Viral strategies for the evasion of immunogenic cell death

The Interplay Between Viral Infection and Cell Death: A Ping-Pong Effect

Programmed cell death (PCD) is a well-studied cellular mechanism that plays a critical role in immune responses, developmental processes, and the maintenance of tissue homeostasis. However, viruses have developed diverse strategies to bypass or manipulate the host apoptotic machinery to enhance their replication and survival. As a result, the interaction between PCD pathways and viruses has garnered increased interest, leading to many studies being published in recent years. This study aims to provide an overview of the current understanding of PCD pathways and their significance in viral infections. We will discuss various forms of cell death pathways, including apoptosis, autophagy, necroptosis, and pyroptosis, as well as their corresponding molecular mechanisms. In addition, we will show how viruses manipulate host PCD pathways to prevent or delay cell death or facilitate viral replication. This study emphasizes the importance of investigating the mechanisms by which viruses control the host's PCD machinery to gain insight into the evolutionary dynamics of host-pathogen interactions and to develop new approaches for predicting and managing viral threats. Overall, we aimed to highlight new research areas in PCD and viruses, including introduction of new targets for the development of new antiviral drugs to modulate the cellular apoptotic machinery and novel inhibitors of host cell death pathways.

MARCH8 Restricts RSV Replication by Promoting Cellular Apoptosis Through Ubiquitin-Mediated Proteolysis of Viral SH Protein

Numerous host factors function as intrinsic antiviral effectors to attenuate viral replication. MARCH8 is an E3 ubiquitin ligase that has been identified as a host restriction factor that inhibits the replication of various viruses. This study elucidated the mechanism by which MARCH8 restricts respiratory syncytial virus (RSV) replication through selective degradation of the viral small hydrophobic (SH) protein. We demonstrated that MARCH8 directly interacts with RSV-SH and catalyzes its ubiquitination at lysine 13, leading to SH degradation via the ubiquitin-lysosomal pathway. Functionally, MARCH8 expression enhances RSV-induced apoptosis through SH degradation, ultimately reducing viral titers. Conversely, an RSV strain harboring the SH-K13R mutation exhibited prolonged SH protein stability and attenuated apoptosis in infected cells, even in the presence of MARCH8. Targeted depletion of MARCH8 enhances cellular survival and potentially increases viral persistence. These findings demonstrate that MARCH8 promotes the early elimination of virus-infected cells by abrogating the anti-apoptotic function of SH, thereby reducing viral transmission. Our study provides novel insights into the interplay between host restriction factors and viral evasion strategies, potentially providing new therapeutic approaches for RSV infections.

Plant extracts modulate cellular stress to inhibit replication of mouse Coronavirus MHV-A59

The Covid-19 infection outbreak led to a global epidemic, and although several vaccines have been developed, the appearance of mutations has allowed the virus to evade the immune response. Added to this is the existing risk of the appearance of new emerging viruses. Therefore, it is necessary to explore novel antiviral therapies. Here, we investigate the potential in vitro of plant extracts to modulate cellular stress and inhibit murine hepatitis virus (MHV)-A59 replication. L929 cells were treated with P2Et (Caesalpinia spinosa) and Anamu SC (Petiveria alliacea) plant extracts during infection and virus production, ROS (reactive oxygen species), UPR (unfolded protein response), and autophagy were assessed. P2Et inhibited virus replication and attenuated both ROS production and UPR activation induced during infection. In contrast, the sustained presence of Anamu SC during viral adsorption and replication was required to inhibit viral infection, tending to induce pro-oxidant effects, and increasing UPR gene expression. Notably, the loss of the PERK protein resulted in a slight decrease in virus yield, suggesting a potential involvement of this UPR pathway during replication. Intriguingly, both extracts either maintained or increased the calreticulin surface exposure induced during infection. In conclusion, our findings highlight the development of antiviral natural plant extracts that differentially modulate cellular stress.

Sneaking into the viral safe-houses: Implications of host components in regulating integrity and dynamics of rotaviral replication factories

The biology of the viral life cycle essentially includes two structural and functional entities—the viral genome and protein machinery constituting the viral arsenal and an array of host cellular components which the virus closely associates with—to ensure successful perpetuation. The obligatory requirements of the virus to selectively evade specific host cellular factors while exploiting certain others have been immensely important to provide the platform for designing host-directed antiviral therapeutics. Although the spectrum of host-virus interaction is multifaceted, host factors that particularly influence viral replication have immense therapeutic importance. During lytic proliferation, viruses usually form replication factories which are specialized subcellular structures made up of viral proteins and replicating nucleic acids. These viral niches remain distinct from the rest of the cellular milieu, but they effectively allow spatial proximity to selective host determinants. Here, we will focus on the interaction between the replication compartments of a double stranded RNA virus rotavirus (RV) and the host cellular determinants of infection. RV, a diarrheagenic virus infecting young animals and children, forms replication bodies termed viroplasms within the host cell cytoplasm. Importantly, viroplasms also serve as the site for transcription and early morphogenesis of RVs and are very dynamic in nature. Despite advances in the understanding of RV components that constitute the viroplasmic architecture, knowledge of the contribution of host determinants to viroplasm dynamicity has remained limited. Emerging evidence suggests that selective host determinants are sequestered inside or translocated adjacent to the RV viroplasms. Functional implications of such host cellular reprogramming are also ramifying—disarming the antiviral host determinants and usurping the pro-viral components to facilitate specific stages of the viral life cycle. Here, we will provide a critical update on the wide variety of host cellular pathways that have been reported to regulate the spatial and temporal dynamicity of RV viroplasms. We will also discuss the methods used so far to study the host-viroplasm interactions and emphasize on the potential host factors which can be targeted for therapeutic intervention in the future.

Induction of cell death in ovarian cancer cells by doxorubicin and oncolytic vaccinia virus is associated with CREB3L1 activation

We have demonstrated that oncolytic vaccinia virus synergizes with doxorubicin (DOX) in inducing immunogenic cell death in platinum-resistant ovarian cancer cells and increases survival in syngeneic and xenograft tumor models. However, the mechanisms underlying the virus- and doxorubicin-mediated cancer cell death remain unknown. In this study, we investigated the effect of the oncolytic virus and doxorubicin used alone or in combination on activation of the cytoplasmic transcription factor CREB3L1 (cyclic AMP [cAMP] response element-binding protein 3-like 1) in ovarian cancer cell lines and clinical specimens. We demonstrated that doxorubicin-mediated cell death in ovarian cancer cell lines was associated with nuclear translocation of CREB3L1 and that the effect was augmented by infection with oncolytic vaccinia virus or treatment with recombinant interferon (IFN)-β used as a viral surrogate. This combination treatment was also effective in mediating nuclear translocation of CREB3L1 in cancer cells isolated from ovarian tumor biopsies at different stages of disease progression. The measurement of CREB3L1 expression in clinical specimens of ovarian cancer revealed lack of correlation with the stage of disease progression, suggesting that understanding the mechanisms of nuclear accumulation of CREB3L1 after doxorubicin treatment alone or in combination with oncolytic virotherapy may lead to the development of more effective treatment strategies against ovarian cancer.

