Rabies virus replication induces Bax-related, caspase dependent apoptosis in mouse neuroblastoma cells

Molecular Mechanisms Associated with Neurodegeneration of Neurotropic Viral Infection

Viral infections of the central nervous system (CNS) cause variable outcomes from acute to severe neurological sequelae with increased morbidity and mortality. Viral neuroinvasion directly or indirectly induces encephalitis via dysregulation of the immune response and contributes to the alteration of neuronal function and the degeneration of neuronal cells. This review provides an overview of the cellular and molecular mechanisms of virus-induced neurodegeneration. Neurotropic viral infections influence many aspects of neuronal dysfunction, including promoting chronic inflammation, inducing cellular oxidative stress, impairing mitophagy, encountering mitochondrial dynamics, enhancing metabolic rewiring, altering neurotransmitter systems, and inducing misfolded and aggregated pathological proteins associated with neurodegenerative diseases. These pathogenetic mechanisms create a multidimensional injury of the brain that leads to specific neuronal and brain dysfunction. The understanding of the molecular mechanisms underlying the neurophathogenesis associated with neurodegeneration of viral infection may emphasize the strategies for prevention, protection, and treatment of virus infection of the CNS.

Phosphoprotein Gene Contributes to the Enhanced Apoptosis Induced by Wild-Type Rabies Virus GD-SH-01 In Vitro

Previous research demonstrated that the matrix protein (M) and glycoprotein (G) of attenuated rabies virus (RABV) strains are involved in the induction of host cell apoptosis. In this work, we show that wild-type (wt) RABV GD-SH-01 induces significantly greater apoptosis than the attenuated strain HEP-Flury. In order to identify the gene(s) accounting for this phenotype, five recombinant RABVs (rRABVs) were constructed by replacing each single gene of HEP-Flury with the corresponding gene of GD-SH-01. By using these rRABVs, we found that not only M and G, but also the phosphoprotein (P) plays an important role in inducing apoptosis. In order to figure out the different role of P gene in inducing apoptosis from the highly divergent background, another rRABV rGDSH-P, which carries the P gene of HEP-Flury in the background of the GD-SH-01 was generated. It was found that infection of NA cells with GD-SH-01 or the recombinant strain rHEP-shP, which carries P gene of GD-SH-01, induced significantly greater apoptosis than HEP-Flury or rGDSH-P in a caspase-dependent pathway that ultimately leads to the activation of the intrinsic apoptotic pathway, which is well characterized with the downregulation of bcl-2, the decrease of mitochondrial membrane potential, the release of mitochondrial cytochrome c, the activation of caspase-9 and caspase-3, and finally the cleavage of poly (ADP-ribose) polymerase. Our results imply that wt P from GD-SH-01 mediates this effect may partly by facilitating viral RNA synthesis but not by viral replication. In sum, we demonstrate a wt RABV strain GD-SH-01 to induce stronger apoptosis than an attenuated RABV HEP-Flury and propose that wt P from GD-SH-01 is involved in this process.

Impact of caspase-1/11, -3, -7, or IL-1β/IL-18 deficiency on rabies virus-induced macrophage cell death and onset of disease

Rabies virus is a highly neurovirulent RNA virus, which causes about 59000 deaths in humans each year. Previously, we described macrophage cytotoxicity upon infection with rabies virus. Here we examined the type of cell death and the role of specific caspases in cell death and disease development upon infection with two laboratory strains of rabies virus: Challenge Virus Standard strain-11 (CVS-11) is highly neurotropic and lethal for mice, while the attenuated Evelyn–Rotnycki–Abelseth (ERA) strain has a broader cell tropism, is non-lethal and has been used as an oral vaccine for animals. Infection of Mf4/4 macrophages with both strains led to caspase-1 activation and IL-1β and IL-18 production, as well as activation of caspases-3, -7, -8, and -9. Moreover, absence of caspase-3, but not of caspase-1 and -11 or -7, partially inhibited virus-induced cell death of bone marrow-derived macrophages. Intranasal inoculation with CVS-11 of mice deficient for either caspase-1 and -11 or -7 or both IL-1β and IL-18 led to general brain infection and lethal disease similar to wild-type mice. Deficiency of caspase-3, on the other hand, significantly delayed the onset of disease, but did not prevent final lethal outcome. Interestingly, deficiency of caspase-1/11, the key executioner of pyroptosis, aggravated disease severity caused by ERA virus, whereas wild-type mice or mice deficient for either caspase-3, -7, or both IL-1β and IL-18 presented the typical mild symptoms associated with ERA virus. In conclusion, rabies virus infection of macrophages induces caspase-1- and caspase-3-dependent cell death. In vivo caspase-1/11 and caspase-3 differently affect disease development in response to infection with the attenuated ERA strain or the virulent CVS-11 strain, respectively. Inflammatory caspases seem to control attenuated rabies virus infection, while caspase-3 aggravates virulent rabies virus infection.

