Bluetongue virus induces apoptosis in cultured mammalian cells by both caspase-dependent extrinsic and intrinsic apoptotic pathways

An updated review on bluetongue virus: epidemiology, pathobiology, and advances in diagnosis and control with special reference to India

Bluetongue (BT) is an economically important, non-contagious viral disease of domestic and wild ruminants. BT is caused by BT virus (BTV) and it belongs to the genus Orbivirus and family Reoviridae. BTV is transmitted by Culicoides midges and causes clinical disease in sheep, white-tailed deer, pronghorn antelope, bighorn sheep, and subclinical manifestation in cattle, goats and camelids. BT is a World Organization for Animal Health (OIE) listed multispecies disease and causes great socio-economic losses. To date, 28 serotypes of BTV have been reported worldwide and 23 serotypes have been reported from India. Transplacental transmission (TPT) and fetal abnormalities in ruminants had been reported with cell culture adopted live-attenuated vaccine strains of BTV. However, emergence of BTV-8 in Europe during 2006, confirmed TPT of wild-type/field strains of BTV. Diagnosis of BT is more important for control of disease and to ensure BTV-free trade of animals and their products. Reverse transcription polymerase chain reaction, agar gel immunodiffusion assay and competitive enzyme-linked immunosorbent assay are found to be sensitive and OIE recommended tests for diagnosis of BTV for international trade. Control measures include mass vaccination (most effective method), serological and entomological surveillance, forming restriction zones and sentinel programs. Major hindrances with control of BT in India are the presence of multiple BTV serotypes, high density of ruminant and vector populations. A pentavalent inactivated, adjuvanted vaccine is administered currently in India to control BT. Recombinant vaccines with DIVA strategies are urgently needed to combat this disease. This review is the first to summarise the seroprevalence of BTV in India for 40 years, economic impact and pathobiology.

Life as a Vector of Dengue Virus: The Antioxidant Strategy of Mosquito Cells to Survive Viral Infection

Dengue fever is a mosquito-borne viral disease of increasing global importance. The disease has caused heavy burdens due to frequent outbreaks in tropical and subtropical areas of the world. The dengue virus (DENV) is generally transmitted between human hosts via the bite of a mosquito vector, primarily Aedes aegypti and Ae. albopictus as a minor species. It is known that the virus needs to alternately infect mosquito and human cells. DENV-induced cell death is relevant to the pathogenesis in humans as infected cells undergo apoptosis. In contrast, mosquito cells mostly survive the infection; this allows infected mosquitoes to remain healthy enough to serve as an efficient vector in nature. Overexpression of antioxidant genes such as superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GPx), glutathione S-transferase (GST), glutaredoxin (Grx), thioredoxin (Trx), and protein disulfide isomerase (PDI) have been detected in DENV2-infected mosquito cells. Additional antioxidants, including GST, eukaryotic translation initiation factor 5A (eIF5a), and p53 isoform 2 (p53-2), and perhaps some others, are also involved in creating an intracellular environment suitable for cell replication and viral infection. Antiapoptotic effects involving inhibitor of apoptosis (IAP) upregulation and subsequent elevation of caspase-9 and caspase-3 activities also play crucial roles in the ability of mosquito cells to survive DENV infection. This article focused on the effects of intracellular responses in mosquito cells to infection primarily by DENVs. It may provide more information to better understand virus/cell interactions that can possibly elucidate the evolutionary pathway that led to the mosquito becoming a vector.

Pathological and immunological characterization of bluetongue virus serotype 1 infection in type I interferons blocked immunocompetent adult mice

Introduction

Wild-type adult mice with intact interferon (IFN) system were neither susceptible to bluetongue virus (BTV) infection nor showed signs of morbidity/mortality. Establishment of immunologically competent wild-type adult mouse model with type I IFNs blockade is necessary to assess the pathogenesis, immune responses and testing of BTV vaccines.

Objectives

Present study aimed to establish and characterize BTV serotype 1 infection in immunocompetent adult mice with type I IFNs blockade at the time of infection by studying immune responses and sequential pathology.

