Trans-complementation of vector replication versus Coxsackie-adenovirus-receptor overexpression to improve transgene expression in poorly permissive cancer cells

Adenoviral Gene Therapy Vectors in Clinical Use-Basic Aspects with a Special Reference to Replication-Competent Adenovirus Formation and Its Impact on Clinical Safety

Adenoviral vectors are commonly used in clinical gene therapy. Apart from oncolytic adenoviruses, vector replication is highly undesired as it may pose a safety risk for the treated patient. Thus, careful monitoring for the formation of replication-competent adenoviruses (RCA) during vector manufacturing is required. To render adenoviruses replication deficient, their genomic E1 region is deleted. However, it has been known for a long time that during their propagation, some viruses will regain their replication capability by recombination in production cells, most commonly HEK293. Recently developed RCA assays have revealed that many clinical batches contain more RCA than previously assumed and allowed by regulatory authorities. The clinical significance of the higher RCA content has yet to be thoroughly evaluated. In this review, we summarize the biology of adenovirus vectors, their manufacturing methods, and the origins of RCA formed during HEK293-based vector production. Lastly, we share our experience using minimally RCA-positive serotype 5 adenoviral vectors based on observations from our clinical cardiovascular gene therapy studies.

Variations in Cell Surface ACE2 Levels Alter Direct Binding of SARS-CoV-2 Spike Protein and Viral Infectivity: Implications for Measuring Spike Protein Interactions with Animal ACE2 Orthologs

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is the causative agent of coronavirus disease 2019 (COVID-19), the most severe pandemic in a century. The virus gains access to host cells when the viral spike protein (S-protein) binds to the host cell surface receptor angiotensin-converting enzyme 2 (ACE2). Studies have attempted to understand SARS-CoV-2 S-protein interactions with vertebrate orthologs of ACE2 by expressing ACE2 orthologs in mammalian cells and measuring viral infection or S-protein binding. Often, these cells only transiently express ACE2 proteins, and the levels of ACE2 at the cell surface are not quantified. Here, we describe a cell-based assay that uses stably transfected cells expressing ACE2 proteins in a bicistronic vector with an easy-to-quantify reporter protein, Thy1.1. We found that both the binding of the S-protein receptor-binding domain (RBD) and infection with a SARS-CoV-2 pseudovirus are proportional to the amount of human ACE2 expressed at the cell surface, which can be inferred by quantifying the level of Thy1.1. We also compared different ACE2 orthologs, which were expressed in stably transfected cells expressing equivalent levels of Thy1.1. When ranked for either viral infectivity or RBD binding, mouse ACE2 had a weak to undetectable affinity for S-protein, while human ACE2 had the highest level detected, and feline ACE2 had an intermediate phenotype. The generation of stably transfected cells whose ACE2 level can be normalized for cross-ortholog comparisons allows us to create a reusable cellular library useful for measuring emerging SARS-CoV-2 variants' abilities to potentially infect different animals. IMPORTANCE SARS-CoV-2 is a zoonotic virus responsible for the worst global pandemic in a century. An understanding of how the virus can infect other vertebrate species is important for controlling viral spread and understanding the natural history of the virus. Here, we describe a method to generate cells stably expressing different orthologs of ACE2, the receptor for SARS-CoV-2, on the surface of a human cell line. We find that both the binding of the viral spike protein receptor-binding domain (RBD) and infection of cells with a SARS-CoV-2 pseudovirus are proportional to the ACE2 levels at the cell surface. This method will allow the creation of a library of stably transfected cells expressing similar levels of different vertebrate ACE2 orthologs, which can be used repeatedly for identifying vertebrate species that may be susceptible to infection with SARS-CoV-2 and its many variants.

Variations in cell-surface ACE2 levels alter direct binding of SARS-CoV-2 Spike protein and viral infectivity: Implications for measuring Spike protein interactions with animal ACE2 orthologs

The severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is the causative agent of COVID-19, the most severe pandemic in a century. The virus gains access to host cells when the viral Spike protein (S-protein) binds to the host cell-surface receptor angiotensin-converting enzyme 2 (ACE2). Studies have attempted to understand SARS-CoV-2 S-protein interaction with vertebrate orthologs of ACE2 by expressing ACE2 orthologs in mammalian cells and measuring viral infection or S-protein binding. Often these cells only transiently express ACE2 proteins and levels of ACE2 at the cell surface are not quantified. Here, we describe a cell-based assay that uses stably transfected cells expressing ACE2 proteins in a bi-cistronic vector with an easy to quantify reporter protein to normalize ACE2 expression. We found that both binding of the S-protein receptor-binding domain (RBD) and infection with a SARS-CoV-2 pseudovirus is proportional to the amount of human ACE2 expressed at the cell surface, which can be inferred by quantifying the level of reporter protein, Thy1.1. We also compared different ACE2 orthologs which were expressed in stably transfected cells expressing equivalent levels of Thy1.1. When ranked for either viral infectivity or RBD binding, mouse ACE2 had a weak to undetectable affinity for S-protein while human ACE2 was the highest level detected and feline ACE2 had an intermediate phenotype. The generation of stably transfected cells whose ACE2 level can be normalized for cross-ortholog comparisons allows us to create a reusable cellular library useful for measuring emerging SARS-CoV-2 variant's ability to potentially infect different animals.

IMPORTANCE

SARS-CoV-2 is a zoonotic virus responsible for the worst global pandemic in a century. An understanding of how the virus can infect other vertebrate species is important for controlling viral spread and understanding the natural history of the virus. Here we describe a method to generate cells stably expressing equivalent levels of different ACE2 orthologs, the receptor for SARS-CoV-2, on the surface of a human cell line. We find that both binding of the viral Spike protein receptor binding domain (RBD) and infection of cells with a SARS-CoV-2 pseudovirus are proportional to ACE2 levels at the cell surface. Adaptation of this method will allow for the creation of a library of stable transfected cells expressing equivalent levels of different vertebrate ACE2 orthologs which can be repeatedly used for identifying vertebrate species which may be susceptible to infection with SARS-CoV-2 and its many variants.

Efficacy and Safety of Clinical-Grade Human Vascular Endothelial Growth Factor-DΔNΔC Gene Therapy Containing Residual Replication-Competent Adenoviruses

Biological bypass through induced angiogenesis by vascular endothelial growth factor D (VEGF-D) gene therapy (GT) is a new concept for the treatment of cardiac ischemia. Serotype 5 adenoviruses are used in the clinical trials for transferring the VEGF-D cDNA into the ischemic myocardium. However, the presence of replication-competent vectors in the adenovirus products is a widely recognized problem that may pose a potential safety risk to the treated patients. We compared three different VEGF-D GT production lots containing different levels of replication-competent adenoviruses (RCA) tested in 3 × 10¹⁰ viral particles (vp): <10 RCA (VEGF-D L-RCA1), 10-100 RCA (VEGF-D H-RCA2), and 100-200 RCA (VEGF-D H-RCA3), as measured by a novel droplet digital polymerase chain reaction (PCR) RCA assay in a preclinical rabbit model (n = 21). β-galactosidase encoding nonclinical-grade preparation was used as a nonangiogenic control. Each preparation was injected into the right semimembranosus muscle using dose of 1 × 10¹¹ vp. Efficacy of the products was tested by the combination of contrast pulse sequencing ultrasound and modified Miles assay as well as quantifying the total cross-sectional area of capillaries. Safety, immunogenicity, toxicity, biodistribution, and shedding were assessed by general histology, serial measurements of C-reactive protein, white blood cell count and body temperature as well as using quantitative real-time PCR with primers targeted to the VEGF-D and replication-permitting E1 sequences. We found no significant differences in the efficacy or safety between the study groups. Most importantly, no detectable presence of RCA-specific E1 sequence was found in any samples tested, indicating that no detectable vector replication took place in vivo. We conclude that relatively low levels of RCA in adenoviral GT products may not be as important major safety issue as previously anticipated.

