Cathepsins are involved in virus-induced cell death in ICP4 and Us3 deletion mutant herpes simplex virus type 1-infected monocytic cells

Role of lysosomes in HSV-induced pathogenesis

HSV can evade host defenses and cause lifelong infection and severe illness. Lysosomes are catabolic organelles that play an important role in the regulation of cellular homeostasis. Lysosomal dysfunction and alterations in the process of autophagy have been identified in a variety of diseases, including HSV infection, and targeting lysosomes is a potential anti-HSV therapeutic strategy. This article reviews the role of lysosomes and lysosome-associated proteins in HSV infection, providing attractive targets and novel strategies for the treatment of HSV infection.

Manipulation of Host Cell Death Pathways by Herpes Simplex Virus

Herpes simplex virus (HSV)-1 and HSV-2 are ubiquitous human pathogens that infect keratinized epithelial surfaces and establish lifelong latent infection in sensory neurons of the peripheral nervous system. HSV-1 causes oral cold sores, and HSV-2 causes genital lesions characterized by recurrence at the site of the initial infection. In multicellular organisms, cell death plays a pivotal role in host defense by eliminating pathogen-infected cells. Apoptosis and necrosis are readily distinguished types of cell death. Apoptosis, the main form of programmed cell death, depends on the activity of certain caspases, a family of cysteine proteases. Necroptosis, a regulated form of necrosis that is unleashed when caspase activity is compromised, requires the activation of receptor-interacting protein (RIP) kinase 3 (RIPK3) through its interaction with other RIP homotypic interaction motif (RHIM)-containing proteins such as RIPK1. To ensure lifelong infection in the host, HSV carries out sophisticated molecular strategies to evade host cell death responses during viral infection. HSV-1 is a well-characterized pathogen that encodes potent viral inhibitors that modulate both caspase activation in the apoptosis pathway and RIPK3 activation in the necroptosis pathway in a dramatic, species-specific fashion. The viral UL39-encoded viral protein ICP6, the large subunit of the virus-encoded ribonucleotide reductase, functions as a suppressor of both caspase-8 and RHIM-dependent RIPK3 activities in the natural human host. In contrast, ICP6 RHIM-mediated recruitment of RIPK3 in the nonnatural mouse host drives the direct activation of necroptosis. This chapter provides an overview of the current state of the knowledge on molecular interactions between HSV-1 viral proteins and host cell death pathways and highlights how HSV-1 manipulates cell death signals for the benefit of viral propagation.

Vaccinium bracteatum Thunb Extract Inhibits HSV-1 Infection by Regulating ER Stress and Apoptosis

Herpes simplex Type 1 (HSV-1) is a neurotropic virus that infects the peripheral and central nervous system. Usually, after primary infection in epithelial cells, HSV-1 migrates retrograde to the peripheral nervous system (PNS), where it establishes a latent infection. HSV-1 can remain latent in the nervous system, and its reactivation in the brain can rarely cause acute HSV-1 encephalitis, often a life-threatening condition, or asymptomatic reactivations that could lead to neuronal damage and ultimately neurodegenerative disorders. Acyclovir and related nucleoside analogs have been used as therapeutic agents for HSV-1 infection, but resistance to the drug can arise, and the protective effect of HSV-1 on brain cells is limited. Therefore, there is an urgent need for research into safe and effective new antiviral agents that can protect brain cells from the damage that is caused by HSV-1 infection. Vaccinium bracteatum Thunb. (VBT) is widely distributed in Korea and China, and has pharmacological actions such as anti-inflammatory, antioxidant, and antidiabetic activity. Studies on the antiviral effect of VBT on HSV-1 infection have not been reported so far. Therefore, we sought to determine the HSV-1 antiviral effect and molecular mechanism of VBT at the cellular level. We confirmed that VBT repressed the VP16 and IE genes in both Vero and SK-N-SH cells. We also found that the generation of HSV-1 virions was inhibited by VBT treatment. VBT inhibited the activities of the HSV-1-induced endoplasmic reticulum (ER) stressors PERK, ATF4, and CHOP. We confirmed that VBT inhibited the activity of apoptosis factors by regulating the expression of death receptor (DR) after HSV-1 infection. As HSV-1 is closely associated with brain diseases, the study of the antiviral drug effects and mechanism of VBT is meaningful. Further studies using animal models of infection will also be performed to determine the potential of VBT as an antiviral agent.

Lysoptosis is an evolutionarily conserved cell death pathway moderated by intracellular serpins

Lysosomal membrane permeabilization (LMP) and cathepsin release typifies lysosome-dependent cell death (LDCD). However, LMP occurs in most regulated cell death programs suggesting LDCD is not an independent cell death pathway, but is conscripted to facilitate the final cellular demise by other cell death routines. Previously, we demonstrated that Caenorhabditis elegans (C. elegans) null for a cysteine protease inhibitor, srp-6, undergo a specific LDCD pathway characterized by LMP and cathepsin-dependent cytoplasmic proteolysis. We designated this cell death routine, lysoptosis, to distinguish it from other pathways employing LMP. In this study, mouse and human epithelial cells lacking srp-6 homologues, mSerpinb3a and SERPINB3, respectively, demonstrated a lysoptosis phenotype distinct from other cell death pathways. Like in C. elegans, this pathway depended on LMP and released cathepsins, predominantly cathepsin L. These studies suggested that lysoptosis is an evolutionarily-conserved eukaryotic LDCD that predominates in the absence of neutralizing endogenous inhibitors.