Subversion of the Immune Response by Rabies Virus

Rabies has affected mankind for several centuries and is one of the oldest known zoonoses. It is peculiar how little is known regarding the means by which rabies virus (RABV) evades the immune response and kills its host. This review investigates the complex interplay between RABV and the immune system, including the various means by which RABV evades, or advantageously utilizes, the host immune response in order to ensure successful replication and spread to another host. Different factors that influence immune responses—including age, sex, cerebral lateralization and temperature—are discussed, with specific reference to RABV and the effects on host morbidity and mortality. We also investigate the role of apoptosis and discuss whether it is a detrimental or beneficial mechanism of the host’s response to infection. The various RABV proteins and their roles in immune evasion are examined in depth with reference to important domains and the downstream effects of these interactions. Lastly, an overview of the means by which RABV evades important immune responses is provided. The research discussed in this review will be important in determining the roles of the immune response during RABV infections as well as to highlight important therapeutic target regions and potential strategies for rabies treatment.

Rabies virus matrix protein induces apoptosis by targeting mitochondria

Apoptosis, as an innate antiviral defense, not only functions to limit viral replication by eliminating infected cells, but also contribute to viral dissemination, particularly at the late stages of infection. A highly neurotropic CVS strain of rabies virus induces apoptosis both in vitro and in vivo. However, the detailed mechanism of CVS-mediated neuronal apoptosis is not entirely clear. Here, we show that CVS induces apoptosis through mitochondrial pathway by dissipating mitochondrial membrane potential, release of cytochrome c and AIF. CVS blocks Bax activation at the early stages of infection; while M protein partially targets mitochondria and induces mitochondrial apoptosis at the late stages of infection. The α-helix structure spanning 67-79 amino acids of M protein is essential for mitochondrial targeting and induction of apoptosis. These results suggest that CVS functions on mitochondria to regulate apoptosis at different stages of infection, so as to for viral replication and dissemination.

Role of apoptosis in rabies viral encephalitis: a comparative study in mice, canine, and human brain with a review of literature

To evaluate the role of apoptosis in rabies encephalitis in humans and canines infected with wild-type street virus, in comparison with rodent model infected with street and laboratory passaged CVS strain, we studied postmortem brain tissue from nine humans, six canines infected with street rabies virus, and Swiss albino mice inoculated intramuscularly (IM) and intracerebrally (IC) with street and CVS strains. Encephalitis and high rabies antigen load were prominent in canine and human brains compared to rodents inoculated with street virus. Neuronal apoptosis was detectable only in sucking mice inoculated with CVS strain and minimal in street virus inoculated mice. In a time point study in suckling mice, DNA laddering was noted only terminally (7 days p.i.) following IC inoculation with CVS strain but not with street virus. In weanling and adult mice, apoptosis was restricted to inflammatory cells and absent in neurons similar to human and canine rabies-infected brains. Absence of neuronal apoptosis in wild-type rabies may facilitate intraneuronal survival and replication while apoptosis in inflammatory cells prevents elimination of the virus by abrogation of host inflammatory response.

The proapoptotic Bcl-2 protein Bax plays an important role in the pathogenesis of reovirus encephalitis

