Vesicular stomatitis virus infection and neuropathogenesis in the murine model are associated with apoptosis

Antiviral Functions of Type I and Type III Interferons in the Olfactory Epithelium

The olfactory neuroepithelium (OE) is one of the few neuronal tissues where environmental pathogens can gain direct access. Despite this vulnerable arrangement, little is known about the protective mechanisms in the OE to prevent viral infection and its antiviral responses. We systematically investigated acute responses in the olfactory mucosa upon exposure to vesicular stomatitis virus (VSV) via RNA-seq. VSVs were nasally inoculated into C57BL/6 mice. Olfactory mucosae were dissected for gene expression analysis at different time points after viral inoculation. Interferon functions were determined by comparing the viral load in interferon receptor knockout (Ifnar1-/- and Ifnlr1-/-) with wildtype OE. Antiviral responses were observed as early as 24 h after viral exposure in the olfactory mucosa. The rapidly upregulated transcripts observed included specific type I as well as type III interferons (Ifn) and interferon-stimulated genes. Genetic analyses demonstrated that both type I and type III IFN signaling are required for the suppression of viral replication in the olfactory mucosa. Exogenous IFN application effectively blocks viral replication in the OE. These findings reveal that the OE possesses an innate ability to suppress viral infection. Type I and type III IFNs have prominent roles in OE antiviral functions.

A tool with many applications: vesicular stomatitis virus in research and medicine

INTRODUCTION

Vesicular stomatitis virus (VSV) has long been a useful research tool in virology and recently become an essential part of medicinal products. Vesiculovirus research is growing quickly following its adaptation to clinical gene and cell therapy and oncolytic virotherapy.

AREAS COVERED

This article reviews the versatility of VSV as a research tool and biological reagent, its use as a viral and vaccine vector delivering therapeutic and immunogenic transgenes and an oncolytic virus aiding cancer treatment. Challenges such as the immune response against such advanced therapeutic medicinal products and manufacturing constraints are also discussed.

EXPERT OPINION

The field of in vivo gene and cell therapy is advancing rapidly with VSV used in many ways. Comparison of VSV's use as a versatile therapeutic reagent unveils further prospects and problems for each application. Overcoming immunological challenges to aid repeated administration of viral vectors and minimizing harmful host-vector interactions remains one of the major challenges. In the future, exploitation of reverse genetic tools may assist the creation of recombinant viral variants that have improved onco-selectivity and more efficient vaccine vector activity. This will add to the preferential features of VSV as an excellent advanced therapy medicinal product (ATMP) platform.

Development of an in-situ hybridization assay using riboprobes for detection of viral haemorrhagic septicemia virus (VHSV) mRNAs in a cell culture model

An in situ hybridization (RNA-ISH) assay has been developed and optimized to detect viral haemorrhagic septicemia virus (VHSV), an OIE listed piscine rhabdovirus, in infected fish cells using fathead minnow (FHM) as a model cell line. Two antisense riboprobes (RNA probes) targeting viral transcripts from a fragment of nucleoprotein (N) and glycoprotein (G) genes were generated by reverse transcription polymerase chain reaction (RT-PCR) using VHSV specific primers followed by a transcription reaction in the presence of digoxigenin dUTP. The synthesized RNA probes were able to detect viral mRNAs in formalin fixed VHSV infected FHM cells at different time points post inoculation (pi). To correlate the signal intensity, a time dependent quantitation of the viral mRNA transcript and infectivity titer was done by quantitative reverse transcriptase polymerase chain reaction (qRT-PCR) and 50% tissue culture infectivity dose (TCID₅₀), respectively, from the infected cells and culture supernatants. Further, we compared the diagnostic sensitivity of ISH assay with immunocytochemistry (ICC). Both the riboprobes used in the ISH assay detected VHSV as early as 6 hpi in the FHM cells inoculated with a multiplicity of infection (moi) of 2. Also, the signal detection in ISH was at an early stage in comparison to ICC, wherein, signal was first detected at 12 hpi. Our results clearly highlight that current ISH assay can be of value as a diagnostic tool to localize and detect VHSV in conjunction with conventional virus isolation in cell culture.

Immune Consequences of in vitro Infection of Human Peripheral Blood Leukocytes with Vesicular Stomatitis Virus

BACKGROUND

Oncolytic vesicular stomatitis virus (VSV) can be delivered intravenously to target primary and metastatic lesions, but the interaction between human peripheral blood leukocytes (PBLs) and VSV remains poorly understood. Our study aimed to assess the overall immunological consequences of ex vivo infection of PBLs with VSV.

