Mutations during the Adaptation of H9N2 Avian Influenza Virus to the Respiratory Epithelium of Pigs Enhance Sialic Acid Binding Activity and Virulence in Mice

Mapping Genetic Markers Associated with Antigenicity and Host Range in H9N2 Influenza A Viruses Infecting Poultry in Pakistan

The aim of the current study was to map the genetic diversity in the haemagglutinin (HA) glycoprotein of influenza A viruses (IAVs) of the H9N2 subtype. Twenty-five H9N2 IAVs were isolated from broiler chickens from March to July 2019. The HA gene was amplified, and phylogenetic analysis was performed to determine the evolutionary relationship. Important antigenic amino acid residues of HA attributed to immune escape and zoonotic potential were compared among H9N2 IAVs. Phylogenetic analysis revealed that sublineage B2 under the G1 lineage in Pakistan was found to be diversified, and newly sequenced H9N2 isolates were nested into two clades (A and B). Mutations linked to the antigenic variation and potential immune escape were observed as G72E (1/25, 4%), A180T (3/25, 12%), and A180V (1/25, 4%). A twofold significant reduction (P < 0.01) in log2 hemagglutination inhibition titers was observed with H9N2 IAV naturally harboring amino acid V180 instead of A180 in HA protein. Moreover, in the last 20 years, complete substitution at residues (T127D, D135N, and L150N) and partial substitution at residues (72, 74, 131, 148, 180, 183, 188, 216, 217, and 249, mature H9 HA numbering) associated with changes in antigenicity were observed. The presence of L216 in all H9N2 IAV isolates and T/V180 in four isolates in the receptor-binding site reveals the potential of these viruses to cross the species barrier to infect human or mammals. The current study observed the circulation of antigenically diverse H9N2 IAV variants that possess potential mutations that can escape the host immune system.

Different populations of A(H1N1)pdm09 viruses in a patient with hemolytic-uremic syndrome

Respiratory viral infections may have different impacts ranging from infection without symptoms to severe disease or even death though the reasons are not well characterized. A patient (age group 5-15 years) displaying symptoms of hemolytic uremic syndrome died one day after hospitalization. qPCR, next generation sequencing, virus isolation, antigenic characterization, resistance analysis was performed and virus replication kinetics in well-differentiated airway cells were determined. Autopsy revealed hemorrhagic pneumonia as major pathological manifestation. Lung samples harbored a large population of A(H1N1)pdm09 viruses with the polymorphism H456H/Y in PB1 polymerase. The H456H/Y viruses replicated much faster to high viral titers than upper respiratory tract viruses in vitro. H456H/Y-infected air-liquid interface cultures of differentiated airway epithelial cells did reflect a more pronounced loss of ciliated cells. A different pattern of virus quasispecies was found in the upper airway samples where substitution S263S/F (HA1) was observed. The data support the notion that viral quasispecies had evolved locally in the lung to support high replicative fitness. This change may have initiated further pathogenic processes leading to rapid dissemination of inflammatory mediators followed by development of hemorrhagic lung lesions and fatal outcome.

The pathogenesis of influenza in intact alveoli: virion endocytosis and its effects on the lung's air-blood barrier

Lung infection by influenza A virus (IAV) is a major cause of global mortality from lung injury, a disease defined by widespread dysfunction of the lung's air-blood barrier. Endocytosis of IAV virions by the alveolar epithelium - the cells that determine barrier function - is central to barrier loss mechanisms. Here, we address the current understanding of the mechanistic steps that lead to endocytosis in the alveolar epithelium, with an eye to how the unique structure of lung alveoli shapes endocytic mechanisms. We highlight where future studies of alveolar interactions with IAV virions may lead to new therapeutic approaches for IAV-induced lung injury.

