Type I interferons produced by hematopoietic cells protect mice against lethal infection by mammalian reovirus

A single mutation in an enteric virus alters tropism and sensitivity to microbiota

Many enteric viruses benefit from the microbiota. In mice, microbiota depletion reduces infection by noroviruses and picornaviruses. However, Reovirales viruses are outliers among enteric viruses. Rotavirus infection is inhibited by bacteria, and we determined that several reovirus strains have enhanced replication following microbiota depletion. Here, we focused on an isogenic pair of reoviruses that have opposing infection outcomes after microbiota depletion. Microbiota depletion reduces infection by reovirus strain T3SA+ but increases infection by strain T3SA-. These strains differ by a single amino acid polymorphism in the σ1 attachment protein, which confers sialic acid binding to T3SA+. Sialic acid binding facilitates T3SA+ infection of intestinal endothelial cells, while T3SA- inefficiently infects intestinal epithelial cells due to restriction by microbiota-driven, host-derived type III interferon responses. This study enhances an understanding of the interactions of enteric viruses, the microbiota, intestinal tropism, and antiviral responses.

Immunoglobulin A controls intestinal virus colonization to preserve immune homeostasis

Immunoglobulin A (IgA) is the predominant immunoglobulin isotype in mammals, primarily secreted at type I mucosal surfaces. Despite its abundance, the precise role of secretory IgA in the intestinal lumen, where it coats a diverse array of commensal microbiota, has remained elusive. Our study reveals that germinal center IgA responses are essential for preventing chronic colonization of the gut by specific viruses. In the absence of IgA, chronic viral colonization triggers an antigen-driven expansion of CD8αβ+ intraepithelial lymphocytes (IELs). Although these IELs are unable to clear the virus, they contribute to maintaining homeostasis by regulating its load and type I interferon responses. Consequently, IgA deficiency increases susceptibility to colitis in genetically susceptible hosts or following chemical induction but only in the presence of viral pathobionts requiring IgA for their clearance. These findings underscore the potential vulnerability of IgA-deficient individuals to immunopathology when exposed to selective viral pathobionts.

Regulation of host/pathogen interactions in the gastrointestinal tract by type I and III interferons

Interferons (IFNs) are an integral component of the host innate immune response during viral infection. Recent advances in the study of type I and III IFNs suggest that though both types counteract viral infection, type III IFNs act predominantly at epithelial barrier sites, while type I IFNs drive systemic responses. The dynamics and specific roles of type I versus III IFNs have been studied in the context of infection by a variety of enteric pathogens, including reovirus, rotavirus, norovirus, astrovirus, and intestinal severe acute respiratory syndrome coronavirus 2, revealing shared patterns of regulatory influence. An important role for the gut microbiota, including the virome, in regulating homeostasis and priming of intestinal IFN responses has also recently emerged.

Indoleamine 2,3-dioxygenase 1 regulates cell permissivity to astrovirus infection

Astroviruses cause a spectrum of diseases spanning asymptomatic infections to severe diarrhea, but little is understood about their pathogenesis. We previously determined that small intestinal goblet cells were the main cell type infected by murine astrovirus-1. Here, we focused on the host immune response to infection and inadvertently discovered a role for indoleamine 2,3-dioxygenase 1 (Ido1), a host tryptophan catabolizing enzyme, in the cellular tropism of murine and human astroviruses. We identified that Ido1 expression was highly enriched among infected goblet cells, and spatially corresponded to the zonation of infection. Because Ido1 can act as a negative regulator of inflammation, we hypothesized it could dampen host antiviral responses. Despite robust interferon signaling in goblet cells, as well as tuft cell and enterocyte bystanders, we observed delayed cytokine induction and suppressed levels of fecal lipocalin-2. Although we found Ido-/- animals were more resistant to infection, this was not associated with fewer goblet cells nor could it be rescued by knocking out interferon responses, suggesting that IDO1 instead regulates cell permissivity. We characterized IDO1-/- Caco-2 cells and observed significantly reduced human astrovirus-1 infection. Together this study highlights a role for Ido1 in astrovirus infection and epithelial cell maturation.

NK cells contribute to reovirus-induced IFN responses and loss of tolerance to dietary antigen

Celiac disease is an immune-mediated intestinal disorder that results from loss of oral tolerance (LOT) to dietary gluten. Reovirus elicits inflammatory Th1 cells and suppresses Treg responses to dietary antigen in a strain-dependent manner. Strain type 1 Lang (T1L) breaks oral tolerance, while strain type 3 Dearing reassortant virus (T3D-RV) does not. We discovered that intestinal infection by T1L in mice leads to the recruitment and activation of NK cells in mesenteric lymph nodes (MLNs) in a type I IFN-dependent manner. Once activated following infection, NK cells produce type II IFN and contribute to IFN-stimulated gene expression in the MLNs, which in turn induces inflammatory DC and T cell responses. Immune depletion of NK cells impairs T1L-induced LOT to newly introduced food antigen. These studies indicate that NK cells modulate the response to dietary antigen in the presence of a viral infection.

Early Events in Reovirus Infection Influence Induction of Innate Immune Response

Mammalian orthoreovirus (reovirus) is a double-stranded RNA (dsRNA) virus which encapsidates its 10 genome segments within a double-layered viral particle. Reovirus infection triggers an antiviral response in host cells which serves to limit viral replication. This antiviral response is initiated by recognition of the incoming viral genome by host sensors present in the cytoplasm. However, how host sensors gain access to the reovirus genome is unclear, as this dsRNA is protected by the viral particle proteins throughout infection. To initiate infection, reovirus particles are endocytosed and the outer viral particle layer is disassembled through the action of host proteases. This disassembly event is required for viral escape into the cytoplasm to begin replication. We show that endosomal proteases are required even late in infection, when disassembly is complete, to induce an immune response to reovirus. Additionally, counter to dogma, our data demonstrate that at least some viral dsRNA genome is exposed and detectable during entry. We hypothesize that some proportion of reovirus particles remain trapped within endosomes, allowing for the breakdown of these particles and release of their genome. We show that rapidly uncoating mutants escape the endosome more rapidly and induce a diminished immune response. Further, we show that particles entering through dynamin-independent pathways evade detection by host sensors. Overall, our data provide new insight into how genomes from entering reovirus particles are detected by host cells. IMPORTANCE Viruses must infect host cells to replicate, often killing the host cell in the process. However, hosts can activate defenses to limit viral replication and protect the organism. To trigger these host defenses to viral infections, host cells must first recognize that they are infected. Mammalian orthoreovirus (reovirus) is a model system used to study host-virus interactions. This study identifies aspects of host and virus biology which determine the capacity of host cells to detect infection. Notably, entry of reovirus into host cells plays a critical role in determining the magnitude of immune response triggered during infection. Mutants of reovirus which can enter cells more rapidly are better at avoiding detection by the host. Additionally, reovirus can enter cells through multiple routes. Entry through some of these routes also helps reovirus evade detection.

Reovirus Activated Cell Death Pathways

Mammalian orthoreoviruses (ReoV) are non-enveloped viruses with segmented double-stranded RNA genomes. In humans, ReoV are generally considered non-pathogenic, although members of this family have been proven to cause mild gastroenteritis in young children and may contribute to the development of inflammatory conditions, including Celiac disease. Because of its low pathogenic potential and its ability to efficiently infect and kill transformed cells, the ReoV strain Type 3 Dearing (T3D) is clinical trials as an oncolytic agent. ReoV manifests its oncolytic effects in large part by infecting tumor cells and activating programmed cell death pathways (PCDs). It was previously believed that apoptosis was the dominant PCD pathway triggered by ReoV infection. However, new studies suggest that ReoV also activates other PCD pathways, such as autophagy, pyroptosis, and necroptosis. Necroptosis is a caspase-independent form of PCD reliant on receptor-interacting serine/threonine-protein kinase 3 (RIPK3) and its substrate, the pseudokinase mixed-lineage kinase domain-like protein (MLKL). As necroptosis is highly inflammatory, ReoV-induced necroptosis may contribute to the oncolytic potential of this virus, not only by promoting necrotic lysis of the infected cell, but also by inflaming the surrounding tumor microenvironment and provoking beneficial anti-tumor immune responses. In this review, we summarize our current understanding of the ReoV replication cycle, the known and potential mechanisms by which ReoV induces PCD, and discuss the consequences of non-apoptotic cell death—particularly necroptosis—to ReoV pathogenesis and oncolysis.

