Innate immune mechanisms dominate elimination of adenoviral vectors following in vivo administration

Towards fibroid gene therapy: adenovirus-mediated delivery of herpes simplex virus 1 thymidine kinase gene/ganciclovir shrinks uterine leiomyoma in the Eker rat model

BACKGROUND/AIMS

The objective of this study was to assess in vivo gene therapy of uterine leiomyomas in the Eker rat model using adenovirus (Ad)-mediated delivery of herpes simplex virus 1 thymidine kinase gene (HSV1TK) followed by ganciclovir (GCV) treatment.

METHODS

We randomized 27 female Eker rats with MRI-confirmed uterine leiomyomas to a single treatment with direct intra-tumor injection of Ad-HSV1TK/GCV, Ad-LacZ/GCV, or medium alone. Samples were collected from tumors, other body organs, and blood at 10, 20, and 30 days after treatment to assess the safety and efficacy of the treatment.

RESULTS

Ad-HSV1TK/GCV treatment significantly decreased uterine fibroid volume by 75 +/- 16, 58.7 +/- 6.3, and 67.5 +/- 27.5%, of the pretreatment volume at days 10, 20, and 30, respectively. Ad-HSV1TK/GCV increased caspase-3 activity, Bax expression, and TUNEL apoptosis marker, and it decreased cyclin D1, PCNA, Bcl2, and PARP protein expressions. Ad transfection induced local CD4+ and CD8+ infiltration and serum anti-Ad antibodies. Additionally, Ad transfection was tumor-localized and safe to non-target tissues.

CONCLUSION

These studies demonstrate a marked efficiency and high safety for the Ad-HSV1TK/GCV therapeutic approach in the context of Eker rat uterine leiomyomas and provide essential preclinical data for the development of Ad-HSV1TK/GCV gene therapy for uterine fibroids.

Dendritic cells and T cells deliver oncolytic reovirus for tumour killing despite pre-existing anti-viral immunity

Reovirus is a naturally occurring oncolytic virus currently in early clinical trials. However, the rapid induction of neutralizing antibodies represents a major obstacle to successful systemic delivery. This study addresses, for the first time, the ability of cellular carriers in the form of T cells and dendritic cells (DC) to protect reovirus from systemic neutralization. In addition, the ability of these cellular carriers to manipulate the subsequent balance of anti-viral versus anti-tumour immune response is explored. Reovirus, either neat or loaded onto DC or T cells, was delivered intravenously into reovirus-naive or reovirus-immune C57Bl/6 mice bearing lymph node B16tk melanoma metastases. Three and 10 days after treatment, reovirus delivery, carrier cell trafficking, metastatic clearance and priming of anti-tumour/anti-viral immunity were assessed. In naive mice, reovirus delivered either neat or through cell carriage was detectable in the tumour-draining lymph nodes 3 days after treatment, though complete clearance of metastases was only obtained when the virus was delivered on T cells or mature DC (mDC); neat reovirus or loaded immature DC (iDC) gave only partial early tumour clearance. Furthermore, only T cells carrying reovirus generated anti-tumour immune responses and long-term tumour clearance; reovirus-loaded DC, in contrast, generated only an anti-viral immune response. In reovirus-immune mice, however, the results were different. Neat reovirus was completely ineffective as a therapy, whereas mDC--though not iDC--as well as T cells, effectively delivered reovirus to melanoma in vivo for therapy and anti-tumour immune priming. Moreover, mDC were more effective than T cells over a range of viral loads. These data show that systemically administered neat reovirus is not optimal for therapy, and that DC may be an appropriate vehicle for carriage of significant levels of reovirus to tumours. The pre-existing immune status against the virus is critical in determining the balance between anti-viral and anti-tumour immunity elicited when reovirus is delivered by cell carriage, and the viral dose and mode of delivery, as well as the immune status of patients, may profoundly affect the success of any clinical anti-tumour viral therapy. These findings are therefore of direct translational relevance for the future design of clinical trials.

Redundant and synergistic mechanisms control the sequestration of blood-born adenovirus in the liver

Human adenovirus (Ad) is a ubiquitous pathogen causing a wide range of diseases. Although the interactions of human Ad serotype 5 (Ad5) with susceptible cells in vitro are known in great detail, host factors controlling the tissue specificity of Ad5 infection in vivo remain poorly understood. Here, we analyzed the mechanisms of sequestration by the liver for blood-born human Ads and Ad5-based vectors. Our data suggest that several known mechanisms that lead to Ad5 sequestration by the liver become engaged in a redundant, sequential, and synergistic manner to ensure the rapid clearance of circulating virus particles from the blood. These mechanisms include (i) trapping of the virus by liver residential macrophages, Kupffer cells; (ii) Ad5 hepatocyte infection via blood factor-hexon interactions; and (iii) Ad5 penton RGD motif-mediated interactions with liver endothelial cells and hepatocytes, mediating virus retention in the space of Disse. More important, we show that when all of these mechanisms are simultaneously inactivated via mutations of Ad5 capsid proteins and pharmacological interventions, virus sequestration by the liver is markedly reduced. Therefore, our study is the first demonstration of the principal possibility of ablating the sequestration of blood-born Ad in the liver via specific inactivation of a defined set of mechanisms that control this process.

INGN 007, an oncolytic adenovirus vector, replicates in Syrian hamsters but not mice: comparison of biodistribution studies

Preclinical biodistribution studies with INGN 007, an oncolytic adenovirus (Ad) vector, supporting an early stage clinical trial were conducted in Syrian hamsters, which are permissive for Ad replication, and mice, which are a standard model for assessing toxicity and biodistribution of replication-defective (RD) Ad vectors. Vector dissemination and pharmacokinetics following intravenous administration were examined by real-time PCR in nine tissues and blood at five time points spanning 1 year. Select organs were also examined for the presence of infectious vector/virus. INGN 007 (VRX-007), wild-type Ad5 and AdCMVpA (an RD vector) were compared in the hamster model, whereas only INGN 007 was examined in mice. DNA of all vectors was widely disseminated early after injection, but decayed rapidly in most organs. In the hamster model, DNA of INGN 007 and Ad5 was more abundant than that of the RD vector AdCMVpA at early times after injection, but similar levels were seen later. An increased level of INGN 007 and Ad5 DNA but not AdCMVpA DNA in certain organs early after injection, and the presence of infectious INGN 007 and Ad5 in lung and liver samples at early times after injection, strongly suggests that replication of INGN 007 and Ad5 occurred in several Syrian hamster organs. There was no evidence of INGN 007 replication in mice. In addition to providing important information about INGN 007, the results underscore the utility of the Syrian hamster as a permissive immunocompetent model for Ad5 pathogenesis and oncolytic Ad vectors.