Cliff Luke et al. report that lysoptosis is a eukaryotic stand-alone regulated cell death pathway. They identify that this new cell death modality predominates in the absence of neutralizing endogenous inhibitors.

Comparison of the Proteomes of Porcine Macrophages and a Stable Porcine Cell Line after Infection with African Swine Fever Virus

African swine fever virus (ASFV), causing an OIE-notifiable viral disease of swine, is spreading over the Eurasian continent and threatening the global pig industry. Here, we conducted the first proteome analysis of ASFV-infected primary porcine monocyte-derived macrophages (moMΦ). In parallel to moMΦ isolated from different pigs, the stable porcine cell line WSL-R was infected with a recombinant of ASFV genotype IX strain “Kenya1033”. The outcome of the infections was compared via quantitative mass spectrometry (MS)-based proteome analysis. Major differences with respect to the expression of viral proteins or the host cell response were not observed. However, cell-specific expression of some individual viral proteins did occur. The observed modulations of the host proteome were mainly related to cell characteristics and function. Overall, we conclude that both infection models are suitable for use in the study of ASFV infection in vitro.

Functional characterization of cathepsin B and its role in the antimicrobial immune responses in golden pompano (Trachinotus ovatus)

Cathepsin B (CTSB) is one of the typical representatives of cysteine protease family. It has the activity of both exopeptidase and endopeptidase. It plays an important role in antigen presentation, degradation, apoptosis, inflammatory response and physiological process of many diseases. In this study, CTSB of Trachinotus ovatus (TroCTSB) was cloned, and its structure and function were analyzed. The results showed that the coding region of TroCTSB was 993 bp, encoding 330 amino acid residues. The homology analysis showed that the amino acid sequence of TroCTSB was similar to that in other teleosts and mammals (68.69%-88.48%). Under normal physiological conditions, TroCTSB was widely distributed in various tissues with the highest expression level in stomach, followed by liver, and the lowest expression level in blood. The optimal pH and temperature of purified recombinant protein rTroCTSB were 5.5 and 40 °C, respectively. The toxicity test of metal ions showed that Fe²⁺, Cu²⁺, Ca²⁺ and Zn²⁺ could all inhibit the activity of TroCTSB, with Zn²⁺ ranking the first. In addition, after Edwardsiella tarda infection, the expression of TroCTSB was significantly up-regulated in liver, spleen and head kidney. The overexpression of TroCTSB significantly inhibited the infection of E. tarda in golden pompano tissues, and the knockdown of TroCTSB remarkably promoted the reproduction of E. tarda in golden pompano tissues in vivo. This study suggests that TroCTSB was involved in the antibacterial immune response of T. ovatus, and provided a reference for further research in elucidating the resistance mechanism of TroCTSB.

Comparative Proteomics of Ostreid Herpesvirus 1 and Pacific Oyster Interactions With Two Families Exhibiting Contrasted Susceptibility to Viral Infection

Massive mortality outbreaks affecting Pacific oysters (Crassostrea gigas) spat/juveniles are often associated with the detection of a herpesvirus called ostreid herpesvirus type 1 (OsHV-1). In this work, experimental infection trials of C. gigas spat with OsHV-1 were conducted using two contrasted Pacific oyster families for their susceptibility to viral infection. Live oysters were sampled at 12, 26, and 144 h post infection (hpi) to analyze host-pathogen interactions using comparative proteomics. Shotgun proteomics allowed the detection of seven viral proteins in infected oysters, some of them with potential immunomodulatoy functions. Viral proteins were mainly detected in susceptible oysters sampled at 26 hpi, which correlates with the mortality and viral load observed in this oyster family. Concerning the Pacific oyster proteome, more than 3,000 proteins were identified and contrasted proteomic responses were observed between infected A- and P-oysters, sampled at different post-injection times. Gene ontology (GO) and KEGG pathway enrichment analysis performed on significantly modulated proteins uncover the main immune processes (such as RNA interference, interferon-like pathway, antioxidant defense) which contribute to the defense and resistance of Pacific oysters to viral infection. In the more susceptible Pacific oysters, results suggest that OsHV-1 manipulate the molecular machinery of host immune response, in particular the autophagy system. This immunomodulation may lead to weakening and consecutively triggering death of Pacific oysters. The identification of several highly modulated and defense-related Pacific oyster proteins from the most resistant oysters supports the crucial role played by the innate immune system against OsHV-1 and the viral infection. Our results confirm the implication of proteins involved in an interferon-like pathway for efficient antiviral defenses and suggest that proteins involved in RNA interference process prevent viral replication in C. gigas. Overall, this study shows the interest of multi-omic approaches applied on groups of animals with differing sensitivities and provides novel insight into the interaction between Pacific oyster and OsHV-1 with key proteins involved in viral infection resistance.

