IRE1α promotes viral infection by conferring resistance to apoptosis

CD81 suppresses NF-κB signaling and is downregulated in hepatitis C virus expressing cells

The tetraspanin CD81 is one of the main entry receptors for Hepatitis C virus, which is a major causative agent to develop liver cirrhosis and hepatocellular carcinoma (HCC). Here, we identify CD81 as one of few surface proteins that are downregulated in HCV expressing hepatoma cells, discovering a functional role of CD81 beyond mediating HCV entry. CD81 was downregulated at the mRNA level in hepatoma cells that replicate HCV. Kinetics of HCV expression were increased in CD81-knockout cells and accompanied by enhanced cellular growth. Furthermore, loss of CD81 compensated for inhibition of pro-survival TBK1-signaling in HCV expressing cells. Analysis of functional phenotypes that could be associated with pro-survival signaling revealed that CD81 is a negative regulator of NF-κB. Interaction of the NF-κB subunits p50 and p65 was increased in cells lacking CD81. Similarly, we witnessed an overall increase in the total levels of phosphorylated and cellular p65 upon CD81-knockout in hepatoma cells. Finally, translocation of p65 in CD81-negative hepatoma cells was markedly induced upon stimulation with TNFα or PMA. Altogether, CD81 emerges as a regulator of pro-survival NF-κB signaling. Considering the important and established role of NF-κB for HCV replication and tumorigenesis, the downregulation of CD81 by HCV and the associated increase in NF-κB signaling might be relevant for viral persistence and chronic infection.

The unfolded protein response components IRE1α and XBP1 promote human coronavirus infection

The cellular processes that support human coronavirus replication and contribute to the pathogenesis of severe disease remain incompletely understood. Many viruses, including coronaviruses, cause endoplasmic reticulum (ER) stress during infection. IRE1α is a component of the cellular response to ER stress that initiates non-conventional splicing of XBP1 mRNA. Spliced XBP1 encodes a transcription factor that induces the expression of ER-related targets. Activation of the IRE1α-XBP1 pathway occurs in association with risk factors for severe human coronavirus infection. In this study, we found that the human coronaviruses HCoV-OC43 (human coronavirus OC43) and SARS-CoV-2 (severe acute respiratory syndrome coronavirus-2) both robustly activate the IRE1α-XBP1 branch of the unfolded protein response in cultured cells. Using IRE1α nuclease inhibitors and genetic knockdown of IRE1α and XBP1, we found that these host factors are required for optimal replication of both viruses. Our data suggest that IRE1α supports infection downstream of initial viral attachment and entry. In addition, we found that ER stress-inducing conditions are sufficient to enhance human coronavirus replication. Furthermore, we found markedly increased XBP1 in circulation in human patients with severe coronavirus disease 2019 (COVID-19). Together, these results demonstrate the importance of IRE1α and XBP1 for human coronavirus infection. IMPORTANCE There is a critical need to understand the cellular processes co-opted during human coronavirus replication, with an emphasis on identifying mechanisms underlying severe disease and potential therapeutic targets. Here, we demonstrate that the host proteins IRE1α and XBP1 are required for robust infection by the human coronaviruses, SARS-CoV-2 and HCoV-OC43. IRE1α and XBP1 participate in the cellular response to ER stress and are activated during conditions that predispose to severe COVID-19. We found enhanced viral replication with exogenous IRE1α activation, and evidence that this pathway is activated in humans during severe COVID-19. Together, these results demonstrate the importance of IRE1α and XBP1 for human coronavirus infection.

The Unfolded Protein Response Sensor IRE1 Regulates Activation of In Vitro Differentiated Type 1 Conventional DCs with Viral Stimuli

