Incoming influenza A virus evades early host recognition, while influenza B virus induces interferon expression directly upon entry

Differential interferon responses to influenza A and B viruses in primary ferret respiratory epithelial cells

Influenza B viruses (IBV) cocirculate with influenza A viruses (IAV) and cause periodic epidemics of disease, yet antibody and cellular responses following IBV infection are less well understood. Using the ferret model for antisera generation for influenza surveillance purposes, IAV resulted in robust antibody responses following infection, whereas IBV required an additional booster dose, over 85% of the time, to generate equivalent antibody titers. In this study, we utilized primary differentiated ferret nasal epithelial cells (FNECs) which were inoculated with IAV and IBV to study differences in innate immune responses which may result in differences in adaptive immune responses in the host. FNECs were inoculated with IAV (H1N1pdm09 and H3N2 subtypes) or IBV (B/Victoria and B/Yamagata lineages) and assessed for 72 h. Cells were analyzed for gene expression by quantitative real-time PCR, and apical and basolateral supernatants were assessed for virus kinetics and interferon (IFN), respectively. Similar virus kinetics were observed with IAV and IBV in FNECs. A comparison of gene expression and protein secretion profiles demonstrated that IBV-inoculated FNEC expressed delayed type-I/II IFN responses and reduced type-III IFN secretion compared to IAV-inoculated cells. Concurrently, gene expression of Thymic Stromal Lymphopoietin (TSLP), a type-III IFN-induced gene that enhances adaptive immune responses, was significantly downregulated in IBV-inoculated FNECs. Significant differences in other proinflammatory and adaptive genes were suppressed and delayed following IBV inoculation. Following IBV infection, ex vivo cell cultures derived from the ferret upper respiratory tract exhibited reduced and delayed innate responses which may contribute to reduced antibody responses in vivo.IMPORTANCEInfluenza B viruses (IBV) represent nearly one-quarter of all human influenza cases and are responsible for significant clinical and socioeconomic impacts but do not pose the same pandemic risks as influenza A viruses (IAV) and have thus received much less attention. IBV accounts for greater severity and deaths in children, and vaccine efficacy remains low. The ferret can be readily infected with human clinical isolates and demonstrates a similar course of disease and immune responses. IBV, however, generates lower antibodies in ferrets than IAV following the challenge. To determine whether differences in initial innate responses following infection may affect the development of robust adaptive immune responses, ferret respiratory tract cells were isolated, infected with IAV/IBV, and compared. Understanding the differences in the initial innate immune responses to IAV and IBV may be important in the development of more effective vaccines and interventions to generate more robust protective immune responses.

Existing Evidence for Influenza B Virus Adaptations to Drive Replication in Humans as the Primary Host

Influenza B virus (IBV) is one of the two major types of influenza viruses that circulate each year. Unlike influenza A viruses, IBV does not harbor pandemic potential due to its lack of historical circulation in non-human hosts. Many studies and reviews have highlighted important factors for host determination of influenza A viruses. However, much less is known about the factors driving IBV replication in humans. We hypothesize that similar factors influence the host restriction of IBV. Here, we compile and review the current understanding of host factors crucial for the various stages of the IBV viral replication cycle. While we discovered the research in this area of IBV is limited, we review known host factors that may indicate possible host restriction of IBV to humans. These factors include the IBV hemagglutinin (HA) protein, host nuclear factors, and viral immune evasion proteins. Our review frames the current understanding of IBV adaptations to replication in humans. However, this review is limited by the amount of research previously completed on IBV host determinants and would benefit from additional future research in this area.

The circRNA circVAMP3 restricts influenza A virus replication by interfering with NP and NS1 proteins

Circular RNAs (circRNAs) are involved in various biological roles, including viral infection and antiviral immune responses. To identify influenza A virus (IAV) infection-related circRNAs, we compared the circRNA profiles of A549 cells upon IAV infection. We found that circVAMP3 is substantially upregulated after IAV infection or interferon (IFN) stimulation. Furthermore, IAV and IFN-β induced the expression of QKI-5, which promoted the biogenesis of circVAMP3. Overexpression of circVAMP3 inhibited IAV replication, while circVAMP3 knockdown promoted viral replication, suggesting that circVAMP3 restricts IAV replication. We verified the effect of circVAMP3 on viral infection in mice and found that circVAMP3 restricted IAV replication and pathogenesis in vivo. We also found that circVAMP3 functions as a decoy to the viral proteins nucleoprotein (NP) and nonstructural protein 1 (NS1). Mechanistically, circVAMP3 interfered with viral ribonucleoprotein complex activity by reducing the interaction of NP with polymerase basic 1, polymerase basic 2, or vRNA and restored the activation of IFN-β by alleviating the inhibitory effect of NS1 to RIG-I or TRIM25. Our study provides new insights into the roles of circRNAs, both in directly inhibiting virus replication and in restoring innate immunity against IAV infection.

Influenza B Virus (IBV) Immune-Mediated Disease in C57BL/6 Mice

Influenza B viruses (IBV) primarily infect humans, causing seasonal epidemics. The absence of an animal reservoir limits pandemic concern, but IBV infections may cause severe respiratory disease, predominantly in young children and the elderly. The IBV disease burden is largely controlled by seasonal influenza vaccination; however, immunity due to vaccination is sometimes incomplete, a feature linked to antigenic mismatches. Thus, understanding the features that contribute to disease pathogenesis is important, particularly immune-mediated versus virus-mediated outcomes. Unexpectedly, C57BL/6 (B6) mice intranasally infected with a low multiplicity of infection of B/Florida/04/2006 developed substantial morbidity and mortality. To address the cause, B6 mice were treated daily with dexamethasone to dampen the immune and pro-inflammatory response to IBV infection, allowing the determination of whether the responses were immune- and/or virus-associated. As expected, dexamethasone (DEX)-treated mice had a lower pro-inflammatory response and reduced lung pathology despite the presence of high viral lung titers, but mortality was comparable to PBS-treated mice, indicating that mortality may be linked to lung virus replication. The results showed that the immune response to IBV is the major cause of morbidity, mortality, lung pathology, and viral clearance. Importantly, the results suggest that a robust lung CTL response and associated leukocyte influx contribute to disease.

