Absence of internal ribosome entry site-mediated tissue specificity in the translation of a bicistronic transgene

Long-Peptide Cross-Presentation by Human Dendritic Cells Occurs in Vacuoles by Peptide Exchange on Nascent MHC Class I Molecules

Cross-presentation enables dendritic cells to present on their MHC class I molecules antigenic peptides derived from exogenous material, through a mechanism that remains partly unclear. It is particularly efficient with long peptides, which are used in cancer vaccines. We studied the mechanism of long-peptide cross-presentation using human dendritic cells and specific CTL clones against melanoma Ags gp100 and Melan-A/MART1. We found that cross-presentation of those long peptides does not depend on the proteasome or the transporter associated with Ag processing, and therefore follows a vacuolar pathway. We also observed that it makes use of newly synthesized MHC class I molecules, through peptide exchange in vesicles distinct from the endoplasmic reticulum and classical secretory pathway, in an SEC22b- and CD74-independent manner. Our results indicate a nonclassical secretion pathway followed by nascent HLA-I molecules that are used for cross-presentation of those long melanoma peptides in the vacuolar pathway. Our results may have implications for the development of vaccines based on long peptides.

Inefficient type I interferon-mediated antiviral protection of primary mouse neurons is associated with the lack of apolipoprotein l9 expression

We examined the antiviral response promoted by type I interferons (IFN) in primary mouse neurons. IFN treatment of neuron cultures strongly upregulated the transcription of IFN-stimulated genes but conferred a surprisingly low resistance to infection by neurotropic viruses such as Theiler's murine encephalomyelitis virus (TMEV) or vesicular stomatitis virus (VSV). Response of primary mouse neurons to IFN treatment was heterogeneous, as many neurons failed to express the typical IFN response marker Mx1 after IFN treatment. This heterogeneous response of primary neurons correlated with a low level of basal expression of IFN-stimulated genes, such as Stat1, that are involved in signal transduction of the IFN response. In addition, transcriptomic analysis identified 15 IFN-responsive genes whose expression was low in IFN-treated primary neurons compared to that of primary fibroblasts derived from the same mice (Dhx58, Gvin1, Sp100, Ifi203 isoforms 1 and 2, Irgm2, Lgals3bp, Ifi205, Apol9b, Ifi204, Ifi202b, Tor3a, Slfn2, Ifi35, Lgals9). Among these genes, the gene coding for apolipoprotein L9b (Apol9b) displayed antiviral activity against Theiler's virus when overexpressed in L929 cells or in primary neurons. Accordingly, knocking down Apol9b expression in L929 cells increased viral replication. Therefore, we identified a new antiviral protein induced by interferon, ApoL9b, whose lack of expression in primary neurons likely contributes to the high sensitivity of these cells to viral infection.

IMPORTANCE

The type I interferon (IFN) response is an innate immune defense mechanism that is critical to contain viral infection in the host until an adaptive immune response can be mounted. Neurons are a paradigm for postmitotic, highly differentiated cells. Our data show that primary mouse neurons that are exposed to type I interferon remain surprisingly susceptible to viral infection. On one hand, the low level of basal expression of some factors in neurons might prevent a rapid response of these cells. On the other hand, some genes that are typically activated by type I interferon in other cell types are expressed at much lower levels in neurons. Among these genes is the gene encoding apolipoprotein L9, a protein that proved to have antiviral activity against the neurotropic Theiler's murine encephalomyelitis virus. Our data suggest important functional differences in the IFN response mounted by specific cell populations.

IFN-ε is constitutively expressed by cells of the reproductive tract and is inefficiently secreted by fibroblasts and cell lines

Type-I interferons (IFNs) form a large family of cytokines that primarily act to control the early development of viral infections. Typical type-I IFN genes, such as those encoding IFN-α or IFN-β are upregulated by viral infection in many cell types. In contrast, the gene encoding IFN-ε was reported to be constitutively expressed by cells of the female reproductive tract and to contribute to the protection against vaginal infections with herpes simplex virus 2 and Chlamydia muridarum. Our data confirm the lack of induction of IFN-ε expression after viral infection and the constitutive expression of IFN-ε by cells of the female but also of the male reproductive organs. Interestingly, when expressed from transfected expression plasmids in 293T, HeLa or Neuro2A cells, the mouse and human IFN-ε precursors were inefficiently processed and secretion of IFN-ε was minimal. Analysis of chimeric constructs produced between IFN-ε and limitin (IFN-ζ) showed that both the signal peptide and the mature moiety of IFN-ε contribute to poor processing of the precursor. Immunofluorescent detection of FLAG-tagged IFN-ε in transfected cells suggested that IFN-ε and chimeric proteins were defective for progression through the secretory pathway. IFN-ε did not, however, act intracellularly and impart an antiviral state to producing cells. Given the constitutive expression of IFN-ε in specialized cells and the poor processing of IFN-ε precursor in fibroblasts and cell lines, we hypothesize that IFN-ε secretion may require a co-factor specifically expressed in cells of the reproductive organs, that might secure the system against aberrant release of this IFN.

