Use of an autonomous parvovirus vector for selective transfer of a foreign gene into transformed human cells of different tissue origins and its expression therein

Evidence for densovirus integrations into tapeworm genomes

BACKGROUND

Tapeworms lack a canonical piRNA-pathway, raising the question of how they can silence existing mobile genetic elements (MGE). Investigation towards the underlying mechanisms requires information on tapeworm transposons which is, however, presently scarce.

METHODS

The presence of densovirus-related sequences in tapeworm genomes was studied by bioinformatic approaches. Available RNA-Seq datasets were mapped against the Echinococcus multilocularis genome to calculate expression levels of densovirus-related genes. Transcription of densovirus loci was further analyzed by sequencing and RT-qPCR.

RESULTS

We herein provide evidence for the presence of densovirus-related elements in a variety of tapeworm genomes. In the high-quality genome of E. multilocularis we identified more than 20 individual densovirus integration loci which contain the information for non-structural and structural virus proteins. The majority of densovirus loci are present as head-to-tail concatemers in isolated repeat containing regions of the genome. In some cases, unique densovirus loci have integrated close to histone gene clusters. We show that some of the densovirus loci of E. multilocularis are actively transcribed, whereas the majority are transcriptionally silent. RT-qPCR data further indicate that densovirus expression mainly occurs in the E. multilocularis stem cell population, which probably forms the germline of this organism. Sequences similar to the non-structural densovirus genes present in E. multilocularis were also identified in the genomes of E. canadensis, E. granulosus, Hydatigera taeniaeformis, Hymenolepis diminuta, Hymenolepis microstoma, Hymenolepis nana, Taenia asiatica, Taenia multiceps, Taenia saginata and Taenia solium.

CONCLUSIONS

Our data indicate that densovirus integration has occurred in many tapeworm species. This is the first report on widespread integration of DNA viruses into cestode genomes. Since only few densovirus integration sites were transcriptionally active in E. multilocularis, our data are relevant for future studies into gene silencing mechanisms in tapeworms. Furthermore, they indicate that densovirus-based vectors might be suitable tools for genetic manipulation of cestodes.

Oncolytic rat parvovirus H-1PV, a candidate for the treatment of human lymphoma: In vitro and in vivo studies

The incidence of lymphomas developing in both immunocompetent and immunosuppressed patients continues to steadily increase worldwide. Current chemotherapy and immunotherapy approaches have several limitations, such as severe side toxicity and selection of resistant cell variants. Autonomous parvoviruses (PVs), in particular the rat parvovirus H-1PV, have emerged as promising anticancer agents. Although it is apathogenic in humans, H-1PV has been shown to infect and suppress various rat and human tumors in animal models. In this study, we demonstrate the capacity of H-1PV for efficiently killing, through necrosis, cell cultures originating from Burkitt's lymphoma (BL), while sparing normal B lymphocytes. The cytotoxic effect was generally accompanied by a productive H-1PV infection. Remarkably, parvovirus-based monotherapy efficiently suppressed established BL at an advanced stage in a severe combined immunodeficient (SCID) mouse model of the disease. The data show for the first time that an oncolytic parvovirus deserves further consideration as a potential tool for the treatment of some non-Hodgkin B-cell lymphomas, including those resistant to apoptosis induction by rituximab.

B1 lymphocytes and myeloid dendritic cells in lymphoid organs are preferential extratumoral sites of parvovirus minute virus of mice prototype strain expression

Due to their oncolytic properties and apathogenicity, autonomous parvoviruses have attracted significant interest as possible anticancer agents. Recent preclinical studies provided evidence of the therapeutic potential of minute virus of mice prototype strain (MVMp) and its recombinant derivatives. In a murine model of hemangiosarcoma, positive therapeutic outcome correlated with high intratumoral expression of MVMp-encoded genes in tumors and lymphoid organs, especially in tumor-draining lymph nodes. The source and relevance of this extratumoral expression, which came as a surprise because of the known fibrotropism of MVMp, remained unclear. In the present study, we investigated (i) whether the observed expression pattern occurs in different tumor models, (ii) which cell population is targeted by the virus, and (iii) the immunological consequences of this infection. Significant MVMp gene expression was detected in lymphoid tissues from infected tumor-free as well as melanoma-, lymphoma-, and hemangiosarcoma-bearing mice. This expression was especially marked in lymph nodes draining virus-injected tumors. Fluorescent in situ hybridization analysis, multicolor fluorescence-activated cell sorting, and quantitative reverse transcription-PCR revealed that MVMp was expressed in rare subpopulations of CD11b (Mac1)-positive cells displaying CD11c+ (myeloid dendritic cells [MDC]) or CD45B (B220+ [B1 lymphocytes]) markers. Apart from the late deletion of cytotoxic memory cells (CD8+ CD44+ CD62L-), this infection did not lead to significant alteration of the immunological profile of cells populating lymphoid organs. However, subtle changes were detected in the production of specific proinflammatory cytokines in lymph nodes from virus-treated animals. Considering the role of B1 lymphocytes and MDC in cancer and immunological surveillance, the specific ability of these cell types to sustain parvovirus-driven gene expression may be exploited in gene therapy protocols.