Epidemiological and Clinical Characteristics of Influenza Outbreaks Among Children in Chongqing, China

Influenza is a global serious public health threat. Seasonal influenza among children in Chongqing has been a heavy health burden. To date, few studies have examined the spatial and temporal characteristics of influenza. This research sheds new light on correlating them with influenza outbreaks with data of over 5 years (2014–2018). All cluster outbreaks among preschool and school-age children reported in Chongqing were collected through the Public Health Emergency Management Information System. The demographical, epidemiological, and clinical data of the cases were analyzed. From 2014 to 2018, a total of 111 preschool- and school-based influenza-like illness outbreaks involving 3,549 cases were identified. Several clinical symptoms that were analyzed in this study showed significant contrast between influenza A and B. Spatial autocorrelation analysis over the 5-year data detected Xiushan district being the most likely cluster. The exploration of the spatial distribution and clinical characteristics of influenza cluster of children in Chongqing could help the effective implementation of health policies. Future studies should be conducted to monitor the outbreaks of influenza among children.

Library-based analysis reveals segment and length dependent characteristics of defective influenza genomes

Found in a diverse set of viral populations, defective interfering particles are parasitic variants that are unable to replicate on their own yet rise to relatively high frequencies. Their presence is associated with a loss of population fitness, both through the depletion of key cellular resources and the stimulation of innate immunity. For influenza A virus, these particles contain large internal deletions in the genomic segments which encode components of the heterotrimeric polymerase. Using a library-based approach, we comprehensively profile the growth and replication of defective influenza species, demonstrating that they possess an advantage during genome replication, and that exclusion during population expansion reshapes population composition in a manner consistent with their final, observed, distribution in natural populations. We find that an innate immune response is not linked to the size of a deletion; however, replication of defective segments can enhance their immunostimulatory properties. Overall, our results address several key questions in defective influenza A virus biology, and the methods we have developed to answer those questions may be broadly applied to other defective viruses.

SARS-CoV-2 Isolates Show Impaired Replication in Human Immune Cells but Differential Ability to Replicate and Induce Innate Immunity in Lung Epithelial Cells

The primary target organ of coronavirus disease 2019 (COVID-19) infection is the respiratory tract. Currently, there is limited information on the ability of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) to infect and regulate innate immunity in human immune cells and lung epithelial cells. Here, we compared the ability of four Finnish isolates of SARS-CoV-2 from COVID-19 patients to replicate and induce interferons (IFNs) and other cytokines in different human cells. All isolates failed to replicate in dendritic cells, macrophages, monocytes, and lymphocytes, and no induction of cytokine gene expression was seen. However, most of the isolates replicated in Calu-3 cells, and they readily induced type I and type III IFN gene expression. The hCoV-19/Finland/FIN-25/2020 isolate, originating from a traveler from Milan in March 2020, showed better ability to replicate and induce IFN and inflammatory responses in Calu-3 cells than other isolates of SARS-CoV-2. Our data increase the knowledge on the pathogenesis and antiviral mechanisms of SARS-CoV-2 infection in human cell systems.

IMPORTANCE With the rapid spread of the coronavirus disease 2019 (COVID-19) pandemic, information on the replication of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) and regulation of innate immunity in human immune cells and lung epithelial cells is needed. In the present study, we show that SARS-CoV-2 failed to productively infect human immune cells, but different isolates of SARS-CoV-2 showed differential ability to replicate and regulate innate interferon responses in human lung epithelial Calu-3 cells. These findings will open up the way for further studies on the mechanisms of pathogenesis of SARS-CoV-2 in human cells.

In vitro production of synthetic viral RNAs and their delivery into mammalian cells and the application of viral RNAs in the study of innate interferon responses

Mammalian cells express different types of RNA molecules that can be classified as protein coding RNAs (mRNA) and non-coding RNAs (ncRNAs) the latter of which have housekeeping and regulatory functions in cells. Cellular RNAs are not recognized by cellular pattern recognition receptors (PRRs) and innate immunity is not activated. RNA viruses encode and express RNA molecules that usually differ from cell-specific RNAs and they include for instance 5'capped and 5'mono- and triphosphorylated RNAs, small viral RNAs and viral RNA-protein complexes called vRNPs. These molecules are recognized by certain members of Toll-like receptor (TLR) and RIG-I-like receptor (RLR) families leading to activation of innate immune responses and the production of antiviral cytokines, such as type I and type III interferons (IFNs). Virus-specific ssRNA and dsRNA molecules that mimic the viral genomic RNAs or their replication intermediates can efficiently be produced by bacteriophage T7 DNA-dependent RNA polymerase and bacteriophage phi6 RNA-dependent RNA polymerase, respectively. These molecules can then be delivered into mammalian cells and the mechanisms of activation of innate immune responses can be studied. In addition, synthetic viral dsRNAs can be processed to small interfering RNAs (siRNAs) by a Dicer enzyme to produce a swarm of antiviral siRNAs. Here we describe the biology of RNAs, their in vitro production and delivery into mammalian cells as well as how these molecules can be used to inhibit virus replication and to study the mechanisms of activation of the innate immune system.

Cellular co-infection can modulate the efficiency of influenza A virus production and shape the interferon response

During viral infection, the numbers of virions infecting individual cells can vary significantly over time and space. The functional consequences of this variation in cellular multiplicity of infection (MOI) remain poorly understood. Here, we rigorously quantify the phenotypic consequences of cellular MOI during influenza A virus (IAV) infection over a single round of replication in terms of cell death rates, viral output kinetics, interferon and antiviral effector gene transcription, and superinfection potential. By statistically fitting mathematical models to our data, we precisely define specific functional forms that quantitatively describe the modulation of these phenotypes by MOI at the single cell level. To determine the generality of these functional forms, we compare two distinct cell lines (MDCK cells and A549 cells), both infected with the H1N1 strain A/Puerto Rico/8/1934 (PR8). We find that a model assuming that infected cell death rates are independent of cellular MOI best fits the experimental data in both cell lines. We further observe that a model in which the rate and efficiency of virus production increase with cellular co-infection best fits our observations in MDCK cells, but not in A549 cells. In A549 cells, we also find that induction of type III interferon, but not type I interferon, is highly dependent on cellular MOI, especially at early timepoints. This finding identifies a role for cellular co-infection in shaping the innate immune response to IAV infection. Finally, we show that higher cellular MOI is associated with more potent superinfection exclusion, thus limiting the total number of virions capable of infecting a cell. Overall, this study suggests that the extent of cellular co-infection by influenza viruses may be a critical determinant of both viral production kinetics and cellular infection outcomes in a host cell type-dependent manner.