Oncolytic virus treatment differentially affects the CD56dim and CD56bright NK cell subsets in vivo and regulates a spectrum of human NK cell activity

Natural killer (NK) cells protect against intracellular infection and cancer. These properties are exploited in oncolytic virus (OV) therapy, where antiviral responses enhance anti-tumour immunity. We have analysed the mechanism by which reovirus, an oncolytic dsRNA virus, modulates human NK cell activity. Reovirus activates NK cells in a type I interferon (IFN-I) dependent manner, inducing STAT1 and STAT4 signalling in both CD56dim and CD56bright NK cell subsets. Gene expression profiling revealed the dominance of IFN-I responses and identified induction of genes associated with NK cell cytotoxicity and cell cycle progression, with distinct responses in the CD56dim and CD56bright subsets. However, reovirus treatment inhibited IL-15 induced NK cell proliferation in an IFN-I dependent manner and was associated with reduced AKT signalling. In vivo, human CD56dim and CD56bright NK cells responded with similar kinetics to reovirus treatment, but CD56bright NK cells were transiently lost from the peripheral circulation at the peak of the IFN-I response, suggestive of their redistribution to secondary lymphoid tissue. Coupled with the direct, OV-mediated killing of tumour cells, the activation of both CD56dim and CD56bright NK cells by antiviral pathways induces a spectrum of activity that includes the NK cell-mediated killing of tumour cells and modulation of adaptive responses via the trafficking of IFN-γ expressing CD56bright NK cells to lymph nodes.

Infection of non-cancer cells: A barrier or support for oncolytic virotherapy?

Oncolytic viruses are designed to specifically target cancer cells, sparing normal cells. Although numerous studies demonstrate the ability of oncolytic viruses to infect a wide range of non-tumor cells, the significance of this phenomenon for cancer virotherapy is poorly understood. To fill the gap, we summarize the data on infection of non-cancer targets by oncolytic viruses with a special focus on tumor microenvironment and secondary lymphoid tissues. The review aims to address two major questions: how do attenuated viruses manage to infect normal cells, and whether it is of importance for oncolytic virotherapy.

DHX15 is required to control RNA virus-induced intestinal inflammation

RNA helicases play critical roles in various biological processes, including serving as viral RNA sensors in innate immunity. Here, we find that RNA helicase DEAH-box helicase 15 (DHX15) is essential for type I interferon (IFN-I, IFN-β), type III IFN (IFN-λ3), and inflammasome-derived cytokine IL-18 production by intestinal epithelial cells (IECs) in response to poly I:C and RNA viruses with preference of enteric RNA viruses, but not DNA virus. Importantly, we generate IEC-specific Dhx15-knockout mice and demonstrate that DHX15 is required for controlling intestinal inflammation induced by enteric RNA virus rotavirus in suckling mice and reovirus in adult mice in vivo, which owes to impaired IFN-β, IFN-λ3, and IL-18 production in IECs from Dhx15-deficient mice. Mechanistically, DHX15 interacts with NLRP6 to trigger NLRP6 inflammasome assembly and activation for inducing IL-18 secretion in IECs. Collectively, our report reveals critical roles for DHX15 in sensing enteric RNA viruses in IECs and controlling intestinal inflammation.

Lymphatic Type 1 Interferon Responses Are Critical for Control of Systemic Reovirus Dissemination

Mammalian orthoreovirus (reovirus) spreads from the site of infection to every organ system in the body via the blood. However, mechanisms that underlie reovirus hematogenous spread remain undefined. Nonstructural protein σ1s is a critical determinant of reovirus bloodstream dissemination that is required for efficient viral replication in many types of cultured cells. Here, we used the specificity of the σ1s protein for promoting hematogenous spread as a platform to uncover a role for lymphatic type 1 interferon (IFN-1) responses in limiting reovirus systemic dissemination. We found that replication of a σ1s-deficient reovirus was restored to wild-type levels in cells with defective interferon-α receptor (IFNAR1) signaling. Reovirus spreads systemically following oral inoculation of neonatal mice, whereas the σ1s-null virus remains localized to the intestine. We found that σ1s enables reovirus spread in the presence of a functional IFN-1 response, as the σ1s-deficient reovirus disseminated comparably to wild-type virus in IFNAR1^-/- mice. Lymphatics are hypothesized to mediate reovirus spread from the intestine to the bloodstream. IFNAR1 deletion from cells expressing lymphatic vessel endothelium receptor 1 (LYVE-1), a marker for lymphatic endothelial cells, enabled the σ1s-deficient reovirus to disseminate systemically. Together, our findings indicate that IFN-1 responses in lymphatics limit reovirus dissemination. Our data further suggest that the lymphatics are an important conduit for reovirus hematogenous spread.IMPORTANCE Type 1 interferons (IFN-1) are critical host responses to viral infection. However, the contribution of IFN-1 responses to control of viruses in specific cell and tissue types is not fully defined. Here, we identify IFN-1 responses in lymphatics as important for limiting reovirus dissemination. We found that nonstructural protein σ1s enhances reovirus resistance to IFN-1 responses, as a reovirus mutant lacking σ1s was more sensitive to IFN-1 than wild-type virus. In neonatal mice, σ1s is required for reovirus systemic spread. We used tissue-specific IFNAR1 deletion in combination with the IFN-1-sensitive σ1s-null reovirus as a tool to test how IFN-1 responses in lymphatics affect reovirus systemic spread. Deletion of IFNAR1 in lymphatic cells using Cre-lox technology enabled dissemination of the IFN-1-sensitive σ1s-deficient reovirus. Together, our results indicate that IFN-1 responses in lymphatics are critical for controlling reovirus systemic spread.

Past, Present and Future of Oncolytic Reovirus

Oncolytic virotherapy (OVT) has received significant attention in recent years, especially since the approval of talimogene Laherparepvec (T-VEC) in 2015 by the Food and Drug administration (FDA). Mechanistic studies of oncolytic viruses (OVs) have revealed that most, if not all, OVs induce direct oncolysis and stimulate innate and adaptive anti-tumour immunity. With the advancement of tumour modelling, allowing characterisation of the effects of tumour microenvironment (TME) components and identification of the cellular mechanisms required for cell death (both direct oncolysis and anti-tumour immune responses), it is clear that a "one size fits all" approach is not applicable to all OVs, or indeed the same OV across different tumour types and disease locations. This article will provide an unbiased review of oncolytic reovirus (clinically formulated as pelareorep), including the molecular and cellular requirements for reovirus oncolysis and anti-tumour immunity, reports of pre-clinical efficacy and its overall clinical trajectory. Moreover, as it is now abundantly clear that the true potential of all OVs, including reovirus, will only be reached upon the development of synergistic combination strategies, reovirus combination therapeutics will be discussed, including the limitations and challenges that remain to harness the full potential of this promising therapeutic agent.

Type III interferons: Balancing tissue tolerance and resistance to pathogen invasion

Type III IFNs, or IFN-λ, are the latest addition to the IFN family. Thanks to a restricted pattern of expression of their receptor and to unique immunomodulatory properties, IFN-λ stimulates pathogen clearance while, at the same time, curbing inflammation to maintain barrier integrity.

Type III IFNs, or IFN-λ, are the newest members of the IFN family and were long believed to play roles that were redundant with those of type I IFNs. However, IFN-λ displays unique traits that delineate them as primary protectors of barrier integrity at mucosal sites. This unique role stems both from the restricted expression of IFN-λ receptor, confined to epithelial cells and to a limited pool of immune cells, and from unique immunomodulatory properties of IFN-λ. Here, we discuss recent findings that establish the unique capacity of IFN-λ to act at the barriers of the host to balance tissue tolerance and immune resistance against viral and bacterial challenges.

Bat mammalian orthoreoviruses cause severe pneumonia in mice

Mammalian orthoreovirus (MRV) infections are ubiquitous in mammals. Increasing evidence suggests that some MRVs can cause severe respiratory disease and encephalitis in humans and other animals. Previously, we isolated six bat MRV strains. However, the pathogenicity of these bat viruses remains unclear. In this study, we investigated the host range and pathogenicity of 3 bat MRV strains (WIV2, 3 and 7) which represent three serotypes. Our results showed that all of them can infect cell lines from different mammalian species and displayed different replication efficiency. The BALB/c mice infected by bat MRVs showed clinical symptoms with systematic infection especially in lung and intestines. Obvious tissue damage were found in all infected lungs. One of the strains, WIV7, showed higher replication efficiency in vitro and vivo and more severe pathogenesis in mice. Our results provide new evidence showing potential pathogenicity of bat MRVs in animals and probable risk in humans.