Construction and Immunogenicity Evaluation of Recombinant Bacillus subtilis Expressing HA1 Protein of H9N2 Avian Influenza Virus

The H9N2 subtype of the avian influenza virus (AIV) is one of the main subtypes of low pathogenic AIV, and it seriously affects the poultry breeding industry. Currently, vaccination is still one of China's main strategies for controlling H9N2 avian influenza. In this study, we selected MW548848.1 on the current popular main branch h9.4.2.5 as the reference strain, and we optimized the amino acid sequence of HA1 to make it suitable for expression in Bacillus subtilis. The B. subtilis expression vector showed good safety and stress resistance; therefore, this study constructed a recombinant B. subtilis expressing H9N2 HA1 protein and evaluated its immunogenicity in mice. The following results were obtained: the sIgA level of HA1 protein in small intestine fluid and the IgG level of PHT43-HA1/B. subtilis in serum were significantly improved (P < 0.01); PHT43-HA1/B. subtilis can cause a special immune response in mice; and cytokine detection interferon-gamma (IFN-γ) (P < 0.05) and Interleukin 2 (IL-2) (P < 0.01) expressions significantly increased. Additionally, the study found that PHT43-HA1/B. subtilis can alleviate the attack of H9N2 AIV in the spleen, lungs, and small intestine of mice. This study was the first to use an oral recombinant B. subtilis-HA1 vaccine candidate, and it provides theoretical data and technical reference for the creation of a new live vector vaccine against H9N2 AIV.

Infection Studies with Airway Organoids from Carollia perspicillata Indicate That the Respiratory Epithelium Is Not a Barrier for Interspecies Transmission of Influenza Viruses

Bats are a natural reservoir for many viruses and are considered to play an important role in the interspecies transmission of viruses. To analyze the susceptibility of bat airway cells to infection by viruses of other mammalian species, we developed an airway organoid culture model derived from airways of Carollia perspicillata. Application of specific antibodies for fluorescent staining indicated that the cell composition of organoids resembled those of bat trachea and lungs as determined by immunohistochemistry. Infection studies indicated that Carollia perspicillata bat airway organoids (AOs) from the trachea or the lung are highly susceptible to infection by two different porcine influenza A viruses. The bat AOs were also used to develop an air-liquid interface (ALI) culture system of filter-grown epithelial cells. Infection of these cells showed the same characteristics, including lower virulence and enhanced replication and release of the H1N1/2006 virus compared to infection with H3N2/2007. These observations agreed with the results obtained by infection of porcine ALI cultures with these two virus strains. Interestingly, lectin staining indicated that bat airway cells only contain a small amount of alpha 2,6-linked sialic acid, the preferred receptor determinant for mammalian influenza A viruses. In contrast, large amounts of alpha 2,3-linked sialic acid, the preferred receptor determinant for avian influenza viruses, are present in bat airway epithelial cells. Therefore, bat airway cells may be susceptible not only to mammalian but also to avian influenza viruses. Our culture models, which can be extended to other parts of the airways and to other species, provide a promising tool to analyze virus infectivity and the transmission of viruses both from bats to other species and from other species to bats. IMPORTANCE We developed an organoid culture system derived from the airways of the bat species Carollia perspicillata. Using this cell system, we showed that the airway epithelium of these bats is highly susceptible to infection by influenza viruses of other mammalian species and thus is not a barrier for interspecies transmission. These organoids provide an almost unlimited supply of airway epithelial cells that can be used to generate well-differentiated epithelial cells and perform infection studies. The establishment of the organoid model required only three animals, and can be extended to other epithelia (nose, intestine) as well as to other species (bat and other animal species). Therefore, organoids promise to be a valuable tool for future zoonosis research on the interspecies transmission of viruses (e.g., bat → intermediate host → human).

Elimination of receptor binding by influenza hemagglutinin improves vaccine-induced immunity

The binding of influenza hemagglutinin (HA) to sialic acid (SA) receptors plays a well-defined role in shaping infection but the impact of such binding on vaccine responses has not yet been explored. We generated a virus-like particle (VLP) vaccine bearing the HA of H1N1 A/California/07/09 that is unable to bind to its α(2,6)-linked SA receptor (H1_Y98F-VLP) and compared its immunogenicity and efficacy to a wild-type H1-VLP (H1_WT-VLP) in mice. The H1_Y98F-VLP elicited significantly stronger and more durable antibody responses (hemagglutination inhibition and microneutralization titers) and greater avidity maturation, likely attributable to improved germinal center formation. H1_Y98F-VLP also resulted in a robust population of IL-2⁺TNFα⁺IFNγ⁻ CD4⁺ T cells that correlated with antibody responses. Compared to H1_WT-VLP vaccination, mice immunized with H1_Y98F-VLP had 2.3-log lower lung viral loads and significantly lower pulmonary inflammatory cytokine levels 5 days post-challenge. These findings suggest that abrogation of HA-SA interactions may be a promising strategy to improve the quality and durability of influenza vaccine-induced humoral responses.

