Early induction of autophagy in human fibroblasts after infection with human cytomegalovirus or herpes simplex virus 1

Crosstalk Between Apoptosis and Autophagy: Environmental Genotoxins, Infection, and Innate Immunity

Autoimmune disorders constitute a major and growing health concern. However, the genetic and environmental factors that contribute to or exacerbate disease symptoms remain unclear. Type I interferons (IFNs) are known to break immune tolerance and be elevated in the serum of patients with autoimmune diseases such as lupus. Extensive work over the past decade has characterized the role of a protein termed stimulator of interferon genes, or STING, in mediating IFN expression and activation in response to cytosolic DNA and cyclic dinucleotides. Interestingly, this STING-dependent innate immune pathway both utilizes and is targeted by the cell's autophagic machinery. Given that aberrant interplay between the apoptotic and autophagic machineries contributes to deregulation of the STING-dependent pathway, IFN-regulated autoimmune phenotypes may be influenced by the combined exposure to environmental carcinogens and pathogenic microorganisms and viruses. This review therefore summarizes recent data regarding these important issues in the field of autoimmunity.

Pseudorabies virus infection inhibits autophagy in permissive cells in vitro

A large number of studies have demonstrated that autophagy is involved in the infection processes of different pathogens. Autophagy is now recognized as an essential component of innate and adaptive immunity. Several herpesviruses have developed various strategies to evade this antiviral mechanism. Pseudorabies virus (PRV) is a swine herpesvirus with a broad host range that causes devastating disease in infected pigs. In this study, we described the interaction between PRV and autophagy for the first time. PRV infection had a dual effect on the cell autophagy response; during the early period of infection, PRV virions induced autophagy without viral replication, and with viral protein expression, PRV reduced the basal level of autophagy in several permissive cells. We observed that inhibit the level of autophagy could increase the titer of infectious PRV. We also found that the conserved alphaherpesvirus US3 tegument protein may reduce the level of autophagy via activation of the AKT/mTOR pathways in PRV infected cells. These findings suggest that autophagy likely contributes to clearance of PRV, and that the virus has evolved strategies to antagonize this pathway.

Analysis of the role of autophagy inhibition by two complementary human cytomegalovirus BECN1/Beclin 1-binding proteins

Autophagy is activated early after human cytomegalovirus (HCMV) infection but, later on, the virus blocks autophagy. Here we characterized 2 HCMV proteins, TRS1 and IRS1, which inhibit autophagy during infection. Expression of either TRS1 or IRS1 was able to block autophagy in different cell lines, independently of the EIF2S1 kinase, EIF2AK2/PKR. Instead, TRS1 and IRS1 interacted with the autophagy protein BECN1/Beclin 1. We mapped the BECN1-binding domain (BBD) of IRS1 and TRS1 and found it to be essential for autophagy inhibition. Mutant viruses that express only IRS1 or TRS1 partially controlled autophagy, whereas a double mutant virus expressing neither protein stimulated autophagy. A mutant virus that did not express IRS1 and expressed a truncated form of TRS1 in which the BBD was deleted, failed to control autophagy. However, this mutant virus had similar replication kinetics as wild-type virus, suggesting that autophagy inhibition is not critical for viral replication. In fact, using pharmacological modulators of autophagy and inhibition of autophagy by shRNA knockdown, we discovered that stimulating autophagy enhanced viral replication. Conversely, inhibiting autophagy decreased HCMV infection. Thus, our results demonstrate a new proviral role of autophagy for a DNA virus.

N-Desmethylclozapine, Fluoxetine, and Salmeterol Inhibit Postentry Stages of the Dengue Virus Life Cycle

Around 10,000 people die each year due to severe dengue disease, and two-thirds of the world population lives in a region where dengue disease is endemic. There has been remarkable progress in dengue virus vaccine development; however, there are no licensed antivirals for dengue disease, and none appear to be in clinical trials. We took the approach of repositioning approved drugs for anti-dengue virus activity by screening a library of pharmacologically active compounds. We identified N-desmethylclozapine, fluoxetine hydrochloride, and salmeterol xinafoate as dengue virus inhibitors based on reductions in the numbers of infected cells and viral titers. Dengue virus RNA levels were diminished in inhibitor-treated cells, and this effect was specific to dengue virus, as other flaviviruses, such as Japanese encephalitis virus and West Nile virus, or other RNA viruses, such as respiratory syncytial virus and rotavirus, were not affected by these inhibitors. All three inhibitors specifically inhibited dengue virus replication with 50% inhibitory concentrations (IC₅₀s) in the high-nanomolar range. Estimation of negative-strand RNA intermediates and time-of-addition experiments indicated that inhibition was occurring at a postentry stage, most probably at the initiation of viral RNA replication. Finally, we show that inhibition is most likely due to the modulation of the endolysosomal pathway and induction of autophagy.

Virion Glycoprotein-Mediated Immune Evasion by Human Cytomegalovirus: a Sticky Virus Makes a Slick Getaway

The prototypic herpesvirus human cytomegalovirus (CMV) exhibits the extraordinary ability to establish latency and maintain a chronic infection throughout the life of its human host. This is even more remarkable considering the robust adaptive immune response elicited by infection and reactivation from latency. In addition to the ability of CMV to exist in a quiescent latent state, its persistence is enabled by a large repertoire of viral proteins that subvert immune defense mechanisms, such as NK cell activation and major histocompatibility complex antigen presentation, within the cell. However, dissemination outside the cell presents a unique existential challenge to the CMV virion, which is studded with antigenic glycoprotein complexes targeted by a potent neutralizing antibody response. The CMV virion envelope proteins, which are critical mediators of cell attachment and entry, possess various characteristics that can mitigate the humoral immune response and prevent viral clearance. Here we review the CMV glycoprotein complexes crucial for cell attachment and entry and propose inherent properties of these proteins involved in evading the CMV humoral immune response. These include viral glycoprotein polymorphism, epitope competition, Fc receptor-mediated endocytosis, glycan shielding, and cell-to-cell spread. The consequences of CMV virion glycoprotein-mediated immune evasion have a major impact on persistence of the virus in the population, and a comprehensive understanding of these evasion strategies will assist in designing effective CMV biologics and vaccines to limit CMV-associated disease.

Interactions between Autophagy and Inhibitory Cytokines

Autophagy is a degradative pathway that plays an essential role in maintaining cellular homeostasis. Most early studies of autophagy focused on its involvement in age-associated degeneration and nutrient deprivation. However, the immunological functions of autophagy have become more widely studied in recent years. Autophagy has been shown to be an intrinsic cellular defense mechanism in the innate and adaptive immune responses. Cytokines belong to a broad and loose category of proteins and are crucial for innate and adaptive immunity. Inhibitory cytokines have evolved to permit tolerance to self while also contributing to the eradication of invading pathogens. Interactions between inhibitory cytokines and autophagy have recently been reported, revealing a novel mechanism by which autophagy controls the immune response. In this review, we discuss interactions between autophagy and the regulatory cytokines IL-10, transforming growth factor-β, and IL-27. We also mention possible interactions between two newly discovered cytokines, IL-35 and IL-37, and autophagy.