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Bat MRVs show wide cell tropism in vivo and in vitro and have a high replication efficiency in lung and intestines.

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Mice infected by bat MRVs showed clinical illness, but without death.

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The higher replication in brain, lung damage and weak innate immune response may be responsible for severe diseases for WIV7.

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The results indicate the potential pathogenicity of bat MRV to human and animals.

Recognition of Reovirus RNAs by the Innate Immune System

Mammalian orthoreovirus (reovirus) is a dsRNA virus, which has long been used as a model system to study host-virus interactions. One of the earliest interactions during virus infection is the detection of the viral genomic material, and the consequent induction of an interferon (IFN) based antiviral response. Similar to the replication of related dsRNA viruses, the genomic material of reovirus is thought to remain protected by viral structural proteins throughout infection. Thus, how innate immune sensor proteins gain access to the viral genomic material, is incompletely understood. This review summarizes currently known information about the innate immune recognition of the reovirus genomic material. Using this information, we propose hypotheses about host detection of reovirus.

Using Diverse Model Systems to Define Intestinal Epithelial Defenses to Enteric Viral Infections

The intestine is an essential physical and immunological barrier comprised of a monolayer of diverse and specialized epithelial cells that perform functions ranging from nutrient absorption to pathogen sensing and intestinal homeostasis. The intestinal barrier prevents translocation of intestinal microbes into internal compartments. The microbiota is comprised of a complex community largely populated by diverse bacterial species that provide metabolites, nutrients, and immune stimuli that promote intestinal and organismal health. Although commensal organisms promote health, enteric pathogens, including a diverse plethora of enteric viruses, cause acute and chronic diseases. The barrier epithelium plays fundamental roles in immune defenses against enteric viral infections by integrating diverse signals, including those from the microbiota, to prevent disease. Importantly, many model systems have contributed to our understanding of this complex interface. This review will focus on the antiviral mechanisms at play within the intestinal epithelium and how these responses are shaped by the microbiota.

Pembrolizumab in Combination with the Oncolytic Virus Pelareorep and Chemotherapy in Patients with Advanced Pancreatic Adenocarcinoma: A Phase Ib Study

PURPOSE

Pelareorep is an intravenously delivered oncolytic reovirus that can induce a T-cell-inflamed phenotype in pancreatic ductal adenocarcinoma (PDAC). Tumor tissues from patients treated with pelareorep have shown reovirus replication, T-cell infiltration, and upregulation of PD-L1. We hypothesized that pelareorep in combination with pembrolizumab and chemotherapy in patients with PDAC would be safe and effective.

PATIENTS AND METHODS

A phase Ib single-arm study enrolled patients with PDAC who progressed after first-line treatment. Patients received pelareorep, pembrolizumab, and either 5-fluorouracil, gemcitabine, or irinotecan until disease progression or unacceptable toxicity. Study objectives included safety and dose-limiting toxicities, tumor response, evaluation for reovirus replication, and immune analysis in peripheral blood and tumor biopsies.

RESULTS

Eleven patients were enrolled. Disease control was achieved in three of the 10 efficacy-evaluable patients. One patient achieved partial response for 17.4 months. Two additional patients achieved stable disease, lasting 9 and 4 months, respectively. Treatment was well tolerated, with mostly grade 1 or 2 treatment-related adverse events, including flu-like symptoms. Viral replication was observed in on-treatment tumor biopsies. T-cell receptor sequencing from peripheral blood revealed the creation of new T-cell clones during treatment. High peripheral clonality and changes in the expression of immune genes were observed in patients with clinical benefit.

CONCLUSIONS

Pelareorep and pembrolizumab added to chemotherapy did not add significant toxicity and showed encouraging efficacy. Further evaluation of pelareorep and anti-PD-1 therapy is ongoing in follow-up studies. This research highlights the potential utility of several pretreatment and on-treatment biomarkers for pelareorep therapy warranting further investigation.

Segmented Filamentous Bacteria Prevent and Cure Rotavirus Infection

Rotavirus (RV) encounters intestinal epithelial cells amidst diverse microbiota, opening possibilities of microbes influencing RV infection. Although RV clearance typically requires adaptive immunity, we unintentionally generated RV-resistant immunodeficient mice, which, we hypothesized, reflected select microbes protecting against RV. Accordingly, such RV resistance was transferred by co-housing and fecal transplant. RV-protecting microbiota were interrogated by heat, filtration, and antimicrobial agents, followed by limiting dilution transplant to germ-free mice and microbiome analysis. This approach revealed that segmented filamentous bacteria (SFB) were sufficient to protect mice against RV infection and associated diarrhea. Such protection was independent of previously defined RV-impeding factors, including interferon, IL-17, and IL-22. Colonization of the ileum by SFB induced changes in host gene expression and accelerated epithelial cell turnover. Incubation of RV with SFB-containing feces reduced infectivity in vitro, suggesting direct neutralization of RV. Thus, independent of immune cells, SFB confer protection against certain enteric viral infections and associated diarrheal disease.

Shared and Distinct Functions of Type I and Type III Interferons

Type I interferons (IFNs) (IFN-α, IFN-β) and type III IFNs (IFN-λ) share many properties, including induction by viral infection, activation of shared signaling pathways, and transcriptional programs. However, recent discoveries have revealed context-specific functional differences. Here, we provide a comprehensive review of type I and type III IFN activities, highlighting shared and distinct features from molecular mechanisms through physiological responses. Beyond discussing canonical antiviral functions, we consider the adaptive immune priming, anti-tumor, and autoimmune functions of IFNs. We discuss a model wherein type III IFNs serve as a front-line defense that controls infection at epithelial barriers while minimizing damaging inflammatory responses, reserving the more potent type I IFN response for when local responses are insufficient. In this context, we discuss current therapeutic applications targeting these cytokine pathways and highlight gaps in understanding of the biology of type I and type III IFNs in health and disease.

Current understanding of reovirus oncolysis mechanisms

Mammalian orthoreovirus (reovirus) is under development as a cancer virotherapy. Clinical trials demonstrate that reovirus-based therapies are safe and tolerated in patients with a wide variety of cancers. Although reovirus monotherapy has proven largely ineffective, reovirus sensitizes cancer cells to existing chemotherapeutic agents and radiation. Clinical trials are underway to test the efficacy of reovirus in combination with chemotherapeutic and radiation regimens and to evaluate the effectiveness of reovirus in conjunction with immunotherapies. Central to the use of reovirus to treat cancer is its capacity to directly kill cancer cells and alter the cellular environment to augment other therapies. Apoptotic cell death is a prominent mechanism of reovirus cancer cell killing. However, reoviruses can also kill cancer cells through nonapoptotic mechanisms. Here, we describe mechanisms of reovirus cancer cell killing, highlight how reovirus is used in combination with existing cancer treatments, and discuss what is known as to how reovirus modulates cancer immunotherapy.

Reovirus-Induced Apoptosis in the Intestine Limits Establishment of Enteric Infection

Several viruses induce intestinal epithelial cell death during enteric infection. However, it is unclear whether proapoptotic capacity promotes or inhibits replication in this tissue. We infected mice with two reovirus strains that infect the intestine but differ in the capacity to alter immunological tolerance to new food antigen. Infection with reovirus strain T1L, which induces an inflammatory immune response to fed antigen, is prolonged in the intestine, whereas T3D-RV, which does not induce this response, is rapidly cleared from the intestine. Compared with T1L, T3D-RV infection triggered apoptosis of intestinal epithelial cells and subsequent sloughing of dead cells into the intestinal lumen. We conclude that the infection advantage of T1L derives from its capacity to subvert host restriction by epithelial cell apoptosis, providing a possible mechanism by which T1L enhances inflammatory signals during antigen feeding. Using a panel of T1L × T3D-RV reassortant viruses, we identified the viral M1 and M2 gene segments as determinants of reovirus-induced apoptosis in the intestine. Expression of the T1L M1 and M2 genes in a T3D-RV background was sufficient to limit epithelial cell apoptosis and enhance viral infection to levels displayed by T1L. These findings define additional reovirus gene segments required for enteric infection of mice and illuminate the antiviral effect of intestinal epithelial cell apoptosis in limiting enteric viral infection. Viral strain-specific differences in the capacity to infect the intestine may be useful in identifying viruses capable of ameliorating tolerance to fed antigen in autoimmune conditions like celiac disease.IMPORTANCE Acute viral infections are thought to be cleared by the host with few lasting consequences. However, there may be much broader and long-lasting effects of viruses on immune homeostasis. Infection with reovirus, a common, nonpathogenic virus, triggers inflammation against innocuous food antigens, implicating this virus in the development of celiac disease, an autoimmune intestinal disorder triggered by exposure to dietary gluten. Using two reovirus strains that differ in the capacity to abrogate oral tolerance, we found that strain-specific differences in the capacity to replicate in the intestine inversely correlate with the capacity to induce apoptotic death of intestinal epithelial cells, providing a host-mediated process to restrict intestinal infection. This work contributes new knowledge about virus-host interactions in the intestine and establishes a foundation for future studies to define mechanisms by which viruses break oral tolerance in celiac disease.

