Reovirus Type 3 Dearing Variants Do Not Induce Necroptosis in RIPK3-Expressing Human Tumor Cell Lines.

Reoviruses are used as oncolytic viruses to destroy tumor cells. The concomitant induction of anti-tumor immune responses enhances the efficacy of therapy in tumors with low amounts of immune infiltrates before treatment. The reoviruses should provoke immunogenic cell death (ICD) to stimulate a tumor cell-directed immune response. Necroptosis is considered a major form of ICD, and involves receptor-interacting protein kinase 1 (RIPK1), RIPK3 and phosphorylation of mixed-lineage kinase domain-like protein (MLKL). This leads to cell membrane disintegration and the release of damage-associated molecular patterns that can activate immune responses. Reovirus Type 3 Dearing (T3D) can induce necroptosis in mouse L929 fibroblast cells and mouse embryonic fibroblasts. Most human tumor cell lines have a defect in RIPK3 expression and consequently fail to induce necroptosis as measured by MLKL phosphorylation. We used the human colorectal adenocarcinoma HT29 cell line as a model to study necroptosis in human cells since this cell line has frequently been described in necroptosis-related studies. To stimulate MLKL phosphorylation and induce necroptosis, HT29 cells were treated with a cocktail consisting of TNFα, the SMAC mimetic BV6, and the caspase inhibitor Z-VAD-FMK. While this treatment induced necroptosis, three different reovirus T3D variants, i.e., the plasmid-based reverse genetics generated virus (T3DK), the wild-type reovirus T3D isolate R124, and the junction adhesion molecule-A-independent reovirus mutant (jin-1) failed to induce necroptosis in HT29 cells. In contrast, these viruses induced MLKL phosphorylation in murine L929 cells, albeit with varying efficiencies. Our study shows that while reoviruses efficiently induce necroptosis in L929 cells, this is not a common phenotype in human cell lines. This study emphasizes the difficulties of translating the results of ICD studies from murine cells to human cells.

The stability of envelope-pseudotyped lentiviral vectors.

Lentiviral vectors have become popular tools for stable genetic modification of mammalian cells. In some applications of lentiviral vector-transduced cells, infectious-lentiviral particles should be absent. Quantification of the free-vector particles that remain from the inoculum can be difficult. Therefore a formula was established that yields an estimation of the 'Reduction Ratio.' This ratio represents the loss of titer based on a number of vector-inactivating effects. In this study, we evaluated several parameters and assumptions that were used in the current formula. We generated new data on the stability and trypsin sensitivity of lentiviral vectors pseudotyped with eight heterologous envelope proteins and the loss of vectors by washing or passaging the cell cultures. Our data demonstrate that the loss of virus titer under the influence of trypsin as well as the half-life of the particles in tissue culture medium is dependent on the vector's envelope protein. While VSV-G-envelope-pseudotyped particles were unsensitive to trypsin, the titer of vectors pseudotyped with other envelope proteins decreased 2-110-fold. The half-life in culture medium ranged from 8 to 40 h for the different envelope-pseudotyped vectors, with 35 h for VSV-G-envelope-pseudotyped vector particles. Additionally, we found that removal of the culture medium from Ø35 mm to Ø10 cm dishes reduces the amount of vector particles in the culture by 50-fold and 20-fold, respectively. Together these data can be used to more precisely estimate the maximum number of free lentiviral vector particles in cell cultures.

Degenerated Cones in Cultured Human Retinas Can Successfully Be Optogenetically Reactivated.

