Construction of a novel bovine papillomavirus vector without detectable transforming activity suitable for gene transfer

Adenovirus-Mediated Gene Delivery: Potential Applications for Gene and Cell-Based Therapies in the New Era of Personalized Medicine

With rapid advances in understanding molecular pathogenesis of human diseases in the era of genome sciences and systems biology, it is anticipated that increasing numbers of therapeutic genes or targets will become available for targeted therapies. Despite numerous setbacks, efficacious gene and/or cell-based therapies still hold the great promise to revolutionize the clinical management of human diseases. It is wildly recognized that poor gene delivery is the limiting factor for most in vivo gene therapies. There has been a long-lasting interest in using viral vectors, especially adenoviral vectors, to deliver therapeutic genes for the past two decades. Among all currently available viral vectors, adenovirus is the most efficient gene delivery system in a broad range of cell and tissue types. The applications of adenoviral vectors in gene delivery have greatly increased in number and efficiency since their initial development. In fact, among over 2,000 gene therapy clinical trials approved worldwide since 1989, a significant portion of the trials have utilized adenoviral vectors. This review aims to provide a comprehensive overview on the characteristics of adenoviral vectors, including adenoviral biology, approaches to engineering adenoviral vectors, and their applications in clinical and pre-clinical studies with an emphasis in the areas of cancer treatment, vaccination and regenerative medicine. Current challenges and future directions regarding the use of adenoviral vectors are also discussed. It is expected that the continued improvements in adenoviral vectors should provide great opportunities for cell and gene therapies to live up to its enormous potential in personalized medicine.

Exploitation of herpesviral transactivation allows quantitative reporter gene-based assessment of virus entry and neutralization

Herpesviral entry is a highly elaborated process requiring many proteins to act in precise conjunction. Neutralizing antibodies interfere with this process to abrogate viral infection. Based on promoter transactivation of a reporter gene we established a novel method to quantify herpesvirus entry and neutralization by antibodies. Following infection with mouse and human cytomegalovirus and Herpes simplex virus 1 we observed promoter transactivation resulting in substantial luciferase expression (>1000-fold). No induction was elicited by UV-inactivated viruses. The response was MOI-dependent and immunoblots confirmed a correlation between luciferase induction and pp72-IE1 expression. Monoclonal antibodies, immune sera and purified immunoglobulin preparations decreased virus-dependent luciferase induction dose-dependently, qualifying this approach as surrogate virus neutralization test. Besides the reduced hands-on time, this assay allows analysis of herpesvirus entry in semi-permissive and non-adherent cells, which were previously non-assessable but play significant roles in herpesvirus pathology.

Shifts in replication timing actively affect histone acetylation during nucleosome reassembly

The entire genome is replicated in a programmed manner, with specific regions undergoing DNA synthesis at different times in S phase. Active genes generally replicate in early S phase, while repressed genes replicate late, and for some loci this process is developmentally regulated. Using a nuclear microinjection system, we demonstrate that DNA sequences originally packaged into nucleosomes containing deacetylated histones during late S become reassembled with acetylated histones after undergoing replication in early S. Conversely, a change from early to late replication timing is accompanied by repackaging into nucleosomes containing deacetylated histones. This is carried out by differential cell-cycle-controlled acetylation and deacetylation of histones H3 and H4. These studies provide strong evidence that switches in replication timing may play a role in the regulation of nucleosome structure during development.

Differential susceptibility of RAE-1 isoforms to mouse cytomegalovirus

The NKG2D receptor is one of the most potent activating natural killer cell receptors involved in antiviral responses. The mouse NKG2D ligands MULT-1, RAE-1, and H60 are regulated by murine cytomegalovirus (MCMV) proteins m145, m152, and m155, respectively. In addition, the m138 protein interferes with the expression of both MULT-1 and H60. We show here that one of five RAE-1 isoforms, RAE-1delta, is resistant to downregulation by MCMV and that this escape has functional importance in vivo. Although m152 retained newly synthesized RAE-1delta and RAE-1gamma in the endoplasmic reticulum, no viral regulator was able to affect the mature RAE-1delta form which remains expressed on the surfaces of infected cells. This differential susceptibility to downregulation by MCMV is not a consequence of faster maturation of RAE-1delta compared to RAE-1gamma but rather an intrinsic property of the mature surface-resident protein. This difference can be attributed to the absence of a PLWY motif from RAE-1delta. Altogether, these findings provide evidence for a novel mechanism of host escape from viral immunoevasion of NKG2D-dependent control.

