The Role of Interferon in Persistent Viral Infection: Insights from Murine Norovirus

Interferons and tuft cell numbers are bottlenecks for persistent murine norovirus infection

Noroviruses (NoVs) are a leading cause of viral gastroenteritis. Despite global clinical relevance, our understanding of how host factors, such as antiviral cytokines interferons (IFNs), modulate NoV population dynamics is limited. Murine NoV (MNoV) is a tractable in vivo model for the study of host regulation of NoV. A persistent strain of MNoV, CR6, establishes a reservoir in intestinal tuft cells for chronic viral shedding in stool. However, the influence of host innate immunity and permissive cell numbers on viral population dynamics is an open question. We generated a pool of 20 different barcoded viruses (CR6BC) by inserting 6-nucleotide barcodes at the 3' position of the NS4 gene and used this pool as our viral inoculum for in vivo infections of different mouse lines. We found that over the course of persistent CR6 infection, shed virus was predominantly colon-derived, and viral barcode richness decreased over time irrespective of host immune status, suggesting that persistent infection involves a series of reinfection events. In mice lacking the IFN-λ receptor, intestinal barcode richness was enhanced, correlating with increased viral intestinal replication. IL-4 treatment, which increases tuft cell numbers, also increased barcode richness, indicating the abundance of permissive tuft cells to be a bottleneck during CR6 infection. In mice lacking type I IFN signaling (Ifnar1-/-) or all IFN signaling (Stat1-/-), barcode diversity at extraintestinal sites was dramatically increased, implicating different IFNs as critical bottlenecks at specific tissue sites. Of interest, extraintestinal barcodes were overlapping but distinct from intestinal barcodes, indicating that disseminated virus represents a distinct viral population than that replicating in the intestine. Barcoded viruses are a valuable tool to explore the influence of host factors on viral diversity in the context of establishment and maintenance of infection as well as dissemination and have provided important insights into how NoV infection proceeds in immunocompetent and immunocompromised hosts.

Production of infectious reporter murine norovirus by VP2 trans-complementation

Human norovirus (HuNoV) causes gastroenteritis, a disease with no effective therapy or vaccine, and does not grow well in culture. Murine norovirus (MNV) easily replicates in cell cultures and small animals and has often been used as a model to elucidate the structural and functional characteristics of HuNoV. An MNV plasmid-based reverse genetics system was developed to produce the modified recombinant virus. In this study, we attempted to construct the recombinant virus by integrating a foreign gene into MNV ORF3, which encodes the minor structural protein VP2. Deletion of VP2 expression abolished infectious particles from MNV cDNA clones, and supplying exogenous VP2 to the cells rescued the infectivity of cDNA clones without VP2 expression. In addition, the coding sequence of C-terminal ORF3 was essential for cDNA clones compensated with VP2 to produce infectious particles. Furthermore, the recombinant virus with exogenous reporter genes in place of the dispensable region of ORF3 was propagated when VP2 was constitutively supplied. Our findings indicate that foreign genes can be transduced into the norovirus ORF3 region when VP2 is supplied and that successive propagation of modified recombinant norovirus could lead to the development of norovirus-based vaccines or therapeutics.IMPORTANCEIn this study, we revealed that some of the coding regions of ORF3 could be replaced by a foreign gene and infectious virus could be produced when VP2 was supplied. Propagation of this virus depended on VP2 being supplied in trans, indicating that this virus could infect only once. Our findings help to elucidate the functions of VP2 in the virus lifecycle and to develop other caliciviral vectors for recombinant attenuated live enteric virus vaccines or therapeutics tools.

Role of type-I and type-III interferons in gastrointestinal homeostasis and pathogenesis

Interferon (IFN) was discovered based on interference with virus production, and three types of IFN are now defined. Since its discovery, IFN's roles have expanded beyond viruses to diverse pathogen types, tissue homeostasis, and inflammatory disease. The gastrointestinal (GI) tract is arguably the tissue where the roles of IFN types are most distinct, with a particularly prominent role for type-III IFN in antiviral protection of the intestinal epithelium. Current studies continue to deepen our understanding of the type- and tissue-specific roles of IFN. This review highlights these advances within the GI tract, including discovery of protective roles for type-III IFNs against nonviral GI pathogens, and discovery of an antiviral homeostatic type-III IFN response within the intestinal epithelium.