Biblical references aside, restoring vision to the blind has proven to be a major technical challenge. In recent years, considerable advances have been made towards this end, especially when retinal degeneration underlies the vision loss such as occurs with retinitis pigmentosa. Under these conditions, optogenetic therapies are a particularly promising line of inquiry where remaining retinal cells are made into "artificial photoreceptors". However, this strategy is not without its challenges and a model system using human retinal explants would aid its continued development and refinement. Here, we cultured post-mortem human retinas and show that explants remain viable for around 7 days. Within this period, the cones lose their outer segments and thus their light sensitivity but remain electrophysiologically intact, displaying all the major ionic conductances one would expect for a vertebrate cone. We optogenetically restored light responses to these quiescent cones using a lentivirus vector constructed to express enhanced halorhodopsin under the control of the human arrestin promotor. In these 'reactivated' retinas, we show a light-induced horizontal cell to cone feedback signal in cones, indicating that transduced cones were able to transmit their light response across the synapse to horizontal cells, which generated a large enough response to send a signal back to the cones. Furthermore, we show ganglion cell light responses, suggesting the cultured explant's condition is still good enough to support transmission of the transduced cone signal over the intermediate retinal layers to the final retinal output level. Together, these results show that cultured human retinas are an appropriate model system to test optogenetic vision restoration approaches and that cones which have lost their outer segment, a condition occurring during the early stages of retinitis pigmentosa, are appropriate targets for optogenetic vision restoration therapies.

Alpha-N-acetyl-neuraminide alpha-2,8-sialyltransferase 1 can support immune responses toward tumors overexpressing ganglioside D3 in mice.

An immunotherapeutic strategy is discussed supporting anti-tumor activity toward malignancies overexpressing ganglioside D3. GD3 can be targeted by NKT cells when derived moieties are presented in the context of CD1d. NKT cells can support anti-tumor responses by secreting inflammatory cytokines and through cytotoxicity toward CD1d+GD3+ tumors. To overexpress GD3, we generated expression vector DNA and an adenoviral vector encoding the enzyme responsible for generating GD3 from its ubiquitous precursor GM3. We show that DNA encoding α-N-acetyl-neuraminide α-2,8-sialyltransferase 1 (SIAT8) introduced by gene gun vaccination in vivo leads to overexpression of GD3 and delays tumor growth. Delayed tumor growth is dependent on CD1d expression by host immune cells, as shown in experiments engaging CD1d knockout mice. A trend toward greater NKT cell populations among tumor-infiltrating lymphocytes is associated with SIAT8 vaccination. A single adenoviral vaccination introduces anti-tumor activity similarly to repeated vaccination with naked DNA. Here, greater NKT tumor infiltrates were accompanied by marked overexpression of IL-17 in the tumor, later switching to IL-4. Our results suggest that a single intramuscular adenoviral vaccination introduces overexpression of GD3 by antigen-presenting cells at the injection site, recruiting NKT cells that provide an inflammatory anti-tumor environment. We propose adenoviral SIAT8 (AdV-SIAT8) can slow the growth of GD3 expressing tumors in patients.

Adipocyte-derived factors impair insulin signaling in differentiated human vascular smooth muscle cells via the upregulation of miR-143.

Cardiovascular complications are common in patients with type 2 diabetes. Adipokines have been implicated in the induction of proliferative and pro-atherogenic alterations in human vascular smooth muscle cells (hVSMC). Other reports demonstrated the importance of the miRNA cluster miR-143/145 in the regulation of VSMC homeostasis and insulin sensitivity. Here we investigated whether the detrimental effects of adipokines on hVSMC function could be ascribed to alterations in miR-143/145 expression. The exposure of hVSMC to conditioned media (CM) from primary human subcutaneous adipocytes increased the expression of smooth muscle α-actin (SMA), and the miR-143/145 cluster, but markedly impaired the insulin-mediated phosphorylation of Akt and its substrate endothelial nitric oxide synthase (eNOS). Furthermore, CM promoted the phosphorylation of SMAD2 and p38, which have both been linked to miR-143/145 induction. Accordingly, the induction of miR-143/145 as well as the inhibition of insulin-mediated Akt- and eNOS-phosphorylation was prevented when hVSMC were treated with pharmacological inhibitors for Alk-4/5/7 and p38 before the addition of CM. The transfection of hVSMC with precursor miR-143, but not with precursor miR-145, resulted in impaired insulin-mediated phosphorylation of Akt and eNOS. This inhibition of insulin signaling by CM and miR-143 is associated with a reduction in the expression of the oxysterol-binding protein-related protein 8 (ORP8). Finally, the knock-down of ORP8 resulted in impaired insulin-mediated phosphorylation of Akt in hVSMC. Thus, the detrimental effects of adipocyte-derived conditioned media on insulin action in primary hVSMC can be ascribed to the Alk- and p38-dependent induction of miR-143 and subsequent downregulation of ORP8.