The infectivity and lytic activity of minute virus of mice wild-type and derived vector particles are strikingly different

Gene therapy vectors have been developed from autonomous rodent parvoviruses that carry a therapeutic gene or a marker gene in place of the genes encoding the capsid proteins. These vectors are currently evaluated in preclinical experiments. The infectivity of the vector particles deriving from the fibroblastic strain of minute virus of mice (MVMp) (produced by transfection in human cells) was found to be far less (approximately 50-fold-less) infectious than that of wild-type virus particles routinely produced by infection of A9 mouse fibroblasts. Similarly, wild-type MVMp produced by transfection also had a low infectivity in mouse cells, indicating that the method and producer cells influence the infectivity of the virus produced. Interestingly, producer cells made as many full vector particles as wild-type particles, arguing against deficient packaging being responsible for the low infectivity of viruses recovered from transfected cells. The hurdle to infection with full particles produced through transfection was found to take place at an early step following entry and limiting viral DNA replication and gene expression. Infections with transfection or infection-derived virus stocks normalized for their replication ability yielded similar monomer and dimer DNA amplification and gene expression levels. Surprisingly, at equivalent replication units, the capacity of parvovirus vectors to kill tumor cells was lower than that of the parental wild-type virus produced under the same transfection conditions, suggesting that beside the viral nonstructural proteins, the capsid proteins, assembled capsids, or the corresponding coding region contribute to the lytic activity of these viruses.

Carrier cell-mediated delivery of oncolytic parvoviruses for targeting metastases

Over the last few years, naturally occurring or genetically manipulated oncolytic viruses gained increasing attention as novel therapeutics for cancer treatment. The present work provides proof of principle that an organotropic cell-based carrier system is suitable to deliver oncolytic parvoviruses to a tissue known to be a target for the formation of metastases. Carrier cells were inactivated by gamma-irradiation after infection, which was found not to affect the production and release of parvoviruses that were capable of lysing cocultured target neoplastic cells. Although systemically administered parvovirus H-1 showed a pronounced therapeutic effect against the development of established Morris hepatoma (MH3924A) lung metastases, the carrier cell strategy offered a number of advantages. Infected carriers were able to sustain H-1 virus expression for 6 days in the lungs of rats affected by metastatic disease and to reduce the spreading of the virus to peripheral organs. Compared to direct virus injection, the carrier cell protocol led to an improved therapeutic effect (metastases suppression) and a lesser generation of virus-neutralizing antibodies. These data support the use of carrier cells to deliver oncolytic viruses and/or viral vectors locally in tumors and, more particularly, metastases.

A nonproliferating parvovirus vaccine vector elicits sustained, protective humoral immunity following a single intravenous or intranasal inoculation

An ideal vaccine delivery system would elicit persistent protection following a single administration, preferably by a noninvasive route, and be safe even in the face of immunosuppression, either inherited or acquired, of the recipient. We have exploited the unique life cycle of the autonomous parvoviruses to develop a nonproliferating vaccine platform that appears to both induce priming and continually boost a protective immune response following a single inoculation. A crippled parvovirus vector was constructed, based on a chimera between minute virus of mice (MVM) and LuIII, which expresses Borrelia burgdorferi outer surface protein A (OspA) instead of its coat protein. The vector was packaged into an MVM lymphotropic capsid and inoculated into naive C3H/HeNcr mice. Vaccination with a single vector dose, either intravenously or intranasally, elicited high-titer anti-OspA-specific antibody that provided protection from live spirochete challenge and was sustained over the lifetime of the animal. Both humoral and cell-mediated Th(1) immunity was induced, as shown by anti-OspA immunoglobulin G2a antibody and preferential gamma interferon production by OspA-specific CD4(+) T cells.