Cancer gene therapy with targeted adenoviruses

BACKGROUND

Clinical experience with adenovirus vectors has highlighted the need for improved delivery and targeting.

OBJECTIVE

This manuscript aims to provide an overview of the techniques currently under development for improving adenovirus delivery to malignant cells in vivo.

METHODS

Primary research articles reporting improvements in adenoviral gene delivery are described. Strategies include genetic modification of viral coat proteins, non-genetic modifications including polymer encapsulation approaches and pharmacological interventions.

RESULTS/CONCLUSION

Reprogramming adenovirus tropism in vitro has been convincingly demonstrated using a range of genetic and physical strategies. These studies have provided new insights into our understanding of virology and the field is progressing. However, there are still some limitations that need special consideration before adenovirus-targeted cancer gene therapy emerges as a routine treatment in the clinical setting.

Gene therapy targeting leiomyoma: adenovirus-mediated delivery of dominant-negative estrogen receptor gene shrinks uterine tumors in Eker rat model

OBJECTIVE

To evaluate the utility of gene therapy for uterine fibroids in the Eker rat model using an adenovirus-mediated delivery of a dominant-negative estrogen receptor gene (Ad-DNER).

DESIGN

Animal study.

SETTING

University animal laboratory.

ANIMAL(S)

Twenty-seven female Eker rats.

INTERVENTION(S)

We randomized Eker rats with magnetic resonance imaging (MRI)-confirmed uterine leiomyomas to a single treatment of direct intrafibroid injection with Ad-DNER, Ad-bacterial ss-galactosidase, or vehicle.

MAIN OUTCOME MEASURE(S)

Tumor volumes were determined by MRI scanning and caliper measurement. Samples of serum, fibroid tumors, and various organs were collected at 8, 15, and 30 days after treatment to assess treatment safety and efficacy.

RESULT(S)

The Ad-DNER treatment significantly decreased uterine fibroid volume by 45%, 80%, and 77.4% of pretreatment volume at days 8, 15, and 30, respectively, and modulated the expression of apoptosis-, proliferation-, and extracellular matrix-related genes' compared with control animals. The Ad-DNER did not produce any toxic effects in nontarget tissues.

CONCLUSION(S)

The Ad-DNER treatment shrinks Eker rats' fibroids, in part, via modulation of several estrogen-regulated genes. This safe gene therapy approach presents a promising conservative treatment option for women with symptomatic uterine fibroids.

Herpes simplex virus type 1 thymidine kinase sequence fused to the lacz gene increases levels of {beta}-galactosidase activity per genome of high-capacity but not first-generation adenoviral vectors in vitro and in vivo

Increased transgene expression per vector genome is an important goal in the optimization of viral vectors for gene therapy. Herein we demonstrate that herpes simplex virus type 1 (HSV1) thymidine kinase (TK) gene sequences (1,131 bp) fused to the 3' end of lacZ increase transgene expression from high-capacity adenoviral vectors (HCAd), but not from first-generation (Ad) vectors. The woodchuck hepatitis virus posttranscriptional regulatory element (WPRE), in contrast, increased transgene expression levels from Ad but not HCAd vectors. The differential activity of the HSV1 TK gene and WPRE sequences was detected both in vitro and in vivo and suggests potentially different mechanisms of action or the interaction of these elements with vector genomic sequences.

Polymers for viral gene delivery

BACKGROUND

The development of viral vectors capable of providing efficient gene transfer in diseased tissues without causing any pathogenic effects is pivotal for overcoming the many challenges facing gene therapy.

OBJECTIVE

Immune responses against viral vectors, inadequate gene expression and inefficient targeting to specific cells in vivo are some of the major problems limiting the clinical utility of viral gene therapy.

METHODS

This review will focus on recent progress in strategic polymer-based modifications to improve the performance and biocompatibility of a variety of viral vectors. We will discuss the preclinical development of four approaches involving injectable polymers, polyelectrolytes, polymer microspheres and polymer-virus conjugates.

RESULTS/CONCLUSION

Much progress has been made in creating 'hybrid' gene delivery vectors that combine the strengths of polymers and viruses. With further optimization, these hybrid vectors, which may be safer and more effective, are likely to succeed in clinical applications.

In vitro and in vivo properties of adenovirus vectors with increased affinity to CD46

Gene transfer vectors containing adenovirus (Ad) serotype 35 (Ad35) fibers have shown promise for cancer and stem cell gene therapy. In this study, we attempted to improve the in vitro and in vivo infection properties of these vectors by increasing their affinity to the Ad35 fiber receptor CD46. We constructed Ad vectors containing either the wild-type Ad35 fiber knob (Ad5/35) or Ad35 knob mutants with 4-fold- and 60-fold-higher affinity to CD46 (Ad5/35+ and Ad5/35++, respectively). In in vitro studies with cell lines, the higher affinities of Ad5/35+ and Ad5/35++ to CD46 did not translate into correspondingly higher transduction efficiencies, regardless of the CD46 receptor density present on cells. However, in vivo, in a mouse model with preestablished CD46(high) liver metastases, intravenous injection of Ad5/35++ resulted in more-efficient tumor cell transduction. We conclude that Ad5/35 vectors with increased affinity to CD46 have an advantage in competing with non-CD46-mediated sequestration of vector particles after intravenous injection.

Adenoviral vector-mediated glucagon-like peptide 1 gene therapy improves glucose homeostasis in Zucker diabetic fatty rats

BACKGROUND

Glucagon-like peptide-1 (GLP-1) is a gut-derived incretin hormone that plays an important role in glucose homeostasis. Its functions include glucose-stimulated insulin secretion, suppression of glucagon secretion, deceleration of gastric emptying, and reduction in appetite and food intake. Despite the numerous antidiabetic properties of GLP-1, its therapeutic potential is limited by its short biological half-life due to rapid enzymatic degradation by dipeptidyl peptidase IV. The present study aimed to demonstrate the therapeutic effects of constitutively expressed GLP-1 in an overt type 2 diabetic animal model using an adenoviral vector system.

METHODS

A novel plasmid (pAAV-ILGLP-1) and recombinant adenoviral vector (Ad-ILGLP-1) were constructed with the cytomegalovirus promoter and insulin leader sequence followed by GLP-1(7-37) cDNA.