Interferon-lambda: New role in intestinal symptoms of COVID-19

The tremendous public health and economic impact of coronavirus disease 2019 (COVID-19), caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), has become a huge challenge globally. There is increasing evidence that SARS-CoV-2 induces intestinal infections. Type III interferon (IFN-λ) has an antiviral role in intestinal infection, with focused, long-lasting, and non-inflammatory characteristics. This review presents a summary of the structure of SARS-CoV-2, including its invasion and immune escape mechanisms. Emphasis was placed on the gastrointestinal impact of SARS-CoV-2, including changes to the intestinal microbiome, activation of immune cells, and inflammatory responses. We also describe the comprehensive functions of IFN-λ in anti-enteric SARS-CoV-2 infection, and discuss the potential application of IFN-λ as a therapeutic agent for COVID-19 with intestinal symptoms.

Early immune and host cell responses to Cryptosporidium infection

Cryptosporidium spp. are opportunistic protozoan parasites that infect epithelial cells of the small intestine and cause diarrheal illness in both immunocompetent and immunodeficient individuals. These infections may be more severe in immunocompromised individuals and young children, especially in children under 2 in developing countries. The parasite has a global distribution and is an important cause of childhood diarrhea where it may result in cognitive impairment and growth deficits. Current therapies are limited with nitazoxanide being the only FDA-approved drug. However, it is not efficacious in immunocompromised patients. Additionally, there are no vaccines for cryptosporidiosis available. While acquired immunity is needed to clear Cryptosporidium parasites completely, innate immunity and early responses to infection are important in keeping the infection in check so that adaptive responses have time to develop. Infection is localized to the epithelial cells of the gut. Therefore, host cell defenses are important in the early response to infection and may be triggered through toll receptors or inflammasomes which induce a number of signal pathways, interferons, cytokines, and other immune mediators. Chemokines and chemokine receptors are upregulated which recruit immune cells such neutrophils, NK cells, and macrophages to the infection site to help in host cell defense as well as dendritic cells that are an important bridge between innate and adaptive responses. This review will focus on the host cell responses and the immune responses that are important in the early stages of infection.

Multifaceted Effects and Mechanisms of N-Acetylcysteine on Intestinal Injury in a Porcine Epidemic Diarrhea Virus-Infected Porcine Model

SCOPE

This study investigates the potential effects of N-acetylcysteine (NAC) on intestinal injury in a porcine epidemic diarrhea virus (PEDV)-infected porcine model.

METHODS AND RESULTS

Thirty-two piglets are randomly assigned to one of four groups: the control, PEDV, NAC, and NAC+PEDV. Piglets in the NAC+PEDV group are orally administrated with NAC (100 mg (kg·BW)^-1  day^-1 ) for 4 consecutive days after 2 days of PEDV infection. The results show that NAC administration decreases the diarrhea rate and improves intestinal morphology. The concentration of diamine oxidase and intestinal fatty-acid binding protein, as well as IL-1β, IL-8, and TNF-α in the plasma, is decreased by NAC. Intriguingly, NAC administration significantly increases the viral load in the jejunum and ileum and down-regulates the expression of interferon-related genes. Microarray and proteomic analyses show that the differentially expressed genes/proteins between NAC+PEDV and PEDV groups are highly enriched in substance transport. Furthermore, aquaporin 8/10 expression is significantly increased by NAC upon PEDV infection.

CONCLUSION

NAC administration alleviates PEDV-induced intestinal injury by inhibiting inflammatory responses and improving substance transport, but promotes viral replication by inhibiting interferon signaling. These results suggest NAC exhibits multifaceted effects upon PEDV infection, and thus caution is required when using NAC as a dietary supplement to prevent viral infection.