METHODS

Phenotypic analysis of lymphocyte subsets and apoptosis were evaluated with flow cytometry. Caspase 3/7 activity was detected by luminescence assay. Virus release was evaluated in a murine cell line (L929). Gene expression and cytokine/chemokine secretion were assessed by real-time PCR and multiplex assay, respectively.

RESULTS

Ex vivo infection of PBLs with VSV elicited upregulated expression of RIG-I, MDA-5, tetherin, IFITM3, and MxA. VSV infection triggered rapid differentiation of blood monocytes into immature dendritic cells as well as their apoptosis, which depended on caspase 3/7 activation. Monocyte differentiation required infectious VSV, but loss of CD14+ cells was also associated with the presence of a cytokine/chemokine milieu produced in response to VSV infection.

CONCLUSIONS

Systemic delivery is a major goal in the field of oncolytic viruses. Our results shed further light on immune mechanisms in response to VSV infection and the underlying VSV-PBL interactions bringing hope for improved cancer immunotherapies, particularly those based on intravenous delivery of oncolytic VSV.

Activating Peripheral Innate Immunity Enables Safe and Effective Oncolytic Virotherapy in the Brain

The oncolytic mutant vesicular stomatitis virus VSVΔ51 achieves robust efficacy in multiple extracranial tumor models. Yet for malignancies of the brain, direct intratumoral infusion of VSVΔ51 causes lethal virus-induced neuropathology. Here, we have developed a novel therapeutic regime that uses peripheral immunization with a single sub-lethal dose of VSVΔ51 to establish an acute anti-viral state that enables the safe intracranial (IC) infusion of an otherwise lethal dose of VSVΔ51 within just 6 hr. Although type I interferons alone appeared insufficient to explain this protective phenotype, serum isolated at early time points from primed animals conferred protection against an IC dose of virus. Adaptive immune populations had minimal contributions. Finally, the therapeutic utility of this novel strategy was demonstrated by peripherally priming and intracranially treating mice bearing aggressive CT2A syngeneic astrocytomas with VSVΔ51. Approximately 25% of animals achieved complete regression of established tumors, with no signs of virus-induced neurological impairment. This approach may harness an early warning system in the brain that has evolved to protect the host against otherwise lethal neurotropic viral infections. We have exploited this protective mechanism to safely and efficaciously treat brain tumors with an otherwise neurotoxic virus, potentially widening the available treatment options for oncolytic virotherapy in the brain.

In Vitro and In Vivo Attenuation of Vesicular Stomatitis Virus (VSV) by Phosphoprotein Deletion

Vesicular stomatitis virus (VSV) is highly immunogenic and able to stimulate both innate and adaptive immune responses. However, its ability to induce adverse effects has held back the use of VSV as a potential vaccine vector. In this study we developed VSV-ΔP, a safe yet potent replication-defective recombinant VSV in which the phosphoprotein (P) gene was deleted. VSV-ΔP replicated only in supporting cells expressing P (BHK-P cells) and at levels more than 2 logs lower than VSV. In vivo studies indicated that the moderate replication of VSV-ΔP in vitro was associated with the attenuation of this virus in the mouse model, whereas mice intracranially injected with VSV succumbed to neurotoxicity. Furthermore, we constructed VSV and VSV-ΔP expressing a variety of antigens including hemagglutinin-neuraminidase (HN) from Newcastle disease virus (NDV), hemagglutinin (HA) from either a 2009 H1N1 pandemic influenza virus (pdm/09) or the avian H7N9. VSV and VSV-ΔP incorporated the foreign antigens on their surface resulting in induction of robust neutralizing antibody, serum IgG, and hemagglutination inhibition (HAI) titers against their corresponding viruses. These results indicated that VSV with P gene deletion was attenuated in vitro and in vivo, and possibly expressed the foreign antigen on its surface. Therefore, the P gene-deletion strategy may offer a potentially useful and safer approach for attenuating negative-sense RNA viruses which use phosphoprotein as a cofactor for viral replication.

