CrmA, a poxvirus-encoded serpin, inhibits cytotoxic T-lymphocyte-mediated apoptosis

Pyroptosis in Antiviral Immunity

Pyroptosis is a form of lytic, programmed cell death that functions as an innate immune effector mechanism to facilitate host defense against pathogenic microorganisms, including viruses. This type of proinflammatory cell death is orchestrated by proteolytic activation of human or mouse caspase-1, mouse caspase-11 and human caspase-4 and caspase-5 in response to infectious and inflammatory stimuli. Induction of pyroptosis requires either a canonical inflammasome responsible for caspase-1 activation or a noncanonical complex composed of caspase-11 in mice or caspase-4 or caspase-5 in humans. Recent studies have identified the pore-forming protein gasdermin D, a substrate of these inflammatory caspases, as an executioner of pyroptosis. The membrane pores formed by gasdermin D facilitate release of proinflammatory cytokines IL-1β and IL-18 and consequent biologic effects of these cytokines together with other released components. Pyroptosis, like other forms of programmed cell death, helps eliminate infected cells and thereby restricts the replicative niche, undermining survival and proliferation of intracellular pathogens. This includes viruses as well as bacteria, where ample evidence supports a critical role for inflammasome effector functions and cell death in host defense. Viruses have evolved their own mechanisms to modulate inflammasome signaling and pyroptosis. Here, we review the current literature regarding the role of pyroptosis in antiviral immune responses.

Genome-Wide Characterization and Comparative Genomic Analysis of the Serpin Gene Family in Microsporidian Nosema bombycis

Microsporidia are ubiquitous in the environment, infecting almost all invertebrates, vertebrates, and some protists. The microsporidian Nosema bombycis causes silkworms pébrine disease and leads to huge economic losses. Parasite secreted proteins play vital roles in pathogen-host interactions. Serine protease inhibitors (serpins), belonging to the largest and most broadly distributed protease inhibitor superfamily, are also found in Microsporidia. In this study, we characterized 19 serpins (NbSPNs) in N. bombycis; eight of them were predicted with signal peptides. All NbSPN proteins contain a typical conserved serpin (PF00079) domain. The comparative genomic analysis revealed that microsporidia serpins were only found in the genus Nosema. In addition to N. bombycis, a total of 34 serpins were identified in another six species of Nosema including N. antheraeae (11), N. granulosis (8), Nosema sp. YNPr (3), Nosema sp. PM-1 (3), N. apis (4), and N. ceranae (5). Serpin gene duplications in tandem obviously occurred in Nosema antheranae. Notably, the NbSPNs were phylogenetically clustered with serpins from the Chordopoxvirinae, the subfamily of Poxvirus. All 19 NbSPN transcripts were detected in the infected midgut and fat body, while 19 NbSPN genes except for NbSPN12 were found in the transcriptome of the infected silkworm embryonic cell line BmE-SWU1. Our work paves the way for further study of serpin function in microsporidia.

Oxeiptosis: a discreet way to respond to radicals

One of the best-studied cellular responses to toxic signals and pathogens is programmed cell death. Over the past years, it became apparent that the specific mechanisms of cell death have tremendous influence at both cellular and organismal level, highlighting the importance of sensors and pathways involved in this decision-making process. Central signalling molecules involved in a variety of cell death pathways are reactive oxygen species (ROS). However, the molecular mechanisms regulating differential responses and cellular fates to distinct ROS levels remain incompletely understood. Recently, we uncovered a caspase-independent cell-death pathway named 'oxeiptosis', which links the ROS sensing capacity of KEAP1 to a cell death pathway involving PGAM5 and AIFM1. Alike apoptosis, oxeiptosis is anti-inflammatory when activated by increased intracellular ROS levels and upon pathogens encounter. Here we discuss the potential impact of oxeiptosis in pathogens clearance and teratogenic cells.

Functional characterization of hesp018, a baculovirus-encoded serpin gene

The serpin family of serine proteinase inhibitors plays key roles in a variety of biochemical pathways. In insects, one of the important functions carried out by serpins is regulation of the phenoloxidase (PO) cascade - a pathway that produces melanin and other compounds that are important in insect humoral immunity. Recent sequencing of the baculovirus Hemileuca sp. nucleopolyhedrovirus (HespNPV) genome revealed the presence of a gene, hesp018, with homology to insect serpins. To our knowledge, hesp018 is the first viral serpin homologue to be characterized outside of the chordopoxviruses. The Hesp018 protein was found to be a functional serpin with inhibitory activity against a subset of serine proteinases. Hesp018 also inhibited PO activation when mixed with lepidopteran haemolymph. The Hesp018 protein was secreted when expressed in lepidopteran cells and a baculovirus expressing Hesp018 exhibited accelerated production of viral progeny during in vitro infection. Expression of Hesp018 also reduced caspase activity induced by baculovirus infection, but caused increased cathepsin activity. In infected insect larvae, expression of Hesp018 resulted in faster larval melanization, consistent with increased activity of viral cathepsin. Finally, expression of Hesp018 increased the virulence of a prototype baculovirus by fourfold in orally infected neonate Trichoplusia ni larvae. Based on our observations, we hypothesize that hesp018 may have been retained in HespNPV due to its ability to inhibit the activity of select host proteinases, possibly including proteinases involved in the PO response, during infection of host insects.

Granzyme M as a novel effector molecule for human cytolytic fusion proteins: CD64-specific cytotoxicity of Gm-H22(scFv) against leukemic cells

Immunotoxins are promising targeted therapeutic agents comprising an antibody-based ligand that specifically binds to diseased cells, and a pro-apoptotic protein. Toxic components from bacteria or plants can trigger a neutralizing immune response, so that human effector molecules are more suitable. In this context, the protease granzyme B has been successfully tested in cytotoxicity assays against different cancer cells in vitro and in vivo. Our aim here was to introduce granzyme M as an alternative and novel component of human cytolytic fusion proteins. We fused it to the humanized single-chain antibody fragment (scFv) H22 which specifically binds to CD64, an FcγRI receptor overexpressed on activated myeloid cells and leukemic cells. We show that the humanized cytolytic fusion protein Gm-H22(scFv) specifically targets the acute myeloid leukemia cell line HL60 in vitro and is cytotoxic with an IC50 between 1.2 and 6.4 nM. These findings were confirmed ex vivo using leukemic primary cells from patients, which were killed by granzyme M despite the presence of the granzyme B inhibitor serpin B9. In conclusion, granzyme M is a promising new cell-death inducing component for hCFPs because it specifically and efficiently kills target cells when fused to a targeting component.

Apoptosis resistance in tumor cells

Various antitumor agents induce apoptotic cell death in tumor cells. Since the apoptosis program in tumor cells plays a critical role in the chemotherapy-induced tumor cell killing, it is suggested that the defect in the signaling pathway of apoptosis could cause a new form of multidrug resistance in tumor cells. This article describes the recent findings concerning the mechanisms of chemotherapy-induced apoptosis and discusses the implication of apoptosis resistance in cancer chemotherapy.

