Transcription factor redundancy ensures induction of the antiviral state

IRF7 from the black flying fox induces a STAT1-independent ISG response in unstimulated cell lines that protects against diverse RNA viruses

Bats are considered unique in their ability to harbor large numbers of viruses and serve as reservoirs for zoonotic viruses that have the potential to spill over into humans. However, these animals appear relatively resistant to the pathogenic effects of many viruses. Mounting evidence suggests that bats may tolerate viral infections due to unique immune features. These include evolutionary innovations in inflammatory pathways and in the molecules involved in viral sensing, interferon induction, and downstream interferon-induced antiviral effectors. We sought to determine whether interferon-stimulated genes (ISGs) from the black flying fox ( Pteropus alecto ) encoded proteins with unique antiviral activity relative to their human orthologs. Accordingly, we compared the antiviral activity of over 50 ISG human-bat ortholog pairs to identify differences in individual effector functions. We identified IRF7 from Pteropus alecto (Pa.IRF7) as a potent and broad-acting antiviral molecule that provides robust antiviral protection without prior activation. We show that Pa.IRF7 uniquely induces a subset of protective ISGs independent of canonical IFN signaling, which leads to protection from alphaviruses, a flavivirus, a rhabdovirus, and a paramyxovirus. In uninfected cells, Pa.IRF7 partially localizes to the nucleus and can directly bind interferon-sensitive regulatory elements (ISREs). Compared to human IRF7, Pa.IRF7 also has additional serines in its C terminal domain that contribute to antiviral activity and may serve as unique phosphorylation hubs for activation. These properties constitute major differences between bat and human IRF7 that offer additional insight into the potential uniqueness of the black flying fox immune system.

Uncovering uncharacterized binding of transcription factors from ATAC-seq footprinting data

Transcription factors (TFs) are crucial epigenetic regulators, which enable cells to dynamically adjust gene expression in response to environmental signals. Computational procedures like digital genomic footprinting on chromatin accessibility assays such as ATACseq can be used to identify bound TFs in a genome-wide scale. This method utilizes short regions of low accessibility signals due to steric hindrance of DNA bound proteins, called footprints (FPs), which are combined with motif databases for TF identification. However, while over 1600 TFs have been described in the human genome, only ~ 700 of these have a known binding motif. Thus, a substantial number of FPs without overlap to a known DNA motif are normally discarded from FP analysis. In addition, the FP method is restricted to organisms with a substantial number of known TF motifs. Here we present DENIS (DE Novo motIf diScovery), a framework to generate and systematically investigate the potential of de novo TF motif discovery from FPs. DENIS includes functionality (1) to isolate FPs without binding motifs, (2) to perform de novo motif generation and (3) to characterize novel motifs. Here, we show that the framework rediscovers artificially removed TF motifs, quantifies de novo motif usage during an early embryonic development example dataset, and is able to analyze and uncover TF activity in organisms lacking canonical motifs. The latter task is exemplified by an investigation of a scATAC-seq dataset in zebrafish which covers different cell types during hematopoiesis.

Human long noncoding RNA, VILMIR, is induced by major respiratory viral infections and modulates the host interferon response

Long noncoding RNAs (lncRNAs) are a newer class of noncoding transcripts identified as key regulators of biological processes. Here we aimed to identify novel lncRNA targets that play critical roles in major human respiratory viral infections by systematically mining large-scale transcriptomic datasets. Using bulk RNA-sequencing (RNA-seq) analysis, we identified a previously uncharacterized lncRNA, named virus inducible lncRNA modulator of interferon response (VILMIR), that was consistently upregulated after in vitro influenza infection across multiple human epithelial cell lines and influenza A virus subtypes. VILMIR was also upregulated after SARS-CoV-2 and RSV infections in vitro. We experimentally confirmed the response of VILMIR to influenza infection and interferon-beta (IFN-β) treatment in the A549 human epithelial cell line and found the expression of VILMIR was robustly induced by IFN-β treatment in a dose and time-specific manner. Single cell RNA-seq analysis of bronchoalveolar lavage fluid (BALF) samples from COVID-19 patients uncovered that VILMIR was upregulated across various cell types including at least five immune cells. The upregulation of VILMIR in immune cells was further confirmed in the human T cell and monocyte cell lines, SUP-T1 and THP-1, after IFN-β treatment. Finally, we found that knockdown of VILMIR expression reduced the magnitude of host transcriptional responses to IFN-β treatment in A549 cells. Together, our results show that VILMIR is a novel interferon-stimulated gene (ISG) that regulates the host interferon response and may be a potential therapeutic target for human respiratory viral infections upon further mechanistic investigation.

Functional comparisons of the virus sensor RIG-I from humans, the microbat Myotis daubentonii, and the megabat Rousettus aegyptiacus, and their response to SARS-CoV-2 infection

IMPORTANCE

A common hypothesis holds that bats (order Chiroptera) are outstanding reservoirs for zoonotic viruses because of a special antiviral interferon (IFN) system. However, functional studies about key components of the bat IFN system are rare. RIG-I is a cellular sensor for viral RNA signatures that activates the antiviral signaling chain to induce IFN. We cloned and functionally characterized RIG-I genes from two species of the suborders Yangochiroptera and Yinpterochiroptera. The bat RIG-Is were conserved in their sequence and domain organization, and similar to human RIG-I in (i) mediating virus- and IFN-activated gene expression, (ii) antiviral signaling, (iii) temperature dependence, and (iv) recognition of RNA ligands. Moreover, RIG-I of Rousettus aegyptiacus (suborder Yinpterochiroptera) and of humans were found to recognize SARS-CoV-2 infection. Thus, members of both bat suborders encode RIG-Is that are comparable to their human counterpart. The ability of bats to harbor zoonotic viruses therefore seems due to other features.

MEF2A suppresses stress responses that trigger DDX41-dependent IFN production

Cellular stress in the form of disrupted transcription, loss of organelle integrity, or damage to nucleic acids can elicit inflammatory responses by activating signaling cascades canonically tasked with controlling pathogen infections. These stressors must be kept in check to prevent unscheduled activation of interferon, which contributes to autoinflammation. This study examines the role of the transcription factor myocyte enhancing factor 2A (MEF2A) in setting the threshold of transcriptional stress responses to prevent R-loop accumulation. Increases in R-loops lead to the induction of interferon and inflammatory responses in a DEAD-box helicase 41 (DDX41)-, cyclic GMP-AMP synthase (cGAS)-, and stimulator of interferon genes (STING)-dependent manner. The loss of MEF2A results in the activation of ATM and RAD3-related (ATR) kinase, which is also necessary for the activation of STING. This study identifies the role of MEF2A in sustaining transcriptional homeostasis and highlights the role of ATR in positively regulating R-loop-associated inflammatory responses.

Delayed engagement of host defenses enables SARS-CoV-2 viremia and productive infection of distal organs in the hamster model of COVID-19

Clinical presentations that develop in response to infection result from interactions between the pathogen and host defenses. SARS-CoV-2, the etiologic agent of COVID-19, directly antagonizes these defenses, leading to delayed immune engagement in the lungs that materializes only as cells succumb to infection and are phagocytosed. Leveraging the golden hamster model of COVID-19, we sought to understand the dynamics between SARS-CoV-2 infection in the airways and the systemic host response that ensues. We found that early SARS-CoV-2 replication was largely confined to the respiratory tract and olfactory system and, to a lesser extent, the heart and gastrointestinal tract but generated a host antiviral response in every organ as a result of circulating type I and III interferons. Moreover, we showed that diminishing the response in the airways by immunosuppression or administration of SARS-CoV-2 intravenously resulted in decreased immune priming, viremia, and increased viral tropism, including productive infection of the liver, kidney, spleen, and brain. Last, we showed that productive infection of the airways was required for mounting an effective and system-wide antiviral response. Together, these data illustrate how COVID-19 can result in diverse clinical presentations in which disease outcomes can be a by-product of the speed and strength of immune engagement. These studies provide additional evidence for the mechanistic basis of the diverse clinical presentations of COVID-19 and highlight the ability of the respiratory tract to generate a systemic immune defense after pathogen recognition.

