Diverse intracellular pathogens activate type III interferon expression from peroxisomes

Identification of a Predominantly Interferon-λ-Induced Transcriptional Profile in Murine Intestinal Epithelial Cells

Type I (α and β) and type III (λ) interferons (IFNs) induce the expression of a large set of antiviral effector molecules via their respective surface membrane receptors. Whereas most cell types respond to type I IFN, type III IFN preferentially acts on epithelial cells and protects mucosal organs such as the lung and gastrointestinal tract. Despite the engagement of different receptor molecules, the type I and type III IFN-induced signaling cascade and upregulated gene profile is thought to be largely identical. Here, we comparatively analyzed the response of gut epithelial cells to IFN-β and IFN-λ2 and identified a set of genes predominantly induced by IFN-λ2. We confirm the influence of epithelial cell polarization for enhanced type III receptor expression and demonstrate the induction of predominantly IFN-λ2-induced genes in the gut epithelium in vivo. Our results suggest that IFN-λ2 targets the epithelium and induces genes to adjust the antiviral host response to the requirements at mucosal body sites.

NLRX1 promotes immediate IRF1-directed antiviral responses by limiting dsRNA-activated translational inhibition mediated by PKR

NLRX1 is unique among nucleotide-binding domain and leucine-rich repeat (NLR) proteins in its mitochondrial localization and capacity to negatively regulate MAVS- and STING-dependent antiviral innate immunity. However, some studies suggest a positive regulatory role for NLRX1 in inducing antiviral responses. We show that NLRX1 exerts opposing regulatory effects on virus activation of the transcription factors IRF1 and IRF3, potentially explaining these contradictory results. Whereas NLRX1 suppresses MAVS-mediated IRF3 activation, NLRX1 conversely facilitates virus-induced increases in IRF1 expression, thereby enhancing control of virus infection. NLRX1 has a minimal effect on NF-κB-mediated IRF1 transcription, and regulates IRF1 abundance post-transcriptionally by preventing translational shutdown mediated by the dsRNA-activated protein kinase PKR, thereby allowing virus-induced increases in IRF1 protein abundance.

Interferon-λs: Front-Line Guardians of Immunity and Homeostasis in the Respiratory Tract

Type III interferons (IFNs), also termed lambda IFNs (IFNλs) or interleukins-28/29, constitute a new addition to the IFN family. They are induced upon infection and are particularly abundant at barrier surfaces, such as the respiratory and gastrointestinal tracts. Although they signal through a unique heterodimeric receptor complex comprising IFNLR1 and IL10RB, they activate a downstream signaling pathway remarkably similar to that of type I IFNs and share many functions with them. Yet, they also have important differences which are only now starting to unfold. Here, we review the current literature implicating type III IFNs in the regulation of immunity and homeostasis in the respiratory tract. We survey the common and unique characteristics of type III IFNs in terms of expression patterns, cellular targets, and biological activities and discuss their emerging role in first line defenses against respiratory viral infections. We further explore their immune modulatory functions and their involvement in the regulation of inflammatory responses during chronic respiratory diseases, such as asthma and chronic obstructive pulmonary disease. Type III IFNs are, therefore, arising as front-line guardians of immune defenses in the respiratory tract, fine tuning inflammation, and as potential novel therapeutics for the treatment of diverse respiratory diseases, including influenza virus infection and asthma.

Innate Immune Receptors as Competitive Determinants of Cell Fate

Infections can cause a multitude of stresses on the host and microbe. To detect potential infections, the mammalian immune system utilizes several families of pattern recognition receptors, which survey the intracellular and extracellular environments for microbial products. Members of each receptor family induce antimicrobial effector responses, which include inflammatory cytokine or interferon expression, downregulation of protein synthesis, or host cell death. In this review, we discuss the benefits of each of these innate immune responses. We highlight how non-infectious bacteria and viruses typically activate a single family of receptors, which results in a predictable host response. Infections with virulent pathogens, in contrast, may activate receptors from distinct families. As each receptor family may induce responses that antagonize or synergize with the activities of another family, cell fate decisions during pathogenic encounters are unpredictable. Understanding the antagonistic antimicrobial activities of the innate immune system should provide insight into how cell fate decisions are made during infections and potentially during other environmental stresses.

The respiratory chain inhibitor rotenone affects peroxisomal dynamics via its microtubule-destabilising activity

Peroxisomes and mitochondria in mammalian cells are closely linked subcellular organelles, which maintain a redox-sensitive relationship. Their interplay and role in ROS signalling are supposed to impact on age-related and degenerative disorders. Whereas the generation of peroxisome-derived oxidative stress can affect mitochondrial morphology and function, little is known about the impact of mitochondria-derived oxidative stress on peroxisomes. Here, we investigated the effect of the mitochondrial complex I inhibitor rotenone on peroxisomal and mitochondrial membrane dynamics. We show that rotenone treatment of COS-7 cells alters peroxisome morphology and distribution. However, this effect is related to its microtubule-destabilising activity rather than to the generation of oxidative stress. Rotenone also induced alterations in mitochondrial morphology, which-in contrast to its effect on peroxisomes-were dependent on the generation of ROS but independent of its microtubule-active properties. The importance of our findings for the peroxisome-mitochondria redox relationship and the interpretation of in cellulo and in vivo studies with rotenone, which is widely used to study Parkinson's disease, are discussed.

IFN-λ suppresses intestinal inflammation by non-translational regulation of neutrophil function

Interferon-λ (IFN-λ) is a central regulator of mucosal immunity; however, its signaling specificity relative to that of type I interferons is poorly defined. IFN-λ can induce antiviral interferon-stimulated genes (ISGs) in epithelia, while the effect of IFN-λ in non-epithelial cells remains unclear. Here we report that neutrophils responded to IFN-λ. We found that in addition to inducing ISG transcription, IFN-λ (but not IFN-β) specifically activated a translation-independent signaling pathway that diminished the production of reactive oxygen species and degranulation in neutrophils. In mice, IFN-λ was elicited by enteric viruses and acted on neutrophils to decrease oxidative stress and intestinal damage. Thus, IFN-λ acted as a unique immunomodulatory agent by modifying transcriptional and non-translational neutrophil responses, which might permit a controlled development of the inflammatory process.

Interferon-λs and Plasmacytoid Dendritic Cells: A Close Relationship

Interferon lambdas (IFNλs) are recently discovered cytokines acting not only at the first line of defense against viral infections but also at the mucosal barriers. In fact, a peculiar feature of the IFNλ system is the restricted expression of the functional IFNλR, which is known to be limited to epithelial cells and discrete leukocyte subsets, including the plasmacytoid dendritic cells (pDCs). In the latter case, current data, discussed in this minireview, indicate that IFNλs positively regulate various pDC functions, including pDC expression of interferon-dependent gene (ISG) mRNAs, production of cytokines, survival, and phenotype. Although the knowledge of the effects on pDCs by IFNλs is still incomplete, we speculate that the peculiar pDC responsiveness to IFNλs provide unique advantages for these innate immune cells, not only for viral infections but also during autoimmune disorders and/or tumors, in which pDC involvement and activation variably contribute to their pathogenesis.