The first characterization of a cystatin and a cathepsin L-like peptidase from Aedes aegypti and their possible role in DENV infection by the modulation of apoptosis

Recently, a salivary gland transcriptome study demonstrated that the transcripts of a putative cystatin gene (SeqID AAEL013287; Aacystatins) from Aedes aegypti were increased in DENV2-infected mosquitoes and that silencing of the Aacystatin gene resulted in an increase in DENV titres. In this work, Aacystatin was biochemically characterized; the purified recombinant inhibitor was able to inhibit typical cysteine proteases with a Ki in the nM range. Pulldown assays using Aag2 cell extracts identified a cathepsin L-like peptidase (AaCatL) as a possible target of Aacystatin. Purified recombinant AaCatL had an optimal pH of 5.0 and displayed a preference for Leu, Val and Phe residues at P2, which is common for other cathepsin L-like peptidases. Transcription analysis of Aacystatin and AaCatL in the salivary glands and midgut of DENV2-infected mosquitoes revealed a negative correlation between DENV2 titres and levels of the inhibitor and peptidase, suggesting their involvement in DENV2-mosquito interactions. Considering that apoptosis may play an important role during viral infections, the possible involvement of Aacystatin in staurosporine-induced apoptosis in Aag2 cells was investigated; the results showed higher expression of the inhibitor in treated cells; moreover, pre incubation with rAacystatin was able to increase Aag2 cell viability.

Necroptosis Signaling Pathways in Stroke: From Mechanisms to Therapies

It has been confirmed that apoptosis, autophagy and necrosis are the three major modes of cell death. For a long time, necrosis is regarded as a deranged or accidental cell demise. In recent years, there is evidence showing that necrotic cell death can be a well regulated and orchestrated event, which is also known as programmed cell death or “necroptosis”. Necroptosis can be triggered by a variety of external stimuli and regulated by a caspase-independent pathway. It plays a key role in the pathogenesis of some diseases including neurological diseases. In the past two decades, a variety of studies have revealed that the necroptosis related pathway is activated in stroke, and plays a crucial role in the pathogenesis of stroke. Moreover, necroptosis may serve as a potential target in the therapy of stroke because genetic or pharmacological inhibition of necroptosis has been shown to be neuroprotective in stroke in vitro and in vivo. In this review, we briefly summarize re-cent advances in necroptosis, introduce the mechanism and strategies targeting necroptosis in stroke, and finally propose some issues in the treatment of stroke by targeting necroptosis

Identification and expression of cathepsin B from the freshwater mussel Cristaria plicata

Cathepsin B plays crucial roles in host immune defense against pathogen infection. In present study, a cathepsin B gene from the freshwater mussel, Cristaria plicata (CpCathB) was cloned and characterized. The full-length cDNA of CpCathB was 1825 bp, and contained a 5' untranslated region (UTR) of 36 nucleotides, an open reading frame (ORF) of 1044 bp and a 3' UTR of 745 bp with a poly (A) tail. The deduced CpCathB protein was encoded as a preproenzyme with 347 amino acid residues and predicted molecular weight of 38.55 kDa. Sequence alignment revealed that CpCathB protein shared 56% - 60.7% identity comparison with other species. The predicted tertiary structure of CpCathB protein was highly similar to that of human. The CpCathB mRNA was expressed in hemocytes, hepatopancreas, adductor muscle, gills and mantle tissues of healthy mussels, and the highest expression level was in hepatopancreas. The transcripts of CpCathB were increased in hemocytes and hepatopancreas from mussels after Aeromonas hydrophila challenge. Moreover, the recombinant CpCathB was expressed in the Escherichia coli Rosetta-gami (DE3) strain. The maximum titer of the anti-CpCathB polyclonal antibodies was 1:640,000.The CpCathB protein had a higher expression in hepatopancreas and mantle and a lower level in hemocytes.

The independence of and associations among apoptosis, autophagy, and necrosis

Cell death is an essential biological process for physiological growth and development. Three classical forms of cell death-apoptosis, autophagy, and necrosis-display distinct morphological features by activating specific signaling pathways. With recent research advances, we have started to appreciate that these cell death processes can cross-talk through interconnecting, even overlapping, signaling pathways, and the final cell fate is the result of the interplay of different cell death programs. This review provides an insight into the independence of and associations among these three types of cell death and explores the significance of cell death under the specific conditions of human diseases, particularly neurodegenerative diseases and cancer.

Cytoplasmic vacuolization in cell death and survival

Cytoplasmic vacuolization (also called cytoplasmic vacuolation) is a well-known morphological phenomenon observed in mammalian cells after exposure to bacterial or viral pathogens as well as to various natural and artificial low-molecular-weight compounds. Vacuolization often accompanies cell death; however, its role in cell death processes remains unclear. This can be attributed to studying vacuolization at the level of morphology for many years. At the same time, new data on the molecular mechanisms of the vacuole formation and structure have become available. In addition, numerous examples of the association between vacuolization and previously unknown cell death types have been reported. Here, we review these data to make a deeper insight into the role of cytoplasmic vacuolization in cell death and survival.

Role of autophagy in oncolytic herpes simplex virus type 1-induced cell death in squamous cell carcinoma cells

Herpes simplex virus type 1 (HSV-1) is one of the most widely studied viruses for oncolytic virotherapy. In squamous cell carcinoma (SCC) cells, the role of autophagy induced by neurovirulence gene-deficient HSV-1s in programmed cell death has not yet been elucidated. The oncolytic HSV-1 strain RH2, which lacks the γ34.5 gene and induces the fusion of human SCC cells, was used. RH2 replicated and induced cell death in SCC cells. RH2 infection was accompanied by the aggregation of microtubule-associated protein 1 light chain 3 (LC3) in the cytoplasm, the conversion of LC3-I to LC3-II and the formation of double-membrane vacuoles containing cell contents. No significant changes were observed in the expression of Bcl-2 or Bax, while a slight decrease was observed in that of Beclin 1. The autophagy inhibitors, 3-methyladenine (3-MA) and bafilomycin A1, did not affect viral replication, but significantly inhibited the cytotoxicity of RH2. The caspase-3 inhibitor z-DEVD-fmk and caspase-1 inhibitor z-YVAD-fmk also reduced the cytotoxicity of RH2. These results demonstrated that γ34.5 gene-deficient HSV-1 RH2 induced autophagic cell death in SCC cells as well as pyroptosis and apoptosis.