Bacterial extracellular vesicles control murine norovirus infection through modulation of antiviral immune responses

Human norovirus is the primary cause of non-bacterial gastroenteritis globally and is the second leading cause of diarrheal deaths in children in developing countries. However, effective therapeutics which prevent or clear norovirus infection are not yet available due to a lack of understanding regarding norovirus pathogenesis. Evidence shows that noroviruses can bind to the surface of commensal bacteria, and the presence of these bacteria alters both acute and persistent murine norovirus infection through the modulation of host immune responses. Interestingly, norovirus-bacterial interactions also affect the bacteria by inducing bacterial stress responses and increasing the production of bacterial extracellular vesicles. Given the established ability of these vesicles to easily cross the intestinal barriers, enter the lamina propria, and modulate host responses, we hypothesized that bacterial extracellular vesicles influence murine norovirus infection through modulation of the antiviral immune response. In this study, we show that murine norovirus can attach to purified bacterial vesicles, facilitating co-inoculation of target cells with both virus and vesicle. Furthermore, we have found that when murine noroviruses and vesicles are used to co-inoculate macrophages, viral infection is reduced compared to virus infection alone. Specifically, co-inoculation with bacterial vesicles results in higher production and release of pro-inflammatory cytokines in response to viral infection. Ultimately, given that murine norovirus infection increases bacterial vesicle production in vivo, these data indicate that bacterial vesicles may serve as a mechanism by which murine norovirus infection is ultimately controlled and limited to a short-term disease.

Synergistic Interferon-Alpha-Based Combinations for Treatment of SARS-CoV-2 and Other Viral Infections

Background: There is an urgent need for new antivirals with powerful therapeutic potential and tolerable side effects. Methods: Here, we tested the antiviral properties of interferons (IFNs), alone and with other drugs in vitro. Results: While IFNs alone were insufficient to completely abolish replication of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), IFNα, in combination with remdesivir, EIDD-2801, camostat, cycloheximide, or convalescent serum, proved to be more effective. Transcriptome and metabolomic analyses revealed that the IFNα–remdesivir combination suppressed SARS-CoV-2-mediated changes in Calu-3 cells and lung organoids, although it altered the homeostasis of uninfected cells and organoids. We also demonstrated that IFNα combinations with sofosbuvir, telaprevir, NITD008, ribavirin, pimodivir, or lamivudine were effective against HCV, HEV, FLuAV, or HIV at lower concentrations, compared to monotherapies. Conclusions: Altogether, our results indicated that IFNα can be combined with drugs that affect viral RNA transcription, protein synthesis, and processing to make synergistic combinations that can be attractive targets for further pre-clinical and clinical development against emerging and re-emerging viral infections.

GII.P16-GII.2 Recombinant Norovirus VLPs Polarize Macrophages Into the M1 Phenotype for Th1 Immune Responses

Norovirus (NoV) is a zoonotic virus that causes diarrhea in humans and animals. Outbreaks in nosocomial settings occur annually worldwide, endangering public health and causing serious social and economic burdens. The latter quarter of 2016 witnessed the emergence of the GII.P16-GII.2 recombinant norovirus throughout Asia. This genotype exhibits strong infectivity and replication characteristics, proposing its potential to initiate a pandemic. There is no vaccine against GII.P16-GII.2 recombinant norovirus, so it is necessary to design a preventive vaccine. In this study, GII.P16-GII.2 type norovirus virus-like particles (VLPs) were constructed using the baculovirus expression system and used to conduct immunizations in mice. After immunization of mice, mice were induced to produce memory T cells and specific antibodies, indicating that the VLPs induced specific cellular and humoral immune responses. Further experiments were then initiated to understand the underlying mechanisms involved in antigen presentation. Towards this, we established co-cultures between dendritic cells (DCs) or macrophages (Mø) and naïve CD4+T cells and simulated the antigen presentation process by incubation with VLPs. Thereafter, we detected changes in cell surface molecules, cytokines and related proteins. The results indicated that VLPs effectively promoted the phenotypic maturation of Mø but not DCs, as indicated by significant changes in the expression of MHC-II, costimulatory factors and related cytokines in Mø. Moreover, we found VLPs caused Mø to polarize to the M1 type and release inflammatory cytokines, thereby inducing naïve CD4+ T cells to perform Th1 immune responses. Therefore, this study reveals the mechanism of antigen presentation involving GII.P16-GII.2 recombinant norovirus VLPs, providing a theoretical basis for both understanding responses to norovirus infection as well as opportunities for vaccine development.