Type 1 conventional dendritic cells (cDC1s) are leukocytes competent to coordinate antiviral immunity, and thus, the intracellular mechanisms controlling cDC1 function are a matter of intense research. The unfolded protein response (UPR) sensor IRE1 and its associated transcription factor XBP1s control relevant functional aspects in cDC1s including antigen cross-presentation and survival. However, most studies connecting IRE1 and cDC1 function are undertaken in vivo. Thus, the aim of this work is to elucidate whether IRE1 RNase activity can also be modeled in cDC1s differentiated in vitro and reveal the functional consequences of such activation in cells stimulated with viral components. Our data show that cultures of optimally differentiated cDC1s recapitulate several features of IRE1 activation noticed in in vivo counterparts and identify the viral analog Poly(I:C) as a potent UPR inducer in the lineage. In vitro differentiated cDC1s display constitutive IRE1 RNase activity and hyperactivate IRE1 RNase upon genetic deletion of XBP1s, which regulates production of the proinflammatory cytokines IL-12p40, TNF-α and IL-6, Ifna and Ifnb upon Poly(I:C) stimulation. Our results show that a strict regulation of the IRE1/XBP1s axis regulates cDC1 activation to viral agonists, expanding the scope of this UPR branch in potential DC-based therapies.

Nuanced role for dendritic cell intrinsic IRE1 RNase in the regulation of antitumor adaptive immunity

In cancer, activation of the IRE1/XBP1s axis of the unfolded protein response (UPR) promotes immunosuppression and tumor growth, by acting in cancer cells and tumor infiltrating immune cells. However, the role of IRE1/XBP1s in dendritic cells (DCs) in tumors, particularly in conventional type 1 DCs (cDC1s) which are cellular targets in immunotherapy, has not been fully elucidated. Here, we studied the role of IRE1/XBP1s in subcutaneous B16/B78 melanoma and MC38 tumors by generating loss-of-function models of IRE1 and/or XBP1s in DCs or in cDC1s. Data show that concomitant deletion of the RNase domain of IRE1 and XBP1s in DCs and cDC1s does not influence the kinetics of B16/B78 and MC38 tumor growth or the effector profile of tumor infiltrating T cells. A modest effect is observed in mice bearing single deletion of XBP1s in DCs, which showed slight acceleration of melanoma tumor growth and dysfunctional T cell responses, however, this effect was not recapitulated in animals lacking XBP1 only in cDC1s. Thus, evidence presented here argues against a general pro-tumorigenic role of the IRE1/XBP1s pathway in tumor associated DC subsets.

Pharmacological Inhibition of IRE-1 Alpha Activity in Herpes Simplex Virus Type 1 and Type 2-Infected Dendritic Cells Enhances T Cell Activation

Herpes simplex virus type 1 (HSV-1) and type 2 (HSV-2) infections are life-long and highly prevalent in the human population. These viruses persist in the host, eliciting either symptomatic or asymptomatic infections that may occur sporadically or in a recurrent manner through viral reactivations. Clinical manifestations due to symptomatic infection may be mild such as orofacial lesions, but may also translate into more severe diseases, such as ocular infections that may lead to blindness and life-threatening encephalitis. A key feature of herpes simplex viruses (HSVs) is that they have evolved molecular determinants that hamper numerous components of the host’s antiviral innate and adaptive immune system. Importantly, HSVs infect and negatively modulate the function of dendritic cells (DCs), by inhibiting their T cell-activating capacity and eliciting their apoptosis after infection. Previously, we reported that HSV-2 activates the splicing of the mRNA of XBP1, which is related to the activity of the unfolded protein response (UPR) factor Inositol-Requiring Enzyme 1 alpha (IRE-1α). Here, we sought to evaluate if the activation of the IRE-1α pathway in DCs upon HSV infection may be related to impaired DC function after infection with HSV-1 or HSV-2. Interestingly, the pharmacological inhibition of the endonuclease activity of IRE-1α in HSV-1- and HSV-2-infected DCs significantly reduced apoptosis in these cells and enhanced their capacity to migrate to lymph nodes and activate virus-specific CD4⁺ and CD8⁺ T cells. These findings suggest that the activation of the IRE-1α-dependent UPR pathway in HSV-infected DCs may play a significant role in the negative effects that these viruses exert over these cells and that the modulation of this signaling pathway may be relevant for enhancing the function of DCs upon infection with HSVs.