Molecular cloning and functional characterization of cathepsin B from the sea cucumber Apostichopus japonicus

Cathepsin B (CTSB), a member of lysosomal cysteine protease, is involved in multiple levels of physiological and biological processes, and also plays crucial roles in host immune defense against pathogen infection in vertebrates. However, the function of CTSB within the innate immune system of invertebrates, particularly in marine echinoderms, has been poorly documented. In this study, the immune function of CTSB in Apostichopus japonicus (designated as AjCTSB), a commercially important and disease vulnerable aquaculture specie, was investigated by integrated molecular and protein approaches. A 2153 bp cDNA representing the full-length of AjCTSB was cloned via overlapping ESTs and RACE fragments. AjCTSB contained an open reading frame of 999 bp encoding a secreted protein of 332 amino acid residues with a predicted molecular mass of 36.8 kDa. The deduced amino acid of AjCTSB shared a typical activity center containing three conserved amino acid residues (Cys¹⁰⁸, His²⁷⁷ and Asn²⁹⁷). Phylogenetic tree analysis also supported that AjCTSB was a new member of CTSB family with clustering firstly with invertebrate CTSBs. Quantitative real time PCR analysis revealed that AjCTSB was ubiquitously expressed in all examined tissues with the highest levels in intestine. The Vibrio splendidus challenged sea cucumber and LPS-exposed coelomocytes could both significantly boost the expression of AjCTSB. Moreover, the purified recombinant AjCTSB exhibited dose-dependent CTSB activities at the concentration ranged from 0 to 0.24 μg μL^-1. Further functional analysis indicated that coelomocytes apoptosis was significantly inhibited by 0.16-fold in vivo and the apoptosis execution Ajcaspase 3 was extremely reduced in Apostichopus japonicus coelomocytes treated with specific AjCTSB siRNA. Collectively, all these results suggested that AjCTSB was an important immune factor and might be served as apoptosis enhancers in pathogen challenged sea cucumber.

HSV-1-induced activation of NF-κB protects U937 monocytic cells against both virus replication and apoptosis

The transcription factor nuclear factor-kappa B (NF-κB) is a crucial player of the antiviral innate response. Intriguingly, however, NF-κB activation is assumed to favour herpes simplex virus (HSV) infection rather than restrict it. Apoptosis, a form of innate response to viruses, is completely inhibited by HSV in fully permissive cells, but not in cells incapable to fully sustain HSV replication, such as immunocompetent cells. To resolve the intricate interplay among NF-κB signalling, apoptosis and permissiveness to HSV-1 in monocytic cells, we utilized U937 monocytic cells in which NF-κB activation was inhibited by expressing a dominant-negative IκBα. Surprisingly, viral production was increased in monocytic cells in which NF-κB was inhibited. Moreover, inhibition of NF-κB led to increased apoptosis following HSV-1 infection, associated with lysosomal membrane permeabilization. High expression of late viral proteins and induction of apoptosis occurred in distinct cells. Transcriptional analysis of known innate response genes by real-time quantitative reverse transcription-PCR excluded a contribution of the assayed genes to the observed phenomena. Thus, in monocytic cells NF-κB activation simultaneously serves as an innate process to restrict viral replication as well as a mechanism to limit the damage of an excessive apoptotic response to HSV-1 infection. This finding may clarify mechanisms controlling HSV-1 infection in monocytic cells.

Immunogenic cell death by oncolytic herpes simplex virus type 1 in squamous cell carcinoma cells

Molecules essential for the induction of immunogenic cell death (ICD) are called damage-associated molecular patterns (DAMPs). The effects of oncolytic herpes simplex virus type 1 (HSV-1) on the production of DAMPs were examined in squamous cell carcinoma (SCC) cells. The cytopathic effects of HSV-1 RH2 were observed in mouse SCCVII cells infected at a high multiplicity of infection (MOI), and the amounts of viable cells were decreased. After being infected with RH2, ATP and high mobility group box 1 (HMGB1) were released extracellulary, while calreticulin (CRT) translocated to the cell membrane. A flow-cytometric analysis revealed an increase in the number of annexin-V and propidium iodide (PI)-stained cells; and the amount of cleaved poly (ADP-ribose) polymerase (PARP) was increased. The killing effect of RH2 was reduced by pan-caspase inhibitor z-VAD-fmk and the caspase-1 inhibitor z-YVAD-fmk, suggesting the involvement of apoptosis and pyroptosis. In C3H mice bearing synergic SCCVII tumors, the growth of tumors injected with the supernatant of RH2-infected cells was less than that of tumors injected with phosphate-buffered saline (PBS). These results indicate that oncolytic HSV-1 RH2 produces DAMPs from SCC cells to induce cell death. This may contribute to the enhancement of tumor immunity by oncolytic HSV-1.

