Structural determinants of insert retention of poliovirus expression vectors with recombinant IRES elements

Development of a Honey Bee RNA Virus Vector Based on the Genome of a Deformed Wing Virus

We developed a honey bee RNA-virus vector based on the genome of a picorna-like Deformed wing virus (DWV), the main viral pathogen of the honey bee (Apis mellifera). To test the potential of DWV to be utilized as a vector, the 717 nt sequence coding for the enhanced green fluorescent protein (eGFP), flanked by the peptides targeted by viral protease, was inserted into an infectious cDNA clone of DWV in-frame between the leader protein and the virus structural protein VP2 genes. The in vitro RNA transcripts from egfp-tagged DWV cDNA clones were infectious when injected into honey bee pupae. Stable DWV particles containing genomic RNA of the recovered DWV with egfp inserts were produced, as evidenced by cesium chloride density gradient centrifugation. These particles were infectious to honey bee pupae when injected intra-abdominally. Fluorescent microscopy showed GFP expression in the infected cells and Western blot analysis demonstrated accumulation of free eGFP rather than its fusions with DWV leader protein (LP) and/or viral protein (VP) 2. Analysis of the progeny egfp-tagged DWV showed gradual accumulation of genome deletions for egfp, providing estimates for the rate of loss of a non-essential gene an insect RNA virus genome during natural infection.

Genetically stable poliovirus vectors activate dendritic cells and prime antitumor CD8 T cell immunity

Viruses naturally engage innate immunity, induce antigen presentation, and mediate CD8 T cell priming against foreign antigens. Polioviruses can provide a context optimal for generating antigen-specific CD8 T cells, as they have natural tropism for dendritic cells, preeminent inducers of CD8 T cell immunity; elicit Th1-promoting inflammation; and lack interference with innate or adaptive immunity. However, notorious genetic instability and underlying neuropathogenicity has hampered poliovirus-based vector applications. Here we devised a strategy based on the polio:rhinovirus chimera PVSRIPO, devoid of viral neuropathogenicity after intracerebral inoculation in human subjects, for stable expression of exogenous antigens. PVSRIPO vectors infect, activate, and induce epitope presentation in DCs in vitro; they recruit and activate DCs with Th1-dominant cytokine profiles at the injection site in vivo. They efficiently prime tumor antigen-specific CD8 T cells in vivo, induce CD8 T cell migration to the tumor site, delay tumor growth and enhance survival in murine tumor models.

Experimental PVSRIPO oncolytic virus therapy of glioblastoma has shown long-term efficacy in a subset of patients. Here the authors engineer the virus to enable incorporation of tumor-specific antigens, and show proof-of-principle evidence that this modification increases anti-tumor immunity and extends survival in mice.

Emergency Services of Viral RNAs: Repair and Remodeling

Reproduction of RNA viruses is typically error-prone due to the infidelity of their replicative machinery and the usual lack of proofreading mechanisms. The error rates may be close to those that kill the virus. Consequently, populations of RNA viruses are represented by heterogeneous sets of genomes with various levels of fitness. This is especially consequential when viruses encounter various bottlenecks and new infections are initiated by a single or few deviating genomes. Nevertheless, RNA viruses are able to maintain their identity by conservation of major functional elements. This conservatism stems from genetic robustness or mutational tolerance, which is largely due to the functional degeneracy of many protein and RNA elements as well as to negative selection. Another relevant mechanism is the capacity to restore fitness after genetic damages, also based on replicative infidelity. Conversely, error-prone replication is a major tool that ensures viral evolvability. The potential for changes in debilitated genomes is much higher in small populations, because in the absence of stronger competitors low-fit genomes have a choice of various trajectories to wander along fitness landscapes. Thus, low-fit populations are inherently unstable, and it may be said that to run ahead it is useful to stumble. In this report, focusing on picornaviruses and also considering data from other RNA viruses, we review the biological relevance and mechanisms of various alterations of viral RNA genomes as well as pathways and mechanisms of rehabilitation after loss of fitness. The relationships among mutational robustness, resilience, and evolvability of viral RNA genomes are discussed.