During influenza A virus (IAV) infection, the number of virions to enter individual cells can be highly variable. Cellular co-infection appears to be common and plays an essential role in facilitating reassortment for IAV, yet little is known about how cellular co-infection influences infection outcomes at the cellular level. Here, we combine quantitative in vitro infection experiments with statistical model fitting to precisely define the phenotypic consequences of cellular co-infection in two cell lines. We reveal that cellular co-infection can increase and accelerate the efficiency of IAV production in a cell line-dependent fashion, identifying it as a potential determinant of viral replication kinetics. We also show that induction of type III, but not type I, interferon is highly dependent upon the number of virions that infect a given cell, implicating cellular co-infection as an important determinant of the host innate immune response to infection. Altogether, our findings show that cellular co-infection plays a crucial role in determining infection outcome. The integration of experimental and statistical modeling approaches detailed here represents a significant advance in the quantitative study of influenza virus infection and should aid ongoing efforts focused on the construction of mathematical models of IAV infection.

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.

Single cell heterogeneity in influenza A virus gene expression shapes the innate antiviral response to infection

Viral infection outcomes are governed by the complex and dynamic interplay between the infecting virus population and the host response. It is increasingly clear that both viral and host cell populations are highly heterogeneous, but little is known about how this heterogeneity influences infection dynamics or viral pathogenicity. To dissect the interactions between influenza A virus (IAV) and host cell heterogeneity, we examined the combined host and viral transcriptomes of thousands of individual cells, each infected with a single IAV virion. We observed complex patterns of viral gene expression and the existence of multiple distinct host transcriptional responses to infection at the single cell level. We show that human H1N1 and H3N2 strains differ significantly in patterns of both viral and host anti-viral gene transcriptional heterogeneity at the single cell level. Our analyses also reveal that semi-infectious particles that fail to express the viral NS can play a dominant role in triggering the innate anti-viral response to infection. Altogether, these data reveal how patterns of viral population heterogeneity can serve as a major determinant of antiviral gene activation.

Innate and adaptive immunity toward influenza B viruses

Despite annual vaccination, influenza B viruses (IBV) cause significant disease with substantial health and socio-economic impacts. Novel vaccination strategies inducing broadly protective and long-lasting immunity across IBV lineages are needed. However, as immune responses toward IBV are largely understudied, host–virus interactions and protective immune mechanisms need to be defined to rationally design such vaccines. Here, we summarize recent advances in our understanding of immunological mechanisms underpinning protection from IBV. We discuss how innate antiviral host factors inhibit IBV replication and the ways by which IBV escapes such restriction. We review the specificity of broadly cross-reactive antibodies and universal T cells, and the mechanisms by which they mediate protection. We highlight important knowledge gaps needing to be addressed to design improved IBV vaccines.

Severity and outcomes of influenza-related pneumonia in type A and B strains in China, 2013-2019

Background

Inconsistencies exist regarding the severity of illness caused by different influenza strains. The aim of this study was to compare the clinical outcomes of hospitalized adults and adolescents with influenza-related pneumonia (Flu-p) from type A and type B strains in China.

Methods

We retrospectively reviewed data from Flu-p patients in five hospitals in China from January 2013 to May 2019. Multivariate logistic and Cox regression models were used to assess the effects of influenza virus subtypes on clinical outcomes, and to explore the risk factors of 30-day mortality for Flu-p patients.

Results

In total, 963 laboratory-confirmed influenza A-related pneumonia (FluA-p) and 386 influenza B-related pneumonia (FluB-p) patients were included. Upon adjustment for confounders, multivariate logistic regression models showed that FluA-p was associated with an increased risk of invasive ventilation (adjusted odds ratio [aOR]: 3.824, 95% confidence interval [CI]: 2.279–6.414; P <  0.001), admittance to intensive care unit (aOR: 1.630, 95% CI: 1.074–2.473, P = 0.022) and 30-day mortality (aOR: 2.427, 95% CI: 1.568–3.756, P <  0.001) compared to FluB-p. Multivariate Cox regression models confirmed that influenza A virus infection (hazard ratio: 2.637, 95% CI: 1.134–6.131, P = 0.024) was an independent predictor for 30-day mortality in Flu-p patients.

Conclusions

The severity of illness and clinical outcomes of FluA-p patients are more severe than FluB-p. This highlights the importance of identifying the virus strain during the management of severe influenza.

Influenza virus NS1- C/EBPβ gene regulatory complex inhibits RIG-I transcription

Influenza virus non-structural protein 1 (NS1) counteracts host antiviral innate immune responses by inhibiting Retinoic acid inducible gene-I (RIG-I) activation. However, whether NS1 also specifically regulates RIG-I transcription is unknown. Here, we identify a CCAAT/Enhancer Binding Protein beta (C/EBPβ) binding site in the RIG-I promoter as a repressor element, and show that NS1 promotes C/EBPβ phosphorylation and its recruitment to the RIG-I promoter as a C/EBPβ/NS1 complex. C/EBPβ overexpression and siRNA knockdown in human lung epithelial cells resulted in suppression and activation of RIG-I expression respectively, implying a negative regulatory role of C/EBPβ. Further, C/EBPβ phosphorylation, its interaction with NS1 and occupancy at the RIG-I promoter was associated with RIG-I transcriptional inhibition. These findings provide an important insight into the molecular mechanism by which influenza NS1 commandeers RIG-I transcriptional regulation and suppresses host antiviral responses.

Next-Generation Sequencing Analysis of Cellular Response to Influenza B Virus Infection

Influenza B virus (IBV) is a respiratory pathogen that infects humans and causes seasonal influenza epidemics. However, cellular response to IBV infection in humans and mechanisms of host-mediated restriction of IBV replication are not thoroughly understood. In this study, we used next-generation sequencing (NGS) to perform transcriptome profiling of IBV-infected human lung epithelial A549 cells at 0, 6, 12, and 24 h post infection (hpi) and characterized the cellular gene expression dynamics. We observed that more than 4000 host genes were differentially regulated during the study period, which included up regulation of genes encoding proteins, having a role in the innate antiviral immune responses, immune activation, cellular metabolism, autophagy, and apoptosis, as well as down regulation of genes involved in mitosis and cell proliferation. Further analysis of RNA-Seq data coupled with RT-qPCR validation collectively showed that double-strand RNA recognition pathways, including retinoic acid-inducible gene I (RIG-I) and Toll-like receptor 3 (TLR3), were substantially activated following IBV infection. Taken together, these results provide important initial insights into the intimate interaction between IBV and lung epithelial cells, which can be further explored towards elucidation of the cellular mechanisms in restriction or elimination of IBV infections in humans.