Enhancing Antigen Cross-Presentation in Human Monocyte-Derived Dendritic Cells by Recruiting the Intracellular Fc Receptor TRIM21

Suboptimal immune responses to pathogens contribute to chronic infections. One way to improve immune responses is to boost Ag presentation. In this study, we investigate the potential of the tripartite motif-containing 21 (TRIM21) pathway. TRIM21 is a ubiquitously expressed cytosolic protein that recognizes the Fc region of Abs. When Abs that are bound to pathogens enter the cell as immune complexes, binding of TRIM21 to Fc initiates downstream inflammatory signaling and targets the immune complexes for proteasomal degradation. In APCs, peptides generated by proteasomes are loaded onto MHC class I molecules to stimulate CD8 T cell responses, which are crucial for effective immunity to pathogens. We hypothesized that increasing the affinity between immune complexes and TRIM21 might markedly improve CD8 T cell responses to Ags processed by the TRIM21 pathway. Using phage display technology, we engineered the human IgG1 Fc to increase its affinity for TRIM21 by 100-fold. Adenovirus immune complexes with the engineered Fc induced greater maturation of human dendritic cells (DC) than immune complexes with unmodified Fc and stimulated increased Ag-specific CD8 T cell proliferation and IFN-γ release in cocultures of DC-PBMC. Thus, by increasing the affinity between Fc and TRIM21, Ags from immune complexes undergo enhanced cross-presentation on DC, leading to greater CD8 T cell responses. Our study reveals an approach that could potentially be used in vaccines to increase cytotoxic T cell responses against Ags that are targeted or delivered by Fc-modified Abs.

Heparan Sulfate Binding Coxsackievirus B3 Strain PD: A Novel Avirulent Oncolytic Agent Against Human Colorectal Carcinoma

Coxsackievirus B3 (CVB3), a single-stranded RNA virus of the picornavirus family, has been described as a novel oncolytic virus. However, the CVB3 strain used induced hepatitis and myocarditis in vivo. It was hypothesized that oncolytic activity and safety of CVB3 depends on the virus strain and its specific receptor tropism. Different laboratory strains of CVB3 (Nancy, 31-1-93, and H3), which use the coxsackievirus and adenovirus receptor (CAR), and the strain PD, which uses N- and 6-O-sulfated heparan sulfate (HS) for entry into the cells, were investigated for their potential to lyse tumor cells and for their safety profile. The investigations were carried out in colorectal carcinoma. In vitro investigations showed variable infection efficiency and lysis of colorectal carcinoma cell lines by the CVB3 strains. The most efficient strain was PD, which was the only one that could lyse all investigated colorectal carcinoma cell lines. Lytic activity of CAR-dependent CVB3 did not correlate with CAR expression on cells, whereas there was a clear correlation between lytic activity of PD and its ability to bind to HS at the cell surface of colorectal carcinoma cells. Intratumoral injection of Nancy, 31-1-93, or PD into subcutaneous colorectal DLD1 cell tumors in BALB/c nude mice resulted in strong inhibition of tumor growth. The effect was seen in the injected tumor, as well as in a non-injected, contralateral tumor. However, all animals treated with 31-1-93 and Nancy developed systemic infection and died or were moribund and sacrificed within 8 days post virus injection. In contrast, five of the six animals treated with PD showed no signs of a systemic viral infection, and PD was not detected in any organ. The data demonstrate the potential of PD as a new oncolytic virus and HS-binding of PD as a key feature of oncolytic activity and improved safety.

The Coxsackievirus and Adenovirus Receptor: Glycosylation and the Extracellular D2 Domain Are Not Required for Coxsackievirus B3 Infection

The coxsackievirus and adenovirus receptor (CAR) is a member of the immunoglobulin superfamily (IgSF) and functions as a receptor for coxsackie B viruses (CVBs). The extracellular portion of CAR comprises two glycosylated immunoglobulin-like domains, D1 and D2. CAR-D1 binds to the virus and is essential for virus infection; however, it is not known whether D2 is also important for infection, and the role of glycosylation has not been explored. To understand the function of these structural components in CAR-mediated CVB3 infection, we generated a panel of human (h) CAR deletion and substitution mutants and analyzed their functionality as CVB receptors, examining both virus binding and replication. Lack of glycosylation of the CAR-D1 or -D2 domains did not adversely affect CVB3 binding or infection, indicating that the glycosylation of CAR is not required for its receptor functions. Deletion of the D2 domain reduced CVB3 binding, with a proportionate reduction in the efficiency of virus infection. Replacement of D2 with the homologous D2 domain from chicken CAR, or with the heterologous type C2 immunoglobulin-like domain from IgSF11, another IgSF member, fully restored receptor function; however, replacement of CAR-D2 with domains from CD155 or CD80 restored function only in part. These data indicate that glycosylation of the extracellular domain of hCAR plays no role in CVB3 receptor function and that CAR-D2 is not specifically required. The D2 domain may function largely as a spacer permitting virus access to D1; however, the data may also suggest that D2 affects virus binding by influencing the conformation of D1.

IMPORTANCE

An important step in virus infection is the initial interaction of the virus with its cellular receptor. Although the role in infection of the extracellular CAR-D1, cytoplasmic, and transmembrane domains have been analyzed extensively, nothing is known about the function of CAR-D2 and the extracellular glycosylation of CAR. Our data indicate that glycosylation of the extracellular CAR domain has only minor importance for the function of CAR as CVB3 receptor and that the D2 domain is not essential per se but contributes to receptor function by promoting the exposure of the D1 domain on the cell surface. These results contribute to our understanding of the coxsackievirus-receptor interactions.

Paradigms lost-an emerging role for over-expression of tight junction adhesion proteins in cancer pathogenesis

Tight junctions (TJ) are multi-protein complexes located at the apicalmost tip of the lateral membrane in polarised epithelial and endothelial cells. Their principal function is in mediating intercellular adhesion and polarity. Accordingly, it has long been a paradigm that loss of TJ proteins and consequent deficits in cell-cell adhesion are required for tumour cell dissemination in the early stages of the invasive/metastatic cascade. However it is becoming increasingly apparent that TJ proteins play important roles in not just adhesion but also intracellular signalling events, activation of which can contribute to, or even drive, tumour progression and metastasis. In this review, we shall therefore highlight cases wherein the gain of TJ proteins has been associated with signals promoting tumour progression. We will also discuss the potential of overexpressed TJ proteins to act as therapeutic targets in cancer treatment. The overall purpose of this review is not to disprove the fact that loss of TJ-based adhesion contributes to the progression of several cancers, but rather to introduce the growing body of evidence that gain of TJ proteins may have adhesion-independent consequences for promoting progression in other cancers.

Transgene delivery to cultured keratinocytes via replication-deficient adenovirus vectors

Transient transgene expression can facilitate investigation of that gene-product function or effect on keratinocyte biology. Several chemical and biologic delivery systems are available, and among them adenoviruses offer particular advantages in efficiency and transgene capacity. Here we describe the advantages of bicistronic adenovirus and inclusion of the polycation hexadimethrine bromide to aid in the detection of positively transduced cells and enhance transduction efficiency.

Expression of the coxsackie adenovirus receptor in neuroendocrine lung cancers and its implications for oncolytic adenoviral infection

Coxsackie adenovirus receptor (CAR) is the primary receptor to which oncolytic adenoviruses have to bind for internalization and viral replication. A total of 171 neuroendocrine lung tumors in form of multitissue arrays have been analyzed resulting in a positivity of 112 cases (65.5%). Immunostaining correlated statistically significant with histopathology and development of recurrence. The subtype small cell lung cancer (SCLC) showed the highest CAR expression (77.6%), moreover the CAR level was correlated to the disease-free survival. Further, high CAR expression level in SCLC cell lines was found in vitro and in vivo when cell lines had been transplanted into immunodeficient mice. A correlation between CAR expression in the primary tumors and metastases development in the tumor model underlined the clinical relevance. Cell lines with high CAR level showed a high infectivity when infected with a replication-deficient adenovirus. Low levels of CAR expression in SCLC could be upregulated with Trichostatin A, a histone deacetylase inhibitor. As a result of the unaltered poor prognosis of SCLC and its high CAR expression it seems to be the perfect candidate for oncolytic therapy. With our clinically relevant tumor model, we show that xenograft experiments are warrant to test the efficiency of oncolytic adenoviral therapy.

Virus-host coevolution in a persistently coxsackievirus B3-infected cardiomyocyte cell line

Coevolution of virus and host is a process that emerges in persistent virus infections. Here we studied the coevolutionary development of coxsackievirus B3 (CVB3) and cardiac myocytes representing the major target cells of CVB3 in the heart in a newly established persistently CVB3-infected murine cardiac myocyte cell line, HL-1(CVB3). CVB3 persistence in HL-1(CVB3) cells represented a typical carrier-state infection with high levels (10(6) to 10(8) PFU/ml) of infectious virus produced from only a small proportion (approximately 10%) of infected cells. CVB3 persistence was characterized by the evolution of a CVB3 variant (CVB3-HL1) that displayed strongly increased cytotoxicity in the naive HL-1 cell line and showed increased replication rates in cultured primary cardiac myocytes of mouse, rat, and naive HL-1 cells in vitro, whereas it was unable to establish murine cardiac infection in vivo. Resistance of HL-1(CVB3) cells to CVB3-HL1 was associated with reduction of coxsackievirus and adenovirus receptor (CAR) expression. Decreasing host cell CAR expression was partially overcome by the CVB3-HL1 variant through CAR-independent entry into resistant cells. Moreover, CVB3-HL1 conserved the ability to infect cells via CAR. The employment of a soluble CAR variant resulted in the complete cure of HL-1(CVB3) cells with respect to the adapted virus. In conclusion, this is the first report of a CVB3 carrier-state infection in a cardiomyocyte cell line, revealing natural coevolution of CAR downregulation with CAR-independent viral entry in resistant host cells as an important mechanism of induction of CVB3 persistence.

