Pandemic H1N1 influenza virus causes a stronger inflammatory response than seasonal H1N1 influenza virus in ferrets

The Intersection of Age and Influenza Severity: Utility of Ferrets for Dissecting the Age-Dependent Immune Responses and Relevance to Age-Specific Vaccine Development

Many factors impact the host response to influenza virus infection and vaccination. Ferrets have been an indispensable reagent for influenza virus research for almost one hundred years. One of the most significant and well-known factors affecting human disease after infection is host age. Another significant factor is the virus, as strain-specific disease severity is well known. Studying age-related impacts on viral infection and vaccination outcomes requires an animal model that reflects both the physiological and immunological changes that occur with human aging, and sensitivity to differentially virulent influenza viruses. The ferret is uniquely susceptible to a plethora of influenza viruses impacting humans and has proven extremely useful in studying the clinical and immunological pictures of influenza virus infection. Moreover, ferrets developmentally have several of the age-related physiological changes that occur in humans throughout infancy, adulthood, old age, and pregnancy. In this review, we discuss ferret susceptibility to influenza viruses, summarize previous influenza studies using ferrets as models of age, and finally, highlight the application of ferret age models in the pursuit of prophylactic and therapeutic agents to address age-related influenza disease severity.

What Does Tolerance Mean for Animal Disease Dynamics When Pathology Enhances Transmission?

Host competence, or how well an individual transmits pathogens, varies substantially within and among animal populations. As this variation can alter the course of epidemics and epizootics, revealing its underlying causes will help predict and control the spread of disease. One host trait that could drive heterogeneity in competence is host tolerance, which minimizes fitness losses during infection without decreasing pathogen load. In many cases, tolerance should increase competence by extending infectious periods and enabling behaviors that facilitate contact among hosts. However, we argue that the links between tolerance and competence are more varied. Specifically, the different physiological and behavioral mechanisms by which hosts achieve tolerance should have a range of effects on competence, enhancing the ability to transmit pathogens in some circumstances and impeding it in others. Because tissue-based pathology (damage) that reduces host fitness is often critical for pathogen transmission, we focus on two mechanisms that can underlie tolerance at the tissue level: damage-avoidance and damage-repair. As damage-avoidance reduces transmission-enhancing pathology, this mechanism is likely to decrease host competence and pathogen transmission. In contrast, damage-repair does not prevent transmission-relevant pathology from occurring. Rather, damage-repair provides new, healthy tissues that pathogens can exploit, likely extending the infectious period and increasing host competence. We explore these concepts through graphical models and present three disease systems in which damage-avoidance and damage-repair alter host competence in the predicted directions. Finally, we suggest that by incorporating these links, future theoretical studies could provide new insights into infectious disease dynamics and host-pathogen coevolution.

A Guide for the Use of the Ferret Model for Influenza Virus Infection

As influenza viruses continue to jump species barriers to cause human infection, assessments of disease severity and viral replication kinetics in vivo provide crucial information for public health professionals. The ferret model is a valuable resource for evaluating influenza virus pathogenicity; thus, understanding the most effective techniques for sample collection and usage, as well as the full spectrum of attainable data after experimental inoculation in this species, is paramount. This is especially true for scheduled necropsy of virus-infected ferrets, a standard component in evaluation of influenza virus pathogenicity, as necropsy findings can provide important information regarding disease severity and pathogenicity that is not otherwise available from the live animal. In this review, we describe the range of influenza viruses assessed in ferrets, the measures of experimental disease severity in this model, and optimal sample collection during necropsy of virus-infected ferrets. Collectively, this information is critical for assessing systemic involvement after influenza virus infection in mammals.

Orally Efficacious Broad-Spectrum Ribonucleoside Analog Inhibitor of Influenza and Respiratory Syncytial Viruses

Morbidity and mortality resulting from influenza-like disease are a threat, especially for older adults. To improve case management, next-generation broad-spectrum antiviral therapeutics that are efficacious against major drivers of influenza-like disease, including influenza viruses and respiratory syncytial virus (RSV), are urgently needed. Using a dual-pathogen high-throughput screening protocol for influenza A virus (IAV) and RSV inhibitors, we have identified N⁴-hydroxycytidine (NHC) as a potent inhibitor of RSV, influenza B viruses, and IAVs of human, avian, and swine origins. Biochemical in vitro polymerase assays and viral RNA sequencing revealed that the ribonucleotide analog is incorporated into nascent viral RNAs in place of cytidine, increasing the frequency of viral mutagenesis. Viral passaging in cell culture in the presence of an inhibitor did not induce robust resistance. Pharmacokinetic profiling demonstrated dose-dependent oral bioavailability of 36 to 56%, sustained levels of the active 5'-triphosphate anabolite in primary human airway cells and mouse lung tissue, and good tolerability after extended dosing at 800 mg/kg of body weight/day. The compound was orally efficacious against RSV and both seasonal and highly pathogenic avian IAVs in mouse models, reducing lung virus loads and alleviating disease biomarkers. Oral dosing reduced IAV burdens in a guinea pig transmission model and suppressed virus spread to uninfected contact animals through direct transmission. Based on its broad-spectrum efficacy and pharmacokinetic properties, NHC is a promising candidate for future clinical development as a treatment option for influenza-like diseases.