Chromosomal integration and homologous gene targeting by replication-incompetent vectors based on the autonomous parvovirus minute virus of mice

The molecular mechanisms responsible for random integration and gene targeting by recombinant adeno-associated virus (AAV) vectors are largely unknown, and whether vectors derived from autonomous parvoviruses transduce cells by similar pathways has not been investigated. In this report, we constructed vectors based on the autonomous parvovirus minute virus of mice (MVM) that were designed to introduce a neomycin resistance expression cassette (neo) into the X-linked human hypoxanthine phosphoribosyl transferase (HPRT) locus. High-titer, replication-incompetent MVM vector stocks were generated with a two-plasmid transfection system that preserved the wild-type characteristic of packaging only one DNA strand. Vectors with inserts in the forward or reverse orientations packaged noncoding or coding strands, respectively. In human HT-1080 cells, MVM vector random integration frequencies (neo(+) colonies) were comparable to those obtained with AAV vectors, and no difference was observed for noncoding and coding strands. HPRT gene-targeting frequencies (HPRT mutant colonies) were lower with MVM vectors, and the noncoding strand frequency was threefold greater than that of the coding strand. Random integration and gene-targeting events were confirmed by Southern blot analysis of G418- and 6-thioguanine (6TG)-resistant clones. In separate experiments, correction of an alkaline phosphatase (AP) gene by gene targeting was nine times more effective with a coding strand vector. The data suggest that single-stranded parvoviral vector genomes are substrates for gene targeting and possibly for random integration as well.

Chimeric and pseudotyped parvoviruses minimize the contamination of recombinant stocks with replication-competent viruses and identify a DNA sequence that restricts parvovirus H-1 in mouse cells

Recent studies demonstrated the ability of the recombinant autonomous parvoviruses MVMp (fibrotropic variant of the minute virus of mice) and H-1 to transduce therapeutic genes in tumor cells. However, recombinant vector stocks are contaminated by replication-competent viruses (RCVs) generated during the production procedure. To reduce the levels of RCVs, chimeric recombinant vector genomes were designed by replacing the right-hand region of H-1 virus DNA with that of the closely related MVMp virus DNA and conversely. Recombinant H-1 and MVMp virus pseudotypes were also produced with this aim. In both cases, the levels of RCVs contaminating the virus stocks were considerably reduced (virus was not detected in pseudotyped virus stocks, even after two amplification steps), while the yields of vector viruses produced were not affected. H-1 virus could be distinguished from MVMp virus by its restriction in mouse cells at an early stage of infection prior to detectable viral DNA replication and gene expression. The analysis of the composite viruses showed that this restriction could be assigned to a specific genomic determinant(s). Unlike MVMp virus, H-1 virus capsids were found to be a major determinant of the greater permissiveness of various human cell lines for this virus.

Production of recombinant H1 parvovirus stocks devoid of replication-competent viruses

Vector and helper plasmids for the production of recombinant H1 (rH1) parvovirus, an oncolytic virus and candidate vector for cancer gene therapy, were constructed with the aim of reducing the contamination of these preparations with replication-competent viruses (RCV). Split-helper plasmids were constructed by manipulating the splicing signals for the capsid proteins such that VP1 and VP2 were expressed from separate plasmids. H1 vectors with similarly mutated splice sites were packaged, using the split-helper plasmids, and the resulting recombinant H1 viruses were completely free of RCV because the generation of recombinants expressing both capsid proteins was prevented. Vector yields of rH1 produced with split-helper plasmids in combination with splice site-modified vectors were similar (in the range of 10(7) replication units/ml) to yields of rH1 produced with the standard vector/helper pair, in which case significant levels of RCV were generated (10(4)-10(5) plaque-forming units/ml). To assess the functionality of this approach in vivo, rH1 was produced that contained the human interleukin 2 (IL-2) transgene and that was devoid of RCV. This IL-2-carrying rH1 vector expressed IL-2 efficiently in human tumor cells (HeLa) in vitro and generated antitumor responses in nude mice xenografted with HeLa cells that had been infected ex vivo with this virus. These results should allow the large-scale production of recombinant oncotropic parvoviruses and their assessment for the gene therapy of cancer in a clinical setting.