PCV2 Triggers PK-15 Cell Apoptosis Through the PLC-IP3R-Ca2+ Signaling Pathway

The endoplasmic reticulum (ER) plays an essential role in Ca²⁺ concentration balance and protein biosynthesis. During infection, the virus needs to complete its life process with the help of ER. At the same time, ER also produces ER stress (ERS), which induces apoptosis to resist virus infection. Our study explored the Ca²⁺ concentration, ERS, and the apoptosis mechanism after porcine circovirus 2 (PCV2) infection. We show here that PCV2 infection induces the increased cytoplasmic Ca²⁺ level and PK-15 cell ER swelling. The colocalization of phospholipase C (PLC) and inositol 1,4,5-trisphosphate receptor (IP3R) in the cytoplasm was observed by laser confocal microscopy. Western blot and quantitative polymerase chain reaction experiments confirmed that PLC and IP3R expression levels increased after PCV2 infection, and Ca²⁺ concentration in the cytoplasm increased after virus infection. These results suggest that PCV2 infection triggers ERS of PK-15 cells via the PLC–IP3R–Ca²⁺ signaling pathway to promote the release of intracellular Ca²⁺ and led to cell apoptosis.

Immunogenic Cell Death (ICD) of Murine H22 Cells Induced by Lentinan

Lentinan can lead to apoptosis of tumor cells and improve immune function. However, limited research focused on the immunogenic death regulation mechanism of lentinan on mouse H22 cells. The study aimed to explore the effect of Lentinan on the expression of immunogenic death-related proteins in mice H22 cells. MTT method was used to detect and evaluate the effect of 200-1000 μg/mL lentinan on the survival rate of H22 cells after 24 h, 48 h, and 72 h, respectively. Flow cytometry was employed to collect the apoptotic rate of lentinan at different concentrations (200-800μg/mL) on H22 cells for 48 h, and obtain the apoptotic rate of 600 μg/mL lentinan at different times (12-72 h). The effect of Lentinan on the expression of H22 Immunogenic Cell Death proteins was analyzed by ELISA and HPLC-MS afterward. Results suggest that lentinan cytotoxic and pro-apoptotic have a concentration-dependent manner with the H22 cells. Moreover, the rate of apoptosis increased significantly (P < 0.05) in 24 h. Lentinan can induce the expression of Calreticulin(CRT), High mobility protein 1(HMGB1), ATP and Heat shock protein 70 (HSP70) .Therefore, the antitumor effect of lentinan may be related to the regulation of immunogenic death-related protein expression, which was beneficial to the future development of liver cancer vaccines.

Bta-miR-98 Suppresses Replication of Caprine Parainfluenza Virus Type 3 Through Inhibiting Apoptosis by Targeting Caspase-3

Caprine parainfluenza virus type 3 (CPIV3) is an emerging respiratory pathogen that affects the sheep and goat industry in China and possibly other countries around the world. Accumulating evidence suggests that microRNAs play important roles in regulating virus-host interactions and can suppress or facilitate viral replication. In this study, we showed that CPIV3 infection induced apoptosis in Madin-Darby bovine kidney (MDBK) cells, as determined by morphological changes and flow cytometry. Caspase activity and the expression of pro-apoptotic genes further indicated that CPIV3 induced apoptosis by activating both the intrinsic and extrinsic pathways. We also demonstrated the involvement of bta-microRNA-98 (bta-miR-98) in regulating CPIV3-induced apoptosis. Bta-miR-98 was downregulated in MDBK cells infected with CPIV3. Overexpression of bta-miR-98 significantly decreased the activities of caspase-3, -8, and -9. Conversely, inhibition of bta-miR-98 had completely opposite effects. Furthermore, our data showed that bta-miR-98 markedly affected CPIV3 replication by regulating apoptosis. Importantly, we found that bta-miR-98 modulated CPIV3-induced apoptosis by targeting caspase-3, an effector of apoptosis. Collectively, our results may suggest that CPIV3 infection induced apoptosis and downregulated the levels of bta-miR-98, and this miRNA regulated viral replication through effected apoptosis. This study contributes to our understanding of the molecular mechanisms underlying CPIV3 pathogenesis.

Current Approaches for Combination Therapy of Cancer: The Role of Immunogenic Cell Death

Cell death resistance is a key feature of tumor cells. One of the main anticancer therapies is increasing the susceptibility of cells to death. Cancer cells have developed a capability of tumor immune escape. Hence, restoring the immunogenicity of cancer cells can be suggested as an effective approach against cancer. Accumulating evidence proposes that several anticancer agents provoke the release of danger-associated molecular patterns (DAMPs) that are determinants of immunogenicity and stimulate immunogenic cell death (ICD). It has been suggested that ICD inducers are two different types according to their various activities. Here, we review the well-characterized DAMPs and focus on the different types of ICD inducers and recent combination therapies that can augment the immunogenicity of cancer cells.

Characterization of virus-mediated immunogenic cancer cell death and the consequences for oncolytic virus-based immunotherapy of cancer

Oncolytic viruses have the potential to induce immunogenic cell death (ICD) that may provoke potent and long-lasting anti-cancer immunity. Here we aimed to characterize the ICD-inducing ability of wild-type Adenovirus (Ad), Semliki Forest virus (SFV) and Vaccinia virus (VV). We did so by investigating the cell death and immune-activating properties of virus-killed tumor cells. Ad-infection of tumor cells primarily activates autophagy, but also activate events of necroptotic and pyroptotic cell death. SFV infection on the other hand primarily activates immunogenic apoptosis while VV activates necroptosis. All viruses mediated lysis of tumor cells leading to the release of danger-associated molecular patterns, triggering of phagocytosis and maturation of dendritic cells (DCs). However, only SFV-infected tumor cells triggered significant T helper type 1 (Th1)-cytokine release by DCs and induced antigen-specific T-cell activation. Our results elucidate cell death processes activated upon Ad, SFV, and VV infection and their potential to induce T cell-mediated anti-tumor immune responses. This knowledge provides important insight for the choice and design of therapeutically successful virus-based immunotherapies.