African swine fever virus pA104R protein acts as a suppressor of type I interferon signaling

This study evaluates the role of the late viral protein, pA104R, in African swine fever virus immunosuppression. ASFV-encoded pA104R is a putative histone-like protein that is highly conserved throughout different virulent and non-virulent isolates. Previous studies have demonstrated that pA104R plays a vital role in the ASFV replication cycle and is a potential target for antiviral therapy. Here, we demonstrated that pA104R is a potent antagonist of type I interferon signaling. IFN-stimulated response element activity and subsequent transcription of co-transfected and endogenous interferon-stimulated genes were attenuated by pA104R treatment in HEK-293 T cells. Immunoprecipitation assay and reciprocal pull-down showed that pA104R does not interact directly with STAT1, STAT2, or IRF9. However, pA104R could inhibit IFN signaling by attenuating STAT1 phosphorylation, and we identified the critical amino acid residues (R/H69,72 and K/R92,94,97) involved through the targeted mutation functional assays. Although pA104R is a histone-like protein localized to the nucleus, it did not inhibit IFN signaling through its DNA-binding capacity. In addition, activation of the ISRE promoter by IRF9-Stat2(TA), a STAT1-independent pathway, was inhibited by pA104R. Further results revealed that both the transcriptional activation and recruitment of transcriptional stimulators by interferon-stimulated gene factor 3 were not impaired. Although we failed to determine a mechanism for pA104R-mediated IFN signaling inhibition other than attenuating the phosphorylation of STAT1, these results might imply a possible involvement of epigenetic modification by ASFV pA104R. Taken together, these findings support that pA104R is an antagonist of type I interferon signaling, which may interfere with multiple signaling pathways.

Initial activation of STAT2 induced by IAV infection is critical for innate antiviral immunity

STAT2 is an important transcription factor activated by interferons (IFNs) upon viral infection and plays a key role in antiviral responses. Interestingly, here we found that phosphorylation of STAT2 could be induced by several viruses at early infection stage, including influenza A virus (IAV), and such initial activation of STAT2 was independent of type I IFNs and JAK kinases. Furthermore, it was observed that the early activation of STAT2 during viral infection was mainly regulated by the RIG-I/MAVS-dependent pathway. Disruption of STAT2 phosphorylation at Tyr690 restrained antiviral response, as silencing STAT2 or blocking STAT2 Y690 phosphorylation suppressed the expression of several interferon-stimulated genes (ISGs), thereby facilitating viral replication. In vitro experiments using overexpression system or kinase inhibitors showed that several kinases including MAPK12 and Syk were involved in regulation of the early phosphorylation of STAT2 triggered by IAV infection. Moreover, when MAPK12 kinase was inhibited, expression of several ISGs was clearly decreased in cells infected with IAV at the early infection stage. Accordingly, inhibition of MAPK12 accelerated the replication of influenza virus in host. These results provide a better understanding of how initial activation of STAT2 and the early antiviral responses are induced by the viral infection.

Tilorone and Cridanimod Protect Mice and Show Antiviral Activity in Rats despite Absence of the Interferon-Inducing Effect in Rats

The synthetic compounds, Tilorone and Cridanimod, have the antiviral activity which initially had been ascribed to the capacity to induce interferon. Both drugs induce interferon in mice but not in humans. This study investigates whether these compounds have the antiviral activity in mice and rats since rats more closely resemble the human response. Viral-infection models were created in CD-1 mice and Wistar rats. Three strains of Venezuelan equine encephalitis virus were tested for the performance in these models. One virus strain is the molecularly cloned attenuated vaccine. The second strain has major virulence determinants converted to the wild-type state which are present in virulent strains. The third virus has wild-type virulence determinants, and in addition, is engineered to express green fluorescent protein. Experimentally infected animals received Tilorone or Cridanimod, and their treatment was equivalent to the pharmacopoeia-recomended human treatment regimen. Tilorone and Cridanimod show the antiviral activity in mice and rats and protect the mice from death. In rats, both drugs diminish the viremia. These drugs do not induce interferon-alpha or interferon-beta in rats. The presented observations allow postulating the existence of an interferon-independent and species-independent mechanism of action.

Respiratory Viral and Bacterial Exacerbations of COPD—The Role of the Airway Epithelium

COPD is a leading cause of death worldwide, with acute exacerbations being a major contributor to disease morbidity and mortality. Indeed, exacerbations are associated with loss of lung function, and exacerbation frequency predicts poor prognosis. Respiratory infections are important triggers of acute exacerbations of COPD. This review examines the role of bacterial and viral infections, along with co-infections, in the pathogenesis of COPD exacerbations. Because the airway epithelium is the initial site of exposure both to cigarette smoke (or other pollutants) and to inhaled pathogens, we will focus on the role of airway epithelial cell responses in regulating the pathophysiology of exacerbations of COPD. This will include an examination of the interactions of cigarette smoke alone, and in combination with viral and bacterial exposures in modulating epithelial function and inflammatory and host defense pathways in the airways during COPD. Finally, we will briefly examine current and potential medication approaches to treat acute exacerbations of COPD triggered by respiratory infections.

The Infection of the Japanese Encephalitis Virus SA14-14-2 Strain Induces Lethal Peripheral Inflammatory Responses in IFNAR Deficiency Mice

The Japanese encephalitis virus (JEV) is a leading cause of mosquito-borne viral encephalitis worldwide. Clinical symptoms other than encephalitis, on the other hand, are substantially more prevalent with JEV infection, demonstrating the relevance of peripheral pathophysiology. We studied the peripheral immunopathogenesis of JEV using IFNAR deficient (IFNAR^–/–) mice infected with the SA14-14-2 strain under the BSL-2. The body weight and survival rate of infected-IFNAR^–/–mice decreased significantly. Infected-IFNAR^–/–mice’s liver and spleen demonstrated obvious tissue damage and inflammatory cell infiltration. There was also extensive viral replication in the organs. IFN-α/β protein expression was dramatically elevated in peripheral tissues and serum, although the related interferon-stimulated genes (ISGs) remained low in the spleen and liver of infected-IFNAR^–/–animals. Consistently, the differentially expressed genes (DEGs) analysis using RNA-sequencing of spleens showed inflammatory cytokines upregulation, such as IL-6, TNF-α, and MCP-1, and IFN-γ associated cytokine storm. The infiltration of macrophages and neutrophils in the spleen and liver of SA14-14-2-infected IFNAR^–/– mice was dramatically elevated. However, there was no significant difference in tissue damage, viral multiplication, or the production of IFNα/β and inflammatory cytokines in the brain. Infection with the JEV SA14-14-2 strain resulted in a lethal peripheral inflammatory response and organ damage without encephalitis in IFNAR^–/– mice. Our findings may help shed light on the peripheral immunopathogenesis associated with clinical JEV infection and aid in developing treatment options.

Influenza-induced Tpl2 expression within alveolar epithelial cells is dispensable for host viral control and anti-viral immunity

Tumor progression locus 2 (Tpl2) is a serine/threonine kinase that regulates the expression of inflammatory mediators in response to Toll-like receptors (TLR) and cytokine receptors. Global ablation of Tpl2 leads to severe disease in response to influenza A virus (IAV) infection, characterized by respiratory distress, and studies in bone marrow chimeric mice implicated Tpl2 in non-hematopoietic cells. Lung epithelial cells are primary targets and replicative niches of influenza viruses; however, the specific regulation of antiviral responses by Tpl2 within lung epithelial cells has not been investigated. Herein, we show that Tpl2 is basally expressed in primary airway epithelial cells and that its expression increases in both type I and type II airway epithelial cells (AECI and AECII) in response to influenza infection. We used Nkx2.1-cre to drive Tpl2 deletion within pulmonary epithelial cells to delineate epithelial cell-specific functions of Tpl2 during influenza infection in mice. Although modest increases in morbidity and mortality were attributed to cre-dependent deletion in lung epithelial cells, no alterations in host cytokine production or lung pathology were observed. In vitro, Tpl2 inhibition within the type I airway epithelial cell line, LET1, as well as genetic ablation in primary airway epithelial cells did not alter cytokine production. Overall, these findings establish that Tpl2-dependent defects in cells other than AECs are primarily responsible for the morbidity and mortality seen in influenza-infected mice with global Tpl2 ablation.

A homeostatic interferon-lambda response to bacterial microbiota stimulates preemptive antiviral defense within discrete pockets of intestinal epithelium

Interferon-lambda (IFN-λ) protects intestinal epithelial cells (IECs) from enteric viruses by inducing expression of antiviral IFN-stimulated genes (ISGs). Here, we find that bacterial microbiota stimulate a homeostatic ISG signature in the intestine of specific pathogen-free mice. This homeostatic ISG expression is restricted to IECs, depends on IEC-intrinsic expression of IFN-λ receptor (Ifnlr1), and is associated with IFN-λ production by leukocytes. Strikingly, imaging of these homeostatic ISGs reveals localization to pockets of the epithelium and concentration in mature IECs. Correspondingly, a minority of mature IECs express these ISGs in public single-cell RNA sequencing datasets from mice and humans. Furthermore, we assessed the ability of orally administered bacterial components to restore localized ISGs in mice lacking bacterial microbiota. Lastly, we find that IECs lacking Ifnlr1 are hyper-susceptible to initiation of murine rotavirus infection. These observations indicate that bacterial microbiota stimulate ISGs in localized regions of the intestinal epithelium at homeostasis, thereby preemptively activating antiviral defenses in vulnerable IECs to improve host defense against enteric viruses.

