Activation of influenza A viruses by host proteases from swine airway epithelium

Establishment of Swine Primary Nasal, Tracheal, and Bronchial Epithelial Cell Culture Models for the Study of Influenza Virus Infection

We established primary porcine nasal, tracheal, and bronchial epithelial cells that recapitulate the physical and functional properties of the respiratory tract and have the ability to fully differentiate. Trans-well cultures demonstrated increased transepithelial electrical resistance over time the presence of tight junctions as demonstrated by immunohistochemistry. The nasal, tracheal, and bronchial epithelial cells developed cilia, secreted mucus, and expressed sialic acids on surface glycoproteins, the latter which are required for influenza A virus infection. Swine influenza viruses were shown to replicate efficiently in the primary epithelial cell cultures, supporting the use of these culture models to assess swine influenza and other virus infection. Primary porcine nasal, tracheal, and bronchial epithelial cell culture models enable assessment of emerging and novel influenza viruses for pandemic potential as well as mechanistic studies to understand mechanisms of infection, reassortment, and generation of novel virus. As swine are susceptible to infection with multiple viral and bacterial respiratory pathogens, these primary airway cell models may enable study of the cellular response to infection by pathogens associated with Porcine Respiratory Disease Complex.

Pigs lacking TMPRSS2 displayed fewer lung lesions and reduced inflammatory response when infected with influenza A virus

Influenza A virus (IAV) infection is initiated by hemagglutinin (HA), a glycoprotein exposed on the virion's lipid envelope that undergoes cleavage by host cell proteases to ensure membrane fusion, entry into the host cells, and completion of the viral cycle. Transmembrane protease serine S1 member 2 (TMPRSS2) is a host transmembrane protease expressed throughout the porcine airway epithelium and is purported to play a major role in the HA cleavage process, thereby influencing viral pathogenicity and tissue tropism. Pigs are natural hosts of IAV and IAV disease causes substantial economic impact on the pork industry worldwide. Previous studies in mice demonstrated that knocking out expression of TMPRSS2 gene was safe and inhibited the spread of IAV after experimental challenge. Therefore, we hypothesized that knockout of TMPRSS2 will prevent IAV infectivity in the swine model. We investigated this hypothesis by comparing pathogenesis of an H1N1pdm09 virus challenge in wildtype (WT) control and in TMPRSS2 knockout (TMPRSS2 -/-) pigs. We demonstrated that TMPRSS2 was expressed in the respiratory tract in WT pigs with and without IAV infection. No differences in nasal viral shedding and lung lavage viral titers were observed between WT and TMPRSS2 -/- pigs. However, the TMPRSS2 -/- pig group had significantly less lung lesions and significant reductions in antiviral and proinflammatory cytokines in the lung. The virus titer results in our direct challenge model contradict prior studies in the murine animal model, but the reduced lung lesions and cytokine profile suggest a possible role for TMPRSS2 in the proinflammatory antiviral response. Further research is warranted to investigate the role of TMPRSS2 in swine IAV infection and disease.

Gene editing of pigs to control influenza A virus infections

Proteolytic activation of the hemagglutinin (HA) glycoprotein by host cellular proteases is pivotal for influenza A virus (IAV) infectivity. Highly pathogenic avian influenza viruses possess the multibasic cleavage site of the HA which is cleaved by ubiquitous proteases, such as furin; in contrast, the monobasic HA motif is recognized and activated by trypsin-like proteases, such as the transmembrane serine protease 2 (TMPRSS2). Here, we aimed to determine the effects of TMPRSS2 on the replication of pandemic H1N1 and H3N2 subtype IAVs in the natural host, the pig. The use of the CRISPR/Cas 9 system led to the establishment of homozygous gene edited (GE) TMPRSS2 knockout (KO) pigs. Delayed IAV replication was demonstrated in primary respiratory cells of KO pigs in vitro. IAV infection in vivo resulted in significant reduction of virus shedding in the upper respiratory tract, and lower virus titers and pathological lesions in the lower respiratory tract of TMPRSS2 KO pigs as compared to WT pigs. Our findings could support the commercial use of GE pigs to minimize (i) the economic losses caused by IAV infection in pigs, and (ii) the emergence of novel IAVs with pandemic potential through genetic reassortment in the "mixing vessel", the pig.