New role of signal peptide peptidase to liberate C-terminal peptides for MHC class I presentation.

The signal peptide peptidase (SPP) is an intramembrane cleaving aspartyl protease involved in release of leader peptide remnants from the endoplasmic reticulum membrane, hence its name. We now found a new activity of SPP that mediates liberation of C-terminal peptides. In our search for novel proteolytic enzymes involved in MHC class I (MHC-I) presentation, we found that SPP generates the C-terminal peptide-epitope of a ceramide synthase. The display of this immunogenic peptide-MHC-I complex at the cell surface was independent of conventional processing components like proteasome and peptide transporter TAP. Absence of TAP activity even increased the MHC-I presentation of this Ag. Mutagenesis studies revealed the crucial role of the C-terminal location of the epitope and "helix-breaking" residues in the transmembrane region just upstream of the peptide, indicating that SPP directly liberated the minimal 9-mer peptide. Moreover, silencing of SPP and its family member SPPL2a led to a general reduction of surface peptide-MHC-I complexes, underlining the involvement of these enzymes in Ag processing and presentation.

Directed adenovirus evolution using engineered mutator viral polymerases.

Adenoviruses (Ads) are the most frequently used viruses for oncolytic and gene therapy purposes. Most Ad-based vectors have been generated through rational design. Although this led to significant vector improvements, it is often hampered by an insufficient understanding of Ad's intricate functions and interactions. Here, to evade this issue, we adopted a novel, mutator Ad polymerase-based, 'accelerated-evolution' approach that can serve as general method to generate or optimize adenoviral vectors. First, we site specifically substituted Ad polymerase residues located in either the nucleotide binding pocket or the exonuclease domain. This yielded several polymerase mutants that, while fully supportive of viral replication, increased Ad's intrinsic mutation rate. Mutator activities of these mutants were revealed by performing deep sequencing on pools of replicated viruses. The strongest identified mutators carried replacements of residues implicated in ssDNA binding at the exonuclease active site. Next, we exploited these mutators to generate the genetic diversity required for directed Ad evolution. Using this new forward genetics approach, we isolated viral mutants with improved cytolytic activity. These mutants revealed a common mutation in a splice acceptor site preceding the gene for the adenovirus death protein (ADP). Accordingly, the isolated viruses showed high and untimely expression of ADP, correlating with a severe deregulation of E3 transcript splicing.

Immunostimulatory Profile of Cancer Cell Death by the AdV-Lumc007-Derived Oncolytic Virus 'GoraVir' in Cultured Pancreatic Cancer Cells.

Pancreatic ductal adenocarcinoma (PDAC) is a highly aggressive malignancy which shows unparalleled therapeutic resistance. Oncolytic viruses have emerged as a new treatment approach and convey their antitumor activity through lysis of cancer cells. The therapeutic efficacy of oncolytic viruses is largely dependent on the induction of immunogenic cell death (ICD) and the subsequent antitumor immune responses. However, the concurrent generation of antiviral immune responses may also limit the a virus' therapeutic window. GoraVir is a new oncolytic adenovirus derived from the Human Adenovirus B (HAdV-B) isolate AdV-lumc007 which was isolated from a gorilla and has demonstrated excellent lytic activity in both in vitro and in vivo models of PDAC. In this study, we characterized the immunostimulatory profile of cancer cell death induced by GoraVir and the concerted cellular antiviral responses in three conventional pancreatic cancer cell lines. While GoraVir was shown to induce late apoptotic/necrotic cell death at earlier time points post infection than the human adenovirus type 5 (HAdV-C5), similar levels of ICD markers were expressed. Moreover, GoraVir was shown to induce ICD not dependent on STING expression and regardless of subsequent antiviral responses. Together, these data demonstrate that GoraVir is an excellent candidate for use in oncolytic virotherapy.

