Vaccinia virus infection disarms the mitochondrion-mediated pathway of the apoptotic cascade by modulating the permeability transition pore

The poxvirus F17 protein counteracts mitochondrially orchestrated antiviral responses

Poxviruses are unusual DNA viruses that replicate in the cytoplasm. To do so, they encode approximately 100 immunomodulatory proteins that counteract cytosolic nucleic acid sensors such as cGAMP synthase (cGAS) along with several other antiviral response pathways. Yet most of these immunomodulators are expressed very early in infection while many are variable host range determinants, and significant gaps remain in our understanding of poxvirus sensing and evasion strategies. Here, we show that after infection is established, subsequent progression of the viral lifecycle is sensed through specific changes to mitochondria that coordinate distinct aspects of the antiviral response. Unlike other viruses that cause extensive mitochondrial damage, poxviruses sustain key mitochondrial functions including membrane potential and respiration while reducing reactive oxygen species that drive inflammation. However, poxvirus replication induces mitochondrial hyperfusion that independently controls the release of mitochondrial DNA (mtDNA) to prime nucleic acid sensors and enables an increase in glycolysis that is necessary to support interferon stimulated gene (ISG) production. To counter this, the poxvirus F17 protein localizes to mitochondria and dysregulates mTOR to simultaneously destabilize cGAS and block increases in glycolysis. Our findings reveal how the poxvirus F17 protein disarms specific mitochondrially orchestrated responses to later stages of poxvirus replication.

Engineering vaccinia virus as an immunotherapeutic battleship to overcome tumor heterogeneity

INTRODUCTION

Immunotherapy is a rapidly evolving area of cancer therapeutics aimed at driving a systemic immune response to fight cancer. Oncolytic viruses (OVs) are at the cutting-edge of innovation in the immunotherapy field. Successful OV platforms must be effective in reshaping the tumor microenvironment and controlling tumor burden, but also be highly specific to avoid off-target side effects. Large DNA viruses, like vaccinia virus (VACV), have a large coding capacity, enabling the encoding of multiple immunostimulatory transgenes to reshape the tumor immune microenvironment. VACV-based OVs have shown promising results in both pre-clinical and clinical studies, including safe and efficient intravenous delivery to metastatic tumors.

AREA COVERED

This review summarizes attenuation strategies to generate a recombinant VACV with optimal tumor selectivity and immunogenicity. In addition, we discuss immunomodulatory transgenes that have been introduced into VACV and summarize their effectiveness in controlling tumor burden.

EXPERT OPINION

VACV encodes several immunomodulatory genes which aid the virus in overcoming innate and adaptive immune responses. Strategic deletion of these virulence factors will enable an optimal balance between viral persistence and immunogenicity, robust tumor-specific expression of payloads and promotion of a systemic anti-cancer immune response. Rational selection of therapeutic transgenes will maximize the efficacy of OVs and their synergy in combinatorial immunotherapy schemes.

Vaccinia Virus Encodes a Novel Inhibitor of Apoptosis That Associates with the Apoptosome

Apoptosis is an important antiviral host defense mechanism. Here we report the identification of a novel apoptosis inhibitor encoded by the vaccinia virus (VACV) M1L gene. M1L is absent in the attenuated modified vaccinia virus Ankara (MVA) strain of VACV, a strain that stimulates apoptosis in several types of immune cells. M1 expression increased the viability of MVA-infected THP-1 and Jurkat cells and reduced several biochemical hallmarks of apoptosis, such as PARP-1 and procaspase-3 cleavage. Furthermore, ectopic M1L expression decreased staurosporine-induced (intrinsic) apoptosis in HeLa cells. We then identified the molecular basis for M1 inhibitory function. M1 allowed mitochondrial depolarization but blocked procaspase-9 processing, suggesting that M1 targeted the apoptosome. In support of this model, we found that M1 promoted survival in Saccharomyces cerevisiae overexpressing human Apaf-1 and procaspase-9, critical components of the apoptosome, or overexpressing only conformationally active caspase-9. In mammalian cells, M1 coimmunoprecipitated with Apaf-1-procaspase-9 complexes. The current model is that M1 associates with and allows the formation of the apoptosome but prevents apoptotic functions of the apoptosome. The M1 protein features 14 predicted ankyrin (ANK) repeat domains, and M1 is the first ANK-containing protein reported to use this inhibitory strategy. Since ANK-containing proteins are encoded by many large DNA viruses and found in all domains of life, studies of M1 may lead to a better understanding of the roles of ANK proteins in virus-host interactions.IMPORTANCE Apoptosis selectively eliminates dangerous cells such as virus-infected cells. Poxviruses express apoptosis antagonists to neutralize this antiviral host defense. The vaccinia virus (VACV) M1 ankyrin (ANK) protein, a protein with no previously ascribed function, inhibits apoptosis. M1 interacts with the apoptosome and prevents procaspase-9 processing as well as downstream procaspase-3 cleavage in several cell types and under multiple conditions. M1 is the first poxviral protein reported to associate with and prevent the function of the apoptosome, giving a more detailed picture of the threats VACV encounters during infection. Dysregulation of apoptosis is associated with several human diseases. One potential treatment of apoptosis-related diseases is through the use of designed ANK repeat proteins (DARPins), similar to M1, as caspase inhibitors. Thus, the study of the novel antiapoptosis effects of M1 via apoptosome association will be helpful for understanding how to control apoptosis using either natural or synthetic molecules.

Vaccinia virus evasion of regulated cell death

Regulated cell death is a powerful anti-viral mechanism capable of aborting the virus replicative cycle and alerting neighbouring cells to the threat of infection. The biological importance of regulated cell death is illustrated by the rich repertoire of host signalling cascades causing cell death and by the multiple strategies exhibited by viruses to block death signal transduction and preserve cell viability. Vaccinia virus (VACV), a poxvirus and the vaccine used to eradicate smallpox, encodes multiple proteins that interfere with apoptotic, necroptotic and pyroptotic signalling. Here the current knowledge on cell death pathways and how VACV proteins interact with them is reviewed. Studying the mechanisms evolved by VACV to counteract host programmed cell death has implications for its successful use as a vector for vaccination and as an oncolytic agent against cancer.

