity against Ad5 hampers consistency of gene transfer, immunological responses, and vector-mediated toxicities. We report the identification of human Ad35 as a virus with low global prevalence and the generation of an Ad35 vector plasmid system for easy insertion of heterologous genes. In addition, we have identified the minimal sequence of the Ad35-E1B region (molecular weight, 55,000 [55K]), pivotal for complementation of fully E1-lacking Ad35 vector on PER.C6 cells. After stable insertion of the 55K sequence into PER.C6 cells a cell line was obtained (PER.C6/55K) that efficiently transcomplements both Ad5 and Ad35 vectors. We further demonstrate that transduction with Ad35 is not hampered by preexisting Ad5 immunity and that Ad35 efficiently infects dendritic cells, smooth muscle cells, and synoviocytes, in contrast to Ad5.

Recombinant micro-genes and dystrophin viral vectors

An effective gene therapy for Duchenne muscular dystrophy ideally relies on the ability to provide long-term expression to muscle tissue of the missing protein, dystrophin. Early work in the mdx mouse using a 6.3 kb mini-dystrophin cDNA, carried out in either adenoviral or retroviral vectors was generally successful, however, expression was only transient. In an attempt to remedy this problem, two approaches are being investigated. The first of these is a hybrid vector system that combines the efficacy of gene transfer into skeletal muscle of adenoviral vectors with the long-term stability of retroviral vectors. The second utilises the inherently efficient transducing properties and stability of the adeno-associated viral delivery system. Using highly truncated micro-dystrophin cDNAs we have shown that both vector systems were able to restore dystrophin and dystrophin-associated protein expression at the plasma membrane of mdx mice for prolonged periods of time. Additionally, evaluation of central nucleation indicated a significant inhibition of degenerative dystrophic muscle pathology. These studies suggest that hybrid adenoviral-retroviral and adeno-associated viral vectors are capable of ameliorating dystrophic pathology at the cellular level and as such are useful tools in the development of a gene therapy for Duchenne muscular dystrophy.

Strategies for muscle-specific targeting of adenoviral gene transfer vectors

Currently, adenoviral transfer of therapeutic genes such as dystrophin is hampered by low transduction efficiency of adult skeletal muscle. This is largely due to the lack of appropriate virus attachment receptors on the myofiber surface. Recent studies in transgenic mice revealed that upregulation of Coxsackie- and adenovirus receptor improves gene transfer efficiency by approximately ten-fold. Conversely, the vector load that needed to be administered to achieve sufficient gene transfer could be lowered significantly. Reduced viral vector loads may help to control virally mediated toxicity and immunogenicity. To date, there are no drugs or methods known to increase Coxsackie- and adenovirus receptor expression in skeletal muscle that would be easily applicable in humans. However, alternative strategies such as vector retargeting are currently being investigated that may allow for an increase in binding of adenoviral vectors to skeletal muscle. Recent experiments have shown that directed mutagenesis of the adenoviral fiber knob allows for a significant reduction in Coxsackie- and adenovirus receptor binding and for introduction of a new binding domain. Therefore, vector retargeting towards efficient and specific infection of skeletal muscle may be achieved by directed genetic alteration of adenoviral capsid proteins.

Adenovirus mediated gene transfer to skeletal muscle

Transfer of therapeutic genes into muscle tissue has promise for the treatment of a variety of muscular dystrophies. Various vectors have been used to deliver genes to skeletal muscle but their application has faced several major limitations including: (1) the lack of transgene persistence caused by the immune rejection of transduced myofibers and/or vector toxicity, and (2) the maturation dependence of viral transduction. While the immunorejection and/or cytotoxic problems are being overcome with the development of new vectors, maturation-dependent viral transduction is still a major hurdle in gene transfer to skeletal muscle. Poor adenoviral transduction in mature myofibers has been attributed to: (1) the extracellular matrix of mature myofibers may form a physical barrier and prevent the passage of large viral particles; (2) viral receptors are down-regulated with muscle maturation; and (3) loss of myoblasts with muscle maturation, which serve as intermediaries in the viral transduction. In this review, we will focus on recent developments in overcoming those hurdles of gene therapy in skeletal muscle, especially to adenovirus (Ad), including: (1) new mutant vectors lacking all viral genes to decrease immunogenicity, and hence, improve persistence of transgene expression in muscle in vivo; (2) using tissue specific promoters to evade immunorejection; (3) permeabilization of the extracellular matrix; (4) modifying the viral receptors in mature myofibers; and (5) myoblast or muscle stem cell mediated ex vivo gene transfer.