RESULTS

The results of an enzyme-linked immunosorbent assay showed significantly elevated levels of GLP-1(7-37) secreted by human embryonic kidney cells transfected with the construct containing the leader sequence. A single intravenous administration of Ad-ILGLP-1 into 12-week-old Zucker diabetic fatty (ZDF) rats, which have overt type 2 diabetes mellitus (T2DM), achieved near normoglycemia for 3 weeks and improved utilization of blood glucose in glucose tolerance tests. Circulating plasma levels of GLP-1 increased in GLP-1-treated ZDF rats, but diminished 21 days after treatment. When compared with controls, Ad-ILGLP-1-treated ZDF rats had a lower homeostasis model assessment for insulin resistance score indicating amelioration in insulin resistance. Immunohistochemical staining showed that cells expressing GLP-1 were found in the livers of GLP-1-treated ZDF rats.

CONCLUSIONS

These data suggest that GLP-1 gene therapy can improve glucose homeostasis in fully developed diabetic animal models and may be a promising treatment modality for T2DM in humans.

GammadeltaT cells initiate acute inflammation and injury in adenovirus-infected liver via cytokine-chemokine cross talk

Emerging studies suggest an important role for the innate immune response in replication-defective adenovirus (Ad)-mediated acute liver toxicity. Specifically, classical innate immune cells (including NK cells, neutrophils, and Kupffer cells) have all been implicated in the development of Ad-mediated acute liver toxicity. The nonclassical innate immune T cell, the gammadeltaT cell, has been implicated in the pathophysiology of several viral infections that predominantly affect the mucosa and brain, but the specific role in the pathology of AdLacZ-mediated acute liver inflammation and injury as well as accompanying vector clearance is largely unknown. In the present study, we demonstrated that a CXCL9-CXCR3-dependent mechanism governed the accumulation of gammadeltaT cells in the livers of mice infected with Ad expressing the Escherichia coli LacZ gene (AdLacZ). We also showed a critical role for gammadeltaT cells in initiating acute liver toxicity after AdLacZ administration, driven in part by the ability of gammadeltaT cells to promote the recruitment of the conventional T cell, the CD8(+) T cell, into the liver. Furthermore, reduced hepatic injury in AdLacZ-infected gammadeltaT-cell-deficient mice was associated with lower hepatic levels of gamma interferon (IFN-gamma) and CXCL9, an IFN-gamma-inducible chemokine. Finally, our study highlighted a key role for IFN-gamma and CXCL9 cross talk acting in a feedback loop to drive the proinflammatory effects of gammadeltaT cells during AdLacZ-mediated acute liver toxicity. Specifically, intracellular IFN-gamma produced by activated hepatic gammadeltaT cells interacts with hepatocytes to mediate hepatic CXCL9 production, with the consequent accumulation of CXCR3-bearing gammadeltaT cells in the liver to cause acute liver damage without vector clearance.

Modified concatemeric oligonucleotide complexes: new system for efficient oligonucleotide transfer into mammalian cells

Antisense oligonucleotides and double-stranded small interfering RNAs have become an important instrument for the manipulation of gene expression in molecular biology experiments and a promising tool for the development of gene-targeted therapeutics. One of the main impediments in the use of oligonucleotide-based therapeutics is their poor uptake by target cells. The formation of supramolecular concatemeric complexes by oligonucleotides was shown to promote their binding to various mammalian cells [Simonova, O.N.,Vladimirova, A.V., Zenkova, M.A., and Vlassov, V.V. (2006). Biochim. Biophys. Acta 1758, 413-418]. We attempted to improve the efficiency of oligonucleotide concatemer delivery into cells by the attachment of lipophilic cholesterol molecules to the components of concatemeric complexes. Uptake, cellular distribution, and biological activity of the supramolecular complexes formed by delivered antisense oligonucleotides and cholesterol-modified "carrier" oligonucleotides were studied. Our results demonstrate that incorporation of an antisense oligonucleotide into the self-assembling concatemeric system promotes its delivery into cells without the addition of any supplementary transfection agents and allows achieving specific inhibition of the target gene expression.

Development of recombinant adeno-associated virus and adenovirus cocktail system for efficient hTERTC27 polypeptide-mediated cancer gene therapy

The low in vivo transduction efficiency of recombinant adeno-associated virus (rAAV) and the undesirably strong immunogenicity of adenovirus (rAdv) have limited their clinical utilization in cancer gene therapy. We have previously demonstrated that intratumoral injection of rAAV expressing a C-terminal polypeptide of human telomerase reverse transcriptase (rAAV-hTERTC27) effectively inhibits the growth of glioblastoma xenografts in nude mice. To further improve its efficacy, we combined rAAV-hTERTC27 with rAdv and investigated the efficiency of the cocktail vectors in vivo. At a nontherapeutic dose (1 x 10(8) plaque-forming units (PFUs)), rAdv-null and rAdv-hTERTC27 were equipotent in enhancing the therapeutic efficacy of rAAV-hTERTC27 (1.5 x 10(11) v.g.), and complete tumor regression was achieved in 25% of the treated animals. Importantly, the combination of rAAV-hTERTC27 and a therapeutic dose (2.5 x 10(9) PFU) of rAdv-hTERTC27 significantly augmented the therapeutic effects and led to a 38% complete tumor regression rate. In vivo optical imaging also showed that rAAV-luc/rAdv-luc cocktail vectors could synergistically enhance the early transient and latent sustained expression of luciferase, as compared to rAdv-luc and rAAV-luc alone. These findings suggest that the combination of rAAV-hTERTC27 and a therapeutic dose of rAdv-hTERTC27 is potentially a promising treatment for glioblastoma, and the rAAV/rAdv cocktail vector system warrants further development for cancer gene therapy.

Water-soluble fullerene (C60) derivatives as nonviral gene-delivery vectors

A new class of water-soluble C60 transfecting agents has been prepared using Hirsch-Bingel chemistry and assessed for their ability to act as gene-delivery vectors in vitro. In an effort to elucidate the relationship between the hydrophobicity of the fullerene core, the hydrophilicity of the water-solubilizing groups, and the overall charge state of the C60 vectors in gene delivery and expression, several different C60 derivatives were synthesized to yield either positively charged, negatively charged, or neutral chemical functionalities under physiological conditions. These fullerene derivatives were then tested for their ability to transfect cells grown in culture with DNA carrying the green fluorescent protein (GFP) reporter gene. Statistically significant expression of GFP was observed for all forms of the C60 derivatives when used as DNA vectors and compared to the ability of naked DNA alone to transfect cells. However, efficient in vitro transfection was only achieved with the two positively charged C60 derivatives, namely, an octa-amino derivatized C60 and a dodeca-amino derivatized C60 vector. All C60 vectors showed an increase in toxicity in a dose-dependent manner. Increased levels of cellular toxicity were observed for positively charged C60 vectors relative to the negatively charged and neutral vectors. Structural analyses using dynamic light scattering and optical microscopy offered further insights into possible correlations between the various derivatized C60 compounds, the C60 vector/DNA complexes, their physical attributes (aggregation, charge) and their transfection efficiencies. Recently, similar Gd@C60-based compounds have demonstrated potential as advanced contrast agents for magnetic resonance imaging (MRI). Thus, the successful demonstration of intracellular DNA uptake, intracellular transport, and gene expression from DNA using C60 vectors suggests the possibility of developing analogous Gd@C60-based vectors to serve simultaneously as both therapeutic and diagnostic agents.