Encephalitis induced by reovirus serotype 3 (T3) strains results from the apoptotic death of infected neurons. Extrinsic apoptotic signaling is activated in reovirus-infected neurons in vitro and in vivo, but the role of intrinsic apoptosis signaling during encephalitis is largely unknown. Bax plays a key role in intrinsic apoptotic signaling in neurons by allowing the release of mitochondrial cytochrome c. We found Bax activation and cytochrome c release in neurons following infection of neonatal mice with T3 reoviruses. Bax(-/-) mice infected with T3 Abney (T3A) have reduced central nervous system (CNS) tissue injury and decreased apoptosis, despite viral replication that is similar to that in wild-type (WT) Bax(+/+) mice. In contrast, in the heart, T3A-infected Bax(-/-) mice have viral growth, caspase activation, and injury comparable to those in WT mice, indicating that the role of Bax in pathogenesis is organ specific. Nonmyocarditic T3 Dearing (T3D)-infected Bax(-/-) mice had delayed disease and enhanced survival compared to WT mice. T3D-infected Bax(-/-) mice had significantly lower viral titers and levels of activated caspase 3 in the brain despite unaffected transneuronal spread of virus. Cytochrome c and Smac release occurred in some reovirus-infected neurons in the absence of Bax; however, this was clearly reduced compared to levels seen in Bax(+/+) wild-type mice, indicating that Bax is necessary for efficient activation of proapoptotic mitochondrial signaling in infected neurons. Our studies suggest that Bax is important for reovirus growth and pathogenesis in neurons and that the intrinsic pathway of apoptosis, mediated by Bax, is important for full expression of disease, CNS tissue injury, apoptosis, and viral growth in the CNS of reovirus-infected mice.

Thai Acanthamoeba isolate (T4) induced apoptotic death in neuroblastoma cells via the Bax-mediated pathway

A Thai Acanthamoeba isolate named AS recovered from a corneal scraping of a keratitis patient was genotypically determined as T4. AS trophozoites were used for studying Acanthamoeba-induced apoptosis in mouse neuroblastoma NA cells during in vitro co-cultivation. The Acanthamoeba-exposed NA cells showed signs of apoptosis including cell shrinkage, nuclear condensation and DNA fragmentation. The effect was confirmed by DNA laddering electrophoresis. Involvement of caspase enzymes and mitochondrial pro- and anti-apoptotic proteins (Bax and Bcl-2) in AS-induced apoptosis was determined. The use of Z-VAD-FMK, a pan-caspase inhibitor, significantly reduced the apoptotic effect, while Bax/Bcl-2 ratio analysis showed a significant increase in the expression of apoptotic proteins in AS-exposed NA cells. These results strongly indicated that apoptosis induced by AS trophozoites is caspase-dependent and is mediated by over-expression of pro-apoptotic proteins in the mitochondrial pathway. This is the first report on the role of Bax in mediating apoptosis induced by Acanthamoeba.

Clinical isolates of dengue virus with distinctive susceptibility to nitric oxide radical induce differential gene responses in THP-1 cells

In the present study, 10 clinical isolates of dengue virus were selected according to their susceptibility to the inhibitory effect of nitric oxide radical, NO. Five of them are nitric oxide-susceptible viruses while the other five are nitric oxide-resistant viruses. These isolates were investigated to identify genetic factors that are responsible for the different phenotypes. Due to the evidence showing that NO suppresses DENV RNA polymerase activity, we, therefore, hypothesized that the RdRp domain of NS5 may responsible for NO inhibition. To answer this question, sequences of NS5 gene of NO-susceptible viruses and NO-resistant viruses were compared. We found that these two groups of viruses contain different amino acid sequence at position 621 to 646 in the active site of NS5. These data suggested that response to the inhibitory effect of nitric oxide radical may, at least in part, be regulated by NS5. The effect of these two different phenotypes of viruses on host cells was studied using cDNA array screening. The cDNA array analysis demonstrated that the nitric oxide-resistant group had a stronger influence on host cells since it induced changes in the expression of a greater number of genes than did the nitric oxide-susceptible group, 97 genes versus 71 genes, respectively. The NO-resistant virus also stimulated cytokines known to be virulent factors, such as IL 6, IL 8, RANTES, and the inflammatory factors. In conclusion, our data demonstrated that dengue viruses isolated from patients show genotypic and phenotypic differences which may correlate with virulence.

Lyssaviruses: current trends

Various technological developments have revitalized the approaches employed to study the disease of rabies. In particular, reverse genetics has facilitated the generation of novel viruses used to improve our understanding of the fundamental aspects of rabies virus (RABV) biology and pathogenicity and yielded novel constructs potentially useful as vaccines against rabies and other diseases. Other techniques such as high throughput methods to examine the impact of rabies virus infection on host cell gene expression and two hybrid systems to explore detailed protein-protein interactions also contribute substantially to our understanding of virus-host interactions. This review summarizes much of the increased knowledge about rabies that has resulted from such studies but acknowledges that this is still insufficient to allow rational attempts at curing those who present with clinical disease.