Precision-cut lung slices: A powerful ex vivo model to investigate respiratory infectious diseases

Respiratory infections are a leading cause of mortality worldwide. Most of the research on the underlying disease mechanisms is based on cell culture, organoid, or surrogate animal models. Although these provide important insights, they have limitations. Cell culture models fail to recapitulate cellular interactions in the lung and animal models often do not permit high‐throughput analysis of drugs or pathogen isolates; hence, there is a need for improved, scalable models. Precision‐cut lung slices (PCLS), small, uniform tissue slices generated from animal or human lungs are increasingly recognized and employed as an ex vivo organotypic model. PCLS retain remarkable cellular complexity and the architecture of the lung, providing a platform to investigate respiratory pathogens in a near‐native environment. Here, we review the generation and features of PCLS, their use to investigate the pathogenesis of viral and bacterial pathogens, and highlight their potential to advance respiratory infection research in the future.

Targeting the glycans: A paradigm for host-targeted and COVID-19 drug design

There is always a need for new approaches for the control of virus burdens caused by seasonal outbreaks, the emergence of novel viruses with pandemic potential and the development of resistance to current antiviral drugs. The outbreak of the 2019 novel coronavirus-disease COVID-19 represented a pandemic threat and declared a public health emergency of international concern. Herein, the role of glycans for the development of new drugs or vaccines, as a host-targeted approach, is discussed where may provide a front-line prophylactic or threats to protect against the current and any future respiratory-infecting virus and possibly against other respiratory pathogens. As a prototype, the role of glycans in the coronavirus infection, as well as, galectins (Gal) as the glycan-recognition agents (GRAs) in drug design are here summarized. Galectins, in particular, Gal-1 and Gal-3 are ubiquitous and important to biological systems, whose interactions with viral glycans modulate host immunity and homeostatic balance.

Complement activation and coagulopathy - an ominous duo in COVID19

INTRODUCTION

COVID-19 has similarities to the Severe Acute Respiratory Syndrome (SARS) and Middle East Respiratory Syndrome (MERS) outbreaks, as severe patients and non-survivors have frequently shown abnormal coagulation profiles. Immune-mediated pathology is a key player in this disease; hence, the role of the complement system needs assessment. The complement system and the coagulation cascade share an intricate network, where multiple mediators maintain a balance between both pathways. Coagulopathy in COVID-19, showing mixed features of complement-mediated and consumption coagulopathy, creates a dilemma in diagnosis and management.

AREAS COVERED

Pathophysiology of coagulopathy in COVID-19 patients, with a particular focus on D-dimer and its role in predicting the severity of COVID-19 has been discussed. A comprehensive search of the medical literature on PubMed was done till May 30th, 2020 with the keywords 'COVID-19', 'SARS-CoV-2', 'Coronavirus', 'Coagulopathy', and 'D-dimer'. Twenty-two studies were taken for weighted pooled analysis of D-dimer.

EXPERT OPINION

A tailored anticoagulant regimen, including intensification of standard prophylactic regimens with low-molecular-weight heparin is advisable for COVID-19 patients. Atypical manifestations and varying D-dimer levels seen in different populations bring forth the futility of uniform recommendations for anticoagulant therapy. Further, direct thrombin inhibitors and platelet inhibitors in a patient-specific manner should also be considered.