An optimized activity-based probe for the study of caspase-6 activation

Although significant efforts have been made to understand the mechanisms of caspase activation during apoptosis, many questions remain regarding how and when executioner caspases get activated. We describe the design and synthesis of an activity-based probe that labels caspase-3/-6/-7, allowing direct monitoring of all executioner caspases simultaneously. This probe has enhanced in vivo properties and reduced cross-reactivity compared to our previously reported probe, AB50. Using this probe, we find that caspase-6 undergoes a conformational change and can bind substrates even in the absence of cleavage of the proenzyme. We also demonstrate that caspase-6 activation does not require active caspase-3/-7, suggesting that it may autoactivate or be cleaved by other proteases. Together, our results suggest that caspase-6 activation proceeds through a unique mechanism that may be important for its diverse biological functions.

Anti-apoptotic effects of SERPIN B3 and B4 via STAT6 activation in macrophages after infection with Toxoplasma gondii

Toxoplasma gondii penetrates all kinds of nucleated eukaryotic cells but modulates host cells differently for its intracellular survival. In a previous study, we found out that serine protease inhibitors B3 and B4 (SERPIN B3/B4 because of their very high homology) were significantly induced in THP-1-derived macrophages infected with T. gondii through activation of STAT6. In this study, to evaluate the effects of the induced SERPIN B3/B4 on the apoptosis of T. gondii-infected THP-1 cells, we designed and tested various small interfering (si-) RNAs of SERPIN B3 or B4 in staurosporine-induced apoptosis of THP-1 cells. Anti-apoptotic characteristics of THP-1 cells after infection with T. gondii disappeared when SERPIN B3/B4 were knock-downed with gene specific si-RNAs transfected into THP-1 cells as detected by the cleaved caspase 3, poly-ADP ribose polymerase and DNA fragmentation. This anti-apoptotic effect was confirmed in SERPIN B3/B4 overexpressed HeLa cells. We also investigated whether inhibition of STAT6 affects the function of SERPIN B3/B4, and vice versa. Inhibition of SERPIN B3/B4 did not influence STAT6 expression but SERPIN B3/B4 expression was inhibited by STAT6 si-RNA transfection, which confirmed that SERPIN B3/B4 was induced under the control of STAT6 activation. These results suggest that T. gondii induces SERPIN B3/B4 expression via STAT6 activation to inhibit the apoptosis of infected THP-1 cells for longer survival of the intracellular parasites themselves.

Fascaplysin exert anti-tumor effects through apoptotic and anti-angiogenesis pathways in sarcoma mice model

Previous studies indicated that fascaplysin derived from marine sponge can induce tumor cell death by apoptosis and possesses anti-angiogenesis activity. In order to verify these two effects in animal model and to identify action mechanisms, we established a sarcoma mice model, and treated mice with fascaplysin for 10 days. The tumor tissues were examined morphologically and immunohistochemically. The differential gene expression was also investigated by mRNA array. Fascaplysin treatment resulted in a significant suppression of tumor growth. Typical apoptotic phenomena were observed by transmission electron microscope and histological detection. Tissue sections were stained with monoclonal antibody directed to proliferating cell nuclear antigen (PCNA) and CD31. The decreased PCNA and CD31 antigen staining indicate the reduction of tumor cell proliferation and tumor vasculature property of fascaplysin in vivo. Microarrays were used to examine the gene expression profiles of tumors on CapitalBio mouse genome oligo array. The regulated genes analyzed from the expression level showed overlapping gene ontology (GO) categories and pathway mapping. Our findings indicate that cell cycle arrest, apoptosis, regulation of actin cytoskeleton, and cell adhesion all play important roles in the onset of fascaplysin. Detailed analysis by real time PCR of key genes confirmed the experimental results of microarrays. From these findings, it can be considered that fascaplysin can inhibit the growth of S180 cell implanted tumor, and the action mechanisms may involve in apoptosis, anti-angiogenesis, or cell cycle arrest.

Metaxin deficiency alters mitochondrial membrane permeability and leads to resistance to TNF-induced cell killing

Metaxin, a mitochondrial outer membrane protein, is critical for TNF-induced cell death in L929 cells. Its deficiency, caused by retroviral insertion-mediated mutagenesis, renders L929 cells resistance to TNF killing. In this study, we further characterized metaxin deficiency-caused TNF resistance in parallel with Bcl-X(L) overexpression-mediated death resistance. We did not find obvious change in mitochondria membrane potential in metaxin-deficient (Met(mut)) and Bcl-X(L)-overexpressing cells, but we did find an increase in the release rate of the mitochondrial membrane potential probe rhodamine 123 (Rh123) that was preloaded into mitochondria. In addition, overexpression of a function-interfering mutant of metaxin (MetaΔTM/C) or Bcl-X(L) in MCF-7.3.28 cells also resulted in an acquired resistance to TNF killing and a faster rate of Rh123 release, indicating a close correlation between TNF resistance and higher rates of the dye release from the mitochondria. The release of Rh123 can be controlled by the mitochondrial membrane permeability transition (PT) pore, as targeting an inner membrane component of the PT pore by cyclosporin A (CsA) inhibited Rh123 release. However, metaxin deficiency and Bcl-X(L) overexpression apparently affect Rh123 release from a site(s) different from that of CsA, as CsA can overcome their effect. Though both metaxin and Bcl-X(L) appear to function on the outer mitochondrial membrane, they do not interact with each other. They may use different mechanisms to increase the permeability of Rh123, since previous studies have suggested that metaxin may influence certain outer membrane porins while Bcl-X(L) may form pores on the outer membrane. The alteration of the mitochondrial outer membrane properties by metaxin deficiency and Bcl-X(L) overexpression, as indicated by a quicker Rh123 release, may be helpful in maintaining mitochondrial integrity.

Toxoplasma gondii inhibits granzyme B-mediated apoptosis by the inhibition of granzyme B function in host cells

Host defense to the apicomplexan parasite Toxoplasma gondii is critically dependent on CD8(+) T cells, whose effector functions include the induction of apoptosis in target cells following the secretion of granzyme proteases. Here we demonstrate that T. gondii induces resistance of host cells to apoptosis induced by recombinant granzyme B. Granzyme B induction of caspase-independent cytochrome c release was blocked in T. gondii-infected cells. Prevention of apoptosis could not be attributed to altered expression of the Bcl-2 family of apoptotic regulatory proteins, but was instead associated with reduced granzyme B-mediated, caspase-independent cleavage of procaspase 3 to the p20 form in T. gondii-infected cells, as well as reduced granzyme B-mediated cleavage of the artificial granzyme B substrate, GranToxiLux. The reduction in granzyme B proteolytic function in T. gondii-infected cells could not be attributed to altered granzyme B uptake or reduced trafficking of granzyme B to the cytosol, implying a T. gondii-mediated inhibition of granzyme B activity. Apoptosis and GranToxiLux cleavage were similarly inhibited in T. gondii-infected cells exposed to the natural killer-like cell line YT-1. The endogenous granzyme B inhibitor PI-9 was not up-regulated in infected cells. We believe these findings represent the first demonstration of granzyme B inhibition by a cellular pathogen and indicate a new modality for host cell protection by T. gondii that may contribute to parasite immune evasion.

