Influenza A H5N1 replication sites in humans

Use of ex vivo and in vitro cultures of the human respiratory tract to study the tropism and host responses of highly pathogenic avian influenza A (H5N1) and other influenza viruses

The tropism of influenza viruses for the human respiratory tract is a key determinant of host-range, and consequently, of pathogenesis and transmission. Insights can be obtained from clinical and autopsy studies of human disease and relevant animal models. Ex vivo cultures of the human respiratory tract and in vitro cultures of primary human cells can provide complementary information provided they are physiologically comparable in relevant characteristics to human tissues in vivo, e.g. virus receptor distribution, state of differentiation. We review different experimental models for their physiological relevance and summarize available data using these cultures in relation to highly pathogenic avian influenza H5N1, in comparison where relevant, with other influenza viruses. Transformed continuous cell-lines often differ in important ways to the corresponding tissues in vivo. The state of differentiation of primary human cells (respiratory epithelium, macrophages) can markedly affect virus tropism and host responses. Ex vivo cultures of human respiratory tissues provide a close resemblance to tissues in vivo and may be used to risk assess animal viruses for pandemic threat. Physiological factors (age, inflammation) can markedly affect virus receptor expression and virus tropism. Taken together with data from clinical studies on infected humans and relevant animal models, data from ex vivo and in vitro cultures of human tissues and cells can provide insights into virus transmission and pathogenesis and may provide understanding that leads to novel therapeutic interventions.

A statistical strategy to identify recombinant viral ribonucleoprotein of avian, human, and swine influenza A viruses with elevated polymerase activity

Objectives

Reassortment of influenza A viruses can give rise to viral ribonucleoproteins (vRNPs) with elevated polymerase activity and the previous three pandemic influenza viruses contained reassorted vRNPs of different origins. These suggest that reassorted vRNP may be one of the factors leading to a pandemic virus. In this study, we reconstituted chimeric vRNPs with three different viral strains isolated from avian, human and swine hosts. We applied a statistical strategy to identify the effect that the origin of a single vRNP protein subunit or the interactions between these subunits on polymerase activity.

Design

Eighty one chimeric vRNPs were reconstituted in 293T cells at different temperatures. Polymerase activity was determined by luciferase reporter assay and the results were analysed by multiway anova and other statistical methods.

Results

It was found that PB2, PB1, NP, PB2‐PB1 interaction, PB2‐PA interaction and PB1‐NP interaction had significant effect on polymerase activity at 37°C and several single subunits and interactions were identified to lead to elevation of polymerase activity. Furthermore, we studied 27 out of these 81 different chimieric vRNPs in different combinations via fractional factorial design approach. Our results suggested that the approach can identify the major single subunit or interaction factors that affect the polymerase activity without the need to experimentally reproduce all possible vRNP combinations.

Conclusions

Statistical approach and fractional factorial design are useful to identify the major single subunit or interaction factors that can modulate viral polymerase activity.

The battle between influenza and the innate immune response in the human respiratory tract

Influenza is a viral infection of the respiratory tract. Infection is normally confined to the upper respiratory tract but certain viral strains have evolved the ability to infect the lower respiratory tract, including the alveoli, leading to inflammation and a disease pattern of diffuse alveolar damage. Factors leading to this sequence of events are novel influenza strains, or strains that have viral proteins, in particular the NS1 protein that allow it to escape the innate immune system. There are three main barriers that prevent infection of pneumocytes - mucin, host defence lectins and cells such as macrophages. Viruses have developed strategies such as neuraminidase and glycosylation patterns that allow this evasion. Though there has been much investment in antiviral drugs, it is proposed that more attention should be directed towards developing or utilizing compounds that enhance the ability of the innate immune system to combat viral infection.

Pathological study of archival lung tissues from five fatal cases of avian H5N1 influenza in Vietnam

Highly pathogenic avian H5N1 influenza virus (H5N1) infection in humans causes acute respiratory distress syndrome, leading to multiple organ failure. Five fatal cases of H5N1 infection in Vietnam were analyzed pathologically to reveal virus distribution, and local proinflammatory cytokine and chemokine expression profiles in formalin-fixed, paraffin-embedded lung tissues. Our main histopathological findings showed diffuse alveolar damage in the lungs. The infiltration of myeloperoxidase-positive and/or CD68 (clone KP-1)-positive neutrophils and monocytes/macrophages was remarkable in the alveolar septa and alveolar spaces. Immunohistochemistry revealed that H5N1 mainly infected alveolar epithelial cells and monocytes/macrophages in lungs. H5N1 replication was confirmed by detecting H5N1 mRNA in epithelial cells using in situ hybridization. Quantitation of H5N1 RNA using quantitative reverse transcription PCR assays revealed that the level of H5N1 RNA was increased in cases during early phases of the disease. We quantified the expression of tumor necrosis factor-alpha (TNF-α), interleukin (IL)-6, IL-8, regulated on activation normal T-cell expressed and secreted (commonly known as RANTES), and interferon-gamma-inducible protein of 10 kDa (IP-10) in formalin-fixed, paraffin-embedded lung sections. Their expression levels correlated with H5N1 RNA copy numbers detected in the same lung region. Double immunofluorescence staining revealed that TNF-α, IL-6, IL-8 and IP-10 were expressed in epithelial cells and/or monocytes/macrophages. In particular, IL-6 was also expressed in endothelial cells. The dissemination of H5N1 beyond respiratory organs was not confirmed in two cases examined in this study.

Oxygen free radical involvement in acute lung injury induced by H5N1 virus in mice

Background

Acute lung injury is an important cause of death in humans infected with H5N1. It has been found that oxygen free radicals (OFRs) are elevated in lung tissue during influenza virus infections. In this study, we used a mouse model to explore the role of OFRs in acute lung injury caused by H5N1 viral infection.

Methods

Four‐ to six‐week‐old male specific pathogen‐free BALB/c mice were inoculated intranasally with 10⁵ 50% tissue culture infective doses (TCID₅₀) of highly pathogenic A/Chicken/Hebei/108/2002 (H5N1) viruses and were then given 1000 IU of lauric acid modified superoxide dismutase (LA‐SOD) by intraperitoneal injection, starting 2 days post‐infection and continuing for 6 days.

Results

The extent of lung injury and the concentration of OFRs were higher, and the SOD activity was lower in H5N1 virus‐infected mice than that in uninfected control mice on days 3, 6, and 7 post‐inoculation. Weak amelioration of clinical signs, a minor decrease in the total mortality and the extent of lung injury, and the lower OFRs concentration were seen in the LA‐SOD treatment group, but a reduction in lung virus titers was not observed in the LA‐SOD treatment at all time points.

Conclusions

The LA‐SOD treatment has a mild inhibitory effect on H5N1 influenza virus infection in mice. OFRs, therefore, might play an important role in the pathogenesis of acute lung injury induced by H5N1 virus.

In situ molecular identification of the influenza A (H1N1) 2009 Neuraminidase in patients with severe and fatal infections during a pandemic in Mexico City

Background

In April 2009, public health surveillance detected an increased number of influenza-like illnesses in Mexico City’s hospitals. The etiological agent was subsequently determined to be a spread of a worldwide novel influenza A (H1N1) triple reassortant. The purpose of the present study was to demonstrate that molecular detection of pandemic influenza A (H1N1) 2009 strains is possible in archival material such as paraffin-embedded lung samples.