Herpes simplex virus interference with immunity: Focus on dendritic cells

Herpes simplex virus type 1 (HSV-1) and type 2 (HSV-2) are highly prevalent in the human population. These viruses cause lifelong infections by establishing latency in neurons and undergo sporadic reactivations that promote recurrent disease and new infections. The success of HSVs in persisting in infected individuals is likely due to their multiple molecular determinants involved in escaping the host antiviral and immune responses. Importantly, HSVs infect and negatively modulate the function of dendritic cells (DCs), key immune cells that are involved in establishing effective and balanced immunity against viruses. Here, we review and discuss several molecular and cellular processes modulated by HSVs in DCs, such as autophagy, apoptosis, and the unfolded protein response. Given the central role of DCs in establishing optimal antiviral immunity, particular emphasis should be given to the outcome of the interactions occurring between HSVs and DCs.

Coronavirus transmissible gastroenteritis virus antagonizes the antiviral effect of the microRNA miR-27b via the IRE1 pathway

Although the functional parameters of microRNAs (miRNAs) have been explored to some extent, the roles of these molecules in coronavirus infection and the regulatory mechanism of miRNAs in virus infection are still unclear. Transmissible gastroenteritis virus (TGEV) is an enteropathgenic coronavirus and causes high morbidity and mortality in suckling piglets. Here, we demonstrated that microRNA-27b-3p (miR-27b-3p) suppressed TGEV replication by directly targeting porcine suppressor of cytokine signaling 6 (SOCS6), while TGEV infection downregulated miR-27b-3p expression in swine testicular (ST) cells and in piglets. Mechanistically, the decrease of miR-27b-3p expression during TGEV infection was mediated by the activated inositol-requiring enzyme 1 (IRE1) pathway of the endoplasmic reticulum (ER) stress. Further studies showed that when ER stress was induced by TGEV, IRE1 acted as an RNase activated by autophosphorylation and unconventionally spliced mRNA encoding a potent transcription factor, X-box-binding protein 1 (Xbp1s). Xbp1s inhibited the transcription of miR-27 and ultimately reduced the production of miR-27b-3p. Therefore, our findings indicate that TGEV inhibits the expression of an anti-coronavirus microRNA through the IRE1 pathway and suggest a novel way in which coronavirus regulates the host cell response to infection.

The molecular mechanism and functional diversity of UPR signaling sensor IRE1

The endoplasmic reticulum is primarily responsible for protein folding and maturation. However, the organelle is subject to varied stress conditions from time to time, which lead to the activation of a signaling program known as the Unfolded Protein Response (UPR) pathway. This pathway, upon sensing any disturbance in the protein-folding milieu sends signals to the nucleus and cytoplasm in order to restore homeostasis. One of the prime UPR signaling sensors is Inositol-requiring enzyme 1 (IRE1); an ER membrane embedded protein with dual enzyme activities, kinase and endoribonuclease. The ribonuclease activity of IRE1 results in Xbp1 splicing in mammals or Hac1 splicing in yeast. However, IRE1 can switch its substrate specificity to the mRNAs that are co-transnationally transported to the ER, a phenomenon known as Regulated IRE1 Dependent Decay (RIDD). IRE1 is also reported to act as a principal molecule that coordinates with other proteins and signaling pathways, which in turn might be responsible for its regulation. The current review highlights studies on IRE1 explaining the structural features and molecular mechanism behind its ribonuclease outputs. The emphasis is also laid on the molecular effectors, which directly or indirectly interact with IRE1 to either modulate its function or connect it to other pathways. This is important in understanding the functional pleiotropy of IRE1, by which it can switch its activity from pro-survival to pro-apoptotic, thus determining the fate of cells.

Role of Endoplasmic Reticulum Stress Sensor IRE1α in Cellular Physiology, Calcium, ROS Signaling, and Metaflammation

Inositol-requiring transmembrane kinase endoribonuclease-1α (IRE1α) is the most prominent and evolutionarily conserved unfolded protein response (UPR) signal transducer during endoplasmic reticulum functional upset (ER stress). A IRE1α signal pathway arbitrates yin and yang of cellular fate in objectionable conditions. It plays several roles in fundamental cellular physiology as well as in several pathological conditions such as diabetes, obesity, inflammation, cancer, neurodegeneration, and in many other diseases. Thus, further understanding of its molecular structure and mechanism of action during different cell insults helps in designing and developing better therapeutic strategies for the above-mentioned chronic diseases. In this review, recent insights into structure and mechanism of activation of IRE1α along with its complex regulating network were discussed in relation to their basic cellular physiological function. Addressing different binding partners that can modulate IRE1α function, UPRosome triggers different downstream pathways depending on the cellular backdrop. Furthermore, IRE1α are in normal cell activities outside the dominion of ER stress and activities under the weather of inflammation, diabetes, and obesity-related metaflammation. Thus, IRE1 as an ER stress sensor needs to be understood from a wider perspective for comprehensive functional meaning, which facilitates us with assembling future needs and therapeutic benefits.