Analysis of the anti-apoptotic activity of four vaccinia virus proteins demonstrates that B13 is the most potent inhibitor in isolation and during viral infection

Vaccinia virus (VACV) is a large dsDNA virus encoding ~200 proteins, several of which inhibit apoptosis. Here, a comparative study of anti-apoptotic proteins N1, F1, B13 and Golgi anti-apoptotic protein (GAAP) in isolation and during viral infection is presented. VACVs strains engineered to lack each gene separately still blocked apoptosis to some degree because of functional redundancy provided by the other anti-apoptotic proteins. To overcome this redundancy, we inserted each gene separately into a VACV strain (vv811) that lacked all these anti-apoptotic proteins and that induced apoptosis efficiently during infection. Each protein was also expressed in cells using lentivirus vectors. In isolation, each VACV protein showed anti-apoptotic activity in response to specific stimuli, as measured by immunoblotting for cleaved poly(ADP ribose) polymerase-1 and caspase-3 activation. Of the proteins tested, B13 was the most potent inhibitor, blocking both intrinsic and extrinsic stimuli, whilst the activity of the other proteins was largely restricted to inhibition of intrinsic stimuli. In addition, B13 and F1 were effective blockers of apoptosis induced by vv811 infection. Finally, whilst differences in induction of apoptosis were barely detectable during infection with VACV strain Western Reserve compared with derivative viruses lacking individual anti-apoptotic genes, several of these proteins reduced activation of caspase-3 during infection by vv811 strains expressing these proteins. These results illustrated that vv811 was a useful tool to determine the role of VACV proteins during infection and that whilst all of these proteins have some anti-apoptotic activity, B13 was the most potent.

Structural insight into BH3 domain binding of vaccinia virus antiapoptotic F1L

Apoptosis is a tightly regulated process that plays a crucial role in the removal of virus-infected cells, a process controlled by both pro- and antiapoptotic members of the Bcl-2 family. The proapoptotic proteins Bak and Bax are regulated by antiapoptotic Bcl-2 proteins and are also activated by a subset of proteins known as BH3-only proteins that perform dual functions by directly activating Bak and Bax or by sequestering and neutralizing antiapoptotic family members. Numerous viruses express proteins that prevent premature host cell apoptosis. Vaccinia virus encodes F1L, an antiapoptotic protein essential for survival of infected cells that bears no discernible sequence homology to mammalian cell death inhibitors. Despite the limited sequence similarities, F1L has been shown to adopt a novel dimeric Bcl-2-like fold that enables hetero-oligomeric binding to both Bak and the proapoptotic BH3-only protein Bim that ultimately prevents Bak and Bax homo-oligomerization. However, no structural data on the mode of engagement of F1L and its Bcl-2 counterparts are available. Here we solved the crystal structures of F1L in complex with two ligands, Bim and Bak. Our structures indicate that F1L can engage two BH3 ligands simultaneously via the canonical Bcl-2 ligand binding grooves. Furthermore, by structure-guided mutagenesis, we generated point mutations within the binding pocket of F1L in order to elucidate the residues responsible for both Bim and Bak binding and prevention of apoptosis. We propose that the sequestration of Bim by F1L is primarily responsible for preventing apoptosis during vaccinia virus infection.

IMPORTANCE

Numerous viruses have adapted strategies to counteract apoptosis by encoding proteins responsible for sequestering proapoptotic components. Vaccinia virus, the prototypical member of the family Orthopoxviridae, encodes a protein known as F1L that functions to prevent apoptosis by interacting with Bak and the BH3-only protein Bim. Despite recent structural advances, little is known regarding the mechanics of binding between F1L and the proapoptotic Bcl-2 family members. Utilizing three-dimensional structures of F1L bound to host proapoptotic proteins, we generated variants of F1L that neutralize Bim and/or Bak. We demonstrate that during vaccinia virus infection, engagement of Bim and Bak by F1L is crucial for subversion of host cell apoptosis.

CD8 T cells use IFN-γ to protect against the lethal effects of a respiratory poxvirus infection

CD8 T cells are a key component of immunity to many viral infections. They achieve this through using an array of effector mechanisms, but precisely which component/s are required for protection against a respiratory orthopox virus infection remains unclear. Using a model of respiratory vaccinia virus infection in mice, we could specifically determine the relative contribution of perforin, TRAIL, and IFN-γ-mediated pathways in protection against virus induced morbidity and mortality. Unexpectedly, we observed that protection against death was mediated by IFN-γ without any involvement of the perforin or TRAIL-dependent pathways. IFN-γ mRNA and protein levels in the lung peaked between days 3 and 6 postinfection. This enhanced response coincided with the emergence of virus-specific CD8 T cells in the lung and the cessation of weight loss. Transfer experiments indicated that CD8 T cell-autonomous expression of IFN-γ restricts virus-induced lung pathology and dissemination to visceral tissues and is necessary for clearance of virus. Most significantly, we show that CD8 T cell-derived IFN-γ is sufficient to protect mice in the absence of CD4 and B-lymphocytes. Thus, our findings reveal a previously unappreciated mechanism by which effector CD8 T cells afford protection against a highly virulent respiratory orthopox virus infection.

Viruses as modulators of mitochondrial functions

Mitochondria are multifunctional organelles with diverse roles including energy production and distribution, apoptosis, eliciting host immune response, and causing diseases and aging. Mitochondria-mediated immune responses might be an evolutionary adaptation by which mitochondria might have prevented the entry of invading microorganisms thus establishing them as an integral part of the cell. This makes them a target for all the invading pathogens including viruses. Viruses either induce or inhibit various mitochondrial processes in a highly specific manner so that they can replicate and produce progeny. Some viruses encode the Bcl2 homologues to counter the proapoptotic functions of the cellular and mitochondrial proteins. Others modulate the permeability transition pore and either prevent or induce the release of the apoptotic proteins from the mitochondria. Viruses like Herpes simplex virus 1 deplete the host mitochondrial DNA and some, like human immunodeficiency virus, hijack the host mitochondrial proteins to function fully inside the host cell. All these processes involve the participation of cellular proteins, mitochondrial proteins, and virus specific proteins. This review will summarize the strategies employed by viruses to utilize cellular mitochondria for successful multiplication and production of progeny virus.