Lychee flower extract inhibits proliferation and viral replication of HSV-1-infected corneal epithelial cells

PURPOSE

Herpes simplex virus type I (HSV-1) is capable of causing a wide array of human ocular diseases. Herpes simplex virus keratitis (HSK)-induced cytopathogenicity together with the chronic immune-inflammatory reaction can trigger stromal scarring, thinning, and neovascularization which may lead to permanent vision impairment. Lychee flower extract (LFE) is known for its antioxidant and anti-inflammatory effects. Therefore, in this study, we investigated the mechanism of the Statens Seruminstitut rabbit corneal (SIRC) epithelial cells infected by HSV-1 and examined the antiviral capabilities of LFE.

METHODS

SIRC cells were pretreated with different concentrations of LFE (0.2, 0.1, and 0.05 μg/ml) and then infected with 1 MOI of HSV-1 for 24 h. The cell viability or morphology was evaluated in this study. In addition, the supernatants and cell extracts were collected for Cell Counting Kit-8 (CCK), plaque assay, and western blotting.

RESULTS

We found that HSV-1-induced cell proliferation is regulated through inhibition of the mammalian target of rapamycin (mTOR) and p70s6k phosphorylation in response to the LFE. In addition, the LFE enhanced the autophagy protein expression (Beclin-1 and light chain 3, LC3) and decreased the viral titers.

CONCLUSIONS

These results showed the antiviral capabilities and the protective effects of LFE. Taken together, our data indicate that LFE has potential as an anti-HSK (herpes simplex keratitis) for HSV-1 infection.

Contribution of autophagy to antiviral immunity

Although identified in the 1960's, interest in autophagy has significantly increased in the past decade with notable research efforts oriented at understanding as to how this multi-protein complex operates and is regulated. Autophagy is commonly defined as a "self-eating" process evolved by eukaryotic cells to recycle senescent organelles and expired proteins, which is significantly increased during cellular stress responses. In addition, autophagy can also play important roles during human diseases, such as cancer, neurodegenerative and autoimmune disorders. Furthermore, novel findings suggest that autophagy contributes to the host defense against microbial infections. In this article, we review the role of macroautophagy in antiviral immune responses and discuss molecular mechanisms evolved by viral pathogens to evade this process. A role for autophagy as an effector mechanism used both, by innate and adaptive immunity is also discussed.

Trehalose, an mTOR-Independent Inducer of Autophagy, Inhibits Human Cytomegalovirus Infection in Multiple Cell Types

Human cytomegalovirus (HCMV) is the major viral cause of birth defects and a serious problem in immunocompromised individuals and has been associated with atherosclerosis. Previous studies have shown that the induction of autophagy can inhibit the replication of several different types of DNA and RNA viruses. The goal of the work presented here was to determine whether constitutive activation of autophagy would also block replication of HCMV. Most prior studies have used agents that induce autophagy via inhibition of the mTOR pathway. However, since HCMV infection alters the sensitivity of mTOR kinase-containing complexes to inhibitors, we sought an alternative method of inducing autophagy. We chose to use trehalose, a nontoxic naturally occurring disaccharide that is found in plants, insects, microorganisms, and invertebrates but not in mammals and that induces autophagy by an mTOR-independent mechanism. Given the many different cell targets of HCMV, we proceeded to determine whether trehalose would inhibit HCMV infection in human fibroblasts, aortic artery endothelial cells, and neural cells derived from human embryonic stem cells. We found that in all of these cell types, trehalose induces autophagy and inhibits HCMV gene expression and production of cell-free virus. Treatment of HCMV-infected neural cells with trehalose also inhibited production of cell-associated virus and partially blocked the reduction in neurite growth and cytomegaly. These results suggest that activation of autophagy by the natural sugar trehalose or other safe mTOR-independent agents might provide a novel therapeutic approach for treating HCMV disease.

IMPORTANCE

HCMV infects multiple cell types in vivo, establishes lifelong persistence in the host, and can cause serious health problems for fetuses and immunocompromised individuals. HCMV, like all other persistent pathogens, has to finely tune its interplay with the host cellular machinery to replicate efficiently and evade detection by the immune system. In this study, we investigated whether modulation of autophagy, a host pathway necessary for the recycling of nutrients and removal of protein aggregates, misfolded proteins, and pathogens, could be used to target HCMV. We found that autophagy could be significantly increased by treatment with the nontoxic, natural disaccharide trehalose. Importantly, trehalose had a profound inhibitory effect on viral gene expression and strongly impaired viral spread. These data constitute a proof-of-concept for the use of natural products targeting host pathways rather than the virus itself, thus reducing the risk of the development of resistance to treatment.

Inhibition of autophagy in EBV-positive Burkitt's lymphoma cells enhances EBV lytic genes expression and replication

Autophagy, an important degradation system involved in maintaining cellular homeostasis, serves also to eliminate pathogens and process their fragments for presentation to the immune system. Several viruses have been shown to interact with the host autophagic machinery to suppress or make use of this cellular catabolic pathway to enhance their survival and replication. Epstein Barr virus (EBV) is a γ-herpes virus associated with a number of malignancies of epithelial and lymphoid origin in which establishes a predominantly latent infection. Latent EBV can periodically reactivate to produce infectious particles that allow the virus to spread and can lead to the death of the infected cell. In this study, we analyzed the relationship between autophagy and EBV reactivation in Burkitt's lymphoma cells. By monitoring autophagy markers and EBV lytic genes expression, we demonstrate that autophagy is enhanced in the early phases of EBV lytic activation but decreases thereafter concomitantly with increased levels of EBV lytic proteins. In a cell line defective for late antigens expression, we found an inverse correlation between EBV early antigens expression and autophagosomes formation, suggesting that early after activation, the virus is able to suppress autophagy. We report here for the first time that inhibition of autophagy by Bafilomycin A1 or shRNA knockdown of Beclin1 gene, highly incremented EBV lytic genes expression as well as intracellular viral DNA and viral progeny yield. Taken together, these findings indicate that EBV activation induces the autophagic response, which is soon inhibited by the expression of EBV early lytic products. Moreover, our findings open the possibility that pharmacological inhibitors of autophagy may be used to enhance oncolytic viral therapy of EBV-related lymphomas.

