Multiscale model of defective interfering particle replication for influenza A virus infection in animal cell culture

Cell culture-derived defective interfering particles (DIPs) are considered for antiviral therapy due to their ability to inhibit influenza A virus (IAV) production. DIPs contain a large internal deletion in one of their eight viral RNAs (vRNAs) rendering them replication-incompetent. However, they can propagate alongside their homologous standard virus (STV) during infection in a competition for cellular and viral resources. So far, experimental and modeling studies for IAV have focused on either the intracellular or the cell population level when investigating the interaction of STVs and DIPs. To examine these levels simultaneously, we conducted a series of experiments using highly different multiplicities of infections for STVs and DIPs to characterize virus replication in Madin-Darby Canine Kidney suspension cells. At several time points post infection, we quantified virus titers, viable cell concentration, virus-induced apoptosis using imaging flow cytometry, and intracellular levels of vRNA and viral mRNA using real-time reverse transcription qPCR. Based on the obtained data, we developed a mathematical multiscale model of STV and DIP co-infection that describes dynamics closely for all scenarios with a single set of parameters. We show that applying high DIP concentrations can shut down STV propagation completely and prevent virus-induced apoptosis. Interestingly, the three observed viral mRNAs (full-length segment 1 and 5, defective interfering segment 1) accumulated to vastly different levels suggesting the interplay between an internal regulation mechanism and a growth advantage for shorter viral RNAs. Furthermore, model simulations predict that the concentration of DIPs should be at least 10000 times higher than that of STVs to prevent the spread of IAV. Ultimately, the model presented here supports a comprehensive understanding of the interactions between STVs and DIPs during co-infection providing an ideal platform for the prediction and optimization of vaccine manufacturing as well as DIP production for therapeutic use.

Incomplete influenza A virus genomes occur frequently but are readily complemented during localized viral spread

Segmentation of viral genomes into multiple RNAs creates the potential for replication of incomplete viral genomes (IVGs). Here we use a single-cell approach to quantify influenza A virus IVGs and examine their fitness implications. We find that each segment of influenza A/Panama/2007/99 (H3N2) virus has a 58% probability of being replicated in a cell infected with a single virion. Theoretical methods predict that IVGs carry high costs in a well-mixed system, as 3.6 virions are required for replication of a full genome. Spatial structure is predicted to mitigate these costs, however, and experimental manipulations of spatial structure indicate that local spread facilitates complementation. A virus entirely dependent on co-infection was used to assess relevance of IVGs in vivo. This virus grows robustly in guinea pigs, but is less infectious and does not transmit. Thus, co-infection allows IVGs to contribute to within-host spread, but complete genomes may be critical for transmission.

Cancer risk in HIV patients with incomplete viral suppression after initiation of antiretroviral therapy

Background

Cancer causes significant morbidity and mortality among HIV patients in the US due to extended life expectancy with access to effective antiretroviral therapy. Low, detectable HIV RNA has been studied as a risk factor for adverse health outcomes, but its clinical impact on cancer risk remains unclear. The objective of this study was to determine whether HIV RNA <1,000 copies/mL six months after starting therapy was associated with 10-year first cancer risk.

Methods

We followed 7,515 HIV therapy initiators from a US-based multicenter clinical cohort from 1998 to 2014. We used nonparametric multiple imputation to account for viral loads that fell below assay detection limits, and categorized viral loads six months after therapy initiation into four groups: <20, 20–199, 200–999, and >999 copies/mL. We calculated estimates of the cumulative incidence of cancer diagnosis, accounting for death as a competing event. Inverse probability of exposure and censoring weights were used to control for confounding and differential loss to follow up, respectively.

Results

Crude 10-year first cancer risk in the study sample was 7.03% (95% CI: 6.08%, 7.98%), with the highest risk observed among patients with viral loads between 200 and 999 copies/mL six months after ART initiation (10.7%). After controlling for baseline confounders, 10-year first cancer risk was 6.90% (95% CI: 5.69%, 8.12%), and was similar across viral load categories.

Conclusion

Overall risk of first cancer was not associated with incomplete viral suppression; however, cancer remains a significant threat to HIV patients after treatment initiation. As more HIV patients gain access to treatment in the current “treat all” era, occurrences of incomplete viral suppression will be observed more frequently in clinical practice, which supports continued study of the role of low-level HIV RNA on cancer development.

Inhibition of infection spread by co-transmitted defective interfering particles

Although virus release from host cells and tissues propels the spread of many infectious diseases, most virus particles are not infectious; many are defective, lacking essential genetic information needed for replication. When defective and viable particles enter the same cell, the defective particles can multiply while interfering with viable particle production. Defective interfering particles (DIPs) occur in nature, but their role in disease pathogenesis and spread is not known. Here, we engineered an RNA virus and its DIPs to express different fluorescent reporters, and we observed how DIPs impact viral gene expression and infection spread. Across thousands of host cells, co-infected with infectious virus and DIPs, gene expression was highly variable, but average levels of viral reporter expression fell at higher DIP doses. In cell populations spatial patterns of infection spread provided the first direct evidence for the co-transmission of DIPs with infectious virus. Patterns of spread were highly sensitive to the behavior of initial or early co-infected cells, with slower overall spread stemming from higher early DIP doses. Under such conditions striking patterns of patchy gene expression reflected localized regions of DIP or virus enrichment. From a broader perspective, these results suggest DIPs contribute to the ecological and evolutionary persistence of viruses in nature.

An Epstein-Barr virus encoded inhibitor of Colony Stimulating Factor-1 signaling is an important determinant for acute and persistent EBV infection

Acute Epstein-Barr virus (EBV) infection is the most common cause of Infectious Mononucleosis. Nearly all adult humans harbor life-long, persistent EBV infection which can lead to development of cancers including Hodgkin Lymphoma, Burkitt Lymphoma, nasopharyngeal carcinoma, gastric carcinoma, and lymphomas in immunosuppressed patients. BARF1 is an EBV replication-associated, secreted protein that blocks Colony Stimulating Factor 1 (CSF-1) signaling, an innate immunity pathway not targeted by any other virus species. To evaluate effects of BARF1 in acute and persistent infection, we mutated the BARF1 homologue in the EBV-related herpesvirus, or lymphocryptovirus (LCV), naturally infecting rhesus macaques to create a recombinant rhLCV incapable of blocking CSF-1 (ΔrhBARF1). Rhesus macaques orally challenged with ΔrhBARF1 had decreased viral load indicating that CSF-1 is important for acute virus infection. Surprisingly, ΔrhBARF1 was also associated with dramatically lower virus setpoints during persistent infection. Normal acute viral load and normal viral setpoints during persistent rhLCV infection could be restored by Simian/Human Immunodeficiency Virus-induced immunosuppression prior to oral inoculation with ΔrhBARF1 or infection of immunocompetent animals with a recombinant rhLCV where the rhBARF1 was repaired. These results indicate that BARF1 blockade of CSF-1 signaling is an important immune evasion strategy for efficient acute EBV infection and a significant determinant for virus setpoint during persistent EBV infection.

Epstein-Barr virus (EBV) is a herpesvirus that persistently infects nearly all humans by adulthood. Acute and persistent phases of EBV infection are associated with a variety of human diseases, including infectious mononucleosis and cancer. To investigate how EBV interacts with the host to successfully establish acute and persistent infection, we combined the power of the rhesus macaque animal model for EBV infection with genetic engineering of the EBV-related herpesvirus, or lymphocryptovirus (LCV), that naturally infects rhesus macaques. We created a recombinant rhLCV carrying a mutated EBV BARF1 homologue, a replication-associated viral protein that is secreted and blocks Colony Stimulating Factor-1 (CSF-1) signaling, a cytokine important for innate immunity. Oral inoculation of rhesus macaques showed that the virus' ability to block CSF-1 was important for achieving the normally high viral loads during acute infection, and surprisingly, was also needed to establish normal levels of virus infection, or viral setpoint, during persistent infection. These studies show that virus-mediated interruption of innate immunity is critical for both acute and persistent phases of EBV infection. Understanding how EBV successfully infects humans and how the natural history of EBV infection can be disrupted will aid in development of vaccines to prevent EBV-associated diseases.

