Susceptibility of human cells to killing by the parvoviruses H-1 and minute virus of mice correlates with viral transcription

Best of most possible worlds: Hybrid gene therapy vectors based on parvoviruses and heterologous viruses

Parvoviruses and especially the adeno-associated virus (AAV) species provide an exciting and versatile platform for the rational design or molecular evolution of human gene-therapy vectors, documented by literature from over half a century, hundreds of clinical trials, and the recent commercialization of multiple AAV gene therapeutics. For the last three decades, the power of these vectors has been further potentiated through various types of hybrid vectors created by intra- or inter-genus juxtaposition of viral DNA and protein cis elements or by synergistic complementation of parvoviral features with those of heterologous, prokaryotic, or eukaryotic viruses. Here, we provide an overview of the history and promise of this rapidly expanding field of hybrid parvoviral gene-therapy vectors, starting with early generations of chimeric particles composed of a recombinant AAV genome encapsidated in shells of synthetic AAVs or of adeno-, herpes-, baculo-, or protoparvoviruses. We then dedicate our attention to two newer, highly promising types of hybrid vectors created via (1) pseudotyping of AAV genomes with bocaviral serotypes and capsid mutants or (2) packaging of AAV DNA into, or tethering of entire vector particles to, bacteriophages. Finally, we conclude with an outlook summarizing critical requirements and improvements toward clinical translation of these original concepts.

The Potential of Cellular- and Viral-Based Immunotherapies for Malignant Glioma-Dendritic Cell Vaccines, Adoptive Cell Transfer, and Oncolytic Viruses

PURPOSE OF REVIEW

Malignant gliomas, including glioblastoma and anaplastic astrocytoma, are the most frequent primary brain tumors and present with many treatment challenges. In this review, we discuss the potential of cellular- and viral-based immunotherapies in the treatment of malignant glioma, specifically focusing on dendritic cell vaccines, adoptive cell therapy, and oncolytic viruses.

RECENT FINDINGS

Diverse cellular- and viral-based strategies have been engineered and optimized to generate either a specific or broad antitumor immune response in malignant glioma. Due to their successes in the preclinical arena, many of these therapies have undergone phase I and II clinical testing. These early clinical trials have demonstrated the feasibility, safety, and efficacy of these immunotherapies. Dendritic cell vaccines, adoptive cell transfer, and oncolytic viruses may have a potential role in the treatment of malignant glioma. However, these modalities must be investigated in well-designed phase III trials to prove their efficacy.

Oncolytic viruses for therapy of malignant glioma

Effective treatment of malignant brain tumors is still an open problem. Location of tumor in vital areas of the brain significantly limits capasities of surgical treatment. The presence of tumor stem cells resistant to radiation and anticancer drugs in brain tumor complicates use of chemoradiotherapy and causes a high rate of disease recurrence. A technological improvement in bioselection and production of recombinant resulted in creation of viruses with potent oncolytic properties against glial tumors. Recent studies, including clinical trials, showed, that majority of oncolytic viruses are safe. Despite the impressive results of the viral therapy in some patients, the treatment of other patients is not effective; therefore, further improvement of the methods of oncolytic virotherapy is necessary. High genetic heterogeneity of glial tumor cells even within a single tumor determines differences in individual sensitivity of tumor cells to oncolytic viruses. This review analyses the most successful oncolytic virus strains, including those which had reached clinical trials, and discusses the prospects for new approaches to virotherapy of gliomas.

Double-faceted mechanism of parvoviral oncosuppression

The H-1 parvovirus (H-1PV) exerts oncosuppressive action that has two components: oncotoxicity and immunostimulation. While many human tumor cells, including conventional drug-resistant ones, can be killed by H-1PV, some fail to support progeny virus production, necessary for infection propagation in neoplastic tissues. This limitation can be overcome through forced selection of H-1PV variants capable of enhanced multiplication and spreading in human tumor cells. In the context of further developing H-1PV for use in cancer therapy, arming it with immunostimulatory CpG motifs under conditions preserving replication and oncolysis enhances its action as an anticancer vaccine adjuvant. A first clinical study of H-1PV treatment in glioma patients has yielded evidence of intratumoral synthesis of the viral oncotoxic protein NS1 and immune cell infiltration.

Current status of gene therapy for breast cancer: progress and challenges

Breast cancer is characterized by a series of genetic mutations and is therefore ideally placed for gene therapy intervention. The aim of gene therapy is to deliver a nucleic acid-based drug to either correct or destroy the cells harboring the genetic aberration. More recently, cancer gene therapy has evolved to also encompass delivery of RNA interference technologies, as well as cancer DNA vaccines. However, the bottleneck in creating such nucleic acid pharmaceuticals lies in the delivery. Deliverability of DNA is limited as it is prone to circulating nucleases; therefore, numerous strategies have been employed to aid with biological transport. This review will discuss some of the viral and nonviral approaches to breast cancer gene therapy, and present the findings of clinical trials of these therapies in breast cancer patients. Also detailed are some of the most recent developments in nonviral approaches to targeting in breast cancer gene therapy, including transcriptional control, and the development of recombinant, multifunctional bio-inspired systems. Lastly, DNA vaccines for breast cancer are documented, with comment on requirements for successful pharmaceutical product development.