H9 Influenza Viruses: An Emerging Challenge

Influenza A viruses (IAVs) of the H9 subtype are enzootic in Asia, the Middle East, and parts of North and Central Africa, where they cause significant economic losses to the poultry industry. Of note, some strains of H9N2 viruses have been linked to zoonotic episodes of mild respiratory diseases. Because of the threat posed by H9N2 viruses to poultry and human health, these viruses are considered of pandemic concern by the World Health Organization (WHO). H9N2 IAVs continue to diversify into multiple antigenically and phylogenetically distinct lineages that can further promote the emergence of strains with pandemic potential. Somewhat neglected compared with the H5 and H7 subtypes, there are numerous indicators that H9N2 viruses could be involved directly or indirectly in the emergence of the next influenza pandemic. The goal of this work is to discuss the state of knowledge on H9N2 IAVs and to provide an update on the contemporary global situation.

COVID-19, SARS and MERS: are they closely related?

BACKGROUND

The 2019 novel coronavirus (SARS-CoV-2) is a new human coronavirus which is spreading with epidemic features in China and other Asian countries; cases have also been reported worldwide. This novel coronavirus disease (COVID-19) is associated with a respiratory illness that may lead to severe pneumonia and acute respiratory distress syndrome (ARDS). Although related to the severe acute respiratory syndrome (SARS) and the Middle East respiratory syndrome (MERS), COVID-19 shows some peculiar pathogenetic, epidemiological and clinical features which to date are not completely understood.

AIMS

To provide a review of the differences in pathogenesis, epidemiology and clinical features of COVID-19, SARS and MERS.

SOURCES

The most recent literature in the English language regarding COVID-19 has been reviewed, and extracted data have been compared with the current scientific evidence about SARS and MERS epidemics.

CONTENT

COVID-19 seems not to be very different from SARS regarding its clinical features. However, it has a fatality rate of 2.3%, lower than that of SARS (9.5%) and much lower than that of MERS (34.4%). The possibility cannot be excluded that because of the less severe clinical picture of COVID-19 it can spread in the community more easily than MERS and SARS. The actual basic reproductive number (R0) of COVID-19 (2.0-2.5) is still controversial. It is probably slightly higher than the R0 of SARS (1.7-1.9) and higher than that of MERS (<1). A gastrointestinal route of transmission for SARS-CoV-2, which has been assumed for SARS-CoV and MERS-CoV, cannot be ruled out and needs further investigation.

IMPLICATIONS

There is still much more to know about COVID-19, especially as concerns mortality and its capacity to spread on a pandemic level. Nonetheless, all of the lessons we learned in the past from the SARS and MERS epidemics are the best cultural weapons with which to face this new global threat.

Avian Influenza A Virus Infects Swine Airway Epithelial Cells without Prior Adaptation

Pigs play an important role in the interspecies transmission of influenza A viruses (IAV). The porcine airway epithelium contains binding sites for both swine/human IAV (α2,6-linked sialic acids) and avian IAV (α2,3-linked sialic acids) and therefore is suited for adaptation of viruses from other species as suggested by the “mixing vessel theory”. Here, we applied well-differentiated swine airway epithelial cells to find out whether efficient infection by avian IAV requires prior adaption. Furthermore, we analyzed the influence of the sialic acid-binding activity and the virus-induced detrimental effects. Surprisingly, an avian IAV H1N1 strain circulating in European poultry and waterfowl shows increased and prolonged viral replication without inducing a strong innate immune response. This virus could infect the lower respiratory tract in our precision cut-lung slice model. Pretreating the cells with poly (I:C) and/or JAK/STAT pathway inhibitors revealed that the interferon-stimulated innate immune response influences the replication of avian IAV in swine airway epitheliums but not that of swine IAV. Further studies indicated that in the infection by IAVs, the binding affinity of sialic acid is not the sole factor affecting the virus infectivity for swine or human airway epithelial cells, whereas it may be crucial in well-differentiated ferret tracheal epithelial cells. Taken together, our results suggest that the role of pigs being the vessel of interspecies transmission should be reconsidered, and the potential of avian H1N1 viruses to infect mammals needs to be characterized in more detail.