Type I interferon antagonistic properties of influenza B virus polymerase proteins

The innate immune system, in particular the type I interferon (IFN) response, is a powerful defence against virus infections. In turn, many if not all viruses have evolved various means to circumvent, resist, or counteract this host response to ensure efficient replication and propagation. Influenza viruses are no exception to this rule, and several viral proteins have been described to possess IFN-antagonistic functions. Although the viral nonstructural protein 1 appears to be a major antagonist in influenza A and B viruses (IAV and IBV), we have previously shown that a specific motif in the IAV polymerase proteins exerts an IFN-suppressive function very early in infection. The question remained whether a similar function would also exist in IBV polymerases. Here, we show that indeed a specific amino acid position (A523) of the PB1 protein in the IBV polymerase complex confers IFN-antagonistic properties. Mutation of this position leads to enhanced activation of the IFN-mediated signalling pathway after infection and subsequent reduction of virus titres. This indicates that inhibition of innate immune responses is a conserved activity shared by polymerase proteins of IAV and IBV.

The Development and Use of Reporter Influenza B Viruses

Influenza B viruses (IBVs) are major contributors to total human influenza disease, responsible for ~1/3 of all infections. These viruses, however, are relatively less studied than the related influenza A viruses (IAVs). While it has historically been assumed that the viral biology and mechanisms of pathogenesis for all influenza viruses were highly similar, studies have shown that IBVs possess unique characteristics. Relative to IAV, IBV encodes distinct viral proteins, displays a different mutational rate, has unique patterns of tropism, and elicits different immune responses. More work is therefore required to define the mechanisms of IBV pathogenesis. One valuable approach to characterize mechanisms of microbial disease is the use of genetically modified pathogens that harbor exogenous reporter genes. Over the last few years, IBV reporter viruses have been developed and used to provide new insights into the host response to infection, viral spread, and the testing of antiviral therapeutics. In this review, we will highlight the history and study of IBVs with particular emphasis on the use of genetically modified viruses and discuss some remaining gaps in knowledge that can be addressed using reporter expressing IBVs.

Cytoplasm and Beyond: Dynamic Innate Immune Sensing of Influenza A Virus by RIG-I

Innate immune sensing of influenza A virus (IAV) requires retinoic acid-inducible gene I (RIG-I), a fundamental cytoplasmic RNA sensor. How RIG-I's cytoplasmic localization reconciles with the nuclear replication nature of IAV is poorly understood. Recent findings provide advanced insights into the spatiotemporal RIG-I sensing of IAV and highlight the contribution of various RNA ligands to RIG-I activation. Understanding a compartment-specific RIG-I-sensing paradigm would facilitate the identification of the full spectrum of physiological RIG-I ligands produced during IAV infection.

Inhibition of Ongoing Influenza A Virus Replication Reveals Different Mechanisms of RIG-I Activation

Pattern recognition receptors provide essential nonself immune surveillance within distinct cellular compartments. Retinoic acid-inducible gene I (RIG-I) is one of the primary cytosolic RNA sensors, with an emerging role in the nucleus. It is involved in the spatiotemporal sensing of influenza A virus (IAV) replication, leading to the induction of type I interferons (IFNs). Nonetheless, the physiological viral ligands activating RIG-I during IAV infection remain underexplored. Other than full-length viral genomes, cellular constraints that impede ongoing viral replication likely potentiate an erroneous viral polymerase generating aberrant viral RNA species with RIG-I-activating potential. Here, we investigate the origins of RIG-I-activating viral RNA under two such constraints. Using chemical inhibitors that inhibit continuous viral protein synthesis, we identify the incoming, but not de novo-synthesized, viral defective interfering (DI) genomes contributing to RIG-I activation. In comparison, deprivation of viral nucleoprotein (NP), the key RNA chain elongation factor for the viral polymerase, leads to the production of aberrant viral RNA species activating RIG-I; however, their nature is likely to be distinct from that of DI RNA. Moreover, RIG-I activation in response to NP deprivation is not adversely affected by expression of the nuclear export protein (NEP), which diminishes the generation of a major subset of aberrant viral RNA but facilitates the accumulation of small viral RNA (svRNA). Overall, our results indicate the existence of fundamentally different mechanisms of RIG-I activation under cellular constraints that impede ongoing IAV replication.IMPORTANCE The induction of an IFN response by IAV is mainly mediated by the RNA sensor RIG-I. The physiological RIG-I ligands produced during IAV infection are not fully elucidated. Cellular constraints leading to the inhibition of ongoing viral replication likely potentiate an erroneous viral polymerase producing aberrant viral RNA species activating RIG-I. Here, we demonstrate that RIG-I activation during chemical inhibition of continuous viral protein synthesis is attributable to the incoming DI genomes. Erroneous viral replication driven by NP deprivation promotes the generation of RIG-I-activating aberrant viral RNA, but their nature is likely to be distinct from that of DI RNA. Our results thus reveal distinct mechanisms of RIG-I activation by IAV under cellular constraints impeding ongoing viral replication. A better understanding of RIG-I sensing of IAV infection provides insight into the development of novel interventions to combat influenza virus infection.

Evaluation of nasal levels of interferon and clinical severity of influenza in children

BACKGROUND

Experimental data show that type I interferon has a key role in innate immune response against influenza infection.

OBJECTIVE

We compared nasal levels of interferon-α2 and β among inpatients and outpatients with influenza.

STUDY DESIGN

Children younger than 5 years of age with influenza-like illness seeking care at the emergency department within the first 72 h of disease onset were prospectively included. Clinical and demographic data and secretions through nasal wash were obtained. Influenza infection was assessed through reverse-transcription polymerase chain reaction and nasal levels of interferon-α2 and β were measured by enzyme-linked immunosorbent assay. All patients followed until the end of the disease.

RESULTS

One hundred patients were included, of which 24 had confirmed influenza infection, and 5 of them were hospitalized. Subtypes A (H3N2) and B were confirmed in 10 and 14 patients, respectively. Seventy-six patients without influenza, including 48% of outpatients, were recruited as controls. All hospitalized patients were significantly younger regardless of influenza status (age <6 months in 59% vs. 23.2%, p < 0.001). All other data were similar among the groups. Comparing median levels of interferon-α2 among children with influenza, levels were significantly higher in outpatients than in hospitalized patients and were 263.2 pg/mL (25-75 interquartile range: 58.3-634) and detectable in only one patient (90 pg/mL), respectively. The levels of interferon-α2 in controls and those of interferon-β in all groups were not detected.

