Application of a Fas ligand encoding a recombinant adenovirus vector for prolongation of transgene expression

Efficient Sequential Gene Regulation via FLP- and Cre-Recombinase Using Adenovirus Vector in Mammalian Cells Including Mouse ES Cells

Site-specific recombinase is widely applied for the regulation of gene expression because its regulatory action is strict and efficient. However, each system can mediate regulation of only one gene at a time. Here, we demonstrate efficient "sequential" gene regulation using Cre-and FLP-expressing recombinant adenovirus (rAd) in two different monitor cell lines, for regulation of one gene (OFF-ON-OFF) and for two genes (ON-OFF and OFF-ON, independently). Generally, serial use of Cre-and FLP-expressing rAd tends to cause significant cytotoxicity, but we here described optimum dose of the rAds for serial regulation. We also established an efficient method of rAd infection to mouse ES cell lines after removing feeder cells, showing that this system is useful for removal of FRT-flanked drug-resistance gene cassette from recombinant ES cells prior to introduction of ES cells into blastocytes for chimeric mice production. Because our sequential gene-regulation system offers efficient purpose-gene regulation and strict OFF-regulation, it is potentially valuable for elucidating not only novel gene functions using cDNA microarray analysis but also for "gene switching" in development and regeneration research.

Practical Range of Effective Dose for Cre Recombinase-Expressing Recombinant Adenovirus without Cell Toxicity in Mammalian Cells

The site-specific recombinase Cre is valuable for regulation of gene expression not only in vitro but also in vivo. We previously reported that replication-deficient recombinant adenovirus (rAd) expressing Cre can mediate efficient and strict regulation in 100% of cultured cells. Recently, the constitutive-expression of Cre using retrovirus or lentivirus vector reportedly inhibited cell-growth, but the effect of transient Cre expression have not yet been examined. Here we showed that an excess amount of Cre produced from Cre-expressing rAd caused a deleterious effect in cells even when Cre was transiently expressed. We used three rAds carrying promoters with different activities: the SV40 early promoter (AxSVENCre), the SR alpha promoter (AxSRCre) and the CAG promoter (AxCANCre). Cell toxicity was clearly caused by Cre itself and was distinguishable from that caused by rAd virions when the cytopathic effects of these rAds were compared with that of a control virus lacking the Cre expression unit. Cre toxicity was strongly correlated with the expression level of Cre. Importantly, AxSRCre and AxCANCre gave a 60-fold range of effective MOIs ("effective range") sufficient for gene activation without causing cell toxicity from either the rAd particles or Cre itself, while AxSVENCre failed to give such a range because the expression level of Cre was too low. When Cre was tagged with a nuclear localization signal (NLS), not only its activity but also Cre toxicity was increased fourfold, and the effective range was unchanged. Therefore, AxSRNCre might be more useful to control cell toxicity from the rAd virions than AxSRCre. Cre-induced cell toxicity can be avoided by pre-examining the "effective range" using the purpose cell lines before starting experiments utilizing the experiment of Cre-expressing rAd.

Inflammation, apoptosis, and necrosis induced by neoadjuvant fas ligand gene therapy improves survival of dogs with spontaneous bone cancer

Fas ligand (FasL) gene therapy for cancer has shown promise in rodents; however, its efficacy in higher mammals remains unknown. Here, we used intratumoral FasL gene therapy delivered in an adenovirus vector (Ad-FasL) as neoadjuvant to standard of care in 56 dogs with osteosarcoma. Tumors from treated dogs had greater inflammation, necrosis, apoptosis, and fibrosis at day 10 (amputation) compared to pretreatment biopsies or to tumors from dogs that did not receive Ad-FasL. Survival improvement was apparent in dogs with inflammation or lymphocyte-infiltration scores >1 (in a 3-point scale), as well as in dogs that had apoptosis scores in the top 50th percentile (determined by cleaved caspase-3). Survival was no different than that expected from standard of care alone in dogs with inflammation scores ≤1 or apoptosis scores in the bottom 50th percentile. Reduced Fas expression by tumor cells was associated with prognostically advantageous inflammation, and this was seen only in dogs that received Ad-FasL. Together, the data suggest that Ad-FasL gene therapy improves survival in a subset of large animals with naturally occurring tumors, and that at least in some tumor types like osteosarcoma, it is most effective when tumor cells fail to express Fas.