Genome replication and postencapsidation functions mapping to the nonstructural gene restrict the host range of a murine parvovirus in human cells

The infection outcome of the Parvoviridae largely relies on poorly characterized intracellular factors modulated by proliferation, differentiation, and transformation of host cells. We have studied the interactions displayed by the highly homologous p and i strains of the murine parvovirus minute virus of mice (MVM), with a series of transformed cells of rat (C6) and human (U373, U87, SW1088, SK-N-SH) nervous system origin, seeking for molecular mechanisms governing parvovirus host range. The MVMp infection of C6 and U373 cells was cytotoxic and productive, whereas the other nervous cells behaved essentially as resistant to this virus. In contrast, MVMi did not complete its life cycle in any of the human nervous cells, though it efficiently killed the astrocytic tumor cells by two types of nonproductive infections: (i) normal synthesis of all viral macromolecules with a late defect in infectious virion maturation and release to the medium in U373; and (ii) high levels of accumulation of the full set of viral messenger RNAs and of both nonstructural (NS-1) and structural (VP-1 and VP-2) proteins, under a very low viral DNA amplification, in U87 and SW1088 cells. Further analyses showed that U87 was permissive for nuclear transport of MVMi proteins, leading to efficient assembly of empty viral capsids with a normal phosphorylation and VP1-to-VP2 ratio. The DNA amplification blockade in U87 occurred after conversion of the incoming MVMi genome to the monomeric replicative form, and it operated independently of the delivery pathway used by the viral particle, since it could not be overcome by transfection with cloned infectious viral DNA. Significantly, a chimeric MVMi virus harboring the coding region of the nonstructural (NS) gene replaced with that of MVMp showed a similar pattern of restriction in U87 cells as the parental MVMi virus, and it attained in U373 cultures an infectious titer above 100-fold higher under equal levels of DNA amplification and genome encapsidation. The results suggest that the activity of complexes formed by the NS polypeptides and recruited cellular factors restrict parvovirus DNA amplification in a cell type-dependent manner and that NS functions may in addition determine MVM host range acting at postencapsidation steps of viral maturation. These data are relevant for understanding the increased multiplication of autonomous parvovirus in some transformed cells and the transduction efficacy of nonreplicative parvoviral vectors, as well as a general remark on the mechanisms by which NS genes may regulate viral tropism and pathogenesis.

Transduction of human MCP-3 by a parvoviral vector induces leukocyte infiltration and reduces growth of human cervical carcinoma cell xenografts

BACKGROUND

The oncosuppressive properties of some autonomous parvoviruses such as H-1 virus, together with their low pathogenicity, make them attractive vectors for tumor-directed gene therapy. Indeed, it was recently shown that these viruses became endowed with an enhanced oncosuppressive activity after they had been engineered to deliver a recognized therapeutic transgene. This prompted us to use a parvoviral vector to analyse the antineoplastic capacity of MCP-3 (monocyte chemotactic protein-3), a CC chemokine which has a broad spectrum of target cells, and can thus be considered to be a promising candidate for cancer treatment.

METHODS

We explored the use of a parvovirus H-1-based vector encoding human MCP-3 for its antitumor potential on human cervical carcinoma cells. HeLa cells were infected in vitro with the recombinant virus hH1/MCP-3 at a low multiplicity [1 replication unit (RU)/cell] and we investigated the effect of parvovirus-mediated MCP-3 transduction on tumor formation and growth upon implantation of HeLa cells in nude mice.

RESULTS

Infection of HeLa cells with hH1/MCP-3 led to secretion of high levels of MCP-3 and to significant retardation of tumor growth in recipient mice, as compared with HeLa cells that were either buffer-treated or infected with a MCP-3-free vector. Tumors from hH1/MCP-3-infected HeLa cells were heavily infiltrated with activated macrophages and showed increased numbers of dendritic cells. In addition, activated natural killer (NK) cells were also recruited into MCP-3-transduced tumors.

CONCLUSION

These observations indicate that parvovirus H-1-transduced MCP-3 is able to exert a significant antitumor activity which is mediated, at least in part, through macrophages and NK cells, under conditions in which activated T cells are lacking.