Methods

Adult mice were administered with anti-mouse IFN-α/β receptor subunit-1 (IFNAR1) blocking antibody (Clone: MAR1-5A3) 24 h before and after BTV serotype 1 infection, and sacrificed at various time points. Sequential pathology, BTV localization by immunohistochemistry and quantification by qRT-PCR, immune cell kinetics and apoptosis by flow cytometry, and cytokines estimation by c-ELISA and qRT-PCR were studied.

Results

IFNAR blocked-infected mice developed clinical signs and typical lesions of BT; whereas, isotype-infected control mice did not develop any disease. The IFNAR blocked-infected mice showed enlarged, edematous, and congested lymph nodes (LNs) and spleen, and vascular (congestion and hemorrhage) and pneumonic lesions in lungs. Histopathologically, marked lymphoid depletion with “starry-sky pattern” due to lymphocytes apoptosis was noticed in the LNs and spleen. BTV antigen was detected and quantified in lymphoid organs, lungs, and other organs at various time points. Initial leukopenia (increased CD4⁺/CD8⁺ T cells ratio) followed by leukocytosis (decreased CD4⁺/CD8⁺ T cells ratio) and significantly increased biochemical values were noticed in IFNAR blocked-infected mice. Increased apoptotic cells in PBMCs and tissues coincided with viral load and levels of different cytokines in blood, spleen and draining LNs and notably varied between time points in IFNAR blocked-infected mice.

Conclusion

Present study is first to characterize BTV serotype 1 infection in immunocompetent adult mouse with type I IFNs blockade. The findings will be useful for studying pathogenesis and testing the efficacy of BTV vaccines.

Virological, immunological and pathological findings of transplacentally transmitted bluetongue virus serotype 1 in IFNAR1-blocked mice during early and mid gestation

Transplacental transmission (TPT) of wild-type Indian BTV-1 had never been experimentally proved. This study was first time investigated TPT of Indian BTV-1 (isolated from aborted and stillborn goat fetal spleens). The sequential pathology, virological and immune cell kinetics (CD4⁺, CD8⁺ T-lymphocytes and NK cells in spleen and PBMCs), and apoptosis in IFNAR1-blocked pregnant mice during early (infected on 1 GD) and mid (infected on 8 GD) gestation have been studied. There was higher rate of TPT during mid stage (71.43%) than early (57.14%) stage. In early stage reduced implantation sites, early embryonic deaths, abortions, and necro-haemorrhagic lesions had observed. Mid stage, congenital defects and neurological lesions in foetuses like haemorrhages, diffuse cerebral edema, necrotizing encephalitis and decreased bone size (Alizarin red staining) were noticed. BTV-1 antigen was first time demonstrable in cells of mesometrium, decidua of embryos, placenta, uterus, ovary, and brain of foetuses by immunohistochemistry and quantified by real-time qRT-PCR. BTV-inoculated mice were seroconverted by 7 and 5 dpi, and reached peak levels by 15 and 9 dpi in early and mid gestation, respectively. CD4⁺ and CD8⁺ cells were significantly decreased (increased ratio) on 7 dpi but subsequently increased on 15 dpi in early gestation. In mid gestation, increased CD8⁺ cells (decreased ratio) were observed. Apoptotic cells in PBMCs and tissues increased during peak viral load. This first time TPT of wild-type Indian BTV-1 deserves to be reported for implementation of control strategies. This model will be very suitable for further research into mechanisms of TPT, overwintering, and vaccination strategies.

Apoptosis induced by Ibaraki virus does not affect virus replication and cell death in hamster lung HmLu-1 cells

Ibaraki virus (IBAV) is an arbovirus that is transmitted by biting midges and causes Ibaraki disease in cattle. IBAV induces apoptosis in several mammalian cell lines, and apoptosis in turn facilitates IBAV replication. In addition, virus-induced apoptosis may contribute to mammalian-specific pathogenicity considering that some arboviruses induce apoptosis in mammalian cells but not in insect cells. In this study, we found that when hamster lung cells (HmLu-1) are used as a virus host, IBAV causes severe cytopathic effects with little induction of apoptosis. Furthermore, pharmacological inhibition of apoptosis did not affect IBAV-induced cytotoxicity. These results indicate the existence of an apoptosis-independent pathway in which IBAV replicates and exerts cytotoxicity in mammalian cells.