Integrated stress response in hepatitis C promotes Nrf2-related chaperone-mediated autophagy: A novel mechanism for host-microbe survival and HCC development in liver cirrhosis

The molecular mechanism(s) how liver damage during the chronic hepatitis C virus (HCV) infection evolve into cirrhosis and hepatocellular carcinoma (HCC) is unclear. HCV infects hepatocyte, the major cell types in the liver. During infection, large amounts of viral proteins and RNA replication intermediates accumulate in the endoplasmic reticulum (ER) of the infected hepatocyte, which creates a substantial amount of stress response. Infected hepatocyte activates a different type of stress adaptive mechanisms such as unfolded protein response (UPR), antioxidant response (AR), and the integrated stress response (ISR) to promote virus-host cell survival. The hepatic stress is also amplified by another layer of innate and inflammatory response associated with cellular sensing of virus infection through the production of interferon (IFN) and inflammatory cytokines. The interplay between various types of cellular stress signal leads to different forms of cell death such as apoptosis, necrosis, and autophagy depending on the intensity of the stress and nature of the adaptive cellular response. How do the adaptive cellular responses decode such death programs that promote host-microbe survival leading to the establishment of chronic liver disease? In this review, we discuss how the adaptive cellular response through the Nrf2 pathway that promotes virus and cell survival. Furthermore, we provide a glimpse of novel stress-induced Nrf2 mediated compensatory autophagy mechanisms in virus-cell survival that degrade tumor suppressor gene and activation of oncogenic signaling during HCV infection. Based on these facts, we hypothesize that the balance between hepatic stress, inflammation and different types of cell death determines liver disease progression outcomes. We propose that a more nuanced understanding of virus-host interactions under excessive cellular stress may provide an answer to the fundamental questions why some individuals with chronic HCV infection remain at risk of developing cirrhosis, cancer and some do not.

IRE1α Promotes Zika Virus Infection via XBP1

Zika virus (ZIKV) is an emergent member of the Flaviviridae family which causes severe congenital defects and other major sequelae, but the cellular processes that support ZIKV replication are incompletely understood. Related flaviviruses use the endoplasmic reticulum (ER) as a membranous platform for viral replication and induce ER stress during infection. Our data suggest that ZIKV activates IRE1α, a component of the cellular response to ER stress. IRE1α is an ER-resident transmembrane protein that possesses a cytosolic RNase domain. Upon activation, IRE1α initiates nonconventional cytoplasmic splicing of XBP1 mRNA. Spliced XBP1 encodes a transcription factor, which upregulates ER-related targets. We find that ZIKV infection induces XBP1 mRNA splicing and induction of XBP1 target genes. Small molecule inhibitors of IRE1α, including those specific for the nuclease function, prevent ZIKV-induced cytotoxicity, as does genetic disruption of IRE1α. Optimal ZIKV RNA replication requires both IRE1α and XBP1. Spliced XBP1 has been described to cause ER expansion and remodeling and we find that ER redistribution during ZIKV infection requires IRE1α nuclease activity. Finally, we demonstrate that inducible genetic disruption of IRE1α and XBP1 impairs ZIKV replication in a mouse model of infection. Together, our data indicate that the ER stress response component IRE1α promotes ZIKV infection via XBP1 and may represent a potential therapeutic target.