[Role of the hepatitis Delta virus on the pathogenesis of hepatic cirrhosis and hepatocellular carcinoma. Recent advances]

Hepatitis Delta virus is a defective RNA virus that requires HBV as helper virus. The two viruses share the same route of transmission, being prevalently transmitted by contaminated blood and body fluids. HDV can infect simultaneously with HBV (coinfection) or in a patient with already established HBV infection (superinfection). A progressive decline in HDV prevalence both as acute or chronic infection has been observed for several years. More recently, several European countries have observed stable HDV prevalence mainly due to migrants from non European countries. Persistent HDV replication has been demonstrated as correlated to cirrhosis development and hepatocellular carcinoma occurrence. Moreover, persistent HDV replication predicts liver related mortality. Early and prolonged treatment may lead to virus eradication, improving long-term prognosis.

Analysis of the selective advantage conferred by a C-E1 fusion protein synthesized by rubella virus DI RNAs

During serial passaging of rubella virus (RUB) in cell culture, the dominant species of defective-interfering RNA (DI) generated contains an in-frame deletion between the capsid protein (C) gene and E1 glycoprotein gene resulting in production of a C-E1 fusion protein that is necessary for the maintenance of the DI [Tzeng, W.P., Frey, T.K. (2006). C-E1 fusion protein synthesized by rubella virus DI RNAs maintained during serial passage. Virology 356 198-207.]. A BHK cell line stably expressing the RUB structural proteins was established which was used to package DIs into virus particles following transfection with in vitro transcripts from DI infectious cDNA constructs. Packaging of a DI encoding an in-frame C-GFP-E1 reporter fusion protein corresponding to the C-E1 fusion protein expressed in a native DI was only marginally more efficient than packaging of a DI encoding GFP, indicating that the C-E1 fusion protein did not function by enhancing packaging. However, infection with the DI encoding the C-GFP-E1 fusion protein (in the absence of wt RUB helper virus) resulted in formation of clusters of GFP-positive cells and the percentage of GFP-positive cells in the culture following infection remained relatively constant. In contrast, a DI encoding GFP did not form GFP-positive clusters and the percentage of GFP-positive cells declined by roughly half from 2 to 4 days post-infection. Cluster formation and sustaining the percentage of infected (GFP-positive) cells required the C part of the fusion protein, including the downstream but not the upstream of two arginine clusters (both of which are associated with RNA binding and association with mitochondrial p32 protein) and the E1 part through the transmembrane sequence, but not the C-terminal cytoplasmic tail. Among a collection of mutant DI constructs, cluster formation and sustaining infected cell percentage correlated with maintenance during serial passage with wt RUB. We hypothesize that cluster formation and sustaining infected cell percentage increase the likelihood of co-infection by a DI and wt RUB during serial passage thus enhancing maintenance of the DI. Cluster formation and sustaining infected cell percentage were found to be due to a combination of attenuated cytopathogenicity of DIs that express the C-E1 fusion protein and cell-to-cell movement of the DI. In infected cells, the C-GFP-E1 fusion protein was localized to potentially novel vesicular structures that appear to originate from ER-Golgi transport vacuoles. This species of DI expressing a C-E1 fusion protein that exhibits attenuated cytopathogenicity and the ability to increase the number of infected cells through cell-to-cell movement could be the basis for development of an attractive vaccine vector.

Herpes simplex virus blocks apoptosis by precluding mitochondrial cytochrome c release independent of caspase activation in infected human epithelial cells

Expression of HSV-1 genes leads to the induction of apoptosis in human epithelial HEp-2 cells but the subsequent synthesis of infected cell protein prevents the process from killing the cells. Thus, viruses unable to produce appropriate prevention factors are apoptotic. We now report that the addition of either a pancaspase inhibitor or caspase-9-specific inhibitor prevented cells infected with an apoptotic HSV-1 virus from undergoing cell death. This result indicated that HSV-1-dependent apoptosis proceeds through the mitochondrial apoptotic pathway. However, the pancaspase inhibitor did not prevent the release of cytochrome c from mitochondria, implying that caspase activation is not required for this induction of cytochrome c release by HSV-1. The release of cytochrome c was first detected at 9 hpi while caspase-9, caspase-3 and PARP processing were detected at 12 hpi. Finally, Bax accumulated at mitochondria during apoptotic, but not wild type HSV-1 infection. Together, these findings indicate that HSV-1 blocks apoptosis by precluding mitochondrial cytochrome c release in a caspase-independent manner and suggest Bax as a target in infected human epithelial cells.

Nef alleles from human immunodeficiency virus type 1-infected long-term-nonprogressor hemophiliacs with or without late disease progression are defective in enhancing virus replication and CD4 down-regulation

Infection with human immunodeficiency virus (HIV)-encoding defective nef variants may contribute to a relatively benign course of disease in a minority of long-term nonprogressors (LTNP). We have examined the functions of nef alleles from six individuals belonging to the same cohort of hemophiliacs infected with HIV-1 prior to 1985 and classified as LTNP in 1995. Three out of six individuals have progressed to HIV disease (late progressors [LP]), whereas the three remainders have maintained their LTNP status at least up to 2003. The nef alleles were obtained from both plasma virus and peripheral blood mononuclear cells of all six individuals in 1995 and 1998. The proportion of sequences containing mutations not yielding Nef expression significantly diminished in 1998 versus that in 1995. Several previously defined functional regions of intact nef alleles were highly conserved. However, the major variant obtained in 1998 from plasma RNA of five out of six individuals significantly reduced HIV infectivity/replication and impaired Nef-mediated CD4 but not major histocompatibility complex class I antigen down-modulation from the cell surface. Thus, functional alterations of the nef gene are present in both LP and LTNP, suggesting that Nef defectiveness in vitro is not necessarily associated with the long-term maintenance of LTNP status. Of interest is the fact that isolates from three out of three LP showed a dual CCR5/CXCR4 coreceptor use (R5X4), in contrast to those from LTNP, which were exclusively R5. Thus, in vivo evolution of gp120 Env to CXCR4 use appears to be associated with HIV disease progression in individuals infected with nef-defective viruses.

Superinfection of human immunodeficiency virus type 1 (HIV-1) to cell clone persistently infected with defective virus induces production of highly cytopathogenic HIV-1

Superinfection with human immunodeficiency virus type 1 (HIV-1) in human subjects, defined as reinfection with a heterologous strain of HIV-1, has become a topic of great interest. To illustrate the significance of this occurrence, we performed HIV-1 superinfection of L-2 cells, which were isolated from MT-4 cells persistently infected with subtype B HIV-1 as a cell clone continuously producing defective HIV-1 particles. L-2 cells carrying provirus with a one-base insertion in the pol protease were superinfected with HIV-1 derived from primary isolates of subtype B or CRF01_AE. The kinetics of the superinfection in L-2 were very slow compared with those of primary infections in MT-4. Interestingly, L-2 shifted after superinfection to become a producer of highly cytopathogenic HIV-1. Molecular characterization revealed that superinfection occurred in only about 10% of the CRF01_AE-superinfected L-2, which carried provirus of both subtypes and produced viral particles containing genomic RNA of both subtypes. Surprisingly, such cytopathogenic HIV-1 showed predominantly the original subtype B phenotype. Thus, the mechanism of the production of cytopathic HIV-1 seemed to be mediated by trans complementation with pol products of superinfected CRF01_AE. These findings suggest the significance of long-lived infected cells as recipients for superinfection that could result in the generation of new HIV-1 variants with high virulence in patients who are off therapy or do not adhere to treatment, and may indicate the need for precautions against such superinfection.