A novel MVMp-based vector system specifically designed to reduce the risk of replication-competent virus generation by homologous recombination

Recent work highlights the potential usefulness of MVM-based vectors as selective vehicles for cancer gene therapy (Dupont et al, Gene Therapy, 2000; 7: 790-796). To implement this strategy, however, it is necessary to develop optimized methods for producing high-titer, helper-free parvovirus stocks. Recombinants of MVMp (rMVMp) are currently generated by transiently co-transfecting permissive cell lines with a plasmid carrying the vector genome and a helper plasmid expressing the capsid genes (replaced with a foreign gene in the vector genome). The resulting stocks, however, are always heavily contaminated with replication-competent viruses (RCV), which precludes their use in vivo and particularly in gene therapy. In the present work we have developed a second-generation MVMp-based vector system specifically designed to reduce the probability of RCV generation by homologous recombination. We have constructed a new MVMp-based vector and a new helper genome with minimal sequence overlap and have used the degeneracy of the genetic code to further decrease vector-helper homology. In this system, the left homologous region was almost completely eliminated and the right sequence overlap was reduced to 74 nt with only 61% homology. We were thus able to substantially reduce ( approximately 200 x), but not completely eliminate, generation of contaminating viruses in medium-scale rMVMp preparations. Since the remaining sequence homology between the new vector and helper genomes is weak, our results suggest that contaminating viruses in this system are generated by nonhomologous recombination. It is important to note, unlike the autonomously replicating helper viruses produced from the first-generation vector/helper genomes, the contaminating viruses arising from the new packaging system cannot initiate secondary infection rounds (so they are not 'replication-competent viruses'). Our findings have important implications for the design of new MVMp-based vectors and for the construction of trans-complementing packaging cell lines.

Expansion of tropism of a feline parvovirus to target a human tumor cell line by display of an alpha(v) integrin binding peptide on the capsid

The autonomous parvoviruses are small, non-enveloped, single strand DNA viruses. They occur in many species and they have oncolytic properties. We are modifying the capsid of feline panleukopenia virus (FPV), a parvovirus which normally infects feline cells, with the goal of targeting human tumor cells for potential cancer therapy. Using recombinant viruses transducing a luciferase reporter, we show that insertion of a cyclically constrained, integrin-binding peptide at an exposed position on the FPV capsid enables transduction of an alpha(v) integrin-expressing human rhabdomyosarcoma cell line (Rh18A). These cells were not transduced by virus with the unmodified FPV capsid. Transduction of Rh18A was specifically inhibited by an alpha(v) integrin blocking antibody. However, other human tumor lines expressing alpha(v) integrins were not transduced by virus with either the modified or unmodified capsid. We conclude that modification of the FPV capsid to bind alpha(v) integrins can contribute to, but is not generally sufficient for, redirecting infection to human tumor cells. The permissiveness of Rh18A cells presumably involves additional factors unique to this line among various human cell lines tested.

Tumor-selective gene transduction and cell killing with an oncotropic autonomous parvovirus-based vector

A recombinant MVMp of the fibrotropic strain of minute virus of mice (MVMp) expressing the chloramphenicol acetyltransferase reporter gene was used to infect a series of biologically relevant cultured cells, normal or tumor-derived, including normal melanocytes versus melanoma cells, normal mammary epithelial cells versus breast adenocarcinoma cells, and normal neurons or astrocytes versus glioma cells. As a reference cell system we used normal human fibroblasts versus the SV40-transformed fibroblast cell line NB324K. After infection, we observed good expression of the reporter gene in the different tumor cell types, but only poor expression if any in the corresponding normal cells. We also constructed a recombinant MVMp expressing the green fluorescent protein reporter gene and assessed by flow cytometry the efficiency of gene transduction into the different target cells. At a multiplicity of infection of 30, we observed substantial transduction of the gene into most of the tumor cell types tested, but only marginal transduction into normal cells under the same experimental conditions. Finally, we demonstrated that a recombinant MVMp expressing the herpes simplex virus thymidine kinase gene can, in vitro, cause efficient killing of most tumor cell types in the presence of ganciclovir, whilst affecting normal proliferating cells only marginally if at all. However, in the same experimental condition, breast tumor cells appeared to be resistant to GCV-mediated cytotoxicity, possibly because these cells are not susceptible to the bystander effect. Our data suggest that MVMp-based vectors could prove useful as selective vehicles for anticancer gene therapy, particularly for in vivo delivery of cytotoxic effector genes into tumor cells.