Distinct host cell fates for human malignant melanoma targeted by oncolytic rodent parvoviruses

The rodent parvoviruses are known to be oncoselective, and lytically infect many transformed human cells. Because current therapeutic regimens for metastatic melanoma have low response rates and have little effect on improving survival, this disease is a prime candidate for novel approaches to therapy, including oncolytic parvoviruses. Screening of low-passage, patient-derived melanoma cell lines for multiplicity-dependent killing by a panel of five rodent parvoviruses identified LuIII as the most melanoma-lytic. This property was mapped to the LuIII capsid gene, and an efficiently melanoma tropic chimeric virus shown to undergo three types of interaction with primary human melanoma cells: (1) complete lysis of cultures infected at very low multiplicities; (2) acute killing resulting from viral protein synthesis and DNA replication, without concomitant expansion of the infection, due to failure to export progeny virions efficiently; or (3) complete resistance that operates at an intracellular step following virion uptake, but preceding viral transcription.

Regression of advanced rat and human gliomas by local or systemic treatment with oncolytic parvovirus H-1 in rat models

Oncolytic virotherapy is a potential treatment modality under investigation for various malignancies including malignant brain tumors. Unlike some other natural or modified viruses that show oncolytic activity against cerebral neoplasms, the rodent parvovirus H-1 (H-1PV) is completely apathogenic in humans. H-1PV efficiently kills a number of tumor cells without harm to corresponding normal ones. In this study, the concept of H-1PV-based virotherapy of glioma was tested for rat (RG-2 cell-derived) and for human (U87 cell-derived) gliomas in immunocompetent and immunodeficient rat models, respectively. Large orthotopic rat and human glioma cell-derived tumors were treated with either single stereotactic intratumoral or multiple intravenous (iv) H-1PV injections. Oncolysis was monitored by magnetic resonance imaging and proven by histology. Virus distribution and replication were determined in brain and organs. In immunocompetent rats bearing RG-2-derived tumors, a single stereotactic intratumoral injection of H-1PV and multiple systemic (iv) applications of the virus were sufficient for remission of advanced and even symptomatic intracranial gliomas without damaging normal brain tissue or other organs. H-1PV therapy resulted in significantly improved survival (Kaplan-Meier analysis) in both the rat and human glioma models. Virus replication in tumors indicated a contribution of secondary infection by progeny virus to the efficiency of oncolysis. Virus replication was restricted to tumors, although H-1PV DNA could be detected transiently in adjacent or remote normal brain tissue and in noncerebral tissues. The results presented here and the innocuousness of H-1PV for humans argue for the use of H-1PV as a powerful means to perform oncolytic therapy of malignant gliomas.

Translation control by protein kinase R restricts minute virus of mice infection: role in parvovirus oncolysis

The relevance of translational control in the gene expression and oncotropism of the autonomous parvoviruses was investigated with MVMp, the prototype strain of minute virus of mice (MVM), infecting normal and transformed rodent and human cells of different tissue origins. Mouse embryo fibroblasts (MEFs) and NIH 3T3 fibroblasts were resistant to MVMp infection, but 3T3 fibroblasts derived from double-stranded RNA (dsRNA)-dependent protein kinase R (PKR) knockout mice (PKR(o/o)) behaved in a manner that was highly permissive to productive MVMp replication. NIH 3T3 resistance correlated with significant phosphorylation of eukaryotic translation initiation factor 2 (eIF2) occurring at early time points after infection. Permissive PKR(o/o) cells were converted to MVMp-restrictive cells after reintroduction of the PKR gene by transfection. Conversely, regulated expression of the vaccinia virus E3 protein, a PKR inhibitor, in MEFs prevented eIF2alpha phosphorylation and increased MVMp protein synthesis. In vitro-synthesized genome-length R1 mRNA of MVMp was a potent activator of PKR. Virus-resistant primary MEFs and NIH 3T3 cells responded to MVMp infection with significant increases in eIF2alpha phosphorylation. In contrast, virus-permissive mouse (PKR(o/o), BHK21, and A9) and human transformed (NB324K fibroblast, U373 glioma, and HepG2 hepatoma) cells consistently showed no significant increase in the level of eIF2alpha phosphorylation following MVMp infection. The synthesis of the viral NS1 protein was inversely correlated with the steady-state PKR levels. Our results show that the PKR-mediated antiviral response is an important mechanism for control of productive MVMp infection, and its impairment in human transformed cells allowed efficient MVMp gene expression. PKR translational control may therefore contribute to the oncolysis of MVMp and other autonomous parvoviruses.

Activation of an antiviral response in normal but not transformed mouse cells: a new determinant of minute virus of mice oncotropism

Parvovirus minute virus of mice (MVMp) is endowed with oncotropic properties so far ascribed only to the dependency of the virus life cycle on cellular factors expressed during S phase and/or modulated by malignant transformation. For other viruses oncotropism relies on their inability to circumvent type I interferon (IFN)-induced innate antiviral mechanisms, the first line of defense triggered by normal cells against viral infections. These agents propagate, therefore, preferentially in transformed/tumor cells, which often lack functional antiviral mechanisms. The present study aimed at investigating whether antiviral processes also contribute to MVMp oncotropism. Our results demonstrate that in contrast to MVMp-permissive transformed mouse A9 fibroblasts, freshly isolated normal counterparts (mouse embryonic fibroblasts [MEFs]) mount, through production and release of type I IFNs upon their infection, an antiviral response against MVMp lytic multiplication. Pretreatment of MEFs with a type I IFN-beta-neutralizing antibody, prior to MVMp infection, inhibits the virus-triggered antiviral response and improves the fulfillment of the MVMp life cycle. Our results also show that part of the A9 permissiveness to MVMp relies on the inability to produce type I IFNs upon parvovirus infection, a feature related either to an A9 intrinsic deficiency of this process or to an MVMp-triggered inhibitory mechanism, since stimulation of these cells by exogenous IFN-beta strongly inhibits the parvovirus life cycle. Taken together, our results demonstrate for the first time that parvovirus infection triggers an innate antiviral response in normal cells and suggest that the MVMp oncotropism depends at least in part on the failure of infected transformed cells to mount such a response.