Identification of sequence elements of tombusvirus-associated defective interfering RNAs required for symptom modulation

Defective interfering (DI) RNAs of tombusviruses are short, non-coding, symptom-modulating RNAs originating from the viral genome. The presence of homologous DI RNA in virus infection attenuates the otherwise lethal viral symptoms. Nicotiana benthamiana plants infected with tomato bushy stunt tombusvirus pepper isolate (TBSV-P) show severe symptoms, which culminate in the death of the plant. In contrast, plants co-inoculated with TBSV-P and TBSV-P-derived DI RNA display attenuated symptoms. However, co-inoculation of TBSV-P with heterologous DI RNA, originating from Carnation Italian ringspot tombusvirus results in development of apical necrotic symptoms. To localize the symptom-determining factors on DI RNA genome, chimeras of protective and non-protective DI RNAs have been constructed. All chimeras were biologically active and accumulated to a high level in the presence of helper virus. We identified a 5' proximal sequence element of the DI RNA as the most important symptom determinant region. However, our results demonstrated that the symptom modulating ability of this region is also influenced by the sequence composition of whole DI RNAs.

Defective infectious particles and rare packaged genomes produced by cells carrying terminal-repeat-negative epstein-barr virus

The Epstein-Barr virus (EBV) lytic program includes lytic viral DNA replication and the production of a viral particle into which the replicated viral DNA is packaged. The terminal repeats (TRs) located at the end of the linear viral DNA have been identified as the packaging signals. A TR-negative (TR(-)) mutant therefore provides an appropriate tool to analyze the relationships between EBV DNA packaging and virus production. Here, we show that supernatants from lytically induced 293 cells carrying TR mutant EBV genomes (293/TR(-)) contain large amounts of viral particles devoid of viral DNA which are nevertheless able to bind to EBV target cells. This shows that viral DNA packaging is not a prerequisite for virion formation and egress. Rather surprisingly, supernatants from lytically induced 293/TR(-) cells also contained rare infectious viruses carrying the viral mutant DNA. This observation indicates that the TRs are important but not absolutely essential for virus encapsidation.

Characterization of P gene-deficient rabies virus: propagation, pathogenicity and antigenicity

The RNA polymerase of rabies virus (RV) is a two-protein complex composed of L (a large catalytic component) and P (a non-catalytic phosphoprotein cofactor) proteins. We generated a gene-deficient RV lacking the entire P gene from HEP-Flury (HEP) strain, one of the most attenuated RV strains, by the method of reverse genetics. This P gene-deficient (def-P) virus could replicate and produce progeny viruses with a slightly retarded rate in the cell lines that constitutively express the P protein. The def-P virus could perform the primary RNA transcription by the virion-associated polymerase even in the infected host without de novo P protein synthesis. However, the def-P virus required the newly synthesized P protein for the secondary RNA transcription and genome RNA replication of virus. No progeny virus was produced in the infected host that did not express P protein. The def-P virus was apathogenic in adult and suckling mice even when inoculated intracranially. On the other hand, inoculation of the def-P virus into mice induced a high titer of virus-neutralizing antibody and protected mice from lethal challenge with the CVS strain. These results demonstrated that the def-P virus could induce strong protective immunity against rabies virus without the production of progeny virus and the severe host damage. The def-P virus would be a potential resource of safe live-attenuated rabies vaccine.

Lytic replication-defective Kaposi's sarcoma-associated herpesvirus: potential role in infection and malignant transformation

Defective viruses often have pivotal roles in virus-induced diseases. Although Kaposi's sarcoma-associated herpesvirus (KSHV) is etiologically associated with Kaposi's sarcoma (KS) and primary effusion lymphoma (PEL), defective KSHV has not been reported. Using differential genetic screening methods, we show that defective KSHV is present in KS tumors and PEL cell lines. To investigate the role of defective viruses in KSHV-induced pathogenesis, we isolated and characterized a lytic replication-defective KSHV, KV-1, containing an 82-kb genomic deletion of solely lytic genes. Cells harboring KV-1 escaped G(0)/G(1) apoptosis induced by spontaneous lytic replication occurred in cells infected with regular KSHV but maintained efficient latent replication. Consequently, KV-1-infected cells had phenotypes of enhanced cell proliferation and transformation potentials. Importantly, KV-1 was packaged as infectious virions by using regular KSHV as helpers, and KV-1-like variants were detected in cultures of two of five KSHV cell lines and 1 of 18 KS tumors. These results point to a potential role for defective viruses in the regulation of KSHV infection and malignant transformation.

Yellow fever 17D virus: pseudo-revertant suppression of defective virus penetration and spread by mutations in domains II and III of the E protein

A yellow fever (YFV) 17D virus variant, which causes persistent infection of mouse neuroblastoma cells associated with defective cell penetration and small plaque size, yielded plaque-revertant viruses from cells transfected with viral transcripts encoding the adaptive mutation (Gly360 in the E protein). Reconstruction of a plaque-purified revertant which contained Gly360 and additional substitutions (Asn for Lys303 and Val for Ala261) yielded a virus whose infectious center size, growth efficiency, and cell penetration rate similar to the parental YF5.2iv virus, whereas viruses with Asn303 or Val261 alone with Gly360 yielded either a small-plaque virus or a parental revertant. These data indicate that the YFV E protein is subject to suppression of mutations in domain III that are deleterious for viral entry and spread by a second-site mutation in domain II. Position 261 lies within the hydrophobic ligand-binding pocket at the domain I-II interface, a site believed to be involved in the hinge-like conformational change of domain II during activation of membrane fusion-activity. Results of this study provide genetic data consistent with findings on flavivirus structure and implicate domain III in functions beyond simply cell surface attachment.

Generation and characterization of P gene-deficient rabies virus

Rabies virus (RV) deficient in the P gene was generated by reverse genetics from cDNA of HEP-Flury strain lacking the entire P gene. The defective virus was propagated and amplified by rescue of virus, using a cell line that complemented the functions of the deficient gene. The P gene-deficient (def-P) virus replicated its genome and produced progeny viruses in the cell lines that constitutively expressed the P protein, although it grew at a slightly retarded rate compared to the parental strain. In contrast, no progeny virus was produced in the infected host when the def-P virus-infected cells that did not express the P protein. However, we found that the def-P virus had the ability to perform primary transcription (by the virion-associated polymerase) in the infected host without de novo P protein synthesis. The def-P virus was apathogenic in adult and suckling mice, even when inoculated intracranially. Inoculation of def-P virus in mice induced high levels of virus-neutralizing antibody (VNA) and conferred protective immunity against a lethal rabies infection. These results demonstrate the potential utility of gene-deficient virus as a novel live attenuated rabies vaccine.

The role of the defective interfering particle DI9c in mucosal disease in cattle

Mucosal disease occurs in cattle persistently infected with a noncytopathogenic strain of bovine viral diarrhoea virus (BVDVnc) following in utero infection. The disease can be initiated by superinfection with a cytopathogenic biotype (BVDVc) of the virus with antigenic "homology" to the persisting virus. A BVDVc isolated from a clinical case of mucosal disease has been discovered to consist of a defective interfering particle, DI9, and an associated BVDVnc helper virus. A defective virus corresponding to DI9 was recently recovered from an infectious cDNA clone and was named DI9c. To evaluate the role of DI9 in the pathogenesis of mucosal disease a two-part experimental study was carried out which included clinical, haematological, pathological and virological investigations. Eight of nine calves persistently infected with BVDVnc were experimentally inoculated with DI9c. The defective virus was propagated in cells preinfected with the same strain of virus used to persistently infect the calves in utero. The calves were euthanased on days 4, 7, 14, 21, 28, 40, 40 or 87 post inoculation. None of the inoculated animals developed classical mucosal disease, neither clinically nor pathologically. DI9c was not found in serum, nasal swab or tissue samples from the calves by observing cytopathogenic effect and/or using a polymerase chain reaction after reverse transcription (RT-PCR) of viral RNA. DI9c did not replicate to a detectable extent in these assays, and its participation in the pathogenesis of mucosal disease could not be proven.