Activation and regulation of DNA-driven immune responses

The innate immune system provides early defense against infections and also plays a key role in monitoring alterations of homeostasis in the body. DNA is highly immunostimulatory, and recent advances in this field have led to the identification of the innate immune sensors responsible for the recognition of DNA as well as the downstream pathways that are activated. Moreover, information on how cells regulate DNA-driven immune responses to avoid excessive inflammation is now emerging. Finally, several reports have demonstrated how defects in DNA sensing, signaling, and regulation are associated with susceptibility to infections or inflammatory diseases in humans and model organisms. In this review, the current literature on DNA-stimulated innate immune activation is discussed, and important new questions facing this field are proposed.

Herpes Simplex Virus-1 Fine-Tunes Host's Autophagic Response to Infection: A Comprehensive Analysis in Productive Infection Models

Herpes simplex virus-1 (HSV-1) infection causes severe conditions, with serious complications, including corneal blindness from uncontrolled ocular infections. An important cellular defense mechanism against HSV-1 infection is autophagy. The autophagic response of the host cell was suggested to be regulated by HSV-1. In this study, we performed a detailed analysis of autophagy in multiple HSV-1-targeted cell types, and under various infection conditions that recapitulate a productive infection model. We found that autophagy was slightly inhibited in one cell type, while in other cell types autophagy maintained its basal levels mostly unchanged during productive infection. This study refines the concept of HSV-1-mediated autophagy regulation to imply either inhibition, or prevention of activation, of the innate immune pathway.

Coxsackievirus A16 elicits incomplete autophagy involving the mTOR and ERK pathways

Autophagy is an important homeostatic process for the degradation of cytosolic proteins and organelles and has been reported to play an important role in cellular responses to pathogens and virus replication. However, the role of autophagy in Coxsackievirus A16 (CA16) infection and pathogenesis remains unknown. Here, we demonstrated that CA16 infection enhanced autophagosome formation, resulting in increased extracellular virus production. Moreover, expression of CA16 nonstructural proteins 2C and 3C was sufficient to trigger autophagosome accumulation by blocking the fusion of autophagosomes with lysosomes. Interestingly, we found that Immunity-related GTPase family M (IRGM) was crucial for the activation of CA16 infection-induced autophagy; in turn, reducing IRGM expression suppressed autophagy. Expression of viral protein 2C enhanced IRGM promoter activation, thereby increasing IRGM expression and inducing autophagy. CA16 infection inhibited Akt/mTOR signaling and activated extracellular signal-regulated kinase (ERK) signaling, both of which are necessary for autophagy induction. In summary, CA16 can use autophagy to enhance its own replication. These results raise the possibility of targeting the autophagic pathway for the treatment of hand, foot, and mouth disease (HFMD).

STING-mediated DNA sensing promotes antitumor and autoimmune responses to dying cells

Adaptive immune responses to Ags released by dying cells play a critical role in the development of autoimmunity, allograft rejection, and spontaneous as well as therapy-induced tumor rejection. Although cell death in these situations is considered sterile, various reports have implicated type I IFNs as drivers of the ensuing adaptive immune response to cell-associated Ags. However, the mechanisms that underpin this type I IFN production are poorly defined. In this article, we show that dendritic cells (DCs) can uptake and sense nuclear DNA-associated entities released by dying cells to induce type I IFN. Remarkably, this molecular pathway requires STING, but not TLR or NLR function, and results in the activation of IRF3 in a TBK1-dependent manner. DCs are shown to depend on STING function in vivo to efficiently prime IFN-dependent CD8(+) T cell responses to tumor Ags. Furthermore, loss of STING activity in DCs impairs the generation of follicular Th cells and plasma cells, as well as anti-nuclear Abs, in an inducible model of systemic lupus erythematosus. These findings suggest that the STING pathway could be manipulated to enable the rational design of immunotherapies that enhance or diminish antitumor and autoimmune responses, respectively.

Macroautophagy Proteins Assist Epstein Barr Virus Production and Get Incorporated Into the Virus Particles

Epstein Barr virus (EBV) persists as a latent herpes virus infection in the majority of the adult human population. The virus can reactivate from this latent infection into lytic replication for virus particle production. Here, we report that autophagic membranes, which engulf cytoplasmic constituents during macroautophagy and transport them to lysosomal degradation, are stabilized by lytic EBV replication in infected epithelial and B cells. Inhibition of autophagic membrane formation compromises infectious particle production and leads to the accumulation of viral DNA in the cytosol. Vice versa, pharmacological stimulation of autophagic membrane formation enhances infectious virus production. Atg8/LC3, an essential macroautophagy protein and substrate anchor on autophagic membranes, was found in virus preparations, suggesting that EBV recruits Atg8/LC3 coupled membranes to its envelope in the cytosol. Our data indicate that EBV subverts macroautophagy and uses autophagic membranes for efficient envelope acquisition during lytic infection.

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Macroautophagic membranes are stabilized during lytic EBV replication.

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Inhibition of macroautophagic membrane formation reduces EBV production.

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Stimulation of macroautophagic membrane formation boosts EBV production.

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Without macroautophagic membranes EBV DNA accumulates in the cytosol.

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Macroautophagic membranes get incorporated into EBV particles.

Regulation of autophagic activation by Rta of Epstein-Barr virus via the extracellular signal-regulated kinase pathway

Autophagy is an intracellular degradation pathway that provides a host defense mechanism against intracellular pathogens. However, many viruses exploit this mechanism to promote their replication. This study shows that lytic induction of Epstein-Barr virus (EBV) increases the membrane-bound form of LC3 (LC3-II) and LC3-containing punctate structures in EBV-positive cells. Transfecting 293T cells with a plasmid that expresses Rta also induces autophagy, revealing that Rta is responsible for autophagic activation. The activation involves Atg5, a key component of autophagy, but not the mTOR pathway. The expression of Rta also activates the transcription of the genes that participate in the formation of autophagosomes, including LC3A, LC3B, and ATG9B genes, as well as those that are involved in the regulation of autophagy, including the genes TNF, IRGM, and TRAIL. Additionally, treatment with U0126 inhibits the Rta-induced autophagy and the expression of autophagy genes, indicating that the autophagic activation is caused by the activation of extracellular signal-regulated kinase (ERK) signaling by Rta. Finally, the inhibition of autophagic activity by an autophagy inhibitor, 3-methyladenine, or Atg5 small interfering RNA, reduces the expression of EBV lytic proteins and the production of viral particles, revealing that autophagy is critical to EBV lytic progression. This investigation reveals how an EBV-encoded transcription factor promotes autophagy to affect viral lytic development.