[Pro-apoptotic function of hepatitis B virus X protein]

Infection of hepatitis B virus (HBV) is a main cause of liver diseases including hepatitis, cirrhosis and hepatocellular carcinoma (HCC). Among the HBV-encoded proteins, the HBV X protein (HBx) has been suspected to be strongly involved in HBV-associated liver pathogenesis. HBx, a virally encoded multifunctional regulator, has been shown to induce apoptosis, anti-apoptosis, proliferation, and transformation of cells depending on the cell lines, model systems used, assay protocols, and research groups. Among the several activities of HBx, the pro-apoptotic function of HBx will be discussed in this review. Given that the disruption of apoptosis pathway by HBx contributes to the liver pathogenesis, a better understanding of the molecular interference in the cellular pro-apoptotic networks by HBx will provide useful clues for the intervention in HBV-mediated liver diseases.

Delivery and therapeutic potential of human granzyme B

Granzyme B (GzmB) is used by cytotoxic lymphocytes as a molecular weapon for the defense against virus-infected and malignantly transformed host cells. It belongs to a family of small serine proteases that are stored in secretory vesicles of killer cells. After secretion of these cytolytic granules during killer cell attack, GzmB is translocated into the cytosol of target cells with the help of the pore-forming protein perforin. GzmB has adopted similar protease specificity as caspase-8, and once delivered, it activates major executioner apoptosis pathways. Since GzmB is very effective in killing human tumor cell lines that are otherwise resistant against many cytotoxic drugs and since GzmB of human origin can be recombinantly expressed, its use as part of a 'magic bullet' in tumor therapy is a very tempting idea. In this review, we emphasize the peculiar characteristics of GzmB that make it suited for use as an effector domain in potential immunoconjugates. We discuss what is known about its uptake into target cells and the trials performed with GzmB-armed immunoconjugates, and we assess the prospects of its potential therapeutic value.

Hepatitis C virus core protein induces apoptosis-like caspase independent cell death

Background

Hepatitis C virus (HCV) associated liver diseases may be related to apoptotic processes. Thus, we investigated the role of different HCV proteins in apoptosis induction as well as their potency to interact with different apoptosis inducing agents.

Methods and Results

The use of a tightly adjustable tetracycline (Tet)-dependent HCV protein expression cell system with the founder osteosarcoma cell line U-2 OS allowed switch-off and on of the endogenous production of HCV proteins. Analyzed were cell lines expressing the HCV polyprotein, the core protein, protein complexes of the core, envelope proteins E1, E2 and p7, and non-structural proteins NS3 and NS4A, NS4B or NS5A and NS5B. Apoptosis was measured mainly by the detection of hypodiploid apoptotic nuclei in the absence or presence of mitomycin C, etoposide, TRAIL and an agonistic anti-CD95 antibody. To further characterize cell death induction, a variety of different methods like fluorescence microscopy, TUNEL (terminal deoxynucleotidyl transferase (TdT)-catalyzed deoxyuridinephosphate (dUTP)-nick end labeling) assay, Annexin V staining, Western blot and caspase activation assays were included into our analysis.

Two cell lines expressing the core protein but not the total polyprotein exerted a strong apoptotic effect, while the other cell lines did not induce any or only a slight effect by measuring the hypodiploid nuclei. Cell death induction was caspase-independent since it could not be blocked by zVAD-fmk. Moreover, caspase activity was absent in Western blot analysis and fluorometric assays while typical apoptosis-associated morphological features like the membrane blebbing and nuclei condensation and fragmentation could be clearly observed by microscopy. None of the HCV proteins influenced the apoptotic effect mediated via the mitochondrial apoptosis pathway while only the core protein enhanced death-receptor-mediated apoptosis.

Conclusion

Our data showed a caspase-independent apoptosis-like effect of the core protein, which seems to be inhibited in the presence of further HCV proteins like the non structural (NS) proteins. This observation could be of relevance for the viral spread since induction of an apoptosis-like cell death by the core protein may have some impact on the release of the HCV particles from the host cell.

Caspase-dependent inhibition of mousepox replication by gzmB

Background

Ectromelia virus is a natural mouse pathogen, causing mousepox. The cytotoxic T (Tc) cell granule serine-protease, granzyme B, is important for its control, but the underlying mechanism is unknown. Using ex vivo virus immune Tc cells, we have previously shown that granzyme B is able to activate several independent pro-apoptotic pathways, including those mediated by Bid/Bak/Bax and caspases-3/-7, in target cells pulsed with Tc cell determinants.

Methods and Findings

Here we analysed the physiological relevance of those pro-apoptotic pathways in ectromelia infection, by incubating ectromelia-immune ex vivo Tc cells from granzyme A deficient (GzmB⁺ Tc cells) or granzyme A and granzyme B deficient (GzmA×B^−/− Tc cell) mice with ectromelia-infected target cells. We found that gzmB-induced apoptosis was totally blocked in ectromelia infected or peptide pulsed cells lacking caspases-3/-7. However ectromelia inhibited only partially apoptosis in cells deficient for Bid/Bak/Bax and not at all when both pathways were operative suggesting that the virus is able to interfere with apoptosis induced by gzmB in case not all pathways are activated. Importantly, inhibition of viral replication in vitro, as seen with wild type cells, was not affected by the lack of Bid/Bak/Bax but was significantly reduced in caspase-3/-7-deficient cells. Both caspase dependent processes were strictly dependent on gzmB, since Tc cells, lacking both gzms, neither induced apoptosis nor reduced viral titers.

Significance

Out findings present the first evidence on the biological importance of the independent gzmB-inducible pro-apoptotic pathways in a physiological relevant virus infection model.

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

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

Near death experiences: poxvirus regulation of apoptotic death

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

Proteomic analysis and the antimetastatic effect ofN-(4-methyl)phenyl-O-(4-methoxy) phenyl-thionocarbamate-induced apoptosis in human melanoma SK-MEL-28 cells

We employed human SK-MEL-28 cells as a model system to identify cellular proteins that accompany N-(4-methyl)phenyl-O-(4-methoxy)phenyl-thionocarbamate (MMTC)-induced apoptosis based on a proteomic approach. Cell viability tests revealed that SK-MEL-28 skin cancer cells underwent more cell death than normal HaCaT cells in a dose-dependent manner after treatment with MMTC. Two-dimensional electrophoresis in conjunction with matrix-assisted laser desorption/ionization-time of flight (MALDI-TOF) mass spectrometry analysis or computer matching with a protein database further revealed that the MMTC-induced apoptosis is accompanied by increased levels of caspase-1, checkpoint suppressor-1, caspase-4, NF-kappaB inhibitor, AP-2, c-Jun-N-terminal kinase, melanoma inhibitor, granzyme K, G1/S specific cyclin D3, cystein rich protein, Ras-related protein Rab-37 or Ras-related protein Rab-13, and reduced levels of EMS (oncogene), ATP synthase, tyrosine-phosphatase, Cdc25c, 14-3-3 protein or specific structure of nuclear receptor. The migration suppressing effect of MMTC on SK-MEL-28 cell was tested. MMTC suppressed the metastasis of SK-MEL-8 cells. It was also identified that MMTC had little angiogenic effect because it did not suppress the proliferation of HUVEC cell line. These results suggest that MMTC is a novel chemotherapeutic and metastatic agents against the SK-MEL-28 human melanoma cell line.