Detection of enterovirus in human skeletal muscle from patients with chronic inflammatory muscle disease or fibromyalgia and healthy subjects

Enterovirus RNA has been found previously in specimens of muscle biopsy from patients with idiopathic dilated cardiomyopathy, chronic inflammatory muscle diseases, and fibromyalgia or chronic fatigue syndrome (fibromyalgia/chronic fatigue syndrome). These results suggest that skeletal muscle may host enteroviral persistent infection. To test this hypothesis, we investigated by reverse transcription-polymerase chain reaction (RT-PCR) assay the presence of enterovirus in skeletal muscle of patients with chronic inflammatory muscle diseases or fibromyalgia/chronic fatigue syndrome, and also of healthy subjects. Three of 15 (20%) patients with chronic inflammatory muscle diseases, 4 of 30 (13%) patients with fibromyalgia/chronic fatigue syndrome, and none of 29 healthy subjects was found positive. The presence of VP-1 enteroviral capsid protein was assessed by an immunostaining technique using the 5-D8/1 monoclonal antibody; no biopsy muscle from any patient or healthy subject was found positive. The presence of viral RNA in some muscle biopsies from patients exhibiting muscle disease, together with the absence of VP-1 protein, is in favor of a persistent infection involving defective viral replication.

Receptor use by vesicular stomatitis virus pseudotypes with glycoproteins of defective variants of measles virus isolated from brains of patients with subacute sclerosing panencephalitis

The vaccine or Vero cell-adapted strains of measles virus (MV) have been reported to use CD46 as a cell entry receptor, while lymphotropic MVs preferentially use the signalling lymphocyte activation molecule (SLAM or CD150). In contrast to the virus obtained from patients with acute measles, little is known about the receptor that is used by defective variants of MV isolated from patients with subacute sclerosing panencephalitis (SSPE). The receptor-binding properties of SSPE strains of MV were analysed using vesicular stomatitis virus pseudotypes expressing the envelope glycoproteins of SSPE strains of MV. Such pseudotype viruses could use SLAM but not CD46 for entry. The pseudotype viruses with SSPE envelope glycoproteins could enter Vero cells, which do not express SLAM. In addition, their entry was not blocked by the monoclonal antibody to CD46, pointing to another entry receptor for SSPE strains on Vero cells. Furthermore, the unknown receptor(s), distinct from SLAM and CD46, may be present on cell lines derived from lymphoid and neural cells. Biochemical characterization of the receptor present on Vero cells and SK-N-SH neuroblastoma cells was consistent with a glycoprotein. Identification of additional entry receptors for MV will provide new insights into the mechanism of spread of MV in the central nervous system and possible reasons for differences between MVs isolated from patients with acute measles and SSPE.

Characterization of classical swine fever virus associated with defective interfering particles containing a cytopathogenic subgenomic RNA isolated from wild boar

Classical swine fever virus (CSFV) strain WB82, isolated from a wild boar in 1982, induced a distinct cytopathic effect (CPE) in primary swine testicle cell culture and in most of the porcine cell lines. This strain of CSFV was found to be composed of two biotypes. cytopathogenic (cp) CSFV, as a minor population, and noncytopathogenic (noncp) CSFV, as a major population. The noncp CSFV (designated strain WB82/E+) was obtained by biological cloning, and it showed the exaltation of Newcastle disease virus phenomenon. In Northern blot analysis and RT-PCR assay, CSFV RNA with a subgenomic (sg) length was detected in addition to full-length viral RNA only in the cells in which a CPE had been revealed. These RNAs represent the genomes of typical defective interfering (DI) particles because of the strict dependence on a complementing helper virus and interference with replication of the helper virus. The sg RNA, which exhibits the genomes of the DI particles, lacked the nucleotides of the viral genomic region from Npro to NS2 (4764 bases). When extracted sg RNA was transfected to the cells infected with the WB82/E+ strain, a distinct CPE was observed. Interestingly, the CPE was observed in cells infected with other heterologous noncp CSFV ALD and GPE- strains by sg RNA transfection. The results suggested that these noncp CSFVs act as helper viruses for the replication of sg RNA (DI particles). It was also shown that the cytopathogenicity of strain WB82 is caused by apoptosis.

Bunyamwera bunyavirus nonstructural protein NSs is a nonessential gene product that contributes to viral pathogenesis

Bunyamwera virus (family Bunyaviridae, genus Bunyavirus) contains a tripartite negative-sense RNA genome. The smallest RNA segment, S, encodes the nucleocapsid protein N and a nonstructural protein, NSs, in overlapping reading frames. We have generated a mutant virus lacking NSs, called BUNdelNSs, by reverse genetics. Compared with the wild-type (wt) virus, BUNdelNSs exhibited a smaller plaque size and generated titers of virus approximately 1 log lower. In mammalian cells, the mutant expressed greatly increased levels of N protein; significantly, the marked inhibition of host cell protein synthesis shown by wt virus was considerably impaired by BUNdelNSs. When inoculated by the intracerebral route BUNdelNSs killed BALB/c mice with a slower time course than wt and exhibited a reduced cell-to-cell spread, and titers of virus in the brain were lower. In addition, the abrogation of NSs expression changed Bunyamwera virus from a noninducer to an inducer of an interferon-beta promoter. These results suggest that, although not essential for growth in tissue culture or in mice, the bunyavirus NSs protein has several functions in the virus life cycle and contributes to viral pathogenesis.

Two point mutations in the U3 region of the long terminal repeat convert a subgroup A transformation-defective rous sarcoma virus to a cytopathic virus

To elucidate the mechanism of cytopathicity of the transformation-defective avian retrovirus tdPH2010, we examined the function of two point mutations we had previously found in the long terminal repeat U3 region of this virus. Our previous studies showed that the U3 region was responsible for the cytopathic effects. These mutations were a G-to-T mutation at position -126 from the transcription start site and a G-to-A mutation at -23. Site-directed mutagenesis was performed on a noncytopathic, wild-type virus BSU to alter the nucleotides at these two positions, one at a time, to those of tdPH2010. Cell growth assay using the altered viruses revealed that host cell growth was retarded only when both of these mutations were present. The two additional mutations previously found in the direct repeat 1-polypurine tract (DR1-PPT) region of tdPH2010 were present also in the noncytopathic strain tdPH2013. Site-directed mutagenesis confirmed that these two mutations had indeed no role in the cytopathic effect of tdPH2010. None of these mutations influenced virus production from the infected cells. We conclude that the cytopathic effects by tdPH2010 are ascribed to the two point mutations in the U3 region.

Targeted foreign gene expression in spinal cord neurons using poliovirus replicons

A hallmark of poliovirus is the propensity to infect and replicate in spinal cord neurons of the central nervous system. Previously, we characterized a poliovirus self-replicating RNA genome (replicon), which encodes firefly luciferase in place of the capsid genes. This replicon is encapsidated into an authentic poliovirion by providing the poliovirus capsid protein in trans. The amount of enzymatically active luciferase in cells infected with this replicon correlated with the infectious dose. To begin to characterize the in vivo infectious potential of replicons, we have inoculated mice transgenic for the human receptor for poliovirus (PVR), either intracranially or intraspinally, with the replicon encoding luciferase. Wild-type poliovirus delivered to PVR mice via intracranial or intraspinal routes resulted in paralysis and death. Replicon preparations were shown by a sensitive biological assay to be free of infectious poliovirus. Neither intracranial nor intraspinal inoculation of the replicon encoding luciferase resulted in any obvious paralysis or disease symptoms. Following intraspinal inoculation with replicons encoding luciferase, luciferase enzyme activity was detected at 4 h post-inoculation, with peak activity at approximately 8 h post-inoculation; by 48 - 72 h, the luciferase activity had returned to background levels. Luciferase activity was detected in spinal cord predominantly near the site of inoculation, although activity was detected anterior and posterior to the site of inoculation, indicating that replicons undergo limited movement within the CNS presumably via the cerebrospinal fluid. In stark contrast to poliovirus though, inoculation of replicons into the spinal cords of PVR mice did not result in noticeable pathogenesis. Using immunofluorescence with antibodies to double-stain for replicons and neurons, we determined that replicons exclusively infect the neurons of the spinal cord, with the expression of the luciferase and replicon proteins confined to the cytoplasm of the infected cells. Replicons, then, possess the identical capacity for infection of spinal cord neurons in vivo as poliovirus. The lack of discernible neuronal destruction following replicon inoculation into the spinal cord suggests that some of the pathogenesis observed during a poliovirus infection might not be due entirely to primary infection of neurons. Finally, the results of this study point to future use of replicons as a means to target recombinant protein expression to neurons in the spinal cord.