Parvovirus H-1 induces cytopathic effects in breast carcinoma-derived cultures

Parvovirus H-1 (H-1 PV) preferentially replicates in malignant cells resulting in their death by cytolysis. It has often been considered a potential candidate for use in novel anticancer therapy. To evaluate its potential in a model of natural tumors, we assayed in vitro the effect exerted by H-1 PV on short-term cultures derived from breast tumor samples freshly excised from patients. Our results show that H-1 PV effectively kills tumor-derived cells, whereas normal tissue-derived cells showed no H-1 PV-induced cytopathic effects (CPE). We also determined that the H-1 PV sensitivity (up to 67% sensitive cultures) is related with the quantities of virus assayed. We further examined the expression and phosphorylation state of the parvoviral nonstructural protein 1 (NS1), known to be associated with parvoviruses-induced CPE. Both appear to be impaired in normal tissue-derived cells and resistant cultures. Finally, we show that H-1 PV sensitivity in cultures correlates significantly with higher tumor grades (Nottingham combined histologic grade 2 or 3). This report confirms that H-1 PV can efficiently induce CPE in primary breast tumor cells in vitro. It identifies tumor characteristics representing potential criteria for recruiting patients for clinical evaluation of H-1 PV antitumor effects.

Oncolytic potential of rodent parvoviruses for cancer therapy in humans: a brief review

Summary Rodent parvoviruses are promising candidates for oncolytic virotherapy of cancer in humans because of their oncotropism (preferential killing of transformed cells) in the absence of pathogenicity. Here, we give an overview concerning the possible application of parvovirus H-1 for cancer therapy, with specific emphasis on malignant brain tumours in humans.

Parvovirus H-1 infection of human glioma cells leads to complete viral replication and efficient cell killing

The extremely poor prognosis of malignant gliomas requires the investigation of other than standard therapies, i.e., the application of oncolytic viruses. In our study, we evaluated the effects of the oncosuppressive parvovirus H-1 on different established glioblastoma cell lines of rat and human origin and on short-term/low-passage cultures of human glioblastoma cells. We observed an efficient and dose-dependent killing of all glioma cell cultures at low multiplicities of infectious particles (MOI) per cell. Southern blot analysis of viral DNA amplification, RT-PCR analysis of viral RNA expression and Western blot analysis of the expression of viral structural (VP-1/VP-2) and nonstructural (NS-1) proteins demonstrated the biosynthesis of these viral macromolecular components in all of the cultures. Moreover, all the glioma cells were proficient for the production of infectious H-1 virus particles. The amount of virus production differed between a several fold increase of the input virus titer in most of the short-term/low-passage cultures up to 1,000-fold in one short-term glioma and in the rat cells. Glioma cells lines and, more importantly, short-term/low-passage cultures of human glioblastomas were found to be highly susceptible target cells for H-1 virus mediated cytotoxicity. The formation of fully infectious progeny particles in infected glioma cells offers the chance for the induction of secondary rounds of infection resulting in an advanced cytotoxic effect. These advantageous characteristics of H-1 virus infection of glioma cells, combined with the known low toxicity of H-1 virus in nontransformed cells, make parvovirus H-1 a promising candidate for oncolytic glioma therapy.

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.

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.

Parvovirus H-1-induced cell death: influence of intracellular NAD consumption on the regulation of necrosis and apoptosis

The autonomous parvovirus H-1 exerts tumor-suppressive effects in living organisms and has been shown to specifically interfere with the survival of transformed cells in culture. The mechanism(s) by which H-1 virus induces death of transformed cells is not yet well understood. It has recently been reported that H-1 virus induces apoptotic cell death in the human monocytic U937 cell line, as assessed by biochemical and morphological changes of infected cells (Rayet, B., Lopez-Guerrero, J.-A., Rommelaere, J., Dinsart, C., 1998. Induction of programmed cell death by parvovirus H-1 in U937 cells: connection with the TNFalpha signalling pathway. J. Virol. 72, 8893-8903). Here we show that parvovirus H-1 infection induced early biochemical changes pointing to apoptotic events also in the transformed human keratinocyte cell line, HeLa, and the transformed rat fibroblast cell line, P1. Morphologic changes, however, and in particular the early breakdown of plasma membrane integrity, suggested that apoptosis did not go to completion, leading to necrotic cell death as the major result of parvovirus infection of HeLa and P1 cells. Parvovirus infection of these, and to a significantly lesser extent of U937 cells, was accompanied by rapid depletion of intracellular NAD stores. Inhibition of NAD-consuming enzymes interfered with parvovirus-induced NAD depletion and increased the proportion of H-1 virus-infected cells displaying apoptotic features of cell death. In contrast, a similar prevention of NAD depletion through stimulation of NAD production had little influence on the cell death pathway, suggesting that NAD-consuming enzymes may promote necrosis in a direct way rather than through inducing the overall drop of intracellular NAD.

A heterogeneous nuclear ribonucleoprotein A/B-related protein binds to single-stranded DNA near the 5' end or within the genome of feline parvovirus and can modify virus replication

Phage display of cDNA clones prepared from feline cells was used to identify host cell proteins that bound to DNA-containing feline panleukopenia virus (FPV) capsids but not to empty capsids. One gene found in several clones encoded a heterogeneous nuclear ribonucleoprotein (hnRNP)-related protein (DBP40) that was very similar in sequence to the A/B-type hnRNP proteins. DBP40 bound specifically to oligonucleotides representing a sequence near the 5' end of the genome which is exposed on the outside of the full capsid but did not bind most other terminal sequences. Adding purified DBP40 to an in vitro fill-in reaction using viral DNA as a template inhibited the production of the second strand after nucleotide (nt) 289 but prior to nt 469. DBP40 bound to various regions of the viral genome, including a region between nt 295 and 330 of the viral genome which has been associated with transcriptional attenuation of the parvovirus minute virus of mice, which is mediated by a stem-loop structure of the DNA and cellular proteins. Overexpression of the protein in feline cells from a plasmid vector made them largely resistant to FPV infection. Mutagenesis of the protein binding site within the 5' end viral genome did not affect replication of the virus.