The Unresolved Role of Interferon-λ in Asthma Bronchiale

Asthma bronchiale is a disease of the airways with increasing incidence, that often begins during infancy. So far, therapeutic options are mainly symptomatic and thus there is an increasing need for better treatment and/or prevention strategies. Human rhinoviruses (HRVs) are a major cause of asthma exacerbations and might cause acute wheezing associated with local production of pro-inflammatory mediators resulting in neutrophilic inflammatory response. Viral infections induce a characteristic activation of immune response, e.g., TLR3, 4, 7, 8, 9 in the endosome and their downstream targets, especially MyD88. Moreover, other cytoplasmic pattern recognition molecules (PRMs) like RIG1 and MDA5 play important roles in the activation of interferons (IFNs) of all types. Depending on the stimulation of the different PRMs, the levels of the IFNs induced might differ. Recent studies focused on Type I IFNs in samples from control and asthma patients. However, the administration of type I IFN-α was accompanied by side-effects, thus this possible therapy was abandoned. Type III IFN-λ acts more specifically, as fewer cells express the IFN-λ receptor chain 1. In addition, it has been shown that asthmatic mice treated with recombinant or adenoviral expressed IFN-λ2 (IL–28A) showed an amelioration of symptoms, indicating that treatment with IFN-λ might be beneficial for asthmatic patients.

Persistent mycobacteria evade an antibacterial program mediated by phagolysosomal TLR7/8/MyD88 in human primary macrophages

Pathogenic mycobacteria reside in macrophages where they avoid lysosomal targeting and degradation through poorly understood mechanisms proposed to involve arrest of phagosomal maturation at an early endosomal stage. A clear understanding of how this relates to host defenses elicited from various intracellular compartments is also missing and can only be studied using techniques allowing single cell and subcellular analyses. Using confocal imaging of human primary macrophages infected with Mycobacterium avium (Mav) we show evidence that Mav phagosomes are not arrested at an early endosomal stage, but mature to a (LAMP1⁺/LAMP2⁺/CD63⁺) late endosomal/phagolysosomal stage where inflammatory signaling and Mav growth restriction is initiated through a mechanism involving Toll-like receptors (TLR) 7 and 8, the adaptor MyD88 and transcription factors NF-κB and IRF-1. Furthermore, a fraction of the mycobacteria re-establish in a less hostile compartment (LAMP1^-/LAMP2^-/CD63^-) where they not only evade destruction, but also recognition by TLRs, growth restriction and inflammatory host responses that could be detrimental for intracellular survival and establishment of chronic infections.

Mycobacterium avium is increasingly reported as a causative agent of non-tuberculous disease in immunocompromised patients and in individuals with underlying disease or using immunosuppressant drugs, with prevalence often higher than the more pathogenic M. tuberculosis in developed countries. Both M. avium and M. tuberculosis cause persistent infections by surviving inside host macrophages. Here, we identify from which compartment M. avium evoke inflammatory signaling in human primary macrophages, and the pattern-recognition receptors involved. In essence, we present three key findings: 1) M. avium phagosomes are not arrested at an early endosomal stage, but rather mature normally into phagolysosomes from where a fraction of the bacteria escape and re-establish in a new compartment. 2) In addition to avoiding degradation in phagolysosomes, by escaping M. avium also evade inflammatory signaling. 3) M. avium unable to escape is degraded in phagolysosomes and recognized by Toll-like receptors 7 and 8. Our results can contribute to new understanding of intracellular infections, and thus have vital clinical implications for development of novel anti-microbial strategies and host-targeted therapy to mycobacterial and other infectious diseases.

Analysis of tick-borne encephalitis virus-induced host responses in human cells of neuronal origin and interferon-mediated protection

Tick-borne encephalitis virus (TBEV) is a member of the genus Flavivirus. It can cause serious infections in humans that may result in encephalitis/meningoencephalitis. Although several studies have described the involvement of specific genes in the host response to TBEV infection in the central nervous system (CNS), the overall network remains poorly characterized. Therefore, we investigated the response of DAOY cells (human medulloblastoma cells derived from cerebellar neurons) to TBEV (Neudoerfl strain, Western subtype) infection to characterize differentially expressed genes by transcriptome analysis. Our results revealed a wide panel of interferon-stimulated genes (ISGs) and pro-inflammatory cytokines, including type III but not type I (or II) interferons (IFNs), which are activated upon TBEV infection, as well as a number of non-coding RNAs, including long non-coding RNAs. To obtain a broader view of the pathways responsible for eliciting an antiviral state in DAOY cells we examined the effect of type I and III IFNs and found that only type I IFN pre-treatment inhibited TBEV production. The cellular response to TBEV showed only partial overlap with gene expression changes induced by IFN-β treatment - suggesting a virus-specific signature - and we identified a group of ISGs that were highly up-regulated following IFN-β treatment. Moreover, a high rate of down-regulation was observed for a wide panel of pro-inflammatory cytokines upon IFN-β treatment. These data can serve as the basis for further studies of host-TBEV interactions and the identification of ISGs and/or lncRNAs with potent antiviral effects in cases of TBEV infection in human neuronal cells.

Tick-Borne Viruses and Biological Processes at the Tick-Host-Virus Interface

Ticks are efficient vectors of arboviruses, although less than 10% of tick species are known to be virus vectors. Most tick-borne viruses (TBV) are RNA viruses some of which cause serious diseases in humans and animals world-wide. Several TBV impacting human or domesticated animal health have been found to emerge or re-emerge recently. In order to survive in nature, TBV must infect and replicate in both vertebrate and tick cells, representing very different physiological environments. Information on molecular mechanisms that allow TBV to switch between infecting and replicating in tick and vertebrate cells is scarce. In general, ticks succeed in completing their blood meal thanks to a plethora of biologically active molecules in their saliva that counteract and modulate different arms of the host defense responses (haemostasis, inflammation, innate and acquired immunity, and wound healing). The transmission of TBV occurs primarily during tick feeding and is a complex process, known to be promoted by tick saliva constituents. However, the underlying molecular mechanisms of TBV transmission are poorly understood. Immunomodulatory properties of tick saliva helping overcome the first line of defense to injury and early interactions at the tick-host skin interface appear to be essential in successful TBV transmission and infection of susceptible vertebrate hosts. The local host skin site of tick attachment, modulated by tick saliva, is an important focus of virus replication. Immunomodulation of the tick attachment site also promotes co-feeding transmission of viruses from infected to non-infected ticks in the absence of host viraemia (non-viraemic transmission). Future research should be aimed at identification of the key tick salivary molecules promoting virus transmission, and a molecular description of tick-host-virus interactions and of tick-mediated skin immunomodulation. Such insights will enable the rationale design of anti-tick vaccines that protect against disease caused by tick-borne viruses.

Interferon-Lambda: A Potent Regulator of Intestinal Viral Infections

Interferon-lambda (IFN-λ) is a recently described cytokine found to be of critical importance in innate immune regulation of intestinal viruses. Endogenous IFN-λ has potent antiviral effects and has been shown to control multiple intestinal viruses and may represent a factor that contributes to human variability in response to infection. Importantly, recombinant IFN-λ has therapeutic potential against enteric viral infections, many of which lack other effective treatments. In this mini-review, we describe recent advances regarding IFN-λ-mediated regulation of enteric viruses with important clinical relevance including rotavirus, reovirus, and norovirus. We also briefly discuss IFN-λ interactions with other cytokines important in the intestine, and how IFN-λ may play a role in regulation of intestinal viruses by the commensal microbiome. Finally, we indicate currently outstanding questions regarding IFN-λ control of enteric infections that remain to be explored to enhance our understanding of this important immune molecule.