A potentially immunologically inert derivative of the reverse tetracycline-controlled transactivator.

The archetypical system for regulating heterologous gene expression in mammalian cells involves tetracycline-activated transactivators (rtTA). Binding of such transactivators to tet-operator-controlled promoters induces transcription. Immune responses directed against the transactivator proteins may limit the applicability of this system in immune-competent hosts. To circumvent such immune responses the immune evasion mechanism of the Epstein-Barr virus Nuclear-Antigen 1 was exploited. Our data show that fusion of the rtTA with the EBNA-1 derived Gly-Ala repeat yielded an efficient transactivator with no detectable activity in absence of inducer. Antigenic peptides of the fusion protein were not presented in MHC class I.

A lentiviral vector-based adenovirus fiber-pseudotyping approach for expedited functional assessment of candidate retargeted fibers.

BACKGROUND: Many studies aimed at retargeting adenovirus (Ad) rationally focus on genetic modification of fiber, which is the primary receptor-binding protein of Ad. Retargeted fibers ultimately require functional validation in the viral context. METHODS: Lentiviral vectors (LV) were used to express fiber variants in cells. Infections with a fiber gene-deleted Ad vector yielded fiber-pseudotyped viruses. An enzyme-linked immunosorbent assay and slot blot-based assays probed target binding-ability of retargeted fibers. Differential treatments with an alkylating agent prior to western blot analysis allowed for examination of intra- and extracellular redox states of fibers. RESULTS: In the present study, LV-based fiber-pseudotyping of Ad is presented as an accelerated means to test new fibers. LV-mediated gene transfer yielded stable and uniform populations of fiber variant-expressing cells. These populations were found to effectively support fiber-pseudotyping of Ad. As a secondary objective of the study, we functionally assessed a chimeric fiber harboring a tumor antigen-directed single-chain antibody fragment (scFv). This fiber was shown to trimerize and achieve a degree of binding to its antigenic target. However, its capsid incorporation ability was impaired and, moreover, it was unable to confer a detectable level of target binding upon Ad. Importantly, subsequent analyses of this fiber revealed the improper folding of its scFv constituent. CONCLUSIONS: LV-based fiber-pseudotyping was established as a convenient method for testing modified fibers for functionality within Ad particles. Furthermore, a new chimeric fiber was found to be inadequate for Ad retargeting. The folding difficulties encountered for this particular fiber might be generally inherent to the use (i.e. for genetic Ad capsid incorporation) of complex, disulfide bridge-containing natural ligands.

Development of a Multicolor Bioluminescence Imaging Platform to Simultaneously Investigate Transcription Factor NF-κB Signaling and Apoptosis.

Here we describe a novel multicolor bioluminescent imaging platform that enables us to simultaneously investigate transcription factor nuclear factor-κB (NF-κB) signalling and apoptosis. We genetically modified the human breast cancer cell line MDA-MB-231 to express green, red, and blue light-emitting luciferases to monitor cell number and viability, NF-κB promoter activity, and to enable specific cell sorting and detection, respectively. Z-DEVD-animoluciferin, the pro-luciferin substrate, was used to determine apoptotic caspase 3/7 activity. We used this multicolored cell line for the in vitro evaluation of natural compounds and in vivo optical imaging of tumor necrosis factor (TNFα)-induced NF-κB activation (Mezzanotte et al., PLoS One 9:e85550, 2014).