Inhibition of mRNA export and dimerization of interferon regulatory factor 3 by Theiler's virus leader protein

Theiler's murine encephalomyelitis virus (TMEV or Theiler's virus) is a neurotropic picornavirus that can persist lifelong in the central nervous system of infected mice, causing a chronic inflammatory demyelinating disease. The leader (L) protein of the virus is an important determinant of viral persistence and has been shown to inhibit transcription of type I interferon (IFN) genes and to cause nucleocytoplasmic redistribution of host proteins. In this study, it was shown that expression of the L protein shuts off synthesis of the reporter proteins green fluorescent protein and firefly luciferase, suggesting that it induces a global shut-off of host protein expression. The L protein did not inhibit transcription or translation of the reporter genes, but blocked cellular mRNA export from the nucleus. This activity correlated with the phosphorylation of nucleoporin 98 (Nup98), an essential component of the nuclear pore complex. In contrast, the data confirmed that the L protein inhibited IFN expression at the transcriptional level, and showed that transcription of other chemokine or cytokine genes was affected by the L protein. This transcriptional inhibition correlated with inhibition of interferon regulatory factor 3 (IRF-3) dimerization. Whether inhibition of IRF-3 dimerization and dysfunction of the nuclear pore complex are related phenomena remains an open question. In vivo, IFN antagonism appears to be an important role of the L protein early in infection, as a virus bearing a mutation in the zinc finger of the L protein replicated as efficiently as the wild-type virus in type I IFN receptor-deficient mice, but had impaired fitness in IFN-competent mice.

Attenuation of rabies virus replication and virulence by picornavirus internal ribosome entry site elements

Gene expression of nonsegmented negative-strand RNA viruses is regulated at the transcriptional level and relies on the canonical 5'-end-dependent translation of capped viral mRNAs. Here, we have used internal ribosome entry sites (IRES) from picornaviruses to control the expression level of the phosphoprotein P of the neurotropic rabies virus (RV; Rhabdoviridae), which is critically required for both viral replication and escape from the host interferon response. In a dual luciferase reporter RV, the IRES elements of poliovirus (PV) and human rhinovirus type 2 (HRV2) were active in a variety of cell lines from different host species. While a generally lower activity of the HRV2 IRES was apparent compared to the PV IRES, specific deficits of the HRV2 IRES in neuronal cell lines were not observed. Recombinant RVs expressing P exclusively from a bicistronic nucleoprotein (N)-IRES-P mRNA showed IRES-specific reduction of replication in cell culture and in neurons of organotypic brain slice cultures, an increased activation of the beta interferon (IFN-beta) promoter, and increased sensitivity to IFN. Intracerebral infection revealed a complete loss of virulence of both PV- and HRV2 IRES-controlled RV for wild-type mice and for transgenic mice lacking a functional IFN-alpha receptor (IFNAR(-/-)). The virulence of HRV2 IRES-controlled RV was most severely attenuated and could be demonstrated only in newborn IFNAR(-/-) mice. Translational control of individual genes is a promising strategy to attenuate replication and virulence of live nonsegmented negative-strand RNA viruses and vectors and to study the function of IRES elements in detail.

A subgenomic segment of Theiler's murine encephalomyelitis virus RNA causes demyelination

The DA strain of Theiler's murine encephalomyelitis virus (TMEV) causes a persistent central nervous system (CNS) infection of mice with a restricted virus gene expression and induces an inflammatory demyelinating disease that is thought to be immune mediated and a model of multiple sclerosis (MS). The relative contribution of virus vis-à-vis the immune system in the pathogenesis of DA-induced white matter disease remains unclear, as is also true in MS. To clarify the pathogenesis of DA-induced demyelination, we used Cre/loxP technology to generate a transgenic mouse that has tamoxifen (Tm)-inducible expression of a subgenomic segment of DA RNA in oligodendrocytes and Schwann cells. Tm-treated young transgenic mice developed progressive weakness leading to death, with abnormalities of oligodendrocytes and Schwann cells and demyelination, but without inflammation, demonstrating that DA virus can play a direct pathogenic role in demyelination. Tm treatment of mice at a later age resulted in milder disease, with evidence of peripheral nerve remyelination and focal fur depigmentation; surviving weak mice had persistent expression of the recombined transgene in the CNS, suggesting that the DA subgenomic segment can cause cellular dysfunction but not death, possibly similar to the situation seen during DA virus persistence. These studies demonstrate that DA RNA or a DA protein(s) is toxic to myelin-synthesizing cells. This Cre/loxP transgenic system allows for spatially and temporally controlled expression of the viral transgene and is valuable for clarifying nonimmune (and immune) mechanisms of demyelination induced by TMEV as well as other viruses.