Conditional gene expression systems to study herpesvirus biology in vivo

Cytomegalovirus (CMV), a prototypic beta-herpesvirus, is an important human pathogen causing protean clinical manifestations in immature and immunocompromised patients. Mechanisms of infection can be studied in a mouse model. Mouse cytomegalovirus (MCMV) resembles in pathogenesis its human counterpart in many ways. Although MCMV infection is studied extensively on the level of organs, the contribution of specific cell types to viral replication in vivo is still elusive. Here we describe our approach based on the the Cre/loxP-system to investigate MCMV infection at the level of cell types in vivo. Using bacterial artificial chromosome (BAC)-technology, we created an MCMV virus containing an enhanced green fluorescent protein (egfp) reporter-gene which is not expressed due to a 'Stop' cassette flanked by two loxP-sites between promoter and coding sequence. Infection of cre-transgenic mice with this reporter virus results in the deletion of the 'Stop' cassette and expression of EGFP in a cell type-specific manner. Using this conditional gene expression system we are able to quantify viral productivity in specific cell types and to determine their contribution to viral dissemination in vivo. Furthermore, the deletion of viral genes can be used to screen for cell type-specificity of viral gene functions. Hence, conditional MCMV mutants allow the study of herpesvirus biology on the level of cell types in vivo.

Fas Ligand Delivery by a Prostate-Restricted Replicative Adenovirus Enhances Safety and Antitumor Efficacy

PURPOSE

Recent studies showed that Fas ligand (FasL) induced apoptosis in tumor cells and suppressed the immune response in several types of tumors. However, the toxicity of FasL limited further administration. This study delivered FasL in prostate cancer cells using an improved prostate-restricted replicative adenovirus (PRRA), thereby improving the antitumor effect while decreasing systemic toxicity.

EXPERIMENTAL DESIGN

We designed a FasL-armed PRRA, called AdIU3, by placing adenoviral E1a and E4 genes, FasL cDNA, and E1b gene under the control of two individual PSES enhancers. Tissue-specific viral replication and FasL expression were analyzed, and the tumor killing effect of AdIU3 was investigated both in vitro and in vivo using androgen-independent CWR22rv s.c. models via local administration and bone models via systemic administration. The safety of systemic administration of AdIU3 was evaluated. AdCMVFasL, in which FasL was controlled by a universal cytomegalovirus (CMV) promoter, was used as a control.

RESULTS

AdIU3 enhanced FasL expression in prostate-specific antigen (PSA)/prostate-specific membrane antigen (PSMA)-positive cells but not in PSA/PMSA-negative cells. It induced apoptosis and killed PSA/PMSA-positive prostate cancer cells but spared normal human fibroblasts, hepatocytes, and negative cells. The increase in killing activity was confirmed to result in part from a bystander killing effect. Furthermore, AdIU3 was more effective than a plain PRRA in inhibiting the growth of androgen-independent prostate cancer xenografts and bone tumor formation. Importantly, systemic administration of AdIU3 resulted in undetectable toxicity, whereas the same doses of AdCMVFasL killed all mice due to multiviscera failure in 16 h.

CONCLUSIONS

AdIU3 decreased the toxicity of FasL by controlling its expression with PSES, with greatly enhanced prostate cancer antitumor efficacy. The results suggested that toxic antitumor factors can be delivered safely by a PRRA.

Mature but not immature Fas ligand (CD95L)-transduced human monocyte-derived dendritic cells are protected from Fas-mediated apoptosis and can be used as killer APC

Several in vitro and animal studies have been performed to modulate the interaction of APCs and T cells by Fas (CD95/Apo-1) signaling to delete activated T cells in an Ag-specific manner. However, due to the difficulties in vector generation and low transduction frequencies, similar studies with primary human APC are still lacking. To evaluate whether Fas ligand (FasL/CD95L) expressing killer APC could be generated from primary human APC, monocyte-derived dendritic cells (DC) were transduced using the inducible Cre/Loxp adenovirus vector system. Combined transduction of DC by AdLoxpFasL and AxCANCre, but not single transduction with these vectors, resulted in dose- and time-dependent expression of FasL in >70% of mature DC (mDC), whereas <20% of immature DC (iDC) expressed FasL. In addition, transduction by AdLoxpFasL and AxCANCre induced apoptosis in >80% of iDC, whereas FasL-expressing mDC were protected from FasL/Fas (CD95/Apo-1)-mediated apoptosis despite coexpression of Fas. FasL-expressing mDC eliminated Fas(+) Jurkat T cells as well as activated primary T cells by apoptosis, whereas nonactivated primary T cells were not deleted. Induction of apoptosis in Fas(+) target cells required expression of FasL in DC and cell-to-cell contact between effector and target cell, and was not dependent on soluble FasL. Induction of apoptosis in Fas(+) target cells required expression of FasL in DC, cell-to-cell contact between effector and target cell, and was not dependent on soluble FasL. The present results demonstrate that FasL-expressing killer APC can be generated from human monocyte-derived mDC using adenoviral gene transfer. Our results support the strategy to use killer APCs as immunomodulatory cells for the treatment of autoimmune disease and allograft rejection.