Interferon responses to norovirus infections: current and future perspectives

Human noroviruses (HuNoVs) are increasingly becoming the main cause of transmissible gastroenteritis worldwide, with hundreds of thousands of deaths recorded annually. Yet, decades after their discovery, there is still no effective treatment or vaccine. Efforts aimed at developing vaccines or treatment will benefit from a greater understanding of norovirus-host interactions, including the host response to infection. In this review, we provide a concise overview of the evidence establishing the significance of type I and type III interferon (IFN) responses in the restriction of noroviruses. We also critically examine our current understanding of the molecular mechanisms of IFN induction in norovirus-infected cells, and outline the diverse strategies deployed by noroviruses to supress and/or avoid host IFN responses. It is our hope that this review will facilitate further discussion and increase interest in this area.

Murine norovirus virulence factor 1 (VF1) protein contributes to viral fitness during persistent infection

Murine norovirus (MNV) is widely used as a model for studying norovirus biology. While MNV isolates vary in their pathogenesis, infection of immunocompetent mice mostly results in persistent infection. The ability of a virus to establish a persistent infection is dependent on its ability to subvert or avoid the host immune response. Previously, we described the identification and characterization of virulence factor 1 (VF1) in MNV, and demonstrated its role as an innate immune antagonist. Here, we explore the role of VF1 during persistent MNV infection in an immunocompetent host. Using reverse genetics, we generated MNV-3 viruses carrying a single or a triple termination codon inserted in the VF1 ORF. VF1-deleted MNV-3 replicated to comparable levels to the wildtype virus in tissue culture. Comparative studies between MNV-3 and an acute MNV-1 strain show that MNV-3 VF1 exerts the same functions as MNV-1 VF1, but with reduced potency. C57BL/6 mice infected with VF1-deleted MNV-3 showed significantly reduced replication kinetics during the acute phase of the infection, but viral loads rapidly reached the levels seen in mice infected with wildtype virus after phenotypic restoration of VF1 expression. Infection with an MNV-3 mutant that had three termination codons inserted into VF1, in which reversion was suppressed, resulted in consistently lower replication throughout a 3 month persistent infection in mice, suggesting a role for VF1 in viral fitness in vivo. Our results indicate that VF1 expressed by a persistent strain of MNV also functions to antagonize the innate response to infection. We found that VF1 is not essential for viral persistence, but instead contributes to viral fitness in mice. These data fit with the hypothesis that noroviruses utilize multiple mechanisms to avoid and/or control the host response to infection and that VF1 is just one component of this.

Insights into Innate Immune Response Against SARS-CoV-2 Infection

The innate immune system is mandatory for the activation of antiviral host defense and eradication of the infection. In this regard, dendritic cells, natural killer cells, macrophages, neutrophils representing the cellular component, and cytokines, interferons, complement or Toll-Like Receptors, representing the mediators of unspecific response act together for both activation of the adaptive immune response and viral clearance. Of great importance is the proper functioning of the innate immune response from the very beginning. For instance, in the early stages of viral infection, the defective interferon response leads to uncontrolled viral replication and pathogen evasion, while hypersecretion during the later stages of infection generates hyperinflammation. This cascade activation of systemic inflammation culminates with cytokine storm syndrome and hypercoagulability state, due to a close interconnection between them. Thus an unbalanced reaction, either under- or over- stimulation of the innate immune system will lead to an uncoordinated response and unfavorable disease outcomes. Since both cellular and humoral factors are involved in the time-course of the innate immune response, in this review we aimed to address their gradual involvement in the antiviral response with emphasis on key steps in SARS-CoV-2 infection.