Expression of Hoxa2 in cells entering chondrogenesis impairs overall cartilage development

Vertebrate Hox genes act as developmental architects by patterning embryonic structures like axial skeletal elements, limbs, brainstem territories, or neural crest derivatives. While active during the patterning steps of development, these genes turn out to be down-regulated in specific differentiation programs like that leading to chondrogenesis. To investigate why chondrocyte differentiation is correlated to the silencing of a Hox gene, we generated transgenic mice allowing Cre-mediated conditional misexpression of Hoxa2 and induced this gene in Collagen 2 alpha 1-expressing cells committed to enter chondrogenesis. Persistent Hoxa2 expression in chondrogenic cells resulted in overall chondrodysplasia with delayed cartilage hypertrophy, mineralization, and ossification but without proliferation defects. The absence of skeletal patterning anomaly and the regular migration of precursor cells indicated that the condensation step of chondrogenesis was normal. In contrast, closer examination at the differentiation step showed severely impaired chondrocyte differentiation. In addition, this inhibition affected structures independently of their embryonic origin. In conclusion, for the first time here, by a cell-type specific misexpression, we precisely uncoupled the patterning function of Hoxa2 from its involvement in regulating differentiation programs per se and demonstrate that Hoxa2 displays an anti-chondrogenic activity that is distinct from its patterning function.

La Crosse bunyavirus nonstructural protein NSs serves to suppress the type I interferon system of mammalian hosts

La Crosse virus (LACV) is a mosquito-transmitted member of the Bunyaviridae family that causes severe encephalitis in children. For the LACV nonstructural protein NSs, previous overexpression studies with mammalian cells had suggested two different functions, namely induction of apoptosis and inhibition of RNA interference (RNAi). Here, we demonstrate that mosquito cells persistently infected with LACV do not undergo apoptosis and mount a specific RNAi response. Recombinant viruses that either express (rLACV) or lack (rLACVdelNSs) the NSs gene similarly persisted and were prone to the RNAi-mediated resistance to superinfection. Furthermore, in mosquito cells overexpressed LACV NSs was unable to inhibit RNAi against Semliki Forest virus. In mammalian cells, however, the rLACVdelNSs mutant virus strongly activated the antiviral type I interferon (IFN) system, whereas rLACV as well as overexpressed NSs suppressed IFN induction. Consequently, rLACVdelNSs was attenuated in IFN-competent mouse embryo fibroblasts and animals but not in systems lacking the type I IFN receptor. In situ analyses of mouse brains demonstrated that wild-type and mutant LACV mainly infect neuronal cells and that NSs is able to suppress IFN induction in the central nervous system. Thus, our data suggest little relevance of the NSs-induced apoptosis or RNAi inhibition for growth or pathogenesis of LACV in the mammalian host and indicate that NSs has no function in the insect vector. Since deletion of the viral NSs gene can be fully complemented by inactivation of the host's IFN system, we propose that the major biological function of NSs is suppression of the mammalian innate immune response.

A glassy-winged sharpshooter cell line supports replication of Rhopalosiphum padi virus (Dicistroviridae)

Rhopalosiphum padi virus (RhPV) (family Dicistroviridae; genus Cripavirus) is an icosahedral aphid virus with a 10kb positive-sense RNA genome. To study the molecular biology of RhPV, identification of a cell line that supports replication of the virus is essential. We screened nine cell lines derived from species within the Lepidoptera, Diptera and Hemiptera for susceptibility to RhPV following RNA transfection. We observed cytopathic effects (CPE) only in cell lines derived from hemipterans, specifically GWSS-Z10 cells derived from the glassy winged sharp shooter, Homalodisca coagulata and DMII-AM cells derived from the corn leaf hopper, Dalbulus maidis. Translation and appropriate processing of viral gene products, RNA replication and packaging of virus particles in the cytoplasm of GWSS-Z10 cells were examined by Western blot analysis, Northern blot hybridization and electron microscopy. Infectivity of the GWSS-Z10 cell derived-virus particles to the bird cherry-oat aphid, R. padi, was confirmed by RT-PCR and Western blot. The GWSS-Z10 cell line provides a valuable tool to investigate replication, structure and assembly of RhPV.