The functional role of S/MARs in episomal vectors as defined by the stress-induced destabilization profile of the vector sequences

The scaffold/matrix attachment regions (S/MARs) are chromosomal elements that participate in the formation of chromatin domains and have origin of replication support functions. Because of all these functions, in recent years, they have been used as part of episomal vectors for gene transfer. The S/MAR of the human beta-interferon gene has been shown to support efficient episome retention and transgene expression in various mammalian cells. In Jurkat and other cells, DNA plasmid vectors containing Epstein-Barr virus origin of replication (EBV OriP) and the EBV nuclear antigen-1 gene mediate prolonged episome retention in the host cell nucleus, which, however, diminishes over time. In order to enhance retention, we combined this system with an S/MAR element. Unexpectedly, this completely eliminated the capacity of episomes to replicate. Calculation of the stress-induced DNA duplex destabilization profile of the vectors suggested that the S/MAR element had created an increase in molecular stability at the OriP site that may have disturbed replicative potential. In contrast, introduction of an alternative initiation of replication region from the beta-globin gene locus, instead of EBV OriP and the EBV nuclear antigen-1 gene, restored replicative capacity and enhanced episome retention mediated by the S/MAR. These effects were associated with a destabilization profile at the initiation of replication region. These data demonstrate a correlation between S/MAR-mediated vector retention and the presence of an unstable duplex at a replication origin, in this particular setting. We consider that the calculation of stress-induced duplex destabilization may be an informative first step in the design of units that replicate extrachromosomally, particularly as the latter present a safer and, therefore, attractive alternative to integrating viral vectors for gene therapy applications.

Replication and partitioning of papillomavirus genomes

Papillomaviruses establish persistent infection in the dividing, basal epithelial cells of the host. The viral genome is maintained as a circular, double-stranded DNA, extrachromosomal element within these cells. Viral genome amplification occurs only when the epithelial cells differentiate and viral particles are shed in squames that are sloughed from the surface of the epithelium. There are three modes of replication in the papillomavirus life cycle. Upon entry, in the establishment phase, the viral genome is amplified to a low copy number. In the second maintenance phase, the genome replicates in dividing cells at a constant copy number, in synchrony with the cellular DNA. And finally, in the vegetative or productive phase, the viral DNA is amplified to a high copy number in differentiated cells and is destined to be packaged in viral capsids. This review discusses the cis elements and protein factors required for each stage of papillomavirus replication.

Surmounting tumor-induced immune suppression by frequent vaccination or immunization in the absence of B cells

Tumor-induced immune suppression is one of the most difficult obstacles to the success of tumor immunotherapy. Here, we show that established tumors suppress CD8 T cell clonal expansion in vivo, which is normally observed in tumor-free mice upon antigen-specific glycoprotein (gp) 96-chaperone vaccination. Suppression of CD8 T-cell expansion by established tumors is independent of tumor-associated expression of the antigen that is recognized by the CD8-T-cell receptor. Vaccination of tumor-bearing mice is associated with increased cellular recruitment to the vaccine site compared with tumor-free mice. However, rejection of established, suppressive tumors required frequent (daily) gp96 vaccination. B cells are known to attenuate T helper cell-1 responses. We found that in B-cell deficient mice, tumor rejection of established tumors can be achieved by a single vaccination. Accordingly, in tumor-free B-cell deficient mice, cognate CD8 cytotoxic T lymphocyte clonal expansion is enhanced in response to gp96-chaperone vaccination. The data have implications for the study of tumor-induced immune suppression and for translation of tumor immunotherapy into the clinical setting. Frequent vaccination with cellular vaccines and concurrent B-cell depletion may greatly enhance the activity of anticancer vaccine therapy in patients.