Genome Sequences of Five Microviruses and a Murine Norovirus

Murine norovirus is a fecal-orally transmitted pathogen in mice which belongs to the same genus as human norovirus. Microviruses are bacteriophages with small circular single-stranded DNA genomes, belonging to the family Microviridae. Here, we report the genome sequences of five microviruses and one murine norovirus obtained from the intestinal content of a lab mouse.

Norovirus evolution in immunodeficient mice reveals potentiated pathogenicity via a single nucleotide change in the viral capsid

Interferons (IFNs) are key controllers of viral replication, with intact IFN responses suppressing virus growth and spread. Using the murine norovirus (MNoV) system, we show that IFNs exert selective pressure to limit the pathogenic evolutionary potential of this enteric virus. In animals lacking type I IFN signaling, the nonlethal MNoV strain CR6 rapidly acquired enhanced virulence via conversion of a single nucleotide. This nucleotide change resulted in amino acid substitution F514I in the viral capsid, which led to >10,000-fold higher replication in systemic organs including the brain. Pathogenicity was mediated by enhanced recruitment and infection of intestinal myeloid cells and increased extraintestinal dissemination of virus. Interestingly, the trade-off for this mutation was reduced fitness in an IFN-competent host, in which CR6 bearing F514I exhibited decreased intestinal replication and shedding. In an immunodeficient context, a spontaneous amino acid change can thus convert a relatively avirulent viral strain into a lethal pathogen.

Isolation and Identification of a Murine Norovirus Persistent Infection Strain in China

Murine Norovirus (MNV) is one of the most known viruses among viruses in mice. Because of the high prevalence of MNV in frequently used laboratory animals in biomedical researches, there is a significant impact of MNV. There may be different prevalence degrees and molecular characteristics of MNV in different regions around the world. Here, we reported an MNV strain "designated HBTS-1806" isolation from commercial mice's feces that caused a detectable cytopathic effect (CPE) in RAW264.7 cells. According to electron microscopy, the virus was 50-70 nm in diameter. The complete genome of HBTS-1806 is 7383 nucleotides with a structure similar to that of MNV reference strains. According to phylogenetic analysis on the basis of the whole genome, HBTS-1806 shared nucleotide sequence identities of 90.2-95.4% with other Chinese isolates reported. Analysis of amino acid sequence on the basis of ORF1 and ORF2 suggested that the isolated strain may be derived from recombination. Although no gross lesions or histopathological changes were found from mice infected with 5 × 10⁵ TCLD₅₀ of MNV by oral gavage inoculation, the intestinal virus loads lasted 12 weeks, suggesting a persistent infection strain of MNV isolate in China.

Selective Interferon Responses of Intestinal Epithelial Cells Minimize Tumor Necrosis Factor Alpha Cytotoxicity

Interferon (IFN) family cytokines stimulate genes (interferon-stimulated genes [ISGs]) that are integral to antiviral host defense. Type I IFNs act systemically, whereas type III IFNs act preferentially at epithelial barriers. Among barrier cells, intestinal epithelial cells (IECs) are particularly dependent on type III IFN for the control and clearance of virus infection, but the physiological basis of this selective IFN response is not well understood. Here, we confirm that type III IFN treatment elicits robust and uniform ISG expression in neonatal mouse IECs and inhibits the replication of IEC-tropic rotavirus. In contrast, type I IFN elicits a marginal ISG response in neonatal mouse IECs and does not inhibit rotavirus replication. In vitro treatment of IEC organoids with type III IFN results in ISG expression that mirrors the in vivo type III IFN response. However, IEC organoids have increased expression of the type I IFN receptor relative to neonate IECs, and the response of IEC organoids to type I IFN is strikingly increased in magnitude and scope relative to type III IFN. The expanded type I IFN-specific response includes proapoptotic genes and potentiates toxicity triggered by tumor necrosis factor alpha (TNF-α). The ISGs stimulated in common by type I and III IFNs have strong interferon-stimulated response element (ISRE) promoter motifs, whereas the expanded set of type I IFN-specific ISGs, including proapoptotic genes, have weak ISRE motifs. Thus, the preferential responsiveness of IECs to type III IFN in vivo enables selective ISG expression during infection that confers antiviral protection but minimizes disruption of intestinal homeostasis.IMPORTANCE Enteric viral infections are a major cause of gastroenteritis worldwide and have the potential to trigger or exacerbate intestinal inflammatory diseases. Prior studies have identified specialized innate immune responses that are active in the intestinal epithelium following viral infection, but our understanding of the benefits of such an epithelium-specific response is incomplete. Here, we show that the intestinal epithelial antiviral response is programmed to enable protection while minimizing epithelial cytotoxicity that can often accompany an inflammatory response. Our findings offer new insight into the benefits of a tailored innate immune response at the intestinal barrier and suggest how dysregulation of this response could promote inflammatory disease.