Gas chromatography-mass spectrometry based plasma metabolomics of H1N1-induced inflammation in mice and intervention with Flos Lonicerae Japonica-Fructus Forsythiae herb pair

Flos Lonicerae Japonica-Fructus Forsythiae herb pair (Yin-Qiao in Chinese, YQ), is used clinically for the treatment of viral pneumonia due to its heat-clearing and detoxifying functions. In the present work, the effect of YQ in H1N1-induced inflammation in mice was investigated by metabolomics based on GC-MS. Body weight and histological results were used to assess the lung injury, while the levels of IL-6 and TNF-α in plasma were used to evaluate the extent of inflammation. The acquired GC-MS data were further subjected to multivariate data analysis, and the significantly altered metabolites identified. After statistical and pathway analysis, 17 significantly altered metabolites and 3 possible metabolic pathways were found in plasma between normal and H1N1-induced pneumonia mice, while 17 significant differential metabolites were identified when YQ treatment group was compared with model group. This work indicates that oral administration of YQ could protect mice from H1N1-induced inflammation partially by ameliorating the associated metabolic disturbances.

Contribution of the Purinergic Receptor P2X7 to Development of Lung Immunopathology during Influenza Virus Infection

An exacerbated immune response is one of the main causes of influenza-induced lung damage during infection. The molecular mechanisms regulating the fate of the initial immune response to infection, either as a protective response or as detrimental immunopathology, are not well understood. The purinergic receptor P2X7 is an ionotropic nucleotide-gated ion channel receptor expressed on immune cells that has been implicated in induction and maintenance of excessive inflammation. Here, we analyze the role of this receptor in a mouse model of influenza virus infection using a receptor knockout (KO) mouse strain. Our results demonstrate that the absence of the P2X7 receptor results in a better outcome to influenza virus infection characterized by reduced weight loss and increased survival upon experimental influenza challenge compared to wild-type mice. This effect was not virus strain specific. Overall lung pathology and apoptosis were reduced in virus-infected KO mice. Production of proinflammatory cytokines and chemokines such as interleukin-10 (IL-10), gamma interferon (IFN-γ), and CC chemokine ligand 2 (CCL2) was also reduced in the lungs of the infected KO mice. Infiltration of neutrophils and depletion of CD11b+ macrophages, characteristic of severe influenza virus infection in mice, were lower in the KO animals. Together, these results demonstrate that activation of the P2X7 receptor is involved in the exacerbated immune response observed during influenza virus infection.IMPORTANCE A hallmark of influenza virus infection is the development of lung pathology induced by an exacerbated immune response. The mechanisms shared by the antiviral host defense required for viral clearance and those required for development of immunopathology are not clearly understood. Purinergic receptors, and in particular the purinergic receptor P2X7 (P2X7r), are involved in activation of the immune response. We used mice lacking the P2X7r (P2X7r KO mice) to better understand the mechanisms that lead to development of lung pathology during influenza virus infection. In our studies, we observed that P2X7r KO mice developed less lung immunopathology and had better survival than the wild-type mice. These results implicate P2X7r in the induction of an exacerbated local immune response to influenza virus and help us to better understand the mechanisms leading to the lung immunopathology observed during severe viral infections.

Neutralizing inhibitors in the airways of naïve ferrets do not play a major role in modulating the virulence of H3 subtype influenza A viruses

Many insights regarding the pathogenesis of human influenza A virus (IAV) infections have come from studies in mice and ferrets. Surfactant protein (SP)-D is the major neutralizing inhibitor of IAV in mouse airway fluids and SP-D-resistant IAV mutants show enhanced virus replication and virulence in mice. Herein, we demonstrate that sialylated glycoproteins, rather than SP-D, represent the major neutralizing inhibitors against H3 subtype viruses in airway fluids from naïve ferrets. Moreover, while resistance to neutralizing inhibitors is a critical factor in modulating virus replication and disease in the mouse model, it does not appear to be so in the ferret model, as H3 mutants resistant to either SP-D or sialylated glycoproteins in ferret airway fluids did not show enhanced virulence in ferrets. These data have important implications for our understanding of pathogenesis and immunity to human IAV infections in these two widely used animal models of infection.

The Effect of Oseltamivir on the Disease Progression of Lethal Influenza A Virus Infection: Plasma Cytokine and miRNA Responses in a Mouse Model

Lethal influenza A virus infection leads to acute lung injury and possibly lethal complications. There has been a continuous effort to identify the possible predictors of disease severity. Unlike earlier studies, where biomarkers were analyzed on certain time points or days after infection, in this study biomarkers were evaluated over the entire course of infection. Circulating proinflammatory cytokines and/or miRNAs that track with the onset and progression of lethal A/Puerto Rico/8/34 (PR8) influenza A virus infection and their response to oseltamivir treatment were investigated up to 10 days after infection. Changes in plasma cytokines (IL-1β, IL-10, IL-12p70, IL-6, KC, TNF-α, and IFN-γ) and several candidate miRNAs were profiled. Among the cytokines analyzed, IL-6 and KC/GRO cytokines appeared to correlate with peak viral titer. Over the selected 48 miRNAs profiled, certain miRNAs were up- or downregulated in a manner that was dependent on the oseltamivir treatment and disease severity. Our findings suggest that IL-6 and KC/GRO cytokines can be a potential disease severity biomarker and/or marker for the progression/remission of infection. Further studies to explore other cytokines, miRNAs, and lung injury proteins in serum with different subtypes of influenza A viruses with varying disease severity may provide new insight into other unique biomarkers.