Recombination within the nonstructural genes of the parvovirus minute virus of mice (MVM) generates functional levels of wild-type NS1, which can be detected in the absence of selective pressure following transfection of nonreplicating plasmids

Recombination within the coding region of the nonstructural genes of minute virus of mice (MVM), which generates functional levels of wild-type NS1, was observed in the absence of selective pressure following cotransfection of nonreplicating plasmids. P38 activity was used as a measure of recombinant NS1 production, which, together with direct detection of recombinant-generated products by RT-PCR, allowed an estimation of recombination efficiency. In addition, we show that very low levels of wild-type NS1 were able to significantly transactivate P38. Given that recombination following cotransfection can generate NS1 at these levels, our observations have implications for the study of parvoviral genetics, the construction of recombinant parvoviral vectors for gene therapy applications, and perhaps other systems using cotransfection of plasmids that share homologous sequences.

Highly efficient transduction and expression of cytokine genes in human tumor cells by means of autonomous parvovirus vectors; generation of antitumor responses in recipient mice

The possible use of recombinant autonomous parvoviruses as vectors to efficiently express therapeutic cytokines in human tumor cells was evaluated in vitro and in vivo. The parvovirus H1 was used to generate recombinant viruses (rH1) that carried transgenes encoding either human interleukin 2 (IL-2) or monocyte chemotactic protein 1 (MCP-1), in replacement of part of the capsid genes. Such rH11 viruses have been shown to retain in vitro the intrinsic oncotropic properties of the parental virus. On infection with the recombinant viruses at an input multiplicity of 1 replication unit (RU) per cell, HeLa cultures were induced to release 4-10 microg of cytokine per 10(6) cells over a period of 5 days. The expression of the rH1-transduced human cytokine/chemokine could also be detected in tumor material recovered from nude mice that had been subcutaneously engrafted with in vitro-infected HeLa cells. The formation of tumors from HeLa xenografts was reduced by 90% compared with wild-type or mock-infected cells as a result of cells preinfected with IL2-expressing virus at an input multiplicity as low as 1 RU per cell. Tumors arising from HeLa cells infected with transgene-free or MCP1-expressing vectors or with wild-type H1 virus were not rejected at this virus dose. Tumors infected with rH1/IL-2 virus displayed markers indicative of their infiltration with NK cells in which the cytocidal program was activated, whereas little NK activity was detected in wild-type virus or mock-infected tumors. Altogether, these data show that the IL-2 expressing H1 vector was a more potent antineoplastic agent than the parental virus, and point to the possible application of recombinant autonomous parvoviruses toward therapy of some human tumors.

cis requirements for the efficient production of recombinant DNA vectors based on autonomous parvoviruses

The replication of viral genomes and the production of recombinant viral vectors from infectious molecular clones of parvoviruses MVMp and H1 were greatly improved by the introduction of a consensus NS-1 nick site at the junction between the left-hand viral terminus and the plasmid DNA. Progressive deletions of up to 1600 bp in the region encoding the structural genes as well as insertions of foreign DNA in replacement of those sequences did not appreciably affect the replication ability of the recombinant H1 virus genomes. In contrast, the incorporation of these genomes into recombinant particles appeared to depend on in cis-provided structural gene sequences. Indeed, the production of H1 viral vectors by cotransfection of recombinant clones and helper plasmids providing the structural proteins (VPs) in trans, drastically decreased when more than 800 bp was removed from the VP transcription unit. Furthermore, titers of viral vectors, in which most of the VP-coding region was replaced by an equivalent-length sequence consisting of reporter cDNA and stuffer DNA, were reduced more than 50 times in comparison with recombinant vectors in which stuffer DNA was not substituted for the residual VP sequence. In addition, viral vector production was restricted by the overall size of the genome, with a mere 6% increase in DNA length leading to an approximately 10 times lower encapsidation yield. Under conditions fulfilling the above-mentioned requirements for efficient packaging, titers of virus vectors from improved recombinant molecular DNA clones amounted to 5 x 10(7) infectious units per milliliter of crude extract. These titers should allow the assessment of the therapeutic effect of recombinant parvoviruses expressing small transgenes in laboratory animals.