IRF3-binding lncRNA-ISIR strengthens interferon production in viral infection and autoinflammation

Interferon regulatory factor 3 (IRF3) is an essential transductor for initiation of many immune responses. Here, we show that lncRNA-ISIR directly binds IRF3 to promote its phosphorylation, dimerization, and nuclear translocation, along with enhanced target gene productions. In vivo lncRNA-ISIR deficiency results in reduced IFN production, uncontrolled viral replication, and increased mortality. The human homolog, AK131315, also binds IRF3 and promotes its activation. More important, AK131315 expression is positively correlated with type I interferon (IFN-I) level and severity in patients with lupus. Mechanistically, in resting cells, IRF3 is bound to suppressor protein Flightless-1 (Fli-1), which keeps its inactive state. Upon infection, IFN-I-induced lncRNA-ISIR binds IRF3 at DNA-binding domain in cytoplasm and removes Fli-1's association from IRF3, consequently facilitating IRF3 activation. Our results demonstrate that IFN-I-inducible lncRNA-ISIR feedback strengthens IRF3 activation by removing suppressive Fli-1 in immune responses, revealing a method of lncRNA-mediated modulation of transcription factor (TF) activation.

Two Interferon-Stimulated Response Elements Cooperatively Regulate Interferon-Stimulated Gene Expression in West Nile Virus-Infected IFNAR-/- Mouse Embryo Fibroblasts

We previously identified a subset of interferon-stimulated genes (ISGs) upregulated by West Nile virus (WNV) infection in wild-type mouse embryo fibroblasts (MEFs) after viral proteins had inhibited type I interferon (IFN)-mediated JAK-STAT signaling and also in WNV-infected RIG-I^-/-, MDA5^-/-, STAT1^-/-, STAT2^-/-, IFNAR^-/-, IRF3^-/-, IRF7^-/-, and IRF3/7^-/- MEFs. In this study, ISG upregulation by WNV infection in IFNAR^-/- MEFs was confirmed by transcriptome sequencing (RNA-seq). ISG upregulation by WNV infection was inhibited in RIG-I/MDA5^-/- MEFs. ISGs were upregulated in IRF1^-/- and IRF5^-/- MEFs but only minimally upregulated in IRF3/5/7^-/- MEFs, suggesting redundant IRF involvement. We previously showed that a single proximal interferon-stimulated response element (ISRE) in the Oas1a and Oas1b promoters bound the ISGF3 complex after type I IFN treatment. In this study, we used wild-type and mutant promoter luciferase reporter constructs to identify critical regions in the Oas1b and Ifit1 promoters for gene activation in infected IFNAR^-/- MEFs. Two ISREs were required in both promoters. Mutation of these ISREs in an Ifit1 promoter DNA probe reduced in vitro complex formation with infected nuclear extracts. An NF-κB inhibitor decreased Ifit1 promoter activity in cells and in vitro complex formation. IRF3 and p50 promoter binding was detected by chromatin immunoprecipitation (ChIP) for upregulated ISGs with two proximal ISREs. The data indicate that ISREs function cooperatively to upregulate the expression of some ISGs when type I IFN signaling is absent, with the binding complex consisting of IRF3, IRF5, and/or IRF7 and an NF-κB component(s) as well as other, as-yet-unknown factors. IMPORTANCE Type I IFN signaling in mammalian cells induces formation of the ISGF3 transcription factor complex, which binds to interferon stimulated response elements (ISREs) in the promoters of interferon-stimulated genes (ISGs) in the cell nucleus. Flavivirus proteins counteract type I IFN signaling by preventing either the formation or nuclear localization of ISGF3. A subset of ISRE-regulated ISGs was still induced in West Nile virus (WNV)-infected mouse embryo fibroblasts (MEFs), indicating that cells have an alternative mechanism for activating these ISGs. In this study, cellular components involved in this ISG upregulation mechanism were identified using gene knockout MEFs and ChIP, and critical promoter regions for gene activation were mapped using reporter assays. The data indicate a cooperative function between two ISREs and required binding of IRF3, IRF5, and/or IRF7 and an NF-κB component(s). Moreover, type I IFN signaling-independent ISG activation requires different additional promoter activation regions than type I IFN-dependent activation.

ETV7 limits antiviral gene expression and control of influenza viruses

The type I interferon (IFN) response is an important component of the innate immune response to viral infection. Precise control of IFN responses is critical because insufficient expression of IFN-stimulated genes (ISGs) can lead to a failure to restrict viral spread, whereas excessive ISG activation can result in IFN-related pathologies. Although both positive and negative regulatory factors control the magnitude and duration of IFN signaling, it is also appreciated that several ISGs regulate aspects of the IFN response themselves. In this study, we performed a CRISPR activation screen to identify previously unknown regulators of the type I IFN response. We identified the strongly induced ISG encoding ETS variant transcription factor 7 (ETV7) as a negative regulator of the type I IFN response. However, ETV7 did not uniformly suppress ISG transcription. Instead, ETV7 preferentially targeted a subset of antiviral ISGs that were particularly important for IFN-mediated control of influenza viruses. Together, our data assign a function for ETV7 as an IFN response regulator and also identify ETV7 as a potential therapeutic target to increase innate antiviral responses and enhance IFN-based antiviral therapies.

Foot-and-Mouth Disease Virus Capsid Protein VP1 Antagonizes TPL2-Mediated Activation of the IRF3/IFN-β Signaling Pathway to Facilitate the Virus Replication

Foot-and-mouth disease (FMD) is a severe, highly contagious viral disease of cloven-hoofed animals. In order to establish an infection, the FMD virus (FMDV) needs to counteract host antiviral responses. Tumor progression locus 2 (TPL2), a mitogen-activated protein kinase, can regulate innate and adaptive immunity; however, its exact mechanisms underlying TPL2-mediated regulation of the pathogenesis of FMDV infection remain unknown. In this study, we confirmed that TPL2 could inhibit FMDV replication in vitro and in vivo. The virus replication increased in Tpl2-deficient suckling mice in association with reduced expression of interferon-stimulated genes interferon-α (IFN-α) and myxovirus resistance (MX2) and significantly reduced expression of C-X-C motif chemokine ligand 10 (CXCL10), interferon regulatory factor 3 (IRF3), and IRF7, while the phosphorylation of IRF3 was not detected. Moreover, the interactions between TPL2 and VP1 were also confirmed. The overexpression of TPL2 promoted IRF3-mediated dose-dependent activation of the IFN-β signaling pathway in association with interactions between IRF3 and TPL2. VP1 also inhibited phosphorylation of TPL2 at Thr290, while Thr290 resulted as the key functional site associated with the TPL2-mediated antiviral response. Taken together, this study indicated that FMDV capsid protein VP1 antagonizes TPL2-mediated activation of the IRF3/IFN-β signaling pathway for immune escape and facilitated virus replication.

Transcriptomic Profiling of Equine and Viral Genes in Peripheral Blood Mononuclear Cells in Horses during Equine Herpesvirus 1 Infection

Equine herpesvirus 1 (EHV-1) affects horses worldwide and causes respiratory disease, abortions, and equine herpesvirus myeloencephalopathy (EHM). Following infection, a cell-associated viremia is established in the peripheral blood mononuclear cells (PBMCs). This viremia is essential for transport of EHV-1 to secondary infection sites where subsequent immunopathology results in diseases such as abortion or EHM. Because of the central role of PBMCs in EHV-1 pathogenesis, our goal was to establish a gene expression analysis of host and equine herpesvirus genes during EHV-1 viremia using RNA sequencing. When comparing transcriptomes of PBMCs during peak viremia to those prior to EHV-1 infection, we found 51 differentially expressed equine genes (48 upregulated and 3 downregulated). After gene ontology analysis, processes such as the interferon defense response, response to chemokines, the complement protein activation cascade, cell adhesion, and coagulation were overrepresented during viremia. Additionally, transcripts for EHV-1, EHV-2, and EHV-5 were identified in pre- and post-EHV-1-infection samples. Looking at micro RNAs (miRNAs), 278 known equine miRNAs and 855 potentially novel equine miRNAs were identified in addition to 57 and 41 potentially novel miRNAs that mapped to the EHV-2 and EHV-5 genomes, respectively. Of those, 1 EHV-5 and 4 equine miRNAs were differentially expressed in PBMCs during viremia. In conclusion, this work expands our current knowledge about the role of PBMCs during EHV-1 viremia and will inform the focus on future experiments to identify host and viral factors that contribute to clinical EHM.