Generation of porcine PK-15 cells lacking the Ifnar1 or Stat2 gene to optimize the efficiency of viral isolation

Because pigs are intermediate or amplifying hosts for several zoonotic viruses, the pig-derived PK-15 cell line is an indispensable tool for studying viral pathogenicity and developing treatments, vaccines, and preventive measures to mitigate the risk of disease outbreaks. However, we must consider the possibility of contamination by type I interferons (IFNs), such as IFNα and IFNβ, or IFN-inducing substances, such as virus-derived double-stranded RNA or bacterial lipopolysaccharides, in clinical samples, leading to lower rates of viral isolation. In this study, we aimed to generate a PK-15 cell line that can be used to isolate viruses from clinical samples carrying a risk of contamination by IFN-inducing substances. To this end, we depleted the IFN alpha and beta receptor subunit 1 (Ifnar1) gene or signal transducer and activator of transcription 2 (Stat2) gene in PK-15 cells using the clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated protein 9 method. Treatment of PK-15 cells lacking Ifnar1 or Stat2 with IFNβ or poly (I:C) resulted in no inhibitory effects on viral infection by a lentiviral vector, influenza virus, and Akabane virus. These results demonstrate that PK-15 cells lacking Ifnar1 or Stat2 could represent a valuable and promising tool for viral isolation, vaccine production, and virological investigations.

The role of influenza-A virus and coronavirus viral glycoprotein cleavage in host adaptation

While receptor binding is well recognized as a factor in influenza-A virus (IAV) and coronavirus (CoV) host adaptation, the role of viral glycoprotein cleavage has not been studied in detail so far. Interestingly, recent studies suggest that host species may differ in their protease repertoire available for cleavage. Furthermore, it was shown for certain bat-derived CoVs that proteolytic activation provides a critical barrier to infect human cells. Understanding the role of glycoprotein cleavage in different species and how IAV and CoVs adapt to a new protease repertoire may allow evaluating the zoonotic potential and risk posed by these viruses. Here, we summarize the current knowledge on the emergence of a multibasic cleavage site (CS) in the glycoproteins of IAVs and CoVs in different host species. Additionally, we discuss the role of transmembrane serine protease 2 (TMPRSS2) in virus activation and entry and a role of neuropilin-1 in acquisition of a multibasic CS in different hosts.

An overview of influenza A virus genes, protein functions, and replication cycle highlighting important updates

The recent research findings on influenza A virus (IAV) genome biology prompted us to present a comprehensive overview of IAV genes, protein functions, and replication cycle. The eight gene segments of the IAV genome encode 17 proteins, each having unique functions contributing to virus fitness in the host. The polymerase genes are essential determinants of IAV pathogenicity and virulence; however, other viral components also play crucial roles in the IAV replication, transmission, and adaptation. Specific adaptive mutations within polymerase (PB2, PB1, and PA) and glycoprotein-hemagglutinin (HA) and neuraminidase (NA) genes, may facilitate interspecies transmission and adaptation of IAV. The HA-NA interplay is essential for establishing the IAV infection; the low pH triggers the inactivation of HA-receptor binding, leading to significantly lower NA activities, indicating that the enzymatic function of NA is dependent on HA binding. While the HA and NA glycoproteins are required to initiate infection, M1, M2, NS1, and NEP proteins are essential for cytoplasmic trafficking of viral ribonucleoproteins (vRNPs) and the assembly of the IAV virions. The mechanisms that enable IAV to exploit the host cell resources to advance the infection are discussed. A comprehensive understanding of IAV genome biology is essential for developing antivirals to combat the IAV disease burden.

Evolutionary history of type II transmembrane serine proteases involved in viral priming

Type II transmembrane serine proteases (TTSPs) are a family of trypsin-like membrane-anchored serine proteases that play key roles in the regulation of some crucial processes in physiological conditions, including cardiac function, digestion, cellular iron homeostasis, epidermal differentiation, and immune responses. However, some of them, in particular TTSPs expressed in the human airways, were identified as host factors that promote the proteolytic activation and spread of respiratory viruses such as influenza virus, human metapneumovirus, and coronaviruses, including SARS-CoV-2. Given their involvement in viral priming, we hypothesized that members of the TTSP family may represent targets of positive selection, possibly as the result of virus-driven pressure. Thus, we investigated the evolutionary history of sixteen TTSP genes in mammals. Evolutionary analyses indicate that most of the TTSP genes that have a verified role in viral proteolytic activation present signals of pervasive positive selection, suggesting that viral infections represent a selective pressure driving the evolution of these proteases. We also evaluated genetic diversity in human populations and we identified targets of balancing selection in TMPRSS2 and TMPRSS4. This scenario may be the result of an ancestral and still ongoing host–pathogen arms race. Overall, our results provide evolutionary information about candidate functional sites and polymorphic positions in TTSP genes.