Sheeppox virus SPPV14 encodes a Bcl-2-like cell death inhibitor that counters a distinct set of mammalian proapoptotic proteins

Many viruses express inhibitors of programmed cell death (apoptosis), thereby countering host defenses that would otherwise rapidly clear infected cells. To counter this, viruses such as adenoviruses and herpesviruses express recognizable homologs of the mammalian prosurvival protein Bcl-2. In contrast, the majority of poxviruses lack viral Bcl-2 (vBcl-2) homologs that are readily identified by sequence similarities. One such virus, myxoma virus, which is the causative agent of myxomatosis, expresses a virulence factor that is a potent inhibitor of apoptosis. In spite of the scant sequence similarity to Bcl-2, myxoma virus M11L adopts an almost identical 3-dimensional fold. We used M11L as bait in a sequence similarity search for other Bcl-2-like proteins and identified six putative vBcl-2 proteins from poxviruses. Some are potent inhibitors of apoptosis, in particular sheeppox virus SPPV14, which inhibited cell death induced by multiple agents. Importantly, SPPV14 compensated for the loss of antiapoptotic F1L in vaccinia virus and acts to directly counter the cell death mediators Bax and Bak. SPPV14 also engages a unique subset of the death-promoting BH3-only ligands, including Bim, Puma, Bmf, and Hrk. This suggests that SPPV14 may have been selected for specific biological roles as a virulence factor for sheeppox virus.

Increased protection from vaccinia virus infection in mice genetically prone to lymphoproliferative disorders

Mutations in the genes that encode Fas or Fas ligand (FasL) can result in poor restraints on lymphocyte activation and in increased susceptibility to autoimmune disorders. Because these mutations portend a continuously activated immune state, we hypothesized that they might in some cases confer resistance to infection. To examine this possibility, the immune response to, morbidity caused by, and clearance of vaccinia virus (VACV) Western Reserve was examined in 5- to 7-week-old Fas mutant (lpr) mice, before an overt lymphoproliferative disorder was observable. On day 6 after VACV infection, C57BL/6-lpr (B6-lpr) mice had decreased morbidity, decreased viral titers, and an increased percentage and number of CD4(+) and CD8(+) T cells. As early as day 2 after infection, B6-lpr mice had decreased liver and spleen viral titers and increased numbers of and increased gamma interferon (IFN-γ) production by several different effector cell populations. Depletion of individual effector cell subsets did not inhibit the resistance of B6-lpr mice. Uninfected B6-lpr mice also had increased numbers of NK cells, γδ(+) T cells, and CD44(+) CD4(+) and CD44(+) CD8(+) T cells compared to uninfected B6 mice. Antibody to IFN-γ resulted in increased virus load in both B6 and B6-lpr mice and eliminated the differences in viral titers between them. These results suggest that IFN-γ produced by multiple activated leukocyte populations in Fas-deficient hosts enhances resistance to some viral infections.

Enhanced tumor therapy using vaccinia virus strain GLV-1h68 in combination with a β-galactosidase-activatable prodrug seco-analog of duocarmycin SA

Breast cancer is the most common cause of cancer-related death worldwide, thus remaining a crucial health problem among women despite advances in conventional therapy. Therefore, new alternative strategies are needed for effective diagnosis and treatment. One approach is the use of oncolytic viruses for gene-directed enzyme prodrug therapy. Here, the lacZ-carrying vaccinia virus (VACV) strain GLV-1h68 was used in combination with a β-galactosidase-activatable prodrug derived from a seco-analog of the natural antibiotic duocarmycin SA. Tumor cell infection with the VACV strain GLV-1h68 led to production of β-galactosidase, essential for the conversion of the prodrug to the toxic compound. Furthermore, drug-dependent cell kill and induction of the intrinsic apoptosis pathway in tumor cells was also observed on combination therapy using the prodrug and the GLV-1h68 strain, despite the fact that VACV strains encode antiapoptotic proteins. Moreover, GI-101A breast cancer xenografts were effectively treated by the combination therapy. In conclusion, the combination of a β-galactosidase-activatable prodrug with a tumor-specific vaccinica virus strain encoding this enzyme, induced apoptosis in cultures of the human GI-101A breast cancer cells, in which a synergistic oncolytic effect was observed. Moreover, in vivo, additional prodrug treatment had beneficial effects on tumor regression in GLV-1h68-treated GI-101A-xenografted mice.

Deerpox virus encodes an inhibitor of apoptosis that regulates Bak and Bax

Poxviruses encode numerous proteins that inhibit apoptosis, a form of cell death critical to the elimination of virally infected cells. Sequencing of the deerpox virus genome revealed DPV022, a protein that lacks obvious homology to cellular members of the Bcl-2 family but shares limited regions of amino acid identity with two unique poxviral inhibitors of apoptosis, M11L and F1L. Given the limited homology, we sought to determine whether DPV022 could inhibit apoptosis. Here we show that DPV022 localized to the mitochondria, where it inhibited apoptosis. We used a Saccharomyces cerevisiae model system to demonstrate that in the absence of all other Bcl-2 family proteins, DPV022 interacted directly with Bak and Bax. We confirmed the ability of DPV022 to interact with Bak and Bax by immunoprecipitation and showed that DPV022 prevented apoptosis induced by Bak and Bax overexpression. Moreover, we showed that DPV022 blocked apoptosis even when all the endogenous mammalian antiapoptotic proteins were neutralized by a combination of selective BH3 ligands. During virus infection, DPV022 interacted with endogenous Bak and Bax and prevented the conformational activation of both of them. Thus, we have characterized a novel poxviral inhibitor of apoptosis with intriguing amino acid differences from the well-studied proteins M11L and F1L.

Myxoma and vaccinia viruses exploit different mechanisms to enter and infect human cancer cells

Myxoma (MYXV) and vaccinia (VACV) viruses have recently emerged as potential oncolytic agents that can infect and kill different human cancer cells. Although both are structurally similar, it is unknown whether the pathway(s) used by these poxviruses to enter and cause oncolysis in cancer cells are mechanistically similar. Here, we compared the entry of MYXV and VACV-WR into various human cancer cells and observed significant differences: 1--low-pH treatment accelerates fusion-mediated entry of VACV but not MYXV, 2--the tyrosine kinase inhibitor genistein inhibits entry of VACV, but not MYXV, 3--knockdown of PAK1 revealed that it is required for a late stage event downstream of MYXV entry into cancer cells, whereas PAK1 is required for VACV entry into the same target cells. These results suggest that VACV and MYXV exploit different mechanisms to enter into human cancer cells, thus providing some rationale for their divergent cancer cell tropisms.