The roles of cell surface attachment molecules and coagulation Factor X in adenovirus 5-mediated gene transfer in pancreatic cancer cells

Transduction of 11 pancreatic cancer cell lines with a replication-deficient adenovirus 5 expressing enhanced green fluorescent protein (Ad5EGFP) was analyzed and variable EGFP levels were observed, ranging from <1% to ∼40% of cells transduced, depending on the cell line. Efficient Ad5EGFP transduction was associated mainly with higher levels of cell surface Coxsackie and adenovirus receptor (CAR) but not with expression of α(v)β(3) and α(v)β(5) integrins and was fiber dependent. Reduction of CAR by RNA interference resulted in a corresponding decrease in Ad5EGFP transduction. Pre-treatment of Ad5EGFP with blood coagulation Factor X increased virus entry even in the presence of low CAR levels generated by RNA interference, suggesting a potential alternative route of Ad5 entry into pancreatic cancer cells. Immunohistochemistry carried out on 188 pancreatic ductal adenocarcinomas and 68 matched controls showed that CAR was absent in 102 (54%) of adenocarcinomas, whereas moderate and strong staining was observed in 58 (31%) and 28 (15%) cases, respectively. Weak or absent CAR immunolabeling correlated with poor histological differentiation of pancreatic cancer. In normal tissue, strong immunolabeling was detected in islet cells and in the majority of inter- and intralobular pancreatic ducts.

Transactivation of human parvovirus B19 gene expression in endothelial cells by adenoviral helper functions

Human parvovirus B19 (B19V) DNA is highly prevalent in endothelial cells lining up intramyocardial arterioles and postcapillary venules of patients with chronic myocarditis and cardiomyopathies. We addressed the question of a possible stimulation of B19V gene expression in endothelial cells by infection with adenoviruses. Adenovirus infection led to a strong augmentation of B19V structural and nonstructural proteins in individual endothelial cells infected with B19V or transfected with an infectious B19V genome. Transactivation was mostly mediated at the level of transcription and not due to adenovirus-mediated induction of second-strand synthesis from the single-stranded parvoviral genome. The main adenoviral functions required were E1A and E4orf6, which displayed synergistic effects. Furthermore, a limited B19V genome replication could be demonstrated in endothelial cells and adenovirus infection induced the appearance of putative dimeric replication intermediates. Thus the almost complete block in B19V gene expression seen in endothelial cells can be abrogated by infection with other viruses.

Inhibition of adenovirus infections by siRNA-mediated silencing of early and late adenoviral gene functions

Adenoviruses are pathological agents inducing mild respiratory and gastrointestinal infections. Under certain circumstances, for example in immunosuppressed patients, they induce severe infections of the liver, heart and lung, sometimes leading to death. Currently, adenoviral infections are treated by palliative care with no curative antiviral therapy yet available. Gene silencing by RNA interference (RNAi) has been shown to be a potent new therapeutic option for antiviral therapy. In the present study, we examined the potential of RNAi-mediated inhibition of adenovirus 5 infection by the use of small interfering (si)RNAs targeting both early (E1A) and late (hexon, IVa2) adenoviral genes. Several of the initially analyzed siRNAs directed against E1A, hexon and IVa2 showed a distinct antiviral activity. Among them, one siRNA for each gene was selected and used for the further comparative investigations of their efficiency to silence adenoviruses. Silencing of the late genes was more efficient in inhibiting adenoviral replication than comparable silencing of the E1A early gene. A combination strategy involving down-regulation of any two or all three of the targeted genes did not result in an enhanced inhibition of viral replication as compared to the single siRNA approaches targeting the late genes. However, protection against adenovirus-mediated cytotoxicity was substantially improved by combining siRNAs against either of the two late genes with the siRNA against the E1A early gene. Thus, an enhanced anti-adenoviral efficiency of RNAi-based inhibition strategies can be achieved by co-silencing of early and late adenoviral genes, with down regulation of the E1A as a crucial factor.

Analysis of adenovirus trans-complementation-mediated gene expression controlled by melanoma-specific TETP promoter in vitro

BACKGROUND

Human adenoviruses (Ads) have substantial potential for clinical applications in cancer patients. Conditionally replicating adenoviruses (CRAds) include oncolytic adenoviruses in which expression of the immediate early viral transactivator protein E1A is controlled by a cancer cell-selective promoter. To enhance efficacy, CRAds are further armed to contain therapeutic genes. Due to size constraints of the capsid geometry, the capacity for packaging transgenes into Ads is, however, limited. To overcome this limitation, the employment of E1A-deleted replication-deficient viruses carrying therapeutic genes in combination with replication-competent CRAd vectors expressing E1A in trans has been proposed. Most trans-complementing studies involved transgene expressions from strong ubiquitous promoters, and thereby relied entirely on the cancer cell specificity of the CRAd vector.

RESULTS

Here we tested the trans-complementation of a CRAd and a replication-deficient transgene vector containing the same cancer cell-selective promoter. Hereto, we generated two new vectors expressing IL-2 and CD40L from a bicistronic expression cassette under the control of the melanoma/melanocyte-specific tyrosinase enhancer tyrosinase promoter (TETP), which we previously described for the melanoma-specific CRAd vector AdDeltaEP-TETP. These vectors gave rise to tightly controlled melanoma-specific transgene expression levels, which were only 5 to 40-fold lower than those from vectors controlled by the nonselective CMV promoter. Reporter analyses using Ad-CMV-eGFP in combination with AdDeltaEP-TETP revealed a high level of trans-complementation in melanoma cells (up to about 30-fold), but not in non-melanoma cells, unlike the AdCMV-eGFP/wtAd5 binary vector system, which was equally efficient in melanoma and non-melanoma cells. Similar findings were obtained when replacing the transgene vector AdCMV-eGFP with AdCMV-IL-2 or AdCMV-CD40L. However, the combination of the novel AdTETP-CD40L/IL-2 vector with AdDeltaEP-TETP or wtAd5 gave reproducible moderate 3-fold enhancements of IL-2 by trans-complementation only.

CONCLUSIONS

The cancer cell-selective TETP tested here did not give the expected enforceable transgene expression typically achieved in the Ad trans-complementing system. Reasons for this could include virus-mediated down regulation of limiting transcription factors, and/or competition for such factors by different promoters. Whether this finding is unique to the particular promoter system tested here, or also occurs with other promoters warrants further investigations.

Role of RGD-containing ligands in targeting cellular integrins: Applications for ovarian cancer virotherapy (Review)

The purpose of this article was to review the current strategies of targeted therapy to integrins and define the best course of future research in ovarian cancer targeting. Cell surface integrin targeting has been used as a strategy for targeted therapy of several diseases with some success. The combination of virotherapy and integrin-targeting shows promise as a method for targeting ovarian cancer. More specifically, targeting of ovarian cancer with integrin-directed adenoviruses may lead to therapy with fewer toxicities and side effects. This article offers a review of the benefits of integrin-specific targeted therapy for several diseases and proposes a unique anti-ovarian cancer strategy involving the combination of the above with virotherapy as a potential anti-ovarian cancer treatment.

hTERT-promoter-dependent oncolytic adenovirus enhances the transduction and therapeutic efficacy of replication-defective adenovirus vectors in pancreatic cancer cells

Adenovirus-mediated gene therapy shows promise for cancer therapy, but transgene expression of replication-defective adenovirus may be low and transient in clinical settings. Recent reports have shown that the use of a conditionally replication-competent adenovirus (CRAd) enhanced the gene transduction of a replication-defective adenovirus vector. The control of tumor-stromal interactions has also been determined to be important in cancer therapy. In this study, we investigated the effect of the human telomerase reverse transcriptase (hTERT)-CRAd, Ad5/3hTERTE1, which possesses the tumor-specific hTERT promoter with the chimeric fiber 5/3, on the transgene expression and therapeutic efficacy of a replication-defective adenovirus vector expressing NK4 under the control of the CMV promoter, Ad-NK4. In addition, we established a new strategy to target both cancer cells and cancer-stromal interactions. Human pancreatic cancer cells were infected with Ad-NK4 and either Ad5/3hTERTE1 (CRAd-combination group) or Ad5/3hTERTLuc (control-combination group). In the CRAd-combination group, Ad-NK4-delivered transgene expression was increased, leading to an enhanced inhibitory effect on the invasion of cancer cells. In in vivo experiments, NK4 expression within tumors and its inhibitory effect on tumor growth, angiogenesis, and metastasis were enhanced in the CRAd-combination group. These results suggest that hTERT-CRAd enhances the transgene expression and therapeutic efficacies of Ad-NK4, possibly through the in-trans replication of Ad-NK4 induced by adenovirus E1 derived from co-infected hTERT-CRAd. This approach may be a promising combination therapy against advanced pancreatic cancer.

