Pathogenesis and transmission of SARS-CoV-2 in golden hamsters.

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), a novel coronavirus with high nucleotide identity to SARS-CoV and to SARS-related coronaviruses that have been detected in horseshoe bats, has spread across the world and had a global effect on healthcare systems and economies1,2. A suitable small animal model is needed to support the development of vaccines and therapies. Here we report the pathogenesis and transmissibility of SARS-CoV-2 in golden (Syrian) hamsters (Mesocricetus auratus). Immunohistochemistry assay demonstrated the presence of viral antigens in nasal mucosa, bronchial epithelial cells and areas of lung consolidation on days 2 and 5 after inoculation with SARS-CoV-2, followed by rapid viral clearance and pneumocyte hyperplasia at 7 days after inoculation. We also found viral antigens in epithelial cells of the duodenum, and detected viral RNA in faeces. Notably, SARS-CoV-2 was transmitted efficiently from inoculated hamsters to naive hamsters by direct contact and via aerosols. Transmission via fomites in soiled cages was not as efficient. Although viral RNA was continuously detected in the nasal washes of inoculated hamsters for 14 days, the communicable period was short and correlated with the detection of infectious virus but not viral RNA. Inoculated and naturally infected hamsters showed apparent weight loss on days 6-7 post-inoculation or post-contact; all hamsters returned to their original weight within 14 days and developed neutralizing antibodies. Our results suggest that features associated with SARS-CoV-2 infection in golden hamsters resemble those found in humans with mild SARS-CoV-2 infections.

Reduced Pathogenicity and Transmission Potential of Omicron BA.1 and BA.2 Sublineages Compared with the Early Severe Acute Respiratory Syndrome Coronavirus 2 D614G Variant in Syrian Hamsters.

BACKGROUND: The epidemiological advantage of Omicron variant is evidenced by its rapid spread and the ability to outcompete prior variants. Among Omicron sublineages, early outbreaks were dominated by BA.1, while BA.2 has gained dominance since February 2022. The relative pathogenicity and transmissibility of BA.1 and BA.2 have not been fully defined. METHODS: We compared viral loads and clinical signs in Syrian hamsters after infection with BA.1, BA.2, or D614G variant. A competitive transmission model and next-generation sequencing were used to compare the relative transmission potential of BA.1 and BA.2. RESULTS: BA.1 and BA.2 caused no apparent clinical signs, while D614G caused more than 10% weight loss. Higher viral loads were detected in nasal wash samples and nasal turbinate and lung tissues from BA.1-inoculated hamsters compared with BA.2-inoculated hamsters. No aerosol transmission was observed for BA.1 or BA.2 under the experimental condition in which D614G transmitted efficiently. BA.1 and BA.2 were able to transmit among hamsters via direct contact; however, BA.1 transmitted more efficiently than BA.2 under the competitive transmission model. No recombination was detected from direct contacts exposed simultaneously to BA.1 and BA.2. CONCLUSIONS: Omicron BA.1 and BA.2 demonstrated attenuated pathogenicity and reduced transmission potential in hamsters compared with early SARS-CoV-2 strains.

A(H1N1)pdm09 Influenza Viruses Replicating in Ferret Upper or Lower Respiratory Tract Differed in Onward Transmission Potential by Air.

BACKGROUND: A(H1N1)pdm09 influenza viruses replicate efficiently in respiratory epithelia and are transmitted via respiratory droplets and aerosols expelled by infected hosts. The relative onward transmission potential of influenza viruses replicating in the upper and lower respiratory epithelial cells has not been fully defined. METHODS: Wild-type and barcoded A(H1N1)pdm09 viruses that differed by 2 synonymous mutations per gene segment were inoculated into ferrets via intranasal and intratracheal routes. Naive recipients were exposed to the exhaled breath of inoculated donors for 8 hours on day 2 postinoculation. Onward transmission potential of wild-type and barcoded genotypes were monitored by next generation sequencing. RESULTS: Transmissible airborne particles were respired from the upper but not the lower respiratory epithelial cells of donor ferrets. There was limited mixing of viral populations replicating in the upper and lower respiratory tissues. CONCLUSIONS: The ferret upper respiratory epithelium was mapped as the anatomic site that generated influenza virus-laden particles mediating onward transmission by air. Our results suggest that vaccines and antivirals should aim to reduce viral loads in the upper respiratory tract for prevention of influenza transmission.