Interplay of autophagy and apoptosis during murine cytomegalovirus infection of RPE cells

PURPOSE

Previous studies have demonstrated that autophagy is involved in the pathogenesis of human cytomegalovirus (HCMV) infection. However, whether autophagy is regulated by murine cytomegalovirus (MCMV) infection has not yet been investigated. The purpose of these studies was to determine how autophagy is affected by MCMV infection of the retinal pigment epithelial (RPE) cells and whether there is a functional relationship between autophagy and apoptosis; and if so, how regulation of autophagy impacts apoptosis.

METHODS

RPE cells were isolated from C57BL/6 mice and infected with MCMV K181. The cells were cultured in medium containing rapamycin, chloroquine, or ammonium chloride. Green fluorescent protein-light chain 3 (GFP-LC3) plasmid was transfected to RPE cells, and the GFP-LC3 positive puncta were counted. Electron microscopic (EM) images were taken to visualize the structure of the autophagic vacuoles. Western blot was performed to detect the expression of related proteins. Trypan blue exclusion assay was used to measure the percentage of viable cells.

RESULTS

Although the LC3B-II levels consistently increased during MCMV infection of RPE cells, administration of chloroquine or ammonium chloride increased LC3B-II expression only at the early stage of infection (6 h post-inoculation [p.i.] and 12 h p.i.), not at or after 24 h p.i. The punctate autophagic vacuoles in the GFP-LC3 transfected RPE cells were counted using light microscopy or by EM examination, the number of autophagic vacuoles was significantly increased in the MCMV-infected RPE cells compared to the uninfected controls. Compared to untreated MCMV-infected control cells, rapamycin treatment resulted in a significant decrease in the cleaved caspase 3 levels as well as a significant decrease in the ratio of phosphorylated mammalian target of rapamycin (mTOR) to total mTOR and in the ratio of phosphorylated P70S6K to total P70S6K. In contrast, chloroquine treatment resulted in a significant increase in the cleaved caspase 3 levels in the MCMV-infected RPE cells.

CONCLUSIONS

Autophagic vacuole accumulation was detected during MCMV infection of RPE cells. In contrast, autophagic flux was greatly decreased at or after 24 h p.i. The results suggest that MCMV might have a strategy for inhibiting or blocking autophagy activity by targeting a later autophagy process, such as the formation of autolysosomes or degradation of their content. Our data also suggest that there is a functional relationship between autophagy and apoptosis, which plays an important role during MCMV infection of the RPE.

Pattern recognition receptors and autophagy

The immune system senses exogenous threats or endogenous stress through specialized machinery known as pattern recognition receptors (PRRs). These receptors recognize conserved molecular structures and initiate downstream signaling pathways to control immune responses. Although various immunologic pathways mediated by PRRs have been described, recent studies have demonstrated a link between PRRs and autophagy. Autophagy is a specialized biological process involved in maintaining homeostasis through the degradation of long-lived cellular proteins and organelles. In addition to this fundamental function, autophagy plays important roles in various immunologic processes. In this review, we focus on the reciprocal influences of PRRs and autophagy in modulating innate immune responses.

Chemotherapy and Oncolytic Virotherapy: Advanced Tactics in the War against Cancer

Cancer is a traitorous archenemy that threatens our survival. Its ability to evade detection and adapt to various cancer therapies means that it is a moving target that becomes increasingly difficult to attack. Through technological advancements, we have developed sophisticated weapons to fight off tumor growth and invasion. However, if we are to stand a chance in this war against cancer, advanced tactics will be required to maximize the use of our available resources. Oncolytic viruses (OVs) are multi-functional cancer-fighters that can be engineered to suit many different strategies; in particular, their retooling can facilitate increased capacity for direct tumor killing (oncolytic virotherapy) and elicit adaptive antitumor immune responses (oncolytic immunotherapy). However, administration of these modified OVs alone, rarely induces successful regression of established tumors. This may be attributed to host antiviral immunity that acts to eliminate viral particles, as well as the capacity for tumors to adapt to therapeutic selective pressure. It has been shown that various chemotherapeutic drugs with distinct functional properties can potentiate the antitumor efficacy of OVs. In this review, we summarize the chemotherapeutic combinatorial strategies used to optimize virally induced destruction of tumors. With a particular focus on pharmaceutical immunomodulators, we discuss how specific therapeutic contexts may alter the effects of these synergistic combinations and their implications for future clinical use.

Interleukin-10 blocks in vitro replication of human cytomegalovirus by inhibiting the virus-induced autophagy in MRC5 cells

BACKGROUND

Interleukin-10 is an important cytokine that regulates immune response. Previous studies have shown that human cytomegalovirus can trigger cell autophagy during the early stages of infection. To our knowledge, whether IL-10 inhibits HCMV-induced autophagy and virus replication has not been studied previously.

OBJECTIVES

We investigated whether IL-10 affects cell viability and autophagy under the conditions of starvation and HCMV infection by using the MRC5 cell line. We also explored the role of IL-10-mediated autophagy on HCMV replication.

RESULTS

Our data showed that IL-10 inhibited the autophagic flux of the MRC5 cells irrespective of starvation or HCMV infection, and suppressed HCMV replication. The promotion of autophagy with either a pharmacological inducer (rapamycin), or a technique to over-express the BECN1 gene reversed the effect of IL-10 on virus replication. Furthermore, the PI3K/Akt signal pathway was activated when the cells were pretreated with IL-10.

CONCLUSIONS

Our results indicated that IL-10 can suppress HCMV replication by inhibiting autophagy in host cells during the early stages of infection.

The impact of macroautophagy on CD8(+) T-cell-mediated antiviral immunity

Macroautophagy is a catabolic recycling pathway, which can be induced by various stress stimuli. Viruses are able to manipulate autophagy in the cells that they infect. The impact of autophagy on the innate immune response to viruses and its stimulatory role in antigen presentation to CD4(+) T cells are well documented. Herein, we present the impact of autophagy on the activation of cytotoxic T lymphocyte (CTL)-mediated antiviral immune responses, which are required for the eradication or control of multiple viruses. We first discuss the general mechanisms by which viruses can either induce or block autophagy in cells. We then explore the cross-talk between autophagy and innate immune processes, which are both first line defenses against viruses; and constitute crucial steps for the initiation of potent adaptive immune responses. We describe the impact of autophagy on the presentation of viral peptide antigens on class I major histocompatibility complex (MHC I), a prerequisite for the priming of CTL responses. In sum, our review highlights the interplay between viruses and three integrated host response pathways - autophagy, innate and adaptive immunity - providing a framework for future mechanistic and pathogenesis-based research.