Host response to the attenuated poxvirus vector NYVAC: upregulation of apoptotic genes and NF-kappaB-responsive genes in infected HeLa cells

NYVAC has been engineered as a safe, attenuated vaccinia virus (VV) vector for use in vaccination against a broad spectrum of pathogens and tumors. Due to the interest in NYVAC-based vectors as vaccines and current phase I/II clinical trials with this vector, there is a need to analyze the human host response to NYVAC infection. Using high-density cDNA microarrays, we found 368 differentially regulated genes after NYVAC infection of HeLa cells. Clustering of the regulated genes identified six discrete gene clusters with altered expression patterns. Clusters 1 to 3 represented 47.5% of the regulated genes, with three patterns of gene activation kinetics, whereas clusters 4 to 6 showed distinct repression kinetics. Quantitative real-time reverse transcription-PCR analysis of selected genes validated the array data. Upregulated transcripts correlated with genes implicated in immune responses, including those encoding interleukin-1 receptor 2 (IL-1R2), IL-6, ISG-15, CD-80, and TNFSF7. NYVAC upregulated several intermediates of apoptotic cascades, including caspase-9, correlating with its ability to induce apoptosis. NYVAC infection also stimulated the expression of NF-kappaB1 and NF-kappaB2 as well as that of NF-kappaB target genes. Expression of the VV host range K1L gene during NYVAC infection prevented NF-kappaB activation, but not the induction of apoptosis. This study is the first overall analysis of the transcriptional response of human cells to NYVAC infection and provides a framework for future functional studies to evaluate this vector and its derivatives as human vaccines.

Targeted expression of the anti-apoptotic gene CrmA to NOD pancreatic islets protects from autoimmune diabetes

The activation of apoptosis is a critical mechanism by which pancreatic beta cells are destroyed in type 1 diabetes (T1DM). Strategies aimed at interfering with the apoptotic pathways could therefore be of potential therapeutic value. To this end, we generated NOD transgenic mice with targeted expression of the anti-apoptotic gene Cytokine response modifier A (CrmA) to pancreatic beta cells using the rat insulin promoter and the reverse tetracycline transactivator to express CrmA in a temporally controlled manner. Two lines of transgenic mice were studied whose expression of CrmA occurred only after feeding doxycycline food. Islet expression of CrmA partially protected pancreatic beta cells from the cytokine-mediated cytotoxicity in vitro and reduced modestly the spontaneous development of diabetes in NOD mice in vivo. In addition, beta cells from NOD CrmA mice were significantly protected from the destruction by diabetogenic T cells after adoptive transfer. More strikingly, NODCrmA mice were significantly resistant to the diabetogenic activity of a potent insulin-specific CD8 T-cell clone. Since these adoptive transfer models mainly represent the effector phase rather than the initiation phase of autoimmune diabetes, our data suggest that the latter is more sensitive to CrmA protection. We conclude that anti-apoptotic genes such as CrmA might be potential candidates to enhance islet graft survival in T1DM.

Viral strategies for evading antiviral cellular immune responses of the host

The host invariably responds to infecting viruses by activating its innate immune system and mounting virus-specific humoral and cellular immune responses. These responses are aimed at controlling viral replication and eliminating the infecting virus from the host. However, viruses have evolved numerous strategies to counter and evade host's antiviral responses. Providing specific examples from the published literature, we discuss in this review article various strategies that viruses have developed to evade antiviral cellular responses of the host. Unraveling these viral strategies allows a better understanding of the host-pathogen interactions and their coevolution. This knowledge is important for identifying novel molecular targets for developing antiviral reagents. Finally, it may also help devise new knowledge-based strategies for developing antiviral vaccines.

Suppressors of a host range mutation in the rabbitpox virus serpin SPI-1 map to proteins essential for viral DNA replication

The orthopoxvirus serpin SPI-1 is an intracellular serine protease inhibitor that is active against cathepsin G in vitro. Rabbitpox virus (RPV) mutants with deletions of the SPI-1 gene grow on monkey kidney cells (CV-1) but do not plaque on normally permissive human lung carcinoma cells (A549). This reduced-host-range (hr) phenotype suggests that SPI-1 may interact with cellular and/or other viral proteins. We devised a genetic screen for suppressors of SPI-1 hr mutations by first introducing a mutation into SPI-1 (T309R) at residue P14 of the serpin reactive center loop. The SPI-1 T309R serpin is inactive as a protease inhibitor in vitro. Introduction of the mutation into RPV leads to the same restricted hr phenotype as deletion of the SPI-1 gene. Second-site suppressors were selected by restoration of growth of the RPV SPI-1 T309R hr mutant on A549 cells. Both intragenic and extragenic suppressors of the T309R mutation were identified. One novel intragenic suppressor mutation, T309C, restored protease inhibition by SPI-1 in vitro. Extragenic suppressor mutations were mapped by a new procedure utilizing overlapping PCR products encompassing the entire genome in conjunction with marker rescue. One suppressor mutation, which also rendered the virus temperature sensitive for growth, mapped to the DNA polymerase gene (E9L). Several other suppressors mapped to gene D5R, an NTPase required for DNA replication. These results unexpectedly suggest that the host range function of SPI-1 may be associated with viral DNA replication by an as yet unknown mechanism.

Maspin overexpression modulates tumor cell apoptosis through the regulation of Bcl-2 family proteins

BACKGROUND

Maspin is a member of serpin family with tumor suppressing activity. Recent studies of maspin in animal models strongly support maspin's role as an inhibitor against the growth of primary tumor sand the process of metastasis. However, the molecular mechanism underlying this inhibition has not been fully elucidated. In this report, we analyze the effect of maspin on tumor cell apoptosis under several stress conditions.

METHODS

Stable clones overexpressing maspin are established in the mouse mammary tumor TM40D cells. They are treated with staurosporine, TNF-alpha, and serum starvation. The rates of cell apoptosis are analyzed by TUNEL assay. Inhibitors against caspase 8 and 9 are used in the apoptosis assay. Western blot analysis and ribonuclease protection assay (RPA) are performed to examine the expression of Bcl2 family genes.