Advances in high-capacity extrachromosomal vector technology: episomal maintenance, vector delivery, and transgene expression

Recent developments in extrachromosomal vector technology have offered new ways of designing safer, physiologically regulated vectors for gene therapy. Extrachromosomal, or episomal, persistence in the nucleus of transduced cells offers a safer alternative to integrating vectors which have become the subject of safety concerns following serious adverse events in recent clinical trials. Extrachromosomal vectors do not cause physical disruption in the host genome, making these vectors safe and suitable tools for several gene therapy targets, including stem cells. Moreover, the high insert capacity of extrachromosomal vectors allows expression of a therapeutic transgene from the context of its genomic DNA sequence, providing an elegant way to express normal splice variants and achieve physiologically regulated levels of expression. Here, we describe past and recent advances in the development of several different extrachromosomal systems, discuss their retention mechanisms, and evaluate their use as expression vectors to deliver and express genomic DNA loci. We also discuss a variety of delivery systems, viral and nonviral, which have been used to deliver episomal vectors to target cells in vitro and in vivo. Finally, we explore the potential for the delivery and expression of extrachromosomal transgenes in stem cells. The long-term persistence of extrachromosomal vectors combined with the potential for stem cell proliferation and differentiation into a wide range of cell types offers an exciting prospect for therapeutic interventions.

Selective down-regulation of the NKG2D ligand H60 by mouse cytomegalovirus m155 glycoprotein

Both human and mouse cytomegaloviruses (CMVs) encode proteins that inhibit the activation of NK cells by down-regulating cellular ligands for the activating NK cell receptor NKG2D. Up to now, three ligands for the NKG2D receptor, named RAE-1, H60, and MULT-1, have been identified in mice. The resistance of mouse strains to murine CMV (MCMV) infection is determined by their ability to generate an effective NK cell response. The MCMV gene m152, a member of the m145 gene family, down-regulates the expression of RAE-1 in order to avoid NK cell control in vivo. Here we report that the m155 gene, another member of the m145 gene family, encodes a protein that interferes with the expression of H60 on the surfaces of infected cells. Deletion of the m155 gene leads to an only partial restoration of H60 expression on the cell surface, suggesting the involvement of another, so far unknown, viral inhibitor. In spite of this, an m155 deletion mutant virus shows NK cell-dependent attenuation in vivo. The acquisition of endo-beta-N-acetylglucosaminidase H resistance and the preserved half-life of H60 in MCMV-infected cells indicate that the m155-mediated effect must take place in a compartment after H60 exits from the ERGIC-cis-Golgi compartment.

NK cell activation through the NKG2D ligand MULT-1 is selectively prevented by the glycoprotein encoded by mouse cytomegalovirus gene m145

The NK cell–activating receptor NKG2D interacts with three different cellular ligands, all of which are regulated by mouse cytomegalovirus (MCMV). We set out to define the viral gene product regulating murine UL16-binding protein-like transcript (MULT)-1, a newly described NKG2D ligand. We show that MCMV infection strongly induces MULT-1 gene expression, but surface expression of this glycoprotein is nevertheless completely abolished by the virus. Screening a panel of MCMV deletion mutants defined the gene m145 as the viral regulator of MULT-1. The MCMV m145-encoded glycoprotein turned out to be necessary and sufficient to regulate MULT-1 by preventing plasma membrane residence of MULT-1. The importance of MULT-1 in NK cell regulation in vivo was confirmed by the attenuating effect of the m145 deletion that was lifted after NK cell depletion. Our findings underline the significance of escaping MULT-1/NKG2D signaling for viral survival and maintenance.

Effective generation of transgenic mice by Bovine papillomavirus type 1 based self-replicating plasmid that is maintained as extrachromosomal genetic element in three generations of animals

The objective of our study was to analyze the efficiency and the properties of the inheritance of the Bovine papillomavirus type 1 (BPV1) replicator-based plasmid used as vector system for generation of transgenic animals. Previously, we have characterized a series of self-replicating plasmid vectors containing all viral factors necessary and sufficient for stable extrachromosomal replication of the BPV1 genome in the tissue culture system. We also demonstrated that the designed replicating vector system has a considerable benefit in the transgene expression, if compared to the regular expression vector. The vector, which showed the highest stability and maintenance function in the tissue culture was chosen for generation of the transgenic mice by pronuclear injections of the circular supercoiled plasmid. This method resulted in successful production of transgenic animals. Transmission efficiency of the vectors into the F(1) generation of animals varied between 0 and 48%, whereas transmission into the F(2) generation was uniformly near 50%. The maintenance of the vector-plasmids in the F(2) generation of transgenic animals as extrachromosomal genetic element was demonstrated by rescue of the plasmid into the Escherichia coli.