Cellular tropism of SARS-CoV-2 in the respiratory tract of Syrian hamsters and B6.Cg-Tg(K18-ACE2)2Prlmn/J transgenic mice.

Several animal models have been developed to study the pathophysiology of SARS-CoV-2 infection and to evaluate vaccines and therapeutic agents for this emerging disease. Similar to infection with SARS-CoV-1, infection of Syrian hamsters with SARS-CoV-2 results in moderate respiratory disease involving the airways and lung parenchyma but does not lead to increased mortality. Using a combination of immunohistochemistry and transmission electron microscopy, we showed that the epithelium of the conducting airways of hamsters was the primary target for viral infection within the first 5 days of infection, with little evidence of productive infection of pneumocytes. At 6 days postinfection, antigen was cleared but parenchymal damage persisted, and the major pathological changes resolved by day 14. These findings are similar to those previously reported for hamsters with SARS-CoV-1 infection. In contrast, infection of K18-hACE2 transgenic mice resulted in pneumocyte damage, with viral particles and replication complexes in both type I and type II pneumocytes together with the presence of convoluted or cubic membranes; however, there was no evidence of virus replication in the conducting airways. The Syrian hamster is a useful model for the study of SARS-CoV-2 transmission and vaccination strategies, whereas infection of the K18-hCE2 transgenic mouse results in lethal disease with fatal neuroinvasion but with sparing of conducting airways.

Defining the sizes of airborne particles that mediate influenza transmission in ferrets.

Epidemics and pandemics of influenza are characterized by rapid global spread mediated by non-mutually exclusive transmission modes. The relative significance between contact, droplet, and airborne transmission is yet to be defined, a knowledge gap for implementing evidence-based infection control measures. We devised a transmission chamber that separates virus-laden particles by size and determined the particle sizes mediating transmission of influenza among ferrets through the air. Ferret-to-ferret transmission was mediated by airborne particles larger than 1.5 µm, consistent with the quantity and size of virus-laden particles released by the donors. Onward transmission by donors was most efficient before fever onset and may continue for 5 days after inoculation. Multiple virus gene segments enhanced the transmissibility of a swine influenza virus among ferrets by increasing the release of virus-laden particles into the air. We provide direct experimental evidence of influenza transmission via droplets and fine droplet nuclei, albeit at different efficiency.

CLEC5A-Mediated Enhancement of the Inflammatory Response in Myeloid Cells Contributes to Influenza Virus Pathogenicity In Vivo.

Human infections with influenza viruses exhibit mild to severe clinical outcomes as a result of complex virus-host interactions. Induction of inflammatory mediators via pattern recognition receptors may dictate subsequent host responses for pathogen clearance and tissue damage. We identified that human C-type lectin domain family 5 member A (CLEC5A) interacts with the hemagglutinin protein of influenza viruses expressed on lentiviral pseudoparticles through lectin screening. Silencing CLEC5A gene expression, blocking influenza-CLEC5A interactions with anti-CLEC5A antibodies, or dampening CLEC5A-mediated signaling using a spleen tyrosine kinase inhibitor consistently reduced the levels of proinflammatory cytokines produced by human macrophages without affecting the replication of influenza A viruses of different subtypes. Infection of bone marrow-derived macrophages from CLEC5A-deficient mice showed reduced levels of tumor necrosis factor alpha (TNF-α) and IP-10 but elevated alpha interferon (IFN-α) compared to those of wild-type mice. The heightened type I IFN response in the macrophages of CLEC5A-deficient mice was associated with upregulated TLR3 mRNA after treatment with double-stranded RNA. Upon lethal challenges with a recombinant H5N1 virus, CLEC5A-deficient mice showed reduced levels of proinflammatory cytokines, decreased immune cell infiltration in the lungs, and improved survival compared to the wild-type mice, despite comparable viral loads noted throughout the course of infection. The survival difference was more prominent at a lower dose of inoculum. Our results suggest that CLEC5A-mediated enhancement of the inflammatory response in myeloid cells contributes to influenza pathogenicity in vivo and may be considered a therapeutic target in combination with effective antivirals. Well-orchestrated host responses together with effective viral clearance are critical for optimal clinical outcome after influenza infections. IMPORTANCE: Multiple pattern recognition receptors work in synergy to sense viral RNA or proteins synthesized during influenza replication and mediate host responses for viral control. Well-orchestrated host responses may help to maintain the inflammatory response to minimize tissue damage while inducing an effective adaptive immune response for viral clearance. We identified that CLEC5A, a C-type lectin receptor which has previously been reported to mediate flavivirus-induced inflammatory responses, enhanced induction of proinflammatory cytokines and chemokines in myeloid cells after influenza infections. CLEC5A-deficient mice infected with influenza virus showed reduced inflammation in the lungs and improved survival compared to that of the wild-type mice despite comparable viral loads. The survival difference was more prominent at a lower dose of inoculum. Collectively, our results suggest that dampening CLEC5A-mediated inflammatory responses in myeloid cells reduces immunopathogenesis after influenza infections.