Modification of a fiber protein in an adenovirus vector improves in vitro gene transfer efficiency to the mouse microglial cell line

In microglia, it is difficult to introduce exogenous genes of interest even by recombinant adenovirus vectors (Ad) which can infect with high efficiency only to the cells expressing coxackievirus and adenovirus receptors (CAR). We found a lack of CAR expression in primary cultured murine microglia (PCMG) and its immortalized cell line MG5 by reverse transcription-polymerase chain reaction. In order to improve the efficiency of gene transfer, we generated a novel Ad (Ad-RGD) by an incorporation of the Arg-Gly-Asp motif (RGD) containing peptide in the HI loop of the viral fiber knob domain, which enables the virus to contact target cells through alpha V integrins which are known to be ubiquitously expressed on the surface of mammalian cells. Ad-RGD showed a remarkable improvement (13-18-fold) in the delivery of Escherichia coli LacZ gene in MG5 cells and a moderate increase in PCMG cells under the treatment with granulocyte-macrophage colony stimulating factor. These results suggest that Ad-RGD may be a potent tool for the delivery of genes to microglia activated by optimum stimulation, and thus analyzing the function of microglia with utilization of MG5 and PCMG cells.

Targeted adenoviral vectors

Replication-defective vectors based on human adenovirus serotypes 2 and 5 (Ad2 and Ad5) possess a number of attributes which favor their use as gene delivery vehicles in gene therapy applications. However, the widespread distribution of the primary cellular receptor for Ad, the coxsackievirus and adenovirus receptor (CAR), allows Ad vectors to infect a broad range of cells in the host. Conversely, a number of tissues which represent important targets for gene therapy, such as the airway epithelium and cancer cells, are refractory to Ad infection due a paucity of CAR. Thus, there is a strong rationale for the development of CAR-independent Ad vectors capable of enhanced specificity and efficiency of gene transfer to target cells. In this article we review the approaches which have been employed to generate tropism-modified Ad vectors. These targeting strategies have led to improvements in the safety and efficacy of Ad vectors and have the potential to yield an increased therapeutic benefit in the human clinical context.

Efficient gene transfer by fiber-mutant adenoviral vectors containing RGD peptide

One of the hurdles to adenovirus (Ad)-mediated gene transfer is that Ad vectors mediate inefficient gene transfer into cells lacking in the primary receptors, Coxsackievirus and adenovirus receptor (CAR). We previously developed a fiber-mutant Ad vector containing the Arg-Gly-Asp (RGD)-containing peptide motif on the HI loop of the fiber knob, and showed that the mutant vector had enhanced gene transfer activity to human glioma cells, which showed little CAR expression, compared to the vector containing wild type fiber. In this study, the feasibility of the Ad vector containing RGD peptide on the fiber knob was examined in a wide variety of cell types: CAR-positive or -negative human tumor cells, mouse cells, and leukemia cells. The mutant vector infected the cells, which lacked CAR expression but showed alpha(v) integrin expression, about 10-1000 times more efficiently than the vector containing wild type fiber via an RGD-integrin (alpha(v)beta3 and alpha(v)beta5)-dependent, CAR-independent cell entry pathway. The results of this study indicate that Ad vector containing RGD peptide on the fiber knob could be of great utility for gene therapy and gene transfer experiments.