Antiviral Innate Immunity: Introduction

The concept of “innate immunity” includes all sorts of measures that exclude, inhibit, or slow down infections with little specificity and without much adaptation or generation of a long-lasting memory. The mammalian innate immune defenses described in this article comprise defensins, the complement system, nonspecific phagocytic and cytolytic leukocytes, and cytokines such as the antivirally active interferons. Since the type I (IFN-α/β) and type III interferon (IFN-λ) systems are our primary defense against viral infections, special attention will be paid to the virus-triggered induction of IFN transcription, the signaling activated by IFNs, and the antiviral factors expressed as a consequence.

Critical role of Syk-dependent STAT1 activation in innate antiviral immunity

The JAK/STAT1 pathway is generally activated by cytokines, providing essential antiviral defense. Here, we identify that STAT1 activation is independent of cytokines and JAKs at the early infection stage of some viruses, including influenza A virus (IAV). Instead, STAT1 is activated mainly through spleen tyrosine kinase (Syk) downstream of retinoic acid-inducible gene-I/mitochondrial antiviral-signaling protein (RIG-I/MAVS) signaling. Syk deletion profoundly impairs immediate innate immunity, as evidenced by the finding that Syk deletion attenuates tyrosine phosphorylation of STAT1 and reduces the expressions of interferon-stimulated genes (ISGs) in vitro and in vivo. The antiviral response to IAV infection is also significantly suppressed in the STAT1^Y701F knockin mice. The results demonstrate that STAT1 activation is dependent on Syk rather than the cytokine-activated JAK signaling at the early stage of viral infection, which is critical for initial antiviral immunity. Our finding provides insights into the complicated mechanisms underlying host immune responses to viral infection.

Rhinovirus-Induced Modulation of Epithelial Phenotype: Role in Asthma

Human rhinoviruses have been linked both to the susceptibility of asthma development and to the triggering of acute exacerbations. Given that the human airway epithelial cell is the primary site of human rhinovirus (HRV) infection and replication, the current review focuses on how HRV-induced modulation of several aspects of epithelial cell phenotype could contribute to the development of asthma or to the induction of exacerbations. Modification of epithelial proinflammatory and antiviral responses are considered, as are alterations in an epithelial barrier function and cell phenotype. The contributions of the epithelium to airway remodeling and to the potential modulation of immune responses are also considered. The potential interactions of each type of HRV-induced epithelial phenotypic changes with allergic sensitization and allergic phenotype are also considered in the context of asthma development and of acute exacerbations.

Interferon Regulatory Factors IRF1 and IRF7 Directly Regulate Gene Expression in Bats in Response to Viral Infection

Bat cells and tissue have elevated basal expression levels of antiviral genes commonly associated with interferon alpha (IFNα) signaling. Here, we show Interferon Regulatory Factor 1 (IRF1), 3, and 7 levels are elevated in most bat tissues and that, basally, IRFs contribute to the expression of type I IFN ligands and high expression of interferon regulated genes (IRGs). CRISPR knockout (KO) of IRF 1/3/7 in cells reveals distinct subsets of genes affected by each IRF in an IFN-ligand signaling-dependent and largely independent manner. As the master regulators of innate immunity, the IRFs control the kinetics and maintenance of the IRG response and play essential roles in response to influenza A virus (IAV), herpes simplex virus 1 (HSV-1), Melaka virus/Pteropine orthoreovirus 3 Melaka (PRV3M), and Middle East respiratory syndrome-related coronavirus (MERS-CoV) infection. With its differential expression in bats compared to that in humans, this highlights a critical role for basal IRF expression in viral responses and potentially immune cell development in bats with relevance for IRF function in human biology.

Cell type- and replication stage-specific influenza virus responses in vivo

Influenza A viruses (IAVs) remain a significant global health burden. Activation of the innate immune response is important for controlling early virus replication and spread. It is unclear how early IAV replication events contribute to immune detection. Additionally, while many cell types in the lung can be infected, it is not known if all cell types contribute equally to establish the antiviral state in the host. Here, we use single-cycle influenza A viruses (scIAVs) to characterize the early immune response to IAV in vitro and in vivo. We found that the magnitude of virus replication contributes to antiviral gene expression within infected cells prior to the induction of a global response. We also developed a scIAV that is only capable of undergoing primary transcription, the earliest stage of virus replication. Using this tool, we uncovered replication stage-specific responses in vitro and in vivo. Using several innate immune receptor knockout cell lines, we identify RIG-I as the predominant antiviral detector of primary virus transcription and amplified replication in vitro. Through a Cre-inducible reporter mouse, we used scIAVs expressing Cre-recombinase to characterize cell type-specific responses in vivo. Individual cell types upregulate unique sets of antiviral genes in response to both primary virus transcription and amplified replication. We also identified antiviral genes that are only upregulated in response to direct infection. Altogether, these data offer insight into the early mechanisms of antiviral gene activation during influenza A infection.

Systematic identification of chicken type I, II and III interferon-stimulated genes

Interferon-stimulated genes (ISGs) play an important role in antiviral innate immune responses. Although many ISGs have been identified in mammals, researchers commonly recognize that many more ISGs are yet to be discovered. Current information is still very limited particularly for the systematic identification of type III ISGs. Similarly, current research on ISGs in birds is still in its infancy. The aim of this study was to systematically identify chicken type I (IFN-α), II (IFN-γ) and III (IFN-λ) ISGs and analyze their respective response elements. RNA sequencing (RNA-Seq) was employed to identify those genes with up-regulated expression following chicken IFN-α, IFN-γ and IFN-λ treatment. Two hundred and five type I ISGs, 299 type II ISGs, and 421 type III ISGs were identified in the chicken. We further searched for IFN-stimulated response elements (ISRE) and gamma-activated sequences (GAS) elements in the promoters region of ISGs. The GAS elements were common in the promoter of type II ISGs and were even detected in type I and III ISGs. However, ISRE were not commonly found in the promoters of chicken ISGs. Furthermore, we demonstrated that ISRE in chicken cells were significantly activated by IFN-α or IFN-λ treatment, and expectedly, that GAS elements were also significantly activated by IFN-γ treatment. Interestingly, we also found that GAS elements were significantly activated by IFN-λ. Our study provides a systematic library of ISGs in the chicken together with preliminary information about the transcriptional regulation of the identified ISGs.

Distinct Cell Transcriptomic Landscapes Upon Henipavirus Infections

Hendra virus (HeV) and Cedar virus (CedV) are henipaviruses, which fall into the Paramyxoviridae family of single-stranded, negative-sense RNA viruses. HeV is classified as a Biosafety Level-4 (BSL-4) agent, as it is highly pathogenic and is often fatal to humans. To date, no HeV prevention or treatment methods for human are available. In contrast, previous experimental infection studies have suggested that CedV is non-pathogenic. Flying foxes (pteropid bats) in Australia are the natural reservoirs of both viruses, but the cellular responses of bats to these viral infections remain unclear. Here, we infected bat and human cells with these viruses. We then examined the total transcriptomic landscapes of the cells at 6 or 24 h post infection. Unexpectedly, despite the close phylogenetic relationship between HeV and CedV, there was a dramatic difference in cellular gene expression patterns in response to the two different infections. It is likely that minor differences in the phosphoprotein (P) gene coding strategy between the two viruses cause the observed incongruence in host transcriptomic divergence and viral lethality. This study greatly expands our understanding of the pathogenic mechanisms of henipaviruses.

Specific enhancer selection by IRF3, IRF5 and IRF9 is determined by ISRE half-sites, 5' and 3' flanking bases, collaborating transcription factors and the chromatin environment in a combinatorial fashion

IRF3, IRF5 and IRF9 are transcription factors, which play distinct roles in the regulation of antiviral and inflammatory responses. The determinants that mediate IRF-specific enhancer selection are not fully understood. To uncover regions occupied predominantly by IRF3, IRF5 or IRF9, we performed ChIP-seq experiments in activated murine dendritic cells. The identified regions were analysed with respect to the enrichment of DNA motifs, the interferon-stimulated response element (ISRE) and ISRE half-site variants, and chromatin accessibility. Using a machine learning method, we investigated the predictability of IRF-dominance. We found that IRF5-dominant regions differed fundamentally from the IRF3- and IRF9-dominant regions: ISREs were rare, while the NFKB motif and special ISRE half-sites, such as 5'-GAGA-3' and 5'-GACA-3', were enriched. IRF3- and IRF9-dominant regions were characterized by the enriched ISRE motif and lower frequency of accessible chromatin. Enrichment analysis and the machine learning method uncovered the features that favour IRF3 or IRF9 dominancy (e.g. a tripartite form of ISRE and motifs for NF-κB for IRF3, and the GAS motif and certain ISRE variants for IRF9). This study contributes to our understanding of how IRF members, which bind overlapping sets of DNA sequences, can initiate signal-dependent responses without activating superfluous or harmful programmes.