RESULTS

We report that maspin expressing tumor cells have increased rate of apoptosis when they are treated with staurosporine and serum starvation. The effect is not through extracellular maspin. Maspin-mediated apoptosis is partially blocked by caspase 8 and 9 inhibitors, and is accompanied by changes in the Bcl-2 family proteins. Maspin-expressing tumor cells have a reduced level of anti-apoptotic protein Bcl-2, and an increased level of pro-apoptotic protein Bax. The regulation is not controlled at the transcriptional level but is through selective control of Bcl-2 and Bax protein stability.

CONCLUSION

Maspin overexpression modulates tumor cell apoptosis through the regulation of Bcl2 family proteins. Such change results in an increased release of cytochrome c from mitochondria, thus the increased apoptosis in maspin-expressing cells. This evidence strongly suggests that the induction of apoptosis in maspin-overexpressing cells represents a major mechanism by which maspin inhibits breast tumor progression.

FADD and caspase-8 are required for cytokine-induced proliferation of hemopoietic progenitor cells

The role of caspase-8 and its adaptor Fas-associated death domain (FADD) in lymphocyte apoptosis is well defined, but their functions in other hemopoietic lineages are not clear. We were unable to generate transgenic mice expressing dominant inhibitors of FADD or caspase-8 in hemopoietic cells, possibly because their expression may have precluded production of vital hemopoietic cells. When using a retroviral gene delivery system, fetal liver stem cells expressing a dominant-negative mutant of FADD (FADD-DN) were unable to generate myeloid or lymphoid cells upon transplantation into lethally irradiated mice. However, fetal liver stem cells expressing very low levels of the caspase-8 inhibitor cytokine response modifier A (CrmA) could reconstitute the hemopoietic system. This level of CrmA expression provided some protection against Fas ligand (FasL)-induced apoptosis and promoted accumulation of myeloid cells in the bone marrow, but it did not inhibit mitogen-induced proliferation of B or T lymphocytes. Using an in vitro colony formation assay, we found that fetal liver stem cells expressing FADD-DN, CrmA, or a dominant-negative mutant of caspase-8 could not proliferate in response to cytokine stimulation. These data demonstrate that the enzymatic activity of caspase-8 and its adaptor FADD are required for cytokine-induced proliferation of hemopoietic progenitor cells.

The 70 kilodalton heat shock protein is an inhibitor of apoptosis in prostate cancer

The 70 kD heat shock protein (HSP70) plays essential cellular roles in mediating intracellular protein folding and protecting cells from proteotoxic stress. This study has examined the role of HSP70 in the expression of apoptosis in prostate carcinoma cells. Apoptosis was negatively correlated with HSP70 expression in PC-3 cells heat shocked in vivo. Further experiments carried out on an in vitro reconstituted system with isolated nuclei and cytoplasm from PC-3 cells showed that purified HSP70 directly inhibits apoptosis in a dose-dependant manner. Therefore, the potential role of depletion of intracellular HSP70 was examined as a means of inducing apoptosis in PC-3 cancer cells. Depletion of HSP70 by two independent strategies, either with anti-sense oligonucleotides directed against HSP70 mRNA or with the bioflavinoid drug quercetin, led to apoptosis in the absence of stress. In addition, quercetin pre-treatment synergistically enhanced apoptosis in combination with heat shock. Thus, HSP70 plays a physiological role in tumour cells as an inhibitor of apoptosis occurring both spontaneously and after stress and is a potential target for apoptosis-based cancer therapy.

Maspin mediates increased tumor cell apoptosis upon induction of the mitochondrial permeability transition

Maspin is a unique serpin with the ability to suppress certain types of malignant tumors. It is one of the few p53-targeted genes involved in tumor invasion and metastasis. With this in mind, we attempted to study the molecular mechanism behind this tumor suppression. Maspin-expressing mammary tumors are more susceptible to apoptosis in both implanted mammary tumors in vivo, a three-dimensional spheroid culture system, as well as in monolayer cell culture under lowered growth factors. Subcellular fractionation shows that a fraction of maspin (in both TM40D-Mp and mutant maspinDeltaN cells) translocates to the mitochondria. This translocation of maspin to the mitochondria is linked to the opening of the permeability transition pore, which in turn causes the loss of transmembrane potential, thus initiating apoptotic degradation. This translocation is absent in the other mutant, maspinDeltaRSL. It fails to cause any loss of membrane potential and also shows decreased caspase 3 levels, proving that translocation to the mitochondria is a key event for this increase in apoptosis by maspin. Suppression of maspin overexpression by RNA interference desensitizes cells to apoptosis. Our data indicate that maspin inhibits tumor progression through the mitochondrial apoptosis pathway. These findings will be useful for maspin-based therapeutic interventions against breast cancer.

Mitochondrial dysfunction in T cells of patients with systemic lupus erythematosus

Activation, proliferation, or programmed cell death of T lymphocytes are dependent on controlled reactive oxygen intermediates (ROI) production and ATP synthesis in mitochondria. The mitochondrial transmembrane potential (Delta Psi(m)) also plays a decisive role in cell survival by controlling activity of redox-sensitive caspases. T lymphocytes of patients with systemic lupus erythematosus (SLE) exhibit mitochondrial hyperpolarization, increased ROI production, diminished intracellular glutathione levels, cytoplasmic alkalinization, and ATP depletion that mediate enhanced spontaneous and diminished activation-induced apoptosis and sensitize lupus T cells to necrosis. These redox and metabolic checkpoints represent novel targets for pharmacological intervention in SLE.

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

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

Perforin and Fas act together in the induction of apoptosis, and both are critical in the clearance of lymphocytic choriomeningitis virus infection

In this report we questioned the current view that the two principal cytotoxic pathways, the exocytosis and the Fas ligand (FasL)/Fas-mediated pathway, have largely nonoverlapping biological roles. For this purpose we have analyzed the response of mice that lack Fas as well as granzyme A (gzmA) and gzmB (FasxgzmAxB(-/-)) to infection with lymphocytic choriomeningitis virus (LCMV). We show that FasxgzmAxB(-/-) mice, in contrast to B6, Fas(-/-), and gzmAxB(-/-) mice, do not recover from a primary infection with LCMV, in spite of the expression of comparable numbers of LCMV-immune and gamma interferon-producing cytotoxic T lymphocytes (CTL) in all mouse strains tested. Ex vivo-derived FasxgzmAxB(-/-) CTL lacked nucleolytic activity and expressed reduced cytolytic activity compared to B6 and Fas(-/-) CTL. Furthermore, virus-immune CTL with functional FasL and perforin (gzmAxB(-/-)) are more potent in causing target cell apoptosis in vitro than those expressing FasL alone (perfxgzmAxB(-/-)). This synergistic effect of perforin on Fas-mediated nucleolysis of target cells is indicated by the fact that, compared to perfxgzmAxB(-/-) CTL, gzmAxB(-/-) CTL induced (i) an accelerated decrease in mitochondrial transmembrane potential, (ii) increased generation of reactive oxygen species, and (iii) accelerated phosphatidylserine exposure on plasma membranes. We conclude that perforin does not mediate recovery from LCMV by itself but plays a vital role in both gzmA/B and FasL/Fas-mediated CTL activities, including apoptosis and control of viral infections.