Methods

In order to detect A (H1N1) virus sequences in archived biological samples, eight paraffin-embedded lung samples from patients who died of pneumonia and respiratory failure were tested for influenza A (H1N1) Neuraminidase (NA) RNA using in situ RT-PCR.

Results

We detected NA transcripts in 100% of the previously diagnosed A (H1N1)-positive samples as a cytoplasmic signal. No expression was detected by in situ RT-PCR in two Influenza-like Illness A (H1N1)-negative patients using standard protocols nor in a non-related cervical cell line. In situ relative transcription levels correlated with those obtained when in vitro RT-PCR assays were performed. Partial sequences of the NA gene from A (H1N1)-positive patients were obtained by the in situ RT-PCR-sequencing method. Sequence analysis showed 98% similarity with influenza viruses reported previously in other places.

Conclusions

We have successfully amplified specific influenza A (H1N1) NA sequences using stored clinical material; results suggest that this strategy could be useful when clinical RNA samples are quantity limited, or when poor quality is obtained. Here, we provide a very sensitive method that specifically detects the neuraminidase viral RNA in lung samples from patients who died from pneumonia caused by Influenza A (H1N1) outbreak in Mexico City.

Cross talk between animal and human influenza viruses

Although outbreaks of highly pathogenic avian influenza in wild and domestic birds have been posing the threat of a new influenza pandemic for the past decade, the first pandemic of the twenty-first century came from swine viruses. This fact emphasizes the complexity of influenza viral ecology and the difficulty of predicting influenza viral dynamics. Complete control of influenza viruses seems impossible. However, we must minimize the impact of animal and human influenza outbreaks by learning lessons from past experiences and recognizing the current status. Here, we review the most recent influenza virology data in the veterinary field, including aspects of zoonotic agents and recent studies that assess the pandemic potential of H5N1 highly pathogenic avian influenza viruses.

Multiple routes of invasion of wild-type Clade 1 highly pathogenic avian influenza H5N1 virus into the central nervous system (CNS) after intranasal exposure in ferrets

Human infections with highly pathogenic avian influenza (HPAI) H5N1 have been associated with central nervous system involvement. The purpose of this study was to examine the route of invasion of wild-type HPAI H5N1 virus into the central nervous system (CNS) using a ferret model of infection. Sixteen ferrets were exposed by the intranasal route to 10(6) TCID(50) of A/Vietnam/1203/04, a Clade 1 strain originally isolated from a fatal human case. The ferrets were euthanased for histological and virological analysis at intervals after challenge at 1, 3, 5, 6 and 7 days post-inoculation (dpi). From 5 dpi encephalitis was seen in all examined ferrets. The detection of antigen in the olfactory epithelium, the olfactory bulb, and related nuclei, in that temporal sequence, supported the contention that this is a major infection route for this virus strain. The detection of antigen in the epithelial cells in the Eustachian tube on 1 dpi, followed by the cochlea and vestibulocochlear nerve on 5 dpi is consistent with a second anterograde route of invasion, namely the vestibulocochlear pathway. There was also antigen in the lining of the ventricles and central canal indicating spread via the cerebrospinal fluid. However, evidence for haematogenous dissemination in the form of antigen in the brain parenchyma surrounding blood vessels was not found. This study provides support to the contention that wild-type HPAI H5N1 virus strains may enter the CNS via cranial nerve pathways and that the ferret is an appropriate model to study preventive and therapeutic procedures involving neural infection with these viruses by this route.

Emergence of HA mutants during influenza virus pneumonia

During the influenza pandemic of 2009, the number of viral pneumonia cases showed a marked increase in comparison with seasonal influenza viruses. Mutations at amino acid 222 (D222G mutations) in the virus hemagglutinin (HA) molecule, known to alter the receptor-recognition properties of the virus, were detected in a number of the more severely-affected patients in the early phases of the pandemic. To understand the background for the emergence of the mutant amino acid D222G in human lungs, we conducted histological examinations on lung specimens of patients from Mexico who had succumbed in the pandemic. Prominent regenerative and hyperplastic changes in the alveolar type II pneumocytes, which express avian-type sialoglycan receptors in the respiratory tract of severely affected individuals, were observed in the Mexican patients. An infection model utilizing guinea pigs, which was chosen in order to best simulate the sialic acid distribution of severe pneumonia in human patients, demonstrated an increase of D222G mutants and a delay in the diminution of mutants in the lower respiratory tract in comparison to the upper respiratory tract. Our data suggests that the predominance of avian-type sialoglycan receptors in the pneumonic lungs may contribute to the emergence of viral HA mutants. This data comprehensively illustrates the mechanisms for the emergence of mutants in the clinical samples.

Zoonotic diseases and human health: The human influenza example

Over the past few decades a large number of new and emerging infectious diseases have been recognised in humans, partly because of improved diagnostic technologies and increased awareness and also, partly because of dynamic ecological changes between human hosts and their exposure to animals and the environment (Coker et al. 2011). Some 177 new pathogenic organisms have been recognised to be ‘emerging’, that is, have newly arisen or been newly introduced into human populations; almost three quarters of these, 130 (73%), have come from zoonotic origins (Cascio et al. 2011; Cutler, Fooks & Van Der Poel 2010; Taylor, Latham & Woolhouse 2001; Woolhouse & Gowtage-Sequeria 2005). One of the most prevalent and important human infectious disease is influenza, a disease responsible globally for a quarter million deaths annually. In the USA alone the toll from influenza is estimated at 36 000 deaths and 226 000 hospitalisations, and it ranks as the most important cause of vaccine preventable mortality in that country (CDC 2010). The epidemiological behaviour of human influenza clearly defines it as an emerging infectious disease and the recent understanding of its zoonotic origins has contributed much to the understanding of its behaviour in humans (Fauci 2006).

Simultaneous investigation of influenza and enteric viruses in the stools of adult patients consulting in general practice for acute diarrhea

BACKGROUND

Gastrointestinal symptoms are not an uncommon manifestation of an influenza virus infection. In the present study, we aimed to investigate the presence of influenza viruses in the stools of adult patients consulting their general practitioner for uncomplicated acute diarrhea (AD) and the proportion of concurrent infections by enteric and influenza viruses.

METHOD

A case-control study was conducted from December 2010 to April 2011. Stool specimens were collected and tested for influenza viruses A (seasonal A/H3N2 and pandemic A/H1N1) and B, and for four enteric viruses (astrovirus, group A rotavirus, human enteric adenovirus, norovirus of genogroups I - NoVGI - and genogroup II - NoVGII).

RESULTS

General practitioners enrolled 138 cases and 93 controls. Of the 138 stool specimens collected, 92 (66.7%) were positive for at least one of the four enteric viruses analysed and 10 (7.2%) tested positive for one influenza virus. None of these 10 influenza positive patients reported respiratory symptoms. In five influenza-positive patients (3.6%), we also detected one enteric virus, with 4 of them being positive for influenza B (2 had co-detection with NoVGI, 1 with NoVGII, and 1 with astrovirus). None of the 93 controls tested positive for one of the enteric and/or other influenza viruses we investigated.