Influence of sequence and size of DNA on packaging efficiency of parvovirus MVM-based vectors

We have derived a vector from the autonomous parvovirus MVM(p), which expresses human IL-2 specifically in transformed cells (Russell et al., J. Virol 1992;66:2821-2828). Testing the therapeutic potential of these vectors in vivo requires high-titer stocks. Stocks with a titer of 10(9) can be obtained after concentration and purification (Avalosse et al., J. Virol. Methods 1996;62:179-183), but this method requires large culture volumes and cannot easily be scaled up. We wanted to increase the production of recombinant virus at the initial transfection step. Poor vector titers could be due to inadequate genome amplification or to inefficient packaging. Here we show that intracellular amplification of MVM vector genomes is not the limiting factor for vector production. Several vector genomes of different size and/or structure were amplified to an equal extent. Their amplification was also equivalent to that of a cotransfected wild-type genome. We did not observe any interference between vector and wild-type genomes at the level of DNA amplification. Despite equivalent genome amplification, vector titers varied greatly between the different genomes, presumably owing to differences in packaging efficiency. Genomes with a size close to 100% that of wild type were packaged most efficiently with loss of efficiency at lower and higher sizes. However, certain genomes of identical size showed different packaging efficiencies, illustrating the importance of the DNA sequence, and probably its structure.

Selective Expansion of Alveolar Macrophages In Vivo by Adenovirus-Mediated Transfer of the Murine Granulocyte-Macrophage Colony-Stimulating Factor cDNA

Based on the hypothesis that genetic modification of freshly isolated alveolar macrophages (AM) with the granulocyte-macrophage colony-stimulating factor (GM-CSF) cDNA would induce AM to proliferate, this study focuses on the ability of adenoviral (Ad) vectors to transfer and efficiently express the murine (m) GM-CSF cDNA in murine AM with consequent expansion in the number of AM in vitro and in vivo. To demonstrate that an Ad vector can effectively transfer and express genes in AM, murine AM recovered by bronchoalveolar lavage from the lung of Balb/c mice were infected with an Ad vector coding for green fluorescent protein (GFP) in vitro and expressed GFP in a dose-dependent fashion. Infection of AM with an Ad vector containing an expression cassette coding for mGM-CSF led to GM-CSF expression and to AM proliferation in vitro. When AM infected with AdGFP were returned to the respiratory tract of syngeneic recipient mice, GFP-expressing cells could still be recovered by bronchoalveolar lavage 2 weeks later. In vitro infection of AM with AdmGM-CSF and subsequent transplantation of the genetically modified AM to the lungs of syngeneic recipients led to GM-CSF expression in vivo. Strikingly, the AM recovered by lavage 5 weeks after transplantation demonstrated an increased rate of proliferation, and the total number of alveolar macrophages was 1.9-fold greater than controls. Importantly, the increase in the numbers of AM was selective (ie, other inflammatory cell numbers were unchanged), and there was no modification to the lung architecture. Thus, it is feasible to genetically modify AM with Ad vectors and to use this strategy to modify the behavior of AM in vivo. Based on the importance of AM in the primary defense of the respiratory epithelial surface, this strategy may be useful in enhancing pulmonary defenses in immunodeficiency states.

Selective Expansion of Alveolar Macrophages In Vivo by Adenovirus-Mediated Transfer of the Murine Granulocyte-Macrophage Colony-Stimulating Factor cDNA

Based on the hypothesis that genetic modification of freshly isolated alveolar macrophages (AM) with the granulocyte-macrophage colony-stimulating factor (GM-CSF) cDNA would induce AM to proliferate, this study focuses on the ability of adenoviral (Ad) vectors to transfer and efficiently express the murine (m) GM-CSF cDNA in murine AM with consequent expansion in the number of AM in vitro and in vivo. To demonstrate that an Ad vector can effectively transfer and express genes in AM, murine AM recovered by bronchoalveolar lavage from the lung of Balb/c mice were infected with an Ad vector coding for green fluorescent protein (GFP) in vitro and expressed GFP in a dose-dependent fashion. Infection of AM with an Ad vector containing an expression cassette coding for mGM-CSF led to GM-CSF expression and to AM proliferation in vitro. When AM infected with AdGFP were returned to the respiratory tract of syngeneic recipient mice, GFP-expressing cells could still be recovered by bronchoalveolar lavage 2 weeks later. In vitro infection of AM with AdmGM-CSF and subsequent transplantation of the genetically modified AM to the lungs of syngeneic recipients led to GM-CSF expression in vivo. Strikingly, the AM recovered by lavage 5 weeks after transplantation demonstrated an increased rate of proliferation, and the total number of alveolar macrophages was 1.9-fold greater than controls. Importantly, the increase in the numbers of AM was selective (ie, other inflammatory cell numbers were unchanged), and there was no modification to the lung architecture. Thus, it is feasible to genetically modify AM with Ad vectors and to use this strategy to modify the behavior of AM in vivo. Based on the importance of AM in the primary defense of the respiratory epithelial surface, this strategy may be useful in enhancing pulmonary defenses in immunodeficiency states.