Specific deletion of autoreactive T cells by adenovirus-transfected, Fas ligand-producing antigen-presenting cells

Immune privilege is a unique strategy developed in several internal organs that can prevent the development of immune attack against these vital organs. One critical mechanism of immune privilege is utilization of Fas/FasL-mediated apoptosis to delete the invading T cells at the immune privilege sites. In this article, we describe the development and application of a unique cell-gene therapy to correct defective FasL-mediated apoptosis and autoimmune disease in autoimmune mice. This cell-gene therapy strategy using antigen-presenting cells (APCs) to express FasL is not only a therapeutic tool, but also has allowed us to understand the complexity of T cell regulation and the concept of eliminating T cells in the spleen, lymph node, or elsewhere in vivo to regulate the homeostasis of the peripheral T cell response. In this regard, the FasL-expressing APCs can be considered as circulating and regulatable immune privilege sites. Our studies provide substantial evidence that FasL-expressing APCs can be introduced exogenously without liver toxicity to eliminate infiltrating T cells and prevent the development of immune attack in lung, liver, kidney, joint, and salivary gland. Therefore, given the hazardous potential of persistent T cell invasion at the local inflammatory site, it is tempting to speculate that such an endogenous control mechanism occurs normally in vivo to limit a chronic T cell inflammatory response.

An adenovirus encoding proapoptotic Bax synergistically radiosensitizes malignant glioma

PURPOSE

We explore the utility of the adenovirus-mediated delivery of proapoptotic Bax for enhancing the cytotoxicity of radiotherapy (RT) in RT-refractory glioma cells.

MATERIALS AND METHODS

Cell lines D54 MG and U87 MG (p53 wild-type), and U251 MG and U373 MG (p53 mutant), and patient-derived astrocytes were evaluated. Cells were irradiated and infected with an inducible adenovirus encoding Bax. Cell proliferation, colony formation assay, quantification of early apoptotic alteration in the plasma membrane by fluorescence-activated cell sorter using annexin V, and nuclear staining with H33258 were used to evaluate apoptosis. The capacity of the combined treatment to induce regression of subcutaneous D54 MG tumors was tested in nude mice. A dose of 5 Gy was administered every other day, four times, for a total dose of 20 Gy. One day after each irradiation, tumors were injected with 1 x 10(9) plaque-forming units (PFU).

RESULTS

Apoptotic death was enhanced by the combination of Ad/Bax and RT. In D54 MG, levels of apoptosis after RT alone, Ad/Bax alone, or the combination were, respectively, 12.3%, 32.1%, and 78.5%. In contrast, treatment of astrocytes did not significantly induce apoptosis. A colony-formation assay showed a 2-log inhibition with respect to controls after combined treatment, irrespective of the endogenous levels of p53. The other apoptosis assays also showed the defining characteristics of apoptosis in the combination group. Remarkably, combined treatment induced regression of tumors in mice.

CONCLUSIONS

Ad/Bax synergistically radiosensitizes glioma, with a seemingly favorable therapeutic index.