Modulation of the extrinsic cell death signaling pathway by viral Flip induces acute-death mediated liver failure

During viral infections viruses express molecules that interfere with the host-cell death machinery and thus inhibit cell death responses. For example the viral FLIP (vFLIP) encoded by Kaposi’s sarcoma-associated herpesvirus interacts and inhibits the central cell death effector, Caspase-8. In order to analyze the impact of anti-apoptotic viral proteins, like vFlip, on liver physiology in vivo, mice expressing vFlip constitutively in hepatocytes (vFlip^AlbCre+) were generated. Transgenic expression of vFlip caused severe liver tissue injury accompanied by massive hepatocellular necrosis and inflammation that finally culminated in early postnatal death of mice. On a molecular level, hepatocellular death was mediated by RIPK1-MLKL necroptosis driven by an autocrine TNF production. The loss of hepatocytes was accompanied by impaired bile acid production and disruption of the bile duct structure with impact on the liver-gut axis. Notably, embryonic development and tissue homeostasis were unaffected by vFlip expression. In summary our data uncovered that transgenic expression of vFlip can cause severe liver injury in mice, culminating in multiple organ insufficiency and death. These results demonstrate that viral cell death regulatory molecules exhibit different facets of activities beyond the inhibition of cell death that may merit more sophisticated in vitro and in vivo analysis.

Orthohantaviruses belonging to three phylogroups all inhibit apoptosis in infected target cells

Orthohantaviruses, previously known as hantaviruses, are zoonotic viruses that can cause hantavirus pulmonary syndrome (HPS) and hemorrhagic fever with renal syndrome (HFRS) in humans. The HPS-causing Andes virus (ANDV) and the HFRS-causing Hantaan virus (HTNV) have anti-apoptotic effects. To investigate if this represents a general feature of orthohantaviruses, we analysed the capacity of six different orthohantaviruses – belonging to three distinct phylogroups and representing both pathogenic and non-pathogenic viruses – to inhibit apoptosis in infected cells. Primary human endothelial cells were infected with ANDV, HTNV, the HFRS-causing Puumala virus (PUUV) and Seoul virus, as well as the putative non-pathogenic Prospect Hill virus and Tula virus. Infected cells were then exposed to the apoptosis-inducing chemical staurosporine or to activated human NK cells exhibiting a high cytotoxic potential. Strikingly, all orthohantaviruses inhibited apoptosis in both settings. Moreover, we show that the nucleocapsid (N) protein from all examined orthohantaviruses are potential targets for caspase-3 and granzyme B. Recombinant N protein from ANDV, PUUV and the HFRS-causing Dobrava virus strongly inhibited granzyme B activity and also, to certain extent, caspase-3 activity. Taken together, this study demonstrates that six different orthohantaviruses inhibit apoptosis, suggesting this to be a general feature of orthohantaviruses likely serving as a mechanism of viral immune evasion.

Chronic intestinal inflammation in mice expressing viral Flip in epithelial cells

Viruses are present in the intestinal microflora and are currently discussed as a potential causative mechanism for the development of inflammatory bowel disease. A number of viruses, such as Human Herpesvirus-8, express homologs to cellular FLIPs, which are major contributors for the regulation of epithelial cell death. In this study we analyzed the consequences of constitutive expression of HHV8-viral FLIP in intestinal epithelial cells (IECs) in mice. Surprisingly, expression of vFlip disrupts tissue homeostasis and induces severe intestinal inflammation. Moreover vFlip^IEC-tg mice showed reduced Paneth cell numbers, associated with excessive necrotic cell death. On a molecular level vFlip expression altered classical and alternative NFκB activation. Blocking of alternative NFκB signaling by deletion of Ikka in vivo largely protected mice from inflammation and Paneth cell loss induced by vFLIP. Collectively, our data provide functional evidence that expression of a single viral protein in IECs can be sufficient to disrupt epithelial homeostasis and to initiate chronic intestinal inflammation.

Critical Interactions between Immunogenic Cancer Cell Death, Oncolytic Viruses, and the Immune System Define the Rational Design of Combination Immunotherapies

Oncolytic viruses (OVs) are multimodal cancer therapeutics, with one of their dominant mechanisms being in situ vaccination. There is a growing consensus that optimal cancer therapies should generate robust tumor-specific immune responses. Immunogenic cell death (ICD) is a paradigm of cellular demise culminating in the spatiotemporal release of danger-associated molecular patterns that induce potent anticancer immunity. Alongside traditional ICD inducers like anthracycline chemotherapeutics and radiation, OVs have emerged as novel members of this class of therapeutics. OVs replicate in cancers and release tumor Ags, which are perceived as dangerous because of simultaneous expression of pathogen-associated molecular patterns that activate APCs. Therefore, OVs provide the target Ags and danger signals required to induce adaptive immune responses. This review discusses why OVs are attractive candidates for generating ICD, biological barriers limiting their success in the clinic, and groundbreaking strategies to potentiate ICD and antitumor immunity with rationally designed OV-based combination therapies.

Species-independent contribution of ZBP1/DAI/DLM-1-triggered necroptosis in host defense against HSV1

Necroptosis complements apoptosis as a host defense pathway to stop virus infection. Herpes simplex virus shows a propensity to trigger necroptosis of mouse cells and mice even though cell death is blocked in human cells through UL39-encoded ICP6. This ribonucleotide reductase large subunit (R1) nucleates RHIM-dependent oligomerization of RIP3 kinase (RIPK3, also known as RIP3) in mouse cells but inhibits activation in cells from the natural human host. By interrogating the comparative behavior of ICP6-deficient viruses in mouse and human cells, here we unveil virus-induced necroptosis mediated by Z-DNA-binding protein 1 (ZBP1, also known as DAI). ZBP1 acts as a pathogen sensor to detect nascent RNA transcripts rather than input viral DNA or viral DNA generated through replication. Consistent with the implicated role of virus-induced necroptosis in restricting infection, viral pathogenesis is restored in Zbp1^−/−, Ripk3^−/− and Mlkl^−/− mice. Thus, in addition to direct activation of RIPK3 via ICP6, HSV1 infection in mice and mouse cells triggers virus-induced necroptosis through ZBP1. Importantly, virus-induced necroptosis is also induced in human HT-29 cells by ICP6 mutant viruses; however, ZBP1 levels must be elevated for this pathway to be active. Thus, our studies reveal a common, species-independent role of this nucleic acid sensor to detect the presence of this virus. HSV1 ICP6 functions as a bona fide RHIM signaling inhibitor to block virus-induced necroptosis in its natural host. Altogether, ZBP1-dependent restriction of herpesvirus infection emerges as a potent antiviral armament of the innate immune system.