Neoplastic transformation-associated stimulation of the in vitro resolution of concatemer junction fragments from minute virus of mice DNA

Minute virus of mice (MVM) shows an oncotropic behavior reflected by its ability to amplify its genome more efficiently in a number of transformed versus normal cells. In vivo and in vitro studies revealed that the major effect of cell transformation on MVM DNA replication occurs at the level of double-stranded replicative-form amplification. In particular, resolution of MVM DNA concatemers into monomers was found to be highly sensitive to neoplastic transformation.

Constitutive Activation of U937 Promonocytic Cell Clones Selected for Their Resistance to Parvovirus H-1 Infection

The human promonocytic cell line U937 is highly sensitive to the lytic effect of the autonomous parvovirus H-1. Rare cell variants that resisted H-1 virus infection could be isolated, of which four (RU1, RU2, RU3, and RU4) were further characterized. In contrast to parental cells, the RU clones sustained an abortive H-1 virus infection. Three of the clones showed a significant decrease in the accumulation levels of the c-Myc oncoprotein and in their capacity for forming tumors in immunodeficient mice. Surprisingly, all RU clones resisted the suppressing effect of 12-O-tetradecanoylphorbol-13-acetate (TPA) on c-myc oncogene expression and cell proliferation. In contrast, RU clones exhibited the TPA-induced changes in membrane surface antigens and nonspecific esterase activities that are characteristic of monocytic differentiation. Studies of the activation steady-state of RU cells demonstrated the constitutive production of significant amounts of nitric oxide (NO) and superoxide anion (O−2⋅ ). Inhibitors of NO and O−2⋅ . production sensitized all RU cells to the killing effect of parvovirus H-1 and increased the production of infectious viral particles. These data argue for the participation of active oxygen species in macrophage defence mechanisms against parvovirus infection. Moreover, the use of parvovirus H-1 as a selective agent in a cell-colony formation assay allowed us to show that expression of defined markers of monocytic differentiation can be uncoupled from suppression of proliferation.

Constitutive Activation of U937 Promonocytic Cell Clones Selected for Their Resistance to Parvovirus H-1 Infection

The human promonocytic cell line U937 is highly sensitive to the lytic effect of the autonomous parvovirus H-1. Rare cell variants that resisted H-1 virus infection could be isolated, of which four (RU1, RU2, RU3, and RU4) were further characterized. In contrast to parental cells, the RU clones sustained an abortive H-1 virus infection. Three of the clones showed a significant decrease in the accumulation levels of the c-Myc oncoprotein and in their capacity for forming tumors in immunodeficient mice. Surprisingly, all RU clones resisted the suppressing effect of 12-O-tetradecanoylphorbol-13-acetate (TPA) on c-myc oncogene expression and cell proliferation. In contrast, RU clones exhibited the TPA-induced changes in membrane surface antigens and nonspecific esterase activities that are characteristic of monocytic differentiation. Studies of the activation steady-state of RU cells demonstrated the constitutive production of significant amounts of nitric oxide (NO) and superoxide anion (O−2⋅ ). Inhibitors of NO and O−2⋅ . production sensitized all RU cells to the killing effect of parvovirus H-1 and increased the production of infectious viral particles. These data argue for the participation of active oxygen species in macrophage defence mechanisms against parvovirus infection. Moreover, the use of parvovirus H-1 as a selective agent in a cell-colony formation assay allowed us to show that expression of defined markers of monocytic differentiation can be uncoupled from suppression of proliferation.

Upstream CREs participate in the basal activity of minute virus of mice promoter P4 and in its stimulation in ras-transformed cells

The activity of the P4 promoter of the parvovirus minute virus of mice (prototype strain MVMp) is stimulated in ras-transformed FREJ4 cells compared with the parental FR3T3 line. This activation may participate in the oncolytic effect of parvoviruses, given that P4 drives a transcriptional unit encoding cytotoxic nonstructural proteins. Our results suggest that the higher transcriptional activity of promoter P4 in FREJ4 cells is mediated at least in part by upstream CRE elements. Accordingly, mutations in the CRE motifs impair P4 function more strongly in the FREJ4 derivative than in its FR3T3 parent. Further evidence that these elements contribute to hyperactivity of the P4 promoter in the ras transformant is the fact that they form distinct complexes with proteins from FREJ4 and FR3T3 cell extracts. This difference can be abolished by treating the FREJ4 cell extracts with cyclic AMP-dependent protein kinase (PKA) or treating original cultures with a PKA activator. These findings can be linked with two previously reported features of ras-transformed cells: the activation of a PKA-inhibited protein kinase cascade and the reduction of PKA-induced protein phosphorylation. In keeping with these facts, P4-directed gene expression can be up- or downmodulated in vivo by exposing cells to known inhibitors or activators of PKA, respectively.