Therapeutic Use of Native and Recombinant Enteroviruses

Research on human enteroviruses has resulted in the identification of more than 100 enterovirus types, which use more than 10 protein receptors and/or attachment factors required in cell binding and initiation of the replication cycle. Many of these “viral” receptors are overexpressed in cancer cells. Receptor binding and the ability to replicate in specific target cells define the tropism and pathogenesis of enterovirus types, because cellular infection often results in cytolytic response, i.e., disruption of the cells. Viral tropism and cytolytic properties thus make native enteroviruses prime candidates for oncolytic virotherapy. Copy DNA cloning and modification of enterovirus genomes have resulted in the generation of enterovirus vectors with properties that are useful in therapy or in vaccine trials where foreign antigenic epitopes are expressed from or on the surface of the vector virus. The small genome size and compact particle structure, however, set limits to enterovirus genome modifications. This review focuses on the therapeutic use of native and recombinant enteroviruses and the methods that have been applied to modify enterovirus genomes for therapy.

Genome characterisation of two Ljungan virus isolates from wild bank voles (Myodes glareolus) in Sweden

Ljungan virus (LV) (family Picornaviridae, genus Parechovirus) is a suspected zoonotic pathogen with associations to human disease in Sweden. LV is a single-stranded RNA virus with a positive sense genome. There are five published Ljungan virus strains, three isolated from Sweden and two from America, and are classified into four genotypes. A further two strains described here were isolated from wild bank voles (Myodes glareolus) caught in Västmanlands county, Sweden in 1994. These strains were sequenced using next generation pyrosequencing technology on the GS454flx platform. Genetic and phylogenetic analysis of the obtained genomes confirms isolates LV340 and LV342 as two new putative members of genotype 2 along with LV145SL, with 92% and 99% nucleotide identities respectively. Only two codon sites throughout the entire genome were identified as undergoing positive selection, both situated within the VP3 structural region, in or near to major antigenic sites. Whilst these two strains do not constitute new genotypes they provide evidence, though weakly supported, which suggests the evolution of Ljungan viruses to be relatively slow, a characteristic unlike other picornaviruses. Additional genomic sequences are urgently required for Ljungan virus strains, particularly from different locations or hosts, to fully understand the evolutionary and epidemiological properties of this potentially zoonotic virus.

Introducing recombinant picornaviral genomes into cells

Picornarviruses comprise a large group of small, nonenveloped, positive-sense, single-stranded RNA viruses. The picornavirus life cycle is usually rapid and exclusively cytoplasmic, without integration into the host cell’s genome or a nuclear phase. Due to their biology and genetic constraints, the utility of picornaviruses for general gene delivery purposes is limited. However, they may prove useful as vaccine vectors. Furthermore, picornavirus-driven expression of various reporter genes or foreign RNA elements is of interest to the picornavirus molecular virologist. Introduction of recombinant picornaviral genomes into the cell relies on the historic observation that the isolated virion RNA is infectious. This property extends to in-vitro-transcribed RNA as long as the authentic viral 5' end is preserved. That said, up to two additional 5'-terminal guanine residues (remnants from the T7 RNA polymerase-based in vitro transcription), although reducing infectivity, can be tolerated. Additional nucleotides at the 3' end are of far less consequence. Thus, any unique restriction site downstream from the poly(A) sequence (preferably as close as possible downstream) can be used to linearize the plasmid containing the viral genome before in vitro transcription.