Transmission of H7N9 Influenza Viruses with a Polymorphism at PB2 Residue 627 in Chickens and Ferrets.

UNLABELLED: Poultry exposure is a major risk factor for human H7N9 zoonotic infections, for which the mode of transmission remains unclear. We studied the transmission of genetically related poultry and human H7N9 influenza viruses differing by four amino acids, including the host determinant PB2 residue 627. A/Silkie chicken/HK/1772/2014 (SCk1772) and A/HK/3263/14 (HK3263) replicated to comparable titers in chickens, with superior oropharyngeal over cloacal shedding; both viruses transmitted efficiently among chickens via direct contact but inefficiently via the airborne route. Interspecies transmission via the airborne route was observed for ferrets exposed to the SCk1772- or HK3263-infected chickens, while low numbers of copies of influenza viral genome were detected in the air, predominantly at particle sizes larger than 4 µm. In ferrets, the human isolate HK3263 replicated to higher titers and transmitted more efficiently via direct contact than SCk1772. We monitored "intrahost" and "interhost" adaptive changes at PB2 residue 627 during infection and transmission of the Sck1772 that carried E627 and HK3263 that carried V/K/E polymorphism at 60%, 20%, and 20%, respectively. For SCk1772, positive selection for K627 over E627 was observed in ferrets during the chicken-to-ferret or ferret-to-ferret transmission. For HK3263 that contained V/K/E polymorphism, mixed V627 and E627 genotypes were transmitted among chickens while either V627 or K627 was transmitted to ferrets with a narrow transmission bottleneck. Overall, our results suggest direct contact as the main mode for H7N9 transmission and identify the PB2-V627 genotype with uncompromised fitness and transmissibility in both avian and mammalian species. IMPORTANCE: We studied the modes of H7N9 transmission, as this information is crucial for developing effective control measures for prevention. Using chicken (SCk1772) and human (HK3263) H7N9 isolates that differed by four amino acids, including the host determinant PB2 residue 627, we observed that both viruses transmitted efficiently among chickens via direct contact but inefficiently via the airborne route. Chicken-to-ferret transmission via the airborne route was observed, along with the detection of viral genome in the air at low copy numbers. In ferrets, HK3263 transmitted more efficiently than SCk1772 via direct contact. During the transmission of SCk1772 that contained E and HK3263 that contained V/K/E polymorphism at PB2 residue 627, positive selections of E627 and K627 were observed in chickens and ferrets, respectively. In addition, PB2-V627 was transmitted and stably maintained in both avian and mammalian species. Our results support applying intervention strategies that minimize direct and indirect contact at the poultry markets during epidemics.

Amino acid substitutions in polymerase basic protein 2 gene contribute to the pathogenicity of the novel A/H7N9 influenza virus in mammalian hosts.