Schmallenberg virus induces apoptosis in Vero cell line via extrinsic and intrinsic pathways in a time and dose dependent manner

Schmallenberg virus (SBV), discovered in 2011 in Germany, is associated with clinical manifestations of fever, diarrhea, reduced milk yield, abortions and congenital malformations in ruminants. Despite many studies performed for SBV, there is no detailed research on in vitro apoptotic effect of SBV. This study is aimed to determine apoptosis pathways and role of pro-apoptotic and anti-apoptotic molecules in Vero cells infected with SBV. The study results showed that SBV induced apoptosis via both extrinsic and intrinsic pathways by activating both caspase-8 and caspase-9, respectively. Expression analyses of pro-apoptotic (Bax, Bak and Puma) and anti-apoptotic (Bcl-2 and Bcl-XL) genes revealed that SBV-induced apoptosis causes upregulation of pro-apoptotic genes, dominantly via Puma gene, whereas Bcl-2 and Bcl-XL genes were downregulated. In conclusion, this is the first detailed report about SBV induced apoptosis in the Vero cells via both extrinsic and intrinsic cascades and apoptosis induction is seem to be regulated by Puma.

The Culicoides sonorensis inhibitor of apoptosis 1 protein protects mammalian cells from apoptosis induced by infection with African horse sickness virus and bluetongue virus

African horse sickness virus (AHSV) and bluetongue virus (BTV) are arboviruses of the genus Orbivirus that are transmitted to their vertebrate hosts by Culicoides biting midges. These orbiviruses exhibit lytic infection (apoptosis) in mammalian cells, but cause persistent infection with no cytopathic effects in Culicoides sonorensis cells. Although regulation of apoptosis could thus be integral for establishing persistent virus infection in midge cells, nothing is known about the presence and function of apoptosis pathways in Culicoides midges and their derived cell lines. Here, we report the cloning and functional characterization of an inhibitor of apoptosis protein (IAP), designated CsIAP1, from C. sonorensis cells. The CsIAP1 protein contains two baculoviral IAP repeat (BIR) domains and a RING domain. Silencing of the Cs iap1 gene in C. sonorensis cells caused apoptosis, indicating that CsIAP1 plays a role in cell survival. Stable expression of the CsIAP1 protein in BSR mammalian cells suppressed apoptosis induced by AHSV-4 and BTV-10 infection, and biochemical data indicated that CsIAP1 is an inhibitor of mammalian caspase-9, an initiator caspase in the intrinsic apoptotic pathway. Mutagenesis studies indicated that the BIR2 and RING domains are required for the anti-apoptotic activity of CsIAP1. The results suggest that the mechanism by which CsIAP1 suppresses apoptosis in insect cells may involve inhibition of a Culicoides caspase-9 homologue through a mechanism that requires both the BIR2 and RING domains. This study provides the first evidence that the CsIAP1 protein is a key negative regulator of apoptosis in C. sonorensis cells.

Pathological Characterization Of IFNAR(-/-) Mice Infected With Bluetongue Virus Serotype 4

Bluetongue virus (BTV) replicates in lymphoid tissues where infected mononuclear leukocytes secrete proinflammatory and vasoactive mediators that can contribute to bluetongue (BT) pathogenesis. Using the well-characterized IFNAR(-/-) mice animal model, we have now studied the histopathology and dynamics of leukocyte populations in different target tissues (spleen, thymus, and lung) during BTV-4 infection by histological and immunohistochemical techniques. The spleen and thymus of BTV-4 infected mice showed severe lymphoid depletion on H&E stained sections. This finding was confirmed by IHC, showing moderate decreased immunopositivity against CD3 in the thymus, and scarce immunoreactivity against CD3 and CD79 in the rest of the white pulp in the spleen, together with an increase in MAC387 immunostaining. BTV-4 infection also induced the expression of active caspase-3 in the spleen, where apoptotic debris was observed by H&E. A dramatic increase in iNOS immunoreactivity associated to necrotic areas of the white pulp was observed, being less noticeable in the thymus and the lung. The induction of pro-inflammatory cytokines in tissues where BTV replicates was evaluated by measuring transcript levels by RT-qPCR. BTV-4 infection led to enhance transcription of IFN-γ, TNF, IL-6, IL-12-p40, and IL-1β mRNA in the thymus, spleen and lung, correlating with the level of virus replication in these tissues. Disease progression and pathogenesis in IFNAR(-/-) mice closely mimics hallmarks of bluetongue disease in ruminants. IFNAR(-/-) mice are a good choice to facilitate a faster advance in the field of orbiviruses.