Hepatic DR5 induces apoptosis and limits adenovirus gene therapy product expression in the liver

A major limitation of adenovirus (Ad) gene therapy product expression in the liver is subsequent elimination of the hepatocytes expressing the gene therapy product. This elimination is caused by both necrosis and apoptosis related to the innate and cell-mediated immune response to the Ad. Apoptosis of hepatocytes can be induced by the innate immune response by signaling through death domain receptors on hepatocytes including the tumor necrosis factor alpha (TNF-alpha) receptor (TNFR), Fas, and death domain receptors DR4 and DR5. We have previously shown that blocking signaling through TNFR enhances and prolongs gene therapy product expression in the liver. In the present study, we constructed an Ad that produces a soluble DR5-Fc (AdsDR5), which is capable of neutralizing TNF-related apoptosis-inducing ligand (TRAIL). AdsDR5 prevents TRAIL-mediated apoptosis of CD3-activated T cells and decreases hepatocyte apoptosis after AdCMVLacZ administration and enhances the level and duration of lacZ transgene expression in the liver. In addition to blocking TRAIL and directly inhibiting apoptosis, AdsDR5 decreases production of gamma interferon (IFN-gamma) and TNF-alpha and decreases NK cell activation, all of which limit Ad-mediated transgene expression in the liver. These results indicate that (i) AdsDR5 produces a DR5-Fc capable of neutralizing TRAIL, (ii) AdsDR5 can reduce activation of NK cells and reduce induction of IFN-gamma and TNF-alpha after Ad administration, and (iii) administration of AdsDR5 can enhance Ad gene therapy in the liver.

Depletion of collagen II-reactive T cells and blocking of B cell activation prevents collagen II-induced arthritis in DBA/1j mice

Collagen II (CII)-induced arthritis in DBA/1j mice is mediated by both CII-reactive T cells and anti-CII Ab-producing B cells. To determine the relative role of these processes in the development of arthritis, we specifically eliminated CII-reactive T cells by treating the mice with CII-pulsed syngeneic macrophages that had been transfected with a binary adenovirus system. These macrophages express murine Fas ligand in a doxycycline-inducible manner with autocrine suicide inhibited by concomitant expression of p35. The mice were treated i.v. with four doses of CII-APC-AdFasLp35Tet or a single dose of AdCMVsTACI (5 x 10(9) PFU), or both simultaneously, beginning 2 wk after priming with CII in CFA. Treatment with CII-APC-AdFasLp35Tet alone or in combination with a single dose of AdCMVsTACI prevented the development of CII-induced arthritis and T cell infiltration in the joint. The elimination of T cells was specific in that a normal T cell response was observed on stimulation with OVA after treatment with CII-APC-AdFasLp35Tet. Treatment with AdCMVsTACI alone prevented production of detectable levels of circulating anti-CII autoantibodies and reduced the severity of arthritis but did not prevent its development. These results indicate that the CII-reactive T cells play a crucial role in the development of CII-induced arthritis and that the anti-CII Abs act to enhance the development of CII-induced arthritis.

Bax-induced apoptosis as a novel gene therapy approach for carcinoma of the cervix

OBJECTIVE

The transfer of tumor suppressor genes has been shown to revert the malignant phenotype. In this regard, bax is a pro-apoptotic molecule that also functions as a tumor suppressor. The purpose of this study was to evaluate bax as a gene therapeutic in the context of cervical cancer.

METHODS

Efficiency of viral transduction in cervical cancer cell lines and primary cervical cancer cells was evaluated with an adenoviral vector encoding green fluorescent protein and luciferase, respectively. We generated a recombinant adenoviral vector that encodes the bax gene under inducible conditions. To this end, expression of this pro-apoptotic gene was controlled by a Cre-LoxP system. Following infection with the recombinant bax adenovirus, the viability of cervical cancer cell lines and primary cervical cancer cells was evaluated using crystal violet staining and FACS analysis. Apoptotic cell death was monitored using annexin V staining.

RESULTS

High levels of viral infection were observed in all cervical cancer cell lines (>85%) and primary cervical cancer cells. Significant cytotoxicity was seen in all cervical cancer cells lines and, more importantly, patient-derived primary cervical cancer cells. Moreover, bax-mediated cell death occurred via an apoptotic pathway.

CONCLUSIONS

Our results indicate that a bax recombinant adenoviral vector causes cell death mediated via an apoptotic pathway in multiple cervical cancer cell lines and primary cervical cancer cells. These data suggest that bax may be a candidate for human gene therapy in the setting of cervical carcinoma.