Cardiovirus leader proteins are functionally interchangeable and have evolved to adapt to virus replication fitness

The leader (L) proteins encoded by picornaviruses of the genus Cardiovirus [Theiler's murine encephalomyelitis virus (TMEV) and Encephalomyocarditis virus (EMCV)] are small proteins thought to exert important functions in virus-host interactions. The L protein of persistent TMEV strains was shown to be dispensable for virus replication in vitro, but crucial for long-term persistence of the virus in the central nervous system of the mouse. The phenotype of chimeric viruses generated by exchanging the L-coding regions was analysed and it was shown that the L proteins of neurovirulent and persistent TMEV strains are functionally interchangeable in vitro and in vivo, despite the fact that L is the second most divergent protein encoded by these viruses after the L* protein. The L protein encoded by EMCV and Mengo virus (an EMCV strain) shares about 35 % amino acid identity with that of TMEV. It differs from the latter by lacking a serine/threonine-rich C-terminal domain and by carrying phosphorylated residues not conserved in the TMEV L protein. Our data show that, in spite of these differences, the L protein of Mengo virus shares, with that of TMEV, the ability to inhibit the transcription of type I interferon, cytokine and chemokine genes and to interfere with nucleocytoplasmic trafficking of host-cell proteins. Interestingly, analysis of viral RNA replication of the recombinant viruses raised the hypothesis that L proteins of TMEV and EMCV diverged during evolution to adapt to the different replication fitness of these viruses.

Age-dependent poliovirus replication in the mouse central nervous system is determined by internal ribosome entry site-mediated translation

Mouse cells are not permissive for the replication of human rhinovirus type 2 (HRV2). To determine the role of the HRV2 internal ribosome entry site (IRES) in determining species specificity, a recombinant poliovirus (P1/HRV2) was constructed by substituting the poliovirus IRES with the IRES from HRV2. This recombinant virus replicated in all human and murine cell lines examined, demonstrating that the HRV2 IRES does not limit viral replication in transformed murine cells. P1/HRV2 replicated in the brain and spinal cord in neonatal but not adult mice transgenic for the poliovirus receptor, CD155. Passage of P1/HRV2 in mice led to selection of a virus that caused paralysis in neonatal mice. To determine the relationship between HRV2 IRES-mediated translation and replication of P1/HRV2 in mice, recombinant human adenoviruses were used to express bicistronic mRNAs in murine organs. The results demonstrate that the HRV2 IRES mediates translation in organs of neonatal but not adult mice. These findings show that HRV2 IRES-mediated translation is a determinant of virus replication in the murine brain and spinal cord and suggest that the IRES determines the species specificity of HRV2 infection.

Development of 2A peptide-based strategies in the design of multicistronic vectors

As science progresses in its understanding of diseases and their treatment, advances have been made in the biotechnology used in disease therapy. Most gene therapy approaches utilise viral vectors to deliver genes of interest. However, multiple proteins are often involved in disease processes and there is often a need to efficiently deliver more than one gene. Researchers have employed several strategies to accomplish this goal. When designing vectors to express multiple genes, there are several factors that need to be taken into account, including cell type, the activity of the protein of interest and subcellular protein localisation. In most cases, it is ideal for each protein to be expressed at comparable levels, a leading issue with traditional strategies for multigene expression. This review describes some of the techniques that have been used to express multiple genes, and will focus on the use of 2A peptides or 2A peptide-like sequences in the design of multicistronic vectors that may alleviate some of these issues.

Poliovirus tropism and attenuation are determined after internal ribosome entry

Poliovirus replication is limited to a few organs, including the brain and spinal cord. This restricted tropism may be a consequence of organ-specific differences in translation initiation by the poliovirus internal ribosome entry site (IRES). A C-to-U mutation at base 472 in the IRES of the Sabin type 3 poliovirus vaccine strain, known to attenuate neurovirulence, may further restrict tropism by eliminating viral replication in the CNS. To determine the relationship between IRES-mediated translation and poliovirus tropism, recombinant human adenoviruses were used to express bicistronic mRNAs in murine organs. The IRESs of poliovirus, the cardiotropic coxsackievirus B3 (CVB3), and the hepatotropic hepatitis C virus (HCV) mediate translation in many organs, including those that do not support viral replication. A translation defect associated with the Sabin type 3 IRES was observed in all organs examined. Poliovirus type 1 and recombinant polioviruses dependent on the IRES of CVB3 or HCV replicate in the CNS of mice and cause paralysis. Although the type 3 Sabin strain is an effective vaccine, polioviruses with a U at base 472 of the IRES cause paralysis in newborn mice. Tropism of wild-type and vaccine strains of poliovirus is therefore determined after internal ribosome entry.

