The mechanism of cell death during West Nile virus infection is dependent on initial infectious dose

The enhanced tumor selectivity of an oncolytic vaccinia lacking the host range and antiapoptosis genes SPI-1 and SPI-2

The ability of cancer cells to evade apoptosis may permit survival of a recombinant vaccinia lacking antiapoptotic genes in cancer cells compared with normal cells. We have explored the deletion of two vaccinia virus host range/antiapoptosis genes, SPI-1 and SPI-2, for their effects on the viral replication and their ability to induce cell death in infected normal and transformed cells in vitro. Indeed, in three paired normal and transformed cell types, the SPI-1 and SPI-2 gene-deleted virus (vSP) preferentially replicates in transformed cells or p53-null cells when compared with their normal counterparts. This selectivity may be derived from the fact that vSP-infected normal cells died faster than infected cancer cells. A fraction of infected cells died with evidence of necrosis as shown by both flow cytometry and detection of high-mobility group B1 protein released from necrotic cells into the culture supernatant. When administered to animals, vSP retains full ability to replicate in tumor tissues, whereas replication in normal tissues is greatly diminished. In a model of viral pathogenesis, mice treated with vSP survived substantially longer when compared with mice treated with the wild-type virus. The mutant virus vSP displayed significant antitumoral effects in an MC38 s.c. tumor model in both nude (P < 0.001) and immunocompetent mice (P < 0.05). We conclude that this recombinant vaccinia vSP shows promise for oncolytic virus therapy. Given its enhanced tumor selectivity, improved safety profile, and substantial oncolytic effects following systemic delivery in murine models, it should also serve as a useful vector for tumor-directed gene therapy.

Host cell killing by the West Nile Virus NS2B-NS3 proteolytic complex: NS3 alone is sufficient to recruit caspase-8-based apoptotic pathway

The West Nile Virus (WNV) non-structural proteins 2B and 3 (NS2B-NS3) constitute the proteolytic complex that mediates the cleavage and processing of the viral polyprotein. NS3 recruits NS2B and NS5 proteins to direct protease and replication activities. In an effort to investigate the biology of the viral protease, we cloned cDNA encoding the NS2B-NS3 proteolytic complex from brain tissue of a WNV-infected dead crow, collected from the Lower Merion area (Merion strain). Expression of the NS2B-NS3 gene cassette induced apoptosis within 48 h of transfection. Electron microscopic analysis of NS2B-NS3-transfected cells revealed ultra-structural changes that are typical of apoptotic cells including membrane blebbing, nuclear disintegration and cytoplasmic vacuolations. The role of NS3 or NS2B in contributing to host cell apoptosis was examined. NS3 alone triggers the apoptotic pathways involving caspases-8 and -3. Experimental results from the use of caspase-specific inhibitors and caspase-8 siRNA demonstrated that the activation of caspase-8 was essential to initiate apoptotic signaling in NS3-expressing cells. Downstream of caspase-3 activation, we observed nuclear membrane ruptures and cleavage of the DNA-repair enzyme, PARP in NS3-expressing cells. Nuclear herniations due to NS3 expression were absent in the cells treated with a caspase-3 inhibitor. Expression of protease and helicase domains themselves was sufficient to trigger apoptosis generating insight into the apoptotic pathways triggered by NS3 from WNV.

Flavivirus activates phosphatidylinositol 3-kinase signaling to block caspase-dependent apoptotic cell death at the early stage of virus infection

Flaviviruses such as dengue virus (DEN) and Japanese encephalitis virus (JEV) are medically important in humans. The lipid kinase, phosphatidylinositol 3-kinase (PI3K) and its downstream target Akt have been implicated in the regulation of diverse cellular functions such as proliferation, and apoptosis. Since JEV and DEN appear to trigger apoptosis in cultured cells at a rather late stage of infection, we evaluated the possible roles of the PI3K/Akt signaling pathway in flavivirus-infected cells. We found that Akt phosphorylation was noticeable in the JEV- and DEN serotype 2 (DEN-2)-infected neuronal N18 cells in an early, transient, PI3K- and lipid raft-dependent manner. Blocking of PI3K activation by its specific inhibitor LY294002 or wortmannin greatly enhanced virus-induced cytopathic effects (CPEs), even at an early stage of infection, but had no effect on virus production. This severe CPE was characterized as apoptotic cell death as evidenced by TUNEL (terminal deoxynucleotidyltransferase-mediated dUTP-biotin nick end labeling) staining and cleavage of caspase-3 and poly(ADP-ribose) polymerase (PARP). Mechanically, the initiator and effector caspases involved are mainly caspase-9 and caspase-6, since only a pan-caspase inhibitor and the inhibitors preferentially target caspase-9 and -6, but not the ones antagonizing caspase-8, -3, or -7 alleviated the levels of PARP cleavage after virus infection and PI3K blockage. Furthermore, Bcl-2 appears to be a crucial mediator downstream of PI3K/Akt signaling, since overexpression of Bcl-2 reduced virus-induced apoptosis even when PI3K activation was repressed. Collectively, our results suggest an anti-apoptotic role for the PI3K/Akt pathway triggered by JEV and DEN-2 to protect infected cells from early apoptotic cell death.