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.

Interferon-λ Receptor Expression: Novel Reporter Mouse Reveals Within- and Cross-Tissue Heterogeneity

Interferon-λ (IFN-λ) plays an important role in mucosal immunity, but reliable information regarding the expression of the IFN-λ receptor in individual cells is still missing. One reason for this knowledge gap is the lack of antibodies that specifically recognize the unique IFNLR1 subunit of the dimeric IFN-λ receptor complex. In this study, we investigated whether a reporter mouse carrying a bacterial β-galactosidase gene inserted into the Ifnlr1 locus could be used to visualize IFN-λ receptor-expressing cells in whole organs. First we confirmed that insertion of the reporter cassette inactivated the Ifnlr1 gene, and that gene function could be restored by removing the β-galactosidase insert by site-specific recombination. When whole tissues were analyzed, prominent β-galactosidase activity was confined to the intestinal tract of reporter mice. However, only the snout expressed β-galactosidase at levels high enough for reliable detection in whole tissue extracts. Interestingly, individual epithelial cells in the upper airways expressed β-galactosidase activity to variable degrees as determined by flow cytometry and histology, suggesting a remarkable heterogeneity in IFNLR1 expression levels. Taken together, our results demonstrate a surprisingly strong within- and cross-tissue heterogeneity of IFNLR1 expression that may have physiological implications.

Viral Infections and Interferons in the Development of Obesity

Obesity is now a prevalent disease worldwide and has a multi-factorial etiology. Several viruses or virus-like agents including members of adenoviridae, herpesviridae, slow virus (prion), and hepatitides, have been associated with obesity; meanwhile obese patients are shown to be more susceptible to viral infections such as during influenza and dengue epidemics. We examined the co-factorial role of viral infections, particularly of the persistent cases, in synergy with high-fat diet in induction of obesity. Antiviral interferons (IFNs), as key immune regulators against viral infections and in autoimmunity, emerge to be a pivotal player in the regulation of adipogenesis. In this review, we examine the recent evidence indicating that gut microbiota uphold intrinsic IFN signaling, which is extensively involved in the regulation of lipid metabolism. However, the prolonged IFN responses during persistent viral infections and obesogenesis comprise reciprocal causality between virus susceptibility and obesity. Furthermore, some IFN subtypes have shown therapeutic potency in their anti-inflammation and anti-obesity activity.

Caspase-mediated cleavage of murine norovirus NS1/2 potentiates apoptosis and is required for persistent infection of intestinal epithelial cells

Human norovirus (HNoV) is the leading cause of acute gastroenteritis and is spread by fecal shedding that can often persist for weeks to months after the resolution of symptoms. Elimination of persistent viral reservoirs has the potential to prevent outbreaks. Similar to HNoV, murine norovirus (MNV) is spread by persistent shedding in the feces and provides a tractable model to study molecular mechanisms of enteric persistence. Previous studies have identified non-structural protein 1 (NS1) from the persistent MNV strain CR6 as critical for persistent infection in intestinal epithelial cells (IECs), but its mechanism of action remains unclear. We now find that the function of CR6 NS1 is regulated by apoptotic caspase cleavage. Following induction of apoptosis in infected cells, caspases cleave the precursor NS1/2 protein, and this cleavage is prevented by mutation of caspase target motifs. These mutations profoundly compromise CR6 infection of IECs and persistence in the intestine. Conversely, NS1/2 cleavage is not strictly required for acute replication in extra-intestinal tissues or in cultured myeloid cells, suggesting an IEC-centric role. Intriguingly, we find that caspase cleavage of CR6 NS1/2 reciprocally promotes caspase activity, potentiates cell death, and amplifies spread among cultured IEC monolayers. Together, these data indicate that the function of CR6 NS1 is regulated by apoptotic caspases, and suggest that apoptotic cell death enables epithelial spread and persistent shedding.