Differential Delivery of Genomic Double-Stranded RNA Causes Reovirus Strain-Specific Differences in Interferon Regulatory Factor 3 Activation

Serotype 3 (T3) reoviruses induce substantially more type 1 interferon (IFN-I) secretion than serotype 1 (T1) strains. However, the mechanisms underlying differences in IFN-I production between T1 and T3 reoviruses remain undefined. Here, we found that differences in IFN-I production between T1 and T3 reoviruses correlate with activation of interferon regulatory factor 3 (IRF3), a key transcription factor for the production of IFN-I. T3 strain rsT3D activated IRF3 more rapidly and to a greater extent than the T1 strain rsT1L, in simian virus 40 (SV40) immortalized endothelial cells (SVECs). Differences in IRF3 activation between rsT1L and rsT3D were observed in the first hours of infection and were independent of de novo viral RNA and protein synthesis. NF-κB activation mirrored IRF3 activation, with rsT3D inducing more NF-κB activity than rsT1L. We also found that IRF3 and NF-κB are activated in a mitochondrial antiviral-signaling protein (MAVS)-dependent manner. rsT1L does not suppress IRF3 activation, as IRF3 phosphorylation could be induced in rsT1L-infected cells. Transfected rsT1L and rsT3D RNA induced IRF3 phosphorylation, indicating that genomic RNA from both strains has the capacity to activate IRF3. Finally, bypassing the normal route of reovirus entry by transfecting in vitro-generated viral cores revealed that rsT1L and rsT3D core particles induced equivalent IRF3 activation. Taken together, our findings indicate that entry-related events that occur after outer capsid disassembly, but prior to deposition of viral cores into the cytoplasm, influence the efficiency of IFN-I responses to reovirus. This work provides further insight into mechanisms by which nonenveloped viruses activate innate immune responses.IMPORTANCE Detection of viral nucleic acids by the host cell triggers type 1 interferon (IFN-I) responses, which are critical for containing and clearing viral infections. Viral RNA is sensed in the cytoplasm by cellular receptors that initiate signaling pathways, leading to the activation of interferon regulatory factor 3 (IRF3) and NF-κB, key transcription factors required for IFN-I induction. Serotype 3 (T3) reoviruses induce significantly more IFN-I than serotype 1 (T1) strains. In this work, we found that differences in IFN-I production by T1 and T3 reoviruses correlate with differential IRF3 activation. Differences in IRF3 activation are not caused by a blockade of the IRF3 activation by a T1 strain. Rather, differences in events during the late stages of viral entry determine the capacity of reovirus to activate host IFN-I responses. Together, our work provides insight into mechanisms of IFN-I induction by nonenveloped viruses.

Distinct Effects of Type I and III Interferons on Enteric Viruses

Interferons (IFNs) are key host cytokines in the innate immune response to viral infection, and recent work has identified unique roles for IFN subtypes in regulating different aspects of infection. Currently emerging is a common theme that type III IFNs are critical in localized control of infection at mucosal barrier sites, while type I IFNs are important for broad systemic control of infections. The intestine is a particular site of interest for exploring these effects, as in addition to being the port of entry for a multitude of pathogens, it is a complex tissue with a variety of cell types as well as the presence of the intestinal microbiota. Here we focus on the roles of type I and III IFNs in control of enteric viruses, discussing what is known about signaling downstream from these cytokines, including induction of specific IFN-stimulated genes. We review viral strategies to evade IFN responses, effects of IFNs on the intestine, interactions between IFNs and the microbiota, and briefly discuss the role of IFNs in controlling viral infections at other barrier sites. Enhanced understanding of the coordinate roles of IFNs in control of viral infections may facilitate development of antiviral therapeutic strategies; here we highlight potential avenues for future exploration.

The Orchestra of Reovirus Cell Entry

Purpose of Review

The ability of viruses to infect host cells is dependent on several factors including the availability of cell-surface receptors, antiviral state of cells, and presence of host factors needed for viral replication. Here, we review findings from in vitro and in vivo studies using mammalian orthoreovirus (reovirus) that have identified an intricate group of molecules and mechanisms used by the virus to attach and enter cells.

Recent Findings

Recent findings provide an improved mechanistic understanding of reovirus cell entry. Of special note is the identification of a cellular mediator of cell entry in neuronal and non-neuronal cells, the effect of cell entry on the outcome of infection and cytopathic effects on the host cell, and an improved understanding of the components that promote viral penetration of cellular membranes.

Summary

A mechanistic understanding of the interplay between host and viral factors has enhanced our view of how viruses usurp cellular processes during infection.

Reovirus infection triggers inflammatory responses to dietary antigens and development of celiac disease

Viral infections have been proposed to elicit pathological processes leading to the initiation of T helper 1 (TH1) immunity against dietary gluten and celiac disease (CeD). To test this hypothesis and gain insights into mechanisms underlying virus-induced loss of tolerance to dietary antigens, we developed a viral infection model that makes use of two reovirus strains that infect the intestine but differ in their immunopathological outcomes. Reovirus is an avirulent pathogen that elicits protective immunity, but we discovered that it can nonetheless disrupt intestinal immune homeostasis at inductive and effector sites of oral tolerance by suppressing peripheral regulatory T cell (pTreg) conversion and promoting TH1 immunity to dietary antigen. Initiation of TH1 immunity to dietary antigen was dependent on interferon regulatory factor 1 and dissociated from suppression of pTreg conversion, which was mediated by type-1 interferon. Last, our study in humans supports a role for infection with reovirus, a seemingly innocuous virus, in triggering the development of CeD.

Type 1 Interferon in the Human Intestine-A Co-ordinator of the Immune Response to the Microbiota

The human gut is in constant complex interaction with the external environment. Although much is understood about the composition and function of the microbiota, much remains to be learnt about the mechanisms by which these organisms interact with the immune system in health and disease. Type 1 interferon (T1IFN), a ubiquitous and pleiotropic family of cytokines, is a critical mediator of the response to viral, bacterial, and other antigens sampled in the intestine. Although inflammation is enhanced in mouse model of colitis when T1IFN signaling is lost, the action of T1IFN is context specific and can be pro- or anti-inflammatory. In humans, T1IFN has been used to treat inflammatory diseases, including multiple sclerosis and inflammatory bowel disease but intestinal inflammation can also develop after the administration of T1IFN. Recent findings indicate that "tonic" or "endogenous" T1IFN, induced by signals from the commensal microbiota, modulates the local signaling environment to prime the intestinal mucosal immune system to determine later responses to pathogens and commensal organisms. This review will summarize the complex immunological effects of T1IFN and recent the role of T1IFN as a mediator between the microbiota and the mucosal immune system, highlighting human data wherever possible. It will discuss what we can learn from clinical experiences with T1IFN and how the T1IFN pathway may be manipulated in the future to maintain mucosal homeostasis.

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.

Engineering Recombinant Reoviruses To Display gp41 Membrane-Proximal External-Region Epitopes from HIV-1

Vaccines to protect against HIV-1, the causative agent of AIDS, are not approved for use. Antibodies that neutralize genetically diverse strains of HIV-1 bind to discrete regions of the envelope glycoproteins, including the gp41 MPER. We engineered recombinant reoviruses that displayed MPER epitopes in attachment protein σ1 (REO-MPER vectors). The REO-MPER vectors replicated with wild-type efficiency, were genetically stable, and retained native antigenicity. However, we did not detect HIV-1-specific immune responses following inoculation of the REO-MPER vectors into small animals. This work provides proof of principle for engineering reovirus to express antigenic epitopes and illustrates the difficulty in eliciting MPER-specific immune responses.