Forced myofiber regeneration promotes dystrophin gene transfer and improved muscle function despite advanced disease in old dystrophic mice

Duchenne muscular dystrophy (DMD) is caused by defects in the dystrophin gene. In young dystrophic mdx mice, immature regenerating myofibers represent the principal substrate for adenovirus vector (AdV)-mediated dystrophin gene transfer. However, in DMD patients immature regenerating myofibers are generally sparse. Such a situation also exists in old mdx mice, which may represent a more realistic model. Therefore, here we have used old mdx mice (of 14- to 17 months of age) to test the hypothesis that one-time administration of a myonecrotic agent can transiently re-establish a population of immature myofibers susceptible to AdV-mediated dystrophin gene transfer. This strategy led to upregulation of the coxsackie/adenovirus attachment receptor by means of induction of regenerating myofibers, significantly augmented AdV-mediated dystrophin gene expression, and enhanced force-generating capacity. In addition, it led to an increased resistance to contraction-induced injury compared with untreated controls. The latter protective effect was positively correlated with the number of dystrophin-expressing myofibers (r=0.83, P<0.05). Accordingly, the risk:benefit ratio associated with the sequential use of forced myofiber regeneration and AdV-mediated dystrophin gene transfer was favorable in old mdx mice despite advanced disease. These findings have implications for the potential applicability of AdV-mediated gene therapy to DMD and other muscle diseases in which immature regenerating myofibers are lacking.

Post-mitotic, differentiated myotubes efficiently produce retroviral vector from hybrid adeno-retrovirus templates

We have examined the ability of proliferating myoblasts and post-mitotic, differentiated myotubes to produce retroviral vector using hybrid adeno-retroviral vectors as templates. We show that production of retroviral vector from myoblasts peaks 48 h after adenoviral infection at 4.8 x 10(4) cfu/ml and is scarcely detectable by 96 h. Both fully and partially differentiated myotubes were able to generate a sustained increase in the levels of retroviral vector compared with myoblasts peaking 48 h at 1.4 x 10(5) cfu/ml and 1.8 x 10(5) cfu/ml, respectively. Addition of the cell cycle inhibitor aphidicolin (5 microg/ml) had no effect on the production of retroviral vector from fully differentiated myotubes, but resulted in an 80% increase in vector production from partially differentiated myotubes. Thus indicating that retroviral vector production is more efficient in post-mitotic myotubes and is independent of muscle cell cycle progression.

Gene therapy for therapeutic angiogenesis in critically ischaemic lower limb - on the way to the clinic

Currently, no effective pharmacological treatment is available for vascularisation defects in lower limbs. Many patients presenting with persistent pain and ischaemic ulcers are not suitable candidates for surgical or endovascular approaches. Further refinement of the available methods will undoubtedly lead to a more active approach towards treatment of peripheral arterial occlusive disease (PAOD). Recently, therapeutic angiogenesis, in the form of recombinant growth factor administration or gene therapy, has emerged as a novel tool to treat these patients. However, improved gene transfer methods and better understanding of blood vessel formation are required to bring therapeutic angiogenesis to clinical practice. Here we review the clinical problem (PAOD), mechanisms of blood vessel formation (angiogenesis, vasculogenesis and arteriogenesis), experimental evidence and clinical trials for therapeutic angiogenesis in critically ischaemic lower limbs. Also, angiogenic growth factors, including vascular endothelial growth factors (VEGFs) and fibroblast growth factors (FGFs), delivery methods, and vectors for gene transfer in skeletal muscle, are discussed. In addition to vascular growth, gene transfer of growth factors may enhance regeneration, survival, and innervation of ischaemic skeletal muscle. Nitric oxide (NO) appears to be a key mediator in vascular homeostasis and growth, and a reduction in its production by age, hypercholesterolemia or diabetes leads to the impairment of ischaemic disorders.