HCMV glycoprotein US6 mediated inhibition of TAP does not affect HLA-E dependent protection of K-562 cells from NK cell lysis

Human cytomegalovirus has evolved multiple strategies to interfere with immune recognition by the host. A variety of mechanisms affect antigen presentation by major histocompatibility complex class I molecules resulting in a reduced class I cell-surface expression. This downregulation is expected to trigger natural killer (NK) cytotoxicity, requiring counteraction by the virus to establish long-term infection. Here we describe that the human cytomegalovirus gpUS6 protein, which has been demonstrated to downregulate the expression of human leukocyte antigen (HLA) class I and the presentation of cytotoxic T lymphocyte epitopes by blocking transporter associated with antigen presentation (TAP function), does not affect the ability of HLA-E to inhibit NK cell mediated lysis of K-562 cells by interaction with CD94/NKG2A expressed on NK cells. Cell surface expression and function of HLA-E is not altered although gpUS6 inhibits TAP-dependent peptide transport by 95%. Moreover, HLA-E molecules presenting HLA class I signal sequence-derived peptides are functionally detectable on transfected TAP-deficient RMA-S cells.

Establishment of transcriptional competence in early and late S phase

In animal cells, the process of DNA replication takes place in a programmed manner, with each gene region designated to replicate at a fixed time slot in S phase. Housekeeping genes undergo replication in the first half of S phase in all cell types, whereas the replication of many tissue specific genes is developmentally controlled, being late in most tissues but early in the tissue of expression. Here we employ nuclear DNA injection as an experimental system to test whether this phenomenon is due to differences in the ability to set up transcriptional competence during S phase. Our results show that, regardless of sequence, exogenous genes are a better template for transcription when injected into nuclei of cells in early as opposed to late S phase, and this expression state, once initiated, is preserved after cell division. DNA injected in late S phase is apparently repressed because it is packaged into chromatin containing deacetylated histones, and the same is true for late replicating chromosomal DNA. These findings suggest a mechanistic connection between replication timing and gene expression that might help to explain how epigenetic states can be maintained in vivo.

Establishment of an oriP/EBNA1-based episomal vector transcribing human genomic beta-globin in cultured murine fibroblasts

A novel oriP/EBNA1-based episomal vector has been constructed that persists episomally in cultured murine fibroblasts. The vector, pBH148, is equipped with the entire 185-kb human beta-globin gene locus. After amplification in bacteria, column-purified episomal pBH148 was transfected into both cultured EBNA1-expressing human D98/Raji positive control fusion cells (DRpBH148) and cultured EBNA1-negative murine fibroblast cells (A9pBH148). Cell cultures were maintained concurrently with and without hygromycin selection for a period of 3 months. We show long-term stable episome maintenance of the full-size 200-kb circular double-stranded pBH148 in both the DRpBH148 cultures and the A9pBH148 cultures, regardless of selective pressure by agarose gel electrophoresis and Southern blot. EBNA1 transgene was detected by PCR in all transfected cultures. In addition, we were able to detect correctly spliced human beta-globin mRNA by RT-PCR in all transfected late-passage DRpBH148 and A9pBH148 cell cultures. These findings illustrate that this oriP/EBNA1-based episomal vector is stable in a previously nonpermissive murine cell line and is a potential vector for human gene therapy.

A bovine papillomavirus-1 based vector restores the function of the low-density lipoprotein receptor in the receptor-deficient CHO-ldlA7 cell line

BACKGROUND

The rationale of using bovine papillomavirus-1 (BPV-1) derived vectors in gene therapy protocols lies in their episomal maintenance at intermediate to high copy number, and stable, high-level expression of the gene products. We constructed the BPV-1 based vector harbouring the human low-density lipoprotein receptor (LDLR) gene cDNA and tested its ability to restore the function of the LDLR in the receptor-deficient cell line CHO-ldlA7.

RESULTS

The introduced vector p3.7LDL produced functionally active LDL receptors in the receptor-deficient cell line CHO-ldlA7 during the 32-week period of observation as determined by the internalisation assay with the labelled LDL particles.