Can we really learn from model pathogens?

Cytolytic T (Tc) lymphocytes are the first order response of the adaptive immune system in the recovery from primary viral infections. These effector cells execute their function either by direct cytotoxicity through the Fas or perforin pathway and/or by the release of cytokines that either directly or indirectly exert antiviral activity. Mice respond to infection by closely related viruses with a vigorous Tc response, which is characterized by extensive crossreactivity on target cells infected with these viruses. However, the action of these cells can be beneficial, detrimental or neutral. From our current state of knowledge, no generalizations as to protective or detrimental effects of cytolytic effector functions in recovery from virus infections can be made. Thus, virus-host immune interactions have to be assessed individually and cannot be generalized.

E1AF/PEA3 reduces the invasiveness of SiHa cervical cancer cells by activating serine proteinase inhibitor squamous cell carcinoma antigen

E1AF/PEA3, a member of the Ets family of transcription factors, is associated with the malignant characteristics of cancer cells. The initial aim of our study was to test whether the invasiveness of SiHa cervical cancer cells could be diminished by transfection with antisense E1AF. Using an in vitro invasion assay in which cells penetrate a layer of Matrigel, we found that this was not the case; indeed, the invasiveness of the transfectants was enhanced. To better understand the mechanism of this enhancement, we used the cDNA microarray technique to search for genes whose expression was altered in the antisense E1AF-transfected SiHa cells. Among several genes affected, we found that expression of squamous cell carcinoma antigen (SCCA), a member of the ovalbumin serine proteinase inhibitor family, was significantly reduced. Forced expression of E1AF enabled activation of SCCA expression, and Luciferase reporter assays revealed that E1AF activates the SCCA promoter. Introduction of antisense SCCA into SiHa cells inhibited production of SCCA protein and markedly increased the invasiveness of the cells. Taken together, these results suggest that E1AF suppresses the invasiveness of SiHa cervical cancer cells through transcriptional activation of the SCCA serine proteinase inhibitor gene.

SPI-CI and SPI-6 cooperate in the protection from effector cell-mediated cytotoxicity

Tumors have several mechanisms to escape from the immune system. One of these involves expression of intracellular anticytotoxic proteins that modulate the execution of cell death. Previously, we have shown that the serine protease inhibitor (serpin) SPI-6, which inactivates the cytotoxic protease granzyme B (GrB), is capable of preventing cytotoxic T lymphocyte (CTL)-mediated apoptosis. Despite its potent antiapoptotic activity, SPI-6 does not prevent membranolysis induced by cytotoxic lymphocytes. We now provide evidence that several colon carcinoma cell lines do resist membranolysis and that this protection is dependent on SPI-6 but also requires expression of a closely related serpin called SPI-CI (serine protease inhibitor involved in cytotoxicity inhibition). Expression of SPI-CI is absent from normal colon but observed in placenta, testis, early during embryogenesis, and in cytotoxic lymphocytes. SPI-CI encodes a chymotrypsin-specific inhibitor and irreversibly interacts with purified granzyme M. Moreover, SPI-CI can protect cells from purified perforin/GrM-induced lysis. Our data therefore indicate that SPI-CI is a novel immune escape molecule that acts in concert with SPI-6 to prevent cytotoxic lymphocyte-mediated killing of tumor cells.

TRAIL and viral infection

Tumor necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL) is a member of the TNF family that can induce apoptosis when binding to either of two receptors bearing an intracellular death domain. The physiologic function of the TRAIL system, which also comprises three receptors not mediating a death signal has just begun to be elucidated. Expression of TRAIL, mostly upon stimulation by interferons, in different cytotoxic immune cells suggested it has a role as an important effector molecule in immune surveillance. In addition to its ability to induce apoptosis in transformed tumor cells, TRAIL has attracted attention for its possibly critical role in the defense against viral infection. Viruses may induce TRAIL expression in host and?or immune cells and sensitize host cells toward TRAIL-mediated apoptosis. On the other hand, viruses have evolved a variety of strategies to prevent TRAIL-mediated host cell death early in infection, which may contribute to allowing their replication and the spread of viral progeny. The knowledge of the molecular mechanisms leading to modification of TRAIL sensitivity in virus-host cell interactions may also impact upon future (virus-based) strategies to increase TRAIL sensitivity of tumor cells.

Cowpox virus CrmA, Myxoma virus SERP2 and baculovirus P35 are not functionally interchangeable caspase inhibitors in poxvirus infections

Cowpox virus (CPV) expresses the serpin (serine proteinase inhibitor) CrmA, an anti-inflammatory, anti-apoptotic protein required for production of red pocks on chicken chorioallantoic membranes (CAMs). In vitro, CrmA inhibits several caspases and granzyme B. Altering the critical P1-aspartate in the CrmA reactive centre loop to alanine resulted in a virus (CPV-CrmA-D303A) that resembled CPV deleted for CrmA (CPVDeltaCrmA : : lacZ); on CAMs it produced white, inflammatory pocks with activated caspase-3 and reduced virus yields, suggesting that CrmA activities are mediated via proteinase inhibition. CrmA in CPV was replaced with SERP2 from Myxoma virus (MYX) or baculovirus P35, which inhibit similar proteinases in vitro. SERP2 and P35 each blocked caspase-3-mediated apoptosis but were unable to control inflammation of CAMs. However, SERP2 and P35 restored virus yields, indicating that the decreased virus titres seen with CPVDeltaCrmA : : lacZ resulted from apoptosis rather than inflammation. To compare the activities of CrmA and SERP2 further, rabbits were infected with MYX recombinant viruses. Intradermal infection of rabbits with MYX was uniformly lethal, generating raised primary lesions and many secondary lesions. In contrast, deletion of SERP2 from MYX (MYXDeltaSERP2 : : lacZ) caused little mortality and produced flat primary lesions with few secondary lesions. Replacement of SERP2 with CrmA (MYXDeltaSERP2 : : CrmA) resulted in partial complementation with flat primary lesions, many secondary lesions and death in 70 % of the rabbits. Therefore, CrmA and SERP2 were not functionally interchangeable during infection of CAMs or rabbits, implying that these serpins have activities that are not evident from biochemical studies with human caspases.

Zoonotic poxvirus infections in humans

PURPOSE OF REVIEW

The 2003 USA monkeypox epidemic caused by imported African rodents, newly emergent poxvirus zoonoses in Brazil and the possible use of variola virus for biological warfare has led to renewed interest in poxviruses and anti-poxviral therapies. Increasing foreign travel and importation of exotic animal species increases the likelihood of poxvirus infections occurring outside their usual geographical range and diagnostic delay has important implications. The present review provides an overview of these rare zoonoses.