UNLABELLED: A novel avian-origin influenza A/H7N9 virus emerged in 2013 to cause more than 130 cases of zoonotic human disease, with an overall case fatality rate of around 30% in cases detected. It has been shown that an E-to-K amino acid change at residue 627 of polymerase basic protein 2 (PB2) occurred frequently in the H7N9 isolates obtained from humans but not in viruses isolated from poultry. Although this mutation has been reported to confer increased mammalian pathogenicity in other avian influenza subtypes, it has not been experimentally investigated in the H7N9 virus. In this study, we determined the contribution of PB2-E627K in H7N9 virus to its pathogenicity in mammalian hosts. In addition, the compensatory role of the PB2 mutations T271A, Q591K, and D701N in H7N9 virus was investigated. We characterized the activity of polymerase complexes with these PB2 mutations and found that they enhance the polymerase activity in human 293T cells. The rescued mutants enhanced growth in mammalian cells in vitro. Mice infected with the H7N9 mutant containing the avian signature protein PB2-627E showed a marked decrease in disease severity (weight loss) and pathology compared to mice infected with the wild-type strain (PB2-627K) or other PB2 mutants. Also, mutants with PB2-627E showed lower virus replication and proinflammatory cytokine responses in the lungs of the virus-infected mice, which may contribute to pathogenicity. Our results suggest that these amino acid substitutions contribute to mouse pathogenicity and mammalian adaptation. IMPORTANCE: A novel avian H7N9 influenza A virus emerged in east China in 2013 to cause zoonotic human disease associated with significant mortality. It is important to understand the viral genetic markers of mammalian adaptation and disease severity in this H7N9 virus. Since many human (but not avian) H7N9 virus isolates have an amino acid substitution at position E627K in the polymerase basic protein 2 (PB2) gene, we investigated the role of this and other functionally related mutations for polymerase activity in vitro, virus replication competence, and pathogenicity in the mouse model. We found that E627K and functionally related mutations are associated with increased polymerase activity, increased viral replication competence, and increased disease severity in mice.

The R292K mutation that confers resistance to neuraminidase inhibitors leads to competitive fitness loss of A/Shanghai/1/2013 (H7N9) influenza virus in ferrets.

BACKGROUND: Neuraminidase (NA) inhibitors are the only licensed therapeutic option for human zoonotic H7N9 infections. An NA-R292K mutation that confers broad-spectrum resistance to NA inhibitors has been documented in H7N9 patients after treatment. METHODS: We evaluated the transmission potential of a human influenza A H7N9 isolate with a NA-R292K mutation in the ferret model followed by genotyping assay to monitor its competitive fitness in vivo. RESULTS: Plaque-purified A/Shanghai/1/2013 wild-type and NA-R292K viruses transmitted at comparable efficiency to direct or respiratory droplet contact ferrets. In ferrets inoculated with the plaque-purified A/Shanghai/1/2013 NA-R292K virus with dominant K292 (94%), the resistant K292 genotype was outgrown by the wild-type R292 genotype during the course of infection. Transmission of the resistant K292 genotype was detected in 3/4 direct contact and 3/4 respiratory droplet contact ferrets at early time points but was gradually replaced by the wild-type genotype. In the respiratory tissues of inoculated or infected ferrets, the wild-type R292 genotype dominated in the nasal turbinate, whereas the resistant K292 genotype was more frequently detected in the lungs. CONCLUSIONS: The NA inhibitor-resistant H7N9 virus with the NA-R292K mutation may transmit among ferrets but showed compromised fitness in vivo while in competition with the wild-type virus.

Highly pathogenic avian influenza A H5N1 and pandemic H1N1 virus infections have different phenotypes in Toll-like receptor 3 knockout mice.

Toll-like receptors (TLRs) play an important role in innate immunity to virus infections. We investigated the role of TLR3 in the pathogenesis of H5N1 and pandemic H1N1 (pH1N1) influenza virus infections in mice. Wild-type mice and those defective in TLR3 were infected with influenza A/HK/486/97 (H5N1) or A/HK/415742/09 (pH1N1) virus. For comparison, mice defective in the gene for myeloid differential factor 88 (MyD88) were also infected with the viruses, because MyD88 signals through a TLR pathway different from TLR3. Survival and body weight loss were monitored for 14 days, and lung pathology, the lung immune-cell profile, viral load and cytokine responses were studied. H5N1-infected TLR3(-/-) mice had better survival than H5N1-infected WT mice, evident by significantly faster regain of body weight, lower viral titre in the lung and fewer pathological changes in the lung. However, this improved survival was not seen upon pH1N1 infection of TLR3(-/-) mice. In contrast, MyD88(-/-) mice had an increased viral titre and decreased leukocyte infiltration in the lungs after infection with H5N1 virus and poorer survival after pH1N1 infection. In conclusion, TLR3 worsens the pathogenesis of H5N1 infection but not of pH1N1 infection, highlighting the differences in the pathogenesis of these two viruses and the different roles of TLR3 in their pathogenesis.