Inactivated Antigen of the H7N9 Influenza Virus Protects Mice from Its Lethal Infection

The H7N9 influenza virus emerged in February 2013 in China, and underlies over 20% of human mortality in the country. Many efforts are being made to develop an effective vaccine against this highly pathogenic virus. We made H7N9 vaccine virus with six internal genes of A/PR/8/34 (H1N1) and two surface genes of hemagglutinin and neuraminidase from A/Anhui/1/2013 (H7N9) by reverse genetics, and the H7N9 vaccine antigens were produced in eggs. Protective antibodies were induced in mice immunized with a single dose (7.5 μg) of the H7N9 antigen. These mice survived lethal infection by the H7N9 virus, although few viruses were found in their lung tissues. However, mice administered with two doses of the H7N9 antigen survived without any viral antigen being detected in their lung tissues. Furthermore, the IgG antibody subtypes were also pronounced in lung tissues of the immunized mice. Therefore, our results suggest that the inactivated whole antigen of the H7N9 influenza virus might protect animals and humans from its lethal infection.

Influenza A (H1N1)

In March 2009, an outbreak of human-infected swine influenza occurred in Mexico, with reports of deaths. Soon afterward, such cases were reported worldwide, namely, a pandemic outbreak. The prevailing disease is then defined as an acute respiratory infectious disease caused by swine-origin influenza A (H1N1) virus, a new variant of influenza virus. Its viral strains contain gene segments of three types of influenza viruses, namely swine influenza virus, avian influenza virus and human influenza virus. And the virus is mainly transmitted among humans via direct and indirect contact or respiratory tract.

Characterization of the Localized Immune Response in the Respiratory Tract of Ferrets following Infection with Influenza A and B Viruses

The burden of infection with seasonal influenza viruses is significant. Each year is typically characterized by the dominance of one (sub)type or lineage of influenza A or B virus, respectively. The incidence of disease varies annually, and while this may be attributed to a particular virus strain or subtype, the impacts of prior immunity, population differences, and variations in clinical assessment are also important. To improve our understanding of the impacts of seasonal influenza viruses, we directly compared clinical symptoms, virus shedding, and expression of cytokines, chemokines, and immune mediators in the upper respiratory tract (URT) of ferrets infected with contemporary A(H1N1)pdm09, A(H3N2), or influenza B virus. Gene expression in the lower respiratory tract (LRT) was also assessed. Clinical symptoms were minimal. Overall cytokine/chemokine profiles in the URT were consistent in pattern and magnitude between animals infected with influenza A and B viruses, and peak expression levels of interleukin-1α (IL-1α), IL-1β, IL-6, IL-12p40, alpha interferon (IFN-α), IFN-β, and tumor necrosis factor alpha (TNF-α) mRNAs correlated with peak levels of viral shedding. MCP1 and IFN-γ were expressed after the virus peak. Granzymes A and B and IL-10 reached peak expression as the virus was cleared and seroconversion was detected. Cytokine/chemokine gene expression in the LRT following A(H1N1)pdm09 virus infection reflected the observations seen for the URT but was delayed 2 or 3 days, as was virus replication. These data indicate that disease severities and localized immune responses following infection with seasonal influenza A and B viruses are similar, suggesting that other factors are likely to modulate the incidence and impact of seasonal influenza.

IMPORTANCE

Both influenza A and B viruses cocirculate in the human population, and annual influenza seasons are typically dominated by an influenza A virus subtype or an influenza B virus lineage. Surveillance data indicate that the burden of disease is higher in some seasons, yet it is unclear whether this is due to specific virus strains or to other factors, such as cross-reactive immunity or clinical definitions of influenza. We directly compared disease severities and localized inflammatory responses to different seasonal influenza virus strains, including the 2009 pandemic strain, in healthy naive ferrets. We found that the disease severities and the cytokine and chemokine responses were similar irrespective of the seasonal strain or the location of the infection in the respiratory tract. This suggests that factors other than the immune response to a particular virus (sub)type contribute to the variable impact of influenza virus infection in a population.