CONCLUSIONS

In this study we showed that the simultaneous detection of influenza and enteric viruses is not a rare event. We have also reported, for the first time in general practice, the presence of seasonal and pandemic influenza viruses in the stools of adult patients consulting for uncomplicated AD. A simultaneous investigation of enteric and influenza viruses in patients complaining of gastrointestinal symptoms could be useful for future studies to better identify the agents responsible for AD.

Development of a murine nose-only inhalation model of influenza: comparison of disease caused by instilled and inhaled A/PR/8/34

Influenza continues to cause widespread disease and death during winter months. In preclinical studies to evaluate the potential efficacy of drugs and vaccines, influenza challenge virus is usually instilled into the noses of animals in the form of large liquid drops. Since inhalation of aerosolized influenza is commonly associated with human transmission, instillation of challenge virus raises uncertainty about the applicability of results. In order to compare the challenge methods, we established conditions to generate influenza aerosols with a mass median aerodynamic diameter (MMAD) of 1 μm that were delivered to mice in a nose-only inhalation system. In this report, we describe the system and compare the 50% lethal dose (LD(50)) of instilled and inhaled A/PR/8/34 (PR8) in BALB/c mice. The estimated LD(50) for inhaled virus was 8.7 plaque forming units (PFU) and the mean time to death was 7.7 days, whereas the estimated LD(50) for instilled virus was 51.6 PFU and the mean time to death was 8.2 days. Our results show that mice are more sensitive to inhaled virus than virus delivered by intranasal instillation. The murine nose-only inhalation model of influenza infection can be used to infect large numbers of animals simultaneously with well-characterized, homogenous PR8 bioaerosol in a controlled and reproducible manner. This model provides the means to evaluate the efficacy of drug and vaccine candidates against the relevant route of challenge, thereby providing data that may better predict clinical outcome.

Antibody pressure by a human monoclonal antibody targeting the 2009 pandemic H1N1 virus hemagglutinin drives the emergence of a virus with increased virulence in mice

In 2009, a novel H1N1 influenza A virus (2009 pH1N1) emerged and caused a pandemic. A human monoclonal antibody (hMAb; EM4C04), highly specific for the 2009 pH1N1 virus hemagglutinin (HA), was isolated from a severely ill 2009 pH1N1 virus-infected patient. We postulated that under immune pressure with EM4C04, the 2009 pH1N1 virus would undergo antigenic drift and mutate at sites that would identify the antibody binding site. To do so, we infected MDCK cells in the presence of EM4C04 and generated 11 escape mutants, displaying 7 distinct amino acid substitutions in the HA. Six substitutions greatly reduced MAb binding (K123N, D131E, K133T, G134S, K157N, and G158E). Residues 131, 133, and 134 are contiguous with residues 157 and 158 in the globular domain structure and contribute to a novel pH1N1 antibody epitope. One mutation near the receptor binding site, S186P, increased the binding affinity of the HA to the receptor. 186P and 131E are present in the highly virulent 1918 virus HA and were recently identified as virulence determinants in a mouse-passaged pH1N1 virus. We found that pH1N1 escape variants expressing these substitutions enhanced replication and lethality in mice compared to wild-type 2009 pH1N1 virus. The increased virulence of these viruses was associated with an increased affinity for α2,3 sialic acid receptors. Our study demonstrates that antibody pressure by an hMAb targeting a novel epitope in the Sa region of 2009 pH1N1 HA is able to inadvertently drive the development of a more virulent virus with altered receptor binding properties. This broadens our understanding of antigenic drift.

Influenza viruses accumulate amino acid substitutions to evade the antibody response in a process known as antigenic drift, making it necessary to vaccinate against influenza annually. Mapping human monoclonal antibody (hMAb) epitopes is a necessary step towards understanding antigenic drift in humans. We defined the specificity of an hMAb that specifically targeted the 2009 pH1N1 virus and describe a novel epitope. In addition, we identified a previously unappreciated potential for antibody escape to enhance the pathogenicity of a virus. The escape mutation that we identified with in vitro immune pressure was independently reported by other investigators using in vivo selection in nonimmune mice. Although in vitro generation of escape mutants is unlikely to recapitulate antigenic drift in its entirety, the data demonstrate that pressure by a human monoclonal antibody targeting a novel epitope in the hemagglutinin of the 2009 pandemic H1N1 virus can inadvertently drive the development of escape mutants, of which a subset have increased virulence and altered receptor binding properties.

Use of praziquantel as an adjuvant enhances protection and Tc-17 responses to killed H5N1 virus vaccine in mice

Background

H5N1 is a highly pathogenic influenza A virus, which can cause severe illness or even death in humans. Although the widely used killed vaccines are able to provide some protection against infection via neutralizing antibodies, cytotoxic T-lymphocyte responses that are thought to eradicate viral infections are lacking.

Methodology/Principal Findings

Aiming to promote cytotoxic responses against H5N1 infection, we extended our previous finding that praziquantel (PZQ) can act as an adjuvant to induce IL-17-producing CD8⁺ T cells (Tc17). We found that a single immunization of 57BL/6 mice with killed viral vaccine plus PZQ induced antigen-specific Tc17 cells, some of which also secreted IFN-γ. The induced Tc17 had cytolytic activities. Induction of these cells was impaired in CD8 knockout (KO) or IFN-γ KO mice, and was even lower in IL-17 KO mice. Importantly, the inoculation of killed vaccine with PZQ significantly reduced virus loads in the lung tissues and prolonged survival. Protection against H5N1 virus infection was obtained by adoptively transferring PZQ-primed wild type CD8⁺ T cells and this was more effective than transfer of activated IFN-γ KO or IL-17 KO CD8⁺ T cells.

Conclusions/Significance

Our results demonstrated that adding PZQ to killed H5N1 vaccine could promote broad Tc17-mediated cytotoxic T lymphocyte activity, resulting in improved control of highly pathogenic avian influenza virus infection.

Infection of pulmonary vascular endothelial cells by H5N1 avian influenza virus and its role in pathogenesis

Evaluation of: Zeng H, Pappas C, Belser JA et al. Human pulmonary microvascular endothelial cells support productive replication of highly pathogenic avian influenza viruses: possible involvement in the pathogenesis of human H5N1 virus infection. J. Virol. 86(2), 667–678 (2012). The H5N1 highly pathogenic avian influenza (HPAI) virus is still a major threat for pandemic influenza. The severity of the infection, with mortality reaching up to 60%, is of particular concern, and the detailed mechanism of this high pathogenicity is not fully understood. While alveolar epithelial cells are considered to be the main target cells in humans, Zeng et al. reported that human pulmonary vascular endothelial cells were more susceptible to the H5N1 HPAI virus than to seasonal, pandemic or low pathogenic avian influenza viruses. Infection induced the endothelial cells to produce a number of proinflammatory cytokines, which raised the possibility of a new virulence mechanism that may link to the systemic dissemination of the infection in humans. However, there are still many unanswered questions regarding the determinants of the higher infectivity of H5N1 HPAI in this cell type and the role of the infection of this cell type in the pathogenesis of H5N1 HPAI.