Transduction and apoptosis induction in the rat prostate, using adenovirus vectors

Proapoptotic adenovirus vectors offer great promise for the treatment of cancer and nonmalignant conditions. Benign prostate hyperplasia (BPH) is a common nonmalignant enlargement of the prostate that involves epithelial, stromal, and smooth muscle components of the gland. We tested the hypothesis that an adenovirus vector expressing Fas ligand can be used to induce apoptosis in the prostate. We analyzed the efficiency of transduction and apoptosis induction in primary cultures of human prostate cells after adenovirus-mediated gene transfer. Efficient transduction was observed in primary prostate epithelial cells. Stromal and smooth muscle cells were more difficult to transduce, as no coxsackie-adenovirus receptor (CAR) expression was detectable on these cells. However, transduction was achieved in these cells when the multiplicity of infection was increased to 100 focal-forming units per cell, or when the vectors were delivered as calcium phosphate precipitates. Infection of all three primary prostate cell types with an adenovirus vector that expresses Fas ligand (AdFasL/G) resulted in rapid apoptosis. Direct injection of the rat prostate with an adenovirus vector carrying luciferase resulted in substantial luciferase expression. TUNEL analysis demonstrated that AdFasL/G administration induced low-level apoptosis in prostatic epithelial cells throughout the gland. As a first step toward enhancing the efficiency of prostate transduction in vivo, we tested an adenovirus vector that was engineered to have an expanded tropism. This vector, AdZ.F2K(pK7), was 10- to 500-fold more efficient than unmodified vectors in transducing prostate epithelial, smooth muscle, and stromal cells in culture. Moreover, AdZ.F2K(pK7) was more efficient than an unmodified vector at transducing the rat prostate in vivo, although the effect was dose dependent.

Treatment of chronic sialadenitis in a murine model of Sjögren's syndrome by local fasL gene transfer

OBJECTIVE

Infection of Fas (Fas/CD95)-mutant C57BL/6 (B6)-lpr/lpr mice with murine cytomegalovirus (MCMV) leads to a chronic sialadenitis similar to that of Sjögren's syndrome (SS). The aim of this study was to evaluate whether chronic sialadenitis would also occur in Fas ligand (FasL/CD95L)-mutant B6-gld/gld mice upon infection with MCMV and whether the expression of FasL by local gene transfer using recombinant adenoviral vectors would be an effective therapeutic strategy.

METHODS

B6-gld/gld mice were infected intraperitoneally with MCMV, and salivary glands were analyzed histologically at different time points. For treatment of sialadenitis, recombinant adenoviral vectors expressing the fasL gene (AdLoxpFasL + AxCANCre) or the lacZ gene (AdCMVLacZ) were locally injected into the salivary glands of MCMV-infected B6-gld/gld mice and uninfected B6-+/+ and B6-gld/gld mice.

RESULTS

Following MCMV infection, B6-gld/gld mice developed an acute and chronic sialadenitis characterized by multiple foci of infiltrating T cells. After local injection of adenoviral vectors, high levels of lacZ or fasL gene expression could be detected in acinar and ductal cells. Treatment of acute and chronic sialadenitis in B6-gld/gld mice with local fasL gene transfer resulted in a significant reduction in the number of inflammatory foci and tissue destruction in salivary glands compared with mice treated with AdCMVLacZ. Despite high levels of FasL expression after injection of recombinant vectors, <5% of ductal and acinar cells were TUNEL positive, demonstrating that, in this model of SS, acinar and ductal cells were not highly sensitive to FasL-mediated apoptosis.

CONCLUSION

Chronic sialadenitis similar to that of SS developed in B6-gld/gld mice after MCMV infection. FasL expression was reconstituted by local gene transfer, resulting in significant reduction of infiltrating mononuclear cells, which indicates that local gene transfer of fasL might be a novel treatment for chronic sialadenitis.

Efficient gene activation in cultured mammalian cells mediated by FLP recombinase-expressing recombinant adenovirus

A recombinant adenovirus (rAd) expressing Cre recombinase derived from bacteriophage P1 has already been extensively used for the conditional gene activation and inactivation strategies in mammalian systems. In this study, we generated AxCAFLP, a rAd expressing FLP recombinase derived from Saccharomyces cerevisiae and carried out quantitative comparisons with Cre-expressing rAd in both in vitro and in cultured cells to provide another efficient gene regulation system in mammalian cells. In the in vitro experiments, the relative recombination efficiency of FLP expressed in 293 cells infected with FLP-expressing rAd was approximately one-thirtieth that of Cre even at 30 degrees C, the optimum temperature for FLP activity, and was approximately one-ninetieth at 37 degrees C. Co-infection experiments in HeLa cells using a target rAd conditionally expressing LacZ under the control of FLP showed that an FLP-expressing rAd, infected at a multiplicity of infection (MOI) of 5, was able to activate the transgene in almost 100% of HeLa cells whereas the Cre-expressing rAd was sufficient at an MOI of 0.2. Since an MOI of 5 is ordinarily used in rAd experiments, these results showed that the FLP-expressing rAd is useful for gene activation strategies and is probably applicable to a sequential gene regulation system in combination with Cre-expressing rAd in mammalian cells.

