Apoptosis regulators from DNA viruses

Cytomegalovirus Biology Viewed Through a Cell Death Suppression Lens

Cytomegaloviruses, species-specific members of the betaherpesviruses, encode an impressive array of immune evasion strategies committed to the manipulation of the host immune system enabling these viruses to remain for life in a stand-off with host innate and adaptive immune mechanisms. Even though they are species-restricted, cytomegaloviruses are distributed across a wide range of different mammalian species in which they cause systemic infection involving many different cell types. Regulated, or programmed cell death has a recognized potential to eliminate infected cells prior to completion of viral replication and release of progeny. Cell death also naturally terminates replication during the final stages of replication. Over the past two decades, the host defense potential of known programmed cell death pathways (apoptosis, necroptosis, and pyroptosis), as well as a novel mitochondrial serine protease pathway have been defined through studies of cytomegalovirus-encoded cell death suppressors. Such virus-encoded inhibitors prevent virus-induced, cytokine-induced, and stress-induced death of infected cells while also moderating inflammation. By evading cell death and consequent inflammation as well as innate and adaptive immune clearance, cytomegaloviruses represent successful pathogens that become a critical disease threat when the host immune system is compromised. This review will discuss cell death programs acquired for mammalian host defense against cytomegaloviruses and enumerate the range of modulatory strategies this type of virus employs to balance host defense in favor of lifelong persistence.

Repositioning of pentoxifylline as an immunomodulator and regulator of the renin-angiotensin system in the treatment of COVID-19

Pentoxifylline (PTX) is a phosphodiesterase inhibitor that increases cyclic adenosine monophosphate levels, which in turn activate protein kinase, leading to a reduction in the synthesis of proinflammatory cytokines to ultimately influence the renin-angiotensin system (RAS) in vitro by inhibiting angiotensin 1 receptor (AT1R) expression. The rheological, anti-inflammatory, and renin-angiotensin axis properties of PTX highlight this drug as a therapeutic treatment alternative for patients with COVID-19 by helping reduce the production of the inflammatory cytokines without deleterious effects on the immune system to delay viral clearance. Moreover, PTX can restore the balance of the immune response, reduce damage to the endothelium and alveolar epithelial cells, improve circulation, and prevent microvascular thrombosis. There is further evidence that PTX can improve ventilatory parameters. Therefore, we propose repositioning PTX in the treatment of COVID-19. The main advantage of repositioning PTX is that it is an affordable drug that is already available worldwide with an established safety profile, further offering the possibility of immediately analysing the result of its use and associated success rates. Another advantage is that PTX selectively reduces the concentration of TNF-α mRNA in cells, which, in the case of an acute infectious state such as COVID-19, would seem to offer a more strategic approach.

Ectromelia virus suppresses expression of cathepsins and cystatins in conventional dendritic cells to efficiently execute the replication process

Background

Cathepsins are a group of endosomal proteases present in many cells including dendritic cells (DCs). The activity of cathepsins is regulated by their endogenous inhibitors – cystatins. Cathepsins are crucial to antigen processing during viral and bacterial infections, and as such are a prerequisite to antigen presentation in the context of major histocompatibility complex class I and II molecules. Due to the involvement of DCs in both innate and adaptive immune responses, and the quest to understand the impact of poxvirus infection on host cells, we investigated the influence of ectromelia virus (ECTV) infection on cathepsin and cystatin levels in murine conventional DCs (cDCs). ECTV is a poxvirus that has evolved many mechanisms to avoid host immune response and is able to replicate productively in DCs.

Results

Our results showed that ECTV-infection of JAWS II DCs and primary murine GM-CSF-derived bone marrow cells down-regulated both mRNA and protein of cathepsin B, L and S, and cystatin B and C, particularly during the later stages of infection. Moreover, the activity of cathepsin B, L and S was confirmed to be diminished especially at later stages of infection in JAWS II cells. Consequently, ECTV-infected DCs had diminished ability to endocytose and process a soluble antigen. Close examination of cellular protein distribution showed that beginning from early stages of infection, the remnants of cathepsin L and cystatin B co-localized and partially co-localized with viral replication centers (viral factories), respectively. Moreover, viral yield increased in cDCs treated with siRNA against cathepsin B, L or S and subsequently infected with ECTV.

Conclusions

Taken together, our results indicate that infection of cDCs with ECTV suppresses cathepsins and cystatins, and alters their cellular distribution which impairs the cDC function. We propose this as an additional viral strategy to escape immune responses, enabling the virus to replicate effectively in infected cells.

Electronic supplementary material

The online version of this article (10.1186/s12866-019-1471-1) contains supplementary material, which is available to authorized users.

Wnt5b regulates apoptosis in Litopenaeus vannamei against white spot syndrome virus

The Wnt signaling mediated by Wnt proteins that orchestrate and influence a myriad of cellular processes, such as cell proliferation, differentiation, tumorigenesis, apoptosis, and participation in immune defense during microbe infection. Wnt5b is one of the Wnt signaling molecules that initiate the cascade. In this study, we cloned and characterized a Wnt5b homolog from Litopenaeus vannamei designed as LvWnt5b. The full length of LvWnt5b transcript was 1726 bp with an 1107 bp open reading frame that encoded a 368 aa protein, which contained 24 discontinuous and highly conserved cysteine. Real-time quantitative PCR showed that the transcriptional level of LvWnt5b was down-regulated when infected with white spot syndrome virus (WSSV). Knock-down of LvWnt5b resulted in inhibition of the transcriptional level of WSSV gene ie1, indicating that LvWnt5b mediated signaling pathway may play an important role in defense against WSSV infection. When LvWnt5b was silenced, caspase3/7 activity in hemocytes was increased significantly, and the transcription of viral gene was decreased as well. Moreover, overexpression of LvWnt5b in HEK293T cells led to inhibition of caspase3/7 activity, which further proved the role of LvWnt5b in restraining apoptosis. The study showed that the shrimp may decrease the expression of LvWnt5b initiatively to act as an immune defense mechanism against WSSV infection via promoting apoptosis. It will be helpful for understanding the function of Wnt signaling pathway in virus invasion and host defense.

Inhibition of translation initiation factor eIF4A is required for apoptosis mediated by Microplitis bicoloratus bracovirus

Apoptotic hemocytes induced by Microplitis bicoloratus parasitism have been reported, and M. bicoloratus bracovirus (MbBV) is known to be the apoptosis inducer. However, the mechanism how MbBV regulates apoptosis remains unclear. eIF4A, one of translation initiation factors, was found from a Spodoptera litura transcriptome, the expression of which in the parasitized hemocytes of S. litura was inhibited in RT-qPCR analysis. The western blot also illustrated eIF4A at 6-day post-parasitization was inhibited in hemocytes. For testing interaction of MbBV-eIF4A-apoptosis, a cDNA clone encoding 1,266 bp of eIF4A was obtained from S. litura hemocytes and sequenced. Then, a 48 kDa V5-fusion protein of the eIF4A was detected by using the anti-V5 antibody at 72-h post-transfection in the High Five cells, which is located in the cell cytoplasm. In vitro, overexpression of eIF4A rescued the apoptotic High Five cells induced by MbBV. Conversely, in vivo, loss of eIF4A proteins by dsRNA feeding increased apoptosis of hemocytes. Furthermore, RNAi and parasitism significantly increased apoptosis of hemocytes in S. litura. These findings suggested that MbBV inhibited the expression of eIF4A, which was required for apoptosis mediated by MbBV. This study will contribute to biological pest control and enhance our understanding of molecular mechanisms underlying polydnavirus-parasitoid-host interaction.