Molecular Characterization and Comparative Pathogenicity of Goose Parvovirus Isolated from Jilin Province, Northeast China

Goose parvovirus (GPV) is a highly contagious disease caused by GPV in goslings and young Muscovy ducklings. In recent years, frequent GPV outbreaks have occurred in many regions of Jilin Province, China. In this study, to understand the immune-related characteristics of the currently prevailing GPV strains in some regions of Jilin Province, six GPV strains were isolated from six different regions of Jilin Province in 2016-2018. The results of phylogenetic analysis, clinical signs, and histopathologic analysis showed that four strains were virulent and two strains were attenuated. Specifically, we found that the two attenuated strains have the same amino acid mutation at the same position in the virus protein 3 (VP3) gene, and the virulent strains have more mutation sites than the attenuated strains. This finding suggests that changes in these sites may result in GPV replication or reduction in the immune response in goslings, thereby producing strong pathogenicity, and that attenuated strains are more conservative than virulent strains.

Infection Studies in Pigs and Porcine Airway Epithelial Cells Reveal an Evolution of A(H1N1)pdm09 Influenza A Viruses Toward Lower Virulence

We analyzed the virulence of pandemic H1N1 2009 influenza A viruses in vivo and in vitro. Selected viruses isolated in 2009, 2010, 2014, and 2015 were assessed using an aerosol-mediated high-dose infection model for pigs as well as air-liquid interface cultures of differentiated airway epithelial cells. Using a dyspnea score, rectal temperature, lung lesions, and viral load in the lung as parameters, the strains from 2014–2015 were significantly less virulent than the strains isolated in 2009–2010. In vitro, the viruses from 2009–2010 also differed from the 2014–2015 viruses by increased release of infectious virus, a more pronounced loss of ciliated cells, and a reduced thickness of the epithelial cell layer. Our in vivo and in vitro results reveal an evolution of A(H1N1)pdm09 viruses toward lower virulence. Our in vitro culture system can be used to predict the virulence of influenza viruses.

Poultry trading behaviours in Vietnamese live bird markets as risk factors for avian influenza infection in chickens

Vietnamese poultry are host to co‐circulating subtypes of avian influenza viruses, including H5N1 and H9N2, which pose a great risk to poultry productivity and to human health. AIVs circulate throughout the poultry trade network in Vietnam, with live bird markets being an integral component to this network. Traders at LBMs exhibit a variety of trading practices, which may influence the transmission of AIVs. We identified trading practices that impacted on AIV prevalence in chickens marketed in northern Vietnamese LBMs. We generated sequencing data for 31 H9N2 and two H5N6 viruses. Viruses isolated in the same LBM or from chickens sourced from the same province were genetically closer than viruses isolated in different LBMs or from chickens sourced in different provinces. The position of a vendor in the trading network impacted on their odds of having AIV‐infected chickens. Being a retailer and purchasing chickens from middlemen was associated with increased odds of infection, whereas odds decreased if vendors purchased chickens directly from large farms. Odds of infection were also higher for vendors having a greater volume of ducks unsold per day. These results indicate how the spread of AIVs is influenced by the structure of the live poultry trading network.

Flexibility In Vitro of Amino Acid 226 in the Receptor-Binding Site of an H9 Subtype Influenza A Virus and Its Effect In Vivo on Virus Replication, Tropism, and Transmission

A single amino acid change at position 226 in the hemagglutinin (HA) from glutamine (Q) to leucine (L) has been shown to play a key role in receptor specificity switching in various influenza virus HA subtypes, including H9. We tested the flexibility of amino acid usage and determined the effects of such changes. The results reveal that amino acids other than L226 and Q226 are well tolerated and that some amino acids allow for the recognition of both avian and human influenza virus receptors in the absence of other changes. Our results can inform better avian influenza virus surveillance efforts as well as contribute to rational vaccine design and improve structural molecular dynamics algorithms.