Homeostatic regulation of STING protein at the resting state by stabilizer TOLLIP

STING (stimulator of interferon genes) is an important innate immune protein, but its homeostatic regulation at the resting state is unknown. Here, we identified TOLLIP as a stabilizer of STING through direct interaction to prevent its degradation. Tollip deficiency results in reduced STING protein in nonhematopoietic cells and tissues, and renders STING protein unstable in immune cells, leading to severely dampened STING signaling capacity. The competing degradation mechanism of resting-state STING requires IRE1α and lysosomes. TOLLIP mediates clearance of Huntington's disease-linked polyQ protein aggregates. Ectopically expressed polyQ proteins in vitro or endogenous polyQ proteins in Huntington's disease mouse striatum sequester TOLLIP away from STING, leading to reduced STING protein and dampened immune signaling. Tollip-/- also ameliorates STING-mediated autoimmune disease in Trex1-/- mice. Together, our findings reveal that resting-state STING protein level is strictly regulated by a constant tug-of-war between 'stabilizer' TOLLIP and 'degrader' IRE1α-lysosome that together maintain tissue immune homeostasis.

Activation of the Unfolded Protein Response Pathway in Cytotoxic T Cells: A Comparison Between in vitro Stimulation, Infection, and the Tumor Microenvironment

IRE1α is an extremely conserved intracellular receptor that regulates one branch of the unfolded protein response (UPR). Homologs of IRE1α are found virtually throughout all eukaryotes. This receptor plays a pivotal role in a cell's reaction to stress, determining whether to take compensatory measures and survive or undergo apoptosis and die. While the role of the unfolded protein response in lower organisms and secretory cells has been comprehensively studied, the precise role of IRE1α in the context of cytotoxic T cells has only begun to be elucidated within the past decade. This review discusses what is known about IRE1α and the unfolded protein response in cytotoxic T cells within the context of development, pathogen response, and cancer cell growth.

Understanding the Role of the Unfolded Protein Response Sensor IRE1 in the Biology of Antigen Presenting Cells

The unfolded protein response (UPR) is an adaptive response that maintains the fidelity of the cellular proteome in conditions that subvert the folding capacity of the cell, such as those noticed in infection and inflammatory contexts. In immunity, the UPR sensor IRE1 (Inositol-requiring enzyme 1-alpha) has emerged as a critical regulator of the homeostasis of antigen presenting cells (APCs). In the past few years, it has become clear that IRE1 plays canonical and non-canonical roles in APCs, many of which intersect with key features of these cells, including the initiation of inflammation, antibody production, and antigen presentation. The aims of the present review are to provide recent insights on the mechanisms by which IRE1 regulates the diversity of APC functions and to highlight its relevance in the coordination of innate and adaptive immunity.

IRE1α Activation in Bone Marrow-Derived Dendritic Cells Modulates Innate Recognition of Melanoma Cells and Favors CD8+ T Cell Priming

The IRE1α/XBP1s signaling pathway is an arm of the unfolded protein response (UPR) that safeguards the fidelity of the cellular proteome during endoplasmic reticulum (ER) stress, and that has also emerged as a key regulator of dendritic cell (DC) homeostasis. However, in the context of DC activation, the regulation of the IRE1α/XBP1s axis is not fully understood. In this work, we report that cell lysates generated from melanoma cell lines markedly induce XBP1s and certain members of the UPR such as the chaperone BiP in bone marrow derived DCs (BMDCs). Activation of IRE1α endonuclease upon innate recognition of melanoma cell lysates was required for amplification of proinflammatory cytokine production and was necessary for efficient cross-presentation of melanoma-associated antigens without modulating the MHC-II antigen presentation machinery. Altogether, this work provides evidence indicating that ex-vivo activation of the IRE1α/XBP1 pathway in BMDCs enhances CD8⁺ T cell specific responses against tumor antigens.

BST2 inhibits infection of influenza A virus by promoting apoptosis of infected cells

BST2 is an antiviral factor that inhibits the release of enveloped virus at the plasma membrane via an unusual topology in which its N-terminal is in the cytosol while its C-terminal is anchored by glycophosphatidylinositol (GPI). BST2-deficient cells showed substantially higher release of virions than wild type cells. Influenza-infected BST2-deficient cells showed greatly reduced cytopathic effect (CPE) than wild type cells despite their generally robust virus production. This finding prompted us to determine whether BST2 was involved in the apoptotic process of virus-infected host cells. Our results revealed that BST2 might be involved in IRE1α-mediated ER stress pathway by increasing spliced form XBP-1. Consequently, levels of cytochrome C, caspase-3, caspase-9, and PARP as representative molecules of apoptosis were significantly increased in wild type cells than those in BST2-deficient cells. These results suggest that BST2 might participate in innate host defense by augmenting ER-stress-induced apoptotic signaling to inhibit the replication and spread of virus.