Expression of the immediate early IE180 protein under the control of the hTERT and CEA tumor-specific promoters in recombinant pseudorabies viruses: Effects of IE180 protein on promoter activity and apoptosis induction

Since the pseudorabies virus (PRV) genome encodes for a single immediate-early protein, IE180, we reasoned that this strong transactivating protein could represent a key regulatory switch that could be genetically manipulated in order to alter its tropism towards cancer cells. We therefore initiated studies to test whether the human telomerase reverse transcriptase (hTERT) and carcinoembryonic antigen (CEA) tumor promoters could functionally replace the IE180 promoter. We show that both promoters can functionally substitute the IE180 promoter in plasmid constructs and recombinant viruses, and observed that IE180 differentially auto-regulated each promoter tested, with PRV IE180 negatively regulating the hTERT promoter but positively hyper-activating the CEA promoter. Interestingly, we also observed that the recombinant PRV-TER and PRV-CEA viruses preferentially replicated in diverse cancer cell lines compared to control non-cancer cells, and the PRV-CEA was capable of additionally inducing a profound apoptotic phenotype which we correlated to the overexpression of IE180.

The UL13 and US3 Protein Kinases of Herpes Simplex Virus 1 Cooperate to Promote the Assembly and Release of Mature, Infectious Virions

Herpes simplex virus type 1 (HSV-1) encodes two bona fide serine/threonine protein kinases, the US3 and UL13 gene products. HSV-1 ΔUS3 mutants replicate with wild-type efficiency in cultured cells, and HSV-1 ΔUL13 mutants exhibit <10-fold reduction in infectious viral titers. Given these modest phenotypes, it remains unclear how the US3 and UL13 protein kinases contribute to HSV-1 replication. In the current study, we designed a panel of HSV-1 mutants, in which portions of UL13 and US3 genes were replaced by expression cassettes encoding mCherry protein or green fluorescent protein (GFP), respectively, and analyzed DNA replication, protein expression, and spread of these mutants in several cell types. Loss of US3 function alone had largely negligible effect on viral DNA accumulation, gene expression, virion release, and spread. Loss of UL13 function alone also had no appreciable effects on viral DNA levels. However, loss of UL13 function did result in a measurable decrease in the steady-state levels of two viral glycoproteins (gC and gD), release of total and infectious virions, and viral spread. Disruption of both genes did not affect the accumulation of viral DNA, but resulted in further reduction in gC and gD steady-state levels, and attenuation of viral spread and infectious virion release. These data show that the UL13 kinase plays an important role in the late phase of HSV-1 infection, likely by affecting virion assembly and/or release. Moreover, the data suggest that the combined activities of the US3 and UL13 protein kinases are critical to the efficient assembly and release of infectious virions from HSV-1-infected cells.

Manipulation of apoptosis and necroptosis signaling by herpesviruses

Like apoptosis, necroptosis is an innate immune mechanism that eliminates pathogen-infected cells. Receptor-interacting protein kinase (RIP)3 (also called RIPK3) mediates necrotic death by phosphorylating an executioner protein, MLKL, leading to plasma membrane leakage. The pathway is triggered against viruses that block caspase 8. In murine CMV, the viral inhibitor of caspase 8 activation prevents extrinsic apoptosis but also has the potential to unleash necroptosis. This virus encodes the viral inhibitor of RIP activation to prevent RIP homotypic interaction motif (RHIM)-dependent signal transduction and necroptosis. Recent investigations reveal a similar mechanism at play in the human alpha-herpesviruses, herpes simplex virus (HSV)1 and HSV2, where RHIM competitor function and caspase 8 suppression are carried out by the virus-encoded large subunit of ribonucleotide reductase (R1). In human cells, R1 inhibition of caspase 8 prevents TNF-induced apoptosis, but sensitizes to TNF-induced necroptosis. The RHIM and caspase 8 interaction domains of R1 collaborate to prevent RIP3-dependent steps and enable both herpesviruses to deflect host cell death machinery that would cut short infection. In mouse cells, HSV1 infection by itself triggers necroptosis by driving RIP3 protein kinase activity. HSV1 R1 contributes to the activation of RIP3 adaptor function in mice, a popular host animal for experimental infection. Based on these studies, infection of RIP3-kinase inactive mice should be explored in models of pathogenesis and latency. The necrotic death pathway that is suppressed during infection in the natural host becomes a cross-species barrier to infection in a non-natural host.

Necroptosis: The Trojan horse in cell autonomous antiviral host defense

Herpesviruses suppress cell death to assure sustained infection in their natural hosts. Murine cytomegalovirus (MCMV) encodes suppressors of apoptosis as well as M45-encoded viral inhibitor of RIP activation (vIRA) to block RIP homotypic interaction motif (RHIM)-signaling and recruitment of RIP3 (also called RIPK3), to prevent necroptosis. MCMV and human cytomegalovirus encode a viral inhibitor of caspase (Casp)8 activation to block apoptosis, an activity that unleashes necroptosis. Herpes simplex virus (HSV)1 and HSV2 incorporate both RHIM and Casp8 suppression strategies within UL39-encoded ICP6 and ICP10, respectively, which are herpesvirus-conserved homologs of MCMV M45. Both HSV proteins sensitize human cells to necroptosis by blocking Casp8 activity while preventing RHIM-dependent RIP3 activation and death. In mouse cells, HSV1 ICP6 interacts with RIP3 and, surprisingly, drives necroptosis. Thus, herpesviruses have illuminated the contribution of necoptosis to host defense in the natural host as well as its potential to restrict cross-species infections in nonnatural hosts.