Subverting Host Cell P21-Activated Kinase: A Case of Convergent Evolution across Pathogens

Intracellular pathogens have evolved a wide range of strategies to not only escape from the immune systems of their hosts, but also to directly exploit a variety of host factors to facilitate the infection process. One such strategy is to subvert host cell signalling pathways to the advantage of the pathogen. Recent research has highlighted that the human serine/threonine kinase PAK, or p21-activated kinase, is a central component of host-pathogen interactions in many infection systems involving viruses, bacteria, and eukaryotic pathogens. PAK paralogues are found in most mammalian tissues, where they play vital roles in a wide range of functions. The role of PAKs in cell proliferation and survival, and their involvement in a number of cancers, is of great interest in the context of drug discovery. In this review we discuss the latest insights into the surprisingly central role human PAK1 plays for the infection by such different infectious disease agents as viruses, bacteria, and parasitic protists. It is our intention to open serious discussion on the applicability of PAK inhibitors for the treatment, not only of neoplastic diseases, which is currently the primary objective of drug discovery research targeting these enzymes, but also of a wide range of infectious diseases.

Immunogenic cell death in cancer and infectious disease

Immunogenicity depends on two key factors: antigenicity and adjuvanticity. The presence of exogenous or mutated antigens explains why infected cells and malignant cells can initiate an adaptive immune response provided that the cells also emit adjuvant signals as a consequence of cellular stress and death. Several infectious pathogens have devised strategies to control cell death and limit the emission of danger signals from dying cells, thereby avoiding immune recognition. Similarly, cancer cells often escape immunosurveillance owing to defects in the molecular machinery that underlies the release of endogenous adjuvants. Here, we review current knowledge on the mechanisms that underlie the activation of immune responses against dying cells and their pathophysiological relevance.

Contribution of RIP3 and MLKL to immunogenic cell death signaling in cancer chemotherapy

Chemotherapy can reinstate anticancer immunosurveillance through inducing tumor immunogenic cell death (ICD). Here, we show that anthracyclines and oxaliplatin can trigger necroptosis in murine cancer cell lines expressing receptor-interacting serine-threonine kinase 3 (RIP3) and mixed lineage kinase domain-like (MLKL). Necroptotic cells featured biochemical hallmarks of ICD and stimulated anticancer immune responses in vivo. Chemotherapy normally killed Rip3 (-/-) and Mlkl (-/-) tumor cells and normally induced caspase-3 activation in such cells, yet was unable to reduce their growth in vivo. RIP3 or MLKL deficiency abolished the capacity of dying cancer cells to elicit an immune response. This could be attributed to reduced release of ATP and high mobility group box 1 (HMGB1) by RIP3 and MLKL-deficient cells. Measures designed to compensate for deficient ATP and HMGB1 signaling restored the chemotherapeutic response of Rip3 (-/-) and Mlkl (-/-) cancers. Altogether, these results suggest that RIP3 and MLKL can contribute to ICD signaling and tumor immunogenicity.

eIF2α phosphorylation as a biomarker of immunogenic cell death

Cancer cells exposed to some forms of chemotherapy and radiotherapy die while eliciting an adaptive immune response. Such a functionally peculiar variant of apoptosis has been dubbed immunogenic cell death (ICD). One of the central events in the course of ICD is the activation of an endoplasmic reticulum (ER) stress response. This is instrumental for cells undergoing ICD to emit all the signals that are required for their demise to be perceived as immunogenic by the host, and culminates with the phosphorylation of eukaryotic translation initiation factor 2α (eIF2α). In particular, eIF2α phosphorylation is required for the pre-apoptotic exposure of the ER chaperone calreticulin (CALR) on the cell surface, which is a central determinant of ICD. Importantly, phosphorylated eIF2α can be quantified in both preclinical and clinical samples by immunoblotting or immunohistochemistry using phosphoneoepitope-specific monoclonal antibodies. Of note, the phosphorylation of eIF2α and CALR exposure do not necessarily correlate with each other, and neither of these parameters is sufficient for cell death to be perceived as immunogenic. Nonetheless, accumulating data indicate that assessing the degree of phosphorylation of eIF2α provides a convenient parameter to monitor ICD. Here, we discuss the role of the ER stress response in ICD and the potential value of eIF2α phosphorylation as a biomarker for this clinically relevant variant of apoptosis.

Emerging Roles for RIPK1 and RIPK3 in Pathogen-Induced Cell Death and Host Immunity

Receptor-interacting protein kinases 1 and 3 (RIPK1 and RIPK3 ) are homologous serine-threonine kinases that were recognized for their roles in directing programmed necrotic cell death or necroptosis under a broad range of pathologic settings. Emerging evidence suggests new physiologic roles for RIPK1 and RIPK3 in mediating cell death of innate immune responses. Our review discusses current evidence on the mechanisms and the impact of RIPK1- and/or RIPK3-dependent cell death in responses to a variety of viral and bacterial pathogens. Furthermore, the discussion also summarizes emerging roles for RIPK1 and RIPK3 in other facets of host immunity, including the maintenance of epithelial barrier function and pro-inflammatory processes that may, in some cases, manifest independent of cell death. Finally, we briefly consider the therapeutic opportunities in targeting RIPK1- and RIPK3-dependent processes in infection and immunity.

Avian reovirus-triggered apoptosis enhances both virus spread and the processing of the viral nonstructural muNS protein

Avian reovirus non-structural protein muNS is partially cleaved in infected chicken embryo fibroblast cells to produce a 55-kDa carboxyterminal protein, termed muNSC, and a 17-kDa aminoterminal polypeptide, designated muNSN. In this study we demonstrate that muNS processing is catalyzed by a caspase 3-like protease activated during the course of avian reovirus infection. The cleavage site was mapped by site directed mutagenesis between residues Asp-154 and Ala-155 of the muNS sequence. Although muNS and muNSC, but not muNSN, are able to form inclusions when expressed individually in transfected cells, only muNS is able to recruit specific ARV proteins to these structures. Furthermore, muNSC associates with ARV factories more weakly than muNS, sigmaNS and lambdaA. Finally, the inhibition of caspase activity in ARV-infected cells does not diminish ARV gene expression and replication, but drastically reduces muNS processing and the release and dissemination of progeny viral particles.

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Avian reovirus-triggered apoptosis promotes cleavage of the viral nonstructural muNS protein.

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muNS processing is catalyzed by a caspase 3-like protease activated during avian reovirus infection.

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Cleavage occurs between residues Asp-154 and Ala-155 of the muNS sequence.

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Avian reovirus-induced apoptosis enhances the release and dissemination of progeny viral particles.