The role of cell tropism for the pathogenesis of influenza in humans

Seasonal, pandemic and avian influenza viruses are able to infect humans, but the disease outcome often differs, ranging from mild upper respiratory tract disease to fatal pneumonia. The cell tropism of influenza viruses is thought to be an important determinant of these factors. Therefore, this review focuses on the factors that, together, determine the cell tropism of influenza viruses. These include: the receptor specificity of the viral hemagglutinin and the distribution of these receptors in the respiratory tract; the presence of inhibitory factors in the fluid lining the respiratory mucosa; and the requirement for host cell proteases that can cleave the precursor hemagglutinin of influenza viruses. Finally, we will discuss how the route of inoculation influences the cell types infected by influenza viruses and associated pathogenesis.

Mast cell-induced lung injury in mice infected with H5N1 influenza virus

Although an important role for mast cells in several viral infections has been demonstrated, its role in the invasion of highly pathogenic H5N1 influenza virus is unknown. In the present study, we demonstrate that mast cells were activated significantly by H5N1 virus (A/chicken/Henan/1/2004) infection both in vivo and in vitro. Mast cells could possibly intensify the lung injury that results from H5N1 infection by releasing proinflammatory mediators, including histamine, tryptase, and gamma interferon (IFN-γ). Lung lesions and apoptosis induced by H5N1 infection were reduced dramatically by treatment with ketotifen, which is a mast cell degranulation inhibitor. A combination of ketotifen and the neuraminidase inhibitor oseltamivir protected 100% of the mice from death postinfection. In conclusion, our data suggest that mast cells play a crucial role in the early stages of H5N1 influenza virus infection and provide a new approach to combat highly pathogenic influenza virus infection.

The TLR4-TRIF pathway protects against H5N1 influenza virus infection

Prestimulation of the TLR4 pathway with lipopolysaccharide (LPS) protects mice from lethal infection with H5N1 influenza virus. Here, we reveal that the TLR4-TRIF pathway is required for this protective effect by using mice whose TLR4-related molecules were knocked out. Microarray analysis of primary mouse lung culture cells that were LPS pretreated and infected with an H5N1 virus indicated that TLR3 mRNA was upregulated. Primary lung culture cells of TLR3 knockout mice showed no response to LPS pretreatment against H5N1 virus infection, suggesting that TLR3 is also involved in the preventive effect of LPS. Our data suggest that the TLR4-TRIF axis has an important role in stimulating protective innate immunity against H5N1 influenza A virus infection and that TLR3 signaling is involved in this pathway.

Viral load of the highly pathogenic avian influenza H5N1 virus in infected human tissues

The highly pathogenic avian influenza A (H5N1) virus is a virulent virus that causes an acute febrile respiratory disease with high mortality in humans. To gain a better insight of H5N1 viral distributions in infected human tissues, the levels of viral RNA were determined in the autopsy tissues from two patients who were infected with H5N1 virus by using real-time reverse transcription-polymerase chain reaction. In one patient who died on day 6 of the illness, the viral load in the lung was extremely high, whereas the levels of viral RNA in the other organs were more than 6 log lower. In the other patient who died on day 17 of the illness, the viral load was similar in the lung and other organs, and was comparable to the viral load in the extra-pulmonary tissues of the first patient. These results suggested that while the H5N1 virus can cause disseminated infection in humans, the lung is still the major site of viral replication, and viral replication in the lung in the later stages may decrease as a result of the depletion of the available target cells. In addition, the mRNA levels of the tumor necrosis factor-α (TNF-α) were found to be associated with the viral titers.

Human intestinal epithelial cells are susceptible to influenza virus subtype H9N2

Avian influenza viruses (AIV) replicate efficiently in guts of birds, and virus shedding is critical to viral transmission among birds and from birds to other species. In this study, we showed that an H9N2 viral strain, isolated from a human patient, caused typical influenza-like signs and illness including loss of body weight in Balb/c mice, and that viral RNA could be detected in intestinal tissues. We demonstrated that human intestinal epithelial cell line HT-29 was susceptible to the virus, and the infected cells went apoptotic at the early stage post infection. Compared to a pandemic (H1N1) 2009 influenza isolate, we found that the human H9N2 virus induced more severe apoptotic and stronger innate immune responses. Both extrinsic and intrinsic apoptotic pathways were activated in human intestinal epithelial cells, and the levels of FasL and TNF-α were induced up to hundreds-fold in response to the H9N2 infection. Interestingly, Bcl-2 family member Bid was cleaved during the course of infection, and the truncated Bid (tBid) appeared to play a role in the initiation of the intrinsic apoptosis with increased release of cytochrome c in cytosol. As for pro-inflammatory responses in H9N2-infected intestinal epithelial cells, RANTES and IP10 were induced significantly and may have played a major role in intestinal pathogenicity. Moreover, TLR-8, MyD88, and MDA-5 were all up-regulated in the infection, critical in the induction of IFN-β and host innate immunity against the H9N2 virus. Our findings have demonstrated a unique pattern of host responses in human gut in response to H9N2 subtype influenza viruses, which will broaden our understanding of the pathogenesis of AIV infection in both humans and animals.

Immune responses to influenza virus infection

Influenza viruses cause annual outbreaks of respiratory tract infection with attack rates of 5-10%. This means that humans are infected repeatedly with intervals of, on average, 10-20 years. Upon each infection subjects develop innate and adaptive immune responses which aim at clearing the infection. Strain-specific antibody responses are induced, which exert selective pressure on circulating influenza viruses and which drive antigenic drift of seasonal influenza viruses, especially in the hemagglutinin molecule. This antigenic drift necessitates updating of seasonal influenza vaccines regularly in order to match the circulating strains. Upon infection also virus-specific T cell responses are induced, including CD4+ T helper cells and CD8+ cytotoxic T cells. These cells are mainly directed to conserved proteins and therefore display cross-reactivity with a variety of influenza A viruses of different subtypes. T cell mediated immunity therefore may contribute to so-called heterosubtypic immunity and may afford protection against antigenically distinct, potentially pandemic influenza viruses. At present, novel viral targets are identified that may help to develop broad-protective vaccines. Here we review the various arms of the immune response to influenza virus infections and their viral targets and discuss the possibility of developing universal vaccines. The development of such novel vaccines would imply that also new immune correlates of protection need to be established in order to facilitate assessment of vaccine efficacy.

Distribution of sialic acid receptors and influenza A virus of avian and swine origin in experimentally infected pigs

Background

Pigs are considered susceptible to influenza A virus infections from different host origins because earlier studies have shown that they have receptors for both avian (sialic acid-alpha-2,3-terminal saccharides (SA-alpha-2,3)) and swine/human (SA-alpha-2,6) influenza viruses in the upper respiratory tract. Furthermore, experimental and natural infections in pigs have been reported with influenza A virus from avian and human sources.

Methods

This study investigated the receptor distribution in the entire respiratory tract of pigs using specific lectins Maackia Amurensis (MAA) I, and II, and Sambucus Nigra (SNA). Furthermore, the predilection sites of swine influenza virus (SIV) subtypes H1N1 and H1N2 as well as avian influenza virus (AIV) subtype H4N6 were investigated in the respiratory tract of experimentally infected pigs using immunohistochemical methods.