Infection with a Mouse-Adapted Strain of the 2009 Pandemic Virus Causes a Highly Severe Disease Associated with an Impaired T Cell Response

Despite a relatively low fatality rate, the 2009 H1N1 pandemic virus differed from other seasonal viruses in that it caused mortality and severe pneumonia in the young and middle-aged population (18–59 years old). The mechanisms underlying this increased disease severity are still poorly understood. In this study, a human isolate of the 2009 H1N1 pandemic virus was adapted to the mouse (MAp2009). The pathogenicity of the MAp2009 virus and the host immune responses were evaluated in the mouse model and compared to the laboratory H1N1 strain A/Puerto Rico/8/1934 (PR8). The MAp2009 virus reached consistently higher titers in the lungs over 14 days compared to the PR8 virus, and caused severe disease associated with high morbidity and 85% mortality rate, contrasting with the 0% death rate in the PR8 group. During the early phase of infection, both viruses induced similar pathology in the lungs. However, MAp2009-induced lung inflammation was sustained until the end of the study (day 14), while there was no sign of inflammation in the PR8-infected group by day 10. Furthermore, at day 3 post-infection, MAp2009 induced up to 10- to 40-fold more cytokine and chemokine gene expression, respectively. More importantly, the numbers of CD4⁺ T cells and virus-specific CD8⁺ T cells were significantly lower in the lungs of MAp2009-infected mice compared to PR8-infected mice. Interestingly, there was no difference in the number of dendritic cells in the lung and in the draining lymph node. Moreover, mice infected with PR8 or MAp2009 had similar numbers of CCR5 and CXCR3-expressing T cells, suggesting that the impaired T cell response was not due to a lack of chemokine responsiveness or priming of T cells. This study demonstrates that a mouse-adapted virus from an isolate of the 2009 pandemic virus interferes with the adaptive immune response leading to a more severe disease.

Ferret models of viral pathogenesis

Emerging and well-known viral diseases remain one the most important global public health threats. A better understanding of their pathogenesis and mechanisms of transmission requires animal models that accurately reproduce these aspects of the disease. Here we review the role of ferrets as an animal model for the pathogenesis of different respiratory viruses with an emphasis on influenza and paramyxoviruses. We will describe the anatomic and physiologic characteristics that contribute to the natural susceptibility of ferrets to these viruses, and provide an overview of the approaches available to analyze their immune responses. Recent insights gained using this model will be highlighted, including the development of new prophylactic and therapeutic approaches. To provide decision criteria for the use of this animal model, its strengths and limitations will be discussed.

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Ferrets as models for respiratory virus pathogenesis.

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Ferrets as models for vaccine and drug efficacy assessment.

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Immunological tools for ferrets.

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Housing and handling of ferrets.

Animal models for influenza viruses: implications for universal vaccine development

Influenza virus infections are a significant cause of morbidity and mortality in the human population. Depending on the virulence of the influenza virus strain, as well as the immunological status of the infected individual, the severity of the respiratory disease may range from sub-clinical or mild symptoms to severe pneumonia that can sometimes lead to death. Vaccines remain the primary public health measure in reducing the influenza burden. Though the first influenza vaccine preparation was licensed more than 60 years ago, current research efforts seek to develop novel vaccination strategies with improved immunogenicity, effectiveness, and breadth of protection. Animal models of influenza have been essential in facilitating studies aimed at understanding viral factors that affect pathogenesis and contribute to disease or transmission. Among others, mice, ferrets, pigs, and nonhuman primates have been used to study influenza virus infection in vivo, as well as to do pre-clinical testing of novel vaccine approaches. Here we discuss and compare the unique advantages and limitations of each model.

Protective efficacy of a single dose of baculovirus hemagglutinin-based vaccine in chickens and ducks against homologous and heterologous H5N1 virus infections

Outbreaks of the highly pathogenic H5N1 virus in poultry and humans are ongoing. Vaccination is an efficient method for prevention of H5N1 infection. Using chickens and ducks, we assessed the efficacy of a vaccine comprising H5N1 hemagglutinin (HA) protein produced in a baculovirus expression system. The immunized chickens and ducks were protected against lethal infection by H5N1 in an antigen dose-dependent manner. Complete protection against homologous challenge and partial protection against heterologous challenge were achieved in chickens immunized with 5 μg HA protein and in ducks immunized with 10 μg HA protein. The IgG antibody subtype was mainly detected in the sera and tissues, including the lungs. The neuraminidase (NA) inhibition assay was negative in immunized chickens and ducks. Our results indicated that the expressed HA protein by baculovirus was immunogenic to both chickens and ducks, and the immunized chickens and ducks were protected from the lethal infections of highly pathogenic H5N1 influenza virus, though ducks required more HA protein than chickens to be protected. Also, baculovirus HA-vaccinated poultry can be differentiated from infected poultry by NA inhibition assay.

TaqMan real time RT-PCR assays for detecting ferret innate and adaptive immune responses

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The ferret model is used to study human disease and physiology.

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TaqMan realtime RT-PCR assays for ferret cytokine and chemokine mRNA were developed.

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Cytokine and chemokine patterns in ferret cells were similar to other mammals.

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A comprehensive panel of mRNAs can be measured in samples of limited quantity.