The gp41 membrane-proximal external region (MPER) is a target for broadly neutralizing antibody responses against human immunodeficiency virus type 1 (HIV-1). However, replication-defective virus vaccines currently under evaluation in clinical trials do not efficiently elicit MPER-specific antibodies. Structural modeling suggests that the MPER forms an α-helical coiled coil that is required for function and immunogenicity. To maintain the native MPER conformation, we used reverse genetics to engineer replication-competent reovirus vectors that displayed MPER sequences in the α-helical coiled-coil tail domain of viral attachment protein σ1. Sequences in reovirus strain type 1 Lang (T1L) σ1 were exchanged with sequences encoding HIV-1 strain Ba-L MPER epitope 2F5 or the entire MPER. Individual 2F5 or MPER substitutions were introduced at virion-proximal or virion-distal sites in the σ1 tail. Recombinant reoviruses containing heterologous HIV-1 sequences were viable and produced progeny yields comparable to those with wild-type virus. HIV-1 sequences were retained following 10 serial passages in cell culture, indicating that the substitutions were genetically stable. Recombinant viruses engineered to display the 2F5 epitope or full-length MPER in σ1 were recognized by purified 2F5 antibody. Inoculation of mice with 2F5-containing vectors or rabbits with 2F5- or MPER-containing vectors elicited anti-reovirus antibodies, but HIV-1-specific antibodies were not detected. Together, these findings indicate that heterologous sequences that form α-helices can functionally replace native sequences in the α-helical tail domain of reovirus attachment protein σ1. However, although these vectors retain native antigenicity, they were not immunogenic, illustrating the difficulty of experimentally inducing immune responses to this essential region of HIV-1.

IMPORTANCE Vaccines to protect against HIV-1, the causative agent of AIDS, are not approved for use. Antibodies that neutralize genetically diverse strains of HIV-1 bind to discrete regions of the envelope glycoproteins, including the gp41 MPER. We engineered recombinant reoviruses that displayed MPER epitopes in attachment protein σ1 (REO-MPER vectors). The REO-MPER vectors replicated with wild-type efficiency, were genetically stable, and retained native antigenicity. However, we did not detect HIV-1-specific immune responses following inoculation of the REO-MPER vectors into small animals. This work provides proof of principle for engineering reovirus to express antigenic epitopes and illustrates the difficulty in eliciting MPER-specific immune responses.

Sequential Activation of Two Pathogen-Sensing Pathways Required for Type I Interferon Expression and Resistance to an Acute DNA Virus Infection

Toll-like receptor 9 (TLR9), its adaptor MyD88, the downstream transcription factor interferon regulatory factor 7 (IRF7), and type I interferons (IFN-I) are all required for resistance to infection with ectromelia virus (ECTV). However, it is not known how or in which cells these effectors function to promote survival. Here, we showed that after infection with ECTV, the TLR9-MyD88-IRF7 pathway was necessary in CD11c(+) cells for the expression of proinflammatory cytokines and the recruitment of inflammatory monocytes (iMos) to the draining lymph node (dLN). In the dLN, the major producers of IFN-I were infected iMos, which used the DNA sensor-adaptor STING to activate IRF7 and nuclear factor κB (NF-κB) signaling to induce the expression of IFN-α and IFN-β, respectively. Thus, in vivo, two pathways of DNA pathogen sensing act sequentially in two distinct cell types to orchestrate resistance to a viral disease.

Leukocyte-derived IFN-α/β and epithelial IFN-λ constitute a compartmentalized mucosal defense system that restricts enteric virus infections

Epithelial cells are a major port of entry for many viruses, but the molecular networks which protect barrier surfaces against viral infections are incompletely understood. Viral infections induce simultaneous production of type I (IFN-α/β) and type III (IFN-λ) interferons. All nucleated cells are believed to respond to IFN-α/β, whereas IFN-λ responses are largely confined to epithelial cells. We observed that intestinal epithelial cells, unlike hematopoietic cells of this organ, express only very low levels of functional IFN-α/β receptors. Accordingly, after oral infection of IFN-α/β receptor-deficient mice, human reovirus type 3 specifically infected cells in the lamina propria but, strikingly, did not productively replicate in gut epithelial cells. By contrast, reovirus replicated almost exclusively in gut epithelial cells of IFN-λ receptor-deficient mice, suggesting that the gut mucosa is equipped with a compartmentalized IFN system in which epithelial cells mainly respond to IFN-λ that they produce after viral infection, whereas other cells of the gut mostly rely on IFN-α/β for antiviral defense. In suckling mice with IFN-λ receptor deficiency, reovirus replicated in the gut epithelium and additionally infected epithelial cells lining the bile ducts, indicating that infants may use IFN-λ for the control of virus infections in various epithelia-rich tissues. Thus, IFN-λ should be regarded as an autonomous virus defense system of the gut mucosa and other epithelial barriers that may have evolved to avoid unnecessarily frequent triggering of the IFN-α/β system which would induce exacerbated inflammation.

Virus-induced interferon consists of two distinct families of molecules, IFN-α/β and IFN-λ. IFN-α/β family members are key antiviral molecules that confer protection against a large number of viruses infecting a wide variety of cell types. By contrast, IFN-λ responses are largely confined to epithelial cells due to highly restricted expression of the cognate receptor. Interestingly, virus resistance of the gut epithelium is not dependent on IFN-α/β but rather relies on IFN-λ, questioning the prevailing view that receptors for IFN-α/β are expressed ubiquitously. Here we demonstrate that the IFN-α/β system is unable to compensate for IFN-λ deficiency during infections with epitheliotropic viruses because intestinal epithelial cells do not express functional receptors for IFN-α/β. We further demonstrate that virus-infected intestinal epithelial cells are potent producers of IFN-λ, indicating that the gut mucosa possesses a compartmentalized IFN system in which epithelial cells predominantly respond to IFN-λ, whereas other cells of the gut mainly rely on IFN-α/β for antiviral defense. We suggest that IFN-λ may have evolved as an autonomous virus defense system of the gut mucosa to avoid unnecessarily frequent triggering of the IFN-α/β system which, due to its potent activity on immune cells, would induce exacerbated inflammation.

Complement receptor C5aR1/CD88 and dipeptidyl peptidase-4/CD26 define distinct hematopoietic lineages of dendritic cells

Differential display of the integrins CD103 and CD11b are widely used to distinguish two major dendritic cell (DC) subsets in nonlymphoid tissues. CD103(+) DCs arise from FLT3-dependent DC precursors (preDCs), whereas CD11b(hi) DCs can arise either from preDCs or FLT3-independent monocytes. Functional characterization of these two lineages of CD11b(hi) DCs has been hindered by the lack of a widely applicable method to distinguish between them. We performed gene expression analysis of fractionated lung DCs from C57BL/6 mice and found that monocyte-derived DCs (moDCs), including CD11b(hi)Ly-6C(lo) tissue-resident and CD11b(hi)Ly-6C(hi) inflammatory moDCs, express the complement 5a receptor 1/CD88, whereas preDC-derived conventional DCs (cDCs), including CD103(+) and CD11b(hi) cDCs, express dipeptidyl peptidase-4/CD26. Flow cytometric analysis of multiple organs, including the kidney, liver, lung, lymph nodes, small intestine, and spleen, confirmed that reciprocal display of CD88 and CD26 can reliably distinguish FLT3-independent moDCs from FLT3-dependent cDCs in C57BL/6 mice. Similar results were obtained when DCs from BALB/c mice were analyzed. Using this novel approach to study DCs in mediastinal lymph nodes, we observed that most blood-derived lymph node-resident DCs, as well as tissue-derived migratory DCs, are cDCs. Furthermore, cDCs, but not moDCs, stimulated naive T cell proliferation. We anticipate that the use of Abs against CD88 and CD26 to distinguish moDCs and cDCs in multiple organs and mouse strains will facilitate studies aimed at assigning specific functions to distinct DC lineages in immune responses.