Binding properties, cell delivery, and gene transfer of adenoviral penton base displaying bacteriophage

The penton base of adenovirus mediates viral attachment to integrin receptors and particle internalisation, properties that can be exploited to reengineer prokaryotic viruses for the infection of mammalian cells. We report that filamentous phage displaying either the full-length penton base gene or a central region of 107 amino acids on their surface were able to bind, internalise, and transduce mammalian cells expressing integrin receptors. Both phage bound alphavbeta3, alphavbeta5, alpha3beta1, and alpha5beta1 integrin subtypes. Cell-binding was shown by electron microscopy; internalisation was investigated by immunofluorescence and confirmed by micropanning. As it has been described for adenovirus, pharmacologic disruption of phosphoinositide-30H kinase, but not of myosin light-chain kinase, inhibited phage internalisation. Recombinant phage encoding an eukaryotic expression cassette was able to mediate gene expression in mammalian cells. Taken together, these data open insights for the exploit of recombinant phage for integrin-targeted gene delivery.

An adenovirus encoding proapoptotic Bax induces apoptosis and enhances the radiation effect in human ovarian cancer

Overexpression of proapoptotic Bax favors death in cells resistant to ionizing radiation. We hypothesized that expression of Bax via adenoviral-mediated gene delivery could sensitize radiation-refractory cells to radiotherapy. An inducible Bax recombinant adenovirus (Ad/Bax) had been generated using the Cre/loxp system. Human ovarian cancer cell lines and primary, patient-derived cancer cells from ascites were irradiated and infected with the Ad/Bax and an expression-inducing vector, Ad/Cre. Cell death was evaluated by crystal violet staining, fluorescence-activated cell sorter analysis of Annexin V, and colony formation assay (cell lines only). To further characterize the mechanism of death, cell morphology was examined by nuclear staining with Hoechst 33258. Lastly, to evaluate the capacity of the combined treatment to inhibit tumor growth, mice were injected subcutaneously with ovarian cancer cells exposed to Bax, radiation therapy (RT), or both, and tumor size was measured periodically. Infection of the cancer cell lines and primary cells with both Ad/Bax and Ad/Cre significantly enhanced sensitivity to ionizing radiation, achieving high levels of cell killing in short-term assays. In addition, the combination of Bax and radiotherapy reduced the survival fraction of cell lines 2 logs in standard colony-forming assays. Investigation into the involved mechanism suggests that Bax-mediated radiosensitization occurs through both apoptosis and necrosis pathways. Further, mice subcutaneously injected with ovarian tumor cells previously treated with radiation, or with radiation and irrelevant viruses, consistently developed tumor nodules. In addition, approximately 80% of injections were followed by tumor formation after treatment with Ad/Bax and Ad/Cre alone. In contrast, tumor formation was completely inhibited after combined treatment with Ad/Bax and Ad/Cre and radiation. Augmentation of the effect of radiotherapy on human ovarian cancer cells and primary cancer cells from patients via a recombinant adenovirus encoding Bax is feasible.

Adenoviral vector which delivers FasL-GFP fusion protein regulated by the tet-inducible expression system

Fas ligand (FasL) is a member of the tumor necrosis family and when bound to its receptor, Fas, induces apoptosis. It plays important roles in immune response, degenerative and lymphoproliferative diseases, development and tumorigenesis. It is also involved in generation of immune privilege sites in the eye and testis. Harnessing the power of this molecule is expected to lead to a powerful chemotherapeutic. We describe the construction and characterization of replication-deficient adenoviral vectors that express a fusion of murine FasL and green fluorescent protein (GFP). FasL-GFP retains full activity of wild-type FasL, at the same time allowing for easy visualization and quantification in both living and fixed cells. The fusion protein is under the control of a tetracycline-regulated gene expression system. Tight control of expression is achieved by creating a novel 'double recombinant' Ad vector, in which the tet-responsive element and the transactivator element are built into the opposite ends of the same vector to avoid enhancer interference. Expression can be conveniently regulated by tetracycline or its derivatives in a dose-dependent manner. The vector was able to deliver FasL-GFP gene to cells in vitro efficiently, and the expression level and function of the fusion protein was modulated by the concentration of doxycycline. This regulation allows us to produce high titers of the vector by inhibiting FasL expression in an apoptosis-resistant cell line. Induction of apoptosis was demonstrated in all cell lines tested. These results indicate that our vector is a potentially valuable tool for FasL-based gene therapy of cancer and for the study of FasL/Fas-mediated apoptosis and immune privilege.