Isolation of a fully infectious variant of parvovirus H-1 supplanting the standard strain in human cells

A variant H-1 virus, designated H-1 dr virus, was isolated from stock of the standard H-1 virus strain propagated in the newborn human kidney cell line NB-E. Molecular cloning and sequence analysis revealed an in-frame deletion at map positions 39 to 41. This deletion affects the open reading frames encoding the nonstructural proteins NS-1 and NS-2 and the untranslated leader sequence of the R3 transcripts encoding the capsid proteins. In addition, H-1 dr virus harbors a 58-nucleotide duplication inboard from the right-hand terminal palindrome. Internal deletions and terminal reiterations are hallmarks of H-1 virus type I variants that typically are defective interfering particles. Indeed, H-1 dr virus was found to progressively supplant the standard strain in serially coinfected NB-E cell cultures. However, H-1 dr virus differed from previously described type I variants in its full infectivity, as was apparent from its ability to give yields of replication and progeny virus production that were similar to those of the standard virus strain in NB-E cells. Hence, the interference of H-1 dr virus in the propagation of standard H-1 virus in coinfected cells was not accompanied by a drop in the titer of infectious virus. Moreover, H-1 dr virus proved to induce the same pathogenic effects in newborn hamsters as the standard virus strain did.

The evolution of small DNA viruses of eukaryotes: past and present considerations

Historically, viral evolution has often been considered from the perspective of the ability of the virus to maintain viral pathogenic fitness by causing disease. A predator-prey model has been successfully applied to explain genetically variable quasi-species of viruses, such as influenza virus and human immunodeficiency virus (HIV), which evolve much faster rates than the host. In contrast, small DNA viruses (polyomaviruses, papillomaviruses, and parvoviruses) are species specific but are stable genetically, and appear to have co-evolved with their host species. Genetic stability is attributable primarily to the ability to establish and maintain a benign persistent state in vivo and not to the host DNA proofreading mechanisms. The persistent state often involves a cell cycle-regulated episomal state and a tight linkage of DNA amplification mechanisms to cellular differentiation. This linkage requires conserved features among viral regulatory proteins, with characteristic host-interactive domains needed to recruit and utilize host machinery, thus imposing mechanistic constrains on possible evolutionary options. Sequence similarities within these domains are seen amongst all small mammalian DNA viruses and most of the parvo-like viruses, including those that span the entire spectrum of evolution of organisms from E. coli to humans that replicate via a rolling circle-like mechanism among the entire spectrum of organisms throughout evolution from E. coli to humans. To achieve benign inapparent viral persistence, small DNA viruses are proposed to circumvent the host acute phase reaction (characterized by minimal inflammation) by mechanisms that are evolutionarily adapted to the immune system and the related cytokine communication networks. A striking example of this is the relationship of hymenoptera to polydnaviruses, in which the crucial to the recognition of self, development, and maintenance of genetic identity of both the host and virus. These observations in aggregate suggest that viral replicons are not recent "escapies" of host replication, but rather provide relentless pressure in driving the evolution of the host through cospeciation.

Transcriptional activation by the parvoviral nonstructural protein NS-1 is mediated via a direct interaction with Sp1

The nonstructural protein NS-1, encoded by the parvovirus minute virus of mice, is a potent regulator of viral gene expression. NS-1 does not bind DNA in a sequence-specific manner, and the mechanism by which it modulates viral promoter function is unclear. We have used Gal4-NS-1 fusion protein constructs to identify and characterize an activating domain encoded within the C-terminal 88 amino acids of NS-1 which competes effectively with the acidic activator domain of the herpes simplex virus VP16 protein. DNA affinity chromatography and immunoprecipitation experiments demonstrate that protein-protein interactions between the transcription factor Sp1 and NS-1 are required to bind NS-1 to promoter DNA in vitro. Cotransfection of Gal4-NS-1 and Sp1-VP16 acidic activator constructs into Drosophila melanogaster Schneider cells, which lack endogenous Sp1, stimulates transcription from a minimal promoter containing five Gal4 binding sites, while single-construct transfections do not. Cotransfection of Schneider cells with wild-type NS-1 and Sp1 constructs activates transcription from a simian virus 40 promoter 10- to 30-fold over that of either construct alone. Thus, Sp1-NS-1 interactions in vivo can stimulate transcription from a heterologous promoter containing Sp1 binding sites.

Characterization of cell lines that inducibly express the adeno-associated virus Rep proteins

The replication (rep) gene of adeno-associated virus (AAV) is involved in AAV DNA replication, gene regulation, and inhibition of cellular transformation induced by various oncogenes. To study the rep gene's antiproliferative effects, we have developed cell lines which express the replication proteins under the control of an inducible mouse metallothionein transcription promoter. The Rep78 protein produced in these cell lines binds to the AAV terminal repeat sequences in vitro and supports AAV DNA replication and trans activation of the AAV p40 transcription promoter in vivo. These cell lines are capable of assembling infectious viruses containing a mutant rep gene or a vector bearing a heterologous gene. Growth rate and colony formation efficiency assays indicated that rep gene expression substantially altered cellular proliferation. Long-term induction of the cell lines followed by removal of the inducing agent suggested that constitutive expression of the Rep proteins does not necessarily result in cell death and that the cells can recover from the cytostatic effects. Flow cytometry analysis indicated that the presence of the Rep proteins increased the population of cells in the S phase of the cell cycle. Thus the rep gene's antiproliferative effects may be realized by interference with cellular DNA replication.

Induced expression of the conditionally cytotoxic herpes simplex virus thymidine kinase gene by means of a parvoviral regulatory circuit

As a step toward the achievement of targeted expression of toxic genes, we have established a model system using the selective trans-activation of the late promoter P38 of Minute Virus of Mice (MVMp) by the parvoviral nonstructural protein NS-1. The conditionally toxic herpes simplex virus type 1 thymidine kinase (tk) gene (HSV1-tk) was cloned under the control of the P38 promoter and transfected into NIH-3T3 TK- cells. Treatment of the stably transfected cells with acyclovir (ACV) followed by infection with MVMp reduced cell survival by 3.5- to 5-fold compared to the toxic effects of ACV or MVMp alone. These results indicate that it should be possible to combine the genuine cytopathic action of parvoviruses with a specific activation of toxic genes driven by parvoviral promoters, to achieve the targeted destruction of parvovirus-expressing (in particular tumor) cells.