Role of type I interferon signaling in human metapneumovirus pathogenesis and control of viral replication

Type I IFN signaling, which is initiated through activation of the alpha interferon receptor (IFNAR), regulates the expression of proteins that are crucial contributors to immune responses. Paramyxoviruses, including human metapneumovirus (HMPV), have evolved mechanisms to inhibit IFNAR signaling, but the specific contribution of IFNAR signaling to the control of HMPV replication, pathogenesis, and adaptive immunity is unknown. We used IFNAR-deficient (IFNAR(-/-)) mice to assess the effect of IFNAR signaling on HMPV replication and the CD8(+) T cell response. HMPV-infected IFNAR(-/-) mice had a higher peak of early viral replication but cleared the virus with kinetics similar to those of wild-type (WT) mice. However, IFNAR(-/-) mice infected with HMPV displayed less airway dysfunction and lung inflammation. CD8(+) T cells of IFNAR(-/-) mice after HMPV infection expressed levels of the inhibitory receptor programmed death 1 (PD-1) similar to those of WT mice. However, despite lower expression of inhibitory programmed death ligand 1 (PD-L1), HMPV-specific CD8(+) T cells of IFNAR(-/-) mice were more functionally impaired than those of WT mice and upregulated the inhibitory receptor Tim-3. Analysis of the antigen-presenting cell subsets in the lungs revealed that the expansion of PD-L1(low) dendritic cells (DCs), but not PD-L1(high) alveolar macrophages, was dependent on IFNAR signaling. Collectively, our results indicate a role for IFNAR signaling in the early control of HMPV replication, disease progression, and the development of an optimal adaptive immune response. Moreover, our findings suggest an IFNAR-independent mechanism of lung CD8(+) T cell impairment.

IMPORTANCE

Human metapneumovirus (HMPV) is a leading cause of acute respiratory illness. CD8(+) T cells are critical for clearing viral infection, yet recent evidence shows that HMPV and other respiratory viruses induce CD8(+) T cell impairment via PD-1-PD-L1 signaling. We sought to understand the role of type I interferon (IFN) in the innate and adaptive immune responses to HMPV by using a mouse model lacking IFN signaling. Although HMPV titers were higher in the absence of type I IFN, virus was nonetheless cleared and mice were less ill, indicating that type I IFN is not required to resolve HMPV infection but contributes to pathogenesis. Further, despite lower levels of the inhibitory ligand PD-L1 in mice lacking type I IFN, CD8(+) T cells were more impaired in these mice than in WT mice. Our data suggest that specific antigen-presenting cell subsets and the inhibitory receptor Tim-3 may contribute to CD8(+) T cell impairment.

The role of type I interferons in intestinal infection, homeostasis, and inflammation

Type I interferons are a widely expressed family of effector cytokines that promote innate antiviral and antibacterial immunity. Paradoxically, they can also suppress immune responses by driving production of anti-inflammatory cytokines, and dysregulation of these cytokines can contribute to host-mediated immunopathology and disease progression. Recent studies describe their anti-inflammatory role in intestinal inflammation and the locus containing IFNAR, a heterodimeric receptor for the type I interferons has been identified as a susceptibility region for human inflammatory bowel disease. This review focuses on the role of type I IFNs in the intestine in health and disease and their emerging role as immune modulators. Clear understanding of type I IFN-mediated immune responses may provide avenues for fine-tuning existing IFN treatment for infection and intestinal inflammation.

Viral infection. Prevention and cure of rotavirus infection via TLR5/NLRC4-mediated production of IL-22 and IL-18

Activators of innate immunity may have the potential to combat a broad range of infectious agents. We report that treatment with bacterial flagellin prevented rotavirus (RV) infection in mice and cured chronically RV-infected mice. Protection was independent of adaptive immunity and interferon (IFN, type I and II) and required flagellin receptors Toll-like receptor 5 (TLR5) and NOD-like receptor C4 (NLRC4). Flagellin-induced activation of TLR5 on dendritic cells elicited production of the cytokine interleukin-22 (IL-22), which induced a protective gene expression program in intestinal epithelial cells. Flagellin also induced NLRC4-dependent production of IL-18 and immediate elimination of RV-infected cells. Administration of IL-22 and IL-18 to mice fully recapitulated the capacity of flagellin to prevent or eliminate RV infection and thus holds promise as a broad-spectrum antiviral agent.

Study of the virulence of serotypes 4 and 9 of African horse sickness virus in IFNAR(-/-), Balb/C and 129 Sv/Ev mice

African horse sickness virus (AHSV) is a double-stranded RNA virus which belongs to the family Reoviridae, genus Orbivirus. Recent studies have focused on the interferon-α/β receptor knock-out mice (IFNAR(-/-)) as a small animal laboratory for the development of AHSV vaccines. The aim of this work was to study in vivo the virulence of two strains of AHSV and to compare the outcome of the infection of three mouse strains. To address this, AHSV serotypes 4 (AHSV-4) and 9 (AHSV-9) were inoculated subcutaneously (SC) and intranasally (IN) in two immunocompetent mouse strains (Balb/C and 129 Sv/Ev (129 WT)) as well as IFNAR(-/-) mice (on 129 Sv/Ev genetic background). In IFNAR(-/-) mice, fatality up to 50% was measured and significantly more clinical signs were observed in comparison with SC inoculated immunocompetent mice. The observed clinical signs were significantly more severe after AHSV-4 infection, in particular in immunocompetent mice inoculated by IN route. Considering RNAemia, significantly higher viral loads were measured following AHSV-4 infection. In the organs of 129 WT inoculated by IN route, significantly higher viral loads were detected after AHSV-4 infection. Together the results support a higher virulence for AHSV-4 compared to AHSV-9 and a higher clinical impact following infections in IN inoculated mice, at least in the investigated strains. The study also brought indirect evidences for type I IFN involvement in the control of AHSV infection.

Alpha/beta interferon receptor signaling amplifies early proinflammatory cytokine production in the lung during respiratory syncytial virus infection

Type I interferons (IFNs) are produced early upon virus infection and signal through the alpha/beta interferon (IFN-α/β) receptor (IFNAR) to induce genes that encode proteins important for limiting viral replication and directing immune responses. To investigate the extent to which type I IFNs play a role in the local regulation of inflammation in the airways, we examined their importance in early lung responses to infection with respiratory syncytial virus (RSV). IFNAR1-deficient (IFNAR1^−/−) mice displayed increased lung viral load and weight loss during RSV infection. As expected, expression of IFN-inducible genes was markedly reduced in the lungs of IFNAR1^−/− mice. Surprisingly, we found that the levels of proinflammatory cytokines and chemokines in the lungs of RSV-infected mice were also greatly reduced in the absence of IFNAR signaling. Furthermore, low levels of proinflammatory cytokines were also detected in the lungs of IFNAR1^−/− mice challenged with noninfectious innate immune stimuli such as selected Toll-like receptor (TLR) agonists. Finally, recombinant IFN-α was sufficient to potentiate the production of inflammatory mediators in the lungs of wild-type mice challenged with innate immune stimuli. Thus, in addition to its well-known role in antiviral resistance, type I IFN receptor signaling acts as a central driver of early proinflammatory responses in the lung. Inhibiting the effects of type I IFNs may therefore be useful in dampening inflammation in lung diseases characterized by enhanced inflammatory cytokine production.

IMPORTANCE The initial response to viral infection is characterized by the production of interferons (IFNs). One group of IFNs, the type I IFNs, are produced early upon virus infection and signal through the IFN-α/β receptor (IFNAR) to induce proteins important for limiting viral replication and directing immune responses. Here we examined the importance of type I IFNs in early responses to respiratory syncytial virus (RSV). Our data suggest that type I IFN production and IFNAR receptor signaling not only induce an antiviral state but also serve to amplify proinflammatory responses in the respiratory tract. We also confirm this conclusion in another model of acute inflammation induced by noninfectious stimuli. Our findings are of relevance to human disease, as RSV is a major cause of infant bronchiolitis and polymorphisms in the IFN system are known to impact disease severity.

Systems biology unravels interferon responses to respiratory virus infections

Interferon production is an important defence against viral replication and its activation is an attractive therapeutic target. However, it has long been known that viruses perpetually evolve a multitude of strategies to evade these host immune responses. In recent years there has been an explosion of information on virus-induced alterations of the host immune response that have resulted from data-rich omics technologies. Unravelling how these systems interact and determining the overall outcome of the host response to viral infection will play an important role in future treatment and vaccine development. In this review we focus primarily on the interferon pathway and its regulation as well as mechanisms by which respiratory RNA viruses interfere with its signalling capacity.