Targeting of adenovirus serotype 5 pseudotyped with short fiber from serotype 41 to c-erbB2-positive cells using bispecific single-chain diabody

The purpose of the current study was to alter the broad native tropism of human adenovirus for virus targeting to c-erbB2-positive cancer cells. First, we engineered a single-chain antibody (scFv) against the c-erbB2 oncoprotein into minor capsid protein IX (pIX) of adenovirus serotype 5 (Ad5) in a manner commensurate with virion integrity and binding to the soluble extracellular c-erbB2 domain. To ablate native viral tropism and facilitate binding of the pIX-incorporated scFv to cellular c-erbB2, we replaced the Ad5 fiber with the Ad41 short (41s) fiber devoid of all known cell-binding determinants. The resultant Ad5F41sIX6.5 vector demonstrated increased cell binding and gene transfer as compared to the Ad5F41s control; however, this augmentation of virus infectivity was not c-erbB2 specific. Incorporation of a six-histidine (His(6)) peptide into the C-terminus of the 41s fiber protein resulted in markedly increased Ad5F41s6H infectivity in 293AR cells, which express a membrane-anchored scFv against the C-terminal oligohistidine tag, as compared to the Ad5F41s vector and the parental 293 cells. These data suggested that a 41s-fiber-incorporated His(6) tag could serve for attachment of an adapter protein designed to guide Ad5F41s6H infection in a c-erbB2-specific manner. We therefore engineered a bispecific scFv diabody (scDb) combining affinities for both c-erbB2 and the His(6) tag and showed its ability to provide up to 25-fold increase of Ad5F41s6H infectivity in c-erbB2-positive cells. Thus, Ad5 fiber replacement by a His(6)-tagged 41s fiber coupled with virus targeting mediated by an scDb adapter represents a promising strategy to confer Ad5 vector tropism for c-erbB2-positive cancer cells.

Coxsackievirus and adenovirus receptor expression in human endometrial adenocarcinoma: possible clinical implications

The coxsackievirus and adenovirus receptor (CAR) is a crucial receptor for the entry of both coxsackie B viruses and adenoviruses into host cells. CAR expression on tumor cells was reported to be associated with their sensitivity to adenoviral infection, while it was considered as a surrogate marker for monitoring and/or predicting the outcome of adenovirus-mediated gene therapy. The aim of the present study was to evaluate the clinical significance of CAR expression in endometrial adenocarcinoma. CAR expression was assessed immunohistochemically in tumoral samples of 41 endometrial adenocarcinoma patients and was statistically analyzed in relation to various clinicopathological parameters, tumor proliferative capacity and patient survival. CAR positivity was noted in 23 out of 41 (56%) endometrial adenocarcinoma cases, while high CAR expression in 8 out of 23 (35%) positive ones. CAR intensity of immunostaining was classified as mild in 11 (48%), moderate in 10 (43%) and intense in 2 (9%) out of the 23 positive cases. CAR positivity was significantly associated with tumor histological grade (p = 0.036), as well differentiated tumors more frequently demonstrating no CAR expression. CAR staining intensity was significantly associated with tumor histological type (p = 0.016), as tumors possessing squamous elements presented more frequently intense CAR immunostaining. High CAR expression showed a trend to be correlated with increased tumor proliferative capacity (p = 0.057). Patients with tumors presenting moderate or intense CAR staining intensity were characterized by longer survival times than those with mild one; however, this difference did not reach statistical significance. These data reveal, for the first time, the expression of CAR in clinical material obtained from patients with endometrial adenocarcinoma in relation to important clinicopathological parameters for their management. As CAR appears to modulate the proliferation and characteristics of cancer cells, its expression could be considered of possible clinical importance for future (gene) therapy applications.

Coxsackievirus B3 and adenovirus infections of cardiac cells are efficiently inhibited by vector-mediated RNA interference targeting their common receptor

As coxsackievirus B3 (CoxB3) and adenoviruses may cause acute myocarditis and inflammatory cardiomyopathy, isolation of the common coxsackievirus–adenovirus-receptor (CAR) has provided an interesting new target for molecular antiviral therapy. Whereas many viruses show high mutation rates enabling them to develop escape mutants, mutations of their cellular virus receptors are far less likely. We report on antiviral efficacies of CAR gene silencing by short hairpin (sh)RNAs in the cardiac-derived HL-1 cell line and in primary neonatal rat cardiomyocytes (PNCMs). Treatment with shRNA vectors mediating RNA interference against the CAR resulted in almost complete silencing of receptor expression both in HL-1 cells and PNCMs. Whereas CAR was silenced in HL-1 cells as early as 24 h after vector treatment, its downregulation in PNCMs did not become significant before day 6. CAR knockout resulted in inhibition of CoxB3 infections by up to 97% in HL-1 cells and up to 90% in PNCMs. Adenovirus was inhibited by only 75% in HL-1 cells, but up to 92% in PNCMs. We conclude that CAR knockout by shRNA vectors is efficient against CoxB3 and adenovirus in primary cardiac cells, but the efficacy of this approach in vivo may be influenced by cell type-specific silencing kinetics in different tissues.

A bidirectional Tet-dependent promotor construct regulating the expression of E1A for tight control of oncolytic adenovirus replication

Tight regulation of oncolytic adenoviruses (oAdV) represents an important requirement for their safe application. Here we describe a new doxycycline (Dox)-dependent oAdV with a bidirectional expression cassette, which drives the expression of the reverse tetracycline-controlled transactivator (rtTA(s)-M2) from a lung tumor-specific promoter and, in the opposite direction, the expression of the adenoviral E1A gene from a second generation TetO(7) sequence linked to an isolated TATA box. In H441 lung cancer cells, this oAdV showed a strictly Dox-dependent E1A expression, adenoviral replication, cell killing activity and a 450-fold induction of progeny virus production. The virus could be shut off again by withdrawal of Dox and, in contrast to a control oAdV expressing E1A directly from the SP-B promoter, did not replicate in non-target cells. However, the absolute values of virus production and the cell killing activity in the presence of the inducer were still reduced as compared to the control oAdV. The results demonstrate, for the first time, Dox-dependent oAdV replication from a single adenoviral vector genome. Future improvement of the Dox-dependent E1A regulation cassette should lead to the generation of an oAdV well suited to meet the demands for a highly regulated and efficient oncolytic virus for in vivo applications.

Highly efficient and specific modulation of cardiac calcium homeostasis by adenovector-derived short hairpin RNA targeting phospholamban

Impaired function of the phospholamban (PLB)-regulated sarcoplasmic reticulum Ca(2+) pump (SERCA2a) contributes to cardiac dysfunction in heart failure (HF). PLB downregulation may increase SERCA2a activity and improve cardiac function. Small interfering (si)RNAs mediate efficient gene silencing by RNA interference (RNAi). However, their use for in vivo gene therapy is limited by siRNA instability in plasma and tissues, and by low siRNA transfer rates into target cells. To address these problems, we developed an adenoviral vector (AdV) transcribing short hairpin (sh)RNAs against rat PLB and evaluated its potential to silence the PLB gene and to modulate SERCA2a-mediated Ca(2+) sequestration in primary neonatal rat cardiomyocytes (PNCMs). Over a period of 13 days, vector transduction resulted in stable > 99.9% ablation of PLB-mRNA at a multiplicity of infection of 100. PLB protein gradually decreased until day 7 (7+/-2% left), whereas SERCA, Na(+)/Ca(2+) exchanger (NCX1), calsequestrin and troponin I protein remained unchanged. PLB silencing was associated with a marked increase in ATP-dependent oxalate-supported Ca(2+) uptake at 0.34 microM of free Ca(2+), and rapid loss of responsiveness to protein kinase A-dependent stimulation of Ca(2+) uptake was maintained until day 7. In summary, these results indicate that AdV-derived PLB-shRNA mediates highly efficient, specific and stable PLB gene silencing and modulation of active Ca(2+) sequestration in PNCMs. The availability of the new vector now enables employment of RNAi for the treatment of HF in vivo.