Diverse mechanisms evolved by DNA viruses to inhibit early host defenses

In mammalian cells, early defenses against infection by pathogens are mounted through a complex network of signaling pathways shepherded by immune-modulatory pattern-recognition receptors. As obligate parasites, the survival of viruses is dependent on the evolutionary acquisition of mechanisms that tactfully dismantle and subvert the cellular intrinsic and innate immune responses. Here, we review the diverse mechanisms by which viruses that accommodate DNA genomes are able to circumvent activation of cellular immunity. We start by discussing viral manipulation of host defense protein levels by either transcriptional regulation or protein degradation. We next review viral strategies used to repurpose or inhibit these cellular immune factors by molecular hijacking or by regulating their post-translational modification status. Additionally, we explore the infection-induced temporal modulation of apoptosis to facilitate viral replication and spread. Lastly, the co-evolution of viruses with their hosts is highlighted by the acquisition of elegant mechanisms for suppressing host defenses via viral mimicry of host factors. In closing, we present a perspective on how characterizing these viral evasion tactics both broadens the understanding of virus-host interactions and reveals essential functions of the immune system at the molecular level. This knowledge is critical in understanding the sources of viral pathogenesis, as well as for the design of antiviral therapeutics and autoimmunity treatments.

Innate defense mechanisms against HSV-1 infection in the target tissues, skin and brain

Herpes simplex virus type 1 (HSV-1) initiates productive infection in mucocutaneous tissues to cause cold sores and establishes latent infection in the trigeminal ganglia. Under certain circumstances, HSV-1 may cause encephalitis. Here, we compared host innate defenses against HSV-1 in the two clinically relevant tissues, skin and brain, using a unique ex vivo system of organ culture. Upon HSV-1 infection and spread, apoptosis induction was observed in the skin, but not in brain tissues. While the two tissues elicited interferon (IFN-β) response upon HSV1 infection, IFN induction was more robust in the skin compared to the brain. Moreover, antiviral response to exogenous IFNβ treatment was much stronger in the skin compared to brain tissues. This observation was not related to the availability of the IFN receptor on cells' surface. Taken together, our study demonstrates differential innate antiviral responses to HSV-1 infection that may be exploited in future development of selective and tissue-specific anti-viral treatments.

Porcine reproductive and respiratory syndrome (PRRS): an immune dysregulatory pandemic

Porcine reproductive and respiratory disease syndrome (PRRS) is a viral pandemic that especially affects neonates within the “critical window” of immunological development. PRRS was recognized in 1987 and within a few years became pandemic causing an estimated yearly $600,000 economic loss in the USA with comparative losses in most other countries. The causative agent is a single-stranded, positive-sense enveloped arterivirus (PRRSV) that infects macrophages and plasmacytoid dendritic cells. Despite the discovery of PRRSV in 1991 and the publication of >2,000 articles, the control of PRRS is problematic. Despite the large volume of literature on this disease, the cellular and molecular mechanisms describing how PRRSV dysregulates the host immune system are poorly understood. We know that PRRSV suppresses innate immunity and causes abnormal B cell proliferation and repertoire development, often lymphopenia and thymic atrophy. The PRRSV genome is highly diverse, rapidly evolving but amenable to the generation of many mutants and chimeric viruses for experimental studies. PRRSV only replicates in swine which adds to the experimental difficulty since no inbred well-defined animal models are available. In this article, we summarize current knowledge and apply it toward developing a series of provocative and testable hypotheses to explain how PRRSV immunomodulates the porcine immune system with the goal of adding new perspectives on this disease.

The miR-100-mediated pathway regulates apoptosis against virus infection in shrimp

The process of programmed cell death process or apoptosis can be regulated by microRNAs, 21-25 nt short non-coding RNAs. However, a comprehensive view of apoptosis-associated miRNAs has not been intensively characterized. In this study, the shrimp miRNA microarray data showed that 199 miRNAs were involved in the regulation of apoptosis, among which 8 miRNAs were evolutionarily conserved in animals. The loss-of-function experiments in vivo in shrimp revealed that miR-100 was served as an anti-apoptosis miRNA through targeting the mRNA of trypsin gene. The results indicated that the silencing of miR-100 expression resulted in the increase of apoptotic activity of shrimp hemocytes and further led to the decreases of virus genome copies in shrimp and virus-infected shrimp mortality compared with the controls. The findings showed that miR-100-trypsin signaling pathway played an important role in the antiviral immunity by regulating apoptosis. Therefore, our study presented a novel miR-100-mediated pathway in the regulation of apoptosis.

Construction and identification of recombinant replication-defective adenovirus vector Αd/CMV/V5-DEST-TNFα-scFv

AIM: To construct a recombinant replication-defective adenovirus vector carrying TNFα-scFv and to obtain high-purity virus solution by viral packaging, purification and titration.

METHODS: The TNFα-scFv gene was amplified from the PUC57-Αmp vector and cloned into the shuttle plasmid pDONR221. The resulting pDONR221-TNFα-scFv was identified by DNA sequencing and then co-transfected into bacteria carrying the adenoviral backbone plasmid pΑd-CMV-V5-DEST to generate an adenoviral plasmid carrying TNFα-scFv (pΑd/CMV/V5-DEST-TNFα-ScFv) by homologous recombination in bacteria. After the pΑd/CMV/V5-DEST-TNFα-ScFv vector was transfected into 293 cells, the transfected 293 cells were infected with adenoviruses. The expression of TNFα-ScFv was detected by cytopathic effect and Western blot.

RESULTS: PCR amplification, restriction analysis and DNA sequencing verified that both the recombinant shuttle plasmid pDONR221-TNFα -scFv and the recombinant adenovirus vector pΑd/CMV/V5-DEST-TNFα-scFv were correctly constructed. After amplification and purification, the titer of recombinant adenovirus was 2.5×10¹¹ TCID 50/mL after proliferation in 293 cells. Western blot analysis demonstrated that TNFα-scFv was expressed efficiently in 293 cells after infection.

CONCLUSION: The recombinant adenovirus vector Αd/CMV/V5-DEST-TNFα-scFv has been successfully constructed, which lays a foundation for further study of gene function and therapy.