Human Norovirus infection is highly contagious and the most common cause of acute gastroenteritis. Norovirus can be persistently shed after resolution of symptoms, perpetuating or initiating new outbreaks. Murine norovirus (MNV) is also persistently shed, enabling study of host and viral determinants of norovirus pathogenesis. We previously identified a critical role for MNV non-structural protein 1 (NS1), in persistence. Herein we find that regulation of NS1 by host apoptotic caspases is required for infection of intestinal epithelial cells, but not for extra-intestinal spread. Additionally, we demonstrate that NS1 reciprocally promotes cell death and spread among epithelial cells. These data identify regulation of NS1 by host proteases and suggest that apoptotic death is a determinant of epithelial spread and persistence.

Type III IFNs: Beyond antiviral protection

The unexpected discovery of a novel family of antiviral mediators, type III IFNs or IFN-λs, challenged the widely accepted primacy of type I IFNs in antiviral immunity, and it is now well recognized that the IFN-λ-based antiviral system plays a major role in antiviral protection of epithelial barriers. The recent characterization of previously unknown IFN-λ-mediated activities has prompted further reassessment of the role of type I IFNs in innate and adaptive immune and inflammatory responses. Since type I and type III IFNs are co-produced in response to a variety of stimuli, it is likely that many physiological processes are simultaneously and coordinately regulated by these cytokines in pathological conditions, and likely at steady state, as baseline expression of both IFN types is maintained by microbiota. In this review, we discuss emerging differences in the production and signaling of type I and type III IFNs, and summarize results of recent studies describing the involvement of type III IFNs in anti-bacterial and anti-fungal, as well as antiviral, defenses.

Ifit1 regulates norovirus infection and enhances the interferon response in murine macrophage-like cells

Background: Norovirus, also known as the winter vomiting bug, is the predominant cause of non-bacterial gastroenteritis worldwide. Disease control is predicated on a robust innate immune response during the early stages of infection. Double-stranded RNA intermediates generated during viral genome replication are recognised by host innate immune sensors in the cytoplasm, activating the strongly antiviral interferon gene programme. Ifit proteins (interferon induced proteins with tetratricopeptide repeats), which are highly expressed during the interferon response, have been shown to directly inhibit viral protein synthesis as well as regulate innate immune signalling pathways. Ifit1 is well-characterised to inhibit viral translation by sequestration of eukaryotic initiation factors or by directly binding to the 5' terminus of foreign RNA, particularly those with non-self cap structures. However, noroviruses have a viral protein, VPg, covalently linked to the 5' end of the genomic RNA, which acts as a cap substitute to recruit the translation initiation machinery. Methods: Ifit1 knockout RAW264.7 murine macrophage-like cells were generated using CRISPR-Cas9 gene editing. These cells were analysed for their ability to support murine norovirus infection, determined by virus yield, and respond to different immune stimuli, assayed by quantitative PCR. The effect of Ifit proteins on norovirus translation was also tested in vitro. Results: Here, we show that VPg-dependent translation is completely refractory to Ifit1-mediated translation inhibition in vitro and Ifit1 cannot bind the 5' end of VPg-linked RNA. Nevertheless, knockout of Ifit1 promoted viral replication in murine norovirus infected cells. We then demonstrate that Ifit1 promoted interferon-beta expression following transfection of synthetic double-stranded RNA but had little effect on toll-like receptor 3 and 4 signalling. Conclusions: Ifit1 is an antiviral factor during norovirus infection but cannot directly inhibit viral translation. Instead, Ifit1 stimulates the antiviral state following cytoplasmic RNA sensing, contributing to restriction of norovirus replication.