Results

SIV antigen was widely distributed in bronchi, but was also present in epithelial cells of the nose, trachea, bronchioles, and alveolar type I and II epithelial cells in severely affected animals. AIV was found in the lower respiratory tract, especially in alveolar type II epithelial cells and occasionally in bronchiolar epithelial cells. SA-alpha-2,6 was the predominant receptor in all areas of the respiratory tract with an average of 80-100% lining at the epithelial cells. On the contrary, the SA-alpha-2,3 was not present (0%) at epithelial cells of nose, trachea, and most bronchi, but was found in small amounts in bronchioles, and in alveoli reaching an average of 20-40% at the epithelial cells. Interestingly, the receptor expression of both SA-alpha-2,3 and 2,6 was markedly diminished in influenza infected areas compared to non-infected areas.

Conclusions

A difference in predilection sites between SIV and AIV virus was found, and this difference was in accordance with the distribution of the SA-alpha-2,6 and SA-alpha-2,3 receptor, respectively. The results indicated that the distribution of influenza A virus receptors in pigs are similar to that of humans and therefore challenge the theory that the pig acts as a mixing vessel between human and avian influenza viruses. Furthermore, it was shown that AIV prefers to infect alveolar type II epithelial cells in pigs. This corresponds with findings in humans emphasising the resemblance between the two species.

The avian influenza virus NS1 ESEV PDZ binding motif associates with Dlg1 and Scribble to disrupt cellular tight junctions

The influenza A virus NS1 protein contains a conserved 4-amino-acid-residue PDZ-ligand binding motif (PBM) at the carboxyl terminus that can function as a virulence determinant by targeting cellular PDZ proteins. The NS1 proteins from avian and human viral isolates have consensus PBM sequences ESEV and RSKV, respectively. Currently circulating highly pathogenic H5N1 viruses contain the ESEV PBM which specifically associates with the PDZ proteins Scribble, Dlg1, MAGI-1, MAGI-2, and MAGI-3. In this study, we found NS1 proteins from viral isolates that contain the PBM sequence RSKV, KSEV, or EPEV are unable to associate with these PDZ proteins. Other results showed that the ESEV PBM mediates an indirect association with PDZ protein, Lin7C, via an interaction with Dlg1. Infection with a virus that expresses a NS1 protein with the ESEV PBM results in colocalization of NS1, Scribble, and Dlg1 within perinuclear puncta and mislocalization of plasma membrane-associated Lin7C to the cytoplasm. Infection of polarized MDCK cells with the ESEV virus additionally results in functional disruption of the tight junction (TJ) as measured by altered localization of TJ markers ZO-1 and Occludin, decreased transepithelial electrical resistance, and increased fluorescein isothiocyanate (FITC)-inulin diffusion across the polarized cell monolayer. A similar effect on the TJ was observed in MDCK cells depleted for either Scribble or Dlg1 by small interfering RNA (siRNA). These findings indicate that ESEV PBM-mediated binding of NS1 to Scribble and Dlg1 functions to disrupt the cellular TJ and that this effect likely contributes to the severe disease associated with highly pathogenic H5N1 influenza A viruses.

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.

A case of hemorrhagic colitis after influenza A infection

The pandemic (H1N1) influenza virus continues to be the predominant circulating virus in both the northern and southern hemispheres. In February 2009, during the early stage of the worldwide H1N1 influenza virus (influenza A) pandemic, we experienced a case of hemorrhagic colitis after infection with influenza A. A 21-year-old man with no serious disease in his past history visited our hospital with chief complaints of a high body temperature and pharyngeal pain. A diagnosis of influenza A was made using a rapid diagnosis kit (Capilia Flu A+B), and the patient was admitted to our hospital. After admission, the patient complained of a lower abdominal pain, diarrhea, and hematochezia. An emergency colonoscopy revealed active bleeding colitis from the sigmoid to descending colon. Hemorrhagic colitis was confirmed by the pathological findings of a punch biopsy specimen. After the administration of an antiviral drug, zanamivir hydrate (10 mg/d), the patient's general condition and colonoscopic findings improved significantly. The findings of both sequential colonoscopies and intestinal histology strongly suggested that infection with influenza A could induce hemorrhagic colitis, though the incidence is quite low.

Highly pathogenic avian influenza virus H5N1 infects alveolar macrophages without virus production or excessive TNF-alpha induction

Highly pathogenic avian influenza virus (HPAIV) of the subtype H5N1 causes severe, often fatal pneumonia in humans. The pathogenesis of HPAIV H5N1 infection is not completely understood, although the alveolar macrophage (AM) is thought to play an important role. HPAIV H5N1 infection of macrophages cultured from monocytes leads to high percentages of infection accompanied by virus production and an excessive pro-inflammatory immune response. However, macrophages cultured from monocytes are different from AM, both in phenotype and in response to seasonal influenza virus infection. Consequently, it remains unclear whether the results of studies with macrophages cultured from monocytes are valid for AM. Therefore we infected AM and for comparison macrophages cultured from monocytes with seasonal H3N2 virus, HPAIV H5N1 or pandemic H1N1 virus, and determined the percentage of cells infected, virus production and induction of TNF-alpha, a pro-inflammatory cytokine. In vitro HPAIV H5N1 infection of AM compared to that of macrophages cultured from monocytes resulted in a lower percentage of infected cells (up to 25% vs up to 84%), lower virus production and lower TNF-alpha induction. In vitro infection of AM with H3N2 or H1N1 virus resulted in even lower percentages of infected cells (up to 7%) than with HPAIV H5N1, while virus production and TNF-alpha induction were comparable. In conclusion, this study reveals that macrophages cultured from monocytes are not a good model to study the interaction between AM and these influenza virus strains. Furthermore, the interaction between HPAIV H5N1 and AM could contribute to the pathogenicity of this virus in humans, due to the relative high percentage of infected cells rather than virus production or an excessive TNF-alpha induction.

Alveolar macrophages (AM), which reside in the alveolar lumen, usually dampen down the host immune response to incoming pathogens. However, they are thought to increase inflammation during highly pathogenic avian influenza virus (HPAIV) H5N1 infections, which cause severe and often fatal disease in humans. This is based on experiments with human macrophages cultured from monocytes rather than with human AM. Here we show that human AM, collected via broncho-alveolar lavage from healthy volunteers, can become infected with HPAIV H5N1. However, this results in neither induction of the pro-inflammatory cytokine TNF-alpha nor virus production. Therefore, AM are most likely not responsible for the excessive cytokine response or high viral load during human HPAIV H5N1 infections as assumed previously. These data significantly changes our insight into the pathogenesis of HPAIV H5N1 pneumonia in humans, indicating that other cells than AM must be responsible for the excessive pro-inflammatory cytokine profile observed during HPAIV H5N1 infections.