Proteomic analysis of sheep primary testicular cells infected with bluetongue virus

Bluetongue virus (BTV) causes a non‐contagious, arthropod‐transmitted disease in wild and domestic ruminants, such as sheep. In this study, we used iTRAQ labeling coupled with LC‐MS/MS for quantitative identification of differentially expressed proteins in BTV‐infected sheep testicular (ST) cells. Relative quantitative data were obtained for 4455 proteins in BTV‐ and mock‐infected ST cells, among which 101 and 479 proteins were differentially expressed at 24 and 48 h post‐infection, respectively, indicating further proteomic changes during the later stages of infection. Ten corresponding genes of differentially expressed proteins were validated via real‐time RT‐PCR. Expression levels of three representative proteins, eIF4a1, STAT1 and HSP27, were further confirmed via western blot analysis. Bioinformatics analysis disclosed that the differentially expressed proteins are primarily involved in biological processes related to innate immune response, signal transduction, nucleocytoplasmic transport, transcription and apoptosis. Several upregulated proteins were associated with the RIG‐I‐like receptor signaling pathway and endocytosis. To our knowledge, this study represents the first attempt to investigate proteome‐wide dysregulation in BTV‐infected cells with the aid of quantitative proteomics. Our collective results not only enhance understanding of the host response to BTV infection but also highlight multiple potential targets for the development of antiviral agents.

Apoptosis, autophagy and unfolded protein response pathways in Arbovirus replication and pathogenesis

Arboviruses are pathogens that widely affect the health of people in different communities around the world. Recently, a few successful approaches toward production of effective vaccines against some of these pathogens have been developed, but treatment and prevention of the resulting diseases remain a major health and research concern. The arbovirus infection and replication processes are complex, and many factors are involved in their regulation. Apoptosis, autophagy and the unfolded protein response (UPR) are three mechanisms that are involved in pathogenesis of many viruses. In this review, we focus on the importance of these pathways in the arbovirus replication and infection processes. We provide a brief introduction on how apoptosis, autophagy and the UPR are initiated and regulated, and then discuss the involvement of these pathways in regulation of arbovirus pathogenesis.

Apoptin as a potential viral gene oncotherapeutic agent

The use of viruses for treatment of cancer overcomes the bottlenecks of chemotherapy and radiotherapy. Several viruses and their proteins have been evaluated for oncolytic effect. The VP3 protein (apoptin) of chicken anemia virus is one such protein with an inherent ability to lyse cancer and transformed cells while leaving normal cells unharmed. In the present study, the apoptosis inducing potential of VP3 protein of CAV was evaluated in human cervical cancer cell line (HeLa). It was found that in VP3-induced apoptosis, caspase-dependent intrinsic pathway plays an important role with the cleavage of poly (ADP-ribose) polymerase (PARP) and there was no evidence of involvement of death receptor-mediated extrinsic pathway. The results of this study provide intuitive information and strengthen the candidacy of apoptin as a viral oncotherapeutic agent.

Comparative ultrastructural characterization of African horse sickness virus-infected mammalian and insect cells reveals a novel potential virus release mechanism from insect cells

African horse sickness virus (AHSV) is an arbovirus capable of successfully replicating in both its mammalian host and insect vector. Where mammalian cells show a severe cytopathic effect (CPE) following AHSV infection, insect cells display no CPE. These differences in cell death could be linked to the method of viral release, i.e. lytic or non-lytic, that predominates in a specific cell type. Active release of AHSV, or any related orbivirus, has, however, not yet been documented from insect cells. We applied an integrated microscopy approach to compare the nanomechanical and morphological response of mammalian and insect cells to AHSV infection. Atomic force microscopy revealed plasma membrane destabilization, integrity loss and structural deformation of the entire surface of infected mammalian cells. Infected insect cells, in contrast, showed no morphological differences from mock-infected cells other than an increased incidence of circular cavities present on the cell surface. Transmission electron microscopy imaging identified a novel large vesicle-like compartment within infected insect cells, not present in mammalian cells, containing viral proteins and virus particles. Extracellular clusters of aggregated virus particles were visualized adjacent to infected insect cells with intact plasma membranes. We propose that foreign material is accumulated within these vesicles and that their subsequent fusion with the cell membrane releases entrapped viruses, thereby facilitating a non-lytic virus release mechanism different from the budding previously observed in mammalian cells. This insect cell-specific defence mechanism contributes to the lack of cell damage observed in AHSV-infected insect cells.