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

In this work, we report the transduction of a chloramphenicol acetyltransferase (CAT) reporter gene into a variety of normal and transformed human cells of various tissue origins. The vector used was MVM/P38cat, a recombinant of the prototype strain of the autonomous parvovirus minute virus of mice (MVMp). The CAT gene was inserted into the capsid-encoding region of the infectious molecular clone of MVMp genome, under the control of the MVM P38 promoter. When used to transfect permissive cells, the MVM/P38cat DNA was efficiently replicated and expressed the foreign CAT gene at high levels. By cotransfecting with a helper plasmid expressing the capsid proteins, it was possible to produce mixed virus stocks containing MVM/P38cat infectious particles and variable amounts of recombinant MVM. MVM/P38cat viral particles were successfully used to transfer the CAT gene and to express it in a variety of human cells. Both viral DNA replication and P38-driven CAT expression were achieved in fibroblasts, epithelial cells, T lymphocytes, and macrophages in a transformation-dependent way, but with an efficiency depending on the cell type. In transformed B lymphocytes, however, the vector was not replicated, nor did it express the CAT gene.

Interconnection between thyroid hormone signalling pathways and parvovirus cytotoxic functions

Nonstructural (NS) proteins of autonomous parvoviruses can repress expression driven by heterologous promoters, an activity which thus far has not been separated from their cytotoxic effects. It is shown here that, in transient transfection assays, the NS-1 protein of the parvovirus minute virus of mice (MVMp) activates the promoter of the human c-erbA1 gene, encoding the thyroid hormone (T3) receptor alpha. The endogenous c-erbA1 promoter is also a target for induction upon MVMp infection. Moreover, T3 was found to up-modulate the level of cell sensitivity to parvovirus attack. These data suggest an interconnection between T3 signalling and NS cytotoxic pathways.

Protein species of the parvovirus minute virus of mice strain MVMp: involvement of phosphorylated VP-2 subtypes in viral morphogenesis

The pattern of induced protein species of the prototype strain of the parvovirus minute virus of mice was determined in permissive A9 mouse fibroblast cells by high-resolution two-dimensional gel electrophoresis. Identities of the viral proteins in the gels were assigned by probing two-dimensional blots with antisera raised against either purified capsids (recognizing VP-1 and VP-2) or specific coding regions of the nonstructural proteins (NS-1 and NS-2) expressed as beta-galactosidase fusion products in bacteria. All viral proteins showed posttranslational modifications, phosphate being a common substituent. The NS-1 protein migrated as a basic polypeptide in the pI range of 7.4 to 7.8 with multiple stages of modification and as a likely minor but hyperphosphorylated component in the neutral region of the gel. The NS-2 isoforms were resolved at a pI value close to 5.5 as three groups of unevenly phosphorylated polypeptides, each composed of at least two protein species. Both VP-1 and VP-2 structural polypeptides were induced as heterogeneous phosphoproteins. The major VP-2 protein could be resolved in the form of a consistent pattern of three abundant (a to c), two intermediate (d and e), and one meager (f) neutral isoelectric focusing species or subtypes. This posttranslational modification precedes and is uncoupled from viral assembly, and all of the VP-2 subtypes are packaged into empty capsids at the induced stoichiometry. However, intracellular full virions harbored additional phosphorylated subtypes (g to l) and a subtle rearrangement in the whole VP-2 composition, while mature virions purified from lysed cultures lacked these subtypes, coordinately with the emergence of six neutral VP-3 subtypes. Thus, the virion coat undergoes a chemical transition entailed by genome encapsidation, in which phosphates seem to play a major role, triggering the preferential proteolytic cleavage of the more acidic VP-2 subtypes to VP-3. Parvoviruses, with small coding capacity, may regulate some morphogenetic steps, such as assembly, genome encapsidation, and maturation, by posttranslational modifications of their structural proteins.

Mutational analysis of the adeno-associated virus rep gene

The replication (rep) gene of the human parvovirus adeno-associated virus (AAV) is a pleiotropic effector of numerous viral functions and experts profound effects on cellular transformation. Of the four Rep proteins, the primarily nuclear Rep78 and Rep68 direct AAV DNA replication, trans activation of the capsid (cap) gene promoter, and inhibition of cellular proliferation mediated by various oncogenes. In an initial attempt to define functional domains in Rep78, we have constructed a comprehensive set of XhoI linker insertion and deletion mutations in the rep gene. Each of the mutant genes has been expressed in cell culture and assayed for the following functions: (i) nuclear localization, (ii) AAV DNA replication, (iii) trans activation of the AAV capsid gene transcription promoter, and (iv) suppression of cellular transformation mediated by the adenovirus E1a and an activated ras oncogene pair. Modest disruptions in the normal conformation of Rep78 inactivated its AAV DNA replication function and trans activation of the cap gene promoter. Linker insertion mutations in the amino-terminal one-third of the protein inactivated Rep78's ability to suppress oncogene-mediated cellular transformation. The transformation suppression domains are not limited to the amino-terminal regions, however, since deletions throughout the protein altered its suppression capabilities. A putative nuclear localization signal that is essential for each of the above functions was found in the Rep proteins. These results provide a preliminary screening of the functional domains in the AAV Rep proteins and pave the way for more subtle mutational analysis.