Human herpesvirus 8 interferon regulatory factor-mediated BH3-only protein inhibition via Bid BH3-B mimicry

Viral replication efficiency is in large part governed by the ability of viruses to counteract pro-apoptotic signals induced by infection of host cells. For HHV-8, viral interferon regulatory factor-1 (vIRF-1) contributes to this process in part via inhibitory interactions with BH3-only protein (BOP) Bim, recently identified as an interaction partner of vIRF-1. Here we recognize that the Bim-binding domain (BBD) of vIRF-1 resembles a region (BH3-B) of Bid, another BOP, which interacts intramolecularly with the functional BH3 domain of Bid to inhibit it pro-apoptotic activity. Indeed, vIRF-1 was found to target Bid in addition to Bim and to interact, via its BBD region, with the BH3 domain of each. In functional assays, BBD could substitute for BH3-B in the context of Bid, to suppress Bid-induced apoptosis in a BH3-binding-dependent manner, and vIRF-1 was able to protect transfected cells from apoptosis induced by Bid. While vIRF-1 can mediate nuclear sequestration of Bim, this was not the case for Bid, and inhibition of Bid and Bim by vIRF-1 could occur independently of nuclear localization of the viral protein. Consistent with this finding, direct BBD-dependent inactivation by vIRF-1 of Bid-induced mitochondrial permeabilization was demonstrable in vitro and isolated BBD sequences were also active in this assay. In addition to Bim and Bid BH3 domains, BH3s of BOPs Bik, Bmf, Hrk, and Noxa also were found to bind BBD, while those of both pro- and anti-apoptotic multi-BH domain Bcl-2 proteins were not. Finally, the significance of Bid to virus replication was demonstrated via Bid-depletion in HHV-8 infected cells, which enhanced virus production. Together, our data demonstrate and characterize BH3 targeting and associated inhibition of BOP pro-apoptotic activity by vIRF-1 via Bid BH3-B mimicry, identifying a novel mechanism of viral evasion from host cell defenses.

Viruses possess mechanisms of subverting host cell defenses against infection and virus replication; these mechanisms are essential to the virus life cycle. Here, we identify and characterize a novel mechanism of HHV-8 mediated inhibition of virus-induced programmed cell death (apoptosis). This function is specified by viral interferon regulator factor homologue vIRF-1, which binds to and directly inhibits pro-death activities of so-called BH3-only proteins (BOPs), induced and activated by stress signals such as those occurring in infected cells. The BH3 domains of BOPs mediate their pro-apoptotic functions, and it is these domains that are targeted by vIRF-1, via a region resembling a BH3-interacting and -inhibitory domain, termed BH3-B, present in one of the vIRF-1 targeted BOPs, Bid. The targeted BOP BH3 domains share characteristic and conserved features. As shown previously for Bim, depletion of Bid leads to enhanced HHV-8 productive replication, demonstrating that Bid, also, is a biologically significant negative regulator of virus replication and suggesting that its control by vIRF-1 is of functional importance. To our knowledge, this is the first report of viral targeting and inhibition of BOP activity via Bid BH3-B mimicry; our studies therefore expand the known mechanisms of viral evasion from antiviral defenses of the host.

The ubiquitin-proteasome pathway is important for dengue virus infection in primary human endothelial cells

Dengue hemorrhagic fever (DHF) and dengue shock syndrome (DSS) are the most severe forms of dengue virus infection with hemorrhage and plasma leakage. However, pathogenic mechanisms of DHF and DSS remain poorly understood. We therefore investigated host responses as determined by changes in the cellular proteome of primary human endothelial cells upon infection with dengue virus serotype 2 (DEN-2) at a multiplicity of infection (MOI) of 10 for 24 h. Two-dimensional PAGE and quantitative intensity analysis revealed 38 significantly altered protein spots (16 upregulated and 22 downregulated) in DEN-2-infected cells compared to mock controls. These altered proteins were successfully identified by mass spectrometry, including those involved in oxidative stress response, transcription and translation, cytoskeleton assembly, protein degradation, cell growth regulation, apoptosis, cellular metabolism, and antiviral response. The proteomic data were validated by Western blot analyses [upregulated ubiquitin-activating enzyme E1 (UBE1) and downregulated annexin A2] and an immunofluorescence study (upregulated MxA). Interestingly, we found that MxA was colocalized with DEN-2 viral capsid protein, strengthening its role as an antiviral protein. Moreover, we also identified upregulation of a proteasome subunit. Our functional study revealed the significant role of ubiquitination in dengue infection and UBE1 inhibition by its specific inhibitor (UBEI-41) caused a significant reduction in the level of viral protein synthesis and its infectivity. Our findings suggest that various biological processes were triggered in response to dengue infection, particularly antiviral IFN and ubiquitin-proteasome pathways.

Analysis of vaccinia virus-host protein-protein interactions: validations of yeast two-hybrid screenings

Vaccinia virus, a large double-stranded DNA virus, is the prototype of the Orthopoxvirus genus, which includes several pathogenic poxviruses of humans, such as monkeypox virus and variola virus. Here, we report a comprehensive yeast two-hybrid (Y2H) screening for the protein-protein interactions between vaccinia and human proteins. A total of 109 novel vaccinia-human protein interactions were detected among 33 viral proteins. To validate subsets of those interactions, we constructed an ORFeome library of vaccinia virus strain WR using the Gateway plasmid cloning system. By co-expressing selected vaccinia and host proteins in a variety of expression systems, we found that at least 17 of the Y2H hits identified between vaccinia and human proteins can be verified by independent methods using GST pull-down assays, representing a 63% validation rate for the Y2H hits examined (17/27). Because the cloned ORFs are conveniently transferable from the entry vectors to various destination expression vectors, the vaccinia ORFeome library will be a useful resource for future high-throughput functional proteomic experiments.

The ubiquitin-proteasome pathway in viral infections

The cellular biological function of the ubiquitin-proteasome pathway as a major intracellular protein degradation pathway, and as an important modulator for the regulation of many fundamental cellular processes has been greatly appreciated over the last decade. The critical role of the ubiquitin-proteasome pathway in viral pathogenesis has become increasingly apparent. Many viruses have been reported to evolve different strategies to utilize the ubiquitin-proteasome pathway for their own benefits. Here, we review the general background and function of the ubiquitin-proteasome pathway, summarize our current understanding of how viruses use this pathway to target cellular proteins, and finally, discuss the roles of this pathway in enteroviral infection, and the potential therapeutic application of proteasome inhibition in myocarditis.

Spodoptera litura multicapsid nucleopolyhedrovirus inhibits Microplitis bicoloratus polydnavirus-induced host granulocytes apoptosis

Baculoviruses and parasitoids are critically important biological control agents in integrated pest management (IPM). They have been simultaneously and sequentially used to target insect pests. In this study, we examined the impacts of both baculovirus and polydnavirus (PDV) infection on the host cellular immune response. Larvae of the lepidopteran Spodoptera litura were infected by Spodoptera litura multicapsid nucleopolyhedrovirus (SpltMNPV) and then the animals were parasitized by the braconid wasp Microplitis bicoloratus. The fate of the parasitoids in the dually infected hosts was followed and encapsulation of M. bicoloratus first instar larvae was observed. Hemocytes of S. litura larvae underwent apoptosis in naturally parasitized hosts and in non-parasitized larvae after injection of M. bicoloratus ovarian calyx fluid (containing MbPDV) plus venom (CFPV). However, assessments of the percentages of cells undergoing apoptosis under different treatments indicated that SpltMNPV could inhibit MbPDV-induced apoptosis in hemocytes when hosts were first injected with SpltMNPV budded virus (BV) followed by injection with M. bicoloratus CFPV. As the time of injection with SpltMNPV BV increased, the percentages of apoptosis in hemocytes population declined. Furthermore, in vitro, the percentages of apoptosis showed that SpltMNPV BV could inhibit MbPDV-induced granulocytes apoptosis. The occurrence of MbPDV-induced host granulocytes apoptosis was inhibited in the dually infected hosts. As hemocytes apoptosis causes host immunosuppression, the parasitoids are normally protected from the host immune system. However, in larvae infected with both baculovirus and PDV, the parasitoids underwent encapsulation in the host hemocoel.