Muscle-specific overexpression of the adenovirus primary receptor CAR overcomes low efficiency of gene transfer to mature skeletal muscle

Significant levels of adenovirus (Ad)-mediated gene transfer occur only in immature muscle or in regenerating muscle, indicating that a developmentally regulated event plays a major role in limiting transgene expression in mature skeletal muscle. We have previously shown that in developing mouse muscle, expression of the primary Ad receptor CAR is severely downregulated during muscle maturation. To evaluate how global expression of CAR throughout muscle affects Ad vector (AdV)-mediated gene transfer into mature skeletal muscle, we produced transgenic mice that express the CAR cDNA under the control of the muscle-specific creatine kinase promoter. Five-month-old transgenic mice were compared to their nontransgenic littermates for their susceptibility to AdV transduction. In CAR transgenics that had been injected in the tibialis anterior muscle with AdVCMVlacZ, increased gene transfer was demonstrated by the increase in the number of transduced muscle fibers (433 +/- 121 in transgenic mice versus 8 +/- 4 in nontransgenic littermates) as well as the 25-fold increase in overall beta-galactosidase activity. Even when the reporter gene was driven by a more efficient promoter (the cytomegalovirus enhancer-chicken beta-actin gene promoter), differential transducibility was still evident (893 +/- 149 versus 153 +/- 30 fibers; P < 0.001). Furthermore, a fivefold decrease in the titer of injected AdV still resulted in significant transduction of muscle (253 +/- 130 versus 14 +/- 4 fibers). The dramatic enhancement in AdV-mediated gene transfer to mature skeletal muscle that is observed in the CAR transgenics indicates that prior modulation of the level of CAR expression can overcome the poor AdV transducibility of mature skeletal muscle and significant transduction can be obtained at low titers of AdV.

The role of receptors in the maturation-dependent adenoviral transduction of myofibers

One of the major hurdles facing the application of adenoviral gene transfer to skeletal muscle is the maturation-dependent transduction of muscle myofibers. It was recently proposed that the viral receptors (Coxsackie and adenovirus receptor (CAR) and the integrins alphavbeta3/beta5) play a major role in the poor adenoviral transduction of mature myofibers. Here we report the findings of morphological studies designed to determine experimentally the role of receptors in the adenoviral transduction of mature myofibers. First, we observed that the expression of both attachment and internalization receptors did not change significantly during muscle development. Second, when an extended tropism adenoviral vector (AdPK) that attaches to heparan sulfate proteoglycan (HSP) is used, a significant reduction of adenoviral transduction still occurs in mature myofibers despite HSP's high expression in mature skeletal muscle fibers. Third, when the adeno-associated virus (AAV) is used, which also utilizes HSP as a viral receptor, muscle fibers at different maturities can be highly transduced. Fourth, the pre-irradiation of the skeletal muscle of newborn mice to inactivate myoblasts dramatically decreased the transduction level of Ad and AdPK, but had no effect on AAV-mediated viral transduction of immature myofibers. These results taken together suggest that the viral receptor(s) is not a major determinant in maturation-dependent adenoviral transduction of myofibers.

Ovine adenovirus vectors mediate efficient gene transfer to skeletal muscle

Ovine adenovirus (OAV) vectors represent a promising tool for human gene therapy since these vectors overcome the problem of pre-existing immunity against human adenovirus vectors. In this report we investigated the in vivo characteristics of this novel vector system with respect to its potential for gene transfer into skeletal muscle. We found that moderate doses of an OAV-derived vector expressing the human alpha1-antitrypsin gene (OAVhaat) infected skeletal muscle in mice very efficiently resulting in high serum hAAT levels. The infection was restricted to skeletal muscle, but gene expression was transient and vector DNA was rapidly cleared. Vector clearance was also observed with a vector that lacked the transgene. The loss of vector DNA was accompanied by a cellular immune response in the infected muscle but was not connected with detectable expression of early or late genes of the viral backbone as analyzed by RT-PCR. A very low dose of OAVhaat (3x 10(7) infectious particles) was sufficient to produce reasonable amounts (>100 ng/ml) of serum hAAT, and this was accompanied by a weak immune response to the vector. Under these conditions, a second intramuscular injection of the same recombinant OAV vector was successful. Our study expands the known tissue tropism of OAV-derived vectors in vivo and points to the possible utility of the vector for muscle gene transfer and vaccination.