RECENT FINDINGS

Three genera of Poxviridae are known to cause human zoonoses: orthopoxviruses, parapoxviruses and yatapoxvirus. Most cases are occupational, sporadic and have few cutaneous lesions with low morbidity. The exception is monkeypox, similar to smallpox, with significant morbidity and childhood mortality. Molecular characterization using polymerase chain reaction (PCR) amplification and other methods provides accurate phylogenetic identification and suggests that a cowpox-like virus is the probable ancestor of variola and other zoonotic poxviruses. DNA genomic sequencing of the Brazilian Cantagalo and Araçatuba viruses shows a close relationship to vaccinia virus. Poxviruses have potential in cancer immunotherapy and their ability to evade host-cell immune responses may provide a basis for new antipoxvirus therapies. Other agents, particularly nucleoside phosphonates such as cidofovir, show therapeutic action against poxviruses.

SUMMARY

Human zoonotic poxvirus infections are rare but increasingly encountered outside their usual geographical range. The 2003 USA monkeypox outbreak emphasizes the importance of early accurate diagnosis, particularly because increasing numbers of immunosuppressed individuals increases the potential for severe or fatal infections. PCR methodology enables accurate phylogenetic typing and has identified new diseases, but rapid, reliable methods must be made available for clinicians. More research into therapeutic agents for the prevention and treatment of poxvirus infections is required.

Poxvirus infection and apoptosis

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

Granzyme B-induced cell death

Granzyme B (GrB) is a serine protease that is released by cytotoxic lymphocytes to kill virus-infected and tumor cells. Recent advances in the understanding of GrB have stressed the importance of reassessing the mechanisms by which GrB accomplishes its death functions. These include the uptake and trafficking of GrB within target cells, pathways used to trigger cell death, and the mechanism(s) controlling its killing activity. In addition, the role that GrB plays in human pathologies is still to be defined. The purpose of this review is to discuss recent insights into the biology of GrB and to evaluate its functional significance in health and disease.

CrmA gene expression protects mice against concanavalin-A-induced hepatitis by inhibiting IL-18 secretion and hepatocyte apoptosis

Activated cytotoxic T-cell-mediated hepatocyte apoptosis via Fas/Fas-ligand and perforin/granzyme pathways are believed to involve the model of concanavalin A (ConA)-induced hepatitis. The purpose of the present study is to investigate whether the cytokine response modifier A (crmA) gene effectively inhibits the hepatocyte apoptosis of ConA-induced hepatitis. We examined survival rates, liver pathology, immune histological changes, and cytokine profiles from mice receiving the recombinant adenovirus vectors containing cre and/or crmA genes, transferred to the liver 3 days before ConA injection, and a crmA gene nonexpression control group. Injection of ConA into mice rapidly led to massive hepatocyte apoptosis, and infiltration of leukocytes, especially CD11b(+) inflammatory cells. In contrast, liver damage was dramatically reduced in the mice that expressed the crmA gene. However, infiltration by CD4(+) cells was not affected. The survival of the mice increased significantly to 100% in the treated group versus the control group. Furthermore, we demonstrated that interleukin (IL)-18 plays an important role in ConA-induced hepatitis, and that crmA expression significantly inhibited IL-18 secretion. Our results showed that the crmA gene effectively inhibits apoptosis induced by ConA hepatitis. This indicates a potential therapeutic usage of crmA for protection from cellular damage due to hepatitis.

Poxviruses and immune evasion

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

Cell-mediated cytotoxicity in recovery from poxvirus infections

The availability of mutant and gene targeted knockout mice with defects in components of cellular cytotoxicity mediated by either the Fas or the exocytosis pathway permitted an analysis of their role in recovery from poxvirus infections. Ectromelia (EV), a natural mouse pathogen causing mousepox, the closely related orthopoxviruses cow pox (CPV) and vaccinia virus (VV), each encode serpins that inhibit Fas mediated apoptosis and lysis of target cells. Nevertheless, distinct differences were seen when the three viruses were inoculated into perforin-deficient mice: highly resistant C57Bl/6 mice became susceptible to low doses of EV; resistance to CPV increased whereas there was no effect on VV infections. Absence of the cytolytic granule associated granzymes (gzm) A and B rendered C57Bl/6 mice increasingly more susceptible to EV infections. Lack of both gzms rendered them as susceptible as perforin deficient mice, despite the presence of functionally active perforin. Elevated EV titres in liver and spleen of gzmA x B deficient mice, early after infection and before cytotoxic T cells were detectable, strongly suggests that these two gzms exert an antiviral effect by a mechanism distinct from effector molecules of NK and cytotoxic T cells.

Subcellular localization of CrmA: identification of a novel leucine-rich nuclear export signal conserved in anti-apoptotic serpins

The cowpox virus-encoded anti-apoptotic protein cytokine response modifier A (CrmA) is a member of the serpin family that specifically inhibits the cellular proteins caspase 1, caspase 8 and granzyme B. In this study, we have used Flag- and yellow fluorescent protein (YFP)-tagged versions of CrmA to investigate the mechanisms that regulate its subcellular localization. We show that CrmA can actively enter and exit the nucleus and we demonstrate the role of the nuclear export receptor CRM1 in this shuttling process. CrmA contains a novel leucine-rich nuclear export signal (NES) that is functionally conserved in the anti-apoptotic cellular serpin PI-9. Besides this leucine-rich export signal, additional sequences mapping to a 103-amino-acid region flanking the NES contribute to the CRM1-dependent nuclear export of CrmA. Although YFP-tagged CrmA is primarily located in the cytoplasm, shifting its localization to be predominantly nuclear by fusion of a heterologous nuclear localization signal did not impair its ability to prevent Fas-induced apoptosis. We propose that nucleocytoplasmic shuttling would allow CrmA to efficiently target cellular pro-apoptotic proteins not only in the cytoplasm, but also in the nucleus, and thus to carry out its anti-apoptotic function in both compartments.

Two newly characterized germinal center B-cell-associated genes, GCET1 and GCET2, have differential expression in normal and neoplastic B cells

A group of genes are highly expressed in normal germinal center (GC) B cells and GC B-cell-derived malignancies based on cDNA microarray analysis. Two new genes, GCET1 (germinal center B-cell expressed transcript 1) and GCET2, were cloned from selected expressed sequence tags (IMAGE clone 1334260 and 814622, respectively). GCET1 is located on chromosome 14q32 and has four splicing isoforms, of which the longest one is 1787 bp and encodes a 435-amino acid protein. GCET2 is located on 3q13.13, and the cloned fragment is 3270 bp, which encodes a protein of 178 amino acids. Blast search showed that GCET1 has a highly conserved serine proteinase inhibitor (SERPIN) domain and is located on a chromosomal locus containing seven other SERPIN family members. GCET2 is a likely homologue of the mouse gene M17, a GC-expressed transcript. Analysis of the GCET2 protein sequence indicated that it may be involved in signal transduction in the cytoplasm. Northern blot and real-time polymerase chain reaction analyses confirmed that GCET1 is highly restricted to normal GC B cells and GCB-cell-derived cell lines. Although GCET2 is also a useful marker for normal and neoplastic GC B cells, it has a wider range of expression including immature B and T cells. Real-time polymerase chain reaction assay showed that both GCET1 and GCET2 are preferentially expressed in follicular lymphoma and diffuse large B-cell lymphoma with GC B-cell differentiation, confirming previous microarray gene expression analysis, but neither one is entirely specific. Multiple markers are necessary to differentiate the GCB from the activated B-cell type of diffuse large B-cell lymphoma with a high degree of accuracy.