Cell-to-Cell Variation in Defective Virus Expression and Effects on Host Responses during Influenza Virus Infection

Defective influenza virus particles generated during viral replication carry incomplete viral genomes and can interfere with the replication of competent viruses. These defective genomes are thought to modulate the disease severity and pathogenicity of an influenza virus infection. Different defective viral genomes also introduce another source of variation across a heterogeneous cell population. Evaluating the impact of defective virus genomes on host cell responses cannot be fully resolved at the population level, requiring single-cell transcriptional profiling. Here, we characterized virus and host transcriptomes in individual influenza virus-infected cells, including those of defective viruses that arise during influenza A virus infection. We established an association between defective virus transcription and host responses and validated interfering and immunostimulatory functions of identified dominant defective viral genome species in vitro. This study demonstrates the intricate effects of defective viral genomes on host transcriptional responses and highlights the importance of capturing host-virus interactions at the single-cell level.

Virus and host factors contribute to cell-to-cell variation in viral infections and determine the outcome of the overall infection. However, the extent of the variability at the single-cell level and how it impacts virus-host interactions at a system level are not well understood. To characterize the dynamics of viral transcription and host responses, we used single-cell RNA sequencing to quantify at multiple time points the host and viral transcriptomes of human A549 cells and primary bronchial epithelial cells infected with influenza A virus. We observed substantial variability in viral transcription between cells, including the accumulation of defective viral genomes (DVGs) that impact viral replication. We show (i) a correlation between DVGs and virus-induced variation of the host transcriptional program and (ii) an association between differential inductions of innate immune response genes and attenuated viral transcription in subpopulations of cells. These observations at the single-cell level improve our understanding of the complex virus-host interplay during influenza virus infection.

VAMP8 Contributes to the TRIM6-Mediated Type I Interferon Antiviral Response during West Nile Virus Infection

Several members of the tripartite motif (TRIM) family of E3 ubiquitin ligases regulate immune pathways, including the antiviral type I interferon (IFN-I) system. Previously, we demonstrated that TRIM6 is involved in IFN-I induction and signaling. In the absence of TRIM6, optimal IFN-I signaling is reduced, allowing increased replication of interferon-sensitive viruses. Despite having evolved numerous mechanisms to restrict the vertebrate host's IFN-I response, West Nile virus (WNV) replication is sensitive to pretreatment with IFN-I. However, the regulators and products of the IFN-I pathway that are important in regulating WNV replication are incompletely defined. Consistent with WNV's sensitivity to IFN-I, we found that in TRIM6 knockout (TRIM6-KO) A549 cells, WNV replication is significantly increased and IFN-I induction and signaling are impaired compared to wild-type (wt) cells. IFN-β pretreatment was more effective in protecting against subsequent WNV infection in wt cells than TRIM6-KO, indicating that TRIM6 contributes to the establishment of an IFN-induced antiviral response against WNV. Using next-generation sequencing, we identified VAMP8 as a potential factor involved in this TRIM6-mediated antiviral response. VAMP8 knockdown resulted in reduced JAK1 and STAT1 phosphorylation and impaired induction of several interferon-stimulated genes (ISGs) following WNV infection or IFN-β treatment. Furthermore, VAMP8-mediated STAT1 phosphorylation required the presence of TRIM6. Therefore, the VAMP8 protein is a novel regulator of IFN-I signaling, and its expression and function are dependent on TRIM6 activity. Overall, these results provide evidence that TRIM6 contributes to the antiviral response against WNV and identify VAMP8 as a novel regulator of the IFN-I system.IMPORTANCE WNV is a mosquito-borne flavivirus that poses a threat to human health across large discontinuous areas throughout the world. Infection with WNV results in febrile illness, which can progress to severe neurological disease. Currently, there are no approved treatment options to control WNV infection. Understanding the cellular immune responses that regulate viral replication is important in diversifying the resources available to control WNV. Here, we show that the elimination of TRIM6 in human cells results in an increase in WNV replication and alters the expression and function of other components of the IFN-I pathway through VAMP8. Dissecting the interactions between WNV and host defenses both informs basic molecular virology and promotes the development of host- and virus-targeted antiviral strategies.

Virus- and Interferon Alpha-Induced Transcriptomes of Cells from the Microbat Myotis daubentonii

Antiviral interferons (IFN-alpha/beta) are possibly responsible for the high tolerance of bats to zoonotic viruses. Previous studies focused on the IFN system of megabats (suborder Yinpterochiroptera). We present statistically robust RNA sequencing (RNA-seq) data on transcriptomes of cells from the “microbat” Myotis daubentonii (suborder Yangochiroptera) responding at 6 and 24 h to either an IFN-inducing virus or treatment with IFN. Our data reveal genes triggered only by virus, either in both humans and Myotis (CCL4, IFNL3, CH25H), or exclusively in Myotis (STEAP4). Myotis cells also express a series of conserved IFN-stimulated genes (ISGs) and an unusually high paralog number of the antiviral ISG BST2 (tetherin) but lack several ISGs that were described for megabats (EMC2, FILIP1, IL17RC, OTOGL, SLC24A1). Also, in contrast to megabats, we detected neither different IFN-alpha subtypes nor an unusually high baseline expression of IFNs. Thus, Yangochiroptera microbats, represented by Myotis, may possess an IFN system with distinctive features.

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Virus- and IFN-responsive transcriptomes of the microbat Myotis daubentonii

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CCL4, IFNL3, CH25H, STEAP4 are IFNB-like genes triggered by virus only

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Microbats encode more paralogs of BST2 (tetherin) than any other mammal

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Clear differences between the IFN systems of microbats and megabats

Rhinovirus replication and innate immunity in highly differentiated human airway epithelial cells

Background

Human rhinovirus (HRV) infections are the primary cause of the common cold and are a major trigger for exacerbations of lower airway diseases, such as asthma and chronic obstructive pulmonary diseases. Although human bronchial epithelial cells (HBE) are the natural host for HRV infections, much of our understanding of how HRV replicates and induces host antiviral responses is based on studies using non-airway cell lines (e.g. HeLa cells). The current study examines the replication cycle of HRV, and host cell responses, in highly differentiated cultures of HBE.

Methods

Highly differentiated cultures of HBE were exposed to initial infectious doses ranging from 10⁴ to 10¹ 50% tissue culture-infective dose (TCID₅₀) of purified HRV-16, and responses were monitored up to 144 h after infection. Viral genomic RNA and negative strand RNA template levels were monitored, along with levels of type I and II interferons and selected antivirals.

Results

Regardless of initial infectious dose, relatively constant levels of both genomic and negative strand RNA are generated during replication, with negative strand copy numbers being10,000-fold lower than those of genomic strands. Infections were limited to a small percentage of ciliated cells and did not result in any overt signs of epithelial death. Importantly, regardless of infectious dose, HRV-16 infections were cleared by HBE in the absence of immune cells. Levels of type I and type III interferons (IFNs) varied with initial infectious dose, implying that factors other than levels of double-stranded RNA regulate IFN induction, but the time-course of HRV-16 clearance HBE was the same regardless of levels of IFNs produced. Patterns of antiviral viperin and ISG15 expression suggest they may be generated in an IFN-independent manner during HRV-16 infections.

Conclusions

These data challenge a number of aspects of dogma generated from studies in HeLa cells and emphasize the importance of appropriate cell context when studying HRV infections.

Global virus outbreaks: Interferons as 1st responders

Outbreaks of severe virus infections with the potential to cause global pandemics are increasing. In many instances these outbreaks have been newly emerging (SARS coronavirus), re-emerging (Ebola virus, Zika virus) or zoonotic (avian influenza H5N1) virus infections. In the absence of a targeted vaccine or a pathogen-specific antiviral, broad-spectrum antivirals would function to limit virus spread. Given the direct antiviral effects of type I interferons (IFNs) in inhibiting the replication of both DNA and RNA viruses at different stages of their replicative cycles, and the effects of type I IFNs on activating immune cell populations to clear virus infections, IFNs-α/β present as ideal candidate broad-spectrum antivirals.