Studies on the pathology, especially brain hemorrhage and angioendotheliomas, induced by two new mos-containing viruses

Recombinant virus 7 (R7), a spontaneous deletion mutant of SV7, which is itself a molecular clone of Moloney murine sarcoma virus 124 (MoMuSV 124), induces brain lesions and tumors of the subcutaneous tissue and spleen in all infected mice. In contrast, SV7 only induces tumors of the spleen and subcutaneous tissues. One of the genetic differences between R7 and SV7 is that R7 encodes a Gag-Mos protein whereas SV7 encodes an Env-Mos protein. To investigate whether the novel R7 gag-mos oncogene is required for brain lesion induction, two viruses (SV7d1 and SVM1) were constructed in which the R7 gag-mos sequences and the adjacent 53 bp of the 5' noncoding sequence were replaced by either the SV7 or myeloproliferative sarcoma virus (MPSV) env-mos oncogenes, respectively. Like R7, SV7d1 and SVM1 induced brain lesions and tumors in the spleen and subcutaneous tissues. A prominent component of R7-, SV7d1-, and SVM1-induced tumors of the brain, subcutaneous tissues, and spleen was the presence of abnormally enlarged cells with eccentric nuclei lining vessels, scattered singly or in small clusters. Their size, localization to the luminal surface of distended vessels, and binding to Bandeiraea simplicifolia (BS-1) lectin, an endothelial cell (EC) marker, suggest that they are most likely transformed ECs. Our findings therefore indicate that the induction of brain lesions is not limited to the expression of the R7 Gag-Mos protein. However, our findings also indicate that expression of the different forms of the Mos protein results in differences in the relative abundance of ECs in brain angioendotheliomas and subcutaneous and spleen tumors induced by these viruses.

Leader switching occurs during the rescue of defective RNAs by heterologous strains of the coronavirus infectious bronchitis virus

A defective RNA (D-RNA), CD-61, derived from the Beaudette strain of the avian coronavirus infectious bronchitis virus (IBV), was rescued (replicated and packaged) using four heterologous strains of IBV as helper virus. Sequence analysis of the genomic RNA from the four heterologous IBV strains (M41, H120, HV10 and D207) identified nucleotide differences of up to 17% within the leader sequence and up to 4.3% within the whole of the adjacent 5' untranslated region (UTR). Analysis of the 5' ends of the rescued D-RNAs showed that the Beaudette leader sequence, present on the initial CD-61, had been replaced with the corresponding leader sequence from the helper IBV strain but the adjacent 5' UTR sequence of the rescued D-RNAs corresponded to the original CD-61 Beaudette sequence. These results demonstrated that the phenomenon of leader switching previously identified for the coronaviruses murine hepatitis virus and bovine coronavirus (BCoV) also occurred during the replication of IBV D-RNAs. Three predicted stem-loop structures were identified within the 5' UTR of IBV. Stem-loop I showed a high degree of covariance amongst the IBV strains providing phylogenetic evidence that this structure exists and is potentially involved in replication, supporting previous observations that a BCoV stem-loop homologue was essential for replication of BCoV defective interfering RNAs.

Retroviruses in autoimmune liver disease: genetic or environmental agents?

Retroviruses have been implicated in the pathogenesis of several human autoimmune conditions including Sjögren's syndrome, primary biliary cirrhosis, immune mediated diabetes, and multiple sclerosis. The human intracisternal A type particle derived from Sjögren's syndrome patients' salivary glands was the first retrovirus to be isolated from a human autoimmune disorder but the agent has yet to be cloned. In primary biliary cirrhosis patients, virus like particles have been observed by electron microscopy in biliary epithelium, endogenous retroviral sequences have been cloned from liver samples, and antibody reactivity to the human intracisternal A type particle has been observed in the majority of patients tested. However, there is no evidence to link the endogenous retroviral sequences in primary biliary cirrhosis patients to the retroviral antibody reactivity or virus like particles. In other patients with liver disease, reactivity to the human intracisternal A type particle was observed in a small but significant proportion of patients with hepatitis C virus infection. If the intracisternal A type particle is an endogenous retrovirus, it is interesting to speculate that hepatitis C virus infection may modulate the endogenous retroviral expression, as chronic hepatitis C has been linked with the development of Sjögren's syndrome. Furthermore, many patients with chronic hepatitis C virus infection have reactivity to an autoantigen of unknown significance known as GOR that has protein sequence homology with both hepatitis C virus nucleocapsid protein as well as HTLV-1 gag. This may be an another example of an endogenous retroviral protein acting as an autoantigen in liver disease patients. At this time, there is little evidence to suggest that endogenous retroviruses are infectious agents that cause autoimmune disease but they may be implicated as either genetic elements or antigens. Further studies will be required to characterize the role that both exogenous and endogenous retroviruses play in the pathogenesis of autoimmune liver diseases.

New defective RNAs from citrus tristeza virus: evidence for a replicase-driven template switching mechanism in their generation

Defective RNAs (D-RNAs) ranging in size from 1968 to 2759 nt were detected in four citrus tristeza virus (CTV) isolates by hybridization of electroblotted dsRNAs with two probes specific for the 5'- and 3'-terminal genomic regions. The RNAs that hybridized with both probes were eluted, cloned and sequenced. Comparison with the sequences of the corresponding genomic regions of the helper virus showed, in all cases, over 99% nucleotide identity and direct repeats of 4-5 nt flanking or in the vicinity of the junction sites. The presence of the repeats from two separate genome locations suggests a replicase-driven template switching mechanism for the generation of these CTV D-RNAs. Two of the CTV isolates that differed greatly in their pathogenicity contained an identical D-RNA, suggesting that it is unlikely that this D-RNA is involved in symptom modulation, which may be caused by another factor.

Recombinant, replication-defective adenovirus gene transfer vectors induce cell cycle dysregulation and inappropriate expression of cyclin proteins

First-generation adenovirus (Ad) vectors that had been rendered replication defective by removal of the E1 region of the viral genome (DeltaE1) or lacking the Ad E3 region in addition to E1 sequences (DeltaE1DeltaE3) induced G2 cell cycle arrest and inhibited traverse across G1/S in primary and immortalized human bronchial epithelial cells. Cell cycle arrest was independent of the cDNA contained in the expression cassette and was associated with the inappropriate expression and increase in cyclin A, cyclin B1, cyclin D, and cyclin-dependent kinase p34(cdc2) protein levels. In some instances, infection with DeltaE1 or DeltaE1 DeltaE3 Ad vectors produced aneuploid DNA histogram patterns and induced polyploidization as a result of successive rounds of cell division without mitosis. Cell cycle arrest was absent in cells infected with a second-generation DeltaE1Ad vector in which all of the early region E4 except the sixth open reading frame was also deleted. Consequently, E4 viral gene products present in DeltaE1 or DeltaE1 DeltaE3 Ad vectors induce G2 growth arrest, which may pose new and unintended consequences for human gene transfer and gene therapy.