Molecular mechanisms of West Nile virus pathogenesis in brain cell

We analyzed the response of human glioma cells to West Nile virus infection by investigating host transcriptional changes. Changes in expression of 23 genes showed similarities to those in other neurodegenerative diseases. These changes may be useful as potential biomarkers and elucidate novel mechanisms behind the neuropathology of infection with this virus.

High-mobility group box 1 protein (HMGB1): nuclear weapon in the immune arsenal

High-mobility group box 1 protein (HMGB1), which previously was thought to function only as a nuclear factor that enhances transcription, was recently discovered to be a crucial cytokine that mediates the response to infection, injury and inflammation. These observations have led to the emergence of a new field in immunology that is focused on understanding the mechanisms of HMGB1 release, its biological activities and its pathological effects in sepsis, arthritis, cancer and other diseases. Here, we discuss these features of HMGB1 and summarize recent advances that have led to the preclinical development of therapeutics that modulate HMGB1 release and activity.

Adaptation of West Nile virus replicons to cells in culture and use of replicon-bearing cells to probe antiviral action

Flaviviruses are emerging threats to public health worldwide. Recently, one flavivirus, West Nile virus (WNV), has caused the largest epidemic of viral encephalitis in US history. Like other flaviviruses, WNV is thought to cause a persistent infection in insect cells, but an acute cytopathic infection of mammalian cells. To study adaptation of WNV to persistently replicate in cell culture and generate a system capable of detecting antiviral compounds in the absence of live virus, we generated subgenomic replicons of WNV and adapted these to persistently replicate in mammalian cells. Here we report that adaptation of these replicons to cell culture results in a reduction of genome copy number, and demonstrate that hamster, monkey, and human cells that stably carry the replicons can be used as surrogates to detect the activity of anti-WNV compounds. Additionally, we have used these cells to investigate the interaction of WNV genomes with interferon (IFN). These studies demonstrated that IFN can cure cells of replicons and that replicon-bearing cells display lower responses to IFN than their IFN-cured derivatives.

Mechanism of cell death during infectious salmon anemia virus infection is cell type-specific

Infectious salmon anemia virus (ISAV) is a very important fish virus in the Northern hemisphere and there is continued interest in understanding the mechanisms of its pathogenesis and persistence in fish. In this study, the permissive fish cell lines SHK-1, CHSE-214 and TO were used to determine whether ISAV-induced cytopathic effect (CPE) is due to apoptosis or necrosis. Characteristic apoptotic DNA fragmentation was observed only in ISAV-infected SHK-1 and CHSE-214 cells. Apoptosis in ISAV-infected SHK-1 cells was confirmed by fragment end-labelling assay, suggesting that CPE in these cells is associated with apoptosis. ISAV-infected TO cells did not undergo apoptosis, but showed leakage of high-mobility group 1 (HMGB1) protein from the nucleus, which is characteristic of cells undergoing necrosis; this suggests that CPE in these cells is associated with necrosis. ISAV-infected SHK-1 cells did not show leakage of HMGB1 protein. Infection with two different strains of ISAV showed that induction of apoptosis was correlated with the appearance of CPE in SHK-1 cells. ISAV-induced apoptosis was inhibited by a pan-caspase inhibitor, Z-VAD-fmk, indicating a caspase-activation pathway. The ISAV putative PB2 protein and proteins encoded by RNA segment 7 bound caspase-8 specifically in vitro, suggesting that these viral proteins may have a role in ISAV-induced apoptosis. These findings demonstrate for the first time that the mechanism of cell death during ISAV infection is dependent on the cell type, which may have implications for ISAV pathogenesis and persistence.

New insights on the neuropathology of West Nile virus

West Nile virus (WNV) is a mosquito-borne disease that emerged in North America where it caused in 2002 te largest arboviral meningoencephalitis outbreak ever recorded in this area. The viral variant responsible for this outbreak has been found to share 99.7% identity over the entire genome with the viral variant that caused the epizootic in Israel in 1998 and has been referred as "Isr98/NY99". It has been shown to exhibit an increased neurovirulence in humans, as well as in experimental infections in different animal models. Mouse model has allowed to demonstrate the preferential infection of neurons within the central nervous system and to point out the genetic determinism of host susceptibility to WNV. In murine neural cell cultures, the selective infection of neurons was accompanied by physiopathological changes and a cytopathic effect, showing the direct effect of infection of neurons as one of the causes of WNV neuropathogenicity.