Epizootic hemorrhagic disease virus induces and benefits from cell stress, autophagy, and apoptosis

The mode and timing of virally induced cell death hold the potential of regulating viral yield, viral transmission, and the severity of virally induced disease. Orbiviruses such as the epizootic hemorrhagic disease virus (EHDV) are nonenveloped and cytolytic. To date, the death of cells infected with EHDV, the signal transduction pathways involved in this process, and the consequence of their inhibition have yet to be characterized. Here, we report that the Ibaraki strain of EHDV2 (EHDV2-IBA) induces apoptosis, autophagy, a decrease in cellular protein synthesis, the activation of c-Jun N-terminal kinase (JNK), and the phosphorylation of the JNK substrate c-Jun. The production of infectious virions decreased upon inhibition of apoptosis with the pan-caspase inhibitor Q-VD-OPH (quinolyl-valyl-O-methylaspartyl-[-2,6-difluorophenoxy]-methyl ketone), upon inhibition of autophagy with 3-methyladenine or via the knockout of the autophagy regulator Atg5, or upon treatment of infected cells with the JNK inhibitor SP600125 or the cyclin-dependent kinase (CDK) inhibitor roscovitine, which also inhibited c-Jun phosphorylation. Moreover, Q-VD-OPH, SP600125, and roscovitine partially reduced EHDV2-IBA-induced cell death, and roscovitine diminished the induction of autophagy by EHDV2-IBA. Taken together, our results imply that EHDV induces and benefits from the activation of signaling pathways involved in cell stress and death.

Novel virostatic agents against bluetongue virus

Bluetongue virus (BTV), a member in the family Reoviridae, is a re-emerging animal disease infecting cattle and sheep. With its recent outbreaks in Europe, there is a pressing need for efficacious antivirals. We presented here the identification and characterization of a novel virostatic molecule against BTV, an aminothiophenecarboxylic acid derivative named compound 003 (C003). The virostatic efficacy of C003 could be improved via chemical modification, leading to a de novo synthesized compound 052 (C052). The 50% effective concentrations (EC(50)) of C003 and C052 were determined at 1.76 ± 0.73 µM and 0.27 ± 0.12 µM, respectively. The 50% cytotoxicity concentration (CC(50)) of C003 was over 100 µM and the CC(50) of C052 was at 82.69 µM. Accordingly, the 50% selective index (SI(50)) of C003 and C052 against BTV was over 57 and 306, respectively. The inhibitory effect of C003/C052 on BTV-induced apoptosis was also confirmed via the inhibition of caspase-3/-7 activation post BTV infection. C003/C052 could inhibit BTV induced CPE even when added as late as 24 h.p.i., indicating that they might act at late stage of viral life-cycle. C003/C052 could reduce over two-logs of both the progeny virus production and the number of genomic viral RNA copies. Interestingly, both the activation of host autophagy and viral protein expression were inhibited post BTV infection when cells were treated with C003 and C052, suggesting that C003/C052 might act as virostatic agents via inhibiting host autophagy activation. Although further investigations might be needed to pin down the exact mechanism of C003/C052, our finding suggested that these compounds might be potent lead compounds with potential novel mechanism of action against BTV.