Hepatic insulin gene therapy normalizes diurnal fluctuation of oxidative metabolism in diabetic BB/Wor rats

Previous studies of hepatic insulin gene therapy (HIGT) focused on glycemic effects of insulin produced from hepatocytes. In this study, we extend the observations of glycemic control with metabolically regulated HIGT to include systemic responses and whole-body metabolism. An insulin transgene was administered with an adenoviral vector [Ad/(GlRE)(3)BP1-2xfur] to livers of BB/Wor rats made diabetic with polyinosinic polycytidilic acid (poly-I:C) (HIGT group), and results compared with nondiabetic controls (non-DM), and diabetic rats receiving different doses of continuous-release insulin implants (DM-low BG and DM-high BG). Blood glucose and growth normalized in HIGT, with lower systemic insulin levels, elevated glucagon, and increased heat production compared with non-DM. Minimal regulation of systemic insulin levels were observed with HIGT, yet the animals maintained normal switching from carbohydrate to lipid metabolism determined by respiratory quotients (RQs), and tolerated 24-hour fasts without severe hypoglycemia. HIGT did not restore serum lipids as we observed increased triglycerides (TGs) and increased free fatty acids, but reduced weight of visceral fat pads despite normal total body fat content and retroperitoneal fat depots. HIGT favorably affects blood glucose, normalizes metabolic switching in diabetic rats, and reduces intra-abdominal fat deposition.

A critical role for type I IFN-dependent NK cell activation in innate immune elimination of adenoviral vectors in vivo

Recombinant adenoviruses have been used widely for gene therapy due to their high transduction efficiency in vivo. However, the attendant innate immune response to adenoviral vectors has limited their applications for in vivo gene therapy. Recent studies have shown that adenoviruses activate the innate immunity through both Toll-like receptor-dependent (TLR-dependent) and TLR-independent pathways, leading to the production of type I interferons (IFNs) and other inflammatory cytokines. Furthermore, type I IFNs play a pivotal role in innate immune elimination of adenoviral vectors in vivo. It remains to be defined how type I IFNs regulate innate immune clearance of adenoviral vectors. In this study, we showed in vivo that natural killer (NK) cells were activated and accumulated in the liver upon intravenous administration of adenoviral vectors, leading to the loss of adenoviral genome and the reduction of transgene expression. We further demonstrated that type I IFNs were critical for the activation of NK cells. This was achieved by direct action of type I IFNs on NK cells. Overall, our observations reveal a critical role for type I IFN-dependent NK cell activation in innate immune elimination of adenoviral vectors in vivo and may help design effective strategies to improve the outcome of adenovirus-mediated gene therapy.

Adenovirus-mediated interleukin-13 gene therapy attenuates acute kidney allograft injury

BACKGROUND

Kidney transplantation is possible by virtue of systemic immunosuppression, which is in turn accompanied by serious side effects. The search for novel therapeutic agents and strategies is ongoing. Here we investigate the effects of adenovirus-mediated gene therapy with interleukin (IL)-13, which is a cytokine with strong immunomodulatory properties, on acute renal allograft injury. In addition, we compare the effects of local (intrarenal) and systemic (intramuscular) IL-13 gene therapy in kidney transplantation.

METHODS

The experiments were performed in a rat Fisher to Lewis acute rejection model of kidney transplantation. An adenovirus-IL-13 or adenovirus-luciferase was injected either into the donor kidney before transplantation (local treatment) or into the hind leg muscle of recipient rats (systemic treatment). A group with no treatment served as control. No additional immunosuppressive therapy was applied. The rats were sacrificed after 8 days and inflammatory markers and renal pre-fibrosis were assessed.

RESULTS

Efficient gene transfer was confirmed by ELISA, immunohistochemistry and real-time PCR. IL-13 gene therapy diminished graft infiltration with macrophages and cytotoxic T cells and limited up-regulation of mRNA levels of the adhesion molecule E-selectin and pro-inflammatory cytokines TNF-alpha and IFN-gamma. Moreover, reduced renal interstitial pre-fibrosis was found in the rats receiving IL-13 gene therapy. The effects of local and systemic therapy were similar.

CONCLUSIONS

This study demonstrates that IL-13 gene therapy in the graft significantly attenuates acute renal allograft damage, suggesting local therapy with IL-13 as a strategy to reduce the need for systemic immunosuppressive medication and thereby its side effects.

Mesenchymal stem cells expressing TRAIL lead to tumour growth inhibition in an experimental lung cancer model

Lung cancer is a major public health problem in the western world, and gene therapy strategies to tackle this disease systemically are often impaired by inefficient delivery of the vector to the tumour tissue. Some of the main factors inhibiting systemic delivery are found in the blood stream in the form of red and white blood cells (WBCs) and serum components. Mesenchymal stem cells (MSCs) have been shown to home to tumour sites and could potentially act as a shield and vehicle for a tumouricidal gene therapy vector. Here, we describe the ability of an adenoviral vector expressing TRAIL (Ad.TR) to transduce MSCs and show the apoptosis-inducing activity of these TRAIL-carrying MSCs on A549 lung carcinoma cells. Intriguingly, using MSCs transduced with Ad.enhanced-green-fluorescent-protein (EGFP) we could show transfer of viral DNA to cocultured A549 cells resulting in transgenic protein production in these cells, which was not inhibited by exposure of MSCs to human serum containing high levels of adenovirus neutralizing antibodies. Furthermore, Ad.TR-transduced MSCs were shown not to induce T-cell proliferation, which may have resulted in cytotoxic T-cell-mediated apoptosis induction in the Ad.TR-transduced MSCs. Apoptosis was also induced in A549 cells by Ad.TR-transduced MSCs in the presence of physiological concentrations of WBC, erythrocytes and sera from human donors that inhibit or neutralize adenovirus alone. Moreover, we could show tumour growth reduction with TRAIL-loaded MSCs in an A549 xenograft mouse model. This is the first study that demonstrates the potential therapeutic utility of Ad.TR-transduced MSCs in cancer cells and the stability of this vector in the context of the blood environment.