Paclitaxel and vincristine potentiate adenoviral oncolysis that is associated with cell cycle and apoptosis modulation, whereas they differentially affect the viral life cycle in non-small-cell lung cancer cells

Chemotherapy, including microtubule (MT)-interacting agents, can enhance the tumor-eradicating activity of replication-competent adenoviruses. The purpose of this study was to obtain more insight into the mechanism underlying this enhancement that may be exploited for the development of improved therapy. Two MT-interacting agents with opposite activity, paclitaxel (PTX) that stabilizes and vincristine (VCR) that destabilizes MTs, were found to synergistically enhance adenoviral oncolysis in non-small-cell lung cancer (NSCLC) cells. To explore the possibility that these drugs affect the viral life cycle by modulating adenoviral gene expression, we used a quantitative reverse transcription-polymerase chain reaction assay and found that PTX, but not VCR, increased the expression of E1A13S, ADP and Penton genes, which correlated with an increase in viral particle assembly and release. Next, the effect of combined treatment on cell-cycle progression was studied. Both drugs suppressed adenovirus-induced S-phase arrest and instead caused G2/M arrest, which was accompanied by an increase in apoptotic cells. Taken together, the enhancement of oncolysis by MT-interacting drugs appears not to require specific MT transport or scaffold functions. Our findings suggest that MT-interacting drug-induced cellular signals that modulate cell-cycle arrest and apoptosis are primarily on the basis of their oncolysis-enhancing activity.

Coxsackievirus protein 2BC blocks host cell apoptosis by inhibiting caspase-3

Virus infection may induce host cell death by apoptosis, but some DNA viruses are capable of preventing this process. RNA viruses were thought not to display anti-apoptotic activities, as their spread appears to benefit from a rapid induction of cell death. Here, we report an antiapoptotic activity in the Picornavirus Coxsackievirus B4 (CVB4). CVB4 infection of HeLa cells induced negligible apoptosis over a period of 10 h. However, infected cells developed resistance to drug-induced apoptosis using staurosporine and actinomycin D and to death receptor-induced apoptosis using tumor necrosis factor-related apoptosis-inducing ligand. Despite this resistance, the apoptotic machinery was nonetheless fully activated in these drug-treated infected cells because the levels of pro-caspase-3 processing to its active form were similar to control cells. However, the DEVDase (Asp-Glu-Val-Asp protease) activity of the processed caspase was significantly inhibited in the virus-infected staurosporine-treated cells compared with drug treatment alone. Likewise, extracts of CVB4-infected cells suppressed recombinant caspase-3 activity in vitro. Immunoprecipitation of activated caspase-3 from radiolabeled virus-infected cells revealed the co-precipitation of a 48-kDa protein that was tentatively identified as viral protein 2BC. Recombinant caspase-3 was found to co-precipitate with virus protein 2BC. Finally, when protein 2BC was expressed in HeLa cells, both staurosporine-induced apoptosis and in vitro caspase-3 DEVDase activity were significantly reduced. Taken together these data imply that CVB4 infection suppresses apoptosis through virus protein 2BC associating with caspase-3 and inhibiting its function. Thus, 2BC is the first reported RNA virus inhibitor of apoptosis protein.

Contribution of GADD45 family members to cell death suppression by cellular Bcl-xL and cytomegalovirus vMIA

Mammalian cells and viruses encode inhibitors of programmed cell death that localize to mitochondria and suppress apoptosis initiated by a wide variety of inducers. Mutagenesis was used to probe the role of a predicted alpha-helical region within the hydrophobic antiapoptotic domain (AAD) of cytomegalovirus vMIA, the UL37x1 gene product. This region was found to be essential for cell death suppression activity. A screen for proteins that interacted with the AAD of functional vMIA but that failed to interact with mutants identified growth arrest and DNA damage 45 (GADD45alpha), a cell cycle regulatory protein activated by genotoxic stress, as a candidate cellular binding partner. GADD45alpha interaction required the AAD alpha-helical character that also dictated GADD45alpha-mediated enhancement of death suppression. vMIA mutants that failed to interact with GADD45alpha were completely nonfunctional in cell death suppression, and any of the three GADD45 family members (GADD45alpha, GADD45beta/MyD118, or GADD45gamma/OIG37/CR6/GRP17) was able to cooperate with vMIA; however, none influenced cell death when introduced into cells alone. GADD45alpha was found to increase vMIA protein levels comparably to treatment with protease inhibitors MG132 and ALLN. Targeted short interfering RNA knockdown of all three GADD45 family members maximally reduced vMIA activity, and this reduction was abrogated by additional GADD45alpha. Interestingly, GADD45 family members were also able to bind and enhance cell death suppression by Bcl-xL, a member of the Bcl-2 family of cell death suppressors, suggesting a direct cooperative link between apoptosis and the proteins that regulate the DNA damage response.

A real-time RT-PCR assay for the quantitative determination of adenoviral gene expression in tumor cells

Oncolytic adenoviruses are exploited as possible anticancer agents in clinical trails. To monitor adenoviral gene expression, a real-time RT-PCR method with a LightCycler was developed that allows the rapid and easy quantification of a number of early and late adenoviral genes in infected tumor cells. Primers were designed that can amplify the spliced forms of the genes encoding E1A13S, DNA polymerase (Pol), pre-terminal protein (pTP), adenoviral death protein (ADP), Hexon (Hex) and Penton (Pent) genes. Standard curves were generated using two-fold serial dilutions of cDNAs derived from non-small cell lung cancer (NSCLC) H460 cells infected for 24h with wild-type adenovirus serotype 5. For all genes correlation coefficients of the standard curves of 0.984 or higher were obtained. The dynamic range of the assay was sufficient to allow the quantitative determination of adenoviral gene expression during a lytic cycle. This RT-PCR assay could be used as a research tool to study the effect of host-cell factors or exogenous treatments on adenoviral gene expression. As example, it is shown that the procedure is suitable to detect changes in adenoviral gene expression in infected H460 cells treated with paclitaxel that is known to enhance the antitumor effect of oncolytic adenoviruses.

Apoptosis and its modulation during infection with Toxoplasma gondii: molecular mechanisms and role in pathogenesis

Infection with the obligate intracellular protozoan Toxoplasma gondii leads to lifelong persistence of the parasite in its mammalian hosts including humans. Apoptosis plays crucial roles in the interaction between the host and the parasite. This includes innate and adaptive defense mechanisms to restrict intracellular parasite replication as well as regulatory functions to modulate the host's immune response. Not surprisingly, however, T. gondii also extensively modifies apoptosis of its own host cell or of uninfected bystander cells. After infection, apoptosis is triggered in T lymphocytes and other leukocytes, thereby leading to suppressed immune responses to the parasite. T cell apoptosis may be largely mediated by Fas engagement but also occurs independently of Fas under certain conditions. Depending on the magnitude of T cell apoptosis, it is either associated with unrestricted parasite replication and severe pathology or facilitates a stable parasite-host-interaction. However, T. gondii has also evolved strategies to inhibit host cell apoptosis. Apoptosis is blocked by indirect mechanisms in uninfected bystander cells, thereby modulating the inflammatory response to the parasite. In contrast, inhibition of apoptosis in infected host cells by direct interference with apoptosis-signaling cascades is thought to facilitate the intracellular development of T. gondii. Blockade of apoptosis by intracellular parasites may be achieved by different means including interference with the caspase cascade, increased expression of antiapoptotic molecules by infected host cells, and a decreased activity of the poly(ADP-ribose) polymerase. The intriguing dual activity of T. gondii to both promote and inhibit apoptosis requires a tight regulation to promote a stable parasite host-interaction and establishment of persistent toxoplasmosis.