Additive protection by antioxidant and apoptosis-inhibiting effects on mosquito cells with dengue 2 virus infection

Cytopathic effects (CPEs) in mosquito cells are generally trivial compared to those that occur in mammalian cells, which usually end up undergoing apoptosis during dengue virus (DENV) infection. However, oxidative stress was detected in both types of infected cells. Despite this, the survival of mosquito cells benefits from the upregulation of genes related to antioxidant defense, such as glutathione S transferase (GST). A second defense system, i.e., consisting of antiapoptotic effects, was also shown to play a role in protecting mosquito cells against DENV infection. This system is regulated by an inhibitor of apoptosis (IAP) that is an upstream regulator of caspases-9 and -3. DENV-infected C6/36 cells with double knockdown of GST and the IAP showed a synergistic effect on activation of these two caspases, causing a higher rate of apoptosis (>20%) than those with knockdown of each single gene (∼10%). It seems that the IAP acts as a second line of defense with an additional effect on the survival of mosquito cells with DENV infection. Compared to mammalian cells, residual hydrogen peroxide in DENV-infected C6/36 cells may signal for upregulation of the IAP. This novel finding sheds light on virus/cell interactions and their coevolution that may elucidate how mosquitoes can be a vector of DENV and probably most other arboviruses in nature.

This study demonstrated an idea that mosquito cells can survive dengue virus (or other arboviruses) infection through antioxidant defense and an additional effect by induction of IAP expression for protection of infection. It makes mosquito eligible to support virus replication efficiently, leading to a goal which is important to explain how mosquitoes can be a vector even when they have been seriously infected by the virus. Our findings opened an avenue for studies on virus/vector co-evolution that benefits for both virus replication and its transmission to humans or susceptible hosts.

Detection of a fourth orbivirus non-structural protein

The genus Orbivirus includes both insect and tick-borne viruses. The orbivirus genome, composed of 10 segments of dsRNA, encodes 7 structural proteins (VP1–VP7) and 3 non-structural proteins (NS1–NS3). An open reading frame (ORF) that spans almost the entire length of genome segment-9 (Seg-9) encodes VP6 (the viral helicase). However, bioinformatic analysis recently identified an overlapping ORF (ORFX) in Seg-9. We show that ORFX encodes a new non-structural protein, identified here as NS4. Western blotting and confocal fluorescence microscopy, using antibodies raised against recombinant NS4 from Bluetongue virus (BTV, which is insect-borne), or Great Island virus (GIV, which is tick-borne), demonstrate that these proteins are synthesised in BTV or GIV infected mammalian cells, respectively. BTV NS4 is also expressed in Culicoides insect cells. NS4 forms aggregates throughout the cytoplasm as well as in the nucleus, consistent with identification of nuclear localisation signals within the NS4 sequence. Bioinformatic analyses indicate that NS4 contains coiled-coils, is related to proteins that bind nucleic acids, or are associated with membranes and shows similarities to nucleolar protein UTP20 (a processome subunit). Recombinant NS4 of GIV protects dsRNA from degradation by endoribonucleases of the RNAse III family, indicating that it interacts with dsRNA. However, BTV NS4, which is only half the putative size of the GIV NS4, did not protect dsRNA from RNAse III cleavage. NS4 of both GIV and BTV protect DNA from degradation by DNAse. NS4 was found to associate with lipid droplets in cells infected with BTV or GIV or transfected with a plasmid expressing NS4.

African horse sickness virus induces apoptosis in cultured mammalian cells

Infection of mammalian cell cultures with African horse sickness virus (AHSV) is known to result in dramatic cytopathic effects (CPE), but no CPE is observed in infected insect cell cultures despite productive virus replication. The basis for this phenomenon has not yet been investigated, but is suggestive of apoptosis being induced following virus infection of the mammalian cells. To investigate whether AHSV can induce apoptosis in infected mammalian cells, Culicoides variipennis (KC) insect cells and BHK-21 mammalian cells were infected with AHSV-9 and analyzed for morphological and biochemical hallmarks of apoptosis. In contrast to KC cells, infection of BHK-21 cells with AHSV-9 resulted in ultrastructural changes and nuclear DNA fragmentation, both of which are associated with the induction of apoptosis. Results also indicated that AHSV-9 infection of BHK-21 cells resulted in activation of caspase-3, a key agent in apoptosis, and in mitochondrial membrane depolarization. Cumulatively, the data indicate that the intrinsic pathway is activated in AHSV-induced apoptosis.