Host-pathogen interactions during Mycobacterium tuberculosis infections

The intimate and persistent connection between Mycobacterium tuberculosis and its human host suggests that the pathogen has evolved extensive mechanisms to evade eradication by the immune system. In particular, the organism has adapted to replicate within phagocytic cells, especially macrophages, which are specialized to kill microbes. Over the past decade of M. tuberculosis research, the means to manipulate both the organism and the host has ushered in an exciting time that has uncovered some of the mechanisms of the innate macrophage-pathogen interactions that lie at the heart of M. tuberculosis pathogenesis, though many interactions likely still await discovery. In this chapter, we will delve into some of these advances, with an emphasis on the interactions that occur on the cellular level when M. tuberculosis cells encounter macrophages. In particular, we focus on two major aspects of M. tuberculosis biology regarding the proximal physical interface between the bacterium and host, namely the interactions with the phagosomal membrane as well as the distinctive mycobacterial cell wall. Importantly, some of the emerging paradigms in M. tuberculosis pathogenesis and host response represent common themes in bacterial pathogenesis, such as the role of host cell membrane perforation in intracellular survival and host response. However, the array of unique bacterial lipid mediators and their interaction with host cells highlights the unique biology of this persistent pathogen.

Cycloheximide inhibits starvation-induced autophagy through mTORC1 activation

Protein synthesis inhibitors such as cycloheximide (CHX) are known to suppress protein degradation including autophagy. The fact that CHX inhibits autophagy has been generally interpreted to indicate that newly synthesized protein is indispensable for autophagy. However, CHX is also known to increase the intracellular level of amino acids and activate mTORC1 activity, a master negative regulator of autophagy. Accordingly, CHX can affect autophagic activity through inhibition of de novo protein synthesis and/or modulation of mTORC1 signaling. In this study, we investigated the effects of CHX on autophagy using specific autophagy markers. We found that CHX inhibited starvation-induced autophagy but not Torin1-induced autophagy. CHX also suppressed starvation-induced puncta formation of GFP-ULK1, an early-step marker of the autophagic process which is regulated by mTORC1. CHX activated mTORC1 even under autophagy-inducible starvation conditions. Finally, the inhibitory effect of CHX on starvation-induced autophagy was cancelled by the mTOR inhibitor Torin1. These results suggest that CHX inhibits starvation-induced autophagy through mTORC1 activation and also that autophagy does not require new protein synthesis at least in the acute phase of starvation.

Checks and balances between human cytomegalovirus replication and indoleamine-2,3-dioxygenase

Despite a rigorous blockade of interferon-γ (IFN-γ) signalling in infected fibroblasts as a mechanism of immune evasion by human cytomegalovirus (HCMV), IFN-γ induced indoleamine-2,3-dioxygenase (IDO) has been proposed to represent the major antiviral restriction factor limiting HCMV replication in epithelial cells. Here we show that HCMV efficiently blocks transcription of IFN-γ-induced IDO mRNA both in infected fibroblasts and epithelial cells even in the presence of a preexisting IFN-induced antiviral state. This interference results in severe suppression of IDO bioactivity in HCMV-infected cells and restoration of vigorous HCMV replication. Depletion of IDO expression nonetheless substantially alleviated the antiviral impact of IFN-γ treatment in both cell types. These findings highlight the effectiveness of this IFN-γ induced effector gene in restricting HCMV productivity, but also the impact of viral counter-measures.

Autophagy in infection, inflammation and immunity

Autophagy is a fundamental eukaryotic pathway that has multiple effects on immunity. Autophagy is induced by pattern recognition receptors and, through autophagic adaptors, it provides a mechanism for the elimination of intracellular microorganisms. Autophagy controls inflammation through regulatory interactions with innate immune signalling pathways, by removing endogenous inflammasome agonists and through effects on the secretion of immune mediators. Moreover, autophagy contributes to antigen presentation and to T cell homeostasis, and it affects T cell repertoires and polarization. Thus, as we discuss in this Review, autophagy has multitiered immunological functions that influence infection, inflammation and immunity.

Autophagy is redundant for the host defense against systemic Candida albicans infections

Autophagy has been demonstrated to play an important role in the immunity against intracellular pathogens, but very little is known about its role in the host defense against fungal pathogens such as Candida albicans. Therefore, the role of autophagy for the host defense against C. albicans was assessed by complementary approaches using mice defective in autophagy, as well as immunological and genetic studies in humans. Although C. albicans induced LC3-II formation in macrophages, myeloid cell-specific ATG7(-/-) mice with defects in autophagy did not display an increased susceptibility to disseminated candidiasis. In in vitro experiments in human blood mononuclear cells, blocking autophagy modulated cytokine production induced by lipopolysaccharide, but not by C. albicans. Furthermore, autophagy modulation in human monocytes did not influence the phagocytosis and killing of C. albicans. Finally, 18 single-nucleotide polymorphisms in 13 autophagy genes were not associated with susceptibility to candidemia or clinical outcome of disease in a large cohort of patients, and there was no correlation between these genetic variants and cytokine production in either candidemia patients or healthy controls. Based on these complementary in vitro and in vivo studies, it can be concluded that autophagy is redundant for the host response against systemic infections with C. albicans.

Autophagy and the effects of its inhibition on varicella-zoster virus glycoprotein biosynthesis and infectivity

Autophagy and the effects of its inhibition or induction were investigated during the entire infectious cycle of varicella-zoster virus (VZV), a human herpesvirus. As a baseline, we first enumerated the number of autophagosomes per cell after VZV infection compared with the number after induction of autophagy following serum starvation or treatment with tunicamycin or trehalose. Punctum induction by VZV was similar in degree to punctum induction by trehalose in uninfected cells. Treatment of infected cells with the autophagy inhibitor 3-methyladenine (3-MA) markedly reduced the viral titer, as determined by assays measuring both cell-free virus and infectious foci (P < 0.0001). We next examined a virion-enriched band purified by density gradient sedimentation and observed that treatment with 3-MA decreased the amount of VZV gE, while treatment with trehalose increased the amount of gE in the same band. Because VZV gE is the most abundant glycoprotein, we selected gE as a representative viral glycoprotein. To further investigate the role of autophagy in VZV glycoprotein biosynthesis as well as confirm the results obtained with 3-MA inhibition, we transfected cells with ATG5 small interfering RNA to block autophagosome formation. VZV-induced syncytium formation was markedly reduced by ATG5 knockdown (P < 0.0001). Further, we found that both expression and glycan processing of VZV gE were decreased after ATG5 knockdown, while expression of the nonglycosylated IE62 tegument protein was unchanged. Taken together, our cumulative results not only documented abundant autophagy within VZV-infected cells throughout the infectious cycle but also demonstrated that VZV-induced autophagy facilitated VZV glycoprotein biosynthesis and processing.