Role of the ubiquitin-proteasome pathway in the diagnosis of human diseases

The ubiquitin-proteasome pathway constitutes the major system for nuclear and extralysosomal cytosolic protein degradation in eukaryotic cells. A plethora of cell proteins implicated in the maintenance and regulation of essential cellular processes undergoes processing and functional modification by proteolytic degradation via the ubiquitin-proteasome pathway. Deregulations of the pathway have been shown to contribute to the pathogenesis of several human diseases, such as cancer, neurodegenerative, autoimmune, genetic and metabolic disorders, most of them exhibiting abnormal accumulation and altered composition of components of the pathway that is suitable for diagnostic proceedings. While the ubiquitin-proteasome pathway is currently exploited to develop novel therapeutic strategies, it is less regarded as a diagnostic area. Future research should lead to an improved understanding of the pathophysiology of the ubiquitin-proteasome pathway with the aim of allowing the development of subtle diagnostic strategies.

Baculovirus P35 interacts with a subunit of human RNA polymerase II and can enhance promoter activity in human cells

The early protein P35 from the baculovirus Autographa californica nucleopolyhedrovirus is a direct inhibitor of caspases and can block apoptosis in a wide variety of systems. In addition, it has been linked to the regulation of viral gene expression, shut-down of protein synthesis in infected insect cells and malignant transformation of mouse fibroblasts. By yeast-two-hybrid screening we identified the RPB11a subunit of human RNA polymerase II as an interaction partner of P35. Specificity of the interaction was confirmed by affinity blotting. By immunocytology, P35 was in part found in the nucleus of transfected cells. Homology searches further revealed that P35 has structural similarity with RPB3, the subunit of RNA polymerase II that has been demonstrated to interact directly with RPB11a. When transfected into human colon carcinoma cells, P35 was able to enhance the activity of E-cadherin and beta-actin promoters by about a factor of two as measured by luciferase reporter assay. P35 and hRPB11a together enhanced the E-cadherin activity about three- to fourfold. These data suggest an additional role for P35 in the regulation of cellular transcription.

Protection by herpes simplex virus glycoprotein D against Fas-mediated apoptosis: role of nuclear factor kappaB

Signals involved in protection against apoptosis by herpes simplex virus 1 (HSV-1) were investigated. Using U937 monocytoid cells as an experimental model, we have demonstrated that HSV-1 rendered these cells resistant to Fas-induced apoptosis promptly after infection. UV-inactivated virus as well as the envelope glycoprotein D (gD) of HSV-1, by itself, exerted a protective effect on Fas-induced apoptosis. NF-kappaB was activated by gD, and protection against Fas-mediated apoptosis by gD was abolished in cells stably transfected with a dominant negative mutant I-kappaBalpha, indicating that NF-kappaB activation plays a role in the antiapoptotic activity of gD in our experimental model. Moreover, NF-kappaB-dependent protection against Fas-mediated apoptosis was associated with decreased levels of caspase-8 activity and with the up-regulation of intracellular antiapoptotic proteins.

Construction of replication-deficient recombinant adenoviral vector carrying HBV S and C region gene by homologous recombination in bacteria and its expression in vitro

AIM

To construct recombinant adenoviral vector carrying HBV S and C region gene by homologous recombination in bacteria and to detect its expression in vitro.

METHODS

HBV pre-S2/S genes and pre-C/C genes were amplified by PCR and were cloned to adenoviral shuttle plasmid pAdTrack-CMV, respectively. Then the resultant pAd Track-CMV-HBs or pAdTrack-CMV-HBe was cotransfected into BJ5183 bacteria with the plasmid pAdeasy-1. The adenoviral plasmid carrying HBV S and C gene (pAd-HBs and pAd-HBe) was generated with homologous recombination in bacteria and the adenoviruses were produced in 293 cells. Both 293 and Vero cells were infected with adenoviruses and the expression of HBsAg and HBeAg was detected by RT-PCR and ELISA in vitro.

RESULTS

The titer of Ad-HBs and Ad-HBe adenoviruses was up to 5×10¹² pfu/L after proliferation in 293 cells. HBsAg and HBeAg were expressed efficiently in 293 and Vero cells after infection.

CONCLUSION

The recombinant adenoviruses expressing HBsAg or HBeAg were constructed successfully and can be used further in gene therapy of HBV.

Host-pathogen interactions during apoptosis

Host pathogen interaction results in a variety of responses, which include phagocytosis of the pathogen, release of cytokines, secretion of toxins, as well as production of reactive oxygen species (ROS). Recent studies have shown that many pathogens exert control on the processes that regulate apoptosis in the host. The induction of apoptosis upon infection results from a complex interaction of parasite proteins with cellular host proteins. Abrogation of host cell apoptosis is often beneficial for the pathogen and results in a successful host invasion. However, in some cases, it has been shown that induction of apoptosis in the infected cells significantly imparts protection to the host from the pathogen. There is a strong correlation between apoptosis and the host protein translation machinery: the pathogen makes all possible efforts to modify this process so as to inhibit cell suicide and ensure that it can survive and, in some cases, establish latent infection. This review discusses the significance of various pathways/steps during virus-mediated modulation of host cell apoptosis.

Apoptosis in calf pneumonia induced by endobronchial inoculation with bovine adenovirus type 3 (BAV-3)

Three calves aged 1 week (group 1), three aged 6 weeks (group 2) and three aged 6 weeks (having been pretreated with dexamethasone) (group 3) were infected endobronchially with bovine adenovirus type 3 (BAV-3). All calves had received colostrum. The histopathological, immunohistochemical, ultrastructural and TUNEL features were examined on post-inoculation day (PID) 3, 5 and 7. Viral replication and intranuclear inclusions were frequently observed in groups 1 and 3, but not in group 2. The lesions became progressively severe on PID 5 and 7 in group 1. In group 3, however, the cellular injury caused by BAV-3 was of short duration and the lesions began to resolve at PID 7. Numerous apoptotic cells were seen in the PID 3 calves of all three groups, and in the PID 7 calves of groups 2 and 3; however, the PID 5 and 7 calves of group 1 showed only a few apoptotic cells in the alveolar septa. The results indicated that (1) the durability of BAV-3 infection in the lung was closely related to apoptosis, and (2) the host defence mechanism that induced apoptosis in infected cells was age-related.