Unbiased selection of bone marrow derived cells as carriers for cancer gene therapy

BACKGROUND

There is currently great interest in development of cell-based carriers for delivery of viral vectors to metastatic tumors. To date, several cell carriers have been tested based largely upon their predicted tumor-localizing properties. However, cell types may exist which can be mobilized from the circulation by a tumor which have not yet been identified. Here we use an unbiased screen of bone marrow (BM) cells to identify cells which localize to tumors and which might serve as effective candidate cell carriers without any prior prediction or selection.

METHODS

Unsorted BM cells from green fluorescent protein (GFP)-transgenic donor mice were adoptively transferred into C57Bl/6 mice bearing pre-established subcutaneous B16 melanoma tumors. Forty-eight hours and eight days later, tumors, organs and blood were analyzed for GFP-expressing cells by flow cytometry. The phenotype of GFP cells in organs was determined by co-staining with specific cell surface markers.

RESULTS

CD45(+) hematopoietic cells were readily detected in tumor, spleen, bone marrow, blood and lung at both time points. Within these CD45(+) cell populations, preferential accumulation in the tumor was observed of cells expressing Sca-1, c-kit, NK1.1, Thy1.2, CD14, Mac-3 and/or CD11c. Lymphodepletion increased homing to spleen and bone marrow, but not to tumors.

CONCLUSIONS

We have used an in vivo screen to identify populations of BM-derived donor cells which accumulate within tumors. These studies will direct rational selection of specific cell types which can be tested in standardized assays of cell carrier efficiency for the treatment of metastatic tumors.

Wild-type adenoviruses from groups A-F evoke unique innate immune responses, of which HAd3 and SAd23 are partially complement dependent

Alternative human and non-human Ad serotype vectors are currently studied for gene therapy and/or vaccine applications to capitalize upon their likely ability to avoid pre-existing immunity to HAd5. However, relatively little attention has been given to the nature and scope of innate immune responses generated by alternative Ad serotypes. In this study, we characterized several innate immune responses after intravenous administration of wild-type Ad serotypes HAd31, HAd3, HAd5, HAd37, SAd23 and HAd41, representing groups A-F, respectively. Notably, biodistribution studies revealed significant differences between the serotypes, with high levels of HAd3 genomes found in the liver and lung, and HAd37 genomes found in the spleen after systemic administration. Relative to similar treatments with other Ad serotypes, HAd3 and SAd23 induced altered innate immune responses, illustrated by induction of higher levels of cellular gene transcription in several tissues, and higher plasma levels of cytokines and chemokines. We also investigated whether complement interactions have a role in HAd3- and SAd23-induced responses. We confirmed complement dependent gene transcription, plasma cytokine/chemokine responses, and liver toxicities incurred after administration of HAd3 and SAd23. This study highlights the potential benefits and/or limitations to the proposed use of alternative Ad serotypes for gene therapy or vaccine applications.

Oncolytic viruses: do they have a role in anti-cancer therapy?

Oncolytic viruses are replication competent, tumor selective and lyse cancer cells. Their potential for anti-cancer therapy is based upon the concept that selective intratumoral replication will produce a potent anti-tumor effect and possibly bystander or remote cell killing, whilst minimizing normal tissue toxicity. Viruses may be naturally oncolytic or be engineered for oncolytic activity, and possess a host of different mechanisms to provide tumor selectivity. Clinical use of live replicating viruses is associated with a unique set of safety issues. Clinical experience has so far provided evidence of limited efficacy and a favourable toxicity profile. The interaction with the host immune system is complex. An anti-viral immune response may limit efficacy by rapidly clearing the virus. However, virally-induced cell lysis releases tumor associated antigens in a 'dangerous' context, and limited evidence suggests that this can lead to the generation of a specific anti-tumor immune response. Combination therapy with chemotherapy or radiotherapy represents a promising avenue for ongoing translation of oncolytic viruses into clinical practice. Obstacles to therapy include highly effective non-specific host mechanisms to clear virus following systemic delivery, immune-mediated clearance, and intratumoral barriers limiting virus spread. A number of novel strategies are now under investigation to overcome these barriers. This review provides an overview of the potential role of oncolytic viruses, highlighting recent progress towards developing effective therapy and asks if they are a realistic therapeutic option at this stage.

The liver sinusoidal endothelial cell: a cell type of controversial and confusing identity

A look through the literature on liver sinusoidal endothelial cells (LSECs) reveals that there are several conflicts among different authors of what this cell type is and does. Major controversies that will be highlighted in this review include aspects of the physiological role, the characterization, and the protocols of isolation and cultivation of these cells. Many of these conflicts may be ascribed to the fact that the cell was only recently established as a distinct cell type and that researchers from different disciplines tend to define their structure and function differently. This field is in need of a common platform to obtain a sound communication and a unified understanding of how to interpret novel research results. The aim of this review is to encourage scientists not to ignore the fact that there are, indeed, different opinions in the literature on LSECs. We also hope that this review will point out to the reader that some issues that may seem well established regarding our knowledge about the LSECs, in reality, are still unresolved and, indeed, controversial.

Lentivirus-mediated superoxide dismutase1 gene delivery protects against oxidative stress-induced liver injury in mice

BACKGROUND

The exposure of liver to hepatotoxins, and their subsequent metabolism, results in increased reactive oxygen species (ROS), one of the major culprits in causing both acute liver cell injury and chronic liver diseases. The aim of this present study is to investigate the protective effects of lentiviral vector-mediated copper-zinc superoxide dismutase (LV-SOD1) gene transfer against ROS-induced cytotoxicity in Hep G2 cells and liver injury in mice.

METHODS

In vitro SOD1 efficacy was tested against two ROS-generating systems: hypoxanthine/xanthine oxidase (HX/XO) and hydroxyethyl radicals (HER), whereas in vivo SOD1 efficacy was evaluated in carbon tetrachloride (CCl4)-induced liver injury in C57BL/6 mice.

RESULTS

LV-SOD1 transduction in Hep G2 cells resulted in a significant increase in SOD activity in cell lysates, and it significantly decreased the toxicity induced by HX/XO and HER. High SOD1 expression in the liver was achieved via portal vein injection of LV-SOD1 in mice and these high levels were observed for 30 days, the length of the experiment to date. SOD1 overexpression significantly decreased the toxicity and restored liver function in the CCl4-treated mice.