Involvement of Interferon Regulatory Factor 7 in Nicotine's Suppression of Antiviral Immune Responses

Nicotine, the active ingredient in tobacco smoke, suppresses antiviral responses. Interferon regulatory factors (IRFs) regulate transcription of type I interferons (IFNs) and IFN-stimulated genes (ISGs) in this response. IRF7 is a key member of the IRF family. Expression of Irf7 is elevated in the brains of virus-infected animals, including human immunodeficiency virus-1 transgenic (HIV-1Tg) rats. We hypothesized that IRF7 affects nicotine's modulation of antiviral responses. Using CRISPR/Cas9 system, IRF7-mutant cell lines were created from human embryonic kidney 293FT cells in which 16 nicotinic acetylcholine receptors (nAChRs) were detected. Decreased expression of IRF7 was confirmed at both the mRNA and protein levels, as was IRF7-regulated cell growth in two IRF7-mutant cell lines, designated IRF7-Δ7 and IRF7-Δ11. In IRF7-Δ7 cells, expression of two nAChR genes, CHRNA3 and CHRNA9, changed modestly. After stimulation with polyinosinic-polycytidylic acid (poly I:C) (0.25 μg/ml) for 4 h to mimic viral infection, 293FT wild-type (WT) and IRF7-Δ7 cells were treated with 0, 1, or 100 μM nicotine for 24 h, which increased IFN-β expression in both types of cells but elevation was higher in WT cells (p < 0.001). Expression was significantly suppressed in WT cells (p < 0.001) but not in IRF7-Δ7 cells by 24-h nicotine exposure. Poly I:C stimulation increased mRNA expression of retinoic-acid-inducible protein I (RIG-I), melanoma-differentiation-associated gene 5 (MDA5), IFN-stimulated gene factor 3 (ISG3) complex, and IFN-stimulated genes (IRF7, ISG15, IFIT1, OAS1); nicotine attenuated mRNA expression only in WT cells. Overall, IRF7 is critical to nicotine's effect on the antiviral immune response. Graphical Abstract Involvement of IRF7 in nicotine's suppression of poly I:C-induced antiviral immune responses. PAMPs, such as a synthetic viral analogue of dsRNA poly I:C attack cells, will be recognized by PRRs, and the host innate immunity against viral infection will be activated. PRRs signaling trigger phosphorylation of IRF7 and IRF3 to induce their translocation to the nucleus and result in the production of type I IFNs. Then IFNs bind to IFNAR to activate the transcription factor ISGF3, a complex consisting of STAT1, STAT2, and IRF9. Further, it induces the expression of ISGs, including IFIT1, OAS1, IRF7, ISG15, etc. Nicotine suppresses the immune responses stimulated by poly I:C. In the IRF7-mutant cells, nicotine's suppressive effects on poly I:C-stimulated immune responses were restrained.

Interferon-Stimulated Genes-Mediators of the Innate Immune Response during Canine Distemper Virus Infection

The demyelinating canine distemper virus (CDV)-leukoencephalitis represents a translational animal model for multiple sclerosis. The present study investigated the expression of type I interferon (IFN-I) pathway members in CDV-induced cerebellar lesions to gain an insight into their role in lesion development. Gene expression of 110 manually selected genes in acute, subacute and chronic lesions was analyzed using pre-existing microarray data. Interferon regulatory factor (IRF) 3, IRF7, signal transducer and activator of transcription (STAT) 1, STAT2, MX protein, protein kinase R (PKR), 2'-5'-oligoadenylate synthetase (OAS) 1 and interferon-stimulated gene (ISG) 15 expression were also evaluated using immunohistochemistry. Cellular origin of STAT1, STAT2, MX and PKR were determined using immunofluorescence. CDV infection caused an increased expression of the antiviral effector proteins MX, PKR, OAS1 and ISG15, which probably contributed to a restricted viral replication, particularly in neurons and oligodendrocytes. This increase might be partly mediated by IRF-dependent pathways due to the lack of changes in IFN-I levels and absence of STAT2 in astrocytes. Nevertheless, activated microglia/macrophages showed a strong expression of STAT1, STAT2 and MX proteins in later stages of the disease, indicating a strong activation of the IFN-I signaling cascade, which might be involved in the aggravation of bystander demyelination.

Early transcriptome profile of goat peripheral blood mononuclear cells (PBMCs) infected with peste des petits ruminant's vaccine virus (Sungri/96) revealed induction of antiviral response in an interferon independent manner

Sungri/96 vaccine strain is considered the most potent vaccine providing long-term immunity against peste des petits ruminants (PPR) in India. Previous studies in our laboratory highlighted induction of robust antiviral response in an interferon independent manner at 48 h and 120 h post infection (p.i.). However, immune response at the earliest time point 6 h p.i. (time taken to complete one PPRV life cycle), in PBMCs infected with Sungri/96 vaccine virus has not been investigated. This study was taken up to understand the global gene expression profiling of goat PBMCs after Sungri/96 PPRV vaccine strain infection at 6 h post infection (p.i.). A total of 1926 differentially expressed genes (DEGs) were identified with 616 - upregulated and 1310 - downregulated. TLR7/TLR3, IRF7/IRF1, ISG20, IFIT1/IFIT2, IFITM3, IL27 and TREX1 were identified as key immune sensors and antiviral candidate genes. Interestingly, type I interferons (IFNα/β) were not differentially expressed at this time point as well. TREX1, an exonuclease which inhibits type I interferons at the early stage of virus infection was found to be highly upregulated. IL27, an important antiviral host immune factor was significantly upregulated. ISG20, an antiviral interferon induced gene with exonuclease activity specific to ssRNA viruses was highly expressed. Functional profiling of DEGs showed significant enrichment of immune system processes with 233 genes indicating initiation of immune defense response in host cells. Protein interaction network showed important innate immune molecules in the immune network with high connectivity. The study highlights important immune and antiviral genes at the earliest time point.

Attenuation of interferon regulatory factor 7 activity in local infectious sites of trachea and lung for preventing the development of acute lung injury caused by influenza A virus

The excessive activation of interferon regulatory factor 7 (IRF7) promotes the development of acute lung injury (ALI) caused by influenza A virus (IAV). However, the deficiency of IRF7 increases the susceptibility to deadly IAV infection in both humans and mice. To test whether the attenuation rather than the abolishment of IRF7 activity in local infectious sites could alleviate IAV-induced ALI, we established IAV-infected mouse model and trachea/lung-tissue culture systems, and designed two IRF7-interfering oligodeoxynucleotides, IRF7-rODN M1 and IRF7-rODN A1, based on the mouse and human consensus sequences of IRF7-binding sites of Ifna/IFNA genes, respectively. In the model mice, we found a close relationship between the IAV-induced ALI and the level/activity of IRF7 in local infectious sites, and also found that the reduced IRF7 level or activity in the lungs of mice treated with IRF7-rODN M1 led to decreased mRNA levels of Ifna genes, reduced neutrophil infiltration in the lungs and prolonged survival of mice. Furthermore, we found that the effects of IRF7-rODN M1 on alleviating IAV-induced ALI could be correlated to the reduced translocation of IRF7, caused by the IRF7-rODN M1, from cytosol to nucleus in IAV-infected cells. These data suggest that the proper attenuation of IRF7 activity in local infectious sites could be a novel approach for treating IAV-induced ALI.

Comparison of Anti-Viral Activity of Frog Skin Anti-Microbial Peptides Temporin-Sha and [K³]SHa to LL-37 and Temporin-Tb against Herpes Simplex Virus Type 1

Temporins are anti-microbial peptides synthesized in the skin of frogs of the Ranidae family. The few studies to date that have examined their anti-viral properties have shown that they have potential as anti-viral therapies. In this work, we evaluated the anti-herpes simplex virus type 1 (HSV-1) activity of the temporin-SHa (SHa) and its synthetic analog [K³]SHa. Human cathelicidin LL-37 and temporin-Tb (Tb), previously demonstrated to have anti-HSV-1 properties, were used as positive controls. We observed that SHa and [K³]SHa significantly inhibit HSV-1 replication in human primary keratinocytes when used at micromolar concentrations. This anti-viral activity was equivalent to that of Tb, but lower than that of LL-37. Transcriptomic analyses revealed that SHa did not act through the modulation of the cell innate immune response, but rather, displayed virucidal properties by reducing infectious titer of HSV-1 in suspension. In contrast, pre-incubation of the virus with LL-37 suggests that this peptide does not act directly on the viral particle at non-cytotoxic concentrations tested. The anti-HSV-1 activity of LL-37 appears to be due to the potentiation of cellular anti-viral defenses through the induction of interferon stimulated gene expression in infected primary keratinocytes. This study demonstrated that SHa and [K³]SHa, in addition to their previously reported antibacterial and antiparasitic activities, are direct-acting anti-HSV-1 peptides. Importantly, this study extends the little studied anti-viral attributes of frog temporins and offers perspectives for the development of new anti-HSV-1 therapies.

The Molecular Basis of Viral Inhibition of IRF- and STAT-Dependent Immune Responses

The antiviral innate immunity is the first line of host defense against virus infections. In mammalian cells, viral infections initiate the expression of interferons (IFNs) in the host that in turn activate an antiviral defense program to restrict viral replications by induction of IFN stimulated genes (ISGs), which are largely regulated by the IFN-regulatory factor (IRF) family and signal transducer and activator of transcription (STAT) family transcription factors. The mechanisms of action of IRFs and STATs involve several post-translational modifications, complex formation, and nuclear translocation of these transcription factors. However, many viruses, including human immunodeficiency virus (HIV), Zika virus (ZIKV), and herpes simplex virus (HSV), have evolved strategies to evade host defense, including alteration in IRF and STAT post-translational modifications, disturbing the formation and nuclear translocation of the transcription complexes as well as proteolysis/degradation of IRFs and STATs. In this review, we discuss and summarize the molecular mechanisms by which how viral components may target IRFs and STATs to antagonize the establishment of antiviral host defense. The underlying host-viral interactions determine the outcome of viral infection. Gaining mechanistic insight into these processes will be crucial in understanding how viral replication can be more effectively controlled and in developing approaches to improve virus infection outcomes.