Hijacking of host calreticulin is required for the white spot syndrome virus replication cycle

We have previously shown that multifunctional calreticulin (CRT), which resides in the endoplasmic reticulum (ER) and is involved in ER-associated protein processing, responds to infection with white spot syndrome virus (WSSV) by increasing mRNA and protein expression and by forming a complex with gC1qR and thereby delaying apoptosis. Here, we show that CRT can directly interact with WSSV structural proteins, including VP15 and VP28, during an early stage of virus infection. The binding of VP28 with CRT does not promote WSSV entry, and CRT-VP15 interaction was detected in the viral genome in virally infected host cells and thus may have an effect on WSSV replication. Moreover, CRT was detected in the viral envelope of purified WSSV virions. CRT was also found to be of high importance for proper oligomerization of the viral structural proteins VP26 and VP28, and when CRT glycosylation was blocked with tunicamycin, a significant decrease in both viral replication and assembly was detected. Together, these findings suggest that CRT confers several advantages to WSSV, from the initial steps of WSSV infection to the assembly of virions. Therefore, CRT is required as a "vital factor" and is hijacked by WSSV for its replication cycle. Importance: White spot syndrome virus (WSSV) is a double-stranded DNA virus and the cause of a serious disease in a wide range of crustaceans that often leads to high mortality rates. We have previously shown that the protein calreticulin (CRT), which resides in the endoplasmic reticulum (ER) of the cell, is important in the host response to the virus. In this report, we show that the virus uses this host protein to enter the cell and to make the host produce new viral structural proteins. Through its interaction with two viral proteins, the virus "hijacks" host calreticulin and uses it for its own needs. These findings provide new insight into the interaction between a large DNA virus and the host protein CRT and may help in understanding the viral infection process in general.

Role of type I interferons in inflammasome activation, cell death, and disease during microbial infection

Interferons (IFNs) were discovered over a half-century ago as antiviral factors. The role of type I IFNs has been studied in the pathogenesis of both acute and chronic microbial infections. Deregulated type I IFN production results in a damaging cascade of cell death, inflammation, and immunological host responses that can lead to tissue injury and disease progression. Here, we summarize the role of type I IFNs in the regulation of cell death and disease during different microbial infections, ranging from viruses and bacteria to fungal pathogens. Understanding the specific mechanisms driving type I IFN-mediated cell death and disease could aid in the development of targeted therapies.

Vaccinia virus induces programmed necrosis in ovarian cancer cells

The mechanisms by which oncolytic vaccinia virus induces tumor cell death are poorly understood. We have evaluated cell death pathways following infection of ovarian cancer cells with both wild-type and thymidine kinase-deleted (dTK) Lister strain vaccinia. We show that death does not rely upon classical apoptosis despite the appearances of some limited apoptotic features, including phosphatidylserine externalization and appearance of sub-G1 DNA populations. Vaccinia infection induces marked lipidation of LC3 proteins, but there is no general activation of the autophagic process and cell death does not rely upon autophagy induction. We show that vaccinia induces necrotic morphology on transmission electron microscopy, accompanied by marked by reductions in intracellular adenosine triphosphate, altered mitochondrial metabolism, and release of high mobility group box 1 (HMGB1) protein. This necrotic cell death appears regulated, as infection induces formation of a receptor interacting protein (RIP1)/caspase-8 complex. In addition, pharmacological inhibition of both RIP1 and substrates downstream of RIP1, including MLKL, significantly attenuate cell death. Blockade of TNF-α, however, does not alter virus efficacy, suggesting that necrosis does not result from autocrine cytokine release. Overall, these results show that, in ovarian cancer cells, vaccinia virus causes necrotic cell death that is mediated through a programmed series of events.

Oncolytic virotherapy and immunogenic cancer cell death: sharpening the sword for improved cancer treatment strategies

Oncolytic viruses are novel immunotherapeutics with increasingly promising outcomes in cancer patient clinical trials. Preclinical and clinical studies have uncovered the importance of virus-induced activation of antitumor immune responses for optimal therapeutic efficacy. Recently, several classes of chemotherapeutics have been shown to cause immunogenic cancer cell death characterized by the release of immunomodulatory molecules that activate antigen-presenting cells and thus trigger the induction of more potent anticancer adaptive immune responses. In preclinical models, several oncolytic viruses induce immunogenic cell death, which is associated with increased cross-priming of tumor-associated antigens. In this review, we discuss the recent advances in immunogenic cancer cell death as induced by chemotherapeutic treatments, including the roles of relevant danger-associated molecular patterns and signaling pathways, and highlighting the significance of the endoplasmic reticulum (ER) stress response. As virtually all viruses modulate both ER stress and cell death responses, we provide perspectives on future research directions that can be explored to optimize oncolytic viruses, alone or in combination with targeted drug therapies, as potent immunogenic cancer cell death-inducing agents. We propose that such optimized virus-drug synergistic strategies will improve the therapeutic outcomes for many currently intractable cancers.

Role of autophagy in aging

Constitutive autophagy is important for the control of the quality of proteins and organelles to maintain cell function. Damaged proteins and organelles accumulate in aged organs. The level of autophagic activity decreases with aging. Autophagic activity is regulated by many factors, such as the insulin receptor-signaling pathway, the TOR pathway, Sirt1, and caloric restriction. Autophagy-related genes are known to be essential for the lifespan extension of flies, nematodes, and mice. The inhibition of autophagy decreases the lifespan, and on the other hand, the induction of autophagy can prolong the lifespan. Pharmacological intervention to extend the lifespan has demonstrated a crucial role for autophagy. Heart failure is an age-related disease, as the incidence increases with age. The autophagic activity of the heart decreases during aging. Cardiac-specific autophagy-deficient mice have shown no obvious phenotype up to 10 weeks of age. However, these mice began to die after the age of 6 months, with a significant increase in the left ventricular dimensions and a decrease in the fractional shortening of the left ventricle compared with control mice. This indicates that continuous constitutive autophagy during aging has a crucial role in maintaining cardiac structure and function.

Inflammasome-mediated pyroptotic and apoptotic cell death, and defense against infection

Cell death is an effective strategy to limit intracellular infections. Canonical inflammasomes, including NLRP3, NLRC4, and AIM2, recruit and activate caspase-1 in response to a range of microbial stimuli and endogenous danger signals. Caspase-1 then promotes the secretion of IL-1β and IL-18 and a rapid form of lytic programmed cell death termed pyroptosis. A second inflammatory caspase, mouse caspase-11, mediates pyroptotic death through an unknown non-canonical inflammasome system in response to cytosolic bacteria. In addition, recent work shows that inflammasomes can also recruit procaspase-8, initiating apoptosis. The induction of multiple pathways of cell death has probably evolved to counteract microbial evasion of cell death pathways.

A calreticulin/gC1qR complex prevents cells from dying: a conserved mechanism from arthropods to humans

The crossroad between cell death and proliferation is a general target for viral infections because viruses need to obstruct apoptosis to use cells for their own replication. Inducing immunogenic cell death in proliferating cells is also an important aim of anticancer chemotherapy. The C1q-binding proteins calreticulin (CRT) and gC1qR are highly conserved ubiquitous proteins, which are putative targets for viral manipulation and are associated with cancer. Here we show that these proteins form a complex in the cytoplasm as a response to viral infection resulting in apoptosis prevention. The formation of a cytosolic CRT/gC1qR complex prevents cell death by reducing gC1qR translocation into the mitochondria, and we provide evidence that this mechanism is conserved from arthropods to human cancer cells. Furthermore, we show that it is possible to prevent this complex from being formed in cancer cells. When the peptides of the complex proteins are overexpressed in these cells, the cells undergo apoptosis. This finding shows a causal link between virus and cancer and may be used to develop new tools in anticancer or antiviral therapy.