TMPRSS2 Activates Hemagglutinin-Esterase Glycoprotein of Influenza C Virus

Influenza C virus (ICV) has only one kind of spike protein, the hemagglutinin-esterase (HE) glycoprotein. HE functions similarly to hemagglutinin (HA) and neuraminidase of the influenza A and B viruses (IAV and IBV, respectively). It has a monobasic site, which is cleaved by some host enzymes. The cleavage is essential to activating the virus, but the enzyme or enzymes in the respiratory tract have not been identified. This study investigated whether the host serine proteases, transmembrane protease serine S1 member 2 (TMPRSS2) and human airway trypsin-like protease (HAT), which reportedly cleave HA of IAV/IBV, are involved in HE cleavage. We established TMPRSS2- and HAT-expressing MDCK cells (MDCK-TMPRSS2 and MDCK-HAT). ICV showed multicycle replication with HE cleavage without trypsin in MDCK-TMPRSS2 cells as well as IAV did. The HE cleavage and multicycle replication did not appear in MDCK-HAT cells infected with ICV without trypsin, while HA cleavage and multistep growth of IAV appeared in the cells. Amino acid sequences of the HE cleavage site in 352 ICV strains were completely preserved. Camostat and nafamostat suppressed the growth of ICV and IAV in human nasal surface epithelial (HNE) cells. Therefore, this study revealed that, at least, TMPRSS2 is involved in HE cleavage and suggested that nafamostat could be a candidate for therapeutic drugs for ICV infection. IMPORTANCE Influenza C virus (ICV) is a pathogen that causes acute respiratory illness, mostly in children, but there are no anti-ICV drugs. ICV has only one kind of spike protein, the hemagglutinin-esterase (HE) glycoprotein on the virion surface, which possesses receptor-binding, receptor-destroying, and membrane fusion activities. The HE cleavage is essential for the virus to be activated, but the enzyme or enzymes in the respiratory tract have not been identified. This study revealed that transmembrane protease serine S1 member 2 (TMPRSS2), and not human airway trypsin-like protease (HAT), is involved in HE cleavage. This is a novel study on the host enzymes involved in HE cleavage, and the result suggests that the host enzymes, such as TMPRSS2, may be a target for therapeutic drugs of ICV infection.

Hemagglutinins of avian influenza viruses are proteolytically activated by TMPRSS2 in human and murine airway cells

Cleavage of the influenza A virus (IAV) hemagglutinin (HA) by host proteases is indispensable for virus replication. Most IAVs possess a monobasic HA cleavage site cleaved by trypsin-like proteases. Previously, the transmembrane protease TMPRSS2 was shown to be essential for proteolytic activation of IAV HA subtypes H1, H2, H7 and H10 in mice. In contrast, additional proteases are involved in activation of certain H3 IAVs, indicating that HAs with monobasic cleavage site can differ in their sensitivity to host proteases. Here, we investigated the role of TMPRSS2 in proteolytic activation of avian HA subtypes H1 to H11 and H14 to H16 in human and mouse airway cell cultures. Using reassortant viruses carrying representative HAs, we analysed HA cleavage and multicycle replication in (i) lung cells of TMPRSS2-deficient mice and (ii) Calu-3 cells and primary human bronchial cells subjected to morpholino oligomer-mediated knockdown of TMPRSS2 activity. TMPRSS2 was found to be crucial for activation of H1 to H11, H14 and H15 in airway cells of human and mouse. Only H9 with an R-S-S-R cleavage site and H16 were proteolytically activated in the absence of TMPRSS2 activity, albeit with reduced efficiency. Moreover, a TMPRSS2-orthologous protease from duck supported activation of H1 to H11, H15 and H16 in MDCK cells. Together, our data demonstrate that in human and murine respiratory cells, TMPRSS2 is the major activating protease of almost all IAV HA subtypes with monobasic cleavage site. Furthermore, our results suggest that TMPRSS2 supports activation of IAV with monobasic cleavage site in ducks. Importance Human infections with avian influenza A viruses upon exposure to infected birds are frequently reported and have received attention as a potential pandemic threat. Cleavage of the envelope glycoprotein hemagglutinin (HA) by host proteases is a prerequisite for membrane fusion and essential for virus infectivity. In this study, we identify the transmembrane protease TMPRSS2 as the major activating protease of avian influenza virus HAs of subtypes H1 to H11, H14 and H15 in human and murine airway cells. Our data demonstrate that inhibition of TMPRSS2 activity may provide a useful approach for the treatment of human infections with avian influenza viruses that should be considered for pandemic preparedness as well. Additionally, we show that a TMPRSS2-orthologous protease from duck can activate avian influenza virus HAs with a monobasic cleavage site and thus represents a potential virus-activating protease in waterfowl, the primary reservoir for influenza A viruses.