Role of cellular caspases, nuclear factor-kappa B and interferon regulatory factors in Bluetongue virus infection and cell fate

Background

Bluetongue virus (BTV) infection causes haemorrhagic disease in ruminants and induces cell death. The pathogenesis in animals and in cell culture has been linked to BTV-induced apoptosis.

Results

In this report, we investigated BTV-induced apoptosis in cell culture in depth and show that both extrinsic (caspase-8 activation) and intrinsic (caspase-9 activation) pathways play roles in BTV apoptosis. Further, by using chemical inhibitors and knock-out cell lines, we show that these pathways act independently of each other in BTV infected cells. In addition to activation of caspase-8, -9 and executioner caspase-3, we also identified that BTV infection causes the activation of caspase-7, which results in the cleavage of poly (ADP-ribose) polymerase (PARP). BTV-induced cell death appears to be due to apoptosis rather than necrosis, as the HMBG-1 was not translocated from the nucleus. We also examined if NF-κB response is related to BTV-induced apoptosis as in reovirus. Our data suggests that NF-κB response is not linked to the induction of apoptosis. It is controlled by the degradation of only IκBα but not IκBβ, resulting in a rapid transient response during BTV infection. This was supported using an NF-κB dependent luciferase reporter gene assay, which demonstrated early response, that appeared to be suppressed by the late stage of BTV replication. Furthermore, virus titres were higher in the presence of NF-κB inhibitor (SN50), indicating that NF-κB has a role in initiating an antiviral environment. In addition, we show that BTV infection induces the translocation of interferon regulatory factors (IRF-3 and IRF-7) into the nucleus. The induction of IRF responses, when measured by IRF dependent luciferase reporter gene assay, revealed that the IRF responses, like NF-κB response, were also at early stage of infection and mirrored the timing of NF-κB induction.

Conclusion

BTV triggers a wide range of caspase activities resulting in cell apoptosis. Although both NF-κB and IRF responses are induced by BTV infection, they are not sustained.

Apoptosis and immuno-suppression in sheep infected with bluetongue virus serotype-23

The role of apoptosis in pathogenesis of bluetongue (BT) has been suggested from various in vitro studies. However, to date, no clear data are available regarding BTV-induced apoptosis and its consequences in natural host, sheep. In the present study, bluetongue virus (BTV)-induced apoptosis was studied in sheep blood and splenic mononuclear cells by analyzing annexin(+)-propidium iodide(-) early apoptotic cells, DNA ladder pattern, and caspase-3 gene expression. The onset of apoptosis and lymphocyte depletion in viraemic phase and IFN-alpha response indicated the involvement of BTV and IFN-alpha in the pathogenesis of BT. The development of Pasteurella pneumonia in 4 of 7 infected sheep during the experiment pointed to possible BTV-induced immuno-suppression and predisposition to secondary microbial infections. These results have significant implications not only in understanding immuno-pathological consequences but also in studying interactions of BTV with host cells.

Cysteine protease activation and apoptosis in Murine norovirus infection

Background

Noroviruses are the leading cause of viral gastroenteritis. Because a suitable in vitro culture system for the human virus has yet to be developed, many basic details of the infection process are unknown. Murine norovirus (MNV) serves as a model system for the study of norovirus infection. Recently it was shown that infection of RAW 264.7 cells involved a novel apoptotic pathway involving survivin.

Results

Using a different set of approaches, the up-regulation of caspases, DNA condensation/fragmentation, and membrane blebbing, all of which are markers of apoptosis, were confirmed. Live cell imaging and activity-based protein profiling showed that activation of caspase-like proteases occurred within two hours of infection, followed by morphological changes to the cells. MNV infection in the presence of caspase inhibitors proceeded via a distinct pathway of rapid cellular necrosis and reduced viral production. Affinity purification of activity-based protein profiling targets and identification by peptide mass fingerprinting showed that the cysteine protease cathepsin B was activated early in infection, establishing this protein as an upstream activator of the intrinsic apoptotic pathway.

Conclusion

This work adds cathepsin B to the noncanonical programmed cell death induced by MNV, and provides data suggesting that the virus may induce apoptosis to expand the window of time for viral replication. This work also highlights the significant power of activity-based protein profiling in the study of viral pathogenesis.