Apoptosis in lymph nodes and changes in lymphocyte subpopulations in peripheral blood of pigs infected with porcine rubulavirus

In a first experiment, five pigs were inoculated intranasally with porcine rubulavirus (PoRV) at 5 days of age and killed 7 days post-infection (pi). In a second experiment, four pigs were infected with the same virus at 17 days of age and killed at 9 or 15 days pi. Control piglets in each experiment received uninfected cell culture supernate. All PoRV-infected pigs developed respiratory and nervous signs, and histological lesions of non-suppurative encephalitis and interstitial pneumonia. All control pigs remained clinically normal and did not have histological lesions. Significantly increased numbers of apoptotic cells were detected by terminal deoxynucleotidyl transferase biotin-dUTP nick end labelling (TUNEL) in tonsil and lymph nodes of the pigs infected at 7 days of age and killed at 7 days pi. Significantly increased percentages of CD2(+) and CD8(+) T lymphocytes were also found in peripheral blood of these animals at this time, while the percentages of CD4(+) and MHC class II lymphocytes were significantly reduced. Significantly increased numbers of apoptotic cells were detected in lymphoid tissues of the pigs infected at 17 days of age and killed at 9 days pi. The percentages of CD2(+), CD8(+) and MHC class II lymphocytes in peripheral blood were also significantly increased at this time; the percentage of MHC class II lymphocytes remained elevated at 15 days pi. These results indicate that induction of apoptosis is an important mechanism in the pathogenesis of PoRV infection in young pigs, and that this virus induces changes in lymphocyte subpopulations in peripheral blood.

HIV-1 protease processes procaspase 8 to cause mitochondrial release of cytochrome c, caspase cleavage and nuclear fragmentation

Infection of T cells with HIV-1 induces apoptosis and modulates apoptosis regulatory molecules. Similar effects occur following treatment of cells with individual HIV-1 encoded proteins. While HIV-1 protease is known to be cytotoxic, little is known of its effect on apoptosis and apoptosis regulatory molecules. The ability of HIV-1 protease to kill cells, coupled with the degenerate substrate specificity of HIV-1 protease, suggests that HIV-1 protease may activate cellular factor(s) which, in turn, induce apoptosis. We demonstrate that HIV-1 protease directly cleaves and activates procaspase 8 in T cells which is associated with cleavage of BID, mitochondrial release of cytochrome c, activation of the downstream caspases 9 and 3, cleavage of DFF and PARP and, eventually, to nuclear condensation and DNA fragmentation that are characteristic of apoptosis. The effect of HIV-1 protease is not seen in T cell extracts which have undetectable levels of procaspase 8, indicating a specificity and requirement for procaspase 8.

Vaccinia virus induces apoptosis of infected macrophages

Vaccinia virus (VV) infects a broad range of host cells, and while it usually causes their lysis (i.e. necrosis), the nature of the cell-death phenomenon is not well understood. In this study, we show that VV induces apoptosis of cells of the murine macrophage line J774.G8, as revealed by morphological signs, DNA ladder formation, changes of mitochondrial membrane potential and annexin-V positivity. Apoptosis occurred in both untreated and IFN-gamma-pretreated macrophages, and could not be inhibited by aminoguanidine, a relatively specific inhibitor of inducible nitric oxide synthase. Inhibition of VV DNA synthesis and late gene expression by cytosine arabinoside also did not prevent apoptosis, while heat- or psoralen/UV-inactivated VV did not cause any apoptosis. Thus, VV early gene expression seems to be required for induction of apoptosis. At the cellular level, infection with VV induced a decrease in the levels of Bcl-x(L), an anti-apoptotic member of the Bcl-2 family. The importance of loss of Bcl-x(L) was demonstrated by prevention of VV-mediated apoptosis on expression of Bcl-2, a functional homologue of Bcl-x(L). Our findings provide evidence that induction of apoptosis by VV in macrophages requires virus early gene expression, does not involve nitric oxide, induces a decrease in mitochondrial membrane potential and is associated with altered levels of Bcl-x(L).

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.

Production of viral progeny in insect cells undergoing apoptosis induced by a mutant Anticarsia gemmatalis nucleopolyhedrovirus

The Anticarsia gemmatalis nucleopolyhedrovirus (AgMNPV) is the most successful viral biopesticide in use worldwide. We have demonstrated that despite widespread apoptosis and no protein synthesis at 48 h p.i., UFL-AG-286 cells infected with a mutant of AgMNPV (vApAg), produced significant amounts of budded virus (BVs) and viral DNA late in infection. However, a different susceptible cell line (BTI-Tn5B 1-4) showed no signs of apoptosis and produced 3.5 times more budded virus when infected with vApAg. A comparison of DNA from AgMNPV and vApAg digested with the same restriction enzymes showed differences in the restriction pattern, indicating that the vApAg phenotype might be due to a mutation in a gene or genes responsible for directly or indirectly inhibiting apoptosis in UFL-AG-286 cells.

Apoptosis during ectromelia orthopoxvirus infection is DEVDase dependent: in vitro and in vivo studies

Ectromelia virus (EV), which causes mousepox, is a member of the orthopoxviruses that are defined as being able to suppress apoptosis. Caspase-3 is one of the key effector proteases which regulates the apoptotic cascade and which is responsible for DNA fragmentation observed during apoptosis. It is well known that viruses, especially poxviruses, can inhibit caspase activity. Here, we report that EV can regulate apoptosis in vitro, suppressing the activity of caspases recognizing the DEVD (Asp-Glu-Val-Asp) motif (caspase-3 and -7) before successful virus replication is completed. Caspase-3 activity measurement showed that an increase in caspase-3 activity preceded the peak of DNA fragmentation demonstrated by TUNEL staining of L929 and RK-13 cells. By using specific caspase inhibitors (Ac-DEVD-CHO, Ac-IETD-CHO and zVAD-fmk), we showed that caspase-3 and -7 (DEVDases) are major effector caspases during EV-induced apoptosis in permissive L929 and RK-13 cell cultures. Apoptosis in vivo seems to play an important role during viraemia as well as during the clearance of EV from genetically susceptible BALB/c (H-2(d)) mice. However, as shown by measurement of caspase-3 activity, caspase-3 protein detection and M30-antibody staining, both DEVDases seem to play an important role during EV clearance from draining lymph nodes and conjunctivae at 15 days p.i. up to 20 days p.i., whereas in the liver and spleen DNA fragmentation coexisted with viral multiplication and secondary viraemia. Apoptosis was DEVDase dependent only in the liver, while spleen DNA fragmentation observed between 5 and 10 days p.i. was caspase independent. Therefore, we conclude that DEVDase- (caspase-3- and caspase-7-) dependent apoptosis is an important mechanism regulating the resolution of EV infection.

The epizootic haematopoietic necrosis virus (Iridoviridae) induces apoptosis in vitro

The epizootic haematopoietic necrosis virus (EHNV) is an iridovirus causing severe disease in different fish species. We investigated the induction of apoptosis during EHNV infection of the epithelioma carp papulosum (EPC) cell line. Apoptosis reveals several characteristic morphological changes, such as chromatin condensation, nuclear fragmentation, cytoplasm membrane disorientation, or mitochondrial changes. During EHNV infection of EPC cells the occurrence of apoptosis was analysed using a fluorescein-isothiocyanate (FITC) conjugate of annexin-V to detect phosphatidylserines that have changed cytoplasm membrane localization. Annexin-V labelling was obvious 12 h after infection. At 54 h after EHNV infection 39% of the investigated EPC cells exhibited fluorescence. Furthermore, EHNV-infected cells were stained with 4'-6'-diamidino-2-phenylindole (DAPI) to detect pycnotic nuclei. Appearance of DAPI-positive nuclei was found beginning at 18 h after infection. At 54 h after EHNV infection approximately 56% of the EPC cells showed fragmented nuclei. Assays to inhibit a protein kinase-dependent (e.g. double-stranded RNA-dependent protein kinase) apoptosis pathway with 2-aminopurine revealed a reduction of EHNV titres, e.g. titres were reduced 1000-fold in the presence of 100 and 200 mM 2-aminopurine. Apoptosis takes place during iridovirus infection in vitro and it seems to involve the activation of protein kinases.