Immune sensing of DNA

Although it has been appreciated for some years that cytosolic DNA is immune stimulatory, it is only in the past five years that the molecular basis of DNA sensing by the innate immune system has begun to be revealed. In particular it has been described how DNA induces type I interferon, central in antiviral responses and a mediator of autoimmunity. To date more than ten cytosolic receptors of DNA have been proposed, but STING is a key adaptor protein for most DNA-sensing pathways, and we are now beginning to understand the signaling mechanisms for STING. In this review we describe the recent progress in understanding signaling mechanisms activated by DNA and the relevance of DNA sensing to pathogen responses and autoimmunity. We highlight new insights gained into how and why the immune system responds to both pathogen and self DNA and define important questions that now need to be addressed in the field of innate immune activation by DNA.

mTOR-independent autophagy counteracts apoptosis in herpes simplex virus type 1-infected U251 glioma cells

We investigated the role of autophagy, a stress-inducible lysosomal self-digestion of cellular components, in modulation of herpes simplex virus type 1 (HSV-1)-triggered death of U251 human glioma cells. HSV-1 caused apoptotic death in U251 cells, characterized by phosphatidylserine externalization, caspase activation and DNA fragmentation. HSV-1-induced apoptosis was associated with the induction of autophagic response, as confirmed by the conversion of cytosolic LC3-I to autophagosome-associated LC3-II, increase in intracellular acidification, presence of autophagic vesicles, and increase in proteolysis of the selective autophagic target p62. HSV-1-triggered autophagy was not associated with the significant increase in the expression of proautophagic protein beclin-1 or downregulation of the major autophagy suppressor mammalian target of rapamycin (mTOR). Moreover, the phosphorylation of mTOR and its direct substrate p70 S6 kinase was augmented by HSV-1 infection, while the mTOR stimulator Akt and inhibitor AMPK-activated protein kinase (AMPK) were accordingly activated and suppressed, respectively. An shRNA-mediated knockdown of the autophagy-essential LC3β, as well as pharmacological inhibition of autophagy with bafilomycin A1 or 3-methyladenine, markedly accelerated apoptotic changes and ensuing cell death in HSV-1-infected glioma cells. These data indicate that AMPK/Akt/mTOR-independent autophagy could prolong survival of HSV-1-infected U251 glioma cells by counteracting the coinciding apoptotic response.

Axin expression enhances herpes simplex virus type 1 replication by inhibiting virus-mediated cell death in L929 cells

Herpes simplex virus type 1 (HSV-1) replicates in various cell types and induces early cell death, which limits viral replication in certain cell types. Axin is a scaffolding protein that regulates Wnt signalling and participates in various cellular events, including cellular proliferation and cell death. The effects of axin expression on HSV-1 infection were investigated based on our initial observation that Wnt3a treatment or axin knockdown reduced HSV-1 replication. L929 cells expressed the axin protein in a doxycycline-inducible manner (L-axin) and enhanced HSV-1 replication in comparison to control cells (L-EV). HSV-1 infection induced cell death as early as 6 h after infection through the necrotic pathway and required de novo protein synthesis in L929 cells. Subsequent analysis of viral protein expression suggested that axin expression led to suppression of HSV-1-induced premature cell death, resulting in increased late gene expression. In analysis of axin deletion mutants, the regulators of the G-protein signalling (RGS) domain were involved in the axin-mediated enhancement of viral replication and reduction in cell death. These results suggest that viral replication enhancement might be mediated by the axin RGS domain.

Targeting autophagy to enhance oncolytic virus-based cancer therapy

INTRODUCTION

Autophagy is a conserved catabolic process crucial in maintaining cellular homeostasis. On infection, oncolytic viruses (OVs) perturb the cellular autophagy machinery in infected tumor cells both in vitro and in vivo. Currently, pharmacological modulation of autophagy in OV-infected tumor cells has been shown to augment OV-mediated antitumor effects in preclinical studies. Combination of OVs with autophagy modulators can, therefore, have many potential applications in the future research on targeting autophagy and novel anticancer therapies.

AREAS COVERED

This review provides a detailed description of known interactions between OVs and autophagy and summarizes the roles of autophagy in OV replication and cell lysis. The recent literature on targeting autophagy with either the autophagy inducers, such as rapamycin, or autophagy inhibitors, such as chloroquine, to increase OV-induced cytotoxicity is reviewed to help researchers in further investigations. The major challenge for investigators is to understand the molecular mechanism underlying the interplay between OV and the autophagy machinery and its effect on oncolysis.

EXPERT OPINION

Targeting the cellular autophagy machinery could be explored as a new therapeutic strategy to enhance OV-mediated antitumor effects in the future.

Inhibition of the host translation shutoff response by herpes simplex virus 1 triggers nuclear envelope-derived autophagy

Macroautophagy is a cellular pathway that degrades intracellular pathogens and contributes to antigen presentation. Herpes simplex virus 1 (HSV-1) infection triggers both macroautophagy and an additional form of autophagy that uses the nuclear envelope as a source of membrane. The present study constitutes the first in-depth analysis of nuclear envelope-derived autophagy (NEDA). We established LC3a as a marker that allowed us to distinguish between NEDA and macroautophagy in both immunofluorescence and flow cytometry. NEDA was observed in many different cell types, indicating that it is a general response to HSV-1 infection. This autophagic pathway is known to depend on the viral protein γ34.5, which can inhibit macroautophagy via binding to beclin-1. Using mutant viruses, we were able to show that binding of beclin-1 by γ34.5 had no effect on NEDA, demonstrating that NEDA is regulated differently than macroautophagy. Instead, NEDA was triggered in response to γ34.5 binding to protein phosphatase 1α, an interaction used by the virus to prevent host cells from shutting off protein translation. NEDA was not triggered when late viral protein production was inhibited with acyclovir or hippuristanol, indicating that the accumulation of these proteins might stress infected cells. Interestingly, expression of the late viral protein gH was sufficient to rescue NEDA in the context of infection with a virus that otherwise does not support strong late viral protein expression. We argue that NEDA is a cellular stress response triggered late during HSV-1 infection and might compensate for the viral alteration of the macroautophagic response.

Peste des petits ruminants virus exploits cellular autophagy machinery for replication

Peste des petits ruminants virus (PPRV) is an important pathogen that seriously influences the productivity of small ruminants worldwide. Although PPRV is known to induce apoptosis in infected cells, the interaction between PPRV and permissive cells requires further elucidation. Here, we provide the first evidence that PPRV infection triggered autophagy in Vero cells based on the appearance of abundant double- and single-membrane vesicles, the accumulation of LC3 fluorescent puncta, the enhancement of LC3-I/-II conversion, and autophagic flux. We further demonstrated that induction of autophagy with rapamycin significantly increased PPRV progeny yield and nucleocapsid (N) protein expression, while inhibition of autophagy with siRNA targeting ATG7 resulted in diametrically opposite results. Our data indicate that PPRV exploits the autophagy machinery to facilitate its own replication in host cells, thus the production efficiency of live attenuated PPRV vaccines may be improved by targeting the autophagic pathway.