Vaccinia virus F1L interacts with Bak using highly divergent Bcl-2 homology domains and replaces the function of Mcl-1

The Bcl-2 family regulates induction of apoptosis at the mitochondria. Essential to this regulation are the interactions between Bcl-2 family members, which are mediated by Bcl-2 homology (BH) domains. Vaccinia virus F1L is a unique inhibitor of apoptosis that lacks significant sequence similarity with the Bcl-2 family and does not contain obvious BH domains. Despite this, F1L inhibits cytochrome c release from mitochondria by preventing Bak and Bax activation. Although F1L constitutively interacts with Bak to prevent Bak activation, the precise mechanism of this interaction remains elusive. We have identified highly divergent BH domains in F1L that were verified by the recent crystal structure of F1L (Kvansakul, M., Yang, H., Fairlie, W. D., Czabotar, P. E., Fischer, S. F., Perugini, M. A., Huang, D. C., and Colman, P. M. (2008) Cell Death Differ. 15, 1564-1571). Here we show that F1L required these BH domains to interact with ectopically expressed and endogenous Bak. The interaction between F1L and Bak was conserved across species, and both F1L and the cellular antiapoptotic protein Mcl-1 required the Bak BH3 domain for interaction. Moreover, F1L replaced Mcl-1 during infection, as the Bak x Mcl-1 complex was disrupted during vaccinia virus infection. In contrast to UV irradiation, vaccinia virus infection did not result in rapid degradation of Mcl-1, consistent with our observation that vaccinia virus did not initiate a DNA damage response. Additionally, Mcl-1 expression prevented Bak activation and apoptosis during infection with a proapoptotic vaccinia virus devoid of F1L. Our data suggest that F1L replaces the antiapoptotic activity of Mcl-1 during vaccinia virus infection by interacting with Bak using highly divergent BH domains.

Detection of herpes simplex virus dependent apoptosis

Subversion of the host response to virus infection is a universal theme of virology and viral immunology. Multiple mechanisms are in place to limit virus spread on behalf of the host, yet through evolution, viruses have adapted to either weaken or eliminate the effects of these host factors. Cell death or apoptosis is one such example of a host response to viral infection. As such, experimental techniques that enable analysis of viruses (and viral genes) involved in triggering, blocking, or perhaps augmenting this process represent important tools for virologists, immunologists, and cell biologists. Presented here are a series of techniques developed in our lab for the analysis of apoptosis that occurs as a consequence of herpes simplex virus type 1 infection.

Lytic infection with vaccinia virus activates caspases in a Bcl-2-inhibitable manner

Vaccinia virus (VV) is considered to cause lytic infection of most cells, with lysis being regarded equivalent to necrosis. Activation of caspases has not been associated with necrosis. However, we observed the activation and activity of caspases in epithelial cells HeLa G and BSC-40 lytically infected with VV. Using three different flow-cytometric approaches, we characterized the distinct stages of caspase cascade in VV-infected cells: a cleaved, activated form of caspases detected using a fluorescent pan-caspase inhibitor; caspase activity assayed by cleavage of a non-fluorescent substrate into a fluorescent product; caspase-specific cleavage of death substrates characterized by a fluorescent antibody detecting a neo-epitope in cytokeratin-18. All of these approaches yielded an increased fluorescent signal in VV-infected cells compared to mock-infected controls. Additionally, the signal was decreased by the expression of Bcl-2. The cleavage of cytokeratin-18 was confirmed by western blotting, but another key protein involved in apoptosis, PARP, was not cleaved in VV-infected lytic cells. The necrotic phenotype of the cells was confirmed by increased cell membrane permeability and/or decreased mitochondrial membrane potential. In conclusion, our data suggest that VV infection of the epithelial cells HeLa G and BSC-40 initiates the apoptotic program, however, apoptosis is not completed and switches into necrosis.

Fowlpox virus encodes a Bcl-2 homologue that protects cells from apoptotic death through interaction with the proapoptotic protein Bak

Poxviruses are renowned for encoding numerous immunomodulatory proteins capable of undermining potent immune defenses. One effective barrier against infection is apoptosis, a process controlled at the mitochondria by pro- and antiapoptotic members of the highly conserved Bcl-2 family of proteins. Although poxviruses are known to encode an array of effective inhibitors of apoptosis, members of the Avipoxvirus genus, which includes fowlpox virus, encode proteins with Bcl-2 homology. Here, we show that FPV039, a fowlpox virus protein with limited Bcl-2 homology, inhibited apoptosis in response to a variety of cytotoxic stimuli, including virus infection itself. Similar to other antiapoptotic Bcl-2 proteins, FPV039 localized predominantly to the mitochondria in both human and chicken cells and protected human cells from tumor necrosis factor alpha-induced loss of the mitochondrial membrane potential. In addition, coimmunoprecipitation revealed that FPV039 interacted constitutively with the proapoptotic Bcl-2 protein, Bak, in both human and chicken cells. Concordantly, FPV039 also inhibited apoptosis induced by the transient overexpression of Bak. To confirm these results in the context of virus infection, we generated a recombinant vaccinia virus lacking F1L, the endogenous apoptotic inhibitor in vaccinia virus, and expressing FPV039. In the context of vaccinia virus infection, FPV039 retained the ability to localize to the mitochondria and interacted with Bak. Moreover, FPV039 prevented the activation of Bak and protected infected cells from apoptosis induced by staurosporine and virus infection. Together, our data indicate that FPV039 is a functional Bcl-2 homologue that inhibits apoptosis by neutralizing the proapoptotic Bcl-2 family member Bak.