Comparison of the eIF-2alpha homologous proteins of seven ranaviruses (Iridoviridae)

The alpha-subunit of the eukaryotic initiation factor 2 (eIF-2alpha) is a key component of the translation machinery of the cell. In response to cellular stress such as viral infections, eIF-2alpha is phosphorylated by double-stranded RNA-dependent protein kinase (PKR) leading to the inhibition of cellular protein synthesis. The importance of eIF-2alpha as a regulatory mechanism for protein synthesis is illustrated by the wide variety of strategies employed by viruses to down-regulate PKR. Thus, Vaccinia virus encodes K3L protein, which resembles eIF-2alpha and acts as a pseudo-substrate inhibitor of PKR. Nucleotide sequencing of the genome of epizootic haematopoietic necrosis virus (EHNV), a member of the genus ranavirus of Iridoviridae, has revealed an eIF-2alpha equivalent gene. We have cloned and sequenced eIF-2alpha genes of several iridoviruses of fishes and frogs. The eIF-2alpha open reading frames and deduced proteins of the iridoviruses investigated exhibit a high degree of homology of both nucleotide and amino acid sequences. At the N-terminus, the iridoviral eIF-2alpha shows significant homology to the N-termini of cellular initiation factor 2-alpha of various species, to full-length poxviral eIF-2alpha proteins, and to the S1 domain of ribosomal proteins. Comparison of amino acid sequences of corresponding iridoviral proteins with eIF-2alpha homologous proteins of poxviruses and eukaryotes has revealed a high conservation of motifs. A phylogenetic analysis of eukaryotic eIF-2alpha and poxvirus and iridovirus eIF-2alpha sequences has demonstrated the relationship of these iridoviruses. In order to investigate the role of the eIF-2alpha equivalent, respective genes have been expressed in prokaryotic and eukaryotic (insect, fish and chicken cell) systems. The iridoviral eIF-2alpha protein has a molecular weight of 31 kDa and is cytoplasmic. The cellular and viral protein synthesis of iridoviruses is probably regulated by a mechanism similar to that of Vaccinia virus. Frog-virus 3, the type species of the genus ranavirus of Iridoviridae, has a unique translational efficiency and, moreover, down-regulates the cellular protein synthesis of infected cells.

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

Many viruses have evolved strategies that target crucial components within the apoptotic cascade. One of the best studied is the caspase 8 inhibitor, crmA/Spi-2, encoded by members of the poxvirus family. Since many proapoptotic stimuli induce apoptosis through a mitochondrion-dependent, caspase 8-independent pathway, we hypothesized that vaccinia virus would encode a mechanism to directly modulate the mitochondrial apoptotic pathway. In support of this, we observed that Jurkat cells, which undergo Fas-mediated apoptosis exclusively through the mitochondrial route, were resistant to Fas-induced death following infection with a crmA/Spi-2-deficient strain of vaccinia virus. In addition, vaccinia virus-infected cells subjected to the proapoptotic stimulus staurosporine exhibited decreased levels of both cytochrome c released from the mitochondria and caspase 3 activation. In all cases we found that the loss of the mitochondrial membrane potential, which occurs as a result of opening the multimeric permeability transition pore complex, was prevented in vaccinia virus-infected cells. Moreover, vaccinia virus infection specifically inhibited opening of the permeability transition pore following treatment with the permeability transition pore ligand atractyloside and t-butylhydroperoxide. These studies indicate that vaccinia virus infection directly impacts the mitochondrial apoptotic cascade by influencing the permeability transition pore.

Inhibition of host cell apoptosis by Toxoplasma gondii is accompanied by reduced activation of the caspase cascade and alterations of poly(ADP-ribose) polymerase expression

The obligate intracellular protozoan parasite Toxoplasma gondii has been shown to protect different cell types from apoptosis induced by a variety of pro-apoptotic treatments. However, the precise cell biological mechanisms of this inhibition remained unknown. As shown in this study, apoptosis in human-derived HL-60 and U937 cells induced by treatment with actinomycin D or TNF-α in combination with cycloheximide, respectively, was indeed dose-dependently downregulated by prior infection with T. gondii, as determined by DNA fragmentation assays. Cleavage of caspase 3 and caspase 9 after treatment with pro-apoptotic stimuli was considerably diminished by T. gondii. Furthermore, release of mitochondrial cytochrome c during apoptosis in HL-60 cells was prevented by intracellular parasites and this was correlated with the absence of DNA strand breaks on the single cell level. Inhibition of cytochrome c release coincided with a twofold upregulation of Mcl-1 protein levels in HL-60 and U937 cells, while Bcl-2 expression did not increase after infection. Parasitic interference with the caspase cascade led to a reduced proteolytic cleavage of the nuclear target molecule protein kinase Cδ. In parallel, poly(ADP-ribose) polymerase protein levels were prominently downregulated by T. gondii, irrespective of whether HL-60 and U937 cells had been treated with pro-apototic stimuli or left untreated. However, poly(ADP-ribose) polymerase mRNA levels remained unchanged after infection as determined by RT-PCR analyses. These observations suggest that T. gondii has evolved different mechanisms that may contribute to downregulation of host cell apoptosis, namely inhibition of cytochrome c release and subsequent caspase activation as well as downregulation of poly(ADP-ribose) polymerase protein levels.

Intracellular protozoan parasites and apoptosis: diverse strategies to modulate parasite-host interactions

Programmed cell death (apoptosis) is an important regulator of the host's response during infection with a variety of intracellular protozoan parasites. Parasitic pathogens have evolved diverse strategies to induce or inhibit host-cell apoptosis, thereby modulating the host's immune response, aiding dissemination within the host or facilitating intracellular survival. Here, we review the molecular and cell-biological mechanisms of the pathogen-induced modulation of host-cell apoptosis and its effects on the parasite-host interaction and the pathogenesis of parasitic diseases. We also discuss the previously unrecognized phenomenon of apoptotic cell death in (unicellular) protozoan parasites and its potential implications.

Viral serine proteinase inhibitor (SERP-1) effectively decreases the incidence of graft vasculopathy in heterotopic heart allografts

BACKGROUND

Graft vascular disease (GVD) is the most common cause of late graft failure in solid organ transplantation. Recent studies have shown good efficacy of a novel nontoxic viral-derived serine proteinase inhibitor (SERP-1) in preventing postangioplasty restenosis. The current study was designed to test whether short-term treatment with SERP-1 was effective in reducing the incidence of GVD in a solid organ transplant.

METHODS

Piebald-Virol-Glaxo (PVG) donor hearts were transplanted into August-Copenhagen-Irish (ACI) recipients and observed for 90 days. All recipients (n=60) were treated with microemulsion cyclosporine (CsA) 7.5 mg/kg per gavage from day 0 to day 9 and randomized into 4 groups. SERP-1 was given intravenously. Group I received CsA monotherapy; group II, CsA+SERP-1 1 ng/g (postoperative days 0-9); group III, CsA+SERP-1 10 ng/g (postoperative days 0-9); and group IV, CsA+SERP-1 10 ng/g (postoperative days 0-9, 30, and 60). Graft viability was monitored by palpation, and GVD was assessed by morphometry.