Type I interferon limits the capacity of bluetongue virus to infect hematopoietic precursors and dendritic cells in vitro and in vivo

Hematopoietic stem cells (HSCs) give rise to progenitors with potential to produce multiple cell types, including dendritic cells (DCs). DCs are the principal antigen-presenting cells and represent the crucial link between innate and adaptive immune responses. Bluetongue virus (BTV), an economically important Orbivirus of the Reoviridae family, causes a hemorrhagic disease mainly in sheep and occasionally in other species of ruminants. BTV is transmitted between its mammalian hosts by certain species of biting midges (Culicoides spp.) and is a potent alpha interferon (IFN-α) inducer. In the present report, we show that BTV infects cells of hematopoietic origin but not HSCs in immunocompetent sheep. However, BTV infects HSCs in the absence of type I IFN (IFN-I) signaling in vitro and in vivo. Infection of HSCs in vitro results in cellular death by apoptosis. Furthermore, BTV infects bone marrow-derived DCs (BM-DCs), interfering with their development to mature DCs in the absence of type I IFN signaling. Costimulatory molecules CD80 and CD86 and costimulatory molecules CD40 and major histocompatibility complex class II (MHC-II) are affected by BTV infection, suggesting that BTV interferes with DC antigen-presenting capacity. In vivo, different DC populations are also affected during the course of infection, probably as a result of a direct effect of BTV replication in DCs and the production of infectious virus. These new findings suggest that BTV infection of HSCs and DCs can impair the immune response, leading to persistence or animal death, and that this relies on IFN-I.

Confounding roles for type I interferons during bacterial and viral pathogenesis

Although type I interferons (IFN-I) were initially defined as potent antiviral agents, they can also cause decreased host resistance to some bacterial and viral infections. The many antiviral functions of the IFN-I include direct suppression of viral replication and activation of the immune response against viruses. In addition to their antiviral effects, IFN-I are also protective against several extracellular bacterial infections, in part, by promoting the induction of TNF-α and nitric oxide. In contrast, there is a negative effect of IFN-I on host resistance during chronic infection with lymphocytic choriomeningitis virus (LCMV) and acute infections with intracellular bacteria. In the case of LCMV, chronic IFN-I signaling induces adaptive immune system suppression. Blockade of IFN-I signaling removes the suppression and allows CD4 T-cell- and IFN-γ-mediated resolution of the infection. During acute intracellular bacterial infection, IFN-I suppress innate immunity by at least two defined mechanisms. During Francisella infection, IFN-I prevent IL-17 upregulation on γδ T cells and neutrophil recruitment. Following Listeria infection, IFN-I promote the cell death of macrophages and lymphocytes, which leads to innate immune suppression. These divergent findings for the role of IFN-I on pathogen control emphasize the complexity of the interferons system and force more mechanistic evaluation of its role in pathogenesis. This review evaluates IFN-I during infection with an emphasis on work carried out IFN-I-receptor-deficient mice.

Interferon α/β receptor knockout mice as a model to study bluetongue virus infection

Bluetongue is an arthropod-borne disease caused by a virus of the genus Orbivirus, the bluetongue virus (BTV), which affects ruminant livestock such as cattle, sheep, and goats and wild ruminants such as deer, and camelids. Recently, adult mice with gene knockouts of the interferon α/β receptor (IFNAR-/-) have been described as a model of lethal BTV infection. IFNAR(-/-) mice are highly susceptible to BTV-1, BTV-4 and BTV-8 infection when the virus is administered intravenously or subcutaneosuly. Disease progression and pathogenesis closely mimics signs of bluetongue disease in ruminants. In the present paper we review the studies where IFNAR(-/-) mice have been used as an animal model to study BTV transmission, pathogenesis, virulence, and protective efficacy of inactivated and new recombinant marker BTV vaccines. Furthermore, we report new data on protective efficacy of different strategies of BTV vaccination and also on induction of interferon α/β and proinflammatory immune responses in IFNAR(-/-) mice infected with BTV.

Comparative analysis of innate immune response following in vitro stimulation of sheep and goat peripheral blood mononuclear cells with bluetongue virus - serotype 23

Bluetongue is an infectious disease caused by bluetongue virus (BTV), which affects sheep, goat, cattle and certain wild ruminants. However severe clinical signs are usually seen with significant mortality in sheep than cattle and goat. To date, comparative studies on innate immune responses of sheep and goat infected with BTV is lacking. In this study, we compared the innate immune response of sheep and goat by infecting the peripheral blood mononuclear cells (PBMCs) with BTV serotype 23. In our study, we observed that sheep PBMCs supports higher virus replication than goat PBMCs. To delineate the role of innate immune response in differential viral replication observed in this study, we examined TLR3 (Receptor for dsRNA virus) mRNA expression and cytokine profiles (IL-1β, Il-6, IL-8, Il-10, IL-12p40, TNF-α, IFN-γ and IFN-α) following Poly I:C (TLR3 ligand) stimulation and BTV 23 infection. In our present study, sheep PBMCs had significantly higher TLR3 mRNA levels, TLR3 specific ligand (Poly I:C) stimulation resulted in increased levels of IFN-γ at transcriptional and translational levels along with IL-8 and IL-10 at transcriptional levels. Whereas, the levels of TNF-α was higher in goat PBMCs at transcriptional levels. BTV infected sheep PBMCs expressed significantly higher levels of IFN-γ at transcriptional and translational levels along with IL-6, IL-8 and IL-10 at transcriptional levels. Whereas the expression levels of TNF-α and IFN-α at transcriptional and translational levels were significantly high in goat PBMCs. To examine the potential factor for consistent increase in the expression of TNF-α, we sequenced the promoter region of TNF-α and identified a total of five single nucleotide polymorphisms (SNP) and one indel in goat TNF-α promoter region. Luciferase assay for transcriptional activity of the promoter showed that goat TNF-α has significantly enhanced transcriptional activity in comparison with sheep TNF-α promoter. Altogether, our data suggests that the expression levels of TNF-α and IFN-α and/or IL-10 plays crucial role in replication of BTV 23.

Anti-influenza virus effects of both live and non-live Lactobacillus acidophilus L-92 accompanied by the activation of innate immunity

The antiviral effects of both a live and non-live Lactobacillus acidophilus strain L-92 (L-92) were investigated by oral administration (10 mg/mouse per d) daily for 21 d in a mouse model infected intranasally with influenza virus (H1N1). Virus titres in the lung of mice administered either live or non-live L-92 cells daily for 15 d were repressed 6 d after virus infection compared with the control group. Natural killer (NK) activity in the orally administered non-live L-92 group was higher compared with that of the control group before virus infection and on day 6. In contrast, NK activity in the live L-92 group compared with the control group was not significantly changed on both days, but was significantly higher on day 1. In contrast, live L-92 showed a greater repression of virus proliferation compared with non-live L-92, 6 d after the infection. Live L-92 decreased the number of neutrophils in the lung and suppressed lung weight, leading to the consequent deterioration of consolidation scores of the lung. These results indicated that pretreatment of live or non-live L-92 cells had protective effects against influenza virus infection. Among the measured cytokines and chemokines, eotaxin, macrophage colony-stimulating factor, IL-1b, RANTES (regulated on activation, normal T cell expressed and secreted) and interferon-a were significantly increased in the lung: IL-17 was significantly increased in Peyer’s patch of the live L-92 group compared with the control group. A mechanistic study suggested that the enhancement of NK activity in the lung caused by stimulating various antiviral cytokines and chemokines after the oral administration of L-92 cells might be important in protecting against virus infection.

The cell biology of Chikungunya virus infection

Chikungunya virus (CHIKV) infection causes a disease which appears to affect multiple cell types and tissues. The acute phase is manifested by a non-fatal febrile illness, polyarthralgia and maculopapular rashes in adults, but with recurrent arthralgia that may linger for months during convalescence. The issue of cellular and tissue tropism of CHIKV has elicited interest primarily because of this lingering incapacitating chronic joint pain, as well as clear encephalopathy in severe cases among neonates during the re-emergence of the virus in recent epidemics. The principle cell types productively infected by CHIKV are skin fibroblasts, epithelial cells and lymphoid tissues. There is controversy as to whether CHIKV productively infects haematopoietic cells and neurones/glia. CHIKV infection triggers rapid and robust innate immune responses which quickly clears the acute phase infection. However, significant acute as well as chronic infection of less obvious cell types, such as monocytes, neurones/glia or even CNS neural progenitors may conceivably occur. There is therefore a need to ascertain the full range potential of CHIKV tropism, fully understand the cellular responses triggered during the acute the convalescent phases, and explore possible cell types that might be the source of chronic problems associated with CHIKV infection.