Proteinases and their inhibitors in the immune system

The most important roles of proteinases in the immune system are found in apoptosis and major histocompatibility complex (MHC) class II-mediated antigen presentation. A variety of cysteine proteinases, serine proteinases, and aspartic proteinases as well as their inhibitors are involved in the regulation of apoptosis in neutrophils, monocytes, and dendritic cells, in selection of specific B and T lymphocytes, and in killing of target cells by cytotoxic T cells and natural killer cells. In antigen presentation, endocytosed antigens are digested into antigenic peptides by both aspartic and cysteine proteinases. In parallel, MHC class II molecules are processed by aspartic and cysteine proteinases to degrade the invariant chain that occupies the peptide-binding site. Proteinase activity in these processes is highly regulated, particularly by posttranslational activation and the balance between active proteinases and specific endogenous inhibitors such as cystatins, thyropins, and serpins. This article discusses the regulation of proteolytic processes in apoptosis and antigen presentation in immune cells and the consequences of therapeutic interference in the balance of proteinases and their inhibitors.

Cell death induced by granzyme C

Although the functions of granzymes A and B have been defined, the functions of the other highly expressed granzymes (Gzms) of murine cytotoxic lymphocytes (C, D, and F) have not yet been evaluated. In this report, we describe the ability of murine GzmC (which is most closely related to human granzyme H) to cause cell death. The induction of death requires its protease activity and is characterized by the rapid externalization of phosphatidylserine, nuclear condensation and collapse, and single-stranded DNA nicking. The kinetics of these events are similar to those caused by granzyme B, and its potency (defined on a molar basis) is also equivalent. The induction of death did not involve the activation of caspases, the cleavage of BID, or the activation of the CAD nuclease. However, granzyme C did cause rapid mitochondrial swelling and depolarization in intact cells or in isolated mitochondria, and this mitochondrial damage was not prevented by cyclosporin A pretreatment. These results suggest that granzyme C rapidly induces target cell death by attacking nuclear and mitochondrial targets and that these targets are distinct from those used by granzyme B to cause classical apoptosis.

Induction of apoptosis by paramyxovirus simian virus 5 lacking a small hydrophobic gene

Simian virus 5 (SV5) is a member of the paramyxovirus family, which includes emerging viruses such as Hendra virus and Nipah virus as well as many important human and animal pathogens that have been known for years. SV5 encodes eight known viral proteins, including a small hydrophobic integral membrane protein (SH) of 44 amino acids. SV5 without the SH gene (rSV5deltaSH) is viable, and growth of rSV5deltaSH in tissue culture cells and viral protein and mRNA production in rSV5deltaSH-infected cells are indistinguishable from those of the wild-type SV5 virus. However, rSV5deltaSH causes increased cytopathic effect (CPE) and apoptosis in MDBK cells and is attenuated in vivo, suggesting the SH protein plays an important role in SV5 pathogenesis. How rSV5deltaSH induces apoptosis in infected cells has been examined in this report. Tumor necrosis factor alpha (TNF-alpha), a proinflammatory cytokine, was detected in culture media of rSV5deltaSH-infected cells. Apoptosis induced by rSV5deltaSH was inhibited by neutralizing antibodies against TNF-alpha and TNF-alpha receptor 1 (TNF-R1), suggesting that TNF-alpha played an essential role in rSV5deltaSH-induced apoptosis in a TNF-R1-dependent manner. Examination of important proteins in the TNF-alpha signaling pathway showed that p65, a major NF-kappaB subunit whose activation can lead to transcription of TNF-alpha, was first translocated to the nucleus and was capable of binding to DNA and then was targeted for degradation in rSV5deltaSH-infected cells while expression levels of TNF-R1 remained relatively constant. Thus, rSV5deltaSH induced cell death by activating TNF-alpha expression, possibly through activation of the NF-kappaB subunit p65 and then targeting p65 for degradation, leading to apoptosis.

Nrf2 is an inhibitor of the Fas pathway as identified by Achilles' Heel Method, a new function-based approach to gene identification in human cells

Here we describe the Achilles' Heel Method (AHM), a new function-based approach for identification of inhibitors of signaling pathways, optimized for human cells. The principle of AHM is the identification of 'sensitizing' cDNAs based on their decreased abundance following selection. As a proof of principle, we have employed AHM for the identification of Fas/CD95/APO-1 pathway inhibitors. HeLa cells were transfected with an antisense cDNA expression library in an episomal vector followed by selection with a suboptimal dose of the apoptotic inducer. Antisense inactivation of Fas inhibitors rendered the cells more sensitive to apoptosis resulting in their preferential death and consequent loss of their sensitizing episomes that were identified by subtraction. We show that the resulting products were enriched for sensitizing cDNAs as seven out of eight candidates tested were confirmed as inhibitors of Fas-induced killing either by transfection or by pharmacological inhibition. Furthermore, we demonstrate by multiple approaches that one candidate, NF-E2 related factor 2 (Nrf2), is an inhibitor of Fas-induced apoptosis. Inactivation of Nrf2 by antisense or by a membrane permeable dominant-negative polypeptide sensitized cells while overexpression of Nrf2 protected cells from Fas-induced apoptosis. In addition, dicumarol, an inhibitor of the phase II detoxifying enzyme NQO1, a downstream target of Nrf2, sensitized cells. Nrf2 induces the production of Glutathione (GSH) and we demonstrated that N-acetyl L-cysteine (NAC), a precursor to GSH, protected cells from Fas-mediated killing. Taken together, AHM is a powerful approach for the identification of inhibitors of a signaling pathway with a low rate of false positives that opens new avenues for function profiling of human genes and discovery of new drug targets.

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.

Antigen-dependent release of IFN-gamma by cytotoxic T cells up-regulates Fas on target cells and facilitates exocytosis-independent specific target cell lysis

Effector cytolytic T (Tc) lymphocytes, deficient in the exocytosis-mediated pathway of target cell lysis, induce Fas on target cells and, in turn, delayed cell death and apoptosis via the Fas ligand-Fas interaction. The induction of Fas can be blocked by anti- IFN-gamma Abs. This Fas up-regulation on initially Fas-negative target cells is not mediated by TCR-MHC/peptide signaling per se, but by secreted IFN-gamma from Tc cells after Ag engagement. The Fas up-regulation by Tc cells can be mimicked by treatment of target cells with rIFN-gamma. Tc cells from IFN-gamma knockout mice do not induce Fas expression on target cells. Tc cell-mediated Fas expression on third party, bystander, target cells does not enhance their susceptibility to lysis by these nominal effector cells. The results are discussed as to the possible relevance of the phenomenon in efficiency and regulation of the Tc cell response to infections by viruses.