Current developments in adenovirus-based cancer gene therapy

Adenovirus (Ad)-based cancer gene therapy is a promising, novel approach for treating cancer resistant to established treatment modalities. Unfortunately, the efficacy of nonreplicative first generation Ads was low and data from clinical trials were disappointing. To address this problem, conditionally replicating Ads have been constructed. Infection of tumor cells with conditionally replicating Ads results in tumor-specific replication, subsequent oncolysis and release of the virus progeny. Recently, it has been suggested that the low expression of the coxsackie-Ad receptor is the rate-limiting factor for infectivity with serotype 5 (Ad5). Unfortunately, coxsackie-Ad receptor expression is highly variable and often low on many tumor types. Consequently, molecular strategies have been applied for the development of coxsackie-Ad receptor-independent oncolytic Ads. This review describes recent developments of Ad-based cancer gene therapy, including novel engineering techniques of the Ad capsid for efficient tumor targeting, as well as targeting techniques, to restrict transgene expression to cancer cells.

Treatment of coxsackievirus-B3-infected BALB/c mice with the soluble coxsackie adenovirus receptor CAR4/7 aggravates cardiac injury

Coxsackie adenovirus receptor (CAR) is involved in immunological processes, and its soluble isoforms have antiviral effects on coxsackievirus B3 (CVB3) infection in vitro. We explored in this study the impact of CAR4/7, a soluble CAR isoform, on CVB3-induced myocarditis in BALB/c mice. BALB/c mice were treated daily with recombinant CAR4/7, beta-galactosidase (beta-Gal; as control protein) or buffer for 9 days. Half of each group was infected with CVB3 on day 3, and all mice were killed on day 9. Myocardial CVB3 titer, histology, and serology were analyzed. Treatment with CAR4/7 led to a significant reduction of myocardial CVB3 titer, whereas the application of beta-Gal had no detectable effect on the myocardial virus load. CAR4/7 application, however, resulted in increased myocardial inflammation and tissue damage in CVB3-infected hearts, whereas beta-Gal caused a degree of cardiac inflammation and injury similar to that in buffer-treated CVB3-infected control animals. CAR4/7 and beta-Gal treatment induced the production of antibodies against the respective antigens. CAR4/7-, but not beta-Gal-specific, virus-negative sera reacted against myocardial tissue and cellular membranous CAR, and significantly inhibited CVB3 infection in vitro. Thus, CAR4/7 suppressed CVB3 infection in vivo, supporting the concept of receptor analog in antiviral therapy. However, CAR4/7 treatment also leads to an aggravation of myocardial inflammation and injury most likely secondary to an autoimmune process.

An improved Tet-On regulatable FasL-adenovirus vector system for lung cancer therapy

Gene therapy is a new therapeutic approach for the treatment of human cancers. Gene expression systems that can be regulated by drugs have been developed to improve the safety and efficacy of therapeutic transgene delivery. One of the most promising systems is the tetracycline (Tet)-responsive system in the Tet-On configuration. A major problem of the Tet-On system if used in viral vectors is the high basal activity of the Tet response element (TRE) promoter leading to leaky expression of transgenes under uninduced conditions. We therefore evaluated novel TRE promoters for controlling gene expression in an adenovirus vector (AdV) Tet-On system and further investigated them for expression of the pro-apoptotic CD95/Fas ligand (FasL) in human epithelial carcinoma cell line (HeLa) and lung cancer cells. Plasmid-based reporter gene assays showed that modifications within the tetO (7) and minimal immediate early cytomegalovirus promoter (CMV)(min) sequence of the TRE promoter reduced its leakiness and led to a markedly improved regulatability by doxycycline. Among several TRE promoters tested, a new construct (TRE-Tight1) containing modifications of both the tetO (7) sequence and the CMV(min) showed 11-fold reduced leakiness and 1.5-fold increased absolute transgene expression levels after induction, as compared to the original TRE. Under induced conditions, a TRE-Tight1 promoter-dependent AdV expressing the pro-apoptotic CD95L/FasL induced apoptosis and cell lysis in HeLa cells as efficiently as an AdV containing the original TRE promoter. In contrast to the latter, however, the vector with the modified TRE promoter left cells totally unaffected in the absence of the inducer. Stringently regulated induction of apoptosis and cell death by TRE-Tight1-AdV was also demonstrated in three human lung cancer cell lines. These data show that the novel TRE-Tight1 promoter has a high potential for closely controlled and efficient expression of cytotoxic genes in AdV-based anti-cancer approaches.

Viral and nonviral factors causing nonspecific replication of tumor- and tissue-specific promoter-dependent oncolytic adenoviruses

Restricted replication-competent adenoviruses (RRCAs) using tumor- and tissue-specific promoters (ttsP's) are new tools for cancer gene therapy. In this study we investigated viral and nonviral factors affecting "leakiness" of several ttsP's and their relevance for nonspecific ttsP-dependent RRCA (ttsP-RRCA) replication. The leakiness of the ttsP's in nontarget cells was per se highly variable and correlated with levels of nonspecific ttsP-RRCA replication. Transcriptional regulator elements fused to ttsP's showed variable effects: a hypoxic response element reduced leakiness of an alpha-fetoprotein promoter. In contrast, a mouse tyrosinase enhancer increased leakiness of a tyrosinase promoter, although it was not affected by a human tyrosinase enhancer. Furthermore, leakiness of ttsP's was enhanced by 5'-terminal adenoviral E1A enhancers, and adenoviral E1A-13S was found to be a strong transactivator of ttsP's, leading to "autoactivation" of leaky ttsP-RRCAs. In a proof-of-principle study, ttsP-RRCA replication was shown to be inhibited by a tetracycline-controlled transcriptional silencer via direct ttsP silencing. This opens up the prospect of pharmacological regulation of ttsP-RRCAs. Together, these data indicate that leakiness of ttsP's induced by several factors is a major cause of nonspecific ttsP-RRCA replication. Consideration of these factors may help optimize ttsP-dependent RRCA vectors and may thereby improve their safety.

Tamoxifen-regulated adenoviral E1A chimeras for the control of tumor selective oncolytic adenovirus replication in vitro and in vivo

Pharmacological control is a desirable safety feature of oncolytic adenoviruses (oAdV). It has recently been shown that oAdV replication may be controlled by drug-dependent transcriptional regulation of E1A expression. Here, we present a novel concept that relies on tamoxifen-dependent regulation of E1A activity through functional linkage to the mutated hormone-binding domain of the murine estrogen receptor (Mer). Four different E1A-Mer chimeras (ME, EM, E(DeltaNLS)M, MEM) were constructed and inserted into the adenoviral genome under control of a lung-specific surfactant protein B promoter. The highest degree of regulation in vitro was seen for the corresponding oAdVs Ad.E(DeltaNLS)M and Ad.MEM, which exhibited an up to 100-fold higher oAdV replication in the presence as compared with the absence of 4-OH-tamoxifen. Moreover, destruction of nontarget cells was six- and 13-fold reduced for Ad.E(DeltaNLS)M and Ad.MEM, respectively, as compared with Ad.E. Further investigations supported tamoxifen-dependent regulation of Ad.E(DeltaNLS)M and Ad.MEM in vivo. Induction of Ad.E(DeltaNLS)M inhibited growth of H441 lung tumors as efficient as a control oAdV expressing E1A. E(DeltaNLS)M and the MEM chimeras can be easily inserted into a single vector genome, which extends their application to existing oAdVs and strongly facilitates in vivo application.

PCI-enhanced adenoviral transduction employs the known uptake mechanism of adenoviral particles

The development of methods for efficient and specific delivery of therapeutic genes into target tissues is an important issue for further development of in vivo gene therapy. In the present study, the physical targeting technique, photochemical internalization (PCI), has been used together with adenovirus. The combination of PCI and adenoviral transduction has previously been shown to be favorable compared to adenovirus used alone, and the aim of this study was to verify the role of the adenoviral receptors and identify the uptake pathway used by adenoviral particles in photochemically treated cells. All examined cell lines showed augmented transduction efficiency after PCI-treatment, with a maximum of 13-fold increase in transgene expression compared to conventionally infected cells. Blocking of CAR induced a complete inhibition of PCI-enhanced transgene expression. However, photochemical treatment managed to enhance the transduction efficiency of the retargeted virus AdRGD-GFP showing also that the virus-CAR interaction is not vital for obtaining a photochemical effect on adenoviral transduction. Blocking the alpha(V)-integrins reduced the gene expression significantly in photochemically treated cells. Subjecting HeLa cells expressing negative mutant-dynamin to light treatment after infection gave no significant increase in gene transfer, while the gene transfer were enhanced seven-fold in cells with wild-type dynamin. Furthermore, chlorpromazine inhibited photochemical transduction in a dose-dependent manner, whereas Filipin III had no effect on the gene transfer. In summary, the data presented imply that adenoviral receptor binding is important and clathrin-mediated endocytosis is the predominant uptake mechanism for adenoviral particles in photochemically treated cells.