The clinically used serine protease inhibitor nafamostat reduces influenza virus replication and cytokine production in human airway epithelial cells and viral replication in mice

The effects of the clinically used protease inhibitor nafamostat on influenza virus replication have not been well studied. Primary human tracheal (HTE) and nasal (HNE) epithelial cells were pretreated with nafamostat and infected with the 2009 pandemic [A/Sendai-H/108/2009/(H1N1) pdm09] or seasonal [A/New York/55/2004(H3N2)] influenza virus. Pretreatment with nafamostat reduced the titers of the pandemic and seasonal influenza viruses and the secretion of inflammatory cytokines, including interleukin-6 and tumor necrosis factor-α, in the supernatants of the cells infected with the pandemic influenza virus. HTE and HNE cells exhibited mRNA and/or protein expression of transmembrane protease serine 2 (TMPRSS2), TMPRSS4, and TMPRSS11D. Pretreatment with nafamostat reduced cleavage of the precursor protein HA0 of the pandemic influenza virus into subunit HA1 in HTE cells and reduced the number of acidic endosomes in HTE and HNE cells where influenza virus RNA enters the cytoplasm. Additionally, nafamostat (30 mg/kg/day, intraperitoneal administration) reduced the levels of the pandemic influenza virus [A/Hyogo/YS/2011 (H1N1) pdm09] in mouse lung washes. These findings suggest that nafamostat may inhibit influenza virus replication in human airway epithelial cells and mouse lungs and reduce infection-induced airway inflammation by modulating cytokine production.

Evolution of A(H1N1) pdm09 influenza virus masking by glycosylation

Introduction: As the pathogen that caused the first influenza virus pandemic in this century, the swine-origin A(H1N1) pdm09 influenza virus has caused continuous harm to human public health. The evolution of hemagglutinin protein glycosylation sites, including the increase in number and positional changes, is an important way for influenza viruses to escape host immune pressure. Based on the traditional influenza virus molecular monitoring, special attention should be paid to the influence of glycosylation evolution on the biological characteristics of virus antigenicity, transmission and pathogenicity. The epidemiological significance of glycosylation mutants should be analyzed as a predictive tool for early warning of new outbreaks and pandemics, as well as the design of vaccines and drug targets.Areas covered: We review on the evolutionary characteristics of glycosylation on the HA protein of the A(H1N1)pdm09 influenza virus in the last ten years.Expert opinion: We discuss the crucial impact of evolutionary glycosylation on the biological characteristics of the virus and the host immune responses, summarize studies revealing different roles of glycosylation play during host adaptation. Although these studies show the significance of glycosylation evolution in host-virus interaction, much remains to be discovered about the mechanism.

Biomedical nanoparticle design: What we can learn from viruses

Viruses are nanomaterials with a number of properties that surpass those of many synthetic nanoparticles (NPs) for biomedical applications. They possess a rigorously ordered structure, come in a variety of shapes, and present unique surface elements, such as spikes. These attributes facilitate propitious biodistribution, the crossing of complex biological barriers and a minutely coordinated interaction with cells. Due to the orchestrated sequence of interactions of their stringently arranged particle corona with cellular surface receptors they effectively identify and infect their host cells with utmost specificity, while evading the immune system at the same time. Furthermore, their efficacy is enhanced by their response to stimuli and the ability to spread from cell to cell. Over the years, great efforts have been made to mimic distinct viral traits to improve biomedical nanomaterial performance. However, a closer look at the literature reveals that no comprehensive evaluation of the benefit of virus-mimetic material design on the targeting efficiency of nanomaterials exists. In this review we, therefore, elucidate the impact that viral properties had on fundamental advances in outfitting nanomaterials with the ability to interact specifically with their target cells. We give a comprehensive overview of the diverse design strategies and identify critical steps on the way to reducing them to practice. More so, we discuss the advantages and future perspectives of a virus-mimetic nanomaterial design and try to elucidate if viral mimicry holds the key for better NP targeting.

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.