The Coronavirus Transmissible Gastroenteritis Virus Evades the Type I Interferon Response through IRE1α-Mediated Manipulation of the MicroRNA miR-30a-5p/SOCS1/3 Axis

In host innate immunity, type I interferons (IFN-I) are major antiviral molecules, and coronaviruses have evolved diverse strategies to counter the IFN-I response during infection. Transmissible gastroenteritis virus (TGEV), a member of the Alphacoronavirus family, induces endoplasmic reticulum (ER) stress and significant IFN-I production after infection. However, how TGEV evades the IFN-I antiviral response despite the marked induction of endogenous IFN-I has remained unclear. Inositol-requiring enzyme 1 α (IRE1α), a highly conserved ER stress sensor with both kinase and RNase activities, is involved in the IFN response. In this study, IRE1α facilitated TGEV replication via downmodulating the host microRNA (miR) miR-30a-5p abundance. miR-30a-5p normally enhances IFN-I antiviral activity by directly targeting the negative regulators of Janus family kinase (JAK)-signal transducer and activator of transcription (STAT), the suppressor of cytokine signaling protein 1 (SOCS1), and SOCS3. Furthermore, TGEV infection increased SOCS1 and SOCS3 expression, which dampened the IFN-I antiviral response and facilitated TGEV replication. Importantly, compared with mock infection, TGEV infection in vivo resulted in decreased miR-30a-5p levels and significantly elevated SOCS1 and SOCS3 expression in the piglet ileum. Taken together, our data reveal a new strategy used by TGEV to escape the IFN-I response by engaging the IRE1α-miR-30a-5p/SOCS1/3 axis, thus improving our understanding of how TGEV escapes host innate immune defenses.IMPORTANCE Type I interferons (IFN-I) play essential roles in restricting viral infections. Coronavirus infection induces ER stress and the interferon response, which reflects different adaptive cellular processes. An understanding of how coronavirus-elicited ER stress is actively involved in viral replication and manipulates the host IFN-I response has remained elusive. Here, TGEV inhibited host miR-30a-5p via the ER stress sensor IRE1α, which led to the increased expression of negative regulators of JAK-STAT signaling cascades, namely, SOCS1 and SOCS3. Increased SOCS1 or SOCS3 expression impaired the IFN-I antiviral response, promoting TGEV replication. These findings enhance our understanding of the strategies used by coronaviruses to antagonize IFN-I innate immunity via IRE1α-mediated manipulation of the miR-30a-5p/SOCS axis, highlighting the crucial role of IRE1α in innate antiviral resistance and the potential of IRE1α as a novel target against coronavirus infection.

The Human Cytomegalovirus Endoplasmic Reticulum-Resident Glycoprotein UL148 Activates the Unfolded Protein Response

Eukaryotic cells are equipped with three sensors that respond to the accumulation of misfolded proteins within the lumen of the endoplasmic reticulum (ER) by activating the unfolded protein response (UPR), which functions to resolve proteotoxic stresses involving the secretory pathway. Here, we identify UL148, a viral ER-resident glycoprotein from human cytomegalovirus (HCMV), as an inducer of the UPR. Metabolic labeling results indicate that global mRNA translation is decreased when UL148 expression is induced in uninfected cells. Further, we find that ectopic expression of UL148 is sufficient to activate at least two UPR sensors: the inositol-requiring enzyme-1 (IRE1), as indicated by splicing of Xbp-1 mRNA, and the protein kinase R (PKR)-like ER kinase (PERK), as indicated by phosphorylation of the α subunit of eukaryotic initiation factor 2 (eIF2α) and accumulation of activating transcription factor 4 (ATF4). During wild-type HCMV infection, increases in Xbp-1 splicing, eIF2α phosphorylation, and accumulation of ATF4 accompany UL148 expression. UL148-null infections, however, show reduced levels of these UPR indicators and decreases in XBP1s abundance and in phosphorylation of PERK and IRE1. Small interfering RNA (siRNA) depletion of PERK dampened the extent of eIF2α phosphorylation and ATF4 induction observed during wild-type infection, implicating PERK as opposed to other eIF2α kinases. A virus with UL148 disrupted showed significant 2- to 4-fold decreases during infection in the levels of transcripts canonically regulated by PERK/ATF4 and by the ATF6 pathway. Taken together, our results argue that UL148 is sufficient to activate the UPR when expressed ectopically and that UL148 is an important cause of UPR activation in the context of the HCMV-infected cell.IMPORTANCE The unfolded protein response (UPR) is an ancient cellular response to ER stress that is of broad importance to viruses. Certain consequences of the UPR, including mRNA degradation and translational shutoff, would presumably be disadvantageous to viruses, while other attributes of the UPR, such as ER expansion and upregulation of protein folding chaperones, might enhance viral replication. Although HCMV is estimated to express well over 150 different viral proteins, we show that the HCMV ER-resident glycoprotein UL148 contributes substantially to the UPR during infection and, moreover, is sufficient to activate the UPR in noninfected cells. Experimental activation of the UPR in mammalian cells is difficult to achieve without the use of toxins. Therefore, UL148 may provide a new tool to investigate fundamental aspects of the UPR. Furthermore, our findings may have implications for understanding the mechanisms underlying the effects of UL148 on HCMV cell tropism and evasion of cell-mediated immunity.