Comparable fitness and transmissibility between oseltamivir-resistant pandemic 2009 and seasonal H1N1 influenza viruses with the H275Y neuraminidase mutation.

Limited antiviral compounds are available for the control of influenza, and the emergence of resistant variants would further narrow the options for defense. The H275Y neuraminidase (NA) mutation, which confers resistance to oseltamivir carboxylate, has been identified among the seasonal H1N1 and 2009 pandemic influenza viruses; however, those H275Y resistant variants demonstrated distinct epidemiological outcomes in humans. Specifically, dominance of the H275Y variant over the oseltamivir-sensitive viruses was only reported for a seasonal H1N1 variant during 2008-2009. Here, we systematically analyze the effect of the H275Y NA mutation on viral fitness and transmissibility of A(H1N1)pdm09 and seasonal H1N1 influenza viruses. The NA genes from A(H1N1)pdm09 A/California/04/09 (CA04), seasonal H1N1 A/New Caledonia/20/1999 (NewCal), and A/Brisbane/59/2007 (Brisbane) were individually introduced into the genetic background of CA04. The H275Y mutation led to reduced NA enzyme activity, an increased K(m) for 3'-sialylactose or 6'-sialylactose, and decreased infectivity in mucin-secreting human airway epithelial cells compared to the oseltamivir-sensitive wild-type counterparts. Attenuated pathogenicity in both RG-CA04(NA-H275Y) and RG-CA04 × Brisbane(NA-H275Y) viruses was observed in ferrets compared to RG-CA04 virus, although the transmissibility was minimally affected. In parallel experiments using recombinant Brisbane viruses differing by hemagglutinin and NA, comparable direct contact and respiratory droplet transmissibilities were observed among RG-NewCal(HA,NA), RG-NewCal(HA,NA-H275Y), RG-Brisbane(HA,NA-H275Y), and RG-NewCal(HA) × Brisbane(NA-H275Y) viruses. Our results demonstrate that, despite the H275Y mutation leading to a minor reduction in viral fitness, the transmission potentials of three different antigenic strains carrying this mutation were comparable in the naïve ferret model.

Amino acid residues 253 and 591 of the PB2 protein of avian influenza virus A H9N2 contribute to mammalian pathogenesis.

We investigated the tropism, host responses, and virulence of two variants of A/Quail/Hong Kong/G1/1997 (H9N2) (H9N2/G1) with D253N and Q591K in the PB2 protein in primary human macrophages and bronchial epithelium in vitro and in mice in vivo. Virus with PB2 D253N and Q591K had greater polymerase activity in minireplicon assays, induced more tumor necrosis factor alpha (TNF-α) in human macrophages, replicated better in differentiated normal human bronchial epithelial (NHBE) cells, and was more pathogenic for mice. Taken together, our studies help define the viral genetic determinants that contribute to pathogenicity of H9N2 viruses.

Limited onward transmission potential of reassortment genotypes from chickens co-infected with H9N2 and H7N9 avian influenza viruses.