New-generation multicistronic expression platform: pTRIDENT vectors containing size-optimized IRES elements enable homing endonuclease-based cistron swapping into lentiviral expression vectors

Capitalizing on a proven multicistronic expression vector platform we have designed novel pTRIDENT vectors which (1). enable coordinated expression of three desired transgenes, (2). are size-optimized, (3). take advantage of small highly efficient internal ribosome entry sites of the GTX or Rbm3 type, (4). harbor various sites specific for homing endonucleases facilitating promoter/multicistronic expression unit/polyadenylation site swapping as well as (5). straightforward integration into human HIV-l-based lentiviral expression vectors tailored to contain compatible homing endonucleases. Multicistronic expression profiles of novel pTRIDENT vectors engineered for different tricistronic expression configurations encoding human low-molecular-weight urokinase-type plasminogen activator (u-PA(LMW)) or Bacillus stearothermophilus-derived alpha-amylase (SAMY), human vascular endothelial growth factor (hVEGF), and human placental secreted alkaline phosphatase (SEAP) have been quantified in Chinese hamster ovary cells (CHO-K1), mouse fibroblasts (NIH/3T3), and/or human fibrosarcoma (HT-1080) cells. In addition, a pTRIDENT-derived SAMY-VEGF-SEAP expression cassette transferred into a compatible lentiviral expression vector enabled simultaneous high-level transgene expression following transduction of transgenic lentiviral particles into primary human chondrocytes.

Characterization of interferon-alpha 13, a novel constitutive murine interferon-alpha subtype

Type-I interferons (IFNs), also called IFNs-alpha/beta, are a family of cytokines induced by viral infection and are primarily involved in antiviral defense of the cells. IFNs-alpha/beta were also reported to be produced constitutively at low levels in mouse and human cells. These so-called endogenous or constitutive IFNs are thought to exert important homeostatic functions in the uninfected host. By searching IFN genes that were not repressed by the leader protein of Theiler's virus, we identified three uncharacterized IFN-alpha genes that are constitutively expressed in uninfected mouse cells, in vitro and in vivo. Two of these genes corresponded to pseudogenes and were tentatively called IFN-alpha(psi2) and IFN-alpha(psi3). IFN-alpha(psi2) transcripts are the most abundant IFN-alpha transcripts detected in several mouse organs in the absence of viral infection. The third gene codes for a new IFN-alpha subtype called IFN-alpha13, which exhibits acid-stable antiviral activity against Theiler's virus, Mengo virus, and vesicular stomatitis virus. IFN-alpha13 displays unusual characteristics, suggesting that it might have a particular function. Firstly, it is transcribed constitutively, independent of viral infection and of interferon regulatory factor-7 induction. Secondly, it contains two N-glycosylation sites, in contrast to other murine IFN-alpha subtypes that contain either one or no N-glycosylation site. In addition to the genes described here above, several other IFN-alpha subtype genes, including a new gene (IFN-alpha14), were expressed in tissues of uninfected mice. In contrast to IFN-alpha13, IFN-alpha14 was found to lack N-glycosylation and have its expression induced in response to viral infection.

Functionality of the 5'- and intergenic IRES elements of cricket paralysis virus in a range of insect cell lines, and its relationship with viral activities

Cricket paralysis virus (Dicistroviridae: Cripavirus) (CrPV) naturally has a wide range of insect hosts which is reflected in its ability to infect several cultured insect cell lines. The expression of viral gene products is controlled by two kinds of internal ribosome entry site (IRES) elements, 5' and intergenic (IG). Using seven cultured cell lines we tested the functionality of both IRES elements by transfection with bi-cistronic RNA constructs. In six of the seven cell lines, expression initiated from both IRES's was significantly higher than that from a control construct and in five of these six lines the expression from the 5'-IRES was higher than that from the IG-IRES. Permissiveness of each of the cell lines for replication of CrPV was tested by infection with purified virions and transfection with viral RNA. Only three of the cell lines were fully permissive for CrPV replication and no correlation between permissiveness and IRES activity was apparent. These results suggest that while IRES function is required for permissiveness, additional cellular and/or viral factors, involved in processing of viral products, packaging of viral particles and interacting with the cap-dependent translation machinery of host cells, are necessary for CrPV to be able to replicate in any given cell.