Type I interferon response impairs differentiation potential of pluripotent stem cells

Unlike all differentiated cells, pluripotent stem cells do not elicit a productive antiviral response when infected by a pathogen. This observation seems at odds with the importance of pluripotent stem cells given their absolute requirement for the development of life. Here we investigate why this antiviral response is not utilized in these unique cells. We find that the factors required to maintain pluripotency are incompatible with those involved in eliciting the canonical interferon-based response to virus infection.

Upon virus infection, pluripotent stem cells neither induce nor respond to canonical type I interferons (IFN-I). To better understand this biology, we characterized induced pluripotent stem cells (iPSCs) as well as their differentiated parental or rederived counterparts. We confirmed that only iPSCs failed to respond to viral RNA, IFN-I, or viral infection. This lack of response could be phenocopied in fibroblasts with the expression of a reprogramming factor which repressed the capacity to induce canonical antiviral pathways. To ascertain the consequences of restoring the antiviral response in the context of pluripotency, we engineered a system to engage these defenses in iPSCs. Inducible expression of a recombinant virus-activated transcription factor resulted in the successful reconstitution of antiviral defenses through the direct up-regulation of IFN-I–stimulated genes. Induction of the antiviral signature in iPSCs, even for a short duration, resulted in the dysregulation of genes associated with all three germ layers despite maintaining pluripotency markers. Trilineage differentiation of these same cells showed that engagement of the antiviral defenses compromised ectoderm and endoderm formation and dysregulated the development of mesodermal sublineages. In all, these data suggest that the temporal induction of the antiviral response primes iPSCs away from pluripotency and induces numerous aberrant gene products upon differentiation. Together these results suggest that the IFN-I system and pluripotency may be incompatible with each other and thus explain why stem cells do not utilize the canonical antiviral system.

Distinct antiviral signatures revealed by the magnitude and round of influenza virus replication in vivo

Influenza virus has a broad cellular tropism in the respiratory tract. Infected epithelial cells sense the infection and initiate an antiviral response. To define the antiviral response at the earliest stages of infection we used a series of single-cycle reporter viruses. These viral probes demonstrated cells in vivo harbor a range in magnitude of virus replication. Transcriptional profiling of cells supporting different levels of replication revealed tiers of IFN-stimulated gene expression. Uninfected cells and cells with blunted replication expressed a distinct and potentially protective antiviral signature, while cells with high replication expressed a unique reserve set of antiviral genes. Finally, we used these single-cycle reporter viruses to determine the antiviral landscape during virus spread, which unveiled disparate protection of epithelial cell subsets mediated by IFN in vivo. Together these results highlight the complexity of virus-host interactions within the infected lung and suggest that magnitude and round of replication tune the antiviral response.

Strand-Specific Dual RNA Sequencing of Bronchial Epithelial Cells Infected with Influenza A/H3N2 Viruses Reveals Splicing of Gene Segment 6 and Novel Host-Virus Interactions

Host-influenza virus interplay at the transcript level has been extensively characterized in epithelial cells. Yet, there are no studies that simultaneously characterize human host and influenza A virus (IAV) genomes. We infected human bronchial epithelial BEAS-2B cells with two seasonal IAV/H3N2 strains, Brisbane/10/07 and Perth/16/09 (reference strains for past vaccine seasons) and the well-characterized laboratory strain Udorn/307/72. Strand-specific RNA sequencing (RNA-seq) of the infected BEAS-2B cells allowed for simultaneous analysis of host and viral transcriptomes, in addition to pathogen genomes, to reveal changes in mRNA expression and alternative splicing (AS). In general, patterns of global and immune gene expression induced by the three IAVs were mostly shared. However, AS of host transcripts and small nuclear RNAs differed between the seasonal and laboratory strains. Analysis of viral transcriptomes showed deletions of the polymerase components (defective interfering-like RNAs) within the genome. Surprisingly, we found that the neuraminidase gene undergoes AS and that the splicing event differs between seasonal and laboratory strains. Our findings reveal novel elements of the host-virus interaction and highlight the importance of RNA-seq in identifying molecular changes at the genome level that may contribute to shaping RNA-based innate immunity.IMPORTANCE The use of massively parallel RNA sequencing (RNA-seq) has revealed insights into human and pathogen genomes and their evolution. Dual RNA-seq allows simultaneous dissection of host and pathogen genomes and strand-specific RNA-seq provides information about the polarity of the RNA. This is important in the case of negative-strand RNA viruses like influenza virus, which generate positive (complementary and mRNA) and negative-strand RNAs (genome) that differ in their potential to trigger innate immunity. Here, we characterize interactions between human bronchial epithelial cells and three influenza A/H3N2 strains using strand-specific dual RNA-seq. We focused on this subtype because of its epidemiological importance in causing significant morbidity and mortality during influenza epidemics. We report novel elements that differ between seasonal and laboratory strains highlighting the complexity of the host-virus interplay at the RNA level.

Topical application of aminoglycoside antibiotics enhances host resistance to viral infections in a microbiota-independent manner

Antibiotics are widely used to treat infections in humans. However, the impact of antibiotic use on host cells is understudied. Here we identify an antiviral effect of commonly used aminoglycoside antibiotics. We show that topical mucosal application of aminoglycosides prophylactically increased host resistance to a broad range of viral infections including herpes simplex viruses, influenza A virus and Zika virus. Aminoglycoside treatment also reduced viral replication in primary human cells. This antiviral activity was independent of the microbiota as aminoglycoside treatment protected germ-free mice. Microarray analysis uncovered a marked upregulation of transcripts for interferon-stimulated genes (ISGs) following aminoglycoside application. ISG induction was mediated by TLR3, and required TIR-domain-containing adapter-inducing interferon-β (TRIF), signaling adaptor, and interferon regulatory factors 3 (IRF3) and IRF7, transcription factors that promote ISG expression. XCR1+ dendritic cells, which uniquely express TLR3, were recruited to the vaginal mucosa upon aminoglycoside treatment and were required for ISG induction. These results highlight an unexpected ability of aminoglycoside antibiotics to confer broad antiviral resistance in vivo.

Reduction of Marek's Disease Virus Infection by Toll-Like Receptor Ligands in Chicken Embryo Fibroblast Cells

Evolutionarily conserved pattern recognition receptors, including Toll-like receptors (TLRs) recognize pathogen-associated molecular patterns (PAMPs) that are present in microbes. PAMPs induce several pathways downstream of TLRs that lead to induction of antiviral responses. The objective of this study was to investigate the stimulatory effect of various PAMPs (in the form of TLR ligands) in reducing Marek's disease virus (MDV) infection in chicken embryo fibroblast cells (CEFs). To this end, CEFs were pretreated with Pam3CSK4, Poly(IC), lipopolysaccharide (LPS), and CpG ODN as TLR2, TLR3, TLR4, and TLR21 ligands, respectively for 24 h followed by infection with MDV. The results indicated that pretreatment with Poly(IC) resulted in a robust reduction (by about 81%) of MDV infection in CEFs at 96 h postinfection while a moderate reduction was observed with treatment of Pam3CSK4 (35%), LPS (26%), and CpG ODN (23%) PAMPs. Transcriptional analysis of gene expression in CEFs demonstrated that all TLR ligand treatments and MDV infection significantly increased the expression of type I interferons, interleukin (IL)-1β, interferon regulatory factor 7 (IRF7), interferon induced protein with tetratricopeptide repeats 5 (IFIT5), and myxoma-resistance protein (Mx). Further studies are needed to explore the mechanism by which PAMPs, particularly the TLR3 ligands could reduce MDV infection in CEFs, which may play an important role in controlling the replication of MDV in chicken.

IFN Regulatory Factors and Antiviral Innate Immunity: How Viruses Can Get Better

The interferon regulatory factor (IRF) family consists of transcriptional regulators that exert multifaceted and versatile functions in multiple biological processes. Their crucial role as central mediators in the establishment and execution of host immunity in response to pathogen-derived signals downstream pattern recognition receptors (PRRs) makes IRFs a hallmark of the host antiviral response. They function as hub molecules at the crossroad of different signaling pathways for the induction of interferon (IFN) and inflammatory cytokines, as well as of antiviral and immunomodulatory genes even in an IFN-independent manner. By regulating the development and activity of immune cells, IRFs also function as a bridge between innate and adaptive responses. As such, IRFs represent attractive and compulsive targets in viral strategies to subvert antiviral signaling. In this study, we discuss current knowledge on the wide array of strategies put in place by pathogenic viruses to evade, subvert, and/or hijack these essential components of host antiviral immunity.