Mechanistic insights into the oncolytic activity of vesicular stomatitis virus in cancer immunotherapy

Immunotherapy and oncolytic virotherapy have both shown anticancer efficacy in the clinic as monotherapies but the greatest promise lies in therapies that combine these approaches. Vesicular stomatitis virus is a prominent oncolytic virus with several features that promise synergy between oncolytic virotherapy and immunotherapy. This review will address the cytotoxicity of vesicular stomatitis virus in transformed cells and what this means for antitumor immunity and the virus’ immunogenicity, as well as how it facilitates the breaking of tolerance within the tumor, and finally, we will outline how these features can be incorporated into the rational design of new treatment strategies in combination with immunotherapy.

Creation of matrix protein gene variants of two serotypes of vesicular stomatitis virus as prime-boost vaccine vectors

To take advantage of live recombinant vesicular stomatitis viruses (rVSVs) as vaccine vectors for their high yield and for their induction of strong and long-lasting immune responses, it is necessary to make live vaccine vectors safe for use without losing their immunogenicity. We have generated safer and highly efficient recombinant VSV vaccine vectors by combining the M51R mutation in the M gene of serotype VSV-Indiana (VSVInd) with a temperature-sensitive mutation (tsO23) of the VSVInd Orsay strain. In addition, we have generated two new serotype VSV-New Jersey (VSVNJ) vaccine vectors by combining M48R and M51R mutations with G22E and L110F mutations in the M gene, rVSVNJ(G22E M48R M51R) [rVSVNJ(GMM)] and VSVNJ(G22E M48R M51R L110F) [rVSVNJ(GMML)]. The combined mutations G21E, M51R, and L111F in the M protein of VSVInd significantly reduced the burst size of the virus by up to 10,000-fold at 37°C without affecting the level of protein expression. BHK21 cells and SH-SY5Y human neuroblastoma cells infected with rVSVInd(GML), rVSVNJ(GMM), and rVSVNJ(GMML) showed significantly reduced cytopathic effects in vitro at 37°C, and mice injected with 1 million infectious virus particles of these mutants into the brain showed no neurological dysfunctions or any other adverse effects. In order to increase the stability of the temperature-sensitive mutant, we have replaced the phenylalanine with alanine. This will change all three nucleotides from UUG (leucine) to GCA (alanine). The resulting L111A mutant showed the temperature-sensitive phenotype of rVSVInd(GML) and increased stability. Twenty consecutive passages of rVSVInd(GML) with an L111A mutation did not convert back to leucine (UUG) at position 111 in the M protein gene.

IMPORTANCE

Recombinant vesicular stomatitis viruses as live vaccine vectors are very effective in expressing foreign genes and inducing adaptive T cell and B cell immune responses. As with any other live viruses in humans or animals, the use of live rVSVs as vaccine vectors demands the utmost safety. Our strategy to attenuate rVSVInd by utilizing a temperature-sensitive assembly-defective mutation of L111A and combining it with an M51R mutation in the M protein of rVSVInd significantly reduced the pathogenicity of the virus while maintaining highly effective virus production. We believe our new temperature-sensitive M gene mutant of rVSVInd(GML) and M gene mutants of rVSVNJ(GMM) and rVSVNJ(GMML) add excellent vaccine vectors to the pool of live viral vectors.

Re-engineering vesicular stomatitis virus to abrogate neurotoxicity, circumvent humoral immunity, and enhance oncolytic potency

As cancer treatment tools, oncolytic viruses (OV) have yet to realize what some see as their ultimate clinical potential. In this study, we have engineered a chimeric vesicular stomatitis virus (VSV) that is devoid of its natural neurotoxicity while retaining potent oncolytic activity. The envelope glycoprotein (G) of VSV was replaced with a variant glycoprotein of the lymphocytic choriomeningitis virus (LCMV-GP), creating a replicating therapeutic, rVSV(GP), that is benign in normal brain but can effectively eliminate brain cancer in multiple preclinical tumor models in vivo. Furthermore, it can be safely administered systemically to mice and displays greater potency against a spectrum of human cancer cell lines than current OV candidates. Remarkably, rVSV(GP) escapes humoral immunity, thus, for the first time, allowing repeated systemic OV application without loss of therapeutic efficacy. Taken together, rVSV(GP) offers a considerably improved OV platform that lacks several of the major drawbacks that have limited the clinical potential of this technology to date.

hsp70-dependent antiviral immunity against cytopathic neuronal infection by vesicular stomatitis virus