Gene therapy for prostate cancer

Prostate cancer remains one of the most significant challenges in clinical oncology, yet present therapies provide incomplete treatment in many cases. Innovative and practical gene therapy-based approaches will prove invaluable in filling the gaps that now exist in the treatment of localized and distant disease. Although multiple potential strategies have been developed, early clinical trials in prostate cancer gene therapy are now in the phase I/II stage of development. Novel preclinical and early clinical data should be considered optimistically, yet cautiously, as this field emerges from its infancy.

Replicating viruses as selective cancer therapeutics

Replication-competent viruses are used as selective cancer therapeutics and the mechanisms leading to tumor-specific replication and antitumoral efficacy are now becoming apparent. The specific viruses in development include tumor-targeting herpes simplex viruses, autonomous parvoviruses, Newcastle disease viruses and adenovirus. Information is also available on antiviral immunology and viral defenses against host-mediated immunity. This approach has many potential attributes, in addition to potential hurdles that must be overcome.

Expression of parvovirus LuIII NS1 from a Sindbis replicon for production of LuIII-luciferase transducing virus

In order to develop an alternative packaging system for recombinant parvoviruses, the gene for the major nonstructural protein (NS1) of parvovirus LuIII was inserted into a Sindbis replicon vector. Cells infected with recombinant SinNS1 virus produced NS1 RNA from the Sindbis 26S promoter and expressed NS1 protein which was able to transactivate a parvovirus P38 promoter. Co-transfections of Sindbis-NS1 RNA together with a packageable LuIII transducing genome and a coat protein expression plasmid generated detectable levels of LuIII-luciferase transducing virus. These levels could be increased by a capsid expression plasmid that was also capable of expressing NS2. These results show that a multi-functional parvovirus protein expressed from a Sindbis RNA molecule can be used to produce recombinant parvoviruses.

Method for concentrating and purifying recombinant autonomous parvovirus vectors designed for tumour-cell-targeted gene therapy

Recent work has highlighted the use of parvoviruses as potential vectors for tumour-cell-targeted gene therapy. The oncotropic properties of the prototype strain of minute virus of mice (MVMp) suggest that this virus might be a useful vehicle for introducing selectively therapeutic genes, e.g. lymphokine or suicide genes, into tumour cells and preferentially expressing them. But the low titre of recombinant virus stocks (10(5)-10(6) infectious units per ml) and their high level of contamination by cell proteins make it practically impossible to evaluate their efficacy in in vivo systems. A technique is described for producing cellular contaminant-free stocks of recombinant virus particles, with titres up to 5 x 10(8) IU/ml.

Parvovirus particles as platforms for protein presentation

Empty capsids of the human pathogenic parvovirus B19 can be produced in a baculovirus system. B19 capsids are composed mainly of major capsid protein (VP2) and a small amount of minor capsid protein (VP1); VP1 is identical to VP2 but contains an additional 227-aa N-terminal region ("unique" region). A portion of that region of VP1 is external to the capsid, and VP1 is not required for capsid formation. We substituted the unique region with a sequence encoding the 147 aa of hen egg white lysozyme (HEL) and constructed recombinant baculoviruses with variable amounts of retained VP1 sequence joined to the VP2 backbone. After cotransfection with VP2 baculovirus and expression in insect cells, capsids were purified by density sedimentation. Purified recombinant capsids contained HEL. External presentation of HEL was demonstrated by immunoprecipitation, ELISA, and immune electron microscopy using anti-lysozyme monoclonal antibodies or specific rabbit antisera. Empty particles showed enzymatic activity in a micrococcal cell wall digestion assay. Rabbits inoculated with capsids made antibodies to HEL. Intact heterologous protein can be incorporated in B19 particles and presented on the capsid surface, properties that may be useful in vaccine development, cell targeting, and gene therapy.

Lack of a detectable effect of capsid proteins on the cell-dependent activity of parvovirus MVMp promoters

Deletion of upstream elements from the early P4 promoter of parvovirus MVM (minute virus of mice, prototype strain MVMp) has a differential effect on its activity, depending on the host cell considered. This indicates that upstream motifs participate in the control of promoter P4 functioning and are responsive to factors which are, at least in part, cell-type specific. In contrast with other viral systems, the capsid proteins of MVMp had no detectable effect on gene expression driven by either the early P4 or late P38 promoter of MVMp in permissive and non-permissive cells.