Necroptosis, in vivo detection in experimental disease models

Over the last decade, our picture of cell death signals involved in experimental disease models totally shifted. Indeed, in addition to apoptosis, multiple forms of regulated necrosis have been associated with an increasing number of pathologies such as ischemia-reperfusion injury in brain, heart and kidney, inflammatory diseases, sepsis, retinal disorders, neurodegenerative diseases and infectious disorders. Especially necroptosis is currently attracting the attention of the scientific community. However, the in vivo identification of ongoing necroptosis in experimental disease conditions remains troublesome, mainly due to the lack of specific biomarkers. Initially, Receptor-Interacting Protein Kinase 1 (RIPK1) and RIPK3 kinase activity were uniquely associated with induction of necroptosis, however recent evidence suggests pleiotropic functions in cell death, inflammation and survival, obscuring a clear picture. In this review, we will present the last methodological advances for in vivo necroptosis identification and discuss past and recent data to provide an update of the so-called "necroptosis-associated pathologies".

Improving immunogenicity and efficacy of vaccines for genital herpes containing herpes simplex virus glycoprotein D

No vaccines are approved for prevention or treatment of genital herpes. The focus of genital herpes vaccine trials has been on prevention using herpes simplex virus type 2 (HSV-2) glycoprotein D (gD2) alone or combined with glycoprotein B. These prevention trials did not achieve their primary end points. However, subset analyses reported some positive outcomes in each study. The most recent trial was the Herpevac Trial for Women that used gD2 with monophosphoryl lipid A and alum as adjuvants in herpes simplex virus type 1 (HSV-1) and HSV-2 seronegative women. Unexpectedly, the vaccine prevented genital disease by HSV-1 but not HSV-2. Currently, HSV-1 causes more first episodes of genital herpes than HSV-2, highlighting the importance of protecting against HSV-1. The scientific community is conflicted between abandoning vaccine efforts that include gD2 and building upon the partial successes of previous trials. We favor building upon success and present approaches to improve outcomes of gD2-based subunit antigen vaccines.

Therapeutic exploitation of necroptosis for cancer therapy

Evasion of programmed cell death represents one of the hallmarks of cancers that contributes to tumor initiation, progression and treatment resistance. This calls for novel therapeutic concepts to reactivate cell death programs in human malignancies. Since necroptosis represents a regulated form of necrosis that is under the control of defined signal transduction pathways, it offers molecular targets for rational therapeutic intervention. Indeed, there is mounting evidence showing that many currently used anticancer agents can engage necroptotic signaling pathways and thereby elicit cell death in malignant cells. A better understanding of the signaling networks regulating necroptosis in cancer cells is expected to speed up the development of anticancer drugs for therapeutic exploitation of necroptosis for cancer therapy.

The combined effects of irradiation and herpes simplex virus type 1 infection on an immortal gingival cell line

BACKGROUND

Oral mucosa is frequently exposed to Herpes simplex virus type 1 (HSV-1) infection and irradiation due to dental radiography. During radiotherapy for oral cancer, the surrounding clinically normal tissues are also irradiated. This prompted us to study the effects of HSV-1 infection and irradiation on viability and apoptosis of oral epithelial cells.

METHODS

Immortal gingival keratinocyte (HMK) cells were infected with HSV-1 at a low multiplicity of infection (MOI) and irradiated with 2 Gy 24 hours post infection. The cells were then harvested at 24, 72 and 144 hours post irradiation for viability assays and qRT-PCR analyses for the apoptosis-related genes caspases 3, 8, and 9, bcl-2, NFκB1, and viral gene VP16. Mann-Whitney U-test was used for statistical calculations.

RESULTS

Irradiation improved the cell viability at 144 hours post irradiation (P = 0.05), which was further improved by HSV-1 infection at MOI of 0.00001 (P = 0.05). Simultaneously, the combined effects of infection at MOI of 0.0001 and irradiation resulted in upregulation in NFκB1 (P = 0.05). The combined effects of irradiation and HSV infection also significantly downregulated the expression of caspases 3, 8, and 9 at 144 hours (P = 0.05) whereas caspase 3 and 8 significantly upregulated in non-irradiated, HSV-infected cells as compared to uninfected controls (P = 0.05). Infection with 0.0001 MOI downregulated bcl-2 in non-irradiated cells but was upregulated by 27% after irradiation when compared to non-irradiated infected cells (P = 0.05). Irradiation had no effect on HSV-1 shedding or HSV gene expression at 144 hours.

CONCLUSIONS

HSV-1 infection may improve the viability of immortal cells after irradiation. The effect might be related to inhibition of apoptosis.