The segmented genome of influenza A virus has conferred significant evolutionary advantages to this virus through genetic reassortment, a mechanism that facilitates the rapid expansion of viral genetic diversity upon influenza co-infections. Therefore, co-infection of genetically diverse avian influenza viruses in poultry may pose a significant public health risk in generating novel reassortants with increased zoonotic potential. This study investigated the reassortment patterns of a Pearl River Delta-lineage avian influenza A(H7N9) virus and four genetically divergent avian influenza A(H9N2) viruses upon co-infection in embryonated chicken eggs and chickens. To characterize "within-host" and "between-host" genetic diversity, we further monitored the viral genotypes that were subsequently transmitted to contact chickens in serial transmission experiments. We observed that co-infection with A(H7N9) and A(H9N2) viruses may lead to the emergence of novel reassortant viruses in ovo and in chickens, albeit with different reassortment patterns. Novel reassortants detected in donor chickens co-infected with different combinations of the same A(H7N9) virus and different A(H9N2) viruses showed distinct onward transmission potential to contact chickens. Sequential transmission of novel reassortant viruses was only observed in one out of four co-infection combinations. Our results demonstrated different patterns by which influenza viruses may acquire genetic diversity through co-infection in ovo, in vivo, and under sequential transmission conditions.

Neutralizing Monoclonal Antibodies That Target the Spike Receptor Binding Domain Confer Fc Receptor-Independent Protection against SARS-CoV-2 Infection in Syrian Hamsters.

The severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) spike protein is the main target for neutralizing antibodies. These antibodies can be elicited through immunization or passively transferred as therapeutics in the form of convalescent-phase sera or monoclonal antibodies (MAbs). Potently neutralizing antibodies are expected to confer protection; however, it is unclear whether weakly neutralizing antibodies contribute to protection. Also, their mechanism of action in vivo is incompletely understood. Here, we demonstrate that 2B04, an antibody with an ultrapotent neutralizing activity (50% inhibitory concentration [IC50] of 0.04 µg/ml), protects hamsters against SARS-CoV-2 in a prophylactic and therapeutic infection model. Protection is associated with reduced weight loss and viral loads in nasal turbinates and lungs after challenge. MAb 2B04 also blocked aerosol transmission of the virus to naive contacts. We next examined three additional MAbs (2C02, 2C03, and 2E06), recognizing distinct epitopes within the receptor binding domain of spike protein that possess either minimal (2C02 and 2E06, IC50 > 20 µg/ml) or weak (2C03, IC50 of 5 µg/ml) virus neutralization capacity in vitro. Only 2C03 protected Syrian hamsters from weight loss and reduced lung viral load after SARS-CoV-2 infection. Finally, we demonstrated that Fc-Fc receptor interactions were not required for protection when 2B04 and 2C03 were administered prophylactically. These findings inform the mechanism of protection and support the rational development of antibody-mediated protection against SARS-CoV-2 infections. IMPORTANCE The ongoing coronavirus disease 2019 (COVID-19) pandemic, caused by SARS-CoV-2, has resulted in the loss of millions of lives. Safe and effective vaccines are considered the ultimate remedy for the global social and economic disruption caused by the pandemic. However, a thorough understanding of the immune correlates of protection against this virus is lacking. Here, we characterized four different monoclonal antibodies and evaluated their ability to prevent or treat SARS-CoV-2 infection in Syrian hamsters. These antibodies varied in their ability to neutralize the virus in vitro. Prophylactic administration of potent and weakly neutralizing antibodies protected against SARS-CoV-2 infection, and this effect was Fc receptor independent. The potent neutralizing antibody also had therapeutic efficacy and eliminated onward aerosol transmission. In contrast, minimally neutralizing antibodies provided no protection against infection with SARS-CoV-2 in Syrian hamsters. Combined, these studies highlight the significance of weakly neutralizing antibodies in the protection against SARS-CoV-2 infection and associated disease.

Influenza virus directly infects human natural killer cells and induces cell apoptosis.

Influenza is an acute respiratory viral disease that is transmitted in the first few days of infection. Evasion of host innate immune defenses, including natural killer (NK) cells, is important for the virus's success as a pathogen of humans and other animals. NK cells encounter influenza viruses within the microenvironment of infected cells and are important for host innate immunity during influenza virus infection. It is therefore important to investigate the direct effects of influenza virus on NK cells. In this study, we demonstrated for the first time that influenza virus directly infects and replicates in primary human NK cells. Viral entry into NK cells was mediated by both clathrin- and caveolin-dependent endocytosis rather than through macropinocytosis and was dependent on the sialic acids on cell surfaces. In addition, influenza virus infection induced a marked apoptosis of NK cells. Our findings suggest that influenza virus can directly target and kill NK cells, a potential novel strategy of influenza virus to evade the NK cell innate immune defense that is likely to facilitate viral transmission and may also contribute to virus pathogenesis.