The major inducible 70-kDa heat shock protein (hsp70) protects against measles virus (MeV) neurovirulence in the mouse that is caused by a cell-associated noncytolytic neuronal infection. Protection is type I interferon (IFN) dependent, and we have established a novel axis of antiviral immunity in which hsp70 is released from virus-infected neurons to induce IFN-β in macrophages. The present work used vesicular stomatitis virus (VSV) to establish the relevance of hsp70-dependent antiviral immunity to fulminant cytopathic neuronal infections. In vitro, hsp70 that was constitutively expressed in mouse neuronal cells caused a modest increase in VSV replication. Infection induced an early extracellular release of hsp70 from viable cells, and the release was progressive, increasing with virus-induced apoptosis and cell lysis. The impact of this VSV-hsp70 interaction on neurovirulence was established in weanling male hsp70 transgenic and nontransgenic mice. Constitutive expression of hsp70 in neurons of transgenic mice enhanced viral clearance from brain and reduced mortality, and it was correlated with enhanced expression of type I IFN mRNA. Nontransgenic mice were also protected against neurovirulence and expressed increased type I IFN mRNA in brain when hsp70 was expressed by a recombinant VSV (rVSV-hsp70), indicating that hsp70 in the virus-infected cell is sufficient for host protection. In vitro data confirmed extracellular release of hsp70 from cells infected with rVSV-hsp70 and also showed that viral replication is not enhanced when hsp70 is expressed in this manner, suggesting that hsp70-mediated protection in vivo is not dependent on stimulatory effects of hsp70 on virus gene expression.

Matrix protein of vesicular stomatitis virus: a potent inhibitor of vascular endothelial growth factor and malignant ascites formation

Malignant ascites is common in various types of cancers and is difficult to manage. Vascular endothelial growth factor (VEGF) has a pivotal role in malignant ascites. The matrix protein of vesicular stomatitis virus (VSVMP) has been shown to inhibit host gene expression and induce the apoptosis of cancer cells. The present study was designed to determine whether VSVMP suppresses the formation of ascites in ascites-producing peritoneal carcinomatosis. BALB/c female mice, 6-8 weeks old, bearing peritoneal tumors of H22 or MethA cells received an intraperitoneal administration of 50 μg VSVMP/250 μg liposome complexes, 50 μg empty plasmid/250 μg liposome complexes or 0.9% NaCl solution, respectively, every 2 days for 3 weeks. Administration of VSVMP resulted in a significant inhibition in ascites formation, improvement in health condition and prolonged survival of the treated mice. Decreased peritoneum osmolarity and reduced tumor vascularity coincided with dramatic reductions in the VEGF level in ascites fluid and plasma. Examination of floating tumor cells collected from the peritoneal wash revealed an apparently increased number of apoptotic cells and profound downregulation of VEGF mRNA in the VSVMP-treated mice. Our data indicate for the first time that in BALB/c mice bearing H22 or MethA cell peritoneal tumors, VSVMP may inhibit VEGF production and suppress angiogenesis, consequently abolishing ascites formation.

Immune clearance of attenuated rabies virus results in neuronal survival with altered gene expression

Rabies virus (RABV) is a highly neurotropic pathogen that typically leads to mortality of infected animals and humans. The precise etiology of rabies neuropathogenesis is unknown, though it is hypothesized to be due either to neuronal death or dysfunction. Analysis of human brains post-mortem reveals surprisingly little tissue damage and neuropathology considering the dramatic clinical symptomology, supporting the neuronal dysfunction model. However, whether or not neurons survive infection and clearance and, provided they do, whether they are functionally restored to their pre-infection phenotype has not been determined in vivo for RABV, or any neurotropic virus. This is due, in part, to the absence of a permanent “mark” on once-infected cells that allow their identification long after viral clearance. Our approach to study the survival and integrity of RABV-infected neurons was to infect Cre reporter mice with recombinant RABV expressing Cre-recombinase (RABV-Cre) to switch neurons constitutively expressing tdTomato (red) to expression of a Cre-inducible EGFP (green), permanently marking neurons that had been infected in vivo. We used fluorescence microscopy and quantitative real-time PCR to measure the survival of neurons after viral clearance; we found that the vast majority of RABV-infected neurons survive both infection and immunological clearance. We were able to isolate these previously infected neurons by flow cytometry and assay their gene expression profiles compared to uninfected cells. We observed transcriptional changes in these “cured” neurons, predictive of decreased neurite growth and dysregulated microtubule dynamics. This suggests that viral clearance, though allowing for survival of neurons, may not restore them to their pre-infection functionality. Our data provide a proof-of-principle foundation to re-evaluate the etiology of human central nervous system diseases of unknown etiology: viruses may trigger permanent neuronal damage that can persist or progress in the absence of sustained viral antigen.