Viral replication and innate host responses in primary human alveolar epithelial cells and alveolar macrophages infected with influenza H5N1 and H1N1 viruses

Highly pathogenic influenza H5N1 virus continues to pose a threat to public health. Although the mechanisms underlying the pathogenesis of the H5N1 virus have not been fully defined, it has been suggested that cytokine dysregulation plays an important role. As the human respiratory epithelium is the primary target cell for influenza viruses, elucidating the viral tropism and innate immune responses of influenza H5N1 virus in the alveolar epithelium may help us to understand the pathogenesis of the severe pneumonia associated with H5N1 disease. Here we used primary cultures of differentiated human alveolar type II cells, alveolar type I-like cells, and alveolar macrophages isolated from the same individual to investigate viral replication competence and host innate immune responses to influenza H5N1 (A/HK/483/97) and H1N1 (A/HK/54/98) virus infection. The viral replication kinetics and cytokine and chemokine responses were compared by quantitative PCR (qPCR) and enzyme-linked immunosorbent assay (ELISA). We demonstrated that influenza H1N1 and H5N1 viruses replicated productively in type II cells and type I-like cells although with different kinetics. The H5N1 virus replicated productively in alveolar macrophages, whereas the H1N1 virus led to an abortive infection. The H5N1 virus was a more potent inducer of proinflammatory cytokines and chemokines than the H1N1 virus in all cell types. However, higher levels of cytokine expression were observed for peripheral blood monocyte-derived macrophages than for alveolar macrophages in response to H5N1 virus infection. Our findings provide important insights into the viral tropisms and host responses of different cell types found in the lung and are relevant to an understanding of the pathogenesis of severe human influenza disease.

Systemic dissemination of H5N1 influenza A viruses in ferrets and hamsters after direct intragastric inoculation

Although oral exposure to H5N1 highly pathogenic avian influenza viruses is a risk factor for infection in humans, it is unclear how oral exposure to these virus results in lethal respiratory infections. To address this issue, we inoculated ferrets and hamsters with two highly pathogenic H5N1 strains. These viruses, inoculated directly into the stomach, were isolated from the large intestine and the mesenteric lymph nodes within 1 day of inoculation and subsequently spread to multiple tissues, including lung, liver, and brain. Histopathologic analysis of ferrets infected with virus via direct intragastric inoculation revealed lymph folliculitis in the digestive tract and mesenteric lymph nodes and focal interstitial pneumonia. Comparable results were obtained with the hamster model. We conclude that, in mammals, ingested H5N1 influenza viruses can disseminate to nondigestive organs, possibly through the lymphatic system of the gastrointestinal tract.

PB1-mediated virulence attenuation of H5N1 influenza virus in mice is associated with PB2

H5N1 avian influenza viruses demonstrate different phenotypes, such as pathogenicity after one or serial passages in mammalian hosts or cells. To establish the molecular basis of these phenotypes, we cloned isolates from the lungs of mice infected with human A/Vietnam/1194/2004 (H5N1) influenza virus. Large-plaque isolates were less pathogenic to mice than small-plaque isolates. Genome sequencing revealed that the small-plaque and large-plaque isolates differed in several amino acids. In order to assess their effects on pathogenicity in mice, two amino acid changes common to attenuated isolates, one in PB2 (I63T) and the other in PB1 (T677M), were inserted into a wild-type recombinant virus construct. The PB2 (I63T) or PB1 (T677M) mutations alone did not alter the phenotype of H5N1 virus, whereas recombinant virus with both mutations was less pathogenic than the wild-type recombinant virus. Furthermore, the PB1 (T677M) mutation showed a lower replication efficiency, although it had higher polymerase activity. The recombinant virus with the PB2 (63T) mutation replicated as well as the wild-type recombinant virus. These results suggest that the C terminus of PB1 of H5N1 influenza virus mediates virulence attenuation of H5N1 influenza virus in mice, associating with the N terminus of PB2. However, the role of the N terminus of PB2 in virulence attenuation in mice remains unclear.

Cytokine production by primary human macrophages infected with highly pathogenic H5N1 or pandemic H1N1 2009 influenza viruses

Highly pathogenic H5N1 avian influenza viruses have caused infection in humans, with a high mortality rate, since 1997. While the pathogenesis of this infection is not completely understood, hypercytokinaemia and alveolar macrophages are thought to play a role. To gain further insight into the cytokine-mediated pathogenesis of this infection in humans, we measured various cytokines produced by primary human macrophages infected with H5N1, pandemic H1N1 or seasonal influenza viruses. We found that many cytokines were produced at higher levels on infection with the H5N1 strains tested compared with seasonal influenza viruses. Interestingly, the extent of cytokine induction varied among the H5N1 strains and did not correlate with replicative ability in macrophages. Further, a pandemic H1N1 virus induced higher levels of several cytokines compared with seasonal viruses and some H5N1 strains. Our results demonstrate that high cytokine induction is not a universal feature of all H5N1 viruses.

Suppression of innate immune pathology by regulatory T cells during Influenza A virus infection of immunodeficient mice

The viral infection of higher vertebrates elicits potent innate and adaptive host immunity. However, an excessive or inappropriate immune response also may lead to host pathology that often is more severe than the direct effects of viral replication. Therefore, several mechanisms exist that regulate the magnitude and class of the immune response. Here, we have examined the potential involvement of regulatory T (Treg) cells in limiting pathology induced by influenza A virus (IAV) infection. Using lymphocyte-deficient mice as hosts, we showed that Treg cell reconstitution resulted in a significant delay in weight loss and prolonged survival following infection. The adoptively transferred Treg cells did not affect the high rate of IAV replication in the lungs of lymphocyte-deficient hosts, and therefore their disease-ameliorating effect was mediated through the suppression of innate immune pathology. Mechanistically, Treg cells reduced the accumulation and altered the distribution of monocytes/macrophages in the lungs of IAV-infected hosts. This reduction in lung monocytosis was associated with a specific delay in monocyte chemotactic protein-2 (MCP-2) induction in the infected lungs. Nevertheless, Treg cells failed to prevent the eventual development of severe disease in lymphocyte-deficient hosts, which likely was caused by the ongoing IAV replication. Indeed, using T-cell-deficient mice, which mounted a T-cell-independent B cell response to IAV, we further showed that the combination of virus-neutralizing antibodies and transferred Treg cells led to the complete prevention of clinical disease following IAV infection. Taken together, these results suggested that innate immune pathology and virus-induced pathology are the two main contributors to pathogenesis during IAV infection.

A systematic molecular pathology study of a laboratory confirmed H5N1 human case

Autopsy studies have shown that human highly pathogenic avian influenza virus (H5N1) can infect multiple human organs other than just the lungs, and that possible causes of organ damage are either viral replication and/or dysregulation of cytokines and chemokines. Uncertainty still exists, partly because of the limited number of cases analysed. In this study, a full autopsy including 5 organ systems was conducted on a confirmed H5N1 human fatal case (male, 42 years old) within 18 hours of death. In addition to the respiratory system (lungs, bronchus and trachea), virus was isolated from cerebral cortex, cerebral medullary substance, cerebellum, brain stem, hippocampus ileum, colon, rectum, ureter, aortopulmonary vessel and lymph-node. Real time RT-PCR evidence showed that matrix and hemagglutinin genes were positive in liver and spleen in addition to positive tissues with virus isolation. Immunohistochemistry and in-situ hybridization stains showed accordant evidence of viral infection with real time RT-PCR except bronchus. Quantitative RT-PCR suggested that a high viral load was associated with increased host responses, though the viral load was significantly different in various organs. Cells of the immunologic system could also be a target for virus infection. Overall, the pathogenesis of HPAI H5N1 virus was associated both with virus replication and with immunopathologic lesions. In addition, immune cells cannot be excluded from playing a role in dissemination of the virus in vivo.