CONCLUSIONS

Higher levels of interferon-α2 in patients with less severe influenza reinforce experimental evidence about the protective role of interferon-α2 against influenza infection.

Comparative Outcomes of Adults Hospitalized With Seasonal Influenza A or B Virus Infection: Application of the 7-Category Ordinal Scale

Background

The objective of this study was to investigate the difference in disease severity between influenza A and B among hospitalized adults using a novel ordinal scale and existing clinical outcome end points.

Methods

A prospective, observational study was conducted over the 2016-2018 influenza seasons in a central hospital. The primary outcome was the rate of clinical improvement, defined as a decline of 2 categories from admission on a 7-category ordinal scale that ranges from 1 (discharged with normal activity) to 7 (death), or hospital discharge up to day 28.

Results

In total, 574 eligible patients were enrolled, including 369 (64.3%) influenza A cases and 205 (35.7%) influenza B cases. The proportion of patients with a worse ordinal scale at admission was higher in influenza A than influenza B (P = .0005). Clinical improvement up to 28 days occurred in 82.4% of patients with influenza A and 90.7% of patients with influenza B (P = .0067). The Cox model indicated that influenza B patients had a higher clinical improvement probability than influenza A cases (adjusted hazard ratio [HR], 1.266; 95% confidence interval [CI], 1.019-1.573; P = .0335). A similar pattern was observed in weaning oxygen supplement (adjusted HR, 1.285; 95% CI, 1.030-1.603; P = .0261). In-hospital mortality for influenza A was marginally higher than influenza B (11.4% vs 6.8%; P = .0782).

Conclusions

Our findings indicated that hospitalized patients with influenza A were more ill and had delayed clinical improvement compared with those with influenza B virus infection.

Non-lytic clearance of influenza B virus from infected cells preserves epithelial barrier function

Influenza B virus (IBV) is an acute, respiratory RNA virus that has been assumed to induce the eventual death of all infected cells. We and others have shown however, that infection with apparently cytopathic viruses does not necessarily lead to cell death; some cells can intrinsically clear the virus and persist in the host long-term. To determine if any cells can survive direct IBV infection, we here generate a recombinant IBV capable of activating a host-cell reporter to permanently label all infected cells. Using this system, we demonstrate that IBV infection leads to the formation of a survivor cell population in the proximal airways that are ciliated-like, but transcriptionally and phenotypically distinct from both actively infected and bystander ciliated cells. We also show that survivor cells are critical to maintain respiratory barrier function. These results highlight a host response pathway that preserves the epithelium to limit the severity of IBV disease.

Infection of a cell with influenza B virus (IBV) often results in cell death and the role of surviving cells in pathogenesis is unclear. Here, Dumm et al. generate a recombinant IBV that activates a host-cell reporter to permanently label infected cells, and show that surviving cells are important to preserve epithelial barrier function.

Human Blood and Tonsil Plasmacytoid Dendritic Cells Display Similar Gene Expression Profiles but Exhibit Differential Type I IFN Responses to Influenza A Virus Infection

Influenza A virus (IAV) infection constitutes an annual health burden across the globe. Plasmacytoid dendritic cells (PDCs) are central in antiviral defense because of their superior capacity to produce type I IFNs in response to viruses. Dendritic cells (DCs) differ depending on their anatomical location. However, only limited host-pathogen data are available from the initial site of infection in humans. In this study, we investigated how human tonsil PDCs, likely exposed to virus because of their location, responded to IAV infection compared with peripheral blood PDCs. In tonsils, unlike in blood, PDCs are the most frequent DC subset. Both tonsil and blood PDCs expressed several genes necessary for pathogen recognition and immune response, generally in a similar pattern. MxA, a protein that renders cells resistant to IAV infection, was detected in both tonsil and blood PDCs. However, despite steady-state MxA expression and contrary to previous reports, at high IAV concentrations (typically cytopathic to other immune cells), both tonsil and blood PDCs supported IAV infection. IAV exposure resulted in PDC maturation by upregulation of CD86 expression and IFN-α secretion. Interestingly, blood PDCs secreted 10-fold more IFN-α in response to IAV compared with tonsil PDCs. Tonsil PDCs also had a dampened cytokine response to purified TLR ligands compared with blood PDCs. Our findings suggest that tonsil PDCs may be less responsive to IAV than blood PDCs, highlighting the importance of studying immune cells at their proposed site of function.

Efficient Inhibition of Avian and Seasonal Influenza A Viruses by a Virus-Specific Dicer-Substrate Small Interfering RNA Swarm in Human Monocyte-Derived Macrophages and Dendritic Cells

Influenza A viruses (IAVs) are viral pathogens that cause epidemics and occasional pandemics of significant mortality. The generation of efficacious vaccines and antiviral drugs remains a challenge due to the rapid appearance of new influenza virus types and antigenic variants. Consequently, novel strategies for the prevention and treatment of IAV infections are needed, given the limitations of the presently available antivirals. Here, we used enzymatically produced IAV-specific double-stranded RNA (dsRNA) molecules and Giardia intestinalis Dicer for the generation of a swarm of small interfering RNA (siRNA) molecules. The siRNAs target multiple conserved genomic regions of the IAVs. In mammalian cells, the produced 25- to 27-nucleotide-long siRNA molecules are processed by endogenous Dicer into 21-nucleotide siRNAs and are thus designated Dicer-substrate siRNAs (DsiRNAs). We evaluated the efficacy of the above DsiRNA swarm at preventing IAV infections in human primary monocyte-derived macrophages and dendritic cells. The replication of different IAV strains, including avian influenza H5N1 and H7N9 viruses, was significantly inhibited by pretransfection of the cells with the IAV-specific DsiRNA swarm. Up to 7 orders of magnitude inhibition of viral RNA expression was observed, which led to a dramatic inhibition of IAV protein synthesis and virus production. The IAV-specific DsiRNA swarm inhibited virus replication directly through the RNA interference pathway although a weak induction of innate interferon responses was detected. Our results provide direct evidence for the feasibility of the siRNA strategy and the potency of DsiRNA swarms in the prevention and treatment of influenza, including the highly pathogenic avian influenza viruses.IMPORTANCE In spite of the enormous amount of research, influenza virus is still one of the major challenges for medical virology due to its capacity to generate new variants, which potentially lead to severe epidemics and pandemics. We demonstrated here that a swarm of small interfering RNA (siRNA) molecules, including more than 100 different antiviral RNA molecules targeting the most conserved regions of the influenza A virus genome, could efficiently inhibit the replication of all tested avian and seasonal influenza A variants in human primary monocyte-derived macrophages and dendritic cells. The wide antiviral spectrum makes the virus-specific siRNA swarm a potentially efficient treatment modality against both avian and seasonal influenza viruses.