Remdesivir, lopinavir, emetine, and homoharringtonine inhibit SARS-CoV-2 replication in vitro.

An escalating pandemic by the novel SARS-CoV-2 virus is impacting global health and effective therapeutic options are urgently needed. We evaluated the in vitro antiviral effect of compounds that were previously reported to inhibit coronavirus replication and compounds that are currently under evaluation in clinical trials for SARS-CoV-2 patients. We report the antiviral effect of remdesivir, lopinavir, homorringtonine, and emetine against SARS-CoV-2 virus in Vero E6 cells with the estimated 50% effective concentration at 23.15 µM, 26.63 µM, 2.55 µM and 0.46 µM, respectively. Ribavirin or favipiravir that are currently evaluated under clinical trials showed no inhibition at 100 µM. Synergy between remdesivir and emetine was observed, and remdesivir at 6.25 µM in combination with emetine at 0.195 µM may achieve 64.9% inhibition in viral yield. Combinational therapy may help to reduce the effective concentration of compounds below the therapeutic plasma concentrations and provide better clinical benefits.

Toll-like receptors, chemokine receptors and death receptor ligands responses in SARS coronavirus infected human monocyte derived dendritic cells.

BACKGROUND: The SARS outbreak in 2003 provides a unique opportunity for the study of human responses to a novel virus. We have previously reported that dendritic cells (DCs) might be involved in the immune escape mechanisms for SARS-CoV. In this study, we focussed on the gene expression of toll-like receptors (TLRs), chemokine receptors (CCRs) and death receptor ligands in SARS-CoV infected DCs. We also compared adult and cord blood (CB) DCs to find a possible explanation for the age-dependent severity of SARS. RESULTS: Our results demonstrates that SARS-CoV did not modulate TLR-1 to TLR-10 gene expression but significantly induced the expression of CCR-1, CCR-3, and CCR-5. There was also strong induction of TNF-related apoptosis-inducing ligand (TRAIL), but not Fas ligand gene expression in SARS-CoV infected DCs. Interestingly, the expressions of most genes studied were higher in CB DCs than adult DCs. CONCLUSION: The upregulation of chemokines and CCRs may facilitate DC migration from the infection site to the lymph nodes, whereas the increase of TRAIL may induce lymphocyte apoptosis. These findings may explain the increased lung infiltrations and lymphoid depletion in SARS patients. Further explorations of the biological significance of these findings are warranted.

Avian influenza virus directly infects human natural killer cells and inhibits cell activity.

Natural killer (NK) cell is a key component of innate immunity and plays an important role in host defense against virus infection by directly destroying infected cells. Influenza is a respiratory disease transmitted in the early phase of virus infection. Evasion of host innate immunity including NK cells is critical for the virus to expand and establish a successful acute infection. Previously, we showed that human influenza H1N1 virus infects NK cells and induces cell apoptosis, as well as inhibits NK cell activity. In this study, we further demonstrated that avian influenza virus also directly targeted NK cells as an immunoevasion strategy. The avian virus infected human NK cells and induced cell apoptosis. In addition, avian influenza virion and HA protein inhibited NK cell cytotoxicity. This novel strategy has obvious advantages for avian influenza virus, allowing the virus sufficient time to expand and subsequent spread before the onset of the specific immune response. Our findings provide an important clue for the immunopathogenesis of avian influenza, and also suggest that direct targeting NK cells may be a common strategy used by both human and avian influenza viruses to evade NK cell immunity.

Pathogenicity of the novel A/H7N9 influenza virus in mice.