STING Is Involved in Antiviral Immune Response against VZV Infection via the Induction of Type I and III IFN Pathways

Varicella zoster virus (VZV) is a human-restricted α-herpesvirus that exhibits tropism for the skin. The VZV host receptors and downstream signaling pathways responsible for the antiviral innate immune response in the skin are not completely understood. Here, we show that STING mediates an important host defense against VZV infection in dermal cells including human dermal fibroblasts and HaCaT keratinocytes. Inhibition of STING using small interfering-RNA or short hairpin RNA-mediated gene disruption resulted in enhanced viral replication but diminished IRF3 phosphorylation and induction of IFNs and proinflammatory cytokines. Pretreatment with STING agonists resulted in reduced VZV glycoprotein E expression and viral replication. Additionally, using RNA sequencing to analyze dual host and VZV transcriptomes, we identified several host immune genes significantly induced by VZV infection. Furthermore, significant up-regulation of IFN-λ secretion was observed after VZV infection, partly through a STING-dependent pathway; IFN-λ was shown to be crucial for antiviral defense against VZV in human dermal cells. In conclusion, our data provide an important insight into STING-mediated induction of type I and III IFNs and subsequent antiviral signaling pathways that regulate VZV replication in human dermal cells.

MicroRNAs upregulated during HIV infection target peroxisome biogenesis factors: Implications for virus biology, disease mechanisms and neuropathology

HIV-associated neurocognitive disorders (HAND) represent a spectrum neurological syndrome that affects up to 25% of patients with HIV/AIDS. Multiple pathogenic mechanisms contribute to the development of HAND symptoms including chronic neuroinflammation and neurodegeneration. Among the factors linked to development of HAND is altered expression of host cell microRNAs (miRNAs) in brain. Here, we examined brain miRNA profiles among HIV/AIDS patients with and without HAND. Our analyses revealed differential expression of 17 miRNAs in brain tissue from HAND patients. A subset of the upregulated miRNAs (miR-500a-5p, miR-34c-3p, miR-93-3p and miR-381-3p), are predicted to target peroxisome biogenesis factors (PEX2, PEX7, PEX11B and PEX13). Expression of these miRNAs in transfected cells significantly decreased levels of peroxisomal proteins and concomitantly decreased peroxisome numbers or affected their morphology. The levels of miR-500a-5p, miR-34c-3p, miR-93-3p and miR-381-3p were not only elevated in the brains of HAND patients, but were also upregulated during HIV infection of primary macrophages. Moreover, concomitant loss of peroxisomal proteins was observed in HIV-infected macrophages as well as in brain tissue from HIV-infected patients. HIV-induced loss of peroxisomes was abrogated by blocking the functions of the upregulated miRNAs. Overall, these findings point to previously unrecognized miRNA expression patterns in the brains of HIV patients. Targeting peroxisomes by up-regulating miRNAs that repress peroxisome biogenesis factors may represent a novel mechanism by which HIV-1 subverts innate immune responses and/or causes neurocognitive dysfunction.

The Peroxisome-Mitochondria Connection: How and Why?

Over the past decades, peroxisomes have emerged as key regulators in overall cellular lipid and reactive oxygen species metabolism. In mammals, these organelles have also been recognized as important hubs in redox-, lipid-, inflammatory-, and innate immune-signaling networks. To exert these activities, peroxisomes must interact both functionally and physically with other cell organelles. This review provides a comprehensive look of what is currently known about the interconnectivity between peroxisomes and mitochondria within mammalian cells. We first outline how peroxisomal and mitochondrial abundance are controlled by common sets of cis- and trans-acting factors. Next, we discuss how peroxisomes and mitochondria may communicate with each other at the molecular level. In addition, we reflect on how these organelles cooperate in various metabolic and signaling pathways. Finally, we address why peroxisomes and mitochondria have to maintain a healthy relationship and why defects in one organelle may cause dysfunction in the other. Gaining a better insight into these issues is pivotal to understanding how these organelles function in their environment, both in health and disease.

Programming and memory dynamics of innate leukocytes during tissue homeostasis and inflammation

The field of innate immunity is witnessing a paradigm shift regarding "memory" and "programming" dynamics. Past studies of innate leukocytes characterized them as first responders to danger signals with no memory. However, recent findings suggest that innate leukocytes, such as monocytes and neutrophils, are capable of "memorizing" not only the chemical nature but also the history and dosages of external stimulants. As a consequence, innate leukocytes can be dynamically programmed or reprogrammed into complex inflammatory memory states. Key examples of innate leukocyte memory dynamics include the development of primed and tolerant monocytes when "programmed" with a variety of inflammatory stimulants at varying signal strengths. The development of innate leukocyte memory may have far-reaching translational implications, as programmed innate leukocytes may affect the pathogenesis of both acute and chronic inflammatory diseases. This review intends to critically discuss some of the recent studies that address this emerging concept and its implication in the pathogenesis of inflammatory diseases.

Contribution of type III interferons to antiviral immunity: location, location, location

Type I interferons (IFN-α/β) and the more recently identified type III IFNs (IFN-λ) function as the first line of defense against virus infection and regulate the development of both innate and adaptive immune responses. Type III IFNs were originally identified as a novel ligand-receptor system acting in parallel with type I IFNs, but subsequent studies have provided increasing evidence for distinct roles for each IFN family. In addition to their compartmentalized antiviral actions, these two systems appear to have multiple levels of cross-regulation and act coordinately to achieve effective antimicrobial protection with minimal collateral damage to the host.

Genetic control of autophagy underlies pathogenesis of inflammatory bowel disease

Autophagy contributes to cellular homeostasis in the face of nutrient deprivation and other cellular stresses. Cell type-specific functions for autophagy are critical in maintaining homeostasis at both the tissue level and at the whole-organism level. Recent work has highlighted the ways in which human genetic variants modulate autophagy to alter epithelial and immune responses in inflammatory bowel disease.

Type I and Type III Interferons Display Different Dependency on Mitogen-Activated Protein Kinases to Mount an Antiviral State in the Human Gut

Intestinal epithelial cells (IECs) are constantly exposed to commensal flora and pathogen challenges. How IECs regulate their innate immune response to maintain gut homeostasis remains unclear. Interferons (IFNs) are cytokines produced during infections. While type I IFN receptors are ubiquitously expressed, type III IFN receptors are expressed only on epithelial cells. This epithelium specificity strongly suggests exclusive functions at epithelial surfaces, but the relative roles of type I and III IFNs in the establishment of an antiviral innate immune response in human IECs are not clearly defined. Here, we used mini-gut organoids to define the functions of types I and III IFNs to protect the human gut against viral infection. We show that primary non-transformed human IECs, upon viral challenge, upregulate the expression of both type I and type III IFNs at the transcriptional level but only secrete type III IFN in the supernatant. However, human IECs respond to both type I and type III IFNs by producing IFN-stimulated genes that in turn induce an antiviral state. Using genetic ablation of either type I or type III IFN receptors, we show that either IFN can independently restrict virus infection in human IECs. Importantly, we report, for the first time, differences in the mechanisms by which each IFN establishes the antiviral state. Contrary to type I IFN, the antiviral activity induced by type III IFN is strongly dependent on the mitogen-activated protein kinases signaling pathway, suggesting a pathway used by type III IFNs that non-redundantly contributes to the antiviral state. In conclusion, we demonstrate that human intestinal epithelial cells specifically regulate their innate immune response favoring type III IFN-mediated signaling, which allows for efficient protection against pathogens without producing excessive inflammation. Our results strongly suggest that type III IFN constitutes the frontline of antiviral response in the human gut. We propose that mucosal surfaces, particularly the gastrointestinal tract, have evolved to favor type III IFN-mediated response to pathogen infections as it allows for spatial segregation of signaling and moderate production of inflammatory signals which we propose are key to maintain gut homeostasis.