The ferret is an excellent model for many human infectious diseases including influenza, SARS-CoV, henipavirus and pneumococcal infections. The ferret is also used to study cystic fibrosis and various cancers, as well as reproductive biology and physiology. However, the range of reagents available to measure the ferret immune response is very limited. To address this deficiency, high-throughput real time RT-PCR TaqMan assays were developed to measure the expression of fifteen immune mediators associated with the innate and adaptive immune responses (IFNα, IFNβ, IFNγ, IL1α, IL1β, IL2, IL4, IL6, IL8, IL10, IL12p40, IL17, Granzyme A, MCP1, TNFα), as well as four endogenous housekeeping genes (ATF4, HPRT, GAPDH, L32). These assays have been optimized to maximize reaction efficiency, reduce the amount of sample required (down to 1 ng RNA per real time RT-PCR reaction) and to select the most appropriate housekeeping genes. Using these assays, the expression of each of the tested genes could be detected in ferret lymph node cells stimulated with mitogens or infected with influenza virus in vitro. These new tools will allow a more comprehensive analysis of the ferret immune responses following infection or in other disease states.

Animal models for influenza virus pathogenesis, transmission, and immunology

In humans, infection with an influenza A or B virus manifests typically as an acute and self-limited upper respiratory tract illness characterized by fever, cough, sore throat, and malaise. However, influenza can present along a broad spectrum of disease, ranging from sub-clinical or even asymptomatic infection to a severe primary viral pneumonia requiring advanced medical supportive care. Disease severity depends upon the virulence of the influenza virus strain and the immune competence and previous influenza exposures of the patient. Animal models are used in influenza research not only to elucidate the viral and host factors that affect influenza disease outcomes in and spread among susceptible hosts, but also to evaluate interventions designed to prevent or reduce influenza morbidity and mortality in man. This review will focus on the three animal models currently used most frequently in influenza virus research - mice, ferrets, and guinea pigs - and discuss the advantages and disadvantages of each.

Influenza virus-induced lung inflammation was modulated by cigarette smoke exposure in mice

Although smokers have increased susceptibility and severity of seasonal influenza virus infection, there is no report about the risk of 2009 pandemic H1N1 (pdmH1N1) or avian H9N2 (H9N2/G1) virus infection in smokers. In our study, we used mouse model to investigate the effect of cigarette smoke on pdmH1N1 or H9N2 virus infection. Mice were exposed to cigarette smoke for 21 days and then infected with pdmH1N1 or H9N2 virus. Control mice were exposed to air in parallel. We found that cigarette smoke exposure alone significantly upregulated the lung inflammation. Such prior cigarette smoke exposure significantly reduced the disease severity of subsequent pdmH1N1 or H9N2 virus infection. For pdmH1N1 infection, cigarette smoke exposed mice had significantly lower mortality than the control mice, possibly due to the significantly decreased production of inflammatory cytokines and chemokines. Similarly, after H9N2 infection, cigarette smoke exposed mice displayed significantly less weight loss, which might be attributed to lower cytokines and chemokines production, less macrophages, neutrophils, CD4⁺ and CD8⁺ T cells infiltration and reduced lung damage compared to the control mice. To further investigate the underlying mechanism, we used nicotine to mimic the effect of cigarette smoke both in vitro and in vivo. Pre-treating the primary human macrophages with nicotine for 72 h significantly decreased their expression of cytokines and chemokines after pdmH1N1 or H9N2 infection. The mice subcutaneously and continuously treated with nicotine displayed significantly less weight loss and lower inflammatory response than the control mice upon pdmH1N1 or H9N2 infection. Moreover, α7 nicotinic acetylcholine receptor knockout mice had more body weight loss than wild-type mice after cigarette smoke exposure and H9N2 infection. Our study provided the first evidence that the pathogenicity of both pdmH1N1 and H9N2 viruses was alleviated in cigarette smoke exposed mice, which might partially be attributed to the immunosuppressive effect of nicotine.

Red Ginseng-containing diet helps to protect mice and ferrets from the lethal infection by highly pathogenic H5N1 influenza virus

The highly pathogenic (HP) H5N1 influenza virus is endemic in many countries and has a great potential for a pandemic in humans. The immune-enhancing prowess of ginseng has been known for millennia. We aimed to study whether mice and ferrets fed with Red Ginseng could be better protected from the lethal infections of HP H5N1 influenza virus than the infected unfed mice and ferrets. We fed mice and ferrets with Red Ginseng prior to when they were infected with HP H5N1 influenza virus. The mice and ferrets fed with a 60-day diet containing Red Ginseng could be protected from lethal infections by HP H5N1 influenza virus (survival rate of up to 45% and 40%, respectively). Interferon-α and -γ antiviral cytokines were significantly induced in the lungs of mice fed Red Ginseng, compared to mice fed an unsupplemented diet. These data suggest that the diet with the immune-enhancing Red Ginseng could help humans to overcome the infections by HP H5N1 influenza virus.

Severe pathogenesis of influenza B virus in pregnant mice

The study on pathogenesis of influenza B virus during pregnancy is limited. Here, we showed using a mouse model that influenza B virus could cause severe disease including death during pregnancy. Infected pregnant mice resulted in 40% mortality, but infected age-matched non-pregnant mice did not show any death. Infected pregnant mice contained high viral loads in lungs with the elevated inductions of inflammatory cytokines and chemokines than infected non-pregnant mice. Infected pregnant mice delivered lower number of neonates than uninfected pregnant mice, suggesting adverse effects of influenza B virus on fetuses. Progesterone which is important for maintaining pregnancy was reduced in uteruses of infected pregnant mice than in those of uninfected pregnant mice. Taken together, our results suggest that influenza B virus can cause severe disease during pregnancy, and that preventive measures including vaccination may be important for protecting women during pregnancy.