Critical roles of glucocorticoid-induced leucine zipper in infectious bursal disease virus (IBDV)-induced suppression of type I Interferon expression and enhancement of IBDV growth in host cells via interaction with VP4

Infectious bursal disease (IBD) is an acute, highly contagious, and immunosuppressive avian disease caused by IBD virus (IBDV). Although IBDV-induced immunosuppression has been well established, the underlying exact molecular mechanism for such induction is not very clear. We report here the identification of IBDV VP4 as an interferon suppressor by interaction with the glucocorticoid-induced leucine zipper (GILZ) in host cells. We found that VP4 suppressed the expression of type I interferon in HEK293T cells after tumor necrosis factor alpha (TNF-α) treatment or Sendai virus (SeV) infection and in DF-1 cells after poly(I·C) stimulation. In addition, the VP4-induced suppression of type I interferon could be completely abolished by knockdown of GILZ by small interfering RNA (siRNA). Furthermore, knockdown of GILZ significantly inhibited IBDV growth in host cells, and this inhibition could be markedly mitigated by anti-alpha/beta interferon antibodies in the cell cultures (P < 0.001). Thus, VP4-induced suppression of type I interferon is mediated by interaction with GILZ, a protein that appears to inhibit cell responses to viral infection.

Identification and functional analysis of the novel ORF4 protein encoded by porcine circovirus type 2

Porcine circovirus type 2 (PCV2) is the primary causative agent of porcine circovirus-associated diseases in pigs. To date, viral proteins Cap, Rep, Rep', and ORF3, encoded by the PCV2 genome, have been described. Here, transcription and translation of a novel viral gene within the PCV2 genome (designated ORF4) was determined and functionally analyzed in vitro and in vivo. Northern blot analysis indicated that the RNA transcribed from the ORF4 gene is about 180 bp in length and overlaps ORF3 in the same direction. Site-directed mutagenesis confirmed that the viral ORF4 protein is not essential for virus replication in PK-15 cells and in mice infected with an ORF4-deficient PCV2 (PCV2Δ). PCV2Δ triggered higher activity levels of caspase-3 and -8 than wild-type PCV2 (wPCV2) in PK-15 cells. The antigenic epitopes of two mouse monoclonal antibodies (MAbs) raised against the viral ORF4 protein were mapped to the same 19KSSASPR25 peptide. Expression of ORF4 was confirmed using the specific MAbs in wPCV2-infected PK-15 cells and mice. Mice infected with PCV2Δ had a higher serum viral load (genomic copies) and more severe lymphoid tissue damage in the spleen than those infected with wPCV2. Meanwhile, flow-cytometric analysis indicated that the PCV2Δ infection caused a significant decrease of CD4(+) and CD8(+) T lymphocytes. Our results demonstrate that ORF4 is a newly discovered viral protein that is not essential for PCV2 replication but plays a role in suppressing caspase activity and regulating CD4(+) and CD8(+) T lymphocytes during PCV2 infection.

Infectomics Screening for Novel Antiviral Drug Targets

Preclinical Research

Copyright 2012 Wiley-Liss, Inc., A Wiley Company

Infectomics, a novel way to globally and comprehensively understand the interactions between microbial pathogens and their hosts, has significantly expanded understanding of the microbial infections. The infectomics view of viral–host interactions on the viral perspective principally focuses on gene acquisition, deletion, and point mutation, while traditional antiviral drug discovery concentrates on viral encoding proteins. Recently, high‐throughput technologies, such as mass spectrometry‐based proteomics, activity‐based protein profiling, microarray analysis, yeast two‐hybrid assay, small interfering RNA screening, and micro RNA profiling, have been gradually employed in the research of virus–host interactions. Besides, signaling pathways and cellular processes involved in viral–host interactions provide new insights of infectomics in antiviral drug discovery. In this review, we summarize related infectomics approaches in the studies of virus–host interactions, which shed light on the development of novel antiviral drug targets screening.

Untangling the Roles of Anti-Apoptosis in Regulating Programmed Cell Death using Humanized Yeast Cells

Genetically programmed cell death (PCD) mechanisms, including apoptosis, are important for the survival of metazoans since it allows, among things, the removal of damaged cells that interfere with normal function. Cell death due to PCD is observed in normal processes such as aging and in a number of pathophysiologies including hypoxia (common causes of heart attacks and strokes) and subsequent tissue reperfusion. Conversely, the loss of normal apoptotic responses is associated with the development of tumors. So far, limited success in preventing unwanted PCD has been reported with current therapeutic approaches despite the fact that inhibitors of key apoptotic inducers such as caspases have been developed. Alternative approaches have focused on mimicking anti-apoptotic processes observed in cells displaying increased resistance to apoptotic stimuli. Hormesis and pre-conditioning are commonly observed cellular strategies where sub-lethal levels of pro-apoptotic stimuli lead to increased resistance to higher or lethal levels of stress. Increased expression of anti-apoptotic sequences is a common mechanism mediating these protective effects. The relevance of the latter observation is exemplified by the observation that transgenic mice overexpressing anti-apoptotic genes show significant reductions in tissue damage following ischemia. Thus strategies aimed at increasing the levels of anti-apoptotic proteins, using gene therapy or cell penetrating recombinant proteins are being evaluated as novel therapeutics to decrease cell death following acute periods of cell death inducing stress. In spite of its functional and therapeutic importance, more is known regarding the processes involved in apoptosis than anti-apoptosis. The genetically tractable yeast Saccharomyces cerevisiae has emerged as an exceptional model to study multiple aspects of PCD including the mitochondrial mediated apoptosis observed in metazoans. To increase our knowledge of the process of anti-apoptosis, we screened a human heart cDNA expression library in yeast cells undergoing PCD due to the conditional expression of a mammalian pro-apoptotic Bax cDNA. Analysis of the multiple Bax suppressors identified revealed several previously known as well as a large number of clones representing potential novel anti-apoptotic sequences. The focus of this review is to report on recent achievements in the use of humanized yeast in genetic screens to identify novel stress-induced PCD suppressors, supporting the use of yeast as a unicellular model organism to elucidate anti-apoptotic and cell survival mechanisms.