GPATCH3 negatively regulates RLR-mediated innate antiviral responses by disrupting the assembly of VISA signalosome

Upon viral infection, retinoic acid–inducible gene I–like receptors (RLRs) recognize viral RNA and trigger a series of signaling events, leading to the induction of type I interferons (IFNs). These processes are delicately regulated to prevent excessive and harmful immune responses. In this study, we identified G patch domain-containing protein 3 (GPATCH3) as a negative regulator of RLR-mediated antiviral signaling pathways. Overexpression of GPATCH3 impaired RNA virus- triggered induction of downstream antiviral genes, whereas its knockdown had opposite effects and attenuated viral replication. In addition, GPATCH3-deficient cells had higher IFNB1 mRNA level compared with control cells after RNA virus infection. Mechanistically, GPATCH3 was recruited to VISA in a viral infection dependent manner and the assembly of VISA/TRAF6/TBK1 signalosome was impaired in GPATCH3-overexpressing cells. In contrast, upon viral infection, the recruitment of TRAF6 and TBK1 to VISA was enhanced in GPATCH3 deficient cells. Taking together, our findings demonstrate that GPATCH3 interacts with VISA and disrupts the assembly of virus-induced VISA signalosome therefore acts as a negative regulator of RLR-mediated innate antiviral immune responses.

Virus infection triggers the host cells to produce type I IFNs and proinflammatory cytokines, which are secreted proteins important for the host to clear viruses. Previously, we identified VISA (also named as MAVS, IPS-1 and Cardif) as a critical adaptor of virus-triggered, RLR-mediated induction of innate antiviral responses. In this study, we further found that GPATCH3, a functionally uncharacterized protein, interacted with mitochondria-localized VISA upon virus infection and disrupted the assembly of VISA-signalosome. Therefore, GPATCH3 acts as a negative regulator of VISA and functions as a brake of RLR-mediated antiviral innate responses. This discovery helps to understand how the innate antiviral responses are delicately regulated.

Activation and pathogenic manipulation of the sensors of the innate immune system

The innate immune system detects the presence of microbes through different families of pattern-recognition receptors (PRRs). PRRs detect pathogens of all origins and trigger signaling events that activate innate and adaptive immunity. These events need to be tightly regulated in order to ensure optimal activation when required, and minimal signaling in the absence of microbial encounters. This regulation is achieved, at least in part, through the precise subcellular positioning of receptors and downstream signaling proteins. Consequently, mislocalization of these proteins inhibits innate immune pathways, and pathogens have evolved to alter host protein localization as a strategy to evade immune detection. This review describes the importance of subcellular localization of various PRR families and their adaptors, and highlights pathogenic immune evasion strategies that operate by altering immune protein localization.

The Impact of the Interferon/TNF-Related Apoptosis-Inducing Ligand Signaling Axis on Disease Progression in Respiratory Viral Infection and Beyond

Interferons (IFNs) are well described to be rapidly induced upon pathogen-associated pattern recognition. After binding to their respective IFN receptors and activation of the cellular JAK/signal transducer and activator of transcription signaling cascade, they stimulate the transcription of a plethora of IFN-stimulated genes (ISGs) in infected as well as bystander cells such as the non-infected epithelium and cells of the immune system. ISGs may directly act on the invading pathogen or can either positively or negatively regulate the innate and adaptive immune response. However, IFNs and ISGs do not only play a key role in the limitation of pathogen spread but have also been recently found to provoke an unbalanced, overshooting inflammatory response causing tissue injury and hampering repair processes. A prominent regulator of disease outcome, especially in-but not limited to-respiratory viral infection, is the IFN-dependent mediator TRAIL (TNF-related apoptosis-inducing ligand) produced by several cell types including immune cells such as macrophages or T cells. First described as an apoptosis-inducing agent in transformed cells, it is now also well established to rapidly evoke cellular stress pathways in epithelial cells, finally leading to caspase-dependent or -independent cell death. Hereby, pathogen spread is limited; however in some cases, also the surrounding tissue is severely harmed, thus augmenting disease severity. Interestingly, the lack of a strictly controlled and well balanced IFN/TRAIL signaling response has not only been implicated in viral infection but might furthermore be an important determinant of disease progression in bacterial superinfections and in chronic respiratory illness. Conclusively, the IFN/TRAIL signaling axis is subjected to a complex modulation and might be exploited for the evaluation of new therapeutic concepts aiming at attenuation of tissue injury.

Type I and III Interferon in the Gut: Tight Balance between Host Protection and Immunopathology

The intestinal mucosa forms an active interface to the outside word, facilitating nutrient and water uptake and at the same time acts as a barrier toward the highly colonized intestinal lumen. A tight balance of the mucosal immune system is essential to tolerate harmless antigens derived from food or commensals and to effectively defend against potentially dangerous pathogens. Interferons (IFN) provide a first line of host defense when cells detect an invading organism. Whereas type I IFN were discovered almost 60 years ago, type III IFN were only identified in the early 2000s. It was initially thought that type I IFN and type III IFN performed largely redundant functions. However, it is becoming increasingly clear that type III IFN exert distinct and non-redundant functions compared to type I IFN, especially in mucosal tissues. Here, we review recent progress made in unraveling the role of type I/III IFN in intestinal mucosal tissue in the steady state, in response to mucosal pathogens and during inflammation.

The dynamic interacting landscape of MAPL reveals essential functions for SUMOylation in innate immunity

Activation of the innate immune response triggered by dsRNA viruses occurs through the assembly of the Mitochondrial Anti-Viral Signaling (MAVS) complex. Upon recognition of viral dsRNA, the cytosolic receptor RIG-I is activated and recruited to MAVS to activate the immune signaling response. We here demonstrate a strict requirement for a mitochondrial anchored protein ligase, MAPL (also called MUL1) in the signaling events that drive the transcriptional activation of antiviral genes downstream of Sendai virus infection, both in vivo and in vitro. A biotin environment scan of MAPL interacting polypeptides identified a series of proteins specific to Sendai virus infection; including RIG-I, IFIT1, IFIT2, HERC5 and others. Upon infection, RIG-I is SUMOylated in a MAPL-dependent manner, a conjugation step that is required for its activation. Consistent with this, MAPL was not required for signaling downstream of a constitutively activated form of RIG-I. These data highlight a critical role for MAPL and mitochondrial SUMOylation in the early steps of antiviral signaling.

Herpes simplex virus 1 infection dampens the immediate early antiviral innate immunity signaling from peroxisomes by tegument protein VP16

Background

Herpes simplex virus 1 (HSV-1) is an archetypal member of the alphaherpesvirus subfamily with a large genome encoding over 80 proteins, many of which play a critical role in virus-host interactions and immune modulation. Upon viral infections, the host cells activate innate immune responses to restrict their replications. Peroxisomes, which have long been defined to regulate metabolic activities, are reported to be important signaling platforms for antiviral innate immunity. It has been verified that signaling from peroxisomal MAVS (MAVS-Pex) triggers a rapid interferon (IFN) independent IFN-stimulated genes (ISGs) production against invading pathogens. However, little is known about the interaction between DNA viruses such as HSV-1 and the MAVS-Pex mediated signaling.