Insights into the pathogenesis of herpes simplex encephalitis from mouse models

A majority of the world population is infected with herpes simplex viruses (HSV; human herpesvirus types 1 and 2). These viruses, perhaps best known for their manifestation in the genital or oral mucosa, can also cause herpes simplex encephalitis, a severe and often fatal disease of the central nervous system. Antiviral therapies for HSV are only partially effective since the virus can establish latent infections in neurons, and severe pathological sequelae in the brain are common. A better understanding of disease pathogenesis is required to develop new strategies against herpes simplex encephalitis, including the precise viral and host genetic determinants that promote virus invasion into the central nervous system and its associated immunopathology. Here we review the current understanding of herpes simplex encephalitis from the host genome perspective, which has been illuminated by groundbreaking work on rare herpes simplex encephalitis patients together with mechanistic insight from single-gene mouse models of disease. A complex picture has emerged, whereby innate type I interferon-mediated antiviral signaling is a central pathway to control viral replication, and the regulation of immunopathology and the balance between apoptosis and autophagy are critical to disease severity in the central nervous system. The lessons learned from mouse studies inform us on fundamental defense mechanisms at the interface of host–pathogen interactions within the central nervous system, as well as possible rationales for intervention against infections from severe neuropathogenic viruses.

A virus-acquired host cytokine controls systemic aging by antagonizing apoptosis

Aging is characterized by degeneration of unique tissues. However, dissecting the interconnectedness of tissue aging remains a challenge. Here, we employ a muscle-specific DNA damage model in Drosophila to reveal secreted factors that influence systemic aging in distal tissues. Utilizing this model, we uncovered a cytokine—Diedel—that, when secreted from muscle or adipose, can attenuate age-related intestinal tissue degeneration by promoting proliferative homeostasis of stem cells. Diedel is both necessary and sufficient to limit tissue degeneration and regulate lifespan. Secreted homologs of Diedel are also found in viruses, having been acquired from host genomes. Focusing on potential mechanistic overlap between cellular aging and viral-host cell interactions, we found that Diedel is an inhibitor of apoptosis and can act as a systemic rheostat to modulate cell death during aging. These results highlight a key role for secreted antagonists of apoptosis in the systemic coordination of tissue aging.

Aging in multicellular organisms is characterized by a progressive decline in the proper function of organs. This deterioration of organ function is a risk factor for many diseases. However, it is unlikely that organs age in isolation, as damage in one organ can presumably impact aging of other organs through either beneficial or detrimental cross-talk. Our work attempts to explore this aspect of aging using fruit flies as a model system. We uncovered that damaged fly muscle can protect against aging in other organs, such as the intestine, through the secretion of a blood-borne factor named Diedel. This blood-borne factor presumably allows damaged organs to communicate with each other during aging. Related factors are also found in certain viruses, which have been hijacked from insect genomes to promote viral spreading during infection. Using this information, we found that viral Diedel inhibits death of infected cells, allowing viruses to spread. Similarly, host (insect) Diedel also blocks cell death in organs during aging, thus limiting deterioration of organ function and extending the organism’s lifespan.