The p38 mitogen-activated protein kinase inhibitor SB203580 reduces glucose turnover by the glucose transporter-4 of 3T3-L1 adipocytes in the insulin-stimulated state.

Insulin induces a profound increase in glucose uptake in 3T3-L1 adipocytes through the activity of the glucose transporter-4 (GLUT4). Apart from GLUT4 translocation toward the plasma membrane, there is also an insulin-induced p38 MAPK-dependent step involved in the regulation of glucose uptake. Consequently, treatment with the p38 MAPK inhibitor SB203580 reduces insulin-induced glucose uptake by approximately 30%. Pretreatment with SB203580 does not alter the apparent K(m) of GLUT4-mediated glucose uptake but reduces the maximum velocity by approximately 30%. Insulin-induced GLUT4 translocation and exposure of the transporter to the extracellular environment was not altered by pretreatment with SB203580, as evidenced by a lack of effect of the inhibitor on the amount of GLUT4 present in the plasma membrane, as assessed by subcellular fractionation, the amount of GLUT4 that is able to undergo biotinylation on intact adipocytes and the level of extracellular exposure of an ectopically expressed GLUT-green fluorescence protein construct with a hemagglutinin tag in its first extracellular loop. In contrast, labeling of GLUT4 after insulin stimulation by a membrane-impermeable, mannose moiety-containing, photoaffinity-labeling agent [2-N-4(1-azido-2,2,2-trifluoroethyl)benzoyl-1,3-bis(d-mannose-4-yloxy)-2-propylamine] that binds to the extracellular glucose acceptor domain was markedly reduced by SB203580, although photolabeling with this compound in the absence of insulin was unaffected by SB203580. These data suggest that SB203580 affects glucose turnover by the insulin-responsive GLUT4 transporter in 3T3-L1 adipocytes.

Optimizing reporter constructs for in vivo bioluminescence imaging of interferon-γ stimulated mesenchymal stromal cells.

Mesenchymal stromal cells (MSCs) are a promising treatment modality for a variety of diseases. Strategies to investigate the fate of MSCs in vivo are important to unravel their therapeutic mechanisms. However, currently available techniques are hampered by their low sensitivity. We therefore aimed to optimize in vivo bioluminescence imaging of MSCs. We compared MSCs transduced with firefly luciferase (Fluc) and transmembrane-bound Gaussia luciferase driven by the human cytomegalovirus, spleen focus-forming virus (SFFV), and elongation factor 1-α (EF1α) promoters. Although cytomegalovirus-transmembrane-bound Gaussia luciferase-transduced MSCs showed the highest light intensity in vitro, the signal was almost undetectable in vivo. Spleen focus-forming virus-Fluc-transduced MSCs revealed a bright signal in vivo, but transgene expression was silenced upon in vitro stimulation with interferon (IFN)-γ. Therefore, the SFFV promoter was replaced by the EF1α promoter. Light emission of Fluc under the control of EF1α was similar to SFFV-Fluc. Although EF1α-Fluc light emission was decreased tenfold in the presence of IFN-γ when compared with unstimulated MSCs, the bioluminescent signal could still be detected and was clearly distinguishable from untransduced MSCs. Furthermore, stimulation of MSCs with tumor necrosis factor-α hardly affected transgene expression in EF1α-Fluc-transduced MSCs. Thus, the use of the EF1α promoter partially overcomes silencing and allows in vivo bioluminescence imaging of IFN-γ-stimulated MSCs.

Activin A impairs insulin action in cardiomyocytes via up-regulation of miR-143.