Results

HSV-1 could activate the MAVS-Pex signaling pathway at a low multiplicity of infection (MOI), while infection at a high MOI dampens MAVS-Pex induced immediately early ISGs production. A high-throughput screen assay reveals that HSV-1 tegument protein VP16 inhibits the immediate early ISGs expression downstream of MAVS-Pex signaling. Moreover, the expression of ISGs was recovered when VP16 was knockdown with its specific short hairpin RNA.

Conclusion

HSV-1 blocks MAVS-Pex mediated early ISGs production through VP16 to dampen the immediate early antiviral innate immunity signaling from peroxisomes.

A New Immunomodulatory Role for Peroxisomes in Macrophages Activated by the TLR4 Ligand Lipopolysaccharide

Peroxisomes are proposed to play an important role in the regulation of systemic inflammation; however, the functional role of these organelles in inflammatory responses of myeloid immune cells is largely unknown. In this article, we demonstrate that the nonclassical peroxisome proliferator 4-phenyl butyric acid is an efficient inducer of peroxisomes in various models of murine macrophages, such as primary alveolar and peritoneal macrophages and the macrophage cell line RAW264.7, but not in primary bone marrow-derived macrophages. Further, proliferation of peroxisomes blocked the TLR4 ligand LPS-induced proinflammatory response, as detected by the reduced induction of the proinflammatory protein cyclooxygenase (COX)-2 and the proinflammatory cytokines TNF-α, IL-6, and IL-12. In contrast, disturbing peroxisome function by knockdown of peroxisomal gene Pex14 or Mfp2 markedly increased the LPS-dependent upregulation of the proinflammatory proteins COX-2 and TNF-α. Specifically, induction of peroxisomes did not affect the upregulation of COX-2 at the mRNA level, but it reduced the half-life of COX-2 protein, which was restored by COX-2 enzyme inhibitors but not by proteasomal and lysosomal inhibitors. Liquid chromatography-tandem mass spectrometry analysis revealed that various anti-inflammatory lipid mediators (e.g., docosahexaenoic acid) were increased in the conditioned medium from peroxisome-induced macrophages, which blocked LPS-induced COX-2 upregulation in naive RAW264.7 cells and human primary peripheral blood-derived macrophages. Importantly, LPS itself induced peroxisomes that correlated with the regulation of COX-2 during the late phase of LPS activation in macrophages. In conclusion, our findings identify a previously unidentified role for peroxisomes in macrophage inflammatory responses and suggest that peroxisomes are involved in the physiological cessation of macrophage activation.

Mast cells are permissive for rhinovirus replication: potential implications for asthma exacerbations

BACKGROUND

Human rhinoviruses (HRVs) are a major trigger of asthma exacerbations, with the bronchial epithelium being the major site of HRV infection and replication. Mast cells (MCs) play a key role in asthma where their numbers are increased in the bronchial epithelium with increasing disease severity.

OBJECTIVE

In view of the emerging role of MCs in innate immunity and increased localization to the asthmatic bronchial epithelium, we investigated whether HRV infection of MCs generated innate immune responses which were protective against infection.

METHODS

The LAD2 MC line or primary human cord blood-derived MCs (CBMCs) were infected with HRV or UV-irradiated HRV at increasing multiplicities of infection (MOI) without or with IFN-β or IFN-λ. After 24 h, innate immune responses were assessed by RT-qPCR and IFN protein release by ELISA. Viral replication was determined by RT-qPCR and virion release by TCID₅₀ assay.

RESULTS

HRV infection of LAD2 MCs induced expression of IFN-β, IFN-λ and IFN-stimulated genes. However, LAD2 MCs were permissive for HRV replication and release of infectious HRV particles. Similar findings were observed with CBMCs. Neutralization of the type I IFN receptor had minimal effects on viral shedding, suggesting that endogenous type I IFN signalling offered limited protection against HRV. However, augmentation of these responses by exogenous IFN-β, but not IFN-λ, protected MCs against HRV infection.

CONCLUSION AND CLINICAL RELEVANCE

MCs are permissive for the replication and release of HRV, which is prevented by exogenous IFN-β treatment. Taken together, these findings suggest a novel mechanism whereby MCs may contribute to HRV-induced asthma exacerbations.

A paradox of transcriptional and functional innate interferon responses of human intestinal enteroids to enteric virus infection

The intestinal epithelium can limit enteric pathogens by producing antiviral cytokines, such as IFNs. Type I IFN (IFN-α/β) and type III IFN (IFN-λ) function at the epithelial level, and their respective efficacies depend on the specific pathogen and site of infection. However, the roles of type I and type III IFN in restricting human enteric viruses are poorly characterized as a result of the difficulties in cultivating these viruses in vitro and directly obtaining control and infected small intestinal human tissue. We infected nontransformed human intestinal enteroid cultures from multiple individuals with human rotavirus (HRV) and assessed the host epithelial response by using RNA-sequencing and functional assays. The dominant transcriptional pathway induced by HRV infection is a type III IFN-regulated response. Early after HRV infection, low levels of type III IFN protein activate IFN-stimulated genes. However, this endogenous response does not restrict HRV replication because replication-competent HRV antagonizes the type III IFN response at pre- and posttranscriptional levels. In contrast, exogenous IFN treatment restricts HRV replication, with type I IFN being more potent than type III IFN, suggesting that extraepithelial sources of type I IFN may be the critical IFN for limiting enteric virus replication in the human intestine.

IFN-lambda preferably inhibits PEDV infection of porcine intestinal epithelial cells compared with IFN-alpha

In contrast to type I interferons that target various types of cells and organs, interferon lambda (IFN-L) primarily acts on mucosal epithelial cells and exhibits robust antiviral activity within the mucosal surface. Porcine epidemic diarrhea virus (PEDV), which causes high morbidity and mortality in piglets, is an enteropathogenic coronavirus with economic importance. Here, we demonstrated that both recombinant porcine IFN-L1 (rpIFN-L1) and rpIFN-L3 have powerful antiviral activity against PEDV infection of both Vero E6 cells and the intestinal porcine epithelial cell line J2 (IPEC-J2). Both forms of rpIFN-L inhibited two genotypes of PEDV (strain CV777 of genotype 1 and strain LNCT2 of genotype 2). rpIFN-L1 primarily controlled viral infection in the early stage and had less antiviral activity in IPEC-J2 than in rpIFN-L3 cells infected with PEDV. In addition, rpIFN-L1 exhibited greater antiviral activity against PEDV infection of IPEC-J2 cells than that of porcine IFN-alpha. Consistent with this finding, rpIFN-L1 triggered higher levels of certain antiviral IFN-stimulated genes (ISGs) (ISG15, OASL, and MxA) in IPEC-J2 cells than porcine IFN-alpha. Although IPEC-J2 cells responded to both IFN-alpha and lambda, transcriptional profiling of ISGs (specifically ISG15, OASL, MxA, and IFITMs) differed when induced by either IFN-alpha or rpIFN-L. Therefore, our data provide the experimental evidence that porcine IFN-L suppresses PEDV infection of IPEC-J2 cells, which may offer a promising therapeutic for combating PED in piglets.