Molecular characterization of a novel Ljungan virus (Parechovirus; Picornaviridae) reveals a fourth genotype and indicates ancestral recombination

Ljungan virus (LV) was discovered 20 years ago in Swedish bank voles (Myodes glareolus, previously referred to as Clethrionomys glareolus) during the search for an infectious agent causing lethal myocarditis in young athletes. To date, the genomes of four LV isolates, including the prototype 87-012 strain, have been characterized. Three of these LV strains were isolated from bank voles trapped in Sweden. Sequence analysis of an American virus (M1146), isolated from a montane vole (Microtus montanus) in western USA, indicates that this strain represents a genotype that is different from the Swedish strains. Here, we present genomic analyses of a fifth LV strain (64-7855) isolated from a southern red-backed vole (Myodes gapperi) trapped during arbovirus studies in New York state in the north-eastern USA in the 1960s. Sequence analysis of the 64-7855 genome showed an LV-like genome organization and sequence similarity to other LV strains. Genetic and phylogenetic analyses of the evolutionary relationship between the 64-7855 strain and other viruses within the family Picornaviridae, including previously published LV strains, demonstrated that the 64-7855 strain constitutes a new genotype within the LV species. Analyses also showed that different regions of the 64-7855 genome have different phylogenetic relationships with other LV strains, indicating that previous recombination events have been involved in the evolution of this virus.

Evaluation of IRES-mediated, cell-type-specific cytotoxicity of poliovirus using a colorimetric cell proliferation assay

PVS-RIPO is a recombinant oncolytic poliovirus designed for clinical application to target CD155 expressing malignant gliomas and other malignant diseases. PVS-RIPO does not replicate in healthy neurons and is therefore non-pathogenic in rodent and non-human primate models of poliomyelitis. A tetrazolium salt dye-based cellular assay was developed and qualified to define the cytotoxicity of virus preparations on susceptible cells and to explore the target cell specificity of PVS-RIPO. In this assay, PVS-RIPO inhibited proliferation of U87-MG astrocytoma cells in a dose-dependent manner. However, HEK293 cells were much less susceptible to cell killing by PVS-RIPO. In contrast, the Sabin type 1 live attenuated poliovirus vaccine strain (PV(1)S) was cytotoxic to both HEK293 and U87-MG cells. The correlation between expression of CD155 and cytotoxicity was also explored using six different cell lines. There was little or no expression of CD155 and PVS-RIPO-induced cytotoxicity in Jurkat and Daudi cells. HEK293 was the only cell line tested that showed CD155 expression and resistance to PVS-RIPO cytotoxicity. The results indicate that differential cytotoxicity measured by the colorimetric assay can be used to evaluate the cytotoxicity and cell-type specificity of recombinant strains of poliovirus and to demonstrate lot to lot consistency during the manufacture of viruses intended for clinical use.

A baculovirus-expressed dicistrovirus that is infectious to aphids

Detailed investigation of virus replication is facilitated by the construction of a full-length infectious clone of the viral genome. To date, this has not been achieved for members of the family Dicistroviridae. Here we demonstrate the construction of a baculovirus that expresses a dicistrovirus that is infectious in its natural host. We inserted a full-length cDNA clone of the genomic RNA of the dicistrovirus Rhopalosiphum padi virus (RhPV) into a baculovirus expression vector. Virus particles containing RhPV RNA accumulated in the nuclei of baculovirus-infected Sf21 cells expressing the recombinant RhPV clone. These virus particles were infectious in R. padi, a ubiquitous aphid vector of major cereal viruses. The recombinant virus was transmitted efficiently between aphids, despite the presence of 119 and 210 vector-derived bases that were stably maintained at the 5' and 3' ends, respectively, of the RhPV genome. The maintenance of such a nonviral sequence was surprising considering that most RNA viruses tolerate few nonviral bases beyond their natural termini. The use of a baculovirus to express a small RNA virus opens avenues for investigating replication of dicistroviruses and may allow large-scale production of these viruses for use as biopesticides.