AIMS: Enhanced activin A release from epicardial adipose tissue (EAT) has been linked to the development of cardiac dysfunction in type 2 diabetes (T2D). This study examined whether the inhibition of insulin action induced by epicardial adipokines in cardiomyocytes can be ascribed to alterations in miRNA expression. METHODS AND RESULTS: Expression levels of miRNAs were assessed by real-time PCR in primary adult rat cardiomyocytes (ARC) exposed to conditioned media generated from EAT biopsies (CM-EAT) from patients with and without T2D. CM-EAT-T2D altered the expression of eight miRNAs in ARC vs. CM-EAT from patients without T2D. Of these, only expression of the miR-143/145 cluster was affected by activin A in the same direction as CM-EAT-T2D. Accordingly, activin A neutralizing antibodies prevented the induction of the miR-143/145 cluster by CM-EAT-T2D. Subsequently, the impact of the miR-143/145 cluster on insulin action was investigated. Transfection of HL-1 cells with precursor-miR-143 (pre-miR-143), but not pre-miR-145, blunted the insulin-mediated phosphorylation of Akt and its substrate proline-rich Akt substrate of 40 kDa (PRAS40), and reduced insulin-stimulated glucose uptake. Also lentivirus-mediated expression of pre-miR-143 in ARC reduced insulin-induced Akt phosphorylation. These effects were ascribed to down-regulation of the miR-143 target and regulator of insulin action, the oxysterol-binding protein-related protein 8 (ORP8) in both ARC and HL-1 cells. Finally, LNA-anti-miR-143 protected against the detrimental effects of CM-EAT-T2D on insulin action in ARC. CONCLUSION: Activin A released from EAT-T2D inhibits insulin action via the induction of miR-143 in cardiomyocytes. This miRNA inhibits the Akt pathway through down-regulation of the novel regulator of insulin action, ORP8.

A system for efficient generation of adenovirus protein IX-producing helper cell lines.

BACKGROUND: The adenovirus 14.3 kDa hexon-associated protein IX (pIX) functions in the viral capsid as 'cement' and assembles the hexons in stable groups-of-nine (GONs). Although viruses lacking pIX do not form GONs, and are less heat-stable than wild-type (wt) viruses, they can be propagated with the same kinetics and yields as the wt viruses. To facilitate 'pseudotyping' of adenoviral vectors we have set up an efficient system for the generation of pIX-producing helper cell lines. METHODS: With a lentiviral pIX-expression cassette, monoclonal and polyclonal helper cell lines were generated, which express wt or modified pIX genes at levels equivalent to wt HAdV-5 infected cells. The incorporation efficiency into pIX gene deleted viruses was examined by Western analysis, immuno-affinity electron microscopy, and heat-stability assays. RESULTS: Immuno-affinity electron microscopy on viruses lacking the pIX gene demonstrated that more than 96% of the particles contain pIX protein in their capsids after propagation on the pIX-expressing helper cell lines. In addition, the pIX level in the helper cells was sufficient to generate heat-stable particles. Finally, the ratio between pIX and fiber was equivalent to that found in wt particles. The pIX-producing cell lines are very stable, demonstrating that pIX is not toxic to cells. CONCLUSION: These data demonstrate that lentivirus vectors can be used for the establishment of pIX-complementing helper cell lines.

The coiled-coil domain of the adenovirus type 5 protein IX is dispensable for capsid incorporation and thermostability.

The 14.4-kDa hexon-associated protein IX (pIX) acts as a cement in the capsids of primate adenoviruses and confers a thermostable phenotype. Here we show that deletion of amino acids 100 to 114 of adenovirus type 5 pIX, which eliminates the conserved coiled-coil domain, impairs its capacity to self-associate. However, pIXDelta100-114 is efficiently incorporated into the viral capsid, and the resulting virions are thermostable. Deletion of the central alanine-rich domain, as in pIXDelta60-72, does not impair self-association, incorporation into the capsid, or the thermostable phenotype. These data demonstrate, first, that the self-association of pIX is dispensable for its incorporation into the capsid and generation of the thermostability phenotype and, second, that the increased thermostability results from pIX monomers binding to different hexon capsomers rather than capsid stabilization by pIX multimers.