•

Porcine IFN-lambda robustly inhibited PEDV in both Vero E6 and IPEC-J2.

•

IFN-lambda exhibited more anti-PEDV activity and induced a better antiviral response in IPEC-J2 than IFN-alpha.

•

Porcine IFN-lambda might represent a novel therapeutic agent for PEDV infection in the future.

Pex35 is a regulator of peroxisome abundance

Peroxisomes are cellular organelles with vital functions in lipid, amino acid and redox metabolism. The cellular formation and dynamics of peroxisomes are governed by PEX genes; however, the regulation of peroxisome abundance is still poorly understood. Here, we use a high-content microscopy screen in Saccharomyces cerevisiae to identify new regulators of peroxisome size and abundance. Our screen led to the identification of a previously uncharacterized gene, which we term PEX35, which affects peroxisome abundance. PEX35 encodes a peroxisomal membrane protein, a remote homolog to several curvature-generating human proteins. We systematically characterized the genetic and physical interactome as well as the metabolome of mutants in PEX35, and we found that Pex35 functionally interacts with the vesicle-budding-inducer Arf1. Our results highlight the functional interaction between peroxisomes and the secretory pathway.

Innate Viral Receptor Signaling Determines Type 1 Diabetes Onset

Heritable susceptibility of the autoimmune disorder, type 1 diabetes (T1D), only partially equates for the incidence of the disease. Significant evidence attributes several environmental stressors, such as vitamin D deficiency, gut microbiome, dietary antigens, and most notably virus infections in triggering the onset of T1D in these genetically susceptible individuals. Extensive epidemiological and clinical studies have provided credibility to this causal relationship. Infection by the enterovirus, coxsackievirus B, has been closely associated with onset of T1D and is considered a significant etiological agent for disease induction. Recognition of viral antigens via innate pathogen-recognition receptors induce inflammatory events which contribute to autoreactivity of pancreatic self-antigens and ultimately the destruction of insulin-secreting beta cells. The activation of these specific innate pathways and expression of inflammatory molecules, including type I and III interferon, prime the immune system to elicit either a protective regulatory response or a diabetogenic effector response. Therefore, sensing of viral antigens by retinoic acid-inducible gene I-like receptors and toll-like receptors may be detrimental to inducing autoreactivity initiated by viral stress and resulting in T1D.

STAT1 is essential for the inhibition of hepatitis C virus replication by interferon-λ but not by interferon-α

Interferon-α (IFN-α) and IFN-λ are structurally distinct cytokines that bind to different receptors, but induce expression of similar sets of genes through Janus kinase (JAK)-signal transducers and activators of transcription (STAT) pathways. The difference between IFN-α and IFN-λ signaling remains poorly understood. Here, using the CRISPR/Cas9 system, we examine the role of STAT1 and STAT2 in the inhibition of hepatitis C virus (HCV) replication by IFN-α and IFN-λ. Treatment with IFN-α increases expression of IFN-stimulated genes (ISGs) such as double-stranded RNA-activated protein kinase (PKR) and decreases viral RNA and protein levels in HCV-infected Huh-7.5 human hepatoma cells. These responses are only partially attenuated by knockout of STAT1 but are abolished by knockout of STAT2. In contrast, the inhibition of HCV replication by IFN-λ is abolished by knockout of STAT1 or STAT2. Microarray analysis reveals that IFN-α but not IFN-λ can induce expression of the majority of ISGs in STAT1 knockout cells. These findings suggest that IFN-α can inhibit HCV replication through a STAT2-dependent but STAT1-independent pathway, whereas IFN-λ induces ISG expression and inhibits HCV replication exclusively through a STAT1- and STAT2-dependent pathway.

Early local immune defences in the respiratory tract

The respiratory immune response consists of multiple tiers of cellular responses that are engaged in a sequential manner in order to control infections. The stepwise engagement of effector functions with progressively increasing host fitness costs limits tissue damage. In addition, specific mechanisms are in place to promote disease tolerance in response to respiratory infections. Environmental factors, obesity and the ageing process can alter the efficiency and regulation of this tiered response, increasing pathology and mortality as a result. In this Review, we describe the cell types that coordinate pathogen clearance and tissue repair through the serial secretion of cytokines, and discuss how the environment and comorbidity influence this response.

HIV-1 Tat Recruits HDM2 E3 Ligase To Target IRF-1 for Ubiquitination and Proteasomal Degradation

In addition to its ability to regulate HIV-1 promoter activation, the viral transactivator Tat also functions as a determinant of pathogenesis and disease progression by directly and indirectly modulating the host anti-HIV response, largely through the capacity of Tat to interact with and modulate the activities of multiple host proteins. We previously demonstrated that Tat modulated both viral and host transcriptional machinery by interacting with the cellular transcription factor interferon regulatory factor 1 (IRF-1). In the present study, we investigated the mechanistic basis and functional significance of Tat−IRF-1 interaction and demonstrate that Tat dramatically decreased IRF-1 protein stability. To accomplish this, Tat exploited the cellular HDM2 (human double minute 2 protein) ubiquitin ligase to accelerate IRF-1 proteasome-mediated degradation, resulting in a quenching of IRF-1 transcriptional activity during HIV-1 infection. These data identify IRF-1 as a new target of Tat-induced modulation of the cellular protein machinery and reveal a new strategy developed by HIV-1 to evade host immune responses.

Current therapies have dramatically reduced morbidity and mortality associated with HIV infection and have converted infection from a fatal pathology to a chronic disease that is manageable via antiretroviral therapy. Nevertheless, HIV-1 infection remains a challenge, and the identification of useful cellular targets for therapeutic intervention remains a major goal. The cellular transcription factor IRF-1 impacts various physiological functions, including the immune response to viral infection. In this study, we have identified a unique mechanism by which HIV-1 evades IRF-1-mediated host immune responses and show that the viral protein Tat accelerates IRF-1 proteasome-mediated degradation and inactivates IRF-1 function. Restoration of IRF-1 functionality may thus be regarded as a potential strategy to reinstate both a direct antiviral response and a more broadly acting immune regulatory circuit.

Potential therapeutic targets for inflammation in toll-like receptor 4 (TLR4)-mediated signaling pathways

Inflammation is set off when innate immune cells detect infection or tissue injury. Tight control of the severity, duration, and location of inflammation is an absolute requirement for an appropriate balance between clearance of injured tissue and pathogens versus damage to host cells. Impeding the risk associated with the imbalance in the inflammatory response requires precise identification of potential therapeutic targets involved in provoking the inflammation. Toll-like receptors (TLRs) primarily known for the pathogen recognition and subsequent immune responses are being investigated for their pathogenic role in various chronic diseases. A mammalian homologue of Drosophila Toll receptor 4 (TLR4) was shown to induce the expression of genes involved in inflammatory responses. Signaling pathways via TLR4 activate various transcription factors like Nuclear factor kappa-light-chain-enhancer (NF-κB), activator protein 1 (AP1), Signal Transducers and Activators of Transcription family of transcription factors (STAT1) and Interferon regulatory factors (IRF's), which are the key players regulating the inflammatory response. Inhibition of these targets and their upstream signaling molecules provides a potential therapeutic approach to treat inflammatory diseases. Here we review the therapeutic targets involved in TLR-4 signaling pathways that are critical for suppressing chronic inflammatory disorders.