Transformation-dependent expression of interleukin genes delivered by a recombinant parvovirus

The prototype strain of minute virus of mice [MVM(p)] is an autonomous parvovirus with a tropism for cells expressing a neoplastically transformed phenotype. To generate gene transfer vectors for tumor-specific gene expression, human interleukin-2 (IL-2) and murine interleukin-4 (IL-4) genes were cloned under the control of the p38 late promoter of MVM(p). Upon transfection into permissive cells, the recombinant MVMIL2 or MVMIL4 DNA was excised, amplified, and, in the presence of a helper plasmid, packaged into recombinant viral particles. The recombinant viruses were able to transfer fully functional IL-2 and IL-4 genes to permissive target cells and retained the oncotropic host range properties of the parental virus. Following infection with MVMIL2, nontransformed fibroblasts of rodent (FR3T3) or human (MRC-5) origin produced minimal IL-2 compared with the high levels of IL-2 production observed in their transformed derivatives (FREJ4 and MRC-5V1).

Restriction of porcine parvovirus replication in nonpermissive cells

Swine testicle (ST) cells and Madin-Darby canine kidney (MDCK) cells differ in their ability to support replication of porcine parvovirus (PPV). Viral replication events in ST cells, a permissive cell type, and MDCK cells, a nonpermissive cell type, were compared in an attempt to elucidate putative mechanisms of restrictive virus replication. Radiolabeled PPV bound to the cell surface of both cell types equally well and the binding was shown to be PPV specific, indicating that the restriction was not at the cell surface level. In contrast, profound differences in intracellular events in PPV replication were observed between these two cell types. Synthesis of viral DNA was limited in MDCK cells in that the percentage of cells with replicative-form DNA as determined by strand-specific probe in situ hybridization was approximately 100-fold lower in MDCK cells than in ST cells at the same multiplicity of infection. Northern (RNA) blot analysis, using oligonucleotide probes derived from both structural and nonstructural protein-coding regions of the PPV genome, revealed four PPV mRNA transcripts from infected ST cells. Comparatively, RNA species from the structural protein coding region were actively transcribed in MDCK cells, but synthesis of RNA species from the nonstructural protein coding region was negligible. Immunoprecipitation of viral polypeptides revealed the three characteristic structural polypeptides, VP1, VP2, and VP3, along with the nonstructural polypeptide, NS-1 from ST cells. In contrast, neither viral structural or nonstructural polypeptides nor progeny virions were produced from MDCK cells. The data suggest that mechanisms controlling permissiveness of cells to PPV infection are associated with the level of viral DNA replication, RNA transcription, and viral antigen expression but not absorption to the cell surface.

Influence of adeno-associated virus on adherence and growth properties of normal cells

It has been shown previously that infection of newly established cell cultures from malignant human tumors with adeno-associated parvovirus type 2 or type 5 results in growth arrest and cell death. Here we report that the additionally observed antiproliferative effect on diploid human fibroblasts is transient and is connected to a reduced number of cells in S phase. Progression through the cell cycle is disturbed either in G0/G1 or at the G1/S boundary, but an additional arrest in G2 cannot be excluded. DNA synthesis and cell proliferation are resumed when cells are recultured after loosening of cell-matrix adhesions by trypsin treatment. In contrast, they are not resumed by solely providing growth factors via higher amounts of fetal calf serum. The results suggest that cell adherence is altered in adeno-associated parvovirus-infected human embryo fibroblasts.

Initiation of transcription from the minute virus of mice P4 promoter is stimulated in rat cells expressing a c-Ha-ras oncogene

Transformation of FR3T3 rat fibroblasts by a c-Ha-ras oncogene but not by bovine papillomavirus type 1 is associated with an increase in the abundance of mRNAs from prototype strain MVMp of infecting minute virus of mice, an oncosuppressive parvovirus. This differential parvovirus gene expression correlates with the reported sensitization of ras- but not bovine papillomavirus type 1-transformed cells to the killing effect of MVMp (N. Salomé, B. van Hille, N. Duponchel, G. Meneguzzi, F. Cuzin, J. Rommelaere, and J. Cornelis, Oncogene 5:123-130, 1990). Experiments were performed to determine at which level parvovirus expression is up-regulated in ras transformants. An MVMp "attenuation" sequence responsible for the premature arrest of RNA elongation was either placed or not placed in front of the chloramphenicol acetyltransferase gene and brought under the control of MVMp early promoter P4. Although the MVMp attenuator reduced P4-driven chloramphenicol acetyltransferase expression, the extent of attenuation was similar in normal and ras-transformed cells. Moreover, the analysis of P4-directed viral RNAs in MVMp-infected cultures by RNase protection and nuclear run-on assays also revealed a transcription elongation block of a similar amplitude in both types of cells. In addition, the stabilities of the three major parvoviral mRNAs did not vary significantly between normal and ras-transformed cells. Hence, it is concluded that the ras-induced increase in the accumulation of parvoviral mRNAs is mainly controlled at the level of transcription. Consistently, the TATA motif of the P4 promoter proved to have a differential photoreactivity when tested by in vivo UV footprinting assays in ras-transformed versus normal cells.