Dynein mediates the localization and activation of mTOR in normal and human cytomegalovirus-infected cells

Activation of stress signaling pathways normally leads to inhibition of the mammalian target of rapamycin complex 1 (mTORC1); however, human cytomegalovirus (HCMV) infection maintains mTORC1 activity in the presence of numerous types of stress. We previously demonstrated that HCMV infection maintains mTORC1 activity during amino acid deprivation through a Ras-related GTP-binding (Rag) protein-independent mechanism. This depends on the colocalization of mTOR and its activator, Rheb (Ras homology enriched in brain)-GTP, to a perinuclear position that corresponds to the viral cytoplasmic assembly compartment (AC). The data presented here show that the HCMV-induced, amino acid depletion-resistant perinuclear localization and activation of mTORC1 occurs as early as 8 h post-infection, prior to AC formation. We show that the molecular motor dynein is required for perinuclear localization of mTORC1 in both uninfected and HCMV-infected cells. Association between dynein and mTOR is shown by coimmunoprecipitation, and inhibition of dynein function using RNAi or the small molecule inhibitor ciliobrevin A inhibits mTORC1 activity in both uninfected and HCMV-infected cells. The data suggest that mTORC1 activation requires dynein-dependent transport to a position in the cell where it can be activated. Thus, the HCMV commandeers a cellular dynein-dependent mTORC1 activation mechanism to maintain stress-resistant mTORC1 activity during infection and to form the AC.

Extracellular M. tuberculosis DNA targets bacteria for autophagy by activating the host DNA-sensing pathway

Eukaryotic cells sterilize the cytosol by using autophagy to route invading bacterial pathogens to the lysosome. During macrophage infection with Mycobacterium tuberculosis, a vacuolar pathogen, exogenous induction of autophagy can limit replication, but the mechanism of autophagy targeting and its role in natural infection remain unclear. Here we show that phagosomal permeabilization mediated by the bacterial ESX-1 secretion system allows cytosolic components of the ubiquitin-mediated autophagy pathway access to phagosomal M. tuberculosis. Recognition of extracelluar bacterial DNA by the STING-dependent cytosolic pathway is required for marking bacteria with ubiquitin, and delivery of bacilli to autophagosomes requires the ubiquitin-autophagy receptors p62 and NDP52 and the DNA-responsive kinase TBK1. Remarkably, mice with monocytes incapable of delivering bacilli to the autophagy pathway are extremely susceptible to infection. Our results reveal an unexpected link between DNA sensing, innate immunity, and autophagy and indicate a major role for this autophagy pathway in resistance to M. tuberculosis infection.

Evasion of oncogene-induced senescence by gammaherpesviruses

A common feature of herpesvirus infection is activation of DNA damage responses (DDRs) that are essential for efficient lytic replication. Latent infection with Epstein-Barr virus (EBV) and Kaposi's sarcoma-associated herpesvirus (KSHV) also elicit DDRs via the action of latent viral oncoproteins that deregulate cell proliferation and initiate a host anti-proliferative defense known as oncogene-induced senescence (OIS). These viruses encode auxiliary latent proteins that undermine OIS to allow the ongoing proliferation of infected cells despite robust DDR signaling. Persistent DDRs have also been linked to the aberrant secretion of pathogenetically important inflammatory mediators from infected cells. The accumulating evidence indicates that herpesviruses have evolved ways to co-opt DDR signaling to manage both latent and lytic phases of infection, and that DDR subversion may contribute to herpesvirus-associated disease states.

Foot-and-mouth disease virus induces autophagosomes during cell entry via a class III phosphatidylinositol 3-kinase-independent pathway

Autophagy is an intracellular pathway that can contribute to innate antiviral immunity by delivering viruses to lysosomes for degradation or can be beneficial for viruses by providing specialized membranes for virus replication. Here, we show that the picornavirus foot-and-mouth disease virus (FMDV) induces the formation of autophagosomes. Induction was dependent on Atg5, involved processing of LC3 to LC3II, and led to a redistribution of LC3 from the cytosol to punctate vesicles indicative of authentic autophagosomes. Furthermore, FMDV yields were reduced in cells lacking Atg5, suggesting that autophagy may facilitate FMDV infection. However, induction of autophagosomes by FMDV appeared to differ from starvation, as the generation of LC3 punctae was not inhibited by wortmannin, implying that FMDV-induced autophagosome formation does not require the class III phosphatidylinositol 3-kinase (PI3-kinase) activity of vps34. Unlike other picornaviruses, for which there is strong evidence that autophagosome formation is linked to expression of viral nonstructural proteins, FMDV induced autophagosomes very early during infection. Furthermore, autophagosomes could be triggered by either UV-inactivated virus or empty FMDV capsids, suggesting that autophagosome formation was activated during cell entry. Unlike other picornaviruses, FMDV-induced autophagosomes did not colocalize with the viral 3A or 3D protein. In contrast, ∼50% of the autophagosomes induced by FMDV colocalized with VP1. LC3 and VP1 also colocalized with the cellular adaptor protein p62, which normally targets ubiquitinated proteins to autophagosomes. These results suggest that FMDV induces autophagosomes during cell entry to facilitate infection, but not to provide membranes for replication.

Dysregulation of autophagy in murine fibroblasts resistant to HSV-1 infection

The mouse L cell mutant, gro29, was selected for its ability to survive infection by herpes simplex virus type 1 (HSV-1). gro29 cells are fully susceptible to HSV-1 infection, however, they produce 2000-fold less infectious virus than parental L cells despite their capacity to synthesize late viral gene products and assemble virions. Because productive HSV-1 infection is presumed to result in the death of the host cell, we questioned how gro29 cells might survive infection. Using time-lapse video microscopy, we demonstrated that a fraction of infected gro29 cells survived infection and divided. Electron microscopy of infected gro29 cells, revealed large membranous vesicles that contained virions as well as cytoplasmic constituents. These structures were reminiscent of autophagosomes. Autophagy is an ancient cellular process that, under nutrient deprivation conditions, results in the degradation and catabolism of cytoplasmic components and organelles. We hypothesized that enhanced autophagy, and resultant degradation of virions, might explain the ability of gro29 to survive HSV-1 infection. Here we demonstrate that gro29 cells have enhanced basal autophagy as compared to parental L cells. Moreover, treatment of gro29 cells with 3-methyladenine, an inhibitor of autophagy, failed to prevent the formation of autophagosome-like organelles in gro29 cells indicating that autophagy was dysregulated in these cells. Additionally, we observed robust co-localization of the virion structural component, VP26, with the autophagosomal marker, GFP-LC3, in infected gro29 cells that was not seen in infected parental L cells. Collectively, these data support a model whereby gro29 cells prevent the release of infectious virus by directing intracellular virions to an autophagosome-like compartment. Importantly, induction of autophagy in parental L cells did not prevent HSV-1 production, indicating that the relationship between autophagy, virus replication, and survival of HSV-1 infection by gro29 cells is complex.