RESULTS

Two animals in group I rejected their allografts on postoperative days 7 and 14, 1 animal in group II rejected the allograft (postoperative day 31), and none in group III and IV rejected the allografts. At 90 days postoperative, 23.8% of all coronary vessels showed evidence of GVD in group I, 18.4% in group II, 12.9% in group III, and 11.8% in group IV. The difference in incidence of GVD was significant between groups I and III (P<0.05) and groups I and IV (P<0.05). Treatment with SERP-1 was well tolerated, and all animals regained weight quickly postsurgery.

CONCLUSIONS

Treatment of allograft recipients with SERP-1 in combination with CsA early after transplantation significantly decreases the incidence of GVD when compared to grafts treated with only CsA. These results demonstrate the clinical potential for this novel serine protease inhibitor to prevent GVD in solid organ transplantation.

Natural biology of polyomavirus middle T antigen

"It has been commented by someone that 'polyoma' is an adjective composed of a prefix and suffix, with no root between--a meatless linguistic sandwich" (C. J. Dawe). The very name "polyomavirus" is a vague mantel: a name given before our understanding of these viral agents was clear but implying a clear tumor life-style, as noted by the late C. J. Dawe. However, polyomavirus are not by nature tumor-inducing agents. Since it is the purpose of this review to consider the natural function of middle T antigen (MT), encoded by one of the seemingly crucial transforming genes of polyomavirus, we will reconsider and redefine the virus and its MT gene in the context of its natural biology and function. This review was motivated by our recent in vivo analysis of MT function. Using intranasal inoculation of adult SCID mice, we have shown that polyomavirus can replicate with an MT lacking all functions associated with transformation to similar levels to wild-type virus. These observations, along with an almost indistinguishable replication of all MT mutants with respect to wild-type viruses in adult competent mice, illustrate that MT can have a play subtle role in acute replication and persistence. The most notable effect of MT mutants was in infections of newborns, indicating that polyomavirus may be highly adapted to replication in newborn lungs. It is from this context that our current understanding of this well-studied virus and gene is presented.

Human cytomegalovirus pp65- and immediate early 1 antigen-specific HLA class I-restricted cytotoxic T cell responses induced by cross-presentation of viral antigens

Dendritic cells (DCs) play a pivotal role in the development of anti-viral CD8(+) CTL responses. This is straightforward if they are directly infected with virus, but is less clear in response to viruses that cannot productively infect DCS: Human CMV (HCMV) shows strain-specific cell tropism: fibroblast (Fb)-adapted laboratory strains (AD169) and recent clinical isolates do not infect DCs, whereas endothelial cell-adapted strains (TB40/E) result in productive lytic DC infection. However, we show here that uninfected DCs induce CD8(+) T cell cytotoxicity and IFN-gamma production against HCMV pp65 and immediate early 1 Ags following in vitro coculture with HCMV-AD169-infected Fbs, regardless of the HLA type of these FBS: CD8(+) T cell stimulation was inhibited by pretreatment of DCs with cytochalasin B or brefeldin A, indicating a phagosome/endosome to cytosol pathway. HCMV-infected Fbs were not apoptotic as measured by annexin V binding, and induction of apoptosis of infected Fbs in vitro did not augment CTL induction by DCs, suggesting a mechanism other than apoptosis in the initiation of cross-presentation. Furthermore, HCMV-infected Fbs provided a maturation signal for immature DCs during coculture, as evidenced by increased CD83 and HLA class II expression. Cross-presentation of HCMV Ags by host DCs enables these professional APCs to bypass some of the evasion mechanisms HCMV has developed to avoid T cell recognition. It may also serve to explain the presence of immediate early 1 Ag-specific CTLs in the face of pp65-induced inhibition of Ag presentation at the level of the infected cell.

X protein of hepatitis B virus inhibits Fas-mediated apoptosis and is associated with up-regulation of the SAPK/JNK pathway

The X protein from a chronic strain of hepatitis B virus (HBx) was determined to inhibit Fas-mediated apoptosis and promote cell survival. Fas-mediated apoptosis is the major cause of hepatocyte damage during liver disease. Experiments demonstrated that cell death caused by anti-Fas antibodies was blocked by the expression of HBx in human primary hepatocytes and mouse embryo fibroblasts. This effect was also observed in mouse erythroleukemia cells that lacked p53, indicating that protection against Fas-mediated apoptosis was independent of p53. Components of the signal transduction pathways involved in this protection were studied. The SAPK/JNK pathway has previously been suggested to be a survival pathway for some cells undergoing Fas-mediated apoptosis, and kinase assays showed that SAPK activity was highly up-regulated in cells expressing the HBx protein. Normal mouse fibroblasts expressing HBx were protected from death, whereas identical fibroblasts lacking the SEK1 component from the SAPK pathway succumbed to Fas-mediated apoptosis, whether HBx was present or not. Assays showed that caspase 3 and 8 activities and the release of cytochrome c from mitochondria were inhibited, in the presence of HBx, following stimulation with anti-Fas antibodies. Coprecipitation and confocal immunofluorescence microscopy experiments demonstrated that HBx localizes with a cytoplasmic complex containing MEKK1, SEK1, SAPK, and 14-3-3 proteins. Finally, mutational analysis of HBx demonstrated that a potential binding region for 14-3-3 proteins was essential for induction of SAPK/JNK activity and protection from Fas-mediated apoptosis.

Immunomodulatory proteins of myxoma virus

Poxviruses collectively encode an impressive collection of diverse immunomodulatory proteins. In this review we draw attention to some of the new open reading frames (ORFs) discovered during the sequencing of the myxoma virus DNA genome [Cameron C, Hota-Mitchell S, Chen L, Barrett J, Cao J-X, Macaulay C, Willer D, Evans D, McFadden G (1999) The complete DNA sequence of myxoma virus. Virology 264:298-318] that may function to subvert the host immune system. Most of these predicted functions are speculative but some of the deduced primary amino acid sequences contain intriguing similarities to known cellular and viral proteins in the public domain for which immunomodulatory functions have been assigned.

Adeno-associated virus type 2 Rep78 induces apoptosis through caspase activation independently of p53

Adeno-associated virus (AAV) type 2 Rep78 is a multifunctional protein required for AAV DNA replication, integration, and gene regulation. The biochemical activities of Rep78 have been described, but the effects of Rep proteins on the cell have not been characterized. We have analyzed Rep-mediated cytotoxicity. We demonstrated that Rep78 expression is sufficient to induce cell death and disruption of the cell cycle. Cell death was found to be mediated by apoptosis. Rep78 expression resulted in the activation of caspase-3, a terminal caspase directly involved in the execution of cell death. A peptidic inhibitor of caspase-3, Z-Asp-Glu-Val-Asp-fluoromethylketone (Z-DEVD-FMK), abrogated Rep78-induced apoptosis, indicating that Rep78-mediated apoptosis is caspase-3 dependent. Rep78 induced apoptosis in wild-type p53-containing human embryonal carcinoma NT-2 cells and in p53-null promyelocytic human HL-60 cells, indicating that at least one pathway of Rep78-induced apoptosis is p53 independent. Apoptosis was shown to occur during the G(1) and early S phases of the cell cycle. By analyzing the effects of Rep78 mutations on cell viability, the cause of cell death was attributed in part to two biochemical activities of Rep78, DNA binding and ATPase/helicase activity. The endonuclease activity of Rep78 did not contribute to apoptosis induction.