CONCLUSION

Bovine papillomavirus type-1 (BPV-1)-derived vectors could be suitable for gene therapy due to their episomal maintenance at intermediate to high copy number and stable, high-level expression of the gene products. The constructed BPV-1 based vector p3.7LDL produced functionally active LDL receptors in the LDLR-deficient cell line CHO-ldlA7 during the 32-week period of observation. In vivo experiments should reveal, whether 1-5% transfection efficiency obtained in the current work is sufficient to bring about detectable and clinically significant lowering of the amount of circulating LDL cholesterol particles.

LCR-mediated, long-term tissue-specific gene expression within replicating episomal plasmid and cosmid vectors

Locus control regions (LCRs) are transcriptional regulatory elements, which possess a dominant chromatin remodelling and transcriptional activating capability conferring full physiological levels of expression on a gene linked in cis, when integrated into the host cell genome. Using the human beta-globin LCR (betaLCR) as a model, we show that this class of control element can drive high levels of tissue-specific gene expression in stably transfected cultured cells from within an Epstein-Barr virus-based plasmid REV. Furthermore, a 38-kb betaLCR minilocus-REV cosmid vector was efficiently retained and maintained therapeutic levels of beta-globin transgene expression in the absence of drug selective pressure over a 2-month period of continuous culture equivalent to at least 60 generations. This demonstrates for the first time the feasibility of using REVs for gene therapy of the haemoglobinopathies. Importantly, our results demonstrate that as in the case of integrated transgenes, expression from within REVs is prone to silencing but that the inclusion of the betaLCR prevented this repression of gene function. Therefore, appropriate control elements to provide and maintain tissue-specific gene expression, as well as the episomal status of REVs is a crucial feature in vector design. Our data suggest that LCRs can contribute to this vital function.

The glycoprotein gp48 of murine cytomegalovirusL proteasome-dependent cytosolic dislocation and degradation

Degradation of misfolded or unassembled proteins that are co-translationally inserted into the endoplasmic reticulum involves the cytosolic proteasome system. Different principles may exist for the export of proteins into the cytosol for proteasomal degradation. Here we studied the degradation pathway of the viral glycoprotein gp48, a type I transmembrane protein, encoded by the m06 gene of murine cytomegalovirus. In cells stably transfected with the cytomegalovirus m06 gene or infected with the virus itself, two populations of gp48 can be distinguished that have different fates. Complexes of gp48 and the major histocompatibility complex (MHC) class I molecule, are transported to the lysosome for degradation. Unassembled gp48 is degraded by the cytosolic proteasome. Proteasomal inhibitors stabilize the unassembled gp48 in its core-glycosylated and membrane-associated form in the endoplasmic reticulum (ER)-Golgi intermediate compartment. This implicates that both endoplasmic reticulum and ER-Golgi intermediate compartment export gp48 and that degradation is coupled to a functional proteasome. Analysis of gp48 mutants revealed that the cytosolic part of gp48 was not responsible for the proteasome-dependent substrate transport out of the ER-Golgi intermediate compartment. Thus an indirect interaction between the proteasome and its substrate has to be discussed.

Efficient transfection of novel bovine papillomavirus 1 expression plasmids

A series of novel bovine papillomavirus type 1 (BPV-1)-based expression plasmids was constructed and characterized in vitro as a starting point for the development of an in vivo gene therapeutic method. The order of transfection efficiency for different pBPVlacZ plasmids was pCGalBPV > pTKBPV > pSRalphaBPV in CV1-P cells. In the absence of selection pressure, the expression of pCGalBPVlacZ and pTKBPVlacZ was associated with long-term maintenance. In a comparison of pBPVlacZ with pSVlacZ, expression was maintained up to 12-17 and 8-12 days, respectively. The transfection of pBPVlacZ plasmids was efficient in secondary and primary, dividing and nondividing, neural and nonneural, and human cells and, furthermore, independent of the cell cycle as seen in growing as well as resting cells. All these characteristics are likely to be relevant for in vivo conditions, under which the percentage of proliferating cells could be quite low. In conclusion, the pBPV plasmids were efficiently delivered and expressed in different host cells, and therefore their performance in gene therapy is worth testing.