Nuclear-resident RIG-I senses viral replication inducing antiviral immunity

The nucleus represents a cellular compartment where the discrimination of self from non-self nucleic acids is vital. While emerging evidence establishes a nuclear non-self DNA sensing paradigm, the nuclear sensing of non-self RNA, such as that from nuclear-replicating RNA viruses, remains unexplored. Here, we report the identification of nuclear-resident RIG-I actively involved in nuclear viral RNA sensing. The nuclear RIG-I, along with its cytoplasmic counterpart, senses influenza A virus (IAV) nuclear replication leading to a cooperative induction of type I interferon response. Its activation signals through the canonical signaling axis and establishes an effective antiviral state restricting IAV replication. The exclusive signaling specificity conferred by nuclear RIG-I is reinforced by its inability to sense cytoplasmic-replicating Sendai virus and appreciable sensing of hepatitis B virus pregenomic RNA in the nucleus. These results refine the RNA sensing paradigm for nuclear-replicating viruses and reveal a previously unrecognized subcellular milieu for RIG-I-like receptor sensing.

Schlafen 14 (SLFN14) is a novel antiviral factor involved in the control of viral replication

Schlafen (SLFN) proteins have been suggested to play important functions in cell proliferation and immune cell development. In this study, we determined the antiviral activities of putative RNA-helicase domain-containing SLFN14. Murine SLFN14 expression was specifically induced by TLR3-mediated pathways and type I interferon (IFN) in RAW264.7 mouse macrophages. To examine the role of SLFN during viral infection, cells were infected with either wild-type PR8 or delNS1/PR8 virus. SLFN14 expression was specifically induced following influenza virus infection. Overexpression of SLFN14 in A549 cells reduced viral replication, whereas knockdown of SLFN14 in RAW264.7 cells enhanced viral titers. Furthermore, SLFN14 promoted the delay in viral NP translocation from cytoplasm to nucleus and enhanced RIG-I-mediated IFN-β signaling. In addition, SLFN14 overexpression promoted antiviral activity against varicella zoster virus (VZV), a DNA virus. In conclusion, our data suggest that SLFN14 is a novel antiviral factor for both DNA and RNA viruses.

Robust Lys63-Linked Ubiquitination of RIG-I Promotes Cytokine Eruption in Early Influenza B Virus Infection

Influenza A and B virus infections both cause a host innate immunity response. Here, we report that the robust production of type I and III interferons (IFNs), IFN-stimulated genes, and proinflammatory factors can be induced by influenza B virus rather than influenza A virus infection in alveolar epithelial (A549) cells during early infection. This response is mainly dependent on the retinoic acid-inducible gene I (RIG-I)-mediated signaling pathway. Infection by influenza B virus promotes intense Lys63-linked ubiquitination of RIG-I, resulting in cytokine eruption. It is known that the influenza A virus NS1 protein (NS1-A) interacts with RIG-I and TRIM25 to suppress the activation of RIG-I-mediated signaling. However, the present results indicate that the influenza B virus NS1 protein (NS1-B) is unable to interact with RIG-I but engages in the formation of a RIG-I/TRIM25/NS1-B ternary complex. Furthermore, we demonstrate that the N-terminal RNA-binding domain (RBD) of NS1-B is responsible for interaction with TRIM25 and that this interaction blocks the inhibitory effect of the NS1-B C-terminal effector domain (TED) on RIG-I ubiquitination. Our findings reveal a novel mechanism for the host cytokine response to influenza B virus infection through regulatory interplay between host and viral proteins.

IMPORTANCE

Influenza B virus generally causes local mild epidemics but is occasionally lethal to individuals. Existing studies describe the broad characteristics of influenza B virus epidemiology and pathology. However, to develop better prevention and treatments for the disease, determining the concrete molecular mechanisms of pathogenesis becomes pivotal to understand how the host reacts to the challenge of influenza B virus. Thus, we aimed to characterize the host innate immune response to influenza B virus infection. Here, we show that vigorous Lys63-linked ubiquitination of RIG-I and cytokine eruption dependent on RIG-I-mediated signal transduction are induced by virus infection. Additionally, TRIM25 positively regulates RIG-I-mediated signaling by ablating the inhibitory function of NS1-B on RIG-I ubiquitination.

Knowns and unknowns of influenza B viruses

Influenza B viruses (IBVs) circulate annually along with influenza A (IAV) strains during seasonal epidemics. IBV can dominate influenza seasons and cause severe disease, particularly in children and adolescents. Research has revealed interesting aspects of IBV and highlighted the importance of these viruses in clinical settings. Yet, many important questions remain unanswered. In this review, the clinical relevance of IBV is emphasized, unique features in epidemiology, host range and virology are highlighted and gaps in knowledge pinpointed. Multiple aspects of IBV epidemiology, evolution, virology and immunology are discussed. Future research into IBV is needed to understand how we can prevent severe disease in high-risk groups, especially children and elderly.

RIG-I Signaling Is Essential for Influenza B Virus-Induced Rapid Interferon Gene Expression

Influenza B virus causes annual epidemics and, along with influenza A virus, accounts for substantial disease and economic burden throughout the world. Influenza B virus infects only humans and some marine mammals and is not responsible for pandemics, possibly due to a very low frequency of reassortment and a lower evolutionary rate than that of influenza A virus. Influenza B virus has been less studied than influenza A virus, and thus, a comparison of influenza A and B virus infection mechanisms may provide new insight into virus-host interactions. Here we analyzed the early events in influenza B virus infection and interferon (IFN) gene expression in human monocyte-derived macrophages and dendritic cells. We show that influenza B virus induces IFN regulatory factor 3 (IRF3) activation and IFN-λ1 gene expression with faster kinetics than does influenza A virus, without a requirement for viral protein synthesis or replication. Influenza B virus-induced activation of IRF3 required the fusion of viral and endosomal membranes, and nuclear accumulation of IRF3 and viral NP occurred concurrently. In comparison, immediate early IRF3 activation was not observed in influenza A virus-infected macrophages. Experiments with RIG-I-, MDA5-, and RIG-I/MDA5-deficient mouse fibroblasts showed that RIG-I is the critical pattern recognition receptor needed for the influenza B virus-induced activation of IRF3. Our results show that innate immune mechanisms are activated immediately after influenza B virus entry through the endocytic pathway, whereas influenza A virus avoids early IRF3 activation and IFN gene induction.