CONCLUSIONS

These findings demonstrate for the first time that LV transduction led to the long-term expression of fully functional transgene expression in both in vitro and in vivo systems.

Adenovirus vector induced innate immune responses: impact upon efficacy and toxicity in gene therapy and vaccine applications

Extensively characterized, modified, and employed for a variety of purposes, adenovirus (Ad) vectors are generally regarded as having great potential by many applied virologists who wish to manipulate and use viral biology to achieve beneficial clinical outcomes. Despite widespread functional prominence and utility (i.e., Ad-based clinical trials have begun to progress to critical Phase III levels, it has recently become apparent that investigations regarding the innate immune response to Ads may reveal not only reasons behind previous failures, but also reveal novel insights that will allow for safer, more efficacious uses of this important gene transfer platform. Insights gained by the exploration of Ad induced innate immune responses will likely be most important to the fields of vaccine development, since Ad-based vaccines are regarded as one of the more promising vaccine platforms in development today. Adenovirus is currently known to interact with several different extracellular, intracellular, and membrane-bound innate immune sensing systems. Past and recent studies involving manipulation of the Ad infectious cycle as well as use of different mutants have shed light on some of the initiation mechanisms underlying Ad induced immune responses. More recent studies using microarray-based analyses, genetically modified cell lines and/or mouse mutants, and advanced generation Ad vectors have revealed important new insights into the scope and mechanism of this cellular defensive response. This review is an attempt to synthesize these studies, update Ad biologists to the current knowledge surrounding these increasingly important issues, as well as highlight areas where future research should be directed. It should also serve as a sobering reality to researchers exploring the use of any gene transfer vector, as to the complexities potentially involved when contemplating use of such vectors for human applications.

Expression of pIX Gene Induced by Transgene Promoter: Possible Cause of Host Immune Response in First-Generation Adenoviral Vectors

First-generation (FG) adenoviral vectors (AdVs) have been widely used not only for gene therapy but also for basic studies. Because vectors of this type lack the E1A gene that is essential for the expression of other viral genes, their expression levels in target cells have been considered low. However, we found that the viral pIX gene, located immediately downstream of the inserted expression unit of the transgene, was significantly coexpressed with the transgene in cells infected with FG AdV. Whereas CAG and SRalpha promoters activated the pIX promoter considerably through their enhancer effects, the EF1alpha promoter hardly did. Moreover, when the expression unit was inserted in the rightward orientation, not only the pIX protein but also a fusion protein consisting of the N-terminal part of transgene product and pIX were sometimes coexpressed with the transgene product through an aberrant splicing mechanism. In in vivo experiments, a LacZ-expressing AdV bearing the CAG promoter caused an elevation of alanine aminotransferase, but an AdV bearing the EF1alpha promoter produced no detectable levels. Whereas the FG AdV expressing human growth hormone under the control of the CAG promoter maintained a high hormone level for less than 1 month, the FG AdV under the control of the EF1alpha promoter maintained a high level for at least 6 months. These results suggest that pIX coexpression may be one of the main causes of AdV-induced immune responses, and that the EF1alpha promoter is probably valuable for the long-term expression of FG AdV. Thus, the in vivo utility of FG AdV should be reevaluated.

A quantitative assay for measuring clearance of adenovirus vectors by Kupffer cells

Kupffer cells are a major barrier to systemic adenovirus (Ad) gene therapy because they rapidly and efficiently clear virions from the circulation. The lack of a straightforward quantitative technique for selectively measuring uptake of Ad by Kupffer cells has made it difficult to study the mechanisms by which they recognize Ad. A new method was developed that relies on immunofluorescent detection of Ad within Kupffer cells in mouse liver sections, followed by confocal microscopy and computerized image analysis. The method is sensitive, quantitative and reproducible, with a linear range spanning two orders of magnitude. As an example of the utility of this method, it was found that pre-injecting mice with polyinosinic acid reduces accumulation of Ad in Kupffer cells by approximately 90%.

Interactions between human plasma components and a xenogenic adenovirus vector: reduced immunogenicity during gene transfer

By the time we are adolescents most of us have been in contact with several of the >50 human adenovirus (HAd) serotypes. These common subclinical infections lead to an efficient anti-adenovirus cross-reacting adaptive immunity. During gene therapy, the ubiquitous anti-adenovirus humoral response and complement activation will modify and dictate vector biodistribution, as well as the response to the virion and transgene(s). In this study, we assayed the interactions of a xenogenic adenovirus derived from canine serotype 2 (CAV-2) with naturally occurring human antibodies (Abs) and the complement system. In our cohort, we found class G immunoglobulins (Igs) that recognized the intact CAV-2 virion and the external virion proteins. However, the majority of donors had low or no neutralizing Abs, class A, or class M Igs. Purified anti-HAd serotype 5 Abs also recognized CAV-2 virion proteins. In addition, in spite of the presence of anti-CAV-2 IgGs, CAV-2 poorly activated the classical and alternative complement cascades. This atypical response was due to a block upstream of the component 3 (C3) convertase and interplay between the component 1 (C1) inhibitor, the C1q-C1r2-C1s2 complex and CAV-2. Our data demonstrate that some xenogenic adenovirus vectors, like CAV-2, could lead to notably different outcomes following systemic delivery.

Rapid upregulation of interferon-regulated and chemokine mRNAs upon injection of 108 international units, but not lower doses, of adenoviral vectors into the brain

The innate immune response, characterized by the rapid induction of proinflammatory genes, plays an important role in immune responses to viral vectors utilized in gene therapy. We demonstrate that several innate proinflammatory mRNAs, i.e., those coding for the interferon (IFN)-regulated proteins interferon regulatory factor 1, 2',5'-oligoadenylate synthetase, and double-stranded-RNA-dependent protein kinase as well as those coding for the chemokines RANTES, IFN-gamma-inducible protein 10, and monocyte chemoattractant protein 1, were all increased in a statistically significant manner in response to 1 x 10(8) IU, but not lower doses, of a first-generation adenovirus injected into the naïve brain. This indicates the presence of a threshold dosage of adenovirus needed to elicit an acute innate inflammatory response.