Efficient downregulation of major histocompatibility complex class I molecules in human epithelial cells infected with cytomegalovirus

Liver and intestinal epithelial cells are a major target of infection by cytomegaloviruses (CMV), causing severe disease in affected organs of immunocompromised patients. CMV downregulates major histocompatibility complex class I (MHC-I) molecule expression in fibroblasts in order to avoid lysis by CD8(+) cytotoxic T lymphocytes. However, MHC-I expression in human cytomegalovirus (HCMV)-infected hepatic tissue was reported to be increased. As it is unclear at present whether HCMV affects MHC-I expression in epithelial cells, new cell culture models for HCMV infection of differentiated hepatobiliary cell lines were established. HCMV immediate early gene expression was achieved in 60 to 95% of cells. Progression of the HCMV replication cycle differed from prototypic infection of fibroblasts, since structural early and late proteins were produced at low levels and HCMV progeny yielded much lower titres in hepatobiliary cells. In contrast, HCMV glycoproteins, gpUS2, gpUS3, gpUS6 and gpUS11, that downregulate MHC-I expression were synthesized with temporal kinetics and in a similar quantity to that seen in fibroblasts. As a result, HCMV infection led to a drastic and selective downregulation of MHC-I expression in epithelial cells and was uniformly observed irrespective of the hepatic or biliary origin of the cells. The new models document for the first time a stealth function of HCMV in epithelial cells and indicate that the downregulation of MHC-I expression by HCMV can occur in the virtual absence of virus replication.

Episomal vectors for gene expression in mammalian cells

An important reason for preferring mammalian cells for heterologous gene expression is their ability to make authentic proteins containing post-translational modifications similar to those of the native protein. The development of expression systems for mammalian cells has been ongoing for several years, resulting in a wide variety of effective expression vectors. The aim of this review is to highlight episomal expression vectors. Such episomal plasmids are usually based on sequences from DNA viruses, such as BK virus, bovine papilloma virus 1 and Epstein-Barr virus. In this review we will mainly focus on the improvements made towards the usefulness of these systems for gene expression studies and gene therapy.

Expression vector with DNA of bovine papilloma virus 1 for keratinocyte gene therapy

Although there are several methods for introducing the genes to keratinocytes in vivo, expression of transgene does not last long enough for effective keratinocyte gene therapy. In this study, we added bovine papilloma virus 1 (BPV) DNA into expression vectors with the lacZ gene driven by metallothionein and keratin 10 promoters, and we transferred them into keratinocytes in vivo using the naked DNA method, and measured beta-gal activity in keratinocytes. The results showed that beta-galactosidase activity of vectors with the BPV DNA was clearly higher than that without the DNA. Moreover, time-course experiment disclosed that the activity of the BPV vector declined at a lower rate than that of the control vector, suggesting this fragment prolonged transgene expression. These results should prove useful for understanding gene regulation in keratinocytes in vivo and for developing potential expression vectors for keratinocyte gene therapy.

The luminal part of the murine cytomegalovirus glycoprotein gp40 catalyzes the retention of MHC class I molecules

Murine cytomegalovirus (MCMV) interferes with the MHC class I pathway of antigen presentation. The type I transmembrane glycoprotein gp40, encoded by the gene m152, retains major histocompatibility complex (MHC) class I complexes in the endoplasmic reticulum (ER)-Golgi intermediate compartment (ERGIC)/cis-Golgi. These MHC class I complexes are stable, show an extended half-life and do not exchange beta(2)-microglobulin, whereas gp40 reaches an endosomal/lysosomal compartment where it subsequently is degraded. The analysis of regions within the viral protein that are essential for MHC class I retention revealed that a secreted form of gp40, lacking the cytoplasmic tail and the transmembrane region, still has the capacity to retain MHC class I complexes. Continuous expression of gp40 is not required for MHC class I retention. Our data indicate that the retention of MHC class I complexes in the ERGIC/cis-Golgi is triggered by gp40 and does not require the further presence of the viral protein.

Viral gene delivery

Experimental studies of viral gene delivery generally support the principle that virus-mediated gene transfer is indeed possible. However, the field of gene therapy has not yet been realised as a practicable clinical intervention. The delay in translation of laboratory work to clinical utility largely reflects the inability of gene delivery vectors to convey adequate genetic material to a desired location, with adequate durability and low enough toxicity to be effective. Current studies of viral gene therapy vehicles have focused on re-engineering viruses being tested as vectors at present, treating the host to facilitate viral gene transfer and the development of new vectors. Initial enthusiasm for oncoretroviral and adenoviral vectors has cooled, while adeno-associated virus and lentiviral vectors are attracting more interest. Experimental studies with modified SV40-based vectors have also been very promising. The future of gene therapy will probably entail using an array of gene delivery vehicles, each with its own strengths and weaknesses. The vector systems will probably be as diverse as the applications to which they will be put.