Towards gene therapy in prosthesis loosening: efficient killing of interface cells by gene-directed enzyme prodrug therapy with nitroreductase and the prodrug CB1954.

BACKGROUND: Loosening is a major complication in prosthesis surgery. To stabilize loosened orthopedic implants, the interface tissue surrounding the implant must be removed. As an alternative to manual removal, we explored the possibility of removing the tissue by gene-directed enzyme prodrug therapy. In the current study we investigated whether interface cells can be transduced by an HAdV-5 vector carrying the E.coli-derived nitroreductase gene and sensitized to the prodrug CB1954. METHODS: The gene transfer efficiency into cultures of diploid human interface cells was tested by exposing these cells to various concentrations of Ad.CMV.LacZ. Subsequently, we studied the susceptibility of cells to the NTR/CB1954 combination. RESULTS: X-gal staining of the Ad.CMV.LacZ-transduced cell cultures revealed that, at 200 plaque-forming units (pfu)/cell, 74% of the cells expressed the LacZ gene. Infection with an NTR construct in interface cell lines resulted in a 60-fold sensitization to the prodrug CB1954. In addition we observed that iotrolan (Isovist) contrast medium had no effect on viability of the cells. However, the presence of the contrast medium completely inhibited adenovirus-mediated gene transfer. CONCLUSIONS: From these data we conclude that HAdV-5-based vectors carrying nitroreductase can be used to sensitize interface tissue. Instead of contrast medium the clinical protocol will use an alternative visualization procedure.

Efficient mobilization of E1-deleted adenovirus type 5 vectors by wild-type adenoviruses of other serotypes.

Mobilization of replication-deficient adenovirus vectors can lead to spread and shedding of the vector. Here we show that in cultured HepG2 cells wild-type (wt) adenoviruses of subgroup A (Ad12), B (Ad7, 11 and 16), C (Ad1, 2 and 5) and E (Ad4) can efficiently mobilize Ad5CMVluc, a DeltaE1DeltaE3-Ad5 vector carrying the firefly luciferase gene as reporter. In addition, we show that Ad5CMVluc can be propagated on Ad12E1-transformed human embryonic retinoblasts. This provides evidence that expression of the E1 region of Ad12 is sufficient for mobilizing DeltaE1-Ad5-derived vectors. Thus, in therapeutic applications of replication-defective Ad vectors any active Ad infection is of potential concern, independent of the serotype involved. To prevent vector mobilization by wt Ads, new vectors should be developed in which essential functions such as the initiation of DNA replication and genome packaging are restricted.

Spacers increase the accessibility of peptide ligands linked to the carboxyl terminus of adenovirus minor capsid protein IX.

The efficiency and specificity of gene transfer with human adenovirus (hAd)-derived gene transfer vectors would be improved if the native viral tropism could be modified. Here, we demonstrate that the minor capsid protein IX (pIX), which is present in 240 copies in the Ad capsid, can be exploited as an anchor for heterologous polypeptides. Protein IX-deleted hAd5 vectors were propagated in hAd5 helper cells expressing pIX variants, with heterologous carboxyl-terminal extensions of up to 113 amino acids in length. The extensions evaluated consist of alpha-helical spacers up to 75 A in length and to which peptide ligands were fused. The pIX variants were efficiently incorporated into the capsids of Ad particles. On intact particles, the MYC-tagged-pIX molecules were readily accessible to anti-MYC antibodies, as demonstrated by electron microscopic analyses of immunogold-labeled virus particles. The labeling efficiency improved with increasing spacer length, suggesting that the spacers lift and expose the ligand at the capsid surface. Furthermore, we found that the addition of an integrin-binding RGD motif to the pIX markedly stimulated the transduction of coxsackievirus group B and hAd receptor-deficient endothelioma cells, demonstrating the utility of pIX modification in gene transfer. Our data demonstrate that the minor capsid protein IX can be used as an anchor for the addition of polypeptide ligands to Ad particles.