Dengue virus (DENV) antibody-dependent enhancement of infection upregulates the production of anti-inflammatory cytokines, but suppresses anti-DENV free radical and pro-inflammatory cytokine production, in THP-1 cells

The immunopathogenesis of dengue haemorrhagic fever and dengue shock syndrome is thought to be mediated by a variety of host factors. Enhancing antibodies are one of the key regulating molecules. These antibodies, via antibody-dependent enhancement (ADE) of infection, are able to facilitate dengue virus (DENV) growth in Fc-bearing host cells. The mechanism of ADE-enhanced DENV production is believed to be mediated through increasing the infected-cell mass. In the present work, the effect of ADE infection was explored further, focusing on the post-entry events of ADE infection. It was hypothesized that the higher virus production in ADE infection compared with DENV infection may be due to the ability of this infection pathway to suppress key antiviral molecules. Therefore, the influence of ADE infection on pro- and anti-inflammatory cytokines, including interleukin-12 (IL-12), gamma interferon (IFN-gamma), tumour necrosis factor alpha (TNF-alpha), IL-6 and IL-10, was investigated and it was found that DENV infection via the Fc receptor-mediated pathway was able to suppress the transcription and translation of IL-12, IFN-gamma and TNF-alpha. In contrast, infection via this route facilitated expression and synthesis of the anti-inflammatory cytokines IL-6 and IL-10. Moreover, this study demonstrates that the ADE infection pathway also suppresses an innate anti-DENV mediator, nitric oxide radicals, by disrupting the transcription of the iNOS gene transcription factor, IRF-1, and blocking the activation of STAT-1. In conclusion, ADE infection not only facilitates the entry process, but also modifies innate and adaptive intracellular antiviral mechanisms, resulting in unrestricted DENV replication in THP-1 cells.

Rabies virus-induced apoptosis involves caspase-dependent and caspase-independent pathways

Previously, it has been shown that the laboratory attenuated rabies virus CVS-B2C, but not the wild-type virus SHBRV, induces apoptosis in mice and the induction of apoptosis is mediated by viral glycoprotein. Induction of apoptosis by CVS-B2C limits the spread of the virus in the CNS. In the present study, we characterized the pathways by which CVS-B2C induces apoptosis. BSR cells were infected with CVS-B2C or SHBRV and harvested at different time points for detection of apoptosis by immunofluorescence and flow cytometry. Apoptosis was detected only in cells infected with CVS-B2C, but not SHBRV. Caspase activity and expression of several apoptotic proteins were analyzed by fluorometric assay and Western blotting. Activation of caspase-8 and -3, but not of caspase-9, was observed in CVS-B2C-infected cells. In addition, the level of expression of Apaf-1 did not change. Furthermore, PARP was cleaved confirming activation of downstream caspases. All these data suggest that CVS-B2C infection activates the extrinsic, but not the intrinsic, apoptotic pathway. In addition, AIF, a caspase-independent apoptotic protein was up-regulated and translocated from the cytoplasm to the nucleus post-infection, suggesting that apoptosis induced by CVS-B2C also involves the activation of a caspase-independent pathway.

Screening of pro-apoptotic genes upregulated in an experimental street rabies virus-infected neonatal mouse brain

Rabies virus (RABV) is able to induce apoptotic death of target cells. The molecular pathway of RABV-induced cell death is partially known. In the present study, cDNA array analysis was used as a tool to screen for pro-apoptotic genes that may be involved in RABV induction. RNA was extracted from the infected CNS and from mock-infected controls. When the mean gene expression was compared between the infected group and controls, 21 potential apoptotic genes were identified that exhibited more than 2.5-fold difference in their expression levels. These 21 genes can be grouped into two groups, those genes that participate in the commitment phase and those that play a role as executioners. Examples of genes in commitment phase were death receptors (Fas-L receptor, TNF-receptor), lysosomal proteases, calpain, caspase-1, signaling molecules (ERK, p38MAPK) and bcl-2 family members. Cytochrome c and caspase-3 were representatives of executioners. Based on types of genes activated during the commitment phase, two independent apoptotic mechanisms may be activated in response to the RV infection. The first is immune-mediated death which may operate through the receptor-ligand pathway activated by caspase-1 and the pro-inflammatory cytokine, IL-1beta. The other mechanism is a protease-mediated process which involves lysosomal proteases and calcium-dependent neutral proteases. These two stimulating pathways were followed by Bad, Bak, Bid activation and subsequently the upregulation of cytochrome c and caspase-3. In addition, mobilization of K+ ion and other accessory apoptotic genes such as annexins and clusterin were also upregulated.