Mice lacking both TNF and IL-1 receptors exhibit reduced lung inflammation and delay in onset of death following infection with a highly virulent H5N1 virus

BACKGROUND

Highly pathogenic avian influenza viruses of the H5N1 subtype continue to cross the species barrier to infect humans and cause severe disease. It has been suggested that an exaggerated immune response contributes to the pathogenesis of H5N1 virus infection in mammals. In particular, H5N1 virus infections are associated with a high expression of the proinflammatory cytokines, including interleukin-1 (IL-1) and tumor necrosis factor alpha (TNF-α).

METHODS

We investigated the compounding affects of both cytokines on the outcome of H5N1 virus disease by using triple mutant mice deficient in 3 signaling receptors, TNF-R1, TNF-R2, and IL-1-RI.

RESULTS

Triple mutant mice exhibited reduced morbidity and a significant delay in mortality following lethal challenge with a lethal H5N1 virus, whereas no such differences were observed with the less virulent A/PR/8/34 (H1N1) virus. H5N1-infected triple mutant mice displayed diminished cytokine production in lung tissue and a quantifiable decrease of macrophages and neutrophils in the lungs postinfection. Moreover, morphometric analysis of airway sections revealed less extensive inflammation in H5N1-infected triple mutant mice, compared with infected wild-type mice.

CONCLUSIONS

The combined signaling from the TNF or IL-1 receptors promotes maximal lung inflammation that may contribute to the severity of disease caused by H5N1 virus infection.

Enhanced susceptibility of nasal polyp tissues to avian and human influenza viruses

BACKGROUND

Influenza viruses bind and infect respiratory epithelial cells through sialic acid on cell surface. Differential preference to sialic acid types contributes to host- and tissue-tropism of avian and seasonal influenza viruses. Although the highly pathogenic avian influenza virus H5N1 can infect and cause severe diseases in humans, it is not efficient in infecting human upper respiratory tract. This is because of the scarcity of its receptor, α2,3-linked sialic acid, in human upper airway. Expression of sialic acid can be influenced by various factors including inflammatory process. Allergic rhinitis and nasal polyp are common inflammatory conditions of nasal mucosa and may affect expression of the sialic acid and susceptibility to influenza infection.

METHODOLOGY/PRINCIPAL FINDING

To test this hypothesis, we detected α2,3- and α2,6-linked sialic acid in human nasal polyp and normal nasal mucosal tissues by lectin staining and infected explants of those tissues with avian influenza viruses H5N1 and seasonal influenza viruses. We show here that mucosal surface of nasal polyp expressed higher level of α2,3- and α2,6-linked sialic acid than normal nasal mucosa. Accordingly, both H5N1 avian influenza viruses and seasonal influenza viruses replicated more efficiently in nasal polyp tissues explants.

CONCLUSIONS/SIGNIFICANCE

Our data suggest a role of nasal inflammatory conditions in susceptibility to influenza infection, especially by avian influenza viruses, which is generally inefficient in infecting human upper airway. The increased receptor expression may contribute to increased susceptibility in some individuals. This may contribute to the gradual adaptation of the virus to human population.

Highly pathogenic avian influenza virus H7N7 isolated from a fatal human case causes respiratory disease in cats but does not spread systemically

Highly pathogenic avian influenza viruses (HPAIV) of the H5 and H7 subtypes primarily infect poultry but are occasionally transmitted to humans and other mammalian species, often causing severe disease. Previously we have shown that HPAIV H5N1 causes severe systemic disease in cats. In this study, we investigated whether HPAIV H7N7 isolated from a fatal human case is also able to cause disease in cats. Additionally, we compared the cell tropism of both viruses by immunohistochemistry and virus histochemistry. Three domestic cats were inoculated intratracheally with HPAIV H7N7. Virus excretion was restricted to the pharynx. At necropsy, 7 days post inoculation, lesions were restricted to the respiratory tract in all cats. Lesions consisted of diffuse alveolar damage and colocalized with virus antigen expression in type II pneumocytes and nonciliated bronchiolar cells. The attachment patterns of HPAIV H7N7 and H5N1 were similar: both viruses attached to nonciliated bronchiolar epithelial cells, type II pneumocytes, as well as alveolar macrophages. These data show for the first time that a non-H5 HPAIV is able to infect and cause respiratory disease in cats. The failure of HPAIV H7N7 to spread beyond the respiratory tract was not explained by differences in cell tropism compared to HPAIV H5N1. These findings suggest that HPAIV H5N1 possesses other characteristics that allow it to cause systemic disease in both humans and cats.

Antiviral immune responses in H5N1-infected human lung tissue and possible mechanisms underlying the hyperproduction of interferon-inducible protein IP-10

Information on the immune response against H5N1 within the lung is lacking. Here we describe the sustained antiviral immune responses, as indicated by the expression of MxA protein and IFN-alpha mRNA, in autopsy lung tissue from an H5N1-infected patient. H5N1 infection of primary bronchial/tracheal epithelial cells and lung microvascular endothelial cells induced IP-10, and also up-regulated the retinoic acid-inducible gene-I (RIG-I). Down-regulation of RIG-I gene expression decreased IP-10 response. Co-culturing of H5N1-infected pulmonary cells with TNF-alpha led to synergistically enhanced production of IP-10. In the absence of viral infection, TNF-alpha and IFN-alpha also synergistically enhanced IP-10 response. Methylprednisolone showed only a partial inhibitory effect on this chemokine response. Our findings strongly suggest that both the H5N1 virus and the locally produced antiviral cytokines; IFN-alpha and TNF-alpha may have an important role in inducing IP-10 hyperresponse, leading to inflammatory damage in infected lung.

Pandemic influenza A (H1N1) virus infection and avian influenza A (H5N1) virus infection: a comparative analysis

The 2009 H1N1 and H5N1 influenza viruses are newly (re-) emerged influenza A viruses (2009 A(H1N1) and A(H5N1), respectively) that have recently posed tremendous health threats in many regions worldwide. With the 2009 outbreak of H1N1 influenza A, the world witnessed the first influenza pandemic of the 21st century. The disease has rapidly spread across the entire globe, and has resulted in hundreds of thousands of cases with confirmed infection. Although characterized by high transmissibility, the virulence and fatality of the 2009 A(H1N1) influenza virus have thus far remained relatively low. The reverse holds true for A(H5N1) influenza; at a fatality rate that exceeds 60%, it is known to cause severe damage to the human respiratory system, but is not presently capable of efficient transmission from human to human. Apart from the clear differences between the two types of influenza, there are some significant similarities that warrant attention. In particular, the more severe and fatal 2009 A(H1N1) influenza cases have shown symptoms similar to those reported in cases of A(H5N1) influenza. Histopathological findings for these cases, to the extent available, also appear to have similarities for both diseases in terms of damage and severity. Here we review important recent publications in this area, and we discuss some of the key commonalities and contrasts between the two influenza A types in terms of their biology, origins, clinical features, pathology and pathogenesis, and receptors and transmissibility.