Amino acids in the capsid protein of tomato yellow leaf curl virus that are crucial for systemic infection, particle formation, and insect transmission

A functional capsid protein (CP) is essential for host plant infection and insect transmission in monopartite geminiviruses. We studied two defective genomic DNAs of tomato yellow leaf curl virus (TYLCV), Sic and SicRcv. Sic, cloned from a field-infected tomato, was not infectious, whereas SicRcv, which spontaneously originated from Sic, was infectious but not whitefly transmissible. A single amino acid change in the CP was found to be responsible for restoring infectivity. When the amino acid sequences of the CPs of Sic and SicRcv were compared with that of a closely related wild-type virus (TYLCV-Sar), differences were found in the following positions: 129 (P in Sic and SicRcv, Q in Sar), 134 (Q in Sic and Sar, H in SicRcv) and 152 (E in Sic and SicRcv, D in Sar). We constructed TYLCV-Sar variants containing the eight possible amino acid combinations in those three positions and tested them for infectivity and transmissibility. QQD, QQE, QHD, and QHE had a wild-type phenotype, whereas PHD and PHE were infectious but nontransmissible. PQD and PQE mutants were not infectious; however, they replicated and accumulated CP, but not virions, in Nicotiana benthamiana leaf discs. The Q129P replacement is a nonconservative change, which may drastically alter the secondary structure of the CP and affect its ability to form the capsid. The additional Q134H change, however, appeared to compensate for the structural modification. Sequence comparisons among whitefly-transmitted geminiviruses in terms of the CP region studied showed that combinations other than QQD are present in several cases, but never with a P129.

Delayed progression of murine AIDS in C57BL/6 mice pre-immunized with a highly antigenic 10-mer peptide encoded by the murine AIDS defective virus gag p12 gene

C57BL/6 (B6) mice were immunized with a highly antigenic 10-mer peptide (P12-10), which is encoded by the murine AIDS (MAIDS) defective virus gag p12 gene, emulsified in incomplete Freund's adjuvant (ICFA). One week later, the mice were inoculated with the MAIDS virus to see if the immunization affects progression of MAIDS. It was demonstrated that the immunization significantly delayed progression of MAIDS, although it failed to induce appreciable cytotoxic T lymphocyte (CTL) responses against the P12-10 antigen. In contrast, immunization of B6 mice with the P12-10 coupled with liposome induced substantial CTL responses but failed to protect the mice against MAIDS development. This segregation between CTL activity and in vivo protection efficacy might be worth considering when we exploit vaccines for augmenting cellular immunity mediated by CD8+ T cells.

The polymerase chain reaction for the detection of defective interfering canine parvovirus particles

The PCR assay was used to amplify a portion of the genome of virulent and vaccinal canine parvovirus strains and of a vaccinal feline panleukopenia virus strain. A DNA fragment corresponding to the gene that encodes the VP1/VP2 proteins was amplified. The size of the PCR products was 2.2 Kbp except for CPV vaccinal 17-80 strain. The PCR product of 17-80 was 1.1 Kbp leading to the hypothesis of the presence of defective particles. All the restriction enzymes digested the 2.2 Kbp amplified products giving restriction fragments of the expected size whereas Hind III, Hpa II and Pvu II did not digest the PCR fragment of 1.1 Kbp.

Mechanistic independence of Nef and cyclophilin A enhancement of human immunodeficiency virus type 1 infectivity

Optimal HIV-1 infectivity requires the presence of both the viral factor Nef and the cellular protein cyclophilin A (CyPA) during virion assembly. These two proteins are integral components of HIV-1 particles. Both CyPA and Nef facilitate a step in the viral life cycle occurring between penetration and reverse transcription, suggesting a common mechanism of action. Experiments were performed to test the potential interplay of Nef- and CyPA-mediated enhancement of HIV-1 infectivity. In single-cycle infection assays, nef-defective virions were partially resistant to cyclosporin A (CsA), a drug that inhibits the binding of CyPA to the HIV-1 Gag precursor and CyPA incorporation into virions. Genetic dissection of the relative contributions of Nef and the cyclophilin A-Gag interaction to HIV-1 infectivity demonstrated the independence of these two effects. Nef was not required for incorporation of CyPA into HIV-1 virions and vice-versa. Surprisingly, CyPA-deficient virions remained sensitive to inhibition by CsA, in a manner that depended strongly on the presence of a functional nef gene. These results demonstrate that Nef and CyPA act independently to render HIV-1 particles fully infectious. They further suggest that in addition to blocking the CyPA-Gag interaction, CsA can also inhibit HIV-1 replication through a novel mechanism involving suppression of Nef-directed enhancement of virus infectivity.

Establishment of MAIDS-defective virus-infected B cell lines and their characterization

Mice inoculated with the murine AIDS (MAIDS)-defective virus develop severe B and T cell dysfunctions. The primary event in the development of this disease is the infection and polyclonal expansion of the target cells of this defective virus, which have been reported to belong to the B cell lineage. To further study the central role that these cells play in the development of MAIDS, we attempted to establish MAIDS-defective virus-infected B cell lines in vitro. We succeeded in establishing two cell lines, SD1 and CSTB5, from the enlarged organs of C57BL/6 mice inoculated with helper-free stocks of the MAIDS-defective virus. Both cell lines are not transplantable in syngeneic C57BL/6 mice or in nude or CD8-/- mice and are apparently not malignant. They both belong to the B lineage, as their immunoglobulin (Ig) genes, but not the T cell receptor (TcR) beta locus, are rearranged, suggesting that they are relatively mature B cells. However, analysis of cell surface marker expression by FACS revealed a surface phenotype similar to that of pre-B cells (MHC I+, MHC II+, B7.2+, sIgM-, sIgG-, kappa-, B220-, CD5-, Thy1.2-, TcR-, CD3-, CD4-, CD8-, Mac-1-, 33D1-). Additionally, the CSTB5 cells express CD40 and the SD1 cells express CD43. Both cell lines contain the MAIDS-defective provirus and express the expected 4.2-kb viral RNA and the corresponding Pr60gag protein. The CSTB5 cells are nonproducer, while the SD1 cell line produces what appears to be an endogenous MuLV. The phenotype of these cell lines is very similar to what is known about the target B cells of this virus in vivo. These new established cell lines are likely to be useful in elucidating the mechanism(s) by which the MAIDS-defective virus causes its target B cells to proliferate and induce T cell anergy in infected animals.

Expression of hemagglutinin/esterase by a mouse hepatitis virus coronavirus defective-interfering RNA alters viral pathogenesis

A defective-interfering (DI) RNA of mouse hepatitis virus (MHV) was developed as a vector for expressing MHV hemagglutinin/esterase (HE) protein. The virus containing an expressed HE protein (A59-DE-HE) was generated by infecting cells with MHV-A59, which does not express HE, and transfecting the in vitro-transcribed DI RNA containing the HE gene. A similar virus (A59-DE-CAT) expressing the chloramphenicol acetyltransferase (CAT) was used as a control. These viruses were inoculated intracerebrally into mice, and the role of the HE protein in viral pathogenesis was evaluated. Results showed that all mice infected with parental A59 or A59-DE-CAT succumbed to infection by 9 days postinfection (p.i.), demonstrating that inclusion of the DI did not by itself alter pathogenesis. In contrast, 60% of mice infected with A59-DE-HE survived infection. HE- or CAT-specific subgenomic mRNAs were detected in the brains at days 1 and 2 p.i. but not later, indicating that the genes in the DI vector were expressed only in the early stage of viral infection. No significant difference in virus titer or viral antigen expression in brains was observed between A59-DE-HE- and A59-DE-CAT-infected mice, suggesting that virus replication in brain was not affected by the expression of HE. However, at day 3 p.i. there was a slight increase in the extent of inflammatory cell infiltration in the brains of the A59-DE-HE-infected mice. Surprisingly, virus titers in the livers of A59-DE-HE-infected mice were 3 log10 lower than that of the A59-DE-CAT-infected mice at day 6 p.i. Also, substantially less necrosis and viral antigen were detected in the livers of the A59-DE-HE-infected mice. This may account for the reduced mortality of these mice. The possible contribution of the host immune system to this difference in pathogenesis was analyzed by comparing the expression of four cytokines. Results showed that both tumor necrosis factor-alpha and interleukin-6 mRNAs increased in the brains of the A59-DE-HE-infected mice at day 2 p.i., whereas interferon-gamma and interleukin-1 alpha mRNAs were similar between A59-DE-HE- and A59-DE-CAT-infected mice. These data suggest that the transient expression of HE protein enhances an early innate immune response, possibly contributing to the eventual clearance of virus from the liver. This study indicates the feasibility of the DI expression system for studying roles of viral proteins during MHV infection.