Viral infection and the evolution of caspase 8-regulated apoptotic and necrotic death pathways

Pathogens specifically target both the caspase 8-dependent apoptotic cell death pathway and the necrotic cell death pathway that is dependent on receptor-interacting protein 1 (RIP1; also known as RIPK1) and RIP3 (also known as RIPK3). The fundamental co-regulation of these two cell death pathways emerged when the midgestational death of mice deficient in FAS-associated death domain protein (FADD) or caspase 8 was reversed by elimination of RIP1 or RIP3, indicating a far more entwined relationship than previously appreciated. Thus, mammals require caspase 8 activity during embryogenesis to suppress the kinases RIP1 and RIP3 as part of the dialogue between two distinct cell death processes that together fulfil reinforcing roles in the host defence against intracellular pathogens such as herpesviruses.

Advances in virus-like particle vaccines for filoviruses

Ebola virus (EBOV) and Marburg virus (MARV) are among the deadliest human pathogens, with no vaccines or therapeutics available. Multiple vaccine platforms have been tested for efficacy as prophylactic pretreatments or therapeutics for prevention of filovirus hemorrhagic fever. Most successful vaccines are based on a virus-vectored approach expressing the protective glycoprotein (GP); protein-based subunit and DNA vaccines have been tested with moderate success. Virus-like particle (VLP) vaccines have realized promising results when tested in both rodents and nonhuman primates. VLPs rely on the natural properties of the viral matrix protein (VP) 40 to drive budding of filamentous particles that can also incorporate ≥ 1 other filovirus protein, including GP, VP24, and nucleoprotein (NP). Filovirus VLP vaccines have used particles containing 2 or 3 (GP and VP40, with or without NP) viral proteins generated in either mammalian or insect cells. Early studies successfully demonstrated efficacy of bivalent VLP vaccines in rodents; more recent studies have shown the ability of the VLP vaccines containing GP, NP, and VP40 to confer complete homologous protection against Ebola virus and Marburg virus in a prophylactic setting against in macaques. This review will discuss published work to date regarding development of the VLP vaccines for prevention of lethal filovirus hemorrhagic fever.

Programmed necrosis: backup to and competitor with apoptosis in the immune system

Programmed cell death is essential for the development and maintenance of the immune system and its responses to exogenous and endogenous stimuli. Studies have demonstrated that in addition to caspase-dependent apoptosis, necrosis dependent on the kinases RIP1 and RIP3 (also called necroptosis) is a major programmed cell-death pathway in development and immunity. These two programmed cell-death pathways may suppress each other, and necroptosis also serves as an alternative when caspase-dependent apoptosis is inhibited or absent. Here we summarize recent advancements that have identified the molecular mechanisms that underlie necroptosis and explore the mechanisms that regulate the interplay between apoptosis and necroptosis.

A physical interaction network of dengue virus and human proteins

Dengue virus (DENV), an emerging mosquito-transmitted pathogen capable of causing severe disease in humans, interacts with host cell factors to create a more favorable environment for replication. However, few interactions between DENV and human proteins have been reported to date. To identify DENV-human protein interactions, we used high-throughput yeast two-hybrid assays to screen the 10 DENV proteins against a human liver activation domain library. From 45 DNA-binding domain clones containing either full-length viral genes or partially overlapping gene fragments, we identified 139 interactions between DENV and human proteins, the vast majority of which are novel. These interactions involved 105 human proteins, including six previously implicated in DENV infection and 45 linked to the replication of other viruses. Human proteins with functions related to the complement and coagulation cascade, the centrosome, and the cytoskeleton were enriched among the DENV interaction partners. To determine if the cellular proteins were required for DENV infection, we used small interfering RNAs to inhibit their expression. Six of 12 proteins targeted (CALR, DDX3X, ERC1, GOLGA2, TRIP11, and UBE2I) caused a significant decrease in the replication of a DENV replicon. We further showed that calreticulin colocalized with viral dsRNA and with the viral NS3 and NS5 proteins in DENV-infected cells, consistent with a direct role for calreticulin in DENV replication. Human proteins that interacted with DENV had significantly higher average degree and betweenness than expected by chance, which provides additional support for the hypothesis that viruses preferentially target cellular proteins that occupy central position in the human protein interaction network. This study provides a valuable starting point for additional investigations into the roles of human proteins in DENV infection.

Encephalomyocarditis virus 2A protein is required for viral pathogenesis and inhibition of apoptosis

The encephalomyocarditis virus (EMCV), a Picornaviridae virus, has a wide host spectrum and can cause various diseases. EMCV virulence factors, however, are as yet ill defined. Here, we demonstrate that the EMCV 2A protein is essential for the pathogenesis of EMCV. Infection of mice with the B279/95 strain of EMCV resulted in acute fatal disease, while the clone C9, derived by serial in vitro passage of the B279/95 strain, was avirulent. C9 harbored a large deletion in the gene encoding the 2A protein. This deletion was incorporated into the cDNA of a pathogenic EMCV1.26 strain. The new virus, EMCV1.26Δ2A, was capable of replicating in vitro, albeit more slowly than EMCV1.26. Only mice inoculated with EMCV1.26 triggered death within a few days. Mice infected with EMCV1.26Δ2A did not exhibit clinical signs, and histopathological analyses showed no damage in the central nervous system, unlike EMCV1.26-infected mice. In vitro, EMCV1.26Δ2A presented a defect in viral particle release correlating with prolonged cell viability. Unlike EMCV1.26, which induced cytopathic cell death, EMCV1.26Δ2A induced apoptosis via caspase 3 activation. This strongly suggests that the 2A protein is required for inhibition of apoptosis during EMCV infection. All together, our data indicate that the EMCV 2A protein is important for the virus in counteracting host defenses, since Δ2A viruses were no longer pathogenic and were unable to inhibit apoptosis in vitro.

The ribonucleotide reductase R1 subunits of herpes simplex virus types 1 and 2 protect cells against TNFα- and FasL-induced apoptosis by interacting with caspase-8

We previously reported that HSV-2 R1, the R1 subunit (ICP10; UL39) of herpes simplex virus type-2 ribonucleotide reductase, protects cells against apoptosis induced by the death receptor (DR) ligands tumor necrosis factor-alpha- (TNFα) and Fas ligand (FasL) by interrupting DR-mediated signaling at, or upstream of, caspase-8 activation. Further investigation of the molecular mechanism underlying HSV-2 R1 protection showed that extracellular-regulated kinase 1/2 (ERK1/2), phosphatidylinositol 3-kinase (PI3-K)/Akt, NF-κB and JNK survival pathways do not play a major role in this antiapoptotic function. Interaction studies revealed that HSV-2 R1 interacted constitutively with caspase-8. The HSV-2 R1 deletion mutant R1(1-834)-GFP and Epstein-Barr virus (EBV) R1, which did not protect against apoptosis induced by DR ligands, did not interact with caspase-8, indicating that interaction is required for protection. HSV-2 R1 impaired caspase-8 activation induced by caspase-8 over-expression, suggesting that interaction between the two proteins prevents caspase-8 dimerization/activation. HSV-2 R1 bound to caspase-8 directly through its prodomain but did not interact with either its caspase domain or Fas-associated death domain protein (FADD). Interaction between HSV-2 R1 and caspase-8 disrupted FADD-caspase-8 binding. We further demonstrated that individually expressed HSV-1 R1 (ICP6) shares, with HSV-2 R1, the ability to bind caspase-8 and to protect cells against DR-induced apoptosis. Finally, as the long-lived Fas protein remained stable during the early period of infection, experiments with the HSV-1 UL39 deletion mutant ICP6∆ showed that HSV-1 R1 could be essential for the protection of HSV-1-infected cells against FasL.