Norovirus antivirals: Where are we now?

Human noroviruses inflict a significant health burden on society and are responsible for approximately 699 million infections and over 200 000 estimated deaths worldwide each year. Yet despite significant research efforts, approved vaccines or antivirals to combat this pathogen are still lacking. Safe and effective antivirals are not available, particularly for chronically infected immunocompromised individuals, and for prophylactic applications to protect high-risk and vulnerable populations in outbreak settings. Since the discovery of human norovirus in 1972, the lack of a cell culture system has hindered biological research and antiviral studies for many years. Recent breakthroughs in culturing human norovirus have been encouraging, however, further development and optimization of these novel methodologies are required to facilitate more robust replication levels, that will enable reliable serological and replication studies, as well as advances in antiviral development. In the last few years, considerable progress has been made toward the development of norovirus antivirals, inviting an updated review. This review focuses on potential therapeutics that have been reported since 2010, which were examined across at least two model systems used for studying human norovirus or its enzymes. In addition, we have placed emphasis on antiviral compounds with a defined chemical structure. We include a comprehensive outline of direct-acting antivirals and offer a discussion of host-modulating compounds, a rapidly expanding and promising area of antiviral research.

Persistence of Systemic Murine Norovirus Is Maintained by Inflammatory Recruitment of Susceptible Myeloid Cells

Viral persistence can contribute to chronic disease and promote virus dissemination. Prior work demonstrated that timely clearance of systemic murine norovirus (MNV) infection depends on cell-intrinsic type I interferon responses and adaptive immunity. We now find that the capsid of the systemically replicating MNV strain CW3 promotes lytic cell death, release of interleukin-1α, and increased inflammatory cytokine release. Correspondingly, inflammatory monocytes and neutrophils are recruited to sites of infection in a CW3-capsid-dependent manner. Recruited monocytes and neutrophils are subsequently infected, representing a majority of infected cells in vivo. Systemic depletion of inflammatory monocytes or neutrophils from persistently infected Rag1-/- mice reduces viral titers in a tissue-specific manner. These data indicate that the CW3 capsid facilitates lytic cell death, inflammation, and recruitment of susceptible cells to promote persistence. Infection of continuously recruited inflammatory cells may be a mechanism of persistence broadly utilized by lytic viruses incapable of establishing latency.

A systematic approach to the development of a safe live attenuated Zika vaccine

Zika virus (ZIKV) is a flavivirus that can cause congenital disease and requires development of an effective long-term preventative strategy. A replicative ZIKV vaccine with properties similar to the yellow fever 17D (YF17D) live-attenuated vaccine (LAV) would be advantageous, as a single dose of YF17D produces lifelong immunity. However, a replicative ZIKV vaccine must also be safe from causing persistent organ infections. Here we report an approach to ZIKV LAV development. We identify a ZIKV variant that produces small plaques due to interferon (IFN)-restricted viral propagation and displays attenuated infection of endothelial cells. We show that these properties collectively reduce the risk of organ infections and vertical transmission in a mouse model but remain sufficiently immunogenic to prevent wild-type ZIKV infection. Our findings suggest a strategy for the development of a safe but efficacious ZIKV LAV.

Virus-Bacteria Interactions: Implications and Potential for the Applied and Agricultural Sciences

Eukaryotic virus-bacteria interactions have recently become an emerging topic of study due to multiple significant examples related to human pathogens of clinical interest. However, such omnipresent and likely important interactions for viruses and bacteria relevant to the applied and agricultural sciences have not been reviewed or compiled. The fundamental basis of this review is that these interactions have importance and deserve more investigation, as numerous potential consequences and applications arising from their discovery are relevant to the applied sciences. The purpose of this review is to highlight and summarize eukaryotic virus-bacteria findings in the food/water, horticultural, and animal sciences. In many cases in the agricultural sciences, mechanistic understandings of the effects of virus-bacteria interactions remain unstudied, and many studies solely focus on co-infections of bacterial and viral pathogens. Given recent findings relative to human viral pathogens, further research related to virus-bacteria interactions would likely result in numerous discoveries and beneficial applications.