A defective interference-like phenomenon of human hepatitis B virus in chronic carriers

Defective interfering (DI) particles have been found in many RNA and DNA viruses of bacteria, plants, and animals since their first discovery in influenza virus. However, this fundamental phenomenon has not been demonstrated in human natural infections. Using a new approach, here we provide the first experimental evidence for the existence of DI-like viruses in human chronic carriers of hepatitis B virus (HBV). Functional characterization of naturally occurring core internal deletion (CID) variants of HBV revealed all of the features of DI particles. When equal amounts of wild-type and CID variant DNAs were cotransfected into a human hepatoma cell line, Huh7, a three- to fivefold enrichment of CID variants was most often observed. The fluctuations of the virus populations between CID variants and helper HBV in three chronic carriers are reminiscent of the cycling phenomenon in other DI viral systems. This finding has important implications for chronic viral hepatitis and other chronic progressive viral diseases.

The coat protein of turnip crinkle virus is involved in subviral RNA-mediated symptom modulation and accumulation

Some satellite (sat-) and defective interfering (DI) RNAs associated with plant viruses intensify or ameliorate the symptoms of the virus. We recently demonstrated that the TCV coat protein (CP) is involved in symptom modulation by sat-RNA C. Two additional subviral RNAs have now been tested for effect of the CP on symptom modulation. DI RNA G, which normally intensifies the symptoms of TCV, is able to attenuate symptoms if the TCV CP is replaced with the CP of cardamine chlorotic fleck virus. DI RNA G had no effect on the symptoms of TCV with a single base alteration in the CP open reading frame, unlike sat-RNA C, which was able to ameliorate the symptoms of the mutant TCV. Using a hybrid sat-RNA constructed from sat-RNA C and TCV (which shares a similar 3'-end region with DI RNA G), the 3'-terminal 53 bases of sat-RNA C were found to be involved in symptom attenuation, which was directly correlated with the lack of detectable viral genomic RNA in whole plants. Sat-RNA D had no effect on the symptoms of mutant or wild-type TCV. The accumulation of TCV subviral RNAs in plants and protoplasts was also found to be strongly influenced by the presence or absence of the wild-type TCV CP.

Concurrent suppression of virus replication and rescue of movement-defective virus in transgenic plants expressing the coat protein of potato virus X

A line of transgenic tobacco expressing the coat protein (CP) of potato virus X (PVX) was resistant against a broad spectrum of PVX strains. Inoculation of leaves and protoplasts with PVX expressing the jellyfish green fluorescent protein reporter gene revealed that this resistance mechanism suppressed PVX replication in the initially infected cell and systemic spread of the virus. Cell-to-cell movement was also slower in the resistant plants. The resistance at the level of replication was effective against wild-type PVX and also against movement-defective isolates with a frameshift mutation or deletion in the CP ORF. However, the cell-to-cell movement defect of the mutant viruses was rescued on the resistant plants. Based on these results it is proposed that the primary resistance mechanism is at the level of replication.

A cDNA clone from a defective RNA of citrus tristeza virus is infective in the presence of the helper virus

A naturally occurring defective RNA of 2379 nt (D2.3) from the VT strain of citrus tristeza closterovirus (CTV) was cloned and sequenced. The D2.3 RNA is a fusion of two regions of 1521 and 858 nt from the 5' and 3' ends of the CTV genome, respectively. A cDNA clone of D2.3 RNA was tagged by the insertion of a 0.47 kb chimeric DNA fragment and the recombinant cDNA was inserted downstream of the cauliflower mosaic virus 35S promoter. The resulting construct was bombarded into CTV-infected tissue, which was then grafted onto virus-free plants. The presence of recombinant RNA in systemically infected leaves was demonstrated by RT-PCR. Sequencing the RT-PCR products synthesized from double-stranded RNA confirmed the presence of the chimeric segment used for tagging. This is the first report of an infectious cDNA molecule derived from CTV D-RNA.

Adenovirus-mediated gene transfer to the brain: methodological assessment

The purpose of this short review is to analyse major advantages and limitations of the adenovirus (Ad), specifically with relevance to its use as a vector for gene transfer to the brain. The characteristics of Ad transduction include: the relative absence of cell type specificity; the limited spatial spread of the virus; and the long-term expression of the transgene. In the central nervous system, in contrast to that which occurs in other organs, Ad transduction in the adult does not systematically provoke cell death. Nevertheless, a proportion of the transduced cells do die, and this represents a conspicuous problem. Mechanisms leading to cell death in the brain may include immune rejection and inflammation-related toxicity, although this would not explain all of the results, and direct toxicity related to either inappropriate preparation or the transduction itself. Taking into account uncertainties concerning the innocuousness of Ad transduction, it may seem unwise to envisage Ad gene therapy for diseases that are not life-threatening and/or benefit from adequate drug or surgical treatments (e.g. Parkinson's disease or epilepsy). Ad vectors may not be easily used either in diseases displaying major immune dysfunction (e.g. multiple sclerosis). In contrast, malignant brain tumors and numerous neurodegenerative diseases (such as Huntington's, Alzheimer's diseases or amyotrophic lateral sclerosis) are directly life-threatening and deprived of any adequate treatment. They may be appropriate targets for Ad-mediated gene therapy, once both the vector and the gene of interest have been defined and optimized.

High levels of gene transduction in human lung tumors following intralesional injection of recombinant adenovirus

To effect gene transfer into large solid malignancies for the purpose of clinical application, new treatment strategies using intralesional administration of adenovirus were studied. Replication-deficient adenovirus Ad5LacZ, containing the Escherichia coli beta-galactosidase (beta-gal) gene (LacZ), was injected directly into 1-cm x 1-cm subcutaneous xenograph tumors of human large cell lung cancers (H460 and H1299). Each tumor received a single injection or three injections of purified virus, diluted in 200 microL of phosphate-buffered saline. The tumors were harvested 3 days after the last injection, serially sectioned, and stained with X-gal. The cells expressing beta-gal were counted by using digital image analysis and the percentage of tumor cells transduced was calculated. After a single viral injection of 1 x 10(9) PFU, 5 x 10(9) PFU, or 1 x 10(10) PFU solid tumor transduction increased significantly with dose escalation. At a dose of 1 x 10(10) PFU, transduction of the H1299 and H460 tumors was 80.2% and 46.7%, respectively. Dividing the viral dose into three injections given on alternating days had no significant effect on viral transduction. These data demonstrate that a large portion of an established human lung cancer cell line tumor undergoes gene transduction after a single intralesional injection of recombinant adenovirus.

The production of recombinant infectious DI-particles of a murine coronavirus in the absence of helper virus

We have studied the production and release of infectious DI-particles in vaccinia-T7-polymerase recombinant virus-infected L cells that were transfected with five different plasmids expressing the synthetic DI RNA MIDI-HD and the four structural proteins (M, N, S, and E) of the murine coronavirus MHV-A59. The DI cDNA contains the hepatitis delta ribozyme sequences to generate in the transfected cells a defined 3' end. In EM studies of transfected cells virus-like particles (VLP) were observed in vesicles. Release of the particles into the medium was studied by immunoprecipitations of proteins released into the culture supernatant. Particle release was independent of S or N, but required M and E. Coexpression of E and M was sufficient for particle release. Coexpression of the structural proteins and the MIDI-HD RNA resulted in the production and release of infectious DI-particles. Infectivity of the DI-particles was determined by adding helper virus MHV-A59 to the medium containing the VLPs and using this mixture to infect new L cells. Intracellular RNA of several subsequent undiluted passages was isolated to detect the MIDI-HD RNA. Passage of the MIDI-HD RNA was dependent on the expression of the structural proteins of MHV-A59 in the transfected cells. In the absence of either E or M, MIDI-HD RNA could not be passaged to fresh L cells. We have thus developed a system in which we can produce coronavirus-like particles and an assay to test their infectivity.