Effective tumor targeted gene transfer using PEGylated adenovirus vector via systemic administration

Conjugation of polyethylene glycol to protein or particles (PEGylation) prolongs their plasma half-lives and promotes their accumulation in tumors due to enhanced permeability and retention (EPR) effect. Although PEGylation of adenovirus vectors (Ads) is an attractive strategy to improve the in vivo kinetics of conventional Ads, the EPR effect of PEGylated Ad (PEG-Ad) had not previously been reported. In this study, we prepared PEG-Ads with PEG at various modification ratios, injected them intravenously into tumor-bearing mice, and determined the blood kinetics, viral distribution, and gene expression patterns, respectively. In addition, we conducted a cancer therapeutic study of PEG-Ad encoding tumor necrosis factor (TNF)-alpha. The plasma half-life of PEG-Ad was longer than that of unmodified-Ad, and accumulation of PEG-Ad in tumor tissue increased as the PEG modification ratio increased. In particular, PEG-Ad with about 90% modification ratio showed higher (35 times) gene expression in tumor and lower (6%) in liver, compared with values for unmodified Ad. Moreover, PEG-Ad encoding TNF-alpha demonstrated not only stronger tumor-suppressive activity but also fewer hepatotoxic side effects compared with unmodified-Ad. PEGylation of Ad achieved tumor targeting through the EPR effect, and these attributes suggest that systemic injection of PEG-Ad has great potential as an anti-tumor treatment.

Suitability of a US3-inactivated HSV mutant (L1BR1) as an oncolytic virus for pancreatic cancer therapy

Recently, the use of oncolytic viruses against cancer has attracted considerable attention. We studied the potential of the US3 locus-deficient herpes simplex virus (HSV), L1BR1, for oncolytic virus therapy. Its high specificity and potency indicate that L1BR1 is a promising candidate as a new oncolytic virus against pancreatic cancer. Moreover, the virus exhibited the unique characteristic of increasing apoptosis when used in combination with anticancer drugs. We assessed the feasibility of using the US3 locus-deficient HSV named L1BR1 as a new replication-competent oncolytic virus for the treatment of pancreatic cancer. The US3 locus of HSV has been shown to be a key gene in producing a multifunctional protein kinase that inhibits apoptosis induced by viral infections, chemicals and ultraviolet (UV) light. L1BR1 has been reported to be more than 10 000-fold less virulent than the parental virus in mice. In this study, we examined the tumor specificity and oncolytic effect of this attenuated replication-competent virus, L1BR1, in pancreatic cancers derived from SW1990, Capan2 and Bxpc-3cells compared with the parent virus and other well-known oncolytic herpes viruses (R3616 and hrR3). We also studied the efficacy of L1BR1 for the induction of apoptosis as an attribute of this virus in combination with the anticancer drugs 5FU and cisplatin. The combined treatment of the pancreatic cancer cells with L1BR1 and these anticancer drugs enhanced apoptosis significantly. More importantly, L1BR1 showed the lowest replication capacity in normal human hepatocytes, but the highest tumor-reducing effect in vivo among the oncolytic herpes viruses tested. In addition, L1BR1 significantly increased the induction of apoptosis of cancer cells when treated in combination with anticancer drugs although the parental virus inhibited the induction of apoptosis. These results suggest that L1BR1 is promising as a new anticancer oncolytic virus.

Fiber-modified adenovirus vectors decrease liver toxicity through reduced IL-6 production

Adenovirus (Ad) vectors are one of the most commonly used viral vectors in gene therapy clinical trials. However, they elicit a robust innate immune response and inflammatory responses. Improvement of the therapeutic index of Ad vector gene therapy requires elucidation of the mechanism of Ad vector-induced inflammation and cytokine/chemokine production as well as development of the safer vector. In the present study, we found that the fiber-modified Ad vector containing poly-lysine peptides in the fiber knob showed much lower serum IL-6 and aspartate aminotransferase levels (as a maker of liver toxicity) than the conventional Ad vector after i.v. administration, although the modified Ad vector showed higher transgene production in the liver than the conventional Ad vector. RT-PCR analysis showed that spleen, not liver, is the major site of cytokine, chemokine, and IFN expression. Splenic CD11c(+) cells were found to secret cytokines. The tissue distribution of Ad vector DNA showed that spleen distribution was much reduced in this modified Ad vector, reflecting reduced IL-6 levels in serum. Liver toxicity by the conventional Ad vector was reduced by anti-IL-6R Ab, suggesting that IL-6 signaling is involved in liver toxicity and that decreased liver toxicity of the modified Ad vector was due in part to the reduced IL-6 production. This study contributes to an understanding of the biological mechanism in innate immune host responses and liver toxicity toward systemically administered Ad vectors and will help in designing safer gene therapy methods that can reduce robust innate immunity and inflammatory responses.

Adenoviral Gene Delivery to Solid Tumors by Recombinant Silk–Elastinlike Protein Polymers

PURPOSE

The purpose of this study was to investigate the potential of silk-elastinlike protein polymers (SELPs) in controlling the release rate of adenoviruses in vitro and in vivo while preserving their bioactivity.

MATERIALS AND METHODS

A hydrogel system composed of SELP/adenovirus mixture was prepared. The release of the adenovirus particles from the hydrogels was quantified by Real Time-PCR and the bioactivity of the released viruses was evaluated using confocal microscopy and beta-galactosidase assay. To demonstrate the ability of SELP in entrapping virus cargo and releasing it over a prolonged period of time in vivo, a SELP/adenovirus mixture was prepared and injected directly into xenograft tumor models of breast and head and neck cancer in mice. At various time points mice were sacrificed, tumors dissected, and tissue sections studied under confocal microscope.

RESULTS

In vitro studies demonstrated that SELP hydrogels release viruses over a period of 4 weeks while preserving their bioactivity. After intratumoral injection, a prolonged and localized expression of adenoviruses was observed.

CONCLUSIONS

These results suggest the potential of SELPs in local adenoviral delivery to solid tumors as an alternative approach to intratumoral virus infusion.

Adenovirus-platelet interaction in blood causes virus sequestration to the reticuloendothelial system of the liver

Intravenous (i.v.) delivery of recombinant adenovirus serotype 5 (Ad5) vectors for gene therapy is hindered by safety and efficacy problems. We have discovered a new pathway involved in unspecific Ad5 sequestration and degradation. After i.v. administration, Ad5 rapidly binds to circulating platelets, which causes their activation/aggregation and subsequent entrapment in liver sinusoids. Virus-platelet aggregates are taken up by Kupffer cells and degraded. Ad sequestration in organs can be reduced by platelet depletion prior to vector injection. Identification of this new sequestration mechanism and construction of vectors that avoid it could improve levels of target cell transduction at lower vector doses.