Chemically inactivated adenoviral vectors that can efficiently transduce target cells when delivered in the form of virus-microbead conjugates

Safe and effective use of viral vectors for gene therapeutics requires versatile control over their delivery to target sites in human subjects. We have developed a strategy for the creation of adenoviral vectors that possess conditional infectivity. The adenoviral vectors used were inactivated chemically such that they had little or no ability to infect cells. However, when such chemically inactivated adenoviral vectors were conjugated to the surfaces of appropriate microbeads and the resulting adenovirus-microbead conjugates were provided with the ability to associate stably with cells, the infectivity of these adenoviral vectors was restored. For certain target cell lines, the infectivity of such adenovirus-microbead conjugates became even higher than that of free, unmodified adenoviral vectors. As a result of the chemical inactivation of viral infectivity, any adenoviral particles that become free from the microbeads should be noninfectious. Thus, these adenoviral vectors have an infectivity that is conditional: They can only infect cells, to which their microbead conjugates come into stable contact. These results lay the groundwork for the creation of targetable adenovirus-microbead conjugates with greater efficacy and safety as delivery agents for gene therapeutics.

Enhanced oncolysis by a tropism-modified telomerase-specific replication-selective adenoviral agent OBP-405 ('Telomelysin-RGD')

Replication-competent oncolytic viruses are being developed for human cancer therapy. We previously reported that an attenuated adenovirus (OBP-301, 'Telomelysin'), in which the hTERT promoter element drives expression of E1A and E1B genes linked with an IRES, could replicate in cancer cells, and causes selective lysis of cancer cells. We further constructed OBP-405 ('Telomelysin-RGD') that contains an RGD motif in the HI loop of the fiber knob. We examined whether OBP-405 could be effective in overcoming the limitations of OBP-301, specifically their inefficient infection into cells lacking the primary receptor, the coxsackievirus and adenovirus receptor (CAR). By flow cytometric analysis, H1299 (lung) and SW620 (colorectal) tumor cells showed high levels of CAR expression, whereas LN444 (glioblastoma), LNZ308 (glioblastoma), and H1299-R5 (lung) tumor cells were negative for CAR expression. A quantitative real-time PCR analysis demonstrated that fiber-modified OBP-405 infected more efficiently than OBP-301, although the intracellular replication rate of both viruses was consistent. The comparative antitumor effect of fiber-modified OBP-405 and unmodified OBP-301 for human cancer cells was evaluated in vitro by XTT assay as well as in vivo by using athymic mice carrying xenografts. OBP-405 had a profound oncolytic effect on human cancer cell lines compared to OBP-301, in particular on cells with low CAR expression. Intratumoral injection of 10(7) plaque-forming units of OBP-405 into CAR-negative H1299-R5 lung tumor xenografts in nu/nu mice resulted in a significant inhibition of tumor growth and long-term survival in all treated mice. Moreover, selective replication of OBP-405 in the distant, uninjected H1299-R5 tumors was demonstrated. Our results suggest that fiber-modified replication-competent adenovirus OBP-405 exhibits a broad target range by increasing infection efficiency, an outcome that has important implications for the treatment of human cancers.

Infectivity enhanced, hTERT promoter-based conditionally replicative adenoviruses are useful for SCLC treatment

Treatment of advanced small-cell lung cancer (SCLC) remains one of the major challenges in current medicine because of the high morbidity and mortality of the disease. Advanced stage lung cancer is refractory to conventional therapies and it also has an extremely poor prognosis. As a result, new therapeutic approaches are needed. Telomere maintenance to the regulation of replicative lifespan strongly implies that alterations in telomere biology play an important role during malignant transformation. Cancers that exhibit high levels of telomerase activity, such as all of the SCLC, were examined in a previous study. In this study, we turned the expression of human telomerase reverse transcriptase (hTERT) by tumors to a therapeutic advantage using a conditionally replication-competent adenovirus (CRAd) in which the expression of E1 (early region 1) is controlled by the hTERT promoter. This virus achieved good levels of viral replication in SCLC cells and induced a substantial anticancer effect in vitro and in vivo. As a further enhancement, the cancer cell killing effect was improved with a tropism modification of the virus to express the knob domain of Ad3 (serotype 3 adenovirus), and this improved infectivity for cancer cells. Conversely, the hTERT promoter has low activity in normal tissues, and the CRAd caused no damage to normal lung fibroblast cells. Since the telomerase activity is common in many types of cancers, these CRAds may be applicable to a wide range of tumors. We concluded that the use of hTERT promoter-based CRAds may be a potentially effective strategy for cancer treatment.

Oncolytic viral therapies

Molecular research has vastly advanced our understanding of the mechanism of cancer growth and spread. Targeted approaches utilizing molecular science have yielded provocative results in the treatment of cancer. Oncolytic viruses genetically programmed to replicate within cancer cells and directly induce toxic effect via cell lysis or apoptosis are currently being explored in the clinic. Safety has been confirmed and despite variable efficacy results several dramatic responses have been observed with some oncolytic viruses. This review summarizes results of clinical trials with oncolytic viruses in cancer.

Use of replicating oncolytic adenoviruses in combination therapy for cancer

Oncolytic virotherapy is the use of genetically engineered viruses that specifically target and destroy tumor cells via their cytolytic replication cycle. Viral-mediated tumor destruction is propagated through infection of nearby tumor cells by the newly released progeny. Each cycle should amplify the number of oncolytic viruses available for infection. Our understanding of the life cycles of cytolytic viruses has allowed manipulation of their genome to selectively kill tumor cells over normal tissue. Because the mechanism of tumor destruction is different, oncolytic virotherapy should work synergistically with current modes of treatment such as chemotherapy and radiation therapy. This article focuses on oncolytic adenoviruses that have been created and tested in preclinical and clinical trials in combination with chemotherapy, radiation therapy, and gene therapy.

High-level expression of the coxsackievirus and adenovirus receptor messenger RNA in osteosarcoma, Ewing's sarcoma, and benign neurogenic tumors among musculoskeletal tumors

PURPOSE

The sensitivity of human tumor tissues to infection with recombinant adenoviruses correlates with the expression of the coxsackievirus and adenovirus receptor (CAR). CAR has been shown to function as the primary receptor for adenoviruses and to play a critical role in adenovirus entry into host cells. It is important for clinical gene therapy to determine the expression level of CAR in tumor tissues.

EXPERIMENTAL DESIGN

We analyzed the expression of CAR mRNA in 154 musculoskeletal tumor tissues from 154 patients and 10 normal mesenchymal tissues from 3 patients using reverse transcription-PCR and real-time quantitative PCR. An adenovirus infection assay was performed in two cell lines that were established from CAR-positive osteosarcoma tissue and CAR-negative malignant fibrous histiocytoma tissue.

RESULTS

Ninety-nine of 154 tumors were detected as CAR positive by reverse transcription-PCR. We found that the expression levels of CAR mRNA varied markedly between different tumors as determined by real-time quantitative PCR. CAR mRNA was expressed at high levels in osteosarcoma, Ewing's sarcoma, neurofibroma, and schwannoma; at intermediate levels in exostosis, giant cell tumor, liposarcoma, synovial sarcoma, malignant peripheral nerve sheath tumor, and hemangioma; and at low levels in alveolar soft part sarcoma and desmoid. Whereas the osteosarcoma cell line that expressed a high level of CAR mRNA, like its parent tumor, had a high efficiency of adenovirus infection, the malignant fibrous histiocytoma cell line with almost undetectable expression of CAR mRNA, like its parent tumor, had a low efficiency of infection.

CONCLUSIONS

Our data showed the great variations in CAR mRNA expression among human musculoskeletal tumors and mesenchymal tissues and implicated the potential usefulness of adenoviral vectors in gene therapy for osteosarcoma, Ewing's sarcoma, neurofibroma, and schwannoma. Efficient transduction with adenovirus for gene therapy could be realized in appropriate, sensitive tumor types.