Neuronal apoptosis in immunodeficient mice infected with the challenge virus standard strain of rabies virus by intracerebral inoculation

The challenge virus standard-11 strain (CVS) of fixed rabies virus produces neuronal apoptosis in widespread areas of the brain of mice after intracerebral inoculation. The role of the adaptive immune response in producing neuronal apoptosis in this model was evaluated by comparing the infections in adult C57BL/6J mice with nude mice (T cell deficient) and Rag1 mice (T and B cell deficient). Both strains of immunodeficient mice showed very similar clinical disease and neuropathological findings, including marked neuronal apoptosis. The adaptive immune response is unlikely of fundamental importance in producing neuronal apoptosis in the brains of mice in this model.

High level of Bcl-2 counteracts apoptosis mediated by a live rabies virus vaccine strain and induces long-term infection

We report here that rabies virus strains, currently used to immunize wildlife against rabies, induce not only caspase-dependent apoptosis in the human lymphoblastoid Jurkat T cell line (Jurkat-vect), but also a caspase-independent pathway involving the apoptosis-inducing factor (AIF). In contrast, a strain of neurotropic RV that does not induce apoptosis did not activate caspases or induce AIF translocation. Bcl-2 overproduction in Jurkat T cells (Jurkat-Bcl-2) abolished both pathways. ERA infection and production were similar in Jurkat-vect and Jurkat-Bcl-2 cells, indicating Bcl-2 has no direct antiviral effects. Bcl-2 production is naturally upregulated by day 3 in ERA-infected Jurkat-vect cultures. The increase in Bcl-2 levels seems to be controlled by the virus infection itself and results in the establishment of long-term, persistently infected cultures that continue to produce virus. Thus, in infections with live RV vaccine strains, infected cells may be productive reservoirs of virus in the long term. This may account for the high efficacy of live rabies vaccines.

Rhabdoviruses and apoptosis

Viral-induced apoptosis is recognized as a common method utilized by viruses to overcome the host. Recent evidence indicates that infection by rhabdoviruses such as vesicular stomatitis virus (VSV), spring viremia of carp virus (SVCV), and rabies virus results in apoptotic cell death. Similar morphological changes and host cell proteins are induced in cells infected with these different viruses; however, the viral proteins responsible for these changes vary. In addition, the molecular mechanism(s) utilized by these viruses to induce apoptosis are on the brink of discovery. This article serves to summarize our current understanding of the apoptotic process during rhabdovirus infection and to illustrate forthcoming areas of study in the field

Vesicular stomatitis virus induces apoptosis at early stages in the viral cycle and does not depend on virus replication

We detected apoptosis induction in the vesicular stomatitis virus (VSV) infected mammalian cell lines Vero-76, Cos-7, and BHK-21. Cell lines were analyzed by chromosomal DNA fragmentation and nuclear morphology. In order to determine the step in the viral cycle at which apoptosis of infected cells is triggered, chemical and physical agents were used to block viral infection at different times and then the apoptotic response of infected cells was examined. The treatment of Vero-76 infected cells with a lysosomotrophic agent, such as NH4Cl, was shown to abrogate virus apoptosis induction. On the other hand, VSV-induced apoptosis was not blocked by the presence of cycloheximide, suggesting that the de novo viral protein synthesis is not required for this process. UV-inactivated viruses were also capable of inducing apoptosis in Vero-76 cells, indicating that the activation of a programmed cell death process by VSV does not require viral replication. We conclude from these findings that VSV induces apoptosis at early stages of infection.