Immune System Cells in Healthy Ferrets

The ferret has emerged as an excellent animal model to characterize several physiologic and pathologic conditions. The distribution and characterization of different types of immune system cells were studied in healthy ferret tissues. Eight primary antibodies were tested for immunohistochemistry in formalin-fixed tissues: anti-CD3, anti-CD79α, anti-CD20, anti-HLA-DR, anti-lysozyme, anti-CD163, anti-SWC3, and anti-Mac387. The anti-CD3 antibody labeled T cells mainly in interfollicular and paracortical areas of lymph nodes, cortex and thymic medulla, and periarteriolar lymphoid sheaths in the spleen. The anti-CD79α and anti-CD20 antibodies immunolabeled B cells located in lymphoid follicles at lymph nodes, spleen, and Peyer patches. The CD79α and CD20 antibodies also labeled cells with nonlymphoid morphology in atypical B-cell locations. The anti-HLA-DR antibody labeled macrophages, some populations of B and T lymphocytes, and different populations of dendritic cells in lymph nodes, Peyer patches, spleen, and thymus. The anti-lysozyme antibody immunolabeled macrophages in the liver, lymph nodes, spleen, and thymus. The Mac-387, CD163, and SWC3 antibodies did not show any positive reaction in formalin-fixed or frozen tissues. To elucidate the origin of the uncommon CD79α/CD20 positive cells, a double immunohistochemistry was carried out using the anti-HLA-DR + the anti-CD79α, the anti-HLA-DR + the anti-CD20, and the anti-lysozyme + the anti-CD79α antibodies. Double labeling was mainly observed when the anti-HLA-DR + the anti-CD79α antibodies were combined. The immunohistologic characterization and distribution of these immune system cells in healthy ferret tissues should be of value in future comparative studies of diseases in ferrets.

The severe pathogenicity of alveolar macrophage-depleted ferrets infected with 2009 pandemic H1N1 influenza virus

The in vivo role of alveolar macrophages in the infections with 2009 pandemic H1N1 influenza virus is not as yet known. Ferret study shows that alveolar macrophages are critical for lowering the risk of severe outcomes in 2009 pandemic H1N1 influenza virus infections. Up to 40% of the infected ferrets depleted of alveolar macrophages died, with elevated body temperature and major loss of body weight in contrast to infected ferrets not depleted of alveolar macrophages. The higher viral titers in the lungs were detected in infected ferrets depleted of alveolar macrophages than infected ferrets not depleted of alveolar macrophages 5 days after infection. The inflammatory chemokines were induced at greater levels in the lungs of infected ferrets depleted of alveolar macrophages than in those of infected ferrets not depleted of alveolar macrophages. Our study implies that alveolar macrophages are important for controlling the infections of 2009 pandemic H1N1 influenza virus.

The association between serum biomarkers and disease outcome in influenza A(H1N1)pdm09 virus infection: results of two international observational cohort studies

BACKGROUND

Prospective studies establishing the temporal relationship between the degree of inflammation and human influenza disease progression are scarce. To assess predictors of disease progression among patients with influenza A(H1N1)pdm09 infection, 25 inflammatory biomarkers measured at enrollment were analyzed in two international observational cohort studies.

METHODS

Among patients with RT-PCR-confirmed influenza A(H1N1)pdm09 virus infection, odds ratios (ORs) estimated by logistic regression were used to summarize the associations of biomarkers measured at enrollment with worsened disease outcome or death after 14 days of follow-up for those seeking outpatient care (FLU 002) or after 60 days for those hospitalized with influenza complications (FLU 003). Biomarkers that were significantly associated with progression in both studies (p<0.05) or only in one (p<0.002 after Bonferroni correction) were identified.

RESULTS

In FLU 002 28/528 (5.3%) outpatients had influenza A(H1N1)pdm09 virus infection that progressed to a study endpoint of complications, hospitalization or death, whereas in FLU 003 28/170 (16.5%) inpatients enrolled from the general ward and 21/39 (53.8%) inpatients enrolled directly from the ICU experienced disease progression. Higher levels of 12 of the 25 markers were significantly associated with subsequent disease progression. Of these, 7 markers (IL-6, CD163, IL-10, LBP, IL-2, MCP-1, and IP-10), all with ORs for the 3(rd) versus 1(st) tertile of 2.5 or greater, were significant (p<0.05) in both outpatients and inpatients. In contrast, five markers (sICAM-1, IL-8, TNF-α, D-dimer, and sVCAM-1), all with ORs for the 3(rd) versus 1(st) tertile greater than 3.2, were significantly (p≤.002) associated with disease progression among hospitalized patients only.