Activation of innate immune responses in the central nervous system during reovirus myelitis

Reovirus infection of the murine spinal cord (SC) was used as a model system to investigate innate immune responses during viral myelitis, including the activation of glia (microglia and astrocytes) and interferon (IFN) signaling and increased expression of inflammatory mediators. Reovirus myelitis was associated with the pronounced activation of SC glia, as evidenced by characteristic changes in cellular morphology and increased expression of astrocyte and microglia-specific proteins. Expression of inflammatory mediators known to be released by activated glia, including interleukin-1β (IL-1β), tumor necrosis factor alpha (TNF-α), chemokine (C-C motif) ligand 5 (CCL 5), chemokine (C-X-C motif) ligand 10 (CXCL10), and gamma interferon (IFN-γ), was also significantly upregulated in the SC of reovirus-infected animals compared to mock-infected controls. Reovirus infection of the mouse SC was also associated with increased expression of genes involved in IFN signaling, including IFN-stimulated genes (ISG). Further, reovirus infection of mice deficient in the expression of the IFN-α/β receptor (IFNAR(-/-)) resulted in accelerated mortality, demonstrating that IFN signaling is protective during reovirus myelitis. Experiments performed in ex vivo SC slice cultures (SCSC) confirmed that resident SC cells contribute to the production of at least some of these inflammatory mediators and ISG during reovirus infection. Microglia, but not astrocytes, were still activated, and glia-associated inflammatory mediators were still produced in reovirus-infected INFAR(-/-) mice, demonstrating that IFN signaling is not absolutely required for these neuroinflammatory responses. Our results suggest that activated glia and inflammatory mediators contribute to a local microenvironment that is deleterious to neuronal survival.

The double-stranded RNA bluetongue virus induces type I interferon in plasmacytoid dendritic cells via a MYD88-dependent TLR7/8-independent signaling pathway

Dendritic cells (DCs), especially plasmacytoid DCs (pDCs), produce large amounts of alpha/beta interferon (IFN-α/β) upon infection with DNA or RNA viruses, which has impacts on the physiopathology of the viral infections and on the quality of the adaptive immunity. However, little is known about the IFN-α/β production by DCs during infections by double-stranded RNA (dsRNA) viruses. We present here novel information about the production of IFN-α/β induced by bluetongue virus (BTV), a vector-borne dsRNA Orbivirus of ruminants, in sheep primary DCs. We found that BTV induced IFN-α/β in skin lymph and in blood in vivo. Although BTV replicated in a substantial fraction of the conventional DCs (cDCs) and pDCs in vitro, only pDCs responded to BTV by producing a significant amount of IFN-α/β. BTV replication in pDCs was not mandatory for IFN-α/β production since it was still induced by UV-inactivated BTV (UV-BTV). Other inflammatory cytokines, including tumor necrosis factor alpha (TNF-α), interleukin-6 (IL-6), and IL-12p40, were also induced by UV-BTV in primary pDCs. The induction of IFN-α/β required endo-/lysosomal acidification and maturation. However, despite being an RNA virus, UV-BTV did not signal through Toll-like receptor 7 (TLR7) for IFN-α/β induction. In contrast, pathways involving the MyD88 adaptor and kinases dsRNA-activated protein kinase (PKR) and stress-activated protein kinase (SAPK)/Jun N-terminal protein kinase (JNK) were implicated. This work highlights the importance of pDCs for the production of innate immunity cytokines induced by a dsRNA virus, and it shows that a dsRNA virus can induce IFN-α/β in pDCs via a novel TLR-independent and Myd88-dependent pathway. These findings have implications for the design of efficient vaccines against dsRNA viruses.

Characterization of intestinal dendritic cells in murine norovirus infection

We have shown that respiratory viral infections drive allergic disease through dendritic cells, whether gastrointestinal viruses induce allergies is not known. Norovirus infections are a major cause of gastroenteritis in humans. We used murine norovirus (MNV) to explore the effect of MNV infection on gastrointestinal conventional DCs (cDCs) and plasmacytoid DCs (pDCs). MNV infection induced disparate effects on cDCs and pDCs in lymphoid tissues of the small intestine and draining mesenteric lymph nodes. FcεRI was transiently expressed on lamina propria cDCs, but not on pDCs. In addition, feeding ovalbumin during the viral infection led to a modest, brief induction of anti-ovalbumin IgE. Together, these data suggest that like with a respiratory viral infection, an intestinal viral infection may be sufficient to induce changes in DCs and the generation of food-specific IgE. Whether this represents a novel mechanism of food allergy remains to be determined.

Type I interferon signaling limits reoviral tropism within the brain and prevents lethal systemic infection

In vivo and ex vivo models of reoviral encephalitis were utilized to delineate the contribution of type I interferon (IFN) to the host's defense against local central nervous system (CNS) viral infection and systemic viral spread. Following intracranial (i.c.) inoculation with either serotype 3 (T3) or serotype 1 (T1) reovirus, increased expression of IFN-α, IFN-β, and myxovirus-resistance protein (Mx1; a prototypical IFN stimulated gene) was observed in mouse brain tissue. Type I IFN receptor deficient mice (IFNAR(-/-)) had accelerated lethality, compared to wildtype (B6wt) controls, following i.c. T1 or T3 challenge. Although viral titers in the brain and eyes of reovirus infected IFNAR(-/-) mice were significantly increased, these mice did not develop neurologic signs or brain injury. In contrast, increased reovirus titers in peripheral tissues (liver, spleen, kidney, heart, and blood) of IFNAR(-/-) mice were associated with severe intestinal and liver injury. These results suggest that reovirus-infected IFNAR(-/-) mice succumb to peripheral disease rather than encephalitis per se. To investigate the potential role of type I IFN in brain tissue, brain slice cultures (BSCs) were prepared from IFNAR(-/-) mice and B6wt controls for ex vivo T3 reovirus infection. Compared to B6wt controls, reoviral replication and virus-induced apoptosis were enhanced in IFNAR(-/-) BSCs indicating that a type I IFN response, initiated by resident CNS cells, mediates innate viral immunity within the brain. T3 reovirus tropism was extended in IFNAR(-/-) brains to include dentate neurons, ependymal cells, and meningeal cells indicating that reovirus tropism within the CNS is dependent upon type I interferon signaling.

Enteric reovirus infection stimulates peanut-specific IgG2a responses in a mouse food allergy model

IgE-mediated food allergies are an important cause of life-threatening hypersensitivity reactions. Orally administered peanut antigens mixed with the mucosal adjuvant cholera toxin (CT) induce a strong peanut extract (PE)-specific serum IgE response that is correlated with T-helper type 1 (Th1) and type 2 (Th2)-like T-cell responses. This study was conducted to determine if respiratory enteric orphan virus (reovirus), a non-pathogenic virus that induces robust Th1-mediated mucosal and systemic responses could modulate induction of PE-specific allergic responses when co-administered with PE. Young mice were orally exposed to PE mixed with CT, reovirus, or both CT and reovirus. As expected, CT promoted PE-specific serum IgE, IgG1, and IgG2a and intestinal IgA production as well as splenic Th1- and Th2-associated cytokine recall responses. Reovirus did not alter PE-specific serum IgE and IgG1 levels, but substantially increased the PE-specific IgG2a response when co-administered with PE with or without CT. Additionally, reovirus significantly decreased the percentage of the Peyer's patch CD8+ T-cells and Foxp3+CD4+ T-regulatory cells when co-administered with PE. These results demonstrate that an acute mucosal reovirus infection and subsequent Th1 immune response is capable of modulating the Th1/Th2 controlled humoral response to PE. The reovirus-mediated increase in the PE-specific IgG2a antibody response may have therapeutic implications as increased levels of non-allergenic PE-specific IgG2a could block PE antigens from binding to IgE-sensitized mast cells.