IMPORTANCE

Recently, a great deal of interest has been paid to identifying the ligands for RIG-I under conditions of natural infection, as many previous studies have been based on transfection of cells with different types of viral or synthetic RNA structures. We shed light on this question by analyzing the earliest step in innate immune recognition of influenza B virus by human macrophages. We show that influenza B virus induces IRF3 activation, leading to IFN gene expression after viral RNPs (vRNPs) are released into the cytosol and are recognized by RIG-I receptor, meaning that the incoming influenza B virus is already able to activate IFN gene expression. In contrast, influenza A (H3N2) virus failed to activate IRF3 at very early times of infection, suggesting that there are differences in innate immune recognition between influenza A and B viruses.

Influenza virus activation of the interferon system

The host interferon (IFN) response represents one of the first barriers that influenza viruses must surmount in order to establish an infection. Many advances have been made in recent years in understanding the interactions between influenza viruses and the interferon system. In this review, we summarise recent work regarding activation of the type I IFN response by influenza viruses, including attempts to identify the viral RNA responsible for IFN induction, the stage of the virus life cycle at which it is generated and the role of defective viruses in this process.

Influenza B viruses: not to be discounted

In contrast to influenza A viruses, which have been investigated extensively, influenza B viruses have attracted relatively little attention. However, influenza B viruses are an important cause of morbidity and mortality in the human population and full understanding of their biological and epidemiological properties is imperative to better control this important pathogen. However, some of its characteristics are still elusive and warrant investigation. Here, we review evolution, epidemiology, pathogenesis and immunity and identify gaps in our knowledge of influenza B viruses. The divergence of two antigenically distinct influenza B viruses is highlighted. The co-circulation of viruses of these two lineages necessitated the development of quadrivalent influenza vaccines, which is discussed in addition to possibilities to develop universal vaccination strategies.

Influenza virus adaptation PB2-627K modulates nucleocapsid inhibition by the pathogen sensor RIG-I

The cytoplasmic RNA helicase RIG-I mediates innate sensing of RNA viruses. The genomes of influenza A virus (FLUAV) are encapsidated by the nucleoprotein and associated with RNA polymerase, posing potential barriers to RIG-I sensing. We show that RIG-I recognizes the 5'-triphosphorylated dsRNA on FLUAV nucleocapsids but that polymorphisms at position 627 of the viral polymerase subunit PB2 modulate RIG-I sensing. Compared to mammalian-adapted PB2-627K, avian FLUAV nucleocapsids possessing PB2-627E are prone to increased RIG-I recognition, and RIG-I-deficiency partially restores PB2-627E virus infection of mammalian cells. Heightened RIG-I sensing of PB2-627E nucleocapsids correlates with previously established lower affinity of 627E-containing PB2 for nucleoprotein and is increased by further nucleocapsid instability. The effect of RIG-I on PB2-627E nucleocapsids is independent of antiviral signaling, suggesting that RIG-I-nucleocapsid binding alone can inhibit infection. These results indicate that RIG-I is a direct avian FLUAV restriction factor and highlight nucleocapsid disruption as an antiviral strategy.

In vitro anti-influenza A activity of interferon (IFN)-λ1 combined with IFN-β or oseltamivir carboxylate

Influenza viruses, which can cross species barriers and adapt to new hosts, pose a constant potential threat to human health. The influenza pandemic of 2009 highlighted the rapidity with which an influenza virus can spread worldwide. Currently available antivirals have a number of limitations against influenza, and novel antiviral strategies, including novel drugs and drug combinations, are urgently needed. Here, we evaluated the in vitro effects of interferon (IFN)-β, IFN-λ1, oseltamivir carboxylate (a neuraminidase (NA) inhibitor), and combinations of these agents against two seasonal (i.e., H1N1 and H3N2) influenza A viruses. We observed that A/California/04/09 (H1N1) and A/Panama/2007/99 (H3N2) isolates were equally sensitive to the antiviral activity of IFN-β and oseltamivir carboxylate in A549 and Calu-3 cells. In contrast, IFN-λ1 exhibited substantially lower protective potential against the H1N1 strain (64-1030-fold ↓, P<0.05), and was ineffective against H3N2 virus in both cell lines. Three dimensional analysis of drug-drug interactions revealed that IFN-λ1 interacted with IFN-β and oseltamivir carboxylate in an additive or synergistic manner, respectively, to inhibit influenza A virus replication in human airway epithelial cells. Overall, the present study demonstrated that anti-influenza agents with different mechanisms of action (e.g., a NA inhibitor combined with IFN-λ1) exerted a significantly greater (P<0.05) synergistic effect compared to co-treatment with drugs that target the same signaling pathway (i.e., IFN-β plus IFN-λ1) in vitro. Our findings provide support for the combined use of interferon plus oseltamivir as a potential means for treating influenza infections.

RIG-I-like receptors and negative-strand RNA viruses: RLRly bird catches some worms

Negative strand RNA viruses with a nonsegmented genome (ns-NSVs) or a segmented genome (s-NSVs) are an important source of human and animal diseases. Survival of the host from those infections is critically dependent on rapidly reacting innate immune responses. Two cytoplasmic RNA helicases, RIG-I and MDA5 (collectively termed RIG-I-like receptors, RLRs), are essential for recognizing virus-specific RNA structures to initiate a signalling cascade, resulting in the production of the antiviral type I interferons. Here, we will review the current knowledge and views on RLR agonists, RLR signalling, and the wide variety of countermeasures ns-NSVs and s-NSVs have evolved. Specific aspects include the consequences of genome segmentation for RLR activation and a discussion on the physiological ligands of RLRs.