Matrix protein mutations contribute to inefficient induction of apoptosis leading to persistent infection of human neural cells by vesicular stomatitis virus

In a model system to study factors involved in the establishment of a persistent viral infection that may lead to neurodegenerative diseases, Indiana and New Jersey variants of vesicular stomatitis virus (VSV) with different capacities to infect and persist in human neural cells were studied. Indiana matrix (M) protein mutants and the wild-type New Jersey strain persisted in the human neural cell line H4 for at least 120 days. The Indiana wild-type virus (HR) and a non-M mutant (TP6), both unable to persist, induced apoptosis more strongly than all the other variants tested, as indicated by higher levels of DNA fragmentation and caspase-3-like activity. Transfection of H4 cells with mRNA coding for the VSV M protein confirmed the importance of this protein in the induction of apoptosis. Furthermore, the pan-caspase inhibitor ZVAD-fmk maintained cell survival to about 80%, whereas inhibition of caspase-8, caspase-9, or both only partially protected the cells against death, consistent with the fact that anti-apoptotic molecules from the Bcl-2 family also protect cells from death only partially. These results suggest that VSV activates many pathways of cell death and that an inefficient induction of caspase-3-related apoptosis participates in the establishment of a persistent infection of human neural cells by less virulent VSV variants.

Neuroblastoma cell lines--a versatile in vitro model in neurobiology

Neuroblastoma (NB) cell lines are transformed, neural crest derived cells, capable of unlimited proliferation in vitro. These cell lines retain the ability of differentiation into neuronal cell types on treatment with various agents. This ability of NB cells to proliferate as well as to differentiate makes it an excellent in vitro system for various studies. This review article focuses on the applications and potential uses of murine and human NB cell lines. NB cells are extensively used for testing neurotoxicity of putative drugs such as antimalarial or anticancer agents. NB cell lines have wide applications in virus research to understand various aspects of virus-host cell interactions at the molecular and cellular levels. They have been used to dissect the relationships between proliferation, differentiation and apoptosis. This feature has been useful in understanding the pediatric cancer--neuroblastoma and for development of newer therapies.

Kinetic studies of the production of nitric oxide (NO) and tumour necrosis factor-alpha (TNF-alpha) in macrophages stimulated with Burkholderia pseudomallei endotoxin

The mechanism by which Burkholderia pseudomallei survives in macrophages is not clearly understood. In this study, we demonstrated that the mouse macrophage cell line (RAW 264.7) treated with lipopolysaccharide (LPS) from B. pseudomallei (BP-LPS) produced significantly less NO and TNF-alpha compared with those stimulated with the LPS from Escherichia coli and Salmonella typhi. The time required for the BP-LPS to trigger substantial NO and TNF-alpha release was at least 30 min, compared with < 5 min for the E. coli-LPS. A time course study of inducible nitric oxide synthase (iNOS) protein expression also indicated that the time required for macrophages stimulated with the BP-LPS to up-regulate iNOS was longer. The longer time lag for the BP-LPS to activate macrophages was probably due to the delay in up-regulation of iNOS and TNF-alpha mRNA transcription. These results indirectly suggest that the delay of the mediators' production may be due to a reduced rate of signal transduction initiated by the interaction of BP-LPS with the macrophage cell surface. The use of MoAb to phosphorylated p38 in a Western blot analysis provided data compatible with the notion that the maximum level of phosphorylated p38 from the cells activated with BP-LPS was attained at a slower rate. These results suggest that the unique structure of BP-LPS exhibits a property which may interfere with macrophage cell activation.

Rabies

Rabies is an important disease in wildlife in the United States and Canada, and dog rabies is still a major public health problem in many developing countries of the world. Rabies virus is transmitted in saliva by animal bites. Bats transmitted most recent cases of human rabies in the United States, often without known exposures. There have been recent developments in our understanding of rabies pathogenesis. Characteristic clinical features should raise the possibility of a diagnosis of rabies and initiation of appropriate diagnostic tests. Therapy of human rabies has been futile except in four patients who were immunized with rabies vaccine prior to the onset of their disease. Rabies can be prevented after an exposure in unimmunized patients with local wound cleansing and administration of rabies vaccine and human rabies immune globulin.

Viruses and apoptosis

Successful viral replication requires not only the efficient production and spread of progeny, but also evasion of host defense mechanisms that limit replication by killing infected cells. In addition to inducing immune and inflammatory responses, infection by most viruses triggers apoptosis or programmed cell death of the infected cell. This cell response often results as a compulsory or unavoidable by-product of the action of critical viral replicative functions. In addition, some viruses seem to use apoptosis as a mechanism of cell killing and virus spread. In both cases, successful replication relies on the ability of certain viral products to block or delay apoptosis until sufficient progeny have been produced. Such proteins target a variety of strategic points in the apoptotic pathway. In this review we summarize the great amount of recent information on viruses and apoptosis and offer insights into how this knowledge may be used for future research and novel therapies.