Understanding orthopoxvirus interference with host immune responses to inform novel vaccine design

Jenner's original vaccine used cowpox virus. Cowpox virus and, subsequently, vaccinia virus, a closely related Orthopoxvirus, provided the means to eradicate smallpox. This history and the unique properties of the virus suggest that vaccinia virus will continue to provide a useful vaccine platform. Yet, surprisingly, it has become apparent that much of the virus genome encodes accessory proteins that interfere with host immune responses to infection. Manipulation of these genes offers the potential for new generations of orthopoxvirus vaccines in which we will have far greater control over key features of the vaccination, including the sites of virus infection, the degree of virus replication, the pathogenicity of the virus and, most importantly, the suppression or induction of immune responses of specific types.

The vaccinia virus protein F1L interacts with Bim and inhibits activation of the pro-apoptotic protein Bax

Vaccinia virus, the prototypic member of the orthopoxvirus genus, encodes the mitochondrial-localized protein F1L that functions to protect cells from apoptotic death and inhibits cytochrome c release. We previously showed that F1L interacts with the pro-apoptotic Bcl-2 family member Bak and inhibits activation of Bak following an apoptotic stimulus (Wasilenko, S. T., Banadyga, L., Bond, D., and Barry, M. (2005) J. Virol. 79, 14031-14043). In addition to Bak, the pro-apoptotic protein Bax is also capable of initiating cytochrome c release suggesting that vaccinia virus infection could also inhibit Bax activity. Here we show that F1L inhibits the activity of the pro-apoptotic protein Bax by inhibiting oligomerization and N-terminal activation of Bax. F1L expression also inhibited the subcellular redistribution of Bax to the mitochondria and the insertion of Bax into the outer mitochondrial membrane. The ability of F1L to inhibit Bax activation does not require Bak, because F1L expression inhibited cytochrome c release and Bax activation in Bak-deficient cells. No interaction between Bax and F1L was detected during infection, suggesting that F1L functions upstream of Bax activation. Notably, F1L was capable of interacting with the BH3-only protein BimL as shown by co-immunoprecipitation, and F1L expression inhibited apoptosis induced by BimL. These studies suggest that, in addition to interacting with the pro-apoptotic protein Bak, F1L also functions to indirectly inhibit the activation of Bax, likely by interfering with the pro-apoptotic activity of BH3-only proteins such as BimL.

Caspase 3 activation during herpes simplex virus 1 infection

During herpes simplex virus 1 (HSV-1) infection, apoptosis is initiated by immediate early gene transcription and is later modulated by proteins synthesized in infected cells. We have previously shown that procaspase 3 levels are reduced during HSV-1 replication. We now demonstrate that a replication-defective HSV-1 recombinant virus which is incapable of packaging viral DNA into capsids activated caspase 3 but retained the ability to prevent the apoptotic process from killing the infected cells. This implies that HSV-1-dependent apoptosis is not merely a response to abortive infection. Maximum accumulation of the active form of caspase 3 accompanied complete HSV-1-dependent apoptosis. Additionally, caspase 7 was found to be activated during HSV-1-dependent apoptosis. Infected MCF-7 cells which ectopically express caspase 3 underwent more efficient apoptosis than their caspase 3-null parental counterparts, confirming that caspase 3 contributes to HSV-1-dependent apoptosis. However, caspase 3 reconstitution did not make the MCF-7 cells as sensitive as HEp-2 cells to HSV-1-dependent apoptosis, suggesting that other cellular factors may be involved in conferring resistance to this process. These results indicate that caspase 3 activation is a consequence of HSV-1 infection and have important implications in our understanding of the interactions of the virus with host cells.

Role of mitochondria in apoptosis induced by the 2-5A system and mechanisms involved

The 2-5A system (2-5OAS/RNaseL) is composed of the 2',5'oligoadenylate synthetase 1 (2-5OAS1) and 2-5A-dependent RNase (RNaseL), enzymes that play a key role in antiviral defence mechanisms. Activation of the 2-5A system by double stranded RNA (dsRNA) induces degradation of ribosomal RNAs and apoptosis in mammalian cells. To obtain further information into the molecular mechanisms by which RNaseL induces apoptosis, we expressed human RNaseL and 2-5OAS in HeLa cells using recombinant vaccinia viruses as vectors and we analysed in detail different biochemical markers of apoptosis. In this expression virus-cell system the activation of RNaseL, as index of rRNA degradation, is an upstream event of apoptosis induction. RNaseL induces apoptosis in a caspase-dependent manner (caspases 8, 9 and 2). At the beginning of apoptosis RNaseL and 2-5OAS are localized in the mitochondria and cytosol fractions, while at the onset of apoptosis both enzymes are largely in mitochondria. The 2-5A system induces the release of Cytochrome c from mitochondria to cytosol in a caspase dependent manner. The onset of apoptosis elicits the disruption of mitochondrial membrane potential (delta psi m), as well as the generation of reactive oxygen species (ROS). Moreover, the activation of RNaseL induces morphological alterations in the mitochondria. Apoptosis induced by the 2-5A system involves mitochondrial proteins, such as the human anti-apoptotic protein Bcl-2, which blocks both the apoptosis and the change of delta psi m induced by the activation of RNaseL. These findings provide new insights into the molecular mechanisms of apoptosis induction by the 2-5A system, demonstrating the importance of mitochondria in 2-5OAS/RNaseL-induced apoptosis.

Vaccinia virus B1R kinase interacts with JIP1 and modulates c-Jun-dependent signaling

Viruses have to adjust to the host cell to guarantee their life cycle and survival. This aspect of the virus-host cell interaction is probably performed by viral proteins, such as serine-threonine kinases, that are present early during infection. Vaccinia virus has an early Ser-Thr kinase, B1R, which, although required for successful viral infection, is poorly characterized regarding its effects on cellular proteins, and thus, its potential contribution to pathogenesis is not known. Signaling by mitogen-activated protein kinase (MAPK) is mediated by the assembly of complexes between these kinases and the JIP scaffold proteins. To understand how vaccinia virus B1R can affect the host, its roles in the cellular signaling by MAPK complexes and c-Jun activation have been studied. Independently of its kinase activity, B1R can interact with the central region of the JIP1 scaffold protein. The B1R-JIP1 complex increases the amount of MAPK bound to JIP1; thus, MKK7 and TAK1 either bind with higher affinity or bind more stably to JIP1, while there is an increase in the phosphorylation state of JNK bound to JIP1. The functional consequence of these more stable interactions is an increase in the activity of transcription factors, such as c-Jun, that respond to these complexes. Furthermore, B1R is also able to directly phosphorylate c-Jun in residues different from those targeted by JNK and, thus, B1R can also cooperate by an independent route in c-Jun activation. Vaccinia virus B1R can thus modulate the signaling of pathways that respond to cellular stress.