Expression of conditional cre recombinase in epithelial tissues of transgenic mice

SUMMARY

Keratin 18 (K18) expression is a defining characteristic of internal epithelial cells of mammals. Here, we used the K18 gene and an internal ribosome entry site (IRES) to express green fluorescent protein, human placental alkaline phosphatase, and a modified Cre recombinase in an epithelial specific pattern in transgenic mice. The K18-driven alkaline phosphatase was expressed in liver, kidney, uterine endometrium, and other internal epithelia. The enzymatic activity of the Cre recombinase-mutant estrogen receptor fusion protein was dependent on tamoxifen administration and resulted in a mosaic pattern in internal epithelia, including bladder, uterus, liver, and kidney. This conditional Cre activity in internal epithelial organs should be valuable for strategies utilizing Cre for activation of gene expression. This study demonstrates that the tissue-specific, position-independent transcriptional activity of the K18 gene is not compromised by the use of an IRES element for the expression of a second protein from a bicistronic mRNA.

Non-AUG-initiated internal translation of the L* protein of Theiler's virus and importance of this protein for viral persistence

Theiler's virus is a neurotropic murine picornavirus which, depending on the strain, causes either acute encephalitis or persistent demyelinating disease. Persistent strains of Theiler's virus (such as DA) produce an 18-kDa protein called L* from an open reading frame overlapping that encoding the viral polyprotein. Neurovirulent strains (such as GDVII) are thought not to produce the L* protein, as the alternative open reading frame of these strains starts with an ACG codon instead of an AUG codon. However, we observed that both persistent and neurovirulent strain derivatives can produce two forms of the L* protein through unusual type II internal ribosome entry site-mediated translation. A full-length 18-kDa protein can be expressed from an ACG or an AUG initiation codon, whereas an N-terminally truncated 15-kDa product can be translated from a downstream AUG initiation codon. The expression of the 18-kDa form is required for efficient persistence of DA virus derivatives in the central nervous system.

Bunyamwera bunyavirus nonstructural protein NSs counteracts the induction of alpha/beta interferon

Production of alpha/beta interferons (IFN-alpha/beta) in response to viral infection is one of the main defense mechanisms of the innate immune system. Many viruses therefore encode factors that subvert the IFN system to enhance their virulence. Bunyamwera virus (BUN) is the prototype of the Bunyaviridae family. By using reverse genetics, we previously produced a recombinant virus lacking the nonstructural protein NSs (BUNdelNSs) and showed that NSs is a nonessential gene product that contributes to viral pathogenesis. Here we demonstrate that BUNdelNSs is a strong inducer of IFN-alpha/beta, whereas in cells infected with the wild-type counterpart expressing NSs (wild-type BUN), neither IFN nor IFN mRNA could be detected. IFN induction by BUNdelNSs correlated with activation of NF-kappaB and was dependent on virally produced double-stranded RNA and on the IFN transcription factor IRF-3. Furthermore, both in cultured cells and in mice lacking a functional IFN-alpha/beta system, BUNdelNSs replicated to wild-type BUN levels, whereas in IFN-competent systems, wild-type BUN grew more efficiently. These results suggest that BUN NSs is an IFN induction antagonist that blocks the transcriptional activation of IFN-alpha/beta in order to increase the virulence of Bunyamwera virus.

Enhanced detection of Theiler's virus RNA copy equivalents in the mouse central nervous system by real-time RT-PCR

Infection of mice by low-neurovirulence Theiler's murine encephalomyelitis virus (TMEV), such as BeAn and DA viruses, provides a relevant experimental animal model for multiple sclerosis (MS). As a step toward determining the kinetics of a persistent central nervous system (CNS) infection that leads to chronic demyelination, we adapted a rapid, accurate and highly specific real-time reverse transcriptase-polymerase chain reaction (RT-PCR) assay for detection and quantitation of BeAn virus RNA copy equivalents in mouse tissues. The assay enabled detection of as few as 20-30 copies of BeAn virus RNA per microg of total RNA from infected mouse tissues and results for spinal cord revealed the same high levels of BeAn RNA as detected by Northern hybridization during the first 4 months of the persistent infection, but also was able to detect virus RNA copies as late as 1 year post-infection. Real-time RT-PCR analysis of BeAn virus RNA copy equivalents in different parts of the CNS, analyses not possible by Northern hybridization, revealed the following cline of virus persistence: spinal cord>brainstem/cerebellum>cerebrospinal fluid (CSF)>cerebral hemispheres. Systemic organs, including heart, intestine and mesenteric lymph nodes of infected mice, showed no evidence of viral persistence at 4 months post-infection.