Antigen presenting cells expressing Fas ligand down-modulate chronic inflammatory disease in Fas ligand-deficient mice

We assessed the effect of modified antigen presenting cells (APCs) expressing high levels of Fas ligand (APC-FasL) on post-viral chronic inflammatory disease. FasL-deficient B6-gld/gld mice infected with murine cytomegalovirus (MCMV) cleared the virus from their lungs, kidneys, and livers within 2 weeks of infection. However, inflammation persisted in these organs for more than 8 weeks, with a chronically increased T-cell response to MCMV-infected APCs and production of autoantibodies. Administration of APC-AdFasL at 4 weeks suppressed this inflammation and diminished the T-cell response and autoantibody production. APC-AdFasL that had been transfected with ultraviolet-irradiated MCMV were more effective than uninfected APC-AdFasL in ameliorating the chronic inflammation. APC-AdFasL migrated preferentially to the spleen, where they triggered apoptosis of lymphocytes in the marginal zone of the spleen. These results confirm that Fas-mediated apoptosis is not required for clearance of virus, but is required for down-modulation of the virally induced chronic inflammatory response. This organwide effect of APC-AdFasL appears to be mediated by elimination of activated T lymphocytes in the spleen before their emigration to the target organs.

Improved production of adenovirus vectors expressing apoptotic transgenes

Adenovirus vectors expressing gene products that can induce apoptosis have potential utility in gene therapy applications ranging from the treatment of proliferative diseases to transplantation. However, adenovirus vectors carrying proapoptotic gene products are difficult to produce, as the apoptotic environment is not conducive to adenovirus gene expression and replication. Production of AdFasL/G, an adenovirus vector that expresses high levels of Fas ligand, was severely reduced in the 293 packaging cell line. Increased yields of AdFasL/G were achieved by inclusion of peptide-based caspase inhibitors in the growth medium. However, use of these inhibitors for large-scale production would be difficult and expensive. A screen for gene products that increase the yield of AdFasL/G in 293 cells revealed that the poxvirus serpin CrmA and the adenovirus 14.7K product were able to increase virus yields significantly. Apoptosis induced by AdFasL/G was attenuated in 293CrmA cell lines and virus titers were increased dramatically. However, serial passage of AdFasL/G on 293CrmA cells resulted in the generation of replication-competent adenovirus. To resolve this problem, the CrmA gene was introduced into AE25 cells, an E1-complementing cell line that has limited sequence identity with the vectors. AdFasL/G titers were increased 100-fold on AE25CrmA cells relative to the AE25 cells and RCA contamination was not detectable. In addition, adenovirus vectors that express FADD, caspase 8, and Fas/APO1 were produced efficiently in AE25CrmA and 293CrmA.

Involvement of Fas system-mediated apoptosis in pathogenesis of viral hepatitis

Enhanced Fas system-mediated hepatocyte apoptosis is observed in hepatitis C virus (HCV)- and hepatitis B virus (HBV)-associated chronic liver diseases. In these forms of viral hepatitis, liver-infiltrating lymphocytes that recognize the viral antigen on hepatocytes become activated and express cytolytic Fas ligand (FasL) molecules. In contrast, hepatocytes exhibit enhanced Fas expression and become susceptible to FasL-mediated death. Augmentation of the Fas system has also been observed in other liver diseases and biliary disorders. Moreover, the Fas system is involved in removing aged hepatocytes. Thus, Fas-mediated apoptosis plays an important role in several liver diseases and in maintaining normal liver homeostasis.