TMPRSS2 Is the Major Activating Protease of Influenza A Virus in Primary Human Airway Cells and Influenza B Virus in Human Type II Pneumocytes

Cleavage of influenza virus hemagglutinin (HA) by host cell proteases is essential for virus infectivity and spread. We previously demonstrated in vitro that the transmembrane protease TMPRSS2 cleaves influenza A virus (IAV) and influenza B virus (IBV) HA possessing a monobasic cleavage site. Subsequent studies revealed that TMPRSS2 is crucial for the activation and pathogenesis of H1N1pdm and H7N9 IAV in mice. In contrast, activation of H3N2 IAV and IBV was found to be independent of TMPRSS2 expression and supported by an as-yet-undetermined protease(s). Here, we investigated the role of TMPRSS2 in proteolytic activation of IAV and IBV in three human airway cell culture systems: primary human bronchial epithelial cells (HBEC), primary type II alveolar epithelial cells (AECII), and Calu-3 cells. Knockdown of TMPRSS2 expression was performed using a previously described antisense peptide-conjugated phosphorodiamidate morpholino oligomer, T-ex5, that interferes with splicing of TMPRSS2 pre-mRNA, resulting in the expression of enzymatically inactive TMPRSS2. T-ex5 treatment produced efficient knockdown of active TMPRSS2 in all three airway cell culture models and prevented proteolytic activation and multiplication of H7N9 IAV in Calu-3 cells and H1N1pdm, H7N9, and H3N2 IAV in HBEC and AECII. T-ex5 treatment also inhibited the activation and spread of IBV in AECII but did not affect IBV activation in HBEC and Calu-3 cells. This study identifies TMPRSS2 as the major HA-activating protease of IAV in human airway cells and IBV in type II pneumocytes and as a potential target for the development of novel drugs to treat influenza infections.IMPORTANCE Influenza A viruses (IAV) and influenza B viruses (IBV) cause significant morbidity and mortality during seasonal outbreaks. Cleavage of the viral surface glycoprotein hemagglutinin (HA) by host proteases is a prerequisite for membrane fusion and essential for virus infectivity. Inhibition of relevant proteases provides a promising therapeutic approach that may avoid the development of drug resistance. HA of most influenza viruses is cleaved at a monobasic cleavage site, and a number of proteases have been shown to cleave HA in vitro This study demonstrates that the transmembrane protease TMPRSS2 is the major HA-activating protease of IAV in primary human bronchial cells and of both IAV and IBV in primary human type II pneumocytes. It further reveals that human and murine airway cells can differ in their HA-cleaving protease repertoires. Our data will help drive the development of potent and selective protease inhibitors as novel drugs for influenza treatment.

Influenza A virus mimetic nanoparticles trigger selective cell uptake

Poor target cell specificity is currently a major shortcoming of nanoparticles (NPs) used for biomedical applications. It causes significant material loss to off-target sites and poor availability at the intended delivery site. To overcome this limitation, we designed particles that identify cells in a virus-like manner. As a blueprint, we chose a mechanism typical of influenza A virus particles in which ectoenzymatic hemagglutinin activation by target cells is a mandatory prerequisite for binding to a secondary target structure that finally confirms cell identity and allows for uptake of the virus. We developed NPs that probe mesangial cells for the presence of angiotensin-converting enzyme on their surface using angiotensin I (Ang-I) as a proligand. This initial interaction enzymatically transforms Ang-I to a secondary ligand angiotensin II (Ang-II) that has the potential to bind in a second stage to Ang-II type-1 receptor (AT1R). The presence of the receptor confirms the target cell identity and triggers NP uptake via endocytosis. Our virus-mimetic NPs showed outstanding target-cell affinity with picomolar avidities and were able to selectively identify these cells in the presence of 90% off-target cells that carried only the AT1R. Our results demonstrate that the design of virus-mimetic cell interactive NPs is a valuable strategy to enhance NP specificity for therapeutic and diagnostic applications. Our set of primary and secondary targets is particularly suited for the identification of mesangial cells that play a pivotal role in diabetic nephropathy, one of the leading causes of renal failure, for which currently no treatment exists.