Genetic determinants of cell type-specific poliovirus propagation in HEK 293 cells

The ability of poliovirus to propagate in neuronal cells can be reduced by introducing appropriate nucleotide substitutions into the viral genome. Specific mutations scattered throughout the poliovirus genome yielded the live attenuated vaccine strains of poliovirus. Neuron-specific propagation deficits of the Sabin strains are partially encrypted within a confined region of the internal ribosomal entry site (IRES), which carries attenuating point mutations in all three serotypes. Recently, high levels of neurovirulence attenuation were achieved with genetically engineered polioviruses containing heterologous IRES elements. This is exemplified with poliovirus recombinants replicating under control of a human rhinovirus type 2 (HRV2) IRES element. We have carried out experiments delineating the genetic basis for neuronal IRES function. Neuronal dysfunction of the HRV2 IRES is determined mainly by IRES stem-loop domain V, the locus for attenuating point mutations within the Sabin strains. Neuronal incompetence associated with HRV2 IRES domain V is substantially more pronounced than that observed with the attenuating IRES point mutation of the Sabin serotype 1 vaccine strain. Mix-and-match recombination of polio and HRV2 IRES domain V suggests that the attenuation phenotype correlates with overall structural features rather than primary sequence. Our experiments have identified HEK 293 cells as a novel system for the study of neuron-specific replication phenotypes of poliovirus. This cell line, originally derived from embryonic human kidney, has recently been described to display neuronal characteristics. We report propagation properties in HEK 293 cells for poliovirus recombinants with attenuated neurovirulence in experimental animals that corroborate this observation.

The polypyrimidine tract binding protein is required for efficient picornavirus gene expression and propagation

Mammalian host factors required for efficient viral gene expression and propagation have been often recalcitrant to genetic analysis. A case in point is the function of cellular factors that trans-activate internal ribosomal entry site (IRES)-driven translation, which is operative in many positive-stranded RNA viruses, including all picornaviruses. These IRES trans-acting factors have been elegantly studied in vitro, but their in vivo importance for viral gene expression and propagation has not been widely confirmed experimentally. Here we use RNA interference to deplete mammalian cells of one such factor, the polypyrimidine tract binding protein, and test its requirement in picornavirus gene expression and propagation. Depletion of the polypyrimidine tract binding protein resulted in a marked delay of particle propagation and significantly decreased synthesis and accumulation of viral proteins of poliovirus and encephalomyocarditis virus. These effects could be partially restored by expression of an RNA interference-resistant exogenous polypyrimidine tract binding protein. These data indicate a critical role for the polypyrimidine tract binding protein in picornavirus gene expression and strongly suggest a requirement for efficient IRES-dependent translation.

Treatment of intracerebral neoplasia and neoplastic meningitis with regional delivery of oncolytic recombinant poliovirus

PURPOSE

Spread to the central nervous system (CNS) and the leptomeninges is a frequent complication of systemic cancers that is associated with serious morbidity and high mortality. We have evaluated a novel therapeutic approach against CNS complications of breast cancer based on the human neuropathogen poliovirus (PV).

EXPERIMENTAL DESIGN

Susceptibility to PV infection and ensuing rapid cell lysis is mediated by the cellular receptor of PV, CD155. We evaluated CD155 expression in several human breast tumor tissue specimens and cultured breast cancer cell lines. In addition, we tested an oncolytic PV recombinant for efficacy in xenotransplantation models of neoplastic meningitis and cerebral metastasis secondary to breast cancer.

RESULTS

We observed that breast cancer tissues and cell lines derived thereof express CD155 at levels mediating exquisite sensitivity toward PV-induced oncolysis in the latter. An association with the immunoglobulin superfamily molecule CD155 renders breast cancer a likely target for oncolytic PV recombinants. This assumption was confirmed in xenotransplantation models for neoplastic meningitis or solitary cerebral metastasis, where local virus treatment dramatically improved survival.

CONCLUSIONS

Our findings suggest oncolytic PV recombinants as a viable treatment option for CNS complications of breast cancer.