Efficient ablation by immunotoxin-mediated cell targeting of the cell types that express human interleukin-2 receptor depending on the internal ribosome entry site

BACKGROUND

Immunotoxin-mediated cell targeting (IMCT) is a technique for conditional genetic ablation of specific cell types. IMCT provides a useful approach for generating animal models for human neurodegenerative disorders. The strategy of IMCT depends on the cytotoxic activity of antiTac-based recombinant immunotoxins that selectively target cells expressing the human interleukin-2 receptor alpha-subunit (IL-2Ralpha). Transgenic mice were generated that express the IL-2Ralpha under the control of an appropriate tissue-specific gene promoter, and they were treated with the recombinant immunotoxins resulting in the ablation of the target cell types. To restrict the expression of IL-2Ralpha transgene in the cell types of interest, it is useful to knock-in the IL-2Ralpha expression cassette into the specific marker gene locus with gene targeting. Moreover, the knock-in of the IL-2Ralpha cassette located downstream of an internal ribosome entry site (IRES) into the 3'-untranslated region of the marker gene enables IL-2Ralpha expression in the restricted cell types while preserving the intact marker gene expression. However, there is a possibility that IRES-dependent expression of the receptor may be less efficient than cap-dependent expression.

METHODS AND RESULTS

The efficiency of IRES-dependent IL-2Ralpha expression and immunotoxin responsiveness of the cells expressing the receptor were examined. The IL-2Ralpha gene fused to green fluorescence protein (GFP) (IL-2R/ GFP) was used as the target receptor. Embryonic stem cell clones were isolated that carry two types of bicistronic vectors in which the IL-2R/GFP fusion gene or the chloramphenicol acetyltransferase gene was connected upstream or downstream of IRES. The expression level of IL-2R/GFP protein in the cell clones was evaluated by GFP fluorescence detection and Western blot analysis. The IRES-dependent expression produced the same level of receptor protein as cap-dependent expression. The immunotoxin responsiveness of the cloned cells was evaluated by measuring the colony-forming efficiency in medium containing various amounts of a recombinant immunotoxin. The colony-forming efficiency of the cells expressing IL-2R/ GFP through IRES-dependent expression was reduced together with increasing immunotoxin concentration in a similar dose-dependent manner to the cells expressing the receptor through cap-dependent expression.

CONCLUSIONS

The present results indicate that it is possible to effectively use the IRES-dependent expression system for IMCT. The system permits expression of the target receptor in selective cell types by introducing the IRES-driven expression cassette into the 3'-untranslated region of the marker gene locus.

The leader protein of Theiler's virus inhibits immediate-early alpha/beta interferon production

Theiler's virus is a picornavirus responsible for a persistent infection of the central nervous system of the mouse, leading to a chronic demyelinating disease considered to be a model for multiple sclerosis. The leader (L) protein encoded by Theiler's virus is a 76-amino-acid-long peptide containing a zinc-binding motif. This motif is conserved in the L proteins of all cardioviruses, including encephalomyocarditis virus. The L protein of Theiler's virus was suggested to interfere with the alpha/beta interferon (IFN-alpha/beta) response (W.-P. Kong, G. D. Ghadge, and R. P. Roos, Proc. Natl. Acad. Sci. USA 91:1796-1800, 1994). We show that expression of the L protein indeed inhibits the production of alpha/beta interferon by infected L929 cells. The L protein specifically inhibits the transcription of the IFN-alpha4 and IFN-beta genes, which are known to be activated early in response to viral infection. Mutation of the zinc finger was sufficient to block the anti-interferon activity, outlining the importance of this motif in the L protein function. In agreement with the anti-interferon role of the L protein, a virus bearing a mutation in the zinc-binding motif was dramatically impaired in its ability to persist in the central nervous system of SJL/J mice.

Co-expression of multiple transgenes in mouse CNS: a comparison of strategies

The introduction of two transgenes into one animal is increasingly common as transgenic experiments become more sophisticated. In this study we examine two strategies for creating double transgenic founders from a single microinjection. In the first approach, two constructs, each with its own promoter element, were coinjected into the pronucleus. In the second approach, both transgenes were cloned into one vector, separated by an internal ribosomal entry site (IRES), and placed under control of a single promoter. Both strategies save time and increase the percentage of double transgenic offspring over the standard method of mating single transgenic lines. However, despite high transgene copy numbers, the bicistronic lines did not show robust expression of either protein. Copy number and protein expression correlated much better in the coinjected lines, with expression levels in one line approaching that observed in some of our best single transgenic controls. Thus we recommend coinjection of individual plasmids for the generation of multiply transgenic founders.