UNLABELLED: A novel avian-origin influenza A/H7N9 virus infecting humans was first identified in March 2013 and, as of 30 May 2013, has caused 132 human infections leading to 33 deaths. Phylogenetic studies suggest that this virus is a reassortant, with the surface hemagglutinin (HA) and neuraminidase (NA) genes being derived from duck and wild-bird viruses, respectively, while the six "internal gene segments" were derived from poultry H9N2 viruses. Here we determine the pathogenicity of a human A/Shanghai/2/2013 (Sh2/H7N9) virus in healthy adult mice in comparison with that of A/chicken/Hong Kong/HH8/2010 (ck/H9N2) virus, highly pathogenic avian influenza (HPAI) A/Hong Kong/483/1997 (483/H5N1) virus, and a duck influenza A H7N9 virus of different genetic derivation, A/duck/Jiangxi/3286/2009 (dk/H7N9). Intranasal infection of mice with Sh2/H7N9 virus doses of 10(3), 10(4), and 10(5) PFU led to significant weight loss without fatality. This virus was more pathogenic than dk/H7N9 and ck/H9N2 virus, which has six internal gene segments that are genetically similar to Sh2/H7N9. Sh2/H7N9 replicated well in the nasal cavity and lung, but there was no evidence of virus dissemination beyond the respiratory tract. Mice infected with Sh2/H7N9 produced higher levels of proinflammatory cytokines in the lung and serum than did ck/H9N2 and dk/H7N9 but lower levels than 483/H5N1. Cytokine induction was positively correlated with virus load in the lung at early stages of infection. Our results suggest that Sh2/H7N9 virus is able to replicate and cause disease in mice without prior adaptation but is less pathogenic than 483/H5N1 virus. IMPORTANCE: An H7N9 virus isolate causing fatal human disease was found to be more pathogenic for mice than other avian H9N2 or H7N9 viruses but less pathogenic than the highly pathogenic avian influenza virus (HPAI) H5N1. Similarly, the ability of Sh2/H7N9 to elicit proinflammatory cytokines in the lung and serum of mice was intermediate to ck/H9N2 and dk/H7N9 on the one hand and HPAI H5N1 on the other. These findings accord with the observed epidemiology in humans, in whom, as with seasonal influenza viruses, H7N9 viruses cause severe disease predominantly in older persons while HPAI H5N1 can cause severe respiratory disease and death in children and young adults.

Experimental challenge of chicken vaccinated with commercially available H5 vaccines reveals loss of protection to some highly pathogenic avian influenza H5N1 strains circulating in Hong Kong/China.

Highly pathogenic avian influenza (HPAI) H5N1 virus continues to circulate in poultry in Asia and Africa posing a threat to both public and animal health. Vaccination, used as an adjunct to improved bio-security and stamping-out policies, contributed to protecting poultry in Hong Kong from HPAI H5N1 infection in 2004-2008 although the virus was repeatedly detected in dead wild birds. The detection of clade 2.3.4 H5N1 viruses in poultry markets and a farm in Hong Kong in 2008 raised the question whether this virus has changed to evade protection from the H5 vaccines in use. We tested the efficacy of three commercial vaccines (Nobilis, Poulvac and Harbin Re-5 vaccine) in specific pathogen free white leghorn chickens against a challenge with A/chicken/Hong Kong/8825-2/2008 (clade 2.3.4) isolated from vaccinated poultry in Hong Kong and A/chicken/Hong Kong/782/2009 (clade 2.3.2). Harbin Re5 vaccine provided the best, albeit not complete protection against challenge with the clade 2.3.4 virus. All three vaccines provided good protection from death and significantly reduced virus shedding following challenge with the clade 2.3.2 virus. Only Harbin Re-5 was able to completely protect chickens from virus shedding as well as mortality. Sera from vaccinated chickens had lower geometric hemagglutination inhibition titers against A/chicken/Hong Kong/8825-2/08, as compared to two other clade 2.3.4 and one clade 0 virus. Alignment of amino-acid sequences of the haemagglutinin of A/chicken/Hong Kong/8825-2/08 and the other H5 viruses revealed several mutations in positions including 69, 71, 83, 95, 133,140, 162, 183, 189, 194 and 270 (H5 numbering) which may correlate with loss of vaccine protection. Our results indicated that the tested HPAI H5N1 (2.3.4) virus has undergone antigenic changes that allow it to evade immunity from poultry vaccines. This highlights the need for continued surveillance and monitoring of vaccine induced immunity, with experimental vaccine challenge studies being done where indicated.