A cytomegalovirus glycoprotein re-routes MHC class I complexes to lysosomes for degradation

Mouse cytomegalovirus (MCMV) early gene expression interferes with the major histocompatibility complex class I (MHC class I) pathway of antigen presentation. Here we identify a 48 kDa type I transmembrane glycoprotein encoded by the MCMV early gene m06, which tightly binds to properly folded beta2-microglobulin (beta2m)-associated MHC class I molecules in the endoplasmic reticulum (ER). This association is mediated by the lumenal/transmembrane part of the protein. gp48-MHC class I complexes are transported out of the ER, pass the Golgi, but instead of being expressed on the cell surface, they are redirected to the endocytic route and rapidly degraded in a Lamp-1(+) compartment. As a result, m06-expressing cells are impaired in presenting antigenic peptides to CD8(+) T cells. The cytoplasmic tail of gp48 contains two di-leucine motifs. Mutation of the membrane-proximal di-leucine motif of gp48 restored surface expression of MHC class I, while mutation of the distal one had no effect. The results establish a novel viral mechanism for downregulation of MHC class I molecules by directly binding surface-destined MHC complexes and exploiting the cellular di-leucine sorting machinery for lysosomal degradation.

Efficient ex vivo inhibition of perforin and Fas ligand expression by chimeric tRNA-hammerhead ribozymes

Graft-versus-host disease (GVHD) is a feared complication of allogeneic bone marrow transplantation. Research in rodent models has linked perforin and Fas ligand (FasL), two components of independent lytic pathways, with the induction of GVHD. In this study we characterized two hammerhead ribozymes that cleave their target perforin and Fas ligand RNAs with high efficiency in CTLL-2 cells. The perforin and Fas ligand ribozymes were expressed from a tRNA-directed RNA polymerase III promoter that was inserted in an episomal multicopy plasmid derived from papilloma virus. Chimeric anti-perforin and anti-FasL tRNA-ribozymes had sequences engineered in order to have specific secondary structure effects. These sequence modifications allow the formation of a 5' --> 3' stem structure and also place the ribozyme in a flexible bulge region that keeps the ribozyme separated from the tRNA domain. Northern and RT in situ PCR analyses showed high levels of transcription and efficient transportation to the cytoplasm. The expression of perforin and FasL in CTLL-2 cells was significantly reduced as assessed by RNA and protein analyses.

Vectors for cancer gene therapy

Many viral and non-viral vector systems have now been developed for gene therapy applications. In this article, the pros and cons of these vector systems are discussed in relation to the different cancer gene therapy strategies. The protocols used in cancer gene therapy can be broadly divided into six categories including gene transfer to explanted cells for use as cell-based cancer vaccines; gene transfer to a small number of tumour cells in situ to achieve a vaccine effect; gene transfer to vascular endothelial cells (VECs) lining the blood vessels of the tumour to interfere with tumour angiogenesis; gene transfer to T lymphocytes to enhance their antitumour effector capability; gene transfer to haemopoietic stem cells (HSCs) to enhance their resistance to cytotoxic drugs and gene transfer to a large number of tumour cells in situ to achieve nonimmune tumour reduction with or without bystander effect. Each of the six strategies makes unique demands on the vector system and these are discussed with reference to currently available vectors. Aspects of vector biology that are in need of further development are discussed in some detail. The final section points to the potential use of replicating viruses as delivery vehicles for efficient in vivo gene transfer to disseminated cancers.

High efficiency, long-term clinical expression of cottontail rabbit papillomavirus (CRPV) DNA in rabbit skin following particle-mediated DNA transfer

The ability of skin to support long lasting expression of genes delivered with a particle-mediated system was evaluated in rabbits inoculated with cottontail rabbit papillomavirus (CRPV) DNA. The optimal delivery force for maximal gene expression in rabbit skin was determined in transient beta-galactosidase assays. Forty-five sites in four rabbits were then inoculated at 350-400 p.s.i. with CRPV DNA. All sites (100%) formed papillomas with multiple papillomas at most sites. These results support the feasibility of using a particle-mediated delivery system for gene therapy and suggest that some papillomavirus features, such an origin of replication, may be well suited for use in vectors to target long term expression to skin.