Rabies is an ancient and fatal neurological disease of animals and humans, caused by infection of the central nervous system (CNS) with Rabies virus (RABV). It is estimated that nearly 55,000 human RABV fatalities occur each year, though this number is likely much higher due to unreported exposures or failure of diagnosis. No treatment has been identified to cure disease after onset of symptoms. Neurovirologists still do not know the cause of rabies' dramatic symptoms and fatality, though it is thought to be due to neuronal loss or dysfunction. Here, we use a novel approach to permanently and genetically tag infected cells so that they can be identified after the infection has been cleared. This allowed us to define neuronal survival time following infection, and to assess neuronal function through gene expression analysis. We found that RABV infection does not lead to loss of neurons, but rather induces a permanent change in gene expression that may be related to the ability of RABV to cause permanent CNS disease. Our study provides evidence that viral infection of the brain can initiate long-term changes that may have consequences for nervous system health, even after the virus has been cleared from the CNS.

Recombinant vesicular stomatitis virus vaccine vectors expressing filovirus glycoproteins lack neurovirulence in nonhuman primates

The filoviruses, Marburg virus and Ebola virus, cause severe hemorrhagic fever with high mortality in humans and nonhuman primates. Among the most promising filovirus vaccines under development is a system based on recombinant vesicular stomatitis virus (rVSV) that expresses an individual filovirus glycoprotein (GP) in place of the VSV glycoprotein (G). The main concern with all replication-competent vaccines, including the rVSV filovirus GP vectors, is their safety. To address this concern, we performed a neurovirulence study using 21 cynomolgus macaques where the vaccines were administered intrathalamically. Seven animals received a rVSV vector expressing the Zaire ebolavirus (ZEBOV) GP; seven animals received a rVSV vector expressing the Lake Victoria marburgvirus (MARV) GP; three animals received rVSV-wild type (wt) vector, and four animals received vehicle control. Two of three animals given rVSV-wt showed severe neurological symptoms whereas animals receiving vehicle control, rVSV-ZEBOV-GP, or rVSV-MARV-GP did not develop these symptoms. Histological analysis revealed major lesions in neural tissues of all three rVSV-wt animals; however, no significant lesions were observed in any animals from the filovirus vaccine or vehicle control groups. These data strongly suggest that rVSV filovirus GP vaccine vectors lack the neurovirulence properties associated with the rVSV-wt parent vector and support their further development as a vaccine platform for human use.

Ebola and Marburg viruses are categorized as Category A priority pathogens by several US Government agencies as a result of their high mortality rates and potential for use as agents of bioterrorism. There are currently no vaccines or therapeutics approved for human use. A replication-competent, recombinant vesicular stomatitis virus (rVSV) vector expressing filovirus glycoproteins (GP), in place of the VSV G protein has shown promise in lethal nonhuman primate models of filovirus infection as both a single-injection preventive vaccine and a postexposure treatment. Replication-competent vaccines that are intended for use in humans usually undergo neurovirulence testing as was done for measles virus, mumps virus, yellow fever virus, and poliovirus vaccines. Here we used a conventional neurovirulence test to evaluate the safety of our rVSV-based Zaire ebolavirus and Lake Victoria marburgvirus GP vaccines in cynomolgus macaques. Importantly, we demonstrate for the first time that these rVSV filovirus GP vectors lack neurovirulence when compared to a rVSV wild-type vector.

Detection of VHSV IVb within the gonads of Great Lakes fish using in situ hybridization

Viral haemorrhagic septicaemia virus (VHSV) genotype IVb was recently detected as the cause of numerous mortality events in Great Lakes fish. In situ hybridization was used to examine the gonads from 13 fish, including freshwater drum Aplodinotus grunniens and muskellunge Esox masquinongy that were infected naturally, as well as rainbow trout Oncorhynchus mykiss and fathead minnows Pimphales promelas, which were experimentally infected. Although the ovaries and testes of fish infected by VHSV IVb had few lesions, viral RNA was present in the ovaries of the rainbow trout and fathead minnow and was abundant in the gonads of muskellunge and in the ovaries of freshwater drum. Viral RNA was present mainly surrounding yolk vacuoles/granules or adjacent to the germinal vesicle, with lesser amounts found within the germinal vesicle, in the mesovarium and/or tunica albuginea and blood vessels of the ovary. Viral RNA was also found in and surrounding primary and secondary spermatocytes of the muskellunge.