Dogs are highly susceptible to H5N1 avian influenza virus

Replication of avian influenza viruses (AIVs) in dogs may facilitate their adaptation in humans; however, the data to date on H5N1 influenza virus infection in dogs are conflicting. To elucidate the susceptibility of dogs to this pathogen, we infected two groups of 6 beagles with 10(6) 50% egg-infectious dose of H5N1 AIV A/bar-headed goose/Qinghai/3/05 (BHG/QH/3/05) intranasally (i.n.) and intratracheally (i.t.), respectively. The dogs showed disease symptoms, including anorexia, fever, conjunctivitis, labored breathing and cough, and one i.t. inoculated animal died on day 4 post-infection. Virus shedding was detected from all 6 animals inoculated i.n. and one inoculated i.t. Virus replication was detected in all animals that were euthanized on day 3 or day 5 post-infection and in the animal that died on day 4 post-infection. Our results demonstrate that dogs are highly susceptible to H5N1 AIV and may serve as an intermediate host to transfer this virus to humans.

Extrapulmonary tissue responses in cynomolgus macaques (Macaca fascicularis) infected with highly pathogenic avian influenza A (H5N1) virus

The mechanisms responsible for virulence of influenza viruses in humans remain poorly understood. A prevailing hypothesis is that the highly pathogenic virus isolates cause a severe cytokinemia precipitating acute respiratory distress syndrome and multiple organ dysfunction syndrome. Cynomolgus macaques (Macaca fascicularis) infected with a human highly pathogenic avian influenza (HPAI) H5N1 virus isolate (A/Vietnam/1203/2004) or reassortants of human influenza virus A/Texas/36/91 (H1N1) containing genes from the 1918 pandemic influenza A (H1N1) virus developed severe pneumonia within 24 h postinfection. However, virus spread beyond the lungs was only detected in the H5N1 group, and signs of extrapulmonary tissue reactions, including microglia activation and sustained up-regulation of inflammatory markers, most notably hypoxia inducible factor-1alpha (HIF-1alpha), were largely limited to this group. Extrapulmonary pathology may thus contribute to the morbidities induced by H5N1 viruses.

Emerging and reemerging influenza virus infections

Influenza A virus infection occurs in many species. Wild waterfowl harbor the widest variety of influenza A viruses and serve as a constant reservoir for the emergence of new viruses. Highly pathogenic avian influenza, or "fowl plague," has been a known poultry disease for more than 130 years. It continues to emerge and reemerge, but global changes in trade and poultry production have expanded the impact and geographic range of these outbreaks. One subtype of highly pathogenic avian influenza, H5N1, has infected poultry on several continents as well as many people, leading to a human disease that is markedly different from seasonal influenza and that is associated with high mortality.

Plasmacytoid dendritic cells in antiviral immunity and autoimmunity

Plasmacytoid dendritic cells (pDCs) represent a unique and crucial immune cell population capable of producing large amounts of type I interferons (IFNs) in response to viral infection. The function of pDCs as the professional type I IFN-producing cells is linked to their selective expression of Toll-like receptor 7 (TLR7) and TLR9, which sense viral nucleic acids within the endosomal compartments. Type I IFNs produced by pDCs not only directly inhibit viral replication but also play an essential role in linking the innate and adaptive immune system. The aberrant activation of pDCs by self nucleic acids through TLR signaling and the ongoing production of type I IFNs do occur in some autoimmune diseases. Therefore, pDC may serve as an attractive target for therapeutic manipulations of the immune system to treat viral infectious diseases and autoimmune diseases.

Viral load in patients infected with pandemic H1N1 2009 influenza A virus

Viral shedding profile of infections caused by the pandemic H1N1 2009 influenza A virus has not been reported. The aim of this study was to determine the viral load in different body sites. Viral loads of pandemic H1N1 virus in respiratory specimens, stool, urine, and serum were determined by quantitative reverse transcriptase‐polymerase chain reaction (RT‐PCR). Respiratory specimens from patients with seasonal influenza were used as historical controls. Initial pre‐treatment viral load were compared between these two groups. Serial respiratory specimens from patients with pandemic H1N1 virus infection were obtained for analysis of viral dynamics. Twenty‐two pandemic H1N1 cases and 44 seasonal influenza historical controls were included. The mean initial viral load before oseltamivir therapy was 1.84 × 10⁸ copies/ml for pandemic H1N1 virus compared with 3.28 × 10⁸ copies/ml in seasonal influenza historical controls (P = 0.085). Among patients with pandemic H1N1 virus infection, peak viral load occurred on the day of onset of symptoms, and declined gradually afterwards, with no virus being detectable in respiratory specimens by RT‐PCR 8 days and by culture 5 days after the onset of symptoms respectively, except in one patient. Pandemic H1N1 virus was detected in stool and in urine from 4/9 and 1/14 patients, respectively. Viral culture was also positive from the stool sample with the highest viral load. Younger age was associated with prolonged shedding in the respiratory tract and higher viral load in the stool. Data from this quantitative analysis of viral shedding may have implications for formulating infection control measures. J. Med. Virol. 82:1–7, 2010. © 2009 Wiley‐Liss, Inc.

Influenza virus infection among pediatric patients reporting diarrhea and influenza-like illness

Background

Influenza is a major cause of morbidity and hospitalization among children. While less often reported in adults, gastrointestinal symptoms have been associated with influenza in children, including abdominal pain, nausea, vomiting, and diarrhea.

Methods

From September 2005 and April 2008, pediatric patients in Indonesia presenting with concurrent diarrhea and influenza-like illness were enrolled in a study to determine the frequency of influenza virus infection in young patients presenting with symptoms less commonly associated with an upper respiratory tract infection (URTI). Stool specimens and upper respiratory swabs were assayed for the presence of influenza virus.

Results

Seasonal influenza A or influenza B viral RNA was detected in 85 (11.6%) upper respiratory specimens and 21 (2.9%) of stool specimens. Viable influenza B virus was isolated from the stool specimen of one case. During the time of this study, human infections with highly pathogenic avian influenza A (H5N1) virus were common in the survey area. However, among 733 enrolled subjects, none had evidence of H5N1 virus infection.

Conclusions

The detection of influenza viral RNA and viable influenza virus from stool suggests that influenza virus may be localized in the gastrointestinal tract of children, may be associated with pediatric diarrhea and may serve as a potential mode of transmission during seasonal and epidemic influenza outbreaks.

Respiratory viral infections in infants: causes, clinical symptoms, virology, and immunology

In global terms, respiratory viral infection is a major cause of morbidity and mortality. Infancy, in particular, is a time of increased disease susceptibility and severity. Early-life viral infection causes acute illness and can be associated with the development of wheezing and asthma in later life. The most commonly detected viruses are respiratory syncytial virus (RSV), rhinovirus (RV), and influenza virus. In this review we explore the complete picture from epidemiology and virology to clinical impact and immunology. Three striking aspects emerge. The first is the degree of similarity: although the infecting viruses are all different, the clinical outcome, viral evasion strategies, immune response, and long-term sequelae share many common features. The second is the interplay between the infant immune system and viral infection: the immaturity of the infant immune system alters the outcome of viral infection, but at the same time, viral infection shapes the development of the infant immune system and its future responses. Finally, both the virus and the immune response contribute to damage to the lungs and subsequent disease, and therefore, any prevention or treatment needs to address both of these factors.