Induction of the unfolded protein response (UPR) during Marek's disease virus (MDV) infection

Marek's disease (MD) is a pathology of chickens associated with paralysis, immune suppression, and the rapid formation of T-cell lymphomas. MD is caused by the herpesvirus, Marek's disease virus (MDV). We examined endoplasmic reticulum (ER) stress and the activation of unfolded protein response (UPR) pathways during MDV infection of cells in culture and lymphocytes in vivo. MDV strains activate the UPR as measured by increased mRNA expression of GRP78/BiP with concomitant XBP1 splicing and induction of its target gene, EDEM1. Cell culture replication of virulent, but not vaccine MDVs, activated the UPR at late in infection. Pathotype-associated UPR activation was induced to a greater level by a vv + MDV. Discrete UPR activation was observed during MDV in vivo infection, with the level of UPR modulation being affected by the MDV oncoprotein Meq. Finally, ATF6 was found to be activated in vv + MDV-induced primary lymphomas, suggesting a possible role in tumor progression.

IRE1α Implications in Endoplasmic Reticulum Stress-Mediated Development and Pathogenesis of Autoimmune Diseases

Inositol-requiring transmembrane kinase/endoribonuclease 1α (IRE1α) is the most prominent and evolutionarily conserved endoplasmic reticulum (ER) membrane protein. This transduces the signal of misfolded protein accumulation in the ER, named as ER stress, to the nucleus as “unfolded protein response (UPR).” The ER stress-mediated IRE1α signaling pathway arbitrates the yin and yang of cell life. IRE1α has been implicated in several physiological as well as pathological conditions, including immune disorders. Autoimmune diseases are caused by abnormal immune responses that develop due to genetic mutations and several environmental factors, including infections and chemicals. These factors dysregulate the cell immune reactions, such as cytokine secretion, antigen presentation, and autoantigen generation. However, the mechanisms involved, in which these factors induce the onset of autoimmune diseases, are remaining unknown. Considering that these environmental factors also induce the UPR, which is expected to have significant role in secretory cells and immune cells. The role of the major UPR molecule, IRE1α, in causing immune responses is well identified, but its role in inducing autoimmunity and the pathogenesis of autoimmune diseases has not been clearly elucidated. Hence, a better understanding of the role of IRE1α and its regulatory mechanisms in causing autoimmune diseases could help to identify and develop the appropriate therapeutic strategies. In this review, we mainly center the discussion on the molecular mechanisms of IRE1α in the pathophysiology of autoimmune diseases.

Cell death and immunity in cancer: From danger signals to mimicry of pathogen defense responses

The immunogenicity of cancer cells is an emerging determinant of anti-cancer immunotherapy. Beyond developing immunostimulatory regimens like dendritic cell-based vaccines, immune-checkpoint blockers, and adoptive T-cell transfer, investigators are beginning to focus on the immunobiology of dying cancer cells and its relevance for the success of anticancer immunotherapies. It is currently accepted that cancer cells may die in response to anti-cancer therapies through regulated cell death programs, which may either repress or increase their immunogenic potential. In particular, the induction of immunogenic cancer cell death (ICD), which is hallmarked by the emission of damage-associated molecular patterns (DAMPs); molecules analogous to pathogen-associated molecular patterns (PAMPs) acting as danger signals/alarmins, is of great relevance in cancer therapy. These ICD-associated danger signals favor immunomodulatory responses that lead to tumor-associated antigens (TAAs)-directed T-cell immunity, which paves way for the removal of residual, treatment-resistant cancer cells. It is also emerging that cancer cells succumbing to ICD can orchestrate "altered-self mimicry" i.e. mimicry of pathogen defense responses, on the levels of nucleic acids and/or chemokines (resulting in type I interferon/IFN responses or pathogen response-like neutrophil activity). In this review, we exhaustively describe the main molecular, immunological, preclinical, and clinical aspects of immunosuppressive cell death or ICD (with respect to apoptosis, necrosis and necroptosis). We also provide an extensive historical background of these fields, with special attention to the self/non-self and danger models, which have shaped the field of cell death immunology.