Modifications of the PSAP region of the matrix protein lead to attenuation of vesicular stomatitis virus in vitro and in vivo

The matrix (M) protein of vesicular stomatitis virus (VSV) is a multi-functional protein involved in virus assembly, budding and pathogenesis. The (24)PPPY(27) late (L) domain of the M protein plays a key role in virus budding, whereas amino acids downstream of the PPPY motif contribute to host protein shut-off and pathogenesis. Using a panel of (37)PSAP(40) recombinant viruses, it has been demonstrated previously that the PSAP region of M does not possess L-domain activity similar to that of PPPY in BHK-21 cells. This study reports the unanticipated finding that these PSAP recombinants were attenuated in cell culture and in mice compared with control viruses. Indeed, PSAP recombinant viruses exhibited a small-plaque phenotype, reduced CPE, reduced levels of activated caspase-3, enhanced production of IFN-beta and reduced titres in the lungs and brains of infected mice. In particular, recombinant virus M6PY>A4-R34E was the most severely attenuated, exhibiting little or no CPE in cell culture and undetectable titres in the lungs and brains of infected mice. These findings indicate an important role for the PSAP region (aa 33-44) of the M protein in the pathology of VSV infection and may have implications for the development of VSV as a vaccine and/or oncolytic vector.

Neurovirulence properties of recombinant vesicular stomatitis virus vectors in non-human primates

Although vesicular stomatitis virus (VSV) neurovirulence and pathogenicity in rodents have been well studied, little is known about VSV pathogenicity in non-human primates. To address this question, we measured VSV viremia, shedding, and neurovirulence in macaques. Following intranasal inoculation, macaques shed minimal recombinant VSV (rVSV) in nasal washes for 1 day post-inoculation; viremia was not detected. Following intranasal inoculation of macaques, wild type (wt) VSV, rVSV, and two rVSV-HIV vectors showed no evidence of spread to CNS tissues. However, macaques inoculated intrathalamically with wt VSV developed severe neurological disease. One of four macaques receiving rVSV developed clinical and histological signs similar to the wt group, while the remaining three macaques in this group and all of the macaques in the rVSV-HIV vector groups showed no clinical signs of disease and reduced severity of histopathology compared to the wt group. The implications of these findings for rVSV vaccine development are discussed.

Resistance to viral infection of super p53 mice

Induction of expression of the tumor suppressor p53 after interferon treatment has been recently demonstrated (Takaoka et al., 2003), suggesting an antiviral activity of the protein. In addition, a direct correlation between p53 levels and tumor resistance has been addressed by generating mice with an extra copy of p53 ('super p53' mice) (Garcia-Cao et al., 2002). Here, we show that vesicular stomatitis virus replication in mouse embryo fibroblasts derived from 'super p53' mice is impaired as a result of apoptosis induction via p53 activation. These findings unequivocally demonstrate an antiviral activity of p53, a process that may contribute to inhibit the spread of the virus in vivo.

Early activation of the mitochondrial apoptotic pathway in Vesicular Stomatitis virus-infected cells

Vesicular Stomatitis Virus (VSV) has been shown to induce apoptosis in a caspase-dependent manner, but the precise apoptotic pathway remains unknown. We found that caspases 9 and 3, but not caspase 8, were activated during VSV-induced apoptosis in infected Vero cells. Since caspase 9 is related to the mitochondrial apoptotic pathway, we analyzed some mitochondrial events such as changes in the mitochondrial transmembrane potential (Deltapsim) and mitochondrial release of apoptogenic proteins such as cytochrome c and the apoptosis inducing factor (AIF). We found that VSV infection triggers the dissipation of the Deltapsim and the release of both cytochrome c and AIF from the mitochondrial intermembrane space very early in the VSV infection. These results indicate that the trigger of apoptosis in VSV-infected cells occurs through the early activation of the mitochondrial apoptotic pathway. On the other hand, intracellular levels of the anti-apoptotic proteins, such as Bcl-2 and Bcl-xL, and the pro-apoptotic protein Bax, were assessed during viral infection. These analyses showed that as viral infection proceeded, the cellular level of Bcl-xL decreased, while the levels of Bax and Bcl-2 remained unaffected. The significance of the Bcl-xL modulation is also discussed.