Soluble IL-6 Receptor and IL-27 Subunit p28 Protein Complex Mediate the Antiviral Response through the Type III IFN Pathway

Previously, we demonstrated that the soluble IL-6R (sIL-6R) plays an important role in the host antiviral response through induction of type I IFN and sIL-6R-mediated antiviral action via the IL-27 subunit p28; however, the mechanism that underlies sIL-6R and p28 antiviral action and whether type III IFN is involved remain unknown. In this study, we constructed a sIL-6R and p28 fusion protein (sIL-6R/p28 FP) and demonstrated that the fusion protein has stronger antiviral activity than sIL-6R alone. Consequently, knockout of sIL-6R inhibited virus-triggered IFN-λ1 expression. In addition, sIL-6R/p28 FP associated with mitochondrial antiviral signaling protein and TNFR-associated factor 6, the retinoic acid-inducible gene I adapter complex, and the antiviral activity mediated by sIL-6R/p28 FP was dependent on mitochondrial antiviral signaling protein. Furthermore, significantly reduced binding of p50/p65 and IFN regulatory factor 3 to the IFN-λ1 promoter was observed in sIL-6R knockout cells compared with the control cells. Interestingly, a novel heterodimer of c-Fos and activating transcription factor 1 was identified as a crucial transcriptional activator of IFN-λ1 The sIL-6R/p28 FP upregulated IFN-λ1 expression by increasing the binding abilities of c-Fos and activating transcription factor 1 to the IFN-λ1 promoter via the p38 MAPK signaling pathway. In conclusion, these results demonstrate the important role of sIL-6R/p28 FP in mediating virus-induced type III IFN production.

Cytokine Regulation of Metastasis and Tumorigenicity

The human body combats infection and promotes wound healing through the remarkable process of inflammation. Inflammation is characterized by the recruitment of stromal cell activity including recruitment of immune cells and induction of angiogenesis. These cellular processes are regulated by a class of soluble molecules called cytokines. Based on function, cell target, and structure, cytokines are subdivided into several classes including: interleukins, chemokines, and lymphokines. While cytokines regulate normal physiological processes, chronic deregulation of cytokine expression and activity contributes to cancer in many ways. Gene polymorphisms of all types of cytokines are associated with risk of disease development. Deregulation RNA and protein expression of interleukins, chemokines, and lymphokines have been detected in many solid tumors and hematopoetic malignancies, correlating with poor patient prognosis. The current body of literature suggests that in some tumor types, interleukins and chemokines work against the human body by signaling to cancer cells and remodeling the local microenvironment to support the growth, survival, and invasion of primary tumors and enhance metastatic colonization. Some lymphokines are downregulated to suppress tumor progression by enhancing cytotoxic T cell activity and inhibiting tumor cell survival. In this review, we will describe the structure/function of several cytokine families and review our current understanding on the roles and mechanisms of cytokines in tumor progression. In addition, we will also discuss strategies for exploiting the expression and activity of cytokines in therapeutic intervention.

Dengue virus infection induces interferon-lambda1 to facilitate cell migration

A marked increase in the rate of dengue virus (DENV) infection has resulted in more than 212 deaths in Taiwan since the beginning of 2015, mostly from fatal outcomes such as dengue hemorrhagic fever and dengue shock syndrome. The pathogenic mechanisms of these fatal manifestations are poorly understood. Cytokines induce an overwhelming immune reaction and thus have crucial roles. Interferon-lambda (IFN-λ), a newly identified IFN subtype, has antiviral effects, but its immunologic effects in DENV infection have not been investigated. In the present study, we show that DENV infection preferentially induced production of IFN-λ1 in human dendritic cells (DCs) and human lung epithelial cells. Virus nonstructural 1 (NS1) glycoprotein was responsible for the effect. DENV-induced production of IFN-λ1 was dependent on signaling pathways involving toll-like receptor (TLR)-3, interferon regulation factor (IRF)-3, and nuclear factor-kappaB (NF-κB). Blocking interaction between IFN-λ1 and its receptor IFN-λR1 through siRNA interference reduced DENV-induced DC migration towards the chemoattractants CCL19 and CCL21, by inhibiting CCR7 expression. Furthermore, IFN-λ1 itself induced CCR7 expression and DC migration. Our study presents the first evidence of the mechanisms and effects of IFN-λ1 induction in DENV-infected DCs and highlights the role of this cytokine in the immunopathogenesis of DENV infection.

Distinct Patterns of Expression of Transcription Factors in Response to Interferonβ and Interferonλ1

After viral infection, type I and III interferons (IFNs) are coexpressed by respiratory epithelial cells (RECs) and activate the ISGF3 transcription factor (TF) complex to induce expression of a cell-specific set of interferon-stimulated genes (ISGs). Type I and III IFNs share a canonical signaling pathway, suggesting that they are redundant. Animal and in vitro models, however, have shown that they are not redundant. Because TFs dictate cellular phenotype and function, we hypothesized that focusing on TF-ISG will reveal critical combinatorial and nonredundant functions of type I or III IFN. We treated BEAS-2B human RECs with increasing doses of IFNβ or IFNλ1 and measured expression of TF-ISG. ISGs were expressed in a dose-dependent manner with a nonlinear jump at intermediate doses. At subsaturating combinations of IFNβ and IFNλ1, many ISGs were expressed in a pattern that we modeled with a cubic equation that mathematically defines this threshold effect. Uniquely, IFNβ alone induced early and transient IRF1 transcript and protein expression, while IFNλ1 alone induced IRF1 protein expression at low levels that were sustained through 24 h. In combination, saturating doses of these 2 IFNs together enhanced and sustained IRF1 expression. We conclude that the cubic model quantitates combinatorial effects of IFNβ and IFNλ1 and that IRF1 may mediate nonredundancy of type I or III IFN in RECs.

Reovirus intermediate subviral particles constitute a strategy to infect intestinal epithelial cells by exploiting TGF-β dependent pro-survival signaling

Intestinal epithelial cells (IECs) constitute the primary barrier that separates us from the outside environment. These cells, lining the surface of the intestinal tract, represent a major challenge that enteric pathogens have to face. How IECs respond to viral infection and whether enteric viruses have developed strategies to subvert IECs innate immune response remains poorly characterized. Using mammalian reovirus (MRV) as a model enteric virus, we found that the intermediate subviral particles (ISVPs), which are formed in the gut during the natural course of infection by proteolytic digestion of the reovirus virion, trigger reduced innate antiviral immune response in IECs. On the contrary, infection of IECs by virions induces a strong antiviral immune response that leads to cellular death. Additionally, we determined that virions can be sensed by both TLR and RLR pathways while ISVPs are sensed by RLR pathways only. Interestingly, we found that ISVP infected cells secrete TGF-β acting as a pro-survival factor that protects IECs against virion induced cellular death. We propose that ISVPs represent a reovirus strategy to initiate primary infection of the gut by subverting IECs innate immune system and by counteracting cellular-death pathways.

Impaired airway epithelial cell responses from children with asthma to rhinoviral infection

BACKGROUND

The airway epithelium forms an effective immune and physical barrier that is essential for protecting the lung from potentially harmful inhaled stimuli including viruses. Human rhinovirus (HRV) infection is a known trigger of asthma exacerbations, although the mechanism by which this occurs is not fully understood.