CONCLUSIONS

In patients presenting with varying severities of influenza A(H1N1)pdm09 virus infection, a baseline elevation in several biomarkers associated with inflammation, coagulation, or immune function strongly predicted a higher risk of disease progression. It is conceivable that interventions designed to abrogate these baseline elevations might affect disease outcome.

Kinetics of viral replication and induction of host responses in ferrets differs between ocular and intranasal routes of inoculation

While influenza viruses are typically considered respiratory pathogens, the ocular system represents a secondary entry point for virus to establish a productive respiratory infection and the location for rare instances of virus-induced conjunctivitis. We used the ferret model to conduct a side-by-side comparison of virus infectivity, kinetics of viral replication, and induction of host responses following inoculation by either the intranasal or ocular routes with two viruses, A/Netherlands/230/03 (H7N7) and A/Panama/2007/99 (H3N2). We show that ocular inoculation resulted in delayed virus replication and reduced levels of proinflammatory cytokine and chemokine transcript in respiratory tract but not ocular tissues compared with intranasally inoculated animals. We identified numerous proinflammatory mediators with known roles in ocular disease elicited in ferret eye tissue following influenza virus infection. These findings provide a greater understanding of the modulation of host responses following different inoculation routes and underscore the risk associated with ocular exposure to influenza viruses.

Functional characterization of ferret CCL20 and CCR6 and identification of chemotactic inhibitors

CCL20 is currently the only known chemokine ligand for the receptor CCR6, and is a mucosal chemokine involved in normal and pathological immune responses. Although nucleotide sequence data are available for ccl20 and ccr6 sequences from multiple species, the ferret ccl20 and ccr6 sequences have not been determined. To increase our understanding of immune function in ferret models of infection and vaccination, we have used RT-PCR to obtain the ferret ccl20 and ccr6 cDNA sequences and functionally characterize the encoded proteins. The open reading frames of both genes were highly conserved across species and mostly closely related to canine sequences. For functional analyses, single cell clones expressing ferret CCR6 were generated, a ferret CCL20/mouse IgG(2a) fusion protein (fCCL20-mIgG(2a)) was produced, and fCCL20 was chemically synthesized. Cell clones expressing ferret CCR6 responded chemotactically to fCCL20-mIgG2a fusion protein and synthetic ferret CCL20. Chemotaxis inhibition studies identified the polyphenol epigallocatechin-3-gallate and the murine γ-herpesvirus 68 M3 protein as inhibitors of fCCL20. Surface plasmon resonance studies revealed that EGCG bound directly to fCCL20. These results provide molecular characterization of previously unreported ferret immune gene sequences and for the first time identify a broad-spectrum small molecule inhibitor of CCL20 and reveal CCL20 as a target for the herpesviral M3 protein.

Influenza A(H1N1)pdm09 virus suppresses RIG-I initiated innate antiviral responses in the human lung

Influenza infection is a major cause of morbidity and mortality. Retinoic acid-inducible gene I (RIG-I) is believed to play an important role in the recognition of, and response to, influenza virus and other RNA viruses. Our study focuses on the hypothesis that pandemic H1N1/09 influenza virus alters the influenza-induced proinflammatory response and suppresses host antiviral activity. We first compared the innate response to a clinical isolate of influenza A(H1N1)pdm09 virus, OK/09, a clinical isolate of seasonal H3N2 virus, OK/06, and to a laboratory adapted seasonal H1N1 virus, PR8, using a unique human lung organ culture model. Exposure of human lung tissue to either pandemic or seasonal influenza virus resulted in infection and replication in alveolar epithelial cells. Pandemic virus induces a diminished RIG-I mRNA and antiviral cytokine response than seasonal virus in human lung. The suppression of antiviral response and RIG-I mRNA expression was confirmed at the protein level by ELISA and western blot. We performed a time course of RIG-I and interferon-β (IFN-β) mRNA induction by the two viruses. RIG-I and IFN-β induction by OK/09 was of lower amplitude and shorter duration than that caused by PR8. In contrast, the pandemic virus OK/09 caused similar induction of proinflammatory cytokines, IL-8 and IL-6, at both the transcriptional and translational level as PR8 in human lung. Differential antiviral responses did not appear to be due to a difference in cellular infectivity as immunohistochemistry showed that both viruses infected alveolar macrophages and epithelial cells. These findings show that influenza A(H1N1)pdm09 virus suppresses anti-viral immune responses in infected human lung through inhibition of viral-mediated induction of the pattern recognition receptor, RIG-I, though proinflammatory cytokine induction was unaltered. This immunosuppression of the host antiviral response by pandemic virus may have contributed to the more serious lung infections that occurred in the H1N1 pandemic of 2009.

Characterization of the 2009 pandemic A/Beijing/501/2009 H1N1 influenza strain in human airway epithelial cells and ferrets

Background

A novel 2009 swine-origin influenza A H1N1 virus (S-OIV H1N1) has been transmitted among humans worldwide. However, the pathogenesis of this virus in human airway epithelial cells and mammals is not well understood.