Membrane-Anchored Serine Proteases: Host Cell Factors in Proteolytic Activation of Viral Glycoproteins

Over one third of all known proteolytic enzymes are serine proteases. Among these, the trypsin-like serine proteases comprise one of the best characterized subfamilies due to their essential roles in blood coagulation, food digestion, fibrinolysis, or immunity. Trypsin-like serine proteases possess primary substrate specificity for basic amino acids. Most of the well-characterized trypsin-like proteases such as trypsin, plasmin, or urokinase are soluble proteases that are secreted into the extracellular environment. At the turn of the millennium, a number of novel trypsin-like serine proteases have been identified that are anchored in the cell membrane, either by a transmembrane domain at the N- or C-terminus or via a glycosylphosphatidylinositol (GPI) linkage. Meanwhile more than 20 membrane-anchored serine proteases (MASPs) have been identified in human and mouse, and some of them have emerged as key regulators of mammalian development and homeostasis. Thus, the MASP corin and TMPRSS6/matriptase-2 have been demonstrated to be the activators of the atrial natriuretic peptide (ANP) and key regulator of hepcidin expression, respectively. Furthermore, MASPs have been recognized as host cell factors activating respiratory viruses including influenza virus as well as severe acute respiratory syndrome (SARS) and Middle East respiratory syndrome (MERS) coronaviruses. In particular, transmembrane protease serine S1 member 2 (TMPRSS2) has been shown to be essential for proteolytic activation and consequently spread and pathogenesis of a number of influenza A viruses in mice and as a factor associated with severe influenza virus infection in humans.

This review gives an overview on the physiological functions of the fascinating and rapidly evolving group of MASPs and a summary of the current knowledge on their role in proteolytic activation of viral fusion proteins.

Different residues in the SARS-CoV spike protein determine cleavage and activation by the host cell protease TMPRSS2

The spike (S) protein of severe acute respiratory syndrome coronavirus (SARS-CoV) mediates viral entry into target cells. Cleavage and activation of SARS S by a host cell protease is essential for infectious viral entry and the responsible enzymes are potential targets for antiviral intervention. The type II transmembrane serine protease TMPRSS2 cleaves and activates SARS S in cell culture and potentially also in the infected host. Here, we investigated which determinants in SARS S control cleavage and activation by TMPRSS2. We found that SARS S residue R667, a previously identified trypsin cleavage site, is also required for S protein cleavage by TMPRSS2. The cleavage fragments produced by trypsin and TMPRSS2 differed in their decoration with N-glycans, suggesting that these proteases cleave different SARS S glycoforms. Although R667 was required for SARS S cleavage by TMPRSS2, this residue was dispensable for TMPRSS2-mediated S protein activation. Conversely, residue R797, previously reported to be required for SARS S activation by trypsin, was dispensable for S protein cleavage but required for S protein activation by TMPRSS2. Collectively, these results show that different residues in SARS S control cleavage and activation by TMPRSS2, suggesting that these processes are more complex than initially appreciated.

The differentiated airway epithelium infected by influenza viruses maintains the barrier function despite a dramatic loss of ciliated cells

Virus-host interactions in the respiratory epithelium during long term influenza virus infection are not well characterized. Therefore, we developed an air-liquid interface culture system for differentiated porcine respiratory epithelial cells to study the effect of virus-induced cellular damage. In our well-differentiated cells, α2,6-linked sialic acid is predominantly expressed on the apical surface and the basal cells mainly express α2,3-linked sialic acid. During the whole infection period, release of infectious virus was maintained at a high titre for more than seven days. The infected epithelial cells were subject to apoptosis resulting in the loss of ciliated cells together with a thinner thickness. Nevertheless, the airway epithelium maintained trans-epithelial electrical resistance and retained its barrier function. The loss of ciliated cells was compensated by the cells which contained the KRT5 basal cell marker but were not yet differentiated into ciliated cells. These specialized cells showed an increase of α2,3-linked sialic acid on the apical surface. In sum, our results help to explain the localized infection of the airway epithelium by influenza viruses. The impairment of mucociliary clearance in the epithelial cells provides an explanation why prior viral infection renders the host more susceptible to secondary co-infection by another pathogen.

Genetic characterization of an adapted pandemic 2009 H1N1 influenza virus that reveals improved replication rates in human lung epithelial cells

The 2009 influenza pandemic originated from a swine-origin H1N1 virus, which, although less pathogenic than anticipated, may acquire additional virulence-associated mutations in the future. To estimate the potential risk, we sequentially passaged the isolate A/Hamburg/04/2009 in A549 human lung epithelial cells. After passage 6, we observed a 100-fold increased replication rate. High-throughput sequencing of viral gene segments identified five dominant mutations, whose contribution to the enhanced growth was analyzed by reverse genetics. The increased replication rate was pinpointed to two mutations within the hemagglutinin (HA) gene segment (HA1 D130E, HA2 I91L), near the receptor binding site and the stem domain. The adapted virus also replicated more efficiently in mice in vivo. Enhanced replication rate correlated with increased fusion pH of the HA protein and a decrease in receptor affinity. Our data might be relevant for surveillance of pre-pandemic strains and development of high titer cell culture strains for vaccine production.