Near death experiences: poxvirus regulation of apoptotic death

Apoptosis, or programmed cell death, plays a critical role in the elimination of virus-infected cells. As a result, a growing number of viruses encode numerous potent anti-apoptotic proteins to counteract apoptosis in an effort to prolong their own survival. This review describes the numerous mechanisms by which poxviruses inhibit apoptosis thereby modulating life and death of the cell.

Interaction of F1L with the BH3 domain of Bak is responsible for inhibiting vaccinia-induced apoptosis

Apoptosis represents an important cellular defence mechanism against viral pathogens by virtue of its ability to remove infected cells. Consequently, many viruses have developed numerous strategies to prevent or delay host cell apoptosis in order to achieve productive replication. Here we report that deletion of the F1L gene from the vaccinia genome results in increased apoptosis during infection. We demonstrate that F1L, which has no sequence homology to Bcl-2 family members, inhibits apoptosis at the level of mitochondria by binding to Bak. As a consequence, F1L prevents Bak activation, oligomerization and interaction with active Bax, all critical steps in the induction of apoptosis. We demonstrate that residues 64-84 of F1L interact directly with the Bcl-2 homology domain 3 (BH3) domain of Bak. This region of F1L has limited sequence similarity to known Bak-interacting BH3 domains. We also find that such additional BH3-like domains exist in the vaccinia genome. We conclude that F1L uses this specific, BH3-like domain to bind and inhibit Bak at the mitochondria.

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.

The vaccinia virus F1L protein interacts with the proapoptotic protein Bak and inhibits Bak activation

Many viruses have evolved strategies to counteract cellular immune responses, including apoptosis. Vaccinia virus, a member of the poxvirus family, encodes an antiapoptotic protein, F1L. F1L localizes to mitochondria and inhibits apoptosis by preventing the release of cytochrome c by an undetermined mechanism (S. T. Wasilenko, T. L. Stewart, A. F. Meyers, and M. Barry, Proc. Natl. Acad. Sci. USA 100:14345-14350, 2003; T. L. Stewart, S. T. Wasilenko, and M. Barry, J. Virol. 79:1084-1098, 2005). Here, we show that in the absence of an apoptotic stimulus, F1L associates with Bak, a proapoptotic member of the Bcl-2 family that plays a pivotal role in the release of cytochrome c. Cells infected with vaccinia virus were resistant to Bak oligomerization and the initial N-terminal exposure of Bak following the induction of apoptosis with staurosporine. A mutant vaccinia virus missing F1L was no longer able to inhibit apoptosis or Bak activation. In addition, the expression of F1L was essential to inhibit tBid-induced cytochrome c release in both wild-type murine embryonic fibroblasts (MEFs) and Bax-deficient MEFs, indicating that F1L could inhibit apoptosis in the presence and absence of Bax. tBid-induced Bak oligomerization and N-terminal exposure of Bak in Bax-deficient MEFs were inhibited during virus infection, as assessed by cross-linking and limited trypsin proteolysis. Infection with the F1L deletion virus no longer provided protection from tBid-induced Bak activation and apoptosis. Additionally, infection of Jurkat cells with the F1L deletion virus resulted in cellular apoptosis, as measured by loss of the inner mitochondrial membrane potential, caspase 3 activation, and cytochrome c release, indicating that the presence of F1L was pivotal for inhibiting vaccinia virus-induced apoptosis. Our data indicate that F1L expression during infection inhibits apoptosis and interferes with the activation of Bak.

Changes in Bcl-2 expression in vaccinia virus-infected human peripheral blood monocytes

In this study, we show that Bcl-2, one of the most important antiapoptotic agents, is expressed in a phase-dependent manner in the human adherent monocytes after vaccinia virus infection, reflecting the viral infection stages. Early viral infection induced Bcl-2 expression in a level higher than in control cells. At 14 h post-infection (p.i.), the Bcl-2 level measured in the whole cell extracts dramatically decreased, followed by the increase at 24 h p.i. The levels of active dephosphorylated Bcl- 2 protein present in the cells reflected the gene expression character, but were much lower than in case of a heat shock. The dramatic increase of Bcl-2 protein level in the nuclear fraction at 4 h p.i. was observed. Changes in Bcl-2 mRNA content in elutriated human blood monocytes isolated from the same donor showed different kinetics, increasing up to 12 h p.i. and diminishing to undetectable level at 24 h p.i. concomitantly with a severe increase in the number of dead cells. The results indicate that virally infected adherent monocytes remain resistant to apoptosis, while freshly isolated monocytes undergo apoptotic cell death. These results throw new light on the apoptotic mechanism in the monocyte-derived cells after vaccinia virus infection in vitro.

Modified vaccinia virus Ankara protein F1L is a novel BH3-domain-binding protein and acts together with the early viral protein E3L to block virus-associated apoptosis

Infection with viruses often protects the infected cell against external stimuli to apoptosis. Here we explore the balance of apoptosis induction and inhibition for infection with the modified vaccinia virus Ankara (MVA), using two MVA mutants with experimentally introduced deletions. Deletion of the E3L-gene from MVA transformed the virus from an inhibitor to an inducer of apoptosis. Noxa-deficient mouse embryonic fibroblasts (MEF) were resistant to MVA-DeltaE3L-induced apoptosis. When the gene encoding F1L was deleted from MVA, apoptosis resulted that required Bak or Bax. MVA-DeltaF1L-induced apoptosis was blocked by Bcl-2. When expressed in HeLa cells, F1L blocked apoptosis induced by forced expression of the BH3-only proteins, Bim, Puma and Noxa. Finally, biosensor analysis confirmed direct binding of F1L to BH3 domains. These data describe a molecular framework of how a cell responds to MVA infection by undergoing apoptosis, and how the virus blocks apoptosis by interfering with critical steps of its signal transduction.