Influenza A viruses (IAVs) remain a significant public health threat, causing more than 300,000 hospitalizations in the United States during the 2015–2016 season alone. While only a few IAVs of avian origin have been associated with human infections, the ability of these viruses to cause zoonotic infections further increases the public health risk of influenza. Of these, H9N2 viruses in Asia are of particular importance as they have contributed internal gene segments to other emerging zoonotic IAVs. Notably, recent H9N2 viruses have acquired molecular markers that allow for a transition from avian-like to human-like terminal sialic acid (SA) receptor recognition via a single amino acid change at position 226 (H3 numbering), from glutamine (Q226) to leucine (L226), within the hemagglutinin (HA) receptor-binding site (RBS). We sought to determine the plasticity of amino acid 226 and the biological effects of alternative amino acids on variant viruses. We created a library of viruses with the potential of having any of the 20 amino acids at position 226 on a prototypic H9 HA subtype IAV. We isolated H9 viruses that carried naturally occurring amino acids, variants found in other subtypes, and variants not found in any subtype at position 226. Fitness studies in quails revealed that some natural amino acids conferred an in vivo replication advantage. This study shows the flexibility of position 226 of the HA of H9 influenza viruses and the resulting effect of single amino acid changes on the phenotype of variants in vivo and in vitro.

IMPORTANCE A single amino acid change at position 226 in the hemagglutinin (HA) from glutamine (Q) to leucine (L) has been shown to play a key role in receptor specificity switching in various influenza virus HA subtypes, including H9. We tested the flexibility of amino acid usage and determined the effects of such changes. The results reveal that amino acids other than L226 and Q226 are well tolerated and that some amino acids allow for the recognition of both avian and human influenza virus receptors in the absence of other changes. Our results can inform better avian influenza virus surveillance efforts as well as contribute to rational vaccine design and improve structural molecular dynamics algorithms.

The Multifaceted Zoonotic Risk of H9N2 Avian Influenza

Poultry-adapted H9N2 avian influenza viruses (AIVs) are commonly found in many countries in Asia, the Middle East, Africa, and Europe, and although classified as low pathogenic viruses, they are an economically important disease. Besides the importance of the disease in the poultry industry, some H9N2 AIVs are also known to be zoonotic. The disease in humans appears to cause primarily a mild upper respiratory disease, and doesn't cause or only rarely causes the severe pneumonia often seen with other zoonotic AIVs like H5N1 or H7N9. Serologic studies in humans, particularly in occupationally exposed workers, show a large number of people with antibodies to H9N2, suggesting infection is commonly occurring. Of the four defined H9N2 poultry lineages, only two lineages, the G1 and the Y280 lineages, are associated with human infections. Almost all of the viruses from humans have a leucine at position 226 (H3 numbering) of the hemagglutinin associated with a higher affinity of binding with α2,6 sialic acid, the host cell receptor most commonly found on glycoproteins in the human upper respiratory tract. For unknown reasons there has also been a shift in recent years of poultry viruses in the G1 and Y280 lineages to also having leucine instead of glutamine, the amino acid found in most avian viruses, at position 226. The G1 and Y280 poultry lineages because of their known ability to infect humans, the high prevalence of the virus in poultry in endemic countries, the lack of antibody in most humans, and the shift of poultry viruses to more human-like receptor binding makes these viruses a human pandemic threat. Increased efforts for control of the virus, including through effective vaccine use in poultry, is warranted for both poultry and public health goals.

Ciliostasis of airway epithelial cells facilitates influenza A virus infection

Porcine precision-cut lung slices (PCLS) were used to analyze the effect of the ciliary activity on infection of airway epithelial cells by influenza viruses. Treatment of slices with 2% NaCl for 30 min resulted in reversible ciliostasis. When PCLS were infected by a swine influenza virus of the H3N2 subtype under ciliostatic conditions, the viral yield was about twofold or threefold higher at 24 or 48 h post-infection, respectively, as compared to slices with ciliary activity. Therefore, the cilia beating not only transports the mucus out of the airways, it also impedes virus infection.

PB2 E627K or D701N substitution does not change the virulence of canine influenza virus H3N2 in mice and dogs

Recently, canine influenza virus H3N2 (CIV H3N2) has circulated continuously in the dog populations of Asia and the United States (US). As humans have close contact with pet dogs, the circulation of CIV H3N2 is a cause for concern. Previous studies have reported that the E627K and D701N substitutions in the PB2 subunit enhanced viral pathogenicity to mammals in various influenza viruses. However, how the E627K and D701N substitutions in the PB2 subunit might affect the virulence of CIV H3N2 is unclear. Here, we constructed recombinant viruses by introducing E627K or D701N into the PB2 gene in the genetic background of A/Canine/Guangdong/02/2011H3N2 using a reverse-genetic system. The results showed that the E627K or D701N substitutions in the PB2 subunit of CIV H3N2 enhanced polymerase activity, but these substitutions did not impact viral pathogenicity in mice or beagles.