Cytosolic double-stranded DNA induces nonnecroptotic programmed cell death in trophoblasts via IFI16

The mechanisms underlying the immune defense by trophoblasts against pathogens remain ill defined. We demonstrated that placental cell death was increased upon in vivo exposure to Listeria monocytogenes. The death of infected cells is an important host innate defense mechanism. Meanwhile, double-stranded DNA (dsDNA) derived from intracellular bacteria or dsDNA viruses is emerging as a potent pathogen-associated molecular pattern recognized by host cells. We sought to characterize trophoblast death in response to cytosolic dsDNA challenge. Our results showed that dsDNA induced caspase-dependent and -independent cell death in human trophoblasts. However, necroptosis, a cell death pathway independent of caspase, could not be induced by dsDNA treatment, even in the presence of exogenously expressed RIPK3. L. monocytogenes-derived genomic DNA triggered a similar cell death pattern. Moreover, the cell death in response to dsDNA was IFI16 dependent. These data suggest that cytosolic dsDNA induces nonnecroptotic cell death in trophoblasts via IFI16, and this could contribute to placental barrier against infection.

The role of necroptosis, an alternative form of cell death, in cancer therapy

Programmed cell death plays an important role in animal development, tissue homeostasis and eliminating harmful or virally infected cells. Necroptosis, a novel form of programmed cell death, is caspase independent but RIPK and RIPK3 dependent. Moreover, it is suggested that necroptosis can be specifically inhibited by small molecular inhibitors such as necrostatin-1. Its signaling pathways have something in common with apoptosis, although the molecular mechanisms of necroptosis need to be further elucidated. Previous evidences suggest that necroptosis has significant effects in regulating various physiological processes and disease, such as ischemic brain injury, immune system disorders and cancer. In this review, the molecular mechanism of necroptosis is described and how it could be manipulated in the treatment of cancer is summarized.

The mechanism of necroptosis in normal and cancer cells

Programmed cell death is a basic cellular process that is critical to maintain tissue homeostasis. Besides apoptosis, necroptosis has more recently been discovered as another form of regulated cell death. Necroptosis plays a pivotal role during normal development and has also been implicated in the pathogenesis of a variety of human diseases. The control of necroptosis by defined signal transduction pathways offers the opportunity to target this cellular process for therapeutic purposes. For example, in cancer necroptosis is often impaired during tumorigenesis and can be engaged by targeted pharmacological approaches. Further insights into the signaling networks involved in the regulation of necroptosis will likely have important implications for the exploitation of this form of programmed cell death for the diagnosis or treatment of many diseases.

Immunological control of herpes simplex virus infections

Herpes simplex virus type 1 (HSV-1) is capable of causing a latent infection in sensory neurons that lasts for the lifetime of the host. The primary infection is resolved following the induction of the innate immune response that controls replication of the virus until the adaptive immune response can clear the active infection. HSV-1-specific CD8⁺ T cells survey the ganglionic regions containing latently infected neurons and participate in preventing reactivation of HSV from latency. The long-term residence and migration dynamics of the T cells in the trigeminal ganglia appear to distinguish them from the traditional memory T cell subsets. Recently described tissue resident memory (T_RM) T cells establish residence and survive for long periods in peripheral tissue compartments following antigen exposure. This review focuses on the immune system response to HSV-1 infection. Particular emphasis is placed on the evidence pointing to the HSV-1-specific CD8⁺ T cells in the trigeminal belonging to the T_RM class of memory T cells and the role of T_RM cells in virus infection, pathogenesis, latency, and disease.

Induction of apoptosis by the Amsacta moorei entomopoxvirus

CF-70-B2 cells derived from the spruce budworm (Choristoneura fumiferana) undergo apoptosis when infected with Amsacta moorei entomopoxvirus (AMEV), as characterized by membrane blebbing, formation of apoptotic bodies, TdT-mediated dUTP nick-end labelling (TUNEL) staining, condensed chromatin and induction of caspase-3/7 activity. The apoptotic response was reduced when cells were infected with UV-inactivated AMEV, but not when infected in the presence of the DNA synthesis inhibitor, cytosine β-d-arabinofuranoside. Hence, only pre-DNA replication events were involved in inducing the antiviral response in CF-70-B2 cells. The virus eventually overcame the host’s antiviral response and replicated to high progeny virus titres accompanied by high levels of caspase-3/7 activity. The CF-70-B2 cells were less productive of progeny virus in comparison to LD-652, a Lymantria dispar cell line routinely used for propagation of AMEV. At late stages of infection, LD-652 cells also showed characteristics of apoptosis such as oligosomal DNA fragmentation, TUNEL staining, condensed chromatin and increased caspase-3/7 activity. Induction of apoptosis in LD-652 cells was dependent on viral DNA replication and/or late gene expression. A significantly reduced rate of infection was observed in the presence of general caspase inhibitors Q-VD-OPH and Z-VAD-FMK, indicating caspases may be involved in productive virus infection.

Role of cathepsin B in dengue virus-mediated apoptosis

Dengue virus (DENV) infection is one of the most important mosquito-borne viral diseases, which is endemic in the tropical and sub-tropical regions. Patients with dengue hemorrhagic fever (DHF) generally present hemorrhagic tendencies, plasma leakage, thrombocytopenia, and hemoconcentration. Hepatic dysfunction is also a crucial feature of DENV infection. Hepatic biopsy specimens obtained from fatal cases of DENV infection show cellular apoptosis, which apparently relate to the pathogenesis. Cathepsins, which are cysteine proteases inside the lysosome, were previously reported to be up-regulated in patients with DHF. However, their functions during DENV infection have not been thoroughly investigated. We show for the first time that DENV induces lysosomal membrane permeabilization. The resulting cytosolic cathepsin B and S contributed to apoptosis via caspase activation. The activity of caspase 3 was significantly reduced in DENV-infected HepG2 cells treatedwith cathepsin B or S inhibitors. Treatment with cathepsin B inhibitor also reduced the activity of caspase 9, suggesting that cathepsin B activates both caspase-9 and caspase-3. Reduced cathepsin B expression, effected by RNA interference, mimicked pharmacological inhibition of the enzyme and confirmed the contribution of cathepsin B to apoptotic events induced by DENV in HepG2 cells.