Influence of the Theiler's virus L* protein on macrophage infection, viral persistence, and neurovirulence

The genome of picornaviruses contains a large open reading frame (ORF) translated as a precursor polypeptide that is processed to yield all the proteins necessary for the viral life cycle. In persistent but not in neurovirulent strains of Theiler's virus, an overlapping ORF encodes an additional 18-kDa protein called L*. We confirmed previous work showing that the L* ORF of persistent strains facilitates the infection of macrophage cell lines, and we present evidence that this effect is due to the L* protein itself rather than to competition for the translation of the two overlapping ORFs. The introduction of an AUG codon to restore the L* ORF of the neurovirulent GDVII strain also enhanced the infection of macrophages, in spite of the divergent evolution of this protein. The presence or the absence of the L* AUG initiation codon had only a weak influence on the neurovirulence of the GDVII strain and on the persistence of the DA1 strain. The results obtained with DA1 in vivo contrast with the results reported previously for DAFL3, another molecular clone of the same virus strain, where the AUG-to-ACG mutation of the L* initiation codon totally blocked viral persistence (G. D. Ghadge, L. Ma, S. Sato, J. Kim, and R. P. Roos, J. Virol. 72:8605-8612, 1998). Thus, a factor that is critical for the persistence of a given clone of Theiler's virus is dispensable for the persistence of a closely related clone, indicating that different adjustments in the expression of persistence determinants occur in related viral strains.

Fibroblast growth factor 2 internal ribosome entry site (IRES) activity ex vivo and in transgenic mice reveals a stringent tissue-specific regulation

Fibroblast growth factor 2 (FGF-2) is a powerful mitogen involved in proliferation, differentiation, and survival of various cells including neurons. FGF-2 expression is translationally regulated; in particular, the FGF-2 mRNA contains an internal ribosome entry site (IRES) allowing cap-independent translation. Here, we have analyzed FGF-2 IRES tissue specificity ex vivo and in vivo by using a dual luciferase bicistronic vector. This IRES was active in most transiently transfected human and nonhuman cell types, with a higher activity in p53 -/- osteosarcoma and neuroblastoma cell lines. Transgenic mice were generated using bicistronic transgenes with FGF-2 IRES or encephalomyocarditis virus (EMCV) IRES. Measurements of luciferase activity revealed high FGF-2 IRES activity in 11-d-old embryos (E11) but not in the placenta; activity was high in the heart and brain of E16. FGF-2 IRES activity was low in most organs of the adult, but exceptionally high in the brain. Such spatiotemporal variations were not observed with the EMCV IRES. These data, demonstrating the strong tissue specificity of a mammalian IRES in vivo, suggest a pivotal role of translational IRES- dependent activation of FGF-2 expression during embryogenesis and in adult brain. FGF-2 IRES could constitute, thus, a powerful tool for gene transfer in the central nervous system.

Comparison of bicistronic retroviral vectors containing internal ribosome entry sites (IRES) using expression of human interleukin-12 (IL-12) as a readout

BACKGROUND

Many gene therapy applications require the co-ordinated delivery of more than one reading frame. We wished to systematically compare IRES in the context of a retroviral vector to determine which was the most effective for protein production and viral titre. To do this we monitored expression of IL-12, as co-ordinated expression of both p35 and p40 subunits is required for production of the active heterodimer.

METHODS

Retroviral vectors were constructed to express human IL-12 in which an IRES initiates translation of the p40 subunit, with the IRES optimally aligned to the initiation codon of p40. Vectors containing an IRES from either polio virus (PV), encephalomyocarditis virus (EMCV), foot and mouth disease virus (FMDV) or murine leukaemia virus (MLV) were compared with a vector expressing IL-12 as a single protein (Flexi-12; in which the two IL-12 subunits are linked by a peptide).

RESULTS

All vectors produced high titre virus and directed synthesis of IL-12 in target cells. The bicistronic vectors containing the IRES from EMCV and PV were the most effective in infected 3T3 cells, producing up to 40 ng IL-12/10(6) cells/48 h, similar to the 50 ng IL-12/10(6) cells/48 h obtained with Flexi-12. The IRES from PV was the most efficient in human melanoma cells.

CONCLUSIONS

Bicistronic retroviral vectors have been constructed that effectively transduce target cells and produce high levels of protein. Target cell specificity of IRES function was observed. The combination of Flexi-12 and the IRES from PV will be useful in the generation of vectors expressing IL-12 with a second protein such as IL-2 for transduction of melanoma cells.