ZIKV infection regulates inflammasomes pathway for replication in monocytes

ZIKV causes microcephaly by crossing the placental barrier, however, the mechanism of trans-placental dissemination of ZIKV remains unknown. Here, we sought to determine whether monocytes, which can cross tissue barriers, assist ZIKV dissemination to the fetus. We determined this by infecting monocytes with two strains of ZIKV: South American (PRVABC59) and Nigerian (IBH30656) and analyzing viral replication. We found that ZIKV infects and replicates in monocytes and macrophages, which results in the modulation of a large number of cellular genes. Analysis of these genes identified multiple pathways including inflammasome to be targeted by ZIKV, which was confirmed by analyzing the transcript levels of the proteins of inflammasome pathways, NLRP3, ASC, caspase 1, IL-1 and IL-18. Interestingly, IFNα and the IFN inducible gene, MxA were not enhanced, suggesting prevention of innate antiviral defense by ZIKV. Also, inhibition of inflammasome led to an increased transcriptional activity of IFNα, MxA and CXCL10. Based on these results we suggest that ZIKV transcription is regulated by inflammasomes.

Vaginal Exposure to Zika Virus during Pregnancy Leads to Fetal Brain Infection

Zika virus (ZIKV) can be transmitted sexually between humans. However, it is unknown whether ZIKV replicates in the vagina and impacts the unborn fetus. Here, we establish a mouse model of vaginal ZIKV infection and demonstrate that, unlike other routes, ZIKV replicates within the genital mucosa even in wild-type (WT) mice. Mice lacking RNA sensors or transcription factors IRF3 and IRF7 resulted in higher levels of local viral replication. Furthermore, mice lacking the type I interferon (IFN) receptor (IFNAR) became viremic and died of infection after a high-dose vaginal ZIKV challenge. Notably, vaginal infection of pregnant dams during early pregnancy led to fetal growth restriction and infection of the fetal brain in WT mice. This was exacerbated in mice deficient in IFN pathways, leading to abortion. Our study highlights the vaginal tract as a highly susceptible site of ZIKV replication and illustrates the dire disease consequences during pregnancy.

Vaccinia Virus Protein C6 Inhibits Type I IFN Signalling in the Nucleus and Binds to the Transactivation Domain of STAT2

The type I interferon (IFN) response is a crucial innate immune signalling pathway required for defense against viral infection. Accordingly, the great majority of mammalian viruses possess means to inhibit this important host immune response. Here we show that vaccinia virus (VACV) strain Western Reserve protein C6, is a dual function protein that inhibits the cellular response to type I IFNs in addition to its published function as an inhibitor of IRF-3 activation, thereby restricting type I IFN production from infected cells. Ectopic expression of C6 inhibits the induction of interferon stimulated genes (ISGs) in response to IFNα treatment at both the mRNA and protein level. C6 inhibits the IFNα-induced Janus kinase/signal transducer and activator of transcription (JAK/STAT) signalling pathway at a late stage, downstream of STAT1 and STAT2 phosphorylation, nuclear translocation and binding of the interferon stimulated gene factor 3 (ISGF3) complex to the interferon stimulated response element (ISRE). Mechanistically, C6 associates with the transactivation domain of STAT2 and this might explain how C6 inhibits the type I IFN signalling very late in the pathway. During virus infection C6 reduces ISRE-dependent gene expression despite the presence of the viral protein phosphatase VH1 that dephosphorylates STAT1 and STAT2. The ability of a cytoplasmic replicating virus to dampen the immune response within the nucleus, and the ability of viral immunomodulators such as C6 to inhibit multiple stages of the innate immune response by distinct mechanisms, emphasizes the intricacies of host-pathogen interactions and viral immune evasion.

In response to a viral infection, infected host cells mount an early, innate immune response to limit viral replication and spread. Type I interferons (IFNs) are produced by a cell when a viral infection is detected and are a crucial aspect of this early immune response. IFNs are released from the infected cell and can act on the infected cell itself or neighbouring cells to initiate a signalling pathway that results in the production of hundreds of anti-viral proteins. In this work we investigated a vaccinia virus protein called C6, a known inhibitor of type I IFN production. Here we show that C6 also inhibits signalling initiated in response to type I IFNs, therefore providing a dual defence against this essential immune response. The results show that, unlike the majority of viral inhibitors of IFN signalling, C6 inhibits the signalling pathway at a late stage once the proteins required for IFN-stimulated gene transcription have reached the nucleus and bound to the DNA. This work illustrates the complex relationship between infecting viruses and the host immune response and further investigation of the mechanism by which C6 inhibits this important immune pathway will likely increase our knowledge of the pathway itself.

Genetic characterization of bovine viral diarrhea virus strains in Beijing, China and innate immune responses of peripheral blood mononuclear cells in persistently infected dairy cattle

To acquire epidemiological data on the bovine viral diarrhea virus (BVDV) and identify cattle persistently infected (PI) with this virus, 4,327 samples from Holstein dairy cows were screened over a four-year period in Beijing, China. Eighteen BVD viruses were isolated, 12 from PI cattle. Based on genetic analysis of their 5'-untranslated region (5'-UTR), the 18 isolates were assigned to subgenotype BVDV-1m, 1a, 1d, 1q, and 1b. To investigate the innate immune responses in the peripheral-blood mononuclear cells of PI cattle, the expression of Toll-like receptors (TLRs), RIG-I-like receptors, interferon-α (IFN-α), IFN-β, myxovirus (influenza virus) resistance 1 (MX1), and interferon stimulatory gene 15 (ISG15) was assessed by qPCR. When compared with healthy cattle, the expression of TLR-7, IFN-α, and IFN-β mRNA was downregulated, but the expression of MX1 and ISG-15 mRNA was upregulated in PI cattle. Immunoblotting analysis revealed that the expression of interferon regulatory factor 3 (IRF-3) and IRF-7 was lower in PI cattle than in healthy cattle. Thus, BVDV-1m and 1a are the predominant subgenotypes in the Beijing region, and the strains are highly divergent. Our findings also suggest that the TLR-7/IRF-7 signaling pathway plays a role in evasion of host restriction by BVDV.

Immunization with Attenuated Equine Herpesvirus 1 Strain KyA Induces Innate Immune Responses That Protect Mice from Lethal Challenge

Equine herpesvirus 1 (EHV-1) is a major pathogen affecting equines worldwide. The virus causes respiratory disease, abortion, and, in some cases, neurological disease. EHV-1 strain KyA is attenuated in the mouse and equine, whereas wild-type strain RacL11 induces severe inflammation of the lung, causing infected mice to succumb at 4 to 6 days postinfection. Our previous results showed that KyA immunization protected CBA mice from pathogenic RacL11 challenge at 2 and 4 weeks postimmunization and that KyA infection elicited protective humoral and cell-mediated immune responses. To investigate the protective mechanisms of innate immune responses to KyA, KyA-immunized mice were challenged with RacL11 at various times postvaccination. KyA immunization protected mice from RacL11 challenge at 1 to 7 days postimmunization. Immunized mice lost less than 10% of their body weight and rapidly regained weight. Virus titers in the lungs of KyA-immunized mice were 1,000-fold lower at 2 days post-RacL11 challenge than virus titers in the lungs of nonimmunized mice, indicating accelerated virus clearance. Affymetrix microarray analysis revealed that gamma interferon (IFN-γ) and 16 antiviral interferon-stimulated genes (ISGs) were upregulated 3.1- to 48.2-fold at 8 h postchallenge in the lungs of RacL11-challenged mice that had been immunized with KyA. Murine IFN-γ inhibited EHV-1 infection of murine alveolar macrophages and protected mice against lethal EHV-1 challenge, suggesting that IFN-γ expression is important in mediating the protection elicited by KyA immunization. These results suggest that EHV-1 KyA may be used as a live attenuated EHV-1 vaccine as well as a prophylactic agent in horses.

IMPORTANCE

Viral infection of cells initiates a signal cascade of events that ultimately attempts to limit viral replication and prevent infection through the expression of host antiviral proteins. In this study, we show that EHV-1 KyA immunization effectively protected CBA mice from pathogenic RacL11 challenge at 1 to 7 days postvaccination and increased the expression of IFN-γ and 16 antiviral interferon-stimulated genes (ISGs). The administration of IFN-γ blocked EHV-1 replication in murine alveolar macrophages and mouse lungs and protected mice from lethal challenge. To our knowledge, this is the first report of an attenuated EHV-1 vaccine that protects the animal at 1 to 7 days postimmunization by innate immune responses. Our findings suggested that IFN-γ serves as a novel prophylactic agent and may offer new strategies for the development of anti-EHV-1 agents in the equine.