The ribonucleotide reductase R1 subunits of herpes simplex virus 1 and 2 protect cells against poly(I · C)-induced apoptosis

We recently provided evidence that the ribonucleotide reductase R1 subunits of herpes simplex virus types 1 and 2 (HSV-1 and -2) protect cells against tumor necrosis factor alpha- and Fas ligand-induced apoptosis by interacting with caspase 8. Double-stranded RNA (dsRNA) is a viral intermediate known to initiate innate antiviral responses. Poly(I · C), a synthetic analogue of viral dsRNA, rapidly triggers caspase 8 activation and apoptosis in HeLa cells. Here, we report that HeLa cells after HSV-1 and HSV-2 infection were quickly protected from apoptosis caused by either extracellular poly(I · C) combined with cycloheximide or transfected poly(I · C). Cells infected with the HSV-1 R1 deletion mutant ICP6Δ were killed by poly(I · C), indicating that HSV-1 R1 plays a key role in antiapoptotic responses to poly(I · C). Individually expressed HSV R1s counteracted caspase 8 activation by poly(I · C). In addition to their binding to caspase 8, HSV R1s also interacted constitutively with receptor-interacting protein 1 (RIP1) when expressed either individually or with other viral proteins during HSV infection. R1(1-834)-green fluorescent protein (GFP), an HSV-2 R1 deletion mutant protein devoid of antiapoptotic activity, did not interact with caspase 8 and RIP1, suggesting that these interactions are required for protection against poly(I · C). HSV-2 R1 inhibited the interaction between the Toll/interleukin-1 receptor domain-containing adaptor-inducing beta interferon (IFN-β) (TRIF) and RIP1, an interaction that is essential for apoptosis triggered by extracellular poly(I · C) plus cycloheximide or TRIF overexpression. TRIF silencing reduced poly(I · C)-triggered caspase 8 activation in mock- and ICP6Δ-infected cells, confirming that TRIF is involved in poly(I · C)-induced apoptosis. Thus, by interacting with caspase 8 and RIP1, HSV R1s impair the apoptotic host defense mechanism prompted by dsRNA.

Viral oncogenesis and its role in nonmelanoma skin cancer

In recent years, the contribution of viruses to cutaneous oncogenesis has steadily gained recognition. The archetype is human herpesvirus 8, which is well established as the causative agent in Kaposi sarcoma. Other viruses believed to play a role in nonmelanoma skin cancer include human papillomavirus and the recently described Merkel cell polyomavirus. We review the mechanisms by which these three viruses interact with the host cell, ultraviolet radiation and immunosuppression to result in carcinogenesis.

The story of DNase II: a stifled death-wish leads to self-harm

DNase II is an endonuclease which plays a fundamental role in the degradation of DNA from both apoptotic cells, and nuclei extruded from red blood cells during erythropoiesis: important tasks, considering that everyday 10(8)-10(9) cells undergo apoptosis, and 10(11) red blood cells are produced in the adult human. The DNase II-null mouse demonstrates embryonic lethality due to type I interferon-mediated erythroid precursor cell death triggered by undegraded nucleic acids. However, the mechanisms leading to such cytotoxicity are poorly understood. A study in the current issue of the European Journal of Immunology investigates the role of the death ligand TRAIL in this process. Although TRAIL is shown to be dispensable for the interferon-induced apoptosis of erythroid cells in DNAse II(-/-) embryos, the authors have developed a useful strategy for further exploring this question in future studies. Interestingly, earlier studies by the same group showed that crossing the DNase II-null mouse with a mouse deficient for the type I interferon receptor can rescue the lethal anaemia observed in the DNase II-null embryos, but only at the cost of developing autoimmunity.

Anti-apoptosis and cell survival: a review

Type I programmed cell death (PCD) or apoptosis is critical for cellular self-destruction for a variety of processes such as development or the prevention of oncogenic transformation. Alternative forms, including type II (autophagy) and type III (necrotic) represent the other major types of PCD that also serve to trigger cell death. PCD must be tightly controlled since disregulated cell death is involved in the development of a large number of different pathologies. To counter the multitude of processes that are capable of triggering death, cells have devised a large number of cellular processes that serve to prevent inappropriate or premature PCD. These cell survival strategies involve a myriad of coordinated and systematic physiological and genetic changes that serve to ward off death. Here we will discuss the different strategies that are used to prevent cell death and focus on illustrating that although anti-apoptosis and cellular survival serve to counteract PCD, they are nevertheless mechanistically distinct from the processes that regulate cell death.

To kill or be killed: how viruses interact with the cell death machinery

A virus (from the Latin virus meaning toxin or poison) is a small infectious agent that can only replicate inside the cells of another organism. Viruses are found wherever there is life and have probably existed since living cells first evolved. Viruses do not have their own metabolism and require a host cell to make new products. The range of structural and biochemical (i.e., cytopathic) effects that viruses have on the host cell is extensive. Most viral infections eventually result in the death of the host cell. The causes of death include cell lysis, alterations to the cell's surface membrane and various modes of programmed cell death. Some viruses cause no apparent changes to the infected cell. Cells in which the virus is latent and inactive show few signs of infection and often function normally. This causes persistent infection and the virus is often dormant for many months or years. Some viruses can cause cells to proliferate without causing malignancy, whereas others are established causes of cancer. Human organisms use a genetically controlled cell death programme that prevents the spreading of viral infection and kills the virus. Between 19 and 21 November 2009, with sponsorship from the Journal of Internal Medicine, the Swedish Research Foundation and the Swedish Cancer Society hosted a conference in Stockholm entitled: 'To kill or to be killed. Viral evasion strategies and interference with cell death machinery'. Four comprehensive reviews from this conference are presented in this issue of the Journal of Internal Medicine. These reviews include descriptions of: the modulation of host innate and adaptive immune defenses by cytomegalovirus; the impact of gamma-chain family cytokines on T cell homoeostasis in HIV-1 infection and the therapeutic implications; approaches to killing tumours by depriving them of the mechanisms for detoxification; and viral strategies for the evasion of immunogenic cell death.