Necroptosis: the release of damage-associated molecular patterns and its physiological relevance

Regulated necrosis, termed necroptosis, is negatively regulated by caspase-8 and is dependent on the kinase activity of RIPK1 and RIPK3. Necroptosis leads to rapid plasma membrane permeabilization and to the release of cell contents and exposure of damage-associated molecular patterns (DAMPs). We are only beginning to identify the necroptotic DAMPs, their modifications, and their potential role in the regulation of inflammation. In this review, we discuss the physiological relevance of necroptosis and its role in the modulation of inflammation. For example, during viral infection, RIPK3-mediated necroptosis acts as a backup mechanism to clear pathogens. Necroptosis is also involved in apparently immunologically silent maintenance of T cell homeostasis. In contrast, the induction of necroptosis in skin, intestine, systemic inflammatory response syndrome, and ischemia reperfusion injury provoke a strong inflammatory response, which might be triggered by emission of DAMPs from necroptotic cells, showing the detrimental side of necroptosis.

Prevention of cellular suicide by cytomegaloviruses

As intracellular parasites, viruses rely on many host cell functions to ensure their replication. The early induction of programmed cell death (PCD) in infected cells constitutes an effective antiviral host mechanism to restrict viral spread within an organism. As a countermeasure, viruses have evolved numerous strategies to interfere with the induction or execution of PCD. Slowly replicating viruses such as the cytomegaloviruses (CMVs) are particularly dependent on sustained cell viability. To preserve viability, the CMVs encode several viral cell death inhibitors that target different key regulators of the extrinsic and intrinsic apoptosis pathways. The best-characterized CMV-encoded inhibitors are the viral inhibitor of caspase-8-induced apoptosis (vICA), viral mitochondrial inhibitor of apoptosis (vMIA), and viral inhibitor of Bak oligomerization (vIBO). Moreover, a viral inhibitor of RIP-mediated signaling (vIRS) that blocks programmed necrosis has been identified in the genome of murine CMV (MCMV), indicating that this cell death mode is a particularly important part of the antiviral host response. This review provides an overview of the known cell death suppressors encoded by CMVs and their mechanisms of action.

Many stimuli pull the necrotic trigger, an overview

The lab of Jürg Tschopp was the first to report on the crucial role of receptor-interacting protein kinase 1 (RIPK1) in caspase-independent cell death. Because of this pioneer finding, regulated necrosis and in particular RIPK1/RIPK3 kinase-mediated necrosis, referred to as necroptosis, has become an intensively studied form of regulated cell death. Although necrosis was identified initially as a backup cell death program when apoptosis is blocked, it is now recognized as a cellular defense mechanism against viral infections and as being critically involved in ischemia-reperfusion damage. The observation that RIPK3 ablation rescues embryonic lethality in mice deficient in caspase-8 or Fas-associated-protein-via-a-death-domain demonstrates the crucial role of this apoptotic platform in the negative control of necroptosis during development. Here, we review and discuss commonalities and differences of the increasing list of inducers of regulated necrosis ranging from cytokines, pathogen-associated molecular patterns, to several forms of physicochemical cellular stress. Since the discovery of the crucial role of RIPK1 and RIPK3 in necroptosis, these kinases have become potential therapeutic targets. The availability of new pharmacological inhibitors and transgenic models will allow us to further document the important role of this form of cell death in degenerative, inflammatory and infectious diseases.

Mycobacterium tuberculosis induces an atypical cell death mode to escape from infected macrophages

Background

Macrophage cell death following infection with Mycobacterium tuberculosis plays a central role in tuberculosis disease pathogenesis. Certain attenuated strains induce extrinsic apoptosis of infected macrophages but virulent strains of M. tuberculosis suppress this host response. We previously reported that virulent M. tuberculosis induces cell death when bacillary load exceeds ∼20 per macrophage but the precise nature of this demise has not been defined.

Methodology/Principal Findings

We analyzed the characteristics of cell death in primary murine macrophages challenged with virulent or attenuated M. tuberculosis complex strains. We report that high intracellular bacillary burden causes rapid and primarily necrotic death via lysosomal permeabilization, releasing hydrolases that promote Bax/Bak-independent mitochondrial damage and necrosis. Cell death was independent of cathepsins B or L and notable for ultrastructural evidence of damage to lipid bilayers throughout host cells with depletion of several host phospholipid species. These events require viable bacteria that can respond to intracellular cues via the PhoPR sensor kinase system but are independent of the ESX1 system.

Conclusions/Significance

Cell death caused by virulent M. tuberculosis is distinct from classical apoptosis, pyroptosis or pyronecrosis. Mycobacterial genes essential for cytotoxicity are regulated by the PhoPR two-component system. This atypical death mode provides a mechanism for viable bacilli to exit host macrophages for spreading infection and the eventual transition to extracellular persistence that characterizes advanced pulmonary tuberculosis.