Nonstructural protein NS2 of parvovirus H-1 is required for efficient viral protein synthesis and virus production in rat cells in vivo and in vitro

We generated a mutation in the gene for the nonstructural protein NS2 of parvovirus H-1 in which the highly conserved dinucleotide AG at the 3' splice acceptor site of NS2 intron 1 was mutated to CG. The mutation does not change the amino acid sequence for NS1. The splice acceptor (SA) mutant gene was introduced into the H-1 virus (H-1SA) and an infectious clone of LuIII (pLuH1SA). The R2 transcripts encoding NS2 were absent by both Northern blot and primer extension analysis in the LuH1SA or H-1SA virus-infected cells and the NS2 protein was undetectable in the infected cell lysate by immunoprecipitation. These NS2 null mutant viruses were capable of lytic growth in cell lines that were derived from human, hamster, and dog, but they produced lower virus titers than wild-type H-1. The H-1SA virus nonproductively infected Rat2 rat fibroblasts and transformed Rat2 cell lines. Analysis of synchronized infections of rat fibroblasts demonstrated that H-1SA viral duplex replicative form DNA replication was reduced and that single-stranded progeny DNA was deficient compared to wild-type H-1. In addition, H-1SA viral protein synthesis was about 10% of wild-type virus and virions were not detectable in rat fibroblasts. However, H-1SA mRNAs R1 and R3 accumulated to wild-type levels. NS2 was also required for productive infection in newborn rats but not in newborn hamsters. These results indicate that NS2 plays an important role in the regulation of viral protein synthesis in rat cells in vivo and in vitro.

Parvovirus NS1 stimulates P4 expression by interaction with the terminal repeats and through DNA amplification

Parvovirus protein NS1 is required for replication of viral DNA and plays a role in the regulation of viral gene expression. NS1 trans-activates the P38 promoter for capsid protein synthesis and has variable effects on other promoters. In this study, we examined the effects of NS1 on the regulation of its own promoter, P4. A number of plasmid constructions were made with the P4 promoter fused to reporter genes. The effects of NS1 on expression from the P4 promoter differed depending on the construction. Plasmids containing viral sequences which could not replicate showed a decrease in P4 expression on cotransfection with the NS1 gene. However, plasmids having replication-proficient viral sequences showed a three- to fivefold increase in P4 expression dependent on NS1. The effect on NS1 on P4 transcription was also evaluated at the steady-state RNA level. An infectious clone of the LuIII viral genome was modified to an NS1-NS2 null mutant (pLu272) that is competent for viral DNA replication by introducing a frameshift mutation at codon 5 of the NS1 open reading frame. The P4 transcripts of pLu272 are four nucleotides longer than the wild type and can therefore be resolved from the wild type by primer extension analysis. pLu272 allows comparison of the constitutive level of steady-state RNA produced by the pLu272 P4 promoter in the absence or presence of a template replication dependent on NS1 supplied in trans. NS1 increased P4 transcripts about six- to eightfold. Expression of P4 transcripts from clones that could not amplify depended on the presence of an intact inverted terminal repeat sequence at the left end. A clone with an intact viral left end and a defective viral right end gave an NS1-dependent threefold increase in P4 expression. Destruction of terminal hairpins at both ends resulted in no significant increase in P4 expression in the presence of NS1. Thus, the positive effect of NS1 on the steady-state levels of P4 transcripts depends on the amplification of gene copy number and the integrity of the terminal repeats.

Cytotoxic infection of hematopoietic stem and committed progenitor cells by the parvovirus minute virus of mice. Propagation of an acute myelosuppression in culture

We have investigated the ability of two strains of the parvovirus minute virus of mice to impair mouse hematopoiesis in vitro. We found that the BFU-E and CFU-GM committed progenitors, CFU-Mix pluripotent progenitor, as well as the CFU-S12d, one of the most primitive hematopoietic precursors of the stem cell compartment detectable by colony technique, were similarly inhibited in their proliferative capacity by the immunosuppressive strain MVMi, but not by the prototype virus MVMp. The inhibitory effect correlated with the input of purified MVMi and was reversed by neutralizing MVM antiserum, showing that cytotoxic mechanisms underlying infectious MVMi replication and not operating in MVMp-infected cells were responsible for the reproductive death of hematopoietic precursors. In agreement with this, myeloid nonadherent cells of long-term bone marrow cultures were selectively permissive for MVMi but not for MVMp replication, as judged by viral DNA synthesis, the expression of the nonstructural cytotoxic NS-1 protein, and virus propagation in these cells. Altogether, the suppressive effects mediated by the MVMi cytotoxic infection define a wide lympho-myelotropism not previously reported for this virus. The MVM-mouse model highlights the role that unsuspected virus-hematopoietic compartment interactions may play in bone marrow failures of immunocompromised animal or human hosts.

Accumulation of MVM gene products is differentially regulated by transcription initiation, RNA processing and protein stability

The accumulation and stability of minute virus of mice (MVMp) RNA and protein as well as comparative strengths of the two viral promoters have been analyzed in highly synchronous infections of murine A9 fibroblasts. Results indicate that there is a temporal phasing of the accumulation of MVM RNA and protein: the RNA products of the P4 promoter appear prior to the products of the P38 promoter and NS1 and NS2 appear prior to the capsid proteins. Total and cytoplasmic spliced RNA accumulate similarly, although there is a lag in cytoplasmic accumulation of about 2 hr. Total RNA contains abundant unspliced R1 and R3 which are confined to the nucleus. There is no detectable difference in the ratio of the various spliced versions of each RNA species throughout infection. R2 accumulates faster and in a greater amount than R1 in both total and cytoplasmic RNA even though they are generated from the same promoter. During this same period, however, NS1 and NS2 accumulate to similar levels during 1-hr pulses. The stabilities of all MVM RNA species produced at both 9 and 12 hr postrelease are equivalent. Late in infection R3 accumulates faster and in greater amounts than the combined products of the P4 promoter, by approximately two- to threefold. This increase can be accounted for by an increase in the frequency of initiation from the P38 promoter, relative to P4, as assayed by nuclear run-on experiments. Therefore, the steady-state levels of the individual viral proteins during infection is controlled by specific regulation at the level of the initiation of transcription, RNA processing, and protein stability.