Novel avian influenza A (H7N9) virus induces impaired interferon responses in human dendritic cells

In March 2013 a new avian influenza A(H7N9) virus emerged in China and infected humans with a case fatality rate of over 30%. Like the highly pathogenic H5N1 virus, H7N9 virus is causing severe respiratory distress syndrome in most patients. Based on genetic analysis this avian influenza A virus shows to some extent adaptation to mammalian host. In the present study, we analyzed the activation of innate immune responses by this novel H7N9 influenza A virus and compared these responses to those induced by the avian H5N1 and seasonal H3N2 viruses in human monocyte-derived dendritic cells (moDCs). We observed that in H7N9 virus-infected cells, interferon (IFN) responses were weak although the virus replicated as well as the H5N1 and H3N2 viruses in moDCs. H7N9 virus-induced expression of pro-inflammatory cytokines remained at a significantly lower level as compared to H5N1 virus-induced “cytokine storm” seen in human moDCs. However, the H7N9 virus was extremely sensitive to the antiviral effects of IFN-α and IFN-β in pretreated cells. Our data indicates that different highly pathogenic avian viruses may show considerable differences in their ability to induce host antiviral responses in human primary cell models such as moDCs. The unexpected appearance of the novel H7N9 virus clearly emphasizes the importance of the global influenza surveillance system. It is, however, equally important to systematically characterize in normal human cells the replication capacity of the new viruses and their ability to induce and respond to natural antiviral substances such as IFNs.

Mechanisms underlying the inhibition of interferon signaling by viruses

A hallmark of the antiviral response is the induction of interferons. First discovered in 1957 by Issac and Lindeman, interferons are noted for their ability to interfere with viral replication. Interferons act via autocrine and paracrine pathways to induce an antiviral state in infected cells and in neighboring cells containing interferon receptors. Interferons are the frontline defenders against viral infection and their primary function is to locally restrict viral propagation. Viruses have evolved mechanisms to escape the host interferon response, thus gaining a replicative advantage in host cells. This review will discuss recent findings on the mechanisms viruses use to evade the host interferon response. This knowledge is important because the treatment of viral infections is a challenge of global proportions and a better understanding of the mechanisms viruses use to persist in the host may uncover valuable insights applicable to the discovery of novel drug targets.

PLC-γ1 signaling plays a subtype-specific role in postbinding cell entry of influenza A virus

Host signaling pathways and cellular proteins play important roles in the influenza viral life cycle and can serve as antiviral targets. In this study, we report the engagement of host phosphoinositide-specific phospholipase γ1 (PLC-γ1) in mediating cell entry of influenza virus H1N1 but not H3N2 subtype. Both PLC-γ1-specific inhibitor and short hairpin RNA (shRNA) strongly suppress the replication of H1N1 but not H3N2 viruses in cell culture, suggesting that PLC-γ1 plays an important subtype-specific role in the influenza viral life cycle. Further analyses demonstrate that PLC-γ1 activation is required for viral postbinding cell entry. In addition, H1N1, but not H3N2, infection leads to the phosphorylation of PLC-γ1 at Ser 1248 immediately after infection and independent of viral replication. We have further shown that H1N1-induced PLC-γ1 activation is downstream of epidermal growth factor receptor (EGFR) signaling. Interestingly, both H1N1 and H3N2 infections activate EGFR, but only H1N1 infection leads to PLC-γ1 activation. Taking our findings together, we have identified for the first time the subtype-specific interplay of host PLC-γ1 signaling and H1N1 virus that is critical for viral uptake early in the infection. Our study provides novel insights into how virus interacts with the cellular signaling network by demonstrating that viral determinants can regulate how the host signaling pathways function in virally infected cells.

Influenza A induced cellular signal transduction pathways

Influenza A is a negative sense single stranded RNA virus that belongs to the Orthomyxoviridae Family. This enveloped virus contains 8 segments of viral RNA which encodes 11 viral proteins. Influenza A infects humans and is the causative agent of the flu. Annually it infects approximately 5% to 15% of the population world wide and results in an estimated 250,000 to 500,000 deaths a year. The nature of influenza A replication results in a high mutation rate which results in the need for seasonal vaccinations. In addition the zoonotic nature of the influenza virus allows for recombination of viral segments from different strains creating new variants that have not been encountered before. This type of mutation is the method by which pandemic strains of the flu arises. Infection with influenza results in a respiratory illness that for most individuals is self limiting. However in susceptible populations which include individuals with pre-existing pulmonary or cardiac conditions, the very young and the elderly fatal complications may arise. The most serious of these is the development of viral pneumonia which may be accompanied by secondary bacterial infections. Progression of pneumonia leads to the development of acute respiratory distress syndrome (ARDS), acute lung injury (ALI) and potentially respiratory failure. This progression is a combined effect of the host immune system response to influenza infection and the viral infection itself. This review will focus on molecular aspects of viral replication in alveolar cells and their response to infection. The response of select innate immune cells and their contribution to viral clearance and lung epithelial damage will also be discussed. Molecular aspects of antiviral response in the cells in particular the protein kinase RNA dependent response, and the oligoadenylate synthetase RNAse L system in relation to influenza infection.

A comparison of the clinical and epidemiological characteristics of adult patients with laboratory-confirmed influenza A or B during the 2011-2012 influenza season in Korea: a multi-center study

BACKGROUND

During the 2011/2012 winter influenza season in the Republic of Korea, influenza A (H3N2) was the predominant virus in the first peak period of influenza activity during the second half of January 2012. On the other hand, influenza B was the predominant virus in the second peak period of influenza activity during the second half of March 2012. The objectives of this study were to compare the clinical and epidemiological characteristics of patients with laboratory-confirmed influenza A or influenza B.

METHODOLOGY/PRINCIPAL FINDINGS

We analyzed data from 2,129 adult patients with influenza-like illnesses who visited the emergency rooms of seven university hospitals in Korea from October 2011 to May 2012. Of 850 patients with laboratory-confirmed influenza, 656 (77.2%) had influenza A (H3N2), and 194 (22.8%) influenza B. Age, and the frequencies of cardiovascular disorders, diabetes, hypertension were significantly higher in patients with influenza A (H3N2) (P<0.05). The frequencies of leukopenia or thrombocytopenia in patients with influenza B at initial presentation were statistically higher than those in patients with influenza A (H3N2) (P<0.05). The rate of hospitalization, and length of hospital stay were statistically higher in patients with influenza A (H3N2) (P<0.05), and of the 79 hospitalized patients, the frequency of diabetes, hypertension, cases having at least one of the comorbid conditions, and the proportion of elderly were significantly higher in patients with influenza A (H3N2) (P<0.05).

CONCLUSIONS

The proportion of males to females and elderly population were significantly higher for influenza A (H3N2) patients group compared with influenza B group. Hypertension, diabetes, chronic lung diseases, cardiovascular disorders, and neuromuscular diseases were independently associated with hospitalization due to influenza. Physicians should assess and treat the underlying comorbid conditions as well as influenza viral infections for the appropriate management of patients with influenza.