Protein transduction domains fused to virus receptors improve cellular virus uptake and enhance oncolysis by tumor-specific replicating vectors

Expression of cellular receptors determines viral tropism and limits gene delivery by viral vectors. Protein transduction domains (PTDs) have been shown to deliver proteins, antisense oligonucleotides, liposomes, or plasmid DNA into cells. In our study, we investigated the role of several PTD motifs in adenoviral infection. When physiologically expressed, a PTD from human immunodeficiency virus transactivator of transcription (Tat) did not improve adenoviral infection. We therefore fused PTDs to the ectodomain of the coxsackievirus-adenovirus receptor (CAR(ex)) to attach PTDs to adenoviral fiber knobs. CAR(ex)-Tat and CAR(ex)-VP22 allowed efficient adenoviral infection in nonpermissive cells and significantly improved viral uptake rates in permissive cells. Dose-dependent competition of CAR(ex)-PTD-mediated infection using CAR(ex) and inhibition experiments with heparin showed that binding of CAR(ex)-PTD to both adenoviral fiber and cellular glycosaminoglycans is essential for the improvement of infection. CAR(ex)-PTD-treated adenoviruses retained their properties after density gradient ultracentrifugation, indicating stable binding of CAR(ex)-PTD to adenoviral particles. Consequently, the mechanism of CAR(ex)-PTD-mediated infection involves coating of the viral fiber knobs by CAR(ex)-PTD, rather than placement of CAR(ex) domains on cell surfaces. Expression of CAR(ex)-PTDs led to enhanced lysis of permissive and nonpermissive tumor cells by replicating adenoviruses, indicating that CAR(ex)-PTDs are valuable tools to improve the efficacy of oncolytic therapy. Together, our study shows that CAR(ex)-PTDs facilitate gene transfer in nonpermissive cells and improve viral uptake at reduced titers and infection times. The data suggest that PTDs fused to virus binding receptors may be a valuable tool to overcome natural tropism of vectors and could be of great interest for gene therapeutic approaches.

Genetic replacement of the adenovirus shaft fiber reduces liver tropism in ovarian cancer gene therapy

Approaches to alter the native tropism of adenoviruses (Ads) are beneficial to increase their efficacy and safety profile. Liver tropism is important with regard to potential clinical toxicity in humans. Ad5/3 chimeras in which the Ad5 knob is substituted by the Ad3 knob, such as Ad5/3luc1, have been recently shown to increase infectivity of ovarian cancer cell lines and primary tumor cells, which express low levels of the coxsackie-adenovirus receptor (CAR), without increasing infectivity of liver cells. A novel strategy to address the problem of liver uptake and improve the tumor/liver ratio is genetic replacement of the Ad fiber shaft. Ad5.Ad3.SH.luc1 is an Ad5-based vector that contains the fiber shaft from Ad serotype 3 but the fiber knob from Ad serotype 5. To compare tumor/liver of Ad5.Ad3.SH.luc1 and Ad5/3luc1 in vivo, we created three different tumor and treatment models of ovarian cancer in mice, simulating intraperitoneal and intravenous administration of tumors. Ad5.Ad3.SH.luc1 displayed the lowest liver tropism of all viruses in all models tested. Intravenous administration of all viruses resulted in higher tumor transduction rates compared to intraperitoneal administration. Genetic shortening of the Ad5 fiber shaft significantly increases relative tumor/liver gene transfer. This could improve the effective tumor dose and reduce side effects, thereby increasing the bioavailability of therapeutic agents.

Tissue-specific promoters for cancer gene therapy

Adenoviral cancer gene therapy approaches have resulted in promising recent results. Following only a decade of intense development, some of the crucial obstacles are now being overcome. Insufficient transduction has been the main limitation of earlier approaches. A new approach for increasing transduction of tumour cells is utilisation of replication-competent oncolytic agents, such as conditionally replicating adenoviruses (CRADs). The anti-tumour effect is caused by replication of the virus per se and, thus, replication must be restricted to tumour cells to protect normal tissues from damage. Tissue-specific promoters (TSPs) represent a powerful tool for decreasing the toxicity of cancer gene therapy to normal tissues and have previously been utilised for specific mutation compensation or delivery of prodrug-converting enzymes. However, TSPs can also be used for controlling crucial viral replication regulators and consequent restriction of replication to tumour cells. Initial clinical trials have demonstrated the safety and suggested efficacy for TSP-controlled CRADs as a novel approach for cancer gene therapy.

Targeted delivery of adenoviral vectors by cytotoxic T cells

Effective targeting of vectors to tumor cells that have metastasized to multiple different tissue sites remains a major challenge for gene therapy. Tumor-specific cytotoxic T lymphocytes (CTLs) have been shown in animal models and in humans to be able to cross tissue barriers and traffic to tumor cells. However, their capacity to eliminate malignancy has been limited by tumor immune evasion strategies. We now use a model of Epstein-Barr virus-mediated malignancy to show that human CTLs themselves may be modified to release therapeutic vectors following engagement of their antigen-specific receptors and that these vectors will effectively transduce and destroy tumor targets. We generated EBV-specific CTLs that were transgenic for the adenoviral E1 gene under the control of the cell activation-dependent CD40 ligand (CD40L) promoter. Following transduction with E1-deficient adenoviral vectors, these CTLs produced infectious virus when exposed to HLA-matched EBV-expressing targets, but not on exposure to major histocompatibility complex (MHC)-mismatched or otherwise irrelevant cells. This approach provides a means of delivering oncolytic/therapeutic vectors not only to locally accessible macroscopic tumors as is presently the case, but also to disseminated metastatic disease, while avoiding the risks associated with systemic administration of large doses of adenoviral vectors.

Optimization of adenoviral vectors to direct highly amplified prostate-specific expression for imaging and gene therapy

Gene expression-based imaging coupled to gene therapy will permit the prediction of therapeutic outcome. A significant challenge for successful gene therapy is to achieve a high-level of specific gene expression; however, tissue-specific promoters are weak. We postulate that if the weak activity of tissue-specific promoters can be amplified to the levels of strong viral promoters, which have been successful in preclinical scenarios, while retaining specificity, the therapeutic index of gene therapy can be greatly augmented. With this in mind, we developed a two-step transcriptional activation (TSTA) system. In this two-tiered system, a modified prostate-specific antigen promoter was employed to drive a potent synthetic transcriptional activator, GAL4-VP2. This, in turn, activated the expression of a GAL4-dependent reporter or therapeutic gene. Here we demonstrate that recombinant adenoviral vectors (Ads) in which we have incorporated prostate-targeted TSTA expression cassettes retain cell specificity and androgen responsiveness in cell culture and in animal models, as measured by noninvasive optical bioluminescence imaging. We investigated the mechanism of TSTA in different adenoviral configurations. In one configuration, both the activator and the reporter components are inserted into a single Ad (AdTSTA-FL). The activity of AdTSTA-FL exceeds that of a cytomegalovirus promoter-driven vector (AdCMV-FL), while maintaining tissue specificity. When the activator and reporter components are placed in two separate Ads, androgen induction is more robust than for the single AdTSTA-FL. Based on these findings, we hope to refine the TSTA Ads further to improve the efficacy and safety of prostate cancer gene therapy.

The Coxsackie-Adenovirus Receptor—A New Receptor in the Immunoglobulin Family Involved in Cell Adhesion

The physiological and cell biological aspects of the Coxsackie-Adenovirus Receptor (CAR) is discussed in this review. The receptor obviously recognizes the group C adenoviruses in vivo, but also fibers from other groups except group B in vitro. The latter viruses seem to utilize a different receptor. The receptor accumulates at, or close to, the tight junction in polarized epithelial cells and probably functions as a cell-cell adhesion molecule. The cytoplasmic tail of the receptor is not required for virus attachment and uptake. Although there is a correlation between CAR and uptake of adenoviruses in several human tumor cells, evidence of an absolute requirement for integrins has not been forthcoming. The implication of these findings for adenovirus gene therapy is discussed.

A novel tetracycline-controlled transactivator-transrepressor system enables external control of oncolytic adenovirus replication

The use of restricted replication-competent adenoviruses (RRCAs) inducing tumor cell-specific lysis is a promising approach in cancer gene therapy. However, the use of RRCAs in humans carries considerable risk, since after injection into the patient, further regulation or inhibition of virus replication from the outside is impossible. Therefore, we have developed a novel system allowing external pharmacological control of RRCA replication. We show here that a tumor-selective E1B-deleted RRCA can be tightly regulated by use of doxycycline (dox)-controlled adenoviral E1A gene expression, which in turn determines vector replication. RRCA replication is switched on by addition and switched off by withdrawal of dox. The system results in efficient tumor cell killing after induction by dox, whereas cells are unaffected by the uninduced system. It was also employed for efficient external control of transgene expression from cotransfected replication-deficient adenovectors. Furthermore, the use of a liver cell-specific human alpha1-antitrypsin (hAAT)-promoter driving a tetracycline-controlled transcriptional silencer allowed specific protection of cells with hAAT-promoter activity in the absence of dox in vitro and in vivo, delineating a new principle of 'tissue protective' gene therapy. The concept of external control of RRCAs may help to improve the safety of cancer gene therapy.