Replication-deficient adenovirus induces host topoisomerase I activity: implications for adenovirus-mediated gene expression

Replication-deficient adenoviruses are useful vectors for the transfer of therapeutic transgenes to malignant and non-malignant tissues. Yet their clinical application is limited by the potential toxicity of viral infection and the transient nature of transgene expression. Although transgene expression from adenovirus vectors is initially higher than expression of transgenes transduced by other viral or non-viral vectors, it is often insufficient to generate a significant therapeutic effect. We addressed this issue by searching for DNA-targeted viral-induced host responses potentially restricting transgene expression. Nuclear protein extracts from livers of rats systemically infected with replication-deficient adenovirus exhibited enhanced topoisomerase I activity compared with extracts from uninfected animals. Consequently, the inhibition of topoisomerase I by the anti-cancer drug topotecan greatly enhanced transgene expression in adenovirus-infected hepatic cells, colon cancer and prostate cancer cell cultures, mouse liver, human ex vivo tumor specimens, and mouse tumor in vivo. The enhancement could not be ascribed to non-specific genotoxic stress, cell death, or cell-cycle perturbation. These findings are significant for gene therapy as they reveal novel aspects of the host anti-adenovirus response and set the stage for the development of a rational molecular-pharmacological approach to increase the effectiveness, and safety, of adenovirus-mediated cancer therapeutics.

Cell-based delivery of oncolytic viruses: a new strategic alliance for a biological strike against cancer

Recent years have seen tremendous advances in the development of exquisitely targeted replicating virotherapeutics that can safely destroy malignant cells. Despite this promise, clinical advancement of this powerful and unique approach has been hindered by vulnerability to host defenses and inefficient systemic delivery. However, it now appears that delivery of oncolytic viruses within carrier cells may offer one solution to this critical problem. In this review, we compare the advantages and limitations of the numerous cell lineages that have been investigated as delivery platforms for viral therapeutics, and discuss examples showing how combined cell-virus biotherapeutics can be used to achieve synergistic gains in antitumor activity. Finally, we highlight avenues for future preclinical research that might be taken in order to refine cell-virus biotherapeutics in preparation for human trials.

Innate immune response to adenoviral vectors is mediated by both Toll-like receptor-dependent and -independent pathways

Recombinant adenoviral vectors have been widely used for gene therapy applications and as vaccine vehicles for treating infectious diseases such as human immunodeficiency virus disease. The innate immune response to adenoviruses represents the most significant hurdle in clinical application of adenoviral vectors for gene therapy, but it is an attractive feature for vaccine development. How adenovirus activates innate immunity remains largely unknown. Here we showed that adenovirus elicited innate immune response through the induction of high levels of type I interferons (IFNs) by both plasmacytoid dendritic cells (pDCs) and non-pDCs such as conventional DCs and macrophages. The innate immune recognition of adenovirus by pDCs was mediated by Toll-like receptor 9 (TLR9) and was dependent on MyD88, whereas that by non-pDCs was TLR independent through cytosolic sensing of adenoviral DNA. Furthermore, type I IFNs were pivotal in innate and adaptive immune responses to adenovirus in vivo, and type I IFN blockade diminished immune responses, resulting in more stable transgene expression and reduction of inflammation. These findings indicate that adenovirus activates innate immunity by its DNA through TLR-dependent and -independent pathways in a cell type-specific fashion, and they highlight a critical role for type I IFNs in innate and adaptive immune responses to adenoviral vectors. Our results that suggest strategies to interfere with type I IFN pathway may improve the outcome of adenovirus-mediated gene therapy, whereas approaches to activate the type I IFN pathway may enhance vaccine potency.

Carrier Cell-based Delivery of an Oncolytic Virus Circumvents Antiviral Immunity

Oncolytic viruses capable of tumor-selective replication and cytolysis have shown early promise as cancer therapeutics. However, the host immune system remains a significant obstacle to effective systemic administration of virus in a clinical setting. Here, we demonstrate the severe negative impact of the adaptive immune response on the systemic delivery of oncolytic vesicular stomatitis virus (VSV) in an immune-competent murine tumor model, an effect mediated primarily by the neutralization of injected virions by circulating antibodies. We show that this obstacle can be overcome by administering virus within carrier cells that conceal viral antigen during delivery. Infected cells were delivered to tumor beds and released virus to infect malignant cells while sparing normal tissues. Repeated administration of VSV in carrier cells to animals bearing metastatic tumors greatly improved therapeutic efficacy when compared with naked virion injection. Whole-body molecular imaging revealed that carrier cells derived from solid tumors accumulate primarily in the lungs following intravenous injection, whereas leukemic carriers disseminate extensively throughout the body. Furthermore, xenogeneic cells were equally effective at delivering virus as syngeneic cells. These findings emphasize the importance of establishing cell-based delivery platforms in order to maximize the efficacy of oncolytic therapeutics.

Minimal hepatic toxicity of Onyx-015: spatial restriction of coxsackie-adenoviral receptor in normal liver

We administered an adenoviral vector, Onyx-015, into the hepatic artery of patients with metastatic colorectal cancer involving the liver. Thirty-five patients enrolled in this multi-institutional phase I/II trial received up to eight arterial infusions of up to 2 × 10¹² viral particles. Hepatic toxicity was the primary dose-limiting toxicity observed in preclinical models. However, nearly 200 infusions of this adenoviral vector were administered directly into the hepatic artery without significant toxicity. Therefore, we undertook this analysis to determine the impact of repeated adenoviral exposure on hepatic function. Seventeen patients were treated at our institution, providing a detailed data set on the changes in hepatic function following repeated exposure to adenovirus. No changes in hepatic function occurred with the first treatment of Onyx-015 among these patients. Transient increases in transaminase levels occurred in one patient starting with the second infusion and transient increases in bilirubin was observed in two patients starting with the fifth treatment. These changes occurred too early to be explained by viral-mediated lysis of hepatocytes. In addition, viremia was observed starting 3–5 days after the viral infusion in half of the patient, but was not associated with hepatic toxicity. To further understand the basis for the minimal hepatic toxicity of adenoviral vectors, we evaluated the replication of adenovirus in primary hepatocytes and tumor cells in culture and the expression of the coxsackie-adenoviral receptor (CAR) in normal liver and colon cancer metastatic to the liver. We found that adenovirus replicates poorly in primary hepatocytes but replicates efficiently in tumors including tumors derived from hepatocytes. In addition, we found that CAR is localized at junctions between hepatocytes and is inaccessible to hepatic blood flow. CAR is not expressed on tumor vasculature but is expressed on tumor cells. Spatial restriction of CAR to the intercellular space in normal liver and diminished replication of adenovirus in hepatocytes may explain the minimal toxicity observed following repeated hepatic artery infusions with Onyx-015.