The serine protease inhibitor camostat inhibits influenza virus replication and cytokine production in primary cultures of human tracheal epithelial cells

BACKGROUND

Serine proteases act through the proteolytic cleavage of the hemagglutinin (HA) of influenza viruses for the entry of influenza virus into cells, resulting in infection. However, the inhibitory effects of serine protease inhibitors on influenza virus infection of human airway epithelial cells, and on their production of inflammatory cytokines are unclear.

METHODS

Primary cultures of human tracheal epithelial cells were treated with four types of serine protease inhibitors, including camostat, and infected with A/Sendai-H/108/2009/(H1N1) pdm09 or A/New York/55/2004(H3N2).

RESULTS

Camostat reduced the amounts of influenza viruses in the supernatants and viral RNA in the cells. It reduced the cleavage of an influenza virus precursor protein, HA0, into the subunit HA1. Camostat also reduced the concentrations of the cytokines interleukin (IL)-6 and tumor necrosis factor (TNF)-α in the supernatants. Gabexate and aprotinin reduced the viral titers and RNA levels in the cells, and aprotinin reduced the concentrations of TNF-α in the supernatants. The proteases transmembrane protease serine S1 member (TMPRSS) 2 and HAT (human trypsin-like protease: TMPRSS11D), which are known to cleave HA0 and to activate the virus, were detected at the cell membrane and in the cytoplasm. mRNA encoding TMPRSS2, TMPRSS4 and TMPRSS11D was detectable in the cells, and the expression levels were not affected by camostat.

CONCLUSIONS

These findings suggest that human airway epithelial cells express these serine proteases and that serine protease inhibitors, especially camostat, may reduce influenza viral replication and the resultant production of inflammatory cytokines possibly through inhibition of activities of these proteases.

Influenza virus activating host proteases: Identification, localization and inhibitors as potential therapeutics

Cellular proteases are reponsible for activation of influenza virus hemagglutinin (HA) in epithelial tissues of the respiratory tract. The trans-Golgi network (TGN) is the main subcellular compartment where HA cleavage occurs during its biosynthesis. The proteolytic HA cleavage is an indispensable prerequisite for the fusion of viral with endosomal membrane and the delivery of the virus genome into the cell. Both, the structure and accessibility of the HA cleavage site determine the responsible host protease(s) for cutting. Most influenza virus strains contain a HA sequence with a single arginine at the cleavage site suitable for processing by the trypsin-like serine proteases human airway trypsin-like protease (HAT) and transmembrane protease serine 2 (TMPRSS2), albeit a minority of viruses possesses HA cleavage site motifs that are processed by other proteases. TMPRSS2-deficient mice demonstrated the relevance of TMPRSS2 for pneumotropism and pathogenicity of H1N1 and H7N9 virus infections. In contrast, H3N2 virus infections are promoted by an additional not yet identified protease. Highly pathogenic avian H5 and H7 viruses are characterized by an enlarged cleavage site loop containing a multibasic amino acid motif, where the eukaryotic subtilases furin or PC5/6 cleave. Their ubiquitous presence in the organism allows a systemic virus infection. Peptidomimetic inhibitors derived from the HA cleavage site inhibit the HA-activating proteases and thus virus propagation.

Equine and Canine Influenza H3N8 Viruses Show Minimal Biological Differences Despite Phylogenetic Divergence

The A/H3N8 canine influenza virus (CIV) emerged from A/H3N8 equine influenza virus (EIV) around the year 2000 through the transfer of a single virus from horses to dogs. We defined and compared the biological properties of EIV and CIV by examining their genetic variation, infection, and growth in different cell cultures, receptor specificity, hemagglutinin (HA) cleavage, and infection and growth in horse and dog tracheal explant cultures. Comparison of sequences of viruses from horses and dogs revealed mutations that may be linked to host adaptation and tropism. We prepared infectious clones of representative EIV and CIV strains that were similar to the consensus sequences of viruses from each host. The rescued viruses, including HA and neuraminidase (NA) double reassortants, exhibited similar degrees of long-term growth in MDCK cells. Different host cells showed various levels of susceptibility to infection, but no differences in infectivity were seen when comparing viruses. All viruses preferred α2-3- over α2-6-linked sialic acids for infections, and glycan microarray analysis showed that EIV and CIV HA-Fc fusion proteins bound only to α2-3-linked sialic acids. Cleavage assays showed that EIV and CIV HA proteins required trypsin for efficient cleavage, and no differences in cleavage efficiency were seen. Inoculation of the viruses into tracheal explants revealed similar levels of infection and replication by each virus in dog trachea, although EIV was more infectious in horse trachea than CIV.