Recruitment of nectin-3 to cell-cell junctions through trans-heterophilic interaction with CD155, a vitronectin and poliovirus receptor that localizes to alpha(v)beta3 integrin-containing membrane microdomains

Nectins present a novel class of Ig superfamily adhesion molecules that, cooperatively with cadherins, establish and maintain cell-cell adherens junctions. CD155, the cognate receptor for poliovirus, undergoes cell-matrix contacts by binding to the extracellular matrix protein vitronectin. The significant homology of nectins with CD155 prompted us to investigate the possibility of their interaction. We determined that nectin-3 binds CD155 and its putative mouse homologue Tage4 in cell-based ligand binding assays. Coculture of nectin-3- and CD155-expressing HeLa cells led to CD155-dependent recruitment of nectin-3 to cell-cell contacts. In a heterologous coculture system with CD155 expressing mouse neuroblastoma cells, HeLa cell-expressed nectin-3 was recruited to contact sites with CD155 bearing neurites. CD155 and nectin-3 colocalized to epithelial cell-cell junctions in renal proximal tubules and in the amniotic membrane. Efficient interaction depended on CD155 dimerization, which appears to be aided by cell type-specific cofactors. We furthermore found CD155 to codistribute with alpha(v) integrin microdomains on the surface of transfected mouse fibroblasts and at amniotic epithelial cell junctions. Our findings demonstrate the possible trans-interaction between the bona fide cell-cell adherens type adhesion system (cadherin/nectin) and the cell-matrix adhesion system (integrin/CD155) by virtue of their nectin-3 and CD155 components, respectively.

Replicative oncolytic adenoviruses in multimodal cancer regimens

The use of replication-competent viruses that have a cytolytic cycle has emerged as a viable strategy (oncolytic virotherapy) to specifically kill tumor cells and the field has advanced to the point of clinical trials. A theoretical advantage of replicative oncolytic viruses is that their numbers should increase via viral replication within infected tumor cells and resulting viral progeny can then infect additional cells within the tumor mass. The life cycle of a virus involves multiple interactions between viral and cellular proteins/genes, which maximize the ability of the virus to infect and replicate within cells. Understanding such interactions has led to the design of numerous genetically engineered adenovirus (Ad) vectors that selectively kill tumor cells while sparing normal cells. These viruses have also been modified to function as therapeutic gene delivery vehicles, thus augmenting their anticancer capacity. In addition, the oncolytic mode of tumor killing differs from that of standard anticancer therapies, providing the possibility for synergistic interactions with other therapies in a multimodal antitumor approach. In this review, we describe the oncolytic Ad vectors tested in preclinical and clinical models and their use in combination with chemo-, radio-, and gene therapies.

A mosaic adenovirus possessing serotype Ad5 and serotype Ad3 knobs exhibits expanded tropism

The efficiency of cancer gene therapy with recombinant adenoviruses based on serotype 5 (Ad5) has been limited partly because of variable, and often low, expression by human primary cancer cells of the primary cellular-receptor which recognizes the knob domain of the fiber protein, the coxsackie and adenovirus receptor (CAR). As a means of circumventing CAR deficiency, Ad vectors have been retargeted by utilizing chimeric fibers possessing knob domains of alternate Ad serotypes. We have reported that ovarian cancer cells possess a primary receptor for Ad3 to which the Ad3 knob binds independently of the CAR-Ad5 knob interaction. Furthermore, an Ad5-based chimeric vector, designated Ad5/3, containing a chimeric fiber proteins possessing the Ad3 knob, demonstrates CAR-independent tropism by virtue of targeting the Ad3 receptor. Based on these findings, we hypothesized that a mosaic virus possessing both the Ad5 knob and the Ad3 knob on the same virion could utilize either primary receptor, resulting in expanded tropism. In this study, we generated a dual-knob mosaic virus by coinfection of 293 cells with Ad5-based and Ad5/3-based vectors. Characterization of the resultant virions confirmed the incorporation of both Ad5 and Ad3 knobs in the same particle. Furthermore, this mosaic virus was able to utilize either receptor, CAR and the Ad3 receptor, for virus attachment to cells. Enhanced Ad infectivity with the mosaic virus was shown in a panel of cell lines, with receptor profiles ranging from CAR-dominant to Ad3 receptor-dominant. Thus, this mosaic virus strategy may offer the potential to improve Ad-based gene therapy approaches by infectivity enhancement and tropism expansion.

In situ transduction of target cells on solid surfaces by immobilized viral vectors

BACKGROUND

For both in vitro and in vivo gene transfer applications, recombinant viral vectors have almost always been used free in solution. Some site-specificity of the delivery of viral vectors can be achieved by applying a solution containing viral particles specifically to the site of interest. However, such site-specificity is seriously limited since viral vectors can diffuse freely in solution after application.

RESULTS

We have developed a novel strategy for in situ transduction of target cells on solid surfaces by viral vectors. In this strategy, adenoviral vectors are attached stably to solid surfaces by using the extremely tight interaction between (strept)avidin and biotin, while maintaining the infectivity of the viral vectors. Target cells are cultured directly on such virus-coated solid surfaces, resulting in the transduction of the cells, in situ, on the solid surface. When compared using an equal number of viral particles present in each well (either immobilized or free), the efficiencies of such in situ transduction on solid surfaces were equivalent to those seen with the adenoviral vectors used free in solution. Since viral particles can be attached at desired locations on solid surfaces in any sizes, shapes, and patterns, the ultimate spatial arrangements of transduced cells on solid surfaces can be predetermined at the time of the preparation of the virus-coated solid surfaces.

CONCLUSIONS

We have devised a method of immobilizing adenoviral vectors, tightly and stably, on solid surfaces, while maintaining their ability to infect cells. Such immobilized viral vectors can infect target cells, in situ, on solid surfaces. This strategy should be very useful for the development of a variety of both in vitro and in vivo applications, including the creation of cell-based expression arrays for proteomics and drug discovery and highly site-specific delivery of transgenes for gene therapy and tissue engineering.

Replication of an integrin targeted conditionally replicating adenovirus on primary ovarian cancer spheroids

Replication competent viruses hold promise for treatment of advanced cancers resistant to available therapeutic modalities. Although preliminary clinical results have substantiated their efficacy, preclinical development of these novel approaches is limited by assay substrates. The evaluation of candidate agents could be confounded by differences between primary tumor cells and tumor cell lines, as discordance in the levels of surface receptors relevant for viral entry has been reported. Since primary tumor cells are difficult to analyze ex vivo for longitudinal observation of virus replication, we developed three-dimensional aggregates or spheroids of unpassaged and purified ovarian cancer cells as a means for prolonging primary tumor cell viability and as a three-dimensional in vitro model for replicative viral infection. Ovarian cancer cells purified from ascites samples were sustained for 30 days while retaining the infection profile with tropism modified and unmodified adenoviruses (Ads). Cell line and primary cell spheroids were used to quantitate the replication and oncolytic potency of replicative Ads in preclinical testing for human ovarian cancer trials. Therefore, spheroids provide a method to sustain purified unpassaged primary ovarian cancer cells for extended periods and to allow evaluation of replicative viruses in a three-dimensional model.

Expression of the coxsackie and adenovirus receptor in human astrocytic tumors and xenografts

The sensitivity of human tissues and tumors to infection with type C adenoviruses correlates with the expression of the human coxsackie B- and adenovirus receptor, hCAR. HCAR is heterogeneously expressed in various tissues and types of human cancer cells, which has implications for the use of adenoviruses as vectors in cancer gene therapy. Using immunoblotting, real-time PCR, FACS-analysis and sensitivity to infection with adenovirus-lacZ, we analyzed the expression level of hCAR in glioma Grade IV cell lines. With real-time PCR, we also analyzed hCAR expression in primary human astrocytomas of different malignancy grades, as well as in their xenograft derivatives. Analysis of a set of 10 cell lines showed great variation in hCAR expression. Susceptibility to Ad5lacZ correlated well with hCAR expression, whereas no correlation was observed with the expression of alphavbeta3/alphavbeta5 integrins, proposed to function as co-receptors for adenoviruses. A great variation of CAR expression was also observed in primary astrocytomas of different malignancy grades. The mean value of CAR expression was significantly lower in 22 Grade IV tumors as compared to the values for 6 Grade II (p = 0.01) and 6 Grade III (p = 0.01) tumors. When the hCAR expression in 11 xenografts derived from Grade IV gliomas were compared to the levels detected in the original parental tumors, a mean 12-fold higher expression was seen in the xenografts (P = 0.01). Two xenografts with low hCAR expression grew considerably faster than the hCAR-expressing cells. Our results have relevance for the use of adenoviruses in gene therapy against astrocytomas.