Tumorigenic poxviruses up-regulate intracellular superoxide to inhibit apoptosis and promote cell proliferation

Tumorigenic leporipoxviruses encode catalytically inactive homologs of cellular Cu-Zn superoxide dismutase (SOD1). The function of the orthologous myxoma virus M131R and Shope fibroma virus S131R gene products is uncertain, but they inhibit SOD1 activity by a process linked to binding its copper chaperone. Using a superoxide-sensitive dye (hydroethidine), we observed that virus infection increased intracellular superoxide levels in an M/S131R-dependent manner. To see whether this effect promotes infection, we deleted the Shope fibroma virus S131R gene and compared the clinical manifestations of wild-type and mutant virus infections in rabbits. S131RDelta virus produced significantly smaller fibroxanthosarcoma-like growths in vivo and, at a point where these growths were already receding, wild-type infections still showed extensive leukocyte infiltration, necrosis, and fibromatous cell proliferation. Coincidentally, whereas Jurkat cells are protected from mitochondria- and Fas-mediated apoptosis by wild-type myxoma virus in vitro, M131RDelta virus could not block Fas-initiated apoptosis as judged by DNA laddering, terminal deoxynucleotidyltransferase-mediated dUTP-fluorescein nick end labeling, and caspase 3 cleavage assays. These data suggest that tumorigenic poxviruses can modulate intracellular redox status to their advantage to stimulate infected cell growth and inhibit programmed cell death.

Fighting Cancer with Vaccinia Virus: Teaching New Tricks to an Old Dog

Vaccinia virus has played a huge part in human beings' victory over smallpox. With smallpox being eradicated and large-scale vaccination stopped worldwide, vaccinia has assumed a new role in our fight against another serious threat to human health: cancer. Recent advances in molecular biology, virology, immunology, and cancer genetics have led to the design of novel cancer therapeutics based on vaccinia virus backbones. With the ability to infect efficiently a wide range of host cells, a genome that can accommodate large DNA inserts and express multiple genes, high immunogenicity, and cytoplasmic replication without the possibility of chromosomal integration, vaccinia virus has become the platform of many exploratory approaches to treat cancer. Vaccinia virus has been used as (1) a delivery vehicle for anti-cancer transgenes, (2) a vaccine carrier for tumor-associated antigens and immunoregulatory molecules in cancer immunotherapy, and (3) an oncolytic agent that selectively replicates in and lyses cancer cells.

Vaccinia virus F1L protein is a tail-anchored protein that functions at the mitochondria to inhibit apoptosis

Members of the poxvirus family encode multiple immune evasion proteins, including proteins that regulate apoptosis. We recently identified one such protein, F1L, encoded by vaccinia virus, the prototypic member of the poxvirus family. F1L localizes to the mitochondria and inhibits apoptosis by interfering with the release of cytochrome c, the pivotal commitment step in the apoptotic cascade. Sequence analysis of the F1L open reading frame revealed a C-terminal motif composed of a 12-amino-acid transmembrane domain flanked by positively charged lysines, followed by an 8-amino-acid hydrophilic tail. By generating a series of F1L deletion constructs, we show that the C-terminal domain is necessary and sufficient for localization of F1L to the mitochondria. In addition, mutation of lysines 219 and 222 downstream of the C-terminal transmembrane domain resulted in altered localization of F1L to the endoplasmic reticulum. Using F1L protein generated in an in vitro transcription-translation system, we found that F1L was posttranslationally inserted into mitochondria and tightly associated with mitochondrial membranes as demonstrated by resistance to alkaline extraction. Sensitivity to protease digestion showed that the N terminus of F1L was exposed to the cytoplasm. Utilizing various F1L deletion constructs, we found that F1L localization to the mitochondria was necessary to inhibit apoptosis, since constructs that no longer localized to the mitochondria had reduced antiapoptotic ability. Our studies show that F1L is a new member of the tail-anchored protein family that localizes to mitochondria during virus infection and inhibits apoptosis as a means to enhance virus survival.

The vaccinia virus B1R kinase induces p53 downregulation by an Mdm2-dependent mechanism

Poxvirus infection has a strong effect on cellular functions. To understand viral pathogenesis, it is necessary to know how viral proteins interact with host proteins. The B1R kinase is an early viral gene required for vaccinia virus DNA synthesis and replication, but no cellular substrate is known for this viral kinase. B1R is able to hyperphosphorylate p53 in several residues in the N-terminal transactivation domain, including Ser15 and Thr18. B1R does not phosphorylate Mdm2. B1R promotes an increase in p53 ubiquitination and a reduction of p53 acetylation by p300. The over-expressed B1R protein induces the degradation of p53 in a concentration-dependent manner and is lost when Ser15 and Th18 are changed to alanine or when the B1R kinase is inactivated by introducing the K149Q substitution. The B1R-induced downregulation of p53 requires Mdm2. The hyperphosphorylated p53 is transcriptionally active, and this activity also falls as B1R increases. The BAX gene promoter is more sensitive to this reduction of transcription than p21 or 14-3-3 gene promoters. This effect of B1R on p53 can be one of the mechanisms by which vaccinia virus exerts its role in infected cells.

Poxvirus infection and apoptosis

The following excellent reviews have been published on poxviruses and apoptosis during the last few years: P.C. Turner and R.W. Moyer, Semin. Virology, 8: 453-469, 1998; J.L. Shisler and B. Moss, Semin. Immunol., 13: 67-72, 2001; and H. Everett and G. McFadden, Curr. Opin. Microbiol., 5: 395-402, 2002. These articles dealt with the viral products and the mechanisms by which they interfere with apoptosis. In this review, we summarize new and old information and also introduce a new approach to explore interactions between the host cell and the replicating virus.