Methodology/Principal Finding

In this study, we showed that a 2009 A (H1N1) influenza virus strain, A/Beijing/501/2009, isolated from a human patient, caused typical influenza-like symptoms including weight loss, fluctuations in body temperature, and pulmonary pathological changes in ferrets. We demonstrated that the human lung adenocarcinoma epithelial cell line A549 was susceptible to infection and that the infected cells underwent apoptosis at 24 h post-infection. In contrast to the seasonal H1N1 influenza virus, the 2009 A (H1N1) influenza virus strain A/Beijing/501/2009 induced more cell death involving caspase-3-dependent apoptosis in A549 cells. Additionally, ferrets infected with the A/Beijing/501/2009 H1N1 virus strain exhibited increased body temperature, greater weight loss, and higher viral titers in the lungs. Therefore, the A/Beijing/501/2009 H1N1 isolate successfully infected the lungs of ferrets and caused more pathological lesions than the seasonal influenza virus. Our findings demonstrate that the difference in virulence of the 2009 pandemic H1N1 influenza virus and the seasonal H1N1 influenza virus in vitro and in vivo may have been mediated by different mechanisms.

Conclusion/Significance

Our understanding of the pathogenesis of the 2009 A (H1N1) influenza virus infection in both humans and animals is broadened by our findings that apoptotic cell death is involved in the cytopathic effect observed in vitro and that the pathological alterations in the lungs of S-OIV H1N1-infected ferrets are much more severe.

The 2009 pandemic H1N1 influenza virus is more pathogenic in pregnant mice than seasonal H1N1 influenza virus

Pregnant women can experience high mortality, high rates of abortion, and severe pneumonia when infected with pandemic influenza viruses. In this context, the severity of the 2009 pandemic H1N1 influenza virus compared with seasonal H1N1 influenza virus is not clear. Presently, in a mouse model of pregnancy, the 2009 pandemic H1N1 influenza virus killed up to 60% of pregnant mice and caused abortion in up to 40%, whereas a circulating seasonal H1N1 influenza virus did not cause any deaths or abortions. Higher viral titers and levels of inflammatory cytokines and chemokines such as interleukin (IL)-1α, IL-6, granulocyte colony-stimulating factor, RANTES, monocyte chemotactic protein, and KC (CXCL1), were detected in the lungs of pregnant mice infected with the 2009 pandemic H1N1 influenza virus, compared with the seasonal H1N1 influenza virus. The results of our study with pregnant mice suggest that the observed higher pathogenesis in pregnant women infected with the 2009 pandemic H1N1 influenza virus than the seasonal H1N1 influenza virus may be due to higher viral replication, elevated induction of inflammatory chemokines, and reduced progesterone.

The fate of influenza A virus after infection of human macrophages and dendritic cells

Airway macrophages (MΦ) and dendritic cells (DC) are important components of the innate host defence. Historically, these immune cells have been considered to play a critical role in controlling the severity of influenza A virus (IAV) infection by limiting virus release, initiating local inflammatory responses and by priming subsequent adaptive immune responses. However, some IAV strains have been reported to replicate productively in human immune cells. Potential amplification and dissemination of IAV from immune cells may therefore be an important virulence determinant. Herein, we will review findings in relation to the fate of IAV following infection of MΦ and DC. Insights regarding the consequences and outcomes of IAV infection of airway MΦ and DC are discussed in order to gain a better understanding of the pathogenesis of influenza virus.

Two years after pandemic influenza A/2009/H1N1: what have we learned?

The world had been anticipating another influenza pandemic since the last one in 1968. The pandemic influenza A H1N1 2009 virus (A/2009/H1N1) finally arrived, causing the first pandemic influenza of the new millennium, which has affected over 214 countries and caused over 18,449 deaths. Because of the persistent threat from the A/H5N1 virus since 1997 and the outbreak of the severe acute respiratory syndrome (SARS) coronavirus in 2003, medical and scientific communities have been more prepared in mindset and infrastructure. This preparedness has allowed for rapid and effective research on the epidemiological, clinical, pathological, immunological, virological, and other basic scientific aspects of the disease, with impacts on its control. A PubMed search using the keywords "pandemic influenza virus H1N1 2009" yielded over 2,500 publications, which markedly exceeded the number published on previous pandemics. Only representative works with relevance to clinical microbiology and infectious diseases are reviewed in this article. A significant increase in the understanding of this virus and the disease within such a short amount of time has allowed for the timely development of diagnostic tests, treatments, and preventive measures. These findings could prove useful for future randomized controlled clinical trials and the epidemiological control of future pandemics.

Influenza-induced innate immunity: regulators of viral replication, respiratory tract pathology & adaptive immunity

Influenza virus infections usually cause mild to moderately severe respiratory disease, however some infections, like those involving the avian H5N1 virus, can cause massive viral pneumonia, systemic disease and death. The innate immune response of respiratory tract resident cells is the first line of defense and limits virus replication. Enhanced cytokine and chemokine production following infection, however, appears to underlie much of the pathology that develops after infection with highly pathogenic strains. A so-called `cytokine storm' can damage the lung tissue and cause systemic disease, despite the control of viral replication. By summarizing current knowledge of the innate responses mounted to influenza infection, this review highlights the importance of the respiratory tract epithelial cells as regulators of innate and adaptive immunity to influenza virus.