IMPORTANCE

Influenza A viruses can cross species barriers and cause severe disease in their new hosts. Infections with highly pathogenic avian H5N1 virus and, more recently, avian H7N9 virus have resulted in high rates of lethality in humans. Unfortunately, our current understanding of how influenza viruses jump species barriers is limited. Our aim was to provide an overview and biological characterization of H3N8 equine and canine influenza viruses using various experimental approaches, since the canine virus emerged from horses approximately 15 years ago. We showed that although there were numerous genetic differences between the equine and canine viruses, this variation did not result in dramatic biological differences between the viruses from the two hosts, and the viruses appeared phenotypically equivalent in most assays we conducted. These findings suggest that the cross-species transmission and adaptation of influenza viruses may be mediated by subtle changes in virus biology.

Identification of a broad-spectrum antiviral small molecule against severe acute respiratory syndrome coronavirus and Ebola, Hendra, and Nipah viruses by using a novel high-throughput screening assay

Severe acute respiratory syndrome coronavirus (SARS-CoV) and Ebola, Hendra, and Nipah viruses are members of different viral families and are known causative agents of fatal viral diseases. These viruses depend on cathepsin L for entry into their target cells. The viral glycoproteins need to be primed by protease cleavage, rendering them active for fusion with the host cell membrane. In this study, we developed a novel high-throughput screening assay based on peptides, derived from the glycoproteins of the aforementioned viruses, which contain the cathepsin L cleavage site. We screened a library of 5,000 small molecules and discovered a small molecule that can inhibit the cathepsin L cleavage of all viral peptides with minimal inhibition of cleavage of a host protein-derived peptide (pro-neuropeptide Y). The small molecule inhibited the entry of all pseudotyped viruses in vitro and the cleavage of SARS-CoV spike glycoprotein in an in vitro cleavage assay. In addition, the Hendra and Nipah virus fusion glycoproteins were not cleaved in the presence of the small molecule in a cell-based cleavage assay. Furthermore, we demonstrate that the small molecule is a mixed inhibitor of cathepsin L. Our broad-spectrum antiviral small molecule appears to be an ideal candidate for future optimization and development into a potent antiviral against SARS-CoV and Ebola, Hendra, and Nipah viruses.

IMPORTANCE

We developed a novel high-throughput screening assay to identify small molecules that can prevent cathepsin L cleavage of viral glycoproteins derived from SARS-CoV and Ebola, Hendra, and Nipah viruses that are required for their entry into the host cell. We identified a novel broad-spectrum small molecule that could block cathepsin L-mediated cleavage and thus inhibit the entry of pseudotypes bearing the glycoprotein derived from SARS-CoV or Ebola, Hendra, or Nipah virus. The small molecule can be further optimized and developed into a potent broad-spectrum antiviral drug.

The hemagglutinin: a determinant of pathogenicity

The hemagglutinin (HA) is a prime determinant of the pathogenicity of influenza A viruses. It initiates infection by binding to cell surface receptors and by inducing membrane fusion. The fusion capacity of HA depends on cleavage activation by host proteases, and it has long been known that highly pathogenic avian influenza viruses displaying a multibasic cleavage site differ in protease sensitivity from low pathogenic avian and mammalian influenza viruses with a monobasic cleavage site. Evidence is increasing that there are also variations in proteolytic activation among the viruses with a monobasic cleavage site, and several proteases have been identified recently that activate these viruses in a natural setting. Differences in protease sensitivity of HA and in tissue specificity of the enzymes are important determinants for virus tropism in the respiratory tract and for systemic spread of infection. Protease inhibitors that interfere with cleavage activation have the potential to be used for antiviral therapy and attenuated viruses have been generated by mutation of the cleavage site that can be used for the development of inactivated and live vaccines. It has long been known that human and avian influenza viruses differ in their specificity for sialic acid-containing cell receptors, and it is now clear that human tissues contain also receptors for avian viruses. Differences in receptor-binding specificity of seasonal and zoonotic viruses and differential expression of receptors for these viruses in the human respiratory tract account, at least partially, for the severity of disease. Receptor binding and fusion activation are modulated by HA glycosylation, and interaction of the glycans of HA with cellular lectins also affects virus infectivity. Interestingly, some of the mechanisms underlying pathogenicity are determinants of host range and transmissibility, as well.