Amino Acid Substitutions HA A150V, PA A343T, and PB2 E627K Increase the Virulence of H5N6 Influenza Virus in Mice

H5N6 avian influenza viruses (AIVs) can cause severe pneumonia and death in humans. However, the molecular determinants of H5N6 influenza virus mammalian adaption are still unclear. Three amino acid substitutions (HA A150V, PA A343T, PB2 E627K) are observed in H5N6 virus A/duck/Zhejiang/6D2/2013 (6D2) in lung-to-lung passage in mice. These substitutions are crucial to the pathogenicity of mouse-adapted virus. In this study, we investigated the contribution of each amino acid substitution in the virus by reverse genetics. The results demonstrate that HA A150V greatly altered the receptor binding preference of 6D2. Virus bearing this substitution acquired increased mortality than mice infected with wild-type 6D2. The PA A343T substitution mildly enhanced viral polymerase activity but the reduced survival rate in mice indicates this substitution may change the immunoreaction of the host. The well-known PB2 E627K substitution increased eight folds the relative polymerase activity compared to PA A343T and resulted in 100% death rate in mice. In addition, we show that PA A343T dramatically exacerbates the effect of PB2 E627K on viral polymerase activity; when combined, these two substitutions work synergistically. However, HA A150V and PA A343T seemed to attenuate PB2 E627K in vivo, which implies the difference between mixed viral populations under natural condition and single population under experiment, specialization and cooperation in quasispecies is important in the process of adaption. This study suggests that HA A150V, PA A343T, and PB2 E627K are crucial in the adaption and increased pathogenicity of H5N6 in mammalian hosts.

Association of Increased Receptor-Binding Avidity of Influenza A(H9N2) Viruses with Escape from Antibody-Based Immunity and Enhanced Zoonotic Potential

We characterized 55 influenza A(H9N2) viruses isolated in Pakistan during 2014-2016 and found that the hemagglutinin gene is of the G1 lineage and that internal genes have differentiated into a variety of novel genotypes. Some isolates had up to 4-fold reduction in hemagglutination inhibition titers compared with older viruses. Viruses with hemagglutinin A180T/V substitutions conveyed this antigenic diversity and also caused up to 3,500-fold greater binding to avian-like and >20-fold greater binding to human-like sialic acid receptor analogs. This enhanced binding avidity led to reduced virus replication in primary and continuous cell culture. We confirmed that altered receptor-binding avidity of H9N2 viruses, including enhanced binding to human-like receptors, results in antigenic variation in avian influenza viruses. Consequently, current vaccine formulations might not induce adequate protective immunity in poultry, and emergence of isolates with marked avidity for human-like receptors increases the zoonotic risk.

Increased virulence of a PB2/HA mutant of an avian H9N2 influenza strain after three passages in porcine differentiated airway epithelial cells

We analyzed the adaptation of influenza viruses to growth in differentiated airway epithelial cells of a new host by passaging an avian H9N2 virus three times in porcine precision-cut lung slices (PCLS). Sequence analysis revealed four mutations: one each in the PB2 and NS1 proteins, and two in the HA protein. In this study, we characterized the PB2 mutation G685R by generating recombinant H9N2 viruses containing the PB2 single mutation alone or in combination with one of the HA mutations (A190V or T212I). When analyzed in porcine cells - a tracheal cell line (NPTr) or PCLS - the PB2-685 mutant did not provide a growth advantage and had no effect on the ciliary activity which is a virulence marker of swine influenza viruses. Pathogenicity for mice was also not increased by the single PB2 mutation. However, both double mutants (HA-190+PB2-685 and HA-212+PB2-685) showed significantly increased virulence in mice. Therefore, the mutations in the HA and PB2 proteins may confer early adaptation of an avian H9N2 virus to a mammalian host. In conclusion, we expect that a broader ensemble of mutations will be required to render an H9N2 virus virulent for pigs.