Immune responses to adenovirus and adeno-associated virus in humans

Vectors based on human adenovirus (Ad) and adeno-associated virus (AAV) are being evaluated for human gene therapy. The response of the host to the vector, in terms of antigen-specific immunity, will play a substantial role in clinical outcome. We have surveyed cohorts of normal subjects and cystic fibrosis patients for pre-existing immunity to these viruses, caused by naturally acquired infections. A number of humoral and cellular assays to adenovirus serotype 5 (Ad5) and adeno-associated virus serotype 2 (AAV2) were performed from serum and peripheral blood mononuclear cells. Virtually all subjects had Ig to Ad5 although only 55% of these antibodies neutralized virus (NAB). Approximately two of three patients demonstrated CD4+ T cells that proliferated to Ad antigens of which most were of the TH1 subset, based on cytokine secretion. A substantially different pattern of immune responses was observed to AAV2. Although virtually all patients had Ig to AAV2, most of these antibodies were not neutralizing (32% NAB) and only 5% of patients had peripheral blood lymphocytes that proliferated in response to AAV2 antigens. These studies demonstrate marked heterogeneity in pre-existing immunity to Ad5 and AAV2 in human populations. The impact of these findings on outcome following gene therapy will require further study.

Adenovirus-mediated gene expression in vivo is enhanced by the antiapoptotic bcl-2 gene

An adenovirus vector encoding the human Bcl-2 gene (hBcl-2) was derived. In vivo expression of hBcl-2 in murine livers enhanced and prolonged adenovirus-mediated gene expression. Furthermore, in the hBcl-2-treated group a significant reduction in the apoptosis induced by the adenovirus vector was observed. Thus, the cytoprotection of the vector-infected cells with antiapoptotic genes appears promising for successful in vivo gene therapy.

Induction of Specific T Cell Tolerance by Fas Ligand- Expressing Antigen-Presenting Cells

Autocrine interaction of Fas and Fas ligand leads to apoptosis of activated T cells, a process that is critical for the maintenance of peripheral T cell tolerance. Paracrine interactions of Fas ligand with T cells also may play an important role in the maintenance of tolerance, as Fas ligand can create immune-privileged sites and prevent graft rejection by inducing apoptosis in T cells. We surmised that APCs that express Fas ligand might directly induce apoptosis of T cells during presentation of Ag to the T cells, thus inducing Ag-specific, systemic T cell tolerance. Here, we show that profound, specific T cell unresponsiveness to alloantigen was induced by treatment of H-2k mice with H-2b APCs that expressed Fas ligand and that profound T cell unresponsiveness specific for the H-Y Ag was induced by treatment of H-2Db/H-Y TCR transgenic female mice with H-2Db/H-Y APCs that expressed Fas ligand. The induction of this systemic T cell tolerance required the expression of Fas ligand on the APCs as well as the expression of Fas on the T cells. The tolerance was restricted to the Ag presented by the APCs. The rapid and profound clonal deletion of the Ag-specific, peripheral T cells mediated by the Fas ligand-expressing APCs contributed to the induction of tolerance. These findings demonstrate that Ag-specific T cell tolerance can be induced by APCs that express Fas ligand and suggest a novel function for APCs in the induction of T cell apoptosis. Furthermore, they indicate a novel immunointervention strategy for treatment of graft rejection and autoantigen-specific autoimmune diseases.

Induction of specific T-cell tolerance by adenovirus-transfected, Fas ligand-producing antigen presenting cells

A major problem associated with adenovirus gene therapy is the T cell-mediated immune response, which is elicited by inoculation of the adenovirus vector and leads to rapid clearance of the virus and loss of transgene expression. In this study, the immune response to adenovirus was prevented by induction of specific T-cell tolerance by pretreatment with adenovirus-infected antigen-presenting cells (APC) that express Fas ligand. Compared with control-treated mice, the tolerized mice showed prolonged expression of lacZ upon administration of AdCMVlacZ 1 week after tolerance induction. In contrast to the control mice, the tolerized mice did not display proliferation of CD3+ T cells in the spleen in response to AdCMVlacZ. Tolerance induction also was indicated by the lower production of interferon-gamma and interleukin-2 by peripheral T cells isolated from AdCMVlacZ-challenged tolerized mice than by AdCMVlacZ-challenged control-treated mice. The T-cell tolerance was specific for the adenovirus as the T-cell responses to irrelative murine cytomegalovirus remained unimpaired. Our results indicate that adenovirus-specific T-cell tolerance can be induced by APCs that coexpress Fas ligand and adenovirus antigens. We propose that this new strategy can be used to induce tolerance to adenovirus vector gene therapy with resultant prolonged expression of the transgene.