OBJECTIVE

To explore the relationship between apoptotic, innate immune and inflammatory responses to HRV infection in airway epithelial cells (AECs) obtained from children with asthma and non-asthmatic controls. In addition, to test the hypothesis that aberrant repair of epithelium from asthmatics is further dysregulated by HRV infection.

METHODS

Airway epithelial brushings were obtained from 39 asthmatic and 36 non-asthmatic children. Primary cultures were established and exposed to HRV1b and HRV14. Virus receptor number, virus replication and progeny release were determined. Epithelial cell apoptosis, IFN-β production, inflammatory cytokine release and epithelial wound repair and proliferation were also measured.

RESULTS

Virus proliferation and release was greater in airway epithelial cells from asthmatics but this was not related to the number of virus receptors. In epithelial cells from asthmatic children, virus infection dampened apoptosis, reduced IFN-β production and increased inflammatory cytokine production. HRV1b infection also inhibited wound repair capacity of epithelial cells isolated from non-asthmatic children and exaggerated the defective repair response seen in epithelial cells from asthmatics. Addition of IFN-β restored apoptosis, suppressed virus replication and improved repair of airway epithelial cells from asthmatics but did not reduce inflammatory cytokine production.

CONCLUSIONS

Collectively, HRV infection delays repair and inhibits apoptotic processes in epithelial cells from non-asthmatic and asthmatic children. The delayed repair is further exaggerated in cells from asthmatic children and is only partially reversed by exogenous IFN-β.

Hepatitis B Virus-Induced Parkin-Dependent Recruitment of Linear Ubiquitin Assembly Complex (LUBAC) to Mitochondria and Attenuation of Innate Immunity

Hepatitis B virus (HBV) suppresses innate immune signaling to establish persistent infection. Although HBV is a DNA virus, its pre-genomic RNA (pgRNA) can be sensed by RIG-I and activates MAVS to mediate interferon (IFN) λ synthesis. Despite of the activation of RIG-I-MAVS axis by pgRNA, the underlying mechanism explaining how HBV infection fails to induce interferon-αβ (IFN) synthesis remained uncharacterized. We demonstrate that HBV induced parkin is able to recruit the linear ubiquitin assembly complex (LUBAC) to mitochondria and abrogates IFN β synthesis. Parkin interacts with MAVS, accumulates unanchored linear polyubiquitin chains on MAVS via LUBAC, to disrupt MAVS signalosome and attenuate IRF3 activation. This study highlights the novel role of parkin in antiviral signaling which involves LUBAC being recruited to the mitochondria. These results provide avenues of investigations on the role of mitochondrial dynamics in innate immunity.

Hepatitis B virus (HBV) chronic infection is one of the major causes of hepatocellular carcinoma. HBV infection is associated with mitochondrial dysfunction. We previously showed that persistent infection of HBV requires rapid clearance of impaired mitochondria by mitophagy, a cellular quality control process that insures survival of HBV infected cells. During the process, Parkin, an RBR E3 ligase, is recruited to mitochondria to induce mitophagy. In this study, we show that the Parkin, plays a critical role in the modulation of innate immune signaling. Using HBV expressing cells, we show that the Parkin recruits linear ubiquitin assembly complex (LUBAC) to the mitochondria and subsequently inhibits downstream signaling of mitochondrial antiviral signaling protein (MAVS). Mitochondrial LUBAC then catalyzes linear ubiquitin chains on MAVS, which abrogates its downstream events such as MAVS-TRAFs interaction and abolishes IRF3 phosphorylation. The results of this study highlight the molecular details explaining how HBV can suppress interferon synthesis implicating a mitophagy-independent role of Parkin. HBV-induced mitochondrial damage serves as the platform for recruitment of Parkin and LUBAC, which together modify MAVS by ubiquitination and cripples its downstream signaling.

Immune evasion of porcine enteric coronaviruses and viral modulation of antiviral innate signaling

•

Enteric coronaviruses have evolved to modulate the host innate immunity.

•

Viral IFN antagonists have been identified and they are mostly redundant.

•

For protection of intestinal epithelia from enteric viruses, type III IFN plays a major role.

Porcine epidemic diarrhea virus (PEDV) and porcine deltacoronavirus (PDCoV) are emerged and reemerging viruses in pigs, and together with transmissible gastroenteritis virus (TGEV), pose significant economic concerns to the swine industry. These viruses infect epithelial cells of the small intestine and cause watery diarrhea, dehydration, and a high mortality in neonatal piglets. Type I interferons (IFN-α/β) are major antiviral cytokines forming host innate immunity, and in turn, these enteric coronaviruses have evolved to modulate the host innate immune signaling during infection. Accumulating evidence however suggests that IFN induction and signaling in the intestinal epithelial cells differ from other epithelial cells, largely due to distinct features of the gut epithelial mucosal surface and commensal microflora, and it appears that type III interferon (IFN-λ) plays a key role to maintain the antiviral state in the gut. This review describes the recent understanding on the immune evasion strategies of porcine enteric coronaviruses and the role of different types of IFNs for intestinal antiviral innate immunity.

Peroxisomes are platforms for cytomegalovirus' evasion from the cellular immune response

The human cytomegalovirus developed distinct evasion mechanisms from the cellular antiviral response involving vMIA, a virally-encoded protein that is not only able to prevent cellular apoptosis but also to inhibit signalling downstream from mitochondrial MAVS. vMIA has been shown to localize at mitochondria and to trigger their fragmentation, a phenomenon proven to be essential for the signalling inhibition. Here, we demonstrate that vMIA is also localized at peroxisomes, induces their fragmentation and inhibits the peroxisomal-dependent antiviral signalling pathway. Importantly, we demonstrate that peroxisomal fragmentation is not essential for vMIA to specifically inhibit signalling downstream the peroxisomal MAVS. We also show that vMIA interacts with the cytoplasmic chaperone Pex19, suggesting that the virus has developed a strategy to highjack the peroxisomal membrane proteins' transport machinery. Furthermore, we show that vMIA is able to specifically interact with the peroxisomal MAVS. Our results demonstrate that peroxisomes constitute a platform for evasion of the cellular antiviral response and that the human cytomegalovirus has developed a mechanism by which it is able to specifically evade the peroxisomal MAVS-dependent antiviral signalling.

Mitochondria and Antiviral Immunity

Mitochondria are unique dynamic organelles that evolved from free-living bacteria into endosymbionts of mammalian hosts (Sagan 1967; Hatefi 1985). They have a distinct ~16.6 kb closed circular DNA genome coding for 13 polypeptides (Taanman 1999). In addition, a majority of the ~1500 mitochondrial proteins are encoded in the nucleus and transported to the mitochondria (Bonawitz et al. 2006). Mitochondria have two membranes: an outer smooth membrane and a highly folded inner membrane called cristae, which encompasses the matrix that houses the enzymes of the tricarboxylic acid (TCA) cycle and lipid metabolism. The inner mitochondrial membrane houses the protein complexes comprising the electron transport chain (ETC) (Hatefi 1985).

Cytosolic Innate Immune Sensing and Signaling upon Infection

Cytosolic sensing of pathogens is essential to a productive immune response. Recent reports have emphasized the importance of signaling platforms emanating from organelles and cytosolic sensors, particularly during the response to intracellular pathogens. Here, we highlight recent discoveries identifying the key mediators of nucleic acid and cyclic nucleotide sensing and discuss their importance in host defense. This review will also cover strategies evolved by pathogens to manipulate these pathways.