Herpesviruses exploit several host compartments for envelopment

Enveloped viruses acquire their host-derived membrane at a variety of intracellular locations. Herpesviruses are complex entities that undergo several budding and fusion events during an infection. All members of this large family are believed to share a similar life cycle. However, they seemingly differ in terms of acquisition of their mature envelope. Herpes simplex virus is often believed to bud into an existing intracellular compartment, while the related cytomegalovirus may acquire its final envelope from a novel virus-induced assembly compartment. This review focuses on recent advances in the characterization of cellular compartment(s) potentially contributing to herpes virion final envelopment. It also examines the common points between seemingly distinct envelopment pathways and highlights the dynamic nature of intracellular compartments in the context of herpesvirus infections.

Autophagy and immunity - insights from human herpesviruses

The herpesviruses are a family of double-stranded DNA viruses that infect a wide variety of organisms. Having co-evolved with their hosts over millennia, herpesviruses have developed a large repertoire of mechanisms to manipulate normal cellular processes for their own benefit. Consequently, studies on these viruses have made important contributions to our understanding of fundamental biological processes. Here we describe recent research on the human herpesviruses that has contributed to our understanding of, and interactions between, viruses, autophagy, and the immune system. The ability of autophagy to degrade proteins located within the nucleus, the site of herpesvirus latency and replication, is also considered.

Autophagy mediates transporter associated with antigen processing-independent presentation of viral epitopes through MHC class I pathway

The endogenous presentation of the majority of viral epitopes through MHC class I pathway is strictly dependent on the transporter associated with antigen processing (TAP) complex, which transfers the peptide products of proteasomal degradation into the endoplasmic reticulum. A small number of epitopes can be presented through the TAP-independent pathway, the precise mechanism for which remains largely unresolved. Here we show that TAP-independent presentation can be mediated by autophagy and that this process uses the vacuolar pathway and not the conventional secretory pathway. After macroautophagy, the antigen is processed through a proteasome-independent pathway, and the peptide epitopes are loaded within the autophagolysosomal compartment in a process facilitated by the relative acid stability of the peptide-MHC interaction. Despite bypassing much of the conventional MHC class I pathway, the autophagy-mediated pathway generates the same epitope as that generated through the conventional pathway and thus may have a role in circumventing viral immune evasion strategies that primarily target the conventional pathway.

Viral mediated redirection of NEMO/IKKγ to autophagosomes curtails the inflammatory cascade

The early host response to viral infections involves transient activation of pattern recognition receptors leading to an induction of inflammatory cytokines such as interleukin-1β (IL-1β) and tumor necrosis factor α (TNFα). Subsequent activation of cytokine receptors in an autocrine and paracrine manner results in an inflammatory cascade. The precise mechanisms by which viruses avert an inflammatory cascade are incompletely understood. Nuclear factor (NF)-κB is a central regulator of the inflammatory signaling cascade that is controlled by inhibitor of NF-κB (IκB) proteins and the IκB kinase (IKK) complex. In this study we show that murine cytomegalovirus inhibits the inflammatory cascade by blocking Toll-like receptor (TLR) and IL-1 receptor-dependent NF-κB activation. Inhibition occurs through an interaction of the viral M45 protein with the NF-κB essential modulator (NEMO), the regulatory subunit of the IKK complex. M45 induces proteasome-independent degradation of NEMO by targeting NEMO to autophagosomes for subsequent degradation in lysosomes. We propose that the selective and irreversible degradation of a central regulatory protein by autophagy represents a new viral strategy to dampen the inflammatory response.

Upon viral infection cells immediately induce an innate immune response which involves the production of inflammatory cytokines. These cytokines activate specific receptors on infected and surrounding cells leading to local signal amplification as well as signal broadcasting beyond the original site of infection. Inflammatory cytokine production depends on transcription factor NF-κB, whose activity is controlled by a kinase complex that includes the NF-κB essential modulator (NEMO). In order to replicate and spread in their hosts, viruses have evolved numerous strategies to counteract innate immune defenses. In this study we identify a highly effective viral strategy to blunt the host inflammatory response: The murine cytomegalovirus M45 protein binds to NEMO and redirects it to autophagosomes, vesicular structures that deliver cytoplasmic constituents to lysosomes for degradation and recycling. By this means, the virus installs a sustained block to all classical NF-κB activation pathways, which include signaling cascades originating from pattern recognition receptors and inflammatory cytokine receptors. Redirection of an essential component of the host cell defense machinery to the autophagic degradation pathway is a previously unrecognized viral immune evasion strategy whose principle is likely shared by other pathogens.

The human cytomegalovirus protein TRS1 inhibits autophagy via its interaction with Beclin 1

Human cytomegalovirus modulates macroautophagy in two opposite directions. First, HCMV stimulates autophagy during the early stages of infection, as evident by an increase in the number of autophagosomes and a rise in the autophagic flux. This stimulation occurs independently of de novo viral protein synthesis since UV-inactivated HCMV recapitulates the stimulatory effect on macroautophagy. At later time points of infection, HCMV blocks autophagy (M. Chaumorcel, S. Souquere, G. Pierron, P. Codogno, and A. Esclatine, Autophagy 4:1-8, 2008) by a mechanism that requires de novo viral protein expression. Exploration of the mechanisms used by HCMV to block autophagy unveiled a robust increase of the cellular form of Bcl-2 expression. Although this protein has an anti-autophagy effect via its interaction with Beclin 1, it is not responsible for the inhibition induced by HCMV, probably because of its phosphorylation by c-Jun N-terminal kinase. Here we showed that the HCMV TRS1 protein blocks autophagosome biogenesis and that a TRS1 deletion mutant is defective in autophagy inhibition. TRS1 has previously been shown to neutralize the PKR antiviral effector molecule. Although phosphorylation of eIF2α by PKR has been described as a stimulatory signal to induce autophagy, the PKR-binding domain of TRS1 is dispensable to its inhibitory effect. Our results show that TRS1 interacts with Beclin 1 to inhibit autophagy. We mapped the interaction with Beclin 1 to the N-terminal region of TRS1, and we demonstrated that the Beclin 1-binding domain of TRS1 is essential to inhibit autophagy.