Assay development and high-throughput antiviral drug screening against Bluetongue virus

Bluetongue virus (BTV) infection is one of the most important diseases of domestic livestock. There are no antivirals available against BTV disease. In this paper, we present the development, optimization and validation of an in vitro cell-based high-throughput screening (HTS) assay using the luminescent-based CellTiter-Glo reagent to identify novel antivirals against BTV. Conditions of the cytopathic effect (CPE)-based assay were optimized at cell density of 5000 cells/well in medium containing 1% FBS and a multiplicity of infection at 0.01 in 384-well plate, with Z'-values > or = 0.70, Coefficient of Variations > or = 5.68 and signal-to-background ratio > or = 7.10. This assay was further validated using a 9532 compound library. The fully validated assay was then used to screen the 194,950 compound collection, which identified 693 compounds with >30% CPE inhibition. The 10-concentration dose response assay identified 185 structures with IC(50) < or =100 microM, out of which 42 compounds were grouped into six analog series corresponding to six scaffolds enriched within the active set compared to their distribution in the library. The CPE-based assay development demonstrated its robustness and reliability, and its application in the HTS campaign will make significant contribution to the antiviral drug discovery against BTV disease.

Newcastle disease virus-induced cytopathic effect in infected cells is caused by apoptosis

The velogenic Newcastle disease virus (NDV) causes highly infectious and economically significant Newcastle disease (ND) in birds of various species. In cell culture NDV induces cytopathic effect (CPE) characterized by rounding, vacuolation, syncytia formation and cell death. Aside from cell to cell fusion caused by the F and HN glycoprotein of the virus molecular events leading to cell death are not known. In the current study, NDV-infected Vero cells, at 48 h p.i., showed nuclear condensation, cytoplasm blebbing, DNA fragmentation, and phosphatidylserine translocation to the cell surface. In addition, virus-infected cells demonstrated decreased DNA content and an increased Bax to Bcl-2 ratio, p53 level and caspase 3, 8, 9 expression compared to mock-infected cells. Based on these results, it was concluded that CPE in NDV-infected cells was caused by to the induction of apoptosis with the involvement of p53 and the Bax, dependent apoptotic pathways.

Small ruminant lentivirus Tat protein induces apoptosis in caprine cells in vitro by the intrinsic pathway

The small ruminant lentiviruses, caprine arthritis-encephalitis virus (CAEV) and maedi visna virus (MVV) naturally cause inflammatory disease in goats and sheep, provoking chronic lesions in several different organs. We have previously demonstrated that in vitro infection of caprine cells by CAEV induces apoptosis through the intrinsic pathway (Rea-Boutrois, A., Pontini, G., Greenland, T., Mehlen, P., Chebloune, Y., Verdier, G. and Legras-Lachuer, C. 2008). In the present study, we used Tat deleted viruses and SLRV Tat-expression vectors to show that the SRLV Tat proteins are responsible for this apoptosis. We have also studied the activation of caspases-3, -8 and -9 by fluorescent assays in caprine cells expressing SRLV Tat proteins, and the effects of transfected dominant negative variants of these caspases, to show that Tat-associated apoptosis depends on activation of caspases-3 and -9, but not -8. A simultaneous disruption of mitochondrial membrane potential indicates an involvement of the mitochondrial pathway.

Induction of apoptosis in Vero cells by Newcastle disease virus requires viral replication, de-novo protein synthesis and caspase activation

Newcastle disease virus causes (NDV) apoptotic death of infected cells. In the present study, the stimulus that provoked the induction of apoptosis in infected cells was examined. Vero cells infected with NDV developed apoptosis as characterized by DNA fragmentation and decreased DNA content. In presence of ammonium chloride, infected cells did not show reduced DNA content indicating the requirement of virus entry for the induction of apoptosis. UV-inactivated NDV did not induce apoptosis in cells suggesting the need of virus replication. Although cycloheximide blocked NDV-induced apoptosis, actinomycin-D did not, suggesting that de-novo viral protein synthesis was critical for the induction of apoptosis. In addition, activation of caspases was also detected by flowcytometry, indirect fluorescent and colorimetric assays. Based on the results, it was concluded that NDV-induced apoptosis in Vero cells required virus replication, de-novo protein synthesis and caspase activation.