Safety of adenovirus-mediated transfer of the human cystic fibrosis transmembrane conductance regulator cDNA to the lungs of nonhuman primates

To define the toxicity of cystic fibrosis transmembrane conductance regulator gene (CFTR) gene therapy with a replication-deficient recombinant adenovirus (Av1Cf2) in a nonhuman primate model, 10(10) plaque forming units (pfu) were instilled directly through a bronchoscope into the right lung of 5 macaques, and a lower dose of 4 x 10(6) pfu was administered to the right lung of 1 macaque. One sham-treated control received phosphate-buffered saline (PBS). The macaques were evaluated sequentially by clinical examination, vital signs, weight, hematology, blood chemistry, chest radiography, pulse oximetry, and bronchoalveolar lavage (BAL) at baseline and 3-28 days post-treatment. After the period of observation, macaques were sacrificed for autopsy and histological examination. The macaques tolerated the experimental therapy clinically with no changes in body temperature, oxygen saturation, heart rate, body weight, or blood pressure. However, 1 macaque with visible evidence of aspiration at the time of initial bronchoscopy developed tachypnea with right lower lobe (RLL) pneumonia on chest radiograph and by histology. There were no changes in Hgb, Wbc, BUN, plasma electrolytes, bilirubin, or hepatic transaminases. In the macaques that received 10(10) pfu, there was a progressive increase in the number of CD8+ lymphocytes in BAL that was maximal at 28 days. Histological examination of the treated lungs of the high-dose macaques at 3 days showed marked peribronchial and perivascular cuffing by inflammatory cells and alveolar accumulation of neutrophils and macrophages. The alveolitis appeared to be resolving at 28 days, although the perivascular and peribronchial aggregates of mononuclear cells were still present. In the high-dose macaques, BAL interleukin-8 (IL-8) was increased at all time points (256-388 pg/ml versus 1-84 pg/ml at baseline and in control), whereas IL-1 beta was increased only at days 21 and 28 (341-852 pg/ml versus 30-92 pg/ml at baseline and in control). There were no increases in BAL cell counts, IL-1 beta or IL-8, and histological changes were mild in the macaque that received 4 x 10(6) pfu. Evaluation for Av1Cf2-derived human CFTR expression using RS-PCR demonstrated expression at 3, 10, and 21, but not 28 days in macaques treated with 10(10) pfu of Av1Cf2. In situ hybridization analysis demonstrated human CFTR mRNA in the alveolar regions of the lobes that received the vector at 10 and 21 days. There was no evidence of expression after treatment with 4 x 10(6) pfu. This study showed that high-dose adenoviral vector administration to the lung achieved CFTR gene transfer and expression but was associated with increased concentrations of cytokines in BAL and alveolar inflammation. A low dose, equivalent to the maximum clinical dose currently proposed for phase I trials in human subjects, was not associated with cellular or cytokine evidence of inflammation, and histological abnormalities were mild.

Adenoviral delivery of recombinant DNA into transgenic mice bearing hepatocellular carcinomas

To evaluate the applicability of recombinant adenoviral vectors in gene transfer to liver cancers, we infused the recombinant adenoviruses AD5CMV-LacZ and Ad5CMV-p53 through the portal veins into two lines of transgenic mice, one bearing the SV40 T antigen and the other the human hepatitis B viral envelope protein. These transgenic animals develop hepatocellular carcinomas (HCC) with predictable pathological manifestations. The levels of expression of the transgenes were dependent upon the viral doses. In all cases, high levels of expression were detected within 2 or 3 days after infusion, but were drastically reduced 7 days after infusion. Significant toxicities were found in the infused animals: > 80% of them died within 7 days after infusion with 10(10) pfu, and transgenic animals bearing HCC apparently were more sensitive to viral toxicity. Although a lower dose (10(9) pfu/animal) produced less toxicity, the levels of expression were substantially reduced (only about 10% of that in animals infused with 10(10) pfu). When Ad5CMV-p53 was infused into animals with nodular hyperplastic stage, the expression of the reporter gene seemed to distribute preferentially at the peripheries of the tumor nodules, and low levels of transgene expression were seen inside the nodules. In tumors in which necrotic lesions were evident, p53 was also expressed at the perpheries of the lesions. These distribution patterns were seen in both tumor models. There was no apparent suppression of tumor growth in the Ad5CMV-p53-infused animals. Our results suggest that alternative methods for gene therapy for HCC need to be explored.

Systematic analysis of repeated gene delivery into animal lungs with a recombinant adenovirus vector

Adenovirus-based vectors are promising candidates for genetic therapy of cystic fibrosis (CF). Because adenoviruses naturally infect airway cells, they grow to very high titers, and the transgenes carried by the adenoviruses are expressed at high levels. In addition, adenoviruses are relatively safe because the disease caused by the wild-type virus is self-limiting. One disadvantage of adenovirual vectors is that the transgene expression would be transient because adenoviruses do not integrate their DNA into the genome of the host cells. Adenoviral gene delivery into the lungs is also complicated by the anatomy of the airways and the defense mechanisms of the recipient. To assess the feasibility of adenovirus-mediated gene therapy for CF, a recombinant adenovirus carrying a lacZ gene was delivered into animal lungs to study the efficiency and cellular distribution of gene transfer, the duration of gene expression, the possible histopathology of the lungs after gene transfer, and the efficacy of repeated administrations of the viral agent. The results of these studies demonstrate that (i) efficient gene transfer into animal lungs can be achieved; (ii) a near-homogenous delivery of the vectors can be achieved by airway instillation, although the pattern of transduction varies among individual animals; (iii) pathological effects are generally mild in CD1 mice; (iv) gene expression is transient; (v) repetitive gene transfer is achievable, but becomes progressively less efficient, and (vi) immune responses are induced against both the viral and transgene products.

Quinolinic acid levels in a murine retrovirus-induced immunodeficiency syndrome

Mice infected with the retrovirus mixture designated LP-BM5 murine leukemia virus (MuLV) develop an immunosuppressive disease. Quinolinic acid (QUIN) is an endogenous neurotoxic N-methyl-D-aspartate agonist that may contribute to the pathogenesis of HIV-associated neurologic disease. In the present study, the levels of QUIN in brain and blood were measured in mice infected with LP-BM5 MuLV and compared with those in uninfected mice and mice infected with the nonpathogenic strain of ecotropic MuLV (helper component of LP-BM5 MuLV). Infection with LP-BM5 MuLV resulted in progressive increases in blood QUIN levels beginning 2 weeks after inoculation that peaked by 16 weeks postinfection. QUIN levels were also increased in cerebral cortex, hippocampus, and striatum. In systemic tissues, QUIN levels were increased in lung, liver, and spleen. In contrast, infection with the ecotropic viral component of the LP-BM5 MuLV mixture was not associated with any changes in brain, blood, or systemic tissue QUIN levels, even though helper virus burdens were comparable to those in mice infected with LP-BM5 MuLV. Treatment of LP-BM5 MuLV-infected mice with the antiretroviral agent zidovudine (azidothymidine) significantly reduced blood and brain QUIN levels in association with reductions in viral load in brain and spleen. These observations suggest that elevated QUIN production is not attributable to productive infection with retrovirus per se but occurs in response to an agent or agents, such as cytokines, that are produced by the host in response to virus infection.