Vaccinia virus encodes a previously uncharacterized mitochondrial-associated inhibitor of apoptosis

To circumvent apoptotic death, many viruses encode Bcl-2 homologous proteins that function at the mitochondria. Vaccinia virus, the prototypic member of the Poxviridae family, does not encode a Bcl-2 homolog but inhibits the mitochondrial arm of the apoptotic cascade by an unknown mechanism. We now report that F1L, a previously unidentified protein in vaccinia virus, is responsible for the inhibition of apoptosis. Cells infected with vaccinia virus are resistant to staurosporine-mediated cleavage of poly(ADP-ribose) polymerase, caspases 3 and 9, and release of cytochrome c. In contrast, a vaccinia virus deletion mutant, VV811, was unable to inhibit apoptosis; however, the antiapoptotic function was restored by expression of the F1L ORF, which is absent in VV811. Although F1L displays no homology to members of the Bcl-2 family, it localizes to the mitochondria through a C-terminal hydrophobic domain. We show that expression of F1L interferes with apoptosis by inhibiting the loss of the inner mitochondrial membrane potential and the release of cytochrome c.

Extracellular matrix proteins protect pancreatic cancer cells from death via mitochondrial and nonmitochondrial pathways

BACKGROUND AND AIMS

Pancreatic cancer is a very aggressive malignancy. Normal cells die through apoptosis when detached from extracellular matrix (ECM), but the role of ECM in cancer cell survival is poorly understood. Here, we determined the effects of ECM proteins on death responses and underlying signaling pathways in human pancreatic cancer cells.

METHODS

We measured apoptosis and necrosis, caspase activation, and mitochondrial dysfunction in MIA PaCa-2 and PANC-1 pancreatic carcinoma cells both detached and attached to ECM proteins.

RESULTS

Detachment of pancreatic cancer cells from ECM did not induce classic apoptosis, as it does in normal cells, but induced necrosis and apoptosis associated with secondary necrosis. It caused a pronounced mitochondrial depolarization and release of cytochrome c and Smac/DIABLO. However, as different from normal cells, cytochrome c release did not result in downstream caspase activation. Executioner caspases were activated in detached pancreatic cancer cells independent of cytochrome c. Laminin and fibronectin, but not collagen I, markedly increased pancreatic cancer cell survival by inhibiting both mitochondrial dysfunction (leading to inhibition of necrosis) and caspase activity (leading to decreased apoptotic DNA fragmentation).

CONCLUSIONS

ECM proteins greatly protect pancreatic cancer cells from death by mechanisms different from those operating in normal cells. The results suggest ECM proteins and their receptors as potential targets for treatment of pancreatic cancer.

Poxviruses and immune evasion

Large DNA viruses defend against hostile assault executed by the host immune system by producing an array of gene products that systematically sabotage key components of the inflammatory response. Poxviruses target many of the primary mediators of innate immunity including interferons, tumor necrosis factors, interleukins, complement, and chemokines. Poxviruses also manipulate a variety of intracellular signal transduction pathways such as the apoptotic response. Many of the poxvirus genes that disrupt these pathways have been hijacked directly from the host immune system, while others have demonstrated no clear resemblance to any known host genes. Nonetheless, the immunological targets and the diversity of strategies used by poxviruses to disrupt these host pathways have provided important insights into diverse aspects of immunology, virology, and inflammation. Furthermore, because of their anti-inflammatory nature, many of these poxvirus proteins hold promise as potential therapeutic agents for acute or chronic inflammatory conditions.

Macrophages and dendritic cells use the cytosolic pathway to rapidly cross-present antigen from live, vaccinia-infected cells

Professional APCs (pAPC) can process and present on their own MHC class I molecules Ags acquired from Ag donor cells (ADC). This phenomenon of cross-presentation is essential in the induction of CD8(+) T cell responses to viruses that do not infect pAPC and possibly contributes to the induction of CD8(+) responses to many other viruses. However, little is known about the mechanisms underlying this process. In this study, we show that dendritic cells and macrophages cross-present a model Ag supplied by vaccinia virus-infected ADC via the cytosolic route. Strikingly, we also found that cross-presentation of Ags provided by vaccinia-infected cells occurs within a couple of hours of pAPC/ADC interaction, that the duration of cross-presentation lasts for only 16 h, and that cross-presentation can occur at early times of infection when the ADC are still alive.

Poxviruses and apoptosis: a time to die

All known apoptosis modulators in poxviruses have been shown to function as inhibitors. The mechanistic classes of these poxvirus-encoded inhibitors are quite diverse, and indicate that a wide variety of distinct host proteins in cellular apoptotic pathways have been targeted for inhibition by individual poxviruses.

A time to kill: viral manipulation of the cell death program

Many viruses have as part of their arsenal the ability to modulate the apoptotic pathways of the host. It is counter-intuitive that such simple organisms would be efficient at regulating this the most crucial pathway within the host, given the relative complexity of the host cells. Yet, viruses have the potential to initiate or stay the onset of programmed cell death through the manipulation of a variety of key apoptotic proteins. It is the intention of this review to provide an overview of viral gene products that are able to promote or inhibit apoptotic death of the host cell and to discuss their mechanisms of action. It is not until recently that the depth at which viruses exploit the apoptotic pathways of their host has been seen. This understanding may provide a great opportunity for future therapeutic ventures.

Cytotoxic T lymphocytes: all roads lead to death

Cytotoxic T lymphocytes (CTLs) provide potent defences against virus infection and intracellular pathogens. However, CTLs have a dark side--their lytic machinery can be directed against self-tissues in autoimmune disorders, transplanted cells during graft rejection and host tissues to cause graft-versus-host disease, which is one of the most serious diseases related to CTL function. Although this duplicitous behaviour might seem contradictory, both beneficial and detrimental effects are the result of the same effector proteins. So, an understanding of the mechanisms that are used by CTLs to destroy targets and a knowledge of pathogen immune-evasion strategies will provide vital information for the design of new therapies.

Visualizing priming of virus-specific CD8+ T cells by infected dendritic cells in vivo

The rational design of vaccines that elicit CD8+ T cell responses requires knowledge of the identity of the antigen-presenting cell (APC), the location and time of presentation and the nature of the antigen presented by the APC. Here we address these questions for an antigen encoded by a recombinant vaccinia virus. We found that, following local infection, vaccinia virus infected macrophages and dendritic cells in draining lymph nodes. However, only the dendritic cells presented antigen to naïve CD8+ T cells, as determined by direct visualization of sectioned nodes by confocal microscopy. Presentation occurred as rapidly as 6 h after inoculation and quickly declined in parallel with the number of infected cells present in the nodes. These data provide direct evidence that virus-infected APCs prime naïve CD8+ T cells in vivo.